Kotronia Antonia, Asfaw Habtom D, Tai Cheuk-Wai, Hahlin Maria, Brandell Daniel, Edström Kristina
Department of Chemistry-Ångström Laboratory, Uppsala University, Uppsala SE-75121, Sweden.
Department of Materials and Environmental Chemistry, Stockholm University, Stockholm SE-10691, Sweden.
ACS Appl Mater Interfaces. 2021 Jan 27;13(3):3867-3880. doi: 10.1021/acsami.0c18586. Epub 2021 Jan 12.
Dual-ion batteries (DIBs) generally operate beyond 4.7 V vs Li/Li and rely on the intercalation of both cations and anions in graphite electrodes. Major challenges facing the development of DIBs are linked to electrolyte decomposition at the cathode-electrolyte interface (CEI), graphite exfoliation, and corrosion of Al current collectors. In this work, X-ray photoelectron spectroscopy (XPS) is employed to gain a broad understanding of the nature and dynamics of the CEI built on anion-intercalated graphite cycled both in highly concentrated electrolytes (HCEs) of common lithium salts (LiPF, LiFSI, and LiTFSI) in carbonate solvents and in a typical ionic liquid. Though Al metal current collectors were adequately stable in all HCEs, the Coulombic efficiency was substantially higher for HCEs based on LiFSI and LiTFSI salts. Specific capacities ranging from 80 to 100 mAh g were achieved with a Coulombic efficiency above 90% over extended cycling, but cells with LiPF-based electrolytes were characterized by <70% Coulombic efficiency and specific capacities of merely ca. 60 mAh g. The poor performance in LiPF-containing electrolytes is indicative of the continual buildup of decomposition products at the interface due to oxidation, forming a thick interfacial layer rich in LiPF, POF, LiPOF, and organic carbonates as evidenced by XPS. In contrast, insights from XPS analyses suggested that anion intercalation and deintercalation processes in the range from 3 to 5.1 V give rise to scant or extremely thin surface layers on graphite electrodes cycled in LiFSI- and LiTFSI-containing HCEs, even allowing for probing anions intercalated in the near-surface bulk. In addition, Raman, SEM and TEM characterizations revealed the presence of a thick coating on graphite particles cycled in LiPF-based electrolytes regardless of salt concentration, while hardly any surface film was observed in the case of concentrated LiFSI and LiTFSI electrolytes.
双离子电池（DIBs）相对于锂金属/锂的工作电压通常超过4.7V，并且依赖于阳离子和阴离子在石墨电极中的嵌入。DIBs发展面临的主要挑战与阴极-电解质界面（CEI）处的电解质分解、石墨剥落以及铝集流体的腐蚀有关。在这项工作中，采用X射线光电子能谱（XPS）来广泛了解在常见锂盐（LiPF、LiFSI和LiTFSI）的高浓度电解质（HCEs）中的碳酸盐溶剂以及典型离子液体中循环的阴离子嵌入石墨上形成的CEI的性质和动态。尽管铝金属集流体在所有HCEs中都具有足够的稳定性，但基于LiFSI和LiTFSI盐的HCEs的库仑效率要高得多。在长时间循环中，实现了80至100 mAh g的比容量，库仑效率高于90%，但基于LiPF的电解质的电池的库仑效率低于70%，比容量仅约为60 mAh g。含LiPF的电解质中的不良性能表明由于氧化，界面处分解产物持续积累，形成了富含LiPF₂、POF₃、LiPO₂F₂和有机碳酸盐的厚界面层，XPS证明了这一点。相比之下，XPS分析的结果表明，在3至5.1V范围内的阴离子嵌入和脱嵌过程在含LiFSI和LiTFSI的HCEs中循环的石墨电极上产生的表面层很少或极其薄，甚至可以探测到近表面主体中嵌入的阴离子。此外，拉曼光谱、扫描电子显微镜和透射电子显微镜表征表明，无论盐浓度如何，在基于LiPF的电解质中循环的石墨颗粒上都存在厚涂层，而在浓LiFSI和LiTFSI电解质的情况下几乎没有观察到任何表面膜。
