Simulation Study On The Application Of O/S Functionalized MXenes In Alkali Metal Ion Batteries And Lithium-Sulfur Batteries

To satisfy the huge requirements of electrochemical energy storage,on the one hand,the performance of conventional lithium-ion batteries(LIBs)needs to be improved,on the other hand,the development of the next generation of alkali metal ion batteries needs to be accelerated,and even high energy density lithium-sulfur batteries(LSBs).However,the traditional graphite anode can not be used in the next generation of alkali metal ion batteries.The large-scale application of LSBs is hindered by capacity loss after many cycles owing to lithium polysulfides(Li PSs)dissolution into electrolyte(shuttle effect).It is one of the key problems in the field of electrochemical energy storage to find new anode materials for alkali metal-ion batteries and anchoring materials(AMs)for LSBs.MXenes were investigated as anode materials of alkali metal-ion batteries to enhance the electrochemical performance by first principles calculations.The O-and S-functionalized M₂C(M=Ti,V,Cr,Mn)MXenes were investigated as anchoring materials(AMs)of cathode for LSBs to suppress shuttle effect.The following results were obtained:(1).Configuration,stability and conductivity of M₂CO₂ and M₂CS₂:Except for Nb₂C and Ta₂C,the stable configurations of M₂CO₂ and M₂CS₂ are same.For M of M₂CT₂ in a same period,formation energies become more negative with the decrease of valence electrons(Ne)of M.Except for Ti₂CO₂,Zr₂CO₂,Hf₂CO₂,Mn₂CO₂,Nb₂CS₂,Ta₂CS₂,and Mn₂CS₂ are semiconductors,the rest of MXenes exhibit metallic conductivity.(2).M₂CT₂(T=O,S)as anode materials for LIBs:For M elements of MXenes in a same period and Ne increase from four to six,adsorption strength of Li-ion on M₂CO₂increases with the increase of Ne,and the diffusion barrier decreases with the increase of Ne.The trend of adsorption strength and diffusion barrier on M₂CS₂ is opposite to that on M₂CO₂.Some S-terminated MXenes exhibit lower OCVs,higher theoretical specific capacities,and lower diffusion barriers than corresponding O-terminated MXenes.In general,Cr₂CS₂ and Mo₂CS₂ MXenes exhibit good overall performance as anode materials of LIBs.(3).M₂CT₂(T=O,S)as anode materials for NIBs and KIBs:Similar to the case of LIBs,when the Ne of M increases from 4 to 6 in the same period,the adsorption of sodium and potassium ions on M₂CO₂ becomes stronger with the increase of Ne,while the adsorption on M₂CS₂ becomes weaker with the increase of Ne.For M in the same period and Ne increases from four to six,the diffusion barrier of Na-ion and K-ion on M₂CO₂ decreases with the increase of Ne.However,the diffusion barrier on M₂CS₂ is opposite to that of Li-ion,and also decreases with the increase of Ne.The adsorption order of three alkali metal ions on M₂CO₂ and M₂CS₂ surfaces is Na<Li<K.The diffusion barriers of Na-ion and K-ion on M₂CS₂ are smaller than those on the corresponding M₂CO₂.The theoretical specific capacities of MXenes as anode materials of KIBs are low.However,MXenes exhibit low OCV,low diffusion barrier and high capacity as anode of NIBs.(4).M₂CT₂(T=O,S)as AMs for LSBs:The adsorption order of lithium polysulfide on M₂CT₂(M=Ti,V,Cr,Mn;T=O,S)is Ti₂CO₂<V₂CO₂<Cr₂CO₂;Ti₂CS₂>V₂CS₂>Cr₂CS₂,respectively.When the Ne of M increases from 4 to 6,the trend of decomposition barrier of Li₂S with Ne is opposite to the trend of adsorption strength with Ne.In general,Ti₂CS₂,V₂CS₂ and Cr₂CS₂ exhibit good electrical conductivity,electrocatalysis and anchoring behavior,while Ti₂CO₂ and Mn₂CS₂exhibit good electrocatalysis and anchoring behavior except poor electrical conductivity.