Energy storage systems (ESS) are increasingly vital due to the rise of renewable energy sources. However, current ESS technology faces challenges related to fire safety, sustainability, and inflexible energy and power design. In this context, the EU-funded SMHYLES project will develop sustainable hybrid energy storage systems (HESS) by combining two low critical raw material storage technologies. The project will design, construct, deploy and demonstrate two types of HESS (aqueous-based and salt-based) and demonstrate storage capacity extension of an existing HESS. These will be used for various applications, including islanded grids, industrial microgrids, provision of grid services, and electric vehicle charging, across demonstration sites in Portugal and Germany. Additionally, the project will develop digital twins to enhance system monitoring and optimisation.