Structural Optimization Design And Electrochemical Performances Of Layered Cathode Materials For Sodium-ion Battery

The national"dual carbon"strategy urgently necessitates the development of new and efficient energy storage devices to accelerate the transformation of the energy structure.Among those electrochemical storage technologies,especially secondary-ion batteries that are not limited by time or space and provide a stable and reliable power supply,have been one of the most efficient and predominant methods.However,the limited lithium resources cannot support the rapid development of electric vehicles and large-scale energy storage in our country due to the continuously increasing demand for lithium-ion batteries,forcing us to seek new secondary-ion battery storage technologies.Compared to lithium,sodium resources offer the advantages of low cost,abundant reserves,and uniform distribution.Thereforce,sodium-ion batteries hold promise for widespread application in large-scale energy storage,serving as a powerful complement and even alternative to lithium-ion batteries.Among those typical types of cathode materials,layered oxide cathodes,have garnered significant research interest in recent years and are gradually being commercialized,woing to their high energy density,diverse compositions,ease of preparation,and environmental friendliness.However,there are the scientific issues including slow ion transport kinetics,irreversible structural evolution,complex interface side reactions,and poor air stability,due to the larger radius of sodium ions than that of lithium ions.These issues significantly impact the rate performance,cycle life,thermal stability,and production and storage.Therefore,this disertation aims to alliviate the problems of adverse phase transitions,lattice strain,slow ion transport kinetics,poor interface stability,and poor air stability,focuses on the rational design and preparation of four types of layered oxide cathode materials,and achieving boosting the comprehensive electrochemical performances of the layered oxides involved in terms of thermodynamics,kinetics and interfacial chemistry.The four main research contents are presented as follows.（1）Structural design and thermodynamic stability of P-type layered cathode materialsP-type layered oxides provide open transmission paths that facilitate Na⁺diffusion,resulting in excellent rate performance.However,side reactions between the cathode material surface and the electrolyte at high voltages severely affect its cycle performance,leading to adverse effects on safety.Therefore,the relationship between the transition metal elements of the cathode material and its intrinsic thermodynamics,and the construction of sodium-ion battery cathode materials with high voltage thermodynamic stability necessitates further in-depth exploration.In this work,P-type binary manganese-based materials were taken as the research model.Theoretical calculations show that,compared to Na_0.78Ni_0.33Mn_0.67O₂ and Na_0.78Fe_0.33Mn_0.67O₂,the Na_0.78Cu_0.33Mn_0.67O₂ material,owing to the weaker catalytic effect of Cu³⁺,exhibits optimal thermodynamic stability in the high-voltage sodium deintercalated state.Mg/Ti co-doping was carried out to improve the thermodynamic stability of the resultant P2/P3-Na_0.78Cu_0.28Mg_0.05Mn_0.57Ti_0.1O₂ cathode material with enhanced structural and thermal stability.X-ray diffraction and transmission electron microscopy confirm the P2 and P3 biphase structures at the macroscopic,microscopic,and atomic scales.In-situ X-ray diffraction results deomonsrate that the Na_0.78Cu_0.28Mg_0.05Mn_0.57Ti_0.1O₂ exhibits a highly reversible P2/P3 solid solution behavior with the minimal lattice parameter changes when charged to 4.2 V.Therefore,the modified Na_0.78Cu_0.28Mg_0.05Mn_0.57Ti_0.1O₂achieves capacity retention of 91.7%after 500 cycles at 1C,significantly improving cycling performance.In particular,the combined results of high energy-X-ray diffraction,thermogravimetric analysis,and differential scanning calorimetry test confirm that Na_0.78Cu_0.28Mg_0.05Mn_0.57Ti_0.1O₂ has the excellent structural and thermal stability,evidenced by the highest exothermic peak starting temperature（269.5°C）and the lowest exothermic enthalpy value(106.8 J g^-1)when reacting with the electrolyte,demonstrating Mg/Ti doping further enhances the intrinsic thermal stability of the material.This work reveals the catalytic decomposition capability of high-valence transition metal ions on the electrolyte in the charged state of layered cathode materials.It offers insights into evaluating the safety of sodium-ion batteries based on the intrinsic thermal stability of layered materials,and guiding further optimization,design,and preparation of high-thermal-stability P-type layered oxide cathodes.