Li-Substituted Layered-Spinel Cathode Material for Sodium-Ion Batteries

Abstract

Sodium-ion battery (SIB) has been considered as an attractive technology for the next-generation, large-scale energy storage systems (EES) in support of reliable, robust and cost-effective electrical power grids.1-4 The O3-type layered Na(NixFeyMnz)O2 (0 < x, y, z< 1) cathode materials are of great interest in sodium-ion batteries due to the abundance and cost of raw materials and their high specific capacities. However, the cycling stability and rate capability at high voltages (>4.0V) of these materials remains an issue. Herein, we successfully synthesized a Li-substituted layered-tunneled (O3-spinel) intergrowth cathode (LS-NFM) to address these issues. The remarkable structural compatibility and connectivity of the two phases were confirmed by X-ray diffraction (XRD), selected area electron diffraction (SAED) and high resolution transmission electron microscopy (HRTEM). LS-NFM electrode reached a discharge capacity 107 mAh g-1 with a capacity retention of 95% after 50 cycles at a current rate of 100 mA g-1 in a voltage window of 2.0 - 4.2 V. Moreover, the LS-NFM cathode exhibited an enhanced rate capability in comparison to the un-doped layered cathode (NFM). The enhanced rate capability of LS-NFM can be explained by the significantly increased effective Na+ diffusivity measured by galvanostatic intermittent titration technique (GITT) compared to the un-doped control NFM cathode, which can be ascribed to the improved charge transport kinetics through shortened diffusion path by direct connection between the 3D channels in the spinel phase and 2D channels in the layered phase. The results from ex situ hard/soft X-ray adsorption spectroscopy (XAS) suggest that the capacity of LS-NFM cathode is mainly associated with the Ni2+/Ni4+ redox couple, and slightly from the Fe3+/Fe4+ redox couple. The voltage profile of the LS-NFM cathode exhibited a reversible plateau above 4.0 V, indicating great stability at high voltages and structural stabilization by the spinel phase. In addition to the substitution of various transition metals, or the modification of the stoichiometry of each transition metal, this study provides a new strategy to improve electrochemical performance of layered cathode materials for sodium ion batteries.