Composite solid electrolyte structure with gradient flexible buffer layer and its application in solid-state batteries

Abstract

This invention discloses a composite solid-state electrolyte structure with a gradient flexible buffer layer and its application in solid-state batteries, belonging to the field of solid-state battery technology. The invention employs a composite structure with a rigid, high-conductivity core and a gradient flexible buffer layer. By limiting the modulus ratio to 0.05·E_SE~0.3·E_SE and the thickness ratio to 0.05~0.5, and combining this with a gradient design where the modulus decreases continuously towards the electrode side, the solid-solid interface shear stress is gradually released layer by layer, suppressing interface microcracks and contact delamination. Simultaneously, to address the differentiated volume deformation of the positive and negative electrodes, the gradient parameters of the buffer layers on both sides are independently adjusted to precisely adapt to different electrode strain conditions. This invention effectively reduces the cycle decay rate of interface integrity, achieving a capacity retention rate of ≥85% after 1000 cycles under standard operating conditions, solving the technical bottleneck of traditional solid-state electrolytes that cannot simultaneously achieve high conductivity and high interface flexibility. The process of this invention is mild and compatible with mass production, significantly improving the cycle life and service safety of solid-state batteries, and possessing extremely high industrial application value.

Description

Technical Field

[0001] This invention relates to the field of core material structure design technology for solid-state lithium-ion batteries, specifically to a flexible buffer layer composite solid electrolyte structure with a continuous gradient modulus. It is particularly suitable for all-solid-state batteries and semi-solid-state batteries with micro-liquid phase wetting. It can effectively solve technical problems such as stress concentration at the solid-solid interface, microcrack initiation, interface peeling, and continuous impedance increase, achieving long-term stability of the battery interface state and improving the cycle life and service safety margin of the power battery. Background Technology

[0002] 2.1 Current Status of Technology All-solid-state batteries abandon traditional liquid electrolytes, using solid electrolytes to achieve lithium-ion conduction and physical isolation between the positive and negative electrodes. They possess advantages such as high energy density, wide temperature range operation, resistance to thermal runaway, and no leakage risk, making them the mainstream technology route for next-generation high-safety, long-life power batteries. However, the solid-solid rigid contact mode of all-solid-state batteries has inherent interface failure defects: during battery charging and discharging, the positive and negative electrode active materials undergo periodic volume strain. Specifically, the volume expansion rate of silicon-based negative electrodes can reach 300%, and that of lithium metal negative electrodes can reach 100%. The positive electrode material also exhibits a trace volume deformation of no more than 10%. Repeated expansion and contraction of the electrodes continuously exert compression, shearing, and peeling effects on the electrolyte interface, inducing interfacial microcracks, delamination, and disruption of effective ion pathways. Ultimately, this leads to a sharp increase in interfacial contact impedance, rapid capacity decay, and a significant reduction in cycle life. Under extreme conditions, it can cause dendrite penetration, local short circuits, and the risk of thermal runaway. Semi-solid batteries improve initial interface adhesion by introducing trace amounts of liquid phase, but cannot eliminate stress concentration problems under long-cycle and high-rate conditions. The core technical pain points of interface degradation and performance degradation have not been eradicated.

