Preparation Of Bimetallic Phosphide/Biomass Carbon Composites And Their Lithium Battery Properties

In recent years,metal phosphide has attracted wide attention from researchers due to its advantages of high specific capacity,abundant resources and high electrochemical activity.It has the potential to become the next generation of lithium ion battery anode materials with excellent performance and low cost.But in the cycle process,the volume expansion effect of phosphide becomes the primary problem to be solved.Volume expansion will lead to rapid capacity attenuation,poor stability and poor rate performance,which limits the further development of metal phosphide.At present,combining with carbon material is an effective way to solve the above problems.Biomass carbon materials as heteroatomic self-enriching carbon precursors have a wide range of sources,diverse varieties and rich active groups,which in line with the requirements of economic and environmental protection,are excellent choice of carbon materials.In this paper,the combination of metal phosphide and biomass carbon materials is expected to alleviate the volume expansion,poor electrical conductivity and poor stability of phosphides in the cycle process.The morphologies of the samples were analyzed by transmission electron microscope（TEM）and scanning electron microscope（SEM）.X-ray photoelectron spectroscopy（XPS）and X-ray diffraction（XRD）were used to test the structure and composition of the samples.The optimal carbon content and the optimal metal ratio were determined,and the lithium storage performance of the samples was compared and analyzed.（1）The natural lignin was selected as biomass to be carbonized,and potassium hydroxide was used to make holes in the lignin.By comparing the long cycle performance,the lignin carbon with the best morphology was selected.Using one step solid synthesis method,phytic acid,iron nitrate,cobalt nitrate and lignin carbon were mixed evenly after lyophilization,and then processed via high temperature calcination.The iron and cobalt bimetallic phosphide coated with lignin-derived porous carbon（Fe-Co-P/LPC）composites were successfully prepared by making use of P atoms decomposed at high temperature in phytic acid easy to combine with free metal ions.The effects of different carbon content and metal ratio on the structure of Fe-Co-P/LPC and the electrochemical performance of lithium ion batteries were investigated.The experimental results showed that Fe-Co-P/LPC-1:1 prepared with biomass carbon content of 15%and Fe/Co metal ratio of 1:1 had abundant microporous and mesoporous channels.Under the current density of 2 A g^-1,the capacity still remained at 659.6 m Ah g^-1after testing 1000 laps.（2）Available citrate acid was used as biological source and treated with special concentration of acid,the hollow carbon shells were prepared.Sodium hypophosphate（Na₂HPO₃）was used as phosphorus source,after low temperature phosphating,the cross-networked Fe-Ni-P/CAC composite coated with citrate acid carbon was designed and synthesized.The carbon content and Fe/Ni metal ratio of the sample were adjusted.The results show that Fe-Ni-P/CAC-1:1 material has the best performance when the carbon content is 10%and the Fe/Ni ratio is 1:1.When the current density is 2 A g^-1,the capacity reaches to 1222 m Ah g^-1after 1000 cycles,and 459.11 m Ah g^-1after 1000 cycles even when the current density increases to 5 A g^-1.This is due to the fact that the cross-networked carbon structure inhibits the expansion of metal phosphide,thus improving the cycling stability.