A composite interfacial layer material and its application in the preparation of all-solid-state lithium batteries

Abstract

The present invention provides a composite interfacial layer material and its application in the preparation of all-solid-state lithium batteries. The composite interfacial layer material is obtained by mixing and grinding soft carbon, lithium difluorooxalate borate, and Li6PS5Cl sulfide solid electrolyte. In the present invention, lithium difluorooxalate borate is embedded in the SC matrix, and a LiF nanolayer with high interfacial energy can be in-situ generated during the cycling process. LiF forms a fast ion diffusion network between soft carbon particles. Combining with the high ionic conductivity of LiDFOB itself, the overall ion transport efficiency of the interfacial layer is significantly improved. At the same time, the electron conduction of SC and the ionic conduction of LiDFOB / LiF act synergistically to promote the homogenization of Li+ flux 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.

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. Solid electrolytes not only fundamentally solve the safety hazards of easy combustion and leakage in liquid systems, but also their high mechanical strength characteristics make it possible to achieve 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 energy density of the battery.

[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 suppressing 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 object 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 interfacial layer material and its application in the preparation of all-solid-state lithium batteries.

[0007] 1. A composite interfacial 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 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 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 foregoing composite interface layer material in the preparation of all-solid-state lithium batteries.

[0022] 3. An all-solid-state lithium battery, including a metallic lithium negative electrode and a sulfide solid electrolyte, and a composite positive electrode and a composite interface layer made of the foregoing composite interface layer material are respectively provided on both sides of the sulfide solid electrolyte.

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

[0024] As one of the further preferred technical solutions, the NCM composite positive electrode 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 positive electrode material and the composite interface layer material on both sides of the electrolyte sheet respectively, carry out die-casting treatment to form a composite positive electrode and a composite interface layer, then gently attach the smooth surface of the cut lithium foil to the surface of the composite interface 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 fully fit.

[0026] As one of the preferred technical solutions, the tabletting conditions are: maintaining a pressure of 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: maintaining a pressure of 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:

[0031] Soft carbon (SC), with its partially graphitized layered structure, exhibits unique mechanical adaptability. The sliding or bending characteristics between its lamellae can effectively absorb the stress of electrode volume deformation and avoid 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.

[0032] The present invention embeds lithium difluoro(oxalato)borate (LiDFOB) into 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, it significantly improves the overall ion transport efficiency of the interfacial layer. At the same time, the electron 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 collaborative regulation strategy provides a new solution for the interfacial engineering of high-performance all-solid-state lithium batteries.

[0033] 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 all-solid-state 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 decomposition of LiDFOB constructs a LiF-rich SEI layer.

[0034] In addition to the purposes, features and advantages described above, the present invention has other purposes, features and advantages. The following will refer to the drawings to further elaborate on the present invention in detail. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] 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:

[0036] Figure 1 : Equivalent circuit in electrochemical impedance spectroscopy.

[0037] Figure 2 : XRD patterns of the composite interface composition raw materials and the composite interface.

[0038] Figure 3 : SEM morphology of SC.

[0039] 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.

[0040] Figure 5 : CCD test results of lithium symmetric batteries assembled with (a) SLL and (b) LPSC.

[0041] Figure 6 : GITT curves of (a) NCM / LPSC / Li and NCM / LPSC / SLL-Li. Figure 6 (b, c) Li + Diffusion coefficient.

[0042] Figure 7 : First-cycle differential capacity curves of NCM / LPSC / SLL-Li and NCM / LPSC / Li batteries.

[0043] Figure 8 : First-cycle charge-discharge efficiency of NCM / LPSC / SLL-Li and NCM / LPSC / Li batteries at 0.1 C.

[0044] Figure 9 : Rate performance of NCM / LPSC / SLL-Li and NCM / LPSC / Li batteries.

[0045] Figure 10 : Charge-discharge curves of NCM / LPSC / SLL-Li (b) and NCM / LPSC / Li (a) batteries at different rates.

[0046] Figure 11 : Cycling performance of NCM / LPSC / SLL-Li and NCM / LPSC / Li batteries at 0.5 C.

[0047] Figure 12 : (a) Differential capacity plots of NCM811 / LPSC / Li and (b) NCM / LPSC / SC-Li batteries at different cycles during 0.5C charge-discharge cycling.

[0048] Figure 13 : Long-term cycling performance of NCM / LPSC / SLL-Li and NCM / LPSC / Li batteries at 1C.

