Reassessing the structure of the sorbent with nominal composition Li$_{1.6}$Mn$_{1.6}$O$_4$ used for direct Lithium extraction

Lithium-rich manganese spinels have attracted sustained interest because they promise high lithium-ion-sieve capacity. Although the material with nominal composition Li$_{1.6}$Mn$_{1.6}$O$_4$ presents better performance than those belonging to the Li$_{(1+δ)}$Mn$_{(2-δ)}$O$_4$ series, yet its exact crystal chemistry and structure remains controversial. In the present experimental and theoretical study, the material was synthesized by spray-drying followed by calcination, obtaining the target stoichiometry in which Mn is present with the 4+ oxidation state. Rietveld refinements were performed to synchrotron X-Ray diffraction patterns using Full Prof and FAULTS programs to model stacking-fault-induced reflections broadening. The refinement revealed a two-phase system, formed by ~80 wt% of cubic spinel Li$_{1.33}$Mn$_{1.67}$O$_{4}$ and ~ 20 wt% of monoclinic-layered Li$_2$MnO$_3$, together reproducing the overall Li$_{1.6}$Mn$_{1.6}$O$_{4}$ composition. 7Li solid-state NMR confirmed that lithium occupies 8a and 16d sites in the spinel and the presence of the Li$_2$MnO$_3$ phase. Furthermore, density functional theory calculations revealed that a slight stoichiometric imbalance in the Li/Mn ratio around 1 in Li$_{1.6}$Mn$_{1.6}$O$_{4}$ could lead to metastable spinel phases, with Li$_2$MnO$_3$ as main secondary product.