Evaluating 3D printed mesh geometries in ceramic LiB electrodes

Additive manufacturing techniques have the potential to promote a paradigmatic change in the electrode fabrication processes for lithium-ion batteries (LiBs) as they may offer alternative component designs to boost their performance or to customise the application. The present research work explores the use of low-cost fused filament fabrication (FFF) 3D printing to fabricate Li4Ti5O12 (LTO) mesh electrodes in the search for enlarged electrochemically active areas. Using different nozzle diameters (ND), we have 3D printed several mesh electrodes that after sintering allow an increase in the surface to volume ratio by up to ≈290% compared to conventional flat cylindrical geometries. As the conventional route to produce 3D printed meshes, i.e. stacking of consecutive layers with a 90° rotation, leads to problems of vertical misalignment that may affect the electrical contact, we have developed a new compact design that maximises the contact between layers. All the 3D printed mesh electrodes with thicknesses of 400 and 800 μm, exhibit electrochemical performance very close to those of thin (70 μm) electrodes, e.g. 175 mAh g−1 at C/2 in the case of ND = 100 μm, which is the theoretical capacity value for LTO. At higher C-rates, 800 μm-thick mesh electrodes with larger ND exhibit a marked drop in the reversible capacity (28 mAh g−1 at 8 C), although the values obtained improve notably those of the equivalent thick solid electrode (almost null at 8 C). The compact design demonstrated superior performance at high C-rates, improving by ≈70% the results of the best conventional mesh electrode at 8 C for 800 μm electrodes. These results highlight the potential of FFF-3D printing to generate novel high aspect ratio geometries and the impact of design and printing parameters on the performance of LiB electrode materials. Exploring alternative efficient geometries may facilitate the integration of thick electrodes in high energy density LiBs.


