Multi-electron redox phenazine for ready-to-charge organic batteries

Article abstract of DOI:10.1039/c7gc00849j

Organic redox compounds represent an emerging class of cathode materials in rechargeable batteries for low-cost and sustainable energy storage. However, the low operating voltage (<3 V) and necessity of using lithium-containing anodes have significantly limited their practical applicability to battery systems. Here, we introduce a new class of p-type organic redox centers based on N,N′-substituted phenazine (NSPZ) to build ready-to-charge organic batteries. In the absence of lithium-containing anodes, NSPZ cathodes facilitate reversible two-electron transfer at 3.7 and 3.1 V accompanying anion association, which results in a specific energy of 622 Wh kg^-1 in dual-ion batteries.

Full text of DOI:10.1039/c7gc00849j

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Paper
Multi-electron redox phenazine for ready-to-charge organic
batteries
Received 00th January 20xx,
Accepted 00th January 20xx
Minah Leea†‡, Jihyun Hongb†, Byungju Leeb, Kyojin Kub, Sechan Leeb, Chan Beum Parka*, Kisuk
Kangb,c
*
DOI: 10.1039/x0xx00000x
Organic redox compounds represent an emerging class of cathode materials in rechargeable batteries for low-cost and
sustainable energy storage. However, the low operating voltage (< 3 V) and necessity of using lithium-containing anodes
have significantly limited their practical applicability to battery systems. Here, we introduce a new class of p-type organic
redox centers based on N, N’-substituted phenazine (NSPZ) to build ready-to-charge organic batteries. In the absence of
lithium-containing anodes, NSPZ cathodes facilitate reversible two-electron transfer at 3.7 and 3.1 V accompanying anion
www.rsc.org/
association,
which
results
in
a
specific
energy
of
622
Wh
kg-1
in
dual-ion
batteries.
24
Organic compounds with p-type redox activities and high
1
Introduction
25 redox potentials can provide new opportunities to create
26 practically viable organic-based rechargeable LIBs. Unlike, n-
27 type organic cathodes using alkali metal cations as charge
2
3
4
5
6
7
8
9
In designing high-performance and sustainable batteries, the
major technical barriers lie in developing new electrode
materials can outperform current electrode materials.1,
28
2
carriers, p-type electrodes utilize molecular anions. Thus, they
29 do not require a lithium-containing anode in LIBs, thereby
inorganic
Unlike
conventional
transition-metal-based
30 presenting
31 anode materials such as graphite can be used.
a significant advantage because conventional
electrodes, organic compounds, which can be obtained from
earth abundant elements with minimum carbon footprint, are
highly desirable for making sustainable electrodes.3-5 Thus,
considerable efforts have been made to identify organic
16, 17
Moreover,
32 p-type cathodes are insensitive in their function to the
33 selection of cations in the electrolyte, offering freedom in
34 building battery systems using various types of cations (e.g.,
35 Li+, Na+, K+, Mg2+, and molecular ions) for the anodes.18, 19
36 Although research on p-type cathodes is in its infancy, recent
10 molecules and polymers that can reversibly react with lithium
11 or sodium ions in rechargeable batteries. Various redox-active
12 organic materials (e.g., conducting polymers, carbonyls,
13 organosulfurs, and biological cofactors) have been explored
14 thus far;6-11 however, they still cannot rival state-of-the-art
15 inorganic electrodes in terms of energy densities. The redox
16 potentials rarely exceed 3 V resulting in a low-voltage cell
37 studies have reported
a
few p-type radical polymers,17
38 graphite,16, 20 and heterocyclic compounds containing sulfur or
39 nitrogen atoms.21-24 Even though the feasibility of p-type
17 which requires additional inactive parts in battery pack design
More importantly,
12-14
18 to meet the voltage specification.7,
19 most organic compounds do not contain lithium in their
20 natural state. Therefore, their use as cathodes in lithium-ion
21 batteries (LIBs) must be accompanied by lithium-containing
22 anodes (e.g., lithium metal), which raises another difficult
23 issue. 7, 15
Figure 1. a) Chemical structure and redox mechanism of NSPZ. b) Schematic
illustration of energy storage in dual-ion battery using DMPZ as a p-type cathode.
c) HOMO plots of DMPZ and its oxidized forms. Galvanostatic charge/discharge
profiles of a lithium rechargeable battery with d) DMPZ cathode and Li metal
anode and e) DMPZ cathode and Li4Ti5O12 anode at a current density of 50 mA g-1
.
