Multilayer Electrode Coatings

Robert Spotnitz

STAR

KOREA CONFERENCE

2015

Overview

Extrusion Coating of Foils

Coextrusion

– Options

– Benefits

Simulation Studies

– Objective: maximize useable energy for HEV

application

– Effect of segregating particle size

– Effect of electrode thickness

Conclusions

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Knife

http://www.coatema.de/eng/presse_news/fachartikel/2009_08_06_97595399_fachartikel.php?navanchor=1010010

Comma Reverse

Coating Processes

Trend of Coating Processes

K. Omori, AABC10, Orlando, Fl

(2010)

Slot Die

Use of slot dies enables coextrusion.

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Co-extrusion manufacturing process of thin film

electrochemical cell for lithium polymer batteries

EP 1568090 B1 Pub Date: 10/13/2010

Coextrusion has been used to make electrodes.

Some Options for Multilayer Extrusion

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Different particle size in each layer

Different materials in each layer

Different materials, particle sizes, and layer thickness

Coextrusion enables multiple layers with different thickness, varying compositions, and particle size distributions.

Mixtures of different materials, particle sizes, and layer thickness

1)

2)

3)

4)

Multilayer Positive Coating: Low-rate material coating high-rate material

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J.F. Whitacre, K. Zaghib, W.C. West, B.V. Ratnakumar “Dual active material composite cathode structures for Li-ion batteries”, J. Power Sources 177 (2008) 528–536

LiFePO4

Li[Li0.17Mn0.58Ni0.25]O2

“All cell current was forced to pass through the Li[Li0.17Mn0.58Ni0.25]O2

material.”

50:50 wt% Blend

50:50 wt% Layers

Energy above 2 V, mWh/g

C-Rate

Multilayer Positive Coating: High-rate, high-capacity material coating high-rate material

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J.F. Whitacre, K. Zaghib, W.C. West, B.V. Ratnakumar “Dual active material composite cathode structures for Li-ion batteries”, J. Power Sources 177 (2008) 528–536

LiCoO2/LiFePO4 multilayer

LiFePO4

Energy above 2 V, mWh/g

C-Rate

“If the carbon-coated LiFePO4 was layered with the higher rate LiCoO2, conditions were improved significantly”

Multi-layered positive coating provides improved 4C-rate capacity.

Example

C-rate capacity (mAh/g) Capacity Retention (%) 0.5 C 3 C 4 C

5:5 LMO/NMC 124.7 100 60 91.1

3:7 LMO/NMC 133.5 106.5 63.6 91.5

NMC 134.2 111.8 46.4 90.1

LMO 102.3 83.7 49.2 91.2

Blend LMO/NMC 120 98.5 46.2 82.6

Power layer - LMO

Energy layer – NMC

LiMn2O4, D ~ 10-7 cm2/s LiNi0.4Mn0.4Co0.2O2 , D ~10-8cm2/s

Power Layer

Energy Layer

Current Collector

Simulation Study: Optimization of Porosity and Particle Size for Discharge Rate

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S. Golmon, K. Maute, M. Dunn, “A design optimization methodology for Li+ batteries” J. Power Sources 253 (2014) 239-250

e=0.3

r=5

“suggests that functionally grading the electrode can improve the overall battery performance allowing for greater usable energy density in a battery.”

Capacity, Ahm-2

Rate, Am-2

Distance Across Battery, mm

Summary – Previous Work

Focus on discharge behavior

Improve discharge rate without loss of capacity by placing

high power cathode coating on top of high energy coating

Can adjust power/energy curve by varying distribution of

particle size and porosity across electrode

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Importance of Surface Area on Lithium Loss

Experiments show that

first cycle loss is

proportional to surface

area of graphite

Simulation predicts

lithium loss due to SEI

growth is proportional to

surface area*

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Irrev.

Cap.

Loss

(Ah/g)

Surface Area (m2/g)

Li/graphite cell

R. Fong, U. von Sacken, and J. R. Dahn,

J. Electrochem. Soc., 137 (7) 1990

* Based on SEI growth model of H. Ploehn et al.

J. Electrochem. Soc., 151 (3) A456-A462 (2004)

Increasing rate capability by increasing surface area may reduce capacity and life.

Surface area determined by particle size distribution

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Computed surface area of spherical

graphite particles m = 6 mm

s = 3 mm

Surface area of mixture is significantly larger than that corresponding to average particle size.

