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

Structural characterization of Li 2– x MP 2 O 7 (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 P 2 1 / c for both materials with a = 11.0192 (4) Å, b = 9.7488 (3) Å, c = 9.8057 (4) Å, and β = 101.569 (3)° for Li 2– x FeP 2 O 7 and a = 10.9574 (3), b = 9.6921 (3), c = 9.7611 (3), and β = 101.776 (2)° for Li 2– x CoP 2 O 7 . XRD analysis revealed partial occupancy of iron and cobalt in the structures of Li 2– x FeP 2 O 7 and Li 2– x CoP 2 O 7, respectively. Also, ND identified lithium positions and partial occupancies in five different Li sites of Li 2– x MP 2 O 7 (M = Fe, Co). Further ex situ XRD showed that the charging/discharging of Li 2– x FeP 2 O 7 occurred primarily via a two-phase reaction with a slight solid solution behavior. We also demonstrated for the first time that Li 2– x CoP 2 O 7 electrodes are electrochemically active, with a redox potential of ∼5 V (versus Li).