有機高分子薄膜電化學性能量測及其於矽碳負極上之應用

Electrochemical Measurements of Organic Polymer Membrane and Its Application on Silicon-carbon Anodes

蘇琳雅(Lin-Ya Su)

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/NTU201901271

摘要

為了滿足人們對高能量密度儲能裝置不斷增長的需求，人們不僅迫切尋找高能量密度材料，在實現鋰離子電池（LIB）的高安全標準方面也越來越受到關注。 近年來，有已發表的論文聲稱在陽極上披覆PVDF塗層能有效地抑制鋰枝晶的生長，使得鋰金屬電池即使在大電流密度下充放電也能保有高庫侖效率，增強全電池的容量密度並提高鋰電池的安全性能。[1] 而這種具有柔韌性和高離子導電率的特性的特殊的塗層，被稱為人造固體電解質界面（ASEI），也是這本論文中最主要會著重的重點部分。
為了精密量測高分子聚合物塗層的離子導電率值，我們設計並製作了一種新型的四點探針電化學阻抗譜（EIS）測量系統，用以測量在液態電解液環境中，各種高分子聚合物膜的阻抗，進而推算其離子導電度。 藉由四點探針測量其本身的特殊性質，可以忽略兩端電極阻抗的貢獻，得到比傳統二極式量測法更精確且不受介面影響的阻抗值。除了單純阻抗的量測，我們還利用系統周遭溫度的調控，由高分子離子導電度隨溫度的變化而求得鋰離子通過聚合物膜所需的活化能。
離子轉移數愈大，可以讓充、放電步驟期間減少電解質的濃度極化現象，從而產生更高的功率密度，因此，對於鋰離子電池來說，盡可能的提高鋰離子的遷移數，使得其在電解質體系中接近1是最為理想的。在這個研究中，我們還建立了一個完整的量測系統，以充分分析鋰離子在各種聚合物中移動率的性質，期望進一步建立一個判斷最佳ASEI的選擇標準。
在第一部分，我們簡單說明了四點探針EIS分析的特點，並比較了每種測量方法的差異和優勢。 經過不斷的設計與嘗試，我們最終建立了一個成熟的量測設置，可用於測量液體電解質環境中聚合物膜的離子導電率。於是我們現在能夠獲得每種聚合物的精確離子導電率值，因此這個物理性質成為我們選擇聚合物候選物適合與否，而進一步使用的有效參數。
在第二部分中，我們探討另一種常見的聚合物物理性質的測量，離子遷移率，並建立了其系統的設置、數據的分析和計算等等。我們發現，將PVDF和Nafion®兩種不同的高分子混合，可以達到鋰離子在高分子中的遷移數提升。在這篇論文中，我們除了分析不同聚合物的遷移數差異外，在較後期的實驗中，我們還比較了在高分子薄膜中，添加一些氧化物添加物對遷移數的影響，尤其是添加氧化鋁的高分子，遷移數都有提升的現象，也成為了我們後期選用高分子時值得一試的選擇。
在研究的最後一部分中，我們將所有測量方法結合在一起，並將ASEI這個想法應用在矽碳陽極電極中，藉由以上的物理量測方法，我們可以初步的對於聚合物塗層候選物進行篩選或改良，再由特殊的手法將高分子均勻而深入的批覆在負極電極上。最終得到的結論是，這層人工SEI可以減少矽碳活物對電解液的直接接觸，減緩劇烈的SEI生長，且在結構上對含矽負極產生穩固和保護效果，進而增強陽極材料的性能以及長久電容量的穩定性。

關鍵字

矽碳陽極 ； 高分子表面改質 ； 人工固體電解質界面 ； 四點探針EIS分析 ； 鋰離子遷移數量測

並列摘要

To meet the ever increasing demand for high energy density storage devices, not only the high energy density materials are eagerly required, but also is there an increasing attention in achieving a high safety standard for lithium-ion batteries (LIBs). Recently, there have been published paper that claimed PVDF coating on the anode is proved to effectively suppress the growth of Li dendrite, leading to high Coulombic efficiency even under large current densities, enhanced full-cell capacity density and improved safety for LIBs.[1] This special coating is named artificial solid-electrolyte interphase (ASEI), which ideally to have the characteristic of flexible and high ionic conductivity.
In order to measure the specific ionic conductivity value of the polymer coating layer, we design a novel four-probe electrochemical impedance spectroscopy (EIS) measuring system, which is allowed to measure the impedance of various polymer membrane in a liquid electrolyte filled atmosphere. With the nature of four probe measurement, the impedance contributed by the electrodes can be ignored in this case, meaning the equivalent circuit is just a resistor, corresponding to the ionic resistance of the electrolyte between the two references. Moreover, the activation energies of each polymer film is also calculated under various temperatures statement applied.
Besides, a large transference number can reduce concentration polarization of electrolytes during charge–discharge steps, and thus produce higher power density. It is highly desirable that the transference number of lithium ions approaches 1 in an electrolyte system. In this work, we also set up a complete system to fully analyze the mobility property of lithium ion for various polymers and to further reach to a possibly best polymer candidate for LIBs as an ASEI.
In the first part of my work, we introduce the characteristic of four-probe EIS analysis, and compare the differences and advantages of each measuring method. With a lot of approaches, we finally came up with a mature setup which can be used to measure the ionic conductivity of polymer membrane in the liquid electrolyte atmosphere. Since we are now able to measure the precise conductivity value of each polymer, it become an efficient means for us to discuss the suitability of a polymer candidate for further use.
In the second part, we setup up the standard operation process and calculation of another commonly seen ionic mobility analysis for polymer physical property measurement, which is the transference number. We found out the blending PVDF and Nafion® together can efficiently increase the transference number. Besides the transference number differences of different polymer, in this work, we also compare the changes of transference number while some kinds of additives are added in polymer structure. The addition of Al2O3 in polymer also surprisingly leads to an increase in tLi+, which is another interesting idea that worth a try.
In the last part, combing all the measuring method together, we apply the polymer coating candidates onto the silicon-carbon anode electrodes with a special coating method and expect this layer of artificial SEI could lead to the effect of protecting silicon material from directly exposing to the electrolyte and do some improvement to the performance of the anode part. By analyzing the effect of this artificial SEI layer in many points of view, we finally achieved some polymers, such as PVDF and PVDF/Nafion®, which has been proved to improve the capacity retention of silicon-carbon anodes by suppressing the violently growing of SEI and reducing the volume expansion problem.

並列關鍵字

Silicon-carbon anode ； Polymer impregnation coating ； Artificial solid electrolyte interphase ； Four-probe EIS analysis ； Lithium ion transference number

參考文獻

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