A Gold Micro-Reference Electrode for Impedance and Potential Measurements in Lithium Ion Batteries

Lithium iron phosphate (LiFePO₄, LFP) electrodes were prepared by mixing LFP (BASF SE, Germany), carbon black (Super C65, Timcal), and polyvinylene diflouride (PVDF, Kynar) in a mass ratio of 93:3:4 with NMP (N-methyl pyrrolidone, anhydrous, Sigma-Aldrich, Germany) in a planetary mixer (Thinky Corp.) for 15 min. The resulting ink was coated on carbon-coated aluminum foil (MTI) with a doctor blade mounted on an automatic coater and dried at 50°C in a convection oven for at least 3 h. The final LFP coating had a loading of 11.7 mg_LFP/cm² (≡ 2.0 mAh/cm² based on 170 mAh/g_LFP). Electrodes with a diameter of 11 mm were punched out and pressed to 35% porosity (2 × 60 s at 260 MPa) with a KBr press (Mauthe, PE-011). Graphite electrodes were prepared by mixing graphite (T311, SGL Carbon GmbH) and PVDF in a mass ratio of 95:5 with NMP, following the same procedure. The graphite ink was doctor-blade coated on copper foil (MTI) and dried in a convection oven at 50°C for at least 3 h. The final loading of the graphite coating was 5.9 mg_graphite/cm² (≡ 2.2 mAh/cm² based on 372 mAh/g_graphite) at a porosity of 40%. Both types of electrodes were dried under dynamic vacuum at 120°C overnight and transferred to an Argon-filled glove box (MBraun, Germany) without exposure to air.

The reference electrode current collector of a 3-electrode Swagelok T-cell (see Figure 1a) was modified to be able to host the GWRE. To this purpose, a small hole (1 mm diameter, 2.5 mm depth) was drilled into the flat front side of the reference current collector. To fix the GWRE wire, a thread was cut into the side of the reference current collector at approximately 2 mm distance from the front edge. For the actual reference electrode, a gold wire with a core diameter of 50 μm, coated with a 7 μm thick polyimide insulation (Goodfellow Cambridge Ltd., United Kingdom), was cut into pieces of ∼1.5 cm. The last 3 mm of one end of the wire was slightly scratched with a scalpel to allow good electrical contact of the wire to the reference electrode current collector. The scratched end of the wire was then inserted into the hole of the reference current collector and fixed with a small set screw. During cell assembly, the GWRE was inserted through a hole in the polymer lining of the T-cell (green lines in Fig. 1) and cushioned between two glassfiber separators (see Figure 1b); note that the insulation at the wire perimeter was not removed and that the only segment of the wire accessible to the electrolyte is the cut cross-section at the tip of the wire (see Figure 1c). The SEM image of the wire tip in Figure 1c shows that the polyimide insulation is almost completely intact around the edge of the cut cross-section, and that the exposed gold surface is relatively smooth. As the sealing and all other cell components are left unchanged compared to the conventional T-cell design, we could omit any benchmarking and air permeation tests that are normally required when developing a new electrochemical cell for the lithium ion chemistry. T-cells with GWRE were assembled with graphite as anode, LFP as cathode, and 2 glassfiber sheets (Whatman) as separator soaked with 60 μL electrolyte.

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As standard electrolyte, 1 M lithium hexafluorophosphate (LiPF₆) in a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) at a weight ratio of 3:7 was used (LP57, BASF SE, Germany). The water content of this electrolyte was determined via Karl-Fischer-Titration to be <10 ppm. Vinylene carbonate (VC, BASF SE, Germany) was added in weight ratios of 0.17% and 0.52% to the standard electrolyte. These concentrations were chosen as they yielded g_VC/Ah_Cell ratios equal to 2% and 6% VC additive (same solvent/salt) in 225 mAh full-cells used in a study on the anode and cathode impedance growth in the presence and absence of VC by Burns et al.³² For stability measurements of the gold wire electrode, symmetrical lithium/lithium cells with a GWRE were built using 11 mm lithium discs (450 μm thickness, Rockwood, USA) as both cathode and anode.

