Preparation Of Metal Oxide-based Separator Modification Materials And Their Application In Lithium-sulfur Batteries

Lithium-sulfur batteries have gained huge attention because of their high theoretical specific capacity(1675 m Ah g^-1)and giant energy density(2600 Wh kg^-1).At the same time,sulfur as a cathode material has natural advantages such as abundant reserves,non-toxicity and harmlessness.However,the shuttle effect caused by the dissolution of long-chain polysulfides and the slow reaction kinetics caused by the insulation of elemental sulfur and sulfide seriously affect the cycling performance of lithium-sulfur batteries.Therefore,researchers are focusing on the rational design of lithium-sulfur batteries with excellent performance,starting from different structures（anode,cathode,separator and electrolyte）.Commercial polyolefin separators have a rich pore structure that cannot block lithium polysulfide on the cathode side.To address this issue,constructing a catalytic modification layer that is both low-cost and efficient is considered to be a feasible strategy.Metal oxides are often used in separator modification layers due to their polar surface and catalytic properties to promote the adsorption and conversion of lithium polysulfide.However,metal oxides typically have lower conductive which limits their application in lithium-sulfur batteries.Moreover,under harsh conditions such as high sulfur loading and low electrolyte dosage,metal oxide catalysts have limited ability to inhibit lithium polysulfide shuttle.In order to address these issues,this thesis proposes designing metal oxide-based catalysts from the perspective of regulating the electronic structure by using mixed valence states and anion doping.Two efficient separator modification layer materials for lithium-sulfur batteries have been developed:（1）The electronic structure of the two-dimensionalδ-MnO₂ nanosheet was modified by a simple thermal annealing step and used as a separator modification layer for lithium-sulfur batteries.The thermal annealed MnO₂ nanosheets not only retain a two-dimensional morphology,which can provide rich lithium polysulfide anchor points,but also improve the electronic conductivity of manganese dioxide with its unique Mn⁴⁺/Mn³⁺mixed valence state.Therefore,the separator modification layer can effectively prevent the shuttle of lithium polysulfide and promote its catalytic conversion.The corresponding lithium-sulfur battery exhibits good cycle stability,rate performance and high load performance.Under high sulfur loading(10.91 mg cm^-2)and low E/S ratio(6.46μL mg^-1),the full cell provides areal capacity of 7.0 m Ah cm^-2at a areal current of 0.56 m A cm^-2,and remains at 4.0 m Ah cm^-2 after 100 cycles at a higher current(1 m A cm^-2).（2）The P-MoO₂@C material was synthesized by using phosphomolybdenum acid as a precursor,and used as a separator modification layer for lithium-sulfur batteries.In the P-MoO₂@C structure,polarized MoO₂ can form polar bonds with lithium polysulfide,and doped P could serve as an electron-rich donor to further boost the adsorption capacity,which can inhibit the shuttle effect of lithium polysulfide.In addition,the carbon-coated layer on the surface of P-MoO₂ can also play a physical adsorption role.Compared with MoO₂@C coatings,P doping can effectively reduce the interfacial charge transfer resistance,thereby improving the utilization of active sulfur.Lithium-sulfur batteries equipped with P-MoO₂@C modified separator have a high initial specific capacity of 980.1 m Ah g^-1 at 1C,and a reversible capacity of 628.4m Ah g^-1 after 400 cycles.At higher current density（2C）,the cell still has a high specific capacity of 791.3 m Ah g^-1,showing good battery capacity and cycle stability.