Lithium-Conducting

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Efficient Transport Networks in a Dual Electron/LithiumConducting Polymeric Composite for Electrochemical Applications Michael McDonald, and Paula T. Hammond ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01519 • Publication Date (Web): 16 Apr 2018 Downloaded from http://pubs.acs.org on April 16, 2018

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ACS Applied Materials & Interfaces

Efficient Transport Networks in a Dual Electron/Lithium-Conducting Polymeric Composite for Electrochemical Applications Michael B. McDonald and Paula T. Hammond* Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, MA 02139 USA Keywords: Mixed Conductors, Conductive Binders, Lithium Battery Electrodes, Solid State Electrolytes, Conducting Polymers, PEDOT

ABSTRACT:

In this work, an all-functional polymer material composed of the electrically

conductive poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonic acid) (PEDOT:PSS) and lithium-conducting poly(ethylene oxide) (PEO) was developed to form a dual conductor for three-dimensional electrodes in electrochemical applications. The composite exhibits enhanced ionic conductivity (~10-4 S cm-1) and, counterintuitively, electronic conductivity (~45 S cm-1) with increasing PEO proportion, optimal at a monomer ratio of 20:1 PEO:PEDOT. Microscopy reveals a unique morphology, where PSS interacts favorably with PEO, destabilizing PEDOT to associate into highly branched, interconnected networks that allow for more efficient electronic transport despite relatively low concentration. Thermal and x-ray techniques affirm the PSS-PEO domain suppresses crytallinity, explaining the high ionic conductivity. Electrochemical

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experiments in lithium cell environments indicate stability as a function of cycling and improved overpotential due to dual transport characteristics despite known issues with both individual components.

1. INTRODUCTION

material

components

that

facilitate

Electrochemical technologies, such as those

movement of charges throughout them.

the

for production of value-added chemicals (e.g.

In addition to enhancing the intrinsic

CH3OH from CO2 reduction1 and H2 from

conductivities of the components contained in

electrolysis of H2O2) and especially energy

the electrode and electrolyte phases of a cell,

storage (e.g. batteries3 and fuel cells4), require

the number and composition of non-active

innovations that improve portability, energy

components in a cell should be minimized. In

efficiency/capability and cost.5 The usage of

particular, an electrode phase that is three-

electrochemical

dimensional

technology

has

become

(thick)

requires

efficient

increasingly prolific, with mainly batteries

transport of both electrons and ions, unlike

utilized in mobile telephones and laptop

the electrolyte and electric circuit phases.

computers,6 and emerging integration in

Examples of this requirement are present in

electric vehicles7 and the renewable energy

such systems as proton exchange membrane

grid.8

is

fuel cells (PEMFCs), where several different

fundamentally distinguished from regular

layers compose the electrode and incorporate

chemical reactions by the spatial mitigation of

gas

both electrons and ions from the reactions for

components nanostructured to conductive

control of the flow of energy. Henceforth,

carbon-proton exchange polymer (inactive)

there is required advancement in the various

components

Electrochemical

technology

diffusion

at

and

the

catalyst

(active)

electrode-electrolyte

interface for electron and ion transport.9 Here,


2

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this inactive component is essentially a dual

materials selected will determine performance

electron/proton conductor, without which the

characteristics;

electrode kinetics would be severely limited.

materials are generally resistive to electron

These dual conductive materials are also of

and

significant interest for solid oxide fuel cell

composed entirely of storage materials are

(SOFC) electrodes,10 and are also studied

therefore not possible, and so the storage

more generally in a variety of fields.11

materials must be in particulate form in a

lithium

however,

ion

LIB

transport.13

storage

Electrodes

been

conductive-structural matrix to permit short

extensively innovated to match reactants with

diffusion lengths for electrons and ions

both electrons and ions in PEMFCs and

between

SOFCs, three-dimensional electrodes are also

collection (Figure 1a).14 Since the binder

desirable

battery

phase is necessary for effective transport of

technology, such as the lithium-ion battery

charge, it therefore determines power density,

(LIB), which is selected to power the vast

but also adds inactive mass, and so also

majority of emerging portable electronics due

controls energy density (capacity).

While

these

interfaces

for

have

lithium-based

to its high energy density and excellent rechargeability

properties.12

intercalation/deintercalation

and

In LIBs and other similar electrode

active

systems, electronic conductivity is usually

component in an LIB is the storage material

achieved through conductive carbon (CC)

(e.g.

LiFePO4,

additives which, at sufficient concentrations

LiNiMnCoO2) in the electrodes, which holds

(5 – 30 wt%), form a critical number of

and

via

percolative pathways.15 These pathways,

The

along with the storage material particles, must

graphite,

The

LiCoO2,

releases


charge of

Li+.

combination of the anode and cathode storage


3


Page 4 of 34

Figure 1. (a) Conventional LIB electrode containing storage material particles (purple) and conductive carbon particles (black) in a polymer binder. Electrons and Li ions move between the storage material, and collector and electrolyte, respectively. Electron transport relies on pathways randomly formed between conductive carbon particles; (b) chemical structure of PEDOT:PSS; (c) chemical structure of PEO with oxygen lone electron pairs interacting with Li ions. themselves be suspended in a structural

