Effects of Zn2+and H+association with naphthalene diimide electrodes for aqueous Zn-ion batteries

Article abstract of DOI:10.1021/acs.chemmater.0c02357

Organic electrodes have been extensively developed to enhance cycling performance in aqueous zinc (Zn)-ion cells. However, little is known about an ion-association process, which caused insufficient diagnoses of various cyclability results. Protons (H+) are charge carriers alongside Zn2+ ions in mildly acidic electrolyte solutions and preferentially participate in the storage process. In addition, dissociation of water can supply the additional H+, and the increased pH yields the precipitate of zinc hydroxy sulfate. Here, we demonstrated the critical effect of the H+ for cycling stability of Zn-ion cells, using a 1,4,5,8-naphthalene diimide (NDI) electrode. Stepwise electron transfer of the NDI electrode proceeded via surface association of the charge carriers and subsequent solid-state diffusion. The H+ intercalation to the NDI electrode became pronounced with an increasing current rate and led to the dissolution of the NDI molecules. We exhibited stable 1000 cycles with a capacity retention of 93.8% by suppression of the H+ impact and suggested a solution-mediated NDI reassembly mechanism as the core reason for the capacity fading.

Full text of DOI:10.1021/acs.chemmater.0c02357

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Article
2+
+
Effects of Zn and H Association with Naphthalene
Diimide Electrodes for Aqueous Zn-ion Batteries
Moony Na, Yusik Oh, and Hye Ryung Byon
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.0c02357 • Publication Date (Web): 13 Jul 2020
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Chemistry of Materials
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Effects of Zn2+ and H+ Association with Naphthalene Diimide Electrodes
for Aqueous Zn-ion Batteries
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Moony Na1,2, Yusik Oh,1,2 and Hye Ryung Byon1,2,*
1Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu,
Daejeon 34141, Republic of Korea
2Advanced Battery Center, KAIST Institute for NanoCentury, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of
Korea
ABSTRACT: Organic electrodes have been extensively developed to enhance cycling performance in aqueous zinc (Zn)-ion
cells. However, little is known about an ion-association process, which caused insufficient diagnoses of various cyclability
results. Protons (H+) are charge carriers alongside Zn2+ ions in mildly acidic electrolyte solutions and preferentially participate
in the storage process. In addition, dissociation of water can supply the additional H+, and the increased pH yields the
precipitate of zinc hydroxy sulfate. Here, we demonstrated the critical effect of the H+ for cycling stability of Zn-ion cells, using
a 1,4,5,8-naphthalene diimide (NDI) electrode. Stepwise electron transfer of the NDI electrode proceeded via surface
association of the charge carriers and subsequent solid-state diffusion. The H+ intercalation to the NDI electrode became
pronounced with an increasing current rate and led to the dissolution of the NDI molecules. We exhibited stable 1000 cycles
with a capacity retention of 93.8% by suppression of the H+ impact, and suggested a solution-mediated NDI re-assembly
mechanism as the core reason for the capacity fading.
14
Introduction
Zn2+ coordination.13,
However, evidence for Zn2+
association from elemental analyses should be considered
carefully, because the zinc hydroxy sulfate, which often
Aqueous batteries have regained the spotlight due to the
increasing desire for low-cost batteries that provide safe
energy storage.1, 2 Much attention has been directed to the
multivalent ion-based system to compensate for low energy
density, which arises from the narrow potential window of
water.3 Zn-ion batteries have been extensively investigated,
because a Zn foil can be directly employed as a negative
electrode.4-6 However, the electrochemistry of the positive
electrode does not involve the desired Zn2+ ion only.
Protons (H+) are charge carriers alongside Zn2+ ions in
mildly acidic solutions.7, 8 H+ has unrivaled ionic mobility via
a unique proton hopping process that occurs in aqueous
media, called the Grotthuss mechanism, and they often
precipitates out during H+ uptake, overlaps the Zn–O
signal.15,
The possible H+ association to the reduced
16
carbonyl group has been rarely investigated despite the
well-known H+ transfer reactions for carbonyl groups such
as quinone/hydroquinone redox couple.17 In contrast,
polyaniline (PANI) and phenazine electrodes recently
showed the predominant H+ uptake, indicating the strong
interaction between the amine group and H+.18-20 The
preferential ion association may rely on surface
functionalities of organic electrodes, electrolyte
concentrations, and applied current rates. However, the
effect of H+ association correlated with the aqueous cell
performance has not been studied yet.
