Composite negative electrode sheet, preparation method thereof, and battery using the same

Abstract

This invention provides a composite negative electrode sheet, its preparation method, and a battery using the composite negative electrode sheet. The composite negative electrode sheet comprises a metal layer and a mullite fiber layer, wherein the metal layer and the mullite fiber layer are composited; the metal layer comprises lithium metal. Applying the composite negative electrode sheet provided by this invention to lithium metal batteries can improve the cycle stability, safety, and cycle life of lithium metal batteries.

Description

Technical Field

[0001] This invention relates to the field of battery technology, specifically to a composite negative electrode sheet and its preparation method, and a battery using the composite negative electrode sheet. Background Technology

[0002] Since the commercialization of lithium-ion batteries based on coke anodes, lithium cobalt oxide cathodes, and carbonate electrolytes, energy devices represented by lithium-ion batteries have been widely used in 3C electronic products, energy storage power stations, and new energy electric vehicles. In recent years, the battery industry has achieved rapid development. In order to maintain the long-term stable development of battery-related fields, there is an urgent need to develop battery technologies that combine high energy density and power density with safety.

[0003] Since the commercialization and popularization of rechargeable batteries, the demand for high-energy-density batteries has continued to rise. Among them, lithium metal batteries, with their high energy density characteristics, have shown great potential to lead the trend of battery technology innovation and are expected to drive leapfrog progress in electric vehicles, consumer electronics and even the energy storage industry.

[0004] Lithium metal batteries are battery systems that use elemental lithium metal as the negative electrode (i.e., lithium metal anode). Their energy density is much higher than that of lithium-ion batteries. However, the energy storage mechanism, structural composition, and conditions for lithium dendrite formation during electrodeposition / stripping processes differ from those of lithium-ion batteries. The core mechanism of sodium metal batteries is the deposition / dissolution of metallic lithium. Their negative electrode active material is metallic lithium itself. During charging, lithium ions gain electrons from the positive electrode and are deposited on the negative electrode surface in the form of elemental lithium. During discharging, the metallic lithium loses electrons and returns to the electrolyte and positive electrode as lithium ions.

[0005] Although lithium metal batteries have great potential and market due to their high energy density, sodium metal batteries face the following challenges in practical applications: During the cycling process of lithium metal batteries, uncontrollable lithium dendrites are generated during the deposition and stripping of lithium metal. These lithium dendrites can easily puncture the separator. At the same time, the lithium metal anode will inevitably experience volume expansion during cycling due to the repeated deposition and stripping of sodium metal, which will cause repeated damage and reconstruction of the solid electrolyte interphase (SEI) membrane. Furthermore, lithium metal that melts from the root can easily detach from the conductive system and form "dead lithium" that is difficult to reuse. All of these factors will lead to a significant decrease in the safety performance and cycle stability of lithium metal batteries, and greatly shorten the cycle life of lithium metal batteries. Summary of the Invention

[0006] To address the issues of decreased safety performance of existing lithium metal batteries due to lithium dendrite formation during cycling, and decreased cycle stability due to volume expansion of the lithium metal anode and the formation of dead lithium, this invention provides a composite anode sheet, its preparation method, and a battery using the composite anode sheet.

[0007] According to a first aspect of the present invention, a composite negative electrode is provided, the composite negative electrode comprising a metal layer and a mullite fiber layer, wherein the metal layer and the mullite fiber layer are composite; the metal layer comprises lithium metal.

[0008] The composite negative electrode provided by this invention comprises a metal layer and a mullite fiber layer that are composited together. The metal layer contains lithium metal, and the mullite fiber layer is composed of mullite (3Al2O3·2SiO2) fibers. The mullite fiber layer acts as a composite material for current collectors in the composite negative electrode. Firstly, the surface of the mullite fibers has abundant Lewis acid sites (exposed Al2O3 on the surface). 3 + The mullite exhibits excellent lithium-affinity properties. Its surface lithium-affinity sites effectively reduce the nucleation overpotential / barrier of lithium ions at the electrode or electrolyte interface, and guide lithium ions to preferentially and uniformly adsorb and deposit on the surface of the three-dimensional fiber network of the mullite fiber layer. Secondly, the mullite fibers in the mullite fiber layer intertwine to form a porous and stable three-dimensional network structure. Its abundant pores provide buffer space for volume changes during lithium metal deposition or stripping, effectively mitigating the volume changes of the composite negative electrode (i.e., the lithium metal negative electrode) during cycling, while reducing the structural stress of the entire composite negative electrode and maintaining its structural integrity. Furthermore, its three-dimensional fiber network provides ample deposition space for lithium ions, allowing lithium metal to adhere to the surface of the three-dimensional fiber network. The process involves several key aspects. First, the mullite fiber layer deposits on its surface. Second, the three-dimensional network structure formed by the mullite fibers in the layer helps to homogenize the lithium-ion flow and current density distribution on the surface of the composite negative electrode, inhibiting lithium dendrite growth from both thermodynamic and kinetic perspectives and reducing the risk of the separator being punctured by lithium dendrite growth. Third, the combination of the mullite fiber layer and the lithium-containing metal layer forms a stable fiber-lithium contact interface, which is beneficial for forming a uniform solid electrolyte interphase (SEI) film during cycling. Simultaneously, the fiber network provides a certain physical constraint on the deposited lithium, effectively reducing the generation and accumulation of dead lithium on the surface of the composite negative electrode, thus maintaining a high lithium-ion diffusion flux and reversible capacity during cycling. Therefore, applying the composite negative electrode provided by this invention to lithium metal batteries can improve the cycle stability, safety, and cycle life of lithium metal batteries.

[0009] Preferably, the composite negative electrode further includes a functional layer, which includes a lithium-loving metal material; the lithium-loving metal material includes gallium.

[0010] Preferably, one surface of the metal layer is composited with a mullite fiber layer, and the other surface of the metal layer is composited with a functional layer.

[0011] Preferably, the thickness of the metal layer is 20~100 μm.

