Sadowski, Marcel (2017)
Ab-initio calculations of structure and properties of lithium thiophosphates (V2).
Technische Universität Darmstadt
Masterarbeit, Bibliographie

Kurzbeschreibung (Abstract)

Batteries, and in particular lithium ion batteries, are widely used energy storage and supply systems. Recently, the conventional lithium ion battery that uses a liquid electrolyte is under revision due to safety issues. As an alternative, liquid electrolytes can be substituted with solid electrolytes. A potential class of solid electrolytes are lithium thiophosphates of the xLi2:(100-x)P_2S_5 system. These materials are usually prepared as glasses or glassceramics. Depending on the preparation method, a glass-ceramic can have improved or reduced transport properties compared to its mother glasses. The reason for this that either highly or poorly ion conductive crystalline phases can be formed. In the 67Li_2:33P_2S_5 (Li_4P_2S_7) glass system, the formation of the crystalline Li_4P_2S_6 phase results in a severe loss of ionic conductivity. In a previous investigation, we analyzed the defect thermodynamics and migration barriers of bulk Li_4P_2S_6 in detail by applying Density Functional Theory calculations and explained that its low ionic conductivity is mainly due to high defect formation energies. Furthermore, thermodynamic considerations and explicit interface models indicated the instability against metallic lithium. In the present wotk, in order to obtain a deeper understanding of lithium thiophosphates we wanted to model glass-ceramics. Therefore, we first focussed on the glassy part. After analyzing the crystalline model structure c-Li_4P_2S_7, we prepared glass-models by a meltquenching protocol. By melting at high temperatures and applying a slow quenching rate, energetically reasonable glass models were obtained. However, their structures differ considerably from what is reported by experimental findings: alongside the usually reported P_2S_7^4-,PS_4^3- and P_2S_6^4- structural motifs (“usual units”) we found a variety of different units (“unusual units”). Idealized glass models with increased relative stabilty compared to the melt-quenched model could be generated. But also for the idealized glass models, the structural motifs vary from the ideal usual units in the sense that the usual units are crosslinked via S-S bonds. This is in line with a simple charge argument. By comparing the transport properties of all glass models it was found that their ionic conductivities at room temperature are very similiar. The averaged diffusion coefficients show an excellent linearity in the Arrhenius plot. Using the averaged data, an ionic conductivity of 1.43 x 10^-2 S/cm was calculated. This value is two to three orders of magnitude larger than what is reported from experiments. From stacking fault and grain boundary models of Li_4P_2S_6 we concluded that these planar defects can enhance the ionic conductivity of Li4P2S6. But the effect is minor and compared to the glassy matrix Li_4P_2S_6 can still be regarded as a non-conductive phase. Finally, our modelling of glass-ceramic interfaces showed the preference of the (001)-orientation of the Li_4P_2S_6 over the (100)-orientation. Both orientation seem to be stable. For the (001)-orientation, glass and crystal are separated by an accumulation layer of lithium ions. For the (100)-orientation, lithium ions of Li4P2S6 close to the interface leave their initial site and migrate into the interface. At the same time, they leave behind a vacant site.

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