Ion Migration Mechanism Of NASICON-type Compounds

NASICON-type compounds have received widespread attention as a class of solid electrolytes or electrode materials due to their superior thermal stability,chemical stability and fast and simple synthesis processes.The ionic conductivity is the key property.A comprehensive understanding of the ionic migration mechanism from atomic scale can lead to rational design rules for improving the ionic conductivity of the NASICON-type compounds,but this understanding is challenging to attain due to the structural complexity of NASICON.In this dissertation,the ionic migration channels and the ion migration mechanisms of three typical NASICON-type compounds were thoroughly investigated by combining multiple theoretical and experimental methods.The main research contents and results are summarized as follows:1.We identified the migration channels and bottlenecks in the rhombohedral NASICON-type compound NaZr₂(PO₄)₃ and unraveled the preferential migration channel for Na⁺ion diffusion by combining crystal structure analysis,bond valence energy landscape(BVEL)analysis,and ab initio molecular dynamics(AIMD)simulations.The Na⁺ions preferentially migrate through the Na1-Na3-Na2-Na3-Na1 channels rather than through the Na2-Na3-Na3-Na2 channels.Moreover,two types of concerted migration mechanisms were found:two Na⁺ions located at the adjacent low-energy Na1and high-energy Na2 sites can migrate either along the same or almost orthogonal directions.The Coulomb interactions between these two Na⁺ions lead to the concerted migration,which in turn lowers the migration barrier due to the potential energy conversion during the Na⁺ions migration process.Additionally,we developed a code which can do the correlated jump analysis of AIMD simulation results for fast ion conductors which have the skeleton structure.The site occupancies of different sites,the jump events and the percentage of correlated jumps can be obtained by using this code.The percentage of correlated jumps(i.e.the ratio of correlated jumps over all jump events)quantify the degree of the concerted migration.The higher percentage of the correlated jumps,indicate more ions diffuse in the concerted migration mechanism.When increasing the Na⁺concentration,more Na⁺ions tend to occupy the high energy Na2 site and the number of Na⁺configurations of“Na2+Na1+Va(Na2)”increases,which further facilitates the concerted migration,decreases the overall activation energy and thus promotes the mobility of Na⁺ions.2.We identified an additional,never reported,Na5 site in the monoclinic NASICON(Na₃Zr₂Si₂PO₁₂)by crystal structure analysis and NPD data-based Rietveld refinement.All the possible Na⁺ion diffusion channels and bottlenecks in the channels of monoclinic NASICON(Na₃Zr₂Si₂PO₁₂)were found by combining the crystal structure analysis,BVEL analysis and AIMD simulations.In particular for the monoclinic NASICON-type structure,the Na1-Na3-Na1channels and the Na1-Na2-Na1 channels provide the Na⁺ion transport while the local channels around Na1 site(Na4-Na5,Na4-Na6 and Na5-Na6 channels)are not percolating.Moreover,the diffusion of Na⁺ions is anisotropic:Na⁺ions tend to migrate though the Na1-Na3-Na1 channels in the bc plane(i.e.(100)plane)than through the Na1-Na2-Na1 channels along the[101]direction.In addition,the concerted migration plays the dominate role in this system while the concerted migration along the Na1-Na3-Na1 channel will decrease the migration barrier.Furthermore,the bulk conductivity of monoclinic NASICON-type ceramics can be further improved by tailoring the bottlenecks to the suitable size(2.35?)mostly by enlarging the bottlenecks B in the Na1-Na2-Na1 migration channels or by increasing the site occupancy of sites in the Na1-Na3-Na1 channel(especially the Na3 and Na5 sites)if the size of all the bottlenecks are larger than the suitable size.The increase of the bulk conductivity of this system can be also achieved by increasing the correlated jump rate via increasing the Na⁺content.3.We demonstrated the viability of NaV₂(PO₄)₃ as a cathode material for Zn ion aqueous batteries.The aberration-corrected STEM was used to directly visualize Zn²⁺distribution in Zn_xNaV₂(PO₄)₃ with atomic resolution for the first time and confirm the mixed occupancy of Na⁺/Zn²⁺at both M1 and M2 sites in the Zn_xNaV₂(PO₄)₃.The fully consistent results of both experimental and theoretical methods(STEM,BVEL,CI-NEB,AIMD)revealed that this phenomenon is enabled by the concerted migration of Na⁺/Zn²⁺ions,which also activates the(de)intercalation of Zn²⁺in NaV₂(PO₄)₃.