Electrolyte and secondary battery using same

Abstract

The invention provides an electrolyte, the electrolyte comprises a fluorine-containing ester additive, the proportion of the fluorine-containing ester additive in the electrolyte is 2%-9% according to the mass percentage, and the fluorine-containing ester additive comprises 2, 2-ethyl difluoroacetate, difluoroethylene carbonate and trifluoroethyl methyl carbonate. In the scheme, the DFEA, the DFEC and the FEC are matched to participate in component construction of the electrolyte, so that the electrolyte provided by the scheme can effectively reduce reaction heat between the electrolyte and a positive electrode active material / negative electrode active material, can effectively reduce reaction heat production and combustion heat of the secondary battery, is beneficial to preventing thermal runaway of the secondary battery, and improves the service life of the secondary battery. A certain flame-retardant effect is achieved in the working process of the secondary battery, and the safety performance of the secondary battery is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery materials, and in particular, relates to an electrolyte and a secondary battery using the same. Background Art

[0002] In recent years, secondary battery technology has developed rapidly and has gradually become the core driving force leading the electrification of automobiles and new energy storage. As a result, users have higher requirements for the energy density of secondary batteries. In order to improve the energy density of secondary batteries, the industry has made various attempts, such as the innovation of positive and negative electrode active materials. The application of high-nickel positive electrode active materials and silicon-based negative electrode active materials are effective ways to improve the energy density of secondary batteries. However, the development direction of large capacity and high energy density increases the energy of thermal runaway of secondary battery cells and increases the risk of flammability and explosion of battery cells. The safety hazards of flammability and explosion have shaken users' confidence in the use of high-energy-density secondary batteries and have become one of the main bottlenecks restricting the large-scale industrial application of high-energy-density secondary batteries. Summary of the invention

[0003] In order to reduce the flammability and explosion risks of high energy density secondary batteries and improve the safety performance of secondary batteries, the present invention provides an electrolyte and a secondary battery using the same.

[0004] According to the first aspect of the present invention, an electrolyte is provided, the electrolyte includes a fluorinated ester additive, and the fluorinated ester additive accounts for 2% to 9% of the electrolyte according to the mass percentage. The fluorinated ester additive includes 2,2-difluoroethyl acetate (DFEA), bisfluoroethylene carbonate (DFEC) and trifluoroethyl methyl carbonate (FEMC). In this scheme, DFEA, DFEC and FEMC are matched to participate in the component construction of the electrolyte, so that the electrolyte provided by this scheme can effectively reduce the reaction heat between the electrolyte and the positive electrode active material / negative electrode active material, and can effectively reduce the reaction heat generation and combustion heat of the secondary battery, which is conducive to preventing the secondary battery from thermal runaway, playing a certain flame retardant role in the working process of the secondary battery, and improving the safety performance of the secondary battery. According to the mass percentage, the proportion of fluorinated ester additives in the electrolyte can be 2%, 4%, 5%, 7%, 9%, etc., but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0005] Preferably, in the electrolyte, calculated by mass ratio, DFEA: DFEC: FEMC = 0.5-3: 1-3: 1-3. Based on the consideration of improving safety performance, the ratio between DFEA, DFEC and FEMC is optimized. The mass ratio of DFEA, DFEC and FEMC DFEA: DFEC: FEMC can be 0.5: 1: 1, 3: 1: 1, 1: 1: 1, 2: 1.5: 1.5, 0.5: 3: 3, etc., but is not limited to the listed values, and other values ​​not listed in the numerical range are also applicable.

[0006] Preferably, in the electrolyte, calculated by mass ratio, DFEA: DFEC: FEMC = 1.2 to 1.8: 1.2 to 1.8: 0.8 to 1.1. By further optimizing the ratio of DFEA, DFEC, and FEMC in the electrolyte provided by this solution, the heat generated by the reaction between the electrolyte and the positive electrode active material and the negative electrode active material can be further reduced, thereby giving full play to the advantages of the electrolyte provided by this solution in improving the safety performance of secondary batteries. The mass ratio of DFEA, DFEC, and FEMC DFEA: DFEC: FEMC can be 1.2: 1.2: 0.8, 1.8: 1.8: 1.1, 1.5: 1.5: 1, 1.5: 1.2: 0.8, 1.2: 1.8: 0.8, etc., but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0007] Preferably, the proportion of the fluorinated ester additive in the electrolyte is 2% to 5% by mass. The proportion of the fluorinated ester additive in the electrolyte can be 2%, 3%, 4%, 4.5%, 5%, etc., but is not limited to the listed values. Other values ​​not listed in the range are also applicable.

