Composite interface layer material and application thereof in preparation of all-solid-state lithium battery

Abstract

The invention provides a composite interface layer material and application thereof in preparation of an all-solid-state lithium battery. The composite interface layer material is prepared by mixing and grinding soft carbon, lithium difluoro (oxalato) borate and Li6PS5Cl sulfide solid electrolyte. Lithium difluoro (oxalato) borate is embedded into an SC substrate, a LiF nano layer with high interface energy can be generated in situ in the circulation process, LiF forms a rapid ion diffusion network among soft carbon particles, and the overall ion transmission efficiency of the interface layer is remarkably improved by combining the high ion conductivity of LiDFOB; meanwhile, through the synergistic effect of electron conduction of SC and ion conduction of LiDFOB / LiF, Li < + > flux homogenization is promoted, the local current density is reduced, and therefore growth of lithium dendrites is inhibited. The multi-scale cooperative regulation strategy provides a new solution for interface engineering of the high-performance solid-state lithium battery.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium batteries, and in particular, to a composite interface layer material and its application in the preparation of all-solid-state lithium batteries. Background Art

[0002] As the core power source of modern mobile electronic devices and electric vehicles, the development of lithium-ion batteries is facing dual constraints of safety and energy density. In traditional liquid lithium-ion battery systems, the flammability of electrolytes, the out-of-control growth of lithium dendrites, and the resulting problems such as capacity decay and shortened cycle life have become the key bottlenecks restricting the development of high-energy-density batteries. Against this background, the all-solid-state lithium battery system with solid electrolytes and lithium metal anodes shows revolutionary breakthrough potential. The solid electrolyte not only fundamentally eliminates the safety hazards of easy combustion and leakage in the liquid system, but also its high mechanical strength characteristics provide the possibility for achieving higher energy density. At the same time, the lithium metal anode with a theoretical capacity of up to 3860 mAh g -1 and a reduction potential as low as -3.04 V (vs. SHE) is regarded as the "holy grail" material to break through the existing specific energy limit, and its application will promote a leapfrog increase in the battery energy density.

[0003] Among many solid electrolytes, sulfide solid electrolytes stand out due to their high ionic conductivity (>1.0 mS cm -1 ), excellent mechanical strength and low-cost advantages. However, the interfacial compatibility problem between sulfide solid electrolytes and lithium anodes needs to be solved urgently. Sulfide solid electrolytes are prone to side reactions with lithium metal, resulting in an increase in interfacial impedance and a decline in battery performance. On the other hand, the volume fluctuations and uneven lithium deposition during the lithium deposition / stripping process will induce lithium dendrites to penetrate the electrolyte, leading to battery short circuit.

[0004] To improve the interfacial performance of sulfide electrolytes / lithium anodes, current research focuses on three directions: element doping, interface buffer layer design, and interface modification. Among them, constructing an interface buffer layer has a significant effect on inhibiting lithium dendrites. The principle is that the high interfacial energy characteristics of the buffer layer can inhibit the nucleation of lithium dendrites, and at the same time, it can also delay the reduction of the electrolyte and promote Li + diffusion kinetics. However, the currently developed inorganic interface layers are too rigid, which makes it difficult for them to adapt to the volume deformation during the lithium deposition / stripping process, and are prone to fracture and peeling, ultimately leading to interface failure. Summary of the Invention

[0005] The purpose of the present invention is to provide a composite interface layer material and its application in the preparation of all-solid-state lithium batteries to solve the technical problems that the existing inorganic interface layers are too rigid and prone to fracture and peeling.

[0006] To achieve the above object, the present invention provides a composite interface layer material and its application in the preparation of all-solid-state lithium batteries.

[0007] 1. A composite interface layer material is obtained by mixing and grinding soft carbon (SC), lithium difluorooxalate borate (LiDFOB), and Li6PS5Cl sulfide solid electrolyte (LPSC) in a mass ratio of 8:0.5 - 1.5:0.5 - 1.5.

[0008] As one of the preferred technical solutions, the mass ratio of soft carbon (SC), lithium difluorooxalate borate (LiDFOB), and Li6PS5Cl sulfide solid electrolyte (LPSC) is 8:1:1.

[0009] As one of the preferred technical solutions, the mixing and grinding time is 30 minutes.

[0010] As one of the preferred technical solutions, the soft carbon is prepared by the following method: First, mix and ball-mill asphalt and xylene, dry, grind to obtain a mixture, then mix and stir the mixture with 1 mol / L phosphoric acid solution and absolute ethanol, dry, pyrolyze, and naturally cool to room temperature to obtain it.

[0011] As one of the further preferred technical solutions, the dosage ratio of asphalt to xylene is 3 g:20 mL; the dosage ratio of the mixture to phosphoric acid and absolute ethanol is 1 g:0.35 mL:4 mL.

[0012] As one of the further preferred technical solutions, the process conditions of ball-milling are: ball-mill at 300 rpm for 3 hours.

[0013] As one of the further preferred technical solutions, the drying conditions are: dry at 60°C for 12 hours; the grinding time is 10 minutes.

[0014] As one of the further preferred technical solutions, the mixing and stirring time is 2 hours.

[0015] As one of the further preferred technical solutions, the process conditions of pyrolysis are: heat to 800°C at 5°C / min and keep pyrolyzing for 3 hours.

