A H-bond stabilized quinone electrode material for Li-organic batteries: The strength of weak bonds

Article abstract of DOI:10.1039/c8sc02995d

Small organic materials are generally plagued by their high solubility in battery electrolytes. Finding approaches to suppress solubilization while not penalizing gravimetric capacity remains a challenge. Here we propose the concept of a hydrogen bond stabilized organic battery framework as a viable solution. This is illustrated for 2,5-diamino-1,4-benzoquinone (DABQ), an electrically neutral and low mass organic chemical, yet with unusual thermal stability and low solubility in battery electrolytes. These properties are shown to arise from hydrogen bond molecular crystal stabilization, confirmed by a suite of techniques including X-ray diffraction and infrared spectroscopy. We also establish a quantitative correlation between the electrolyte solvent polarity, molecular structure of the electrolyte and DABQ solubility-then correlate these to the cycling stability. Notably, DABQ displays a highly reversible (above 99%) sequential 2-electron electrochemical activity in the solid phase, a process rarely observed for similar small molecular battery chemistries. Taken together, these results reveal a potential new strategy towards stable and practical organic battery chemistries through intramolecular hydrogen-bonding crystal stabilization.

Full text of DOI:10.1039/c8sc02995d

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A H-bond stabilized quinone electrode material for
Li–organic batteries: the strength of weak bonds†
Cite this: Chem. Sci., 2019, 10, 418
a
Louis Sieuw,a Alia Jouhara,b E´ric Quarez, b Chloe Auger,b Jean-François Gohy,
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
´
b
a
*
*
Philippe Poizot
and Alexandru Vlad
Small organic materials are generally plagued by their high solubility in battery electrolytes. Finding
approaches to suppress solubilization while not penalizing gravimetric capacity remains a challenge.
Here we propose the concept of a hydrogen bond stabilized organic battery framework as a viable
solution. This is illustrated for 2,5-diamino-1,4-benzoquinone (DABQ), an electrically neutral and low
mass organic chemical, yet with unusual thermal stability and low solubility in battery electrolytes. These
properties are shown to arise from hydrogen bond molecular crystal stabilization, confirmed by a suite of
techniques including X-ray diffraction and infrared spectroscopy. We also establish a quantitative
correlation between the electrolyte solvent polarity, molecular structure of the electrolyte and DABQ
solubility – then correlate these to the cycling stability. Notably, DABQ displays a highly reversible (above
99%) sequential 2-electron electrochemical activity in the solid phase, a process rarely observed for
similar small molecular battery chemistries. Taken together, these results reveal a potential new strategy
towards stable and practical organic battery chemistries through intramolecular hydrogen-bonding
crystal stabilization.
Received 6th July 2018
Accepted 9th October 2018
DOI: 10.1039/c8sc02995d
rsc.li/chemical-science
radicals.9,13–15 Belonging to the carbonyl family, quinone-based
compounds have been extensively explored and a rich library
for quinone-battery chemistries is already available.11,16
However, limiting the solubility of these molecules in battery
electrolytes while maintaining high gravimetric capacities
remains a persistent challenge.
Introduction
From an economic point of view, the future of battery materials
research is bright, as it is crucial to the continuous development
of energy-consuming electronic devices such as smartphones
and laptops.1,2 On another account, the emergence of new,
higher performing energy storage technologies is also required
to keep up with the transition towards renewable energy sour-
ces, which presents the downside of being intermittent.
