Quinone-Functionalized Carbon Black Cathodes for Lithium Batteries

Apr 29, 2015 - conductive additives in virtually all current lithium battery technologies,6 these .... electron couples centered at E1/2 = 2.72 and E1...
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Quinone-Functionalized Carbon Black Cathodes for Lithium Batteries with High Power Densities Adam Jaffe,† Abraham Saldivar Valdes,† and Hemamala I. Karunadasa* Department of Chemistry, Stanford University, Stanford, California 94305, United States S Supporting Information *

fficient 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 storage mechanism limits attainable energy densities.1,2 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.2,3 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.4,5 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,6 these lightweight and inexpensive materials are a natural choice as the conductive platform. We covalently functionalized these sp2 carbon surfaces through spontaneous reaction with diazonium salts7−9 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

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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 75 mA·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.10 To address these problems, organic redox centers have been polymerized or grafted onto conductive backbones,10 incorporated into insoluble frameworks,11,12 and/or embedded into conductive hosts.13 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 sp2 carbon surfaces, liberating N2 and forming carbon−carbon bonds between the molecules and the surface.7−9 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.10,12,14−17 Carbon materials covalently bound to quinones have also been used in aqueous supercapacitors.18,19 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 13 000 W·kg−1.19 We selected PAQ as a well-studied, inexpensive small molecule with reversible redox couples at high potentials vs Li+/0. Composite electrodes with unattached PAQ and carbon have previously been investigated as Li battery cathodes, but they showed poor cycling performance.16,20 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)

Scheme 1. Carbon Functionalization Strategy and a Schematic Representation of Quinones Covalently Bound to an sp2 Carbon Surfacea

Received: March 17, 2015 Revised: April 29, 2015

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PAQ: 9,10-phenanthrenequinone, AQ: 9,10-anthraquinone, and PYT: pyrene-4,5,9,10-tetraone. © XXXX American Chemical Society

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DOI: 10.1021/acs.chemmater.5b00990 Chem. Mater. XXXX, XXX, XXX−XXX

Communication

Chemistry of Materials

1500 mA·g−1 showed good capacity retention even at the highest cycling rates (Figure 2B). In long-term stability studies, these

electrodes by immersing them in an acetonitrile solution of (PAQ-N2)PF6 and (Bu4N)PF6.9 We characterized the functionalized GC electrodes by cyclic voltammetry in a three-electrode electrochemical cell containing 0.7 M lithium bis(trifluoromethanesulfonyl) imide (Li(TFSI)) in propylene carbonate with lithium metal as counter and reference electrodes. The redox waves of the surface-bound quinone (Figure 1A) are

Figure 1. (A) Cyclic voltammograms of PAQ-GC at scan rates from 20 to 1000 mV·s−1. (B) Laviron plot of the oxidative and reductive peak potentials for the redox couple at E1/2 = 2.72 V vs Li+/0 showing divergence above 20 V·s−1. Inset: linear relationship between peak current density (Jp) and scan rate for the oxidative peak of this redox couple.

Figure 2. (A) Discharge (solid) and charge (dashed) profiles at a rate of 74 mA·g−1 for PAQ-KB (red), AQ-KB (green), and PYT-KB (blue). (B) Specific capacities of PAQ-KB at various discharge rates. (C) Coulombic (circle) and energy (triangle) efficiencies for PAQ-KB. (D) Relative capacity retention for PAQ-KB (red) and for unattached PAQ (black) electrodes cycled at 74 mA·g−1.

qualitatively similar to those of PAQ in solution (SI Figure S3), and the linear dependence of current on scan rate for PAQ-GC (Figure 1B inset and SI Figures S4 and S7) confirms that the redox centers are attached to the carbon surface. Cyclic voltammograms (CVs) of PAQ-GC show two reversible oneelectron couples centered at E1/2 = 2.72 and E1/2 = 2.43 V vs Li+/0 with differences between oxidation and reduction peak potentials (ΔEp−p) of 110 and 240 mV, respectively. In Laviron plots21 (Figure 1B), which show the variation of peak potential with scan rate, we observe that the oxidative and reductive peak potentials for PAQ-GC diverge only above 20 V·s−1, indicating fast electron transfer kinetics. We determined the surface coverage of molecules on the GC electrodes by integrating the CVs of PAQ-GC. We achieve a maximum electroactive surface concentration of 2(1) × 10−9 mol·cm −2 for PAQ-GC (SI Figure S10). The surface concentration of molecules attached through aryldiazonium functionalization is limited in this case by electron transfer through the oligomeric surface layer, which we estimate to be 4− 5 monolayers thick (SI Figure S2).22 Given this surface concentration value and the surface area of KB, we can calculate the maximum quinone loading and therefore the maximum specific capacity (MSC) achievable for KB in the absence of other limiting factors such as inaccessible pores (described later). At a functionalization density of 2.0 × 10−9 mol·cm−2, PAQ-KB has a MSC of 220 mAh·g−1. This value corresponds to Faradaic charge only and includes the mass of the carbon substrate as well as the quinones. We assembled standard CR2032-type coin cells with PAQ-KB coated on aluminum-foil current collectors as positive electrodes, lithium metal negative electrodes, and 1 M Li(TFSI) electrolyte in propylene carbonate. Typical PAQ-KB loadings were 0.3−0.4 mg·cm−2. The CVs obtained in a coin cell mimic those of the derivatized GC electrodes, albeit with lower electroactive surface concentrations (SI Figure S11). Coin cells were cycled galvanostatically between discharge and charge potential limits of 2.1 and 3.1 V vs Li+/0, respectively. The cells rapidly achieve a stable cycling capacity of 75 mAh·g−1 (including the mass of carbon) at 30 mA·g−1. Rate studies at current densities from 30 to

