A thermostable lithium-ion battery and formation method thereof

Abstract

The present application relates to the technical field of lithium-ion batteries, and specifically discloses a thermostatic lithium-ion battery and a formation method thereof. A thermostatic lithium-ion battery includes an inner layer of a SEI membrane and an outer layer of a SEI membrane, wherein the inner layer of the SEI membrane is formed by electrolyte A, and the outer layer of the SEI membrane is formed by electrolyte B; the electrolyte A is 8-18wt% of a lithium salt, 78-91.5wt% of a solvent, and 0.5-4wt% of an additive; the electrolyte B is 6-21wt% of a lithium salt, 75.5-93.5wt% of a solvent, and 0.5-6.0wt% of a sulfur-containing additive; the sulfur-containing additive is one of vinyl sulfate, sulfopropionic anhydride, phenyl sulfone, dimethyl sulfite, or any combination thereof, or any combination thereof with vinylene carbonate. The thermostatic lithium-ion battery formation method of the present application can be used for battery production, and has the advantage of increasing the service life of low-temperature lithium-ion batteries under normal and high temperature environments.

Description

Technical Field

[0001] The present application relates to the technical field of lithium-ion batteries, and more specifically, to a thermostatic lithium-ion battery and a formation method thereof. Background Art

[0002] With the popularization of electronic products and new energy vehicles, the application environment of lithium-ion batteries, the main mobile power source, has gradually extended to the high-latitude cold regions in Northeast my country. In addition, special operations such as exploration and investigation in extremely cold environments such as mountains, polar regions, and space have put forward higher requirements for the performance of lithium-ion batteries at ultra-low temperatures. Therefore, the research and development of low-temperature and ultra-low-temperature lithium-ion batteries has received widespread attention.

[0003] The development of special electrolytes for use at low or ultra-low temperatures is key. At present, the electrolytes used at low or ultra-low temperatures generally include lithium hexafluorophosphate (LiPF6) as lithium salts, vinylene carbonate (VC) and fluoroethylene carbonate (FEC) as film-forming additives, and low-melting-point cyclic carbonates, chain carbonates, and carboxylates as solvents or a mixture of several as co-solvents to achieve the electrochemical performance of lithium-ion batteries in ultra-low temperature environments.

[0004] However, the electrolyte formula and corresponding formation system designed above make the SEI film (solid-electrolyte interface film) formed on the negative electrode surface only suitable for good electrochemical performance in low temperature environment. Its structure is unstable at room temperature or high temperature, and it is easy to break and cause continuous reorganization, causing the capacity of low-temperature lithium-ion batteries to decay seriously with the increase of cycle number when used at room temperature or high temperature, and the performance is far inferior to that of conventional lithium-ion batteries. In actual use, the ambient temperature switches back and forth between low temperature, room temperature and even high temperature, which leads to severe performance decay of the existing low-temperature lithium-ion batteries, low service life, and frequent battery replacement, which brings huge inconvenience and economic losses. Summary of the invention

[0005] In order to increase the service life of a low-temperature lithium-ion battery in normal temperature and high temperature environments, the present application provides a temperature-adaptable lithium-ion battery and a formation method thereof.

[0006] In a first aspect, the present application provides a thermostatic lithium-ion battery, which adopts the following technical solution:

[0007] A thermostatic lithium-ion battery, comprising an inner SEI membrane layer and an outer SEI membrane layer, wherein the inner SEI membrane layer is formed by electrolyte A, and the outer SEI membrane layer is formed by electrolyte B;

[0008] The electrolyte A is 8-18wt% of lithium salt, 78-91.5wt% of solvent and 0.5-4wt% of additive;

[0009] The electrolyte B is 6-21wt% of lithium salt, 75.5-93.5wt% of solvent and 0.5-6.0wt% of sulfur-containing additive;

[0010] The sulfur-containing additive is one of vinyl sulfate, sulfopropionic anhydride, phenyl sulfone, dimethyl sulfite or any combination thereof, or any combination thereof with vinylene carbonate.

[0011] By adopting the above technical scheme, the addition amounts of lithium salt, solvent and additives in electrolyte A are controlled within this range, so that the inner layer of the formed SEI film has low impedance, which helps the battery to exert good low-temperature performance; the addition amounts of lithium salt, solvent and additives in electrolyte B are taken within this range, and the electrolyte B formed after mixing is chemically formed to obtain the outer layer of the SEI film, and because the outer layer of the SEI film contains sulfur, it has antioxidant properties, reduces the oxidation of the electrolyte at high temperature, protects the SEI film with good structural stability, and makes the SEI film have a smaller impedance, which facilitates lithium ions to pass through the outer layer of the SEI film to enter the interior of the SEI film, thereby improving the performance of ultra-low temperature lithium-ion batteries at normal or high temperatures.