Inorganic compounds have long thrived both as research
and commercial battery materials.3–6 However, the redox
chemistry of organic molecules is increasingly attracting
interest as it is rich and has the potential to produce electrode
material candidates combining high capacities and practical
redox potentials.7–12 Moreover, organic materials allow for
transition-metal free battery compositions making possible the
fabrication of all-organic batteries. Several families of organic
redox compounds have been investigated so far including
carbonyls, amines, sulfur compounds and nitroxide
Until now, three main strategies have been explored to
address this issue:9,11,16 polymerization,17–23 introduction of
ionic groups7,10,24–32 and graing on mesoporous solid supports
(e.g. carbon or silica nanoparticles).33–36 While successfully
reducing the solubility of the quinone materials, these strate-
gies add to synthesis complexity, manufacturing costs and
molecular weight that is inversely proportional to the gravi-
metric storage capacity. This calls for new paradigms in choice,
design and synthesis of new organic battery materials. For
instance, taking inspiration from nature, where hydrogen
bonding (H-bonding) creates strong interactions and stabilizes
complementary bases on opposite strands of DNA,37 H-bonding
could also be explored to create stable molecular crystal mate-
rials and electrodes for battery applications. Few attempts in
this direction exist and H-bonding seems indeed to be suitable
to organic battery materials as well. For example, Luo et al., have
combined di-sodium rhodizonate with a hydroxyl-rich sodium
alginate binder and detailed a self-healing mechanism of the
electrode during electrochemical cycling.38 However, rhodizo-
nate is an intrinsically insoluble battery chemical, so that the
impact of H-bonding was on the macroscopic structural
stability of the electrode rather than on the molecular level
a
`
´
Institut de la Matiere Condense et des Nanosciences (IMCN), Universite Catholique de
Louvain, Place L. Pasteur 1, 1348 Louvain-la-Neuve, Belgium. E-mail: alexandru.
vlad@uclouvain.be
b
´
´
Institut des Materiaux Jean Rouxel (IMN), UMR CNRS 6502, Universite de Nantes, 2
`
rue de la Houssiniere, B.P. 32229, 44322 Nantes Cedex 3, France. E-mail: philippe.
poizot@cnrs-imn.fr
† Electronic supplementary information (ESI) available: Additional experimental
details and protocols. CCDC 1854000. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c8sc02995d
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solubility effect. Thus, our study goes a step further by
demonstrating that the solubility of electrically neutral small
organic compounds can be controlled through hydrogen
intermolecular bonding, and that consequently the electro-
chemical cycling improved, without the need of a binder in the
electrode formulation.
Herein we prove that the unusual thermal stability and low
solubility in battery electrolytes of 2,5-diamino-1,4-
benzoquinone (DABQ) arise from hydrogen bond molecular
crystal stabilization. Electronic delocalization polarizes the
amino and carbonyl groups, which are at the origin of the
strong H-bond network that stabilizes the crystal. DABQ solu-
bility is found to be strongly dependent on the nature of the
electrolyte solvent and we also observe a clear correlation
between the dipolar momentum of the solvent and the solu-
bility. Consequently, stable cycling with high coulombic effi-
ciency is achieved in the solid state. By combining the H-
bonding properties of this molecule with the use of low
polarity battery electrolytes, we intend to initiate a new
approach towards the insolubilization of quinone materials,
a concept that could be generalized to other organic battery
materials as well.
Furthermore, the electrochemistry of diamino-quinone and
-anthraquinone derivatives has so far been investigated only as
part of a larger molecular system, through covalent xation
within a covalent organic framework.39 The stable electro-
chemical cycling was assigned to the rigid xation at the
expense of lower gravimetric capacity since a larger molecular
mass COF unit cell was used. It is also worth mentioning that
the introduction of amine groups has been shown to affect the
cycling stability of phenazine derivatives,40 however the origin
of this observation was not detailed and there was no specic
mention of any particular intra-/inter-molecular interaction.
Fig. 1 Ab initio structure determination of DABQ. (A) X-ray powder
diffraction pattern (final Rietveld refinement) of crystalline DABQ with
the molecular structure depicted in the inset. (B) View of the solved
DABQ crystal structure (yellow dotted line: intramolecular H-bonds;
green dotted line: intermolecular H-bonds).
Results and discussion
DABQ was obtained through the oxidation of 2,5-diaminohy-
droquinone dihydrochloride in aqueous medium, following the
procedure published by Das et al.41 A pure purple-red powder
was obtained (Fig. S1†).
