Pre-film-forming negative plate, preparation method thereof and lithium battery applying pre-film-forming negative plate

Abstract

The invention provides a preformed film negative plate, a preparation method thereof and a lithium battery applying the preformed film negative plate, and the preparation method of the preformed film negative plate comprises the following steps: S1, adding a pre-lithiation reagent into the negative plate for pre-lithiation treatment to prepare a pre-lithiated negative plate; the pre-lithiation reagent comprises organic lithium and fluorine-containing lithium salt, and the fluorine-containing lithium salt comprises at least one of lithium hexafluorophosphate, lithium bis (fluorosulfonyl) imide and lithium tetrafluoroborate; s2, adding a pre-film-forming reagent for pre-film-forming treatment to prepare a pre-film-forming negative plate; the pre-film-forming reagent comprises a pre-film-forming additive, and the pre-film-forming additive comprises at least one of lithium bis (oxalato) borate and lithium difluoro (oxalato) borate. According to the invention, the pre-lithiation treatment and the pre-film-forming treatment are sequentially carried out on the negative plate, so that the formation of a good SEI film on the surface of the negative plate is promoted, the chemical composition of the SEI film is regulated and controlled, a negative electrode interface is protected, the consumption of active lithium in the charging and discharging process is reduced, and the first coulombic efficiency and the cycling stability of the battery are improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium batteries, and in particular, relates to a pre-filmed negative electrode sheet and a preparation method thereof, and a lithium battery using the same. Background Art

[0002] Lithium batteries have attracted much attention due to their high energy density and long cycle life. However, during the first charge and discharge process of lithium batteries, a solid electrolyte interface (SEI) will form on the negative electrode surface, consuming a large amount of active lithium and causing irreversible capacity loss. Pre-lithiation is a method to increase the total amount of lithium source in lithium batteries, improve the initial coulombic efficiency of lithium batteries, and extend the cycle life of lithium batteries. Therefore, pre-lithiation technology has been widely studied. However, the pre-lithiation electrode is relatively active and easily reacts with water and oxygen, resulting in failure of the pre-lithiation electrode, thereby affecting the initial coulombic efficiency and cycle performance of the pre-lithiation negative electrode. The main method at present is to control the atmosphere and strictly control the humidity during the production process to prevent the negative electrode from failing during subsequent operation and assembly. However, this will further increase the process cost of pre-lithiation. Summary of the invention

[0003] The object of the present invention is to provide a pre-filmed negative electrode sheet and a preparation method thereof and a lithium battery using the same, wherein the pre-filmed negative electrode sheet has good first coulombic efficiency and excellent structural stability and cycle stability.

[0004] According to one aspect of the present invention, there is provided a method for preparing a pre-filmed negative electrode sheet, comprising the following operations: S1. adding a pre-lithiation agent to a negative electrode sheet coated with a negative electrode active material for pre-lithiation treatment to obtain a pre-lithiation negative electrode sheet; the pre-lithiation agent comprises organic lithium and a fluorine-containing lithium salt, and the fluorine-containing lithium salt comprises at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, and lithium tetrafluoroborate; S2. adding a pre-filming agent to the pre-lithiation negative electrode sheet for pre-filming treatment to obtain a pre-filmed negative electrode sheet; the pre-filming agent comprises a pre-filming additive, and the pre-filming additive comprises at least one of lithium dioxalate borate and lithium difluorooxalate borate.

