Lithium Dynamics in ${\mathrm{LiMn}}_2{O}_4$ Probed Directly by Two-Dimensional ${}^7\mathrm{Li}$ NMR

${\mathrm{LiMn}}_2{\mathrm{O}}_4$ has been studied using magic-angle-spinning nuclear magnetic resonance (MAS NMR). 1D MAS NMR shows three Li resonances assigned to different crystallographic sites. At low temperatures an extra peak appears, indicating charge ordering of ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Mn}}^{4+}$. Direct observation of the lithium dynamics was possible using rotor-synchronized 2D exchange NMR. A millisecond time scale exchange of lithium starts around 285 K between the $8a$ and the $16c$ site. At 380 K lithium even starts to hop between more than two sites. The activation energies and Li jump rates are derived and are in agreement with those determined macroscopically.