DABQ was rst studied by powder X-ray diffraction (PXRD)
using synchrotron radiation at the European Synchrotron
Radiation Facility (ESRF, Grenoble) with a wavelength of
DABQ can be regarded as a molecular hybrid of 1,4-benzo-
quinone (BQ) and para-phenylenediamine (p-PDA) (Fig. 2A). The
Fourier-transform infrared (FTIR) spectrum of DABQ contains
the stretching peak of the carbonyl groups (C]O) in BQ, as well
as the stretching peaks of C–N and bending peaks of N–H in p-
PDA (Fig. 2B). However, the C]O stretching peak interestingly
undergoes a bathochromic shi (about 100 cmꢁ1) in the case of
DABQ as compared to BQ. This shi can be explained by the
weakening of the C]O bond, which further supports the
aforementioned intermolecular H-bond formation, reinforced
by the electronic density delocalization.
The thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC) further conrm that the intermo-
lecular interactions translate into a considerable gain of
stability for DABQ. Indeed, DABQ shows no melting behavior
with direct thermal decomposition at a temperature above
300 ꢀC. This is unusually high for a neutral organic low
molecular mass compound. For instance, BQ and p-PDA both
˚
0.74580 A making the ab initio structure determination
possible. The Rietveld renement of the diffractogram is dis-
played in Fig. 1A, and allowed us to determine the structure of
DABQ (Fig. 1B) in the P21/n space group. The unit cell has
3
˚
a volume of 299.88 (4) A and the following parameters: a ¼
˚
˚
˚
5.2532 (4) A, b ¼ 5.3210 (3) A, c ¼ 10.9671 (7) A, b ¼ 101.9740
(17)ꢀ (ESI Tables 1–4†). The individual molecules assemble into
parallel layers stacked along the b-axis with adjacent layers
being perpendicular to each other (Fig. S2†). Intermolecular H-
bonding is observed between the polarized carbonyl and amino
groups. However, these bonds only form between perpendicular
molecules and between molecules of nearest parallel layers, not
between those in the same layer. This results in particles with
lamellar structures, as evidenced by SEM analysis (Fig. S3†).
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Fig. 2 Stabilization of the DABQ molecular crystal through H-bonding. (A) Molecular structure of DABQ and of the reference compounds (i.e.,
BQ and p-PDA) and their corresponding (B) FTIR spectra as well as (C) thermogravimetry (TG) and differential scanning calorimetry (DSC) plots.
The stars mark the melting point in the DSC plots.
display a decomposition onset around 120 ꢀC and 170 ꢀC
respectively; and for both compounds, melting precedes and galvanostatic cycling was measured. These formulations
vaporization. are based on lithium bis(triuoromethylsulfonyl)imide (LiTFSI)
Different electrolyte formulations were tested for solubility
According to the above-mentioned analyses, the crystalline and lithium hexauorophosphate (LiPF6) supporting salts in
phase of DABQ is determined to have intermolecular hydrogen various mixtures of carbonates or glymes in 1 : 1 volume ratio
bonding involving the N–Hd+ and dꢁO–C end groups. Any (1 : 1 v/v): ethylene carbonate (EC)/dimethyl carbonate (DMC),
chemical modication or steric hindering is expected to weaken 1,2-dimethoxyethane (DME)/1,3-dioxolane (DOL) and tetra-
the H-bond strength and thus affect the crystal stability, as ethylene glycol dimethyl ether (TEGDME)/1,3-dioxolane (DOL).
discussed above. Indeed, we synthesized two N-substituted alkyl The effect of salt concentration as well as the associated impact
derivatives of DABQ (N-methyl and N-ethyl DABQ) and their on electrochemical cycling efficiency were also assessed. The
solubility was increased, as also evidenced by faster capacity results are summarized in Fig. 3 and discussed in the following.
decay during galvanostatic cycling (Fig. S4†). Since the cycling
performances of these derivatives were considerably dimin- salt, the solubility of DABQ was found to be higher in carbon-
ished, we did not proceed to further extended analysis. ates when compared to glyme based electrolytes (Fig. 3A):
Considering electrolytes with a 1 M concentration in a carrier
When considering the application of DABQ as a battery elec- approximately 200 mg Lꢁ1 in EC/DMC (1 : 1 v/v) for both LiPF6
trode material, it is important to note that a solvent which can and LiTFSI salts, and in the range of 90 mg Lꢁ1 for DME/DOL
interfere with the molecule’s crystal structure through H-bonding (1 : 1 v/v) and TEGDME/DOL (1 : 1 v/v) electrolytes at 1 M
or electrostatic interactions could lead to a different degree of LiTFSI. To place these values in perspective, BQ displays
solvation and consequently to partial or total dissolution of a solubility of about 200 g Lꢁ1 in TEGDME/DOL (1 : 1 v/v), and is
DABQ. The polarity of the electrolyte solvent is thus expected to thus 2.000 times more soluble than DABQ in the same elec-
play an important role and control the saturation solubility of trolyte formulation.
