Internal Electric Field Induced by Superexchange Interaction on Mn 4+ -O 2- -Ni 2+ Unit Enables Highly Efficient Hybrid Capacitive Deionization.

Internal electric field (IEF) construction is an innovative strategy to regulate the electronic structure of electrode materials to promote charge transfer processes. Despite the wide use of IEF in various applications, the underlying mechanism of its formation in an asymmetric TM-O-TM unit still remains poorly understood. Herein, the essential principles for the IEF construction at electron occupancy state level and explore its effect on hybrid capacitive deionization (HCDI) performance is systematically investigated. By triggering a charge separation in Ni-MnO 2 via superexchange interactions in a coordination structure unit of Mn 4+ -O 2- -Ni 2+ , the formation of an IEF that can enhance charge transfer during the HCDI process is demonstrated. Experimental and theoretical results confirm the electrons transfer from O 2p orbital to TM (Ni 2+ and Mn 4+ ) e g orbital via superexchange interactions in the basic Mn 4+ -O 2- -Ni 2+ coordination unit. As a result of the charge redistribution, the IEF endows Ni-MnO 2 with superior electron and ion transfer property. This work presents a unique material design strategy that activates the electrochemical performance, and provides insights into the formation mechanism of IEF in an asymmetric TM-O-TM unit, which has potential applications in the construction of other innovative materials.