An electrolyte containing a nitrile additive and a lithium-ion battery

Abstract

The present invention discloses an electrolyte containing a nitrile additive. The electrolyte includes a lithium salt, an organic solvent, and a nitrile additive. The nitrile additive is selected from at least one of succinonitrile, glutaronitrile, adiponitrile, trans-butenedinitrile, trans-hexenedinitrile, 1,2-bis(cyanoethoxy)ethane, 1,3,6-hexanetricarbonitrile, and 1,2,3-tris(cyanoethoxy)propane, and the content of olefin nitrile impurities in the electrolyte containing the nitrile additive is ≤ 100 ppm. The present invention has the advantages of improving the battery cycle performance and storage stability performance.

Description

Technical Field

[0001] The present invention relates to an electrolyte for a lithium-ion battery, and particularly to an electrolyte containing a nitrile additive with acrylonitrile impurities content ≤ 100 ppm and a lithium-ion battery. Background Art

[0002] In order to improve the energy density of a lithium-ion battery, increasing the charging cut-off potential of the positive electrode active material is an effective solution. However, as the charging voltage of the positive electrode material continuously increases, the oxidation reaction of the non-aqueous electrolyte solvent on the surface of the positive electrode material also continuously intensifies, resulting in problems such as battery gas generation and increased internal resistance, deteriorating the battery cycle stability. In addition, with the dissolution of transition metal ions in the positive electrode material, the solvent oxidation is aggravated on the positive electrode side, and the SEI is damaged by reduction deposition on the negative electrode side, resulting in the deterioration of the overall battery performance.

[0003] Sony Patent JP2010073367A discloses that nitrile compounds can effectively complex transition metal ions and improve the oxidation resistance of the electrolyte. LG Chem Patent KR1195931B1 discloses a polynitrile compound containing an ether bond, which, as an electrolyte additive, can effectively improve the high-temperature cycle and storage performance of the battery. That is to say, nitrile compounds are commonly used in the art to improve the high-temperature performance of the battery. However, those skilled in the art also know well that the use of nitrile additives will also bring problems such as an increase in the internal resistance of the battery cell and deterioration of the normal-temperature and low-temperature cycles.

[0004] Aiming at the defect of deterioration of the normal-temperature and low-temperature cycles caused by nitrile additives, the industry usually adds impedance-reducing additives such as lithium difluorophosphate and vinylene sulfate to the electrolyte containing nitrile additives to improve the performance of the battery cell. However, lithium difluorophosphate will increase the gas generation during the storage of the battery cell, and vinylene sulfate has problems with the storage stability of the electrolyte. Summary of the Invention

[0005] The inventor of the present invention has found through research that adding nitrile additives with different purities to the electrolyte has different degrees of deterioration of the normal-temperature performance of the battery cell. Further research has found that acrylonitrile impurities in nitrile additives are the main factors affecting the difference in the use performance of nitrile additives. Specifically, acrylonitrile compounds contain double bonds in their structures and are prone to obtaining electrons and undergoing reduction decomposition on the electrode surface. Tests show that acrylonitrile compounds have a relatively high film formation impedance. Therefore, when the content of acrylonitrile impurities in the electrolyte is relatively high, the electrode interface impedance increases, the battery internal resistance rises, the cycle attenuation intensifies, and the normal-temperature and low-temperature performance of the battery deteriorates.

[0006] Based on the above research, in order to solve the above technical problems, the present invention proposes an electrolyte containing a nitrile additive with the content of acrylonitrile-based impurities ≤ 100 ppm. When using this electrolyte, the battery cycle performance and storage stability performance are effectively improved. Moreover, without adding any other room-temperature additives, the high-temperature performance of the nitrile additive is efficiently utilized, and at the same time, the room-temperature performance and low-temperature performance of the battery will not deteriorate.

[0007] The object of the present invention is achieved by the following technical solutions:

[0008] An electrolyte containing a nitrile additive, comprising a lithium salt, an organic solvent, and a nitrile additive, wherein the content of acrylonitrile-based impurities in the electrolyte is ≤ 100 ppm. When the content of acrylonitrile-based impurities in the nitrile additive is ≤ 100 ppm, the battery performance does not show obvious differences with the decrease of the impurity content. More preferably, the content of acrylonitrile-based impurities in the electrolyte containing a nitrile additive is ≤ 50 ppm.

