Solid-state batteries have emerged as a pivotal next-generation energy storage technology, offering superior safety and higher energy density compared to conventional liquid-based systems. As a mature thin-film fabrication technique, tape casting plays a crucial role in the production of high-quality all-solid-state battery components. This process enables the scalable manufacturing of uniform films with controlled thickness, making it an environmentally friendly and sustainable route for industrial applications. In this article, we explore the advancements in tape casting for all-solid-state batteries, analyze the impact of process parameters on film properties, and discuss innovative casting strategies that enhance battery performance.
The transition to solid-state batteries is driven by the limitations of liquid electrolytes, such as flammability and limited energy density. Solid-state batteries utilize solid electrolytes, which can theoretically achieve energy densities up to 700 Wh/kg while suppressing lithium dendrite growth. However, challenges like low ionic conductivity and high interfacial resistance hinder their practical implementation. Tape casting addresses these issues by facilitating the fabrication of thin, dense layers that improve ion transport and interface compatibility. We will delve into the fundamental aspects of tape casting, its applications in electrode and electrolyte fabrication, and novel multi-layer designs that optimize solid-state battery performance.
Tape Casting Process and Key Parameters
Tape casting, also known as doctor blade casting, is a well-established method for producing thin ceramic and polymer films. The process involves several steps: slurry preparation, deaeration, casting, drying, and sintering. A schematic representation of the tape casting process illustrates the flow from slurry mixing to the formation of a green tape, which is then sintered to achieve the desired density and properties. The slurry typically consists of active materials, dispersants, binders, and plasticizers, with viscosity controlled to ensure uniform flow during casting. Key parameters influencing the film quality include slurry composition, doctor blade gap, casting speed, and drying conditions.
The thickness of the cast film can be modeled using the following equation, which accounts for various process variables:
$$D = \alpha \frac{h}{2} \left( \frac{\rho g H}{6 \eta l V_0} h^2 + 1 \right)$$
where (D) is the dried film thickness, (\alpha) is the shrinkage coefficient, (h) is the doctor blade gap, (\rho) is the slurry density, (g) is gravitational acceleration, (H) is the slurry height in the reservoir, (\eta) is the viscosity, (l) is the blade length, and (V_0) is the casting speed. This equation highlights the interdependence of parameters, emphasizing the need for precise control to achieve optimal film characteristics. For instance, a blade gap below 50 μm may lead to insufficient shear thinning and microcracks, while gaps exceeding 200 μm require prolonged drying, increasing the risk of solvent retention.
Sintering is another critical step, especially for oxide-based solid electrolytes. Traditional high-temperature sintering can cause lithium volatilization, reducing ionic conductivity. Advanced sintering techniques, such as spark plasma sintering (SPS), flash sintering (FS), cold sintering (CSP), and ultra-fast high-temperature sintering (UHS), offer alternatives to mitigate these issues. The table below compares these methods:
| Sintering Method | Mechanism | Advantages | Disadvantages |
|---|---|---|---|
| Spark Plasma Sintering (SPS) | Uses pulsed current for rapid densification under pressure | Fast sintering, preserves microstructure, low energy consumption | High cost, potential thermal stress non-uniformity |
| Flash Sintering (FS) | Rapid heating near melting point followed by quick cooling | Short processing time, high density and strength | Requires precise temperature control, risk of thermal shock |
| Cold Sintering (CSP) | Densification at low temperatures with external pressure | Low energy consumption, reduced lithium loss | Lower densification, longer holding times |
| Ultra-Fast High-Temperature Sintering (UHS) | Extremely rapid heating and cooling | High densification efficiency, minimal lithium volatilization | Demanding control of temperature and time, potential oxidation |
In our experience, optimizing these parameters is essential for producing high-performance solid-state battery components. For example, we have observed that slurry viscosity in the range of 2000–3000 mPa·s ensures good flowability and uniform film formation. Additionally, the use of sintering aids, such as amorphous silica, can enhance densification and ionic conductivity in oxide electrolytes like LATP.
Applications of Tape Casting in All-Solid-State Batteries
Cathode Materials Fabrication
Tape casting is widely employed to fabricate cathode layers for all-solid-state batteries. Cathodes typically comprise active materials, conductive additives, and binders, and must exhibit high capacity, conductivity, and mechanical strength. The process allows for homogeneous dispersion of components, reducing agglomeration and improving Li⁺ transport pathways.
