Electrochemical and structural study of layered P2-type Na_2/3Ni_1/3Mn_2/3O₂ as cathode material for sodium-ion battery

P2-type Na_2/3Ni_1/3Mn_2/3O₂ was synthesized by a controlled co-precipitation method followed by a high-temperature solid-state reaction and was used as a cathode material for a sodium-ion battery (SIB). The electrochemical behavior of this layered material was studied and an initial discharge capacity of 151.8 mAhg^-1 was achieved in the voltage range of 1.5-3.75 V versus Na⁺/Na. The retained discharge capacity was found to be 123.5 mAhg^-1 after charging/dis charging 50 cycles, approximately 81.4% of the initial discharge capacity. In situ X-ray diffraction analysis was used to investigate the sodium insertion and extraction mechanism and clearly revealed the reversible structural changes of the P2-Na_2/3Ni_1/3Mn_2/3O₂ and no emergence of the O2-Ni_1/3Mn_2/3O₂ phase during the cycling test, which is important for designing stable and high-performance SIB cathode materials.