Sustainable Energy Technologies
"Iron-based cathode materials for lithium-ion batteries"
Presented by Akshaya Padhi
Monday, April 22, 2013, 11 am
Bldg. 815, 1st floor conference room
Hosted by: J. P. Looney
Lithium intercalation compounds such as layered rock salt oxides, LiCoO2 and LiNiO2, as well as, the manganese spinel framework system, LiMn2O4 (with M4+/M3+ redox couples, M = Co, Ni or Mn) are commonly used as cathode materials in rechargeable lithium batteries. However, future energy storage systems, especially for electric vehicles, necessitates search for materials that are safe, less expensive and environmentally benign. Iron-based materials present such an alternative. In LiFeO2, (isostructural with LiCoO2), the Fe4+/Fe3+ lies too far below and Fe3+/Fe2+ lies too close to the Li/Li+ couple, resulting in either a thermodynamically unstable or a low voltage system, respectively. This study is undertaken to understand various factors that affect the position of redox couples and to possibly tuning them to obtain a high voltage, yet a stable energy storage system. Lithium intercalation is performed on oxides containing tetrahedral polyanions and octahedral transition-metal cations in 3D-framework host structures to determine the position of the redox couples for different transition-metal cations with respect to the Fermi energy of lithium metal and understand how these positions vary with changes in host structure and/or choice of polyanion. The position of a redox couple, Mn+/M(n-1)+, of a metal cation in an oxide primarily depends on two factors: (a) the electrostatic Madelung stabilization energy of the structure, and (b) the covalent contribution to the M-O bonding, which may be modulated by a counter cation through the inductive effect. To understand these factors, a series of compounds with polyanions (XO4)y+ (X = S, P , y = 2 or 3) are investigated. The use of (PO4)3- and (SO4)2- are shown to stabilize the structure and lower the Fe3+/Fe2+ redox energy to useful levels. The presence of the polyanion XO4)y+ with strong X-O covalency stabilizes the antibonding Fe3+/Fe2+ state through an Fe-O-X inductive effect to generate an