Design And Performance Study Of Functional Separators And Multifunctional Binders For High-performance Lithium-sulfur Batteries

The sulfur cathode material has a high theoretical specific capacity(1675 m Ah g^-1).Its low environmental pollution and low preparation cost make the Li-S battery system considered as a better performance energy storage system.However,the practical application of Li-S batteries is limited by its own problems that need to be solved urgently,which mainly include the following three aspects:（1）the bare electrical conductivity of sulfur;（2）the soluble intermediate product lithium polysulfide generated during the working process induced shuttle effect;（3）unavoidable volume expansion during working process.Furthermore,in order to qualify for commercial applications,it is necessary to have a higher area capacity of the Li-S battery to better achieve the commercial application conditions,which requires a higher sulfur loading.However,with the increase of sulfur loading,the above-mentioned constraints also intensify,which hinders the commercialization of Li-S batteries.In response to the above problems,this paper mainly adopts the following two measures:screening and optimization of adhesives;modification of diaphragms.The main research contents of this paper are as follows:（1）Usually,manufacture of traditional oily PVDF binders requires the use of toxic and expensive NMP solvents.NMP not only pollutes the environment but also harms human health.Therefore,we used hydrophilic alginic acid（Alginic Acid,Alg）and isosorbide（Isosorbide,IS）to prepare a three-dimensional network binder c-Alg-IS with low environmental pollution,high wettability and high modulus through a one-step condensation reaction.The binder can effectively maintain the integrity of the high-loaded electrode and fast lithium-ion transport,so that it can be applied to high-energy-density lithium-sulfur batteries with better cycle stability and rate capability.When the sulfur loading was increased to 9.1 mg cm^-2,the Li-S battery with this binder could still achieve60 stable cycles at a high current density of 2.4 m A cm^-2 and maintain a high current density of 7.2 m Ah cm^-2 surface specific capacity.（2）During the exploration of Li-S batteries,it was found that high energy density can be achieved by modifying the separator of batteries,and the shuttle of polysulfides can be well inhibited,which prompts us to do research on modified material applied on the separator of batteries.Firstly,Ni Co₂O₄ bimetallic oxides were synthesized by hydrothermal method,and Ni Co₂S₄ nanotubes were obtained by further vulcanization.Coating this material on one side of the commercial PP separator not only achieves the inhibition of polysulfide shuttle,but also alleviates the problem of uncontrollable growth of lithium dendrites.The cycle performance test results show that,compared with oxide-modified separators and PP separators,the separators modified by this vulcanized material can significantly improve the performance of Li-S batteries.At a current density of 1 C,the discharge capacity of the Li-S battery with the Ni Co₂S₄ modified separator can reach 894.9m Ah g^-1 after 200 cycles,which is significantly higher than that of the Li-S battery with the ordinary separator(583.4 m Ah g^-1).（3）NiSe₂-Co Se₂ heterostructure was obtained by one-step selenization of hydrothermal-synthesis Ni Co₂S₄ nanotubes.Nanotubes with this structure is used as an effective polysulfide barrier coating on common separators.Studies have shown that lithium-sulfur batteries with separators based on the heterostructures exhibit better cycling stability and rate performance than ordinary separators.The Li-S battery using the modified separator maintain a discharge capacity of 557.2 m Ah g^-1after 1000 cycles at a current density of 1 C,with a capacity decay rate of 0.04%per cycle（except for the first cycle）;Under the current density of 4 C,it still has a specific discharge capacity of 796.4 m Ah g^-1.