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The Mg/S battery is attractive because of its high theoretical energy density and the abundance of Mg and S on the earth. However, its development is hindered by the lack of understanding to the underlying electrochemical reaction mechanism of its charge−discharge processes. Here, using a unique in situ X-ray absorption spectroscopic tool, we systematically study the reaction pathways of the Mg/S cells in Mg(HMDS) 2 −AlCl 3 electrolyte. We find that the capacity degradation is mainly due to the formation of irreversible discharge products, such as MgS and Mg 3 S 8 , through a direct electrochemical deposition or a chemical disproportionation of intermediate polysulfide. In light of the fundamental understanding, we propose to use TiS 2 as a catalyst to activate the irreversible reaction of loworder MgS x and MgS, which results in an increased discharging capacity up to 900 mAh•g −1 and a longer cycling life.

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Metallic lithium electrode with high capacity of 3860 mA h g−1 is the most promising candidate for rechargeable batteries. However, some inherent problems such as dendrite growth, uneven solid electrolyte interphase (SEI), and high manufacturing risk restraint its practical application. Herein, distinct from the conventional mosaic structure, a facile fabrication of in situ self‐assembled organic/inorganic hybrid SEI with ordered dual‐layer structure on the lithium surface to suppress dendrite formation is proposed. With the aid of moderate active fluoric‐containing ionic liquid, the as‐formed lithium fluoride and robust ordered organic moieties are in situ self‐assembled on the metallic lithium surface. The evolution process of the dual‐layered structure is revealed by X‐ray spectroscopy, in situ sum frequency generation spectroscopy, and atomic force microscopy. The formed “double protection” ordered hybrid interphase layer also exhibits the surprising ability against the corrosion of carbonate electrolyte or dry air. As a consequence, the pretreated metallic lithium electrode represents excellent stripping/plating reversibility of ≈99% and a long lifespan up to 1200 h without formation of dendrite, and remains high performance at a current density of 10 mA cm−2, which is much higher than most reports, showing the facilitating promising to the future utilization.

Oxygen defect-rich iron oxide (ODFO) nanoparticle catalyst on nanocarbon is in situ synthesized with the assistance of multi-ion modulation in one pot. The nanoparticle catalyst is employed to propel electrochemical kinetics in lithium/sulfur batteries. Electrochemical analysis and theoretical simulation evidently verify the critical role of defect sites on catalyzing conversion reactions of sulfur species and reducing energy barriers. As a consequence, the ODFO-enhanced sulfur cathode exhibits a high specific capacity of 1489 mA h g −1 at 0.1 C, an excellent rate performance of 644 mA h g −1 at 10 C, and a superior cycling stability with an average capacity fading rate of as low as 0.055% per cycle under an ultrahigh rate of 10 C. More importantly, even with a high sulfur loading of 11.02 mg cm −2 , the Li/S cell can still deliver an areal capacity of 8.7 mA h cm −2 at 0.5 C (9.23 mA cm −2 ). Such performance is the highest among reported metal oxide-catalyzed sulfur cathodes. This work opens a new route to boosting conversion reaction kinetics by introduction of active oxygen defect sites in electrodes of various emerging ultrafast batteries.