Understanding the Microscopic Structure of a “Water-in-Salt” Lithium Ion Battery Electrolyte Probed with Ultrafast IR Spectroscopy

• Published in: Journal of physical chemistry. C Vol. 124; no. 16; pp. 8594 - 8604
• Main Authors: Zhang, Miaomiao, Hao, Hongxing, Zhou, Dexia, Duan, Yiyi, Wang, Yuan, Bian, Hongtao
• Format: Journal Article
• Language: English
• Published: American Chemical Society 23-04-2020
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.0c00937

“Water-in-salt” electrolytes have been demonstrated to have potential applications in the field of high-voltage aqueous lithium ion batteries (LIBs). However, the basic understanding of the structure and dynamics of the concentrated “water-in-salt” electrolytes at the molecular level is still lacking. In this report, the structural dynamics of the concentrated lithium bis­(tri­fluoro­methane sulfonyl)­imide (LiTFSI) aqueous solutions were investigated using Fourier transform infrared (FTIR) spectroscopy and ultrafast IR spectroscopy. The OD stretches of the water molecule and the thiocyanate (SCN–) anionic solute were utilized as the IR probes to monitor the local structure of the concentrated LiTFSI aqueous solutions. The concentration-dependent IR spectra and vibrational relaxation dynamics of the SCN– anion were systematically measured and analyzed. It was found that the vibrational population of the SCN– anion decayed at a much faster rate as the LiTFSI bulk concentration increased. From the static IR spectra and vibrational relaxation dynamics of SCN–, it was firmly confirmed that the SCN– anion interacted with the Li+ cations surrounded with domains of bulk-like water in the concentrated LiTFSI aqueous solutions. The rotational dynamics and spectral diffusion measurements of the SCN– anion further showed that the rotation of the SCN– anionic probe is strongly confined and restricted by the specific network structure formed by a framework of the TFSI– anions and the associated interfacial water molecules. Macroscopic conductivity and viscosity measurements were also performed for the LiTFSI aqueous solutions in order to elucidate a full picture of the liquid structure. The rotational dynamics of the SCN– anion decoupled from the viscosity of the solution are suggested to be segregated from the dynamics of the heterogeneous domains formed by the extended ion network of the TFSI– anions. All of these results demonstrated that the structural dynamics of the concentrated LiTFSI aqueous solutions can be revealed from the viewpoint of the anionic solute. And the heterogeneous structures in the LiTFSI solutions proposed by the molecular dynamic (MD) simulation were supported by experiment. The results in this work are expected to provide a basic understanding of the microscopic structure and transport mechanism of lithium ions in the water-in-salt electrolytes of LIBs.