DABQ. Overall, we observed limited solubility of DABQ in various
The measured solubility of DABQ correlates with the dipole
organic and inorganic solvents, already indicative of stronger moments of the aprotic electrolyte solvents used for these
interactions between DABQ molecules than between DABQ and experiments. The ethers, with low values for dipole moments of
solvent molecules. The synthesis protocol of DABQ is based on its 1.25 D for DOL, 1.15 D for DME and 2.45 D for TEGDME, disrupt
intrinsic poor solubility in water (DABQ precipitates instanta- the H-bond based crystallinity of DABQ to a lesser extent than
neously) but also on methanol and ethers (used for purication). EC-based electrolytes (m ¼ 4.61 D for EC).43 EC is considerably
Since our interest is in the applicability of DABQ as a battery more polar than the other electrolyte constituents, enhancing
electrode material, we next focused our attention on the quanti- the formation of H-bonds between its carbonyl group and the
cation of DABQ solubility in conventional battery electrolytes amine groups of DABQ, thus leading to increased solubility.
and subsequent correlation with cycling stability and efficiency. Interesting to note is that DMC has the lowest dipole
The solubility of DABQ in a series of electrolytes with varying momentum amongst all tested electrolyte solvents, of 0.76D
dipolar moments was determined through UV-vis titration and correspondingly, DABQ also displays the lowest solubility
following an experimental approach recently reported42 (refer to in this solvent, of about 65 mg Lꢁ1 (Fig. S6†).
Fig. S5† and associated content for experimental details). Given
The impact of DABQ solubility on the electrochemical
the concentration of the solute being directly proportional to cycling stability is clearly observed in Fig. 3B. It is to be stressed
the absorbance of the solution, the solubility of DABQ in the that low rate cycling (1Li+ or 1eꢁ exchanged per 1 molecule of
electrolytes can be calculated with relative precision, using the DABQ in 10 hours) was deliberately chosen here to emphasize
DABQ strong absorption band at 330 nm.
the impact of solubility on cycling stability over a long time
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Fig. 3 Solubility of DABQ in different electrolyte formulations and consequences for cycling reversibility. (A) Saturation concentration of DABQ
in a series of representative electrolytes and (B) the corresponding half-cell cycling stability at a lithium insertion/extraction rate of 1Li+/1eꢁ in
10 h (i.e. equivalent to 40 hours per one full cycle). (C) Solubility of DABQ in TEGDME/DOL electrolyte as a function of LiTFSI supporting salt
concentration and (D) the associated potential–composition traces for the 1st cycle.
interval. Fast cycling, although leading to an apparently greater suppress the polysulde shuttle effect in lithium–sulfur cells.44
cycle number stability, implies also shorter test times (a lower However, a progressively increasing carrier salt concentration
measurement time per cycle) which can mask the slow solubi- had the opposite effect on DABQ solubility, as a non-linear
lisation of the materials as well as underlying redox-reactions upward trend was observed. Starting from
a value of
between reactive lithium and dissolved material. Cycling 72 mg Lꢁ1 for the pure TEGDME/DOL solvent, the DABQ solu-
stability analysis revealed that while all electrolyte formulations bility remained constant – around 90 mg Lꢁ1 – for LiTFSI
yielded an initial capacity close to the theoretical value of concentrations ranging from 0.25 M to 1.5 M (Fig. 3C). Inter-
388 mA h gꢁ1 (corresponding to a two-electron redox process), estingly, an abrupt increase in solubility was observed for salt
the ether electrolytes showed enhanced stability with much concentrations higher than 1.5 M, leading to two distinct
lower capacity loss over cycling, with similar degradation regions/regimes in the solubility diagram (Fig. 3C, regions I
tendencies (as displayed by the slope of the capacity decay and II).
