Choosing the cathode binder:
How much difference can it make?
Matthew J. Lacey
Fabian Jeschull, Kristina Edström and Daniel Brandell
Department of Chemistry – Ångström Laboratory, Uppsala University
matthew.lacey@kemi.uu.sehttp://www.iws.fraunhofer.de/malisu

The target:
Optimisation of carbon structure for energy
density and stability
Polymer materials/electrolyte additives forredox shuttle/self-discharge suppression
Anode stabilisation, cell balancing
Prototype cell of 400 Wh/kg(3 mg (S) cm-2, 60% S in cathode, 1 Ah g-1 (S))

Why binder? Why would it matter?
Slurry casting: convenient existing process using active
material, binder and other additives for making stablecomposite electrodes
But, compared to Li-ion, we think the binder in a Li-S
electrode can be significant in the following ways:
Pore blocking: i.e., inaccessibility of surface area/porevolume à reduced capacity, rate capability

Functionality: beneficial interactions with intermediates (or evenend products?)

Experimental overview
Binders:
Two types of cell:nF

2025 coin cellFFFnF FFmCF3O nROO

(Kynar FLEX 2801)(Aldrich, Mw ~4M)

OROn

CMC:SBROR

1 M LiTFSI, 0.25 M LiNO3, DME:DOL 1:1

PVdF-HFPPEO“Coffee-bag” pouch cell

Electrolyte:PVdF(Aldrich)(Leclanché, Targray)

R = H or CH2CO2H

nN

O

PVP
(Aldrich, Mw ~360k)

Simple recipe for high capacity:

surface area, pore volumesolid à solubleS8 à S62soluble à solid

S62- à S42- - - -> Li2S
Surface area for kinetics, delayed passivation
Pore volume space for discharge products…and some sort of strategy to minimise capacity lossOptimised host structures are
well-investigatedBinders: a literature snapshot(2013/14)
Other aq.,
5%
PEO, 4%
CMC or
CMC:SBR,9%

PVdF-HFP,1%

PVdF
81%

Pore filling in carbon black

PVdF

Loss of:surface area
pore volume

PVdF-HFP

S.A.
m2/g

pore volcm3 g-1

µpore
S.A.

µpore
volCB

1100

1.76

376

0.16

C:PVdF

175

0.53

00.00

C:(-HFP)

119

0.55

00.00

C:PEO

170.11

00.00

Pores of all sizes filled between 1.7 and 100 nm;
significant reduction of surface area and pore volume.
Lacey et al, J. Phys. Chem. C, in press
N.B. pore volume measured up to ~75 nm pore width

“Swellabilityseries”
PEO > PVdF-HFP > PVdF-HFP
Exaggerated cathode composition:
60% S, 25% C, 15% binder
6 µL/mg (S) electrolyte, C/20 rate
Unusual increase in capacity in first few
cycles because of swelling
No observable trend in capacity withquality of coating

Lacey et al, J. Phys. Chem. C, in pressContrast - at C/10
Upper plateau
stays the same…

Lacey et al, J. Phys. Chem. C, in press

Lower plateau
increases over 3 cycles!

Not increased utilisation of S8,but increasing pore volume?

Even more pronounced effect with

pre-infiltration of sulfur
Pre-infiltration of S by mixing with C and heating to155 °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, J. Phys. Chem. C,
in press

C/20

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)à 57.3% w/w S in cathodePVdF homopolymer shows
consistently poorer performancecompared to PVdF-HFP overextended period and over a rangeof rates
Lacey et al, J. Phys. Chem. C, in press

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 ofmicroporosity blocking from water-based slurries

PEO shows better performance

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

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

Higher capacity, reduced hysteresis, lower impedance atcharge/discharge limits

Voltage profiles at 1 C

C/5 rate

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

20 mm dia. cathodes
100 µL/mg electrolyte

Background:

PEO as a polysulfide trap?
Functionalised mesoporous carbon
surface with PEG-250
X. Ji, K.T. Lee, L.F. Nazar, Nat. Mater. 8 (2009) 500

PEG coating
No PEG coating“We believe that the effect of the PEG-functionalized surface
is twofold. First, it serves to trap the polysulphide
species by providing a highly hydrophilic surface chemical
gradient that preferentially solubilizes them in relation to theelectrolyte. Second, by limiting the concentration of thepolysulphide anions in the electrolyte, the redox shuttle
mechanism is curtailed to a large degree.

