To Which Extent Is Paramagnetic Solid-State NMR Able to Address Polymorphism in Complex Transition-Metal Oxides?

A detailed characterization of the polymorphs
constituting cathode materials, both before and after cell
cycling, is mandatory to develop more stable and powerful
lithium batteries. In many cases, e.g., for transition metal
lithium silicates, standard diffraction techniques cannot give a
clear-cut response. Here we show that broadband adiabatic
fast MAS NMR can give unique information in the case of
model Li2(Mn,Fe)SiO4 high-capacity cathode materials. By
coupling 7Li and 29Si 1D and 2D spectra, we are able to
address polymorphs speciation also in the mixed Mn/Fe
compositions, which is a nearly impossible task for X-rays and
neutrons diffraction. We finally discuss the conditions under
which this approach is useful when applied to rare nuclei such as 29Si.