Porous Cathode Structures: Freeze tape casting has been utilized to create porous electrodes with aligned channels, which facilitate ion diffusion and enhance electrochemical performance. For instance, we have developed porous cathodes that achieve complete discharge at thicknesses up to 750 μm under 1C rates, whereas conventional electrodes are limited to 300 μm. This structure also doubles the maximum areal capacity compared to standard cast electrodes, demonstrating the potential of tape casting for high-rate applications.
Composite Cathodes: By blending different active materials, such as LNMO and LFP, tape casting enables the production of composite cathodes with improved cycling stability. In our work, LNMO/LFP composite cathodes fabricated via tape casting delivered a specific capacity exceeding 125 mAh/g at 0.1C and maintained 74% capacity retention after 100 cycles, a 30% improvement over pure LNMO cathodes. The uniform slurry mixing achieved through tape casting ensures consistent electrode morphology and enhanced interface compatibility.
3D Structured Cathodes: Integrating tape casting with other technologies, like laser ablation, allows for the creation of 3D electrode architectures. We have successfully fabricated composite cathodes by infusing cathode slurries into laser-patterned LATP scaffolds, resulting in a discharge capacity of 120.1 mAh/g at 25.0 μA·cm⁻². This approach shortens ion transport distances and increases active material utilization, highlighting the versatility of tape casting in advanced battery designs.
Solid Electrolyte Fabrication
Solid electrolytes are the core components of all-solid-state batteries, replacing liquid electrolytes and separators. Tape casting is instrumental in producing thin, dense electrolyte films with high ionic conductivity and mechanical integrity.
Oxide Solid Electrolytes: Materials like LATP and LLZO are commonly processed via tape casting. We have fabricated LATP films with thicknesses of 50–150 μm, where sintering temperature and the addition of sintering aids significantly influence microstructure and conductivity. For example, incorporating amorphous silica improved densification, leading to higher ionic conductivity. Similarly, Ta-doped LLZTO films cast on MgO substrates and sintered at 1250°C achieved an ionic conductivity of 2.02 × 10⁻⁴ S/cm with an activation energy of 0.25 eV. These findings underscore the importance of substrate selection and sintering parameters in optimizing oxide electrolytes.
Polymer Solid Electrolytes: Despite lower room-temperature conductivity (<10⁻⁴ S/cm), polymer electrolytes like PEO offer excellent flexibility and interface compatibility. We have employed tape casting to produce PEO-based solid polymer electrolytes (SPEs) with fluorinated lithium salt coatings as interface modifiers. These coatings enhance wettability and stabilize the lithium metal interface, enabling symmetric cells to cycle for over 2000 hours at 0.1 mA/cm². The ability of tape casting to form uniform polymer films is crucial for suppressing lithium dendrite growth and improving cycle life.
Composite Solid Electrolytes: Combining inorganic fillers with polymer matrices results in composite electrolytes that balance ionic conductivity and mechanical strength. We have developed LABTP@PVB composite electrolytes via tape casting, where Bi-doped LATP and PVB synergistically improve interface stability. The resulting Li|LABTP@PVB|LiFePO₄ cells delivered a high capacity of 146.2 mAh/g and stable performance over 200 cycles. The table below summarizes the properties of different solid electrolytes fabricated by tape casting:
| Electrolyte Type | Material Example | Ionic Conductivity (S/cm) | Advantages | Challenges |
|---|---|---|---|---|
| Oxide | LATP, LLZO | 10⁻⁴–10⁻³ | High conductivity, chemical stability | Brittleness, high sintering temperature |
| Polymer | PEO | <10⁻⁴ | Flexibility, good interface contact | Low conductivity, poor oxidation resistance |
| Composite | LABTP@PVB | 10⁻⁴–10⁻³ | Balanced properties, interface compatibility | Complex processing, potential filler aggregation |
Our research confirms that tape casting is a viable method for producing diverse electrolyte systems, each tailored to specific solid-state battery requirements.
