Composite negative electrode all-solid-state lithium battery and preparation method thereof

Abstract

The invention belongs to the technical field of electrochemical energy storage, and discloses a composite negative electrode all-solid-state lithium battery and a preparation method thereof.The composite negative electrode all-solid-state lithium battery comprises a negative electrode current collector, a composite negative electrode, an artificial solid electrolyte interface layer, a gradient function interface layer, a block solid electrolyte layer, a positive electrode and a packaging buffer layer. The flexible buffer layer can form ultra-tight physical contact with the rough or volume-variable silicon-based negative electrode and the lithium metal negative electrode, gradient components of the gradient transition layer can effectively buffer chemical potential difference, the inorganic connection layer and the bulk solid electrolyte have good compatibility, the three layers cooperate to greatly reduce interface impedance, and the lithium metal negative electrode can be used for solving the problem of poor interface stability. The gradient component design of the gradient transition layer is combined with the inorganic particles subjected to stabilization treatment, and the pre-constructed artificial solid electrolyte interface layer is matched, so that harmful reaction between the solid electrolyte and a negative electrode active material can be effectively inhibited, interface defects are reduced, and excellent interface stability is guaranteed.

Description

Technical Field

[0001] This invention relates to the field of electrochemical energy storage technology, and in particular to a composite negative electrode all-solid-state lithium battery and its preparation method. Background Technology

[0002] All-solid-state lithium batteries are considered the next generation of lithium-ion battery technology due to their potential high energy density, high safety and long cycle life.

[0003] The commercialization of all-solid-state lithium batteries still faces many key technological challenges, mainly in terms of poor solid-solid interface contact, poor interface stability, and poor adaptability to volume changes. Poor solid-solid interface contact is due to poor physical contact between the rigid solid electrolyte and electrode materials, especially the negative electrode, resulting in high interface impedance and uneven current distribution. Poor interface stability is due to the fact that side reactions easily occur between the electrode active materials, especially high-capacity silicon-based and lithium metal negative electrodes, and the rigid solid electrolyte, forming harmful interface layers (such as lithium dendrites and chemical decomposition layers), which deteriorate the interface and lead to capacity decay and safety hazards. Poor adaptability to volume changes is due to the fact that the huge volume changes of silicon-based negative electrodes or lithium metal during charging and discharging can easily damage the brittle rigid solid electrolyte layer, leading to interface contact failure.

[0004] In summary, regarding the core issues of poor solid-solid interface contact, poor interface stability, and poor adaptability to volume changes in the existing technologies, existing solutions such as single-layer solid electrolyte structures, simple electrode surface coatings, and mixed solid-liquid electrolyte systems are all unable to effectively solve the problems of interface contact, stability, and volume change adaptability at the same time. Moreover, some solutions have drawbacks such as complex processes, high costs, or sacrifice of the core advantages of all-solid-state lithium batteries, such as intrinsic safety and high energy density, which restricts the industrial application of all-solid-state lithium batteries.

[0005] Application number CN112133967A discloses an all-solid-state lithium-sulfur battery, comprising, from top to bottom, a positive electrode layer, a positive electrode buffer layer, a sulfur-based solid electrolyte layer, a negative electrode buffer layer, and a negative electrode layer. The preparation method of the all-solid-state lithium-sulfur battery includes the following steps: Step 1: Preparing the positive electrode material; Step 2: Preparing the solid electrolyte sheet; Step 3: Preparing the deposited solid electrolyte sheet; Step 4: Battery assembly. The all-solid-state lithium-sulfur battery prepared by this invention, by adding a positive electrode buffer layer and a negative electrode buffer layer between the positive and negative electrodes and the solid electrolyte layer, can effectively overcome the interfacial reaction between the electrolyte and the lithium metal negative electrode, effectively suppressing the increase in battery internal resistance, thereby significantly improving battery cycle performance. Furthermore, the use of positive and negative electrode buffer layers can effectively prevent the formation of lithium dendrites during battery electrochemical cycling; therefore, the all-solid-state battery of this invention has better electrochemical cycle stability.

[0006] Current all-solid-state lithium batteries generally have an energy density below 350Wh / kg and a cycle life of less than 500 cycles with an 80% capacity retention rate, failing to meet the demands of commercial applications. The aforementioned solutions are all difficult to simultaneously and effectively address the issues of interface contact, stability, and volume change adaptation. Furthermore, some solutions suffer from complex processes, high costs, or sacrifice the inherent safety and high energy density of all-solid-state lithium batteries, thus hindering their industrial application. Summary of the Invention

[0007] This invention provides a composite negative electrode all-solid-state lithium battery. By constructing a gradient functional interface layer at the negative electrode or electrolyte interface and combining it with a composite negative electrode structure, this invention significantly improves interface contact, suppresses side reactions, and adapts to volume changes, thereby enhancing battery performance.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: a composite negative electrode all-solid-state lithium battery, wherein the battery is stacked in sequence from negative electrode to positive electrode, including: negative electrode current collector, composite negative electrode, artificial solid electrolyte interface layer, gradient functional interface layer, bulk solid electrolyte layer, positive electrode and encapsulation buffer layer.

[0009] The gradient functional interface layer consists of a flexible buffer layer, a gradient transition layer, and an inorganic bonding layer in sequence from the negative electrode side to the positive electrode side, which is used to improve the contact between the electrode and the electrolyte interface and enhance the interface stability.

