Nanostructured Si Alloys Via Ethanol Delithiation

Abstract

Introduction 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.