|Institution:||University of Michigan|
|Keywords:||Electrochemical and local structure analysis of Li ion battery materials using X-ray technique.; Chemistry; Science|
|Full text PDF:||http://hdl.handle.net/2027.42/111390|
Efficient inexpensive energy storage is essential for widespread adoption of alternatives to fossil fuels. Lithium-ion batteries are a promising technology for energy storage. This dissertation describes the electrochemical and local structural analysis of the Li-ion cathode/anode materials Li4Ti5O12, LiMn2O4, Li3V2(PO4)3, and Mg-doped Li3V2(PO4)3) using synchrotron-based X-ray Absorption Spectroscopy (XAS). For the well-known anode material Li4Ti5O12, XAS provides an explanation for the ??????Zero-strain?????? characteristic of the material; there is, as expected, an increase in Ti-O distance reduction but no change in Ti-Ti distance, consistent with the added lithium pulling the oxide anions closer together, allowing the Ti-O distance to expand, with negligible change in the lattice parameter. XAS showed that when the spinel LiMn2O4 cathode is annealed, there is a slight increase in the average Mn oxidation state. In-situ XAS studies of another cathode material, Li3V2(PO4)3, showed the presence of significant kinetic effects such that the measured electrochemical behavior does not represent the bulk vanadium. There are only two distinct vanadium species when the cathode is cycled between 3.0 and 4.5 V. However, when the potential exceed 4.5 V a third vanadium species is formed, consistent with the formation of V5+. The data suggest that a portion of the V5+ can migrate to empty Li sites, indicating a possible explanation for capacity loss at high potential. If a portion of the V is replaced with Mg giving Li3V(2-2/3x)Mgx(PO4)3, the electrochemical performance, cycling retention and stability increase up to X~0.30. In-situ XAS of Li3V(2-2/3x)Mgx(PO4)3, x=0.15, 0.30, and 0.45 was used to characterize the structural and electronic consequences of Mg doping. In general, the doped cathodes behave similarly to the undoped parent material, with oxidation of V3+ to V4+ over 3 ??? 4.5 V and for the oxidation to V5+ at higher potential. As in the undoped material, we see substantial migration of V5+ to Li sites, and find evidence, especially for x = 0.15, that this tetrahedral V can be reduced to V4+ without returning to the octahedral V site. The most important consequence of doping seems to be a decrease in the kinetic lag that is seen in the undoped material.