[0016] As one of the preferred technical solutions, the Li6PS5Cl sulfide solid electrolyte is prepared by the following method: Under an argon atmosphere, take raw materials Li2S, P2S5, and LiCl, grind and mix them evenly in a molar ratio of 5:1:2 to obtain a premix, ball-mill, press into tablets, seal in a glass tube, and then quickly transfer it to a tube furnace filled with argon, heat and sinter, and grind and sieve to obtain it.

[0017] As one of the further preferred technical solutions, the grinding and mixing time is 10 minutes; the ball milling conditions are: the ball-to-material ratio is 1:40, and ball milling is carried out at 500 rpm for 40 hours.

[0018] As one of the further preferred technical solutions, the tabletting conditions are: the pressure is 300 MPa and the time is 3 minutes.

[0019] As one of the further preferred technical solutions, the process conditions for heat sintering are: heating to 500 °C at a rate of 2 °C / min and holding for sintering for 5 hours.

[0020] As one of the further preferred technical solutions, after grinding, it is sieved through a 400-mesh sieve.

[0021] 2. Application of the aforementioned composite interfacial layer material in the preparation of all-solid-state lithium batteries.

[0022] 3. An all-solid-state lithium battery, including a metallic lithium anode and a sulfide solid electrolyte, and a composite cathode and a composite interfacial layer made of the aforementioned composite interfacial layer material are respectively provided on both sides of the sulfide solid electrolyte.

[0023] As one of the preferred technical solutions, the composite cathode is made of an NCM composite cathode material, and the mass ratio of the NCM composite cathode material to the composite interfacial layer material is 1:1.

[0024] As one of the further preferred technical solutions, the NCM composite cathode material is prepared by the following method: under an argon atmosphere, NCM, vapor-grown carbon fiber, and Li6PS5Cl sulfide solid electrolyte are mixed and ground for 30 minutes according to a mass ratio of 70:5:25 to obtain.

[0025] 4. A method for preparing an all-solid-state lithium battery, the specific steps are as follows: under an argon atmosphere, first tablet the sulfide solid electrolyte to form an electrolyte sheet, then uniformly coat the composite cathode material and the composite interfacial layer material on both sides of the electrolyte sheet respectively, carry out die-casting treatment to form a composite cathode and a composite interfacial layer, then gently attach the smooth surface of the cut lithium foil to the surface of the composite interfacial layer, and attach the copper foil to the outside of the lithium foil, carry out pressure treatment, and stand still to make the battery interface fit fully.

[0026] As one of the preferred technical solutions, the tabletting conditions are: holding the pressure at 200 MPa for 3 minutes.

[0027] As one of the preferred technical solutions, the die-casting conditions are: die-casting at 400 MPa for 5 minutes.

[0028] As one of the preferred technical solutions, the pressure treatment conditions are: holding the pressure at 50 MPa for 3 minutes.

[0029] As one of the preferred technical solutions, the fixing nut is tightened to 2 Nm and left standing for 6 hours.

[0030] The present invention has the following beneficial effects: Due to its partially graphitized layered structure, soft carbon (SC) exhibits unique mechanical adaptability. The sliding or bending characteristics between its lamellae can effectively absorb the stress of electrode volume deformation, avoiding the phenomenon of interfacial brittle fracture. At the same time, the dense structure of the SC material can physically isolate the direct contact between lithium metal and sulfide solid electrolyte, significantly inhibiting interfacial side reactions. In addition, the conductivity of the SC material can guide Li + to achieve uniform deposition, providing guarantee for the interfacial electrochemical stability.

[0031] In the present invention, lithium difluorooxalate borate is embedded in the SC matrix. During the cycling process, a LiF nanolayer with high interfacial energy can be in-situ generated. LiF forms a fast ion diffusion network between soft carbon particles. Combining with the high ionic conductivity of LiDFOB itself, the overall ionic transport efficiency of the interfacial layer is significantly improved. At the same time, the electronic conduction of SC and the ionic conduction of LiDFOB / LiF act synergistically to promote Li + flux homogenization and reduce the local current density, thereby inhibiting the growth of lithium dendrites. This multi-scale synergistic regulation strategy provides a new solution for the interfacial engineering of high-performance solid-state lithium batteries.

[0032] The present invention constructs an interfacial protection layer and a stable solid electrolyte interface by using SC and LiDFOB. The composite interfacial layer combines structural flexibility, electron / ion double conduction ability and chemical stability. It can not only relieve the volume expansion during cycling, but also achieve uniform deposition of lithium metal through multiple mass transfer regulations. The results show that the constructed gradient protection layer has a lithium ion transference number as high as 0.93, enabling the symmetric battery to achieve a critical current density of 2 mA cm⁻ 2 . The NCM / Li full battery assembled based on this system has a capacity retention rate of 70.5% after 500 cycles at 1 C. Through GITT and DRT analysis, it is found that the battery with the composite interfacial layer has better Li + diffusion kinetics. XPS depth profiling confirms the existence of LiF in the SEI, and its spatial gradient distribution characteristics effectively inhibit the growth of lithium dendrites. This composite interfacial engineering strategy realizes the synergistic improvement of the high-rate and long-cycle performance of all-solid-state batteries through a dual-action mechanism - the three-dimensional carbon network guides the uniform deposition of lithium ions and the in-situ construction of a LiF-rich SEI layer by the decomposition of LiDFOB.

