Li15Si4 Formation in Thin Film Si Negative Electrodes for Li-ion Batteries

Abstract

In this work, a better understanding behind, and the implications of Li15Si4 formation in Si containing negative electrodes for rechargeable Li-ion batteries during electrochemical cycling was sought. Si in the form of thin films, powder, nanoparticles, recovered film flakes, and films deposited onto compliant layers were fabricated and studied as negative electrodes in Li half-cells. Through a study of the electrochemical behaviour of Si thin films, a model for Li15Si4 formation was proposed. The model hypothesizes that Si under high stress loadings does not form Li15Si4 due to shifting in the voltage curve caused by stress-potential coupling. Later in the study, the formation of Li15Si4 was confirmed to have harmful effects on electrochemical cycling. Overall, the formation of Li15Si4 in Si-based negative electrodes during electrochemical cycling was shown to have potential as an indicator for poorly attached silicon in an alloy or composite coating. A second study in this work investigated the effect of the binder in electrode coatings on electrochemical performance for rechargeable Li-ion batteries. In this work, coatings using polyimide binder were found to exhibit the most Li15Si4 suppression and least capacity fade during electrochemical cycling. Additionally, composite coatings incorporating polyimide or lithium polyacrylate binder were found to be capable of maintaining electrical contact to poorly attached silicon during extensive cycling, while coatings using polyvinylidene difluoride binder showed capacity fade and an inability to maintain electrical contact to poorly attached Si. Lastly, the use of compliant conducting layers between the substrate and sputtered Si film in negative electrodes for Li-ion batteries was investigated. This investigation sought to attain similar electrochemical behaviour in Si films to that shown in Si powder based composite coatings when incorporated into Li half-cells. Ultimately, the use of a compliant layer was shown to allow Si films and a binary Si-Fe thin film to exhibit significantly more similar electrochemical behaviour to Si powder containing composite coatings than films deposited directly onto a metallic substrate.