polymers
Article
Study of Water-Based Lithium Titanate Electrode
Processing: The Role of pH and Binder
Molecular Structure
Diogo Vieira Carvalho 1,2 , Nicholas Loeffler 1,2 , Guk-Tae Kim 1,2, *, Mario Marinaro 3 ,
Margret Wohlfahrt-Mehrens 3 and Stefano Passerini 1,2, *
1
2
3
*
Helmholtz Institute Ulm (HIU), Helmholtzstrasse 11, 89081 Ulm, Germany; [email protected] (D.V.C.);
[email protected] (N.L.)
Karlsruhe Institute of Technology (KIT), P.O. Box 3640, 76021 Karlsruhe, Germany
Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg (ZSW), Helmholtzstrasse 8,
89081 Ulm, Germany; [email protected] (M.M.);
[email protected] (M.W.-M.)
Correspondence: [email protected] (G.-T.K.); [email protected] (S.P.);
Tel.: +49-0731-50-34102 (S.P.); Fax: +49-0731-50-34199 (S.P.)
Academic Editor: Antonio Pizzi
Received: 30 June 2016; Accepted: 22 July 2016; Published: 2 August 2016
Abstract: This work elucidates the manufacturing of lithium titanate (Li4 Ti5 O12 , LTO) electrodes
via the aqueous process using sodium carboxymethylcellulose (CMC), guar gum (GG) or pectin as
binders. To avoid aluminum current collector dissolution due to the rising slurries’ pH, phosphoric
acid (PA) is used as a pH-modifier. The electrodes are characterized in terms of morphology, adhesion
strength and electrochemical performance. In the absence of phosphoric acid, hydrogen evolution
occurs upon coating the slurry onto the aluminum substrate, resulting in the formation of cavities
in the coated electrode, as well as poor cohesion on the current collector itself. Consequently,
the electrochemical performance of the coated electrodes is also improved by the addition of PA
in the slurries. At a 5C rate, CMC/PA-based electrodes delivered 144 mAh¨g´1 , while PA-free
electrodes reached only 124 mAh¨g´1 . When GG and pectin are used as binders, the adhesion of the
coated layers to the current collector is reduced; however, the electrodes show comparable, if not
slightly better, electrochemical performance than those based on CMC. Full lithium-ion cells, utilizing
CMC/PA-made Li[Ni0.33 Mn0.33 Co0.33 ]O2 (NMC) cathodes and LTO anodes offer a stable discharge
capacity of ~120 mAh¨g´1 (NMC) with high coulombic efficiencies.
Keywords: lithium batteries; lithium titanate LTO; water-soluble binders; CMC; guar gum; pectin
1. Introduction
The worldwide trend to develop light weight and high performance devices, such as smartphones,
personal computers and tablets, demands designable and reliable power supplies. In this respect,
lithium-ion batteries (LIBs) are well-established systems to power such devices. Additionally, LIBs are
also used as the main or secondary energy supply, respectively, in pure electric or hybrid vehicles,
thus reducing the dependence of fossil fuels.
Nowadays, a major requirement of new battery technologies is, besides key factors, like high
performance, safety and low cost, the sustainability and environmental friendliness of the used
components [1,2]. From this point of view, aqueous processing of LIB electrodes, i.e., utilizing
only water-soluble binders, is an extremely promising approach. The binder is a fundamental
electrode component, since it ensures a firm binding of the active material and the conductive
carbon, as well as of the electrode layer to the current collector. Moreover, binders have
Polymers 2016, 8, 276; doi:10.3390/polym8080276
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Polymers 2016, 8, 276
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the additional function to provide appropriate slurry rheology for coating. The state of the
art binders for LIBs are poly(vinylidene-di-fluoride) (PVdF) for cathodes and styrene-butadiene
rubber (SBR)/sodium-carboxymethylcellulose (CMC) mixtures for graphite-based anode electrodes.
Despite being well established in commercial electrodes, the synthetic PVdF binder exhibits some
drawbacks, for instance the higher cost compared to natural binders and the necessity of using toxic
solvents, particularly N-methyl-2-pyrrolidon (NMP), during electrode processing. As an alternative,
aqueous processed electrodes are under development, offering significantly-reduced electrode costs
and an environmentally-friendly process without compromising the battery performance [3].
Aside from graphite, spinel-structured lithium titanate Li4 Ti5 O12 (LTO) is commercially used as
the anode for LIBs, providing remarkable advantages, such as stable cycling performance even at high
C-rates, low toxicity and enhanced safety [4]. Those characteristics render LTO suitable for large-scale
LIBs as those used in electric and hybrid vehicles. Moreover, the combination of the LTO desirable
properties with the aqueous electrode manufacturing would lead to LIB anodes with enhanced safety
and performance properties. Nevertheless, the alkaline nature of aqueous LTO slurries favors the
corrosion of the aluminum current collector upon electrode coating [5]. To avoid this issue, the use of
mild acids, i.e., phosphoric (PA) and formic (FA) acids, as pH modifiers of Li[Ni0.33 Mn0.33 Co0.33 ]O2
(NMC) electrodes has been earlier investigated in our laboratories [6]. It was reported that both
acids can reduce the pH of the electrode slurry to values of about nine, thereby preventing corrosion.
Moreover, better cyclability was shown for electrodes prepared with the addition of PA due to the
formation and deposition of highly insoluble phosphate compounds on the active material surface of
NMC, which reduce the transition metal leaching and enhance the electrochemical performance.
As an alternative to SBR/CMC and PVdF binders, natural polysaccharides, such as guar gum
(GG), have been investigated as potential binders for graphite [7], silicon [8,9] and LTO [10] electrodes.
