Negative plate and battery

Abstract

The invention relates to a negative plate and a battery. The negative plate comprises a negative current collector and a negative coating arranged on at least one side surface of the negative current collector; the negative electrode coating comprises a first active layer and a second active layer which are stacked, and the first active layer is arranged on the surface of the negative electrode current collector; wherein the second active layer comprises a fast ion conductor, and the chemical formula of the fast ion conductor is Li < 1 + x-y + z > Al < x > Ti < 2-x > P < 3 > O < 12-y-z > F < y > N < z >; wherein 0.1 < = x < = 0.4, 0 < = y < = 0.2, 0 < = z < = 0.2, and y and z are not 0 at the same time. According to the scheme provided by the invention, the internal resistance of the battery can be reduced, lithium precipitation of the battery is inhibited, and the battery shows excellent cycle performance and rate capability.

Description

Technical Field

[0001] This application relates to the field of battery technology, and in particular to negative electrode sheets and batteries. Background Technology

[0002] Lithium-ion batteries are widely used in 3C digital products, power tools, aerospace, energy storage, and electric vehicles due to their advantages such as high specific energy, no memory effect, and long cycle life. The rapid development of electronic information technology and consumer products has placed higher demands on the electrochemical performance of lithium-ion batteries.

[0003] The lithium-ion transport rate in the negative electrode has a significant impact on the electrochemical performance of the battery. During charging and discharging, if the migration speed of lithium ions in the negative electrode is slow, it will exacerbate the problem of complex lithium-ion transport paths and increase the internal resistance of the battery. This leads to uneven or obstructed lithium-ion migration, causing some lithium ions to preferentially reach the negative electrode surface. When the subsequent supply is insufficient, they are prone to accumulating and depositing there. Severe lithium plating will shorten battery life and reduce the battery's rate performance.

[0004] Therefore, there is an urgent need to develop a negative electrode that can improve the internal resistance and lithium plating problems of batteries, as well as enhance cycle performance and rate performance. Summary of the Invention

[0005] To solve or partially solve the problems existing in related technologies, this application provides a negative electrode sheet and a battery that can reduce the internal resistance of the battery, suppress lithium plating in the battery, and enable the battery to exhibit excellent cycle performance and rate performance.

[0006] A first aspect of this application provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode coating disposed on at least one surface of the negative electrode current collector; the negative electrode coating comprises a first active layer and a second active layer stacked thereon, the first active layer being disposed on the surface of the negative electrode current collector; wherein: The second active layer includes a fast ion conductor, the chemical formula of which is Li. 1+x-y+z Al x Ti 2- x P3O 12-y-z F y N z Where 0.1≤x≤0.4, 0≤y≤0.2, 0≤z≤0.2, and y and z are not both 0.

[0007] As described in the first aspect, the fast ion conductor in the second active layer has a mass percentage of a%, 0 < a ≤ 5.

[0008] As described in the first aspect, the negative electrode sheet includes a first active layer comprising a first active material, the first active material comprising a carbon material; and a second active layer comprising a second active material, the second active material comprising a carbon material and a silicon-containing material.

[0009] As described in the first aspect, the thickness ratio of the first active layer to the second active layer is k, where k ≥ 1.

[0010] The negative electrode as described in the first aspect, wherein the negative electrode satisfies the following relationship: 0.3≤a / k≤1.0.

[0011] The negative electrode sheet as described in the first aspect, wherein the silicon element in the negative electrode coating has a mass percentage content of 5% to 40%.

[0012] The negative electrode as described in the first aspect, wherein the median particle size Dv50 of the fast ion conductor is 0.4 nm to 0.8 nm.

[0013] A second aspect of this application provides a battery comprising a negative electrode as described in the first aspect.

[0014] The battery as described above, wherein the battery includes an electrolyte, the electrolyte includes an electrolyte additive, and the electrolyte additive includes fluoroethylene carbonate and / or difluoroethylene carbonate.

[0015] In the battery described above, the electrolyte additive has a mass percentage content of 0.5% to 2% in the electrolyte.