[0003] 2.2 Core Defects of Existing Technologies Current single-system solid electrolytes generally exhibit performance contradictions, failing to simultaneously meet the engineering requirements of high ionic conductivity, interfacial deformation adaptability, and long-term structural stability: Sulfide and halide solid electrolytes exhibit room-temperature ionic conductivity on the order of 10⁻³ S / cm, demonstrating excellent ionic conduction performance. However, after molding, their Young's modulus reaches as high as 20–80 GPa, resulting in extreme rigidity and a lack of plastic deformation capability. This makes them unsuitable for adapting to large-rate volume expansion and contraction of electrodes, and they are prone to interfacial cracking and contact failure during cycling. Polymer solid electrolytes offer good flexibility and excellent interfacial adhesion, but their intrinsic ionic conductivity is low, and their electrochemical window is narrow, making them difficult to adapt to high-voltage, high-rate power battery operating conditions. Oxide solid electrolytes possess high chemical stability, but they are brittle, have poor impact and deformation resistance, and are prone to breakage and failure under cyclic stress. Existing composite solid electrolytes mostly employ simple blending, homogeneous single-layer coating, and conventional stacking composite processes. These methods merely achieve the superposition of material components without addressing the refined matching design of modulus, thickness, and gradient variations to accommodate the differentiated deformation characteristics of the positive and negative electrodes. This fails to fundamentally eliminate interfacial stress concentration. Traditional homogeneous buffer layers can only achieve weak stress release on the surface, easily leading to localized stress peaks and rapid degradation of interfacial integrity under long-term cycling. More critically, current technologies only focus on material modification and simple stacking, failing to establish a quantitative coupling relationship between electrolyte structural parameters and interfacial aging states. They lack constraints on modulus ratio, thickness ratio, interfacial decay rate, and critical threshold for interfacial failure. Structural design lacks quantitative theoretical support, resulting in limited lifespan extension and poor adaptability.

[0004] 2.3 Industry Technology Gaps and Bottlenecks In summary, existing technologies generally suffer from four major technology gaps: First, the lack of a layered coupling structure between a rigid high-conductivity core and a flexible gradient buffer layer; second, the absence of a dual quantitative matching range for modulus and thickness; third, the lack of a continuous and smooth gradient modulus design, with only stepped multilayer composites or homogeneous coatings existing; and fourth, the failure to achieve differentiated gradient adaptation design for the positive and negative electrode buffer layers. The industry urgently needs a gradient composite solid-state electrolyte structure that is quantifiable and controllable, possesses strong stress relief capabilities, and balances high conductivity and high interface flexibility to overcome the industrialization bottlenecks of long-cycle and high-safety solid-state batteries. Summary of the Invention

[0005] 3.1 Purpose of the Invention This invention addresses the shortcomings of existing technologies, such as the contradictory performance of single materials, insufficient stress relief in homogeneous composite structures, lack of quantitative design standards, and rapid interface degradation. It provides a composite solid-state electrolyte structure with a gradient flexible buffer layer and its application in solid-state batteries, aiming to achieve the following technical objectives: 1. Constructing a composite system of a rigid, high-conductivity inorganic core and a gradient flexible organic buffer layer, simultaneously considering high ionic conductivity and interface deformation adaptability, overcoming the performance limitations of single electrolyte materials; 2. Precisely releasing interfacial shear stress generated by periodic electrode deformation through dual quantitative parameter constraints of modulus ratio and thickness ratio, inhibiting microcrack initiation and interfacial delamination from a mechanistic perspective; 3. Employing a continuous and smooth gradient modulus structure to eliminate the stress concentration problem caused by abrupt modulus changes in traditional multilayer composite structures, further improving interface stability; 4. Setting differentiated gradient parameters for the deformation differences between the positive and negative electrodes to achieve precise adaptation of high and low strain electrodes, maximizing the interface stress relief effect; 5. The process is compatible with existing mass production lines. Low-temperature molding can avoid the risk of electrolyte decomposition at high temperatures. It can be equipped with an interface state monitoring system to form full life cycle interface control, which significantly improves battery cycle life and service safety.

[0006] 3.2 Technical Solution 3.2.1 Composite Solid Electrolyte Overall Structure A composite solid electrolyte with a gradient flexible buffer layer includes a high-conductivity core layer and a flexible buffer layer; the flexible buffer layer is attached to one or both sides of the surface of the high-conductivity core layer; when both sides are set, the buffer layer on the positive electrode side and the buffer layer on the negative electrode side are independent of each other, and the composition, thickness and gradient parameters can be configured differently according to the electrode deformation characteristics.

