Recyclable 3D‐Printed Aqueous Lithium‐Ion Battery

Additive manufacturing, or 3D printing, in energy storage devices such as batteries has the potential to create new form factor small cells that are incorporated into the shape of the device at the design stage. With large‐scale proliferation, sustainable and recyclable materials are needed to avoid used cell waste accumulation, and the cells should have performance metrics that match or exceed existing cells. Inspired by safe aqueous battery chemistries and development in stereolithographic photopolymerization printing methods such as vat polymerization (Vat‐P), a 3D‐printed aqueous lithium‐ion battery developed, using sustainable active cathode and anode materials of LiMn2O4 and FePO4·2H2O, which can be fully recycled using a simple combustion method. This battery is designed to allow a stable cycling, higher energy density option compared to a metallic cell of similar construction, and to ensure better intraelectrode electrical conductivity and rigidity necessary for a viable cell, avoiding brittleness sometimes found in all‐in‐one composite‐printed electrodes. The printed cell has a stable cell‐level capacity of 1.86 mAh, better than that of a comparable metallic coin cell of similar internal chemistry, with an average cell voltage just over 1.0 V. Following combustion, the crystalline phase of LiMn2O4 and a mixed phase of some Fe2O3 mixed with a dominant composition of FePO4 are recovered. All inorganic materials are recovered after combustion.

of interconnected 3D networks that can be fabricated up to a thickness of 100 μm and can be served as a porous substrate for lithium-ion batteries.These structures not only could provide multiple electrical pathways for charge transfer but also allows space for lithium accessibility.Starting from the first attempts to incorporating SLA to print a full 3D-printed battery, [25] the method is gaining interest due to the fidelity of the printed structures and the possibility for multimaterial or direct composite printing with support or active materials.In one example, a trilayer structure in which Li-AlO 2 -PEO-based quasi-solid electrolyte was sandwiched between LFP cathode and LTO anode.Despite severe capacity fading, the authors showed that the areal density of the full cell was 3Â the commercial planar thin film batteries.That work also highlighted the potential of SLA-based printing for constructing an ultrathin and mechanical robust lithium-ion battery (LIB).More recently, Martinez et al. showed [26] that Vat-P can be used to directly print battery electrodes containing LiNi 1/3 Mn 1/3 Co 1/3 O 2 (NMC 111) by carefully choosing the precursor and accommodating changes to light scattering in the composite resin to ensure good print quality.
Besides the selection of the 3D printing method and the associated influence on the quality, porosity, mechanical strength, and conductivity of the material, the compatibility of active material with printing methods needs to be optimized before realizing a fully functional 3D-printed battery.Additional requirements include safety, cost, and recyclability which can be achieved using safe alternative chemistries, such an aqueous chemistry, so long as the voltage and power requirements are suitable for the end use.12a] For example, aqueous lithium-ion batteries have been proposed to replace toxic and potentially hazardous traditional batteries especially in medical applications [27] and other wearable medical devices where the power requirements are small and the cell form factor and footprint requires a bespoke design to be unobtrusive the use of the sensor or device.Aqueous Li-ion batteries [28] are capable of high-rate charging, are lower in cost, and safer due to the use of environmentally friendly aqueous electrolytes. [29]ome use water-in-salt approaches, [30] and others are capable of higher voltage (%4.0 V) operation. [31]Careful selection of active materials also allows low-cost manufacturing in tandem with recyclability.27b,32] Recently, Wang et al. [33] reported an industrially available amorphous FePO 4 •2H 2 O as an anode material for their aqueous LIB.It gave a stable capacity of 82 mAh g À1 and maintained 92% capacity after 500 cycles at a current rate of 0.2C.When paired with LiMn 2 O 4 in an acidic LiTFSI-based aqueous electrolyte, a full cells can avoid cathodic oxygen evolution, while the higher equilibrium voltage of FePO 4 avoids hydrogen evolution making it stable for use in an aqueous Li-ion battery cell.
