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 Ti0.88Nb0.88O4-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 Ti0.88Nb0.88O4-x@C exhibits rapid Na+ diffusion kinetics at 25 and -40 degrees 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 degrees C. More importantly, even at -40 degrees C, the mesoporous Ti0.88Nb0.88O4-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.