Lithium replenishment method, pre-lithiated negative electrode plate, secondary battery and electronic device

Abstract

This application provides a lithium replenishment method, a pre-lithiated negative electrode sheet, a secondary battery, and an electronic device. The lithium replenishment method for the negative electrode sheet includes the following steps: providing a negative electrode sheet and a lithium replenishment composite layer; bonding the lithium replenishment composite layer and the negative electrode sheet; subjecting the lithium replenishment composite layer and the negative electrode sheet to a discharge treatment, wherein the discharge treatment is divided into a first stage, a second stage, and a third stage in chronological order, with different discharge currents and times in each stage; after the discharge treatment, peeling the lithium replenishment composite layer from the negative electrode sheet to obtain a first pre-lithiated negative electrode sheet; and storing the first pre-lithiated negative electrode sheet to obtain a second pre-lithiated negative electrode sheet. The second pre-lithiated negative electrode sheet prepared using the lithium replenishment method of this application, when applied to a secondary battery, can improve the initial coulombic efficiency and energy density of the secondary battery and reduce cycle capacity decay.

Description

Technical Field

[0001] This application relates to the field of electrochemical technology, and in particular to a lithium replenishment method, a pre-lithiated negative electrode, a secondary battery, and an electronic device. Background Technology

[0002] Secondary batteries, such as lithium-ion batteries, possess advantages such as high energy density, high power, and long cycle life, and are widely used in consumer electronics, electric bicycles, and electric vehicles. As their application scope continues to expand, the requirements for energy density and cycle performance of lithium-ion batteries are constantly increasing. Currently, graphite, the commonly used negative electrode active material for lithium-ion batteries, can no longer meet the energy density requirements. Although silicon-carbon and silicon-oxygen negative electrode active materials have high theoretical specific capacity and are ideal materials to replace graphite and improve the energy density of lithium-ion batteries, they have not been widely adopted due to their low initial coulombic efficiency and poor cycle life. Existing methods to improve the initial coulombic efficiency and reduce cycle capacity decay of lithium-ion batteries containing silicon-carbon or silicon-oxygen negative electrode active materials involve pre-replenishing the negative electrode with lithium to compensate for the irreversible capacity consumed during the first charge, discharge, and cycle, thereby improving the initial coulombic efficiency of lithium-ion batteries containing silicon-carbon or silicon-oxygen negative electrode active materials and ultimately increasing the energy density of the lithium-ion battery.

[0003] Existing lithium replenishment methods for negative electrode sheets mainly include lithium powder replenishment, lithium strip replenishment, and electrochemical replenishment. However, these three methods all have certain problems in terms of environmental control, replenishment uniformity, and post-replenishment side reactions. Therefore, there is an urgent need to provide a lithium replenishment method for negative electrode sheets to improve the initial coulombic efficiency of lithium-ion batteries containing silicon-carbon or silicon-oxygen negative electrode active materials, reduce cycle capacity decay, and increase the energy density of lithium-ion batteries. Summary of the Invention

[0004] The purpose of this application is to provide a lithium replenishment method, a pre-lithiated negative electrode, a secondary battery, and an electronic device to improve the initial coulombic efficiency of the secondary battery, reduce cycle capacity decay, and increase the energy density of the secondary battery. The specific technical solution is as follows:

[0005] The first aspect of this application provides a method for lithium replenishment of a negative electrode sheet, which includes the following steps:

[0006] (1) A negative electrode sheet is provided, the negative electrode sheet including a negative current collector and a negative electrode material layer disposed on at least one surface of the negative current collector.

[0007] (2) Provide a lithium replenishment composite layer, the lithium replenishment composite layer including a support layer, a lithium replenishment layer and a pre-impregnated electrolyte separator, the lithium replenishment layer being disposed between the pre-impregnated electrolyte separator and the support layer.

[0008] (3) The lithium replenishment composite layer and the negative electrode sheet are bonded together, so that the separator pre-impregnated with electrolyte comes into contact with the negative electrode material layer. The lithium replenishment composite layer and the negative electrode sheet are then subjected to discharge treatment. The discharge treatment is divided into three stages in time sequence: the first stage, the second stage, and the third stage. The constant current discharge current of the first stage is 0.1C to 0.3C, and the discharge time is 0.008h to 0.28h. The constant current discharge current of the second stage is 1C to 2C, and the discharge time is 0.032h to 0.30h. The constant current discharge current of the third stage is 0.3C to 0.5C, and the discharge time is 0.027h to 0.058h.

[0009] (4) After the discharge treatment is completed, the lithium replenishment composite layer is peeled off from the negative electrode to obtain the first pre-lithiated negative electrode.

[0010] (5) The first pre-lithiated negative electrode is stored in an environment of 20°C to 30°C and humidity ≤1.7% for 12h to 72h to obtain the second pre-lithiated negative electrode.

[0011] The lithium replenishment method of this application replenishes lithium on the negative electrode sheet, which can replenish the active lithium lost during the first charge and discharge process of the secondary battery due to the formation of the solid electrolyte interphase (SEI) film, as well as the active lithium lost during cycling. Furthermore, by controlling the discharge treatment conditions, the first stage of discharge enables the formation of a dense inorganic SEI film, the second stage enables the formation of a loose organic SEI film, shortens the lithium replenishment time, and improves the lithium replenishment efficiency. The third stage of discharge ensures the integrity and stability of the SEI film, thereby forming a dense SEI film on the surface of the negative electrode sheet. Using the lithium replenishment method of this application and selecting the electrolyte of this application in the pre-impregnated electrolyte separator can also improve the stability of the SEI film on the surface of the negative electrode sheet in the environment, further facilitating the formation of a dense and stable SEI film on the negative electrode sheet surface, and improving the stability and integrity of the SEI film. Simultaneously, since a dense and stable SEI film has already been formed during the above discharge and storage treatments, the formation step can be omitted in the subsequent secondary battery preparation process. Therefore, the second pre-lithiated negative electrode obtained by the lithium supplementation method of this application can improve the environmental stability of the negative electrode. Applying it to secondary batteries can improve the initial coulombic efficiency of secondary batteries, reduce cycle capacity decay, increase the energy density of secondary batteries, simplify the preparation process of secondary batteries, improve production efficiency, and reduce preparation costs.

[0012] In some embodiments of this application, the first coulombic efficiency of the first pre-lithiated negative electrode applied to the secondary battery is a first initial coulombic efficiency E1, and the first initial coulombic efficiency of the second pre-lithiated negative electrode applied to the secondary battery is a second initial coulombic efficiency E2, where 90% ≤ E2 / E1 ≤ 100%. When the value of E2 / E1 is within the above range, the secondary battery has a higher initial coulombic efficiency, lower cycle capacity decay, and higher energy density.

[0013] In some embodiments of this application, 90% ≤ E1 ≤ 120%. When the value of E1 is within the above range, the secondary battery has a higher initial coulombic efficiency, lower cycle capacity decay, and higher energy density.

[0014] In some embodiments of this application, the lithium fluoride coverage on the surface of the first pre-lithiated negative electrode is 45% to 85%, and the lithium fluoride coverage on the surface of the second pre-lithiated negative electrode is 45% to 95%. When the lithium fluoride coverage on the surfaces of the first and second pre-lithiated negative electrodes is within the above ranges, the SEI film integrity and stability on the surface of the negative electrode are high, the ionic conductivity is also high, and the secondary battery exhibits high initial coulombic efficiency, low cycle capacity decay, and high energy density.

[0015] In some embodiments of this application, the mass content of hydrogen fluoride on the surface of the first pre-lithiated negative electrode is from 10 ppm to 450 ppm, and the mass content of hydrogen fluoride on the surface of the second pre-lithiated negative electrode is from 10 ppm to 500 ppm. The fact that the mass content of hydrogen fluoride on the surfaces of the first and second pre-lithiated negative electrodes is within the above range indicates that fewer side reactions occur on the negative electrode during the discharge treatment and lithium replenishment process and during storage, resulting in higher integrity of the SEI film on the surface of the negative electrode. Consequently, the secondary battery exhibits higher initial coulombic efficiency, lower cycle capacity decay, and higher energy density.

[0016] In some embodiments of this application, the electrolyte includes additives, including non-fluorinated additives, such as lithium hexamethyldisilazane, lithium thiodicarbonate, or lithium bis(oxalato)borate. The mass percentage of the non-fluorinated additive is 0.1% to 10% based on the mass of the electrolyte. Including the aforementioned non-fluorinated additive in the electrolyte, and controlling its mass percentage within the above range, is beneficial for forming a stable SEI film on the surface of the negative electrode, improving the stability of the SEI film on the negative electrode surface, and improving the environmental stability of the negative electrode when placed in air for extended periods. This, in turn, helps to improve the initial coulombic efficiency of the secondary battery, reduce cycle capacity decay, and increase the energy density of the secondary battery.

[0017] In some embodiments of this application, the electrolyte includes additives, including fluorinated additives, such as lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethyl)sulfonyl)imide, lithium fluorosulfonyl(trifluoromethyl)imide, lithium bis(pentafluoroethane)sulfonylimide, lithium (trifluoromethyl)(nonafluorobutyl)sulfonylimide, or lithium 1,3-perfluoropropanedisulfonylimide; the mass percentage of the fluorinated additive is 0% to 5% based on the mass of the electrolyte. Including the above-mentioned fluorinated additives in the electrolyte, and controlling their mass percentage within the above range, is beneficial to improving the hydrolysis resistance of the electrolyte, improving the stability of the SEI film on the surface of the negative electrode, thereby improving the initial coulombic efficiency of the secondary battery, reducing cycle capacity decay, and increasing the energy density of the secondary battery.

[0018] In some embodiments of this application, the electrolyte includes a non-aqueous organic solvent, including ethylene carbonate; based on the mass of the electrolyte, the mass percentage of ethylene carbonate is 32.5% to 91%. Including ethylene carbonate in the electrolyte and controlling its mass percentage within the above range is beneficial for dissolving lithium salts, improving the ionic conductivity of the electrolyte, and also beneficial for forming a stable SEI film on the surface of the negative electrode, improving the stability of the SEI film on the surface of the negative electrode, and improving the environmental stability of the negative electrode when placed in air for a long time. This, in turn, helps to improve the initial coulombic efficiency of the secondary battery, reduce cycle capacity decay, and increase the energy density of the secondary battery.

[0019] In some embodiments of this application, the discharge treatment temperature is between 25°C and 90°C. By controlling the discharge treatment temperature within the above range, it is beneficial to shorten the lithium replenishment time, improve the lithium replenishment efficiency, and achieve a good lithium replenishment effect, thereby improving the initial coulombic efficiency of the secondary battery, reducing cycle capacity decay, and increasing the energy density of the secondary battery.

[0020] In some embodiments of this application, the interfacial pressure per unit area between the lithium replenishment composite layer and the negative electrode is 0.1 MPa to 0.6 MPa. By adjusting the interfacial pressure per unit area between the lithium replenishment composite layer and the negative electrode within the above range, the distance between the lithium replenishment composite layer and the negative electrode can be shortened, reducing the impedance of lithium ion transport in the lithium replenishment composite layer and the negative electrode, shortening the lithium replenishment time, and improving the lithium replenishment efficiency. This is beneficial for improving the initial coulombic efficiency of the secondary battery, reducing cycle capacity decay, and increasing the energy density of the secondary battery.

[0021] In some embodiments of this application, the areal density of the electrolyte on the pre-impregnated electrolyte separator is 3 g / mm². 2 Up to 60g / mm 2By controlling the areal density of the electrolyte on the pre-impregnated electrolyte separator within the aforementioned range, it is beneficial to improve the transport rate of lithium ions in the lithium replenishment composite layer and the negative electrode material layer, thereby improving the initial coulombic efficiency of the secondary battery, reducing cycle capacity decay, and increasing the energy density of the secondary battery.

