3D printing has gradually become an important processing method for solid electrolytes due to its low cost, fast speed, and good formability. This technology primarily manufactures required material components based on three-dimensional CAD data through a layer-by-layer material accumulation method.
The application of 3D printing in the energy storage field provides new opportunities for expanding the manufacturing of multi-dimensional/multi-scale complex structures and high-performance flexible wearable devices. Typically, the physical or chemical deposition manufacturing methods commonly used in the battery industry limit the complexity and diversity of batteries in terms of size and shape. 3D printing can meet the customization requirements of batteries, adapting to the demands for battery shapes and sizes in future complex scenarios.
Direct ink writing (DIW) and stereolithography (SLA) are the most commonly used technologies in the research of 3D printed solid electrolytes. DIW is an extrusion-based 3D printing technology that manufactures free-form shapes by dispensing ink through a movable nozzle controlled by pneumatic or mechanical pump devices. SLA is a 3D printing technology based on photopolymerization.
Due to the poor air stability of sulfide and halide solid electrolytes, their application in 3D printing is limited. However, oxide solid electrolytes and polymer solid electrolytes have sparked significant research interest in the field of 3D printing.
Oxide-based solid electrolytes are widely used in all-solid-state batteries due to their non-flammability and good electrochemical stability, such as garnet-type LLZO. However, the electrolytes produced by traditional powder pressing methods are almost all planar shapes, resulting in a larger thickness and higher bulk resistance. Additionally, poor electrode-electrolyte interface contact leads to high area-specific resistance (ASR), resulting in a higher overall resistance value for the battery. To address this issue, Dennis et al. printed various structures of LLZO solid electrolytes using ink. This thin and complex electrolyte membrane structure effectively reduces the area-specific resistance of the battery. Furthermore, the continuous layering and structuring of the electrolyte structure also effectively prevents dendrite propagation.
In addition to oxide-based solid electrolytes, 3D printing technology has also been widely researched in polymer solid electrolytes. However, the performance of pure polymer electrolytes produced by 3D printing is not outstanding and needs to be combined with other materials to form composite polymer electrolytes to better leverage the advantages of different components, thereby enhancing the overall performance of solid electrolytes.
Cheng et al. used the direct ink writing (DIW) method to manufacture composite polymer electrolytes based on a PEO matrix, using silane-treated hexagonal boron nitride (S-hBN) sheets as fillers. Due to the strong hydrogen bonding interactions, the silane coupling agent can improve the compatibility between the filler and the polymer matrix. Thanks to the addition of S-hBN, the thermal conductivity of the solid electrolyte was effectively enhanced. Before printing, the well-dispersed S-hBN filler PEO slurry ink was placed in a syringe at 40°C. During the printing process, the ink was extruded, and the high shear force aligned the S-hBN sheets. Finally, the electrolyte with S-hBN filler printed on the substrate was cured under ultraviolet light, completing the preparation of PEO/S-hBN.
In addition to controlling material formulations and internal microstructures, the external geometries such as the shape and thickness of solid electrolytes can also be adjusted using 3D printing. Cheng et al. achieved 3D printing of PVDF-co-HFP-based composite solid electrolytes using high-temperature DIW. The composite solid electrolyte consists of a polymer matrix PVDF-co-HFP, ionic liquid electrolyte, and TiO2 filler. The polymer electrolyte ink based on PVDF-co-HFP is stored in a syringe and kept in a molten state at 120°C in a heating chamber. During the printing process, the three-axis platform moves according to a pre-written printing program. By adjusting the air pressure, printing speed, and nozzle size, electrolytes of different widths and thicknesses can be printed. A small cylindrical nozzle extrudes the PVDF-co-HFP/TiO2 slurry ink under set pressure, and the slurry solidifies upon reaching the substrate.
In terms of enhancing the interfacial performance of electrolytes with electrodes, 3D printing technology has also played a corresponding role. Reza et al. used direct ink writing technology to prepare ceramic-polymer-ionic liquid composite solid electrolytes at high temperatures. This electrolyte forms a thin interface in close contact with the electrode, thereby reducing interfacial resistance and achieving an ionic conductivity of 0.78×10−3 S·cm−1.
Additionally, Sang et al. printed gel composite electrolytes (GCEs) using UV curing-assisted printing, which exhibited excellent flexibility, cycling performance, and non-flammability (showing significant improvements in flame-retardant performance compared to carbonate electrolytes), and significantly reduced the interfacial resistance of bipolar batteries.
Durstock et al. achieved controlled and uniform porosity in solid electrolytes using a dry phase conversion method, employing a unique mixed solvent system for pore formation and introducing alumina nanoparticles into the PVDF matrix, ensuring that the composite electrolyte maintains good rate performance while also exhibiting good wettability and thermal stability. Due to the universality of this processing technology, this method can also be extended to the preparation of printed electrodes.
References:
[1] Su Hang et al.: Progress in the Preparation Technology of Solid Lithium/Sodium Ion Electrolyte Membranes, Institute of Physics, Chinese Academy of Sciences
[2] Li Changgang: Research on All-Solid-State Lithium Batteries Based on 3D Printed PEO-Based Composite Solid Electrolytes, China University of Geosciences
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