Correlation Between Separator Ionic Conductivity and Cell Cycle Stability

Electrode Characterization

Updated on 2026/10/27

Abstract

Separator ionic conductivity, a quantitative metric (mS cm⁻¹) describing lithium-ion transport through the electrolyte-filled separator, contributes to a cell’s overall impedance. This study compared two polypropylene-based separators produced using different coating processes. Ionic conductivity was measured by electrochemical impedance spectroscopy (EIS) using the IEST EIC2400M Multi-Channel Ionic Conductivity Test System. Separator A had an ionic conductivity of 1.3182 mS cm⁻¹, compared with 1.0176 mS cm⁻¹ for Separator B, which is 29.5% higher than Separator B. LiFePO₄‖C coin cells assembled with the two separators were then evaluated by EIS and cycled for 100 cycles at 0.2C. The Separator A cell retained 97.11% of its capacity, while the Separator B cell retained 93.23%, a difference of 3.88 percentage points. Within this two-separator comparison, the separator with higher ionic conductivity was also associated with lower cell impedance and better capacity retention. The results suggest that ionic conductivity may be useful as an early-stage separator screening indicator, to be followed by cell-level electrochemical validation.

1. Background: Why Separator Ionic Conductivity Determines Battery Performance

The separator is the only cell component that must perform two inherently conflicting functions: (1) electrically insulate the cathode from the anode to prevent internal short circuits, and (2) ionically conduct lithium ions through its electrolyte-filled pore network to enable charge and discharge. The effect of separator coating on electrochemical performance is therefore a first-order design consideration: every coating modification that alters pore structure, surface chemistry, or electrolyte affinity directly impacts both ionic transport efficiency and ultimately battery cycle life.

Separator Ionic Conductivity ($\sigma$) — A quantitative measure (mS cm$^{-1}$) describing how readily lithium ions migrate through the electrolyte-filled separator membrane. Calculated from EIS: $\sigma = L / (R_{bulk} \times A)$,

where $L$ is the single-layer separator thickness (cm), $R_{bulk}$ is the bulk ionic resistance ($\Omega$) extracted from the Nyquist high-frequency intercept, and $A$ is the electrode contact area (cm$^2$). Higher $\sigma$ directly reduces cell internal resistance and concentration polarization.

A separator ionic conductivity is not an isolated material property — it is the net result of three coupled factors: (a) porosity — the volume fraction available for electrolyte uptake; (b) pore connectivity — whether electrolyte-filled pores form continuous ion pathways between cathode and anode; and (c) electrolyte wettability — the separator surface’s affinity for the electrolyte, which determines how completely the pores are filled. The separator wettability and electrolyte uptake for Li-ion batteries are particularly critical for coated separators, where the coating layer must enhance — not compromise — the base material’s wetting characteristics.

Table 1. Separator properties, their direct impact on cell performance, and measurement methods.
Separator Property	Direct Impact on Cell Performance	Measurement Method
Ionic Conductivity ($\sigma$)	Determines bulk ionic resistance ($R_{bulk}$) — higher $\sigma$ → lower internal impedance → lower voltage drop under load → better rate capability	EIS on multi-layer separator stacks (100 kHz–10 kHz); IEST EIC2400M
MacMullin Number ($N_M$)	Ratio of electrolyte-filled separator resistance to free electrolyte resistance — closer to 1 = more efficient ion transport	Derived from σ and free electrolyte conductivity
Electrolyte Wetting Rate	Faster wetting reduces manufacturing filling time and ensures uniform Li⁺ flux during first-cycle formation	Contact angle; time-resolved EIS; IEST EWS/ETS/CHT wetting test system
Tortuosity ($\tau$)	Describes the effective path length of ion transport — higher τ increases resistance for the same porosity	EIS + porosity measurement; IEST EIC Electrode tortuosity tester

Table 1 is critical separator properties linking coating modification to cell-level electrochemical performance. Ionic conductivity is the primary screening metric because it captures the combined effects of porosity, pore connectivity, separator wettability and electrolyte uptake, and surface chemistry in a single measurable value.

Note: the methodology described here applies to both ceramic-coated vs polymer-coated separator performance comparisons.

