Functional, water-soluble binders
for improved capacity and stabilityof lithium-sulfur batteries(… and how we came to realise how important the binder is!)Matthew J. Lacey, Fabian Jeschull, Kristina Edströmand Daniel Brandell

Overview
Compared to Li-ion, the choice of binder in a Li-S electrode canbe significant in the following ways:

The effect of pore blocking by the binder
New study – how the binder can limit cathode energy density

Functional capacity and stability enhancement
Continuing work – how functional binders can enhance performanceSimple recipe for high discharge capacity:

surface area and pore volume
In DME:DOL-based electrolytes…

Upper plateau (faster)
Limited by access andamount of electrolyte

ECEC

Pore volume
for maximum sulfur loading
Surface areafor greatest utilisation, electron transfer…

Lower plateau (slower)
Limited by pore volume, surface area,kinetics, mass transport…
Lacey et al, Electrochem. Comm. 46, 91 (2014)
Cuisinier et al, J. Phys. Chem. Letts. 4, 3227 (2014)
Lu et al, J. Phys. Chem. C 118, 5733 (2014)

Favoured binders for Li-S
Other aq.,
5%
PEO, 4%CMC or
CMC:SBR,9%

PVDF
81%

PVDF-HFP,1%…according to 79 recent publications (2013-2014)
where electrodes were casted from slurries

Porosity blocking in carbon black

PVdF

PVdF-HFP

Comparison of PVdF and PVdF-HFP ina cathode with high S.A. carbon black –only binder solubility in the electrolyte
differs.
S.A.
m2/g

pore volcm3 g-1

µpore
S.A.

µpore
volCB

1100

2.07

376

0.16

C:PVdF

175

0.53

00.00

C:(-HFP)1190.55

00.00

C:PEO

170.11

00.00

Loss of:surface area
pore volume

Pores of all sizes filled between 1.7 and 100 nm,
significant reduction of S.A. and pore volume

Lacey et al, submitted

Trend in binder “swellability”
Exaggerated cathode composition:
60% S, 25% C, 15% binder ~ 1 mg cm-2
6 µL mg-1 (S) electrolyte, C/20 rate
1 M LiTFSI, 0.25 M LiNO3, 1:1 DME:DOL
same electrolyte for all experiments“Swellability series”:– PEO > PVdF-HFP > PVdF

Unusual increasing capacity overfirst few cycles because of swelling
No observable trend in capacitywith quality of coating– Note: PVdF-HFP = Kynar Flex 2801
2025 coin cell

Lacey et al, submitted

Effect is even more pronouncedwith pre-infiltration of sulfur
Pre-infiltration of S by mixing with C and heating
to 155 °C
Binder fills remaining poresElectrochemistry even worse, except if binder isnot included at all!
~1100 mAh g-1 with 70.6% S in cathode –extremely high!

Lacey et al, submitted

Almost total
pore blocking!

2025 coin cell

PVdF* is not a good binder for Li-S
* Disclaimer: only guaranteed for homopolymer PVdF in DME:DOL electrolytes
with high S-loading into highly porous carbon hosts prepared from slurries in NMP!

“Realistic” composition:
86% (2:1 S:C, melt infiltr.)
7% Super P
7% binder (from NMP) 57.3% w/w S in cathode2025 coin cell

PVdF homopolymer shows
consistently poorer performancecompared to PVdF-HFP overextended period and over a rangeof rates

Reminder…
Other aq.,
5%
PEO, 4%
CMC or
CMC:SBR,9%

PVDF-HFP,1%

PVDF
81%

CMC:SBR and PEO
CMC:SBR is a decent alternative to PVdF
Stable binder system with reduced degree of microporosity
blocking from water-based slurries

PEO shows better performance
Higher capacity, reduced hysteresis, lower impedance atcharge/discharge limits

Note! Older results with different cathodecomposition and cell construction.
Capacities and capacity fade cannot be compared
between coin cell and coffee-bag cells

50% S, 40% Super P, 10% binder

Voltage profiles at 1 C

Lacey et al, Chem. Commun. 49, 8531 (2013)“Coffee-bag” cell
20 mm dia. cathodes
100 µL/mg (S) electrolyte

Local electrolyte additive effect of PEO
Lacey et al, Chem. Commun. 49, 8531 (2013) – Hot article for Aug 2014!
Motivated by reports of PEO/PEGbased cathode “barriers” or“polysulfide traps”
Unification of several literaturestudiesCommon beneficial effect ofpolyethers – as a binder, a cathodecoating, or electrolyte additive
Higher capacity (sulfur utilisation)and reduced hysteresis“Coffee-bag” cell

PEO as a binder: best performance
Lower impedance with PEO binder

Reduced overpotential atcharge/discharge limits

Lacey et al, J. Power Sources 264, 8–14 (2014)80

 Reduced passivation of electrodesurface (e.g., effect of Li+ softening)?