Here, we investigated the Zn2+ and H+ storing process
using an 1,4,5,8-naphthalene diimide (NDI) electrode and
demonstrated the significant impact of H+ association on
cycling stability. The NDI formed the anion radical, followed
by the dianion through two-electron (2e–) transfer
processes. We revealed the stepwise electron transfer in a 1
M Zn2+ aqueous solution, where the H+ uptake by the NDI
particle became significant with an increasing current rate.
In particular, the H+ diffusion inside the NDI particle
triggered the solvation of NDI, which resulted in the loss of
NDI molecules. The stable 1000 cycles for the Zn–NDI cells
were only achieved by suppressing the H+ intercalation.
9
occupy the active sites in the electrode.7,
This H+
engagement significantly influences cell performance.
Several intercalation and conversion electrodes, such as
those in the MnO2, V2O5, and V2(PO4)3 families, showed the
co-insertion of H+, Zn2+, and water, which caused chemical
conversion, volume expansion, and phase transformation.10
Compared to those of metal oxide, organic electrodes are
more flexible in accommodating these charge carriers,11 and
13
afford prolonged cyclability.12,
However, little is still
known about the Zn2+ and H+ uptake in the organic
electrodes, and the effect of their association for cell
performance. Organic electrodes containing carbonyl
groups, such as p-chloranil, calix[4]quinone, pyrene-
4,5,9,10-tetraone, and phenanthrenequinone, exhibited
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This report discloses the critical features of the organic
mildly acidic conditions. Therefore, we consider two
positive charge carriers, Zn2+ and H+, for the charge storing
process. These charge carriers’ valence states, hydrations,
electrode to alleviate H+ intercalation.
Results and Discussions
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and diffusivities significantly dictate the electron transfer
Electrochemical analyses of an NDI electrode
containing Zn2+ and H+ ions. NDI is an excellent electron-
acceptor and n-type organic semiconductor.21 The imide
group is tolerant of hydrolysis, while the carbonyl groups
25
process of the NDI electrode.24,
The electrochemical
behaviors of the NDI electrodes were firstly examined by
cyclic voltammetry (CV) using three-electrode cells. The
NDI with 1 M of Zn(OTf)2(aq) typically showed stepwise
cathodic waves at –0.44 V (C1) and –0.68 V (C2) vs. Ag/AgCl
(Figure 1b). The corresponding anodic responses were
split into three waves, –0.37 V (A2–1), –0.28 V (A2–2), and –
0.17 (A1). The two smaller peaks belonged to A2, the
response to C2, while the intense anodic signal was assigned
to A1 (Figure S3). In addition, we could measure the
numbers of electron transfer from the capacity of the
aqueous Zn–NDI cells, where the NDI electrode was
assembled from metallic Zn foil and 1 M ZnSO4(aq)
electrolyte solution (Figure 1c). The galvanostatic profile of
aqueous Zn–NDI cells is consistent with the CV results.13, 26
The C2 capacity was almost equivalent to that of the C1, and
the total capacity (C1 + C2) at ~185 mAh g–1NDI was ~92% of
the theoretical value (201 mAh g–1NDI). This behavior
verified a stepwise 1e– transfer of the NDI at the C1 and C2
regions.
participate in
a sequential electrochemical reduction
reaction (Figure 1a). In addition, NDI has strong
intermolecular interactions via short-range π–π stacking
and hydrogen bonding, which are responsible for its
solvation resistance in aqueous electrolyte solutions.22 As-
synthesized NDI particles had a triclinic crystal structure
with a submicron size polyhedron shape (Figures S1–2)
and were insoluble in the aqueous electrolyte solution.
These NDI particles (60 wt%) were mixed with Super P
carbon (30 wt%) and PVdF binder (10 wt%) to prepare NDI
electrodes on the porous carbon paper.
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To identify the C1 reaction, we switched the electrolyte
solution to 1 M of HOTf(aq), where H+ is the sole positive
charge carrier. The CV curve showed two cathodic waves
successively at –0.34 and –0.37 V, around the C1 potential
region (Figure 1b). The H+ provided the most positive
potentials among other monovalent ions, such as K+ (–0.40,
–0.43 V, Figure S4) and Li+ (–0.50, –0.54 V) due to its fast
mobility in the aqueous solution, while the hydrogen
evolution reaction occurred below –0.5 V due to the
strongly acidic condition. The single C1 wave from the Zn2+
aqueous solution had 70 mV more negative than that from
H+. Furthermore, the C1 potential from Zn2+ fell between
those of the K+ and Li+ solutions, which was in line with the
Zn2+ mobility in an infinitely dilute solution (Table S1). This
comparison seemingly indicates the predominant Zn2+
association with the radical anion of NDI– during the C1.
