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Modeling Mechanical Failure in Lithium-Ion Batteries

Shriram Santhanagopalan, Chao Zhang, Chuanbo Yang, Andy (Zenan) Wu, Lei Cao and Ahmad Pesaran

Transportation and Hydrogen Systems CenterNational Renewable Energy Laboratory, Golden CO 80401http://www.nrel.gov/vehiclesandfuels/energystorage/

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Temperature Stress Build-Up

Temperature

Ohmic heat generation Total heat generation rate

Velocity field

Commercially available tools implementing rigorous electrochemistry

Realistic geometries, CAD capabilities, validated models

Active participation from cell-manufacturers, OEMs

A. Pesaran, et al., DOE AMR 2013

S. Santhanagopalan, Batteries 2008, Niece, Oct. 2008

Computer Aided Engineering for Electric Drive Vehicle Batteries (CAEBAT)

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Background• Numerical simulation of vehicle crash is a fairly well-established practice in the auto industry, with

significant cost reductions and faster delivery.• For PEVs, it is difficult to build predictive models: state-of-the art design tools do not consider the

battery as an energized entity.• The industry has a strong interest in having models for simulating crush of PEVs with batteries as

energized entities.• As part of CAEBAT, NREL has focused on developing tools to couple electrochemical and thermal

response of PEV batteries with mechanical deformation.

Structural additions for battery pack crash protection (Purple)

State-of-the Art Battery Crash Protection is currently limited to Mechanical AspectsM. Bartolo, Electric Vehicle Safety Technical Symposium, 2012.

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From Crush to Potential Thermal Runaway

Heat GenerationHeat generation without rejection

Temperature increase

Spontaneous reactionsThermal Runaway

Reach reaction onset temperature

Battery CrushDamaged Zone created

Failure of Separator, current collector, etc.

Local shortCurrent Flow

Electrode contacts initiated

Smoke and Fire“may lead to” (depending on many factors) A. Pesaran et al., Battery Safety (2016)

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Sample Output:• Current distribution among the different

cells within the module• Localized heat generation rates far away

from damage zone• Stress distribution across multiple parts of

the battery module

Validate against Module-Level Data

Displacement under Crush

Current density under short-circuit

Mechanical Modeling ApproachScale to Module-Level

Simulate Cell-Level Response for Multiple Cases

Explicit simulations parameterizematerial response

Sample Input:• Stress-strain curves for cell components

(separator, current collector, etc.)• Failure strengths for particles• Mechanical data for cell packaging• Temperature Vs C-rate for cell• Abuse reaction data from calorimetry

for specific chemistries

Start with Cell-Level Test Results as Input

Predicts cell temperatures to +10oC

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Example 1: Mechanism of Cell Failure

Cathode-anode shortFailure of Copper Foil

Copper foil fails before separator ruptures

Shear failure of active material layers within a battery

Cell-level crush tests used to have a “pass” or “fail”

H. Wang, Battery Safety 2015

Copper foil Layer 1 Anode Layer 4 Cathode Layer 6

Side

faci

ng

inde

nter

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Example 2: Estimating Short-Circuit Resistance

Anode-to-Cathode Short

Tmax= 224oC

Anode-to-Aluminum Short

Tmax= 1458oC

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Component-level Mechanical Models feed into Cell-Level Safety Simulations

Evolution of Short Circuit Area

Time 0.0016 seconds

Time 0.0004 seconds

• Criteria for short-circuit not just based on mechanical failure.

• Changes in local resistivity and temperature effects determine outcome of mechanical crush events.

Time 0.0008 seconds

Dots: Experimental DataLine: Simulation

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Example 3: Interpreting Test Results

Sahraei et al. Journal of Power Sources, 2014

C. Zhang et al. Journal of Power Sources, 2015

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Example 4: 1S4P Cell-String – Lightweighting

Deformation of packaging material Deformation of the cells

Simulation results predict that:1. The packaging can prevent deformation of the cells by as much as 50% under these crush test

conditions.2. There is a significant scope to light-weight the pack, even after the safety threshold is met.

Simulation Results - Displacements

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Example 5: Failure Propagation

Face impact – failure through electrode layer compression

Edge impact– failure through buckling of electrodes

von Mises Stress Contours for Side Vs Edge Impact on Module

Cell Level Tests performed at SNL

Failure Propagation Simulations in a Module

Module Crash test at SNL

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Displacement (m)

Stress (kPa)

Cell Venting

Example 6: Multiple Failure Modes

Swelling of a Battery Cell

Santhanagopalan et al., 214th ECS Meeting SFO 2013Cellphone Drop Test Stress Evolution on Passive Components

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Mechanical Deformation of Module Components

Multiple Failure Modes across Length Scales

Electrical Response of Individual Cells

Component level stress-strain response

• A comprehensive tool that covers multiple physics across length scales does not exist in the industry.

• CAEBAT models address this gap by working with different software companies, cell vendors and OEMs.

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Summary

• Mechanical provide reliable insights into failure mechanisms and mitigation tools for lithium ion batteries.

• These tools are being successfully used by battery makers and OEMs to study application specific limitations in battery packs.

• These software also serve as repositories for material properties, as resources for exchange of relevant performance information across different scales among the key players at each level.

• Test methods to evaluate battery safety are constantly being improved based on feedback from end-users and lessons-learned on the field. Theoretical insights reduce the time for such build-and-break cycles.

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Acknowledgements

Collaborations and Coordination

Funding

The NREL TeamMatt Keyser Mitch PowellKandler Smith Aron Saxon