Toward 3D Thin-Film Batteries: Optimal Current-Collector Design and

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Towards 3D Thin-Film Batteries: Optimal Current-Collector Design and Scalable Fabrication of TiO Thin-Film Electrodes 2

Sébastien Moitzheim, Joan Elisabeth Balder, Riina Ritasalo, Satu Ek, Paul Poodt, Sandeep Unnikrishnan, Stefan De Gendt, and Philippe Vereecken ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01905 • Publication Date (Web): 12 Feb 2019 Downloaded from http://pubs.acs.org on February 13, 2019

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ACS Applied Energy Materials

Towards 3D Thin-Film Batteries: Optimal Current-Collector Design and Scalable Fabrication of TiO2 Thin-Film Electrodes S. Moitzheima,b,, J.E. Balderc, R. Ritasalod, S. Ekd, P. Poodtc, S. Unnikrishnanc, S. De Gendta,e, P. M. Vereeckena,b,* imec, Kapeldreef 75, B-3001, Belgium KU Leuven, Department of Microbial and Molecular Systems, Kasteelpark Arenberg 23, B-3001 Heverlee, Belgium c TNO-Holst Centre, High Tech Campus 31, 5656 AE Eindhoven, The Netherlands d Picosun Oy, Masalantie 365, FI-02430 Kirkkonummi, Finland e KU Leuven, Department of Chemistry, Celestijnenlaan 200f, B-3001 Heverlee, Belgium a

b

*Corresponding author: [email protected]

Abstract Three-dimensional (3D) thin-film solid-state batteries are an interesting concept for microstorage, promising high footprint capacity, fast charging, safety and long lifetime. However, to realize their commercialization, several challenges still need to be overcome. In this work, we focus on two issues: the conformal coating and the high throughput deposition of thin-film layers. First, to facilitate conformal deposition, a design based on 3D micropillars is chosen. Although such a design has been suggested in the past, we calculate for the first time what (footprint) capacities can be expected when using fully optimized pillar geometries, while taking practical manufacturability into consideration. Next, spatial atomic layer deposition (S-ALD) is investigated as a scalable and conformal deposition technique. As proof-of-concept, 100 nm Cl-doped am-TiO2 thin-film electrodes are deposited by S-ALD on TiN-coated silicon micropillars. The influence of deposition parameters (i.e. exposure time and temperature) on the conformality and uniformity across the micropillar substrate is investigated. The results are discussed in terms of precursor diffusion and depletion, which is supported by an analytical model developed for our micropillar array. Furthermore, the Li-ion insertion properties of 3D electrodes fabricated by S-ALD and conventional ALD are compared. This research highlights the challenges and promises of 3D microbatteries and guides future S-ALD development to enable conformal and high-throughput thin-film deposition. Keywords: 3D current-collector, thin-film, Li-ion battery, atomic layer deposition, titania

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Introduction The fast development of portable electronic devices, such as smartphones, autonomous sensors, and wearable electronics, leads to increasing demands for integrated energy storage. Currently, for many novel applications, the size determining factor is the battery.1,2 Inherently, battery capacity scales with active material volume, and consequently, decreasing the battery size will inevitable come at the cost of capacity. A battery that can potentially overcome this, is the (rechargeable) thin-film solid-state Li-ion battery. Planar thin-film solid-state batteries (TFB) are already commercially available for microstorage and backup power.3 Benefits of TFBs include fast charging, long cycle life, and enhanced safety. However, the main drawback of TFBs is the low (footprint) capacity. Compared to conventional coin cell batteries (CB), TFBs deliver about 100 times less capacity (i.e. 10 mAh cm−2 vs 0.1 Ah cm−2, see Figure S1). Ideally, the capacity of TFBs is increased by increasing the mass loadings of the active electrodes per area, or in other words, increasing the film thickness. Unfortunately, due to kinetic (i.e. low ionic and electronic conductivity) and structural (e.g. film delamination) reasons, the thickness, and in turn capacity, is limited. To increase the footprint capacity of TFBs without increasing the film thickness, the concept of the three-dimensional (3D) TFB can be applied. In this case, the capacity is increased by depositing a thin-film battery stack on a large surface area current collector.3–6 The capacity is mainly enhanced by increasing the surface area of the 3D current-collector substrate. Unfortunately, 3D TFBs have proven technologically very challenging to fabricate. Arguably, the main challenge to overcome is conformal coating of a full thin-film battery stack on a high-aspect ratio structure. Various 3D TFBs designs have been introduced such as microsponges,3 microtrenches,7 microholes8 and micropillars.8 These give different levels of area enhancement and require different levels of complexity to fully coat in a conformal way. In general, the more open the geometry, the easier subsequent layers can be deposited. Although the use of micropillar structures has been previously suggested and tested as a substrate for 3D thin-film electrodes,9–15 a theoretical investigation of the highest achievable capacity is still lacking. Therefore, in the first section of this paper, we report on the capacity that can be achieved when simultaneously optimizing film thicknesses, pillar diameter, and inter-pillar


