Masatoshi Nagayama

Electrochemistry and Structural Chemistry of LiNiO2 (R3̅m) for 4 Volt Secondary Lithium Cells

The synthesis and characterization of LiNiO[sub 2] for a 4 V secondary lithium cell was done. The LiNiO[sub 2] was prepared by ten different methods and characterized by X-ray diffraction and electrochemical methods. LiNiO[sub 2] prepared from LiNO[sub 3] and NiCO[sub 3] [or Ni(OH)[sub 2]] exhibited more than 150 mAh/g of rechargeable capacity in the voltage range between 2.5 and 4.2 V in 1M LiClO[sub 4] propylene carbonate solution. The reaction mechanism was also examined and explained in terms of topotactic reaction. Lithium nickelate (III) (R3m; a[equals]2.88 [angstrom], c[equals]14.18 [angstrom] in hexagonal setting) was oxidized to nickel dioxide (R3m; a[equals]2.81 [angstrom], c[equals]13.47 [angstrom]) via Li[sub 1[minus]x]NiO[sub 2] (0.25(2) [le]x[le]0.55(2)) having a monoclinic lattice (C2/m). The nickel dioxide could be reversibly reduced to lithium nickelate(III). Factors affecting the electrochemical reactivity of LiNiO[sub 2] are given and the possibility of using LiNiO[sub 2] for 4 V secondary lithium cells is described.

Comparative study of LiCoO2, LiNi12Co12O2 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.