Minimizing Sulfur Cathode Catalysts Based On Ti And La Oxides

With the rapid development of electrical vehicles and the clean energy industry,traditional Li⁺-intercalation/de-intercalation Li-ion batteries would not meet the ever-growing demands in our future life.Thus,the development of more advanced,efficient and compatible electrochemical energy-storage devices has become a research hotspot nowadays.Among current energy-storage systems,Li-sulfur batteries are regarded as the most promising next-generation candidates due to their high theoretical energy density(2600 Wh kg^-1),good environmental friendliness and abundant sulfur resources.However,in practical research on Li-sulfur batteries,there are still many challenges remained,including the poor conductivity of sulfur cathodes,the severe volume expansion of sulfur-based actives among redox reactions,Li polysulfide dissolution in ether-based electrolytes and their serious shuttling effect,etc.Particularly,major reasons for the performance degradation of cathodes involve the hysteresis of sulfur-based phase transition reactions and the polysulfide shuttling effect.Normally,that the above two issues are reported to be effectively addressed using transitional metal oxide catalyst-loaded conductive carbon frameworks.However,current catalyst/carbon host materials suffer from several limitations.On the one hand,the overall content of transition metal oxide catalysts is higher than 10%in weight,which limits the content of sulfur actives in cathodes and degrades the battery energy density.On the other hand,these catalysts are unstable enough during battery operation,which are prone to suffer from sulfurization or passivation reactions,compromising their catalytic activity.To address the aforementioned issues about sulfur cathodes,it is proposed to use Ti and La-based transition metal oxide catalysts with the minimized usage amount,a high efficiency and stable physicochemical properties in this thesis.The structure and electrochemical behavior of as-built electrodes are thoroughly characterized to clarify the stability of Ti/La oxide catalysts during cathode deep cycling processes.The main research content is listed as follows:1.Synthesis of little-amount Ti O nano clusters/mesoporous carbon（L-Ti O@CH）hybrid hosts for sulfur cathodes:C₄K₂O₉Ti·2H₂O,PVP and KNO₃ are mixed in solution and frozen/dried to form solid precursors,which are then carbonized to obtain L-Ti O@CH hosts.Herein,the molten KNO₃ salt serves as both a hard template and a pore-forming agent.With the rise of heating temperature,C₄K₂O₉Ti·2H₂O salt gradually decomposes,and the resultant Ti O nano clusters are evenly distributed in the inner region of mesoporous carbon.Such Ti O nano clusters with abundant（200）facets can firmly confine Li polysulfides within cathode regions through physicochemical adsorptions,meanwhile catalytically promoting the phase transitions of Li polysulfides.It is found that only 1.48%of Ti O is contained in L-Ti O@CH hosts.After being infused by S₈actives,the as-formed S₈@L-Ti O@CH cathodes exhibit a high specific capacity(initial capacity:1331 m Ah g^-1 at 0.2 C)and an excellent cycling stability（a capacity decay of0.044%per cycle over 600 cycles）.In addition,we further affirm the outstanding physicochemical stability of Ti O in deep cycling periods aided by multiple in-situ/ex-situ characterization techniques.2.Synthesis of little-amount porous La₂Ti₂O₇（LTO）nanocrystals/carbon black（KB）hosts for sulfur cathodes:The hybrids of LTO fixed in a conductive KB matrix（LTO@KB）can be readily made on a large scale via simple hydrothermal treatments.The LTO nanocrystals generated are surrounded by KB nanoparticles,which can significantly enhance the overall electron transfer property of cathode films.The LTO nanocrystals exhibit a well-defined porous structure,with a pore size ranging from 5 to 50nm.This feature quite benefits electrolyte absorption and infiltration.To guarantee the electrode density and weight ratio of S₈ actives,the total content of LTO in LTO@KB hybrids is controlled at 4.05%.Compared with S₈@KB counterparts,our constructed S₈@LTO@KB cathodes show a better electrochemical activity,a greater rate performance and a higher capacity retention.Furthermore,the ex-situ postmortem characterization and analysis also demonstrate the excellent physicochemical stability of LTO during the whole cycling stage.