Electrolyte for lithium manganese iron phosphate battery and application of electrolyte

Abstract

The invention provides an electrolyte for a lithium iron manganese phosphate battery and application of the electrolyte. The electrolyte for the lithium iron manganese phosphate battery comprises electrolyte lithium salt, a polyfunctional group compound additive, a film-forming additive and an organic solvent. The polyfunctional group compound additive with a specific structure is added into the electrolyte, so that the high-voltage performance and the high-temperature storage performance of the battery can be improved, the dissolution of manganese in the positive electrode material can be inhibited, and the cycle performance of the positive electrode material is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery materials, and relates to an electrolyte for a lithium iron manganese phosphate battery and its application. Background Art

[0002] With the development of battery energy storage, higher requirements have been put forward for the volumetric energy density of lithium-ion batteries. Lithium iron manganese phosphate (LiMn x Fe 1-x PO4, 0 < x < 1) with an olivine structure has the characteristic of a high working voltage and is a promising new cathode material for the next generation. However, it also poses new challenges to battery materials.

[0003] Under high-voltage conditions, the oxidation reaction between the cathode and the electrolyte intensifies. In addition, metal ions in the high-valent state will also migrate under the action of an electric field and undergo a reduction reaction at the negative electrode, resulting in metal precipitation and irreversible capacity loss, which will have a serious impact on the high-temperature storage and cycling of the battery. As an important component of a lithium-ion battery, the electrolyte has become one of the most important factors affecting the electrical performance of a lithium iron manganese phosphate battery.

[0004] CN109473721A discloses a high-voltage electrolyte additive, and the additive is a nitrile compound grafted on an unsaturated five-membered heterocycle, which can form a dense and stable interfacial film on the surface of the cathode material, effectively inhibiting the oxidation decomposition of the electrolyte, thereby improving the cycling performance and high-temperature storage performance of the battery.

[0005] CN104979589A discloses a high-voltage electrolyte and a lithium-ion battery using the electrolyte. The high-voltage electrolyte contains a carboxylic ester solvent for improving the electrode / electrolyte interface. By combining with various additives such as LiBOB, fluoroethylene carbonate, and propionitrile, excellent cycling performance and high-temperature storage performance can be ensured for high-voltage batteries.

[0006] When the high-voltage electrolytes and additives described in the above solutions are applied to lithium iron manganese phosphate batteries, the improvement of the performance of lithium iron manganese phosphate batteries is not obvious. Therefore, there is an urgent need to develop an electrolyte that can improve the cycling, high-temperature, and high-voltage performance of lithium iron manganese phosphate batteries. Summary of the Invention

[0007] The purpose of the present invention is to provide an electrolyte for a lithium iron manganese phosphate battery and its application. The present invention adds a multi-functional group compound additive with a specific structure to the electrolyte, which can not only improve the high-voltage performance and high-temperature storage performance of the battery, but also inhibit the dissolution of manganese in the cathode material and enhance the cycling performance of the cathode material.

[0008] To achieve the purpose of this invention, the following technical solutions are adopted:

[0009] In a first aspect, the present invention provides an electrolyte for a lithium iron manganese phosphate battery. The electrolyte for the lithium iron manganese phosphate battery includes an electrolyte lithium salt, a multi-functional group compound additive, a film-forming additive, and an organic solvent. The structural formula of the multi-functional group compound additive is shown in the following formula I:

[0010]

[0011] The present invention adds a multi-functional group compound additive with a specific structure to the electrolyte. The benzene ring in the multi-functional group compound additive can increase the battery voltage and prevent the continuous side reaction between the electrolyte and the positive electrode material at a high voltage above 4.5V. The sulfonate group in the multi-functional group compound additive can be reduced to an interface film containing a sulfur-oxygen structure on the surface of the negative electrode, reducing the Li + diffusion resistance and improving the cycling performance. The nitrogen atom in the amide group of the multi-functional group compound additive can form a coordination bond with Mn 2 + to inhibit the dissolution of Mn in the positive electrode material, reduce the concentration of free metal ions in the electrolyte. In addition, this group can form a protective layer on the surface of the positive electrode, reducing the oxidative decomposition of the electrolyte at a high voltage and improving the high-temperature stability, thereby improving the high-temperature storage and cycling performance.