[0033] In addition to the purposes, features and advantages described above, the present invention has other purposes, features and advantages. The present invention will be further described in detail below with reference to the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The accompanying drawings, which form a part of this application, are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings: Figure 1 : Equivalent circuit in electrochemical impedance spectroscopy.

[0035] Figure 2 : XRD patterns of the composite interface before and after the composition of the composite interface raw materials.

[0036] Figure 3 : SEM morphology of SC.

[0037] Figure 4 : Electrochemical impedance spectra of (a) Li-SLL / LPSC / SLL-Li and (c) Li / LPSC / Li before and after polarization. (b) Current-time curves of Li-SLL / LPSC / SLL-Li and (d) Li / LPSC / Li at 10 mV voltage.

[0038] Figure 5 : CCD test results of lithium symmetric batteries assembled with (a) SLL and (b) LPSC.

[0039] Figure 6 : GITT curves of (a) NCM / LPSC / Li and NCM / LPSC / SLL-Li. Figure 6 (b, c) Li + Diffusion coefficient.

[0040] Figure 7 : First-cycle differential capacity curves of NCM / LPSC / SLL-Li and NCM / LPSC / Li batteries.

[0041] Figure 8 : First-cycle charge-discharge efficiency of NCM / LPSC / SLL-Li and NCM / LPSC / Li batteries at 0.1 C.

[0042] Figure 9 : Rate performance of NCM / LPSC / SLL-Li and NCM / LPSC / Li batteries.

[0043] Figure 10 : Charge-discharge curves of NCM / LPSC / SLL-Li (b) and NCM / LPSC / Li (a) batteries at different rates.

[0044] Figure 11 : Cycle performance of NCM / LPSC / SLL-Li and NCM / LPSC / Li batteries at 0.5 C.

[0045] Figure 12: Differential capacity plots of (a) NCM811 / LPSC / Li and (b) NCM / LPSC / SC-Li batteries at different cycles under 0.5C charge-discharge cycling.

[0046] Figure 13 : Performance of NCM / LPSC / SLL-Li and NCM / LPSC / Li batteries during long-term cycling at 1 C.

[0047] Figure 14 : In-situ Nyquist plots of NCM / LPSC / Li and NCM / LPSC / SLL-Li batteries.

[0048] Figure 15 DRT curves obtained by in-situ impedance analysis of (a) NCM / LPSC / Li and (b) NCM / LPSC / SLL-Li batteries. (c, d) 2D intensity color maps corresponding to the DRT curves.

[0049] Figure 16 : Electrochemical impedance and DRT test results of batteries with different interfacial layers. (a) Electrochemical impedance spectra of NCM / LPSC / SLL-Li batteries at different cycles. (b) DRT curves fitted based on the electrochemical impedance spectra; (c) Electrochemical impedance spectra of NCM / LPSC / Li batteries, (d) DRT curves fitted based on the electrochemical impedance spectra.

[0050] Figure 17 : After 300 cycles of NCM / LPSC / Li and NCM / LPSC / SLL-Li batteries, XPS spectra of (a, d) F 1s, (b, e) B 1s, and (c, f) S 2p were collected on the negative electrode interfaces of the two.

[0051] Figure 18 : Cycling performance of NCM / LPSC / SLL-Li, NCM / LPSC / LL-Li, NCM / LPSC / SL-Li, and NCM / LPSC / Li batteries at 0.5 C. Detailed implementation mode

[0052] The following will describe the embodiments of the present invention in detail with reference to the accompanying drawings. However, the present invention can be implemented in many different ways defined and covered by the claims.

[0053] The reagents and instruments involved in the present invention are shown in Tables 1 and 2.

[0054] Table 1. Reagents Reagent Name Specification (Purity) Manufacturer <![CDATA[Lithium sulfide (L2S)]]> 99.9% Aladdin <![CDATA[Phosphorus pentasulfide (P2S5)]]> 99.9% Aladdin Lithium Chloride (LiCl) 99.99% Aladdin Lithium difluoro(oxalato)borate (LiDFOB) 99.99% Aladdin Asphalt 99.99% Maclean Vapor Grown Carbon Fiber (VGCF) ≥99% Aladdin Tetrahydrofuran Spectral Grade Aladdin Absolute Ethanol >99.7% Anhui Ant Biochem Co., Ltd. NCM Analytical Reagent Kelude High Purity Lithium Ribbon (Li) Battery Grade Tianjin Zhongneng Lithium Industry Co., Ltd. Copper Foil (Cu) Battery Grade Kelude

[0055] Table 2. Main experimental instruments Equipment Name Model Manufacturer Glove Box UNIVERSAL Shanghai Mikairoana Mechatronics Technology Co., Ltd. Electrochemical Workstation CHI760E Shanghai Chenhua Instrument Co., Ltd. Powder Press HY-15 Tianjin Tianguang Optical Instrument Co., Ltd. Solid State Battery Mold JYG-10 Beijing Zhongke Wanyuan Technology Co., Ltd. Battery Test Cabinet CT-4008-5V 20mA Shenzhen Neware Electronic Co., Ltd. CNC Ultrasonic Cleaner KQ-500DA Kunshan Ultrasonic Instruments Co., Ltd. Planetary Ball Mill MSK-SFM-1S Hefei Kejing Materials Technology Co., Ltd. Vacuum Drying Oven 101-1H Zhengzhou Keheng Co., Ltd. Tube Furnace OTF-1200X-S Hefei Kejing Materials Technology Co., Ltd. Electronic Balance Pmk224zh Ohaus X-ray Powder Diffractometer Bruker D8 Advance Bruker, Germany X-ray Photoelectron Spectrometer Gammadata-scienta 2002 Shimadzu Corporation, Japan Scanning Electron Microscope NanoScope200 FEI Company, USA Raman Spectrometer InVia Raman microscope Renishaw, U.K.

