Entering Electrochemistry | GITT Analyzes The Diffusion Kinetics Of Lithium Batteries

1. Preface

Galvanostatic Intermittent Titration Technique (GITT) is a transient measurement technique that quantifies the diffusion capacity of lithium ions in the electrode material through the relationship of potential change with time.^{[1] [2]} 

A lithium-ion battery is a rocking chair secondary battery that operates primarily on the movement of lithium ions between the cathode and anode electrodes (Figure 1). During the charging process, lithium ions are dislodged from the positive electrode and embedded in the negative electrode through the electrolyte. At this point, electrons flow from the positive electrode to the negative electrode through an external circuit, creating a current. In the discharge process, the opposite is true, lithium ions are dislodged from the negative electrode and return to the positive electrode through the electrolyte, while electrons flow from the negative electrode to the positive electrode through the external circuit, generating a current for external power supply. Therefore, the diffusion speed and efficiency of lithium ions are directly related to the battery’s charging/discharging multiplication, cycle life and high and low temperature performance.

Figure 1. Schematic diagram of the working principle of lithium-ion battery

2. Basic Overview of the Galvanostatic Intermittent Titration Technique (GITT)

The overall GITT process consists of a series of “pulse-constant current-relaxation” processes (Figure 2). A set of “pulse-constant current-relaxation” process is to charge/discharge the battery by applying a constant current for a certain period of time, and then disconnecting the current and recording the voltage change of the whole process, the key of the test is the constant current and the accurate voltage. In the relaxation phase after disconnecting the current, it is necessary to let the lithium ions diffuse fully inside the active material, and the diffusion coefficient is further calculated by the relationship between voltage and time. In order to satisfy the assumption of the GITT method that “the diffusion process mainly occurs in the surface layer of the solid-phase material”, it is necessary to make certain limitations on the test conditions:

• The time t of the constant current pulse should be relatively short and satisfy t<<L²/D, where L is the characteristic length of the material and D is the diffusion coefficient of the material.
• The relaxation time should be long enough to allow Li⁺ to diffuse sufficiently inside the active material and reach an equilibrium state, which can be determined by keeping the voltage stable.

Figure 2. (a) GITT test curve and (b) localized zoomed-in schematic

3. The core formula of the Galvanostatic Intermittent Titration Technique (GITT)

The GITT test data allows for further calculation of the corresponding diffusion coefficients with the following equations:

D is the lithium ion diffusion coefficient, m_B is the mass of active material, V_m is the molar volume of electrode material, M_B is the relative molecular mass of the material, S is the effective surface area of the electrode in contact with the electrolyte, τ is the relaxation time, ΔE_t is the change of cell voltage during the charging/discharging process, ΔE_s is the change of voltage during the relaxation phase, t is the pulse time, and L is the thickness of the electrode. The diffusion coefficient of lithium ions is obtained by substituting the physical parameters of the corresponding materials and the ΔE_s and ΔE_t in each set of “pulse-relaxation” cells into Eq.
In general, the voltage changes obtained during the test include not only the surface diffusion values, but also the voltage changes that reflect the SOC changes. Theoretically, the testing accuracy of GITT can be improved by reducing the pulse time, but as the pulse time becomes smaller, the change of ΔE_s will become very small, which requires equipment with high measuring accuracy to reduce noise. IEST self-developed charging and discharging equipment can be configured with 8 test channels of 0.01% accuracy, thus obtaining more accurate test results.

4. Application Cases

4.1 Li-ion diffusion coefficient in different SOC states

The authors investigated the changes of Li-ion diffusion coefficient during the charging and discharging process of LiNi_0.8Co_0.1Mn_0.1O₂₍NCM811) by means of GITT test^[3]. The value of lithium ion diffusion coefficient D_Li+ varies significantly in different SOC states. The values of D_Li+ were 10^-8~10^-9cm² s^-1 during charging and 10^-7~10^-11cm² s^-1 during discharging. At the beginning of charging, D_Li+ increased with the release of lithium ions, reached a maximum value at a lithium content of~0.5, and then gradually decreased. The diffusion coefficient decreases rapidly when the lithium content is lower than 0.2. In addition, during the discharge process, D_Li+ was extremely high at the beginning; after that, the value decreased slightly and remained at a high level with the insertion of lithium ions. When the Li-ion embedding content reaches 0.8, the D_Li+ drops sharply by three orders of magnitude. This very low Li-ion embedding kinetics explains the capacity loss in the first cycle.

Figure 3. First turn GITT curve and Li ion diffusion coefficient of NCM811

4.2 Effect of material modification on ion diffusion coefficient

he authors introduced high entropy elements (Cr, Mn, Fe, Zn, Al) into the NASICON structure Na₃V₂(PO₄)₃ (NVP) to obtain NNa₃V_1.8(CrMnFeZnAl)_0.2(PO₄)₃(HE-NVP-0.2) in order to realize the material’s crystal structure tuning and diffusion ability [4]. As shown in Figures 4a and 4b, the GITT results show that the HE-NVP-0.2 electrode exhibits better Na ion diffusion kinetics after the introduction of high entropy elements. After NVP and HE-NVP-0.2 were assembled into half-cells and the rate performance was tested, it was found that the rate performance of HE-NVP-0.2 was significantly better than that of the NVP sample (Figure 4c).

Figure 4. (a) GITT curves and corresponding Na ion diffusion coefficients for NVP and (b) HE-NVP-0.2

5. Summary

The diffusion behavior of lithium ions within the active material reflects the microscopic kinetic performance of the battery and greatly affects the overall performance of the battery. Segmented studies of electrochemical reactions at different charging and discharging depths can effectively find the key factors affecting the polarization of the battery at each stage, and GITT can effectively determine the diffusion coefficient of lithium ions, D, and thus study the kinetic process of the battery.

Based on the importance of GITT to the study of lithium-ion batteries, IEST has independently developed a high-precision electrochemical performance analyzer, which integrates the GITT test into the conventional charging and discharging equipment, templates the work steps, makes the setup simple, and makes the operation easy to improve the testing efficiency. The device also integrates CV (Cyclic Voltammetry), EIS (Electrochemical impedance spectroscopy) and LSV (linear scanning voltammetry) modules to enable R&D personnel to quickly conduct relevant electrochemical performance studies (Figure 5). In addition, the meta-energy electrochemical analyzer is equipped with advanced data processing and analysis software that enables real-time processing and multi-dimensional analysis of complex electrochemical data.

Figure 5. IEST Electrochemical performance analyzer and preview of work step

6. References