Water-soluble densely functionalized poly(hydroxycarbonylmethylene) binder for higher-performance hard carbon anode-based sodium-ion batteries

Abstract

Hard carbon (HC) is a promising anode material in developing sodium-ion batteries (SIBs). However, due to diminished ion diffusion kinetics, low initial coulombic efficiency (ICE) and continuous electrolyte decomposition, HC-based SIBs have led to sluggish rate capability and specific capacity. Herein, we utilised a polymer with dense functional groups containing carboxylic acid in its side chains as a binder that demonstrates ion transport, defect passivation and better mechanical stability. The consecutive polar functional groups provide ion transfer channels and enhanced adhesion to electrode components. The poly(hydroxycarbonylmethylene) (PFA) binder for the HC electrode achieved the highest ICE of 80.8% and a specific capacity of 288 mA h g⁻¹ and 254 mA h g⁻¹ at 30 mA g⁻¹ and 60 mA g⁻¹, respectively, which are superior to those for HC electrodes containing PAA and PVDF binders. Anodic half-cells containing the PFA binder showed high capacity retention of 85.4% (250 cycles) and 91.6% (200 cycles) at current densities of 60 mA g⁻¹ and 30 mA g⁻¹ respectively. In addition, the presence of dense polar groups boosted the diffusion kinetics and lowered the Na⁺ activation energy. XPS and SEM studies further verified that the dense functional groups influence the formation of a thin, stable and inorganic-rich solid–electrolyte interface and crack-free electrodes. Therefore, polymers with dense functional groups will help in the early adoption of SIBs in the market.

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Introduction

The growing demand for energy storage systems has led to a search for low-cost rechargeable batteries based on abundant resources.¹ SIBs have become a promising alternative to lithium ion batteries,² potassium ion batteries³ and zinc ion batteries⁴ due to the unlimited sodium resources in seawater and salt deposits^5,6 and safe shipping at 0 V when Al is used as a current collector on the anode.⁷ Moreover, the chemistry of lithium and sodium is similar; sodium is the lightest and smallest alkali metal after lithium.⁸ The key components of SIBs that have been explored in the last decade to deliver long cycles and a stable and thin solid–electrolyte interface (SEI) are positive electrodes (transition metal oxides,^9,10 Prussian blue analogues^11,12), negative electrodes (hard carbon¹³ (HC), TiO₂ polymorphs,¹⁴ Na–Sn alloy system¹⁵) and electrolytes. Hard carbon is a promising anode for practical applications; however, its commercialisation has been hindered due to low ICE and poor rate capability in traditional organic electrolytes.¹⁶ It has been observed that defect-rich hard carbon leads to the formation of an uneven and thick SEI with poor mechanical strength due to increased electrolyte consumption, which lowers the cycling stability and reaction kinetics.^17–22 Much research has focused on electrolytes,²³ defect optimization and introducing oxygen functional groups in HC,²⁴ artificial SEI,^25,26 presodiation,²⁷ HC/MoS₂/NC nanocomposites²⁸etc., to obtain a stable, uniform, and thin SEI. However, such methods are complicated and challenging for large-scale applications. Therefore, there is a need for a solution that is easy, low-cost and can be considered for practical application in SIBs.

The factors influencing the electrochemical performance of HC depend on the electrolyte composition and concentration, active material properties, choice of binder, electrode morphology, operating conditions, cycling rate, electrolyte additives, electrolyte solvent choice, interface stability and passivation layer composition.^15,29–31 Binders play an essential role in the mechanical integrity of the adhesion of the anode material, impedance, cycling stability and SEI formation.^32–34 In addition, functional binders participate between the metal ion-solvated structures and interfaces, which affects the decomposition of ester electrolytes and induces SEI formation and compositions.^35,36 The factor affecting the dynamic performance of SIBs is the desolvation kinetics; reducing the energy barrier of sodium ions from the solvated sphere will help to form a stable SEI and initiate faster Na⁺ transport.^37,38 However, a few reports on functional binders for HC in SIBs have been published. Komaba et al. reported that sodium carboxymethyl cellulose (CMC) demonstrated superior binding, reversibility and cycling ability compared to a poly(vinylidene fluoride) (PVDF) binder. Surface analysis revealed a different surface and passivation chemistry due to the presence of carboxylate polyanions in the CMC binder, which improves the sodiation reversibility. The use of monofluoroethylene carbonate as an electrolyte additive is essential for PVDF electrodes to improve cyclability.³⁹ Li et al. demonstrated a trifunctional SA/PEO binder exhibiting a better rate capability and cycling stability with a higher ICE than traditional PVDF binders. The structural design of the SA/PEO binder leads to the formation of a uniform SEI film, which inhibits the slow continuous electrolyte decomposition. The ionic SA/PEO enhances the diffusion kinetics of Na⁺ by reducing the impedance and facilitating ion transport.⁴⁰ Naylor et al. reported a sustainable biopolymer LgSA binder for HC in SIBs.⁴¹ They fabricated full cells with a Prussian white cathode, which showed better cycling stability over 40 cycles with a 122 mA h g⁻¹ specific capacity at 60 mA g⁻¹ in 1 M NaPF₆ EC:DEC electrolyte. This might be due to the functional group (sodium sulfonate) present in the lignin polymer, which helps to form a stable and uniform SEI and affects the diffusion kinetics of Na⁺. Shen et al. reported using a composite binder consisting of chondroitin sulfate A and polyethylene oxide for HC in SIBs. The anodic half-cell exhibits 84% ICE and long cyclability with 94% retention capacity for 150 cycles at 50 mA g⁻¹. The –SO₃– and –CO–NH– functional groups of chondroitin sulfate A are involved in the fast transportation of Na⁺, reducing electrolyte decomposition and helping in the formation of inorganic-rich SEI.⁴² Trivedi et al. reported that the use of inorganic binders such as (NaPO₃)₃, (NaPO₃)₆ and (NaPO₃)_n in HC electrodes resulted in better capacity than organic binders due to enhanced electronic and ionic conductivity.³¹ These binder studies indicated that not only do the functional groups form good coordination with hard carbon, but they also reduce electrolyte consumption and build better SEIs than PVDF.⁴³ However, there are no studies on countering the poor rate capability and specific capacity due to the slow Na⁺ diffusion kinetics and facilitating Na⁺ transmission by providing ion channels. Therefore, there is an urgent need for a dense functional group binder to improve the electrochemical rate capability performance by influencing the surface chemistry of SEI and enhancing the Na⁺ transport in the electrode domain.

