Synthesis And Electrochemical Property Of NASICON Phosphate Cathode Material For Sodium-ion Batteries

Sodium-ion batteries(SIBs)have been expected to be a promising alternative to the current lithium-ion batteries(LIBs)on many occasions due to the advantages of low cost and high abundance of sodium resources,especially in the field of low-speed electric vehicle and large-scale energy storage.Therefore,the development of highperformance SIBs electrode materials with fast charge/discharge speed,high safety,long lifespan,low cost and environmentally friendly properties have become a research hotspot in recent years.As a core component of SIBs,the property of cathode material plays a determining role in the electrochemical performance of the batteries.The current cathode materials can be classified as transition metal oxides,Prussian blue analogues,and polyanionic compounds.Among them,Na-ion superionic conductor(NASICON)phosphates,a type of polyanionic compounds,possess rapid ion diffusion kinetics and robust structural stability due to their three-dimensional ion channels and strong covalent bond between anion units,and exhibit excellent cycle stability and safety.However,NASICON phosphates still have some problems to be solved for the practical application,such as poor electronic conductivity,unsatisfactory rate performance and specific capacity,expensive raw material and environmental problem.Aiming at these problems,we carried out a series of basic researches on performance improving and internal mechanism exploration of two typical cathode materials Na3V2(PO4)3 and Na4MnV(PO4)3.The electrochemical performance of the materials was improved by optimizing the preparation conditions,elemental doping,morphology regulation,crystallinity regulation and carbon coating methods.Moreover,the full cell assembled with the prepared cathode and proper anode materials showed outstanding performance with much higher initial Coulombic efficiency and specific capacity.The main content of this thesis includes the following aspects:1.Na3V2(PO4)3 cathode materials with different Zr doping contents were prepared by sol-gel method,and the change of the electronic and ionic conductivities induced by Zr-doping was determined by ion blocking electrode method.Compared to the pristine material,both the electronic and ionic conductivities of the Zr-doped Na3V2xZrx(PO4)3/C samples are improved,but the increase of the electronic conductivity is more significant.The larger enhancement of the electronic conductivity can match the high ionic conductivities of Na3V2(PO4)3 better,obtaining an optimized cycle and rate performance.The x=0.1 sample exhibits a specific capacity of 91 mAh g-1 at 10 C with capacity retention of 93.1%after 1000 cycles.Its discharge capacity at 50 C is 71.9 mAh g-1,which is about 4.5 times larger than that of the undoped sample.2.Na3V2(PO4)3 material with disordered structure was prepared by electrostatic spray deposition method,the M1 site sodium ions constrained by the crystal structure were effectively activated and reversible three sodium ions insertion/extraction was achieved in Na3V2(PO4)3.Both V4+/V3+and V5+/V4+ redox pairs could participate in the reaction,so that the material shows very excellent electrochemical performance.The disordered Na3V2(PO4)3 sample delivered an ultra-high specific capacity of 179.6 mAh g-1 at 0.2 C.In addition,there also existed a pseudocapacitive charge storage mechanism induced by the disordered structure in this Na3V2(PO4)3 material.3.Double-carbon-layer coated Na4MnV(PO4)3 material(Na4MnV(PO4)3/C@C)was synthesized by one-step method.The coated double-carbon-layer provides a conductive network and protective layer for Na4MnV(PO4)3,which improves the electrical conductivity of the active material and protects the active material from the electrolyte corrosion.Therefore,Na4MnV(PO4)3/C@C possesses robust structural stability and rapid electrochemical reaction kinetics,leading to a high specific capacity of 99.8 mAh g-1 at 1 C with 94.3%capacity retention after 500 cycles,and a high rate capacity of 81.3 mAh g-1 at 10 C.4.The practicability of the prepared Na3V2(PO4)3 and Na4MnV(PO4)3 was verified by assembling full-cell with the self-developed biomass hard carbon material.The biomass hard carbon material has much rich micropores,which can provide sufficient sites for sodium-ion storage.Moreover,a particularly beneficial graphite layer was constructed on the surface of the particle,which effectively reduced the sodium loss of the hard carbon in the charge and discharge processes.So,the initial Coulombic efficiency of the hard carbon material as high as 90.7%can be achieved,and the fullcell composed of the Na3V2(PO4)3 and Na4MnV(PO4)3 cathode and the hard carbon anode also showed prominent performance.The two full-cell delivered high discharge capacity of 91 and 89.9 mAh g-1 at 0.1 A g-1 with initial Coulombic efficiency of 82.3%and 84.4%,respectively.The excellent full-cell performance shows that the obtained Na3V2(PO4)3 and Na4MnV(PO4)3 electrode materials in present work possess good application prospects.