[0008] Preferably, the electrolyte further comprises vinylene carbonate (VC).

[0009] Preferably, the electrolyte also includes an organic solvent, and the organic solvent includes dimethyl carbonate (DMC). Calculated by mass percentage, dimethyl carbonate accounts for 52% to 59% of the electrolyte. Based on the application of DFEA, DFEC, and FEMC, DMC is further used in combination with the above materials to achieve the effect of taking into account the safety performance and cycle performance of the secondary battery using the electrolyte, thereby ensuring the service life of the secondary battery using the electrolyte. Calculated by mass percentage, the proportion of DMC in the electrolyte can be 52%, 54%, 55%, 57%, 59%, etc., but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0010] Preferably, in the electrolyte, calculated by mass ratio, DMC: fluorinated ester additive = 55-60:2-5. Calculated by mass ratio, DMC: fluorinated ester additive can be 55:2, 60:2, 55:5, 60:5, 57:4, etc., but is not limited to the listed values, and other values ​​not listed in the numerical range are also applicable.

[0011] Preferably, the organic solvent also includes propylene carbonate (PC) and fluoroethylene carbonate (FEC). Based on the application of DFEA, DFEC and FEMC, the composition of the organic solvent of the electrolyte is further optimized, thereby achieving the effect of further improving the cycle performance of the electrolyte.

[0012] Preferably, in the electrolyte, calculated by mass ratio, PC: FEC: DMC = 11 to 20: 5 to 15: 52 to 59. Calculated by mass ratio, PC: FEC: DMC can be 11: 5: 52, 20: 15: 59, 11: 15: 52, 11: 5: 59, 20: 5: 52, 15: 10: 55, etc., but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0013] According to a second aspect of the present invention, a secondary battery is provided. The secondary battery includes a positive electrode, a negative electrode and the above electrolyte.

[0014] Preferably, the above-mentioned secondary battery satisfies at least one of the following a and b: a. the positive electrode active material in the secondary battery includes a high-nickel positive electrode active material, in which the total amount of metal elements other than lithium is A, the amount of nickel element is B, and 0.6≤B / A<1; b. the negative electrode active material in the secondary battery includes a silicon-based negative electrode active material. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 DSC test results of soft-pack batteries in battery systems using electrolytes with formula D1;

[0016] Figure 2 DSC test results of soft-pack batteries in battery systems using electrolytes in Formula 1;

[0017] Figure 3 DSC test results of soft-pack batteries in the battery system using the electrolyte of Formula 2. DETAILED DESCRIPTION

[0018] In order to enable those skilled in the art to better understand the solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only embodiments of a part of the present invention, rather than all embodiments.

[0019] Example 1

[0020] 1. Prepare electrolyte

[0021] According to Table 1, this embodiment needs to prepare electrolytes with formula numbers of Formula 1, Formula 2, Formula D1, Formula D2, Formula D3, Formula D4, Formula D5, Formula D6, and Formula D7 in Table 1, and prepare materials according to the electrolyte formula components shown in Table 1. After the preparation is completed, according to different formulas, the required materials are mixed in amount and fully dissolved to form a uniform solution to complete the preparation of the electrolyte.

[0022] Table 1. Electrolyte formula required for preparation in Example 1

[0023]

[0024] 2. Preparation of soft pack batteries

[0025] The electrolyte prepared in this example was used to prepare soft-pack batteries with a battery capacity of 3.3Ah. The specific operations are as follows:

[0026] S1. Prepare materials according to the mass ratio of ternary material NCM811: binder polyvinylidene fluoride (PVDF): conductive agent = 97:1.6:1.4, use the above materials to prepare positive electrode slurry, use aluminum foil as the positive electrode current collector, apply the positive electrode slurry on the surface of the aluminum foil, and dry it to convert the positive electrode slurry coating into a positive electrode active material layer, thereby preparing a soft-pack positive electrode.

