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

LiNi[sub 1/2]Co[sub 1/2]O[sub 2] (R[bar 3]m; a = 2.84 [angstrom], c = 14.1 [angstrom] in hexagonal setting) whose parent structure was a solid solution of LiNiO[sub 2] and LiCoO[sub 2] was prepared and examined in a nonaqueous lithium cell. The oxidation of LiNi[sub 1/2]Co[sub 1/2]O[sub 2] and reduction of [open square]Ni[sub 1/2]Co[sub 1/2]O[sub 2] 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 LiNi[sub 1/2]Co[sub 1/2]O[sub 2], the c-axis dimension elongated from 14.1 to 14.5 [angstrom] and the a-axis shortened from 2.84 to 2.80 [angstrom] almost linearly as a function of x in Li[sub 1[minus]x]Ni[sub 1/2]Co[sub 1/2]O[sub 2] until x reached about 0.5. Consequently, the lattice volume remained almost invariable at 98.5 A[sup 3]. Further oxidation resulted in the shrinkage of the c-axis dimension and the elongation of the a-axis. Detained open-circuit voltage measurements also were carried out. The open-circuit voltages varied as a function of x in Li[sub 1[minus]x]Ni[sub 1/2]Co[sub 1/2]O[sub 2] 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 similaritiesmore » of the solid-state redox reactions among LiNiO[sub 2], LiNi[sub 1/2]Co[sub 1/2]O[sub 2], and LiCoO[sub 2] were discussed.« less