An air-stable lithiated cathode material based on a 1,4-benzenedisulfonate backbone for organic Li-ion batteries

Article abstract of DOI:10.1039/c8ta07097k

To meet current market demands as well as emerging environmental concerns there is a need to develop less polluting battery technologies. Organic electrode materials could offer the possibility of preparing electrode materials from naturally more abundant elements and eco-friendly processes coupled with simplified recycling management. However, the potential use of organic electrode materials for energy storage is still challenging and a lot of developments remain to be achieved. For instance, promoting high-energy Li-ion organic batteries inevitably requires the development of lithiated organic electrode materials which are able to be charged (delithiated) at a high enough potential (>3 V vs. Li⁺/Li⁰)-a challenging point rarely discussed in the literature. Here, we evaluate tetralithium 2,5-dihydroxy-1,4-benzenedisulfonate as an air-stable lithiated cathode material for the first time and its reversible Li⁺ electrochemical extraction. Quite interestingly, in comparison with the dicarboxylate counterpart, it was observed that the theoretical two-electron reaction is readily reached with this organic structure and at an average operating potential of 650 mV higher.

Full text of DOI:10.1039/c8ta07097k

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J. Mater. Chem. A, 2018, DOI: 10.1039/C8TA07097K.
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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
rsc.li/materials-a

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Air-stable lithiated cathode material based on 1,4-benzenedisulfonate
backbone for organic Li-ion batteries
A.E. Lakraychi,a,b,e,f E. Deunf,c K. Fahsi, a,b,e,f P. Jimenez,c J.-P. Bonnet,b,e,f F. Djedaïni-Pilard,a,e,f
M. Bécuwe,b,e,f P. Poizot,c,d,* F. Dolhema,e,f,
*
[a]
Laboratoire des Glucides, des Antimicrobiens et des Agroressources (LG2A), UMR CNRS 7378,
Université de Picardie Jules Verne, 33 rue Saint-Leu, 80039 Amiens Cedex, France.
[b]
Laboratoire de Réactivité et Chimie des Solides (LRCS), UMR CNRS 7374, Université de Picardie
Jules Verne, 33 rue Saint-Leu, 80039 Amiens Cedex, France.
[c]
Institut des Matériaux Jean Rouxel (IMN), UMR CNRS 6502, Université de Nantes, 2 rue de la
Houssinière, B.P. 32229, 44322 Nantes Cedex 3, France.
[d] Institut Universitaire de France (IUF), 1 rue Descartes, 75231 Paris Cedex 05, France.
[e] Institut de Chimie de Picardie (ICP), FR CNRS 3085, 33 rue Saint-Leu, 80039 Amiens, France.
[f] Réseau sur le Stockage Électrochimique de l’Énergie (RS2E), FR CNRS 3459, France.
AUTHOR INFORMATION
Corresponding Authors:
*e-mail adresses: franck.dolhem@u-picardie.fr; philippe.poizot@cnrs-imn.fr
Abstract
To meet the current market demands as well as the emerging environmental concerns there
is a need to develop low polluting battery technologies. Organic electrode materials could
offer the possibility of preparing electrode materials from naturally more abundant elements
and eco-friendly processes coupled with a simplified recycling management. However, the
potential use of organic electrode materials for energy storage is still challenging and a lot of
developments remain to be achieved. For instance, promoting high-energy Li-ion organic
batteries inevitably require the development of lithiated organic electrode materials able to
be charged (delithiated) at high enough potential (> 3 V vs. Li+/Li0); a challenging point rarely
discussed in the literature. Here, we evaluate tetralithium 2,5-dihydroxy-1,4-
benzenedisulfonate as air-stable lithiated cathode material for the first time and its
reversible Li+ electrochemical extraction. Quite interestingly, in comparison with the
dicarboxylate counterpart, it was observed that the theoretical two-electron reaction is
1

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readily reached with this organic structure and at an overage operation potential of 300 mV
higher.
