Patent Application
20200020980
This application is based on and claims priority to Chinese Patent Application No. 201710486002.X filed on Jun. 23, 2017, which is incorporated herein by reference in its entirety.
The present application relates to the field of batteries, and more particularly, to a lithium iron phosphate battery.
Lithium-iron secondary batteries are widely used in electric vehicles and consumer electronics because of their high energy density, high output power, long cycle life and small environmental pollution. Lithium iron phosphate is one of the most commonly used positive materials in power battery, due to its high cycle life, good safety and low price and other characteristics. The disadvantage of lithium iron phosphate batteries is that their energy density is low. To improve the energy density, one way is to increase the capacity per gram of the positive material and negative material, and the other way is to increase the press density of the positive electrode film and negative electrode film. However, it is difficult to diffuse the lithium irons after increasing the press density, and the wettability of the electrode plate in the electrolyte is deteriorated, so that the cycle life of the lithium iron phosphate battery is reduced. Therefore, there is a need to improve the performance of lithium iron phosphate batteries having a high press density electrode plate system from the perspective of electrolyte.
In view of the problems as mentioned in the background art, it is an object of the present application to provide a lithium iron phosphate battery capable of solving the problem of poor wettability of an electrode plate having high press density in an electrolyte, to improve the low-temperature performance and the cycle performance at normal temperature and high temperature of a lithium iron phosphate battery, and to effectively prolong the service life of lithium iron phosphate battery.
In order to achieve the above objects, the present application provides a lithium iron phosphate battery comprising: a positive electrode plate comprising a positive current collector and a positive electrode film provided on the surface of the positive current collector; a negative electrode plate comprising a negative current collector and a negative electrode film provided on the surface of the negative current collector; a separator provided between the positive electrode plate and the negative electrode plate; and an electrolyte comprising an organic solvent, a lithium salt and an electrolyte additive. The positive active material in the positive electrode plate comprises lithium iron phosphate; and the negative active material in the negative electrode plate comprises graphite. The electrolyte additive comprises a cyclic carbonate containing a double bond and a cyclic disulfonate represented by the Formula I; in Formula I, A and B are each independently selected from an alkylene group having 1 to 3 carbon atoms.
Compared with the prior art, the present application has the following advantages: the present application can solve the problem that the electrode plate with high press density has poor wettability in the electrolyte, so that the low temperature performance and the cycle performance at normal temperature and high temperature of the lithium iron phosphate battery are improved, and the service life of the lithium iron phosphate battery is prolonged effectively.
The lithium iron phosphate battery according to the present application will be described in details below.
The lithium iron phosphate battery according to the present application comprises a positive electrode plate comprising a positive current collector and a positive electrode film provided on the surface of the positive current collector; a negative electrode plate comprising a negative current collector and a negative electrode film provided on the surface of the negative current collector; a separator provided between the positive electrode plate and the negative electrode plate; and an electrolyte comprising an organic solvent, a lithium salt and an electrolyte additive. The positive active material in the positive electrode plate comprises lithium iron phosphate; and the negative active material in the negative electrode plate comprises graphite. The electrolyte additive comprises a cyclic carbonate containing a double bond and a cyclic disulfonate represented by the formula I; in formula I, A and B are each independently selected from an alkylene group having 1 to 3 carbon atoms.
In the lithium iron phosphate battery according to the present application, the cyclic carbonate containing a double bond can improve the capacity retention rate of the lithium iron phosphate battery in the high temperature environment, but the unavoidable problem is that the SEI film impedance is increased, which will affect the use of lithium iron phosphate battery in the low temperature environment. The cyclic disulfonate shown in Formula I can reduce the SEI film impedance. The combination of the cyclic carbonate containing a double bond and the cyclic disulfonate used in the electrolyte can improve the low temperature performance and the cycle performance at normal temperature and high temperature of the lithium iron phosphate battery and effectively prolong the service life of the lithium iron phosphate battery.
In the lithium iron phosphate battery according to the present application, the cyclic carbonate containing a double bond may be selected from one or both of vinylene carbonate (VC) and vinyl ethylene carbonate (VEC).
