Jeong Heonjae, Nguyen Dan-Thien, Yang Yingjie, Buchholz D Bruce, Evmenenko Guennadi, Guo Jinghua, Yang Feipeng, Redfern Paul C, Hu Jian Zhi, Mueller Karl T, Klie Robert, Murugesan Vijayakumar, Connell Justin, Prabhakaran Venkateshkumar, Cheng Lei
The Joint Center for Energy Storage Research (JCESR), Argonne National Laboratory, Lemont, Illinois 60439, United States.
Materials Science Division, Argonne National Laboratory, Lemont, Illinois 60439, United States.
ACS Appl Mater Interfaces. 2023 Oct 18;15(41):48072-48084. doi: 10.1021/acsami.3c07875. Epub 2023 Oct 8.
Predictive understanding of the molecular interaction of electrolyte ions and solvent molecules and their chemical reactivity on electrodes has been a major challenge but is essential for addressing instabilities and surface passivation that occur at the electrode-electrolyte interface of multivalent magnesium batteries. In this work, the isolated intrinsic reactivities of prominent chemical species present in magnesium bis(trifluoromethanesulfonimide) (Mg(TFSI)) in diglyme (G2) electrolytes, including ionic (TFSI, [Mg(TFSI)], [Mg(TFSI):G2], and [Mg(TFSI):2G2]) as well as neutral molecules (G2) on a well-defined magnesium vanadate cathode (MgVO) surface, have been studied using a combination of first-principles calculations and multimodal spectroscopy analysis. Our calculations show that nonsolvated [Mg(TFSI)] is the strongest adsorbing species on the MgVO surface compared with all other ions while partially solvated [Mg(TFSI):G2] is the most reactive species. The cleavage of C-S bonds in TFSI to form CF is predicted to be the most desired pathway for all ionic species, which is followed by the cleavage of C-O bonds of G2 to yield CH or OCH species. The strong stabilization and electron transfer between ionic electrolyte species and MgVO is found to significantly favor these decomposition reactions on the surface compared with intrinsic gas-phase dissociation. Experimentally, we used state-of-the-art ion soft landing to selectively deposit mass-selected TFSI, [Mg(TFSI):G2], and [Mg(TFSI):2G2] on a MgVO thin film to form a well-defined electrolyte-MgVO interface. Analysis of the soft-landed interface using X-ray photoelectron, X-ray absorption near-edge structure, electron energy-loss spectroscopies, as well as transmission electron microscopy confirmed the presence of decomposition species (e.g., MgF, carbonates) and the higher amount of MgF with [Mg(TFSI):G2] formed in the interfacial region, which corroborates the theoretical observation. Overall, these results indicate that Mg desolvation results in electrolyte decomposition facilitated by surface adsorption, charge transfer, and the formation of passivating fluorides on the MgVO cathode surface. This work provides the first evidence of the primary mechanisms leading to electrolyte decomposition at high-voltage oxide surfaces in multivalent batteries and suggests that the design of new, anodically stable electrolytes must target systems that facilitate cation desolvation.
对电解质离子与溶剂分子的分子相互作用及其在电极上的化学反应性进行预测性理解一直是一项重大挑战，但对于解决多价镁电池电极-电解质界面处发生的不稳定性和表面钝化问题至关重要。在这项工作中，我们结合第一性原理计算和多模态光谱分析，研究了二甘醇二甲醚（G2）电解质中双（三氟甲磺酰）亚胺镁（Mg(TFSI)）中主要化学物种的孤立本征反应性，这些物种包括离子型（TFSI、[Mg(TFSI)]、[Mg(TFSI):G2]和[Mg(TFSI):2G2]）以及中性分子（G2）在明确的钒酸镁阴极（MgVO）表面上的反应性。我们的计算表明，与所有其他离子相比，非溶剂化的[Mg(TFSI)]是MgVO表面上吸附最强的物种，而部分溶剂化的[Mg(TFSI):G2]是反应性最强的物种。预测TFSI中C-S键的断裂形成CF是所有离子物种最有利的途径，其次是G2的C-O键断裂生成CH或OCH物种。与本征气相解离相比，发现离子电解质物种与MgVO之间的强稳定作用和电子转移显著有利于这些表面分解反应。在实验中，我们使用了最先进的离子软着陆技术，将质量选择的TFSI、[Mg(TFSI):G2]和[Mg(TFSI):2G2]选择性地沉积在MgVO薄膜上，以形成明确的电解质-MgVO界面。使用X射线光电子能谱、X射线吸收近边结构、电子能量损失谱以及透射电子显微镜对软着陆界面进行分析，证实了分解物种（如MgF、碳酸盐）的存在，并且在界面区域形成的[Mg(TFSI):G2]产生的MgF量更高，这证实了理论观察结果。总体而言，这些结果表明，Mg去溶剂化导致电解质分解，这是由表面吸附、电荷转移以及在MgVO阴极表面形成钝化氟化物所促进的。这项工作首次证明了多价电池中高压氧化物表面电解质分解的主要机制，并表明新型阳极稳定电解质的设计必须针对促进阳离子去溶剂化的体系。
