Structural Origin of Reversible Li Insertion in Guest-Free, Type-II Silicon Clathrates

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.