[0049] Figure 14 : In-situ Nyquist plots of NCM / LPSC / Li and NCM / LPSC / SLL-Li batteries.

[0050] Figure 15 DRT curves obtained from 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.

[0051] Figure 16 : Electrochemical impedance and DRT test results of batteries with different interlayers. (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.

[0052] 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 at the negative electrode interfaces of the two.

[0053] Figure 18 : Cycling performance of NCM / LPSC / SLL-Li, NCM / LPSC / LL-Li, NCM / LPSC / SL-Li, and NCM / LPSC / Li batteries at 0.5C. Detailed implementation manners

[0054] The following describes the embodiments of the present invention in detail with reference to the accompanying drawings, but the present invention can be implemented in many different ways defined and covered by the claims.

[0055] The present invention relates to reagents and instruments as shown in Tables 1 and 2.

[0056] Table 1. Reagents

[0057] 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 Pitch 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 Keluode High-Purity Lithium Ribbon (Li) Battery Grade Tianjin Zhongneng Lithium Industry Co., Ltd. Copper Foil (Cu) Battery Grade Keluode

[0058] Table 2. Main experimental instruments

[0059] Equipment Name Model Manufacturer Glove Box UNIVERSAL Shanghai Micro-Nano Optoelectronic 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. Digital Control 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 Corporation, 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.

[0060] 1. Material Preparation

[0061] (1) Preparation of LPSC Sulfide Solid Electrolyte:

[0062] Synthesize the Li6PS5Cl sulfide solid electrolyte by high - energy ball milling and solid - phase sintering method. First, weigh the raw materials Li2S, P2S5, and LiCl in a glove box under an 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 mill at a speed of 500 rpm for 40 h. Subsequently, transfer the obtained material to the glove box, press it into a sheet 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 °C at a rate of 2 °C / min and hold for 5 h. Finally, grind and sieve the sintered electrolyte sheet (400 mesh) to obtain the Li6PS5Cl sulfide solid electrolyte material. -1

[0063] (2) Preparation of NCM Composite Cathode Material

[0064] Weigh NCM, vapor - grown carbon fiber, and sulfide solid electrolyte powder in a glove box under an argon atmosphere in a mass ratio of 70:5:25. Then transfer them to an agate mortar and grind for 30 min to obtain the composite cathode material.

[0065] 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, thereby suppressing side reactions and protecting the lithium negative electrode. During the discharging process, due to the suppression of side reactions by the composite interface layer, the loss of lithium ions is reduced, enabling more lithium ions to return to the positive electrode, thus significantly improving the initial Coulomb efficiency.

[0066] The mass ratio of the composite cathode material to the composite interface layer material is 1:1.

[0067] (3) Preparation of Soft Carbon Material

[0068] Add a mixture containing 3 g of pitch and 20 mL of xylene to an agate jar. The mixture is milled at 300 rpm for 3 h. After drying and grinding, stir 1 g of the mixture with 0.35 mL of phosphoric acid and 4 mL of absolute ethanol magnetically 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 °C, argon gas flow rate 40 mL / min) -1 ​) was pyrolyzed for 3 h at a heating rate of 5 °C min -1 , and finally cooled naturally to room temperature.

[0069] (4) Preparation of composite interface layer

[0070] In an argon atmosphere glove box, 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).

[0071] 2. Electrochemical performance test

[0072] (1) AC impedance test

[0073] 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 boundary on charge transfer, and the low-frequency region mainly characterizes the polarization phenomenon at the electrode / electrolyte interface. In addition, it was found in the AC impedance test that the curve will always deviate from the trajectory of the semicircle more or less, showing the shape of a circular arc, also known as a capacitive reactance arc. This process is defined as a "diffusion effect". The main reasons for its occurrence are 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 as follows: 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 indicates the impact of ion diffusion in solid-state batteries on battery performance, usually occurs in the low frequency band.

[0074] (2) GITT test

[0075] To collect GITT data, the freshly assembled cells were connected to a Neware battery test system (CT-4008T5V20mA-164) and charged and discharged for three cycles at 0.1 C. 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 the 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. + The diffusion coefficient of ions (D Li+) It can be calculated by formula (S1), and formula (S1) is:

[0076] ,

[0077] where m B is the mass of the active material; V m and M B are 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.

