Solid-State Defect Engineering of Nickel Titanate with Crumpled Graphene for High-Energy, Long-Life Asymmetric Supercapacitors

Nickel titanate (NiTiO3) is a promising pseudocapacitive electrode material; however, its electrochemical performance is limited by low electrical conductivity and slow ion transport. In this work, a solvent-free solid-state NaBH4 reduction strategy is developed to introduce oxygen vacancies (OVs) into NiTiO3, producing reduced NiTiO3 (R-NTO) with a three-dimensional (3D) cauliflower-like morphology and controllable defect density. The optimized R300-NTO-1.2 electrode delivers a 207.8% increase in specific capacitance (505 F g–1) compared with pristine NiTiO3, along with improved wettability and stable cycling performance (89.2% retention after 10,000 cycles), owing to its OV-rich structure and hierarchical porosity. By coupling R300-NTO-1.2 with reduced crumpled graphene oxide (rCGO) as the negative electrode, a high-performance asymmetric supercapacitor (ASC) is constructed, combining Faradaic charge storage at the positive electrode with an electric double-layer capacitance at the negative electrode. Benefiting from effective electrode potential matching, the R300-NTO-1.2//rCGO-ASC operates stably over a 2.0 V voltage window, achieving an energy density of 44.0 Wh kg–1 at 1000 W kg–1 and retaining 34.2 Wh kg–1 at 6000 W kg–1, together with 94.9% capacitance retention and 100% Coulombic efficiency. This study demonstrates that solid-state NaBH4-induced OV engineering, when combined with a structurally robust carbon electrode, offers an effective and scalable approach for developing high-energy, long-life aqueous ASCs.