Structure/Kinetic Regulation And Sodium Storage Properties Of Sodium-Ion Layered Cathode Materials

Renewable clean energy can effectively solve the problem of environmental pollution caused by traditional fossil energy.Energy storage technology is of great significance to solve the problem of new energy grid connection and realize the"dual carbon"strategic goal.Electrochemical energy storage technology represented by lithium-ion batteries（LIBs）is widely regarded as one of the most efficient energy storage methods because it is not limited by time and space.With the significant increase in demand for LIBs,lithium resources are facing the severe situation of raw material shortage and high cost.Sodium-ion batteries（SIBs）technology is expected to be widely used in large-scale energy storage,due to the abundant sodium resources and the low cost of sodium salt.Layered oxides for SIBs have been considered as the most promising cathode materials for commercialization due to the advantages of simple preparation process,high specific capacity and environmental friendliness.However,such a type of cathode materials generally has difficult problems such as slow ion transport kinetics,irreversible structure evolution and air instability during deep sodium removal/insertion process and humid air storage conditions,owing to the large radius of sodium ion,which seriously affect the rate performance,capacity and cycle life of the materials.Therefore,this dissertation focuses on the two basic scientific issues of Na⁺transport kinetics and structural stability of layered oxides,aiming at improving the electrochemical performance and structural stability of P-and O-type layered oxides.The Na⁺transport kinetics of P-type layered oxide was investigated,and the influence of different P-type layered oxide structures on ion migration behavior was elucidated.Furthermore,a series of high-performance layered oxide cathode materials with excellent dynamic performance and stable structure were developed using the strategies of composite structure construction,structure disorder regulation and transition metal layer composition optimization.The specific content of the dissertation is presented in the following parts.（1）Na⁺kinetic during sodium storage process in P-type layered cathode materialsP-type layered oxides have an open Na⁺transport path,the Na⁺can be transferred directly between two adjacent triangular prism sites,and the Na⁺transport kinetics is faster within the structure.However,the structure of P-type layered oxides is chemically complex,and there are two heterogeneous structures,P2-type and P3-type.The effects of different coordination environments of Na⁺on the electrochemical properties and ion transport kinetics of the materials remain unclear.Here,a pair of layered oxides with isomers were prepared by controlling the calcination temperature:P2-and P3-Na_2/3Ni_1/3Mn_2/3O₂.The results of electrochemical testing show that the two compounds exhibit different electrochemical behaviors.The P3-type material has better apparent Na⁺diffusion rate,but with lower cyclic stability and rate performance than those of P2-type material.Further theoretical results demonstrate that Na⁺in P2-type material along the Na_f-Na_e-Na_f migration path has smaller migration energy barrier than those in the P3-type material along the Na_e/f-Na_e/f migration path,which confirms that the P2-type material has more excellent intrinsic Na⁺diffusion rate.In addition,the structural damage caused by P2-O2 phase transition and P3-O′3 phase in the high-voltage region are the main factors that limits the high-voltage cycle stability of P2 and P3materials,respectively.The result reveals the relationship between the crystal structure and electrochemical properties of two typical P-type layered oxides and provides a new guideline for optimal design and preparation of high performance P-type layered oxide cathode electrodes.