[0012] Preferably, the injection amount of the electrolyte A is 65%-95% of the total amount of the electrolyte, and the injection amount of the electrolyte B is 5%-35% of the total amount of the electrolyte.

[0013] By adopting the above technical solution, the injection amount of electrolyte A and electrolyte B is prepared within this range, the impedance value of the SEI film after formation is smaller, and the actual performance of the battery is better.

[0014] Preferably, the lithium salt in the electrolyte A is lithium hexafluorophosphate, lithium tetrafluoroborate or a mixture thereof.

[0015] The solvent in the electrolyte A is one of ethylene carbonate, propylene carbonate, ethyl acetate, methyl acetate or any combination thereof.

[0016] The additive in the electrolyte A is one of vinylene carbonate, fluoroethylene carbonate and lithium bis(oxalatoborate) or any combination thereof.

[0017] By adopting the above technical scheme, lithium hexafluorophosphate is selected as the lithium salt of electrolyte A to provide lithium ions in the electrolyte, and lithium tetrafluoroborate has good low-temperature performance and can enhance the wetting performance of the electrolyte on the electrode; ethylene carbonate is selected as the solvent to promote the dissolution of lithium hexafluorophosphate, and propylene carbonate, ethyl acetate and methyl acetate are used as low-melting-point electrolyte solvents to improve the low-temperature performance of the battery;

[0018] Selecting one or a combination of vinylene carbonate, fluoroethylene carbonate, and lithium dioxalatoborate as the film-forming additive can stably form a film on the negative electrode surface. The SEI film formed by fluoroethylene carbonate has good flexibility and can improve the stability of the solvent under high voltage. The electrolyte A formed by mixing the additive with the lithium salt and the solvent can form a dense SEI film inner layer after formation, ensuring that the battery has excellent low-temperature performance.

[0019] Preferably, the lithium salt, solvent and additive in the electrolyte A are in liquid state at a temperature below 35°C.

[0020] By adopting the above technical scheme, since the ethylene carbonate in the solvent is solid when the temperature is less than 35°C, the ethylene carbonate needs to be heated to a liquid state before use when preparing the electrolyte A; and when ethylene carbonate is mixed with other solvents, the melting point of ethylene carbonate will be reduced, so that the system of the electrolyte A is liquid, thereby ensuring the low-temperature performance of the electrolyte A after formation; the liquid electrolyte prepared by the lithium salt, solvent and additive in the electrolyte A at a temperature less than 35°C can improve the low-temperature performance of the battery.

[0021] Preferably, the lithium salt in the electrolyte B is one of lithium hexafluorophosphate, lithium difluorooxalatoborate, lithium bis(trifluoromethylsulfonyl)imide and lithium tetrafluoroborate, or any combination thereof.

[0022] The solvent in the electrolyte B is one of diethyl carbonate, methyl butyrate, ethylene carbonate, ethyl methyl carbonate or any combination thereof.

[0023] By adopting the above technical scheme, one or any combination of lithium hexafluorophosphate, lithium difluorooxalatoborate, lithium bis(trifluoromethylsulfonylimide) and lithium tetrafluoroborate is selected as the lithium salt, which has good film-forming properties and good low-temperature performance, and can inhibit the oxidation of the electrolyte; one or a mixture of diethyl carbonate, methyl butyrate, ethylene carbonate, and ethyl methyl carbonate is selected as the solvent to improve the conductivity of the electrolyte at low temperatures and improve the low-temperature performance of the battery; and the electrolyte B formed by mixing the solvent of the electrolyte B with the lithium salt and the sulfur-containing additive under the ratio of the present application, the outer layer of the SEI film obtained after the formation has good high temperature oxidation resistance and structural stability due to the presence of the sulfur-containing component.

[0024] The electrolyte used to prepare a single layer of membrane is either biased towards low temperature or high temperature. It is difficult to combine electrolytes with two different properties into the same electrolyte, and the performance of the SEI membrane formed is also either one or the other, and it cannot meet the two different performances of high and low temperature at the same time. The two electrolytes A and B here are different in lithium salt, additives and solvents, and form the inner and outer layers of the battery SEI membrane with different usage amounts and formation systems, so that two SEI membranes with different performance and functions can be formed on the negative electrode surface.

[0025] Preferably, the lithium salt, solvent and additive in the electrolyte B are in liquid state at a temperature below 35°C.

[0026] By adopting the above technical scheme, since the ethylene carbonate in the solvent is solid when the temperature is less than 35°C, the ethylene carbonate needs to be heated to a liquid state for use when preparing the electrolyte B; and when ethylene carbonate is mixed with other solvents, the melting point of ethylene carbonate will be reduced, so that the system of the electrolyte B is liquid, thereby ensuring the high temperature performance of the electrolyte B after formation; the liquid electrolyte prepared by the lithium salt, solvent and additive in the electrolyte B at a temperature less than 35°C can ensure the high temperature performance of the battery.