[0016] The technical solution provided in this application can include the following beneficial effects: On the one hand, the addition of a fast ion conductor in the second active layer, which has high ionic conductivity, can provide more efficient transport channels for lithium ions in the second active layer, reduce the path tortuosity and transport resistance during lithium ion migration, and promote the transport of lithium ions in the second active layer, thereby increasing the transport rate of lithium ions in the negative electrode. This not only reduces the internal resistance during battery charging and discharging, but also alleviates the problem of local aggregation of lithium ions caused by migration lag, effectively improving the risk of lithium plating in the battery, thereby extending the cycle life of the battery and improving the rate performance of the battery. On the other hand, the doping of fluorine and / or nitrogen elements in the fast ion conductor can achieve directional control of the SEI film through interfacial reaction, forming an SEI film rich in LiF and / or Li3N on the surface of the negative electrode active material. This can significantly improve the mechanical strength and lithium conduction performance of the SEI film, avoid the SEI film from cracking and reconstructing during charge and discharge cycles, reduce the loss of active lithium caused by SEI film failure, and optimize the overall lithium ion transport efficiency, resulting in lower internal resistance of the battery and further improving the cycle performance and rate performance of the battery.

[0017] It should be understood that the above general description and the following detailed description are exemplary and explanatory only, and do not limit this application. Detailed Implementation

[0018] To facilitate understanding of this application, it will be described in detail below. However, before describing this application in detail, it should be understood that this application is not limited to the specific embodiments described. It should also be understood that the terminology used herein is for the purpose of describing specific embodiments only and is not intended to be restrictive.

[0019] Where a numerical range is provided, it should be understood that every intermediate value between the upper and lower limits of the range and any other specified or intermediate value within the specified range is covered within this application. The upper and lower limits of these smaller ranges may be independently included in the smaller range and are also covered within this application, subject to any explicitly excluded limits within the specified range. Where the specified range includes one or two limits, the range excluding any or both of those included limits is also included within this application.

[0020] Unless otherwise defined, all terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. While the methods and materials described herein, or any equivalent methods and materials, may also be used in the implementation or testing of this application, preferred methods and materials are now described.

[0021] The lithium-ion transport rate in the negative electrode has a significant impact on the electrochemical performance of the battery. During charging and discharging, if the migration speed of lithium ions in the negative electrode is slow, it will exacerbate the problem of complex lithium-ion transport paths and increase the internal resistance of the battery. This leads to uneven or obstructed lithium-ion migration, causing some lithium ions to preferentially reach the negative electrode surface. When the subsequent supply is insufficient, they are prone to accumulating and depositing there. Severe lithium plating will shorten battery life and reduce the battery's rate performance.

[0022] To address the aforementioned problems, this application provides a negative electrode sheet, which includes a negative current collector and a negative electrode coating disposed on at least one surface of the negative current collector; the negative electrode coating includes a first active layer and a second active layer stacked thereon, the first active layer being disposed on the surface of the negative current collector; the second active layer includes a fast ion conductor, the fast ion conductor having the chemical formula Li. 1+x-y+z Al x Ti 2-x P3O 12-y-z F y N z Where 0.1≤x≤0.4, 0≤y≤0.2, 0≤z≤0.2, and y and z are not both 0.

[0023] This application does not limit the selection of the negative electrode current collector, and it can be selected according to actual needs, such as copper foil. The negative electrode coating of this application includes two layers, namely a first active layer and a second active layer. The first active layer is disposed on the surface of the negative electrode current collector, and the second active layer is disposed on the surface of the first active layer away from the current collector.

[0024] The second active layer of this application includes a fast ion conductor, the chemical formula of which is Li. 1+x-y+z Al x Ti 2- x P3O 12-y-z F y N z Where 0.1≤x≤0.4, for example, x can be 0.1, 0.15, 0.2, 0.25, 0.3, 0.35 or 0.4, etc.; 0≤y≤0.2, for example, y can be 0, 0.05, 0.1, 0.15 or 0.2, etc.; 0≤z≤0.2, for example, z can be 0, 0.05, 0.1, 0.15 or 0.2, etc.; y and z are not both 0.