Introduction
Rechargeable (or secondary) batteries are nowadays considered as essential energy storage
technologies able to power a diverse and always increasing range of applications (electronic
devices, electric vehicles, integration of renewable electricity on and off the grid, ...). This
accelerating worldwide demand for powerful, safer and greener batteries urges researchers
to explore new battery chemistries and cell technologies.1–4 To this respect, the last decade
has notably seen a renewed interest in investigating and promoting redox-active organic
compounds since they can bring real added value to the existing rechargeable systems as
demonstrated by the series of recent reviews on the topic.5–16 In fact, organic electroactive
materials could pave the way towards all-organic batteries notably for some low-cost
applications (e.g., from disposable devices or grid energy storage) as recently reviewed.17
One particularly attractive point is that the redox chemistry of organic materials offers
access to two different electrochemical storage mechanisms: (i) n-type systems where the
organic structure evolves from a neutral state to a negatively charged one with a cation
uptake (and release), and conversely (ii) p-type systems where the structure evolves from a
neutral state to a positively charged one with anion uptake (and release).18,19 Note that p-
type organic structures have already offered to the positive organic electrode family some
successful materials in the late 1980s with all the examples of ‘doped’ conducting polymers
−
and finally the first commercialized Li-organic cells based on polyaniline (PANI) and BF4 as
counter ion.20–22 More recently great achievements have also been realized with so-called
Organic Radical Batteries (ORBs),23 especially thanks to R&D efforts from NEC and Nishide’s
group,24–29 even if this technology is still in development stages and not commercial yet.30 In
this case, the formal p-type redox center of the positive electrode in ORBs deals with the
nitroxide radical/oxoammonium cation reversible system. Nonetheless, the development of
metal-ion organic batteries requires, in principle, the use of n-type system for both
electrodes (except when an ionic self-compensation mechanism is involved at the molecular
level)31,32. Hence the assembly of an all-organic Li-ion battery in its discharged state
obviously needs a negative electrode material able to accommodate Li+ at low potentials
(such as conjugated dicarboxylates)33–41 together with a lithiated positive electrode material.
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For the latter, examples are scarce in the literature and the large majority of reported
positive organic materials consist in oxidized compounds (charged state) electrochemically
tested in Li-half cell,7,8,16 as a result, very few examples of all-organic Li-ion cells have been
reported.17 Having rapidly recognized this bottleneck, our group have designed and
elaborated several lithiated host materials over the past few years. First, we reported on the
tetralithium salt of tetrahydroxyquinone (Li4C6O6) which can be reversibly oxidized to Li2C6O6
but at relatively low potential (1.8 V vs. Li+/Li0).42,43 Substitution with methoxy groups at the
2 and 5 positions giving the dilithium salt of 3,6-dihydroxy-2,5-dimethoxy-p-benzoquinone
(Li2DHDMQ), increased the average potential at 3 V vs. Li+/Li, however this compound is
hampered by an important polarization phenomenon.44 More recently, we demonstrated
that dilithium (2,5-dilithium-oxy)-terephthalate (Li4-p-DHT) lead to a robust lithiated material
exhibiting quite good electrochemical behavior upon cycling with good reversibility and low
polarization for an average operating potential at 2.55 V vs. Li+/Li0.45 Note that such
performances were confirmed not long after by Chen’s group while demonstrating the full
capacity of this material can be achieved when using nanosheet particles (∼220 mAh/g for
the theoretical 2-electron electrode reaction).46 Finally, a voltage gain was attained by
switching from a para to an ortho-dihydroxyterephthaloyle system giving dilithium (2,3-
dilithium-oxy)-terephthalate (Li4-o-DHT) for which the reversible Li deinsertion/insertion
reaction occurs at 2.85 V vs. Li+/Li0).19 Nevertheless, most of these materials were found
unstable under air (autoxidation) due to a working potential below 3 V vs. Li+/Li0
complicating their processing.
To overcome this issue while keeping in mind the efficiency of the para-benzoquinoid
system, we were interested to switch from the carboxylate functional groups present in Li4-
p-DHT45 to the sulfonate substituent comforted by a recent study from Zhou et al.47 which
demonstrated the benefic effect of the sulfonate group on the reversible reduction of the
anthraquinone backbone vs. Li. Therefore, we were interested in studying tetralithium 2,5-
dihydroxy-1,4-benzenedisulfonate (namely Li4-p-DHBDS) as the sulfonated analogue
compound of Li4-p-DHT. Herein, we report the synthesis and the characterization of this
organic Li-salt as well as its electrochemical behavior evaluated in Li half-cell. Interestingly,
the lithium extraction occurs at high operating potential (<E> ∼3.25 V vs. Li+/Li0) making it
the first organic and lithiated positive material processable under dried air (i.e., discharged
of state the material).
