以臨場掃描式電子顯微鏡分析技術研究鋰離子電池陽極材料於鋰化反應時之微結構變化

Abstract

鋰離子電池是近幾年應用最為廣泛的儲能裝置，提高鋰電池的電量與循環壽命是現在電池研究中非常重要的議題。 為了有效提升電池的效能，了解電極材料的反應與形貌變化成了非常關鍵的資訊。因此，在本研究中，我們開發出新型的臨場掃描式電子顯微鏡觀察方法，用於分析電極材料在不同電壓下的結構變化，以了解其電量衰退的機制。我們以化學氣相沉積法成長出單層且不連續的二硫化鉬薄膜作為電極材料並進行臨場觀察，實驗發現單層二硫化鉬薄膜在當測試電壓由開路電位下降至1.1 V時，薄膜的形貌出現了巨大的轉變，薄膜材料不再平整，取而代之的是許多塊狀的生成物產生。接著將電壓降至0.5 V時，出現大量的島狀結構且表面更加粗糙。此外，我們也試圖以穿透式電子顯微鏡、歐傑電子能譜分析儀以及電化學測試的方法對多層二硫化鉬薄膜進行鋰化過程的觀察以了解其反應機制。我們認為二硫化鉬在鋰電池中1.1V的反應不只有過去研究所提到的離子插層反應而已，而是有更劇烈的相轉變過程，這樣的變化很可能是導致二硫化鉬電極壽命降低的主要原因之一。 本研究也以臨場掃描式電子顯微鏡觀察矽奈米線在鋰電池中的反應。觀察後發現，矽奈米線在10 mV反應10小時後，直徑的部分有約150%的膨脹率。接著，在10 mV反應25小時後，我們觀察到矽奈米線會因為過度的膨脹而在表面出現裂痕，最終導致破裂。這樣的現象也同樣的影響了電極的穩定性。因此，我們試圖利用原子層沉積法在矽奈米線上鍍上一層TiO2保護層以減低矽奈米線膨脹所造成的問題。雖然許多文章聲稱TiO2具有一定的可塑性因此可以用以承受矽奈米線膨脹所產生的應力，但在實驗後發現實際上並沒有這麼理想，在10 mV反應10小時的條件下，這個TiO2保護層就出現碎裂的問題，仍然無法有效解決矽奈米線因為膨脹導致碎裂的問題。本研究希望能透過對於鋰電池電極材料的觀察來了解電極的反應與變化以利未來在電池修飾上有更明確的方向。

Lithium ion battery is the most popular and widely used energy storage system. To increase the capacity and cycle stability is an important issue for the new generation of lithium ion battery. In order to improve the performance and efficiency of lithium ion battery, understanding the reaction mechanisms and morphology changes of electrode materials is essential. In this work, we develop a new in-situ scanning electron microscopy (SEM) technique to observe the morphological evolution of the electrode materials for understanding the mechanism of capacity decay of lithium ion batteries. We apply CVD method to synthesize monolayer MoS2 for the anode material and analyze the anode using in-situ SEM. The monolayer MoS2 has significant morphological changes after discharging from the open circuit voltage to 1.1 V. When the cell is discharged to 0.5 V, the MoS2 anode becomes rough and loses the thin-film morphology. We also use Transmission electron microscopy, Auger electron spectroscopy and electrochemical analysis on multilayer MoS2 thin films in order to understand the lithiation mechanism thoroughly. Based on the results, we believe the reaction at 1.1 V is not only Li+ ion intercalation but also accompanied by a phase transformation, which could be the reason of capacity fading of MoS2 anode. In this work, Si nanowires are also studied by the in-situ SEM observation. The Si nanowires as the anode material exhibit obvious expansion when it is lithiated at 10 mV for 10 h. Further lithiation for 25 h, the Si nanowires are even cracked due to excessive expansion. Crack is one of the main causes that can reduce the electrochemical performance of Si anode. Therefore, we coat a TiO2 protective layer on Si nanowires to decrease the effect of extensive expansion. Many reports suggest the TiO2 layer is an elastic material that can accommodate the volume expansion of the Si anode, but we find that, after lithiation of 10 mV for 10 h, the TiO2 protective layer is still cracked. These results also show that the in-situ SEM observation method developed in this study is useful for understanding the reaction mechanisms in lithium ion battery and the effectiveness of material modification.