Differentiating the Synergistic Interactions Between Li+ Salts and Cyclic to Linear Carbonate Ratios to Enable Wide‐Temperature Performance of Lithium‐Ion Batteries

Lithium‐ion batteries (LIBs) operate without significant degradation between a temperature range of +15 and +35 °C. In the design of electrolytes, the selection of Li+ salt and solvents determine the composition of solid‐electrolyte interphase (SEI) and cathode‐electrolyte interphase (CEI) films. This work identifies the resultant interactions of Li+ salt anions and electrolyte cosolvents across a wide‐temperature range of −40 to +60 °C. Specifically, this work selects lithium hexafluorophosphate (LiPF6), lithium difluoro(oxolato)borate (LiDFOB), and lithium bis(fluorosulfonyl)imide (LiFSI) for testing in varied ethylene carbonate (EC) and ethyl methyl carbonate (EMC) ratios in Li1.02Ni0.8Mn0.1Co0.1O2 (NMC811) cathode and graphite anode full cells. Electrochemical impedance spectroscopy (EIS) analysis indicates that electrolytes with EC > 20 v% exhibit charge‐transfer resistance (Rct) growth, while electrolytes with EC < 20 v% experience larger growth in the high‐frequency region associated with SEI and CEI resistance. Symmetric cells of electrodes cycled in single‐salt EC‐free electrolytes indicate that high impedance growth occurs on the cathode in LiPF6 electrolytes and the anode in LiFSI electrolytes, while LiDFOB passivates both electrodes, improving the retention of low‐temperature performance after high‐temperature cycling. By identifying Li+ salt and solvent combinations favorable for wide‐temperature performance, this work suggests new electrolyte formulas designed to assist in extending the temperature range LIBs can reliably operate.