[0005] The present invention uses organic lithium and specific fluorine-containing lithium salts as pre-lithiation agents to perform pre-lithiation treatment on the negative electrode sheet, which can enable lithium ions to be quickly and evenly embedded in the negative electrode sheet, and provide sufficient active lithium ions for the subsequent introduction of pre-filming additives for pre-filming treatment. At the same time, the organic lithium can promote specific fluorine-containing lithium salts such as lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium tetrafluoroborate (LiBF4) to generate reduction product LiF particles on the surface of the negative electrode sheet. The inorganic LiF, as an inner layer component of the SEI film of the negative electrode sheet, has the advantages of high mechanical strength and low solubility, which is beneficial to improving the structural stability of the SEI film. Based on this, by introducing specific pre-filming additives into the pre-lithiation negative electrode sheet, lithium dioxalatoborate (LiBOB) and lithium difluorooxalatoborate (LiDFOB) in the pre-filming additives are rich in B element, which can utilize lithium ions to form boron-containing organic alkyl lithium carbonate on the surface of the negative electrode sheet. The organic boron-containing organic alkyl lithium carbonate, as the outer layer component of the SEI film of the negative electrode sheet, has the advantages of low high-temperature solubility and good long-term stability, which is beneficial to protect the negative electrode interface from being destroyed, reduce the loss of active lithium ions, inhibit the consumption of electrolyte components and the thickening of the SEI film during the formation process, and solve the problem of instability of the pre-lithiation electrode sheet, thereby effectively improving the initial efficiency of the lithium battery and extending the cycle life of the lithium battery.

[0006] In summary, the present invention can form a good SEI film on the surface of the negative electrode sheet before the negative electrode sheet is assembled into a battery and formed by sequentially performing pre-lithiation treatment and pre-filming treatment on the negative electrode sheet, which can prevent the negative electrode sheet from failing during the subsequent assembly and formation process. The chemical composition of the SEI film is regulated, so that an SEI film with an inner layer rich in inorganic substances and an outer layer rich in organic substances is formed on the surface of the negative electrode sheet, which effectively protects the negative electrode interface and reduces the consumption of active lithium during the charge and discharge process, so that the first coulomb efficiency, structural stability, and cycle stability of the pre-filmed negative electrode sheet and the lithium battery using the same are improved.

[0007] Preferably, the pre-filming additive includes lithium difluorooxalate borate. Lithium difluorooxalate borate (LiDFOB) contains both B and F elements, and can form an outer layer of SEI film with organic matter encapsulating LiF nanoparticles on the surface of the negative electrode. Therefore, by using LiDFOB as a pre-filming additive, the rigidity and flexibility of the SEI film can be effectively improved, and the risk of SEI film rupture and shedding caused by the volume change of the negative electrode can be reduced, thereby improving the cycle characteristics of the lithium battery.

[0008] Preferably, the pre-filming additive also includes lithium dioxalate borate, and the molar ratio of lithium dioxalate borate to lithium difluorooxalate borate is (1-2): (1-2). By combining LiBOB and LiDFOB as pre-filming additives and reasonably setting the feed ratio of LiBOB and LiDFOB, LiBOB and LiDFOB can play a synergistic role, control the ratio of inorganic and organic matter in the outer layer of the SEI film, and improve the film-forming properties of the SEI film. On the one hand, it is beneficial to improve the density and uniformity of the SEI film on the surface of the negative electrode sheet; on the other hand, it is beneficial to improve the mechanical properties of the SEI film on the surface of the negative electrode sheet, thereby enabling the SEI film to play a good protective role on the negative electrode sheet, reduce the occurrence of side reactions, reduce the loss of active lithium ions, fully utilize the battery capacity, and further improve the structural stability of the negative electrode sheet and the cycle stability of the lithium battery using it.

[0009] Preferably, the pre-filming reagent further includes a pre-filming solvent, and the pre-filming solvent includes a five-membered cyclic ether. The combination of the pre-filming solvent and the specific pre-filming additive is conducive to further promoting the formation of a dense and uniform SEI film on the surface of the negative electrode sheet, and in situ regulating the chemical composition of the SEI film, inhibiting the formation of inert lithium, avoiding negative electrode lithium deposition and battery polarization caused by the accumulation of inert lithium, and further improving the structural stability and cycle stability of the negative electrode sheet.

[0010] Preferably, the five-membered cyclic ether includes at least one of tetrahydrofuran and dimethyltetrahydrofuran.

