Electrochemical Performance of Si Modified TiO2 Nanotube Arrays Investigated in Li-Ion Containing Ionic Liquid Electrolyte

Widely researched in the recent years, Si offers one of the possibilities for significant increase of the capacity of Li-ion battery anode. During its lithiation Si undergoes series of phase transitions related to formation of a number of Li-Si alloy phases with a total capacity of about 4200 mAhg^-1[1-3]. Consequently, the electrochemical lithiation of Si is associated with a drastic volume increase, usually resulting in rapid capacity fade and mechanical decomposition of the structure [2]. A number of different approaches, including nanostructuring [3], application of amorphous Si [1,2,4-8], limitation of state of charge of Si anodes by means of capacity or voltage control [8-10] composites with carbon [2] and more recently with TiO₂[11-12], has been undertaken in order to improve the cycling stability of Si based electrodes.

This paper will discuss recent results from the author's labs on the application of TiO₂nanotube arrays as templates for magnetron sputtering of Si. Tuning of the deposition parameters (power and time) allowed obtaining a specific particulate Si morphology with sufficient free space, suitable for application as anode material in Li ion batteries (Fig.1).

Fig1. Scanning electron microscopy (SEM) image of the Si-modified TiO2 nanotube layer.

The electrochemical performance of the Si containing structures elucidates the importance of the Si surface morphology for the cycling stability of the anodes. The structures deposited at 50 W are favorable for Si stabilization during electrochemical cycling in Li ion electrolyte.

The nanostructured Ti/TiO₂layers were electrochemically tested for Li-ion exchange in 1-butyl-1-methylpyrrolidinium bis (trifluoromethyl) sulfonylimide ([BMP][TFSI]) containing 1M Li[TFSI]. The results show that the ionic liquid [BMP][TFSI] is a promising electrolyte for batteries with silicon anodes.

The substrate type significantly influences the long term galvanostatic cycling of the samples. It was found that the structures deposited at 50 W on amorphous TiO₂ exhibit a superior constant current cycling, finishing the 200^th galvanostatic cycle with discharge capacity value of 1150 mAh g^-1, with a tendency for further stabilization of the cycling. The good electrochemical performance of this sample type was attributed to the specific morphology of the Si deposit and structural stability of the amorphous TiO₂ nanotubes.

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