Polyethylene Glycol Surface Modification and Polythiophene Side-Chain Chemistry: A Combined Strategy toward High-Capacity Lithium-Ion Battery Anodes

Abstract

In the development of high-capacity lithium-ion batteries (LIBs), the combined optimization of active material interfaces and polymer binder chemistry plays a critical role in improving electrode performance and longevity. This work explores a dual design strategy incorporating polyethylene glycol (PEG) surface modification and carboxylated polythiophene side-chain tailoring to enhance the electrochemical behavior of magnetite (Fe₃O₄)-based anodes. PEG is employed to improve interfacial stability, while carboxylated polythiophene binders with varying alkyl side-chain lengthspoly­[3-(potassium-4-butanoate)­thiophene-2,5-diyl] (P3KBT), poly­[3-(potassium-5-pentanoate)­thiophene-2,5-diyl] (P3KPT), and poly­[3-(potassium-6-hexanoate)­thiophene-2,5-diyl] (P3KHT)are used to modulate molecular interactions and ion transport. Among these three analogs, the PEG–Fe₃O₄–P3KHT electrode exhibits superior ion-transfer kinetics, the highest capacity retention, and the lowest charge-transfer resistance after extended cycling. Compared to their non-PEG analogs, PEG-coated electrodes demonstrate enhanced structural integrity and electrochemical behavior, emphasizing the synergistic effects of surface modification and side-chain chemistry. These findings highlight the importance of interfacial interactions and molecular design in achieving robust and high-performance composite anodes for next-generation LIBs.

1. Introduction

The need to reduce greenhouse gas emissions and address the global energy crisis has accelerated the urgent demand for advanced, high energy density, sustainable storage systems. Due to their extended cycle life and reliability, lithium-ion batteries (LIBs), which typically rely on graphite-based anodes, dominate the portable electronics and electric vehicle markets. Graphite, however, possesses a relatively low theoretical specific capacity (372 mAh g^–1) that significantly limits its potential for applications requiring higher energy storage capabilities. Thus, alternative high-capacity anode materials, particularly magnetite (Fe₃O₄), nickel oxide (NiO), manganese oxide (MnO), and silicon (Si), have attracted considerable attention owing to their substantially higher theoretical capacities (∼926 mAh g^–1 for Fe₃O₄, ∼718 mAh g^–1 for NiO, ∼756 mAh g^–1 for MnO, ∼4200 mAh g^–1 for Si), , natural abundance, and environmental compatibility. Nevertheless, the practical implementation of metal oxides for anode applications remains challenging because of their poor intrinsic electronic conductivity and severe volumetric expansion/contraction (∼200% for Fe₃O₄) during lithium insertion/extraction cycles, leading to rapid capacity fade and electrode disintegration.

To address these challenges, recent strategies have focused on incorporating conductive polymers into composite electrode designs, aiming to mitigate volume fluctuations and enhance both ion and electron transport. Previous studies have demonstrated that the use of thiophene-based conductive polymers, such as poly­[3-(potassium-4-butanoate)­thiophene-2,5-diyl] (P3KBT), poly­[3-(potassium-5-pentanoate)­thiophene-2,5-diyl] (P3KPT), and poly­[3-(potassium-6-hexanoate)­thiophene-2,5-diyl] (P3KHT), as binder components in Fe₃O₄-based composite electrodes leads to enhanced capacity retention, improved ion transport, and superior structural integrity, with P3KHT exhibiting the most favorable performance across all three aspects. Furthermore, surface modification strategies, including the use of polyethylene glycol (PEG) as an active material coating, have proven beneficial in stabilizing interfaces. It was presumed that the incorporation of PEG facilitated lithium-ion transport , and reduced electrode resistance, thereby contributing to significantly improved electrochemical behavior.

Motivated by reported results surrounding the impact of alkyl carboxylate side-chain length on the electrochemical performance of magnetite-based anodes, coupled with the potential for added benefits regarding incorporation of PEG, this work systematically investigates synergies derived from the use of carboxylated polythiophene functionalized binders coupled with PEG as a surface modification agent for Fe₃O₄. Specifically, composite electrodes comprising PEG-coated Fe₃O₄ particles and carbon were fabricated with alternative carboxylated polythiophene binders featuring varying alkyl side-chain lengths (collectively termed P3KAT (A = B, P, H)). The interplay between the PEG coating and carboxyalkyl side-chain length of the polythiophene binders was systematically explored through a combination of structural, interfacial, and morphological characterization along with electrochemical evaluation. The results demonstrate that the incorporation of PEG consistently enhances overall electrode performance compared to non-PEG systems and further reveals that seemingly minor changes in the alkyl side-chain length of the carboxylated polythiophene binders markedly affect ion transport, structural stability, and capacity retention.

