Research Article pubs.acs.org/journal/ascecg
Oxygen Reduction Reactions in Aprotic Ionic Liquids Based Mixed Electrolytes for High Performance of Li−O2 Batteries Asim Khan and Chuan Zhao* School of Chemistry, The University of New South Wales, Sydney, NSW 2052, Australia S Supporting Information *
ABSTRACT: The kinetics and mechanisms of oxygen reduction reactions (ORRs) and oxygen evolution reactions (OERs) in the absence and presence of Li+ have been systematically investigated in five ionic liquid (IL)/dimethyl sulfoxide (DMSO) mixed electrolytes for potential applications for high performance Li−O2 batteries. The diffusion coefficient and solubility of oxygen in these mixed electrolytes are determined by nonlinear fitting of potential-step chronoamperometry obtained at a platinum microelectrode. The mixed electrolytes exhibit enhanced oxygen diffusion coefficient (6.2−11.3 × 10−6 cm2 s−1), compared to neat ILs, and solubility (2.9−6.8 mM), compared to DMSO. Cyclic voltammetry (CV) shows that all the IL-mixed electrolytes display good stability against superoxide (O2•−) except for 1-butyl-3-methyl-pyridinium bis(trifluoromethylsulfonyl)imide [BMPy][NTf2], where the generated superoxide is found to be unstable and reacts with the cations of the IL. Using digital simulation of cyclic voltammograms for ORR, the heterogeneous rate constant, khet, for reduction of O2 to O2•− in these mixed electrolytes is obtained (khet = 3.6−3.9 × 10−3 cm s−1). The presence of Li+ in the mixed electrolytes shows minor influence on the khet. The cations of ILs show strong interaction with O2•− thus stabilizing the ORR products. The role of anions of ILs on the stability of electrolytes and reversibility of ORR−OER processes is also investigated. KEYWORDS: Oxygen reduction reactions, Oxygen evolution reactions, Ionic liquids, Li−O2 batteries, Electrochemical energy conversion and storage
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solvents like dimethyl sulfoxide (DMSO),10,11 acetonitrile,4,10,12 and dimethylformamide (DMF).10,12 DMSO has displayed better stability against reduction products.13,14 However, there is still some decomposition of DMSO takes place which forms different side products and accumulates onto the carbon cathode during charge/discharge thus decreasing the cycling efficiency of Li−O2 batteries.15−17 Additionally, the formation of unstable solid electrolyte interphase (SEI) by DMSO in contact with Li metal results in poor cycling efficiency. Stable formation of SEI is important for the better rechargeability of Li anode.18,19 Because of their unique properties like good thermal stability, good conductivity, low vapor pressure, and wide electrochemical potential window,20 ionic liquids (ILs) have drawn attentions as an electrolyte for Li−O2 batteries.21−23 Several studies on oxygen (O2) solubility, diffusion coefficients, and kinetics and mechanism of ORR in pure ILs have been reported in detail.3,24−27 The addition of Li+ in ILs has shown a significant effect on the ORR mechanism making it electro-
INTRODUCTION The rapid depletion of fossil fuel reserves and concerns about environmental pollution and global warming have promoted the need for alternative, renewable, and clean energy sources. Li−O2 batteries are promising candidates for future energy storage and conversion, and can offer high energy densities at relatively low cost. However, the technology is still at research and development stages and extensive efforts are required to address problems related to discharge/charge cycles in order to make it commercially successful.1,2 In Li−O2 batteries, the cathode reaction, i.e. the oxygen reduction reaction (ORR), is of great importance and is greatly influenced by the type of electrocatalysts and electrolytes.3 In particular, stability of the electrolyte toward the generated ORR species is crucial for the rechargeable nonaqueous Li−O2 battery. In aprotic electrolyte based Li−O2 batteries, it is supposed that Li2O2 is formed at the cathode during discharge, which is oxidized back to Li+ and O2 on charge.4 The spectroelectrochemical analyses have shown that the common electrolytes, like organic carbonates and ethers, are not suitable for Li−O2 batteries as they are decomposed by the reduction products to form side products, such as Li2CO3 and lithium alkyl carbonates.5−8 Better understanding of the ORR mechanism and kinetics in nonaqueous electrolytes will be helpful for the formation of desired products during the operation of a typical Li−O2 battery.9 The ORR has been extensively studied in organic © XXXX American Chemical Society