[0027] S2. Prepare materials according to the mass ratio of silicon-carbon negative electrode material (silicon doping accounts for 20wt.%): aqueous binder: conductive agent = 96:3:1, use the above materials to prepare negative electrode slurry, use copper foil as the negative electrode current collector, apply the negative electrode slurry on the surface of the copper foil, and dry it to convert the negative electrode slurry coating into a negative electrode active material layer (negative electrode gram capacity is 600mAh / g), thereby preparing the soft-pack battery negative electrode.

[0028] S3. A PE film with a ceramic coating on the surface is used as a separator, and the soft-pack positive electrode, separator, and soft-pack battery negative electrode are stacked in sequence to make a battery cell. The compaction density of the obtained battery cell is 1.4g / cm 3 .

[0029] S4. Place the battery cell obtained in S3 into a soft-pack shell, inject the electrolyte obtained in this embodiment, and obtain a soft-pack battery through vacuum packaging, standing, forming, shaping and other processes.

[0030] 3. Needle puncture resistance test and high temperature resistance test

[0031] The soft-pack battery prepared in this embodiment is used as the test object for the needle puncture resistance test and the high temperature resistance test. Based on the difference in the composition of the electrolyte used, 50 repetitions are set for each soft-pack battery tested, and each repetition is a soft-pack battery. The specific test methods of the needle puncture resistance test and the high temperature resistance test are shown below.

[0032] (1) Test method

[0033] Needle puncture test: After the test object is fully charged, use a Φ3mm high-temperature resistant steel needle (the cone angle of the needle tip is 45-60°, the needle surface is smooth, free of rust, oxide layer and oil), at a speed of (25±5)mm / s, to penetrate from a direction perpendicular to the plate of the soft-pack battery. The penetration position should be close to the geometric center of the punctured surface. The steel needle stays in the soft-pack battery for 1 hour, observe the fire situation of the test object and count and calculate the number of test objects that caught fire.

[0034] High temperature resistance test: Place the test object to be tested in a 150°C hot box for 30 minutes, observe the fire situation of the test object and count and calculate the number of test objects that caught fire.

[0035] (2) Test results

[0036] The test results are shown in Table 2. From the test results shown in Table 2, it can be seen that in the needle puncture resistance test and the high temperature resistance test, the soft-pack batteries measured in the corresponding proportions can reach 0% only with the soft-pack batteries using the electrolyte of Formula 1 and the electrolyte of Formula 2. The soft-pack batteries made with the electrolytes of Formula D1, Formula D2, Formula D3, Formula D4, Formula D5, Formula D6, and Formula D7 all caught fire during the test.

[0037] Formula D1 does not contain DFEA, DFEC and FEMC. From the test results, we can see that the soft-pack batteries using the electrolyte of Formula D1 basically caught fire in the needle puncture resistance test and the high temperature resistance test. Formula D5, Formula D6 and Formula D7 only contain one of DFEA, DFEC and FEMC. By comparing the test results of the corresponding electrolytes of the above formulas with the test results of the corresponding electrolyte of Formula D1, the results show that the introduction of one or two materials of DFEA, DFEC and FEMC into the electrolyte cannot effectively improve the safety performance and heat release of the battery.

[0038] Formula D2, Formula D3, and Formula D4 lack one of DFEA, DFEC, and FEMC, respectively. Compared with soft-pack batteries using Formula D1 electrolyte, Formula D5 electrolyte, Formula D6 electrolyte, and Formula D7 electrolyte, the proportion of fire-catching soft-pack batteries corresponding to the needle puncture test and high temperature test of soft-pack batteries using Formula D2 electrolyte, Formula D3 electrolyte, and Formula D4 electrolyte has been reduced to a certain extent, but the proportion of fire-catching soft-pack batteries in the needle puncture test and high temperature test is still relatively high.

[0039] In summary, the test results in Table 2 show that the use of DFEA, DFEC and FEMC as fluorinated ester additives in the preparation of the electrolyte can significantly improve the safety performance of the soft-pack battery.

