Effect of Polysulfide Speciation on Mg Anode Passivation in Mg–S Batteries

Abstract

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, highcapacity S 8 cathode. However, S 8 -based cathodes exhibit the "polysulfide shuttle" effect, wherein soluble partially reduced S x 2− 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 S 8 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 S 8 .

Manipulation of Ferrofluid‐Wrapped Drops: Translation, Coalescence, and Release

“…The generation process of the compound droplet remains the same, as discussed in Section 2. Owing to the sufficient kinetic energy, the droplet pierces through the interfacial layer, where it overcomes the interfacial barrier offered by the ferrofluid-water interface and results in the formation of the encapsulated droplet as reported by Misra et al [65,66] In both situations, the interface undergoes a similar evolution process, as evident from the high-speed timestamped images of Figure 2a,b, respectively. The color images of the final encapsulated droplets are shown in Figure 2a (rightmost) and Figure 2b (rightmost).…”

“…We first enclose the EG droplet inside a carrier droplet of laser oil using a Y-junction flow arrangement and then encapsulate the laser oil -EG compound droplet with a ferrofluid shell layer using the same impact-driven liquid-liquid encapsulation technique. [65,66] Subsequently, we use the controlled motion of a permanent magnet to manipulate the compound cargo and demonstrate on-demand coalescence, allowing controlled mixing of the inner EG core without perishing in the surrounding aqueous medium. Finally, we show the release of the EG core inside the water bath using the magnetic field.…”

“…The encapsulation of the core liquid (laser oil) inside the oil-based ferrofluid is achieved by using the robust impact-driven liquid-liquid encapsulation technique developed by our group. [65,66] In those works, Misra et al [65,66] exploited the thermodynamically favorable tendency of a liquid-triplet to demonstrate impact-driven stable and ultrafast encapsulation by impinging a target core analyte on the interfacial layer of a shell-forming liquid floating on the pool of another host liquid. This technique is further utilized by Yin et al [67] for the fabrication of triple-layered encapsulated droplets, indicating the robustness of the technique.…”

Polysulfides in Magnesium‐Sulfur Batteries

“…S 8 in specific ratios, depending on the chain length of the target polysulfides. [43,51,52] However, dissociation in solution can still generate shorter-chain polysulfides and result in a mixture of different polysulfide species, complicating spectroscopic measurements and the corresponding data analysis. [53] For more accurate spectroscopic characterization in a battery setting, in situ UV-vis methods can monitor the evolution of polysulfide species during the battery charging/discharging process, thus avoiding issues induced by post-battery processing.…”

“…For instance, there is a debate on the number of polysulfide intermediates formed during redox processes and pathways for polysulfide conversion. [ 43 ] In addition, Mg polysulfides can occur either as solids or as dissolved species in the battery electrolyte, which likely has a significant impact on reaction mechanisms. [ 44 ] Knowledge of Mg polysulfide behavior can be exploited to improve battery design and resolve key challenges related to sulfur chemistry in the Mg‐S system.…”