Various compositions of tin-rich, non-stoichiometric, solid solutions of Sn4+ δ P3 are synthesized by a mechanochemical method. The materials are tested as anodes for lithium secondary batteries to enhance reversible capacity and cycleability. Investigative analyses show that the region for the solid-solution formation of Sn4+ δ P3 is 0≤ δ ≤1. The reaction mechanism of the tin-rich solid solutions is similar to that of stoichiometric Sn4P3, except for the absence of a topotactic lithium insertion reaction during the first cycle. As the tin content is increased, tin-rich phosphide exhibits better cycleability and retains a higher reversible capacity, namely, about 20% more than that of stoichiometric Sn4P3.