Phosphate-based Polyanionic Cathode Materials For Lithium/Sodium-ion Batteries

In the past three decades,lithium-ion batteries（LIBs）have successfully occupied the new energy market owing to their high energy density and excellent cycle stability,and have widely been used in people’s daily lives,such as in mobile phones,notebook computers,and electric vehicles.It is widely acknowledged that,the cathode materials of alkali ion batteries play a decisive role in determining the energy density of whole cells.In terms of battery performance,both sodium-ion batteries（SIBs）and LIBs are committed to the pursuit of high safety,long cycle life and fast charging speed.Among cathode materials for LIBs/SIBs,compared with layered transition metal oxide materials,phosphate-based materials have attracted increasing attention due to their advantages of high working voltage,excellent cycle stability,high safety and stable structure during cycling.However,there still exist some problems in the phosphatebased cathodes for SIBs,such as poor electronic conductivity,poor rate performance,and serious side reactions between electrode materials and electrolytes.Therefore,in this thesis,a series of basic research have been carried out to investigate several phosphate-based cathode materials,including Na₄Fe₃（PO₄）₂（P₂O₇）,Na₇V₄（P₂O₇）₄（PO₄）,Na_3.36FeV（PO₄）₃ and Na₄MnCr（PO₄）₃ for SIBs and NaLi₃Fe₃（PO₄）₂（P₂O₇）for LIBs.We mainly investigate the influences of the materials’ morphology,carbon content,coating amount and other factors on their lithium/sodium storage performance,and study the reaction mechanism of the electrode materials during the charging/discharging,which may lay the foundation for their practical applications.In Chapter 1,the composition and working principle of LIBs/SIBs are introduced.And the research progress of the widely studied cathode and anode materials for SIBs is briefly summarized.Finally,the research background and research content of this thesis are described.In Chapter 2,the main reagents,synthesis methods,structural characterization of the electrode materials and equipment used in electrochemical performance testing are introduced.In Chapter 3,Na₄Fe₃（PO₄）₂（P₂O₇）（NFPP）with hollow sphere structure is synthesized by spray drying method,which can produce electrode materials’ powders on a large scale in industry.In this chapter,we study the effect of glucose content on the morphology and electrochemical performance of NFPP materials.The results show that the in-situ coated carbon by thermal decomposition of the glucose on the surface of NFPP particles can inhibit the growth and agglomeration of the particles,and can improve the sodium storage capacity of the electrode material.The initial discharge capacity of the NFPP electrode at 10 C is 88.8 mAh g-1,and the capacity retains 92%after 1500 cycles.In addition,ex-situ XRD analysis shows that NFPP electrode material is a solid solution reaction mechanism during the charge and discharge process,and the volume change ratio is less than 3%.In Chapter 4,based on Chapter 3,the active Na sites in Na₄Fe₃（PO₄）₂（P₂O₇）structure are engineered by electrochemical ion exchange method to synthesize NaLi₃Fe₃（PO₄）₂（P₂O₇）（NLFPP）cathode material for LIBs.The lithium storage mechanism and the performance of the novel electrode material are investigated.It is found that the electrode has an ultra-high rate performance.The reversible capacity of the electrode material can reach 81.5 mAh g-1 at 30 C.The in-situ XRD analysis shows that the volume change rate of NLFPP is larger than that of NFPP during charge and discharge process.In Chapter 5,the Na₇V₄（P₂O₇）₄（PO₄）（NVPP）electrode powder with submicron rod-like structure is prepared with a high temperature solid-phase method.Using polyvinyl alcohol（PVA）as the carbon source,VOx is used to catalyze the in-situ conversion of PVA to graphene-like layers,which is coated on the surface of NVPP particles.Therefore,the electronic conductivity of the electrode is increased,and the electrochemical performance of the material is improved.In addition,ex-situ XRD proves that the cathode material NVPP follows a solid solution reaction mechanism during charge/discharge process,and the volume change rate was less than 4%.Finally,the electrode materials are assembled into a symmetric full cell with an energy density of 185.7 Wh kg^-1 at 0.2 C.In Chapter 6,the Na_3.36FeV（PO₄）₃（NFVP）cathode material with sodium superionic conductor（NASICON）structure is prepared by a sol-gel method.The electronic conductivity of NFVP is improved by controlling the thickness of the carbon layer coated on the surface of NFVP particles,thus improving its electrochemical performance.The initial discharge capacity of the NFVP electrode is 90.4 mAh g-1 at 10 C,and the capacity retention rate is 80%after 5000 cycles.In addition,ex-situ XRD analysis shows that the volume change rate of NFVP electrode material is less than 3%during charging and discharging.In Chapter 7,the NASICON structured SIB cathode Na₄MnCr（PO₄）₃（NMCP）with a high working voltage is prepared by a sol-gel method.The capacity of the pristine material decays rapidly due to the Jahn-Teller effect of Mn3+.Al2O3 and Na₄Fe₃（PO₄）₂（P₂O₇）are selected to coat the material aiming to solve this problem.In addition,the effect of coating amount on the properties of NMCP is explored.It tures out that,after 150 cycles at a current rate of 2 C,the capacity retention of Na4MnCr（PO4）/C@1%Al2O3 and Na4MnCr（PO4）/C@4%Na₄Fe₃（PO₄）₂（P₂O₇）electrodes are 70%and 71.6%,which increases 20.2%and 21.8%compared with NMCP/C electrode,respectively.In Chapter 8,we briefly summarize the innovations and shortcomings of the work in this thesis.Some suggestions are given for related research work that can be carried out in the future.