Restrict radius from 0.5 to 11.5 mm

rSa

3

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3 Electrode Designs with 3 Particles Sizes (same total particle size distribution)

Single layer Ternary mixture

Two layers Binary mixture Single particle size

Three layers Single size particle in each

Lowest porosity for packing of single size spherical particles is ~25%. Particle mixtures can give higher packing densities, but keep porosity constant at 25%.

Electrode Designs based on Sorting Particle Size into Layers

18650 Cell Design

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Computed

Electrode Properties

NCM

Positive

Value

Graphite

Negative

Value

Average Voltage, V 3.85 0.074

Stoichiometry at formation

0.473 0.896

Unit Capacity, mAh/cm² 1.58 1.70

Loading, mg/cm² 25.4 10.8

Thickness(w/collector), µ 98.0 78.8

Coating Thickness, µ 40.0 33.4

Coating Porosity, % 25.0 25.0

Coated Length*, cm 204.7 206.0

Total Length, cm 104.5 107.3

Cell Properties Value

Capacity, Ahr 1.8

Energy, Whr 6.9

Energy Density

Whr/kg 154

Whr/liter 416

Weight, g 44.7

Active Area, m² 0.116

Simulations use 1D

model; does not account

for current distribution

along collectors.

Use same particle size distribution for positive and negative electrodes.

Battery Design Studio Settings

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Simulation of lithium loss during calendar aging

Aging of all three electrode designs is identical because total particle size distributions are identical.

3% capacity loss after 1 year at 30°C

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Simulated HPPC Testing with Fresh Cells

~ 13 mV difference

10% SOC

90% SOC

~ 10 mV difference

The difference in voltage drop on discharge between cell with 3 layers and cell with 1 layer is ~10-13 mV during pulses.

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Polarization Analysis – what causes voltage losses?

Polarization %Difference

Between L1 and L3 Diffusion Electrolyte 48% Ohmic Drop Electrolyte 37% Pos Activation 12% Neg Activation 1% Pos Solid-Phase Diffusion 1%

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Major difference between one layer and three layers are voltage losses in electrolyte.

Most significant sources of voltage loss are electrolyte diffusion and ohmic drop, and activation overpotentials at negative and positive electrodes.

Profiles at end of first discharge pulse

In multilayer electrodes, liquid-phase current density drops more rapidly in layer near separator so lower current density throughout most of electrode. This lower current density results in lower liquid-phase potential drop and concentration gradient.

3 Layers

3 Layers

1 Layer 1 Layer

Negative Positive Separator Negative Positive Separator

3 Layers

1 Layer Negative Positive Separator

Liquid-Phase Potential

Liquid-Phase Concentration

Liquid-Phase Current Density

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Hybrid Pulse Power Characterization Test

By putting fine particles on layer near separator, reduce battery size factor compared to single layer case by ~6% Segregating particles further has very minimal impact.

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Segregating fine particles near

electrode/separator interface reduces # of

cells required to meet goals from 249 to 233.

Simulation of constant current discharge for cells with thin (40 mm) electrodes, 10 C rate, ambient 25°C

Putting small particles near electrode/separator interfaces increases power output slightly initially, but effect is small <1 %.

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Simulation of constant current discharge for cells with thin (40 mm) electrodes, 20 C rate, ambient 25°C

Putting small particles near electrode/separator interfaces increases power output slightly initially, effect ~2 % at short times.

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Simulation of constant current discharge for cells with thin (40 mm) electrodes, 30 C rate, ambient 25°C

Putting small particles near electrode/separator interfaces increases power output slightly initially, effect ~4 % and extends time before lower voltage limit reached by ~2 s.

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Simulation of discharge for cells with thick (80 mm) electrodes

Putting small particles near electrode/separator interfaces increases power output slightly initially, but effect is small < 1 %.

4A rate, 25°C 6A rate, 25°C

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Putting small particles near electrode/separator interfaces increases power output slightly initially, but effect is small. At very high rate, single layer electrode gives longer discharge because cell temperature is higher.

10A, ambient 25°C 20A, ambient 25°C

Simulation of constant current discharge for cells with thick (80 mm) electrodes

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Summary/Conclusions

Extrusion coating enables use of multilayer coating.

Thin (40 micron) multilayer coatings provides a small (~5%) advantage over single layers for pulse power applications. For constant current discharges the multilayer electrode is only advantageous at very high rates.

Thick (80 micron) multilayer coatings provide a small benefit in power for short discharge times at very high rates. However for a complete constant current discharge, the multilayer coating may give a shorter runtime due to operating at a lower temperature than a multilayer coatings.

Capability to simulate multilayer coatings available in Battery Design Studio and STAR CCM+/BSM.

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