Abstract
The guest‐free, type‐II Si clathrate (Si136) is an open cage polymorph of Si with structural features amenable to electrochemical Li storage. However, the detailed mechanism for reversible Li insertion and migration within the vacant cages of Si136 is not established. Herein, X‐ray characterization and density functional theory (DFT) calculations are used to understand the structural origin of electrochemical Li insertion into the type‐II clathrate structure. At low Li content, instead of alloying with Si, topotactic Li insertion into the empty cages occurs at ≈0.3 V versus Li/Li+ with a capacity of ≈231 mAh g−1 (corresponding to composition Li32Si136). A synchrotron powder X‐ray diffraction analysis of electrodes after lithiation shows evidence of Li occupation within the Si20 and Si28 cages and a volume expansion of 0.22%, which is corroborated by DFT calculations. Nudged elastic band calculations suggest a low barrier (0.2 eV) for Li migration through interconnected Si28 cages, whereas there is a higher barrier for Li migration into Si20 cages (2.0 eV). However, if Li is present in a neighboring cage, a cooperative migration pathway with a barrier of 0.65 eV is possible. The results show that the type‐II Si clathrate displays unique electrochemical properties for potential applications as Li‐ion battery anodes.
Highlights
The guest-free, type-II Si clathrate (Si136) is an open cage polymorph of Si with structural features amenable to electrochemical Li storage
In our previous density functional theory (DFT) study focused on guest-free, type-I clathrates, we found that Li could migrate into the Si20 cage of the type-I Si46 clathrate through a different pathway that involved temporary Si─Si bond breaking.[17]
The structural origins of the electrochemical properties of type-II Si clathrate were investigated with synchrotron X-ray characterization and DFT calculations
Summary
To understand the role of guest atom occupancy on the electrochemical reaction of Li with type-II Si clathrates, two samples with different Na contents were prepared (see Figure S1, Supporting Information) and characterized with synchrotron powder XRD (PXRD). The Rietveld refinement (Figure 2b, and Table S2, Supporting Information) of this sample resulted in a composition of Na10.7(1)Si136 (we refer to this as the “Na11” sample) with a lattice parameter of 14.6544(2) Å and 1.4 wt% impurity of Na8Si46. When the EDS spectrum was collected from a thicker area near the center of a larger particle, the Na content was higher than in the Na1 sample, which is consistent with the higher average Na content of Na11 This variation of the Na content based on the particle size is likely due to Na evaporation during the synthesis under vacuum, where smaller particles are expected to lose Na more quickly because of the higher surface area and shorter diffusion lengths
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