Inspired by the safety and performance of aqueous lithiumion batteries for application that do not require high voltages, we show a stable rechargeable Li-ion battery cell that combines Vat-P printing and standard slurry/separator application that is completely recyclable.The tandem approach allows the use of printed plastic parts instead of stainless steel with control over the internal electrical wiring and porosity, while keeping the rigidity of a traditional coin cell that is sometimes a limitation of multimaterial or all-in-one printed active electrodes when full cells are assembled and tested.We compared the printed cell to an equivalently mass-loaded traditional coin cell and show that the printed cell is capable of faster rate response and overall cell-level capacity, without adverse effect of internal cell pressure on the active material components.Active material slurries are "housed" in porous compartments in each current collector that are metallized.The metal coating and all active materials is fully recovered after combustion.

Fabrication and Design of 3D-Printed Electrodes and Cells
12a] This method allows fidelity in printed layer thickness on the order of %25 μm giving high contrast delineation and smooth features in comparison to FDM, which uses thermosetting plastics such as ABS and PLA.With Vat-P printing, we designed a coin cell casing with an internal ordered porous structure based on open diamond cubic and body-centered cubic structures to house the active material.This internal lattice, when metallized, provides cocoons of fixed volume for active material, reduces variation in internal cell pressure by maintaining a fixed and constant thickness of the material-filled casing, and provides electrical wiring through the electrode.Figure 1a shows the two types of 3D lattice we implemented for the current collectors and casings.To ensure planar morphology, the 3D structure of these kinds of lattices required flattening at design stage prior to printing (see Figure S1, Supporting Information).Version I includes a 3D-printed seal gasket, with Version II implementing a click-seal design.These cell casings were metallized with either Ti or Au (Figure 1c).SEM images in Figure 1d confirm the high fidelity print of the struts of the microlattice current collector structures, and EDS mapping confirms that the sputtered Au metallization on the surface and inside of the DC lattice (Figure 1e-g).Further details are found in Figure S2, Supporting Information.

Metallization of Printed Cells for Aqueous Battery Chemistry
Nickel and gold were tested as coating materials for the 3D-printed conductive current collectors and cell casings.Adhesion to the photopolymerized resin was an important factor, and stability with slurry coating, sealing pressure, and aqueous electrochemistry were tested to ensure reliable and consistent data, charge storage processes, and as will be shown later, for recyclability without modifying the active materials.Electrical conductivity measurements on Au-coated printed casings showed low resistance values typical in the range 1.6-2.9Ω compared to Ni-coated casings (%30-100 Ω).In case of Au-coated current collectors, no significant difference was observed between body-centered cubic (BCC) and diamond cubic (DC) lattices across a similar nominal casing diameter.However, for Ni-coated casings, the DC lattice showed a slightly higher resistance than the BCC structured lattice when measurements were made across the internal porous side (current collector) and also through the thickness of the casing (Figure 2a,b).The choice of a BCC lattice Au-coated current collector and coin cell casing was more compelling when both Ni-and Au-coated substrates were subjected to erosion tests where each were immersed in 10 M LiTFSi-based aqueous electrolyte for 24 h.Electrical measurements conducted after this test (Figure 2b) showed a stable, similarly low resistance for Au and in most cases, complete loss of conductance for Ni-coated-printed structures.Photographs of the casings showed significant oxidation and delamination of the sputtered Ni film, while Au remained pristine.The delamination occurred predominantly around the nonplanar strut features of the current collector lattice, removing all electrical connectivity to where active material would be infilled.
We observed no significant difference in resistance for Aucoated casings and current collectors printed from two different polymer resins (Figure 2b).The uniform and stable coating by Au was confirmed with Raman scattering spectra in which modes at 2939 and 1463 cm À1 associated with the polymeric material were attenuated due to thick layer of Au and Ni (Figure 2d).After immersion in the electrolyte for 24 h, the modes of the polymer were observed in the Ni coated casings, while the Au-coated material remained unchanged, attenuating the vibrational modes.Therefore, we chose Au-coated current collectors/casings with internal porous BCC lattice for further experiments in the aqueous battery cell.

