Structural and transport properties of V6O13 insertion electrodes

Polycrystalline V6O13 with an apparent oxygen excess stoichiometry was synthesised by controlled thermal decomposition of ammonium metavanadate and chemically lithiated with n-butyl lithium. X-ray diffraction was used to characterise the prepared powders and lattice parameters were determined as a function of lithium content. Examination of the unlithiated host material by high resolution electron microscopy revealed a complicated array of shear and twin defects. Single crystals of V6O13 were grown by chemical vapour transport and lithiated by coulometric titration in solid state electrochemical cells. From the shape of the coulometric titration curves a single phase region was identified for LixV6O13 between x = 0 and x 0.17 at 120 deg. C. Chemical diffusion coefficients were estimated over this composition range in two perpendicular crystallographic directions. The figures obtained were independent of composition with mean values of 3.5 x 10-8 and 4.9 x 10-9 cm2 s-1 at 120 deg. C perpendicular to the (010) and the (001) crystal planes respectively. The variation in cell voltage between 80 and 130 deg. C was investigated and found to show two linear regions with a change in slope occurring between 110 and 100 deg. C. Values of the partial molar free energy of lithium were calculated at 120 deg. C. The thermodynamics of the Li-V6O13 system between x = 0 and x = 7.7 were examined by measuring the emf of cells, constructed with chemically lithiated polycrystalline cathodes, between 65 and 135 deg. C. High partial molar enthalpies (from -270 to -215 kJ mol-1) and low partial molar entropies (around -25 kJ mol-1 K-1) were determined. All cells were constructed using lithium trifluoromethane sulphonate poly(ethylene oxide) electrolyte which was first characterised by ac impedance spectroscopy. Lithium ion transport numbers at the compositions O/Li = 4, 4.5 and 8 were found to be only about 0.45. (author)