# 3D Printing of Highly Porous Polypropylene Separators for Lithium-Ion Batteries Using Fused Deposition Modeling and Thermally Induced Phase Separation

Abraham Enchinton, Ana Martinez, Katherine Gonzalez, Christian Fernandez, Sivasai Balivada, Laura Merrill, Jorge Cardenas, Stéphane Panier, Alexis Maurel

► To cite this version:

Abraham Enchinton, Ana Martinez, Katherine Gonzalez, Christian Fernandez, Sivasai Balivada, et al.. 3D Printing of Highly Porous Polypropylene Separators for Lithium-Ion Batteries Using Fused Deposition Modeling and Thermally Induced Phase Separation. Advanced Materials Technologies, 2025, 10.1002/admt.202500912. hal-05244453

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# 3D Printing of Highly Porous Polypropylene Separators for Lithium-Ion Batteries Using Fused Deposition Modeling and Thermally Induced Phase Separation

Abraham Enchinton, Ana C. Martinez, Katherine R. Gonzalez, Christian A. Fernandez, Sivasai Balivada, Laura C. Merrill, Jorge A. Cardenas, Stéphane Panier, and Alexis Maurel*

Appearing as one of the key-components of lithium-ion batteries (LIBs), this work specifically focuses on the additive manufacturing (AM) of custom-shape separators, facilitated by the filament material extrusion process, also called fused deposition modeling (FDM). The development and optimization of composite thermoplastic filament feedstocks combining polypropylene and paraffin wax, followed by the 3D printing of the separator membranes is shown. A post-processing step, based on thermal induced phase separation (TIPS), is introduced to promote porosity formation through removal of the paraffin wax sacrificial phase within the 3D printed items. Separators with different polypropylene/paraffin wax ratios are developed and the impact on printability, mechanical strength, porosity, and electrochemical performances, is thoroughly discussed. X-ray micro-computed tomography is employed to assess the geometric fidelity and to detect printing defects in a complex 3D lattice structure. The performance of the 3D printed porous separators is also compared to a commercial separator. This pioneering research establishes a foundation for the creation of porous separators that can adapt to and conform into 3D printed battery architectures with novel form factors, and also creates opportunities for the use of FDM and TIPS for a wide range of applications that employ porous structures beyond the energy storage field.

electric and hybrid vehicles, and even large-scale energy storage systems.[1–3] While their application breadth and demand continues to grow, commercial battery geometry has remained relatively unchanged: a planar separator, impregnated with liquid electrolyte, is sandwiched between an anode and cathode with current collectors allowing the flow of electrons between the electrodes and the external circuit. These compact, multilayer assemblies are typically encased in formats such as coin cells, rectangular prismatic cells, pouch cells, or cylindrical cells.[4] However, these standardized geometries impose limitations, making it challenging to integrate batteries seamlessly into unconventional applications, often resulting in unused or dead-volumes.[5,6] In an attempt to deviate from 2D battery architectures, the use of additive manufacturing (AM) technologies has been investigated, as it potentially enables the development of batteries that maximize spatial efficiency while meeting the specific

needs of nontraditional applications by providing enhanced power performances.

A considerable amount of recent research efforts on the topic have tried to demonstrate the 3D printing of nonspecific, shape-conformable batteries with adequate electrochemical

## 1. Introduction

Since their commercialization in 1991, lithium-ion batteries (LIBs) have grown in interest with applications including, but not limited to, modern portable electronics, aerospace systems,

A. Enchinton, A. C. Martinez, K. R. Gonzalez, A. Maurel
Metallurgical, Materials and Biomedical Engineering Department
University of Texas at El Paso
El Paso, TX 79968, USA
E-mail: amaurel@utep.edu

K. R. Gonzalez, C. A. Fernandez
Department of Aerospace and Mechanical Engineering
University of Texas at El Paso
El Paso, TX 79968, USA

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/admt.202500912

© 2025 The Author(s). Advanced Materials Technologies published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/admt.202500912

S. Balivada
Department of Biological Sciences
The University of Texas at El Paso
El Paso, TX 79968, USA

S. Balivada
Imaging & Behavioral Neuroscience (IBN) facility
The University of Texas at El Paso
El Paso, TX 79968, USA

L. C. Merrill, J. A. Cardenas
Sandia National Laboratories
Albuquerque, NM 87123, USA

S. Panier
Université Picardie Jules Verne
Laboratoire des Technologies Innovantes
Avenue des Facultés, Amiens 80000, France
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Schematic illustration of the step-by-step protocol used to fabricate porous polymer separators by combining fused deposition modeling (FDM) 3D printing with thermally induced phase separation (TIPS). The process includes filament feedstock preparation via melt blending of polypropylene (PP) and paraffin wax (PW), extrusion into a printable filament, 3D printing of desired geometries, and subsequent solvent extraction to remove the PW phase, resulting in a porous PP structure.</figcaption></figure>

performance. In this context, material extrusion techniques,[2,3,7-13] as well as vat photopolymerization,[6,14-16] or also powder bed fusion processes,[17,18] have been investigated. Most reported studies predominantly concentrate on the development of material feedstocks for the printers, with particular emphasis on electrode design and fabrication. Although the separator is a critical component in batteries, research on the AM of separators remains until now limited due to the requirements for it to be a highly porous electronic insulator with high ionic conductivity.

In commercial LIBs, porous separator membranes such as those from the Celgard brand are made from polyolefins like polyethylene (PE) and polypropylene (PP). Standard separators are less than 30 µm thick, with pore sizes ranging from 0.03 to 0.1 µm.[19] In monolayer separators made of either PE or PP, extreme heat causes the polymer to melt, closing the pores. This pore closure acts as an internal safety mechanism, reducing the risk of thermal runaway in the battery.[20] To enhance this safety feature, tri-layer separators were developed, consisting of a PE layer sandwiched between two PP layers.[21] In this configuration, the PE layer melts at 130 °C and seals the pores during overheating, effectively shutting down the battery. The outer PP layers, with a higher melting temperature of 150 °C, remain solid, providing a more robust physical barrier compared to monolayer de-

signs. Commercially available separators are often manufactured by means of an extrusion process where the pores are created through the subsequent mechanical stretching of the thinly extruded polymer membrane. To date, commercially available separators from Celgard represent the standard for porosity, so their pore uniformity and shape establishes a performance baseline by which AM separators are compared.[19]

However, the traditional mechanical stretching method used to create pores in conventional 2D separators is not feasible for 3D printed separators. Unlike 2D extruded polymer membranes, which are uniform and continuous, 3D printed structures are formed layer by layer, resulting in inherent anisotropy and weaker interlayer bonding. Attempting to mechanically stretch a 3D printed separator to induce porosity would likely lead to deformation, delamination, or even fracture of the printed item. Furthermore, the high precision and intricate geometries enabled by 3D printing make it incompatible with post-processing techniques like stretching, as these processes could compromise the dimensional accuracy and mechanical integrity of the separator. Alternative methods for introducing porosity, such as incorporating sacrificial materials, controlled extrusion parameters, or selective curing strategies during the printing process must be employed to achieve highly porous 3D printed separators.