Infilling of Anode and Cathode Slurries
One of the most critical steps to ensure good cell performance involves electrode fabrication in which slurries of active materials, binders, and conductive agents are deposited on a conductive substrate.Due to the 3D architecture of our 3D-printed cell casings, slurry optimization was inevitable for homogenous packing of the active material.Ouyang et al. [34] recently showed that slurries with lower viscosities give more uniform deposition of active material.And in 3D-printed cells using various space filling [35] and fractal geometries, the infilling of active material that is accessible and without excessive internal resistance is important for a reliable cell.Here have been recent reports of Vat-P printing [24,26] of electrodes with active materials directly within the resins where resin-soluble precursors approaches are used, and this shows the sensitive relationship between the printed material quality and the mechanical properties.This trade-off can render directly printed materials, in some case, to be brittle particularly when porous.Hence, our approach sacrifices the one-step printed capability to improve rigidity and allow porosity to reduce electrical resistance in a plastic cell.In our case, waterdiluted slurries (low viscosity) were beneficial and relatively easy to work with and they allowed faster and even material infilling.The water-based slurries were optimized by mixing active material (FPO for anode and LMO for cathode), CMC-SBR binder, and conductive carbon (see Experimental section) with different volumes of deionized water.As expected, we observed that viscous slurries prepared in deionized water with a solid content greater than 5% (w/w) were difficult to infiltrate inside the 3D architecture of open BCC lattice current collector (changing lattice configuration to DC did not alleviate this problem).This behavior can be rationalized as a result of large contact angles formed by the slurry droplets [36] when prepared using a high surface tension solvent (72.75 mN m À1 [37] for H 2 O at 20 °C).However, slurries prepared using a mixture of ethanol and water (3:7 v/v) with overall lower surface tension (56.41 mN m À1 [37] at 20 °C) allowed faster imbibition even at higher solid content (20% w/w).shearing rates increase, suggesting shear thinning behavior which is representative of non-Newtonian fluid with agglomerates. [34]t has been reported that viscosities at lower sheer rates represent the stability of the slurry, which is the measure of solid sedimentation behavior, whereas viscosity at high shear rate is the measure of slurry processibility. [38]Our data showed similar viscosities at higher sheer rates for water and Et-OH/H 2 O-based slurries.It suggests that slurries with high solid content can be prepared using Et-OH/H 2 O as their processibility is similar to slurries obtained with lower solid content using only deionized water.Moreover, viscosities at lower shear rates increase as the solid content increases from 5% to 20% to improve slurry stability and denser packing of active material inside 3D porous lattice current collector.Figure 3b shows Au-coated-printed current collector lattice before and after infiltration of active material (FePO 4 -containing anode slurry in this example).The slurry densely packs each compartment of the current collector, completely filling the electrode.EDS mapping in Figure 3c of infilled FPO-based electrode confirm the uniform distribution of active material (Fe and P, and C also from the printed resin) between the 3D architecture of conductive current collector lattice.Similar overall behavior was observed for slurries prepared with NMC and graphite-based active materials for comparison.

Electrochemical Response of the Printed Full Cell
The electrochemical response of the Vat-P 3D-printed Li-ion aqueous battery was testing using galvanostatic cycling.Since there is no commercially available rechargeable Li-ion coin cell using this aqueous cell chemistry, we also made traditional coin cells using identical electrode materials and loading pressed onto titanium mesh to mimic the porous-printed current collector and achieve a similar overall electrode.
