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

4Dr. Peter Keil | High Precision Coulometry

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

5Dr. Peter Keil | High Precision Coulometry

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

6Dr. Peter Keil | High Precision Coulometry

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

7Dr. Peter Keil | High Precision Coulometry

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

8Dr. Peter Keil | High Precision Coulometry

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

9Dr. Peter Keil | High Precision Coulometry

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

10Dr. Peter Keil | High Precision Coulometry

NMC, 3.4 Ah

Check-UpCheck-Up

Cycling

High Precsion Coulometry – Summary

11Dr. Peter Keil | High Precision Coulometry

+ 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)

peter.keil@battery-dynamics.de

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