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Reports: DNI1055708-DNI10: Syntheses of Polycarbonyl Aromatic Compounds for New Redox Materials

Xiulei Ji, PhD, Oregon State University

The ACS PRF grant has made a tremendous impact on our resrach, which helps identify a new direction for the PI's research as well as for the graduate students. This project integrates two fields into the single picture: organic chemistry with petroleum products and sustainable energy storage. Our graudate students, over the last year, made a leap in terms of their understanding in fundamental research, where they are highly excited with great passion on this new direction of research. This is an interdisciplinary project, where the students have pursued knowledge and solutions in terms of properties of organic solids, electrochemical studies, and simulation studies.


We have demonstrated, for the first time, a polycyclic aromatic hydrocarbon (PAH) - crystalline and readily available coronae - exhibits highly reversible anion-storage properties. Conventional graphite anion insertion electrodes operate at potentials > 4.5 V vs Li+/Li, requiring additives and the use of ionic liquids. The coronene electrode shows flat plateaus at 4.2 V (charge) and 4.0 V (discharge) in a standard alkyl carbonate electrolyte and delivers a reversible discharge capacity of ~ 40 mA h g-1. The discovery of the reversible anion-storage properties of coronene may open new avenues toward dual-ion batteries based on PAHs as electrodes.

Figure 1a inset shows the first-cycle charge/discharge potential profiles of the coronene electrode in electrolyte of LiPF6 (1.0 M) solvated in ethylene carbonate (EC) and diethyl carbonate (DEC) (v/v 1:1). Furthermore, to better understand PF6- incorporation into the coronene electrodes, we investigated the evolution of crystal structure and chemical bonding as a function of the state of charge. Figure 1a inset shows the selected points of state of charge at which ex situ measurements were performed (Figure 1a,b).

Figure 1. a) Charge/discharge potential profiles (at a current density of 20 mA g-1) with Roman numerals corresponding to different state of charge for ex situ XRD measurements. Pattern of point IV is indicated in red, which is corresponding to the fully charged electrode. b) Ex situ XRD patterns of coronene electrodes showing 2_ regions from 7-13¼, 16-20¼, and 20-30¼ to demonstrate peak formations and peak fading. The Roman numerals in a and b correspond to the same state of charge.

Figure 2a shows the charge/discharge profiles for the 1st, 2nd, 3rd, and 10th cycles, where the plateau behavior is well maintained. Cyclic voltammetry (CV) results show high reversibility in the first 10 cycles (Figure 4b). Figure 2c shows the rate capability, where coronene is able to deliver ~21 mA h g-1, even at a high current density of 500 mA g-1, despite its low conductivity. The coronene electrode exhibits impressive long-term cyclability, where after 960+ cycles, the capacity retention is 92% (Figure 2d). Furthermore, coulombic efficiency in the long-term cycling study rises from 67.1% in the 1st cycle to 97.3% in the 100th cycle, 98.5% in the 300th cycle, and 98.6% in the 900th cycle.

Figure 2. a) Charge/discharge potential profiles for the 1st, 2nd, 3rd, and 10th cycles. b) Cyclic voltammetry (CV) measurements for the 1st, 2nd, 3rd, and 10th cycles at a rate of 0.1 mV/s. c) Rate capability measurements, cycled 10 times at 20 mA g-1 for conditioning and 5 times each at 10, 20, 50, 100, 200, 500, and 100 mA g-1, followed by continued cycling at 20 mA g-1. d) Long-term cycling of the half-cell for over 960 cycles, along with coulombic efficiency (%).

Scanning electron microscopy (SEM) studies were conducted on coronene electrodes before and after insertion of PF6- anions (Figure 3). The macro-scale structural changes of coronene are evident, where the rod-like crystals of over 5 µm long and ~3 µm wide (Figure 3a) crack upon the first charge (Figure 3b) and turn into porous crystals after 400 cycles (Figure 3c). This explains why the capacity only drops in the first 50 cycles and is stable henceforth. The surprisingly stable cycling of coronene may be associated with the formation of the more porous structure with a reduced strain for PF6- anion insertion.

Figure 3. Scanning electron microscopy imaging. a) SEM image of a pristine coronene electrode showing a clear image of the coronene crystals. b) SEM image of the coronene electrode once fully charged. It is evident that upon charging, the morphology of the crystals has changed, showing a cracked, porous structure. c) SEM image of coronene electrodes after 400 cycles showing a completely amorphous structure.

Figure 4. Ex situ spectroscopy measurements. FT-IR characterization corresponding to the states of charge/discharge and XRD patterns I-VII (Figure 2). I-VII indicate different charging/discharging times, where P indicates the pristine electrode, and P.W. indicates a pristine electrode soaked in the 1 M electrolyte LiPF6 /ED:DEC (1:1).

Ex situ nuclear magnetic resonance (NMR) performed upon full charge confirms the absence of new bonding in coronene (Figure 4). Ex situ Fourier-Transform Infrared (FTIR) spectra were also used to characterize changes in chemical bonding in coronene upon PF6- incorporation (Figure 5). Excluding electrolyte peaks, it is clear that no new peaks arise upon charging, suggesting that new chemical bonds formed are not formed during charging.

Figure 5. Ex situ NMR measurements. The peak at ~ 8.75 ppm is the main and most important peak representative of pure coronene that is the emphasis of these spectra (NMR not shown due to a single peak present; 1H NMR (700 MHz, C6D6) _ 8.75 (s, 12H)), the rest are due to solvent and partial impurities, including the binder used to make the electrode. a) NMR spectrum of the electrode before cycling (pristine); 1H NMR (700 MHz, C6D6) _ 8.75 (s, 12H), 5.52 (s, 2H), 2.90 (s, 1H), 2.12 (t, J = 0.7 Hz, 2H), 1.40 - 1.21 (m, 20H), 0.89 (s, 2H) ppm. b) NMR spectrum of the electrode after a full charge. These spectra remained the same for all other ex situ measurements, hence omitted; 1H NMR (700 MHz, C6D6) _ 8.75(s, 12H), 5.51 (s, 1H), 3.19 (s, 3H), 2.92 (s, 17H), 2.12 (t, J = 0.7 Hz, 2H), 1.73 (t, J = 0.7, 1H), 1.40 - 1.21 (m, 6H), 1.01 (s, 2H) 0.89 (s, 1H) ppm.

Furthermore, we, for the first time, report that 3,4,9,10-perylenetetracaboxilicdianhydride (PTCDA), exhibits high rate performance in Mg-ion storage and stable cycling, as shown in Figure 6.

Figure 6. a) Rate capability measurements at 20, 50, 100, 200, 500, and 200 mA g-1. b) Galvanostatic magnesiation/demagnesiation potential profiles of the PTCDA electrode at different current densities from 20-500 mA g-1 in a potential range of -0.8 - 0.4 V. The profiles shown are the profiles of the 4th cycle at each specific current density. c) Cycling performance obtained in two-electrode coin cells at a current density of 20 mA g-1, along with a coulombic efficiency curve. The plot omits the first cycle.