[0056] 1. Material Preparation (1) Preparation of LPSC Sulfide Solid Electrolyte: Synthesize Li6PS5Cl sulfide solid electrolyte by high - energy ball milling and solid - phase sintering method. First, weigh raw materials Li2S, P2S5 and LiCl in a glove box under argon atmosphere according to a certain molar ratio. After initially mixing the materials in a mortar for 10 min, transfer them to an agate ball - milling jar (ball - to - material ratio 1:40) and ball - mill at a speed of 500 rpm for 40 h. Subsequently, transfer the obtained material to the glove box, press it into sheets under a pressure of 300 MPa, seal it in a glass tube, and quickly transfer it to a tubular furnace filled with argon. In the tubular furnace, heat it to 500℃ at a rate of 2℃ / min -1 and hold for 5 h. Finally, grind and sieve the sintered electrolyte sheet (400 mesh) to obtain Li6PS5Cl sulfide solid electrolyte material.

[0057] (2) Preparation of NCM Composite Cathode Material Weigh NCM, vapor - grown carbon fiber and sulfide solid electrolyte powder in a glove box under argon atmosphere at a mass ratio of 70:5:25, and then transfer them to an agate mortar and grind for 30 min to obtain the composite cathode material.

[0058] There is a dynamic coupling relationship between the composite cathode material and the composite interface layer during the charge - discharge process of all - solid - state lithium batteries. During the charging process, an interface layer rich in LiF with high interface energy will be formed at the negative electrode interface, thus inhibiting side reactions and protecting the lithium negative electrode. During the discharging process, due to the composite interface layer inhibiting side reactions and reducing lithium - ion loss, more lithium ions can return to the positive electrode, thus significantly improving the initial Coulomb efficiency.

[0059] The mass ratio of the composite cathode material to the composite interface layer material is 1:1.

[0060] (3) Preparation of Soft Carbon Material Add a mixture containing 3 g of pitch and 20 mL of xylene to an agate jar. Ball - mill the mixture at 300 rpm for 3 h. After drying and grinding, magnetically stir 1 g of the mixture with 0.35 mL of phosphoric acid and 4 mL of absolute ethanol for 2 h. Place the stirred material in a forced - air oven to dry to obtain soft carbon. Subsequently, place it in a tubular furnace (800℃, argon gas flow rate 40 mL / min -1 ) and pyrolyze for 3 h, with a heating rate of 5℃ / min -1 , and finally cool it to room temperature naturally.

[0061] (4) Preparation of Composite Interface Layer In a glove box with an argon atmosphere, SC, LiDFOB and sulfide solid electrolyte were weighed in a mass ratio of 8:1:1, then transferred to an agate mortar and ground for 30 min to obtain a composite interface layer material, namely SC+LiDFOB+LPSC (SLL).

[0062] 2. Electrochemical performance test (1) AC impedance test Based on the AC impedance spectrum of all-solid-state batteries, the grain resistance, grain boundary resistance and grain boundary capacitance of the electrolyte can be systematically characterized and analyzed. In the Nyquist spectrum, the high-frequency region corresponds to the ion migration behavior inside the grains, the medium and high-frequency regions reflect the blocking effect of the grain boundaries on charge transfer, and the low-frequency region mainly characterizes the polarization phenomenon at the electrode / electrolyte interface. In addition, in the AC impedance test, it was found that the curve will always deviate from the trajectory of the semicircle more or less, presenting the shape of a circular arc, also known as a capacitive reactance arc. This process is defined as a "diffusion effect", which is mainly caused by the uneven electrode surface, the poor conductivity of the adsorption layer on the electrode surface and the solution. It is used to describe the properties of the electrode double layer deviating from the ideal capacitor. The equivalent circuit diagram corresponding to the test system is shown below. Figure 1 As shown, R e : The volume resistance of the electrolyte under high frequency response, which is mainly related to the conductivity of the electrolyte; R sf : Solid electrolyte / electrode interface resistance, which describes the contact resistance at the interface; R ct : Charge transfer resistance, which characterizes the ease of charge transfer at the electrode / electrolyte interface; W: Warburg impedance, which represents the impact of ion diffusion in solid-state batteries on battery performance, usually occurs in the low frequency band.

[0063] (2) GITT test To collect GITT data, the freshly assembled cells were connected to a Neware battery test system (CT-4008T5V20mA-164) and charged and discharged at 0.1 C for three cycles. Subsequently, each cell was charged at 0.1 C for 10 min and then left to stand for 30 min to reach steady state. This process was repeated until an upper cutoff voltage of 4.3 V was reached, after which the test switched to discharge mode with the same pulse and relaxation duration until a lower cutoff voltage of 2.8 V was reached. Li + The diffusion coefficient of the ion (D Li+ ) can be calculated by formula (S1), which is: , Where m B is the mass of active material; V m and M Bare the molar volume and molar weight, respectively; S is the area of the electrode; τ is the duration of a single current pulse; E s and E τ are the voltage changes during rest and pulse duration; L is the diffusion length of Li + in the solid material.

