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

Electrochemical reduction of a SnO 2 electrode for a lithium ion cell is known to result in formation of Li 4.4 Sn alloy+2Li 2 O. In order to determine to which extent such an electrode can be considered as reversible, we studied the electrochemical oxidation of a previously reduced SnO 2 electrode, using in situ 119 Sn Mossbauer spectroscopy. Contrary to what could be expected, the first step does not consist in extraction of lithium from Li 4.4 Sn for β -Sn to be obtained. In fact, simple lithium extraction proceeds only down to the Li 1.4 Sn composition. Further oxidation (second step) involves formation of unusual species (Sn(0) and oxygen-surrounded Sn(II), both probably in interaction with Li 2 O). 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.