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spacing for a micropillar-based 3D TFBs. For this study, we chose a design based on micropillar arrays, as this offers an open geometry, and can be fabricated in a controlled manner using standard microfabrication techniques. To successfully realize 3D TFBs, thin-film deposition techniques need to be explored. Atomic layer deposition (ALD) is ideally suited to deposit thin-films on high aspect ratio structures. Unfortunately, the typically slow deposition rate makes ALD unpractical for film thicknesses beyond a few tens of nanometers. For this reason we investigate spatial ALD (S-ALD), or atmospheric pressure ALD, as it can significantly speed up deposition times and can potentially be integrated in roll-to-roll fabrication process.16,17 S-ALD relies on the spatial, rather than the conventional temporal distribution of reagent precursors; with this technique, gas precursors are continuously dosed (and purged) over a moving substrate, while the different half-reactions are separated in space by an inert gas curtain.17 Using this design, precursors are more efficiently exposed to the surface, while the purging time is significantly decreased. This effectively translates in much shorter deposition times compared to (low-pressure) ALD. However, because of the fast deposition rates with S-ALD, precursor diffusion into high-aspect ratio structures needs to be addressed. Therefore, in the second part of the paper, we develop an analytical model for precursor diffusion (i.e. for TiCl4) into micropillar structures, and predict the minimal exposure dose needed for conformal coating. To validate our theoretical (exposure) assumptions, we deposit Cl-doped TiO2 thin-film electrodes on Si micropillars using S-ALD. We have previously reported on the fabrication of Cl-doped amTiO2 using S-ALD, and have shown high capacity for Li-ion insertion and excellent rate performance.14 Chlorine is incorporated during low-temperature deposition (≤ 115 °C) of TiO2 when using TiCl4 and H2O as the precursors. Electrodes with stoichiometry of TiO1.91Cl0.18 reached a capacity of 855 and 268 mAh cm−3 at a rate of 1 and 50 C, respectively. In this work, the influence of deposition parameters on the coating conformality and on the chemical composition over the micropillar substrate is analyzed using SEM and EDX. A 100 nm thick Cl-doped am-TiO2 film is deposited on Si micropillars with an aspect ratio of 25. Although this thickness and aspect ratio are not yet optimized for high footprint capacity, it facilitates the thickness characterization by



SEM and allows comparison with our previous results. Also, the relatively small film thickness, enables high-rate performance. In the final section, we analyze the electrochemical properties (cyclic voltammetry, rateperformance and cycle life) of 3D Cl-doped TiO2 thin-film electrodes that were deposited using different deposition parameters. The performance between 3D electrodes deposited by ALD and S-ALD are compared. This work aims at critically addressing the challenges and benefits of allsolid-state 3D thin-film batteries.