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Experimental
Chemicals
Acetonitrile, methanol, acetone, dimethyl carbonate (DMC) and anhydrous diethyl ether
solvents were purchased from Sigma Aldrich, anhydrous N,N-dimethylformamide (DMF) was
purchased from Alfa Aesar. Oleum (23% in SO3), lithium hydride, lithium methoxide,
hydrogen peroxide solution (30% v/v in H2O) were purchased from Sigma Aldrich,
hydroquinone from Alfa Aesar and 1,4-benzenedithiol from TCI Europe. All solvents and
reactants were used as received.
Analytical and characterization techniques
Infrared spectra were recorded in transmission mode on a FTIR spectrometer at room
temperature (Nicolet AVATAR 370 DTGS). Air-free pellets were prepared in an argon-filled
glovebox by mixing the sample with spectroscopic-grade potassium bromide at 1 wt.%.
1
Spectra were measured over range of 4000 to 400 cm−1 with a resolution of 2 cm−1. H and
13C NMR spectra were recorded on a BRUKER 400 MHz Advance III HD spectrometer (at 400
and 100 MHz, respectively) at 298 K. Chemical shifts (δ) are given in ppm relative to TMS.
Deuterated dimethyl sulfoxide and deuterium oxide were purchased from Sigma Aldrich
(higher than 99.5% purity). Thermal analyses were performed on a Netzsch thermal analyzer
STA 449C Jupiter equipped with a differential analysis microbalance coupled with a mass
spectrometer QMS 403 Aëolos (heating rate: 5°C min−1 under argon flow). Lithium
stoichiometry was determined by atomic absorption spectroscopy using a Perkin-Elmer
AAnalyst 300 (hollow-cathode lamp: Li, wavelength: 670.8 nm, oxidant/fuel: air/acetylene.
The morphology of the powders was observed by scanning electron microscopy (SEM) using
QUANTA 200 F (FEI). Temperature-resolved X-ray powder diffraction (TRXRPD) patterns
were recorded with a Bruker D8 Advance diffractometer equipped with an Anton Parr
XRK900 high-temperature chamber (purged under N2) using a heating rate of 0.1°C s−1. Data
were collected in the Bragg-Brentano geometry with a Cu-anode X-ray source operating at
40 kV and 40 mA. The Cu Kβ radiation was filtered by means of Ni foil. The experiments were
systematically performed in the 5-45° 2θ range with a step of 0.016° and an acquisition time
of 1.2 s per step.
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Synthetic procedures
Preparation of 2,5-dihydroxy-1,4-benzenedisulfonic acid (H4-p-DHBDS). The synthesis was
adapted from a procedure by Wan et al.48 8 g of 1,4-dihydroxybenzene were added by
portions to 28 mL of oleum (23%) under stirring. The green solution was heated to 120°C for
8 h and then cooled to room temperature (rt). The green obtained paste was filtered off to
give a white precipitate which was washed three times with acetonitrile (3 × 50 mL). The
white solid was then dried under vacuum overnight at 120°C giving pure 2,5-dihydroxy-1,4-
1
benzenedisulfonic acid (8 g, 60% yield). H NMR (400 MHz, D2O) δ (ppm) 7.14 (s, 2H, Harom.).
13C NMR (100 MHz, D2O) δ (ppm) 145.53 (C-OH), 131.17 (C-SO3H), 115.93 (CH). IR (KBr pellet)
υ
max/cm-1: 3430-3195 (υ O-H, υ C-H), 1421 (υ C=C), 1213, 1108 (υ SO3H), 1226, 1030 (υ C-O),
882, 813 (δ O-H), 660, 539, 473.
Preparation of tetralithium 2,5-dihydroxy-1,4-benzenedisulfonate as solvate (Li4-p-
DHBDS⋅2DMF). The lithiation of 2,5-dihydroxy-1,4-benzenedisulfonic acid (3 g, 11.1 mmol)
was performed in anhydrous N,N-dimethylformamide (50 mL) with a stoichiometric amount
of lithium hydride (0.35 g, 44.4 mmol). The solution was stirred at room temperature over 3
days inside an argon-filled glovebox containing less than 1 ppm of oxygen and water. After
filtration of the precipitate, washing with diethyl ether and drying step at 60°C under
vacuum overnight, a pale yellow powder was recovered (3.52 g, 80% yield). The complete
chemical formula of the compound was Li3.988±0.005-p-DHBDS⋅2DMF (solvate) as deduced by
AAS for the Li content and thermogravimetric measurement (TG) for the solvation degree.