Figure 4a shows the charge discharge profile of traditional aqueous coin cell with the 1 st cycle discharge capacity reaching 1.76 mAh cell À1 .The capacity of the cell briefly increases during first five cycles reaching 1.8 mAh cell À1 after the 5 th cycle followed by a steady decay in cell capacity to 1.40 mAh cell À1 after its 50 th cycle (Figure 4b).Contrary to the traditional coin cell, our 3D-printed Li-ion aqueous cell showed an initial discharge capacity of 0.76 mAh cell À1 , which increased to a stable capacity of 1.86 mAh cell À1 after 15 cycles (Figure 4a,b).The low coulombic efficiency obtained in the first cycle of 3D-printed battery suggest significant charge transfer used to form the SEI layer on the packed active materials. [31,39]Once the coulombic efficiency equilibrates, we note a gradual increase in capacity to a final stable value of 1.86 mAh cell À1 .We surmise the gradual increase to the final capacity is caused by accessibility to more of the active material over the first 15 cycles.Once the cell capacity reaches 1.86 mAh cell À1 , it remains stable for the rest of the cycling period.Differential capacity plots for both the traditional and 3D-printed Li-ion batteries, in Figure 4c, showed two prominent oxidation and reduction peaks.32d,33] On the cathode side, the response describes the two-phase transformations of LiMn 2 O 4 /Li 0.5 Mn 2 O 4 and Li 0.5 Mn 2 O 4 /MnO 2 . [40]he delithiation and lithiation peaks at 0.88 and 0.78 V, respectively, matched well in both types of batteries.However, peaks associated with second-phase transformation (Li 0.5 Mn 2 O 4 to MnO 2 ) were shifted to higher voltages for the traditional Liion aqueous battery using the Ti mesh current collector.After 50 cycles, the oxidation potentials (1.09 and 1.14 V) remain within electrochemical stability window of our electrolyte system showing minimum polarization and degradation of Au-coated substrates.Better electrical conductivity through 3D-printed architecture was also confirmed using rate testing where 3Dprinted Li-ion aqueous battery outperformed the traditional lithium-ion coin cell.We believe that high-rate capability of 3Dprinted Li-ion aqueous battery is the result of highly interconnected network of slurry "pockets" electrically connected by the metallized current collector lattice that provides multiple pathways for electron transfer.
Table 1 compares some performance metrics of both traditional and 3D-printed aqueous Li-ion cells.Besides gravimetric capacities (when calculated using mass of active material), the 3D-printed battery provides an improvement over a similarly sized traditional coin-cell for cell-level metrics.This is due to the fact that 3D-printed Li-ion cells is %2Â lighter than the traditionally configured cell and provides higher cell-level capacities at faster rates.Nevertheless, dense and thick electrodes of 3D-printed aqueous cell limited complete utilization of active material, which might be the result of incomplete wetting of the 3D-printed electrode.

Recycling the 3D-Printed Lithium-Ion Battery
One of the main advantages of our 3D-printed LIB lies in its recyclability using a simple combustion method.Once the battery life is complete, it can be burned in air and the components of active materials can be recovered while the cell casing and current collectors are fully decomposed.The 3D-printed LIB consists of printed substrate (polymer resins), coating material (Au), active materials (LMO-cathode, FPO-anode), conductive carbon (Super-P), Whatman GF/D Glass Microfiber separator, LiTFSI salt, and trace amount of binder (CMC-SBR).The printed substrate and conductive carbon can be burned in air without any traces.However, a small bead of gold was obtained for Au-coated 3D-printed substrates.
Thermogravimetric analysis was conducted using high-temp (HT) and clear resins, as well as combinations of the active electrode slurries with cells made from either resin.Using the HT resin, we find a 76% total mass loss when, as a current collector, it contains the LMO cathode material (Figure 5a) and for comparison the HT and clear resins alone showed a total mass loss of 88% and 96%, respectively (Figure 5b).While both resins are usable within the printed cells, the standard clear resin formulation is more susceptible to near complete combustion.Although the composition of these proprietary polymer resins is unknown, based on FTIR spectra of both resins shown in Figure 5d, the HT resin can withstand higher temperatures (HDT = 238 °C), consistent with the reduced weight loss measured by TGA under identical conditions to the photopolymerized clear resin, whose HDT is just 60 °C.
