First-principles Design Of Light Metal-based Hydrides For Sodium/Potassium Storage And Catalytic Application

Light metal-based hydride is a kind of environment-friendly material.It not only has excellent performance in hydrogen storage,but also has a lot of progress in exploration of other functional applications.In recent years,countries around the world are constantly paying attention to the application of light metal-based hydride in different fields.In this article,based on first principles,we explored the light metal based hydride LiAl(NH2)4 and its doping system,Mg7TiH16 and its doping system as sodium/potassium ion battery anode materials,including their crystal structure,electronic structure and the electrochemical properties.We also studied the TiH2(111)surface as the ammonia synthesis catalyst of its reaction mechanism.The concrete research contents and main results are as follows:The theoretical specific capacity of Li4Al4(N4H8)4 as sodium storage anode material is 1093.77 mAh/g,and the average embedded voltage of sodium ions is 0.29 V.It is found that the electrochemical performance is improved obviously when the doping atom concentration of B element is 5.36 at%.When the optimal doping system is used as the anode material for sodium storage,the theoretical specific capacity is increased to 1249.57 mAh/g,and the average embedded voltage is reduced to 0.09 V.The diffusion barrier of sodium ions in the lattice of Li4Al4(N4H8)4 is 0.45 eV,and that in the lattice of Li4AlB3(N4H8)4 is 0.31 eV.The average embedded voltage of Li4Al4(N4H8)4 is 0.23 V,which is the same as the theoretical specific capacity of Li4Al4(N4H8)4 as the potassium storage anode material.With the doping of B element and the increase of doping concentration,the average embedded voltage of potassium ions decreases to 0.15 V.In the diffusion process,potassium ions need to overcome the diffusion barrier of 1.46 eV in the lattice of Li4Al4(N4H8)4 and 1.25 eV in the lattice of Li4AlB3(N4H8)4.By comparing the electrochemical performance of Li4Al4(N4H8)4 and its doping system as sodium storage materials and potassium storage materials,we found that Li4Al4(N4H8)4 and its doping system as sodium storage materials and potassium storage materials have higher theoretical specific capacity,lower average embedded voltage,the doping of B element can efectively improve the electrochemical performance of LiAl(NH2)4 system and improve the diffusion properties of sodium/potassium ions in the lattice.The theoretical specific capacity of Mg7TiH16 as sodium storage anode material is 1832.56 mAh/g,and the average embedded voltage is 0.414 V.It was found that when Li element was selected as the dopant and occupied the Mgl position,there are the most obvious performance improvement effect.When Li-Mgl doping system is used as anode material of sodium ion battery,the theoretical specific capacity is increased to 1978.26 mAh/g,and the average embedded voltage is reduced to 0.408 V.The diffusion energy barrier of 0.04 eV is needed for the diffusion of sodium ions on the Mg7TiH16 surface,and the diffusion energy barrier of 0.01 eV is needed for the Li-Mg1 doped system surface.The average embedded voltage of Mg7TiH16 as potassium storage anode material is 0.32 V,and the average embedded voltage decreases to 0.31 V when Li atom occupies the position of Mgl.In the diffusion process,potassium ions need to overcome the energy barrier of 0.07 eV on the surface of Mg7TiH16 and 0.02 eV on the surface of Li-Mgl doping system.When Mg7TiH16 is used as sodium storage material and potassium storage material,the theoretical specific capacity is higher and the average embedded voltage is lower.The doping of Li element can effectively improve the electrochemical performance of Mg7TiH16 system and improve the diffusion properties of sodium/potassium ions in the lattice.It was found in the study of TiH2 as catalyst for ammonia synthesis that on the TiH2(111)plane,the stable adsorption position of N2 molecule is that one N atom is above fcc site and the other N atom is above hcp site.The stable adsorption position of N atom on the TiH2(111)plane is fcc site,and the dissociation energy barrier of N2 molecule on the TiH2(111)plane is 0.64 eV.When using L-H mechanism to investigate the ammonia synthesis process of N2 and H2 on TiH2(111)surface,the potential barrier of N*+H*=NH*+*is higher,which is 1.97 eV,indicating that it is difficult to form NH*by adding H atoms directly on the surface.When N2 and H2 synthesize ammonia on TiH2(111)surface,the activation energy of the dissociated N atom is still high during hydrogenation.When N2 and H2 synthesize ammonia at the TiH2(111)plane,the H atom at hcp site needs to overcome the activation energy of 1.35 eV to combine with the N atom at the surface to form NH*,and the H atom at fcc site needs to overcome the activation energy of 1.69 eV to combine with the N atom at the surface to form NH*.When the number of H atoms is increased both on the surface and in the interior,the protonation of N atoms on the surface needs to overcome a small energy barrier.The above studies are of great significance for exploring the application of TiH2 and other metal hydrides in ammonia synthesis catalyst.