In Silico Studies Show that -OBO Units Bound to Stable Boron Cages in Y B 11 ( O B O ) 12 - $$ \mathbf{Y} {\mathbf{B}}_{11} ( \mathbf{O} \mathbf{B} \mathbf{O} {)}_{12}^{-} $$ (Y = C/Si) Anions Provide a Desirable Borate-Rich Solid Electrolyte Interface in Ca-Ion Batteries.

Abstract

The development of calcium-ion batteries (CIBs) as potential successors to lithium-ion batteries has been hindered by the lack of suitable electrolytes. Conventional electrolytes often decompose on the Ca anode, forming calcium-ion impermeable passivation layers that impede reversible calcium plating and stripping. Therefore, the design of stable electrolytes or those capable of forming Ca²⁺ permeable passivation layers remains a critical challenge. The present study investigates calcium salts of ${\text{YB}}_{11}{X}_{12}^{-}$ (Y = C/Si and X = -BO/OBO) anions using density functional theory (DFT)-based simulations for CIBs. DFT calculations emphasize the oxidative stability of anions, while ab initio molecular dynamics (AIMD) simulations reveal their reductive behavior on the Ca anode. Time-dependent charge transfer analysis and projected density of states provide an atomistic perspective on electron transfer between the Ca surface and anions, aligning with observations from frontier orbital analyses. Decomposition of ${\text{YB}}_{11}{\left(\text{OBO}\right)}_{12}^{-}$ (Y = C/Si) anions during AIMD simulation reveals the formation of borate-based species as part of the solid electrolyte interface, suggesting their potential as electrolytes that enable effective calcium plating and stripping. Overall, this work paves the way for designing efficient electrolytes, offering a fresh perspective to identify optimal CIB electrolytes, and moving beyond ensuring electrolyte stability on the Ca anode.