The gold wire reference was lithiated by applying a current of 150 nA between the working electrode (LFP or lithium) and the gold wire reference electrode using a potentiostat (VMP300, BioLogic, France). Please note that the selected current range of 10 μA has an accuracy of 0.1%, which leads to an error of ∼10 nA. LFP/graphite cells were cycled between cell voltages of 2 and 4 V using a BioLogic potentiostat and a CCCV charge/CC discharge procedure with a C/20 current cutoff to end the CV phase. During cycling, the cells were placed inside a climatic chamber with a constant temperature of 25°C or 40°C. Electrochemical impedance spectroscopy (EIS) measurements were conducted either potential controlled with a perturbation of 5 mV at OCV (referred to as PEIS, with the AC voltage perturbation applied between working and reference electrode) or current controlled with a perturbation of 0.5 mA (referred to as GEIS), both in a frequency range of 100 kHz–0.1 Hz. The impedance measurements were conducted at 50% SOC and 25°C or 10°C. Prior to the measurement, the cells were allowed to rest at OCV and thermally equilibrate for 15 min.

As a first step, the potential stability of a lithiated GWRE was investigated in symmetric lithium-lithium T-cells with our modified design (see Fig. 1b). The GWRE was lithiated by applying a current of 150 nA for 1 h between one of the lithium electrodes and the GWRE. The black curve in Figure 2a shows the potential of the GWRE vs. Li/Li⁺ during galvanostatic lithiation at 25°C. The potential drops briefly below 0 V vs. Li/Li⁺ and then stays constant at ∼0.2 V vs. Li/Li⁺ during the entire lithiation procedure, which is similar to the first potential plateau observed during the electrochemical lithiation of gold thin films.²⁵ The overpotential at the first moments of lithiation have been attributed to the reduction of surface oxides¹⁹ or the nucleation of the lithium-gold alloy phase.²⁵ During the subsequent OCV at 25°C (see black curve in Figure 2b), the potential of the GWRE shoots up to 0.318 V and then quickly relaxes to ∼0.311 V vs. Li/Li⁺, which corresponds to the OCV potential of a Li_xAu alloy with 0 < x < ∼1.2.²⁵ The lithiated GWRE potential remains stable for more than 500 h, varying by less than 2 mV after the initial 20 h of the OCV period. This means that the lithiated GWRE might not be suitable for highly accurate potential measurements during initial cycles, but is sufficient for tracking electrode potentials during prolonged cycling. Further, no morphological changes of the wire could be observed visually after disassembly of the cells.

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As many battery cycling tests are performed at higher temperatures to accelerate aging and to reflect more realistic operating conditions, it is desirable that the GWRE also functions at higher temperatures. However, if the GWRE is lithiated at 25°C and the cell temperature is then increased to 40°C for OCV measurements, the gold wire potential starts to drift to more positive values after less than 10 hours (see orange curve in Figures 2a and 2b). This is in accordance with Abraham et al.,⁶ who reported that the potential of a lithiated tin wire is substantially less stable at elevated temperatures, where the rate of SEI growth and the concomitant self-delithiation is generally enhanced. Once the cell is heated to 40°C, this effect must lead to a rather rapid depletion of lithium at the wire's tip, resulting in the observed potential drift. Interestingly, a re-lithiation of the wire with the same procedure at 25°C restored a stable GWRE potential of 0.311 V vs. Li/Li⁺, as long as the cell was kept at 25°C. We also observed that the GWRE potential stability over long time was limited in combination with high voltage cathodes (>4.7 V vs. Li/Li⁺), and also here the GWRE could be relithiated.³³

After further investigations, we found that if the gold wire lithiation is conducted at 40°C (see green curve in Figures 2a and 2b), the GWRE shows the same stability during OCV at 40°C as was observed at 25°C, only shifted downwards by 1–2 mV. It is reported that an SEI formed at higher temperatures contains more inorganic species,³⁴ which we hypothesize might form a more effective surface film on the lithium-gold alloy. While high temperature SEI formation was shown to lead to inferior capacity retention on graphite anodes during cycling,³⁴ the more inorganic SEI could be advantageous in the absence of cycling-induced volume changes, i.e., for reference electrodes. However, the exact mechanism behind this enhanced stability by lithiation at higher temperatures is not clear at this point. We further believe that the stable potential of the GWRE for over hundreds of hours at up to 40°C is partly due to the fact that, contrary to previous micro-electrode designs,^6,7 the reference electrode area exposed to the electrolyte is limited to the cross-sectional area of the tip (see Figure 1c), minimizing side reactions with the electrolyte. The stable potential over 500 h indicates that the lithium diffusion along the wire (i.e., away from the tip) must be sufficiently slow to prevent a significant depletion of lithium at the tip.

