Insights into unrevealing the effects of the monovalent cation substituted tunnel-type cathode for high-performance sodium-ion batteries

Tunnel-type Na0.44MnO2 (NMO) has attracted extensive concern as a prospective cathode for sodium-ion batteries due to the advantages of facile synthesis and cost-effectiveness. Nonetheless, the low capacity, unsatisfactory rate performance, and sluggish kinetics of NMO impede its further development for commercial application. Herein, a series of Li-substituted Na0.44Mn1-xLixO2 samples are synthesized to address the abovementioned issues. Results demonstrate that Li substitution can not only stabilize the crystal structure but also facilitate the Na+ diffusion behavior during the charging/discharging process. Density functional theory (DFT) calculations reveal that Li introduction can effectively strengthen the metallicity of NMO, which is conducive to electron transferability. Accordingly, the optimized Na0.44Mn0.94Li0.06O2 (NMOL0.06) can exhibit an improved reversible capacity of 106.1 mAh g−1 at 5 C, accompanying extraordinary cycling durability with 80.6 % capacity retention after 700 cycles at 10 C. More importantly, the full cell combined with the NMOL0.06 cathode and the commercial hard carbon anode displays a high energy density of 257.2 Wh kg−1, and superior capacity retention of 83.6 % after 300 cycles at 1 C, demonstrating its great potential for practical applications. This work not only provides some insights into the influence of Li substitution on tunnel-type materials but also elucidates the construction of ultra-stable and high-performance cathodes for sodium-ion batteries.