Patent Application
20160181591
The disclosure of Japanese Patent Application No. 2014-259619 filed on Dec. 23, 2014 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to a method of manufacturing a lithium ion secondary battery.
2. Description of Related Art
One of the secondary batteries is a lithium ion secondary battery. The lithium ion secondary battery includes: positive and negative electrodes capable of reversibly storing and releasing lithium ions; and a separator that is interposed between the positive and negative electrodes. Recently, a secondary battery such as a lithium ion secondary battery has been used as a motor-driving power supply or an auxiliary power supply for an electric vehicle, a hybrid electric vehicle, or a fuel cell vehicle.
As a method of manufacturing a lithium ion secondary battery, various methods have been disclosed. For example, Japanese Patent Application Publication No. 2013-097980 (JP 2013-097980 A) discloses a method of reducing the internal pressure of a battery case in order to make an electrolytic solution spread over the entire area of an electrode body after injection of the electrolytic solution. Japanese Patent Application Publication No. 2014-154279 (JP 2014-154279 A) discloses a nonaqueous electrolyte secondary battery in which styrene-butadiene rubber (SBR) is used as a binder of a negative electrode, and lithium bis(oxalato)borate (LiBOB) is added to a nonaqueous electrolytic solution.
The charging-discharging of a lithium ion secondary battery is controlled by determining maximum current value in order to suppress lithium deposition and to obtain the maximum battery output. Lithium deposition needs to be minimized because it causes deterioration in, for example, the capacity or output of a battery. Therefore, the maximum current value is desirably a value corresponding to a limit current value which is determined based on the resistance value of a battery. The limit current value is a current value at which lithium deposition is started.
The technique disclosed in JP 2014-154279 A relates to a nonaqueous electrolyte secondary battery in which superior battery characteristics are realized. In many cases, the battery system of a nonaqueous electrolyte secondary battery such as a lithium ion secondary battery contains a large amount of sodium (Na) which is an impurity. In this case, when LiBOB is used as an additive of an electrolytic solution, LiBOB and Na react with each other, and a reactant between LiBOB and Na precipitates. At this time, the electrolytic solution is impregnated into both ends of an electrode body. Therefore, LiBOB precipitates on the end portions of the electrode body, and LiBOB is likely to be lean at the center of the electrode body. Therefore, the resistance value at the center of the electrode body is likely to increase. Accordingly, the limit current value is dependent on the resistance value at the center of the electrode body. In the technique disclosed in JP 2013-097980 A, the internal pressure of a battery case is reduced in order to make an electrolytic solution spread over the entire area of an electrode body after injection of the electrolytic solution. However, in the above-described method of injecting an electrolytic solution, the LiBOB distribution in the electrode body of the obtained battery is not stable, and the resistance value at the center of the electrode body is unstable as described above. As a result, the resistance value varies depending on each battery, and there is a variation in limit current value. Accordingly, a gap is generated between the limit current value and the maximum current value. The reason for this is presumed to be that, due to a reduction in pressure after the injection (a reduction in pressure in a state where half of the electrode body is soaked in the electrolytic solution), the electrolytic solution is not impregnated into both ends of the electrode body and does not spread over the entire region of the electrode body. Specifically, when the limit current value is higher than the maximum current value, the battery performance cannot be maximized, and the maximum output of the battery cannot be obtained. Conversely, when the limit current value is lower than the maximum current value, the current value exceeds the limit current value, and thus lithium deposition may not be suppressed.
The invention provides a method of manufacturing a lithium ion secondary battery in which a nonaqueous electrolytic solution contains LiBOB, the method capable of reducing a variation in resistance value and making battery characteristics uniform.
