We seek to establish how Li+ intercalation affects the atomic and electronic structure of Li-Mn-O spinels used in Li secondary batteries. We use a variety of XAS and electrochemical techniques with the intent of developing a solid-state reaction mechanism based upon the aforementioned types of structural change. This information will be used to identify beneficial material parameters providing increased capacity, improved cyclability, and/or higher rate capacities.
MnO2 spinel-based electrodes are the most promising for Li+-intercalation batteries when considering a combination of specific energy, cost, availability, toxicity, and electrode potential. However, batteries derived from these materials display reaction-rate limitations (which affect the battery specific power) and capacity fading that circumvent their present usefulness and commercial viability.
Upon Li+ intercalation, the Mn sites in MnO2-spinel based host materials are reduced from Mn(IV) to Mn(III), and the structure changes to accommodate the Li intercalate at empty tetrahedral or octahedral sites. Therefore, changes in the atomic structure as well as the Mn 3d states occur during the reaction. Previous research on MnO2-spinel based electrodes materials combined electrochemical characterization with structural information obtained from X-ray and/or neutron diffraction studies. Investigators found that, upon discharging, when the average Mn-oxidation state is reduced below 3.5 (x > 1.0 in LixMn2O4), a cooperative Jahn-Teller distortion takes place, and lowers the spinel’s symmetry from cubic to tetragonal symmetry and expands the unit cell volume by 5-6%. Measruements of the open-circuit voltage as a function of Li+ content in LixMn2O4 for 0 < x < 2 show two voltage plateaus when the Li+ content is varied over the full range, the 4 volt plateau for 0 < x< 1, and the 3 volt plateau for 1 < x < 1.8. Introduction of dopants and cation vacancies into LiMn2O4 has yielded improved Li+ intercalation properties. Dopants were shown to be superior to cation vacancies for improving cell cycling stability. Due to the nature of XRD and neutron diffraction techniques, these studies have provided a long-range atomic structural picture as well as an indirect approach to interpreting electronic structure information.
We use electrochemical characterization techniques to help interpret XAS spectra. The electrochemical techniques are cyclic voltammetry, repetitive galvanostatic cycling, and potential-step voltammetry. The XAS techniques employed in collaboration with Prof. S. Cramer of UC Davis can be grouped into three main categories: 1) X-ray Absorption Near Edge Spectroscopy (XANES), 2) Extended X-ray Absorption Fine Structure (EXAFS), and 3) Kb Emission Spectroscopy. K-edge XANES and EXAFS give information on the local atomic structure about the absorbing atom, which can also be used to interpret the electronic configuration of the absorber. The absorbing atom's electronic structure is directly determinable by Kb Emission Spectroscopy and L2,3-edge XANES as these techniques detect transitions involving the absorbing element's valence states.
We have characterized the base material LiMn2O4. Physical & chemical characterization of this material included atomic absorption, B.E.T. surface area, XRD, and SEM. Electrochemical characterization was carried out in swagelok-type cells within a He glove box and included galvanostatic cycling, cyclic voltammetry, and potential step voltammetry. Additionally, a series of XAS measurements was performed on electrochemically intercalated compositions LixMn2O4, 0 < x < 2.2. The electrochemical intercalation was performed slowly and the electrodes were allowed to reach open-circuit potentials after the calculated charge was passed to achieve the desired lithiated state, x. The XAS measurements included K-edge XANES & EXAFS, Kb emission, and L2,3-edge absorption. We measured the Mn K-edge XANES data set. Along the 4 volt plateau, the spectra all correspond to octahedral symmetry of the oxygen ligands about the Mn. Small distortions of the Mn local environment occur due to Li+ intercalation and are reflected by the inflection seen in the edge spectra. The observations are consistent with a cubic spinel structure. However, along the 3V plateau a step develops in the absorption edge which is indicative of octahedral distortion caused by the Jahn-Teller effect and the resultant tetragonal symmetry of the highly lithiated phase. The Mn K-edge main peak result primarily from 1s ` 4p transitions. As the Jahn-Teller distortion occurs, the oxygen octahedra surrounding the Mn elongate along the z-axis. The larger Mn-O distance along the z-axis splits the degeneracy of the Mn 4p states by lowering the energy of the 4pz orbital relative to the 4px and 4py giving the observed step in the XANES.
Doped spinels and related structures of the Li-Mn-O system have been synthesized, Li+ intercalated within electrochemical cells, and analyzed by the above described XAS techniques. Atomic and electronic structural changes can then be correlated with electrochemical performance via the techniques described above to identify how the modified spinels behave compared to LiMn2O4. Based on this information, spinels with alternate dopant concentration or type can be synthesized to obtain the parameters for optimum cell performance.
We expect to obtain an atomic-level understanding of how the Mn 3d orbitals adapt to Li+ intercalation in various MnO2-spinel materials, and to establish relationships between the electrochemical information, the L2,3-edge, and K-edge XAS results. This information should allow us to deduce the important compositional and structural properties necessary for the most complete and reversible reactions of lithium with MnO2-spinel and related materials and point the way towards synthesizing improved battery materials.