Oxidative Stability and Initial Decomposition Reactions of Carbonate, Sulfone, and Alkyl Phosphate-Based Electrolytes

The oxidative stability and initial oxidation-induced decomposition reactions of common electrolyte solvents for batteries and electrical double layer capacitors were investigated using quantum chemistry (QC) calculations. The investigated electrolytes consisted of linear (DMC, EMC) and cyclic carbonate (EC, PC, VC), sulfone (TMS), sulfonate, and alkyl phosphate solvents paired with BF₄ ^–, PF₆ ^–, bis­(fluorosulfonyl)­imide (FSI^–), difluoro-(oxalato)­borate (DFOB^–), dicyanotriazolate (DCTA^–), and B­(CN)₄ ^– anions. Most QC calculations were performed using the M05-2X, LC-ωPBE density functional and compared with the G4MP2 results where feasible. The calculated oxidation potentials were compared with previous and new experimental data. The intrinsic oxidation potential of most solvent molecules was found to be higher than experimental values for electrolytes even after the solvation contribution was included in the QC calculations via a polarized continuum model. The presence of BF₄ ^–, PF₆ ^–, B­(CN)₄ ^–, and FSI^– anions near the solvents was found to significantly decrease the oxidative stability of many solvents due to the spontaneous or low barrier (for FSI^–) H- and F-abstraction reaction that followed the initial electron removal step. Such spontaneous H-abstraction reactions were not observed for the solvent complexes with DCTA^– or DFOB^– anions or for VC/anion, TMP/PF₆ ^– complexes. Spontaneous H-transfer reactions were also found for dimers of the oxidized carbonates (EC, DMC), alkyl phosphates (TMP), while low barrier H-transfer was found for dimers of sulfones (TMS and EMS). These reactions resulted in a significant decrease of the dimer oxidation potential compared to the oxidation potential of the isolated solvent molecules. The presence of anions or an explicitly included solvent molecule next to the oxidized solvent molecules also reduced the barriers for the oxidation-induced decomposition reaction and often changed the decomposition products. When a Li⁺ cation polarized the solvent in the EC_<i>n</i>/LiBF₄ and EC_<i>n</i>/LiPF₆ complexes, the complex oxidation potential was 0.3–0.6 eV higher than the oxidation potential of EC_<i>n</i>/BF₄ ^– and EC_<i>n</i>/PF₆ ^–