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<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 Li<sub>2</sub>Sn<sub>5</sub> 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<sub><i>x</i></sub>Sn<sub><i>y</i></sub> phases, including the Li<sub>22</sub>Sn<sub>5</sub> 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