Disentangling Li-ion Transportation And Phase Transition In Li₁₀GeP₂S₁₂ Solid Electrolyte By In-operando High-pressure And High-resolution NMR Spectroscopy

Solid-state battery is a greatly promising system for the next generation batteries due to the advantages of highly safety and higher energy density.Li₁₀GeP₂S₁₂ superionic conductor,with room temperature ionic conductivities rivaling its liquid counterparts,is one of the most promising candidates for next-generation Li-ion batteries.Li-ion transportation and phase transition of solid electrolyte are critical and fundamental issues governing the rate and cycling performances of the solid-state devices.Whereas,the scientific problems including the Li-ion transportation and phase transformation in the Li₁₀GeP₂S₁₂ electrolyte,especially under operating conditions are rarely studied and reported,due to the limitation of current in-situ characterization techniques,which are highly important for the fundamental study and to further improvement of the performance.At present,nuclear magnetic resonance（NMR）in-situ cells for solid-state batteries are mostly pressurized by threads or springs,which are uncontrollable and cannot meet the needs of solid-state batteries.Herein,we report the design and prototype of a homemade in-operando high-pressure NMR for solid-state battery that can sustain a stack pressure up to 400 MPa,which enables the NMR study of solid-state batteries under true-to-life operation conditions.The application of in-operando high-pressure NMR combined with ⁶Li-tracer and high-resolution NMR spectroscopy,provides a previously lacking insights into the Li-ion transportation and phase transition of Li₁₀GeP₂S₁₂ solid electrolyte.The results indicate that at high current densities（e.g.,current density exceeding 0.5 m A cm-²）,Li₁₀GeP₂S₁₂undergoes a more disorderly phase transition.A significant amount of the conductive metastable phaseβ-Li₃PS₄ in the electrolyte transforms into a less conductive phase,primarily composed ofγ-Li₃PS₄,resembling a glassy matrix.Under high current conditions,the transport of lithium ions exhibits a trend of synergistic conduction between the boundary phase and Li₁₀GeP₂S₁₂.In other words,under low current,Li₁₀GeP₂S₁₂ solid electrolyte primarily relies on the crystalline phase for ion transport.As the current density increases,the proportion of ion transport through grain boundaries also increases.This is because higher current allows lithium ions to overcome higher energy barriers and transport within components with lower ion conductivity.Simultaneously,we propose a reasonable explanation for the phase transition of the grain boundary phase in Li₁₀GeP₂S₁₂ solid electrolyte under high current.Due to the intensified Li-In alloying process under high current,severe morphological changes occur at the electrode,leading to variations in stress distribution at the interface and inducing structural changes that trigger the phase transition of the grain boundary phase.We also hypothesize the existence of local differences in lithium-ion diffusion coefficient caused by localized changes in lithium-ion concentration at the interface.In other words,the formation of a space charge layer,hindering the transport of lithium ions,occurs at the interface due to localized changes in lithium-ion concentration.Because the ionic conductivity ofβ-Li₃PS₄ is higher（10-³S cm-¹）and transformed intoγ-Li₃PS₄ phase with lower ionic conductivity（10-⁷S cm-¹）at high current,we speculate that this phase transition will affect the electrochemical performance of the material as a whole,and this conjecture is confirmed by electrochemical impedance spectroscopy.At the same time,the local lithium-ion diffusion coefficient difference（space charge layer）at the interface between the glassy grain boundary phase and the crystalline Li₁₀GeP₂S₁₂ will also lead to the decrease of the ionic conductivity and the deterioration of the high-rate performance.This study reports,for the first time,the new findings regarding lithium-ion transport and phase transition in Li₁₀GeP₂S₁₂ solid electrolyte under high pressure and high current density.In previous studies,researchers mostly attributed the poor high-rate performance of Li₁₀GeP₂S₁₂ solid electrolyte to interface issues.This research focuses on the internal aspects of Li₁₀GeP₂S₁₂ solid electrolyte,suggesting that the poor high-rate performance may also be closely related to the internal phase transition of the electrolyte.These findings will contribute to a better understanding of structural design and provide new insights for designing new high-performance solid electrolyte materials.