[0025] According to the above-mentioned scheme provided in this application, after the negative electrode is applied to the battery, the battery has low internal resistance, excellent cycle performance and rate performance, and low lithium plating probability. The applicant analyzed the principle and believes the reasons are as follows: Firstly, the addition of a fast-ion conductor in the second active layer, with its high ionic conductivity, provides more efficient transport channels for lithium ions within the second active layer, reducing path tortuosity and transport resistance during lithium ion migration. This facilitates lithium ion transport in the second active layer, thereby increasing the transport rate of lithium ions in the negative electrode. This not only reduces the internal resistance during battery charging and discharging but also alleviates the problem of localized lithium ion aggregation caused by migration lag, effectively improving the risk of lithium plating and thus extending the battery's cycle life and improving its rate performance. Secondly, the doping of fluorine and / or nitrogen elements in the fast-ion conductor can achieve directional control of the SEI film through interfacial reactions, forming a LiF- and / or Li3N-rich SEI film on the surface of the negative electrode active material. This significantly improves the mechanical strength and lithium conductivity of the SEI film, preventing SEI film rupture and reconstruction during charge-discharge cycles, reducing active lithium loss due to SEI film failure, and optimizing overall lithium ion transport efficiency, resulting in lower battery internal resistance and further improving the battery's cycle performance and rate performance.

[0026] In one specific embodiment, the mass percentage of the fast ion conductor in the second active layer is a%, where 0 < a ≤ 5. For example, a can be 0.1, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, or 5, etc. When the mass percentage of the fast ion conductor in the second active layer is within the above range, the fast ion conductor can better perform its function, providing more transport channels for lithium ions in the second active layer, while also significantly improving the mechanical strength and lithium conductivity of the SEI film. This further reduces the battery's internal resistance and suppresses lithium plating, resulting in a longer cycle life and higher rate performance.

[0027] In one specific embodiment, the first active layer includes a first active material, which includes a carbon material; the second active layer includes a second active material, which includes both a carbon material and a silicon-containing material. This application does not limit the selection of the carbon material and can choose according to actual needs, such as graphite, graphene, mesophase carbon microspheres, carbon black, hard carbon, soft carbon, etc. This application does not limit the selection of the silicon-containing material and can choose according to actual needs, such as silicon-carbon composite materials, silicon oxide, elemental silicon, pre-lithiated silicon oxide, pre-magnesiumized silicon oxide, etc. The carbon materials in the first and second active materials of this application can be the same or different, depending on actual needs.

[0028] The first active layer contains only carbon materials, which increases the conductive connection between the negative electrode coating and the negative electrode current collector, facilitating electron transport and reducing the internal resistance of the negative electrode sheet. The second active layer contains both carbon and silicon-containing materials, concentrating the silicon-containing material on top of the negative electrode coating. This shortens the migration distance of lithium ions in the negative electrode, significantly reducing the transport resistance and minimizing polarization caused by long-distance migration. This reduces the probability of lithium plating and improves the battery's cycle and rate performance. Furthermore, the SEI film rich in LiF and / or Li3N, formed with the participation of fast ion conductors in the second active layer, inhibits the expansion of silicon-containing materials, preventing the negative electrode coating from pulverizing and peeling off, further enhancing the battery's cycle and rate performance.

[0029] In one specific embodiment, the ratio of the thickness H1 of the first active layer to the thickness H2 of the second active layer is k, where k ≥ 1. For example, k can be 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, or 2, etc. When the thickness ratio of the first active layer and the second active layer is within the above range, a thicker first active layer can further enhance electron transport efficiency and provide a more stable substrate for the second active layer, offsetting the stress caused by the expansion of the silicon-containing material in the second active layer; while a thinner second active layer can shorten the transport path of lithium ions in the silicon-based material, reduce migration resistance, alleviate the impact of the volume expansion of the silicon-containing material on the coating integrity, and the fast ion conductor in the second active layer can provide more transport channels for lithium ions, better improve the transport rate of lithium ions in the negative electrode, reduce the internal resistance of the battery, and optimize the cycle performance and rate performance of the battery.

[0030] In one specific embodiment, the negative electrode sheet satisfies the following relationship: 0.3 ≤ a / k ≤ 1.0, for example, a / k can be 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, or 1, etc. When the ratio of the mass percentage of fast ion conductors to the thickness of the active layer is within the above range, sufficient fast ion conductors in the second active layer can provide more transport channels for lithium ions and further shorten the migration path of lithium ions, effectively reducing lithium ion transport resistance, improving the internal resistance of the battery, and enhancing the cycle performance and rate performance of the battery. At the same time, the SEI film formed after adding fast ion conductors has better mechanical properties and lithium conductivity, further ensuring the integrity of the negative electrode coating and ensuring the electrochemical performance of the battery.

