Solid-state NMR studies of lithium manganese oxides cathode materials for rechargeable batteries

Lithium manganese oxides have been studied by 6Li and 7Li solid state NMR spectroscopy, X-ray, and neutron diffraction. Lithium manganese oxides are promising cathode materials for rechargeable batteries. However, they suffer from poor cycling behavior after repeated charging and discharging cycles. Knowledge concerning the local electronic structure is crucial to the understanding their electrochemical properties and the improvement of their battery performance. The NMR spectra of these paramagnetic materials are dominated by the Hyperfine interaction between lithium and the unpaired electron spins on the manganese. This coupling has been demonstrated to be sensitive to the local atomic and electronic structure. In the study of model compounds containing various Mn oxidation states from 3+ to 4+, distinctive Li NMR shifts have been obtained depending on the local coordination environment and the manganese oxidation state. For example, the Li cations in the octahedral site and the tetrahedral site are distinguished. The mechanism to rationalize the size and the direction of the shifts has been developed.; The information obtained from the model compounds can be used as a fingerprint to investigate more complicated systems including the materials during electrochemical cycling, various cation-substituted (Ni, Zn, Li) phases and oxygen-deficient derivatives. Two separate resonances are obtained for stoichiometric LiMn ₂O₄ during charging, which is consistent with two-phase behavior observed by X-ray diffraction. In contrast, a gradual shift in the peak position is observed for the metal doped spinels, indicating single-phase behavior. This must be correlated with the difference in the electronic band structure.; The cation defects, oxygen vacancy, and the metal-doping have been shown to perturb the Mn energy levels and create localized electron holes or electrons. This result in a new series of resonances in Li NMR spectra. At high temperature, this fine structure was observed to collapse due to the electron delocalization and/or the Li ion motion.; The charge ordering between Mn3+ and Mn4+ are observed at low temperature, for stoichiometric LiMn₂O ₄ and oxygen deficient materials. Various spin correlations have been investigated: two-dimensional and three-dimensional antiferromagnetic ordering and their transition, ferromagnetic coupling in LiZn_0.5Mn _1.5O₄, and ferrimagnetic ordering in LiNi_0.5Mn _1.5O₄.