[0012] Preferably, the thickness of the mullite fiber layer is 15~50 μm.

[0013] Preferably, the thickness of the functional layer is 1~15 μm.

[0014] According to a second aspect of the present invention, a method for preparing the above-mentioned composite negative electrode sheet is provided, comprising the following steps: rolling a metal foil and a mullite fiber sheet to composite the metal foil and the mullite fiber sheet, thereby obtaining a composite negative electrode sheet.

[0015] In the method for preparing the composite negative electrode sheet provided in this scheme, metal foil and mullite fiber sheets are stacked and then rolled to combine them to obtain the composite negative electrode sheet. Firstly, the surface of mullite fibers has abundant Lewis acid sites, exhibiting excellent lithium-affinity properties. These lithium-affinity sites can effectively reduce the nucleation overpotential / barrier of lithium ions at the electrode or electrolyte interface and guide lithium ions to preferentially and uniformly adsorb and deposit on the surface of its three-dimensional fiber network. Secondly, the mullite fibers in the mullite fiber sheet interweave to form a porous and stable three-dimensional network structure. The abundant pores within this structure provide buffer space for volume changes during lithium metal deposition or stripping, effectively mitigating the volume change of the composite negative electrode sheet (i.e., the lithium metal negative electrode) during cycling, while also reducing the structural stress of the entire composite negative electrode sheet and maintaining its structural integrity. The composite anode exhibits several advantages. First, its structural integrity is enhanced by its three-dimensional fiber network, which provides ample deposition space for lithium ions, allowing lithium metal to deposit along the surface of the mullite fiber layer. Second, the three-dimensional network structure formed by the mullite fibers in the mullite fiber sheet helps to homogenize the lithium ion flow and current density distribution on the surface of the composite anode, inhibiting the localized preferential growth of lithium dendrites from both thermodynamic and kinetic perspectives, thus reducing the risk of lithium dendrites puncturing the separator. Third, the combination of mullite fiber sheets and metal foil forms a stable fiber-lithium contact interface, which is beneficial for forming a uniform solid electrolyte interphase (SEI) film. Simultaneously, the fiber network provides a certain physical constraint on deposited lithium, effectively reducing the generation and accumulation of dead lithium on the surface of the composite anode, thereby maintaining a high lithium ion diffusion flux and reversible capacity during cycling. Applying the composite anode prepared using the method provided in this scheme to lithium metal batteries can improve the cycle stability, safety, and cycle life of lithium metal batteries.

[0016] Preferably, the above-mentioned mullite fiber sheet is prepared by the following steps: preparing a slurry containing polyacrylamide, silica sol and short mullite fibers, drying the slurry to obtain a blank, and sintering the blank at 1300~1500℃ for 1~4 h to obtain a mullite fiber sheet; the mass ratio of polyacrylamide, silica sol and short mullite fibers is (0.5~2):(1~5):(0.4~4).

[0017] Preferably, in the preparation process of mullite fiber sheets, the sintering of the blank includes the following two stages: the blank is first sintered at 1300~1350℃ for 1~2 h, and then sintered at 1450~1500℃ for 1~2 h.

[0018] Preferably, the short mullite fibers have a length of 50~300 nm and a diameter of 30~100 nm.

[0019] Preferably, after the metal foil is composited with the mullite fiber sheet, the process further includes coating the surface of the metal foil away from the mullite fiber sheet with a dispersion of a lithium-philic metal material and drying it to form a functional layer on the surface of the metal foil.

[0020] Preferably, the lithium-ion metal material dispersion comprises a lithium-ion metal material and a dispersion medium; the lithium-ion metal material comprises gallium, and the dispersion medium comprises tetrahydrofuran, isopropanol, or water; the mass ratio of the lithium-ion metal material to the dispersion medium is (0.05~2):(10~20).

[0021] Preferably, the coating thickness of the lithium-loving metal material dispersion is 1~30 μm.

[0022] Preferably, the concentration of the lithiophilic metal material in the lithiophilic metal material dispersion is 0.05~5 g / L.

[0023] Preferably, the lithium-loving metal material dispersion further includes a dispersant, which includes at least one of xanthan gum, guar gum, and dodecyl mercaptan; the concentration of the dispersant in the lithium-loving metal material dispersion is 0.05~10 g / L.

[0024] According to a third aspect of the present invention, a battery is provided, the battery comprising the above-described composite negative electrode sheet, or a composite negative electrode sheet prepared by the above-described method for preparing the composite negative electrode sheet.

[0025] Applying the composite negative electrode sheet provided by this invention or the composite negative electrode sheet prepared by the preparation method provided by this invention to batteries can improve the cycle stability, cycle life, safety performance and rate performance of batteries. Detailed Implementation

[0026] The technical features of the technical solution provided by the present invention will be further clearly and completely described below with reference to specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0027] To address the issues of decreased safety performance of existing lithium metal batteries due to lithium dendrite formation during cycling, and decreased cycle stability due to volume expansion of the lithium metal anode and the formation of dead lithium, this invention provides a composite anode sheet, its preparation method, and a battery using the composite anode sheet.

[0028] According to a first aspect of the present invention, a composite negative electrode is provided, the composite negative electrode comprising a metal layer and a mullite fiber layer, wherein the metal layer and the mullite fiber layer are composite; the metal layer comprises lithium metal.

