# Unveiling the reference electrode's artefacts: competition between polarization and delay

Sylvain Rey, Justin Bouvet, Eric Woillez, Eric Mayousse, Sebastien Martinet, Willy Porcher

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Sylvain Rey, Justin Bouvet, Eric Woillez, Eric Mayousse, Sebastien Martinet, et al. Unveiling the reference electrode's artefacts: competition between polarization and delay. Journal of The Electrochemical Society, 2026, 173, pp.030502. 10.1149/1945-7111/ae3860. cea-05513818

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Submitted on 16 Feb 2026

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# Unveiling the Reference Electrode's Artifacts: Competition between Polarization and Delay

Sylvain Rey,<fnref n="2" /> ID, Justin Bouvet,¹ ID, Eric Woillez,¹ ID, Eric Mayousse,¹ ID, Sébastien Martinet,¹ ID and Willy Porcher<fnref n="2" /> ID

Univ. Grenoble Alpes, CEA, Liten, F-38000 Grenoble, France

Integration of reference electrodes (REs) in lithium-ion batteries is a critical step toward enabling in situ diagnostics, real-time monitoring of electrode potentials, and development of advanced battery management systems. However, practical implementation of REs remains hampered by the presence of artifacts that distort local potential measurements and compromise diagnostic accuracy. This study investigated the origin and impact of such artifacts through a combined experimental and simulation-based approach. Using six cell designs incorporating lithium foil or lithium iron phosphate (LFP) mesh REs, we show that the blocking effect of the RE induces (1) localized polarization due to increased local resistance, particularly pronounced for Li REs, and (2) delayed lithiation on the positive electrode (PE) and negative electrode (NE) areas facing the RE, highlighted by post-mortem analysis. A pseudo-3D Newman model elucidates the behavior of the RE and its surroundings. Polarization and delay phenomenon induced by the RE are shown to compete: the former leads to an underestimation of the NE potential, while the latter leads to its overestimation during charge. These findings provide a comprehensive understanding of the origin of RE artifacts, suggest design guidelines for minimizing them, and highlight the necessity of interpreting RE measurements with an awareness of such artifacts. © 2026 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/1945-7111/ae3860]

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Manuscript submitted October 17, 2025; revised manuscript received December 22, 2025. Published January 29, 2026.

Supplementary material for this article is available online

In response to climate and air pollution challenges, the electrification of transportation has gained considerable momentum, driven by a global push for energy transition. Electric vehicles, powered by lithium-ion batteries (LIBs), have emerged as a promising solution due to their high energy efficiency and potential for decarbonization.¹ Despite their widespread adoption, current LIBs still present limitations in terms of safety, cost, environmental impact, and performance under fast-charging conditions.<fnref n="2" /> Addressing these challenges necessitates advanced diagnostic tools to elucidate the internal electrochemical processes in battery cells. One such tool is the reference electrode (RE), which enables in situ monitoring of individual electrode potentials and could pave the way for intelligent battery management strategies aimed at enhancing longevity, reliability, and safety.³

In standard two-electrode configurations, solely the overall cell voltage is accessible. However, some critical degradation mechanisms are strongly influenced by the potential of a single electrode. For instance, lithium plating is closely related to the negative electrode (NE) potential vs Li⁺/Li, although other factors such as local current density, temperature and state of charge (SoC) also play a role. This phenomenon is particularly critical during fast charging, where high current densities may depress the NE potential below 0 V vs Li⁺/Li, rendering metallic lithium deposition thermodynamically favorable. Such plating not only leads to irreversible lithium loss but also increases the risk of dendrite formation and thermal runaway. Integrating a RE into the cell enables independent monitoring of the electrode potentials, thus allowing real-time detection of unsafe conditions during aggressive charging.⁴ This capability is essential for the development of smart charging protocols that push performance limits without compromising safety. Additionally, REs open the door to advanced diagnostic techniques on each electrode independently, such as electrochemical impedance spectroscopy (EIS).⁵

However, technical challenges have hampered the adoption of REs in commercial lithium-ion cells. First, while metallic lithium, lithium alloys, and insertion compounds have been proposed as RE materials, each presents limitations: lithium metal forms a solid electrolyte interphase (SEI) that can lead to potential drift over time

and is sensitive to electrolyte composition; alloy-based REs require precise control of the lithiation level and suffer from temperature-dependent potential variations and mechanical stability issues; insertion materials like LiFePO₄ (LFP) and Li₄Ti₅O₁₂ offer more stable potentials without SEI formation, but are generally more hindering and may require pre-conditioning or activation that limits their use in early cycling stages.⁶⁻⁸ Furthermore, the physical integration of a RE between the NE and the positive electrode (PE), which is the most promising location reported in the literature,⁶,⁹,¹⁰ introduces geometric and electrochemical complexities: spatial current inhomogeneities, potential gradients, and disrupted lithium-ion transport can introduce measurement artifacts and alter the cell performances.¹¹ On the one hand, the RE has been shown to induce a localized voltage drop, which increases with its hindrance: the more difficult it is for ions to pass through, the higher the resistance will be, and the larger the voltage drop will become under similar current densities. This leads to increased local polarization, reflected by a greater potential difference between the RE and the other electrodes during charge.⁴ On the other hand, the blocking effect leads to a longer path to join the areas of the PE and NE. This results in a local delay in lithiation: as it is more difficult for ions to access these areas, lithiation proceeds more slowly than in other, more accessible regions.¹²,¹³ However, to the best of our knowledge, a correlation between the delay and the polarization has not yet been demonstrated.

Nevertheless, recent developments point toward promising pathways for practical implementation. Composite insertion electrodes combining lithiated and delithiated phases of LFP can provide a stable potential from the onset of cycling, enabling immediate diagnostic capability.¹⁴ Moreover, advances in electrode geometries, such as thin and sparse mesh-type RE structures or wire configurations, minimize their impact on ionic transport and cell performance.¹⁵ Although complete isolation from parasitic effects remains elusive, these strategies enable reliable in situ diagnostics, especially when combined with model-based artifact correction approaches.¹⁶,¹⁷ Together, these innovations offer a pathway toward integrating REs in research and potentially in future commercial battery systems.

