Investigation of Mn and Fe Substitution Effects on the Characteristics of High-Voltage LiCo1- xMxPO4 (x = 0.1, 0.4) Cathodes Prepared by Sol-gel Route

Herein, we provide a fundamental study revealing the substantial changes promoted by manganese and iron substitution for cobalt in a high-voltage LiCoPO4 olivine cathode. Therefore, LiCoPO4, LiCo0.9Fe0.1PO4, LiCo0.6Fe0.4PO4, LiCo0.9Mn0.1PO4, and LiCo0.6Mn0.4PO4 are synthesized by a sol-gel pathway and comparatively investigated in terms of structure, morphology, and electrochemical features in lithium battery. Besides the observed effects on structure, particle size, and metals distribution, the work reveals a gradually enhancing electrode reaction by increasing the Fe content in LiCo0.9Fe0.1PO4 and LiCo0.6Fe0.4PO4, with Co3+/Co2+ and Fe3+/Fe2+ signatures at 4.8 and 3.5 V vs Li+/Li, respectively. On the other hand, the introduction of Mn leads to a progressive electrode deactivation in LiCo0.9Mn0.1PO4 and LiCo0.6Mn0.4PO4 due to an intrinsic hindering of the Mn3+/Mn2+ process at 4.1 V vs Li+/Li. The reasons accounting for such an intriguing behavior are investigated in detail using electrochemical impedance spectroscopy within the potential range of the redox processes. The study reveals that manganese and iron substitutions in the high-voltage olivine have opposite effects on the charge transfer resistance, i.e., detrimental for the former while beneficial for the latter, with remarkable enhancement of the reversible capacity, the Coulombic efficiency, and the cycle life. Such results provide to the scientific community useful information on possible strategies to enhance the emerging LiCoPO4 high-voltage electrode by transition metal substitution.