Negative pole piece and secondary battery

Abstract

The invention discloses a negative pole piece and a secondary battery, and belongs to the technical field of electrochemical energy storage, according to the negative pole piece, carbon fibers jointly modified by lithium alloy and polydopamine serve as a conductive agent to be compounded with silicon-containing particles, the volume effect of the silicon-containing particles can be effectively restrained, meanwhile, the electrical conductivity of the whole material is improved, and the service life of the battery is prolonged. And when the negative electrode plate is applied to a sulfide solid-state battery, the negative electrode plate and a solid-state electrolyte can form an effective bicontinuous conductive path, the interface side reaction degree of the negative electrode plate and the solid-state electrolyte during lithium deintercalation is low, and excellent electrochemical performance can be realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electrochemical energy storage, in particular to a negative electrode material, a negative electrode sheet and a secondary battery. BACKGROUND

[0002] Silicon-based materials have obvious energy density advantages when used as negative electrode materials of battery cells. The theoretical capacity is high, but the volume expansion rate during lithium intercalation and deintercalation is as high as about 300%, which sharply weakens the cycle performance of the battery cell. In addition, the electrical conductivity of silicon-based materials is low, making it difficult to achieve ideal capacity performance. In order to overcome this defect, people use silicon particle refinement, such as nanocrystallization of the currently common micron-sized silicon particles. However, such a method is expensive and difficult to mass-produce in a factory. In addition, when silicon-based materials are applied to new generation solid-state batteries, such as sulfide solid-state batteries, in addition to the expansion effect, the traditional silicon-based materials and conductive agents in the negative electrode active material layer are difficult to form a continuous and volume-stable dual-continuous conductive path with the solid-state electrolyte. In addition, highly lithiated silicon will have an interface side reaction with the sulfide solid-state electrolyte, increasing the interface impedance, and ultimately failing to achieve ideal electrochemical performance. SUMMARY

[0003] The purpose of the present application is to overcome the deficiencies in the prior art and provide a negative electrode sheet. The carbon fibers modified by lithium alloy and polydopamine are used as conductive agents and are compounded with silicon-containing particles. This not only effectively suppresses the volume effect of silicon-containing particles, but also improves the electrical conductivity of the overall material. In addition, when the negative electrode sheet is applied to a sulfide solid-state battery, it can form an effective dual-continuous conductive path with the solid-state electrolyte. The interface side reaction between the two during lithium intercalation and deintercalation is low, and excellent electrochemical performance can be achieved.

[0004] To achieve the above-mentioned purpose, in a first aspect of the present application, the present application provides a negative electrode sheet, comprising a negative electrode active material layer, wherein the negative electrode active material layer comprises an active material and a conductive agent. The active material comprises silicon-containing particles. The conductive agent comprises carbon fibers, and the surface of the carbon fibers is provided with a first functional layer and a second functional layer, wherein the first functional layer is arranged between the carbon fibers and the second functional layer. The first functional layer comprises polydopamine, and the second functional layer comprises a lithium alloy.

[0005] In some embodiments, the silicon-containing particles comprise at least one of silicon, silicon oxide, silicon alloy and silicon-carbon composite material.

[0006] In some embodiments, the average particle size of the silicon-containing particles is 1-5 μm.

[0007] In some embodiments, the average thickness of the first functional layer is 1-500 nm.

[0008] In some embodiments, the average thickness of the second functional layer is 10~100nm.

[0009] In some embodiments, the lithium alloy includes at least one of lithium indium alloy, lithium aluminum alloy, lithium zinc alloy, lithium silver alloy, and lithium magnesium alloy.

[0010] In some embodiments, the carbon fiber has an average length of 10-50 μm and an average diameter of 0.2-2 μm.

[0011] In some embodiments, the average aspect ratio of the carbon fiber is 10 to 200.

[0012] In some embodiments, the mass ratio of the active material to the conductive agent is (80~99):(1~20).

[0013] In a second aspect, this application provides a solid-state secondary battery, including the negative electrode sheet described in this application.

