Electrolyte and lithium ion battery thereof

Abstract

The application discloses an electrolyte and a lithium ion battery thereof, and the electrolyte comprises a lithium salt, a solvent and an additive, wherein the additive comprises 1,3-propanesulfonic acid lactone, lithium difluorophosphate and hexamethyldisilazane; the percentage content of 1,3-propanesulfonic acid lactone in the electrolyte is 1-3% by mass percentage; the percentage content of lithium difluorophosphate in the electrolyte is 0.5-2%; and the mass percentage content of hexamethyldisilazane in the electrolyte is 0.1-2%. The lithium ion battery electrolyte of the application obtains the most suitable lithium salt concentration by screening the types of solvents and optimizing the proportion of solvents, and then adds the additive suitable for high-concentration electrolyte, thereby saving the lithium salt cost to a certain extent, retaining the performance of the high-concentration electrolyte, and significantly improving the low-temperature and fast-charging performance of the battery.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of secondary batteries, in particular to a lithium ion battery electrolyte and a lithium ion battery. BACKGROUND

[0002] Lithium ion batteries have the advantages of high specific energy, no memory effect, green environmental protection, etc., and are currently widely used in consumer electronic products and power batteries. With the expansion of the market scale of lithium ion batteries, higher demands are put forward for their comprehensive performance. The high-temperature cycle life and high-temperature storage performance of the battery have become important indicators for measuring the comprehensive performance of the lithium ion battery.

[0003] The performance of a lithium ion battery is a key factor related to whether it can be popular in the market. The performance of a lithium ion battery is comprehensively affected by multiple indicators, and the low-temperature and fast-charging performance of the lithium ion battery needs to be improved at present. SUMMARY

[0004] The application aims to provide an electrolyte and a lithium ion battery thereof, and improve the low-temperature and fast-charging performance of the lithium ion battery.

[0005] The application discloses an electrolyte, which comprises a lithium salt, a solvent and an additive, and the additive comprises 1,3-propane sultone, lithium difluorophosphate and hexamethyldisilazane.

[0006] The percentage content of 1,3-propane sultone in the electrolyte is 1%-3% by mass percentage; the percentage content of lithium difluorophosphate in the electrolyte is 0.5%-2% by mass percentage; and the mass percentage content of hexamethyldisilazane in the electrolyte is 0.1%-2% by mass percentage.

[0007] Optionally, the structural formula of 1,3-propane sultone is formula I, the structural formula of lithium difluorophosphate is formula II, and the structural formula of hexamethyldisilazane is formula III.

[0008]

[0009] Optionally, the lithium salt is selected from one or two or more of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium difluoro(oxalato)borate, lithium bis(oxalato)borate and lithium tetrafluoroborate, wherein at least one is lithium bis(fluorosulfonyl)imide.

[0010] Optionally, the concentration of the lithium salt in the high-concentration electrolyte is 2M-3M.

[0011] Optionally, the solvent is selected from two or two or more of ethylene glycol dimethyl ether, acetonitrile, butyronitrile and fluorinated ethylene carbonate, wherein at least one is a nitrile solvent.

[0012] Optionally, the mass ratio of the nitrile solvent to ethylene glycol dimethyl ether is 1:(0.5-5); and / or the mass ratio of the nitrile solvent to fluoroethylene carbonate is 1:(0.5-5).

[0013] The application further discloses a lithium ion battery comprising a positive electrode sheet, a negative electrode sheet, a diaphragm and the electrolyte.

[0014] Optionally, the positive electrode sheet comprises a positive electrode current collector and a positive electrode diaphragm, the positive electrode diaphragm comprises a positive electrode active material, a conductive agent and a binder, and the positive electrode active material is LiNi 1-x-y-z Co x Mn y Al z O2; wherein 0<=x<=1, 0<=y<=1, 0<=z<=1 and 0<=x+y+z<=1.

