Electrochemistry and structural evolution of monoclinic NaNiO2 as a cathode material for Na-ion battery is reported. The initial charge capacity reached 160 mA h g−1 and the following discharge capacity of 114.6 mA h g−1, within the voltage range of 4.0–1.5 V at C/10. The multiple phase transition leading to O′3, P′3, P″3, O″3, and O‴3 stacking types (NaNiO2, Na0.91NiO2, Na0.84NiO2, Na0.81NiO2 and Na0.79NiO2 transitions, respectively, according to a previous report) during the 1st charge/discharge process is analysed using ex situ and in situ XRD techniques, and the stoichiometry of each phase is herein revised. The charge/discharge profile shows a highly reversible nature of the cathode, except that fully sodiated phase could not be achieved at the subsequent discharge. Two new phases have been discovered: a monoclinic O3 structure (designated as O⁗3) at the beginning of the charge (and end of discharge) and a P3 structure (designated as P‴3) at 3.38 V that appeared only during the charge process. The composition of the new O⁗3-phase corresponds to Na0.83NiO2, which is the closest to the fully sodiated phase at room temperature achieved during the discharge process reported up to date, and the composition of the new P‴3-phase corresponds approximately to Na0.50NiO2.