Department of Chemistry and Biotechnology, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan.
Phys Chem Chem Phys. 2019 Feb 27;21(9):5097-5105. doi: 10.1039/c9cp00425d.
Recent studies have suggested that a Li ion hopping or ligand- or anion-exchange mechanism is largely involved in Li ion conduction of highly concentrated liquid electrolytes. To understand the determining factors for the Li ion hopping/exchange dominant conduction in such liquid systems, ionic diffusion behavior and Li ion coordination structures of concentrated liquid electrolytes composed of lithium bis(fluorosulfonyl)amide (Li[FSA]) and keto ester solvents with two carbonyl coordinating sites of increasing intramolecular distance (methyl pyruvate (MP), methyl acetoacetate (MA), and methyl levulinate (ML)) were studied. Diffusivity measurements of MP- and MA-based concentrated electrolytes showed faster Li ion diffusion than the solvent and FSA anion, demonstrating that Li ion diffusion was dominated by the Li ion hopping/exchange mechanism. A solvent-bridged, chain-like Li ion coordination structure and highly aggregated ion pairs (AGGs) or ionic clusters e.g. Lix[FSA]y(y-x)- forming in the electrolytes were shown to contribute to Li ion hopping conduction. By contrast, ML, with greater intramolecular distance between the carbonyl moieties, is more prone to form a bidentate complex with a Li cation, which increased the contribution of the vehicle mechanism to Li ion diffusion even though similar AGGs and ionic clusters were also observed. The clear correlation between the unusual Li ion diffusion and the solvent-bridged, chain-like structure provides an important insight into the design principles for fast Li ion conducting liquid electrolytes that would enable Li ion transport decoupled from viscosity-controlled mass transfer processes.
最近的研究表明，锂离子跳跃或配体-或阴离子交换机制在高浓度液态电解质中锂离子传导中起着重要作用。为了理解在这种液态体系中锂离子跳跃/交换主导传导的决定因素，研究了由双（氟磺酰基）酰胺（Li[FSA]）和具有两个羰基配位位点的酮酯溶剂组成的高浓度液态电解质的离子扩散行为和锂离子配位结构，羰基配位位点的分子内距离逐渐增大（丙酮酸甲酯（MP）、乙酰丙酮甲酯（MA）和戊二酸二甲酯（ML））。MP 和 MA 基高浓度电解质的扩散率测量结果表明，锂离子的扩散速度比溶剂和 FSA 阴离子快，这表明锂离子的扩散主要由锂离子跳跃/交换机制主导。溶剂桥接的链状锂离子配位结构和高度聚集的离子对（AGGs）或离子簇例如 Lix[FSA]y(y-x)-在电解质中形成，这有助于锂离子跳跃传导。相比之下，由于羰基部分之间的分子内距离更大，ML 更倾向于与锂离子形成双齿配合物，这增加了载流子机制对锂离子扩散的贡献，尽管也观察到了类似的 AGGs 和离子簇。不寻常的锂离子扩散与溶剂桥接的链状结构之间的明显相关性为设计快速锂离子传导液态电解质提供了重要的见解，这将使锂离子传输与粘度控制的传质过程解耦。