Herein, we propose using the polymethylene-based polymer poly(hydroxycarbonylmethylene), also known as poly(fumaric acid) (PFA), which features dense carboxylic acid functional groups, as a polymer binder. PFA is synthesised through a novel synthetic route with the precursor diethyl fumarate that uses cost-effective and non-hazardous chemicals.⁵⁴ Diethyl fumarate can be obtained from the biomolecule fumaric acid, which is a product of the citric acid cycle in mitochondria and can be isolated from plants and fungi.^55–57 In contrast, PVDF synthesis involves the use of costly fluorinated precursors, which affects the environment. PAA and styrene–butadiene rubber require acrylic acid and styrene–butadiene monomer from petrochemical sources, impacting their sustainability, environment and cost-effectiveness.^58–60 Polymethylene-based polymers are less flexible at their polymer backbone and are expected to show different properties than polyethylene polymers.^44,45 PFA as a binder can passivate the surface defects of hard carbon through hydrogen bonding between the dense carboxylic acid polar functional groups of the binder and the oxygen functional groups present on the surface defects of HC, thereby preventing the defects from being exposed to the electrolyte due to greater amount of functional groups covering the defects and reducing the ability of the defects to decompose the electrolyte and form a thick SEI.⁴² Moreover, cross-linkable and intermolecular-hydrogen-bonding hybrid binders have shown stable electrode–electrolyte interphases, increased ionic conductivity, reduced electrolyte decomposition, mitigated transition metal dissolution and enhanced rate capability.⁴⁶ The polar carboxylic functional groups interacted with Na⁺ and reduced the activation energy of sodium ions from the solvation sphere.^47,48 The binder PFA offers superior water solubility and non-toxicity compared to the binder PVDF, which requires N-methyl-2-pyrrolidone, a hazardous and expensive organic solvent.⁴⁹ At the same time, the carboxylic acid group provides better adhesion with the electrode material through hydrogen bonding, ion–dipole and chemical bonds, which are absent in PVDF as it contains an inactive C–F functional group.⁵⁰ Introducing functional groups at every carbon of the polymer chain can provide a directional Na⁺ transmission path and thereby counter the sluggish diffusion kinetics of Na⁺ due to its large ionic radius and atomic mass.⁵¹ Furthermore, the dense carboxylic acid groups will reduce the agglomeration effect due to electrostatic repulsion, which makes the binder wettable and uniformly covers the surface of the hard carbon particles.^40,52 The binder performance of PFA was evaluated using cyclic voltammetry, impedance and galvanostatic charge–discharge measurements and compared with those of PAA and PVDF. In addition, the impact of the binders on the rheology and surface chemistry of the electrodes was investigated. Our findings offer a new perspective on choosing a binder and shed light on the charge–discharge mechanism and interfacial Na⁺ transport.

Experimental

Preparation of PFA polymer

The PFA was prepared via the ester hydrolysis of poly(ethoxycarbonylmethylene). The poly(ethoxycarbonylmethylene) was derived from the radical bulk polymerisation of diethyl fumarate (Sigma-Aldrich, 98%) in a vacuum Schlaker reaction tube using 2,2′-azoisobutyronitrile (AIBN, Sigma-Aldrich, >98%) as a radical initiator at 70 °C for 24 h. It was further purified by washing with excess hexane and dried overnight under vacuum. Poly(ethoxycarbonylmethylene) was base-hydrolysed using an excess of KOH (Wako) in an EtOH : H₂O (2 : 1) solvent mixture under reflux.⁵³ After hydrolysis, the mixture was acidified using 1 N HCl (Sigma-Aldrich, 12 N) and dialysed using a cellulose acetate tube (Fisherbrand). Water was evaporated from the aqueous polymer after dialysis to obtain PFA. The yield of PFA was 82% with respect to diethyl fumarate.