2. Experimental Section

2.1. Materials

Fe₃O₄ nanoparticles (∼10 nm) were synthesized by a previously reported coprecipitation process using an aqueous solution of iron­(III) chloride hexahydrate, iron­(II) chloride hexahydrate, and triethylamine. , P3KBT (M _w: 19 kDa, polydispersity: 2.4, regioregularity: 89%), P3KPT (M _w: 28 kDa, polydispersity: 2.0, regioregularity: 89%), and P3KHT (M _w: 34 kDa, polydispersity: 2.3, regioregularity: 89%) were purchased from Rieke Metals Inc. PEG (M _r: 1500, 50% w/v) was purchased from Sigma-Aldrich. Carbon black (Super P) was purchased from MTI. CR2032 coin cell shell was purchased from AME Energy. Lithium metal foil was purchased from Honzo Chemical Corp. 1 M Lithium hexafluorophosphate (LiPF₆) in a mixture of ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1 v/v %) was purchased from Sigma-Aldrich.

2.2. Preparation of PEG-Fe₃O₄ Particles

For the preparation of Fe₃O₄ particles coated with PEG (PEG-Fe₃O₄), 1 g of Fe₃O₄ powder in 10 g of deionized (DI) water (purchased from Sigma-Aldrich) was immersed in an ice bath and sonicated for 1.5 min with an ultrasonic probe (3 pulses of 30 s each, operated at 40% amplitude, Fisher Scientific Sonic Dismembrator Model 500). PEG 1500 solution (2 mL; 50% w/v, Sigma-Aldrich) was added with sonication for 30 s, and this process was repeated four times until the total amount of PEG 1500 was 8 mL. The PEG-Fe₃O₄ powders were washed and extracted using acetone by centrifuge separation (Beckman Coulter Optima XPN-90 Ultracentrifuge) 3 times at a speed of 6000 rpm for 5 min.

2.3. Materials Characterization

Thermogravimetric analysis (TGA) of PEG-Fe₃O₄ was conducted by using a TA Instruments TGA Q500 analyzer. Measurements were performed from 20 to 700 °C at a heating rate of 10 °C/min under a nitrogen atmosphere with a flow rate of 70 mL/min. An isothermal step at 100 °C was maintained for 15 min to ensure complete removal of residual moisture. The PEG-Fe₃O₄ particles comprised 9 wt % PEG. Fourier transform infrared (FT-IR) spectra were collected using a Thermo Scientific Nicolet iS50 FT-IR spectrometer. Data were collected by 32 scan cycles from 750 cm^–1 to 4000 cm^–1 with a resolution of 0.4 cm^–1. X-ray photoelectron spectroscopy (XPS) was performed using a Specs XPS system with an Al Kα monochromatic beam (1486.6 eV). XPS spectra were fitted using an XPSPEAK 41. Ultraviolet–visible (UV–vis) spectra of polymer films were obtained using an Agilent Cary 5000 spectrophotometer, which has a 175 to 3300 nm spectrophotometer wavelength and a 0.01 to 5 nm UV–vis spectral bandwidth. Raman spectra of polymer films were obtained by using a Horiba LabRAM Odyssey confocal Raman microscope equipped with an air-cooled Ar-ion laser operating at 633 nm. The contact angle of polymer films was measured by a Rame-Hart instrument 210. The corresponding polymer films were obtained by spray coating (Iwata Eclipse HP-CS airbrush, gravity feed) aqueous solutions of the polymers onto precleaned glass slides for measurements. The solutions of either P3KBT, P3KPT, or P3KHT were prepared in DI water (2 mg mL^–1) and stirred overnight at room temperature. For blends of PEG and carboxylated polythiophene, solutions were prepared in DI water (total weight of 2 mg mL^–1 with a PEG-P3KAT weight ratio of 1:2) and stirred overnight at room temperature. To calculate the Hansen solubility parameters (HSPs), 10 mg of a polymer and 5 mL of a test solvent were placed into a vial. The vial was sonicated for 1 h and then placed in an exhaust hood to stand overnight. The pictures were taken after sonication and after standing overnight to determine compatibility of the polymer and solvent. In the case of the polymer, solvents were considered poor if they were unable to dissolve the polymer after the dissolution process and good if they were able to dissolve the polymer. Based on the visual examination, HSPs (δ_D, δ_P, and δ_H) and the radius (R _o ) of the sphere of interaction were calculated and fitted by HSPiP software (Hansen Solubility Parameters in Practice sixth edition).