In this work, we systematically dissect the artifacts introduced by REs in lithium-ion batteries, focusing on their impact on cell behavior and the accuracy of local potential measurements. We

<footnote n="2">²E-mail: sylvain.rey@cea.fr; willy.porcher@cea.fr</footnote>
employ a dual approach combining experimental characterization and physics-based modeling. Experimentally, we fabricated and tested six distinct cell designs, incorporating both lithium foil and LFP mesh REs in “Usual” (3 × 3 mm²) and “Full surface” (or “Full surf”) configurations. The “Full surface” designs, in which the REs occupy the entire geometric area of the cell, serve as extreme cases to amplify their impact, enabling a deeper understanding of their underlying mechanisms. Through EIS, cycling at different rates, and post-mortem analysis, we quantify how RE geometry and material properties influence local polarization, lithiation heterogeneity, and active surface area. Complementarily, we develop a pseudo-3D Newman model to simulate the coupled electrochemical and transport phenomena in the vicinity of the RE. This model resolves the competition between polarization artifacts and delay artifacts, while also capturing transient current density fluctuations induced by the RE. By correlating experimental observations with simulation predictions, we identify key design parameters, such as mesh aperture for instance, that mitigate artifacts without sacrificing diagnostic capability. Ultimately, this study provides actionable insights for optimizing RE integration, with implications for fast-charging protocols, state-of-health estimation, and the development of model-based artifact correction strategies.

## Materials and Methods

**Experimental.**—Electrodes.—The NE was composed of 96.5 wt% of blend graphite (Nippon Carbon S12 86.85 wt% + Imerys C-NERGY™ QX2 9.65 wt%), 3 wt% of binder (1.7 wt% of latex (ENEOS TRD105A) + 1.3 wt% of Na-CMC (Ashland)) and 0.5 wt% of carbon black (Imerys, SuperP C45). The slurry was coated on a 10 µm-thick copper current collector foil with a loading of 14 mg cm⁻². Then, the electrode was calendared to a porosity of 30%, corresponding to a thickness of about 100 µm (including the current collector).

The PE was composed of 96 wt% of $LiNi_{0.8}Mn_{0.1}Co_{0.1}O_2$ (NMC811, Targray), 2 wt% of binder (Solvay 5130 PVDF (poly-vinylidene fluoride)) and 2 wt% of electronic conductors (1.5 wt% of carbon black (Imerys, SuperP C65) + 0.5 wt% of vapor grown carbon fibers (VGCF from Showa Denko)). The slurry was coated on a 20 µm-thick aluminum current collector foil with a loading of 22.5 mg cm⁻². Then, the electrode was calendared to a porosity of 33%, corresponding to a thickness of about 95 µm (including the current collector).

The lithium RE was prepared from a Li foil with a thickness of 50 µm. For the “Usual RE Li” design, the Li foil was sandwiched around a copper support mesh and the lithium RE thickness was approximately 100 µm. For the “Full surface RE Li” design, the Li foil with a thickness of 50 µm was manually pressed on the copper support mesh.

The LFP RE was composed of 90.5 wt% of LFP (Prayon Grade 2B) with 45.25 wt% pristine and 45.25 wt% chemically delithiated using $NO_2BF_4$ (Aldrich), 4.5 wt% of binder (Solvay 5130 PVDF) and 5 wt% of VGCF (Showa Denko). The slurry was deposited on a 25 µm-thick aluminum current collector mesh with a loading of ~1 mg cm⁻² on each side. Then, the electrode was pressed to a total thickness of about 30 µm. The final mesh aperture is approximately 40%, which is similar to what is usually used in literature.¹⁸,¹⁹ Optical microscope images can be seen in Fig. S1 (supporting information).

**Pouch cells.**—Cells were assembled in single layer pouch format by stacking a 35 × 35 mm² graphite electrode, a 40 × 40 mm² separator with a porosity of 49% (Senior Material), and a 32 × 32 mm² NMC electrode, for the “without RE” design, as depicted in Fig. 1a. The separator material is a PE base film with 2 µm aluminum oxide coating on both sides, resulting in a total thickness of 16.5 µm. The “Triple separator” (or “Triple sep”) design is similar except the stacking of three 40 × 40 mm² separators between electrodes, as depicted in Fig. 1d. The aluminum and

copper current collectors were welded respectively to an aluminum and a nickel tab for electrical connection. For “Usual RE” designs, the RE functional surface was of 3 × 3 mm² positioned at 3 mm from the PE edge. An additional separator of 7 × 7 mm² was added to the PE side, as depicted in Figs. 1b and 1c (not to scale for visibility purposes). For the “Full surface Li” design depicted in Fig. 1e, the RE surface was of 40 × 40 mm² as the additional separator. Finally, two RE sizes have been used for the “Full surface LFP” design depicted in Fig. 1f: 40 × 40 mm² for renormalization measurements (Fig. 6), the overhang ensuring that the entirety of the electrodes was covered, and 30 × 30 mm² for other measurements, in order to not be influenced by the NE overhang when measuring potentials. With regard to electrical connections, the copper support mesh of the Li REs was welded to a nickel tab, whereas a non-coated part of the aluminum support mesh of the LFP REs was welded to an aluminum tab. Each design was realized in 3 to 5 replicates to ensure reproducibility.

The pouch cells were vacuum sealed in a glove box after the addition of 250 µl of electrolyte. The electrolyte used was a commercial mixture (UBE) of dimethyl carbonate, ethyl carbonate and ethyl methyl carbonate in equal proportions (DMC:EC:EMC 1:1:1 weight) with 1 M $LiPF_6$ and 2 wt% vinyl carbonate (VC), denoted as LPX+2%VC.

**Electrochemical tests.**—A pressure of 1 bar was applied to the cells. Except the symmetric ones (see Symmetric cells below), all cells were cycled with the same protocol. The formation cycle was performed in a climatic chamber at 45 °C (Binder BD 53). It began with a first charge at C/20 for 5 min before a rest for 6 h to ensure good impregnation of the electrolyte. Then, the charge at C/20 continued until the cell voltage reached 4.2 V, and was completed by a constant voltage step at 4.2 V until the current fell below C/50. After a 10 min rest, the cell was discharged at D/20ª until the cell voltage reached 2.7 V.

The cells were then transferred to a climatic chamber at 25 °C (Liebherr FKU1800) for potentiostatic EIS (PEIS) from 500 kHz to 5 mHz with 8 frequencies per decade and 5 mV amplitude. This was realized at 6 different $SoC$ from 0% to 100%: in between the PEISs, the cell was charged at C/5 during 1 h with 1 h of rest before and 4 h of rest after. Regarding EIS reproducibility, a typical maximum deviation on the order of 0.2 Ω.cm² was observed among replicates of a given design, except for “Full surface LFP,” which will be addressed when discussing Fig. 6.

To continue, discharges at different rates were realized at 25 °C: a first charge at C/5 was realized until the cell voltage reached 4.2 V, followed by a constant voltage step at 4.2 V. After a 30 min rest, a constant current discharge at D/N was applied until the cell voltage reached 2.7 V, followed by another 30 min rest. This sequence was repeated from the charge at C/5 for N taking successive values 10, 5, 3, 2, 1, 0.67, 0.5, 10. Finally, charges at different rates were realized at 25 °C: a first discharge at D/5 was realized until the cell voltage reached 2.7 V. After a 30 min rest, a constant current charge at C/N was applied until the cell voltage reached 4.2 V. Then, a constant voltage step at 4.2 V was applied, followed by another 30 min rest. This sequence was repeated from the discharge at D/5 for N taking successive values 10, 5, 3, 2, 1, 0.67, 0.5, 10.