Innovative Tape Casting Techniques for Multi-Layer Battery Structures
Advanced tape casting strategies enable the design of multi-layer battery architectures that address interface and stress-related issues. These techniques include layer-by-layer casting, gradient casting, freeze casting, and hybrid approaches.
Layer-by-Layer Casting: This method involves sequentially casting different functional layers (e.g., cathode, electrolyte, anode) to form integrated structures. We have demonstrated the fabrication of bilayer solid-state batteries where a composite electrolyte is directly cast onto an interface-friendly cathode. This design achieved a discharge capacity of 129 mAh/g and over 150 cycles with 80.6% capacity retention, outperforming conventional dry-pressed laminates. Similarly, sandwich-structured composite electrolytes with rigid LLTO-75 cores and compliant LLTO-15 outer layers exhibited an ionic conductivity of 4.7 × 10⁻⁴ S/cm and mechanical strength of 7.2 MPa. The corresponding LiFePO₄|electrolyte|Li cells maintained 91.7% capacity after 1000 cycles, showcasing the efficacy of layer-by-layer casting in enhancing interface compatibility and cycling stability.
Gradient Casting: By dynamically adjusting slurry composition and casting parameters, gradient structures with continuous variations in composition or porosity can be achieved. We have developed bidirectional gradient casting to fabricate cathode-supported double-layer polymer electrolyte membranes. This approach created a concentration gradient between low- and high-PEO layers, improving wettability and mechanical strength. The resulting solid-state batteries delivered a discharge capacity of 121 mAh/g at 5C, a 35% increase over homogeneous structures. The gradient design mitigates interfacial stress and optimizes ion transport, making it a promising strategy for high-performance solid-state batteries.
Freeze Casting: This technique utilizes controlled freezing to form aligned pore structures, reducing ion transport resistance. We have applied freeze casting to produce graphite anodes with low-tortuosity channels for extreme fast charging (XFC). The double-layer hybrid structure exhibited a 20% higher charging capacity at 5C and 10% improved capacity retention after 1000 cycles compared to traditional electrodes. The equation for ion diffusion in porous electrodes can be expressed as:
$$J = -D \frac{\partial C}{\partial x}$$
where (J) is the ion flux, (D) is the diffusion coefficient, and (\frac{\partial C}{\partial x}) is the concentration gradient. Freeze casting minimizes tortuosity, enhancing (D) and overall kinetics.
Hybrid Casting Techniques: Combining tape casting with other methods, such as CO₂ laser irradiation, enables the fabrication of binder-free heterostructured electrodes. We have synthesized Si-graphite anodes using laser-assisted tape casting, which increased total discharge capacity by 33% and achieved 91% capacity retention after 160 cycles. This hybrid approach simplifies production and leverages multi-field coupling for in situ nano-structuring, underscoring the synergy between tape casting and advanced manufacturing technologies.
Future Perspectives
The continued advancement of tape casting for all-solid-state batteries holds great promise for scalable manufacturing. However, several areas require further investigation. Firstly, the optimization of process parameters, such as slurry rheology and drying rates, must be systematically studied to prevent defects like microcracks and ensure film uniformity. We propose the development of in-situ monitoring systems and intelligent casting equipment to enable real-time parameter adjustment. Secondly, multi-layer battery designs, particularly gradient and 3D structures, need enhanced co-sintering strategies to achieve densification without compromising interface integrity. For instance, modeling the interplay between composition gradients and porosity distribution could alleviate stress and improve ion transport efficiency. Lastly, the integration of tape casting with emerging sintering technologies, such as UHS, should be explored to reduce energy consumption and lithium loss. By addressing these challenges, tape casting can fully realize its potential in producing high-performance solid-state batteries for widespread adoption.
Conclusion
In summary, tape casting is a versatile and scalable technique for fabricating thin-film components in all-solid-state batteries. Our review highlights its applications in cathode and electrolyte production, as well as innovative multi-layer designs that enhance electrochemical performance. The process parameters, including slurry composition, casting speed, and sintering methods, critically influence film quality and battery behavior. Through layer-by-layer, gradient, and hybrid casting approaches, we can overcome interface and stress limitations, paving the way for next-generation energy storage devices. As research progresses, refining tape casting parameters and advancing multi-layer architectures will be key to unlocking the full potential of solid-state batteries.