Spinel LiMn2O4 and LiMn1.95Ni0.05O4 powders with sub-micron, narrow-size-distribution, and phase-pure particles are synthesized by a sol-gel method. The effects of heat treatment on the physicochemical properties of the spinel LiMn2O4 powder are examined with X-ray diffractometry, the Brunauer-Emmett-Teller method and scanning electron microscopy. For lithium/polymer battery applications, the LiMn2O4 and LiMn1.95Ni0.05O4 electrodes are characterized electrochemically by charge-discharge experiments and a.c.-impedance spectroscopy. Although the Ni-doped electrode has a smaller initial capacity of 126 mA h g(-1), it exhibits better cycling performance than the conventional electrode which delivers a higher initial capacity of 145 mA h g(-1). The improvement in cycling performance of the former electrode is attributed to stabilization of the spinel structure by the presence of nickel ion. The cycling performance of a Li/polymer electrolyte/LiMn1.95Ni0.05O4 cell at various temperatures is discussed in terms of interfacial resistance and lithium-ion diffusion determined by a.c.-impedance spectroscopy.