Reducing Voltage Hysteresis in Li-Rich Sulfide Cathodes by Incorporation of Mn.

Conventional intercalation-based cathode materials in Li-ion batteries are based on charge compensation of the redox-active cation and can only intercalate one mole of electron per formula unit. Anion redox, which employs the anion sublattice to compensate charge, is a promising way to achieve multielectron cathode materials. Most anion redox materials still face the problems of slow kinetics and large voltage hysteresis. One potential solution to reduce voltage hysteresis is to increase the covalency of the metal-ligand bonds. By substituting Mn into the electrochemically inert Li1.33Ti0.67S2 (Li2TiS3), anion redox can be activated in the Li1.33-2y/3Ti0.67-y/3Mn y S2 (y = 0-0.5) series. Not only do we observe substantial anion redox, but the voltage hysteresis is significantly reduced, and the rate capability is dramatically enhanced. The y = 0.3 phase exhibits excellent rate and cycling performance, maintaining 90% of the C/10 capacity at 1C, which indicates fast kinetics for anion redox. X-ray absorption spectroscopy (XAS) shows that both the cation and anion redox processes contribute to the charge compensation. We attribute the drop in hysteresis and increase in rate performance to the increased covalency between the metal and the anion. Electrochemical signatures suggest the anion redox mechanism resembles holes on the anion, but the S K-edge XAS data confirm persulfide formation. The mechanism of anion redox shows that forming persulfides can be a low hysteresis, high rate capability mechanism enabled by the appropriate metal-ligand covalency. This work provides insights into how to design cathode materials with anion redox to achieve fast kinetics and low voltage hysteresis.

Effect of Polysulfide Speciation on Mg Anode Passivation in Mg-S Batteries.

Mg-S batteries are a promising next-generation system for beyond conventional Li-ion chemistry. The Mg-S architecture pairs a Mg metal anode with an inexpensive, high-capacity S8 cathode. However, S8-based cathodes exhibit the "polysulfide shuttle" effect, wherein soluble partially reduced Sx2- species generated at the cathode diffuse to and react with the anode. While dissolved polysulfides may undergo reactions to form Li+-permeable layers in Li-S systems, the interfaces on Mg anodes are passivating. In this work, we probe the reactivity of various Mg polysulfide solutions at the Mg anode interface. Mg polysulfide solutions are prepared without any chelating agents to closely mimic conditions in a Mg-S cell. The polysulfides are synthesized by reacting Mg metal and S8 in electrolyte, and the speciation is controlled by varying the Mg:S precursor ratio. S-poor precursor ratios produce magnesium polysulfide solutions with a higher proportion of short-chain polysulfides that react at the Mg anode faster than the longer-chain analogues. Anode passivation can be slowed by shifting the polysulfide equilibria toward longer-chain polysulfides through addition of S8.

Mg Anode Passivation Caused by the Reaction of Dissolved Sulfur in Mg-S Batteries.

As Li-ion battery optimization approaches theoretical limits, interest has grown in designing next-generation batteries from low-cost earth-abundant materials. Mg-S batteries are promising candidates, exhibiting widespread abundance of elemental precursors and a relatively large theoretical energy density albeit at lower cell voltage. However, Mg-S batteries exhibit poor reversibility, in part due to interactions between dissolved polysulfides and the Mg anode. Herein, we employ electrochemical experiments using Ag2S quasi-reference electrodes to probe the interactions between Mg anodes and dissolved polysulfides. We show that Mg2+ reduction (charging) is impeded in the presence of polysulfides, while Mg metal oxidation (discharging) remains facile. Large reduction overpotentials arise due to the formation of a passivation layer on the anode surface, likely composed primarily of MgS. The passivation layer is removed under oxidative conditions but quickly reforms during reduction. We discover that dissolved S8 influences the rate of MgS formation by shifting the polysulfide disproportionation equilibria. Shorter-chain polysulfides react more readily than longer-chain polysulfides at the Mg electrode, and thus, film formation is mediated by the electrochemical generation of shorter-chain polysulfide species.