Layered NaNixMn1-xO2 (0 <= x <= 1) oxides were prepared via solid state reactions. Different reaction conditions were required to obtain phase pure samples, depending on the value of x. The 0 <= x <= 0.1 compositions were prepared in an inert argon atmosphere at 700 degrees C and had a monoclinically distorted O'3 type structure. The 0.25 <= x <= 0.33 compositions were prepared in air at 850 degrees C and had a P2-type structure. Compositions in the range of 0.5 <= x <= 0.66 were synthesized in air at 850 degrees C and had an O3-type structure. Lastly, compositions with 0.9 <= x <= 1 were prepared in an oxygen atmosphere at 700 degrees C and had a monoclinically distorted O'3 type structure. Electrochemical experiments were performed on pure phase samples. All showed reversibility of sodium ions and high capacities. The highest reversible capacity was achieved for x = 0.66, with a capacity of similar to 190 mAh/g and an average discharge voltage of 3.07 V, corresponding to a high energy density of 2705 Wh/L. This is among the highest reported volumetric energy densities for Na-ion battery electrodes. (C) 2015 The Electrochemical Society. All rights reserved.