[0027] In a second aspect, the present application provides a lithium ion battery formation method, which adopts the following technical solution:

[0028] A formation method for a lithium ion battery, wherein an outer layer of a SEI film formed after formation of an electrolyte B wraps an inner layer of a SEI film formed after formation of an electrolyte A, and the inner layer of the SEI film and the outer layer of the SEI film form a complete SEI film.

[0029] Preferably, the electrolyte A is first formed to form the inner layer of the SEI film, and the electrolyte B is then formed to form the outer layer of the SEI film.

[0030] More preferably, the method comprises the following steps: a primary formation: first injecting electrolyte A into the battery shell containing the electrode group, letting it stand, then charging at a current density of 0.01-0.1C for 30-120min, and then charging at a current density of 0.05-0.2C for 60-180min to form an inner layer of SEI film at the negative electrode; a secondary formation: injecting electrolyte B containing sulfur element in the additive, and then charging at a current density of 0.05-0.5C for 60-180min until not less than 75-95% of the total capacity of the battery is charged, stopping charging, and forming an outer layer of SEI film at the negative electrode.

[0031] By adopting the above technical scheme, according to the SEI film components suitable for functioning in different temperature zones, electrolytes with different formulations are injected twice in accordance with the film-forming order, and SEI films with different performances are obtained by differentially adjusting the current and voltage parameters of the two formations; first, electrolyte A is injected for battery formation, so that the inner layer of the SEI film formed at the negative electrode first has a lower lithium ion passage impedance at low temperature, that is, it has a smaller charge and discharge polarization, which is conducive to the performance of the battery at low temperature; then, electrolyte B is injected for secondary formation, so that it forms an integrated structure with the inner layer of the SEI film formed first at the molecular or molecular cluster scale by relying on van der Waals force or bond energy; on the one hand, due to the secondary formation, The formation current is large and the voltage is high, so the density of the outer layer of the SEI film formed later is lower than the density of the inner layer of the SEI film formed earlier. On the other hand, since the outer layer of the SEI film formed later contains sulfur, the impedance of the outer layer of the SEI film formed later is smaller, making it easier for lithium ions to pass through the SEI film formed later and enter its interior, which is more conducive to performance at room temperature or high temperature, and has good structural stability. It can also effectively reduce the damage of solvent molecules to the composite heterogeneous SEI film at room temperature or high temperature and the graphite layer peeling caused by entering the negative electrode through the composite heterogeneous SEI film, thereby solving the problem of poor cycle performance of thermocompatible lithium-ion batteries at room temperature or high temperature.

[0032] In summary, this application has the following beneficial effects:

[0033] 1. Since the present application uses electrolyte A to be formed once after standing, a current density of 0.01-0.1C is used for charging for 30-120min, and then a current density of 0.05-0.2C is used for charging for 60-180min, an inner layer of SEI film is formed at the negative electrode, so that the inner layer of SEI film formed at the negative electrode first has a lower lithium ion passage impedance at low temperature, that is, it has a smaller charge and discharge polarization, which is beneficial to the performance of the battery at low temperature.

[0034] 2. In the present application, it is preferred to use sulfur-containing electrolyte B for secondary formation, and charge for 60-180 min at a current density of 0.05-0.5C until not less than 75-95% of the total capacity of the battery is charged, and charging is stopped to form an outer layer of the SEI film at the negative electrode. Since the current of the secondary formation is large and the voltage is high, the density of the outer layer of the SEI film formed later is lower than the density of the inner layer of the SEI film formed earlier, making it easier for lithium ions to pass through the later formed SEI film and enter its interior, which is more conducive to the performance at room temperature or high temperature, and has good structural stability. It can also effectively reduce the damage of solvent molecules to the composite heterogeneous SEI film at room temperature or high temperature and the graphite layer peeling caused by entering the interior of the negative electrode through the composite heterogeneous SEI film, thereby solving the problem of poor cycle performance of thermocompatible lithium-ion batteries at room temperature or high temperature.

[0035] 3. The method of the present application forms an inner layer of the SEI film on the surface of the negative electrode by injecting electrolyte A for a primary formation, and forms an outer layer of the SEI film by injecting electrolyte B for a secondary formation. The inner layer of the SEI film and the outer layer of the SEI film are bonded by chemical bonds and intermolecular forces to form a composite heterogeneous structure SEI film, thereby reducing the capacity attenuation of thermocompatible lithium-ion batteries at room temperature or high temperature. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 Schematic diagram of the microstructure of the composite heterogeneous SEI film of Example 1A.

[0037] Figure 2 1A and Comparative Examples 1-2 are discharge curves at different temperatures. DETAILED DESCRIPTION

[0038] The technical solution of the present invention is further illustrated below through specific implementation methods.

