Preparation And Electrochemical Performance Of Metal Telluride Catalytic Materials For Lithium-sulfur Batteries

Lithium-sulfur batteries with high theoretical capacity and energy density are highly promising energy storage systems.However,many critical issues,especially the dissolution and shuttling of the intermediate lithium polysulfides and their slow conversion kinetics,severely limit the practical application of lithium-sulfur batteries.The use of catalytic materials to accelerate the redox reactions of the cathode can increase the utilization of active materials and improve the cycling stability of batteries.The development of efficient cathode catalytic materials has become a hot research topic in the field of lithium-sulfur batteries.Metal tellurides have good electrical conductivity and show excellent electrocatalytic properties in reactions such as oxygen reduction/evolution.Therefore,they are potential catalytic materials for lithium-sulfur batteries.In this thesis,several metal telluride catalytic materials were prepared to investigate their positive effects on the electrochemical performance of lithium-sulfur batteries.Bi₂Te₃ catalytic material is prepared by a solvothermal method to study their effects on the electrochemical performance of lithium-sulfur batteries.The prepared Bi₂Te₃ catalytic material has nanoflower morphology and is assembled from ultrathin nanosheets.X-ray photoelectron spectroscopy tests and first-principles calculations show that Bi₂Te₃ can chemically adsorb soluble lithium polysulfides by forming Bi-S and Te-Li bonds.The results of electrochemical test show that Bi₂Te₃ catalytic material can accelerate both the charging process and the discharging process of the cathode and reduce the polarization during charging and discharging.First-principles calculations reveal that the decomposition energy barrier of Li₂S on the Bi₂Te₃ surface is only 0.86 e V,which is significantly lower than that on the graphene surface（1.88e V）,indicating that Bi₂Te₃ has a catalytic function on the cathode charge reactions.Meanwhile,it is found that rapid charge transfer between Bi₂Te₃ and the surface adsorbed lithium polysulfides can be realized,which is conducive to the efficient catalysis of lithium polysulfide conversion reactions.This study has demonstrated the feasibility of using Bi₂Te₃ catalytic materials in lithium-sulfur batteries.CoTe₂-based catalytic material is prepared using ZIF-67 as a precursor to study its effects on the electrochemical performance of lithium-sulfur batteries.It is shown that CoTe₂ nanoparticles in the as-prepared catalytic materials are uniformly loaded on hollow porous carbon spheres.Metallic Co-based catalytic material is prepared with the same ZIF-67 precursor for performance comparison,and the measurement results show that CoTe₂ has higher catalytic activity for lithium polysulfide conversion reactions compared with metallic Co.The results of spectroscopic experiments and molecular dynamics simulations show that sulfur dissociates into S atoms upon contact with the surface of metallic Co.These S atoms are firmly adsorbed on the surface of metallic Co,burying the highly active atomic Co sites,namely,metallic Co undergoes sulfur poisoning.First-principles calculations reveal that the reason why metallic Co is susceptible to sulfur poisoning is that its surface exhibits strong adsorption for S atoms,and the distance between adsorption sites and S-S bond length does not match.In contrast,CoTe₂ surface has moderate binding energy for S atoms,which can avoid sulfur poisoning and chemically interact with lithium polysulfides,thus accelerating the conversion reactions of lithium polysulfides when used in lithium-sulfur batteries,leading to suppressed shuttle effect.The average capacity decay rate of battery during 1000 cycles is 0.030%.In addition,molecular dynamics simulations show that Ni Te₂ and Fe Te₂ are also resistant to sulfur poisoning,while the corresponding metallic Ni and metallic Fe are both susceptible to sulfur poisoning.N-doped CoTe₂ catalytic material is prepared by combining hydrothermal reaction with heat treatment in ammonia atmosphere.It is shown that the N content in the material and the catalytic activity increase gradually with the increase of heat treatment time.However,excessive heat treatment time leads to the formation of metallic Co in the material and the catalytic activity decreases.Experimental results confirm that N doping can enhance the lithiophilicity of the material.It can also reduce the charge density of Co atoms in CoTe₂,leading to enhanced electrophilicity and thus the Co atoms in N-doped CoTe₂ are more likely to bind S atoms with lone pair electrons in lithium polysulfides.Meanwhile,it is found that Co atoms in N-doped CoTe₂ tend to bind to the terminal sulfur in lithium polysulfides because the terminal sulfur has stronger nucleophilicity than the bridged sulfur.The results of first-principles calculations show that N doping increases the adsorption energy of CoTe₂ to lithium polysulfides,which enables it to suppress the shuttle effect more effectively and enhance the cycling stability of batteries.The measurement results show that lithium-sulfur batteries using N-doped CoTe₂ catalytic materials have excellent electrochemical performance,delivering a capacity of 1273 m Ah g^-1 at a current density of 0.2 C,and a high discharge capacity of 758 m Ah g^-1 is still avaliable at a current density of 4 C.Moreover,the battery can stably work for 1000 cycles with an average capacity decay rate of only 0.021%per cycle.The battery shows a high surface capacity of 6.4 m Ah cm^-2 when increasing the sulfur loading to 6.8 mg cm^-2 and it can stably operate for 100 cycles.