Heavily n-dopable π-conjugated redox polymers with ultrafast energy storage capability

Article abstract of DOI:10.1021/jacs.5b02290

We report here the first successful demonstration of a π-conjugated redox polymer simultaneously featuring a π-conjugated backbone and integrated redox sites, which can be stably and reversibly n-doped to a high doping level of 2.0 with significantly enhanced electronic conductivity. The properties of such a heavily n-dopable polymer, poly{[N,N′-bis(2-octyldodecyl)-1,4,5,8-naphthalenedicarboximide-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (P(NDI2OD-T2)), were compared vis-à-vis to those of the corresponding backbone-insulated poly{[N,N′-bis(2-octyldodecyl)-1,4,5,8-naphthalenedicarboximide-2,6-diyl]-alt-5,5′-[2,2′-(1,2-ethanediyl)bithiophene]} (P(NDI2OD-TET)). When evaluated as a charge storage material for rechargeable Li batteries, P(NDI2OD-T2) delivers 95% of its theoretical capacity at a high rate of 100C (72 s per charge-discharge cycle) under practical measurement conditions as well as 96% capacity retention after 3000 cycles of deep discharge-charge. Electrochemical, impedance, and charge-transport measurements unambiguously demonstrate that the ultrafast electrode kinetics of P(NDI2OD-T2) are attributed to the high electronic conductivity of the polymer in the heavily n-doped state.

Full text of DOI:10.1021/jacs.5b02290

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Communication
Heavily n-Dopable #-Conjugated Redox Polymers
with Ultrafast Energy Storage Capability
Yanliang Liang, Zhihua Chen, Yan Jing, Yaoguang Rong, Antonio Facchetti, and Yan Yao
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b02290 • Publication Date (Web): 31 Mar 2015
Downloaded from http://pubs.acs.org on April 1, 2015
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Heavily n-Dopable π-Conjugated Redox PolymerswithUltrafast
Energy Storage Capability
Yanliang Liang,†Zhihua Chen,§Yan Jing,†YaoguangRong,† Antonio Facchetti,*, §Yan Yao*,†,¶
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† Department of Electrical and Computer Engineering, University of Houston, Houston, Texas 77204, USA.
§ Polyera Corporation, 8045 Lamon Avenue, Skokie, Illinois 60077, USA.
¶Texas Center of Superconductivity, University of Houston, Houston, Texas 77204, USA.
Supporting Information Placeholder
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1a).Pioneering attempts to construct such a polymer were limited
ABSTRACT: We report here the firstsuccessful demonstration
ofa “πꢀconjugated redox polymer”simultaneously featuring a πꢀ
conjugated backbone and integrated redox sites, whichcan be
stably and reversibly nꢀdoped to a high doping level of 2.0 with
significantly enhanced electronic conductivity. The properties of
suchaheavily nꢀdopable polymer, poly{[N,N’ꢀbis(2ꢀoctyldodecyl)ꢀ
1,4,5,8ꢀnaphthalenedicarboximideꢀ2,6ꢀdiyl]ꢀaltꢀ5,5’ꢀ(2,2’ꢀ
bithiophene)} (P(NDI2ODꢀT2)), were compared vis-a-visto those
of the corresponding backboneꢀinsulated poly{[N,N’ꢀbis(2ꢀ
octyldodecyl)ꢀ1,4,5,8ꢀnaphthalenedicarboximideꢀ2,6ꢀdiyl]ꢀaltꢀ
by insufficient reversibility of the doping process,6 unknown dopꢀ
ing level,6b, c, 7and/or uninvestigated/poor conductivity of the nꢀ
doped state6ꢀ7.Thus, it remains a major challenge to develop
heavily nꢀdopable πꢀconjugated polymers with high electronic
conductivity.
π-Conjugated units
Redox sites
a)
e−
Non-conjugated bondings
5,5’ꢀ[2,2’ꢀ(1,2ꢀethanediyl)bithiophene]}
(P(NDI2ODꢀTET)).