Yoon et al. reported the use of pectin as a binder for silicon anodes and compared their electrochemical
performance with CMC and amylose-based electrodes correlating the electrochemical performance,
in particular cycle life, with the polysaccharides’ backbone structure [11]. In a different approach,
this work focuses on the development of an aqueous process to prepare LTO electrodes, using CMC
as the binder, but with the addition of phosphoric acid (PA) as the pH modifier. Additionally, we
investigated different polysaccharides, i.e., guar gum (GG, branched mannose:galactose 2:1 chain)
and pectin (α-linking galacturonic acid chain), as binders for LTO electrodes, assessing the adhesion
strength, thermal stability, electrode morphology and electrochemical performance. Finally, we report
our investigation of full cells comprising NMC as the cathode and LTO as the anode, both prepared
via aqueous processing using natural polymeric binders.
2. Materials and Methods
2.1. Electrode Processing
Lithium titanate (Li4 Ti5 O12 , Hombitec LTO5; average primary particle size: 250 nm, Huntsman,
Duisburg, Germany) and Li[Ni0.33 Mn0.33 Co0.33 ]O2 (NMC; average particle size d90 = 10 µm,
TODA, Battle Creek, MI, USA) were used as, respectively, anode and cathode active materials.
Sodium carboxymethylcellulose (Walocel CRT 2000 PPA 12, Dow Wolff Cellulosics, Bomlitz, Germany)
with a degree of substitution of 1.2, guar gum (GG, Lamberti SpA, Albizzate, Italy) and pectin (from
citrus peel, Alfa Aesar, Ward Hill, MA, USA) were used as binders for the anodes. The conducting
carbon black was always C-NERGY Super C45 (Imerys, Bironico, Switzerland). NMC electrodes
were prepared using only CMC as the binder, but with the addition of PA (Bernd Kraft GmbH,
Duisburg, Germany) as the pH modifier [6]. All electrodes (NMC or LTO) were prepared with
the same methodology. At first, the binder was dissolved in deionized water by magnetic stirring,
and subsequently, a predetermined amount of Super C45 was added. After 3 h of continuous mixing,
the active material was added, and the electrode slurries were stirred for 2 h. Further dispersion at
medium stirring speed (5000 rpm) was performed with a high-speed mixer (4000-4/65, DREMEL,
Polymers 2016, 8, 276
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Mount Prospect, IL, USA). For all electrode formulations (NMC, LTO-CMC-PA, LTO-pectin-PA and
LTO-GG-PA),
1%IL,ofUSA).
PA by
weight
of active
material (LTO)
was
added in theLTO-pectin-PA
slurry to prevent
Mount
Prospect,
For
all electrode
formulations
(NMC,
LTO-CMC-PA,
and
aluminum current
corrosion
[5,6].
To evaluate
the added
influence
ofslurry
PA, atoPA-free,
LTO-GG-PA),
1% of collector
PA by weight
of active
material
(LTO) was
in the
prevent LTO-CMC
aluminum
slurry was
also prepared.
The slurries
werethe
cast
on aluminum
(thickness:
20 µm)
using
current
collector
corrosion [5,6].
To evaluate
influence
of PA, afoil
PA-free,
LTO-CMC
slurry
wasa
laboratory-scale
doctor
blade
coater
a wet thickness
ranging 20
between
150 aand
180 µm. The
also
prepared. The
slurries
were
cast with
on aluminum
foil (thickness:
µm) using
laboratory-scale
coated aluminum
foils
were
pre-dried
in an
atmospheric
oven
80 °C,
vacuum
at 180 °C
doctor
blade coater
with
a wet
thickness
ranging
between
150atand
180then
µm.under
The coated
aluminum
˝
˝
for
12
h
prior
to
performing
electrochemical
investigations.
The
electrode
formulation
was
88 wt%
foils were pre-dried in an atmospheric oven at 80 C, then under vacuum at 180 C for 12 h prior
to
active electrode
material (LTO
or NMC), 7 The
wt%electrode
Super C45
and 5 wt%was
binder.
To homogenize
the
performing
electrochemical
investigations.
formulation
88 wt%
active electrode
−2
surface, reduce
thickness
and,
therefore,
the5porosity,
the electrodes
were pressed
at 10reduce
tons·cm
material
(LTO orthe
NMC),
7 wt%
Super
C45 and
wt% binder.
To homogenize
the surface,
the
´2 using
using a manual
press (Atlas
manual hydraulic
presswere
15T,pressed
Specac,atOrpington,
for 30
s. This
thickness
and, therefore,
the porosity,
the electrodes
10 tons¨cmUK)
a manual
process
alsomanual
enhanced
the adhesion
of the
active Orpington,
material layer
the30current
press
(Atlas
hydraulic
press 15T,
Specac,
UK)tofor
s. Thiscollector.
process also enhanced
the adhesion of the active material layer to the current collector.
2.2. Electrode Characterization
2.2. Electrode Characterization
The thermal properties of CMC, GG, pectin and LTO were evaluated by thermogravimetric
The (TGA).
thermalThe
properties
of CMC, GG,
pectin
and LTO
were
evaluated
thermogravimetric
analysis
TGA experiments
were
carried
out on
a Q
5000 IR by
TGA
instrument (TA
analysis
(TGA).