[0011] Preferably, the molar ratio of organic lithium to fluorine-containing lithium salt is (1-9): 1. By controlling the feed ratio of organic lithium to fluorine-containing lithium salt in the pre-lithiation reagent, the synergistic combination between organic lithium and lithium salt can be promoted. On the one hand, it is conducive to promoting the formation of a dense and uniform inner layer of SEI film rich in inorganic substances on the surface of the negative electrode sheet, so that the structural stability of the negative electrode sheet is further improved; on the other hand, it is conducive to the timely replenishment of lithium ions, so that the surface of the negative electrode sheet is uniformly embedded with lithium, providing sufficient active lithium ions for subsequent pre-filming treatment, and further promoting the formation of a good outer layer of SEI film on the surface of the inner layer of SEI film.

[0012] Preferably, the organic lithium includes at least one of naphthalene lithium, biphenyl lithium, and pyrene lithium; the fluorine-containing lithium salt includes lithium bis(fluorosulfonyl)imide (LiFSI) and lithium tetrafluoroborate (LiBF4). The above-mentioned organic lithium and fluorine-containing lithium salt have good film-forming ability and can work together to pre-form a LiF-rich SEI film inner layer with a stable structure, dense and uniform on the surface of the negative electrode sheet.

[0013] Preferably, the temperature of the pre-lithiation treatment in S1 is 20°C to 30°C and the time is 4 minutes to 10 minutes; the temperature of the pre-filming treatment in S2 is 25°C to 50°C and the time is 10 minutes to 60 minutes. By controlling the temperature and time of the pre-lithiation treatment and the pre-filming treatment respectively, it is beneficial to regulate the thickness, density and uniformity of the SEI film, so that the SEI film formed on the surface of the negative electrode sheet has a stable structure with an inner layer rich in inorganic substances and an outer layer rich in organic substances.

[0014] According to another aspect of the present invention, a pre-filmed negative electrode sheet is provided. The pre-filmed negative electrode sheet is prepared by the above-mentioned method for preparing a pre-filmed negative electrode sheet. The pre-filmed negative electrode sheet includes an artificial SEI film, and the artificial SEI film contains LiF and boron-containing organic alkyl lithium carbonate.

[0015] Preferably, the negative electrode active material contained in the pre-filmed negative electrode sheet includes graphite.

[0016] According to another aspect of the present invention, a lithium battery is provided, comprising the above-mentioned pre-filmed negative electrode sheet. The lithium battery provided by the present invention has good first coulombic efficiency and excellent structural stability and cycle stability. DETAILED DESCRIPTION

[0017] 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.

[0018] Example 1

[0019] This embodiment provides a lithium battery, and the preparation method thereof includes the following operations:

[0020] 1. Preparation of pre-filmed negative electrode sheet

[0021] S1. Preparation of pre-lithiation negative electrode sheet

[0022] Prepare a negative electrode sheet coated with a negative electrode active material: mix the negative electrode active material graphite, the conductive agent conductive carbon black, and the binder sodium carboxymethyl cellulose in a mass ratio of 92:4:4, disperse them in deionized water to obtain a negative electrode slurry, coat the negative electrode slurry on a copper foil, and dry it in a vacuum environment at 100°C for 12 hours to obtain a negative electrode sheet coated with a negative electrode active material.

[0023] Preparation of a pre-lithiation reagent: Organic lithium (biphenyl lithium) and a fluorine-containing lithium salt (the fluorine-containing lithium salt is composed of LiFSI and LiBF4 in a molar ratio of 1:1) are mixed according to a molar ratio of organic lithium to fluorine-containing lithium salt of 7:1 to obtain a pre-lithiation reagent.

[0024] Preparation of pre-lithiation negative electrode sheet: Add 20 mL of pre-lithiation reagent into a reaction container, then add the above-mentioned negative electrode sheet coated with negative electrode active material, and perform pre-lithiation treatment at 25°C for 5 minutes; take out the negative electrode sheet after the reaction is completed and wash it to obtain the pre-lithiation negative electrode sheet.

