Li-Substituted Co-Free Layered P2/O3 Biphasic Na0.67Mn0.55Ni0.25Ti0.2–xLixO2 as High-Rate-Capability Cathode Materials for Sodium Ion Batteries

The effects of Li substitution for Ti on the structure and electrochemical performances of Co-free Na0.67Mn0.55Ni0.25Ti0.2–xLixO2 (x = 0, 0.1, 0.2) layered cathode materials for sodium ion batteries have been comprehensively investigated. X-ray diffraction and Rietveld refinement results demonstrate that Li mainly occupies TM (TM = transition metal) sites in the crystal structure to maintain the P2 structure majority and a small amount of Li atoms enter Na sites to generate some O3 phase. The discharge voltage, reversible capacity, rate capability, cycling performance, and Coulombic efficiency all have been improved by Li substitution, which can be largely attributed to the integration of P2 and O3. Li substitution also raises the average discharge voltage from 2.6 to 3.1 V. Na0.67Mn0.55Ni0.25Li0.2O2 (L02) can deliver an initial capacity of about 158 mA h g–1 at 0.05C (12 mA g–1) in comparison with the Li-free sample (147 mA h g–1). Even at the high rates of 480 (2C), 1200 (5C), and 1920 (8C) mA g–1, L02 can also display ca. 93, 65, and 38 mA h g–1 discharge capacities, respectively. The rate capability is higher than what is reported in the previous Li-substituted cathode materials. In addition, Li substitution in transition-metal sites generates more defects to maintain the charge neutrality, which enhances the electronic conductivity and also has a positive effect on the Na ion diffusion coefficient. The electronic conductivity and Na ion diffusion coefficient have been enhanced by 122% and 29%, respectively, with the substitution of Li for Ti. Our results also show that the oxidation peaks become sharper with increasing Li content, which indicates the feasibility of Na ion intercalation/deintercalation in the integrated P2/O3 phase. This study also offers some new insights into designing high-performance cathode materials for sodium ion batteries.