2.4. Electrode Fabrication

Composite electrode slurries were prepared by mixing PEG-Fe₃O₄ (0.14 g), Super P (0.03 g), and either P3KBT, P3KPT or P3KHT in DI water (10 wt % solution, 0.3 g) with a weight ratio of PEG-Fe₃O₄/carbon/polymer = 70:15:15. The 15 wt % carbon additive loading aligns with typical settings used to explore properties of new electrode materials. The composite electrodes were fabricated by blade coating (MTI) the slurry onto copper foil (MTI), and the samples were dried in ambient at room temperature for 1 h, followed by 110 °C for 12 h in a vacuum oven (Thermo Fisher Scientific) at 20 kPa. The active material mass loading was 0.7–1.2 g/cm². Fe₃O₄–P3KAT electrodes were prepared following the method described in the literature.

2.5. Electrochemistry

CR2032 coin cells were used for the electrochemical measurements. Lithium metal was used as the counter electrode, and 1 M LiPF₆ in EC/DEC (1:1 v/v %) was adopted as the electrolyte. During the assembly, 80 μL of the electrolyte was added to the cell and a propylene membrane was used as the separator (Celgard 2400). All assembly was conducted inside an argon-filled glovebox (H₂O and O₂ less than 0.5 ppm). The as-assembled cells were stored in air overnight before measurement. For all electrochemistry characterization, the C rate was calculated using the theoretical capacity of magnetite (926 mAh g^–1) and the active material composition (∼63.7 wt %) of the total electrode mass. Note that the proportion of Fe₃O₄ in the composite was determined by TGA analysis of PEG-Fe₃O₄ (vide supra). Constant current measurements were performed using an MZTC Multi-Chamber purchased from Arbin. Before electrochemical testing, all coin cells were charged and discharged at a rate of 0.01 C to conduct one formation cycle (or two cycles for cycling performance). The tests then proceeded for cycling or rate capability. All cycling performance (300 cycles) was conducted at a rate of 0.3 C in a voltage range of 0.01–3 V vs Li/Li⁺, whereas rate capability tests were conducted at varying charge/discharge rates (0.1, 0.3, 0.5, 1.0, and 2.0 C). Cyclic voltammetry (CV) was performed in the potential range of 0.01–3 V versus Li/Li⁺ at scan rates of 0.1, 0.3, 0.5, 0.7, 1.0, 1.5, and 2.0 mV s^–1. Electrochemical impedance spectroscopy (EIS) measurements were conducted using a PMC 200 (AMETEK) in the frequency range of 0.1 MHz to 0.1 Hz with open-circuit potential as direct current voltage and 10 mV root-mean-square (RMS) alternating current amplitude. Nyquist plots were fitted using ZView 3.

2.6. Electron Microscopic Characterization

Surface and cross-sectional field-emission scanning electron microscopy (FE-SEM) images of the electrodes were obtained using a Hitachi 4300 SE/N FE-SEM at an accelerating voltage of 10 kV in the high-vacuum mode at room temperature. FE-SEM images of electrodes after cycling were obtained from cells disassembled in an argon-filled glovebox. Samples were rinsed with 5 mL of anhydrous DEC (99%; Fisher Scientific) and subsequently dried overnight inside the glovebox. Transmission electron microscopy (TEM) images were collected using a JEOL JEM2100 instrument operating at an accelerating voltage of 200 kV. Samples for TEM analysis were prepared by drop-casting aqueous suspensions of Fe₃O₄ or PEG-Fe₃O₄ onto ultrathin holey carbon films supported on copper mesh TEM grids (TED Pella, 400 mesh Cu), followed by drying overnight under vacuum at 20 kPa.

