Three Dimensional Graphene Based Composite Material for Electrode Application in Li-ion Battery

• Other Authors: Huang, Yuan (author.)
• Format: Others
• Language: English; Chinese
• Published: 2016
• Online Access: http://repository.lib.cuhk.edu.hk/en/item/cuhk-1292199

本論文致力於優化活性物質/石墨烯複合電極的構型，並將其應用於鋰離子電池。我們以磷酸鐵鋰/還原后的氧化石墨烯複合物為例子，首先研究電極的集流體選擇問題。相比常用的二維集流體鋁箔，使用三維集流體可以使磷酸鐵鋰正極的電化學性能得到提升。這是源於使用三維集流體可以降低電極的電荷轉移電阻，同時提高鋰離子的擴散速率。如果進一步引入還原后的氧化石墨烯作為原有三維集流體的二級結構，磷酸鐵鋰正極的電化學性能將有進一步的提升。但是，當電極已有足夠的導電網絡結構，這種增益效應將趨於飽和。 === 為了進一步提升磷酸鐵鋰/還原后的氧化石墨烯複合電極的電化學性能，我們在複合電極中添加鎂離子，使其倍率性能得到明顯的提升（比如在20C倍率下可達到78mA hg-1）。這是因為鎂離子在氧化石墨烯存在的情況下可以促進磷酸鐵鋰中的二價鐵離子還原成單質鐵，提高複合電極的導電性。此外，經過優化，我們發現磷酸鐵鋰和鎂的質量比為130/1時複合電極可以得到最佳的倍率性能。 === 接下來，我們利用電泳沉積技術實現磷酸鐵鋰和還原后的氧化石墨烯複合電極的製備，得到了不添加粘結劑及其它導電劑的鋰離子電池正極。經過優化，這種複合電極含有7.5%的氧化石墨烯，1%的鎂，退火溫度為700oC時，該複合電極具有最高的比容量。我們系統地研究了複合電極製備過程中的關鍵參數，如鎂離子的濃度，退火溫度，還原后氧化石墨烯的含量，并探討他們各自的作用。相對傳統的磷酸鐵鋰電極以及通過機械混合方法得到的複合電極，通過電泳方法得到的複合電極具有更好的電化學性能。 === 還原后的氧化石墨烯導電性不佳，利用化學氣相沉積法生長的石墨烯成為另一種選擇。在本論文中，我們利用電鍍技術實現了在化學氣相沉積法製備的石墨烯泡沫上沉積四氧化三鐵納米顆粒，得到了不添加粘結劑及其它導電劑的鋰離子電池負極。X射線吸收近邊結構揭示四氧化三鐵納米顆粒與石墨烯泡沫形成化學鍵合。這種電鍍複合電極可在1Ag-1倍率下顯示出~1220mA hg-1的容量；在5Ahg-1倍率下可釋放出~500mA hg-1的容量。同時，我們也觀察到這種複合電子在循環過程中容量上升的現象，並對其可能的機理進行討論。 === In the thesis, we attempt to optimize configurations for graphene-based electrodes for lithium ion batteries (LIBs) application, aiming at binder/additive free electrode with high capacity and rate performance. A first attempt is made on the choice of the current collector for LiFePO4 (LFP)/reduced graphene oxide (rGO) composite. Largely improved electrochemical properties are achieved when three-dimensional (3D) current collector (e.g. carbon cloth) serves as the current collector, as compared to those employing conventional Al foils. The reduced charge transfer resistance and more effective Li+ diffusion in the3D electrode architectures are responsible for their improved rate performance. The rGO serves as secondary structure in addition to the primary 3D current collector, contributing to the improvement of both capacity and rate performance of the electrode. The enhancement effect eventually saturate when adequate interconnecting network is built up. === Next, we demonstrate that introducing Mg2+ to LFP/rGO composite via mechanical mixing and annealing leads to further improvement in the rate performance of the cathode (on carbon cloth substrate). X-ray photoelectron spectroscopy unravels that the enhanced reduction of Fe2+ to Fe0 occurs in the simultaneous presence of Mg2+ and rGO, which is beneficial for the improvement of electronic conductivity of LFP/rGO composite. === With the understanding on the effect of the 3D current collector and the Mg2+ ion introduction, we further develop a fabrication method for LFP/rGO composite using electrophoresis that results in a binder/additive free composite cathode on carbon cloth with extremely high LFP mass ratio (91.5 wt%). The quasi-spherical LFP particles are uniformly connected to and/or wrapped by three-dimensional networks of rGO nanosheets. Enhanced capacity is achieved in the electrophoretic composite cathode, when compared to either a conventional one or composite cathode formed by mechanically mixing LFP and rGO. === The major problem associated with rGO-based composite is the limited electronic conductivity of rGO itself. Graphene grown by chemical vapor deposition (CVD) shows superior electronic conductivity when compared to rGO. Replacing rGO with the graphene foam, we develop an alternative strategy to achieve the binder/additive free electrode. In this specific work, we fabricate a binder/additive free CVD graphene/Fe3O4 composite electrode via electroplating process. X-ray absorption near edge structure unravels that Fe3O4 nanoparticles are chemically bonded with graphene in the composite. A high capacity of ~1220 mA h g-1 is achieved at 1 A g-1 up to 500 cycles in the electroplated composite anode. Moreover, the composite electrode delivers excellent rate capability of ~500 mA h g-1 at 5 A h g-1. Cycling induced capacity increase is observed in the composite electrode, and the mechanisms for the capacity increase is also discussed. === Huang, Yuan. === Thesis Ph.D. Chinese University of Hong Kong 2016. === Includes bibliographical references (leaves ). === Abstracts also in Chinese. === Title from PDF title page (viewed on …). === Detailed summary in vernacular field only. === Detailed summary in vernacular field only. === Detailed summary in vernacular field only. === Detailed summary in vernacular field only.