[0022] In some embodiments of this application, the areal density of the electrolyte on the pre-impregnated electrolyte separator is 5 g / mm². 2 Up to 30g / mm 2 By controlling the areal density of the electrolyte on the pre-impregnated electrolyte separator within the aforementioned range, it is beneficial to further improve the transport rate of lithium ions in the lithium replenishment composite layer and the negative electrode material layer, thereby further improving the initial coulombic efficiency of the secondary battery, reducing cycle capacity decay, and increasing the energy density of the secondary battery.

[0023] The second aspect of this application provides a pre-lithiated negative electrode sheet prepared according to the lithium supplementation method in any of the foregoing embodiments. When applied to a secondary battery, the pre-lithiated negative electrode sheet of this application can improve the initial coulombic efficiency of the secondary battery, reduce cycle capacity decay, increase the energy density of the secondary battery, and also simplify the manufacturing process of the secondary battery, improve production efficiency, and reduce manufacturing costs.

[0024] A third aspect of this application provides a secondary battery comprising a pre-lithiated negative electrode as described in any of the foregoing embodiments. The secondary battery of this application exhibits high initial coulombic efficiency, low cycle capacity decay, and high energy density.

[0025] A fourth aspect of this application provides an electronic device that includes a secondary battery as described in any of the foregoing embodiments. The electronic device of this application has a long service life and good performance.

[0026] The beneficial effects of this application are:

[0027] This application provides a lithium replenishment method, a pre-lithiated negative electrode, a secondary battery, and an electronic device. This lithium replenishment method replenishes the active lithium lost during the first charge-discharge process due to the formation of the solid electrolyte interphase (SEI) film, as well as the active lithium lost during cycling. By controlling the discharge treatment conditions, a first-stage discharge enables the formation of a dense inorganic SEI film, a second-stage discharge enables the formation of a loose organic SEI film, shortens the lithium replenishment time, and improves the lithium replenishment efficiency. A third-stage discharge ensures the integrity and stability of the SEI film, thereby forming a dense SEI film on the surface of the negative electrode. Using this lithium replenishment method and selecting the electrolyte of this application in the pre-impregnated electrolyte separator further improves the stability of the SEI film on the surface of the negative electrode in the environment, which is more conducive to the formation of a dense and stable SEI film on the surface of the negative electrode, improving the stability and integrity of the SEI film. Furthermore, since a dense and stable SEI film has already been formed during the above discharge and storage treatments, the formation step can be omitted in the subsequent secondary battery preparation process. Therefore, the second pre-lithiated negative electrode obtained by the lithium supplementation method of this application can improve the environmental stability of the negative electrode. Applying it to secondary batteries can improve the initial coulombic efficiency of secondary batteries, reduce cycle capacity decay, increase the energy density of secondary batteries, simplify the preparation process of secondary batteries, improve production efficiency, and reduce preparation costs.

[0028] Of course, implementing any product or method of this application does not necessarily require achieving all of the advantages described above at the same time. Attached Figure Description

[0029] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other embodiments can be obtained based on these drawings.

[0030] Figure 1 This is a schematic diagram of the negative electrode sheet during the lithium replenishment process according to one embodiment of this application.

[0031] Reference numerals: negative electrode 110, negative electrode current collector 111, negative electrode material layer 112, lithium replenishment composite layer 120, separator 121 pre-impregnated with electrolyte, lithium replenishment layer 122, support layer 123. Detailed Implementation

[0032] The technical solutions of this application will be clearly and completely described below with reference to the embodiments and accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0033] It should be noted that, in the specific embodiments of this application, lithium-ion batteries are used as an example of secondary batteries to explain this application, but the secondary batteries in this application are not limited to lithium-ion batteries.

[0034] The first aspect of this application provides a method for lithium replenishment of a negative electrode sheet, which includes the following steps:

[0035] (1) A negative electrode sheet is provided, the negative electrode sheet including a negative current collector and a negative electrode material layer disposed on at least one surface of the negative current collector.

[0036] (2) Provide a lithium replenishment composite layer, the lithium replenishment composite layer including a support layer, a lithium replenishment layer and a pre-impregnated electrolyte separator, the lithium replenishment layer being disposed between the pre-impregnated electrolyte separator and the support layer.

[0037] (3) The lithium replenishment composite layer and the negative electrode sheet are bonded together, so that the separator pre-impregnated with electrolyte comes into contact with the negative electrode material layer. The lithium replenishment composite layer and the negative electrode sheet are then subjected to discharge treatment. The discharge treatment is divided into three stages in time sequence: the first stage, the second stage, and the third stage. The constant current discharge current of the first stage is 0.1C to 0.3C, and the discharge time is 0.008h to 0.28h. The constant current discharge current of the second stage is 1C to 2C, and the discharge time is 0.032h to 0.30h. The constant current discharge current of the third stage is 0.3C to 0.5C, and the discharge time is 0.027h to 0.058h.

[0038] For example, the constant current discharge current in the first stage can be 0.1C, 0.12C, 0.15C, 0.18C, 0.2C, 0.23C, 0.25C, 0.28C, 0.3C, or any combination of two of the above values; the discharge time can be 0.008h, 0.03h, 0.05h, 0.07h, 0.1h, 0.13h, 0.15h, 0.18h, 0.20h, 0.22h, 0.25h, 0.28h, or any combination of two of the above values; the constant current discharge current in the second stage can be 1C, 1.1C, 1.2C, 1.3C, 1.4C, 1.5C, 1.6C, 1.7C, 1.8C, 1.9C, 2C, or any combination of two of the above values. The discharge time can be 0.032h, 0.05h, 0.07h, 0.1h, 0.13h, 0.15h, 0.18h, 0.20h, 0.22h, 0.25h, 0.28h, 0.30h, or any two of the above values; the constant current discharge current in the third stage can be 0.3C, 0.34C, 0.38C, 0.4C, 0.43C, 0.45C, 0.48C, 0.5C, or any two of the above values, and the discharge time can be 0.027h, 0.030h, 0.035h, 0.040h, 0.042h, 0.045h, 0.048h, 0.052h, 0.058h, or any two of the above values.

[0039] (4) After the discharge treatment is completed, the lithium replenishment composite layer is peeled off from the negative electrode to obtain the first pre-lithiated negative electrode.

[0040] (5) The first pre-lithiated negative electrode is stored in an environment of 20°C to 30°C and humidity ≤1.7% for 12h to 72h to obtain the second pre-lithiated negative electrode.

[0041] For example, the storage temperature of the first pre-lithiated negative electrode sheet can be 20°C, 21°C, 22°C, 23°C, 24°C, 25°C, 26°C, 27°C, 28°C, 29°C, 30°C, or any combination of two of the above values; the humidity can be 0%, 0.2%, 0.4%, 0.6%, 0.8%, 1.0%, 1.2%, 1.4%, 1.7%, or any combination of two of the above values; and the storage time can be 12h, 20h, 25h, 30h, 35h, 40h, 45h, 50h, 55h, 60h, 65h, 72h, or any combination of two of the above values.

[0042] The lithium replenishment method of this application is used to replenish lithium in the negative electrode sheet. The lithium replenishment layer and the negative electrode material layer can form an external short circuit. The lithium atoms in the lithium replenishment layer undergo an oxidation reaction and lose electrons to generate lithium ions. The lithium ions are transported to the surface or interior of the negative electrode material layer through the separator pre-impregnated with electrolyte. The electrons are transported to the negative electrode current collector through the external circuit and then to the surface or interior of the negative electrode material layer. The electrons and lithium ions undergo a reduction reaction in the negative electrode material layer to form a lithium intercalation compound, thereby achieving pre-lithiation of the negative electrode material layer to replenish the active lithium lost during the first charge and discharge process due to the formation of the SEI film and the active lithium lost during cycling.

[0043] Because a pre-impregnated electrolyte separator is provided between the lithium replenishment layer and the negative electrode material layer, the separator has good ion conduction capability, which can reduce the probability of the lithium replenishment layer adhering to the surface of the negative electrode material layer, reduce the impact of side reactions that may occur in the negative electrode material layer due to the high activity of lithium metal in the lithium replenishment layer on the electrochemical performance of the secondary battery, and also reduce the impact of silicone oil residue on the surface of the lithium replenishment layer entering the secondary battery and affecting the electrochemical performance of the secondary battery.

[0044] Within the scope of this application, the lithium replenishment composite layer and the negative electrode are subjected to discharge treatment, and the discharge treatment conditions are controlled. The first stage discharge treatment enables the formation of a dense inorganic SEI film; the second stage discharge treatment enables the formation of a loose organic SEI film, shortening the lithium replenishment time and improving the lithium replenishment efficiency; and the third stage discharge treatment ensures the integrity and stability of the SEI film, thereby forming a dense SEI film on the surface of the negative electrode, reducing the probability of damage to the negative electrode active material due to solvent molecule co-intercalation, improving lithium-ion transport efficiency, and reducing side reactions between the negative electrode and the electrolyte. After the discharge treatment, the lithium replenishment composite layer is peeled off from the negative electrode to obtain the first pre-lithiated negative electrode. The above peeling process can reduce the impact of side reactions occurring after the lithium metal element in the lithium replenishment layer remains on the surface of the negative electrode material and enters the secondary battery, thus affecting the electrochemical performance of the secondary battery.

[0045] However, the SEI film formed on the surface of the first pre-lithiated negative electrode after discharge treatment has poor stability when left in air for a long time. It easily reacts with CO2 and H2O to form inorganic lithium salts such as Li2CO3 and Li2O. At the same time, lithium in the SEI film can also react with O2 to form various strongly nucleophilic oxides, and react with organic molecules and hemicarbonates to form carbonates and alkoxides. However, by using the lithium replenishment method of this application and selecting the electrolyte of this application in the separator pre-impregnated with electrolyte, the stability of the SEI film on the surface of the negative electrode can be improved in air environment. This is more conducive to the formation of a dense and stable SEI film on the surface of the negative electrode, improving the stability and integrity of the SEI film. At the same time, since a dense and stable SEI film has been formed during the above discharge and storage treatments, the formation step can be omitted in the subsequent preparation of the secondary battery, reducing the consumption of electrolyte during the formation process and increasing the overall electrolyte content in the secondary battery.

[0046] When the constant current discharge current in the first stage of discharge treatment is too small, for example, less than 0.1C, the formed inorganic SEI film is too dense, which is not conducive to lithium-ion transport and results in poor kinetics of the secondary battery. When the constant current discharge current in the first stage of discharge treatment is too large, for example, greater than 0.3C, the formed inorganic SEI film is too porous. Although it is conducive to lithium-ion transport, the stability of the SEI film is poor. When the discharge time in the first stage of discharge treatment is too small, for example, less than 0.008h, the formed inorganic SEI film is too thin, resulting in poor SEI film stability. When the discharge time in the first stage of discharge treatment is too large, for example, greater than 0.28h, the formed inorganic SEI film is too thick, which is not conducive to lithium-ion transport and results in poor kinetics of the secondary battery. When the constant current discharge current in the second stage of discharge treatment is too small, for example, less than 1C, the formed organic SEI film is too dense, which is not conducive to lithium-ion transport and results in poor kinetics of the secondary battery. When the constant current discharge current in the second stage of discharge treatment is too large, for example, greater than 2C, the formed organic SEI film is too porous, resulting in poor SEI film stability. When the discharge time in the second-stage discharge treatment is too short, for example, less than 0.032 h, the resulting organic SEI film is too thin, leading to poor SEI film stability. When the discharge time in the second-stage discharge treatment is too long, for example, greater than 0.30 h, the resulting organic SEI film is too thick, which is detrimental to lithium-ion transport and results in poor kinetics of the secondary battery. When the constant current discharge current in the third-stage discharge treatment is too short, for example, less than 0.3 C, the resulting SEI film is too dense, resulting in poor kinetics of the secondary battery. When the constant current discharge current in the third-stage discharge treatment is too long, for example, greater than 0.5 C, the resulting SEI film is too porous, resulting in poor integrity and stability. When the discharge time in the third-stage discharge treatment is too short, for example, less than 0.027 h, the resulting SEI film is too thin, resulting in poor integrity and stability. When the discharge time in the third-stage discharge treatment is too long, for example, greater than 0.058 h, the resulting SEI film is too thick, resulting in poor kinetics of the secondary battery.

