| 摘要: | 無鈷尖晶石正極材料LiNi0.5Mn1.5O4(LNMO)由於低成本、高工作電壓、優異的能量密度(690 Wh/kg)，以及良好的熱穩定性而受到廣泛的關注。然而，其高工作電壓(4.7 V vs Li/Li+)會加速商用電解液劣化，容易導致長時間循環壽命衰減，使得實際可行性備受挑戰。為了解決這些問題，我們提出一種創新的方式結合乙醇輔助水熱和金屬有機框架(基於PTA的MOFs)作為前驅物，合成出LNMO複合正極。如此一來，金屬離子便可以成功與有機配體排列整齊，同時乙醇輔助水熱法進一步改善LNMO顆粒的分散性與降低顆粒尺寸。而為了實現最佳電化學性能，因此我們使用不同的水熱溫度(T=100、120、140、160 °C)來定義出最佳參數，並加以合成。接著，透過在 MOF 的 Ni-Mn 前驅體中摻雜 Fe，以三金屬前驅體煅燒形成 Fe 摻雜 LNMO (LiNi0.5-xMn1.5-xFe2xO4 (x=0, 0.03, 0.05 , 0.1, 0.15))來增加結構穩定性。結果證實，這種新穎的合成方法是製備高性能LNMO正極之可行策略，不同水熱溫度讓LNMO前軀物擁有不同的顏色和粒徑大小。此外，合成之LNMO表面會自然生成出無定質的Li2CO3層(厚度約1 nm)，以保護材料免受電解液之過度反應，並有效輔助Li+擴散進行。在所有樣品中，LNMO-120°C具有最小的顆粒大小和均勻的粒徑分佈，在0.2 C下，具有最佳的142.5 mAh/g之實際電容量，並在1 C速率下第200次循環後之電容保持率為80.1%。與原始LNMO正極不同，摻雜Fe之尖晶石材料減少了常見的LixNi1-xO雜質相，並具有令人滿意的循環穩定性， LiNi0.45Fe0.1Mn1.45O4樣品顯示出最出色的循環穩定性(在1 C下200圈循環後之電容保持率為96.7%)，成功大幅改善了商用LNMO所存在的結構穩定性問題。另外，也具備極佳的倍率性能，即使在5 C下，其容量仍達到118.9 mAh/g，並通過GC-MS證實Fe摻雜能有效去抑制並減緩電解液中有機酯EC與DEC劣化成其他副產物。;The cobalt-free spinel positive electrode LiNi0.5Mn1.5O4 (LNMO) is receiving extensive attention for lithium-ion batteries due to its low cost, high operating voltage (4.7 V vs. Li/Li+), superior energy density (690 Wh/kg), and good thermal stability. However, its high operating voltage hampers its stability with commercial electrolytes, leading to capacity decay during long-term charge-discharge cycling. This makes its practical viability challenging.
To solve these problems, we combined ethanol-assisted hydrothermal and metal−organic frameworks (PTA-based MOFs) as precursors to synthesize high-voltage LNMO composite positive electrode. In this way, the metal ions can be successfully aligned by organic ligands, meanwhile the ethanol-assisted hydrothermal process improves the dispersity of LNMO particles with reduced particle size. Since selecting the appropriate hydrothermal temperature is crucial to achieve optimal electrochemical performance, we used different hydrothermal temperatures (T=100, 120, 140, 160°C) for synthesis. In order to further enhance structural stability, Fe was doped in the Ni-Mn precursors of MOF, and the trimetallic precursor is calcinated to form Fe-doped LNMO (LiNi0.5-xMn1.5-xFe2xO4 (x=0, 0.03, 0.05, 0.1, 0.15)).
The results confirm that the novel synthesis method is a feasible strategy to fabricate high-performance LNMO positive electrodes. Various hydrothermal temperatures give rise to different colors and particle size of the LNMO precursors. In addition, it is confirmed by XPS that an amorphous Li2CO3 layer formed on the surface of the as-synthesized LNMO, which can protect the material from overreaction and effectively assist Li+ diffusion. Among the samples, the LNMO-120°C is the smallest with uniform size distribution and has the best discharge capacity (142.5 mAh/g at 0.2 C) and cycling stability with a capacity retention of 80.1 % after 200 cycles at 1 C. Different from the parent LNMO material, the Fe-doped spinel one has reduced rock-salt-type impurity phase and satisfactory cycling stability and rate performance. Among all the samples, the LiNi0.45Fe0.1Mn1.45O4 sample shows the best cycling stability (capacity retention of 96.7 % after 200 cycles at 1 C) and rate capability with a capacity of 118.9 mAh/g even at 5 C. |
