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NREL Report Lithium-Ion Battery Safety Study Using Multi-Physics Internal Short-Circuit Model 2009


Lithium-Ion Battery Safety Study Using Multi-Physics Internal Short-Circuit Model
The 5th Intl. Symposium on Large Lithium-Ion Battery Technology and Application in Conjunction with AABC09 June 9-10, 2009, Long Beach, CA

Gi-Heon Kim, Kandler Smith, Ahmad Pesaran National Renewable Energy Laboratory Golden, Colorado
gi-heon.kim@nrel.gov
This research activity is funded by US DOE’s ABRT program (Dave Howell)
NREL/PR-540-45856
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Background
NREL’s Li-ion thermal abuse modeling study was started under the Advanced Technology Development (ATD) program; it is currently funded by Advanced Battery Research for Transportation (ABRT) program. NREL’s previous model study focused on understanding the interaction between heat transfer and exothermic abuse reaction propagation for a particular cell/module design, and provided insight on how thermal characteristics and conditions can impact safety events of lithium-ion batteries.
Total Volumetric Heat Release from Component Reactions

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Focus Here: Internal Short-Circuit
Li-Ion thermal runaway due to internal short-circuit is a major safety concern. Other safety concerns may be controlled by electrical and mechanical methods. Initial latent defects leading to later internal shorts may not be easily controlled, and evolve into a hard short through various mechanisms: separator wear-out, metal dissolution and deposition on electrode surface, or extraneous metal debris penetration, etc. Thermal behavior of a lithium-ion battery system for an internal shortcircuit depends on various factors such as nature of the short, cell capacity, electrochemical characteristics of a cell, electrical and thermal designs, system load, etc. Internal short-circuit is a multi-physics, 3-dimentional problem related to the electrochemical, electro-thermal, and thermal abuse reaction kinetics response of a cell.
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Approach:
Understanding of Internal Short Circuit Through Modeling
Perform 3D multi-physics internal short simulation study to characterize an internal short and its evolution over time Expand understanding of internal shorts by linking and integrating NREL’s electrochemical cell, electro-thermal, and abuse reaction kinetics models
Electro-Thermal Model
Temperature Current Density

Abuse Kinetics Model

Electrochemical Model
soc [%] 62 61.5 61 60.5 60 59.5 150 100 50 Y(mm) 0 0 50 100 X(mm) 150 200

T

soc

Internal Short Model Study
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Research Focus Is on …
Understanding electrochemical response for short Understanding heat release for short event Understanding exothermic reaction propagations Understanding function and response of mitigation technology designs and strategies

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Heating Pattern Change
A multi-physics model simulation demonstrates that heating patterns at short events depend on the nature of the short, cell characteristics such as capacity and rate capability.

Heating Pattern at Different Resistance-Shorts

7 Short Can slow down the evolution? Can suppress? 3 min 6min 9 min
Affected by external thermal conditions
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30 m Short

4s
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8s

12s
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Undetectable from external probes

Understanding Heating Response Differences
Heating from Short Circuit = Heat from Cell Discharge + Joule Heat at Short Joule Heat at Short
Heat for Cell Discharge [oC/sec]
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Ro Vocv Rs

Qd Vocv Ro = M (Ro + Rs )2 M
cell size increase

2

Short Heating =
Global

+
Local

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0 -4 10

10 10 Short Resistance [ ]
Ro= 100m (0.4Ah) Ro= 1m (40Ah)

-2

0

10

2

Small cell Large cell

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Understanding Heating Response Differences
Heating from Short Circuit = Heat from Cell Discharge + Joule Heat at Short Joule Heat at Short
Heat for Cell Discharge [oC/sec]
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Ro Vocv Rs

Qd Vocv Ro = M (Ro + Rs )2 M
cell size increase

2

Short Heating =
Global

+
Local

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Qualitative Representation for Heating Pattern 10 m Short 10 Short

0 -4 10

10 10 Short Resistance [ ]
Ro= 100m (0.4Ah) Ro= 1m (40Ah)

-2

0

10

2

Small cell Large cell
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Heat at Short [W]

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V R Qs = ocv s 2 (Ro + Rs )

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0

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-2

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-4

10 10 Short Resistance [ ]

-2

0

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2

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Understanding Heating Response Differences
Heating from Short Circuit = Heat from Cell Discharge + Joule Heat at Short Joule Heat at Short
Heat for Cell Discharge [oC/sec]
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Ro Vocv Rs

Qd Vocv Ro = M (Ro + Rs )2 M
cell size increase

2

Short Heating =
Global

+
Local

10

5

Qualitative Representation for Heating Pattern 10 m Short 10 Short

0 -4 10

10 10 Short Resistance [ ]
Ro= 100m (0.4Ah) Ro= 1m (40Ah)

-2

0

10

2

Small cell Large cell
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10

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Heat at Short [W]

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V R Qs = ocv s 2 (Ro + Rs )

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0

Large cell: localized heating Small cell: global heating
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-2