curves, Fig. 3B) for DME/DOL and TEGDME/DOL. This is
Whereas the common ion effect could account for the lower
consistent with the close DABQ solubility values in these elec- solubility of polysuldes in high molarity electrolytes, clearly
trolytes. Regardless of the nature of the carrier salt, galvano- another mechanism should be considered to explain the
static cycling of DABQ in carbonate electrolytes (except for the increase in DABQ solubility with the increase in salt concen-
DMC LiTFSI 1 M) systematically resulted in important capacity tration. For instance, the solubility can be correlated to the
loss, assigned to a higher solubility of DABQ in the EC con- solvent (TEGDME/DOL) to salt (LiTFSI) molar ratio (MR) in the
taining electrolytes. Clear evidence of active material dissolu- electrolyte (Fig. 3C) and thus to the molecular structure of the
tion and electrode morphology change are also provided by electrolyte. As shown in Fig. 3C, the solubility remains nearly
post-cycling cell inspection and SEM analysis of the electrode constant for LiTFSI concentrations ranging from 0 to 1.5 M.
(Fig. S7†).
This corresponds to electrolytes with the salt-to-solvent MR
In an effort to further understand and control the dissolu- above 4.25. Below this ratio, a rapid increase in solubility is
tion process of DABQ in glyme electrolytes and thus reduce observed – notably, the solubility of DABQ has about doubled
capacity loss during cycling, we assessed the variation of the for a 2.68 MR in the 2 M LiTFSI electrolyte.
solubility with changing carrier salt (LiTFSI) concentrations.
Through the study of isolated Li+ solvates, Henderson et al.
Indeed, electrolytes with high salt concentrations – ‘solvent-in- determined that the coordination number of Li+ in glyme–Li
salt electrolytes’ – are known to decrease the solubility of elec- salt binary mixtures was in the range of 4 to 6.45–48 Although the
troactive species; notably, these have been extensively used to TFSIꢁ counter anions also play a role in the occupation of the
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Fig. 4 Monitoring the reversible solid-state electrochemical Li insertion/deinsertion process occurring in DABQ using TEGDME : DOL 1 : 1 LiTFSI
1 M as the electrolyte. (A) A typical potential–composition curve for a Li half-cell using DABQ as the positive electrode cycled in GITT mode at
a rate of 1Li+/10 h for Dx ¼ 0.2 followed by an OCV period of 2 hours. (B) Ex situ FTIR of the pristine (P), two-electron reduced (R2) and two-
electron oxidized (O2) DABQ. (C) In situ X-ray diffraction powder patterns of an electrode made of DABQ collected during the first cycle. The cell
was cycled in the 2.0–2.8 V potential range using an intermittent galvanostatic mode. More specifically, a cycling rate of 1Li+/10 h was applied for
a period of 2 h, separated by resting periods of 1 h during which the XRD patterns were collected. Note that XRD patterns labelled (P) and (O2)
formally correspond to the DABQ crystallized form, (R1) and (O1) to the monolithiated semiquinone radical form Li1C6H6N2O2, and (R2) to the
fully lithiated Li2C6H6N2O2 phase.
free Li+ coordination sites,49 it is reasonable to assume that, at ionic conductivity of the electrolyte, rendered insufficient by the
a specic threshold of MR – close to 4 – the coordination decrease in electrolyte salt concentration.
environment will not be saturated and the respective free sites
Having established a correlation between the solubility of
may interact with the DABQ molecules. In the process, this can DABQ and the cycling stability, we undertook a mechanism
facilitate the solubilization of DABQ in the electrolyte, thus study to conrm and understand the solid-phase electro-
explaining the increasing solubility values of DABQ for LiTFSI chemical process occurring with DABQ in TEGDME/DOL 1 : 1
concentrations beyond approximately 1.5 M.