TGA
were
outspecimen
on a Q from
500030IR
instrument
Instruments,
NewThe
Castle,
DE,experiments
USA) by heating
thecarried
respective
°C TGA
up to 500
°C with
˝C
−1 under
(TA
Instruments,
Castle,
DE,aUSA)
by heating
the
respective
from
30 ˝ C up pans.
to 500The
a heating
rate of 5New
°C·min
nitrogen
gas flow
(25
mL·min−1specimen
) using open
aluminum
˝ C¨min
´1 underwithout
´1 )electrode
with
a heating
rate
of 5were
a nitrogen
gaspre-treatment.
flow (25 mL¨min
using open
aluminum pans.
samples
(10–20
mg)
evaluated
any
The
morphology
was
investigated
ZEISS
1550VP
Field Emission
SEM (CarlThe
Zeiss,
Oberkochen,
Germany),
The
samples using
(10–20the
mg)
wereLEO
evaluated
without
any pre-treatment.
electrode
morphology
was
while energyusing
dispersive
X-ray
spectroscopy
(EDX)
experiments
wereZeiss,
performed
using an
EDX Xinvestigated
the ZEISS
LEO
1550VP Field
Emission
SEM (Carl
Oberkochen,
Germany),
2
MaxNenergy
(50 mm
), 10 kV (Oxford
Instruments,
Abingdon
Oxfordshire,
England).using an EDX X-MaxN
while
dispersive
X-ray spectroscopy
(EDX)
experiments
were performed
2 ),electrode
The
adhesion
strength was
evaluated
using a Z2.5
Zwick/Roell machine (Zwick Roell,
(50 mm
10 kV (Oxford
Instruments,
Abingdon
Oxfordshire,
England).
Ulm,The
Germany).
Figure
1 displays
thewas
basic
measurement
Briefly, an
electrode
of defined
electrode
adhesion
strength
evaluated
using aprinciple.
Z2.5 Zwick/Roell
machine
(Zwick
Roell,
2
area (6.45
cm ) is fixed
between
twothe
planar
and
parallel plates
with theBriefly,
help ofan
double-sided
Ulm,
Germany).
Figure
1 displays
basic
measurement
principle.
electrode ofadhesive
defined
2 ) isafixed
tape (6.45
(3M).cm
After
startbetween
phase, intwo
which
the and
specimen
is plates
approached
before
is established,
the
area
planar
parallel
with the
helpcontact
of double-sided
adhesive
compression
phase
takesphase,
place.in
Within
the compression
stress
until a
tape
(3M). After
a start
whichthe
thecompression
specimen is phase,
approached
before contact
is rises
established,
defined
pressurephase
level (2000
is achieved
andcompression
then kept constant
during
dwell time
(120rises
s), inuntil
order
the
compression
takes N)
place.
Within the
phase, the
compression
stress
a
−1) order
to allowpressure
the adhesive
contact
electrode.
the pull-off
phase
(1000
takes
defined
level to
(2000
N) isthe
achieved
andAfterwards,
then kept constant
during
dwell
timemm·min
(120 s), in
1 ) using
place,
and
maximum
tensilethe
force
is detected.
The adhesion
strength
σ (1000
is calculated,
by
to
allow
thethe
adhesive
to contact
electrode.
Afterwards,
the pull-off
phase
mm¨min´
takes
Equation
the maximum
tensile
|Ft, max
| oradhesion
pull-off force
related
tocalculated,
the sampleby
area
A.
place,
and(1),
thefrom
maximum
tensile force
is force
detected.
The
strength
σn is
using
Equation (1), from the maximum tensile force |Ft, max | or pull-off force related to the sample area A.
σ =
|
,
|
|Ft,max |
σn “
A
(1)
(1)
Figure 1. Representation of the adhesion strength measurement.
2.3. Electrochemical Characterization
Half-cells, i.e., cells made with the Li metal anode (Rockwood Lithium, battery grade, Frankfurt
am Main, Germany), were assembled in pouch bag configuration in order to evaluate the effect of PA
Polymers 2016, 8, 276
4 of 9
2.3. Electrochemical Characterization
Half-cells, i.e., cells made with the Li metal anode (Rockwood Lithium, battery grade, Frankfurt
8, 276
of 9PA
amPolymers
Main,2016,
Germany),
were assembled in pouch bag configuration in order to evaluate the effect4 of
and the different binders (CMC, GG and pectin) on the LTO electrodes’ performance. The cells were
and the different binders (CMC, GG and pectin) on the LTO electrodes’ performance. The cells were
assembled in a dry-room (R.H. <0.01% at 20 ˝ C ˘ 1 ˝ C), using commercial electrolyte consisting of a
assembled in a dry-room (R.H. <0.01% at 20 °C ± 1 °C), using commercial electrolyte consisting of a 1
1 mol solution of lithium hexafluorophosphate (LiPF6 ) in a mixture of ethylene carbonate and dimethyl
mol solution of lithium hexafluorophosphate (LiPF6) in a mixture of ethylene carbonate and dimethyl
carbonate
(EC:DMC (1:1 w/w)) (LP30, BASF, Ludwigshafen, Germany). The porous polyethylene
carbonate (EC:DMC (1:1 w/w)) (LP30, BASF, Ludwigshafen, Germany). The porous polyethylene
membrane
from
Asahi
wasused
usedasasthe
theseparator.
separator.Full-cells
Full-cells
were
membrane
from
AsahiKasei
Kasei(Hipore
(HiporeSV718,
SV718, Tokyo,
Tokyo, Japan)
Japan) was
were
also
assembled
to
evaluate
the
potential
of
the
aqueous
processed
electrodes
in
Li-ion
cells.
For
such
also assembled to evaluate the potential of the aqueous processed electrodes in Li-ion cells. For such
tests,
coin
cells
(2032)
glove box
box (O
(O22<<0.1
0.1ppm,
ppm,HH
< 0.1
ppm)
2O
tests,
coin
cells
(2032)were
wereassembled
assembledin
inan
an argon-filled
argon-filled glove
2O
< 0.1
ppm)
22). As the separator, a glass felt (GF/D, Whatman,
using
NMC
and
LTO
electrode
discs
(area
=
1.13
cm
using NMC and LTO electrode discs (area = 1.13 cm ). As the separator, a glass felt (GF/D, Whatman,
Maidstone,
placedbetween
betweenthe
the
electrodes
soaked
the electrolyte
Maidstone,England)
England)was
was placed
electrodes
andand
soaked
with with
the electrolyte
(LP30).(LP30).