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40 organic cathodes was successfully demonstrated therei
41 these cathodes generally exhibited limited specific capacities
42 of less than 100 mAh g-1. 17, 21-23, 25 To achieve higher capaciti
es
43 in p-type cathodes, it is necessary to identify organic materials
9
9
9
9
n
4
,
association reaction. Charge/discharge profiles of DMPZ, mp-
DPPZ and pp-DPPZ powder samples were measured versus a Li
metal foil (Hohsen, Japan) in coin-type cells (CR2032). The
7 electrodes were fabricated by mixing 50% w/w active
5
6
44 that are lighter and exhibit multi-electron redox capability. 98 materials, 35% w/w conductive carbon (Super P) and 15% w/w
45
To that end, here we present a new class of multi-electro
46 p-type organic cathodes based on N, N’-substituted phenazin
47 (NSPZ) derivatives and reveal underlying mechanisms of th
48 redox reactions with molecular anions. The phenazines ar
49 family of heterocyclic nitrogen-containing metabolites in liv
50 organisms, which are known to scavenge harmful radicals10in
51 biological systems and are involved with various physiologic
52 charge-transfer reactions.26-30 We demonstrate a reversib
53 two-electron redox reaction in NSPZ cathodes in LIBs for th
54 first time, which exhibits an average redox potential of 3.
55 vs. Li/Li+ and a theoretical capacity of 254.9 mAh g-1. Combin
56 electrochemical and structural studies reveal that anio
57 interactions govern the reversible redox reaction of the NSP
58 electrodes. We further demonstrate that the number11
59 electrons and anions that reversibly react with the NSP
salt
9
0e
n
9
polytetrafluoroehylene (PTFE) binder, which are self-standing
without using any current collector. The weight of active
compounds were adjusted within the range of 2.5 ± 0.5 mg per
cell. A porous glassy microfiber filters (GF/F Whatman, UK)
were used as separator after washed thoroughly in acetone
and dried overnight at 120 °C. The composition of electrolyte
salts were altered to compare the potentials of redox reaction;
Electrolytes containing LiPF6, (1 M), LiClO4 (1 M), LiTFSI (from
0.1 M to 4.5 M) in tetraethylene glycol dimethyl ether
(TEGDME) were utilized for galvanostatic measurements. The
cells were assembled in an Ar-filled glove box under inert
atmosphere (<0.5 ppm O2, H2O). Galvanostatic tests were
performed at 50 mA g-1 current density on a battery cycler
(Won-A tech, Korea).
1
1
0
1
e
0
ir
1e0a2
1in0g3
4
10a5l
1l0e6,
10e7
140V8
1e0d9
11n0
11Z1
o
2f
11Z3
1
60 electrode can be tuned based on the electrolyte type and
61 concentration.
1
4
Ex situ electrode characterization
115 The electrodes at different states of charge were prepared by
116 disassembling coin cells (as-prepared, half-charged, fully
117 charged to 4.1 V, half-discharged, and fully discharged to 2.5
118 V) followed by rinsing the electrodes with diethylene glycol
119 dimethyl ether (diglyme). To prevent air contamination, all
62 Experimental
63 Preparation of materials
120 samples were prepared and sealed in an Ar-filled glove box.
64 5,10-Dihydro-5,10-dimethyl phenazine (DMPZ) was purchased
121 XPS spectra were collected with a Thermo VG Scientific Sigma
65 from TCI chemicals (Japan). Other chemicals were purchased
122 Probe spectrometer (UK) equipped with
a microfocus
66 from Sigma-Aldrich (UK) and utilized without further
123 monochromated X-ray source (90ꢀW). Binding energies were
67 purification. mP-DPPZ and pP-DPPZ were synthesized
68 according to the literature.31 A phenazine solution (1 M in o-
124 referenced to the C–C bond of the C 1s region at 284.5ꢀeV.
