Preprint of "Tailored Silicon Nanostructures in Conductive-based Hydrogel Binders: Role of Size, Structure, and Surface Chemistry in Enhancing Li-Ion Battery Performance"

1 Tailored Silicon Nanostructures in Conductive-based Hydrogel Binders: Role of Size, Structure, and Surface Chemistry in Enhancing Li-Ion Battery Performance Gabriela Soukupová 1, Filip Matějka 1,2, Zuzana Vlčková Živcová 3, Abdelghani Laachachi 4, Pavel Galář 2, Miloslav Lhotka 5, Otakar Frank 3, Jiří Červenka 2, Fatima Hassouna 1* (1) Faculty of Chemical Engineering, University of Chemistry and Technology, Prague, 166 28 Prague 6, Czech Republic (2) FZU - Institute of Physics of the Czech Academy of Sciences, Cukrovarnická 10/112, Prague, 162 00 Prague 6, Czech Republic (3) J. Heyrovsky Institute of Physical Chemistry, Czech Academy of Sciences, Dolejskova 21553, Prague 18223 8, Czech Republic (4) Luxembourg Institute of Science and Technology (LIST), 5, rue Bommel, L-4940 Hautcharage, Luxembourg (5) Faculty of Chemical Technology, University of Chemistry and Technology, Prague, 166 28 Prague 6 , Czech Republic Corresponding author: E-mail addresses: [email protected] (Assoc. prof. Fatima Hassouna) This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=5189743 Preprint not peer reviewed 2 Abstract Silicon (Si) is a promising high-capacity anode material for Li-ion batteries (LIBs), but its practical application is limited by severe volume expansion during lithiation/delithiation, leading to poor cycling stability. While Si nanostructuring mitigates this issue, it remains only a partial solution. This study systematically investigates the effects of Si particle size (6, 20, 55, and 100 nm), surface chemistry (oxide type and degree), and solid-state properties (amorphous vs. crystalline) on the electrochemical performance of Si-based anodes using polypyrrole (PPy) hydrogel binder. In situ PPy polymerization around Si nanoparticles forms a 3D interconnected conductive network within PPy/Si anodes, effectively accommodating volume changes and maintaining electrical contact during the galvanostatic charge-discharge cycling. The particle size dependence shows that larger Si nanoparticles provide higher initial charge capacity (2975 mAh/g), whereas smaller ones improve cycling stability (85 % capacity retention after 100 cycles). Amorphous Si exhibits lower specific capacity but superior capacity retention (~100% after 100 cycles) compared to crystalline Si. Cyclic voltammetry and electrochemical impedance spectroscopy demonstrate that integrating 6 and 20 nm Si nanocrystals into the PPy network significantly enhances anode performance. These findings highlight the importance of optimizing Si material properties in designing conductive hydrogel-based anodes for high-performance LIBs. Keywords Li-ion battery, Si nanoparticles, 3D network, electrically conductive polymer, electrochemical properties This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=5189743 Preprint not peer reviewed 3 1 Introduction In recent years, the demand for cost-effective, high-performance rechargeable batteries with extended cycle life has grown significantly, driven by their applications in electronic devices, electric vehicles, and large-scale energy storage systems. Among all, Li-ion batteries (LIB) have attracted significant attention due to their exceptional energy density, high power density, and long cycle life [1, 2]. The commercially used graphitic anode offers only limited theoretical specific capacity (~370 mAh/g) [2, 3], which has prompted extensive research into alternative anode materials. Si is considered one of the most promising anode materials for LIB due to its high theoretical specific lithiation capacity (~3579 mAh/g for Li15Si4), low discharge potential (∼0.4 V vs Li/Li+), environmental sustainability, and high elemental abundance [1, 4, 5]. However, the practical application of Si as an anode material in LIB is significantly limited due to its poor electrical conductivity and substantial volume expansion (~300 % for Li15Si4 [6]) during lithiation. This expansion leads to severe issues such as material cracking, anode pulverization, overgrowth of a solid electrolyte interphase (SEI), and capacity fading [4, 5, 7]. To address these challenges, several strategies have been explored. One promising approach is the nanostructuring of Si (e.g., nanowires or nanoparticles), which helps accommodate the volume changes, thereby enhancing the mechanical stability of the material and improving its cycling performance [8-10]. It has been reported that when the particle size is reduced below a critical threshold of 150 nm, the cracking of the material is significantly reduced [11-13]. Although nanostructuring improves the mechanical properties of Si anodes, several significant challenges remain. These include the higher surface activity of Si, which can lead to particle agglomeration, the formation of a thicker SEI layer, and the high cost associated with the preparation process [11, 14, 15]. To overcome the poor electrical conductivity of nanostructured Si and facilitate more efficient Li-ions diffusion, conductive carbon (C) additives such as carbon black (Super P), carbon nanotubes, and graphene are commonly incorporated. The addition of these C materials not only helps to buffer the volume expansion during lithiation but also