Reduced graphene oxide as an additive in advanced lead-acid batteries

Reduced graphene oxide synthesis

The rGO was synthesized using the improved Hummers method as developed by Abdolhosseinzadeh et al. ²⁵ The process involves three main steps: oxidation (graphite powder is oxidized to graphite oxide), oxidation/exfoliation (graphite oxide is exfoliated to yield GO), and reduction (GO is reduced to rGO). The reagents used for synthesis included 99% graphite powder (Sigma-Aldrich, USA), concentrated sulfuric acid (98%), potassium permanganate PA, sodium hydroxide in pellets, ascorbic acid PA, and hydrochloric acid (36%). The water used in this process was demineralized, with conductivity of 5–8 µS cm⁻¹. The initial stage is carried out at a controlled temperature of a maximum of 10 °C, by maintaining the system in an ice bath. A total of 3.0 g of graphite powder was added to a beaker containing concentrated sulfuric acid (150 mL) under stirring. Subsequently, 9.0 g of potassium permanganate was slowly incorporated. After the addition of potassium permanganate, a sequence of 25-min stirring was initiated, followed by 5 min of sonication. This stirring/sonication sequence was repeated 12 times. Prior to the reduction stage, demineralized water (600 mL) was added, followed by the gradual addition of NaOH pellets until the pH was between 6 and 7. After neutralization, the mixture was subjected to sonication for 2 h. In the final stage of the synthesis, chemical reduction was carried out by adding an ascorbic acid solution (60 g/600 mL), and the reaction was sustained with stirring for 1 h at 95 °C. The resulting rGO underwent vacuum filtration process, washing with 1 mol/L HCl solution and demineralized water until the pH stabilized between 5 and 6. The drying process occurred at 60 °C for 48 h. Starting from 3.0 g of graphite, 2.81 g of rGO were obtained, corresponding to a gravimetric yield of 93.7%. Since the reduction of graphene oxide by L-ascorbic acid is not complete and the exact chemical composition of the obtained rGO cannot be accurately determined, the synthesis yield was calculated based on the recovered mass of the final material.

Reduced graphene oxide microstructural characterization

To confirm the phase composition, X-ray diffraction (XRD) was performed using a Bruker D8 Advance ECO device equipped with a copper target (CuKα, λ = 0.1514 nm) at 40 kV and 25 mA. Measurements were taken over an angle range of 2θ = 5–60 °C and scan speed of 2°/minute. Fourier transform infrared spectroscopy (FTIR) was used to analyze the oxygen functional groups in the sample, utilizing a Bruker Vertex 70 setup. Samples were analyzed in transmittance mode using KBr tablets with a wavelength range of 3500–500 cm⁻¹ and a resolution of 4 cm⁻¹.

Raman spectroscopy was conducted to evaluate structural defects in the synthesized rGO structure using a Micro RAMAN spectrometer (LabRAM HR, Horiba, USA) with a Nd:YAG laser (λ = 632.8 nm, 19 mW power). Additional information on electronic levels and their relationship with the efficiency of the reduction process efficiency was obtained through photoluminescence spectroscopy using a spectrofluorometer (Fluorolog-3 FL3-122, Horiba, USA). The rGO sample was prepared by dispersing 0.02 mg cm⁻³ in demineralized water and sonicated for 5 min. The spectrum was obtained using an excitation source (laser) at 325 nm and an emission wavelength of 370–850 nm.

Morphological structure analysis was performed through scanning electron microscopy (SEM) with FEI equipment featuring a FEG filament (Magellan 400 L, FEI, USA) at an accelerating voltage of 25.0 kV. Surface areas were measured by obtaining adsorption isotherms using nitrogen gas at −195.5 °C (77 K) with an automatic physisorption equipment (ASAP 2020 Plus, Micromeritics, USA) within a relative pressure range (P/P₀) of 0–0.995. Prior to measurements, all samples were pretreated at 70 °C for 12 h to eliminate volatile compounds and then subjected to a vacuum of 10 µm Torr. The specific surface area (S_BET) was determined by the Brunauer-Emmett-Teller (BET) method using adsorption data within the P/P⁰ range of 0.05–0.30. The mean pore diameter (d_P) was calculated using the Barrett-Joyner-Halenda method (d_P = 4V_P/S_BET, where V_P is the total pore volume). Total pore volume was determined by converting the amount of adsorbed N₂ gas at a P/P₀ of 0.995 to the volume of the liquid adsorbate.

