The electrochemical extraction and insertion of Na in P2-Na-2/3[Ni1/3Mn2/3]
O-2 was studied by in situ X-ray diffraction. All the original Na can be ex
tracted from P2-Na-2/3[Ni1/3Mn2/3]O-2 and it can be reversibly inserted aga
in. When x > 1/3 in Na-x[Ni1/3Mn2/3]O-2, the compound remains in the P2 str
ucture. For x approximate to 1/3, a small amount of O-2-type stacking fault
s are introduced into the structure. When x < 1/3, the electrode exists as
two coexisting phases. These are P2-Na-1/3[Ni1/3Mn2/3]O-2 (with some O-2-ty
pe stacking faults) and [Ni1/3Mn2/ 3]O-2. [Ni1/3Mn2/3]O-2 adopts the O-2 st
ructure with stacking faults. As sodium is reinserted in [Ni1/3Mn2/3]O-2 to
make Na-1/3[Ni1/3Mn2/3]O-2 the P2 structure forms again. The evolution of
the lattice parameters of P2-Na-x[Ni1/3Mn2/3]O-2 (1/3 < x < 2/3) with x and
the lattice constants of [Ni1/3Mn2/3]O-2 are also reported. (C) 2001 The E
lectrochemical Society.