Exploring the bottlenecks of anionic redox in Li-rich layered sulfides

Anionic redox chemistry has emerged as a new paradigm to design higher-energy lithium ion-battery cathode materials such as Li-rich layered oxides. However, they suffer from voltage fade, large hysteresis and sluggish kinetics, which originate intriguingly from the anionic redox activity itself. To fundamentally understand these issues, we decided to act on the ligand by designing new Li-rich layered sulfides Li1.33 – 2y/3Ti0.67 – y/3FeyS2, among which the y = 0.3 member shows sustained reversible capacities of ~245 mAh g−1 due to cumulated cationic (Fe2+/3+) and anionic (S2−/Sn−, n < 2) redox processes. Moreover, its negligible initial cycle irreversibility, mitigated voltage fade upon long cycling, low voltage hysteresis and fast kinetics compare positively with its Li-rich oxide analogues. Moving from the oxygen ligand to the sulfur ligand thus partially alleviates the practical bottlenecks affecting anionic redox, although it penalizes the redox potential and energy density. Overall, these sulfides provide chemical clues to improve the holistic performance of anionic redox electrodes, which may guide us to ultimately exploit the energy benefits of oxygen redox. The utilization of oxygen redox is a promising way of designing high-energy cathode materials for batteries. Here, Tarascon and colleagues report a class of Li-rich layered sulfides and unravel the potential of sulfur redox.