Dynamics of Li4Ti5O12/sulfone-based electrolyte interfaces in lithium-ion batteries

Binary mixtures of cyclic (TMS) or acyclic sulfones (MIS, EIS and EMS) with EMC or DMC have been used in electrolytes containing LiPF6 (1 M) in both Li4Ti5O12/Li half-cells and Li4+xTi5O12/Li4Ti5O12 symmetric cells and compared with standard EC/EMC or EC/DMC mixtures. In half-cells, sulfone-based electrolytes cannot be satisfactorily cycled owing to the formation of a resistive layer at the lithium interface, which is not stable and generates species (RSO2− and RSO3−) able to migrate toward the titanate electrode interface. Potentiostatic and galvanostatic tests of Li4Ti5O12/Li half-cells show that charge transfer resistance increases drastically when sulfones are used in the electrolyte composition. Moreover, cyclability and coulombic efficiency are low. Conversely, when symmetric Li4+xTi5O12/Li4Ti5O12 cells are used, it is demonstrated that MIS-(methyl isopropyl sulfone) and TMS-(tetra methyl sulfone) based electrolytes exhibit reasonable electrochemical performances as compared to the EC/DMC or EC/EMC standard mixtures. Surface analysis by XPS of both the Li4+xTi5O12 (partially oxidized) and Li7Ti5O12 (reduced) electrodes taken from symmetric cells reveals that sulfones do not participate in the formation of surface layers. Alkylcarbonates (EMC or DMC), used as co-solvents in sulfone-based binary electrolytes, ensure the formation of surface layers at the titanate interfaces. Therefore, EMC reduction at the two Li4+xTi5O12/electrolyte interfaces in symmetric cells leads to the formation of carbonates, ethers and mineral compounds such as ROCO2Li and Li2CO3. Finally, huge amounts of LiF are detected at the titanate electrode surface, resulting in an increase in the resistivity of symmetric cells and capacity losses.