Carbon Nanotubes for High-Performance Lithium-Ion Batteries

CarboPower

David Ensling1, Alberto Varzi2, Martin Kreis3, Corina Täubert2, Sebastian Schebesta1

1 VARTA Microbattery GmbH2 FutureCarbon GmbH3 Zentrum für Sonnenenergie- und Wasserstoff-Forschung (ZSW)

Outline

• Introduction

• Project

• Lithium-Ion Battery

• Processing

• Material Evaluation

• Anode

• Cathode

• Conclusions

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� Evaluation of Carbon Nanotubes (CNT) as conductive agent for next-generation

lithium ion batteries• Improved energy density due to smaller amounts of conductive additives• Increased power density due to high conductivity

CarboPower

FutureCarbon

Preparation, purification, functionalization and dispersion of CNTs

Varta Microbattery

Slurry modification, electrode preparation,

test / full cells

ZSW

Characterization of raw materials and electrodes

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CNT Manufacturing

and Refinement Processes

CCVD Process for Base Material Synthesis (Catalytic Assisted Chemical Vapor Deposition)

Current Production Capacity: ~ 1 ton per yearShort-term upscaleable to >10 tons per year, depending on demand

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CNT Properties

SEM (electrode)TEM

- MWCNTs (5 to 9 walls)

- average outer diameter: 13 nm

- length: 1-100 µm

- BET: ~ 290 m2/g

� CNT-Dispersions

- Water (CMC-binder)

- NMP / NEP (PVDF-binder)

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Preparation of Electrodes

for Lithium-Ion Batteries

Coating Cutting

Process steps:

1. Mixing of components

2. Coating on Al- or Cu-foil (10 – 20 µm)

3. Cutting of electrodes

4. Calendaring

Mixing

Composite Electrodes:

• Active material, i.e. LiCoO2, Graphite (90%)

• Conductive agents, i.e. carbon black, CNTs (5%)

• Binder, i.e. PVDF, CMC (5%)

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Typical processes during

cell fabrication

Stacking

(Winding)

TaggingPacking

Finishing

Activating

Example: stacked cell

1. Stacking of single electrodes

and separator

2. Tagging of current collectors

3. Packing in Pouch

4. Filling with electrolyte

5. Formation (first charging)

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dch.

CNT: active anode materialCNT: conductive agent

in anode and cathode

Graphite

a bx y

z

Layered TM-Oxide

Composite electrodes:

active material/binder/conductive agentLithium-ion battery

Solid-electrolyte

interphase (SEI)

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Negative Electrodes

- MWCNTs as Lithium Host

Graphite (BET: ca. 4 m2 g-1)

0 100 200 300 400 500 6000

1

2

3

E v

s.

Li/Li+

/ V

Specific Capacity / mAh g-1

SEI

Irrev. loss

0 200 400 600 800 1000 1200 1400 1600 18000

1

2

3

E v

s.

Li/Li+

/ V

Specific Capacity / mAh g-1

MWCNTs (BET: ca. 300 m2 g-1)

SEI

Irrev. loss

� Different insertion mechanism compared to graphite (absence of a charge/discharge

plateau) – still unclear due to difficulty in determining the Li storage sites

� Huge irreversible capacity due to Solid Electrolyte Interphase (SEI) → proportional to

surface area!

Not suited as active anode material !

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Negative Electrodes

- MWCNTs as Conductive Agent

1C

Long-term cycling

NMC/CNT vs. Graphite/CNT

Graphite anode0.05 – 0.35V

Initial losses

Ref.