The
Thecells
cells
were
tested
using
a MACCOR
Battery
tester
(Tulsa,
USA)
at controlled
temperature
were
tested
using
a MACCOR
Battery
tester
43004300
(Tulsa,
OK,OK,
USA)
at controlled
temperature
in
˝ C.
in climatic
climaticchambers
chambers(Binder
(Binder
400)
at°C
20±˝0.1
C˘
The galvanostatic
charge/discharge
tests
KBKB
400)
at 20
°C.0.1
The
galvanostatic
charge/discharge
tests of LTO
+ . The galvanostatic
were performed
betweenbetween
1.0 V and
vs. 2.5
Li/Li
TheLi/Li
galvanostatic
tests were performed
of half-cells
LTO half-cells
were performed
1.02.5VVand
V+.vs.
tests were
at different
(0.1C,
0.5C,(0.1C,
1C, 2C,
3C 1C,
and2C,
5C).3C
The
full5C).
cellThe
(NMC/LTO)
tests were carried
performed
at C-rates
different
C-rates
0.5C,
and
full cell (NMC/LTO)
testsout
were
between
V and 1.3
2.8 V.
carried
out 1.3
between
V and 2.8 V.
Resultsand
andDiscussion
Discussion
3. 3.Results
3.1.3.1.
Thermal
Stability
Thermal
Stability
Figure
2 displaysthe
theTGA
TGAresults
resultsin
inN
N22 atmosphere
LTO
particles
Figure
2 displays
atmosphereof
ofLTO,
LTO,CMC,
CMC,GG
GGand
andpectin.
pectin.
LTO
particles
stable
500˝ C°Csince
sinceno
nomaterial
materialdegradation
degradation is detected
detected up
three
areare
stable
to to
500
up to
to this
this temperature.
temperature.The
The
three
˝
binders,
on
the
other
hand,
show
comparable
weight
loss
profiles
up
to
200
°C.
The
weight
decrease
binders, on the other hand, show comparable weight loss profiles up to 200 C. The weight decrease at
200
is related
to water
desorption
the polymers
they were
not pre-dried.
Above
200at˝ C
is °C
related
to water
desorption
from from
the polymers
since since
they were
not pre-dried.
Above
200 ˝ C,
200 °C,
pectinaexhibits
a sharp decomposition,
thus showing
a lower
thermal
stability
than
CMC
and
pectin
exhibits
sharp decomposition,
thus showing
a lower
thermal
stability
than
CMC
and
GG,
GG,
in
line
with
previous
reports
[12].
In
contrast,
CMC
and
GG
start
to
decompose
only
at
in line with previous reports [12]. In contrast, CMC and GG start to decompose only at temperatures
temperatures above 250 °C [10,13]. The decomposition mechanism of all three polymers is
above 250 ˝ C [10,13]. The decomposition mechanism of all three polymers is determined by the
determined by the breakdown of the main polymer chain [7,10,12–14]. Overall, all binders showed
breakdown of the main polymer chain [7,10,12–14]. Overall, all binders showed thermal stability at
thermal stability at least up to 200 °C, which allows the high temperature (180 °C) drying of the coated
least up to 200 ˝ C, which allows the high temperature (180 ˝ C) drying of the coated electrode without
electrode without thermal decomposition.
thermal decomposition.
Figure
2. 2.
TGA
gum (GG)
(GG)and
andpectin
pectinwith
witha aheating
heating
rate
Figure
TGAweight
weightloss
lossprofiles
profilesof
of LTO,
LTO, CMC,
CMC, guar
guar gum
rate
of of
5 ˝5C/min
°C/minininNN2 2atmosphere.
atmosphere.
3.2. Electrode Surface Characterization
SEM images from the unpressed electrodes are shown in Figure 3. Panel (a) shows the
micrograph of the LTO-CMC electrode prepared without PA addition. The electrode surface is
Polymers 2016, 8, 276
5 of 9
3.2.
Electrode Surface Characterization
Polymers 2016, 8, 276
5 of 9
SEM images from the unpressed electrodes are shown in Figure 3. Panel (a) shows the micrograph
dominated
by cavities
generated
by without
the gas evolution,
which
from theis reaction
of the
of
the LTO-CMC
electrode
prepared
PA addition.
Theoriginates
electrode surface
dominated
by
alkaline
slurry
with
the
aluminum
current
collector
during
the
casting
and
drying
step.
In
fact,
cavities generated by the gas evolution, which originates from the reaction of the alkaline slurry with
without
the addition
PA, the
slurrythe
achieves
pHdrying
of 11.4step.
(seeIn
Figure
S1), leading
to aluminum
the
aluminum
current of
collector
during
casting aand
fact, without
the addition
of PA,
(current
collector)
corrosion
and
H
2 bubble formation [15]. For a detailed investigation of the Al
the slurry achieves a pH of 11.4 (see Figure S1), leading to aluminum (current collector) corrosion and
current
collector corrosion, energy dispersive X-ray spectroscopy EDX experiments were performed
H
2 bubble formation [15]. For a detailed investigation of the Al current collector corrosion, energy
on
several
the LTO-CMC
electrode.
Tablewere
S1 shows
the elemental
composition
andLTO-CMC
Figure S2
dispersive spots
X-rayof
spectroscopy
EDX
experiments
performed
on several
spots of the
the SEM micrograph
of thethe
spots
evaluated.
