Abstract: This study introduces the development of a novel sodium anode filament specifically designed for Fused Filament Fabrication (FFF) using a combination of Polylactic Acid (PLA) and hard carbon as the active material. The filament is enhanced with Polyethylene Glycol dimethyl ether and Carbon Black to improve both plasticity and conductivity. The research involves optimizing the anode slurry, producing the filament, and analyzing its electrochemical performance through various testing techniques, including cyclic voltammetry and scanning electron microscopy (SEM). Rheological tests confirmed that the developed filaments possess shear-thinning properties, making them well-suited for 3D printing. Although the 3D-printed samples exhibited a 15% lower capacity compared to conventional slurry-coated films, this reduction is primarily attributed to the PLA encapsulating the active material during printing, which reduces electrolyte access and impairs ion diffusion.
1. Introduction: Batteries play a crucial role in modern energy storage systems, with applications ranging from portable electronics to electric vehicles (EVs). Advances in battery technology, particularly in the area of energy-to-weight and power-to-weight ratios, have allowed electric vehicles to compete effectively with traditional gasoline-powered vehicles. The introduction of lithium-ion batteries (LIBs) in the 1990s revolutionized the portable electronics industry due to their high energy density and performance. However, the flammability of liquid electrolytes and the environmental impact of lithium mining have sparked interest in alternative battery chemistries.
Sodium-ion batteries (SIBs) have emerged as a promising alternative, offering significant advantages over LIBs. Sodium is abundant and inexpensive, making SIBs more cost-effective and environmentally friendly. SIBs also exhibit better thermal stability, making them safer for high-temperature applications. This has led to significant growth in the global battery market, with the industry projected to reach $92.2 billion by 2024. However, the large-scale production of conventional batteries poses environmental challenges, particularly during the manufacturing and disposal phases.
Traditional battery manufacturing methods, such as slurry casting, involve mixing active materials, conductive additives, and binders with solvents like N-methyl-2-pyrrolidone (NMP), a chemical known for its harmful environmental and health effects. This has driven research into more sustainable manufacturing methods, such as additive manufacturing (AM). AM, particularly Fused Filament Fabrication (FFF), allows for the creation of complex, porous structures that enhance electrode reactions and ion transport. These 3D-printed battery designs reduce internal resistance and enable more efficient energy storage.
This study aims to develop a sodium anode filament for FFF, combining PLA with hard carbon, polyethylene glycol, and carbon black. The research explores the potential of this filament for 3D-printed sodium-ion batteries and evaluates its electrochemical performance.
2. Experimental Methodology:
2.1 Materials: The materials used in this study include PLA pellets as the polymer matrix due to their low toxicity and printability. Hard carbon was chosen as the active material for its availability and high electrochemical performance. Polyethylene glycol dimethyl ether (PEG) was added to enhance plasticity, while Carbon Black was included to improve conductivity. Dichloromethane (DCM) was used as a solvent to dissolve the PLA, taking advantage of its fast evaporation and low toxicity.
2.2 Slurry Preparation: The PLA was preheated at 80°C to reduce moisture content before being dissolved in DCM under continuous magnetic stirring. Once fully dissolved, PEG was added to the solution, followed by a mixture of hard carbon and carbon black, which was ground together using a mortar and pestle for uniform mixing. The resulting thick paste was coated onto aluminum sheets using a doctor blade, and the coated samples were left to air dry for 24 hours to ensure complete evaporation of DCM.
2.3 Filament Production: The dried film specimens were cut into small pieces and fed into a laboratory-scale screw extruder to produce filaments with a 2.85 mm diameter. The temperature of the extruder was set based on the results of Differential Scanning Calorimetry (DSC), and the filament was wound onto a spool after solidification. The filaments were printed using an FFF printer to produce the 3D-printed battery anodes.
