Preparation And Application Of Metal Selenium/Phosphorus-based Compounds For Lithium Sulfur Batteries

The development and utilization of clean energy will increase in future under the promotion of China’s“double carbon”policy,and it is crucial to realize the efficient utilization of clean energy by vigorously developing green,efficient,stable and inexpensive energy storage system.Among them,lithium-sulfur（Li-S）batteries are considered as the next-generation of battery systems with great application potential because of the advantages of high energy density,low cost and environmental friendliness.However,Li-S batteries still face some challenges in practical applications.For example,the insulating nature of sulfur and discharge products in sulfur cathode leads to sluggish redox reaction,thus reducing the sulfur utilization.The shuttling effect of soluble lithium polysulfides causes a rapid capacity decay,which seriously affects the actual energy density of cells.Moreover,the uncontrollable growth of dendrite in lithium metal anode may puncture the separator,leading to the safety hazards such as short-circuiting of battery.The volume change in the cathode or anode during cycling may cause the collapse of electrode structure.Transition metal compounds play a positive role in anchoring and catalyzing polysulfides,as well as inducing uniform Li⁺transport and deposition.In this study,different metal selenides/phosphides were designed to explore the potentials in improving the electrochemical performance by combining theoretical calculations and experiments,aiming to address the key problems of the sulfur cathode and lithium metal anode from the perspectives of separator modification and cathode/anode host.The main studies are as follows:（1）Heterostructure Zn Se-Sn O₂/C was prepared by a self-templated co-precipitation method as well as an annealing process,and coated on the commercial separator.Heterostructure Zn Se-Sn O₂/C,combining the advantages of metal oxides and selenides,effectively improves the cell’s performance.Experimental results show that the Zn Se-Sn O₂/C-modified separator has good electrolyte wettability,accelerating ion transport and promoting the reaction kinetics.Thanks to the strong sulfophilicity of Zn Se-Sn O₂/C,the separator modification layer plays a role of anchoring and catalyzing polysulfides,which can enhance the utilization of active material.Furthermore,the Zn Se-Sn O₂/C-modified separator can guide the uniform deposition of Li⁺and lower the lithium nucleation overpotential,thus realizing a stable cycling of lithium anode.As a result,the Li||Li symmetric cell with Zn Se-Sn O₂/C-modified separator achieves a stable cycling over 1000 hours with a low overpotential of 24 m V.And the Li-S cell maintains a reversible capacity of 470 m A h g^-1 after 600 cycles at 5 C.（2）Lithophilic Zn Se nanoparticles/N-doped carbon nanosheets embellished 3D flexible carbon cloth（Zn Se/NC@CC）was developed to act as lithium host by a room-temperature crystallization method and selenization processes.The 3D flexible framework with high specific surface area not only reduces the local current density and inhibits the growth of lithium dendrites,but also remits the volume change of lithium,realizing a stable cycling of lithium anode.Based on the experimental and theoretical calculations,Zn Se/NC@CC with strong lithophilicity can induce uniform lithium nucleation and inhibit the irregular growth of lithium dendrites.Zn Se nanoparticles,N-doped sites and the high ionic/electronic conduction mixture Li₂Se-Zn derived from the lithiation of Zn Se act as lithophilic sites,which can facilitate the homogeneous Li⁺transport/deposition and accelerate the reaction kinetics.Therefore,the Li Fe PO₄ cell based on Li@Zn Se/NC@CC electrode exhibits an excellent cycling and rate performances.Meanwhile,the cycling stability of the assembled Li-S cell is also significantly improved.（3）Bi-functional materials Sn SSe/r GO with anionic vacancies was synthesized using a simple one-step hydrothermal method,and then coated onto the commercial separator and Cu foil as the separator modification layer and lithium host,respectively.Experimental and theoretical calculations show that the vacancy-rich Sn SSe/r GO possesses a strong sulfophilic and lithophilic property,which not only adsorb polysulfides and catalyze their rapid conversion,but also regulate the lithium deposition behavior and inhibit the growth of lithium dendrites.As a result,the Li-S half-cells and lithium-metal anodes containing Sn SSe/r GO exhibit improved electrochemical performance.Moreover,the Li-S full cells based on Sn SSe/r GO achieve a stable cycling performance and excellent rate performance.And it demonstrates a remarkable area capacity of about 8.0 m A h cm^-2,even when subjected to lean electrolyte(9.0μL mg^-1)and low N/P ratio（about 1.3）with high sulfur loading(11.2 mg cm^-2).（4）Free-standing integrated electrode Ni Co P@CC was prepared by hydrothermal method and low-temperature phosphorization process,aiming to act as both sulfur and lithium host to mitigate the shuttling effect of polysulfides and inhibit the irregular growth of lithium dendrites.Experiments indicate that Ni Co P@CC has a strong adsorption capacity and high catalytic activity towards polysulfides,which can significantly reduce the nucleation and decomposition barriers of Li₂S,promoting the redox kinetics of polysulfides.In addition,theoretical calculations and experiments indicate that there is a strong interaction between Ni Co P and lithium atoms.And Ni Co P acts as deposition sites to induce homogeneous Li⁺distribution and deposition for inhibiting lithium dendrites.Ultimately,the Li-S full cell based on the Ni Co P@CC integrated electrode achieves a long-term cycling stability with a capacity decay rate of about 0.04%per cycle at 1 C for 750 cycles under a low N/P ratio and sulfur loading of 2.5 mg cm^-2.