[0010] As a further improvement of the present invention: the composite negative electrode is coated-dried-compacted onto the upper surface of the negative electrode current collector through a coating-drying-compacting process, and the two form an integrated structure. The composite negative electrode is composed of an active material, a conductive agent, and a binder. The active material is selected from either a silicon-based composite material or a pre-lithiated composite lithium metal material. The silicon-based composite material is a silicon oxide carbon or silicon-carbon composite material. The pre-lithiated composite lithium metal material is a composite structure formed by depositing lithium on a three-dimensional porous current collector. The thickness of the composite negative electrode is 30-50 micrometers. This thickness range is the balance range between electrode energy density and structural stability. When the thickness is less than 30 micrometers, the capacity density is insufficient. When the thickness is greater than 50 micrometers, the interface peeling is easily caused by volume changes.

[0011] As a further improvement of the present invention: the conductive agent in the composite negative electrode is selected from superconducting carbon black or Ketjen black, with a mass fraction of 2% to 10%. This range is based on the balance requirement between the formation of conductive network and the retention of ion channels. When it is less than 2%, the conductivity is insufficient, and when it is more than 10%, it will block lithium ion transport. The binder is selected from sodium carboxymethyl cellulose-styrene-butadiene rubber composite binder or polyvinylidene fluoride, with a mass fraction of 1% to 5%. This range is suitable for the requirements of electrode density and interface adhesion. When it is less than 1%, it cannot effectively bind the components, and when it is more than 5%, it will reduce the electrode porosity. When the active material of the composite negative electrode is a silicon-based composite material, the oxygen-silicon atomic ratio in silicon oxide carbon is 0.5 to 1.5. This ratio is the range of suitable conditions for the degree of silicon oxidation, structural stability, and capacity. When the oxygen-silicon atomic ratio is less than 0.5, the volume expansion rate of silicon is likely to exceed 200%, and when it is greater than 1.5, the capacity density is less than 300 mAh / g. The mass fraction of silicon in the silicon-carbon composite material is 30% to 70%. This range is the synergistic range of conductive network construction and volume expansion suppression. When the silicon mass fraction is less than 30%, the capacity improvement is not significant, and when it is greater than 70%, a stable conductive framework cannot be formed. The negative electrode current collector is a copper foil with a thickness of 8 to 15 micrometers. This thickness is within the range of electron conduction efficiency and mechanical strength. When it is less than 8 micrometers, the support is insufficient, and when it is greater than 15 micrometers, the proportion of inactive mass of the battery increases. When the active material of the composite negative electrode is a pre-lithiated composite lithium metal material, the three-dimensional porous current collector is a three-dimensional porous copper current collector with a pore size of 1 to 10 micrometers. This pore size range is the balance range between capillary action and mechanical support capability. When the pore size is less than 1 micrometer, the lithium deposition channel is blocked, and when it is greater than 10 micrometers, lithium dendrites cannot be effectively suppressed. The lithium deposition amount is 0.5 to 2 mg / cm². This range is designed according to the capacity matching of the positive electrode. When the deposition amount is less than 0.5 mg / cm², lithium deficiency will occur, leading to capacity decay. When it is greater than 2 mg / cm², there is a risk of lithium dendrite penetration.

[0012] As a further improvement of the present invention: the artificial solid electrolyte interface layer is directly laminated onto the upper surface of the composite negative electrode by magnetron sputtering or atomic layer deposition, with a thickness of less than 50 nanometers. The artificial solid electrolyte interface layer is a dense and ionically conductive thin film, and the material is selected only from one of lithium fluoride, lithium nitride, or lithium nitrophosphate, with an ionic conductivity of not less than [value missing]. S / cm.

[0013] As a further improvement of the present invention: the flexible buffer layer is based on a flexible polymer electrolyte or ionic liquid, and uniformly disperses an electronic conductive agent and a lithium salt. The polymer electrolyte is polyethylene oxide-bis(trifluoromethanesulfonyl)imide lithium, and the molecular weight of polyethylene oxide is 10,000 to 100,000. This molecular weight range is the balance range between chain segment mobility and ion conduction efficiency. When the molecular weight is below 10,000, the ion migration rate is significantly reduced, and when the molecular weight is above 100,000, the mechanical stability is insufficient. The ionic liquid is 1-propyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt, the electronic conductive agent is superconducting carbon black, and the thickness of the flexible buffer layer is 5-20 micrometers. This thickness is the appropriate range for buffer capacity and ion conduction efficiency. When it is less than 5 micrometers, it cannot completely absorb the volume change of the negative electrode, and when it is greater than 20 micrometers, the interfacial impedance increases significantly.

[0014] As a further improvement of the present invention: In the flexible buffer layer, the mass ratio of polymer electrolyte to ionic liquid is 10:0 to 0:10. This ratio is determined by the synergistic regulation of flexibility and ionic conductivity, which can adapt to the volume change characteristics of different negative electrode materials. The mass fraction of electronic conductive agent is 0.5% to 5%. This range is based on the balance requirements of electronic conduction and ion channels. When it is below 0.5%, an effective conductive network cannot be formed, and when it is above 5%, ion transport will be blocked. The molar concentration of lithium salt is 0.5 to 2 mol / L. This concentration is determined based on ionic conductivity and solubility. When it is below 0.5 mol / L, the ion concentration is insufficient, and when it is above 2 mol / L, lithium salt precipitation is likely to occur. The lithium salt is selected only from lithium bis(trifluoromethanesulfonyl)imide or lithium bis(fluorosulfonyl)imide. The selected lithium salt has good compatibility with polymer electrolyte and ionic liquid and will not undergo decomposition reaction.

