Correlating Ionic Conductivity, Oxygen Transport and ORR with Structure of Dialkylacetamide-Based Electrolytes for Lithium-Air Batteries

Abstract

Chemical structures of lithium and tetrabutylammonium (TBA) salt solutions in N,N-dimethylacetamide (DMAc) and N,N-diethylacetamide (DEAc), two high Donor Number organic solvents, have been studied. In LiX salt solutions (where X = PF 6 − , CF 3 SO 3 − , ClO 4 − and NO 3 − ), solvation occurs when the Li + bonds with the solvent's carbonyl group forming Li + [O=C(CH 3 )N(CH 3 ) 2 ] n X − ion pairs. Infrared and 13 C-NMR spectra are consistent with the ion pair being solvent-separated when the anion is PF 6 − , ClO 4 − or NO 3 − , and a contact ion pair in the case of CF 3 SO 3 − . Chemical interactions between TBA + and the solvents to form conducting solutions appeared to be dipolar in nature. Ionic conductivities of TBA + and Li + electrolytes were measured and correlated with their viscosities. In 0.1M TBAPF 6 /DMAc, the O 2 solubility and diffusion coefficient (3.09 × 10 −6 mol/cm −3 and 5.09 × 10 −5 cm 2 s −1 , respectively) measured using microelectrode technique are typical of values measured in several TBA + solutions. Microelectrode voltammetry revealed steady-state limiting current behavior for oxygen reduction reactions (ORR) in TBAX/DMAc electrolytes indicating a reversible ORR process. Conversely, microelectrode current-voltage data for ORR in LiX/DMAc electrolytes revealed irreversible behavior mainly ascribed to the blockage of the electrode surface by insoluble ORR products. The ORR in DMAc correlated with its high Donor Number and the overall process conformed to the Hard-Soft Acid-Base theory.

Overlooked Factors Required for Electrolyte Solvents in Li–O2 Batteries: Capabilities of Quenching 1O2 and Forming Highly‐Decomposable Li2O2

“…These peaks disappeared after the cells were charged. A peak attributed to Li 2 CO 3 was also observed in the pattern for the sample discharged in the DMA‐based electrolyte, in agreement with previously reported results [19, 27, 28] …”

“…A peak attributed to Li 2 CO 3 was also observed in the pattern for the sample discharged in the DMA-based electrolyte, in agreement with previously reported results. [19,27,28] The cathode surfaces after discharge were subsequently inspected by scanning electron microscopy (SEM). When a porous carbon electrode based on Ketjenblack (KB) was used, the Li 2 O 2 deposits could not be identified because the sizes of the single KB particles and the deposited Li 2 O 2 crystals were similar (Figure S5).…”

Understanding the role of electrolytes and electrodes on Li-air batteries through molecular dynamics simulation

“…Lithium ions are highly coordinated with four oxygens in DMSO solutions of LiPF 6 . This result is in agreement with the observation that lithium ion is solvated by four DMSO molecules forming Li + (DMSO) 4 PF -6 ion pair 117,118 . Yamada et al 10 have also shown, with Raman spectroscopy, the Li + (DMSO) 4 coordination in 1 mol•kg −1 solution, as can be seen in 3.11 (a).…”

Design Rules for Selecting Fluorinated Linear Organic Solvents for Li Metal Batteries

“…Significant efforts have been focused on exploring the large design space of liquid electrolytes − for improved operational temperature window, − electrochemical stability, , ionic transport property, − and the stability of electrode–electrolyte interfaces. , In particular, modifying liquid solvents using fluorination is considered one of the most convenient and scalable manufacturing methods. , The benefits of using the fluorinated solvents are three-fold. First, the fluorine substitution reduces the highest occupied molecular orbital (HOMO), which is necessary but not sufficient descriptor for the oxidative stability of the electrolyte. , The decreased HOMO level increases oxidation stability, , thus broadening the allowed open-circuit threshold for high-voltage cathodes. , Second, the simultaneous decrease in LUMO causes the fluorinated organic solvents to reduce before their nonfluorinated prototypes …”