Phosphate-based Electrode Materials For Lithium/Sodium Ion Batteries

Since the first lithium-ion batteries(LIBs)commercialized by Sony Corporation in 1991,they have achieved un-rivaled success in the markets of portable consumer electronics.Due to their high energy density,LIBs have been widely used in portable laptops,mobile phones,cameras,electric vehicles and electric buses.Nevertheless,considering the limited reserves of Li resources on the earth,sodium and potassium ion batteries(SIBs and PIBs)have been recognized as the alternative devices for the future applications in large-scale energy storage systems.The most concerned features of a battery are its cycle life,charging speed and safety,which are largely determined by its electrode materials.Compared with oxide electrode materials(such as LiCoO2,Na0.67MnO2,etc),phosphate-based electrodes have attracted much attention because of their stable structure,excellent cycling stability,small volume change during charge/discharge process and high safety.However,these materials have a low intrinsic electronic conductivity,which leads to poor rate and low-temperature performances.Therefore,the study of this thesis aims to systematically investigate the crucial issues of several phosphate-based electrode materials(Li3V2(PO4)3 and LiFePO4 for LIBs,Na3V2(PO4)3,NaTi2(PO4)3 and TiP207 for SIBs),including the synthesis,particle morphology,carbon coatings,and lattice doping.The results of the study may be used to improve the electrochemical performances of these phosphate electrode materials and provide an experimental basis for the future practical applications.Chapter 1 introduces the composition,working mechanism and main characteristics of LIBs/SIBs.Then a brief overview of literature reported cathode and anode materials of SIBs is given.Finally,the research background and the scope of this thesis are described.Chapter 2 lists the main experimental reagents,equipments and characterization of the materials.Then the assembly process of lithium/sodium ion batteries and the measure methods of electrochemical performance are introduced in detail.In Chapter 3,two carbon-coated Li3V2(POa)3(LVP)powders,i.e.LVP@C and LVP@G with citric acid and polyvinyl alcohol(PVA)respectively as the carbon source,are synthesized through a two-step solid state reaction process.The effects of different carbon sources on the morphology and electrochemical performance of LVP are investigated.It is found that citric acid as a comparative carbon source leads to less amount of carbon residue than PVA.The graphene is formed in-situ from PVA or citric acid as a carbon source with the VOx-containing intermediate product as a catalyst.Under the condition of similar carbon residue,the carbon coating in LVP@G is primarily in the form of graphene-like layers,while it is in the form of mixed carbon in LVP@C.The existence of graphene-like carbon largely improves the electronic conductivity of LVP@G.Hence,PVA is a more suitable carbon source than citric acid for preparing carbon-coated electrode materials.In Chapter 4,based on Chapter 3,the composite electrode materials(1-x)LiFePO4·xLi3V2(PO4)3(x=0,0.02,0.05,0.1)are synthesized by a solid-state reaction process with PVA as the carbon source.The effects of different amount of LVP precursor and carbon form are investigated.It is found that,with a small amount of LVP in the synthesis of LiFePO4(LFP)-based active powders,the form of carbon coating on the primary particles changes partially from sp3 to sp2 hybridization in the carbon-carbon covalent bonding.The change in the form of carbon coating on the surface of LiFePO4 plays an important role to improve the electronic conductivity,which contributes to its excellent rate and low-temperature performances.In Chapter 5,a series of NASICON-type Na3V2-xGax(PO4)3(x=0,0.1,0.2,0.4 and 0.6)are synthesized via a solid-state reaction process.The catalytic mechanism of VOx is further clarified.Compared with the baseline Na3V2(PO4)3(NVP),the Ga-doped samples Na3V2-xGax(PO4)3(x=0.1,0.2 and 0.4)show an extra redox couple V4+/V5+(4.0 V vs.Na+/Na)in addition to V3+/V4+(3.4 V vs.Na+/Na).Also,Ga3+increases the proportion of sp2-type carbon leading to the higher energy density and power density of the material.In Chapter 6,another NASICON-type NaTi2(PO4)3(NTP)powder is synthesized by a hydrothermal process.The coated carbon form is optimized by adding a small amount of NVP intermediate powder during the heating treatment.It is found that the NVP intermediate powder also has a catalytic effect on the carbonation of PVA at 600?,which enhances the electronic conductivity of NTP.The optimized sample has long cycle life,excellent rate and low temperature performances.In Chapter 7,nanosized carbon-coated NTP particles with a high yield(more than 95%)are synthesized by a reflux method in ethylene glycol(EG).The effects of three organic solvents(EG,carbitol and glycerine)with different boiling points and viscosities on morphology and carbon coating of NTP are clarified.The results show that under the condition of similar carbon residue and carbon form,the NTP-EG sample has the most suitable carbon-coating layer,which effectively improves the electronic conductivity and rate performance.In Chapter 8,3D porous NaTi2(PO4)3 thin films decorated with reduced graphene oxide(NTP@rGO)on a nickel foam substrate are synthesized by electrostatic spray deposition technique.The effects of organic solvent and experimental conditions are explored.The results show that when the solvent is 1,2-propylene glycol/acetylacetone(v/v,9/1),3D porous NTP thin films are obtained.The interconnected pore networks and highly conductive carbon frameworks not only facilitate the kinetics of sodium-ion diffusion but also provide abundant active interfaces.The NTP@rGO electrode shows superb rate performance with a charge capacity of 109.4 mAh g-1 at 100C,i.e.,96.8%retention of its capacity at 0.5 C.Even at-10?,NTP@rGO still delivers 92.5 mAh g-1 at 100C.Such a composite anode combines the advantages of batteries and supercapacitors,showing great potential applicability for high-energy and high-power sodium-ion storage.In Chapter 9,the layered Ti(HPO4)2·H2O(THP)is synthesized by a solvothermal method.The effect of annealing temperature on THP sample is studied.It is found that,when the heat treatment temperature is increased from 500 ? to 800?,the structure of TiP2O7(TP)is transformed from layered to cubic phase.For the first time,the electrochemical performances of layered TP are explored.The results show that this layered phase follows a solid-solution mechanism for the Li-,Na-and K-intercalation reactions,while the cubic phase follows a two-phase co-existence mechanism for Li-intercalation.Also,the cubic phase is electrochemically inactive for Na-storage.As a new cathode material for rechargeable alkali metal batteries,the layered TP shows a fairly reversible capacity close to 100 mAh g-1 in both Li and Na cells and 46.9 mAh g-1 in K cells.In Chapter 10,we briefly summarize the innovations and deficiencies of this thesis.Some suggestions for the future work on high energy density electrode materials are also presented.Finally,based on the research of the residual carbon content and carbon form of electrode materials in this thesis,appendix A introduces the catalytic effects of high-valent transition metal oxides(VO2,Nb2O5,Ta2O5 and WO3)on different carbonization behaviors of PVA.