[0015] As a further improvement of the present invention: the thickness of the inorganic bonding layer is 5 to 15 micrometers. The thickness is determined according to the bonding requirements with the bulk solid electrolyte. When it is less than 5 micrometers, the interface defects increase, and when it is greater than 15 micrometers, the interface impedance increases. The inorganic bonding layer is mainly composed of inorganic solid electrolyte nanoparticles that are compatible with the bulk solid electrolyte layer, accounting for more than or equal to 90% by mass, and contains a very small amount of polymer binder. This proportion is determined based on the compatibility with the bulk solid electrolyte and the mechanical strength requirements. If it is less than 90%, good bonding with the bulk solid electrolyte cannot be guaranteed. In the gradient transition layer, the inorganic solid electrolyte nanoparticles are selected from one of the following: lithium phosphide-sulfur chloride, garnet-type lithium lanthanum zirconium oxide, or phosphate-type lithium aluminum titanium phosphorus. The particle size is 50-500 nanometers. This particle size range is determined based on dispersibility and ion conduction efficiency. When the particle size is less than 50 nanometers, it is easy to agglomerate. When the particle size is greater than 500 nanometers, it is impossible to form a continuous gradient. The surface coating layer is lithium phosphate or aluminum oxide. The coating layer thickness is 1-10 nanometers. This thickness is determined by balancing compatibility improvement and ion conduction. When the thickness is less than 1 nanometer, the coating is incomplete. When the thickness is greater than 10 nanometers, it will block ion transport.

[0016] As a further improvement of the present invention: the polymer binder in the inorganic bonding layer is selected from one of polyethylene oxide, polyvinylidene fluoride-hexafluoropropylene copolymer, polymethyl methacrylate or polyvinylpyrrolidone, and has no chemical reaction with the bulk solid electrolyte layer; The bulk solid electrolyte layer is a sulfide solid electrolyte, oxide solid electrolyte, or composite solid electrolyte, with a thickness of 50–200 micrometers and an ionic conductivity of not less than [value missing]. S / cm, formed by pre-sintering, with a sintering temperature of 300~800℃.

[0017] As a further improvement of the present invention: the positive electrode is composited onto the surface of the bulk solid electrolyte layer by a coating-drying-hot pressing process, and is composed of active material, composite solid electrolyte, conductive agent and binder. The positive electrode active material is selected from at least one of high nickel nickel cobalt manganese oxide, lithium-rich manganese-based material or lithium cobalt oxide, wherein the high nickel nickel cobalt manganese oxide is NCM811 or NCM622, and the lithium-rich manganese-based material is a composite system of lithium manganese oxide and lithium nickel cobalt manganese oxide. The mass fraction of the composite solid electrolyte is 10% to 30%. This range is based on the balance requirements between ion conduction and electrode mechanical strength. Below 10%, ion conduction is insufficient, and above 30%, the positive electrode capacity density will be reduced. The mass fraction of the conductive agent is 2% to 8%, and the mass fraction of the binder is 1% to 5%. The thickness of the positive electrode is 40 to 100 micrometers. This thickness is the appropriate range for capacity density and ion conduction efficiency. When the thickness is less than 40 micrometers, the capacity is insufficient, and when the thickness is greater than 100 micrometers, the interfacial impedance increases. The positive current collector is an aluminum foil with a thickness of 10-20 micrometers, and the encapsulation buffer layer has a thickness of 10-50 micrometers. This thickness is determined based on the encapsulation structure size and the volume change of the battery during charging and discharging, ensuring that there is no interface compression failure after encapsulation.

[0018] A method for preparing a composite anode all-solid-state lithium battery, comprising the above-mentioned composite anode all-solid-state lithium battery, includes the following steps: S1. Negative electrode pretreatment: The composite negative electrode slurry is uniformly coated on the negative electrode current collector, and then dried and compacted to obtain the composite negative electrode. Optionally, an artificial solid electrolyte interface layer is deposited on the surface of the composite negative electrode by magnetron sputtering or atomic layer deposition. S2. Preparation of gradient functional interface layer: In the order from negative electrode side to positive electrode side, a flexible buffer layer, a gradient transition layer and an inorganic bonding layer are prepared sequentially on the surface of artificial solid electrolyte interface layer or composite negative electrode. S3, Bulk Electrolyte Integration: The pre-sintered bulk solid electrolyte layer is bonded to the upper surface of the inorganic bonding layer by hot pressing (60-200℃, 5-30MPa, 10-60 minutes) or solvent-assisted bonding. S4. Positive electrode integration: A positive electrode slurry is coated on the side of the bulk solid electrolyte layer away from the gradient functional interface layer, dried at 80-120℃ for 2-6 hours, and hot-pressed at 100-180℃ and 5-20MPa for 10-30 minutes to form the positive electrode. S5. Encapsulation buffer layer setting: A packaging buffer layer is reserved on the top layer of the positive electrode, and the battery is encapsulated in a dry argon atmosphere with a water and oxygen content of less than or equal to 10ppm.

[0019] Compared with the prior art, the advantages and positive effects of the present invention are as follows: 1. This invention addresses the problem of poor solid-solid interface contact. The flexible buffer layer can form an ultra-close physical contact with the rough or volume-changing silicon-based anode and lithium metal anode. The gradient component of the gradient transition layer can effectively buffer the chemical potential difference. The inorganic connecting layer has good compatibility with the bulk solid electrolyte. The three work together to significantly reduce the interface impedance.