[0014] The beneficial effects of this application are as follows: This application provides a negative electrode sheet, which is compounded with silicon-containing particles by using carbon fibers modified with lithium alloy and polydopamine as conductive agents. This not only effectively suppresses the volume effect of silicon-containing particles, but also improves the overall conductivity of the material. Furthermore, when the negative electrode sheet is applied to solid-state batteries such as sulfide batteries, it can form an effective dual continuous conductive path with the solid electrolyte. The degree of interfacial side reactions between the two during lithium insertion / extraction is low, which can achieve excellent electrochemical performance. Detailed Implementation

[0015] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0016] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0017] In this application, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0018] The present application is further illustrated below with specific embodiments: A negative electrode sheet includes a negative electrode active material layer, wherein the negative electrode active material layer includes an active material and a conductive agent; The active material includes silicon-containing particles; The conductive agent includes carbon fiber, and a first functional layer and a second functional layer are disposed on the surface of the carbon fiber, with the first functional layer disposed between the carbon fiber and the second functional layer. The first functional layer comprises polydopamine; the second functional layer comprises a lithium alloy.

[0019] To effectively overcome the volume expansion problem of silicon-based materials when applied to solid-state batteries, especially sulfide all-solid-state batteries, and to form a continuous dual-continuous conductive path with the conductive agent and solid electrolyte while maintaining high interfacial stability, this application uses silicon-containing particles as the active material and carbon fibers as the conductive agent. A three-dimensional conductive network of the negative electrode active material is constructed through the fiber structure, achieving efficient ion / electron transport. In this network structure, a layer of polydopamine is disposed on the surface of the carbon fibers. Besides effectively connecting the outer alloy layer, it also provides an elastic buffer effect for the volume expansion of the silicon-containing particles, effectively suppressing the expansion amplitude and maintaining the structural integrity of the overall network. Outside the polydopamine layer, a specific lithium alloy layer is disposed. This material, as a fast ion conductor, has high conductivity and can achieve a "highway" for ion transport through the fiber structure. It also acts as an interfacial buffer layer, reducing direct contact between the silicon-containing particles and the sulfide electrolyte and suppressing interfacial side reactions. By applying the conductive components, the negative electrode can achieve high energy density and ion conduction efficiency without introducing sulfide electrolytes. This results in high coulombic efficiency and good cycle stability when the product is applied to sulfide solid-state batteries. At the same time, the capacity of the silicon particles is fully utilized, resulting in high specific capacity, and ideal rate performance can also be achieved.

[0020] However, if other materials are used instead of polydopamine or lithium alloy layers during structural construction, the product will not achieve the same technical effect.

[0021] In some embodiments, the average particle size of the silicon-containing particles is 1~5 μm.

[0022] In some embodiments, the average particle size of the silicon-containing particles can be a range of one or any two of 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, and 5 μm.

[0023] It should be noted that the average particle size of the silicon-containing particles described in this application can be confirmed by, but is not limited to, the following methods: the negative electrode sheet is disassembled from the discharged secondary battery, then soaked in dimethyl carbonate for 2 hours, dried, and the negative electrode active material layer powder is scraped off. The powder is dispersed in ethanol and then observed using a scanning electron microscope (SEM). The SEM is performed at a magnification of 10X to 100X using a JEOL SM-74190UEC model. The distribution of silicon-containing particles is located and confirmed by elemental analysis (silicon element). At least 20 silicon-containing particles are measured using measurement software at the maximum magnification, and the average value is calculated, which is the average particle size of the silicon-containing particles.

[0024] More preferably, the silicon-containing particles include at least one of elemental silicon, silicon oxide, silicon alloy, and silicon-carbon composite material.

[0025] In this application, the type of silicon-containing particles can be selected according to actual needs and is not limited to the above types. Any type that can achieve the characteristics of conventional silicon-based materials is acceptable.

[0026] In some implementations, the average thickness of the first functional layer is 1 to 500 nm.