70

260-Z'' /  cm

Lacey et al, Chem. Commun. 49, 8531 (2013)

ChargedDischarged

CMC:SBR

50403020100020406080

Z' /  cm

100120140160

280
70

PEO

260-Z'' /  cm

ChargedDischarged

50
4030
20
100020406080

Z' /  cm“Coffee-bag” cell

100120140160

2However: PEO is actually not a very good binder...
difficult to coat from water, poor adhesion

Amides/lactams
strong interactions with PS! Can it be a real barrier?
Lacey et al, J. Power Sources 264, 8–14 (2014)
Li2S6 in
DME:DOL (1:1)

Seh et al, Chem. Sci. 4, 3673 (2013)

Increased stability of Li2S-based cathodes
with PVP binder – less PS in electrolyte,
therefore less active mass loss to the anode

Addition of PS solution
to a solution of PVP

Our observation – dark red, insoluble, stablecomplex formed between Li2S6 and PVP
Is the effect retained with S-based cathodes?
Can we pair it with PEO for increased
capacity and stability?

PEO:PVP
a functional, cooperative binder system
1400

Lacey et al, J. Power Sources 264, 8–14 (2014)

50% S, 40% Super P, C/5

13001000 mAh g-1 after 50 cycles

1200

Optimal 4:1 mixture outperforms
individual components

1100

Qdischarge / mAh g-1

1000
900

PEO increases capacity, PVP stabilises

800

PVP reduces slurry viscosity enablingwater-based cathode preparation

700
600
500

OCV retention over 1 week

400
300

2:3 CMC:SBR
PEO
PVP
4:1 PEO:PVP

200
100001020

1:1 PEO:PVP
7:3 PEO:PVP
9:1 PEO:PVP
30

cycle number“Coffee-bag” cell

40500102030

cycle number

4050

Very promising results withoptimised PVP-based binders
1100 mAh g-1, 59% S in cathode, commercial materials only!

PVP-based binder mixtures
59 % SHigh capacity @ high S loading,water solubility, compatible withhigh S.A. carbons

68 % S
58% S(CMC:SBR)

No exotic materials or techniques

Optimised binder in this casematched to optimised carbon
with slightly higher S.A. and porevolumeC/10, ~ 1 mg cm-2 (S)2025 coin cellVery promising results with

optimised PVP-based binders
2025 coin cellMax. capacity loss ~270 mAh/g- Mostly reversible- Doesn’t seem to be significantly
changed over time

Filled points – cycle begins after a wait at OCV – number of days indicated by number

Rate of self-discharge clearly slowed by PVP binder
With optimised carbon/binder – double capacity after 3 months!

Besides the very high energy density…
…perhaps the next best advantage of Li-S
is that it is potentially cheap

So it is surely important that strategies totackle the drawbacks are cheap and scalable

Sulfur stacks from oil sands in Fort McMurray, AB, Canada
Photo credit: globalforestwatch.caConclusions• The binder can be considered as a functional, local
electrolyte additive• Polyethers can be used to increase capacity, PVP canbe used to stabilise capacity– Can investigate cooperative and water-soluble binder
combinations based on this concept• Certain binder/solvent combinations can be
detrimental to performance – PVdF/NMP is a notable
example• Self-discharge is still a considerable problem with thissystem which deserves more attention

Thank you for your attention!
Kind acknowledgements:– Martin Oschatz, TU Dresden– Dr Martin Cadek, OrionEngineered Carbons GmbH
– Era Net Transport project“MaLiSu”– Vinnova, Sweden

For further information:
The effect of PEO:
Lacey et al, Chem Commun. 49, 8531 (2013)
PEO:PVP binder:
Lacey et al, J. Power Sources 264C, 8 (2014)
Porosity blocking:Lacey et al, in submission
Our poster – a Li anode study (s05-054)
Before it gets taken down this evening!

Correspondence to: matthew.lacey@kemi.uu.se