[0020] 2. This invention addresses the problem of poor interface stability by using a gradient transition layer with gradient components designed and stabilized inorganic particles, combined with a pre-constructed artificial solid electrolyte interface layer. This effectively suppresses harmful reactions between the solid electrolyte and the negative electrode active material, reduces interface defects, and ensures excellent interface stability.

[0021] 3. This invention addresses the problem of poor adaptability to volume changes. The high deformability of the flexible buffer layer and the modulus gradient of the gradient transition layer can fully absorb the stress of volume changes in the negative electrode, preventing cracking of the solid electrolyte layer. Furthermore, the synergistic effect of the high-modulus inorganic particles in the gradient transition layer and inorganic bonding layer with the gradient design can physically block lithium dendrite penetration, achieving good adaptation to volume changes and effective suppression of lithium dendrites. In addition, the polymer electrolyte or ionic liquid inside the gradient functional interface layer (especially the flexible buffer layer and gradient transition layer) provides continuous ion channels. Combined with the gradient-changing inorganic particles, it ensures smooth lithium ion migration from the negative electrode to the bulk solid electrolyte, resulting in high ion transport efficiency.

[0022] 4. Through the synergistic design of composite negative electrode and gradient functional interface layer, the present invention achieves a battery energy density >400Wh / kg and a capacity retention rate of ≥83% after 1000 cycles, which is far superior to existing all-solid-state lithium batteries, and balances high safety and long cycle stability. Attached Figure Description

[0023] Figure 1 This invention presents a schematic diagram of the overall three-dimensional structure of a composite negative electrode all-solid-state lithium battery and its preparation method. Figure 2 This is a side-view three-dimensional structural diagram of a composite negative electrode all-solid-state lithium battery and its preparation method proposed in this invention; Figure 3 This is a process flow diagram of the method for preparing a composite negative electrode all-solid-state lithium battery according to the present invention.

[0024] Legend: 1. Negative electrode current collector; 2. Composite negative electrode; 3. Artificial solid electrolyte interface layer; 4. Gradient functional interface layer; 401. Flexible buffer layer; 402. Gradient transition layer; 403. Inorganic bonding layer; 5. Bulk solid electrolyte; 6. Positive electrode; 7. Encapsulation buffer layer. Detailed Implementation

[0025] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. 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.

[0026] Example 1 like Figures 1 to 2 As shown, the present invention provides a composite negative electrode all-solid-state lithium battery, which is stacked sequentially from negative electrode to positive electrode, including: negative electrode current collector 1, composite negative electrode 2, artificial solid electrolyte interface layer 3, gradient functional interface layer 4, bulk solid electrolyte layer 5, positive electrode 6, and encapsulation buffer layer 7; the gradient functional interface layer 4 is composed of a flexible buffer layer 401, a gradient transition layer 402, and an inorganic bonding layer 403 in sequence from the negative electrode side to the positive electrode side, which is used to improve the contact between the electrode and electrolyte interface and enhance the interface stability.

[0027] Furthermore, such as Figures 1 to 2 As shown, the composite negative electrode 2 is coated, dried and compacted onto the upper surface of the negative electrode current collector 1 through a coating-drying-compacting process, and the two form an integrated structure. The composite negative electrode 2 is composed of active material, conductive agent and binder. The active material is selected from silicon-based composite material or pre-lithiation composite lithium metal material. The silicon-based composite material is silicon oxide carbon or silicon carbon composite material. The pre-lithiation composite lithium metal material is a composite structure formed by lithium deposition on a three-dimensional porous current collector. The thickness of the composite negative electrode 2 is 30 to 50 micrometers.

[0028] Furthermore, such as Figures 1 to 2 As shown, the conductive agent in the composite negative electrode 2 is selected from superconducting carbon black or Ketjen black, with a mass fraction of 2% to 10%. The binder is selected from sodium carboxymethyl cellulose-styrene-butadiene rubber composite binder or polyvinylidene fluoride, with a mass fraction of 1% to 5%. The negative electrode current collector 1 is copper foil with a thickness of 8 to 15 micrometers. When the active material of the composite negative electrode 2 is a silicon-based composite material, the oxygen-silicon atomic ratio in silicon oxide carbon is 0.5 to 1.5, and the mass fraction of silicon in the silicon-carbon composite material is 30% to 70%. When the active material of the composite negative electrode 2 is a pre-lithiated composite lithium metal material, the three-dimensional porous current collector is a three-dimensional porous copper current collector with a pore size of 1 to 10 micrometers, and the lithium deposition amount is 0.5 to 2 mg / cm².

[0029] Furthermore, such as Figures 1 to 2 As shown, the artificial solid electrolyte interface layer 3 is directly laminated onto the upper surface of the composite negative electrode 2 via magnetron sputtering or atomic layer deposition, with a thickness of less than 50 nanometers. The artificial solid electrolyte interface layer 3 is a dense and ionically conductive thin film, and its material is selected from only one of lithium fluoride, lithium nitride, or lithium nitrophosphate, with an ionic conductivity of not less than [value missing]. S / cm.

[0030] Furthermore, such as Figures 1 to 2 As shown, the flexible buffer layer 401 uses a flexible polymer electrolyte or ionic liquid as a substrate, and uniformly disperses an electronic conductive agent and lithium salt. The polymer electrolyte is polyethylene oxide-bis(trifluoromethanesulfonyl)imide lithium, with a molecular weight of 10,000 to 100,000 for polyethylene oxide. The ionic liquid is 1-propyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt, and the electronic conductive agent is superconducting carbon black. The thickness of the flexible buffer layer 401 is 5 to 20 micrometers. The flexible buffer layer 401 balances the buffer capacity and ion conduction efficiency, avoiding the inability to absorb volume changes due to excessive thinness or the increase in interfacial impedance due to excessive thickness. It has excellent deformation ability and can closely adhere to the negative electrode surface, especially the irregular surface of silicon particles or lithium metal, buffering volume changes, providing good initial wetting and interfacial contact, and providing a continuous channel for lithium-ion transport.