[0040] Table 2. Statistics of the needle puncture resistance test results and high temperature resistance test results of the soft-pack battery in Example 1

[0041]

[0042] 4.DSC test

[0043] Among the soft-pack batteries prepared in this embodiment, the soft-pack batteries using the electrolyte of Formula 1, the electrolyte of Formula 2, and the electrolyte of Formula D1 were used as the test objects of the DSC test.

[0044] (1) Test method

[0045] The specific operation of DSC test is as follows:

[0046] S1. Disassemble the fully charged soft-pack battery as the test object and pour out the electrolyte for later use;

[0047] S2. Separate the positive electrode and the negative electrode, wash them with DMC and dry them, and then use a ceramic knife to scrape the positive electrode active material layer and the negative electrode active material layer to obtain the positive electrode / negative electrode active material powder for standby use;

[0048] S3. Set the relevant test parameters of the DSC tester as follows: temperature range: 25-500°C; atmosphere: argon-Ar; heating rate: 5°C / min;

[0049] S4 records the subsequent empty crucible mass of the aluminum crucible used to load the sample to be tested, and then put the aluminum crucible into the DSC tester, and perform DSC test under the test conditions set in S3 to obtain a reference curve of the empty crucible;

[0050] S5. Load the positive electrode active material powder, the negative electrode active material powder and the soft pack battery into the aluminum crucible used in S4, cover the open mouth of the aluminum crucible with the aluminum cover of the aluminum crucible, press to seal, and then insert the hole to perform DSC test under the test conditions set in S3.

[0051] (2) Test results

[0052] Figure 1 The DSC test results of the soft-pack battery of the battery system using the formula D1 electrolyte are shown, and the heat release of the DSC test is 2114 J / g. Figure 2 The DSC test results of the soft-pack battery of the battery system using the electrolyte of Formula 1 are shown, and the heat release of the DSC test is 1276 J / g. Figure 3 The DSC test results of the soft-pack battery of the battery system using the electrolyte of Formula 2 are shown, and the heat release of the DSC test is 1328 J / g. Obviously, compared with the electrolyte of Formula D1, the heat generated by the reaction between the electrolyte of Formula 1 and Formula 2 and the positive electrode active material and the negative electrode active material is significantly lower. Based on this, the safety performance of the soft-pack battery using the electrolyte of Formula 1 and the soft-pack battery using the electrolyte of Formula 2 is better, which is consistent with the test results of the above-mentioned needle puncture resistance test and high temperature resistance test in this embodiment.

[0053] Example 2

[0054] 1. Prepare electrolyte

[0055] According to Table 3, this embodiment needs to prepare electrolytes with formula numbers of Formula 3, Formula 4, Formula 5, Formula 6, Formula 7, and Formula 8 in Table 3, and prepare materials according to the electrolyte formula component composition shown in Table 3. For ease of comparison, Table 3 also shows the component composition of Formula 1 electrolyte, Formula 2 electrolyte, and Formula D1 electrolyte prepared in Example 1. After the preparation is completed, according to different formulas, the required materials are mixed according to the amount, fully dissolved to form a uniform solution, and the preparation of the electrolyte is completed.

[0056] Table 3. Electrolyte formula required for preparation in Example 2

[0057]

[0058]

[0059] 2. Preparation of soft pack batteries

[0060] The electrolyte prepared in this example was used to prepare soft-pack batteries with a battery capacity of 3.3 Ah. Except for the difference in the electrolyte used, the operations for preparing the soft-pack batteries in this example were consistent with the related operations for preparing the soft-pack batteries in Example 1.

[0061] 3. Needle puncture resistance test and high temperature resistance test

[0062] The soft-pack battery prepared in this embodiment is used as the test object for the needle puncture resistance test and the high temperature resistance test. Based on the difference in the composition of the electrolytes used, 50 repetitions are set for each soft-pack battery tested, and each repetition is a soft-pack battery.

[0063] (1) Test method

[0064] Except for the difference in the test objects, the specific test methods of the needle puncture resistance test and the high temperature resistance test in this embodiment are consistent with the relevant operations of the needle puncture resistance test and the high temperature resistance test in Example 1.

[0065] (2) Test results

[0066] The test results are shown in Table 4. For ease of comparison, Table 4 also shows the test results of the needle puncture resistance test and high temperature resistance test of the soft-pack batteries using Formula 1 electrolyte, Formula 2 electrolyte, and Formula D1 electrolyte in Example 1.

