Muon Spin Relaxation Study of Spinel Lithium Manganese Oxides

The muon spin relaxation technique has been used to study spinel lithium manganate insertion compounds. Two spinel compositions of lithium manganese oxide have been studied: mixed valence (MnIII/MnIV) LiMn2O4, where the lithium extraction process is mainly by redox reaction, and Li1.33Mn1.67O4, where all the manganese is MnIV and lithium extraction is a lithium by proton ion exchange. Extraction of lithium from the former gives a spinel manganese oxide without lithium or protons (λ-MnO2), while from the latter a protonated manganese oxide is obtained (H+-MnO2). Muons are quasi-static in these compounds, and they are located both in “regular” lithium and proton sites and in interstitial sites of the spinel structure, these latter being used during diffusion of lithium ions and protons. Results clearly reflect the presence of protons in H+-MnO2 and the movement of lithium ions in both lithium manganates. The lithium-rich phase Li1.33Mn1.67O4 is characterized by an increase of the number of implanted muons localized in interstitial sites, as well as by a decrease of the onset temperature for lithium diffusion.