For this purpose, VMP3 and MPG2 potentiostats from Bio-Logic have been used. Cell voltages during the formation step are available in Fig. S2 (supporting information). Table S1 reports the corresponding capacities of 43.5 ± 1.5 mAh. The “Full surface Li” design has a significantly lower irreversible capacity (relative to balancing), which will be addressed when discussing this design Fig. 5. With this exception, irreversible capacities varied by no more than 4% and the evolution of cell voltages as a function of the capacity during formation step is very similar for all cells regardless of the design. This illustrates the high reproducibility of the assembled cells.

<footnote>ªThrough this article, we use the notation D/N to denote a constant-current discharge rate such that the full theoretical capacity is extracted in N hours.</footnote>
<figure><img src="image_5.png" /><figcaption>**Figure 1.** Schematic representation of the studied cell designs: aluminum (Al) and copper (Cu) current collectors, PE, RE, NE and separator (Sep.). Control designs without RE with (a) one or (d) 3 separators, denoted as “Without RE” and “Triple separators” respectively. Designs with usual 3x3 mm² RE made of (b) a Li foil and denoted as “Usual RE Li” or (c) a LFP mesh and denoted as “Usual RE LFP”. Designs with a wide RE covering all the cell’s geometric surface area, made of (e) a Li foil and denoted as “Full surface Li” or (f) a LFP mesh and denoted as “Full surface LFP”. These representations are not to scale for visibility purposes, and the colors used for the REs are purely illustrative and consistently reused throughout the figures of this article to plot the corresponding curves.</figcaption></figure>

*Symmetric cells.*—Positive/positive and negative/negative symmetric cells (not represented here) have also been assembled in single layer pouch format using the electrodes from two “Without RE” cells after their formation step and a charge to 60% SoC at C/5. They underwent a single PEIS measurement from 200 kHz to 2 mHz with 8 frequencies per decade and 10 mV amplitude.

*Simulation.*—A Doyle-Fuller-Newman model²⁰ at constant temperature and with concentrated solution theory has been implemented in COMSOL Multiphysics 6.3 in order to simulate the behavior of the “Usual RE LFP” design and its RE. As depicted in Fig. 2, it is actually composed of two electrically-coupled submodels. The first one is a pseudo-3D (P3D) model corresponding to the RE and its surroundings, denoted as the “reference side” in the rest of this article (Fig. 2a). As a simplifying hypothesis, the RE geometry is considered invariant along the y-axis. Symmetry and periodicity of the mesh allows to reduce the reference side geometry to the light area of the figure, composed of a half mesh wire cross section covered with LFP, sandwiched by separators and with NMC and graphite on opposite sides. The second one is a pseudo-2D (P2D) model representing the rest of the cell, which is not perturbated by the RE, with invariance along the x and y axis. It will be denoted as the “main side” in the rest of this article (Fig. 2b). These two submodels are connected in parallel: 0 V is imposed on the NE collectors (potential reference) and PE collectors are subjected to

the same potential. The constant current imposed to the entire cell is distributed between them. The governing equations and parameters are given in Table S2–S4 and Fig. S3 (supporting information).

A spherical particles model has been used for the active surface density of electrodes. Open circuit voltage of LFP comes from Safari et al.²¹ Electrolyte parameters have been adapted from Landesfeind et al.²² although the electrolyte used in our experiments was different: conductivity has been rescaled to match the experimental one measured at 1 M, and diffusivity has been adjusted to fit the experimental cell voltage. The agreement between experiment and simulation is shown in Fig. S4 (supporting information). Although simulated and experimental cell voltage are quite in agreement, there are significant differences on the probed NE potential. Nevertheless, the model provides a representative description of the system, and one can reasonably rely on its general trends, if not its quantitative predictive behavior.

With regard to the potentials extracted from the simulation, we get $V_{neg}$ the potential of the NE collector, $V_{ref}$ the potential of the aluminium mesh, $V_{pos}$ the potential of the PE collector, and $V_{el}^{main}$ or $V_{el}^{ref}$ the potentials of the electrolyte at the center of the separator (main side) or at the center of the mesh aperture (reference side) respectively. Then, the cell voltage and the electrode potentials can be obtained as follows (in green in Fig. 2): $U = V_{pos} - V_{neg}$ the cell voltage; $U_{neg}^{RE} = 3.424 \text{ V} - V_{ref} + V_{neg}$ the NE potential probed by
<figure><img src="image_6.png" /><figcaption>**Figure 2.** Schematization of the simulation. A pseudo-3D model describes the area of the cell perturbated by the RE (a) whereas a simpler pseudo-2D model describes the rest of the cell (b). These two sub-models are electrically coupled in parallel.</figcaption></figure>

the RE, 3.424 V being the plateau potential of the open circuit voltage (OCV) of the LFP; $U_{pos}^{RE} = U + U_{neg}^{RE}$ the PE potential probed by the RE; $U_{neg}^{k} = V_{neg} - V_{el}^{k}$ the actual potential experienced by the NE ($k$ = main or ref) and $U_{pos}^{k} = V_{pos} - V_{el}^{k}$ the actual potential experienced by the PE.

## Results and Discussion

Six NMC811 vs graphite cell designs, depicted in Fig. 1, were assembled in single layer pouch format. The “Without RE” design is a standard cell without any RE. The “Usual RE Li” and “Usual RE LFP” designs contain a small RE (100 µm-thick Li foil or 30 µm-thick LFP mesh), similar to REs commonly used in literature and covering less than 1% of the PE surface area. The “Full surface Li” and “Full surface LFP” designs include the same REs except these ones are deliberately larger in order to emphasize their impact in the cell. Even though their measurements may possibly not differ from that of the “Usual RE” designs, their overall impact on the entire cell is significantly increased, if not proportionally. Finally, the “Triple separator” is another control design like “Without RE,” but three separators instead of one have been used in order to get a similar distance between electrodes as “Full surface LFP” design.

With regard to the electrochemical characteristics of these cells, observations are consistent with the designs. First, raw cell EIS spectra in Nyquist representation are shown in Fig. 3a. All spectra exhibit a profile characterized by an offset between the origin and the high frequencies attributable to electrolyte and electrical resistances, a capacitive semi-circle associated with charge transfer phenomena, and a straight line at low frequencies associated with diffusion phenomena.²³ We can see that the “Without RE” design and the two “Usual RE” ones are almost perfectly superimposed. Thus, the impact of the small 3 × 3 mm² RE on the cell behavior is negligible.

