Delithiation/lithiation behavior of LiNi<inf>0.5</inf>Mn<inf>1.5</inf>O<inf>4</inf> studied by in situ and ex situ ^6,7Li NMR spectroscopy

Abstract

Delithiation and lithiation behaviors of ordered spinel LiNi<inf>0.5</inf>Mn<inf>1.5</inf>O<inf>4</inf> and disordered spinel LiNi<inf>0.4</inf>Mn<inf>1.6</inf>O<inf>4</inf> were investigated by using in situ (in operando) ⁷Li NMR and ex situ ⁶Li MAS NMR spectroscopy. The in situ ⁷Li monitoring of the ordered spinel revealed a clear appearance and subsequent disappearance of a new signal from the well-defined phase Li<inf>0.5</inf>Ni<inf>0.5</inf>Mn<inf>1.5</inf>O<inf>4</inf>, suggesting the two-phase reaction processes among Li<inf>1.0</inf>Ni<inf>0.5</inf>Mn<inf>1.5</inf>O<inf>4</inf>, Li<inf>0.5</inf>Ni<inf>0.5</inf>Mn<inf>1.5</inf>O<inf>4</inf>, and Li<inf>0.0</inf>Ni<inf>0.5</inf>Mn<inf>1.5</inf>O<inf>4</inf>. Also, for the disordered spinel, Li<inf>0.5</inf>Ni<inf>0.4</inf>Mn<inf>1.6</inf>O<inf>4</inf> was identified with a broad distribution in Li environment. High-resolution ⁶Li 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<inf>0.0</inf>Ni<inf>0.5</inf>Mn<inf>1.5</inf>O<inf>4</inf> can accommodate a small amount of Li ions in its structure. The tetragonal phases Li<inf>2.0</inf>Ni<inf>0.5</inf>Mn<inf>1.5</inf>O<inf>4</inf> and Li<inf>2.0</inf>Ni<inf>0.4</inf>Mn<inf>1.6</inf>O<inf>4</inf>, 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<inf>0.5</inf>Mn<inf>1.5</inf>O<inf>4</inf> but also to the valence changes of Ni and Mn on charge-discharge process.