Figure 1. Electrochemical analysis of NDI electrodes in
aqueous electrolyte solutions. (a) Schematic illustration of two-
electron transfer for NDI molecule. (b) Cyclic voltammograms
(CVs) of NDI electrodes with 1 M of Zn(OTf)2 (top) and 1 M
HOTf (bottom) aqueous solutions. The scan rates were 0.2 mV
s-1. C1 (A1) and C2 (A2) indicate the first and second cathodic
A similar C2 wave was observed from a non-aqueous
(water-free) Zn2+ electrolyte solution. The C2 plateau at
(anodic) reactions, respectively. (c-d) Galvanostatic profiles of
~0.3
V
(vs. Zn/Zn2+, unless otherwise noted) was
Zn–NDI cell at 0.2 C with (c) 1 M ZnSO4 in water, (d) 1 M
Zn(OTf)2 in acetonitrile (ACN). (e–f) Comparisons of charging
capacity ratios with (e) different electrolyte solutions, 1 M
ZnSO4 + H2SO4(aq) at a pH of 2.3 (blue), 1 M ZnSO4(aq) (black),
and 3 M ZnSO4(aq) (red), and with (f) different current rates,
0.2 C (green), 1 C (black), and 5 C (orange).
distinguishable from the C1 potential at ~0.6 V, whereas it
overlapped with the single potential plateaus from the non-
aqueous Zn–NDI cells (1 M Zn(OTf)2/acetonitrile (ACN),
Figures 1c–d and S5–6). However, corresponding charging
processes for A2 were entirely different from the two cells.
The A2–1 (0.55 V) and A2–2 (0.7 V) plateaus emerged in the
aqueous Zn–NDI cells, whereas the single A2 plateau (0.47
V) appeared in the ACN electrolyte solution. Furthermore,
the charging capacities of A2–1 and A2–2 were governed by
the Zn2+ and H+ concentrations in the aqueous electrolyte
solutions. In the standard 1 M ZnSO4(aq) solution (pH = 5.2),
the capacity ratios of A2–1 and A2–2 were 23 and 28%,
respectively, at 0.2 C (Figures 1e–f). The total capacity of A2
(i.e., A2–1 + A2–2) was comparable to that of A1 (49%). When
increasing the H+ concentration, by adjusting the pH to 2.8,
the A2–1 capacity ratio increased to 35% while that of A2–2
shrank by 16% (Figures 1e and S7a). In contrast, the 3 M
ZnSO4(aq) solution (pH = 4.3) increased the A2–2 capacity to
A Zn2+ aqueous solution with 1–3 M of electrolyte salt
typically has a pH of 4–5 because the multivalent ion acts as
a weak acid through acid hydrolysis, according to Eq. 1,23
푴(퐻2푂)푥푛 + ⇄푴(퐻2푂)푥 ― 1(푂퐻)(푛 ― 1) + + 퐻 +
[Eq. 1]
where Mn+ is the metal ion, n is the multivalent state, and
x is the number of hydrated water molecules. The Zn2+ is
coordinated to the weakly dissociating base, while the
separated H+ ion (10–4–10–5 M with a pH of 4–5) causes the
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Chemistry of Materials
45% and reduced that of A2–1 to 7%. Consequently, the A2–1
dianhydride (NTCDA). The NDI electrode containing Super
P carbon and PVdF binder displayed the additional C–OH
(532.8 eV) and CF2 moieties (291.4 eV, Figure S9), analyzed
after resting at open circuit potential (OCP).28, 29 When the
C1 terminated at 0.2 C, the Zn–O signal emerged at 531.4 eV.
Moreover, a new shoulder appeared in the N 1s region at a
lower BE (399.3 eV) by the Zn2+ coordination to the amine
group (Zn2+…NHC=O).30, 31 This Zn2+ coordination peak was
pronounced after the C2, demonstrating the Zn2+ uptake. For
the charging process, these coordination signals were
attenuated and eliminated at the end of A1. The Zn2+ uptake
was also supported by elemental mapping images of NDI
particles. A scanning transmission electron microscopy
(STEM)–linked energy dispersive X-ray spectrometer (EDS)
mapped the uniform distribution of Zn in the polyhedral
NDI particles after C1 and C2 (Figures 2b–c). In contrast, S,
from the sulfate (SO42–) ions, was not visible in the NDI
particles. We note that these NDI particles were separated
from byproducts, zinc hydroxy sulfate, having flake or
granular shape (see below).
