Lithium Metal Halide Solid Electrolytes for All-Solid-State Batteries

All-solid-state batteries (ASSBs) have emerged as promising alternatives to conventional Li-ion batteries for electrical energy storage, owing to their anticipated enhanced safety and higher energy densities. ASSBs are founded on high performance superionic conducting solid electrolytes, where the search for new, advanced materials hinges on determining the factors that dictate facile Li-ion (or Na-ion) transport. Incorporating them into highly functional ASSBs, however, relies on mastering the interface of the solid electrolyte with the electrode materials. While there has been much focus on the relative stability of various solid electrolytes against lithium metal as the negative electrode, the cathode-electrolyte interface has received comparatively less attention.<br/> <br/>A fundamental understanding of the interaction of the surface of cathode materials with solid electrolytes is crucial to design practical ASSBs. Although sulfides (i.e., thiophosphates) are widely explored, they lack electrochemical stability above ∼ 2.7 V vs Li⁺/Li, whereas a new class of lithium metal chloride (Li-M-Cl) solid electrolytes are receiving rapidly growing scrutiny owing to their very high oxidative stability (&gt; 4.2 V) in combination with good ionic conductivity and ductility. Because Li-M-Cl electrolytes typically contain resource-limited metals (M) such as indium or rare earths, work has focused on substituting M with more abundant elements such as zirconium and iron. However, a unified understanding of the materials design principles for such substitutions is still lacking, both with respect to voltage stability and ion/electron transport. This presentation will explore both concepts. It will focus on examination of the dynamic evolution of the interphase (via a combination of theory and experiment, including <i>operando</i> impedance measurements) at different upper cutoff potentials and degrees of delithiation at the surface of Ni-rich NCM cathode particles. The cathode is coupled to a range of different Li-(M₁,M₂)-Cl catholytes to assess the role of the central metal and the contribution of partial electronic conductivity in determining both chemical stability and high voltage electrochemical stability. Finally, the effect of crystal structure (layered vs spinel, etc) on ion transport will be presented. Overall, this approach establishes a platform for the metrics that can be utilized to efficiently evaluate new halide SEs in SSB cells.