A Lithium-Ion Battery Electrolyte and Its Preparation Method and Application

Abstract

The present invention provides a lithium-ion battery electrolyte, a preparation method thereof and an application. The lithium-ion battery electrolyte includes a lithium salt, an organic solvent and an additive, and the organic solvent includes a nitrile solvent; the room-temperature conductivity σ of the lithium-ion battery electrolyte satisfies the following relationship: 15 ≤ (K × ε × γ) / μ ≤ 30, where K = 0.02 - 0.07, ε is the dielectric constant, γ is the mass fraction of the nitrile solvent in the lithium-ion battery electrolyte, and μ is the viscosity of the lithium-ion battery electrolyte at room temperature. This electrolyte can effectively inhibit the reductive decomposition of the nitrile solvent, has the characteristics of weak solvation binding energy, and still has a high ionic conductivity at low temperature, showing good fast charging performance; at the same time, the battery prepared based on this has excellent low-temperature capacity retention rate, effectively improving the safety, energy density, power, rate and cycle performance of the battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion battery materials, and particularly relates to a lithium-ion battery electrolyte, a preparation method thereof, and an application thereof. Background Art

[0002] Lithium-ion batteries play an important role in the fields related to environment and energy. Improving the ionic conductivity of electrolytes is crucial for enhancing the performance of next-generation batteries. When a high-conductivity electrolyte is added, the DCR of the battery cell can be significantly reduced, and great changes will occur in the charge and discharge performance. Such electrolytes are extremely important in the fields of low temperature and thick electrodes. Since the battery performance depends not only on the resistance of the electrolyte but also on the resistance of the electrode / electrolyte interface, nitrile solvents, such as non-aqueous electrolytes containing acetonitrile (AN), have an excellent performance balance between relative dielectric constant and viscosity, so their potential applications in electric vehicle (EV) batteries have received extensive attention. However, on the graphite negative electrode, nitrile solvents themselves are not stable to electrochemical reduction. Overcoming this problem can commercialize nitrile solvents in the power battery industry.

[0003] How to suppress the reductive decomposition of non-aqueous electrolytes containing nitriles under low-concentration lithium salt conditions and achieve battery performance with high ionic conductivity in a wide temperature range is a huge challenge. Methods for suppressing the reduction of AN include: 1. Yamada et al. improved the reduction stability of AN by dissolving a high concentration of lithium salt in AN (more than 2 mol / L); 2. Peng et al. reported the development of high-power lithium metal batteries by using a high concentration of aniline-based electrolyte and ethylene carbonate (VC) as additives; 3. An effective method for suppressing the reductive decomposition of AN is to use a negative electrode active material to insert lithium ions at a potential higher than the reduction potential of AN. For example, researchers at Toshiba reported a battery that can prevent the reductive decomposition of AN, which uses lithium titanate as the negative electrode.

[0004] However, the prior art reduces the reductive decomposition of nitrile solvents by increasing the lithium salt concentration, which is not advantageous in today's low-cost electrolytes, and when the lithium salt concentration increases, the viscosity of the electrolyte increases, which will relatively reduce the ionic conductivity to a certain extent, resulting in a decline in the cycle and rate performance of the battery cell.

[0005] Therefore, how to effectively suppress the reductive decomposition of nitrile solvents, while maintaining a high lithium ion conductivity, and having excellent cycle and rate performance of the battery cell is a technical problem that urgently needs to be studied at present. Summary of the Invention

[0006] In view of the deficiencies of the prior art, the purpose of the present invention is to provide a lithium-ion battery electrolyte, a preparation method thereof, and an application. The present invention designs a lithium-ion battery electrolyte that satisfies the above relationship. This type of electrolyte can not only effectively inhibit the reductive decomposition of nitrile solvents, has the characteristic of weak solvation binding energy, but also has a high ionic conductivity at low temperatures. At high rates, solvated lithium ions can be quickly desolvated, showing good fast charging performance. At the same time, the fast lithium ion transport and desolvation at low temperatures can ensure that the battery has excellent low-temperature capacity retention rate, effectively improving the safety, energy density, power, rate, and cycle performance of the battery.

[0007] To achieve this purpose, the present invention adopts the following technical solutions:

[0008] In the first aspect, the present invention provides a lithium-ion battery electrolyte, which includes a lithium salt, an organic solvent, and an additive. The organic solvent includes a nitrile solvent;

[0009] The room temperature conductivity σ of the lithium-ion battery electrolyte satisfies the following relationship:

[0010] 15 ≤ (K × ε × γ) / μ ≤ 30, where K = 0.02 - 0.07, ε is the dielectric constant, γ is the mass fraction of the nitrile solvent in the lithium-ion battery electrolyte, and μ is the viscosity of the lithium-ion battery electrolyte at room temperature.