[0027] In some implementations, the average thickness of the first functional layer is a range of one or any two of the following: 1 nm, 5 nm, 10 nm, 20 nm, 50 nm, 80 nm, 100 nm, 150 nm, 200 nm, 250 nm, 300 nm, 350 nm, 400 nm, 450 nm, and 500 nm.

[0028] In some implementations, the average thickness of the first functional layer is 5 to 250 nm.

[0029] More preferably, the average thickness of the first functional layer is 10~100nm.

[0030] Increasing the thickness of the polydopamine layer can improve the bonding strength between the lithium alloy layer and the carbon fiber to a certain extent, and can also further improve the buffering effect of the carbon fiber on the volume expansion of silicon-containing particles. However, as the thickness increases further, it will also cause the carbon fibers to stick together to a certain extent, reducing the overall fiber structure dispersion. When the thickness of the polydopamine is further preferably within the above range, the dispersion of the carbon fiber structure is better, and the interlayer stability of the functional layers on the carbon fiber surface is better, resulting in a better buffering effect on silicon-containing particles. Consequently, the rate performance and cycle performance of the secondary battery can be further improved to a certain extent.

[0031] It should be noted that the average thickness of the first functional layer in this application can be confirmed by, but is not limited to, the following methods: the negative electrode sheet is disassembled from the discharged secondary battery, dried, and the negative electrode active material layer powder is scraped off. The powder is dispersed in an ethanol-water solution (V:V=50:50), and then observed using a scanning electron microscope (SEM). The SEM is used with a JEOL SM-74190UEC model at a magnification of 100X to 1000X. The distribution of polydopamine is confirmed by the color depth of the image area and elemental analysis. At least 10 cross-sections where polydopamine coating can be seen are selected at the maximum magnification, and the thickness of polydopamine is measured using measurement software. The average value of the 10 data points is calculated as the average thickness of the polydopamine.

[0032] In some embodiments, the lithium alloy includes at least one of lithium indium alloy, lithium aluminum alloy, lithium zinc alloy, lithium silver alloy, and lithium magnesium alloy.

[0033] In the technical solution of this application, the selection of lithium alloy can be based on actual use and production needs, and is not subject to specific limitations. As long as it can be used as a fast ion conductor and can achieve the expected effect of lithium alloy, it is acceptable.

[0034] In some embodiments, the lithium alloy is a lithium-indium alloy, wherein the mass ratio of indium to lithium in the lithium-indium alloy is 1:5 to 146:1.

[0035] In some embodiments, the average thickness of the second functional layer is 10~100nm.

[0036] The test method for the average thickness of the second functional layer in this application is similar to that for the first functional layer. The only difference is that the distribution of the layer is confirmed by the color intensity of the image area and the location of the element (lithium element) during observation, which will not be described in detail here.

[0037] In some embodiments, the second functional layer includes lithium alloy particles that are partially embedded in the first functional layer.

[0038] It should be noted that when the lithium alloy in the second functional layer is introduced into the first functional layer in a partially embedded form, the bonding strength of the two interface layers is higher, and the two functional layers improve the ion conduction efficiency more. The setting of the functional layer can be achieved by in-situ synthesis method, but is not limited to this. Those skilled in the art can also combine the two functional layers by other means, such as PVD method, solid phase mixing method, etc., in which case there is no partial embedding between the functional layers. As long as the technical effect claimed by the product described in this application can be achieved, it is acceptable.

[0039] In some embodiments, the carbon fiber has an average length of 10-50 μm and an average diameter of 0.2-2 μm.

[0040] In some embodiments, the average length of the carbon fiber is a range of one or any two of the following: 10 μm, 12 μm, 15 μm, 18 μm, 20 μm, 22 μm, 25 μm, 28 μm, 30 μm, 32 μm, 35 μm, 38 μm, 40 μm, 42 μm, 45 μm, 48 μm, and 50 μm.

[0041] In some embodiments, the average diameter of the carbon fiber is a range of one or both of 0.2 μm, 0.5 μm, 0.8 μm, 1 μm, 1.2 μm, 1.5 μm, 1.8 μm, and 2 μm.

