In Situ 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. In situ 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 ab initio 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 LixSny 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.