In this study, a new class of contenders for high-voltage and high-capacity positive electrode materials with the composition Na x Li0.7−x Ni1−y MnyO2 (0.03 < x ≤ 0.25, 0.5 ≤ y ≤ 0.8) was synthesized by thermal treatment; these positive electrode materials synthesized by Na+/Li+ exchange using P3Na0.7Ni1−y Mn y O2 (0.5 ≤ y ≤ 0.8) as the precursor exhibited a mixture of layered and spinel structures. The (dis)charge voltage-capacity curve of materials with these compositions significantly varied according to the residual Na and Mn content. Notably, HT-Na x Li0.7−x Ni1−y Mn y O2 (x = 0.093, y = 0.67) exhibited a maximum discharge capacity of 261 mA h g−1 at an average voltage of 3.36 V at 25 °C (between 2.0 and 4.8 V), which translates to an energy density of 943 W h kg−1. The obtained electrochemical performance is rationalized by the phase fractions of layered and spinel structures, which is triggered by the residual Na and Mn content in Na x Li0.7−x Ni1−y Mn y O2.