[0031] If a / k < 0.3, two scenarios may occur: First, insufficient addition of fast ion conductors cannot effectively improve ion transport in the second active layer, leading to difficulty in releasing the capacity of silicon-containing materials, a significant decrease in ion transport efficiency, and reduced cycle life and rate performance of the battery. Second, an excessively large thickness ratio between the first and second active layers (with the second active layer being too thin) results in insufficient total silicon-containing material, significantly reducing the overall battery capacity. If a / k > 1, two scenarios may also occur: First, excessive addition of fast ion conductors reduces the effective proportion of carbon and silicon-containing materials, leading to a significant decrease in the overall battery capacity. Simultaneously, excessive doping of fluorine and nitrogen elements can cause an excessively thick SEI film, hindering lithium-ion insertion and affecting the battery's kinetic performance, resulting in reduced rate performance. Second, an excessively small thickness ratio between the first and second active layers (with the second active layer being too thick) significantly increases the volume expansion stress of the silicon-containing material, exceeding the support capacity of the first active layer, causing coating cracking and peeling, severely impairing structural stability. Simultaneously, the lithium-ion migration path becomes longer, reducing transport efficiency and significantly lowering the battery's cycle and rate performance.

[0032] In one specific embodiment, the mass percentage of silicon in the negative electrode coating is 5% to 40%, for example, the mass percentage of silicon in the negative electrode coating can be 5%, 10%, 15%, 20%, 25%, 30%, 35%, or 40%, etc. When the mass percentage of silicon in the negative electrode coating is within the above range, it can ensure the high capacity of the battery while avoiding problems such as powdering and peeling of the negative electrode coating caused by the volume expansion of silicon, thus ensuring the structural integrity of the negative electrode sheet, thereby enabling the battery to achieve both high capacity density and high cycle performance.

[0033] In one specific embodiment, the median particle size Dv50 of the fast ion conductor is 0.4 nm to 0.8 nm. For example, the median particle size Dv50 of the fast ion conductor can be 0.4 nm, 0.45 nm, 0.5 nm, 0.55 nm, 0.6 nm, 0.65 nm, 0.7 nm, 0.75 nm, or 0.8 nm. When the Dv50 of the fast ion conductor is within the above range, the small particle size of the fast ion conductor can shorten the diffusion distance of lithium ions, further reduce the lithium ion transport resistance, and more easily participate in the formation of the SEI film, improving the mechanical strength and lithium conductivity of the SEI film. It can also optimize the coating process of the second active layer, ensure the integrity of the negative electrode coating, and reduce the problems of powdering and peeling of the negative electrode coating, thereby ensuring the high cycle performance and rate performance of the battery, and also alleviating the lithium plating problem of the battery.

[0034] In one specific embodiment, the first active layer and the second active layer further include a conductive agent, which includes at least one of carbon nanotubes (CNTs), conductive carbon black (SP), graphene, vapor-grown carbon fiber (VGCF), and sheet graphite.

[0035] In one specific embodiment, the first active layer and the second active layer further include an adhesive, which includes at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), polyacrylamide (PAM), polyacrylonitrile (PAN), polyurethane (PU), polyethylene oxide (PEO), polyacryl alcohol (PVA), polyimide (PI), sodium carboxymethyl cellulose (CMC), and polyvinylidene fluoride (PVDF).

[0036] This application also provides a battery comprising the aforementioned negative electrode. This battery exhibits low internal resistance, high cycle performance, and high rate performance, while also having a low risk of lithium plating.

[0037] In one specific embodiment, the battery includes an electrolyte, which in turn includes electrolyte additives, including fluoroethylene carbonate and / or difluoroethylene carbonate. When fluoroethylene carbonate and / or difluoroethylene carbonate are added to the electrolyte, the electrolyte additives can participate in the formation of the SEI film, further improving the mechanical strength and lithium conductivity of the SEI film, thereby inhibiting electrolyte decomposition, reducing lithium dendrite growth, protecting the structural integrity of the negative electrode, and simultaneously improving the lithium ion insertion / extraction efficiency. This, in turn, reduces the battery's internal resistance, improves the battery's cycle performance and rate performance, and mitigates the risk of lithium plating.

[0038] In one specific embodiment, the electrolyte additive has a mass percentage content of 0.5% to 2% in the electrolyte. For example, the mass percentage content of the electrolyte additive in the electrolyte can be 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, or 2%, etc. When the mass percentage content of the electrolyte additive is within the above range, the SEI film has better mechanical strength and lithium conductivity, which can further improve the cycle performance and rate performance of the battery, and reduce the internal resistance and lithium plating risk of the battery.

