M.N. Danchevskaya, Yu.D. Ivakin, G.P. Panasyuk, I.L. Voroshilov, G.P. Budova et al.
3-rd International Conference on Solvothermal Reactions ICSTR - 3, Bordeaux, July 19-23, 1999, p. 110.
ABSTRACT. In the recent paper spinel LiMn2O4 and the phase Li2MnO3 were prepared by hydrothermal treatment of MnO2 with solutions of LiOH at 170 C under autogenic pressure after 24 days. In the present work were studied the products of solid state reaction of powdered mixture of gamma-MnO2 or beta-MnO2 and LiOH or Li2CO3 with a molar ratio Li:Mn (0.85-1.60):2 during thermovaporous treatment at 400 C, P H2O = 20 MPa, 1-2 days. The following methods were used: lazer dispersion analysis, X-ray phase analysis, microscopy, thermogravimetry, NMR, EPR and IR spectroscopy. At a ratio Li:Mn ( 1:2 spinel LiMn2O4, sp. gr. Fd3m (ASTM, inorg. 18-736) was a single crystal phase with the size of crystal grains 1-70 mkm. At change of this ratio in the charge from 1:2 up to 1.6:2 the lattice constant a(c) grew from 0.817 till 0.820 nm. At a ratio Li:Mn < 1:2 in the charge a prevailling phase in a product was spinel with a(c) =0.814-0.815 nm, and besides the impurity of beta-MnO2 and beta-Mn2O3 were present. On the heating curve up to 1400 C the endothermic peaks at 130 and 530 C were present, peculiar to hydrothermal spinels, and also additional endothermic peak at 230 C was present. Besides, endothermic peaks at 990 and 1100 C were present, peculiar to spinels, obtained by solid-state high temperature reaction. In NMR (7)Li spectra of spinels two lines with chemical shift 500 - 1400 ppm and 100 -350 ppm are observed. Two signals also there are in EPR spectra: a symmetric one without thin structure and nonsymmetric one with thin structure. The conclusion was made, that the parameters of the spectrum are determined by size and sign of pseudo-contact and dipol-dipol interaction (7)Li: - e- and that they depend on the magnitude of electronic conductivity. The spinel, obtained in the conditions of monophase synthesis, have been used for manufacturing a cathodes in scale models of the rechargeable lithium cells Li/1M LiClO4-propilene carbonate ( dimethoxyethan (7:3)/LixMn2O4. The cells discharged in the potential range 2.8-3.5V. The specific capacity was 100-140 mAh/g. The charge-discharge curves are high stable during the cycling, and the specific characteristics of cathode have a slight dependence on the load in sufficiently wide interval of current densities. This research was made with financial support of RFBR pr. 98-03-32645a.