| As solid-state cells including solid oxide fuel cells(SOFCs)and all-solid-state sodium-ion batteries(SIBs)show their respective advantages,solid-state cells have received a lot of attentions in recent years.As one key part of all-solid-state cells,electrolytes play a role in transporting ions.Reported works about solid-state electrolytes investigated in depth the mechanism of both the defective stabilization and the ionic migration by many advanced techniques for characterizing properties of materials.Although theoretical researches of solid-state electrolytes have unraveled the mechanism of both defective stabilization and the ionic migration in detail,the mechanism of both defective stabilization and the ionic migration was not analyzed systematically by the chemical bond based on electronic structures.As a matter of fact,the ionic migration is driven by the chemical bond.In this work,effects of chemical bond based on electronic structures on ionic conduction of tetrahedron-based ionic conductors were investigated.(1)Formation energies of oxygen vacancies of LnBO3-based(Ln=Gd,Y)materials were discussed,and the chemical bonding interaction between boron and oxygen ions was studied and the mechanism of the long-range oxygen-ion diffusion was revealed by first-principles calculations and simulations.The crystal structure of LnBO3(Ln=Gd,Y)is formed by the alternative stacking of the B3O9-ring layer and the Gd3+/Y3+layer along the crystallographic c axis,and a B3O9 ring is comprised of three BO4 tetrahedra sharing the vertex.O1,O2 and O3 are the terminal oxygen,while O4 and O5 the bridging one.As the bond length of B1-O5 in BO4 tetrahedron is the longest one,the formation energy of oxygen vacancies at the O5 site is the lowest one,suggesting that the B3O8 chain is more stable.The crystal orbital Hamilton population(COHP)analysis on B-O bonds of BO4 tetrahedron shows that the B1-O5 bond adopts the lowest bond strength in accordance with the calculations of formation energies for oxygen vacancies.Ab initio molecular dynamic(AIMD)simulations were performed on Zn-doped LnBO3(Ln=Gd,Y),and two-dimensional oxygen-ion diffusion in the ab plane was observed.B3O9 rings were broken into B3O8 chains,while B3O8 chains were reformed into B3O9 rings,which is the origin of the long-range diffusion of oxygen ions in Zn-doped LnBO3(Ln=Gd,Y).The chemical-bonding strength between bridging O and B in all of tetrahedral anionic groups of BO4 is relatively weak,therefore,oxygenion vacancies were stabilized at positions of the bridging O,however,B and O form the covalent interaction,suggesting that the chemical bond B-O is difficult to be broken,thus oxygen-ion conductors of borates show the low ionic conductivity.This work first revealed the mechanism of the vacancy stability and long-range diffusion of oxygen ions by first-principles calculations and simulations.(2)A series of materials of Pb3-xBixGa2Ge4O14+0.5x(x=0.0,0.1,0.2,0.3,0.4,0.5,0.6,0.7)were synthesized by the solid-state reaction method and atomic positions of both Ga and Ge were confirmed by the Rietveld refinement on both NPD and XRD data,and electrical properties were characterized for all of materials.The chemical bonding interaction among ions of Pb3Ga2Ge4O14-based materials was investigated and the mechanism of the long-range oxygen-ion diffusion of Bi3+-doped materials was unveiled.The Rietveld refinements on both XRD and NPD data revealed that the crystal structure of the hexagonal Pb3Ga2Ge4O14 is formed by the alternative stacking of the 6member-ring layer and the Pb layer along the crystallographic c axis.Ga and Ge of Pb3Ga2Ge4O14 are disordered at the Wyckoff position of 3f,while those of Bi3+-doped materials ordered,suggesting that the Ga/Ge ordering can be stabilized by interstitial oxygen ions which reside in 6-member-ring voids.Electrochemical impedance spectroscopies showed that Bi3+-doped materials are oxygen-ion conductors.and the composition x=0.6 exhibits the best conductivity(～5.7×10-3 S/cm)at 900℃.Electronic structures of both Pb3Ga2Ge4O14 and Bi3+-doped Pb3Ga2Ge4O14 were calculated,and AIMD simulations on Pb2.5Bi0.5Ga2Ge4O14.25 were carried out.The COHP analysis reveals that antibonding interactions are formed between cations with lone-pair electrons and skeleton oxygen ions,and interstitial and bridging oxygen ions(O2 and O3)also form antibonding interactions.Electronic localization function(ELF)calculations of Bi3+-doped Pb3Ga2Ge4O14 show that there exists the strong columbic repulsion between interstitial oxygen ions and cations with lone-pair electrons,leading to the lone-pair electrons towards 6-member-ring voids only.AIMD simulations of Pb2.5Bi0.5Ga2Ge4O14.25 reveal the essence of two-dimensional(ab plane)diffusion of oxygen ions,and interstitial oxygen ions exhibit the long-range diffusion more remarkable than skeleton oxygen ions due to antibonding interactions between interstitial oxygen ions and other ions.Ga/Ge and O form the bonding interaction which stabilizes interstitial oxygen ions in 6-member-ring voids,and the long-range oxygenion migration was propelled by antibonding interactions between interstitial and skeleton oxygen ions and also by the coulombic repulsion between interstitial oxygen ions and Pb2+/Bi3+with lone-pair electrons.This work first revealed the mechanism of the interstitial oxygen-ion stability and long-range oxygen-ion diffusion of Pb3Ga2Ge4O14-based materials by first-principles calculations and simulations as well as techniques for characterizing properties of materials.