# A polydihydroxystyrene (PDHS) mussel-mimetic binder: toward sustainable hard carbon electrodes with outstanding performance in sodium-ion batteries

Adrian Beda, Jason Beaufrez, Charles Cougnon, Camélia Matei Ghimbeu

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Adrian Beda, Jason Beaufrez, Charles Cougnon, Camélia Matei Ghimbeu. A polydihydroxystyrene (PDHS) mussel-mimetic binder: toward sustainable hard carbon electrodes with outstanding performance in sodium-ion batteries. Sustainable Energy & Fuels, 2025, 9 (13), pp.3591-3595. 10.1039/d5se00403a. hal-05356043

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# A polydihydroxystyrene (PDHS) mussel-mimetic binder: toward sustainable hard carbon electrodes with outstanding performance in sodium-ion batteries

Adrian Beda,<sup>a,bc</sup> Jason Beaufrez,<sup>d</sup> Charles Cougnon and Camélia Matei Ghimbeu*<sup>abc</sup>

E-mail: camelia.ghimbeu@uha.fr;

This work reports a fluorine-free polymer binder (polydihydroxystyrene, PDHS) for the preparation of hard carbon (HC) anodes. Very small amounts (3%) can be used to obtain HC electrodes with high initial coulombic efficiency, iCE (91%) and outstanding reversible capacity (~360 mA h g⁻¹). The obtained performance is better than that of their PVDF/CMC counterparts, due to enhanced adhesion and the formation of a stable, conductive SEI layer.

<figure><img src="image_2.png" /></figure>

Hard carbon (HC) anode represents the state-of-the-art negative electrode for Na-ion batteries (NIBs).¹ Its unique properties ensure high specific capacity, long-term stability, and high rate capabilities, among others. Although significant improvements in performance have been achieved in the recent years, this technology still lags behind Li-ion batteries. This is mainly due to the HC anode microstructure which limits Na⁺ diffusion and cycling stability.² Therefore, numerous strategies are being explored to further develop this technology, including optimization of HC properties,³,⁴ electrode formulation/preparation,⁵,⁶ cell assembly⁷ or testing conditions.⁸ In particular, the improvement of the electrode preparation step might allow to enhance the electrochemical performance, i.e., initial coulombic efficiency (iCE), reversible capacity, rate capability and cycle life. The electrode design involves selecting the binder, determining its ratio with the HC active material and carbon black additive, as well as choosing an appropriate solvent. In addition, the mixing process for all these components, along with the coating of the resulting slurry onto the

current collector, plays an important role in the quality of the electrode. Among these steps, the choice of binder is of paramount importance⁹ as it holds all the involved constituents together, ensures adhesion to the surface of the current collector and provides mechanical stability. Optimal slurry adhesion limits the interphase resistance,⁵ reduces the mechanical stress during Na-ion insertion-extraction, and prevents the electrode delamination, thereby improving battery capacity and cycle life. For the HC active material, a good contact between particles favours electron flow and minimises internal resistance. Na⁺ diffusion is also enhanced by shortening the electron pathway, improving the iCE and the reversible capacity. In addition to performance-related factors, environmental considerations have also been taken into account for the binder selection.¹⁰ Currently, fluorinated binders, such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), which are formulated in organic solvents, are the most commonly used for carbon electrode manufacture, despite their recyclability and toxicity issues. For example, N-methyl-2-pyrrolidone (NMP) solvent used for PVDF dissolution is toxic, and European regulations impose stricter controls on its use.¹¹ Therefore, finding sustainable binders that can be paired with green solvents is of great interest. Such binders have been proposed in the literature, e.g. Na-carboxymethylcellulose (CMC),¹² Na-alginate,¹³ Na-lignosulfonate,¹⁴ etc. Some of them are claimed to form an artificial passivating solid electrolyte interphase (SEI) film on the carbon surface, which suppresses electrolyte decomposition and provides higher iCE and capacity.¹³

Scientists have also found inspiration from nature and designed synthetic polymer binders that can mimic the natural adhesive produced by mussels. Remarkably, sea mussels possess superior ability to strongly anchor on different surfaces,¹⁵ even on very hydro-phobic ones (PTFE).¹⁶ This advantage is due to the release of several proteins, containing the amino acid 3,4-dihydroxy-phenyl-L-alanine (L-DOPA), which plays an important role in the cross-linking reactions that lead to its exceptional adhesive properties.¹⁷ In particular, polydihydroxystyrene (PDHS)

<footnote><sup>a</sup>Université de Haute-Alsace, Institut de Science des Matériaux de Mulhouse (IS2M), CNRS, UMR 7361, F-68100 Mulhouse, France.</footnote>

