The synthesis and characterization of LiNiO2 for a 4 V secondary lithium cell was done. The LiNiO2 was prepared by ten different methods and characterized by x-ray diffraction and electrochemical methods. LiNiO8 prepared from LiNO3 and NiCO3 [or Ni(OH)2] exhibited more than 150 mAh 9 g-1 of rechargeable capacity in the voltage range between 2.5 and 4.2 V in 1M LiC104 propylene carbonate solution. The reaction mechanism was also examined and explained in terms of to~otactic reaction. Lithium nickelate(III) (R3m; a = 2.88 A, c = 14.18 A in hexagonal setting) was oxidized to nickel dioxide (R3m; a = 2.81/~, c = 13.47 A) via Lil_=NiO2 (0.25(2) _-< x _-< 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 LiNiO2 are given and the possibility of using LiNiO2 for 4 V secondary lithium cells is described.

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Lithium nickel manganese oxides Li[Ni x Li (1/3À2x/3) Mn (2/3Àx/3) ]O 2 (x ¼ 1/2, 2/7, and 1/5) are prepared and characterized by XRD and FT-IR, and the samples are examined in non-aqueous lithium cells at room temperature and 55 C. Among these materials LiNi 1/2 Mn 1/2 O 2 (x ¼ 1/2) shows the highest operating voltage and the smallest polarization with a rechargeable capacity of ca. 230 mA h g À1 and Li[Li 1/5 Ni 1/5 Mn 3/5 ]O 2 (x ¼ 1/5) shows the lowest operating voltage and the largest polarization with a rechargeable capacity more than 300 mA h g À1 . Extraordinarily large rechargeable capacity of Li[Li 1/5 Ni 1/5 Mn 3/5 ]O 2 together with an anomalously long voltage plateau at 4.5 V only observed at first charging process is examined by window-opening charge and discharge, continuous charge and discharge combined with differential chronopotentiometry at room temperature and at 55 C, and possible mechanisms are discussed in terms of lithium insertion scheme.

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