In the lithium iron phosphate battery according to the present application, in the electrolyte the content of the cyclic carbonate containing a double bond may be 0.5% to 4% by mass. If the content is low, SEI film will be unstable and the cycle performance at high temperature of the lithium iron phosphate battery will be deteriorated; if the content is high, it will result in too thick SEI film and Li plating will occur after the lithium iron phosphate battery cycles, which will lead to cycle capacity diving. Preferably, the content of the cyclic carbonate containing a double bond is 0.5% to 3% by mass.
In the lithium iron phosphate battery according to the present application, the cyclic disulfonate may be selected from one or more of methylene methane disulfonate (MMDS), ethylene ethane disulfonate and propylene methane disulfonate.
In the lithium iron phosphate battery according to the present application, the content of the cyclic disulfonate in the electrolyte may be 0.2% to 2% by mass. If the content is too low, the effect on the improvement of the SEI film impedance is very little, and if the content is too high, such cyclic disulfonate is easy to crystallize and precipitate in the electrolyte. At the same time, because of its poor high-temperature stability, the high addition amount is more likely to deteriorate the electrochemical performance of the lithium iron phosphate battery. Preferably, the cyclic disulfonate is present in an amount of from 0.2% to 1% by mass.
In the lithium iron phosphate battery according to the present application, the charge cut-off voltage of the lithium iron phosphate battery may not exceed 3.8 V, and preferably, the charge cut-off voltage of the lithium iron phosphate battery may not exceed 3.6 V. This is because that the performance of vinylene carbonate (VC), vinyl ethylene carbonate (VEC) and cyclic disulfonate at high voltage is unstable, easily decomposed by oxidation, and the probability of side reaction in the electrolyte is higher. In addition, even in the case that the added amount is very small the side reaction product will deteriorate the performance of the SEI film formed on the surface of the negative electrode, so that the charging cut-off voltage should not be too high. Under such conditions, the SEI film of the electrolyte according to the present application is better and the impedance is smaller, and the wettability of the electrode plate in the electrolyte is excellent. Thus the low temperature performance and the cycle performance at normal temperature and high temperature of the lithium iron phosphate battery containing the high-press density electrode plate can be more remarkably improved.
In the lithium iron phosphate battery according to the present application, the press density of the negative electrode plate may be 1.4 g/cm3 to 1.8 g/cm3. If the press density is too low, the contact resistance between the powder particles will increase, and the overall energy density of the lithium iron phosphate battery will be too low; if the press density is too high, the electrode plate will be easily crushed and the cycle performance of the lithium iron phosphate battery will be deteriorated.
In the lithium iron phosphate battery according to the present application, the press density of the positive electrode plate may be 2 g/cm3 to 2.5 g/cm3. If the press density is too low, the contact resistance between the powder particles will increase, and the overall energy density of the lithium iron phosphate battery will be too low; if the press density is too high, the electrode plate will be easily crushed and the cycle performance of the lithium iron phosphate battery will be deteriorated.
On the surface of the negative electrode plate having a high press density, the electrolyte according to the present application can form a denser and more stable SEI film than the conventional electrolyte, and the SEI film has small impedance and the electrode plate has good wettability in the electrolyte. Thus in the lithium iron phosphate battery containing a negative electrode plate having a high press density, the electrolyte according to the present application can make the lithium iron phosphate battery have good low temperature performance and good cycle performance at normal temperature and high temperature.
In the lithium iron phosphate battery according to the present application, the electrolyte has a conductivity of 8 mS/cm to 11 mS/cm at 25° C. If the conductivity is too low, the dynamic performance of the electrolyte will be poor, and the lithium iron phosphate battery's polarization is great, affecting the cycle performance at normal temperature and low temperature performance; if the conductivity is too high, the thermal stability of the electrolyte will be poor, resulting in the lithium iron phosphate battery having poor cycle performance at high temperature.
In the lithium iron phosphate battery according to the present application, the viscosity of the electrolyte at 25° C. may be ranging from 2 mPa·s to 4 mPa·s. If the viscosity is too high, on one hand the dynamic performance of the electrolyte will be poor, on the other hand the ability of the electrolyte for infiltrating the electrode plate will decline, which will deteriorate the comprehensive performance of the lithium iron phosphate battery; if the viscosity is too low, the thermal stability of the electrolyte will be poor, resulting in the lithium iron phosphate battery having poor cycle performance at high temperature.
