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Anthraquinone Oligomers as Anode-Active Material in Rechargeable Nickel/Oligomer Batteries with Aqueous Electrolyte Casper Clausen, Emil Draževi#, Anders Søndergaard Andersen, Martin Lahn Henriksen, Mogens Hinge, and Anders Bentien ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00009 • Publication Date (Web): 11 Jan 2018 Downloaded from http://pubs.acs.org on January 13, 2018

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ACS Applied Energy Materials

Anthraquinone Oligomers as Anode-Active Material in Rechargeable Nickel/Oligomer Batteries with Aqueous Electrolyte Casper Clausen, Emil Draºevi¢,

∗

Anders Søndergaard Andersen, Martin Lahn

Henriksen, Mogens Hinge, and Anders Bentien

∗

Department of Engineering, Aarhus University, Hangøvej 2, 8200 Aarhus N, Denmark E-mail: [email protected]; [email protected]

Keywords: Rechargeable batteries, oligomer batteries, anthraquinone, anthraquinone batteries, organic batteries, nickel batteries, electrical energy storage, grid storage, chemical storage.

Abstract This paper is an initial investigation of the possible use of redox active anthraquinone as anode material in combination with Ni(OH)2 in a secondary battery with aqueous KOH electrolyte. Three dierent anode materials are investigated: an anthraquinone monomer and two dierent anthraquinone based oligomers. All batteries are rechargeable and almost 100 % of the theoretical capacity can been accessed in the rst cycle, after which signicant capacity loss occurs. The capacity loss is attributed mainly to swelling of the electrodes and solubility of anthraquinones in their charged (reduced) state because reduced anthraquinone is deprotonated and ionised in alkaline solutions. Nonetheless, batteries based on one of the oligomerized anthraquinones (oligo[benzene-1,4-dithiol-alt -(1,5- dichloroanthraquinone)] anode) show the best performance and retains almost 50 % of the discharge capacity after 100 cycles. The better 1

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performance is attributed to the reduced solubility caused by the oligomerization. It is anticipated that measures to decrease the solubility further could lead to much improved capacity retention. Introduction of renewable and intermittent photovoltaic and wind electricity sources in the utility grid will increase the demand for stationary electrical energy storage technologies. In such applications high energy density has less importance, while a low cost has highest priority. This can be quantied by the levelized-cost-of-electricity-storage (LCES), 1,2 and is given by LCES [e kWh−1 cycle−1 ] = CC [e kWh−1 ]/(N · η), where LCES expresses the cost of storage and discharge of one kWh. CC are the capital costs per kWh, N the number of cycles during lifetime and η is the cycle eciency. The US Department of Energy set a longterm goal of CC below $150 kWh−1 and LCES below ¢10 kWh−1 cycle−1 as a break through for batteries in grid-storage applications. 3,4 However, this is currently not met by any of the state-of-the-art batteries like Li-ion, lead-acid, nickel-iron, nickel-zinc and nickel-metal hydride (Ni-MH) because of either high capital costs, short lifetime or a combination of both. In Ni-MH batteries the metal hydride (MH) takes up an estimated cost share of 45 % of the total battery cost 5 making it the single most costly component of the battery, considerably higher than the Ni(OH)2 electrode that only takes up about 25 % of the cost. Furthermore, long-term capacity degradation of Ni-MH batteries can to a large part be related to the repeated charge and discharge of the MH anodes 6 and alternative low-cost materials that replace the MH and improve the lifetime could lead to new battery technologies for grid-scale electricity storage. Iron has been used as anode material in alkaline batteries for more than a century and the Ni-Fe battery still has superior long-term durability. However, low energy density and eciency are some of the obstacles for a more widespread use of Ni-Fe batteries and ongoing research is taking place to improve the performance of the iron anode. 79 Zinc is another promising inorganic material used as anodes in alkaline nickel batteries. With less than a decade as a commercial technology the Ni-Zn battery still faces some challenges regarding reliability, self-discharge, and capacity loss but recent research is working to modify 2