[0047] When the storage temperature of the first pre-lithiated negative electrode is too low, for example below 20°C, although the degree of side reactions in the resulting second pre-lithiated negative electrode is reduced, the cost of negative electrode storage and processing increases, making practical applications difficult. When the storage temperature of the first pre-lithiated negative electrode is too high, for example above 30°C, the SEI film formed on the surface of the negative electrode is prone to side reactions, leading to a decrease in the density and stability of the SEI film. When the humidity during storage of the first pre-lithiated negative electrode is too high, for example greater than 1.7%, the SEI film formed on the surface of the negative electrode is prone to side reactions, leading to a decrease in the density and stability of the SEI film. When the storage time of the first pre-lithiated negative electrode is too short, for example, less than 12 hours, although the degree of side reactions in the resulting second pre-lithiated negative electrode is reduced, the production process is more difficult, meaning it is difficult to prepare a secondary battery within a short time after the negative electrode is replenished with lithium. When the storage time of the first pre-lithiated negative electrode is too long, for example, greater than 72 hours, the SEI film formed on the surface of the negative electrode is prone to side reactions, leading to a decrease in the density and stability of the SEI film. Therefore, any of the constant current discharge current and discharge time in the first, second, and third stages of discharge treatment, as well as any of the temperature, humidity, and time of storage treatment, which are not within the scope of this application, will affect the initial coulombic efficiency, cycle performance, and energy density of the secondary battery, or make the process more difficult to implement.

[0048] Therefore, the second pre-lithiated negative electrode obtained by the lithium supplementation method of this application can improve the environmental stability of the negative electrode. Applying it to secondary batteries can improve the initial coulombic efficiency of secondary batteries, reduce cycle capacity decay, increase the energy density of secondary batteries, simplify the preparation process of secondary batteries, improve production efficiency, and reduce preparation costs.

[0049] In this application, the water content of the negative electrode sheet in step (1) is ≤500ppm. The negative electrode sheet includes a negative current collector and a negative electrode material layer disposed on at least one surface of the negative current collector. The aforementioned "negative electrode material layer disposed on at least one surface of the negative current collector" means that the negative electrode material layer can be disposed on one surface of the negative current collector along its own thickness direction, or it can be disposed on two surfaces of the negative current collector along its own thickness direction. It should be noted that the "surface" here can be the entire area of ​​the negative current collector or a part of the negative current collector. This application has no particular limitation, as long as the purpose of this application can be achieved. It is understood that the above-mentioned negative electrode sheet refers to a negative electrode sheet that has not undergone pre-lithiation.

[0050] In some embodiments of this application, the electrolyte includes additives, including non-fluorinated additives, which include at least one of lithium hexamethyldisilazane (LiHMDS), lithium thiodicarbonate (LiTDI), or lithium bis(oxalatoborate) (LiBOB). Based on the mass of the electrolyte, the mass percentage of the non-fluorinated additive is from 0.1% to 10%. Exemplarily, the mass percentage of the non-fluorinated additive can be 0.1%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or a range consisting of any two of the above values. The inclusion of the aforementioned non-fluorinated additives in the electrolyte, and the control of their mass percentage within the above range, is beneficial for forming a stable SEI film on the surface of the negative electrode, improving the stability of the SEI film on the surface of the negative electrode, improving the environmental stability of the negative electrode when placed in an air environment for a long time, and improving the integrity of the SEI film. This, in turn, helps to improve the initial coulombic efficiency of the secondary battery, reduce cycle capacity decay, and increase the energy density of the secondary battery.

[0051] In some embodiments of this application, the electrolyte includes an additive, which includes a fluorinated additive, comprising at least one of lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethyl)sulfonyl)imide (LiTFSI), lithium fluorosulfonyl(trifluoromethyl)imide (LiFTFSI), lithium bis(pentafluoroethane)sulfonylimide (LiBETi), lithium (trifluoromethyl)(nonafluorobutyl)sulfonylimide, or lithium 1,3-perfluoropropanedisulfonylimide; the mass percentage of the fluorinated additive is 0% to 5% based on the mass of the electrolyte. Exemplarily, the mass percentage of the fluorinated additive may be 0%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, or a range consisting of any two of the above values. The electrolyte includes the aforementioned fluorinated additives, and the mass percentage of these additives is controlled within the aforementioned range. This is beneficial for improving the electrolyte's resistance to hydrolysis, reducing side reactions between the residual electrolyte in the negative electrode material layer and moisture in the environment during storage, improving the stability of the SEI film on the surface of the negative electrode, improving the environmental stability of the negative electrode when placed in the air for a long time, and improving the integrity of the SEI film. This is beneficial for improving the initial coulombic efficiency of the secondary battery, reducing cycle capacity decay, and increasing the energy density of the secondary battery.

[0052] In some embodiments of this application, the electrolyte includes a non-aqueous organic solvent, including ethylene carbonate (EC); based on the mass of the electrolyte, the mass percentage of ethylene carbonate is between 32.5% and 91%. Exemplarily, the mass percentage of ethylene carbonate can be 32.5%, 40%, 50%, 60%, 70%, 80%, 91%, or a range consisting of any two of the above values. Including ethylene carbonate in the electrolyte and controlling its mass percentage within the above range is beneficial for dissolving lithium salts, improving the ionic conductivity of the electrolyte, and also beneficial for forming a stable SEI film on the surface of the negative electrode, improving the stability of the SEI film on the surface of the negative electrode, improving the environmental stability of the negative electrode when placed in air for a long time, and improving the integrity of the SEI film. This, in turn, helps to improve the initial coulombic efficiency of the secondary battery, reduce cycle capacity decay, and increase the energy density of the secondary battery.

[0053] In some embodiments, the electrolyte further includes other non-aqueous organic solvents, the mass percentage of which is 0% to 50% based on the mass of the electrolyte. Exemplarily, the mass percentage of other non-aqueous organic solvents can be 0%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, or a range of any two of the above values. This application does not particularly limit the types of other non-aqueous organic solvents, as long as they achieve the purpose of this application. For example, other non-aqueous organic solvents may include at least one of carbonate compounds, carboxylic acid ester compounds, ether compounds, or other electrolyte organic solvents. The aforementioned carbonate compounds may include, but are not limited to, at least one of chain carbonate compounds, cyclic carbonate compounds, or fluorocarbonate compounds. The aforementioned chain carbonate compounds may include, but are not limited to, at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), or methyl ethyl carbonate (EMC). The aforementioned cyclic carbonate compounds may include, but are not limited to, at least one of vinylene carbonate, propylene carbonate (PC), butylene carbonate (BC), or vinylene carbonate (VEC). The aforementioned fluorocarbonate compounds may include, but are not limited to, at least one of fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, or trifluoromethylethylene carbonate. The aforementioned carboxylic acid ester compounds may include, but are not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, or propyl propionate. The aforementioned ether compounds may include, but are not limited to, at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. Other electrolyte organic solvents may include, but are not limited to, at least one of 1,3-propanesulfonyl lactone, dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolium ketone, N-methyl-2-pyrrolidone, formamide, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, trioctyl phosphate, or phosphate esters.

[0054] In some embodiments, the electrolyte further includes a lithium salt, wherein the mass percentage of the lithium salt is 8% to 20% based on the mass of the electrolyte. Exemplarily, the mass percentage of the lithium salt can be 8%, 10%, 12%, 13%, 14%, 16%, 18%, 20%, or a range consisting of any two of the above values. This application does not particularly limit the type of lithium salt; lithium salts known in the art can be used. Exemplarily, the lithium salt can be, but is not limited to, at least one of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiN(CF3SO2)2, LiTFSI), lithium bis(fluorosulfonyl)imide (Li(N(SO2F)2), LiFSI), lithium difluorophosphate (LiPO2F2), lithium bis(oxalato)borate (LiB(C2O4)2, LiBOB), or lithium difluorooxalatoborate (LiBF2(C2O4), LiDFOB).

[0055] In some embodiments, the expansion rate of the separator in the lithium replenishment composite layer after immersion in electrolyte at 25°C to 90°C for 24 hours is 1% to 10%. Exemplarily, the expansion rate of the separator in the lithium replenishment composite layer after immersion in electrolyte at 25°C to 90°C for 24 hours can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or a range of any two of the above values. When the expansion rate of the separator in the lithium replenishment composite layer after immersion in electrolyte at 25°C to 90°C for 24 hours is within the above range, the distance for lithium ions to transport from the lithium replenishment layer to the negative electrode material layer is within a preferred range. This is beneficial for reducing the impedance of lithium ion transport in the lithium replenishment composite layer and the negative electrode sheet, shortening the lithium replenishment time, and improving the lithium replenishment efficiency. This, in turn, is beneficial for improving the initial coulombic efficiency of the secondary battery, reducing cycle capacity decay, and increasing the energy density of the secondary battery. In this application, the expansion rate refers to the volume expansion rate.

[0056] This application does not impose any particular limitation on the method of controlling the expansion rate of the separator after immersion in the electrolyte, as long as the purpose of this application can be achieved. For example, at least one surface of the separator substrate layer of this application may be provided with a coating containing a polymer. The expansion rate of the separator after immersion in the electrolyte can be controlled by adjusting the type or particle size of the polymer in the separator coating and the coating amount. Generally, when other conditions remain unchanged, the larger the expansion rate of the polymer in the coating, the larger the expansion rate of the separator after immersion in the electrolyte; the smaller the expansion rate of the polymer in the coating, the smaller the expansion rate of the separator after immersion in the electrolyte. When other conditions remain unchanged, the larger the particle size of the polymer in the coating, the larger the expansion rate of the separator after immersion in the electrolyte; the smaller the particle size of the polymer in the coating, the smaller the expansion rate of the separator after immersion in the electrolyte. When other conditions remain unchanged, increasing the coating amount increases the expansion rate of the separator after immersion in the electrolyte; decreasing the coating amount decreases the expansion rate of the separator after immersion in the electrolyte. The type of separator and the composition ratio of the electrolyte also usually affect the expansion rate of the separator after immersion in the electrolyte.

[0057] In some embodiments of this application, the areal density of the electrolyte on the pre-impregnated electrolyte separator is 3 g / mm². 2 Up to 60g / mm 2 Preferably, the areal density of the electrolyte on the pre-impregnated separator is 5 g / mm². 2 Up to 30g / mm 2 For example, the areal density of the electrolyte on the pre-impregnated separator can be 3 g / mm². 2 5g / mm 2 10g / mm 2 15g / mm 2 20g / mm 2 25g / mm 2 30g / mm 2 40g / mm 2 50g / mm 2 60g / mm 2 Or it can be a range consisting of any two of the above values. By adjusting the areal density of the electrolyte on the pre-impregnated electrolyte separator within the above range, the separator contains an appropriate amount of electrolyte, which can reduce the risk of electrolyte entering the negative electrode material layer and the uncontrollable lithium replenishment process due to excessive electrolyte content, improve the operability of the lithium replenishment process, and at the same time help to improve the transport rate of lithium ions in the lithium replenishment composite layer and the negative electrode material layer, increase the amount of lithium replenished in the negative electrode material layer, improve the integrity and stability of the SEI film, thereby helping to improve the initial coulombic efficiency of the secondary battery, reduce cycle capacity decay, and increase the energy density of the secondary battery.

[0058] This application does not impose any particular limitation on the method of controlling the areal density of the electrolyte on the pre-impregnated separator membrane, as long as the purpose of this application can be achieved. For example, the areal density of the electrolyte on the pre-impregnated separator membrane can be controlled by adjusting the amount of electrolyte dripped or sprayed onto the separator membrane surface and the immersion time of the separator membrane in the electrolyte. Generally, when other conditions remain unchanged, increasing the amount of electrolyte dripped or sprayed onto the separator membrane surface increases the areal density of the electrolyte on the pre-impregnated separator membrane; decreasing the amount of electrolyte dripped or sprayed onto the separator membrane surface decreases the areal density of the electrolyte on the pre-impregnated separator membrane. When other conditions remain unchanged, increasing the immersion time of the separator membrane in the electrolyte increases the areal density of the electrolyte on the pre-impregnated separator membrane; decreasing the immersion time of the separator membrane in the electrolyte decreases the areal density of the electrolyte on the pre-impregnated separator membrane. The type of separator and the composition ratio of the electrolyte usually affect the areal density of the electrolyte on the separator pre-impregnated with electrolyte.

