Structure and ion dynamics of mechanosynthesized oxides and fluorides

Abstract

Abstract:In many cases, limitations in conventional synthesis routes hamper the accessibility to materials with properties that have been predicted by theory. For instance, metastable compounds with local non-equilibrium structures can hardly be accessed by solid-state preparation techniques often requiring high synthesis temperatures. Also other ways of preparation lead to the thermodynamically stable rather than metastable products. Fortunately, such hurdles can be overcome by mechanochemical synthesis. Mechanical treatment of two or three starting materials in high-energy ball mills enables the synthesis of not only new, metastable compounds but also of nanocrystalline materials with unusual or enhanced properties such as ion transport. In this short review we report about local structures and ion transport of oxides and fluorides mechanochemically prepared by high-energy ball-milling.

Multi-anionic and -cationic compounds: new high entropy materials for advanced Li-ion batteries

“…1b shows the 7 Li-and 19 F-MAS-NMR spectra of both Li(HEO)F and a physical mixture of HEO and LiF (see ref. [40][41][42] for reviews of 7 Li-and 19 F-NMR on energy-related materials). The incorporation of Li + and F À into the HEO leads to a different chemical environment compared to the LiF reference material.…”

Fluoride ion batteries – past, present, and future

“…This observation within tysonite-type LBF fluoride ion conductors is explained by the differences in the grain boundary structure, which can result in a partial blocking of the migration pathways, and has been observed for various anion conducting compounds. 41 Thus, sintering treatments are viable methods to increase the ionic conductivity at the cost of the ease of cell fabrication. It is also worth acknowledging that the grain boundary influence can be compensated without increasing the grain size: In the case of a co-precipitated electrolyte, a slight increase of conductivity was attributed to reduced amounts of impurities during the synthesis process as compared to mechanical milling.…”

Spatial confinement – rapid 2D F−diffusion in micro- and nanocrystalline RbSn2F5

“…The incorporation of Rb for Ba in the fast 2D ionic conductor BaSnF 4 increases ionic conductivity; 59 most likely aliovalent doping affects, however, the slower interlayer exchange process rather than fast intralayer diffusion. 59,91 For the layer-structured form of LiBH 4 , 27,92 crystallizing with hexagonal symmetry and showing 2D diffusion, downsizing the mean crystallite diameter through ball-milling has only small effect on Li ion conductivity. 92 On the other hand, the poorly conducting orthorhombic form of LiBH 4 with 3D diffusion, clearly benefits from nanostructuring.…”

“…25 These dynamic processes might be involved in also causing the huge increase in conductivity seen for the aforementioned nanocrystalline oxides, fluorides and carbonates prepared by milling. 91,94 Prominent examples include, for example, LiXO 3 (X = Ta, Nb), 78,79 and Li 2 CO 3 , 94 as mentioned above. Here, the subtle changes when going from the micro scale to nm-sized crystallites are, however, hardly comparable with the boosts in 3D ionic conductivity seen for the disordered systems.…”