[0011] The present invention designs a lithium-ion battery electrolyte that satisfies the above relationship. This type of electrolyte can not only effectively inhibit the reductive decomposition of nitrile solvents, has the characteristic of weak solvation binding energy, but also has a high ionic conductivity at low temperatures. At high rates, solvated lithium ions can be quickly desolvated, showing good fast charging performance. At the same time, the fast lithium ion transport and desolvation at low temperatures can ensure that the battery has excellent low-temperature capacity retention rate, effectively improving the safety, energy density, power, rate, and cycle performance of the battery.

[0012] The electrolyte provided by the present invention has good compatibility with the negative electrode, will not damage the negative electrode, and can form a SEI film on the negative electrode, inhibit the reaction between the electrolyte and the negative electrode material interface, reduce the generation of HF, and improve problems such as rapid battery swelling and capacity decay during cycling / high-temperature storage.

[0013] In the present invention, the room temperature conductivity σ of the lithium-ion battery electrolyte satisfies the relationship 15 ≤ (K × ε × γ) / μ ≤ 30. The viscosity of the electrolyte can be changed by adjusting the content of the low-viscosity organic solvent, thereby improving the dissociation ability of lithium ions, increasing the conductivity, and meeting the fast charging requirements.

[0014] Conductivity is one of the most practically significant parameters in the electrolyte of lithium-ion batteries. The conductivity of the electrolyte has a great influence on the internal resistance and rate performance of the battery, and can point the way for the design of the electrolyte. From the perspective of the solvent, the main factors affecting the conductivity of the electrolyte are the dielectric constant and viscosity of the solvent. By reducing the solvent viscosity and increasing the solvent dielectric constant, the conductivity of the electrolyte can be increased without the need to additionally increase the lithium salt.

[0015] It should be noted that the present invention does not specifically limit the room temperature. Exemplarily, for example, it can be 25±5°C, including 20°C, 25°C or 30°C, etc.

[0016] In the present invention, K = 0.02 - 0.07, for example, it can be 0.02, 0.03, 0.04, 0.05, 0.06 or 0.07, etc.

[0017] As a preferred technical solution of the present invention, the room temperature conductivity σ is 15 - 30 mS·cm -1 , for example, it can be 15 mS·cm -1 , 20 mS·cm -1 , 25 mS·cm -1 or 30 mS·cm -1 etc.

[0018] Preferably, the μ is 1.5 - 2.1 mPa·s, for example, it can be 1.5 mPa·s, 1.6 mPa·s, 1.7 mPa·s, 1.8 mPa·s, 1.9 mPa·s, 2 mPa·s or 2.1 mPa·s, etc.

[0019] Preferably, the ε is 20 - 40, for example, it can be 20, 25, 30, 35 or 40, etc.

[0020] Preferably, the γ is 15 - 50 wt%, for example, it can be 15 wt%, 20 wt%, 25 wt%, 30 wt%, 35 wt%, 40 wt%, 45 wt% or 50 wt%, etc.

[0021] In the present invention, if the mass fraction of the nitrile solvent in the lithium-ion battery electrolyte is too small, the viscosity and conductivity of the electrolyte are too low, which is not conducive to the desolvation of lithium ions, resulting in a decline in the fast charging performance; if the mass fraction of the nitrile solvent in the lithium-ion battery electrolyte is too large, stripping of the graphite negative electrode will cause cycling gas generation.

[0022] As a preferred technical solution of the present invention, the structural formula of the nitrile solvent is Wherein, R 1 is -C n H 2n+1 or R 1 is -C 6 H5 , where n = 1 - 4. Exemplarily, it can be -CH 3 or -C 2 H 5 etc.

[0023] Preferably, the nitrile solvent includes any one or a combination of at least two of acetonitrile, propionitrile, methoxypropionitrile, isopropyl cyanide, succinonitrile, butyronitrile, adiponitrile, valeronitrile, glutarodinitrile, sebacodinitrile, acrylonitrile, cyclohexyl cyanide or phthalonitrile.

[0024] Preferably, the organic solvent further includes a carbonate solvent.

[0025] Preferably, the carbonate solvent includes any two or a combination of at least three of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate or dimethyl carbonate.

[0026] As a preferred technical solution of the present invention, the additive includes any one or a combination of at least two of pyridine additives, lithium salt additives or ester additives, preferably a combination of pyridine additives, lithium salt additives and ester additives.

[0027] In the present invention, the addition of pyridine compounds can preferentially form a dense and stable protective film on the negative electrode side, inhibit the redox reaction of the electrolyte on the surfaces of the positive and negative electrodes, stabilize the positive and negative active materials, improve the cycle storage performance, reduce the gas generation amount of the battery cell at high temperature, and improve the safety performance of the battery cell.