[0031] Furthermore, such as Figures 1 to 2 As shown, in the flexible buffer layer 401, the mass ratio of polymer electrolyte to ionic liquid is 10:0 to 0:10, the mass fraction of electronic conductive agent is 0.5% to 5%, the molar concentration of lithium salt is 0.5 to 2 mol / L, and the lithium salt is selected only from lithium bis(trifluoromethanesulfonyl)imide or lithium bis(fluorosulfonyl)imide. The thickness of the gradient transition layer 402 is 10 to 30 micrometers, and it is composed of polymer electrolyte, ionic liquid, and surface-modified inorganic solid electrolyte nanoparticles mixed in a gradient ratio. The mass fraction of inorganic solid electrolyte nanoparticles near the flexible buffer layer 401 is 10% to 30%, and the mass fraction near the inorganic connecting layer 403 is 60% to 80%. By buffering the chemical potential difference between the negative electrode and the bulk electrolyte and suppressing side reactions, and by gradually changing the modulus, the interlayer performance is smoothly transitioned, avoiding interface defects caused by sudden changes in performance.

[0032] Furthermore, such as Figures 1 to 2 As shown, the inorganic connecting layer 403 has a thickness of 5–15 micrometers. The inorganic connecting layer 403 is mainly composed of inorganic solid electrolyte nanoparticles with good compatibility with the bulk solid electrolyte layer 5, accounting for more than or equal to 90% by mass, and contains a very small amount of polymer binder. In the gradient transition layer 402, the inorganic solid electrolyte nanoparticles are selected from one of lithium phosphide sulfide, garnet-type lithium lanthanum zirconium oxide, or phosphate-type lithium aluminum titanium phosphide, with a particle size of 50–500 nanometers. The surface coating layer is lithium phosphate or aluminum oxide with a thickness of 1–10 nanometers, which improves the interfacial compatibility with the bulk electrolyte, reduces the interfacial impedance, and at the same time, the high modulus of the inorganic particles synergistically inhibits the penetration of lithium dendrites into the bulk solid electrolyte 5.

[0033] Furthermore, such as Figures 1 to 2As shown, the polymer binder in the inorganic connecting layer 403 is selected from one of polyethylene oxide, polyvinylidene fluoride-hexafluoropropylene copolymer, polymethyl methacrylate, or polyvinylpyrrolidone. The bulk solid electrolyte layer 5 is a sulfide solid electrolyte, oxide solid electrolyte, or composite solid electrolyte, with a thickness of 50–200 micrometers and an ionic conductivity of not less than [value missing]. S / cm, formed by pre-sintering at a temperature of 300-800℃, the bulk solid electrolyte layer 5 is the core channel for lithium-ion transport inside the battery, and the inorganic connecting layer 403 ensures the continuity of ion transport between the bulk electrolyte and the gradient functional interface layer.

[0034] Furthermore, such as Figures 1 to 2 As shown, the positive electrode 6 is composited onto the upper surface of the bulk solid electrolyte layer 5 through a coating-drying-hot pressing process. It consists of active materials, composite solid electrolyte, conductive agent, and binder. The positive electrode active material is selected from at least one of high-nickel lithium nickel cobalt manganese oxide, lithium-rich manganese-based materials, or lithium cobalt oxide. The mass fraction of the composite solid electrolyte is 10% to 30%, the mass fraction of the conductive agent is 2% to 8%, and the mass fraction of the binder is 1% to 5%. The thickness of the positive electrode 6 is 40 to 100 micrometers. The positive electrode current collector is aluminum foil with a thickness of 10 to 20 micrometers. The thickness of the encapsulation buffer layer 7 is 10 to 50 micrometers. The positive electrode 6 serves as a carrier for lithium-ion insertion and extraction, working synergistically with the composite negative electrode 2 to achieve high energy density. The encapsulation buffer layer provides space for volume changes in the battery as a whole, ensuring the structural integrity of the battery during cycling.

[0035] Specific parameters and test performance of this embodiment: In this embodiment, the composite negative electrode 2 uses silicon-carbon composite material with a silicon mass fraction of 50% as the active material, superconducting carbon black with a mass fraction of 5% as the conductive agent, sodium carboxymethyl cellulose-styrene-butadiene rubber composite binder with a mass fraction of 3%, and a thickness of 40 micrometers. The negative electrode current collector 1 is a 12-micrometer thick copper foil, and the artificial solid electrolyte interface layer 3 is a 30-nanometer thick lithium fluoride thin film prepared by magnetron sputtering with an ionic conductivity of 1.2 × 10⁻⁶ m / s. The flexible buffer layer 401, with a thickness of 10 micrometers, is based on a 5:5 mass ratio of polyethylene oxide-lithium bis(trifluoromethanesulfonyl)imide to 1-propyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt, containing 2% superconducting carbon black and 1 mol / L lithium bis(trifluoromethanesulfonyl)imide. The gradient transition layer 402 has a thickness of 20 micrometers and uses Li6PS5Cl nanoparticles with a particle size of 200 nanometers. The coating thickness is 5 nm, prepared by mixing in gradient proportions of 20%, 45%, and 70%. The inorganic bonding layer 403 is 10 μm thick and consists of 95% Li6PS5Cl nanoparticles and 5% PEO binder. The bulk solid electrolyte layer 5 is 100 μm thick pre-sintered Li6PS5Cl, sintered at 600℃, with an ionic conductivity of 1.5 × 10⁻⁶. S / cm, positive electrode 6 uses NCM811 as active material, contains 20% composite sulfide solid electrolyte, 5% superconducting carbon black, 3% PVDF binder, and has a thickness of 60 micrometers. The positive electrode current collector is a 15-micrometer thick aluminum foil. The encapsulation buffer layer 7 has a thickness of 30 micrometers.

