Secondary battery liquid injection method, electrolyte and secondary battery

Abstract

The present invention provides a secondary battery liquid injection method, an electrolyte and a secondary battery, the secondary battery liquid injection method comprises the following operations: S1, a first electrolyte is utilized to perform primary liquid injection on a battery cell, a positive plate included in the battery cell satisfies that a positive active material contains transition metal elements, the first electrolyte comprises vinylene carbonate, and the second electrolyte comprises vinylene carbonate; after liquid injection is completed, formation is carried out, the cut-off voltage is first voltage, and the first voltage is 3.5-4.0 V; and S2, secondary liquid injection is carried out on the battery cell by using a second electrolyte, the second electrolyte comprises (trimethylsilane) phosphate and lithium difluoro (oxalato) borate, formation is carried out after liquid injection is completed, the cut-off voltage is a second voltage, and the second voltage is 4.0-4.5 V. By adopting the liquid injection method, the problems of large impedance of a secondary battery system and insufficient positive and negative electrode film formation protection are solved, and the cycle performance of the secondary battery is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and in particular, relates to a secondary battery injection method, an electrolyte, and a secondary battery. Background Art

[0002] As a green and environmentally friendly new energy, lithium-ion batteries have the advantages of high safety, small size and light weight. They have been widely used in digital products, electric vehicles, military products and other fields. With the country's strong support for the new energy field, the development of lithium-ion batteries is in full swing, but people's requirements for the service life and safety of lithium-ion batteries are getting higher and higher. At present, lithium-ion batteries are also developing towards long life, high rate and high cycle capacity retention rate.

[0003] Cathode active materials containing transition metal elements show unique advantages in the field of lithium-ion batteries, but they also face some inherent technical challenges. Specifically, the Jahn-Teller effect is particularly significant in these cathode active materials containing transition metal elements. This effect originates from the geometric distortion of transition metal ions in a specific oxidation state. This distortion further promotes the dissolution of metal ions, the so-called "transition metal dissolution" phenomenon. The dissolution of transition metals not only consumes active substances, but may also form a short-circuit path inside the battery. In severe cases, it will cause damage to the internal structure of the battery and accelerate the decay of battery performance.

[0004] Therefore, for the battery system using the above-mentioned positive electrode active material containing transition metal elements as the positive electrode active material, it is necessary to add an additive that can form a protective layer on the electrode surface to the electrolyte to reduce the occurrence of side reactions and improve the cycle life. As an electrolyte additive, vinylene carbonate has good electrochemical stability and thermal stability, can reduce unexpected reactions on the electrode surface, and improve the cycle performance of the battery. However, when a single injection method is used in a battery containing the above-mentioned positive electrode active material to inject an electrolyte containing vinylene carbonate, although vinylene carbonate can form an SEI film on the negative electrode surface, it will also participate in the film formation on the positive electrode surface, resulting in an increase in the positive electrode impedance, and vinylene carbonate has no obvious effect on the protective effect of the positive electrode, so that the battery has a large impedance, insufficient film protection of the positive and negative electrodes, etc., resulting in a decrease in the battery's cycle power performance. Summary of the invention

[0005] The purpose of the present invention is to provide a secondary battery injection method, electrolyte, and secondary battery. By adopting the injection method, the problems of large impedance of the secondary battery system and insufficient film protection of the positive and negative electrodes are solved, and the cycle performance of the secondary battery is improved.

[0006] According to a first aspect of the present invention, a secondary battery injection method is provided, comprising the following operations: S1. injecting a first electrolyte into a battery cell once, wherein the battery cell comprises a positive electrode sheet satisfying that the positive electrode active material contains a transition metal element, the first electrolyte comprises vinylene carbonate, and after the injection is completed, formation is performed, and the cut-off voltage is a first voltage, and the first voltage is 3.5-4.0V; S2. injecting a second electrolyte into the battery cell twice, the second electrolyte comprises at least one of (trimethylsilyl) phosphate and lithium difluorooxalatoborate, and after the injection is completed, formation is performed, and the cut-off voltage is a second voltage, and the second voltage is 4.0-4.5V.

