Lattice strain induced d-band centre engineering enabled pseudocapacitive energy storage in 2D hypo–hyper electronic V-NiCo ₂ O ₄ for asymmetric supercapacitors

Understanding the role of fundamental structural engineering of materials in unravelling the underlying rudimentary electronic structure-dependent charge storage mechanisms is crucial for developing new strategic approaches toward high-performance electrochemical energy storage devices. Here, we demonstrate the role of strain engineering by V doping-induced lattice contraction in NiCo₂O₄ for increasing the energy density and power density of aqueous asymmetric hybrid supercapacitors. For application in energy storage, we demonstrate the influence of electron-deficient V^4+/5+ doping in electron-rich Ni²⁺ sites, which has been found to result in the formation of a hypo-hyper electronically coupled cation pair causing a shift in the d-band and O 2p band centres and distortion of CoO₆ octahedra. Optimization of V doping to 3 mol%, achieved by a binder-free one-step hydrothermal method, has yielded a 96% increase in specific capacitance of up to 2316 F g^-1 from 1193 F g^-1 in pristine materials at 1 A g^-1 in a three-electrode configuration with a coulombic efficiency (η%) of 94% and a 24% increase in rate capacity. A two-fold increase in specific capacitance in the pouch cell device, fabricated with a functionalized carbon nanosphere positive electrode, has been observed for the V-doped samples at 1 A g^-1 with a η% of 97% and a maximum energy density of 96.3 W h g^-1 and a maximum power density of 8733.6 W g^-1 which are 41% and 24.3% higher than the pristine device, respectively. Excellent cycling stability of 95.4% capacitance retention has been observed after 6000 cycles. DFT calculations have been carried out to understand the previously unexplored effect of lattice strain on charge transport and quantum capacitance, and ultimately its effect on the transition state kinetics of energy storage faradaic reaction mechanisms. The aim of this work is to establish a fresh perspective on developing a deep understanding of the fundamental electronic and structural properties of materials by drawing in concepts from descriptor models in electrocatalysis to reveal the role of lattice strain and d-band centre tailoring in enabling pseudocapacitive energy storage.