Spots 1 (Spectrum
and
(Spectrum
2) are in
electrode.
Table S1 shows
elemental
composition
and Figure1)S2
the2 SEM
micrograph
of the
the depth
spots
of
the
cavity,
while
Spot
3
(Spectrum
3)
is
on
the
electrode
surface.
The
high
fraction
of
Al
detected
evaluated. Spots 1 (Spectrum 1) and 2 (Spectrum 2) are in the depth of the cavity, while Spot 3
on Spots 1 3)
(87.69%)
and
2 (66.12%)
reveals
thehigh
exposure
of of
theAlcurrent
collector
at the
bottom and
of the
(Spectrum
is on the
electrode
surface.
The
fraction
detected
on Spots
1 (87.69%)
2
observed
cavities.
In
Spot
3,
a
small
fraction
of
Al
was
detected
(1.2%),
which
is
a
side
product
of
the
(66.12%) reveals the exposure of the current collector at the bottom of the observed cavities. In Spot 3,
corrosion,
mainly
2O3, which after solubilization from the current collector, is redeposited onto
aAlsmall
fraction
of Al Al
was
detected (1.2%), which is a side product of the Al corrosion, mainly Al2 O3 ,
the
solid
electrode
components
solvent
evaporation.
Thus,onto
the high
pH electrode
value of the
aqueous
which after solubilization from theafter
current
collector,
is redeposited
the solid
components
LTO solvent
slurry corrodes
the aluminum
traces of Al
can
be detected
inthe
the aluminum
composite
after
evaporation.
Thus, the current
high pHcollector,
value of and
the aqueous
LTO
slurry
corrodes
electrode
layer.
Its
influence
will
be
discussed
in
the
next
sections.
current collector, and traces of Al can be detected in the composite electrode layer. Its influence will be
On the
other
hand,
the electrodes prepared by adding 1 wt% PA (Figure 3b; LTO-CMC-PA)
discussed
in the
next
sections.
showOn
nothe
cavities
due
to
the
pH adjustment
at by
values
around
~6.7,
which avoids
Al corrosionshow
(see
other hand, the electrodes
prepared
adding
1 wt%
PA (Figure
3b; LTO-CMC-PA)
Figure
S1).
Moreover,
Figure
3c,
d
displays
the
surface
of
the
electrodes
prepared
using
pectin
no cavities due to the pH adjustment at values around ~6.7, which avoids Al corrosion (see Figure and
S1).
GG
as
binders.
Cracks
can
be
observed
on
the
surface
of
the
LTO-pectin-PA
electrode.
The
Moreover, Figure 3c,d displays the surface of the electrodes prepared using pectin and GG as binders.
LTO-GG-PA
much of
more
that ofThe
LTO-pectin-PA,
small
Cracks
can beelectrode
observedsurface
on the is
surface
the homogeneous
LTO-pectin-PAthan
electrode.
LTO-GG-PA but
electrode
defects
can
also
be
detected.
The
defects
on
the
electrode
are
associated
with
the
binder
shrinkage
surface is much more homogeneous than that of LTO-pectin-PA, but small defects can also be detected.
during
the final
drying
step.are associated with the binder shrinkage during the final drying step.
The
defects
on the
electrode
Figure
LTO
electrodes
using:
(a) CMC
as the
(LTO-CMC);
(b) CMC
Figure 3.
3. SEM
SEMimages
imagesofofunpressed
unpressed
LTO
electrodes
using:
(a) CMC
asbinder
the binder
(LTO-CMC);
(b)
as
the binder
phosphoric
acid (PA) (LTO-CMC-PA);
(c) pectin as the
CMC
as theand
binder
and phosphoric
acid (PA) (LTO-CMC-PA);
(c) binder
pectinand
as PA
the(LTO-pectin-PA);
binder and PA
(d)
and guar gum as
and PA
(LTO-GG-PA).
(LTO-pectin-PA);
(d)the
andbinder
guar gum
as the
binder and PA (LTO-GG-PA).
3.3.
3.3. Adhesion Strength
Electrode adhesion
adhesionstrength
strengthis aisrelevant
a relevant
factor
for development
LIBs’ development
The
coated
factor
for LIBs’
[16]. The[16].
coated
composite
composite
electrode
must,
in fact,
stand the mechanical
stress
upon
cutting, cell
winding,
cell assembling
electrode must,
in fact,
stand
the mechanical
stress upon
cutting,
winding,
assembling
processes
processes
and for
battery
cycle life.
Figurethe
4 displays
adhesion
strength
of the LTO
and for battery
cycle
life. Figure
4 displays
adhesionthe
strength
of the
LTO electrodes.
Theelectrodes.
adhesion
The
adhesion
strength
of the
electrodewithout
prepared
PA (LTO-CMC)
not be determined,
strength
of the
electrode
prepared
PAwithout
(LTO-CMC)
could notcould
be determined,
due to
due
to cohesion-failure
the pull-off
step,parts
i.e., parts
the electrode
remained
on both
cohesion-failure
duringduring
the pull-off
step, i.e.,
of theofelectrode
layerlayer
remained
on both
the
the
adhesive
current
collector.This
Thisisisan
anextremely
extremely negative
negative phenomenon
phenomenon because the
adhesive
tapetape
andand
thethe
current
collector.
the
delamination
of of
thethe
electrode
layer
during
processing
would
reduce
the electrode
active
delaminationofofsmall
smallareas
areas
electrode
layer
during
processing
would
reduce
the electrode
layer,
affectingaffecting
the cell performance.
As depicted
Figure 3a
the electrode’s
activestrongly
layer, strongly
the cell performance.
