The Li7MnN4 structural response upon the first Li extraction–insertion cycle is highlighted using in operando XRD experiments. A 3-phases mechanism involving two biphasic regions for 0.1 ≲ x ≲ 0.8 and 0.8 ≲ x ≲ 1.2 in Li7−x MnN4 and a solid solution behaviour (1.2 ≲ x ≲ 1.5) explains its electrochemical fingerprint. These successive structural transitions do not change the cubic symmetry of the cell and induce a limited cell contraction (∼7%) associated to a reversible mechanical strain. This finding partly explains the excellent cycle life of this promising negative electrode for Li-ion batteries.