<figure><img src="image_4.png" /><figcaption>**Figure 2.** a) Differential Scanning Calorimetry (DSC) curves obtained under a N₂ atmosphere and b) Thermogravimetric Analysis (TGA) curves obtained under an air atmosphere for pure polypropylene (PP), paraffin wax (PW), and composite samples containing 30:70 and 60:40 wt.% PP/PW ratios.</figcaption></figure>
For 3D printing separators, the most commonly explored AM technique to date remains a form of material extrusion known as direct ink writing (DIW) or Liquid Deposition Modeling. Despite being cost-effective and highly-loadable, DIW faces serious architectural restrictions due to the effects of gravity on low viscosity inks. As a result, DIW cannot produce battery architectures with complex geometries in the z-axis. In spite of this, DIW does possess the unique advantage of solvent evaporation to create porosity, a feature critical to the success of battery separators.[22] Blake et al.[23] reported the DIW 3D printing of a composite separator ink containing poly(vinylidene fluoride) (PVDF), N-methyl-2-pyrrolidone (NMP) and glycerol. Al₂O₃ nanoparticles were added in a 70:30 Al₂O₃/PVDF wt.% ratio for thermal stability. During printing, each layer was dried at 90 °C to ensure the evaporation of glycerol and NMP generated porosity. The resulting 100 µm thick separator membrane reported an ionic conductivity of 8.2 × 10⁻⁴ S cm⁻¹ at room temperature upon soaking in a 1 M LiPF₆ in 1:1 ethylene carbonate/diethylene carbonate (EC/DEC) wt.% electrolyte. Similarly, Liu et al.[24] developed a custom separator ink containing a 2:9:3 wt.% ratio of PVDF-co-hexafluoropropylene, N,N-dimethylformamide, and boron nitride nanosheets (employed as the polymer, solvent, and thermal stabilizer, respectively) in which pore formation was induced by solvent evaporation after printing. The resulting porous separator membranes were 30 µm thick. Ding et al.[25] also developed a custom separator ink composed of poly(methyl methacrylate), silicon dioxide, and N,N-dimethylformamide in a 16:7:77 wt.% ratio; a Li₄Ti₅O₁₂-based negative electrode ink, and a LiFePO₄-based positive electrode ink. The three inks were then loaded and simultaneously printed using a DIW printer equipped with three independent dispensers. Printed samples showed porosity only after drying.

On the other hand, Fused Deposition Modeling (FDM) or Fused Filament Fabrication, appears as an option of choice for 3D printing shape-conformable batteries and geometrically-complex separators due to its ability to print 3D structures with less limitations in the z-axis. Another advantage is the affordable cost of desktop printers and the wide commercialization of multi-material options that could enable the printability of the full battery in a single step in the future.[18,26] In spite of its many алuring features as a 3D printing technique, FDM's most glaring drawback, in the context of manufacturing battery separators, is the inability to produce porosity as a result of printed feedstock cohering to previous passes during its layer-by-layer application of filament. As a result, the application of FDM to print battery separators remains an underexplored field of research.

Previous groups attempted the additive manufacturing of separators through FDM.[2,3] Reyes et al.[3] and Maurel et al.[2] employed a poly(lactic acid) (PLA) polymer matrix. Both teams, while novel in their approach, focused more directly on filament formulation as opposed to emphasizing the development of a technique for porosity formation. Reyes et al. prepared a pure PLA filament to 3D print separator membranes. The printed separator discs (150 µm thick) where subsequently infused 24 hours within a 1 M LiClO₄ in a 50:50 vol% mixture of ethyl methyl carbonate/propylene carbonate (EMC/PC) leading to an ionic conductivity of 8.5 × 10⁻⁵ S cm⁻¹ at 20 °C. On the other hand, Maurel et al. reported the formulation of a PLA filament containing SiO₂ and a polyethylene glycol dimethyl ether Mn 500 (PEGDME500)

plasticizer (PLA/SiO₂/PEGDME500 66:7:27 wt.%) as additives. SiO₂ ceramic fillers were added for thermal stability, mechanical strength and ionic conductivity. The separator discs of 150 µm thick exhibited an ionic conductivity of 1.20 × 10⁻⁴ S cm⁻¹ at room temperature using a 1 M LiPF₆ in EC/DEC 1:1 weight ratio electrolyte. To go further, through manipulation of the 3D printer's infill pattern capabilities, the authors were able to achieve macroporosity that enhanced the separator's liquid electrolyte uptake during the 3D printing of the complete battery. Despite these improvements, the long-term stability of the PLA polymer matrix within the liquid electrolyte remained a concern as the oxygen presence in PLA chains interacting with lithium cations might lead to undesired phenomenon such as capacity fading or even short-circuiting in the long-term.[27] To address this issue, the authors suggested employing chemically inert polyolefins, such as PP or PE, as alternative separator materials better suited for long-term electrochemical stability. In this context, achieving a high density of submicron pores (mesopores and micropores) is desirable to facilitate liquid electrolyte impregnation and to achieve an adequate ionic conductivity. Since these pores act as channels for the ions to diffuse between both electrodes, the inherently nonporous nature of FDM-printed structures can hinder ion mobility, leading to ion trapping and capacity fading.

A viable solution to FDM's limitations is the use of the thermally induced phase separation (TIPS) method. This thermally driven mechanism involves cooling a homogeneous high-temperature solution composed of a polymer and diluent, causing phase separation into a solid two-phase structure.[28] As the solution cools below a binodal solubility temperature and enters a metastable region in its phase diagram, differences in surface energy will cause the polymer and diluent to demix, where nucleation and growth becomes the driving mechanism for the formation of a dispersed diluent-rich phase in a polymer-rich matrix. Once phase separation occurs, the diluent-rich phase can be selectively removed using an extractant, yielding a porous polymer membrane. By carefully tuning parameters such as composition, cooling rate, and the specific polymer/diluent system, a range of reproducible pore morphologies can be achieved.[29] Among recent works, growing interest surrounds applying TIPS to produce highly porous membranes for battery separators. Although not 3D printed, TIPS was reported with great success by Li et al.[30] to produce porous trilayer LIB separators using multilayer coextrusion as a manufacturing process. Using a 55:45 mass ratio of PP to paraffin wax (PW), as the polymer and diluent respectively, Li et al. produced a 25 µm thick membrane with a calculated porosity of 54.6% rivaling commercial Celgard's 36.7% porosity as a reference. The resulting separator reported a 1.46 × 10⁻³ S cm⁻¹ ionic conductivity at 20 °C using a 1 M LiPF₆ in a 1:1:1 EMC/EC/dimethyl carbonate (DMC) vol.% electrolyte. Nevertheless, multilayer coextrusion is strictly limited to fabricating 2D sheets, making the technique a poor manufacturing choice with regards to nontraditional battery geometries.

Based on this framework, this article aims to investigate the production and optimization of a FDM-3D printable biphasic filament, specifically designed to be used as a separator in a LIB after being 3D printed and subjected to a post-processing step promoting the formation of pores leveraging the TIPS method. While studies have explored FDM and TIPS independently for
separator fabrication, none have combined their advantages to enable the straightforward production of a highly porous, 3D printable battery separator with complex 3D geometries. This study introduces and investigates an innovative process that integrates both techniques to overcome existing limitations. The well-characterized PP/PW system reported by Li et al.[30] is here adapted for use in a 3D-printable filament, aiming to explore whether the benefits of TIPS-based porosity can be translated to additive manufacturing processes such as FDM. Two distinct PP/PW filament compositions are introduced, and the impact on their microstructure and porosity, when 3D printed into separators, is analyzed. The resulting morphology is characterized using Scanning Electron Microscopy (SEM), while mechanical properties are assessed through tensile testing, and electrochemical performance is evaluated via electrochemical impedance spectroscopy, linear sweep voltammetry and galvanostatic cycling. Finally, the ability of this protocol to fabricate complex 3D geometries is demonstrated through the production of a highly porous separator with an intricate 3D lattice design, whose geometric fidelity and printing defects are evaluated using X-ray micro-computed tomography (μCT).

