Optimizing Lithium‐Ion Storage in Single‐Crystalline Cu 2 Nb 34 O 87 Anode via Aliovalent In 3+ Doping: Enhanced Diffusion Kinetics and Cycling Stability Toward Lithium‐Ion Capacitors

Cu 2 Nb 34 O 87 (CNO) with the Wadsley–Roth monoclinic structure has emerged as a promising candidate for lithium storage due to its considerable theoretical capacity and unique lithium‐ion transport mechanism. However, its poor electronic conductivity and slow lithium‐ion diffusion kinetics hinder its practical application as an anode material. To address these challenges, an In 3+ ion doping strategy is implemented here to optimize the performance of CNO, yielding the single‐crystalline In‐doped CNO (ICNOx) compounds with In 3+ substitutes Cu 2+ lattice sites. Electrochemical characterizations reveal that the optimized ICNO5 delivers the superior rate performance and maintains an exceptional capacity retention of 95.7% after 1000 cycles at 2.0 A g –1 . Theoretical calculation demonstrates the optimized electronic structure and lower Li + diffusion energy barriers of ICNOx. Furthermore, lithium‐ion capacitor configured with ICNO5 as the anode and activated carbon as the cathode obtains an energy density of 24.75 Wh kg –1 at a high‐power density of 4.30 kW kg –1 . The findings here underscore the potential of lattice‐engineered niobium‐based oxides for high‐performance, long‐life energy storage applications.