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10 10 Short Resistance [ ]

-2

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Understanding Heating Response Differences
Heating from Short Circuit = Heat from Cell Discharge + Joule Heat at Short Joule Heat at Short
Heat for Cell Discharge [oC/sec]
15

Ro Vocv Rs

Qd Vocv Ro = M (Ro + Rs )2 M
cell size increase

2

Short Heating =
Global

+
Local

10

5

Qualitative Representation for Heating Pattern 10 m Short 10 Short

0 -4 10

10 10 Short Resistance [ ]
Ro= 100m (0.4Ah) Ro= 1m (40Ah)

-2

0

10

2

Small cell Large cell
2

10

4

Heat at Short [W]

10

2

V R Qs = ocv s 2 (Ro + Rs )

10

0

Large cell: localized heating Same local heating, and Small cell: global heating Negligible global heating for both
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10 10 Short Resistance [ ]

-2

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Observations from Literature:
Various Short Resistances
small resistance short (metal to metal) medium resistance short (bypassing cathode) large resistance short (flow through cathode)

John Zhang, Celgard, AABC08

Joule Heat at Short

*Small Cell with Shut-Down Separator

Heat for Discharge

Short Resistance

cell shut-down

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Observations from Literature:
Various Short Resistances
small resistance short (metal to metal) medium resistance short (bypassing cathode) large resistance short (flow through cathode)

John Zhang, Celgard, AABC08

Joule Heat at Short

*Small Cell with Shut-Down Separator Peter Roth, SNL, ATD presentation Delayed Separator Breakdown
200 180 160
Temperature (C)

With No Cell Runaway thermal runaway short observed without (1 amp current)

Heat for Discharge

Gen3 Electrodes Celgard Separator EC:EMC\LiPF6

30 25
Cell Voltage (V)

140 120 100 80 60 40 20 0 0 500 1000 Time (s) 1500 Pos. Term. Temp OCV Voltage

20 15 10 5 0 2000

Short Resistance

cell shut-down

large resistance short (>5)

Literature cases with wide range of internal short resistances are observed
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Prismatic Stack Cell Short Simulation
Cu Cu Cu Al
20 Ah P/E ~ 10 h-1 Stacked prismatic Form factor: 140 mm x 200 mm x 7.5 mm Layer thickness: (Al-Cathode-Separator-Anode-Cu) 15 m-120 m-20 m-135 m-10 m Multi-physics model parameters Electrochemistry model: a set evaluated at NREL Exothermic kinetics: Hatchard and Dahn (1999) Electronic conductivity: Srinivasan and Wang (2003) Shorted Spot Negative Tab 140 mm Positive Tab 7.5 mm 200 mm
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Al

Modeling Objectives Characterize short natures Predict cell responses Predict onset of thermal runaway Assumptions Short remains same Structurally intact No venting and no combustion

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Short Between Al & Cu Current Collector Foils
Shorted area: 1 mm x 1 mm
e.g., metal debris penetration through electrode & separator layers contact between outermost bare Al foil and negative-bias can

Cu Al

Rshort ~ 10 m Ishort ~ 300 A (15 C-rate)

current density field near short

electric potential distribution at shorted metal foil layers
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diverging current converging current at Al foil at Cu foil
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Short Between Al & Cu Current Collector Foils
Joule Heat for Short Temperature @10 sec after short 800oC

surface temperature

internal temperature
25oC



Joule heat release is localized for converging current near short Localized temperature rise is observed. Temperature of Al tab appears to reach its melting temperatures (~600oC)
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Short Between Al & Cu Foils: Reaction Propagation
Exothermic Reaction Heat [kW] 12 10 8 6 4 2 0 0 10 20 30 40 Time [sec] 50 60

10 sec
T

20 sec
T

Q
T : Temperature Q: Reaction Heat

Q

30 sec
T

40 sec
T

50 sec
T

60 sec
T

Q

Q

Q

Q

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Short Between Cathode and Anode Electrodes
Shorted area: 1 mm x 1 mm
e.g., separator puncture separator wearout under electrochemical environment
Cathode

Anode

potential near short
Anode Cathode

Rshort ~ 20 Ishort ~ 0.16 A (< 0.01 C-rate)

current density field near short
Electron current is still carried mostly by metal current collectors Short current should get through the resistive electrode layers Potential drop occurs mostly across positive electrode
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Short Between Electrodes: Temperature
Temperature (oC)

Temperature at Short

Temperatures at 20 min after Short
43.1°C

Cell Average Temperature
0 200 400 600 800 1000 1200

Time (sec)

surface temperature

internal temperature
25.5°C



Thermal signature of the short is hard to detect from the surface The short for simple separator puncture is not likely to lead to an immediate thermal runaway
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Observations: Simple Separator Puncture
Celina Mikolajczak, Exponent, NASA Aerospace Battery Workshop 2008



Wear and puncture or degradation of separator Local degradation of electrode materials