[0039] In one specific embodiment, the electrolyte of this application further includes lithium salt and organic solvent. The lithium salt and organic solvent are lithium salts and organic solvents known in the art that can be used in electrolytes and make the battery electrochemical performance excellent. The specific configuration can be set as needed.

[0040] In one specific embodiment, the battery of this application further includes a positive electrode sheet, which comprises a positive current collector and a positive electrode coating coated on the positive current collector. The positive electrode coating comprises a positive electrode active material, which comprises at least one of lithium iron phosphate, lithium cobalt oxide, and lithium nickel cobalt manganese oxide. When the above-mentioned compounds are selected as the positive electrode active material, the positive electrode active material can fully exert its performance and improve the electrochemical performance of the lithium-ion battery.

[0041] In this application embodiment, there is no particular limitation on the type of positive electrode current collector; it can be any known material suitable for use as a positive electrode current collector. In one embodiment, the positive electrode current collector includes metallic materials such as aluminum, stainless steel, nickel plating, titanium, and tantalum, as well as carbon materials such as carbon cloth and carbon paper. Preferably, the positive electrode current collector is a metallic material.

[0042] In one specific embodiment, the positive electrode coating further includes a conductive agent and a binder. The conductive agent includes at least one carbon material selected from acetylene black, needle coke, carbon nanotubes, graphene, and vapor-grown carbon fiber (VGCF). The binder includes at least one selected from polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, nitrocellulose, polyvinylidene fluoride, and polytetrafluoroethylene.

[0043] In one specific embodiment, the lithium-ion battery further includes a separator. The embodiments of this application do not have any particular restrictions on the material and shape of the separator, as long as it does not significantly impair the effect of this application. It may include porous sheet-like or non-woven fabric-like materials with excellent liquid retention. The materials of the resin or glass fiber separator include, but are not limited to, polyolefins, aromatic polyamides, polytetrafluoroethylene, polyethersulfone, etc., and can be set according to needs.

[0044] In one embodiment, the battery may include an outer packaging that can be used to encapsulate the electrode assembly and electrolyte.

[0045] In one specific embodiment, the outer packaging of the battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch-type soft pack. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0046] This application does not impose any particular restrictions on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape.

[0047] This application does not impose any particular restrictions on the application fields of lithium-ion batteries, and they can be used in fields such as consumer batteries, power batteries for new energy vehicles, and energy storage batteries.

[0048] The present application will be further described in detail below through specific embodiments.

[0049] Example 1 1. Preparation of negative electrode sheet The active material (graphite), dispersant (CMC), and binder (SBR) are mixed in a ratio of 98:1:1, and then water is added to adjust the solid content to 55%. The first slurry is obtained under the action of a vacuum mixer. The active material (25% silicon-carbon material + 75% graphite) and the fast ion conductor (Li) are used. 1.05 Al 0.1 Ti 1.9 P3O 11.85 F 0.05The dispersant (CMC), binder (SBR), and conductive agent (carbon nanotubes) are mixed in a mass ratio of 96.2:0.5:0.5:2.5:0.3, and then water is added to adjust the solid content to 55%. The mixture is then stirred in a vacuum mixer to obtain a second slurry. The first slurry is uniformly coated on one surface of a negative electrode current collector copper foil with a thickness of 6 μm to obtain a first active layer. Then, a second slurry is coated on the surface of the first active layer to obtain a second active layer. The above operation is repeated on the other surface of the negative electrode current collector copper foil to obtain a negative electrode sheet.

[0050] Where x is 0.1, y is 0.05, and z is 0; The mass percentage (a%) of the fast ion conductor in the second active layer is 0.5%. The first active layer H1 is 50 μm, the second active layer H2 is 50 μm, and k = H1 / H2 = 1; a / k=0.5; The Dv50(b) of the fast ion conductor is 0.6 nm; The mass percentage of silicon in the negative electrode coating is 8%.

