Neutron and X-ray Diffraction Study of Pyrophosphate-Based Li2-xMP2O7 (M = Fe, Co) for Lithium Rechargeable Battery Electrodes

Structural characterization of Li2-xMP2O7 (M = Fe, Co) was carried out using neutron diffraction (ND) and X-ray diffraction (XRD) analyses to elucidate structural information and structural changes during an electrochemical reaction. The crystal system and space group were determined to be monoclinic P2(l)/c for both materials with a = 11.0192 (4) angstrom, b = 9.7488 (3) angstrom, c = 9.8057 (4) angstrom, and beta = 101.569 (3)degrees for Li2-xFeP2O7 and a = 10.9574 (3), b = 9.6921 (3), c = 9.7611 (3), and beta = 101.776 (2)degrees for Li2-xCoP2O7. XRD analysis revealed partial occupancy of iron and cobalt in the structures of Li2-xFeP2O7 and Li2-xCoP2O7, respectively. Also, ND identified lithium positions and partial occupancies in five different Li sites of Li2-xMP2O7 (M = Fe, Co). Further ex situ XRD showed that the charging/discharging of Li2-xFeP2O7 occurred primarily via a two-phase reaction with a slight solid solution behavior. We also demonstrated for the first time that Li2-xCoP2O7 electrodes are electrochemically active, with a redox potential of similar to 5 V (versus Li).