Zhenpeng Yao1 Chun Zhan2 Jun Lu2 Liang Li2 Eungje Lee2 Maria Chan2 Christopher Johnson2 Michael Thackeray2 Khalil Amine2 Christopher Wolverton1

1, Northwestern University, Evanston, Illinois, United States
2, Argonne National Laboratory, Lemont, Illinois, United States

Lithium ion batteries (LIBs) have been the most prominent electrochemical energy storage technology over the past decades and enabled the wireless evolution of portable electronic devices. Conventional cathode materials used are typically lithium-containing transition metal oxides and phosphides (e.g., LiCoO2, LiFePO4, LiMn2O4) which store (release) electrical energy via (de-)insertion of Li+ ions, accompanied by redox reactions of the transition metal cation. The specific capacity of the cathode is therefore limited by the number of electrons per transition metal cation that can participate in the redox reaction. This exclusive dependence on the transition metal cations as the redox center has been challenged by the recent discovery of oxygen redox reactivity in Li-excess cathode materials. The opportunity has thus arisen to boost the capacity and energy density of LIBs if the anionic and cationic redox activity can be enabled at the same potential. However, it is challenging to develop anionic-redox-based cathodes with acceptable cycle performance. Here in this study, we report the realization of simultaneous anionic and cationic redox in the super lithium-rich Li5FeO4 based cathode materials.1 Highly reversible anionic redox reactivity with no obvious oxygen release was realized in this low-cost and earth-abundant iron oxide under a controlled voltage range. We then present a clear and quantitative picture of the structural, composition, and oxidation state evolution by first-principles density functional theory (DFT) calculations and various experimental measurements. We identify one special anionic redox active Li-excess configuration “Li6-O” by exclusively examining the local atomistic environments of O ions involved in the anionic redox. The O2-/O1- ions in the “Li6-O” configurations can give/uptake one extra electron on further oxidation/reduction enabling the reversible O2-↔ O1- during the charging/discharging. Our findings provide insights into the electrochemical oxygen redox reactivity and shed light on the design of low-cost transition metal oxide-based high-energy-density cathode materials.
(1) Zhan, C.; Yao, Z.; Lu, J.; Ma, L.; Maroni, V. A.; Li, L.; Lee, E.; Alp, E. E.; Wu, T.; Wen, J.; Ren, Y.; Johnson, C.; Thackeray, M. M.; Chan, M. K. Y.; Wolverton, C.; Amine, K. Nat. Energy 2017, 2 (12), 963.