First-Principles Study To Investigate Potential Electrode Materials For Sodium-Ion Batteries

With the continuous development of science and technology,human’s demand for energy is increasing day by day.At present,people’s energy consumption mainly comes from nonrenewable fossil fuels.With the continuous exploitation and application of fossil fuels,human beings are facing serious energy crisis and environmental pollution on the road of sustainable development.The key to solving such problems is the development and use of clean and renewable energy.At present,clean and renewable energy includes solar energy,wind energy,tidal energy and so on.However,due to the common intermittency and limitations of such clean renewable energy,it is difficult to use it directly.Therefore,it is particularly important to develop effective energy storage devices to store and convert energy.Secondary battery is an important energy storage device,which has been widely used in human daily life.It plays an irreplaceable role in smart grids,electric vehicles,and small portable electronic devices.Secondary batteries are also known as rechargeable batteries.Compared with primary batteries,they can convert electrical and chemical energy based on reversible chemical reactions.The types of secondary batteries have been developed from leadacid,nickel-metal hydride,nickel-cadmium batteries.To the present,new secondary batteries have been developed with relatively more extensive research and application,including lithium-ion batteries and other metal-ion battery systems,lithium-sulfur batteries,lithium-air batteries,etc.Among them,metal-ion batteries are the most common systems at present.Such novel secondary batteries have the following advantages.For example,the specific energy is large,the self-discharge rate is small,the cycle life is long,and there is no environmental pollution.The basic structure of the new secondary battery includes positive and negative electrodes and an electrolyte.The mechanism of its operation is that the metal ions move back and forth between the positive electrode and the negative electrode.Since the binding energy of ions in different materials is different,the back and forth movement of ions will cause the chemical energy of the entire battery system to change.We can use these changes to store electrical energy.Lithium-ion batteries（LIBs）are currently the rechargeable batteries with the best comprehensive performance,which have greatly promoted the development of energy storage technology and powered various portable electronic devices.In addition,many lithium-ion batteries are also used as power stations,and new energy power generation such as solar energy and wind energy have also begun to be applied.However,the reserves of lithium resources in the earth’s crust are very limited.In contrast,the amount of sodium in the earth’s crust is 4 – 5orders of magnitude higher than that of lithium.Meanwhile,both sodium and lithium have similar physical and chemical properties.The standard electrode potential of metallic sodium differs from metallic lithium by only 0.3 V.Therefore,sodium-ion batteries（NIBs）are considered as a new type of secondary batteries that can replace lithium-ion batteries.It is worth pointing out that the energy density of Na-ion batteries is lower than that of Li-ion batteries due to the larger relative atomic weight of Na.The energy density,rate capability,thermal stability,cycle life and other properties of secondary ion batteries are closely related to the intrinsic properties of electrode materials.Therefore,the development of new materials for new secondary batteries,or the study of chemical reactions,is critical to improve battery performance.For example,performance is critical in terms of energy density,the intrinsic number of reaction electrons and electrode potential of electrode materials are inversely proportional to energy density.Whereas,the performance in terms of power density,chemical reaction rates are inversely proportional to the reaction activation energy of electrode materials.Therefore,it is of great significance to deeply understand the sodium ion storage mechanism of electrode materials.Traditional research is mainly based on experiments.However,with the continuous improvement of people’s requirements for various properties of materials,the understanding of the reaction mechanism is deepened,and the spatial scale of material research is also reduced.Research on batteries has been unable to meet the needs of modern development.Therefore,the method based on first-principles research can study the intrinsic physical and chemical properties of electrode materials from the atomic scale,and at the same time predict their potential values in different metal-ion battery systems,which is of great research significance.In