Importance of Reduction and Oxidation Stability of High Voltage Electrolytes and Additives

Abstract

The electrolyte is a critical component for rechargeable Li-ion batteries, especially batteries containing high voltage cathodes. A series of electrolyte salts, solvents and additives was investigated via cyclic voltammetry (CV) on glassy carbon (GC) electrodes. Quantum chemistry (QC) calculations were used for prediction of oxidation and reduction stability of electrolyte components such as ethylene carbonate (EC), dimethyl carbonate (DMC), fluoroethylene carbonate (FEC), vinylene carbonate (VC), tris(trimethylsilyl) phosphate (TMSP), 1,3-propane sultone (PS), tris(hexafluoroisopropyl) phosphate (HFiPP) and lithium salts such as LiPF6, LiBF4, lithium difluoro(oxalato)borate (LiDFOB), lithium 4,5-dicyano-2-trifluoromethyl-imidazolide (LiTDI), and lithium bis(trifluoromethanesulfonimide) (LiTFSI). QC calculations predicted that defluorination of LiPF6 and LiTFSI aggregates coupled with electron transfer significantly increased their reduction potential, while H-transfer upon oxidation lowered oxidation potential for many solvents. The composition of the Li+ cation solvation shell was estimated from the binding energies for the Li+–solvent complexes using cluster-continuum calculations and was used to provide insight into the experimental data on electrolyte reduction. Full coin cell data was acquired using LiNi0.5Mn1.5O4 (LNMO) cathodes with graphite anodes at 25°C and 55°C. Differential capacity plots (dQ/dE vs. E) and electrochemical impedance spectroscopy (EIS) results. The electrolyte with the most desirable performance was 1M LiPF6 in 3:7 EC:EMC (wt%) with 1wt% TMSP. Oxidative CV experiments show that the TMSP containing electrolyte has a slightly lower oxidation stability compared to the baseline, which is consistent with the order of oxidation stability of electrolyte components from QC calculations. The EIS measurements showed that the TMSP containing electrolyte had the lowest impedance after cycling and the dQ/dE plots show that the redox reactions retained their peak shape and area more so than an electrolyte without TMSP, indicating a greater capacity retention. Density functional theory calculations of TMSP oxidation on the LNMO surface were performed in order to provide insight into the additive role in improving cell performance.