3. Results and Discussion

3.1. PEG-Fe₃O₄ Preparation and Materials Characterization

To build on prior studies that demonstrated the impact of P3KAT alkyl spacer length on composite anode performance and possible synergistic benefits associated with the incorporation of PEG as an ion conducting active material coating, composite Fe₃O₄-based anodes were fabricated and characterized to determine their electrochemical performance. Facile introduction of PEG onto the surface of Fe₃O₄ to create an ion conducting shell surrounding well-dispersed aggregates of the active material was performed as presented schematically in Figure a. The presence of an amorphous PEG coating on the Fe₃O₄ surface was confirmed by TEM (Figure b,c). Note the appearance of an additional layer on the Fe₃O₄ surface pointed to by red arrows (Figure c) after sonication of the aqueous Fe₃O₄ dispersion in the presence of PEG. XPS O 1s spectra provided evidence of direct interactions between Fe₃O₄ and PEG (Figure d). In the presence of PEG, the Fe–O binding energy experienced a slight shift from ∼529.5 eV to ∼529.8 eV, suggesting changes in the Fe–O bond length in Fe₃O₄ vs PEG-Fe₃O₄. The observed phenomenon likely arises from electron redistribution due to coordination between Fe atoms and the O atoms in PEG. FTIR spectra (Figure S1) provided further support for the potential binding of PEG with Fe₃O₄. As expected, C–H and C–O stretching vibrations are absent for Fe₃O₄, however, both are clearly visible in PEG-Fe₃O₄. Specifically, the PEG C–H band at 2883 cm^–1 underwent a red shift to 2869 cm^–1, while the PEG C–O stretch blue-shifted from 1101 cm^–1 to 1096 cm^–1 upon introduction of the oligomer as a coating on the Fe₃O₄ surface. The observed spectral changes suggest differences in the C–O and C–H bond length that may arise from binding of PEG to the nanoparticle surface. , TGA analysis (Figure S2) indicated that the PEG coating accounts for approximately 9 wt % of PEG-Fe₃O₄, providing a quantitative baseline to determine active material loading for subsequent electrochemical characterization.

1.

(a) Schematic of the PEG-Fe₃O₄ particle preparation. TEM images of (b) Fe₃O₄ particles and (c) PEG-Fe₃O₄ particles (red outline represents the PEG coating). (d) XPS O 1S spectra of Fe₃O₄ and PEG-Fe₃O₄.

3.2. Electrochemical Characterization

The electrochemical performance of PEG-Fe₃O₄ was characterized using the three P3KAT binders, namely, P3KBT, P3KPT, and P3KHT (Figure S3), for comparison with electrodes fabricated with Fe₃O₄ in conjunction with the same binders in the absence of PEG.

CV (Figure a–c) was performed in the potential window of 0.01 to 3 V versus Li/Li⁺ for 4 cycles to characterize PEG-Fe₃O₄–P3KAT electrochemical behavior. The first cycle showed specific Fe₃O₄ anodic and cathodic peaks. When P3KBT was introduced as the binder, the cathodic peak gradually broadened and the peak current decreased with increasing cycle number, suggesting a deterioration in electrochemical kinetics. In contrast, the CV curves of the P3KHT analogue retained sharp cathodic peaks with stable peak current, pointing to more robust electrochemical behavior. Electrode rate performance (Figure d) was determined via cycling at current rates, ranging from 0.1 to 2 C. All three PEG-Fe₃O₄–P3KAT electrodes displayed comparable specific capacity ∼1100 mAh g^–1 at 0.1 C. However, distinct capacity differences emerged at higher current rates. Specifically, the PEG-Fe₃O₄–P3KHT electrode maintained a considerable capacity of 720 mAh g^–1 at 2 C compared with its P3KBT (606 mAh g^–1) and P3KPT (624 mAh g^–1) counterparts.

2.

CV profiles of (a) PEG-Fe₃O₄–P3KBT, (b) PEG-Fe₃O₄–P3KPT, and (c) PEG-Fe₃O₄–P3KHT electrodes in the potential window of 0.01 to 3 V versus Li/Li⁺ collected at the rate of 0.1 mV/s. (d) Specific capacity of PEG-Fe₃O₄ electrodes with different binders. (e) Plot of log­(I _pc) vs log­(ν), which were plotted from the results of anodic peak currents of CV curves with different scan rates (ν) of PEG-Fe₃O₄ electrodes with different binders.