[0067] The soft-pack batteries using the electrolyte of Formula 1, the electrolyte of Formula 2 and the electrolyte of Formula 3 did not catch fire in the needle puncture resistance test and the high temperature resistance test involved in this embodiment, which shows that the safety performance of the above three electrolytes is relatively good. The ratio of DFEA, DFEC and FEMC contained in the electrolyte of Formula 1 and the electrolyte of Formula 3 is the same. In addition, the ratio of DFEA, DFEC and FEMC contained in the electrolyte of Formula 8 is also the same as the above two electrolytes, but because the total amount of DFEA, DFEC and FEMC contained in Formula 8 is relatively low, the safety performance of the soft-pack batteries using the electrolyte of Formula 8 is not as good as that of the soft-pack batteries using the electrolyte of Formula 1 and the electrolyte of Formula 3.

[0068] Comparing the electrolytes of Formula 2, Formula 3, Formula 4, Formula 5, Formula 6, and Formula 7, the total amount of DFEA, DFEC, and FEMC contained in these formulas is the same, and the difference lies in the ratio between the above three materials. From the test results of the soft-pack batteries using the above electrolytes, when the mass ratio of DFEA, DFEC, and FEMC in the electrolyte formula satisfies DFEA: DFEC: FEMC = 0.5-3: 1-3: 1-3, it is beneficial to further improve the safety performance of the battery.

[0069] Table 4. Statistics of the needle puncture resistance test results and high temperature resistance test results of the soft-pack battery in Example 2

[0070]

[0071] 4. Cycle performance test

[0072] Soft-pack batteries based on Formula 1 electrolyte, Formula 2 electrolyte, Formula 3 electrolyte, Formula 8 electrolyte, and Formula D1 electrolyte were used as test objects for the cycle performance test.

[0073] (1) Test method

[0074] The soft-pack battery as the test object is placed in a constant temperature box, and charged to 4.2V at 1.0C at a temperature of (25±2)℃, and then charged to 0.05C at a constant voltage. After being set aside for 10 minutes, it is discharged to 2.5V at 1.0C in preparation for a normal temperature cycle test. Before the normal temperature cycle test of the soft-pack battery, and every 50 cycles during the cycle test of the soft-pack battery, the capacity retention rate of the soft-pack battery as the test object is tested.

[0075] (2) Test results

[0076] As shown in Table 5, under the same test conditions, among the test objects participating in the cycle performance test, the soft-pack battery using the electrolyte of Formula 1 has the highest cycle capacity retention rate. As mentioned above, the ratio of DFEA, DFEC and FEMC contained in the electrolyte of Formula 1, the electrolyte of Formula 3 and the electrolyte of Formula 8 is the same, and the difference between the three lies in the total content of DFEA, DFEC and FEMC in the electrolyte. From the cycle performance test results of the soft-pack batteries using the above three electrolytes, it can be seen that with the increase of the total content of DFEA, DFEC and FEMC in the electrolyte, the cycle stability of the soft-pack battery shows a trend of first getting better and then getting worse. Among the above three electrolytes, the electrolyte of Formula 1, in which the total content of DFEA, DFEC and FEMC in the electrolyte reaches 4%, has the best measured cycle stability of the soft-pack battery.

[0077] In addition, as mentioned above, the total amount of DFEA, DFEC and FEMC contained in the electrolyte of Formula 2 and the electrolyte of Formula 3 is the same, and the difference lies in the ratio between the above three materials. Based on the test results shown in Table 5, in the above two electrolytes, the mass ratio of DFEA, DFEC and FEMC reaches DFEA: DFEC: FEMC = 1.5:1.5:1 for the electrolyte of Formula 3, and the corresponding battery cycle stability is better.