and A2–2 correspond to the H+ and Zn2+ concentrations,
respectively. In addition, the A2 capacity is influenced by the
current rates. When increasing the rate from 0.2 to 1 C, the
A2–1 capacity also increased; namely, the H+ uptake is
dominant for the C2 at rates of 1 C (Figures 1f and S7b).20
On the contrary, the Zn2+ insertion was prominent at lower
current rates, as supported by the negligible A2–1 in the CV
curve at 0.05 mV s–1 (Figure S8). These results demonstrate
that two positive charge carriers participated in the ion
association during the C2, and the test conditions
determined the preferential charge carrier.
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Further, the H+ association with the NDI could be verified
by the zinc hydroxy sulfate byproducts. The
ZnSO4[Zn(OH)2]3∙nH2O is yielded at high local pH of the
electrolyte solution, where the zinc hydroxide coordinates
2–
to SO4 and settles into the electrode.32 This local pH rise
can be created by the loss of H+ from the electrolyte solution.
The amine/imide-containing organic electrodes and MnO2
cells showed the predominant H+ uptake in precipitating the
zinc hydroxy sulfate,8, 18-20 which made the pH of the entire
solution constant.18 In our system, the air-drying NDI
electrodes had an intense 001 reflection of
ZnSO4[Zn(OH)2]3∙4H2O at 2θ = 8.55º from X-ray diffraction
(XRD, Figure S10). In addition, the pentahydrate of
ZnSO4[Zn(OH)2]3 appeared after the 2nd discharge (Figure
S11). Attenuated total reflectance-infrared (ATR-IR)
spectra also exhibited the symmetric stretching vibrations
2–
of the SO4 group at 1114.3 and 1157 cm–1 after C1, which
became very pronounced after C2 (Figures 3a and S12).32
At the end of C2, the electrode surface turned white, and
micron flake-shaped precipitates covered the surface
(Figures 3b–d). The numerous precipitates implicate that
the resource of H+ is not limited by the mildly acidic solution
that contains only ~10–5 M of H+, but is supplied by
dissociation of water.8 The dissociation of water, forming H+
and OH– under the applied electric field, was observed in the
vicinity of the charged surface group from ion-exchange
membranes.33-35 Besides, the hydration shell of the divalent
Figure 2. Evidence of Zn2+ uptake and extraction by using XPS
and elemental mapping images. The NDI electrodes were
examined with 1 M ZnSO4(aq) at 0.2 C and dried in a vacuum.
(a) XPS analysis of NDI electrodes in O 1s (left) and N 1s (right)
binding energy (BE) regions at different discharging and
charging stages. The OCP indicates an open-circuit potential
state. (b–d) STEM-EDS mapping images of NDI particles (b)
after C1, and (c) after C2. Orange, blue, and green dots are
nitrogen, zinc, and sulfur element, respectively. The scale bars
are 100 nm.
Zn2+ containing the weakly dissociating base (Eq. 1) can
35, 36
promote the dissociation of water further.
Thus, it is
suggested that the negatively charged NDIs intimately
interact with water molecules, followed by H+ transfer
before the Zn2+ uptake. This process is supposed to be
different from the H+ electrochemistry showing the CV
profile with 1 M HOTf(aq) (Figure 1b). During the charging
process, the H+ was extracted, and the zinc hydroxy sulfate
was dissolved. The SO42– modes were attenuated in the ATR-
IR spectra after A2 and vanished at the end of A1. In addition,
the color of the electrode surface returned to black.
However, the XRD pattern of ZnSO4[Zn(OH)2]3∙4H2O was
still intense after A2. Furthermore, the remaining XRD signal
after A1 reflected incomplete H+ extraction (Figure S10).
The XPS signals of ZnSO4[Zn(OH)2]3∙4H2O may overlay
Chemical evidence for Zn2+ and H+ uptake in aqueous
Zn-NDI cells. We investigated the Zn2+ and H+ uptake of
discharged and charged NDI electrodes using various
chemical analyses. The Zn2+ uptake was studied by X-ray
photoelectron spectroscopy (XPS). As-synthesized NDI
(Figure 2a) showed the C=O signal (532.0 eV in the O 1s
binding energy (BE) region) and N–C=O from the imide
group (400.8 eV in the N 1s BE region).27 The marginal
signal in O 1s spectrum (light pink, 533.8 eV, <5 at.%
quantity) is assigned to the carboxyl group, arising from the
hydrolyzed precursor, 1,4,5,8-naphthalene tetracarboxylic
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those of Zn2+ and OH. The increased Zn–O signal is also
related to the substantial formation of the zinc hydroxy
sulfate (Figure 2a), which is supported by the increase in
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the O atomic % during discharge (Figure S13). This
byproduct could not be completely removed even after
washing and drying processes.