[0059] In some embodiments, the elongation of the separator under a tensile force of 2N / 10mm to 4N / 10mm is 0% to 1%. Exemplarily, the elongation of the separator under a tensile force of 2N / 10mm to 4N / 10mm can be 0%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, or a range of any two of the above values. Separators meeting these characteristics have high mechanical strength and are less prone to breakage during lithium replenishment, which is beneficial for improving lithium replenishment efficiency.

[0060] In some implementations, the tensile strength of the separator in the belt-moving (MD) direction is 4000 kg / cm². 2 Up to 7000 kg / cm 2 The tensile strength perpendicular to the TD direction is 2000 kg / cm². 2 Up to 4000 kg / cm 2 The MD direction mentioned above refers to the length direction of the separator, and the TD direction refers to the width direction of the separator. For example, the tensile strength of the separator in the belt-running (MD) direction can be 4000 kg / cm². 2 4500kg / cm 2 5000kg / cm 2 5500kg / cm 2 6000kg / cm 2 6500kg / cm 2 7000kg / cm 2 Or it can be a range consisting of any two of the above values; the tensile strength of the separator membrane in the direction perpendicular to the conveyor belt (TD) can be 2000 kg / cm². 2 2500kg / cm 23000kg / cm 2 3500kg / cm 2 4000kg / cm 2 Or it can be a range consisting of any two of the above values. A separator that meets these characteristics has high mechanical strength and is less prone to breakage during lithium replenishment, which helps improve lithium replenishment efficiency.

[0061] The separator in this application is reusable, provided it remains undamaged after lithium replenishment. For example, the separator can be reused more than 5 times. The initial coulombic efficiency of the second pre-lithiated negative electrode sheet prepared by the first use of the separator sheet in a secondary battery is a1, and the initial coulombic efficiency of the second pre-lithiated negative electrode sheet prepared by the i-th use (1≤i≤15) in a secondary battery is a. i , a i The / a1 ratio of 99% to 100% indicates that the reusable separator has little impact on the initial coulombic efficiency of the secondary battery and can be maintained at a high level, which helps to reduce the cost of the preparation process.

[0062] This application does not impose any particular limitation on the thickness of the separator membrane, as long as it achieves the purpose of this application. For example, the thickness of the separator membrane can be from 3 μm to 30 μm. This application does not impose any particular limitation on the material of the separator membrane, as long as it achieves the purpose of this application. For example, the material of the separator membrane can include, but is not limited to, at least one of polyethylene (PE), polyolefins (PO) mainly composed of polypropylene (PP), polyester (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, or aramid; the type of separator membrane can include at least one of woven membrane, nonwoven membrane, microporous membrane, composite membrane, rolled membrane, or spun membrane.

[0063] In some embodiments, the lithium replenishment layer comprises at least one of lithium foil or lithium alloy foil, wherein the material of the lithium alloy foil may include, but is not limited to, at least one of lithium-aluminum alloy, lithium-silicon alloy, lithium-magnesium alloy, lithium-tin alloy, or lithium-nickel alloy. Based on the mass of the lithium replenishment layer, the mass percentage of lithium in the lithium replenishment layer may be from 97% to 100%.

[0064] In some embodiments, the thickness of the lithium replenishment layer is from 5 μm to 200 μm, preferably from 10 μm to 100 μm. Exemplarily, the thickness of the lithium replenishment layer can be 5 μm, 10 μm, 20 μm, 50 μm, 80 μm, 100 μm, 120 μm, 150 μm, 180 μm, 200 μm, or a range of any two of the above values. By controlling the thickness of the lithium replenishment layer within the above range, the lithium replenishment layer can better meet the lithium replenishment requirement while reducing the manufacturing cost, thereby improving the initial coulombic efficiency of the secondary battery, reducing cycle capacity decay, and increasing the energy density of the secondary battery.

[0065] In some embodiments, the tensile strength at break along the substrate direction (MD) of the lithium replenishment layer is from 0.5 N / 10 mm to 200 N / 10 mm, preferably from 1 N / 10 mm to 100 N / 10 mm. Exemplarily, the tensile strength at break along the substrate direction (MD) of the lithium replenishment layer can be 0.5 N / 10 mm, 1 N / 10 mm, 20 N / 10 mm, 50 N / 10 mm, 80 N / 10 mm, 100 N / 10 mm, 120 N / 10 mm, 150 N / 10 mm, 180 N / 10 mm, 200 N / 10 mm, or a range consisting of any two of the above values. When the tensile strength at break along the substrate direction (MD) of the lithium replenishment layer is within the above range, the lithium replenishment layer has high mechanical strength and is less prone to breakage during lithium replenishment, which is beneficial for achieving a good lithium replenishment effect. This, in turn, helps to improve the initial coulombic efficiency of the secondary battery, reduce cycle capacity decay, and increase the energy density of the secondary battery.

[0066] In some embodiments, the support layer includes at least one selected from metal foil, polyethylene terephthalate film, polypropylene film, or polyethylene film, wherein the metal foil includes copper foil, nickel foil, steel foil, or copper-nickel alloy foil. By selecting the above-mentioned support layer, the support layer can provide better support for the lithium replenishment layer and the separator pre-impregnated with electrolyte, improving the mechanical strength of the lithium replenishment composite layer. This allows the lithium replenishment composite layer to be easily and completely peeled off from the surface of the negative electrode material layer after lithium replenishment, reducing the possibility of breakage of the lithium replenishment composite layer. This is beneficial to improving the lithium replenishment efficiency of the negative electrode sheet, thereby improving the initial coulombic efficiency of the secondary battery, reducing cycle capacity decay, and increasing the energy density of the secondary battery. This application does not have a particular limitation on the thickness of the support layer, as long as it achieves the purpose of this application. For example, the thickness of the support layer can be from 3 μm to 50 μm, preferably from 5 μm to 20 μm.

[0067] This application does not impose any particular limitation on the preparation method of the lithium replenishment layer, as long as it achieves the purpose of this application. For example, in some embodiments, lithium metal powder slurry is coated onto a support layer and dried and rolled to form the lithium replenishment layer; in other embodiments, lithium foil and / or lithium alloy foil is rolled onto a support layer to form the lithium replenishment layer; in still other embodiments, lithium or lithium alloy molten slurry is coated onto a support layer and cooled and rolled to form the lithium replenishment layer. This application does not impose any particular limitation on the rolling pressure during the preparation of the above-mentioned lithium replenishment layer, as long as it achieves the purpose of this application. For example, the rolling pressure can be from 0.1T / 10mm to 2T / 10mm.

[0068] In this application, step (3) involves bonding the lithium-filled composite layer and the negative electrode sheet, bringing the pre-impregnated electrolyte separator into contact with the negative electrode material layer. Specifically, as shown... Figure 1As shown, for ease of understanding, the thickness direction of the negative electrode and the lithium replenishment composite layer is defined as Y. The negative electrode 110 includes a negative current collector 111 and a negative electrode material layer 112 disposed on both surfaces of the negative current collector 111. The lithium replenishment composite layer 120 includes a support layer 123, a lithium replenishment layer 122, and a pre-impregnated electrolyte separator 121, with the lithium replenishment layer 122 disposed between the pre-impregnated electrolyte separator 121 and the support layer 123. The lithium replenishment composite layer 120 and the negative electrode 110 are bonded together, so that the pre-impregnated electrolyte separator 121 contacts the negative electrode material layer 112. It can be understood that the above-mentioned negative electrode refers to a negative electrode that has not undergone pre-lithiation.

[0069] In this application, a voltage control system is used to discharge the lithium replenishment composite layer and the negative electrode. The positive terminal of the voltage control system is connected to the negative electrode, and the negative terminal is connected to the lithium replenishment layer. During this discharge process, the potential of the negative electrode is higher than that of the lithium replenishment layer. The negative electrode acts as the "positive electrode," and the lithium replenishment layer acts as the "negative electrode." Therefore, to achieve lithium replenishment on the negative electrode, allowing lithium ions to transfer from the lithium replenishment layer to the negative electrode, a discharge process is required.

[0070] In some embodiments of this application, the discharge treatment temperature is between 25°C and 90°C. Exemplarily, the discharge treatment temperature can be 25°C, 35°C, 45°C, 55°C, 65°C, 75°C, 85°C, 90°C, or a range of any two of the above values. During the discharge treatment, lithium atoms in the lithium replenishment layer lose electrons to form lithium ions. The lithium ions are transferred to the surface or interior of the negative electrode material layer through the pre-impregnated electrolyte separator. Electrons are transferred to the negative electrode current collector via the external circuit and then to the surface or interior of the negative electrode material layer. By controlling the discharge temperature within the aforementioned range, it is beneficial to increase the transport speed of lithium ions in the lithium replenishment composite layer and the negative electrode material layer, shorten the lithium replenishment time, improve the lithium replenishment efficiency, and reduce the safety risks caused by excessively high temperatures during the lithium replenishment process. At the same time, it is beneficial to form a dense and stable SEI film on the surface of the negative electrode sheet, eliminating the formation step in the subsequent secondary battery preparation process, reducing electrolyte consumption during the formation process, and increasing the overall electrolyte retention in the secondary battery. This is beneficial to improving the initial coulombic efficiency of the secondary battery, reducing cycle capacity decay, increasing the energy density of the secondary battery, simplifying the secondary battery preparation process, improving production efficiency, and reducing preparation costs.

[0071] In some embodiments of this application, the interfacial pressure per unit area between the lithium replenishment composite layer and the negative electrode is between 0.1 MPa and 0.6 MPa. Exemplarily, the interfacial pressure per unit area between the lithium replenishment composite layer and the negative electrode can be 0.1 MPa, 0.15 MPa, 0.2 MPa, 0.25 MPa, 0.3 MPa, 0.35 MPa, 0.4 MPa, 0.45 MPa, 0.5 MPa, 0.55 MPa, 0.6 MPa, or a range consisting of any two of the above values. By adjusting the interfacial pressure per unit area between the lithium replenishment composite layer and the negative electrode within the above range, the distance between the lithium replenishment composite layer and the negative electrode can be shortened, reducing the impedance of lithium ion transport in the lithium replenishment composite layer and the negative electrode, shortening the lithium replenishment time, and improving the lithium replenishment efficiency. This is beneficial for improving the initial coulombic efficiency of the secondary battery, reducing cycle capacity decay, and increasing the energy density of the secondary battery.

[0072] This application does not impose any particular restrictions on the bonding method between the lithium-filled composite layer and the negative electrode sheet, as long as the purpose of this application can be achieved. For example, the bonding method between the lithium-filled composite layer and the negative electrode sheet can be winding into a roll or pressing into a sheet. This application does not impose any particular restrictions on the method for controlling the interfacial pressure per unit area between the lithium-filled composite layer and the negative electrode sheet, as long as the purpose of this application can be achieved. For example, when the lithium-filled composite layer and the negative electrode sheet are wound into a roll, the interfacial pressure per unit area between the lithium-filled composite layer and the negative electrode sheet can be controlled by adjusting the winding tension. When other conditions remain unchanged, increasing the winding tension increases the interfacial pressure per unit area; decreasing the winding tension decreases the interfacial pressure per unit area. For example, when the lithium-filled composite layer and the negative electrode sheet are pressed into a sheet, the interfacial pressure per unit area between the lithium-filled composite layer and the negative electrode sheet can be controlled by adjusting the pressing pressure. When other conditions remain unchanged, increasing the pressing pressure increases the interfacial pressure per unit area; decreasing the pressing pressure decreases the interfacial pressure per unit area.