cells were continuously cycled at current densities of 74 mA·g−1. The cells exhibit high Coulombic and round-trip energy efficiencies of 99.8(3)% and 96.1(3)%, respectively, over more than 500 cycles (Figure 2C). They are also capable of charging and discharging for 500 cycles with minimal capacity decay to 71% of the initial cycling capacity. Surface functionalization dramatically improves the cycling stability of the electrode. Compared to PAQ-KB, the capacity for unattached PAQ and KB decays rapidly over time (Figure 2D). As expected for highsurface-area carbon, we see a significant non-Faradaic contribution to the total stored charge. This is apparent from the slope of the galvanostatic charge and discharge curves (Figure 2A) as well as the charging current baseline in coin-cell CV experiments (SI Figure S11). By extrapolating from the non-Faradaic region of the discharge curves, measuring capacity in control experiments using pristine KB electrodes, and determining the non-Faradaic contribution to current in CV experiments, we estimate a capacitive contribution from KB to the cathode capacity of 15(5) mAh·g−1 (SI Figures S11 and S12). After subtracting non-Faradaic charge storage, the experimentally determined specific capacity for PAQ-KB is lower than the MSC estimated from CVs obtained on PAQ-GC by a factor of 3−4. We estimated the amount of PAQ molecules in electronic contact with the carbon in PAQ-KB by comparing the mass difference for pristine KB and PAQ-KB (including the mass of the PF6− counterions) with the charge passed in galvanostatic cycling experiments. This calculation shows that ca. 80% of PAQ molecules in PAQ-KB are electroactive, implying that the lower capacities are not mainly due to lack of electronic or ionic accessibility to the redox centers. The reduced capacity is more likely due to a lower surface concentration of PAQ in PAQ-KB compared to PAQ-GC. We measured nitrogen adsorption isotherms on KB and PAQ-KB and used nonlocal density functional theory models to simulate the isotherms and B

DOI: 10.1021/acs.chemmater.5b00990 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials determine the pore size distribution. These measurements (SI Figures S13 and S14) show a significant fraction of solventaccessible mesopores in PAQ-KB. Similar to a previous report,23 we observe a decrease in total pore volume without a change in size distribution, which could result from predominant functionalization at pore edges blocking access to the pore. Alternatively, faster attachment of free quinones to surfacebound quinones compared with attachment to KB would promote the formation of extended polymeric clusters24 in some of the pores. This is consistent with the broad peaks in CV measurements of PAQ-GC at submonolayer surface coverage.25 Since we still see unfilled pores, the limit to functionalization density may be electrochemical. Surface functionalization requires electron transfer from KB to the aryldiazonium cation.9,26 Therefore, the reaction will stop when the Fermi level of the carbon is lowered significantly below the reduction potential of the aryldiazonium salt.26 To evaluate this possibility, we estimated the density of states of KB near the Fermi level using differential capacitance measurements and surface-area data.27,28 By using the density of states per carbon atom and the mass of attached PAQ, we estimate that during the reaction the Fermi level of KB decreases between 1−2 eV (see SI for details), which should limit surface functionalization density. Synthetic fine-tuning of the redox-active molecules should allow us to improve on these hybrid electrodes. To test the generality of this method, we covalently grafted PYT15,16 and AQ (9,10-anthraquinone) to KB using the same procedure (Scheme 1). We selected PYT to improve on capacity and voltage as it can store up to four electrons per molecule, and its reduction potential is slightly higher than that of PAQ (2.8 V vs Li+/0 for the first redox process, Figure 2A, SI Figures S16 and S17). Furthermore, PYT shows a higher surface coverage on PYTGC of 5(1) × 10−9 mol·cm−2 (SI Figure S10), affording a MSC of 385 mAh·g−1 for PYT-KB. We achieved a capacity of 123 mAh· g−1 (at cycle 5) for PYT-KB with almost twice the Faradaic charge storage of PAQ-KB, as well as good cycling-rate behavior and efficiency (Table 1 and SI Figures S18 and S19).