[0028] Preferably, the content of the pyridine additive accounts for 1 - 9 wt% of the content of the nitrile solvent, and for example, it can be 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt% or 9 wt% etc.

[0029] In the present invention, pyridine ring compounds can form a dense and stable protective film in the negative electrode of the lithium ion battery, inhibit the volume expansion of the negative electrode active material during charge and discharge. If the proportion of the content of the pyridine additive in the content of the nitrile solvent is too small, the film forming quality is too poor, and the nitrile solvent peels off the graphite, resulting in a cycle drop; if the proportion of the content of the pyridine additive in the content of the nitrile solvent is too large, it will lead to too high a film forming thickness, increased impedance and decreased cycle performance.

[0030] Preferably, the structural formula of the pyridine additive is where R 2 is -H, -F, -C n H 2n+1 or -C n H 2n-1 any one of them, and n = 1 - 3. Exemplarily, it can be -CH 3 or -CH 2etc.

[0031] Preferably, the lithium salt additive includes any one or a combination of at least two of oxaloborate, oxalophosphate or difluorophosphate.

[0032] Preferably, the ester additive includes a combination of any three or at least four of vinylene carbonate, ethylene vinylene carbonate, fluorinated ethylene carbonate, ethylene sulfite or methylene methanedisulfonate.

[0033] It should be noted that vinylene carbonate needs to supplement and stabilize the negative electrode interface through double injection to improve the cycling performance.

[0034] Preferably, the content of the lithium salt additive accounts for 0.1-0.8 wt% of the total amount of the lithium ion battery electrolyte, and can be, for example, 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt%, 0.7 wt% or 0.8 wt% etc.

[0035] Preferably, the content of the ester additive accounts for 2-7 wt% of the total amount of the lithium ion battery electrolyte, and can be, for example, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt% or 7 wt% etc.

[0036] As a preferred technical solution of the present invention, the lithium salt includes lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide.

[0037] Preferably, in the lithium ion battery electrolyte, the concentration of the lithium salt is 0.8-2.5 mol / L, and can be, for example, 0.8 mol / L, 1 mol / L, 1.5 mol / L, 2 mol / L or 2.5 mol / L etc.

[0038] Preferably, the content of the lithium salt accounts for 10-20 wt% of the total amount of the lithium ion battery electrolyte, and can be, for example, 10 wt%, 12 wt%, 14 wt%, 16 wt%, 18 wt% or 20 wt% etc.

[0039] In a second aspect, the present invention provides a preparation method of the lithium ion battery electrolyte as described in the first aspect, and the preparation method includes the following steps:

[0040] Mix the lithium salt, additive and organic solvent to obtain the lithium ion battery electrolyte;

[0041] Wherein, the organic solvent includes nitrile solvents.

[0042] As a preferred technical solution of the present invention, the organic solvent further includes a carbonate solvent. Preferably, the volume ratio of the carbonate solvent to the nitrile solvent is (70 - 50):(30 - 50), where the selection range of the carbonate solvent "70 - 50" can be, for example, 70, 60 or 50, etc., and the selection range of the nitrile solvent "30 - 50" can be, for example, 30, 40 or 50, etc.

[0043] Preferably, the mixing method includes:

[0044] Mix the carbonate solvent and the nitrile solvent, then add a lithium salt for mixing, and then add an additive for mixing.

[0045] As a preferred technical solution of the present invention, the preparation method includes the following steps:

[0046] (1) Stir and mix ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate and acetonitrile in a volume ratio of (15 - 35):(15 - 45):(0 - 30):(30 - 50) to obtain an organic solvent;

[0047] (2) Add lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide with a total content of 14.5 - 18 wt% to the organic solvent and stir and mix to obtain a mixed solution;

[0048] (3) Add a pyridine-based additive, a lithium salt-based additive and an ester-based additive to the mixed solution and stir and mix to obtain the lithium-ion battery electrolyte.

[0049] In a third aspect, the present invention provides a lithium-ion battery, which includes a positive electrode sheet, a negative electrode sheet and the lithium-ion battery electrolyte as described in the first aspect.

[0050] As a preferred technical solution of the present invention, the preparation method of the positive electrode sheet includes:

[0051] Add a positive electrode active material, a conductive agent (such as conductive carbon black), a binder (such as PVDF, acrylonitrile-acrylate or PTFE, etc.) and a dispersant (such as CGI-QC, etc.) to an organic solvent (such as N-methylpyrrolidone), stir and mix evenly to obtain a positive electrode slurry with a solid content of 50 - 60% (such as 50%, 52%, 54%, 56%, 58% or 60%, etc.); coat the positive electrode slurry on the positive electrode current collector aluminum foil to obtain a single-sided positive electrode sheet surface density of 15 - 20 mg / cm 2 (such as 15 mg / cm 2 、16 mg / cm 2 、17 mg / cm 2 、18 mg / cm2 , 19 mg / cm 2 or 20 mg / cm 2 etc.) of the positive electrode pole piece roll, and the positive electrode pole piece roll is dried and roll-pressed and slit to obtain the positive electrode pole piece.