[0073] In some implementations, after the discharge treatment, the peel force between the pre-impregnated electrolyte separator and the lithium replenishment layer is 1 N / m to 5 N / m. Exemplarily, the peel force between the pre-impregnated electrolyte separator and the lithium replenishment layer can be 1 N / m, 1.5 N / m, 2 N / m, 2.5 N / m, 3 N / m, 3.5 N / m, 4 N / m, 4.5 N / m, 5 N / m, or a range of any two of the above values. When the peel force between the pre-impregnated electrolyte separator and the lithium replenishment layer is within the above range, the separator is more easily separated from the lithium replenishment layer after the discharge treatment. This reduces the probability of lithium metal remaining on the surface of the negative electrode material layer, lowers the possibility of side reactions of lithium metal on the negative electrode material layer, and also reduces the probability of the lithium replenishment layer remaining on the separator surface, increasing the number of times the separator can be used, reducing costs, and improving surface lithium deposition on the negative electrode material layer. This is beneficial for improving the initial coulombic efficiency of the secondary battery, reducing cycle capacity decay, and increasing the energy density of the secondary battery.

[0074] In some embodiments, after the discharge treatment, the peel force between the pre-impregnated electrolyte separator and the negative electrode material layer is 1 N / m to 8 N / m. Exemplarily, the peel force between the pre-impregnated electrolyte separator and the negative electrode material layer can be 1 N / m, 1.5 N / m, 2 N / m, 2.5 N / m, 3 N / m, 3.5 N / m, 4 N / m, 4.5 N / m, 5 N / m, 6 N / m, 7 N / m, 8 N / m, or a range consisting of any two of the above values. When the peel force between the pre-impregnated electrolyte separator and the negative electrode material layer is within the above-mentioned range, the separator is more easily separated from the negative electrode material layer after the discharge treatment is completed. This reduces the probability that lithium metal in the lithium replenishment layer remains on the surface of the negative electrode material layer, lowers the possibility of side reactions of lithium metal in the negative electrode material layer, and also reduces the probability that the negative electrode active material remains on the surface of the separator, increasing the number of times the separator can be used and reducing costs. At the same time, it improves the surface lithium deposition of the negative electrode material layer, thereby helping to improve the initial coulombic efficiency of the secondary battery, reduce cycle capacity decay, and increase the energy density of the secondary battery.

[0075] In some embodiments of this application, the initial coulombic efficiency of the first pre-lithiated negative electrode applied to the secondary battery is a first initial coulombic efficiency E1, and the initial coulombic efficiency of the second pre-lithiated negative electrode applied to the secondary battery is a second initial coulombic efficiency E2, where 90% ≤ E2 / E1 ≤ 100%. Exemplarily, the value of E2 / E1 can be 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, 100%, or a range consisting of any two of the above values. When the value of E2 / E1 is within the above range, during storage, the first pre-lithiated negative electrode has fewer side reactions between the residual electrolyte in the negative electrode material layer and moisture in the environment. The resulting second pre-lithiated negative electrode has better integrity and stability of the SEI film on its surface, which is beneficial for improving the environmental stability of the negative electrode. Therefore, the secondary battery has a higher initial coulombic efficiency, lower cycle capacity decay, and higher energy density.

[0076] In some embodiments of this application, 90% ≤ E1 ≤ 120%. Exemplarily, the value of E1 can be 90%, 95%, 100%, 102%, 105%, 108%, 110%, 112%, 115%, 118%, 120%, or a range of any two of the above values. When the value of E1 is within the above range, the secondary battery exhibits higher initial coulombic efficiency, lower cycle capacity decay, and higher energy density.

[0077] In some implementations, 81% ≤ E2 ≤ 120%. Exemplarily, the value of E2 can be 81%, 85%, 90%, 95%, 100%, 102%, 105%, 108%, 110%, 112%, 115%, 118%, 120%, or a range of any two of the above values. When the value of E2 is within the above range, the secondary battery exhibits higher initial coulombic efficiency, lower cycle capacity decay, and higher energy density.

[0078] In some embodiments of this application, the lithium fluoride coverage on the surface of the first pre-lithiated negative electrode is 45% to 85%, and the lithium fluoride coverage on the surface of the second pre-lithiated negative electrode is 45% to 95%. Exemplarily, the lithium fluoride coverage on the surface of the first pre-lithiated negative electrode can be 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, or a range consisting of any two of the above values, and the lithium fluoride coverage on the surface of the second pre-lithiated negative electrode can be 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or a range consisting of any two of the above values. The lithium fluoride on the surface of the negative electrode is typically generated during the lithium replenishment process of the negative electrode discharge treatment and during storage. The presence of lithium fluoride is beneficial for improving the mechanical strength, chemical stability, and ionic conductivity of the SEI film, and reducing further decomposition reactions between the electrolyte and the SEI film. The lithium fluoride coverage on the surfaces of the first and second pre-lithiated negative electrode sheets is within the above range. The SEI film on the surface of the negative electrode sheet has high integrity and stability, and high ionic conductivity. As a result, the secondary battery has high initial coulombic efficiency, low cycle capacity decay, and high energy density.

[0079] In some embodiments of this application, the mass content of hydrogen fluoride on the surface of the first pre-lithiated negative electrode is from 10 ppm to 450 ppm, and the mass content of hydrogen fluoride on the surface of the second pre-lithiated negative electrode is from 10 ppm to 500 ppm. Exemplarily, the mass content of hydrogen fluoride on the surface of the first pre-lithiated negative electrode can be 10 ppm, 50 ppm, 100 ppm, 150 ppm, 200 ppm, 250 ppm, 300 ppm, 350 ppm, 400 ppm, 450 ppm, or a range consisting of any two of the above values, and the mass content of hydrogen fluoride on the surface of the second pre-lithiated negative electrode can be 10 ppm, 50 ppm, 100 ppm, 150 ppm, 200 ppm, 250 ppm, 300 ppm, 350 ppm, 400 ppm, 450 ppm, 500 ppm, or a range consisting of any two of the above values. Hydrogen fluoride on the surface of the negative electrode is typically generated during the discharge treatment, lithium replenishment, and storage processes. Hydrogen fluoride corrodes the SEI film, negatively impacting its stability. The hydrogen fluoride content on the surfaces of the first and second pre-lithiated negative electrode sheets falls within the aforementioned range, indicating fewer side reactions occurring during discharge treatment, lithium replenishment, and storage. This suggests better environmental stability and higher integrity of the SEI film on the negative electrode surface, resulting in a higher initial coulombic efficiency, lower cycle capacity decay, and higher energy density in the secondary battery.

[0080] This application does not impose any particular restrictions on the environmental parameters in steps (1) to (4), as long as they can achieve the purpose of this application. For example, the environmental parameters in step (1) can be temperature ≤30℃ and humidity ≤1.7%, the environmental parameters in step (2) can be temperature ≤30℃ and humidity ≤1.7%, the environmental parameters in step (3) can be humidity ≤1.7%, and the environmental parameters in step (4) can be temperature ≤30℃ and humidity ≤1.7%.

[0081] In some implementation schemes, lithium replenishment of the negative electrode sheet can be achieved by the following methods: (1) impregnating the separator with electrolyte to form a separator pre-impregnated with electrolyte. (2) unwinding the support layer with lithium replenishment layer, the separator pre-impregnated with electrolyte, and the negative electrode sheet, and then winding them into a roll in the above order; or stacking the support layer with lithium replenishment layer, the separator pre-impregnated with electrolyte, and the negative electrode sheet in the above order. (3) connecting the positive electrode of the voltage control system to the negative electrode sheet and the negative electrode of the voltage control system to the lithium replenishment layer, and performing discharge treatment on the lithium replenishment composite layer and the negative electrode sheet. The discharge treatment is divided into a first stage, a second stage, and a third stage in chronological order. (4) After the discharge treatment is completed, unwinding the roll of the support layer with lithium replenishment layer, the separator pre-impregnated with electrolyte, and the negative electrode sheet, and winding them up separately; or disassembling the support layer with lithium replenishment layer, the separator pre-impregnated with electrolyte, and the negative electrode sheet to obtain the first pre-lithiated negative electrode sheet. (5) The first pre-lithiated negative electrode is stored in an environment of 20°C to 30°C and humidity ≤1.7% for 12h to 72h to obtain the second pre-lithiated negative electrode.

[0082] The second aspect of this application provides a pre-lithiated negative electrode sheet prepared according to the lithium supplementation method in any of the foregoing embodiments. The aforementioned pre-lithiated negative electrode sheet refers to a second pre-lithiated negative electrode sheet. The second pre-lithiated negative electrode sheet of this application exhibits good environmental stability. Its application in secondary batteries can improve the initial coulombic efficiency of the secondary battery, reduce cycle capacity decay, increase the energy density of the secondary battery, and also simplify the secondary battery manufacturing process, improve production efficiency, and reduce manufacturing costs.

[0083] The pre-lithiated negative electrode sheet of this application includes a negative electrode material layer, which includes a negative electrode active material. This application does not impose any particular limitation on the type of negative electrode active material, as long as it can achieve the purpose of this application. For example, the negative electrode active material may include, but is not limited to, natural graphite, artificial graphite, mesophase microcarbon spheres, hard carbon, soft carbon, silicon, silicon-carbon composites, Li-Sn alloys, Li-Sn-O alloys, Sn, SnO, SnO2, and spinel-structured lithiated TiO2-Li4Ti5O. 12 Or at least one of Li-Al alloys.

[0084] In this application, the negative electrode material layer further includes a negative electrode binder and a negative electrode conductive agent. This application does not particularly limit the type of negative electrode binder, as long as it achieves the purpose of this application. For example, the negative electrode binder may include, but is not limited to, at least one of polyacrylate, polyimide, polyamide, polyamide-imide, polyvinylidene fluoride, polystyrene-butadiene copolymer (styrene-butadiene rubber, SBR), sodium alginate, polyvinyl alcohol, polytetrafluoroethylene, polyacrylonitrile, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, lithium carboxymethyl cellulose, lithium polyacrylate, sodium carboxymethyl cellulose, or potassium carboxymethyl cellulose. This application does not particularly limit the type of negative electrode conductive agent, as long as it achieves the purpose of this application. For example, the negative electrode conductive agent may include, but is not limited to, at least one of conductive carbon black, carbon nanotubes (CNTs), carbon fibers, flake graphite, graphene, metallic materials, or conductive polymers. The aforementioned metallic materials may include, but are not limited to, metal powder and / or metal fibers. Specifically, the metal may include, but is not limited to, at least one of copper, nickel, aluminum, or silver. The aforementioned conductive polymer may include, but is not limited to, at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene, or polypyrrole. This application does not impose any particular limitation on the mass ratio of the negative electrode active material, negative electrode binder, and negative electrode conductive agent in the negative electrode material layer; those skilled in the art can select these according to actual needs, as long as the purpose of this application can be achieved.

[0085] This application does not impose any particular restrictions on the negative electrode current collector, as long as it can achieve the purpose of this application. For example, it may include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, foamed nickel, foamed copper, or composite current collectors (such as lithium copper composite current collectors, carbon copper composite current collectors, nickel copper composite current collectors, titanium copper composite current collectors, etc.).

[0086] This application does not impose any particular limitation on the thickness of the negative electrode current collector, as long as the purpose of this application can be achieved. For example, the thickness of the negative electrode current collector can be from 4 μm to 20 μm. This application also does not impose any particular limitation on the thickness of the negative electrode material layer, as long as the purpose of this application can be achieved. For example, the thickness of a single-sided negative electrode material layer can be from 30 μm to 250 μm.

[0087] A third aspect of this application provides a secondary battery comprising a pre-lithiated negative electrode as described in any of the foregoing embodiments. The aforementioned pre-lithiated negative electrode refers to a second pre-lithiated negative electrode. The secondary battery of this application exhibits high initial coulombic efficiency, low cycle capacity decay, and high energy density.