π-Conjugated Polymers
When evaluated as a charge storage material for rechargeable Li
batteries, P(NDI2ODꢀT2) delivers 95% of its theoretical capacity
at a high rate of 100C (72 s per charge−discharge cycle) under
practical measurement conditions, as well as 96% capacity retenꢀ
tion after 3000 cycles of deep discharge−charge.Electrochemical,
impedance, and charge transport measurements unambiguously
demonstrate that the ultrafast electrode kinetics of P(NDI2ODꢀT2)
are attributed to the high electronic conductivity of the polymerꢀ
inthe heavily nꢀdoped state.
e−
e−
e−
π-Conjugated Redox Polymers
e−
Redox Polymers
b)
Organicπꢀconjugated polymers are emergingas a materials class
for energyꢀrelated applications enabling a path to a more sustainaꢀ
ble energy landscape without the need of energyꢀintensive, expenꢀ
sive, and sometimes toxic metalꢀbased compounds.1Furthermore,
the possibility to fabricate lightweight and mechanically flexible
devices makes polymeric materials even more attractive. Holeꢀ
transporting(semi)conducting polymers with substantial redox
activity and electronic conductivity have been long recognized as
electrode materials for batteries, supercapacitors, and thermoelecꢀ
trics.2 However, allꢀpolymer devices of this type have been diffiꢀ
cult to realize due to the limitations of electronꢀtransporting polꢀ
ymers.3Two general classes of electronꢀtransporting polymers are
known: πꢀconjugated polymers and nonꢀconjugated redox polyꢀ
mers.2aElectronꢀtransporting(or nꢀtype) πꢀconjugated polymers
typically need multiple repeating units in the backbone to stabilize
injected electrons. Thus, the inability to stably and reversibly store
these chargeslimits thenꢀdoping level,4 which in turn reduces the
amount of free electrons and therefore restricts the electronic conꢀ
ductivity. On the other hand, nonꢀconjugated redox polymers have
dedicated redox active sites accepting one or more electrons per
repeating unit,5 but lack a πꢀconjugated backbone as found in πꢀ
conjugated polymers, which is crucial for efficient electron conꢀ
duction. Since the advantages of the two classes of polymers perꢀ
fectly complement each other, the weakness of both polymers
could be addressed bya rational combination of the characteristics
from each class,such as a “πꢀconjugated redox polymer” (Figure
P(NDI2OD-TET)
P(NDI2OD-T2)
Figure 1. (a) Graphical illustration of the structural characteristics
of πꢀconjugated polymers, redox polymers, and πꢀconjugated
redox polymers. (b) Molecular structure of the nonꢀconjugated
P(NDI2ODꢀTET) and the πꢀconjugated P(NDI2ODꢀT2).
Herein we report the first demonstration ofa πꢀconjugated reꢀ
dox
polymer
inpoly{[N,N’ꢀbis(2ꢀoctyldodecyl)ꢀ1,4,5,8ꢀ
naphthalenedicarboximideꢀ2,6ꢀdiyl]ꢀaltꢀ5,5’ꢀ(2,2’ꢀbithiophene)}
(P(NDI2ODꢀT2)),whichcan be stably and reversibly nꢀdoped to a
high doping level (2.0) and exhibits significantelectronic conducꢀ
tivity, enabling the realization of ultraꢀfast rechargeable Li batterꢀ
ies (Figure 1b). P(NDI2ODꢀT2) is an electronꢀtransporting πꢀ
conjugated polymer extensively studied in organic electronics, but
only in its intrinsic and slightly nꢀdoped states (doping level ≤
0.1).8Our results demonstrate that P(NDI2ODꢀT2) meets key criꢀ
teria for an efficient πꢀconjugated redox polymer: a linear πꢀ
conjugated alternating naphthalene−bithiophene backbone providꢀ
ing an electron transport pathway and a naphthalenedicarboximide
(NDI) unit, which is a highly reversible twoꢀelectron accepꢀ
tor,5bensuring a high nꢀdoping level. The advantage of a πꢀ
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conjugated redox polymer as an energy storage material over a
nonꢀconjugated one is highlighted by the design of a control polꢀ
ymer, poly{[N,N’ꢀbis(2ꢀoctyldodecyl)ꢀ1,4,5,8ꢀ
naphthalenedicarboximideꢀ2,6ꢀdiyl]ꢀaltꢀ5,5’ꢀ[2,2’ꢀ(1,2ꢀ
ethanyl)bithiophene]} (P(NDI2ODꢀTET, for synthetic details see
SI)), where a saturated –CH2ꢀCH2ꢀgroup is inserted in between
the two thienylunits to break polymer backboneπꢀ
conjugation,leaving all the other P(NDI2ODꢀT2) structural comꢀ
ponents unaltered (Fig. 1b).
