High-capacity Li_1+x(Ni_0.3Mn_0.7)_1-xO₂, (0 < x < 1/3) samples were synthesized by the coprecipitation–calcination method. Both electrochemical cycle and high-rate performances were drastically improved by selecting an N₂ atmosphere as final calcination. Scanning transmission electron microscopy—energy dispersive X-ray spectroscopy analysis showed that the sample calcined in an N₂ atmosphere had a more homogeneous transition metal distribution into primary particles than that calcined in air. The solid-state ⁷Li nuclear magnetic resonance data showed that electrochemically inactive domains were only diminished for the sample calcined in an N₂ atmosphere after electrochemical activation. X-ray Rietveld analysis revealed that the suitable transition metal distribution and content of the samples were different from those of typical layered rock-salt materials. Only that calcined in an N₂ atmosphere had no spinel formation during charging and no oxide ion insertion reaction during discharging. No positive Co substitution effect was observed under the optimized preparation conditions. At the 100th cycle, the discharge capacity was 216 mAh g⁻¹, which corresponds to 87% of the initial capacity (251 mAh g⁻¹) at optimizing synthetic condition.