The influence of low-concentration additives of dimethyl sulfoxide and formamide on the stability and performance of lithium bis(trifluoromethanesulfonyl)imide-based electrolyte for lithium-ion batteries

The development of advanced electrolytes is essential for improving the stability and safety of lithium-ion batteries (LIBs). This study systematically investigated the effect of low concentrations of two polar additives, dimethyl sulfoxide (DMSO) and formamide (FA), in a lithium bis(trifluoromethane-sulfonyl)imide (LiTFSI) electrolyte with ethylene carbonate and diethylene carbonate (EC/DEC, 1:1 v/v). Electrolytes containing 2.5 wt% of each additive were prepared and evaluated for ionic conductivity, electrochemical stability window (ESW), Li-ion transference number, and cycling performance in graphite||Li+ half-cells relative to the blank electrolyte. Ionic conductivity measurements showed that both DMSO and FA reduced conductivity due to higher viscosity and stronger Li⁺ solvation. Linear sweep voltammetry (LSV) indicated that while the blank electrolyte exhibited a wide ESW of 5.45 V, the addition of 2.5 wt% DMSO and FA slightly narrowed the window to 5.15 V and 4.98 V, respectively. Both additives increased the Li-ion transference number compared to the blank. Cyclic voltammetry (CV) revealed that DMSO improved interfacial reversibility, whereas FA induced quasi-capacitive behavior with suppressed faradaic processes. However, galvanostatic cycling demonstrated that both additives led to poor coulombic efficiency and unstable cycling, likely due to incompatibility with the graphite anode. Galvanostatic charge-discharge (GCD) results further indicated that low concentrations of DMSO and FA did not enhance long-term cycling stability, probably due to irregular solid electrolyte interphase (SEI) formation, although they may hold potential for high-voltage cathode applications.