Study of C-coated LiFe0.33Mn0.67PO4 as positive electrode material for Li-ion batteries

Commercial C-LiFe0.33Mn0.67PO4 positive electrode material has been investigated by 57Fe Mössbauer Spectroscopy (MS), X-ray Photoelectron Spectroscopy (XPS) and X-ray Absorption Spectroscopy (XAS). The combined use of these experimental techniques provides a better understanding of the electrochemical reaction involved during cycling. 57Fe MS is very efficient to directly follow oxidation state of Fe in the electrode, and gives surprisingly indirect information on the oxidation state of Mn as observed by XAS and XPS. The electrochemical mechanism is proposed based from in situ and operando investigations using both MS and XAS, and is consistent with XPS surface studies. XPS analysis of the electrodes at the end of charge (4.4 V) reveals enhanced electrode/electrolyte interface reactivity at this high potential. Aging of C-LiFe0.33Mn0.67PO4/Li cells after 50 cycles at 60 °C indicates a rather good electrochemical behavior (low capacity fading) of the electrode material. Both 57Fe MS and XPS (Mn 2p and Fe 2p) clearly show no modification on Fe and Mn oxidation state compared to fresh electrode confirming the good electrochemical performances.