To accelerate the energy transition there is an urgent need for energy storage both for electricity and heat due to the intermittency of renewable resources. As a large part of our energy consumption is in form of thermal energy (78% in the built environment), and as there is a large amount of unused low temperature waste heat (8900 PJ in Europe), thermal energy storage offers route towards bridging availability and demand. Long term loss-free compact heat storage is the missing link, and Thermo-Chemical Energy Storage (TCES) has the potential to play this role. Unfortunately, present TCES concepts lack flexibility as a precise fit between the phase diagram of TCES-materials, the system design and the temperature demand of an application is needed. Therefore, most of the R&D for applying TCES is very specific for a particular application and switching to another application requires that much of the R&D process has to be redone. 4TunaTES will deliver a groundbreaking flexible TCES technology that can be easily adapted to different applications (variable in- and output temperatures) and thereby reduce the development costs by 90% as the R&D the process does not have be redone repeatedly. 4TunaTES develops a TCES-prototype that can be used for domestic use cases, which addresses three challenges: 1) radically new TCES materials with tunable phase diagrams by using a second gas or dopants, 2) heat exchanging components with a high degree of manufacturing flexibility, and 3) revolutionary systems with electricity adapted thermodynamic cycles. CNR and DLR will integrate the key findings in their high-TRL project portfolio, and Cellcius (CEL) will integrate the successful materials and designs in their technology. This will unlock the potential thermal energy storage. For this purpose, a strong consortium has been built which brings together computational material scientists (VUB), physical-chemists (TUE, CIIAE), mechanical engineers (CNR, DLR) and heat battery developer (CEL).