Study On Electrochemical Properties Of Lithium-sulfur Battery With Separator Modified By Metal Sulfide/Oxide Compounds

Lithium-sulfur battery?LSB?is one of the potential candidate technologies in advanced electrochemical energy storage devices.This is mainly attributed to the high theoretical capacity(1675 m Ah g^-1)and energy density(2600 Wh kg^-1),extremely low cost and non-toxicity of sulfur-based cathodes.To date,despite their promising properties,there is some distance left before LSB develop to commercialization due to the inherent problems,such as the low utilization of sulfur and the shuttling of dissolved lithium polysulfides?Li₂Sn,n=4?8?species.Separator is an important component of LSB.Commercial polyolefin separator is unable to restrain shuttle effect of Li₂Sn in LSB since its large number of nanoscale pores are much larger than the size of Li₂Sn.To solve thisproblem,functional material modification of polyolefin separators is a simple and effective strategy.In addition to being able to suppress the shuttle effect,the functional material should have certain conductivity and catalytic activity to solve the problem of low sulfur utilization of LSB.Therefore,in this work,we focus on the multifunctional design of the modified layer of the LSB separator,which ensures the energy density of the battery and improves its performance.This paper mainly consists of three parts,TiO,ZnS and Sn S are selected as adsorbents on the separator,respectively.These materials all have unsaturated metal centers and anion vacancies,which are conducive to the adsorption of polysulfides.The main works are listed as follows:1.In this work,the separator was modified by the combination of commercial titanium monoxide and multi-walled carbon nanotubes?TiO/MWCNTs?for the first time,which is a low-cost and simple preparation process.TiO has a high density of titanium and oxygen vacancies on its surface,which makes it have a stronger adsorption effect of polysulfide than TiO₂.TiO and MWCNT were mixed and coated on the surface of separator.The strong adsorption effect of TiO on polysulfides?Li PS?and the high conductivity of MWCNT were utilized.The synergistic effect made the initial capacity of LSB reaches a relatively high discharge capacity of 1527.2 m Ah g^-1 at 0.5 C,and excellent cyclic stability can be obtained in 1000 cycles,while the attenuation rate of each cycle is only 0.057%.The anti-self-discharge behavior is also significantly improved.2.Herein,we first report a structure in which the ZnS nanoparticles were embedded in the nitrogen-doped carbon nanosheet skeleton to modify the separator of LSB.We also incorporated some carbon nanotubes?CNTs?into the carbon material to improve the overall conductivity of the material,which is beneficial to reuse of the active material and diffusion of lithium ions.ZnS does not have a large number of anion vacancy,and although sulfide has a certain ability to catalyze the conversion of Li PS,the adsorption effect is often weaker than oxide.During the high-temperature carbonization of carbon material,some sphalerite-type ZnS undergoes a phase transition from sphalerite to wurtzite,which generates anion vacancies(S^2-)and unsaturated Zn centres.This is beneficial for enhancing the adsorption effect of Li PS.Nitrogen doping in the carbon nanosheet not only enhances the conductivity of carbon material,but also produces a polar adsorption effect with Li+ of Li PS.The cell with an areal sulfur loading of 6 mg cm^-2 can also achieve a stable cycle at 0.5 C and excellent rate performance,which can be achieved by using the ZnS/NCNS modified separator.The homogeneously distributed N heteroatoms can also help to achieve a stable cycle for 500 h with a lithium anode.3.Here,for the first time,Sn S was used as Li PS adsorbent on separator of LSB.The band gap of Sn S is narrower than SnS₂,so the conductivity is better.At the same time,the inherent S vacancies of Sn S is favorable for the adsorption of Li PS.The results of density functional theory?DFT?show that Sn S absorbs Li PS more effectively than SnS₂.We loaded Sn Snanosheet onto the ultrathin porous carbon nanosheet skeleton?PCNS?,which can be matched with a high loading cathode.Meanwhile,the abundant pore structure of the carbon nanosheet also facilitates the diffusion of lithium ions.These merits contribute to excellent electrochemical performance of the LSB.Therefore,the cell with high sulfur loading(6 mg cm^-2)was also tested at 0.5 C,and after charge–discharge 100 cycles,the discharge capacity was maintained at 739.9 m Ah g^-1.Even at 5 C,the cell with Sn S/PCNS/Celgard separator can still stably cycle for 800 cycles.The synthetic strategy confirmed in this work would stimulate further capability on designing novel architectures for various energy-related device applications.