Electrochemical tests

The performance of rGO was evaluated at three mass loadings: 0.2%, 0.75%, and 1.4%. These experimental conditions were systematically compared with a control plate formulated with carbon black PBX-135, which has a BET area of 120–180 m² g⁻¹ (Cabot Corporation, USA). PBX-135 was incorporated at 0.75%, as recommended by the manufacturer. This selection is justified by the material's designation as an advanced carbon specifically formulated for enhanced performance in advanced lead-acid batteries (Enhanced Flooded Battery and AGM), providing a high-performance benchmark for assessing the rGO variants. The influence of these materials on the performance of negative plates (5.7 × 6.1 cm) was investigated using a 2 V PNP electrochemical cell (one negative plate in the center and two oversized positive plates at the ends). The composition of the negative pastes consisted of lead oxide (75% PbO and 25% Pb), water, lignin (Vanisperse A, Borregaard LignoTech, Norway), barium sulfate (Solvay, Belgium), and one of the three carbonaceous additives. Prior to incorporation into the paste formulation, the carbon materials were subjected to a preliminary ultrasonic dispersion step in a suspension containing the corresponding amount of lignin and water. This procedure ensured adequate wetting and homogenization of the carbonaceous additives, as lignin acts as an effective dispersing agent. The amount of water added was sufficient to achieve the consistency required for adequately applying the mass to the grid. Each plate contained 15.5 g of negative paste. The plates were cured in an industrial oven with automatic temperature and humidity control under the following conditions: 2 h at 85 °C with 95% relative humidity, 30 h at 60 °C with 70% relative humidity, followed by 10 h at 75 °C with 0% relative humidity. After the curing process, the electrochemical cells were assembled, and the formation process was continued. The electrolyte volume was 315 mL. A soaking period of 2 h was performed at an H₂SO₄ concentration of 1.0 mol L⁻¹. After soaking, the electrolyte was replaced with a 4.6 mol L⁻¹ H₂SO₄ solution, and the formation process began according to the formation profile Ic = 1.58 mA cm⁻² for 36 h, followed by Ic = 3.16 mA cm⁻² for 10 h.

The HRPSoC cycle life test began by bringing the negative plate to 50% state of charge through a discharge current of 1600 mA for 30 min. Immediately after this period, a micro-charge of 1600 mA was applied for 2 min, followed by a rest of 10 s, a micro-discharge of 1600 mA for 2 min, and another rest of 10 s. This sequence of micro-charges and discharges was repeated until the cell voltage reached 1.83 V. This test protocol was adapted based on Pavlov et al. ²⁹ Figure 1 presents a diagram of this HRPSoC cycle.

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Figure 1.

Profile of a micro-cycle of the HRPSoC cycling test.

The specific capacitance of carbonaceous materials was calculated using cyclic voltammetry, electrochemical impedance spectroscopy, and electrochemical capacitance spectroscopy, as outlined in the literature.^26–28 In addition to rGO and PBX-135, the electrochemical properties of the graphite used as the precursor in rGO synthesis were also studied using a potentiostat (AUT8684, Metrohm, Switzerland). The working electrode was made of glassy carbon with a geometric area of 0.3 cm², onto which 12 µL of a suspension containing 0.5% (w/w) of the carbonaceous materials under study was applied. This suspension comprised isopropyl alcohol (19.3%), Nafion (1.0%), and demineralized water (79.2%). The container containing the suspension was subjected to ultrasound for 30 min to ensure adequate dispersion of the materials. The counter electrode used was platinum, and the reference electrode was Hg/Hg₂SO₄, H₂SO₄ 4.6 mol L⁻¹. The specific capacitance (C_S) was calculated using Equation (1):

C s = Q a + | Q c | 2 Δ V m v

(1)

The results from this method show that the charge associated with the electric double layer is obtained by extrapolating the voltammetric charge to zero from the graph of q⁻¹ vs. v^1/2, while the total charge is obtained by extrapolating the voltammetric charge to infinity from the graph of q vs. v^−1/2. The pseudocapacitive charge is represented by the difference between total charge and charge associated with the electric double layer. The respective specific capacitances (C_P, C_DL, and C_T) are obtained by dividing the corresponding charge values ​​by the ΔV used in cyclic voltammetry. The electrochemical impedance spectroscopy measurements used sinusoidal disturbance signal parameters of 10 mV amplitude, applied over the frequency range of 0.1–100 mHz with 50 steps/dec, at an open-circuit potential of 160 mV. The impedance (Z) values were converted into capacitance (C) through C′′ = φZ′ and C′ = φZ′′. The redox capacitance was represented through the Nyquist capacitive diagram.

