Preparation Of Metal Phosphide Nanocomposites And Study On Anchoring And Catalytic Properties Of Polysulfides

Lithium-sulfur battery is considered as one of the most promising low-cost energy storage battery systems in the next generation due to its high energy density（2600 Wh kg-1）and low price of active material sulfur.However,lithium-sulfur battery is still subject to the shuttle effect of lithium polysulfide and slow energy storage dynamics,resulting in low utilization rate of sulfur cathode,low capacity retention rate and poor rate performance,which seriously restricts the commercialization of lithium-sulfur battery.Taking into account the good conductivity of metal phosphide,as well as strong chemisorption and catalytic conversion to lithium polysulfide,which can not only enhance the charge transfer dynamics of the whole battery,but also reduce the redox energy barrier of lithium polysulfide,catalyze the redox process of soluble lithium polysulfide to suppress the shuttle effect effectively.Based on the advantages of metal phosphides,two different metal phosphides,Ni₂P/TiO₂/C and Fe₃O₄/Fe P were developed in this paper,where Ni₂P/TiO₂/C acts as the host material of sulfur with the soluble lithium polysulfide firmly fixed on the cathode side,while Fe₃O₄/Fe P was used as the PP membrane modification material to prevent the soluble lithium polysulfide from shuttling to the anode through the membrane.The results of research are as follows:（1）Taking hexagonal nickel titanium bimetallic glycol compound Ni-Ti-EG micro rod prepared by solvothermal method as precursor,Ni₂P/TiO₂/C micro rod composites were successfully prepared by further high-temperature annealing-phosphating in argon atmosphere.Under the synergetic effect of Ni₂P catalyst with good conductivity,strong adsorbent TiO₂ and porous carbon material,Ni₂P/TiO₂/C micron rod composite,as the host material of sulfur cathode,effectively anchored the soluble lithium polysulfide and accelerated the conversion of polysulfide as well as the charge transfer dynamics,which contribute to reducing battery polarization and suppressing the shuttle effect effectively.Therefore,at the current density of 0.1 C,the initial capacity of the lithium-sulfur battery assembled with S@Ni₂P/TiO₂/C composite cathode material is 1267.6 m Ah g^-1,and the sulfur utilization rate is 75.67%;at a high current density of 3 C,the specific capacity of 302.08 m Ah g^-1can still be obtained after 750 cycles,and the coulomb efficiency is maintained at about 100%,showing good cycle stability.In addition,the preparation process of Ni₂P/TiO₂/C composite is simple with a high yield,showing a good application prospect.（2）Taking Fe₂O₃ nanoparticles prepared by solvothermal method as the precursor,Fe₃O₄/Fe P heterojunction composites were successfully prepared by further high-temperature annealing phosphating method.Then,Fe₃O₄/Fe P powder,Ketjenblack and PVDF are compounded for pulping and coated on polypropylene（PP）separator as a modified layer.The Fe₃O₄/Fe P modified separators can not only effectively adsorb soluble lithium polysulfide through strong chemical adsorption,but also accelerate the conversion of soluble lithium polysulfide intermediates.At present,the lithium storage performance of lithium-sulfur battery with Fe₃O₄/Fe P modified separator has been significantly improved compared with that of lithium-sulfur battery with original PP separator:at 0.1 C current density,the initial capacity is up to 1399.82 m Ah g^-1,and the sulfur utilization rate is up to 83.57%.At the high current density of 2 C,after 900 cycles,the capacity retention rate is as high as 91.62%,and the capacity attenuation rate per cycle is only 0.0093%.