（2）High-voltage phase transition mechanism and kinetics/cycle stability of O3-type layered cathode materialsO3-type layered cathodes have received extensive attention due to their high-capacity advantage compared to P2-type layered cathode materials,and are well considered as one of the potential cathode materials for achieving high energy density in sodium-ion batteries.However,most O3-type cathode materials typically undergo complex phase transitions during charging and discharging process,especially the P–O phase transition process at the high-voltage（>4.1 V）,which can lead to significant volume changes.The continuous accumulation of structural stress during long cycling can damage the crystal structure of the material,leading to a decline in cycle stability.Moreover,the inherently narrow sodium layer spacing of the O3 phase result in slow ions diffusion kinetics.Here,sodium site doping(Ca²⁺)and transition metal site doping（Li⁺）were utilized to synergistically improve both the high-voltage phase transition issue and ion transport kinetics of the O3-Na Ni_0.4Fe_0.15Mn_0.35Ti_0.1O₂ material,and calcination process optimization was used to enhance the purity of the cathode material.The Ca,Li co-doped O3-Na_0.96Ca_0.02Ni_0.4Fe_0.15Mn_0.32Li_0.03Ti_0.1O₂ cathode material has an increased interlayer spacing and stable crystal structure,which enhances the rate performance and cycle stability.In-situ X-ray diffraction results show that the formation of the OP2 phase at a high charge voltage of 4.2 V is significantly alleviated and coexists with the P3 phase,distinctly different from the complete P3-OP2 phase evolution presented by the unmodified sample,thereby effectively alleviating the structural stress during the charging and discharging process.Ex-situ X-ray diffraction,X-ray photoelectron spectroscopy and electrochemical impedance results after 100 cycles confirm the robust structural and interfacial stability of the target sample.Therefore,O3-Na_0.96Ca_0.02Ni_0.4Fe_0.15Mn_0.32Li_0.03Ti_0.1O₂ exhibits a high reversible capacity of144.2 m Ah g^-1 at 0.1C(1C=240 m A g^-1),and maintains capacity retention of80.1%after 200 cycles at 1C,and still has capacity retention of 78.4%after 500cycles at 5C.The synergistic doping stratrgy of sodium and transition metal sites can effectively suppress the adverse phase transition behavior of O3 phase at high voltages,which is crucial for designing O3-type layered cathode materials with a high capacity and wide working voltage range.（3）Orientation control of the{010}crystal plane and kinetics of O3-type layered cathode materialsCompared to expanding ion transport channels,special morphological modification design is another effective way to improve the Na⁺diffusion rate in O3-type oxides.Since the{010}active crystal facets of O3-type materials can provide sodium ion transport channels,controlling the exposure of{010}active crystal facets can effectively enhance the sodium ion diffusion kinetics.Herein,Bi-doping is introduced to adjust the{010}facets of O3-Na Ni_0.5Mn_0.5O₂ microspherical cathode materials with a morphology structure composed of radially exposed{010}crystal facets,which is distinctive different from the wet chemical methods used commonly.The bulk doping of trace heavy element Bi successfully induces the formation of large primary nanosheets with the radial orientation exposing{010}active facets,which can offer pathways for sodium ions to diffuse directly from the electrochemically active facets to the electrolyte,achieving rapid sodium-ion diffusion kinetics.At the same time,bulk doping of Bi can optimize the crystal structure,enhancing structural stability during charging and discharging process.Consequently,the Bi-doped O3-Na Ni_0.5Mn_0.5O₂ cathode exhibits excellent cycle stability after 300 cycles at1C,with a capacity retention of 72.5%,and a capacity retention of 45.0%after1000 cycles at a high current of 5C,far surpassing those of the original Na Ni_0.5Mn_0.5O₂ cathode.Notably,the Bi doping strategy proposed in this work is effective at preparing other O3-type Ni Mn Ti-and Ni Fe Mn-based layered oxides,offering a novel method for developing high-performance oxide cathode materials.（4）Interface engineering and air stability of high-capacity P2-type layered cathode materialsThe interface stability of layered oxides during cycling and storage cannot be ignored.P2-type manganese-based layered cathode materials（Na_xMn O₂）are widely investigated owing to their high theoretical specific capacity across a broad voltage range,abundant elemental reserves,low cost,and low toxicity.However,the Jahn-Teller effect of Mn³⁺leading to Mn dissolution and poor air stability increases the cost of preparation,storage,and usage.Herein,this work combines ion doping and surface engineering strategy to prepare K-doped and Ce O₂-coated P2-Na_0.62K_0.05Mn O₂ cathode material:Ce O₂@Na_0.62K_0.05Mn O₂.K⁺doping at the sodium site expands the sodium layer spacing,benefiting ion transport kinetics,and acts as"pillar"to prevent the collapse of the crystal structure under high pressure.The Ce O₂ coating layer effectively isolates the material from moisture in the air,and the shortened Na-O bond length hinders water molecule insertion,thus enhancing the air stability.The synergistic effect of K⁺doping and Ce O₂ coating significantly suppresses phase transitions and volume strains caused by Jahn-Teller distortion.During charging and discharging,the modified material exhibits only a delayed P2-P2′phase transition at the low voltage during discharge process,and the OP4,P′2 phase transitions during charging and discharging are completely suppressed.It maintains a capacity retention of 84%after 200 cycles at a 1C,and delivers a high capacity of 67.95 m Ah g^-1 at 10C rate（about 41.0%of 0.1C）,demonstrating improved rate performance and cycle stability.At the same time,after air/water stability testing,the modified sample maintains its crystal structure and morphology,and the aged electrochemical performance does not exhibit a significant degradation,indicating the good air stability.The result highlights that the combined strategies of ion doping and interface engineering can simultaneously improve rate performance and air stability,offering a novel approach to designing easy-to-store and high-performance P2-type manganese-based cathode materials.