1. Introduction
Lithium-ion batteries (LIBs) have revolutionized energy storage technologies, powering applications ranging from portable electronics to electric vehicles. However, the energy density of conventional LIBs, primarily limited by graphite anodes (theoretical capacity: 372 mAh/g), is approaching its theoretical ceiling. To meet the growing demand for higher energy densities, researchers are exploring next-generation electrode materials such as silicon-based anodes (3579 mAh/g) and lithium metal anodes (3860 mAh/g). Despite their potential, these materials face challenges like severe volume expansion, unstable electrode-electrolyte interfaces, and dendrite formation. Scaffold materials—three-dimensional (3D) frameworks with tunable porosity, mechanical strength, and chemical stability—offer promising solutions to these issues. This article reviews recent advancements in scaffold materials and their design strategies to enhance the performance of high-energy lithium-ion batteries.
2. Classification and Properties of Scaffold Materials
Scaffold materials are broadly categorized into organic and inorganic frameworks, each offering distinct advantages for LIB applications (Table 1).
| Category | Subtypes | Key Features |
|---|---|---|
| Organic Scaffolds | Metal-Organic Frameworks (MOFs) | High porosity, tunable pore size, dual redox activity (metal nodes + organic linkers). |
| Covalent Organic Frameworks (COFs) | Crystalline, π-conjugated structures, redox-active functional groups. | |
| Hydrogen-Bonded Frameworks (HOFs) | Self-healing properties, reversible hydrogen bonding. | |
| Inorganic Scaffolds | Oxide Frameworks | High thermal/chemical stability, ion-conductive pathways. |
| Metallic Frameworks | High electrical conductivity, mechanical robustness. |
Key Equations:
- Porosity (P) of scaffold materials:P=VporesVtotal×100%P=VtotalVpores×100%
- Ionic Conductivity (σ) in MOF-based electrolytes:σ=n⋅q⋅μσ=n⋅q⋅μwhere nn = charge carrier density, qq = charge, μμ = mobility.
3. Applications of Scaffold Materials in Lithium-Ion Batteries
3.1 Cathode Materials and Protection
MOF/COF-Based Cathodes:
- MOFs: Redox-active metal nodes (e.g., Fe, Cu, V) and organic linkers enable dual lithium storage mechanisms. For example, FeFe(CN)66 MOFs deliver 160 mAh/g with 90% capacity retention after 300 cycles.
- COFs: Conjugated structures with C=O or C=N groups achieve theoretical capacities up to 773 mAh/g. A BQ1-COF network demonstrated an energy density of 1033 Wh/kg, surpassing conventional LiCoO22.
Challenges:
- Low electronic conductivity (<10−5<10−5 S/cm) and compaction density limit volumetric energy density.
- Strategies: Hybridization with conductive additives (e.g., carbon nanotubes) or doping with heteroatoms.
Cathode Protection:
- MOF/COF coatings suppress transition metal dissolution (e.g., Ni, Co, Mn) in high-voltage cathodes. For instance, NH22-MIL-53(Ti) coatings reduce capacity fade to 19% after 500 cycles in NCM622 cathodes.
3.2 Separators
Scaffold-modified separators enhance safety and ion transport:
- MOF-Coated Separators: ZIF-8-modified polypropylene (PP) separators increase Li++ transference number (tLi+=0.80tLi+=0.80) and suppress polysulfide shuttling in Li-S batteries.
- COF-Based Separators: Pyr-2D COFs improve thermal stability (>200°C) and mechanical strength.
Key Metrics:
- Li++ Transference Number:tLi+=σLi+σtotaltLi+=σtotalσLi+
- Ion Diffusion Coefficient (D):D=RTn2F2AπσD=n2F2AπσRT
3.3 Solid-State Electrolytes
Scaffold materials address the low ionic conductivity and dendrite growth in solid-state electrolytes:
- MOF/Polymer Hybrids: CuBTC-PEO composites achieve σ=8.76×10−4σ=8.76×10−4 S/cm at 30°C.
- COF-Based Electrolytes: PEG-modified COFs exhibit σ=0.153σ=0.153 mS/cm and inhibit Li dendrites for >1800 hours.
Advantages:
- Flame retardancy (e.g., TPP-loaded PAN fibers).
- High Li++ migration number (tLi+=0.97tLi+=0.97) in single-ion conductors.
3.4 Anodes and Current Collectors
Silicon Anodes:
- Yolk-Shell Structures: Carbon-encapsulated Si nanoparticles (e.g., pomegranate-inspired designs) retain 97% capacity after 1000 cycles.
- Compaction Strategies: Si@SiO22@C microspheres balance volume expansion and energy density.
Lithium Metal Anodes:
- 3D Hosts: Porous CuZn alloys and Ni foam reduce volume strain and guide uniform Li deposition.
- Current Collector Modifications: SiO22-gridded Cu foils lower nucleation overpotential (η=15η=15 mV).
Key Equation for Li Deposition:η=RTαFln(jj0)η=αFRTln(j0j)
where jj = current density, j0j0 = exchange current density.
4. Challenges and Future Directions
4.1 Persistent Challenges
- Low Conductivity: Organic scaffolds (MOFs/COFs) suffer from poor electronic/ionic conductivity.
- Scalability: Complex synthesis routes (e.g., solvothermal methods) hinder industrial adoption.
- Volume Energy Density: Porous structures reduce active material loading.
4.2 Emerging Solutions
- Multifunctional Scaffolds: Integrating conductive polymers (e.g., PEDOT:PSS ) with MOFs/COFs.
- Machine Learning: Accelerating material discovery for optimized pore size and redox activity.
- Hybrid Architectures: Combining organic and inorganic scaffolds (e.g., MOF@TiO22) for synergistic effects.
Future Metrics:
- Target ionic conductivity: >1>1 mS/cm at 25°C.
- Cycle life: >1000>1000 cycles with <0.1<0.1% capacity decay per cycle.
5. Conclusion
Scaffold materials represent a transformative approach to overcoming the limitations of high-energy lithium-ion batteries. By engineering porosity, mechanical resilience, and redox activity, these frameworks enhance stability across cathodes, separators, electrolytes, and anodes. While challenges in conductivity and scalability persist, advancements in hybrid designs and computational modeling promise to unlock their full potential. As the demand for sustainable energy storage grows, scaffold materials will play a pivotal role in realizing lithium-ion batteries with energy densities exceeding 500 Wh/kg, paving the way for a carbon-neutral future.