Structurally Regulating Cation Migration Suppresses Voltage Fade and Redox Asymmetry in Lithium-Rich Layered Oxides

Abstract

Although lithium-rich layered oxides hold the greatest promise to transcend energy-density limitations in rechargeable lithium-ion batteries because of their high reversible capacity (exceeding 250 mAh g−1) and high-voltage anionic redox chemistry, the inevitable voltage decay, or gradual decrease in the average discharge voltage during cycling remains one some of the most pernicious problems jeopardizing their real-world application. Of paramount importance in understanding the fundamentals of the voltage decay is that its essential determinant is not the transition metal (TM) migration itself but the resulting confinement of TM ions in the Li layer. Therefore, a substantive key lies in improving the intra-cycle reversibility of TM migration. In this work, reversible intra-cycle TM migration is first demonstrated by modifying the oxygen lattice of lithium-rich layered oxides. O2-type stackings inherently allow reversible intra-cycle TM migration, thus delivering outstanding voltage retention over extended cycling and far outperforming their O3-phase counterparts and other lithium-rich layered 3d metal oxides. Suppressed voltage decay arose from the retention of the pristine layered structure with highly reversible TM migration over extended cycling was revealed by various structural characterizations with the aid of first-principles calculations. Our findings indicate that tailoring the migration path of TM ions provides a viable strategy to address the critical issues of voltage decay, which may help rejuvenate the research field of lithium-rich layered oxides.