Selenium and sulfur, both as chalcogen elements, show similar volumetric capacity as cathode material for both lithium and sodium ion batteries. [1] Additionally selenium has notable higher electrical conductivity than sulfur. [2] In this work, we have investigated the kinetics of sodiation reaction in selenium nanotube as the cathode material for sodium ion battery. We have monitored the microstructure evolution and interface dynamics using in situ TEM during sodiation process. A three steps

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... action mechanism appears to explain the sodiation process ( Figure 1 ). In the first step, single crystalline selenium nanotube rapidly transforms to an amorphous Na x Se alloy phase. In the second step with continued charging, the amorphous phase recrystallizes to a polycrystal Na 2 Se 2 phase. In the final step near full sodiation, polycrystalline Na 2 Se 2 appears to completely transform into Na 2 Se phase with high content of Na. Intriguingly, the reaction front region movement is found to be quite different in the different sodiation stages. The solid-state amorphization process quickly finishes due to the high diffusion of sodium ions inside Se nanotube, with the highest nominal speed of ~2.8 nm/s, and the recrystallization processes has a speed of ~1.0 nm/s (Figure 2 ). Moreover the speed of solid-state amorphization process is nearly 10 times higher than lithation process when selenium nanotube were tested in lithiation reaction. Molecule Dynamics (MD) calculation shows all the intermediate phases produced in sodiation are good conductor of both electrons and ions. These observations can not only reveal the reaction mechanism and reaction process, but also to provide insights to design novel nanostructure of electrodes with excellent electrochemical performance. 826