First-principles Study On The Energy Storage Properties Of Two-dimensional NbS₂ And NbS₂/WS₂ Heterojunction

In recent years,due to the crisis of traditional fossil energy and environmental pollution problems,the search for sustainable,low-cost,low-pollution and high-performance energy storage materials has become the main research direction in the field of new energy development.Two-dimensional transition metal dichalcogenides（TMDs）has stable voltage window,high theoretical capacity,excellent electrical conductivity and high migration rate when used as anode materials,so it has a wide range of practical application prospects in energy storage.In this paper,first-principles calculations are used to theoretically simulate and predict the performance of NbS₂ and NbS₂/WS₂heterojunctions as anode materials for alkali metal ion batteries.The first chapter mainly summarizes the research status of alkali metal ion batteries and two-dimensional materials and heterojunctions.The second chapter mainly introduces the theoretical basis and methods of the software for first-principles calculations.The third chapter mainly explores the electrochemical performance of two-dimensional NbS₂monolayer as anode material for alkali metal ion batteries,the adsorption and migration properties of Li,Na,K,Mg,Ca and Al ions which are investigated by employing first-principles calculations.The NbS₂ monolayer exhibits high structural stability upon metal ion adsorption as well as retains low metal ion diffusion barriers of 0.20（Li）,0.09（Na）,0.06（K）,0.51（Mg）,0.37（Ca）,and 0.39 e V（Al）.A NbS₂ monolayer could adsorb two layers of Li,Na and Mg,one layer of K and Ca,and 1/9 layer of Al ions,result in theoretical capacities of 580.14,430.56,843.23,227.87,451.98 and 55.83 m Ah?g^-1,respectively.These results suggesting that the NbS₂ monolayer could be suitable anode material for Li,Na,K,Mg and Ca ion batteries with average open circuit voltages of 1.34,1.06,1.38,0.28 and 0.84 V.The fourth chapter mainly studies the electrochemical performance of NbS₂/WS₂ van der Waals heterojunction as anode material of lithium-ion battery and magnesium-ion battery.The results show that the constructed NbS₂/WS₂ heterojunction has thermodynamic stability,metallic conductivity and high Mg ion mobility.Through researching the adsorption of metal ions on the surface of the NbS₂/WS₂ heterostructure,confirming that the formation of the heterojunction retains the ions stable adsorption energy on the NbS₂ layer.In addition,the diffusion energy barriers of Li and Mg ions on the WS₂ surface in the heterojunction are 0.25and 0.18 e V,where the diffusion energy barrier of Li ion is similar to the NbS₂ monolayer,while that of Mg ion is significantly lower.The NbS₂/WS₂ heterojunction can stably adsorb six layers of Li and four layers of Mg with theoretical capacities of 359.95 and 428.18m A?h?g^-1.Meanwhile,both Li and Mg exhibite lower average open circuit voltages of 1.33and 0.55 V,respectively.In this paper,we systematically explore the energy storage properties of TMDs monolayer materials and its van der Waals heterojunction applied to battery anode materials from theoretical perspective.The interactions between these materials and metal ions are analyzed from the views of electronic structure and migration mechanism,which provides a useful reference value for the application of such materials and heterojunctions in new generation of metal ion batteries.