Patent Application
20240294392
This application relates to the technical field of secondary batteries, and in particular to a positive electrode active material, a preparation method of positive electrode active material, a positive electrode plate, a secondary battery, a battery module, a battery pack, and an electric apparatus.
In recent years, as secondary batteries are increasingly widely used, secondary batteries have been widely used in energy storage power supply systems such as hydroelectric power plants, thermal power plants, wind power plants, and solar power plants, and many other fields including electric tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, and aerospace. Along with the great development of secondary batteries, higher requirements are imposed on energy density, cycling performance, safety performance, and the like of secondary batteries. Lithium manganese phosphate, as a positive electrode active material being used for secondary batteries, is prone to Li/Mn anti-site defects and large dissolving-out amount of manganese during charge and discharge, which affects the gram capacity of the secondary battery and leads to deterioration of the safety performance and cycling performance of the secondary battery.
This application is carried out in view of the preceding matter with the purpose to provide a positive electrode active material, a preparation method of positive electrode active material, a positive electrode plate, a secondary battery, a battery module, a battery pack, and an electric apparatus, so as to solve the Li/Mn anti-site defects and large dissolving-out amount of manganese that are easily produced in the lithium manganese phosphate positive electrode active material in use during charge and discharge, thereby solving the problems of low capacity and poor safety and cycling performance of secondary batteries.
To achieve the preceding purpose, a first aspect of this application provides a positive electrode active material having a core-shell structure, where the positive electrode active material includes a core and a shell enveloping the core; the core including Li1+xMn1−yAyP1−zRzO4, where x is any value in the range of −0.100 to 0.100, y is any value in the range of 0.001 to 0.600, z is any value in the range of 0.001 to 0.100, A is selected from one or more elements of Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb, and Ge, and optionally selected from one or more elements of Fe, V, Ni, Co, and Mg, and R is selected from one or more of B, Si, N, and S, and optionally selected from one or more elements of Si, N, and S; and
the shell including a first coating layer enveloping the core, a second coating layer enveloping the first coating layer, and a third coating layer enveloping the second coating layer; where the first coating layer contains a crystalline pyrophosphate LiaMP2O7 and/or Mb(P2O7)c, where a is greater than 0 and less than or equal to 2, b is any value in the range of 1 to 4, c is any value in the range of 1 to 3, and each M in the crystalline pyrophosphates LiaMP2O7 and Mb(P2O7)e is independently selected from one or more elements of Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, and Al, and optionally selected from one or more elements of Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, and Al;
the second coating layer contains a crystalline oxide M′dOe, where d is greater than 0 and less than or equal to 2, e is greater than 0 and less than or equal to 5, M′ is selected from one or more elements of alkali metals, alkaline earth metals, transition metals, group IIIA elements, group IVA elements, lanthanide elements, and Sb, optionally selected from one or more elements of Li, Be, B, Na, Mg, Al, Si, P, S, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Sr, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Te, W, La, and Ce, and more optionally selected from one or more elements of Mg, Al, Si, Ti, V, Ni, Cu, Zr, and W; and the third coating layer contains carbon.
The inventors of this application have found in actual operation that the lithium manganese phosphate positive electrode active material is prone to Li/Mn anti-site defects and large dissolving-out amount of manganese during deep charge and discharge. The dissolved manganese is reduced to metal manganese after migrating to the negative electrode. The metal manganese produced is equivalent to “catalyst”, which can catalyze the decomposition of SEI (solid electrolyte interphase, solid electrolyte interphase) on the surface of the negative electrode. Part of the by-products produced is gas, which can easily lead to the swelling of the battery and affect the safety performance of the secondary battery. The other part of the by-products produced is deposited on the surface of the negative electrode, which obstructs the passage of lithium ions into and out of the negative electrode, causing an increase in the impedance of the secondary battery and affecting the kinetic performance and cycling performance of the battery. In addition, to supplement the lost SEI, the electrolyte and the active lithium inside the battery are continuously consumed, which has irreversible effects on the capacity retention rate of the secondary battery.
Based on this, the applicant fortuitously found that a new type of active material having a core-shell structure is provided by doping element A at the manganese site and element R at the phosphorus site of lithium manganese phosphate to obtain a doped lithium manganese phosphate core and performing three coatings on the core surface, which can greatly reduce the Li/Mn anti-site defects produced, significantly reduce the dissolving-out amount of manganese and lattice change rate, and increase the compacted density. Applying such material to a secondary battery can increase the capacity of the secondary battery and improve the cycling performance, high-temperature storage performance, and safety performance of the secondary battery. The crystalline oxide in the second coating layer has high structural stability and low surface activity, so with the second coating layer coated, the interfacial side reactions can be effectively reduced, thereby improving the high-temperature cycling performance and high-temperature storage performance of the battery.
In this specification, the crystalline means that the crystallinity is higher than 50%, to be specific, 50% to 100%. A crystallinity lower than 50% is called glassy state.
The crystalline pyrophosphate of this application has a crystallinity of 50% to 100%. The pyrophosphate with a specific crystallinity is not only conducive to giving full play to the ability of the pyrophosphate coating layer to hinder the manganese dissolution and reducing the interfacial side reactions, but also enables the pyrophosphate coating layer and the oxide coating layer to be better lattice matched, such that a tight bond between the coating layers can be achieved.
Unless otherwise stated, in the chemical formula Li1+xMn1−yAyP1−zRzO4, under the condition that A is more than two elements, the above-mentioned limitation on the range of y is not only a limitation on the stoichiometric number of each element as A, but also a limitation on the sum of the stoichiometric numbers of each element as A. For example, under the condition that A is more than two elements A1, A2, . . . , An, the stoichiometric numbers y1, y2, . . . , yn of each of A1, A2, . . . , An each shall fall within the range defined by this application for y, and the sum of y1, y2, . . . , yn shall also fall within the range. Similarly, under the condition that R is more than two elements, the limitation on the range of the stoichiometric numbers of R in this application also has the preceding meaning.
Unless otherwise stated, in the chemical formula Mb(P2O7)e, under the condition that M is more than two elements, the above-mentioned limitation on the range of b is not only a limitation on the stoichiometric number of each element as M, but also a limitation on the sum of the stoichiometric numbers of each element as M. For example, under the condition that M is more than two elements M1, M2, . . . , Mn, the stoichiometric numbers bi, b2, . . . , bn of each of M1, M2, . . . , Mn each shall fall within the range defined by this application for y, and the sum of b1, b2, . . . , bn shall also fall within the range. Similarly, under the condition that M′ in the chemical formula M′dOe is more than two elements, the limitation on the range of the stoichiometric numbers d of M′ in this application also has the preceding meaning.
In any embodiment of the first aspect, the crystalline pyrophosphate in the first coating layer has an interplanar spacing in the range of 0.293 nm to 0.470 nm and an included angle in the range of 18.00° to 32.00° in the [111] crystal orientation;
optionally, the crystalline pyrophosphate in the first coating layer has an interplanar spacing in the range of 0.297 nm to 0.462 nm; and/or
optionally, the crystalline pyrophosphate in the first coating layer has an included angle in the range of 19.211° to 30.846° in the [111] crystal orientation.
The first coating layer in the positive electrode active material of this application uses crystalline substances whose interplanar spacing and included angle range are within the foregoing ranges. In this way, the heterophase structures in the coating layer can be effectively avoided, thereby increasing the gram capacity of the material and improving the cycling performance and rate performance of the secondary battery.
In any embodiment of the first aspect, a ratio of y to 1−y in the core ranges from 1:10 to 1:1, and optionally from 1:4 to 1:1. This further improves the cycling performance and rate performance of the secondary battery.
In any embodiment of the first aspect, a ratio of z to 1−z in the core ranges from 1:9 to 1:999, and optionally from 1:499 to 1:249. This further improves the cycling performance and rate performance of the secondary battery.
In any embodiment of the first aspect, carbon in the third coating layer is a mixture of SP2 carbon and SP3 carbon; and optionally a molar ratio of SP2 carbon to SP3 carbon is any value in the range of 0.07 to 13, more optionally any value in the range of 0.1 to 10, and further optionally any value in the range of 2.0 to 3.0.
This application improves the comprehensive performance of the secondary battery by limiting the molar ratio of SP2 carbon to SP3 carbon to the foregoing range.
In any embodiment of the first aspect, based on a weight of the core, a coating amount of the first coating layer is greater than 0 and less than or equal to 6 wt %, optionally greater than 0 and less than or equal to 5.5 wt %, and more optionally greater than 0 and less than or equal to 2 wt %; and/or
based on the weight of the core, a coating amount of the second coating layer is greater than 0 and less than or equal to 6 wt %, optionally greater than 0 and less than or equal to 5.5 wt %, and more optionally from 2 wt % to 4 wt %; and/or
based on the weight of the core, a coating amount of the third coating layer is greater than 0 and less than or equal to 6 wt %, optionally greater than 0 and less than or equal to 5.5 wt %, and more optionally greater than 0 and less than or equal to 2 wt %.
In the positive electrode active material having the core-shell structure of this application, the coating amounts of the three coating layers are preferably within the foregoing ranges, and therefore the core can be fully enveloped and the kinetic performance and safety performance of the secondary battery can be further improved without reducing the gram capacity of the positive electrode active material.
In any embodiment of the first aspect, a thickness of the first coating layer ranges from 2 nm to 10 nm. In this application, under the condition that the thickness of the first coating layer is in the range of 2 nm to 10 nm, the dissolution and migration of transition metal ions can be further reduced and the kinetic performance of the secondary battery can be improved.
In any embodiment of the first aspect, a thickness of the second coating layer ranges from 3 nm to 15 nm. Under the condition that the thickness of the second coating layer is in the range of 3 nm to 15 nm, the second coating layer has a stable surface structure and small side reactions with the electrolyte, such that the interfacial side reactions can be effectively mitigated, thereby improving the high-temperature performance of the secondary battery.
In any embodiment of the first aspect, a thickness of the third coating layer ranges from 5 nm to 25 nm. Under the condition that the thickness of the third coating layer is in the range of 5 nm to 25 nm, the electrical conductivity of the material can be improved and the compacted density performance of the battery electrode prepared using the positive electrode active material can be increased.
In any embodiment of the first aspect, based on a weight of the positive electrode active material,
a content of element manganese is in the range of 10 wt % to 35 wt %, optionally in the range of 15 wt % to 30 wt %, and more optionally in the range of 17 wt % to 20 wt %;
a content of element phosphorus is in the range of 12 wt % to 25 wt %, and optionally in the range of 15 wt % to 20 wt %; and
optionally, a weight ratio of element manganese to element phosphorus is in the range of 0.90 to 1.25, and more optionally in the range of 0.95 to 1.20.
