3.1.1. PVdF
PVdF is a widely used polymer binder in secondary batteries, known for its thermal and electrochemical stability, making it suitable for various battery applications. However, when used as a binder for Si-based electrodes, PVdF does have certain limitations. One limitation of PVdF stems from its nonfunctional linear chain structure, which can result in low adhesive force and weak interaction with Si-based materials, potentially leading to degradation in battery performance [
53]. Additionally, PVdF exhibits low mechanical characteristics and flexibility, making it susceptible to the breakage of adhesive bonds during the volume changes occurring during the charging and discharging of Si-based electrodes, further impacting battery performance [
77,
78,
89]. The non-reactive C-F structure of PVdF may also result in weak interactions between electrode materials, hindering the formation of conducting channels [
99,
100]. The high viscosity of PVdF in solvents like
N-methyl-2-pyrrolidone (NMP) used in the slurry process can lead to accumulation with the active material, potentially blocking charge-transfer pathways and degrading charge-transfer rates. However, researchers have demonstrated that adjusting the molecular weight of PVdF can influence its performance as a binder. Increasing the molecular weight of PVdF can lead to higher binding strength and improved long-term cycling performance of the battery electrode [
100].
Efforts have also been made to address the limitations of PVdF by developing composite materials. For example, synthesizing PVdF with other materials, such as lithium lanthanum titanate, has been explored to achieve improved conductivity for solid-state electrolytes [
101]. In the broader context, it is crucial to carefully consider the characteristics of the polymer binder and select an appropriate one based on the specific requirements of the battery electrode and electrolyte system. Additionally, researchers are actively working on addressing environmental concerns associated with the use of solvents like NMP in the slurry process and exploring more sustainable alternatives [
102]. Understanding both the properties and limitations of PVdF, as well as other binder materials, is essential for designing high-performance batteries with improved electrochemical performance and environmental sustainability [
6]. Ongoing research and development in this area will contribute to the advancement of battery technologies.
3.1.3. CMC/SBR
CMC is a linear polymer derivative of natural cellulose extensively investigated as a binder for Si electrodes across various industries [
105,
106]. CMC, being water-soluble, has the capability to establish robust hydrogen and covalent bonds with surfaces containing hydroxyl groups, such as Si. This unique property enables CMC to maintain high mechanical integrity within the battery cell without undergoing expansion in liquid electrolytes. In terms of solubility, hardness, and elastic modulus in electrolyte solvents, CMC exhibits behavior akin to PAA [
86]. To enhance the flexibility of CMC as a binder, researchers have explored combinations with other polymers, such as SBR. The utilization of a CMC/SBR mixed binder has demonstrated an increase in maximum elongation, leading to improved adhesion strength. Lee's findings indicate that, while the tensile strength and modulus of the CMC/SBR binder may be slightly lower compared to PVdF, the maximum elongation and adhesive force are elevated. This mitigated the volume expansion of Si, resulting in significantly enhanced cycle performance, highlighting the benefits of employing CMC as a binder for Si electrodes [
107]. Further studies, such as those conducted by He et al., have extended the application of CMC/SBR as an efficient binder for lithium-sulfur (Li-S) batteries, showcasing improved cycling performance with higher capacity retention compared to PVdF binder. The electrode utilizing the CMC/SBR binder exhibited a capacity retention of 580 mA h g
-1 after 60 cycles, outperforming the 370 mA h g
-1 retention achieved with PVdF binder. Additionally, it was noted that the anode using CMC/SBR exhibited lower resistance and charge transfer impedance, contributing to a more stable interface structure and an efficient electron transport network [
108]. However, a challenge associated with binders like PAA and CMC/SBR is the potential uneven coverage of the active material on the electrode surface, leading to localized mechanical stress and potential particle breakage [
51]. Therefore, achieving uniform adhesion of the binder remains crucial, and ongoing research is dedicated to developing strategies to address this issue.
