Highly Efficient Sodium Storage in Iron Oxide Nanotube Arrays Enabled by Built-In Electric Field

High-power sodium-ion batteries capable of charging and discharging rapidly and durably are eagerly demanded to replace current lithium-ion batteries. However, poor activity and instable cycling of common sodium anode materials represent a huge barrier for practical deployment. A smart design of ordered nanotube arrays of iron oxide (Fe₂ O₃ ) is presented as efficient sodium anode, simply enabled by surface sulfurization. The resulted heterostructure of oxide and sulfide spontaneously develops a built-in electric field, which reduces the activation energy and accelerates charge transport significantly. Benefiting from the synergy of ordered architecture and built-in electric field, such arrays exhibit a large reversible capacity, a superior rate capability, and a high retention of 91% up to 200 cycles at a high rate of 5 A g^-1 , outperforming most reported iron oxide electrodes. Furthermore, full cells based on the Fe₂ O₃ array anode and the Na_0.67 (Mn_0.67 Ni_0.23 Mg_0.1 )O₂ cathode deliver a specific energy of 142 Wh kg^-1 at a power density of 330 W kg^-1 (based on both active electrodes), demonstrating a great potential in practical application. This material design may open a new door in engineering efficient anode based on earth-abundant materials.