[0009] The nitrile additives that can be used in the present invention are selected from at least one of succinonitrile, glutarodinitrile, adiponitrile, trans-butenedinitrile, trans-hexenedinitrile, 1,2-bis(cyanoethoxy)ethane, 1,3,6-hexanetricarbonitrile, 1,2,3-tris(cyanoethoxy)propane. Preferably, the nitrile additive is selected from at least one of trans-butenedinitrile, trans-hexenedinitrile, 1,2-bis(cyanoethoxy)ethane, 1,3,6-hexanetricarbonitrile, 1,2,3-tris(cyanoethoxy)propane. More preferably, the nitrile additive is 1,3,6-hexanetricarbonitrile and / or 1,2,3-tris(cyanoethoxy)propane.

[0010] The content of the nitrile additive used in the present invention accounts for 0.1% - 5.0% of the total mass of the electrolyte, preferably 1.0% - 3.0%.

[0011] The preparation of nitrile additives often uses acrylonitrile or 4-pentenenitrile as raw materials. For example, US Patent US20120197025A discloses a method for preparing adiponitrile using 4-pentenenitrile as a raw material, the article (Tetrahedron Letters, vol. 46, #42, p. 7225 - 7228) discloses a method for preparing trans-butenedinitrile using acrylonitrile as a raw material, US Patent US2014018567A discloses a method for preparing 1,2-bis(cyanoethoxy)ethane using acrylonitrile as a raw material, US Patent US20020007081A discloses a method for preparing 1,3,6-hexanetricarbonitrile using acrylonitrile as a raw material, and the article (New Journal of Chemistry, 2015, vol. 39, #12, p. 9155 - 9161) discloses a method for preparing 1,2,3-tris(cyanoethoxy)propane using acrylonitrile as a raw material.

[0012] During the preparation of the nitrile additive, the generated nitrile impurities may include: acrylonitrile, acrylonitrile dimer, acrylonitrile polymer, 4-pentenenitrile, 4-pentenenitrile dimer, 4-pentenenitrile polymer, and the acrylonitrile polymer / 4-pentenenitrile polymer includes but is not limited to trimers, tetramers, and pentamers.

[0013] Generally, commercially available nitrile additives all contain nitrile impurities, and their purity is between 98% and 99%. In order to obtain a nitrile additive that meets the requirements for use in the lithium-ion battery electrolyte, the present invention further purifies the commercially available nitrile additive or the crude nitrile additive obtained by column chromatography, recrystallization, rectification or other methods.

[0014] The lithium salt of the present invention can be a conventional lithium salt in the electrolyte. Preferably, the lithium salt is selected from at least one of lithium hexafluorophosphate, lithium difluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium perchlorate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium difluoro bis(oxalato)phosphate, and lithium tetrafluoro(oxalato)phosphate, and the concentration of the lithium salt in the electrolyte is 0.3 M to 2 M. More preferably, the lithium salt includes lithium hexafluorophosphate, and its concentration in the electrolyte is 0.8 to 1.5 M.

[0015] The organic solvent of the present invention can be a commonly used solvent in the electrolyte. Preferably, the organic solvent is selected from at least one of ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, 1,4-butyrolactone, methyl propionate, methyl butyrate, ethyl acetate, ethyl propionate, and propyl propionate.

[0016] In order to further improve the performance of the lithium-ion battery in all aspects, the electrolyte further includes a basic additive, and the basic additive is selected from at least one of cyclic carbonates containing carbon-carbon double bonds, fluorinated cyclic carbonates, and sulfonic acid lactone compounds, and the mass percentage of any one of the basic additives in the total mass of the electrolyte is 0.1% to 5.0%.

[0017] The present invention also provides a lithium-ion battery, which includes a positive electrode, a negative electrode, a separator, and the electrolyte described in any one of the above.

[0018] The positive electrode can be a commonly used positive electrode in the lithium battery field. Preferably, the positive electrode active material is selected from one of nickel-manganese-cobalt or nickel-manganese-aluminum ternary materials, lithium nickel manganate, lithium cobaltate, lithium-rich manganese-based solid solution, lithium manganate, or lithium iron phosphate.

[0019] The negative electrode can be a commonly used negative electrode in the lithium battery field. Preferably, the negative electrode active material is selected from one of artificial graphite, coated natural graphite, silicon-carbon negative electrode, silicon negative electrode, or lithium titanate.

