SOLID-STATE REDOX REACTIONS OF LINI1/2CO1/2O2 (R(3)-OVER-BAR-M) FOR 4-VOLT SECONDARY LITHIUM CELLS

LiNi1/2Co1/2O2 (R3mBAR; a = 2.84 angstrom, c = 14.1 angstrom in hexagonal setting) whose parent structure was a solid solution of LiNiO2 and LiCoO2 was prepared and examined in a nonaqueous lithium cell. The oxidation of LiNi1/2Co1/2O2 and reduction of squareNi1/2Co1/2O2 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 LiNi1/2Co1/2O2, 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 Li1-xNi1/2Co1/2O2 until x reached about 0.5. Consequently, the lattice volume remained almost invariable at 98.5 angstrom3. 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 Li1-xNi1/2Co1/2O2 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 LiNiO2, LiNi1/2Co1/2O2, and LiCoO2 were discussed.