improves the overall conductivity, thereby providing an effective electronic pathway for electron transfer [4, 14]. Generally, polymer binders such as carboxymethyl cellulose, polyamide imide, or poly(acrylic acid) (PAA) are This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=5189743 Preprint not peer reviewed 4 incorporated into Si/C to enhance mechanical stability and hence, the cycling performance of the anode material [4, 16]. Effective binders for Si-based anodes must meet several critical requirements. They must be capable of forming uniform mixtures with Si and C, establishing strong binding interactions with both Si and C to ensure stable electrical pathways to the current collector and demonstrating electrochemical stability within the operational potential window. Moreover, they should exhibit high elastic modulus values to accommodate the significant volume expansion of Si, resist excessive swelling in the electrolyte, facilitate the formation of a stable and thin SEI layer, and ensure a robust connection between the anode material and the current collector throughout cycling. In addition to these technical characteristics, an ideal binder should also be economically feasible, straightforward to manufacture in large quantities, affordable, and compatible with existing production techniques [16]. The use of the aforementioned insulating polymer binders may result in a weak interface between Si and C, leading to contact loss during cycling [16]. In this context, replacing the insulating conventional binders with electrically conductive polymers presents a promising alternative, as it would eliminate the weak interface between Si and C. The conductive polymer would not only serve as a binder but also enhance the conductivity of the anode material. The use of conductive polymers, such as polyaniline, polypyrrole (PPy), and poly(3,4-ethylenedioxythiophene), offers several advantages, including mechanical flexibility, cost-effectiveness, ease of synthesis and processing, environmental friendliness, high electrical conductivity, and electrochemical activity [17-19]. Although conductive polymers have received significant interest for their application in LIB, relatively limited research has been conducted on integrating them with Si to develop commercially viable anodes for LIB [1-3]. A viable and promising strategy for integrating Si into conductive polymer binders involves a hydrogel-based preparation technique. In this approach, in situ polymerization and crosslinking of the conductive polymer take place in the presence of Si, often accompanied by an additional electrically conductive additive. The process results in the formation of a three-dimensional (3D) network, wherein Si is uniformly coated and interconnected by the conductive polymer matrix [1-3, 5, 8, 16, 20, 21]. This 3D interconnected network improves electrochemical performance through several beneficial features. The hydrogel framework functions as both a conductivity enhancer and a conductive binder, improving the particle-toThis preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=5189743 Preprint not peer reviewed 5 particle contact. Its porous structure helps buffer the volume changes of Si during lithiation [21], while the 3D conductive network facilitates improved electron and ion diffusion [8]. Furthermore, embedding Si within the framework promotes the formation of a more stable SEI during lithiation by better isolating Si from the electrolyte [20]. Despite their importance, the impact of key factors such as the size, surface chemistry, and solid-state properties of Si on the electrochemical performance of resulting anodes remains poorly understood and has not been systematically investigated in Si/conductive polymer-based anodes, including those prepared using hydrogelbased approaches. Previous studies investigating the size effect of Si in anode materials have primarily focused on physical blending methods, where Si particles or carbonized core-shell Si particles (ranging in size from 30 nm to 5 μm) were mechanically mixed with a binder and carbon additive [12, 15, 22-24], or prepared via gas deposition techniques [25]. The studies on Si anodes using conductive hydrogel binders have used different types and sizes of Si, including nanoparticles with average diameters ranging from 40 to 300 nm [1, 3, 5, 8, 16, 21, 26, 27], micro-sized porous Si particles [28], and Si dendrites [20]. However, the selection of Si type and size has often been arbitrary, lacking a systematic methodology. Furthermore, these studies predominantly used commercial Si materials with uncontrolled surface chemistry. This lack of systematic investigation extends to the promising potential of Si quantum dots (SiQD). Despite their advantages, such as high surface area, shorter diffusion pathways, and reduced volume expansion, which make SiQD promising for LIB, only a limited number of studies have focused on their application [29-31]. These studies have demonstrated encouraging electrochemical performances [32-35]. However, challenges persist in synthesizing monodisperse SiQD and in mitigating side reactions that arise due to their large active surface area, which complicate their practical use. To the best of our knowledge, no comprehensive study has systematically examined the influence of Si particle size, surface chemistry, and