[0052] Preferably, the positive electrode active material includes lithium iron phosphate positive electrode material.

[0053] As a preferred technical solution of the present invention, the preparation method of the negative electrode sheet includes:

[0054] Dissolve the negative electrode active material, a conductive agent (for example, it can be conductive carbon black), a binder (for example, it can be polyacrylonitrile and / or styrene-butadiene rubber, etc.) and a thickening agent (for example, it can be sodium carboxymethylcellulose, etc.) in the solvent deionized water, and after mixing evenly, obtain a negative electrode slurry with a solid content of 45-50% (for example, it can be 45%, 46%, 47%, 48%, 49% or 50%, etc.); coat the negative electrode slurry on the negative electrode current collector copper foil, and after drying and roll-pressing and slit, obtain the negative electrode pole piece.

[0055] Preferably, the negative electrode material includes artificial graphite.

[0056] As a preferred technical solution of the present invention, the particle size D50 of the negative electrode active material in the negative electrode sheet is 13-14 μm, for example, it can be 13 μm, 13.2 μm, 13.4 μm, 13.6 μm, 13.8 μm or 14 μm, etc.

[0057] Preferably, the BET specific surface area of the negative electrode active material is 0.5-2 m 2 / g, for example, it can be 0.5 m 2 / g, 1 m 2 / g, 1.5 m 2 / g or 2 m 2 / g, etc.

[0058] It should be noted that the separator is a conventional commercially available polypropylene film coated with a ceramic layer or a PVDF layer on both sides.

[0059] The numerical ranges described in the present invention not only include the point values listed above, but also include any point values between the above numerical ranges not listed. Due to space limitations and for the sake of brevity, the present invention does not exhaustively list the specific point values included in the range.

[0060] Compared with the prior art, the present invention has the following beneficial effects:

[0061] (1) The present invention designs a lithium-ion battery electrolyte that satisfies the above relationship. This type of electrolyte can not only effectively inhibit the reductive decomposition of nitrile solvents, has the characteristics of weak solvation binding energy, but also has a high ionic conductivity at low temperatures. At high rates, solvated lithium ions can quickly desolvate, showing good fast charging performance. At the same time, the fast lithium-ion transport and desolvation at low temperatures can ensure that the battery has excellent low-temperature capacity retention rate, effectively improving the safety, energy density, power, rate, and cycle performance of the battery.

[0062] (2) The electrolyte provided by the present invention has good compatibility with the negative electrode, will not damage the negative electrode, and can form a SEI film on the negative electrode, inhibit the reaction between the electrolyte and the interface of the negative electrode material, reduce the generation of HF, and improve problems such as rapid battery swelling and capacity decay during cycling / high-temperature storage. Detailed implementation mode

[0063] The technical solution of the present invention will be further described below through specific implementation modes. Those skilled in the art should understand that the embodiments are only helpful for understanding the present invention and should not be regarded as specific limitations of the present invention.

[0064] It should be noted that in the following implementation modes, room temperature refers to 25 °C.

[0065] Example 1

[0066] This example provides a lithium-ion battery electrolyte. The lithium-ion battery electrolyte includes a lithium salt, an organic solvent, and an additive. The organic solvent includes a nitrile solvent and a carbonate solvent;

[0067] The room temperature conductivity σ of the lithium-ion battery electrolyte satisfies the following relationship:

[0068] 15 ≤ (K × ε × γ) / μ ≤ 30, where K = 0.06, ε is the dielectric constant, γ is the mass fraction of the nitrile solvent in the lithium-ion battery electrolyte, μ is the viscosity of the lithium-ion battery electrolyte at room temperature, and the room temperature conductivity σ is 17.5 mS·cm -1 , the μ is 1.89 mPa·s, the ε is 30.44, and the γ is 18.17 wt%;

[0069] The nitrile solvent is acetonitrile. The carbonate solvents include ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate. The additives include pyridine additives, lithium salt additives, and ester additives. The content of the pyridine additives accounts for 5.5 wt% of the content of the nitrile solvent. The pyridine additive is pyridine. The content of the lithium salt additives accounts for 0.5 wt% of the total amount of the lithium-ion battery electrolyte. The lithium salt additive is difluorooxalate borate. The content of the ester additives accounts for 4 wt% of the total amount of the lithium-ion battery electrolyte. The ester additives include vinylene carbonate, fluoroethylene carbonate, and methylene methanedisulfonate in a mass ratio of 2:1:1;

[0070] The lithium salts include lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide in a mass ratio of 11.88:3.08. In the lithium-ion battery electrolyte, the concentration of the lithium salt is 1.5 mol / L, and the content of the lithium salt accounts for 14.95 wt% of the total amount of the lithium-ion battery electrolyte.