Introduction
Lithium-ion batteries (LiBs) are at the forefront of the energy storage technologies in several key industrial applications such as portable electronics, electric vehicles (EVs), or hybrid systems based on renewables.Although in general terms LiBs exhibit superior energy density, reliability, and cycling performance compared to other energy storage devices, the transition towards a sustainable energy economy and the recent advancements in microelectromechanical systems place on LiBs increasingly larger demands for higher energy densities in addition to more rigorous safety specifications [1][2][3].Therefore, novel battery chemistries have been explored in the search for specific capacities beyond those of conventional LiB materials [4][5][6][7][8][9].Nevertheless, several other alternative strategies have also been developed to achieve this goal such is the broadening of the electrochemically active potential window [10,11] and the use of alternative storage devices based on new electrochemical reactions [12,13].Battery performance may also be enhanced through strategies that optimise the use of conventional materials.Thus, higher energy densities can be obtained by maximising the active electrode materials to inactive components (i.e.current collectors, separators and battery case) ratio by using binder-free thick electrodes [14][15][16][17].This is particularly relevant in the case of portable electronics such as mobile phones or laptops that typically include 20-100 µm thick film LiB electrodes.Increasing the electrode thickness may be considered as a potential strategy to increase the active material loading and hence the gravimetric capacity/energy density, whilst decreasing the manufacturing costs [18,19] according to recent models that anticipate a 25% cost reduction by doubling the electrode thickness [20].
Unfortunately, such approach usually results in severe capacity losses at high discharge rates, poor cyclability and electrode cracking in the long-term because of the longer pathways for Li-ion diffusion and the increased resistance associated to thicker electrodes [15,17,[21][22][23][24].This becomes a major disadvantage in the case of conventional electrodes that include polymeric binders in the formulation.In such cases, the non-active polymers constitute 10-30 vol% of the electrode [25] that do not contribute to the Li-ion diffusion process.Moreover, traditional manufacturing methods such as tape casting or die pressing usually generate pores parallel to the electrode plane, i.e. perpendicular to the theoretical path that Li ions must follow from one electrode to the other [26].All in all, increasing the electrode thickness contributes to more tortuous Li-ion pathways and also leads to an increase of the ohmic resistance that combined with the presence of non-active materials may restrict the use of those devices in high energy density applications [27][28][29].
Nevertheless, the use of binder-free electrodes is in the spotlight for searching alternative routes to achieve high-energy-density batteries.Thus, LiCoO 2 (LCO) ceramic cathodes up to 800 µm were obtained by uniaxially pressing and sintering, showing two levels of densification, i.e. 74% and 87%, which resulted in a significant performance improvement, with approximately 100% of the theoretical capacity at C/10 and more than 80% at C/3 [30].To reduce diffusion paths of lithium ions, several other authors have generated additional macroscopic porosity via the use of pore formers [31][32][33].Sander et al reported thick LCO electrodes with ≈10 µm diameter pores oriented in the thickness direction, displaying up to 128 mAh g −1 (93% compared to theoretical) in dynamic discharge [34].Elango et al studied LiFePO 4 (LFP) and Li 4 Ti 5 O 12 (LTO) electrodes fabricated by spark plasma sintering (SPS), using NaCl pore formers to produce anisotropic pores (7-9 µm in diameter) which led to specific capacities close to the theoretical values at low C-rates (≈140 and ≈167 mAh g −1 at C/20), with increasing losses at higher C-rates (≈50 and ≈15 mAh g −1 at C/2) [32].Porous LTO and LFP electrodes (500 µm) fabricated through infiltration in polydimethylsiloxane (PDMS) sponge-like structures exhibited reversible capacities of 135 and 140 mAh g −1 at C/10, respectively [35].
In addition to the porosity generated during sintering or via the use of pore formers, the limitations in Li-ion diffusion in thick electrodes could also be overcome through the use of electrodes exhibiting grid-shaped or mesh geometries.
In this context, additive manufacturing (AM) have emerged as alternative processing technologies to produce non-conventional geometries that may provide added-value in the performance of devices for energy and environmental applications [36].AM technologies have undergone phenomenal progress and have potential application fields such as biomedical scaffolds for tissue engineering, adsorbents, filters, catalysts, and even electrodes for LiBs and other electrochemical applications [37][38][39][40][41][42][43].Indeed, in the last few years there are several reports on the fabrication of flexible architectures and precise designs (figure 1) for energy storage applications [44][45][46][47][48].
In the search for high surface to volume ratio structures, grid geometries produced via robocasting have been explored in 1 mm thick LTO electrodes for high performance LiBs rendering over 140 mAh g −1 at 1 C [49] or through the use of composites with Ag nanowires 121 mAh g −1 at 10 C and stability after 100 cycles [50].On the other hand, fused filament fabrication (FFF) has become the most popular 3D printing technique worldwide primarily due to the accessibility and low cost (e.g. a filament desktop 3D printer costs just a few hundred euro), it is an environmentally friendly AM technology, with a wide range of commercially available materials, etc… [51].Although there are some reports of FFF applied to the fabrication of LiB components, including binder free electrodes [52], to the best of our knowledge, there are no previous reports on the production of binder-free ceramic mesh LiB electrodes using this 3D printing technology.In a recent work, we reported the use of FFF as a valuable method to obtain fully ceramic electrodes with high electrochemical performance, achieving up to 96% and 94% of the theoretical capacity in LTO and LCO, respectively [53].In this work, we further analyse the impact of different mesh geometries [54] on the electrochemical performance of binder-free LTO electrodes.

