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Thermodynamic Explanation of Orientation-Dependend Expansion of Lithiated Si Nanowires
The highest capacites have been achieved for Si nanowires, wires consisting of a crystalline core and an amorphous shell.
During first-cycle lithiation, both the amorphous shell and the crystalline core transform into amorphous LixSi where x may reach a maximum value of ~4.4 at full lithiation.
During first lithiation, it has been observed that Si nanowires expand differently into different directions [1].
In particular, expansion into <110> is largest while expansion into <111> is smallest.
It has been suggested that this behaviour is due to an orientation-dependent diffusion rate of the interface
between lithiated Si (amorphous LixSi) and the crystalline core [1,2].
In this work we use ab initio density functional theory calculations to study interfaces between lithiated Si and crystalline Si.
In particular we focus on interfaces between Si(100), Si(110) and Si(111) with amorphous Li2Si (a-Li2Si).
a-Li2Si is chosen as model for lithiated Si since it has recently been suggested to coexist with crystalline Si [3].
We find that a-Li2Si/Si{111} interfaces are energetically most favourable,
while a-Li2Si/Si{110} interfaces are the least favourable (among the three considered ones).
Since expansion along <111> generates a-Li2Si/Si{110} interfaces (<1-10> is orthogonal to <111>)
and expansion along <110> generates a-Li2Si/Si{111} interfaces,
our calculations naturally explain experimental observations on a thermodynamic basis.
For completeness, we also study the kinetics of Li across the interfaces.
Preliminary results indicate that Li diffusion barriers are independent of the orientation of the interface.
This implies in particular that kinetics does not hinder a thermodynamic evolution of the system.
References:
[1] H. Yang et al., Nano Lett. 12, 1953 (2012).
[2] M.K.Y. Chan, C. Wolverton and J.P. Greeley, J. Am. Chem. Soc. 134, 14362 (2012)
[3] J. Rohrer and K. Albe, J. Phys. Chem. C 117, 18796 (2013).