Half cells and full cells
A new material is usually first tested in a half cell, a coin cell against lithium metal, which acts as both counter and reference electrode. Lithium is in large excess, so the test shows what the material can do. A full cell pairs a real positive and negative electrode and shows how the material behaves in a device, where lithium lost to side reactions is not replaced. Chen and co-workers (2019) showed how much coin-cell results depend on electrolyte amount, lithium excess and loading.
C-rates and galvanostatic cycling
The C-rate expresses current relative to capacity. 1C fully charges or discharges the cell in one hour, C/10 in ten hours, and 2C in half an hour. Most tests use galvanostatic cycling between voltage limits, often with a constant-voltage step at the end of charge.
A rate capability test cycles a few times at each of several C-rates, then returns to the slow rate to check that the capacity recovers.
Coulombic efficiency and capacity retention
Coulombic efficiency (CE) compares the charge that comes out with the charge that went in, in one cycle. It is the most sensitive single number for side reactions. A CE of 99.9% sounds high, but in a full cell, where lithium is not replaced, it can still mean a loss of about 10% of the capacity over 100 cycles.
What to report
| Item | Why it matters |
|---|---|
| Mass loading (mg/cm²) and areal capacity (mAh/cm²) | Thin electrodes look better than practical ones. Commercial electrodes carry a few mAh/cm². |
| Composition of the electrode | Share of active material, conductive carbon and binder, and whether capacity is per active mass or per electrode |
| Electrolyte and its amount | A flooded coin cell hides electrolyte consumption |
| Voltage window and C-rate definition | Which capacity defines 1C, and the cut-off voltages |
| First-cycle and average CE | Lithium losses that decide full-cell life |
| Number of cells tested | Results from one coin cell can mislead |
| Full-cell data with N/P ratio | Shows behaviour in a device |
Other common techniques
- CV at several scan rates shows the redox potentials and, with the b-value, how much of the charge is capacitive (Lesson 3.1).
- GITT, the galvanostatic intermittent titration technique of Weppner and Huggins (1977), uses short current pulses and rests to estimate the diffusion coefficient of lithium in the electrode at each state of charge.
- EIS at several states of charge separates the resistance of the electrolyte, the SEI and charge transfer, and shows how they grow with ageing (Lesson 1.3).
Check your understanding
Quiz for lesson 6.2
Pick one answer per question. You see the explanation straight away.
References and further reading
- S. Chen et al., Critical parameters for evaluating coin cells and pouch cells of rechargeable Li-metal batteries, Joule 3 (2019) 1094–1105. doi:10.1016/j.joule.2019.02.004
- W. Weppner, R. A. Huggins, Determination of the kinetic parameters of mixed-conducting electrodes and application to the system Li3Sb, J. Electrochem. Soc. 124 (1977) 1569–1578.
- Y. Gogotsi, P. Simon, True performance metrics in electrochemical energy storage, Science 334 (2011) 917–918. doi:10.1126/science.1213003
- J. B. Goodenough, K.-S. Park, The Li-ion rechargeable battery, a perspective, J. Am. Chem. Soc. 135 (2013) 1167–1176.