Reduced graphene oxide microstructural properties

The synthesized rGO was characterized for its macrostructural properties using XRD, FTIR, Raman, and photoluminescence measurements (Figure 2), which confirmed that the exfoliation and reduction processes were effective in obtaining rGO. The XRD analysis showed that the graphite used in the synthesis exhibited a crystalline peak at 2θ = 26.3°, corresponding to the basal plane 002 of its hexagonal structure. For rGO, the graphite crystalline disappeared and a halo formed at 2θ = 24.7°, with low intensity and a wide base. The disappearance of the characteristic graphite peak, as well as the absence of other well-defined peaks, indicated that the reduction of GO to rGO was efficient. Additionally, the shift in the 2θ value in the rGO diffractogram relative to graphite suggests increased interplanar distance. The calculated interplanar distances for graphite and rGO were 3.3 Å and 3.6 Å, respectively. This increase is attributed to the presence of residual functional groups resulting from the oxidation of graphite to GO. FTIR was used to characterize these functional groups.

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Figure 2.

Microstructural characterizations: (a) X-ray diffraction diffractograms of reduced graphene oxide (rGO) and graphite, (b) Fourier-transform infrared (FTIR) spectrum of rGO and graphite; (c) Raman spectrum of rGO andgraphite; (d) photoluminescence spectrum of rGO.

The rGO spectrum showed low-intensity peaks; notable peaks include the sp² structure of C = C in aromatics at 1550 cm⁻¹, the C-O stretching likely from ester groups at 1194 cm⁻¹, and hydroxyls (O-H) from carboxylic acids at 1007 cm⁻¹. The low intensity of the peaks supports the XRD discussion that the GO to rGO reduction process was effective. Some studies have investigated the effects of these functional groups on the capacitive properties of carbonaceous materials and suggested the existence of significant effects.^30–32 Evidence suggests that the introduction of pseudocapacitance from these oxygenated functional groups occurs through a mechanism of reversible Faradaic redox reactions and, in the case of acidic media, alkaline functional groups, especially carbonyls and quinones, react with protons (H+).^33,34

The Raman spectra showed D (1322 cm⁻¹), G (1579 cm⁻¹), and 2D (2648 cm⁻¹) bands. The D band, related to the A_1g vibrational mode, is associated with disorder and defects in the material structure; a strong signal indicates a high degree of defects and disorder. The G band, related to the E_2g vibrational mode, is associated with the quality of the graphitic network; more intense signals suggest a higher degree of order. Based on the Raman spectra, the I_D/I_G intensity ratio, widely used to evaluate structural defects,^35–37 was significantly higher in rGO (1.48) than in graphite (0.27), indicating an increased degree of structural disorder. This suggests successful exfoliation of the highly ordered graphitic structure (indicated by the high intensity in the graphite G band), resulting in a material with greater structural disorder due to exfoliated graphene layers. ³⁵ A prominent intensity peak was observed in the graphite 2D band, significantly higher than in the rGO spectrum. This also indicates effective exfoliation, as the highest intensity peak in graphite is linked to increased resistance for electron movement due to distortions from van der Waals interactions between graphite layers.^36,38

The band gap in rGO permits light absorption and emission (photoluminescence), a property that can be used to characterize the degree of reduction. In its oxidized form (GO), the photoluminescence spectrum presents emission in the yellow-red region of the UV-vis spectrum, with a broad-based peak centered at approximately 600 nm. After reduction, the rGO spectrum exhibits fluorescence in the blue region of the UV-vis spectrum, with a narrow-width peak centered at approximately 420 nm. This shift in the photoluminescence spectrum occurs in rGO due to the restoration of sp² domains, resulting from the removal of oxygenated groups during reduction.^39,40

Morphological analyses were conducted using SEM (Figure 3), revealing that the synthesized material exhibited a morphology similar to other studies, ²⁵ with a nanometric thickness of rGO layers and a crumpled, flexible aspect. Quantitative analyses of the micrographs revealed that the rGO sheets possessed a nanometric thickness of 25–35 nm. This thickness is significantly higher than the theoretical few-layer rGO, possibly indicating the high degree of restacking or aggregation of the individual rGO sheets. Despite restacking being evident, the crumpled morphology may have prevented complete stacking, ensuring the formation of sufficient inter-sheet voids and channels. These channels are essential for maintaining the solvated electrolyte ions’ accessibility during cell operation.

