Porocarb®

Carbon Functional Additive

In recent years, new cell designs have been developed specifically for large scale applications in the automotive and stationary storage sectors. Such batteries require a different balance between energy and power density. Furthermore, cell degradation and safety are important aspects to consider due to the needed long useful life and large battery formats.

For new applications in electro-mobility and energy storage, such as battery systems for electric drivetrains or decentralized storage systems for renewable energies, the energy and power capabilities are similar to consumer cells but require a calendar life of approximately 10 years and a cyclic life of more than 3000 cycles.

Introduction

Porocarb® batteries combine the advantages of power-oriented and

energy-oriented cell designs.

Higher cycling stability and capacity retention

in Porocarb® batteries.

Electric vehicles do, however, face significant battery-related challenges:

Driving range, Recharge time, Durability, and cycle life.

New concepts for improved and advanced battery technologies are required in order to increase driving range and durability, while simultaneously decreasing recharging time, weight, and cost. These factors will ultimately determine the future of electric vehicles.

2

3

Improvement in cycle life and safety ultimately leads to a trade-off in energy densities and cost with standard cell designs. A new electrode concept is to supplement with Heraeus Porocarb® porous particles as an “ionic conductive additive” to assure optimal mass transport of lithium ions by locally enhancing the effective diffusivity in the volume of the calendared electrode. Thus, it will be possible to achieve higher energy densities with improved cycle life and durability.

Heraeus Porocarb® is an electrochemically inert carbon with low specific surface that can be used similarly to other conductive carbons in both electrodes of lithium-ion cells. Its unique porosity profile with distinct pore size regimes comprising of fully interconnected macropores for rapid ion transport and mesopores for local electrolyte storage not only promotes mass transport within the electrode, but also improves the pulse response of a cell.

Porocarb® – New Electrode Concepts for advanced Lithium-ion Batteries

Porocarb®’s fully interconnected macropores enable

rapid ion transport.

Porocarb® enables energy-cell like loadings for power-oriented

electrodes.

4

Percolation vs. Imprinting

Typically, high-energy electrodes show porosities well below 20 % after calendaring, where more than half of the remaining porosity is given by smaller pores with average diameters less than 400 nm. These electrodes show substantially reduced effective mass diffusivity caused by low residual porosity, a high degree of twisting for the electrolyte path (tortuosity) and increased average viscosity as a result of higher mean proximity to pore walls compared to larger pores in uncalendared electrodes.

Porocarb® particles are designed to withstand the mechanical pressure of the calendaring process and retain their inner porosity and pore size distribution. Therefore porosity, which is usually negatively influenced by the calendaring process, can be locally conserved within the Porocarb® particles.

Porocarb® can be implemented into lithium-ion-battery electrodes in two different ways. More conventionally, a percolation-type approach can be taken for Porocarb® particles that are smaller than the active material. In this case, we recommend to scale the Porocarb® particles in a way that their d50 diameter is approximately one third that of the d50 of the active material (e.g. Porocarb® d50 = 3 µm and d90 = 6 µm for an active material with d50 = 10 µm).

A percolation curve for such electrodes is shown in Figure 1, indicating the onset of percolation at 3 wt.- % of Porocarb®. Due to the fully interconnected and surface-accessible pore network in Porocarb®, ionic percolation sets in at the same time as electronic transport, and local improvement of the porosity (no-percolation case, Figure 2a) evolves into a globally interconnected transport pathway for Li+ ions with low tortuosity (Figure 2b).

The use of larger Porocarb® particles [d50 (Porocarb®) > d50 (Active material), d90 (Porocarb®) << dry film thickness] enables a second electrode concept (Figure 2c), where the active material is not only slightly imprinted but to a certain degree embedded in the Porocarb®

after calendaring. Due to the large contact area, very low resistances can be achieved in between the carbon and the active material. The elasticity of the carbon furthermore accommodates expansion of the active material, which alleviates contact resistance degradation and delamination, thereby extending cycle lifetime of the electrode.

Figure 2:

Electrode concepts and percolation behavior with Porocarb®.

Non-percolating Porocarb® particles (low wt. fraction)

Percolating Porocarb® particles (higher wt. fraction)

Active material

Large Porocarb® particle

a b cPorocarb® Fraction [wt .-%]

Resi

stan

ce [Ω

/cm

2 ]

250

200

150

100

50

043210 5 6 7

Figure 1:

Electrical percolation for Porocarb® LD2N at 40 % porosity.

This percolation curve has been recorded without the addition of any

other carbon. Addition of 1 – 2 % of carbon black further reduces

the electronic percolation threshold, and is furthermore required for

setting the rheological profile of the slurry.