[0007] 3.2.2 High Conductivity Core Layer The high conductivity core layer is a rigid ion-conducting host layer, and the material is selected from sulfide solid electrolytes, halide solid electrolytes, and their element-doped modified derivatives. Among them, the sulfide electrolyte includes Li 10 GeP2S 12 The electrolyte comprises Li6PS5Cl and Sn, Ge, O, and Cl doped and modified products; the halide electrolyte includes Li3YCl6, Li3ErCl6, and rare earth and halogen doped and modified products. The high-conductivity core layer is formed by cold isostatic pressing at room temperature at 200–500 MPa, resulting in a dense structure and continuous ion conduction pathways, with an ionic conductivity ≥1×10⁻³ S / cm at 25℃. The Young's modulus E_SE of the core layer is the actual modulus measured by nanoindentation or ultrasonic echo method of the formed electrolyte sheet, with a value range of 20–80 GPa, which is more in line with engineering application conditions compared to the theoretical bulk modulus.

[0008] 3.2.3 Flexible Buffer Layer and Core Quantitative Proportioning Relationship The flexible buffer layer is directly bonded between the high-conductivity core layer and the electrode active material, and is composed of a polymer matrix and inorganic nanofillers. The polymer matrix is ​​selected from one or more of PEO, PC, PAN and their copolymers; the inorganic nanofillers are selected from one or more of LLZO, Al2O3, and TiO2, and the amount of inorganic nanofillers added is 0.5–20 wt% of the total mass of the buffer layer. This invention has obtained two sets of core quantitative proportioning ranges through extensive mechanical simulation and accelerated aging tests, which are necessary technical constraints for achieving long-term interface stability in this invention. (1) Modulus ratio relationship: 0.05·E_SE ≤ E_buf ≤ 0.3·E_SE If E_buf<0.05·E_SE, the stiffness of the buffer layer is too low, and wrinkles, collapses and pore closure are likely to occur during cyclic deformation, blocking lithium ion conduction and causing a rapid increase in interface impedance; if E_buf>0.3·E_SE, the stiffness of the buffer layer approaches that of the core layer, the deformation relaxation ability is basically lost, and it cannot relieve the interface shear stress, and the stress concentration and interface cracking problems recur.

[0009] (2) Thickness ratio: 0.05 ≤ t_buf / t_SE ≤ 0.5. If t_buf / t_SE < 0.05, the buffer layer is too thin and cannot cover the micro-deformation gaps and interface defects of the electrode, resulting in stress relief failure; if t_buf / t_SE > 0.5, the proportion of flexible layer is too high, which will reduce the overall ion conduction efficiency and battery volumetric energy density, and impair rate performance. The overall thickness of the composite solid electrolyte is 20-200 μm, which is suitable for the design requirements of different specifications of solid cell.

[0010] 3.2.4 Continuous Gradient-Sized Core Structure: The flexible buffer layer features a continuous and smooth gradient: along the direction away from the high-conductivity core layer and towards the electrode, the polymer matrix mass fraction continuously increases, while the inorganic nanofiller particle size continuously decreases, causing the Young's modulus of the buffer layer to gradually decrease from the inside out. This structure enables layer-by-layer stress release, completely avoiding the modulus abrupt changes and secondary stress concentration defects present in traditional multi-layer stepped composite structures. Furthermore, a differentiated gradient adaptation design for the positive and negative electrodes is adopted: for the low strain characteristics of the positive electrode with a volume expansion rate <10%, a low gradient rate and small modulus-sized gradient structure is used, balancing interface stability and ion conductivity; for the ultra-high volume expansion rates of silicon-based and lithium metal negative electrodes (100%–300%), a high gradient rate and large modulus-sized gradient structure is used to maximize the absorption of negative electrode deformation stress.

[0011] 3.2.5 Solid-state battery stacked structure: The stacking sequence of the battery cells equipped with this composite solid-state electrolyte is as follows: positive electrode current collector → positive electrode active material layer → positive electrode side flexible buffer layer → high conductivity core layer → negative electrode side flexible buffer layer → negative electrode active material layer → negative electrode current collector. The buffer layer on one side can be omitted depending on the operating conditions, and it is compatible with graphite, silicon-based, lithium metal anodes, and all-solid-state and semi-solid-state battery systems.