[0012] Preferably, the electrolyte lithium salt includes any one or at least two combinations of lithium hexafluorophosphate, lithium difluorophosphate, or lithium bis(fluorosulfonyl)imide. Typical but non-limiting combinations include the combination of lithium hexafluorophosphate and lithium difluorophosphate, the combination of lithium difluorophosphate and lithium bis(fluorosulfonyl)imide, or the combination of lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide, etc.

[0013] Preferably, the mass concentration of the electrolyte lithium salt in the electrolyte for the lithium iron manganese phosphate battery is 10% to 15%, such as: 10%, 11%, 12%, 13%, 14%, or 15%, etc. It is not limited to the listed values, and other unlisted values within this numerical range are equally applicable.

[0014] Preferably, the mass concentration of the multi-functional group compound additive in the electrolyte for the lithium iron manganese phosphate battery is 1.5% to 4%, such as: 1.5%, 2%, 2.5%, 3%, 3.5%, or 4%, etc. It is not limited to the listed values, and other unlisted values within this numerical range are equally applicable.

[0015] Preferably, the film-forming additive includes any one or at least two combinations of vinylene carbonate, ethylene sulfate, or fluoroethylene carbonate. Typical but non-limiting combinations include the combination of vinylene carbonate and ethylene sulfate, the combination of ethylene sulfate and fluoroethylene carbonate, or the combination of vinylene carbonate and fluoroethylene carbonate, etc.

[0016] Preferably, the mass concentration of the film-forming additive in the electrolyte for the lithium iron manganese phosphate battery is 1% to 3%, for example: 1%, 1.5%, 2%, 2.5% or 3%, etc., not limited to the listed values, and other unlisted values within this numerical range are equally applicable.

[0017] Preferably, the organic solvent includes any one or a combination of at least two of ethylene carbonate, propylene carbonate, ethyl methyl carbonate or dimethyl carbonate. Typical but non-limiting combinations include a combination of ethylene carbonate, propylene carbonate and ethyl methyl carbonate, a combination of propylene carbonate, ethyl methyl carbonate and dimethyl carbonate, or a combination of ethylene carbonate, propylene carbonate and dimethyl carbonate, etc. Preferably, it is ethylene carbonate, propylene carbonate, ethyl methyl carbonate and dimethyl carbonate.

[0018] Preferably, the mass ratio of ethylene carbonate, propylene carbonate, ethyl methyl carbonate and dimethyl carbonate is (20 - 35):(5 - 10):(30 - 55):(20 - 25), for example: 20:5:30:20, 25:8:32:20, 30:10:40:25, 30:10:55:25 or 35:10:55:25, etc., not limited to the listed values, and other unlisted values within this numerical range are equally applicable.

[0019] Preferably, the mass fraction of the organic solvent in the electrolyte for the lithium iron manganese phosphate battery is 78% - 87.5%, for example: 78%, 80%, 82%, 85% or 87.5%, etc., not limited to the listed values, and other unlisted values within this numerical range are equally applicable.

[0020] In a second aspect, the present invention provides a lithium iron manganese phosphate battery, and the lithium iron manganese phosphate battery includes the electrolyte for the lithium iron manganese phosphate battery as described in the first aspect.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] (1) By adding a multi-functional group compound additive with a specific structure to the electrolyte in the present invention, not only can the high-voltage performance and high-temperature storage performance of the battery be improved, but also the dissolution of manganese in the positive electrode material can be inhibited, and the cycle performance of the positive electrode material can be enhanced.

