The chemical changes occurring upon cycling of a SnO2 negative electrode for lithium ion cell: In situ Mössbauer investigation

Electrochemical reduction of a SnO2 electrode for a lithium ion cell is known to result in formation of Li4.4Sn alloy+2Li2O. In order to determine to which extent such an electrode can be considered as reversible, we studied the electrochemical oxidation of a previously reduced SnO2 electrode, using in situ 119Sn Mössbauer spectroscopy. Contrary to what could be expected, the first step does not consist in extraction of lithium from Li4.4Sn for β-Sn to be obtained. In fact, simple lithium extraction proceeds only down to the Li1.4Sn composition. Further oxidation (second step) involves formation of unusual species (Sn(0) and oxygen-surrounded Sn(II), both probably in interaction with Li2O). Then (third step), red SnO-like Sn(II) species are formed, along with some Sn(IV). Especially during the second and third steps, the working electrode is far from thermodynamic equilibrium despite the low oxidation rate. This non-equilibrium behavior is probably related to the ultrafine particle size resulting from electrochemical grinding.