The total charge storage within electrodes can be categorized into three components: a Faradaic contribution resulting from Li⁺-ion insertion, a Faradaic contribution arising from charge-transfer reactions at surface atoms (commonly referred to as pseudocapacitance), and a non-Faradaic contribution from electrical double-layer effects. When particle size is reduced to the nanoscale, both capacitive contributions (pseudocapacitance and double-layer capacitance) may become significant due to the substantial increase in the specific surface area. These capacitive effects can be characterized by analyzing CV data collected at various scan rates (Figure S4), as detailed below:

$$i_{\mathrm{p}\mathrm{c}}=a{v}^b$$ 1

The b value in eq was derived from the slope of the log­(i) versus log­(ν) plot for the anodic peaks (Figure e). Qualitatively, the charge storage kinetics mechanism can be inferred based on the b value: a b value of 0.5 indicates that the current i is proportional to the square root of the scan rate, signifying control by semi-infinite linear diffusion, whereas a b value of 1 indicates that the current is surface controlled. In general, a higher b value suggests enhanced surface-controlled charge transfer and more pronounced pseudocapacitive behavior, implying improved ion transport kinetics. The b values for the respective composite electrodes (Table S1) are in the following order: PEG-Fe₃O₄–P3KBT (0.536) < PEG-Fe₃O₄–P3KPT (0.796) < PEG-Fe₃O₄–P3KHT (0.840), suggesting that in the case of PEG-Fe₃O₄ electrodes, P3KHT more effectively facilitates ion transport compared to its shorter side-chain analogs. The observed trend where the b value increases with increasing alkyl spacer length is comparable to that reported for electrodes fabricated in the absence of PEG (see Table S2); however, for all side-chain analogs, incorporation of the PEG coating led to notably higher b values. The results support the hypothesis that the incorporation of PEG helps promote ion transport kinetics and may synergistically work with the P3KAT binders to improve the composite electrode performance.

Electrode cycling stability was evaluated at 0.3 C, after activation for 2 cycles at 0.1 C (Figure a), with individual charge–discharge profiles and Coulombic efficiency provided in Figure S5 and the initial Coulombic efficiency (ICE) of the electrodes shown in Table S3. The ICE of the PEG-coated electrodes was slightly higher than that of non-PEG analogs, suggesting that the PEG coating may help improve interfacial stability. Notably, PEG-Fe₃O₄–P3KHT exhibited stable performance over 300 cycles vs its lower alkyl chain length analogs. The P3KHT electrodes maintained a stable, high reversible capacity, in addition to a slightly increasing specific capacity over 300 cycles. As suggested by the electrochemical test results provided in Figure a, incorporation of PEG provided improved electrode stability vs the non-PEG Fe₃O₄-based systems (Table S4). It is noted that the capacity gradually increases during cycling for PEG-Fe₃O₄–P3KHT, a phenomenon previously observed in the long-term cycling of metal oxide electrodes. − Two possible explanations have been previously proposed: (i) electrode pulverization reduces the nanoparticle size through an electrochemical grinding effect, increasing surface area and thereby exposing more active sites for lithium storage; (ii) the extra lithium storage capacity may arise from spin-polarized surface capacitance associated with magnetic transitions in Fe⁰ nanoparticles, as electrochemically reduced Fe nanoparticles can store a large number of spin-polarized electrons, contributing to excess capacity. ,

3.

(a) Cycling performance of PEG-Fe₃O₄ and Fe₃O₄ electrodes with different binders at 0.3 C between 0.01 and 3 V versus Li/Li⁺. Adapted from Han et al. Polythiophene Side Chain Chemistry and Its Impact on Advanced Composite Anodes for Lithium-Ion Batteries. Chemical Communications 2024 with permission from the Royal Society of Chemistry. EIS Nyquist plots for corresponding electrodes before and after 300 cycles in the frequency range from 0.1 MHz to 0.1 Hz of (b) PEG-Fe₃O₄–P3KBT, (c) PEG-Fe₃O₄–P3KPT, and (d) PEG-Fe₃O₄–P3KHT. Top-view FE-SEM images of (e) PEG-Fe₃O₄–P3KBT, (f) PEG-Fe₃O₄–P3KPT, (g) PEG-Fe₃O₄–P3KHT electrodes before cycling; (h) PEG-Fe₃O₄–P3KBT, (i) PEG-Fe₃O₄–P3KPT, and (j) PEG-Fe₃O₄–P3KHT electrodes after cycling (red circle shows the crack area); (k) Fe₃O₄–P3KBT, (l) Fe₃O₄–P3KPT, and (m) Fe₃O₄–P3KHT electrodes after cycling.

