Preparation And Electrochemical Performance Study Of FeP-based Compounds Catalytic Materials For Lithium-sulfur Batteries Cathode

Lthium-sulfur batteries have been regarded as one of the potential energy storage systems,due to their excellent theoretical capacity(1675 mAh g-1),high energy density(2600 Wh kg-1)and low price.However,a number of key issues,especially the shuttle effect of lithium polysulfide(LiPS),have posed a fatal impact on the electrochemical performance of Li-S batteries.The situation is even more serious for high sulfur-loaded,which is the essential component for practical Li-S batteries.So far,in order to solve such issues,the use of metal compounds as catalytic materials in lithium-sulfur systems is considered to be the most effective strategy.Various metal compounds with unique catalytic activity(such as metal oxides,metal sulfides,metal nitrides and metal carbides,etc.)can accelerate the redox reaction while adsorbing LiPS,thereby fundamentally inhibiting the shuttle effect and promoting the performance of Li-S batteries.In recent years,metal phosphides have shown great potential in the research and industrial production of Li-S catalytic materials,owing to their excellent electrical conductivity,high electrocatalytic activity and relatively easy synthesis process.Neverthelss,there are still some shortcomings in the research of metal phosphides in Li-S batteries,such as the lack of the catalytic mechanism,the research mainly focusing on high value metal elements like Co and Ni as well as complex preparation processes.To ameliorate these shortcomings,this dissertation takes cheap FeP-based compounds as the research object.Comprehensive researches from the aspects of material design,catalytic mechanism exploration and application are as follows:Based on reasonable material design,a gradient porous carbon microsphere embedded with FeP nanoparticles composite material(PCM/FeP)was prepared,which was used as a cathode sulfur host for high-performance Li-S batteries.The sulfur host has both sulfur immobilization and redox promotion effects,which can improve the high-rate performance and cycle stability of the Li-S batteries.PCM/FeP possesses micro-,meso-and macroporous structure,which provides effective adsorption of LiPS and accommodation of volume-expanded Li2S.The embedding of FeP nanoparticles in the porous carbon matrix ensures maximum exposure of catalytic sites and improves the interaction with sulfur species under confinement effect of carbon nanopores.FeP with moderate adsorption ability offers a balance between the effectively LiPS trapping and elevated conversion kinetics for the liquid-solid transitions,which significantly improves the sulfur utilization during the deposition and decomposition of Li2S.The rationality and working mechanism of this material structure are explained in detail through electrochemical measurements and density functional theory(DFT)calculations.PCM/FeP ensures the cycling performance of the battery at a high rate(4 C).Besides,Li-S pouch celles with a high areal(8.6 mAh cm-2)and a total(414 mAh)capacies were assembled and tested.Further,FeP nanoparticles were introduced into the carbon black conductive agent that is essential in the slurry-coated cathode,so as to achieve the purpose of improving the electrochemical performance of lithium-sulfur batteries,in which cathodes were composed of commonly used sulfur hosts(carbon nanotubes and nanofibers).At a low addition of SP/FeP of 10 wt%,SP/FeP exhibits strong sulfur immobilizations and effective electrocatalysis for the liquid-liquid and liquid-solid transitions.The cell with modified additive agent achieved high specific capacities(1518 mAh g-1 at 0.2 C),excellent rating performance(728 mAh g-1 at 5 C)and stable long-term cycling.Through electrochemical measurement and DFT calculation,the catalytic mechanism of SP/FeP conductive agent is revealed.Besides,Li-S pouch celles with a high areal(8.0 mAh cm-2)and a total(387 mAh)capacies were assembled and tested.The transition of FeP nanoparticles from the crystalline state to the amorphous state was realized through the induction of auto-oxidation under hearment after phosphorization.Two sulfur hostes,crystalline FeP and amorphous FeP modified carbon nanotubes(CNT/FeP and CNT/aFeP)were prepared respectively.This amorphous transformation greatly improves the adsorption and catalytic performance of FeP on LiPS.LiPS adsorption test and subsequent XPS analysis results show that CNT/aFeP exhibits stronger adsorption and electron exchange with LiPS.Compared with CNT/FeP,the catalytic effects of CNT/aFeP on each step of Li-S redox reactions,especially the nucleation and deposition of Li2S,were significantly enhanced.Amorphous FeP maxinizes the exposes of adsorption and catalytic sites due to its isotropy.In addition,the unsaturated P atoms in amorphous FeP can form P-S bonds with S in LiPS,enhancing the interaction and electron exchange between the catalyst and LiPS,thereby enhancing adsorption and catalysis.The CNT/aFeP sulfur host ensures the cell high specific capacity(1509 mAh g-1 at 0.2 C)and high rate(5 C)long-term cycling stability(83%capacity retention after 400 cycles).Through electrochemical measurements and DFT calculations,the reasons for the improvement of the catalytic effect of amorphous FeP over crystalline FeP have been revealed.Through molecular frontier orbital analysis,the energy gap between the HOMO and LUMO energy levels of the system(aFeP adsorbing Li2S4)is greatly reduced compared to Li2S4,suggesting promotion of the redox reaction kinetics in both charging and discharging stages.By assembling a Li-S pouch cell with CNT/aFeP sulfur host,high areal capacity(6.4 mAh cm-2)and long cycle life(90%capacity retention after 100 cycles)at high current density(5.0 mA cm-2)were achieved.Finally,using recycled waste corn stalks as carbon source,a simple one-step method is used to synthesize a porous carbon sheet embedded with FeP/Fe(PO3)3 nanoparticles,which is used as a cathode sulfur host for high-performance Li-S batteries.This preparation method not only greatly reduces the preparation cost of Li-S batteries cathode materials and simplifies the preparation steps,but also has a great benefit on solid waste recycling.This porous carbon sheet possesses micro-,meso-and macroporous structure,which can effectively contain and confine sulfur species and satisfy the requirements of high sulfur-loaded in application.FeP/Fe(PO3)3 nanoparticles can effectively adsorb and catalytically convert LiPS,especially the reaction kinetics of Li2S deposition is significantly improved.This sulfur host ensures the high specific capacity and long cycle stability of the battery.Besides,Li-S pouch celles with a high areal(6.9 mAh cm-2)and a total(332 mAh)capacies were assembled and tested.