[0039] The raw materials of electrolyte A used in this application: lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), ethylene carbonate (EC), propylene carbonate (PC), ethyl acetate (EA), methyl acetate (MA), vinylene carbonate (VC), fluoroethylene carbonate (FEC), lithium dioxalatoborate (LiBOB) and the raw materials of electrolyte B: lithium hexafluorophosphate (LiPF6), lithium difluorooxalatoborate (LiDFOB), lithium bis(trifluoromethylsulfonyl imide) (LiTFSI), lithium tetrafluoroborate (LiBF4), diethyl carbonate (DEC), methyl butyrate (MB), ethylene carbonate (EC), ethyl methyl carbonate (EMC), vinyl sulfate (DTD), sulfopropionic anhydride (SPA), vinylene carbonate (VC), phenyl sulfone (PS), and dimethyl sulfite (DMS) can all be obtained commercially.

[0040] Example 1A

[0041] A thermostatic lithium-ion battery comprises an inner SEI membrane layer and an outer SEI membrane layer, wherein the inner SEI membrane layer is formed by electrolyte A, and the outer SEI membrane layer is formed by electrolyte B;

[0042] Electrolyte A is 12wt% lithium salt, 86wt% solvent and 2wt% additive;

[0043] Electrolyte B is 13wt% lithium salt, 82.5wt% solvent and 4.5wt% sulfur-containing additive;

[0044] The lithium salt in the electrolyte A is lithium hexafluorophosphate (LiPF6); the solvent in the electrolyte A is ethylene carbonate (EC); the additive in the electrolyte A is vinylene carbonate (VC);

[0045] The lithium salt in the electrolyte B is lithium hexafluorophosphate (LiPF6), the solvent in the electrolyte B is diethyl carbonate (DEC), and the additive in the electrolyte B is diethyl sulfate (DTD);

[0046] A thermostatic lithium-ion battery formation method comprises the following steps:

[0047] Primary formation: first inject electrolyte A into the battery shell containing the electrode group, the injection amount of electrolyte A is 82% of the total electrolyte, let it stand for 12 hours, then charge it at a current density of 0.1C for 120 minutes, and then charge it at a current density of 0.2C for 60 minutes to form an inner layer of SEI film at the negative electrode;

[0048] Secondary formation: inject sulfur-containing electrolyte B again, the injection amount of electrolyte B is 18% of the total electrolyte, and then charge for 120 minutes at a current density of 0.3C until not less than 90% of the total battery capacity is charged, stop charging, and form an outer layer of SEI film on the negative electrode;

[0049] The inner layer of the SEI film and the outer layer of the SEI film are combined by chemical bonds and intermolecular forces to form a composite heterogeneous structure SEI film. Figure 1 As shown;

[0050] A lithium ion battery also includes an electrode group consisting of a positive electrode sheet, a negative electrode sheet and a separator. The electrode group is located in a battery shell. The negative electrode sheet includes a copper foil current collector and a graphite negative electrode material arranged on the current collector.

[0051] The thermostatic lithium-ion battery is manufactured by the above-mentioned formation method.

[0052] Comparative Example 1

[0053] A formation method for a lithium ion battery, which is different from that of Example 1A in that only electrolyte A is used for primary formation and secondary formation to obtain a SEI film, the formula of electrolyte A is the same as that of Example 1A, and the formation method comprises: injecting electrolyte A, standing for 12 hours, then charging for 120 minutes at a current density of 0.1C, then charging for 60 minutes at a current density of 0.2C, and finally charging for 120 minutes at a current density of 0.3C, until not less than 90% of the total capacity of the battery is charged, and then stopping charging.

[0054] Comparative Example 2

[0055] A formation method for a lithium ion battery, which is different from that in Example 1A in that only electrolyte B is used for primary formation and secondary formation to obtain a SEI film, the formula of electrolyte B is the same as that in Example 1A, and the formation method comprises: injecting electrolyte B, standing for 12 hours, then charging for 60 minutes at a current density of 0.1C, then charging for 120 minutes at a current density of 0.2C, and finally charging for 600 minutes at a current density of 0.3C, until not less than 90% of the total capacity of the battery is charged, and then stopping charging.

[0056] Comparative Example 3

[0057] A formation method for a lithium ion battery is different from that of Example 1A in that only electrolyte A is injected and then the formation is performed according to a primary formation condition, and the formula of electrolyte A and the primary formation condition are the same as those of Example 1A.

[0058] Comparative Example 4

[0059] A formation method for a lithium ion battery, which is different from Example 1A in that only electrolyte B is injected and then the formation is performed according to secondary formation conditions, and the formula of electrolyte B and secondary formation conditions are the same as those of Example 1A.

[0060] Performance testing

[0061] Tests include

[0062] 1. The lithium ion batteries of Example 1A and Comparative Examples 1-4 were respectively subjected to discharge tests at a low temperature of -40°C. The test results are as follows: Figure 2 shown.

