In Situ EXAFS-Derived Mechanism of Highly Reversible Tin Phosphide/Graphite Composite Anode for Li-Ion Batteries

A novel Sn4P3/graphite composite anode material with superior capacity and cycling performance (651 mA h g(-1) after 100 cycles) is investigated by in situ X-ray absorption spectroscopy. Extended X-ray absorption fine structure modeling and detailed analysis of local environment changes are correlated to the cell capacity and reveal the mechanism of lithiation/delithiation process. Results show that in the first two lithiation/delithiation cycles crystalline Sn4P3 is fully converted to an amorphous SnPx phase, which in further cycles participates in reversible conversion and alloying reactions. The superior reversibility of this material is attributed to the highly dispersed SnPx in the graphite matrix, which provides enhanced electrical conductivity and prevents aggregation of Sn clusters during the lithiation/delithiation process. The gradual capacity fading in long-term cycling is attributed to the observed increase in the size and the amount of metallic Sn clusters in the delithiated state, correlated to the reduced recovery of the SnPx phase. This paper reveals the mechanism responsible for the highly reversible tin phosphides and provides insights for improving the capacity and cycle life of conversion and alloying materials.