A method of manufacturing a lithium ion secondary battery according to the invention has the following configurations. In the method of manufacturing a lithium ion secondary battery, a battery system contains sodium (Na), the lithium ion secondary battery includes a battery case which accommodates an electrode body and a nonaqueous electrolytic solution, the electrode body includes a positive electrode and a negative electrode, the positive electrode includes a positive electrode active material layer containing a positive electrode active material, the negative electrode includes a negative electrode active material layer containing a negative electrode active material, and the nonaqueous electrolytic solution contains lithium bis(oxalato)borate. This method includes reducing an internal pressure of the battery case; and injecting the nonaqueous electrolytic solution after the reduction in the internal pressure.
According to this method, the nonaqueous electrolytic solution is injected after the reduction in the internal pressure of the battery case. Therefore, the nonaqueous electrolytic solution is impregnated into both ends of the electrode body and can spread over the entire region of the electrode body. Thus, the resistance value at the center of the electrode body can be increased, and battery characteristics can be stabilized. Specifically, the limit current value can be stabilized. Due to the above-described configuration, even in a lithium ion secondary battery in which a battery system contains Na and in which LiBOB is added to the nonaqueous electrolytic solution, a variation in resistance value can be reduced, and battery characteristics can be made to be uniform. As a result, safety and performance as a battery can be secured.
According to an aspect of the method disclosed herein, the negative electrode may contain styrene-butadiene rubber (SBR) as a binder.
SBR contains a large amount of Na and is highly reactive with LiBOB. Therefore, the resistance at the center of the electrode body is likely to increase. As a result, a lithium ion secondary battery having reduced variation in resistance value and uniform battery characteristics can be manufactured. According to another aspect of the method disclosed herein, in the positive electrode active material, a full width at half maximum β of a diffraction peak of a (003) plane may satisfy 0.055≦β≦0.097. Unless specified otherwise, “the full width at half maximum β of a diffraction peak of a (003) plane” refers to a full width at half maximum obtained from X-ray diffraction.
In the positive electrode active material, crystallinity is optimized. Thus, the resistance value at the center of the electrode body is stable, and battery characteristics can be stabilized. For example, when the full width at half maximum β is 0.055 or lower and crystallinity is low, a layered structure is disordered. Therefore, metal is likely to be eluted from a positive electrode, and the resistance value is likely to increase. Thus, the resistance value varies depending on each battery, and a lithium ion secondary battery having uniform battery characteristics cannot be obtained. On the other hand, for example, when the full width at half maximum β is 0.097 or higher and crystallinity is high, the resistance value increases, and the resistance value is likely to vary depending on each battery. The reason for this is presumed to be that, the conductivity of the positive electrode active material is reduced due to high crystallinity, and contact between a conductive material and the positive electrode active material is not likely to occur. According to still another aspect of the method disclosed herein, during the reduction in the internal pressure, a vacuum degree may be 1 kPa·abs to 40 kPa·abs. Since the vacuum degree as a pressure reduction condition is adjusted to be within the above-described range, the electrolytic solution is impregnated into both ends of the electrode body and can spread over the entire region of the electrode body. Thus, the resistance value at the center of the electrode body can be increased, and battery characteristics can be stabilized. For example, when the vacuum degree is 1 kPa·abs or lower, the internal pressure of the battery system is excessively low, and the electrolytic solution is boiled. On the other hand, for example, when the vacuum degree is 40 kPa·abs or higher, pressure reduction is insufficient. Therefore, the electrolytic solution cannot spread over the entire region of the electrode body, and battery characteristics cannot be stabilized.
Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:
Preferred embodiments of the invention are described below. Matters necessary to implement the embodiments of the invention other than those specifically referred to in this description may be understood as design matters based on the related art in the pertinent field for a person of ordinary skill in the art. The invention can be practiced based on the contents disclosed in this specification and common technical knowledge in the pertinent field.
In the following drawings, parts or portions having the same function are represented by the same reference numerals, and repeated descriptions will not be made or will be simplified. In each drawing, a dimensional relationship (for example, length, width, or thickness) does not reflect the actual dimensional relationship.