As in
depicted
in (LTO-CMC),
Figure 3a (LTO-CMC),
the
surface
morphology
is not homogeneous
due to the due
aluminum
corrosion. corrosion.
Even afterEven
electrode
electrode’s
surface morphology
is not homogeneous
to the aluminum
after
compression
(image not
shown),
the solid
in thein
composite
electrode
do not
adhere
to
electrode compression
(image
not shown),
theparticles
solid particles
the composite
electrode
do not
adhere
each
other.
On
the
other
hand,
the
adhesion
strength
of
electrodes
coated
from
PA-containing
slurries
to each other. On the other hand, the adhesion strength of electrodes coated from PA-containing
was
successfully
measured.
The LTO-CMC-PA
electrodeselectrodes
exhibitedexhibited
the highest
of adhesion
slurries
was successfully
measured.
The LTO-CMC-PA
thevalue
highest
value of
adhesion strength (>1100 kPa) compared to the LTO-pectin-PA (>600 kPa) and LTO-GG-PA
(>450 kPa) electrodes. The higher adhesion strength of CMC compared to GG electrodes can be
explained by the linear β-linkage polymer chain geometry. The interchain hydrogen bonds between
the cellulose chains are stronger than between the galactomannan branched chains [10]. Moreover,
Polymers 2016, 8, 276
6 of 9
strength (>1100 kPa) compared to the LTO-pectin-PA (>600 kPa) and LTO-GG-PA (>450 kPa) electrodes.
The higher adhesion strength of CMC compared to GG electrodes can be explained by the linear
Polymers 2016, 8, 276
β-linkage polymer chain geometry. The interchain hydrogen bonds between the cellulose chains6 of
are9
stronger than between the galactomannan branched chains [10]. Moreover, the linear α-linkage pectin
the linear α-linkage pectin galacturonic acid chain seems to be weaker than the β-linkage of the CMC
galacturonic acid chain seems to be weaker than the β-linkage of the CMC molecule. In fact, it has
molecule. In fact, it has been reported that α-linkage polysaccharides are flexible, whereas β-linkage
been reported that α-linkage polysaccharides are flexible, whereas β-linkage polymers are stiff [11].
polymers are stiff [11].
1200
Adhesion strenght (kPa)
1000
800
600
400
200
0
LTO-CMC-PA
LTO-GG-PA
LTO-Pectin-PA
Binder
fabricated
using
PAPA
as an
and and
CMC,
guar guar
gum
Figure 4. Adhesion
Adhesion strength
strengthofofLTO
LTOelectrodes
electrodes
fabricated
using
as additive
an additive
CMC,
and pectin
as theasbinder.
gum
and pectin
the binder.
3.4. Electrochemical
Electrochemical Characterization
In order to evaluate
evaluate the influence
influence of
of the
the polymeric
polymeric binders
binders in
in LTO
LTO electrodes’
electrodes’ performance,
performance,
half-cells
half-cells were
were assembled
assembled and
and tested at several C-rates. In
In the
the rate capability
capability test, different current
densities are applied to the
the electrode
electrode under
under investigation,
investigation, and
and the
the corresponding
corresponding delivered
delivered capacities
are recorded.
at at
low
(0.1C
and
0.5C)
andand
high
(2C,(2C,
3C and
5C) C-rates
(the
recorded. Thus,
Thus,the
thecells
cellswere
weretested
tested
low
(0.1C
and
0.5C)
high
3C and
5C) C-rates
C-rate
is commonly
used in
battery
testing,testing,
becausebecause
it is independent
of the electrode
active material
(the C-rate
is commonly
used
in battery
it is independent
of the electrode
active
mass;
1Cmass;
rate 1C
corresponds
to a specific
current
leading
to to
thethefull
material
rate corresponds
to a specific
current
leading
fulldischarge/charge
discharge/chargeof
of the
electrochemical cell in one hour).
In Figure 5a are depicted the discharge capacities of half-cells assembled with lithium metal and
LTO electrodes, these latter
latter incorporating
incorporating different
differentbinders
binders(CMC,
(CMC,CMC-PA,
CMC-PA,GG-PA
GG-PAand
andpectin-PA).
pectin-PA).
During
During the
the first five cycles at low C-rate (0.1C), the electrodes made
made from PA-containing slurries
showed
showed similar
similar capacity
capacity values,
values, while
while slightly
slightly lower
lower discharge
discharge capacities
capacities were
were observed
observed for the
electrode
density
increases
to 0.5C
andand
subsequently
to 1C,
electrode made
made without
withoutPA.
PA.Once
Oncethe
thecurrent
current
density
increases
to 0.5C
subsequently
to the
1C,
difference
in cycling
performance
between
CMC
and
CMC-PA
the difference
in cycling
performance
between
CMC
and
CMC-PAelectrodes
electrodesbecomes
becomesvery
veryobvious.
obvious.
While both electrodes show a stable cycling performance, higher discharge capacities are delivered
from the
theCMC-PA
CMC-PA
electrode.
At high
C-rates
(2C,
5C), the difference
is pronounced.
even more
electrode.
At high
C-rates
(2C, 3C
and3C
5C),and
the difference
is even more
´1 and
pronounced.
(Figure
cycle),
144 124¨mAh
mAh·g−1 and
124·mAh
g−1 were
delivered
by and
the
At 5C (FigureAt
5a;5C
30th
cycle),5a;
14430th
mAh¨g
g´1 were
delivered
by the
CMC-PA
CMC-PA
and CMC
electrodes,
Thecapacities
lower discharge
capacities electrode
of the CMC-based
CMC electrodes,
respectively.