Electrode preparation

Hard carbon (Kuranode Type 2 (5 μm), Kuraray), Super P carbon (Alfa Aesar, 99+%) and PFA were mixed in water in a weight ratio of 80 : 10 : 10 to obtain a homogenous slurry. The slurry was cast onto copper foil and dried overnight. The coated copper was calendared and punched into 13 mm diameter discs. CR-2025 coin cells were assembled using the obtained HC electrode as the anode, sodium metal disc as the counter electrode, glass fibre from Whatman as a separator and 1 M NaClO₄ in EC : PC (1 : 1, by vol) as an electrolyte in an argon-filled glove box with O₂ and H₂O levels of <0.5 ppm. Before electrochemical measurements, the coin cells were kept at rest for 24 h to allow them to reach open circuit voltage. Various weight percentages (wt%) of PFA binders (3, 5 and 15 wt%) along with the corresponding HC amounts, keeping the wt% of Super P carbon at 10%, were fabricated. In addition, PAA (polyacrylic acid, Sigma-Aldrich) and PVDF (Sigma-Aldrich, MW: 540 000) binders were used to fabricate HC electrodes using the above procedure, and CR2025 coin cells were fabricated for comparative studies.

Characterisation

PFA was characterized using ¹H NMR, and ¹³C NMR using a Bruker Advance II-400 MHz spectrometer in D₂O solvent. FT-IR was performed using a PerkinElmer 100 FT-IR spectrometer. Thermogravimetric analysis (TGA) was recorded using a Hitachi STA7200 instrument at a 10 °C min⁻¹ heating rate under a 200 ml min⁻¹ N₂ flow rate. The rheology of the HC-coated electrode was tested using a peel test (Instron 3342). Galvanostatic charge–discharge tests were carried out using an Electrofield ABE 1024 in the 0.01–2.5 V vs. Na/Na⁺ voltage range. Cyclic voltammetry (CV) measurements were performed on Bio-Logic Science Instruments with scan rates of 0.1, 0.2, 0.4, 0.6, 0.8 and 1 mV s⁻¹. The electrochemical impedance spectroscopy (EIS) measurements were tested in the frequency range from 1.0 MHz to 0.1 Hz with an AC amplitude of 10 mV. Using the EIS method, the cycled electrodes were further used for dynamic electrochemical impedance spectroscopy (DEIS) measurement and activation energy evaluation at different voltage perturbations and temperatures, respectively. Postmortem studies were carried out to determine the surface morphology structure and elemental composition distribution of HC after prolonged cycling using a scanning electron microscope (SEM, Hitachi S-4500) and X-ray photoelectron spectroscopy (XPS, Fisons instruments S-probe TM 2803), respectively.

Results and discussion

PFA, which features a high density of functional groups, was utilised based on its desirable physicochemical properties, such as good adhesion, self-healing ability, rate capability and ionic conductivity in SIBs. The reaction method for synthesising PFA utilising radical bulk polymerisation and base-catalyzed ester hydrolysis is depicted in Fig. 1. To examine the purity of the PFA obtained, NMR and IR analyses were executed. Fig. S1(a)† shows that the peak at 3.25 ppm in the ¹H NMR spectrum corresponds to the polymer chain methylene proton and is consistent with the reported literature. In the ¹³C NMR spectrum (Fig. S1(b)†), the peaks at 170.8 ppm and 62.4 ppm were assigned to the carbonyl of carboxylic acid and the methylene carbon, respectively. The synthesis of PFA was further confirmed using FT-IR spectroscopy, as presented in Fig. S2.† The strong broad peak at 3500 cm⁻¹ is attributed to the O–H stretching vibration of carboxylic acid. The peaks at 1715 and 1090 cm⁻¹ are assigned to the carboxylic acid carbonyl and ether. The MALDI-TOF data reveal the molecular weight of the PFA polymer to be 3000 m/z (Fig. S3†). The thermal stability of PFA was evaluated using TGA. Fig. S4† shows the TG and DTG profiles of PFA. The initial weight loss at ∼100 °C corresponds to the vaporization of the moisture absorbed by the hydrophilic carboxylic group. PFA is shown to be stable up to ∼235 °C, after which a steep degradation slope is observed, making it suitable as a binder for high-temperature SIB. These observations reveal that the PFA polymer was formed through the new synthetic route.

Fig. 1 Schematic representation of the synthesis of poly(fumaric acid).