## 2. Solvent-Free Filament Feedstock Preparation

The complete porous separator fabrication process is illustrated in **Figure 1**. The first step consists of preparing the composite PP/PW filament feedstocks (30:70 and 60:40 wt.%) to feed the FDM 3D printer. While the solvent method is commonly regarded as the simplest approach for achieving good homogeneity of the 3D printable filament at the laboratory scale, it was not used in this study. Instead, a solvent-free extrusion method where PP and PW pellets are mixed and extruded together was chosen to avoid the use of toxic xylene, which is often employed for PP dissolution.[31]

The temperature of the extruder is a crucial parameter, as it significantly influences the distribution of the PP and PW phases in the resulting filament's microstructure. **Figure 2a** displays differential scanning calorimetry (DSC) curves of the as-received PP and PW pellets, as well as of the two composite formulations. The as-received PP exhibits an endothermic peak corresponding to the melting temperature (Tm) at 156 °C in good agreement with literature.[32] On the other hand, the DSC curve of the as-received PW exhibits two distinct peaks. The primary peak at 60 °C corresponds to the Tm, marking the transition from solid to liquid. The smaller peak at 32 °C represents a solid-solid phase transition (Tt), where PW undergoes a structural rearrangement within the solid state, before the material fully melts.[33] Based on these results, it was decided to maintain the extruder temperature at 160 °C for the filament preparation of all composite PP/PW samples. This temperature ensures the complete melting of PP while preserving the thermal characteristics of PW, which could play a role in the filament's structural integrity and performance during processing. Indeed, as shown in the thermogravimetric analysis (TGA) results performed under air atmosphere for the all samples (Figure 2b), the extruder temperature's upper limit is established at 160 °C to prevent the degradation of the PW. Finally, the DSC curves of the composite formulations exhibit both melting temperatures, reflecting the coexistence of PP and PW phases (Figure 2a).

During the composite filaments preparation, the PP and PW pellets move through the heated barrel, melt and form a solvent-free homogeneous solution that cools into a 1.75 mm diameter filament as it passes through the extruder's die (Figure 3a). When extruding the filament at temperature >160 °C, a phenomenon coined as "wax bleed out" would occur during the filament fabrication, where the wax content would become too liquidous to produce harvestable filament. This likely stems from the extruding temperature excessively exceeding the melting point of PW (60 °C). On the other hand, any extruding temperature below 160 °C, resulted in the preparation of an inhomogeneous filament with visibly unmelted PP pellets.

The heating and cooling of the polymer/diluent system during extrusion of the filament feedstock is the first instance during the separator production process where TIPS occurs. Within the extruder's heated barrel at 160 °C, the PP and PW form a homogeneous solution and the phase separation occurs as soon as the filament exits the extrusion die when the solution's temperature drops below its compositional binodal line. The resulting microstructure of the filament, formed through this thermally driven mechanism, is characterized by a dense dispersion of PW flakes embedded within a continuous PP matrix, as visualized in the SEM images in Figure 3b,c. As expected, the PP/PW 60:40 sample, which has the highest PP content, shows a noticeable reduction in interspersed PW flakes within its microstructure compared to the 30:70 PP/PW sample.

## 3. Optimization of the 3D Printing Parameters

After the successful filament production, the second step in the porous separator fabrication involves feeding the filament to a FDM 3D printer to print any desired shape. As the filament passes through the heated nozzle, it is heated a few degrees above its melting temperature to enable smooth extrusion and precise 3D printing. This heating process is the second instance of TIPS. The sudden temperature difference between the printer nozzle and the filament's deposition site cools the solution into a metastable state, where the PW nucleates into smaller flakes dispersed within the PP matrix.

Dealing with custom composition filaments can introduce complexity in printing parameters, particularly when combining two constituents like PP and PW, which have significantly different melting temperatures. As such, determining optimal 3D printing parameters became the subject of interest following the preparation of the custom filaments. The first attempt at printing a single-layer sheet from the PP/PW 60:40 filament followed the standard printing parameters for a commercial PP-based filament (Table 1). As a result, visually apparent macrotears or decohesions between adjacent passes of the PP/PW 60:40 filament began forming unpredictably during printing (Figure 4a,b). Consequently, viable separator membranes could not be punched from the printed sheet due to the high short circuit probability of the cell during electrochemical testing due to the apparent gaps.

To improve the sheet's print quality, a number of printing parameters such as the nozzle diameter, nozzle temperature, bed temperature, solid infill print speed, and filament extrusion multiplier, were experimentally adjusted within the printer's software. The altered printing parameters, outlined in Table 1, led to a higher quality product by various means. For example, PP has
<figure><img src="image_5.png" /><figcaption>**Figure 3.** a) Optical image of the extruded PP/PW 30:70 wt.% filament, showing uniform quality and diameter. b) Cross-sectional SEM image of the PP/PW 30:70 filament, and c) cross-sectional SEM image of the PP/PW 60:40 filament, both revealing the internal microstructure consisting of paraffin wax (PW) domains dispersed within the polypropylene (PP) matrix as a result of thermally induced phase separation during extrusion.</figcaption></figure>

a well-documented propensity for shrinking and warping during printing, resulting in poor print bed adhesion.[34] To prevent sheets from detaching, the bed's temperature was increased from 60 °C to 80 °C, a value further corroborated by other works.[35] The filament extrusion multiplier value was also changed from 1.0× to 1.4×. Increasing this parameter accounted for inhomogeneities in the custom filament's diameter, compared against commercial filaments, ensuring adequate feed rate to prevent macrotears and under-extrusion. Adjusting the extrusion multiplier parameter allows control over the amount of material extruded.[36] Typically, standard filament extrusion multiplier is set to 1 because using a lower value can result in under-extrusion, leading to insufficient material deposition; while values greater than 1 can cause over-extrusion, resulting in overly dense prints and material overflow.[36] The nozzle diameter was also increased to 0.6 mm instead of standard 0.4 mm with a view to counteract the under-extrusion effect through facilitating the material's flow (also preventing clogging issues). The nozzle's speed was also reduced to reduce residual stress,[37] a hypothesized contributing factor to macrotears formation. The combined efforts of all the aforementioned altered printing parameters successfully led to the suppression of macrotears as a defect, as visualized in Figure 4c.

Another critical printing parameter investigated in this work was the nozzle temperature of the 3D printer.[38] The optimization was conducted using the sample with the highest PP amount (PP/PW 60:40 wt.%), as the higher polymer matrix content made it the most extrudable. While a temperature of 160 °C was originally employed during the filament feedstock preparation, such temperature could not be employed for the FDM 3D printing pro-

cess. Indeed, it was experimentally discovered that 3D printing at any temperature below 180 °C led to recurrent nozzle clogging and extrusion issues. Such difference in the required temperature between the filament extrusion process and the 3D printing process is not surprising as this parameter is often machine-specific, as reported in our previous study.[2] A key factor contributing to this difference is the presence of cooling fans on the 3D printer's extruder, which are designed to rapidly solidify the printed material as it exits the nozzle while also influencing the local temperature of the filament feedstock. Given these considerations, the nozzle temperature range of 180–220 °C was selected based on two factors: 180 °C was the lowest temperature that enabled stable extrusion without clogging, while 220 °C represented the upper limit to avoid significant thermal degradation and maintain material integrity. This range allowed systematic exploration of the temperature's effect on print quality and surface porosity. Replicas of a printed 10 × 10 × 2.5 mm coupon were printed at 180 °C, 190 °C, 200 °C, 210 °C, and 220 °C and studied to understand the nozzle temperature's effect on print quality. Figure 5 highlights via SEM the differences in surface porosity between coupons printed at different nozzle temperatures before and after soaking. Interestingly, the presence of pores prior to soaking the samples in petroleum ether to remove the PW phase is indicative of a secondary pore formation mechanism. Printing-induced pore formation is the introduced term for the phenomenon by which a mild amount of pores form prior to solvent extraction as a direct result of the printer's nozzle melting PW on a printed sheet's surface. In doing so, the presence of nascent surface pores introduces more surface area for petroleum ether in the subsequent soaking step. The concluded