Materials and methods
3D printable filaments including LTO + carbon black were produced following a patented process [55].First, the organic binder component of the filament was obtained by mixing dibutyl phthalate, ethylene glycol, and polyvinyl butyral (Merck KGaA, DE) in a 1:2:2 ratio.Next, 70 wt% of LTO ceramic powders (D 50 = 0.9-1.8µm, specific surface area ⩽ 6.0 m 2 g −1 , tap density ⩾ 0.65 g cm −3 -Targray, Kirkland, USA-) and carbon additive Super C45 (CB, IMERYS, BE) were added under continuous stirring until homogenisation.The resulting mixture was then left to consolidate at room temperature to produce a viscoelastic green body, which was subsequently cut into pellets and extruded at 70 • C using a Filastruder Kit (USA) to produce a ≈1.75 mm diameter filament.
FFF-3D printing was carried out using a conventional Ender-3 V2 3D printer (Creality, Shenzhen, CH).All specimens were printed at 210 • C based on previous results [43].Cilindrical solid and mesh electrodes were produced with a constant diameter of 6 mm and two different electrode thicknesses (or height): 0.4 mm and 0.8 mm.The mesh electrodes were printed using nozzle diameters (ND) of 0.10, 0.20 and 0.40 mm and layer heights (LH) of 0.05, 0.10 and 0.20 mm respectively to keep the ND/LH ratio at 2.0 as shown in figure 2.
Initially, the mesh electrodes were produced by following a conventional pattern .Generally, the user defines the nozzle path as a continuous line to prevent problems related to non-continuous extrusion, such as retraction.Therefore, consecutive layers are usually defined as depicted in figure 3(a), with a 90 • rotation between them.As discussed below, such procedure results in vertical misalignment in the mesh electrode that may have a negative impact in the electrical contact between layers.Therefore, in the present work, a novel compact design has been defined to stack identical consecutive layers (figure 3(b)), thus improving contact between them in the search for enhanced electrochemical performance.
The G-code files of the solid electrodes were generated by Ultimaker Cura (Ultimaker, ED Utrecht, NL) with concentric infill pattern.The G-codes files of the porous electrodes were written by extracting the coordinates of paths drawn in AutoCAD (Autodesk, San Rafael, US-CA), as described in figure 3. Further, the printing flow was adjusted to reach the same final weight of printed material in every electrode, i.e. ≈12 mg in 0.4 mm thick electrodes and ≈24 mg in 0.8 mm thick electrodes.
Debinding and sintering of the 3D printed LTO electrodes was performed by heating up to 900 • C at 1 K min −1 for 6 h under N 2 atmosphere to avoid the decomposition of the carbon additives.
X-ray diffraction (XRD) was used to evaluate phase stability upon FFF and further sintering.The experiments were performed on a PANalytical's X'Pert PRO MRD (PANalytical, Almelo, NL) diffractometer operating with monochromatic Cu K α1 radiation (λ = 1.5406Å) operating at 45 kV and 40 mA in the 10-100 • 2θ range.The resulting diffraction patterns were analysed with the FullProf software [56].
Scanning electron microscopy (SEM) was used to study the microstructure of the 3D printed electrodes.SEM images were recorded in a JEOL JSM 6490LV electron microscope operating in the 0.5-30 kV range.The microscope is equipped with secondary and backscattered electron detectors for image generation and also with an EDS detector (Oxford Link) to monitor the chemical composition.
Analysis of the 3D printed electrodes real geometry was performed by using Autodesk AutoCAD 2023 (Autodesk, Inc., USA).SEM and digital single lens reflex (DSLR) images (using a Nikon D3200 digital camera) of the sintered electrodes were segmented to calculate the projected surface.The 3D models of the printed and sintered electrodes were made via segmentation and measurement of the thickness from top-view DSLR and SEM images, respectively.The surface area and volume measurements of both 3D printed electrodes and theoretical models were obtained using the CAD software.
The electrochemical characterisation of the 3D printed LTO ceramic electrodes was carried out using Swagelok half-cell test configuration [57].Cells were assembled in an argon glovebox, following a single-cell configuration using LTO as working electrode and lithium metal (Alfa Aesar, 99.9%) as counter and reference electrode, both soaked in 1M LiPF 6 in EC/DMC 1:1 liquid electrolyte solution (Merck KGaA,

