Improvement of electrochemical activity of LiMnPO4-based cathode by surface iron enrichment

Abstract

Abstract LiMnPO 4 has attracted massive interests due to its appropriate redox potential and the success of its iron comparative in the lithium ion batteries. The bulk substitution has been widely used to address the poor electrochemical activity of LiMnPO 4 , which is much lower than that of LiFePO 4 . In this work, we compare the performance of the core-shell structure and the homogeneous substitution with the same Mn/Fe molar ratio of LiMn 0.8 Fe 0.2 PO 4 . The core-shell phosphate material after carbon coating is composed of a core part of quasi-single LiMnPO 4 phase, and a 3–4 nm shell layer of quasi-single LiFePO 4 -phase, separated by the phase boundary with 1–2 nm thickness. It is interesting to reveal that the core-shell samples exhibit capacities of 156.4, 144.5, 128.2 mAh g −1 under 0.1C, 1C and 5C respectively, which are 5–10% higher than that of the homogenous substituted LiMn 0.8 Fe 0.2 PO 4 at the corresponding rates, while both of these samples present excellent cyclic stability, still retaining ∼95% of the initial capacities after 1000 cycles under 1C discharging rate. Our results demonstrate that the main reason for LiMnPO 4 's poor electrochemical activity should be emphasized on the surface polarization, whereas the tardiness on bulk transportation is not as serious as it was presumed.