2. Test Conditions and Methodology

2.1 Test Equipment

Two IEST systems were used in this study. The EIC2400M measured separator ionic conductivity using EIS on multi-layer separator stacks. The ERT7008 was used for coin-cell EIS and charge–discharge cycling.

Figure 1. IEST EIC2400M Multi-Channel Separator Ion Conductivity Test System used for separator EIS measurements and conductivity calculation.

Figure 2. IEST ERT7008 Electrochemical Property Analyzer used for LiFePO₄‖C coin-cell EIS and cycling tests.

2.2 Test Samples

Separator ionic conductivity test: Separator A and Separator B were polypropylene-based separators produced using different coating processes.
Cell electrochemical test: LiFePO₄‖C coin cells were assembled with Separator A and Separator B, respectively. One cell was tested for each separator.

2.3 Test Conditions

EIS for separator ionic conductivity measurement uses the multi-layer stack method to isolate the intrinsic separator impedance from electrode interface contributions:

Table 2. Test configurations for measuring lithium-ion battery separator ionic conductivity and cell-level electrochemical performance evaluation.
Test Type	Equipment	Configuration	Parameters
Separator Ionic Conductivity	IEST EIC2400M	One- to four-layer separator stacks; blocking-electrode SS‖SS symmetric cell	EIS: 100 kHz – 10 kHz; voltage amplitude: 10 mV
Cell EIS	IEST ERT7008	LiFePO₄\|\|C coin cell	EIS: 100 kHz – 0.1 Hz; voltage amplitude: 10 mV
Coin-cell cycle stability	IEST ERT7008	LiFePO₄\|\|C coin cell	0.2C charge/discharge; 100 cycles; 2.5–3.65 V

3. Results and Analysis

3.1 Separator Ionic Conductivity Measured by Multi-Layer EIS

Figure 3. Nyquist plots for one- to four-layer stacks of (a) Separator A and (b) Separator B.

The EIS results in Figure 3 show that the measured bulk resistance increases as the number of separator layers increases from one to four. Linear fitting of resistance against layer count gives R² values of 0.9910 for Separator A and 0.9950 for Separator B. This linear relationship indicates that the slope represents the resistance contributed by a single separator layer, while reducing the influence of layer-independent contact and interfacial resistance.

For a separator with single-layer thickness L, contact area A, and through-thickness resistance per layer R_sep, the ionic conductivity is calculated as:

\[\sigma = \frac{L}{R_{bulk} \cdot A}\]

where $\sigma$ is the ionic conductivity (mS cm$^{-1}$), $L$ is the single-layer separator thickness (cm), $R_{bulk}$ is the bulk resistance per layer ($\Omega$), and A is the electrode area (cm$^2$).

Table 3. Resistance measurement data and ionic conductivity for Separator A and Separator B (area: 2.01 cm²; thickness: 12.00 µm).
Parameter	Separator A	Separator B
Area/cm²	2.01
Thickness/μm	12.00
1	0.4266	0.6391
2	0.9079	1.1145
3	1.4361	1.7327
4	1.7600	2.3888
R/Ω	0.4529	0.5867
R²	0.9910	0.9950
σ/ mS/cm	1.3182	1.0176

Table 4. Ionic conductivity results measured by IEST EIC2400M.
Separator	Ionic Conductivity (mS cm⁻¹)	Relative Advantage	Coating Modification Implication
Separator A	1.3182	+29.5%	Higher ionic conductivity under the tested conditions, indicating more efficient through-thickness ion transport
Separator B	1.0176	Baseline	Coating may partially occlude pore openings or reduce effective porosity, increasing tortuosity

3.2 Cell-Level EIS: Comparing Conductivity and Impedance

Figure 4. EIS spectra of LiFePO$_4$||C coin cells assembled with Separator A and Separator B.

Coin cell EIS (Figure 4) confirms that the separator-level ionic conductivity difference propagates to measurable cell-level impedance. Two observations are notable: (1) the high-frequency intercept ($R_{bulk}$) is lower for the Separator A cell — consistent with its higher σ; and (2) the mid-frequency semicircle ($R_{ct}$) is also smaller. This second observation is mechanistically significant: it demonstrates the effect of separator on battery charge transfer resistance — better ionic transport through the separator promotes more uniform Li⁺ flux distribution at the electrode interfaces, reducing localized charge-transfer bottlenecks that contribute to capacity fade.