this paper,by means of first-principles calculations,the geometric structure,stability,physical and chemical intrinsic properties,ion storage and transport mechanisms of several typical sodium-ion battery cathode and anode materials are systematically studied.Theoretical studies have explored the relationship between the crystal microstructure and intrinsic properties as well as the macroscopic properties of electrode materials.The main contents of the paper are as follows:（1）Two-dimensional transition metal dichalcogenide WX₂（X = Se,Te）Two-dimensional energy storage materials are favored by researchers in the study of sodium-ion batteries due to their abundant ion storage active sites and open ion migration space.There are many kinds of two-dimensional materials,from elemental to ternary compounds,mainly including carbon-based materials,transition metal chalcogenides,MXene materials,etc.For a long time,transition metal chalcogenides such as VS2,Mo S2,and Mn O2 have been a typical class of two-dimensional energy storage materials.Recently,it has been found that tungsten-based chalcogenides have long cycle life,suitable operating voltage,and large charge-discharge capacity,and WSe₂ has been proven an effective electrode material in sodium-ion battery.Unlike most transition metal dichalcogenides,the discharge process of WSe₂ involves both intercalation and switching electrochemical reaction mechanisms,and can maintain a reversible capacity of up to 200 m A h g-1 at 20 m Ag-1.In order to deeply understand the electrochemical energy storage mechanism of this type of materials,this thesis used firstprinciples calculations to study the storage behavior of WX₂（X = Se,Te）bulk structure for sodium ions and the underlying electrochemical mechanism.WX₂ had a 2H hexagonal geometry,P63/mmc space group,and its lattice parameters were a = b = 3.327 ?,c = 12.997 ?（WSe₂）and a = b = 3.55 ?,c = 14.965 ?（WTe2）.There were three types of unequal Na ion active sites in the WX₂ structure,namely octahedral（Oh）site,tetrahedral（Th）site and triangular（Tr）site.According to the comparison of the binding energies of Na ions at different sites,we determined that the Oh site was the most energetically stable Na ion active site,followed by the Th site,and finally the Tr site.On this basis,this chapter simulated the structural changes of Na_yWX₂（y = 0,0.5,1,1.5,2,2.5,3）under different Na ion content,and observed the phase transition process of the material during Na ion intercalation.The optimized structure showed that the material stored two Na ion layers between adjacent WX₂ layers when the sodium content did not exceed y = 2.At this concentration,the WX₂ layer remained stable in the molecular dynamics simulation at 300 K,and the sodium storage process belonged to the intercalation reaction stage.When the sodium content exceeded to y = 2,a large number of sodium ions reacted with the anions in the structure to form Na_yX and destroyed the WX₂ layered framework.At this concentration,the crystal structure of Na_yWX₂ changed from trigonal to triclinic,and the electrode went from the intercalation reaction stage to the conversion reaction stage.The theoretical voltage calculation results showed that the operating voltage range of Na_yWSe₂ was 2.05 – 0.48 V,while that of Na_yWTe2 was 2.5 – 0.65 V.The density of states results showed that the pure WX₂ material was a semiconductor.When the sodium ions were embedded in the material,the material gradually exhibited metallicity due to the influence of the charge transfer of the sodium ions.The results of ionic charge calculation showed that when the content of sodium ions was low,the number of charge transfer was higher,and with the increase of the content of sodium ions,the average charge transfer number decreased gradually.Clearly,the charge transfer number of sodium ions was inversely proportional to its concentration.The change of the charge of W ions had different rules.During the intercalation reaction,the charge transfer of W ions decreased with the increase of sodium ion concentration.After entering the conversion reaction stage,the number of charge transfer increased.Such anomalous charge transfer changes indicated a dramatic change in the material’s structure,further confirming the phase transition process in the switching reaction.Using the CI-NEB method,this section simulated the migration behavior of sodium ions in the WX₂ structure.The migration model showed that sodium ions had a zigzag periodic migration channel in the WX₂ structure,in which sodium ions started from an octahedral site of Na X6 and passed through a tetrahedral site of Na X4 to another equivalent Na X6 octahedral site.Among them,WTe2 showed a small sodium ion migration energy barrier of 0.40 e V,while Na+ in WSe₂ manifested a large migration energy barrier of about 0.60 e V.In contrast,the sodium ion migration barrier in Mo S2 electrode material with similar structure was 0.68 e V.This indicated that bulk WX₂ exhibited suitable channels for sodium ion transport,and was a potential electrode material.