[0036] Test performance: The battery in this embodiment has an energy density of 452Wh / kg, an interface impedance of 42Ω・cm², a capacity retention of 85% after 1000 cycles, a silicon-based anode volume expansion rate of 110%, no lithium dendrite penetration, and excellent safety performance.

[0037] Compared with existing technologies: all-solid-state lithium batteries with a single-layer solid electrolyte structure.

[0038] Battery structure: Negative electrode current collector copper foil, 12 micrometers - Silicon-carbon negative electrode silicon mass fraction 50%, thickness 40 micrometers - Li6PS5Cl bulk solid electrolyte 100 micrometers - Positive electrode NCM811, thickness 60 micrometers - Positive electrode current collector aluminum foil, 15 micrometers, no artificial solid electrolyte interface layer and gradient functional interface layer.

[0039] Test performance: Energy density 358Wh / kg, interface impedance 168Ω・cm², capacity retention rate 68% after 500 cycles, silicon-based anode volume expansion rate 210%, battery failure due to electrolyte layer cracking and lithium dendrite penetration after 620 cycles.

[0040] In summary, this invention solves the core problems of poor solid-solid interface contact, poor stability, and poor adaptability to volume changes in existing all-solid lithium batteries through the synergistic design of composite negative electrode, gradient functional interface layer, and artificial solid electrolyte interface layer. Ultimately, it achieves the synergistic advantages of high capacity, energy density >400Wh / kg, low impedance, high stability, capacity retention rate ≥83% after 1000 cycles, and long cycle life.

[0041] Example 2 like Figure 3 As shown, the present invention provides a method for preparing a composite negative electrode all-solid-state lithium battery, including the following steps.

[0042] S1. Negative electrode pretreatment: The composite negative electrode slurry is uniformly coated on the negative electrode current collector 1, and then dried and compacted to obtain the composite negative electrode 2. Optionally, an artificial solid electrolyte interface layer 3 is deposited on the surface of the composite negative electrode 2 by magnetron sputtering or atomic layer deposition.

[0043] Specifically, the composite negative electrode slurry is prepared by mixing active materials, conductive agents, binders, and solvents in a mass ratio of 85–95:2–10:1–5:10–20. The active material is a silicon-carbon composite material with a silicon mass fraction of 50%; the conductive agent is superconducting carbon black; the binder is a sodium carboxymethyl cellulose-styrene-butadiene rubber composite binder; and the solvent is deionized water. The slurry is uniformly coated onto the surface of an 8–15 micrometer thick copper foil current collector at a coating speed of 0.3–0.8 m / s, with a coating surface density of 10–20 mg / cm². After coating, the composite negative electrode 2 is dried in a hot air drying oven at 80-120℃ for 1-4 hours to remove the solvent. After that, it is cold-pressed at a pressure of 5-15MPa to obtain a composite negative electrode 2 with a thickness of 30-50 micrometers. If an artificial solid electrolyte interface layer 3 is deposited, a magnetron sputtering process is used with a power of 100-300W and an argon atmosphere pressure of 0.3-0.8Pa to deposit a lithium fluoride film with a thickness controlled at 20-40 nanometers. Alternatively, an atomic layer deposition process can be used to deposit a lithium nitrogen phosphate film with a precursor pulse time of 0.1-1 seconds and a film thickness of less than 50 nanometers.

[0044] S2. Preparation of gradient functional interface layer: In the order from negative electrode side to positive electrode side, a flexible buffer layer 401, a gradient transition layer 402 and an inorganic connecting layer 403 are prepared sequentially on the surface of artificial solid electrolyte interface layer 3 or composite negative electrode 2.

[0045] Specifically, the flexible buffer layer 401 is prepared by mixing polyethylene oxide-bis(trifluoromethanesulfonyl)imide lithium PEO (molecular weight 50000) with 1-propyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt at a mass ratio of 5:5, adding bis(trifluoromethanesulfonyl)imide lithium (molar concentration 1 mol / L) and superconducting carbon black (mass fraction 2%), dissolving in acetonitrile to prepare a dispersion with a solid content of 40%, spraying the dispersion onto the target surface at a spraying rate of 0.1–0.3 mL / min, and drying and curing at 70°C for 4 hours to form a flexible buffer layer with a thickness of 10 micrometers. Low-temperature curing can avoid degradation of polymer electrolyte segments. Preparation of gradient transition layer 402: Three groups of Li6PS5Cl nanoparticles with different proportions and a particle size of 200 nm were prepared. A 5-nanometer-thick PEO-LiTFSI slurry with particle mass fractions of 20%, 45%, and 70% was coated onto the surface of a flexible buffer layer using a three-channel co-spraying technology with a channel flow difference of 10%. The slurry was then treated at 50°C for 6 hours to form a gradient transition layer with a thickness of 20 micrometers. Preparation of inorganic bonding layer 403: 95% of Li6PS5Cl nanoparticles were mixed with 5% of PEO binder, and acetonitrile was added to prepare a slurry with a solid content of 70%. The slurry was then coated onto the surface of the gradient transition layer, dried at 60°C for 3 hours, and compacted with a pressure of 8 MPa to form an inorganic bonding layer with a thickness of 10 micrometers.

