The GITT procedure consists of a series of current pulses, each followed by a relaxation time, in which no current passes through the cell. The current is positive during charge and negative during discharge.
During a positive current pulse, the cell potential quickly increases to a value proportional to the iR drop
where R= uncompensated resistance, Run + the charge transfer resistance, Rct
Afterwards, the potential slowly increases, due to the galvanostatic charge pulse, in order to maintain a constant concentration gradient.
When the current pulse is interrupted, i.e., during the relaxation time, the composition in the electrode tends to become homogeneous by Li-ions diffusion. Consequently, the potential first suddenly decreases to a value proportional to the iR drop, and then it slowly decreases until the electrode is again in equilibrium (i.e., when dE/dt ~ 0) and the open circuit potential (Voc) of the cell is reached. Then, the galvanostatic pulse is applied again, followed by current interruption. This sequence of charge pulse followed by a relaxation time is repeated until the battery is fully charged.
During a negative current pulse, the opposite holds. The cell potential quickly decreases to a value proportional to iR. Then, the potential slowly decreases, due to the galvanostatic discharge pulse. During the relaxation time, the potential suddenly increases by a value proportional to iR, and then it slowly increases, until the electrode is again in equilibrium (i.e., when dE/dt ~ 0) and the Voc of the cell is reached. Then, the following galvanostatic pulse is applied, followed by current interruption. This sequence of discharge pulse followed by a relaxation time is repeated until the battery is fully discharged. The following picture shows the process. Reference: Autolab Application Note BAT03
To illustrate it clearer, 2 cycles are enlarged as the following picture:
Based on the data, two important properties can be calculated: chemical diffusion coefficient and resistance.
Chemical diffusion coefficient
The chemical diffusion coefficient can be calculated at each step, with the following formula:
If sufficient small currents are applied for short time intervals, dE/d sqrt t can be considered linear and the coulometric titration curve can be also considered linear over the composition range involved in that step, Equation 1.1 can be simplified into[1]:
Area Specific Resistance (ASR)
- Resistance can be calculated from the iR drop by R=delta Voltage/current.
ASR=resistance *cell active area.
[1] https://www.ecochemie.nl/download/Applicationnotes/Autolab_Application_Note_BAT03.pdf