3D printing and SEM morphology
Figure 2 shows the 3D printed electrodes tested in the present work after sintering: flat electrode and different grid-shaped electrode in the search for the maximisation of the electrolyte-electrode contact area.At the macroscopic level, they were replicas of the 3D models.The SEM images recorded in the sintered electrodes show that in all cases the resulting geometry matched the corresponding initial model and the wall thickness for each mesh was in good agreement with the ND.In the cross-section SEM images, the specimens exhibited a good layer to layer contact and no large defects or delamination processes were detected (figure 4).However, it should be noted that the layer-by-layer growth of the mesh is not perfectly aligned, and consecutive layers may result in zigzag patterns as can be observed in the corresponding cross-section images.This effect is a consequence of the conventional methodology applied when performing FFF-3D printing.Typically, when printing ceramic or metal loaded filaments, the user defines a continuous nozzle path to avoid problems related to retraction, etc.Consequently, a mesh is typically produced by stacking alternative layers as previously described (figure 3) and misalignment may appear in certain points because part of the printing process is overhung.Extrusion flow may be adjusted to minimise that problem, but it may persist.In the context of electrode 3D printing, such misalignment may have a negative impact on the electrical contact between layers and to overcome such limitations, a novel compact design has been defined to stack identical consecutive layers (figure 3(b)) and indeed, the SEM images corresponding to the cross-section view of the compact electrode reveal a more compact structure, with a better-defined and more homogeneous contact between layers.
The geometrical analysis of the sintered electrodes revealed that, as expected, the use of mesh geometries results in a gradual enhancement of the surface to volume ratios and hence the electrolyte-electrode contact areas are increased with the potential positive impact on battery performance as discussed below.The experimental values depicted in table 1 show values very close to those of the CAD model.The minor deviations observed are related to gravity effects on the softened filament during 3D printing.As a consequence, the resulting struts are not perfect cylinders, but ellipsoids as can be observed in the SEM cross-section view.This effect combined with the different behaviour of the filament when deposited on top of solid material or overhung, causes that the mesh walls are not 100% square and indeed a certain degree of curvature can be observed as well as a variation in wall thickness.In any case, it should be noted that the increase of the electrolyte-electrode contact area is fairly close to that expected from the CAD models (see table 1).The 3D printed meshes produced using the smallest ND diameter (ND01, 100 µm) i.e. smallest windows and thinnest walls, exhibit a 287% ratio increase in area compared to the 3D printed solid electrode.It should also be noted that the compact CAD design has fairly close parameters to those of ND01 regarding the enhancement in the area to volume ratio.On the other hand, when analysing the sintered electrodes, the compact design exhibits the largest deviation compared to CAD models, possibly due to the very use of identical printing paths: the addition of consecutive layers of highly loaded filament, causes the bottom layers to be flattened and consequently the experimental S/V decreases.This occurs for all the mesh electrodes as can be observed in the SEM cross-section images.This effect is more pronounced in the novel compact Table 1.CAD and 3D printed electrode geometrical analysis, where S is the surface, V the volume, S/V is the surface to volume ratio according to the CAD model ((S/V)CAD) and the 3D printed and sintered electrodes ((S/V) printed ).(SN0X/S Thick ) printed corresponds to the ratio between the 3D printed and sintered surface for each nozzle diameter (SN0X) and the surface of the thick solid electrode (S Thick ).

S CAD (mm 2 )
V CAD (mm 3 )  because there is full contact between consecutive layers and therefore the pressure made during nozzle extrusion is higher in the entire layers.Nevertheless, as discussed below, despite the lower experimental S/V ratio, such improved contact between layers is beneficial in terms of the electrochemical response.SEM performed at higher magnifications (figure 5) reveal that the microstructure of all the grids produced in the present study after the corresponding debinding and sintering processes was fairly similar, i.e. 75% relative density with mostly equiaxial submicron pores (diameter 0.3-0.5 µm) in good agreement with previous results [53].LTO grains in the sintered electrodes are in the 0.5-1.5 µm range, which is very close to the initial particle size of the LTO powders used during filament preparation, i.e. 0.2-1.0µm.