The other designs exhibit a higher horizontal offset between the origin and the high-frequencies corresponding to a higher resistance, which is consistent with the increased distance between the PE and NE. Finally, their capacitive semi-circles are also dissimilar: compared to that of the “Without RE” design, “Full surface LFP” and “Full surface Li” exhibit semi-circles 1.5 and 10.7 times larger respectively, suggesting that the charge transfer phenomena involved in these designs are different. Moreover, Fig. 3b shows, for each design, the cell voltage during a charge at C/2 after being discharged at D/5. Once again, the “Without RE” design and the two “Usual RE” ones are almost perfectly superimposed, confirming the negligible impact of the small RE. On the other hand, the other designs exhibit greater cell polarization, especially the “Full surface Li,” but it should be noted that due to this same polarization, the preceding D/5 discharge have stopped earlier, leading to a non-identical initial state for the C/2 charge. This bias has been corrected by shifting the curves horizontally to account for this missing discharge capacity.

“Usual RE Li” and “Usual RE LFP” designs are firstly compared in order to highlight first evidences of polarization and delay artifacts when using a RE. As a starting point, the NE potential of the “Usual RE” designs during charge as a function of specific capacity is shown Fig. 4a for several C-rates. As expected, when the charging rate increases the probed potential is lower due to polarization and may drop below 0 V for the highest C-rates at a capacity $Q^{0V}$, posing a risk of lithium plating. This plating prediction is synthetized in Fig. 4b where $Q^{0V}$ is plotted vs C-rate for the two “Usual RE” designs. The area associated with a risk of lithium plating is located in the upper right corner. The capacity $Q^{0V}$ is systematically higher for “Usual RE LFP” than for “Usual RE Li.” Thus, the later indicates that plating may happen earlier. This is due to its higher hindrance, leading to a larger polarization, thereby inducing a lower probed NE

<figure><img src="image_7.png" /><figcaption>**Figure 3.** Overview of the characteristics of the studied cell designs. (a) Full cell EIS spectra at 60 % SoC and 25 °C. (b) Cell voltage during a C/2 charge at 25 °C.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Figure 4.** Comparison of the “usual RE” designs. (a) NE potential measurements of the “Usual RE” designs during C-rate. (b) Charge capacity reached when the different REs measure a NE potential below 0 V as a function of C-rate. Cell voltage (c) and NE potential measurements (d) during a D/2 discharge for all designs. (e) Comparison between the EIS of “Without RE” on the one hand, and the one reconstructed from negative/negative and positive/positive symmetric (sym) cells on the other hand. (f) PE EIS for “Usual RE Li” and “Usual RE LFP” designs measured using RE, along with the PE EIS measured in positive/positive symmetric (pos sym) cell. These latter EIS spectra have been translated horizontally in order to facilitate the comparison of the semi-circles and the diffusion part: the local minimum located between the charge transfer semi-circle and the diffusion part of each spectrum has been placed at the origin.</figcaption></figure>

potential. In the literature, several authors have already shown that a more obstructive RE induces a higher local polarization. This causes the NE potential to be underestimated during charge because of the subsequent voltage drop, and conversely during discharge.¹² Thus, our results are consistent with published studies. The lithium foil RE, due to its thicker and opaque geometry, is more hindering than the LFP mesh RE, and therefore probes a lower and underestimated potential than the latter. This highlights the polarization effect.

Then, we can consider the cell voltage during a D/2 discharge as a function of specific capacity for the “Without RE,” “Usual RE” and “Full surface” designs in Fig. 4c. The cell voltages for “Without RE” and the two “Usual RE” designs are almost superimposed: the impact of the RE on the overall cell in the latter designs is negligible. In contrast, “Full surface Li” exhibit a downward offset of about 100 mV which may be explained by polarization. On the other hand, the cell voltage of the “Full surface LFP” design appears as if it was
shifted to the left by approximately 10 mAh g⁻¹, leading to a lower final specific capacity which was not observed at low regimes or during formation (see Table S1). While polarization may contribute, the mesh geometry of the RE, which occupies the entire cell surface, suggests that this effect primarily stems from the reduced geometric active area of the cell. Indeed, the areas facing the mesh openings are more accessible to Li ions than those beneath the mesh wires. Together with limited in-plane homogenization of the electrodes, this leads to a reduced apparent capacity at high C-rates and a delayed lithiation beneath the mesh wires. This highlights what will be referred to throughout this article as the delay phenomenon: the more hindered a region of the cell is, the greater the lithiation delay experienced by the electrodes in that region.

In spite of their similar cell voltages, even the “Usual RE” designs exhibit dissimilar probed NE potentials as shown in Fig. 4d which thus can only be explained by the RE itself. As expected, the NE potential increases at the end of discharge due to delithiation of the graphite electrode, but this increase is less pronounced for the lithium REs than for the LFP ones. At the end of discharge, one can observe a probed potential difference of more than 100 mV between the two “Usual RE” designs. The most hindering RE is the lithium one as previously established (Fig. 4b). Thus, its larger polarization effect should induce a higher probed NE potential. However, the opposite trend is observed here, which undermine this explanation. Nevertheless, as mentioned previously, a RE can also induce a lithiation delay: it is more difficult for ions to access the area face to the RE mesh wires due to their blocking effect. In average, lithiation then proceeds more slowly near the RE than on the rest of the cell, and the RE surroundings undergo a local lithiation delay. Furthermore, this local lithiation delay is probed by the RE, thereby providing a delayed potential compared to the real NE potential elsewhere in the cell. Consequently, this delay in the measured potential increases with the RE hindrance. This is consistent with Fig. 4d where the potential measured by the more hindering lithium RE is to the right of that measured by the LFP RE. Thus, the abrupt NE potential increase at the end of discharge is not captured by the lithium RE (Fig. 4d). However, this NE potential increase must occur elsewhere in the cell as it represents the only reasonable explanation for the cell voltage drop at this same moment (Fig. 4c). This observation once again underscores the occurrence of lithiation delay induced by the RE. Further discussion about this figure, especially regarding the “Full surface” designs, is available in supporting information (section Discussion about the delay in “Full surface” designs). It is important to note that this is in competition with polarization which tends to increase the NE potential during discharge. The aforementioned phenomena will be supported by the following of this article.

Finally, electrodes EIS measured using the RE are analyzed in Figs. 4e and 4f. Symmetric positive/positive or negative/negative cells were assembled with electrodes from “Without RE” cells after their formation and conditioning to 60% SoC. The credibility of these symmetric cells is underlined by Fig. 4e where the EIS of the “Without RE” design is shown along with the one reconstructed by adding the half-impedances of negative/negative and positive/positive symmetric cells on the other hand. Apart from a small horizontal offset attributable to electrolyte and electrical resistances, the two spectra are in good agreement both in shape and characteristic frequencies associated with charge transfer and diffusion phenomena. This supports the reliability of our EIS measurements in symmetric cells, which will therefore be used as the reference (i.e., the “expected”) EIS for each electrode throughout this article.

