Markiewitz Daniel M, Goodwin Zachary A H, McEldrew Michael, Pedro de Souza J, Zhang Xuhui, Espinosa-Marzal Rosa M, Bazant Martin Z
Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA.
Faraday Discuss. 2024 Oct 25;253(0):365-384. doi: 10.1039/d4fd00021h.
Ionic liquids (ILs) are an extremely exciting class of electrolytes for energy storage applications. Upon dissolving alkali metal salts, such as Li or Na based salts, with the same anion as the IL, an intrinsically asymmetric electrolyte can be created for use in batteries, known as a salt-in-ionic liquid (SiIL). These SiILs have been well studied in the bulk, where negative transference numbers of the alkali metal cation have been observed from the formation of small, negatively charged clusters. The properties of these SiILs at electrified interfaces, however, have received little to no attention. Here, we develop a theory for the electrical double layer (EDL) of SiILs where we consistently account for the thermoreversible association of ions into Cayley tree aggregates. The theory predicts that the IL cations first populate the EDL at negative voltages, as they are not strongly bound to the anions. However, at large negative voltages, which are strong enough to break the alkali metal cation-anion associations, these IL cations are exchanged for the alkali metal cation because of their higher charge density. At positive voltages, we find that the SiIL actually becomes more aggregated while screening the electrode charge from the formation of large, negatively charged aggregates. Therefore, in contrast to conventional intuition of associations in the EDL, SiILs appear to become more associated in certain electric fields. We present these theoretical predictions to be verified by molecular dynamics simulations and experimental measurements.
离子液体（ILs）是一类极具吸引力的用于储能应用的电解质。在溶解碱金属盐（如锂盐或钠盐）时，若其阴离子与离子液体相同，就能制备出一种用于电池的本征不对称电解质，即离子液体中的盐（SiIL）。这些SiILs在本体中已得到充分研究，通过形成带负电的小簇，观察到碱金属阳离子的负迁移数。然而，这些SiILs在带电界面处的性质却很少受到关注，甚至无人问津。在此，我们针对SiILs的双电层（EDL）建立了一种理论，该理论始终考虑离子热可逆地缔合成凯莱树状聚集体的情况。该理论预测，由于离子液体阳离子与阴离子的结合不强，它们会在负电压下首先占据双电层。然而，在足够大的负电压下，强大到足以打破碱金属阳离子 - 阴离子缔合时，由于其更高的电荷密度，这些离子液体阳离子会被碱金属阳离子取代。在正电压下，我们发现SiIL实际上会变得更加聚集，同时通过形成大的带负电聚集体来屏蔽电极电荷。因此，与双电层中缔合的传统直觉相反，SiILs在某些电场中似乎会变得更加缔合。我们展示这些理论预测结果，以待通过分子动力学模拟和实验测量进行验证。