Hereinafter, a lithium ion secondary battery 100 (hereinafter, also referred to simply as “battery”) according to a preferred embodiment of the invention will be described.
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The positive electrode active material layer 54 according to the embodiment contains a positive electrode active material as a major component.
As the positive electrode active material, one material or two or more materials selected from materials which are used for a lithium ion secondary battery in the related art may be used without any particular limitation. Examples of the positive electrode active material include oxides (lithium transition metal composite oxides) containing lithium and a transition metal element as constituent metal elements, such as lithium nickel composite oxide (for example, LiNiO2), lithium cobalt composite oxide (for example, LiCoO2), and lithium manganese composite oxide (for example, LiMn2O4); and phosphates containing lithium and a transition metal element as constituent metal elements, such as lithium manganese phosphate (LiMnPO4) and lithium iron phosphate (LiFePO4).
The positive electrode active material is not particularly limited, but, for example, a lithium transition metal composite oxide powder which is substantially formed of secondary particles having a particle size of 1 μm to 25 μm (typically, 2 μm to 10 μm; for example, 6 μm to 10 μm) can be preferably used as the positive electrode active material, in which the particle size corresponds to a cumulative value of 50% (median size: D50) in a volume particle size distribution obtained using a general laser diffraction particle size distribution analyzer. In this specification, unless specified otherwise, “particle size” refers to a median size in a volume particle size distribution obtained using a general laser diffraction particle size distribution analyzer.
The positive electrode active material layer 54 may further contain components other than the positive electrode active material as the above-described major component, for example, a conductive material or a binder. As the conductive material, for example, a carbon material such as carbon black (for example, acetylene black (AB)) or graphite may be preferably used. As the binder, for example, polyvinylidene fluoride (PVdF) may be used.
The negative electrode active material layer 64 contains at least a negative electrode active material. As the negative electrode active material, for example, a carbon material such as graphite, hard carbon, or soft carbon may be used. The negative electrode active material layer 64 may further contain components other than the active material, for example, a binder or a thickener. As the binder, for example, styrene-butadiene rubber (SBR) may be used. As the thickener, for example, carboxymethyl cellulose (CMC) may be used.
Examples of the separator 70 include a porous sheet (film) formed of a resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, or polyamide. The porous sheet may have a single-layer structure or a laminate structure including two or more layers (for example, a three-layer structure in which a PE layer is laminated on both surfaces of a PP layer).
In the nonaqueous electrolytic solution, typically, an organic solvent (nonaqueous solvent) may contain a predetermined supporting electrolyte and predetermined additives.
As the nonaqueous solvent, various organic solvents which can be used in an electrolyte of a general lithium ion secondary battery 100, for example, carbonates, ethers, esters, nitriles, sulfones, and lactones can be used without any limitation. Specific examples of the nonaqueous solvent include ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC). Among these nonaqueous solvents, one kind can be used alone, or two or more kinds can be appropriately used in combination.
Alternatively, fluorine-based solvents, for example, fluorinated carbonates such as monofluoroethylene carbonate (MFEC), difluoroethylene carbonate (DFEC), and trifluoro dimethyl carbonate (TFDMC) can be preferably used. For example, a mixed solvent containing MFEC and TFDMC at a volume ratio of 1:2 to 2:1 (for example, 1:1) has high oxidation resistance and thus can be preferably used in combination with a high-potential electrode.
As the supporting electrolyte, for example, a lithium salt such as LiPF6, LiBF4, or LiClO4 can be preferably used. Among these supporting electrolytes, for example, LiPF6 is particularly preferably used. The concentration of the supporting electrolyte is preferably 0.7 mol/L to 1.3 mol/L and is more preferably about 1.0 mol/L.