The same thermal treatment was applied to 3D-printed substrates packed with active materials (FPO and LMO), resulting in bright-red and black powders, respectively, along with a tiny bead of gold (Figure 5e).After the gold (all of which forms a single, solid bead) is separated from the powders, X-ray diffraction (XRD) analysis was performed to examine their crystallinity and phase purity.Figure 6a shows negligible change in the XRD reflections of crystalline LMO before and after heat treatment confirming its successful recycling and recovery from the cell.However, the amorphous nature of FPO and its combusted product made it difficult to confirm its purity via XRD.FTIR data in Figure 5d does indicate a dehydrated form of FePO 4 for the recovered bright red powder.Imaging after combustion showed no visible traces of printed resins (clear resin and HT resin) when thermally treated at 600 °C for 24 h.
To analyze the anode FPO material post combustion, XPS analysis was carried out on the pristine material, after cycling the 3D-printed cell, and finally after combustion and recovery of the powder.The XPS analysis of pristine FPO reveals the presence of two characteristic Fe 2p 1/2 and Fe 2p 3/2 peaks at 725 and 712 eV from spin-orbit coupling, [41] respectively, in Figure 6b.This is in agreement with the values illustrated in the literature. [42]Deconvolution of Fe 2p 3/2 peak showed two peaks at 711.7 and 713.9 eV, while deconvolution of Fe 2p 1/2 showed two peaks centered at 725.6 and 727.6 eV consistent with Fe(III) in FePO 4 .Similar core-level emission was observed in FPO after charge-discharge cycling.Here we notice an additional peak at 709.6 eV that is linked to Fe(II) from Li x FePO 4 . [43]oreover, the small satellite peak obtained at 718.8 eV is distinguishable in both the cycled and combusted samples indicating Fe 3þ existing as a minute quantity of Fe 2 O 3 .Exposure to the atmosphere post-cycling and from its time in vicinity to water likely promoted localized oxidation, which is also plausible during combustion in the ambient atmosphere.This is also confirmed by the core-level emission from O 1s spectra, where oxide is clearly seen post-cycling in addition to oxygen from (PO 4 ) 3À phosphate groups and the presence of oxide of Fe.Mass of coin cell = 2.98 g and mass of 3D-printed cell = 1.34 g; b) Nominal voltage of cell = 1.0 V.
The C 1s spectra remain consistent as shown in Figure S3, Supporting Information, in all cases, and only the cycled case do we find and expected signal from F 1s (PVDF), absent post-combustion (see Figure S4, Supporting Information).Looking at the P 2p core levels, we observe a peak at 138.6 eV uniquely in the cycled FPO material that is related to P─F bonds from the PVDF binder.Post combustion, however, the phase pure material is obtained and fully recycled.While LMO is recycled and recovered close to its original form, the bright red powder of recovered FePO 4 can be converted back to pristine hydrated phase FePO 4 •2H 2 O by treating it with phosphoric acid. [44]As for the remaining two components of our 3D-printed battery, LiTFSI also decomposes after 300 °C without any traces, [45] while the Whatman GF/D separator remains stable for quite an extended range of temperatures as expected of fiber glass (borosilicate fibers). [46]SEM images of pre-and postcombusted fiber glass separators can be found in Figure S5, Supporting Information, with EDX compositional analysis of pristine separators.Overall, the 3D aqueous-printed LIB is completely recyclable using industrially common heat treatment methods, with complete recovery of the Au metallization and active electrode materials and the disintegration of the plastic cell components.

Conclusions
We have developed a 3D-printed aqueous lithium-ion aqueous battery using sustainable active cathode and anode materials of LiMn 2 O 4 and FePO 4 •2H 2 O, which can be fully recycled using a simple combustion method.This battery was designed to allow a stable cycling, higher energy density option compared to a metallic cell of similar construction and to ensure better intraelectrode electrical conductivity and rigidity necessary for a viable cell, which can be difficult to achieve with all-in-one multimaterial printing approaches.The printed cell has a stable cell-level capacity of 1.86 mAh, better that that of a comparable metallic coin cell of similar internal chemistry, with an average cell voltage just over 1.0 V. Following combustion, the crystalline phase of LiMn 2 O 4 is recovered, and a mixed phase of some Fe 2 O 3 mixed with a dominant composition of FePO 4 .All gold used for electrical contacts is fully recovered.
