P2-Na0.66Li0.18Fe0.12 Mn0.7O2, a High-Performance Cathode Material for Sodium Ion Batteries
Authors: Yang Lufeng , Li Xiang , Liu Jue , Xiong Shan , Ma Xuetian , Liu Pan , Bai Jianming , Xu Wenqian , Tang Yuanzhi , Hu Yan-Yan , Liu Meilin , Chen Hailong 
Published: February 2020

Room temperature Na-ion batteries (NIBs) hold great promises in grid-scale electric energy storage1. An in-depth and fundamental understanding of the structure-property relationship in the electrode materials is pivotal to the rational design of electrodes and batteries with improved performance. As a group of high capacity cathode materials for NIBs, P2-NaxFeyMn1-yO2 compounds typically undergo a phase transition from P2 to O2 (or OP4) phase accompanied by the creation of defects and dislocations at the grain/domain boundaries, upon sodium extraction up to 4.2 V2
-4. Such phase transition greatly hinders the kinetics of sodium insertion/extraction and leads to mechanical damage of the cathode materials and thus poor cycling stability of the battery.
Herein, a novel cathode material for sodium ion batteries, namely Na0.66Li0.18Fe0.12Mn0.7O2 was designed based on the strategy of using lithium to stabilize the deeply charged structure and was successfully synthesized. This material shows high discharge capacity of ~210 mA g-1 and the excellent cycle stability. Analysis based on in situ XRD and and ex situ solid state NMR results revealed that two factors are key to the achievement of the high performance: 1) the irreversible P2-O2 phase transition is eliminated within a wide voltage range of 1.5–4.5 V, so to achieve a highly reversible phase evolution pathway and small volume change. 2) lithium is effectively kept in the lattice as Li ions can reversibly migrate between the transition metal layer and alkaline metal layer in the charge-discharge cycle. The high performance of Na0.66Li0.18Fe0.12Mn0.7O2 demonstrated the effectiveness of Li doping and opens up opportunities of designing layered oxide cathodes with higher capacity and longer cycle life.

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Figure 1