Study on Electrochemical Performance of Hollow Carbon Sphere and Graphene Nanostructures as Sulfur Cathode for Lithium-sulfur Batteries

Author:Liu Shuang Ke

Supervisor:xie kai hong xiao bin


Degree Year:2016





Lithium sulfur(Li-S)battery has attracted researchers’wide attentions both at home and abroad,and is regarded as one of the most promising high energy density rechargeable batteries for next generation,due to its the high theoretical energy density(2600 Wh kg-1),high theoretical specific capacity(1675 mAh g-1),low cost and environmental friendliness.However,the low eletronic conductivity,shuttle effects and volume expansion of sulfur cathode result in low discharge capacity,poor rate performance,short cycle life and low coulombic efficiency of Li-S battery.Furthermore,the low sulfur content in sulfur cathode composite and low sulfur areal density in electrode directly lead to the practical energy density of Li-S battery is far from its theoretical energy density and even lower than the energy density of lithium-ion battery,which hinders its practical application.To address these issues,novel micro-nano structure design and reasonable combination of graphene and hollow carbon sphere were proposed to improve the discharge capacity,cycle life and rate performance of sulfur cathode.At the same,the sulfur content and sulfur areal density of the electrode are also supposed to be improved with the improvement of the tap density and manufacturability of the high sulfur loading cathode material,thus providing practicable strategy to realize high energy density lithium sulfur battery.(1)The effects of sulfur content,sulfur loading approach,particle size,annealing atmosphere and pore creation on the structure and electrochemical performance of the hollow carbon sphere-sulfur(HCS-S)cathode were systematically investigated.HCS-S cathodes with different sulfur content were prepared using melt-diffusion method,and the results indicate the lower sulfur content,the higher conductivity of the sulfur cathode and the better electrochemical performance.HCS-S cathodes were prepared by melt-diffusion method and solution deposition method,the results indicate the solution deposition method leads to smaller sulfur particles,more homogenous distribution of sulfur,which result in better rate performance and long cycle life.HCS-S cathodes with different particle size were prepared,and the results reveal that smaller particle size causes higher specific surface area and pore volume,better contact between sulfur and carbon,thus result in higher discharge capacity and better rate performance.The HCS precursors were annealed under Ar and Ar+5%H2 atmosphere,respectively,the obtained porous shelled HCS annealed under Ar+5%H2 atmosphere has abundant pores and lower oxygen content,which enables stronger polysulfides absorption,thus effectively improve the cycling performance of Li-S battery.The capacity retention could reach 63.6%after 1000 cycles at 2C rate.The EIS plots,SEM images and XPS results before and after cycles further indicate the porous shelled HCS electrode has more stable resistance and better structural stability,the corresponding lithium anode has smaller structural failure and volume expansion,and lower polysulfides and their oxidation products generated during cycling were observed on the surface.A porous honeycomb-like carbon with large pore volume(3.687 cm3 g-1)and high specific surface area(1099.5 m2g-1)were prepared as sulfur cathode by a chemical activation process using ZnCl2 and FeCl3 as porogen and catalyst,which effectively improved the electrochemical performance of the sulfur cathode with high sulfur content.The SEM images of the electrode before and after cycles indicate the importance of large pore volume and specific surface area of the carbon for improving sulfur loading and buffering volume expansion during cycles for sulfur cathode with high sulfur content.(2)The GO coated sulfur(GO-S)cathode and rGO-S cathode were prepared by solution deposition method,the structure and electrochemical performance of the two cathode were studied and compared.Due to the low electronic conductivity of GO and large particles of sulfur,the GO-S cathode has a low discharge capacity;the physical coating and chemical bonding effects of the GO layer could suppress sulfur dissolution and shuttle effects to some extent,thus the cycle performance of the GO-S cathode could be effectively improved.The SEM and XPS analysis of the electrodes during cycling indicate despite the GO coating and chemical bonding effects,the sulfur dissolution and shuttle effects still exist during the long cycling process,moreover,the deposit of the sulfur cathode transfer from polysulfides to sulfur oxide species,the corresponding lithium anode generates lithium dendrite and uneven volume changes,which may be the main reason of the capacity loss during long cycle life.The rGO-S cathode has smaller sulfur particles and the rGO has good electronic conductivity,thus it delivers higher discharge capacity and rate performance.However,the rGO fails to form coating structure and has lower polysulfides absorption ability than the GO,thus the rGO-S has a worse cycle performance than the GO-S cathode。(3)The novel GO coated HCS-S cathode and 3D rGO-HCS/S cathode were designed and prepared,their structure and electrochemical performance were also studied.A novel nanostructure of GO coated HCS-S cathode was prepared by a two-step process,the HCS could provide abundant pores to accommodate sulfur and fast electronic transportation,the GO coating structure could effectively suppress the shuttle effect of polysulfides,thus this nanostructure design could effectively improve the discharge capacity and cycle life.A novel 3D nanostructure of rGO-HCS was prepared by hydrothermal self-assembly method and high temperature carbonization process.The 3D conductive rGO network provides fast electronic transportation and solid frame construction,the HCS could effectively accommodate sulfur and buffer volume expansion,and both could effectively improve the rate performance and cycling stability.The discharge capacity is as high as 770 mAhg-1 at 4C rate,the capacity retention is 77.8%after 400 cycles at 1C rate with a low capacity decay rate of 0.052%per cycle.(4)The HCS micro-cluster and 3D rGO-HCS were used as sulfur host to prepare high sulfur loading cathode to realize high energy density of Li-S batteries.The HCS micro-cluster was prepared as sulfur host by a simple binder self-assembly method,which could effectively improve the tap density of the cathode.There are no cracks on the electrode under a high sulfur areal density and the cathode electrode could still maintain good electrochemical performance.When the sulfur areal density is 4.5 g cm-2,the discharge capacity could reach 1058.4 mAhg-1 at 0.2C rate,and the capacity retention is 57.8%after 115 cycles.The energy density of the Li-S battery is calculated to be 421 Wh kg-1 according to the proposed equation.A 3D rGO-HCS/S cathode with high sulfur content was prepared by melt diffusion method,the tap density of the cathode increased markedly with the increase of sulfur content,so do the specific capacity and energy density based on the whole mass of the electrode.A 3D rGO-HCS/S cathode with high sulfur content of 79%was also prepared by a solution deposition method,when sulfur areal density reaches 5.5 mg cm-2,the discharge capacity could reach 921.2 mAhg-1 at 0.2C,and the capacity retention is 84%after 60cycles.The energy density of the Li-S battery is calculated to be 470 Wh kg-1 according to the proposed equation.