Reconstruction of LiF-rich interphases through an anti-freezing electrolyte for ultralow-temperature LiCoO₂ batteries

The lowest operational temperature of commercial graphite||LiCoO2 (LCO) batteries is limited to similar to-20 degrees C due to the high reaction energy barrier of Li+ in the interlayers of the graphite anode and the unstable solid electrolyte interphase (SEI) forming at low temperatures. Lithium (Li) metal with ideally host-less nature is expected to support the low-temperature operation of the LCO cathode, but low-temperature applications of Li||LCO batteries are severely challenged with disastrous issues of conventional electrolytes including the high solvation structure of Li+, low desolvation energy, low Li+ saturation concentration, and LiF-barren SEI and cathode electrolyte interphase (CEI) (below 7%) with a small Li+ conductivity and diffusion coefficient. Here, using iso-butyl formate (IF) as an anti-freezing agent with an ultralow melting point of -132 degrees C and an ultralow viscosity of 0.30 Pa s, a fluorine-sulfur electrolyte is designed to achieve a low-coordination number (0.07), high desolvation energy (-27.97 eV) and high Li+ saturation concentration (1.40 x 10(-10) mol s(-1)) electrolyte, which enables efficient reversible transport of Li+ and formation of abundant F radicals to construct stable LiF-rich SEI (10.48%) and CEI (17.91%) layers with large Li+ conductivities (1.00 x 10(-5) mS cm(-1) and 6.65 x 10(-5) mS cm(-1)) and large diffusion coefficients (1.10 x 10(-21) m(2) s(-1) and 2.07 x 10(-20) m(2) s(-1)). With the electrolyte, Li||LCO batteries deliver unprecedented cyclic performances at -70 degrees C including a stable capacity of 110 mA h g(-1) over 170 cycles. The work provides an opportunity for developing ultralow-temperature LCO batteries.