[0029] The composite negative electrode provided by this invention comprises a metal layer and a mullite fiber layer that are composited together. The metal layer contains lithium metal, and the mullite fiber layer is composed of mullite (3Al2O3·2SiO2) fibers. The mullite fiber layer acts as a composite material for current collectors in the composite negative electrode. Firstly, the surface of the mullite fibers has abundant Lewis acid sites (exposed Al2O3 on the surface). 3 +The mullite exhibits excellent lithium-affinity properties. Its surface lithium-affinity sites effectively reduce the nucleation overpotential / barrier of lithium ions at the electrode or electrolyte interface, and guide lithium ions to preferentially and uniformly adsorb and deposit on the surface of the three-dimensional fiber network of the mullite fiber layer. Secondly, the mullite fibers in the mullite fiber layer intertwine to form a porous and stable three-dimensional network structure. Its abundant pores provide buffer space for volume changes during lithium metal deposition or stripping, effectively mitigating the volume changes of the composite negative electrode (i.e., the lithium metal negative electrode) during cycling, while reducing the structural stress of the entire composite negative electrode and maintaining its structural integrity. Furthermore, its three-dimensional fiber network provides ample deposition space for lithium ions, allowing lithium metal to adhere to the surface of the three-dimensional fiber network. The process involves several key aspects. First, the mullite fiber layer deposits on its surface. Second, the three-dimensional network structure formed by the mullite fibers in the layer helps to homogenize the lithium-ion flow and current density distribution on the surface of the composite negative electrode, inhibiting lithium dendrite growth from both thermodynamic and kinetic perspectives and reducing the risk of the separator being punctured by lithium dendrite growth. Third, the combination of the mullite fiber layer and the lithium-containing metal layer forms a stable fiber-lithium contact interface, which is beneficial for forming a uniform solid electrolyte interphase (SEI) film during cycling. Simultaneously, the fiber network provides a certain physical constraint on the deposited lithium, effectively reducing the generation and accumulation of dead lithium on the surface of the composite negative electrode, thus maintaining a high lithium-ion diffusion flux and reversible capacity during cycling. Therefore, applying the composite negative electrode provided by this invention to lithium metal batteries can improve the cycle stability, safety, and cycle life of lithium metal batteries.

[0030] The aforementioned "dead lithium" refers to metallic lithium that detaches from the lithium metal anode, loses its electrical connection, and can no longer participate in subsequent electrochemical reactions during charge-discharge cycles. It mainly consists of lithium-containing compounds in the SEI film and metallic lithium encapsulated by the SEI film. The formation of dead lithium leads to a significant decrease in the coulombic efficiency and lifespan of the battery, severely limiting the development and application of batteries.

[0031] Preferably, the composite negative electrode further includes a functional layer, which includes a lithium-loving metal material; the lithium-loving metal material includes gallium.

[0032] Preferably, one surface of the metal layer is composited with a mullite fiber layer, and the other surface of the metal layer is composited with a functional layer.

[0033] The composite negative electrode provided in this solution has a functional layer containing a lithium-loving metal material deposited on the surface of the metal layer away from the mullite fiber layer. This lithium-loving metal material can spontaneously alloy with lithium metal, thereby constructing a lithium-loving metal-lithium (Li) alloy layer at the interface between the metal layer and the functional layer. This lithium-loving metal-Li alloy layer can induce lithium ions (Li... +Uniform deposition can suppress the formation of lithium dendrites in batteries using this composite negative electrode during cycling, and can also improve the wettability of the electrolyte to the composite negative electrode and promote the rapid transport of lithium ions.

[0034] Preferably, the thickness of the metal layer is 20~100 μm.

[0035] Preferably, the thickness of the mullite fiber layer is 15~50 μm.

[0036] Preferably, the thickness of the functional layer is 1~15 μm.

[0037] By controlling the thicknesses of the metal layer, mullite fiber layer, and functional layer in the composite negative electrode within the aforementioned range, firstly, a metal layer thickness between 20 and 100 μm ensures the energy density of the composite negative electrode while reducing the risk of structural pulverization or detachment from the mullite fiber layer due to excessive volume changes during cycling caused by an excessively thick metal layer. This improves the cycle stability and cycle life of the composite negative electrode. Secondly, a thickness between 15 and 50 μm... The mullite fiber layer with a thickness between μm ensures its ability to homogenize lithium-ion flow and electric field distribution, which is more conducive to the uniform deposition of lithium ions on its surface, suppresses the nucleation and growth of lithium dendrites, and significantly improves the safety performance of batteries using this composite negative electrode. Moreover, the pore volume of the mullite fiber layer within this thickness range matches the volume change of the metal layer within the same thickness range during cycling, which can effectively absorb stress, maintain the interfacial contact between the mullite fiber layer and the metal layer, reduce the impact of the overall expansion and contraction of the composite negative electrode on the battery during cyclic charging and discharging, and also improve the problem of excessively long lithium-ion transport paths and sharp increases in interfacial impedance caused by excessively thick mullite fiber layers, thereby improving the rate performance of the composite negative electrode. Thirdly, the functional layer with a thickness between 1 and 15 μm not only ensures the energy density of the composite negative electrode, but also further improves the uniformity of lithium-ion deposition on its surface to better suppress the formation of lithium dendrites. In summary, by controlling the thickness of the metal layer, mullite fiber layer, and functional layer to within the aforementioned range, the composite anode sheet possesses high energy density characteristics and synergistically improves the cycle stability, cycle life, safety performance, and rate performance of batteries using this composite anode sheet.

[0038] According to a second aspect of the present invention, a method for preparing the above-mentioned composite negative electrode sheet is provided, comprising the following steps: rolling a metal foil and a mullite fiber sheet to composite the metal foil and the mullite fiber sheet, thereby obtaining a composite negative electrode sheet.

