The pursuit of higher energy density in lithium-ion batteries has led to the widespread adoption of thick electrode designs. By increasing the mass loading of active materials, thick electrodes effectively reduce the proportion and cost of inert components like current collectors and separators within the battery pack. However, this strategy introduces significant challenges, primarily concerning the electrode’s wettability by the liquid electrolyte. Inadequate wettability in thick electrodes restricts electrolyte penetration, limits the accessible interfacial area for electrochemical reactions, and creates inhomogeneous current and lithium-ion flux distributions. These issues collectively lead to compromised rate capability, accelerated capacity fade, and increased risks of localized heating and thermal runaway. Therefore, improving the wettability of thick electrodes is paramount for unlocking the full potential of high-energy-density lithium-ion battery systems. This review systematically examines recent advancements from three key perspectives: rational electrode structure design, strategic modification of electrode slurry components, and the application of external field assistance.
Fundamentals and Influencing Factors of Electrode Wettability
Wettability describes the tendency of a liquid to spread on a solid surface, governed by the interplay of interfacial tensions. The contact angle ($\theta$), formed at the solid-liquid-vapor triple-phase line, is the primary metric. A smaller $\theta$ indicates better wettability. The relationship is defined by Young’s equation for an ideal, smooth, homogeneous surface:
$$ \gamma_{SV} = \gamma_{SL} + \gamma_{LV} \cos\theta $$
where $\gamma_{SV}$, $\gamma_{SL}$, and $\gamma_{LV}$ represent the solid-vapor, solid-liquid, and liquid-vapor interfacial tensions, respectively. For the rough and porous surfaces characteristic of battery electrodes, two primary models are applicable:
- Wenzel State: The liquid completely penetrates the surface roughness, amplifying the intrinsic wetting behavior. The apparent contact angle $\theta_W$ is related to the intrinsic angle $\theta_Y$ and the surface roughness factor $r$ ($r > 1$) by: $$ \cos\theta_W = r \cos\theta_Y $$
- Cassie-Baxter State: The liquid rests on a composite surface of solid and trapped air, leading to a larger apparent contact angle. The relationship is: $$ \cos\theta_{CB} = f_s \cos\theta_Y + f_s – 1 $$ where $f_s$ is the solid fraction in contact with the liquid.
In practical lithium-ion battery electrodes, a mixed or “coexistence” state is often observed. The key factors influencing wettability in thick electrodes are multifaceted.
1. Electrode Structural Parameters
The microstructure of a thick electrode critically determines electrolyte infiltration and ion transport.
- Porosity ($\varepsilon_0$): This is the volume fraction of void space within the electrode, calculated as:
$$ \varepsilon_0 = 1 – \frac{\sum V_i}{V} $$
where $V_i$ is the volume of each solid component (active material, binder, conductive agent) and $V$ is the total electrode volume. Higher porosity provides more space for electrolyte storage and easier pathways for infiltration, directly enhancing wettability and accessible surface area. - Tortuosity ($\tau$): This parameter quantifies the convolutedness of the pore pathways. It is defined as the ratio of the actual mean path length ($L_e$) a diffusing species travels to the straight-line thickness of the electrode ($L$): $$ \tau = \frac{L_e}{L} $$ High tortuosity, common in randomly packed thick electrodes, severely hinders ionic transport, increases effective resistance, and exacerbates electrolyte depletion within the electrode bulk, effectively reducing its operable wettability.
2. Electrode Slurry Component Properties
The chemical nature and physical interactions of each constituent profoundly affect the final electrode’s interfacial energy with the electrolyte.
- Active Material: The surface chemistry (e.g., functional groups, coatings) dictates intrinsic affinity with electrolyte solvents. For instance, carbonaceous materials can be modified to improve polarity matching.
- Electrolyte: Its viscosity ($\eta$), surface tension ($\gamma_{LV}$), and solvation structure determine its ability to wick into pores. Lower viscosity and surface tension generally promote faster wetting.
- Binder: Traditionally hydrophobic binders like PVDF can create poor electrolyte-electrode interfaces. Hydrophilic or functionalized binders can significantly improve wetting.
- Separator: Its wettability controls the initial electrolyte supply rate to the electrode. Modified separators with ceramic coatings or surface treatments can enhance capillary-driven electrolyte flow.
