We report in the context of lithium-rich antiperovskite cathode materials outstanding electrochemical properties of (Li₂Fe)SeO, which for the first time was synthesized via direct ball-milling. The unique structured material displays an electrochemical cycling performance of 250 mA h g⁻¹ at 0.1C when used as a cathode in lithium-ion batteries. Comprehensive electrochemical analysis combined with detailed transmission electron microscopy studies reveal that, above 2.5 V, the multi electron storage mechanism involves conversion of (Li₂Fe)SeO to Fe_1−xSe_x. Our results furthermore demonstrate the general relevance of our findings to the whole class of antiperovskite cathode materials and present a route to strongly enhance their cell performance by avoiding the degradation path deciphered by our studies.