The

limitations

of

this

conventional

binder, often an inert polymer.16 This polymer

approach to access the active portion of

binder adds inactive mass and also blocks CC

electrodes present an opportunity to integrate

transport pathways. Because the pathways are

new electron and ion-conducting materials in

limited by length, efficient transport is

place of conductive carbon-inert polymer

ensured by casting thin layers17 of slurry

composites. To reduce the number of

containing all components on metal foil

components, an ideal material will possess

current collectors.18 Ionic conductivity is

dual conductivity (e.g. transporting both

gained via the wetting of porous regions of

electrons and Li+), as well as adhesive

the electrode with liquid electrolyte, and is

properties to bind active material particulates,

often limited by the electrode morphology.

in a single material. Although analogous to


4

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modern PEMFC electrodes that interface the

1b), which helps solubilize PEDOT and

active component to the electrolyte and

compensates

circuit, bulk carbon fiber is not practical or

PEDOT:PSS has recently been studied as a

efficient to form the electron transport

functional binder in LIB electrodes,22 and was

network, and lithium conductivity has not

shown to decrease the porosity of the

been

electrode (inactive space between storage

specifically

considered

for

such

its

charged

backbone.

material particles) and the cell overpotential

materials. One method to replace CC percolation is

for

lithium

intercalation/deintercalation,

also

enhancing rate capability (~80% at 5C).23

conductive. This ‘functional binder’ approach

This system outperformed the conventional

attempts

and

CC/polyvinylidene fluoride (PVDF), a result

and so in principle the

of the longer-range, continuous conductive

inactive material can be reduced and energy

networks formed from a polymer. Unlike CC

density increased. One class of material that

particles that are incorporated based on

has recently been investigated for this

percolation thresholds, conductive polymers

approach

make up most of the bulk of conductive

with

structural

binders

to

conductivity,19,

combine 20

is

polymers.

that

adhesion

electronically Of

are

conducting

these,

poly(3,4-

ethylenedioxythiophene) (PEDOT) is the most

attractive

conductivity,

due

chemical

to

its

excellent

stability,

binders,

leading

to

continuous

charge

pathways throughout the material. Specific Li+ transport is typically ignored in

and

both battery electrode binders as well as dual

processability.21 PEDOT is commercially

conductors as a whole. This can also be

available in an aqueous dispersion with

specifically addressed in a functional binder

poly(4-styrenesulfonic acid) (PSS) (Figure

using polymers. Polyethylene oxide (PEO,


5


Page 6 of 34

Figure 1c) is the most widely studied Li+

In this work, quantities of PEO are

conductive polymer for lithium battery solid

combined with PEDOT:PSS to form a dual

electrolytes.24

conductive,

Its

conductivity

in

the

all-polymer

composite

for

amorphous state arises from ion-dipole

applications requiring both electronic and Li+

interactions between Li+ and lone pair

conductivity, as well as mechanical properties

electrons on PEO oxygen, along with its

including adhesion, structure and flexibility,

flexible chains that allow ion mobility.25 PEO

such as a highly functional binder in lithium-

has previously been combined with PEDOT

based battery electrodes. Furthermore, the

with

electronic

processing is water-based, eliminating toxic,

conductivity both in small quantities,26 and in

high-cost organic solvents.30 The scope of

large quantities to, for example, improve

this work involves the thorough materials

mechanical properties for electro-spinning

characterization of the composite, followed

applications27 and form a mixed conductor for

by an electrochemical analysis of the

the

intent

electrochemical

to

enhance

supercapacitors.28

In

composite,

its

parent

materials,

and

addition, PEO has also been combined

conventional binder material under vigorous

previously with another conducting polymer,

LIB cell conditions to evaluate potential for

poly(pyrrole), as a coating for LIB cathode

lithium cell electrode/binder applications (in

storage materials to successfully access

the absence of storage materials). The

greater capacity.29 Exploiting the greater

development

mechanical

of

(“PEDOT-PEO”) has potential to minimize

PEDOT:PSS longer-chain PEO in a bulk

the binder-active material ratio while also

matrix compared to a coating approach is the

improving

next logical progression.

properties simultaneously to enhance energy

and

transport

properties

of

this

electron

material

and

ion

concept

transport


6

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density, power density and cyclability in

PEDOT:PSS and PEO stock solutions were

electrochemical systems. In addition to

combined

conductivity

resulting

vigorously for ~3 min using a vortex mixer.

morphologies are thoroughly characterized to

Solutions could be drop-cast onto the desired

unravel the mechanism of dual charge

substrate (glass, conductive glass, metal

transport. This novel all-polymer electrode

spacers) and dried at room temperature

binder is (elemental) carbon-free, and is

overnight, followed by an additional 4 hr at

shown to have special dual conductive

100°C under vacuum. Solutions were always

properties.

drop-cast immediately after mixing. The

optimization,

the

in

sample

vials

and

mixed

2. METHODS

conventional electrode matrix consisting of

2.1. Synthesis of materials. PEDOT:PSS

CC-binder was fabricated by combining

was purchased from Ossila Ltd. under the

Super P (Alfa Aesar) with PVDF (Alfa

trade name Heraeus CleviosTM PH 1000,

Aesar)

which came as a 1.0–1.3 wt% (1:2.5

methylpyrrolidone (NMP) to form a solution

PEDOT:PSS ratio) aqueous dispersion. PEO

of the same weight percent as the polymer

(MW=5 MDa, “PEO-5M”) was purchased

mixtures herein, followed by 20 min pulsed

from Sigma-Aldrich and was dissolved in

ultrasonication (Misonix).