In the positive electrode active material having a core-shell structure of this application, limiting the content of element manganese in the foregoing range can effectively improve the structural stability and density of the positive electrode active material, thus improving the performance of the secondary battery such as cycling, storage, and compacted density; and can maintain a specific voltage plateau height, thus increasing the energy density of the secondary battery.
In the positive electrode active material having a core-shell structure of this application, limiting the content of element phosphorus in the foregoing range can effectively improve the conductivity of the positive electrode active material and can enhance the structural stability of the positive electrode active material.
In the positive electrode active material having a core-shell structure of this application, limiting the weight ratio of element manganese to element phosphorus in the foregoing range can reduce the dissolution of transition metal so as to improve the stability of the positive electrode active material and the cycling and storage performance of the secondary battery, and can maintain a specific discharge voltage plateau height so as to increase the energy density of the secondary battery.
In any embodiment of the first aspect, a lattice change rate of the positive electrode active material before and after complete deintercalation of lithium is lower than 50%, optionally lower than 4%, more optionally lower than 3.8%, and further optionally from 2.0% to 3.8%.
The positive electrode active material having a core-shell structure of this application can achieve a low lattice change rate before and after deintercalation of lithium. Therefore, the use of the positive electrode active material can improve the gram capacity and rate performance of the secondary battery.
In any embodiment of the first aspect, a Li/Mn anti-site defect concentration of the positive electrode active material is lower than 5.3%, optionally lower than 4%, more optionally lower than 2.2%, and further optionally from 1.5% 10 to 2.2%. Limiting the Li/Mn anti-site defect concentration in the foregoing range can motivate the Li+ transport, increase the gram capacity of the positive electrode active material, and improve the rate performance of the secondary battery.
In any embodiment of the first aspect, a compacted density of the positive electrode active material under 3 tons is greater than 1.95 g/cm3, optionally greater than 2.2 g/cm3, further optionally greater than 2.2 g/cm3 and less than 2.8 g/cm3, and more further optionally greater than 2.2 g/cm3 and less than 2.65 g/cm3. Therefore, increasing the compacted density increases the weight of the positive electrode active material per unit volume, which is conducive to increasing the volumetric energy density of the secondary battery.
In any embodiment of the first aspect, a surface oxygen valence of the positive electrode active material is lower than −1.89, and optionally from −1.90 to −1.98. Therefore, limiting the surface oxygen valence of the positive electrode active material in the foregoing range can mitigate the interfacial side reactions between the positive electrode material and the electrolyte, thereby improving the performance of the cell in terms of cycling, high temperature storage, and the like and suppressing gas production.
A second aspect of this application provides a preparation method of positive electrode active material, including the following steps: a step of providing a core material, the core material including Li1+xMn1−yAyP1−zRzO4, where x is any value in the range of −0.100 to 0.100, y is any value in the range of 0.001 to 0.600, z is any value in the range of 0.001 to 0.100, A is selected from one or more elements of Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb, and Ge, and optionally selected from one or more elements of Fe, V, Ni, Co, and Mg, and R is selected from one or more of B, Si, N, and S, and optionally selected from one or more elements of Si, N, and S; and
a first coating step: providing a first mixture including a pyrophosphate LiaMP2O7 and/or Mb(P2O7)e, and mixing the core material and the first mixture for drying and sintering to obtain a material coated by a first coating layer, where a is greater than 0 and less than or equal to 2, b is any value in the range of 1 to 4, c is any value in the range of 1 to 3, and each M in the pyrophosphates LiaMP2O7 and Mb(P2O7)e is independently selected from one or more elements of Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, and Al, and optionally selected from one or more elements of Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, and Al;
a second coating step: providing a second mixture including an oxide M′dOe, and mixing the material coated by the first coating layer and the second mixture for drying and sintering to obtain a material coated by two coating layers, where d is greater than 0 and less than or equal to 2, e is greater than 0 and less than or equal to 5, M′ is selected from one or more elements of alkali metals, alkaline earth metals, transition metals, group IIIA elements, group IVA elements, lanthanide elements, and Sb, optionally selected from one or more elements of Li, Be, B, Na, Mg, Al, Si, P, S, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Sr, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Te, W, La, and Ce, and more optionally selected from one or more elements of Mg, Al, Si, Ti, V, Ni, Cu, Zr, and W; and
a third coating step: providing a third mixture including a source of carbon, and mixing the material coated by the two coating layers and the third mixture for drying and sintering to obtain a positive electrode active material; where the positive electrode active material has a core-shell structure including the core and a shell enveloping the core, the core including Li1+xMn1−yAyP1−zRzO4, the shell including the first coating layer enveloping the core, a second coating layer enveloping the first coating layer, and a third coating layer enveloping the second coating layer, the first coating layer containing a crystalline pyrophosphate LiaMP2O7 and/or Mb(P2O7)c, the second coating layer containing a crystalline oxide M′dOe, and the third coating layer containing carbon.
Based on this, the applicant provides a new type of positive electrode active material having a core-shell structure by doping element A at the manganese site and element R at the phosphorus site of lithium manganese phosphate to obtain a doped lithium manganese phosphate core and performing three coatings on the core surface, which can greatly reduce the Li/Mn anti-site defects produced, significantly reduce the dissolving-out amount of manganese and lattice change rate, and increase the compacted density. Applying such material to a secondary battery can increase the capacity of the secondary battery and improve the cycling performance, high-temperature storage performance, and safety performance of the secondary battery.
In any embodiment of the second aspect of this application, the step of providing a core material includes the following steps:
step (1): mixing a source of manganese, a source of element A, and an acid to obtain a mixture; and
step (2): mixing the mixture obtained in step (1) with a source of lithium, a source of phosphorus, a source of element R, and an optional solvent, and sintering under the protection of an inert gas to obtain a core material containing Li1+xMn1−yAyP1−zRzO4.
In any embodiment of the second aspect of this application, step (1) is performed at 20° C. to 120° C., and more optionally at 40° C. to 120° C.; and/or, in the step (1), mixing is performed by stirring at 400 rpm to 700 rpm for 1 h to 9 h.
In any embodiment of the second aspect of this application, in step (2), mixing is performed at a temperature of 20° C. to 120° C., optionally 40° C. to 120° C., for 1 h to 10 h.
In any embodiment of the second aspect of this application,
in the first coating step, a first mixture is obtained by mixing a source of element M, a source of phosphorus, an acid, an optional source of lithium, and an optional solvent; and/or
in the second coating step, a second mixture is obtained by mixing a source of element M′ and a solvent; and/or
in the third coating step, a third mixture is obtained by mixing a source of carbon and a solvent.
In any embodiment of the second aspect of this application, in the first coating step, the source of element M, the source of phosphorus, the acid, the optional source of lithium, and the optional solvent are mixed at room temperature for 1 h to 5 h, then heated to 50° C. to 120° C. and kept at this temperature for 2 h to 10 h, the above mixing all being performed at a pH 3.5 to 6.5.
In any embodiment of the second aspect of this application, in the second coating step, the source of element M′ and the solvent are mixed at room temperature for 1 h to 10 h, then heated to 60° C. to 150° C. and kept at this temperature for 2 h to 10 h.
In any embodiment of the second aspect of this application, the source of element A is selected from one or more of monomer, carbonate, sulfate, halide, nitrate, organic acid salt, oxide, and hydroxide of element A; and/or
the source of element R is selected from one or more of inorganic acid, organic acid, sulfate, halide, nitrate, organic acid salt, oxide, and hydroxide element R.
In any embodiment of the second aspect of this application, in the first coating step, the sintering is performed at 650° C. to 800° C. for 2 hours to 8 hours; and/or in the second coating step, the sintering is performed at 400° C. to 750° C. for 6 hours to 10 hours; and/or in the third coating step, the sintering is performed at 600° C. to 850° C. for 6 hours to 10 hours.
A third aspect of this application provides a positive electrode plate including a positive electrode current collector and a positive electrode film layer provided on at least one surface of the positive electrode current collector, where the positive electrode film layer includes the positive electrode active material according to the first aspect of this application or a positive electrode active material prepared by the preparation method according to the second aspect of this application; and optionally, based on a total weight of the positive electrode film layer, a content of the positive electrode active material in the positive electrode film layer is 90 wt % to 99.5 wt %, and more optionally 95 wt % to 99.5 wt %.
A fourth aspect of this application provides a secondary battery, including the positive electrode active material according to the first aspect of this application or the positive electrode active material prepared by using the preparation method according to the second aspect of this application or the positive electrode plate according to the third aspect of this application.
A fifth aspect of this application provides a battery module, including the secondary battery according to the fourth aspect of this application.
A sixth aspect of this application provides a battery pack, including the battery module according to the fifth aspect of this application.
A seventh aspect of this application provides an electric apparatus, including at least one selected from the secondary battery according to the fourth aspect of this application, the battery module according to the fifth aspect of this application, and the battery pack according to the sixth aspect of this application.
1. battery pack; 2. upper box body; 3. lower box body; 4. battery module; 5. secondary battery; 51. housing; 52. electrode assembly; and 53. cover plate.
The following specifically discloses embodiments of a positive electrode active material, a preparation method of positive electrode active material, a positive electrode plate, a secondary battery, a battery module, a battery pack, and an electric apparatus in this application with appropriate reference to detailed descriptions of accompanying drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, detailed descriptions of well-known matters and repeated descriptions of actually identical structures have been omitted. This is to avoid unnecessarily prolonging the following description, for ease of understanding by persons skilled in the art. In addition, the accompanying drawings and the following descriptions are provided for persons skilled in the art to fully understand this application and are not intended to limit the subject matter recorded in the claims.
“Ranges” disclosed in this application are defined in the form of lower and upper limits. A given range is defined by one lower limit and one upper limit selected, where the selected lower and upper limits define boundaries of that particular range. Ranges defined in this method may or may not include end values, and any combinations may be used, meaning any lower limit may be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are provided for a specific parameter, it is understood that ranges of 60-110 and 80-120 can also be envisioned. In addition, if minimum values of a range are given as 1 and 2, and maximum values of the range are given as 3, 4, and 5, the following ranges can all be envisioned: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, a value range of “a-b” is a short representation of any combination of real numbers between a and b, where both a and b are real numbers. For example, a value range of “0-5” means that all real numbers in the range of “0-5” are listed herein, and “0-5” is just a short representation of a combination of these values. In addition, a parameter expressed as an integer greater than or equal to 2 is equivalent to disclosure that the parameter is, for example, an integer among 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, and so on.
The ranges “less/lower than a value” and “greater/higher than a value” in this application indicate the range limited by the value as an upper or lower limit.
Unless otherwise specified, all the embodiments and optional embodiments of this application can be combined with each other to form new technical solutions.
Unless otherwise specified, all the technical features and optional technical features of this application can be combined with each other to form new technical solutions.
