| Summary: | 碩士 === 元智大學 === 先進能源碩士學位學程 === 103 === The dissertation investigates the effect of operating temperature on electrochemical performance of cathode materials for Li-ion batteries. This study can be qualitatively divided into three parts: (i) LiFePO4 (LFP), (ii) Li(Ni1/3Co1/3Mn1/3)O2 (LNCM), (iii) Li(Ni0.8Co0.15Al0.05)O2 (LNCA).
(i) LiFePO4
The delithiation-lithiation process of C-coated olivine LFP (LFP/C) cathodes with the temperature range between 25 and 55℃ has been investigated using cyclic voltammetry (CV), charge-discharge cycling, and ac impedance spectroscopy. Using polyethylene glycol as carbon precursor, the LFP/C powders are synthesized by an efficient calcination/pyrolysis method at 700℃. The experimental results reveal that the high-temperature operation of LFP/C cathodes shows an improved capacity at 0.1−5C but negative effect on high-rate cyclic stability. The ac impedance spectroscopy incorporated with equivalent circuit indicates the decrease in equivalent series resistance and the increase in diffusion coefficient (DLi) with operating temperature. The DLi value achieved is as high as 1.49 #westeur024# 10-12 cm2 s-1 at 55℃ according to the Randles plot. On the basis of the results, the improved performance is attributed to high ionic conductivity, high ionic migration rate in solid electrolyte interphase film, high electronic conductivity, and high diffusion rate.
(ii) Li(Ni1/3Co1/3Mn1/3)O2
This study examines the effect of operating temperature on electrochemical performance of LNCM cathode, prepared by rheological method combined withmedium-frequency induction heating (MFIH) technique. The MFIH route is capable of preparing highly-crystalline LNCM powders, confirmed by X-ray diffraction study. The performance of as-prepared LNCM cathodes were well characterized using CV and charge-discharge cycling at 25 and 55℃. The carbon coating, deposited from glucose additive, onto LNCM powders displays a crucial factor in facilitating the specific capacity, rate capability, and cyclic stability, especially for high-temperature operation. The diffusion coefficients in oxidation and reduction states of C-coated LNCM (LNCM/C) cathode can achieve as high as 5.98 #westeur024# 10-7 and 2.67 #westeur024# 10-7 cm2 s-1 at 55℃ according to the calculation of Randles-Sevcik equation. With increasing the operating temperature, the electrode polarization becomes minor and the diffusion kinetics is significantly improved. The discharge capacities of LNCM/C cathode can reach to 183.9 and 203.2 mAh g-1 at 25 and 55℃, respectively. The LNCM/C cathode still remains high capacity retention of 90% at 55℃ after 50 cycles at 1C, as compared with the LNCM cathode without carbon coating. This enhanced performance can be ascribed to fast Li+ diffusion in the solid solution and electron jumping across the LNCM cathode at 55℃, with aid of carbon layer over the LNCM cathode.
(iii) Li(Ni0.8Co0.15Al0.05)O2
This study adopts an efficient technique to synthesize LNCA cathode materials for Li-ion batteries by rheological method combined with MFIH. The electrochemical performance of LNCA cathodes is well characterized by using CV and charge-discharge cycling at 25 and 55℃. The MFIH route is capable of synthesizing layered LNCA crystals at 700℃ within a shorter calcination period of 6hr, as compared with traditional heating methods. For 0.1C charge-discharge cycling, the LNCA cathode delivers high discharge capacity of 200.7 mAh g-1 at 55℃, showing an increased capacity of 18% as compared with the 25℃ operation. Analyzed by the Randles-Sevcik equation, the ionic diffusion kinetics in the LNCA cathode is obviously enhanced when operating at 55℃. For the 55℃ operation, both the diffusion coefficients in Li-extraction and Li-insertion states are increased up to 1.43 #westeur024# 10-7 and 0.85 #westeur024# 10-7 cm2 s-1, respectively. Without any surface modification, the as-prepared LNCA cathode still remained higher discharge capacity of 115.2 mAh g-1 at 55℃ after 50 cycles under 1C rate. Accordingly, the high-temperature operation promotes not only the Li occupancy fraction but also the chemical kinetics in the LNCA crystals. The MFIH route turns on a commercial feasibility to synthesize LNCA cathode materials for high-performance Li-ion batteries, favoring the development of electric vehicles.
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