Special Issue: Ionic Liquids at the Interface of Chemistry and Engineering Received: August 17, 2015 Revised: September 23, 2015
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DOI: 10.1021/acssuschemeng.5b00890 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering chemically irreversible.28−30 Allen et al.31 investigated the ORR in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][NTf2]) and 1-methyl-1-butylpyrrolidinium bis(trifluoromethylsulfonyl)imide [BMP][NTf2] in the presence of Li salt. The correlation between the O2 reduction products and cations of ILs has been explained in terms of the Lewis acidity scale, derived from the 13C NMR chemical shifts and spin−lattice relaxation times of 13CO. Herranz et al.32 have used rotating ring disk electrode (RRDE) voltammetry to quantify the stability of superoxide in propylene carbonate (PC) and [BMP][NTf2]. The [BMP][NTf2] was found to be stable against the attack of superoxide while PC showed poor stability as evidenced by the three-order magnitude slower rate constant for [BMP][NTf2]. However, the remarkably slow diffusion of O2 in IL could result in the poor discharge capacity of Li−O2.29,33,34 Mixing the common organic solvents (PC, tetraethylene glycol dimethyl ether (TEGDME), and DMSO, etc.) with ILs has shown to be a promising approach for minimizing the drawbacks of each solvent and combining the best features of each electrolyte.35,36 Li et al.36 have investigated the effect of complexing cations on the ORR in DMSO. The addition of complexing cations in DMSO resulted in a 6-fold enhancement in the heterogeneous rate of the reduction O2 to O2•− and promoted the further reduction to O22−, both in the presence and absence of Li+. Cecchetto et al.37 reported a mixed electrolyte of [BMP][NTf2]/TEGDME. Additions of [BMP][NTf2] improved the conductivity, kinetics, and reversibility of the ORR and lowered the overpotential for the charge reactions. Recently, we reported a mixed electrolyte based on [BMP][NTf2] and DMSO. It was found that mixing [BMP][NTf2] with DMSO can combine advantages of each solvent and results in enhanced O2 solubility, enhanced current density, and better cycling of the oxidation−reduction processes.35 However, relation between the structures of ILs and the properties of the mixed electrolytes such as oxygen solubility and diffusivity and their impact on the ORR and the reversibility of the ORR−OER processes in the presence of Li+ are yet to be established. Here, we carried out a systematic study of ORR−OER in a range of ILs based electrolytes mixed with DMSO. Aprotic ILs containing different combinations of cations and anions (Scheme 1) are combined with DMSO. The O2 solubility, diffusion coefficient and electron transfer rates have been determined, for the first time, in these ionic liquid based mixed electrolytes using chronoamperometry, cyclic voltammetry and digital simulation. Furthermore, the ORR−OER processes are studied in the presence of Li+ and the role of ILs on the stability
of electrolytes and discharge/charge cycling ability of ORR− OER is established.
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EXPERIMENTAL SECTION
Materials. 1-Butyl-3-methyl-imidazolium tetrafluoroborate ([BMIM][BF4], >99%), 1-butyl-3-methyl-imidazolium hexafluorophosphate ([BMIM][PF6]) (>99%), 1-butyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide ([BMIM][NTf2]) (>99%), 1-butyl3-methyl-pyridinium bis(trifluoromethylsulfonyl)imide ([BMPy][NTf2]) (>99%), 1-butyl-1-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP][NTf2]) (99%), and lithium bis(trifluoromethylsulfonyl)imide (LiNTf2) (99%) were purchased from io-li-tec (Germany). Anhydrous grade DMSO (99.9%), acetonitrile (CH 3 CN), lithium hexafluorophosphate (LiPF 6 ) (>99.9% battery grade), and tetrabutylammonium hexafluorophosphate (TBAPF6) (>99.0%) were purchased from Sigma-Aldrich (Australia). Ferrocene ((C6H5)2Fe) was obtained from BDH (Australia). All the chemicals were stored in an MBraun glovebox filled with argon (Ar) with moisture and O2 levels