intrinsic solid-state properties on the electrochemical performance of Si-based anodes with conductive hydrogel binders in LIB. To address this knowledge gap and develop a fundamental understanding of structure-property relationships, this study investigated the performance of Si-based anode materials for LIB. These materials were prepared via the in situ polymerization of a conductive PPy hydrogel using an environmentally friendly, waterborne approach. Phytic acid (PhA), a naturally occurring molecule, was employed This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=5189743 Preprint not peer reviewed 6 as a crosslinking agent for the PPy chains, while PAA served as a stabilizing agent for the Si nanoparticles. The PPy hydrogel functioned as a high-performance conductive binder, conformally coating the Si nanoparticles. The study examined the effects of size, surface chemistry, and solidstate properties (amorphous versus crystalline) of both commercial and lab-synthesized Si nanoparticles, including SiQD (particle size below 10 nm), on the structural, morphological, and electrochemical performance of the Si-based anode materials. These insights enabled the optimization of Si-based anodes for enhanced electrochemical performance. A clear correlation was established between Si nanoparticle size, surface chemistry, solid-state properties, and the resulting electrochemical performance of the developed anodes. 2 Experimental part 2.1 Materials Two types of Si nanocrystals (SiNC100 with 100 nm average diameter, stock keeping unit NG04CO28095; SiNC55 with 55 nm average diameter, stock keeping unit NG04EO1804) were purchased from Nanografi. Two other types of SiNC (SiNC6 with 6 nm average diameter; SiNC20 with 20 nm average diameter) and one type of amorphous Si nanoparticles (SiNA20 with 20 nm average diameter) were synthesized in the frame of this study. Diluted silane (1% in argon, Linde, Ar 5.0, SiH4 5.0, UN1954), hydrogen (H2 7.0, Linde, UN1049), argon (Ar, Ar 6.0, UN1006), and pure silane (SiH4, UN2203) were used for the Si synthesis. Conductive carbon filler Super P (40 nm average diameter) was kindly donated by Imerys S.A. Pyrrole monomer (reagent grade, 98 %, Mw = 67.09), phytic acid solution (PhA, 50 % (w/w) in H2O, Mw = 660.04), poly(acrylic acid) (PAA, Mw = 450,000), ammonium persulfate (APS, ≥98.0%, Mw = 228.20), lithium hexafluorophosphate solution in ethylene carbonate and dimethyl carbonate (1.0 M LiPF6 in EC/DMC = 50/50 (v/v), battery grade), and Li-metal foil (thickness: 0.6 mm, 99.9 %) were purchased from Merck. Current collector copper foil (thickness: 25 µm, 99.8 %) was purchased from Thermo Fisher. Deionized (DI) water was used as an aqueous medium in all the experiments. This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=5189743 Preprint not peer reviewed 7 2.2 Materials Preparation 2.2.1 Synthesis of SiNC SiNC20, SiNC6, and SiNA20 were synthesized using a non-commercial non-thermal plasma flowthrough reactor operated under low pressure. The system was created by adapting the apparatus of Korsthagen et al [36]. The operational pressures were within 10 Pa in the standby mode and lower than 500 Pa in the synthesis mode. The plasma was generated with a radiofrequency (RF) power source (Coaxial Power Systems) operating at 13.56 MHz. For the synthesis of 6 mm crystalline particles (SiNC6), a narrow glass tube reactor was used, with an internal diameter of 0.8 cm. The plasma discharge was generated using planar electrodes (dimensions 5 to 12 cm). The output RF power was 150 W, and the SiNC were synthesized using a 1% diluted silane (flow: 80 sccm) and hydrogen (flow: 10 sccm). For the synthesis of the 20 nm crystalline particles (SiNC20) glass reactor with internal diameter of 2.1 cm attached with ring electrodes (height 2.7 cm) was used. The reaction mixture was composed of argon (flow: 80 sccm) and pure silane (flow: 2 sccm), and the output RF power was set to 107 W. The 20 nm amorphous particles (SiNA20) were synthesized using a two-stage reactor. The first stage consisted of a narrow glass tube reactor with an internal diameter of 0.8 cm and a length of 50 cm. Planar electrodes (measuring 5 cm by 12 cm) were attached to the reactor. A flow of diluted silane (80 sccm) was introduced into this stage, and the RF output power was set to 150 W. The second stage was implemented using a broad glass tube with an internal diameter of 2.1 cm and a length of 50 cm. Double-helix electrodes were attached to the tube, with one electrode grounded, and the wires spaced 1 cm apart. During this stage, flows of pure silane (flow: 12 sccm) and argon (flow: 48 sccm) were introduced, and the second RF power output was set to 250 W. 2.2.2 Synthesis of PPy hydrogel-based Si composites and anode preparation PPy hydrogel-based composites were prepared via in situ oxidative polymerization of pyrrole monomer using APS in the presence of Si nanoparticles (SiNC or SiNA). The composites were prepared according to the following procedure. Firstly, 11 µl of pyrrole monomer was transferred to a small glass vial with 35.4 µl of PhA and DI water. The solution was mixed using a magnetic stirrer. Next, 38.4 mg of Si nanoparticles were grounded in a mortar for 10 min. Subsequently, 15.9 mg of Super P was added and mixed with the Si nanoparticles for an additional 10 