Remarkable Charge-Discharge Mechanism for a Large Capacity in Fe-Containing Li2MnO3 Cathodes

Abstract

Fe-containing lithium-rich layered manganese oxides are remarkable cathode materials, which can fabricate the high energy density and low-cost lithium-ion batteries. Here, we investigated thoroughly the charge-discharge mechanism for the large capacity in 0.7Li2MnO3-0.3LiFeO2. X-ray absorption spectroscopy (XAS) and soft-XAS measurements using synchrotron radiation revealed that the large capacity in 0.7Li2MnO3-0.3LiFeO2 was achieved mainly by the collaborative participation of Mn and oxygen ions in charge compensation. Mn ions changed to the valence states of +2 and/or +3 for discharged states near the surface after oxygen and Li had been removed simultaneously at 4.4 V. Charge compensation for charged states was also accomplished by the oxidizing oxide ions such as those in the formation of O22−. Redox reactions in Fe ions mainly occurred with initial charge-discharge. The crystal structure has been changed partially in these processes from layered rock-salt to spinel-like structures for the Li deintercalation, which was confirmed by selected area electron diffraction (SAED). We also found that the change of electronic states with valence modifications and drastic structural changes happened preferentially on the surface of the cathode electrode.