Design And Synthesis Of Phosphorus-Based Electrode Materials For Electrochemical Performance

Phosphorus-based materials,mainly red phosphorus and metal phosphides have shown very promising applications in energy-related fields such as lithium/sodium batteries because of the high theoretical specific capacity and low price.However,red phosphorus and metal phosphides still suffers from such as large volume variation during charge and discharge cycling and poor self-conductivity,leading to low capacity,short cycle life and rapid capacity decay problems in their applications.Nanomaterials have the advantages of being small,having a large specific surface area,and having a faster electron/ion transport rate,which can effectively relieve the stress of volume change and improve the electrical conductivity of electrode materials,and are significantly effective for improving battery performance.Based on the foregoing,we developed several methods for the synthesis of phosphorus-based nanomaterials and investigated the conformational relationship between the improvement of electrochemical performance and electrode structure when phosphorus-based materials with hollow nanosphere structure and nanofiber structure are used as anode in sodium batteries;we analyzed the mechanism and reasons for the effect of metal phosphide nanocomposites on the reaction kinetics in lithium-sulfur batteries.The following are the main research contents of this thesis.1.A red phosphorus nanomaterial with hollow spheres was synthesized in a ternary molten salt system of NaCl-KCl-AlCl3 under mild reaction conditions,and the growth mechanism of the hollow structure was attributed to the Kirkendall effect by experiment characterization.Benefiting from hollow nanosphere structure can not only adapt to volume change and alleviate the volume expansion problem during electrochemical cycling when red phosphorus is used as the anode of sodium ion batteries,but also shorten the ion transport path.The hollow red phosphorus nanospheres exhibited reversible capacities of 624 mA h g-1 at 4 C and 737 mA h g-1 at 1 C,respectively,when used as the anode of sodium ion batteries.In addition,the reversible capacity decay rate of the prepared hollow red phosphorus nanospheres was only 0.06%per cycle after 600 cycles at a rate of 1 C.2.Here,we present a simple selenium-induced method to generate fibrous nano red phosphorus.It turns out that a small amount of selenium can not only induce highquality fibrous nano red phosphorus from commercial red phosphorus,but also benefit qualitatively to the sodium ionic conductivity.Interestingly,the selenium dopant can also manipulate the chemical state of phosphorus leading to valence tunability which is able to tolerate structural stability under the Na-rich or Na-poor condition.As a result,the optimal fibrous nano red phosphorus used as anode outperforms over the traditional red phosphorus～6 and～10 times in terms of rate performance and cycle stability,respectively.Furthermore,as employed for sodium metal anode protection,the symmetrical cell cycling maintains long stable at a low polarization voltage for over 1000 hours.The full cell performs excellent cycling stability of 1500 cycles at 3 C and high-rate performance up to 30 C.3.Herein,cobalt phosphide/graphene composite materials were synthesized by in situ growth method,and the kinetic behavior in lithium-sulfur chemistry as sulfur cathode was systematically investigated,and compounds with fixed metal cation and different non-metal anions were synthesized for comparison.The experimental combined with DFT theoretical calculations deciphered the variations of electrochemical properties between different non-metallic anions and the intrinsic modulation essence on the kinetics of lithium-sulfur batteries,which is due to the shift of the p-band centers regulating the interfacial electron transfer dynamics.Because of the effective promotion of cobalt phosphide composite on Li-S kinetics.Among the Cobased compounds,CoP exhibits the lowest overpotential for polysulfide transformation.Even at 40 C,S@CoP/rGO still delivers a high capacity of 417.3 mA h g-1 and an unprecedented power density of 137.3 kW kg-1,representing the best rate performance at that time.This work could unlock the potential of band engineering for Li-S batteries and beyond.4.A self-supporting three-dimensional skeleton was synthesized by secondary graphene cladding method,which can not only achieve uniform distribution of cobalt phosphide on three-dimensional graphene,but also,synergistically with lithiophilic deposition sites and high specific surface deposition skeleton,which is a more ideal substrate for lithium deposition.The experiments show that the symmetric cell can stably cycle for 1100 h at 0.5 mA cm-2,1 mA h cm-2.In addition,the matching performance with sulfur cathode material was investigated.The introduction of N doping and S vacancies in MoS2 resulted in N-MoS2-x/rGO can be used as a carrer material for the sulfur cathode.Finally,using an anode made of pre-lithiated selfsupported cobalt phosphide material and a cathode made of N-MoS2-x/rGO@S as the full cell yields a capacity of 968.4 mA h g-1 after 150 stable cycles at 0.5 C.