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 is the sum of the uncompensated resistance R_u and the charge transfer resistance R_ct. 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 (OCP) 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 the iR drop. Then, the potential slowly decreases, due to the galvanostatic discharge pulse. During the relaxation time, the potential suddenly increases by a value proportional to the iR drop, and then it slowly increases, until the electrode is again in equilibrium (i.e., when dE/dt ≈ 0) and the OCP 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 chemical diffusion coefficient can be calculated at each step, with the following formula:[1-3]

Where i (A) is the current; V_m (cm³/mol) is the molar volume of the electrode; z_A is the charge number; F (96485 C/mol) is the Faraday’s constant and S (cm²) is the electrode area. Besides, dE/𝑑𝛿 is the slope of the coulometric titration curve, found by plotting the steady state voltages E(V) measured after each titration step 𝛿 and dE/𝑑√𝑡 is the slope of the linearized plot of the potential E(V) during the current pulse of duration t (s). In Figure 1, an example of potential versus the square root of time plot is shown. Using the linear regression tool provided in NOVA, information about dE/d√t can be obtained from the slopes of the galvanostatic pulses versus the square root of time [4].

If sufficient small currents are applied for short time intervals, then dE/d√t can be considered linear, as well as the coulometric titration curve over the composition range involved in that step. With these conditions, Equation (1) can be simplified to:

Here, 𝜏 (s) is the duration of the current pulse; n_m (mol) is the number of moles; V_m (cm³/mol) is the molar volume of the electrode; S (cm²) is the electrode area; ΔE_s (V) is the steady-state voltage change, due to the current pulse and ΔE_t (V) is the voltage change during the constant current pulse, eliminating the iR drop.

For the experiments, an Autolab PGSTAT302N was employed, together with a commercial 2.2 Ah Li-ion battery from Enix Energies, with a nominal voltage of 3.75 V and a nominal energy of 8.25 Wh.

Figure 2 shows the complete GITT potential profile. The procedure starts at OCP ≈ 3.62 V. Then, GITT charge pulses are applied; each followed by a relaxation period. Here, it can be noticed the potential drops between the pulses and the relaxation times, and that the overall potential increases until 4.2 V. After charging, the potential decreases, due to the galvanostatic discharge pulses, each followed by the relaxation time, until potential of 2.8 V is reached.

In order to shed more light on the GITT steps, in Figure 3 the first two charge pulses are shown.

Here, it is assumed that the currents are so small that dE/d𝛿 and dE/d√t holds and Equation (2) can be exploited. Note the increasing potential and ΔE_𝑡 value can be calculated. Afterwards, the 10 minute relaxation step is applied. Here, it should be noted the sudden potential decrease, due to the iR drop. Then, the potential slowly decreases. After the relaxation time, a sudden increase in the potential occurs. This is again due to the iR drop of the cell. Another 10 minutes galvanostatic potential step is then applied. Here, it can be better noticed the linear region. After the iR drop, the relaxation step is finally applied, and the ΔE_s value can be calculated.

This application note showed how AUTOLAB and NOVA could be used to perform GITT tests on a Li-ion battery. Here, galvanostatic charge pulses are applied, each followed by relaxation time, until the upper potential limit is reached. Then, discharge pulses are applied, followed by equilibration time, until the lower potential limit is reached. From the potential vs. time plot, important information to calculate the diffusion coefficient and thermodynamics parameters quantities can be obtained.

1. C.J. Wen, B.A. Boukamp and R.A. Huggins, J. Electrochem. Soc. Vol. 126, No. 12, 2258 (1979);
2. W. Weppner and R.A. Huggins, J. Electrochem. Soc. Vol. 124, No. 10, 1569, (1977);
3. Y. Zhu and C. Wang, J. Phys. Chem. Vol. 114, No. 6, 2830, (2010);
4. Z. Shen, L. Cao, C.D. Rahn and C.-Y. Wang, J. Electrochem. Soc. Vol. 160, No. 10, A1842, (2013)