[0007] Adopting the above-mentioned first electrolyte and second electrolyte to carry out secondary liquid injection to the battery core, can not only enhance the migration ability of lithium ions in the electrolyte, accelerate the transfer rate of charge, effectively improve the conductivity inside the battery, but also reduce the contact resistance between the electrolyte and the electrode material, reduce impedance, reduce internal energy loss, improve the cycle stability of the battery, and extend its service life. On the one hand, by introducing vinylene carbonate as an additive to the first electrolyte, and using the first voltage as the cut-off voltage for the first formation, vinylene carbonate can be promoted to generate SEI film only on the negative electrode surface after the first formation, and will not form a film on the positive electrode surface, thereby effectively reducing internal impedance and improving the power output of the secondary battery. On the other hand, by introducing (trimethylsilane) phosphate and / or difluoro oxalate borate lithium as an additive to the second electrolyte, and using the second voltage as the cut-off voltage for the second formation, (trimethylsilane) phosphate and difluoro oxalate borate lithium can be generated on the positive electrode surface. Stable CEI film.

[0008] The strategy of using the first electrolyte and the second electrolyte to perform multiple injection and multiple formation of the battery cell is helpful to form a more stable SEI film and CEI film, which not only provides effective protection for the positive and negative electrodes and reduces the side reactions between the electrolyte and the electrodes; it can also reduce the impedance of lithium ions between the positive and negative electrodes, improve the high-temperature storage performance and capacity recovery rate of the battery, and at the same time improve the uniformity of electrolyte distribution, reduce the local concentration unevenness caused by electrode reaction or diaphragm absorption of electrolyte, thereby improving the battery's discharge capacity, cycle life and other performance indicators.

[0009] Preferably, the first voltage is 3.7V.

[0010] Preferably, the second voltage is 4.2V.

[0011] Preferably, the second electrolyte includes vinylene carbonate, (trimethylsilane) phosphate and lithium difluorooxalate borate, and the mass ratio of vinylene carbonate, tri(trimethylsilane) phosphate and lithium difluorooxalate borate is 10-20: 0.1-0.3: 0.5-1.0. The above-mentioned additives are used in the second electrolyte. First, vinylene carbonate, (trimethylsilane) phosphate and lithium difluorooxalate borate are matched according to the above-mentioned mass ratio, which can not only improve the film-forming effect and the protection of the positive and negative electrodes, but also reduce the acidic substances in the electrolyte and improve the stability of the electrolyte. Secondly, vinylene carbonate continues to form a film on the surface of the negative electrode during the cycle of the secondary battery, thereby repairing the SEI film, thereby maintaining the integrity of the SEI film, improving the cycle stability and energy efficiency of the battery, and reducing the side reactions during the battery cycle, thereby improving the cycle life and energy density of the battery. In addition, in the formation process with the second voltage as the cut-off voltage, vinylene carbonate is not easy to form a film on the positive electrode surface, so vinylene carbonate will still form a film on the negative electrode surface. If vinylene carbonate forms a film on the positive electrode, it will increase the impedance of the positive electrode, and the vinylene carbonate formed on the positive electrode has no obvious protective effect on the positive electrode.

[0012] Preferably, the mass proportion of vinylene carbonate in the first electrolyte is 1-3wt%; the total mass proportion of vinylene carbonate, tris(trimethylsilyl)phosphate and lithium difluorooxalatoborate in the second electrolyte is 10-15wt%. By adjusting the amount of the additives added, it is easy for the additives to form a structurally stable, low-impedance interface film on the surface of the negative electrode or the positive electrode, so that the secondary battery has a lower battery impedance and excellent cycle performance.

[0013] Preferably, the first electrolyte also includes a first lithium salt, the first lithium salt includes lithium hexafluorophosphate and bisfluorosulfonyl imide lithium salt, and in the first lithium salt, the mass ratio of lithium hexafluorophosphate and bisfluorosulfonyl imide lithium salt is 7 to 9: 1 to 3. When the first lithium salt adopts lithium hexafluorophosphate and bisfluorosulfonyl imide lithium salt with the above mass ratio, it can not only improve the transmission performance of lithium ions in the secondary battery, but also help to overcome the problem of low conductivity of the secondary battery, achieve the effect of reducing impedance, and reduce the side reactions caused by the hydrolysis of lithium hexafluorophosphate, laying a solid foundation for the overall improvement of battery performance. It should be noted here that, generally speaking, bisfluorosulfonyl imide lithium salt is easy to form an irregular and loose surface structure of aluminum foil in a high temperature environment or a high voltage environment, aggravating the corrosion degree of aluminum foil, resulting in increased impedance and decreased battery cycle capacity retention rate. By adjusting the type and ratio of the first lithium salt, the bisfluorosulfonyl imide lithium salt is not easy to corrode the aluminum foil in the battery at the first voltage.

