Thermodynamic Analysis of the Hydrolysis of Borate-Based Lithium Salts by Density Functional Theory

Abstract

Here we discuss the thermodynamics of the hydrolysis of three borate-based lithium salts commonly used in aprotic electrolytes for lithium-ion batteries: lithium tetrafluoroborate (LiBF4), lithium difluoro(oxalate)borate (LiODBF) and lithium bis(oxalate)borate (LiBOB). We performed density functional theory calculations at the ωB97M-V/6–31++G** level to compute the thermodynamic stability of reagents, intermediate and products in solution phase. The variations of Gibbs free energy in heterogeneous reactions were evaluated by coupling DFT values with thermochemical cycles. LiBF4 and LiODBF can be easily hydrolyzed by the direct reaction with water at room temperature: the salts degradation is driven by the precipitation of LiF(s) and by the release of HF. On the contrary, LiBOB is much more stable and only weakly reacts with water: it is therefore more resilient to hydrolyzation and therefore suitable for application in Li-ion battery electrolytes.