Formation and Evolution of the Solid Electrolyte Interphase at Calcium Surfaces

The solid electrolyte interphase (SEI) plays a crucial role in the reversible metal-ion deposition at electrodes, impacting battery performance and lifespan. SEI formation results from the decomposition of salts and solvents at the electrode surface via redox reactions, with its growth governed by dynamic interactions between the electrode, electrolyte, and decomposed products. These interactions are difficult to capture experimentally. Here, using large-scale ab initio molecular dynamics simulations, we explored the formation and evolution of SEIs at the calcium anode under varying solvent, salt, and temperature conditions over 100 ps. Our simulations are inspired by several recent experiments, which show reversible calcium-ion deposition only with a few selected salts under specific experimental conditions. Our work not only deciphers these experiments but also provides detailed microscopic insights into (i) the solvent decomposition, (ii) the order of salt/solvent decomposition in the cell, (iii) the impact of electrode passivation on salt’s stability, and (iv) the role of nuclear dynamics and coordination geometry in anion decomposition, thereby influencing the reversible deposition of cations. These findings offer critical insights into the formation and evolution of SEIs at metal anodes and provide guidance for designing electrolytes to enhance the performance and longevity of metal anode batteries.