Direct Prelithiation of Silicon-Based Composite Electrodes via Island-like Thermal Evaporation

Irreversible losses of Li during solid electrolyte interface (SEI) conditioning in batteries are a key contributor to the lower specific capacities observed in silicon-containing Li-ion batteries. Herein, thermal evaporation of between 1 and 20 μm of Li onto Si-based composite anodes has been investigated as a prelithiation method to compensate for such losses. Additionally, to account for mechanical strain caused by Li-Si alloying and electrode expansion during the deposition, a stainless-steel mesh is applied to the electrodes before prelithiation to form “island-like” deposition on the electrode surface. The open circuit potential was also found to decrease as a function of increased Li evaporation, consistent with the potentials of electrochemically prepared Li_xSi alloys. Prelithiating to compensate for irreversible Li losses to SEI formation resulted in full cells with a 15.8% increase in initial Coulombic efficiency and a 47.8% reduction in irreversible capacity loss after SEI formation cycling. Subsequent C/3 cycling showed up to a 62.9% increase in the specific capacity in prelithiated cells. X-ray photoelectron spectroscopy (XPS) revealed differences in the SEI composition that was formed by electrochemical cycling and reactively formed in prelithiated cells upon exposure to the Gen2 + 3% FEC electrolyte. The reactively formed SEI from the spontaneous reaction with lithiated silicon was carbonate-rich, while the electrochemical SEI formation showed significantly more LiPF_x species, which could play a role in overall cycling performance.