（2）NASICON material Na₃Cr₂（PO₄）₃Since the Na/Na+ redox pair has a higher redox potential than Li/Li+,theoretically,the operating voltage of Na-ion batteries is lower than that of Li-ion batteries.To overcome this shortcoming,the development of high-voltage cathode materials for Na-ion batteries is an important research direction.Polyanionic materials have an inductive effect,which can increase the battery voltage by increasing the redox potential of transition metals.In addition,the strong covalent bonding of polyanions can effectively improve the structural stability and electrochemical cycling performance of the materials.Among many polyanionic cathode materials,Na_x M2（PO₄）₃（M stands for transition metal）is an important class of sodium ion fast ion conductors（NASICON）.Such materials have the advantages of high voltage,low volume change,good cycling performance,and stable structure.When the transition metal M is Cr,Na_x Cr₂（PO₄）₃（NCP）exhibits a reversible capacity of 79 m A h g-1 in previous experimental studies,and the Cr4+/Cr3+ redox pair has a redox potential as high as 4.5V.The properties are excellent in polyanionic materials.In order to further study the sodium storage performance of this material;this chapter studied its operating voltage,structural change process,ion and electron transport and the mechanism behind it by first-principles methods.The optimized structure showed that Na₃Cr₂（PO₄）₃ had the space group R3c with corresponding lattice constants a = b = 8.641 ? and c = 21.621 ?,which were in good agreement with the experimental observations.The Na ions in the material had two different active sites,denoted as Na1 and Na2.In order to determine the preferential binding of sodium ions at the two sites,we calculated the binding energies of sodium ions at different sites and compared them.The results showed that the Na1 site was more stable than the Na2 site during the charging process,indicating that the Na ions at the Na2 site would be released first.On this basis,we gradually and reasonably removed the sodium ions from the structure and optimized the structure,to determine the phase transition process of the material for sodium removal.The optimized structure showed that the structure of the material was undergone a transition from space group R3c to 3c1when 1/3 of the Na ions were extracted from the Na₃Cr₂（PO₄）₃ structure.With the continuous extraction of Na ions,the material reverted to the R3c space group again.The thermal stability of the Na₃Cr₂（PO₄）₃ structures was evaluated by molecular dynamics calculations,which showed that all Na_x Cr₂（PO₄）₃ structures were stable at 1000 K under different sodium ion contents.In addition,the thermodynamic stability of the intermediate structure of Na_x Cr₂（PO₄）₃ during de-sodiumization was evaluated by the calculation of formation energy.Using Cr₂（PO₄）₃ and Na_x Cr₂（PO₄）₃ as references,we plotted the convex hulls of the materials under different sodium ion contents,and found that Na Cr₂（PO₄）₃ was the most stable phase during the sodium removal process.Based on all thermodynamically stable phases in the convex hull diagram,we calculated the Na_x Cr₂（PO₄）₃ charging voltage.The results showed that the process of three moles of Na ions in Na₃Cr₂（PO₄）₃ de-structured corresponded to the theoretical voltages of 3.65 V,4.50 V and 4.70 V,respectively.We calculated the density of states structure of the material and analyzed the interaction between the Cr-3d and O-2p orbitals.It turned out that at the Na ion concentration of x = 3,the initial Na₃Cr₂（PO₄）₃ belonged to the semiconducting material with a band gap of about 1.0 e V.During the de-sodiuming process,the poor electrical conductivity was caused by the strong covalent bonding between Cr and O due to the obvious hybridization of Cr-3d and O-2p orbitals.In addition,the charge transfer process during Na ion extraction was quantitatively analyzed by calculating the Bader charge.The results showed that the charge number of each Na ion and P ion remained stable during the Na ion extraction process,while the valence state of O ion decreases,and the charge transfer number of Cr ion increases only slightly.This result indicated that the PO₄ tetrahedral charge distribution in the material was very stable,while the Cr-O covalent bond in the Cr O6 octahedron was gradually strengthened.Finally,the diffusion properties of Na ions in Na₃Cr₂（PO₄）₃ were investigated by CI-NEB method.There were three unequal Na ion migration paths in the Na₃Cr₂（PO₄）₃ structure,and the corresponding migration energy barriers range from 0.17 to 0.42 e V.These three pathways formed a periodic migration channel,enabling Na ions to achieve long-range diffusion in the material.