Characterization of Phospho-olivines as Materials for Li-ion Cell Cathodes

Solid State Reaction was employed to prepare phospho-olivines LiMPO(4) (where M = Fe, Co) as pure phase and LiNiPO(4) in presence of foreign phases, as cathodic materials for lithium-ions batteries. The relationship between structural, morphological and electrochemical properties were investigated in the case of LiFePO(4). Structural investigation has been carried out by means of X-ray powder diffraction (XRPD) and Rietveld refinement. The influence on the morphology of annealing temperature, different flowing gas mixture and addition of ascorbic acid during the synthesis, has been analysed via scanning electron microscopy. The electrochemical cycling performances on LiFePO(4) showed to be positively affected by the modifications of the experimental conditions. Cyclic voltammetry showed a good reversibility during insertion-extraction mechanism, in particular in presence of additives. LiCoPO(4) and LiNiPO(4) are interesting as high voltage cathode materials for Li-ion batteries and have been taken into account, but their electrochemical operating conditions are still to be optimised. In the case of LiNiPO(4) it is very difficult to obtain, by solid state synthesis, suitable purity powders, having a grain size small enough to exploit it usefully as cathodic material for Li-ion cells.