Engineering Diffusion-Capacitive Charge Storage through Synthesis-Induced Structural Evolution in Layered Na0.6MnO2 Cathodes for Sodium−Ion Battery

Abstract Layered sodium manganese oxide (NaxMnO2) is one of the promising cathodes for sodium−ion batteries. It is known that the Na+ charge storage in NaxMnO2 is primarily by means of Na+ intercalation/de-intercalation. Off late, there has been an occurrence of Na+ charge storage which is governed by the canopy of NaxMnO2. As the morphology can strongly depend on the synthetic routes, it is important to probe the influence of synthetic routes on the electrochemical performance of NaxMnO2 and to understand the capacitive storage. Herein, the Na0.6MnO2 (NMO) cathode was synthesized using the sol−gel method (NMO-SG), self-combustion method (NMO-SC), and freeze-drying route (NMO-FD). The synthesized samples were characterized structurally and morphologically. The X-ray diffraction and Raman results confirmed the formation of layered NMO with significant co-operative Jahn−Teller distortion. The extent of the Jahn−Teller effect in Na0.6MnO2 is strongly associated with the synthetic routes. The lyophilization-derived NMO sample had a less pronounced Jahn−Teller effect. Sodium−ion battery performance of the NMO cathode was examined in the form of a CR2032 coin cell using sodium as the counter electrode. The cathodes exhibited a discharge capacity of 100, 120, and 145 mAh g−1 for the NMO-SC, NMO-SG, and NMO-FD cathode, respectively, at 0.1 C. The detailed charge storage modes of the NMO synthesized through different method are presented. Dunn’s analyses confirmed that the Na+ charge storage is dominated by the diffusive mode at the peak potential region and the capacitive mode at the non-peak potential regions. The synthesis route had a profound influence on the microstructure, extent of Jahn−Teller effect, and the battery performances.