[0064] (3) Relaxation time distribution analysis The electrochemical impedance spectrum is deeply analyzed by using the relaxation time distribution analysis method. The kinetic characteristics of different electrochemical processes can be quantitatively evaluated by calculating the characteristic relaxation time. Compared with the traditional Nyquist plot analysis method, the DRT technique extracts the characteristic frequencies in the impedance spectrum through mathematical deconvolution, and can accurately reveal the relaxation time distribution law of each dynamic process in the system

[136] . Since there are significant differences in the time required for different electrochemical processes (such as charge transfer, mass diffusion, surface adsorption, etc.) to reach a steady state, their corresponding relaxation times are significantly separated in the time domain dimension. Based on this, DRT analysis can achieve precise decoupling and quantitative characterization of multiple coupled electrochemical processes by constructing a relaxation time distribution function, providing an important technical means for the mechanism research of complex electrode systems.

[0065] (4) Li + mobility coefficient test The lithium ion transference number (T Li+ ) of the electrolyte was determined by using the combined technique of electrochemical impedance spectroscopy and DC polarization. The specific operation is as follows: First, a lithium symmetric battery was constructed, and electrochemical impedance spectroscopy was tested using a CHI760E electrochemical workstation. The test frequency range was from 1 MHz to 0.1 Hz, and an AC perturbation signal of 10 mV was applied. Subsequently, a DC constant polarization voltage of 10 mV was applied to the battery, and the change law of the current response with time was monitored in real time. After the system current reached a steady state, electrochemical impedance spectroscopy characterization was performed again. The Li + transference number was calculated by formula (S2), which comprehensively considered the impedance change parameters and steady-state current values of the system before and after polarization. Formula (S2) is: , where I0 and I S are the initial and steady-state currents, respectively, and R0 and R S are the initial and steady-state interfacial resistances between the electrolyte and the Li electrode, respectively, and V is the polarization voltage applied to the battery.

[0066] (5) Charge-discharge test of all-solid-state lithium batteries The assembled all-solid-state battery was connected to a Neware battery test system (CT-4008T5V20mA-164) for battery performance testing. First, a constant current charge-discharge test at 0.1 C for 3 cycles was carried out in the voltage range of 2.8 - 4.3 V as the full activation of the battery. The charging process of constant current first and then constant voltage was used to allow the ternary cathode material to fully release lithium ions. Then, in the voltage range of 2.8 - 4.3 V (vs. Li / Li + ), the cycling performance of the battery was evaluated at 0.5 C and 1 C, and the rate performance of the battery was tested at 0.1, 0.2, 0.5, 1, and 2 C.

[0067] (6) In-situ impedance test The in-situ impedance test was carried out through a CHI760E electrochemical workstation. The electrochemical impedance spectroscopy test was carried out in the frequency range of 1 MHz to 0.01 Hz, and the AC perturbation signal was 10 mV. By controlling the time interval for sampling, the impedance spectra at different voltage stages were tested.

[0068] 3. Assembly of all-solid-state battery A die-pressed tablet battery was used for testing, and the entire assembly process was carried out in a glove box. 100 mg of electrolyte powder was poured into a ceramic mold sleeve with a diameter of 10 mm. After installing the stainless steel stud, it was kept under pressure of 200 MPa for 3 min under a powder press. Then, the composite cathode was coated on one side of the electrolyte sheet, and the composite interface layer powder was coated on the other side. After gently shaking to ensure uniform distribution of the two, it was die-cast at 400 MPa for 5 min. Next, the smooth surface of the cut circular lithium foil was gently attached to the other side of the electrolyte sheet, and the copper foil was pasted on the outermost side of the lithium foil and kept under pressure of 50 MPa for 3 min. Finally, the fixing nut was tightened to 2 N m using a digital display torque wrench and left standing in an incubator for 6 h to allow the battery interface to fully fit.

[0069] 4. Results and discussion 4.1 Material characterization and electrochemical behavior analysis (1) Phase characterization The interface made of the mixture of the three materials was characterized by XRD ( Figure 2 ). SC did not show specific characteristic peaks in the XRD image, so it has the characteristics of an amorphous state. Obvious characteristic peaks were shown in the XRD of LiDFOB, which has the characteristics of a crystalline state, and the characteristic peaks belonging to it could be observed in the XRD of the mixture of the three materials. Thus, it can be inferred that the three were successfully mixed. In addition, the morphology of SC was examined by SEM analysis ( Figure 3 ). SC has a partially graphitized layered structure, and these lamellae can slide or bend slightly under mechanical stress, thus absorbing the stress generated by volume changes.

[0070] (2) Lithium ion migration coefficient After the applicant assembled the lithium symmetric battery, the electrochemical impedance spectroscopy and DC polarization combined method was used to conduct a preliminary electrochemical analysis on the composite interface layer and the original LPSC ( Figure 4 ) and perform T Li+ calculation. According to formula (S2), the T Li+ values of SLL and LPSC were calculated, which were 0.93 and 0.76 respectively. The closer T Li+ is to 1, the stronger the Li + diffusion ability, indicating that SLL can effectively improve the T Li+ of the interface, thereby improving the rate performance of the battery and enabling the battery to have a high power density.

