In Situ X-Ray Diffraction Study of P 2 ­ Na2 / 3 [ Ni1 / 3Mn2 / 3 ] O 2

The electrochemical extraction and insertion of Na in P2-Na 2/3 ][Ni 1/3 Mn 2/3 ]O 2 was studied by in situ X-ray diffraction. All the original Na can be extracted from P2-Na 2/3 ][Ni 1/3 Mn 2/3 ]O 2 and it can he reversibly inserted again. When x > 1/3 in Na x [Ni 1/3 Mn 2/3 ]O 2 . the compound remains in the P2 structure. For x 1/3, a small amount of 02-type stacking faults are introduced into the structure. When x < 1/3, the electrode exists as two coexisting phases. These are P2-Na 1/3 [Ni 1/3 Mn 2/3 ]O 2 (with some 02-type stacking faults) and [Ni 1/3 Mn 2/3 ]O 2 . [Ni 1/3 Mn 2/3 ]O 2 adopts the 02 structure with stacking faults. As sodium is reinserted in [Ni 1/3 Mn 2/3 ]O 2 to make Na 1/3 [Ni 1/3 Mn 2/3 ]O 2 the P2 structure forms again. The evolution of the lattice parameters of P2-Na x [Ni 1/3 Mn 2/3 ]O 2 (1/3 < x < 2/3) with x and the lattice constants of [Ni 1/3 Mn 2/3 ]O 2 are also reported.