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Nanostructured solid electrolytes for safe high performance batteries
TThe development of high-energy density solid-state rechargeable lithium batteries is vital to cater the increasing need of safe, cost-efficient, low-weight energy sources for new mobile technologies, for hybrid cars or the efficient use of renewable energy sources. The transition from the presently used liquid to solid state electrolytes in batteries would both enhance the safety and allow for an easier miniaturization, but requires solid lithium electrolytes with higher conductivities than those presently available. Here, we could reveal by a combination of X-ray diffraction, electrochemical characterization and computer simulations how interface effects in nanostructured disordered systems can be employed to boost the ionic conductivities in the solid state. The achievements of this project include from the application point of view the identification of Li7P3S11 as the phase that is responsible for the high conductivity of thiophosphate glass ceramics, the determination of its crystal structure and the analysis of the ion transport mechanism. Today Li7P3S11 is the solid electrolyte with the highest known Li+ ion conductivity at room temperature. From the basic science point of view, the most important finding is that our MD simulations could provide the first atomic scale verification of the redistribution of mobile ions in nanostructured heterolayers consisting of two ionic conductors, leading to a violation of the local electroneutrality for entire nanocrystals as postulated in the mesoscopic multiphase effect model. For nanostructured stacks of alternating ion conductor layers (BaF2 and CaF2), we could demonstrate a statistically significant enrichment of the mobile ions in the CaF2 phase and thereby of vacancies in the BaF2 phase. The mechanism of the experimentally observed conductivity enhancement can thus be understood. It could also be shown how misfit dislocations influence the local transport processes. Thereby we could significantly contribute to a better understanding of the “anomalously high” conductivities of nanostructured composites containing ion conductor and insulator phases or two moderate ionic conductors. For glass ceramics we found that a pronounced conductivity increase occurs during the initial stages of crystallization of moderately ion conducting glasses. In-situ powder diffraction monitoring of the crystallization process reveals that that this conductivity increase is an interface effect and we could demonstrate that this conductivity enhancement can be retained at room temperature if the crystallization process is interrupted at the nucleation stage, due to the high conductivity of the glass-crystal interface area. In a subsequent detailed study of the changes in structure and transport properties induced by halide doping in oxysalt glasses and glass ceramics we applied our specific bond valence tools to analyze the ion transport pathway topology and local pathway dimensionality and to derive better predictions of transport properties from the structure.
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Fig. 1: Rietveld refinement of synchrotron diffraction data for the thiophosphate Li7P3S11 , the solid electrolyte with the highest known room temperature conductivity, and the refined triclinic crystal structure with a superimposed BV model of Li+ ion transport pathways..
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Fig. 2 (a) Detail from a dynamic bond valence model of F- ion transport pathways at a CaF2(top):BaF2(bottom) interface. (b) Bottom: Variation of the MD simulated diffusion coefficient for F- as a function of the distance from the BaF2 : CaF2 interface at T= 1000K. The broken lines indicate the simulated conductivities for the individual bulk materials.
Contact Person : Assoc Prof S. Adams
E-mail: mseasn@nus.edu.sg
Tel : 6874 6869
Fax: 6776 3604