Design And Application Of Functional Sulfur-containing Polymer Composites In Lithium Sulfur Battery

Lithium-sulfur（Li-S）battery has the great advantages of high theoretical specific energy(2,600 Wh kg^-1)and low cost of sulfur,which make it a promising candidate in the field of secondary battery.At present,the understanding of basic issues such as the charging and discharging mechanism of Li-S battery has dramatically improved.However,there are still some limiting factors for its commercialization,such as the low conductivity of sulfur and its discharge products,the shuttle effect caused by the high solubility of polysulfide intermediates,the large volume change of the electrode during cycling,the growth of lithium dendrites and the interfacial side reactions,etc.The structure of polymer materials is ever-changing;their physical and chemical properties have great regulatory potential.In this thesis,a series of functional sulfur-containing polymer materials are designed and optimised.According to the structural characteristics,these materials can be used in Li-S batteries as new high-performance composite cathodes and multi-functional electrolyte additives,respectively.Via deeply studing on the structure-activity relationships,the underlying mechanisms are elucidated,which can provide innovative ideas for the improvement of the comprehensive performance of Li-S batteries.The main research contents are as follows:（1）A new kind of sulfur-triallyl isocyanurate（STI）composite is designed and synthesized as a high-performance Li-S battery cathode material,in which the sulfur content can be as high as 90 wt%.The existence of C-S bond in STI composite not only improves the activity of sulfur on the molecular level,but also reduces the dissolution and diffusion of polysulfides in the electrolyte.Besides,by controlling the preparation conditions,a special monoclinic sulfur phase can be generated,which can further provide more space for the diffusion of lithium ions.The STI cathode exhibits excellent rate capabilities and good cycle stabilities:the capacity retention can reach 94%after 350 cycles at 1 C.（2）The structure of STI is further regulated on the basis of the previous chapter;and the relationship between the structure and the reaction mechanism is established.With increasing synthesis temperature,the existing form of sulfur atoms in the structure of STI composites is gradually transformed into small sulfur molecules,and the discharge curves of the Li-S cells based on these materials in ether electrolyte will gradually change from two-plateau to one-plateau.The pouch cell based on optimized STI material shows almost no capacity decay for 125 cycles under the conditions of high sulfur loading(4.5 mg cm^-2)and lean electrolyte(8μL mgs^-1).In addition,based on the in-depth understanding of the solid-phase sulfur conversion reaction mechanism of STI materials,two kinds of organic sulfur polymer materials are further rationally designed,including sulfur-triallyl phosphate（STP）and sulfur-triallylamine（STA）.These two kinds of materials show similar active structure and reaction mechanism to the optimized STI,and have high lithium storage potential.（3）To further construct the high-performance Li-S full battery,a selenium substituted lithium polythioctic acid（PTA-Se）is developed as a bifunctional electrolyte additive via in-situ polymerization method,which can improve the performance of cathode and anode simultaneously.During discharge process,in the side of cathode,PTA-Se additives can react reversibly with polysulfides to form a solid organosulfur polymer species,thus effective immobilize polysulfides within the cathode,its active selenium sites can further promote the conversion of polysulfides and the uniform nucleation of lithium sulfides.In the side of anode,the PTA-Se additives can homogenize the lithium deposition at the interface,and faciliatate the formation of a stable solid electrolyte phase interface（SEI）film.The introduction of PTA-Se electrolyte additive enables a stable cycling up to 4500 h at a high capacity of 40 m Ah cm^-2 in the Li-Li symmetric cell,and the Li-S battery based on STI cathode can achieve a stable cycle of 900 cycles at 2 C.