[0042] In some embodiments, the average aspect ratio of the carbon fiber is 10 to 200.

[0043] In some embodiments, the average aspect ratio of the carbon fiber can be a range of one or both of the following: 10, 20, 25, 30, 40, 50, 60, 80, 100, 120, 150, 180, 200.

[0044] In some embodiments, the average aspect ratio of the carbon fiber is 12 to 115.

[0045] More preferably, the average aspect ratio of the carbon fiber is 20 to 60.

[0046] As a key component in constructing a three-dimensional conductive network structure, the aspect ratio of carbon fiber affects the stability of the structure, the buffering effect of silicon particle volume expansion, and the length of the lithium-ion transport path. If the aspect ratio is too small, the overall size of the carbon fiber is more granular, increasing the overall difficulty of ion transport. Furthermore, its supporting strength for the structure and its buffering effect on silicon particles are reduced, failing to fully utilize the effect of polydopamine. If the aspect ratio is too large, the supporting strength of the structure is higher, but the dispersion uniformity is lower and agglomeration is more likely. At the same time, the difference in the rate of lithium ion transport along the long and short axes of the fiber is too large, which can easily cause concentration polarization. Also, there will be some positions where the two functional layers cannot contact the silicon particles, resulting in poor isolation of silicon particles, reduced buffering effect, and lower interface stability. When the aspect ratio of the carbon fiber is further optimized within the above range, not only can better structural uniformity be achieved, but also high supporting strength can be achieved. The two functional layers can fully exert their gain effect, further optimizing the cycle performance and rate performance of the negative electrode sheet when applied to sulfide solid-state batteries.

[0047] It should be noted that the average length and average diameter of the carbon fiber described in this application can be confirmed by, but not limited to, the following methods: The negative electrode sheet is disassembled from the discharged secondary battery, dried, and the negative electrode active material layer powder is scraped off. The powder is dispersed in an ethanol-water solution (V:V=50:50), and then observed using a transmission electron microscope (TEM). Observation is performed under a scanning electron microscope at a magnification of 100X~1000X. The carbon fiber is identified by shape. At least 10 structurally intact carbon fibers are selected and identified under the first functional layer coating by elemental analysis (nitrogen element) at maximum magnification. The diameter and length of the carbon fiber are measured (the diameter is measured using a three-point method, measuring the diameter values ​​at the leftmost, middle, and rightmost ends of the carbon fiber, and calculating the average value as the diameter of that carbon fiber). The average length and average diameter of the 10 measured data are calculated, which are the average length and average diameter of the carbon fiber. The average length-to-diameter ratio is then calculated as the average length / average diameter.

[0048] In some embodiments, the mass ratio of the active material to the conductive agent is (80~99):(1~20).

[0049] In some embodiments, the negative electrode active material layer of the negative electrode sheet also includes a binder.

[0050] More preferably, the adhesive includes at least one of sodium carboxymethyl cellulose, polyisobutylene, styrene-butadiene rubber, hydrogenated nitrile rubber, nitrile rubber, and styrene-ethylene-butene-styrene block copolymer.

[0051] More preferably, the binder content in the negative electrode active material layer is 1~10wt%.

[0052] It should be noted that the binder in the negative electrode active material layer described in this application is used to improve the adhesion between the active material and the conductive agent and the adhesion between the negative electrode active material layer and the current collector. Any binder can be used without particular limitation, as long as it has suitable binder properties and does not significantly cause adverse chemical changes in the battery.

[0053] In some embodiments, the method for preparing the negative electrode sheet may include, but is not limited to, the following: Carbon fibers were dispersed in an aqueous solution of polydopamine and stirred for 0.5 to 10 hours, then washed to obtain coated carbon fiber I. Coated carbon fiber I and nano-metal powder were mixed in a solvent, sheared, and dried to obtain coated carbon fiber II; The coated carbon fiber II is mixed with silicon-containing particles and binder, and lithium powder is added to prepare a slurry. The slurry is coated on the current collector and heated and rolled to obtain the negative electrode sheet.