min. Then, This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=5189743 Preprint not peer reviewed 8 15.4 mg of PAA was incorporated into the Si and Super P mixture and blended for another 10 min. The resulting powder mixture was transferred into a small glass vial, and DI water was added. The powder mixture was dispersed in DI water using a sonication bath (PS 3000, PowerSonic) and further mixed with a magnetic stirrer to achieve a homogeneous dispersion. Once the powders were fully dispersed and a uniform ink was obtained, the solution of pyrrole with PhA and DI water was added to the ink. This mixture was stirred for 10 minutes with a magnetic stirrer, and the vial was labeled as solution A. In parallel, an APS solution was prepared by dissolving 11 mg of APS in DI water. Both solutions, solution A and APS solution, were cooled separately in an ice bath for 10 minutes. Finally, the APS solution was poured into solution A under continuous stirring in the ice bath, initiating the in situ polymerization of the pyrrole monomer. After 4 hours, the resulting ink was cast onto copper foil and allowed to dry at room temperature. The obtained thin films were then pressed for 1 min at 60 kPa and dried in a vacuum oven at 60°C overnight. The prepared anodes were labeled as PPy/Si, specifying the type of Si used, i.e., SiNC or SiNA. 2.3 Characterization methods The chemical structure of all Si nanoparticles was analyzed by Raman spectroscopy (633 nm, 1.96 eV, objective 100x, He-Ne laser; LabRAM HR spectrometer Horiba Jobin-Yvon integrated with an Olympus microscope), and Fourier-transform infrared spectroscopy (FTIR, Nicolet iS50 ABX). The surface chemistry was examined using X-ray photoelectron spectroscopy (XPS, X-ray beam Al Kα with E = 1486.6 eV and power of 60 W, photoelectron emission take-off angle of 0°; Thermofisher Nexsa G2). The obtained XPS spectra were deconvoluted using the CasaXPS software. The micro/nanostructure was visualized using scanning electron microscopy (SEM, Mira3 LMH, Tescan, secondary electrons at 3 kV) and high-resolution transmission electron microscopy (TEM, EFTEM Jeol 2200 FS, copper mesh). The crystallinity of the Si nanoparticles was characterized using X-ray diffraction (XRD, Cu lamp; PANalytical X’Pert PRO with PIXcel1D_1D detector). The specific surface area was analyzed using the nitrogen physisorption technique on a 3Flex analyzer (Micromeritics, Norcross). To assemble Li-ion half-coin cell batteries, the anode materials prepared in this work were dried at 80°C in a vacuum oven for 12 hours and cut into 1.5 cm diameter circles. Each cut circle was This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=5189743 Preprint not peer reviewed 9 weighed and transferred to an Ar-filled glovebox for the battery assembly. Li metal foil was used as the reference and counter electrode, and 1.0 M LiPF6 in EC/DMC served as the electrolyte. The electrochemical performance of hydrogel-based anodes was evaluated through galvanostatic charge/discharge (GCD) cycling within the potential range of 0.05-1 V versus Li/Li+. This was performed using a battery tester (Neware BTS-4008-5V50mA) at a charging rate of 0.1 C (0.4 A/g), calculated for each sample individually based on the mass of Si. Further characterization involved cyclic voltammetry (CV) using a potentiostat (µAutolab, Metrohm), and electrochemical impedance spectroscopy (EIS) in the discharged state (0.05 V), with frequencies ranging from 80 kHz to 0.03 Hz (Ivium CompactStat, Ivium Technologird B. V.). 3 Results and discussion The PPy hydrogel-based anodes were synthesized through a straightforward in situ oxidative polymerization process in an aqueous medium (Figure 1). This method facilitates the formation of a 3D interconnected network, wherein the conductive polymer embeds, connects, and stabilizes Si nanoparticles and Super P. PPy was selected due to its excellent electrical conductivity, mechanical flexibility, and its ability to form a conductive, 3D crosslinked network when combined with a crosslinking agent such as PhA. Different types of Si nanoparticles with varying sizes and physicochemical properties were integrated within the anodes to examine the effect of Si particle size, surface chemistry, and intrinsic solid-state properties on the electrochemical performance of Si-based anodes with conductive hydrogel binders in LIB. This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=5189743 Preprint not peer reviewed 16 experimental procedure. Although the measured capacity (Figure S4) was inherently low, given that SiNC is the primary active material in this system, no capacity degradation was observed over 500 cycles. Instead, the capacity gradually increased. This emphasizes the significant role of SiNC on the overall capacity decay. To further confirm the beneficial role of the 3D interconnected PPy structure in combination with SiNC, an anode material composed of non-crosslinked PPy and SiNC20 (a representative type of SiNC) was prepared. The GCD cycling of the resulting anode, labeled as PPynon-crosslinked/SiNC20 (Figure S5), reveals poor cycling stability, with the specific capacity dropping from approximately 800 mAh/g to nearly 0 mAh/g after 300 cycles. These findings highlight the importance of combining small SINC with a 3D crosslinked network to achieve good capacity retention. The GCD profiles were measured at