Unless otherwise specified, all the steps in this application can be performed in the order described or in random order, preferably, in the order described. For example, a method including steps (a) and (b) indicates that the method may include steps (a) and (b) performed in order or may include steps (b) and (a) performed in order. For example, the above-mentioned method may further include step (c), which indicates that step (c) may be added to the method in any ordinal position, for example, the method may include steps (a), (b), and (c), steps (a), (c), and (b), steps (c), (a), and (b), or the like.
Unless otherwise specified, “include” and “contain” mentioned in this application are inclusive or may be exclusive. For example, terms “include” and “contain” can mean that other unlisted components may also be included or contained, or only listed components are included or contained.
Unless otherwise specified, in this application, the term “or” is inclusive. For example, the phrase “A or B” means “A, B, or both A and B”. More specifically, any one of the following conditions satisfies the condition “A or B”: A is true (or present) and B is false (or not present); A is false (or not present) and B is true (or present); or both A and B are true (or present).
If there is no special description, in this application, the term “coating layer” is a material layer enveloping the core, the material layer can completely or partially envelop the core, and the use of “coating layer” is only for ease of description but not intended to limit the present invention.
If there is no special description, in this application, the term “thickness of the cladding layer” is a thickness of the coating layer in the radial direction of the core, and the definition of the term “coating layer” is the same as above.
Secondary batteries, also referred to as rechargeable batteries or storage batteries, are batteries whose active material can be activated for continuous use through charging after the batteries are discharged.
Generally, a secondary battery includes a positive electrode plate, a negative electrode plate, a separator, and an electrolyte. During charge and discharge of the battery, active ions (such as lithium ions) intercalate and deintercalate back and forth between the positive electrode plate and the negative electrode plate. The separator is sandwiched between the positive electrode plate and the negative electrode plate to mainly prevent short circuit between the positive and negative electrodes and to allow active ions to pass through. The electrolyte is between the positive electrode plate and the negative electrode plate, playing a role of conducting active ions.
This application provides a positive electrode active material having a core-shell structure. The positive electrode active material includes a core and a shell enveloping the core;
the core including Li1+xMn1−yAyP1−zRzO4, where x is any value in the range of −0.100 to 0.100, y is any value in the range of 0.001 to 0.600, z is any value in the range of 0.001 to 0.100, A is selected from one or more elements of Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb, and Ge, and optionally selected from one or more elements of Fe, V, Ni, Co, and Mg, and R is selected from one or more of B, Si, N, and S, and optionally selected from one or more elements of Si, N, and S; and
the shell including a first coating layer enveloping the core, a second coating layer enveloping the first coating layer, and a third coating layer enveloping the second coating layer; where
the first coating layer contains a crystalline pyrophosphate LiaMP2O7 and/or Mb(P2O7)c, where a is greater than 0 and less than or equal to 2, b is any value in the range of 1 to 4, c is any value in the range of 1 to 3, and each M in the crystalline pyrophosphates LiaMP2O7 and Mb(P2O7)e is independently selected from one or more elements of Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, and Al, and optionally selected from one or more elements of Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, and Al;
the second coating layer contains a crystalline oxide M′dOe, where d is greater than 0 and less than or equal to 2, e is greater than 0 and less than or equal to 5, M′ is selected from one or more elements of alkali metals, alkaline earth metals, transition metals, group IIIA elements, group IVA elements, lanthanide elements, and Sb, optionally selected from one or more elements of Li, Be, B, Na, Mg, Al, Si, P, S, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Sr, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Te, W, La, and Ce, and more optionally selected from one or more elements of Mg, Al, Si, Ti, V, Ni, Cu, Zr, and W; and
the third coating layer contains carbon.
The positive electrode active material of this application can improve the gram capacity, cycling performance, and safety performance of the secondary battery. Although the mechanism is still unclear, it is presumed that the positive electrode active material of this application is a core-shell structure. Doping element A and element R respectively on the manganese site and phosphorus site of the lithium manganese phosphate core can not only effectively reduce the dissolving-out amount of manganese, reduce the migration of manganese ions to the negative electrode, reduce the consumption of the electrolyte due to the decomposition of the SEI, and improve the cycling performance and safety performance of the secondary battery, but also promote the adjustment of the Mn—O bond, reduce the migration barrier of lithium ions, promote the migration of lithium ions, and improve the rate performance of the secondary battery. Enveloping the core with the first coating layer including crystalline pyrophosphate can further increase the migration resistance of manganese, reduce the dissolving-out amount of manganese, decrease the content of impurity lithium on the surface, and reduce the contact between the core and the electrolyte, thereby reducing interfacial side reactions, reducing gas produced, and improving high-temperature storage performance, cycling performance, and safety performance of the secondary battery. Further, coating the crystalline oxide coating layer with high stability can effectively reduce the interfacial side reactions on the surface of the positive electrode active material, thus improving the high-temperature cycling and storage performance of the secondary battery. Further, coating the carbon layer as the third coating layer can further improve the safety performance and kinetic performance of the secondary battery. In addition, the element A doping at the manganese site of lithium manganese phosphate in the core helps to reduce the lattice change rate of lithium manganese phosphate in the process of lithium deintercalation, improve the structural stability of the positive electrode material, greatly reduce the dissolving-out amount of manganese, and reduce the oxygen activity on the surface of the particles. The element R doping at the phosphorus site also helps to change difficulty of the change of the Mn—O bond length, thereby improving the electronic conductivity, reducing the migration barrier of lithium ions, promoting the migration of lithium ions, and improving the rate performance of the secondary battery.
Unless otherwise stated, in the chemical formula Li1+xMn1−yAyP1−z,RzO4, under the condition that A is more than two elements, the above-mentioned limitation on the range of y is not only a limitation on the stoichiometric number of each element as A, but also a limitation on the sum of the stoichiometric numbers of each element as A. For example, under the condition that A is more than two elements A1, A2, . . . , An, the stoichiometric numbers y1, y2, . . . , yn of each of A1, A2, . . . , An each shall fall within the range defined by this application for y, and the sum of y1, y2, . . . , yn shall also fall within the range. Similarly, under the condition that R is more than two elements, the limitation on the range of the stoichiometric numbers of R in this application also has the preceding meaning.
In some embodiments, under the condition that A is selected from one, two, three, or four elements of Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb, and Ge, Ay is Qn1Dn2En3Kn4, where n1+n2+n3+n4=y, and n1, n2, n3, and n4 are all positive and not both zero, and Q, D, E, K are each independently selected from one of Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb, and Ge, and optionally at least one of Q, D, E, and K is Fe. Optionally, one of n1, n2, n3, n4 is zero and the rests are not zero; more optionally, two of n1, n2, n3, n4 are zero and the rests are not zero; and further optionally, three of n1, n2, n3, n4 are zero and the rest is not zero. In the core Li1+xMn1−yAyP1−zRzO4, it is advantageous to dope one, two, three, or four of the preceding elements A at the manganese site, and optionally dope one, two, or three of the preceding elements A; and furthermore, it is advantageous to dope one or two of elements R at the phosphorus site, which facilitates a uniform distribution of the doping elements.
In some embodiments, the values of x, y, and z satisfy the condition that the entire core is made electrically neutral.
In the core Li1+xMn1−yAyP1−zRzO4, the magnitude of x is influenced by the magnitude of the valences of A and R and the magnitudes of y and z so as to ensure that the entire system is electrically neutral. If the value of x is too small, the content of lithium of the entire core system decreases, which affects the gram capacity extractability of the material. The value of y limits the total content of all doping elements. If y is too small, that is, too little doping amount, the doping elements do not function, and if y is more than 0.5, the content of Mn in the system is decreased, which affects the voltage plateau of the material. The element R dopes at the P site. The P—O tetrahedron is more stable and the value of z is too large, which affects the stability of the material. Therefore, the value of z is limited in the range of 0.001 to 0.100.
In addition, the entire core system is made electrically neutral, which can ensure that there are as few defects and heterophase structures in the positive electrode active material as possible. If there is an excess of transition metal (for example, manganese) present in the positive electrode active material, the excess transition metal is likely to precipitate out as an elementary substance or form heterophase structures inside the lattice due to the inherently stable structure of the material system. Therefore, making the system electrically neutral can minimize such heterophase structures. In addition, ensuring the electrical neutrality of the system can also lead to lithium vacancies in the material in some cases, resulting in better kinetic performance.
Process control (for example, mixing and grinding of the material sources) can ensure that the elements are uniformly distributed in the lattice without aggregation. The positions of the main characteristic peaks in the XRD plots of lithium manganese phosphate doped with element A and element R are consistent with those of undoped LiMnPO4, indicating that no heterophase structure was introduced in the doping process. Therefore, the improvement on the performance of the core mainly comes from element doping rather than heterophase structures. After preparing the positive electrode active material, the inventors of this application cut out the middle area of the prepared positive electrode active material particles through focused ion beam (FIB for short), tested it through transmission electron microscopy (TEM for short) and X-ray energy spectroscopy (EDS for short), and found that the elements were uniformly distributed and did not appear to be aggregated.
In some embodiments, the values of a, b, and c satisfy the condition that the crystalline pyrophosphate LiaMP2O7 or Mb(P2O7)e is made electrically neutral.
In some embodiments, the values of d and e satisfy the condition that the crystalline M′dOe is made electrically neutral.
In some embodiments, the crystalline means that the crystallinity is higher than 50%, to be specific, 50% to 100%. A crystallinity lower than 50% is called glassy state.
In some embodiments, the crystalline pyrophosphate of this application has a crystallinity of 50% to 100%. The pyrophosphate with a specific crystallinity is not only conducive to giving full play to the ability of the pyrophosphate coating layer to hinder the manganese dissolution and reducing the interfacial side reactions, but also enables the pyrophosphate coating layer and the oxide coating layer to be better lattice matched, such that a tighter bond between the coating layers can be achieved.
In some embodiments, the crystallinity of the crystalline pyrophosphate in the first coating layer material of the positive electrode active material can be tested by using conventional technical means in the art, for example, by using density, infrared spectroscopy, differential scanning calorimetry, and nuclear magnetic resonance absorption methods, and also by, for example, X-ray diffraction.
A specific method of testing the crystallinity of the crystalline pyrophosphate in the first coating layer of the positive electrode active material by using X-ray diffraction can include the following steps:
taking a specific amount of positive electrode active material powder; measuring a total scattering intensity by X-ray, where the total scattering intensity is a sum of scattering intensities of substances in the entire space and is only related to intensities of primary rays, a chemical structure of the positive electrode active material powder, the total number of electrons participating in the diffraction, that is, mass, but not related to the order state of the sample; and separating the crystalline scattering from the non-crystalline scattering in the diffractogram, where the crystallinity is a ratio of the intensity of the crystalline scattering part to the total scattering intensity.
It should be noted that in some embodiments, the crystallinity of the pyrophosphate in the coating layer can be adjusted, for example, by adjusting the process conditions of the sintering process, for example, sintering temperature and sintering time.
