聚偏二氟乙烯於鋰離子電池電極材料之表面修飾

Surface Modification of Lithium-ion Battery Electrode Materials with Polyvinylidene Difluoride

張瑜修(Yu-Hsiu Chang)

指導教授 ： 吳乃立

國立臺灣大學/工學院/化學工程學研究所/碩士(2019年)

https://doi.org/10.6342/NTU201903417

摘要

具有高能量密度、優秀的循環壽命和高安全性的鋰離子電池（LIB），是目前用於電動車輛之儲能設備中最有前景的材料之一。傳統的石墨負極具有有限的速率能力和安全性問題。因此，開發具有長時間的循環穩定性、優異的充放電表現和高安全性的高效電極材料，對於高功率鋰離子電池是相當重要的。尖晶石Li4Ti5O12（LTO）由於其高安全性、優異的速率性能和極長的循環穩定性，而成為高功率LIB中具有競爭力的負極材料。然而，在充電及放電循環和儲存期間，皆能觀察到嚴重的氣體產生，並成為LTO於大規模應用於LIB時的主要障礙。改善LTO電池中的嚴重產氣反應是非常必要的，因為它不僅嚴重惡化了它們循環穩定性，還導致嚴重的安全問題。迄今為止，一些研究報告提到了LTO電極的產氣現象及概略的改善方法。然而，對於LTO產氣的機制並沒有詳細的研究。因此，本研究嘗試以高分子材料──聚偏二氟乙烯(PVDF)──建構出人工的固液界面層，改善LTO的產氣問題。並利用氣相層析質譜儀(GC-MS, Gas Chromatography-Mass Spectrometry)觀測LTO半電池於充放電時的即時產氣情況，並交差比對電性、壓力、氣體成份等數據，釐清電池內的實際反應狀況。
首先，我們希望對現成的LTO極片直接進行加工，因此將LTO、黏著劑及助導劑製作成電極片後，再透過刮刀塗布法(Blade Coating)將PVDF塗層於製備完成的LTO電極上。然而，僅僅在極片表面上建造保護層，並無法提供很好的保護。因此，我們將LTO極片含浸於PVDF的溶液中，並搭配負壓的效果將氣體抽出，促成只有固態及液態的環境，使溶液能與LTO表面完全地接觸，盡可能地將PVDF包覆於每個LTO顆粒上。由於PVDF具有強烈的疏水性及偶極矩，除了能避免殘餘的水份與LTO表面反應外，鋰離子經由此界面層進入LTO時，周圍的電解液分子能被隔絕在外，而不會與LTO接觸並反應。
結果的部分，除了一系列的電性測量外，透過GC-MS及壓力觀測來分析人工固態電解質界面(ASEI)對產氣反應的實際效果，並間接推敲出產氣現象的機制及順序。即時的GC-MS數據能提供各種氣體成份於充放電時的消長，以及電池內各成份的相對比例。比對各氣體成份的趨勢變化後，可以判斷出產氣反應主要的反應物來源是電解液中的何種成份。另外，直接的壓力觀測最能表現出此表面修飾於產氣問題的效果。綜合以上的結果，使我們逐漸瞭解產氣的前因後果，以及伴隨著產氣反應，會有何種產物在電池內生成。未來的研究者便能更輕易地應用LTO於各種產業，促使此材料成功地商業化。

關鍵字

鋰離子電池 ； 鈦酸鋰 ； 鋰鈦氧 ； 產氣 ； 聚偏二氟乙烯 ； 人工固態電解質界面

並列摘要

Lithium-ion battery (LIB) with high energy density, excellent cycle life and high safety is one of the most promising materials currently used in energy storage devices for electric vehicles. Conventional graphite anodes have limited rate capabilities and safety issues. Therefore, development of a highly efficient electrode material with long-term cycle stability, excellent charge and discharge performance, and high safety is quite important for a high-power LIB. Spinel Li4Ti5O12 (LTO) is a competitive anode material for high-power LIB due to its high safety, excellent rate performance and extremely long cycle stability. However, severe gas evolution can be observed during charging and discharging and storage. Then it becomes a major obstacle to the large-scale application of LTO to LIB. It is necessary to improve the severe gassing reaction in LTO batteries because it not only seriously deteriorates their cycle stability, but also causes serious safety problems. So far, some research reports have mentioned gassing phenomena and a few improvement methods for LTO electrodes. However, there is no detailed study on the mechanism of LTO gassing reactions. Therefore, this study attempts to construct an artificial solid-electrolyte-interface (SEI) layer with a polymeric material, polyvinylidene fluoride (PVDF), to improve the gassing problem of LTO. Gas chromatography-mass spectrometry (GC-MS) was used to observe the in-situ gassing phenomenon of LTO half-cells during charge and discharge. Electrochemical performance, pressure change, gas composition, and other data was compared to clarify the actual reaction situation inside.
First, we want to directly process the already-made LTO electrode. Therefore, after preparing LTO electrode, PVDF was coated on the LTO electrode by blade-coating. However, constructing a protective layer only on the surface of the electrode did not provide good protection. Therefore, we impregnated the LTO electrode in a solution of PVDF, and with the effect of negative pressure, the gas was extracted, which promoted two-phase-only condition, so that the solution can be closely contact with the surface of LTO, while PVDF was coated as much as possible on each LTO particle. Since PVDF has strong hydrophobicity and dipole moment, it can avoid residual moisture from reacting with the LTO surface. Moreover, when lithium-ions enter the LTO through this interface, the surrounding electrolyte molecules will be isolated, and unable to react with LTO surface.
In addition to a series of electrochemical measurements, the results of GC-MS and the pressure monitor were used to analyze the exact effect of the artificial solid-electrolyte-interface (ASEI) on gassing reactions. Then, investigate into the mechanism and the sequence of gassing reactions. The operando in-situ GC-MS data provides the performance of various gas components during charging and discharging, as well as the relative proportions of the gaseous components in the battery. After comparing the trends of the gaseous components, it can be judged which component of the electrolyte dominated the gas reaction. In addition, the pressure monitor can show the effect of this surface modification on gassing problems. Based on the above results, we gradually understand the cause and effect of gas production, and compounds produced during gassing. It will be more easily for researchers to apply LTO to a variety of applications in the future. And facilitate the successful commercialization of this material.

並列關鍵字

Lithium-ion batteries ； Lithium titanate ； Lithium titanium oxide ； Gassing ； Polyvinylidene difluoride ； Artificial solid electrolyte interphase

參考文獻

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