Then, Fig. 4f shows the PE EIS measurements made by the lithium RE and the LFP one in “Usual RE” designs, along with the expected EIS which was measured in positive/positive symmetric cell. EIS spectra have been translated horizontally in order to facilitate the comparison of the semi-circles and the diffusion part: the local minimum located between the charge transfer semi-circle and the diffusion part of each spectrum has been placed at the origin. Furthermore, the raw impedance in ohms was multiplied by the

active surface area of the cell, resulting in Ω.cm², to eliminate size-related effects and enable comparison with other cell formats. Since the full cell EIS of the “Without RE” and “Usual RE” designs are superimposed (see Fig. 3a), the differences observed in Fig. 4f can only be explained by the RE itself. In particular, focusing on the semi-circle part, the PE EIS measured by the “Usual RE Li” differs significantly from the expected EIS obtained on a symmetric cell. The differences concern the shape, with two semi-circles instead of a single predominant one; the width, with typical charge transfer resistances of 1.5 Ω.cm² rather than 3.5 Ω.cm²; and the characteristic frequencies. Conversely, the “Usual RE LFP” PE EIS measurement is quite similar to the expected one. On the other hand, the straight line associated with diffusion phenomena is closer to the expected one with the “Usual RE Li” design than with the “Usual RE LFP” one. It is worth noting that the “Usual RE LFP” design seems also more accurate for the NE EIS on both charge transfer and diffusion (see supporting information Fig. S5). However, the sum of the PE and NE EIS measured using the RE necessarily corresponds to the cell EIS. Thus, the absolute deviation between the expected EIS and the one measured by the RE is identical whether considering the PE or NE. The error is nevertheless more visible on the EIS of the PE (Fig. 4f) because of its lower impedance, leading to a higher relative deviation compared to the NE (Fig. S5). In conclusion, RE introduces deviation from the expected results obtained with the symmetric cell configuration and these deviations are higher for “Usual RE Li” with regard to the charge transfer phenomena.

In spite of its lithium foil covering the entire cell area, it has been shown that the “Full surface Li” design has a nominal capacity similar to the other cells (see Table S1 in supporting information) and can be cycled under standard conditions. A lower irreversible capacity is however observed and will be addressed below. Since the lithium foil overhangs prevent ions from bypassing the lithium foil, ionic transport through the cell can only be explained by lithium plating and stripping on each side of the RE as depicted in Fig. 5a. This has already been highlighted by few studies.¹⁸,²⁴ This cell is thus equivalent to two half-cells in series: NMC vs lithium on the one hand, lithium vs graphite on the other hand, which are electrically connected through the lithium foil.²⁵ Post-mortem analysis (Figs. 5b and 5c) corroborate this hypothesis: a brittle foam has formed on the lithium foil area facing the PE and NE after several cycling, contrary to the lithium overhangs which are clean and easy to remove, particularly visible in Fig. 5c. These traces are attributed to the electrochemical activity (plating and stripping) of the lithium foil.²⁶

Moreover, Fig. 5d shows the EIS at 60% SoC of the “Triple separator” and “Full surface Li” designs. The latter exhibits very wide charge transfer semi-circle approximately 10 times larger than that of the former. This can also be attributed to lithium plating/ stripping on both side of the lithium foil, which is supported by the difference in characteristic frequency compared to the “Triple separator” design, 500 Hz vs 119 Hz. In addition, we can estimate the charge transfer resistance of the lithium foil considering an exchange current of $j_0 \sim 10 \text{ A m}^{-2}$:

$$r_{ct} = \frac{RT}{j_0 F} = \frac{8.31 \cdot 298}{10 \cdot 96485} \approx 0.0026 \text{ Ω.m}^2 \\ \approx 26 \text{ Ω.cm}^2 \text{ at } T = 298 \text{ K.}$$

This equation derives from linearization of the Butler-Volmer equation for $\eta \ll RT/F$. Since charge transfer occurs on both sides of the lithium foil, the total extra resistance should be about 50 Ω.cm² in order of magnitude. This simple calculation is consistent with the observed 80 Ω.cm²-wide semi-circle on EISᵇ. In addition,

<footnote>ᵇThe charge transfer resistance in the EIS of the “Triple separator” design, approximately 10 Ω.cm², is due to PE and NE charge transfer, and contributes to the “Full surface Li” EIS. Then, the additional charge transfer resistance between the “Full surface Li” and “Triple separator” designs is approximately 70 Ω.cm² and must be explained by the presence of the Li foil. The difference with the calculated 50 Ω.cm² arises from order-of-magnitude considerations: $j_0$ was not measured and may be lower, in particular due to SEI effects.</footnote>
<figure><img src="image_9.png" /><figcaption>**Figure 5.** Observations and measurements on “Full surface Li” design. (a) Representation of the cell operation. (b) and (c) Post-mortem observations of the lithium foil. A is the lithium foil RE overhang, B corresponds to the area of NE overhang, and C corresponds to the geometric active area of the cell. d) Full cell EIS spectra and e) cell voltage during discharges at different rates of “Full surface Li” and “Triple separator” designs for comparison. EIS spectra have been translated horizontally in order to facilitate the comparison of the semi-circles and the diffusion part: the local minimum located between the charge transfer semi-circle and the diffusion part of each spectrum has been placed at the origin.</figcaption></figure>

Fig. 5e shows the cell voltage of the “Full surface Li” during charge for different C-rates, along with the “Triple separator” for comparison. A strong cell polarization can be observed on discharge for the “Full surface Li” design. Once again, this is due to this additional charge transfer induced by the presence of the lithium foil.

Finally, it was previously noted that the “Full surface Li” design exhibit a higher specific capacity and a lower irreversible capacity loss corrected by cell balancing during the first cycle (see Table S1). When a lithium ion extracted from the PE is plated on the lithium foil during charge, another lithium ion is stripped from anywhere on the other lithium foil side, even from the lithium overhang part as depicted in Fig. 5a. In particular, it allows the NE overhang to be lithiated which may influence the cell operation. Thus, the total NE capacity available may be increased. This provides a first possible explanation for the observed capacity differences. However, this phenomenon does not seem predominant: in post-mortem observations presented in Figs. 5b and 5c, areas A (lithium foil overhang) and B (NE overhang) of lithium foil exhibit much less traces of lithium plating/stripping than area C (geometric active area). Another explanation for the lower irreversible capacity of the “Full surface Li” design may lie in the fact that cross-talk between PE and NE is blocked by the lithium foil.²⁵

This plating/stripping phenomenon probably also occurs on small lithium REs such as the one used in the “Usual RE Li” design (see supporting information Fig. S6). Indeed, during cycling, a potential difference necessarily appears on either side of the lithium foil RE, which inevitably results in plating/stripping. This phenomenon can be regarded as a shortcut for lithium ions, thereby rendering the RE

bypass less favorable for them. However, this does not mitigate the aforementioned lithiation delay face to the RE, the charge transfer resistance of the lithium foil being significant.