[0039] In the method for preparing the composite negative electrode sheet provided in this scheme, metal foil and mullite fiber sheets are stacked and then rolled to combine them to obtain the composite negative electrode sheet. Firstly, the surface of mullite fibers has abundant Lewis acid sites, exhibiting excellent lithium-affinity properties. These lithium-affinity sites can effectively reduce the nucleation overpotential / barrier of lithium ions at the electrode or electrolyte interface and guide lithium ions to preferentially and uniformly adsorb and deposit on the surface of its three-dimensional fiber network. Secondly, the mullite fibers in the mullite fiber sheet interweave to form a porous and stable three-dimensional network structure. The abundant pores within this structure provide buffer space for volume changes during lithium metal deposition or stripping, effectively mitigating the volume change of the composite negative electrode sheet (i.e., the lithium metal negative electrode) during cycling, while also reducing the structural stress of the entire composite negative electrode sheet and maintaining its structural integrity. The composite anode exhibits several advantages. First, its structural integrity is enhanced by its three-dimensional fiber network, which provides ample deposition space for lithium ions, allowing lithium metal to deposit along the surface of the mullite fiber layer. Second, the three-dimensional network structure formed by the mullite fibers in the mullite fiber sheet helps to homogenize the lithium ion flow and current density distribution on the surface of the composite anode, inhibiting the localized preferential growth of lithium dendrites from both thermodynamic and kinetic perspectives, thus reducing the risk of lithium dendrites puncturing the separator. Third, the combination of mullite fiber sheets and metal foil forms a stable fiber-lithium contact interface, which is beneficial for forming a uniform solid electrolyte interphase (SEI) film. Simultaneously, the fiber network provides a certain physical constraint on deposited lithium, effectively reducing the generation and accumulation of dead lithium on the surface of the composite anode, thereby maintaining a high lithium ion diffusion flux and reversible capacity during cycling. Applying the composite anode prepared using the method provided in this scheme to lithium metal batteries can improve the cycle stability, safety, and cycle life of lithium metal batteries.

[0040] Preferably, the above-mentioned mullite fiber sheet is prepared by the following steps: preparing a slurry containing polyacrylamide, silica sol and short mullite fibers, drying the slurry to obtain a blank, and sintering the blank at 1300~1500℃ for 1~4 h to obtain a mullite fiber sheet; the mass ratio of polyacrylamide, silica sol and short mullite fibers is (0.5~2):(1~5):(0.4~4).

[0041] The mullite fiber sheet involved in this scheme is prepared by drying a slurry made of polyacrylamide, silica sol, and short mullite fibers in a specific mass ratio, and then sintering the resulting blank at 1300~1500℃ for 1~4 h. Firstly, in the slurry preparation process, polyacrylamide, silica sol, and short mullite fibers are used as raw materials. Polyacrylamide and silica sol can remove impurities from the short mullite fibers, preventing these impurities from affecting the performance of the resulting composite negative electrode sheet. Furthermore, the introduction of polyacrylamide promotes uniform dispersion of the short mullite fibers, while silica sol plays a crucial connecting role in the subsequent sintering process. Controlling the mass ratio of the three components within the aforementioned range ensures that the slurry has good dispersibility and viscosity, which is beneficial for the subsequent forming of the blank. Secondly, controlling the sintering temperature and time within the aforementioned range during the blank sintering process allows the silica sol to be fully converted into silicon dioxide and combined with the short mullite fibers. Localized sintering or the formation of strong chemical bonds on the surface of mullite fibers occurs. This sintering process preserves the lithium-affinity properties of the short mullite fibers while enabling better connection between the short mullite fibers and silica, greatly enhancing the overall mechanical strength and structural integrity of the three-dimensional network in the mullite fiber sheet. When the mullite fiber sheet obtained in this way is rolled and laminated with metal foil, the three-dimensional network can withstand greater pressure without collapsing, ensuring a tight and strong interfacial bond between the mullite fiber sheet and the metal foil. Furthermore, the mullite fiber sheet obtained under the above-mentioned sintering temperature and time has a moderate pore size distribution and porosity, thereby significantly improving the buffering performance of the composite negative electrode sheet against volume expansion during cyclic charging and discharging.

[0042] Preferably, in the preparation process of mullite fiber sheets, the sintering of the blank includes the following two stages: the blank is first sintered at 1300~1350℃ for 1~2 h, and then sintered at 1450~1500℃ for 1~2 h.

[0043] Preferably, the short mullite fibers have a length of 50~300 nm and a diameter of 30~100 nm.

[0044] In the preparation of mullite fiber sheets, short mullite fibers with a moderate aspect ratio are used. Firstly, this prevents them from precipitating in the slurry due to excessive size and avoids entanglement and agglomeration due to excessive length. This allows them to form a uniform and stable slurry with polyacrylamide and silica sol, which is beneficial for the subsequent preparation of the blank. Secondly, it can increase the specific surface area and interfacial activity of the prepared mullite fiber sheets, enhance the overall lithium affinity of the mullite fiber sheets, and further improve the uniformity of lithium ion deposition on their surface. Thirdly, it can optimize the pore structure of the prepared mullite fiber sheets, which can efficiently accommodate the small local volume expansion caused by lithium deposition and ensure sufficient electrolyte wetting and rapid lithium ion transport, thereby improving the structural stability and rate performance of the final composite negative electrode sheet.

[0045] Preferably, after the metal foil is composited with the mullite fiber sheet, the process further includes coating the surface of the metal foil away from the mullite fiber sheet with a dispersion of a lithium-philic metal material and drying it to form a functional layer on the surface of the metal foil.

[0046] Preferably, the lithium-ion metal material dispersion comprises a lithium-ion metal material and a dispersion medium; the lithium-ion metal material comprises gallium, and the dispersion medium comprises tetrahydrofuran, isopropanol, or water; the mass ratio of the lithium-ion metal material to the dispersion medium is (0.05~2):(10~20).

[0047] After rolling a metal foil and a mullite fiber sheet to composite them, a lithium-ion metal material dispersion prepared from a lithium-ion metal material and a dispersion medium is coated on the surface of the metal foil away from the mullite fiber sheet. Utilizing the cavitation effect generated by high-speed stirring and ultrasound, the lithium-ion metal undergoes a near-emulsification process at a temperature of 20–30°C, thereby forming a stable nanoscale dispersion near its melting point (Ga: 29.8°C) (Reference: Reversible Size Control of Liquid-Metal Nanoparticles under Ultrasonication, Akihisa Yamaguchi, Angew. Chem. Int. Ed. 2015, 54, 12809-12813). After drying, the lithium-ion metal material dispersion forms a functional layer. During the coating process, the lithium-ion metal material in the dispersion can rapidly and uniformly alloy with the lithium metal in the metal foil, thereby forming an alloy layer (i.e., a functional layer) on the surface of the metal foil. This functional layer can induce Li... + Uniform deposition suppresses lithium dendrite formation in batteries using this composite negative electrode during cycling and also improves the wettability of the electrolyte to the composite negative electrode, promoting rapid ion transport, thereby improving the safety and rate performance of the final composite negative electrode.

