High-energy X-ray powder diffraction and atomic-pair distribution-function studies of charged/discharged structures in carbon-hybridized Li ₂MnSiO₄ nanoparticles as a cathode material for lithium-ion batteries

The stable cycling performance with a high discharge capacity of ∼190 mAh g^-1 in a carbon-hybridized Li₂MnSiO₄ nanostructured powder has prompted an experimental investigation of the charged/discharged structures using synchrotron-based and laboratory-based X-rays and atomic-pair distribution-function (PDF) analyses. A novel method of in-situ spray pyrolysis of a precursor solution with glucose as a carbon source enabled the successful synthesis of the carbon-hybridized Li ₂MnSiO₄ nanoparticles. The XRD patters of the discharged (lithiated) samples exhibit a long-range ordered structure characteristic of the (β) Li₂MnSiO₄ crystalline phase (space group Pmn2₁) which dissipates in the charged (delithiated) samples. However, upon discharging the long-range ordered structure recovers in each cycle. The disordered structure, according to the PDF analysis, is mainly due to local distortions of the MnO₄ tetrahedra which show a mean Mn-O nearest neighbor distance shorter than that of the long-range ordered phase. These results corroborate the notion of the smaller Mn³⁺/Mn ⁴⁺ ionic radii in the Li extracted phase versus the larger Mn ²⁺ ionic radius in Li inserted phase. Thus Li extraction/insertion drives the fluctuation between the disordered and the long-range ordered structures. © 2014 Elsevier B.V. All rights reserved.