Bioactive sol-gel glasses at the atomic scale: the complementary use of advanced probe and computer modelling methods

Sol-gel synthesised bioactive glasses may be formed via a hydrolysis condensation reaction, silica being introduced in the form of tetraethyl orthosilicate (TEOS) and calcium is typically added in the form of calcium nitrate. The synthesis reaction proceeds in an aqueous environment; the resultant gel is dried, before stabilisation by heat treatment. These materials, being amorphous, are complex at the level of their atomic-scale structure, but their bulk properties may only be properly understood on the basis of that structural insight. Thus, a full understanding of their structure : property relationship may only be achieved through the application of a coherent suite of leading-edge experimental probes, coupled with the cogent use of advanced computer simulation methods. Using as an exemplar a calcia-silica sol-gel glass of the kind developed by Larry Hench, to whose memory this paper is dedicated, we illustrate the successful use of high-energy x-ray and neutron scattering (diffraction) methods, magic-angle spinning solid state NMR, and molecular dynamics simulation as components to a powerful methodology for the study of amorphous materials

Anion Redox Chemistry in the Cobalt Free 3d Transition Metal Oxide Intercalation Electrode Li[Li_0.2Ni_0.2Mn_0.6]O₂

Conventional intercalation cathodes for lithium batteries store charge in redox reactions associated with the transition metal cations, e.g., Mn^3+/4+ in LiMn₂O₄, and this limits the energy storage of Li-ion batteries. Compounds such as Li­[Li_0.2Ni_0.2­Mn_0.6]­O₂ exhibit a capacity to store charge in excess of the transition metal redox reactions. The additional capacity occurs at and above 4.5 V versus Li⁺/Li. The capacity at 4.5 V is dominated by oxidation of the O^2– anions accounting for ∼0.43 e^–/formula unit, with an additional 0.06 e^–/formula unit being associated with O loss from the lattice. In contrast, the capacity above 4.5 V is mainly O loss, ∼0.08 e^–/formula. The O redox reaction involves the formation of localized hole states on O during charge, which are located on O coordinated by (Mn⁴⁺/Li⁺). The results have been obtained by combining operando electrochemical mass spec on ¹⁸O labeled Li­[Li_0.2Ni_0.2­Mn_0.6]­O₂ with XANES, soft X-ray spectroscopy, resonant inelastic X-ray spectroscopy, and Raman spectroscopy. Finally the general features of O redox are described with discussion about the role of comparatively ionic (less covalent) 3d metal–oxygen interaction on anion redox in lithium rich cathode materials