（3）Polyanionic compound Na VPO₄FFluorophosphates,as another type of typical polyanionic materials,also have stable structures and high working voltages.Its good structural stability originates from the strong PO covalent bonding in the firm PO₄ tetrahedra.Na VPO₄ F obtained by introducing vanadium transition metal into fluorophosphate material exhibits good and efficient working performance in Na-ion battery system,with a theoretical capacity of 143 m A?h?g-1 and a high working capacity of 2.5 – 4.5 V voltage.However,the low electronic conductivity also limits the electrochemical performance of Na VPO₄ F in Na-ion batteries.In response to this problem,researchers have adopted methods such as nanomaterial preparation,surface coating,and ion doping as solutions.However,the intrinsic properties of the material and its sodium storage mechanism has not been reported in detail.In this chapter,first-principles methods were mainly used to study the structure,electronic properties and ion diffusion kinetics of Na VPO₄ F.The crystal structure of Na_xVPO₄ F was similar to that of Na Al PO₄ F,which belonged to the C2/c space group.This chapter simulated the crystal structure of Na_x VPO₄F（x = 0,0.25,0.5,0.75,1）at different sodium content x.The results showed that the space group of the material shifted from C2/c to P2/c when half of the sodium ions were extracted from Na VPO₄ F.And when all the sodium ions were removed,the space group of the material changed from C2/c back to P2/c.The optimized results showed that the Na_x VPO₄ F structures of the C2/c and P2/c space groups had topologically identical VPO₄ F storage frameworks,and the materials of different space groups had only small differences in bond lengths and bond angles.Furthermore,the thermodynamic stability of the intermediate structure of Na_x VPO₄ F during de-sodiumization was evaluated by the calculation of the formation energy.Using VPO₄ F and Na VPO₄ F as references,we plotted the convex hull of the material at different sodium ion content,and the results showed that all intermediate structures of Na_x VPO₄ F were thermodynamically stable in the process of gradually decreasing sodium ion content,which meant that the sodium removal process from the material was a homogeneous reaction.Based on these intermediate structures,the theoretical voltage plateau of Na_x VPO₄ F was predicted to be 3.4 – 4.3 V,and such a high voltage was mainly derived from the transition of V3+ to V4+ in the material.In addition,this section evaluated the electronic conductivity variation trend of Na_x VPO₄ F during the Na ion extraction process through the computational evaluation of the density of states.The results showed that except for the discharge product Na VPO₄ F and the charge product VPO₄ F,the rest of the intermediate phases were metallic.In the process of sodium ion desorption,V4+ and V3+ coexisted in Na_x VPO₄ F,which caused the density of states corresponding to the V-3d orbital to pass through the Fermi level,making the material appear metallic.In addition,this chapter analyzed the charge transfer behavior of Na_x VPO₄ F in the process of sodium ion desorption through Bader charge calculation.The results showed that under different sodium ion content,the number of charges carried by sodium ions was almost the same,about +0.83 e.While the number of charges on the transition metal V gradually decreased during desorption of Na ions,which indicated that V ions were redox centers in the material.During the sodium removal process,the Bader charge values of the anions O and F also decreased.This meant that the covalent bond between the anion and the V ion was gradually enhanced,reflecting the charge transfer bridge role of the anion in the process of sodium removal.Finally,the diffusion properties of Na ions in Na_x VPO₄ F were investigated by CI-NEB method.There were four unequal Na ion migration paths in the Na_x VPO₄ F structure,and the highest migration energy barriers of these paths were quite different,the highest being 3.9 e V and the lowest being 0.85 e V.From this,it was determined that sodium ions achieved a long-range diffusion process in a one-dimensional channel.In summary,in this paper,first-principles calculations were used to determine the crystal structure,electronic structure,sodium ion storage and transport of typical twodimensional transition metal sulfides,NASICON structural compounds,and polyanionic compounds for sodium ion battery cathode and anode materials.A systematic and in-depth theoretical study of the mechanism was carried out for the relationship between the crystal microstructure,intrinsic properties and their sodium ion storage properties was discussed.