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Figure 3.

Scanning electron microscopy images of an rGO structure: (a) 100,000 x; (b) 50,000 x; (c) and (d) 200,000 x.

Regarding the BET surface area, the isotherms in Figure 4 indicate that, since there was no capillary condensation (stabilization of the adsorbed volume at high P/P₀ values), the resulting isotherm can be described as a mixture of type II and V. This is typical of a type II macroporous material but includes slit-shaped mesopores, which are associated with type V isotherms, characteristic of porous solids with low surface activity. The average pore diameter of the material is within the 2–50 nm range, indicating the presence of mesopores, a structure limited between microporous and microporous.

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Figure 4.

Brunauer-Emmett-Teller analysis of nitrogen adsorption-desorption isotherm of rGO.

Although both the presence of mesopores and the high degree of structural disorder observed by Raman spectroscopy (I_D/I_G = 1.48) would typically be associated with higher BET surface areas, the measured BET value was 17.25 m² g⁻¹. This relatively low value may be attributed to partial restacking and agglomeration of graphene sheets during the reduction and drying processes, as suggested by the SEM images, which can reduce the accessibility of internal surfaces to N₂ during BET measurements. Despite this relatively low BET area value for carbonaceous materials with capacitive properties, ¹⁰ yet it did not hinder the electrochemical performance of the rGO, which exhibited a capacitance of 127 F g⁻¹ (Table 2). This can be attributed to a possible synergy between the material's capacitive nature and its electrochemical surface area, which is likely related to its highly defective structure. A detailed analysis showed that 38% (48 F g⁻¹) is derived from the C_P (Table 2), a Faradaic charge storage mechanism associated with the functional groups remaining in the rGO structure, which does not directly depend on a high BET area. The remaining capacitance, representing 79 F g⁻¹, suggests that the electrochemical surface area is, in practice, significantly larger than the measured BET area. This implies that, under experimental conditions of ambient temperature and applied potential, the solvated electrolyte ions can effectively penetrate the stacking regions of the rGO layers that are inaccessible to N₂ gas in the BET measurement.

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Table 2.

Total specific capacitance, double layer capacitance, and pseudocapacitance values.

Sample	CT (F g−1)	CDL (F g−1)	CP (F g−1)	CP/CT (%)
rGO	127	79	48	38
Graphite	16	2	14	88
Carbon black	12	2	9	81

Effect of reduced graphene oxide addition in negative plate formation and capacity

Figure 5 shows the influence of rGO on the NAM formation process. During the first formation stage, occurred at −0.90 V, no significant differences were observed between the various plate versions. This stage corresponds to the formation of a primary Pb structure (structural), followed by a secondary Pb structure (energetic), derived from the reduction of PbSO₄. Figure 5(b) highlights the transition from the first to the second potential stage, which initiates between −1.310 V and −1.450 V and is associated with the hydrogen evolution reaction. In this stage, the rGO/1.4 version remained closer to the region of the first stage for longer, indicating improved efficiency of the formation reactions. In versions with lower rGO concentrations and the control, no significant differences were observed.

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Figure 5.

(a) Formation curves of the negative plates; (b) formation curves in the transition region from the first to second potential level; (c) formation curves in the region of the second potential level; (d) specific capacity C₂₀.

The most notable characteristic during the formation process was observed in the second stage, where a potential shift to less negative values is evident in the plates containing rGO. This shift is proportional to the concentration of rGO and is more pronounced in plates with higher concentrations of this material (Figure 5(c)). The reduction in the overpotentials required to reach the second stage suggests that the hydrogen evolution reaction is favored in the presence of this material. This phenomenon may indicate a lower efficiency of the recharge reactions throughout the plate's life cycle and, depending on the effect's magnitude, may result in lower performance during charge and discharge cycles. Less efficient recharging could lead to a greater lead sulfate accumulation in the negative material, especially HRPSoC. Supporting this observation, an increase in the H₂ rate occurred in the last hour of the formation process of the plate with the highest rGO concentration (1.40%), where potential instability was observed, likely due to a significant evolution of hydrogen bubbles. This favoring of hydrogen evolution has also been reported elsewhere, ³⁰ suggesting that carbon additives can promote hydrogen evolution, as carbonaceous materials generally exhibit lower overpotential for hydrogen evolution, particularly in acidic environments (e.g., lead-acid batteries). ⁴¹