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Figure 3. Characterizations of ZnSO4[Zn(OH)2]3∙4H2O byproduct. The NDI electrodes were examined with 1 M ZnSO4(aq) at 0.2 C
and dried in air. (a) ATR-IR spectra of NDI electrodes with different discharging and charging stages. (b) Digital photo of NDI
electrodes at OCP (as prepared), after C1, and after C2. (c) SEM image of NDI electrodes after C2. The scale bars indicate 2 μm. (d)
Solid-state (ss) 1H NMR spectra of NDI electrodes at OCP (black) and after the C2 (orange). The inset indicates each moiety labeled
to Ha~Hc. The bottom panel shows ss 1H NMR of the standard references.
Noteworthy is that the electrochemical reduction of the
NDI was not explicitly exhibited by the IR vibration
spectroscopy. Typically, the carbonyl group of the NDI
weakens its stretching band, while the enolate band
appeared and became enhanced during the discharge.
Previous studies showed the increasing enolate signals by
adding the Li+, K+, or Mg2+ ions.37-39 However, the NDI
electrodes in the Zn cells only showed the attenuated
νC=O,anti–sym band without the enolate signal after the C2
(Figure S12). A similar characteristic was reported for a
3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA)
electrode and was ascribed to the screening of H+ by the
water cluster.37 Indeed, we also observed the water cluster
inside the NDI electrodes by using solid-state 1H NMR
spectroscopy. The as-prepared NDI electrode that was
rested at OCP had three resonances, the methylene group of
the PVdF binder (Ha, 2.4 ppm), the aromatic ring (Hc, 7.3
ppm) of the NDI, and the N–H group (Hb, at 11.8 ppm)
(Figure 3d).40, 41 After the C2, a new signal emerged at 5.1
ppm, which was assigned to the water cluster inside the
NDI. This 1H resonance was distinct from the water signals
from ZnSO4[Zn(OH)2]3∙4H2O (1.5 ppm), and the hydrated
ZnSO4 (ZnSO4∙xH2O, 6.5–9.5 ppm). In comparison with the
bulk water peak (4.95 ppm),16 the discharged NDI electrode
had the proton downfield and a broader shape. This
characteristic was typically observed from the confined
water cluster.42 The insertion of the water cluster into the
NDI particle might be attained by the H+ uptake and their
hydrogen bonding.
reveals the effect of ion association. During the C1, the NDI
structure was marginally changed. The XRD reflections of
the triclinic NDI structure slightly broadened (Figure 4a),
and scanning electron microscopy (SEM) images displayed
a negligible deformation of the NDI shape and size (Figures
4b–c). These characteristics are attributed to the surface
reaction of the NDI.43 The surface association and
dissociation processes of the positive charge carriers are
fast without the perturbation of electrode structure, and
minimally dependent on the test conditions (Figures 1e–f).
The proposed, solution-mediated re-assembly
mechanism. The evolution of the NDI structure and size
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Figure 4. Characterizations of NDI particle structures. The NDI
aqueous Zn–NDI cells, where the intimate interaction
between the NDI molecules and the ACN solvent sharply
reduced the cycling capacity (Figure S15). Similarly, the
capacity faded by half (~100 mAh g–1NDI) for 50 cycles at 1 C
in the aqueous medium (Figures 5a and S16). At 1 C, the H+
intercalation into the NDI particle is superior to that of the
Zn2+ ion (Figure 1f); thereby, the solvation of NDI2–
molecules was expedited. The increasing current rate
exacerbated the cycling stability (Figure 5b), and the IR
vibrations of the NDI molecules were observed from the
separator (Figure S17). The Zn–NDI cells with 3 M
ZnSO4(aq) also showed no improvements in the cycling
stability at 1 C rate, exhibiting >65% capacity fading for 50
cycles (Figure S18).