[0014] Preferably, the concentration of the first lithium salt in the first electrolyte is 0.8 to 1.2 mol / L. When the first lithium salt is used at this concentration, the bis(fluorosulfonyl)imide lithium salt can improve the conductivity of the electrolyte and help the Li + Migration, concentration polarization is weakened, thermal stability is good and it is not easy to hydrolyze.

[0015] Preferably, the second lithium salt is lithium hexafluorophosphate. When the second lithium salt is lithium hexafluorophosphate, it reacts with trace water to generate an acidic substance, which corrodes Al2O3 in the aluminum foil to form an AlF3 passivation layer to prevent LiFSI from corroding the aluminum foil at high voltage.

[0016] Preferably, the concentration of the second lithium salt in the second electrolyte is 0.8-1.2 mol / L.

[0017] Preferably, the first electrolyte includes a first solvent, the first solvent includes ethylene carbonate, and the amount of ethylene carbonate added is 20 to 30 wt% of the mass of the first solvent; the second electrolyte includes a second solvent, the second solvent includes ethylene carbonate, and the amount of ethylene carbonate added to the second solvent is greater than the amount of ethylene carbonate added to the first solvent. By regulating the amount of ethylene carbonate added to the first solvent and the second solvent, the viscosity of the first electrolyte is moderate, the fluidity is good, and lithium ions can be effectively transferred, the charge and discharge efficiency and fast charging capability of the battery are improved, and the compatibility of the second electrolyte is improved, which is conducive to the dissolution of lithium salts, improves the ionic conductivity of the electrolyte, and broadens the electrochemical window of the secondary battery.

[0018] Preferably, the first solvent includes ethylene carbonate, propylene carbonate, dimethyl carbonate and ethyl methyl carbonate, and the second solvent includes ethylene carbonate, propylene carbonate, dimethyl carbonate and ethyl methyl carbonate. By selecting the above-mentioned composite organic solvent as the first solvent and the second solvent, not only can the compatibility between the additives, lithium salts and solvents in the electrolyte be improved, but it is also conducive to forming an electrolyte system with a high dielectric constant and good fluidity.

[0019] Preferably, in the first solvent, the mass ratio of ethylene carbonate, propylene carbonate, dimethyl carbonate and ethyl methyl carbonate is 25:5:20:55.

[0020] Preferably, in the second solvent, the mass ratio of ethylene carbonate, propylene carbonate, dimethyl carbonate and ethyl methyl carbonate is 30:5:20:50.

[0021] Preferably, the weight ratio of the first electrolyte to the second electrolyte injected into the battery cell is 80-95:20-5.

[0022] Preferably, the positive electrode active material includes lithium manganese iron phosphate.

[0023] According to a second aspect of the present invention, there is provided an electrolyte for a secondary battery, comprising the first electrolyte and the second electrolyte used in the secondary battery injection method.

[0024] According to a third aspect of the present invention, there is provided a secondary battery which is injected with liquid using the above secondary battery injection method. DETAILED DESCRIPTION

[0025] In order to enable those skilled in the art to better understand the technical solutions of the present invention, the technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0026] Example 1

[0027] This embodiment provides a secondary battery (soft pack), including a battery cell, a first electrolyte and a second electrolyte.

[0028] The composition of the first electrolyte is shown in Table 1:

[0029] Table 1. Formula composition of the first electrolyte

[0030]

[0031] The first electrolyte further includes a first lithium salt, which is selected from lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI) in a mass ratio of 8:2, and the concentration of the first lithium salt is 1 mol / L.

[0032] The composition of the second electrolyte is shown in Table 2:

[0033] Table 2. Formula composition of the second electrolyte

[0034]

[0035] The second electrolyte also includes a second lithium salt, the second lithium salt is lithium hexafluorophosphate (LiPF6), and the concentration of the second lithium salt is 1 mol / L.

