REVISITING THE ELECTROCHEMICAL STABILITY WINDOW OF SOLID ELECTROLYTES FOR THE DEVELOPMENT OF BULK-TYPE ALL-SOLID-STATE LITHIUM BATTERIES
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Bulk-type all-solid-state lithium-ion batteries (ASSLIBs) are being considered as the ultimate solution for safe lithium-ion batteries due to the replacement of volatile and flammable liquid electrolytes by nonflammable inorganic solid electrolytes (SEs). Significant advances have been made in achieving superionic SEs with a wide electrochemical stability window (ESW) from 0 to 5 V. The ESW of solid electrolytes was usually measured from the Li/SE/inert metal semi-blocking electrode. Because of the wide ESW, solid electrolytes hold great promise for high energy density batteries with high columbic efficiency and long cycle life. In this dissertation, we challenge the claimed ESW of solid electrolytes. The conventional method to measure ESW provides an overestimated value because the kinetics of the electrochemical decomposition reaction is limited in the semi-blocking electrode. A novel experimental method using Li/SE/SE+carbon cell is proposed to approach the intrinsic stability window of solid electrolytes. The ESWs of Li10GeP2S12 (LGPS) and Li7La3Zr2O12 (LLZO), the most promising SE for sulfide and oxide electrolytes respectively, are examined using the novel experimental method. The results suggest that both SEs have much narrower electrochemical stability window than what was previously claimed. The cathodic and anodic decomposition products for both electrolytes are also characterized. The measured stability window and the decomposition products agree well with the calculated results from first principles. The reversible decompositions of LGPS at both high and low voltages enable the realization of a battery made from a single material. The electrochemical decompositions of the SEs in ASSLIBs can lead to large interfacial resistances between electrode and electrolyte. The interfacial resistances arising from the decomposition of SEs have been ignored in previous research efforts because the batteries are cycled within the “claimed” stable window of SEs. Suppressing the (electro)chemical reactions between LiCoO2 cathode and LLZO electrolyte by engineering their interphase enables a high performance all-ceramic lithium battery. By taking advantage of the electrochemical decomposition of SEs, an effective approach to suppress Li dendrite formation in sulfide electrolyte is also demonstrated.