A Flame-Retardant and Insoluble Inorganic–Organic Hybrid Cathode Material Based on Polyphosphazene with Pyrene-Tetraone for Lithium Ion Batteries

A cathode material based on polyphosphazene with pyrene-4,5,9,10-tetraone (PTO) units as electroactive groups with a high specific capacity in the side chain, poly[(bis(2-amino-4,5,9,10-pyrenetetraone)]phosphazene (PPAPT), is synthesized. The structural characterization of PPAPT is carried out by using appropriate standard spectroscopic methods such as 31P NMR spectroscopy, FT-IR, DSC, and TGA. The material is found to be an insoluble and halogen-free flame retardant in accordance with the results of the simple flame test and solubility control in electrolyte solutions accompanied by UV–vis analysis. The electrochemical performance of PPAPT is evaluated as a Li–ion battery cathode material. The fabricated cells demonstrate immensely good capacity retention with 72% after 500 discharge–charge cycles at a high current density of 20 C. In comparison with the pristine PTO, introducing a PTO unit into the side chain of the polyphosphazene leads to substantially improved performance because of the lowered LUMO energy levels of PPAPT. In order to investigate the reversibility of carbonyl groups as an electroactive side with respect to their chemical composition, complementary chemical post-mortem analyses are performed by FT-IR, X-ray photoelectron spectroscopy (XPS) analysis. Density functional theory (DFT) calculations are also proposed to determine HOMO–LUMO levels and investigate the lithiation mechanism of PPAPT.