[0048] Preferably, the coating thickness of the lithium-loving metal material dispersion is 1~30 μm.

[0049] Preferably, the concentration of the lithiophilic metal material in the lithiophilic metal material dispersion is 0.05~5 g / L.

[0050] Preferably, the lithium-loving metal material dispersion further includes a dispersant, which includes at least one of xanthan gum, guar gum, and dodecyl mercaptan; the concentration of the dispersant in the lithium-loving metal material dispersion is 0.05~10 g / L.

[0051] In addition to the lithiophilic metal material and the dispersion medium, a specific dispersant is added during the preparation of the lithiophilic metal material dispersion, and the concentration of the dispersant in the lithiophilic metal material dispersion is controlled within the above-mentioned range. Xanthan gum and guar gum can encapsulate the lithiophilic metal material particles through steric hindrance, while dodecyl mercaptan can act as a surfactant to reduce the surface energy of the lithiophilic metal material through adsorption, thereby reducing the risk of agglomeration and sedimentation of the lithiophilic metal material in the dispersion medium, forming a stable and uniform lithiophilic metal material dispersion. This ensures the uniformity and density of the functional layer formed after coating, while also enhancing the bonding force between the functional layer and the metal layer, thus ensuring the structural stability of the final composite negative electrode sheet and extending the cycle life of the composite negative electrode sheet.

[0052] According to a third aspect of the present invention, a battery is provided, the battery comprising the above-described composite negative electrode sheet, or a composite negative electrode sheet prepared by the above-described method for preparing the composite negative electrode sheet.

[0053] Applying the composite negative electrode sheet provided by this invention or the composite negative electrode sheet prepared by the preparation method provided by this invention to batteries can improve the cycle stability, cycle life, safety performance and rate performance of batteries.

[0054] Example 1 A composite negative electrode sheet includes a functional layer, a metal layer and a mullite fiber layer sequentially laminated together, wherein the metal layer includes lithium metal, the functional layer includes a lithium-loving metal material, the thickness of the metal layer is 50 μm, the thickness of the mullite fiber layer is 20 μm, and the thickness of the functional layer is 10 μm. The composite negative electrode sheet provided in this embodiment is prepared through the following steps: S1. Mix 2.5 g of polyacrylamide and 10 g of silica sol and disperse them in 25 mL of pure water. Stir until the solution is uniform. Then add 5 g of short mullite fibers with a length of 50~300 nm and a diameter of 30~100 nm (purchased from Miya Refractory Materials Direct Sales Store, product name M-1600 polycrystalline mullite fiber cotton, white in color, melting point 1760℃, Al2O3+SiO2+ZrO2=100%) to the solution and continue stirring until uniform to prepare a slurry. The mass ratio of polyacrylamide, silica sol and short mullite fibers is 1:4:2, and the solid-liquid ratio (w / v) of polyacrylamide to pure water is 1 g:10 mL. S2. Pour the prepared slurry into the mold, let it stand for 1 hour, filter out the excess solution, and then put the mold into a 60°C oven to dry overnight (12 hours) to obtain the blank; S3. The dried blank is cut into the size of a conventional negative electrode sheet (area size of 3.5 cm × 4.5 cm) using a punching die. Finally, the cut blank is sandwiched between the sintering plates and placed in a tube furnace and sintered at 1300℃ and 1500℃ for 2 h respectively to obtain a mullite fiber sheet with a thickness of 18 μm. S4. The metal foil (pure lithium strip) and the mullite fiber sheet are combined by mechanical rolling to obtain a semi-finished product, which includes a lithium metal layer and a mullite fiber layer that are combined with each other. S5. According to the mass ratio of gallium (Ga) as a lithiophilic metal material, dodecanethiol as a dispersant, and tetrahydrofuran (THF) as a dispersion medium, Ga was added to THF. After stirring at 800 rpm for 12 h, the mixture was sonicated for 5 h to obtain a uniformly dispersed nanoscale Ga dispersion, which is a lithiophilic metal material dispersion. S6. Use a pipette to measure the prepared lithiophilic metal material dispersion and uniformly coat it on the surface of the lithium metal layer of the semi-finished product. Let it stand for 1 hour to ensure that the lithiophilic metal material dispersion and the surface of the lithium metal layer are in full contact to form a Li-Ga alloy layer (i.e., a functional layer) to obtain a composite negative electrode sheet.

[0055] Example 2 This embodiment provides a composite negative electrode sheet. Compared with Example 1, the difference in composition is that in step S1 of preparing the composite negative electrode sheet, the amount of polyacrylamide is 2.5 g, the amount of silica sol is 10 g, and the amount of short mullite fiber is 1 g (the mass ratio of polyacrylamide, silica sol, and short mullite fiber is 1:4:0.4).

[0056] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are consistent with those in Example 1.

[0057] Example 3 This embodiment provides a composite negative electrode sheet. Compared with Example 1, the difference in composition is that in step S1 of preparing the composite negative electrode sheet, the amount of polyacrylamide is 2.5 g, the amount of silica sol is 10 g, and the amount of short mullite fiber is 2.5 g (the mass ratio of polyacrylamide, silica sol, and short mullite fiber is 1:4:1).

[0058] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are consistent with those in Example 1.

[0059] Example 4 This embodiment provides a composite negative electrode sheet. Compared with Example 1, the difference in composition is that in step S1 of preparing the composite negative electrode sheet, the amount of polyacrylamide is 2.5 g, the amount of silica sol is 10 g, and the amount of short mullite fiber is 7.5 g (the mass ratio of polyacrylamide, silica sol, and short mullite fiber is 1:4:3).

