Investigation of Solvation Structure and Battery Performance of Highly-Concentrated Aqueous Meo-TEMPO Catholyte
Authors: Yang Ruidong , Takechi Kensuke , Zhang Yong , Maginn Edward J. 
Published: February 2020

A type of high-energy-density redox-active catholyte using 4-methoxy-2,2,6,6-tetra-methypiperidine 1-oxyl (MT) as a redox compound has been reported. MT melts with the plasticizing lithium salt (lithium bis(trofluoromethanesulfonyl) imide or LT), and the molecules coordinate Li cations (Li+) forming “solvate ionic liquid”. With addition of water, the catholytes express improved properties (e.g. conductivity and viscosity), making them suitable for practical battery application. Nevertheless, no bulk water behaviors (such as icing) are observed in the differential scanning calorimetry (DSC) analysis at lower water content (< 29 wt%), implying the resulting aqueous catholytes maintain the nature of “solvate ionic liquid”. Because mechanistic understandings of the unique feature of the catholytes are very important to improve the performance of the battery, we investigated the solvation state in these liquids from both theoretical and experimental aspects.

Molecular simulation was conducted to understand the dynamic properties of the MT/LT/water mixtures. In the dry MT/LT mixture, Li+ are found to be adjacent to the MT ring, and TFSI- are surrounding the N-O• group. With the addition of water, Li+ does not have a well-defined preferred location but tends to be uniformly distributed at its bulk density. Water was observed to locate around the N-O• group, while the probability of finding TFSI- in the same area decreased significantly. A small amount of water is able to break up MT-TFSI- and MT-Li+associations. The maximum amount of water held up by MT molecules is found to be 5-6 to LT/MT. The liquid phase structure is also consistent with the self-diffusion coefficient calculated using the Einstein relation; a sharp increase in MT diffusivity occurs upon addition of water up to a water molar ratio of 5 and then it slows down thereafter.

The solvation structure of catholyte system is also investigated by Raman spectroscopy. Observed from the region of TFSI- vibration, the ordered contact ion pair of Li+ and TFSI- was interrupted by increased amount of water, and complete solvation was achieved at water ratio close to 6. Water is also found to solvate Li+ and the coordination number is around 4 even with the existence of MT in the liquid. On the N-O• group, three types of vibrations were discovered indicating complex interactions among MT, Li+ and water. By increasing the concentration of water, the amount of MT coupling with Li+was reduced, while increased fraction bonding with water was observed. The bonding capacity is determined at ~5, which shows good consistency with molecular simulation results.

Ionic conductivity and viscosity of the resulting liquids were measured and these properties are found to be greatly dependent on the water content. Higher amount of water is favorable to improve these physical properties. On the other hand, when employing these liquids as catholytes in lithium ion based batteries, usage efficiency and discharge voltage are found to more rapidly increase before water molar ratio reaches to ~5. MT-water interactions are believed to play a dominating role on the battery performance. The interactions reduce the stronger bonding forces between MT and the ions, resulting in faster diffusion of the redox active material (MT). The accelerated MT diffusion enhances the utility efficiency, and also reduces the distance of diffusion layer on the electrode resulting higher discharge voltages.