[0036] The secondary battery injection method comprises the following operations:

[0037] S1. The battery cell is injected with a first electrolyte, the battery cell comprising a positive electrode sheet with lithium manganese iron phosphate as a positive electrode active material and a negative electrode sheet with graphite as a negative electrode active material. After the injection is completed, the formation is performed, and the cut-off voltage is a first voltage, and the first voltage is 3.7V;

[0038] S2. The battery cell is injected with the second electrolyte for a second time, and then formed with the cut-off voltage being the second voltage, which is 4.2 V. The weight ratio of the first electrolyte to the second electrolyte injected into the battery cell is 90:10.

[0039] In other embodiments, the type and / or content of the first lithium salt, the first additive, and the first solvent can be adjusted according to actual needs; or the type and content of the second lithium salt, the second additive, and the second solvent can be adjusted; or the injection ratio of the first electrolyte to the second electrolyte can be adjusted; or the charging current of the first charge and the second charge, the first charging cut-off voltage, and the second charging cut-off voltage can be adjusted.

[0040] Example 2

[0041] This embodiment refers to the secondary battery injection method provided in Example 1 to prepare a secondary battery. The difference between this embodiment and Example 1 is that the second solvent of equal mass is used to replace the vinylene carbonate in the second additive, that is, the second additive only includes tris(trimethylsilyl)phosphate and lithium difluorooxalatoborate. The rest of the raw material ratios and preparation methods are strictly consistent with those in Example 1.

[0042] Example 3

[0043] This embodiment refers to the injection method provided in Example 1 to prepare a secondary battery. The difference between this embodiment and Example 1 is that during the first formation process, the first additive in Example 1 is replaced by a second additive of equal mass as a new first electrolyte. The rest of the raw material ratios and preparation methods are strictly consistent with those in Example 1.

[0044] Example 4

[0045] This embodiment refers to the secondary battery injection method provided in Example 1 to prepare a secondary battery. The difference between this embodiment and Example 1 is that in the second formation process, an equal mass of tris(trimethylsilyl)phosphate is used to replace the lithium difluorooxalate borate in Example 1 as a new second electrolyte. The rest of the raw material ratios and preparation methods are strictly consistent with those in Example 1.

[0046] Example 5

[0047] This embodiment refers to the secondary battery injection method provided in Example 1 to prepare a secondary battery. The difference between this embodiment and Example 1 is that during the second formation process, an equal mass of lithium difluorooxalatoborate is used to replace tris(trimethylsilyl)phosphate in Example 1 as a new second electrolyte. The rest of the raw material ratios and preparation methods are strictly consistent with those in Example 1.

[0048] Example 6

[0049] This embodiment refers to the secondary battery injection method provided in Example 1 to prepare a secondary battery. The difference between this embodiment and Example 1 is that lithium hexafluorophosphate of equal mass is used to replace the bis(fluorosulfonyl)imide lithium salt in Example 1, that is, the first lithium salt is lithium hexafluorophosphate. The rest of the raw material ratios and preparation methods are strictly consistent with those in Example 1.

[0050] Example 7

[0051] This embodiment refers to the secondary battery injection method provided in Example 1 to prepare a secondary battery. The difference between this embodiment and Example 1 is that the amount of bisfluorosulfonyl imide lithium salt added in the first electrolyte is reduced, and lithium hexafluorophosphate and bisfluorosulfonyl imide lithium salt are used as the second lithium salt, so that the total amount of bisfluorosulfonyl imide lithium salt added to the secondary battery is strictly consistent with the total amount of bisfluorosulfonyl imide lithium salt added to the secondary battery in Example 1. The rest of the raw material ratios and preparation methods are strictly consistent with Example 1.

[0052] Example 8

[0053] This embodiment refers to the secondary battery injection method provided in Example 1 to prepare a secondary battery. The difference between this embodiment and Example 1 is that the first solvent in Example 1 is replaced by a second solvent of equal mass, that is, in this embodiment, the first solvent is composed of ethylene carbonate, propylene carbonate, dimethyl carbonate and ethyl methyl carbonate in a mass ratio of 30:5:20:50, that is, the amount of ethylene carbonate added to the second solvent is equal to the amount of ethylene carbonate added to the first solvent. The rest of the raw material ratios and preparation methods are strictly consistent with Example 1.