The full cell EIS of the “Full surface LFP” design at 60% SoC is shown in Fig. 6a in comparison to that of the “Triple separator” design under the same conditions (red and pink circles). The spectra have been shifted horizontally in order to align the right sides of the capacitive semi-circles on the origin for ease of comparison, as done previously. Both spectra exhibit the same behavior, namely a semi-circle with a characteristic frequency of 119 Hz associated with charge transfer phenomena and a straight line at frequencies below 1.18 Hz whose slope is similar. “Full surface LFP” has a wider apparent charge transfer semi-circle on this representation of 11.7 vs 8.23 Ω.cm². However, these EIS spectra have been normalized by the geometric area of the PE in order to obtain Ω.cm² on both axes. Thus, if the effective active surface area is actually smaller than the geometric area, this would result in wider semi-circles in this representation. This is the case here: the presence of the LFP mesh shown Fig. 6c acts as a mask and obstructs the lithium ions path, resulting in a smaller active surface area for electrodes. Lithium ions are thus concentrated towards the parts of the electrodes facing the holes of the mesh. This is supported by the fact that lithium plating occurs preferentially at these specific places as visible in post-mortem observations Fig. 6b. For this reason, the normalization surface area used for “Full surface LFP” in Fig. 6a is wrong: the active surface area $S^{act}$, denoting the geometric area which is electrochemically active and not screened by the RE, is lower than the geometric area of the cathode $S^{geo}$. Therefore, this EIS graph
<figure><img src="image_10.png" /><figcaption>**Figure 6.** Study of the “Full surface LFP” design. (a) Full cell EIS spectra of “Triple separator” and “Full surface LFP” before and after surface area renormalization. EIS spectra have been translated horizontally in order to facilitate the comparison of the semi-circles and the diffusion part: the local minimum located between the charge transfer semi-circle and the diffusion part of each spectrum has been placed at the origin. (b) Post mortem observations of the NE from “Full surface LFP” design. (c) Optical microscope image of the mesh covered by LFP. (d) and (e) Image processing to estimate the typical diffusion length under a mesh wire.</figcaption></figure>

allows an active surface area measurement. To achieve this, one can adjust the normalization area of the “Full surface LFP” design until its EIS spectrum superimposes to the “Triple separator” one, as shown by the black stars in Fig. 6a^c. Thus, the active surface area $S^{act}$ is the normalization area when superposition occurs. This method reveals an effective aperture $\beta^{eff} = S^{act}/S^{geo} \approx 63\%$. By doing so with the other “Full surface LFP” cells, we get an average of $\beta^{eff} = 68\%$ with a standard deviation of 5 percentage points (absolute) which can be attributed to mesh manufacturing reproducibility. Indeed, the LFP deposition was performed manually, leading to possible loading heterogeneities, and subsequent aperture heterogeneities during the pressing operation (see Fig. S1). This value of 63% can be compared with the optical aperture of the coated mesh used in this same cell $\beta^{opt}$, defined as the open area divided by the total area. Using optical microscopy (see Fig. 6c) and image analysis, we plotted the greyscale intensity and observed two peaks, the first one corresponding to bright pixels (apertures) and the second one to dark pixels (mesh). Thus, we computed the ratio of bright pixels to the total number of pixels and obtained $\beta^{opt} \approx 42\%$. The same measurement was performed on another coated mesh, and resulted in $\beta^{opt} \approx 44\%$. Here again, the deviations can be attributed to mesh manufacturing reproducibility. The optical aperture is thus lower than the effective aperture, which is consistent with the fact

that lithium ions can pass through the porous LFP partially covering the aluminum mesh holes, and migrate or diffuse under the mesh wires.

Furthermore, it is possible to estimate an order of magnitude of the typical length of this migration by image analysis (see Fig. 6d). To do so, once $\beta^{eff} \approx 63\%$ and $\beta^{opt} \approx 42\%$ have been measured, a gaussian blur filter has been applied to smooth the black and white microscopy image, then pixels whose greyscale level was between the 42^th^ and 63^th^ percentiles of the entire image have been colored in red. By doing so, we get a typical length of about 50 µm (Fig. 6e), meaning that a mesh made of wires finer than 50 µm in order of magnitude should have a reduced local impact. However, it is important to note that this measurement probably depends on the charging rate, whose impact has not been studied here. In addition, in the same way as they can plate and strip on both sides of a lithium foil, lithium ions can simultaneously be inserted on one side and disinserted on the other side of an LFP mesh wire, through the exchange of an electron. This phenomenon was not taken into account in the active surface area measurement and further investigations are underway on this topic. Thus, the active surface area should lie between $S^{geo}$ and $S^{act}$ calculated from the EIS normalization.

Finally, the two “Full surface” designs allow to get an insight into the local phenomenon which can occur around the RE. By the presence of a LFP mesh, Li ions are deflected through the holes of the latter, leading to a reduction of the active surface area of electrodes and delayed lithiation face to the RE wires, along with increased polarization. On the other hand, ionic transport is possible through Li

<footnote>^cActually, a more direct and rigorous but equivalent method was used to measure $\beta^{eff}$: Re($Z_{ref}$) was plotted versus Re($Z_{mes}$), $Z_{ref}$ and $Z_{mes}$ being the impedances of “Triple sep” and “Full surface LFP” respectively. Then, a linear regression in the range 2 Hz — 9 kHz corresponding to the frequencies of the charge transfer semi-circle was performed. The slope of this regression corresponds to $\beta^{eff}$.</footnote>
<figure><img src="image_11.png" /><figcaption>**Figure 7.** Simulation results. NE potential $U_{neg}^{RE}$ during discharge (a) and charge (b) for different RE apertures. (c) Average lithiation state of graphite on the main and the reference sides during a C/2 charge, 1 h rest, and a D/2 discharge cycle. (d) Graphite lithiation state on the reference side at the end of a D/2 discharge.</figcaption></figure>

foil RE by plating/stripping on each side, at the cost of a high charge transfer resistance. Despite this mechanism, the lithium RE imposes significantly greater hindrance than the LFP mesh, which was already foreseen with the “Usual RE” designs comparison.

## Simulation Study

In order to get insight into internal perturbation by the RE and expand understanding of its operation, a model has been implemented on a numerical simulation software. Competition between polarization and delay will be revealed along with the evolution of the inhomogeneities induced by the LFP RE.