Therespectively.
lower discharge
of the CMC-based
are mostly
electrode
related to theoflower
conductivity ofcoated
the loosely-packed
coated layer.
related to are
the mostly
lower conductivity
the loosely-packed
layer.
Upon the
theaddition
addition
of inPA
the(Figure
slurry5a),
(Figure
5a), theexhibit
electrodes
exhibit
remarkable
of PA
the in
slurry
the electrodes
remarkable
electrochemical
electrochemical
performance,
even at
high C-rates.the
Additionally,
theexcellent
cells showed
excellent
capacity
performance, even
at high C-rates.
Additionally,
cells showed
capacity
retention
after
retention
at 5C and cycles
the subsequent
cycles the
at 1C.
However, the
LTO-CMC-PA
five cyclesafter
at 5Cfive
andcycles
the subsequent
at 1C. However,
LTO-CMC-PA
electrode
shows a
electrode
a lower
rate capability
compared
to those
based
GG-PA and
pectin-PA.
could
lower rateshows
capability
compared
to those
based on
GG-PA
andon
pectin-PA.
This
could beThis
related
to
be
the higher
electrolyte
uptake
of, e.g.,
guar gum
compared
In fact,
therelated
higher to
electrolyte
uptake
of, e.g.,
guar gum
compared
to CMC
[9,10].toInCMC
fact, [9,10].
the motion
of the
+ ions in
motion
of the galactomannan
and the
ether oxygens’
of the GGLimolecule
galactomannan
and the ether oxygens’
lone-pair
electrons lone-pair
of the GG electrons
molecule coordinate
+
coordinate
Licomparable
ions in its to
structure,
comparable
to polyethylene
oxide (PEO)
in solid
electrolytes,
its structure,
polyethylene
oxide (PEO)
in solid electrolytes,
which
accounts
for the
which accounts for the improvement in electrochemical performance [17,18]. However, CMC-PA
electrodes show the highest value of adhesion strength, due to the optimal binding of the solid
material components resulting from the homogeneous binder distribution, i.e., the optimal electrode
preparation. Thus, it is reasonable to assume that the better electrochemical performance of LTO
electrodes made with guar gum and pectin is rather related to increased ionic conductivities than
Polymers 2016, 8, 276
7 of 9
improvement in electrochemical performance [17,18]. However, CMC-PA electrodes show the highest
value of adhesion strength, due to the optimal binding of the solid material components resulting from
the homogeneous binder distribution, i.e., the optimal electrode preparation. Thus, it is reasonable
to assume that the better electrochemical performance of LTO electrodes made with guar gum and
Polymers 2016, 8, 276
7 of 9
pectin is rather related to increased ionic conductivities than solid particle adhesion. Additionally,
pectin-PA
and GG-PA electrodes show comparable electrochemical performance at low C-rates of 0.1C,
solid particle adhesion. Additionally, pectin-PA and GG-PA electrodes show comparable
0.5C
and
1C.
However,
at 2C, 3C
and C-rates
especially
5C, electrodes
using
pectin at
as2C,
the 3C
binder
showed the
electrochemical
performance
at low
of 0.1C,
0.5C and 1C.
However,
and especially
highest
capacity. using
As thepectin
adhesion
strength
obviously
directly
correlated
with
the electrochemical
5C, electrodes
as the
binderisshowed
the not
highest
capacity.
As the
adhesion
strength is
properties
of
the
herein
presented
electrodes,
the
best
performance
of
pectin
electrodes
has to be
obviously not directly correlated with the electrochemical properties of the herein presented
related
to
its
different
molecular
structure
and
its
affinity
towards
the
electrolyte
favoring
lithium
electrodes, the best performance of pectin electrodes has to be related to its different molecular
ionstructure
transport.
and its affinity towards the electrolyte favoring lithium ion transport.
Figure
5b5bdisplays
the coulombic
coulombicefficiency
efficiencyofofa acathode-limited,
cathode-limited,
Figure
displaysthe
thedischarge
discharge capacity
capacity and the
lithium-ion
cell
consisting
of
NMC
and
LTO
electrodes,
both
made
using
CMC
as
the
binder
and
lithium-ion cell consisting of NMC and LTO electrodes, both made using CMC as the binder
and
PAPA
pH-modifier.Besides
Besidesthe
thefirst
first cycle
cycle at
at low
low current
current density
at at
as as
thethe
pH-modifier.
density(0.1C),
(0.1C),the
thetest
testwas
wasperformed
performed
constant
charge/discharge current
current densities
cells
showed
an average
discharge
capacity
of
constant
charge/discharge
densitiesofof1C.
1C.The
The
cells
showed
an average
discharge
capacity
1 in
~120 mAh·g
the
cycles
at aat1C
rate.
Moreover,
high
of ~120
mAh¨g−1´in
thecourse
courseofof190
190consecutive
consecutivecharge/discharge
charge/discharge
cycles
a 1C
rate.
Moreover,
high
values
of
coulombic
efficiency
(~99.8%)
were
achieved.
These
results
confirm
the
validity
of
making
values of coulombic efficiency (~99.8%) were achieved. These results confirm the validity of making
NMC
LTO
electrodes
using
CMC
andasPA
the binder
and
the pH-modifier,
respectively,
NMC
andand
LTO
electrodes
using
CMC
and PA
theasbinder
and the
pH-modifier,
respectively,
resulting
resulting
in
fully-aqueous
processed
LIBs
with
remarkable
performance.
in fully-aqueous processed LIBs with remarkable performance.