[0015] Optionally, the negative electrode sheet comprises a negative electrode current collector and a negative electrode diaphragm, the negative electrode diaphragm comprises a negative electrode active material, a conductive agent and a binder, and the negative electrode active material is selected from graphite, silicon or SiO w w is a silicon-carbon composite material composed of graphite and silicon, or pure lithium metal; wherein 1

[0016] The electrolyte of the lithium ion battery of the application obtains the most suitable lithium salt concentration by screening the types of solvents and optimizing the proportion of solvents, and further adding an additive suitable for high-concentration electrolyte, thereby saving the cost of lithium salt to a certain extent, retaining the performance of high-concentration electrolyte, and significantly improving the low-temperature and fast-charging performance of the battery. DETAILED DESCRIPTION

[0017] It should be understood that the terms used herein, the specific structural and functional details disclosed, are only for the purpose of describing specific embodiments, and are representative, but the application can be embodied in many alternative forms, and should not be interpreted as being limited to the embodiments set forth herein.

[0018] The application will be described in detail below with reference to optional embodiments.

[0019] As an embodiment of the application, the application discloses an electrolyte, which comprises a lithium salt, a solvent and an additive, and the additive comprises 1,3-propane sultone, lithium difluorophosphate and hexamethyldisilazane.

[0020] The percentage content of 1,3-propane sultone in the electrolyte is 1%-3% by mass percentage; the percentage content of lithium difluorophosphate in the electrolyte is 0.5%-2% by mass percentage; and the percentage content of hexamethyldisilazane in the electrolyte is 0.1%-2% by mass percentage.

[0021] Specifically, the structural formula of 1,3-propanesultone is the following formula I, the structural formula of lithium difluorophosphate is the following formula II, and the structural formula of hexamethyldisilazane is the following formula III:

[0022]

[0023] Specifically, the lithium salt is selected from one or more of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium difluoro(oxalato)borate, lithium bis(oxalato)borate, lithium tetrafluoroborate, at least one of which is lithium bis(fluorosulfonyl)imide.

[0024] Specifically, the concentration of the lithium salt in the high-concentration electrolyte is 2M-3M.

[0025] Specifically, the solvent is selected from two or more of ethylene glycol dimethyl ether, acetonitrile, butyronitrile, and fluoroethylene carbonate, at least one of which is a nitrile solvent.

[0026] Specifically, the mass ratio of the nitrile solvent to ethylene glycol dimethyl ether is 1: (0.5-5); and / or the mass ratio of the nitrile solvent to fluoroethylene carbonate is 1: (0.5-5).

[0027] The application also discloses a lithium ion battery comprising a positive electrode sheet, a negative electrode sheet, a separator, and the above electrolyte.

[0028] Specifically, the positive electrode sheet comprises a positive electrode current collector and a positive electrode diaphragm, the positive electrode diaphragm comprises a positive electrode active material, a conductive agent, and a binder, and the positive electrode active material is LiNi 1-x-y-z Co x Mn y Al z O2; wherein 0≤x≤1, 0≤y≤1, 0≤z≤1 and 0≤x+y+z≤1.

[0029] Specifically, the negative electrode sheet comprises a negative electrode current collector and a negative electrode diaphragm, the negative electrode diaphragm comprises a negative electrode active material, a conductive agent, and a binder, and the negative electrode active material is selected from graphite, silicon or SiO w a silicon-carbon composite material composed of graphite and silicon, and pure lithium metal; wherein 1

[0030] Preferably, non-limiting examples of the binder include polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene butadiene rubber, acrylated styrene butadiene rubber, epoxy resin, nylon, and the like.

[0031] The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer on the positive electrode current collector, wherein the positive electrode active material layer includes a positive electrode active material, a positive electrode binder and a positive electrode conductive agent. The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer on the negative electrode current collector, wherein the negative electrode active material layer includes a negative electrode active material, a negative electrode binder and a negative electrode conductive agent.