5

MatErIaL

Porocarb® Material Properties

Porocarb® is a product family of synthetic porous carbon powders tailored specifically for demanding electrochemical applications where the needs for a designed porosity and good kinetic accessibility intersect. Porocarb® porous carbon powders open the path to improved electrochemical systems which were not achievable in the past using standard carbon conductive additives off the shelf.

Porocarb® LD2N grade is especially suitable as a functional carbon additive for lithium-ion batteries.

Porocarb® HD3 grade is a porous species, especially suitable as a highly porous network particle in conversion batteries, as catalyst support in fuel cells and as a surface enhancer for electrodes.

For all our synthetic grades, we can offer a variety of customized post treatments in order to meet your systems requirements such as graphitization, particle sizing, surface treatments and more.

Pore Diameter [µm]dV

/dlo

gD [c

m3 g-1

]

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

0.10.010.001 1.0

Porocarb® LD2N

Pore Diameter [µm]

dV/d

logD

[cm

3 g-1

]

3.0

2.5

2.0

1.5

1.0

0.5

0

0.10.010.001 1.0

Porocarb® HD3

20 µm

20 nm

Figure 3: Pore size distribution of Porocarb® LD2N.

Figure 4: Pore size distribution of Porocarb® HD3.

SEM picture of

a Porocarb® particle

shows the unique

pore structure.

Aerated Density [g/cm3]

Tapped bulk density [g/cm3]

Pressed Density (75 kg/cm2) [g/cm3]

Real Density (Pycnometry) [g/cm3]

Surface Area (BET) [m2/g]

Ash Residue [wt.- %]

Humidity [wt.- %]

Electrical Conductivity (75 kg/cm2) [S/cm]

Oil Absorption Number (TP130P) [ml/100 g]

ASTM D6393 (2008)

ASTM D6393 (2008)

internal Heraeus method

ASTM B923-02

ISO 9277:10

ASTM D2866-94

ASTM D2867-09

internal Heraeus method

ASTM D2414-13a

0.331

0.475

0.510

1.651

55 – 60

< 0.2

< 2

0.019

184.7

0.190

0.248

0.270

1.697

529

< 0.02

< 2

0.038

371.6

Method Porocarb® LD2N Porocarb® HD3Property

6

Slurry Processing and Coating

Typically, slurries for lithium-ion electrodes consist of a solvent, the anode or cathode active material, carbon black or graphite to ensure the electrical conductivity and a binder for the cohesion between the particles and the adhesion of the electrode layer to the current collector. By partially substituting the conventional conductive additive (carbon black, graphite) by Porocarb®, the electrode loading can be increased while maintaining the performance, specifically the performance at high current rates.

Figure 5 shows viscosity data for cathode slurry before and after the addition of Porocarb®. One can clearly see that shear viscosity after addition increases approximately by a factor of 2, which can clearly be attributed to solvent absorption into the pore system of the Porocarb® particles. In a typical slurry process, the solvent that is absorbed in the pores would furthermore contain dissolved binder, which – after drying – could clog smaller pores.

Shear rate [s-1]

Shea

r vis

cosi

ty [P

as]

1000

100

10

1.0

0

0.1 10.01 10010

before addition of Porocarb®

after addition of 4% Porocarb®

Fortunately, the Porocarb® synthetic carbon is very easy to disperse and does not need high-shear energy for deagglomation and dispersion. Therefore we recommend to add the Porocarb® at the very end of the dispersion process. In order to allow maximum performance of the Porocarb® electrode, without clogged pores and reduced binder content, we further recommend to use pre-dispersed pastes prepared by pre-wetting Porocarb® with pure NMP under vacuum with solid contents of 60 wt.- %. This allows the wetting of the pores and avoids binder migration and absorption through the pore system.

A detailed application note of the slurry preparation of cathode slurries including Porocarb® is available on request.

Figure 5: Adding pre-wetted Porocarb® is recommended in order

to avoid solvent absorption and dynamic viscosity increase.

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In order to increase the cell level energy density, inactive materials need to be minimized in batteries and cells. Increasing the thickness of electrodes is one method of reducing inactive materials such as the current collector, but it further drives the problems of transport of mass and charge and requires a highly engineered porosity. The addition of Porocarb® maintains the ionic pathways and electrolyte supply at high electrode loadings and at high press densities. The interconnected network of pores allows for fast electrolyte penetration into the bulk of the electrode compared to electrodes containing conventional conductive additives (Figure 6).

Bending radius [mm] 0.1 0.5 1.0 2.5

Electrode (Ref.) --- ± +++ +++

Electrode Porocarb® --- – +++ +++

Electrode* (ref.)

