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)
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