NiCo₂O₄@TiN Core-shell Electrodes through Conformal Atomic Layer Deposition for All-solid-state Supercapacitors

Highlights: • NiCo₂O₄ nanostructures are prepared via a simple hydrothermal method. • Outer shell of TiN is then grown through conformal atomic layer deposition. • Electrodes exhibit significantly enhanced rate capability with TiN coating. • Solid-state polymer electrolyte is employed to improve cycling stability. • Full devices show a stack power density of 58.205 mW cm⁻³ at 0.061 mWh cm⁻³. - Abstract: Ternary transition metal oxides such as NiCo₂O₄ show great potential as supercapacitor electrode materials. However, the unsatisfactory rate performance of NiCo₂O₄ may prove to be a major hurdle to its commercial usage. Herein, we report the development of NiCo₂O₄@TiN core–shell nanostructures for all-solid-state supercapacitors with significantly enhanced rate capability. We demonstrate that a thin layer of TiN conformally grown by atomic layer deposition (ALD) on NiCo₂O₄ nanofiber arrays plays a key role in improving their electrical conductivity, mechanical stability, and rate performance. Fabricated using the hybrid NiCo₂O₄@TiN electrodes, the symmetric all-solid-state supercapacitor exhibited an impressive stack power density of 58.205 mW cm⁻³ at a stack energy density of 0.061 mWh cm⁻³. To the best of our knowledge, these values are the highest of any NiCo₂O₄-based all-solid-state supercapacitor reported. Additionally, the resulting NiCo₂O₄@TiN all-solid-state device displayed outstanding cycling stability by retaining 70% of its original capacitance after 20,000 cycles at a high current density of 10 mA cm⁻². These results illustrate the promise of ALD-assisted hybrid NiCo₂O₄@TiN electrodes within sustainable and integrated energy storage applications.