[0012] 3.2.6 Preparation Method This invention employs a low-temperature compatible mass production process, with the following specific steps: S1. High-conductivity core molding: Sulfide / halide electrolyte powder is placed in a mold and cold isostatically pressed at room temperature (200–500 MPa) to obtain a dense and flat high-conductivity core sheet; S2. Gradient slurry preparation: The polymer matrix and 0.5–20 wt% inorganic nanofillers are dispersed in an organic solvent, and after high-speed stirring and ultrasonic homogenization, a composite slurry with uniform composition and suitable for gradient coating is obtained; S3. Gradient coating and curing: A step-by-step gradient coating process is used to form a buffer layer wet film with continuously varying components on one or both sides of the high-conductivity core. This is then cured by thermosetting or UV curing at a temperature ≤120 ℃ to avoid high-temperature decomposition and failure of the electrolyte; S4. Secondary densification: Optional isostatic pressing at 50–200 MPa can be used to improve interlayer bonding strength and the actual contact area at the interface, reducing the initial interface impedance; S5. Cell stacking: The cells are matched and stacked with positive and negative electrode sheets and packaged to prepare a solid-state battery cell.

[0013] 3.2.7 Technical Effects and Performance Quantitative Indicators This invention, through gradient structure optimization and dual quantitative parameter constraints, significantly suppresses interface crack propagation and contact peeling, greatly reducing the interface integrity decay rate. Under standard charge / discharge conditions of 25℃, 2.5–4.3V, and 1C / 1C, the interface integrity decay slope of this invention is ≥40% lower than that of a single electrolyte without a buffer layer, the capacity retention rate after 1000 cycles is ≥85%, and the interface impedance increase is ≤150%. Under the same conditions, the capacity retention rate of the traditional electrolyte control group is ≤65%, and the impedance increase is ≥400%, demonstrating a significant improvement in interface stability and cycle life.

[0014] Beneficial effects 1. Overcoming the contradiction in material performance: The rigid, high-conductivity core ensures ultra-high ionic conductivity, while the gradient flexible buffer layer provides deformation adaptability, simultaneously solving the industry pain points of traditional electrolytes that are "highly conductive but not flexible, or flexible but with low conductivity".

[0015] 2. Mechanistic stress relief: The continuous and smooth gradient modulus structure enables stress to be released layer by layer, eliminating local stress peaks caused by abrupt changes in modulus, and suppressing interface microcracks and delamination failure from the root.

[0016] 3. Precise and differentiated adaptation: Independent gradient parameter design for positive and negative electrodes can be adapted to different chemical conditions such as low strain positive electrode and ultra-high strain negative electrode, and is compatible with the entire series of positive and negative electrode material systems.

[0017] 4. Quantitative and controllable with strong stability: By using precise range constraints of modulus and thickness, the structural parameters are standardized and quantifiable, avoiding the defects of traditional structural design, which are characterized by high arbitrariness and poor stability.

[0018] 5. High industrial adaptability: The entire process is low-temperature molding, and the process is compatible with existing solid-state battery mass production lines. It can be adapted to both all-solid-state and semi-solid-state technology routes, and has extremely strong engineering application value. Attached Figure Description

[0019] Figure 1 The schematic diagram of the cross-sectional layered structure of the composite solid electrolyte of the present invention shows the stacked matching structure of a high-conductivity core layer and a double-sided gradient flexible buffer layer. Figure 2 A schematic diagram of the gradient variation curve of the buffer layer modulus along the thickness direction shows the continuous and gradual decrease in modulus from the core side to the electrode side. Figure 3 A comparison curve of the interface integrity cycling decay of the present invention and that of a traditional non-gradient electrolyte; Figure 4 A schematic diagram of the exploded structure of a solid-state battery cell stack equipped with the composite electrolyte of the present invention. Detailed Implementation

[0020] The following embodiments are used to illustrate the technical solution of the present invention in detail, but are not intended to limit the scope of protection of the present invention. Conventional adjustments made by those skilled in the art within the parameter range of the present invention are all within the scope of protection of the present invention. Comparative examples are also provided to highlight the inventiveness and progress of the present invention.

