Sulfone‐Based Low Fluorine Electrolytes for High‐Voltage Cobalt‐Free Cathodes

Replacing commercial cathode active materials with lithium nickel manganese oxide (LNMO, spinel) has the potential to increase the energy density of lithium‐ion batteries. However, to successfully utilize this material, an alternative to conventional EC‐based electrolytes needs to be found, as this solvent is unstable at potentials above 4.7 V vs. Li + /Li. In this context, a novel electrolyte formulation consisting of lithium difluoro (oxalato) borate (LiDFOB) and ethyl isopropyl sulfone (EiPS) is examined. The LiDFOB salt is investigated at various concentrations and demonstrates a good solubility in the EiPS solvent as well as the ability to passivate the aluminum current collector at high potentials. Additionally, EiPS exhibits a high dielectric constant and outstanding thermal and electrochemical stability. In combination with LNMO electrodes, the 1 M LiDFOB in EiPS electrolyte shows the best performance stability among all tested electrolytes with an average 82 mAh g −1 and a discharge capacity retention of 89%, after 100 cycles at 0.33 C. A subsequent ex situ X‐ray photoelectron spectroscopy analysis reveals a thinner cathode‐electrolyte interface forming during cycling when using EiPS‐based electrolytes compared to a benchmark formulation, as well as a lower LiF content, indicating lower impedance of the CEI.