[0063] Combined with Example 1A and Comparative Examples 1-2 and Figure 2 It can be seen that the thermostatic lithium-ion battery obtained by using electrolyte A and electrolyte B and performing two formations in Example 1A of the present application has the highest discharge voltage and the highest discharge capacity. However, the discharge voltage and discharge capacity of the thermostatic lithium-ion battery obtained by using electrolyte A alone in Comparative Example 1 and electrolyte B alone in Comparative Example 2 are significantly lower. This shows that the use of electrolyte A and electrolyte B in Example 1A of the present application through two formations has a positive effect on the excellent low-temperature discharge performance of the thermostatic lithium-ion battery.

[0064] However, working SEI films cannot be obtained for Comparative Examples 3 and 4. The reason for this is that the battery formation mechanism requires a small current to start, a densified core, a large current, and a certain current to end. However, Comparative Example 3 was formed only once, and Comparative Example 4 was formed only twice. Therefore, an effective SEI film cannot be formed, and the performance test data of Comparative Examples 3 and 4 cannot be tested.

[0065] The lithium ion battery of Example 1A was subjected to discharge tests at low temperature, room temperature and high temperature. The test results are as follows: Figure 2 shown.

[0066] Combining Example 1A and Figure 2 It can be seen that when discharged at room temperature of 25°C and high temperature of 55°C, the temperature-adaptable lithium-ion battery obtained by compounding and re-forming electrolyte A and electrolyte B in Example 1A exhibits stronger discharge performance.

[0067] The discharge performance of the lithium ion battery of Example 1A at low temperature is better than that of Comparative Examples 1-2 because the lithium ion battery of Example 1A has a diff and the battery body impedance R b In addition to showing smaller values ​​in both aspects, the resistance R sei and charge transfer resistance R ct Both aspects have smaller values, so the discharge capacity at low temperature of -40°C is higher; while Comparative Examples 1 and Comparative Examples 2 can only diff and the battery body impedance R b Both aspects show small values, but lithium ions pass through the SEI film impedance R sei and the charge transfer resistance R ct The high values ​​of both aspects are due to the thicker SEI film formed by only electrolyte A or electrolyte B, resulting in a higher SEI film impedance R sei and charge transfer resistance R ct The impedance values ​​of Example 1A and Comparative Examples 1-2 are shown in the following table.

[0068] Rdiff(Ω) Rb(Ω) Rsei(Ω) Rct(Ω) Example 1A 0.15 0.12 0.2 0.2 Comparative Example 1 0.16 0.12 0.39 0.37 Comparative Example 2 0.15 0.12 0.38 0.36

[0069] Examples 1A-1L

[0070] As shown in Table 1, the main difference between Examples 1A-1L is the different composition of the electrolyte A.

[0071] Table 1 Raw material composition of Examples 1A-1L

[0072]

[0073]

[0074] Examples 2A-2L

[0075] As shown in Table 2, the main difference between Examples 2A-2L is the different composition of the electrolyte A.

[0076] Table 2 Raw material composition of Examples 2A-2L

[0077]

[0078]

[0079] Examples 3A-3L

[0080] As shown in Table 3, the main difference between Examples 3A-3L is the different composition of the electrolyte A.

[0081] Table 3 Raw material composition of Examples 3A-3L

[0082]

[0083]

[0084] Examples 4A-4L

[0085] A thermostable lithium-ion battery, based on Example 1A, the main difference between Examples 4A-4L is that the composition of electrolyte A is different, as shown in Table 4.

[0086] Table 4 Raw material composition of Examples 4A-4L

[0087]

[0088]

[0089] Embodiment (5-1)-(5-28)

[0090] A thermostable lithium-ion battery, based on Example 1A, the main difference between Examples (5-1)-(5-28) is that the composition of electrolyte B is different, as shown in Table 5.

[0091] Table 5 Raw material composition of Examples (5-1)-(5-28)

[0092]

[0093]

[0094]

[0095]

[0096] Embodiment (6-1)-(6-27)

[0097] A thermocompatible lithium-ion battery, based on Example 1A, the main difference between Examples (6-1)-(6-27) is that the composition of the electrolyte B is different, the lithium salt is 15wt% LiPF6 or LiDFOB or LiTFSI or LiBF4, the solvent is 82.5wt% DEC or MB or EC or EMC, and the sulfur-containing additive is 2.5wt% DTD or SPA or PS or DMS.

[0098] Embodiment (7-1)-(7-28)

[0099] A thermocompatible lithium-ion battery, based on Example 1A, the main difference between Examples (7-1)-(7-28) is that the composition of the electrolyte B is different, the lithium salt is 21wt% LiPF6 or LiDFOB or LiTFSI or LiBF4, the solvent is 75.5wt% DEC or MB or EC or EMC, and the sulfur-containing additive is 3.5wt% DTD or SPA or PS or DMS.