Spin-coatedPEG-20000“barrier”

J.-Q. Huang, Q. Zhang, S.-M. Zhang, X.-F. Liu, W.
Zhu, W.-Z. Qian, F. Wei, Carbon N. Y. (2013) 1.

Alternative interpretation?
Ji et al: “the kinetics of the last reaction step has a role in capacity limitation…There is
progressively more limited accessibility of Li+ ions and electrolyte to the sulphur mass towards
the end of discharge because the pores become filled with insoluble LixS (x =1–2)”

C. Barchasz, J.-C. Leprêtre, S. Patoux, F. Alloin, Electrochim. Acta 89 (2013) 737.

Barchasz et al: “PEGDME solvents proved to be key components for Li/S electrolytes, as
preventing the fast electrode passivation and extending the length of the second discharge
plateau.”Ji et al: “Deposition of insoluble sulphur species on the surface of the Li electrode andformation of irreversible Li2S on the [PEG-modified] cathode surface are strongly inhibited”

Local electrolyte additiveeffect of PEO
Lacey et al, Chem. Commun. 49, 8531 (2013)
Motivated by reports of PEO/PEGbased cathode “barriers” or“polysulfide traps”
Unification of several literaturestudies
Common beneficial effect ofpolyethers – as a binder, acathode coating, or electrolyte
additiveHigher capacity (sulfur utilisation)and reduced hysteresis

PEO as a binder: best performance
Lower impedance with PEO binder
Reduced overpotential at charge/
discharge limits

Lacey et al, J. Power Sources
264, 8–14 (2014)	  8040	  2-­‐Z ''	  /	   Ω	  c m

50

3020
10002040

60

80

Z '	  /	   Ω	  c m

100

120

14016028070	  C ha rg e d	  D is c ha rg e d

PEO

60
2

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

60-­‐Z ''	  /	   Ω	  c m

à Reduced passivation of electrodesurface (e.g., effect of Li+ softening)?	  C ha rg e d	  D is c ha rg e d

CMC:SBR

7050403020100020406080

Z '	  /	   Ω	  c m

100

120

140

2

However: PEO is actually not a very good binder...
difficult to coat from water, poor adhesion160

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?

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

PEO:PVP

a functional, co-operative binder system	  1400

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

13001000 mAh g-1 after 50 cycles

1200

Optimal 4:1 mixture outperforms
individual components

1100

PEO increases capacity, PVP stabilises

900

PVP reduces slurry viscosity enablingwater-based cathode preparation

800
700	  Q dis c ha rg e 	  /	  m A h	  g-­‐11000

600

OCV retention over 1 week

500
400300	  2:3	  C MC :S B R	  P E O	  P V P	  4:1	  P E O :P V P

200
100001020	  1:1	  P E O :P V P	  7:3	  P E O :P V P	  9:1	  P E O :P V P
30c yc le 	  num be r

4050

0102030c yc le 	  num be r

4050

PVP – high impedance on its own,
kept low in combination with PEO
1st cycle

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

To answer a question from earlier:self-discharge reduced, rate capability preserved
1200
1100
1000

Q dis c ha rg e 	  /	  m A h	  g-­‐1900
800
700
600
500
400300	  2:3	  C MC :S B R	  P E O	  P V P	  4:1	  P E O :P V P

200
10000204060

80100120c yc le 	  num be r

140

160

180200Very promising results with

optimised 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 ortechniques

Optimised binder in this casematched to optimised carbon
with slightly higher S.A. andpore volume

C/10, ~ 1 mg cm-2 (S)Very promising results with

optimised PVP-based binders
2025 coin cell

Irreversible capacity loss
Reversible capacity loss ~270 mAh/g

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 isthat 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!
Kind acknowledgements:

For further information:– Martin Oschatz, TU Dresden– Dr Martin Cadek, Orion
Engineered Carbons GmbHThe effect of PEO:
Lacey et al, Chem Commun. 49, 8531
(2013)– Era Net Transport project“MaLiSu”– Vinnova, Sweden

PEO:PVP binder:
Lacey et al, J. Power Sources 264C, 8(2014)
Porosity blocking:Lacey et al, J. Phys. Chem. C,
DOI: 10.1021/jp508137m

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