Studies of lithium-silicon negative electrode materials for batteries

Abstract

Silicon is a promising negative electrode material for lithium-ion batteries because of its high specific capacity which is ten times higher than graphite. Lithium silicides are also the negative electrode material for thermal batteries. As a result, lithium silicides are of significant interest to the battery community. There are four thermodynamically stable phases in the lithium-silicon system: Li₂₁Si₅, Li₁₃Si₄, Li₇Si₃, Li₁₂Si₇ and this was determined electrochemically. The coulometric titration curve for lithium silicides was obtained at 415°C by Wen and Huggins. This work collected electrochemical data on lithium silicides as the negative electrode materials for thermal batteries before carrying out neutron diffraction on lithium-silicon phases. Galvanostatic discharges of FeS₂/LiCl-KCl/Li-Si cells were carried out to identify the voltage plateaux. Payne et al observed that no new lithium-silicon phases formed during the discharge of the thermal batteries with NiS₂/Li₁₃Si₄ and CoS₂/Li₁₃Si₄ chemistries. The question remained if a loss in crystallinity occurred due to Li₁₃Si₄ becoming amorphous during lithium removal at high temperature. This work studied if high temperature amorphisation occurs in Li₁₃Si₄ and Li₇Si₃ phases because electrochemically driven amorphisation was seen in the silicon electrode in lithium-ion cells at room temperature. The Li-Si phases were probed at room temperature and at high temperature (500°C). The thermal expansion coefficients were obtained from the neutron data. The samples retained crystallinity and did not become amorphous at high temperature. Both phases were remarkably stable at high temperature. The second part of this work studied the silicon clusters that exist within lithium silicides. The silicon clusters, which behave like molecules, likely control the electrochemistry of the batteries. Structural, magnetic and electronic transitions of the lithium silicides were investigated to find which temperature points to collect longer and good quality structural information with total scattering neutron diffraction.