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.