125 Fourier transform infrared spectra were obtained with pellets
69 xylene) was added to a phenyllithium solution (1.9 M in dibutyl
126 consisted of the mixture of electrodes and KBr powder on an
70 ether) and mixed for 3h. The reaction mixture was quenched
127 FT/IR-4200 (Jasco, Japan) at a resolution of 2ꢀcm−1. Sulfur to
71 by adding cold deaerated distilled water and the organic layer
128 nitrogen ratio was measured with FLASH 2000 series CHNS
72 was separated for following reaction. The resulting solution
129 elemental analyzer of Thermo Scientific (UK). The results of
73 (2.5 mmol), which was stored with dry sodium sulfate, was
130 elemental analysis were averaged after five times tests. The
74 further mixed with m-diiodobenzene (1 mmol), sodium tert-
131 absorption spectrum of each sample was measured in
75 butoxide (3.18 mmol), bis(tri-tert-butylphosphine) palladium
132 TEGDME by a V/650 spectrophotometer (Jasco, Inc., Tokyo,
76 (0.017 mmol) at 120 °C for 1 hour. After cooling to room
133 Japan).
77 temperature, the mixture was filtered and the filtrate was
78 evaporated. The residue was subjected to silica
79 chromatography (hexane-toluene 3:1) followed by
80 recrystallization (tetrahydrofuran-methanol), resulting a pale
81 yellow solid of mP-DPPZ. pP-DPPZ was prepared according t
82 similar procedure as for the synthesis of mP-DPPZ using13
83 diiodobenzene instead of m-diiodobenzene. The synthesis w13as
=
134 Results and discussion
1
o
3
a
5
Among the phenazines, NSPZ is of particular interest because
of its multi-electron redox capability, as it is known to undergo
two successive one-electron transfer reactions (Fig. 1a).32 In
p6-
7
84 performed under nitrogen atmosphere.
85
86 Electrochemistry
138 our continuing effort to utilize a conjugated diazabutadiene
139 motif in a heterocyclic system to store energy, for example,
140 flavins9, 33 and pteridines,7 we note that NSPZ also shares the
141 diazabutadiene motif in the heterocyclic system. Unlike flavins
87 A three-electrode system (Pt counter electrode, Ag/AgNO3
142 and pteridines, the motif in NSPZ is in its reduced form and is
88 reference electrode, glassy carbon working electrode) was
143 therefore “ready-to-charge”. DMPZ, the simplest form of
89 employed to measure solution-based cyclic voltammograms of
−1
144 NSPZ, is a phenazine that is methylated at both nitrogen atoms
90 DMPZ molecules (1 mM) at the scan rate of 100ꢀmVꢀs . We
145 in the pyrazine ring. Note the difference between phenazine34
91 used various electrolytes including LiClO4, LiTFSI, LiPF6, NaClO4,
146 and DMPZ, which are oxidized and reduced, respectively, in
92 and MgClO4 in different types of solvents to test the
147 pristine states. We fabricated a dual-ion rechargeable battery
93 electrolyte salt and solvent dependence of the anion-
2 | Green Chem., 2017, 00, 1-3
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148 system based on DMPZ as a cathode, as schematic
149 illustrated in Fig. 1b, where the as-assembled cell was i
150 discharged state. The charging process leads to the oxidat
151 of the DMPZ cathode via anion association and simultaneou
th
th
nc
1
1n
1io
1
1
al
9
a
ly
0
affected by the type of electrolyte solvent. In electrolytes using
low-donor-number solvents, i.e., weak Lewis bases, such as
acetonitrile (ACN, 14.1)37 and tetraglyme (TEGDME, 16.6),38
the CV curves of DMPZ exhibited two clear pairs of redox
peaks, indicative of two stable and reversible successive one-
electron redox reactions. In high-donor-number solvents such
as dimethyl sulfoxide (DMSO, 29.8) and dimethylformamide
(DMF, 26.6), in contrast, the second cathodic peaks at 0.5 V
9
9
9
9
9
9
1
2
3
4
5
6
7
n
s
152 reduction of the anode via cation insertion. During
153 discharge process, the reverse reactions occur. Therefore,
e
1
e
154 p-type DMPZ is capable of being charged even in the abse
1
e
155 of a lithium (or sodium) reservoir in a counter electrode
156 lithium- (or sodium-) ion batteries.