LiTFSI 1 M, as this electrolyte formulation delivered the best
The correlation between the electrolyte salt concentration results. Given the noticeable polarization values observed
and cycling efficiency is also found to be non-linear, as other during the regular galvanostatic cycling (up to 450 mV for the
factors inuence the extent of material utilization and cycling rst electron and 300 mV for the second at a rate of 1Li+/10 h,
stability. Increasing the LiTFSI content from 1 M to 2 M leads to Fig. 3D), a cell was cycled using a galvanostatic intermittent
a considerable solubility increase that is indeed translated into titration technique (GITT). Experimentally, a constant current
a lower rst cycle efficiency and consequently quicker loss of equivalent to a cycling rate of 1Li+/10 h was applied over 2 hours
capacity with cycling (Fig. 3D and S7†). Conversely, while an (i.e., Dx ¼ 0.2), followed by a relaxation period of 2 hours at zero
electrolyte with only 0.25 M LiTFSI slightly reduces the solu- current (Fig. 4A). Basically, this measurement conrms that the
bility, the electrochemical cycling stability and efficiency are overall electrochemical process occurs according to two
considerably affected, with poor material utilization at the rst successive phase transitions (two separated plateaus), each
cycle and high cell polarization detected. This is ascribed to the involving one electron per molecule. The rst phase transition
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process (1st plateau) appears signicantly more sluggish than Li2C6H6N2O2). By contrast, whereas the reduced form of ben-
the second one (2nd plateau). Indeed, the quasi-equilibrium zoquinone becomes poorly soluble in organic electrolytes due to
state of this biphasic system is not reached, which is not the the formation of
a delocalized organic polyanion (Li2–
case for the second phase transition. However, by extrapolation C6H4O2),7,42 the oxidized form (benzoquinone, BQ, C6H4O2) is
we can estimate the quasi-equilibrium potential values for the highly soluble due to the weaker van der Waals intermolecular
two biphasic systems at ꢂ2.4 V and 2.35 V vs. Li+/Li, respectively. interactions. In fact, the solubility of BQ is so high (measured in
The lower (as compared to BQ) electrochemical reduction excess of 200 g Lꢁ1, Fig. S8†) that the solid-state electrochemical
potential of DABQ is the inevitable result of the richer electronic cycling is impossible to carry out. As such, the exceptional H-
environment of the carbonyl double bond due to the electron bonding feature grants DABQ far superior cycling
delocalization in DABQ, as also conrmed by the n(CO) vibra- performances.
tion band shi in FTIR analysis (Fig. 2).
To get further insight into the electrochemical mechanism
and the reversibility of the DABQ redox process, ex situ FTIR was
Conclusion
carried out (Fig. 4B). Therefore, three samples of the cycled
In this work, a new conceptual strategy for small quinone
DABQ electrode material were retrieved and denoted as follows
battery material insolubilization is presented. The intermolec-
(Fig. 4): pristine electrode (P), aer a 2-electron reduction (R2),
ular hydrogen bonding in 2,5-diamino-1,4-benzoquinone
then aer a full reversible 2-electron oxidation (O2). Consistent
(DABQ), reinforced by the strongly polarized amino and
with the carbonyl group reduction mechanism, the C]O
carbonyl groups, is found to be robust enough to result in an
stretching band disappeared upon the 2-electron reduction of
unusually low solubility in organic battery electrolytes. We
DABQ (R2), accompanied by the appearance of a phenolate
further provide a correlation between the electrolyte solvent
stretching band at 1220 cmꢁ1. This reaction was found to be
polar momentum, the solvation state of the electrolyte and the
perfectly reversible, as evidenced by the spectrum of the re-
solubility of DABQ, directly inuencing the cycling stability of
oxidized DABQ (O2) being identical to that of the pristine
the solid electrode cells. With an optimized electrolyte formu-
material (P).
lation we achieved full material utilization under galvanostatic
Finally, in situ XRD patterns were collected to monitor the
cycling (theoretical capacity of 388 mA h gꢁ1 for a reversible 2-
structural changes during the lithium insertion/deinsertion
electron process) and remarkable cycling stability when
process in the DABQ electrodes. The gradual fading of most
compared to organic battery materials similar in composition
of the reections accompanied by the appearance of new peaks
(e.g., 1,4-benzoquinone). Furthermore, through ex situ FTIR and
indicates a biphasic reaction for each sequentially exchanged
in situ PXRD, the chemical and structural changes undergone by
electron. In agreement with the GITT measurement (Fig. 4a),
DABQ upon electrochemical cycling are shown to be perfectly
the 2-electron reduction thus proceeds through two distinct
reversible, attesting the stability of the electrochemical process
phase transitions with three related phases: pristine C6H6N2O2
and the unusual robustness of this peculiar molecular crystal.
