Cathode-solid electrolyte interfaces

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A major challange in all-solid-state batteries is to design cathodes with sufficient compliance that high stresses are avoided when active particles swell or contract, so that contact is maintained upon charge and discharge. Practical cathodes like NMC are strongly oxidising, and so generally degrade solid electrolytes at interfaces. LiTaO3 and LiNbO3 coatings have been investigated to suppress reactivity, but the mechanical problems remain. Ceramic coatings are not compliant, and tend to detach as active particles charge or discharge. To understand the impact of volume changes within composite electrodes the student will compare directly zero-strain active materials, such as Li4Ti5O12, with volume-change materials, e.g. LixNb16W5O55 and LixTiS2. This will decouple volume effects from surface phenomena. The project will focus on sulphide solid electrolytes because of the higher ionic conductivities required for the longer path lengths in cathodes (80-100 mu m) compared with the separator (10 mu m). Comparing LixNb16W5O55 and LixTiS2 will show how electrode hardness affects interfacial integrity. The cathode/solid electrolyte interface will be studied as a function of charge state, cycling, current density, temperature, and pressure. The student will be trained on electrochemical characterisation (3-electrodes cells) and impedance spectroscopy, morphology studies by plasma-FIB SEM, sample thinning and TEM, in-operando X-ray tomography, XPS, and EDX. Isostatic pressing will be combined with electrochemistry to establish the mechanical responses of composites.EPSRC Research Area Theme: Energy