[0025] S2. Preparation of pre-filmed negative electrode sheet

[0026] Preparation of a pre-filming reagent: dissolving a pre-filming additive (the pre-filming additive consists of LiBOB and LiDFOB in a molar ratio of 1:1) into a pre-filming solvent (tetrahydrofuran) to prepare a pre-filming reagent with a concentration of 1 M.

[0027] Preparation of pre-filmed negative electrode sheet: soak the pre-lithiated negative electrode sheet in 20 mL of pre-filming reagent and perform pre-filming treatment at 40° C. for 30 minutes; take out the negative electrode sheet after the reaction is completed and wash it to obtain the pre-filmed negative electrode sheet.

[0028] 2. Preparation of lithium batteries

[0029] The pre-filmed negative electrode sheet, lithium iron phosphate positive electrode sheet and separator are stacked and assembled into a soft-pack battery cell. 1.0M-LiPF6+DMC electrolyte is injected into the battery cell, and the lithium battery is prepared through vacuum packaging, static, formation and constant capacity processes.

[0030] Example 2

[0031] This embodiment prepares a lithium battery with reference to the embodiment 1. The difference between this embodiment and the embodiment 1 is that the pre-filming additive used in this embodiment only includes LiBOB. In addition to the above differences, the materials and process operations used in this embodiment are strictly consistent with those in the embodiment 1.

[0032] Example 3

[0033] This embodiment prepares a lithium battery with reference to the embodiment 1. The difference between this embodiment and the embodiment 1 is that the pre-film forming additive used in this embodiment only includes LiDFOB. In addition to the above differences, the materials and process operations used in this embodiment are strictly consistent with those in the embodiment 1.

[0034] Example 4

[0035] This example prepares a lithium battery with reference to Example 1. The difference between this example and Example 1 is that the mass ratio of LiBOB and LiDFOB in the pre-filming additive used in this example is 1:3. In addition to the above differences, the materials and process operations used in this example are strictly consistent with those in Example 1.

[0036] Example 5

[0037] This example prepares a lithium battery with reference to Example 1. The difference between this example and Example 1 is that the mass ratio of LiBOB and LiDFOB in the pre-filming additive used in this example is 3:1. Except for the above difference, the materials and process operations used in this example are strictly consistent with those in Example 1.

[0038] Example 6

[0039] This embodiment prepares a lithium battery with reference to the embodiment 1. The difference between this embodiment and the embodiment 1 is that in the process of preparing the pre-filmed negative electrode sheet, the tetrahydrofuran in the embodiment 1 is replaced with ethylene carbonate of the same mass as the pre-filmed solvent. Except for the above difference, the materials and process operations used in this embodiment are strictly consistent with those in the embodiment 1.

[0040] Example 7

[0041] This embodiment prepares a lithium battery with reference to the embodiment 1. The difference between this embodiment and the embodiment 1 is that in the process of preparing the pre-filmed negative electrode sheet, the tetrahydrofuran in the embodiment 1 is replaced with dimethyl carbonate of the same mass as the pre-filmed solvent. Except for the above difference, the materials and process operations used in this embodiment are strictly consistent with those in the embodiment 1.

[0042] Example 8

[0043] This embodiment refers to Example 1 to prepare a lithium battery. The difference between this embodiment and Example 1 is that in the process of preparing the pre-filmed negative electrode sheet in this embodiment, the pre-filming treatment temperature is 20° C. In addition to the above differences, the materials and process operations used in this embodiment are strictly consistent with those in Example 1.

[0044] Example 9

[0045] This embodiment refers to Example 1 to prepare a lithium battery. The difference between this embodiment and Example 1 is that in the process of preparing the pre-filmed negative electrode sheet in this embodiment, the pre-filming treatment temperature is 60° C. In addition to the above differences, the materials and process operations used in this embodiment are strictly consistent with those in Example 1.