1
198
f
9
or
(
vs. Ag/AgNO3, the grey dotted circle) disappeared, which
suggests that the multi-electron transfer in DMPZ is no longer
reversible. Instead, new redox reaction appeared as
157
Before the experiments, we examined the redox capab
158 of DMPZ by calculating the highest occupied molecular orbi
159 (HOMO) using density functional theory (DFT) calculations20to
160 investigate the electronic stability of DMPZ during th
161 oxidation reaction (Fig. 1c). Upon the successive removal of
162 to two electrons from DMPZ molecules, the electrons w
163 effectively delocalized in the conjugated structure, imply
1
2ta
ili
9
ls
ty
9
0
1
0
a
shoulders of the first redox peaks at −0.3 to −0.2 V, which is
attributed to demethylation of DMPZ during the second
oxidation process in strong nucleophiles forming of 5-
methylphenazinium-type products.32 Nevertheless, when we
20e2
2u0p3
2e
0re
0
4
5
2in
g
36
constrained the potential window from −0.8 to 0.1
V
164 the structural stablilty after two-electron oxidation.352, 06 facilitating only the first redox reaction, the redox couple was
165 Inspired by this theoretical feasibility, the electrochem
166 performance of DMPZ cathodes was investigated in the du
167 ion batteries based on lithium ions and molecular anion
168 [bis(trifluoromethane) sulfonamide, TFSI-] at 50 mA g-1,21as
169 illustrated in Fig. 1d and e. The charge–discharge profile21
170 DMPZ in Fig. 1d reveals well-defined voltage plateaus t at
171 correspond to average potentials of 3.7 and 3.1 V (vs. Li/L2+
172 respectively. The specific capacities were 217 and 191 mAh
173 for the initial charge and discharge, respectively, wh
174 strongly indicates two-electron redox reaction th
175 theoretical capacity of DMPZ for a one-electron reaction21is
176 127.4 mAh g-1). We further examined the performance o
2
2a0l8-
20s9
ic
0
a
7
l
fully reversible even in DMSO and DMF (Fig. 2a, black dotted
lines). These results suggest that the selection of an
appropriate solvent for the electrolyte is important to fully
utilize the multi-electron redox activity of the DMPZ electrode.
We further attempted to verify how the charge carriers
affect the redox potential of the DMPZ electrode. As observed
i1)3, in Fig. 2b, we obtained identical CV curves in two different
0
o1f
2h12
2
g1-14 electrolytes with LiClO4 and NaClO4 of the same molarity,
2ic
(21e6
1
h
5
suggesting the cation selection (either Li+ or Na+) would not
alter the electrochemical response of DMPZ in rechargeable
batteries. The CV curve of DMPZ in electrolytes of a divalent
a
7
8
-
2f
1
a
cation (Mg2+) with the same ClO4 anion reveals reversible,
177 DMPZ//Li4Ti5O12-based full cell (Fig. 1e). The full cell exhibited
178 specific capacities of 220 mAh g-1 (charge) and 163 mAh g-1
179 (discharge) with voltage plateaus at 2.2 and 1.6 V, respectively.
180 This corresponds well with the result of Fig. 1d, considering the
181 redox potential of the Li4Ti5O12 electrode (~1.5 V vs. Li/Li+). The
182 reversible cycling of the full cell suggests that the dual-ion
183 rechargeable battery system using p-type organic compounds
184 is practically feasible.
185
To better understand the redox mechanism based on
186 anion-association reaction, we investigated electrochemical
187 activities of DMPZ by altering the electrolyte conditions.
