Synergistic F-/Ti4+ Codoping Modulation of P2-Na0.67Ni0.33Mn0.67O2 for High-Performance Capacitive Deionization

Capacitive deionization is an electrochemical desalination technology characterized by low energy consumption and holds promise for addressing global water scarcity, and electrode materials are key to determining its desalination performance. Layered sodium-manganese oxides, such as P2-Na0.67Ni0.33Mn0.67O2, exhibit relatively high theoretical capacities. However, their practical application in capacitive deionization is hindered by intrinsically low electronic conductivity and structural instability during operation. To overcome these limitations, this study reports a series of modified P2-type sodium-manganese oxides synthesized via a synergistic F-/Ti4+ codoping strategy using a high-temperature solid-state reaction. The resulting materials were systematically characterized with respect to crystal structure, electrochemical behavior, and capacitive deionization desalination performance. It was found that F- doping effectively suppresses the formation of NiO impurities and enhances electronic conductivity, whereas Ti4+ doping expands the interlayer spacing, disrupts the long-range ordering of Na+/vacancy, and thereby improves Na+ diffusion kinetics. The optimized P2-Na0.67Ni0.33Mn0.57Ti0.1O1.9F0.1 electrode delivers a high specific capacitance of 244.8 F g-1 and retains 86.9% of its initial capacity after 300 cycles. When used as a capacitive deionization cathode, it achieves a salt adsorption capacity of 47.91 mg g-1 and a maximum desalination rate of 21.41 mg g-1 min-1, both metrics markedly surpassing those of the undoped counterpart. This work not only establishes F-/Ti4+ codoping as an effective strategy for stabilizing layered manganese oxide structures but also provides a facile and scalable route to develop high-performance electrode materials for practical capacitive deionization applications.