cm5009448_si_002.mpg (3.95 MB)
In Situ TEM on the Reversibility of Nanosized Sn Anodes during the Electrochemical Reaction
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posted on 2014-07-22, 00:00 authored by Qianqian Li, Peng Wang, Qiong Feng, Minmin Mao, Jiabin Liu, Scott X. Mao, Hongtao WangExcellent 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.