Ceramic Stereolithography of Li₇La₃Zr₂O₁₂ Micro-Embossed Sheets for Solid Electrolyte Applications

Abstract

Lithium-ion batteries (LIBs) have significantly advanced portable electronics, yet their reliance on flammable organic solvents and lithium dendrite formation pose safety risks. Solid-state batteries (SSBs), utilizing solid electrolytes, offer a safer alternative with higher energy and power densities. This study explores the fabrication of solid electrolytes using ceramic stereolithography, focusing on lithium lanthanum zirconate (LLZ) due to its high ionic conductivity and chemical stability. A photosensitive paste containing 40–43 vol% LLZ was suitable for processing by stereolithography, and optimized processing parameters of 100 mW laser power and 1000 mm/s laser scanning speed with a 50 μm laser spot size were identified for sufficient material curing and interlayer lamination of LLZ. Thin embossed sheets were designed to enhance ion exchange and reduce internal resistance and were fabricated by the ceramic stereolithography method. The effect of cold isostatic pressing (CIP) on the sintered microstructure was investigated, and the potential for CIP to promote solid-phase diffusion during sintering was demonstrated, particularly at 67 MPa. The resulting LLZ-embossed sheets exhibited dense ceramic microstructures. These findings support the potential application of ceramic stereolithography for fabricating efficient solid electrolytes for next-generation telecommunications and mobile devices.

1. Introduction

As demand for sustainable energy rises, development of efficient electrical energy storage systems is becoming increasingly important to mitigate the variability of renewable energy sources [1]. While lithium-ion batteries (LIBs) have had a significant impact on society, enabling the development of portable electronics, from laptop computers to electric cars, their merits and limitations must be considered [2]. LIBs boast high energy density, which allows greater energy storage capacity per unit volume, and are also designed to have good cycling stability, offering an increasing number of charge/discharge cycles [3,4]. However, the relatively low ionic resistance of the electrolyte and the use of flammable organic solvents have led to safety concerns over fires and explosions caused by leaks and heat generation [5,6]. Furthermore, although lithium metal anodes have the highest theoretical specific capacity (3860 mAhg⁻¹) and lowest redox potential (−3.040 V vs. a standard hydrogen electrode), they have been replaced by carbonaceous anodes [7]. This is because lithium metal anodes with liquid electrolytes potentially cause fires due to the formation of lithium dendrites during cycling, which can penetrate the separator and lead to a short-circuit [8]. These limitations are driving research towards efficient solid-state batteries to eliminate the safety concerns of these electrochemical batteries.

Unlike traditional LIBs, that typically use a liquid electrolyte between a metal anode and cathode, a solid-state battery utilizes a solid electrolyte plate between the electrodes. Typically, transition metal oxides such as lithium cobalt oxide can be used as cathode materials, while graphite and lithium metal can be employed as anode materials. For the solid electrolyte, oxide and sulfide ceramics with ionic conductivity are leading candidates. Lithium ions move through the electrolyte between the electrodes, generating electrical flow. Solid-state batteries can operate safely at high temperatures, enabling rapid charging, even in high current circuits with significant heat generation. Because the electrolyte is solid, non-flammable and non-volatile, solid-state batteries are expected to counter the safety issues of LIBs. Aqueous batteries also offer a safer alternative to LIBs, utilizing non-volatile and environmentally friendly water-based electrolytes [9]. However, limitations of aqueous batteries such as the issues of self-discharge, the limited number of materials that can operate within the narrow electrochemical stability window of aqueous electrolytes, and their unsuitability for operation in high temperature environments are significant challenges to overcome [10].

Furthermore, solid-state batteries also promise higher energy densities and power densities thanks to the possibility of bipolar stacking and the potential use of lithium metal or silicon anodes. Additionally, solid-state batteries can avoid the issue of electrode crosstalk seen in LIBs, where degraded electrodes can cause a cascade of side reactions at opposing electrodes, reducing cell capacity and battery lifetime [11,12]. Various practical applications are being considered for these batteries, such as in electric vehicles for faster charging and extended range, and consumer electronics with longer battery life.