[0051] 2. Preparation of electrolyte In an environment with a water content of less than 10 ppm, diethyl carbonate (DEC) and ethylene carbonate (EC) are mixed at a mass ratio of 2:1. Based on the total mass of the electrolyte, lithium hexafluorophosphate (LiPF6) is added to the solvent, dissolved, and mixed evenly. Then, electrolyte additives are added to obtain the electrolyte. The mass percentage of LiPF6 in the electrolyte is 12.5%, and the electrolyte additives include fluoroethylene carbonate (FEC) and difluoroethylene carbonate (Difluorocarbonate). The mass percentages of FEC and Difluoroethylene carbonate are both 0.5%, meaning the mass percentage (c) of the electrolyte additives in the electrolyte is 1.0%.

[0052] 3. Preparation of positive electrode sheet Lithium cobalt oxide (CCO), conductive carbon black (SP), and polyvinylidene fluoride (PVDF) binder were mixed uniformly at a mass ratio of 97.4:2.0:0.6. The mixture was then thoroughly stirred in N-methylpyrrolidone solvent to prepare a slurry with a solid content of 73%, which was then stirred evenly to obtain the positive electrode slurry. The positive electrode slurry was uniformly coated onto one surface of a 12 μm thick aluminum foil, dried at 90°C, and cold-pressed to obtain a positive electrode sheet with a coating thickness of 110 μm. The above steps were then repeated on the other surface of the same positive electrode sheet to obtain a positive electrode sheet with a double-sided coating. The positive electrode sheet was cut into 76 mm × 851 mm dimensions and tabs were welded on for later use.

[0053] 4. Manufacturing of lithium-ion batteries The positive and negative electrode sheets are stacked in sequence, and the stacked electrode sheets are wound with a separator to obtain an electrode assembly. The electrode assembly is placed in a pre-formed aluminum-plastic film and dehydrated at 80°C. The prepared electrolyte is then injected, and the lithium-ion battery is obtained through vacuum sealing, settling, formation, and shaping processes.

[0054] The main differences between Examples 2-43, Comparative Examples 1-5 and Example 1 are the parameters of the first active layer, the second active layer, and the electrolyte. Please refer to Tables 1, 2, and 3.

[0055] Table 1

[0056] Table 2

[0057] Table 3

[0058] Test case The following performance tests were performed on the batteries prepared in the examples and comparative examples: 1. DCIR test In a 25℃ constant temperature chamber, charge the battery cell to 4.53V at 0.5C, cut off at 0.05C, and let it stand for 2 hours; then discharge it at 0.2C for 30 seconds, and read the discharge end voltage V1 and current I1; then discharge it at 2C for 10 seconds, and read the discharge end voltage V2 and current I2; calculate DCIR=(V1-V2) / (I2-I1).

[0059] 2. Lithium plating test The battery cell was charged to 4.53V with constant current and constant voltage at different rates (0.6C~3.0C), with a cutoff current of 0.05C, and then discharged to 2.8V at 1.0C. After 10 cycles, the battery cells with different charging currents were disassembled and the interface condition of the negative electrode was observed. The maximum rate current that ensures that the negative electrode does not deposit lithium can be obtained.

[0060] 3. Ratio Performance Test Rate performance test: In a constant temperature chamber at 25±2℃, the lithium-ion batteries are charged to 4.53V at a constant current and constant voltage of 4.0C, cut off current of 0.05C, and then discharged to 2.8V at 1.0C. The charge and discharge cycles are performed under the above conditions, and the cycle discharge capacity of the batteries is recorded. There are 5 batteries in each group.

[0061] High-rate charging capacity retention rate (%) = 4.0C discharge capacity (mAh) / 1.0C rate cycle discharge capacity (mAh) × 100% Five batteries were used in each group. The average capacity retention rate at 4.0C was taken, and the results are recorded in Tables 4 to 6.

[0062] 4. Cyclic performance test In a constant temperature chamber at (25±2)℃, the lithium-ion battery was charged at a constant current and constant voltage of 1.2C to 4.53V, then charged at a constant voltage to 0.05C, and after resting for 5 minutes, discharged at 1.0C to 2.8V. The capacity obtained in this step was taken as the initial capacity. Cyclic tests were performed using 1.2C charge / 1.0C discharge, and the capacity retention rate of the battery after 200 cycles was calculated.

[0063] Cycle capacity retention (%) = Discharge capacity at 200th cycle (mAh) / Discharge capacity at first cycle (mAh) × 100% The average cycle life of each group of 5 batteries is recorded in Tables 4 to 6.