[0078] (3) Relaxation time distribution analysis

[0079] The relaxation time distribution analysis method is used to deeply analyze the electrochemical impedance spectrum, and 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 technology 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.

[0080] (4) Li + Migration coefficient test

[0081] The lithium ion transference number (T Li+ ) of the electrolyte was determined by using the combined technology of electrochemical impedance spectroscopy and DC polarization. The specific operation is as follows: First, a lithium symmetric battery is constructed, and the electrochemical impedance spectroscopy is tested by using a CHI760E electrochemical workstation. The test frequency range is from 1 MHz to 0.1 Hz, and an AC perturbation signal of 10 mV is applied. Subsequently, a DC constant polarization voltage of 10 mV is applied to the battery, and the variation law of the current response with time is monitored in real time. After the system current reaches a steady state, the electrochemical impedance spectroscopy characterization is carried out again. The Li + transference number is calculated by formula (S2). This formula combines the impedance change parameters and the steady-state current value of the system before and after polarization. Formula (S2) is:

[0082] ,

[0083] where I0 and I Sare the initial and steady-state currents, 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.

[0084] (5)Charge and discharge tests of all-solid-state lithium batteries

[0085] The assembled all-solid-state battery was connected to a Neware battery test system (CT-4008T5V20mA-164) for battery performance testing. First, three cycles of constant current charge and discharge tests at 0.1 C were carried out in the voltage range of 2.8 - 4.3 V as the full activation of the battery. The working steps of constant current charging first and then constant voltage charging were 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 cycle 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.

[0086] (6)In-situ impedance test

[0087] 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.

[0088] 3. Assembly of all-solid-state batteries

[0089] 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 a stainless steel stud, it was kept under pressure at 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. It was gently shaken to ensure uniform distribution of the two, and 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 at 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 fit well.

[0090] 4. Results and discussion

[0091] 4.1 Material characterization and electrochemical behavior analysis

[0092] (1)Phase characterization

[0093] The interface formed by mixing the three materials was characterized by XRD ( Figure 2), SC did not exhibit 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, thereby absorbing the stress generated by volume changes.

[0094] (2) Lithium ion migration coefficient

[0095] 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 of 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.

[0096] (3) Critical current density test

[0097] 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 at a step size of 0.1 mA cm -2 increasing 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, and 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.

[0098] In order 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 6Figure (a) shows the complete time-voltage curves of NCM / LPSC / SLL-Li and NCM / LPSC / Li batteries. The SLL-based battery exhibits higher capacity and smaller voltage fluctuations during titration, indicating that the composite interface layer has good reaction transport kinetics and reduces Li + loss during charge and discharge. The diffusion coefficient of Li⁺ ( Figure 6 Figures (b) and (c)) was further calculated. The diffusion coefficient of Li⁺ in SLL is greater than that in LPSC during charge and discharge, directly demonstrating that the battery assembled with SLL has better Li + transport performance.

[0099] 4.2 Battery performance analysis

[0100] Figure 7 Figure shows 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. The peak values of the oxidation / reduction peaks in the figure correspond to the potentials at which phase transitions occur, similar to cyclic voltammograms. The peaks at approximately 3.7, 4.0, and 4.2 V during charging (on the dashed line) reflect the phase transitions of H1→M, M→H2, and H2→H3, respectively, and the peaks during discharging reflect the reduced 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, suggesting its better Li storage capacity and electrochemical activity.

[0101] 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 Figure 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 Coulomb efficiency (ICE) is 82.18%, which is significantly greater than the ICE (77.83%) of NCM / LPSC / Li. 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 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 Coulomb efficiency.

[0102] The rate performance of NCM / LPSC / SLL-Li and NCM / LPSC / Li batteries was tested, and the results are as Figure 9 shown. The battery assembled with SLL performed better than the battery without the intermediate layer in this test, having the highest discharge capacity at the same rate, especially the capacity difference was most significant at the high rate of 2 C. When the rate dropped to 0.1 C, the discharge capacity of the battery assembled based on SLL recovered to a level comparable to that before high-rate cycling. From the charge-discharge curves at different rates ( Figure 10 ), it can be seen that as the rate increased, the discharge platforms of all batteries showed an increasing trend, and at the same time, the applicant could see that the polarization change of the battery assembled based on SLL was the smallest, indicating that the battery assembled with SLL had good reversibility.