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the anode to overcome these challenges. 1012 Here we present a preliminary study on alkaline batteries where low-cost organic oligomerized anthraquinone redox pairs are used as the electroactive anode material. Anthraquinones have for the past years been investigated for uses in aqueous redox ow batteries. 13,14 A recent study 15 on the water solubility and redox potentials of many anthraquinones found that most are insoluble in water while their potential in alkaline solutions ranged from -0.6 to -0.7 VNHE . If combined with Ni(OH)2 cathodes this results in cell potentials up to 1.2 V, which together with estimated cost of about $10-$15 kAh−1 for unsubstituted anthraquinone 15 (see SI 1) underlines the potential as anode material. Poly(vinylanthraquinone) (PVAQ), see Scheme 1, has previously been investigated as anode material in polymer/air batteries in aqueous alkaline electrolyte. 16 With an anthraquinone anode and Ni(OH)2 as cathode the battery has a theoretical specic energy of about 143 Wh kg−1 (see SI 1). When including the additional mass of e.g. separators, carbon black, binder, and electrolyte it is still realistic to obtain specic energies comparable to those of NiCd (50-80 Wh kg−1 ). 5,17 The battery performance of monomer 2,6-dihydroxyanthraquinone (DHAQ) and two different oligomerized anthraquinones; oligo(vinylanthraquinone) (OVAQ) and oligo[benzene1,4-dithiol-alt -(1,5-dichloro-anthraquinone)] (OBDTAQ) with Ni(OH)2 as cathode was tested and compared. The half-cell reactions are shown in Scheme 1.

DHAQ is commercially available (AK Scientic) and was used without further processing. OVAQ and OBDTAQ were synthesized and the structures shown in Scheme 1 were conrmed by solid-state NMR and FTIR (SI 2). Furthermore, elemental (CHNS) analysis (SI 3) showed average oligomer lengths of 5.1 and 20.2 units for OVAQ and OBDTAQ, respectively. Figure 1 shows the cyclic voltammograms (CVs) of DHAQ, OVAQ, OBDTAQ measured in 13 M KOH supporting electrolyte at 50 mV s−1 (details in SI 4). A CV of Ni(OH)2 measured in 6 M KOH at 100 mV s−1 is also shown. In 13 M KOH only OBDTAQ displays quasi reversible voltammograms while DHAQ and OVAQ have irreversible peaks. DHAQ 3



2 Ni(OH)2 (s) + 2 OH- (aq)

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2 NiO(OH) (s) + 2 H2O (l) + 2 eO-

O O+2

O-

13 M KOH e-

-O

-O

O-

O

O

O-

n +2

n

13 M KOH e-

O-

O n

n

S O

S

S + 2 e-

13 M KOH

O-

S

O-

O

Scheme 1: The electrochemical half-cell reactions involved in the three dierent battery types; DHAQ (red), OVAQ (blue), and OBDTAQ (green).

4


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has a relatively small reduction peak compared to the one for oxidation while OVAQ shows two reduction and oxidation peaks. The two CV peaks for anthraquinones derive from two single-electron transfers (instead of one two-electron transfer). In the presence of water these two peaks merge into one but when water accessibility is low they separate. 18 It is speculated that the low water accessibility is a result of a the high KOH concentration that binds almost all water molecules. Furthermore, a potential reason why this is only seen for OVAQ could be related to its higher hydrophobicity. In 6 M KOH both OVAQ and DHAQ show quasi reversible cyclic voltammograms as presented in SI 4. In addition, DHAQ shows completely reversible CVs in 1 M KOH at all scan rates. 15 The dashed lines show the expected nominal standard potentials of −0.78 VNHE for DHAQ, −0.57 VNHE for OBDTAQ, and −0.50 VNHE for OVAQ.

1.19 V 0.98 V 0.91 V

Current / A.U.