[0023] (2) The lithium iron manganese phosphate battery prepared from the electrolyte of the present invention has a thermal thickness change rate of within 2.71% after being stored at 60 °C for 30 days, an internal resistance change rate of within 3.58%, a capacity retention rate of over 93.2%, a capacity recovery rate of over 96%, and a capacity retention rate of over 91% after 1000 cycles of 1C / 1C at 25 °C. By adjusting the addition amounts of the additives, the lithium iron manganese phosphate battery has a thermal thickness change rate of within 2.51% after being stored at 60 °C for 30 days, an internal resistance change rate of within 3.31%, a capacity retention rate of over 95.4%, a capacity recovery rate of over 98.2%, and a capacity retention rate of over 93.1% after 1000 cycles of 1C / 1C at 25 °C. Detailed Embodiments

[0024] The technical solution of the present invention will be further described below through specific embodiments. Those skilled in the art should understand that the embodiments are only for helping to understand the present invention and should not be regarded as specific limitations on the present invention.

[0025] Example 1

[0026] This example provides an electrolyte for a lithium iron manganese phosphate battery. The composition of the electrolyte for the lithium iron manganese phosphate battery is as follows:

[0027] The mass fraction of the organic solvent is 84%, the mass fraction of lithium hexafluorophosphate is 12%, the mass fraction of vinylene carbonate is 2.5%, and the mass fraction of the multi-functional group compound additive is 2.5%;

[0028] Among them, the organic solvent is a mixed organic solvent of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, and dimethyl carbonate, and the mass ratio of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, and dimethyl carbonate is 20:5:55:20.

[0029] Example 2

[0030] This example provides an electrolyte for a lithium iron manganese phosphate battery. The composition of the electrolyte for the lithium iron manganese phosphate battery is as follows:

[0031] The mass fraction of the organic solvent is 86.5%, the mass fraction of lithium bis(fluorosulfonyl)imide is 10%, the mass fraction of ethylene sulfate is 1.5%, and the mass fraction of the multi-functional group compound additive is 1.5%;

[0032] Among them, the organic solvent is a mixed organic solvent of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, and dimethyl carbonate, and the mass ratio of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, and dimethyl carbonate is 25:10:40:25.

[0033] Example 3

[0034] This example provides an electrolyte for a lithium iron manganese phosphate battery. The composition of the electrolyte for the lithium iron manganese phosphate battery is as follows:

[0035] The mass fraction of the organic solvent is 78.5%, the mass fraction of lithium difluorophosphate is 15%, the mass fraction of fluoroethylene carbonate is 3%, and the mass fraction of the multi-functional group compound additive is 4.0%;

[0036] Among them, the organic solvent is a mixed organic solvent of ethylene carbonate, propylene carbonate, ethyl methyl carbonate and dimethyl carbonate, and the mass ratio of ethylene carbonate, propylene carbonate, ethyl methyl carbonate and dimethyl carbonate is 30:10:35:25.

[0037] Example 4

[0038] This example provides an electrolyte for a lithium iron manganese phosphate battery. The composition of the electrolyte for the lithium iron manganese phosphate battery is as follows:

[0039] The mass fraction of the organic solvent is 85%, the mass fraction of lithium hexafluorophosphate is 12%, the mass fraction of vinylene carbonate is 2.5%, and the mass fraction of the multi-functional group compound additive is 1.5%;

[0040] Among them, the organic solvent is a mixed organic solvent of ethylene carbonate, propylene carbonate, ethyl methyl carbonate and dimethyl carbonate, and the mass ratio of ethylene carbonate, propylene carbonate, ethyl methyl carbonate and dimethyl carbonate is 20:5:55:20.

[0041] Example 5

[0042] The difference between this example and Example 1 is only that the mass fraction of the multi-functional group compound additive is 5.5%, the mass fraction of the organic solvent is 80%, and other conditions and parameters are exactly the same as those in Example 1.