[0021] Example 1: Sulfide core + PEO / LLZO gradient buffer layer (all-solid-state battery) The high-conductivity core uses a Li6PS5Cl sulfide electrolyte, cold isostatically pressed at 300 MPa, with a thickness of 80 μm and a measured modulus of 45 GPa. A PEO + 5 wt% LLZO gradient buffer layer is prepared on both sides, with a single-sided thickness of 15 μm and a thickness ratio of 0.1875. The measured modulus of the buffer layer is 6 GPa, meeting the modulus range of 0.05·E_SE to 0.3·E_SE. The buffer layer achieves a continuous modulus gradient by increasing polymer content and decreasing filler particle size from the inside out. It is matched with an NCM811 positive electrode and a silicon-carbon negative electrode, and undergoes a secondary densification treatment at 100 MPa. Standard operating condition testing shows: 89% capacity retention after 1000 cycles, 142% increase in interfacial impedance, a 45% decrease in the interface integrity decay slope, and an interface integrity of 0.72 after cycling, exceeding the critical failure threshold.

[0022] Example 2: Micro-liquid semi-solid adaptation system Based on the structure of Example 1, ≤5 wt% ionic liquid was introduced into the cell as a trace wetting liquid phase to construct a semi-solid system. The trace liquid phase fills the microscopic gaps at the interface, further reducing the initial interface impedance. Test results: After 1500 long cycles, the interface integrity remained above 0.70, with no interface delamination or capacity drop, further improving long-cycle stability.

[0023] Example 3: Halogenated core + PAN / Al2O3 high-pressure adaptation system The high-conductivity core uses an Er-doped Li3YCl6 halide electrolyte with a measured modulus of 55 GPa. The buffer layer uses PAN + 3wt% Al2O3 with a thickness ratio of 0.15 and a measured modulus of 8 GPa, both falling within the parameter range of this invention. The PAN matrix exhibits excellent high-voltage resistance, making it suitable for 4.6 V high-voltage cathode systems. This system shows extremely low interface degradation under high-rate, long-cycle conditions, making it suitable for high-voltage, high-energy-density solid-state battery scenarios. Comparative Example 1: A pure solid-state electrolyte without a buffer layer uses the same Li6PS5Cl core as in Example 1, without any buffer layer structure. Under the same operating conditions, the capacity retention rate is 67% after 1000 cycles, the interface impedance increases by 410%, and the interface integrity decays rapidly, falling below the critical failure threshold after 800 cycles. Comparative Example 2: A homogeneous, gradient-free buffer layer structure uses the same modulus and thickness parameters as in Example 1, with the buffer layer employing a homogeneous blend structure and no gradient. The capacity retention rate after 1000 cycles was 76%, the interface impedance increased by 285%, stress concentration was significant, and the interface decay rate was significantly higher than that of the gradient structure of this invention. Comparative Example 3 uses a reverse gradient design with a higher electrode-side modulus in the buffer layer, with other parameters consistent with Example 1. The capacity retention rate after 1000 cycles was 72%, but the interface was prone to modulus abrupt changes and stress concentration, resulting in rapid crack initiation and significantly inferior performance compared to the forward gradient structure of this invention.

[0024] Using the same Li6PS5Cl core as in Example 1, without any buffer layer structure, the capacity retention rate was 67% after 1000 cycles under the same operating conditions, the interface impedance increased by 410%, the interface integrity decayed rapidly, and it fell below the critical failure threshold after 800 cycles.

[0025] Using the same modulus and thickness parameters as in Example 1, the buffer layer adopts a homogeneous blend structure without gradient transition. After 1000 cycles, the capacity retention rate is 76%, the interface impedance increases by 285%, stress concentration is significant, and the interface decay rate is significantly higher than that of the gradient structure of this invention.

[0026] The buffer layer adopts a reverse gradient design with a higher modulus on the electrode side, and the other parameters are the same as in Example 1. The capacity retention rate is 72% after 1000 cycles. The interface is prone to modulus abrupt change and stress concentration, and the crack initiation speed is fast. Its performance is significantly worse than the forward gradient structure of this invention.

