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 Si136is 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 Si20and Si28cages 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 Si28cages, whereas there is a higher barrier for Li migration into Si20cages (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.
Formation of Type II Silicon Clathrate with Lithium Guests through Thermal Diffusion
At low guest atom concentrations, Si clathrates can be viewed as semiconductors, with the guest atoms acting as dopants, potentially creating alternatives to diamond Si with exciting optoelectronic and spin properties. Studying Si clathrates with different guest atoms would not only provide insights into the
electronic structure of the Si clathrates but also give insights into the unique properties that each guest can bring to the Si clathrate structure. However, the synthesis of Si clathrates with guests other than Na is challenging. In this study, we have developed an alternative approach, using thermal diffusion into type II Si
clathrate with an extremely low Na concentration, to create Si clathrate with Li guests. Using time-of-flight secondary-ion mass spectroscopy, X-ray diffraction, and Raman scattering, thermal diffusion of Li into the nearly empty Si clathrate framework is detected and characterized as a function of the diffusion temperature and time. Interestingly, the Si clathrate exhibits reduced structural stability in the presence of Li, converting to polycrystalline or disordered phases for anneals at temperatures where the starting Na guest Si clathrate is quite stable. The Li atoms inserted into the Si clathrate lattice contribute free carriers, which can be detected in Raman scattering through their effect on the strength of Si−Si bonds in the framework. These carriers can also be observed in electron paramagnetic resonance (EPR). EPR shows, however, that Li guests are not simple analogues of Na guests. In particular, our results suggest that Li atoms, with their smaller size, tend to doubly occupy cages, forming “molecular-like” pairs with other Li or Na atoms. Results of this work provide a deeper insight into Li guest atoms in Si clathrate. These findings are also relevant to understanding how Li moves through and interacts with Si clathrate anodes in Li-ion batteries. Additionally, techniques presented in this work demonstrate a new method for filling the Si clathrate cages, enabling studies of a broad range of other guests in Si clathrates.
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- Award ID(s):
- 1810463
- NSF-PAR ID:
- 10470216
- Publisher / Repository:
- American Chemical Society
- Date Published:
- Journal Name:
- Inorganic Chemistry
- Volume:
- 62
- Issue:
- 18
- ISSN:
- 0020-1669
- Page Range / eLocation ID:
- 6882 to 6892
- Subject(s) / Keyword(s):
- Silicon Clathrate Lithium Diffusion, Thermal Diffusion, Li-ion Battery
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1021/acs.inorgchem.2c03703
