Structural polymorphism in Li₂CoSiO₄ intercalation electrodes: a combined diffraction and NMR study

Li2CoSiO4 was prepared in three polymorphic forms. The βII (Pmn21) polymorph was obtained by hydrothermal synthesis (150 °C), and subsequent heat treatments yielded the βI (Pbn21) form (700 °C) and the γ0 (P21/n) form (1100° then quenching from 850 °C). Rietveld refinement of X-ray and neutron powder diffraction patterns reveal considerable Li/Co mixing for βII, very moderate mixing for βI, and no mixing for γ0. 7Li MAS NMR spectra have been recorded for the three forms. The mechanism of the Fermi contact interaction that leads to negatively shifted signals is as yet unexplained, but the nature and the number of signals were analyzed in relation to the site occupancies for each compound. The agreement is good for βII, although the extent of disorder leads to very poorly defined NMR signals; it is reasonable (although not fully quantitative) for βI, where well-defined NMR signals can be assigned to definite environments; finally, the γ0 sample surprisingly leads to a single rather broad NMR signal, whereas two well-defined and rather different environments are present in the structure deduced from diffraction