Tuning redox activity through delithiation induced protective layer and Fe-O coordination for Li-rich cathode with improved voltage and cycle performance

Abstract

Li-rich layered transition metal oxides are one of the most promising cathode materials for their high energy density. However, the cathodes usually suffer from severe potential dropping and capacity fading during cycling, which are associated with the surface oxygen release and accompanied by cation densification and structural collapse. Herein, an integrative approach of simultaneous constructing uniform 3d Fe-ion doping in the transition metal layer and Li-rich Li5FeO4 shell to grab the oxygen and prevent interfacial side reactions is proposed. The introduction of Fe induces higher redox potential and stronger 3d Fe-O 2p covalent bond, triggering reversible anionic redox via a reductive coupling mechanism. And the delithiated product of Li-rich Li5FeO4 not only acts as a protective layer alleviating the side reactions but also enhances the surface kinetic property. With the benefit of promoted reversibility of oxygen redox and enhanced surface stability, the cathode exhibits high reversible capacity and superior cycle performance. Density function theory calculation indicates that the O 2p non-bonding state in the cathode incorporated with Fe sits at a lower energy band, resulting in higher energy storage voltage and improved oxygen stability. Consequently, the modified cathode exhibits a discharge specific capacity of 307 mA h g−1 (1C = 250 mA g−1), coulombic efficiency of 82.09% in the initial cycle at 0.1C and 88.34% capacity retention after 100 cycles at 1C. The work illustrates a strategy that could simultaneously enhance oxygen redox reversibility and interface stability by constructing lattice bond coordination and delithiation induced protective layer to develop Li-rich materials with high reversible capacity and long lifespan.