[0071] This example also provides a method for preparing the above lithium-ion battery electrolyte. The preparation method includes the following steps:

[0072] In a nitrogen glove box, ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate, and acetonitrile are mixed evenly in a volume ratio of 25:25:20:30. Subsequently, lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide are added. After mixing evenly, vinylene carbonate, fluoroethylene carbonate, methylene methanedisulfonate, difluorooxalate borate, and pyridine are added, and after mixing evenly, the lithium-ion battery electrolyte is obtained.

[0073] Example 2

[0074] The difference between this example and Example 1 is that the volume ratio of ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate, and acetonitrile is adjusted to 25:25:10:40, so that K = 0.038, σ is 18.5 mS·cm -1 , μ is 1.72 mPa·s, ε is 33.27, and γ is 24.9 wt%.

[0075] The remaining preparation methods and parameters are the same as those in Example 1.

[0076] Example 3

[0077] The difference between this example and Example 1 is that the carbonate solvents include ethylene carbonate and ethyl methyl carbonate, and the volume ratio of ethylene carbonate, ethyl methyl carbonate, and acetonitrile is 25:25:50, so that K = 0.029, σ is 20.4 mS·cm -1 , μ is 1.68 mPa·s, ε is 36.27, and γ is 32.03 wt%.

[0078] The remaining preparation methods and parameters are the same as those in Example 1.

[0079] Example 4

[0080] The difference between this example and Example 1 is that the carbonate solvent includes ethylene carbonate and ethyl methyl carbonate, and the volume ratio of ethylene carbonate, ethyl methyl carbonate and acetonitrile is 25:15:60, so that K = 0.023 and σ is 21.6 mS·cm -1 , μ is 1.62 mPa·s, ε is 39.24, and γ is 39.34 wt%.

[0081] The remaining preparation methods and parameters are the same as those in Example 1.

[0082] Example 5

[0083] The difference between this example and Example 3 is that the content of the pyridine additive accounts for 1.55 wt% of the content of the nitrile solvent.

[0084] The remaining preparation methods and parameters are the same as those in Example 3.

[0085] Example 6

[0086] The difference between this example and Example 3 is that the content of the pyridine additive accounts for 6.23 wt% of the content of the nitrile solvent.

[0087] The remaining preparation methods and parameters are the same as those in Example 3.

[0088] Example 7

[0089] The difference between this example and Example 1 is that the volume ratio of ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate and acetonitrile is adjusted to 25:45:15:15, so that K = 0.125 and σ is 12.37 mS·cm -1 , μ is 2.38 mPa·s, ε is 26.71, and γ is 8.82 wt%.

[0090] The remaining preparation methods and parameters are the same as those in Example 1.

[0091] Example 8

[0092] The difference between this example and Example 1 is that the carbonate solvent is ethylene carbonate, and the volume ratio of ethylene carbonate and acetonitrile is 25:75, so that K = 0.016 and σ is 23.48 mS·cm -1 , μ is 1.54 mPa·s, ε is 43.95, and γ is 50.97 wt%.

[0093] The remaining preparation methods and parameters are the same as those in Example 1.

[0094] Example 9

[0095] The difference between this example and Example 3 is that no pyridine additive is added.

[0096] The remaining preparation methods and parameters are the same as those in Example 3.

[0097] Example 10

[0098] The difference between this example and Example 3 is that the content of the pyridine additive accounts for 0.62 wt% of the content of the nitrile solvent.

[0099] The remaining preparation methods and parameters are the same as those in Example 3.

[0100] Example 11

[0101] The difference between this example and Example 3 is that the content of the pyridine additive accounts for 27.38 wt% of the content of the nitrile solvent.

[0102] The remaining preparation methods and parameters are the same as those in Example 3.

[0103] Comparative Example 1

[0104] The difference between this comparative example and Example 1 is that no acetonitrile is added, that is, ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate are mixed in a volume ratio of 25:45:30, so that K = 0.02, σ is 10.54 mS·cm -1 , μ is 2.38 mPa·s, the ε is 22.99, and γ is 0.

[0105] The remaining preparation methods and parameters are the same as those in Example 1.

[0106] Performance Test

[0107] (1) The electrolytes prepared in the above examples and comparative examples were subjected to conductivity tests at different temperatures, and the test results are shown in Table 1.

[0108] Table 1

[0109]

[0110]

[0111] Analysis:

[0112] As can be seen from Table 1, when the low-viscosity solvent acetonitrile is added, the conductivity of the electrolyte can be significantly improved, and it contributes more to the low-temperature conductivity.