[0071] (3) Critical current density test After the applicant assembled the lithium symmetric battery, the CCD test was carried out on it using the Neware battery test system. During the test, a stepped increasing current density was applied to the battery with a step size of increasing 0.1 mA cm -2 every 2 h. It was observed that the overpotential showed a continuous upward trend with the increase of the current density. Finally, the overpotential suddenly decreased irreversibly, indicating that the growth of lithium dendrites caused an internal short circuit. At this time, the corresponding current density value was defined as the CCD of the electrolyte. The specific CCD curve is as Figure 5 shown, in which the symmetric battery assembled with SLL obtained the maximum critical current density of 2.0 mA cm -1 . This further proves the improvement effect of adding the SLL interface layer on the uniform Li + deposition.

[0072] To further reveal the reason behind the ability of the composite interface to improve the battery performance rate, GITT studied the electrochemical kinetics of LPSC and SLL batteries. The GITT measurement and Li + diffusion coefficient calculation are as Figure 6 shown. Figure 6 In (a), the complete time-voltage curves of NCM / LPSC / SLL-Li and NCM / LPSC / Li batteries are shown. The SLL-based battery showed higher capacity and smaller voltage fluctuations during the titration process, indicating that the composite interface layer has good reaction transport kinetics and reduces the loss of Li + during the charge and discharge process. The diffusion coefficient of Li⁺ was further calculated ( Figure 6 in (b) and (c)). During the charge and discharge process, the diffusion coefficient of Li⁺ in SLL is greater than that in LPSC, directly proving that the battery assembled with SLL has better Li + transport performance.

[0073] 4.2 Battery performance analysis Figure 7 The differential capacity plots (dQ / dV-V) obtained from the first charge-discharge curves of NCM / LPSC / SLL-Li and NCM / LPSC / Li batteries at 0.1 C are shown. The peak values of the oxidation / reduction peaks in the figure correspond to the potentials at which phase transitions occur, similar to cyclic voltammetry curves. During the charging process (on the dashed line), the peak values at approximately 3.7, 4.0, and 4.2 V respectively reflect the phase transitions of H1→M, M→H2, and H2→H3, and the peak values during the discharging process reflect the reduction phase transitions. In addition, it can be seen that the peak positions of the two batteries are almost the same, indicating that they exhibit almost the same electrochemical behavior at the beginning of the constant current charge-discharge test. However, it is still not difficult to find that the NCM / LPSC / SLL-Li battery material has a larger integral area, which implies better Li storage capacity and electrochemical activity.

[0074] The applicant analyzed the first charge-discharge test results of NCM / LPSC / SLL-Li and NCM / LPSC / Li batteries at 0.1 C. Figure 8 Shows the initial charge-discharge curves of the two batteries at 0.1 C (1 C = 200 mA g –1 )). The first-cycle discharge capacity of NCM / LPSC / SLL-Li is 175.4 mAh g –1 , and the initial Coulombic efficiency (ICE) is 82.18%, which is significantly greater than the ICE of NCM / LPSC / Li (77.83%). In addition, it can also be clearly seen that there is no obvious difference in the first charging platform of the two batteries, indicating that the introduction of the composite layer does not increase the first polarization of the electrode. Obviously, the initial discharge capacity of the battery with the added composite layer is improved compared to the original material, which is mainly attributed to the inhibitory effect of the SLL composite layer on the initial capacity loss. Specifically, this composite layer effectively improves the compactness and stability of the SEI film on the electrode surface, reduces the loss of lithium ions caused by irreversible side reactions, and thus significantly improves the first Coulombic efficiency.

[0075] The rate performance tests were carried out on NCM / LPSC / SLL-Li and NCM / LPSC / Li batteries, and the results are as Figure 9 shown. The battery assembled with SLL performs better than the battery without the intermediate layer in this test, having the highest discharge capacity at the same rate, especially the capacity difference is most significant at the high rate of 2 C. When the rate drops to 0.1 C, the discharge capacity of the battery assembled based on SLL recovers to a level comparable to that before high-rate cycling. From the charge-discharge curves at different rates ( Figure 10It can be seen that as the magnification increases, the discharge platforms of all batteries show an increasing trend. At the same time, the applicant can observe that the polarization change of the battery assembled with SLL is the smallest, indicating that the battery assembled with SLL has good reversibility.

[0076] To further evaluate the cycling performance of all-solid-state lithium batteries, the applicant compared and analyzed the long-term cycling characteristics of the NCM / LPSC / SLL-Li and NCM / LPSC / Li systems at 0.5 C ( Figure 11 ). The experimental data show that the initial capacity of the NCM811 / LPSC / SLL-Li battery is 153.1 mAh g -1 , and the capacity retention rate is 88% after 200 cycles, demonstrating excellent cycling stability. In contrast, the initial capacity of the NCM811 / LPSC / Li battery is 123.1 mAh g -1 , and the capacity decay is significant (the retention rate drops sharply to 55% after 200 cycles).

[0077] To further explore the evolution law of electrode reactions during cycling, the applicant carried out differential capacity analysis. As shown in (a) of Figure 12 , the oxidation peak of the NCM / LPSC / Li battery shows a trend of shifting to higher voltages with increasing cycle number, and the reduction peak intensity weakens and shifts towards lower potential, indicating the deterioration of Li + insertion / extraction kinetics and the aggravation of interfacial polarization. On the contrary, for the NCM / LPSC / SLL-Li battery ( Figure 12 shown in (b)), its differential capacity curve shows a high degree of coincidence, confirming that this system has a stable lithium-ion transport channel and a reversible electrode reaction process, which is consistent with the long-term cycling performance results. The above results indicate that the composite interface layer optimizes the interfacial transport efficiency during cycling and significantly improves the cycling durability of all-solid-state lithium batteries.