EIS measurements performed before and after cycling provided insights into PEG-Fe₃O₄–P3KAT electrode charge-transfer kinetics in the electrode. As shown in Figure S6, the semicircle in the high-frequency regions represents the charge-transfer resistance (R _ct), while the intercept on the real axis can be assigned to the ohmic resistance (R _s). The equivalent circuit used for fitting is shown in Figure S7, with the corresponding fitting results summarized in Tables S5 and S6. While the PEG-Fe₃O₄–P3KHT electrode exhibited the highest R _ct before cycling, after 300 cycles it showed the lowest R _ct (41.4 Ω) vs the P3KBT (125.9 Ω) and P3KPT (119.1 Ω) analogs. Separate inspection of the EIS data (Figure b–d) reveals that PEG-Fe₃O₄–P3KBT exhibited an increase in R _ct (from 115.2 to 125.9 Ω) after cycling, indicating structural degradation of the electrode. In contrast, the PEG-Fe₃O₄–P3KHT alternative showed a substantial decrease in R _ct, suggesting that it maintained a highly integrated structure during long-term cycling consistent with its superior and increasing cycling capacity.

FE-SEM characterization was used to examine the electrode morphology before and after electrochemical cycling. As presented in Figure e–g, prior to cycling, the PEG-Fe₃O₄–P3KHT electrode exhibited a smoother surface with fewer cracks compared with the shorter P3KAT counterparts. After cycling, all PEG-Fe₃O₄-based electrode morphologies (Figure h–j) displayed a more uniform solid electrolyte interphase (SEI) and fewer cracks than those of the non-PEG, Fe₃O₄-based analogs (Figure k–m), suggesting better structural stability. These results indicate that the presence of PEG plays a key role in maintaining electrode morphology and structural integrity. The differences among the PEG-based electrodes with three different binders after cycling did not appear significant; small cracks on the SEI surface can be observed with higher-magnification images on the surface of the P3KBT and P3KPT analogs (highlighted in red circles). Overall, evaluating capacity retention over extended cycling, charge-transfer kinetics, and structural stability, PEG-Fe₃O₄ electrodes demonstrated highly improved performance vs the Fe₃O₄ counterparts. Specifically, PEG-Fe₃O₄–P3KHT appears to benefit from the synergistic effect of its longer alkyl side-chain and PEG coating, leading to the best structural and electrochemical behavior.

3.3. Elucidating the Origins of PEG-P3KAT Synergistic Impact

The electrochemical results presented above point to favorable synergistic impacts associated with the incorporation of a PEG coating on the active material surface that warrants further analysis. To deconvolute the underlying material chemical origins of the synergies, PEG-Fe₃O₄–P3KAT composites were subjected to FT-IR and XPS analysis. Subsequently, PEG-P3KAT interactions were interrogated using HSP analysis, contact angle measurements, and UV–vis and Raman spectroscopy.

