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WIR SCHAFFEN WISSEN – HEUTE FÜR MORGEN

Enhanced rate performance in electrode materials for lithium-ion batteries

Juliette Billaud, Florian Bouville, Tommaso Magrini, Fabio Bargardi, Claire Villevieille, André R. Studart

International Battery Seminar & Exhibit

March 20-23, 2017

SLS

SINQ

The Paul Scherrer Institut

New, better performing materials?

Engineering of existing materials?

Reduced charging times

More autonomy

Low cost

Lifetime

Safety

New, better performing materials?

Engineering of existing materials?

Lithium-ion batteries

Lithium-ion batteries

LiCoO2

Graphite

Specific energy

= Specific charge

* Voltage

Slow rates

Low loadings

The example of graphite

ca. 95% of commercial rechargeable Li-ion batteries

Low volumetric expansion (10%)

Reasonable specific charge

Low potential

Graphite vs. fast cycling

M. Hess, PhD Thesis No. 21240, ETH Zürich Electrodes: SFG6 graphite - ca. 50 µm - 4 mg

Limitation from

stage dense 2 to 1

Limitation from stage 1-2 but

compensated by following transitions

50 mV

overpotential

Graphite vs. different electrode thicknesses?

M. Hess, PhD Thesis No. 21240, ETH Zürich

Despite graphite’s:

low cost,

good reversibility,

high energy density

Graphite does not limit discharge performance but

does limit charge performance

Problem: diffusion of Li+ through thick

electrodes

Buqa et al., Journal of The Electrochemical Society, 152, A474 (2005)

Graphite vs. particle size & electrode thickness

What improvements have been made so far?

Diffusion of Li in graphite:

very anisotropic

(only within the

crystallographic basal

plane)

SFG 44 (ca. 40 µm) SFG 6 (2-5 µm)

Thin

electrodes

Thick

electrodes High tortuosity

Use of smaller graphite flakes

Improvements for graphite so far

Tran et al., Journal of applied electrochemistry (1996)

Aurbach et al., Solid State Ionics (2002)

Use of particles with different morphologies

Other approach, cheaper? Reduction of tortuosity?

Tortuosity?

𝜏 =𝐿′

𝐿

Graphics: M. Magrini

Ebner et al., Journal of the Electrochemical Society, 162, A3064 (2015)

Tortuosity = 1

Tortuosity > 1

L’ : Path of a Li+ that intercalates

through an electrode

L : Thickness of the electrode

Average ratio of the length of the actual path taken between

two points and the straight distance between them

Lower tortuosity believed to increase the accessibility of

the intercalation planes of graphite to the Li+

Tortuosity – homogeneisation strategy

Doyle et al., Journal of the Electrochemical Society (1996)

Credit: Tommaso Magrini

𝐷𝑒𝑓𝑓 =𝜀

𝜏𝐷0

(diffusion coefficient) (effective diffusion

coefficient)

Use of magnetic alignment with

sacrificial solids

How to reduce the tortuosity?

Freeze-casting

Sander et al., Nature Energy (2016) Bahrami et al., Green Chem. (2017)

Co-extrusion with a sacrificial pore former

Bae et al., Advanced Materials (2013)

Can we work directly on the active materials to optimise the architecture?

Reduction of the tortuosity by magnetic alignment

Fe3O4 nanoparticles

Only 0.01 – 0.5 vol. % Fe3O4 needed

Studart, Erb, Libanori, Patent WO2011120643, 2011

Erb et al. Science. 2012

Principle of the magnetic alignment

Erb et al. Science. 2012

Can we apply this to battery materials…?

Bouville, Le Ferrand et al. Nature Materials. 2015

Approach to reduce the tortuosity

Li+

Li+

Aligning perpendicular to the current collector:

- Enhances accessibility of intercalation planes

- Reduces tortuosity of electrolyte in intercalation direction

Shorter and easier pathway

for Li diffusion, even in

thick/highly loaded

electrodes

Simple, scalable and

inexpensive

technique consisting

in applying a low

magnetic field during

fabrication of anode

x 20

Hrot

Electrodes preparation

Slurry preparation:

80% graphite flakes

10% PVP

10% super P

Magnet: 100 mT

J. Billaud, F. Bouville et al., Nature Energy, 1, 16097 (2016)

Graphite used: SFG44

XRD

Degree of alignment probed by tracking the intensity of the

(002) Bragg peak (corresponding to the basal plane spacing)

C. E. Banks and R. G. Compton, The Analyst, 131, 15 (2006)

(002)

In-plane alignment of the flakes

Cross-section SEM

Aligned electrodes Non-aligned electrodes

Orientation not totally random…

FIB-tomography

𝐷𝑒𝑓𝑓 =𝜀

𝜏𝐷0

Non-aligned electrodes Aligned electrodes

Rate capability tests

Galvanostatic profiles

Galvanostatic profiles

Overpotential increased by two times more for non-aligned electrodes

Aligned Non-aligned

Long term cycling on thick electrodes

Current collector

Current collector

27% Qth

18% Qth

Effect of the calendering on the alignment

Intensity of the (002) peak does not increase

until the thickness is reduced of 30 %

Compression with SEM

Initial

thickness

Conclusions and outlook

Simple and cheap method to align graphite particles and decrease tortuosity of the lithium

paths

High loading electrodes for fast rate cycling obtained

Performance improved solely by changing the inner architecture of the electrode

But:

- large graphite flakes

- high porosity electrodes calendering allows densification by 30%, tests in progress

- optimisation of the process (carbon, binder, ..)

Many more materials to look at!

Acknowledgements

BatMat group at the PSI

Complex materials group at the ETHZ

Thank you for your

attention!

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