(DABQ), monolithiated semiquinone radical form Li1C6H6N2O2,
Our work paves the way to new designs of H-bond stabilized
and then the fully lithiated Li2C6H6N2O2.
organic molecular compounds for battery applications. If
These major structural changes are the result of H-bond
combined with non-polar electrolyte solvents, H-bond inter-
disruption in DABQ driven by reduction of the carbonyl
acting binders and/or conductive carbon frameworks, this
groups. Since the electronic as well as the coordination envi-
could lead to a new strategy towards insoluble small organic
ronment of the O atoms is changing during carbonyl bond
battery materials with practical impact.
reduction accompanied by lithium insertion (as also conrmed
by ex situ FTIR measurements, Fig. 4b), the intermolecular H-
bonds are disrupted. The stabilization of the newly formed
Experimental methods
Synthesis of 2,5-diaminobenzoquinone (DABQ)
crystalline phase takes place through coordination of the Li+
cations in the vicinity of newly formed phenolate groups.
Remarkably, this two-step reduction is fully reversible, the Commercially available 2,5-diaminohydroquinone dihydro-
diffraction patterns of Li1C6H6N2O2 (O1) and C6H6N2O2 (O2) in chloride (>90%, Sigma Aldrich) (213 mg, 1 mmol) was dissolved
the re-oxidation route matching those of Li1–C6H6N2O2 (R1) and in 20 mL of Milli-Q water and an excess of triethylamine (Sigma
C6H6N2O2 (P) obtained during the reduction, respectively. Aldrich) (1.4 mL, 10 mmol) was added as a proton scavenger
Furthermore, no broadening of the peaks is observed, which while stirring the mixture in air at room temperature. The
means no amorphization of the material. The structure is thus addition was followed by a change of color from violet to dark
exible and strong enough to allow for subsequent lithium red and formation of a dark purple precipitate. The suspension
insertions (deinsertions) and H-bond regeneration. The stable was stirred for 30 minutes and the precipitate was ltered off
intermediate Li1C6H6N2O2 structure (R1 or O1) probably and washed with water, methanol and nally with ether. Aer
contributes to the material’s strength.
drying under vacuum, the reaction afforded an 80% yield of
1
An important aspect to be highlighted here is the peculiar pure DABQ (110 mg). H NMR (300 MHz, DMSO-d6, TMS): d ¼
properties of H-bond stabilized DABQ in that the electrochem- 7.76 (br, 2H; NHax), d ¼ 7.02 (br, 2H; NHeq), d ¼ 5.30 (s, 2H;
istry of this solid material proceeds exclusively via intrinsically aromatic CH) (Fig. S1†). FTIR: 3348 cmꢁ1, 3158 cmꢁ1 (nN–H);
insoluble phases (DABQ–C6H6N2O2 via H-bonding and 1663 cmꢁ1, 1653 cmꢁ1 (dN–H); 1563 cmꢁ1 (nC]O); 1522 cmꢁ1
,
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1402 cmꢁ1, 1356 cmꢁ1 (nC]C); 1259 cmꢁ1 (nC–N) 1065 cmꢁ1 (nC– powder ground with Ketjenblack EC600JD carbon black (67 : 33
O) (Fig. 2 and S1†).
in mass ratio) was collected aer cell disassembly in an argon-
lled glovebox before cycling, aer the rst reduction and aer
the rst full cycle (rst discharge/charge). The collected samples
were gently washed with DMC, centrifuged to remove any trace
of LiTFSI then placed in a drying glass oven (Buchi B-585 glass-
Characterization techniques
The crystal structure of DABQ was solved from PXRD data
collected using the ESRF synchrotron (Grenoble) with a wave-
length l ¼ 0.74580 A. FTIR spectroscopy was carried out on
ꢀ
oven Kugelrohr) and heated at 120 C for 3 hours. Finally, the
˚
FTIR analyses were carried out on the resulting dried powders
using a Bruker Alpha P spectrometer with a Single reection
ATR module installed in the glovebox.