1
Li4-p-DHBDS⋅2DMF: H NMR (400 MHz, D2O) δ (ppm) 7.87 (s, H, DMF), 6.92 (s, 2H, Harom.),
2.95, 2.80 (s, 2CH3, DMF). 13C NMR (100 MHz, D2O) δ (ppm) 164.94 (C=O, DMF.) 150.72 (C-
OH), 132.74 (C-SO3H), 118.24 (CH), 36.91, 31.39 (2CH3, DMF). IR (KBr pellet) υmax/cm-1: 1669
(υ C=O, DMF), 1452 (υ C=C), 1300 (DMF) 1230, 1095 (υ SO3H), 1208, 1043 (υ C-O), 900
(DMF) 847 (δ O-H), 673, 512, 456.
Desolvation of Li4-p-DHBDS⋅2DMF. 100 mg of previously grounded powder of Li4-p-
DHBDS⋅2DMF were heated in a Büchi glass oven (B-585 Kugelrohr) installed inside an argon-
filled glovebox at a temperature of 300°C over 20 h. The final Li4-p-DHBDS compound was
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obtained (70 mg scale, quantitative yield). The efficacy of the desolvation process was
monitored by thermal analysis, and the absence of DMF traces was confirmed by NMR and
IR spectra. Li4-p-DHBDS: grey powder; IR (KBr pellet) υmax/cm-1: 1452 (υ C=C), 1230, 1095 (υ
SO3H), 1208, 1043 (υ C-O), 847 (δ O-H), 673, 512, 456.
Preparation of 1,4-benzenedisulfonic acid (H2-p-BDS). 0.65 g of 1,4-benzenedithiol was
dissolved in 9 mL of methanol at 40°C, followed with addition of 7.5 mL of H2O2 (30% v/v in
H2O).49 The suspension was stirred at room temperature (20°C) for 16 h and dried under
1
vacuum overnight at 70°C giving H2-p-BDS as a white solid (0.9 g, 84% yield). H NMR
(400 MHz, D2O) δ (ppm) 7.72 (s, 4H, Harom.). 13C NMR (100 MHz, D2O) δ (ppm) 143.83 (C-
SO3H), 126.15 (CH). IR (KBr pellet): υmax/cm-1 1452 (υ C=C), 1243, 1130 (υ SO3H), 673, 512,
456 cm-1.
Preparation of dilithium 1,4-benzenedisulfonate (Li2-p-BDS). The lithiation reaction of 1,4-
benzenedisulfonic acid (H2-p-BDS) (0.3 g, 1.1 mmol) was performed in methanol (10 mL) with
lithium methoxide (0.175 g, 2.5 mmol). After refluxing at 90°C for 20 h, the precipitated
compound was washed 3 times with acetone and dried under vacuum for 6 h at 60°C. The
final product (Li2-p-BDS) was obtained as white powder (0.29 g, 93% yield). 1H NMR
(400 MHz, D2O) δ (ppm) 7.88 (s, 4H, Harom.). 13C NMR (100 MHz, D2O) δ (ppm) 144.83 (C-
SO3H), 126.20 (CH). IR (KBr pellet): υmax/cm-1 1434 (υ C=C), 1234, 1056 (υ SO3H), 686, 585,
496 cm-1. The Li content was measured by AAS giving rise to the following chemical formula:
Li1.992±0.005-p-BDS.
Electrochemical study
The electrochemical performance of the materials was tested vs. lithium in Swagelok®-type
cells using a Li metal disc as the negative electrode and fiberglass separators (Whatman®)
with a 1 M LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 in vol.)
electrolyte. The positive electrodes were prepared in an argon-filled glovebox by grinding
powder of organic active materials with 30 wt.% of carbon black (Ketjenblack EC-600JD,
AkzoNobel) and 5 wt.% of polytetrafluoroethylene (PTFE) with a pestle in a mortar. The
typical loading of the cells was 5-7 mg of active material. The cells were cycled in
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galvanostatic mode using a MacPile automatic/data recording system (Bio-Logic SAS, Claix,
France) in the potential range of 2.5-4.0 V vs. Li+/Li at a typical rate of one lithium ion
exchanged per ring in 10 h (i.e., 1 Li+/10 h ≡ C/20 cycling rate).