[0078] To further verify the ability of the composite interface to protect the lithium anode during high-rate charge and discharge, the applicant further demonstrated the long-term cycling performance of the NCM / LPSC / SLL-Li and NCM811 / LPSC / Li batteries at 1 C. The NCM / LPSC / SLL-Li battery can exhibit an initial discharge specific capacity of 127.0 mAh g Figure 13 at 1 C and a capacity retention rate of 70.5% after 500 cycles. Obviously, the composite interface layer can greatly reduce the side reactions on the lithium anode, thus achieving excellent cycling stability at high rates. -1

[0079] 4.3 Mechanism Study (1) Interfacial SEI Analysis ​To deeply analyze the dynamic formation and evolution mechanism of the interfacial film in all-solid-state lithium batteries, the applicant adopted the method of in-situ electrochemical impedance spectroscopy combined with relaxation time distribution analysis. The experimental design is as follows: During the battery cycling process (such as the first-week formation, the cycling stable period, and the capacity fade stage), the charge and discharge are paused at a specific state of charge (SOC), the electrochemical impedance spectroscopy test is carried out in the frequency range of 1 MHz to 0.01 Hz, the AC perturbation signal is 10 mV, and the impedance spectrum data at multiple time nodes are collected.

[0080] The in-situ impedance test results of NCM / LPSC / Li and NCM / LPSC / SLL-Li batteries are as Figure 14 shown. The in-situ impedance test results of both materials show that as the voltage increases, the impedance of the electrode material first decreases and then increases. It is not difficult to see that both materials exhibit the smallest impedance at 4.0 V, and the NCM / LPSC / SLL-Li battery has a smaller impedance compared to the NCM / LPSC / Li battery, which means that the composite interfacial layer has better kinetic performance and diffusion coefficient. This conclusion is consistent with the above conclusion, indicating that the addition of SC and LiDFOB can effectively improve the rate performance and cycling stability of the electrode material. To obtain more detailed kinetic data, the relaxation time distribution curve (DRT) needs to be used to further analyze the EIS data for a deeper understanding of the electrochemical kinetic process.

[0081] By deconvoluting the complex impedance data through the DRT algorithm, the solid electrolyte grain boundary signal (R s ) (τ≈10 -6 ~10 -5 s), the negative electrode interface impedance R SEI (τ≈10 -5 ~10 -4 s), and the charge transfer impedance R ct (in the low-frequency region τ>10 -3 s) and R d diffusion impedance (in the low-frequency region τ>10 0 s) and other characteristic peaks of the kinetic process are separated. Quantitatively characterize the growth trend of the SEI film impedance with the number of cycles and verify the ion transport characteristics of the SEI film analyzed by DRT.

[0082] From the DRT curves ( Figure 15 (a) and (b) in) analyzed by in-situ impedance, it can be obtained that the R SEI of the NCM / LPSC / Li battery gradually increases with the increase of voltage, indicating that more Li + participates in side reactions. To more intuitively identify the evolution law of the DRT curve, the DRT curve is analyzed into a 2D intensity color map ( Figure 15In (c) and (d)). In the figure, the applicant can clearly find that the most obvious difference between the two is in the red dotted line frame, where the R of the NCM / LPSC / Li battery SEI is larger and the corresponding color is deeper.

[0083] To further explore the influence of the evolution of the composite interface layer during cycling on the battery, electrochemical impedance and DRT analyses were performed on these NCM / LPSC / SLL-Li and NCM / LPSC / Li batteries after cycling to distinguish the impedance response changes at different interfaces of the battery. The semi-quantitative distribution of the DRT analysis was further used to determine the detailed electrochemical contributions of the main time constants in the EIS. Figure 16 In (a) and (c) are the EIS of the NCM / LPSC / SLL-Li and NCM / LPSC / Li batteries after 100, 200, and 300 cycles, respectively. The resolved DRT results are as Figure 16 shown in (b) and (d), where the peaks at 10 -6 and 10 -5 s correspond to the solid electrolyte grain boundary signal (R s ), respectively. The peaks at 10 -5 and 10 -4 s belong to the solid electrolyte interface (R SEI ), and the peak at 10 -3 s is related to R ct .

[137] .

[0084] The results show that R SEI in the NCM / LPSC / Li battery changes significantly with cycling, which means that there are obvious diffusion barriers at the battery interface, mainly due to the more intense reaction between the electrolyte and the lithium negative electrode at high current density, and the generated by-products passivate the interface, reducing the interlayer Li + solid-state diffusion kinetics. Encouragingly, the NCM / LPSC / SLL-Li battery exhibits a lower solid-state diffusion peak, indicating that the composite interface layer works together to modify and stabilize the layered structure of the material, enabling Li + to have better diffusion kinetics.