（2）P2/O3 composite layered oxide and structural stability mechanismTo solve the problem of reduced cycle stability caused by irreversible phase transition of P2-type materials when charging to high-voltage,constructing composite structure strategy is effective.Composite structure cathodes generally exhibit better electrochemical performance than single phase cathodes,but the structure evolution and synergistic effect of the biphasic structure during charging to high voltage（>4.2 V,vs.Na⁺/Na）are still unclear.Herein,a structure-stable P2/O3 composite layered oxide-Na_0.85Ni_0.34Mn_0.33Ti_0.33O₂（P2:O3=24.8%:75.2%）was prepared by adjusting the Ti content in Na_0.85Ni_0.34Mn_0.66-xTi_xO₂.The XRD and TEM results confirm that the Ti substitution can promote the transition from the thermodynamically stable P2-type structure to O3-type structure at the macroscopic and microscopic scales.In-situ XRD results show that the material successfully inhibits the formation of O2 phase at the high-voltage region and undergoes a highly reversible P2/O3–P2/P3–OP4/OP2 phase transition when charged to 4.4V.The"interlocking effect"generated by the two structures at the phase boundary during the cycling effectively alleviates the structural stress and transition metal layer gliding,inhibits the large volume change,and thus improves the structural stability during the high-voltage cycles.The P2/O3-Na_0.85Ni_0.34Mn_0.33Ti_0.33O₂ cathode exhibits a capacity retention of 80.6%after200 cycles at 1C.Furthermore,the full cells assembled with hard carbon anode exhibit an energy density of 294.6 Wh kg^-1.The result emphasizes the excellent electrochemical performance of cathode materials with composite phase structure,illustrates the reason of composite phase structure keeps stable in the process of high-voltage cycling,and provides a new idea for the design of advanced composite structure materials.（3）Cationic disorder regulation and kinetic properties of high-voltage O3-type layered cathode materialsThe antimony-based O′3-Na₃Ni₂Sb O₆ model material is a suitable foundation for developing high energy layered cathode materials with a high operating voltage of 3.4 V.However,the cationic arrangement in each slab creates an ordered honeycomb network that results in complex phase transitions and Na-vacancy ordering,therefore slowing Na⁺transport kinetics and shorten cycle life.By adjusting the cationic ratio in the transition metal layer,we were able to prepare a cation-disordered O3-Na_0.8Ni_0.6Sb_0.4O₂.This altered composition randomly co-edges the Na O₆ octahedron with either the Ni O₆ or Sb O₆ octahedron,relieving charge ordering and Na⁺/vacancy ordering problems.Our experimentation and theoretical calculations indicate that the Na⁺diffusion energy barrier is low and the material exhibits an excellent apparent Na⁺diffusion rate.In-situ XRD analysis reveals that the material undergoes a reversible O3–P3 phase transition process,with the volume variation being only1.0%.The O3-Na_0.8Ni_0.6Sb_0.4O₂ cathode material demonstrates smooth charge/discharge curves with a reversible specific capacity of 106 m Ah g^-1 at0.1 C.Moreover,the electron state density results indicate that Sb⁵⁺has electron-filled 4d orbitals,which reduces the hybridization of O 2p orbitals while improving the Ni–O bonding ionicity,resulting in the material having a high average operating voltage of 3.5 V.The novel O3 cathode material with cationic disordered structure and high operating voltage has great potential in realizing high rate and high-voltage sodium-ion batteries.（4）Structural design and sodium storage stability for high energy O3-type layered cathode materialsAchieving high energy density sodium-ion batteries can be done through not only utilizing high-voltage cathode materials,but also high-capacity cathodes.One such potential cathode material is the O3-type layered oxide,which possesses enough initial sodium content to qualify as a high-capacity cathode material.However,those materials typically undergo complex phase transitions from irreversible transition metal layer gliding during cycling and structural degradation during humid air storage.To combat these challenges,a Fe/Ti co-substitution strategy by high redox potential dopant(Fe³⁺)and inactive stable dopant(Ti⁴⁺)is used to enhance the electrochemical performance and storage stability of O3-Na Ni_0.5Mn_0.5O₂ cathode material.The co-substituted Na_0.95Ni_0.40Fe_0.15Mn_0.3Ti_0.15O₂ phase transition morphs from the complex O3-O′3-P3-P′3-P3′-O1 into a highly reversible O3-P3-OP2.This process results in a volume change of only 2.8%during cycling,a reversible specific capacity of 161.6 m Ah g^-1,an energy density of 530 Wh kg^-1,and capacity retention of 81.8%after 200 cycles at 5C.In addition,the spontaneous Na⁺removal phenomenon is effectively inhibited due to the increase in redox potential and the reduction of sodium layer spacing.The structure and morphology of Fe/Ti co-substituted Na_0.95Ni_0.40Fe_0.15Mn_0.3Ti_0.15O₂ remain intact even after exposure to air for 7 days and soaking in water for 1 hour.These improvements provide a new strategy for enhancing the structural and storage stability of O3-type layered oxides.