[0113] (2) The electrochemical performance of the battery was tested.

[0114] The positive electrode sheet, the separator and the negative electrode sheet are arranged in sequence, and the electrode group is obtained by using the rapid stacking technology. The electrode group is welded into the shell (aluminum-plastic film) through the positive and negative electrode ears, and the side of the top is sealed and dried. After that, the above-mentioned electrolyte (4Ah, injection coefficient 3.3g / Ah) is injected, and the lithium-ion battery is obtained after static, pre-charging, secondary injection, formation and other processes.

[0115] The preparation method of the positive electrode sheet includes: adding lithium iron phosphate positive electrode material, conductive carbon black, binder (polyvinylidene fluoride binder) and dispersant (LIB-D300) to N-methylpyrrolidone (NMP) at a mass ratio of 96.9%:1.5%:1.5%:0.1%, and stirring and mixing to obtain a positive electrode slurry (solid content 58%); coating the positive electrode slurry evenly on the positive electrode current collector aluminum foil to obtain a single-sided positive electrode sheet with a surface density of 18 mg / cm 2 The positive electrode coil is dried, rolled and cut to obtain the positive electrode sheet.

[0116] The preparation method of the negative electrode sheet comprises: preparing the negative electrode active material artificial graphite (particle size D50 is 13.5 μm; BET specific surface area of ​​the material is 1 m 2 / g), conductive carbon black, binder (polyacrylonitrile and styrene-butadiene rubber) and thickener sodium carboxymethyl cellulose are dissolved in deionized water as a solvent in a weight ratio of 95.7:1.0:(2.32+0.58):0.4, and the negative electrode slurry (solid content is 48%) is prepared after being evenly mixed; the negative electrode slurry is evenly coated on the negative electrode current collector copper foil, and the negative electrode sheet is obtained after drying, rolling and cutting.

[0117] The isolation film is a conventional commercially available polypropylene film coated with ceramic layers on both sides.

[0118] The second injection electrolyte used for the second injection is prepared, and the specific steps include: mixing ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate in a volume ratio of 30:40:30, ethylene carbonate accounts for 14% of the mass of the second injection electrolyte, and then adding lithium hexafluorophosphate with a concentration of 12.5%, mixing evenly, to obtain the second injection electrolyte; during the second injection, the mass ratio of the first injection electrolyte to the second injection electrolyte is 9:1.

[0119] 1) The rate performance test was performed on the lithium ion battery prepared above, and the capacity retention rate was recorded as XC charging capacity / 0.33C discharge capacity, where X is 1, 2, 3, 4 and 5.

[0120] The test results are shown in Table 2.

[0121] Table 2

[0122]

[0123]

[0124] Analysis:

[0125] As can be seen from Table 2, after using acetonitrile-based solvents, the viscosity is reduced, the dielectric constant is increased, the degree of lithium ion dissociation is increased, and the conductivity of the electrolyte is improved. Its rate performance is significantly better than that of carbonate solvents, and with the increase of the acetonitrile content, the rate performance is correspondingly improved.

[0126] As can be seen from Example 1 and Examples 7-8, if the mass fraction of nitrile solvents in the lithium-ion battery electrolyte is too small, the viscosity of the electrolyte is slightly high, the ability to dissociate lithium ions is poor, the conductivity is too low, and the rate performance deteriorates; if the mass fraction of nitrile solvents in the lithium-ion battery electrolyte is too large, the conductivity is significantly improved and the rate performance is significantly excellent, but stripping of the graphite negative electrode will cause cycling gas generation.

[0127] As can be seen from Example 3 and Example 9, if pyridine-based additives are not added, the negative electrode protection effect is poor, and acetonitrile is reduced and stripped from the graphite negative electrode, resulting in poor rate performance.

[0128] As can be seen from Example 3 and Examples 10-11, if the ratio of the content of pyridine-based additives to the content of nitrile solvents is too small, the impedance is low and the rate performance is slightly improved; if the ratio of the content of pyridine-based additives to the content of nitrile solvents is too large, the SEI film formation is relatively dense, the SEI impedance of the battery cell increases, resulting in a decrease in rate performance.

[0129] 2) The above-prepared lithium-ion battery was subjected to a high-temperature storage experiment after formation. First, the battery was charged and discharged 3 times at 0.33C / 1C (3.65 / 2.0V) at room temperature (the last discharge capacity was recorded as DC0), and then the fully charged (DOD = 100 SOC%) battery cell was placed in an oven at 60 °C for 60 days. After taking it out, it was left standing at room temperature for 4 h, and then discharged at 1C at room temperature (recorded as DC1), and then charged and discharged 3 times, and the last discharge capacity was recorded as DC2. The capacity retention rate and recovery rate of the lithium-ion battery were calculated.