3.1.4. Binders for Si/Graphite (Si/G) Anodes
The utilization of nickel-rich materials as cathodes in LIBs has garnered attention due to their high energy density and cost-effectiveness compared to cobalt-based counterparts [
4]. However, these nickel-rich cathodes often encounter challenges such as capacity retention issues and the development of an unstable cathode-electrolyte interface layer [
109,
110,
111]. To address these concerns, extensive research is underway to enhance the performance of nickel-rich cathodes [
112]. In parallel, researchers are exploring alternative anode materials to replace graphite, which has a limited theoretical capacity of 370 mA h g
-1 [
113,
114,
115,
116]. Si has emerged as a promising anode material due to its abundance, low operating voltage (~0.2 V vs Li/Li
+), and high theoretical capacity (3,572 mA h g
-1) [
117,
118]. Nevertheless, However, Si undergoes significant volume expansion (approximately 300%) during the intercalation/deintercalation process of lithium ions, in stark contrast to the volume expansion of graphite (around 10%). The substantial volume expansion of Si during cycling brings about structural alterations in the electrode, leading to reduced adhesive force between the substrate and active material, electrode peeling, and the formation of an unstable SEI layer [
77,
78]. These challenges may result in capacity decay, decreased CE, and heightened internal resistance of the electrode. Consequently, for Si to be successfully commercialized as a high-capacity anode material, it is imperative to restrict volume expansion to within 10% to minimize structural transformations, with a compression density of approximately 1.65 g cm
-3. Moreover, the Si anode should exhibit a performance of ≥500 mA h g
-1, achieve ~99% CE, and maintain ≥80% capacity retention even after 500 cycles to meet the criteria for large-scale energy storage systems [
119].
Recently, both industry and academia have directed their research efforts toward the design of Si anodes, employing a combination of Si and graphite along with suitable binders and electrolytes. This strategy aims to enhance battery performance and tackle the inherent challenges associated with Si anodes [
121,
122,
123,
124]. The anticipated outcome of this approach is the development of LIBs characterized by high performance, cost-effectiveness, and safety, making them well-suited for large-scale energy storage applications.
Table 1 provides a summary of the electrochemical properties of Si/G composite materials, detailing the Si/G ratio, binder utilized, and electrolyte [
73,
74,
75,
76,
99,
100,
125,
126,
127,
128,
129,
130]. The data indicates that an increase in Si content leads to higher capacity but lower CE and cycle retention. However, it is observed that the currently employed commercial binders and electrolytes may not be optimal for Si/G anodes, lacking the necessary stability during cycling for viable commercialization. This underscores the necessity for further research and development endeavors aimed at identifying and optimizing suitable binders and electrolytes tailored specifically for Si/G composite anodes. This may involve the creation of novel binders or electrolytes designed to address the challenges associated with Si-based anodes, such as significant volume changes during cycling and the formation of an unstable SEI layer [
105,
106]. The quest for appropriate binders and electrolytes capable of ensuring stable cycling performance for Si/G composite anodes is pivotal for the successful commercialization of LIBs featuring Si-based anode materials. Continued research efforts in this domain can significantly contribute to the progress of Si/G composite anodes, addressing the challenges inherent in Si-based anode materials [
97,
131]. Ultimately, this research may bring us closer to the realization of LIBs characterized by high capacity, low cost, and suitability for large-scale energy storage applications [
130]. The challenges associated with the volume changes during lithiation and delithiation of Si and graphite in a Si/G composite anode can indeed impact the stability and cycling performance of LIBs. The substantial volume expansion of Si and the moderate volume expansion of graphite can induce mechanical stress, cracks, and the loss of electrical contact between active materials and conducting agents [
73,
75,
125,
126,
132]. These issues result in elevated internal resistance and irreversible cycling. Additionally, the repeated cracking and regeneration of the SEI layer during cycling may lead to the formation of an unstable and thick SEI, further compromising the battery's performance and capacity. This phenomenon can also contribute to the consumption of lithium ions, leading to a gradual depletion of the usable electrolyte over time. To tackle these challenges, researchers are actively exploring the use of different binder materials with diverse polymer characteristics in Si/G composite anodes [
76,
99,
100,
127,
128,
129]. The binder assumes a crucial role in upholding the structural integrity of the composite electrode, mitigating volume changes, and enhancing the cycling stability of the battery [
77,
78]. The meticulous selection and optimization of binder materials aim to minimize the stress and cracks induced by volume changes, enhance the electrical contact between active materials and conducting agents, and facilitate the formation of a stable and thin SEI layer [
119,
120]. This strategic approach can alleviate the issues of irreversible cycling and capacity fading linked with Si/G composite anodes, paving the way for stable cycling performance, even with higher Si content (up to 10%) in real-world commercial applications. The ongoing development of advanced binder materials and their effective integration into Si/G composite anodes remains a dynamic area of research [
122,
123]. Continued progress in this direction holds the potential to usher in the commercialization of high-performance LIBs featuring Si-based anode materials, effectively addressing the challenges associated with volume changes and SEI formation.