However, although solid-state batteries are considered as an advanced alternative for LIBs, poor compatibility of solid electrolytes and electrodes was found to be a major hindrance to developing all-solid-state lithium batteries. For instance, poor contact between the solid electrolyte and electrode is caused by poor wettability, inappropriate microstructures and stress cracking [13]. Given this background, improving the structure of electrolytes has attracted attention in research. One promising approach to addressing poor contact is designing effective Li-ion and electron conductive interlayers [13]. Additionally, the electrical resistance inside a solid-state battery increases in proportion to the distance between the positive and negative electrodes, so the thickness of the solid electrolyte can be optimized to create short ion pathways. To improve the energy density of the battery, increasing the contact area of the solid electrolyte is expected to enhance ion conduction. To meet these requirements, an embossed sheet is proposed to increase the specific surface area of the solid electrolyte [14].

Various methods have been considered for fabrication of solid electrolytes, such as dry pressing and wet molding of powder materials [15]. The resulting powder compacts must be heat-treated, sintered, and finished by post-processing such as cutting and grinding. However, these techniques can result in cracks and fine pores in the sintered material due to the non-uniform distribution of powder particles, leading to part damage and fracture [16]. Tape casting is another established method for fabrication of ceramic thin films. Ceramic slurries are cast into thin layers and subsequently dried and sintered to densify. This method offers scalability but requires careful optimization of materials, suffers from a limited material selection of suitable solvents and binders, and it can prove challenging to achieve fully dense, defect-free parts [17,18]. All of these methods are limited in terms of geometrical complexity and are often restricted to simple shapes like flat plates or discs. Additive manufacturing (AM) of embossed sheet solid electrolytes is proposed as an alternative fabrication method. AM technologies enable high-speed fabrication of complex three-dimensional structures by selective material joining by layer-wise processing of 2D cross-sectional data [19]. These techniques also benefit from high resolution and low material waste due to the selective processing, often allowing excess material to be reused or recycled.

Stereolithography is an AM technique that joins material by selective curing of photosensitive pastes and resins. In the ceramic stereolithography method, ceramic particles are dispersed in a UV-sensitive resin and mixed to create a thixotropic paste with suitable viscosity [20]. The paste is spread into thin layers of a defined thickness across a build platform by a blade. A UV laser is used to selectively cure the paste layer-by-layer, as directed by the computer-aided design (CAD) data, joining the material by photopolymerization. The composite green part is then thermally post-processed to de-bind the resin material and sinter the part to a dense ceramic. This method has been reported for fabrication of solid electrolytes using yttria-stabilized zirconia, working towards the development of high-performance electrical devices [21]. Highly dense ceramic parts with customized geometries and high resolution can be fabricated by this method, but limitations such as the small catalog of optimized materials, lengthy sintering steps, and unsuitability for large scale production need to be addressed in future if it is to be firmly adopted in industry [22].

In this research, Lithium Lanthanum Zirconate (LLZ) was selected due to the potential benefits of improving energy density as an oxide-based solid electrolyte, with its relatively high ionic conductivity and good chemical stability against Li-metal anodes [8]. LLZ also benefits from high thermal stability, enabling operation in high-temperature environments. Optimization of paste composition and mixing parameters, laser processing parameters, and post-processing conditions was carried out by systematic investigations. Fabrication of concept designs for LLZ solid electrolyte embossed sheets was carried out using the ceramic stereolithography method. The use of ceramic stereolithography for fabrication of solid electrolytes is expected to increase component energy density and reduce internal resistance of the solid-state battery, enabling applications in next-generation telecommunications equipment and mobile devices.

2. Materials and Methods

LLZ (Li₇La₃Zr₂O₁₂), with a sintered ionic conductivity of 0.8 mS cm⁻¹ at room temperature, was ball-milled and ground to fine particles with an average diameter of 1 μm (DSZ-4, Daiichi Kigenso Kagaku Kogyo Co. Ltd., Osaka, Japan). 1-octanol (O0036, Tokyo Kasei Kogyo, Tokyo, Japan) was added to coat the particles with a surface film. The materials were mixed with photosensitive acrylic resin (KC1287, JSR Corporation, Tokyo, Japan) at volume ratios between 30–50% and mechanically combined by centrifugal planetary mixing (SK-350T, Shashin Kagaku, Shiga, Japan). Mixing speeds of 300 rpm revolution and 700 rpm rotation were used for a total mixing time of 15 min.