[0130] The experimental results are shown in Table 3.

[0131] Table 3

[0132]

[0133]

[0134] Analysis:

[0135] As can be seen from Table 3, the lithium-ion battery prepared by the present invention has a capacity recovery rate greater than 91.5% after 60 days of high-temperature storage. Optimizing the electrolyte formulation can achieve the addition of a large amount of acetonitrile without deteriorating the storage performance.

[0136] As can be seen from Example 1 and Examples 7-8, if the mass fraction of the nitrile solvent in the lithium-ion battery electrolyte is too small, the compatibility with the negative electrode is stronger and the storage performance is slightly improved; if the mass fraction of the nitrile solvent in the lithium-ion battery electrolyte is too large, the pyridine film formation is insufficient to inhibit the reduction of acetonitrile at the negative electrode, resulting in a decrease in the storage capacity retention rate and an increase in gas generation.

[0137] As can be seen from Example 3 and Example 9, if no pyridine-based additive is added, the capacity retention rate and the capacity recovery rate are significantly reduced, and the gas generation amount is high.

[0138] As can be seen from Example 3 and Examples 10-11, if the ratio of the content of the pyridine-based additive to the content of the nitrile solvent is too small or too large, the electrochemical performance of the lithium-ion battery will be deteriorated.

[0139] 3) Under the conditions of 25 °C and 45 °C, the above-prepared lithium-ion battery is subjected to a 15-min step cycle at 20-80% SOC to 3.65 V, and then discharged at a constant current of 1C to 2.0 V for a cycle test.

[0140] The test results are shown in Table 4.

[0141] Table 4

[0142]

[0143]

[0144]

[0145] Analysis:

[0146] As can be seen from Table 4, after using acetonitrile as the solvent, the cycle life is increased compared with the scheme without acetonitrile, and the gas generation is less. When using only the carbonate solvent, the electrolyte viscosity and conductivity are low, and the cell kinetics is insufficient, resulting in insufficient room-temperature charging ability and lithium plating and voltage drop.

[0147] As can be seen from Example 1 and Examples 7-8, if the mass fraction of the nitrile solvent in the lithium-ion battery electrolyte is too small, the electrolyte viscosity is slightly high, the ability to dissociate lithium ions is low, the conductivity is low, and the fast-charging performance decreases; if the mass fraction of the nitrile solvent in the lithium-ion battery electrolyte is too large, the pyridine film formation is insufficient to inhibit the reduction of acetonitrile at the negative electrode, and the repair of the SEI film during the cycle is insufficient, resulting in a slightly lower cycle capacity retention rate and an increase in gas generation.

[0148] As can be seen from Example 3 and Example 9, when using acetonitrile as the solvent, it is necessary to add a pyridine-based additive to protect the negative electrode. The pyridine ring compound can participate in polymerization in the negative electrode of the lithium-ion battery to form a dense and stable protective film, improve the stability of the SEI film, inhibit the volume expansion of the negative electrode active material during charge and discharge, and improve the cycle performance.

[0149] As can be seen from Example 3 and Examples 10-11, if the amount of pyridine additives added is too high, the DCR increases too much and the room-temperature cycle deteriorates significantly. If the amount of pyridine additives added is too low, the reduction of acetonitrile at the graphite negative electrode cannot be completely inhibited, resulting in an increase in gas production and cycle deterioration.

[0150] In summary, when using nitrile solvents, it is necessary to use an appropriate amount of pyridine additives in combination, which can fully protect the negative electrode, improve the compatibility between acetonitrile and the negative electrode, and thus improve the cycle performance and reduce gas production.

[0151] The applicant declares that the present invention uses the above embodiments to illustrate the technical solutions of the present invention, but the present invention is not limited to the above embodiments, that is, it does not mean that the present invention must rely on the above embodiments to be implemented. Those skilled in the art should understand that any improvement of the present invention, the equivalent replacement of each raw material of the product of the present invention, the addition of auxiliary components, and the selection of specific methods, etc., all fall within the protection scope and the disclosure scope of the present invention.

Claims

1. A lithium-ion battery electrolyte solution, characterized in that, the lithium-ion battery electrolyte solution comprises a lithium salt, an organic solvent and an additive, and the organic solvent comprises a nitrile solvent; the room temperature conductivity σ of the lithium-ion battery electrolyte solution satisfies the following relational expression: 15 ≤ (K × ε × γ) / μ ≤ 30, where K = 0.02 - 0.07, ε is the dielectric constant, γ is the mass fraction of the nitrile solvent in the lithium-ion battery electrolyte solution, and μ is the viscosity of the lithium-ion battery electrolyte solution at room temperature; the μ is 1.5 - 2.1 mPa•s; The additive includes pyridine additives, and the structural formula of the pyridine additives is , where R 2 is any one of -H, -F, -C n H 2n+1 or -C n H 2n-1 , and n = 1 - 3.

