Electrochemical study of Na 0.66 Ni 0.33 Mn 0.67−x Mo x O 2 as cathode material for sodium-ion battery

P2-type Na0.66Ni0.33Mn0.67−xMoxO2 (x = 0, 0.03, 0.05, 0.07) were prepared using a conventional solid state method and for the first time developed as promising cathode materials for sodium-ion batteries. The XRD patterns show that Mo6+ ions are successfully incorporated into the lattice of the Na-Ni-Mn-O system and the P2-type structure remains unchanged after substitution. The introduction of Mo6+ in the Na-Ni-Mn-O system can significantly improve capacity retention compared to the unsubstituted material during cycling. In addition, an additional charge/discharge profile can be observed between 3.0 and 3.2 V for Mo-substituted samples, demonstrating that Na+/vacancy ordering can be suppressed during sodium insertion/extraction. Na0.66Ni0.33Mn0.62Mo0.05O2 can deliver an initial capacity of 112 mAhg−1 at 34 mAg−1 with a high average voltage of 3.6 V and a capacity retention of 87% after 50 cycles. EIS measurements demonstrate that Mo-substitution is an effective way to hinder the increase of inter-particle contact resistance by suppressing any possible irreversible phase transformation found at low sodium contents.