Application And Performance Of Phosphorus-Containing Compounds In Lithium-Sulfur Batteries

In today’s mobile Internet era,mobile devices are developing rapidly in various fields,and corresponding battery systems require higher energy density.Lithium-sulfur（Li-S）battery has become a promising next generation of high-energy battery due to its theoretical energy density up to 2600 Wh?kg^-1,low cost and pollution-free cathode material sulfur.However,Li-S batteries have some problems,such as the"shuttle effect"generated by intermediate lithium polysulfides（Li PSs）,and safety issues caused by flammable electrolytes,which hinder their commercial application.Based on the flame retardancy of phosphorus-containing compounds,this paper solves the shuttle effect and lithium dendrites on the anode while preventing the combustion and fire of electrolytes,so as to realize a synergistic improvement in the safety and cycle performance of Li-S batteries.The main contents are as follows:First,considering the poor thermal stability of the commonly used polyolefin separators（polyethylene PE or polypropylene PP separators）,a heat-resistant and fire-resistant separator was designed.The ceramic-coated separator（CCS）was first modified with phenolic resin（PFR）to obtain CCS@PFR,and then ammonium polyphosphate（APP）was coated on the CCS@PFR separator to obtain the APP-CCS@PFR separator.PFR improves the stability of the CCS separator after heating,making the CCS@PFR act as a thermal support layer.APP can form a dense polyphosphoric acid layer on the surface of the separator to block air,and release non-flammable gases such as nitrogen,to achieve the purpose of fire prevention.Benefiting from the adsorption of APP to Li PSs,the Li-S battery with APP-CCS@PFR separator exhibits high cycling stability and high rate performance.The capacity retention after 300 cycles at a current density of 0.5 C is 70.5%.Then,in order to further study the effect of P and N composite flame retardants on the performance of Li-S batteries,hexafluorocyclotriphosphazene（HFPN）was selected as the electrolyte additive.HFPN can not only improve the flame retardancy of the electrolyte,but also the supported solid electrolyte interface has excellent mechanical properties and lithium ion conductivity due to the unique molecular structure of HFPN.HFPN can also buffer the volume expansion of the lithium anode and guide the uniform deposition of lithium ions.Meanwhile,based on density functional theory calculations,HFPN is structurally stable and has a relatively strong affinity for lithium metal.The cycle life and Coulombic efficiency of Li-S batteries using HFPN additive are significantly improved,and the Coulombic efficiency remains around 98%for 200 cycles at 0.5 C.Finally,the phosphazene flame retardants were complexed with cobalt atoms to form cobalt-phosphazene complexes（Co-HFAPP）.Co-HFAPP not only maintains the flame retardant properties of phosphazene,but also has electrochemical catalytic effect,which can enhance the redox reaction kinetics between Li PSs and inhibit the shuttle of soluble intermediate Li PSs in Li-S batteries.Afterwards,the carbonized fibre paper interlayer modified with Co-HFAPP acts as a flame-retardant"secondary electrode".The porous network-structured carbon matrix could effectively accommodate Li PSs to promote the utilization of active materials.The corresponding cells exhibit excellent cycling stability with the capacity retention rate of 68.9%after 500 cycles at a current density of 1 C.