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

• Published in: Journal of solid state chemistry 2012-08, Vol.192, p.201-209
• Main Authors: Perea, A., Castro, L., Aldon, L., Stievano, L., Dedryvère, R., Gonbeau, D., Tran, N., Nuspl, G., Bréger, J., Tessier, C.
• Format: Article
• Language: English
• Subjects: 57Fe Mössbauer; Applied sciences; Chemical Sciences; Chemistry; Direct energy conversion and energy accumulation; Electrical engineering. Electrical power engineering; Electrical power engineering; Electrochemical conversion: primary and secondary batteries, fuel cells; Electrochemistry; Electrodes: preparations and properties; Exact sciences and technology; General and physical chemistry; Lithium-ion batteries; Manganese; Material chemistry; Olivine; X-ray absorption spectroscopy; XPS
• ISSN: 0022-4596; 1095-726X
• DOI: 10.1016/j.jssc.2012.04.016

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 (4.4V) reveals enhanced electrode/electrolyte interface reactivity at this high potential. Aging of C-LiFe0.33Mn0.67PO4/Li cells after 50cycles 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. Quantitative evaluation of the Fe3+ and Mn3+ content during the first charge/discharge cycle obtained from K-edge XANES spectra of C-LiFe0.33Mn0.67PO4 recorded upon cell operation at RT with C/10 rate. During the charge co-existence of Fe and Mn oxidation is observed between points 2 and 4 of the potential curve. At the end of the charge the cut-off voltage limits the oxidation at about 93%. [Display omitted] ► C-LiFe0.33Mn0.67PO4 electrode material upon cycling vs. metallic lithium. ► 57Fe Mössbauer spectroscopy is a (in)direct probe for Fe(Mn) oxidation state. ► Both K–Fe and K–Mn edges XAS show a simultaneous oxidation of Fe2+ and Mn2+ in a small range of compositions. ► Surface analysis from XPS allows reveals slight differences at the surface of the electrode with respect to the bulk.