in

a

2:1

ratio

with

N-

distilled water by heating and stirring to make

2.2. Physical Characterization. Electronic

a 0.01 g mL-1 50/50 v/v water/methanol

conductivity was measured on samples drop-

solution. PEO of other molecular weights

cast on glass pieces using a Signatone S3042

were also purchased from Sigma-Aldrich and

four point probe with Keithley SCS4200

were used in select confirmation experiments

current source and voltage measurement

(e.g. Figure S2). Desired proportions of the


7


Page 8 of 34

digital interface, and calculated using the

analyzer, and was oscillated with a 10 mV ac

standard equation (1),

perturbation (no dc control) from 105–10-1 Hz. The ionic resistance was deduced from

= 1/[( /ln (2)) × (/)

the width of the 45° high-frequency region of

× ] (1) where σ is the conductivity; V/I is the inverse

the resulting Nyquist plots (Figure S1)

slope of the resulting I-V curve by applying

between the high- and (extrapolated) low-

current, I, across the outer two probes and

frequency intercepts (Zreal), and accounting

measuring the voltage drop, V, between the

for electrode area (A) and thickness (t),

inner two probes; and t is the film thickness.

calculated from equation (2).31

Ionic conductivity was measured on samples drop-cast on a 1 cm2 masked area of

= /(3(() − (ℎℎ))

conductive glass electrode pieces (“TEC 15”,

× ) (2)

Hartford Glass Inc., Hartford, IN). Sample open

The distance on the impedance plane between

electrochemical cell containing 0.5 M LiClO4

the origin and the high frequency intercept is

in propylene carbonate to mimic the inert

commonly

electrolyte used in LIB cells, with the

resistance, which is not included in the

working electrode lead attached to the

calculation.

electrodes

were

placed

in

an

attributed

to

the

solution

and

Film thicknesses were measured using a

reference/counter electrode lead attached to

Dektak 150 Surface Profiler. Transmission

an epoxy-sealed platinum foil in a 2-electrode

electron microscopy (TEM) images were

configuration. The cell was connected to a

taken from a FEI Tecnai G2 Spirit Twin at

Solartron

120 keV accelerating voltage with a Gatan

exposed

conductive

1255B

glass,

frequency

response


8

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CCD camera. TEM samples were prepared by

Differential scanning Calorimetry (DSC)

drop-casting thick films of polymers and

thermograms were collected on a TA

cutting them into small pieces with a razor

Instruments Discovery calorimeter from 25°C

blade, which were then glued onto a stub

– 225°C in aluminum T-zero pans containing

head. The stub was placed in a Leica UC7

drop-cast

ultramicrotome

cryochamber

substrates and cut into small pieces with a

accessory. Glass knives were cut with a Leica

razor blade. Wide-angle x-ray diffraction

EM KMR3 Knife Maker. The chamber, knife,

(WAXD) samples were prepared by drop-

and sample were allowed to equilibrate to -45

casting materials on glass pieces, and were

°C (near the glass transition temperature for

measured on a Bruker D8 General Area

PEO31) for 15 min before advancing the knife

Detector Diffraction System (GADDS) with a

stage at the sample. The sample was rocked at

0.5 mm collimator.

with

FC7

samples

removed

from

glass

a rate of 0.1 cm s-1, and the knife was

2.3. Electrochemical Characterization.

advanced to cut 40 nm thick slices. Sample

Samples were assembled into cathodes using

flakes were collected from the knife edge

CR2016 coin cell parts (Pred Materials

using a loop with water, followed by

International, Inc. New York) by drop-casting

subsequent

lacey

0.5 mL sample solutions onto coin cell

carbon/copper grids (Ted Pella). Atomic force

spacers directly. Spacers were dried overnight

microscopy (AFM) images of materials drop-

at room temperature, followed by drying

cast on glass were gathered on a Veeco

overnight at 100°C under vacuum to remove

Nanoscope V with a Dimension 3100 D3005-

all water possible. Spacers were assembled

1 detector using a Bruker cantilever (k = 40 N

into coin cells in an argon glove box

m-1) in tapping mode at 4 μm s-1 scan rate.

(MBRAUN) (water-free) with Celgard 2400

deposition

onto


9


Page 10 of 34

polyethylene separator and 1 M LiPF6 in 1:1

from 0.5 to 300 PEO:PEDOT by monomer

ethylene

carbonate

from their aqueous solutions. Upon vigorous

solvent electrolyte system (BASF), in the

mixing, the homogeneous aqueous solutions

order of: 20 μL electrolyte, separator, 20 μL

could be easily cast onto the desired substrate

electrolyte, separator, 20 μL electrolyte,

under ambient conditions. A library of

followed by a lithium metal counter electrode

varying PEO:PEDOT composites were cast

(0.75 mm thickness, 99.9%, Alfa Aesar).

on conductive and non-conductive glass to

Galvanostatic cycling was carried out on a

measure

Solartron 1470E battery cycler, allowing

electrochemical

charging for 12 hr/5 V and indefinite

(EIS) and electronic conductivity by four

discharge time to 1.5 V. Cyclic voltammetry

point probe, respectively. The values obtained

(CV) was performed on the coin cells using a

are shown in Figure 2. The ionic conductivity

EG&G Princeton Applied Research 263A

was calculated from the resulting Nyquist

potentiostat from 1.5 V to 4 V at a scan rate

plots (Figure S1) using an established EIS

of 0.1 mV s-1. All potentials reported herein

model for mixed ionic-electronic conductors,

are relative to Li/Li+.

