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

Abstract

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 Li_4+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 Li_4+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 Li_4+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 Li_4+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.