Quinone-Functionalized Carbon Black Cathodes for Lithium Batteries with High Power Densities

Abstract

E and rapid energy storage is essential for sustainable energy delivery. Fast transfer of charges stored at the surface of supercapacitor electrodes can provide high power densities (fast (dis)charge rates). However, their exclusively non-Faradaic energy storagemechanism limits attainable energy densities. In contrast, electrochemical reactions in intercalation or conversion electrodes can afford high energy densities in batteries, but slow reaction kinetics often limits their power densities. Nanostructured pseudocapacitor electrodes with high surface area and redox-active components have shown the potential to bridge the gap between high-power and high-energy electrodes, yet they often require complex material engineering. Using wellestablished methods, we coupled organic molecules with fast redox kinetics to inexpensive high-surface-area conductive substrates to access high energy and power densities in scalable lithium battery electrodes. Because various types of carbon black are already employed as conductive additives in virtually all current lithium battery technologies, these lightweight and inexpensive materials are a natural choice as the conductive platform. We covalently functionalized these sp carbon surfaces through spontaneous reaction with diazonium salts of redox-active quinones. To the extent of our knowledge, these electrodes show the largest energy densities at high power of reported quinone electrodes. High-surface-area conductive Ketjenblack (KB, BET surface area: 1220 m2·g−1) covalently functionalized with PAQ (PAQ = 9,10-phenanthrenequinone, Scheme 1) exhibits a gravimetric capacity per total mass of quinone and carbon of 75 mAh·g−1 at a cycling rate of 30 mA·g−1. This electrode maintains a capacity of 58 mAh·g−1 even at cycling rates as high as 1500 mA·g−1. It exhibits a Coulombic efficiency of 99.8(3)% and a round-trip energy efficiency (energy extracted during discharge/energy stored during charge) of 96.1(3)% over 500 (dis)charge cycles. Importantly, the electrode maintains high energy density and efficiency at exceptional power densities of over 80 000 W·kg−1. We show that electrode capacity and voltage can be improved by substituting other redox-active molecules. For example, substitution of PYT (PYT = pyrene-4,5,9,10-tetraone) for PAQ increases the electrode’s energy density at 75mA·g−1 from 160 to over 300 Wh·kg−1. Redox-active organic materials have been extensively investigated as lightweight and low-cost electrodes that are amenable to synthetic design. However, serious shortcomings impede their use as electrodes: solubility in common electrolytes, slow redox kinetics, and poor electrical conductivity. To address these problems, organic redox centers have been polymerized or grafted onto conductive backbones, incorporated into insoluble frameworks, and/or embedded into conductive hosts. In order to access facile electron transfer, we covalently attached molecules with reversible redox couples to conductive carbon substrates. Covalent attachment prevents dissolution of the molecules, improves electron-transfer kinetics, and increases cycle life. Many strategies are available for tethering molecules to various forms of carbon. In a robust and versatile reaction, aryldiazonium salts react with bare sp carbon surfaces, liberating N2 and forming carbon−carbon bonds between the molecules and the surface. We chose this method for its ability to yield electroactive multilayers (Supporting Information (SI) Figure S2) of a wide range of small organic molecules for which amine precursors can be synthesized. Quinones with well-documented reversible electrochemistry have recently shown promise as organic electrodes. Carbon materials covalently bound to quinones have also been used in aqueous supercapacitors. Here, low quinone loadings and the narrow potential window of the aqueous electrolyte limit the energy density to a maximum of ca. 45 Wh·kg−1, which declines sharply at power densities above 13 000W·kg−1.We selected PAQ as a well-studied, inexpensive small molecule with reversible redox couples at high potentials vs Li. Composite electrodes with unattached PAQ and carbon have previously been investigated as Li battery cathodes, but they showed poor cycling performance. We synthesized the diazonium derivative of PAQ (PAQ-N2 ) from 2-amino-9,10-phenanthrenequinone (PAQ-NH2) through reaction with (NO)PF6 in acetonitrile at −30 °C. 8,9 We functionalized both glassy-carbon (GC) and Ketjenblack (KB)