部分碳化聚乙烯吡咯烷酮黏著劑應用於高電壓鋰離子電池正極之研究;Partially Carbonized Polyvinylpyrrolidone as Binder for High-Voltage Li-ion Battery Cathode

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Title:	部分碳化聚乙烯吡咯烷酮黏著劑應用於高電壓鋰離子電池正極之研究;Partially Carbonized Polyvinylpyrrolidone as Binder for High-Voltage Li-ion Battery Cathode
Authors:	薛仲軒;Hsueh, Chung-Hsuan
Contributors:	化學工程與材料工程學系
Keywords:	高電壓正極;聚乙烯吡咯烷酮;部分碳化;黏著劑;表面修飾;鋰離子電池;high-voltage;cathode;partially carbonized;polymer binder;surface modification;lithium ion battery
Date:	2022-09-15
Issue Date:	2022-10-04 11:00:58 (UTC+8)
Publisher:	國立中央大學
Abstract:	LiNi0.5Mn1.5O4 (LNMO)正極材料在商業化上是令人期待的，因其具有目前正極材料中相當突出的操作電壓(4.7 V vs. Li/Li+)和可觀的電容量（140 mAh g-1）。然而，商用電解液（六氟磷酸鋰（LiPF6）溶解於有機碳酸酯類當中）在高工作電壓（> 4.5V）會加速劣化，導致電池性能衰退。另一方面，商業鋰離子電池大多以鋁箔以及聚偏二氟乙烯(PVDF)當作正極的集電體及黏著劑，在高工作電壓下會加速鋁箔集電體的溶解，加上PVDF的黏著性能有限，可能會導致電極與活性材料分層的問題，造成活性物質加速脫落。為了解決這些問題，我們在本研究中提出了兩項策略：首先，我們將鋁箔集電體換成碳纖維集電體，碳纖維所具有的3D結構能夠有效強化鋰離子擴散的能力，並且能夠使活性物質鑲入其中，防止活性物質和集電體產生結構分層，導致電化學性能受到影響。再來，本研究透過部分碳化聚乙烯吡咯烷酮(PVP)取代原先的PVDF黏著劑。這項策略不僅能夠提升整體電極的導電性，並且碳化PVP會使其在LNMO表面上形成一層具有含氮官能基的碳塗層。根據文獻指出，這些含氮官能基或許有減輕電解液於高電壓下劣化的現象。除此之外，為了改善集電體與活性物質之間脫落的問題，我們使用水以及乙醇當作活性物質漿料之溶劑，這會影響活性物質與集電體彼此之間的貼合度，最終可能會影響到電池的充放電性能，所以在本次實驗中選用了乙醇及水作為漿料溶劑進行了比較。本研究透過SEM切面圖及接觸角測量儀了解不同的漿料溶劑與碳紙集電體之間的貼合度，並且透過SEM及XPS分析不同退火時間下的LNMO表面碳塗層的微觀結構以及表面官能基組成，利用TEM分析不同退火時間下的LNMO碳塗層變化。最後，以改良式電極與鋰金屬片組裝的半電池進行電化學性能測試。由結果顯示，以水/乙醇(50 / 50 wt%)當作溶劑並且在2小時的退火程序下的樣品具有最高的放電容量與優異的快速充放電及循環穩定性表現。在1C的初始放電容量高達122.5mAh g-1，經過100圈循環後仍然有93.4%的容量保持率，明顯優於碳纖維集電體與鋁箔集電體製作而成之商用電極的循環穩定性(容量保持率分別為79.2%及65.6%)。由此研究發現利用部分碳化聚乙烯吡咯烷酮以及碳紙集電體，製作出的改良式電極能夠有效提升高電壓鋰鎳錳氧正極在電池中的表現，改善放電容量以及循環穩定性等效果。 ;LiNi0.5Mn1.5O4 (LNMO) has the highest operating voltage (4.7 V vs. Li/Li+) and moderate capacity (140 mAh g-1), which lead to a favorable energy density. However, the high operating voltage (>4.5V) will cause the decomposition of commercial electrolyte (lithium hexafluorophosphate (LiPF6) dissolved in organic carbonates) easily. It would be accelerated by high-voltage cathode side, resulting in battery failure. On the other hand, the electrode composition will also cause some problem at high operating voltage. For example, the aluminum is used as a current collector for the positive electrode. It accelerates the dissolution of the current collector and even the shedding of active substances at high voltage. Besides, the polymer binder PVDF used in electrodes has the characteristic of insulation, hindering electron transfer from the active materials to the current collector and degrading the whole performance of the electrode. To solve these problems, we provide two strategies in the study. First, we used carbon paper (CP) to replace the aluminium current collector. This is because the CP has the 3D structure to enhanced the diffusion of Li+ and also avoid the shedding of active substances. Second, we carbonized polyvinylpyrrolidone (PVP) and used it to replace the PVDF binder. This tactic not only increased the electrical conductivity of electrode, but also prevented insulating binder which may induce some side reactions with the electrolyte. Besides, when we carbonized the PVP, a carbon coating layer can be formed on LNMO. This coating layer may protect the activity material from the attack of corrosive products of electrolyte degradation and mitigate the electrolyte decomposition. In addition, the properties of the PVP binders changed drastically because of the interactions with the different solvents (water, ethanol), significantly affecting the capacity and electrochemical cycle stability of the electrodes. Among all of the samples, water /ethanol (50 / 50 wt%) as a solvent and annealed for 2 hours have the highest discharge capacity (122.5mAh g-1) and excellent rate capability and cycle stability (93.4%). The modified electrode has better cycle stability than the traditional electrodes made of carbon paper and aluminum foil current collectors (rate capability: 79.2% and 65.6%).
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