In situ infrared subtractive normalized Fourier transform infrared spectroscopy (SNIFTIRS) experiments performed simultaneously with the electroreduction of oxygen on gold and platinum cathodes in LiPF 6 dimethyl sulfoxide (DMSO) electrolyte have shown that the solvent is stable with respect to nucleophilic attack by the electrogenerated superoxide radical anion. However, long-term experiments with KO 2 solutions in DMSO have shown a slow formation of dimethyl sulfone. Evidence of dimethyl sulfone formation by anodic oxidation of DMSO above 4.2 V (Li/Li + ) in the presence of trace water has been obtained on gold. On platinum, this unwanted reaction in the charging cycle of a lithium−air battery takes place at lower potentials, i.e., 3.5 V.

We have employed the rotating ring disk electrode (RRDE) technique to study the oxygen reduction reaction (ORR) on gold and glassy carbon cathodes in dimethyl sulfoxide (DMSO) electrolytes containing lithium salts. At the gold ring electrode at 3.0 V vs. Li/Li + (0.1 M LiPF 6 ) soluble superoxide radical anion undergoes oxidation to O 2 under convective-diffusion conditions. For both glassy carbon and gold cathodes, typical oxygen reduction current-potential curves are sensitive to rotation speed and undergo a maximum and further electrode passivation by formation of Li 2 O 2 while the Au ring electrode currents follow the same peak shape with detection of soluble superoxide at the ring downstream in the electrolyte solution. Unlike the behavior in acetonitrile-lithium solutions, LiO 2 is more stable in DMSO and can diffuse out in solution and be detected at the ring electrode. While in cyclic voltammetry both time and potential effects are convoluted, we have carried out RRDE chrono-amperometry experiments at the disk electrode with detection of superoxide at the Au ring so that thus potential and time effects were clearly separated. The superoxide oxidation ring currents exhibit a maximum at 2.2 V due to the interplay of O 2 − formation by one-electron O 2 reduction, Li 2 O 2 disproportionation and two-electron O 2 reduction.

The solid electrolyte interphase (SEI) is the most critical yet least understood component to guarantee stable and safe operation of a Li-ion cell. Herein, the early stages of SEI formation in a typical LiPF 6 and organic carbonate-based Li-ion electrolyte are explored by operando surface-enhanced Raman spectroscopy, on-line electrochemical mass spectrometry, and electrochemical quartz crystal microbalance. The electric double layer is directly observed to charge as Li + solvated by ethylene carbonate (EC) progressively accumulates at the negatively charged electrode surface. Further negative polarization triggers SEI formation, as evidenced by H 2 evolution and electrode mass deposition. Electrolyte impurities, HF and H 2 O, are reduced early and contribute in a multistep (electro)chemical process to an inorganic SEI layer rich in LiF and Li 2 CO 3 . This study is a model example of how a combination of highly surface-sensitive operando characterization techniques offers a step forward to understand interfacial phenomena in Li-ion batteries.

The challenges facing metal-air batteries have prompted fundamental studies of non-aqueous electrochemistry. However it appears that contributors in the field are not aware that the potentials of Li/Li + , Na/Na + , K/K + , and Mg/Mg 2+ electrodes depend on the nature of solvent due to the cation solvation. Therefore, it is imperative to define a clear potential scale that can be correlated in different solvents. Here we report on the strong effect of the solvent on the Li/Li + redox potential and discuss the use of the ferrocene/ferrocenium couple as internal or external standard for the measurements in non-aqueous solvents in lithium-ion and lithium-0 2 battery systems. © The Author The Li-air or Li-O 2 system has captured a scientific attention worldwide due to its high theoretical energy density, however many challenges in the electrochemistry of this system still remain to reach commercialization. [1][2][3][4] Those challenges have given rise to a fundamental understanding of the oxygen reduction reaction (ORR) mechanistic paths in lithium containing aprotic solvent systems. Interestingly, almost all electrochemical measurements relevant to the Li-air battery in the literature report a potential scale versus Li/Li + potential, very often without specifying the solvent, electrolyte salt and lithium concentration.Unlike in aqueous electrochemistry where the normal hydrogen electrode potential is the reference electrode of choice, in the lithium battery community the electrode potentials are referred to the Li/Li + system since in a lithium battery either the anode is Li metal or lithium intercalated in graphite with a redox potential very close to the metal Li/Li + electrode. Due to the strong solvation of the small lithium cation and the different electron donor capacity of different solvents, the electrode potential of Li/Li + couple strongly depends on the solvent used and it could vary by as much as half of a volt between dimethyl sulfoxide and acetonitrile. 5 Furthermore, in many reports while the potential scale is referred versus the Li/Li + , Li metal is not actually used as the reference electrode, but a different reference electrode such as Ag/Ag + is employed and then converted into the Li/Li + scale often without specifying how this was done. Current StatusBrowsing through the Li-air literature we can find that in 2009 Laoire et al. used aqueous Ag/AgCl reference electrode and repored their data versus this reference. However they also indicated that Ag/AgCl gives a potential of 2.93 V versus Li/Li + , as measured using a Li foil reference electrode in a LiPF 6 solution in organic carbonates. 6This is an example of the very rare case in the Li-air literature, where the results were reported versus actual reference employed as we can see later since almost all the literature refer to the Li/Li + scale. In 2010, the same authors used the Pt mesh as the reference electrode and reported their data versus Li/Li + . They argued that the Pt electrode was calibrated with reference to the ferrocenium ion/f...

In situ infrared subtractive normalized Fourier transform infrared spectroscopy (SNIFTIRS) experiments were performed simultaneously with the electrochemical experiments relevant to Li-air battery operation on gold cathodes in ionic liquid PYR14TFSI based electrolyte. Ionic liquid anion was found to be stable, while the cation PYR14+ was found to decompose in studied conditions. In oxygen saturated LiTFSI containing PYR14TFSI electrolyte carbon dioxide and water were formed at potential 4.3 V either with or without previous oxygen electro-reduction reaction. However in deoxygenated LiTFSI contacting ionic liquid no formation of CO2 or water was observed, suggesting oxygen presence to be crucial in carbon dioxide production.

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This review provides an accessible analysis of the processes on reference electrodes and their applications in Li-ion and next generation batteries research. It covers fundamentals and definitions as well as specific practical applications and is intended to be comprehensible for researchers in the battery field with diverse backgrounds. It covers fundamental concepts, such as two- and three-electrodes configurations, as well as more complex quasi- or pseudo- reference electrodes. The electrode potential and its dependance on the concentration of species and nature of solvents are explained in detail and supported by relevant examples. The solvent, in particular the cation solvation energy, contribution to the electrode potential is important and a largely unknown issue in most the battery research. This effect can be as high as half a volt for the Li/Li+ couple and we provide concrete examples of the battery systems where this effect must be taken into account. With this review, we aim to provide guidelines for the use and assessment of reference electrodes in the Li-ion and next generation batteries research that are comprehensive and accessible to an audience with a diverse scientific background.

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