The new battery electrode materials being developed today are extremely complex, and it is vital to be able to characterize the redox and structural changes taking place during their lifetime. One of the techniques of choice today is electron paramagnetic resonance (EPR) spectroscopy. EPR, or magnetic spectroscopy, is a major tool for characterizing paramagnetic centers containing a transition metal in model compounds and materials, as well as the active sites of metalloenzymes and organic radicals generated by auto-oxidation or photochemical processes. EPR information on the composition, structure or environment directly related to the paramagnetic center is obtained by analysis of the Landé factor (electronic structure and system geometry), hyperfine coupling, and sometimes by quadrupole coupling, coupling resulting from interactions between the electronic spin and the nuclear spin associated with either the ligand nuclei or those of its immediate environment, e.g. 1H(I = 1/2), 13C(I = 1/2), 14N(I = 1) 31P(I = 1/2), 19F(I = 1/2)..
The main aim of studying paramagnetic centers in EPR is therefore to determine the spin coupling values of the nuclei surrounding the paramagnetic center (hyperfine coupling and quadrupolar coupling). Precise determination of these values gives precise information on the chemical, electronic and molecular structure of the compound under study. Thus, for solid samples - powder or frozen solutions - which are referred to as polycrystalline or disordered, the EPR spectra obtained are envelopes containing a sum of elementary resonance lines corresponding to all possible orientations of the paramagnetic center with respect to the external magnetic field. The profile of an EPR spectrum is determined by two main parameters: the g tensor (Landé factor) and the hyperfine anisotropy tensor A, as well as the line width of the spectrum; non-zero quadrupolar interactions for nuclear spins I > 1/2 have second-order effects. The values of these different parameters are reflected in the EPR spectrum by the appearance of couplings.
Electron paramagnetic resonance (EPR) spectroscopy has proven its effectiveness in characterizing such paramagnetic centers, both in terms of quantifying them and determining their chemical nature. However, despite the wealth of information available from this spectroscopic tool, what about the distribution of these species in materials ? The recent development of EPR imaging has made it possible to spatially locate these centers, providing us with information on their spatial distribution and enabling us to distinguish between those of different natures.
