Fundamental Research On Sulfide-Based Solid-State Electrolytes And Their Cathode-Electrolyte Interfaces

All-solid-state lithium batteries have the advantages of high safety,high energy density,and long cycle life,and are considered to be the most potential next-generation battery system to replace traditional liquid lithium-ion batteries.Among them,sulfide solid-state electrolytes are favored due to their high ionic conductivity,low grain boundary resistance,and good mechanical properties.Therefore,sulfide-based all-solid-state lithium batteries have become a research hotspot.However,due to the poor comprehensive performance of solid-state electrolytes and the poor stability of the interface of the all-solid-state lithium batteries,which greatly limit the performance of all-solid-state lithium batteries.Based on this,sulfide-based all-solid-state lithium batteries were selected as the research object in this paper.A systematic research were conducted on the preparation of sulfide solid electrolytes and the interfaces between common cathode materials and sulfide solid-state electrolytes.The main research contents and results are as follows:To study the effects of different preparation methods on the structure,morphology and electrochemical performance of sulfide solid electrolytes.The most typical pseydi-binary glass-ceramic Li₇P₃S₁₁（LPS）was selected as the research object,and it was prepared by solid-phase synthesis and liquid-phase synthesis,respectively.Among them,the S-LPS electrolyte prepared by the solid-phase synthesis has no impurity phase,and the L-LPS electrolyte prepared by the liquid-phase synthesis has smaller and uniform particles,and particle size of 1-5μm.At the same time,the optimal heat treatment temperature for both solid-phase synthesis and liquid-phase synthesis is260°C.In contrast,the S-260 electrolyte prepared by the solid-phase synthesis exhibits the best comprehensive performance.S-260 electrolyte show extremely high ionic conductivity of 7.46×10^-4 S cm^-1 at room temperature and a low reaction activation energy of 30.61 KJ mol^-1.The electrochemical stability window up to 5 V was demonstrated.S-260 electrolyte exhibits excellent lithium deposition/dissolution properties in symmetric batteries,stable cycling for more than 700 h at a current of 0.1m A cm^-2,and a critical current density greater than 5 m A cm^-2.To investigate the failure behavior of interfacial physical contact between metal sulfide cathodes and sulfide electrolytes.The typical metal sulfide Fe S₂ was selected as a model cathode material to test its electrochemical performance in all-solid-state battery systems and liquid battery systems.By comparison,it was found that the solid-state battery system exhibits severe voltage hysteresis and low first coulombic efficiency.Using synchrotron radiation imaging technology,combined with synchrotron radiation X-ray absorption spectroscopy,a visual study of the microstructural evolution of Fe S₂ active particles in solid-state battery systems was performed,revealing the interface physical contact failure in solid-state battery systems,which was attributed to the heterogeneous phase conversion of Fe S₂ along with solid-solid contact interface.Further,the large volume expansion of Fe S₂ and the dense Fe phase on the surface during the charging and discharging process seriously hinder the transport of Li⁺,resulting in high interfacial resistance.In light of the performance degradation mechanism,porous Fe S₂ microsphere structure were designed and synthesized to buffer the volume effect.At the same time,the solid-solid close contact interface of the positive electrode/electrolyte was constructed by in-situ growth of the electrolyte on the surface of the positive electrode material.The interface contact loss caused by the stress and volume change of the cathode material is reduced,the electrochemical-mechanical induced capacity fading is alleviated,and the electrochemical performance of the all-solid-state battery is improved.To investigate the interfacial chemical degradation behavior between layered oxide cathodes and sulfide electrolytes.High-nickel single-crystalline Li Ni_0.6Co_0.2Mn_0.2O₂（SCNCM）was selected as a model layered oxide cathode material.surface Ti Nb₂O₇-coated and bulk Ti-doped single crystal NCM cathodes（DC-TNO@SCNCM）were successfully synthesized by using a typical wet chemical approach and subsequent thermally driven diffusion method.To the best of our knowledge,this is the first time that a doping-coating synergistic modification strategy has been proposed in a high-voltage sulfide-based all-solid-state battery.The results show that the uniform TNO coating layer with thermodynamic/electrochemical stability and electronic insulation can avoid the decomposition of the sulfide electrolyte.Meanwhile,Ti doping in SCNCM to form a strong Ti-O bond can effectively stabilize the lattice oxygen and prevent the formation of oxygen-containing species at the interface.Synergistic modification of the SCNCM through surface and bulk fundamentally stabilizes the interface,reduces the interface resistance,and significantly improves the electrochemical performance of sulfide-based all-solid-state batteries.Therefore,DC-TNO@SCNCM cathode exhibits a high discharging capacity of 180.3 m Ah g^-1 and encouraging capacity retention of 92.2%after 140 cycles at a high cut-off voltage of 4.4V.