shown inFigure 3a,the capacity retention at 10C (12 min per cyꢀ
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cle) is 99% and 80% for P(NDI2ODꢀT2) and P(NDI2ODꢀTET),
respectively, whereas at the ultrafast rate of 500C (14 s per cycle)
the capacity difference between the two polymers is larger (79%
and 43%, respectively). The potential difference between charge
and dischargecurves is also smaller for P(NDI2ODꢀT2) at all Cꢀ
rates(Figure S1 in SI). Clearly, P(NDI2ODꢀT2) exhibits much
faster electrode kinetics than that of P(NDI2ODꢀTET).
a)
b)
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P(NDI2OD-TET)
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Specific capacity (mAh g−
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Doping level
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Dopant ratio (mol.%)
Doping level
Doping level
Figure 3.Electrochemical measurements for the indicated polyꢀ
mers.(a) Capacity retention of Li cells at increased
charge−discharge rates. (b) Nyquist plot from EIS measurements
of Li cells at a depthꢀofꢀdischarge of 25%. (c) Solidꢀstate Li diffuꢀ
sivity measured with GITT at different doping levels. (d) Elecꢀ
tronic conductivity of polymer films without carbon additiveindifꢀ
ferent chemically nꢀdoped states.
Figure 2. (a) Twoꢀstep nꢀdoping/lithiation mechanism of the
naphthalene diimide unit. (b,c) Potential profile of P(NDI2ODꢀ
TET) (b) and P(NDI2ODꢀT2) (c) during nꢀdoping−undoping at
1C (2 h per cycle).
The vis-à-visnꢀdopability of the two polymers was investigated
using an in situ electrochemical doping method. This method
enables precise control of the doping process and monitoring of
the property change of the polymer. Li is an idealdoping reagent
because the small size of Li+prevents phase segregation (vide
infra). For this purpose, coinꢀtype rechargeable Li cells were fabꢀ
ricated with a polymer/conductive carbon mixture as the working
electrode and metallic Li as the counter and reference electrode
(see SI). Polymer/carbon black composite films containing 60
wt% of polymer with an active mass of 0.3 mg cm−2 was galꢀ
vanostaticallycycled in a Li+ꢀcontaining ethereal electrolyte soluꢀ
tion to realize the redox process. Every NDI unit of our polymers
contains four carbonyl groups, two of which are expected to be
reversibly reduced via a twoꢀstep twoꢀlithium addition reaction
(Figure 2a; further reduction of more carbonyl groups leads
toirreversible transformation of the NDI structure).9During the
twoꢀcarbonyl reduction, two electrons will be delocalizedin the
polymer backbone repeating units, corresponding to an nꢀdoping
level of ~2.0. Figure 2b,c shows the potential profile during the nꢀ
doping−undoping process, demonstrating that both polymers unꢀ
dergo twoꢀstep reduction. The two stages are less discernable for
P(NDI2ODꢀT2), reflecting the charge delocalization by the πꢀ
conjugated backbone.Judging from the charge injected during nꢀ
doping, both polymers roughly complete the twoꢀelectron reducꢀ
tion, leading to high nꢀdoping levels of 1.8 and 2.0 for
P(NDI2ODꢀTET) and P(NDI2ODꢀT2), respectively.If considered
as a Li storage material, the specific capacity of P(NDI2ODꢀT2) is
54.2 mAh g−1, or 100% of its theoretical capacity (54.2 mAh g−1;
calculated for a 2ꢀelectron redox reaction), whereas the specific
capacity of P(NDI2ODꢀTET) is 47.4 mAh g−1, or 90% of its theoꢀ
retical capacity(52.7 mAh g−1).Thus,the πꢀconjugated backbone of
P(NDI2ODꢀT2) does not decrease the nꢀdopability.