<footnote><sup>b</sup>Université de Strasbourg, F-67081 Strasbourg, France</footnote>

<footnote><sup>c</sup>Réseau sur le Stockage Electrochimique de l'Energie (RS2E), HUB de l'Energie, FR CNRS 3459, 80039 Amiens, France <sup>d</sup>Laboratoire MOLTECH-Anjou, Université d'Angers, UMR, CNRS 6200, 2 Bd Lavoisier, 49045 Angers Cedex, France</footnote>
combines the benefits of natural, reactive and functional polymer binders in a single material, mainly due to its hydroxyl groups, which are able to create specific interaction with the surface of carbon. Therefore, PDHS is expected to have a positive effect in increasing the cohesion of the electrode components and the mechanical stability of the electrode.

In this work, we propose a nature-inspired fluorine-free binder that can be formulated in a non-toxic solvent (Et-OH) and used in low amounts (3%) to prepare HC electrodes that deliver high electrochemical performance in NIBs (iCE, reversible capacity and rate capability). More precisely, the mussel-inspired adhesive was used as a binder to prepare HC electrodes, as illustrated in Fig. 1. The PDHS binder (Fig. S1†) was obtained through demethylation of polydimethoxystyrene (PDMS) with boron tribromide (BBr₃),¹⁸ according to the procedure described in the ESI.† To our knowledge, this is the first work reporting such a binder for HC anodes used in NIBs. Based on our previous findings,⁶,¹⁹ an optimized HC, which presents high performance vs. Na/Na⁺, was selected.

The PDHS binder is mixed with the HC active material, the conductive carbon black and the solvent (Et-OH) to obtain a homogeneous slurry, which is then coated on aluminium foil (more details can be found in the ESI†). Most of the reported articles on HC electrodes use a binder amount of 5-10% (Table S1†) as the “optimum” to obtain homogeneous coatings, mainly PVDF and then CMC. Although the binder plays a crucial role in ensuring electrode cohesion, mechanical strength and electrical conductivity, it is generally considered as electrochemically inactive as charge-storing material and acts as an electrical insulator. However, its properties can indirectly impact the electrochemical performance metrics,²⁰ such as iCE, reversible capacity, and energy density. Therefore, reducing the amount used in electrode formulation would be beneficial to performance; however, care must be taken to avoid compromising its adhesive properties. Considering the exceptional binding properties of mussel-inspired polymers, the binder content was reduced to 3% in an attempt to further enhance the iCE and the reversible capacity of the HC electrode.

The obtained electrode was investigated by SEM (Fig. S2†), and the images show a homogeneous surface with carbon particles (~3 µm) that are well interconnected to each other. The binder and carbon black nanoparticles can also be seen as small entities (10-300 nm), uniformly distributed among the

<figure><img src="image_3.png" /><figcaption>**Fig. 1** Mussel-inspired binder for hard carbon electrode preparation, including: mussel-mimetic adhesive representation, the structural formula of PDHS and a graphical illustration of the HC-PDHS electrode.</figcaption></figure>

HC particles, ensuring cohesion and electronic conductivity (Fig. S2b†). No cracks or mechanical defects were observed. To strictly assess the potential interest of this new binder and for comparison purposes, HC electrodes were also prepared under similar conditions (3% binder) using the more common PVDF-NMP and CMC-H₂O binder-solvent systems. Peel tests (Fig. S3†) confirmed the best adhesion to the current collector for the PDHS binder, followed by the PVDF binder and lastly CMC (the latter two show deep detachment, revealing the Al foil). The catechol hydroxyl groups in PDHS enable strong adhesion to the Al current collector *via* bridging bonds.

First discharge-charge profiles of the PDHS-HC electrode in a Na-half-cell (Fig. 2a) reveal the typical behaviour of Na⁺ storage in HC materials: a sloping region at high voltage (>0.1 V) and a plateau region below 0.1 V.²¹⁻²³ This profile is very similar to those obtained for both CMC- and PVDF-HC electrodes. However, it can be seen that the shape of the sloping region is different for the three electrodes. Within the first cycle, this region is mainly linked to electrolyte decomposition and SEI formation. In contrast to the PVDF electrode, the slope of CMC and particularly the PDHS electrode drops steeply (~0.85 V for PVDF, 0.65 V for CMC and 0.45 V for PDHS) before significant oxidation occurs, indicating earlier SEI formation in case of PVDF.