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electrodes were examined with 1 M ZnSO4(aq) at 0.2 C and
dried in air for (a, f) or vacuum for (b–e) samples. (a) XRD
patterns of NDI electrodes. The facets of triclinic NDI crystal are
indicated. The asterisk (*) and hash (#) symbols denote the
reflections
of
porous
carbon
substrate
and
ZnSO4[Zn(OH)2]3∙4H2O, respectively. (b–e) SEM images of NDI
electrodes (b) as-prepared, (c) after C1, (d) after C2, and (e)
after A1. The scale bars indicate 1 μm. (f) Schematic illustration
for solution-mediated assembly of NDI during the discharging
process.
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By comparison, the C2 has a large polarization in the
galvanostatic profile (Figure 1c). It is attributed to the
intercalation and solid-state diffusion of hydrated H+ and
Zn2+ inside of the NDI particles. The single reduction wave
at C2 suggests the negligible energy difference for H+ and
Zn2+ intercalation, although the prominent ions are
determined from the current rate (or scan rate). In addition,
this bulk process accompanies the dissolution of the NDI
molecules. The H+ and water extend the interlayered and
intermolecular distance between the NDI molecules. The
loose π–π interaction for stacking direction and the weaken
hydrogen bonding for the in-plain direction of NDI
molecules promote the facile solvation of dianionic NDI
(NDI2–) molecules. Then, the solvated NDI molecules
containing hydrated H+ or Zn2+ are assembled and re-
precipitated through π–π interaction, indicated as the
solution-mediated assembly process. As a result, the NDI
particles become enlarged, revealing ~1 μm of size (Figure
4d). The XRD pattern showed the broad crystalline
reflections of the NDI particles at the end of C2. The
intensities were significantly attenuated (Table S2),
reflecting the formation of mostly amorphous NDI particles
after C2. This solution-mediated assembly process is
depicted in Figure 4f.
A viable method to ameliorate the cycling performance is
preventing the H+ and water intercalation into the NDI
electrode during the C2. The cutoff potential by the C1
reaction rendered a marginal change of NDI particle size
caused by the surface association of charge carriers.
Moreover, the byproduct was moderately formed on the
electrode surface, as revealed by the black color of the
electrode surface (Figure 3b). We demonstrated that the
discharging stage, limited by C1 (0.4–1.0 V), was stable for
1000 cycles at 1 C (Figure 5e). The total capacity retention
and Coulombic efficiency (CE) were estimated to be 93.8%
and 99.1%, respectively. In addition, the voltage
polarization and capacity drop were moderate as the
current rate increased, which was the crucial advantage of
the surface reaction (Figure S19). The rate capability was
excellent at 2 C, and the original capacity level at 1 C was
regained, even after testing at 10 C. (Figure 5d). At 2 C, the
Zn–NDI cells also successfully performed over 600 cycles
(Figure S20). In addition, after 100 cycles at 5 C, the NDI
vibrations were not detected from the separator (Figure
S17), verifying the insignificant NDI solvation in the C1–A1
region. The surface implementation of Zn2+ and H+ is fast
and does not deteriorate the NDI particles. The improved
cycling performance of Zn–NDI cells demonstrated that the
key processes of NDI electrochemical reactions are the
mitigation of the H+ intercalation to the inside of the NDI
particle. On the other hand, this mechanistic study suggests
the direction of future research for molecular design.
Increasing intermolecular interaction is needed to suppress
the molecular solubility during the electrochemical
reactions. This approach will enhance the capacity and
stability of long-term cyclability.
During the charging process, two potential regions of A2–1
and A2–2 emerge during the A2 process. It is caused by the
extraction of the Zn2+ and the H+ with different solid-state
diffusions. The extraction of hydrated H+ and Zn2+ from
inside of the NDI particle moderately reduces the particular
size at A2 (Figure S14). However, the subsequent surface
reaction for A1, i.e., the extraction of H+ and Zn2+ from the
surface of the NDI, does not visibly change the particular
size (Figure 4e). The crystalline reflections became intense
during the charging process (Figures 4a and Table S2). In
comparison with the as-prepared NDI electrodes, the 110,
11
002, and
2 become pronounced after the A2 and A1
(Figure 4a). We anticipate that this structural transition was
mostly established during the C2 through the solution-
mediated assembly process, while the intercalated charge
carriers made the featureless pattern. After the bulk
extraction process at the A2, the crystalline pattern appears.
Overall, there is a negligible solution-meditated process
during A2~A1.