In reference to the C₂₀ capacity measurements, Figure 5(d) presents the capacity values expressed as specific capacity (mAh g⁻¹), considering the exact mass of active material. Notably, plate versions containing rGO exhibited a progressive increase from the first capacity measurement, except for the rGO/1.40 plate. The rGO/0.20 version stood out, which presented a 6.2% increase between the first and fourth capacity measurements, a significant value compared to the specific capacity values of the other versions. This increasing trend may suggest that rGO, when added in specific concentrations, can enhance the efficiency of the kinetics of the NAM reactions. As for the observed decrease in capacity over the measurements for the rGO/1.40 plate, a plausible explanation is its high rGO concentration, results in an elevated rate of hydrogen evolution, thereby reducing the efficiency of recharges post capacity measurements. Conversely, the rGO/1.40 plate presented the highest value for the initial capacity measurement (indicative of formation efficiency), supporting the observation that its formation curve remained at a potential close to the initial level longer than the other versions. The rGO/0.75 plate did not show a tendency for decreasing capacity, although its increase was limited to 2.1% between the first and fourth measurements, lower than the rGO/0.20 plate, possibly influenced by a higher hydrogen evolution rate. The control plate exhibited a 1.0% increase between the first and second capacity measurements, followed by a progressive 1.2% decrease between the second and fourth measurements. The initial increase can be attributed to capacity stabilization after formation, while the subsequent decline was possibly related to the lower contribution of carbon black compared to rGO in enhancing the efficiency of the kinetics of NAM reactions. Considering the last capacity measurements, the best-performing plate (rGO/0.20) exhibited a specific capacity value 6.1% higher than the control. This outcome aligns with other studies that have added different types of rGO to NAM.^21,22

Electrochemical performance

The voltammograms (Figure 6) obtained for rGO, graphite, and carbon black exhibited a quasi-rectangular shape, typical of materials with pseudocapacitive properties, representing an intermediary behavior between capacitors that store energy purely in the electric double layer and batteries.^42,43 The synthesized rGO presented a broad peak in the range of 0.5–0.6 V, indicative of pseudocapacitive character associated with faradaic charge transfer. This is due to a rapid sequence of reversible “faradaic redox” processes, defined by three different mechanisms: adsorption, redox, and intercalation.^42,43 Unlike an ideal capacitor, the current values presented dependence on the scanning speed, often attributed to the double layer structure, ion adsorption, and the material's porous structure influencing electrolyte diffusion. These findings suggest that rGO films have binding sites capable of facilitating faradaic redox reactions, which can be explained by the remaining oxygenated groups in the rGO structure, as identified in FTIR measurements.

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Figure 6.

Cyclic voltammograms of (a) rGO, graphite and carbon black (300 mV s⁻¹ scan rate); (b) rGO at different scanning speeds; (c) graphite at different scanning speeds; (d) carbon black at different scanning speeds.

The calculated values of specific capacitance for the materials (rGO, graphite, and carbon black) at different scanning speeds are presented in Figure 7(a). As previously inferred from the magnitude differences in the current values of the respective cyclic voltammograms (Figure 6(a)), rGO presented a specific capacitance (calculated by Equation (1)) significantly higher than those of graphite and carbon black (approximately 10 times higher). These values suggest that the synthesized rGO has the potential to exert a capacitive effect, consistent with findings in the literature for other types of rGO produced from different synthetic routes.^21,44–48 Data were then treated (Figure 7(b) and (c)), from which the values resulting from the C_DL and C_P were calculated. These values are listed in Table 2, in which one can observe that rGO significantly contributed its pseudocapacitive properties, representing 38% of the calculated C_T. The C_T value of 127 F g⁻¹ aligns closely with values calculated from Equation (1), represented in Figure 7(a), underscoring the data's consistency.

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Figure 7.

(a) Specific capacitance values ​of rGO, graphite and black at each of the scanning speeds, representation of the dependence of b q vs v^−1/2 and c q⁻¹ vs v^1/2 and d nyquist capacitive diagram for the rGO coated electrode.