Experimental methods Planar and micropillar substrates Planar substrates. A 200 mm n-type phosphorous doped crystalline Si wafer was coated with 60 nm TiN by physical vapor deposition and diced in 2 × 2 cm2 pieces. The diced pieces were subsequently used as-is for ALD and S-ALD of Cl-doped am-TiO2. TiN was chosen as it functions as current collector and diffusion barrier for lithium into the underlying Si.18 An n-type Si wafer was used as it allows electrical contact to be made to the TiN layer via the back side of the Si sample (see electrochemical methods) A good ohmic contact between TiN and n-type Si is achieved by an HF clean directly before TiN deposition. Silicon micropillar substrate. A 300 mm n-type phosphorous doped Si wafer was used for the Si micropillar fabrication. Standard photolithographic patterning, combined with deep reactive ion etching were employed to fabricate the pillar structures. Si micropillar arrays were defined on the 300 mm wafer in 1 × 1 cm2 squares, with 1 cm spacing in-between the arrays. Pillars were ordered in a square lattice, with a diameter and inter-pillar spacing of 2 µm, and a nominal height of 50 µm given an expected area enhancement of 21X. A 23 nm TiN current collector was deposited on the pillars by a standard plasma-based ALD process at 370 °C. From previous experience, the thickness of 23 nm was known to be sufficient to act as a current collector and effectively block Li diffusion into the Si micropillars. The wafers containing the micropillar arrays were diced in 2 × 2 cm2 pieces, with a 1 × 1 cm2 micropillar array in the center of each diced piece. ALD and S-ALD of Cl-doped am-TiO2 was subsequently performed on the diced pieces. Derivations of the equations for the theoretical capacity and plotting of the optimized capacity were done using the Mathematica


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software.19 N-type Si was used as this enables the use of a back-contact for electrochemical testing (see electrochemical methods).

Spatial Atomic Layer Deposition of TiO2 A custom built rotary type reactor was used which allows up to 152 mm round substrates to be mounted. Precursor inlets are surrounded by exhaust zones and are incorporated in a 150 mm diameter round reactor head situated above the sample. The inlets are surrounded by gas bearing planes, which separate the different reaction zones and prevent precursor intermixing. Gas bearing is formed by flowing pressurized N2 through holes located on the gas bearing surface. The sample table can be rotated at different rotation frequencies. Frequencies of 20, 30 and 40 rpm were investigated, which correspond to 136, 91 and 68 ms exposure time (here denoted as 140, 90 and 70 ms, respectively) at the center of the sample. The entire construction is mounted in a convection oven which controls the deposition temperature. More details on the reactor used can be found in ref. 10. Depositions were done simultaneously on four 2 × 2 cm2 samples which were mounted on a holder and loaded into the S-ALD reactor. For each deposition run, two planar and two 3D substrates were loaded. The deposition precursors were TiCl4 and H2O, which were evaporated from a bubbler using 50 sccm and 500 sccm (standard cm3 per min) flow through the bubblers, respectively. Both flows were further diluted by N2, resulting in total volume flows (N2 + precursor) of 1 slm (standard liter per min). The temperature of the precursor bottles was controlled outside the reactor and kept at room temperature for TiCl4, and at 50 °C for H2O. Depositions were done at 100 °C (with 70 ms exposure time) and at 115 °C (with 140, 90 and 70 ms exposure time). The growth per cycle (GPC) rate was determined in a previous report on planar samples.14 For 100 and 115 °C deposition, the GPC is 0.085 and 0.080 nm/cycle, respectively, and is independent of rotation frequency (i.e. conditions are in the growth saturation regime). It is known for the TiCl4-based process, that a lower deposition temperature leads to a higher GPC, as more stable surface –OH groups are available during the TiCl4 pulse.21 In a previous report, we have shown that films deposited at 100 and 115 °C using the same ALD process, are amorphous with a density of 2.9 g cm−3.14 A total of 1155 and 1225 substrate rotation cycles were adopted, which corresponds to a nominal thickness of 98 nm for 100 °C and 115 °C, respectively.