Figure
5. 5.Delivered
dischargecapacity
capacity
LTO
half-cells
as the and
binder
and
CMC,
Figure
Delivered discharge
of of
(a) (a)
LTO
half-cells
usingusing
CMC CMC
as the binder
CMC,
pectin
pectin
as the
binder
additive
several
current
densities;
LTO
mass
loading:
and and
GG GG
as the
binder
andand
PA PA
as as
an an
additive
at at
several
current
densities;
LTO
mass
loading:
´2−2; electrolyte: 1 mol of LiPF in ethylene carbonate and dimethyl carbonate (EC:DMC
3.6–4.1
mg¨cm
3.6–4.1
mg·cm
; electrolyte: 1 mol LiPF66 in ethylene carbonate and dimethyl carbonate (EC:DMC
(1:1(1:1
w/w));
and
(b)(b)
cathode-limited
Mn
Co0.33
full-cell
using
CMC
w/w));
and
cathode-limitedLi[Ni
Li[Ni
0.33
Mn0.33
0.33Co
]O]O
2 (NMC)/LTO
full-cell
using
CMC
as as
thethe
0.33
0.33
2 (NMC)/LTO
−2; ;electrolyte:
binder
and
additive
1C;
NMCmass
massloading:
loading:~4.3
~4.3mg¨cm
mg·cm´2
6 in
binder
and
PAPA
as as
anan
additive
at at
1C;
NMC
electrolyte:1 1mol
molofofLiPF
LiPF
6 in
EC:DMC
(1:1
w/w).
EC:DMC
(1:1
w/w).
Conclusions
4. 4.Conclusions
The
reportedresults
results prove
prove the
polymers
from
renewable
sources
as binders
for
The
reported
the applicability
applicabilityofof
polymers
from
renewable
sources
as binders
LTO electrodes, using PA as the pH-modifier. A small addition of the latter, in fact, leads to a great
for LTO electrodes, using PA as the pH-modifier. A small addition of the latter, in fact, leads to a
performance improvement due to optimal composite electrode cohesion and adhesion to the
great performance improvement due to optimal composite electrode cohesion and adhesion to the
current collector.
current collector.
In addition to carboxymethylcellulose, two natural binders, guar gum and pectin, were
In addition to carboxymethylcellulose, two natural binders, guar gum and pectin, were evaluated
evaluated for making LTO electrodes. Regarding the thermal stability, the decomposition of pectin
forwas
making
LTO near
electrodes.
the thermal
stability,
thetodecomposition
of pectin
was detected
detected
200 °C,Regarding
while GG and
CMC are
stable up
250 °C. Thus, all
three binders
can
˝ C, while GG and CMC are stable up to 250 ˝ C. Thus, all three binders can support high
near
200
support high temperature drying (180 °C). Electrodes prepared using GG showed the lowest
˝
temperature
drying (180
Electrodes
prepared
usingpolymer
GG showed
lowest electrodes
adhesion strength,
adhesion strength,
mostlyC).
due
to its branched
mannose
chain.the
CMC-PA
(linear
mostly
due
to
its
branched
mannose
polymer
chain.
CMC-PA
electrodes
(linear
cellulose
chain)
were
cellulose chain) were more adhesive than pectin (linear galacturonic acid chain). The effect of the
more
adhesive
than
pectin
(linear
galacturonic
acid
chain).
The
effect
of
the
polymeric
chain
was
also
polymeric chain was also evident in the electrochemical test: guar gum- and pectin-based electrodes
showed a slightly superior rate capability compared to CMC electrodes. As discussed, the GG
molecule offers a better affinity for the organic electrolyte than CMC. Additionally, the binder ability
to coordinate Li+ affects the overall electrochemical performance. The Li+ coordination by GG and
pectin polysaccharides may be higher than that of the CMC molecule due to the lower motion of the
linear cellulose chain. Moreover, the pectin polymer might also have good affinity to organic
Polymers 2016, 8, 276
8 of 9
evident in the electrochemical test: guar gum- and pectin-based electrodes showed a slightly superior
rate capability compared to CMC electrodes. As discussed, the GG molecule offers a better affinity
for the organic electrolyte than CMC. Additionally, the binder ability to coordinate Li+ affects the
overall electrochemical performance. The Li+ coordination by GG and pectin polysaccharides may be
higher than that of the CMC molecule due to the lower motion of the linear cellulose chain. Moreover,
the pectin polymer might also have good affinity to organic electrolytes, such as the GG molecule,
due to the α-linking between the galacturonic acid rings, resulting in higher electrolyte absorption.
Finally, full lithium-ion cells were manufactured using NMC and LTO electrodes both prepared
via the aqueous process using CMC as the binder and PA as the pH-modifier. The full-cell delivered
a stable and remarkable discharge performance of ~120 mAh¨g´1 at 1C over 190 cycles with high
coulombic efficiency (99.8%). These results reaffirm the suitability of making LIB electrodes by simple
and inexpensive aqueous processes.
Supplementary Materials: Supplementary Materials can be found at www.mdpi.com/2073-4360/8/8/276/s1.
Acknowledgments: The authors thank the European Commission within the FP7 Projects Advanced eco-design
and manufacturing processes for batteries and electrical (GREENLION) (Grant Agreement No. 285268) for
financial support. Huntsman, Imerys, TODA and Rockwood Lithium are kindly acknowledged for supplying
LTO, C-NERGY Super C45, NMC and lithium metal, respectively.
Author Contributions: Guk-Tae Kim and Stefano Passerini conceived of and designed this study.
Diogo Vieira Carvalho performed the experiments. Mario Marinaro and Margret Wohlfahrt-Mehrens carried out
the adhesion test. Diogo Vieira Carvalho, Nicholas Loeffler and Mario Marinaro interpreted the data and wrote
the manuscript. Guk-Tae Kim, Margret Wohlfahrt-Mehrens and Stefano Passerini edited the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
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