[0054] Example 9

[0055] This embodiment refers to the secondary battery injection method provided in Example 1 to prepare a secondary battery. The difference between this embodiment and Example 1 is that: the mass ratio of ethylene carbonate in the first solvent is adjusted so that the amount of ethylene carbonate added in the second solvent is less than the amount of ethylene carbonate added in the first solvent. Specifically, ethylene carbonate, propylene carbonate, dimethyl carbonate and ethyl methyl carbonate in a mass ratio of 35:5:20:45 are used as the first solvent of this embodiment. The rest of the raw material ratios and preparation methods are strictly consistent with Example 1.

[0056] Comparative Example 1

[0057] This comparative example refers to the injection method provided in Example 1 to prepare a secondary battery. The difference between this comparative example and Example 1 is that this comparative example adopts a single injection, and the other raw material ratios and preparation methods are strictly consistent with Example 1.

[0058] The specific operation of single injection is as follows:

[0059] The battery core is injected with a composite electrolyte, wherein the composite electrolyte is composed of the first electrolyte and the second electrolyte in Example 1 in a mass ratio of 90:10. After the injection is completed, the battery core is formed, and the cut-off voltage is 4.2V.

[0060] Comparative Example 2

[0061] This comparative example refers to the injection method provided in Example 1 to prepare a secondary battery. The difference between this comparative example and Example 1 is that during the first formation process, the first charge cutoff voltage is adjusted to 3.0 V. The rest of the raw material ratios and preparation methods are strictly consistent with Example 1.

[0062] Comparative Example 3

[0063] This comparative example refers to the injection method provided in Example 1 to prepare a secondary battery. The difference between this comparative example and Example 1 is that during the first formation process, the first charge cut-off voltage is adjusted to 4.5 V. The rest of the raw material ratios and preparation methods are strictly consistent with Example 1.

[0064] Comparative Example 4

[0065] This comparative example refers to the injection method provided in Example 1 to prepare a secondary battery. The difference between this comparative example and Example 1 is that during the second formation process, the second charge cut-off voltage is adjusted to 3.5 V. The rest of the raw material ratios and preparation methods are strictly consistent with Example 1.

[0066] Comparative Example 5

[0067] This comparative example refers to the injection method provided in Example 1 to prepare a secondary battery. The difference between this comparative example and Example 1 is that during the second formation process, the second charge cut-off voltage is adjusted to 5.0 V. The rest of the raw material ratios and preparation methods are strictly consistent with Example 1.

[0068] Comparative Example 6

[0069] This comparative example refers to the injection method provided in Example 1 to prepare a secondary battery. The difference between this comparative example and Example 1 is that in the second formation process, the first additive of equal mass is used to replace the tris(trimethylsilyl)phosphate and lithium difluorooxalate borate in Example 1 as a new second electrolyte. The rest of the raw material ratios and preparation methods are strictly consistent with those in Example 1.

[0070] Test Example 1

[0071] Test objects: Secondary batteries (soft packs) provided in Examples 1 to 9 and Comparative Examples 1 to 6

[0072] Test method:

[0073] (1) Impedance: The test object is subjected to impedance test, and the test method is as follows: In an environment of 25°C, the secondary battery as the test object is subjected to the following test operations: stand for 5 minutes, charge to 4.2V with 0.5C (C is the nominal capacity of the battery) constant current and constant voltage, stand for 5 minutes, discharge with 0.5C constant current, cycle three times and record the capacity of the last cycle as the rated capacity Q; ① stand for 5 minutes; ② charge to 4.2V with 0.5Q constant current and constant voltage, with a cut-off current of 0.05Q; ③ stand for 5 minutes, ④ discharge with 0.5Q constant current, with a cut-off capacity of 50% Q, ④ stand for 60 minutes; ⑤ then discharge to 30 seconds with I1=Q, and measure the voltage U1; ⑥ stand for 5 minutes, and record the voltage as U2. DC internal resistance = (U1 U2) / I1.

