First Principles Calculation Of The Adsorption Behavior Of Lithium Polysulfide/Sodium Polysulfide On Co₉S₈

Lithium-ion batteries have an increasingly difficulty to satisfy the market development requirements because of high-cost lithium and limited energy density.In comparison,lithium-sulfur and sodium-sulfur batteries are expected to be ideal choices for energy storage systems due to their high theoretical energy density.Researchers have sought to address such issues as slow reaction kinetics,which is caused by solubility of long-chain polysulfides into electrolyte,and electronic insulation of cathode sulfide and its negative effect on charge/discharge efficiency.To suppress adsorption of polysulfides and increase electronic conductivity,suitable materials as sulfur hosts have been proposed.Co₉S₈,a transition metal sulfide with excellent electrical conductivity,in this thesis is to be investigated for its adsorption and catalytic performances on polysulfides in lithium-sulfur and sodium-sulfur batteries.The main contents are as follows:1.Co₉S₈ is utilized as an anchoring material for polysulfides in lithium-sulfur batteries,the VASP software which bases on DFT framework is employed to calculate electron structures such as the band structure and density of states of Co₉S₈ as well as other properties such as the ground state energy,the change of bond length along with bond angle,charge transfer quantity,charge density difference,density of states,Gibbs free energy,and the energy barrier of the reaction as well as other aspects of lithium polysulfides（including Li₂S_x,x=1,2,4,6,8 and S₈ ring）on Co₉S₈（202）characteristic surface by utilizing the first principles calculation.The results demonstrate that the Co₉S₈’s band gap is close to 0 e V,which indicates its obvious metal conductivity,and the conductivity remains after adsorption of lithium polysulfides.The adsorption energy of long chain lithium polysulfides Li₂S₈ is the highest in polysulfides species absorbed on Co₉S₈（202）,reaching-6.069 e V,which plays an important role in suppressing the effective mitigation of the"shuttle effect".For reaction from S₈to Li₂S₂,Co₉S₈ can make the reduction of Gibbs free energy towards negative,promoting the discharge reaction of lithium-sulfur batteries and improving the electrochemical performance.Co₉S₈ can also reduce the decomposition barrier of Li₂S and the diffusion barrier of Li atom to 0.71 e V and 0.698 e V,respectively,indicating that Co₉S₈ can promote the conversion of lithium polysulfide and the whole electrochemical process.2.The adsorption energy of sodium polysulfides（including Na₂S_x,x=1,2,4,6,8 and S₈ring）,the geometry optimization of crystals,charge density difference,charge transfer quantity,density of states,Gibbs free energy and energy barrier are calculated.The results show that Co₉S₈ has the highest adsorption energy with Na₂S₈,reaching-4.576 e V,which can effectively alleviate the shuttle effect of sodium-sulfur batteries.After adsorption of sodium polysulfides by Co₉S₈,the conductivity of the system is expressed by the density diagram of states,which improves electrical conductivity of sulfur cathode.The Gibbs free energy of the reaction from short-chain Na₂S₄ to Na₂S₂ is 0.2 e V lower than that of lithium-sulfur.Decomposition and diffusion barriers on surface of Co₉S₈ are lower than those of lithium-sulfur,thus demonstrating that it has a superior catalytic and decomposition effect on sodium polysulfides,thereby increasing the charging reaction and reaction rate of sodium-sulfur batteries.In this paper,it is considered that transition metal sulfide Co₉S₈ can effectively enhance the conductivity of sulfur cathode and adsorb long chain polysulfides.