The Sn[BPO4]0.4 composite material is composed of three main constituents: the electrochemically active Sn0 species, the BPO4 buffer matrix, and an interfacial amorphous SnII borophosphate phase which acts as a link between the other two, improving the cohesion of the whole composite. In this paper, we report an investigation of the effect of the reaction time on structure and size of this interfacial layer formed between the Sn0 and the BPO4 particles. 119Sn Mössbauer spectroscopy shows an increase in the amount of the oxidized SnII species with the increase of the reaction time following a quasi-kinetic profile. Operando Mössbauer spectroscopy allowed the study of the transformation of the interface at the beginning of the first electrochemical cycle, indicating that the big irreversible loss during the first discharge is closely related to the increase of the amount of SnII in the amorphous interface.