Insights Into Ni ²⁺ ‐Dominated Phase Regulation of Accordion‐Shaped CuNb ₂ O ₆ Framework Toward Lithium‐Ion Capacitors

ABSTRACT The monoclinic CuNb 2 O 6 (M‐CNO) with a columbite structure is a potential anode candidate for advanced lithium‐ion capacitors (LICs). However, its practical application is plagued seriously by the irreversible phase transformation and sluggish kinetics. For this, herein, we develop an adjustable Cu‐site Ni substituting strategy to regulate the phase transition (i.e., Ni x Cu 1‐ x Nb 2 O 6 ) from M‐CNO to orthorhombic NiNb 2 O 6 . With the fine optimization in Ni substitution, the stress/thermal‐induced phase transformation and the initial electrochemical conversion are effectively prevented in the optimal Ni 0.5 Cu 0.5 Nb 2 O 6 (55NCNO). Moreover, elaborate experimental observation and theoretical calculations synergistically authenticate that the orthorhombic 55NCNO phase is endowed intrinsically with both superb Li + and electronic conductivities, thanks to its large two‐dimensional Li + diffusion channel and narrow band gap. Benefiting from such appealing merits, 55NCNO with the solid‐solution Li + ‐storage mechanism obtains attractive high‐rate capacities and cycling stability, when evaluated as a competitive anode for LICs. Besides, the 55NCNO constructed LICs display a striking energy density of 49.5 Wh kg −1 at 10.8 kW kg −1 along with long‐span cycle life (just 0.0065% capacitance decay per cycle). More significantly, the heteroatomic substitution methodology here will guide future design of advanced anodes for next‐generation LICs, and propel their practical advancement.