Bi@C nanosphere anode with Na+-ether-solvent cointercalation behavior to achieve fast sodium storage under extreme low temperatures

The low ion transport is a major obstacle for low-temperature (LT) sodium-ion batteries (SIBs). Herein, a core-shell structure of bismuth (Bi) nanospheres coated with carbon (Bi@C) is constructed by utilizing a novel Bi-based complex (1,4,5,8-naphthalenetetracarboxylic dianhydride as the ligand) as the precursor, which provides an effective template to fabricate Bi-based anodes. At −40°C, the Bi@C anode achieves a high capacity, which is equivalent to 96% of that at 25°C, benefitting from the core-shell nanostructured engineering and Na⁺-ether-solvent cointercalation process. The special Na⁺-diglyme cointercalation behavior may effectively reduce the activation energy and accelerate the Na⁺ diffusion kinetics, enabling the excellent low-temperature performance of the Bi@C electrode. As expected, the fabricated Na₃V₂(PO₄)₃//Bi@C full-cell delivers impressive rechargeability in the ether-based electrolyte at −40°C. Density functional theory calculations and electrochemical tests also reveal the fast reaction kinetic mechanism at LT, thanks to a much lower diffusion energy barrier (167 meV) and a lower reaction activation energy (32.2 kJ mol⁻¹) of Bi@C anode in comparison with that of bulk Bi. This work provides a rational design of Bi-based electrodes for rechargeable SIBs under extreme conditions.