Comparative study of LiCoO2, LiNi Co O2 and LiNiO2 for 4 volt secondary lithium cells

The preparation and characterization of LiNi1−xCoxO2 compounds (0 ⩽ x ⩽ 1) having a space group R3m, for 4 volt secondary lithium cells were investigated. By developing processing methods, homogeneous LiNi1−xCoxO2 samples were obtained and characterized by XRD, ir and magnetic susceptibility measurements. When increasing x in LiNi1−xCoxO2, the unit cell dimensions a and c in hexagonal setting, decreased almost linearly as a function of x. Magnetic susceptibility measurements indicated that LiNi1−xCoxO2 consists of low-spin states of Co3+ (t62g e0g) and Ni3+ (t62ge1g). All samples may be used as positive electrodes in nonaqueous lithium cells. Of these, LiCoO2 showed the highest working voltage and about 120 mAh g−1 of rechargeable capacity, and LiNi12Co12O2 showed the lowest working voltage and about 130 mAh g−1 of rechargeable capacity in the voltage range 2.5–4.2 V in 1 M LiClO4 propylene carbonate solution. LiNiO2 has more than 150 mAh g−1 of rechargeable capacity with working voltages above 3.5 V. Secondary lithium ion cells which consisted of LiNi1−xCoxO2 cathodes and natural graphite anodes, were also examined and the specific problems of establishing an innovative secondary lithium cell were discussed.