Hollow Multi-Shelled Structures TiO₂ As Sulfur Carriers And Their Applications In Li-S Battery

Materials scientists are devoting a huge effort for developing the next generation of high-performance energy storage materials.Based on the advantages of sulfur such as discharge capacity of 1675 mAh g^-1 when combined with lithium anode,abundant and environmentally friendly characteristics,lithium-sulfur batteries have received extensive attention.Currently,some serious problems in lithium-sulfur batteries have hindered their commercial applications.First,sulfur is not electrically conductive,and the conductivity of the electrode material is critical to the rate performance of the battery.Second,lithium polysulfides generated during discharge will cause irreversible side reactions at the positive and negative electrodes,resulting in capacity loss.Third,during the cycling performance,the sulfur cathode will undergo a dramatic volume change,which affects the durability and safety of the battery.Therefore,in this Ph.D.research,titanium-based oxides with hollow multi-shelled structures?HoMSs?were designed and synthesized by sequential template approach?STA?,and used as sulfur carriers to inhibit the shuttle effect of lithium polysulfide in order to improve the cycling performance of lithium-sulfur batteries.In addition,the physicochemical properties of titanium oxides HoMSs were investigated in depth,and their effect as sulfur carriers on improving the stability of sulfur cathode circulation were also discussed.The main novelty of this thesis is that titanium oxides HoMSs were used as sulfur carriers for the first time in the cathode materials of lithium-sulfur batteries.Initially,titanium oxides HoMSs were used as sulfur carrier materials because of its good chemical stability.In fact,in order to maintains the mechanical strength of sulfur cathode during long cycling,to achieve efficient adsorption of polysulfide anions and rapid migration of electrons and ions in multiple channels,the structural design of the sulfur carrier material is required.Therefore,we firstly designed and synthesized TiO₂ HoMSs with uniform morphology and studied their properties as sulfur carriers.The results have shown that sulfur cathode using triple-shelled TiO₂ HoMSs as sulfur carier exhibited excellent electrochemical performances,with delivered a discharge specific capacity of 623 mAh g^-1 after 160 cycles at 0.5 C rate.In order to improve the intrinsic conductivity of TiO₂ HoMSs,hollow multi-shelled structures TiO_2-x with controllable morphology and uniform size were synthesized and used as sulfur carriers.The TiO_2-x HoMSs were effectively inhibited the shuttle of polysulfide through the synergistic effect of physical trapping and chemisorption,wherein HoMSs can shorten the charge transfer route in sulfur cathode.When the triple-shelled TiO_2-x was used as sulfur carrier,the obtained sulfur cathode exhibited a discharge capacity of 903 mAh g^-1 at 0.5 C rate,and the capacity retention rate after 1000 cycles was 79%with a high coulomb efficiency of 97.5%.The excellent electrochemical performance of sulfur cathode using the triple-shelled TiO_2-x as sulfur carrier should be attributed to the unique physical spatial confinement ability of HoMSs,the chemical adsorption capability for lithium polysulfide,and good electrical conductivity of TiO_2-x.This special HoMSs can shorten the charge transfer path which also contributes to the realization of excellent electrochemical performance.We further designed and synthesized C@TiO_2-x core-shelled structure as sulfur carrier with uniform size and excellent properties.When sulfur loading is up to 56 wt%,the discharge specific capacity of the obtained sulfur cathode is as high as 1456 mAh g^-1 at 0.1 C rate,and the capacity retention after 100 cycles is 97%,which was benefited from uniform core-shelled structure,more polar active sites and a relatively large specific surface area.