Observation on Ferric Pyrophosphate and New Inquiry on Mechanisms of Capacity Degradation in Lithium Iron Phosphate Cathode Materials

The hierarchical carbon-coated lithium iron phosphate (LiFePO4) nano-grain microspheres are prepared by chemical co-precipitation method. The microstructure and cycling performance are compared for samples treated at 975 K and 1025 K. Transmission electron micrograph (TEM) analysis confirms the existence of ferric pyrophosphate phase of the general formula Fe4(P2O7)3. Reasons for the formation of the crystalline Fe4(P2O7)3 phase are intensively discussed. It is considered that a little amount of Fe4(P2O7)3 phase has no distinct detrimental effect on the cycling capacity of samples. Although there is a big decrease from 132 to 74.5 mAh/g in discharge capacity at a current rate of 1C for the sample treated at 975 K from 400 to 500 cycles. The capacities for the sample were observed to be recovered to its theoretical value in discharge at a current rate of 0.1C after 500 cycles. It is demonstrated that the degradation behavior for the sample treated at 975 K is attributed to the increase of diffusion resistance of the lithium ions into the bulk of LiFePO4 electrode materials, which is not reported before in the literatures.