Probing the Failure Mechanism of SnO₂ Nanowires for Sodium-Ion Batteries

Nonlithium metals such as sodium have attracted wide attention as a potential charge carrying ion for rechargeable batteries. Using in situ transmission electron microscopy in combination with density functional theory calculations, we probed the structural and chemical evolution of SnO₂ nanowire anodes in Na-ion batteries and compared them quantitatively with results from Li-ion batteries (Huang, J. Y.; et al. Science 2010, 330, 1515−1520). Upon Na insertion into SnO₂, a displacement reaction occurs, leading to the formation of amorphous Na_<i>x</i>Sn nanoparticles dispersed in Na₂O matrix. With further Na insertion, the Na_<i>x</i>Sn crystallized into Na₁₅Sn₄ (<i>x</i> = 3.75). Upon extraction of Na (desodiation), the Na_<i>x</i>Sn transforms to Sn nanoparticles. Associated with the dealloying, pores are found to form, leading to a structure of Sn particles confined in a hollow matrix of Na₂O. These pores greatly increase electrical impedance, therefore accounting for the poor cyclability of SnO₂. DFT calculations indicate that Na⁺ diffuses 30 times slower than Li⁺ in SnO₂, in agreement with in situ TEM measurement. Insertion of Na can chemomechanically soften the reaction product to a greater extent than in lithiation. Therefore, in contrast to the lithiation of SnO₂ significantly less dislocation plasticity was seen ahead of the sodiation front. This direct comparison of the results from Na and Li highlights the critical role of ionic size and electronic structure of different ionic species on the charge/discharge rate and failure mechanisms in these batteries