various current densities (Figure S6) of 0.4, 0.7, 1.8, and 3.6 A/g and compared for two selected representative anodes, PPy/SiNC20 and PPy/SiNC100. The charge capacity of PPy/SiNC20 ranges from ~1280 to 165 mAh/g, demonstrating good stability and reversibility, especially at lower current rates. After returning to 0.4 A/g, it recovers its initial capacity values. In contrast, the charge capacity of PPy/SiNC100 varies from 2698 to 69 mAh/g. Interestingly, this anode does not show the same level of reversibility and stability as the one with a smaller SiNC20. While the reversibility of PPy/SiNC100 remains fairly high, it does not fully recover its initial capacity upon returning to 0.4 A/g. This observation further emphasizes the beneficial role of small-sized nanostructured SiNC (20 nm) in enhancing cycling stability and capacity retention. The electrochemical performance of PPy/SiNC20 and PPy/SiNA20, both containing Si nanoparticles of the same size (Figure S7) was examined to evaluate the influence of Si solid-state properties (crystalline vs. amorphous) on electrochemical behavior. Although PPy/SiNA20 exhibits remarkable capacity retention (78 % after 500 cycles), its specific capacity is significantly lower than that of PPy/SiNC20. This indicates the importance of Si crystallinity in achieving higher specific capacities. Notably, with prolonged cycling, the capacity of PPy/SiNC20 gradually approaches that of PPy/SiNA20, likely due to progressive amorphization. To investigate the structural and morphological evolution of the anode materials after the GCD cycling, representative anodes were analyzed by Raman spectroscopy and SEM before and after cycling. SEM images (Figure S8) of anode materials on their cross-sections before and after the cycling show a loss of the structure. The Raman spectra of PPy/SiNC100 and PPy/SiNC20 before This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=5189743 Preprint not peer reviewed 17 and after the cycling are compared to those of PPy/SiNA20 (Figure S9). All anodes display characteristic peaks of PPy [59, 60] along with the distinct Si peak. Prior to cycling, the Si peak appears at 515 cm-1 for both PPy/SiNC100 and PPy/SiNC20, corresponding to the crystalline Si. However, after cycling, a noticeable shift of the SiNC peak to lower wavenumbers (~472 cm-1) is observed, aligning with the peak position of PPy/SiNA20. These shifts confirm the amorphization of SiNC after cycling. Table S3 presents the electrochemical performance of our developed anodes containing SiNC6 and SiNC20 (i.e., PPy/SiNC6 and PPy/SiNC20) in comparison with the most relevant hydrogelbased conducting polymer/Si anodes reported in the literature. It is worth mentioning that two key factors must be considered before making any direct comparison. First of all, the Si used in those studies varies significantly in terms of particle size and surface chemistry. In many cases, properties such as crystallinity and surface chemistry are neither analyzed nor discussed. Often, commercially available Si is selected without thorough examination and used as is, which can introduce significant variability in the results. As demonstrated in our study, any surface modification, pronounced oxidation, or alteration in the crystalline structure can have a tremendous impact on the resulting electrochemical properties. Secondly, the conditions under which the electrochemical measurements are performed, such as the potential window, current rate, electrolyte used, and the number of cycles, also vary across studies. In addition, important details, such as whether the reported initial specific capacity refers to discharge or charge capacity or whether the stated capacity retention is related to the 1st, 2nd, or later cycles, are not always clearly specified. Therefore, direct comparisons of these results should be made with caution, as they may conceal several potential pitfalls. Both anodes (PPy/SiNC6 and PPy/SiNC20) presented in this study exhibit promising characteristics and properties across various parameters. The preparation procedure was conducted in an aqueous medium using a conducting polymer as a binder, both of which are environmentally friendly. As shown in Table S3, these anodes achieved initial discharge capacities of 3291 mAh/g for PPy/SiNC6 and 2978 mAh/g for PPy/SiNC20, which are among the highest values reported for hydrogel-based conducting polymer/Si anode materials. Although their initial charge capacities are relatively modest (1014 mAh/g for PPy/SiNC6 and 1273 mAh/g for PPy/SiNC20), they remain significant and are comparable to those reported in the This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=5189743 Preprint not peer reviewed 18 literature [3, 5, 21]. Furthermore, the obtained capacity retentions of this work are satisfactory and comparable with previously reported results (Table S3). Figure 4: Electrochemical performance in potential window 0.05-1 V at current density 0.4 A/g of PPy hydrogel-based anodes containing SiNC: a) GCD cycling for 500 cycles, b) zoom of the GCD cycling up to 100 cycles, c) galvanostatic voltage profiles from 1st cycle, d) coulombic 0 100 200 300 400 500 0 1000 2000 3000 4000 5000 0 20 40 60 80 100 0 1000 2000 3000 4000 5000 0 1000 2000 3000 4000 5000 0,0 0,5 1,0 1,5 2,0 2,5 3,0 0 100 200 300 400 500 0 20 40 60 80 100 120 SiNC100 SiNC55 