2.4 Electrochemical Testing: The electrochemical performance of the filaments was evaluated using Swagelok-type cells, with metallic sodium serving as the counter/reference electrode. A 1M NaClO4 solution in propylene carbonate and ethylene carbonate (1:1 weight ratio) was used as the electrolyte. Galvanostatic cycling tests were performed at various current densities within a voltage range of 1.5 to 4 V (vs Na/Na+). Cyclic voltammetry (CV) tests were also conducted to assess the electrochemical activity of the filaments.
2.5 Structural and Rheological Characterization: SEM and Raman spectroscopy were employed to analyze the surface morphology and elemental distribution of the filaments. The rheological properties of the filaments were measured using a parallel plate rheometer to determine their elastic (storage) and viscous (loss) modulus. A dynamic amplitude sweep test and frequency sweep test were conducted to evaluate the shear-thinning behavior and viscoelastic properties of the filaments.
3. Results and Discussion:
3.1 Capacity Measurements: The electrochemical performance of three samples (S-I, S-II, and S-III) was evaluated. Sample S-II, which contained 58% hard carbon, demonstrated the highest capacity at 273 mAh/g. This superior performance can be attributed to the optimized balance between the active material concentration and the reduced PLA content. The presence of more active material facilitated better sodium-ion accommodation, leading to higher capacity. In contrast, samples with higher PLA content, such as S-I, exhibited lower capacities due to the reduced availability of active material.
3.2 Cyclic Voltammetry Analysis: The CV results indicated that sample S-II had the highest peak current during both oxidation and reduction processes, suggesting enhanced electrochemical activity. This aligns with the capacity measurements, where S-II consistently outperformed the other samples. The shape of the CV curves also indicated that all samples underwent similar redox processes, likely involving sodium-ion intercalation and de-intercalation in the anode material.
3.3 Filament Homogeneity and Structural Analysis: SEM images revealed that sample S-II had a smooth surface with closely packed particles and minimal voids, indicating good material homogeneity. This uniformity likely contributed to its superior electrochemical performance. In contrast, sample S-III exhibited a rougher surface with larger particles and noticeable voids, which may have hindered its performance. Raman spectroscopy confirmed the presence of carbon-based materials in all samples, with sample S-II showing a higher degree of graphitization and crystallinity, which likely enhanced its conductivity and stability.
3.4 Thermal and Rheological Behavior: DSC analysis of the filaments showed that the melting point of PLA in the composite was around 156°C, with significant energy absorption during the thermal decomposition of PEG and PLA. The presence of carbon materials provided thermal stability to the composite. Rheological testing revealed that the developed filaments exhibited shear-thinning behavior, making them well-suited for extrusion in the FFF process. The reduced viscosity of the composite filaments compared to pure PLA suggests that the addition of hard carbon and PEG improved the material’s processability, which is essential for 3D printing applications.
3.5 Comparison of 3D Printed and Slurry-Coated Samples: The electrochemical performance of 3D-printed samples was compared to conventional slurry-coated samples. A 15% reduction in capacity was observed in the 3D-printed samples, which achieved 230 mAh/g compared to 273 mAh/g in the slurry-coated films. The 3D-printed samples exhibited a higher discharge voltage profile and a less distinct voltage plateau, suggesting higher internal resistance and less efficient sodium-ion intercalation. SEM analysis revealed that the 3D printing process caused the PLA to encapsulate active carbon particles, reducing their accessibility and contributing to the reduced electrochemical performance.
4. Conclusion: This study successfully developed a hard carbon-based sodium anode filament for FFF and demonstrated its potential for 3D-printed sodium-ion batteries. Sample S-II exhibited the highest capacity and electrochemical activity due to its optimized material composition, including a high concentration of hard carbon and a reduced PLA content. However, the 3D printing process introduced challenges, such as material encapsulation and reduced porosity, which resulted in a 15% reduction in capacity compared to slurry-coated samples. Future research should focus on optimizing the 3D printing process to minimize material encapsulation and enhance the uniformity of active material distribution, potentially improving the performance of 3D-printed battery electrodes.