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 Na0.67 (Mn0.67 Ni0.23 Mg0.1 )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.