[0064] Table 4

[0065] Table 5

[0066] Table 6

[0067] As shown in Tables 4-6, based on the comparison between Examples 1-11 and Comparative Examples 1-5, when the chemical formula of the fast ion conductor is Li... 1+x-y+z Al x Ti 2-x P3O 12-y-z F y N z Where 0.1≤x≤0.4, 0≤y≤0.2, 0≤z≤0.2, and y and z are not both 0, fast ion conductors can improve the transport rate of lithium ions in the negative electrode, while improving the mechanical strength and lithium conduction performance of the SEI film, ultimately reducing the internal resistance of the battery, improving the cycle performance and rate performance of the battery, and reducing the risk of lithium plating.

[0068] According to the comparison of Examples 8 and Examples 12-18, when 0 < a ≤ 5, the fast ion conductor can better play its role, resulting in lower battery internal resistance and lithium plating risk, as well as longer battery cycle life and higher rate performance.

[0069] Based on the comparison of Examples 8 and 12 to 30, it can be seen that when k≥1, the migration resistance of lithium ions is smaller and the transmission rate is higher, thereby reducing the internal resistance of the battery and optimizing the cycle performance and rate performance of the battery.

[0070] According to the comparison of Examples 8 and 12 to 30, when 0.3≤a / k≤1.0, the lithium-ion transport rate and the mechanical and lithium-conducting properties of the SEI film can be improved simultaneously, thereby improving the rate performance and cycle performance of the battery.

[0071] According to the comparison of Examples 8 and Examples 31-34, when the Dv50 of the fast ion conductor is 0.4nm~0.8nm, the lithium ion transport resistance is lower, and the mechanical strength and lithium conduction performance of the SEI film are higher, thereby enabling the battery to exhibit high cycle performance and rate performance, and alleviating the lithium plating problem of the battery.

[0072] According to the comparison of Examples 8 and Examples 35-39, when the mass percentage of silicon in the negative electrode coating is 5% to 40%, the internal resistance of the battery can be reduced, and the battery can exhibit high cycle performance and rate performance.

[0073] According to the comparison of Examples 8 and Examples 40-43, when the mass percentage of electrolyte additive in the electrolyte is 0.5% to 2%, the mechanical strength and lithium conduction performance of the SEI film can be further improved, thereby improving the battery's internal resistance, lithium plating risk, cycle performance and rate performance to a greater extent.

[0074] The various embodiments of this application have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen to best explain the principles, practical application, or improvement of the technology in the market, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. A negative electrode sheet, characterized in that, The present invention includes a negative electrode current collector and a negative electrode coating disposed on at least one surface of the negative electrode current collector; the negative electrode coating includes a first active layer and a second active layer stacked thereon, the first active layer being disposed on the surface of the negative electrode current collector; wherein: The second active layer includes a fast ion conductor, the chemical formula of which is Li. 1+x-y+z Al x Ti 2-x P3O 12-y- z F y N z Where 0.1≤x≤0.4, 0≤y≤0.2, 0≤z≤0.2, and y and z are not both 0.

2. The negative electrode sheet according to claim 1, characterized in that, The mass percentage of the fast ion conductor in the second active layer is a%, 0 < a ≤ 5.

3. The negative electrode sheet according to claim 1 or 2, characterized in that, The first active layer includes a first active material, which includes a carbon material; the second active layer includes a second active material, which includes a carbon material and a silicon-containing material.

4. The negative electrode sheet according to claim 3, characterized in that, The thickness ratio of the first active layer to the second active layer is k, where k ≥ 1.

5. The negative electrode sheet according to claim 2 or 4, characterized in that, The negative electrode sheet satisfies the following relationship: 0.3≤a / k≤1.

0.

6. The negative electrode sheet according to claim 3, characterized in that, The silicon element in the negative electrode coating has a mass percentage content of 5% to 40%.

7. The negative electrode sheet according to claim 1, characterized in that, The median particle size Dv50 of the fast ion conductor is 0.4 nm to 0.8 nm.

8. A battery comprising the negative electrode as described in any one of claims 1 to 7.

9. The battery according to claim 8, characterized in that, The battery includes an electrolyte, the electrolyte includes electrolyte additives, and the electrolyte additives include fluoroethylene carbonate and / or difluoroethylene carbonate.

10. The battery according to claim 9, characterized in that, The electrolyte additive has a mass percentage content of 0.5% to 2% in the electrolyte.

Images