Studies On The Synthesis And Electrochemical Properties Of Fe-Based Electrode Materials For Lithium-Ion Batteries

It is well known that olivine LiFePO4 is one of the most promising electrode materials for power batteries due to its high theoretical capacity (170 mAh·g-1), long cyclability, intrinsic thermal safety, abundant precursors and environmental compatibility. However, the main weaknesses of LiFePO4, such as low tap density, low electronic conductivity and low lithium ion diffusivity, need to be overcome for a large-scale application purpose. Insofar as the 1D diffussion of Li+ions within LiFePO4 orthorhombic lattices is concerned, computational and experimental studies have demonstrated that Li+ion migration occurs preferentially via one dimensional channels oriented along [010] direction, however, as we know this could hardly be electrochemically visualized so far. Furthermore, in order to reduce the cost for the high conditions of synthesis, storage and the electrode coating, many studies have focused on the synthesis of iron(III) phosphate and its derivatives.In this dissertation, we fouces on the controlling syntheses and structural characterizations of (010)-defective LiFePO4 hollow platelets and iron(III) hydroxide phosphate dehydrate (Fes(PO4)4(OH)3·2H2O) at first, then on the electrochemical visualization the 1D migration of Li+ions along [010] direction, and then on the bifunctional electrochemical performances of Fes(PO4)4(OH)3·2H2O as both anode and cathode active substances for lithium ion batteries. The main experimental results are divided into two parts, shown as below.1. To visualize the one-dimensional (ID) migration of Li+ions along [010] direction, two kinds of (010)-defective LiFePO4 platelets with different hollow interiors are prepared using the polyethylene glycol (PEG)-assisted hydrothermal reaction of LiOH, FeSO4 and H3PO4 at the molar ratio of 3:1:1. Owing to the acidic and viscous reaction circumstance in the presence of polymeric PEG, the relatively high concentration of PEG induces the formation of (010)-defective LiFePO4 platelets with a small average length and a small hollow interior. As a Li-ion battery cathode, (010)-defective crystallites with a small hollow interior exhibit the higher reversible capacity at each charge-discharge cycle, the smaller concentration polarization and charge transfer resistance and the bigger Coulombic efficiency and Li-ion diffusion coefficient than (010)-defective LiFePO4 platelets with a large hollow interior. Furthermore, (010)-manifest LiFePO4 micro-rhombohedra and their surface-etched derivatives could be treated as a comparative couple to prove the probable formation mechanism of (010)-defective LiFePO4 platelets and to visualize their sluggish charge transfers along [010] direction.2. Fe5(PO4)4(OH)3·2H2O tetragonal bipyramids with a solid or hollow interior can be uniquely prepared using the solvothermal reaction of FeCl3 and H3PO4 in the mixed solvents of 1,2-propanediol and ultrapure water at 140℃ for 48 h. Time-dependent results show that disc-like FePO4·2H2O crystallites initially form and their dissolution-recrystallization indice the uniform formation of Fe5(PO4)4(OH)3·2H2O tetragonal bipyramids at the solvothermal incubation interval of 48 h. When applied as lithium-ion battery cathodes within a high voltage range of 4.5 and 1.5 V, at 50 mA·g-1 the working electrode of Fe5(PO4)4(OH)3·2H2O tetragonal bipyramids could deliver an initial discharge capacity of 44.1 mAh·g-1, still remaining a relatively high value of 70.8 mAh·g-1 over 200 charge-dischage cycles. Interestingly, Fe5(PO4)4(OH)3·2H2O tetragonal bipyramids can also be applied as anode active substances within the low potential range between 3.0 and 0.01 V, at a current density of 500 mA·g-1 the working electrode delivers a discharge capacity of 591.6 mAh·g-1 over 600 cycles. Comparing with the lithium-storage capability of FePO4, those of cathode and anode Fes(PO4)4(OH)3·2H2O are not high and may be assigned to both the poor conductivity and the absence of a conductive coating. However, the good cycling stability of Fe5(PO4)4(OH)3·2H2O tetragonal bipyramids implies that they deserve to be conducted in future.