Structural characterisation
Figure 6 shows the XRD patterns for LTO mesh electrodes after the corresponding debinding and sintering processes.
The Le Bail patterns fitting were fully indexed and matched nicely with previous reports [53,58].LTO exhibits a cubic unit cell (s.g.Fd-3 m) with a = b = c = 8.40290(2) Å and V = 593.317(3)Å 3 , in perfect agreement with the cell parameters of the starting LTO commercial powder, i.e. a = b = c = 8.40341(2) Å and V = 593.425(13)Å 3 , which implies volume change below 0.11%.Therefore, no evidence of decomposition or secondary phase formation was detected and hence one may assume that the processing route, i.e. 3D printing and further debinding and sintering thermal treatments did not cause degradation that may have a negative impact on the electrochemical performance of the active material.

Electrochemical tests
Figure 7 shows the electrochemical profiles corresponding to the first discharge/charge cycles at different C-rates obtained for the different 3D printed electrodes.As expected, thick solid LTO electrodes exhibited a significant loss of capacity compared to the theoretical value (175 mAh g −1 ), independently of the C-rate tested.
At the lowest rate, C/2, the capacity values were approximately 125 and 112 mAh/g for 3D printed solid electrodes with thicknesses of 400 and 800 µm respectively, which corresponds to capacity losses of ≈29 and 36%.Similar trends were observed for all the C-rates tested.
In the case of mesh electrodes, the electrochemical performance was significantly improved, especially when decreasing the ND.Thus, electrodes produced with ND = 100 µm exhibited the highest electrochemical performance and lowest polarisation, matching the theoretical capacity (≈175 mAh g −1 ) at C/2 for both electrode thickness, i.e. 400 and 800 µm (figures 7(g) and (h)).As the ND increases, the maximum capacity decreased, especially at higher C-rates.This was observed for both electrode thicknesses.The electrochemical performance at 8 C was ≈54 and 34mAh g −1 with ND = 100 µm, and it decreased down to ≈32 and 8 mAh g −1 when using the largest ND, i.e. 400 µm, for 400 and 800 µm electrodes, respectively.
Regarding the impact of the electrode thickness on the electrochemical performance, it is obvious that thicker electrodes result in higher performance losses, larger polarisation values and reduction in the rate capability.From these results, the production of high-performance thick electrodes must consider the use of high aspect ratio structures as is the case of mesh electrodes, trying to reduce wall thickness as much as possible.
The rate capability and the corresponding coulombic efficiency tests of the 3D printed electrodes in half-cell configuration (figure 8) revealed that thin electrodes exhibited much higher reversible capacities compared to thick electrodes, e.g. over 99% of the theoretical capacity at C/2 and approximately 90% at 4 C.Although the C-rates performance was better for thinner electrodes, i.e. 400 µm, especially for rates above 2 C, it should be highlighted that in all cases the mesh electrodes exhibited highly competitive values compared to the 70 µm (thin) solid electrode used as reference.Thick solid electrodes exhibited problems related to poor reversibility even at low C-rates, and at 8 C the reversible capacity was null.However, in the case of the mesh electrodes with thickness of 400 and 800 µm, the response was considerably higher.Thus, 400 µm mesh electrodes yielded ≈132 and 52 mAh g −1 vs. 33 and 13 mAh g −1 in the case of the equivalent solid electrode, which implies up to 300% capacity enhancement at 4 C and 8 C rates, respectively.Similarly, in the case of 800 µm thick mesh electrode, the reversible capacity was 106 and 30 mAh g −1 vs. 13 and  3 mAh g −1 in the case of the equivalent solid electrode, that is, up to 715% and 900% capacity improvement for 4 C and 8 C rates, respectively.
A further advantage of the grid-shaped electrodes is that after cycling at 8 C, the specific capacity is fully recovered, contrary to the case of the thick solid electrodes.These results are in agreement with the charge-discharge profiles at the different C-rates (figure 7).At C/2 and C, the discharge/charge profile are almost identical for mesh electrodes (ND01) and thin electrodes, although at 4 C and 8 C rates the response in thin electrodes is far more stable.Nevertheless, the responses of mesh electrodes are always superior to the equivalent thick solid electrodes.The improvement achieved in thick mesh electrodes compared to the thick solid electrodes is quite remarkable, e.g.≈50% at C/2 and 350% at 2 C (figure 8(b)), and highlights the enormous potential of using novel hollow geometries in the development of binder-free electrodes for high energy density devices.