188 According to solution-based cyclic voltammetry (CV) analyses
189 (Fig. 2a and Table S1), the reversibility was significantly
Figure 3 Ex-situ analyses of DMPZ electrodes at different states of charge in a
DMPZ//Li cell. a) Sulfur to nitrogen ratio according to CHNS elemental analysis. b)
UV/Vis absorption spectra. c) and d) XPS spectra of N 1s and C 1s local scan,
respectively. e) FTIR spectra.
Figure 2. Combined electrochemical studies in liquid and solid states. CVs of DMPZ
This journal is © The Royal Society of Chemistry 20xx
molecules in a) 0.1 M LiClO4 in TEGDME, ACN, DMF, and DMSO, b) 0.1 M LiClO4 in
Green Chem., 2017, 00, 1-3 | 3
TEGDME and 0.1 M NaClO4 in TEGDME, c) 0.1 M LiTFSI, 0.1 M LiPF6, and 0.1 M
LiClO4 in TEGDME. d) Relative sizes of DMPZ and molecular anions including
electron clouds. e) dq/dV curves of solid DMPZ electrodes in
1 M LiTFSI in
TEGDME. f) Average potential of the first redox reactions measured for solid DMPZ
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219 two-step redox reactions with similar potential values as th
220 in Fig. 2b. (Fig. S1) The higher polarization might be due to
221 strong solvation between ions and solvent molecu
222 Nevertheless, this result demonstrates the possibility25
2
2
2
o
4
e
se
th
le
8
9
0.
The anion association during battery operation is proved
by the redox potential dependence on the salt concentration
in electrolytes (Fig. 2f). In dilute conditions (< 1 M), the redox
potential decreases linearly with the salt concentration
following the Nernst equation described below (Eq. 1, 2),
4
5
s
o
1f
2t
223 utilizing DMPZ in rechargeable batteries based on multival
2
e
5
n
224 cations such as Mg2+ and Ca2+.39
253 where the activity (a[anion-]) is proportional to the mole
225
In contrast, the redox potential of DMPZ varied nota
2
b
5
ly
4
concentration. On the other hands, the redox potential should
more rapidly decrease with salt concentration for high-
concentration conditions (> 1 M) because the short-range ion-
solvent interactions become non-negligible, resulting in a rapid
increase in the activity coefficients of ions in such
25g9
concentrated solution.41-43
-
226 with the type of anion species: TFSI-, PF6 , or ClO4- (Fig. 2c). T
2h5e5
227 redox potential of DMPZ was the highest in the electrol2y5te6
228 containing TFSI- ions and lowest in the one having ClO4 io
229 We speculate that the potential change originates from th
230 different sizes of the anions (Fig. 2d) and the resultin
231 difference in the stability of the ([DMPZ]+-anion-) complexes.
2n5s7.
-
25e8
260 [DMPZ] + anion- ↔ [DMPZ]+anion- + e-
Eq. 1
232 Large anions tend to exhibit weaker electrostatic interaction
233 with [DMPZ]+ compared with smaller anions because of the
261 E[DMPZ]/[DMPZ]+ = E0,[DMPZ]/[DMPZ]+ + 2.30RT/F ⅹ log(1/a[anion-]) Eq. 2
234 diffuse (or delocalized) nature of charge.40 In addition, the
235 steric effect becomes stronger as the size of anions gets
236 bigger, which increases structural distortion and destabilizes
262
We examined the structural changes and the electron
263 transfers in DMPZ molecules during the battery operation
237 the complex, thus resulting in a higher voltage in the
264 using ex situ UV/Vis absorption spectroscopy (Fig. S2b), X-ray
238 electrochemical cell. This effect becomes more severe for the
265 photoelectron spectroscopy (Fig. S2c-d, and S3), and Fourier-
239 second redox reaction, where two anions must be associated
266 transform infrared (FTIR) spectroscopy (Fig. S2e). We
240 with one molecule to compensate for the charge of [DMPZ]2+.
267 measured UV/Vis spectra of DMPZ electrodes at as-prepared,
241 This description is consistent with the observation in Fig. 2c,
268 half-charged by one-electron oxidation, fully charged by two-
242 where the variation of the second redox potential (~ 0.5 V vs.
