| Lithium-sulfur(Li-S)battery,as a kind of lithium metal battery,is one of the most promising next-generation high-capacity energy storage systems due to its high theoretical specific capacity and energy density,low sulfur price and environmental friendliness.However,the shuttle effect of polysulfides in Li-S batteries usually leads to a series of problems such as loss of sulfur active materials,rapid capacity decline,and poor cycle life.Along with the common dendrite growth and safety problems in lithium metal batteries,these issues seriously hinder its commercial application.Due to the structural complexity,diversity and high adjustability of polymer materials,modern interdisciplinary integration injects new vitality into the development of Li-S batteries.In Li-S batteries,polymer materials can be applied to many aspects,such as sulfur cathodes(polymer sulfur cathodes),electrolytes(polymer solid electrolytes)and so on.Through the design and synthesis of molecular structure,different functionalities can be given to polymer materials to meet the different requirements of Li-S batteries.Therefore,functional polymer materials have considerable prospects in the field of Li-S batteries.We take the applications of functional polymers in Li-S batteries as the research object,mainly focusing on polymer sulfur cathode materials and polymer solid electrolytes.Specific research contents are as follows:(1)The design and preparation of organosulfur polymer cathode materials with adjustable sulfur chain:Small molecules with adjustable sulfur atoms in the structure are selected as precursors to synthesize organosulfur polymer cathode materials with adjustable sulfur chain length.Among them,polydiallyl tetrasulfide(PDATtS)has a high capacity of 700 mA h g-1 and a high capacity retention of 85%after 300 cycles.The electrode can realize a high mass loading of 10.3 mg cm-2 with an areal capacity up to 4 mA h cm-2.Through insitu UV/Vis spectroscopy,nuclear magnetic resonance(NMR)studies and density functional theory(DFT)calculations,it is confirmed that PDATtS exhibits a different reaction mechanism from traditional sulfur cathodes:thiolates(RSLi)and insoluble lithium sulfides(Li2S)or slightly soluble lithium disulfide(Li2S2)are generated because Li ions react with radicals arisen from the homolytic cleavage of S-S bond in PDATtS,which avoids the production of long-chain polysulfides,and fundamentally solves the key issues of polysulfide dissolution and shuttle effect in traditional Li-S batteries.(2)Development of selenium-doped organosulfur polymer cathode materials to improve the volumetric/areal capacity and Coulombic efficiency:The designed and synthesized selenium-doped organosulfur cathode material has a unique molecular structure with four sulfur atoms and one selenium atom(as a heteroatom).Compared with pure sulfur electrodes,the doping of selenium significantly increases the conductivity and tap density of battery electrodes,improves lithium ion transmission,thus greatly increasing the volumetric/areal capacity.This structure also completely limits the generation of long-chain lithium polysulfides,leading to Coulombic efficiency as high as 100%.These advantages make it possible to achieve a high specific gravimetric capacity of 700 mA h g-1(approaching to the theoretical specific capacity),an extraordinary volumetric capacity as high as 2457 mA h cm-3,and capacity retention of 92%after 400 cycles.DFT calculations and in-situ UV/Vis spectroscopy both confirm the molecular structure produces novel chemical reaction that no long-chain polysulfides are formed when cycling,resulting in～100%Coulombic efficiency and satisfactory cycle stability.(3)The construction of a new type of gel polymer electrolyte(SHGP)with ultra-high ionic conductivity and strong polysulfide adsorption:The SHGP molecular structure with polar nitrogen heteroatoms and ether chains is conducive to lithium ion migration.Combined with the liquid electrolyte in the gel polymer electrolyte,the SHGP exhibits high ionic conductivity(2.2×10-3 S cm-1 at 60℃,0.75×10-3 S cm-1 at 30℃).On the other hand,the polar functional groups in SHGP have strong adsorption and fixation effects for lithium polysulfides through chemical adsorption,which has been theoretically confirmed by DFT calculation and molecular dynamincs(MD)simulation,experimentally by in-situ UV/Vis spectroscopy.This SHGP-based quasi-solid-state Li-S battery exhibits high specific capacity of 950 mA h g-1 at 0.2 C,and excellent cycling performance of 400 cycles at 1.5 C,which is far beyond the traditional polyethylene oxide-based quasi-solid-state Li-S battery.(4)The design of thermoresponsive solid polymer electrolyte with high ionic conductivity to achieve high-safety all-solid-state Li-S batteries:Beyond a specific temperature,the thermoresponsive solid polymer electrolyte effectively suppresses the ionic conduction between the electrodes,thus intelligently realizing automatic shutdown caused by thermal runaway to reduce safety risks at high temperature.A self-protected all-solidstate Li-S battery with intelligent thermal management function could be developed to ensure the safety of the battery packs.This solid electrolyte has high ionic conductivity(2×10-4 S cm-1 at 30℃).The all-solid-state Li-S battery exhibits good electrochemical and thermoresponsive performance.In addition,this solid electrolyte can also be effectively applied in other lithium battery system,indicating the wide applicability of this thermoresponsive solid polymer electrolyte. |
