We demonstrate that a microelectrode can be used as a diagnostic tool to optimize the properties of electrolytes for non-aqueous Li-air batteries, and to elucidate the influence of ion-conducting salts on O 2 reduction reaction mechanisms. Oxygen reduction/evolution reactions on carbon microelectrode have been studied in dimethyl sulfoxide-based electrolytes containing Li + and tetrabutylammonium((C 4 H 9 ) 4 N + ) ions. Analysis of chronoamperometric current-time transients of the oxygen reduction reactions (ORR) in the series of tetrabutylammmonium (TBA) electrolytes, TBAPF 6 , TBAClO 4 , TBACF 3 SO 3 , TBAN(CF 3 SO 2 ) 2 in DMSO revealed that the anion of the salt exerts little influence on oxygen transport. Whereas steady-state ORR currents(sigmoidal-shaped) were observed in TBA-based electrolytes, peak-shaped current-voltage profiles were seen in the electrolytes containing their Li salt counterparts. The latter response results from the combined effects of the electrostatic repulsion of the superoxide intermediate as it is reduced further to peroxide (O 2 2− ) at low potentials, and the formation of passivation films at the electrode. Raman spectroscopic data confirmed the formation of Li 2 O 2 and Li 2 O on the microelectrode surface at different reduction potentials in Li salt solutions. Out of the four lithium electrolytes, namely LiPF 6 , LiClO 4 , LiCF 3 SO 3 , or LiN(CF 3 SO 2 ) 2 in DMSO, the LiCF 3 SO 3 /DMSO solution revealed the most favorable ORR kinetics and the least passivation of the electrode by ORR products.

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

The electrocatalysis of oxygen reduction reactions (ORR) in non-aqueous electrolytes is coupled to the ability of the solvents to modulate the Lewis acidity of Li + . This is accomplished through chemical interactions of Li + with the solvent to form acid-base complexes of the general formula, Li(solvent) n + , which determine the relative stability of the ORR intermediates and the final products formed. In high Donor Number solvents such as dimethyl sulfoxide (DMSO), the ORR proceeds via an outer Helmholtz plane (OHP) reaction pathway, conforming to a homogeneous catalysis of the reaction, irrespective of the presence of a catalyst in the cathode. In low Donor Number solvents exemplified by tetraethylene glycol dimethyl ether (TEGDME) and CH 3 CN, catalysts such as cobalt phthalocyanine (CoPC), Pt and Au promote heterogeneous electrocatalysis at the inner Helmholtz plane (IHP) of the electrical double layer on the electrode. The catalysis in this case involve the adsorption of O 2 as well as the ORR intermediates on the catalyst surface leading to lower activation energy of the reactions and increases in the discharge voltages of Li-air cells compared to uncatalyzed cells. The heterogeneous catalysis at the IHP may promote the full electrochemical reduction of O 2 to O 2− .

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Solid-phase catalysts prepared by pyrolysis of Iron(II) phthalocyanine (FePC) embedded in high-surface carbons were evaluated for the catalysis of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in Li +-conducting non-aqueous electrolytes. The ORR mechanism in high donor number (DN) dimethyl sulfoxide (DMSO)-based electrolytes is markedly different from that occurs in low DN acetonitrile(MeCN)-based electrolytes. The ORR is catalyzed by the reduced Fe(I) state of Fe(II)PC. Consequently, the Fe(II)PC/Fe(I)PC redox potential relative to O 2 reduction potential in each electrolyte is important for ORR catalysis. In MeCNbased electrolytes, the Fe(I)PC catalyst is formed at a higher potential than the ORR potential. Hence the catalyzed ORR occurs at the inner-Helmholtz plane of the electrode, stabilizing the superoxide ion (O 2 −) formed by one-electron reduction of O 2 , as Fe(I)PC-O 2 −. Indeed, LiO 2 was identified in the Raman spectra of cathodes from discharged Li-O 2 battery cells. In DMSO-based electrolytes, the Fe(I)PC formation potential occurs below the ORR potential and accordingly LiO 2 is more stable in its solvated state in the electrolyte solution as the Li(DMSO) nO 2 − ion pair. This drives the ORR at the outer-Helmholtz plane of both catalyzed and uncatalyzed electrodes in DMSO-based electrolytes. The FePC embedded carbon electrode doubled the cycle life of Li-O 2 cells utilizing low DN electrolytes.