Solid solution samples of P2-type Na2/3Ni1/3-xMn2/3CuxO2 (0 <= x <= 1/9) are successfully synthesized and their electrochemical performance are examined in non-aqueous Na cells. Non-substituted Na2/3Ni1/3Mn2/3O2 delivers the highest reversible capacity compared to those of the solid solution samples; however the capacity rapidly decays during cycles. Partial substitution of Cu for Ni in Na2/3Ni1/3Mn2/3O2 effectively improves its cyclability and rate capability. In particular, a Na//Na2/3Ni1/4Mn2/3Cu1/12O2 cell delivers reversible capacity of 138 mAh g(-1) and the average discharge voltage reaches 3.54 V on initial discharge with good cycle stability. Operando XRD reveals that the original P2 phase transforms to P2-O2 intergrowth phase having stacking faults during sodium extraction, as is supported by XRD data and simulation with DIFFaX software. Although non-substituted Na2/3Ni1/3Mn2/3O2 shows P2-O2 phase transition as a two-phasic reaction, P2-type Cu-substituted material does not transform to O2-type one and P2-type layers with wider interlayer distance than that of the O2-type Ni1/3Mn2/3O2 are randomly and partly retained in the desodiated phase because Cu is distributed in the initial phase at random and Na in interslab space surrounding the electrochemically inactive Cu is not extracted from the structure. The suppressed volume change during charge/discharge results in the excellent electrode performance compared to non-substituted Na2/3Ni1/3Mn2/3O2. (C) 2017 The Electrochemical Society. All rights reserved.