<i>In Situ</i> TEM on the Reversibility of Nanosized Sn Anodes during the Electrochemical Reaction

Excellent reversibility is crucial for the storage capacity and the cycle life of anode materials in high-performance lithium ion batteries, which has not been observed in alloy-type materials such as Si or Ge. <i>In situ</i> transmission electron microscopy reveals a sequential phase transformation in individual Sn nanowires during Li insertion, which is in a reverse order during Li extraction. Both the bright field image and the electron diffraction show a two-step reversible crystalline–crystalline phase transformation. It is noted that the crystalline tin has a more open lattice to readily accommodate Li up to the Li₂Sn₅ phase while retaining the crystallinity, which distinguishes Sn from its metalloid counterparts. The connected interstices along [001] inside lattice form a helix pipe for fast Li diffusion, indicating the openness of the Sn lattice. The <i>ab initio</i> simulations reveal facile Li diffusion along [001] with a low migration barrier of 0.014 eV. No phase boundary is visible in this step. In the second step, the Li_<i>x</i>Sn_<i>y</i> phases, including the Li₂₂Sn₅ phase, nucleated with grain refinement and enormous volume expansion. The broaden phase boundary indicates that the further alloying is rate-limited not by the diffusion of Li but by the interfacial conversion reaction. The pulverization occurs during delithiation by agglomeration of regular-shape voids, showing a different mechanism from the cracking-dominated fracture in Si. These results elucidate the structural evolution and the phase transformation during the electrochemical cycling, which sheds light on engineering Sn anode materials