It was previously considered that a hindering RE induces delay at the end of discharge and a lower NE potential during charge (Figs. 4a, 4b and 4d). We tried to reproduce these trends in our simulation. Figure 7 shows the NE potential probed by the simulated RE during discharge (a) and charge (b) for different RE apertures $\beta_{ref}$. The probed NE potential during charge decreases when RE aperture decreases, and is lower at the very end of discharge. Thus, it shows similar trends as experimental. However, the probed NE potential is higher during the rest of the discharge for low apertures, whereas the one probed by the lithium RE, which is supposed to be more hindering, is lower in experiments (see Fig. 4d). One can see here the limits of this comparison. Actually, this could be due to a stronger delay induced by the lithium RE, leading to a lower measured potential.

Figure 7c shows the spatially-averaged lithiation state of graphite on the reference side and on the main side as a function of time

during a charge at C/2, 1 hour rest, then a discharge at D/2. During charge, lithiation at the reference side is below lithiation at the main side. During the rest, the two coupled parts of the cell are still electrochemically active. The main side is at a higher SoC than the reference side and thus exchange current with the reference side such that both parts of the cell tend to equilibrate. When the discharge begins, equilibrium has not been reached and the lithiation state at the reference side is still slightly below that at the main side. However, the reference side is discharged at a lower rate than the main side and the lithiation difference reverses during the discharge. The simulation thus illustrates that the reference side of the cell charges and discharges with a delay compared to the main side. The delay can be explained because the impedance of the reference side is higher than the impedance of the main side. A constant current is imposed at the cell level while the potential difference for both sides is the same. The current density flowing in the reference side is thus smaller than in the main side. The current distributions between the reference side and the main side are further studied in Fig. 9.

Figure 7d illustrates more precisely the lithiation distribution at the reference side. The figure displays the lithiation state of graphite on the reference side at the end of the discharge. The lithiation state is strongly heterogeneous in the electrode, being larger below the mesh wire and lower below the mesh aperture. The average lithiation is of $\bar{x}_{Li} = 0.029$ face to the aperture (right side of the figure) and $\bar{x}_{Li} = 0.26$ face to the RE mesh wire (left side). This significant difference shows that the Li ions preferentially flow in the electrode part below the aperture. The $Li^+$ ions originating from the electrode part below the wire have to flow around the wire to insert in the PE
<figure><img src="image_12.png" /><figcaption>**Figure 8.** Comparison of the NE potentials. True NE potential on the main side $U_{neg}^{main}$ (blue), on the reference side $U_{neg}^{ref}$ (green), and probed by the RE $U_{neg}^{RE}$ (red) during a C/2 charge and rest (a) or during a D/2 discharge (b).</figcaption></figure>

and have thus a much longer electrochemical path to cross. Consequently, the phenomenon is emphasized when the mesh wire width increases: the same simulation with a RE mesh wire width $l_{wire}$ of 400 µm instead of 200 µm results in average lithiations of $\bar{x}_{Li} = 0.024$ and 0.68 face to the aperture and the RE mesh wire respectively.

Simulation enables comparison between the electrode potentials probed by the RE within the simulation, namely $U_{pos}^{RE}$ and $U_{neg}^{RE}$ and the actual experienced ones, namely $U_{pos}^{main}$ and $U_{neg}^{main}$ on the main side and $U_{pos}^{ref}$ and $U_{neg}^{ref}$ on the reference side. Figure 8 shows $U_{neg}^{main}$ and $U_{neg}^{ref}$ along with $U_{neg}^{RE}$ during charge, rest and discharge. Most of the time, $U_{neg}^{ref}$ and $U_{neg}^{RE}$ are almost superimposed, except at the beginning of the discharge where differences of about 20 mV can be observed. However, they deviate from $U_{neg}^{main}$ by almost 50 mV at the end of charge. This demonstrates that the mere presence of the RE is detrimental to the measurement: the latter perturbates the cell operation in its vicinity while measuring the local perturbated potential at the same time. Under optimal conditions, the RE may accurately measure $U_{neg}^{ref}$, however the potential of practical relevance is $U_{neg}^{main}$. Thus, it is necessary to design a finer and more open mesh, less hindering, in order to perform more accurate potential measurements. As shown in Figs. 7a and 7b, with an aperture of 90%, the difference between $U_{neg}^{main}$ and $U_{neg}^{RE}$ never exceeds 20 mV, which is much more acceptable. Additionally, decreasing the width of the mesh wires further improves the measurement, and the error tend towards 0.

The deviations observed between $U_{neg}^{RE}$ and $U_{neg}^{main}$ are the result of both polarization and delay, as shown experimentally and with simulation. Up to this point, these two phenomena have been assumed to be in competition, their prevalence depending on the situation. To support this hypothesis, an illustrative model has been simulated, as shown in Fig. 9. In this simulation, the electrode OCVs have been arbitrarily chosen to illustrate the RE behavior in different situations: the PE OCV is a perfectly regular slope (a) while the NE OCV is a succession of steps (b). The obtained NE potentials $U_{neg}^{RE}$ and $U_{neg}^{main}$ along with the OCVs on the main and reference side $\phi_{Gr}^{OCV}(\bar{x}_{Li,Gr}^{main})$ and $\phi_{Gr}^{OCV}(\bar{x}_{Li,Gr}^{ref})$, where $\bar{x}_{Li,Gr}^{k}$ is the spatially-averaged graphite lithiation over the electrode volume, are shown in Figs. 9c and 9d during charge, rest and discharge. Several observations should be noted. First, as already discussed in Fig. 7c, lithiation $\bar{x}_{Li,Gr}^{ref}$ and thus OCV variations $\phi_{Gr}^{OCV}(\bar{x}_{Li,Gr}^{ref})$ on the reference side are

always late compared to the main side. However, $U_{neg}^{RE}$ and $U_{neg}^{main}$ regularly intersect each other: in spite of this OCV delay, $U_{neg}^{RE}$ does not always appear late. More precisely, it is slightly delayed when the NE OCV undergo a sharp increase, but polarization dominates on potential plateaus, which is highlighted in the close-up of Fig. 9g. This demonstrates and illustrates the true competition between polarization and delay: the first one prevails on potential plateaus of the OCV, where a small delay has no impact on potential, whereas the second one prevails on steep slopes of the OCV.

Moreover, Figs. 9e and 9f show the current densities on the main and reference sides along with the same OCV curves as previously. The current applied to the overall cell is constant during charge or discharge, however, important current densities variations are observed on the reference side, indicating lithiation rates variations. In particular, a current peak is systematically observed when the OCV slope increases sharply, even exceeding the current density of the main side. This highlights an interesting phenomenon: although the reference side is accumulating delays during charge and discharge, the current density flowing through it may temporarily be greater than the one on the main side, indicating a higher lithiation rate during a short period of time. At first sight, this may be surprising given the greater impedance of the reference side. However, this is perfectly consistent since these current density peaks happen precisely on the OCV slopes of the NE. Indeed, the previously accumulated delay is being made up at this precise moment, driven by the sudden voltage variation of the main side. Due to the crossing of the OCV step, a small increase of lithiation in the main side induces a large voltage variation, which must be followed by the reference side since they are connected in parallel. However, the delayed reference side is still on the plateau: only a high current, inducing a high lithiation rate, can induce a similar fast voltage variation.

