Sixteen years after the first detection of the cosmic positron excess, its origin remains uncertain. Understanding this phenomenon is crucial not only for astrophysics but also for fundamental physics, as it represents one of the most compelling potential signatures of dark matter. To clarify the nature of the positron excess, it is essential to independently extend the measurements of electron and positron fluxes beyond a few hundreds of GeV. In the near term, this cannot be achieved with the conventional approach based on a magnetic spectrometer, but it is necessary to develop an innovative charge-sign discrimination technique suitable for calorimeter-based space experiments. The Electron Positron Space Instrument (EPSI) is a two-year R\&D project designed to explore the feasibility of distinguishing electrons from positrons in space by exploiting synchrotron photons emitted as charged particles propagate through Earth’s magnetic field. Achieving this goal requires the development of a cost-effective X-ray detector optimized for high efficiency in the low-energy range, while remaining scalable to large detection areas. This contribution outlines the project’s core concept and provides an overview of the status of the ongoing activities.