The nonaqueous electrolyte may further contain optional components other than the nonaqueous solvent and the supporting electrolyte within a range where the effects of the invention do not deteriorate. These optional components are used for one or two or more of the purposes including: improvement of battery output performance; improvement of storability (prevention of a decrease in capacity during storage); and improvement of initial charge-discharge efficiency. Examples of the optional components other than LiBOB include a gas producing agent such as biphenyl (BP) or cyclohexylbenzene (CHB); and various additives, for example, a film forming agent such as an oxalato complex compound containing a boron atom and/or a phosphorus atom, vinylene carbonate (VC), or fluoroethylene carbonate (FEC), a dispersant, and a thickener.
Next, a method of manufacturing the lithium ion secondary battery 100 according to the embodiment will be described.
Next, the process proceeds to a step of preparing the positive electrode 50 and the negative electrode 60 which constitute the electrode body (S102). The manufacturing step S102 will be described below in detail.
First, the positive electrode 50 will be described. The above-described positive electrode active material (for example, LiNi0.5Mn1.5O4 which is the high-potential positive electrode active material), and other optional materials (for example, the binder and the conductive material) are dispersed in an appropriate solvent (when PVdF is used as the binder, N-methyl-2-pyrrolidone (NMP) is preferable) to prepare a paste (slurry) composition. Next, an appropriate amount of the composition is applied to a surface of the positive electrode current collector 52 and then is dried to remove the solvent. As a result, the positive electrode active material layer 54 having desired characteristics can be formed on the positive electrode current collector 52, and thus the positive electrode 50 can be formed. In addition, by optionally performing an appropriate pressing treatment, the characteristics (for example, the average thickness, the active material density, or the porosity of the active material layer) of the positive electrode active material layer 54 can be adjusted.
Next, the negative electrode 60 will be described. The negative electrode 60 can be manufactured, for example, using the same method as in the positive electrode 50. That is, the negative electrode active material and other optional materials are dispersed in an appropriate solvent (for example, ion exchange water) to prepare a paste-like (slurry-like) composition. Next, an appropriate amount of the composition is applied to a surface of the negative electrode current collector 62 and then is dried to remove the solvent. As a result, the negative electrode can be formed. In addition, by optionally performing an appropriate pressing treatment, the characteristics (for example, the average thickness, the active material density, or the porosity of the active material layer) of the negative electrode active material layer 64 can be adjusted.
Referring to the manufacturing steps of
After the formation of the electrode body (S103), the process proceeds to a step of constructing the battery (S104). Here, the battery is constructed using the battery case 30 and the electrode body (for example, the wound electrode body 20) described above. The wound electrode body 20 is accommodated in the battery case 30 to construct the lithium ion secondary battery 100. After the construction of the battery, the process proceeds to a step of reducing the internal pressure of the battery case 30 (S105). Next, after the reduction in the internal pressure, the process proceeds to a step of injecting the nonaqueous electrolytic solution in a state where the battery case 30 is sealed (S106). After completion of the injection, the process proceeds to a step of sealing the battery case 30 with the lid 34 (S107). In the pressure reduction step (S105), the internal pressure of the battery case 30 is reduced by providing the battery case 30 in a large closed space and reducing the internal pressure of the closed space. At this time, a vacuum degree as a pressure reduction condition is 1 kPa·abs to 40 kPa·abs. The lower limit of the vacuum degree is very close to 0; however, when the vacuum degree is lower than 1 kPa·abs, the internal pressure of the battery system is excessively low, and the electrolytic solution is boiled. The minimum vacuum degree is 40 kPa·abs or lower. When the vacuum degree is 40 kPa·abs or higher, pressure reduction is insufficient. Therefore, the electrolytic solution cannot spread over the entire region of the electrode body, and battery characteristics cannot be stabilized. The vacuum degree is more preferably 25 kPa·abs or lower and still more preferably 15 kPa·abs or lower. In the liquid injection step (S106), in a state where the internal pressure of the battery case 30 is reduced in the pressure reduction step (S105), a nonaqueous electrolytic solution containing LiBOB is injected into the battery case 30 using a liquid injecting device provided inside the closed space. Next, the pressure-reduced state is released, and the battery case 30 is sealed with the lid 34. As a result, a square battery is constructed.