[0100] Embodiment (8-1)-(8-28)

[0101] A thermocompatible lithium-ion battery, based on Example 1A, the main difference between Examples (8-1)-(8-28) is that the composition of the electrolyte B is different, the lithium salt is 10wt% LiPF6 or LiDFOB or LiTFSI or LiBF4, the solvent is 84wt% DEC or MB or EC or EMC, and the sulfur-containing additive is 6wt% DTD or SPA or PS or DMS.

[0102] Example 9

[0103] A formation method for a thermophilic lithium-ion battery is different from that of Example 1A in that, in one formation, only a current density of 0.01C is used for charging for 120 minutes, and other steps are the same as those of Example 1A.

[0104] Example 10

[0105] A formation method for a thermophilic lithium-ion battery, which is different from Example 1A in that a current density of 0.7C is used for charging for 30 minutes in the secondary formation, and the other steps are the same as those in Example 1A.

[0106] Embodiment 11

[0107] A formation method for a thermocompatible lithium-ion battery, which is different from Example 1A in that the injection amount of electrolyte A is 65% of the total amount of the electrolyte, and the injection amount of electrolyte B is 35% of the total amount of the electrolyte.

[0108] Example 12

[0109] A thermostatic lithium-ion battery, which is different from Example 1A in that the injection amount of electrolyte A is 95% of the total amount of the electrolyte, and the injection amount of electrolyte B is 5% of the total amount of the electrolyte.

[0110] Comparative Example 5

[0111] A formation method for a thermophilic lithium-ion battery, which differs from Example 1A in that electrolyte B is first added for primary formation, and then electrolyte A is added for secondary formation, and electrolyte A, electrolyte B, and primary formation and secondary formation condition parameters are the same as those in Example 1A.

[0112] Comparative Example 6

[0113] A thermostatic lithium-ion battery, which is different from Example 1A in that the additive of electrolyte B does not contain sulfur.

[0114] Performance Testing

[0115] 2. The thermocompatible lithium-ion batteries of Examples 1A-1L, 2A-2L, 3A-3L, 4A-4L, (5-1)-(5-28), (6-1)-(6-27), (7-1)-(7-28), (8-1)-(8-28), 9-12, Comparative Examples 1-2, and 5 were tested for capacity retention at low temperature of -40°C and high temperature of 55°C; test conditions: 1.0C rate discharge, 1.0C rate charge, voltage range 2.00-3.65V; after 100 cycles under the above conditions, the capacity retention rate was tested, 100 times capacity retention rate = 100th cycle discharge specific capacity / first discharge specific capacity.

[0116] The test results are shown in Table 6:

[0117] Table 6 Capacity retention rate of lithium-ion batteries under low and high temperature environments

[0118]

[0119]

[0120] As shown in Table 6, the capacity retention rates of the lithium ion batteries of Examples 1A-1L in low-temperature and high-temperature environments are the same, namely, the capacity retention rate in the low-temperature environment is 90.9% and the capacity retention rate in the high-temperature environment is 98.5%.

[0121] The capacity retention rates of the lithium ion batteries of Examples 2A-2L in low-temperature and high-temperature environments were the same, with the capacity retention rate in the low-temperature environment being 90.1% and the capacity retention rate in the high-temperature environment being 98.1%.

[0122] The capacity retention rates of the lithium-ion batteries of Examples 3A-3L in low-temperature and high-temperature environments are the same, with the capacity retention rate in the low-temperature environment being 90.6% and the capacity retention rate in the high-temperature environment being 98.3%.

[0123] The capacity retention rates of the lithium ion batteries of Examples 4A-4L in low-temperature and high-temperature environments are the same, with the capacity retention rate in the low-temperature environment being 90.8% and the capacity retention rate in the high-temperature environment being 98.4%.

[0124] The capacity retention rates of the lithium-ion batteries of Examples (5-1) to (5-28) in low-temperature and high-temperature environments are the same, with the capacity retention rate in low-temperature environments being 90.2% and the capacity retention rate in high-temperature environments being 97.8%;

[0125] The capacity retention rates of the lithium-ion batteries of Examples (6-1) to (6-27) in low-temperature and high-temperature environments are the same, with the capacity retention rate in the low-temperature environment being 90.9% and the capacity retention rate in the high-temperature environment being 98.5%.

[0126] The capacity retention rates of the lithium-ion batteries of Examples (7-1) to (7-28) in low-temperature and high-temperature environments are the same, with the capacity retention rate in the low-temperature environment being 90.4% and the capacity retention rate in the high-temperature environment being 98%;

[0127] The capacity retention rates of the lithium-ion batteries of Examples (8-1) to (8-28) in low-temperature and high-temperature environments are the same, with the capacity retention rate in the low-temperature environment being 90.3% and the capacity retention rate in the high-temperature environment being 97.9%.