To build on the insights derived from the PEG-Fe₃O₄ FT-IR and XPS results presented above, a series of Fe₃O₄-based electrodes with P3KAT were systematically characterized in the presence and absence of PEG. FT-IR spectra (Figure a–c) of P3KAT and PEG-P3KAT blends confirmed that the C–O stretching bands near 1100 cm^–1 are associated with PEG. The P3KAT carboxylate (O–C–O) asymmetric and symmetric stretching bands are visible around 1560 cm^–1 and 1400 cm^–1, respectively. In the Fe₃O₄–P3KAT electrodes, the asymmetric band of the O–C–O narrowed and experienced a red-shift, with the shift being less pronounced in the PEG-Fe₃O₄–P3KAT case. This observation suggests that the PEG coating may modify the binding environment between the P3KAT carboxylate group and Fe₃O₄. Specifically for the PEG-Fe₃O₄-based electrodes, the P3KBT O–C–O asymmetric stretch shifted from 1557 cm^–1 to 1538 cm^–1; for P3KPT and P3KHT, the band shifted from 1557 cm^–1 to 1542 cm^–1 and 1558 cm^–1 to 1552 cm^–1, respectively. The decreasing extent of red-shift with increasing side-chain length suggests a change in how the carboxylate groups interact with PEG and/or Fe₃O₄. Conceivably, the longer side-chain polythiophenes such as P3KHT have greater affinity for PEG vs the active material, and their engagement may be attributed to this preferential interaction. XPS C 1s spectra (Figure d–f) confirmed the presence of chemical interactions between PEG, P3KAT, and the Fe₃O₄ surface. The increased intensity of the C–C peak at 284.4 eV suggests an enriched level of polymer at the electrode surface. The C–C peak was more intense in PEG-Fe₃O₄–P3KAT electrodes vs the non-PEG counterparts, confirming the presence of PEG on the Fe₃O₄ surface, which may facilitate ion transport and improve structural integrity of electrodes. In comparison to the parent polymers, all electrodes exhibited shifts in the CO and C–O binding energies, pointing to chemical bonding between the polythiophene carboxylate groups and the Fe₃O₄ surface.

4.

FT-IR spectra of materials (upper lines: P3KATs, PEG-P3KAT) and electrodes (bottom lines: Fe₃O₄–P3KAT, PEG-Fe₃O₄–P3KAT): (a) P3KBT, PEG-P3KBT, Fe₃O₄–P3KBT, and PEG-Fe₃O₄–P3KBT; (b) P3KPT, PEG-P3KPT, Fe₃O₄–P3KPT, and PEG-Fe₃O₄–P3KPT; (c) P3KHT, PEG-P3KHT, Fe₃O₄–P3KHT, and PEG-Fe₃O₄–P3KHT. XPS C 1S spectra of (d) P3KBT, Fe₃O₄–P3KBT, and PEG-Fe₃O₄–P3KBT; (e) P3KPT, Fe₃O₄–P3KPT, and PEG-Fe₃O₄–P3KPT; (f) P3KHT, Fe₃O₄–P3KHT, and PEG-Fe₃O₄–P3KHT.

HSP spheres, as shown in Figure a–c, provided insights into the physical affinity between PEG and P3KAT. The dispersive, polar, and hydrogen-bonding parameters are denoted as δ_D, δ_P, and δ_H, respectively, and their calculated values are summarized in Tables S7–S10. Typically, a significant overlap between solubility spheres points to strong physical attraction between components. The overlapping volume between each pair of spheres (V _int) was calculated, and the overlap percentage (V _int/V _PEG) was used as an estimate of the physical affinity. It should be noted that HSP testing may involve some degree of variation due to differences in experimental protocols, material batches, and subjective interpretation of dispersion quality, which can introduce some uncertainty into the results. As such, this overlap percentage is best regarded as a qualitative approximation of physical affinity. The overlap percentage between three carboxylated polythiophenes and PEG is as follows: PEG-P3KHT (72%) > PEG-P3KPT (70%) > PEG-P3KBT (66%). The results suggest that the affinity of P3KHT to PEG is more favorable than the shorter alkyl chain alternatives. Contact angle measurements (Figure S8) facilitated quantitative determination of the P3KAT-water and P3KAT-PEG solution affinity. Most likely due to its longer alkyl side chain, P3KHT displayed a relatively higher contact angle with water (∼54°) than either P3KPT (∼42°) or P3KBT (∼28°). Alternatively, the reverse trend was observed using a 50 wt % PEG 1500 aqueous solution: the longer-side-chain carboxylated polythiophenes exhibited noticeably reduced contact angles, reflecting enhanced PEG affinity with P3KHT, in agreement with HSP predictions.

5.

HSP spheres of interaction for (a) PEG and P3KBT, (b) PEG and P3KPT, (c) PEG and P3KHT. % V _int refers to the superimposed volume portions in which PEG is occupied by the other polymer. Raman spectra of (d) P3KBT and PEG-P3KBT, (e) P3KPT and PEG-P3KPT, (f) P3KHT and PEG-P3KHT. UV–vis absorbance spectra of (g) P3KBT, P3KPT, P3KHT, and (h) their blend films with PEG.