[0060] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are consistent with those in Example 1.

[0061] Example 5 This embodiment provides a composite negative electrode sheet. Compared with Example 1, the difference in composition is that in step S1 of preparing the composite negative electrode sheet, the amount of polyacrylamide is 2.5 g, the amount of silica sol is 10 g, and the amount of short mullite fiber is 10 g (the mass ratio of polyacrylamide, silica sol, and short mullite fiber is 1:4:4).

[0062] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are consistent with those in Example 1.

[0063] Example 6 This embodiment provides a composite negative electrode sheet. Compared with Example 1, the difference in composition is that in step S1 of preparing the composite negative electrode sheet, the amount of polyacrylamide is 2.5 g, the amount of silica sol is 10 g, and the amount of short mullite fiber is 0.5 g (the mass ratio of polyacrylamide, silica sol, and short mullite fiber is 1:4:0.2).

[0064] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are consistent with those in Example 1.

[0065] Example 7 This embodiment provides a composite negative electrode sheet. Compared with Example 1, the difference in composition is that in step S1 of preparing the composite negative electrode sheet, the amount of polyacrylamide is 2.5 g, the amount of silica sol is 10 g, and the amount of short mullite fiber is 12.5 g (the mass ratio of polyacrylamide, silica sol, and short mullite fiber is 1:4:5).

[0066] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are consistent with those in Example 1.

[0067] Example 8 This embodiment provides a composite negative electrode sheet. Compared with Embodiment 1, the difference in composition is that the thickness of the metal layer is 15 μm, the thickness of the mullite fiber layer is 55 μm, and the thickness of the functional layer is 10 μm.

[0068] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are consistent with those in Example 1.

[0069] Example 9 This embodiment provides a composite negative electrode sheet. Compared with Embodiment 1, the difference in composition is that the thickness of the metal layer is 50 μm, the thickness of the mullite fiber layer is 10 μm, and the thickness of the functional layer is 20 μm.

[0070] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are consistent with those in Example 1.

[0071] Example 10 This embodiment provides a composite negative electrode sheet. Compared with Embodiment 1, the difference in composition is that in the preparation step S1 of the composite negative electrode sheet, the short mullite fibers used have a length of 20~40 nm and a diameter of 110~130 nm.

[0072] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are consistent with those in Example 1.

[0073] Example 11 This embodiment provides a composite negative electrode sheet. Compared with Embodiment 1, the difference in composition is that in the preparation step S1 of the composite negative electrode sheet, the short mullite fibers used have a length of 320~340 nm and a diameter of 10~20 nm.

[0074] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are consistent with those in Example 1.

[0075] Example 12 This embodiment provides a composite negative electrode sheet. Compared with Embodiment 1, the difference in composition is that in step S3 of the preparation of the composite negative electrode sheet, the sintering temperature of the blank is 1500℃ and the sintering time is 4 h.

[0076] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are consistent with those in Example 1.

[0077] Example 13 This embodiment provides a composite negative electrode sheet. Compared with Embodiment 1, the difference in composition is that in step S3 of the preparation of the composite negative electrode sheet, the sintering temperature of the blank is 1300℃ and the sintering time is 4 h.

[0078] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are consistent with those in Example 1.

[0079] Example 14 This embodiment provides a composite negative electrode sheet. Compared with Embodiment 1, the difference in structure is that the preparation step S5 of the composite negative electrode sheet is different, as detailed below: S5. According to the mass ratio of lithiophilic metal material gallium (Ga): dispersant xanthan gum: dispersion medium water = 1:1:20, Ga and xanthan gum are added to water, stirred at 800 rpm for 12 h, and then sonicated for 5 h to obtain a uniformly dispersed nanoscale Ga dispersion, that is, a lithiophilic metal material dispersion.

[0080] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are consistent with those in Example 1.

[0081] Example 15 This embodiment provides a composite negative electrode sheet. Compared with Embodiment 1, the difference in structure is that the preparation step S5 of the composite negative electrode sheet is different, as detailed below: S5. Add Ga to THF at a mass ratio of 1:20 for the lithiophilic metal material gallium (Ga): the dispersion medium tetrahydrofuran (THF). Stir at 800 rpm for 12 h and then sonicate for 5 h to obtain a uniformly dispersed nanoscale Ga dispersion, i.e., a lithiophilic metal material dispersion.

[0082] Apart from the differences mentioned above, the materials, formulation ratios, and preparation operations used in this embodiment are consistent with those in Example 1.

[0083] Comparative Example 1 A composite negative electrode sheet includes a functional layer and a metal layer that are composited with each other, wherein the metal layer includes lithium metal, the functional layer includes a lithium-loving metal material, the thickness of the metal layer is 50 μm, and the thickness of the functional layer is 10 μm. The composite negative electrode sheet provided in this embodiment is prepared through the following steps: S1. Add Ga to THF at a mass ratio of 1:20 for the lithiophilic metal material gallium (Ga): the dispersion medium tetrahydrofuran (THF). Stir at 800 rpm for 12 h and then sonicate for 5 h to obtain a uniformly dispersed nanoscale Ga dispersion, i.e., a lithiophilic metal material dispersion. S2. Using a pipette, measure the prepared lithiophilic metal material dispersion and uniformly coat it onto the surface of the lithium-copper composite strip (as a lithium metal layer, i.e., a lithium metal layer is plated on the surface of the copper foil). Let it stand for 1 hour to ensure that the lithiophilic metal material dispersion and the surface of the lithium metal layer are in full contact to form a Li-Ga alloy layer (i.e., a functional layer), thus obtaining a composite negative electrode sheet.

[0084] Test case 1. Participants This test example applies the composite negative electrode sheets prepared in Examples 1-15 and Comparative Example 1 to lithium metal batteries and performs relevant performance tests on the lithium metal batteries.

