In Situ Electron Microscopy Study On Failure Mechanism Of All-solid-state Lithium Batteries

Solid-state batteries（SSBs）have great potential as high energy density Li battery systems for electrical vehicle and grid energy storage applications.Solid-state batteries（SSBs）using a solid electrolyte show potential for providing improved safety as well as higher energy and power density compared with conventional Li-ion batteries.However,the application of SSLMBs is hampered by the new multifold problems from the solid-solid contact to dendrites to deleterious interface reactions between lithium and the SSEs.To understand the fundamental science of these emerging problems,it is critical to develop state-of-the-art characterization tools to understand the failure mechanism of the solid-state battery.Unfortunately,lithium dendrite growth and the interphase formation are buried in the Li/SSE interfaces or the bulky SSEs,rendering them difficult to directly characterize.Another practical challenge that constrains the dendrite and interface characterization arises from the extreme reactivity of lithium metal and the SSEs with oxygen,moisture,and organic species,making sample preparation and transfer extremely difficult.In this work,we design a novel mesoscale electrochemical device in a focused ion beam-scanning electron microscopy（FIB-SEM）system,which enables real-time observations of the lithium stripping/plating process and Li/SSE interface evolutions at nanometer resolution.Meanwhile,the reliable analysis of battery materials can be realized in FIB-SEM and TEM by home-made vacuum transfer system and cryo-electron microscopy system.Based on the above advanced instruments,we used in situ electron microscopy to investigate important failure mechanisms in solidstate batteries.More specific experiments and conclusions are as follows:（1）Lithium（Li）penetration through solid electrolytes（SEs）induces short circuits in Li solid-state batteries（SSBs）and is a critical issue that hinders the development of high energy density SSBs.While cracking in ceramic SEs has been often shown to accompany Li penetration,the interplay between Li deposition and cracking remains elusive.Here we constructed a mesoscale SSB inside a focused ion beam-scanning electron microscope（FIB-SEM）for in situ observation of Li deposition-induced cracking in SEs at nanometer resolution.Our results revealed that Li propagated predominantly along transgranular cracks in a garnet Li_6.4La₃Zr_1.4Ta_0.6O₁₂（LLZTO）.Cracks appeared to initiate from the interior of LLZTO beneath the electrode surface and then propagated by curving towards the LLZTO surface.The resulting bowl-shaped cracks resemble those from hydraulic fractures caused by high fluid pressure on the surface of internal cracks,suggesting that the Li deposition-induced pressure is the major driving force of crack initiation and propagation.The high pressure generated by Li deposition is further supported by in situ observation of the flow of filled Li between the crack flanks,causing crack widening and propagation.This work unveils the dynamic interplay between Li deposition and cracking in SEs,and provides insight into the mitigation of Li dendrite penetration in SSBs.（2）The very high ionic conductivity of Li₁₀Ge P₂S₁₂（LGPS）solid electrolyte（SE）makes it a promising candidate SE for solid-state batteries in electrical vehicles.However,chemomechanical failure,whose mechanism remains unclear,has plagued its widespread applications.Herein we report in situ imaging lithiation induced failure of LGPS SE.We revealed a strong size effect in the chemomechanical failure of LGPS particles:when the particle size is less than 1μm,no chemomechanical failure was observed.This strong size effect is interpreted by the interplay between elastic energy storage and dissipation.We also found a strategy to create a chemical passivation layer on the electrolyte surface.Using this strategy,the strongly reacting interface between LGPS and Li metal was partially stabilized via electrolyte hydration achieved by exposing the electrolyte pellet to ambient air.Our finding has important implications for the design of high-performance LGPS SE,for example,by reducing the particle size to less than 1μm or creating passivation layers on their surfaces,the chemomechanical failure of LGPS SE can be mitigated.（3）For another class of Li_1.3Al_0.3Ti_1.7（PO₄）₃（LATP）electrolytes that are unstable to lithium metal,we found that they exhibit different mechanical stability from LGPS electrolytes during electrochemical failure.The electrochemical reaction of LATP electrolyte with lithium metal results in volume expansion,but no powdering occurs,and even cracks are rare.The mechanical properties of LATP nanorods were tested before and after lithiation through nano-mechanical experiments,and we found that the original LATP electrolyte had typical brittleness as ceramic materials,while after lithiation,the LATP nanorods both maintained a certain strength and had good plasticity.Based on the mechanical stability of LATP electrolytes,we propose to apply the LATP electrolyte in the field of positive electrode interface coating to release stress in solid-state batteries.（4）The addition of a layer of indium between the metal lithium and the electrolyte can effectively prevent the formation of lithium dendrites,but the detailed mechanism and the microstructure evolution of Li-In after many cycles are still unknown.Herein,we designed a nano battery in the TEM to study the changes in Li and In during the repeated charge and discharge process.We found that single crystal In can gradually transform into Li-In alloy during discharge,which is also a single crystal,and this has never been found for other materials in the in-situ electron microscopic study.Secondly,due to the volume expansion and contraction of the Li-In alloy during repeated charge and discharge,voids were also found after repeated cycles.Third,metal In is originally a very soft material,but it becomes particularly hard after reacting with Li.The reaction speed of Li-In is very fast,and if it is not controlled well,it may lead to the generation of Li In dendrites.Combined with the macroscopic experimental results,we believe that the In materials have many advantages in lithium batteries,but we also need to pay attention to preventing the occurrence of Li In dendrites,which is essential for improving the stability of solid metal lithium batteries.