which assumes the electronic conductivity is

carbonate/dimethyl

the

significantly

ionic

conductivity

impedance

greater

than

by

spectroscopy

the

ionic

3. RESULTS AND DISCUSSION

conductivity.33-35 According to Figure 2, this

3.1. Charge Transport Properties. It is

assumption is valid, and therefore it is

expected that blending a conductor and an

reasonable to consider the values calculated

insulator for a given carrier will lower the

from the EIS method reflect only ion

conduction of the other carrier. PEO and

transport. In addition, it can be assumed that

PEDOT:PSS were combined in varying ratios

the observed impedance is due exclusively to


10

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compensating the excess sulfonic acid groups on PSS are driven out of the composite matrix when the material (PEDOT) is positively polarized because PSS is immobilized, and are likewise drawn into the matrix upon negative polarization (transport of anions is excluded). The ionic conductivity of pure Figure 2. Electronic (squares) and ionic

PEDOT:PSS is measured to be 1×10-5 S cm-

(triangles) conductivities of varying PEDOT-

1

PEO polymer composite ratios (light blue),

conductors.37 No other studies were found

PEDOT:PSS (dark blue), and 2:1 Super P-

that analyzed Li+ transport in PEDOT. When

PVDF (black).

PEO is incorporated, the ionic conductivity

the working electrode, which is relatively

steadily increases, and peaks at 8×10-5 S cm-

thick and porous compared to the planar

1

, which is comparable to known Li+

for the 20:1 PEO:PEDOT monomer

platinum counter electrode. While ClO4- is

combination. This ratio amounts to 64 wt%

also present in the test electrolyte, it is

PEO, and is in good agreement with reported

expected to be repelled by the large amount

values for pure PEO in its conductive state.25

of negatively charged PSS immobilized in the

However, beyond this point, the conductivity

polymer matrix, and so it is assumed that Li+

is not proportional to the amount of PEO

will be the dominant charges transported in

present, as the value declines for higher

this matieral.36 Here, acidic PSS charge-

loadings. The 300:1 PEO:PEDOT composite

compensated PEDOT in its native, positively

is 97 wt% PEO, and thus the ionic

charged conductive state. Cations (Li+)

conductivity with a

large presence of


11


Page 12 of 34

PEDOT:PSS is greater than nearly pure PEO

increase of more than 2 orders of magnitude.

in these conditions.

The conductivity decreases by half but pure

remains within this range when the ratio is yet

PEDOT:PSS was found to be 4.2×10-1 S cm-

again increased to 85:1, which yields a

1

material that contains 89 wt% insulating

The

electronic

conductivity

of

, which is in good agreement with literature as-

PEO. It is not until the PEO loading is

purchased PEDOT:PSS is in its conductive

increased to 97 wt% (only trace amounts of

state, validating the assumption of large

PEDOT) that the conductivity decreases by 5

electronic

ionic

orders of magnitude (< 1×10-4 S cm-1). This

conductivity for the EIS model (above). A

trend is reproducible for other molecular

standard conductive binder mixture, Super P

weights of PEO (400 kDa, 4 kDa, and 400

CC and PVDF structural polymer, were cast

Da- liquid at RT) (Figure S2). This electronic

from a 2:1 ratio in NMP and the material was

conductivity for the 20:1 composite is

found to have a lower conductivity than pure

improved by 2.5 orders of magnitude

PEDOT:PSS. As PEO is incorporated with

compared to the conventional matrix material

PEDOT:PSS, it is logical that the electronic

(1.6×10-1 S cm-1). Therefore, both the ionic

conductivity

will

the

and electronic conductivities are maximized

concentration

of

decreases,

at the 20:1 (64 wt%) PEO:PEDOT ratio. The

diminishing continuous transport pathways.

formation of a mixed conductor between

However, the conductivity is practically

PEDOT and PEO is in agreement with

unaffected when PEO:PEDOT < 10:1. When

previous work,28 and these measurements

the ratio increases to 20:1, the electronic

quantify the optimal values and expose a non-

conductivity surges to nearly 50 S cm-1, an

intuitive trend of improved ionic and

reports.21

This

also

indicates

conductivity

that

versus

decrease PEDOT

as


12

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Figure 3. TEM images with inset AFM-phase images (1 μm2) of a) PEDOT:PSS, b) 4:1 (25 wt%) composite, c) 20:1 (64 wt%) composite, and d) 85:1 (89 wt%) composite. The thick black lines in the TEM images are the lacey carbon grid upon which sample slices are suspended.


13


Page 14 of 34

electronic conductivity with a large proportion of

studies and has been interpreted as conductive

PEO incorporated, which is highly desirable in a

PEDOT globular domains encapsulated in a PSS

dual conductive material.

matrix,38,39 where the conductive and non-

3.2. Morphological and Structural Analysis.

conductive domains appear as bright and dark respectively.40

The 20:1 polymer composite possesses attractive

regions

but unexpected properties. Therefore, it is

Conversely, the more electron absorbing PEDOT

critical to understandthe resulting structural

represents the dark areas in the case of TEM

features underlying the enhanced functionality;

imaging. The concentration of PEDOT domains

specifically, why Li+ transport is higher in

is sufficient for forming long-range pathways for

PEDOT-PEO than pure PEO and why electron

conductivity throughout bulk films.

transport is improved when less electronic conductor

is

present.