[0085] Lithium metal batteries are prepared according to the following steps: (1) The positive electrode active material LiNi 0.6 Co 0.1 Mn 0.3 O2 (NCM613) was mixed with binder polyvinylidene fluoride (PVDF), conductive carbon black and conductive graphite in a mass ratio of 97.6:0.9:0.5:1.1 and then dispersed in N-methylpyrrolidone (NMP) to prepare a positive electrode slurry. The positive electrode slurry was coated on both surfaces of the positive electrode current collector copper foil to form a positive electrode active material layer. After drying, a positive electrode sheet was obtained and cut into a size of 3.2 cm * 4.2 cm. (2) The composite negative electrodes prepared in Examples 1-15 and Comparative Example 1 were cut into sizes of 3.5 cm * 4.5 cm respectively; (3) A polyethylene (PE) film with a thickness of 12 μm and a size of 4 cm * 5 cm is used as the separator; (4) Ethylene glycol dimethyl ether (DME) and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) were mixed at a volume ratio of 1:1 to obtain an organic solvent. Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was added to the organic solvent to prepare an electrolyte with a lithium salt concentration of 1 M. (5) The battery is assembled in the order of “positive electrode shell - positive electrode sheet - separator - composite negative electrode sheet - negative electrode shell”, injected with electrolyte, and after encapsulation and formation steps, a soft-pack lithium metal battery is obtained.

[0086] 2. Test Content (1) Cyclic performance The test temperature was 25℃, and the test voltage range was 2.8~4.25 V. The lithium metal batteries using the composite negative electrode sheets of Examples 1~16 and Comparative Example 1 were connected to the charging cabinet. First, they were charged with a constant current of 0.1C to 4.3 V, and then left to stand for 5 min. After that, they were discharged with a constant current of 0.1C to 2.8 V and left to stand for 5 min. This step was repeated twice, and then left to stand for 30 min to complete the formation step. Then, they were charged with a constant current of 0.33C to 4.25 V and then charged with a constant voltage until the current was ≤0.05C. After that, they were left to stand for 5 min. After that, they were discharged with a constant current of 0.5C to 2.8 V and left to stand for 5 min. This step was repeated until the capacity retention rate of the lithium metal battery was 80%, and the number of cycles was recorded.

[0087] (2) Safety performance To maintain the consistency of lithium metal battery cycling, the lithium metal battery is placed in a glass clamp. The glass clamp does not affect the test results. First, the thickness of the initial lithium metal battery is measured. Referring to the cycling performance test method in (1), the thickness of the lithium metal battery is measured again after cycling. The expansion rate of the battery during cycling is calculated according to the following formula: Expansion rate of lithium metal battery during cycling = (thickness of lithium metal battery after cycling - thickness of lithium metal battery before cycling) / thickness of battery before cycling. The expansion rate of lithium metal battery during cycling is used to characterize the safety performance of lithium metal battery. The smaller the expansion rate, the better the safety performance of lithium metal battery. Conversely, the larger the expansion rate, the worse the safety performance of lithium metal battery.

[0088] (3) Ratio performance The following steps were used to conduct a rate discharge test on the lithium metal battery: The test temperature was 25℃, and the test voltage range was 2.8~4.25 V. The lithium metal battery was connected to the charging cabinet and first charged with a constant current of 0.1C to 4.3 V. After resting for 5 minutes, it was discharged with a constant current of 0.1C to 2.8 V and then rested for 5 minutes. This step was repeated twice, and then the battery was rested for 30 minutes to complete the formation step. After charging with a constant current of 0.1C to 4.25 V, it was charged with a constant voltage until the current was ≤0.05 C and then rested for 5 minutes. After discharging with a constant current of 0.1C to 2.8 V, it was rested for 5 minutes. After each charge and discharge cycle, the charging current density was kept constant, and the discharge current density was increased sequentially to 0.2C / 0.5C / 1C / 3C / 5C. After the 5C discharge cycle, the rate charge and discharge test was completed. The rate discharge efficiency of the lithium metal battery was calculated according to the following formula: Rate discharge efficiency = Discharge capacity of lithium metal battery at different discharge rates / Charge capacity of lithium metal battery at the corresponding charging rate.

[0089] 3. Experimental Results Table 1. Performance test results of lithium metal batteries

[0090] The composite negative electrode sheets prepared in Examples 1-15 and Comparative Example 1 were applied to lithium metal batteries, and the cycle performance, safety performance and rate performance of the prepared lithium metal batteries were tested. The results are shown in Table 1.

[0091] The composite negative electrode sheet provided in Comparative Example 1 is directly prepared by combining a functional layer and a metal layer, without containing a mullite fiber layer. Compared with Comparative Example 1, the composite negative electrode sheets provided in Examples 1-14 include a functional layer, a metal layer, and a mullite fiber layer sequentially combined. Test results show that the lithium metal battery using the composite negative electrode sheets provided in Examples 1-7 has higher cycle performance and rate discharge efficiency than Comparative Example 1. This is mainly because, in the composite negative electrode sheets provided in Examples 1-14, on the one hand, the mullite fibers in the mullite fiber layer interweave to form a porous and stable three-dimensional network structure. The abundant pores inside can provide buffer space for volume changes during lithium metal deposition or stripping, effectively mitigating the volume changes of the composite negative electrode sheet during cycling. Furthermore, the composite of the mullite fiber layer and the lithium-containing metal layer forms a stable fiber-lithium contact interface, which is beneficial for the formation of a uniform solid electrolyte interphase (SEI) film during cycling. Simultaneously, the fiber network provides physical confinement for deposited lithium, effectively reducing the generation and accumulation of dead lithium on the surface of the composite negative electrode. This allows the lithium metal to maintain a high lithium-ion diffusion flux and reversible capacity during cycling. On the other hand, a functional layer containing a lithium-philic metal material is placed on the surface of the metal layer away from the mullite fiber layer. This lithium-philic metal material can spontaneously alloy with the lithium metal, thus constructing a lithium-philic metal-lithium (Li) alloy layer at the interface between the metal layer and the functional layer. This lithium-philic metal-Li alloy layer can induce lithium-ion (Li... + Uniform deposition can suppress the formation of lithium dendrites in batteries using this composite negative electrode during cycling, and can also improve the wettability of the electrolyte to the composite negative electrode and promote the rapid transport of lithium ions. Thus, through the above two effects, the cycle performance and rate performance of lithium metal batteries using the composite negative electrode provided in Examples 1 to 14 are improved.

