Delithiation/Lithiation Behavior of LiNi0.5Mn1.5O4 Studied by In Situ and Ex Situ 6,7Li NMR Spectroscopy

Delithiation and lithiation behaviors of ordered spinel LiNi 0.5 Mn 1.5 O 4 and disordered spinel LiNi 0.4 Mn 1.6 O 4 were investigated by using in situ (in operando) 7 Li NMR and ex situ 6 Li MAS NMR spectroscopy. The in situ 7 Li monitoring of the ordered spinel revealed a clear appearance and subsequent disappearance of a new signal from the well-defined phase Li 0.5 Ni 0.5 Mn 1.5 O 4, suggesting the two-phase reaction processes among Li 1.0 Ni 0.5 Mn 1.5 O 4, Li 0.5 Ni 0.5 Mn 1.5 O 4, and Li 0.0 Ni 0.5 Mn 1.5 O 4 . Also, for the disordered spinel, Li 0.5 Ni 0.4 Mn 1.6 O 4 was identified with a broad distribution in Li environment. High-resolution 6 Li MAS NMR spectra were also acquired for the delithiated and lithiated samples to understand the detailed local structure around Li ions. We suggested that the nominal Li-free phase Li 0.0 Ni 0.5 Mn 1.5 O 4 can accommodate a small amount of Li ions in its structure. The tetragonal phases Li 2.0 Ni 0.5 Mn 1.5 O 4 and Li 2.0 Ni 0.4 Mn 1.6 O 4, which occurred when the cell was discharged down to 2.0 V, were very different in the Li environment from each other. It is found that 6,7 Li NMR is highly sensitive not only to the Ni/Mn ordering in LiNi 0.5 Mn 1.5 O 4 but also to the valence changes of Ni and Mn on charge–discharge process.