The role of Zn substitution in P2-type Na0.67Ni0.23Zn0.1Mn0.67O2 cathode for inhibiting the phase transition at high potential and dissolution of manganese at low potential

Abstract

P2-type Na2/3Ni1/3Mn2/3O2 has attracted particular attention as a cathode for sodium-ion batteries. However, its cycling stability in the potential range of 1.5–4.3 V is poor due to the phase transition at high potential and dissolution of manganese in the electrolyte solution at low potential. Herein, Zn substitution in Na0.67Ni0.23Zn0.1Mn0.67O2 is found to significantly improve the cycling stability, especially in the potential range of 1.5–4.3 V. XRD analysis and EDS elemental mapping indicate that Zn2+ ions take part in the crystallization of P2-type layered structure. The SEM measurement demonstrates that Zn substitution is beneficial for the growth of the P2-type layered structure. Charge/discharge profiles and ex-situ XRD provide evidences that Na0.67Ni0.23Zn0.1Mn0.67O2 exhibits a solid solution process at Ni4+/Ni3+ redox reaction and remains P2-type structure at potential up to 4.4 V. Zn substitution efficiently improves the cycling performances in the potential range of both 2.5–4.3 V and 1.5–4.3 V. When cycling in 1.5–4.3 V at a current density of 1 C (173 mA g−1), the specific capacity of Na0.67Ni0.23Zn0.1Mn0.67O2 changes from 142.2 mAh g−1 at 1st cycle to 129.0 mAh g−1 at 50th cycle, corresponding to 90.7% capacity retention. Zn substitution inhibits the phase transition at high potential and thus decreases the bulk manganese ions exposure to the electrolyte solution, so dissolution of surface manganese is efficiently suppressed at low potential. Na0.67Ni0.23Zn0.1Mn0.67O2 delivers an initial discharge capacity of 176.3 mAh g−1 at 0.1 C and a capacity of 86.3 mAh g−1 at 5 C, exhibiting enhanced rate capability.