Microstructural related residual stress distributions in solid state ceramic battery electrolytes processed by low energy methods

Solid-state batteries are of intense interest for industrial development due to their promise of safer operation, higher energy density, and durability compared to conventional liquid electrolyte batteries. The viability of production of ceramic solid-state batteries is constrained by the challenges of processing the materials where the ceramic components require densification temperatures significantly higher than the melting temperatures of the metal parts. With the addition environmental and cost disadvantages of conventional high temperature ceramic sintering techniques, in recent years significant progress has been made in the development of low temperature, low energy sintering routes including flash sintering and cold sintering. In flash sintering the overall energy use in the sintering process is reduced by passing current through the sample during heating, such that densification occurs at low (<1000^oC) temperatures and in short durations of time (a few seconds to minutes compared to many hours). This can be done using attached electrodes, or in a contactless setup where a plasma is used to induce the current in the material. In cold sintering the use of a water transient is exploited to enable diffusion and densification in ceramics under uniaxial pressure at very low temperatures of 100-300^oC.

Previous work in the group has shown that both techniques can produce dense (>90%) samples of solid-state battery electrolyte materials at small scale. However, we have identified some inhomogeneities in the materials which produce regions of residual stress due to variations in the local density of the samples. Understanding how these affect the mechanical durability of the sample and the electrochemical performance is important to optimise the processing route. This project will greatly increase our understanding of these factors by focusing on characterisation routes which can provide spatially resolved information about different parts of the samples, and correlating the information obtained. Samples will be produced using a variety of well-controlled techniques at the industrial partner Lucideon, based in Stoke-on-Trent. The microstructural and chemical variations in the samples will be studied by electron microscopy, including using hybrid SEM/FIB/SIMS which allows the simultaneous collection of microstructure, porosity, and chemical data in 3-dimensional regions of the sample. Spatially resolved X-ray diffraction and microhardness measurements will be used to understand variations in crystallography and mechanical properties, respectively. In addition, a significant part of the project will involve exploring the use of Raman microspectroscopy to measure localised residual stresses in the samples and to correlate these to the presence of processing-induced impurity phases. This will require the development of a technique to correlate the residual stresses in the ceramic electrolyte samples to the changes in the Raman spectrum peaks, a relationship which is known for some ceramics but not those used for electrolytes. Finally, electrochemical performance will be determined using facilities available at WMG and correlated to microstructural properties. Overall, this project will contribute significantly to our understanding of the causes and effects of microstructural deviations in ceramic electrolyte materials and enable us to determine optimised processing conditions to achieve the best electrochemical performance.

Start date: Monday, 26 September 2022

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