Peeling tests were performed to test the effect of the binder on the adhesion between the electrode components and the current collector. To improve the cycle life of the SIBs, a stronger adhesion force of electrode material to copper is needed. The peeling force vs. displacement curves are shown in Fig. 2. The peeling force of the PFA-binder-containing HC electrode was 12.5 N, which was higher than those of the PAA (11.5 N) and PVDF (9.8 N) electrodes due to the density of –COOH groups present in PFA. PAA and PVDF have low carboxylic group content and no polar group for binding to copper, respectively, which result in poor adhesion capability. The photographs of the tape in the insets show the amount of HC peeled from the peel tests, indicating the better binding of PFA to copper and the significant amounts of the HC containing the PAA and PVDF binder that adhered to the tape.

Fig. 2 Force vs. displacement profiles from the peel tests for the coated electrodes with the different binders.

To determine the influence of the binders on the electrochemical performances, CV and galvanostatic charge–discharge studies were conducted. The HC-based anodic half cells using PFA, PAA and PVDF binders are denoted as PFA/HC, PAA/HC and PVDF/HC, respectively. NaClO₄ in EC–PC (1 : 1) was used as an electrolyte for our studies because of its better cycling performance among the various electrolyte solutions for HC reported by Ponrouch et al.,³⁰ as also confirmed by the studies of Komaba et al.^15,54 In addition, PC does not co-intercalate with Na⁺ due to the unique structure of the HC containing defects, nanovoids and random graphene layer orientation, which restrains graphene layer exfoliation.⁵⁵ All the CV curves of anodic half cells reveal a broad cathodic peak between 0.3 V and 1.0 V in the first cycle, which is ascribed to the decomposition of the electrolyte and SEI formation (Fig. 3(a)–(c)). The second and third curves reveal no large additional irreversible electrolyte consumption, and an SEI film is formed only in the first cycle. The two main redox peaks at ∼0.01 and ∼0.3 V correspond to the intercalation and deintercalation of Na⁺ in HC. It is also necessary to consider the overpotential of the batteries due to internal and interfacial resistances. The CV profiles at different potential rates showed overpotentials of 0.200 V, 0.221 V and 0.237 V for PFA/HC, PAA/HC and PVDF/HC, respectively. These results suggest that PFA/HC has less overpotential than PAA/HC and PVDF/HC due to the presence of dense polar groups on the surface and in the electrode domain which is helpful for facile sodium ion transport. In addition, it prevents continuous side reactions between the electrolyte and active material.^34,56

Fig. 3 Cyclic voltammogram (CV) curves of (a) PFA/HC, (b) PAA/HC and (c) PVDF/HC. Insets: CV at 0.1 mV s⁻¹. (d) EIS profiles before and after CV for PFA/HC, PAA/HC and PVDF/HC. (e) Arrhenius plots at various temperatures. (f) Warburg plots deduced from the low-frequency portion of (d).

Furthermore, to investigate the Na⁺ storage kinetics in the HC anode and SEI layer formation during cycling, electrochemical impedance spectroscopy (EIS) was performed. The Nyquist plots of before and after CV are depicted in Fig. 3(d). The charge transfer from the electrolyte into the active material through SEI is in the mid-frequency range (∼1 kHz to 10 mHz), represented by a semicircular arch in the impedance spectrum.⁵⁷ A smaller semicircle diameter corresponds to a lower charge transfer resistance (R_CT). All the anodic half-cells showed lower charge transfer resistance after the CV impedance than the before the CV cycle impedance. The SEI formed shows the lowest R_SEI (18.1 Ω) and R_CT (673.4 Ω) in PFA/HC compared to those for PAA/HC (R_SEI = 22.3 Ω, R_CT = 4051 Ω) and PVDF/HC (R_SEI = 31.6 Ω, R_CT = 412.2 Ω) by circuit fitting of EIS after the CV cycles (Table S1†). This implies that the dense carboxylic acid groups might influence the formation of a thin SEI layer on the surface of the HC electrode consisting of PFA binder and will enhance the sodium-ion transport process across the electrode–electrolyte interface. To understand the effect of the binder on Na⁺ desolvation and ion crossing in the SEI, the activation energy was calculated using the following equation:

where σ corresponds to the conductivity or R_CT⁻¹, A denotes the pre-exponential entropic factor, R is the gas constant (8.314 J K⁻¹ mol⁻¹), E_a denotes the activation energy and T is temperature. Using Arrhenius's law, the activation energies of the anodic half-cells were calculated from the R_CT obtained from circuit fitting (Table S2†) of the EIS data measured at different temperatures (Fig. S5† and 3(e)). The activation energy of PFA/HC is 50.7 kJ mol⁻¹, which is lower than those of PAA/HC (52.9 kJ mol⁻¹) and PVDF/HC (58.6 kJ mol⁻¹). Further, the sodium-ion diffusion coefficient was calculated by analysing the EIS after CV at low frequency through the following equation:where D is the diffusion coefficient, R is the universal gas constant, T is the temperature, A is the surface area of the electrode, n is the number of electrons taken per Na⁺, F is Faraday's constant, C is the Na⁺ electrolyte concentration and σ is the slope obtained by fitting ω^−1/2vs. Z_real. The Na⁺ ion diffusion coefficient for PFA/HC is estimated to be 1.90 × 10⁻¹³ cm² s⁻¹ compared to 1.75 × 10⁻¹³ cm² s⁻¹ for PAA/HC and 8.80 × 10⁻¹⁴ cm² s⁻¹ for PVDF/HC (Fig. 3(f)). The dense polar carboxylic acid functional groups in the PFA polymer offer reversible weak bonding with Na⁺ during sodiation/desodiation in the HC electrodes and act as a self-healing component by hydrogen bonding during volume change. Thereby, the continuous carboxylic acid groups facilitate Na⁺ transport within the polymer domain via a surface hopping mechanism, resulting in a high diffusion coefficient and lower activation energy at the surface of the electrode.^58,59 The R_SEI, R_CT, E_a and D_Na⁺ values reveal the formation of a thin SEI due to the influence of the dense carboxylic acid groups present on the surface of the HC electrodes.

The galvanostatic charge–discharge curves of the first two cycles at 10 mA g⁻¹ are depicted in Fig. 4(a)–(c). The first sodiation capacities are 441.9 mA h g⁻¹, 404.7 mA h g⁻¹ and 273.1 mA h g⁻¹ with ICE values of 80.8%, 75.7% and 76.3% for PFA/HC, PAA/HC and PVDF/HC, respectively. The low ICE values for PAA/HC and PVDF/HC indicate that a large amount of SEI was accumulated in the first cycle. As expected, two sodium storage regions are observed in the sloping region (>0.1 V) and plateau region (<0.1 V). In some recent articles, the sloping region is ascribed to Na intercalation into a misaligned graphene layer, whereas the plateau region corresponds to the insertion of Na into HC closed pores.⁶⁰ The sloping region exhibits a lack of reversibility during the initial cycle, indicating that it possesses a high surface area that develops a SEI, resulting in a low ICE. This SEI film formation mainly impacts the specific capacity in the sloping region. Moreover, it contains large defects and oxygen-related functional groups that could entrap sodium. PFA/HC exhibits the highest plateau capacity with better reversibility. Both slope and plateau capacity are considered for calculating coulombic efficiency. The impact of the irreversible capacity on the ICE at high voltage was mitigated by the improved plateau capacity of PFA/HC. A low discharge voltage and longer plateau region are important for high-output-voltage, high-ICE and high-energy-density batteries. However, the capacity from the slope region limits energy density.¹⁹ To further understand the impact of the plateau region on the ICE, the percentage and capacity of the plateau region in the initial sodiation cycle were determined and found to be 52.4% and 214.7 mA h g⁻¹ for PFA/HC, 48.4% and 196 mA h g⁻¹ for PAA/HC and 46.4% and 126.8 mA h g⁻¹ for PVDF/HC. The values indicate that the high percentage of plateau region resulting from sodiation in the PFA/HC helps in increasing the ICE and minimising the impact of SEI from the slope region.²⁰ In addition, the high capacity in the plateau region obtained by PFA/HC is due to Na⁺ transport to all the edges of the graphene layers and enhanced intercalation of ions. An electrolyte reduction peak was observed in the CV curve between 0.7 and 0.01 V in the first cycle. A similar irreversible capacity peak occurs in the first sodiation in all the anodic half-cells containing different binders.⁶¹

Fig. 4 Galvanostatic charge–discharge profiles at 10 mA g⁻¹ of (a) PFA/HC, (b) PAA/HC and (c) PVDF/HC. (d) Rate performances at different current densities of the different anodic half cells. Long-cycle performances at current densities of (e) 30 mA g⁻¹ and (f) 60 mA g⁻¹. dQ/dV curves at different cycles for (g) PFA/HC, (h) PAA/HC and (i) PVDF/HC.