To gain deeper insight into the molecular basis for the apparent PEG-polythiophene interactions, aqueous solutions of PEG and P3KAT (1:2 weight ratio, which approximates the weight ratio of PEG to P3KAT within the electrode), in addition to an aqueous solution of P3KAT, were prepared. Samples were then spray coated onto glass slides and allowed to dry at room temperature to access uniform polymer films for further interrogation of the PEG-P3KAT interactions. Raman spectra of both the PEG-P3KAT blends and P3KAT films are displayed in Figure d–f. All P3KAT samples exhibited characteristic peaks at approximately 1380 cm^–1 and 1450 cm^–1, corresponding to C–C and CC stretching vibrations of the thiophene ring, respectively. In the presence of PEG, the CC stretching peak exhibited a red-shift, indicating a lower degree of molecular ordering. These findings suggest that interactions between PEG and the carboxylated polythiophenes disrupt π–π stacking, reduce P3KAT organization/crystallinity, and may contribute to the creation of amorphous domains that facilitate ion transport. UV–vis absorption spectroscopy provided additional insights into the extent of PEG-induced perturbation of π–π stacking across different P3KAT series. As shown in Figure g, the P3KAT films exhibited distinct vibronic absorption bands near 610 and 530 nm, corresponding to the 0–0 and 0–1 transitions, respectively, indicating the existence of an ordered phase. In contrast, the PEG-P3KAT films (Figure h) displayed significant spectral changes. Both PEG-P3KBT and PEG-P3KPT experienced a substantial decrease in the intensity of the 530 and 610 nm vibronic bands that are typically attributed to extended conjugation resulting from π–π stacking, with concomitant increase in the absorption near 420 nm, which resembles solution phase spectra of alkyl-substituted polythiophenes. Thus, it appears that the presence of PEG may help solubilize the two polythiophenes, disrupting π–π stacking, reducing aggregation, and conceivably contributing to the amorphous content. While this phenomenon may enhance ion transport in the system, electronic transport pathways will be compromised. , PEG-P3KHT, on the other hand, exhibited more intense vibronic features at ∼530 and ∼610 nm vs P3KHT, suggesting that π–π stacking of the hexanoate was impacted to a lesser extent. The apparent favorable interfacial interactions between PEG and P3KHT may lead to retention of P3KHT electronic transport characteristics while maintaining the favorable ion transport characteristics associated with the PEG surface coating.

4. Conclusion

This work explored the critical role of introducing a PEG coating in conjunction with tailoring the carboxylated polythiophene side-chain length in optimizing the structural and electrochemical properties of Fe₃O₄ composite electrodes for LIBs. The PEG coating markedly improved electrode stability and rate performance through physicochemical interactions with both the polythiophene and the active material surface. Specifically, the longer alkyl side-chain carboxylated polythiophene, P3KHT, preferentially interacted with PEG instead of the Fe₃O₄ surface, leading to enhanced ion transport properties and better durability toward the effects of volumetric expansion.

Further, CV, rate performance, cycling performance, and EIS confirmed that the presence of PEG significantly enhanced electrode functionality, as evidenced by improved ion transport kinetics, better capacity retention at high rate, stabilized cycling stability, and reduced charge-transfer resistance. Among the PEG-Fe₃O₄ and Fe₃O₄ with P3KAT analogs, the PEG-Fe₃O₄–P3KHT electrode consistently outperformed other systems, emphasizing the favorable synergistic interactions between PEG and the polythiophene having a longer carboxylalkyl side-chain. These insights provide valuable guidance for advancing the design of advanced composite electrodes with balanced electron and ion transport pathways, optimized structural flexibility, and superior durability to meet the demands of high-performance and long-lifetime energy storage systems.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsaem.5c01648.

• FT-IR spectra of materials; TGA profile of PEG-Fe₃O₄; chemical structures of P3KAT; CV profiles; b value of Fe₃O₄-based electrodes; initial Coulombic efficiency of Fe₃O₄-based electrodes; cycling performance of PEG-Fe₃O₄ electrodes; capacities of Fe₃O₄-based electrodes; EIS Nyquist plots for PEG-Fe₃O₄ electrodes; equivalent circuit diagrams for EIS fitting and results; solubility tests for HSP of PEG, P3KBT, P3KPT, and P3KHT; and contact angle measurements (PDF)

The authors declare no competing financial interest.

Published as part of ACS Applied Energy Materials special issue “70th Anniversary of Materials Science at Donghua University”.