On the electrochemistry of tin oxide coated tin electrodes in lithium-ion batteries

As tin based electrodes are of significant interest in the development of improved lithium-ion batteries it is important to understand the associated electrochemical reactions. In this work it is shown that the electrochemical behavior of SnO₂ coated tin electrodes can be described based on the SnO₂ and SnO conversion reactions, the lithium tin alloy formation and the oxidation of tin generating SnF₂. The CV, XPS and SEM data, obtained for electrodeposited tin crystals on gold substrates, demonstrates that the capacity loss often observed for SnO₂ is caused by the reformed SnO₂ layer serving as a passivating layer protecting the remaining tin. Capacities corresponding up to about 80 % of the initial SnO₂ capacity could, however, be obtained by cycling to 3.5 V vs. Li⁺/Li. It is also shown that the oxidation of the lithium tin alloy is hindered by the rate of the diffusion of lithium through a layer of tin with increasing thickness and that the irreversible oxidation of tin to SnF₂ at potentials larger than 2.8 V vs. Li⁺/Li is due to the fact that SnF₂ is formed below the SnO₂ layer. This improved electrochemical understanding of the SnO₂/Sn system should be valuable in the development of tin based electrodes for lithium-ion batteries.