In the lithium iron phosphate battery according to the present application, the type of the lithium salt is not limited and can be selected according to the actual demands. Preferably, the lithium salt may be selected from one or more of LiPF6, LiBF4, LiBOB, LiAsF6, LiCF3SO3, LiFSI and LiTFSI.
In the lithium iron phosphate battery according to the present application, the organic solvent may be selected from one or more of ethylene carbonate, propylene carbonate, butylene carbonate, pentylene carbonate, 1,2-butylene glycol carbonate, 2,3-butylene glycol carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl ethyl carbonate, methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate and ethyl butyrate.
In the lithium iron phosphate battery according to the present application, it is preferable that the organic solvent comprises a mixed solvent of a cyclic carbonate and a chain carbonate. Such mixed solvent can be conducive to the preparation of an electrolyte having better conductivity, viscosity and other comprehensive performance.
In the lithium iron phosphate battery according to the present application, it is further preferable that the organic solvent comprises a chain carbonate having a methyl group. Still more preferably, the content of the chain carbonate having a methyl group may be 40% or more by mass based on the total mass of the organic solvent of the electrolyte. The chain carbonate having a methyl group is preferably selected from one or two of dimethyl carbonate and methyl ethyl carbonate. Chain carbonate having a methyl group helps to further improve the anti-overcharge performance of the electrolyte. When the content of the chain carbonate having a methyl group is 40% or more by mass, the conductivity, viscosity and other comprehensive performance of the electrolyte are better.
In the lithium iron phosphate battery according to the present application, it is preferable that the organic solvent further comprises a carboxylic acid ester and the content of the carboxylic acid ester is less than 30% by mass based on the total mass of the organic solvent of the electrolyte. The addition of the carboxylic acid ester can further improve the conductivity and viscosity of the electrolyte, improve the wettability of the electrode plate having high press density in the electrolyte, and further improve the low temperature performance and other electrochemical performance of the lithium iron phosphate battery. However, if the content of the carboxylic acid ester is too high, it will affect the stability at high temperature of the electrolyte and deteriorate the cycle performance at high temperature of the lithium iron phosphate battery. Meanwhile, due to that the oxidation potential of the carboxylic acid ester is lower than that of the cyclic carbonate and the chain carbonate, adding too much carboxylic acid ester may increase the gas production of lithium iron phosphate battery.
The present application will be described in further detail with reference to the following examples, in order to make the object of the present application, the technical solution and the advantageous technical effects clearer. It should be understood that the embodiments described in this specification are merely for the purpose of explaining the disclosure and are not intended to limit the scope of the disclosure. The formulation of the examples, the proportions, and the like may have no substantial effect on the results.
Examples 1-15 and Comparative Examples 1-13 were prepared according to the following methods.
1. Preparation of Positive Electrode Plate
The positive active material lithium iron phosphate, the binder PVDF and the conductive agent acetylene black were mixed in a mass ratio of 98:1:1. Then. N-methylpyrrolidone was added and the mixture was stirred uniformly under a vacuum stirrer to obtain a positive electrode paste. The positive electrode paste was evenly coated on the aluminum foil, and the aluminum foil was dried at room temperature and then transferred to a blast drying oven at 120° C. for 1 hour. Then, the positive electrode plate was obtained after cold pressing and slitting.
2. Preparation of Negative Electrode Plate
The negative active material graphite, the conductive agent acetylene black, the thickening agent sodium carboxymethyl cellulose (CMC) solution and the binder styrene-butadiene rubber emulsion were mixed in the mass ratio of 97:1:1:1. Then deionized water was added and the mixture was stirred uniformly under a vacuum stirrer to obtain a negative electrode paste. The negative electrode paste was evenly coated on the copper foil, and the copper foil was dried at room temperature and then transferred to a blast drying oven at 120° C. for 1 hour. Then, the negative electrode plate was obtained after cold pressing and slitting.