The impedance of pristine electrodes significantly decreases after the first discharge/charge cycle, and then gradually increases upon cycling (figure 9).This trend has been observed for the mesh electrodes independent of the ND, with somewhat lower impedance values when reducing the ND.
The EIS spectroscopy analysis was performed using the CNLS fitting routine of ZView 4 to obtain the corresponding equivalent circuits and thus monitor the intrinsic resistance and diffusion characteristics of the LTO vs. Li cell on cycling.According to Boukamp's equivalent circuit notation, they can be depicted as W, which provided the best fitting for both mesh electrodes after 15 cycles in the charged state, i.e. at 3.0 V (see figure 10 and table 2) [59].
In this circuit, R O refers to the ohmic resistance (R ohm ) while R 1 and R 2 constitute the polarization resistance (R p ). Q elements refers the pseudocapacitance of the constant phase elements that accounts for the distribution of the relaxation frequency in electrochemical processes.The true capacitance value related with Q elements are obtained by C = R (1−n)/n Q 1/n , where R is the resistance in parallel to the corresponding Q and n-coefficient considers the deviation degree with respect to a pure capacitor (in which n = 1).Finally, W  element represents the infinite form of Warburg diffusion element, which is the solution for a one-dimensional diffusion phenomenon and therefore, can be related to the Li + ion diffusion in the electrolyte-electrode interphase where the electrochemical reduction/oxidation processes occur via charge transfer.
According to R ohm , no significant changes occurred during cycling, with values close to 1 Ω cm 2 in all cases.As this parameter is mainly due to the electrolyte resistance, it can be inferred that no degradation took place during the test.For R p , both electrodes exhibit a marked resistance drop after the first cycle, which is in good agreement with the very low conductivity of the starting LTO [60] and the improvement during the first charging process is related to Ti 4+ to Ti 3+ reduction [61].
On the other hand, the polarisation resistance increases for both electrodes on cycling, although slightly faster for the printed electrode with ND = 400 µm compared to ND = 200 µm; this agrees with the improved reversibility of electrodes fabricated with lower ND as mentioned above, which in turn can be related to the charge transfer process during cycling, more impeded in the case of thicker ND.
Next, we evaluated the performance of the novel compact design for FFF-3D printed LTO electrodes.As can be observed in figure 11, the compact electrodes exhibit reversible capacities that are even closer to that of thin electrodes.At low to moderate C-rates (from C/2 to 2 C) the responses are almost identical, but at  higher C-rates (4 C-8 C) the compact mesh electrode exhibited improved performances for both 400 µm and 800 µm thick electrodes, which implies an improvement of around 60% for the reversible capacity at 8 C.
Figure 12 shows the impact of the different mesh geometries evaluated in the present work on the electrochemical performance compared to that of thin solid LTO electrodes.All mesh electrodes produced with ND = 100 µm exhibited very high reversibility as the coulombic efficiency was above 98%-99% in most cases.At moderate C-rates, i.e. up to 2 C, the specific reversible capacities are almost identical in all cases, though at higher C-rates (i.e. 4 C and 8 C), electrodes fabricated with ND = 100 µm exhibited better responses, especially in the case of the compact design, possibly due to the improved contact between consecutive layers in the mesh.These results highlight that the electrochemical performance can be improved via geometrical designs that maximise the contact area of the electrolyte with the electrode active material, but also by defining printing patterns that enhance the contact between layers.

