Understanding Electrochemical Intercalation of Al³⁺ Cation into the WO₃ Electrochromic Electrode from Solid Electrolyte Interphase and Mass Changes

Multivalent chemistry has drawn extensive research interests because it provides intriguing benefits to develop beyond-lithium-ion electrochromic and energy storage technologies. Among the multivalent candidates, the aluminum (Al)-based electrolyte offers an attractive high capacity for designing multivalent-ion electrochromic batteries and devices. However, the understanding of complex electrochemical and mechanical interactions of the electrode/electrolyte interface during cycling is extremely limited. Herein, we develop an Al³⁺-based half-cell that consisted of a WO₃ thin film cathode, Al­(ClO₄)₃-PC electrolyte, and Au counter electrode. Electrochemical quartz crystal microbalance (EQCM) observations suggest that there is a link between the interfacial degradation of WO₃/electrolyte and the formation of chelate-like compounds in a high-concentration electrolyte environment. The intercalation/de-intercalation of the Al³⁺ cations in WO₃ electrodes was directly evaluated through in situ real-time EQCM. As indicated by the results of EQCM, the formation of chelate-like compounds leads to the irreversible process of intercalation and de-intercalation in the WO₃ thin film electrode and the instability of the solid electrolyte interphase. The validated EQCM-based analysis provides a correlation between solution concentration and cycle stability, which would be expected to reveal new insights into the electrochemical and electrochromic behavior in many other multivalent systems.