[0046] Example 10

[0047] This embodiment prepares a lithium battery with reference to the embodiment 1. The difference between this embodiment and the embodiment 1 is that in the process of preparing the pre-lithiation negative electrode sheet in this embodiment, the temperature of the pre-lithiation treatment is 40° C. In addition to the above differences, the materials and process operations used in this embodiment are strictly consistent with those in the embodiment 1.

[0048] Embodiment 11

[0049] This embodiment prepares a lithium battery with reference to the embodiment 1. The difference between this embodiment and the embodiment 1 is that in the process of preparing the pre-lithiation negative electrode sheet, the fluorine-containing lithium salt in the embodiment 1 is replaced by lithium bis(fluorosulfonyl)imide (LiFSI) of the same mass. Except for the above difference, the materials and process operations used in this embodiment are strictly consistent with those in the embodiment 1.

[0050] Example 12

[0051] This embodiment prepares a lithium battery with reference to Example 1. The difference between this embodiment and Example 1 is that the mass ratio of organic lithium to fluorine-containing lithium salt in the pre-lithiation reagent used in this embodiment is 0.5:1. Except for the above difference, the materials and process operations used in this embodiment are strictly consistent with those in Example 1.

[0052] Embodiment 13

[0053] This embodiment prepares a lithium battery with reference to the embodiment 1. The difference between this embodiment and the embodiment 1 is that the mass ratio of the organic lithium to the fluorine-containing lithium salt in the pre-lithiation reagent used in this embodiment is 10:1. Except for the above difference, the materials and process operations used in this embodiment are strictly consistent with those in the embodiment 1.

[0054] Comparative Example 1

[0055] This comparative example refers to Example 1 to prepare a lithium battery. The difference between this comparative example and Example 1 is that in the process of preparing the lithium battery, this comparative example omits the operation of adding a pre-filming agent to the pre-lithiated negative electrode sheet for pre-filming treatment. Except for the above differences, the materials and process operations used in this comparative example are strictly consistent with those in Example 1.

[0056] Comparative Example 2

[0057] This comparative example prepares a lithium battery with reference to Example 1. The difference between this comparative example and Example 1 is that in the process of preparing the pre-filmed negative electrode sheet, the pre-filmed additive in Example 1 is replaced by lithium hexafluorophosphate (LiPF6) of the same mass. Except for the above differences, the materials and process operations used in this comparative example are strictly consistent with those in Example 1.

[0058] Comparative Example 3

[0059] This comparative example prepares a lithium battery with reference to Example 1. The difference between this comparative example and Example 1 is that in the process of preparing the pre-filmed negative electrode sheet, the pre-filmed additive in Example 1 is replaced by vinylene carbonate (VC) of the same mass. Except for the above difference, the materials and process operations used in this comparative example are strictly consistent with those in Example 1.

[0060] Comparative Example 4

[0061] This comparative example prepares a lithium battery with reference to Example 1. The difference between this comparative example and Example 1 is that the pre-lithiation reagent used in this comparative example only includes organic lithium. In addition to the above differences, the materials and process operations used in this comparative example are strictly consistent with those in Example 1.

[0062] Comparative Example 5

[0063] This comparative example prepares a lithium battery with reference to Example 1. The difference between this comparative example and Example 1 is that the pre-lithiation reagent used in this comparative example only includes fluorine-containing lithium salt. In addition to the above differences, the materials and process operations used in this comparative example are strictly consistent with those in Example 1.

[0064] Test Example 1

[0065] 1. Test object:

[0066] Examples 1 to 13 and comparative examples 1 to 5 were used as test objects of this test example.

[0067] 2. Test items:

[0068] (1) First coulombic efficiency: At 25°C, charge to 3.65 V at 0.2C constant current and constant voltage, let stand for 10 min, discharge to 2.5 V at 0.2C constant current, let stand for 10 min, and record the first coulombic efficiency of the battery. First coulombic efficiency = total capacity of the battery for the first discharge at 0.2C / total capacity of the battery for the first charge at 0.2C*100%.

[0069] (2) Cycle capacity retention rate: The battery was charged and discharged by Xinwei equipment at 25℃, 45℃, and 60℃ test environments respectively; it was charged to 3.65V at a constant current and constant voltage of 1C, left to stand for 10 minutes, and then discharged to 2.5V at a constant current of 1C. The final result is expressed as the capacity retention rate of 500 cycles of 1C / 1C charge and discharge, and the calculation formula is as follows: Capacity retention rate (%) = discharge capacity per cycle / first discharge capacity × 100%.

[0070] 3. Test results:

[0071] Table 1 Performance test results of lithium batteries of Examples 1 to 13 and Comparative Examples 1 to 5

[0072]

[0073] The test results are shown in Table 1. By comparing the performance test results corresponding to Example 1 and Comparative Example 1, it can be found that the first coulombic efficiency and high and low temperature cycle capacity retention rate of the lithium battery prepared in Comparative Example 1 are significantly lower than those in Example 1. The reason is that, under the same conditions of other materials and operations for preparing lithium batteries, Comparative Example 1 omits the operation of adding a pre-filming agent to the pre-lithiated negative electrode sheet for pre-filming treatment, resulting in the inability to form an outer layer of the SEI film containing boron organic alkyl lithium carbonate on the surface of the negative electrode sheet, which is not conducive to effectively protecting the negative electrode interface, thereby significantly deteriorating the first coulombic efficiency and high and low temperature cycle stability of the battery.

[0074] The performance test results of Example 1 are compared with those of Comparative Examples 2 to 3. As shown in Table 1, under the same conditions of other materials and operations for preparing lithium batteries, Comparative Examples 2 and 3 respectively use LiPF6 and VC as pre-filming additives in the process of preparing pre-filmed negative electrode sheets, and the first coulombic efficiency and high and low temperature cycle capacity retention rate of the lithium battery obtained are lower than those of Example 1. This shows that, compared with Comparative Examples 2 to 3, Example 1 is rich in B elements through the pre-filming additives used, and can use lithium ions to form boron-containing organic alkyl lithium carbonate on the surface of the negative electrode sheet. The organic boron-containing organic alkyl lithium carbonate is used as the outer layer component of the negative electrode sheet SEI film, which has the advantages of low high-temperature solubility and good long-term stability, which is conducive to protecting the negative electrode interface from being destroyed, reducing the loss of active lithium ions, inhibiting the consumption of electrolyte components and the thickening of the SEI film during the formation process, and solving the problem of instability of the pre-lithiation electrode sheet, thereby improving the first coulombic efficiency and high and low temperature cycle stability of the battery.

[0075] By comparing the performance test results corresponding to Example 1 and Comparative Examples 4 to 5, it can be found that the initial coulombic efficiency and high and low temperature cycle capacity retention rate of the lithium batteries prepared in Comparative Examples 4 to 5 are lower than those in Example 1. The reason is that under the same conditions of other materials and operations for preparing lithium batteries, the pre-lithiation reagents used in Comparative Examples 4 and 5 only include organic lithium and fluorine-containing lithium salts, respectively, which is not conducive to the rapid and uniform embedding of lithium ions into the negative electrode sheet, and is not conducive to providing sufficient active lithium ions for the subsequent introduction of pre-filming additives for pre-filming treatment, thereby deteriorating the initial coulombic efficiency and high and low temperature cycle stability of the battery.