The following table summarizes the key influencing factors and their impact on thick electrode wettability and performance.
| Factor Category | Specific Parameter / Component | Impact on Wettability & Performance |
|---|---|---|
| Electrode Structure | Porosity ($\varepsilon_0$) | Higher porosity facilitates electrolyte infusion and increases reactive surface area, improving rate capability and active material utilization. |
| Electrode Structure | Tortuosity ($\tau$) | Lower tortuosity shortens and straightens Li+ diffusion paths, reducing concentration polarization and enabling high-power performance in thick electrodes. |
| Electrode Structure | Pore Size Distribution | A hierarchical structure with well-connected macro/meso-pores enhances bulk infiltration, while micro-pores increase surface area. |
| Slurry Components | Active Material Surface | Polar surface groups or hydrophilic coatings reduce the solid-liquid interfacial tension ($\gamma_{SL}$), decreasing contact angle. |
| Slurry Components | Electrolyte Formulation | Low $\eta$ and $\gamma_{LV}$ improve kinetics of pore filling. Additives can modify SEI formation and interfacial compatibility. |
| Slurry Components | Binder Chemistry | Hydrophilic binders (e.g., CMC, PAA) improve wetting in aqueous processing and enhance adhesion, crucial for thick electrodes. |
| Slurry Components | Separator Modification | Coatings (e.g., ceramic, gel) improve electrolyte affinity and reservoir capacity, ensuring sustained supply to the electrode. |
Strategies for Improving Wettability in Thick Electrodes
1. Electrode Structure Design and Engineering
This approach focuses on architecting the electrode’s microstructure to create low-tortuosity, high-porosity pathways for efficient electrolyte penetration and ion transport.
a) Template-Assisted Methods: These involve incorporating sacrificial porogens into the electrode slurry that are removed during drying or post-processing, leaving behind designed pores.
- Gas-Foaming Agents: Compounds like ammonium bicarbonate ($NH_4HCO_3$) decompose upon heating during electrode drying, releasing gases ($NH_3$, $CO_2$, $H_2O$) that create pores. By controlling drying conditions, vertically aligned channel-like pores can be formed, significantly reducing through-plane tortuosity. For example, Zhang et al. fabricated vertically channeled silicon-based composite (VC-SC) electrodes using this method, which demonstrated markedly improved rate performance compared to conventional dense electrodes.
- Ice-Templating (Freeze Casting): The electrode slurry is frozen, causing solvent (usually water) to form ice crystals that template the pore structure. Subsequent sublimation leaves behind aligned pores replicating the ice crystal morphology. This method is excellent for creating long-range vertical channels.
While cost-effective and scalable, template methods can sometimes lead to pore collapse or non-uniform distribution if not carefully controlled.
b) Vapor Deposition Techniques: These methods allow for atomic-level modification of electrode surfaces or the growth of nanostructures within pores.
- Chemical Vapor Deposition (CVD): Can be used to grow conformal coatings or nanostructures like vertical graphene nanosheets on active material particles. Ge et al. combined spray drying with CVD to grow vertical graphene (VGs) on Si/C composite particles. The VGs created a highly porous, conductive network with abundant mesopores, dramatically increasing the electrode’s specific surface area and enhancing electrolyte permeability and ion transport kinetics.
These techniques offer precise control over surface chemistry and nano-porosity but may be less effective for creating the large, percolating macro-pores needed for bulk electrolyte transport in very thick electrodes.
c) 3D Printing (Additive Manufacturing): This represents a paradigm shift, enabling the direct, digital fabrication of electrodes with pre-designed, optimized architectures.
- Direct Ink Writing (DIW): This extrusion-based 3D printing method allows for the layer-by-layer construction of electrodes with customized geometries, such as grid, interdigitated, or gyroid structures. The primary advantage is the unparalleled control over pore architecture. Researchers can design electrodes with vertically aligned channels, gradient porosity, or bio-inspired fractal patterns that minimize tortuosity ($\tau \rightarrow 1$ for ideal straight channels).
