battery degradation analysis with high precision coulometry...cycling of high-energy 18650 cells...

Battery Degradation Analysis with

High Precision Coulometry

Peter Keil, Robert Burrell, Thibaut Fortin, Yi Li, Alana Zülke, Denes Csala, Harry Hoster

Future Powertrain Conference 2019

Typical Test Procedure

Charge-discharge cycles, usually performed with low currents

Degradation Indicators

• Coulombic Efficiency (ampere-hour efficiency) per cycle

• Slippage of charging end point & discharging end point

2Dr. Peter Keil | High Precision Coulometry

Introduction: Coulometry

Cell voltage reconstruction based on anode and cathode half-cell potential

➢ Discharging end point: dominated by voltage increase of delithiated anode

➢ Charging end point: dominated by voltage increase of delithiated cathode

End Point Conditions

Dr. Peter Keil | High Precision Coulometry 3P. Keil et al., Journal of The Electrochemical Society, 163 (9), A1872 (2016).

Evaluation of end point slippages

solely anodic side reactions => capacity fade

solely cathodic side reactions => reversible self-discharge + capacity increase


Identification of Side Reactions

P. Keil et al., Journal of The Electrochemical Society, 164 (1), A6066 (2017).

Cycling of high-energy 18650 cells

Variation of

• Charge voltage (4.1 - 4.2 V), discharge voltage (2.5 - 3.4 V), charge current (1.0 - 2.5A)

Evaluation of

• Coulombic efficiency, end point slippage

HPC Experiment


Check-Up

Cycling

24 cycles

NMC, 3.4 Ah

temperature: 25°C

discharge current: 1.0A

Cycling of high-energy 18650 cells

Observations

• CE values are approaching a stable value of about 99.92 % for cycling between 4.1 V and 3.1 V

• Stable CE values are reached after the about 50 cycles

• CE values are starting at a notably lower value (<99.7%) due to anode overhang effects

• Temperature fluctuations in the climate chamber affect the stability of the CE values

Coulombic Efficiency


NMC, 3.4 Ah

Cycling after long-term storage at different SoCs

• Local potential gradients between overhang areas and active electrode areas make the overhang areas

act as a source or sink for cyclable lithium, depending on the voltage history

• The extent of this effect depends on the size of the inactive anode overhang areas


Effects of Anode Overhang Areas

J. Wilhelm et al., Journal of Power Sources, 365, 327-338 (2017).

LFP, 1.1 Ah

End points of check-ups and cycling periods

• Cycling endpoints illustrate different capacities available for different cycling windows

• Discharge end points exhibit more pronounced slippage than charge end points

• For degradation analysis: end points from checkups should be compared

End Point Slippage


NMC, 3.4 Ah

End points of check-ups for the four extreme cases

• Slight slippage of the charge end points indicate some cathodic side reactions at the beginning

• Discharge end point slippage dominant => anodic side reactions as main cause for capacity fade

• Lithium movement to anode overhang areas + electrolyte reduction reactions

End Point Slippage


NMC, 3.4 Ah

Cycling with different charging currents

• High charging currents can cause a lithium metal deposition instead of a lithium intercalation

• Some plated lithium reacts irreversibly with the electrolyte, which consumes cyclable lithium

• Discharge end point slippage reveals lithium plating

End Point Slippage


NMC, 3.4 Ah

Check-UpCheck-Up

Cycling

High Precsion Coulometry – Summary


+ Aging results obtained faster

+ Identification of anodic and cathodic

side reactions possible

+ Detection of lithium plating helps

developing fast charging protocols

• High precision voltage and current

measurements

• Long-term stable charge balance

• Effects of anode overhang areas

• Changes in environmental temperature

Advantages Challenges

• New battery lab to focus on high precision battery testing

• Improving test setups to minimize distortions

• Method development with 18650 and 21700 cells

• Wide range of test channels:

from coin cells to large-format automotive cells

Outlook

Faster and more precise battery diagnostics with High Precision Coulometry!

Battery Dynamics GmbH

Lichtenbergstr. 8

85748 Garching b. München (Germany)

[email protected]

www.battery-dynamics.de

Supporting research and developement processes Benefits

Battery Testers for High Precision Coulometry

• Development of long-life electrode materials

• Development of superior battery systems

• Optimization of operational strategies

• Speeding up development processes

• Shortening cycle life experiments by up to 80%

• Additional aging insights without post mortem analysis

HPC-M20

• up to 20A

• ideal for 18650, 21700 cells

HPC-L200

• up to 100A / 200 A

• ideal für large automotive cells

battery degradation analysis with high precision coulometry...cycling of high-energy 18650 cells...

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