[0076] The performance test results of Example 1 are compared with those of Examples 2 to 5. As shown in Table 1, under the same conditions of other materials and operations for preparing lithium batteries, the pre-filming additives in Examples 2 and 3 only include LiBOB and LiDFOB, respectively, and the mass ratios of LiBOB and LiDFOB in the pre-filming additives in Examples 4 and 5 are respectively lower than (1 to 2): (1 to 2) and higher than (1 to 2): (1 to 2), and the first coulombic efficiency and high and low temperature cycle capacity retention rate of the lithium batteries obtained therefrom are lower than those in Example 1. This shows that, compared with Examples 2 to 5, Example 1 uses LiBOB and LiDFOB as pre-film additives and reasonably sets the feed ratio of LiBOB and LiDFOB, so that LiBOB and LiDFOB can play a synergistic role, control the ratio of inorganic and organic matter in the outer layer of the SEI film, and improve the film-forming properties of the SEI film. On the one hand, it is beneficial to improve the density and uniformity of the SEI film on the surface of the negative electrode sheet; on the other hand, it is beneficial to improve the mechanical properties of the SEI film on the surface of the negative electrode sheet, thereby making the SEI film play a good protective role on the negative electrode sheet, reducing the occurrence of side reactions, reducing the loss of active lithium ions, and fully utilizing the battery capacity, thereby improving the first coulombic efficiency and high and low temperature cycle stability of the battery.

[0077] The performance test results of Example 1 are compared with those of Examples 6 to 7. As shown in Table 1, under the same conditions of other materials and operations for preparing lithium batteries, the pre-filming solvents used in Examples 6 and 7 are ethylene carbonate and dimethyl carbonate, respectively, and the first coulombic efficiency and high and low temperature cycle capacity retention rate of the lithium batteries obtained are lower than those of Example 1. This shows that, relative to Examples 6 to 7, Example 1 uses a five-membered cyclic ether as a pre-filming solvent, and the combination of the pre-filming solvent and a specific pre-filming additive can further promote the formation of a dense and uniform SEI film on the surface of the negative electrode sheet, and in situ regulate the chemical composition of the SEI film, inhibit the formation of inert lithium, and avoid the negative electrode lithium precipitation and battery polarization caused by the accumulation of inert lithium, thereby improving the first coulombic efficiency and high and low temperature cycle stability of the battery.

[0078] The performance test results of Example 1 are compared with those of Examples 8 to 10. As shown in Table 1, under the same conditions of other materials and operations for preparing lithium batteries, the temperatures of the pre-filming treatment in Examples 8 and 9 are lower than 25°C and higher than 50°C, respectively, and the temperature of the pre-lithiation treatment in Example 10 is higher than 30°C, and the first coulombic efficiency and high and low temperature cycle capacity retention rate of the lithium battery obtained are lower than those of Example 1. This shows that, compared with Examples 8 to 10, Example 1 is beneficial to regulating the thickness, density and uniformity of the SEI film by controlling the temperature and time of the pre-lithiation treatment and the pre-filming treatment, respectively, so that the SEI film formed on the surface of the negative electrode sheet presents a stable structure with an inner layer rich in inorganic substances and an outer layer rich in organic substances, thereby improving the first coulombic efficiency and high and low temperature cycle stability of the battery.

[0079] The performance test results of Example 1 and Example 11 are compared. As shown in Table 1, under the same conditions of preparing other materials and operations for lithium batteries, Example 11 only uses LiFSI as a fluorine-containing lithium salt, and the first coulombic efficiency and high and low temperature cycle capacity retention rate of the lithium battery obtained are lower than those of Example 1. This shows that, relative to Example 11, Example 1 uses LiFSI and LiBF4 as fluorine-containing lithium salts, and the fluorine-containing lithium salts and organic lithium can work together to pre-form a structurally stable, dense and uniform LiF-rich SEI film inner layer on the surface of the negative electrode sheet, thereby improving the first coulombic efficiency and high and low temperature cycle stability of the battery.

