For energy storage, nowadays, Li-ion and Na-ion batteries are major technologies for mobility applications and also for large-scale storage of energy and their integration in the grid. In the scope of finding new positive electrode materials with improved performances, the deep understanding of the link between their structure, electronic structure and electrochemical behavior is crucial. As the presence of defects or disorder may play a critical role, a local characterization of the materials is highly required. To that extent, Magic Angle Spinning Nuclear Magnetic Resonance (MAS-NMR) appeared to be a key tool. For paramagnetic materials, it allows to probe both, the local structure and the local electronic structure thanks to the Fermi contact interaction. In order to assign the signals and understand the spin transfer mechanism through the chemical bonds, we have been developed the use of ab initio calculations for some years.

Some recent studies of the characterization of defects or disorder in layered oxides or V-phosphate materials will be presented. Using MAS-NMR we showed that several phosphate materials as LiVPO₄F, or Na₃V₂(PO₄)₂F₃, which are promising materials for positive electrode application in Li-ion and Na-ion batteries respectively, exhibit some O-defects leading to the formation of V⁴⁺ ions locally. These V⁴⁺ ions are forming a vanadyl-type bond with the defect O, and affect the electrochemical cycling performances.