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

Abstract

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.

Silicate cathodes for lithium batteries: alternatives to phosphates?

“…1(d) The third member of the orthosilicate family of cathode materials, Li 2 CoSiO 4 , has received comparatively less attention. [32][33][34] Three polymorphic forms (b II , b I , and g 0 ) have been prepared and investigated with powder diffraction and Li MAS NMR. 32,33 The b II (Pmn2 1 ) polymorph was obtained by hydrothermal synthesis (150 C), and subsequent heat treatments yielded the b I (Pbn2 1 ) form (700 C) and the g 0 (P2/n) form (1100 C then quenching from 850 C).…”

“…[32][33][34] Three polymorphic forms (b II , b I , and g 0 ) have been prepared and investigated with powder diffraction and Li MAS NMR. 32,33 The b II (Pmn2 1 ) polymorph was obtained by hydrothermal synthesis (150 C), and subsequent heat treatments yielded the b I (Pbn2 1 ) form (700 C) and the g 0 (P2/n) form (1100 C then quenching from 850 C). Rietveld refinement of X-ray and neutron powder diffraction patterns reveal antisite disorder for b II with Li on the Co site and Co sharing the Li site (essentially an inverse b II structure analogous to inverse spinels), very moderate Li/Co mixing for b I , and no mixing for g 0 .…”

“…7 Li MAS NMR spectra have been recorded for the three forms, and the nature and number of signals were analyzed in relation to the site occupancies for each compound. 33 …”

Hydrothermal Synthesis and Electrochemical Properties of Li₂Fe<i>_x</i>Mn<i>_x</i>Co_1&minus;2<i>_x</i>SiO₄/C Cathode Materials for Lithium-ion Batteries

“…All the diffraction lines were indexed on the basis of the orthorhombic unit cell with the Pmn2 1 space group symmetry. 21,22 The Miller indices for some of the most intense reflections are noted in Fig. 2.…”

Structural Phase Transition of Li₂MnSiO₄

“…5,6) The homogeneity of the ordered structure may have significant effect on the electrode properties. Several investigations on the details of polymorphism have been reported [7][8][9][10][11][12][13] , mainly for the hydro-thermally prepared and heat-treated samples. However, the phase relations among polymorphs and the microstructures are still unclear.…”