[0046] S3, Bulk Electrolyte Integration: The pre-sintered bulk solid electrolyte layer 5 is bonded to the upper surface of the inorganic bonding layer 403 by hot pressing or solvent-assisted bonding.

[0047] Specifically, a pre-sintered Li6PS5Cl bulk solid electrolyte with a thickness of 100 micrometers was selected, sintered at a temperature of 600℃, and an ionic conductivity of [missing information]. S / cm, cut to the size matching the electrode, wipe the surface clean with anhydrous ethanol. If hot pressing is used, press at 120℃ and 15MPa for 30 minutes under argon protection to make the bulk electrolyte and inorganic bonding layer tightly bonded. If solvent-assisted bonding is used, add 0.5μL / cm² of isopropanol to wet the surface of the inorganic bonding layer. After bonding the bulk electrolyte, dry at 80℃ for 2 hours to remove the solvent and achieve interfacial bonding.

[0048] S4. Positive electrode integration: A positive electrode slurry is coated on the side of the bulk solid electrolyte layer 5 away from the gradient functional interface layer 4, dried at 80-120℃ for 2-6 hours, and hot-pressed at 100-180℃ and 5-20MPa for 10-30 minutes to form the positive electrode 6.

[0049] Specifically, the positive electrode slurry is prepared by mixing NCM811, composite sulfide solid electrolyte, superconducting carbon black, PVDF binder and NMP solvent in a mass ratio of 72:20:5:3:20. The slurry is coated on the surface of the bulk electrolyte with a coating density of 18-22 mg / cm². It is then dried at 90°C for 4 hours to remove NMP. Subsequently, it is hot-pressed at 100°C and 10MPa pressure for 20 minutes to form a positive electrode with a thickness of 60 micrometers. A 15-micrometer thick aluminum foil is then hot-pressed onto the surface of the positive electrode as the positive electrode current collector.

[0050] S5. Encapsulation buffer layer setting: A packaging buffer layer 7 is reserved on the top layer of the positive electrode 6 to complete the battery assembly.

[0051] Specifically, a 30-micron-thick high-temperature resistant buffer ring is placed on the top layer of the positive electrode 6, reserving space for the encapsulation buffer layer. The assembled battery core is placed in an aluminum-plastic film and transferred to a dry argon glove box with a water and oxygen content of less than or equal to 10 ppm. A heat sealer is used to heat seal for 5 seconds at 150°C and 0.5 MPa to complete the aluminum-plastic film encapsulation, ultimately forming a composite negative electrode all-solid-state lithium battery containing the encapsulation buffer layer 7.

[0052] The preparation method of this invention adopts existing mature processes such as coating, spraying, and hot pressing, without the need for additional special production equipment. It can be directly adapted to existing lithium-ion battery production lines for large-scale preparation and has good industrialization prospects.

[0053] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A composite negative electrode all-solid-state lithium battery, characterized in that, The battery is stacked in sequence from negative electrode to positive electrode, including: negative electrode current collector (1), composite negative electrode (2), artificial solid electrolyte interface layer (3), gradient functional interface layer (4), bulk solid electrolyte layer (5), positive electrode (6) and encapsulation buffer layer (7). The gradient functional interface layer (4) is composed of a flexible buffer layer (401), a gradient transition layer (402) and an inorganic bonding layer (403) in sequence from the negative electrode side to the positive electrode side, which is used to improve the contact between the electrode and the electrolyte interface and enhance the interface stability.

2. The composite negative electrode all-solid-state lithium battery according to claim 1, characterized in that: The composite negative electrode (2) is coated, dried and compacted onto the upper surface of the negative electrode current collector (1) to form an integrated structure. The composite negative electrode (2) is composed of active material, conductive agent and binder. The active material is selected from silicon-based composite material or pre-lithiation composite lithium metal material. The silicon-based composite material is silicon oxide carbon or silicon carbon composite material. The pre-lithiation composite lithium metal material is a composite structure formed by lithium deposition on a three-dimensional porous current collector. The thickness of the composite negative electrode (2) is 30 to 50 micrometers.

3. The composite negative electrode all-solid-state lithium battery according to claim 2, characterized in that: The conductive agent in the composite negative electrode (2) is selected from superconducting carbon black or Ketjen black, with a mass fraction of 2% to 10%. The binder is selected from sodium carboxymethyl cellulose-styrene-butadiene rubber composite binder or polyvinylidene fluoride, with a mass fraction of 1% to 5%. The negative electrode current collector (1) is copper foil with a thickness of 8 to 15 micrometers. When the active material of the composite negative electrode (2) is a silicon-based composite material, the oxygen-silicon atomic ratio in silicon oxide carbon is 0.5 to 1.5, and the mass fraction of silicon in the silicon-carbon composite material is 30% to 70%. When the active material of the composite negative electrode (2) is a pre-lithiated composite lithium metal material, the three-dimensional porous current collector is a three-dimensional porous copper current collector with a pore size of 1 to 10 micrometers and a lithium deposition amount of 0.5 to 2 mg / cm².