A CAD package (Fusion 360, Autodesk, San Francisco, CA, USA) was used to design a solid electrolyte component as a thin plate with total dimensions of 10 mm × 10 mm × 0.3 mm. A pattern of 24 × 24 square holes of 200 μm × 200 μm × 200 μm was inlaid into the plate, as shown in Figure 1. A larger 50 mm × 50 mm × 0.75 mm double-sided etched plate was also designed with 2 mm × 2 mm × 0.25 mm hole dimensions (Figure 1B). The hole dimensions were selected to balance the benefit of increased surface area with mechanical stability and processability by stereolithography. The CAD files were converted to a triangular mesh in STL file format composed of sliced data with a thickness of 50 μm (Magics, Materialise NV, Leuven, Belgium). A square test sample with dimensions of 5 mm × 5 mm with a 1 mm diameter hole in the center was also designed and prepared in the same way. To fabricate test sample films, a single layer of the sample geometry was scanned on a 1 mm-thick paste layer. Using this method, the thin film was observed by digital optical microscopy (DOM-VH-Z100, Keyence, Osaka, Japan) to measure the curing depth (sample thickness) and dimensional error (reduced hole diameter) at 1000 mm/s scan speed and varying laser powers between 50–200 mW.

For sample fabrication, the paste material was delivered and spread across the base plate of the stereolithography machine (SZ 2500c, SK-Fine, Shiga, Japan) to form layers of 50 μm thickness. A 355 nm wavelength UV laser with a 50 μm spot size diameter was used to selectively scan the surface of the paste layers as directed by the CAD data (Figure 2). Parameters were informed by the results of test sample measurements and used to fabricate LLZ-embossed sheets for solid-state batteries.

Thermal post-processing of the larger embossed samples was carried out in air by electric furnace (S7-2025D, Motoyama Corporation, Osaka, Japan) according to the treatment schedule shown in Figure 3. The samples were heated to 600 °C at a heating rate of 2 °C/min with an isothermal hold of two hours to de-bind the acrylic resin. The temperature was then raised to 1050 °C at a heating rate of 0.5 °C/min with a five-hour isothermal hold, before cooling to room temperature at a cooling rate of 5 °C/min. Sintered LLZ samples were examined by X-ray Diffraction (XRD) to visualize crystal phase (XRD-6100, Shimadzu, Kyoto, Japan) and compared to data available in the literature [23,24]. Cold isostatic pressing (CIP) was investigated as a post-processing step for the small etched plates to allow solid phase dispersion of particles in the composite green parts. Isostatic pressing was carried out at hydrostatic pressures of 67–100 MPa using a hydraulic CIP machine (DRY-CIP, Kobe Steel, Ltd., Kobe, Japan). The samples were then thermally treated in a nitrogen atmosphere at 1050 °C to sinter the ceramic. Scanning electron microscopy (SEM, JSM-6060, JEOL Ltd., Tokyo, Japan) was used to evaluate the microstructure of the sintered embossed sheets.

3. Results and Discussion

The maximum solid volume of LLZ photosensitive pastes was determined by systematically altering the solid-to-resin ratio. When the solid volume ratio exceeded 45%, the viscosity of the paste became too high to achieve smooth spreading for stereolithography. A suitable solid volume was found to be between 40–43%. Premature curing was observed in the as-mixed ceramic paste, thought to be caused by elution of lithium ions, chemically altering the resin solvent. The addition of 1-octanol prevented premature curing without changing the viscosity of the paste, allowing spreading and processing by stereolithography. The photosensitive paste was sensitive to temperature, with viscosity increasing after as little as ten minutes at room temperature. Even with the addition of dispersants of varying pH level, the stability of paste viscosity was not improved. It was therefore important to minimize the heat generation during paste mixing, utilizing a slower rotation speed and shorter mixing time. For small sample fabrication, requiring a small number of paste layers, the optimized LLZ pastes were suitable for processing. For fabrication of larger samples, the increasing viscosity of the paste at room temperature is expected to be problematic.

