First-principles Calculations Of The Energy Storage Mechanisms Of The Cathode Materials For Sodium Ion Batteries

With the rapid growth of energy demand,people pay more attention to the development of sustainable energy,such as solar energy and wind energy.The effective utilization of sustainable energy depends on efficient energy storage and conversion technology.Large scale energy storage system has become an important research field in recent years.Due to the advantages of abundant raw materials,low cost and high technical similarity with lithium-ion battery,sodium ion battery has become the most potential new secondary battery system.However,the low energy density,poor cycle stability and low power performance of sodium ion battery hinder the development of commercial application of sodium ion battery to a large extent.Therefore,it is of great theoretical significance to study the basic scientific problems of sodium ion battery materials and explore new electrode materials through the first principle calculation based on density functional theory.Based on the above research background,through the first principle calculation method based on the Density Functional Theory,we take some typical positive materials of sodium ion battery as the research object,and discuss the basic scientific issues such as the crystal structure of electrode materials,physical and chemical properties,ion storage and transport mechanism.At the atomic scale,the structural characteristics of the battery materials and the kinetic properties of the electrochemical reaction and ion migration during the charging and discharging process were studied.The relationship between microstructure and electrochemical properties is discussed and predicted by theoretical calculation method,which provides a feasible theoretical basis for further understanding the electrochemical mechanism of electrode materials and exploring new positive materials for sodium ion batteries.The main research results of this paper are as follows:?1?In the presentwork,the structural and thermal stability,and the anionic oxygen redox mechanism of Na₂MnO₃ were studied by using first-principle calculations based on density functional theory.About 1.75 Na⁺per formula unit were extracted from Na_2-xMnO₃ through partial O^2-oxidation while the local structure remained intact,resulting in a large theoretical capacity of 315 mA?h?g^-1.Surface Na⁺was extracted before bulk Na⁺,which might have led to oxygen loss and structural transformation from the surface to the material bulk.Given the importance of high capacity in practical applications,Na₂MnO₃ and its derived Na-rich layered oxides might be considered promising cathode materials for sodium ion batteries.?2?In this study,the phase transformation,charge transfer property,and sodium ion?Na⁺?diffusion kinetics of Na₄MnV?PO₄?₃ during desodiation were studied by first-principles calculations combined with experimental studies using magnetic property measurements,nuclear magnetic resonance,and galvanostatic intermittent titration technique.Na_4-xMnV?PO₄?₃ underwent a solid-solution reaction in the desodiation process of 0?x?1,followed by a biphasic transition in 1<x?2.Charge transfer reactions occurred successively on V³⁺and Mn²⁺cations,resulting in two voltage plateaus at 3.4 and 3.6 V,respectively.In addition,the open NASICON structure provided low energy barriers for Na⁺migration,which were beneficial for the rate capability of Na₄MnV?PO₄?₃ in SIBs.In this paper,the sodium ion secondary battery material is studied from many angles by using the first principles.The basic scientific problems of secondary battery materials,such as crystal structure,phase change process,conductivity,ion storage and migration mechanism,were explored from the atomic scale.Through systematic study,on the atomic scale to determine the relationship between material structure and electrochemical properties,to explain the mechanism of sodium ion battery material and structure design of new type of electrode material provides the reliable theory basis,makes every effort to improve the existing system of secondary battery energy density and cycle life,promote the development of secondary batteries.