Figure

3

shows

in

AFM

imaging,

Upon introduction of 25 wt% PEO, light (dark) regions in the TEM (AFM) expand, and

transmission electron microscopy

occupy a greater amount of space (Figure 3b).

(TEM) and atomic force microscopy AFM

This is the PEO favorably associating with the

(phase) imaging performed on composites of

PSS-rich phase, which is expected given their

varying PEO:PEDOT ratios in order to extricate

similar solvation properties and potential for

the morphological structure of the blends that

favorable ion-dipole interactions. The agreeable

might be responsible for transport phenomena.

interaction between PSS and PEO has been

PEDOT:PSS shows obvious phase separation

studied before in a similar system, and has been

using both techniques, indicating that although

shown to drive phase separation of PEDOT from

electrostatically bound, the PEDOT and PSS

the electronically insulating PSS chains when a

have unfavorable mixing parameters and form

small amount of low molecular weight PEO is

~100 nm globular-shaped conductive and non-

used as a dopant.26 These features are extended

conductive

This

when PEO is loaded to 64 wt% (Figure 3c), the

morphology is in agreement with previous

ratio giving the highest electronic and ionic

domains

(Figure

3a).


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conductivities. While the presence of the

at the lowest weight fractions of PEDOT,

PEDOT phase noticeably diminishes as PEO is

resulting still in enhanced conductivity, although

added, its domains are denser and more

reduced compared to the highly continuous

interconnected than pristine PEDOT:PSS. This is

morphology of the 20:1 composite.

especially evident in the 20:1 composite, where

It has been shown using numerous approaches

PEDOT ultimately forms a webbed morphology

that the electronic conductivity of PEDOT:PSS

throughout the frame examined. This structure

is enhanced with a small introduction of dopant

creates a completely connected pathway for the

in the form of ions, surfactants, ionic liquids and

flow of charge. Additionally, the increased

solvents.21 While the precise mechanism is not

electron density observed in TEM for the

fully elucidated, most studies attribute the

PEDOT domains at these concentrations suggest

phenomenon to effects by the PSS-dopant

denser and therefore more efficient packing of

interaction, which increases the unfavorable

PEDOT chains, which will also lower flow

solvation space and in turn removes steric and

resistance. When PEO is loaded to a 85:1 ratio

ionic

(89 wt%), the PSS-PEO phase dominates (Figure

formation.41 In cases where ethylene glycol and

3d). While there is a clear shift to a more

poly(ethylene glycol) (low molecular weight

disperse morphology in the TEM image, the

PEO) are included as the dopant, this effectively

AFM image shows that the remaining PEDOT is

allows for the ordering of PEDOT chains to a

further densely packed into a thin yet highly

more extended and linear secondary structure,

continuous

However,

and drives PEDOT domains into the randomly

compared to the 20:1 composite, the web

branched web morphology observed in Figure

structure is more branched, and, due to the very

3.42,43 This intensifies molecular interactions

low PEDOT concentration, more branch termini

between PEDOT chains which, forced to closer

are

proximity, induce hydrogen bonding and π–π

present.

web

These

morphology.

images

illustrate

that

interconnected phase domains are present even

shielding

stacking

among

of

the

PEDOT

during

conjugated

film

thiophene

15 ACS Paragon Plus Environment


Page 16 of 34

backbone. The linear PEDOT also prefers the quinoid conformation to the benzoid, common in pristine PEDOT:PSS, which has a higher degree of

electron

delocalization

and

hence

conductivity.44 Along with morphological domain structure, the degree of crystallinity strongly impacts the ion conductivity of PEO.25 This semi-crystalline polymer has a low glass transition temperature well below room temperature, but is typically highly crystalline in the neat form at room temperature, with a melting point of about 60° to 70°C.45 To gain further insight into the structurefunction

relationship,

thermal

and

x-ray

diffraction analyses were performed to determine the impact to the crystallinity and melt behavior of the resulting composite by blending with PEDOT:PSS (Figure 4). Differential scanning calorimetry (DSC) thermograms (Figure 4a)

Figure 4. a) DSC thermograms of 4:1 (light blue), 20:1 (green), 85:1 (orange), 300:1 (red) polymer composites, and PEO-5M (grey) (inset: magnification of 4:1 and 20:1 composite); b) wide-angle x-ray diffractograms of varying compositions of polymer composites.

were used to determine the (peak) melting

20:1 composite. This is a meaningful drop in

temperatures (Tm) (Table 1). PEO-5M has a

melt point and is anticipated due to blending of

measured (Tm) of 67.7 °C, which is in agreement

PEO with PSS chains. PEDOT:PSS alone does

with the literature value.46 Upon blending, Tm

not exhibit features in its thermogram as it is

steadily decreases in correlation with the amount

fully amorphous,47 which is in agreement with

of PEDOT:PSS, ultimately to 59.9 °C for the

previous analyses.48


Page 17 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Thermal properties of Polymer

1. Because there is a discrepancy for standard

Composites.

heat of fusion (ΔHf°) given a strong dependence on molecular weight,32 the values here are

PEO:PEDOT PEO monomer content ratio (wt%)

Tm (°C)

