wang hay kan - Direct synthesis of nanocrystalline Li0.90FePO4: observation of phase segregation of anti-site defects on delithiation

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  Shri-Prakash Badi, Marnix Wagemaker, Brian L. Ellis, Deepak P.   Singh, Wouter J. H. Borghols, Wang Hay Kan, D. H. Ryan, Fokko M.   Mulder and Linda F. Nazar

  J. Mater. Chem., 2011,   21, 10085-10093


  Solid solutions of   LixFePO4 are   of tremendous interest because of a proposed increase in ion   transport properties, but the formation of these solutions at   high temperatures is difficult if not impossible and direct   synthesis is difficult and rarely reported. Here we report   modified polyol syntheses which produce nanocrystalline   Li1−yFePO4  directly, where the maximum Li substoichiometry on the M1 site   sustained at synthesis temperatures of 320 °C is about 10%. High   target lithium vacancy concentrations promote the increase in   anti-site disorder of Li+ and   Fe2+, as this process is driven by   vacancy stabilization. Combined neutron and X-ray diffraction on   partial delithiated substoichiometric olivines reveals segregated   defect-free (where Li is extracted) and defect-ridden (where Li   remains) regions. This proves (1) that the anti-site defects   obstruct Li+ diffusion explaining the   detrimental electrochemistry and (2) that the anti-site defects   form clusters. Finally, preferential anisotropic strain   broadening in the bc-plane indicates the existence of a   coherent interface between the Li-poor and Li-rich phases. Along   with the size broadening upon delithiation this proves that in   nano-sized   LixFePO4 the   two phases coexist within a single particle, which is not   expected based on thermodynamics arguments due to the energy   penalty associated with the coherent interface. Thereby, these   results give important and unique insight and understanding in   the properties of nano sized LiFePO4.

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