Atomic Layer Deposition of TiO2 ALD was performed at 100 °C under reduced pressure (under N2 atmosphere) of 3 mbar in a PicosunTM R-200 Advanced reactor. The TiCl4 and H2O precursors were vaporized from Picosolution source bottles at room temperature. Several 3D substrate pieces were placed on a Si pocket wafer together with a reference Si wafer piece and loaded into the ALD reactor. To ensure conformal ALD coating, three consecutive TiCl4 pulses (100 ms each) and one H2O pulse (100 ms) was used, together with a long purging time between the different precursors. The thickness of the Cl-doped am-TiO2 film was determined by spectroscopic ellipsometry on the Si reference piece.

Thickness and conformality characterization The thickness and surface morphology of am-TiO2 films deposited on TiN-coated Si micropillars was examined in a NOVA 200 (FEI) scanning electron microscope (SEM). The film thickness was determined by SEM at the trench wall of the recessed pillar array, and at the bottom in-between the pillars. All reported thicknesses were measured at magnification of 160kX to 200kX. Energy dispersive X-ray spectroscopy (EDX) was performed at 10 kV with an INCA Pentafet x3 detector (Oxford Instruments) mounted on the Nova 200 SEM. Elemental analysis was performed using the INCA Microanalysis Suite (Issue 18b).

Electrochemical methods A custom-made three-electrode polytetrafluoroethylene (PTFE) cell was used, which was clamped onto the substrate using a Kalrez O-ring, with an exposed surface area of 1.1 and 1.79 cm2 for planar and 3D substrates, respectively. The larger exposed surface for the 3D substrates was needed to ensure that the whole of the pillar array was enclosed. The cell contains two compartments, the main compartment in contact with the sample also comprised a Li metal foil as a counter electrode and a smaller side compartment comprised a Li metal foil as a reference electrode. The compartment with the Li reference electrode was connected to the main compartment through a Luggin capillary close to the surface of the working electrode (at 4 mm). The cell was filled with 10–15 mL liquid electrolyte solution. Note that, because of the design of


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our electrochemical cell, the O-ring seal prevents the electrolyte solution from contacting the Si substrate and TiN current collectors. This prevents any insertion or alloying reactions to occur with these components. See Figure S2 for a schematic of the electrochemical cell and more information. All experiments were performed at room temperature (21 °C) using a LiClO4 in propylene carbonate (PC) as the electrolyte solution. This model electrolyte was used rather than the commercial LiPF6 based electrolytes for the reason of potential corrosion of the thin films by HF byproducts which was observed in these electrolytes. For ease of preparation, an ampoule containing LiClO4 (100 g, battery grade, dry, 99.99%, Sigma Aldrich) was dissolved in propylene carbonate (100 mL, 99.7%, Sigma Aldrich), which resulted in a 0.94 M solution. Measurements were done in an Ar filled glove box with O2 and H2O kept below 1 ppm. Electrical contact was made to the back of the samples by scratching with a diamond tip and applying gallium indium eutectic (Alfa Aesar). The Ga-In coated backside of the Si sample was placed on a Cu foil mounted on a solid support. The electrochemical cell was controlled through a PGSTAT101 Autolab (Metrohm) potentiostat/galvanostat, using the Nova 1.10 software. To activate and stabilize the response of am-TiO2, five cyclic voltammogram (CV) cycles were recorded at 10 mV s−1 in the range of 0.1 V to 3.2 V. Afterwards, galvanostatic lithiation and delithiation experiments were carried out with cut-off voltages of 3.0 and 0.1 V. Between CVs and lithiation/delithiation experiments, the electrode was relaxed at 3.0 V until a 1/50 C cut-off current was reached. For the planar samples, a current of 5, 10, 25, 50, 100, 250, 2.5 and 5 µA was consecutively applied. For the 3D samples, 0.1, 0.2, 0.5, 1, 2 and 5, 0.05 and 0.1 mA were applied in sequence. The lowest current was the second to last in the sequence, as a low current might lead to more decomposition of the electrolyte solution which could be detrimental to the rate-performance (e.g. if a resistive surface film is formed). The last cycle was a repeat of the first cycle to test if the capacity was stable. For the 3D samples, the current was normalized by the geometric or footprint area of the pillar array (i.e. 1 cm2). Hence the effective current density is about a factor 21 lower, taking into account the area enhancement of the pillars. Since most of the active material is contained within the micropillar array, it is safe to neglect the planar area contacting the electrolyte solution outside of the micropillar array (i.e. 0.79 cm2 compared to ~21 cm2) as the exact area enhancement is not known (sloped pillar profile and slight variations in height of pillar



array). A (footprint) current density of 10.3 µA cm−2 and 0.2 mA cm−2 corresponds to a rate of 1 C for the planar and 3D samples, respectively. For the cycling experiment, a current density of 0.5 mA cm−2 was applied for 50 charge/discharge cycles. All voltages are given versus Li+/Li.