To compare the electrochemical properties and structural stability of HC containing different binders, rate capabilities and long cycling tests were further conducted. The rate capability of the HC electrodes was evaluated at different current densities from 10 to 200 mA g⁻¹ (Fig. 4(d)). The discharge capacity of PFA/HC reaches 332 mA h g⁻¹ at a current density of 10 mA g⁻¹, which decreases to 313, 303, 251, and 132 mA h g⁻¹ at 20, 30, 60, and 150 mA g⁻¹, respectively. When the current density is 20 times the initial current density, a capacity of 119 mA h g⁻¹ is achieved. For comparison, PAA/HC and PVDF/HC showed poorer rate performance (Table S3†). At a current density of 10 mA g⁻¹, the rate capacity of PAA/HC and PVDF/HC are 316 and 208 mA h g⁻¹ respectively, which decline to 105 and 67 mA h g⁻¹, respectively, at the current density of 200 mA g⁻¹ (Table S3†) (Fig. 4(d)). The exceptional rate performances of PFA/HC are mainly due to the dense polar carboxylic acid groups accelerating the ion transport rate. The PFA binder achieved good results in terms of ICE and rate capability, which are challenging to achieve in HC, due to the significant effect of the dense carboxylic acid groups in the formation of crosslinking via hydrogen bonding, influencing the formation of a thin SEI and enhancing the ion transmission. To evaluate the service lifespan of the HC electrode, Fig. 4(e) and (f) display the long cycling capacities at current densities of 30 mA g⁻¹ and 60 mA g⁻¹, respectively. PFA/HC achieved higher stable reversible specific capacities of 288 mA h g⁻¹ and 254 mA h g⁻¹ at 30 mA g⁻¹ and 60 mA g⁻¹, respectively, than PAA/HC (260 mA h g⁻¹ at 30 mA g⁻¹ and 224 mA h g⁻¹ at 60 mA g⁻¹) and PVDF/HC (171 mA h g⁻¹ at 30 mA g⁻¹ and 145 mA h g⁻¹ at 60 mA g⁻¹). After 250 cycles, the capacity retentions of PFA/HC, PAA/HC and PVDF/HC at 60 mA g⁻¹ are 85.4%, 46.1% and 74%, respectively. At 30 mA g⁻¹, the capacity retentions of PFA/HC, PAA/HC and PVDF/HC after 200 cycles are 91.6%, 90.3% and 85.1% respectively. The differential capacity profiles of the different cycles for the sodiation of the anodic half-cells clearly indicate the loss of active material from the HC electrode in PAA/HC (Fig. 4(h)) and PVDF/HC (Fig. 4(i)), as the peak height decreases at approximately constant voltage. In contrast, the dQ/dV peaks of PFA/HC (Fig. 4(g)) remain almost constant, which is attributed to the lack of loss of electrical contact in the electrode. The dense hydrogen bonding within the electrode system holds the electrode materials together and enhances the stability of the HC electrode during cycling.^62,63 To evaluate the influence of binder loading on specific capacity and stability, charge–discharge cycles were carried out using 3, 5 and 15 wt% of PFA, as shown in Fig. 5(a). Interestingly, increasing the binder loading to 15 wt% results in higher specific capacity, showing a capacity of 267 mA h g⁻¹, which is higher than that for 10 wt% PFA. In contrast, decreasing the binder content to 5 and 3 wt% led to a decrease in capacity to 249 mA h g⁻¹ and 231 mA h g⁻¹, respectively, compared to 10 wt% PFA. This signifies that increasing the amount of binder led to obtaining higher specific capacity, as increasing the binder content does facilitate Na⁺ transport to additional activation sites, whereas decreasing binder content lowers the ion transport to several active sites. All wt% of PFA showed higher capacity than the 10 wt% PVDF and 10 wt% PAA binders. However, a decrease in stability was observed in the case of 3 wt% after 25 cycles due to insufficient binder content to hold the electrode material together. The compatibility of a binder with various types of electrolytes is important to understand the adaptability of the binder to various SIB chemistries. Fig. 5(b) shows the charge–discharge performance of 10 wt% PFA binder in various electrolytes, namely, 1 M NaClO₄ in EC:DMC, 1 M NaTFSI in EC:PC, 1 M NaTFSI in EC:DMC, 1 M NaTFSI in EC:DEC and 1 M NaClO₄ in EC:DEC. The use of the electrolytes 1 M NaTFSI in EC:PC, 1 M NaTFSI in EC:DEC and 1 M NaClO₄ in EC:DEC showed better capacity and stability than the electrolytes containing EC:DMC. In both Komaba et al.'s study and ours, the EC:DMC solvent system results in reduced capacity due to severe decomposition of DMC, leading to the formation of Na alkoxide and Na alkyl on the HC.⁵⁴ The results suggest that the PFA-containing electrodes showed better performance and compatibility with the solvents EC:PC and EC:DEC containing NaClO₄ and NaTFSI salts than with the EC:DMC solvent system. These galvanostatic charge–discharge results show that the dense functional groups inhibit the loss of irreversible capacity and promote high capacity retention during long cycling. The PFA binder helps Na⁺ to percolate into every graphitic domain of the HC and increases the specific capacity through ion conduction by the consecutive presence of carboxylic acid in the polymer backbone. Moreover, PFA keeps the electronic conductivity intact during long cycling and facilitates Na⁺ transport, which has been proven to be an effective approach for low-temperature performance.⁶⁴ Therefore, using the PFA binder led to better rate capability, electrochemical stability and specific capacity.

Fig. 5 (a) Long cycling of PFA/HC containing various amounts of PFA binder. (b) Long cycling of PFA/HC in different electrolyte systems. R_SEIvs. potential plots of the (c) sodiation and (d) desodiation of cycled anodic half cells.