As described above, all the operations after the pressure reduction step (S105) are performed in the closed space. Therefore, the nonaqueous electrolytic solution is impregnated into both ends of the electrode body and can spread over the entire region of the electrode body. Thus, the resistance value at the center of the electrode body can be increased, and battery characteristics can be stabilized. The reduction in the internal pressure and the liquid injection can be performed using a method other than the above-described method. Specifically, a method may be adopted including: bringing a container, which is filled with the electrolytic solution, into contact with a liquid injection hole of the constructed battery cell; and reducing the internal pressure of the container.
In the above-described method of manufacturing the lithium ion secondary battery 100 according to the embodiment, the nonaqueous electrolytic solution is injected after the reduction in the internal pressure of the battery case 30. Therefore, the nonaqueous electrolytic solution can spread over the entire region of the electrode body. Thus, the resistance value at the center of the electrode body can be increased, and battery characteristics can be stabilized. Even in the lithium ion secondary battery 100 in which SBR is used as the binder of the negative electrode and in which LiBOB is added to the nonaqueous electrolytic solution, a variation in resistance value can be reduced, and battery characteristics can be made to be uniform. Accordingly, in the lithium ion secondary battery 100, safety and reliability as a battery can be secured.
In the method of manufacturing the lithium ion secondary battery 100 according to the embodiment, the electrode body is formed after the formation of the battery case. However, the battery case may be formed after the formation of the electrode body. That is, the manufacturing steps S102 and S103 may be performed before the manufacturing step S101.
The lithium ion secondary battery 100 disclosed herein can be used in various applications. For example, the lithium ion secondary battery 100 can be preferably used as a driving power supply mounted in a vehicle such as a plug-in hybrid vehicle (PHV), a hybrid vehicle (HV), or an electric vehicle (EV).
Hereinafter, test examples relating to the invention will be described. However, the descriptions of these test examples are not intended to limit the technical scope of the invention.
In order to prepare a positive electrode mixture, a layered positive electrode active material, acetylene black (conductive material), and PVdF (binder) were mixed with each other such that a weight ratio thereof was 89:8:3. The obtained mixture was dissolved in NMP as a solvent to prepare a slurry composition. The layered positive electrode active material used herein was LiNi0.5Mn1.5O4 and had an average particle size of 13 μm. This positive electrode mixture slurry was applied to aluminum foil (positive electrode current collector) having a thickness of 15 μm, was dried to form a positive electrode active material layer thereon, and was roll-pressed. As a result, a positive electrode was manufactured.
In order to prepare a negative electrode mixture, graphite (negative electrode active material; average particle size: 20 μm, graphitization degree≧0.9), CMC (thickener), and SBR (binder) were mixed with each other such that a weight ratio thereof was 98:1:1. The obtained mixture was dissolved in water as a solvent to prepare a slurry. This negative electrode mixture slurry was applied to copper foil (negative electrode current collector) having a thickness of 10 μm, was dried to form a negative electrode active material layer thereon, and was roll-pressed. As a result, a negative electrode was manufactured.
LiBOB as an additive was added to a mixed solvent containing EC, EMC, and DMC at a volume ratio of 3:4:3 to prepare a nonaqueous electrolytic solution. The content of LiBOB was adjusted to be 1 wt % with respect to 100 wt % of the content of the negative electrode active material.
A separator (porous PE/PP/PE three-layer sheet) having an appropriate size was cut out and was impregnated with the nonaqueous electrolytic solution. The positive electrode and the negative electrode were laminated with the separator interposed therebetween to obtain a laminate, and this laminate was wound. As a result, an electrode body was formed. The electrode body was accommodated in the battery case, the internal pressure of a battery case was reduced, the nonaqueous electrolytic solution was injected into the battery case, and the battery case was sealed with a lid. As a result, a square battery was constructed. During the reduction in the internal pressure, the vacuum degree was 10 kPa·abs.