2. The lithium-ion battery electrolyte solution according to claim 1, characterized in that, The room-temperature conductivity σ is 15-30 mS·cm -1 .

3. The lithium-ion battery electrolyte solution according to claim 1, characterized in that, the ε is 20 - 40.

4. The lithium-ion battery electrolyte solution according to claim 1, characterized in that, the γ is 15 - 50 wt%.

5. The lithium-ion battery electrolyte solution according to claim 1, characterized in that, The structural formula of the nitrile solvent is , where R 1 is -C n H 2n+1 or R 1 is -C 6 H 5 , and n = 1 - 4.

6. The lithium-ion battery electrolyte solution according to claim 1, characterized in that, the organic solvent further comprises a carbonate solvent.

7. The lithium-ion battery electrolyte solution according to claim 6, characterized in that, the carbonate solvent comprises any two or at least three combinations of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate or dimethyl carbonate.

8. The lithium-ion battery electrolyte solution according to claim 1, characterized in that, the additive further comprises a lithium salt additive and / or an ester additive.

9. The lithium-ion battery electrolyte solution according to claim 8, characterized in that, the additive is a combination of a pyridine additive, a lithium salt additive and an ester additive.

10. The lithium-ion battery electrolyte solution according to claim 1, characterized in that, the content of the pyridine additive accounts for 1 - 9 wt% of the content of the nitrile solvent.

11. The lithium-ion battery electrolyte solution according to claim 8, characterized in that, the lithium salt additive comprises any one or at least two combinations of oxalate borate, oxalate phosphate or difluorophosphate.

12. The lithium-ion battery electrolyte solution according to claim 8, characterized in that, the ester additive comprises any three or at least four combinations of vinylene carbonate, ethylene vinylene carbonate, fluorinated ethylene carbonate, ethylene sulfite or methylene methane disulfonate.

13. The lithium-ion battery electrolyte solution according to claim 8, characterized in that, the content of the lithium salt additive accounts for 0.1 - 0.8 wt% of the total amount of the lithium-ion battery electrolyte solution.

14. The lithium-ion battery electrolyte solution according to claim 8, characterized in that, the content of the ester additive accounts for 2 - 7 wt% of the total amount of the lithium-ion battery electrolyte solution.

15. The lithium-ion battery electrolyte solution according to claim 1, characterized in that, the lithium salt comprises lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide.

16. The lithium-ion battery electrolyte solution according to claim 1, characterized in that, in the lithium-ion battery electrolyte solution, the concentration of the lithium salt is 0.8 - 2.5 mol / L.

17. The lithium-ion battery electrolyte according to claim 1, characterized in that the content of the lithium salt accounts for 10-20 wt% of the total amount of the lithium-ion battery electrolyte.

18. The lithium-ion battery electrolyte according to claim 17, characterized in that the content of the lithium salt accounts for 14.5-18 wt% of the total amount of the lithium-ion battery electrolyte.

19. A method for preparing a lithium-ion battery electrolyte according to any one of claims 1-18, characterized in that the preparation method comprises the following steps: mixing a lithium salt, an additive and an organic solvent to obtain the lithium-ion battery electrolyte; wherein, the organic solvent comprises a nitrile solvent.

20. The preparation method according to claim 19, characterized in that the organic solvent further comprises a carbonate solvent.

21. The preparation method according to claim 20, characterized in that the volume ratio of the carbonate solvent to the nitrile solvent is (70-50):(30-50).

22. The preparation method according to claim 19, characterized in that the mixing method comprises: mixing the carbonate solvent and the nitrile solvent, then adding the lithium salt for mixing, and subsequently adding the additive for mixing.

23. The preparation method according to claim 19, characterized in that the preparation method comprises the following steps: (1) Stirring and mixing ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate and acetonitrile according to a volume ratio of (15-35):(15-45):(0-30):(30-50) to obtain an organic solvent; (2) Adding lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide with a total content of 14.5-18 wt% into the organic solvent and stirring and mixing to obtain a mixed solution; (3) Adding a pyridine-based additive, a lithium salt-based additive and an ester-based additive into the mixed solution and stirring and mixing to obtain the lithium-ion battery electrolyte.

24. A lithium-ion battery, characterized in that the lithium-ion battery comprises a positive electrode sheet, a negative electrode sheet and the lithium-ion battery electrolyte according to any one of claims 1-18.

25. The lithium-ion battery according to claim 24, characterized in that the particle size D50 of the negative active material in the negative electrode sheet is 13-14 μm.

26. The lithium-ion battery according to claim 25, characterized in that The BET specific surface area of the negative electrode active material is 0.5 - 2 m 2 / g.

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