Fifteen year later the first detection of the cosmic positron excess, the origin of this structure is still unclear. Its understanding is crucial not only for astrophysics but also for fundamental physics, since it is one of the most striking signature that could be possibly associated with dark matter origin. In order to shed some light on the nature of the positron excess, it is necessary to separately extend the measurement of electron and positron fluxes above a few hundreds of GeV. In a relatively short time scale, this can be performed by developing an innovative charge sign discrimination technique that could be mounted aboard calorimeter-based experiments. The Electron Positron Space Instrument (EPSI) project is a two year R&D aiming to study the feasibility of electron/positron separation in space by exploiting the synchrotron photons emitted as the charge particles travel in the geomagnetic field. This goal requires the development of a X-ray detector optimized to have a high detection efficiency in the low energy region, while keeping the cost low enough to make it scalable to a large area. In this contribution, the general idea of the project is presented, together with the description of the on-going laboratory activities.