Solid‐State Redox Reactions of LiNi1 / 2Co1 / 2 O 2 ( R 3̄m ) for 4 Volt Secondary Lithium Cells

( in hexagonal setting) whose parent structure was a solid solution of and was prepared and examined in a nonaqueous lithium cell. The oxidation of and reduction of proceeded reversibly in the voltage range above 3.5 V. X‐ray diffractional examinations indicated the reaction proceeded in a topotactic manner. During the oxidation of , the dimension elongated from 14.1 to 14.5 Å and the shortened from 2.84 to 2.80 Å almost linearly as a function of until reached about 0.5. Consequently, the lattice volume remained almost invariable at 98.5 Å 3 . Further oxidation resulted in the shrinkage of the dimension and the elongation of the . Detained open‐circuit voltage measurements also were carried out. The open‐circuit voltages varied as a function of following a hyperbolic tangent curve, not an S‐shaped Nernstian curve. The composition‐dependent open‐circuit voltage curve together with the XRD observations indicated that the reaction proceeded in a homogeneous phase. Differences and similarities of the solid‐state redox reactions among , , and were discussed.