- Synergy with Other Techniques: 3D printing can be combined with other methods. For instance, Li et al. used DIW followed by a unidirectional freeze-drying process to create hierarchical porous structures in ultra-thick LTO and LFP electrodes. This combination yielded electrodes with extremely high active mass loadings (~74 mg/cm²) and low tortuosity, resulting in exceptional areal capacity and rate performance.
The freedom in structural design offered by 3D printing directly addresses the core wettability challenges of thick electrodes in lithium-ion battery technology by engineering efficient ion highways. It also facilitates the integration of electrode and current collector, reducing interfacial resistance.
2. Modification of Electrode Slurry Components
This strategy focuses on altering the chemical and physical properties of the electrode’s constituents to improve their intrinsic affinity with the electrolyte.
a) Active Material Surface Modification:
- Heteroatom Doping: Introducing atoms like nitrogen (N), oxygen (O), or phosphorus (P) into carbon matrices (e.g., graphene, porous carbon) changes their surface electronic structure and introduces polar sites. Xu et al. showed that N-doping of graphene removed oxygenated groups and increased the material’s affinity for organic carbonate electrolytes, leading to improved electrochemical performance in lithium-ion battery anodes.
- Conformal Coating: Applying ultrathin, uniform coatings of carbon, polymers, or metal oxides on active material particles can serve multiple purposes. For example, a carbon coating on LiFePO₄ or on Bi₂O₃ nanoparticles (as shown by Fang et al.) not only enhances electronic conductivity but can also be engineered to present a more electrolyte-philic surface, improving wettability and stabilizing the electrode-electrolyte interface.
b) Electrolyte Engineering:
- Low-Viscosity Solvents & Additives: Formulating electrolytes with solvents like fluorinated ethers or linear carboxylates can lower overall viscosity ($\eta$) and surface tension ($\gamma_{LV}$), promoting faster capillary infiltration into thick electrodes. Li et al. proposed a “superwettable” electrolyte engineering approach using an ultra-low concentration electrolyte to achieve uniform SEI formation on graphite anodes, which is particularly beneficial for thick electrodes where wetting is challenging.
- Wetting Agents: Specific additives, such as small amounts of surfactants or compounds with both hydrophobic and hydrophilic moieties, can be added to the electrolyte to reduce the interfacial tension ($\gamma_{SL}$) between the electrode and the electrolyte, effectively decreasing the contact angle.
c) Binder Functionalization: Moving beyond inert binders like PVDF to functional polymers can drastically improve electrode wettability and adhesion.
- Hydrophilic Binders: For aqueous battery systems (e.g., Zn-ion), but also relevant for solvent processing, binders like carboxymethyl cellulose (CMC), sodium alginate, or polyacrylic acid (PAA) are inherently hydrophilic. Lee et al. demonstrated that sulfonating PVDF (introducing -SO₃H groups) transformed it from hydrophobic to hydrophilic, greatly enhancing electrode wettability in aqueous Zn-ion batteries.
- Multifunctional Binders: Modern binder design incorporates self-healing, conductive, or cross-linkable properties that not only improve wetting but also enhance mechanical integrity and electronic connectivity in thick electrodes, mitigating cracking and delamination.
d) Separator Functionalization: Since the separator is the conduit for electrolyte to reach the electrode, its properties are crucial.
- Ceramic/Polymer Coatings: Coating commercial polyolefin separators with a layer of ceramic nanoparticles (e.g., Al₂O₃, SiO₂) or hydrophilic polymers creates a composite separator with higher surface energy, better thermal stability, and increased electrolyte uptake. Jeon et al., through multiphase lattice Boltzmann simulations and experiments, elucidated that the ceramic coating layer (CCL) provides additional nano-porous space for electrolyte storage and, more importantly, generates strong capillary forces that actively pull electrolyte towards the electrode side, significantly accelerating the overall battery wetting process.