To evaluate if the GWRE in the modified T-cell design is suitable for impedance measurements of individual electrodes, we also measured the impedance of a symmetrical lithium/lithium cell with a GWRE (see Figure 2c). Arbitrarily, one of the lithium electrodes was designated as working electrode (WE), while the other was designated as counter electrode (CE). Prior to the impedance measurement in the lithium/lithium cell, the GWRE was lithiated at 25°C as described above from the lithium electrode designated as WE. The high frequency resistance (see inset) is identical for both lithium electrodes, which indicates that the GWRE is located centrally between the electrodes. Hence, a first precondition for an artefact-free measurement is fulfilled.¹⁴ Both lithium electrodes show a large semicircle in the high-frequency region (100 kHz–20 Hz, with the apex at ≈1.3 kHz), followed by a smaller semicircle at frequencies between 20 and 0.1 Hz (with the apex at ≈1 Hz), as reported previously for lithium metal electrodes.^35,36 While the high-frequency semicircle has been ascribed to the SEI resistance, the semicircle in the low-frequency region is thought to represent the charge transfer resistance.³⁶ Interestingly, both semicircles of the electrode used for the lithiation of the GWRE (designated as WE, see red line in Figure 2c) are about 35% smaller compared to the other electrode (≡ CE, s. blue line). We believe that this originates from the stripping of lithium from the WE electrode during lithiation of the GWRE, as this would cause a roughening of the lithium surface, leading to higher surface area and thus smaller impedance.

As a next step, the use of the GWRE in a LFP/graphite full-cell is tested and evaluated. Here, we also want to assess whether lithiation of the reference electrode is necessary for impedance measurements, i.e., whether the non-lithiated Au wire can be used as pseudo-GWRE. To this purpose, we built identical LFP/graphite cells with GWRE: in one case, we lithiated the GWRE with 150 nA for 1 h at 25°C from the LFP electrode (note that the 150 nAh needed for lithiation of the GWRE are negligible compared to the LFP cathode capacity of 1.95 mAh); in the other case, we did not lithiate the GWRE. Subsequently, both cells underwent one formation cycle (at a rate of C/10) at 25°C and then were charged to 50% SOC. Figure 3a shows the potential of the LFP cathodes vs. the non-lithiated pseudo-GWRE and vs. the lithiated GWRE during 30 seconds of OCV prior to the impedance measurement. As the potential of the LFP electrode does not change significantly during the measurement, all potential changes can be ascribed to changes in the GWRE potential. While the LFP potential vs. the non-lithiated GWRE drifts about 20 mV during 30 seconds (black curve in Figure 3a), the LFP potential vs. lithiated GWRE remains stable within 0.3 mV (green curve in Figure 3a). In the subsequent potential-controlled impedance measurement (PEIS; 5 mV amplitude, 100 kHz–0.1 Hz) at OCV, the cell with the non-lithiated GWRE (see Figure 3b) shows significant distortions at frequencies near/below 1 Hz: i) the graphite impedance (blue line) displays an inductive loop; ii) the LFP impedance (red line) bends toward lower Re(Z) values; and, iii) even the full-cell impedance (black line) shows an irregular sharp peak. These distortions appear at frequencies near or below 1 Hz, where the average potential drift of 0.67 mV/s of the non-lithiated pseudo-GWRE (see black line in Figure 3a) is no longer significantly lower than the change of the AC voltage amplitude of 5 mV. In contrast, the impedance spectra of the cell with the lithiated GWRE (Figure 3c) do not show these distortions, as the reference potential drift is almost two orders of magnitude lower in this case (0.01 mV/s). Our measurements are in agreement with simulations by Victoria et al.,¹⁷ who showed that linear potential drifts on the order of 0.1 mV/s during impedance measurements can lead to these types of artefacts below 1 to 0.1 Hz, depending on the excitation amplitude. The potentiostatic impedance measurement mode used here (see Figure 4a), where the potential between WE and RE is controlled, leads to a particularly detrimental effect: As the base potential between RE and WE is fixed, the WE potential has to drift in the same way as the RE, which leads to a bias current between WE and CE. This continuously increasing current renders the full system nonlinear and time-variant, leading to the full cell impedance artefacts observed at low frequencies. While normally the full cell impedance should be unaffected by artefacts related to the reference electrode,¹⁴ this comparison shows that it is crucial to use a reference electrode with a stable and defined potential for WE - RE potential controlled impedance measurements at low frequencies. To avoid the effects of a drifting pseudo-reference electrodes on the full cell impedance, one could either use a current-controlled measurement mode (GEIS, see Figure 4b), or control the potential between WE and CE during the impedance measurement (Figure 4c). Yet, artefacts of a non-stable RE will still be visible in the half cell impedance in these measurement setups.