In some embodiments, metal ions are difficult to migrate in the pyrophosphate, and therefore the pyrophosphate, as the first coating layer, can effectively isolate doping metal ions from the electrolyte. The crystalline pyrophosphate has a stable structure, so coating with the crystalline pyrophosphate can effectively suppress the dissolution of transition metals and improve the cycling performance.
In some embodiments, the bond between the first coating layer and the core is similar to a heterojunction, and the firmness of the bond is limited by the degree of lattice match. Under the condition that the lattice mismatch is below 5%, the lattice match is better and the two are easily bound tightly. The tight bond can ensure that the coating layer does not detach from the core in the subsequent cycle process, which is beneficial to ensure the long-term stability of the material. The degree of bond between the first coating layer and the core is mainly measure by calculating the degree of mismatch between the core and each lattice constant of the coating. In this application, compared with the core doped with no elements, the core doped with elements A and R has an increased match with the first coating layer, and the core can be more tightly bonded to the pyrophosphate coating layer.
The crystalline oxide is selected as the second coating layer because, firstly, it has a better lattice match with the first coating layer crystalline pyrophosphate (the mismatch is only 3%), and secondly, the crystalline oxide served as the second coating layer has better stability than the pyrophosphate. Therefore, using the crystalline oxide for enveloping the pyrophosphate is beneficial to improve the stability of the material. The crystalline oxide served as the second coating layer has a stable structure, and therefore using the crystalline oxide for coating can effectively reduce the interfacial side reactions on the surface of the positive electrode active material, thereby improving the high-temperature cycling and storage performance of the secondary battery. The lattice match between the second coating layer and the first coating layer is, for example, similar to the bond between the first coating layer and the core. Under the condition that the lattice mismatch is below 5%, the lattice match is better, and the second coating layer and the first coating layer are easily bonded tightly.
The main reason for selected carbon as the third coating layer is that the carbon layer has better electronic conductivity. Because the electrochemical reactions of carbon applied in secondary batteries requires the participation of electrons, to promote the electron transport between particles and the electron transport at different positions on the particles, carbon with excellent electrical conductivity can be used for coating. Carbon coating can effectively improve the electrical conductivity and desolvation of the positive electrode active material.
In some embodiments, the crystalline pyrophosphate in the first coating layer has an interplanar spacing in the range of 0.293 nm to 0.470 nm and an included angle in the range of 18.00° to 32.000 in the [111] crystal orientation;
optionally, the crystalline pyrophosphate in the first coating layer has an interplanar spacing in the range of 0.297 nm to 0.462 nm; and/or
optionally, the crystalline pyrophosphate in the first coating layer has an included angle in the range of 19.211° to 30.846° in the [111] crystal orientation.
The crystalline pyrophosphate in the first coating layer may be characterized by means of conventional technical means in the art, or may be characterized, for example, by means of transmission electron microscopy (TEM). Under the TEM, the core and the coating layer can be distinguished by measuring the interplanar spacing.
The specific method of measuring the interplanar spacing and included angle of the crystalline pyrophosphate in the first coating layer can include the following steps:
taking a specific amount of coated positive electrode active material sample powder in a test tube, injecting the test tube with a solvent such as alcohol, and stirring and dispersing the mixture thoroughly; then taking an appropriate amount of the resulting solution with a clean disposable plastic pipette, and dropping the solution on a 300-mesh copper grid, part of the powder remaining on the copper grid at this point; and transferring the copper grid together with the sample to the TEM sample cavity for testing, obtaining an original picture under the TEM test, and saving the original picture.
The original picture obtained in the TEM test is opened in the diffractometer software, Fourier transform is performed to get a diffraction pattern, a distance from a diffraction spot to the center of the diffraction pattern is measured to get the interplanar spacing, and the included angle is calculated according to the Bragg equation.
The crystalline pyrophosphate having the interplanar spacing and included angle in the preceding ranges can more effectively suppress the lattice change rate and Mn dissolution of the lithium manganese phosphate during deintercalation of lithium, thereby enhancing the high-temperature cycling performance, rate performance, cycling stability, and high-temperature storage performance of the secondary battery.
In some embodiments, a ratio of y to 1−y in the core ranges from 1:10 to 1:1, and optionally from 1:4 to 1:1. Herein, y denotes the sum of the stoichiometric numbers of element A doping at Mn site. The energy density, rate performance, and cycling performance of the secondary battery using positive electrode active material can be further improved when the preceding conditions are met.
In some embodiments, a ratio of z to 1−z in the core ranges from 1:9 to 1:999, and optionally from 1:499 to 1:249. Herein, z denotes the sum of the stoichiometric numbers of element R doping at P site. The energy density, rate performance, and cycling performance of the secondary battery using positive electrode active material can be further improved when the preceding conditions are met.
In some embodiments, carbon in the third coating layer is a mixture of SP2 carbon and SP3 carbon; and optionally a molar ratio of SP2 carbon to SP3 carbon is any value in the range of 0.07 to 13, more optionally any value in the range of 0.1 to 10, and further optionally any value in the range of 2.0 to 3.0.
In some embodiments, a molar ratio of SP2 carbon to SP3 carbon may be about 0.1, about 0.2, about 03, about 0.4, about 0.5, about 0.6, about 0.7, about 0.8, about 0.9, about 1, about 2, about 3, about 4, about 5, about 6, about 7, about 8, about 9, or about 10, or in any range of any of these values.
In this application, a value of “about” indicates a range of ±10% of that value.
By selecting the morphology of carbon in the carbon coating layer, the overall electrical performance of the secondary battery can be improved. Specifically, by using a mixture of SP2 carbon and SP3 carbon and limiting the ratio of SP2 carbon to SP3 carbon to a certain range, good electrical conductivity can be achieved and the passage of lithium ions can be ensured, so it is beneficial to realize functions and cycling performance of the secondary battery.
The mixing ratio of SP2 carbon to SP3 carbon of the third coating layer can be controlled by sintering conditions such as sintering temperature and sintering time. For example, under the condition that sucrose is used as the source of carbon to prepare the third coating layer, the sucrose is deposited on the second coating layer after pyrolysis, and under the action of high temperature, a carbon coating layer with both SP3 carbon and SP2 carbon is produced. The ratio of SP2 carbon to SP3 carbon can be controlled by selecting the pyrolysis and sintering conditions.
The structure and characteristics of the third coating layer carbon can be measured by Raman (Raman) spectroscopy, and the specific measurement method is as follows: splitting the energy spectrum of the Raman test to obtain Id/Ig (where Id is a peak intensity of SP3 carbon and Ig is a peak intensity of SP2 carbon), and then determining the molar ratio therebetween.
In some embodiments, based on a weight of the core, a coating amount of the first coating layer is greater than 0 and less than or equal to 6 wt %, optionally greater than 0 and less than or equal to 5.5 wt %, and more optionally greater than 0 and less than or equal to 2 wt %; and/or based on the weight of the core, a coating amount of the second coating layer is greater than 0 and less than or equal to 6 wt %, optionally greater than 0 and less than or equal to 5.5 wt %, and more optionally from 2 wt % to 4 wt %; and/or based on the weight of the core, a coating amount of the third coating layer is greater than 0 and less than or equal to 6 wt %, optionally greater than 0 and less than or equal to 5.5 wt %, and more optionally greater than 0 and less than or equal to 2 wt %.
In this application, the coating amount of each layer is not zero.
In the positive electrode active material having the core-shell structure of this application, the coating amounts of the three coating layers are preferably within the foregoing ranges, and therefore the core can be fully enveloped and the kinetic performance and safety performance of the secondary battery can be further improved without reducing the gram capacity of the positive electrode active material.
Limiting the coating amount of the first coating layer in the foregoing range can reduce the dissolving-out amounts of transition metals and ensure the smooth migration of lithium ions, thereby improving the rate performance of the positive electrode active material.
Limiting the coating amount of the second coating layer in the foregoing range can maintain a specific platform voltage of the positive electrode active material and ensure the coating effect.
For the third coating layer, the carbon coating mainly promotes the electron transfer between the particles. However, because the structure also contains a large amount of amorphous carbon, the density of carbon is low. The compacted density of the electrode plate can be ensured by limiting the coating amount in the foregoing range.
In some embodiments, a thickness of the first coating layer ranges from 2 nm to 10 nm; and/or
a thickness of the second coating layer ranges from 3 nm to 15 nm; and/or
a thickness of the third coating layer ranges from 5 nm to 25 nm.
In some embodiments, the thickness of the first coating layer may be about 2 nm, about 3 nm, about 4 nm, about 5 nm, about 6 nm, about 7 nm, about 8 nm, about 9 nm, or about 10 nm, or in any range of any of these values.
In some embodiments, the thickness of the second coating layer may be about 3 nm, about 4 nm, about 5 nm, about 6 nm, about 7 nm, about 8 nm, about 9 nm, about 10 nm, about 11 nm, about 12 nm, about 13 nm, about 14 nm or about 15 nm, or in any range of any of these values.
In some embodiments, the thickness of the third coating layer may be about 5 nm, about 6 nm, about 7 nm, about 8 nm, about 9 nm, about 10 nm, about 11 nm, about 12 nm, about 13 nm, about 14 nm, about 15 nm, about 16 nm, about 17 nm, about 18 nm, about 19 nm, about 20 nm, about 21 nm, about 22 nm, about 23 nm, about 24 nm, or about 25 nm, or in any range of any of these values.
Under the condition that the thickness of the first coating layer is in the range of 2 nm to 10 nm, the dissolving-out amounts of the transition metals can be effectively reduced and the kinetic performance of the secondary battery can be ensured.
Under the condition that the thickness of the second coating layer is in the range of 3 nm to 15 nm, the second coating layer has a stable surface structure and small side reactions with the electrolyte, such that the interfacial side reactions can be effectively mitigated, thereby improving the high-temperature performance of the secondary battery.
Under the condition that the thickness of the third coating layer is in the range of 5 nm to 25 nm, the electrical conductivity of the material can be improved and the compacted density performance of the battery electrode prepared using the positive electrode active material can be increased.
The thickness of the coating layer is measured mainly by FIB, and the specific method can include the following steps: randomly selecting a single particle from the positive electrode active material powder to be tested, cutting a thin slice with a thickness of about 100 nm from the middle of the selected particle or near the middle, and performing TEM test on the thin slice to measure the thickness of the coating layer where the measurement is performed for 3 to 5 positions and an average value is taken.
In some embodiments, based on the weight of the positive electrode active material, a weight content of element manganese is in the range of 10 wt % to 35 wt %, optionally in the range of 15 wt % to 30 wt %, and more optionally in the range of 17 wt % to 20 wt %; a content of element phosphorus is in the range of 12 wt % to 25 wt %, and optionally in the range of 15 wt % to 20 wt %; and a weight ratio of element manganese to element phosphorus is in the range of 0.90 to 1.25, and optionally in the range of 0.95 to 1.20.