The kinetics of asolidꢀstate batteryelectrode isdetermined by
three major parameters: (1) reaction rate of the redox site, (2)
solidꢀstate ion (Li+)transport, and (3) electronic conduction. Thus,
we havemeasured these parameters for our polymers to underꢀ
stand the origin of the fast electrode kinetics of P(NDI2ODꢀT2).
First, the redox activity was probed with electrochemical impedꢀ
ance spectroscopy (EIS). The obtained Nyquist plots (Figure 3b)
for the two polymers at a depthꢀofꢀdischarge of 25% (correspondꢀ
ing to a doping level of 0.5) show semicircles of about the same
size in the lowꢀ to midꢀfrequency region, which can be fitted to
yield almost identical charge transfer resistance (Rct = 206.7 and
210.4 ꢀ for P(NDI2ODꢀT2) and P(NDI2ODꢀTET), respectively).
The Rct values for the two polymers remain very similar(235.5
and 223.8 ꢀ) at a depthꢀofꢀdischarge of 75% (corresponding to a
doping level of 1.5). SinceRctis a measure of the electrochemical
reactivity of an electrode material at a given discharged state,the
almost identical values indicate no obvious difference in the reacꢀ
tion rate for the two polymers inany nꢀdoped state. Second, the
solidꢀstate Li diffusivity was assessed with galvanostatic intermitꢀ
tent titration technique (GITT), and the calculated diffusivity valꢀ
ues are plotted against doping level (Figure 3c). The Li diffusiviꢀ
ty−doping level dependence is similar for both polymers. The
average Li diffusivitiesare1.45 × 10−11 and 3.38 × 10−11 cm2 s−1
for P(NDI2ODꢀTET) and P(NDI2ODꢀT2), respectively, again
very close to each other. These similarities are not unexpected
given the largely identical functional groups in the two polymers.
The slight advantage of P(NDI2ODꢀTET) over P(NDI2ODꢀT2) in
Li diffusivity may be due to the less efficient stacking
ofP(NDI2ODꢀTET) compared with P(NDI2ODꢀT2) (Figure S2 in
SI), which allows faster Li diffusion.Finally, the film electronic
conductivity of our polymers was measured forchemically doped
samples. Note, conductive carbon was not used in these measureꢀ
The electrode kinetics of P(NDI2ODꢀT2) and P(NDI2ODꢀTET)
were compared viafast charge−discharge measurements. As
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mentsto extract the intrinsic conductivity of the polymers. Both
need for the construction of porous polymer structures, incorporaꢀ
pristine polymers exhibit a conductivity of ~10−8 S cm−1. Howevꢀ
er, when reduced with progressively larger amounts of the molecꢀ
ular dopant 4ꢀ(1,3ꢀdimethylꢀ2,3ꢀdihydroꢀ1Hꢀbenzimidazolꢀ2ꢀyl)ꢀ
N,Nꢀdiphenylaniline (NꢀDPBI),10 the conductivity of P(NDI2ODꢀ
T2) rapidly increases by five orders of magnitude to ~10−3 S cm−1,
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tion of nanostructured 1ꢀ/2ꢀ/3ꢀdimensional conducting reagents, or
nanoscale morphology control, which are strategies usually necesꢀ
sary to achieve fast kinetics for organic electrode materials.5c, 11
The fact that P(NDI2ODꢀT2) exhibits the best electrode kinetics
despite the presence of significant amount (~57% by mass; >65%
by volume from DFT computations) of redoxꢀinactive/insulating
alkyl chains in the molecular structure fully demonstrates the
potential of this molecular design for energy storage purposes.
