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 (O2p) redox at <2 V by constructing a V2O5/V2CO2p heterostructure. Unlike the synthetic strategy based on excess Li/Na, we develop a barrier strategy based on an oxidative nucleophilic reaction using V2CFx as a precursor to induce the formation of heterolattice oxygen in V2O5/V2CO2p 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 O2p 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)/V2CO2p heterostructure. This strategy provides insights into the development of high-performance CDI materials with oxygen redox based on lattice oxygen‒metal configurations. Activation of lattice oxygen‒metal configurations in transition metal oxide-based capacitive deionization systems can enhance their performance but requires high voltages. Here the authors enhanced the capacitive deionization performance by developing a V2O5/V2CO2p heterostructure that can activate oxygen redox at low voltage (<2 V).