| In the process of human social development,energy occupies a pivotal position,but due to the massive consumption of non-renewable energy by human beings,the development of new renewable energy sources is very urgent.In recent years,wind,solar and tidal energy have been considered as renewable energy sources with wide application prospects.However,these natural energy sources are characterized by randomness,intermittency and fluctuation,which may cause serious energy waste.Therefore,mankind is actively developing an environmentally friendly,convenient,safe,low-cost,stable capacity and long-life energy storage device to achieve efficient use of energy.In recent years,lithium-ion batteries（LIBs）have been widely used because of their high quality energy density,no memory effect and green environmental protection,promoting the development of cell phones and portable notebooks,as well as being the dominant power source for future electric vehicles.However,the current energy density of commercial LIBs is low and cannot meet the higher requirements of society for power batteries,and there is an urgent need to develop new secondary batteries with high energy density and low cost.Sulfur cathode has a high theoretical specific capacity(1675 m Ah g-1)and is environmentally friendly and resource-rich.In addition,lithium-sulfur batteries also have a high theoretical specific energy(2600 Wh kg-1),which is expected to meet the demand for high energy density development.However,the problems of polysulfide"shuttle effect"and slow kinetics of redox reaction during the cycle of lithium-sulfur batteries lead to the loss of active material and rapid decay of capacity,which limit their wide application.Therefore,the development of catalysts that can both effectively adsorb polysulfides and promote the rapid conversion reaction of polysulfides is the key to commercialize lithium-sulfur batteries.The main research of this paper is as follows:（1）In lithium-sulfur batteries,controlling the uniform deposition of the discharge product Li2S is the key to achieve high-performance lithium-sulfur batteries.In this paper,the homogeneous deposition of Li2S is achieved by regulating the phase of cobalt selenide,and different phases of Co Se2 grown on carbon cloth,i.e.,orthogonal phase（o-Co Se2）,cubic phase（c-Co Se2）and two-phase mixed phase（o/c-Co Se2）,are synthesized by a two-step method of hydrothermal and then low-temperature selenization.It was found that the Li2S morphology on the o-Co Se2 surface was more uniform after depositing the same volume of Li2S,which belonged to the 3DI deposition mode.The reason why the crystal structure affects the morphology of Li2S deposition was investigated by first-nature principle calculations,namely,different crystal surfaces have different adsorption and catalytic abilities for polysulfides;the d-band centers of c-Co Se2,o/c-Co Se2 and o-Co Se2gradually move up,and according to the d-band center theory,o-Co Se2 has the strongest adsorption ability for polysulfides.Meanwhile,o-Co Se2 is more likely to promote Li-S bond breaking and reduce the decomposition potential of Li2S,increasing the polysulfide conversion rate.The results of the subsequent adsorption experiments and kinetic performance tests were consistent with the theoretical calculations.Therefore,when o-Co Se2/CC was used as the catalyst for the lithium-sulfur cell,the cell maintained a specific capacity of 839.55 m Ah g-1 after 400 cycles at a current density of 1 C.（2）Transition metal phosphides are promising catalysts.When another metal is introduced and a bimetallic phosphide is formed,the coordination between the two metals can provide bi-directional enriched active centers and change the electronic structure and coordination environment of the monometallic phosphide,which in turn affects the d-band center and valence state of the metal compounds and reduces the band gap to improve the electrical conductivity,so the bimetallic phosphide will show superior catalytic activity.In this paper,Mo P was used as a model to form bimetallic phosphides by introducing a second transition metal atom such as Fe,Co,and Ni,namely Fe Mo P,Co Mo P,and Ni Mo P.The effects of introducing different metals on the electronic structure of Mo P were investigated to establish the conformational relationships between different bimetallic phosphides and the catalytic decomposition of polysulfides and the oxidation of Li2S.Three Mo-based bimetallic phosphides with three-dimensional ordered porous structures,3DOP MMo P（M=Fe,Co,Ni）,were prepared by sol-gel method.It was found by theoretical calculations that the d-band center gradually shifted upward with the increase of atomic number,indicating that Ni Mo P has stronger adsorption effect on polysulfides,which is consistent with the adsorption experimental results.In addition,the 3D porous morphology also provides a fast channel for ion transport,which significantly improves the power performance of the cell.Thus,when 3DOP Ni Mo P was used as the catalyst for the lithium-sulfur battery,the battery maintained a reversible specific capacity of 775.01 m Ah g-1 at a current density of 1 C for 300 cycles at a sulfur loading of 1.5 mg cm-2.The electrochemical performance of the three bimetallic phosphides was ranked as Ni Mo P>Co Mo P>Fe Mo P.（3）Compared with metal compounds,polyacids can provide more polar adsorption sites that can effectively adsorb and catalyze polysulfides,providing an effective solution to eliminate the shuttle effect.In this paper,polyacids,namely[Co3（4-NH2-trz）6][V6O18]·3H2O（4-NH2-trz=4-amino-1,2,4-triazole）,abbreviated as Co,V-POM,were synthesized by a hydrothermal method and compounded with reduced graphene to obtain Co,V-POM@r GO composites.Co,V-POM@r GO can effectively peg polysulfides and promote conversion reaction of polysulfides,so that when Co,V-POM@r GO was used as a catalyst for lithium-sulfur batteries,the batteries could maintain a specific capacity of 729.9m Ah g-1 after 570 cycles at a current density of 0.5 C. |
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