whereas that of P(NDI2ODꢀTET) only increases slightly (~10−7
S
cm−1). This data indicate that intraꢀchain charge transport within
the πꢀconjugated backbone of P(NDI2ODꢀT2) dramatically enꢀ
hances electronic conductivity whereas that of P(NDI2ODꢀTET)
must rely only on the less efficient interchain charge hopping.In
our lithium cells, although the addition of conductive carbon will
increase the apparent conductivity of electrodes based on both
polymers, the electron transport within the polymer domain will
be still dominated by the intrinsic conductivity of the doped polꢀ
ymers. The combined results demonstrate thatthe presence of the
πꢀinsulating ethanylgroup in P(NDI2ODꢀTET) does not alter the
redox activity nor the ionic conductivity, and that the highly conꢀ
ducting nature of the heavily nꢀdoped P(NDI2ODꢀT2) is unamꢀ
biguously responsible for theultrafast energy storage capability.
Table1. Comparison of the rate capability of P(NDI2OD-T2)
with those of state-of-the-art electrode materials.
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Doping
type
Active Capacity
Compound
Ref.
ratioa
60%
80%
60%
55%
50%
retentionb
95% (100C)
81% (10C)
28% (80C)
78% (100C)
71% (50C)
This
work
[12]
[11a]
[11b]
P(NDI2ODꢀT2)
n
PTCDAꢀPI2
PVK
PTMA
n
p
p
Li4C6O6
n
80%
27% (6C)
[13]
aOnly the results obtained with an active mass ratio of ≥ 50% andnonꢀ
b
thinꢀfilm devices (e.g. thickness > 1 ꢁm) are included. Relative to the
highest capacity recorded at low rates. Only the results obtained at ≥ 5C
(for active ratio ≥ 50%) or ≥ 50C (for active ratio ≥ 80%) are included.
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
105
100
95
P(NDI2OD-T2), 10C
Figure 4. AFM height images of (a) pristine P(NDI2ODꢀT2) thin
film, (b) P(NDI2ODꢀT2) thin film doped with 28 mol% of Nꢀ
DPBI, and (c) P(NDI2ODꢀT2) thin film doped with 200 mol% of
Li ion. Scale bar: 500 nm. The rms roughness for three films is
1.07, 1.33, and 5.14 nm, respectively.
90
85
80
0
500 1000 1500 2000 2500 3000
Cycle number
We believe that the electronic conductivity saturation observed
in Figure 3d for P(NDI2ODꢀT2)atNꢀDPBI molar doping ratio >
15mol%is due to inefficiencies of the dopant rather than an intrinꢀ
sic limitation of the polymer. Indeed, Figure 4 compares the morꢀ
phology of the pristine P(NDI2ODꢀT2) film, the chemically
doped polymer film with28 mol% of NꢀDPBI, and the electroꢀ
chemically doped polymer film with200 mol% of Liꢀion. The
pristine film shows fiberꢀlike structure as previously reported.8aNꢀ
DPBIꢀdoped film at 28 mol% shows grainy aggregates, indicatꢀ
ingsignificant phase segregation of the molecular dopant. This
observation is in agreement with previous studiesshowing similar
phase segregation of molecular dopants in the polymer host films
for NꢀDPBI dopant ratio >10 mol%,8b thus precluding further
doping. In contrast, the electrochemically Li+ꢀdoped P(NDI2ODꢀ
T2) filmexhibit negligible phase segregation and the increased
surface roughness is due to the exceptionally large(200
mol%)dopant content(Figure 4c). Because of the much higher
doping level, it is expected that Liꢀionꢀdoped filmsshould exhibit
similar trends but with far higher conductivities than those measꢀ
ured for the NꢀDPBIꢀdoped films in Figure 3d.