[0074] (2) Battery cycle performance: At 25°C, stand for 5 minutes, charge to 4.2V at 0.5C (C is the nominal capacity of the battery) constant current and constant voltage, stand for 5 minutes, discharge at 0.5C constant current, cycle three times and record the capacity of the last cycle as the rated capacity Q; charge the secondary battery as the test object to 4.2V at 1Q constant current and constant voltage, cut off at 0.05Q, stand for 10 minutes, discharge at 1Q constant current to 2.5V, stand for 10 minutes, cycle for 1000 weeks, and record the battery 1C / 1C cycle 1000 cycle capacity retention rate.

[0075] Test results: The test results in this test case are shown in Table 3.

[0076] Table 3. Performance results of each test object

[0077]

[0078]

[0079] Result analysis:

[0080] From the test performance shown in Table 3, compared with the secondary battery injection method provided in Comparative Examples 1 to 6, the batteries prepared by the secondary battery injection method provided in Examples 1 to 9 exhibit lower impedance and higher cycle stability.

[0081] Comparing the test results of Example 1 and Comparative Example 1 shown in Table 3, it can be found that the comprehensive performance of the battery prepared by the secondary battery injection method provided in Example 1 is better than that of the secondary battery provided in Comparative Example 1, which is reflected in that the battery provided in Example 1 has lower impedance and higher cycle capacity retention rate. Among them, Comparative Example 1 adopts a single injection method, so that the film-forming additive in the composite electrolyte forms a film at the positive electrode and the negative electrode at the same voltage, and there is a problem that the electrolyte side reaction is violent, and the impedance of the formed CEI film and SEI film is large, which is reflected in that the impedance of the battery provided in Comparative Example 1 is 115mΩ and the capacity retention rate of the battery after 1000 cycles is only 89.5%.

[0082] By comparing the test results of the battery provided by Example 1 with those of Comparative Examples 2 to 5, it can be found that the secondary battery provided by Example 1 has lower impedance and better cycle stability than the secondary battery provided by Comparative Examples 2 to 5. Although both Example 1 and Comparative Examples 2 to 5 adopt the method of secondary injection, the first voltage and the second voltage adopted by Example 1 are highly matched with the first electrolyte and the second electrolyte, respectively, which is conducive to reducing the battery impedance and improving the battery cycle capacity retention rate. After exploration and testing, it was found that when the first voltage is 3.5 to 4.0V and the second voltage is 4.0 to 4.5V, it is helpful to form a more stable SEI film and CEI film, which can improve the overall performance of the battery. Among them, in the injection method provided in Comparative Example 2, the first voltage is lower than 3.5V, resulting in the incomplete reaction of vinylene carbonate (VC) during the first formation, and the thickness of the generated SEI film is small and uneven. In the subsequent battery cycle process, the SEI film is easily damaged, resulting in the consumption of more active lithium, which is reflected in the cycle capacity retention rate of only 86.3%. In the injection method provided in Comparative Example 3, the first voltage is higher than 4.0V, resulting in part of VC forming a film on the positive electrode during the first formation process, which not only increases the impedance of the positive electrode, but also has no obvious effect on the protection of the positive electrode, and also causes the SEI film to insufficiently protect the negative electrode, which is reflected in the fact that the cycle capacity retention rate measured in Comparative Example 3 is only 86.4%. In the injection method provided in Comparative Example 4, the second voltage is lower than 4.0V, which will cause the second additive to have a poor film-forming effect on the surface of the positive electrode, affecting the protective effect on the positive electrode. In the injection method provided in Comparative Example 5, the second voltage is higher than 4.5V, which will increase the consumption of the second additive in the second formation process, and consume more active lithium in the electrode material, and bis(fluorosulfonyl)imide lithium salt (LiFSI) is easy to corrode aluminum foil at a higher voltage, further deteriorating the cycle performance, which is reflected in the fact that the battery impedance provided in Comparative Example 5 is large and the cycle capacity retention rate is small.