In this application, under the condition that only the core of the positive electrode active material contains manganese, the content of manganese may correspond to the content of the core.
In this application, limiting the content of element manganese in the foregoing range can ensure the stability and high density of the positive electrode active material, thus improving the performance of the secondary battery such as cycling, storage, and compacted density; and can maintain a specific voltage plateau height, thus increasing the energy density of the secondary battery.
In this application, limiting the content of element phosphorus in the foregoing range can effectively improve the conductivity of the material and can enhance the overall stability of the material.
In this application, limiting the weight ratio of manganese to phosphorus in the foregoing range can effectively reduce the dissolving-out amount of transition metal manganese, improve the stability and gram capacity of the positive electrode active material, thereby improving the cycling performance and storage performance of the secondary battery, and can help to reduce the heterophase structure in the material and maintain the discharge voltage plateau height of the material, thereby increasing the energy density of the secondary battery.
The measurement of element manganese and element phosphorus can be performed by means of conventional technologies in the art. In particular, the following method is used to determine the content of element manganese and element phosphorus: dissolving the material in dilute hydrochloric acid (with a concentration of 10% to 30%), measuring the content of each element in the solution using ICP, and then performing measurement and conversion for the content of element manganese to obtain its weight ratio.
In some embodiments, a lattice change rate of the positive electrode active material having a core-shell structure before and after complete deintercalation of lithium is lower than 50%, optionally lower than 4%, more optionally lower than 3.8%, and further optionally from 2.0% to 3.8%.
The process of deintercalation of lithium in lithium manganese phosphate (LiMnPO4) is a two-phase reaction. The interfacial stress of the two phases is determined by the magnitude of the lattice change rate before and after deintercalation of lithium, where a smaller lattice change rate indicates a lower interfacial stress and easier Li+ transfer. Therefore, reducing the lattice change rate of the core is beneficial to increase the Li+ transport capacity, thereby improving the rate performance of the secondary battery. The positive electrode active material having a core-shell structure of this application can achieve a low lattice change rate before and after deintercalation of lithium, so the use of the positive electrode active material can improve the rate performance of the secondary battery. The lattice change rate can be measured by methods known in the art, for example, X-ray diffraction mapping (XRD).
In some embodiments, a Li/Mn anti-site defect concentration of the positive electrode active material having a core-shell structure is lower than 5.3%, optionally lower than 4%, more optionally lower than 2.2%, and further optionally from 1.5% to 2.2%.
The Li/Mn anti-site defects of this application mean the interchange of the sites of Li+ and Mn2+ in the LiMnPO4 lattice. Accordingly, the Li/Mn anti-site defect concentration refers to a percentage of Li+ interchanged with Mn2+ in the total amount of Li+. In this application, the Li/Mn anti-site defect concentration can be, for example, tested according to JIS K 0131-1996.
The positive electrode active material having a core-shell structure of this application can achieve the aforementioned low Li/Mn anti-site defect concentration. Although the mechanism has not been understood yet, the inventors of this application speculate that because the sites of Li+ and Mn2+ in the LiMnPO4 lattice are interchanged and the transport passage of Li+ is a one-dimensional passage, Mn2+ in the passage of Li+ is difficult to migrate and thus hinders the transport of Li+. Therefore, the positive electrode active material having a core-shell structure of this application can reduce Mn2+ hindering the transport of Li+ due to the low Li/Mn anti-site defect concentration, which is in the foregoing range, and increase the gram capacity extractability and rate performance of the positive electrode active material.
In some embodiments, a compacted density of the positive electrode active material under 3 tons is greater than 1.95 g/cm3, optionally greater than 2.2 g/cm3, further optionally greater than 2.2 g/cm3 and less than 2.8 g/cm3, and more further optionally greater than 2.2 g/cm3 and less than 2.65 g/cm3. A higher compacted density indicates a large weight of the active material per unit volume. Therefore, increasing the compacted density is conducive to increasing the volumetric energy density of the cell. The compacted density can be measured according to GB/T 24533-2009.
In some embodiments, a surface oxygen valence of the positive electrode active material is lower than −1.89, and optionally from −1.90 to −1.98.
The stable valence of oxygen is originally −2. The valence closer to −2 indicates a stronger electron accepting ability, in other words, a higher oxidability. Typically, the surface valence of oxygen is below −1.7. Limiting the surface oxygen valence of the positive electrode active material of this application in the foregoing range can mitigate the interfacial side reactions between the positive electrode material and the electrolyte, thereby improving the performance of the cell in terms of cycling, high temperature storage, and the like and suppressing gas production.
The surface oxygen valence can be measured by methods known in the art, for example, measured by electron energy loss spectroscopy (EELS).
Selecting the doping elements within the foregoing ranges is beneficial to enhance the doping effect, thereby further reducing the lattice change rate, suppressing the manganese dissolution, and decreasing the consumption of electrolyte and active lithium, and also beneficial to lower the surface oxygen activity and reduce the interfacial side reactions between the positive electrode active material and the electrolyte, thereby improving the cycling performance and high-temperature storage performance of the secondary battery.
In some embodiments, x in the core of the positive electrode active material is any value in the range of −0.005 to 0.002, for example, −0.004, −0.003, −0.002, −0.001, 0, 0.001, and 0.002.
In some embodiments, y in the core of the positive electrode active material may be, for example, 0.001, 0.002, 0.3, 0.35, 0.4, and 0.5.
In some embodiments, z in the core of the positive electrode active material may be, for example, 0.001, 0.002, 0.003, 0.005, and 0.1.
In some embodiments, in the second coating layer of the positive electrode active material, d is any value in the range of 1 to 2.
In some embodiments, in the second coating layer of the positive electrode active material, e is any value in the range of 1 to 5
This application provides a preparation method of positive electrode active material, including the following steps:
a step of providing a core material, the core material including Li1+xMn1−yAyP1−zRzO4, where x is any value in the range of −0.100 to 0.100, y is any value in the range of 0.001 to 0.600, z is any value in the range of 0.001 to 0.100, A is selected from one or more elements of Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb, and Ge, and optionally selected from one or more elements of Fe, V, Ni, Co, and Mg, and R is selected from one or more of B, Si, N, and S, and optionally selected from one or more elements of Si, N, and S; and
a first coating step: providing a first mixture including a pyrophosphate LiaMP2O7 and/or Mb(P2O7)e, and mixing the core material and the first mixture for drying and sintering to obtain a material coated by a first coating layer, where a is greater than 0 and less than or equal to 2, b is any value in the range of 1 to 4, c is any value in the range of 1 to 3, and each M in the pyrophosphates LiaMP2O7 and Mb(P2O7)e is independently selected from one or more elements of Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, and Al, and optionally selected from one or more elements of Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, and Al;
a second coating step: providing a second mixture including an oxide M′dOe, and mixing the material coated by the first coating layer and the second mixture for drying and sintering to obtain a material coated by two coating layers, where d is greater than 0 and less than or equal to 2, e is greater than 0 and less than or equal to 5, M′ is selected from one or more elements of alkali metals, alkaline earth metals, transition metals, group IIIA elements, group IVA elements, lanthanide elements, and Sb, optionally selected from one or more elements of Li, Be, B, Na, Mg, Al, Si, P, S, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Sr, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Te, W, La, and Ce, and more optionally selected from one or more elements of Mg, Al, Si, Ti, V, Ni, Cu, Zr, and W; and
a third coating step: providing a third mixture including a source of carbon, and mixing the material coated by the two coating layers and the third mixture for drying and sintering to obtain a positive electrode active material;
where the positive electrode active material has a core-shell structure including the core and a shell enveloping the core, the core including Li1+xMn1−yAyP1−zRzO4, the shell including the first coating layer enveloping the core, a second coating layer enveloping the first coating layer, and a third coating layer enveloping the second coating layer, the first coating layer containing a crystalline pyrophosphate LiaMP2O7 and/or Mb(P2O7)e, the second coating layer containing a crystalline oxide M′dOe, and the third coating layer containing carbon. A, R, M, M′, x, y, z, a, b, c, d, and e are defined as previously described.
Based on this, the applicant provides a new type of positive electrode active material having a core-shell structure by doping element A at the manganese site and element R at the phosphorus site of lithium manganese phosphate to obtain a doped lithium manganese phosphate core and performing three coatings on the core surface, which can greatly reduce the Li/Mn anti-site defects produced, significantly reduce the dissolving-out amount of manganese and lattice change rate, and increase the compacted density. Applying such material to a secondary battery can increase the capacity of the secondary battery and improve the cycling performance, high-temperature storage performance, and safety performance of the secondary battery.
In some embodiments, the step of providing a core material includes the following steps:
step (1): mixing a source of manganese, a source of element A, and an acid to obtain a mixture; and
step (2): mixing the mixture obtained in step (1) with a source of lithium, a source of phosphorus, a source of element R, and an optional solvent, and sintering under the protection of an inert gas to obtain a core material containing Li1+xMn1−yAyP1−zRzO4. A and R are defined as previously described.
In some embodiments, step (1) is performed at 20° C. to 120° C., and optionally at 40° C. to 120° C. (for example, about 30° C., about 50° C., about 60° C. about 70° C., about 80° C., about 90° C., about 100° C., about 110° C., or about 120° C.); and/or in step (1), the mixing is performed by stirring at 400 rpm to 700 rpm for 1 h to 9 h (optionally for 3 h to 7 h, for example, about 2 h, about 3 h, about 4 h, about 5 h, about 6 h, about 7 h, about 8 h, or about 9 h).
In some embodiments, in step (2), the mixing is performed at 20° C. to 120° C., and optionally 40° C. to 120° C. (for example, about 30° C., about 50° C., about 60° C., about 70° C., about 80° C., about 90° C., about 100° C., about 110° C., or about 120° C.) for 1 h to 10 h (for example, about 2 h, about 3 h, about 4 h, about 5 h, about 6 h, about 7 h, about 8 h, about 9 h, about 10 h, about 11 h, or about 12 h).
Under the condition that the temperature and time during the preparation of the core particles are in the foregoing ranges, the cores obtained from the preparation and the positive electrode active material made therefrom have fewer lattice defects, which is conducive to suppressing manganese dissolution and reducing interfacial side reactions between the positive electrode active material and the electrolyte, thereby improving the cycling performance and safety performance of the secondary battery.
In some embodiments, in step (2), the mixing is performed at pH 3.5 to pH 6, optionally pH 4 to pH 6, more optionally pH 4 to pH 5. It should be noted that the pH may be adjusted in this application by methods commonly used in the art, for example, by adding an acid or alkali.
In some optional embodiments, the mixture obtained in step (1) is filtered, dried, and sanded to obtain element A-doped manganese salt particles with a particle size of 50 nm to 200 nm, and the element A-doped manganese salt particles are used in step (2) to mix with a source of lithium, a source of phosphorus, a source of element R, and optionally a solvent.