Once the sloping region starts, several fluctuations can be observed for both CMC and PDHS electrodes, while for PVDF, this region is rather smooth. This suggests that the electrolyte decomposition and the SEI formation are progressive and gradual (stepwise) in the first two cases, while its growth is more spontaneous and favoured by the PVDF binder, as suggested by its higher sloping region. In the 2nd cycle (Fig. S4†), the shape of the curve changes in the sloping region, PVDF is steeper now, in line with other studies,²⁴ while CMH and PDHS still exhibit similar behaviour. This is confirmed by the iCE values observed for the three electrodes (Fig. 2b): the lowest value of 81% for PVDF, an increased value of 90.5% for CMC and the highest value of 91.1% for PDHS. It has been reported in the literature that the CMC binder can act as an artificial SEI layer²⁵,²⁶ which prevents excessive electrolyte decomposition by limiting irreversible reactions. Considering the similarities noticed between the CMC and PDHS sloping regions and iCE, it can be assumed that the new binder proposed herein has a similar protective effect.

The plateau region (<0.1 V) contributes significantly to the total capacity of the material (slope + plateau) and, according to current knowledge,¹ is mainly governed by Na⁺ intercalation between HC's graphitic domains and/or closed pore filling. A rather similar behavior can be observed among the HC electrodes, as illustrated in Fig. 2c. In terms of reversible capacity delivered (Fig. 2b), the PDHS electrode could reach the highest value of 359 mA h g⁻¹, one of the highest reported values in the literature, to our knowledge (Fig. S5†), and which is very close to the theoretical capacity of graphite in LIBs (372 mA h g⁻¹). It is followed by the CMC electrode (334.5 mA h g⁻¹) and last, PVDF (325 mA h g⁻¹). This trend is somehow surprising, because the PVDF electrode could reach the highest discharge capacity, exceeding 400 mA h g⁻¹. However, as the irreversible reactions have an important contribution to this initial discharge capacity gain, its reversible capacity is the lowest. The significantly high
<figure><img src="image_4.png" /><figcaption>Fig. 2 First discharge-charge cycle (a) and the reversible capacity and iCE (b) of HC electrodes obtained with three different binders: PDHS, CMC and PVDF (3%). Sloping and plateau capacities obtained for the three types of electrodes, along with their electronic conductivity (c). XPS C 1s high-resolution deconvoluted spectra of (d) pristine and (e) cycled HC-PDHS electrodes. (f) Symmetric C-rate tests of the HC-PDHS electrode.</figcaption></figure>

reversible capacity obtained for the PDHS electrode (Fig. 2b) can be related to several factors. Firstly, it can be linked to improved electrode properties (better conductivity between particles), while PVDF is known to have rather poor electronic conductivity.²⁷ To confirm this hypothesis, electronic conductivity measurements were performed on the bulk electrodes. The results presented in Fig. 2c show indeed that the PDHS electrode has an electronic conductivity, approximately twice than that of PVDF and CMC electrodes (74 mS cm⁻¹ vs. 37 and 26 mS cm⁻¹, respectively).

Secondly, PDHS might promote the formation of a robust and flexible SEI layer, which limits irreversible reactions and accommodates the volume changes, thereby enhancing the iCE and reversible capacity. To confirm SEI formation, both SEM and XPS analyses were performed. SEM images of the cycled electrodes (Fig. S6†) clearly show the presence of a thin layer on the surface of the electrodes. To check its composition, XPS was carried out on both the pristine and cycled PDHS electrodes, and C 1s deconvoluted spectra are shown in Fig. 2d and e. The signals corresponding to both C sp² at 284.2 eV (from the HC powder) and C–OR at 285.8 eV (from the PDHS binder) in the pristine electrode (Fig. 2d), disappear in the cycled electrode (Fig. 2e), thus confirming the formation of the SEI layer covering the HC electrode surface. In addition, four noticeable contributions are observed: (i) the most intense peak at 285 eV is related to C–C/C–H bonds in aliphatic chains; (ii) the second intense peak at 286.3 eV is assigned to C–O bonds; (iii) the contribution of CO₂ species at 287.8 eV is due to the degradation of the solvent (EC: DMC) by C–O bond cleavage and (iv) the CO₃ species at 290.5 eV are related to Na₂CO₃ and/or Na alkyl carbonates (ROCO₂Na).²⁸ Moreover, O 1s spectrum presents an

intense peak at 532.3 eV associated to C=O and C–O bonds in CO, CO₂ and CO₃ species (Fig. S7†), such as sodium alkyl carbonate, sodium carboxylate and sodium carbonate, which are specific to the SEI layer containing both organic and inorganic species.²⁹ To confirm its protective role, impedance measurements were performed, under different conditions (Fig. S8†). Before cycling (Fig. S8a†), all electrodes have a similar resistance, which decreases significantly after 5 cycles at C/10 (Fig. S8b†), due to the formation of the SEI. It is worth noting that at this stage, CMC and PDHS electrodes exhibit very similar behavior, although they did not induce the smallest resistance. However, after a few cycles at 1C (Fig. S8c†), when the SEI layer has completely formed and stabilized, the CMC and PDHS electrodes still behave similarly, but show lower resistance compared to PVDF electrode.