Mochizuki et al. and Wang et al. conducted research focusing on enhancing the performance of Si/G composite electrodes in LIBs using different binder materials [
100,
133]. In their study, Mochizuki et al. employed lithium poly-γ-glutamate (Li-PGlu) and four natural polymers as binders in Si/G composite electrodes with a mass loading of 1 mg cm
-2 [
100]. The initial reversible capacities achieved ranged from 800 to 1200 mA h g
-1 (1.3 mA h cm
-2), with a maximum CE of 51% to 79%. Li-PGlu emerged as an effective binder for suppressing electrolyte decomposition, providing uniform coverage of the active material surface. The binder was intentionally designed with a robust structure featuring polarized functional groups (-COOH and -NH-CO) (
Figure 3a-e), promoting smooth lithium-ion movement in the electrolyte/electrode interface through coordination functional effects of oxygen and nitrogen atoms. Surface chemical and binding characteristics were confirmed using hard X-ray photoelectron spectroscopy (XPS) in
Figure 3f-h. On the other hand, Luo et al. utilized UV-cured urushiol monomers as binders in a Si/G composite electrode with a Si/G ratio of 90:10 and a mass loading of 0.8 mg cm
-2 (
Figure 4) [
73]. The resulting electrode demonstrated a capacity of 603.3 mA h g
-1 and excellent capacity retention of 96.1% even after 400 cycles. The binder facilitated Si-O-C bond formation on the surface of Si powder, and strong interaction between the surface of Si particles and phenolic hydroxyl groups improved adhesion, limiting volume change and mitigating capacity loss and electrode degradation during volume expansion [
119,
120]. In another approach, Liu et al. synthesized a functional aqueous binder, PAA-vinyl triethoxy silane (VTEO), using lithium acrylate and VTEO. This binder was utilized in a composite electrode with a Si/G ratio of 97:3 and a mass loading of 0.8 mg cm
-2 [
74]. The PAA-VTEO binder exhibited a specific capacity of 470 mA h g
-1 and high cycle retention of 99% after 100 cycles. XPS analysis confirmed the formation of a strong 3D cross-linked network between the binder and silanol groups on the surface of Si nanoparticles, resulting in the formation of Si-OH groups and Si-O-Si covalent bonds. This highlighted the solid mechanical and binding properties of the PAA-VTEO binder, effectively mitigating volume expansion and enhancing electrochemical cycling stability.
Each of these studies underscores the crucial role of binder materials in enhancing the performance and stability of Si/G composite electrodes in LIBs [
119]. Binders possessing robust mechanical strength, elasticity, and effective adhesion to both Si and graphite particles are essential to suppress the volume expansion of Si during cycling and maintain the structural integrity of the electrode [
73,
74,
125]. The careful selection and design of binders, considering appropriate functional groups and chemical characteristics, can significantly influence the electrochemical performance of composite electrodes. This impact extends to key aspects such as capacity retention, cycling stability, and the adhesion between active materials and conducting agents [
99,
100,
127,
128]. Continued research and optimization of binder materials hold the promise of advancing the development of high-performance Si/G composite electrodes for the batteries of the future. It's crucial to recognize that the interaction not only with Si but also with graphite plays a pivotal role in determining the overall performance of Si/G composite electrodes [
77,
78]. Further exploration is necessary to gain a comprehensive understanding of the intricate interactions between different binders and both Si and graphite particles. This understanding will shed light on their effects on the mechanical and electrochemical properties of composite electrodes [
97,
131,
134,
135,
136]. Through meticulous optimization of binder selection and properties, the cycle stability and overall performance of Si/G composite electrodes in advanced LIBs can be improved.