(3)The position of interstitial fluorine ions of the mixed-anionic conductor La1+xSr1xGa3O7+0.5(x-δ)Fδ was confirmed,and the chemical bonding interaction among ions were researched and the mechanism of the long-range diffusion of both oxygen ions and fluorine ions was also revealed by first-principles calculations and simulations.Formation energies of interstitial F ions show that interstitial F ions reside in either 5member-ring or octahedral voids.The COHP analysis of chemical bonds La-(O/F),Sr(O/F),Ga-(O/F)and(O/F)-(O/F)reveals that the strength of chemical bonds between F and other ions is weak,leading to weak interactions on F from chemical bonds.AIMD simulations show that oxygen ions exhibit the two-dimensional diffusion in the ab plane while fluorine ions three-dimensional diffusion.The interstitial anions adopt the diffusive coefficient much higher than that of skeleton anions,and the diffusive coefficient of interstitial fluorine ions is higher than that of interstitial oxygen ions,however,the total ionic conductivity is mainly from the ionic conductivity of skeleton anions.Interstitial fluorine ions were stabilized at 5-member-ring voids by the bonding interaction between Ga and interstitial F,and the long-range fluorine-ion migration was propelled by the antibonding interaction between interstitial fluorine ions and skeleton oxygen ions.This work first revealed the mechanism of stability and long-range diffusion of interstitial F ions in the mixed-anionic conductor La1+xSr1-xGa3O7+0.5(x-δ)Fδby first-principles calculations and simulations.(4)Electronic structures of Na3(P/V)S4 of both type Ⅰ(type Ⅰ-t and type Ⅰ-c denote tetragonal and cubic structures,respectively)and Ⅱ(tetragonal structure different from the type Ⅰ-t one)were calculated,and the chemical bonding effect between P/V and S of the tetrahedral anionic group of(P/V)S4 was discussed and the difference of the longrange diffusive ability of sodium ions for all of materials was unveiled by firstprinciples calculations and simulations.The isolated tetrahedral anionic group of(P/V)S4 is aligned along the crystallographic a,b and c axes in both type Ⅰ-t and Ⅰ-c structures,forming large three-dimensional channels for the sodium-ion conduction,while the c axis only in the type Ⅱ structure,forming small ones,and type Ⅰ-t,Ⅰ-c andⅡ structures adopt two,two and four Wyckoff positions,respectively,among which sodium ions are aligned along three-dimensional channels in the type Ⅰ-c structure.Comparison of both electronic structures and migration tunnels in Na3(P/V)S4 for each structural type reveals that the superior sodium-ion diffusion ability in type Ⅰ Na3PS4 is determined by the strong covalent interaction in the tetrahedral anionic group PS4,the weak interaction between Na and S,and the large size and high dimensionality of the sodium-ion diffusive channels blocked by the alignment of the tetrahedral anionic group of PS4.The chemical-bonding interaction between Na and S was weakened by the covalent interaction between P and S of the tetrahedral anionic group PS4,and the sodium-ion diffusive channels of the type Ⅰ structure are larger than that of the type Ⅱone due to the alignment of the tetrahedral anionic group PS4 in the type Ⅰ structure,therefore,type Ⅰ Na3PS4 shows the excellent room-temperature conductivity.This work first revealed the structural stability of Na3(P/V)S4 and factors leading to the big contrast of the room-temperature conductivity between Na3PS4 and Na3VS4 materials by first-principles calculations and simulations.(5)The chemical bonding interaction among ions in Na3MgHfP3O12 were investigated by first-principles calculations,and the mechanism of the long-range diffusion of sodium ions was studied by molecular dynamic simulations based on the atomic potential method.A series of materials of Na3+2xMg1+xHf1-xP3O12(x=-0.05,0.00,0.05,0.10,0.15,0.20,0.25)were synthesized by the solid-state reaction method,and the influence of the non-stoichiometric method on the crystal structure and the conductivity was studied.The skeleton of NASICON structures is formed by both tetrahedra and octahedra sharing the vertex,and sodium ions reside in threedimensional channels.Electronic structures calculations of Na3MgHfP3O12 reveal that Na-O and Mg-O form ionic bonds,while P-O and Hf-O covalent bonds.Interstitial Na ions were introduced into Na3MgHfP3O12 by the non-stoichiometric method,resulting in Na3+2xMg1+xHf1-xP3O12,and a phase transition from trigonal(R3cH)to monoclinic(C12/cl)took place at x≈0.05.Generally,the minimum bond length of Na-O for the trigonal composition is larger than that for the monoclinic composition,or the local structure of sodium ions in the trigonal composition is larger than that in the monoclinic one,therefore,the trigonal composition Na3.1Mg1.05Hf0.95P3O12 shows optimized bulk conductivity at the specif1c temperature(9.88×10-6 S/cm at room temperature).The bond length of Nal-Na2 is shorter than that of Na2-Na2 and the local structure of Nal is bigger than that of Na2,therefore,the migration of from Nal to Na2 is the mechanism of the long-range sodium-ion migration of Na3MgHfP3O12,and the trigonal Na3.1Mg1.05Hf0.95P3O12 shows the best room-temperature conductivity due to the big local structure of sodium ions.This work first revealed the mechanism of the long-range sodium-ion diffusion of NASICON-type Na3MgHfP3O12 by molecular dynamic simulations based on the atomic potential method and the relationship between the minimum bond length of Na-O and the room-temperature conductivity of materials by techniques for characterizing properties of materials.The chemical-bonding strength of tetrahedral anionic groups is reduced by the ionicbonding interaction in such groups,therefore,ionic tetrahedral anionic groups are expected to promote the migration of anions at tetrahedral vertexes.The chemicalbonding interaction between anions of tetrahedral anionic groups and cations beyond such groups is weakened by the covalent-bonding interaction in such groups,therefore,covalent tetrahedral anionic groups are expected to promote the migration of cations beyond such groups. |
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