Finally, the third aspect that may explain the high performance of the PDHS electrode is related to the lower binder content (3%) used, compared to the values reported in the literature (Table S1†). This reduction allow a higher amount of HC active material in the electrode, which likely contributes to the increased capacity observed. To our knowledge, there are no previous reports on HC electrodes that can deliver both high reversible capacity and iCE simultaneously, as illustrated in Fig. S5,† which compares the highest-performing electrodes reported in the literature to date. A lower binder percentage, coupled with the high reversible capacity achieved, could significantly enhance the energy density of the NIBs. Attempts have been made to lower even more the binder amount (Fig. S9†), and very good results were obtained for 2% PDHS (i.e., 91% iCE and 352 mA h g⁻¹ reversible capacity). However,
the mechanical stability of the electrodes was rather limited, as delamination was observed (inset, Fig. S9†). Therefore, further optimization is needed for high-quality electrodes.

Nevertheless, it should be reminded that the electrode was tested at a relatively low C-rate (C/10). This raises the question of whether a 3% binder would provide sufficient electrode stability at higher current densities. To investigate this aspect, symmetric C-rate tests were performed (Fig. 2f), and it can be seen that there is a small drop in capacity when the C-rate increases from C/10 to 1C, 3C, 5C, and 10C. However, a reversible capacity as high as 255 mA h g⁻¹ can still be achieved at 10C, which corresponds to a high current density of 3.72 A g⁻¹. Furthermore, when returning from 10C to 1C-rate, most of the initial capacity is recovered, i.e., 290 mA h g⁻¹ vs. 275 mA h g⁻¹, suggesting that the electrode is rather stable. Advantageously, the iCE of the HC electrode reaches ~100% no matter the cycling conditions. The PVDF electrode presents good stability during the C-rate test (Fig. S10a†), while the CMC electrode is initially stable but at high C-rates (5C/10C) presents important problems (Fig. S10b†), probably due to its poor adhesion properties. Finally, the cycling performance of the HC-PDHS electrode has been evaluated at a rate of 5C (Fig. S11a†). Prior to cycling at 5C, 5 cycles were performed at C/10, followed by 2 cycles at 1C, in order to ensure good SEI layer formation. A capacity as high as 290 mA h g⁻¹ is obtained at 5C, which progressively decreases over 100 cycles, reaching 230 mA h g⁻¹ at the end. This corresponds to a capacity retention value of ~80%, which appears as moderate compared to the results of other studies using the same C-rate.³⁰ However, the capacity delivered here is significantly higher than that in ref. 30 (230 vs. 100 mA h g⁻¹, cycle 100), despite the higher current density applied (372 mA g⁻¹ for 1C vs. 200 mA g⁻¹ for 1C). The other two electrodes exhibit stability issues even at 1C (Fig. S11b and c†). The cause behind this instability can be attributed to the weaker adhesion between the electrode material and the current collector, which follows the same trend: CMC < PVDF < PDHS.

In conclusion, this work demonstrates that it is possible to use fluorine-free binders and non-toxic solvents to obtain HC electrodes. Thanks to its “mussel” bonding ability, the PDHS binder used in low quantities (3%) provided HC electrodes with high performance vs. Na/Na⁺, exceeding those of their PVDF and CMC counterparts. The delivered performance was outstanding: high iCE (91%), very high reversible capacity (~360 mA h g⁻¹), long cycling behavior at 5C and high rate capability. The formation of a more stable and conductive SEI layer likely contributed to the excellent iCE and reversible capacity, while the good adhesion to the current collector likely provided better cycling behavior.

## Data availability

The data supporting this article have been included as part of the ESI.†

## Conflicts of interest

There are no conflicts to declare.

## Acknowledgements

This work was performed in the framework of RS2E (French research network on electrochemical energy storage). The authors gratefully acknowledge funding support from the ANR (ANR-10-LABX-76-01, STORE-EX, ANR-22-PEBA-0003, PEPR Battery – HipoHyBat and ANR-22-CE05-0011). The authors acknowledge Dr Frédéric Gohier, Dr Samar Garreau and Julia Coffre for their help with the polymer preparation, XPS analysis and scheme illustration, respectively.

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