269 electron oxidation, half-discharge, and fully discharged states
243 Ag/AgNO3) is more significant. In coin cells using solid-state
270 by recollecting electrodes from coin cells disassembled at each
244 DMPZ electrodes, a similar dependence of the DMPZ redox
271 state. The detailed analyses and discussions for the
245 potential on the anion type was observed (Fig. 2e), indicating
272 spectroscopic results are described in Supplementary
246 the analogous electrochemical mechanism of the DMPZ in
273 Information section 2.1. The evolution of UV/Vis spectra is
274 attributed to the step-wise transformation of neutral DMPZ
275 into radical and divalent cations, which is fully reversible
276 during the cycling. The reversible shift of the peaks in the N 1s
277 and C 1s XPS spectra indicates that the diazabutadiene motif is
247 solid state.
278 the key redox center for the anion association reactions similar
33
279 to n-type flavin9,
and pteridine36 systems. FTIR spectra
280 verifies the reversible evolution of local bond structures in
281 DMPZ, especially those related to C-N-C and C=C bonds, which
282 agrees well with the XPS results. DFT calculations also support
283 our experimental observations that the diazabutadiene motif
284 is responsible to the reversible anion-coupled electron transfer
285 reaction of DMPZ (Fig. S4). Overall, we confirmed that the
286 structural evolution of DMPZ during oxidation/reduction is
287 highly reversible and stable at molecular level.
288
To improve electrochemical performances of the redox
289 active unit, we further synthesized two different dimers of
290 NSPZ with distinct symmetry by coupling two monosubstituted
291 5,10-dihydrophenazine units with
m
- or
p
-diiodobenzene (Fig.
-phenylene-linked
DPPZ) are both
31
292 4).
The
resulting
m
-
or
p
293 diphenylphenazine derivatives (
m
P-
DPPZ,
p
P
-
294 electrochemically active with two redox peaks in the CV curves
295 in analogy to DMPZ (Fig. 4a-c). The dimerization strategy
296 improved the cycle stability of the electrode due to suppressed
Figure 4. (a-d) Dimerization of NSPZ using phenyl groups for high performance.2a)97 dissolution of the molecules. This confirms that the redox
Molecular structure of two different isomeric P-DPPZs. b) CV curves of DMPZ, m2P-98 motif of NSPZ can be versatilely utilized in various forms of
DPPZ, and pP-DPPZ molecules in 0.1
M LiTFSI in TEGDME. c) and d)
Charge/discharge profiles of P-DPPZ electrodes for five electrochemical cycles (5
mA g-1). e) Voltage profile and capacity retention of DMPZ for the single-electron
2
0
99 organic compounds, providing room for designing additional
300 NSPZ-based cathodes. We note that the limited capacity
redox reaction. f) Capacity retention of DMPZ in 5 M electrolyte with an additive3. 01 retention of the DMPZ electrode was dominantly observed for
302 the second plateau above 3.5 V, indicating that [DMPZ]2+ is
4 | Green Chem., 2017, 00, 1-3
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303 most soluble. As shown in Figure 4e, about 97% capacity of
304 initial cycle was still available in the small potential window35
305 2.6 to 3.5 V corresponding to the single-electron redo
306 reaction. From the voltage profiles in various electrol te
307 conditions, we hypothesized that the solubility of DMPZ35is
308 insensitive to anion types (Fig. S4a), and is higher in EC/D
309 than TEGDME electrolyte (Fig. S4b). Alternatively, increas
310 salt concentration in electrolytes improved the cy
311 performance of DMPZ (Fig. 4f), analogous to the solvent-in-sa
312 approach for suppresing polysulfide dissolution in Li/Sulfur
313 batteries.44 With high salt concentration electrolyte contain
in
314 DMPZ as additive, the dissolution of DMPZ can
315 suppressed. We expect that polymerization of the redox mo
316 or utilization of solid electrolytes would further improve cy
317 life of the NSPZ-based organic cathodes. In additi
318 optimizing electrode architecture would improve practic
319 applicability, for examples, encapsulating active materials36in
320 porous scaffolds for longer cycle life, and ensuring effectiv
er
3
th
5
e
3
5
6
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338 This study was supported by the National Research Foundation
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