Investigation of a Porous NiSi2/Si Composite Anode Material Used for Lithium-Ion Batteries By X-Ray Absorption Spectroscopy
Authors: Zhou Dong , Jia Haiping , Rana Jatinkumar , Placke Tobias , Winter Martin , Banhart John , Schumacher Gerhard 
Published: February 2020

As one of the most promising anode materials for the next generation high performance lithium-ion batteries, silicon exhibits not only a low lithiation potential of 0.2V vs. Li/Li+, but also the highest theoretical capacity (4200 mA h g-1), which is more than one order of magnitude higher than the current commercial graphite anode material (~370 mA h g-1). However, despite of the advantages, there are several crucial problems such as, tremendous volume changes (≥300%) during Li+ insertion/extraction and low electronic conductivity which impede the commercial application of silicon anodes [1-2].
In order to moderate the drastic volume changes of silicon and some other draw backs, three main approaches have been developed. The first important approach is the use of nanoscaled particles instead of normal materials. Meanwhile, another useful strategy is to design new structures (porous, nanoweb, and hierarchical) which can supply enough free space for the expansion of silicon. Additionally, some groups [3-5] tried to introduce a volumetric stable high electrical conductivity second phase into the host matrix, which can act as a buffer to reduce the volume changes of silicon and maintain the electrode integrity. For example, Jia et al[6], proposed a porous NiSi2/Si/Carbon core-shell structured anode material by using a facile ball milling and chemical vapor deposition (CVD) method, which resulted in a stable capacity of 1272 mA h g-1 for 200 cycles (at 1C) and a reversible capacity of 740 mA h g-1 (at 5C). However, despite of the superior electrical performance, the exact role of the NiSi2phase in the composite during the discharge/charge process is still unclear.
As a unique elementary selective technique, X-ray absorption spectroscopy experiments were performed on porous NiSi2/Si composite electrodes in various states of charge during the 1st lithiation and de-lithiation steps. It is observed that the NiSi2 phase shows a strong metal-metal bond character and no clear changes can be observed in XANES in fig.1 during lithiation and de-lithiation. The variation of the number of nearest neighbors of the Ni atom for the 1st coordinate Ni-Si shell and σ2 in the 1st cycle, both determined by refinement, demonstrates that NiSi2 can partially react with lithium during discharge and charge. A partially reversible non-stoichiometric compound NiSi2-y is formed during cell operation, the crystal structure of which is the same as that of the NiSi2 phase. It can be concluded that NiSi2in the composite not only accommodates the pronounced volume changes caused by the lithium uptake into silicon, but also contributes to the reversible capacity of the cell.
Reference
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Figure 1