P2-type Na 0.66 Ni 0.33–x Zn x Mn 0.67 O 2 as new high-voltage cathode materials for sodium-ion batteries

P2-type Na0.66Ni0.33–xZnxMn0.67O2 (x = 0, 0.07, 0.14) are prepared using a conventional solid state method and for the first time developed as promising cathode materials for high-voltage sodium-ion batteries. The XRD patterns show that Zn2+ 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 Zn2+ in the Na–Ni–Mn–O system can effectively overcome the drawback of voltage decay when charged to a higher cutoff voltage (>4.0 V), and significantly improve capacity retention compared to the unsubstituted material during cycling. In addition, a smoother charge/discharge profile can be observed between 3.0 and 4.0 V for Zn-substituted samples, demonstrating that Na+/vacancy ordering can be suppressed during sodium insertion/extraction. Na0.66Ni0.26Zn0.07Mn0.67O2 can deliver an initial capacity of 132 mAh g−1 at 12 mA g−1 with a high average voltage of 3.6 V and a capacity retention of 89% after 30 cycles. EIS measurements demonstrate that Zn-substitution is an effective way to limit the increase of inter-particle contact resistance by suppressing any possible irreversible phase transformation found at low sodium contents.