Insight into the competitive reaction between LiDFP and LiFSI in lithium-ion battery at low temperature

Lithium-ion batteries (LIBs) suffering from severe performance degradation because of the unstable solid electrolyte interphase (SEI) on the anode at low temperature restricts their practical applications. Herein, lithium difluorophosphate (LiDFP) as the additive is introduced into lithium bis(fluorosulfonyl) imide (LiFSI) based electrolyte to improve the electrochemical performance of graphite/Li half-cells at low temperature. Contrary to the popular perception, we reveal at low temperature that LiDFP attenuates the decomposition of LiFSI by competing the hydrolysis reaction with LiFSI to generate a SEI film rich in LiF and Li3PO4 during prolonged cycling, rather than preferential decomposition. Additionally, the possible reaction equations and the interaction mechanism between LiDFP and LiFSI are proposed by combining in situ electrochemical impedance electrochemical (PRIs-EIS) tests, spectroscopic characterization techniques and density functional theory (DFT) calculations. It is demonstrated that the strong binding energy between LiF (from the decomposition of LiFSI) and LiDFP makes LiDFP easier deposit on the electrode surface. This work demonstrates the synergistic role combining the complementary advantage of film-forming additives and main lithium salts to improve the interfacial stability of LIBs at low temperature. Additionally, it can also pave the new pathway for the design of low temperature electrolytes in LIBs.