Influence of oxides on the stress evolution and reversibility during SnO_x conversion and Li-Sn alloying reactions

The effect of varying the oxygen content in Sn and SnO_x films during potential dependent SnO_x conversion reactions and Li y Sn alloying relevant to Li ion battery anodes is examined. For metallic Sn films, the stresses and stability of the films are controlled by Li alloying reactions. Small, non-contacting separated Sn particles exhibit higher electrochemical stability relative to more continuous polycrystalline films with larger particles. Metallic Sn particles develop tensile stress during Li_ySn de-alloying as porous structures are formed. The amount of stress associated with lithiation and delithiation of well-separated metallic particles decreases as a porous, easy to lithiate, material forms with cycling. During the lithiation of oxides, conversion reactions (SnO_x→Sn) and the lithiation of the metallic Sn control the stress responses of the films, leading to highly potential-dependent stress development. In particular, evidence for a multistep electrochemical mechanism, in which partially reversible lithiation of the oxygen-containing phases is conjoined with a fully reversible lithiation of the metallic phases of the Sn, is found. The electrochemical stress analysis provides new insight into these mechanisms and delineates the extent of the reversibility of lithiation and conversion reactions of oxides.