[0088] In this application, the secondary battery further includes a positive electrode sheet. The positive electrode sheet includes a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector. The phrase "positive electrode material layer disposed on at least one surface of the positive current collector" means that the positive electrode material layer can be disposed on one surface of the positive current collector along its thickness direction, or on two surfaces of the positive current collector along its thickness direction. It should be noted that the "surface" here can be the entire area of ​​the positive current collector or only a portion of it; this application does not have any particular limitation, as long as the purpose of this application is achieved.

[0089] This application does not impose any particular restrictions on the positive electrode current collector, as long as it can achieve the purpose of this application. For example, it may include aluminum foil, aluminum alloy foil, or composite current collector (such as aluminum-carbon composite current collector).

[0090] The positive electrode material layer of this application includes a positive electrode active material, which comprises a substance capable of reversibly inserting and extracting active ions such as lithium ions. The positive electrode material layer can be one or more layers, and each layer in a multilayer positive electrode material layer can contain the same or different positive electrode active materials. This application does not impose any particular limitation on the positive electrode active material, as long as it can achieve the purpose of this application. For example, the positive electrode active material can include, but is not limited to, at least one of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide (LiCoO2), lithium manganese oxide, lithium manganese iron phosphate, or lithium titanate. The aforementioned lithium nickel cobalt manganese oxide can include LiNi... 0.95 Co 0.03 Mn 0.02 O2(Ni95), LiNi 0.91 Co 0.03 Mn 0.06 O2(Ni91), LiNi 0.8 Co 0.1 Mn 0.1 O2(NCM811), LiNi 0.6 Co 0.2 Mn 0.2 O2(NCM622), LiNi 0.5 Co 0.2 Mn 0.3 O2 (NCM523) or LiNi 1 / 3 Co 1 / 3 Mn 1 / 3At least one of O2 (NCM111). The positive electrode material layer of this application also includes a positive electrode conductive agent and a positive electrode binder. This application does not have any particular limitations on the positive electrode conductive agent and the positive electrode binder in the positive electrode material layer, as long as the purpose of this application can be achieved. For example, the positive electrode conductive agent in the positive electrode material layer may include at least one of the above-mentioned negative electrode conductive agents; the positive electrode binder in the positive electrode material layer may include at least one of the above-mentioned negative electrode binders. This application does not have any particular limitations on the mass ratio of the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder in the positive electrode material layer. Those skilled in the art can choose according to actual needs, as long as the purpose of this application can be achieved.

[0091] This application does not impose any particular limitation on the thickness of the positive electrode current collector, as long as the purpose of this application can be achieved. For example, the thickness of the positive electrode current collector can be from 6 μm to 25 μm. This application also does not impose any particular limitation on the thickness of the positive electrode material layer, as long as the purpose of this application can be achieved. For example, the thickness of a single-sided positive electrode material layer can be from 25 μm to 250 μm.

[0092] In this application, the secondary battery also includes an electrolyte. The electrolyte in the secondary battery can be the same as or different from the electrolyte pre-impregnated in the separator of the lithium replenishment composite layer. The electrolyte in the secondary battery includes a lithium salt. This application does not particularly limit the type of lithium salt; lithium salts known in the art can be used. Exemplarily, the lithium salt can include, but is not limited to, at least one of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiN(CF3SO2)2, LiTFSI), lithium bis(fluorosulfonyl)imide (Li(N(SO2F)2), LiFSI), lithium difluorophosphate (LiPO2F2), lithium bis(oxalatoborate) (LiB(C2O4)2, LiBOB), or lithium difluorooxalatoborate (LiBF2(C2O4), LiDFOB). This application does not particularly limit the mass percentage of lithium salt in the electrolyte, as long as the purpose of this application is achieved. The electrolyte in the secondary battery also includes a non-aqueous organic solvent. This application does not impose any particular limitation on non-aqueous organic solvents, as long as they can achieve the purpose of this application. For example, the non-aqueous organic solvent may include at least one of carbonate compounds, carboxylic acid ester compounds, ether compounds, or other organic solvents. The aforementioned carbonate compounds may include, but are not limited to, at least one of chain carbonate compounds, cyclic carbonate compounds, or fluorinated carbonate compounds. The aforementioned chain carbonate compounds may include, but are not limited to, at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), or methyl ethyl carbonate (EMC). The aforementioned cyclic carbonate compounds may include, but are not limited to, at least one of ethylene carbonate (EC), vinylene carbonate, propylene carbonate (PC), butylene carbonate (BC), or vinyl ethylene carbonate (VEC). The aforementioned fluorocarbonate compounds may include, but are not limited to, at least one of fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, or trifluoromethylethylene carbonate. The aforementioned carboxylic acid ester compounds may include, but are not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, or propyl propionate. The aforementioned ether compounds may include, but are not limited to, at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran.The other organic solvents mentioned above may include, but are not limited to, at least one of 1,3-propanesulfonyl lactone, dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolium ketone, N-methyl-2-pyrrolidone, formamide, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, trioctyl phosphate, or phosphate esters. This application does not impose any particular limitation on the mass percentage of non-aqueous organic solvents in the electrolyte, as long as the purpose of this application is achieved.

[0093] In this application, the secondary battery also includes a separator. The separator is used to separate the positive electrode and the pre-lithiated negative electrode, preventing internal short circuits in the secondary battery, allowing electrolyte ions to pass freely, and not affecting the electrochemical charge-discharge process. The separator in the secondary battery can be the same as or different from the separator in the lithium replenishment composite layer. This application does not impose any particular limitation on the separator in the secondary battery, as long as it can achieve the purpose of this application. For example, the material of the separator can include, but is not limited to, at least one of polyethylene (PE), polyolefins (PO) mainly composed of polypropylene (PP), polyester (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, or aramid; the type of separator can include at least one of woven membrane, nonwoven membrane, microporous membrane, composite membrane, rolled membrane, or spun membrane.

[0094] In this application, the separator membrane may include a substrate and a surface treatment layer. The substrate may be a nonwoven fabric or composite membrane with a porous structure, and the material of the substrate may include at least one of polyethylene, polypropylene, polyethylene terephthalate, or polyimide. Optionally, a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane may be used. Optionally, a surface treatment layer is provided on at least one surface of the substrate. The surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by mixing polymers and inorganic materials. For example, the inorganic layer includes inorganic particles and a separator membrane adhesive. This application does not particularly limit the aforementioned inorganic particles, and may include at least one of alumina, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. This application does not particularly limit the aforementioned separator membrane adhesive, and may include at least one of the aforementioned negative electrode adhesives. The polymer layer contains a polymer, and the polymer material may include, but is not limited to, at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyvinylpyrrolidone, polyvinyl ether, polyvinylidene fluoride, or polyvinylidene fluoride-hexafluoropropylene copolymer.

[0095] The secondary battery of this application also includes a packaging bag for containing the positive electrode, separator, pre-lithiated negative electrode, electrolyte, and other components known in the art for secondary batteries. This application does not limit the aforementioned other components. This application does not impose any particular limitation on the packaging bag; it can be any packaging bag known in the art, as long as it achieves the purpose of this application.

[0096] This application does not impose any particular limitation on the type of secondary battery, which may include any device in which an electrochemical reaction occurs. In this application, the secondary battery may include, but is not limited to: lithium metal secondary batteries, lithium-ion secondary batteries (lithium-ion batteries), lithium polymer secondary batteries, or lithium-ion polymer secondary batteries (lithium-ion polymer batteries), etc.

[0097] The preparation process of the secondary battery described in this application is well known to those skilled in the art, and this application has no particular limitations. For example, it may include, but is not limited to, the following steps: stacking the positive electrode, separator, pre-lithiated negative electrode, and separator in sequence, and performing operations such as winding and folding as needed to obtain a wound electrode assembly; placing the electrode assembly in a packaging bag; injecting electrolyte into the packaging bag and sealing it to obtain a secondary battery; or stacking the positive electrode, separator, pre-lithiated negative electrode, and separator in sequence, and then fixing the four corners of the entire stacked structure with tape to obtain a stacked electrode assembly; placing the electrode assembly in a packaging bag; injecting electrolyte into the packaging bag and sealing it to obtain a secondary battery. In addition, overcurrent protection components, conductive plates, etc., may be placed in the packaging bag as needed to prevent the internal pressure of the secondary battery from rising and overcharging / discharging.

[0098] A fourth aspect of this application provides an electronic device that includes a secondary battery as described in any of the foregoing embodiments. The electronic device of this application has a long service life and good performance.

[0099] This application does not specifically limit the type of electronic device, which can be any electronic device known in the prior art. In some embodiments, the electronic device may include, but is not limited to, laptops, pen input computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium-ion capacitors, etc.

[0100] Example

[0101] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below. Furthermore, unless otherwise specified, "parts" and "%" are quality standards.

[0102] Test methods and equipment:

[0103] Electrolyte surface density test on pre-impregnated separator

[0104] Before the separator is immersed in the electrolyte, weigh the separator and record the mass as M1 g, and measure the area of ​​the separator and record it as S mm. 2 After the separator membrane is soaked in the electrolyte, its mass is measured and recorded as M2 g. The areal density (g / mm²) of the electrolyte on the pre-soaked separator membrane is also measured. 2 ) = (M2-M1) / S.

[0105] Lithium fluoride coverage test on negative electrode surface

[0106] The first pre-lithiated negative electrode sheet was cleaned with dimethyl carbonate (DMC) solvent in an argon atmosphere, dried, and a 10mm × 10mm sample was cut and mounted on the sample stage. Scanning electron microscopy (SEM) was used to acquire the morphology image of the negative electrode sheet surface. Clear images were obtained by adjusting the SEM accelerating voltage and working distance. Based on the SEM images, elemental analysis was performed using energy-dispersive X-ray spectroscopy (EDS). The presence of lithium fluoride was confirmed by detecting the characteristic X-ray peak of fluorine. A surface scan was performed on the sample perpendicular to the thickness direction to obtain the distribution information of fluorine, i.e., lithium fluoride, on the negative electrode sheet surface. The distribution area of ​​lithium fluoride on the negative electrode sheet surface was statistically calculated based on the distribution information. The ratio of the lithium fluoride distribution area to the surface area of ​​the negative electrode sheet sample perpendicular to the thickness direction is the lithium fluoride coverage of the first pre-lithiated negative electrode sheet surface. The lithium fluoride coverage of the second pre-lithiated negative electrode sheet surface was measured using the same testing method.

[0107] Mass content test of hydrogen fluoride on the surface of negative electrode sheet

[0108] Remove loose particles and free liquid from the surface of the first pre-lithiated negative electrode to keep the surface of the negative electrode material layer clean. Place the first pre-lithiated negative electrode in the detection window of a reflectance Fourier transform infrared spectroscopy (FTIR) instrument and use an attenuated total reflectance accessory to improve the sensitivity of the test. Scan the first pre-lithiated negative electrode to obtain the reflectance infrared spectrum. Locate the characteristic peak of hydrogen fluoride in the infrared spectrum, which is typically around 3000 cm⁻¹. -1 Up to 4000cm -1The infrared spectrum described above was compared with a standard spectrum containing known hydrogen fluoride content to calculate the mass content of hydrogen fluoride on the surface of the first pre-lithiated negative electrode. The mass content of hydrogen fluoride on the surface of the second pre-lithiated negative electrode was then measured using the same method.

[0109] First initial Coulomb efficiency E1, second initial Coulomb efficiency E2 test

[0110] A negative electrode sheet comprising a negative electrode current collector and a negative electrode material layer is provided. The negative electrode sheet is subjected to a discharge treatment according to the lithium replenishment method of this application. After the discharge treatment is completed, the lithium replenishment composite layer is peeled off from the negative electrode sheet to obtain a first pre-lithiated negative electrode sheet. The first pre-lithiated negative electrode sheet is then stored to obtain a second pre-lithiated negative electrode sheet.

[0111] Based on the design voltage range of the lithium-ion battery, for example, when the design voltage range of the lithium-ion battery is 3.0V to 4.45V, the charging cut-off voltage is 4.45V and the discharging cut-off voltage is 3.0V.

