Synthesis And Performance Of Phosphate-based Electrode Materials For Li-ion And Na-ion Batteries

In this thesis,the research content concerning the exploration, synthesis, structural and electrochemical properties of phosphate-based electrodes for Li-ion and Na-ion batteries. Firstly, the subsolidus phase relation of Li₂O-MO-P₂O₅（M=Fe, Co） systems have been determined by experiment methods and, within these ternary systems, the structure and electrochemical properties of related compound have been systemically studied. Secondly, we have systematically studies the Li-stroage properties of Li₉V₃（P2O7）₃（PO4）2/C electrode, both as cathode and anode for lithium-ion batteries. Thirdly, we have firstly reported the synthesis, structural and related physicochemical properties of two new metaphosphate compounds Na M（PO₃）₃（M=Fe, Co）. Especially, the electrochemical properties of Na M（PO₃）₃（M=Fe, Co） compounds have been investigated and the results shows that these metaphosphate can be used as cathode for sodium-ion batteries. The mainly research content concerning:1. By using the powder XRD data of 65 specimens, the subsolidus phase relation of Li₂O-Fe O-P₂O₅ ternary system under the reducing atmosphere（95 %Ar+5%H2） has been firstly determined. The results show that, within this ternary system, there exist 8 binary compounds, 4 ternary compounds, 2 two-phase regions and 17 three-phase regions. Also, the Li1-xFe1-xPO4 solid solution phase with the homogeneous range of（-0.15≤x≤0.06） is determined within this system and their corresponding lattice parameters are obtained by the refinement results.2. By using the powder XRD data of 52 specimens, the subsolidus phase relation of Li₂O-Co O-P₂O₅ ternary system under the reducing atmosphere（95 %Ar+5%H2） has been firstly determined. The results show that, within this ternary system, there exist 6 binary compounds, 5 ternary compounds and 17 three-phase regions. Within this ternary system, the electrochemical properties of related Co-based phosphate have been systemically studied. We found that Li6Co5（P2O7）4 compound as lithium-ion batteries cathode is electrochemically active and at the current rate of 5m Ag-1, a well-kept reversible capacity and oxidation/reduction plateau can still be observed after 30 cycles within the voltage of 3.0-4.9V.3. A series of single-phase Co-doped Li₉V₃-xCox（P2O7）₃（PO4）2/C（x=0.00, 0.04, 0.06, 0.08, 0.10） compounds have been successfully prepared by sol-gel method. The fitting results of X-ray photoelectron spectroscopy data suggest that the oxidation state of cobalt is +2 due to the 70% Ar + 30% H2 atmosphere used in our experiments, while the fitting curves of XPS data for V 2p in Li₉V₃-xCox（P2O7）₃（PO4）2/C show evidence of the coexistence of V4+ and V³⁺ ion when Co²⁺ ion doping is presented. Their electrochemical properties have been investigated. The results show that the Li9V0.96Co0.04（P2O7）₃（PO4）2/C compound presents the best electrochemical rate and cyclic ability. The best electrochemical performance of Li9V0.96Co0.04（P2O7）₃（PO4）2/C compound can be ascribed to enhance specific surface area, which facilitates the lithium ion diffusion.4. We have prepared the carbon-coated Li₉V₃（P2O7）₃（PO4）2（LVPP/C） composite via a simple carbon thermal reduction and its lithium-storage properties are systematically investigated, both as cathode and anode electeodes. LVPP/C cathode shows two-pair redox peaks and keeps a well cycle performance within the voltage range of 2.5V-4.6V. As an anode, LVPP/C electrode shows an intercalation reaction mechanism and a well rate performance. Meanwhile, taking advantage of the big voltage difference between LVPP/C（cahode） and LVPP/C（anode）, a symmetric cell using LVPP/C composite as both cathode and anode electrodes have been designed and investigated. The results show that LVPP（cathode）//LVPP（anode） symmetric cell has two-pair redox couples at around 2.38V/2.37 V and 2.75V/2.68 V and can deliver a well-kept reversible capacity after 30 cycles.5. We have prepared a new Fe-based metaphosphate Na Fe（PO₃）₃. The XRD refinement results show that this new compound forms the orthorhombic lattice（S.G. P212121） and have the lattice parameters of a=14.3390（3） ?, b=14.3374（2） ?, c=14.3642（1） ?. The structure of Na Fe（PO₃）₃ is constructed via the arrangement of PO4 tetrahedrons and Fe O6 octahedrons, which indicates that this title compound contains infinite（PO₃）- chains along a-axis and the Fe O6 octahedrons show the function of bridging the neighboring chains. The magnetic measurement results indicate that Na Fe（PO₃）₃ compound shows a typical paramagnetic behavior of high-spin Fe²⁺, following the Curie–Weiss law within the temperature range of 10-300 K. The electrochemical properties of Na Fe（PO₃）₃ compound is also measured and only partially reversible capacity is obtained due to its kinetic limitations. More efforts such as developing appropriate post-synthesis treatment techniques need be put on the electrode preparation to future optimize the sodium deinsertion/insertion behavior of Na Fe（PO₃）₃ compound.6. At last, a new Co-base sodium metaphosphate compound, Na Co（PO₃）₃, has been synthesized here by solid-state method and a variety of chemical, spectroscopic and electrochemical analytical techniques are applied to study its physicochemical properties. Especially, the XRD refinement show that this compound is isotypic with the metaphosphates Na Fe（PO₃）₃ and has the lattice parameters of a=14.2453（2） ?, b=14.2306（1） ?, c=14.2603（2） ?. At last, the electrochemically active of Na Co（PO₃）₃ cahode for sodium-ion batteries is demonstrated for the first time and at the current rate of 0.05 C, the Na Co（PO₃）₃ electrode can deliver an initial charge-discharge capacity of 49.5 m Ah/g and 33.8 m Ah/g. After 20 cycles, a well-kept oxidation-reduction plateaus and reversible capacities can still be obtained for Na Co（PO₃）₃ cahode. Meanwhile, a reversible sodium-ion extraction and insertion process, corresponding to the oxidation-reduction process of Co²⁺/Co³⁺, is also observed by ex-situ XRD and XPS.