3.3 Cycle Stability: 100 Cycles at 0.2C

Figure 5. LiFePO₄||C coin cell cycling performance at 0.2C over 100 cycles.

After 100 cycles at 0.2C, Separator A’s cell retains 97.11% versus 93.23% for Separator B — a statistically significant 3.88-percentage-point improvement. This separator ionic conductivity vs battery capacity fade relationship operates through a clear mechanism: higher σ enables more uniform Li⁺ flux distribution during each charge/discharge cycle, reducing localized overpotential regions that accelerate electrolyte decomposition and progressive solid-electrolyte interphase (SEI) growth — the primary drivers of gradual capacity fade in lithium-ion cells.

Table 4. Comparison of separator ionic conductivity and LiFePO₄‖C coin-cell cycling results measured using the IEST EIC2400M and ERT7008.
Cycle Metric	Separator A Cell	Separator B Cell	Δ (A – B)
100-Cycle Capacity Retention	97.11%	93.23%	+3.88%
Average Coulombic Efficiency	~99.7%	~99.3%	+0.4%
Separator σ (mS cm⁻¹)	1.3182	1.0176	+29.5%

4. Conclusions

This study compared two polypropylene-based separators produced using different coating processes. The separators were evaluated first by multi-layer EIS and then in LiFePO₄‖C coin cells. The results support the following conclusions:

Multi-layer EIS provides a quantitative measure of separator ionic conductivity: Separator A has an ionic conductivity of 1.3182 mS cm⁻¹ compared with 1.0176 mS cm⁻¹ for Separator B, which is 29.5% higher than Separator B. Multi-layer EIS using the IEST EIC2400M distinguishes the through-thickness ion-transport performance of the two separators before cell-level cycling evaluation.
The separator-level difference is consistent with the cell-level EIS results: The LiFePO₄‖C coin cell containing Separator A shows a smaller mid-frequency semicircle than the cell containing Separator B. This indicates that the separator with higher ionic conductivity is associated with lower cell impedance under the tested conditions. Because the separators also differ in their coating process, this does not establish that conductivity alone causes the difference.
Higher ionic conductivity is associated with better capacity retention in this comparison: After 100 cycles at 0.2C, the cell containing Separator A retained 97.11% of its initial capacity, while the cell containing Separator B retained 93.23%. This 3.88-percentage-point difference follows the same trend as the separator ionic-conductivity and coin-cell EIS results.
Separator-level testing may improve the efficiency of material screening: The IEST EIC2400M provides separator ionic conductivity data before extended cell cycling. This measurement can help identify candidates for subsequent cell assembly, EIS analysis, and cycle testing, and may reduce the number of separator formulations that need to proceed to longer electrochemical tests.

In separator coating development, ionic conductivity should be evaluated together with mechanical strength, thermal stability, electrolyte wettability, and other relevant properties. A larger set of separators and repeated cells would be needed to establish how well ionic conductivity predicts cell performance. Combining separator-level measurements with cell-level validation provides a more efficient basis for comparing coating processes and selecting candidates for further development.

⚡ Screen Your Separator Coating Candidates in Minutes, Not Weeks

The IEST EIC2400M Multi-Channel Ion Conductivity Test System measures separator ionic conductivity across up to 8 channels simultaneously — with automated EIS fitting (100 kHz–10 kHz) and one-click σ output — enabling researchers to rapidly quantify separator ionic conductivity and screen separator performance before committing to time-intensive cell assembly and cycling tests.

Explore EIC Series → Explore ERT Series →

5. References

[1] Huang S., Xie D., Bao L., et al. Layer-by-layer self-assembly modified polypropylene separator and its performance study. Plastics Science and Technology, 2026, 54(03): 156–161.

[2] He C. Preparation and battery performance of polypropylene lithium-ion battery separator modified by functionalized nano-SiO₂. Tianjin University of Technology, 2025.