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DHAQ OBDTAQ OVAQ Ni(OH)

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

Potential / V

2

0.0

0.2

0.4

0.6

NHE

Figure 1: Cyclic voltammograms of DHAQ, OVAQ, and OBDTAQ measured in 13 M KOH at 50 mV s−1 and Ni(OH)2 measured in 6 M KOH at 100 mV s−1 . The three dierent anode materials were each mixed with graphite, carbon black, and binders and assembled as 2 mAh anodes with commercial Ni(OH)2 cathodes in coin cells using a 13 M KOH electrolyte. All batteries were characterized by constant current cycling 5



tests with a C-rate of 0.5 (1 mA). Figure 2 shows the voltage-capacity curves for the 1st , 11th , and 112th cycles. The higher Ecell of Ni(OH)2 -DHAQ is expected because the OH groups donate electrons to the conjugated system which generally tends to lower the half-cell redox potential, 15 hereby increasing the total cell potential. The lower Ecell for OVAQ compared to OBDTAQ is most likely because the electron donation to the anthraquinone rings is lower for alkyl groups than for sulde groups. Furthermore, it is noted that the over-potential for discharging appears higher than the over-potential for charging and is increasing as the battery is discharged. Figure 3 displays the discharge capacity as well as coulombic and energy eciencies vs. the cycle number for the 0.5 C cycling test. Similar data for other current densities are shown for the OBDTAQ batteries in SI 5. The data shown are averages of three to six batteries and the small standard deviations show that properties of each of the three battery types are highly reproducible. The charging capacities of the rst cycle are 91.7 %, 88.2 %, and 98.6 % of the theoretical ones (223.2 mAh g−1 , 228.8 mAh g−1 , and 154.9 mAh g−1 ) for the DHAQ, OVAQ, and OBDTAQ batteries, respectively. The capacities decrease continuously with the cycle number and after 100 cycles, they are 17 %, 5 %, and 48 % of the original capacities. The coulombic eciency increases to > 90 % after the rst cycle and stabilizes around 98102 % after 10 cycles for all three battery types. Coulombic eciencies above 100 % are explained by excess charge from previous cycles where the coulombic eciency was low. The energy eciency stabilizes around 74 % and 72 % for the DHAQ and the OBDTAQ batteries, respectively, while it is only about half of this for the OVAQ batteries after 100 cycles. A preliminary study (see SI 6) of the anode performances of DHAQ and OVAQ using 6 M KOH electrolyte showed a signicantly higher capacity retention for OVAQ than observed here. This can likely be related to the better reversibility observed in the CVs in 6 M KOH (SI 4). At 13 M KOH the CVs (Figure 1) showed some irreversibility combined with two single-electron transfers in which case the oligomer rst becomes a radical and then it reacts into its phenol forms. In its radical state it is more reactive and more prone to degradation 6


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Voltage / V

1.2

0.9

DHAQ 0.6

0.3

Voltage / V

1.2

0.9

OVAQ 0.6

0.3

1.2

Voltage / V

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0.9

OBDTAQ 0.6

0.3

0

50

100

Capacity / mAh g

150

200

-1

Figure 2: Voltage vs. specic capacity for the 1st cycle (solid), 11th cycle (dash), and 112th cycle (short dash). The horizontal gray lines indicate the cell potentials expected from the CVs.

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and the larger capacity loss for OVAQ could also be attributed to this. It is clear that the electrolyte concentration is one of many parameters that aect the performance of the batteries and it indicates that dierent anthraquinones show best performance at dierent electrolyte concentrations. To investigate the capacity loss in more detail electrochemical impedance spectroscopy (EIS) was measured on all batteries before battery cycling, after 11 cycles, and after 112 cycles. The EIS data are shown in the Nyquist plot in Figure 4 for the same three batteries as for the voltage vs. capacity data shown in Figure 2. Here, we interpret the DC resistances found from the intercepts with the real impedance (Z 0 ) axis as an ohmic electronic/ionic resistance Re (kHz range). The half arc (Hz range) is interpreted as (quasi ohmic) electrode charge transfer resistance Rct , while the increasing −Z 00 with Z 0 in the mHz range is related to diusion (Warburg) resistance (Rd ). Because the rst EIS spectra were measured before cycling they cannot be compared directly to the subsequent EIS spectra that were measured at the end of a discharge where the battery state-of-charge is a few per cent. Still it can for cycle 0 be seen that the Re is of the order 1-2 Ω cm2 , with the smallest value for OVAQ and the highest for DHAQ. After the 11th cycle this has almost doubled for the OVAQ and OBDTAQ, while it almost remains constant for DHAQ. Furthermore, it is seen that Rct for the DHAQ and OBDTAQ is about 6-8 Ω cm2 , while it is much higher for OVAQ. After the 112th cycle it is only for the OBDTAQ that a clear Rct contribution is seen, for DHAQ and in particular OVAQ this is merged together with the diusive resistance. The ohmic resistances (Re + Rct ) for all batteries and for all cycles, except OVAQ for the 112th cycle, are below 15 Ω. Considering the charge/discharge current of 1 mA in the cycling experiments in Figure 2 and 3 it is evident that the over-potentials which are of the order > 0.1 V (corresponding to > 100 Ω) are almost entirely diusive. This was also seen during the cycling experiments where a clear time dependence of the voltage was seen when the charging/discharging was put to rest. Furthermore, it is also seen that the 'poor' cycling performance of the OVAQ battery 8


