Cycling Stability of Manganese Dioxide Polymorph Supercapacitors at Elevated Temperature

本研究主要探討結晶型二氧化錳超高電容器 (包括cryptomelane (α) MnO2, 及birnessite MnO2)於高溫下的循環壽命以及其電容量衰退機制。 文獻首先藉由循環伏安法了解α-MnO2在常溫及高溫的電容量衰退現象，並觀察出高溫下的電容量衰退速度有上升趨勢。由α-MnO2在分段工作電位下的循環壽命能進一步了解其主要衰退是源自於0.7 V的陽離子遷入/遷出程序。此外，在高溫下充放電途中切換不同掃描電位，電容量衰退隨掃描速率增加而趨於嚴重。此現象代表經過高溫下充放電，α-MnO2微結構已無法提供足量電子導電度或離子導電度。同步輻射X光繞射光譜進一步顯示，經過5000圈重覆充放電，與α-MnO2層狀結構有直接關係的峰值強度降低，於高溫下較明顯。此結果是源自於Jahn-Teller效應的貢獻，該效應主要顯現在陽離子遷入時所造成的Mn原子價數變化的電位 (0.7 V)。而此價數變化能藉由充放電過程中X光吸收光譜觀察出。由於Jahn-Teller效應在高溫充放電過程所引起的α-MnO2晶格重覆扭曲，其電容量損失在高掃描速率下尤其明顯。 研究第二部分著重於birnessite樣品在高溫下的循環穩定性。在使用與α-MnO2相同漿料比例塗佈birnessite樣品條件下，其電容量在室溫及高溫循環5000圈後僅存30%及27%。藉由添加較多比例接著劑，能提升程度上的穩定度。如α-MnO2，經充放電後birnessite樣品在高掃描速率下電容量衰退較明顯。而結合測量出的整體電極電阻率的結果，能了解此樣品電容量損失主要源自於電極整體結構崩解，造成的電子導電度下降。而此結構崩解是由於birnessite MnO2粒子充放電過程中劇烈的膨脹/收縮，進而造成PVdF接著劑的鬆弛現象並使電容量衰減。當工作電位範圍縮小時，限制了陽離子在充放電過程中遷入/遷出的量，故在常溫及高溫下兩者電容量下降的幅度相近。Cycling stability at elevated temperature (50°C) of supercapacitors made of two different MnO2 polymorphs, including cryptomelane (α), birnessite were investigated. In the first part of the thesis, the cycle life of the α-MnO2 was analyzed by the cyclic voltammetry, which was found to be worse at 50°C. The cycling performances obtained from different potential regions reveals that the capacitance fading of the α-MnO2 while cycling at full range is caused by the intercalation/deintercalation process mainly occurs at 0.7 V. In addition, from the cycling tests in alternative scan tare, the capacitance fading of the sample became more dramatic for higher scan rate at 50°C. This means that the structure could not supply well enough electronic or ionic transfer rate after cycled at 50°C for 5000 times. From the synchrotron X-ray diffraction analysis, the peak intensity that related to the layer structure had decreased after 5000 cycles at room temperature, and decreased to an even lower value after cycled at elevated temperature. This result is believed to be caused by the Jahn-Teller effect which mainly presents at the potential (around 0.7 volts) for cations to intercalate into the bulk material. In-situ XAS was further done to investigate the valence change of the Mn atoms which confirmed the intercalation/deintercalation behavior at 0.7 Volts. The repeated volume change of the unit cell attributed from the Jahn-Teller effect during the charge/discharge upon cycling at elevated temperature leads to loss of capacitance, especially at higher scan rate. High-temperature (50°C) cycling stability of birnessite MnO2 was also examined, while different finding comparing to the α-MnO2 was obtained. For the birnessite MnO2, only 30 and 27 percent of initial specific capacitance were remained after 5000 cycles under the same composition of electrode used for α-MnO2 in room temperature and 50°C respectively, which can be improved by adding more binder. Same procedure of changing the scan rate during the cycling test as we did for α-MnO2 samples was carried out for birnessite samples as well. Combining with the resistivity measurements of the electrode, results show that the decay of the capacitance for the birnessite sample at elevated temperature is due to the overall structure breakdown which increases the electronic resistivity of the electrode. The reason of the structure breakdown is the severe swelling and shrinking of the birnessite MnO2 that stretch and loose the PVdF binder, which causes the fatigue of the binder after 5000 cycles. The amount of cations intercalating or deintercalating through the layer structures was limited for smaller potential regions, so the difference between the samples that cycled at room temperature and 50°C was therefore unobvious in these cases.Chapter 1 Introduction…1 1.1 Background…1 1.2 Motivations and Objectives…2 Chapter 2 Theory and Literature Review…3 2.1 Introduction to Electrochemical Capacitors…3 2.1.1 Introduction to Energy Storage Devices…3 2.1.2 Classifications of Electrochemical Capacitors…6 2.1.3 Models and Theory for Electric Double Layer…8 2.1.4 Electrochemical Characteristics of Capacitors…11 2.2 Development of Electrochemical Capacitors…13 2.2.1 Electrode Materials…13 2.2.2 Electrolyte…15 2.3 Introduction to Manganese Dioxide…16 2.3.1 Crystalline Structure and Characteristic…16 2.3.2 Synthesis and Development…18 2.4 Cryptomelane (α) MnO2…22 2.4.1 Structure, Synthesis and Development…22 2.4.2 Charge Storage Mechanism…24 2.4.3 Jahn-Teller Effect…24 2.5 Birnessite MnO2…26 2.5.1 Structure and Development…26 2.5.2 Charge Storage Mechanism…28 2.6 Experiment Technique…29 2.6.1 X-ray Absorption Spectroscopy…29 Chapter 3 Experimental…30 3.1 Synthesis of Electrode Materials…30 3.1.1 Synthesis of Birnessite MnO2…31 3.1.2 Synthesis of α-MnO2…32 3.2 Electrochemical characterization…33 3.2.1 Preparation of Electrodes…33 3.2.2 Cyclic Voltammetry…33 3.3 Structural Characterization…34 3.3.1 Morphology Characterization…34 3.3.2 X-ray diffraction analysis…35 3.3.3 X-ray absorption spectroscopy…36 Chapter 4 Cycling stability of α-MnO2 under elevated temperature…38 4.1 Cycling stability of α-MnO2 in aqueous K2SO4 electrolyte …38 4.2 Structure analysis of α-MnO2 electrode…62 4.3 Explaining the temperature effect on α-MnO2…72 Chapter 5 Cycling stability of birnessite MnO2 under elevated temperature…76 5.1 Cycling stability of birnessite MnO2 in aqueous K2SO4 electrolyte…76 5.2 Structure analysis of α-MnO2 electrode…82 5.3 Explaining the temperature effect on birnessite MnO2 …103 Chapter 6 Conclusions…104 Reference…10