SiNC20 SiNC6 0 20 40 60 80 100 Specific capacity (mAh/g) Number of cycles PPy/SiNC100 PPy/SiNC55 PPy/SiNC20 PPy/SiNC6 a) b) c) d) e) f) Specific capacity (mAh/g) Number of cycles PPy/SiNC100 PPy/SiNC55 PPy/SiNC20 PPy/SiNC6 Potential (V) Specific capacity (mAh/g) PPy/SiNC100 PPy/SiNC55 PPy/SiNC20 PPy/SiNC6 Coulombic efficiency (%) Number of cycles PPy/SiNC100 PPy/SiNC55 PPy/SiNC20 PPy/SiNC6 SiNC100 SiNC55 SiNC20 SiNC6 20 30 40 50 60 70 80 90 Capacity retention after 100 cycles 2nd cycle PPy/SiNC Capacity retention after 100 cycles (%) 1000 1500 2000 2500 3000 Specific capacity (2nd cycle) (mAh/g) Capacity retention after 100 cycles (%) PPy/SiNC Capacity retention After 100 cycles After 200 cycles After 500 cycles This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=5189743 Preprint not peer reviewed 19 efficiency, e) capacity retention after 100 cycles and initial specific capacity (in 2nd cycle), and f) evolution of capacity retention during the cycling for each anode material. The empty squares in e) and f) represent the capacity retention of PPy/SiNC55 calculated based on the highest specific capacity achieved at the 10th cycle. For deeper understanding of the electrochemical performance, CV was conducted on coin halfcell batteries containing PPy hydrogel-based anodes (Figure 5 and Figure S10). Measurements were performed in the potential window of 0.05-1 V at a scan rate of 0.1 mV/s. Figure 5a-c compares 1st, 3rd, and 10th cycles of all the anodes and Figure S10 presents CV of each anode separately. In the 1st cycle, a reduction peak appears at 0.4 V for PPy/SiNC20 and PPy/SiNA20, 0.5 V for PPy/SiNC6, and approximately 0.6 V for PPy/SiNC100 and PPy/SiNC55. This peak disappears in subsequent cycles and corresponds to the irreversible reaction of electrolyte decomposition on the anode surface, and to the formation of the SEI layer [61-63]. Interestingly, PPy/SiNC55 exhibits a broad reduction peak at around 0.9 V, which is significantly more intense than the peak at 0.6 V, suggesting the occurrence of another irreversible reaction. This peak (around 0.9 V and higher) has been previously connected with the irreversible reaction between SiO2 and Li+, resulting in the conversion of SiO2 to Si [64, 65]. This observation aligns well with the earlier analysis of SiNC55, which showed a higher SiO2 content. During continued CV cycling, four peaks emerged. Reduction cathodic peaks at 0.17 V and 0.05 V are linked to the conversion of crystalline Si to amorphous Li-Si phases (a-LixSi) during the lithiation and the subsequent crystallization and formation of c-Li3.75Si [4, 62]. Oxidation anodic peaks at 0.35 V (a-Li3.5Si to a-Li2Si) and 0.5 V (a-Li2Si to a-Si) correspond to the transition of crystalline to an amorphous phase, followed by a dealloying reaction [4, 66, 67]. However, PPy/SiNC6 and PPy/SiNA20 (Figure S10d) initially showed only one reduction and one oxidation peak. This behavior can be expected for PPy/SiNA20 as it contains amorphous Si, meaning there is no transition from crystalline to amorphous Si, and only alloying and dealloying processes occur. As CV cycling continues, a reduction peak around 0.1 V starts to appear, indicating a gradual activation of lithiation sites. In the case of PPy/SiNC6, only one reduction and one oxidation peak are observed until the 7th cycle, when the reduction peak at 0.17 V first appears. The anodic peak at 0.35 V remains barely noticeable throughout the 10 cycles. These observations suggest that the lithiation connected with phase transformation of both PPy/SiNC6 and PPy/SiNA20 is slower compared to those of PPy/SiNC20. The positions of both reduction and oxidation peaks remain stable throughout cycling for all the studied anodes, indicating good reversibility of the Si-Li reactions [66]. CV cycling tests of This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=5189743 Preprint not peer reviewed 20 PPy/SiNC20, PPy/SiNC6, and PPy/SiNC55 reveal increasing peak intensities over time, a phenomenon previously reported in the literature and ascribed to the activation process of the anode material [66]. This activation process is facilitated by the amorphization of Si, which enhances lithiation capacity with each cycle [68]. Notably, this effect is less pronounced for PPy/SiNC100, where peak intensities initially increase but begin to decline after 5 cycles. This behaviour can be attributed to the larger size of SiNC100 due to a limited lithiation time, which restricts the depth of lithiation and stress buildup to the sub-surface regions in the larger nanoparticles. This localized stress can cause nanoparticle fracture, deactivation, and a subsequent decrease in peak intensity [68]. In contrast, the peak intensities of PPy/SiNC55 (Figure S10b) increase slightly over cycling, as a result of the smaller size of SiNC55. Among all the anodes, PPy/SiNC20 demonstrates the best CV performance, which can be linked to the optimal size of the SiNC20. Interestingly, PPy/SiNC6 does not reach similarly high peak intensities, indicating lower electrochemical activity that aligns with its initial lower specific capacity values (Figure 4a). This behaviour may be due to nanoparticle agglomeration, which diminishes the stabilizing advantages typically associated with smaller particle sizes (< 20 nm)[69]. Indeed, as previously observed, SEM images of PPy/SiNC6 (Figrue 