[0054] In the preparation method described in this application, the carbon fiber is first used to construct a polydopamine layer on its surface via liquid-phase dispersion. Then, it is mixed with nano-metal powder, forming a second coating layer on the surface. After mixing with silicon-containing particles, a binder, and lithium powder, and then coating and hot-pressing, the lithium powder forms a lithium alloy in situ with the nano-alloy powder on the carbon fiber surface, ultimately yielding the second functional layer. However, this method is not limited to this; those skilled in the art can also use other methods to construct the negative electrode sheet. For example, the carbon fiber can be pre-modified with a first and second functional layer (a lithium alloy has already formed in the second functional layer), and then directly mixed with silicon-containing particles and other components and coated onto the current collector. Alternatively, the carbon fiber can be pre-immersed in dopamine monomer, followed by further in-situ polymerization to form polydopamine, and then the second functional layer can be constructed. No limitation is made in these methods. Furthermore, the conductive agent can also be a commercially available product.

[0055] In some embodiments, the concentration of polydopamine in the aqueous solution of polydopamine is 0.1~20mM.

[0056] Those skilled in the art can adjust the average thickness of the first functional layer by adjusting the concentration of the polydopamine aqueous solution during product preparation. The higher the concentration, the greater the thickness, but this is not the only method. Those skilled in the art can also increase the thickness by other means, such as increasing the temperature while dispersing carbon fibers in the solution, thereby enhancing the adsorption activity of polydopamine.

[0057] In some embodiments, the nano-metal powder includes nano-indium powder.

[0058] In some embodiments, the mass ratio of the coated carbon fiber I to the nano-metal powder is 1:1 to 60:1.

[0059] Similarly, those skilled in the art can adjust the thickness of the second functional layer by varying the amount of nano-metal powder added during the preparation process. However, they are not limited to this; they can also control the thickness by adjusting the particle size of the nano-metal powder.

[0060] In some embodiments, the average particle size of the nano-metal powder is 10-50 nm.

[0061] In some embodiments, the shearing rate is 100-1000 s. -1 The time is 10-60 minutes.

[0062] In some embodiments, the mass ratio of nano-metal powder to lithium powder in the coated carbon fiber II is 1:5 to 146:1.

[0063] In some embodiments, the temperature during the heating roller pressing is 80~120℃ and the pressure is 50~150kN / m.

[0064] In some embodiments, the negative electrode sheet further includes a current collector, which includes at least one of metal, alloy, and conductive polymer. Specifically, it can be a copper current collector, a copper alloy current collector, a conductive polymer current collector, or a composite structure of the above current collectors, etc., without any special limitation.

[0065] In some embodiments, the compaction density of the negative electrode sheet is 0.9~1.6 g / cm³. 3 .

[0066] In a second aspect, this application provides a solid-state secondary battery, including the negative electrode sheet described in this application.

[0067] In some embodiments, the solid-state secondary battery further includes a sulfide solid electrolyte.

[0068] Furthermore, the sulfide solid electrolyte includes Li6PS5Cl, Li3PS4, and Li7P3S. 11 Li 10 GeP2S 12 Li 7-x PS 6-x Cl x At least one of the following: polymer complex sulfides.

[0069] Based on the structural design of the negative electrode sheet described in this application, it can be effectively applied to secondary batteries with sulfide solid electrolyte systems, while taking into account both high ion conduction efficiency and high stability. As for the sulfide solid electrolyte used, in addition to the types mentioned above, other types commonly known to those skilled in the art can also be selected, and no specific limitation is made in this regard.

[0070] In some embodiments, the solid-state secondary battery further includes a positive electrode sheet, which includes a current collector and a positive active material layer, the positive active material layer including a positive electrode material.

[0071] In some embodiments, the cathode material includes at least one of lithium cobalt oxide, lithium nickel cobalt manganese oxide, and lithium iron phosphate.

