The Structures,Electronic Structures And Na-ion Diffusion Dynamics In Mn-Based Cathode Materials For Na-ion Batteries:A First-principles Study

Because the radius of sodium ion is larger than lithium ion and its mass is larger than lithium ion,the dynamic performance of sodium ion battery materials is usually worse than that of lithium ion battery materials.The experiments have shown that NaMnO₂ cathode materials have excellent multiplier properties and cyclic stability.However,it is not clear what the mechanism is.Therefore,it is necessary to explain this phenomenon from the perspective of basic research from the microscopic view.In this paper,the first-principles calculation method is adopted to study the change of the entire NaMnO₂ material in the process of charging and discharging from the atomic level,and the reason why NaMnO₂ cathode material has such excellent dynamic properties is systematically analyzed.This paper deals with two key structures in the experiment:Na₂MnO₃ and NaMnO₂.We studied their atomic structure,electron properties,sodium ion migration and thermodynamic stability.As the initial structure of charge and discharge reaction,Na₂MnO₃ underwent phase transition and formed more stable NaMnO₂thermodynamically after charge and discharge cycles.1)in terms of atomic structure,Na₂MnO₃ is a Li₂MnO₃-like structure.The radius of sodium ion is larger than that of lithium ion,so the lattice constant is larger than that of Li₂MnO₃.In NaMnO₂,Mn³⁺O₆octahedron will produce two kinds of Jahn-Teller distortion（JT distortion）under the crystal field theory,namely elongated Mn³⁺O₆ octahedron and compressed Mn³⁺O₆octahedron.Since the conditions of the experiment were at room temperature,we observed that the structure of NaMnO₂ material is a mixture of two Jahn-Teller distortions thermodynamically.2)in terms of electronic structure,the expansion of lattice constant of Na₂MnO₃ leads to the weakening of the octahedral field of Mn⁴⁺O₆,making its band gap smaller than that of Li₂MnO₃,which is conducive to the electronic conductivity and suitable for use in batteries.Moreover,the band gap of NaMnO₂ is smaller than that of Na₂MnO₃,even less than 0.14 eV.Compared with Na₂MnO₃materials,the manganese in NaMnO₂ continuously changes from+3 to+4 valence during the cycle of sodium ion removal to complete the chemical reaction.3)in terms of the migration of sodium ions,on the one hand,the sodium ions in Na₂MnO₃ are not easy to migrate out of the transition metal layer,while the migration of sodium ions in the sodium ion layer is not difficult because of the large weight and large radius of sodium ions.On the other hand,sodium ions in NaMnO₂ are easier to migrate due to the presence of JT polaron.In particular,during the migration of sodium ions,JT polaron will also diffuse along with the migration of sodium ions,further assisting the migration of sodium ions,reducing the migration barrier of sodium ions and improving the diffusion rate of sodium ions.In conclusion,we studied the NaMnO₂ structure theoretically by using the first-principles calculation method,explained the reason for the excellent dynamics of the assembled sodium ion battery,and revealed the microscopic mechanism of the Jahn-Teller polaron assisting the migration of sodium ions.