A Study of the Influence of Lithium Salt Anions on Oxygen Reduction Reactions in Li-Air Batteries

Abstract

The influence of lithium salts on O 2 reduction reactions (ORR) in 1, 2-dimethoxyethane (DME) and tetraethylene glycol dimethyl ether (TEGDME) has been investigated. Microelectrode studies in a series of tetrabutylammonium salt (TBA salt)/DME-based electrolytes showed that O 2 solubility and diffusion coefficient are not significantly affected by the electrolyte anion. The ORR voltammograms on microelectrodes in these electrolytes exhibited steady-state limiting current behavior. In contrast, peak-shaped voltammograms were observed in Li + -conducting electrolytes suggesting a reduction of the effective electrode area by passivating ORR products as well as migration-diffusion control of the reactants at the microelectrode. FT-IR spectra have revealed that Li + ions are solvated to form solvent separated ion pairs of the type Li + (DME) n PF 6 − and Li + (TEGDME)PF 6 − in LiPF 6 -based electrolytes. On the other hand, the contact ion pairs (DME) m Li + (CF 3 SO 3 − ) and(TEGDME)Li + (CF 3 SO 3 − ) appear to form in LiSO 3 CF 3 -containing electrolytes. In the LiSO 3 CF 3 -based electrolytes the initial ORR product, superoxide (O 2 − ), is stabilized in solution by forming [(DME) m-1 (O 2 − )]Li + (CF 3 SO 3 − ) and [(TEGDME)(O 2 − )]Li + (CF 3 SO 3 − ) complexes. These soluble superoxide complexes are able to diffuse away from the electrode surface reaction sites to the bulk electrolyte in the electrode pores where they decompose to form Li 2 O 2 . This explains the higher capacity obtained in Li/O 2 cells utilizing LiCF 3 SO 3 /TEGDME electrolytes.The Li-air battery with a theoretical energy density many times higher than that of Li-ion batteries, 1 has gained world-wide attention as a potential energy source for the development of long range electrical vehicles as well as many other energy hungry applications in the defense and civilian sectors. The promise of the high theoretical energy density has not yet been translated into practical devices due to the shortcomings of the electrode and electrolyte materials used in the battery as well as an incomplete understanding of the chemical and electrochemical processes involved in its operation. In our previous publications we have shown the strong effect of nonaqueous solvents on the oxygen reduction reaction (ORR) electrochemistry in Li + -containing electrolytes. [2][3][4][5][6][7] We have also successfully employed a carbon microelectrode to quantify oxygen transport parameters and elucidate the influence of the Li salt on the reversibility of the ORR in tetrabutylammonium and lithium salt-containing electrolytes in dimethyl sulfoxide (DMSO). 8,9 It has become increasingly clear that the ORR in the non-aqueous Li-air battery is highly solvent controlled 10 with a secondary role of the Li salt through the solvates formed between the solvent and the salt. 11

Enhancing electrochemical intermediate solvation through electrolyte anion selection to increase nonaqueous Li–O 2 battery capacity

“…However, there is tremendous scope in tuning the electrolyte anion in low-DN solvents to obtain high discharge capacities. Given that it should be simpler to identify anions stable to the Li-O 2 cathode electrochemistry than high-DN solvents (36,37), anion selection in combination with low-DN solvents potentially provides a route to avoid the unfavorable capacity/stability trade-off observed in high-DN solvents, such as DMSO (14,15,38,39).…”

Effect of Anion in Glyme-based Electrolyte for Li-O2 Batteries: Stability/Solubility of Discharge Intermediate

“…[TFA] ¹ has high donor property, typically exhibits strong ion association, 3 and interacts strongly with lithium cations in the glyme-based electrolytes. 9 Upon association with [TFA] ¹ , the lithium cation becomes a weaker 3 (or softer 18 ) acid, and thus less likely to interact with O 2 •¹ . Therefore, as has been suggested previously, O 2…”

Effects of Li Salt Anions and O2Gas on Li Dissolution/Deposition Behavior at Li Metal Negative Electrode for Non-Aqueous Li-Air Batteries

“…16 These ethers have high oxygen solubility and relatively low electric constants, resulting in lower reactivity toward O 2 − radicals than that of carbonate-based electrolytes. 17 Hence, alternative ways of stabilizing the surface of Li metal NE in glyme electrolytes are currently in demand. Moreover, much more data is required on the Li dissolution/deposition behavior in glyme electrolytes for finding new stabilization methods to enable realization of next-generation LMBs, including LABs.…”