[0032] The specific types of the positive electrode active material, the positive electrode binder, the positive electrode conductive agent, the negative electrode active material, the negative electrode binder and the negative electrode conductive agent are not specifically limited and can be selected according to requirements. The positive electrode active material is preferably a lithium cobalt oxide material, and the negative electrode active material is preferably a silicon-oxygen-carbon composite material.

[0033] The application is further described below by specific examples.

[0034] Example

[0035] The present example is used to illustrate the lithium ion battery and the preparation method thereof disclosed by the present application, which includes the following operation steps:

[0036] Preparation of electrolyte: The electrolyte is prepared according to the electrolyte formula shown in Table 1.

[0037] Preparation of positive electrode sheet: The positive electrode active material LiCoO2, the conductive agent CNT (Carbon Nanotube) and the binder PVDF (Polyvinylidene Fluoride) are mixed in a mass ratio of 97:1.5:1.5 in an N-methylpyrrolidone solvent to form a uniform positive electrode slurry. The slurry is coated on a positive electrode current collector aluminum foil, dried, cold-pressed to obtain a positive electrode sheet.

[0038] Preparation of negative electrode sheet: The negative electrode active material silicon-oxygen-carbon composite material, the conductive agent acetylene black, the binder styrene butadiene rubber and the thickening agent sodium carboxymethyl cellulose are mixed in a mass ratio of 95:2:2:1 in a proper amount of deionized water solvent to form a uniform negative electrode slurry. The slurry is coated on a negative electrode current collector copper foil, dried, cold-pressed to obtain a negative electrode sheet.

[0039] Preparation of lithium ion battery: The PE porous polymer film is used as a separator.

[0040] The positive electrode sheet, the separator and the negative electrode sheet are stacked in order, the separator is located between the positive electrode and the negative electrode to play a separation role, then the stacked electrode sheet and the separator are wound to obtain a winding core. The winding core is placed in an aluminum plastic film bag shaped by punching a shell, the electrolyte prepared above is injected into the dried electrode core, and the lithium ion battery is prepared through vacuum packaging, standing, formation and other processes.

[0041] Table 1

[0042]

[0043] The difference between Examples 2-8 is that the content of lithium salt is different in the preparation of electrolyte, as shown in Table 1.

[0044] The difference between Comparative Examples 1-12 is that the specific formulation is different, as shown in Table 1.

[0045] The lithium ion batteries prepared in the above Examples 1-8 and Comparative Examples 1-12 are subjected to the following performance tests:

[0046] -10℃ cycle test of the battery:

[0047] The test method is as follows: the lithium battery is charged to 4.45V at 1.5C constant current and constant voltage in a constant temperature box at -10±2℃, the cutoff current is 0.05C, and then discharged at 0.5C to 3V, and the above conditions are repeated for multiple charge-discharge cycles. The capacity retention rate and average coulombic efficiency after 200 cycles of the battery are calculated, and each group has 5 batteries.

[0048] Capacity retention rate (%) = discharge capacity (mAh) corresponding to the cycle number / discharge capacity (mAh) of the third week cycle * 100%

[0049] The capacity retention rate of each group of 5 batteries after different cycles is averaged and recorded in Table 2. Table 2

[0050]

[0051] Comparative Example 1, only DME as solvent, the low temperature cycle performance of the battery is the worst, because the ether solvent has poor oxidation resistance, the electrolyte of pure DME solvent has serious decomposition at 4.4V. Comparative Example 2, after using butyronitrile solvent, the capacity retention rate is improved, which is due to the nitrile solvent improving the oxidation resistance of the electrolyte and widening the electrochemical window of the ether solvent, but the coulombic efficiency decreases because the reduction stability of the nitrile solvent is poor; Comparative Example 3, after introducing fluoroethylene carbonate, the capacity retention rate and coulombic efficiency are improved, which is because a large amount of FEC helps to form a SEI film with good performance on the negative electrode, but FEC with high content will react to generate HF to corrode the electrode material when containing trace amount of water. Comparative Example 4, after optimizing the solvent ratio, the capacity retention rate and coulombic efficiency are improved.

