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Lithium nickel manganese oxides, LiNi15Mn5O2÷5, (0 y 0.5) were prepared via a new solution technique. The corresponding mixed nickel manganese hydroxide precursors were synthesized in an oxidative coprecipitation method. Subsequent calcination in the presence of LiOH leads to crystalline products with a partially disordered layered-type cs-NaFeO2 structure. X-ray photoelectron spectroscopic analysis has indicated a strong enrichment of lithium at the surface. The electrochemical performance of these materials as positive electrodes in lithium-ion batteries was evaluated as a function of the calcination temperature and manganese content. A calcination temperature of 700°C leads to the best cycling stability. At this temperature, a sufficiently high degree of crystallinity was achieved, having a strong influence on the cycling stability of these "4 V" materials. The specific charge and cycling stability obtained for the solution-prepared pure lithium nickel oxide, LiNiO2, was low, but was significantly enhanced by replacing some nickel with manganese. With increasing manganese content, the specific charge increased to about 170 mAh g' for materials with a Ni:Mn ratio of about 1:1. Ex situ magnetic susceptibility measurements proved that during lithium deinsertion, the trivalent manganese is preferentially oxidized, and seems to be the more reactive redox center in these oxides. InfroductionCurrently interest is focused on the use of lithium transition metal oxides as positive electrode materials in rechargeable high energy density lithium-ion transfer battery systems.'-3 Elemental lithium has been substituted by some forms of carbon as the negative electrode material for safety reasons. Oxides containing highly mobile lithium function as the lithium source in these types of cells. Attention has focused on lithium metal oxides of the form LiMeO2 (Me = Co, Ni), which have the layered cz-NaFeO2 structure type. This structure is shown in Fig. 1 for LiNiO2. In a distorted cubic-closed-packed oxygen array, the lithium and the transition metal atoms are distributed in the octahedral interstitial sites in such a way that Me02 layers are formed. These layers consist of edge sharing [Me06] octahedra of trigonal symmetry. Between these layers, lithium resides in octahedral coordination sites [Li06], leading to alternating lithium and nickel layers along the [111] direction. In a crystal lattice of the space group R3m, oxy-