A square battery was constructed using the same method as in Example 1, except that the internal pressure of the battery case was reduced after the injection of the nonaqueous electrolytic solution.
A square battery was constructed using the same method as in Example 1, except that no additives including LiBOB were used.
A square battery was constructed using the same method as in Example 1, except that: the internal pressure of the battery case was reduced after the injection of the nonaqueous electrolytic solution; and no additives including LiBOB were used.
The state of charge (SOC) of each of the battery cells according to Examples 1 to 4 was adjusted to 80%, and the battery cell was stored at an environmental temperature of 60° C. for 3 days.
After the conditioning treatment, a durability test was performed on each of the battery cells according to the examples using two methods which were different from each other regarding whether or not an additive was added. The details will be described below. In the two tests, the limit current value was determined by calculating a current value, at which the measured capacity was 96% or lower with respect to the initial capacity, and setting a current value before the calculated current value as a limit current value.
First, the initial capacity was measured. Next, the battery was discharged for 10 minutes, was charged at 55 A for 5 seconds, was discharged for 10 minutes, and was discharged at 55 A for 5 seconds. Next, the capacity was measured. Next, the battery was discharged for 10 minutes, was charged at 60 A for 5 seconds, was discharged for 10 minutes, and was discharged at 60 A for 5 seconds. Next, the capacity was measured, and then the charging-discharging current was increased at intervals of 5 A. These operations were repeated until the measured capacity reached 96% or lower with respect to the initial capacity (limit value: 90 A). The durability test was performed 30 times for each of the examples.
First, the initial capacity was measured. Next, the battery was discharged for 10 minutes, was charged at 20 A for 5 seconds, was discharged for 10 minutes, and was discharged at 20 A for 5 seconds. Next, the capacity was measured. Next, the battery was discharged for 10 minutes, was charged at 25 A for 5 seconds, was discharged for 10 minutes, and was discharged at 25 A for 5 seconds. Next, the capacity was measured, and then the charging-discharging current was increased at intervals of 5 A. These operations were repeated until the measured capacity reached 96% or lower with respect to the initial capacity (limit value: 60 A). The durability test was performed 30 times for each of the examples. Table 1 of
As shown in Table 1 of
A square battery was constructed using the same method as in Example 1, except that the full width at half maximum β of a diffraction peak of a (003) plane of the positive electrode active material was set as 0.048.
A square battery was constructed using the same method as in Example 1, except that the full width at half maximum β of a diffraction peak of a (003) plane of the positive electrode active material was set as 0.055.
A square battery was constructed using the same method as in Example 1, except that the full width at half maximum β of a diffraction peak of a (003) plane of the positive electrode active material was set as 0.086.
A square battery was constructed using the same method as in Example 1, except that the full width at half maximum β of a diffraction peak of a (003) plane of the positive electrode active material was set as 0.097.
A square battery was constructed using the same method as in Example 1, except that the full width at half maximum β of a diffraction peak of a (003) plane of the positive electrode active material was set as 0.114.
A square battery was constructed using the same method as in Example 1, except that the full width at half maximum β of a diffraction peak of a (003) plane of the positive electrode active material was set as 0.125.
As in the case of the battery cells according to Examples 1 to 4, the state of charge (SOC) of each of the battery cells according to Examples 5 to 10 was adjusted to 80%, and the battery cell was stored at an environmental temperature of 60° C. for 3 days.
The durability test was performed on each of the battery cells according to the examples using the same method as in Example 1. Table 2 of
As shown in Table 2 of
Hereinabove, the invention has been described in detail, but the above-described embodiments and examples are merely exemplary. The invention disclosed herein includes various modifications and alternations of the above-described specific examples.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2014-259619 | Dec 2014 | JP | national |