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Next, we take a closer look at the impedance spectra of the LFP and graphite electrodes recorded with a lithiated GWRE (Figure 3c). In contrast to the previous setup with two lithium electrodes, the HFR of both electrodes is not identical here. Gaberscek et al.³⁷ showed that the contact resistance between an aluminum current collector and an LFP electrode composite can be on the order of several Ωcm². Our own measurements confirm that the through-plane resistance of the used LFP electrodes is about 1 Ωcm² higher compared to the graphite electrodes (data not shown). Thus, the ≈1 Ω difference in HFR originates from the higher contact resistance between the LFP coating and the current collector (1 Ωcm² corresponds to ≈1 Ω for our electrode area of 0.95 cm²). The charge transfer semicircle of the LFP electrode is small and almost invisible, which suggests the lack of a resistive cathode film.^38,39 At the same time, the graphite anode shows a clearly distinguishable semicircle. As this semicircle is not visible in graphite electrodes prior to cycling, we attribute it to a combined SEI/charge transfer resistance on the graphite surface.

To further validate the impedance data measured in a full-cell with a lithiated GWRE, we compare its impedance response with that of symmetric cells, which are commonly used for accurate impedance measurements.⁴ Figures 5a and 5b show the comparison of the impedance spectra of a graphite and a LFP electrode measured in a full-cell with lithiated GWRE and in reassembled symmetric LFP/LFP and graphite/graphite cells, all after one C/10 formation cycle at 25°C and subsequent charge to 50% SOC. Note that the impedances of the symmetric cells have been divided by 2 in order to account for the two nominally identical electrodes in the symmetric cells. Apart from a slight shift in HFR, the impedance response of the symmetric cells and the full-cell with the lithiated GWRE are essentially identical for both graphite (Figure 5a) and LFP (Figure 5b) electrodes. The HFR shift is probably introduced by a weaker compression of the glassfiber seperators in the symmetric cells, caused by the slightly different assembly procedure for cells with and without GWRE. The additional high frequency contact resistance feature visible in the impedance spectra of symmetric cells by Dahn's group,⁴ which results from the contact resistance between the cell housing and the electrode coating on the back side, does not appear in our symmetric cell impedance spectra (see Figures 5a and 5b), as we use single-side coated electrodes for both symmetric cells and full cells.

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As a final consistency check, we performed a potential-controlled impedance measurement (PEIS) followed by a current-controlled impedance measurement (GEIS) on the same LFP/graphite full-cell with a lithiated GWRE (see Figure 5c). Mathematically speaking, both measurements should give identical results in a Nyquist plot; hence any differences between them would indicate a biased impedance response.²¹ However, Figure 5c shows that the two methods deliver completely identical impedance spectra. These results confirm that the presented cell setup with the lithiated GWRE is free of measurement artefacts and is suitable for the impedance investigation of individual electrodes in full-cells. In summary, our modified T-cell design with a lithiated GWRE is able to provide accurate impedance measurements of individual electrodes in full-cells in a wide frequency range (100 kHz–0.1 Hz). A stable potential of the GWRE is especially crucial for measurements at low frequencies. If lithiated at elevated temperature, the potential of the GWRE is stable for several weeks at up to 40°C, which we partially attribute to the small area exposed to the electrolyte.

