Disordered Li-rich, Ti-based oxyfluoride cathode with multiple cation and anion redox chemistry
The recent discovery of fast Li-ion transport networks in Li-rich disordered rock-salt (DRX) cathodes expands vastly composition space of high-energy–density cathode materials. Despite numerous Nb, Mo and W based DRX materials have been reported, high energy density DRX materials with composition of inexpensive, eco-friendly and earth abundant elements still urgently need to be developed. Herein, a promising set of Ti based oxyfluorides Li 2 Ti 1/3 Ni 1/3 M 1/3 O 2 F (M = Mn, Fe), which contain high voltage Ni 2+ /Ni 4+ redox couple and low-cost redox couple with Fe and Mn were successfully synthesized. Benefited from the stable Ni 2+ / 4+ and Mn 3+ / 4+ redox and moderated anion redox, Li 2 Ti 1/3 Ni 1/3 Mn 1/3 O 2 F oxyfluoride exhibits high discharge capacities of 285.3 mAh g −1 and energy content of 909.5 Wh kg −1 as well as relatively stable cyclability. Especially, high discharge capacity of 215 mAh g −1 above 3 V acquired by coordination utilization of cation and anion redox for Li 2 Ti 1/3 Ni 1/3 Mn 1/3 O 2 F endows the oxyfluoride with high energy quality, showing enormous potential for practical utilization in the future. For Li 2 Ti 1/3 Ni 1/3 Fe 1/3 O 2 F, the poor reversibility of Ni 2+ /Ni 4+ redox and much higher oxygen charge redox are the main reasons for the much worse capacity retention, and the activation of Fe 2+ /Fe 3+ redox at low voltage partially contributes to the decreased discharge voltage. Density-functional theory calculation reveals that localized oxygen 2p orbital of Li 2 Ti 1/3 Ni 1/3 Fe 1/3 O 2 F is closer to the Fermi level than that of Li 2 Ti 1/3 Ni 1/3 Mn 1/3 O 2 F, which would ultimately lead to much higher content of oxygen redox and relatively lower capacity retention. The synergistic utilization of multiple cation redox reservoirs which consist of high voltage redox couple and low-cost redox couple, and small quantity of O redox centers to construct high energy quality DRX materials would provide new reference for developing other cathodes.
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