[0128] Combining Example 1A-1L, Example 2A-2L, Example 3A-3L and Example 4A-4L with Table 6, it can be seen that the selection of lithium hexafluorophosphate in the lithium salt of the electrolyte A can provide lithium ions in the electrolyte, and lithium tetrafluoroborate has good low-temperature performance, which can enhance the wetting performance of the electrolyte to the electrode; the selection of ethylene carbonate in the solvent can promote the dissolution of lithium hexafluorophosphate, and propylene carbonate, ethyl acetate and methyl acetate are used as low-melting point electrolyte solvents to improve the low-temperature performance of the battery; the selection of vinylene carbonate, fluoroethylene carbonate, lithium dioxalate borate or a combination thereof as a film-forming additive can stably form a film on the negative electrode surface, and the SEI film formed by fluoroethylene carbonate has good flexibility, which can improve the stability of the solvent under high voltage; the electrolyte A formed by mixing the additive with the lithium salt and the solvent can form a dense SEI film inner layer after formation, ensuring that the battery has excellent low-temperature performance;

[0129] Example 1A-1L is better than Example 2A-2L, Example 3A-3L and Example 4A-4L, indicating that the electrolyte A is a better embodiment when the lithium salt addition amount is 12wt%, the solvent addition amount is 86wt% and the additive addition amount is 2wt%. It can form a low-impedance SEI film inner layer at the negative electrode, which helps the lithium-ion battery to exhibit good low-temperature performance.

[0130] In combination with Examples (5-1)-(5-28), Examples (6-1)-(6-27), Examples (7-1)-(7-28), Examples (8-1)-(8-28) and Table 6, it can be seen that the lithium salt selected from lithium hexafluorophosphate, lithium difluorooxalatoborate, lithium bis(trifluoromethylsulfonylimide) and lithium tetrafluoroborate has good film-forming properties and good low-temperature performance, and can inhibit the oxidation of the electrolyte; the solvent selected from diethyl carbonate, methyl butyrate, ethylene carbonate, ethyl methyl carbonate or a mixture thereof can improve the conductivity of the electrolyte at low temperatures and improve the low-temperature performance of the battery; and the electrolyte B formed by mixing the solvent of the electrolyte B with the lithium salt and the sulfur-containing additive under the ratio of the present application, the outer layer of the SEI film obtained after the formation has good high temperature oxidation resistance and structural stability due to the presence of the sulfur-containing component.

[0131] The capacity retention rate of the lithium ion batteries of Examples (6-1)-(6-27) and Examples 1A-1L under low temperature and high temperature environments is the same, and Examples (6-1)-(6-27) are better than Examples (5-1)-(5-28), Examples (7-1)-(7-28), and Examples (8-1)-(8-28), indicating that the electrolyte B is a better embodiment when the lithium salt addition amount is 13wt%, the solvent addition amount is 82.5wt% and the additive addition amount is 4.5wt%. The electrolyte B formed by mixing the lithium salt, solvent and additive addition amount in the electrolyte B, under the formation conditions of the present application, obtains an SEI film outer layer with lower density, has good high temperature performance, and slows down the capacity decay of the battery at high temperature.

[0132] Combining Example 1A with Examples 9-10 and Table 6, it can be seen that Example 1A is better than Examples 9-10, indicating that the primary formation conditions of Example 1A of the present application and the combination of electrolyte A can form a better SEI film inner layer, and the secondary formation conditions and electrolyte B can form a better SEI film outer layer, while ensuring the low temperature performance of the lithium-ion battery, the composite heterogeneous SEI film formed after compounding with the outer side of the SEI film has good cycle performance in a high temperature environment, thereby improving the service life of the thermocompatible lithium-ion battery in a medium or high temperature environment.

[0133] Combining Example 1A with Examples 11-12 and Table 6, it can be seen that Example 1A is superior to Examples 11-12. This indicates that the injection amounts of electrolyte A and electrolyte B in the present application are prepared within the scope of the present application, the impedance value of the SEI film after formation is smaller, and the actual performance of the battery is better.

[0134] Combining Example 1A and Comparative Examples 1-2 and Table 6, it can be seen that the capacity retention rate of Example 1A is better than that of Comparative Examples 1-2, indicating that in the present application, according to the SEI film components suitable for functioning in different temperature zones, electrolytes of different formulations are injected twice in accordance with the film formation order, and the current and voltage parameters of the two formations are adjusted differentially to obtain a composite heterogeneous SEI film with different performances. The structural design of the composite heterogeneous SEI film on the negative electrode surface of the lithium ion battery is to first form an inner layer of the SEI film suitable for working well under low temperature conditions on the electrode surface, so that the battery can perform good battery performance at low temperatures, and then coat the surface of the inner layer of the SEI film with an outer layer of the SEI film suitable for working well under normal or high temperature conditions to protect the inner layer of the SEI film from being easily disintegrated under normal or high temperature conditions, so that the battery can also perform good battery performance at normal or high temperatures; the inner layer of the SEI film and the outer layer of the SEI film are bonded by intermolecular forces such as van der Waals forces and chemical bonds to obtain an integrated structure of the composite heterogeneous SEI film, which has good structural stability and can improve the service life of the thermocompatible lithium ion battery in a high temperature environment.