In some embodiments, optionally, a molar ratio of the mixture or element A-doped manganese salt particles to the source of lithium and the source of phosphorus in step (2) is 1:0.5-2.1:0.5-2.1, and optionally about 1:1:1.
In some embodiments, in step (2), the sintering is performed under an inert gas or a mixture of inert gas and hydrogen at 600° C. to 950° C. for 4 hours to 10 hours; optionally, the protective atmosphere is a mixture of 70 vol % to 90 vol % nitrogen and 10 vol % to 30 vol % hydrogen; optionally, the sintering may be performed at about 650° C., about 700° C., about 750° C., about 800° C., about 850° C., or about 900° C. for about 2 hours, about 3 hours, about 4 hours, about 5 hours, about 6 hours, about 7 hours, about 8 hours, about 9 hours, or about 10 hours; and optionally, the sintering temperature and sintering time may be in any range of any of these values, which can increase the crystallinity of the core, reduce the heterophase structures generated, and maintain a specific granularity of the core, thereby increasing the gram capacity and compacted density of the positive electrode active material, and improving the overall performance of the secondary battery, including the rate performance.
In some optional embodiments, the material mixed in step (2) is dried to obtain a powder, and then the powder is sintered to obtain a core material containing Li1+xMn1−yAyP1−zRzO4.
In some embodiments, in the first coating step, a first mixture is obtained by mixing a source of element M, a source of phosphorus, an acid, an optional source of lithium, and an optional solvent; and/or in the second coating step, a second mixture is obtained by mixing a source of element M′ and a solvent; and/or in the third coating step, a third mixture is obtained by mixing a source of carbon and a solvent.
In some embodiments, the first mixture, the second mixture, and the third mixture may be provided in the form of a suspension.
In some embodiments, in the first coating step, the source of element M, the source of phosphorus, the acid, the optional source of lithium, and the optional solvent are mixed at room temperature for 1 hour to 5 hours (for example, about 1.5 hours, about 2 hours, about 3 hours, about 4 hours, about 4.5 hours, or about 5 hours), and then warmed to 50° C. to 120° C. (for example, about 55° C., about 60° C., about 70° C., about 80° C., about 90° C., about 100° C., about 110° C., or about 120° C.) and mixed at this temperature for 2 hours to 10 hours (for example, about 2 hours, about 3 hours, about 4 hours, about 5 hours, about 6 hours, about 7 hours, about 8 hours, about 9 hours, or about 10 hours), where the mixing is all performed at pH 3.5 to pH 6.5 (for example, pH 4 to pH 6).
In some embodiments, in the second coating step, the source of element M′ is mixed with the solvent at room temperature for 1 hour to 10 hours (for example, 1.5 hours, about 2 hours, about 3 hours, about 4 hours, about 4.5 hours, about 5 hours, about 6 hours, about 7 hours, about 8 hours, about 9 hours, or about 10 hours), and then warmed to 60° C. to 150° C. (for example, about 65° C., about 70° C., about 80° C., about 90° C., about 100° C., about 110° C., about 120° C., about 130° C., about 140° C., or about 150° C.) and mixed at this temperature for 2 hours to 10 hours (for example, about 2 hours, about 3 hours, about 4 hours, about 5 hours, about 6 hours, about 7 hours, about 8 hours, about 9 hours, or about 10 hours).
The preparation method of this application is not particularly limited to the source of the material, and the source of an element may include one or more of monomers, sulfates, halides, nitrates, organic acid salts, oxides, or hydroxides of the element, provided that the source achieves the purpose of the preparation method of this application.
In some embodiments, the source of element A is selected from one or more of monomer, carbonate, sulfate, halide, nitrate, organic acid salt, oxide, and hydroxide of element A; and/or the source of element R is selected from one or more of inorganic acid, organic acid, sulfate, halide, nitrate, organic acid salt, oxide, and hydroxide element R.
In some embodiments, the source of element M is selected from one or more of monomer, carbonate, sulfate, halide, nitrate, organic acid salt, oxide, and hydroxide of element M.
In some embodiments, the source of element M′ is selected from one or more of monomer, carbonate, sulfate, halide, nitrate, organic acid salt, oxide, and hydroxide of element M′.
The addition amounts of the respective sources of elements A, R, M, and M′ depend on the target doping amount, and the ratio of the amounts of the lithium, manganese, and phosphorus sources conforms to the stoichiometric ratio.
In this application, the source of manganese may be a manganese-containing substance known in the art that can be used to prepare lithium manganese phosphate. For example, the source of manganese may be selected from one or more of monomeric manganese, manganese dioxide, manganese phosphate, manganese oxalate, and manganese carbonate.
In this application, the acid may be selected from one or more of organic acids such as hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, silicic acid, metasilicic acid, and oxalic acid. In some embodiments, the acid is a dilute organic acid with a concentration lower than 60 wt %.
In this application, the source of lithium may be a lithium-containing substance known in the art that can be used to prepare lithium manganese phosphate. For example, the source of lithium is selected from one or more of lithium carbonate, lithium hydroxide, lithium phosphate, and lithium dihydrogen phosphate.
In this application, the source of phosphorus may be a phosphorus-containing substance known in the art that can be used to prepare lithium manganese phosphate. For example, the source of phosphorus is selected from one or more of diammonium phosphate, ammonium dihydrogen phosphate, ammonium phosphate, and phosphoric acid.
In this application, for example, the source of carbon is selected from one or more of starch, sucrose, glucose, polyvinyl alcohol, polyethylene glycol, and citric acid.
In some embodiments, in the first coating step, the sintering is performed at 650° C. to 800° C. (for example, about 650° C., about 700° C., about 750° C., or about 800° C.) for 2 hours to 8 hours (about 2 hours, about 3 hours, about 4 hours, about 5 hours, about 6 hours, or about 8 hours); and/or in the second coating step, the sintering is performed at 400° C. to 750° C. (for example, about 400° C., about 450° C., about 500° C., about 550° C., about 600° C., about 700° C., or about 750° C.) for 6 hours to 10 hours (for example, about 6 hours, about 7 hours, about 8 hours, about 9 hours, or about 10 hours); and/or in the third coating step, the sintering is performed at 600° C. to 850° C. (for example, about 600° C., about 650° C., about 700° C., about 750° C., about 800° C., or about 850° C.) for 6 hours to 10 hours (for example, about 6 hours, about 7 hours, about 8 hours, about 9 hours, or about 10 hours).
In the first coating step, the second coating step, and the third coating step described above, the drying is all performed at 80° C. to 200° C., optionally at 80° C. to 190° C., more optionally at 120° C. to 180° C., further more optionally 120° C. to 170° C., and most optionally 120° C. to 160° C., with a drying time of 3 h to 9 h, optionally 4h to 8 h, more optionally 5h to 7 h, and most optionally about 6 h.
This application provides a positive electrode plate including a positive electrode current collector and a positive electrode film layer provided on at least one surface of the positive electrode current collector, where the positive electrode film layer includes the foregoing positive electrode active material or a positive electrode active material prepared by the foregoing preparation method; and optionally, based on a total weight of the positive electrode film layer, a content of the positive electrode active material in the positive electrode film layer is higher than 10 wt %.
In some embodiments, the content of the positive electrode active material in the positive electrode film layer is 90 wt % to 99.5 wt % based on the total weight of the positive electrode film layer. The secondary battery is guaranteed to have a high capacity and better cycling performance, high-temperature storage performance, and safety performance.
In some embodiments, the positive electrode current collector may be a metal foil current collector or a composite current collector. For example, an aluminum foil may be used as the metal foil. The composite current collector may include a polymer material matrix and a metal layer formed on at least one surface of the polymer material matrix. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, or the like) on a polymer material matrix (for example, matrices of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE)).
In some embodiments, the positive electrode film layer may further include other positive electrode active materials for batteries that are well known in the art. For example, the positive electrode active material may include at least one of the following materials: olivine-structured lithium-containing phosphate and respective modified compounds thereof. However, this application is not limited to such materials, and may alternatively use other conventional well-known materials that can be used as positive electrode active materials for batteries. One type of these positive electrode active materials may be used alone, or two or more of them may be used in combination. Examples of the olivine-structured lithium-containing phosphate may include but are not limited to at least one of lithium iron phosphate (for example, LiFePO4 (LFP for short)), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate (for example, LiMnPO4), a composite material of lithium manganese phosphate and carbon.
In some embodiments, the positive electrode film layer further optionally includes a binder. For example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorine-containing acrylic resin.
In some embodiments, the positive electrode film layer further optionally includes a conductive agent. For example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.
The negative electrode plate includes a negative electrode current collector and a negative electrode film layer disposed on at least one surface of the negative electrode current collector, where the negative electrode film layer includes a negative electrode active material.
For example, the negative electrode current collector includes two opposite surfaces in its thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.
In some embodiments, the negative electrode current collector may be a metal foil current collector or a composite current collector. For example, for the metal foil, a copper foil may be used. The composite current collector may include a polymer material matrix and a metal layer formed on at least one surface of the polymer material matrix. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, or the like) on a polymer material matrix (for example, matrices of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE)).
In some embodiments, the negative electrode active material may be a well-known negative electrode active material used for a battery in the art. In an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, a silicon-based material, a tin-based material, and lithium titanate. The silicon-based material may be selected from at least one of elemental silicon, silicon-oxygen compound, silicon-carbon composite, silicon-nitrogen composite, or silicon alloy. The tin-based material may be selected from at least one of elemental tin, tin-oxygen compound, or tin alloy. However, this application is not limited to these materials, but may use other conventional materials that can be used as negative electrode active materials for batteries instead. One of these negative electrode active materials may be used alone, or two or more of them may be used in combination.
In some embodiments, the negative electrode film layer further optionally includes a binder. For example, the binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), polyacrylic acid sodium (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), or carboxymethyl chitosan (CMCS).
In some embodiments, the negative electrode film layer further optionally includes a conductive agent. For example, the conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, or carbon nanofibers.
In some embodiments, the negative electrode film layer may further optionally include other promoters such as a thickener (for example, sodium carboxymethyl cellulose (CMC-Na)).
In some embodiments, the negative electrode plate may be prepared in the following manner: the constituents used for preparing the negative electrode plate, for example, the negative electrode active material, the conductive agent, the binder, and any other constituent, are dispersed in a solvent (for example, deionized water) to form a negative electrode slurry; and the negative electrode slurry is applied onto the negative electrode current collector, followed by processes such as drying and cold pressing to obtain the negative electrode plate.
The electrolyte conducts ions between the positive electrode plate and the negative electrode plate. The electrolyte is not specifically limited to any particular type in this application, and may be selected based on needs. For example, the electrolyte may be in a liquid state, a gel state, or an all-solid state.
In some embodiments, the electrolyte is liquid and includes an electrolytic salt and a solvent.
