First-Principles Studies On The Physical Properties Of Ternary ANiN(A=Li,Na,Mg,Ca) Compounds

Lithium-ion batteries are new type of high-energy and environmentally friendly green energy storage equipment,and they are currently in the leading position in the portable electronic equipment market.However,due to the lack of lithium resources and the demand for large-scale energy storage equipments,it is crucial to develop other green energy storage equipments that can replace lithium-ion batteries.The resources of Na,Mg,and Ca are rich in content and have the closest physical and chemical properties to Li,which have attracted wide attentions at present.However,Na+,Mg2+,and Ca2+cannot deintercalate smoothly by large capacity in the carbon-based anode materials commonly used in current lithium-ion batteries.Therefore,finding and developing new anode materials is a key step in the development of new ion batteries.Due to the variable valence states of the transition metals,they are very important in the research of new battery electrode materials.In the ANiN(A=Li,Na,Mg,Ca)materials,lithium transition metal nitrides exhibit high ionic conductivity and cycle stability,as well as good specific capacity and interesting one-dimensional conductivity.Therefore,we conducted a comparative study on the physical properties of this series of materials by the first-principles calculation:1)When ANiN(A=Li,Na,Mg,Ca)materials are used in batteries,their thermodynamic stability and elastic stability are very important,and the electronic structures(such as the band gap)also play a key role.Therefore,it is meaningful to study the physical properties of ANiN(A=Li,Na,Mg,Ca)materials.We first calculate the equation of states(i.e.,the relationship between binding energy and cell volume)of these materials,and fit the results by using the Murnaghan equation of state.Then,the phonon spectra of the systems are calculated,and the most stable structure of each material is determined under the comprehensive consideration of both the binding energy and phonon spectrum of the system(e.g.,the phonon spectrum should not have imaginary frequencies).The calculations of the elastic constants of ANiN(A=Li,Na,Mg,Ca)materials show that the elastic constants of these materials all satisfy the BornHuang criterion,which indicate that these materials are elastically stable.On the basis of further consideration of the elastic properties,it can be determined that the tetragonal phase is the most stable structural phase of ANiN(A=Li,Na,Mg,Ca)materials.After that,we calculate the properties of the electronic structures for the most stable structure.The results show that both LiNiN and CaNiN materials are half-metals,MgNiN is a spin-polarized magnetic metal,and NaNiN is just a common metal.The Ni-N chain constitutes the main framework of the structure of these materials.The bond between Ni and N atoms is mainly a mixture of ionic and covalent bonds.2)In practice,the conditions(such as temperature and pressure)during material manufacturing may cause defects.The lack of transition metal Ni atoms in the ternary transition metal nitride LiNiN may have a large impact on the properties of the material.Through the first-principles method,we have studied the vacancy formation energy of Ni in LiNixN(x=1.000,0.875,0.750)materials and the electronic structure properties of the corresponding materials.The calculation results show that the vacancy formation energy of Ni in LiNi0.750N(8.732 eV)is more than twice the vacancy formation energy of Ni in LiNi0.875N(3.184 eV).It shows that with the increase of Ni defects in the LiNiN,it is difficult for Ni atoms to leave the system.The decrease in the number of Ni atoms leads to a significant decrease in the total magnetic moment of the system,indicating that Ni atoms play a decisive role in the ferromagnetism of the system.Through the analysis of the band structure and the total density of electronic states,as the decrease of Ni atoms,more bands cross the Fermi level,and the density of electronic states at the Fermi level continues to increase.The analysis of the partial density of states and the deformation charge density shows that the Ni-N bond in the LiNixN(x=1.000,0.875,0.750)system comes from a strong coupling between the d orbital of the Ni atoms and the p orbital of the N atoms.Ni-N bonds also have obvious ionic properties.After the Ni atom is removed,part of the Ni-N straight chain is broken into the N-NiN short chains,resulting in a decrease in the stability of the material.