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Results and discussion Design considerations and theoretical capacity of micropillar structures The first aim of this study is to calculate theoretical capacities achievable with 3D TFBs based on micropillar arrays. A schematic of such a 3D TFB is shown in Figure 1. The full 3D battery consists of: (1) a microstructured current collector, (2) first active electrode (here “cathode”), (3) solidelectrolyte and (4) second electrode (here “anode”). To finish the 3D battery, the structure needs to be filled with a second current collector. Parameters that influence the footprint capacity are: the pillar diameter (d), pillar height (h), inter-pillar spacing (sp), first electrode thickness (l1) and volumetric capacity of the electrode (Cv). A general equation for the footprint capacity (C3D) of this 3D TFB is given by (see SI for derivation):

𝐶𝐶3D =

𝐶𝐶v 𝑙𝑙1 𝐺𝐺

�1 +

πℎ(𝑑𝑑+𝑙𝑙1 ) �𝑑𝑑+𝑠𝑠p �

2

(1)

�

where l1, d, sp, h (in m) and Cv (in Ah m−3) are given above, and G is a geometric constant which depends on the pillar arrangement (i.e. unit cell). For a square lattice, G is equal to 1, and for the more closely packed hexagonal arrangement G is equal to

√3 2

(≈ 0.87).

Figure 1. Schematic of a micropillar array arranged in a square lattice with a height (h). In the top-view (left side) the pillar diameter (d) and inter-pillar spacing (sp) are denoted. The 3D thin-film solid-state battery (right-side), showing the pillar current collector, cathode (1st electrode), solid electrolyte and anode (2nd electrode).



The square lattice is chosen for the remainder of the calculations as it offers a higher amount of open space, which is beneficial for conformal coating. Note that the capacity from eq. 1 is written as a function of the first electrode thickness, which then is the capacity determining electrode. Previously, similar equations have been used to calculate the expected footprint capacity or surface area enhancement for micropillar arrays (see e.g. refs. 10, 12, 13 and 15). Typically, values for the pillar length, height and diameter, and inter-pillar spacing are arbitrarily chosen for illustration purposes (e.g. ref. 15), or to compare experimental values (obtained from 3D TFB halfcells measurements) with theory (e.g. refs. 10, 12, and 13). To our knowledge, the (theoretical) maximum that such a micropillar design can achieve has not yet been investigated. For this, we analyzed eq. 1 in more detail, and aimed to minimize the inter-pillar spacing, while maximizing the amount of active material. The complete derivation can be found in the supporting information. To summarize, a relationship between the (smallest) inter-pillar spacing and (optimal) pillar diameter can be found, for which the footprint capacity is at maximum. The capacity then only depends on the pillar height and the 1st electrode thickness (for a constant value of Cv, and G). For geometric reasons, the inter-pillar spacing and diameter are fixed by the choice of solid-electrolyte thickness and the volumetric capacity of the 2nd electrode (see SI). Additional calculated theoretical capacities as a function of geometric parameters can be found in Figure S4. The result of the optimization study is shown in Figure 2. For clarity, the aspect ratio (AR = h/d) of the micropillar structure is also given in Figure 2. For the calculations, a solid-electrolyte thickness of 100 nm is assumed. Currently, the thinnest functional LiPON solid-electrolyte was shown as thin as 15 nm.22 However, to ensure integrity of the stack across large effective surface areas, it is safer to assume a thickness of 100 nm. For all calculations, an active electrode volumetric capacity (Cv) of 1000 mAh cm−3 is chosen. We have shown previously that such capacity can be achieved with 100 nm thick films of Cl-doped am-TiO2 as the negative electrode14 and LiMn2O4 as the positive electrode.23


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Figure 2. Theoretical calculations of the maximum footprint capacity for 3D micropillar arrays arranged in a square lattice (G = 1). A 100 nm solid-electrolyte, and electrodes with equal volumetric capacity (1000 mAh cm−3) are assumed. Dotted lines denote the corresponding aspect ratio (AR = h/d) of the pillar structure.

