Improved Sodium Storage Performance of Zn-Substituted P3-Na0.67Ni0.33Mn0.67O2 Cathode Materials for Sodium-Ion Batteries

A series of P3-phase Na0.67Ni0.33-xZnxMn0.67O2 (x = 0, 0.06, 0.09, and 0.12) samples have been synthesized and investigated as cathode materials for sodium-ion batteries. The partial substitution of Ni with Zn in the P3-phase Na0.67Ni0.33Mn0.67O2 lattice can markedly improve the electrochemical performance. A Na0.66Ni0.24Zn0.09Mn0.67O2 cathode material with an optimized Zn content of x = 0.09 can deliver an initial reversible discharge specific capacity of 127.4 mA. h g(-1) and Coulomb efficiency of 89.2% at 10 mA.g(-1) in a voltage range of 2.0-4.25 V. When the current density increases to 100 mA.g(-1), Na-0.67Ni0.24Zn0.09Mn0.67O2 delivers an initial reversible discharge specific capacity of 106.1 mA.g(-1), and capacity retention is 68.05% after 50 cycles, which is much higher than those of other samples with different Zn content. The structural characterization reveals that Zn-substituted Na0.67Ni0.24Zn0.09Mn0.67O2 increases the lattice parameters and crystallite size, and decreases the lattice strain, which is beneficial for the structural stability whether before or after the cycle. In addition, the charge-transfer impedance (R-ct) and cathodic electrolyte interphase impedance (R-CEI) of Na0.67Ni0.24Zn0.09Mn0.67O2 are significantly decreased after cycling compared with the undoped Zn electrode, which should be partially responsible for the improvement of the electrochemical performance of the Zn-doped electrode.