Synthesis and characterization of solid electrolytes with high lithium ionic conductivity: Investigation of the influence of stoichiometry on crystal structure and ionic conductivity

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In rechargeable lithium-ion batteries the ion transport between anode and cathode is usually made by lithium salts that are dissolved in organic solvents. Although the organic electrolyte provides a high ionic conductivity, intrinsic disadvantages such as toxicity and flammability are accompanied with problems such as lack of temperature stability and the risk of leakage in case of damage. If it is possible to increase the ionic conductivity of inorganic solid electrolyte, the liquid electrolyte may entirely be replaced with a solid-state electrolyte. Thereby the safety and thus the market acceptance for mobile applications could be increased.In the present project solid ceramic electrolytes with high ionic conductivity shall be identified in conjunction with low electronic conductivity. They will be prepared and extensively characterized. As material system, the class of NZP is addressed. The general structure can be written as (M1)[6] (M2)3[8] [L2[6] (X[4] O4) 3]. With M1 and M2 being interstitials, who are partially or fully occupied with lithium. The places L and X are occupied by titanium or phosphorus, respectively. The aim of this work is giving answers to the following question: How is the NZP structure influenced by various cation substitutions and what are the consequences resulting therefrom in terms of lithium ion conductivity? This should be clarified with reference to the combination of experimental and theoretical work. Furthermore, this question is with regard to a (partial) substitution of the anionic (PO4)3- structural components by (SiO4)4-. By doing so, more Li+ can be brought to the M1 or M2 places. This opens up the opportunity to decouple the structural properties of the amount of intercalated Li up to a certain degree.To investigate the influence of chemical composition and crystallographic structure of NZP-based solid electrolyte on the Li distribution and Li-motion, there are various methods that will be applied in this project. As a fundamental step towards resolution the outstanding issues the diffusion behavior of the alkali metal ions are investigated as a function of crystal structure and composition. The combination of modeling (first-principles calculations and atomistic simulations) and experiment (producing phase-pure, highly compacted materials and their characterization, determination of Li-distribution and the diffusion paths employing neutron diffraction) are to identify the pros and cons of the class of NZP compounds as solid electrolytes. This creates the basis for a scientific strategy to optimize the NZP structure and composition. Finally, it should be possible to say to what extent NZP-based materials are suitable as solid electrolytes.