Introduction
The rapid development of lithium-ion batteries (LIBs) has revolutionized energy storage systems, yet the limitations of conventional graphite anodes (theoretical capacity: 372 mAh·g⁻¹) necessitate exploration of high-capacity alternatives. Silicon (Si), with its ultrahigh theoretical capacity (4200 mAh·g⁻¹), emerges as a promising candidate. However, severe volume expansion (~300%) during lithiation/delithiation cycles leads to electrode disintegration and rapid capacity fade. Traditional polyvinylidene fluoride (PVDF) binders, reliant on weak van der Waals forces, fail to accommodate Si’s mechanical stresses. This study addresses these challenges by designing a water-soluble, eco-friendly binder system based on sodium carboxymethyl cellulose (CMC-Na) and polyethyleneimine (PEI), leveraging electrostatic interactions for enhanced electrode integrity.
Materials and Methodology
Binder Synthesis
The cross-linked binder (C-PEI-10%) was prepared by blending anionic CMC-Na (1.5–3.0 kDa, DS = 0.8–1.2) with cationic PEI (M_w = 70,000) at a 9:1 mass ratio. The mixture underwent vigorous stirring (500 rpm, 2 h) followed by vacuum drying (80°C, 8 h). Electrostatic cross-linking between CMC-Na’s carboxylate (–COO⁻) and PEI’s protonated amines (–NH₃⁺) formed a 3D network (Figure 1).
Electrode Fabrication
Electrode slurries were formulated with:
- Active material: Nano-Si (80–100 nm, 60 wt%)
- Conductive agent: Carbon black (20 wt%)
- Binder: C-PEI-10%, CMC-Na, or PVDF (20 wt%)
Slurries were cast onto Cu foil, dried (80°C, 12 h), and calendared to 50 μm thickness. CR2032 coin cells were assembled using Li foil counter-electrodes and 1 M LiPF₆ in EC:EMC (1:1 v/v) with 10% FEC electrolyte.
Key Findings
1. Mechanical and Adhesive Properties
The 180° peel test revealed superior adhesion of C-PEI-10% (Table 1).
Table 1. Peel strength of Si electrodes with different binders.
| Binder | Average Peel Force (N) | Post-Electrolyte Soaking (N) |
|---|---|---|
| PVDF | 0.57 ± 0.05 | 0.21 ± 0.03 |
| CMC-Na | 1.52 ± 0.11 | 0.89 ± 0.07 |
| C-PEI-10% | 2.42 ± 0.15 | 2.40 ± 0.14 |
The 3D network in C-PEI-10% provided robust interfacial adhesion, resisting electrolyte-induced swelling.
2. Electrochemical Performance
A. Cycling Stability
At 0.2 C (1 C = 3800 mA·g⁻¹), C-PEI-10% exhibited exceptional capacity retention (Figure 2):
Equation 1. Capacity retention after n cycles:Retention (%)=C4Cn×100
where Cn = capacity at cycle n, C4 = capacity at cycle 4 (excluding SEI formation).
| Binder | Initial Capacity (mAh·g⁻¹) | Capacity at 100 Cycles (mAh·g⁻¹) | Retention (%) |
|---|---|---|---|
| PVDF | 1536.2 | 166.9 | 10.86 |
| CMC-Na | 2452.4 | 1430.3 | 58.33 |
| C-PEI-10% | 2724.8 | 2249.1 | 82.54 |
B. Rate Capability
C-PEI-10% maintained high capacities at elevated rates (Figure 3):
Table 2. Rate performance of Si electrodes.
| Current Density (C) | C-PEI-10% (mAh·g⁻¹) | CMC-Na (mAh·g⁻¹) | PVDF (mAh·g⁻¹) |
|---|---|---|---|
| 0.1 | 2414.4 | 2099.3 | 928.9 |
| 1.0 | 2030.5 | 1477.3 | 16.7 |
3. Ionic Transport and SEI Analysis
Electrochemical impedance spectroscopy (EIS) and GITT quantified Li⁺ diffusion coefficients (DLi+):
Equation 2. Li⁺ diffusion coefficient:DLi+=πτ4(MBSmBVm)2(ΔEτΔES)2
where τ=relaxation time, mB=active mass, Vm=molar volume, S=electrode area.
Table 3. DLi+ values during lithiation/delithiation.
| State | C-PEI-10% (cm²·s⁻¹) | CMC-Na (cm²·s⁻¹) |
|---|---|---|
| Discharging | 2.7 × 10⁻¹¹ | 1.2 × 10⁻¹¹ |
| Charging | 3.1 × 10⁻¹¹ | 1.5 × 10⁻¹¹ |
XPS analysis confirmed a LiF-rich SEI layer on C-PEI-10% electrodes (92.05% LiF vs. 69.07% for CMC-Na), enhancing interfacial stability.
Discussion
Mechanistic Insights
The synergy between CMC-Na and PEI arises from:
- Electrostatic Cross-Linking: –COO⁻/–NH₃⁺ interactions create a resilient 3D network.
- Hydrogen Bonding: –OH (CMC-Na) and –NH (PEI) groups further stabilize the matrix.
- Li⁺ Coordination: PEI’s amines act as Lewis bases, facilitating Li⁺ transport.
Comparative Advantages Over PVDF
- Eco-Friendly: Water-soluble processing eliminates toxic NMP solvents.
- Cost-Effective: CMC-Na (5–10/kg)vs.PVDF(50–100/kg).
- Superior Adhesion: 4.2× higher peel strength than PVDF.
Conclusion
The C-PEI-10% binder system demonstrates transformative potential for silicon anodes in lithium-ion batteries. Key achievements include:
- High Capacity Retention: 82.54% after 100 cycles at 0.2 C.
- Robust Rate Performance: 2030.5 mAh·g⁻¹ at 1 C.
- Stable SEI Layer: LiF-dominated interface minimizes parasitic reactions.
This work underscores the viability of biopolymer-derived binders in advancing next-generation lithium-ion batteries, aligning with global sustainability goals.