[0103] To further evaluate the cycling performance of all-solid-state lithium batteries, the applicant comparatively analyzed the long-cycle characteristics of NCM / LPSC / SLL-Li and NCM / LPSC / Li systems at 0.5 C ( Figure 11 ). The experimental data showed that the initial capacity of the NCM811 / LPSC / SLL-Li battery was 153.1 mAh g -1 , and the capacity retention rate was 88% after 200 cycles, demonstrating excellent cycling stability. In contrast, the initial capacity of the NCM811 / LPSC / Li battery was 123.1 mAh g -1 , and the capacity decay was significant (the retention rate dropped sharply to 55% after 200 cycles).

[0104] To further explore in depth 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 showed a trend of moving towards higher voltages with the increase in the number of cycles, the reduction peak intensity weakened and shifted towards lower potentials, 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 in (b)), its differential capacity curve showed a high degree of coincidence, confirming that this system had a stable lithium-ion transport channel and a reversible electrode reaction process, which was mutually verified with the long-cycle performance results. The above results indicate that the composite interface layer optimized the interfacial transport efficiency during cycling and significantly improved the cycling durability of all-solid-state lithium batteries.

[0105] To further verify the ability of the composite interface to protect the lithium anode during high-rate charge and discharge, the applicant carried out the test in Figure 13Further shows the long-cycle performance of 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 -1 and shows 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 obtaining excellent cycle stability at high rates.

[0106] 4.3 Mechanism study

[0107] (1)Interface SEI analysis

[0108] To deeply analyze the dynamic formation and evolution mechanism of the interfacial film in all-solid-state lithium batteries, the applicant adopted in-situ electrochemical impedance spectroscopy combined with relaxation time distribution analysis method. The experimental design is as follows: During the battery cycling process (such as the first-week formation, cycling stable period, and capacity decay 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.

[0109] 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 these two materials both show that with the increase of voltage, the impedance of the electrode material first decreases and then increases. It is not difficult to see that both materials show 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 interface 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 cycle stability of the electrode material. To obtain more detailed kinetic data, it is necessary to further analyze the EIS data using the distribution of relaxation times (DRT) curve to more deeply understand the electrochemical kinetic process.

[0110] The complex impedance data is deconvolved by the DRT algorithm to separate 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 dDiffusion impedance (low frequency region τ>10 0 s) and other characteristic peaks of kinetic processes. Quantitatively characterize the growth trend of SEI membrane impedance with cycle number and verify the SEI membrane ion transport characteristics analyzed by DRT.

[0111] DRT curve analyzed by in-situ impedance ( Figure 15 From (a) and (b) above, it can be concluded that the R SEI As the voltage increases, it gradually becomes larger, which means more Li + Participate in side reactions. In order to more intuitively identify the evolution of the DRT curve, the DRT curve is analyzed into a 2D intensity color map ( Figure 15 In (c), (d)). In the figure, the applicant can clearly see that the red dotted line frame is the place where the difference between the two is most obvious. The R SEI The bigger, the darker the color.

[0112] To further investigate the impact of the evolution of the composite interfacial layer during cycling, post-cycling electrochemical impedance spectroscopy (EIS) and density-transfer (DRT) analyses were performed on the NCM / LPSC / SLL-Li and NCM / LPSC / Li cells to distinguish the impedance responses at different interfaces. The semi-quantitative distribution of DRT analysis was further used to determine the detailed electrochemical contributions of the main time constants in the electrochemical impedance spectroscopy (EIS). Figure 16 (a) and (c) are the EIS of NCM / LPSC / SLL-Li and NCM / LPSC / Li batteries after 100, 200, and 300 cycles, respectively. Figure 16 As shown in (b) and (d), 10 -6 and 10 -5 The peaks at s correspond to the solid electrolyte grain boundary signals (R s ), 10 -5 and 10 -4 The peak at s belongs to the solid electrolyte interface (R SEI ), 10 -3 The peak value of s and R ct related

[137] .

[0113] The results show that the R SEI The change with the cycle is more obvious, which means that there is an obvious diffusion barrier at the battery interface. This is mainly attributed to the fact that at high current density, the reaction between the electrolyte and the lithium negative electrode is more intense, 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 synergistically modifies and stabilizes the layered structure of the material, enabling Li + to have better diffusion kinetics.