Electrode**Porocarb®

Surface weight [mg/cm2] 19.5 19.0

Surface thickness uncal. [µm] 92.1 93.7

Surface thickness cal. [µm] 66.8 67.3

Density cal. [g/cm3] 2.91 2.81

Through the partial imprinting of the active material particles into the Porocarb® particles, as well as the particle size distribution of the Porocarb® itself, the compaction properties are similar to electrodes with conventional conductive additives (Figure 7). The imprinting of the active mass particles results in the mechanical stabilization of the particle-to-particle contacts throughout the electrode which is especially important at higher electrode loadings and when electrode materials exhibit faster or/and larger volume expansion.

The initial adhesion strength (stick winding test) of a Porocarb® containing electrode was demonstrated to be similar to a reference electrode containing carbon black and the same binder concentration (Table 1).

600

500

400

300

200

100

024 26 28 30 32

Liqu

id A

bsor

ptio

n Ti

me

[s]

Porosity [%]

AB 4% AB 2 % : LD2N 2 %

Figure 6: Liquid Absorption rate (propylene carbonate).

600

500

400

300

200

100

024 26 28 30 32

Liqu

id A

bsor

ptio

n Ti

me

[s]

Porosity [%]

AB 4% AB 2 % : LD2N 2 %

Radius

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Dens

ity [g

/cm

3 ]

Press Pressure [kN/cm2]

3.3

3.1

2.9

2.7

2.5

2.3

2.1

1.9

1.7

AB 4% AB 2 % : LD2N 2 %

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Dens

ity [g

/cm

3 ]

Press Pressure [kN/cm2]

3.3

3.1

2.9

2.7

2.5

2.3

2.1

1.9

1.7

AB 4% AB 2 % : LD2N 2 %

Figure 7: Relation between press pressure and electrode density.

Stick winding test for coated electrodes.* Active material 95 % binder 3 %, carbon black 2 %** Active material 91 % binder 3 %, Porocarb® 4 %, carbon black 2 %

SLurry & ELECtroDE

Electrode

Table 1: Stick winding test

8

rate Performance

0 5 20

spec

. dis

char

ge c

apac

ity [m

Ah/g

]

discharge cycle10 15

160

140

120

100

80

60

40

20

0

LD2NBaseline

C/10 C/2

1C

2C

5C

C/2

Figure 8: The use of Porocarb® in power-cells with lower

loadings (120 g/m2) improves the fast charging/

discharging performance at higher C-rates (4C, 5C)

up to 31%.

Loading: 120 g/m2

Formulation: 93 % active material, 3 % binder,

1.5 % Porocarb®, 2.5 % carbon black

Porosity: 37 %

Coating thickness: 38 µm

Figure 9: The drawback of low rate capability in high energy-cells

can be easily overcome by the use of Porocarb®.

Loading: 240 g/m2

Formulation: 93 % active material, 3 % binder,

1.5 % Porocarb®, 2.5 % carbon black

Porosity: 37 %

Coating thickness: 78 µm

0 5 10 15 20

spec

. dis

char

ge c

apac

ity [m

Ah/g

]

discharge cycle

160

140

120

100

80

60

40

20

025

LD2NBaseline

C/10

4C

C/2 1C2C

5C

C/2

9

A higher cycling stability up to 3000 cycles and less degradation

due to homogenous reaction distribution can be achieved by

using Porocarb®.

CELL

Cycle Life

100

90

80

70

60

50

40

30

20

10

0

0 500 1000 1500 2000 2500 3000

Capa

city

Ret

entio

n [%

]

Cycles

Porocarb® LD2N 3% Carbon Black 1% [190g/m2] Graphite 3% Carbon Black 1% [180g/m2] Graphite 3% Carbon Black 1% [130g/m2]

10

Porocarb® Further applications

The carbon functional additive platform Porocarb® fulfills the requirements on a carbon support and kinetic additive for other electrochemical power sources as well.

Porocarb® grades with high amount of mesopores (HD3) have been successfully applied in applications where nucleation and confinement of discharge products is required, such as Lead Acid and Lithium Sulfur cells or conversion electrodes.

Full or partial graphitization leads to Porocarb® grades with extremely high electrochemical corrosion resistance, which are useful in fuel cells as catalyst support, and in redox flow batteries as surface enhancer for the electrode.

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research and Development

Our mission is to develop innovative products and solutions for our customers and to provide the required scientific and technical support into a dynamic and fast growing market. Our team of highly qualified engineers, scientists and technicians are using state of the art instruments and technologies to continuously develop and improve our carbon products and manufacturing processes.

Heraeus New Businesses

Heraeus Deutschland GmbH & Co. KG

Heraeusstr. 12 – 14

63450 Hanau, Germany

Phone +49 (0) 6181. 35 - 6132

porocarb@heraeus.com

www.heraeus-porocarb.com

www.heraeus.com

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