Cell performances relied on the solution-mediated
process. The challenge of this solution-mediated re-
assembly process is the possible loss of NDI molecules due
to dissolution, which leads to swift capacity fading for a
cycling test.37 It is akin to the inferior cyclability of the non-
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Figure 5. Comparative cycling performances of Zn–NDI cells
the aqueous electrolyte solution, 1 M zinc sulfate (ZnSO4,
Sigma Aldrich, 99.9%) was added. These aqueous
electrolyte solutions were purged with Ar gas (Daesung,
99.999%) for 5 min before use. For non-aqueous electrolyte
solution examinations, the same volume of 1 M Zn(OTf)2 in
acetonitrile (Sigma Aldrich, 99.8%) was employed.
1
2
3
4
5
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with 1 M ZnSO4(aq) with different discharging depths. (a, c)
Galvanostatic profiles in a potential range of (a) 0.2–1.0 V and
(c) 0.4–1.0 V, under the condition at 1 C. (b, d) Corresponding
rate-capability results from 0.2 C to 10 C. (e) Prolonged cycling
stability and Coulombic efficiency (CE) in a potential range of
0.4–1.0 V. The current rate was 1 C.
Electrochemical
examinations.
For
cyclic
voltammogram (CV) tests with different aqueous
electrolyte solutions, three-electrode cells were installed.
The three-electrode cells were comprised of the NDI
electrode as the working electrode (WE), a platinum wire as
the counter electrode (CE), and Ag/AgCl as the reference
electrode (RE, 0.197 V vs. SHE, EC-Frontier). The aqueous
electrolyte solutions were prepared from 1 M HOTf (TCI,
98%), LiOTf (TCI, 98%), NaOTf (Sigma Aldrich, 98%), KOTf
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We demonstrated the ion-association process of the
positive charge carries Zn2+ and H+ with the organic NDI
electrode. Our fundamental study showed that stepwise
electron transfer of the NDI determined the approaching
depth of positive charge carriers. The surface-association
process occurring for the first discharging process was fast
and minimally dependent on either Zn2+ or H+. In contrast,
the second discharging region of the NDI underwent solid-
state diffusion of the charge carriers through intercalation.
The H+ intercalation became predominant at 1 C rate, and
caused the dissolution of the NDI molecules. Numerous H+
could be supplied through dissociation of water, supported
by the precipitation of a zinc hydroxy sulfate. We
demonstrated that the suppression of the H+ intercalation
by controlling the discharging depth enhanced the
cyclability over 1000 cycles. This study sheds light on the
dissolution mechanism of organic electrodes and provides
broad implications for developing aqueous batteries with
improved stability.
(TCI, 98%), Mg(OTf)2 (TCI, 98%), or
1 M zinc
trifluoromethanesulfonate (Zn(OTf)2, TCI, 98%) with a
volume of ~7.5 mL, and purged with Ar gas at least 5 min
prior to examinations. The CV tests were conducted using a
single-channel potentiostat (SP-150, Biologic) with mild
purging of Ar gas.
Galvanostatic examinations of Zn–NDI cells were
conducted using battery cycler (Small cell cycler, PNE
solution) after resting at open circuit potential (OCP) for 12
h.
Characterizations. The Zn-NDI cells were disassembled
inside of Ar-filled glovebox after electrochemical tests. The
NDI electrodes were thoroughly washed with DI water and
dried in a glass vacuum oven (B-585, Buchi) at 60 °C for 12
h
for X-ray photoelectron spectroscopy (XPS), field
emission scanning electron microscopy (FE-SEM), and
scanning transmission electron microscopy (STEM)
observations. The electrodes for infrared (IR) spectroscopy,
solid-state 1H nuclear magnetic resonance (NMR)
spectroscopy and X-ray diffraction (XRD) analyses were
dried in ambient air overnight. The XPS analysis of the NDI
electrode surface was attained from Al Kα X-ray
monochromator (K-alpha, Thermo VG Scientific). All
samples spectra were calibrated to sp2 hybridized carbon
at 248.5 eV in C 1s binding energy region. The intensity of
the O 1s spectrum for all galvanostatic tested samples was
normalized from the area of N 1s spectrum. The IR spectrum
was taken from attenuated total reflection IR (ATR‒IR, iS50
built-in diamond ATR module, Thermo Scientific). Solid-
state 1H NMR spectroscopy (Agilent 400 MHz, 54 mm NMR,
DD2 spectrometer) was measured for tested electrodes to
find the water cluster. As the sample preparation, the air-
drying NDI electrode was detached from the carbon paper,
then ground using a mortar and pestle. The sample spinning
was controlled to 30 kHz, and chemical shifts are reported
in delta (δ) unit. Powder XRD using Cu Kα radiation and
D/tex Ultra 250 detector (SmartLab, RIGAKU) showed a
crystalline pattern of NDI and byproduct in the electrodes.