[0043] Example 6

[0044] The difference between this example and Example 1 is only that the mass fraction of the multi-functional group compound additive is 0.5%, the mass fraction of the organic solvent is 85%, and other conditions and parameters are exactly the same as those in Example 1.

[0045] Example 7

[0046] The difference between this example and Example 1 is only that the mass fraction of vinylene carbonate is 3.5%, the mass fraction of the organic solvent is 83%, and other conditions and parameters are exactly the same as those in Example 1.

[0047] Example 8

[0048] The difference between this example and Example 1 is only that the mass fraction of vinylene carbonate is 0.5%, the mass fraction of the organic solvent is 86%, and other conditions and parameters are exactly the same as those in Example 1.

[0049] Comparative Example 1

[0050] The difference between this comparative example and Example 1 is only that the multi-functional group compound additive is not added. The mass fraction of the organic solvent is 85.5%, and other conditions and parameters are exactly the same as those in Example 1.

[0051] Comparative Example 2

[0052] The difference between this comparative example and Example 1 is only that vinylene carbonate is not added, the mass fraction of the organic solvent is 86.5%, and other conditions and parameters are exactly the same as those in Example 1.

[0053] Performance test:

[0054] (1) For the electrolyte samples prepared in Examples 1-8 and Comparative Examples 1-2, inject them into the LiMn 0.5 Fe 0.5 PO4 system lithium-ion battery. Charge the battery at a constant current of 1.0C and a constant voltage of 4.5V at 25°C, let it stand for 5 minutes, then discharge it at 0.1C to 2.5V. The discharged capacity is recorded as the initial capacity. Then charge it at a constant current of 1.0C and a constant voltage of 4.5V again, measure the initial thickness and initial internal resistance. Store the battery under the condition of 60°C ± 2°C, after open-circuit storage for 30 days, take out the battery, test the hot state thickness, test the internal resistance after standing at room temperature for 2 hours, then charge and discharge the battery core at 1.0C, test the remaining capacity and recovery capacity, and then calculate the thermal thickness change rate, internal resistance change rate, capacity retention rate, and capacity recovery rate.

[0055] (2) 1C charge and discharge cycle test at 25°C

[0056] Inject the electrolyte samples prepared in Examples 1-8 and Comparative Examples 1-2 into the LiMn 0.5 Fe 0.5 PO4 system lithium-ion battery. Charge and discharge at 1C at 25°C, and the voltage range is 2.5 - 4.5V. Calculate the capacity retention rate. The test results are shown in Table 1:

[0057] Table 1

[0058]

[0059]

[0060] As can be seen from Table 1, it can be obtained from Examples 1-8 that the lithium iron manganese phosphate battery prepared from the electrolyte for lithium iron manganese phosphate battery of the present invention has a thermal thickness change rate of within 2.71% after being stored at 60°C for 30 days, an internal resistance change rate of within 3.58%, a capacity retention rate of over 93.2%, a capacity recovery rate of over 96%, and a capacity retention rate of over 91% after 1000 cycles of 1C / 1C at 25°C. By adjusting the addition amounts of various additives, the lithium iron manganese phosphate battery has a thermal thickness change rate of within 2.51% after being stored at 60°C for 30 days, an internal resistance change rate of within 3.31%, a capacity retention rate of over 95.4%, a capacity recovery rate of over 98.2%, and a capacity retention rate of over 93.1% after 1000 cycles of 1C / 1C at 25°C.

[0061] From the comparison between Example 1 and Examples 5-6, it can be obtained that in the electrolyte for lithium iron manganese phosphate battery of the present invention, the mass concentration of the multi-functional group compound additive affects its performance. When the mass fraction of the multi-functional group compound additive is controlled within 1.5% - 4%, the performance of the electrolyte for lithium iron manganese phosphate battery is better. If the mass fraction of the multi-functional group compound additive is too high, it will affect the lithium ion transmission and deteriorate the performance. If the mass fraction of the multi-functional group compound additive is too low, its effect is not obvious, thus affecting the performance.