[0020] The voltage of the lithium-ion battery can be selected from the conventional battery voltage range of 2.5 to 4.2V or the high-voltage test range above 4.2V.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0022] 1. By limiting the content of vinyl cyanide impurities in the nitrile additive, the present invention improves the cycle performance of the battery, especially improves the room-temperature and high-temperature cycles of the battery containing the nitrile additive.

[0023] By limiting the content of vinyl cyanide impurities in the nitrile additive, the present invention further improves the storage stability of the battery. Detailed implementation manners

[0024] The present invention will be further described below in conjunction with specific embodiments, but the present invention is not limited to these specific implementation manners. Those skilled in the art should recognize that the present invention covers all alternative solutions, improvement solutions, and equivalent solutions that may be included within the scope of the claims.

[0025] Preparation examples:

[0026] Preparation of nitrile additives:

[0027] The purchased adiponitrile and 1,2,3-tris(cyanoethoxy)propane were purified by column chromatography. The specific steps are as follows:

[0028] The stationary phase of column chromatography was 200 - 300 mesh silica gel, and the eluent was a mixed solution of petroleum ether and ethyl acetate. The eluent of the mixed sample obtained by column chromatography was removed by a rotary evaporator to obtain adiponitrile with a purity of ≥99.9 and 1,2,3-tris(cyanoethoxy)propane with a purity of ≥99.9%.

[0029] The purchased trans-butenedinitrile was purified by recrystallization. The specific steps are as follows:

[0030] Trans-butenedinitrile was added to dimethyl carbonate and heated until it completely dissolved. Then it was cooled until a large amount of crystals precipitated, and it was filtered while it was hot. The solid obtained after filtration was dried to obtain trans-butenedinitrile with a purity of ≥99.9%.

[0031] Preparation of basic electrolyte:

[0032] In a glove box (both H2O and O2 < 0.1 ppm), ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed at a mass ratio of EC:EMC:DEC = 30:20:50, and then lithium hexafluorophosphate with a mass fraction of 14.0 wt% was slowly added to the mixed solution.

[0033] Preparation of lithium-ion battery: Inject the above-prepared lithium-ion battery electrolyte into a 4.45V LiCoO2 / graphite polymer battery that has been fully dried. After the battery is left standing at 45°C, formed, and second-sealed, a grading test is carried out.

[0034] Example 1

[0035] Add the obtained 1,2,3-tris(cyanoethoxy)propane to the basic electrolyte obtained in the preparation example, and the addition amount of the 1,2,3-tris(cyanoethoxy)propane accounts for 2.0 wt% of the total mass of the electrolyte. In this example, the nitrile impurities are mainly acrylonitrile, and the acrylonitrile content in the electrolyte is 10 ppm.

[0036] Example 2

[0037] Add acrylonitrile on the basis of Example 1 to make the content of nitrile impurities in the electrolyte 50 ppm.

[0038] Example 3

[0039] Add acrylonitrile on the basis of Example 1 to make the content of nitrile impurities in the electrolyte 70 ppm.

[0040] Example 4

[0041] Add acrylonitrile on the basis of Example 1 to make the content of nitrile impurities in the electrolyte 100 ppm.

[0042] Example 5

[0043] Add the obtained adiponitrile to the basic electrolyte obtained in the preparation example, and the addition amount of the adiponitrile accounts for 2.0 wt% of the total mass of the electrolyte. In this example, the nitrile impurities are mainly 4-pentenitrile, and the 4-pentenitrile content in the electrolyte is 10 ppm.

[0044] Example 6

[0045] Add 4-pentenitrile on the basis of Example 5 to make the content of nitrile impurities in the electrolyte 50 ppm.

[0046] Example 7

[0047] Add 4-pentenitrile on the basis of Example 5 to make the content of nitrile impurities in the electrolyte 100 ppm.

[0048] Example 8

[0049] The operation of this example is the same as that of Example 1, except that: the addition amount of 1,2,3-tris(cyanoethoxy)propane is reduced to 1.0 wt% of the total mass of the electrolyte, and the acrylonitrile content in the electrolyte is 5 ppm.

[0050] Example 9

[0051] Based on Example 8, acrylonitrile was added to make the content of acrylonitrile impurities in the electrolyte 100 ppm.