6. FAQs

6.1 What is separator ionic conductivity and how is it measured?

Separator ionic conductivity (σ, in mS cm⁻¹) quantifies the rate at which lithium ions migrate through the electrolyte-filled separator. Measuring lithium-ion battery separator ionic conductivity uses EIS for separator ionic conductivity measurement: the separator is stacked in 1–4 layers between blocking electrodes, EIS is recorded from 100 kHz to 10 kHz, and the bulk resistance (R_bulk) at each layer count is extracted from the Nyquist high-frequency intercept. σ is then calculated as σ = L / (R_bulk × A). The IEST EIC2400M automates this process with one-click fitting and conductivity output across up to 8 channels simultaneously.

6.2 How does separator ionic conductivity affect battery cycle life?

The separator ionic conductivity and battery cycle life correlation operates through two linked mechanisms: (1) higher σ reduces bulk ionic resistance, lowering cell internal impedance and operating temperature — both of which slow electrolyte degradation; (2) more uniform Li⁺ flux distribution reduces localized overpotential regions where solvent decomposition and progressive SEI growth concentrate, which are the primary drivers of capacity fade. In this study, a 29.5% higher σ (1.3182 vs. 1.0176 mS cm⁻¹) produced a 4-percentage-point improvement in 100-cycle retention (97.11% vs. 93.23%) — directly validating the separator ionic conductivity vs battery capacity fade relationship.

6.3 What is the difference between ionic conductivity and MacMullin number?

Ionic conductivity (σ, mS cm⁻¹) measures the absolute lithium-ion transport rate through the separator-electrolyte composite. The MacMullin number (N_M) is a dimensionless ratio comparing this value to the conductivity of the free electrolyte alone — N_M = σ_electrolyte / σ_{separator+electrolyte}. An N_M close to 1 indicates that the separator imposes minimal additional resistance beyond the electrolyte itself. Both metrics are useful: σ provides an absolute material benchmark, while N_M isolates the structural contribution of the separator from the electrolyte’s intrinsic properties.

6.4 How can ionic conductivity be used to screen separator performance?

How to screen separator performance by ionic conductivity is a strategic workflow question. In separator coating development, each formulation iteration requires validation. Full-cell assembly and cycling takes weeks per iteration. The alternative — validated by this study — is to measure σ on separator stacks using the IEST EIC2400M, which returns quantitative results in minutes. A σ threshold (e.g., >1.2 mS cm⁻¹ for PP-based separators in standard LiPF₆ electrolyte) can serve as a go/no-go gate: formulations above the threshold advance to cell-level testing; those below are revised or discarded. This σ-first screening approach can reduce total development cycle time by 60–80%.

6.5 What role does separator wettability play in ionic conductivity and charge transfer resistance?

Separator wettability and electrolyte uptake for Li-ion batteries are fundamental prerequisites for ionic conductivity. A separator with poor wettability will trap air in its pore network, reducing effective electrolyte-filled porosity and creating localized dry spots — both of which increase R_bulk. Furthermore, incomplete wetting creates non-uniform current distribution at electrode interfaces, increasing the effect of separator on battery charge transfer resistance (R_ct). This study’s Separator A benefited from a coating modification that simultaneously improved wettability (faster electrolyte uptake) and ionic conductivity (29.5% higher σ), producing lower R_bulk and R_ct compared to Separator B.

6.6 How do ceramic-coated and polymer-coated separators compare in ionic conductivity?

Ceramic-coated vs polymer-coated separator performance depends critically on the coating morphology, not just the coating chemistry. Ceramic coatings (e.g., Al₂O₃, SiO₂) generally improve thermal stability and wettability due to their hydrophilic surface chemistry, but if the ceramic layer is too dense or thick, it may increase tortuosity and reduce ionic conductivity. Polymer coatings (e.g., PVDF, aramid) can form gel-like interfaces with the electrolyte that enhance ion transport, but may swell excessively or block pore openings if applied non-uniformly. In practice, the optimal coating strategy depends on the target application: ceramic coatings are preferred for high-temperature safety, while thin polymer coatings can maximize σ for high-rate applications. The IEST EIC2400M provides the quantitative σ measurement needed to objectively compare coating strategies.

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