[0062] From the comparison between Example 1 and Examples 7-8, it can be obtained that in the electrolyte for lithium iron manganese phosphate battery of the present invention, the mass concentration of the film-forming additive affects its performance. When the mass fraction of the film-forming additive is controlled within 1% - 3%, the performance of the electrolyte for lithium iron manganese phosphate battery is better. If the mass fraction of the film-forming additive is too high, the film formed is too thick, deteriorating the electrical performance. If the mass fraction of the film-forming additive is too low, it is not sufficient to form a dense SEI film, deteriorating the electrical performance.

[0063] From the comparison between Example 1 and Comparative Example 1, it can be obtained that by adding a multi-functional group compound additive with a specific structure to the electrolyte of the present invention, not only can the high-voltage performance and high-temperature storage performance of the battery be improved, but also the dissolution of manganese in the positive electrode material can be inhibited, enhancing the cycle performance of the positive electrode material.

[0064] From the comparison between Example 1 and Comparative Example 2, it can be obtained that by adding a film-forming additive to the electrolyte of the present invention, a stable SEI film can be formed, thereby significantly enhancing the electrical performance.

[0065] The applicant declares that the above description is only the specific implementation manners of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by any person skilled in the art within the technical scope disclosed by the present invention fall within the protection scope and the disclosure scope of the present invention.

Claims

1. An electrolyte for a lithium iron manganese phosphate battery, characterized in that, The electrolyte for the lithium iron manganese phosphate battery includes an electrolyte lithium salt, a multi-functional group compound additive, a film-forming additive, and an organic solvent. The structural formula of the multi-functional group compound additive is shown in the following formula I:

2. The electrolyte for a lithium iron manganese phosphate battery according to claim 1, wherein The electrolyte lithium salt includes any one or a combination of at least two of lithium hexafluorophosphate, lithium difluorophosphate, or lithium bis(fluorosulfonyl)imide.

3. The electrolyte for a lithium iron manganese phosphate battery according to claim 1 or 2, characterized in that The mass concentration of the electrolyte lithium salt in the electrolyte for the lithium iron manganese phosphate battery is 10% - 15%.

4. The electrolyte for a lithium iron manganese phosphate battery according to any one of claims 1 to 3, characterized in that, The mass concentration of the multi-functional group compound additive in the electrolyte for the lithium iron manganese phosphate battery is 1.5% - 4%.

5. The electrolyte for a lithium iron manganese phosphate battery according to any one of claims 1-4, characterized in that, The film-forming additive includes any one or a combination of at least two of vinylene carbonate, ethylene sulfate, or fluoroethylene carbonate.

6. The electrolyte for lithium iron manganese phosphate battery according to any one of claims 1-5, characterized in that The mass concentration of the film-forming additive in the electrolyte for the lithium iron manganese phosphate battery is 1% - 3%.

7. The electrolyte for a lithium iron manganese phosphate battery according to any one of claims 1-6, characterized in that, The organic solvent includes any one or a combination of at least two of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, or dimethyl carbonate, and is preferably ethylene carbonate, propylene carbonate, ethyl methyl carbonate, and dimethyl carbonate.

8. The electrolyte for lithium iron manganese phosphate battery according to claim 7, characterized in that, The mass ratio of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, and dimethyl carbonate is (20 - 35):(5 - 10):(30 - 55):(20 - 25).

9. The electrolyte for a lithium iron manganese phosphate battery according to any one of claims 1-8, characterized in that, The mass fraction of the organic solvent in the electrolyte for the lithium iron manganese phosphate battery is 78% - 87.5%.

10. A lithium iron manganese phosphate battery, characterized in that, The lithium iron manganese phosphate battery includes the electrolyte for the lithium iron manganese phosphate battery according to any one of claims 1 - 9.

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