Suppressing the P2-OP4 phase transition of single-crystal P2-type Ni/Zn/ Mn-based layered oxide for advanced sodium-ion batteries

P2-type layered Na0.66Ni0.26Zn0.07Mn0.67O2 (NNZM) is expected to be a competitive alternative to lithium layered oxide due to its high-energy-density, low production cost, and high-speed Na+ ion transport channels. However, it is still necessary to further improve its air stability and cycle life to meet the needs of practical applications. Single crystals with micron-scale have smaller specific surface area and greater packing density, which can improve air stability and cycle life compared to the traditional polycrystalline. Herein, single-crystal Na0.66Ni0.26Zn0.07Mn0.67O2 (SC-NNZM) is prepared by using Na2MoO4 as the molten salt. Driven by molten salt Na2MoO4, regular hexagonal prism morphology SC-NNZM with a median diameter (D50) of 7.86 mu m and the (001) plane-dominated structure is obtained, larger than that (5.05 mu m) of single-crystal C-NNZM without Na2MoO4, implying that specific surface area of SC-NNZM is smaller than that of C-NNZM, thereby reducing the contact area of SC-NNZM with the electrolyte, which is good for suppressing both harmful interface side-reactions and phase transitions at high voltages. Therefore, SC-NNZM exhibits 95.44 % capacity retention at 100 mA g(-1) after 100 cycles, higher than C-NNZM (86.09 %). Moreover, rate capability of SC-NNZM is also higher than that of C-NNZM. This study provides a new strategy for inducing crystal plane growth that is conducive to the structure stability of single-crystal layered oxide at high voltages.