Electrochemical Performance and Mechanisms of NaSn2(PO4)3/C Composites as Anode Materials for Li-Ion Batteries

NaSn2(PO4)3/C composites obtained by solid-state and pyrolysis reactions present high capacity retention and high-rate capability as anode materials for Li-ion batteries. The structure of NaSn2(PO4)3 was analyzed by combining X-ray diffraction and density functional theory to confirm the R3̅ space group. The composite is formed by submicrometer particles with a cube-like shape coated by pyrolytic carbon that improves the electronic percolation. The 119Sn Mössbauer spectroscopy shows the existence of Sn4+ with a more ionic character than SnO2, which can be related to the inductive effect of the phosphate groups. This technique was used in the operando mode to follow the reaction with lithium during the first discharge that is a crucial step for improving the performance. A two-step mechanism has been identified that consists of the irreversible transformation of the pristine material into Sn0 species for the first half of the discharge followed by reversible Li–Sn alloying reactions. The Mössbauer spectra of the Sn0 species differ from the spectrum of βSn because of their nanosize and the existence of chemical bonds with the sodium phosphate matrix formed during the conversion reaction. The first part of the discharge should be considered as a restructuration of the pristine material leading to the dispersion of Sn0 small particles in strong interaction with the phosphate matrix and providing good cyclability. Such a mechanism strongly differs from the insertion mechanism of NaTi2(PO4)3/C that contains a transition metal with the same oxidation state as Sn4+.