The properties of SEI and its effects on Na⁺ diffusion into electrodes were determined by DEIS. Nyquist impedance plots at different potentials within the range 0.01–2.5 V are plotted as shown in Fig. S6.† The different EIS curves of different anodic half cells at different potentials are due to dynamic interphase performance. Fitting of impedance data to an equivalent circuit was conducted, and the resulting internal resistance (R_int), SEI resistance (R_SEI), charge transfer resistance (R_CT) and diffusion resistance (R_diff) values are tabulated in Tables S4–S9.† The R_SEIvs. potential during sodiation and desodiation are plotted as shown in Fig. 5(c) and (d), and it was observed that PFA/HC has a lower R_SEI compared to PAA/HC and PVDF/HC. This suggests that an SEI with high Na⁺ ion transference is formed on the HC electrode surface, leading to better reversible capacity and rate performance in PFA/HC. The dense polar carboxylic groups influence the R_SEI by lowering the activation energy and resistance due to greater interaction with Na⁺. The R_SEIvs. potential plots showed more stable R_SEI values for PFA/HC compared to PAA/HC and PVDF/HC owing to presence of a uniform layer of SEI on the surface of the PFA/HC electrode, which is non-uniform on the PAA/HC and PVDF/HC electrodes. Elevated R_SEI values were noted for PFA/HC (Fig. 5(d)) within the desodiation voltage range of 0.01–0.5 V. This observation can likely be attributed to the numerous interactions between Na⁺ ions and the densely packed polar functional groups of PFA. These interactions may impede the desodiation process from the SEI within that specific potential range.

PFA enhances the cell performance due to various factors: (a) the dense carboxylic groups in PFA serve as polyanions that cover the surface of HC and act as part of the SEI. (b) The solvated positively charged Na⁺ interacts with the unpaired electrons found in the negatively polarised oxygen atoms of the –COOH group, accelerating the desolvation and intercalation of Na⁺ into HC. These factors induce the formation of a thin and stable SEI from the electrolyte decomposition and enhance the charge transfer of Na⁺ across the electrode–electrolyte interface.⁶⁵ The presence of continuous carboxylic acid groups on the polymer chain enhances Na⁺ diffusion within the electrode, which increases the specific capacity, lowers the polarization and enhances the SIB performance. In addition, the dense carboxylic acid forms many inter/intra-polymer complexes through hydrogen bonding, increasing structural integrity and capacity retention.⁶⁶

The interphase analysis of the HC electrodes would provide details on the dissimilarity of the surface chemistry that has occurred due to the presence of the various binders. The binders might influence the electrolyte decomposition mechanism. Thus, interfacial properties such as SEI composition and electrode surface morphology after cycling might have changed and require analysis. To investigate the interfacial composition chemistry, XPS analysis was performed on cycled HC electrodes with PFA, PAA and PVDF binders. The high-resolution spectra and binding energies of the deconvoluted peaks are presented. The C 1s spectra (Fig. 6(a)–(c)) of all the cycled HC electrodes with various binders showed typical species such as sp³ C–C, C–O, C=O and Na₂CO₃. Here, the presence of C–O and C=O is believed to result from electrolyte decomposition on the electrode surface, as supported by the evidence provided by the O 1s spectra (Fig. S7†).^67,68 The detection of a C=C peak from HC that was found only in the C 1s spectrum of PFA/HC indicates that the X-rays can scan beneath the thin SEI formed in the PFA/HC electrode. The higher concentration of C–O and C=O observed in the cycled electrode with the PFA binder could originate from the dense carboxylic groups present in the PFA binder and from the carbonate species present in the SEI. The C–F peak is only observed for the cycled electrode containing PVDF binder, indicating that the peak might be from the PVDF binder functional group. In addition, PFA/HC has a thick layer of inorganic species (Na₂CO₃, Na₂O and NaCl) as confirmed by the composition percentages (Tables S10 and S11†) of the C 1s, O 1s and Cl 2p spectra (Fig. S7†). The presence of rich inorganic species in the interior of the double layer of SEI facilitates rapid transport of Na⁺ and inhibits continuous electrolyte decomposition.^69,70 The composition percentage of ROCONa, C–O and C=O in O 1s spectra of PAA/HC and PVDF/HC are higher, suggesting that SEI contains a large organic-carbonate-derived component due to constant degradation of the organic solvent (Table S11†).^71,72 A Na KLL auger peak also appears in the O 1s spectra for all the electrodes.⁴¹ The thick inorganic layer beneath and thin outer organic species SEI of PFA/HC contribute towards the robust SEI during the long-term cycling of the HC electrode. As the SEI is dynamic in nature, the outer organic layer is dissolved or stripped during discharge, leaving the inner inorganic layer exposed directly to the electrolyte. Hence, a rich thick inorganic layer with a high modulus strength is needed to prevent continuous electrolyte decomposition and enhance rate capability and Na⁺ transport into the HC.⁷³ A morphology study of the cycled HC electrodes with PFA, PAA and PVDF binders shed light on the electrochemical performances of the respective coin cells. After 300 cycles the coin cells were disassembled, and the electrode surface morphologies were observed under SEM. The freshly prepared HC electrodes with varied binders were shown to have a uniform, porous structure with good adherence to the current collector (Fig. S8(a₁), (b₁) and (c₁)†). The SEM images of the cycled PFA/HC (Fig. S8(a₂) and (a₃)†) electrode indicate uniform SEI formation due to electrolyte reduction and a crack-free electrode surface. In contrast, the PAA/HC (Fig. S8(b₂) and (b₃)†) and PVDF/HC (Fig. S8(c₂) and (c₃)†) electrodes showed cracks from the top view. Note that the thread-like particles observed in the SEM images on top of the cycled electrode surface are from the glass fibre of the separator. The cross-sectional view of the post-cycled PAA/HC (Fig. 6(e)) and PVDF/HC (Fig. 6(f)) electrodes showed massive cracks and detachment/peeling from the current collector compared to pristine electrodes (Fig. S9(b) and (c)†). In contrast, the cycled PFA/HC (Fig. 6(d)) electrode remains crack-free and maintains its structural integrity with intact electrical contact to the current collector compared to the pristine electrode (Fig. S9(a)†), which helps to enhance the cycling performance. The PFA binder firmly holds the electrode materials to the current collector surface and maintains the integrity of the HC electrode. This might be due to the many chemical bonds formed between the polar groups of the binder and the current collector, while only intermolecular forces form between PVDF and the current collector. The interface between the electrode material and SEI film cannot be identified because the thickness of the SEI is ∼8 nm, making the interphase impossible to observe using SEM.⁷⁴ Additionally, the cross-link formation via inter- and intramolecular hydrogen bonding of the PFA binder has a greater modulus and hardness, helping to alleviate volume change stress during sodiation/desodiation of the HC electrode. Changes in the electrode morphology and SEI after charging and discharging were observed. The formation of SEI film on the surface of the electrodes containing different binders can be confirmed by the slight disappearance of the electrode surface roughness compared to the pristine electrodes. Additionally, crack formation on the material after long cycling is observed in PAA/HC and PVDF/HC, but is mitigated in the PFA/HC. Therefore, the dense hydrogen bonding and strong interactions with the current collector preserve the integrity of the electrode material.⁷⁵

