Ultrafast Cointercalation Chemistry for Low-Temperature Sodium-Ion Batteries.

Sodium-ion batteries (SIBs) offer a sustainable and promising solution for large-scale energy storage because of their low cost and abundant element resources, especially in cold environments, where traditional batteries struggle. The cointercalation chemistry for graphite anode presents a potential avenue due to its fast intercalation kinetics, but it faces significant challenges at low temperatures. Herein, we first unravel a previously overlooked desolvation behavior in the cointercalation system, a key factor in performance decay under low temperatures. We propose a novel two-step reaction mechanism involving partial desolvation and interlayer diffusion for the cointercalation chemistry, which demonstrates the challenge of single-solvent solvation structures in achieving overall kinetics. Based on this, we developed an electrolyte composed of solvents with strong and weak solvation capabilities to accelerate the above two dynamic processes. Benefiting from the unique dual-solvent solvation structure, fast partial desolvation is realized by the easy removal of weakly solvating solvents, while rapid interlayer diffusion is driven by solvated Na with strong solvents, verified by solid-state nuclear magnetic resonance (ss-NMR). The assembled battery shows an ultrahigh capacity retention of up to ∼90.0% at -30 °C compared with that at room temperature at 1 C. Under this temperature, the battery still shows excellent rate performance with a high capacity maintenance of ∼ 84% for the rate increasing from 0.1 to 5 C.