To summarize, the reference side exhibits a larger impedance than the main side, leading to a higher local polarization. Thus, as long as the OCVs does not vary sharply, the local current density is lower and the lithiation rate is smaller: a delay arises. When an OCV variation occurs, the main side experiences a sudden voltage change, and undergoes it first due to its advance. Because of the parallel electrical coupling, the reference side must follow this variation. However, the local OCVs does not vary yet because of the accumulated delay. Hence, the local current density increases until polarization and overpotentials compensate the voltage difference between both sides. Then, when the OCV slope decreases, local polarization induces a lower local current density again, and delay is re-established.

Finally, Figs. S7 to S11 in supporting information depict various possible case combinations of step, slope, plateau for PE and NE OCVs, leading to the apparition of different artifacts depending on the combination. In addition to the previously revealed polarization and delay artifacts, it could be noted that the aforementioned current density variations lead to polarization fluctuations which can be observed on these figures, for example on the PE potential measured by the RE ($U_{pos}^{RE}$, Fig. S10) with combination C1 (see Fig. S7 for combination explanation) just before 1 h, and at 4 h. These fluctuations are only due to the current variations in the reference side (Fig. S9), which are themselves caused by OCV variations (of the NE in this case). This demonstrates how the OCV of one electrode affects the potential of the other electrode when measured using the RE. All the depicted cases on these figures can be associated with real material combinations such as graphite, NMC, LFP or other electrode materials, and may help identifying the resulting artifacts.

## Conclusions

This study provides a comprehensive analysis of the artifacts introduced by REs in lithium-ion batteries, elucidating their origins and proposing strategies for mitigation. Through a combination of experimental designs and simulation-based insights, we demonstrate
<figure><img src="image_13.png" /><figcaption>**Figure 9.** Simulation results with arbitrary OCVs for PE (a) and NE (b). NE potential probed by the RE $U_{neg}^{RE}$ (red) and on the main side $U_{neg}^{main}$ (blue) along with the associated NE OCVs during a C/2 charge and rest (c) or during a D/2 discharge (d). (e) and (f) Current densities on the reference (red) and main (blue) sides along with the associated NE OCVs during the same simulation. (g) Close-up of (c).</figcaption></figure>

that REs perturb local electrochemical environments in two primary ways: (a) polarization, arising from increased ionic resistance and charge-transfer limitations, and (b) lithiation delay, caused by heterogeneous current distribution in the vicinity of the RE. The “full surface” RE designs, where the RE occupies the entire geometric area of the cell, reveal the extreme manifestations of these artifacts. Lithium foil REs, for instance, introduce a high charge-transfer resistance (~80 Ω·cm²) due to continuous plating/stripping on both sides of the foil, while LFP mesh REs reduce the active surface area which is quantifiable via EIS normalization. This masking effect deflects lithium ion flux toward mesh apertures, leading to localized plating and accelerated degradation at these sites.

Our pseudo-3D Newman model further clarifies the dynamic interplay between polarization and delay, showing that their relative

dominance depends on the OCV profile of the electrodes. Polarization artifacts prevail during potential plateaus, where minor delays have negligible impact on measured potentials. Conversely, delay artifacts dominate during sharp potential transitions, where even minor lithiation heterogeneities translate into significant potential discrepancies. Notably, the model suggests that the current density on the reference side may temporarily exceed that of the main side, particularly when the reference side catches up to the main side's state of charge. This phenomenon underscores the need for artifact-aware interpretation of RE data, as the probed potential may not reflect the actual electrode behavior.

From a practical standpoint, these findings yield design guidelines for minimizing RE artifacts. With regard to the material selection, the LFP mesh design outperforms lithium foil due to its
lower hindrance and absence of plating/stripping, though its masking effect reduces the active area. Concerning the mesh geometry, finer and more open meshes reduce both polarization and delay. As for the model-based correction, the simulation framework presented here can be adapted to deconvolve artifacts from RE measurements, enabling more accurate diagnostics in real-world applications.

Beyond these technical recommendations, this study reveals a critical yet overlooked aspect of REs: their intrinsic electrochemical activity. Although this phenomenon has been explicitly demonstrated in lithium foil REs through plating/stripping dynamics, our findings show that LFP-based REs exhibit insertion/disinsertion mechanisms too, which will be addressed in a forthcoming publication. To the best of our knowledge, this phenomenon has not previously been investigated. However, unveiling these mechanisms is critical in order to gain a comprehensive understanding of the RE operation and measurements. This could enable safer fast-charging protocols, high-precision state-of-health estimation, and mechanism-specific degradation analysis, thereby accelerating the development of smarter, more reliable battery systems.

## Acknowledgments

The authors gratefully acknowledge Léa Nassoura for her preliminary experimental work, which helped guide the subsequent investigations; Sylvie Geniès and Chau Cam Hoang Tran for providing LFP RE materials and post-mortem capabilities; Lisa Vericel for the coating of the graphite NE; Daniel Carlos Saez Martinez for performing the OCV measurements on the graphite electrode.

## Author Contributions

**Sylvain REY:** Methodology (equal); Investigation (lead); Data curation (lead); Formal analysis (lead); Writing - original draft (lead). **Justin Bouvet:** Conceptualization (equal); Methodology (equal); Investigation (equal); Data curation (equal); Formal analysis (equal); Writing - review & editing (equal); Supervision (equal). **Eric Woillez:** Conceptualization (equal); Methodology (equal); Writing - review & editing (equal); Supervision (equal). **Eric Mayousse:** Writing - review & editing (equal); Conceptualization (equal). **Sébastien Martinet:** Writing - review & editing (equal). **Willy Porcher:** Conceptualization (equal); Methodology (equal); Writing - review & editing (equal); Supervision (lead).

## Data Availability

The data that support the findings of this study are included in the article and its supplementary information. These data are also available from the corresponding authors upon request.

## ORCID

Sylvain Rey https://orcid.org/0009-0005-1382-8880

Justin Bouvet https://orcid.org/0000-0002-3668-776X

Eric Woillez https://orcid.org/0000-0001-9250-6196

Eric Mayousse https://orcid.org/0000-0002-2734-5231

Sébastien Martinet https://orcid.org/0000-0001-6127-3676

Willy Porcher https://orcid.org/0000-0002-4121-8503

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