[0135] Combining Example 1A and Comparative Example 5 and Table 6, it can be seen that Example 1A is better than Comparative Example 5, which shows that the present application first forms an inner layer of SEI film suitable for working well under low temperature conditions on the electrode surface, so that the battery can exert good battery performance at low temperatures, and then coats the surface of the inner layer of the SEI film with an outer layer of SEI film suitable for working well under normal temperature or high temperature conditions to protect the inner layer of the SEI film from being easily disintegrated under normal temperature or high temperature conditions, so that the battery can also exert good battery performance under normal temperature or high temperature; the inner layer of the SEI film and the outer layer of the SEI film are bonded by intermolecular forces such as van der Waals forces and chemical bonds to obtain an integrated composite heterogeneous SEI film with good structural stability, and maintains the excellent low temperature characteristics of the battery, and improves the performance of the battery at normal temperature or high temperature.

[0136] Combining Example 1A and Comparative Example 6 and Table 6, it can be seen that Example 1A is better than Comparative Example 6, indicating that a sulfur-containing additive is added to the electrolyte B, so that the outer side of the SEI film formed after formation has antioxidant properties, reducing the oxidation of the electrolyte in a high temperature environment, playing a role in protecting the SEI film, helping to reduce impedance and increase discharge capacity.

[0137] This specific embodiment is merely an explanation of the present application and is not a limitation of the present application. After reading this specification, those skilled in the art may make modifications to the present embodiment without any creative contribution as needed, but such modifications are protected by the patent law as long as they are within the scope of the claims of the present application.

Claims

1. A thermostable lithium-ion battery, characterized in that: It comprises an inner SEI film layer and an outer SEI film layer, wherein the inner SEI film layer is formed by electrolyte A, and the outer SEI film layer is formed by electrolyte B; The electrolyte A is 8-18wt% of lithium salt, 78-91.5wt% of solvent and 0.5-4wt% of additive; The electrolyte B is 6-21wt% of lithium salt, 75.5-93.5wt% of solvent and 0.5-6.0wt% of sulfur-containing additive; The sulfur-containing additive is one of vinyl sulfate, sulfopropionic anhydride, phenyl sulfone, dimethyl sulfite, or any combination thereof, or any combination thereof and vinylene carbonate; The injection amount of the electrolyte A is 65%-95% of the total amount of the electrolyte, and the injection amount of the electrolyte B is 5%-35% of the total amount of the electrolyte; The lithium salt in the electrolyte A is lithium hexafluorophosphate, lithium tetrafluoroborate or a mixture thereof; The solvent in the electrolyte A is one of ethylene carbonate, propylene carbonate, ethyl acetate, methyl acetate or any combination thereof; The additive in the electrolyte A is one of vinylene carbonate, fluoroethylene carbonate and lithium bis(oxalatoborate) or any combination thereof; The lithium salt, solvent and additive in the electrolyte A are in liquid state when the temperature is less than 35°C; The lithium salt in the electrolyte B is one of lithium hexafluorophosphate, lithium difluorooxalatoborate, lithium bis(trifluoromethylsulfonylimide) and lithium tetrafluoroborate, or any combination thereof; The solvent in the electrolyte B is one of diethyl carbonate, methyl butyrate, ethylene carbonate, ethyl methyl carbonate or any combination thereof; The lithium salt, solvent and additive in the electrolyte B are in liquid state when the temperature is less than 35°C.

2. A formation method for a thermostable lithium-ion battery according to claim 1, characterized in that: The SEI film outer layer formed by the electrolyte B wraps the SEI film inner layer formed by the electrolyte A, and the SEI film inner layer and the SEI film outer layer form a complete SEI film.

3. A formation method for a thermostable lithium-ion battery according to claim 2, characterized in that: The electrolyte A is first formed to form the inner layer of the SEI film, and the electrolyte B is then formed to form the outer layer of the SEI film.

4. A formation method for a thermostable lithium-ion battery according to claim 2 or 3, characterized in that: The following steps are included: Primary formation: first inject the electrolyte A into the battery shell containing the electrode group, let it stand for 6-48 hours, then charge it at a current density of 0.01-0.1C for 30-120 minutes, and then charge it at a current density of 0.05-0.2C for 60-180 minutes to form an inner layer of SEI film at the negative electrode; Secondary formation: Inject electrolyte B containing sulfur as an additive, then charge at a current density of 0.05-0.5C for 60-180min until not less than 75-95% of the total battery capacity is filled, stop charging, and form an outer layer of SEI film at the negative electrode.

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