Lithium-ion batteries (LIBs) have become the primary power source for portable electronics and hybrid and electric vehicles. The rapid growth applications of LIBs require significant improvement in their capacity, energy density and cycle life. Sn-based materials are alternatives to replace the traditional most widely used anode material, graphite, which only offers a capacity of 372 mAh/g. Metallic tin or tin oxide can achieve initial capacities over 1000 mAh/g. However, these materials suffer from volume expansion and formation of irreversible phases during lithiation that degrade the capacity and battery performance rapidly. A layered structure material tin phosphide (Sn₄P₃) is a promising anode material with a high initial capacity of over 1200 mAh/g. Lithiation of Sn₄P₃ starts with a conversion reaction that forms metallic tin and lithium phosphide, followed by a lithium-tin alloying reaction. Although Li₃P is a better lithium-ion conductor than Li₂O, Sn₄P₃ still suffers from a large volume change during the initial lithiation and the irreversible formation of Li₃P.

This study focuses on the characterization of Sn₄P₃ and Sn₄P₃/graphite composites synthesized by high-energy ball milling using red phosphorus and tin nanoparticles under Ar atmosphere. The capacity of pure Sn₄P₃ rapidly decreases to around 200 mAh/g after 15 cycles. In contrast, Sn₄P₃/graphite composites exhibit excellent electrochemical performance and durability with a capacity of 610 mAh/g at 0.4 C after 100 cycles. In addition, the coulombic efficiency was also much higher in the Sn₄P₃/graphite composites, indicating a reversible lithiation/delithiation behavior and more stable SEI layer formation.

In situ extended x-ray absorption fine structure (EXAFS) measurements were performed to study the local environment changes around Sn atoms during the lithiation/delithiation process. EXAFS scans of the cell were taken continuously and were analyzed as a function of different charged and discharged states. It is evident that the local Sn environment in the Sn₄P₃/graphite composite is fully reversible on lithiation/delithiation while the local Sn environment of pure Sn₄P₃ changes irreversibly after several cycles consistent with the electrochemical results. A mechanism for performance degradation of Sn₄P₃ and cycling durability of Sn₄P₃/graphite composites is proposed based on modeling of the EXAFS data.