Electrode/Electrolyte Interface Studies in Lithium Batteries Using NMR

Easy access to portable energy sources has become necessary for the last decades and rechargeable batteries are now omnipresent in everyday tools and devices thanks to their storage capacity and relatively low weight. More recently, the possible use of lithium-ion batteries in full electric and hybrid electric vehicles has attracted considerable attention. However, great challenges still remain in this area of research and the importance of interfaces has become especially obvious in the field of electrochemistry and its applications to energy storage devices.1,2 The solid electrolyte interphase (SEI) between the negative electrode and the electrolyte of a Li-ion battery is known to factor into the overall battery behavior in terms of irreversible capacity loss, charge transfer kinetics, and storage properties.3-6 More than ten years of research in this field have led to excellent control and optimization of the SEI layer on carbon electrodes. Surface formulation and/or coatings of the positive electrode have been shown more recently to influence the battery performance as well.7-11 Interfacial reactions and the growth of a passivation layer at the electrode surface upon cycling have been also highlighted for different positive electrode materials and have been identified to be of paramount importance as they can lead to performance degradation of the battery upon aging and cycling.12,13 The existence of surface reactions at the positive electrode/electrolyte interface has been clearly demonstrated but the experimental conditions of formation, growth and modification, as well as its subsequent influences on the electrochemical performance, remain unclear. The chemical, physical, and structural properties of the interfacial layer at the positive electrode in particular are still not well known, and thus this more recent research area is of great interest.14-18 This article gives an overview of the NMR approach developed to extract and interpret information on the electrode/ electrolyte interphase in lithium battery materials to probe evolution of electrochemical behavior and/or failure mechanisms along electrochemical cycling of lithium batteries. Such approach focuses on ex situ analysis of electrode materials. Recent studies have shown that in situ/operando NMR signals can be obtained during electrochemical processes19-21 but these works mainly focus on the interpretation of bulk electrochemical mechanisms and therefore, are not discussed in the present article. First are summarized the results of works dealing with the extraction and interpretation, from NMR data, of chemical information based on the chemical shift of various nuclei, and of quantitative information based on the integrated intensities of NMR signals. The theory required to interpret NMR spectra of paramagnetic samples and to extract information on the interaction of surface layers with the electrode material bulk is then briefly described in order to make it accessible to the non-NMR audience. An earlier review article gives a more comprehensive description of the chemical and physical information that can be extracted from the presence of unpaired localized electrons.22 This second part is then followed by illustrative examples obtained from stored or cycled electrode materials.