The approach can of course be extended to cells with alternative form factors and internal battery chemistry.While a standard coin cell form factor was used here so that its response could be benchmarked with existing coin cell geometry using the aqueous electrolyte and identical materials, the ability to disintegrate the light plastic components and recover all useful materials is nearly reusable form is potentially useful for applications where many of these cells are used.This would ease the burden of additional waste cell accumulation and provide a circular approach for low power battery cell applications that could benefit from lightweight, alternative form factor batteries.
Modeling and Printing of Electrodes: Two versions of coin-type full cells with architected porous current collectors were designed using SideFX Houdini software and exported as standard tessellation language (STL) files for the subsequent import into the 3D printer slicer software.The STL models were imported into Formlabs PreForm slicer software, scaled, duplicated, and oriented as required.Supports were automatically generated by the software with manual editing as necessary to avoid the attachment of supports to small printed features.Modification to the periodic lattice that ensures an even, flat upper surface is outlined in the Supporting Information.
All objects were printed using Formlabs Form 2 desktop SLA 3D printer with 25 μm layer thickness.Methacrylate-based resins from Formlabs, Clear V4 and High-Temp, were chosen for printing.These two resins have similar PMMA and photoinitiator base composition, but the HT resin has additional undisclosed components that raise the HDT from 60 to 238 °C.After the objects are printed, they are left attached to the build platform and rinsed with isopropyl alcohol (IPA) in the Form Wash automated wash station to remove uncured resin.Washing time is typically 10 and 6 min for clear and HT resins, respectively.After washing, the parts were removed from the build platform together with the supports, dried with an air blower and post-cured under λ = 405 nm light in the Form Cure station from Formlabs.Post-curing time and temperature were 15 min at 60 °C, and 120 min at 80 °C for Clear and HT resin, respectively.
Supports were carefully detached from the post-cured objects, and the latter was polished with sandpaper (800 and 1200 grit) and synthetic leather polishing pad.After rinsing in IPA and drying, the polished cell parts were fully coated with gold or nickel (layer thickness %200 nm) using Quorum Q150T S sputter coater and Ted Pella Au or Ni targets (57 mm Â 0.1 mm).
Materials Characterization: The quality and fidelity of geometric 3D-printed structures were evaluated using scanning electron microscopy (SEM) using an FEI Quanta 650 scanning electron microscope operated at spot size 3 and 20 kV beam voltage.Energy-dispersive X-ray analysis was also performed to observe the morphology and distribution of the sputtercoated gold.Electrical transport measurements were carried out using a 2-probe method on Au-coated electrodes with 1-or 2-layered vertical structures each with cubical or tetragonal geometrical shape.I-V data were recorded using a Keithley 2612B source meter with 50 ms per point integration time between 0.0 and 0.2 V.The distance between the two probes was fixed at 20 mm for all measurements.Measurements were taken across the porous side, across the flat side, and through the thickness of each electrode to ensure consistent electrical connectivity of the printed casings.Raman scattering analysis on the surface of pristine-and goldcoated electrodes was carried out using a Renishaw InVia Raman spectrometer in conjunction with a 30 mW Ar þ laser featuring an excitation wavelength of 514 nm.The laser was focused using a 40Â objective lens and collected using a RenCam charge-coupled device (CCD) camera.
The crystal structure of the pristine active materials and combusted material was confirmed by XRD using a Philips X'pert Pro MPD equipped with a Panalytical Empyrean Cu X-Ray tube and a Philips X'celerator detector.Infrared spectra were recorded on a PerkinElmer Spectrum 2 FT-IR Spectrometer.Perkin-Elmer Spectrum v5.0.1 software was used to perform baseline corrections and spectral evaluation.Each spectrum was scanned between 400 and 4000 cm À1 with a resolution of 4 cm À1 , and a minimum of 64 scans were collected and averaged.Viscosities of electrode slurries were measured using a digital battery slurry viscosity tester (MTI Corporation) at 6, 12, 30, and 60 rpm.Thermogravimetric analysis and mass loss quantification were carried out using a Perkin Elmer TGA 4000 instrument.Samples of SLA printed and cured resins, and printed cell casings with material slurries, were combusted in air at a ramp rate of 5 °C min À1 from room temperature to 600 °C.Samples were then held at 600 °C for a period of 24 h.X-ray photoelectron spectroscopy was acquired using Kratos AXIS ULTRA XPS with a 165 mm mean radius hemispherical electron energy analyzer at a base pressure of 5.0 Â 10 À10 mbar.Core-level scans were acquired with a step size of 0.05 eV, dwell time of 100 ms, with 18 scans averaged at a pass energy of 20 eV from a monochromated Al Kα X-ray source at 300 W power. Data were processed using CasaXPS software, where a Shirley background correction was employed and peaks were fitted to mixed Gaussian-Lorentzian profiles with relative sensitivity factors containing Scofield cross-sections.The analytical area was typically 1 mm 2 .