[0083] Comparing the test results of Examples 1 to 5 with those of Comparative Example 6 in Table 3, it can be found that compared with the battery provided in Comparative Example 6, the batteries provided in Examples 1 to 5 have lower resistance and higher cycle capacity retention. It can be explained that when the first electrolyte includes vinylene carbonate and the second electrolyte includes at least one of (trimethylsilyl) phosphate and lithium difluorooxalate borate, it helps to form a more stable SEI film and CEI film, which can not only provide effective protection for the positive and negative electrodes, but also reduce the battery impedance and extend the battery cycle life. Among them, the battery provided in Example 1 has the best cycle stability. In the injection method provided in Comparative Example 6, only vinylene carbonate (VC) is used as the second additive. Although VC can participate in the reaction of positive electrode film formation during the second formation process, the formed CEI film has poor protection effect on the positive electrode and increased impedance, which is reflected in the larger impedance and poor cycle performance of the battery in Comparative Example 6.

[0084] Comparing the test results of Example 1 with those of Examples 8 to 9 in Table 3, it can be found that as the mass proportion of ethylene carbonate (EC) in the first solvent increases, the impedance measured by the secondary battery shows an increasing trend. When the amount of ethylene carbonate added is 20 to 30 wt% of the mass of the first solvent, and the amount of ethylene carbonate added to the second solvent is greater than the amount of ethylene carbonate added to the first solvent, the viscosity of the first electrolyte is moderate and the fluidity is good, which is conducive to reducing the battery impedance.

[0085] The above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the protection scope of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the essence and scope of the technical solution of the present invention.

Claims

1. A secondary battery injection method, characterized in that: The following operations are included: S1. The battery cell is injected with a first electrolyte, wherein the battery cell includes a positive electrode sheet that satisfies the positive electrode active material containing a transition metal element, the first electrolyte includes vinylene carbonate, and after the injection is completed, a constant current formation is performed, and the cut-off voltage is a first voltage, and the first voltage is 3.5 to 4.0 V; S2. The battery cell is injected with a second electrolyte for a second time, wherein the second electrolyte includes at least one of (trimethylsilyl) phosphate and lithium difluorooxalate borate. After the injection is completed, constant current formation is performed, and the cut-off voltage is the second voltage, which is 4.0 to 4.5V.

2. The secondary battery injection method according to claim 1, characterized in that: The second electrolyte includes vinylene carbonate, trimethylsilyl phosphate and lithium difluorooxalate borate, and the mass ratio of the vinylene carbonate, the tri(trimethylsilyl) phosphate and the lithium difluorooxalate borate is 10-20:0.1-0.3:0.5-1.

0.

3. The secondary battery injection method according to claim 2, characterized in that: The mass percentage of the vinylene carbonate in the first electrolyte is 1 to 3 wt %; The total weight ratio of the vinylene carbonate, the tris(trimethylsilyl)phosphate and the lithium difluorooxalatoborate in the second electrolyte is 10-15 wt %.

4. The secondary battery injection method according to claim 1, characterized in that: The first electrolyte further includes a first lithium salt, which includes lithium hexafluorophosphate and bisfluorosulfonyl imide lithium salt. In the first lithium salt, the mass ratio of lithium hexafluorophosphate to bisfluorosulfonyl imide lithium salt is 7-9:1-3.

5. The secondary battery injection method according to claim 4, characterized in that: The second lithium salt is lithium hexafluorophosphate.

6. The secondary battery injection method according to claim 1, characterized in that: The first electrolyte includes a first solvent, the first solvent includes ethylene carbonate, and the amount of ethylene carbonate added is 20-30 wt % of the mass of the first solvent; The second electrolyte includes a second solvent, the second solvent includes ethylene carbonate, and the amount of ethylene carbonate added to the second solvent is greater than the amount of ethylene carbonate added to the first solvent.

7. The secondary battery injection method according to claim 6, characterized in that: The first solvent includes ethylene carbonate, propylene carbonate, dimethyl carbonate and ethyl methyl carbonate, The second solvent includes ethylene carbonate, propylene carbonate, dimethyl carbonate and ethyl methyl carbonate.

8. The secondary battery injection method according to claim 1, characterized in that: The weight ratio of the first electrolyte to the second electrolyte injected into the battery cell is 80-95:20-5.

9. An electrolyte, characterized in that: The electrolyte includes the first electrolyte and the second electrolyte used in the secondary battery injection method described in any one of claims 1 to 8.

10. A secondary battery, characterized in that: The electrolyte comprises the electrolyte as claimed in claim 9 or is injected using the secondary battery injection method as claimed in any one of claims 1 to 8.

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