| A study of the processing - structure - property - performance interrelationship was performed on several lithium manganese oxides that are candidates for use in lithium rechargeable batteries. The materials studied were LiMn 2O4, Mn-substituted LiMn2O4 (LiMn 2−yMeyO4), and Li1.5Na0.5 MnO2.85I0.12. Processing evaluations pertained to the substitution of Mn by other cations (Me = Li, Co, or Ni). Atomic and electronic structure was investigated using X-ray diffraction, FTIR, 7Li NMR, and a variety of X-ray spectroscopy techniques. The properties investigated included the voltage profile during charge and discharge in a Li metal cell along with the structural changes taking place upon lithium extraction or insertion. Performance evaluation consisted of cycling several of these materials within a Li metal cell.; Among the significant findings of this study are that a dynamic Jahn-Teller distortion takes place in LiMn2O4 and that the Li + inserted derivative (Li2Mn2O4) possess a significantly greater degree of covalency with respect Mn oxides with similar atomic structure. These results are used to construct a structural reaction mechanism for the Li+ extraction and insertion reactions in LiMn2O4 and the phase transformation that accompanies Li+ insertion. In contrast, evidence for increased covalency upon lithium insertion was not apparent in Li1.5Na0.5MnO 2.85I0.12, an amorphous material. Furthermore, it was found that the distortion of the [MnO6] octahedra occurring upon Li + insertion is reversible. This is consistent with the high degree of cycling stability in this material and attributable to the amorphous nature.; It was also found that the degree of covalency increases when substituting Mn with either Li, Co, or Ni. The amount of increased covalency depends on the degree of substitution and the substituent used. It was found that substitution lowers the discharge voltage along with affecting the degree of distortion and volume change that takes place during the cubic to tetragonal phase transformation concomitant with Li+ insertion. Assimilating these results with the increased cyclability of Mn substituted spinels reveals that lowering the discharge voltage and/or the volume change associated with the phase transformation will result in a spinel with improved performance.; The understanding gained in this dissertation leads to guidelines for engineering improved lithium rechargeable battery electrode materials. |