[0092] Furthermore, the performance test results of lithium metal batteries using the composite anode sheets in Examples 1-7 show that when sintered mullite short fiber sheets are used as current collectors, the three-dimensional structure of the mullite short fibers can provide channels for lithium-ion deposition and enhance the mechanical strength of the anode material. However, when there is too much mullite material, Al... 3+ Ions expand in volume during the later stages of cycling, causing lithium metal batteries to expand, reducing battery cycle stability and weakening cycle performance. When there is too little mullite material, the lithium affinity of the current collector decreases, and lithium dendrites cannot be deposited more uniformly in the three-dimensional current collector. As cycling progresses, lithium dendrites will produce a tip effect, increasing battery impedance and deteriorating cycle performance.

[0093] The performance test results of lithium metal batteries using the composite negative electrode sheets in Examples 1, 8, and 9 show that when the thickness of the mullite fiber layer decreases, the difference in cycle performance is not significant. This is mainly because all the internal networks of the fiber layer can be effectively deposited. However, when the thickness of the fiber layer increases, the lower layer of the usable internal three-dimensional network cannot play a role. Instead, the uneven stress inside the mullite fiber layer due to the overfilling of the upper layer and the underfilling of the lower layer leads to phenomena such as breakage of the current collector in the mullite fiber layer.

[0094] Performance test results of lithium metal batteries using composite negative electrode sheets from Examples 1, 10, and 11 show that when the diameter and length of the mullite fiber are too small, it cannot form an effective three-dimensional network. Instead, the complex internal network leads to uneven lithium deposition. When the diameter and length of the mullite fiber are too large, its internal structure has high rigidity, which can provide support and reduce volume expansion in the early stages of cycling. However, as cycling progresses, the uneven deposition of lithium causes excessive rigidity of the mullite fiber current collector, resulting in stress concentration and ultimately negative electrode sheet breakage, leading to battery safety issues.

[0095] The performance test results of the lithium metal battery using the composite negative electrode sheets in Examples 1, 13, and 14 show that when the one-step heating sintering process of mullite fibers is too high or too low, it will cause uneven dispersion or adhesion of mullite fibers, affecting the mechanical properties of mullite fibers and ultimately affecting the cycle performance of the battery.

[0096] The performance test results of the lithium metal battery using the composite negative electrode sheets in Examples 1 and 15 show that when a dispersant is used, the Ga solution can be well dispersed, reducing gallium agglomeration and making the gallium dispersion more uniform. This helps to improve the deposition uniformity of the negative electrode material. Specifically, the thiol groups (-SH) in an appropriate amount of dodecyl mercaptan strongly bind to the gallium surface oxide layer / metal sites to form a dense self-assembled monolayer (SAM), which effectively inhibits droplet aggregation, sedimentation and oxidation thickening.

[0097] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention, but such modifications or substitutions are all within the scope of protection of the present invention.

Claims

1. A composite negative electrode, characterized in that: The composite negative electrode sheet includes a metal layer and a mullite fiber layer, wherein the metal layer is composite with the mullite fiber layer; the metal layer includes lithium metal.

2. The composite negative electrode sheet as described in claim 1, characterized in that: The composite negative electrode also includes a functional layer, which comprises a lithium-loving metal material, including gallium.

3. The composite negative electrode sheet as described in claim 2, characterized in that: One surface of the metal layer is composited with the mullite fiber layer, and the other surface of the metal layer is composited with the functional layer.

4. The composite negative electrode sheet as described in claim 2, characterized in that: The thickness of the metal layer is 20~100 μm, and / or the thickness of the mullite fiber layer is 15~50 μm, and / or the thickness of the functional layer is 1~15 μm.

5. The method for preparing the composite negative electrode sheet according to any one of claims 1 to 4, characterized in that, Includes the following steps: The metal foil and the mullite fiber sheet are rolled together to form a composite negative electrode sheet.

6. The method for preparing the composite negative electrode sheet as described in claim 5, characterized in that, The mullite fiber sheet is prepared by the following steps: preparing a slurry containing polyacrylamide, silica sol and short mullite fibers, drying the slurry to obtain a blank, and sintering the blank at 1300~1500℃ for 1~4 h to obtain the mullite fiber sheet. The mass ratio of the polyacrylamide, the silica sol, and the short mullite fibers is (0.5~2):(1~5):(0.4~4), and / or the short mullite fibers have a length of 50~300 nm and a diameter of 30~100 nm.

7. The method for preparing the composite negative electrode sheet as described in claim 5, characterized in that: After the metal foil is composited with the mullite fiber sheet, the process further includes coating the surface of the metal foil away from the mullite fiber sheet with a dispersion of a lithium-philic metal material and drying it to form a functional layer on the surface of the metal foil.

8. The method for preparing the composite negative electrode sheet as described in claim 7, characterized in that: The lithium-loving metal material dispersion includes a lithium-loving metal material and a dispersion medium; The lithium-loving metal material includes gallium, and the dispersion medium includes tetrahydrofuran, isopropanol, or water; The mass ratio of the lithiophilic metal material to the dispersion medium is (0.05~2):(10~20).

9. The method for preparing the composite negative electrode sheet as described in claim 8, characterized in that: The lithium-loving metal material dispersion further includes a dispersant, which includes at least one of xanthan gum, guar gum, and dodecyl mercaptan. The concentration of the dispersant in the lithium-loving metal material dispersion is 0.05~10 g / L.

10. A battery, characterized in that: The battery includes a composite negative electrode sheet as described in any one of claims 1 to 4, or a composite negative electrode sheet prepared by the method for preparing a composite negative electrode sheet as described in any one of claims 5 to 9.

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