To further investigate the pseudocapacitive behavior of rGO, we utilized electrochemical impedance spectroscopy. Figure 7(d) presents the Nyquist capacitive spectrum obtained for the electrode coated with rGO. The presence of rGO resulted in a response characterized by a well-defined capacitive arc, qualitatively indicating its capacitive nature. The calculated value of the redox capacitance was 28.7 F g⁻¹, comparable to the C_P value calculated via voltammetry, confirming that pseudocapacitance contributes significantly to the total capacitance, although electric double-layer capacitance remains predominant.

Figure 8 shows the curves of the final discharge voltages obtained during the HRPSoC microcycle test. The results demonstrate that the negative plate containing 0.20% rGO exhibited superior performance in the HRPSoC microcycle test, completing 21% more cycles than the control plate (3656 vs. 3023 microcycles). This enhanced performance of the rGO/0.20 version is consistent with the C₂₀ capacity measurements (Figure 5(d)) where this plate version stood out, presenting a significant progressive increase in values compared to other plate versions. The superior performance is hypothesized to stem from the dual electrochemical role of rGO within the NAM: providing enhanced electronic conductivity and a significant capacitive contribution. During high-rate charging, the rGO sheets may act as conductive and capacitive interfaces that temporarily accommodate incoming charge through electric double-layer charging. This effect likely promotes a more homogeneous distribution of charge transfer across the NAM and reduces local polarization, particularly considering the high electronic conductivity of sp²-hybridized carbon materials. In this context, the role of rGO is not to replace the faradaic conversion of PbSO₄ to Pb, but to improve the electrochemical accessibility of PbSO₄ regions. This mechanism not only enhances the overall charge acceptance efficiency but also minimizes the accumulation and growth of large, electrically insulating lead sulfate crystals, which are characteristic of irreversible sulfation. Consequently, the improved efficiency of the charging reaction facilitated by rGO directly leads to a sustained reduction in the concentration of PbSO₄ over cycling. Although the measured pseudocapacitive contribution associated with residual oxygen-containing functional groups may also participate in transient charge storage, the results presented in Table 2 suggest that the predominant contribution to the improved HRPSoC performance arises from the electrochemically accessible surface area of rGO and the corresponding electric double-layer capacitive behavior, promoting rapid charge accommodation and mitigating local polarization within the NAM.

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Figure 8.

HRPSoC microcycle curves of discharge plotted as a function of the final discharge voltages.

Despite the improved performance of the plate containing rGO, as seen in the rGO/0.20 version, the same behavior was not observed in versions produced with higher rGO concentrations. The rGO/0.75 and rGO/1.40 plates completed 20% and 53% fewer microcycles than the control plate, respectively. This can be attributed to the relationship between increased areas containing carbon centers and decreased capacitive effects, favoring enhanced hydrogen evolution during recharges, as indicated by previous analyses of the formation curves and C₂₀ capacity measurements. Figure 9 presents images showing the condition of the plates after the HRPSoC microcycle test, evidencing the more significant damage caused by increased hydrogen evolution in the plates with higher rGO concentrations, as evidenced by the presence of eruptions and signs of mass detachment.

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Figure 9.

Photos of the negative plates after HRPSoC microcycle test.

Although desirable, a direct comparison between results from distinct studies is complex and unfeasible. As seen in Table 1, there is significant heterogeneity in experimental conditions and methods, preventing direct comparison. This complexity mainly arises from differences in cell technology, test protocols, and reference materials (control). Most studies have focused on AGM batteries, which inherently exhibit superior performance in controlled environments and often use distinct HRPSoC protocols. In contrast, our work applied a specific HRPSoC algorithm adapted for flooded-type cells, a technology where optimizing NAM with rGO under these conditions is less explored. Regarding the control material, many studies have used conventional carbon black. However, we utilized the advanced carbon black Cabot PBX-135 for a more rigorous comparison standard. Consequently, the reported performance increase was measured against a high-performing control, enhancing the real impact value of our additive. In addition, the composition used in preparing the active materials must also be considered. The origin and properties of the raw materials, as well as their respective proportions, result in specific conditions of porosity, surface area, paste density, crystal size of the basic sulfates, humidity, and corrosion layer at the active material/current collector interface. As shown in Table 1, the paste formulation differs significantly between the studies compared. Together with other points discussed, this underscores the care needed when comparatively analyzing results from different studies.