In the following, we want to demonstrate the suitability of the lithiated GWRE to investigate the evolution of anode and cathode impedances during extended charge/discharge cycle tests in full-cells. To this purpose, LFP/graphite full-cells with lithiated GWRE were cycled at 25°C for 200 cycles at a rate of 1C after two initial formation cycles at C/10. Impedance measurements were performed at 50% SOC after 5, 10, and each subsequent 10^th cycle at 25°C. Figures 6a and 6b show the potential of the cathode and anode vs. the lithiated GWRE potential (left y-axis) during cycles 10, 50, 100 and 200 (for the sake of clarity, cycles in between were omitted), which can easily be converted into the Li/Li⁺ scale by adding 0.311 V (right y-axis). The LFP charge and discharge plateaus are centered around 3.11 V vs. the lithiated GWRE (see Figure 6a and also Figure 3a), corresponding to a calculated value of 3.42 V vs. Li/Li⁺, which matches well with the true LFP equilibrium potential.⁴⁰ The LFP potential center vs. lithiated GWRE remains constant during cycling, meaning that the lithiated GWRE maintains its stable potential of 0.311 V vs. Li/Li⁺. Throughout cycling, the overpotentials of both electrodes do not change, yet the maximum potential of the graphite anode at the discharge end point moves upwards (see dark blue to light blue lines in Figure 6b). At the same time, the minimum potential of the cathode also moves upwards (see dark red to light red lines in Figure 6a), which indicates that the SOC of both electrodes slip against each other. Figure 6c shows the impedance spectra of both cathode and anode after 10, 50, 100 and 200 cycles. Note that both the cathode and anode impedance decrease slightly from cycle 5 (data not shown) to cycle 10, which could be related to the dissolution of gasses evolved during formation and/or improved wetting over the first cycles. Between cycle 10 and 200, the high frequency resistance of both electrodes increases slightly by about 0.1–0.2 Ω. This could be due to an increased electrical resistance between the electrode coatings and the current collectors, implying a very slow delamination of the composite electrodes, or a higher ionic resistance within the bulk electrolyte. While the cathode impedance shows no further changes during cycling, the anode semicircle increases slightly from ∼1.9 Ω after cycle 10 to ∼2.2 Ω after cycle 200, which indicates a very slow SEI growth. Overall, the potential changes of both electrodes during cycling and the small but measureable impedance growth of the anode can be related to the loss of active lithium due to a slow but steady SEI growth, which has been identified as the dominant aging mechanism in LFP/graphite cells.^41–45

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Vinylene carbonate (VC) is one of the most commonly used electrolyte additives, as it leads to improved SEI stability at elevated temperatures and thus enhanced cycle life of lithium ion cells.^46,47 However, high concentrations of VC have shown to increase the impedance of both anode and cathode,³² which in turn leads to higher overpotentials and heat generation during cycling. Freiberg et al.⁴⁸ recently indicated that the absolute amount of an additive per active material, instead of its concentration, is the crucial parameter when comparing larger cells (e.g. commercial cells) and small lab-scale cells (e.g. coin cells). Therefore, we want to compare the effect of different amounts of VC in LP57 electrolyte on both anode and cathode impedance in LFP/graphite full-cells obtained with a lithiated GWRE to the study by Burns et al.,³² who used 225 mAh LCO/graphite pouch cells with the same electrolyte and examined the effect of VC on the impedance of the individual electrodes via symmetric cell measurements. In Burns' study, it was shown that the charge transfer resistance of a graphite anode decreases slightly from 0% to 0.5% VC in the electrolyte and increases roughly linearly with VC concentration between 1% and 6% VC (see Figure 9b in Ref. 32). At the same time, the impedance of the LCO cathode from Burns' study (see Figure 9a in Ref. 32) decreases about 50% from 0% to 2% VC and then gradually increases again up to VC concentrations of 6% to a value which is still below the 0% VC case. Unfortunately, the exact amount of active material in the cells used by Burns et al. was not given. However, the specific capacities of LCO and LFP are similar, and our anode to cathode capacity ratio of 1.1 is close to a commercial balancing. Hence, we think it is reasonable to assume that the masses of both anode and cathode active materials are proportional to the respective cell capacity. As the ratio of electrolyte to cell capacity (and thus active material) in our lab-scale T-cell design is 11.6 times higher compared to Burns et al.³² (38 g_electrolyte/Ah_Cell vs. 3.3 g_electrolyte/Ah_Cell), we adjusted the amount of VC in the electrolyte accordingly. Thus, our chosen concentrations of 0.17% and 0.52% VC represent the same g_VC/Ah_Cell ratio, namely 0.06 g_VC/Ah_Cell and 0.2 g_VC/Ah_Cell, as cells with 2% and 6% VC in the study by Burns et al.³² After lithiation of the GWRE and one formation cycle at 40°C, the LFP/graphite cells were charged to 50% SOC and the impedance measurements were then conducted at 10°C, i.e., under the same conditions as reported by Burns et al.³²

Figure 7 shows the Nyquist plot of graphite electrodes after formation with different concentrations of VC. For each concentration, two cells are shown to assess the cell to cell variation. Quite clearly, the cells with 0.17% and 0.52% VC show an increased charge transfer resistance of the graphite anodes. These results already indicate that electrolytes cannot be compared without considering the amount of active material, as the anode charge transfer resistance decreases up to a VC concentration of 0.5% in the study by Burns et al.,³² while Figure 7 shows that the anode charge transfer resistance increases substantially within the same VC concentration range.

