Nuclear magnetic resonance (NMR) is a spectroscopic technique for characterizing the chemical environment of spin-bearing nuclei. In particular, 7LiNMR can be used to study the mechanisms at work in battery electrodes from the nanometric to the microscopic scale.
For example, in a material such as LiVPO4F, which appears well-ordered from the point of view of XRD and TEM, the 7Lispectrum shows that 10-20 % of the lithiums belong to a site different from the expected crystallographic site. Two-dimensional dipole recoupling correlation experiments show that these sites are spatially adjacent (separated by a distance of less than a nanometer) and that they are structural defects, the nature of which can be explained by DFT chemical shift calculations. In lithium-mobile materials such as Li3V2( PO4)3, the chemical exchange between the three sites of the structure can be precisely measured by a 2D experiment(Exchange SpectrocopY), and the activation energy of each jump mechanism can be inversely correlated with the size of the space available between the oxygens for lithium to pass through. The use of magnetic field gradients enables the position of the spins to be encoded in their resonance frequency. This makes it possible to obtain spectroscopic images with the 7LiNMR spectrum in one dimension and the position of the emitting spins in the other, with a resolution of 100 mm, and to characterize battery charging and discharging in situ, showing, for example, that the evolution of the electrode is homogeneous for LiCoO2, but that the part of the Li4Ti5O12 electrode close to the separator tends to react first, no doubt due to competition between lithium diffusion through the electrode and its rapid consumption by Li4Ti5O12.
Field gradients can also be used to measure the self-diffusion coefficients of lithium ions or anions ( 19FNMR), particularly in solid electrolytes. Ion diffusion must take place over a length of the order of a millimetre, to obtain transport numbers or the relative contribution of cations and anions to conductivity. In block copolymer electrolytes such as PS-PEO-PS, diffusion coefficient measurements provide information on the tortuosity of the PEO network. Numerous developments in NMR instrumentation(ultra-fast MAS, micro-coils, etc.) will further increase its scope of application, particularly in the field of battery materials.
