Structure Optimization And Electrochemical Performance Of Molybdene-based Electrode Materials

With the increasingly severe problems of environmental pollution and energy crisis,the development of high-performance,low-cost clean energy and efforts to build a clean,low-carbon,safe and efficient energy system is conducive to realizing carbon peak carbon neutrality.Molybdenite has the advantages of environmental protection,abundant resources,high theoretical capacity,and so on,which has the potential to become another choice of electrode materials for commercial lithium-ion batteries.However,the low electronic conductivity and significant volume change during the cycle lead to the electrode material crushing.Based on the structural characteristics of molybdenite,a series of structural optimization strategies were systematically carried out,including crystal shape and morphology control,composite with carbon-based materials,and introduction of MoS₂nanoarray to form a multi-component hierarchical structure to achieve high specific capacity and good cycling stability of electrode materials,and clarify the lithium storage performance enhancement mechanism.This thesis provides a new technical means and strategy for the effective utilization of molybdenite in lithium-ion batteries and makes practical application possible.The main conclusions are as follows:（1）The structure morphology and lithium storage properties of molybdenite concentrate（MC）were investigated and compared with the synthetic flower spherical MoS₂.The results show that MC is 2H-MoS₂and has the characteristics of low electronic conductivity and small specific surface area.MC has better cyclic stability than synthetic MoS2 due to its complete structure and high crystallinity.Still,lamellar packing cannot fully release its inherent capacity,while low conductivity makes its charge transfer resistance large and reaction kinetics poor.After 100 cycles at 100 m A g^-1,MC is only 291 m Ah g^-1.The advantages of flower spherical fluffy structure and high specific capacity of synthetic MoS₂provide research ideas for the subsequent preparation of expanded molybdenite（EM）and the introduction of nano-MoS₂on a molybdenite base.（2）A new expansion process was used to prepare EM with an expansion structure from molybdenite concentrate to release its inherent capacity.The effects of reaction time,reaction temperature,and Na OH concentration on expanded molybdenite’s structure morphology and electrochemical properties were studied.The results show that expanded molybdenite has a 1T@2H MoS₂heterostructures with worm-like structure,many micropores,a specific surface area of 46.91 m²g^-1,and electronic conductivity of 10 S cm^-1.Compared with MC,the expanded molybdenite has significantly improved lithium storage performance,minor charge transfer resistance,and fast reaction kinetics.When the current density is 100 m A g^-1,the specific capacity is 823 m A g^-1after 500 cycles.At the same time,the specific capacity is 727 m A g^-1at 1A g^-1.However,the expanded molybdenite cannot avoid voltage failure after～125cycles,leading to rapid capacity attenuation.The EM with expansion structure has a large specific surface area,many defect sites and 1T@2H MoS₂heterostructures,which shorts the lithium-ion transmission distance and forms a built-in electric field to accelerate the transfer of electrons and ions,and improves the lithium storage performance.（3）The expanded molybdenite@C（EM@C）composite was constructed by carbon coating strategy with expanded molybdenite as raw material to solve the voltage failure and improve the stability of the electrode.The effects of reaction time and dopamine concentration on the structural morphology and electrochemical properties of EM@C composites were studied.The results show that the N element exists in pyridine N and pyrrole N in the carbon layer.The N-doped carbon layer can fix Moatoms and polysulfide ions and help release bulk strain and accelerate electron transport.The electrochemical test showed that EM@C composite had good cyclic stability and further released the inherent capacity of molybdenite.No voltage failure occurred during the cyclic process,and the specific capacity was still 1213 m Ah g^-1after 200cycles.（4）The expanded molybdenite@C@MoS₂（EM@C@MoS₂）was fabricated by in-situ growth of MoS₂nanosheets on the surface of the carbon layer using EM@C as raw material to optimize the specific capacity of molybdenite-based further.The results show that the MoS₂nanosheets grown vertically on the surface of the carbon layer interlace with each other,forming a labyrinth with an expanded lattice spacing of 0.73nm.MoS₂nanosheets increased the specific surface area the number of micropores and exposed more active sites of the hierarchical materials.EM@C@MoS₂hierarchical materials demonstrate excellent electrochemical properties.After 200 cycles of 100 m A g^-1,the specific capacity of 1336 m Ah g^-1is higher than that of the EM@C composite.Under the high current density of 1 A g^-1,the specific capacity is 658 m Ah g^-1.At the same time,EM@C@MoS₂hierarchical materials have lower charge transfer resistance,and the pseudocapacitance dominates the fast reaction kinetics.Under the high current density of 1 A g^-1,the specific capacity is 658 m Ah g^-1.At the same time,EM@C@MoS₂has lower charge transfer resistance,and the pseudocapacitance dominates the fast reaction kinetics.EM@C@MoS₂hierarchical materials in the synergy between the various structures to achieve complementary advantages,shorten the lithium-ion transmission distance,improve the transmission rate and provide more lithium storage space.