Institute for Frontier Materials , Deakin University , Geelong , Victoria 3217 , Australia.
CIC Energigune , Parque Tecnológico de Álava , Albert Einstein 48 , Miñano , 01510 Álava , Spain.
ACS Appl Mater Interfaces. 2019 Nov 20;11(46):43093-43106. doi: 10.1021/acsami.9b12913. Epub 2019 Nov 8.
The interphase layer that forms on either the anode or the cathode is considered to be one of the critical components of a high performing battery. This solid-electrolyte interphase (SEI) layer determines the stability of the electrode in the presence of a given electrolyte as well as the internal resistance of a battery, and hence the overpotential of a cell. In the case of lithium ion batteries where carbonate based electrolytes are used, additives including hexafluorophosphate (PF), bis-trifluoromethylsulfonimide (TFSI), (fluorosulfonyl)(trifluoromethanesulfonyl)imide (FTFSI), and fluorosulfonimde (FSI) are used to obtain favorable SEI layers. Ionic liquids and salts based on anions containing nitrile groups, including dicyanamide (DCA), offer a less expensive alternative to a fluorinated anion and have also been shown to support stable electrochemistry in lithium and sodium systems. However, longer term cycling leads to the eventual passivation of the electrode, presumed to be due to the instability of the DCA anion. We herein consider the use of a fluorinated anion to control the interfacial electrochemistry and provide a more stable SEI in DCA ILs. We investigate the addition of NaDCA, NaFSI, NaTFSI, and NaFTFSI to the methylpropylpyrrolidinium dicyanamide ([C3mpyr]DCA) ionic liquid. NaFSI was found to generate a more stable SEI layer, as evidenced by extended symmetric cell cycling, while the TFSI and FTFSI salts both lead to thicker, highly passivating surfaces. We use molecular dynamics, infrared spectroscopy and X-ray photoelectron spectroscopy to interrogate and discuss the influence of the anion on the bulk electrolyte, the interfacial electrolyte structure, and the formation of the SEI layer, in order to rationalize the contrasting electrochemical observations.
在阳极或阴极上形成的相间层被认为是高性能电池的关键组件之一。这种固体电解质相间（SEI）层决定了给定电解质存在下电极的稳定性以及电池的内阻，因此也是电池的过电势。在使用碳酸酯基电解质的锂离子电池中，添加六氟磷酸盐（PF）、双三氟甲基磺酰亚胺（TFSI）、（氟磺酰基）（三氟甲磺酰基）亚胺（FTFSI）和氟磺酰亚胺（FSI）等添加剂以获得有利的 SEI 层。基于含腈基阴离子的离子液体和盐，包括双氰胺（DCA），为氟化阴离子提供了一种更便宜的替代品，并且还被证明在锂和钠系统中支持稳定的电化学。然而，长期循环会导致电极最终钝化，据推测这是由于 DCA 阴离子的不稳定性。在此，我们考虑使用氟化阴离子来控制界面电化学，并在 DCA IL 中提供更稳定的 SEI。我们研究了将 NaDCA、NaFSI、NaTFSI 和 NaFTFSI 添加到甲基丙基吡咯烷二氰胺（[C3mpyr]DCA）离子液体中。发现 NaFSI 生成了更稳定的 SEI 层，这可以从扩展的对称电池循环中得到证明，而 TFSI 和 FTFSI 盐都导致了更厚的、高度钝化的表面。我们使用分子动力学、红外光谱和 X 射线光电子能谱来研究和讨论阴离子对体电解质、界面电解质结构和 SEI 层形成的影响，以合理化对比的电化学观察结果。