Electrochemical Device Fabrication and Characterization: Slurries for anode and cathode materials were prepared by first ball milling super-P carbon black with FePO 4 ⋅2H 2 O (FPO) and LiMn 2 O 4 (LMO), respectively, using a high-speed 3D ball mill (MTI corporation) with a maximum speed of 1200 rpm.Subsequently, each powder was mixed with carboxymethyl cellulose (average M w = 400 000 g mol À1 )/styrene butadiene rubber (158.24g mol À1 ) (CMC-SBR) and water suspension using a compact vacuum mixer (MTI corporation).The final electrode slurry consists of 80% (w/w) active material (FPO or LMO), 10% (w/w) Super-P carbon black, and 10% (w/w) CMC-SBR binder with solid content ranging from 5% to 10%.Ethanol was used to adjust the surface tension of the slurry mixture in order to make it spreadable inside the 3D geometrical Au-coated electrode casings.Electrodes were prepared by drop casting each slurry into 3D-printed Au-coated substrates and dried inside oven at 60 °C for 12 h.A gasket was attached where applicable (version I cell), and a glass fiber separator (Whatman GF/D) was placed between cathode and anode parts.The electrolyte was a 10 M LiTFSI aqueous solution, and the resulting cell was hermetically sealed around the edges with a two-component epoxy glue.The mass loading of each electrode was between 80 and 100 mg electrode À1 for single-layer geometry and 150-200 mg electrode À1 for double-layer geometry.The cross-section area of each 3D-printed electrode was 3.14 cm 2 , while the loading ratio between FPO-based anode and LMO-based cathode was 2.0.
For comparison, standard electrodes were also prepared by grinding 80% (w/w) active material (FPO or LMO), 10% (w/w) Super-P carbon black, and 10% (w/w) CMC-SBR binder using a mortal/pistol and pressed on a circular titanium mesh (12 mm, diameter) using a hydraulic press applying a pressure of 15 tonnes.The back side of titanium mesh was coated with 100 nm layer of gold to obtain a conductive and smooth surface.The mass loadings of FPO and LMO-based electrodes were 35.4 and 17.7 mg cm À2 , respectively.
Electrochemical testing of standard and 3D-printed cells was performed by sandwiching glass fiber separator (Whatman GF/D) soaked with 10 M LiTFSI aqueous electrolyte solution.LiNO 3 was also added as a supporting electrolyte.The edges of 3D printed cells were sealed with epoxy glue, while standard cells were assembled and tested using two electrode stainless steel split cell (a coin cell assembly that can be disassembled for postmortem analysis).Galvanostatic measurements were performed using biologic VMP3 multichannel potentiostat controlled by EC lab software.Both standard and 3D-printed cells were cycled in the potential ranges of 0-1.30, 0-1.35, and 0-1.40 V at various current densities.

Figure 1 .
