以表面改質和奈米複合材料提升高功率型鈦酸鋰鋰離子負極材料

Improving Cycling Performance of Spinel Lithium Titanate Anode Materials by Surface Modification and Composite Material for High Power Lithium-ion Batteries

藍浚愷(Chun-Kai Lan)

指導教授 ： 杜正恭

國立清華大學/工學院/材料科學工程學系/博士(2016年)

摘要

為了解決排放二氧化碳造成之環境議題以及提升整體儲能系統使用效率，鋰離子二次電池被廣泛應用在油電混合車和大型儲能系統，進而減少廢氣排放以及能源使用效率。因此高功率與高能量的電池需求大增佳，也促使專家學者積極開發新型負極材料。本研究將表面改質與複合材料的概念引入負極材料之開發中，期望能藉由材料的最佳化來改善整體鋰電池的電化學表現。
鈦酸鋰負極材料具備高工作電壓之特性，可避免樹枝狀鋰金屬的生成，避免隔離膜被刺穿引起的爆炸危險。另外其高度結構穩定性，在鋰離子遷入遷出的過程中，體積也無劇烈之變化，因此被認為是下世代動力電池之首選。惟鈦酸鋰陶瓷材料本身導電子與導鋰離子能力不佳，嚴重阻礙其商業化之可行性。由文獻上指出，參雜氮離子到鈦酸鋰粉末會製造出氧空缺，使部分鈦四正離子轉換成鈦三正離子，進而有效的提升整體導電性。因此，本論文第一部分利用本實驗室所開發之大氣電漿裝置直接對鈦酸鋰極片做表面處理，藉由功率與時間的調控，成功地將氮離子參雜到鈦酸鋰極片表面。此電漿處理過後之極片在10 C的充放電速率下，即使到100圈仍然保有132mAh g-1 的表現，表示其快速充放電能力在電將處理後受到顯著改良。由近期內的報導指出，鈦酸鋰負極材料在一伏以上進行充放電，鈦酸鋰表面會和電解液起反應生成不可逆的SEI薄膜，進而影響其高充放電表現。因此，在第二部分研究中，將利用濺鍍的方式在鈦酸鋰極片表面直接鍍覆一層保護碳膜，避免電解液的直接接觸。此外，在這部分中利用大氣電漿處理裝置加工便利，用來對鍍完碳膜的極片進行處理，來調控碳膜的性質。由結果指出，表面有鍍覆碳膜的極片在經過電漿處理過後，即使在10C的充放電速率下充放電300圈仍然可以維持91%　retention。並且由掃描式電子顯微鏡跟XPS depth profile分析，可以觀察到有鍍覆碳膜的樣品表面SEI膜的厚度明顯減少，表示碳膜的引入可以有效地抑制極片與電解液之間的不可逆反應。而在第三個部分，我們藉由前面的結果可以知道，氧缺的產生有利於鈦酸鋰在高出放電速率下的循環穩定性。另外，利用表面碳批覆所製造出來的連續導電網路不僅可以提供更好的導電性也可以抑制SEI薄膜的生成。因此綜合之前的結果，最後一部分將在還原氣氛下合成出奈米鈦酸鋰並利用無毒且便宜的高分子當作多孔性碳基材的前驅物，奈米鈦酸鋰粉末鑲嵌在碳基材裡面避免電解液的直接接觸，並提供連續型的導電網路來大幅提升其在快速充分電的能力。而由結果顯示，其在50C的充放電速率下，展現出極為優異的充放電穩定性，在兩百圈的充放電後仍可以維持92 mAh g-1。並且此鈦酸鋰與多孔隙的碳基複合材也被應用到鈉電池上，一樣獲得十分優異的電化學性能。因此此鈦酸鋰與多孔隙的碳基複合材在鋰電池與鈉電池同時展現出優異的快速充放電能力以及循環穩定性，堪稱下世代高功率電池的希望!

關鍵字

鈦酸鋰 ； 鋰電池 ； 大氣電漿 ； 奈米複合材

並列摘要

To solve the environmental concern for global issue and to enhance the efficiency of energy storage system, lithium-ion batteries have been used for large-scale energy storage system and hybrid electric vehicle (HEV) to save oil and to decrease exhaust emissions. Therefore, the increasing demands for high energy density and high power density of batteries have attracted investigators to develop new materials for lithium-ion batteries. In this study, the concept of surface modification and composite are introduced to explore advanced negative materials.
Spinel Li4Ti5O12 is a promising anode material, due to its stable working voltage and negligible structure change during charge-dscharge process. Nevertheless, the relatively low electronic conductivity will limit the commercialization of spinel Li4Ti5O12. Hereafter, improving electronic conductivity via ion doping approach to promote the rate capability of spinel Li4Ti5O12 anodes is investigated in the first section. Lithium titanate was successfully doped by N3- ions into O2- sites through Ar/N2 plasma irradiation at atmospheric pressure. The electrochemical behavior of plasma-treated lithium titanate will be systematically investigated; it also exhibits a desirable discharge capacity of 132 mAh g-1 with almost 100% capacity retention after 100 cycling life at a high rate of 10C. Afterwards, to suppress irreversible reaction and to greatly accelerate their rate capability, carbon passivation layer is introduced via sputtering process. The carbon overlayer-coated lithium titanate shows desirable rate capability. The reversible capacity at 10 C even remains over 91 % of that at initial cycles. Besides, the carbon passivation layer successfully alleviates the irreversible interfacial reaction between active material and electrolyte. In the last section, the Li4Ti5O12/porous carbon matrices was synthesized under reducing atmosphere, the aim of which was to realize the excellent chemical performance of Li4Ti5O12-based anodes, based on the concept of designing continuous conductive network and inducing oxygen vacancies. Li4Ti5O12/porous carbon matrices can retain both remarkable rate capability and superior cycling stability. The c-CMC-LTO exhibits a superior capacity of 92 mAh g-1 and retains its initial value with no obviously capacity decay over 200 cycles under an ultra-high C rate (50 C). Furthermore, for sodium ion batteries, the c-CMC-LTO also showed an excellent cycling stability with a discharge capacity of 127.6 mA g-1 even after 100 cycles at 1C. In summary, the c-CMC-LTO is expected to be a promising anode material for integrating both ultrahigh rate and extremely stable cycling performance for next-generation Li-ion batteries and Na-ion batteries.

並列關鍵字

Lithium titanate ； Li-ion batteries ； Atmospheric pressure plasma ； nanocomposite

參考文獻

1. B. Dunn, H. Kamath, J.M. Tarascon, “Electrical energy storage for the grid: a battery of choices, Science, 334 (2011) 928- 935.

2. V. R. Subramanian, P. Yu, B.N. Popov, R.E. White, “Modeling Lithium Diffusion in Nickel Composite Graphite, J. Power Sources, 96(2001) 396- 405.

3. T. Ohzuku, A. Ueda, N. Yamamota, “Zero‐Strain Insertion Material of Li4Ti5O12 for Rechargeable Lithium Cells, J. Electrochem. Soc., 142 (1995) 1431- 1435.

4. J. Wolfenstine, J.L. Allen, “Electrical conductivity and charge compensation in Ta doped Li4Ti5O12,” J. Power Sources, 180 (2008) 582- 585.

5. H. Ni, L.Z. Fan, “Nano-Li4Ti5O12 anchored on carbon nanotubes by liquid phase deposition as anode material for high rate lithium-ion batteries,” J. Power Sources, 214 (2012) 195-199.