| 摘要: | 近年來，由於環保意識的提升導致全球的電動車銷售量持續增加，而電動車電池是提升電動車性能的關鍵之一，因此具備高能量密度的鋰離子電池需求量增加。各種鋰離子電池正極材料中，尖晶石 LiNi0.5Mn1.5O4 (LNMO) 的高工作電壓 (4.7 V vs. Li+/Li) 引起許多關注，然而，這種高電壓材料受限於商用電解液在 4.5 V 以上發生的劣化反應。這些劣化反應最終會造成電容不可逆的衰退。
電極中通常會加入導電碳以提升電極中活性物質與黏著劑較低的導電性，而碳黑是其中一種普遍的導電碳且通常被認為是電極中具有電化學惰性的成分。然而，過去研究指出碳黑表面具有多種含氧官能基，可能加速電解液在高電壓的劣化。除此之外，有研究將氮摻雜碳材應用於鋰離子電池負極能夠透過加強鋰離子的嵌入/嵌出來提升電容量。
為了探討碳黑上的含氧官能基與含氮官能基對於高電壓正極電化學表現的影響，此研究中透過對碳黑進行熱處理及硝酸氧化分別達到去除/生成含氧官能基的目的，並以水熱法分別對原始碳黑與氧化碳黑進行氮摻雜。此研究中透過 XPS、氮氣吸附儀及拉曼光譜分析不同改質方式對於碳黑表面官能基、元素組成、比表面積與缺陷程度的影響，以不同碳黑作為 LNMO/Li 半電池中的導電劑進行電化學表現測試。
以原始碳黑進行氮摻雜的樣品具有高放電電容與優異的快速充放電表現及循環表現，在 1 C 的放電電容和原始碳黑相比由 110.1 mAh g-1提升至119.2 mAh g-1，快速充放電測試中在 5 C 的放電電容能夠維持 0.2 C放電電容的 69.4%，另外，儘管氮摻雜碳黑有最高的含氧量，經過 1 C 循環 100 圈後電容保持率仍有 94.4%。SEM 及 TEM 分析結果中，此樣品呈現較少的電極沉積物，將經過電化學劣化的電解液以 LC-MS/MS 分析，發現碳黑上的含氮官能基能減少電解液劣化產物並降低電解液主要成分的消耗量。由此研究發現以原始碳黑進行氮摻雜能有效提升高電壓正極的放電電容及快速充放電表現，並有維持高循環穩定性的效果。
;In recent years, the high energy density of Li-ion battery makes it become a popular battery type. Among various cathode materials, spinel LiNi0.5Mn1.5O4 (LNMO) has been investigated because of its high operating voltage (4.7 V vs. Li+/Li). However, LNMO is limited by the decomposition of commercial electrolyte at high voltage (> 4.5 V). Commercial electrolytes commonly use lithium hexafluorophosphate (LiPF6) dissolved in organic carbonates. However, decomposition of LiPF6 produces highly reactive PF5, which is thermally unstable and sensitive to moisture because of the unstable P-F bond. The instability of PF5 results in side reactions with water and organic carbonates, and eventually lead to capacity decay.
To improve the poor electronic conductivity of active material and binder, conductive additives are added into electrode. Carbon black (CB) is one of the common conductive additives considered to be inactive component. However, various functional groups were found on CB surface. Oxygen-containing functional groups could increase the absorption of water and lead to electrolyte decomposition. Nitrogen-doped carbon materials have been applied on anode materials of Li-ion batteries to improve Li-ion storage capacity, ion diffusion and electrical conductivity.
To study the influences of oxygen-functional groups and nitrogen-functional groups of CB on electrochemical performance of high-voltage cathode. In this work, oxygen-functional groups on CB were modified by thermal treatments and nitric acid treatment. In addition, hydrothermal treatment was applied on CB and oxidized CB for N-doping. Surface functional groups, elemental composition, specific surface area and defects degree of CB were analyzed by XPS, gas sorption analyzer, raman spectroscopy. These CB were used as conductive additives in high voltage cathode and assembled into LNMO/Li half-cell for electrochemical testing.
Among all of the samples, N-doped CB shows high discharge capacity (119.2 mAh g-1), excellent rate capability (Discharge capacity at 5 C retains 69.4% of capacity at 0.2 C) and high cycling stability (94.4% capacity retention after 100 cycles at 1 C). In the results of SEM and TEM, this sample shows less deposition on electrode. After electrochemical aging, the electrolyte decomposition products were analyzed by LC-MS/MS. The results suggested NCB with nitrogen functional groups would reduce amount of decomposition products and the consumption of main compositions of electrolyte. According to this study, N-doped CB can effectively improve discharge capacity and rate performance, even cycling stability of high-voltage cathode. |
