Pushing the limit of layered transition metal oxides with heterolattice oxygen-mediated redox for capacitive deionization.

Abstract

The use of transition metal oxides to achieve capacitive deionization (CDI) via salt adsorption is based mainly on cation electrochemistry. Activating anionic (oxygen) redox chemistry can enable additional salt adsorption on transition metal oxides, but most conventional lattice oxygen‒metal configurations require high voltages (>4 V) for activation and are prone to lattice oxygen loss. Here, we propose a heterolattice oxygen-mediated redox mechanism to activate oxygen (O_2p) redox at <2 V by constructing a V₂O₅/V₂CO_2p heterostructure. Unlike the synthetic strategy based on excess Li/Na, we develop a barrier strategy based on an oxidative nucleophilic reaction using V₂CF_x as a precursor to induce the formation of heterolattice oxygen in V₂O₅/V₂CO_2p heterostructures. Consequently, ultrahigh CDI performance is achieved, including a salt adsorption capacity of 185.8 mg g^-1 at 1.4 V and a salt adsorption rate of 12.1 mg g^-1 min^-1, which exceed those of reported other faradaic materials. Further mechanistic studies reveal that the induced O_2p electrons that dominate the Fermi level provide an additional pathway for electron movement, activating additional oxygen redox processes and forming a sodium-rich vanadate (Na₄V₂O₇)/V₂CO_2p heterostructure. This strategy provides insights into the development of high-performance CDI materials with oxygen redox based on lattice oxygen‒metal configurations.