| Renewable energy generation and large-scale energy storage are the foundation of the global energy Internet.Due to the limitation of space-time discontinuity,renewable energy cannot meet the huge load of smart grid connection for energy storage power generation.Therefore,it is urgent to develop large-scale energy storage technology with a high safety coefficient and long operating lifespan.Among the energy storage systems,lithium-ion batteries have been widely utilized in the current market due to their high energy density and superior power density.However,the rare reserves and uneven geographical distribution of lithium resources hinder their further development in scalable energy storage,which stimulates the upsurge of research on alternative systems.Na-ion batteries exhibit the advantages of abundant resources and low-cost features with similar working principles,which are expected to be the main force of the next generation of large-scale energy storage technologies.The key to the development of Na-ion batteries lies in the exploration of electrode materials,and the synthesis and optimization of cathode materials are the core of obtaining high energy density batteries.Among the cathode materials,polyanionic compounds and layered compounds have shown great potential in industrialization due to their high operating voltage and high specific capacity,respectively.Based on this,this dissertation revolves around the cathode materials with high energy density,including polyanionic compounds and O3/P2 layer oxides.The regulation of morphology,structure,electrochemical performance and practical application prospect in the full cell were systematically investigated.(1)For the to-be-improved electrochemical performance of monoclinic NaVOPO4,we proposed a low-temperature hydrothermal method to regulate the particle morphologies by varying the feeding ratio of sodium source and vanadium source.Besides,the nano-composites were synthesized by optimizing conductive additives and carbon coating via high energy ball milling to further improve its Na-storage properties.The optimized cathode shows a 135.3 mAh g-1 discharge capacity at 0.2 C,with 87.1%capacity retention over 1000 cycles at a high rate.The as-prepared nanocomposite presents superior reversibility during the charge-discharge process with pseudocapacity behavior.The current cathode was applied to the aqueous full-cell system with sodium phosphate titanium as the anode,showing decent performance and application prospect.The structure regulation and application provide feasible strategy and practical experience for Na-ion batteries.(2)Based on the optimized synthesis of NaVOPO4,extended the hydrothermal synthesis to F-containing Na3(VOPO4)2F with high working voltage.The hollow microspheres were obtained by exploring the optimized raw materials and temperatures.Further,the poor electrical conductivity was improved by high-energy mixing with KB.The results show that the dual function of temperature/pH is of great significance to obtain the special micro/nano structure,leading to varied electrochemical behaviors.Besides,the introduction of sodiation agent promotes the construction of a high-power energy storage battery(120 C,12000 cycles,71%capacity retention),and it can be successfully applied to the Na3(VOPO4)2F//NaTi2(PO4)3 full-cell system.(3)Inspired by the solution synthesis,a solvent-free mechanochemical strategy for the fabrication of Na3(VO1-xPO4)2F1+2x(0≤x≤1)and in-situ construction of their nanocomposites was proposed.Benefiting from the nano-crystallization features and extra Na-storage sites achieved in the synthetic process,the as-prepared Na3(VOPO4)2F/KB nanocomposite exhibits a 142 mAh g-1 capacity at 0.1 C,higher than its theoretical capacity(130 mAh g-1).Moreover,a scaled synthesis with 2 kg of the product was conducted and 26650-prototype cells were demonstrated to show decent performance.This work could mark an important step in the industrial application of sodium vanadium fluorophosphates for Na-ion batteries.(4)Considering the high cost of vanadium-based raw materials in polyanionic compounds,high-capacity O3-type cathode materials were developed.Taking cost-effective O3-NaNi0.3Feo.4Mn0.3O2 material as the starting composition,the bulk and surface properties were successively improved.To solve its applicability at high voltage(4.2 V),Cu/Ti co-doping was employed to alleviate the phase transition in the high voltage region and reduce the irreversible capacity loss.For the poor storage stability,a surface modification strategy was utilized to inhibit the formation of residual alkali on the surface,which achieves the improvement of both bulk structure and surface condition.The as-prepared NaCu0.1Ni0.2Fe0.4Mn0.2Ti0.1O2 with H3BO3 modification was matched with hard carbon to construct the full cell and displays a 132.6 mAh g-1 reversible capacity at 0.1 C with 75.8%capacity retention over 100 cycles,which shows cracking application prospect for Na-ion batteries.(5)To further improve the operating voltage of layered oxides,the P2-type oxides with anionic redox properties were designed by regulating the temperature and sodium dosage.Their electrochemical behaviors were compared and the optimized strategy for the full cell fabrication was investigated.To suppress the irreversible energy loss caused by sodium deficiency in the full-cell system,a novel high voltage sodiation agent Na2C3O5 was synthesized,which inherits the high-voltage features of Nao.75Ni0.25Mn0.75O2 with a discharge capacity up to 156.3 mAh g-1.For the sodium-rich P2-Na0.85Ni0.25Mn0.75O2,the full cell can be assembled by adjusting the cut-off voltage,and the initial Coulomb efficiency is 82.9%with a capacity retention rate of 79.0%over 100 cycles at 1 C.This full cell construction strategy including sodiation additives and voltage regulation provides a new idea for the practical application of P2 layered materials and offers more material models for sodiation agents. |
PDF Full Text Request