The table below summarizes the component modification strategies and their primary mechanisms of action.
| Component | Modification Strategy | Primary Mechanism for Improved Wettability | Key Benefit for Thick Electrodes |
|---|---|---|---|
| Active Material | Heteroatom Doping; Conformal Coating | Reduces solid-liquid interfacial tension ($\gamma_{SL}$) by introducing polar surface groups or compatible coating surfaces. | Enhances electrolyte affinity at the primary particle level, improving interfacial charge transfer. |
| Electrolyte | Low-$\eta$/$\gamma_{LV}$ solvents; Wetting additives | Reduces viscous resistance to flow and lowers the contact angle via surface tension modulation. | Enables faster and more complete pore filling throughout the thick electrode bulk. |
| Binder | Use of hydrophilic polymers; Sulfonation | Changes binder surface from hydrophobic to hydrophilic, promoting electrolyte spreading on binder networks. | Improves slurry rheology, electrode adhesion, and creates hydrophilic percolating networks. |
| Separator | Ceramic/Gel Polymer Coating | Increases surface energy and capillary force; acts as an electrolyte reservoir. | Accelerates initial wetting and ensures continuous electrolyte supply during cycling. |
3. External Field Assistance
Beyond material and structural changes, applying external physical fields can provide an additional driving force to overcome the capillary and viscous resistances hindering electrolyte infiltration.
- Pressure & Vacuum: Applying a pressure differential across the electrode stack (e.g., vacuum filling) forces electrolyte into the pores, reducing filling time. However, excessive pressure may damage electrode microstructure.
- Temperature Control: Moderately elevating temperature lowers electrolyte viscosity ($\eta$), according to the Arrhenius-type relationship, facilitating easier flow into pores. It must be balanced against solvent volatility and material stability.
- Ultrasonic Agitation: Ultrasound waves create micro-scale agitation and cavitation bubbles that can help dislodge trapped air and promote electrolyte penetration into small or blind pores.
- Electric Field Assistance (Electrocapillary Effect): This is an emerging and powerful technique. Cui et al. demonstrated that applying a small external voltage during electrolyte filling can induce an electric field within the electrical double layer (EDL) at the electrode-electrolyte interface. This changes the electrostatic interactions and effectively reduces the solid-liquid interfacial tension ($\gamma_{SL}$) via the electrocapillary effect, thereby decreasing the contact angle and enhancing wetting. Their work, validated on commercial LiFePO₄/graphite pouch cells, showed this method could significantly boost electrolyte penetration depth and uniformity in porous electrodes.
Summary and Future Perspectives
Enhancing the wettability of thick electrodes is a critical enabler for next-generation high-energy-density lithium-ion battery technology. The strategies reviewed—structural design, component modification, and external field assistance—each target different aspects of the wettability challenge. Structural engineering (especially via 3D printing) directly optimizes transport pathways, component modification improves intrinsic interfacial compatibility, and external fields provide dynamic wetting enhancement.
Looking forward, several key research directions and challenges remain:
- Multi-strategy Integration and Synergy: The most significant gains will likely come from the intelligent combination of strategies. For example, 3D-printed electrodes with optimized pore structures could be fabricated using functional inks containing hydrophilic binders and surface-modified active materials, followed by electrolyte filling assisted by an electric field. Understanding the synergies between these approaches is crucial.
- Advanced Characterization and Modeling: In-situ and operando techniques (e.g., neutron imaging, X-ray tomography, MRI) are needed to visualize and quantify electrolyte infiltration and distribution within operating thick electrodes. Coupled with multi-physics modeling (combining capillary flow, electrochemistry, and mechanics), these tools will enable predictive design of optimal electrode architectures.
- Scalability and Cost-Effectiveness: While techniques like 3D printing offer unparalleled design freedom, their scalability and throughput for mass production of lithium-ion battery electrodes need substantial improvement. Developing high-speed additive manufacturing or cost-effective templating methods that replicate the benefits of designed structures is essential.
- System-level Compatibility: Wettability improvements must be evaluated in the context of full-cell performance, including long-term cycling stability, SEI/CEI evolution, and gas generation under realistic conditions.
- Beyond Conventional Liquid Electrolytes: Research should extend to solid-state and semi-solid batteries. The wetting concept translates to solid-solid interfacial contact, where strategies to maximize interfacial area and intimacy between solid electrodes and solid electrolytes are equally vital.
In conclusion, overcoming the wettability barrier in thick electrodes requires a holistic approach that merges insights from materials science, electrochemistry, and manufacturing engineering. Continued innovation in this area is fundamental to realizing the promise of safer, cheaper, and higher-energy-density lithium-ion battery systems for widespread applications from electric vehicles to grid storage.