[0052] Example 10

[0053] Trans - butenedinitrile obtained by preparation was added to the basic electrolyte obtained by the preparation example, and the addition amount of adiponitrile accounted for 2.0 wt% of the total mass of the electrolyte. In this example, the main acrylonitrile impurity was 4 - pentenenitrile, and the content of 4 - pentenenitrile in the electrolyte was 10 ppm.

[0054] Example 11

[0055] Based on Example 10, acrylonitrile was added to make the content of acrylonitrile impurities in the electrolyte 70 ppm.

[0056] Example 12

[0057] Based on Example 10, acrylonitrile was added to make the content of acrylonitrile impurities in the electrolyte 100 ppm.

[0058] Comparative Example 1

[0059] The difference from Example 1 was that no nitrile additive was added to the electrolyte.

[0060] Comparative Example 2

[0061] Based on Example 1, acrylonitrile was added, and the content of acrylonitrile impurities in the electrolyte was 200 ppm.

[0062] Comparative Example 3

[0063] The difference from Example 1 was that commercially available 1,2,3 - tris(cyanoethoxy)propane was added, and the content of acrylonitrile impurities in the electrolyte was 300 ppm.

[0064] Comparative Example 4

[0065] The difference from Example 5 was that commercially available adiponitrile was added, and the content of 4 - pentenenitrile impurities in the electrolyte was 200 ppm.

[0066] Comparative Example 5

[0067] The difference from Example 10 was that commercially available trans - butenedinitrile was added, and the content of acrylonitrile impurities in the electrolyte was 400 ppm.

[0068] Comparative Example 6

[0069] Based on Example 8, acrylonitrile was added, and the content of acrylonitrile impurities in the electrolyte was 200 ppm.

[0070] The normal temperature, low temperature and high temperature cycle performance tests of the batteries, as well as the high temperature storage performance test of the batteries, were respectively carried out on Examples 1-12 and Comparative Examples 1-6. The test results are shown in Table 1.

[0071] Table 1 Test Results of Battery Performance

[0072]

[0073] Compared with the examples, for Comparative Example 1, with the introduction of the nitrile additive, the normal temperature and low temperature cycle performance deteriorated, but the high temperature cycle and storage performance were enhanced. By comparing Example 1-4 with Comparative Examples 2 and 3, Example 5-7 with Comparative Example 4, Example 10-12 with Comparative Example 5, and Example 8 and 9 with Comparative Example 6, it can be concluded that when the content of olefin nitrile impurities in the nitrile-containing electrolyte is higher than 100 ppm, the cycle performance and storage stability of the battery cells deteriorate with the increase of the impurity content.

Claims

1. An electrolyte containing a nitrile additive, comprising a lithium salt, an organic solvent, and 1,2,3-tris(cyanoethoxy)propane, characterized in that: The electrolyte further comprises: acrylonitrile and / or 4-pentenenitrile, with a content in the electrolyte of 5 to 100 ppm; 1,2,3-tris(cyanoethoxy)propane accounts for 1.0% to 3.0% of the total mass of the electrolyte.

2. The electrolyte according to claim 1, wherein: The content of acrylonitrile and / or 4-pentenenitrile in the electrolyte is 5 to 50 ppm.

3. The electrolyte according to claim 1, characterized in that: The 1,2,3-tris(cyanoethoxy)propane is prepared by using acrylonitrile or 4-pentenenitrile as a raw material.

4. The electrolyte according to claim 1, characterized in that: The lithium salt is selected from at least one of lithium hexafluorophosphate, lithium difluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium perchlorate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium difluorobis(oxalato)phosphate, and lithium tetrafluoro(oxalato)phosphate, and the concentration of the lithium salt in the electrolyte is 0.3 M to 2 M.

5. The electrolyte according to claim 1, characterized in that: The organic solvent is selected from at least one of ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, 1,4-butyrolactone, methyl propionate, methyl butyrate, ethyl acetate, ethyl propionate, and propyl propionate.

6. The electrolyte according to any one of claims 1-5, characterized in that: The electrolyte further comprises a basic additive, and the basic additive is selected from at least one of cyclic carbonates containing carbon-carbon double bonds, fluorinated cyclic carbonates, sultone compounds, and cyclic sulfates, and any one of the basic additives accounts for 0.1% to 5.0% of the total mass of the electrolyte.

7. A lithium-ion battery, comprising a positive electrode, a negative electrode, and a separator, characterized in that: The lithium ion battery further comprises the electrolyte according to any one of claims 1-6.

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