以超音波霧化熱裂解法製備鋰鐵磷/碳二次鋰離子電池之正極複合材

因橄欖石結構相之LiFePO4擁有低成本、低的環境汙染性、穩定的熱穩定性、較佳的循環壽命與高的理論電容量(170 mAhg-1)，使得LiFePO4是目前鋰離子二次電池之正極材料中，最熱門的研究方向。在市場上，可利用於電子手工具、電動自行車、電動摩托車和油電混合車等大電流充放特性之器材。然而其低的鋰離子擴散速率與低的電子導電率通常會造成鋰鐵磷在商業運用上的一大瓶頸。本論文利用超音波霧化熱裂解法成功地製備純相鋰鐵磷/碳(LiFePO4 /C) 及其摻雜異質元素 (LiFe(1-x)MxPO4 /C, M = Mg, V) 之二次鋰離子電池正極複合材。超音波霧化熱裂解法之製程特點為其可一次性合成摻雜異質元素及在細小且均勻的顆粒表面上均勻披覆助導性物質於表面，進而提升鋰鐵磷正極材料之導電性、鋰離子擴散速率及其電化學特性。此研究中，以經由PVA 熱裂解所得之碳膜批覆(碳含量3.54 wt.%，電子導電率為 2.59 × 10-2 S cm-1) LiFe0.95V0.05PO4/C複合材可達最佳之電化學特性，0.1C和1C分別達134 mAhg-1和122 mAhg-1，並且經由30次充放電循環下，呈幾乎無電容量衰退之現象。這是由於異質元素之摻雜能有效的提高磷酸根分子之結構穩定性，並且使得Li-O鍵之鍵長增加，促使弱化Li-O鍵，造成提高橄欖石結構在充放電過程中，提高了橄欖石結構之穩定性及增進鋰離子的擴散速率。由CV 和EIS結果證明，其鋰離子擴散速分別為1.14 x10-11 cm2s-1 and 1.32 x10-11 cm2s-1，高於純相之鋰鐵磷六個數量級(106)，並且降低了電荷轉移電阻及Warburg 阻抗。由XRD 及FTIR 之結果得知，超音波霧化熱裂解法相較其他不同製程下，確實能得到無雜相之純相鋰鐵磷/碳及其摻雜異質元素之二次鋰離子電池正極複合材。

關鍵字

鋰鐵磷；交流阻抗；超音波霧化；正極材料

並列摘要

Small crystalline pure LiFePO4 and doped-LiFePO4 powders with conducting carbon coating can be synthesized by ultrasonic spray pyrolysis. Cheaper trivalent iron ion is used as the precursor. The powders can be prepared with the duplex process of spray pyrolysis and subsequent heat treatment. From SEM observation, it can be found that the powders are covered with fine pyrolyzed carbon. The evenly-distributed carbon will provide intimate contact among particles to reduce the electrical resistance. Raman spectra indicate that the phase of carbon with higher electrically-conductive phase is predominant when prolonged subsequent heat treatment is carried out. On the other hand, the spherical, dense and solid LiFe(1-x)VxPO4/C (x = 0, 0.01, 0.05, 0.1) composite cathodes were prepared by introducing the polyvinyl alcohol (carbon precursor) in the precursor solution The amount of carbon in LiFe0.95V0.05PO4/C powders is 3.54 wt.% and its electrical conductivity is 2.59 × 10-2 S cm-1. The solid spherical structure provides excellent electrical contact between particles to enhance the discharge capacity and mitigate the fading rate. In contrast to the hollow spherical morphology, the solid spherical morphology of the LiFePO4/C powders will have higher discharge capacity. The cells were tested at C/10 and 1C discharge rate at 30oC. The LiFe0.95V0.05PO4/C sample, prepared at Tsp = 450oC and post heat-treatment 700oC for 8h, exhibits higher specific discharge capacity (122 mAhg−1) and no obvious fading rate after 30 charge-discharge cycles at 1C rate. The kinetics of chemical behavior of the LiFe0.97V0.03PO4/carbon cathode materials were measured by using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The chemical diffusion coefficient of lithium was found by CV and EIS were 1.14 x10-11 cm2s-1 and 1.32 x10-11 cm2s-1, respectively. The phase purity and morphology of LiFe0.97V0.03PO4 were also identified using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). The lower fading rate at high discharge rate can be attributed to the intrinsic doping, which can enhance the diffusion coefficient of Li ions and reduce the Warburg impedance and the charge-transfer resistance.

並列關鍵字

Lithium iron phosphate ； Ac impedance ； ultrasonic spray pyrolysis ； cathode material ； Raman

參考文獻

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以超音波霧化熱裂解法製備鋰鐵磷/碳二次鋰離子電池之正極複合材

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