Figure 3. Ragone plot showing energy density versus power density for quinone-KB electrodes (filled symbols) and several state-of-the-art battery and supercapacitor electrodes. Here, PAQ-KB displays exemplary rate performance. Conventional battery cathodes shown in blue,29,30 supercapacitors shown in gray (CNT = carbon nanotube, HPGC = hierarchical porous graphitic carbon, RGO−Li = high-surfacearea reduced graphite oxide−Li cell),31−33 and metal-oxide pseudocapacitors shown in green.5,34

energy and power densities for quinone-KB electrodes compared to high-performing electrodes in recent literature is shown in Figure 3 (see SI for details). Supercapacitors are capable of high power yet often lack high energy densities due to low voltage limitations and a sloping charge−voltage relationship stemming from an exclusively non-Faradaic energy storage mechanism. By supplementing fast capacitive energy storage with a Faradaic process in a lithium battery, we achieve a flatter charge−voltage profile and higher energy density. PAQ-, AQ-, and PYT-KB exhibit lower energy densities at low power values compared to common Li-ion battery technologies yet sustain their energy densities at power densities that greatly exceed those of conventional batteries.29,30 For example, Li(Ni0.5Mn0.5)O2 shows rapid loss in energy density above ca. 5000 W·kg−1,30 while PAQ-KB maintains its energy density until above 80 000 W·kg−1. Quinone-KB electrodes use scalable chemistry and a carbon substrate that is already produced industrially. Yet they compete with, and in most cases exceed, the energy and power densities of state-of-the-art carbon based electrochemical double-layer capacitors (EDLCs)31−33 and transition-metal based pseudocapacitors.4,5,34 These electrodes also avoid the highly engineered nanomaterials often used in EDLC and pseudocapacitor technologies, which may be difficult to produce on a large scale. We aim to improve on the energy densities of these electrodes by designing lighter molecules with more positive redox potentials. Attaching these molecules to chemically or electrochemically reduced carbon with higher surface area should lead to improved functionalization density. These electrodes are not limited to lithium-based electrochemistry; they can also serve as cathodes in sodium batteries (SI Figures S22 and S23). This century-old aryldiazonium chemistry35 may have the potential to be optimized for large-scale manufacture to meet the increasingly stringent demands for rapid and efficient energy storage delivered at low cost.

Table 1. Average Performance Metrics (for at least five cells) for PAQ-KB, AQ-KB, and PYT-KB Batteries Measured with Potential Windows of 2.1 to 3.1, 1.5 to 3.1, and 1.8 to 3.2 V vs Li+/0, Respectively

PAQ-KB AQ-KB PYT-KB

open-circuit voltage (V)

specific capacity (at cycle 5, mAh·g−1)

Coulombic efficiency (%)

energy efficiency (%)

3.1(1) 3.0(1) 3.1(1)

74(4) 89(2) 116(7)

99.8(3) 99(1) 98.5(8)

96.1(3) 90(1) 94(1)

Furthermore, PYT-KB provided energy densities of ca. 305 Wh·kg−1, almost double that of PAQ-KB. Although AQ has a lower redox potential (2.3 V vs Li+/0) and slower redox kinetics than PAQ (SI Figure S17), it is an inexpensive dye molecule for which the 2-amino derivative is available in industrial quantities. Our initial experiments with these quinones attached to GC and KB electrodes show good redox kinetics and high gravimetric energy and power densities (Figure 3). However, they both showed faster capacity decay compared to PAQ-KB (SI Figures S18 and S20). Our future efforts will focus on improving the cycling stability of these electrodes. High capacitance and rapid charge-transfer kinetics allow quinone-KB electrodes to achieve fast charge−discharge rates while maintaining high energy density. The correlation between

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details, spectra, and electrochemical characterization. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b00990. C

DOI: 10.1021/acs.chemmater.5b00990 Chem. Mater. XXXX, XXX, XXX−XXX

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AUTHOR INFORMATION

Corresponding Author

*(H.I.K.) E-mail: [email protected]. Author Contributions †

(A.J. and A.S.V.) These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the National Science Foundation CAREER award (DMR-1351538). A.J. thanks the Satre Family for the Stanford Interdisciplinary Graduate Fellowship. XPS studies were done at the Stanford Nanocharacterization Lab.

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DOI: 10.1021/acs.chemmater.5b00990 Chem. Mater. XXXX, XXX, XXX−XXX