Investigation of the electrochemical mechanisms
Operando X-ray diffraction (XRD) powder patterns were recorded, during battery operation,
with a Bruker D8 Advance AXS diffractometer operating in Bragg-Brentano geometry with
the Cu Kα radiation using special stainless steel electrochemical cell equipped with a Be
window described elsewhere.50 The positive electrode was prepared by directly depositing
∼40 mg of the composite electrode (carbon additive: 30 wt.% and active material: 70 wt.%)
onto the Be window in an argon-filled glovebox. Li metal was used as the negative electrode
pasted and a Whatman glass fiber sheet separator was soaked with a 1 M LiPF6 in ethylene
carbonate (EC) and dimethyl carbonate (DMC) (1:1 in vol.) electrolyte. The cell was cycled in
the 2.5-4.0 V potential range in galvanostatic mode at a cycling rate of 1 Li+/20 h.
Complementary ex situ characterizations were also performed by both FTIR and 13C liquid
NMR spectroscopies. Composite positive electrodes were recovered by disassembling cells
at different states of charge and rinsed several times with dimethyl carbonate (DMC) in
order to eliminate residual traces of electrolyte and then dried at 100°C for 4 hours under
vacuum in an argon-filled glovebox. The as-obtained samples were either mixed with
potassium bromide at 1 wt.% for FTIR measurement or solubilized in D2O then filtered
through a 2 ꢀm syringe filter to remove carbon for 13C NMR characterization.
Results and discussion
An up-scalable two-step procedure was applied for the synthesis of desired material.
(Scheme 1). The first step was adapted from a procedure by Wan et al. in which the
preparation of the dihydroxybenzenedisulfonic acid was described.48 The acid was
synthesized from the hydroquinone in a straightforward double electrophilic aromatic
1
sulfonation. The dihydroxybenzenedisulfonic acid was characterized by FTIR, liquid H, and
13C NMR (Fig. S1). Afterward, the tetralithium salt was prepared by a neutralization reaction
with a stoichiometric amount of lithium hydride conducted in anhydrous N,N-
dimethylformamide under an inert atmosphere since Wan et al. have already reported the
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failure in using water as the solvent.48 Our non-aqueous synthetic procedure produced a
yellow powder which was characterized with different analytical techniques.
Scheme 1. Two-step synthetic procedure for the preparation of the target lithiated organic
salt
1
First, the deprotonation of the sulfonic functional groups was readily confirmed by H NMR
(Fig. S1) with a noticeable chemical shift of the aromatic proton peak from 7.14 ppm (s,
Harom. acid) to 6.92 ppm (s, Harom. salt) due to the modification of the electronic environment
of the molecule. In addition, the presence of characteristic peaks located at 7.87 ppm (s, H),
2.95, 2.80 ppm (s, 2CH3) indicated the presence of DMF molecules present, which was also
corroborated by FTIR measurements (Fig. S2). SEM imaging of the sample revealed an
acicular and layered morphology a few micrometers in length and hundreds of nanometers
in width (Fig. S3a). The thermal stability of the compound was first probed by thermal
analysis under inert atmosphere. The typical TG curve (Fig. 1a, dark blue curves) shows a first
weight loss of about 30% starting at 150°C associated with an endothermic phenomenon on
the DSC trace which is attributed to the loss of two DMF molecules per ring; the same
1
content being deduced by the H NMR spectroscopy proton integration methodology. After
the desolvation step, a good thermal stability of the sample is observed up to 400°C (stable
TG trace). The desolvation process was then further monitored by TRXRPD under nitrogen
(Fig. 1b). Basically, the pristine compound appeared well-crystallized with the presence of a
particular intense diffraction peak located at low angle (2θ = 9°), which supports the growth
of a lamellar-type phase. Nevertheless, the series of typical Bragg peaks related to Li4-p-
DHBDS⋅2DMF vanish upon heating for producing a poorly-crystalized phase as the
desolvation proceeds. An isotherm step maintained at 300°C during several dozens of hours
does not bring any change.
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Figure 1. (a) Thermal analysis measurements of Li4-p-DHBDS⋅2DMF (dark blue) together with
the TG curve related to desolvated Li4-p-DHBDS (black) obtained under argon flow at a
heating rate of 10°C min-1. (b) TRXRPD patterns recorded from rt to 300°C under N2 flow. The
series of XRD patterns in dark blue are representative of Li4-p-DHBDS⋅2DMF phase (*
corresponds to the diffraction peak of the sample holder made of Al2O3).
