Design Of Key Materials For High Performance Lithium-sulfur Batteries And Their Property Research

The demand for energy increases steadily with time due to population and economic growth and advances in lifestyle.However,the current energy density of the latest lithium-ion batteries has reached the threshold,and it is difficult to have a large room for improvement.Therefore,it is urgent to develop the new energy storage systems with higher power densities.Among various secondary energy storage systems,lithium-sulfur batteries（LSB）have become a strong candidate for next-generation energy storage batteries,with a high theoretical energy density of 2600 Wh kg^-1and a high specific capacity of 1675 m Ah g^-1,and benefiting from sulfur’s earth-abundance and environmental friendliness.However,there are some challenges that hinder their commercial application,such as the insulation of S and Li₂S,the shuttle effect of soluble lithium polysulfides（Li PSs）,sluggish kinetic conversion,and the growth of lithium dendrites.In this paper,we address the critical challenges of lithium-sulfur batteries from the perspective of sulfur host material design,separator modification,and functional electrolyte additive.And discussion of the transition metal single-atom materials on selectively stepwise-catalysis conversion process of sulfur cathode,separator modified materials for dissolution inhibition and catalytic reuse of polysulfides,the effect of functional electrolyte additive on a sulfur redox reaction,and the role of lithium anode interfacial stability.Collectively,enhancement of the kinetics of the electrochemical reaction of sulfur cathode and stabilization of the lithium-metal interface for the construction of high-performance lithium-sulfur batteries.The main research contents and results are as follows:（1）Construction of metal-loaded catalysts M-NC（M=Fe,Co,Ni,Zn）and their selective catalytic effect on sulfur cathode.Various electrochemical experiments have confirmed that Fe-NC catalyst accelerates the conversion of soluble long-chain polysulfides,while Zn-NC catalyst contributes to the solid-phase short-chain sulfide conversion process.DFT calculations further verified that the Fe-NC catalyst has a stronger charge transfer ability with Li PSs,which can effectively restrict Li PSs dissolved and migrated,while the Zn-NC catalyst with lower Li⁺transport energy barrier,which is favorable for the conversion transition of Li₂S.Constructing a lithium-sulfur battery with Fe-NC as a sulfur host and Zn-NC as a separator modified material,S@Fe-NC/Zn cell achieve a significantly enhanced sulfur utilization(specific capacity up to 1620 m Ah g^-1of the initial cycle at 0.05 C),and improved cycling stability(630 m Ah g^-1after 600 cycles at 1.0 C).Even at high sulfur loading of 7.38mg cm^-2,the S@Fe-NC/Zn cell still provides high capacity retention of 770 m Ah g^-1at 0.1 C after 50 cycles.This work provides a guided synergistically combining design of sulfur cathode and separation modification for high-performance lithium-sulfur batteries.（2）MXene-based separator construction and its application in lithium-sulfur batteries.Nickle doped MXene（Ni-MXene）was prepared by the molten salt etching method and as a separator modification material for lithium-sulfur batteries.Experiment results verified that the existence of elemental Ni on Ni-MXene materials can be used as adsorption and catalytic sites for Li PSs to enhance the catalytic conversion process of Li PSs.At the same time,the cell assembled by Ni-MXene modified material can achieve long-term cycling of lithium metal at low polarization voltage.Benefiting from its catalytic effect on the sulfur cathode and the interfacial stability on the lithium anode,the constructed lithium-sulfur battery with Ni-MXene modified separator owes superior sulfur utilization(the specific capacity is 1400m Ah g^-1at 0.05 C)and a stable cycle life(with a capacity retention of 510 m Ah g^-1after 600cycles at 2.0 C).Even at a high sulfur loading mass of 4.5 mg cm^-2,the capacity remained 600m Ah g^-1after 90 cycles at 0.2 C.This work provides an efficient separator modification strategy for accelerating the sulfur redox reaction kinetics.（3）Electrolyte additive dibenzyl carbonate（DBC）as polysulfide inhibitor and lithium anode interfacial stabilizer.The chemical nucleophilic reaction of DBC with soluble long-chain Li PSs was utilized to achieve a bifunctional synergistic effect of shuttling effect suppression of sulfur cathode and interfacial stabilization of lithium anode.Raman and UV-vis spectra results confirmed that the long-chain Li PSs were consumed by the DBC additive.DBC additive has two main roles:on the one hand,it generates insoluble products on the cathode side,which can act as a soluble Li PSs barrier layer and act as nucleation sites for regulating the nucleation process of Li₂S,achieving three-dimensional（3D）deposition morphology and improving the utilization of active materials.On the other hand,the DBC additive positively participates in the formation of the solid electrolyte interface（SEI）of the lithium anode,which can effectively stabilize the lithium metal interface.Using commercially available Ketjen black（KB）carbon material as a sulfur host,the assembled lithium-sulfur batteries have excellent rate performance(specific capacity keep 580 m Ah g^-1at 2.0 C)and long cycle life(the specific capacity is 650 m Ah g^-1after 180 cycles at 1.0 C).（4）High-donor（DN）solvent is used as an electrolyte additive to realize the S₃^·-radicals reaction pathway of sulfur cathode while being compatible with lithium anode.The use of high DN solvent as an additive not only induces tri-sulfur radical intermediate thus 3D nucleation of Li₂S with fast kinetics but also solves incompatibility issues between the high-donor solvent and the lithium anode.Adopting N-methyl-2-pyrrolidone（NMP）as a proof-of-concept.Such strategy is accomplished by the unique solvation structure of the NMP added electrolyte,where the preference of NMP-Li⁺coordination squeezes out partial DME molecules while enriching DOL molecules in the first solvation sheath of Li⁺ions.Such solvation structure forms a stable Li anode interfacial composition while preserving its S₃^·-radicals stabilization ability,which effectively improved the sulfur conversion kinetics and reversibility.As a result,the lithium-sulfur batteries with NMP electrolyte have excellent reversible capacity(the specific capacity is 1250 m Ah g^-1at 0.05 C)and long cycle life(700m Ah g^-1at 0.3 C after 340 cycles).