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To improve the cycle performance of spinel LiMn,O4as the cathode of 4 V class lithium secondary batteries, the quaternary spinel phases LiM5Mn,,04 (M = Co, Cr, Ni; y 1/12, 1/9, 1/6, 1/3) were prepared at 750°C in air. X-ray diffraction and electrochemical studies were carried out using the LilLiM,Mn29O4 cells. These cathodes were more tolerant to repeated lithium extraction and insertion than a standard LiIVIn,04 spinel electrode in spite of a small reduction in the initial capacity. Considering both cycle life and capacity density, LiCo,,0Mn,,,804 showed good cycle performance with an energy density of 370 Wh kg' at the 300th cycle. The improvement in cycling performance is attributed to the stabilization in the spinel structure by the doped metal cations.

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Hereunder presented is an AC impedance spectroscopic study of the lithium-inserted material Li x La 1/3 NbO 3 . Two semicircles were noted in accordance with the plots given as the complex impedance in the frequency region of 0.5 to 5000 Hz at room temperature, and these two semicircles made variation accompanied with the composition x. The dependence of the impedance spectra on the lithium salt concentration and temperature was examined, and it is explained that these two semicircles were in relation to the Butler-Volmer type kinetics. The evolution of impedance spectra for various conditions reveals that the electrochemical insertion process is described in terms of an adatom model; the resistance observed at a higher frequency region includes the adsorption and desolvation process accompanied with electron-transfer reaction. Furthermore, assignment as lattice incorporation, or insertion process at the electrode surface, is made with the other one observed in the lower frequency region.

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