Figure 1.a) Software rendering of two versions of the Vat-P-printed cell.Version I cell was equipped with a 3D-printed gasket, whereas version II was gasket-less.Both version I and II cells were implemented with different ordered porous current collector geometries: b) diamond cubic (DC) and bodycentered cubic with horizontal and vertical edges removed (BCC).The lattices were rotated as appropriate, and the topmost edges were flattened in modeling to ensure good contact between cathode and anode sides and a separator to avoid any rupture between layers.c) Photograph of one side of the printed coin cell electrode coated with Ni (top) and Au (bottom).d) SEM images of Au-coated 3D-printed substrates with a DC (top) and a BCC lattice (bottom).Inset are magnified images of individual geometrical struts and pores.e) Tilted cross-sectional SEM image of a Vat-P-printed BCC lattice coated with Au and its corresponding EDS map of Au.The central of the three image is a false color image showing the Au coating of the texture of the struts of the lattice.f ) SEM image of the Au-coated resin cross-section and g) higher magnification SEM showing the characteristic granular morphology of the Au nominally 200 nm-thick Au coating.

Figure 2 .
Figure 2. a) Representation of the two-probe geometries for I-V measurements.Resistance values are extracted by averaging data from the slope of multiple ohmic I-V curves.b) (Top) Comparison of Au-and Ni-coated 3D-printed casings resistance values when printed with BCC and DC internal lattice structures for each of the three orientations.(Middle) Resistance values for Au-and Ni-coated casings before and after electrolytic erosion tests.(Bottom) Comparison of the resistance of Au-coated clear resin and high temp resin printed with identical dimensions.c) Photographs of Au and Ni-coated current collectors before and after immersion in LiTFSI aqueous electrolyte for 24 h.Delamination of the Ni coating occurs across most of the surface area.d) Raman spectra of 3D-printed polymer substrate (blue), Au-coated substrate (orange), Ni-coated substrate (gray).
Figure 3. a) Viscosity-shear rate curves of FPO and LMO slurries with different solvent and solid contents.b) SEM image of a Au-coated-printed current collector lattice and cross-sectional image of the current collector filled with FePO 4 slurry.c) EDS images and elemental maps of C, Fe, and P for FPO infiltrated current collector.The C is also found uniformly across the slurry form the graphitic conductive additive.

Figure 4 .
Figure4.a) Galvanostatic charge-discharge profile of the traditional coin cell using Ti mesh current collectors (top) and the Vat-P-printed coin cell (bottom), both using similar electrode mass loading.Both types of cell were cycled at 0.5 mA from 0 to 1.3 V over 50 cycles.b) Full cell capacity plotted as a function of cycle number in addition to the coulombic efficiency.c) Differential capacity (dðQ À Q 0 Þ=dE plots derived from chronopotentiometric galvanostatic charge-discharge data.The 1 st and 5 th cycles were used for the traditional coin cell design, while the 1 st , 5 th , and 50 th cycles are shown for the 3D-printed aqueous cell.d) Variable rate response of both types of cells from the 0.6 to 3.0 mA current for the traditional coin cell and 0.5-2.5 mA for the printed cell.

Figure 5 .
Figure 5. a) Thermogravimetric analysis of the high-temp (HT) resin with and without LMO cathode material.For the HT resin, the heat deflection temperature (HDT) is 238 °C.In b), the comparison of the thermal mass loss for the clear resin and the HT resin is shown.The HDT for the clear resin is just 60 °C.c) Comparison of thermogravimetric data from the clear resin on its own, and when filled with LMO cathode material and FPO anode materials.d) FTIR spectra of the clear and HT resins and the FPO anode material after combustion and recovery.e) Pictures of crucibles used for thermal decomposition of the full printed cell components: both resins are completed combusted showing no visible traces.Au-coated clear resin (Au-CL) is also entirely combusted to CO 2 leaving a singular bead of recovered Au, and recovered, combusted powders from LMO and FPO electrode materials.

Figure 6 .
Figure6.a) X-ray diffraction patterns from LMO cathode material as-received, and after combustion.b) Core-level photoemission from X-ray photoelectron spectroscopy measurements of Fe 2p, P 2s, P 2p, and O 1s.Spectra were acquired from pristine hydrated FPO, the cycled FPO material from a printed battery cell (mixed in a PVDF/graphite-containing slurry), and the FPO material recovered after cell combustion.

Table 1 .
Comparison of performance matrix between a coin cell and 3D-printed cell.