A P2@Tunnel Heterostructure Cathode for High‐Performance Sodium‐Ion Batteries.

High‐working potential layered oxides as cathode materials have been considered as an effective way to improve the energy density of Na‐ion batteries (NIBs) to enhance their competitiveness as a promising alternative for lithium ion batteries in large‐scale energy storage applications. However, they generally suffer irreversible phase transition, large volume change and electrolyte corrosion when charged to high voltage (>4.2 V), resulting in poor cyclic stability. In this work, a P2@tunnel heterostructure cathode material is designed and obtained by wet chemistry combined with solid‐state reaction method. Moreover, partly of the Ti4+ ions which are designed for the tunnel structure is also doped into the P2 phase under high temperature annealing process, resulting in expanded crystal parameters and smooth charge‐discharge profiles. The stable tunnel structure layer can effectively protect the layered P2 type cathode material from corrosion by electrolyte upon high voltage and alleviate the crack propagated from the particle surface. Therefore, the P2@tunnel heterostructure cathode can still maintain a discharge capacity of 146.3 mAh g−1 while the discharge capacity of pristine P2 type cathode is only 114.4 mAh g−1 after 100 cycles at 0.25 C, it also reveals superior cyclic stability and rate performance compared to the pristine P2 type cathode. [ABSTRACT FROM AUTHOR]