Solving the Li7La3Zr2O12 electrochemical stability window puzzle

The use of ceramic solid electrolytes (SEs) to produce all-solid-state lithium batteries (ASSLBs) can overcome the safety issues related to conventional liquid electrolytes and enable Li batteries to operate at high voltage, therefore producing high power density. To make this happen, one of the most crucial requirements for the SE is to possess a large electrochemical stability window (ESW). Li7La3Zr2O12 (LLZO) solid electrolyte has attracted a lot of attention given its high ionic conductivity and because it was first reported with a very wide ESW (0–9 V vs. Li+/Li). It was not until recently that this value of the ESW was challenged, mainly proving that the oxidation of LLZO happened at a much earlier potential than 9 V. The present manuscript describes a methodology to accurately determine the ESW of doped M:LLZO (M = Al, Ta and Nb). To do so, M:LLZO is processed into a composite with gold powder to allow a large contact surface between SE and conductive gold particles, which maximizes the redox currents. To characterize the ESW, potentiostatic intermittent titration technique (PITT) was used. PITT allowed to remain close to the thermodynamic equilibrium to observe the redox currents. The ESW of M:LLZO was found to be [1.65–3.7] V vs. Li+/Li for all three dopants. The reduction reaction occurring at 1.65 V was evidenced for Al:LLZO through ex situ 7Li nuclear magnetic resonance (NMR) and operando powder X-ray diffraction (PXRD) analyses.