Understanding the Shifts in the Redox Potentials of Olivine LiM1−y M y PO4 (M = Fe, Mn, Co, and Mg) Solid Solution Cathodes

A facile, high-energy mechanical milling (HEMM) approach has been developed to synthesize carbon-coated olivine LiM1−y M y PO4 (M = Fe, Mn, Co, and Mg) solid solution nanoparticles. A systematic structural and electrochemical characterization of the solid solution series has been carried out by X-ray diffraction (XRD), scanning electron microscopy (SEM), charge−discharge measurements, and galvanostatic intermittent titration technique (GITT). The discharge capacity, voltage profile, and cycle performance of the LiM1−y M y PO4 solid solution cathodes are found to be dependent on the different redox couples involved in the reaction. Equilibrium potentials obtained from GITT and dQ/dV plot reveal a systematic shift in the redox potential of Fe2+/3+, Mn2+/3+, and Co2+/3+ couples in the LiM1−y M y PO4 solid solution compared to their pristine end members (LiMPO4). The shifts in the redox potential are explained on the basis of the changes in the covalency of the M−O bond and M−O−M interaction, and the consequent change in the position of the M2+/3+ redox energy. The self-discharge phenomenon of the Co2+/3+ couple in LiM1−y Co y PO4 has also been investigated by electrochemical impedance spectroscopy.