Investigating structural properties and reaction mechanism of non-stoichiometric spinel LNMO via solid state NMR

In this study, Li-6,Li-7 solid state nuclear magnetic resonance (ssNMR) has been used to characterize local disorder in LiNi0.5Mn1.5O4 (LNMO) spinel cathodes and to understand their implications for the phase transformation mechanism during electrochemical cycling vs. lithium. By comparing NMR spectra of LNMO samples with slightly varying transition metal stoichiometries, we demonstrate the high sensitivity of ssNMR in the indirect detection of transition metal order and its ability to identify disorder induced by stoichiometric variations. Secondly, we investigate the lithiation reaction mechanism of the non-stoichiometric transition metal-ordered LNMO spinel cathode by ex situ Li-7 NMR and highlight that its reaction process involves primarily two successive biphasic reactions. However, our results suggest that stoichiometrically induced transition metal disorder promotes a mixed solid solution and biphasic reaction mechanism. Besides an extended solid solution region, we evince that the biphasic reaction involves the presence of intermediate transient species that make it possible to bridge the miscibility gap between the Li-1 and Li-0.5 phases. These results go beyond our operando XRD results, underlining that local structural analysis by Li-7 ssNMR can provide valuable additional insights into the complex reaction mechanism that are difficult to access by other techniques.