<table><caption>**Table 1.** Summary of the printing parameters modified during the optimization process and their corresponding effects on print quality.</caption><thead><tr><td>Printing Parameter</td><td>Original Value</td><td>Altered Value</td><td>Effect</td></tr></thead><tbody><tr><td>Nozzle Diameter</td><td>0.4 mm</td><td>0.6 mm</td><td>Reduced risks of under-extrusion and clogging issues</td></tr><tr><td>Nozzle Temperature</td><td>215 °C</td><td>220 °C</td><td>Reduced risks of under-extrusion. It promotes print-induced porosity formation</td></tr><tr><td>Bed Temperature</td><td>60 °C</td><td>80 °C</td><td>Reduced risk of print bed detachment and warping</td></tr><tr><td>Solid Infill Print Speed</td><td>80 mm s<sup>-1</sup></td><td>30 mm s<sup>-1</sup></td><td>Prevented print line decohesion and macrotears</td></tr><tr><td>Filament Extrusion Multiplier</td><td>1.0x</td><td>1.4x</td><td>Improved filament flow and print homogeneity, leading to reduced risks of under-extrusion and macrotears</td></tr></tbody></table>
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Optical images of 3D-printed PP/PW 60:40 wt.% samples. a) Single-layer sheet exhibiting macroscopic tearing defects due to suboptimal printing parameters. b) Separator membrane obtained by punching the printed sheet, still showing signs of tearing. c) Final punched separator membrane produced after optimization of printing parameters (e.g., temperature, speed), resulting in improved mechanical integrity and print quality.</figcaption></figure>

relationship notes an increase in printing-induced pores when nozzle temperature also increases. Henceforth, all 3D printing of sheets, tensile specimens, and complex 3D lattice (Figure 6), was performed at 220 °C to best optimize the final product's porosity.

As exhibited in the stress-strain curves presented in Figure S1 (Supporting Information) and data gathered in Table 2, tensile testing of the unsoaked 3D printed separators revealed clear differences in mechanical behavior based on the PP/PW composition. As expected, the PP/PW 60:40 ratio exhibited the highest ultimate tensile strength (UTS) at 9.8 MPa and the highest modulus of elasticity (150.26 MPa), indicating a stronger and stiffer structure than the 30:70 PP/PW formulation with a UTS of 7.05 MPa and a modulus of 110.14 MPa. This can be simply explained by the higher amount of PP polymer matrix, which has better mechanical properties (UTS and Elastic modulus) than the paraffin wax, within the sample with PP/PW 60:40 composition. The higher wax content in the PP/PW 30:70 sample leads to a sample with lower stiffness and reduced tensile strength, in good agreement with observations previously reported in the literature for polyolefin/paraffin wax mixtures.[³⁹]

In this work, the infill structure and printing orientation were kept constant (rectilinear pattern with 100% infill density and printing orientation of 45° as shown in Figure 6d,e) to focus specifically on the impact of separator composition on electrochemical performance of 3D printed separators. While the influence of the printing orientation and infill structure on the mechanical properties was not explored here, both parameters are well known in the literature to influence the mechanical properties of 3D-printed components. Indeed, as reported by Hiremath et al.,[⁴⁰] loading along the direction (0°) was reported to yield to higher tensile strength and elongation compared to other orientations like perpendicular to the loading direction (90°) and inclined to the loading direction (45°). Regarding the infill structure, Qamar Tanveer et al.[⁴¹] demonstrated that an increase in infill density increases the mechanical strength of the part due to inter-layer bonding between consecutive layers, with the honeycomb infill pattern showing better tensile properties at 100% infill than the rectilinear pattern.

µCT with a resolution of 2 µm was used for non-destructive 3D imaging of the internal structure of the printed lattice. **Figure 7**
<figure><img src="image_7.png" /><figcaption>**Figure 5.** SEM images comparing unsoaked (left column) and soaked (right column) PP/PW 60:40 wt.% 3D-printed samples fabricated at various nozzle temperatures: a,b) 180 °C, c,d) 190 °C, e,f) 200 °C, g,h) 210 °C, and i,j) 220 °C.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Figure 6.** Printability assessment of the PP/PW 60:40 wt.% composition using various geometries. The top row shows the standard tessellation language (STL) models used for slicing: a) a single-layer sheet, b) an ASTM D638-22 Type V tensile specimen, and c) a custom geometrically complex lattice. The bottom row displays the corresponding 3D-printed parts: d) printed sheet, e) printed tensile specimen, and f) printed lattice. The comparison highlights the filament's capability to accurately reproduce both simple and complex geometries.</figcaption></figure>

illustrates the internal structure of the 3D-printed geometrically complex lattice (after soaking) using µCT and corresponding slicer software visualizations. A video showing the 3D reconstruction of the printed lattice obtained from the µCT is provided in the Supporting Information. The µCT reconstructions (Figure 7a–i) reveal that the geometry of the lattice was successfully 3D printed, demonstrating the printer's ability to produce intricate, three-dimensional architectures using the 60:40 separator filament as feedstock material. While some localized defects and deviations from the intended path are visible, particularly at sharp transitions or overhangs, the overall dimensional fidelity remains high, with less than ≈5% deviation (X, Y, and Z axis) from the CAD model based on µCT measurements. The slicer-generated nozzle path simulations (Figure 7d–f–l) provide additional context for assessing the print quality, confirming that most of the intended features were accurately reproduced. These results highlight the capability of the system to fabricate complex lattice structures with minimal defects, showcasing its potential for advanced applications in architected materials and functional

components. To improve the printing quality of such complex lattice structures, future work could focus on optimizing further some printing parameters such as the printing speed and extrusion multiplier to minimize defects during fabrication. Due to the resolution limitations of µCT (voxel size ≈6.9 µm), the mesoporosity generated by PW removal (primarily in the sub-micron range) could not be resolved. Higher-resolution SEM imaging and image-based analysis were thus performed to qualitatively and quantitatively assess the pore morphology. In addition, nitrogen adsorption-desorption measurements (BET and DFT analyses) were conducted to confirm the presence of interconnected mesopores and to provide quantitative data on pore size distribution and total pore volume.

