A key aspect of energy security is access to grid storage technologies. One of the most promising cathode materials for application in grid storage is the family of Prussian blue analogues (PBAs). PBAs use earth-abundant elements and operate using K+-ion technology. While PBAs can show competitive energy storage densities (optimised for high K+-ion contents) and extraordinary cyclability (optimised for low K+-ion contents) one of the most important challenges is reconciling these two targets via an appropriate design strategy. This project aims to exploit the interplay of cooperative Jahn-Teller distortions and vacancy distributions as a mechanism of achieving both high energy storage and competitive cyclability in a single PBA material. Doing so will identify new improved PBA-based materials for application in next-generation grid-storage technology. The main aims of the project include: (i) developing coarse-grained microscopic models for determining the interplay of Jahn-Teller (JT) concentration, vacancy concentration, vacancy distribution, and long-range symmetry breaking; (ii) experimental demonstration of the veracity of this model in terms of the behaviour of key PBA families; (iii) the application (if possible) of X-ray free-electron laser (XFEL) technologies to characterising local JT / vacancy order in PBA cathode materials; and (iv) the development of new PBA-based K+-ion cathode materials. In all our studies, we will exploit three-dimensional difference pair distribution function (3D-DPDF) methodologies, which are a recently-developed application of diffuse scattering methods. In tandem, we will be carrying out the very first XFEL-based total scattering measurements as part of this project. If successful, the project will form an important proof-of-concept in applying XFEL total scattering measurements to the study of local structure in functional materials. The project is well aligned to both the physical sciences and energy research themes. Not only does it address key objectives in terms of energy security and storage, but it also seeks to develop a broader science case for the UK's involvement in the European XFEL facility. This project falls within the EPSRC Physical Sciences and Energy research areas.