[0072] In some embodiments, the positive electrode active material layer further includes a solid electrolyte, a binder, and a conductive agent.

[0073] It should be noted that the solid electrolyte in the positive electrode active material layer described in this application may be the same as or different from the solid electrolyte in the secondary battery mentioned above.

[0074] In some embodiments, the mass ratio of the positive electrode material, solid electrolyte, binder, and conductive agent is (50~80):(15~30):(1~10):(1~10).

[0075] In some embodiments, the adhesive includes at least one selected from polyvinylidene fluoride, polyvinyl butyral, polytetrafluoroethylene, ethylene-vinyl acetate copolymer, and polyvinyl alcohol.

[0076] In some embodiments, the conductive agent includes at least one of graphite, superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0077] The present invention is further illustrated below with specific embodiments, which should not be construed as limiting the scope of protection claimed by the present invention: Materials used: Carbon fiber, VGCF series products manufactured by Kleway Nano Carbon Materials Co., Ltd.; Polydopamine, products manufactured by Sinopharm Reagent Platform.

[0078] Example 1 An embodiment of a negative electrode sheet and a secondary battery, wherein the method for preparing the negative electrode sheet includes the following steps: 0.1 g of carbon fiber was dispersed in 100 mL of polydopamine aqueous solution and magnetically stirred for 1 h at 25 °C. The mixture was then washed with deionized water and dried at 60 °C for 24 h to obtain coated carbon fiber I. Carbon fiber I and indium nanoparticles with an average particle size of 10 nm were mixed in 50 mL of ethanol-water solution (V:V=50:50) and stirred for 200 s. -1 Rate shearing treatment for 30 min, drying at 60℃ for 24 h, yielding coated carbon fiber II; In a glove box with water and oxygen monitoring, coated carbon fiber II, elemental silicon particles, and binder polyisobutylene (total slurry material excluding solvent 1 wt%) were mixed in dodecane solvent. Lithium powder with an average particle size of 15 μm was added to prepare a slurry (the mass ratio of coated carbon fiber II and lithium powder to elemental silicon particles was 93.1:6.9, and the mass ratio of nano-indium powder to lithium powder was 25:1). This slurry was coated onto a current collector copper foil and subjected to a rolling process at 80°C and a pressure of 100 kN / m to obtain the negative electrode sheet with a compaction density of 1.2 g / cm³. 3 .

[0079] The positive electrode sheet is prepared as follows: Commercially available lithium cobalt oxide with an average particle size of 3 μm is used as the positive electrode material. It is compounded with Li6PS5Cl solid electrolyte, conductive agent VGCF, and binder polyisobutylene in a mass ratio of 75:20:2:3 and dispersed in xylene to prepare a slurry. This slurry is coated onto aluminum foil, cold-pressed, and slit to obtain the positive electrode sheet. The compacted density of the positive electrode sheet is 3.5 g / cm³. 3 ; The method for preparing the solid-state secondary battery includes the following steps: Using Li6PS5Cl as a solid electrolyte, 100 mg of Li6PS5Cl powder was pressed into an electrolyte layer at 200 MPa. The resulting negative electrode, electrolyte layer, and positive electrode were then placed into a mold and cold-pressed at 500 MPa to form a solid secondary battery with a diameter of 10 mm.

[0080] Examples 2-12 An embodiment of a negative electrode sheet and a secondary battery differs from Embodiment 1 only in that the parameters of the negative electrode sheet during preparation and in the finished product are different. In Embodiment 6, the silicon-containing particles are replaced with silicon oxide with an average particle size of 3.5 μm, and in Embodiment 7, the silicon-containing particles are replaced with silicon-carbon composite material with an average particle size of 4 μm (silicon content of 50 wt%). Those skilled in the art can also adjust these parameters in other ways.

[0081] Comparative Example 1 A negative electrode sheet and a secondary battery differ from Example 1 only in that, in the preparation method of the negative electrode sheet, the coated carbon fiber II is replaced with unmodified carbon fiber raw material, and lithium powder is not added to prepare the slurry.

