Structure Design And Electrochemical Performance Studies Of Cathode Materials For Secondary Batteries

Secondary batteries are the current mainstream energy storage devices,and greatly increasing the energy density of secondary batteries is the main development direction in the field of energy storage.Lithium/sodium ion batteries with high energy density are currently the most concerned secondary battery technologies.As an important component of lithium/sodium ion batteries,cathode materials play a decisive role in their performance.Therefore,high-capacity cathode materials are the research focus of lithium/sodium ion batteries.Among them,the layered oxide cathode materials can introduce additional Li⁺/Na⁺to the transition metal layer for higher specific capacities,while the lithium or sodium-rich cathode materials show the disadvantage of structural instability.The cathode materials with NASICON structure provides a more stable three-dimensional framework for Li⁺/Na⁺diffusion,but their reversible capacities are limited.From the perspective of improving the cycling stability of layered oxides and increasing the capacity of NASICON structural materials,the material design,electrochemical performance and mechanism of Li-rich,Na-rich layered oxide and NASICON structure cathode materials are discussed in this thesis.Based on Li₂MnO₃-LiMO₂ high-capacity Li-rich cathode material,the metal-conductive Li₂RuO₃ was introduced to replace Li₂MnO₃.Li_1.2Ni_0.2Mn_0.2Ru_0.4O₂material was prepared by the traditional solid phase method,and it was coated with Al₂O₃ by liquid phase method to improve its cycle stability.The results of XRD refinement reveal that the prepared sample is more closely matched with the P2₁/m space group structure,which proves the existence of Ru-Ru dimer.In-situ XRD results reveal that the material as a cathode material has different lattice constant changes from the traditional layered material during the first cycle of delithiation,which confirms that the presence of Ru-Ru dimer contributes to the inhibition of the shrinkage of the a-b plane in the delithiation state and the improvement of the structural stability and electrochemical properties of the material.Compared with different proportions of Al₂O₃ coated samples,2%Al₂O₃ coated samples show the best electrochemical performance.The inert Al₂O₃ coating layer protectes the surface of the electrode by weakening the side reaction with electrolyte,which improves the Coulomb efficiency of the 1^st cycle,mitigates the voltage decay problem,and improves the cycle stability and rate performance of the material.A Na-rich material of similar composition,Na_1.2Ni_0.2Mn_0.2Ru_0.4O₂,was prepared by solid phase method.As a cathode material for sodium-ion batteries,the material provides a reversible capacity of¹61.7 mAh g^-1 in the 1^st cycle in the voltage range of1.5⁴.5 V,and a capacity retention rate of 70.5%after 60 cycles.The poor cycle stability is mainly affected by the anion participating in the reaction of providing capacity.By controlling the operating voltage range to 1.5-3.8 V,the capacity retention rate after 60 cycles is increased to 94.4%and the 1^st cycle reversible capacity is increased to¹28 mAh g^-1.In-situ XRD results reveal that the material experiences a phase transition of O3-O3'-P3 during the 1^st cycle of charging and experiences a phase transition of P3-O3'during the 1^st cycle of discharge.The simplified reversible phase transition process contributes to improving the cycle stability of the battery.Partially elements substituted of V³⁺in Na₃V₂?PO₄?₃ with Mn²⁺to introduce a fourth Na⁺into the NASICON structure,and carbon-coated Na₄MnV?PO₄?₃/C was prepared by a simple sol-gel method.The mechanism of sodium storage in different voltage ranges was studied.As a cathode material for sodium-ion batteries,Na₄MnV?PO₄?₃/C can achieve reversible de/intercalation of two Na⁺in the voltage range of 2.5-3.8 V.The reversible charge-discharge curves show a voltage slope at 3.4V and a plateau at 3.6 V.In-situ XRD results reveal that at different stage of the electrochemical process,it undergoes different sodium storage mechanisms,while GITT and PITT indicate that the first Na⁺de/intercalation process with single-phase reaction shows a faster kinetic compared to the second Na⁺de/intercalation process with two-phase reaction.At the same time,the long-range structure characterized by in-situ XRD also exhibits highly reversible structural changes in the 2.5-3.8 V voltage range.In the high voltage range of 2.5-4.5 V,Na₄MnV?PO₄?₃/C can achieve three Na⁺deintercalation during the first charge process,but irreversible capacity loss occurs during the discharge process with poor cycle stability.GITT shows that the material exhibited poor kinetic performance at voltages above 3.8 V,which continues into the next cycle.In-situ XRD results reveal the asymmetric structure evolution of the material during charge and discharge under high voltage conditions and the irreversibility of phase transition explains the main reason for its irreversible capacity loss and poor cycle stability.