A molybdenum doped layer-spinel composite cathode material for sodium-ion battery

Abstract

Sodium-ion batteries are pursued as pragmatic alternative to the Li-ion battery technology having operational similarity along with natural resource abundance. P2-type manganese-rich layered oxides are widely explored exhibiting high capacity along with fast rate kinetics. To improve their electrochemical performance and reduce voltage decay upon cycling and to mitigate irreversible phase transitions, cation doping or hybrid composite integrations have been proposed. Here, we report a unique Mn-rich layer-spinel composite, Na0.7(Li1/18Mn11/18Ni3/18Fe2/18χ1/18)O2–xNa2MoO4, leading to a synergistic effect of layered P2 and spinel phases. This stable layer/spinel biphasic composite was stabilized through Mo doping and its electrochemical activity was studied at different voltage windows. When cycled between 1.5–4.5 V, this composite delivered a high specific capacity of 183 mAh.g–1 involving both cationic and anionic (O2–/O2n–) redox. The structural evolution during (dis)charge was studied by ex-situ X-ray diffraction and cyclic voltammetry. It is observed that mitigating P2-P2′′ phase transition at higher voltage is crucial to improve the electrochemical performance, cycling stability and reduce the voltage hysteresis.