Formation and operating mechanisms of single-crystalline perovskite NaNbO3 nanocubes/few-layered Nb2CTx MXene hybrids towards Li-ion capacitors

Lithium-ion capacitors (LICs), combining the merits of both lithium-ion batteries and supercapacitors, possess high energy/power density and a long-duration lifespan in one device. However, the dynamic imbalance between the positive and negative electrodes greatly limits their further practical application. Herein, we first fabricate single-crystalline perovskite NaNbO3 nanocubes (S-P-NNO NCs) via a simple hydrothermal alkalization strategy by using the few-layered Nb2CTx MXene (f-Nb2CTx) as a precursor, and further assemble the resultant S-P-NNO NCs with f-Nb2CTxvia a freeze-drying process to fabricate S-P-NNO/f-Nb2CTx hybrids. The in-depth understanding of the formation of the S-P-NNO NCs is reasonably unveiled with comprehensive experiments. Benefiting from the compositional optimization and synergistic effects between S-P-NNO NCs and f-Nb2CTx, the optimized S-P-NNO/f-Nb2CTx nanohybrid exhibits a large reversible capacity of ∼157 mA h g−1 at 2.0 A g−1. The S-P-NNO/f-Nb2CTx based LICs are endowed with an energy density of ∼56 Wh kg−1 at an ultrahigh power density of 13 kW kg−1, along with a long-duration cycle stability (∼75% retention after 4000 cycles). The intrinsic Li-storage mechanism of the S-P-NNO/f-Nb2CTx anode is put forward with in/ex situ X-ray diffraction analysis. More meaningfully, the devised synthetic methodology and unique insights here will stimulate the extensive development of novel Nb-based oxides towards next-generation LICs and beyond.