Phase Engineering Of MoSe₂ For High-performance Lithium-sulfur Batteries

Owing to high specific energy,low cost,and environmental friendliness,lithium-sulfur?Li-S?batteries have gained a lot of attention for meeting the increasing demand of advanced energy storage and mitigating environmental problems.However,there are several challenges in the application of Li-S batteries,including their short life and low sulfur utilization.They become more serious when S-loading density is more than 2 mg cm^-2.These problems seriously degrade the performance of Li-S batteries.Transition metal dichalcogenides?TMDs?have a wide range of applications in devices for energy storage and conversion due to their unique electrochemical properties and tunability of band gap and phases.In this thesis,the application of TMDs in Li-S batteries has been studied.The effects of MoSe₂ on the performance of Li-S batteries are investigated by using carbon cloth?CC?modified by MoSe₂ as interlayer?between cathode and separator?and high S-loading material in Li-S batteries.The detailed contents and results are as follows:1.MoSe₂ is decorated on the surface of carbon fiber in CC by hydrothermal method to obtain MoSe₂/CC.The MoSe₂/CC served as the interlayer between cathode and separator of Li-S batteries.CC has a good conductivity and a physical blocking function,and the introduction of MoSe₂ can make CC polar.As a result,MoSe₂/CC between cathode and separator can restrict the diffusion of sulfur to anode to limit the loss of sulfur.The introduction of MoSe₂ can enhance CC's adsorption of polysulfide,so the dissolved polysulfide can be reintroduced into the redox reaction of charging and discharging.The MoSe₂/CC as interlayer can also accelerate the catalytic conversion of polysulfide.Furthermore,the effect of 1T-MoSe₂/CC and 2H-MoSe₂/CC as interlayer on the batteries is also studied.Our results show that 1T-MoSe₂/CC can improve the performance of Li-S batteries more effectively.The batteries with 1T-MoSe₂/CC as interlayer exhibit a high reversible capacity of 1504 mA h g^-1 at 0.2 C,1053 mA h g^-1 at 2 C and 881 mA h g^-1 at4 C.An outstanding cycling stability with maintained capacity at 601 mA h g^-1?4 C?is observed after 2000 cycles,and the capacity attenuation is 0.017%for every cycle.2.Sandwiched structure cathode is achieved by two pieces of 1T-MoSe₂/CC and size containing elemental sulfur and acetylene black,using 1T-MoSe₂/CC as the high S-loading material.The sandwiched structure can act as the high S-loading cathode.The MoSe₂ can enhance the polarity of the two pieces of CC.As the result,the cathode of sandwiched structure can not only limit the loss of sulfur and buffer the volume change of sulfur,but also adsorb a lot of polysulfide and accelerate the conversion of polysulfide to improve the utilization rate of sulfur.The cathodes with high S-loading of 3 mg cm^-2and 4.5 mg cm^-2 are obtained with the sandwiched structure.The batteries(high S-loading of 3 mg cm^-2)with the sandwiched structure exhibit a high reversible capacity of 1132mA h g^-1 at 0.2 C and 748 mA h g^-1 at 2 C.The batteries also present an outstanding cycling stability with a capacity loss of 0.017%for every cycle after 400 cycles?1 C?.The capacity of 4.5 mA h cm^-2 is achieved by the cathode of sandwiched structure with high S-loading of 4.5 mg cm^-2.