[0082] Comparative Example 2 A negative electrode sheet and a secondary battery, differing from Example 1 only in that the method for preparing the negative electrode sheet includes the following steps: 0.1 g of carbon fiber was dispersed in 100 mL of polydopamine aqueous solution and magnetically stirred for 1 h at 25 °C. The mixture was then washed with deionized water and dried at 60 °C for 24 h to obtain coated carbon fiber I. In a glove box with water and oxygen monitoring, carbon fiber I, elemental silicon particles, and polyisobutylene binder are mixed in dodecane solvent to prepare a slurry. This slurry is then coated onto a copper foil current collector and subjected to a rolling process at 80°C and a pressure of 100 kN / m to obtain the negative electrode sheet.

[0083] Comparative Example 3 A negative electrode sheet and a secondary battery, differing from Example 1 only in that, in the preparation method of the negative electrode sheet, when preparing the coated carbon fiber II, the nano-indium powder is replaced with an equal mass of commercially available lithium lanthanum zirconium oxide (LLZO) with an average particle size of 0.5 μm. In a glove box with water and oxygen monitoring, carbon fiber II coated with elemental silicon particles and polyisobutylene binder are mixed in dodecane solvent to prepare a slurry, which is then coated onto copper foil current collector and subjected to a rolling process heated to 80°C and pressured at 100 kN / m to obtain the negative electrode sheet.

[0084] Comparative Example 4 A negative electrode sheet and a secondary battery are disclosed, differing from Example 1 only in that lithium powder is not added during the preparation of the slurry in the method for preparing the negative electrode sheet.

[0085] Comparative Example 5 A negative electrode sheet and a secondary battery are disclosed, differing from Example 1 only in that, in the preparation method of the negative electrode sheet, the carbon fiber is replaced with an equal mass of Merck 806625 graphene nanosheets. Furthermore, testing confirmed that the average length and average diameter of the carbon fibers used in the preparation methods of each embodiment changed by no more than 5% before and after the preparation of the negative electrode sheet. Therefore, it is believed that the original size of the carbon fibers can be retained in the negative electrode sheet. In this application embodiment, the size of the carbon fibers in the negative electrode sheet is controlled by selecting different raw material size conditions, but this is not limited to this method. Those skilled in the art can also use other methods to control the size.

[0086] Table 1 Electrochemical performance tests were performed on the battery products of each embodiment and comparative example: Each solid-state secondary battery was charged at a constant current rate of 0.05C until the voltage reached 4.5V, and then further charged at a constant voltage of 4.5V until the current dropped below 0.02C, bringing it to a fully charged state at 4.5V. Subsequently, it was discharged at a constant current rate of 0.05C until the voltage reached 3.0V. Following this, the activated batteries underwent cycle capacity retention and rate testing.

[0087] First, a first charge and discharge cycle was performed. Constant current and constant voltage charging was conducted at a charging current of 0.1C until the upper limit voltage reached 4.5V. Then, constant current discharging was performed at a discharging current of 0.1C until the final voltage reached 3V. The discharge capacity A0 of the first cycle was recorded. Next, 100 charge and discharge cycles were performed, and the discharge capacity A1 of the 100th cycle was recorded. Finally, five rate charge and discharge cycles were performed at current densities of 0.2C, 0.5C, 1.0C, 1.5C, and 2.0C, respectively, and the discharge capacity B of the final cycle at current densities of 0.2C, 1.0C, and 2.0C was recorded. 0.2 B 1.0 and B 2.0 Subsequently, the cycle capacity retention rate was calculated as 100% × A1 / A0, and the capacity retention rate at a 1.0C rate was calculated as 100% × B. 1.0 / B 0.2 Capacity retention at 2.0C rate = 100% × B 2.0 / B 0.2 ; The test results are shown in Table 2.