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100

Coulombic efficiency

180

70

140

Energy efficiency 60

120

DHAQ

100 80

40

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40

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20

10 0

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OVAQ

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Efficiency / %

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Discharge capacity / mAh g

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Efficiency / %

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OBDTAQ

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0 0

20

40

60

80

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Cycle #

Figure 3: Results from constant current cycling tests at a C-rate of 0.5. The graphs show the specic discharge capacity (left), coulombic and energy eciencies (right) vs. the cycle number. All data points are average values of three to six batteries and standard deviations are shown. For clarity only every 2nd data point is included. The vertical gray line indicates where the batteries were removed for EIS.

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is reected in EIS spectra, in the 112th cycle it appears as if the Rct has become extremely large (> 60 Ω cm2 ).

The capacity loss seen in the cycling experiments can be attributed to three main deactivation mechanisms; chemical degradation, swelling of the electrode, and dissolution of the active species, as illustrated schematically in Figure 5. To test how much of the capacity loss was caused by chemical degradation an additional experiment where the three batteries used in Figure 2 and 4 were reassembled after 200 cycles with a new separator and KOH solution (SI 8) was performed. For all batteries an increased capacity right after the reassembly is seen. For the OBDTAQ it increases from 30 % to 50 % of the rst cycle discharge capacity. The regained capacity is a clear indication that the main fraction of the capacity loss is not related to either diusion of anthraquinone to the Ni(OH)2 side or chemical degradation of the redox active anthraquinone. However, as pointed out earlier the partly irreversible CV of OVAQ in 13 M KOH is an indication that this anthraquinone may be more prone to degradation than the others. Generally, the conjugated system of the anthraquinone is chemically robust although some side groups on anthraquinones are prone to substitution reactions. 15 However, if a substitution reaction had occurred it could potentially be seen as a changed cell potential due to the functional dependence of the voltage. Swelling is caused during charging when potassium ions from the electrolyte solution enter the anode to balance the negative charge of reduced anthraquinone. In this process water molecules coupled to potassium ions cause the swelling of the electrode. In turn, this causes a loss of electrical contact between the micro/nano sized electrically conductive graphite and carbon black particles. We interpret the increasing Re during cycling seen in the EIS measurements (Figure 4) to be a direct eect from this. Although the oxidized/uncharged forms may be insoluble, and considering the relatively short oligomer lengths (see SI 3), the reduced/charged forms will inevitably have some solubility because reduced anthraquinone is deprotonated and ionized in alkaline solutions. In 10


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cm

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DHAQ

-Z'' /

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-Z'' /

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10

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0 0

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Z'

/

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cm

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Figure 4: Nyquist plots showing the EIS results from before the 1st cycle (solid), after the 11th cycle (dash), and after the 112th cycle (short dash).