[0112] The specific testing steps are as follows: Lithium-ion batteries are prepared using the first pre-lithiated negative electrode and the second pre-lithiated negative electrode, respectively. The initial coulombic efficiency of the lithium-ion batteries is tested using the following method: The lithium-ion batteries are charged at a constant current of 0.2C at 25°C to a cutoff voltage of 4.45V, then charged at a constant voltage of 4.45V until the current is less than 0.05C. After standing for 5 minutes, they are discharged at a constant current of 0.2C to a cutoff voltage of 3.0V. The capacity during the above charging process is denoted as C0, and the capacity during the above discharging process is denoted as C1. The initial coulombic efficiency of the lithium-ion batteries is calculated using the following formula: Initial coulombic efficiency (%) = C1 / C0 × 100%. The initial coulombic efficiency of the lithium-ion batteries using the first pre-lithiated negative electrode is E1, and the initial coulombic efficiency of the lithium-ion batteries using the second pre-lithiated negative electrode is E2.

[0113] Cyclic performance test

[0114] The voltage range indicated on the outer packaging of the lithium-ion battery shall prevail. For example, when the voltage range indicated on the outer packaging of the lithium-ion battery is 3.0V to 4.45V, the charging cut-off voltage is 4.45V and the discharging cut-off voltage is 3.0V.

[0115] The specific test steps are as follows: Under 25℃ conditions, the lithium-ion battery undergoes its first charge and discharge cycle. It is charged at a constant current of 0.2C to the cutoff voltage of 4.45V, then charged at a constant voltage of 4.45V until the current is less than 0.05C. After resting for 5 minutes, it is discharged at a constant current of 0.2C to the cutoff voltage of 3.0V. The discharge capacity of the lithium-ion battery is measured as A. Then, in an environment of 25℃, 400 charge and discharge cycles are performed according to the above steps. The discharge capacity of the lithium-ion battery after the 400th cycle is measured as B. The cycle capacity retention rate of the lithium-ion battery is calculated using the following formula: Cycle capacity retention rate (%) = B / A × 100%. The higher the measured cycle capacity retention rate, the better the cycle performance of the lithium-ion battery and the smaller the cycle capacity decay.

[0116] Energy density test

[0117] The voltage range indicated on the outer packaging of the lithium-ion battery shall prevail. For example, when the voltage range indicated on the outer packaging of the lithium-ion battery is 3.0V to 4.45V, the charging cut-off voltage is 4.45V and the discharging cut-off voltage is 3.0V.

[0118] The specific test steps are as follows: Under 25℃ conditions, the lithium-ion battery is charged at a constant current of 0.2C to the cutoff voltage of 4.45V, then charged at a constant voltage of 4.45V until the current is less than 0.05C. After resting for 5 minutes, it is discharged at a constant current of 0.2C to the cutoff voltage of 3.0V, and then rested for 5 minutes. The energy of the above discharge process is recorded as the discharge energy E. Calculate the volume V (mm²) of the lithium-ion battery. 3 = Length × Width × Height. The energy density of a lithium-ion battery is calculated using the following formula: Energy density (Wh / L) = E / V × 10 6 .

[0119] Example 1-1

[0120] <Preparation of pre-lithiated negative electrode>

[0121] (1) The negative electrode active material silicon-carbon material (a composite material of silicon and carbon, with a silicon content of 50% and a carbon content of 50% by mass), the negative electrode conductive agent acetylene black, the negative electrode binder styrene-butadiene rubber (SBR), and the negative electrode binder lithium carboxymethyl cellulose are mixed in a weight ratio of 85:5:5:5. Deionized water is added as a solvent, and the mixture is stirred and mixed evenly to obtain a negative electrode slurry with a solid content of 28 wt%. The negative electrode slurry is uniformly coated on one surface of a 12 μm thick copper foil for the negative electrode current collector and dried at 90 °C to obtain a negative electrode sheet with a single-sided coating of negative electrode material. The above steps are repeated on the other surface of the copper foil to obtain a negative electrode sheet with a double-sided coating of negative electrode material. The negative electrode sheet is dried under vacuum at 90 °C for 1 h, and then cold-pressed, cut, slit, and welded with tabs to obtain a negative electrode sheet with a specification of 80 mm × 1000 mm. The surface density of the single-sided coating of the negative electrode material layer is 2.3 mg / cm³. 2 The thickness of the negative electrode material layer on one side is 23 μm.

[0122] (2) Under the conditions of ambient temperature of 25℃ and humidity of 1.0%, the lithium foil is rolled to a copper foil support layer with a thickness of 14μm to form a lithium replenishment layer. The rolling pressure is 1.5T / 10mm and the thickness of the lithium replenishment layer is 15μm, thus obtaining a lithium replenishment layer / support layer composite structure.

[0123] In an argon-atmosphere glove box with a water content of less than 10 ppm, a base solvent was prepared by mixing non-aqueous organic solvents ethylene carbonate (EC) and dimethyl carbonate (DMC). Then, lithium salt lithium hexafluorophosphate (LiPF6) and non-fluorinated additive lithium hexamethyldisilamide were added and mixed thoroughly to obtain the electrolyte. The mass percentages of lithium salt, non-fluorinated additive, fluorinated additive, and non-aqueous organic solvent, based on the mass of the electrolyte, are shown in Table 2.

[0124] A 5 μm thick porous polypropylene film (provided by Celgard) was placed in the electrolyte solution prepared above. After absorbing part of the electrolyte, it was removed to obtain a pre-impregnated separator membrane with the areal density shown in Table 3. The pre-impregnated separator membrane was then placed on the surface of the lithium replenishment layer to obtain the lithium replenishment composite layer.

[0125] (3) The lithium replenishment composite layer and the negative electrode sheet are bonded together, so that the separator pre-impregnated with electrolyte comes into contact with the negative electrode material layer. The interfacial pressure per unit area between the lithium replenishment composite layer and the negative electrode sheet is shown in Table 3. The lithium replenishment composite layer and the negative electrode sheet are subjected to discharge treatment. The positive terminal of the voltage control system is connected to the negative electrode sheet, and the negative terminal of the voltage control system is connected to the lithium replenishment layer. The discharge treatment is divided into three stages in time sequence: the first stage, the second stage, and the third stage. The constant current discharge current and discharge time of the first stage, the constant current discharge current and discharge time of the second stage, the constant current discharge current and discharge time of the third stage, and the discharge treatment temperature are shown in Table 1.

[0126] (4) After the discharge treatment is completed, the lithium replenishment composite layer is peeled off from the negative electrode to obtain the first pre-lithiated negative electrode.

[0127] (5) The first pre-lithiated negative electrode is stored. The storage temperature, humidity and time are shown in Table 1 to obtain the second pre-lithiated negative electrode.

[0128] <Preparation of the positive electrode>

[0129] Lithium cobalt oxide (LiCoO2), a positive electrode active material, acetylene black, a positive electrode conductive agent, and polyvinylidene fluoride (PVDF), a positive electrode binder, were mixed in a weight ratio of 97:1.4:1.6. N-methylpyrrolidone (NMP) was added as a solvent, and the mixture was stirred until homogeneous to obtain a positive electrode slurry with a solid content of 75 wt%. The positive electrode slurry was uniformly coated onto one surface of a 12 μm thick aluminum foil current collector and dried at 110 °C to obtain a positive electrode sheet with a single-sided coating of the positive electrode material layer. The above steps were repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coating of the positive electrode material layer. After drying under vacuum at 110 °C for 1 hour, the sheet was cold-pressed, cut, slit, and had tabs welded to obtain a positive electrode sheet with dimensions of 77 mm × 995 mm. The areal density of the single-sided coating of the positive electrode material layer was 19.0 mg / cm³. 2 The compaction density during the cold pressing process is 4.15 g / cm³. 3 .

[0130] <Preparation of Electrolyte>

[0131] In an argon-atmospheric glove box with a water content of less than 10 ppm, ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), and ethyl propionate (EP) were mixed in a weight ratio of 3:1:3:3 to obtain a base solvent. Then, lithium hexafluorophosphate (LiPF6) was added and mixed thoroughly to obtain the electrolyte. Based on the mass of the electrolyte, the mass percentage of lithium salt LiPF6 was 12.5%, with the remainder being the base solvent.

[0132] <Preparation of the separating membrane>

[0133] A porous polypropylene film with a thickness of 5 μm (provided by Celgard) was used as the separator.

[0134] <Preparation of Lithium-ion Batteries>

[0135] The positive electrode, separator, second pre-lithiated negative electrode, and separator prepared above are stacked in sequence, with the separator positioned between the positive electrode and the second pre-lithiated negative electrode to provide isolation. The electrode assembly is then wound to obtain an electrode assembly. The electrode assembly is placed in an aluminum-plastic film packaging bag and dried in an 85°C vacuum oven for 12 hours to remove moisture. Electrolyte is then injected, and the battery undergoes vacuum sealing, settling, degassing, edge trimming, and capacity processing to obtain a lithium-ion battery.

[0136] Examples 1-2 to 1-21

[0137] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as in Example 1-1.

[0138] Examples 2-1 to 2-14

[0139] Except for adjusting the composition and content of the electrolyte soaked in the separator in the lithium replenishment composite layer according to Table 2 in the <Preparation of Pre-lithiated Negative Electrode>, the rest is the same as in Example 1-1.

[0140] Examples 3-1 to 3-6

[0141] Except for adjusting the relevant preparation parameters according to Table 3, the rest is the same as in Examples 1-1. The areal density of the electrolyte on the pre-impregnated separator is controlled by adjusting the amount of electrolyte dropped onto the separator surface or the soaking time of the separator in the electrolyte.

[0142] Comparative Example 1

[0143] Except for the following preparation method used in <Preparation of Pre-lithiated Negative Electrode> and the addition of the formation step (charging to 3.5V at a constant current of 0.02C, and then charging to 3.9V at a constant current of 0.1C) in <Preparation of Lithium-ion Battery>, the rest is the same as in Examples 1-1.

[0144] <Preparation of Negative Electrode Sheets>

[0145] A negative electrode active material (silicon-carbon composite material, with silicon and carbon content of 50% by mass and carbon content of 50% by mass), a negative electrode conductive agent (acetylene black), a negative electrode binder (styrene-butadiene rubber (SBR), and a negative electrode binder (lithium carboxymethyl cellulose)) were mixed in a mass ratio of 85:5:5:5. Deionized water was added as a solvent, and the mixture was stirred until homogeneous to obtain a negative electrode slurry with a solid content of 28 wt%. The negative electrode slurry was uniformly coated onto one surface of a 12 μm thick copper foil current collector and dried at 90 °C to obtain a negative electrode sheet with a single-sided coating of the negative electrode material layer. The above steps were repeated on the other surface of the copper foil to obtain a negative electrode sheet with a double-sided coating of the negative electrode material layer. The sheet was dried under vacuum at 90 °C for 1 hour, and then cold-pressed, cut, slit, and had tabs welded to obtain a negative electrode sheet with dimensions of 80 mm × 1000 mm. The surface density of the single-sided coating of the negative electrode material layer is 2.3 mg / cm³. 2 The thickness of the negative electrode material layer on one side is 23 μm.

[0146] Comparative Example 2

[0147] Except for the fact that the discharge treatment in <Preparation of Prelithiated Negative Electrode Sheet> only includes the first stage, and the constant current discharge current and discharge time of the first stage are shown in Table 1, the rest is the same as in Example 1-1.

[0148] Comparative Examples 3 to 17

[0149] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as in Example 1-1.

[0150] The preparation and performance parameters of each embodiment and comparative example are shown in Tables 1 to 3.