[0052] The battery performance is obviously improved with the increase of lithium salt concentration in Example 1-3 and Comparative Examples 4, 5. In Comparative Example 12, when the lithium salt concentration is 3M, the low-temperature cycle performance of the battery is decreased, because the viscosity of the electrolyte is increased and the conductivity is reduced due to the too high lithium salt concentration, and the low-temperature performance is lost. Comparative Examples 6 and 7 show that the use effect of the three additives in the high-concentration electrolyte is better than that in the conventional electrolyte concentration. Comparative Examples 8, 9 and 10 show that the addition of the additives alone can improve the battery performance, but compared with Comparative Example 7, the use effect of the three additives in combination is better. In Comparative Example 11, when the content of lithium difluorophosphate in the electrolyte is higher than 2% and the content of 1,3-propanesulfonic acid lactone is higher than 3% and the content of hexamethyldisilazane is higher than 2%, the battery performance is deteriorated, mainly because the excessive additives increase the electrode interface impedance and deteriorate the battery performance.

[0053] The above is a further detailed description of the present application in combination with specific optional embodiments, and cannot be regarded as a limitation of the specific implementation of the present application. For ordinary skilled persons in the technical field to which the present application belongs, a number of simple deductions or substitutions can be made without departing from the concept of the present application, and all of them should be regarded as falling within the protection scope of the present application.

Claims

1. An electrolyte, characterized by, The electrolyte comprises a lithium salt, a solvent and an additive, wherein the additive comprises 1,3-propane sultone, lithium difluorophosphate and hexamethyldisilazane. The percentage content of the 1,3-propane sultone in the electrolyte is 1-3% by mass percentage; the percentage content of the lithium difluorophosphate in the electrolyte is 0.5-2% by mass percentage; and the percentage content of the hexamethyldisilazane in the electrolyte is 0.1-2% by mass percentage. The lithium salt is selected from one or more than two of lithium bis(fluorosulfonyl)imide, lithium bis(oxalato)borate and lithium difluoro(oxalato)borate, wherein at least one is lithium bis(fluorosulfonyl)imide. The molar concentration of the lithium bis(fluorosulfonyl)imide is 2.0-2.8M, the molar concentration of the lithium bis(oxalato)borate is 0.1-0.2M, and the molar concentration of the lithium difluoro(oxalato)borate is 0.1-0.2M. The solvent is ethylene glycol dimethyl ether, butyronitrile and fluoroethylene carbonate, and the mass ratio of the ethylene glycol dimethyl ether, butyronitrile and fluoroethylene carbonate is 1:(0.5-5):(0.5-5).

2. The electrolyte of claim 1, wherein The 1,3-propane sultone has the following formula I, the lithium difluorophosphate has the following formula II, and the hexamethyldisilazane has the following formula III.

3. A lithium-ion battery, characterized by, The electrolyte comprises a lithium salt, a solvent and an additive, wherein the additive comprises 1,3-propane sultone, lithium difluorophosphate and hexamethyldisilazane.

4. The lithium-ion battery of claim 3, wherein: The positive electrode sheet includes a positive electrode current collector and a positive electrode diaphragm including a positive electrode active material, a conductive agent, and a binder, the positive electrode active material being LiNi 1-x-y- z Co x Mn y AlzO2; 0≤x≤1, 0≤y≤1, 0≤z≤1 and 0≤x+y+z≤1. The electrolyte comprises a lithium salt, a solvent and an additive, wherein the additive comprises 1,3-propane sultone, lithium difluorophosphate and hexamethyldisilazane.

5. The lithium-ion battery of claim 3, wherein: The negative electrode sheet includes a negative electrode current collector and a negative electrode film sheet including a negative electrode active material, a conductive agent, and a binder, the negative electrode active material being selected from graphite, silicon, or SiO w A silicon-carbon composite material composed of graphite and silicon, and pure lithium metal; wherein 1 < w < 2.

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