pristine DABQ powders using a Bruker Alpha P spectrometer
with a single reection ATR module. Thermogravimetry (TG)
and differential scanning calorimetry (DSC) experiments were
carried out under argon with a SENSYSevꢁo1instrument from
Setaram using a heating rate of 5 ꢀC min between 25 and
600 ꢀC. Scanning electron microscopy (SEM) images were
collected using a JEOL JSM-7600F microscope. UV-visible
spectra were recorded using a Shimadzu UV-1700 PharmaS-
pec. Samples were analyzed in 1 cm length quartz cuvettes.
Saturated solutions of DABQ in the studied electrolyte systems
were prepared by adding 2 mg of the powder to 3 mL of the
electrolyte. Aer equilibrium was reached, the resulting super-
natant was carefully ltered through Millipore Millex-HV PVDF
0.45 mm lters to remove excess insolubilized DABQ. The
saturated solution was subsequently diluted by a factor of 100.
Standard solutions were prepared with four for each electrolyte
with DABQ concentrations of 2, 5, 10 and 20 mM. Absorbance
was measured in the 200–800 nm range for all solutions. The
calibration curve was obtained by linking the maximum
absorbance (lmax ¼ 330 nm) of the standard solutions to their
concentration in DABQ using the common Beer–Lambert law:
A ¼ 3lC.
In situ powder XRD
Diffractograms were recorded using a PANanalytical X’Pert Pro
diffractometer operated in Bragg–Brentano reection geometry
with a long line focus Cu-anode X-ray source, and a X’Celerator
RTMS detector. Data were collected in the 10–40ꢀ 2q range with
a step of 0.017ꢀ and an acquisition time of 1.9 s per step.
Measurements were performed with a Multi Purpose Sample
Stage (MPSS) able to accommodate the in situ Swagelok-type cell
described elsewhere.50 A Li metal disk was used as the negative
electrode whereas the positive electrode composition was
70 wt% of DABQ, 25 wt% Ketjenblack EC-600JD, and 5 wt%
PTFE. This mixture was pressed at 1 ton on a stainless steel (AISI
316L) grid current collector.
Conflicts of interest
There are no conict of interest to declare.
Acknowledgements
Electrochemical study
Pristine DABQ powder was ground with Ketjenblack EC600JD
carbon black (AkzoNobel) in a 67 : 33 mass ratio (mtot. ¼ 100
mg) using a planetary ball mill (1 hour; 400 rpm; ve 5 mm steel
balls and stainless steel container). The mixed powders were
LS and AV acknowledge FSR UCL and F.R.S.-FNRS through
grant No. J.0111.16 ‘Equinox’ for nancial support, and wish to
thank Y. Filinchuk and F. Morelle (UCL) for help with XRD data
analysis. LS and JFG are grateful to CfB in the frame of the ARC
BATTAB 14/19-057. AJ and PP wish to thank J. Gaubicher, O.
Crosnier and S. Grolleau (IMN) for their assistance.
ꢀ
dried in a vacuum oven at 70 C for 12 hours and kept under
argon atmosphere aerwards. The electrodes were tested in
half-cell conguration using Li–metal foil (Alfa Aesar) as both
reference and counter electrode. CR2032 (SS316) coin-cells were
assembled in an argon-lled glovebox. One sheet of Whatman
Grade GF/D Glass Microber Filters was used as a separator
between the working electrode and the lithium disk. The
following electrolytes (Solvionic) were used in a 1 : 1 (v/v) ratio:
EC/DMC LiPF6 1 M; EC/DMC LiTFSI 1 M; DME/DOL LiTFSI 1 M
and TEGDME/DOL LiTFSI 1 M. Other electrolyte formulations
(TEGDME/DOL 1 : 1 (v/v) LiTFSI 0.25 M/0.5 M/1.5 M/2 M/3 M)
were homemade using TEGDME (>99%, Sigma Aldrich), DOL
(99.8%, Sigma Aldrich) and high-purity LiTFSI (3 M). The gal-
vanostatic cycling experiments were performed using an Arbin
BT-2043 multichannel potentiostat battery tester.
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