4. The composite negative electrode all-solid-state lithium battery according to claim 2, characterized in that: The artificial solid electrolyte interface layer (3) is directly composited onto the upper surface of the composite negative electrode (2) by magnetron sputtering or atomic layer deposition, with a thickness of less than 50 nanometers. The artificial solid electrolyte interface layer (3) is a dense and ionically conductive thin film, and the material is selected from only one of lithium fluoride, lithium nitride, or lithium phosphate, with an ionic conductivity of not less than 50 nanometers. S / cm.

5. The composite negative electrode all-solid-state lithium battery according to claim 4, characterized in that: The flexible buffer layer (401) is based on a flexible polymer electrolyte or ionic liquid, and uniformly disperses an electronic conductive agent and a lithium salt. The polymer electrolyte is polyethylene oxide-bis(trifluoromethanesulfonyl)imide lithium, the molecular weight of polyethylene oxide is 10,000 to 100,000, the ionic liquid is 1-propyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt, the electronic conductive agent is superconducting carbon black, and the thickness of the flexible buffer layer (401) is 5 to 20 micrometers.

6. The composite negative electrode all-solid-state lithium battery according to claim 5, characterized in that: In the flexible buffer layer (401), the mass ratio of polymer electrolyte to ionic liquid is 10:0 to 0:10, the mass fraction of electronic conductive agent is 0.5% to 5%, the molar concentration of lithium salt is 0.5 to 2 mol / L, and the lithium salt is selected only from lithium bis(trifluoromethanesulfonyl)imide or lithium bis(fluorosulfonyl)imide. The thickness of the gradient transition layer (402) is 10 to 30 micrometers, and it is composed of polymer electrolyte, ionic liquid, and inorganic solid electrolyte nanoparticles with surface coating modification mixed in a gradient ratio. The mass fraction of inorganic solid electrolyte nanoparticles near the flexible buffer layer (401) is 10% to 30%, and the mass fraction near the inorganic connecting layer (403) is 60% to 80%.

7. The composite negative electrode all-solid-state lithium battery according to claim 6, characterized in that: The inorganic connecting layer (403) has a thickness of 5 to 15 micrometers. The inorganic connecting layer (403) is mainly composed of inorganic solid electrolyte nanoparticles that are compatible with the bulk solid electrolyte layer (5), with a mass ratio greater than or equal to 90%, and contains a very small amount of polymer binder. In the gradient transition layer (402), the inorganic solid electrolyte nanoparticles are selected from one of lithium phosphide sulfide, garnet-type lithium lanthanum zirconium oxide, or phosphate-type lithium aluminum titanium phosphide, with a particle size of 50 to 500 nanometers. The surface coating layer is lithium phosphate or aluminum oxide, with a thickness of 1 to 10 nanometers.

8. The composite negative electrode all-solid-state lithium battery according to claim 7, characterized in that: The polymer binder in the inorganic bonding layer (403) is selected from one of polyethylene oxide, polyvinylidene fluoride-hexafluoropropylene copolymer, polymethyl methacrylate, or polyvinylpyrrolidone. The bulk solid electrolyte layer (5) is a sulfide solid electrolyte, oxide solid electrolyte, or composite solid electrolyte with a thickness of 50-200 micrometers and an ionic conductivity of not less than [missing value]. S / cm, formed by pre-sintering, with a sintering temperature of 300~800℃.

9. The composite negative electrode all-solid-state lithium battery according to claim 8, characterized in that: The positive electrode (6) is composited onto the upper surface of the bulk solid electrolyte layer (5) by a coating-drying-hot pressing process. It is composed of active material, composite solid electrolyte, conductive agent and binder. The positive electrode active material is selected from at least one of high nickel nickel cobalt manganese oxide, lithium-rich manganese-based material or lithium cobalt oxide. The mass fraction of the composite solid electrolyte is 10% to 30%, the mass fraction of the conductive agent is 2% to 8%, the mass fraction of the binder is 1% to 5%, the thickness of the positive electrode (6) is 40 to 100 micrometers, the positive electrode current collector is aluminum foil with a thickness of 10 to 20 micrometers, and the thickness of the encapsulation buffer layer (7) is 10 to 50 micrometers.

10. A method for preparing a composite negative electrode all-solid-state lithium battery, characterized in that: The method for preparing the composite negative electrode all-solid-state lithium battery according to claims 1-9 includes the following steps: S1. Negative electrode pretreatment: The composite negative electrode slurry is uniformly coated on the negative electrode current collector (1), and the composite negative electrode (2) is obtained by drying and compaction. Optionally, an artificial solid electrolyte interface layer (3) is deposited on the surface of the composite negative electrode (2) by magnetron sputtering or atomic layer deposition process. S2. Preparation of gradient functional interface layer: In the order from negative electrode side to positive electrode side, a flexible buffer layer (401), a gradient transition layer (402) and an inorganic connection layer (403) are prepared on the surface of artificial solid electrolyte interface layer (3) or composite negative electrode (2). S3, Bulk Electrolyte Integration: The pre-sintered bulk solid electrolyte layer (5) is bonded to the upper surface of the inorganic bonding layer (403) by hot pressing or solvent-assisted bonding. S4. Positive electrode integration: A positive electrode slurry is coated on the side of the bulk solid electrolyte layer (5) away from the gradient functional interface layer (4), dried at 80-120℃ for 2-6 hours, and hot-pressed at 100-180℃ and 5-20MPa for 10-30 minutes to form a positive electrode (6). S5. Encapsulation buffer layer setting: A packaging buffer layer (7) is reserved on the top layer of the positive electrode (6), and the battery is encapsulated in a dry argon atmosphere with a water and oxygen content of less than or equal to 10ppm.

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