Si-based alloys are potential candidates as high energy density negative electrodes in Li-ion batteries.  Such alloys have high theoretical capacity but also have high volume expansion, which can lead to cell fade.  It has been shown previously that amorphous Si (a-Si) synthesized by chemical or physical deposition has improved cycling performance compared to crystalline Si (cr-Si) [1, 2].  Typically, a-Si is made by ball milling or atomic deposition techniques.  Templating methods have also been employed to make nanostructured Si alloys that include void space in which the Si can expand [3, 4].  Such alloys can perform well as negative electrodes in Li cells.  An inexpensive route for synthesizing bulk quantities of a-Si or nanostructured Si alloys is desirable.   

In this study, a new chemical delithiation method employing ethanol as an oxidizing agent was applied to prepare bulk quantities of a-Si from Li-Si compounds.  The a-Si formed was found to have a unique exfoliated layered structure, which has lower volume expansion than cr-Si and improved cycling characteristics.  C-Si and Fe-Si alloys were also synthesized by the delithiation of C-Li-Si and Fe-Li-Si alloys using this chemical delithiation method.  When tested as negative electrodes in Li cells, the C-Si and Fe-Si alloys showed superior electrochemical characteristics and lower volume expansion than cr-Si.

Experimental

Li₁₂Si₇, Li₇Si₃, Li₁₃Si₄ or Li₂₂Si₅ compounds were first prepared in an arc furnace from the elements.  Ethanol was then used to delithiate the Li-Si compounds under an Ar atmosphere.  After the resulting slurry was washed with distilled water, a-Si was recovered with a 70% yield. 

C-Li-Si and Fe-Li-Si precursors with serial C: Si or Fe: Si stoichiometric ratios were prepared by ball milling.  Ethanol was then used to delithiate the C-Li-Si or Fe-Li-Si alloys under an Ar atmosphere.  After the resulting products were washed with distilled water, C-Si and Fe-Si alloys were recovered with 79% and 70% yields, respectively. 

Electrode slurries were prepared by mixing active materials (a-Si, C-Si or Fe-Si), carbon black and polyimide in a volume ratio of 62.5/18/19.5 in N-methyl pyrrolidinone.  Electrode disks were punched from the coating foil and heated in a tube furnace for 3h at 300 °C under an Ar flow.  2325 coin-type cells were assembled in an Ar-filled glovebox with a Li counter/reference electrode.  All cells were cycled between 5~900 mV with a Maccor Series 4000 Automated Test System.

Results

a-Si prepared from ethanol delithiation of Li₁₂Si₇, Li₇Si₃, Li₁₃Si₄, and Li₂₂Si₅ resulted in layered products, except in the case of Li₂₂Si₅, which was composed of dense particles.  Figure 1 shows an SEM image of a-Si prepared from ethanol delithiation of Li₁₂Si₇.  a-Si prepared from ethanol delithiation of Li₁₂Si₇ had the most orderly layered structure, with the layers being highly exfoliated.  All a-Si samples had superior cycling performance and lower volume expansion compared to cr-Si.  Figure 2 shows the cycling performance of an a-Si electrode prepared from ethanol delithiation of Li₁₂Si₇ and a cr-Si electrode.  It is thought that the porous layered structure of these delithiated materials can accommodate the Si volume expansion during lithiation, resulting in low overall particle expansion and improved cycling performance.

Remarkably, C-Si and Fe-Si alloys made by the delithiation of ball milled C-Li-Si or Fe-Li-Si alloys contained very low silicide or carbide content.  Instead the alloys were primarily composed of a nano-composite of elemental C and Si or Fe metal and Si.  This was confirmed by X-ray diffraction and Mössbauer spectroscopy.  These alloys have a completely different nanostructure than conventional ball milled Si-based alloys, in which carbon and Fe are typically completely reacted to form carbides and silicides.

Conclusions

The alcohol delithiation method represents an effective means of producing Si-based alloys as negative electrode materials for Li cells.  The unique nanostructures of these alloys and their electrochemical performance will be discussed.  

References

[1]        L. B. Chen, J. Y. Xie, H. C. Yu, and T. H. Wang, J. Appl. Electrochem., 39, 1157 (2009).

[2]        R. Epur, M. Ramanathan, F. R. Beck, A. Manivannan, and P. N. Kumta, Mat. Sci. Eng. B-Solid, 177, 1157 (2012).

[3]        H. Y. Lee and S. M. Lee, J. Power Sources, 112, 649 (2002).

[4]        M. Yoshio, T. Tsumura, and N. Dimov, J. Power Sources, 146, 10 (2005).