Exploring Pristine And Complex Mg-based Compounds With Potential Lithium-storage Properties:Ab Initio Insight

With the changes in the environment and the consumption of fossil fuels,the demand for green energy in today's society is growing.The energy issue has attracted the attention of experts and scholars around the world.Finding new green energy has become one of most the important issues in the world today.In recent years,renewable energy such as solar energy and wind energy has achieved rapid development,and new energy sources often require a long period from birth to widespread application.In this long period of time,the corresponding advanced energy storage technology is particularly important.Lithium-ion battery technology is one of the representatives.Up to now,the traditional embedded electrode reaction mechanism has limited the further development of lithium storage materials to some extent.With the discovery of new conversion-reactive lithium-ion battery materials such as MgH2 for the first time by Oumellal et al.,complex hydrides(compared to simple hydrides)with richer chemical bond types and potential doping effects have received increasing attention.In this paper,based on the first-principles method,we have calculated,screened and designed magnesium-based complex hydrides as conversion-reactive lithium-storage materials,and tried to provide experimental researchers in the field in the context of the emerging paradigm of the material genome,in-depth guidance and design basis.The main research results are as follows:(1)The performance of Mg2NiH4 acts as potential negative anode of conversion-type Li-ion batterybasedonAb initio insightIn this work,pure and dopant Mg2NiH4 hydrides are explored for potential Li-ion battery conversion anode materials applications from state-of-the-art Density functional theory.The most thermodynamically stable dopant Mg2NiH4 structure is determined by the formation energy.Comparing the electrochemical properties of the different structure,4 kinds of elements are chosen as the dopant.Especially,the Li-doped Mg2NiH4structure is determined to be the most thermodynamically structure which possesses a smaller band gap than pure material and owns a theoretical specific capacity of 975.35mAhg-1 and an average voltage of 0.437 V(vs.Li+/Li0).The Li-dopingalso improves the diffusion behavior of Li-ion in electrode material especially at 300K,which implies the promising rate capability of the device at room temperature whenthe anode material is doped utilizing Li element.The non-empirical values of diffusion coefficients of Li-ion in both pure and Li-doped Mg2NiH4 system are also quantitatively determined from ab initio molecular dynamics.After Li-doping,the diffusion coefficient of Li-ion in the electrodeis evidently increased sharply at 300K.(2)The performance of Mg(AlH4)2 acts as potential negative anode of conversion-type Li-ion batterybasedonAb initio insightAccording to the calculation of the electronic structure of the Mg(AlH4)2 system,it can be concluded that there are two doping sites of Mg atom position and Al atom position in the Mg(AlH4)2 system.In order to properly control the doping concentration,the Mg(AlH4)2 unit cell is expanded to a 2×2×2 supercell.According to the properties of different elements in Mg(AlH4)2 system,seven different doping elements of Li,B,C,Na,Si.K and Ca were used to respectively carry out two doping sites of Mg(AlH4)2 system.The formation energies of 15 systems were calculated separately,and the thermodynamically stable structure of each dopant atom was determined according to the formation energy.By comparing the electrochemical properties of lithium storage,such as specific capacity,band gap and half voltage of these systems,it is determined that the substitution of Mg,Ni and K atoms for the position of Mg atoms is good for improving the electrochemical properties of Mg(AlH4)2 system effectively.All three dopants reduce the bandgap,half-voltage and volumetric rate of the system.On this basis,the electronic structures of these three doping systems were analyzed in depth,and the reasons for the improvement of the performance of these three doping atoms were determined.