Fig. 6 XPS profile of the cycled HC electrodes with different binders: (a) PFA, (b) PAA and (c) PVDF. Cross-sectional SEM of the cycled HC electrodes with (d) PFA, (e) PAA and (f) PVDF binders.

Conclusions

In summary, we utilised the polymer poly(fumaric acid) with dense polar carboxylic acid functional groups as a promising, cost-effective and environmentally friendly binder for HC anodes in sodium-ion batteries. The less-flexible polymer backbone and high concentration of functional groups resulted in better rheology and electrochemical study results owing to the interaction of the carboxylic acid with the electrode component, and more importantly, with Na⁺. PFA/HC exhibits a higher ICE of 80.8% and delivers higher reversible rate capacities of 332, 313, 303, 251, 132 and 119 mA h g⁻¹ at current densities of 10, 20, 30, 60, 150 and 200 mA g⁻¹ compared to PAA/HC and PVDF/HC. Moreover, PFA/HC retains 85.4% of its reversible specific capacity for more than 250 cycles of sodiation/desodiation at a higher current density of 60 mA g⁻¹ with a highest specific capacity achieved of up to 254 mA h g⁻¹. The enhanced specific capacity is closely related to the continuous carboxylic acid groups present in the backbone of the polymer, which form an ion path for augmenting the Na⁺ migration and transporting ions to every edge of the disordered graphite. The PFA binder modifies the SEI, leading to enhanced Na⁺ kinetics at the interface, such as a lower activation energy of 50.7 kJ mol⁻¹, higher Na⁺ ion diffusion coefficient of 1.90 × 10⁻¹³ cm² s⁻¹ and lower R_SEI compared to PAA/HC and PVDF/HC. Due to the passivation of HC defects by the formation of dense hydrogen bonding with the oxygen functional groups at the surface, continuous electrolyte decomposition was suppressed due to the reduced contact between the electrode and electrolyte. The cycling stability was improved owing to inorganic-rich SEI, and the maintenance of the electrode integrity by dense hydrogen bonding within the electrode was confirmed by XPS and SEM, respectively. Hence, the present study on water-soluble PFA binder opens an area of design development using polymer dense functional polar groups for better electrochemical performance in sodium-ion batteries, such as enhanced rate capability and specific capacity, thereby showing great potential for the practical application of sodium-ion batteries.

Author contributions

A. P.: conceptualization, methodology, polymer synthesis, electrochemical data analysis and writing. N. M.: conceptualization, supervision, scientific discussion and writing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The author is thankful to the financial support provided by the Ministry of Education, Culture, Sports, and Technology (MEXT), Japan.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta00285g

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