The attainable specific energy of Li-S cells is largely affected by the electrolyteto-sulfur (E/S) ratio, with a low value [5] thereof being prerequisite to achieve a competitive energy density for a practical cell. However, the most commonly used electrolyte, a mixture of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) (i.e., v/v = 1:1), exhibits low Li 2 S 8 solubility of at most 0.7 m at room temperature. [6] This limited solubility constitutes an obstacle in the way of the full-depth utilization of sulfur under so-called lean electrolyte conditions. Although it could be overcome by using a large amount of electrolyte, this would severely impair the volumetric energy density. This notwithstanding, the DOL/DME system and its analogues served as standard electrolyte solvents in the early stages of investigation, because the stable environment they offer to Li metal anodes contributed greatly to enhancing the cycle life. Nevertheless, identifying electrolyte conditions that allow effective operation at low E/S ratios (i.e., below 2 μL electrolyte mg sulfur −1 in practice) is essential for practical cells. [7] Therefore, the development of alternative electrolyte solvents with high polysulfide solubility remains highly desirable. Electrolytes with a high Gutmann donor number, such as N,N-dimethyl acetamide (DMAc), [7a,8] dimethyl sulfoxide (DMSO), [6,7,9] N,N-dimethyl formamide (DMF), [10] and N-methyl-2-pyrrolidone (NMP) [9c] are good candidates for lean electrolyte conditions in Li-S batteries. High donicity affords an environment that promotes interaction with electrophilic cations, implying that the solvation of Li ions can facilitate the solubility of polysulfides. For example, high donor electrolytes were reported [6] to readily dissolve more than 1.6 m of Li 2 S 8. In the same line, the utilization of sulfur can be enhanced for the given amount of electrolyte introduced in the cell. Apart from the solvation capability, high donor electrolytes confer 3D morphology for the final discharging product, namely Li 2 S, contrary to low donor electrolytes that give rise to a 2D film-like morphology. The 3D morphology is beneficial in that it leaves the conductive electrode surface available for repeated dischargecharge over cycling without passivation. [11] In addition, high donor electrolytes activate reaction routes that involve S 3 •− species (Figure 1a), [8a,12] the availability of which represents diverse reaction pathways for discharge to further enhance the utilization of sulfur. Despite these remarkable advantages, high donor electrolytes are known to have a short cycle life mainly because of their catastrophic reactivity with the Li metal anode. [8a,11c] This problem was subsequently pinpointed by Gupta et al., [7a] who
Lithium–sulfur (Li–S) batteries continue to be considered promising post‐lithium‐ion batteries owing to their high theoretical energy density. In pursuit of a Li–S cell with long‐term cyclability, most studies thus far have relied on using ether‐based electrolytes. However, their limited ability to dissolve polysulfides requires a high electrolyte‐to‐sulfur ratio, which impairs the achievable specific energy. Recently, the battery community found high donor electrolytes to be a potential solution to this shortcoming because their high solubility toward polysulfides enables a cell to operate under lean electrolyte conditions. Despite the increasing number of promising outcomes with high donor electrolytes, a critical hurdle related to stability of the lithium‐metal counter electrode needs to be overcome. This review provides an overview of recent efforts pertaining to high donor electrolytes in Li–S batteries and is intended to raise interest from within the community. Furthermore, based on analogous efforts in the lithium‐air battery field, strategies for protecting the lithium metal electrode are proposed. It is predicted that high donor electrolytes will be elevated to a higher status in the field of Li–S batteries, with the hope that either existing or upcoming strategies will, to a fair extent, mitigate the degradation of the lithium–metal interface.
Lithium–sulfur (Li–S) batteries by far offer higher theoretical energy density than that of the commercial lithium-ion battery counterparts, but suffer predominantly from an irreversible shuttling process involving lithium polysulfides. Here, we report a fluorinated covalent organic polymer (F-COP) as a template for high performance sulfur cathodes in Li–S batteries. The fluorination allowed facile covalent attachment of sulfur to a porous polymer framework via nucleophilic aromatic substitution reaction (SNAr), leading to high sulfur content, e.g., over 70 wt %. The F-COP framework was microporous with 72% of pores within three well-defined pore sizes, viz. 0.58, 1.19, and 1.68 nm, which effectively suppressed polysulfide dissolution via steric and electrostatic hindrance. As a result of the structural features of the F-COP, the resulting sulfur electrode exhibited high electrochemical performance of 1287.7 mAh g–1 at 0.05C, 96.4% initial Columbic efficiency, 70.3% capacity retention after 1000 cycles at 0.5C, and robust operation for a sulfur loading of up to 4.1 mgsulfur cm–2. Our findings suggest the F-COP family with the adaptability of SNAr chemistry and well-defined microporous structures as useful frameworks for highly sustainable sulfur electrodes in Li–S batteries.
Recent research has built a consensus that the binder plays a key role in the performance of high‐capacity silicon anodes in lithium‐ion batteries. These anodes necessitate the use of a binder to maintain the electrode integrity during the immense volume change of silicon during cycling. Here, Zn2+–imidazole coordination crosslinks that are formed to carboxymethyl cellulose backbones in situ during electrode fabrication are reported. The recoverable nature of Zn2+–imidazole coordination bonds and the flexibility of the poly(ethylene glycol) chains are jointly responsible for the high elasticity of the binder network. The high elasticity tightens interparticle contacts and sustains the electrode integrity, both of which are beneficial for long‐term cyclability. These electrodes, with their commercial levels of areal capacities, exhibit superior cycle life in full‐cells paired with LiNi0.8Co0.15Al0.05O2 cathodes. The present study underlines the importance of highly reversible metal ion‐ligand coordination chemistries for binders intended for high capacity alloying‐based electrodes.
The high specific capacity in excess of 200 mAh g–1 and low dependence on cobalt have enhanced the research interest on nickel-rich layered metal oxides as cathode materials for lithium-ion batteries for electric vehicles. Nonetheless, their poor cycle life and thermal stability, resulting from the occurrence of cation mixing between the transition-metal (TM) and lithium ions, are yet to be fully addressed to enable the widespread and reliable use of these materials. Here, we report a two-dimensional (2D) pyrazine-linked covalent organic framework (namely, Pyr-2D) as a coating material for nickel-rich layered cathodes to mitigate unwanted TM dissolution and interfacial reactions. The Pyr-2D coating layer, especially the 2D planar morphology and conjugated atomic configuration of Pyr-2D, protects the electrode surface effectively during cycling without sacrificing the electric conductivity of the host material. As a result, Pyr-2D-coated nickel-rich layered cathodes exhibited superior cyclability, rate performance, and thermal stability. The present study highlights the potential ability of 2D conjugated covalent organic frameworks to improve the key electrochemical properties of emerging battery electrodes.
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