Silicon (Si) is capable of holding greater amounts of lithium compared with common carbon-based materials; as such, it is a target of great focus as an anode material for higher capacity Li-ion batteries. However, in crystalline form, Si possesses a structure that deteriorates during charging cycles, ultimately impacting performance. However, amorphous SiO is resistant to such deterioration.

Using a new methodology, researchers in Japan—including colleagues from Nissan subsidiary Nissan Arc Ltd., a materials analysis and research center—have developed a heterostructure model of the atomic structure of silicon monoxide (SiO). The heterostructure model well explains the distinctive structure and properties of the material, which could play an important role in boosting the capacity of Li-ion batteries. An open-access paper on the work is published in Nature Communications .

The base structure of SiO has been unknown, making it difficult for mass production. The new methodology developed by the team provides an accurate understanding of the amorphous structure of SiO, based on a combination of structural analyses and computer simulations.

Solid silicon monoxide is an amorphous material which has been commercialized for many functional applications. However, the amorphous structure of silicon monoxide is a long-standing question because of the uncommon valence state of silicon in the oxide. It has been deduced that amorphous silicon monoxide undergoes an unusual disproportionation by forming silicon- and silicon-dioxide-like regions. Nevertheless, the direct experimental observation is still missing.

Here we report the amorphous structure characterized by angstrom-beam electron diffraction [ABED], supplemented by synchrotron X-ray scattering [HEXRD] and computer simulations [MD-RMC]. In addition to the theoretically predicted amorphous silicon and silicon-dioxide clusters, suboxide-type tetrahedral coordinates are detected by angstrom-beam electron diffraction at silicon/silicon-dioxide interfaces, which provides compelling experimental evidence on the atomic-scale disproportionation of amorphous silicon monoxide. —Hirata et al.



Reconstructed heterostructure model of amorphous SiO. The inner part corresponds to an amorphous Si cluster and the outer part is amorphous SiO 2 matrix. The blue, red and green circles denote Si and O in amorphous SiO 2 and Si in the Si cluster, respectively. Hirata et al.

The ABED experiment provided direct evidence on the atomic-scale disproportionation in amorphous SiO, predicted by theoretical calculations. The distinctive interfacial structure between amorphous Si and SiO 2 clusters illuminated the structural origins of amorphous SiO, different from the simple mixture of Si and SiO 2 .

Although the RMC approach, which is known to fail in determining definite local atomic structure, is also used to generate the initial structure of amorphous SiO, the uncertainness of the RMC can be overcome by the successful utilization of ABED which provides well-defined local atomic configurations in the heterogeneous SiO. Therefore, the experiment-based MD–RMC approach developed in this study could be a powerful and generic method to model disordered materials with structural heterogeneity based on both local and global diffraction data. —Hirata et al.

The atomic structure of SiO was thought to be inhomogeneous, making its precise atomic arrangements the subject of debate. The new findings show that its structure allows the storage of a larger number of Li ions, in turn leading to higher capacity.

The work is the result of a combined R&D effort between Nissan Arc Ltd., a Nissan subsidiary, Tohoku University, the National Institute for Materials Science (NIMS), the Japan Synchrotron Radiation Research Institute (JASRI), and Japan Science and Technology Agency (JST).

The invention of this new analysis method is essential to further develop the next generation of high-capacity lithium-ion batteries. It will certainly become one of our core technologies. The utilization of this analysis method in our future R&D will surely contribute to extending the cruising range of future zero-emission vehicles. —Takao Asami, Senior Vice President of Nissan Motor Co., Ltd. and President of Nissan Arc Ltd.

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