3i) reveal significant agglomeration of SiNC6, a feature unique to this anode. This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=5189743 Preprint not peer reviewed 21 Figure 5: CV measurements at a scan rate 0.1 mV/s of all the anodes in a) 1st, b) 3rd cycle, and c) 10th cycles. To gain deeper insights into the processes occurring at the anode-electrolyte interface, EIS was carried out on four PPy hydrogel-based anodes. Figure 6 and Figure S11 show Nyquist plots measured at the discharged state (0.05 V) for 5 cycles. The data were fitted using an equivalent circuit (Figure S12), which accounts for the contributions of various components. The fitting procedure is based on the observed shape of Nyquist plots, which feature two distinct semicircles and a linear region at high frequencies. The cell resistance (RS) is represented by the highfrequency intercept and corresponds to the overall resistance of the battery system, including the electrolyte, current collector, and separator [1, 70]. The first semicircle, observed at high to medium frequencies, is attributed to the formation of the SEI layer. This is modeled by a parallel circuit comprising the SEI resistance RSEI and a constant phase element CPESEI [4, 70], which accounts for the non-ideal capacitive behavior rather than an ideal capacitor. The second semicircle, appearing at intermediate frequencies, is associated with charge transfer processes and is represented by the charge transfer resistance RCT in parallel with a constant phase element CPECT [2, 4]. Reportedly, this semicircle is influenced by changes in the surface coating and particle size, 0,0 0,2 0,4 0,6 0,8 1,0 -1,4 -1,2 -1,0 -0,8 -0,6 -0,4 -0,2 0,0 0,2 0,0 0,2 0,4 0,6 0,8 1,0 -1,2 -1,0 -0,8 -0,6 -0,4 -0,2 0,0 0,2 0,4 0,0 0,2 0,4 0,6 0,8 1,0 -2,0 -1,6 -1,2 -0,8 -0,4 0,0 0,4 0,8 1,2 Current density (A/g) Potential vs. Li/Li+ (V) 1st cycle of anode containing SiNC100 SiNC55 SiNC20 SiNC6 Current density (A/g) Potential vs. Li/Li+ (V) 3rd cycle of anode containing SiNC100 SiNC55 SiNC20 SiNC6 Current density (A/g) Potential vs. Li/Li+ (V) 10th cycle of anode containing SiNC100 SiNC55 SiNC20 SiNC6 a) b) c) This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=5189743 Preprint not peer reviewed 22 making it a useful indicator for understanding the reaction processes occurring within the anode [70]. The linear region at low frequencies corresponds to the Warburg impedance, which represents the diffusion of Li ions in the anode [2, 71]. To compare the electrochemical processes in different PPy/SiNC anodes, Nyquist diagrams were plotted, and the evolution of RS, RSEI, and RCT was analyzed (Figure 6 and Figure S11). One can observe that RS remains stable in all anodes at very similar values, consistently ranging between 6-12 Ω. This stability likely reflects the use of the same half-cell setup for each battery [72]. In the 1st cycle of each anode, elevated RSEI values are observed, which can be attributed to the formation of the SEI layer. The recorded values for PPy/SiNC100 (121 Ω), PPy/SiNC55 (134 Ω), PPy/SiNC20 (159 Ω), and PPy/SiNC6 (133 Ω) follow the trend of decreasing Si particle size, with the exception of PPy/SiNC6. The observed increase in RSEI with decreasing Si size suggests enhanced SEI layer formation due to a higher specific surface area, aligning with the GCD results discussed earlier. The lower RSEI observed in the 1st cycle of PPy/SiNC6 can be ascribed to its agglomerated inner structure, as discussed above (Figure 3i), where Si clusters are embedded within the formed PPy hydrogel. In the subsequent cycles of all the anodes, RSEI values decrease, suggesting the formation of a relatively stable SEI layer. This could be explained by a favorable effect of PPy, which promotes the development of a more stable SEI layer [72]. Interestingly, PPy/SiNC100 shows the lowest RSEI (16 Ω) in the 3rd cycle, after which gradually increased (to 35 and 48 Ω). This suggests the ongoing formation of the unstable SEI layer during cycling, most likely due to the cracking of larger SiNC, which exposes fresh SiNC surfaces to the electrolyte. This observation supports the previously presented GCD and CV results in this study, suggesting that there is an incomplete lithiation of larger SiNC, which proceeds to only shallow depths and leads to particle breakage in the later cycling stages [68]. Moreover, this anode shows the least stability across all electrochemical methods presented, which may be connected to the particle fragmentation and the potential formation of a thicker SEI layer in later cycles. A similar behavior is exhibited by PPy/SiNC55, where a slight increase in RSEI is noticed in the 5th cycle (up to 96 Ω). The slower increase in subsequent cycles can be attributed to improved stability as SiNC size decreases. However, it is important to consider that the reported 55 nm diameter represents an average value for this commercial SiNC, meaning there are larger particles present, which could contribute to faster material degradation. The smaller SiNC (6-20 nm) and SiNA exhibit better stability in terms This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=5189743 Preprint not peer reviewed 23 of RSEI, showing a continuous decrease over all 5 cycles. By the 5th cycle, the values reach 89 Ω for PPy/SiNC20, 67 Ω for PPy/SiNC6, and 32 Ω for PPy/SiNA20. The most