In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroborate, lithium bis(fluorosulfonyl)imide, lithium bis-trifluoromethanesulfonimide, lithium trifluoromethanesulfonat, lithium difluorophosphate, lithium difluorooxalatoborate, lithium bisoxalatoborate, lithium difluorobisoxalate phosphate, and lithium tetrafluoro oxalate phosphate.
In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, methyl sulfonyl methane, ethyl methanesulfonate, and diethyl sulfone.
In some embodiments, the electrolyte further optionally includes an additive. For example, the additive may include a negative electrode film-forming additive or a positive electrode film-forming additive, or may include an additive capable of improving some performance of the battery, for example, an additive for improving overcharge performance of the battery, or an additive for improving high-temperature or low-temperature performance of the battery.
In some embodiments, the secondary battery further includes a separator. The separator is not limited to any particular type in this application, and may be any well-known porous separator with good chemical stability and mechanical stability.
In some embodiments, a material of the separator may be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator may be a single-layer film or a multi-layer composite film, and is not particularly limited. When the separator is a multi-layer composite film, all layers may be made of same or different materials, which is not particularly limited.
In some embodiments, the positive electrode plate, the negative electrode plate, and the separator may be made into an electrode assembly through winding or lamination.
In some embodiments, the secondary battery may include an outer package. The outer package may be used for packaging the electrode assembly and the electrolyte.
In some embodiments, the outer package of the secondary battery may be a hard shell, for example, a hard plastic shell, an aluminum shell, or a steel shell. The outer package of the secondary battery may alternatively be a soft pack, for example, a soft bag. Material of the soft pack may be plastic, which, for example, may be polypropylene, polybutylene terephthalate, and polybutylene succinate.
This application does not impose any special limitations on a shape of the secondary battery, and the secondary battery may be cylindrical, rectangular, or of any other shapes. For example,
In some embodiments, referring to
In some embodiments, the secondary battery may be assembled into a battery module, and the battery module may include one or more secondary batteries. The specific quantity may be chosen by persons skilled in the art according to use and capacity of the battery module.
Optionally, the battery module 4 may further include a housing with accommodating space, and the plurality of secondary batteries 5 are accommodated in the accommodating space.
In some embodiments, the battery module may be further assembled into a battery pack, and the battery pack may include one or more battery modules. The specific quantity may be chosen by persons skilled in the art according to use and capacity of the battery pack.
In addition, this application further provides an electric apparatus. The electric apparatus includes at least one of the secondary battery, the battery module, or the battery pack provided in this application. The secondary battery, the battery module, or the battery pack may be used as a power source for the electric apparatus or an energy storage unit of the electric apparatus. The electric apparatus may include a mobile device (for example, a mobile phone or a notebook computer), an electric vehicle (for example, a battery electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf vehicle, or an electric truck), an electric train, a ship, a satellite system, an energy storage system, or the like, but is not limited thereto.
The secondary battery, the battery module, or the battery pack may be selected for the electric apparatus based on requirements for using the electric apparatus.
The following describes examples of this application. The examples described below are illustrative and only used for explaining this application, and cannot be construed as limitations on this application. Examples whose technical solutions or conditions are not specified are made in accordance with technical solutions or conditions described in literature in the field or made in accordance with product instructions. The reagents or instruments used are all conventional products that are commercially available if no manufacturer is indicated.
The raw materials used in the embodiments of this application are sourced as follows:
689.6 g of manganese carbonate, 455.27 g of ferrous carbonate, 4.65 g of cobalt sulfate, and 4.87 g of vanadium dichloride were added into a mixer and mixed thoroughly for 6 h. The resulting mixture was transferred into a reactor, 5 L of deionized water and 1260.6 g of oxalic acid dihydrate were added into the reactor, the reactor was heated to 80° C., and the materials were mixed well for 6 h at 500 rpm until the reaction was terminated without bubbles to obtain a Fe, Co, and V co-doped manganese oxalate suspension. The suspension was filtered, dried at 120° C., and then sanded to obtain manganese oxalate particles with a particle size of 100 nm.
Step S2: Preparation of core
Li0.997Mn0.60Fe0.393V0.004Co0.003P0.997S0.003O4
1793.1 g of manganese oxalate prepared in (1), 368.3 g of lithium carbonate, 1146.6 g of ammonium dihydrogen phosphate, and 4.9 g of dilute sulfuric acid were added into 20 L of deionized water, the mixture was stirred thoroughly, evenly mixed and reacted at 80° C. for 10 h to get a slurry. The slurry was transferred to a spray drying equipment for spray drying and granulation at 250° C. to obtain a powder. The powder was sintered in a roller kiln for 4 h at 700° C. in a protective atmosphere (90% nitrogen and 10% hydrogen) to obtain a core material. The element content of the core material was examined by inductively coupled plasma emission spectrometry (ICP-AES) to obtain a core with the chemical formula as shown above.
Preparation of Li2FeP2O7 solution: 7.4 g of lithium carbonate, 11.6 g of ferrous carbonate, 23.0 g of ammonium dihydrogen phosphate, and 12.6 g of oxalic acid dihydrate were dissolved in 500 mL of deionized water, during which a pH value was controlled to be 5; the mixture was stirred and reacted at room temperature for 2 h to obtain a solution; and then the solution was heated to 80° C. and left standing at this temperature for 4 h to obtain a first coating layer suspension.
1571.9 g of doped lithium manganese phosphate core material obtained in step S2 was added to the first coating layer suspension (having a coating substance of 15.7 g) obtained in step S3, and the mixture was fully mixed and stirred for 6 h. After well mixing, the mixture was transferred to an oven for drying at 120° C. for 6 h, and then sintered at 650° C. for 6 h to obtain a pyrophosphate-coated material.
47.1 g of nanoscale Al2O3(particle size about 20 nm) was dissolved in 1500 mL of deionized water, and the mixture was stirred for 2 h to obtain a second coating layer suspension.
1586.8 g of pyrophosphate-coated material obtained in step S4 was added to the second coating layer suspension (having a coating substance of 47.1 g) obtained in step S5, and the mixture was fully mixed and stirred for 6 h. After well mixing, the mixture was transferred to an oven for drying at 120° C. for 6 h, and then sintered at 700° C. for 8 h to obtain a two-layer-coated material.
37.3 g of sucrose was dissolved in 500 g of deionized water, and then the mixture was stirred and fully dissolved to obtain an aqueous sucrose solution.
1633.9 g of the two-layer-coated material obtained in step S6 was added into the sucrose solution obtained in step S7, the mixture was stirred and mixed for 6 h. After well mixing, the mixture was transferred to an oven for drying at 150° C. for 6 h, and then sintered at 700° C. for 10 h to obtain a three-layer-coated material.
The positive electrode active materials of Examples 2 to 56 and Comparative Examples 1 to 17 were made in a method similar to that of Example 1, and the differences in the preparation of the positive electrode active materials are shown in Tables 1-6.
The positive electrode active materials of Comparative Examples 1 and 2, 4 to 10, and 12 were not coated with the first layer, so there were no steps S3 and S4; and the positive electrode active materials of Comparative Examples 1 to 11 were not coated with the second layer, so there were no steps S5 and S6.
The prepared three-layer-coated positive electrode active material, a conductive agent acetylene black, and a binder polyvinylidene fluoride (PVDF) were added at a weight ratio of 97.0:1.2:1.8 to N-methylpyrrolidone (NMP), stirred and mixed to uniformity to obtain a positive electrode slurry. The positive electrode slurry was applied onto an aluminum foil at 0.280 g/1540.25 mm2, followed by drying and cold pressing, to obtain a positive electrode plate.
A negative electrode active material artificial graphite, a conductive agent super-conductive carbon black (Super-P), a binder styrene butadiene rubber (SBR), and a thickener sodium carboxymethyl cellulose (CMC-Na) were dissolved at a mass ratio of 95%:1.5%:1.8%:1.7% in deionized water, fully stirred, and mixed to uniformity to obtain a negative electrode slurry with a viscosity of 3000 mPa·s and a solid content of 52%. The negative electrode slurry was applied on a 6 m negative electrode current collector copper foil, followed by baking at 100° C. for drying for 4 hours and roll pressing, to obtain a negative electrode plate with a compacted density of 1.75 g/cm3.
A polypropylene film was used.
Ethylene carbonate, dimethyl carbonate, and 1,2-propanediol carbonate were mixed at a volume ratio of 1:1:1, and then LiPF6 was evenly dissolved in such solution, to obtain an electrolyte. In the electrolyte, a concentration of LiPF6 was 1 mol/L.
The positive electrode plate, separator, and negative electrode plate obtained above were stacked in order, so that the separator was placed between the positive and negative electrodes for separation, and winding was performed to obtain a jelly roll. The jelly roll was placed in an outer package, the electrolyte was injected, and the outer package was sealed to obtain a full battery (also referred to as “full battery” hereinafter).
The foregoing positive electrode plate, the negative electrode, and the electrolyte were assembled into a button battery (hereinafter also referred to as “button battery”) in a button box.
I. Performance test of positive electrode active material
At a constant temperature of 25° C., the positive electrode active material samples were placed in an XRD instrument (model: Bruker D8 Discover), and the samples were tested at 1°/min. The test data were organized and analyzed, and the lattice constants a0, b0, c0, and v0 (where a0, b0, and c0 represent the lengths of faces of the unit cell, and v0 denotes the volume of the unit cell, which can be obtained directly from XRD refinement results) at this moment were calculated with reference to standard PDF cards.
The positive electrode active material samples were prepared into button batteries using the foregoing button battery preparation method, and the button batteries were charged at a small rate of 0.05C until the current was reduced to 0.01C. The positive electrode plate was then taken out from the button battery and placed in dimethyl carbonate (DMC) for 8 hours, dried, and scraped for powder, and particles with a particle size less than 500 nm were sifted out from the powder. Samples were taken and subjected to the same way as the fresh samples under test above to calculate their unit cell volumes v1, and (v0−v1)/v0×100% was taken as their lattice change rates (volume change rate of unit cell) before and after complete deintercalation of lithium.
The XRD results tested in “test method of lattice change rate” were compared with the PDF (Powder Diffraction File) card of the standard crystal to obtain the Li/Mn anti-site defect concentration. Specifically, the XRD results tested in the “test method of lattice change rate” were imported into the general structural analysis system (GSAS) software to automatically derive the refinement results, which contain the occupancy of the different atoms, and then the Li/Mn anti-site defect concentration was obtained by reading the refinement results.
5 g of the foregoing prepared positive electrode active material powder was taken and put in a special mold for compaction (CARVER mold, model 13 mm, USA), and then the mold was put on the compacted density apparatus. A pressure of 3 tons was applied, the thickness of the powder under pressure (thickness after pressure relief) on the apparatus was read, and the compacted density was calculated based on p=m/v, where the area value used was the area of standard small picture, 1540.25 mm2.