[0078] Table 5. Statistics of the cycle performance test results of the soft pack battery of Example 2

[0079]

[0080] Example 3

[0081] 1. Prepare electrolyte

[0082] According to Table 6, this embodiment needs to prepare electrolytes with formula numbers of Formula 9, Formula D9, Formula 10, and Formula D10 in Table 6, and prepare the materials according to the electrolyte formula component composition shown in Table 6. For the convenience of comparison, Table 6 also shows the component composition of Formula 1 electrolyte and Formula D1 electrolyte prepared in Example 1. For the convenience of comparison, the electrolyte formulas shown in Table 6 are grouped, and the specific groupings are as follows: Group 1, Formula 1, Formula D1; Group 2, Formula 9, Formula D9; Group 3, Formula 10, Formula D10. In Table 6, DEC refers to diethyl carbonate, and EMC refers to ethyl methyl carbonate. After the preparation is completed, according to different formulas, the required materials are mixed according to the amount, fully dissolved to form a uniform solution, and the preparation of the electrolyte is completed.

[0083] Table 6. Electrolyte formula required for preparation in Example 3

[0084]

[0085] 2. Preparation of soft pack batteries

[0086] The electrolyte prepared in this example was used to prepare soft-pack batteries with a battery capacity of 3.3 Ah. Except for the difference in the electrolyte used, the operations for preparing the soft-pack batteries in this example were consistent with the related operations for preparing the soft-pack batteries in Example 1.

[0087] 3. Needle puncture resistance test and high temperature resistance test

[0088] The soft-pack battery prepared in this embodiment is used as the test object for the needle puncture resistance test and the high temperature resistance test. Based on the difference in the composition of the electrolytes used, 50 repetitions are set for each soft-pack battery tested, and each repetition is a soft-pack battery.

[0089] Except for the difference in the test objects, the specific test methods of the needle puncture resistance test and the high temperature resistance test in this embodiment are consistent with the relevant operations of the needle puncture resistance test and the high temperature resistance test in Example 1.

[0090] 4. Cycle performance test

[0091] The soft-pack battery prepared in this embodiment is used as a test object for the cycle performance test.

[0092] Except for the difference in the test object, the specific test method of the cycle performance test in this embodiment is consistent with the relevant operations of the cycle performance test in Example 2.

[0093] 5. Test results

[0094] The test results are shown in Table 7. For ease of comparison, Table 7 also shows the test results of the needle puncture resistance test and the high temperature resistance test of the soft-pack batteries using the electrolyte of Formula 1 and the electrolyte of Formula D1 in Example 1, as well as the cycle performance test results of the soft-pack batteries using the electrolyte of Formula 1 and the electrolyte of Formula D1 in Example 2.

[0095] Comparing the test results of the soft-pack battery using the electrolyte of formula 9 with the soft-pack battery using the electrolyte of formula D9 in this embodiment, the proportion of the fire soft-pack battery corresponding to the needle puncture resistance test and the high temperature resistance test of the former is significantly reduced, and in the cycle capacity retention rate test, under the same cycle conditions, the cycle capacity retention rate measured by the former is higher, which shows that the former has higher safety performance and cycle stability. Comparing the test results of the soft-pack battery using the electrolyte of formula 1 with the soft-pack battery using the electrolyte of formula D1 in Example 1 and Example 2, and comparing the test results of the soft-pack battery using the electrolyte of formula 10 with the soft-pack battery using the electrolyte of formula D10 in this embodiment, similar conclusions can be obtained. Based on the above comparison results, it is once again explained that the safety performance of the battery can be effectively improved by using DFEA, DFEC and FEMC in combination as electrolyte components, and it can also be explained that the use of DFEA, DFEC and FEMC in combination as electrolyte components can also improve the cycle stability of the battery.

[0096] The difference between Formula 1, Formula 9 and Formula 10 is that the carbonate solvents contained in the electrolyte are different. The carbonate solvent contained in Formula 1 is DMC, the carbonate solvent contained in Formula 9 is DEC, and the carbonate solvent contained in Formula 10 is EMC. Based on the above differences, the soft-pack batteries using the above three kinds of electrolytes have different cycle stability, among which the soft-pack batteries using Formula 1 electrolyte have the best cycle stability.

[0097] The difference between formula D1, formula D9 and formula D10 is that the carbonate solvents contained in the electrolyte are different. The carbonate solvent contained in formula D1 is DMC, the carbonate solvent contained in formula D9 is DEC, and the carbonate solvent contained in formula D10 is EMC. Based on the above differences, the soft-pack batteries using the above three formula electrolytes also have differences in cycle stability. However, the safety performance and cycle stability of the soft-pack battery using the formula D1 electrolyte are not the best.