The curing depth and dimensional error with respect to the UV laser power was measured (Figure 4a). For sufficient inter-layer lamination, a curing depth of 1.5 times the layer thickness is required. For 50 μm layers, to achieve a curing depth of 75 μm, a laser power of 100 mW was required. Under these conditions (100 mW laser power, 1000 mm/s scan speed), the dimensional error of the designed 1 mm hole was measured, indicating the required laser offset to achieve accurate dimensions. A laser offset of 86 μm was applied to adjust the laser beam focus to achieve contour correction. The inter-layer lamination between four processed layers was observed by DOM to confirm that sufficient curing was achieved and there was no evidence of delamination between layers (Figure 4b).

Using the parameters indicated in

Table 1. Laser processing parameters for LLZ photosensitive paste.

Parameter	Value
Laser wavelength	355 nm
Spot size diameter	50 μm
Layer thickness	50 μm
Laser power	100 mW/52 mW
Scan speed	1000 mm/s

, LLZ-embossed sheet samples were fabricated by the stereolithography method as previously described. By etching two sides of the sheet, the specific surface area of the sample was increased by 10%. DOM observation of the side surface of the sheet showed no evidence of delamination, and inter-layer lamination was achieved. The composite green part showed no extreme agglomeration of particles or cavities, with almost homogenous distribution of particles. For the 10 mm × 10 mm × 0.3 mm plate with 200 μm holes, a dimensional accuracy of +/−10 μm was measured by DOM. Inspection of the surface of the green bodies was carried out by DOM, as shown in Figure 5. The input laser power was 100 mW corresponding to an actual laser power output of 52 mW. XRD analysis was carried out on sintered samples and showed characteristic diffraction peaks of cubic LLZ (Figure 6).

The microstructure of the sintered LLZ-embossed sheet is shown in Figure 7. Without CIP, the sintered sheet was observed to have many fine pores distributed throughout. With a CIP treatment at 67 MPa, the dense microstructure formed a continuous phase, although pores could be observed. With the application of CIP, the solid particles in the acrylic resin matrix were brought into close contact, suggesting that solid phase diffusion could be achieved during heat treatment.

CIP treatment appeared to contribute to microstructure densification of the embossed sheets; however, at hydrostatic pressure of 84 and 100 MPa, the microporosity became widely distributed, as shown in Figure 7c,d. As the yield stress of acrylic resin is 80 MPa, hydrostatic pressures above this threshold are thought to cause compressive failure and the formation of microcracks. The presence of microcracks leads to an increase in specific surface area facilitating the decomposition and vaporization of the resin material during heat treatment. This is thought to lead to the resin material being removed at lower temperature, followed by insufficient diffusion bonding during heat treatment, preventing the formation of a dense sintered component.

4. Conclusions

The development of solid-state batteries addresses limitations of lithium-ion batteries, such as safety concerns and limited energy density. In this research, the fabrication of LLZ solid electrolytes using ceramic stereolithography and cold isostatic pressing was investigated for application in solid-state batteries.

As a compound with excellent ionic conductivity, LLZ was selected for stereolithography fabrication of solid electrolytes for solid-state batteries. LLZ pastes were highly sensitive to temperature, with pastes becoming too viscous to achieve smooth spreading when left at room temperature for ten minutes. To improve the outlook of LLZ processing by ceramic stereolithography, further research into paste composition is required to improve the stability of the paste at room temperature.

Concept solid electrolytes of thin embossed sheets were designed with the aim of reducing the distance between positive and negative electrodes, reducing internal electrical resistance, and promoting ion exchange to improve battery energy density. Photosensitive paste composition and mixing parameters were optimized. Addition of 1-octanol was required to prevent the elution of lithium ions into the LLZ paste. Suitable solid dispersion of 40–43 vol% resulted in relatively stable photosensitive pastes that could be processed by the ceramic stereolithography method. Laser processing parameters were optimized and embossed sheets were fabricated.

Cold isostatic pressing was applied to the composite green parts to mechanically adhere the dispersed particles, promoting solid-phase diffusion during heat treatment. LLZ-embossed sheets were successfully sintered with a dense ceramic microstructure. With further development, these methods are expected to improve the microstructural integrity and electrochemical performance of solid electrolytes. This advancement will contribute to the creation of safer and more efficient energy storage systems, invaluable for applications in electric vehicles, consumer electronics, and next-generation telecommunications equipment.