[0114] (2)Interface XPS composition analysis

[0115] To explore the electrochemical environment changes at the negative electrode SEI film interface layer 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 presents the electrochemical environment of the SEI film interface layer of the lithium metal negative electrode after cycling. Figure 17 Among them, (a) and (d) respectively show the F 1s spectra of SLL and LPSC. It can be clearly seen from the figure 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 for 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 Figure 17 shown in (e), further confirming the contribution of LiDFOB to the SEI film, while the B 1s signal in the LPSC material is not detected, as Figure 17 shown in (b).

[0116] The by-products generated at the interface between the solid-state electrolyte and the lithium metal negative electrode 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 negative electrode, XPS was used to analyze the S 2p of the two materials, SLL and LPSC, respectively, as Figure 17 shown in (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 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, and the small peak near 163 eV implies the side reaction product polysulfide S x 0 / S x 2-, the peak at 160.3 eV was attributed to Li2S, and both are the product signals after LPSC was reduced by lithium metal. This indicates that the introduction of LiDFOB effectively inhibits the generation of by-products, enhances the structural stability of the solid electrolyte, reduces the electrode interface impedance, and improves the Li + diffusion coefficient, and finally realizes excellent cycle performance and rate performance of all-solid-state lithium batteries.

[0117] 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 after 200 cycles is 88%, showing excellent cycle stability. The initial capacity of the NCM / LPSC / LL-Li battery is 148.5 mAh g -1 1, and the capacity retention rate after 200 cycles is 76%. The initial capacity of the NCM / LPSC / SL-Li battery is 144.3 mAh g -1 , and the capacity retention rate after 200 cycles is 60%. The initial capacity of the NCM811 / LPSC / Li battery is 123.1 mAh g -1 , and the capacity retention rate after 200 cycles is 55%.

[0118] The above are only the preferred embodiments of the present invention and are not used 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 interface layer material, characterized in that, It is disposed on one side surface of the Li6PS5Cl sulfide solid electrolyte sheet and 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, and embedding lithium difluorooxalate borate into the soft carbon matrix.

2. The composite interface layer material according to claim 1, wherein The mixing and grinding time is 30 minutes.

3. A composite interface layer material according to claim 1, characterized in that, The soft carbon is prepared by the following method: First, pitch and xylene are mixed and ball-milled, dried, and ground to obtain a mixture. Then, the mixture is mixed and stirred with 1 mol / L phosphoric acid solution and absolute ethanol, dried, pyrolyzed, and naturally cooled to room temperature to obtain the soft carbon.

4. A composite interface layer material according to claim 1, characterized in that, The Li6PS5Cl sulfide solid electrolyte is prepared by the following method: Under an argon atmosphere, raw materials Li2S, P2S5, and LiCl are ground and mixed evenly in a molar ratio of 5:1:2 to obtain a premix, ball-milled, pressed into tablets, sealed in a glass tube, and then quickly transferred to a tube furnace filled with argon, heated and sintered, and ground and sieved to obtain the Li6PS5Cl sulfide solid electrolyte.

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

6. All-solid-state lithium battery, characterized in that, It includes a metallic lithium negative electrode and a sulfide solid electrolyte. The sulfide solid electrolyte is the Li6PS5Cl sulfide solid electrolyte, and a composite positive electrode and a composite interfacial layer made of the composite interfacial layer material according to claim 1 are respectively disposed on both sides of the sulfide solid electrolyte.

7. The all-solid-state lithium battery according to claim 6, wherein, The composite positive electrode is made of an NCM composite positive electrode material, and the mass ratio of the NCM composite positive electrode material to the composite interfacial layer material is 1:

1.

8. The all-solid-state lithium battery according to claim 7, characterized in that, The NCM composite positive electrode 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 in a mass ratio of 70:5:25 for 30 minutes to obtain the NCM composite positive electrode material.

9. A method for preparing an all-solid-state lithium battery, characterized in that, The specific steps are as follows: Under an argon atmosphere, first, the sulfide solid electrolyte is pressed into an electrolyte sheet, and then the composite positive electrode material and the composite interfacial layer material according to claim 1 are uniformly coated on both sides of the electrolyte sheet respectively, followed by die-casting treatment to form a composite positive electrode and a composite interfacial layer. Then, the smooth surface of the cut lithium foil is gently attached to the surface of the composite interfacial layer, and a copper foil is pasted on the outside of the lithium foil, followed by pressure treatment and standing to ensure that the battery interface fits well; the sulfide solid electrolyte is the Li6PS5Cl sulfide solid electrolyte.

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