## 4. Porosity Formation and Assessment

In the third and final step of the porous membrane fabrication procedure introduced in Figure 1, pores are generated through soaking as-printed discs in petroleum ether to extract the PW phase. Interestingly, the soaking process had an unexpected effect on the color of printed items. Prior to soaking, thin as-printed sheets appeared waxy with a distinct semi translucence when held up to light. Conversely, as-soaked sheets appear entirely white and opaque upon removal from solvent, which remains consistent with the appearance of the PP pellets used in the filament fabrication process. This further establishes the notion that solvent soaking effectively removes PW content from printed items. As shown in Figure 5, an SEM comparison reveals a higher density of surface pores after soaking the samples in petroleum ether. SEM images of the membranes' cross sections (exhibited

<table><caption>**Table 2.** Average values of ultimate tensile strength (UTS) and modulus of elasticity for 3D-printed tensile specimens fabricated using both PP/PW 60:40 and 30:70 wt.% compositions.</caption><thead><tr><td>Sample composition</td><td>Average Ultimate Tensile Strength, UTS (MPa)</td><td>Average Modulus of Elasticity, E (MPa)</td></tr></thead><tbody><tr><td>PP/PW 30:70</td><td>7.05 ± 2.45</td><td>110.14 ± 15.67</td></tr><tr><td>PP/PW 60:40</td><td>9.83 ± 3.99</td><td>150.26 ± 4.06</td></tr></tbody></table>
<figure><img src="image_9.png" /><figcaption>**Figure 7.** Volume renderings from µCT scans a–c,g–i) showing the internal structure of 3D-printed complex lattice structures fabricated using the PP/PW 60:40 wt.% composition after PW removal via soaking. The corresponding slicer software visualizations d–f,j–l) display the intended nozzle paths at various Z-axis slice levels. The side-by-side comparison highlights the fidelity of the printed structure to the original design.</figcaption></figure>

in **Figure 8**), compared against an unsoaked membrane of corresponding composition, highlight the procedure's ability to generate significant porosity throughout the bulk of every composition.

As shown in Figure 8 through SEM and subsequent image analysis, an interesting pattern appears among the two compositions: PP/PW 60:40, with the highest PP polymer content, results in a larger average pore size (0.143 µm² or diameter of the pore of 0.427 µm), while the other sample 30:70 exhibits smaller pore size (0.039 µm² or diameter of the pore of 0.223 µm). This relationship is most easily identified when comparing the cross

section images of the soaked PP/PW 30:70 sample to the soaked PP/PW 60:40 sample, as the 30:70 sample's largest pore is ≈1 µm in diameter, whereas the 60:40 sample forms pores with diameters as large as 2.5 µm. The microstructure of the prepared separators exhibits a coral-like morphology, characterized by an interconnected porous network. This structural feature is essential for facilitating ion transport when soaked in liquid electrolyte for battery cycling, while also maintaining mechanical stability. Through SEM image analysis, overall porosity percentages of 22% and 27% are observed for samples PP/PW 30:70 and 60:40,
<figure><img src="image_10.png" /><figcaption>**Figure 8.** SEM analysis of porosity evolution in 3D-printed PP/PW composites before and after solvent extraction. a–c) SEM images of the PP/PW 30:70 wt.% sample: (a) before soaking, (b) after soaking in petroleum ether to remove paraffin wax (PW), and (c) image analysis highlighting the resulting porous network. d–f) Corresponding SEM images and image analysis for the PP/PW 60:40 wt.% sample.</figcaption></figure>

respectively. These results are unexpected, as higher initial PW content would typically lead to greater porosity. This discrepancy can be attributed to the limitations of 2D image analysis, which may not capture the full extent of interconnected pores and their 3D shape, as well as the SEM and µCT resolution, which may be insufficient to detect smaller pores.

The porosity of the samples is comparable to that of a commercially available Celgard separator membrane as seen in **Figure 9**, suggesting the potential suitability in electrochemical applications. Serving here as a reference, a porosity percentage of 10% was deduced for the Celgard separator membrane through identical image analysis. Pore size distributions for all custom compositions and the Celgard reference are presented in Figure 9c. In comparison with the 3D printed porous samples, commercial Celgard presents smaller pore size with an average value of 0.004 µm² (or average pore diameter of 0.071 µm), in good agreement with literature.[19,42] Nevertheless, the uniformity of the pores in the printed samples and their distribution indicate effective processing conditions, which could further be optimized to enhance performance.

To complement the 2D image-based porosity analysis and overcome the resolution limitations of SEM, N₂ adsorption-desorption measurements were conducted and analyzed using the Density Functional Theory (DFT) method. The sample with higher PW content (PP/PW 30:70) displayed a higher total pore volume of 0.085 cm³ g⁻¹ and a wider mode pore diameter of 28 nm, compared to the 60:40 sample, which had a lower pore volume of 0.057 cm³ g⁻¹ and a smaller mode pore diameter of 5 nm. Interestingly, both samples exhibited nearly identical specific surface areas of ≈19 m² g⁻¹. These results suggest that although the 60:40 sample forms larger macropores observable by SEM, the 30:70 composition contains a higher population of mesopores and larger internal surface area at the nanoscale. This hierarchi-

cal porosity observed for both samples is beneficial for liquid electrolyte uptake and ion diffusion.

Such morphology is particularly advantageous for applications requiring high ionic conductivity and electrolyte retention, making these separators promising candidates for use in energy storage systems including LIBs, as discussed later in this article. In the future, the porosity morphology of the samples could be tailored by adjusting their composition and processing conditions, leading to structures such as spherulitic, cellular, nodular, or, as presented in this study, coral-like formations. For instance, variations in polymer concentration, solvent evaporation rate, and phase separation techniques can influence the development of these distinct morphologies. Previous studies,[29,43,44] have shown that manipulating these parameters allows for the control of pore structures, enabling the design of membranes with specific properties suited for targeted applications.

## 5. Impact of Separator Composition on Electrochemical Performance of 3D Printed Separators

Coin cells were assembled using the two custom separator compositions (after PW phase removal) and a Celgard reference, with 1 M LiPF₆ in ethylene carbonate and diethyl carbonate (EC/DEC 1:1 vol.%) liquid electrolyte (LP40). The cells were then analyzed using potentiostatic electrochemical impedance spectroscopy (PEIS) to determine their ionic conductivity and the results are illustrated as Nyquist plots in **Figure 10a**. The reason the semicircle obtained from the Celgard separator appears at a smaller resistance value compared to the custom-made separators, while the ionic conductivity of the Celgard separator shows the opposite trend, lies in the fact that the Celgard separator has
<figure><img src="image_11.png" /><figcaption>**Figure 9.** a) SEM image and b) corresponding image analysis of the commercially available Celgard separator membrane, used as a benchmark. c) Pore size distribution comparison between the two custom 3D-printed PP/PW compositions (30:70 and 60:40 wt.%) and the Celgard membrane.</figcaption></figure>

a smaller thickness (Equation (3)). The calculated ionic conductivity values are $3.28 \times 10^{-3}$ S cm$^{-1}$ for the PP/PW 30:70 composition, $3.94 \times 10^{-3}$ S cm$^{-1}$ for 60:40, and $1.18 \times 10^{-3}$ S cm$^{-1}$ for Celgard (Table 3). These findings indicate that variations in porosity among the custom separators do not significantly influence Li$^+$ conductivity, as evidenced by their comparable values. On the other hand, the slight difference in the ionic conductivity delivered by these cells in comparison with the Celgard separator can be explained by the difference in tortuosity.[45,46] These values are highly comparable to the literature, where it was found that pure LP40 electrolyte within a commercial glassy fiber presents an ionic conductivity of $7.5 \times 10^{-3}$ S cm$^{-1}$,[27] and an electrospun porous poly(vinylidene fluoride)-based composite separator using 1M LiPF$_6$ in EC/EMC/DMC 1:1:1 vol. electrolyte exhibited $1.69 \times 10^{-3}$ S cm$^{-1}$.[47]

The electrochemical stability window of the separators in the presence of the LP40 electrolyte within a battery configuration was evaluated via linear sweep voltammetry (LSV) testing. The measurements involved applying a linearly varying potential to the working electrode and measuring the resulting current response. Figure 10b shows that the custom-made porous separators PP/PW 30:70 and 60:40 (after PW phase removal) present excellent electrochemical stability in the 0–5.2 V versus Li/Li$^+$ window, even exceeding the performance of the Celgard separa-

tor, which showed an onset current increase at 5 V. The custom porous separator prepared from the PP/PW 60:40 composition exhibited a minor 5 µA current increase within the 2–3 V Li/Li$^+$ range, however, this increase is attributed to impurities in the electrolyte and not the separator itself.[48] The LSV results demonstrate that these custom separators can be successfully utilized in high-voltage batteries that employ LiPF$_6$-based electrolytes.