[0085] (2) Interface XPS composition analysis To explore the electrochemical environment changes at the interface layer of the negative electrode SEI film before and after electrochemical cycling, the battery after 300 cycles at 1 C was disassembled, and XPS analysis was performed on its negative electrode interface. As Figure 17 shown, the figure shows the electrochemical environment of the interface layer of the SEI film on the lithium metal negative electrode after cycling. Figure 17Figures (a) and (d) respectively show the F 1s spectra of SLL and LPSC. It can be clearly seen from the figures that the F 1s of SLL is divided into three deconvolutions, namely P-F / C-F, Li x PO y F z and LiF. The presence of LiF is beneficial to delaying the reduction of the electrolyte and promoting Li + diffusion. In addition, the signal peak of B 1s in the SLL material at 192 eV is attributed to Li x B y OF z , as shown in Figure 17 (e), further confirming the contribution of LiDFOB to the SEI film, while the B 1s signal in the LPSC material is not detected, as shown in Figure 17 (b).

[0086] The by-products generated at the interface between the solid-state electrolyte and the lithium metal anode during the electrochemical reaction are also one of the factors affecting the electrochemical performance of the battery. To further clarify the influence of LiDFOB on the interface between the solid-state electrolyte and the lithium metal anode, XPS was used to analyze the S 2p of two materials, SLL and LPSC, respectively, as shown in Figure 17 (c) and (f). It is not difficult to see from Figure 17 (c) and (f) that the S 2p spectrum of SLL is refined into two deconvolutions. The main peak located at 161.7 eV is attributed to the PS4 3- group of LPSC, while the S 2p of LPSC is refined into four deconvolutions. The main peak at 161.7 eV corresponds to the PS4 3- group of LPSC. The small peak near 163 eV implies the by-product polysulfide S x 0 / S x 2- , and the peak at 160.3 eV is attributed to Li2S. The two are the product signals after LPSC is reduced by lithium metal. This shows that the introduction of LiDFOB effectively inhibits the generation of by-products, enhances the structural stability of the solid-state electrolyte, reduces the electrode interface impedance, and improves the Li + diffusion coefficient, ultimately achieving excellent cycle performance and rate performance of all-solid-state lithium batteries.

[0087] To further evaluate the influence of different components in the composite interface layer on the cycle performance of all-solid-state lithium batteries, the cycle performance of NCM / LPSC / SLL-Li, NCM / LPSC / LL-Li, NCM / LPSC / SL-Li and NCM / LPSC / Li batteries at 0.5 C was compared and analyzed ( Figure 18). The results show that the initial capacity of the NCM811 / LPSC / SLL-Li battery is 153.1 mAh g -1 , and the capacity retention rate is 88% after 200 cycles, showing excellent cycle stability. The initial capacity of the NCM / LPSC / LL-Li battery is 148.5 mAh g -1 , and the capacity retention rate is 76% after 200 cycles. The initial capacity of the NCM / LPSC / SL-Li battery is 144.3 mAh g -1 , and the capacity retention rate is 60% after 200 cycles. The initial capacity of the NCM811 / LPSC / Li battery is 123.1 mAh g -1 , and the capacity retention rate is 55% after 200 cycles.

[0088] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A composite interfacial layer material, characterized in that, It is obtained by mixing and grinding soft carbon, lithium difluorooxalate borate, and Li6PS5Cl sulfide solid electrolyte in a mass ratio of 8:0.5 - 1.5:0.5 - 1.

5.

2. The composite interfacial layer material according to claim 1, characterized in that, The mixing and grinding time is 30 minutes.

3. The composite interfacial layer material according to claim 1, characterized in that, The soft carbon is prepared by the following method: First, mix and ball-mill asphalt and xylene, dry, and grind to obtain a mixture. Then, mix and stir the mixture with 1 mol / L phosphoric acid solution and absolute ethanol, dry, pyrolyze, and naturally cool to room temperature to obtain it.

4. The composite interfacial layer material according to claim 1, characterized in that, The Li6PS5Cl sulfide solid electrolyte is prepared by the following method: Under an argon atmosphere, take raw materials Li2S, P2S5, and LiCl and grind and mix them in a molar ratio of 5:1:2 to obtain a premix, ball-mill, press into tablets, seal in a glass tube, and then quickly transfer it to a tubular furnace filled with argon, heat and sinter, and grind and sieve to obtain it.

5. Application of the composite interfacial layer material according to any one of claims 1 to 4 in the preparation of all-solid-state lithium batteries.

6. An all-solid-state lithium battery, characterized in that, It includes a metallic lithium anode and a sulfide solid electrolyte, and a composite cathode and a composite interface layer made of the composite interface layer material according to claim 1 are respectively provided on both sides of the sulfide solid electrolyte.

7. The all-solid-state lithium battery according to claim 6, characterized in that, The composite cathode is made of NCM composite cathode material, and the mass ratio of the NCM composite cathode material to the composite interface layer material is 1:

1.

8. The all-solid-state lithium battery according to claim 7, characterized in that, The NCM composite cathode material is prepared by the following method: Under an argon atmosphere, mix and grind NCM, vapor-grown carbon fiber, and Li6PS5Cl sulfide solid electrolyte in a mass ratio of 70:5:25 for 30 minutes to obtain it.

9. A method for preparing the all-solid-state lithium battery according to claim 6, characterized in that, The specific steps are as follows: Under an argon atmosphere, first press the sulfide solid electrolyte into an electrolyte sheet, then evenly coat the composite cathode material and the composite interface layer material on both sides of the electrolyte sheet respectively, perform die-casting treatment to form a composite cathode and a composite interface layer, then gently attach the smooth surface of the cut lithium foil to the surface of the composite interface layer, stick the copper foil on the outside of the lithium foil, perform pressure treatment, and let it stand still to make the battery interface fit well.

Images