鋰離子二次電池電極與電解液介面現象的研究與應用

Studies and applications of the electrode/electrolyte interface in Lithium-ion secondary batteries

李孟倫(Meng-Lun Lee)

指導教授 ： 施漢章 ； 葉均蔚

國立清華大學/工學院/材料科學工程學系/博士(2013年)

https://doi.org/10.6843/NTHU.2013.00493

摘要

鋰離子二次電池(LIB)因為擁有高能量密度、輕量化與環保的特性而被大量應用在可攜式電子產品的電源上。近年來除了針對電極中主要活性物質進行探討之外，許多研究也發現電解液與電極的固液異相介面對電池電性的影響也是至關重要，該介面不只影響電池的長效循環穩定性甚至與電池的安全性有關。在此研究中，三種方式被用來改良此介面的性質: (1)由基礎著手，針對單一活物顆粒進行表面包覆；(2)進一步直接在製作完成的電極片上作鍍膜；(3)從另一方面進行，改變電解液的添加成分再反應到電極表面上。
鋰鈦氧(LTO)包覆石墨顆粒以溶凝膠法合成再經過燒結得到。此核(石墨)殼(鋰鈦氧)結構中石墨顆粒大小為20 µm，而鋰鈦氧的厚度約為60-100 nm。此鋰鈦氧-石墨化合物作為鋰離子負極活物，與原本的石墨比起來可提高充電電流耐受性以及常溫下和55˚C下的循環壽命；電化學阻抗分析(EIS)中顯示，鋰鈦氧-石墨化合物負極在60次循環的過程中明顯抑制了阻抗升高；此外，以拉曼(Raman)圖譜測試在常溫與55˚C的60次循環充放電前後的電極之ID/IG (無序碳/石墨化碳結構)的比值，發現純的石墨負極在60次的循環後，此比值有重大的增加，顯示石墨化結構的大量受破壞；而鋰鈦氧包覆的石墨電極在60次的循環後此比值僅有微量增加。
原子級膜濺鍍法(ALD)為一新的電極加工方式，將二氧化鈦(TiO2)奈米膜層均勻的鍍在先前製備完成的石墨負極上。經由精確的厚度控制(約40 nm)所製備出的二氧化鈦-石墨電極在循環壽命上有優異表現；表面阻抗方面，在100個充放電循環下仍可以有效抑制質傳阻抗(Rct)的增加，且在拉曼圖譜中此二氧化鈦-石墨負極在充放電循環後也顯示出完整石墨結構的保持性。此兩種針對石墨碳材的表面改質由於增加了電解液與電極界面(SEI)的穩定性，使得在充放電過程中維持了電極中石墨結構的穩定性，進而提升了電池的循環壽命。
槲皮素(Quercetin) 是一有機抗氧化劑，在此被添加到鋰離子電池的電解液(LiPF6/EC+ EMC+DMC)中強化了電池的電化學特性，包括循環壽命過充電耐受度及安全性；且在特定的(0.05%)添加量下，對原本電性(C-rate)無不良影響。鋰鈷氧(LiCoO2)與碳組成的全電池在3C-6V過負載充電下，有添加槲皮素的電池與一般電池比較可延長損毀時間高達800秒。此改善現象源自電解液添加劑所誘導之電極表面鈍化微結構的形成，一方面減少電極與電解液的直接接觸，另一方面抑制了表面阻抗的增加。
本研究針對鋰離子電池電解液與電介面，提供可行之改善方法來提升電性效能以滿足電器中高功率、長效循環與安全性的訴求。

關鍵字

鋰離子二次電池 ； 石墨 ； 表面改質 ； 電解液 ； 添加劑

並列摘要

Rechargeable lithium-ion batteries (LIBs) have been widely used as a power source for many portable electronic devices due to the high energy density, light and environmental friendly. In recent years, many researches have pointed out that aside from the active materials dominating the electrochemical properties of a battery, the most attractive topic is the electrolyte and the related heterogeneous electrolyte and electrode interface/interphase. The interface of electrolyte and electrode not only affect the long term cycling stability but also refers to the safety quality of a battery. In this study, three ways are used to modify the circumstance of the interface: (1) the fundamental surface coating on single particle of active material, (2) one step ahead of depositing compound directly onto the finished electrode, and (3) in contrast to modify the electrolyte transforming the reaction back to the electrode surface.
Li4Ti5O12 (LTO)-coated graphite as an anode material for Li-batteries is synthesized. The surface of graphite powders is uniformly coated by the LTO nanoparticles to form a core-shelled structure via a sol-gel process, followed by calcination. The average size of graphite core is 20 µm while the thickness of LTO shell is 60 to 100 nm. We found that LTO-coated graphite has better rate-capability and cycle life at RT and at 55˚C, compared with the pristine graphite. The electrochemical impedance spectroscopy (EIS) results of the cell with LTO-coated graphite anode showed a significant suppression of the impedance rise after 60 cycles. In addition, the Raman spectrum showed that after 60 charge-discharge cycles at 55˚C, the ID/IG ratio of the LTO-coated graphite electrode increased slightly, while that of the pristine graphite electrode increased significantly.
Atomic layer deposition (ALD) acts as a novel process to fabricate TiO2 nano-layer with high uniformity by ALD technique on a completive graphite negative electrode of lithium battery is reported. Under accurate thickness control, a TiO2 plated (~40 nm) graphite electrode shows remarkable performance in cycle life. The surface resistance of the electrode has been suppressed after 100 charge-discharge cycles and the stability of surface graphite layer structure has been maintained after 60 cycles. The deposition strategy directly on the electrode shows a resemble purpose as well as the core-shell particle coating of active materials.
Quercetin, an organic antioxidant, has been employed as an additive in lithium-ion cells to enhance the electrochemical performance to enhance the cycle life and the overcharging characteristics of LiPF6/EC+ EMC+DMC (1 M) when used as an electrolyte. A LiCoO2/graphite full cell with 0.05% quercetin showed a significant improvement in safety associated with overcharging tolerance and thermal stability, without causing damage on electrochemical properties including C-rate and cycle life. Under the 3C-6V charging circumstance, the LIB with quercetin contained has postponed the vented time for more than 800 s, comparing to the normal battery.
Improvements might result from the formation of a passivation microstructure on the electrode’s surface which could both minimize the reaction between electrode and electrolyte and suppress the surface impedance increase of the interface, especially suppresses the increase in the charge-transfer resistance. The studies focused on electrolyte and electrode interface of Li-ion cells provide a viable way to improve the power source for the applications involving electric devices with high rate, long term cycling, and high safety requirements.

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