PEO-5M

100

67.7

--

300:1

97

64.2

+6.5

wt% and 11 wt% for the 300:1 and 85:1

85:1

89

61.9

+15.1

composites, respectively), the crystallinity is

20:1

64

59.9

-95.3

actually greater than pure PEO, which is the

4:1

25

--

-100

Crystallinity change (%)

calculated using PEO-5M as a standard. When PEDOT:PSS is present in small quantities (3

result of the measured values of ΔHf being larger than PEO. This implies that although the melt

It can therefore be inferred that PEDOT:PSS point is depressed, there is increased ordering destabilizes the intermolecular forces that drive within the phase-segregated domains that are crystallization because it interferes with PEOformed in the interconnected morphologies at PEO chain interactions. This is in agreement intermediate concentrations. Thus, more overall with the above morphology experiments, which energy is required to fully melt the material, determined that PSS interacts distinctly with which

strongly

suggests

that

different

PEO due to more favored thermodynamic intermolecular forces are introduced. Given solvation properties. Further, the disruption to previous evidence in this report, it is likely that the thermal stability of the PEO matrix by these are strong interactions between PEO and PEDOT networks on the nanoscale likely adds to the introduced PSS, even at these small the destabilization effect to decrease Tm. concentrations. This is supported by the fact that Because the composite consists of amorphous the larger crystallinity resides with the 85:1 and crystalline components, the crystallinity of composite compared to the 300:1 composite, PEDOT-PEO composites was calculated from which

contains

less

PEDOT:PSS.

When

the heat of fusion (ΔHf) values for melting events additional

PEDOT:PSS

is

introduced,

the

in the thermograms (Figure 4a), shown in Table 17 ACS Paragon Plus Environment


Page 18 of 34

crystallinity is drastically reduced (to 95% for

in the TEM and AFM micrographs in Figure 3.

the 20:1 composite, which still contains 64 wt%

This is also supported by the DSC thermograms,

crystallizable PEO). This suggests that there is a

which show no Tm peak for composites < 20:1

change in the nature of the PEO-PSS interaction,

(Figure 4a). For composites ≥ 20:1, an increasing

leading to very low crystallinity in a composite

excess of PEO crystallizes within the blend as

containing a highly crystallizable polymer. This

the concentration of PSS decreases and PEO is

finding is consistent with a previous report of

in excess. This mechanism is in agreement with

crystallinity depression with low molecular

the more isolated, spherically shaped domains

weight PEO crosslinked to PSS in a similar

observed for the 85:1 composite in the TEM

composite.27 These observations are supported

micrograph in Figure 3d, and is likely related to

using wide angle x-ray diffraction (WAXD)

the formation of spherulites in PEO-containing

(Figure 4b), which shows that the 20:1 ratio is

blends, as reported previously.42

the onset of observable crystallinity, given the

Of the

composites

examined,

both

the

formation of peaks near 23°, 26°, 32° and 36°,

transport properties and structure of the 20:1 (64

which coincide with peaks for pristine PEO.

wt%) ratio are the most interesting, since this

Composites of lower ratios (more PEDOT:PSS-

combination yields both the highest electronic

like) than the 20:1 composite exhibit only an

and ionic conductivity and possesses a structure

amorphous shoulder between 17° and 30°, while

that is nearing the point of crystallization of

composites of ≥ 20:1 ratio contain correlating

PEO. It is apparent that the presence of PEO is

peak intensities characteristic of pristine PEO

necessary to enhance conductivity, but only in

superimposed on the amorphous shoulder. From

the amorphous phase, where PEO and PSS

this trend, it can be inferred that composites

until

20:1),

the

when

onset the

of

ionic

including

this

and

harsh

work

concentrates

discharging

solvents,

on

processes.

high

the

The

potentials,

conductivity begins to decrease (in Figure 2).

reactive salts and exhaustive charge movement

The trends observed in these DSC and WAXD

throughout cycling, are taxing on the electrode

experiments also pointed to these conclusions

materials. Conducting polymers, in particular,

when the same experiment was performed using

are already known to undergo unfavorable



Page 20 of 34

processes at even modest potentials in aqueous

which displays negligible Li capacity, and

conditions, including overoxidation, whereby

PEDOT:PSS, which contains a slight discharge

electrochemical degradation of the polymer leads

plateau that indicates charge storage in the

to loss of electronic conductivity.49 Of this

electroactive thiophene backbone of PEDOT

family, PEDOT is known as the most stable,

(Figure S3).51 The capacity of the composite is

although reports have shown its degradation in

approximately 40% that of pure PEDOT:PSS,

similar conditions.50 Therefore, it is critical to

which is in agreement with the PEDOT:PSS

examine the response of the PEDOT-containing

concentration in this material (36 wt%). The faradaic efficiency of the composite, its

composite in specific LIB conditions. Lithium coin cells with various electrode

parent materials, and conventional binder system

materials were cycled between 1.5 and 5 V, the

(2:1 Super P-PVDF) as a function of cycling was

two extremes of all potential anodic and cathodic

measured in order to reveal electrochemical

storage

and

stability (Figure 5). Initially cycling slowly at 1

LiCoPO4/LixNi0.5Mn1.5O4, respectively),3 which

mA g-1, both the conventional binder and PEO

is a larger window than is typically examined in

are near-unit efficiency. PEDOT:PSS has a

the literature. Because dual conductors are likely

measured efficiency of 200%, suggesting that

to be incorporated into electrochemical systems

degradative processes are likely occurring to the

in a passive (e.g. non-redox) role, figures of

conjugated backbone under the highly oxidative

merit such as charge capacity and faradaic

conditions,50 resulting in freed charges being

efficiency for charge storage purposes are

introduced to the cell. The composite has an

unimportant.

useful

initial efficiency of ~90%, which implies a

indicators of electrochemical stability in a

different electrochemical process associated with

lithium cell environment. The PEDOT-PEO

its PEDOT content resulting in charge lost. Upon

composite exhibits a charge-discharge trace that

further cycling, PEDOT:PSS has < 100%

is qualitatively a compromise between PEO,

efficiency, going as low as 74% after 50 cycles

materials

However,

(graphite

these

are


Page 21 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


composite

exhibit

(extremely

rapid

immeasurable charge

and

cycling

discharge).

