Ca(BH$_4$)$_2$在THF中的Ca二聚体、溶剂分层及主导电化学活性物种
Ca-dimers, solvent layering, and dominant electrochemically active species in Ca(BH$_4$)$_2$ in THF
AI总结：
通过分子动力学模拟揭示Ca(BH$_4$)$_2$/THF电解质中电活性物种为歧化产生的CaBH$_4^+$，其富集受电极表面溶剂分层调控，为二价电池界面设计提供分子尺度指导。
摘要：
二价离子，如Mg、Ca和Zn，正被视为锂离子电化学的竞争性、安全且地壳储量丰富的替代方案。然而，挑战仍然在于匹配能够与这些新配方稳定循环的电极和电解质材料，这主要基于对界面现象的控制。我们通过分子动力学模拟探索了Ca(BH$_4$)$_2$在THF中电解质内电活性物种的形成。自由能分析表明，该电解质中主要存在中性的Ca二聚体和单体，尽管无监督学习技术揭示了多样的分子构象，但可能具有电活性的带电物种（如单价阳离子CaBH$_4^+$）的浓度低一个数量级，我们证明该阳离子是通过中性Ca(BH$_4$)$_2$配合物的歧化反应产生的。电极表面（此处以石墨建模）1 nm内THF分子的致密分层阻碍了可还原物种接近至0.6 nm以内，反而增强了0.7-0.9 nm狭窄中间密度层中物种的局部浓度。在负偏压作用下，该中间层中单价阳离子群体显著增加，这是由局部二聚体歧化反应提供的。我们没有发现任何证据支持完全溶剂化的Ca$^{2+}$在该电解质的电化学活性中发挥功能作用。基于这一分子尺度的见解，讨论了其对性能和替代配方的影响。
英文摘要：
Divalent ions, such as Mg, Ca, and Zn, are being considered as competitive, safe, and earth-abundant alternatives to Li-ion electrochemistry. However, the challenge remains to match electrode and electrolyte materials that stably cycle with these new formulations, based primarily on controlling interfacial phenomena. We explore the formation of electroactive species in the electrolyte Ca(BH$_4$)$_2$ in THF through molecular dynamics simulation. Free-energy analysis indicates that this electrolyte has a majority population of neutral Ca dimers and monomers, albeit with diverse molecular conformations as revealed by unsupervised learning techniques, but with an order of magnitude lower concentration of possibly electroactive charged species, such as the monocation, CaBH$_4^+$ , which we show is produced via disproportionation of neutral Ca(BH$_4$)$_2$ complexes. Dense layering of THF molecules within 1 nm of the electrode surface (modeled here using graphite) hinders the approach of reducible species to within 0.6 nm and instead enhances the local concetration of species in a narrow intermediate-density layer from 0.7-0.9 nm. A dramatic increase in the monocation population in this intermediate layer is induced at negative bias, supplied by local dimer disproportionation. We see no evidence to support any functional role of fully-solvated Ca$^{2+}$ in the electrochemical activity of this electrolyte. The consequences for performance and alternative formulations are discussed in light of this molecular-scale insight.
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