The stability of Li4-p-DHBDS⋅2DMF under ambient air was also checked by FTIR spectroscopy
(Fig. 2). Experimentally, a series of FTIR spectra was recorded coupled with sequential
photography at different exposure times (Fig. 2). Both the starting spectrum and picture
were unchanged after 24 hours indicating a good chemical stability against oxygen unlike Li4-
p-DHT or Li4-o-DHT that exhibit rapid autoxidation in the presence of oxygen.19,45
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Figure 2. FTIR spectra of Li4-p-DHBDS⋅2DMF for different times of exposure to air. Note that
no color change is observed after 24 hours.
Before the electrochemical investigation of Li4-p-DHBDS, the dilithium 1,4-
benzenedisulfonate salt was synthesized and electrochemically evaluated vs. Li within 4.0 -
2.5 V voltage range in order to probe the stability of the sulfonate groups at high potential.
The compound showed no electrochemical response meaning that the sulfonate groups do
not present any electrochemical activity in our experimental conditions (Fig. S4). The
electrochemical performance of Li4-p-DHBDS⋅2DMF was evaluated vs. Li metal in Swagelok®-
type cell. Interestingly, the starting open circuit potential displayed a value of about 3 V
(against 2.5 V for Li4-p-DHT45), which supports well the as-observed stability of the
compound in air reported above. Fig. 3a shows the as-obtained potential-composition trace
upon cycling within 4.0-2.5 V vs. Li+/Li0 potential window at a cycling rate of 1 electron per
molecule exchanged in 10 h (C/20). The first oxidation (charge) is mainly characterized by a
first plateau located at 3.3 V followed by another one ∼100 mV higher corresponding to an
overall extraction of two Li+ per ring as expected (Qtheo. = 122 mAh g-1) unlike the
carboxylate-based analogous compound (namely Li4-p-DHT) that exhibit only half of the
expected specific capacity19,45 except when using nanosheet particles as reported by Chen’s
group.51,52
Figure 3. (a) First two cycles of a Li half-cell using Li4-p-DHBDS⋅2DMF as the positive
electrode material (carbon additive: 30 wt.% and PTFE: 5 wt.%) and galvanostatically cycled
in EC-DMC/LiPF6 1 M at a rate of 1 Li+/10 h (C/20, I = 6.08 mA.g−1). (b) Corresponding
capacity retention curve, together with the coulombic efficiency. (c) Specific capacity vs.
cycle number for current rate changes ranging from C/20 to C per series of five cycles.
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However, only 1.4 Li ions is inserted upon the subsequent discharge leading to a reversible
capacity stabilized at 86 mAh g−1 (i.e., 70% of the theoretical one) whereas the
electrochemical profile is modified into a more complex feature with a sloppy region in the
second part of the reduction process. This new electrochemical profile is then maintained
upon cycling. Quite interestingly, this active electrode material presents a gain in terms of
operating potential by approaching 3.2 V vs. Li+/Li0 compared to the carboxylate-based
analogous compound (2.55 V vs. Li+/Li0 for Li4-p-DHT45), which represents roughly a voltage
gain of +650 mV. The capacity retention curve obtained a rate of 1 Li+/10 h shows a slight
increase in capacity during the ten first cycles up to ∼100 mAh g−1 before stabilizing at
90 mAh g−1 through the 40 subsequent cycles (Fig. 3b). The recovered capacity was also
assessed at different cycling rates (Fig. 3c). The slight increase of the restored specific
capacity value during the beginning of the cycling was also observed during this
electrochemical experiment. A reversible capacity of ∼40 mAh.g−1 was only measured at C-
rate, however, the stabilized capacity value at 90 mAh.g−1 was restored when decreasing the
cycling rate at C/10. To grasp further insight regarding the electrochemical reactivity of this
organic electrode material, we first performed an electrochemical operando XRD
experiment. Figure 4a shows the series of XRD powder patterns collected for every change in
Δx of 0.2 during the 1st charge/discharge of a typical Li4-p-DHBDS⋅2DMF/Li half-cell
galvanostatically cycled at a rate of 1 Li+/10 h. As expected, the two plateaus observed
during the 1st charge correspond to phase transitions as demonstrated by the progressive
disappearance of all diffraction peaks of the pristine material at 3.3 V giving rise to a poorly
crystallized compound (Fig. 4a, light green XRD pattern). Then, new Bragg peaks clearly
emerge during the second electrochemical step at 3.4 V for producing a new crystallized
phase with a better crystallinity compared to the starting material (Fig. 4a, red XRD pattern).