However, this does not mean that parasitic side reactions occur at the high cycle rate that irreversibly

degrade

the

materials,

as

its

conjugation (and therefore conductivity) is in tact, and its capacity does not fade considerably after this cycling treatment (not shown). It Figure 5. Faradaic efficiency at varying rates (in mA g-1) for CR2016 lithium coin cells containing PEO-5M (grey), PEDOT:PSS (blue), polymer composite (64 wt% PEO) (green), and

should be noted that the presence of water can also be the cause of electrochemical side processes that interfere with the charge and discharge processes and hence the faradaic efficiency.53 Although preemptive steps were

2:1 Super P-PVDF (no fill) electrodes.

built in to remove water (heating, vacuum, at

1

mA

-1

g ,

implying

a

change

in processing in argon atmosphere), water is known

electrochemical process to side reactions that

to be locked by H-bonding in PSS,54 explaining

result in a loss of charges. In contrast, PEO and the effects prominent in both the PEDOT:PSS the composite stabilize to near 100% efficiency, and composite samples. The conventional binder which is unexpected due to the reported material

undergoes

very

significant

side

instability of PEO at high potentials,52 and that reactions in response to the higher rates, with a the composite contains a large amount of susceptible PEDOT:PSS. When the rate is

low efficiency (19%) at 10 mA g-1 and > 100% efficiency (192%) at 100 mA g-1, implying

increased to 10 mA g-1, the polymer materials different electrochemical reactive modes that unexpectedly possess near-unit efficiency, and it result in diminished and added charges to the -1

is not until when a rate of 100 mA g is applied cell, respectively. Upon returning to the initial that both the PEO and 21 ACS Paragon Plus Environment


Page 22 of 34

rate of 1 mA g-1, all materials tested express near-unit

efficiency,

signifying

that

side

reactions cease and the materials stabilize. This behavior is akin to the formation of the solid electrolyte interphase (SEI layer), in which a stable layer must form as a result of reactions between the electrode and electrolyte during initial cycling before the electrode is stabilized

Figure 6. Fifth cycle of cyclic voltammograms

for the bulk of the cell lifetime.55 We emphasize

(0.1 mV s-1) of CR2016 lithium coin cells

that this experiment elucidates the stability of the

containing the 2:1 Super P-PVDF conventional

materials (reactivity of charges enterting/exiting

binder (black), pure PEDOT:PSS (blue), and the

the matrix) under electrochemical conditions and

polymer composite (64 wt% PEO) (green).

is not tied to their inherent conductivity. That

used as a storage material in LIB anodes and

these cells continue to operate after vigorous

therefore

cycling conditions implies their conductivity is

electrochemical

not detrimentally affected.

processes.56 The CV for this material exhibits

would

participate

in

the


To more closely examine electrochemical

minimal charging/discharging peak separation

events, cyclic voltammetry (CV) was performed

between 2.7–3.7 V, which presumably indicates

on fresh cells at a rate of 0.1 mV s-1. The

the lithium charge transfer between the material

resulting current response as a function of

and lithium metal electrodes. PEDOT:PSS

applied potential is shown in Figure 6. The

displays good stability in the presence of

conventional

electrolyte at high potentials. The charging peak

matrix

material

possesses

electroactive character from its capacitive-like

occurs at the same potential (3.5 V) as the Super

trace. This is expected, since the CC Super P is

P-PVDF. Because each cell examined possesses

similar to graphite, which is very commonly

the same anode (lithium metal), this charging


Page 23 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


peak is presumably plating of lithium onto the

rather than side reactions. The discharge peak is

counter electrode for all cases. The discharge

not observable in this voltage window, and was

peak that evolves at 2 V can therefore be

found to occur at an even lower potential than

associated with insertion of Li+ into PEDOT to

1.5 V (Figure S4), signifying that the superior

balance the electrochemical reduction of its

ionic and electronic transport pathways of the

conjugated backbone.57 This peak position is 1.5

composite

V lower relative to the charging process,

overpotentials.

meaning the conducting polymer facilitates

conductor will result in improved power density

charge

comparative

in Li-based electrochemical devices, since more

overpotential (this is different than the potential

facile movement of charges throughout the

at the start of discharge, which indicates the

electrode will allow for greater currents.

transfer

with

less

result

in

lower

discharge

This suggests that this dual

reduction potential). This is attributable to superior conductive pathways compared to CC

4. CONCLUSIONS

electronic transport. With the addition of

The electronically conductive polymer system

significant amounts of PEO in the polymer

PEDOT:PSS was combined with the lithium ion

composite, erratic behavior is observed at

conductive polymer PEO by mixing and casting

potentials > 4.3 V. This can potentially be

aqueous solutions at ambient conditions to form

attributed to PEO, which was shown to have

composites of varying PEDOT-PEO ratios. For

stability issues at potentials > 4 V,52 and so

the first time, such a composite of polymers is

electrochemically-induced reactions between the

optimized and examined in terms of application

electrode components, electrolyte, and perhaps

to

unwanted residual water may be occurring that

multifunctional binder that is (elemental) carbon-

cause spikes in current in this region. This was

free and considers Li+ transport. The dual

not observed in Figure 5, and so might be the

conductor was found to have greatly enhanced

result of rearrangement to the polymer matrix

transport

lithium-based

cells

properties,

as

both

an

alternative,

electrically

and



Page 24 of 34

ionically, with the addition of large loadings (up

where

to 89 wt%) of PEO. This system is optimal at the

disrupting this unique morphology.