3. Preparation of Electrolyte
The organic solvent was a mixed organic solvent of ethylene carbonate (EC), methyl ethyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC) and methyl propionate (MP). Lithium salt was LiPF6, wherein the content of LiPF6 was 12.5% by mass, based on the total mass of the electrolyte. Finally the cyclic carbonate containing a double bond and cyclic disulfonate were added. The mass content of each component in the electrolyte was shown in Table 1. By adjusting the adding proportion of each component, the conductivity and viscosity of the electrolyte can be adjusted correspondingly.
4. Preparation of Lithium Iron Phosphate Battery
The positive electrode plate, the negative electrode plate and the separator were wound to obtain a battery core, and the battery core was put into the packaging shell, the electrolyte was injected and sealed. The lithium iron phosphate battery was obtained by the steps of standing, pressing, forming, degassing and the like.
Next to explain the test of lithium iron phosphate battery.
(1) Test of Discharge Capacity at Low Temperature
At 25° C., the lithium iron phosphate battery was firstly discharged to 2.0V with a current of 1 C; and then charged to 3.6V with a constant current of 1 C, and then charged to a current of 0.05 C with a constant voltage, wherein the charge capacity was represented by CC; and then the furnace temperature was adjusted to −10° C., and the battery was discharged to 2.0V with a constant current of 1 C, wherein the discharge capacity was represented by CDT. The ratio of discharge capacity to charge capacity is a discharge capacity retention rate.
The discharge capacity retention rate (%) of lithium iron phosphate battery at −10° C.=CDT/CC×100%.
(2) Cycle Test at Normal Temperature
At 25° C., the lithium iron phosphate battery was firstly discharged to 2.0V with a current of 1 C, and then was subjected to the cycle test. The battery was charged to 3.6V with a constant current of 1 C, and then charged to the current of 0.05 C with a constant voltage, and then discharged to 2.0V with a constant current of 1 C. Charging/discharging cycles were done in such way. Then, the cycle capacity retention rate of the lithium iron phosphate battery of 1000th cycle at 25° C. was calculated.
Cycle capacity retention rate of the lithium iron phosphate battery (%) of 1000th cycle at 25° C.=discharge capacity of the 1000th cycle/discharge capacity at the first cycle×100%.
(3) Cycle Test at High Temperature
At 25° C., the lithium iron phosphate battery was first discharged to 2.0V with a current of 1 C, and then was subjected to the cycle test. The oven was heated to 60° C., and then the battery was charged to 3.6 V with a constant current of 1 C, and then charged to the current of 0.05 C with a constant voltage, and then discharged to 2.0V with a constant current of 1 C. Charging/discharging cycles were done in such way. Then, the cycle capacity retention rate of the lithium iron phosphate battery of 500th cycle at 60° C. was calculated.
Cycle capacity retention rate of the lithium iron phosphate battery (%) of 500th cycle at 60° C. discharge capacity of the 500th cycle/discharge capacity at the first cycle×100%.
As can be seen from Comparative Examples 1-3, if the press density of the positive electrode film and negative electrode film was improved, the performance of the lithium iron phosphate battery was rapidly decreased without adding the MMDS. However, in Examples 1 to 3, the press density of the positive electrode film and negative electrode film was improved, and the downtrend of the lithium iron phosphate battery performance was remarkably changed after the addition of MMDS into the electrolyte, and the cycle life of the lithium iron phosphate battery was prolonged. This shows that in the lithium iron phosphate battery system comprising high press density electrode plate, the low temperature performance and the cycle performance at normal temperature and high temperature of lithium iron phosphate battery can be improved by adjusting the ratio and the amount of the cyclic carbonate containing a double bond and the cyclic disulfonate.
