Fabrication And Thermal Effects Of Lithium-Sulfur Batteries Based On Porous Carbon/Metal Phosphides Composites

The advanced multifunctional carbon materials have been used widely in information materials and devices fields due to their outstanding electronic,thermal and mechanical properties.The rapid growth and development of advanced multifunctional carbon materials promotes diversified electronic devices.In particular,multifunctional carbon materials are capable of making commercial high-energy-density lithium-sulfur batteries.However,still there are several challenges of lithium-sulfur batteries as a power source for large electronic devices,which includes,the low efficiency of sulfur utilization due to the slow redox kinetics of elemental sulfur;volume expansion（80%）severely destabilizing the cathode structure;"Shuttle effect"caused by polysulfide diffusion;the thermal effect during charging and discharging is not clear.In order to rectify of these challenges need to develop new kind of novel and unique materials with different strategies.The purpose of this present work is to guide the synthesis route through green environmental protection concepts and functions,starting from two directions of"physically confined"and"chemically confined"polysulfides.Construction of heteroatom-doped three-dimensional porous carbon materials with high specific surface area and high conductivity,and introduction of nano-metal phosphides.The effects of heteroatom doping on the morphology,structure and electrochemical properties of materials were discussed.The role of metal phosphide in adsorbing lithium polysulfide and promoting electrochemical reaction was studied,and the multi-step electrochemical reaction mechanism and thermal effect during the charging and discharging process of the battery was analyzed.To provide a feasible path for the practical application of high energy density lithium-sulfur batteries,the specific research is as follows:（1）A simple and inexpensive oxidation approach was used to successfully produce guanine-assisted nitrogen-doped ordered mesoporous carbons（NOMCs）as sulphur carriers.Combining organised mesoporous nanoarrays with nitrogen-doped surface assimilation of polysulfides to physically restrict them and examine their microstructures and chemical science transformation mechanisms.The highly structured mesoporous structure would efficiently prevent polysulfide dissolution while increasing the electrical physical phenomena of insulating sulphur,allowing sulphur to be used to its full potential.By using polysulfide adsorption trials and cyclic voltammetry tests,nitrogen doping produces strong binding sites for chemisorption of polysulfides.High electrochemical activity is achieved,facilitating the polysulfide conversion process.At 1 C current density,the NOMC-2/S cathode has a sulphur concentration of 1.2-1.5 mg·cm^-2 and a high-rate capacity of 460.5m Ah·g^-1.（2）Solution chelation and high-temperature pyrolysis were used to successfully prepare nitrogen and phosphorus co-doped three-dimensional porous carbon-encapsulated transition metal phosphides（PCS900@Fe P,PCS900@Co P,and PCS900@Ni₂P）.Different metal phosphides’electrochemical reaction mechanisms were discussed.Lithium polysulfides exhibit a high binding energy to Fe P,according to a combination of density functional theory（DFT）calculations and electrochemical research.As a result,PCS900@Fe P efficiently suppresses the"shuttle effect"and improves the kinetics of electrochemical reactions.The transition metal phosphides are not just nano-sized,according to SEM and TEM examinations.Carbon layers encase it,forming a three-dimensional network structure with a stable catalytic structure and porous carbon nanosheets.PCS900@Fe P exhibits a high reversible capacity of 487.2 m Ah·g^-1 with an average capacity decay rate of 0.046%after500 cycles at 1 C current density.Even at a high sulfur loading of 4.0 mg·cm^-2,the specific capacity remains at 462.5 m Ah·g^-1 for 300 cycles.（3）Nitrogen and phosphorus co-doped three-dimensional porous carbon encapsulated ruthenium diphosphide（PCS900@Ru P₂-2）was successfully prepared by phytate phosphating.The Ru P₂ nanoparticles range in size from 10 to 50 nm and are encapsulated by carbon to have a stable catalytic structure.The nitrogen adsorption and desorption tests show that PCS900@Ru P₂-2/S has high specific surface area and abundant micro/mesoporous structure.The high porosity creates a good buffer space for sulphur volume expansion.PCS900@Ru P₂-2/S can not only efficiently adsorb soluble polysulfides to minimise the"shuttle effect",but also promote the redox reaction between polysulfides and lithium ions,thanks to a combination of density functional theory（DFT）calculations and electrochemical analyses.Active materials’utilisation rate and electrochemical performance have both improved.After 500 cycles at 1 C current density,the reversible capacity of PCS900@Ru P₂-2/S remains at 511.6 m Ah·g^-1,with an average capacity decay rate of 0.047 percent.Even after 500 cycles of heavy sulphur(4.2 mg·cm^-2)loading,a high reversible capacity of 386.6 m Ah·g^-1 was maintained.（4）The porosity and specific surface area of nitrogen and phosphorus co-doped three-dimensional porous carbon nanosheets（PCS900）were regulated by the pyrolysis temperature.The researchers created three-dimensional network-structured carbon materials with high porosity,specific surface area,and atomic doping.PCS900 is abundant in micropores and mesopores,which not only sustain high sulphur loading but also provide multidirectional ion transport channels.When charging and discharging with a small current,the heat released during the charging process is greater than the heat released during the discharging process;when charging and discharging with a large current,the heat released during the discharging process is greater than the heat released during the charging process.The extensive nitrogen and phosphorus doping in the carbon lattice serves to boost the chemisorption between the active interface and lithium polysulfides and improve battery cycling stability,according to DFT calculations and polysulfide adsorption studies.At a current density of 1 C,the PCS900/S electrode has an initial specific capacity of 737m Ah·g^-1,and the average capacity decay rate per 500 cycles is as low as 0.079 percent.