20:1 PEO:PEDOT monomer ratio (64 wt%

The

excess

PEO

begins

electrochemical

to

properties

crystallize,

of

this

PEO), where electronic conductivity is ~45 S

composite were investigated in lithium cells

cm-1 and Li+ conductivity is ~10-4 S cm-1

from 1.5 V to 5 V, and it was found that

(increases of 2 orders of magnitude and 8 fold,

electrochemical processes that occur to polymers

respectively, compared to pristine PEDOT:PSS).

in

The Li+ conductivity is comparable to other

detrimental to transport functions, with high

lithium-based polymer electrolytes at room

faradaic efficiencies stabilizing over cycling and

temperature.24

also

CV showing lower charge transfer overpotential

improved 2.5 orders of magnitude compared to a

due to improved ion and electron transport

conventional

electron

properties. While the evidence from experiments

transport. TEM, AFM, DSC, and WAXD were

performed in this work support this conclusion,

used

structure-function

more rigorous electrochemical testing with

relationship, and it was found that PEO and PSS

emphasis on mechanisms is required in future

interact strongly and drive the formation of

work to fully understand the electrochemical

better-aligned and grouped PEDOT networks

capabilities of this material. We note it will be

such that they form more efficient pathways

important to ensure the complete removal of

throughout the bulk material despite their

water post-casting to decrease electrochemical

decreasing concentration as PEO is added,

side reactions between electrode and electrolyte.

giving to the enhanced electronic conductivity.

Because this preliminary testing is performed

PSS in turn suppresses PEO crystallization,

under purposefully vigorous conditions to

leading to enhanced Li+ conductivity. The

explore the full potential of the material, it is

system is optimal near the saturation point,

expected that the voltages of storage materials to

to

These

binder

expose

conditions

system

the

for

are

high-potential

environments

are

not


Page 25 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


be incorporated will be smaller and therefore the

future work. Because the fabrication is a simple

material will be more stable yet.

addition of components into a processable, device

aqueous solution, this is an attractive alternative

applications, the better ionic and electronic

to toxic processing solvents such as NMP and

conductivity can potentially increase power

vigorous dispersion procedures that are costly

density and energy density. With more efficient

and more time consuming. When combining

charge transport compared to conventional

with active materials, the presence of PSS may

binder materials, and all functionality combined

also have dispersion qualities due to its

into a single material, presumably less of the

surfactant nature to prevent agglomeration and

binder phase would be needed to achieve the

maximize surface area in three-dimensional

required conductivity and so the proportion of

electrodes.58 It is also important to note that the

active mass can be increased. Future work

composite morphology will likely be dependent

should include monitoring of the conductive

on fabriction procedure, and it is possible that

properties as a function of cycling in an

other

electrochemical cell environment, and especially

morphologies are attainable by deviating from

the incorporation of active materials with the

the simple drop-casting and curing in ambient

polymer composite for applications of interest

conditions used in this work. The role of phase

such as LIBs. This will also provide information

separation for enhanced conductivty here can be

regarding the ability of the material to retain its

better understoof using conductive AFM.

In

terms

of

electrochemical

interesting,

higher

functioning

unique morphology in real conditions. In

Polymers are already studied heavily for the

addition to the importance of ionic and electronic

LIB electrolyte phase to produce all-solid

conductivity to binder functionality, it should be

systems,59 so it is reasonable that polymer-based

emphasized that a detailed understanding of the

electrodes will form favorable interfaces for

mechanical

adhesion,

charge transport. Additionally, other desirable

flexibility and strength, should be established in

features of polymers such as flexibility may

properties,

including



make for the ability to adapt to many modules and configurations (e.g. wearables and unique

Page 26 of 34

ACKNOWLEDGMENT This publication is based on work funded by

60

folding for improved packing efficiency).

Lastly, the improved transport may enable thicker architectures, which would

further

increase energy density with less to power density.

the

Skolkovo

of

Science

and

Technology (Skoltech), program name “Center for Research, Education and Innovation for Electrochemical Energy Storage” under contract number

ASSOCIATED CONTENT

Institute

186-MRA.

The

authors

wish

to

acknowledge infrastructure support through the

Supporting Information. Nyquist plots from Koch Institute for Integrative Cancer Research impedance spectroscopy, conductivity as a (MIT), the Center for Materials Science and function of PEO:PEDOT content for PEO of Engineering (MIT) and the Institute for Soldier varying molecular weight, galvanostatic chargeNanotechnologies (MIT). discharge curves, CV of PEDOT-PEO in larger voltage

window

(PDF).

This

Supporting

Information is available free of charge on the ACS Publications website.

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Lithium-Conducting

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