The reaction mechanisms of Li with Sn/BPO4 composites to be used as negative electrode materials for Li-ion batteries were studied during electrochemical cycling by operando Mössbauer spectroscopy and X-ray diffraction using a specifically conceived in situ electrochemical cell. The starting composites consist of three main components: β-Sn particles as the electrochemically active species, an inactive matrix of BPO4 and an amorphous SnII-borophosphate interfacial phase linking the two former components and improving the cohesion of the composite. During the first discharge, the latter Sn(II) species are first reduced to zerovalent tin forming Li-poor Li-Sn alloys. After its complete reduction, the reaction of Li continues with β-Sn leading to Li-Sn alloys increasingly rich in Li, with a final composition between those of Li7Sn2 and Li13Sn5. X-ray diffraction shows a progressive loss of long range order of the composites with the suppression of the diffraction peaks of the initial β-Sn and the formation of an ill-defined mixture of Li-Sn alloys. The evolution of this mechanism is investigated on going from a reference Sn/BPO4 composite prepared by conventional ceramic methods with common micrometric BPO4 to a new improved material prepared by carbothermal synthesis starting from nanometric BPO4. With the new composite prepared by carbothermal synthesis, a significant improvement of the reversible capacity at the first cycle is obtained together with a slight improvement of the cycling behaviour. An additional improvement can be obtained by increasing the rate of the first discharge, and thus hampering the formation of the thermodynamically stable LiSn intermetallic.