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

Abstract

The lowest operational temperature of commercial graphite‖LiCoO₂ (LCO) batteries is limited to ∼−20 °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 °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 × 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 × 10⁻⁵ mS cm⁻¹ and 6.65 × 10⁻⁵ mS cm⁻¹) and large diffusion coefficients (1.10 × 10⁻²¹ m² s⁻¹ and 2.07 × 10⁻²⁰ m² s⁻¹). With the electrolyte, Li‖LCO batteries deliver unprecedented cyclic performances at −70 °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.