[0098] The above comparison shows that in the electrolyte using DFEA, DFEC and FEMC together, further introducing DMC into the electrolyte is conducive to obtaining an electrolyte with higher cycle stability. However, when the electrolyte does not contain DFEA, DFEC and FEMC at the same time, selecting DMC as the organic solvent component of the electrolyte does not necessarily have the effect of improving the cycle stability of the electrolyte.

[0099] Table 7. Statistics of soft pack battery performance test results of Example 3

[0100]

[0101]

[0102] Example 4

[0103] 1. Prepare electrolyte

[0104] According to Table 8, this embodiment needs to prepare electrolytes with formula numbers of Formula 11, Formula D11, Formula 12, and Formula D12 in Table 8, and prepare the materials according to the electrolyte formula component composition shown in Table 8. For the convenience of comparison, Table 8 also shows the component composition of Formula 1 electrolyte and Formula D1 electrolyte prepared in Example 1. For the convenience of comparison, the electrolyte formulas shown in Table 8 are grouped, and the specific groupings are as follows: Group 1, Formula 1, Formula D1; Group 4, Formula 11, Formula D11; Group 5, Formula 12, Formula D12. In Table 8, EC refers to ethylene carbonate. After the preparation is completed, according to different formulas, the required materials are mixed according to the amount, fully dissolved to form a uniform solution, and the preparation of the electrolyte is completed.

[0105] Table 8. Electrolyte formula required for preparation in Example 4

[0106]

[0107]

[0108] 2. Preparation of soft pack batteries

[0109] The electrolyte prepared in this example was used to prepare soft-pack batteries with a battery capacity of 3.3 Ah. Except for the difference in the electrolyte used, the operations for preparing the soft-pack batteries in this example were consistent with the related operations for preparing the soft-pack batteries in Example 1.

[0110] 3. Needle puncture resistance test and high temperature resistance test

[0111] The soft-pack battery prepared in this embodiment is used as the test object for the needle puncture resistance test and the high temperature resistance test. Based on the difference in the composition of the electrolytes used, 50 repetitions are set for each soft-pack battery tested, and each repetition is a soft-pack battery.

[0112] Except for the difference in the test objects, the specific test methods of the needle puncture resistance test and the high temperature resistance test in this embodiment are consistent with the relevant operations of the needle puncture resistance test and the high temperature resistance test in Example 1.

[0113] 4. Cycle performance test

[0114] The soft-pack battery prepared in this embodiment is used as a test object for the cycle performance test.

[0115] Except for the difference in the test object, the specific test method of the cycle performance test in this embodiment is consistent with the relevant operations of the cycle performance test in Example 2.

[0116] 5. Test results

[0117] The test results are shown in Table 9. For the convenience of comparison, Table 9 also shows the test results of the needle puncture resistance test and high temperature resistance test of the soft-pack battery using the electrolyte of Formula 1 and the electrolyte of Formula D1 respectively in Example 1, and shows the cycle performance test results of the soft-pack battery using the electrolyte of Formula 1 and the electrolyte of Formula D1 respectively in Example 2. The test results of the soft-pack battery using the electrolyte of Formula 11 and the soft-pack battery using the electrolyte of Formula D11 in this embodiment are compared. The proportion of the fire soft-pack battery corresponding to the needle puncture resistance test and the high temperature resistance test of the former is significantly reduced, and in the cycle capacity retention rate test, under the same cycle conditions, the cycle capacity retention rate measured by the former is higher, which shows that the former has higher safety performance and cycle stability. The test results of the soft-pack battery using the electrolyte of Formula 1 and the soft-pack battery using the electrolyte of Formula D1 in Examples 1 and 2 are compared, and the test results of the soft-pack battery using the electrolyte of Formula 12 and the soft-pack battery using the electrolyte of Formula D12 in this embodiment are compared, and similar conclusions can be obtained. Based on the above comparison results, it is once again demonstrated that by using DFEA, DFEC and FEMC in combination as electrolyte components, not only the safety performance of the battery can be effectively improved, but also the cycle stability of the battery can be improved.