Half-cell coin cells were assembled for electrochemical cycling testing. Each cell incorporated a 3D printed LiFePO$_4$ working electrode described and characterized by Cardenas et al.,[49] a lithium metal reference and counter electrode, and either a Celgard or custom separator soaked with LP40 liquid electrolyte. While it is true that separators must be kept as thin as possible to minimize internal resistance,[50] the difficulty of printing a filament with a high content of wax inadvertently led to thick separator membranes in comparison with Celgard (30 µm). The 30:70 composition, which contains 50% less polymer content than the 60:40 sample, was challenging to prepare, but the final printed separator thickness of ≈150 µm was comparable to the 60:40 composition of ≈160 µm, which was easier to print. Employing thicker separators diminishes the probability of puncture during cell assembly, thereby mitigating the occurrence of internal short circuits. Although the printed separators are thicker than commercial alternatives, the objective of this work is not to replicate
<figure><img src="image_12.png" /><figcaption>**Figure 10.** a) Fitted Nyquist plots obtained from electrochemical impedance spectroscopy (EIS) measurements for both PP/PW compositions and the Celgard separator. The inset shows the equivalent circuit model used for the fitting; b) Linear sweep voltammetry (LSV) curves from 0 to 5.2 V at a scan rate of 0.1 mV s⁻¹, demonstrating the electrochemical stability window of the custom separators relative to the commercial benchmark.</figcaption></figure>

commercial thickness standards, but to demonstrate the feasibility of using FDM 3D printing for producing functional, mechanically robust, and shape-conformable separator architectures. This versatility could be particularly beneficial in next-generation batteries that require shape-conformable components. As previously reported in the literature, FDM-based additive manufacturing is inherently limited in achieving ultra-thin layers, particularly for the initial layer which typically ranges from 150 to 200 µm.[2] Despite this limitation, the printed membranes exhibited sufficient ionic conductivity and electrochemical stability for battery operation.

The results of the rate capability tests in **Figure 11** show that the cells using a Celgard separator or a custom separator obtained from the PP/PW 60:40 composition (after PW phase removal) exhibited equivalent results with a maximum specific capacity of ≈150 mAh g⁻¹ at C/20. On the other hand, the porous separators obtained from the PP/PW 30:70 composition exhibited a maximum specific capacity of ≈170 mAh g⁻¹ at C/20, thus closely approaching the 170 mAh g⁻¹ theoretical specific capacity of LiFePO₄. Similarly, the cell containing the custom porous separator obtained from the PP/PW 30:70 composition surpassed the performance of the commercially available Celgard counterpart at C/10. As expected, performance drops at the high C-rate of 1C for all compositions due to the bigger overpotential and the lower amount of Li⁺ transferred in low time. Moreover, an important change in trend was however observed as the coin cell contain-

<table><caption>**Table 3.** Parameters used to calculate the ionic conductivity of the separators in the presence of LP40 electrolyte. The measurements were conducted using EIS, and the same contact area of 1.267 cm² was used for all samples. Values include separator thickness, resistance, and resulting ionic conductivity for both custom PP/PW compositions and the commercial Celgard membrane.</caption><thead><tr><th>Separator</th><th>Resistance [Ω]</th><th>Thickness [µm]</th><th>Li⁺ conductivity[S cm⁻¹]</th></tr></thead><tbody><tr><td>PP/PW 30:70</td><td>3.6</td><td>150</td><td>3.28 × 10⁻³</td></tr><tr><td>PP/PW 60:40</td><td>3.2</td><td>160</td><td>3.94 × 10⁻³</td></tr><tr><td>Celgard</td><td>2.0</td><td>30</td><td>1.18 × 10⁻³</td></tr></tbody></table>

<figure><img src="image_13.png" /><figcaption>**Figure 11.** Battery performance comparison of 3D-printed separators (PP/PW 30:70 and 60:40 wt.%) after paraffin wax (PW) removal, and a commercial Celgard separator. Tests were performed in coin cells with a Li⁰/separator/LiFePO₄ configuration and were cycled across a range of C-rates from C/20 to 1C to evaluate rate capability.</figcaption></figure>

ing the PP/PW 30:70 composition (originally exhibiting the best performances at C/20 and C/10) depicted the lower performance at 1C with a retained discharge capacity of only ≈24 mAh g⁻¹. In comparison, the 60:40 composition delivered ≈70 mAh g⁻¹ while the Celgard retained ≈70 mAh g⁻¹ of capacity. When coming back at C/20 for cycle #16, the previously obtained specific capacities were recovered. After 30 cycles, and three different C-rates, the charge capacity fade was only 4.6%, 4.8%, and 3.7% for the 30:70, 60:40, and Celgard separators, respectively. This indicates that all the 3D printed separators exhibit a comparable capacity loss to the commercial separator when using LP40 as liquid electrolyte and a 3D printed LiFePO₄ positive electrode.

The differences in electrochemical performance between the 30:70 and 60:40 compositions can be correlated with their distinct pore morphologies and distributions, as confirmed by both
<figure><img src="image_14.png" /><figcaption>**Figure 12.** Individual charge-discharge voltage profiles at selected cycles (3, 8, 13, and 18) corresponding to C-rates of C/20, C/10, 1C, and a return to C/20, respectively, for separators fabricated from a) PP/PW 30:70 wt.%, b) PP/PW 60:40 wt.%, and c) commercial Celgard membrane.</figcaption></figure>

BET/DFT analysis and SEM image analysis. The 30:70 separator exhibited a significantly higher total pore volume (0.085 cm³ g⁻¹ versus 0.057 cm³ g⁻¹) and a larger mode pore diameter (≈28 nm versus ≈5 nm) than the 60:40 sample, despite both having similar specific surface areas (≈19 m² g⁻¹). This more open and accessible mesoporous structure in the 30:70 composition likely enhances ionic transport by reducing tortuosity and facilitating electrolyte uptake, leading to the observed higher specific capacity of ≈170 mAh g⁻¹ at C/20. While the 30:70 separator's higher pore volume and larger mode pore size appear to enhance electrolyte accessibility and facilitate Li⁺ transport at low C-rates (C/20 and C/10), these same features may lead to less efficient ion conduction pathways under high-rate conditions. The potentially uneven pore connectivity at high current densities could result in greater polarization and limited ion mobility. This may explain the pronounced capacity drop observed at 1C, despite its superior performance at lower C-rates. In contrast, the 60:40 separator, with its narrower pores and lower total pore volume, demonstrated better rate retention under high current loads, possibly due to improved ion transport uniformity and reduced polarization stemming from a tighter and more interconnected pore network.

Individual cycling plots (one cycle per C-rate) were plotted to observe the charge-discharge behavior of 3D printed LiFePO₄ in the presence of the printed separators in comparison to a commercial one (Figure 12). All of the separator present a charge or discharge plateau around 3.4 V versus Li/Li⁺ at C/20 and C/10, which is ascribed to the Li⁺ insertion/extraction and the two-phase transitions between LiFePO₄ and FePO₄ (LiFePO₄ → FePO₄ + Li⁺ + e⁻).[51] Moreover, the cell using a custom separator obtained from the PP/PW 30:70 composition exhibited a pronounced polarization exceeding 0.6 V versus Li/Li⁺ during charge-discharge cycles at C/20 and C/10 (Figure 12a). In comparison, the 60:40 composition and the Celgard, both presented a lower polarization of about 0.5 V versus Li/Li⁺ (Figure 12b,c). The increased polarization of the PP/PW 30:70 composition in comparison with the other samples is probably the reason why at high 1C the specific capacity drops. Finally, at 1C the charge process delivers higher capacity than the discharge process. This means that the battery's coulombic efficiency is less than 100% because the stripping of lithium from the 3D printed LiFePO₄ electrodes is more efficient than lithium intercalation within this specific configuration. Nonetheless, similar to the cycling results, this effect is only seen at 1C and the capacity is recovered at C/20.