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the charged state the oxidized and insoluble anthraquinone is located on the graphite/carbon black electrode resulting in a small over-potential during charging. However, during this process the reduced anthraquinone dissolves o and when the battery is then discharged it has to diuse back to the electrode, causing the larger diusive over-potential observed during discharge in Figure 2. The dierence in the increasing over-potential between charging and discharging is least signicant for the OBDTAQ battery indicating a reduced solubility for this anode. This is backed up by the EIS measurements in Figure 4. The increasing Rct during cycling is mainly attributed to the continuous dissolution of the anthraquinones and the OBDTAQ shows the lowest increase in Rct . This is corroborated by the fact that the increased Rct with cycle number scales reasonably with the loss in capacity as expected. After cycling, the cells were opened and the separators of DHAQ and OVAQ batteries were colored (see SI 7). However, this was not the case for OBDTAQ which is interpreted as further conrmation of a reduced solubility problem for this anode. The longer chains of the OBDTAQ compared to the OVAQ can likely explain the reduced solubility that ultimately leads to a better performance of this anode.

In summary, a new solid state battery technology based on novel oligomer anthraquinone and Ni(OH)2 in aqueous alkaline electrolyte with low-cost potential has been demonstrated. Coulombic eciencies are close to 100 %, indicating that there are no side reactions like hydrogen/oxygen evolution. Oligomerized anthraquinone (OBDTAQ) has better capacity retention than monomer anthraquinone (DHAQ) which is attributed to the lower solubility of the OBDTAQ anode as a result of the larger molecules and a dierent chemical structure. However, the capacity retention is not satisfactory and the main capacity loss is attributed to the repeated swelling of the electrode during charge and discharge and partly to the solubility of the OBDTAQ in the charged form, both leading to an increase of the electrical resistance. Considering the general stability of anthraquinones, chemical degradation is not considered as a major contribution to the capacity loss, although it cannot be ruled out 12


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Figure 5: Schematic of the three main deactivation mechanisms: Chemical degradation, swelling, and dissolution. Drawing is not to scale.

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completely. It is anticipated that the use of an oligomer, or even a polymer, with a higher molecular weight, cross-linking between linear units, or additives in the electrolyte that decrease anthraquinone solubility could result in higher capacity retention.

Experimental Methods In brief (see detailed methods in SI 9), the OVAQ was synthesized from 2-vinylanthraquinone in chloroform in a radical polymerization reaction with benzoyl peroxide used as the initiator. The OBDTAQ was synthesized from benzene-1,4-dithiol (deprotonated by potassium tertbutoxide) and 1,5-dichloroanthraquinone by an amide solvent-promoted nucleophilic substitution of the aryl chloride by the thiolate ion. 19 For battery tests Ni(OH)2 cathodes were retrieved from commercial Fujitsu rechargeable Ni-MH AA batteries while anodes were produced by ball-milling the redox active materials with graphite and carbon black, adding poly(vinylalcohol) and ZEON binder suspensions, followed by homogenization and drying. The anode materials were pressed into pellets that were soaked in 13 M KOH solutions before assembly in 2032 coin-cells with two Cellgaard 5500 seperators in each (details in SI 10). For each of the three anode materials between three and six batteries were assembled with an approximate anode loading of 2 mAh.

Acknowledgement The Aarhus University Research Foundation through the AU Ideas Programme supported C. Clausens work. E. Draºevi¢ is grateful to the Marie Sklodowska-Curie Individual Fellowship (H2020-MSCA-IF-2014, grant number 657041). Søren Duch-Hennings is acknowledged for measuring the CV of Ni(OH)2 .

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Supporting Information Available A Supporting Information document is available free of charge. It contains the following sections: SI 1

Cost and Energy Density Estimations

SI 2

Chemical Analysis of Anthraquinone Oligomers

SI 3

Elemental Analysis and Number of Units Estimation of Anthraquinone Oligomers

SI 4

Cyclic Voltammetry Methods and Results in 6 M KOH

SI 5

Performance of OBDTAQ at Dierent Current Densities

SI 6

Battery tests in 6 M KOH

SI 7

Photos of Disassembled Battery Cells

SI 8

Cycling Tests after Reassembly

SI 9

Synthesis of Anthraquinone Oligomers

SI 10

Preparation of Electrodes and Battery Assembly

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