[0118] The difference between Formula 1, Formula 11, and Formula 12 lies in the different organic solvent components contained in the electrolyte. Compared with Formula 1, Formula 11 uses PC to replace FEC, so that Formula 11 does not contain FEC, while Formula 12 uses EC to replace PC, so that Formula 12 does not contain PC. Based on the above differences, the soft-pack batteries using the above three formula electrolytes also have differences in safety performance and cycle stability. Among them, the soft-pack battery using Formula 1 electrolyte has the best cycle stability.

[0119] The difference between formula D1, formula D11, and formula D12 is that the organic solvent components in the electrolyte are different. Compared with formula D1, formula D11 uses PC to replace FEC, so that formula D11 does not contain FEC, and formula D12 uses EC to replace PC, so that formula D12 does not contain PC. Based on the above differences, the soft-pack batteries using the above three formula electrolytes also have differences in safety performance and cycle stability. However, the cycle stability of the soft-pack battery using the formula D1 electrolyte is not the best.

[0120] The above comparison shows that in the electrolyte using DFEA, DFEC and FEMC together, further introducing a certain amount of FEC and PC into the electrolyte is conducive to obtaining an electrolyte with higher cycle stability. However, when the electrolyte does not contain DFEA, DFEC and FEMC at the same time, introducing a certain amount of FEC and PC into the electrolyte does not necessarily have the effect of improving the cycle stability of the electrolyte.

[0121] Table 9. Statistics of soft pack battery performance test results of Example 4

[0122]

[0123] The above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the protection scope of the present invention. Although the present invention is described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the technical solutions of the present invention can be modified or equivalently replaced, but these modifications or replacements are all within the protection scope of the present invention.

Claims

1. An electrolyte, characterized in that: The electrolyte includes a fluorine-containing ester additive, and the fluorine-containing ester additive accounts for 2% to 9% of the electrolyte in terms of mass percentage. The fluorine-containing ester additive includes ethyl 2,2-difluoroacetate, difluoroethylene carbonate and trifluoroethyl methyl carbonate.

2. The electrolyte according to claim 1, characterized in that: In the electrolyte, calculated by mass ratio, ethyl 2,2-difluoroacetate: difluoroethylene carbonate: trifluoroethyl methyl carbonate = 0.5-3: 1-3: 1-3.

3. The electrolyte according to claim 2, characterized in that: In the electrolyte, calculated by mass ratio, ethyl 2,2-difluoroacetate: difluoroethylene carbonate: trifluoroethyl methyl carbonate = 1.2-1.8: 1.2-1.8: 0.8-1.

1.

4. The electrolyte according to claim 1, characterized in that: Calculated by mass percentage, the fluorinated ester additive accounts for 2% to 5% of the electrolyte.

5. The electrolyte according to claim 1, characterized in that: The electrolyte further includes other additives, and the other additives include at least one of vinylene carbonate, vinyl ethylene carbonate, and fluoroethylene carbonate.

6. The electrolyte according to any one of claims 1 to 5, characterized in that: The electrolyte also includes an organic solvent, and the organic solvent includes dimethyl carbonate. Calculated by mass percentage, the proportion of dimethyl carbonate in the electrolyte is 52% to 59%.

7. The electrolyte according to claim 6, characterized in that: In the electrolyte, calculated by mass ratio, dimethyl carbonate: the fluorinated ester additive = 55-60: 2-5.

8. The electrolyte according to claim 6, characterized in that: The organic solvent also includes propylene carbonate and fluoroethylene carbonate.

9. The electrolyte according to claim 8, characterized in that: In the electrolyte, calculated by mass ratio, propylene carbonate:fluoroethylene carbonate:dimethyl carbonate=11-20:5-15:52-59.

10. A secondary battery, characterized in that: The secondary battery comprises a positive electrode, a negative electrode and the electrolyte according to any one of claims 1 to 9.

11. The secondary battery according to claim 10, characterized in that: The secondary battery satisfies at least one of the following a and b: a. The positive electrode active material in the secondary battery includes a high-nickel positive electrode active material, in which the total amount of metal elements other than lithium is A, the amount of nickel element is B, and 0.6≤B / A<1; b. The negative electrode active material in the secondary battery includes a silicon-based negative electrode active material.

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