| Iron-based fluorides?FeF3 and FeF2?with high voltage and capacity have been considered as a kind of promising cathode materials for secondary batteries.However,poor electronic conductivity seriously restricts their practical application,and the ionic conductivity still needs to be further improved.It has been proposed that the formation of nanosheet is an effective way to solve these problems.However,the storage mechanism of iron-based fluorides nanosheets needs further study.In this paper,first-principles calculations were carried out to investigate systematically the surface structure of iron-based fluorides nanosheets and the electrochemical reaction mechanism with metal atoms at electronic and atomic scales.These results will provide systematic theoretical guidance for designing a new generation of electrode materials with excellent electrochemical properties.The specific contents of this paper can be summarized as follows:?1?The Li storage mechanism of FeF3?012?nanosheet has been investigated.It is found that the surface structure of FeF3?012?nanosheet with F termination?FeF3?012?-F?is much more stable than that of FeF termination?FeF3?012?-FeF?by calculating the surface energy;For the isolated Li adsorption case,we determined the bridge site is the most favored site by calculating the adsorption energy,and its adsorption energy is-3.77 eV;On the basis of isolated Li adsorption,multiple Li atoms adsorption models were constructed.The results indicate that Li atoms are distributed on the FeF3?012?nanosheet in the form of Li dimer;Then,we employed the CI-NEB technique to explore Li-ion diffusion behavior on the FeF3?012?nanosheet,its lowest energy barrier is 0.17 eV and diffusion coefficient is as high as1.06×10-6cm2·s-1;Finally,we calculated the theoretical voltage as a function of theoretical capacity,and FeF3?012?nanosheet shows a more stable voltage profile than bulk FeF3.?2?The Na/K storage mechanisms on FeF3?012?nanosheet have been investigated.First,for the cases of isolated Na/K adsorption,the most stable adsorption sites of Na and K both are hollow site with adsorption energies of-3.55 eV and-3.98 eV,respectively.Subsequently,we constructed multiple adatoms adsorption models.By analysis of these relative energies,it is included that Na/K atoms are distributed on the FeF3?012?nanosheet in the form of Na dimer and K tetramer,respectively.Next,CI-NEB technique was used to study Na/K-ion diffusion kinetics on the FeF3?012?nanosheet,the lowest energy barriers of isolated Na ion and K ion are 0.18 eV and 0.26 eV,respectively.Moreover,the energy barriers of Na dimer and K tetramer are 0.45 eV and 0.43 eV,respectively,the corresponding diffusion coefficients are 3.32×10-10cm2·s-1 and 4.22×10-10cm2·s-1;Compared with NaFeF3nanoplates and sheet-like FeF3,FeF3?012?nanosheet exhibits the most stable voltage plateau during Na/K insertion.Finally,we calculated the in-plane stiffness of FeF3?012?nanosheet as a function of potassium concentration.And it is confirmed that FeF3?012?nanosheet possesses good plasticity and isotropic property,which indicates that FeF3?012?nanosheet can be acted as a flexible electrode material.?3?The Mg storage mechanism on FeF3?012?nanosheet has been investigated.For the isolated Mg adsorption case,from the symmetry analysis of FeF3?012?nanosheet,five possible adsorption sites were discussed.The adsorption energies of adsorption structures were calculated and compared,it is found that the most stable adsorption site is hollow site,and its adsorption energy is-3.55 eV.On the basis of isolated Mg adsorption,we constructed multiple Mg atoms adsorption models.By analysis of these relative energies,it is confirmed that Mg atoms prefer to form Mg dimer adsorbed on FeF3?012?nanosheet.Then,the energy barriers of isolated Mg and Mg dimer were calculated by CI-NEB method.Isolated Mg atom passes through bridge site to adjacent hollow site,possessing the lowest diffusion barrier?0.56 eV?.Morever,the energy barrier of Mg dimer?1.19 eV?is closed to two times that of isolated Mg.?4?The Li/Na/K storage mechanisms on FeF2?110?nanosheet have been investigated.First,considering the surface structure of FeF2?110?nanosheet with F termination?FeF2?110?-F?is much more stable,we constructed surface models of FeF2?110?-F,and its percent change of spacing was calculated.The result shows that the strongest relaxation occurs in the top atomic layer,inside atoms move very little which is close to its bulk state.Subsequently,the most stable adsorption sites of Li/Na/K on FeF2?110?nanosheet were obtained by calculating the adsorption energy,and their adsorption energies are-4.76 eV,-4.47 eV and-4.95 eV,respectively;Finally,the CI-NEB method was used to study the Li/Na/K-ion energy barriers,and they are0.97 eV,0.61 eV and 0.42 eV,respectively.The results indicate that energy barrier of K-ion is the lowest. |
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