In situ XRD measurements were performed using Cu Kα
radiation and image plate system (D/MAX-2500, RIGAKU).
PDXL Application analysis software and Cambridge
crystallographic data center (QEFBUY)45 were used to
identify the crystalline patterns. The FE-SEM, S-4800
(HITACH) and STEM (FEI Tecnai F30 ST, 300 kV, high-angle
annular dark-field (HAADF) detector)-incorporated energy
dispersive spectrometer (EDS) were used to observe the
Methods
Synthesis of NDI molecules. Naphthalene diimide (NDI)
was synthesized by dissolving 1.25
g of 1,4,5,8-
naphthalene-tetracarboxylic dianhydride (NTCDA, Alfa
Aesar, 97%) in 62.5 mL of aqueous ammonia solution
(Junsei, 28 wt%).44 The mixture was stirred at room
temperature under argon (Ar) condition for 6 h. Afterward,
a pale yellow precipitant was filtrated using cellulose
membrane (0.1 µm pore size, Millipore) and washed with
deionized (DI) water (18.2 MΩ). The NDI powder then dried
under 60 °C vacuum overnight.
Preparation of NDI electrodes. To prepare NDI
electrodes, NDI, Super
P
carbon (TIMCAL), and
polyvinylidene fluoride (PVdF, HSV 900, Kynar) binder
were mixed with 60, 30, and 10 wt%, respectively. NDI
powder was firstly ground with Super P carbon, and PVdF
binder that was dissolved in N-methyl-2-pyrrolidone (NMP,
Sigma Aldrich, 99%) was added. This slurry was
subsequently blended using a planetary mixer (THINKY,
ARE-310), and cast on a gas diffusion layer (GDL) of carbon
paper (BC 39, SGL). After drying under vacuum, the
electrode was cut and dried in a vacuum oven at 60 ° C
overnight. The loading weight of the NDI electrode was
typically 1.7─2.8 mg cm-2.
Preparation of Zn–NDI cells. A coin cell was assembled
with a metallic Zn foil (Welcos, 99.99%, thickness: 0.05 mm)
as the negative electrode, a piece of glass fiber (GF-C,
Whatman), and NDI positive electrode in an Ar-filled
glovebox (MOTEK, Ar purity 99.999%). A total of 150 μL of
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Chemistry of Materials
rechargeable zinc batteries. Energy Environ. Sci. 2019, 12 (6), 1999-
morphology of NDI particles and elemental mapping,
respectively.
2009.
16.
1
Yan, M.; He, P.; Chen, Y.; Wang, S.; Wei, Q.; Zhao, K.; Xu,
2
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X.; An, Q.; Shuang, Y.; Shao, Y.; Mueller, K. T.; Mai, L.; Liu, J.; Yang, J.,
Water-Lubricated Intercalation in V2O5·nH2O for High-Capacity and
High-Rate Aqueous Rechargeable Zinc Batteries. Adv. Mater. 2018, 30
(1), 1703725.
ASSOCIATED CONTENT
Supporting Information: Supporting data analyzed by CV,
galvanostatic examinations, XPS, XRD, and SEM (PDF)
17.
Guin, P. S.; Das, S.; Mandal, P. C., Electrochemical Reduction
of Quinones in Different Media: A Review. Int. J. Electrochem. Sci.
2011, 2011, 816202.
AUTHOR INFORMATION
18.
Shi, H.-Y.; Ye, Y.-J.; Liu, K.; Song, Y.; Sun, X., A Long-Cycle-
Life Self-Doped Polyaniline Cathode for Rechargeable Aqueous Zinc
Batteries. Angew. Chem. Int. Ed. 2018, 57 (50), 16359-16363.
Corresponding Author
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Tie, Z.; Liu, L.; Deng, S.; Zhao, D.; Niu, Z., Proton Insertion
* hrbyon@kaist.ac.kr
Chemistry of a Zinc–Organic Battery. Angew. Chem. Int. Ed. 2020, 59
(12), 4920-4924.
ACKNOWLEDGMENT
This work is supported by the Samsung Research Funding &
Incubation Center of Samsung Electronics under Project
Number SRFC-MA1702-05.
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Huang, J.; Wang, Z.; Hou, M.; Dong, X.; Liu, Y.; Wang, Y.;
Xia, Y., Polyaniline-intercalated manganese dioxide nanolayers as a
high-performance cathode material for an aqueous zinc-ion battery.
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