[0151]

[0152]

[0153] As can be seen from Examples 1-1 to 1-21 and Comparative Examples 1 to 17, when the second pre-lithiated negative electrode sheet prepared using the lithium replenishment method provided in this application is applied to a lithium-ion battery, the lithium-ion battery exhibits higher initial coulombic efficiency, cycle capacity retention, and energy density. This indicates that this application can improve the initial coulombic efficiency of lithium-ion batteries, reduce cycle capacity decay, and increase energy density. However, the negative electrode sheet of Comparative Example 1 was not replenished with lithium, the discharge treatment of the negative electrode sheet of Comparative Example 2 only included the first stage, and the constant current discharge current and discharge time of the first, second, and third stages of the discharge treatment of the negative electrode sheets of Comparative Examples 3 to 14 are not within the scope of this application. The temperature, humidity, and time of the storage treatment of the negative electrode sheets of Comparative Examples 15 to 17 are not within the scope of this application. Therefore, the lithium-ion batteries of Comparative Examples 1 to 17 cannot simultaneously possess high initial coulombic efficiency, cycle capacity retention, and energy density, indicating that they cannot simultaneously improve the initial coulombic efficiency, reduce cycle capacity decay, and increase energy density of lithium-ion batteries.

[0154] The constant current discharge current and discharge time in the first, second, and third stages typically affect the initial coulombic efficiency, cycle capacity retention, and energy density of lithium-ion batteries. As can be seen from Examples 1-1 to 1-13 and Comparative Examples 3 to 14, when the constant current discharge current in the first, second, and third stages is too small (e.g., Comparative Examples 3, 7, and 11), the initial coulombic efficiency, cycle capacity retention, and energy density of the lithium-ion battery are lower; conversely, when the constant current discharge current in the first, second, and third stages is too large (e.g., Comparative Examples 4, 8, and 12), the cycle capacity retention of the lithium-ion battery is lower. When the discharge times of the first, second, and third stages are too short, such as in Comparative Examples 5, 9, and 13, the initial coulombic efficiency, cycle capacity retention, and energy density of the lithium-ion battery are lower. When the discharge times of the first, second, and third stages are too long, such as in Comparative Examples 6, 10, and 14, the cycle capacity retention of the lithium-ion battery is lower, indicating that it is impossible to simultaneously improve the initial coulombic efficiency, reduce cycle capacity decay, and increase energy density of the lithium-ion battery. When the constant current discharge current and discharge time of the first, second, and third stages are within the range of this application, the lithium-ion battery exhibits higher initial coulombic efficiency, cycle capacity retention, and energy density, indicating that this application can improve the initial coulombic efficiency, reduce cycle capacity decay, and increase energy density of the lithium-ion battery.

[0155] The temperature, humidity, and time of storage processing typically affect the initial coulombic efficiency, cycle capacity retention, and energy density of lithium-ion batteries. As seen in Examples 1-1, 1-14 to 1-19, and Comparative Examples 15 to 17, when the storage processing temperature is too low, although the initial coulombic efficiency, cycle capacity retention, and energy density of the lithium-ion battery are relatively high, the cost of negative electrode storage processing increases, making practical applications difficult. When the storage processing temperature is too high, such as in Comparative Example 15, the initial coulombic efficiency, cycle capacity retention, and energy density of the lithium-ion battery are even lower. When the humidity during storage processing is too high, such as in Comparative Example 16, the initial coulombic efficiency, cycle capacity retention, and energy density of the lithium-ion battery are even lower. When the storage treatment time is too short, the initial coulombic efficiency, cycle capacity retention, and energy density of the lithium-ion battery are relatively low. However, it is difficult to fabricate a lithium-ion battery in a short time after the negative electrode is replenished with lithium, making industrial application challenging. When the storage treatment time is too long, as in Comparative Example 17, the initial coulombic efficiency, cycle capacity retention, and energy density of the lithium-ion battery are even lower, indicating that it is impossible to simultaneously improve the initial coulombic efficiency, reduce cycle capacity decay, and increase energy density. When the storage treatment temperature, humidity, and time are within the range of this application, the lithium-ion battery exhibits higher initial coulombic efficiency, cycle capacity retention, and energy density, indicating that this application can improve the initial coulombic efficiency, reduce cycle capacity decay, and increase energy density of the lithium-ion battery.

[0156] The temperature of discharge treatment typically affects the initial coulombic efficiency, cycle capacity retention, and energy density of lithium-ion batteries. As can be seen from Examples 1-1, 1-20, and 1-21, when the discharge treatment temperature is within the range specified in this application, the lithium-ion battery exhibits higher initial coulombic efficiency, cycle capacity retention, and energy density. This indicates that this application can improve the initial coulombic efficiency of lithium-ion batteries, reduce cycle capacity decay, and increase energy density.

[0157]

[0158] The mass percentage of non-fluorinated additives typically affects the initial coulombic efficiency, cycle capacity retention, and energy density of lithium-ion batteries. As can be seen from Examples 1-1, 2-1 to 2-5, when the mass percentage of non-fluorinated additives is within the range specified in this application, the lithium-ion batteries exhibit higher initial coulombic efficiency, cycle capacity retention, and energy density. This indicates that this application can improve the initial coulombic efficiency of lithium-ion batteries, reduce cycle capacity decay, and increase energy density.

[0159] The mass percentage of fluorinated additives typically affects the initial coulombic efficiency, cycle capacity retention, and energy density of lithium-ion batteries. As can be seen from Examples 1-1, 2-6 to 2-8, when the mass percentage of fluorinated additives is within the range specified in this application, the lithium-ion batteries exhibit higher initial coulombic efficiency, cycle capacity retention, and energy density. This indicates that this application can improve the initial coulombic efficiency of lithium-ion batteries, reduce cycle capacity decay, and increase energy density.

[0160] The mass percentage of ethylene carbonate typically affects the initial coulombic efficiency, cycle capacity retention, and energy density of lithium-ion batteries. As can be seen from Examples 1-1, 2-9 to 2-12, when the mass percentage of ethylene carbonate is within the range specified in this application, the lithium-ion battery exhibits higher initial coulombic efficiency, cycle capacity retention, and energy density. This indicates that this application can improve the initial coulombic efficiency of lithium-ion batteries, reduce cycle capacity decay, and increase energy density.

[0161] The types of non-fluorinated and fluorinated additives typically affect the initial coulombic efficiency, cycle capacity retention, and energy density of lithium-ion batteries. As can be seen from Examples 1-1, 2-13 to 2-14, the use of non-fluorinated and fluorinated additives within the scope of this application results in lithium-ion batteries exhibiting higher initial coulombic efficiency, cycle capacity retention, and energy density. This demonstrates that this application can improve the initial coulombic efficiency of lithium-ion batteries, reduce cycle capacity decay, and increase energy density.

[0162] Table 3

[0163]

[0164] The interfacial pressure per unit area between the lithium replenishment composite layer and the negative electrode typically affects the initial coulombic efficiency, cycle capacity retention, and energy density of a lithium-ion battery. As can be seen from Examples 1-1, 3-1, and 3-2, when the interfacial pressure per unit area between the lithium replenishment composite layer and the negative electrode is within the range specified in this application, the lithium-ion battery exhibits higher initial coulombic efficiency, cycle capacity retention, and energy density. This indicates that this application can improve the initial coulombic efficiency of lithium-ion batteries, reduce cycle capacity decay, and increase energy density.

[0165] The areal density of the electrolyte on the pre-impregnated separator typically affects the initial coulombic efficiency, cycle capacity retention, and energy density of a lithium-ion battery. As can be seen from Examples 1-1, 3-3 to 3-6, when the areal density of the electrolyte on the pre-impregnated separator is within the range specified in this application, the lithium-ion battery exhibits higher initial coulombic efficiency, cycle capacity retention, and energy density. This indicates that this application can improve the initial coulombic efficiency of lithium-ion batteries, reduce cycle capacity decay, and increase energy density.

[0166] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, or article that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, or article.

[0167] The various embodiments in this specification are described in a related manner. The same or similar parts between the various embodiments can be referred to each other. Each embodiment focuses on describing the differences from other embodiments.

[0168] The above description is only a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A method for lithium replenishment of a negative electrode sheet, comprising the following steps: A negative electrode sheet is provided, the negative electrode sheet comprising a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector; A lithium replenishment composite layer is provided, the lithium replenishment composite layer comprising a support layer, a lithium replenishment layer and a separator membrane pre-impregnated with electrolyte, the lithium replenishment layer being disposed between the separator membrane pre-impregnated with electrolyte and the support layer, the electrolyte comprising a lithium salt, the lithium salt comprising at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium difluorophosphate or lithium difluorooxalate borate; The lithium-replenishing composite layer and the negative electrode sheet are bonded together, and the separator pre-impregnated with electrolyte is brought into contact with the negative electrode material layer. The lithium-replenishing composite layer and the negative electrode sheet are then subjected to a discharge treatment. The discharge treatment is divided into a first stage, a second stage, and a third stage in chronological order. In the first stage, the constant current discharge current is 0.1C to 0.3C and the discharge time is 0.008h to 0.28h. In the second stage, the constant current discharge current is 1C to 2C and the discharge time is 0.032h to 0.30h. In the third stage, the constant current discharge current is 0.3C to 0.5C and the discharge time is 0.027h to 0.058h. After the discharge process is completed, the lithium replenishment composite layer is peeled off from the negative electrode to obtain the first pre-lithiated negative electrode. The first pre-lithiated negative electrode is stored at 20°C to 30°C and humidity ≤1.7% for 12 to 72 hours to obtain the second pre-lithiated negative electrode.

2. The lithium replenishment method according to claim 1, wherein, The first coulombic efficiency of the first pre-lithiated negative electrode applied to the secondary battery is the first initial coulombic efficiency E1, and the first coulombic efficiency of the second pre-lithiated negative electrode applied to the secondary battery is the second initial coulombic efficiency E2, where 90% ≤ E2 / E1 ≤ 100%.

3. The lithium replenishment method according to claim 2, wherein, 90％≤E1≤120％。 4. The lithium replenishment method according to claim 1, wherein, The coverage of lithium fluoride on the surface of the first pre-lithiated negative electrode is 45% to 85%, and the coverage of lithium fluoride on the surface of the second pre-lithiated negative electrode is 45% to 95%. The ratio of the distribution area of ​​lithium fluoride on the surface of the negative electrode to the surface area of ​​the negative electrode sample perpendicular to the thickness direction is the coverage of lithium fluoride on the surface of the pre-lithiated negative electrode.

5. The lithium replenishment method according to claim 1, wherein, The mass content of hydrogen fluoride on the surface of the first pre-lithiated negative electrode is 10 ppm to 450 ppm, and the mass content of hydrogen fluoride on the surface of the second pre-lithiated negative electrode is 10 ppm to 500 ppm.

6. The lithium replenishment method according to any one of claims 1 to 5, wherein, The electrolyte includes additives, which include non-fluorinated additives, and the non-fluorinated additives include at least one of hexamethyldisilazine lithium, lithium thiodicarbonate or lithium dioxaborate. Based on the mass of the electrolyte, the non-fluorinated additive has a mass percentage content of 0.1% to 10%.

7. The lithium replenishment method according to any one of claims 1 to 5, wherein, The electrolyte includes additives, which include fluorinated additives, including at least one of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethyl)sulfonyl)imide, lithium fluorosulfonyl(trifluoromethyl)sulfonyl)imide, lithium bis(pentafluoroethane)sulfonylimide, lithium (trifluoromethyl)(nonafluorobutyl)sulfonylimide, or lithium 1,3-perfluoropropanedisulfonylimide. Based on the mass of the electrolyte, the mass percentage of the fluorinated additive is 0.5% to 5%.

8. The lithium replenishment method according to any one of claims 1 to 5, wherein, The electrolyte includes a non-aqueous organic solvent, which includes ethylene carbonate. Based on the mass of the electrolyte, the mass percentage of ethylene carbonate is between 32.5% and 91%.

9. The lithium replenishment method according to any one of claims 1 to 5, wherein, The temperature of the discharge treatment is between 25°C and 90°C.

10. The lithium replenishment method according to any one of claims 1 to 5, wherein, The interfacial pressure per unit area between the lithium replenishment composite layer and the negative electrode sheet is 0.1 MPa to 0.6 MPa.

11. A pre-lithiated negative electrode sheet prepared by the lithium replenishment method according to any one of claims 1 to 10.

12. A secondary battery comprising the pre-lithiated negative electrode as described in claim 11.

13. An electronic device comprising the secondary battery of claim 12.

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