| dc.description.abstract | Abstract
Globally, the development of high-power lithium-ion batteries is focused on lithium manganese oxide batteries, lithium cobalt nickel manganese batteries and lithium iron phosphate batteries. Lithium iron phosphate is regarded as a practical and popular cathode material for high power lithium-ion batteries due to its high capacity, high rate capability, long cycle life, superior thermal stability and good high-temperature performance. However, the intrinsic disadvantages of pure LiFePO4 material are its poor electronic conductivity, low tap density and low operating voltage. Therefore, we combined a number of innovative concepts and techniques in the fabrication processes to alleviate the aforementioned problems. The main purpose of this study has been divided into three sections:
(1) To improve the manufacturing process of battery cells by enhancing the tap density of LiFePO4 powders.
(2) To enhance the electrochemical performance by doping the metal ions into LiFePO4 crystal.
(3) To increase the average operating voltage by introducing the phosphate-based compounds with high working voltage into LiFePO4 material.
Olivine-structured LiFePO4 cathode materials were prepared via a combination of carbothermal reduction (CR) and molten salt (MS) methods. To enhance the tap density of powders, the LiFePO4/C composite was pressed into pellets and then sintered for at least 1 h at 1028 K in the reaction environment of KCl molten salts. The use of molten salt can effectively influence unit cell volume, morphology and tap density of particles, and consequently change the electrochemical performance of LiFePO4/C. The composites were characterized in detail by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), dynamic light scattering (DLS), Raman spectroscopy and tap density testing. The final product with high tap density of 1.50 g cm−3 contains 4.58 wt.% carbon and exhibits good discharge capacity of 141 mAh g−1 at a 0.2 C-rate in the potential range of 2.8-4.0 V.
These olivine LiFe1-xCaxPO4/C composites (x = 0 ~ 0.014) were synthesized by a solid-state method using sebasic acid as a carbon source. The structure and electrochemical properties of the LiFe1-xCaxPO4/C compounds were studied. The X-ray diffractometer (XRD) results indicated that Ca2+ doping did not affect the structure of the samples, but the unit cell volume of the doped samples was slightly increased. Electrochemical measurements showed that the LiFe0.99Ca0.01PO4/C composite delivered a discharge capacity of 149 mAh g-1 at a 0.2 C-rate between 4.0 and 2.8 V, probably due to the significant improvement in electronic conductivity and Li+ ion diffusion. The cell could also sustain a 20 C-rate, and this rate capability is equivalent to charge or discharge in 3 min.
Phosphate-based compounds with a high working voltage, such as Li3V2(PO4)3, LiVPO4F and LiMnPO4, have been proposed as a new class of cathode materials for lithium-ion batteries. To improve the operating voltage of LiFePO4, we introduced LiVPO4F to the preparation of xLiFePO4‧(1-x)LiVPO4F (LFP-LVPF) composites through an aqueous precipitation and carbothermal reduction method. A series of LFP-LVPF composites have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and total organic carbon (TOC) analysis. The discharge capacities of LFP-LVPF composites for x:(1-x) = 1:0, 0.99:0.01, 0.75:0.25, 0.5:0.5, 0.25:0.75 and 0:1 at a 0.2 C-rate were 153, 160, 132, 106, 92 and 78 mAh g-1, respectively. The discharge capacity decreased with increasing mole fraction of LVPF. Moreover, the operating voltage of LFP-LVPF composites for x:(1-x) = 0.75:0.25, 0.5:0.5 or 0.25:0.75 is higher than that of LFP, and the charge/discharge plateaus around 4.35/4.15 V for LFP-LVPF composites become longer as the value of x decreases.
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