Structural Design And Li/Na/K Storage Mechanism For MoSe₂-based Heterostructure

MoSe₂,as a typical transition metal chalcogenide(TMDs),has attracted much attention due to its unique crystal structure,high specific surface area and low voltage plateau.However,its wide band gap(1.55 e V)results in poor electronic conductivity of MoSe₂,which limits its further development in the field of energy storage.Constructiing conductive material/MoSe₂heterojunction is an effective way to solve this problem,but its structural properties and Li/Na/K storage mechanism need to be further studied.In this paper,the crystal structure,electronic properties and Li/Na/K storage mechanism of four heterojunctions based on MoSe₂(VSe₂/MoSe₂,Graphene/MoSe₂,Nb Se₂/MoSe₂,Ti S₂/MoSe₂)were systematically studied by first principles calculation.It is found that the introduction of appropriate vacancy defects is an important method to improve its Li storage performance,and the internal mechanism was discussed.Its specific content and conclusion are as follows:(1)Mechanism of Li/Na/K storage in VSe₂/MoSe₂heterojunction.Firstly,the optimal configuration of VSe₂/MoSe₂heterojunction was constructed,and the binding energy of Li/Na/K adsorbed in the heterojunction was calculated.It shows that Li/Na/K is more easily adsorbed on the interlayer H_Vsite of the heterojunction,which is located in the center of the six membered ring surrounded by Se in VSe₂layer.Then,the interlayer diffusion barriers of Li,Na and K in the VSe₂/MoSe₂heterostructure are 0.48 e V,0.21 e V and 0.11 e V,respectively.The interlayer diffusion barrier of Li in V_0.92Se_1.84/MoSe₂heterostructure is 0.07 e V.Therefore,the introduction of appropriate VSe₂-type vacancy defect in VSe₂/MoSe₂is an effective method to improve obviously its Li interlayer diffusion.Then,molecular dynamics method was used to further prove that the V_0.92Se_1.84/MoSe₂heterojunction has good thermal stability at 300 K;In addition,the V_0.92Se_1.84/MoSe₂heterojunction has metal characteristics and good electronic conductivity.(2)Mechanism of Li/Na/K storage in Graphene/MoSe₂heterojunction.Firstly,the most stable configuration of Graphene/MoSe₂heterojunction was constructed by calculating the mismatch degree.The binding energy of Li/Na/K in the heterojunction was compared.The results show that the interlayer M_Mosite of the heterojunction(above Mo)are the most stable adsorption sites of Li and Na,while K is stably adsorbed at the H_Gsite of Graphene surface(above the center of C 6-membered ring).Secondly,the interlayer diffusion barriers of Li and Na in the Graphene/MoSe₂heterostructure are 0.36 e V and 0.20 e V,respectively.The diffusion barrier of K ions on the Graphene surface is 0.087 e V.And the interlayer diffusion barrier of Li ions in the heterojunction with single C atom vacancy(SV–Graphene/MoSe₂)is 0.03e V.Therefore,the interlayer Li can diffuse much faster in the SV–Graphene/MoSe₂heterostructure.Meanwhile,SV-Graphene/MoSe₂heterojunction can maintain good metal property and thermal stability.(3)Mechanism of Li/Na/K storage in Nb Se₂/MoSe₂heterojunction.Firstly,the Nb Se₂/MoSe₂heterojunction with stable structure was constructed by single-layer Nb Se₂and single-layer MoSe₂.By comparing the binding energies of Li,Na and K adsorbed on the Nb Se₂/MoSe₂heterojunction,it is determined that the most stable adsorption site of Li and Na is the interlayer M_Hsite(the hollow site of Se-Mo six membered ring),The most stable adsorption site of K is the T_Nbsite on the surface of Nb Se₂(the top of Nb).Then the diffusion barrier of Li/Na/K is calculated by CI-NEB method.The interlayer diffusion barriers of Li and Na are 0.668 e V and 0.390 e V,respectively.The diffusion barrier of K on Nb Se₂surface is 0.049 e V,and the Li interlayer diffusion barrier of Nb_0.92Se_1.84/MoSe₂heterojunction is lower than that of Nb Se₂/MoSe₂heterojunction(0.040 e V vs.0.668 e V).Nb_0.92Se_1.84/MoSe₂heterojunction still has metal conductivity and good thermal stability.(4)Mechanism of Li/Na/K storage in Ti S₂/MoSe₂heterojunction.Firstly,based on the optimal configuration of Ti S₂/MoSe₂heterojunction,the binding energy of Li/Na/K was calculated.It shows that the interlayer M_Sesite(above Se atom)is more conducive to Li/Na intercalation,and K is more easily adsorbed on the surface of Ti S₂(above S atom).And the interlayer diffusion barrier of Li and Na are 0.216 e V and0.157 e V,respectively.The diffusion barrier of K ion on Ti S₂surface is 0.139 e V.On the other hand,the interlayer diffusion barrier of Li in Ti_0.92S_1.84/MoSe₂heterojunction is only 0.07 e V,which is significantly lower than that in Ti S₂/MoSe₂heterojunction.At the same time,the Ti_0.92S_1.84/MoSe₂heterojunction still has metal characteristics.The molecular dynamics method further shows that the Ti_0.92S_1.84/MoSe₂heterojunction has good thermal stability at 300 K.