Description
1. Testing Significance
Tortuosity of Electrodes and Kinetic Performance
- Tortuosity represents the degree of bending in the transport pathways within a porous electrode. It is a critical parameter, alongside porosity, governing transport properties. It characterizes the percolation capability of the electrolyte and the ion migration rate, directly impacting the battery’s capacity utilization and rate capability.
- Measuring the pore tortuosity of an electrode enables performance prediction. This facilitates the rapid correlation of electrode structure with expected performance, accelerating electrode design and process development.
Ionic Conductivity of Separators
- In recent years, separator coating applications have expanded significantly. Coating processes enhance lithium-ion battery separator properties such as puncture resistance, thermal stability, and electrolyte wettability. While improving safety performance, it is equally crucial to ensure stable electrochemical performance. Therefore, measuring ionic conductivity is particularly important for comparing and characterizing separator performance.
2. Testing & Calculation Methods
2.1 Electrode Testing
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Assemble a symmetrical cell and perform Electrochemical Impedance Spectroscopy (EIS) testing.
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As shown in the figure below, perform linear fitting on the high-frequency region and the low-frequency region of the EIS spectrum (Nyquist plot). Three times the difference between the points where the respective fitted curves intersect the real impedance axis (Z’/X-axis) gives the ionic resistance Rion of the electrode coating.
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Calculate the MacMullin number using the formula to indirectly characterize the tortuosity of the electrode.
2.2 Separator Testing
- Measure the impedance of separators with 1 to 4 layers, obtaining values R₁, R₂, R₃, R₄.
- Plot separator resistance (R) versus number of separator layers on the vertical and horizontal axes, respectively.
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Determine the slope of the curve and its linearity of fit.
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The linear regression coefficient (R²) must be ≥0.99.
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- Calculate the ionic conductivity (σ) using the formula: σ = d / (R × S)
3. Creative Solutions
3.1 Creative Solution one
- Determining Electrode Tortuosity via EIS Testing of Symmetric Cells
- Streamlined cell assembly, automated testing and analysis, simplified operation workflow, enhanced testing throughput
- Four-channel synchronous measurement
3.2. Creative Solution Two
Applications
Case 1. Different compaction density of cathode electrodes
Summary:
- The consistency of ElS testing for symmetric battery of electrodes is generally good.
- Within a certain range of compaction density, as the compaction increases, the ionic resistance/MacMullin numb also increases.
Case 2. Different compaction density of anode electrodes
Summary:
- The consistency of ElS testing for symmetric battery of electrodes is generally good.
- Within a certain range of compaction density, as the compaction increases, the ionic resistance/MacMullin numb also increases.
Case 3. The correlation between electrode tortuosity and electrochemical performance(Gr anode electrodes of different thicknesses)
Summary:
- As the thickness of the electrode increases, its tortuosity also increases, However, the rate performance of the battery decreases with increasing thickness.
- This indicates that the rate performance of the battery decreases with increasing tortuosity. There is a certain correlation between electrode tortuosity and rate performance of battery.
Case 4. Ionic Conductivity Comparison of Separators with Four Different Coatings
Summary:
- Test the EIS of 1-4 layers of separators to obtain R1, R2, R3, R4.
- Plot a curve with the number of separator layers as the x-axis and separator resistance as the y-axis. Calculate the slope and linear fitting degree of the curve, with the linear fitting degree ≥0.99.
- Calculate the separator ionic conductivity according to the formula.
Case 5. Correlation Between Electrode Tortuosity and Electrochemical Performance in Different Electrolytes
Summary:
- MacMullin Number Trend: For both cathode and anode electrodes, the MacMullin number across electrolyte formulations follows: Formulation 3 > Formulation 1 > Formulation 2
- Rate Performance at 10C: Formulation 3 exhibits the lowest capacity retention (89%) under 10C discharge conditions.
- Electrolyte formulations significantly impact lithium-ion transport kinetics within electrodes. A higher MacMullin number corresponds to greater ionic migration resistance, consequently degrading the battery’s rate capability.
Case 6. Tortuosity & Wettability of LFP Cathodes with Different Compaction Densities
Summary：
- Electrode with poorer wettability exhibit greater tortuosity.
- As compaction density increases, electrolyte absorption declines, hindering permeation and lithium-ion migration, which elevates ion transport resistance and electrode tortuosity.
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