Design And Electrochemical Na-storage Study Of High-energy NASICON-type Phosphates Cathodes

Lithium-ion batteries(LIBs)have been widely applied into various energy-storage areas,such as portable electronic devices and electric vehicles.However,the limited lithium resources still restrict its use in the large-scale energy storage systems.Sodium-ion batteries(SIBs),which possess similar working mechanisms to that of LIBs,has been considered as one of ideal large-scale energy storage technologies due to the low cost and abundant sodium resources.Whereas SIBs suffer from the issue of low energy density,recently,it has been a research hotspot to find cathode materials with simultaneous high specific capacity and working voltage plateau to achieve elevated energy density.Among numerous cathode materials,Na superionic conductor(NASICON)-type phosphates cathodes have been identified as promising candidates by virtue of its three-dimensional(3D)framework,high ionic conductivity,as well as structural diversity.In this thesis,a series of NASICON-structured phosphate materials are selected as research subject and developed as high-energy cathodes for SIBs.Conjunct studies of synchrotron X-ray diffraction(SXRD),neutron powder diffraction(NPD),in-situ XRD,and X-ray absorption near-edge structure(XANES)spectra were performed to investigate the multi-electron reaction process of electrode and illustrate the structural evolution during Na+extraction/insertion.Combined with the study of density functional theory(DFT)computations,the Na-storage reaction mechanisms of high-performance cathode were elaborately revealed.Firstly,a comprehensive study of Na3Fe0.75V1.25(PO4)3/C as low-cost cathode for SIBs were carried out.As a classical NASICON-structured phosphate,Na3V2(PO4)3 delivers high specific theoretical capacity of 117 mA h g-1,operating voltage of 3.4 V,stable framework structure,as well as superior cyclic stability,enabling it one of the most promising phosphate candidates for SIBs.However,the high toxicity and high cost of Vanadium still restrict its practical application.Herein,suitable amount of iron with low cost and environmental friendliness was introduced into Na3V2(PO4)3 structure to form Na3Fe0.75V1.25(PO4)3(NFVP).The NFVP/C nanocomposites were prepared by facile sol-gel methods.When evaluated as cathode for SIBs,the half-cell can deliver reversible capacity of 115 mA h g-1 at current density of 0.5 C,a specific capacity of 57.4 mA h g-1 can be achieved even at ultrahigh current density of 60 C.Furthermore,the electrodes can present capacity retention of 86.2%after 3000 cycles at current of 20 C.XANES spectroscopy revealed that the redox peaks located at 2.6 V,3.4 V,and 4.0 V correspond to Fe2+/3+,V3+/4+,and V4+/5+,respectively.Moreover,in-situ X-ray diffraction(XRD)demonstrate highly reversible structural evolution with moderate unit cell volume change of 7.5%.Combined with DFT calculation and kinetics study,we find that the low migration barrier and high Na+ diffusion coefficient account for the excellent rate capability and cyclic stability.Moreover,a cathode Na3.5Mn0.5V1.5(PO4)3/C(NVMP/C)nanoparticles were designed and successfully synthesized considering the active Mn2+/Mn3+redox couples with high operating voltage.The electrochemical tests and in-operando XRD measurement indicated that the average operating voltage was significantly improved compared with pristine Na3V2(PO4)3,meanwhile,the detrimental Mn3+Jahn-Teller effect was alleviated due to the incorporation of suitable amount of Mn.Consequently,the electrode enable excellent specific capacity of 92 mA h g-1 at current density of 60 C,and superior capacity retention of 88%after 4000 cycles at 20 C.Ex-situ XANES manifested that Mn2+/3,V3+/4+,and small amount of V4+/5+ involved into the electrochemical reaction,thus contributing to the high voltage and capacity.Finally,the NVMP/C cathode was paired with hard carbon anode to assemble the full cell,the battery can present good capacity retention of 87%after 50 cycles as well as high energy density of 234.63 Wh kg-1,further verifying practical feasibility of present cathode materials.Following the abovementioned works,we have designed a new bimetallic NASICON-type material of Na4MnCr(PO4)3(NMCP).The carbon coated MNCP nanoparticles were successfully synthesized by simple sol-gel methods.When used as cathode for SIBs,the material can exhibit ultrahigh capacity of 160.5 mA h g-1 and average discharge voltage of 3.53 V,achieving a high energy density of 566.5 Wh kg-1,which is even higher than that of commercial LiFePO4 in LIBs.Such value is also the highest among reported phosphate cathode for SIBs so far.Single-phase accompanied by two-phase transition during the Na+extraction/insertion process have been vividly revealed by in-situ XRD patterns.XANES result demonstrate that Mn2+/3+(3.6 V),Mn3+/4+(4.2 V),and Cr3+/4+(4.4 V)three-electron reaction are responsible for the desirable capacity.DFT calculation illustrate that the new compound shows some metallic properties as well as low Na+migration barrier of 0.38 eV.Finally,the hard carbon anode was matched with the NMCP/C cathode to assemble Na-ion full battery,the full battery also obtains a high energy density,showing a great prospect of practical application.