Figure 5.Reversibility of P(NDI2ODT2) during repeated deep nꢀ
doping−undoping at 10C (12 min per cycle).
The stability of P(NDI2ODꢀT2) when heavily nꢀdoped and the
reversibility of the doping process were investigated through reꢀ
peated doping/undoping cycles. Impressively, after 3000 cycles to
a doping level of 2.0 and complete undoping to the intrinsic state,
the dopability of P(NDI2ODꢀT2) remains intact (Figure5), with a
capacity retention of 96% and an average coulombic efficiency >
99.95%. These results demonstrate for the first time that πꢀ
conjugated redox polymers can be stably and reversibly nꢀdoped
to such high levels. As a charge storage material, the specific
capacity of P(NDI2ODꢀT2) (~54 mAh g−1) is lower than that of
inorganic counterparts due tothe unoptimizedlong solubilizing
alkyl chains that do not contribute to redox reaction. Thus, imꢀ
proved molecular design of πꢀconjugated redox polyꢀ
mersshouldresult in significant improvement in energy density.
In summary,we have successfully demonstrated that πꢀ
conjugated redox polymers are a class of heavily nꢀdopable conꢀ
ducting polymers combining the high redox activity of redox polꢀ
ymers andthe high electronic conductivity of πꢀconjugated polyꢀ
mers. A model polymer, P(NDI2ODꢀT2), was stably and reversiꢀ
bly nꢀdoped to a high doping level of 2.0, a significant progressfor
electronꢀtransportingπꢀconjugated polymers. The high electronic
conductivity of the heavily nꢀdoped P(NDI2ODꢀT2) leads to unꢀ
precedented electrode kinetics when evaluated as an organic elecꢀ
trode material for rechargeable Li batteries. With rational molecuꢀ
lar design, πꢀconjugated redox polymers will establish new design
space in polymer chemistry and see wideꢀspread applications
especially in energyꢀrelated ones such as batteries, supercapaciꢀ
tors, and thermoelectrics.14
The improved redox kinetics of P(NDI2ODꢀT2) encouraged us
to explore itsfast charge−discharge capability. Electrodes with
higher loading (up to 1.3 mg cm−2; see Figure S3 in SI) and higher
mass ratio (up to 80 wt% active mass; see Figure S4 in SI) were
fabricated and the results are compared to the stateꢀofꢀtheꢀart orꢀ
ganic electrode materials (Table S1).For relatively “standard”
highꢀrate conditions (~1 mg cm−2, ~60 wt% active mass, ~10C),
very few materials approach the capacity retention level of
P(NDI2ODꢀT2). Under more harsh testing conditions such as an
ultraꢀhighꢀrate of 100C (72 s per cycle) and a high active mass
ratio of ≥80 wt% (unusually high for organic electrode materials),
the superiority of P(NDI2ODꢀT2) becomes even more evident
(Table 1).Note that all these results were achieved without the
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5341.(i) Zhu, Z.; Hong, M.; Guo, D.; Shi, J.; Tao, Z.; Chen, J. J.
Am. Chem. Soc. 2014, 136, 16461.
ASSOCIATED CONTENT
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Supporting Information
(6).(a) Li, Y.; Zhan, H.; Kong, L.; Zhan, C.; Zhou, Y.
Electrochem. Commun. 2007, 9, 1217.(b) Levi, M. D.; Demadrille,
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Details on material synthesis and electrochemical measurements,
detailed electrochemical measurement results.This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
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yyao4@uh.edu (Y.Y.), afacchetti@polyera.com (A.F.)
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Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
We acknowledge the startup funding from the University of Houꢀ
ston. We thank Dr. Yufeng Zhao for performing the AFM measꢀ
urement for the polymer thin film samples.
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