New full batteries prepared from the foregoing examples and comparative examples were left standing for 5 min at 25° C. in a constant temperature environment and discharged to 2.5 V at ⅓C. The full batteries were left standing for 5 min, charged to 4.3 V at ⅓C, and then charged at a constant voltage of 4.3 V to a current less than or equal to 0.05 mA. The full batteries were left standing for 5 min. The charge capacity at this moment was recorded as CO. The full batteries were discharge at ⅓C to 2.5 V, left standing for 5 min, then charged at 3C to 4.3 V, and left standing for 5 min. The charge capacity at this moment was recorded as Cl. The ratio of 3C constant current charge was C1/C0×l00%.
A higher ratio of 3C constant current charge indicates better rate performance of the secondary battery.
Full batteries prepared from the positive electrode active materials of the foregoing examples and comparative examples and cycled at 45° C. until capacity faded to 80% were discharged to a cut-off voltage of 2.0 V at 0.1C. The battery was then disassembled, the negative electrode plate was removed, and 30 discs with a unit area (1540.25 mm2) were randomly taken on the negative electrode plate and tested by using Agilent ICP-OES730 for inductively coupled plasma atomic emission spectrometry (ICP). The amounts of Fe (if the Mn site of the positive electrode active material was doped with Fe) and Mn were calculated from the ICP results, so as to calculate the dissolving-out amount of Mn (and Fe doped at the Mn site) after the cycling. The test standard is in accordance with EPA-6010D-2014.
5 g of the foregoing prepared positive electrode active material sample was taken to prepare into a button battery according to the preparation method of button battery as described above. The button battery was charged at a small rate of 0.05C until the current was reduced to 0.01C. The positive electrode plate was then taken out from the button battery and placed in DMC for 8 hours, then dried, and scraped for powder, and particles with a particle size less than 500 nm were sifted out from the powder. The thus obtained particles were measured by using electron energy loss spectroscopy (EELS; the model of the instrument used was Talos F200S) to obtain energy loss near edge structures (ELNES) that reflect the density of states and energy level distribution of elements. Based on the density of states and energy level distribution, the number of occupying electrons was calculated by integrating the data of the valence band density of states to deduce the valence state of surface oxygen after charging.
5 g of the foregoing prepared positive electrode active material was dissolved in 100 ml of inverse aqua regia (concentrated hydrochloric acid:concentrated nitric acid=1:3) (concentration of concentrated hydrochloric acid is about 37%, and concentration of concentrated nitric acid is about 65%), the content of each element in the solution was tested using ICP, and then the content of element manganese or element phosphorus (amount of element manganese or element phosphorus/amount of positive electrode active material×100%) was measured and converted to get its weight percentage.
Under a voltage of 2.5 V to 4.3 V, the prepared button batteries of the foregoing examples and comparative examples were charged to 4.3 V at 0.1C, then charged at constant voltage of 4.3 V to a current less than or equal to 0.05 mA, left standing for 5 min, and then discharged to 2.0 V at 0.1C. The discharge capacity at this moment was an initial gram capacity and recorded as DO.
9. Cell Swelling Test of Full Battery after 30 Days of Storage at 60° C.:
The prepared full batteries of the foregoing examples and comparative examples at 100% state of charge (SOC) were stored at 60° C. The open circuit voltage (OCV) and alternating current internal resistance (IMP) of the cells were measured before, during, and after storage to monitor the SOCs and the volumes of the cells were measured. The full batteries were taken out after every 48 h of storage and left standing for 1 h. Then the open circuit voltage (OCV) and internal resistance (IMP) were measured, and the cell volumes were measured by a drainage method after the cells were cooled to room temperature. In the drainage method, first a scale that automatically performs unit conversion on dial data was used to separately measure a weight of the cell, denoted as F1, then the cell was completely placed into deionized water (density known to be 1 g/cm3), and the weight of the cell at this moment was measured as F2. The buoyant force experienced by the cell, denoted as Fbuoyant, was calculated as F1-F2. Then, according to Archimedes' principle, Fbuoyant=ρ×g×Vdisplaced, the volume V of the cell could be calculated as V=(F1−F2)/(ρ×g).
It can be learned from the OCV and IMP test results that the batteries of all the examples always maintained an SOC higher than 99% throughout the storage test.
After 30 days of storage, the cell volumes were measured and the percentage increases in the cell volumes after storage relative to the cell volumes before storage were calculated.
The battery was charged to 4.3 V at 1C under a voltage of 2.5 V to 4.3 V at a constant temperature of 45° C., and then charged to a current less than or equal to 0.05 mA at a constant voltage of 4.3 V. After being left standing for 5 min, the battery was discharged to 2.5 V at 1C, and a capacity at this moment was recorded as D. (n=0, 1, 2, . . . ). The previous process was repeated until the capacity was faded (fading) to 80%, and the number of repetitions at this moment was record, which is the number of cycles corresponding to 80% capacity retention rate at 45° C.
1 g of each prepared positive electrode active material powder was taken and placed into a 50 mL test tube, 10 mL of alcohol with a mass fraction of 75% was injected into the test tube, and the substances were then fully stirred and dispersed for 30 minutes. Then an appropriate amount of the solution was taken with a clean disposable plastic pipette and added dropwise on a 300 mesh copper grid. At this moment, part of the powder would remain on the copper grid. The copper grid with the sample was transferred to the TEM (Talos F200s G2) sample cavity for testing, and original TEM test images were obtained and saved in an original image format (xx.dm3).
The original image obtained from the TEM test was opened in DigitalMicrograph software, and Fourier transform was performed (done automatically by the software upon clicking) to obtain a diffraction pattern. A distance from the diffraction spot to the center of the diffraction pattern was measured to obtain the interplanar spacing, and the included angle was calculated according to the Bragg equation.
By comparing the obtained interplanar spacing and corresponding included angle with their respective standard values, the substances and crystalline states of different coating layers could be identified.
The thickness of the coating was tested mainly in the following manner. A thin slice of about 100 nm thickness was cut from the middle of a single particle of the prepared positive electrode active material by FIB, then a TEM test was performed on the thin slice, and original TEM test images were obtained and saved in an original image format (xx.dm3).
The original image obtained from the TEM test was opened in DigitalMicrograph software, the coating was identified by using the information of the lattice spacing and included angle, and the thickness of the coating was measured.
The thicknesses at three locations were measured for the selected particle and the average value was taken.
This test was performed by Raman (Raman) spectroscopy. The energy spectrum of the Raman test was splited to obtain Id/Ig, where Id is a peak intensity of SP3 carbon and Ig is a peak intensity of SP2 carbon, and then the molar ratio therebetween was determined.
An aberration corrected scanning transmission electron microscope (ACSTEM) was used for high spatial resolution characterization of the internal microstructure and surface structure of the positive electrode active material, and the chemical formula of the core and the compositions of different coating layers of the positive electrode active material were obtained in combination with three-dimensional reconstruction techniques.
The performance test results of the positive electrode active materials of the examples and the comparative examples are shown in the following tables.
It can be learned from Tables 7 and 8 that compared with the comparative examples, the examples of this application achieve smaller lattice change rate, smaller Li/Mn anti-site defect concentration, greater compacted density, surface oxygen valence closer to −2 valence, less dissolving-out amount of Mn and Fe after cycles, and better battery performance, for example, higher capacity of button battery, better high-temperature storage performance, higher safety, and better cycling performance.
It can be learned from Table 9 that by doping the manganese and phosphorus sites of lithium manganese iron phosphate (containing 35% manganese and about 20% phosphorus) and three-layer coating, the content of element manganese and the weight content ratio of element manganese to element phosphorus in the positive electrode active material are significantly reduced; furthermore, comparing Examples 1 to 14 with Comparative Examples 3, 4, and 12, it can be learned from Table 7 that the decrease of elements manganese and phosphorus in the positive electrode active material leads to the decrease of the dissolving-out amounts of manganese and iron and suppresses the improvement of the battery performance of the secondary batteries prepared therefrom.
II. Examination of the influence of sintering method of coating layer on performance of positive electrode active material
The preparation method of batteries in the examples and comparative examples in the table below are similar to that of Example 1. See the method parameters in the table below for differences. See Table 10 below for the results.
It can be seen from the above that under the condition that in step S4, the sintering temperature is in the range of 650° C. to 800° C. and the sintering time is 2 hours to 6 hours, in step S6, the sintering temperature is 400° C. to 600° C. and the sintering time is 6 hours to 10 hours, and in step S8, the sintering temperature is 700° C. to 800° C. and the sintering time is 6 hours to 10 hours, smaller lattice change rate, smaller Li/Mn anti-site defect concentration, less dissolving-out amount of elements manganese and iron, better ratio of 3C constant current charge, larger battery capacity, better battery cycling performance, and between high temperature storage stability can be achieved.
In addition, better performance of the positive electrode active material and better battery performance were achieved in Example II-1) in step S4, sintering temperature is 750° C. and sintering time is 4.5 h) compared with those in Example II-16 (in step S4, sintering temperature is 750° C. and sintering time is 4 h), which indicates that when the sintering temperature in step S4 is 750° C. or higher than 750° C., and the sintering time needs to be controlled to be less than 4.5 h.
The preparation of positive electrode active materials and batteries in the examples in the table below are similar to Example 1. See the method parameters in the table below for differences in the preparation of the positive electrode active material. The results are also shown in the following table.
It can be seen from Table 11 that under the condition that the reaction temperature in step S1 is in the range of 60° C. to 120° C., the reaction time is in the range of 2 hours to 9 hours, the reaction temperature in step S2 is in the range of 40° C. to 120° C., and the reaction time is 1 hour to 10 hours, the performance (lattice change rate, Li/Mn anti-site defect concentration, surface oxygen valence, compacted density) of the positive electrode active material powder and the performance (electric capacity, high-temperature cycling performance, high-temperature storage performance) of the prepared batteries were excellent.
It should be noted that this application is not limited to the foregoing embodiments. The foregoing embodiments are merely examples, and embodiments having substantially the same constructions and the same effects as the technical idea within the scope of the technical solutions of this application are all included in the technical scope of this application. In addition, without departing from the essence of this application, various modifications made to the embodiments that can be conceived by persons skilled in the art, and other manners constructed by combining some of the constituent elements in the embodiments are also included in the scope of this application.
This application is a continuation of international application PCT/CN2022/099516, filed Jun. 17, 2022 and entitled “POSITIVE ELECTRODE ACTIVE MATERIAL AND PREPARATION METHOD THEREOF, POSITIVE ELECTRODE PLATE, SECONDARY BATTERY, BATTERY MODULE, BATTERY PACK, AND ELECTRIC APPARATUS”, the entire content of which is hereby incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/CN2022/099516 | Jun 2022 | WO |
| Child | 18527290 | US |
