| 摘要: | 隨著電動汽車的與日俱增，具有高能量密度和功率密度的鋰離子電池愈來愈被重視。近期有文獻證實，透過摻雜定量的過渡金屬元素，能使LiNi0.5Mn1.5O4材料的費米能階提高，進而滿足高電壓的需求。此外為了滿足現代人對3C產品輕薄短小的要求，減少像是黏合劑和導電劑以減少電池的重量，且同時保持電池的容量與活性材料和電極間的黏著性都是常被探討的議題。本實驗結合 LiNi0.5(1-x)Mn1.5(1-x/3)CrxO4 活性物質與碳纖維，透過電沉積法與水熱法，並結合鍛燒與抽氣過濾合成無黏合劑LiNi0.5(1-x)Mn1.5(1-x/3)CrxO4 高電壓複合式正極材料。另一方面，透過電泳沉積與鍛燒開發出無黏合劑 CNT/CF 複合型負極材料。
正極材料的部分，從 XRD 的分析可得知，增加鍛燒時間可有效增強峰值強度，但同時也會因為氧缺失而造成 LixNi1-xO 雜質項的產生，而定量的鉻摻雜 (x≥0.2) 可以有效改善結晶性，進而完全去除雜質項。而從充放電測試中得知摻雜鉻的正極材料 (x=0.1- 0.4) 因 Cr-O 鍵的強結合能，降低在鍛燒時氧氣的損失，以致於在 4.0 V 處 (Mn3+) 展現出較窄的平台，進而抑制電容量快速衰退。其中當 x=0.2 時，LiNi0.5(1-x)Mn1.5(1-x/3)CrxO4 正極材料具有最理想的放電容量 (135 mAh/g)，即使在 5 C 的電流速率下，仍表現出118 mAh/g 的放電容量及 87.4 % 的容量保持率，而在第200個循環後也仍保持 134.7 mAh/g 的放電容量及 97.6 % 的容量保持率，表明定量摻雜鉻確實可優化在高電壓充放電區間的循環性能。
負極材料的部份，根據 EDS 圖上均勻的分佈層可得知，奈米碳管確實均勻的披覆在碳纖維上。且從循環充放電與循環壽命測試可得知，跟原始的碳纖維相比，藉由 CNT 的修飾改善負極材料在高電流速率下容易快速衰退的問題，即便在 0.5 C 的速率下仍保持 283 mAh/g 的放電容量，並在100個循環後保持 344 mAh/g 的放電容量及 86 % 的容量保持率，表明CNT的修飾可以優化負極材料的放電電容，且改善在高電流速率下的電化學穩定性。
;With increasing demands of large-scale electronic applications such as electric vehicles (EVs), lithium ion batteries (LIBs) with high energy density and power density are desired. Recent researches has confirmed that doping quantitative transition metal will increase the Fermi level of the spinel LiNi0.5Mn1.5O4 material, which can improve the cycle performance in high voltage. In addition, to meet the aforementioned requirement, an electrode design that can accommodate more active material loading while reduce additives such as binders and conductive agents without losing any capacity is therefore expected. In the current study, we aim to combine nickel-manganese-chromium based cathode materials and CFs, developing a binder-free LiNi0.5(1-x)Mn1.5(1-x/3)CrxO4 high voltage composite cathode by using electrochemical deposition together with hydrothermal reaction and suction filtration. On the other hand, we develop a binder-free carbon nanotube (CNT)/CF composite anode by using an electrophoretic deposition method.
For the LiNi0.5(1-x)Mn1.5(1-x/3)CrxO4 cathode material. From XRD experiment results, it is found that increasing the calcination time can improve crystal purity, but it also lose oxygen and turn into disproportionate spinel LiNi0.5Mn1.5O4 and LixNi1−xO, so we dope Cr to stabilize the spinel structure of LiNi0.5Mn1.5O4 with increasing crystallinity. The composite cathode demonstrates favorable electrochemical performance against high-voltage operation due to the stronger bonding energy of Cr–O. Among them, the LiCr0.2Ni0.4Mn1.4O4 cathode material exhibits the best cyclic and rate performance. It can deliver the discharge capacity of 135 mAh/g at 0.2 C rate. Even at 5 C rate, it still delivers over 87.4 % capacity retention compared to that of 0.2 C. Through long cycle test, the LiCr0.2Ni0.4Mn1.4O4 cathode material delivers the discharge capacitie of 135.7 mAh/g with 97.6 % capacity retention after 200 cycles.
In addition, the morphology of the CNT/CF composite has been examined using energy-dispersive x-ray spectroscopy, and the results indicate that a CNT layer uniformLy deposites on the CFs. The CNT/CF anode shows better performance than the CF anode in terms of specific capacity, cycling stability, and rate capability. Even at 0.5 C rate, it still delivers discharge capacitie of 283 mAh/g. Through long cycle test, the CNT/CF anode material indicates the discharge capacitiey of 344 mAh/g with 86 % capacity retention after 100 cycles. |
