Galvanostatic Ion Detrapping Rejuvenates Oxide Thin Films

Ion trapping under charge insertion-extraction is well-known to degrade the electrochemical performance of oxides. Galvano-static treatment was recently shown capable to rejuvenate the oxide, but the detailed mechanism remained uncertain. Here we report on amorphous electrochromic (EC) WO3 thin films prepared by sputtering and electrochemically cycled in a lithium-containing electrolyte under conditions leading to severe loss of charge exchange capacity and optical modulation span. Time-of-flight elastic recoil detection analysis (ToF-ERDA) documented pronounced Li+ trapping associated with the degradation of the EC properties and, importantly, that Li+ detrapping, caused by a weak constant current drawn through the film for some time, could recover the original EC performance. Thus, ToF-ERDA provided direct and unambiguous evidence for Li+ detrapping.