<figure><img src="image_15.png" /><figcaption>**Figure 13.** Long-term galvanostatic cycling performance of Li⁰/separator/LiFePO₄ coin cells at a C/20 rate, demonstrating the stability and capacity retention of the battery configuration over extended cycling.</figcaption></figure>

The overall cycling results show the good interfacial compatibility between a 3D printed LiFePO₄ and the printed separators, which is ultimately the foundation step for the production of 3D printed batteries with complex geometries.

To assess long-term electrochemical performance, galvanostatic cycling was conducted at a fixed C-rate of C/20, focusing on stability and capacity retention over extended cycling. Based on prior rate capability results, the PP/PW 30:70 composition demonstrated the most promising performance, closely approaching the theoretical specific capacity of LiFePO₄ (170 mAh g⁻¹). Consequently, only the 30:70 composition was selected for long-term cycling, and its performance was benchmarked against a cell incorporating a commercial Celgard separator.

As shown in Figure 13, the performance trend observed during rate capability testing is maintained during prolonged cycling. The 30:70 separator composition outperforms Celgard in both specific discharge capacity and retention. After 65 cycles, the 30:70 sample delivered a higher specific discharge capacity (147.3 mAh g⁻¹) compared to Celgard (131.9 mAh g⁻¹), along with improved capacity retention (94.8% vs 88.2%).
**6. Conclusion**

This article successfully investigated the production and optimization of an FDM-3D printable separator filament with a novel PP/PW composition, achieving bio-inspired porosity formation through the combined use of FDM and TIPS. While FDM and TIPS have been individually explored for separator manufacturing, this study uniquely leveraged their combined advantages to enable the facile fabrication of highly porous, 3D printable battery separators with any geometries. To the best of the authors' knowledge, no current phase diagram exists showcasing the solubility of PW in PP. As such, this study further examined the viability of this polymer/diluent system by comparing two custom PP/PW ratios. The optimized printing parameters, including nozzle temperature and extrusion multiplier, played a crucial role in achieving defect-free prints and enhancing porosity. The post-processing solvent soaking effectively removed PW, leading to a significant increase in porosity and a noticeable change in material properties. Moreover, it was found that varying the PP/PW ratio influenced the printability and the pore size and distribution, which in turn affected the mechanical strength and electrochemical performance. The separator obtained from the PP/PW 30:70 composition exhibited the best electrochemical performance at the C-rate of C/20 and C/10, surpassing other printed separators compositions and even a commercial Celgard separator. At the highest tested C-rate, the cell containing the porous separator obtained from the PP/PW 60:40 composition delivered the best capacity retention, exceeding the performances observed from the other 30:70 composition and the commercial Celgard separator.

This innovative process, requiring only a single post-processing step, offers a promising alternative to traditional battery manufacturing, with potential applications in one-shot 3D printing of a complete battery and shape-conformable battery production. While this work was focused on adapting the PP/PW system validated by Li et al.,[30] future studies could explore a broader range of polymer/diluent compositions to further improve printability, varying pore morphologies, and electrochemical performance. Incorporating ceramic particles to improve thermal stability and safety during cycling also represents a promising direction. Additionally, investigating dual polymer or diluent phases and alternative phase separation techniques, such as vat photopolymerization induced phase separation, could yield further advancements in this field. The full extent to which this process can be applied has yet to be realized; additional meaningful applications extend beyond AM rechargeable batteries including but not limited to microfiltration/ultrafiltration,[52] or even bone tissue engineering.[53]

**7. Experimental Section**

**Filament Preparation:** PP pellets (GR10SPP grade) and PW pellets were purchased from Braskem and Sigma-Aldrich, respectively. Two distinct PP/PW compositions (30:70 and 60:40 wt.%) were prepared in batches of 20 g (minimum amount to ensure the subsequent preparation of a homogeneous 3D printable filament). Solvent-free filament extrusion of a ≈1.75 mm diameter 3D printable filament was performed using a Filabot Original EX2 extruder (Filabot Triex LLC, United States) by homogeneously feeding a composition batch into the hopper under a fume hood. The extruder's temperature remained fixed at 160 °C during filament preparation, a value found experimentally and further sup-

ported by DSC analysis of the pure PP. The prepared filament was manually spooled using a large beaker covered in masking tape, an alteration made as a result of the filament's wax content not adhering to glass, for ease of handling and storage. Before each batch's extrusion into filament, the extruder's barrel was purged and cleared of any contaminants by allowing the screw to rotate while empty at 160 °C for at least 20 minutes. Purging the barrel ensured each batch's composition could remain as close to its desired ratio as possible without altering intended weight percentages.

**Printing:** FDM was completed under fume hood using an Original Prusa i3 MK3S+ (Prusa Research, Czech Republic). Printed models were designed in Fusion360 (Autodesk, United States) then sliced using the PrusaSlicer (Prusa Research, Czech Republic). To account for the custom filament's unique preparation and properties during 3D printing, the following altered printing parameters were employed: 0.6 mm nozzle diameter, 220 °C nozzle temperature, 80 °C bed temperature, 30 mm s⁻¹ solid infill print speed, and a 1.4× filament extrusion multiplier. Besides the aforementioned alterations, all other printing parameters were consistent with the "0.15 mm quality @0.6MK3" system preset. Additionally, the printing plate was altered by adding masking tape to ensure proper print bed adhesion. Multiple models were developed for testing the two different filament compositions. This includes single layer sheets (≈150 µm thick) from which separator discs (18 mm and 12.7 mm diameter) were obtained by means of mechanical punching, ASTM D638-22 Type V tensile specimens, and a custom geometrically complex 3D lattice. The single layer print was employed to perform subsequent electrochemical battery cycling as well as the ionic conductivity testing. The tensile specimens were used for mechanical tensile testing and the lattice was created to showcase the custom filament's printability of any complex 3D shape.

**Porosity Induction:** To induce porosity, the as-printed separators items were placed in a flask containing petroleum ether (ACS reagent, Sigma-Aldrich) under a fume hood. The separators were left agitating overnight to allow the petroleum ether to preferentially dissolve the PW-rich phase in the disc. Then, the as-soaked separators were removed from the solvent to air dry under the fume hood for 1 hour before being dried overnight in an 80 °C oven to remove residual moisture. The aforementioned procedure was followed exactly on the complex 3D lattice to induce porosity after soaking.

**Thermal, Structural, and Mechanical Characterization:** TGA was performed using a TA Instruments Discovery TGA 5500 conducted in air from 30 °C up to 200 °C. DSC was completed under a nitrogen environment ($N_2$) using a TA Instruments Discovery DSC 2500 from 30 °C up to 300 °C. Both techniques were completed at a temperature ramp rate of 10 °C min⁻¹. As-received PP pellets, PW pellets, and the two custom compositions were analyzed.