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Issue on Structural Integrity of Separator
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Short Between Electrodes: Impact of Short Area
Impact of Separator Structural Integrity
Separator Hole Propagation 1 mm x 1 mm 3 cm x3 cm
Rshort ~ 20 Ishort ~ 0.16 A (< 0.01 C-rate)
Myung Hwan Kim, LG Chem, AABC08

Rshort ~ 30 m Ishort ~ 100 A (5 C)

3cm x 3cm Separator Hole

Exothermic Heat [W]

Joule Heat @ cathode layer Temperature at 1min after short Joule Heat @ foils

Time [sec]
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Observations:


Structurally Reinforced Separator

Y. Baba, Sanyo Electric, PRIME 2008 (214th ECS)

Ceramic coated (one-side) functional separator was tested. Improvements in safety were NOT observed clearly against typical abuse tests. Slight performance improvements were reported.

vs
Myung Hwan Kim, LG Chem, AABC08



Mn-spinel based cathode, ceramic coated separator, and laminated packaging provide good abuse-tolerance against typical abuse tests.
Nail Penetration Test

NOTE: An abuse test such as nail-penetration is not likely to represent the process of formation and evolution of internal short-circuits.

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Rationale: metal plating – lowering Rs
Celina Mikolajczak, Exponent, NASA Aerospace Battery Workshop 2008

Metal plating provides a potential site for low resistance short formation.
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Short Resistance (Rs) Decrease With increase in plating area and depth
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Li plating on Anode Surface

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Short Between Anode and Al Foil
Shorted area: 1 mm x 1 mm e.g., metal particle inclusion in cathode slurry deep copper deposition on cathode during overdischarge
Al

Anode

Temperatures at 1 hr after short

250°C

Rshort ~ 2 Ishort ~ 1.8A (< 0.1C)
Temperature (oC)

Temperature at Short

surface temperatures

Cell Average Temperature

internal temperatures
Time ( x 10sec)



Temperature at short quickly reaches over 200oC. This type of short is likely to evolve into a hard short in relatively short time.
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37°C

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Observations: Cathode Layer Bypassing
Takefumi Inoue, GS YUASA, NASA Aerospace Battery Workshop 2008 Explanation published by SONY and presented by GS YUASA The mechanism of the fire accident on the DELL PC / SONY Lithium-Ion battery published by SONY in “Nikkei Electronics, Nov. 6, 2006”.

Metal particle enters into the triangle zone at the edge of positive electrode where bare Al foil is exposed.

Metal particle dissolves and deposits on anode surfaces. Lithium dendrite grows back on the deposited nickel.

Low resistance short forms.

NOTE: A short formed through or bypassing a resistive cathode layer would result in relatively low resistance short and ,highly likely, evolve quickly into a more severe short leading to a safety incident.
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Small Cell (0.4 Ah) Short: metal to metal
0.4 Ah cell Shorted area: 1 mm x 1 mm
Cu Al

Shorted Spot 35 mm

Rshort ~ 7 m Ishort ~ 34 A (85 C-rate)
140 surface temperature

Large cell (20Ah) Rshort ~ 10 m Ishort ~ 300 A (15 C-rate) mm 40
volume fraction

3 mm

0.2 0.1 0 100 110 120 130 140

130

110

volume fraction

120

0.2

0.1 0 100 110 120 130 140

100
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temperature [oC]

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Shut-Down Separator
STOP
Li+ Large Cell Representation

Thermally triggered Block the ion current in circuit

STOP

Li+

Difficult to apply in Large capacity system High voltage system

Li+

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Thermal Behavior of a Small Cell
Even with a small cell, 213 in some conditions, shutdown separator may not function
800 600 400 200 0 0 2 4 6 8 10 12 14 16 18
130 131 126 126 149 123 244 555 359

without shut-down shut-down functioned

In some conditions, shutdown separator will function

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Summary
NREL performed an internal short model simulation study to characterize an internal short and its evolution over time by linking and integrating NREL’s electrochemical cell, electro-thermal, and abuse reaction kinetics models. Initial heating pattern at short events depends on various physical parameters such as nature of short, cell size, rate capability. Temperature rise for short is localized in large-format cells. Electron short current is carried mostly by metal collectors. A simple puncture in the separator is not likely to lead to an immediate thermal runaway of a cell. Maintaining the integrity of the separator seems critical to delay short evolution. Electrical, thermal, and electrochemical responses of a shorted cell change significantly for different types of internal shorts.
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Future Work
Perform in depth analysis for evaluating recommended safety designs such as structurally intact separators and shutdown featured device/strategy in relation to cell design parameters (materials, electrode thickness, cell capacity, etc.) Design experimental apparatus for model validation through the collaboration with other national labs (Sandia National Laboratory) Partner with cell manufacturers and auto industries to help them design safer lithium-ion battery system that appears critical to realize technologies for green mobility





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Acknowledgments

Vehicle Technology Program at DOE Advanced Battery Research for Transportation program Dave Howell Gary Henriksen

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