Claims

1. A composite solid electrolyte structure with a gradient flexible buffer layer, characterized in that, include: A high-conductivity core layer, wherein the high-conductivity core layer is a sulfide or halide solid electrolyte layer formed by cold isostatic pressing, and E_SE is the measured Young's modulus after the high-conductivity core layer is formed; A flexible buffer layer is attached to at least one side surface of the high-conductivity core layer. The flexible buffer layer is a composite layer of polymer and inorganic nanofiller. E_buf is the measured Young's modulus of the flexible buffer layer after curing. The flexible buffer layer and the high-conductivity core layer satisfy the modulus ratio: 0.05·E_SE ≤ E_buf ≤ 0.3·E_SE; The thickness ratio of the single-sided flexible buffer layer to the high-conductivity core layer is: 0.05 ≤ t_buf / t_SE ≤ 0.

5.

2. The composite solid electrolyte structure with a gradient flexible buffer layer according to claim 1, characterized in that: The flexible buffer layer has a continuously increasing polymer matrix mass fraction and a continuously decreasing inorganic nanofiller particle size along the direction away from the high-conductivity core layer toward the electrode, forming a gradient structure in which the Young's modulus decreases continuously and gently from the inside to the outside.

3. The composite solid electrolyte structure with a gradient flexible buffer layer according to claim 2, characterized in that: The positive electrode side buffer layer attached to the positive electrode and the negative electrode side buffer layer attached to the negative electrode are independently set with gradient change rate and modulus gradient curves to differentiate and adapt to the volume strain difference between the positive and negative electrodes.

4. The composite solid electrolyte structure with a gradient flexible buffer layer according to claim 1, characterized in that: The sulfide solid electrolyte is selected from Li 10 GeP2S 12 The halide solid electrolyte is selected from Li6PS5Cl and its Sn, Ge, O and Cl element-doped and modified derivatives; the halide solid electrolyte is selected from Li3YCl6, Li3ErCl6 and its rare earth and halogen element-doped and modified derivatives. The polymer matrix is ​​selected from one or more of polyethylene oxide, polycarbonate, polyacrylonitrile and their copolymers; The inorganic nanofiller is selected from Li7La3Zr2O 12 One or more of Al2O3 and TiO2 are used, and the amount of inorganic nanofiller added is 0.5 to 20 wt% of the total mass of the buffer layer.

5. The composite solid electrolyte structure with a gradient flexible buffer layer according to claim 1, characterized in that: The high-conductivity core layer is formed by cold isostatic pressing at room temperature with a pressure of 200–500 MPa, and has an ionic conductivity of ≥1×10⁻³ S / cm at 25℃; the overall thickness of the composite solid electrolyte is 20–200 μm.

6. The composite solid electrolyte structure with a gradient flexible buffer layer according to claim 1, characterized in that: Under standard operating conditions of 25℃, 2.5~4.3 V, and 1C / 1C charge / discharge, the interface integrity decay slope of the battery equipped with the composite solid electrolyte after 1000 cycles is reduced by ≥40% compared with the single electrolyte without buffer layer, the capacity retention rate is ≥85%, and the interface impedance increase is ≤150%.

7. A solid-state or semi-solid-state battery cell, characterized in that: It includes the composite solid electrolyte structure according to any one of claims 1 to 6.

8. An electrical appliance, characterized in that: It is equipped with the solid-state or semi-solid-state battery cell as described in claim 7.

9. A method for preparing a composite solid electrolyte, used to prepare the composite solid electrolyte according to any one of claims 1 to 6, characterized in that, include: Dense, high-conductivity core wafers are prepared by cold isostatic pressing of sulfide or halide solid electrolyte powder at 200–500 MPa. A composite slurry containing a polymer matrix and inorganic nanofillers is prepared and coated onto at least one side surface of a high-conductivity core wafer using a gradient coating process. The flexible buffer layer is formed by thermosetting or UV curing process, with a curing temperature ≤120 ℃; Optionally, the composite structure can be subjected to secondary isostatic pressing densification treatment at 50–200 MPa to complete the preparation.

10. The preparation method according to claim 9, characterized in that: The gradient coating process is used to form a gradient buffer layer with continuously varying polymer content and filler particle size, thereby achieving a continuous gradient change in the modulus of the buffer layer along the thickness direction.

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