During the 1st discharge, all the diffraction peaks of the latter progressively vanish in a
continuous manner without changes in the Bragg positions which seems indicate that an
amorphous compound is formed as the reduction proceeds (lithium insertion). Interestingly,
the collection of XRD patterns related to the full second cycle (Fig. 4b, red XRD pattern) show
the occurrence of a single reversible phase transition between the electrochemically
generated crystallized phase and an amorphous state. In other words, the electrochemical
pathway observed during the 1st charge is irreversible.
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Figure 4. Operando XRD investigations during both the first (a) and the second (b) cycle of
Li4-p-DHBDS⋅2DMF as the positive electrode material (carbon additive: 30 wt.%)
galvanostatically cycled vs. Li in EC-DMC/LiPF6 1 M at a rate of 1 Li+/10 h. The XRD powder
patterns were collected for every change in Δx of 0.2.
Complementary ex situ characterizations were also performed by both FTIR and 13C liquid
NMR spectroscopies especially to better understand the origin of this irreversible
transformation and further explore the electrochemical reaction mechanism. Liquid 13C NMR
(Fig. 5a) and FTIR (Fig. 5b) analysis were realized by recovery of the organic electrode
material at various states of charge and discharge. A simple observation of 13C NMR spectra
indicates that the DMF initially present in the pristine material (i.e., carbonyl carbon peak:
164 ppm, methyl carbon peaks: 36 and 31 ppm) is not there anymore at the end of the first
charge (Fig. 5a, black spectrum). However, characteristics peaks related to DMC (i.e.,
carbonyl carbon peak: 157 ppm, methoxy carbon peak: 55 ppm) can be unambiguously
identified after the 1st charge as well as the subsequent discharge (one cycle). Note that a
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blank experiment was performed to make sure that the DMF removal was not related to the
washing step conducted prior NMR analysis. Experimentally, powder of Li4-p-DHBDS⋅2DMF
(5 mg) in contact with 2 mL of pure DMC has been sonicated during 1 hour and finally stirred
for 24 hours under inert atmosphere. The recovered powder was dried and then dissolved in
DMSO-d6 for liquid 13C NMR measurements showing no changes. At the end of the first
charge, the corresponding spectrum show also, as expected, a chemical shift in oxidation
(Fig. 5a, red spectrum) of aromatic carbon peaks from 132 and 118 ppm to 145 and
135 ppm, respectively, due to the switch from aromatic structure to a quinone one which is
confirmed by the appearance of the carbonyl peak at 184 ppm and the disappearance of the
peak at 151 ppm peak from the oxygen-bonded carbon. After the 1st cycle, NMR data
confirm well the recovery of the dihydroxybenzenoate electroactive core (Fig. 5a, dark green
spectrum) but also the presence of DMC.
Figure 5. (a) Ex situ 13C NMR spectrum of the electrode material at different state of charge:
pristine material, after first charging and subsequent discharging at 4.0 and 2.5 V vs. Li+/Li0,
respectively (*denote typical peaks either for DMF or DMC molecule). (b) Corresponding
FTIR response focused within the 1300-1800 cm-1 region including the typical spectrum
obtained during the first charge when using 3.3 V vs. Li+/Li0 as cut-off potential.
The FTIR investigation has readily confirmed the 13C NMR analysis with the disappearance of
all absorption bands relative to DMF at 1669, 1300 and 900 cm−1 (Fig. 5b). In addition, one
observes the appearance of new band at 1552 cm-1, attributed to the formation of the
quinonoid carbonyl groups, which becomes more intense at the end of the charge (Fig. 5b,
red spectrum). After the 1st cycle (Fig. 4c, dark green spectrum), the quinonoid carbonyl
vibration band has partially vanished and all other bands remaining unchanged, pointing out
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the reversibility of the process at the molecular level. To sum up, the solvent exchange
reaction occurring as the first discharge proceeds induces an irreversible phase transition of
the pristine organic electrode material giving rise thereafter to a different electrochemical
profile upon cycling. Note that the peculiar amorphisation step observed during the
beginning of the first charge (Fig. 4, green XRD) could be related to the concomitant DMF
release as the lithium extraction proceeds giving rise to a turbostratic stacking of the pristine
layered structure.
