Poly(phosphazene)-Coatings for Stabilizing Silicon Thin-Film Anodes in Lithium-Ion-Batteries

Silicon is a promising anode material for lithium-ion batteries, but its practical use is limited by several factors: first, upon lithiation, silicon forms highly reduced phases, which themselves reduce the electrolyte unless the surfaces are passivated; second, silicon undergoes large volume changes during charge and discharge. Both processes lead to capacity loss, and their effects need to be mitigated to improve cell lifetime. To address these fundamental roadblocks, in this work, flexible poly(phosphazenes) are explored as Si coatings. Amorphous silicon thin-film electrodes are used to simplify the coating procedure and explore the role of these coatings in limiting electrolyte reduction. Two short-chain linear poly(phosphazenes) were prepared and cross-linked on the surfaces of the electrodes. The electrochemical performance of the electrodes and the accompanying structural changes of the electrode material were investigated. A partially fluorinated phosphazene coating was shown to improve the cycle life of the silicon thin-film anode most, allowing more than 500 cycles to be achieved with improved Coulombic efficiencies compared to the other electrodes. For analytical purposes, the Navani Python package was developed to track lithiation potentials from incremental capacity analysis during cycling. The package was used to measure how the polymer coatings affected lithiation potentials of the silicon electrode, giving information about the changes in mechanical stress during cell aging, and changes in overpotential due to the buildup of the solid electrolyte interface (SEI).