Unusual Mesoporous Titanium Niobium Oxides Realizing Sodium-Ion Batteries Operated at -40 °C.

Sodium-ion batteries (SIBs) are a promising candidate for grid-scale energy storage, however, the sluggish ion-diffusion kinetics brought by the large radius of Na + seriously limits the performance of SIBs, let alone at low temperatures. Herein, a confined acid-base pair self-assembly strategy to synthesize unusual Ti 0.88 Nb 0.88 O 4- x @C for high-performance SIBs operating at room and low temperatures is proposed. The confinement self-assembly of the acid-base pair around the micelles and confined crystallization by the carbon layer realize the formation of ordered and stoichiometric mesoporous frameworks with opened ion channels. Thus, the mesoporous Ti 0.88 Nb 0.88 O 4- x @C exhibits rapid Na + diffusion kinetics at 25 and -40 °C, which are one order higher than that of the nonporous one. A high reversible capacity of 233 mAh g -1 , excellent rate (a specific capacity of 103 mAh g -1 at 50 C), and cycling performances (<0.03% fading per cycle) can be observed at 25 °C. More importantly, even at -40 °C, the mesoporous Ti 0.88 Nb 0.88 O 4- x @C can still deliver the 161 mAh g -1 capacity, a high initial Coulombic efficiency of 60% and outstanding cycling stability (99 mAh g -1 at 0.5 C after 500 cycles). It is believed this strategy opens a new avenue for constructing novel mesoporous electrode materials for low-temperature energy storage.