Conclusions
In the present work, the fabrication of mesh LTO/C electrodes via FFF-3D printing has been reported as a valid strategy to develop relatively thick binder free electrodes for high energy density devices, exhibiting performances close to those of thin electrodes.This has been achieved via the design of mesh electrodes to enlarge the electrochemically active area, that is, through the development of high aspect ratio structures.The conventional route to produce 3D meshes via FFF reveals certain limitations when stacking consecutive layers and consequently vertical misalignment and bad contact may take place.To overcome such drawback, a novel design, so-called compact design, has been envisaged to stack identical consecutive layers and hence improve the electrical contact along the y-axis.This has been achieved by defining optimised nozzle paths to produce compact designs.
Although some deviations occur after 3D printing and further debinding and sintering processes compared to the CAD models, the experimental area enhancement in the mesh electrodes is quite marked, up to 290% for 3D printed mesh electrodes fabricated with the smallest ND, i.e. 100 µm.Such surface to volume ratio increase allows the production of thick LTO electrodes that exhibit electrochemical performance fairly close to those of thin electrodes, e.g. the reversible capacity at 1 C is almost the same and matches the theoretical value of LTO (175 mAh g −1 ).At higher C-rates, the capacity decays although is fairly close to thin electrodes, particularly for compact mesh electrodes, which exhibit up to 70% enhancement compared to conventional meshes.Consequently, this work emphasises the impact of 3D design upon electrochemical performance, as the definition of the nozzle path, which implies a precise control over the printing process, may result in novel designs that, on one hand, maximise the surface to volume ratio and, on other hand, to optimise the contact between consecutive layers to ensure the best electrochemical performance.3D printing paves the way to the production of novel geometries that cannot be produced via conventional processing and offers alternative routes to successfully produce each layer and further stacking in the search for the best results.
Finally, these results demonstrate the extraordinary potential of FFF-3D printing to produce binder-free thick electrodes for the development of high energy density Li batteries.Such thick electrodes do not usually perform correctly at moderate to high charge/discharge rates when considering conventional flat disk geometries.However, in the present work, optimised mesh electrodes up to 800 µm thick have been produced and exhibited performances comparable to those of thin electrodes, in other words performing well (≈70% of the normalised reversible capacity) at rates as high as 4 C.

Figure 1 .
Figure 1.Schematic representation of various designs and geometries that can be produced via AM technologies.

Figure 2 .
Figure 2. 2D designs, 3D CAD models and 3D printed mesh electrodes produced in this work.

Figure 3 .
Figure 3. 3D printing strategies to produce mesh electrodes.(a) conventional mesh slicing that consists in the stacking of consecutive layers with a 90 • rotation between them.(b) compact design that consists in the stacking of identical consecutive layers to maximise contact.

Figure 5 .
Figure 5. SEM micrograph showing the microstructure of sintered LTO electrodes (a) and initial LTO powder used for filament fabrication.

Figure 6 .
Figure 6.X-ray Diffraction patterns and Le Bail fittings of LTO (a) commercial and (b) sintered powders, respectively.Experimental (red circles), calculated (black continuous line), their difference (blue line) and Bragg peaks position (green vertical bars).

Figure 10 .
Figure 10.Nyquist plots and corresponding fittings of mesh electrodes with 400 µm thickness before cycling (fresh) and after 15th cycle for (a) and (b) ND = 400 µm and (c) and (d) ND = 200 µm.Inset in (a) depicts the equivalent circuit employed in all fittings.

Figure
Figure Comparative C-rate test between compact and conventional mesh electrodes for (a) 400 µm and (b) 800 µm thickness.

Table 2 .
Evolution of R ohm and Rp parameters upon cycling.