significant differences between the small and large Si nanoparticles are observed when comparing the evolution of RCT. Anodes containing larger SiNC (PPy/SiNC100 and PPy/SiNC55) show an increase in RCT with continuous cycling. After 5 cycles, RCT increases from 38 to 101 Ω in PPy/SiNC100 and from 100 to 260 Ω in PPy/SiNC55. This indicates the instability of formed SEI followed with a progressive loss of electrical contact between the SiNC and the current collector [2], likely due to material cracking, degradation, and pulverization associated with the use of larger SiNC particles. This was further supported by the evolution of RSEI. In contrast, smaller SiNC (620 nm) show a similar trend for RCT as observed for RSEI, with the highest value in the 1st cycle, followed by a continuous decrease in the subsequent cycles. The RCT of PPy/SiNC20 decreases from 420 to 71 Ω, while the values for PPy/SiNC6 decrease from 150 to 65 Ω. Interestingly, RCT of PPy/SiNA20 shows huge increase from the 1st cycle (184 Ω) to the 2nd cycle (706 Ω) indicating SEI layer formation and slow charge transfer. But, in subsequent cycles, RCT is decreasing (to 375 Ω in the 5th cycle), which might be ascribed to the SEI layer stabilization and potentially to the formation of more active sites (as showed by CV). This suggests that the PPy network coating could be more effective when applied to smaller SiNC, likely for the following reasons, i.e., enhanced stability during cycling and a higher specific surface area, which facilitates charge transfer. Thus, smaller SiNC may interact more effectively with the crosslinked PPy matrix, promoting efficient charge transfer. This synergistic combination ultimately leads to improved charge kinetics [72]. 0 100 200 300 0 50 100 150 200 250 300 1 2 3 4 5 0 50 100 150 200 -Z'' (Ω) Z' (Ω) 1st cycle 2nd cycle 3rd cycle 4th cycle 5th cycle PPy/SiNC100 PPy/SiNC100 RS RSEI RCT Resistance (Ω) Cycle a) This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=5189743 Preprint not peer reviewed 24 Figure 6: Nyquist plots (left charts) from EIS at the discharged state (0.05 V) with fitting (solid line) to the equivalent circuit (Figure S12) and the evolution of resistance RS, RSEI, RCT (right charts) during 5 cycles of a) PPy/SiNC100, b) PPy/SiNC55, c) PPy/SiNC20, and d) PPy/SiNC6. 0 100 200 300 400 500 600 0 50 100 150 200 250 300 1 2 3 4 5 0 100 200 300 400 -Z'' (Ω) Z' (Ω) 1st cycle 2nd cycle 3rd cycle 4th cycle 5th cycle PPy/SiNC55 PPy/SiNC55 b) RS RSEI RCT Resistance (Ω) Cycle 0 100 200 300 400 500 600 0 50 100 150 200 250 300 1 2 3 4 5 0 100 200 300 400 500 -Z'' (Ω) Z' (Ω) 1st cycle 2nd cycle 3rd cycle 4th cycle 5th cycle c) PPy/SiNC20 PPy/SiNC20 RS RSEI RCT Resistance (Ω) Cycle 0 100 200 300 400 500 600 700 0 50 100 150 200 250 300 350 400 1 2 3 4 5 0 50 100 150 200 -Z'' (Ω) Z' (Ω) 1st cycle 2nd cycle 3rd cycle 4th cycle 5th cycle PPy/SiNC6RS RSEI RCT Resistance (Ω) Cycle d) PPy/SiNC6 This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=5189743 Preprint not peer reviewed 25 4 Conclusions In this study, the electrochemical properties of PPy/Si anodes were investigated. These anodes are composed of Si nanoparticles with varying characteristics, including particle size (6, 20, 55, and 100 nm), textural features, solid-state properties (amorphous and crystalline), and surface chemistry (degree and type of surface oxide), in combination with PPy hydrogel, which acts as an electrically conductive binder. The PPy/Si anodes were fabricated using in situ polymerized PPy hydrogel, crosslinked with phytic acid, i.e., a naturally occurring crosslinking agent, and incorporated with Si nanoparticles. The PPy hydrogel formed a conductive network that conformally coats the Si surface and provides porous space to accommodate the volume expansion of Si nanoparticles. The GCD cycling over 500 cycles in the potential window 0.05-1 V at a current density of 0.4 A/g showed that, regardless of the Si nanoparticle type, integration into the 3D conductive network significantly improved electrochemical performance by accommodating Si volume expansion and maintaining conductive contact between particles. GCD results further revealed that larger Si nanoparticles contributed to higher initial charge capacity (e.g., 2975 mAh/g for PPy/SiNC100 and 1013 mAh/g for PPy/SiNC6), whereas smaller Si nanoparticles enhanced cycling stability and capacity retention (e.g., 24 % for PPy/SiNC100 vs. 85 % for PPy/SiNC6 after 100 cycles). The importance of Si crystallinity in achieving higher specific capacities was also demonstrated, with PPy/SiNC20 exhibiting significantly higher specific capacity throughout the entire cycling range compared to PPy/SiNA20. All the GCD, CV, and EIS analyses highlighted the advantages of integrating SiNC with average sizes lower than 55 nm (i.e., SiNC20 and SiNC6) into the PPy crosslinked matrix. This particular dual combination provides enhanced electrochemical performance of the anode. More generally, it establishes a new foundation for future studies on SiNC and conductive polymer binders, aiming to mitigate the challenges associated with the substantial volume changes of Si anodes in LIB. Conflicts of interest There are no conflicts to declare. This preprint research paper has not been peer reviewed. Electronic copy available at: https://ssrn.com/abstract=5189743 Preprint not peer reviewed