Application And Electrochemical Performance Of Sulfide Electrolyte In All-Solid-State Battery

All-solid-state batteries?ASSB?have attracted much attention due to their high safety and ability to match high voltage cathodes and lithium anodes with high theoretical capacity(3860 m Ah g^-1)and low voltage?-3.04 V vs.SHE?.As one of the most critical components of ASSB,the solid-state electrolyte directly affects the performance of the ASSB,so it is vital to find a suitable solid electrolyte.In an ASSB,each component is in solid-solid contact and has a high internal resistance.And the mismatched electrolyte-electrode interface can cause electrolyte decomposition or the formation of unstable mesophase.These results all result in a higher charge transfer impedance,making it challenging to achieve regular operation of ASSB.Based on the narrow electrochemical stability window of the sulfide electrolyte,combined with theoretical calculations,the positive electrode is coated to widen the actual working voltage window of the electrolyte and solve the problem of electrolyte interface compatibility.In this paper,an argyrodites sulfide electrolyte was synthesized.By optimizing the synthesis conditions and modifying the surface of the positive electrode material,the problems of narrow stable voltage window of the sulfide electrolyte and incompatibility with the interface of transition metal oxide are solved,to improve the cycling performance of ASSB.The main research contents of this paper are as follows:?1?The sulfide electrolyte Li_6.25PS_5.25Cl_0.75 was prepared by a two-step method of ball milling and heat treatment.The effects of ball milling time,calcination,and calcination temperature on the synthetic electrolyte were discussed.Calcined uniformly ball-milled electrolyte precursors in the temperature range of 490-550?can effectively obtain sulfide electrolyte.The physicochemical properties of the synthesized sulfide electrolyte were characterized.The results show that it has high ion conductivity,low electron conductivity,and a lithium-ion migration number close to 1,which satisfies its basic conditions as a solid electrolyte.Then the Li/Li_6.25PS_5.25Cl_0.75/Li symmetrical battery test was carried out.The electrochemical test and the Raman characterization results after the test showed that the electrolyte has good compatibility with lithium metal.Through the variable current test of symmetrical batteries,the limiting current density of electrolytes with different capacities was characterized.At a capacity of 0.44 m Ah cm^-2,the limiting current density of the electrolyte is 1.10 m A cm^-2.As the fixed capacity decreases,the limiting current increases slightly.Finally,Li_6.25PS_5.25Cl_0.75 electrolyte was used in Li₂S/SE/Li ASSB,which showed good reversibility.This work proves that the lithium-rich argyrodite electrolyte is a potentially excellent electrolyte material,and has initially demonstrated the feasibility of this electrolyte for ASSB.?2?Based on the optimized electrolyte Li_6.25PS_5.25Cl_0.75,ASSBs were assembled with Li Co O₂ positive electrode and lithium metal negative electrode.By theoretical calculation,it is found that Li Nb O₃ has a wide electrochemical stability window,which can be used to coat the positive electrode to avoid the oxidation of the electrolyte and widen the operating voltage range of the sulfide electrolyte.We used Li Nb O₃-coated Li Co O₂ as the cathode material to study the effect of cathode coating on battery performance.The diffusion coefficient of lithium ions,XRD during charge and discharge,rate performance,cycle performance,and different cut-off voltages were tested.The results show that the coating of the positive electrode can improve the compatibility with the electrolyte,reduce the space-charge layer phenomenon at the interface,the charge transfer impedance of the battery,and improve the discharge medium voltage and discharge capacity of the battery.At the same time,the ASSB of the coated positive electrode shows improved cycle stability and rate performance.After 430 cycles,the capacity retention rate of the battery is 74.11%at 0.4 C.At 3.0 C,the capacity retention rate of the battery is still 61.76%.At a 4.2 V charge cut-off voltage,the battery showed the highest capacity retention rate.By increasing the cut-off voltage of the battery,the high-voltage resistance of the battery is tested.As a result,the battery shows an increased capacity decay rate.Further increase the ratio of active material in the positive electrode to 6:3:1,the discharge specific capacity after 100cycles of the battery is still 55.2 m Ah g^-1.The impedance test of the battery before and after the positive electrode coating found that the charge transfer impedance related to the positive electrode is the leading cause of the battery capacity decay.The impurities on the edge of the electrolyte will increase the tendency of metal lithium deposition,resulting in electrolyte short circuit.This work proves the effectiveness of the positive electrode coating strategy and the feasibility of the application of sulfide electrolyte in the LCO@LNO/SE/Li ASSB.?3?To further improve the energy density of the ASSB,the sulfide electrolyte Li_6.25PS_5.25Cl_0.75 is matched with the high-capacity ternary material to prepare the NCM523-SE-C/SE/Li ASSB.The coating of the positive electrode reduces the charge transfer impedance of the battery significantly.Through the AC impedance and GITT test,it was found that the lithium-ion diffusion coefficient of the coated positive electrode was slightly improved.The NCM523 positive electrode coated with Li Al O₂used in ASSB showed reduced impedance and improved cycle stability.Under the condition of 100?A cm^-2 current density test,the capacity retention rate of the battery was 80%after the 1237 cycle of cycling.When the battery cycled for 1765 cycles,the capacity retention rate was 71.28%,and the cycle capacity decay rate was only 0.016%.At the charge cut-off voltage of 4.3 V,the battery showed the highest capacity retention rate,and as the cut-off voltage increased,the capacity retention rate decreased.Increasing the ratio of active materials to 60%,the discharge specific capacity of the battery in the 100th cycle is still 58.3 m Ah g^-1,and the capacity retention rate is 77.63%.The increase of the cut-off voltage leads to an increase in the charge transfer impedance of the battery and a decrease in the discharge capacity,which has little effect on the film impedance and electrolyte impedance.This research work provides a feasible solution for the application of sulfide electrolyte in NCM523-SE-C/SE/Li ASSB,and also reveals the main law of capacity decay of ASSB.