[0088] Table 2 As shown in Table 2, the negative electrode sheet described in this application, when containing silicon particles, constructs a three-dimensional conductive network of negative electrode active material layer by introducing carbon fiber components with composite functional layers into the conductive agent, thereby achieving efficient ion / electron transport. Simultaneously, in this network structure, a layer of polydopamine is disposed on the surface of the carbon fiber. Besides effectively connecting the outer alloy layer, it also provides an elastic buffer effect for the volume expansion of the silicon particles, effectively suppressing the volume expansion amplitude of the silicon particles and maintaining the structural integrity of the overall network structure. Outside the polydopamine layer, a specific lithium alloy layer is also disposed. This material, as a fast ion conductor, has high conductivity and can achieve a "highway" for ion transport based on the fiber structure. It also serves as an interface buffer layer, reducing direct contact between the silicon particles and the sulfide electrolyte and suppressing interfacial side reactions. By applying the conductive components, the negative electrode can achieve high energy density and ion conduction efficiency without introducing sulfide electrolyte. This results in high coulombic efficiency and good cycle stability when the product is applied to sulfide solid-state batteries. At the same time, the capacity of the silicon particles is fully utilized, resulting in high specific capacity and ideal rate performance. The capacity of the products in each embodiment can reach more than 140 mAh / g, while the cycle stability is high and the capacity retention rate after cycling can reach more than 80%. In terms of rate testing, the capacity retention rate at 2C rate can reach more than 50%.

[0089] In contrast, the carbon fiber in Comparative Example 1 lacks a functional coating layer, failing to effectively balance improving ion conduction efficiency and suppressing volume expansion. Consequently, its capacity is already low, while its cycle and rate performance are extremely poor. Comparative Example 2 lacks a lithium alloy functional layer, Comparative Example 3 uses a conventional fast ion conductor instead of a lithium alloy functional layer, and Comparative Example 4 only has a nano-indium coating layer. These four products also fail to achieve the performance levels of the embodiments. In Comparative Example 5, the carbon fiber is replaced with two-dimensional graphene nanosheets. Although it still exhibits high capacity, its cycle and rate performance do not meet the expected levels of the embodiments. This demonstrates that not any carbon material can be used as a conductive agent and modified with a functional coating to achieve similar effects to the products in this application.

[0090] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this article and are not intended to limit the scope of protection of this article. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this article without departing from the essence and scope of the technical solutions of this article.

Claims

1. A negative electrode sheet, characterized in that, It includes a negative electrode active material layer, wherein the negative electrode active material layer comprises an active material and a conductive agent; The active material includes silicon-containing particles; The conductive agent includes carbon fiber, and a first functional layer and a second functional layer are disposed on the surface of the carbon fiber, with the first functional layer disposed between the carbon fiber and the second functional layer. The first functional layer comprises polydopamine; the second functional layer comprises a lithium alloy.

2. The negative electrode sheet as described in claim 1, characterized in that, The silicon-containing particles include at least one of elemental silicon, silicon oxide, silicon alloy, and silicon-carbon composite materials.

3. The negative electrode sheet as described in claim 1, characterized in that, The average particle size of the silicon-containing particles is 1~5μm.

4. The negative electrode sheet as described in claim 1, characterized in that, The average thickness of the first functional layer is 1~500nm.

5. The negative electrode sheet as described in claim 1, characterized in that, The average thickness of the second functional layer is 10~100nm.

6. The negative electrode sheet as described in claim 1, characterized in that, The lithium alloy includes at least one of lithium indium alloy, lithium aluminum alloy, lithium zinc alloy, lithium silver alloy, and lithium magnesium alloy.

7. The negative electrode sheet as described in claim 1, characterized in that, The carbon fibers have an average length of 10~50μm and an average diameter of 0.2~2μm.

8. The negative electrode sheet as described in claim 1, characterized in that, The carbon fiber has an average aspect ratio of 10 to 200.

9. The negative electrode sheet as described in claim 1, characterized in that, The mass ratio of the active material to the conductive agent is (80~99):(1~20).

10. A solid-state secondary battery, characterized in that, Includes the negative electrode sheet as described in any one of claims 1 to 9.

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