[0080] The performance test results of Example 1 are compared with those of Examples 12 to 13. As shown in Table 1, under the same conditions of other materials and operations for preparing lithium batteries, the mass ratios of organic lithium and fluorine-containing lithium salts in Examples 12 and 13 are respectively lower than (1 to 9): 1 and higher than (1 to 9): 1, and the first coulombic efficiency and high and low temperature cycle capacity retention rate of the lithium battery obtained are lower than those of Example 1. This shows that, relative to Examples 12 to 13, Example 1 can promote the synergistic combination between organic lithium and lithium salts by controlling the feed ratio of organic lithium and fluorine-containing lithium salts in the pre-lithiation reagent. On the one hand, it is conducive to promoting the formation of a dense and uniform inner layer of SEI film rich in inorganic substances on the surface of the negative electrode sheet, so that the structural stability of the negative electrode sheet is further improved; on the other hand, it is conducive to the timely replenishment of lithium ions, so that the surface of the negative electrode sheet is uniformly embedded with lithium, providing sufficient active lithium ions for subsequent pre-filming treatment, and further promoting the formation of a good outer layer of SEI film on the surface of the inner layer of the SEI film, thereby improving the first coulombic efficiency and high and low temperature cycle stability of the battery.

[0081] 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. A method for preparing a pre-filmed negative electrode sheet, characterized in that: The following operations are included: S1. Adding a pre-lithiation agent to a negative electrode sheet coated with a negative electrode active material for pre-lithiation treatment to obtain a pre-lithiation negative electrode sheet; the pre-lithiation agent includes organic lithium and a fluorine-containing lithium salt, and the fluorine-containing lithium salt includes at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, and lithium tetrafluoroborate; S2. Add a pre-filming reagent to the pre-lithiated negative electrode sheet for pre-filming treatment to obtain a pre-filming negative electrode sheet; the pre-filming reagent includes a pre-filming additive, and the pre-filming additive includes at least one of lithium dioxalate borate and lithium difluorooxalate borate.

2. The method for preparing a pre-filmed negative electrode sheet according to claim 1, characterized in that: The pre-film-forming additive includes the lithium difluorooxalatoborate.

3. The method for preparing a pre-filmed negative electrode sheet according to claim 2, characterized in that: The pre-film forming additive further comprises the lithium bis(oxalatoborate), and the molar ratio of the lithium bis(oxalatoborate) to the lithium difluoro(oxalatoborate) is (1-2):(1-2).

4. The method for preparing a pre-filmed negative electrode sheet according to claim 1, characterized in that: The pre-film-forming reagent further includes a pre-film-forming solvent, and the pre-film-forming solvent includes a five-membered cyclic ether.

5. The method for preparing a pre-filmed negative electrode sheet according to claim 4, characterized in that: The five-membered cyclic ether includes at least one of tetrahydrofuran and dimethyltetrahydrofuran.

6. The method for preparing a pre-filmed negative electrode sheet according to claim 1, characterized in that: The molar ratio of the organic lithium to the fluorine-containing lithium salt is (1-9):

1.

7. The method for preparing a pre-filmed negative electrode sheet according to claim 6, characterized in that: The organic lithium includes at least one of naphthalene lithium, biphenyl lithium and pyrene lithium; the fluorine-containing lithium salt includes lithium bis(fluorosulfonyl)imide and lithium tetrafluoroborate.

8. The method for preparing a pre-filmed negative electrode sheet according to any one of claims 1 to 7, characterized in that: The temperature of the pre-lithiation treatment in S1 is 20°C to 30°C and the time is 4 minutes to 10 minutes; The temperature of the pre-film forming treatment in S2 is 25° C. to 50° C. and the time is 10 minutes to 60 minutes.

9. A pre-filmed negative electrode sheet, characterized in that: The pre-filmed negative electrode sheet is prepared by the method for preparing a pre-filmed negative electrode sheet according to any one of claims 1 to 8, and the pre-filmed negative electrode sheet comprises an artificial SEI film, wherein the artificial SEI film contains LiF and boron-containing organic alkyl lithium carbonate.

10. A lithium battery, characterized in that: Including the pre-filmed negative electrode sheet as described in claim 9.

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