P2-Na_2/3[Ni_xMn_y]O₂-based cathode materials are promising candidates for Na-ion batteries. The cycling performance of these cathodes was usually reported to drop at working voltages over 4.2 V, due to the P2/O2 irreversible phase transition. Here we demonstrate a colloidally assisted two-step synthesis to phase pure P2-Na_0.43Ni_0.25Mn_0.75O_1.9. The adopted synthetic route and the chosen Na:Ni:Mn ratio (0.43:0.25:0.75) leads to a cathode material that can withstand long charge/discharge cycles at working voltages up to 4.4 V. The XRD pattern recorded on the de-sodiated electrode (4.4 V) does not evidence the presence of any peak related to the O2 phase, corroborating the effective suppression of the P2/O2 phase transition upon Na⁺ intercalation/deintercalation. The discharge capacity delivered at relatively low current (C/10) is 109 mAh g^-1, with a capacity retention exceeding the 93% in 20 cycles. At the high current of 1C the delivered discharge capacity is 94 mAh g^-1.