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1. Stable, Impermeable Hexacyanoferrate Anolyte for Nonaqueous Redox Flow Batteries

   https://doi.org/10.1021/acsenergylett.4c01351  

   Zhao, Yuyue; Bheemireddy, Sambasiva R; Yue, Diqing; Yu, Zhou; Uddin, Mohammad Afsar; Liu, Haoyu; Li, Zhiguang; Fang, Xiaoting; Lyu, Xingyi; Agarwal, Garvit; et al (September 2024, ACS Energy Letters)

   Kamat, Prashant V (Ed.)

   Redox-active molecules, or redoxmers, in nonaqueous redox flow batteries often suffer from membrane crossover and low electrochemical stability. Transforming inorganic polyionic redoxmers established for aqueous batteries into nonaqueous candidates is an attractive strategy to address these challenges. Here we demonstrate such tailoring for hexacyanoferrate (HCF) by pairing the anions with tetra-n-butylammonium cation (TBA+). TBA3HCF has good solubility in acetonitrile and >1 V lower redox potential vs the aqueous counterpart; thus, the familiar aqueous catholyte becomes a new nonaqueous anolyte. The lowering of redox potential correlates with replacement of water by acetonitrile in the solvation shell of HCF, which can be traced to H-bond formation between water and cyanide ligands. Symmetric flow cells indicate exceptional stability of HCF polyanions in nonaqueous electrolytes and Nafion membranes completely block HCF crossover in full cells. Ion pairing of metal complexes with organic counterions can be effective for developing promising redoxmers for nonaqueous flow batteries. 

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2. Phosphorus-Functionalized Organic Linkers Promote Polysulfide Retention in MOF-Based Li–S Batteries

   https://doi.org/10.1021/acsaem.2c02925  

   Baumann, Avery E.; Anayah, Rasha I.; Thoi, V. Sara (November 2022, ACS Applied Energy Materials)

   Metal–organic frameworks (MOFs) have been an area of intense research for their high porosity and synthetic tunability, which afford them controllable physical and chemical properties for various applications. In this study, we demonstrate that functionalized MOFs can be used to mitigate the so-called polysulfide shuttle effect in lithium–sulfur batteries, a promising next-generation energy storage device. UiO-66-OH, a zirconium-based MOF with 2-hydroxyterephthalic acid, was functionalized with a phosphorus chloride species that was subsequently used to tether polysulfides. In addition, a molecular chlorophosphorane was synthesized as a model system to elucidate the chemical reactivity of the phosphorus moiety. The functionalized MOFs were then used as a cathode additive in coin cell batteries to inhibit the dissolution of polysulfides in solution. Through this work, we show that the functionalization of MOF with phosphorus enhances polysulfide redox and thereby capacity retention in Li–S batteries. While demonstrated here for polysulfide tethering in batteries, we envision this linker functionalization strategy could be more broadly utilized in separations, sensing, or catalysis applications. 

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3. Maleimide-functionalized metal–organic framework for polysulfide tethering in lithium–sulfur batteries

   https://doi.org/10.1039/D1MA00084E  

   Burns, David A.; Benavidez, Angelica; Buckner, Jessica L.; Thoi, V. Sara (May 2021, Materials Advances)

   Lithium–sulfur (Li–S) batteries have great potential as next generation energy storage devices. However, the redox chemistry mechanism involves the generation of solubilized lithium polysulfides, which can lead to leaching of the active material and, consequently, passivated electrodes and diminished capacities. Chemical tethering of lithium polysulfides to materials in the sulfur cathode is a promising approach for resolving this issue in Li–S batteries. Borrowing from the field of synthetic chemistry, we utilize maleimide functional groups in a Zr-based metal–organic framework to chemically interact with polysulfides through the Michael Addition reaction. A combination of molecular and solid-state spectroscopies confirms covalent attachment of Li 2 S x to the maleimide functionality. When integrated into Li–S cathodes, the maleimide-functionalized framework exhibits notable performance enhancements over that of the unfunctionalized material, revealing the promise of polysulfide anchors for Li–S battery cycling. 

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4. Implications of in situ chalcogen substitutions in polysulfides for rechargeable batteries

   https://doi.org/10.1039/D1EE01113H  

   Nanda, Sanjay; Bhargav, Amruth; Jiang, Zhou; Zhao, Xunhua; Liu, Yuanyue; Manthiram, Arumugam (October 2021, Energy & Environmental Science)

   The electrochemical behavior of sulfur-based batteries is intrinsically governed by polysulfide species. Here, we compare the substitutions of selenium and tellurium into polysulfide chains and demonstrate their beneficial impact on the chemistry of lithium–sulfur batteries. While selenium-substituted polysulfides enhance cathode utilization by effectively catalyzing the sulfur/Li 2 S conversion reactions due to the preferential formation of radical intermediates, tellurium-substituted polysulfides improve lithium cycling efficiency by reducing into a passivating interfacial layer on the lithium surface with low Li + -ion diffusion barriers. This unconventional strategy based on “molecular engineering” of polysulfides and exploiting the intrinsic polysulfide shuttle effect is validated by a ten-fold improvement in the cycle life of lean-electrolyte “anode-free” pouch cells. Assembled with no free lithium metal at the anode, the anode-free configuration maximizes the energy density, mitigates the challenges of handling thin lithium foils, and eliminates self-discharge upon cell assembly. The insights generated into the differences between selenium and tellurium chemistries can be applied to benefit a broad range of metal–chalcogen batteries as well as chalcogenide solid electrolytes. 

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5. Chemistry‐Performance Relationships of Polymer Gel‐Electrolytes for Mg−S and Li−S Batteries: Influence of Network Cation Solvation Capacity on Polymer‐Polysulfide Interactions

   https://doi.org/10.1002/cphc.202100881  

   Ford, Hunter_O; He, Peng; Schaefer, Jennifer_L (February 2022, ChemPhysChem)

   Abstract Metal‐sulfur batteries are a promising next‐generation energy storage technology, offering high theoretical energy densities with low cost and good sustainability. An active area of research is the development of electrolytes that address unwanted migration of sulfur and intermediate species known as polysulfides during operation of metal‐sulfur batteries, a phenomenon that leads to low energy efficiency and short life‐spans. A particular class of electrolytes, gel polymer electrolytes, are especially attractive for their ability to repel polysulfides on the basis of structure, electrostatics, and other polymer properties. Herein, within the context of magnesium‐ and lithium‐sulfur batteries, we investigate the impact of gel polymer electrolyte cation solvation capacity, a property related to network dielectric constant and chemistry, on sulfur/polysulfide‐polymer interactions, an understudied property‐performance relationship. Polymers with lower cation solvation capacity are found to permanently absorb less polysulfide active material, which increases sulfur utilization for Li−S batteries and significantly increases charge efficiency and life‐span for Li−S and Mg−S batteries. 

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