Ab Initio Calculations On The Physical Properties Of Sodium-ion Battery Electrode Materials

From 1970s to 1980s,researches on the sodium ion batteries and lithium-ion batteries are almost simultaneously emerged.However,due to the higher energy density of lithium-ion batteries,it is then more suitable for smart portable electronic devices.Therefore,a great deal of efforts have been focused on the field of lithium-ion batteries.In recent years,under the stimulation of large-scale energy storage applications,the development of sodium ion batteries and new electrode materials of sodium ion batteries have begun to recover.For the cathode material,transition metal compounds,MF3(M=Fe,Ti,V,Mn,Co,and Bi),have been extensively investigated as potential electrode materials for Li-ion batteries.In addition,the studies on the lithium orthosilicates Li2MSiO4(M=Mn,Fe,Co)has made great breakthroughs in recent years,especially for the Li2FeSiO4 materials.Thanks for that,transition metal sodium orthosilicates has attracted much attention due to the significant progress being made in its analog of lithium orthosilicates.For the sodium ion batteries,the negative electrode material is one of the most difficult parts in the sodium ion battery,since the graphite,as the typical anode applied in lithium-ion battery,is not applicable for the sodium ion battery,which cannot embedded in any sodium under normal circumstances.Due to the formation of dendrites and interface aging,use of metallic sodium directly as anode material of sodium ion battery is not desirable.Therefore,much attention has been focused on the alloying materials,which can store a large number of ions,have high gravimetric and volumetric energy densities.However,the complexity of alloy process and the large volume expansion,which often leads to pulverization of alloy particles from electrode and a rapid deteriorate in cycle stability,is a key issue for the alloy anode.In this paper,the physical and electrochemical properties of the transition metal fluorides and iron orthosilicate materials,as cathode materials of sodium ion battery,were studied by using the first-principles calculations based on the density functional theory(DFT).Besides,the adaptive genetic algorithm(Adaptive genetic algorithm,AGA)combined with DFT calculations also used to predict and study the Na-Sb intermetallic compounds.In the first part of the thesis,Na-ion battery cathode material NaFeF3 and the corresponding desodiated products were investigated by using first-principles density functional theory calculations within the generalized gradient approximation(GGA)+U framework.Our results show that Na0.5FeF3 is the only energetically stable intermediate phase among the cases considered in the present work,leading to theoretically two voltage plateaus,i.e.,2.63 V(1?x?0.5)and 2.82 V(0.5?x?0)in NaxFeF3,respectively.In addition,we also discussed the crystal structures and electronic structures properties of NaFeF3 and the corresponding desodiated products.Studying from the densities of states,the redox reaction of Fe2+/Fe3+ occurred during the desodiation process.Charge localization into distinct Fe2+ and Fe3+ was found in the intermediate-phase Na0.5FeF3.It also corroborated by the analysis based on the variety of bond lengths in the FeF6 octahedral.In the second part of the thesis,first-principles calculations have been performed on the structural stabilities and electrochemical properties of six Na2FeSiO4 polymorphs and their corresponding desodiated products NaxFeSiO4(x=1,0).Our results indicate that formation energies of all the calculated structures differ substantially upon removal of Na,where SiO4 tetrahedra provide the basic structural stability for these materials.The most stable phases for(x=2,1,0)are different,indicating that phase transformations should happen during the desodiation for different polymorphs.The mean voltage plateau for the removal of the first Na ion is?2 V,while the removal of the second sodium ion exhibit relative high voltage of?4.5 V.Moreover,the oxidation of Fe2+ to Fe3+ ions was observed during the removal of the first Na ion,while both the oxidation of O2-and Fe3+ ions occured at the further desodiation.In the third part of the thesis,we have performed the crystal structure prediction by AGA code combined with DFT calculations for NaxSb(x = 1,1.5,2,2.5,3).Through the analysis and discussion of the formation of energies,it reveals that NaSb is the only energetically stable intermediate phase.Therefore,Na-Sb alloy process can be expressed as:Sb?NaSb ? Na3Sb,the average voltages were 0.68V and 1.1 V,respectively.We also obtained some low-energy structures for NaSb and Na3Sb by AGA,the predicted Immm NaSb proved to possess dynamic stability.