First Principles Study On Ti-based Material As Anode Material Of Sodium-sulfur Battery

Rechargeable batteries are a major factor in the transition to renewable energy systems;however,the high cost of current cathode materials and their relatively low capacity limit their widespread use in reality.For an ideal energy storage battery,high capacity and excellent conductivity are necessary.Due to economic and environmental impacts,sodium sulfur（Na-S）batteries are a promising alternative energy storage device.It is hoped that nanosized sulfur cathodes will compensate for the current disadvantages of polysulfide shuttle effect,volume expansion,Na dendrite growth and slow reaction kinetics.Due to their high surface-volume ratio and unique electronic properties,two-dimensional materials have attracted wide attention.The excellent properties of 2D materials are considered to improve the performance of sodium-sulfur batteries.In this paper,several suitable sulfur cathode materials for sodium-sulfur batteries are systematically investigated using first-principles calculations,including geometric mechanisms and electrochemical properties,and basic scientific issues such as anchoring properties and transport mechanisms of Na PSs and sulfur cathode materials are also discussed.The main contents of the paper are as follows:1.Two-dimensional transition metal sulfide,as a typical energy storage material in two-dimensional materials,can theoretically alleviate the shuttle effect of sodium-sulfur batteries better in practical applications.An attempt is made to suppress the shuttle effect of polysulfides with Ti X₂ monolayer（X=S,Se）to accelerate the kinetic process of low-order polysulfides.The calculation results show that Ti X₂has a suitable adsorption strength for polysulfides,which can not only improve the conductivity,but also promote the oxidative decomposition process of polysulfides and improve the sulfur utilization.Theoretically,it can be proved that Ti S₂ is better than Ti Se₂ in improving the performance of sodium-sulfur batteries and is more suitable as the anchoring material for sodium-sulfur batteries.2.MXenes has proven to be a promising sulfur host for Na-S batteries,whose surface functional groups play a key role in their performance.The electrochemical properties of Ti₂CT₂（T=S,Se）MXenes as sulfur host for Na-S batteries were investigated by density functional theory（DFT）calculation.We found that surface functional groups significantly affect the structural properties and electrochemical properties of MXenes.Ti₂CS₂ and Ti₂CSe₂ have good affinity for soluble sodium polysulfide.In addition,they showed excellent catalytic activity for the decomposition of Na₂S.Overall,Ti₂CS₂ and Ti₂CSe₂ are considered promising sulfur hosts for Na-S cells,and our results can also provide insights into the performance of modulated MXenes in other applications.3.In this paper,a novel two-dimensional Ti S₂/graphene heterostructure was theoretically designed as the anchoring material for sodium-sulfur batteries to inhibit the shuttle effect.The heterogeneous structure formed by graphene superimposed on Ti S₂ monolayer retains the inherent metallic electronic structure.Simulation of polysulfide adsorption and charge transfer analysis show that Ti S₂/graphene heterostructures can effectively anchor polysulfide while maintaining metal properties.The low Na ion diffusion potential suggests that the heterogeneous structure can facilitate electrochemical kinetic processes.Therefore,Ti S₂/graphene heterostructure may be an important candidate for sodium-sulfur battery anchoring materials.