Structural Polymorphism in Li₂CoSiO₄ Intercalation Electrodes: A Combined Diffraction and NMR Study

Li2CoSiO4 was prepared in three polymorphic forms. The beta(II) (Pmn2(1)) polymorph was obtained by hydrothermal synthesis (150 degrees C), and subsequent heat treatments yielded the beta(1) (Pbn2(1)) form (700 degrees C) and the gamma(0) (P-2,/n) form (1100 degrees then quenching from 850 degrees C). Rietveld refinement of X-ray and neutron powder diffraction patterns reveal considerable Li/Co mixing for beta(II), very moderate mixing for beta(I), and no mixing for gamma(0) Li-7 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 beta(II), although the extent of disorder leads to very poorly defined NMR signals, it is reasonable (although not fully quantitative) for beta(I), where well-defined NMR signals can be assigned to definite environments, finally, the gamma(0) 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.