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To quantify the charge transfer resistances, the impedance spectra of cathode and anode of each cell were fitted using a simple electrochemical equivalent circuit composed of: i) a resistor for the electrolyte, ii) a resistor and a constant phase element in parallel to describe the electrolyte/electrode interface resistance, and, iii) a Warburg-type diffusion element in series representing solid state diffusion. This circuit is a simplified version of a model used by Illig et al.³⁵ for LFP electrodes; we omitted the electrode contact resistance and the low frequency capacitor, as both are not visible within our measurement range. Figure 8 shows the average fitted charge transfer resistances (left y-axis), normalized to the geometrical electrode area, of both electrodes at different g_VC/Ah_Cell ratios (lower x-axis). The anode charge transfer resistance is ∼5 Ωcm² for cells without VC and increases to ∼16 Ωcm² and ∼47 Ωcm² for cells with 0.06 g_VC/Ah_Cell (≡0.17% VC) and 0.2 g_VC/Ah_Cell (≡0.52% VC), respectively. In comparison, Burns et al.³² showed an anode charge transfer resistance of ∼30 Ωcm², ∼60 Ωcm² and ∼150 Ωcm² for cells with identical g_VC/Ah_Cell ratios (0%, 2% and 6% VC in their study). The linear increase in charge transfer resistance from 0.033 to 0.2 g_VC/Ah_Cell that has been observed by Burns et al.³² (corresponding to 1%-6% VC in their study) is also found in our results within the same g_VC/Ah_Cell range, although our absolute VC concentrations are completely different (0–0.52% VC). This further proves that the amount of additive per active material (here corresponding to the g_VC/Ah_Cell ratio) determines the effect of an additive on the surface of an electrode, and not its concentration. The differences in absolute resistance values between Burns' study³² and ours could be explained by differences in active material loading and BET surface area of the used electrodes: As the impedance of an electrode is inversely proportional to the electrochemical active area, a higher roughness factor (i.e., electrode surface area per geometric area) will result in an overall lower impedance, even if the surface chemistry is identical. While our electrodes are loaded with 5.9 mg/cm² graphite having a BET surface area of ∼5 m²/g, we can estimate the anodes investigated by Burns et al.³² to have a loading of ∼10 mg/cm² graphite⁴⁹ with a BET surface area of ∼0.7 m²/g.^50,51 In total, this would give a ∼5-fold higher roughness factor in our study, which would fit with the measured ∼4 times lower absolute charge transfer resistance values. However, it is to note that the assumed values for loading and BET were taken from other publications by the Dahn group and not directly from Burns et al.,³² and hence this is only an estimate. A different BET surface area would also affect the amount of additive per unit surface, and thus result in a different charge transfer resistance. To make our data more comparable to future studies, we therefore included the amount of VC per graphite BET surface area (mg_VC/m²_Graphite, upper x-axis) and the charge transfer resistance normalized to the graphite BET surface area (Ω m²_Graphite, right y-axis) in Figure 8. An additional difference between our cells and the study by Burns et al.³² is the different cycling protocol: Our impedance data was recorded after one formation cycle, whereas the cells by Burns et al.³² were disassembled for impedance measurements of symmetric cells after 23 cycles. However, further cycling and impedance measurements of our LFP/graphite cells showed that the impedances of both electrodes does not change significantly with cycle number once the formation cycle is completed.

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The charge transfer resistance of the LFP cathode in our study does not show any dependency on the VC content (see Figure 8). In contrast, Burns et al.³² found that the impedance of an LCO cathode decreases about half by the addition of low concentrations of VC (0.5–2%) and increases again slightly at higher VC concentrations (4–6%). This discrepancy can be understood considering the studies by El Ouatani et al.,^52,53 which showed that LCO cathodes form a surface film of poly(VC) in VC-containing electrolytes, while this film is lacking on LFP cathodes. Thus, the cathode charge transfer resistance remains constant and independent from the VC content in LFP/graphite cells. As VC reacts on the LCO surface,^52,53 one can imagine that slightly less VC is available for SEI formation in LCO/graphite than in LFP/graphite cells. This could in turn also partially explain the deviations of the absolute values for the anode charge transfer resistance at points of equal g_VC/Ah_Cell in our study compared to Burns et al.³² Hence, we can conclude that not only the ratio of additive to active material, but also the cell chemistry of cathode and anode and their reactivity toward the additive is an important aspect to consider when comparing additives across different cell types.