Li-Ion Battery Testing
Li-Ion Battery Testing
Electrochemical testing systems from Ametek can be used to test battery materials in R&D, including cathodes, anodes, electrolytes and separators, as well as complete cells and stacks of multiple cells connected together.
Blue Scientific is the official distributor of potentiostats from Solartron Analytical and Princeton Applied Research in the UK and Ireland. For more information and quotes, please get in touch:
Potentiostats for Battery Testing
- PARSTAT MC – Robust Design, Fast Data Acquisition, Wide Dynamic Current Range
- PARSTAT 4000+ – Measured and Applied Voltage Accuracy
- Solartron Modulab ECS – Market-leading impedance analysis, and the widest voltage and current range available
- Solartron EnergyLab XM – Affordable, small footprint system for battery, fuel cell and super-capicitor research
DC Voltage Accuracy
Accurate voltage is extremely important in battery testing. Both galvanostatic and potentiostatic controlled experiments benefit from a high degree of voltage accuracy.
Common Li-ion battery experiments:
- Charge-Discharge Experiments (Measured Accuracy)
- Voltage-Hold (Measured and Applied Accuracy)
- Potentiostatic EIS at EOC (Measured + Applied + Measured Accuracy)
Electrochemical Impedance Spectroscopy (EIS)
EIS measurements use an AC stimulus. The resulting AC response is affected by resistance, capacitance, inductance and diffusion elements of the sample.
Sweeping frequency allows the total response to be segmented and attributed to different electrochemical processes.
Signal / Response or Response / Signal
Electrochemical Impedance (Z) is the ratio of voltage (VAC) to current (I AC):
Z = VAC / I AC
To provide valid EIS data, the system must be:
- Linear. A = B; then 2A = 2B
- Causal. The response is only do to the signal
- Stable. No significant drift is occurring.
Assuming these validity statements hold true, the measured impedance should be independent of mode of operation.
Problems with 10 mV PEIS Stimulus
Batteries and energy storage devices are low impedance devices, less than 10 Ohms (1E1Ohms) and even often less than 10 milliOhms(1E-2 Ohms). Even a small voltage signal generates large currents at these small resistances. For example, 10 mV on 1 mOhm is 10 Amps.
Reducing the amplitude helps: 1 mV generates 1Amp. However, the DC voltage offsets of the measurement system can still cause very high currents.
Benefits of GEIS
Most energy storage techniques and practical applications use galvanostatic control. Stimulus (AC) and DC levels are defined by the user, so current overloads can be easily avoided. Current offsets are negligible versus signal level. Also, the resolution and accuracy of GEIS allow a cleaner stimulus signal to be applied.
State-of-Charge (SoC) Measurement
The battery indicator on portable devices becomes less reliable as the battery ages. Traditionally, state-of-charge is measured with DC Coulomb Counting. This integrates charge during device operation and compares it with nominal battery capacity.
This method works well when the battery is new, but as the battery life degrades over time, it starts to give inaccurate readouts. This is a serious issue for applications where accurate determination of SoC are critical, for example in electric vehicles. Kalman filer algorithms provide a solution to this problem.
EIS measurements at different SoC
Circuit Modelling: L = cell inductance; Rs= ESR; RSEI = Solid Electrolyte Interface; RCT= Charge transfer resistance; CPE SEI and DL= Constant Phase Element; W = Warburg (Diffusion) Impedance
- GEIS –500mA AC stimulus
- 20kHz to 10mHz
Analysis of RCT and RSEI as a function of SoC
- Both RCT and RSEI showed the most variation with SoC.
- Results are between cells.
- Results were suitable for both single cell and multiple cell packs.
- In this instance, data modelling requires wide bandwidth measurements.
- In this case, single frequency measurements will not be sufficient to determine Impedance versus SoC.
High Currents without External Boosters
A key parameter in Li-On battery testing is the Equivalent Series Resistor (ESR). This requires high frequency EIS data.
To handle (source and sink) high currents, a potentiostat needs higher bandwidth and higher accuracy than a potentiostat that requires external boosters.
On-board high current measurement improves measurements, in addition to the benefits of saving bench space and expense.
In the example above:
- ZRESISTOR= R
- 100 mOhm resistor
Note the much improved bandwidth of the data acquired with the high current potentiostat. With an external booster, the response rolls off at a high frequency. Without the booster, there is a much more useable frequency range.
Testing High Voltage Batteries
The PMC 2000 has unique features for testing high voltage batteries:
- 30 V Polarisation and Compliance – Control a stack of energy storage devices.
- 6th Wire – Measure the impedance of a single cell, and monitor DC Voltage
The open circuit graph below depicts high voltage measurement. The EIS plots show simultaneously acquired Zstack and Zcell, identifying the failing cell:
Recommended Potentiostats for Battery Testing
- Multiple potentiostat models in one chassis
- Data buffer
- User replaceable functional blocks
- Unmatched dynamic current range, supporting nanoscale to stack testing
- Large voltage range as standard
Solartron 1470E, 1451/5
- 4 Amps per channel as standard
- Combine up to 32-Amps
- Auxiliary electrometers available
- EIS hardware from the leaders in FRA technology
If you’d like to know more about any of these instruments, please get in touch: