The solid electrolyte interphase (SEI) is the most critical, yet least understood, component to guarantee a stable and safe operation of the Li-ion cell. Herein, the early stages of SEI formation in a typical LiPF₆ and organic carbonate based Li-ion electrolyte are explored by operando surface enhanced Raman spectroscopy (SERS), online electrochemical mass spectrometry (OEMS), and electrochemical quartz crystal microbalance (EQCM). The electric double-layer is directly observed to charge as Li⁺ solvated by EC progressively accumulates at the negatively charged electrode surface. Further negative polarization triggers SEI formation as evidenced by H₂ evolution and electrode mass deposition. Electrolyte impurities, HF and H₂O, are reduced early and contribute in a multistep electro-/chemical process to an inorganic SEI layer rich in LiF and Li₂CO₃. 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/Mg²⁺ 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-0₂ battery systems.

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.

Electrode/electrolyte interfaces are the most important and least understood components of Li-ion and next-generation batteries. An improved understanding of interphases in batteries will undoubtedly lead to breakthroughs in the field. Traditionally, evaluating those interphases involves using ex situ surface sensitive and/or imaging techniques. Due to their very dynamic and reactive nature, ex situ sample manipulation is undesirable. From this point of view, operando surface sensitive techniques represent a major opportunity to push boundaries in battery development. While numerous bulk spectroscopic, scattering, and imaging techniques are well established and widely used, surface sensitive operando techniques remain challenging and, to a larger extent, restricted to the model systems. Here, we give a perspective on techniques with the potential to characterize solid/liquid interfaces in both model and realistic battery configurations. The focus is on techniques that provide chemical and structural information at length and time scales relevant for the solid electrolyte interphase (SEI) formation and evolution, while also probing representative electrode areas. We highlight the following techniques: vibrational spectroscopy, X-ray photoelectron spectroscopy (XPS), neutron and X-ray reflectometry, and grazing incidence scattering techniques. Comprehensive overviews, as well as promises and challenges, of these techniques when used operando on battery interphases are discussed in detail.

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.