CNT

� Very good cycling stability, butinacceptable initial losses (<1V)

� Capacity is retained up to 80% after1000 cycles in the potential window 1-3V

Long-term cycling

TiO2/CNT vs. Li

TiO2 anode1.0 - 1.8V

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Positive Electrodes

- MWCNTs as Conductive Agent

Capacity Retention SEM

CNT /Binder

NCM

Model

Long-term cycling stability of half cells (NMC vs. Li)

� MWCNTs avoid the contact loss between the particlesproviding a capacity retention up to 75% after 250 cycles at 2C

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� Promising results for CNT-cathodes

� combination with standard anode in full cell

Experiment

� Variation of conductive additives

(CNT, carbon black)

Full-cells

Anode Cathode

A Graphite NMC + carbon / reference

B Graphite NMC + „more“ carbon

C Graphite NMC + CNT

CB

CB++

CNT

CB = carbon black

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Positive Electrodes - MWCNTs as Conductive Agent

Rate Capability of full cells (NMC/CNT vs. C)

BA

C

� improved rate performance for CNT electrodes

� only slight improvement with high carboncontent over reference

Capacity 3C 5C 8C

(A) Reference 74% 45% 14%

(B) High Carbon 77% 56% 22%

(C) CNT 82% 61% 36%

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C-rate:C (mA) = Cap. (mAh)/h

Positive Electrodes - MWCNTs as Conductive Agent

Long-term cycling stability of full cells (NMC/CNT vs. C)

B

A

C

� better cycling stability for CNT electrodes thanfor reference

� comparable to high carbon content electrodes(energy density)

Capacity 250 500 750

(A) Reference 92% 89% 86%

(B) High Carbon 94% 91% 89%

(C) CNT 96% 93% 90%

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The Strategy: functionalization

Improved particle-

particle connection(better dispersibility in

polar solvents)

Chemical oxidation

MWCNTs-COOH

CNTs unbundling

pristine MWCNTs

(highly agglomerated)

• further improve the electrochemical

performance

• completely replace common additives with

the lowest CNT amount possible

The aim was to

In principle

• use of CNTs result in lower percolation

threshold than carbon black and/or graphite

In practice

• MWCNTs have strong tendency to

agglomerate leading to poor connection of

the active materials particles

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NCM-based cathodes (~ 10µm)

90 wt% active material, 2 wt% Conductive Agent, 8 wt% Binder

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LiFePO4-based cathodes (~ 500nm)

90 wt% active material, 2 wt% Conductive Agent, 8 wt% Binder

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NCM-based cathodes LiFePO4-based cathodes

0 20 40 60 80 100 120 140 160

3.0

3.2

3.4

3.6

3.8

4.0

4.2

4.4 with MWCNTs-COOH with MWCNTswith carbon black

E v

s.

Li/L

i+ / V

Specific Capacity / mAh g-1

C/D rate: 1C

0 20 40 60 80 100 120 140

0

20

40

60

-Zim

/ O

hm

-Z

Zreal

/ Ohm

surface passivation

0 20 40 60 80 100 120 140 160 180

2.8

3.0

3.2

3.4

3.6

3.8

4.0

4.2

4.4

with MWCNTs-COOH

with MWCNTs

with carbon black

E v

s. L

i/L

i+ / V

Specific Capacity / mAh g-1

C/D rate: 1C

� electrochemical performance appears to be affectedby surface passivation reaction

� improved connection network among the active particles,,however dramatic fade of electrochemical performance

� MWCNTs-COOH provide an enhanced connectionnetwork among the nano-sized LFP particles thusimproving the capacity

� for LFP cathodes the electrode kinetics appearsto be mainly dominated by resistivity issues

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Conclusions

Cathodes

• Significant increase of long-term cycling stability

• Improvement of rate capability (power density)

• Increase of rate capability

• Increase of long-term cycling stability

• However, large initial losses for graphite due to SEI formation

• Stable above 1V vs. Li/Li+ (e.g. TiO2); Problem with graphite

Anodes

• More difficult than “classic” conductive agents (e.g. carbon black, graphite particles)

• Use of CNTs requires modification of processing

Processability

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Comparison

• CNT as conductive additive

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consumer,

EV

Power tools,

HEV

Automotive,

Grid storage

Thank you!

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