To complete our study and with the aim at improving the intrinsic theoretical specific
capacity value of this compound, Li4-p-DHBDS⋅2DMF was desolvated and then
electrochemically assessed vs Li. Basically, Li4-p-DHBDS was readily prepared at the gram-
scale by thermal annealing of the solvate at 300°C for 20 hours under Ar. The corresponding
TG/DSC traces related to the annealed compound confirm the complete removal of DMF
molecules present in the material (Fig. 1a, black curves). Figure S3b shows a selected SEM
image of the sample after this thermal annealing that exhibits alteration of the morphology
of the pristine particles. The particles length and size remained the same but the lamellar
stacking is clearly revealed since a beginning of exfoliation is visible due to the extraction of
the DMF molecules. Consequently, the as-obtained material appeared not enough
crystallized (Fig. S5) to perform again an Operando XRD investigation. The electrochemical
investigation of Li4-p-DHBDS (Fig. 6) was then performed according to a similar procedure
compared to Li4-p-DHBDS⋅2DMF (Fig. 3) for the sake of comparison. When the non-solvated
phase Li4-p-DHBDS was oxidized up to 4 V, it was also possible to extract two Li+ at an
oxidation potential of about 3.35 V vs. Li+/Li0, corresponding to a theoretical capacity of
180 mAh.g-1 (Fig. 6a). Basically, the recorded electrochemical cycling curve is quite similar
whatever the considered active material (i.e., Li4-p-DHBDS or Li4-p-DHBDS⋅2DMF) as shown
in Figure S6. However, to our surprise and on the contrary of our previous studies reporting
the electrochemical performance of several desolvated electroactive organic lithiated
phases,42,43,53 Li4-p-DHBDS presented a poorer retention capability (Fig. 6b) compared to the
pristine solvated phase (Fig. 3). Basically, the typical retention curves exhibit a quasi-linear
capacity fading upon cycling. In the present case, the exfoliation process of the pristine
particles due to the thermal desolvation seems very detrimental for getting long stable
cycling; this situation has never been encountered with other of our tested n-type organic
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electrode materials. A better electrochemical stability upon cycling being anticipated by an
appropriated formulation of the composite electrode, a dedicated study to address this issue
is currently in progress.
Figure 6. (a) First two cycles of a Li half-cell using Li4-p-DHBDS as the positive electrode
material (carbon additive: 30 wt.% and PTFE: 5 wt.%) and galvanostatically cycled in EC-
DMC/LiPF6 1 M at a rate of 1 Li+/10 h (C/20, I = 9.11 mA.g−1). (b) Corresponding capacity
retention curve, together with the coulombic efficiency. (c) Specific capacity vs. cycle
number for current rate changes ranging from C/20 to C per series of five cycles
Conclusions
Continuing our effort to promote high-energy organic LIBs, the disulfonate analog of
tetralithium 2,5-dihydroxy-1,4-benzenedicarboxylate was synthesized and electrochemically
assessed in Li half-cell as a novel lithiated insertion organic material for positive electrode. In
its solvated form (Li4-p-DHBDS⋅2DMF), this redox-active organic structure exhibits a stable
cycling profile at 100 mAh.g−1 but also an average operating potential of ∼3.25 V vs. Li+/Li0
making this lithiated positive material processable under dried air. Based on several
characterization techniques it was demonstrated that DMF molecules incorporated in the
crystal structure of the pristine disulfonate were irreversibly exchanged with DMC of the
electrolyte during the first charge. Regarding the desolvated phase (Li4-p-DHBDS), the
electrochemical performance upon cycling was amazingly inferior in comparison probably
due to the damage caused by the desolvation reaction; the latter exfoliating a lot the pristine
lamellar particles.
Acknowledgments
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This work was partially funded by the Conseil Régional de Picardie (Financial support for
A.E.L.) and by a public grant (VOLTA) overseen by the French National Research Agency
(ANR) as part of the program “Investissements d'Avenir” (ANR-13-PRGE-0012) also labelled
by the Pôle de Compétitivité S2E2. The authors wish to thank O. Ouari and C. Frayret
(“Sustainable Storage” Research team-RS2E) and T. Gutel (CEA-Liten) for the fruitful
discussions.
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