Investigation of P2-Na2/3Mn1/3Fe1/3Co1/3O2 for Na-Ion Battery Positive Electrodes

The preparation and electrochemical properties of a new P2-Na2/3Mn1/3Fe1/3Co1/3O2 phase is reported. This material has a high reversible capacity of 173 mAh/g and an average voltage of 3.00 V, resulting in an estimated 520 mWh/g (2240 Wh/L) when discharged from 4.5 V. Using ex-situ Mossbauer spectroscopy, it is found that all Fe is 3+ in the starting material. Fe3+ -> Fe4+ proceeds linearly during charge between OCV and 4.1 V and continues at high voltages, and suggests simultaneous mixed metal oxidation during removal of Na. At 4.1 V (91 mAh/g first charge) an estimated 40% of the iron present is 4+. Excellent cycling characteristics with very low voltage hysteresis can be obtained using this cutoff voltage. Access of a high voltage plateau at voltages above 4.1 V results in decreased capacity retention. (C) 2014 The Electrochemical Society. All rights reserved.