Contriving a gel polymer electrolyte to drive quasi-solid-state high-voltage Li metal batteries at ultralow temperatures†

Gel polymer electrolytes (GPEs) synergizing the benefits of solid and liquid electrolytes are promising electrolyte candidates for future lithium metal batteries (LMBs). However, the poor performance of GPEs in subzero temperatures (particularly in extremely cold conditions) limits their practical applications. Here, we contrived a new and simple GPE recipe for low-temperature operation using only common electrolyte components, viz. a single-solute LiBF₄ (lithium salt and initiator), polymerized monomer 1,3-dioxolane (DOL), and high-dielectric-constant solvent fluoroethylene carbonate (FEC). We demonstrated the unique multiple functions of each component and achieved fast kinetics and interfacial stability at −60 °C to −20 °C. New insights into how the solvation structures evolve during in situ polymerization were proposed. Distinctive Li⁺ solvation structures involving poly-DOL, BF₄⁻ and FEC regulated stable solid electrolyte interphases, which contained robust LiF and Li⁺-conducting Li_xBO_yF_z. Consequently, Li|GPE|LiNi_0.8Co_0.1Mn_0.1O₂ and Li|GPE|LiCoO₂ cells released impressive capacities of ∼142 and ∼155 mA h g⁻¹ at −20 °C, with excellent capacity retention of 94% (200 cycles) and 96% (350 cycles), respectively. Notably, the GPE enabled Li//LiCoO₂ pouch cells to operate as low as −60 °C, delivering a high capacity of ∼112 mA h g⁻¹, which represents the lowest operating temperature and competitive performance (capacity and cycling life) for high-voltage LMBs reported in the GPE field.