Electrical Properties Of BSB/BP Semiconductor Monolayers As Cathodes For Lithium-sulfur Batteries

With today’s rapid technological development,along with the need for energy and environmental protection,we need to reduce our reliance on traditional fossil fuels.In recent years,the lithium-sulphur battery（Li-S battery）has gradually come into the limelight,as its theoretical specific energy density can reach 2600 Wh·kg^-1,far exceeding the battery capacity of conventional lithium-ion electrons,a demand for energy storage that cannot be met in conventional batteries.Over the past decades of development,lithium-ion batteries have come close to the limit of their theoretical capacity.As a result,lithium-sulphur batteries are widely considered to be a new type of energy storage material.In lithium-sulphur batteries,sulphur is used as the cathode material,which is abundant in nature,relatively cheap and less polluting to the environment.However,there are many challenges in the development of lithium-sulphur batteries,such as low Coulomb efficiency and low cycle times,which have hindered the commercialisation of lithium-sulphur batteries.One of the most important challenges is the dissolution of polysulphides into the electrolyte during charging and discharging,which is known as the"shuttle effect".In order to solve this problem,scientists have been searching for new materials or advanced technologies to solve this problem.In this paper,we have explored two new group III-V two-dimensional porous materials,BSb and BP,as adsorbent materials for lithium-sulphur battery cathodes based on first principles and density flooding theory.The main studies are as follows:（1）In the BP and BSb unit ring structures we compared the effect of atom substitution,after replacing the P atoms with Sb atoms.After structural optimisation of the BSb monolayer,and thus considering the various initial adsorption sites of Li₂S_n on the BSb monolayer,the optimal adsorption sites were determined after structural optimisation.Next,the properties of charge transfer,physical adsorption chemisorption,density of states and diffusion potential barriers were analysed.The results obtained from the calculations can show that BSb has improved in adsorption energy and electrical conductivity after replacing non-metallic atoms with metallic atoms,thus it can better adsorb polysulfides and avoid polysulfides from dissolving into the electrolyte,therefore,BSb monolayer material is expected to be an anode 2D material for lithium-sulfur batteries.（2）On the basis of BP unit ring we considered the effect of structural change from the original unit ring structure to multi ring structure.The multiple ring structure provides more adsorption sites for the adsorption of polysulphide,which is expected to improve the adsorption performance.On this basis,we carried out structural optimisation of the polycyclic BP monolayer so that various initial adsorption sites for Li₂S_n on the BP monolayer were considered.For each polysulphide,we tested 8to 10 different initial positions and,after structural optimisation,identified the best adsorption site with the highest adsorption energy.Next,the properties of charge transfer,physical adsorption chemisorption,density of states and diffusion potential were analysed.The results of the calculations show that the polycyclic BP monolayer has significantly improved adsorption energy,electrical conductivity and diffusion properties compared to the unitary ring structure.The BP polycyclic monolayer has stronger adsorption properties and can effectively avoid the dissolution of polysulphides in the electrolyte.At the same time,the polycyclic BP has a lower diffusion barrier,which ensures the rapid diffusion of polysulphides along the BP monolayer.Therefore,the BP polycyclic ring structure can alleviate the shuttle effect and improve the performance of lithium-sulphur batteries,and is a promising cathode 2D material for lithium-sulphur batteries.In particular,the structural change from a unitary ring to a polycyclic ring provides a new idea to improve the adsorption performance of electrode materials.