| 摘要: | 第一部份旨在研究超臨界流體合成SnO2/graphene nanosheets (GNS)的電化學特性。相比於在空氣下合成的SnO2/GNS，利用超臨界二氧化碳(SCCO2)合成出SnO2/GNS的複合材能夠均勻分散在石墨烯上，且原本堆疊的石墨烯易被SCCO2撐開，並在層與層之間沉積SnO2，避免石墨烯堆疊回去。當流體密度增加時，黏滯度上升、擴散速度變慢，SnO2的沉積部分分散、部分聚集。而在注入SCCO2前將反應腔體進行抽真空，有助於SnO2緊密排列形成一網狀骨架。且在電性表現上，以抽真空條件的SnO2/GNS有最高電性表現，在100 mA g−1電流密度下可達到~800 mAh g−1的初始放電電容值，在6000 mA g−1高速下，可得到475 mAh g−1放電電容值，不論是在低速還是在高速表現上，皆優於未抽真空製得的SnO2/GNS的電容值。
第二部份利用SCCO2作為前驅物合成CoCO3/GNS複合材，並應用於鋰離子電池與鈉離子電池。利用SCCO2的特性(類氣體的擴散性、極低黏滯度、接近於零的表面張力)，CoCO3粒子可以均勻且緊密分散在GNS上。相比於一般的合成方法，利用SCCO2反應溫度僅需50 ºC、反應時間僅需2小時，大幅減少合成過程中所消耗的能源及成本。利用SCCO2可合成出極小的CoCO3奈米粒子(~20 nm)，且具備獨特的多孔狀結構，增加了電極活性位置、利於電解液滲入、較短的擴散路徑、舒緩在充放電過程中產生的應變。CoCO3/GNS不僅可逆電容值可達1105 mAh g−1，也展現了絕佳的高速充放電能力(7.5 min內達745 mAh g−1)及良好的循環壽命(200圈充放電後保有98%電容維持率)。
第三部份在利用電化學法沉積出具有雙層排列的V2O5，並應用於鈉離子電池。電極製備過程中不需添加其他導電劑、黏著劑，即可直接沉積在基材上。在添加NaCH3COO的VOSO4鍍液中沉積之V2O5可改善氧化物沉積在基材上的均勻性。在本實驗中改變釩氧化物的沉積電位，並探討不同的電位對表面形貌、結構、電化學性質的影響。利用FESEM觀察其表面形貌，在低電位下沉積之V2O5具有多孔狀的結構，而當沉積電位提高時，V2O5表面會越來越緻密，直到水的分解電位發生才會在表面形成較大的孔洞。利用XRD分析得知，沉積出來的V2O5並不會隨沉積電位不同而改變其結構，且具有非常大的層間距，約11.6 Å，足以提供比Li+還大的Na+進出。而在放電電容值、不同定電流充放電、循環壽命等儲鈉特性表現上，以在最低沉積電位出來的V2O5具有最好的表現，在20 mA g−1的定電流值下，可達到220 mAh g−1之放電電容值，且以50 mA g−1的定電流充放電之500圈循環壽命測試上，仍可達到92%的壽命維持率。
;Supercritical CO2 (SCCO2) fluid, which has gas-like diffusivity, extremely low viscosity, and near-zero surface tension, is used to synthesize SnO2 nanoparticles (a 3-nm diameter is achievable), which are uniformly dispersed and tightly anchored on graphene nanosheets (GNSs). Comparatively, the conventional process (in the absence of SCCO2) produces aggregated SnO2 clusters. This study also tunes the SCCO2 pressure (and thus its fluid density) and vacuum the autoclave before injecting into CO2. The results show that these factors crucially affect the distribution of the produced SnO2 nanoparticles on GNSs, determining the resulting electrochemical properties. Increasing the pressure leads to an enhancement of SCCO2 density (and viscosity), decreasing the transport of SnO2 precursors through out the sample. On the other hand, vacuuming the autoclave before injecting into CO2, the SnO2 particle decoration density on GNSs increased. The discharge capacity, rate capability, and cyclic stability of the synthesized SnO2/GNS nanocomposites are compared. The SnO2/GNS electrode which synthesized during the vacuum process has the best electrochemical performance, which can deliver ~800 mAh g−1 at 100 mA g−1 and retain 60% of this of this capacity when the rate was increased to 6000 mA g−1.
An eco-efficient synthesis route of high-performance carbonate anodes for Li+ and Na+ batteries is developed. With supercritical CO2 (SCCO2) as the precursor, which has gas-like diffusivity, extremely low viscosity, and near-zero surface tension, CoCO3 particles are uniformly formed and tightly connected on graphene nanosheets (GNSs). This synthesis can be conducted at 50 °C, which is considerably lower than the temperature required for conventional preparation methods, minimizing energy consumption. The obtained CoCO3 particles (~20 nm in diameter), which have a unique interpenetrating porous structure, can increase the number of electroactive sites, promote electrolyte accessibility, shorten ion diffusion length, and readily accommodate the strain generated upon charging/discharging. With a reversible capacity of 1105 mAh g−1, the proposed CoCO3/GNS anode shows an excellent rate capability, as it is able to deliver 745 mAh g−1 in 7.5 min. More than 98% of the initial capacity can be retained after 200 cycles. These properties are clearly superior to those of previously reported CoCO3-based electrodes for Li+ storage, indicating the merit of our SCCO2 synthesis, which is facile, green, and easily scaled up for mass production.
Nanocrystalline V2O5 with a bilayer structure is directly grown on a steel substrate electrochemically in VOSO4-based solution as a cathode for sodium-ion batteries. No complicated slurry preparation procedure, involving polymer binder and conducting agent additions, is needed for this electrode synthesis. The incorporation of NaCH3COO in the VOSO4 solution promotes oxide growth and improves oxide film uniformity. The interlayer distance between two-dimensional V2O5 stacks in the structure is as large as ~11.6 Å, which is favorable for accommodating Na+ ions. The growth potential is critical to determine the oxide architectures and thus the corresponding Na+ storage properties (in terms of capacity, high-rate capability, and cycling stability). An optimal charge–discharge capacity of 220 mAh g1 is achieved for V2O5 grown in an activation-controlled potential region (i.e., 0.8 V vs. an Ag/AgCl reference electrode). This V2O5 electrode shows only 8% capacity decay after 500 cycles. |
