Research On Design,Preparation And Performance Of Solid Electrolyte Interface Toward Solid-State Li-Metal Batteries

The limited specific energy and energy density of traditional lithium-ion batteries based on graphite anode cannot meet the endurance requirement of fast-developing electric vehicles.In addition,the toxic and flammable organic liquid electrolyte used in traditional lithium-ion batteries leads to safety issues.Lithium metal anode simultaneously meets the advantages of high specific capacity(3800 m A h g^-1)and the lowest reductive potential（-3.04 V vs.SHE）.The substitution of liquid electrolytes by non-flammable solid electrolytes can effectively alleviate safety issues,but there will be some other challenges brought by the introduction of solid electrolytes.Firstly,reductive Li metal could react with solid electrolyte,resulting in interfacial degradation.In addition,in sandwiched battery configuration,the solid electrolyte cannot immerge into the cathode,leading to poor Li⁺conduction in cathode and large interfacial resistance between cathode and solid electrolyte.Finally,some solid electrolytes could be electrochemically oxidized under high potential and high voltage cathode cannot be adopted.Referring to these challenges,we have made some improvements and investigations based on Li_1.5Al_0.5Ge_1.5（PO₄）₃（LAGP）and gel electrolyte in our work.Based on solid electrolyte materials,a three-dimensional porous framework is prepared,in which the cathode material and the electronic conductor are loaded,to ensure continuous Li⁺conduction and electronic conduction inside the cathode.Integrating the porous cathode with the solid electrolyte membrane through high-temperature sintering can facilitate the interfacial Li⁺conduction between solid electrolyte and cathode.Based on LAGP,a dense LAGP layer was integrated with a porous LAGP layer through one-step pressing and one-step sintering.The porous layer acted as the framework of cathode materials.The overall Li⁺conductivity at room temperature of the integrated dual LAGP layer is 2×10^-5 S cm^-1.The pore diameter is more than 5μm.The integrated assembly was adopted in all-solid-state Li-air battery and the porous cathode can facilitate the diffusion of gaseous reactants and the accumulation of solid products.The porous layer was loaded with carbon material of 1.67 mg cm^-2 and the battery delivered a capacity of0.48 m A h cm^-2 at 5μA cm^-2 and 0.38 m A h cm^-2 at 10μA cm^-2 under pure oxygen atmosphere.And the battery can sustain 6 cycles at a fixed capacity of 0.08 m A h cm^-2under the current of 10μA cm^-2.Not only design of porous cathode framework but also introduction of gel electrolyte between cathode and solid electrolyte can support Li⁺conduction in cathode and at the interface.A high-concentration lithium salt hydrogel（HG）was introduced between LAGP and cathode,and quasi-solid-state Li-sulfur battery was assembled to investigate the reduction of interfacial impedance and improvement of battery performance.The solid-state lithium-sulfur battery with HG can obtain a capacity of1400 m A h g^-1 at 60°C and 0.05 C,while the battery without HG showed little capacity.Otherwise,the detrimental reactions between Li and LAGP lead to the increase of interfacial impedance and rapid degradation of battery performance.A PEO-based gel polymer electrolyte（GPE）was introduced between Li and LAGP to alleviate side reactions.The Li-Li symmetrical battery with GPE can cycle for more than 600 h at a current of 0.1 m A cm^-2 and a capacity of 0.1 m A h cm^-2.Meanwhile,solid-state lithium-sulfur batteries with both GPE and HG showed a capacity retention of 70%after 100cycles at 60°C and 0.1 C.We have also assembled high-voltage solid-state batteries based on Li Ni_0.5Mn_1.5O₄（LNMO）cathode to investigate the electrochemical stability of PEO-based solid electrolyte and LAGP at high voltage.LAGP and LLZTO were incorporated in PEO-Li TFSI system to enhance the electrochemical window.It was found that the incorporation of LAGP could not improve the electrochemical oxidation stability and resulted in unstable Li/PEO interface.Although the incorporation of LLZTO improved the electrochemical oxidation stability,it was still lower than the voltage range in which LNMO works.Based on LAGP only,a high-voltage ionic liquid electrolyte was introduced to improve the cathode/LAGP interface.However,it was found that LAGP was electrochemically oxidized above 4.5 V,resulting in increased cathode/LAGP interfacial impedance and inferior battery performance.These results are discrepant with literatures and whether solid-state electrolyte will be chemically oxidized by delithiated LNMO at high voltage also needs further investigation and testing.