In Comparative Example 7, when using a high-press density positive electrode film and negative electrode film, it was difficult to infiltrate the electrode plate with the electrolyte, resulting in a low capacity retention rate of the lithium iron phosphate battery after cycles at normal temperature and at high temperature, which will in turn affect the service life at normal temperature and high temperature. In Comparative Example 3 and Comparative Example 6, VC and VEC were added, respectively, which can significantly improve the cycle performance at normal temperature and high temperature of lithium iron phosphate battery, but the unavoidable problem was that the SEI film impedance was increased, which will affect the use of the lithium iron phosphate battery in low temperature environment. As can be seen from Comparative Examples 3-5, the high-temperature cycle performance of the lithium iron phosphate battery was improved with the increase of the VC content, but the low temperature performance and the cycle performance at normal temperature were deteriorated due to that the impedance of the solid electrolyte interface film (SEI film) was increased. In Examples 4 and 8-11, 0.5% of the cyclic disulfonate was added into the electrolyte, with the synergistic effect of the cyclic disulfonate and the cyclic carbonate containing a double bond, the SEI film impedance was effectively reduced, so that the low temperature and the cycle performance at normal temperature of the electrode plate having a high press density have been significantly improved. In Comparative Example 8, the addition amount of VC was too low and the SEI film was unstable, and the improvement of the high-temperature cycle performance of the lithium iron phosphate battery was not obvious. In Comparative Example 9, the content of VC was too high, and even the addition of MMDS cannot suppress the increase of the SEI film resistance, thus the performance of the lithium iron phosphate battery was deteriorated.
In Examples 4-6, as the added amount of MMDS was increased, the normal-temperature cycle performance and the low temperature performance of the lithium iron phosphate battery were significantly improved; however, the effect of improving the high-temperature cycle performance was poor. In Comparative Example 10, the added amount of MMDS was too high, and it was easily precipitated in the electrolyte to affect the quality of the electrolyte, meanwhile it was not consumed at the beginning of the cycle, then it was decomposed into by-products due to its own instability, which will deteriorate the performance of the lithium iron phosphate battery instead, especially for high-temperature cycle performance.
As can be seen from Examples 12-15, it is possible to improve the viscosity and the conductivity of the electrolyte by adjusting the composition of the organic solvent, thereby improving the low temperature performance and the cycle performance at normal temperature and high temperature of the lithium iron phosphate battery. For example, in Examples 12-14, the content of the cyclic carbonate was controlled to 30%, and the total amount of the chain carbonate was 70%. Since only the chain carbonic acid esters having a methyl group EMC and DMC were used in Example 14 without adding DEC, so the conductivity, viscosity and other comprehensive performance of the electrolyte were better, and then the comprehensive performance of the lithium iron phosphate battery was also better. In Example 15, a carboxylic acid ester was further added as an organic solvent which could further improve the conductivity and viscosity of the electrolyte and improve the wettability of the high press density electrode plate in the electrolyte, but it would inevitably affect the high-temperature cycle performance of the lithium iron phosphate battery. In Comparative Examples 11 and12, the conductivity of the electrolyte was too low, and the viscosity was too high, resulting in that the electrode plate has poor wettability in the electrolyte, which will deteriorate the power performance of the lithium iron phosphate battery and deteriorate the low-temperature discharge capacity and normal-temperature cycle performance. In Comparative Example 13, the content of the chain carbonate having a methyl group was low (only 20%), and the content of the carboxylic acid ester was high (up to 50%), which will result in that the conductivity of the electrolyte is too high and that the viscosity is too low. The stability of the electrolyte will be worse, which will seriously deteriorate the high-temperature cycle performance of the lithium iron phosphate battery. So the conductivity and viscosity of the electrolyte also need to be controlled in a certain range in order to make that the low temperature performance and the cycle performance at normal temperature and high temperature of lithium iron phosphate battery have been improved.
In summary, the present application can be used to improve the performance of a lithium iron phosphate battery comprising a high-press density electrode plate by adjusting the amount of the cyclic carbonate containing a double bond and the cyclic disulfonate and adjusting the composition of the organic solvent system. Then an electrolyte having better comprehensive performance can be obtained, and the low temperature performance and the cycle performance at normal temperature and high temperature of the lithium iron phosphate battery have been improved.
It will be apparent to those skilled in the art that the present application may be modified and varied in accordance with the above teachings. Accordingly, the present application is not limited to the specific embodiments disclosed and described above, and modifications and variations of the present application are intended to be included within the scope of the claims of the present application. In addition, although some specific terminology is used in this specification, these terms are for convenience of illustration only and are not intended to limit the present application in any way.
| Number | Date | Country | Kind |
|---|---|---|---|
| 201710486002.X | Jun 2017 | CN | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/CN2017/098784 | 8/24/2017 | WO | 00 |
