Internal and External Cultivation Design of Zero-Strain Columbite-Structured MNb2O6 toward Lithium-Ion Capacitors as Competitive Anodes

Modest rate behaviors and structural collapse of battery-type anodes limit the commercial application of lithium-ion capacitors (LICs). For this, rational design of advanced anodes with both structural stability and high ionic/electronic conductivities becomes essential to advanced LICs. Herein, a general avenue is developed to construct a series of single-crystal nano-blocks assembled as "zero-strain" columbite-structured MNb2O6 (M = Cd, Co, Zn, Mn, Mg, Ca) accordion frameworks toward LICs. The intrinsic Li+ (de)insertion involves a solid-solution charge storage mechanism with a volumetric change of <0.59% over (de)lithiation as established with systematical in(ex) situ analysis. The MNb2O6 exhibits M-dependent electron/ion transport capabilities, along with the highest electronic and Li+ diffusion rates and the smallest volume change (0.32%) for CdNb2O6. Thanks to its robust structure and superb electron/ion conductivities originating from the "internal (i.e., ionic/electronic transport optimization) and external (i.e., structural design) cultivation" design, the CdNb2O6 shows the optimum electrochemical behaviors with a capacity of 102.8 mAh g(-1) at 10 A g(-1) and capacity retention of 80.3% after 20 000 cycles at 5 A g(-1). The assembled CdNb2O6-based LICs display high-rate energy density and long-duration stability. More importantly, the devised strategy provides meaningful guidance for optimum design of next-generation LICs.