Molecular unraveling of Li⁺-solvent interaction reversal and its impact on low-temperature desolvation for weakly solvated electrolytes

Weakly-solvated electrolytes (WSEs) enable rapid desolvation, yet their rational design for low-temperature lithium-ion batteries (LIBs) remains hindered by poor understanding of solvation structure evolution and its impact on desolvation kinetics. Conventional WSEs primarily focus on weakening Li⁺-solvent interactions at room temperature. However, such interactions inevitably strengthen upon cooling, making desolvation increasingly difficult at low temperatures. Herein, in situ liquid secondary ion mass spectrometry was employed to differentiate the dipole interaction and electrostatic interaction in fluoroethylene carbonate (FEC) and lithium difluoro(oxalato)borate (LiDFOB)-based WSE at operando temperatures. Notably, we reveal for the first time a reversal of Li⁺-dipole interaction at low temperature with direct molecular evidence, marked by a transition to a weak solvent FEC-dominated primary solvation sheath (PSS). The transition is induced by the stronger dipole interaction of FEC to Li⁺, after DFOB⁻ electron delocalizes onto the FEC. The increased presence of FEC in the PSS moderates cation-anion aggregation, thereby effectively suppressing salt precipitation at low temperature. This is achieved by the equilibrium established between the Li⁺-FEC ion-dipole interaction and the Li⁺-DFOB⁻ electrostatic interaction, moreover it speeds up the interfacial Li⁺ desolvation process at low temperature. Our findings provide novel molecular insights on low-temperature solvation restructuring and establish design principles for extreme-condition LIBs.