Towards a Better Reversibility of the Electrochemical Li-Si Reaction through the Use of Partially Oxidized Silicon Particles and Alternative Electrolyte Salt/Additives

Silicon is one of the most promising candidates as anode material for Li-ion batteries because it offers the largest gravimetric and volumetric capacities^[1]. However, the most challenging problem associated with the Li-Si reaction is the drastic particles swelling (up to + 300% in volume) which results in degradation of the electrode cohesion and in capacity loss upon cycling^[2]. Accordingly, the choice of the binder is crucial as it dictates most of the performances of the Si-based electrode^[3] and also drives the nature of the solvent used to dissolve it. CMC (Carboxy-Methyl-Cellulose) is known to be a very efficient binder to maintain electronic percolation and mechanical integrity of Si-based electrodes^[4]. Also, CMC is water soluble hence environment friendly compared to other polymeric binders that could necessitate hazardous solvents, such as NMP.

However, the large-scale production of Si/CMC/C composite electrode is troublesome since it generally leads to a large H₂ bubbling within the slurry, later making impossible the fabrication of uniform electrodes. This results from the oxidation of Si by water (Si + 2H₂O à SiO₂ + 2H₂), and we will here show how we mastered and finally avoided this reaction through partial and controlled oxidation of Si by pre-reaction with either water (Si_water) or air at moderated temperatures (Si_air).

By coupling volumetric, chemical, XRD, TEM/EELS, TGA/DSC, IR and gas adsorption data, the texture and chemistry of the resulting silica-based coating layers were found to be highly dependent on the oxidation path (Figure 1), while the level of oxidation is tuned by the time-temperature of the treatment. Although fully oxidized samples are totally inactive vs. Li., a mild oxidation (e.g. 10-15wt% SiO₂) protects the Si core from water during the electrode processing, while ensuring very stable capacity upon extended cycling (Figure 2). 

The electrochemical properties of the various resulting oxidized materials will be here deeply discussed. In addition, we will describe the effects of 1) alternative salts (LiPF₆ and LiTDI), 2) various additives (FEC and VC), and 3) different procedures for the electrode making (e.g. powder mixing, drying conditions) onto the reaction of these Si/SiO₂composite materials with Li. 

[1] G. C. L. B.A. Boukamp, and R.A Huggins, J. Electrochem. Soc: Electrochem Sicence and Thechnology 1981, 128, 725-729.

[2] U. Kasavajjula, C. Wang and A. J. Appleby, Journal of Power Sources 2007, 163, 1003-1039.

[3] N. S. Hochgatterer, M. R. Schweiger, S. Koller, P. R. Raimann, T. Wöhrle, C. Wurm and M. Winter, Electrochemical and Solid-State Letters 2008, 11, A76.

[4] B. Lestriez, S. Bahri, I. Sandu, L. Roue and D. Guyomard, Electrochemistry Communications 2007, 9, 2801-2806.