Scanning electron microscopy was performed using a Hitachi SU3500 (Hitachi, Ltd., Japan) under high vacuum, operating between 5–10 kV, using a secondary electron detector. All samples were coated with gold using an ion sputtering coater (MSE Supplies, United States). Cross sectional analysis of the separators and filaments required submerging samples in liquid $N_2$ and subsequently splitting normal to the print line direction and normal to the filament's length to reveal the sheet's undeformed profile and the filament cross section, respectively.

µCT imaging and 3-D reconstruction was also performed. The internal architecture of the 3D-printed lattice sample was imaged using a SkyScan 1276 CMOS edition µCT scanner (Bruker, Billerica, MA). The sample was positioned on a low-density foam support (thermocol) to minimize X-ray attenuation artifacts during scanning. Radiographic projections were acquired at an X-ray source voltage of 42 kV and a current of 200 µA, with a full 360° rotation performed at angular steps of 0.2°. No beam filter was applied during acquisition. The acquired radiographs were reconstructed into 16-bit TIFF image stacks using NRecon software 2.2.0.6 (Bruker), incorporating corrections for beam hardening and ring artifacts. The reconstructed volumes were reoriented along the principal XYZ axes using DataViewer 1.7.0 (Bruker), and the aligned datasets were exported as 8-bit PNG image stacks. These image stacks were then converted into .ims files using the Imaris File Converter (Andor, Oxford Instruments, United
Kingdom). The final 3-D volume was rendered at an isotropic voxel size of 6.9 µm. Segmentation and surface rendering were performed in Imaris version 10.2 (Andor, Oxford Instruments, United Kingdom), utilizing its integrated machine learning-based segmentation tool. This tool employs a trainable pixel classification algorithm to delineate geometries within volumetric datasets.

Nitrogen adsorption-desorption isotherms were measured at 77 K using a Quantachrome Autosorb iQ gas sorption analyzer. Samples (≈13 mg) were degassed at 60 °C for approximately 24 hours prior to analysis. The surface area was calculated by the Brunauer-Emmett-Teller (BET) method, while pore size distribution and volume were calculated using the Density Functional Theory (DFT) method applying the carbon cylindrical pore model.

Tensile testing was performed using a MTS Universal Testing Machine (C44-105 model, San Diego Transit Corp.) and standard MTS EM extensometer at a constant crosshead speed of 10 mm min⁻¹ in accordance with ASTM D638-22. The software used during testing was the MTS Test-Suite TW Elite. In total, 8 tensile specimens were 3D printed (4 samples for each custom composition) and tested to analyze their UTS and modulus of elasticity. These mechanical properties were averaged and compared for each composition. Equation (1) was used to calculate the UTS (MPa, σ<sub>UTS</sub>), where F<sub>max</sub> (N) was the maximum applied force and A (mm²) was the specimen's nominal cross sectional area. Equation (2), known as Hooke's Law, was used to calculate Young's modulus of elasticity (E) where σ (MPa) was the stress and ε (mm mm⁻¹) was the corresponding strain within the initial linear region of the stress-strain curve.

$$ \sigma_{UTS} = \frac{F_{max}}{A} \qquad (1) $$

$$ E = \frac{\sigma}{\varepsilon} \qquad (2) $$

**Electrochemical Characterization:** Cell preparation took place in an argon-filled glove box (Mbraun, United States) under an environment with < 0.1 ppm H₂O and O₂. Prior to assembly, the 3D printed porous separator samples (discs of 18 mm and 12.7 mm diameter) were immersed overnight into a LP40 liquid electrolyte.

For half-cell galvanostatic cycling, 18 mm diameter discs were used as separators, metallic lithium (99.9% purity, 0.38 mm thick, Sigma Aldrich, USA) was employed as the counter/reference electrode and 3D printed LiFePO₄ electrodes of 12.7 mm diameter were employed as positive electrodes. The LiFePO₄ electrodes were 3D printed onto battery grade aluminum foil using DIW 3D printing as described in the previous study using NMP (Sigma Aldrich, United States) as a solvent, then dried overnight at 70 °C to yield a film with ≈4.5 mg cm⁻² of active material.[49] The LiFePO₄ electrode contained 80 wt.% of active material (Skyspring Nanomaterials, United States), 10 wt.% PVDF (Solvay, United States), and 10 wt.% carbon black (Super P, TIMCAL, MSE Supplies, United States) in the dried state. 150 µL of LP40 were added before sealing the cells with 1 ton of pressure using a Compact Digital Pressure Controlled Electric Crimper-MSDK-160E (MTI Corporation, United States). A resting time of 20 hours was performed before launching the cycling. For rate capability testing, the coin cells were galvanostatically charged and discharged between 2.5 and 3.8 V versus Li/Li⁺ using a LBT galvanostat (Arbin, United States) for 5 cycles at C/20, C/10, and 1C, then 15 cycles at C/20.

For the LSV tests, as-soaked separators (18 mm diameter and ≈150 µm thick) or Celgard 2325 (18 mm diameter and 30 µm thick) were placed between a stainless steel 316L spacer serving as the working electrode and a lithium metal serving as the counter/reference electrode. The experiments were done in coin cells between 0–5.2 V at a scan rate of 0.1 mV s⁻¹ using a MSTAT potentiostat/galvanostat (Arbin, United States).

PEIS testing was performed by loading a 12.7 mm diameter (≈150 µm thick) as-impregnated separator into a coin cell between two 316L stainless steel current collectors. An Interface 1010E (Gamry, United States) was used to measure ionic conductivity (σ<sub>Li⁺</sub>) across a frequency range from 2 × 10⁶ to 0.1 Hz with an AC amplitude of 50 mV, a DC offset of 5 mV, an AC peak of 70.7 mV and 20 points per decade. Using Equation (3), σ<sub>Li⁺</sub>

(S cm⁻¹) was calculated, where A (cm²) was the area of contact between the separator and electrodes, t (cm) was the separator membrane's thickness, and R (Ω) was the resistance. The resistance values were extracted from fitted Nyquist plots using ZView 3.2b software.

$$ \sigma_{Li+} = \frac{1}{R} \cdot \frac{t}{A} \qquad (3) $$

## Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

## Acknowledgements

The work at The University of Texas at El Paso was supported by the ES-TRELLA group and the Imaging and Behavioral Neuroscience Facility (IBN facility) at the Interdisciplinary Research Building. The authors would like to thank Dr. Eric MacDonald, Dr. Christopher Bradley and Dr. Stephen Stafford for the valuable discussions. Research reported in this publication was supported by the Office of The Director, National Institutes of Health (NIH), under award number C06OD032074. The content was solely the responsibility of the authors and does not necessarily represent the official views of the NIH. The work at Sandia National Laboratories was supported by the Laboratory-Directed Research and Development (LDRD) Programs. Sandia National Laboratories was a multi-mission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC (NTESS), a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy's National Nuclear Security Administration (DOE/NNSA) under contract DE-NA0003525. This written work was authored by an employee of NTESS. The employee, not NTESS, owns the right, title and interest in and to the written work and was responsible for its contents. Any subjective views or opinions that might be expressed in the written work do not necessarily represent the views of the U.S. Government. The publisher acknowledges that the U.S. Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this written work or allow others to do so, for U.S. Government purposes. The DOE will provide public access to results of federally sponsored research in accordance with the DOE Public Access Plan.

## Conflict of Interest

The authors declare that this study's contents were previously submitted to the United States Patent and Trademark Office as a provisional patent (Application Number: 63/766116) on March 3rd, 2025 by the following co-inventors: Alexis Maurel, Abraham Enchinton, Katherine R. Gonzalez, and Ana C. Martinez.

## Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

## Keywords

3D printing, batteries, lithium-ion, porosity, separator

Received: April 28, 2025

Revised: August 5, 2025

Published online: September 4, 2025
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