We shall now consider two specific cases in detail. First, a micropillar arrangement that can achieve a footprint capacity of 1 mAh cm−2 is evaluated, as this is near the lower limit of coin cells, but still about 10 times that planar thin-film batteries (see Figure S1). From Figure 2, it follows that different geometries can achieve such capacity. For example, using a pillar structure with AR of 200 (i.e. d = 0.3 µm and h = 60 µm) together with an 80 nm (first) electrode, leads to the target of 1 mAh cm−2. Such a relatively thin electrode will ensure ultra-fast charging; for example, a 75 nm thin-film of LiMn2O4 can be charged and discharged at 50 C (in about 1 min) with 80% of its maximum capacity accessible (i.e. 880 mAh cm−3, as both the 3 V and 4 V regions are available for LiMn2O4 thin-films).23 Technologically, however, such high-aspect-ratio structures will be extremely challenging for the conformal coating of the full battery stack, and could lead to issues with mechanical integrity (e.g. collapsing structures during fabrication).24 On the other hand, decreasing the AR will come at the penalty of an increased electrode thickness and thus decreased rate-performance. The structure with AR of 50 (h = 40 µm, d = 0.8 µm) and electrode thickness of ~370 nm shows a good trade-off in terms of AR and film thickness. If we assume a substrate and packaging thickness of 200 µm, a volumetric capacity of ~40 mAh cm−3 is obtained; i.e. matching that of coin cells and 4 to 16 times that of planar thin-film batteries.



Secondly, when looking at the highest footprint capacity that can be reached with this technology, up to 3 mAh cm−2 is shown to be feasible in Figure 2. In principle, taller pillar structures (> 100 µm) and thicker electrodes (> 1 µm) will increase the footprint capacity even further. However, fabricating pillars with such height and aspect ratio (e.g. AR of 200), together with the required pillar density (up to 1 × 108 cm−2) already poses a significant challenge (see e.g. ref 17). Also, using electrodes thicker than 1 µm will likely result in poor rate-performance, which cancels out the major benefit of thin-film batteries. Therefore, to reach footprint capacities higher than 3 mAh cm−2, a strategy where 3D TFB cells are stacked is likely required, as is also done for conventional powder cells. For example, to reach 10 mAh cm−2, which is the average value of CBs, three to four 3D TFB cells would need to be stacked. Note that, to achieve such capacity with planar TFBs, 60 to 120 stacked cells would be needed. The volumetric capacity of the stacked 3D design would amount to about 150 mAh cm−3, when assuming a substrate thickness of 50 µm (per cell) and a packaging thickness of 200 µm (i.e. a total battery thickness of 650 µm). Hence, using the micropillar-based 3D TFB architecture, a volumetric capacity of almost 3 times that of coin cells can be fabricated. Although these calculations give a good indication on the upper limits of such a 3D TFB design, ultimately, the practical capacity will depend on the (technological) realization of the desired micropillar structure, the degree of conformal coating of the active materials, and the actual volumetric capacity of the electrodes. Nevertheless, these calculations reveal a clear trade-off between faster charging (i.e. for ultra-thin films with high AR pillars) and lower technological complexity (i.e. for “thick” films with low AR pillars).

Theoretical conditions for conformal coating To deposit thin-films on high aspect-ratio micropillar substrates, (theoretical) limitations of the fabrication technique needs to be assessed first. As mentioned before, we explore S-ALD as a scalable technique to coat these micropillar structures. To understand the required exposure time for conformal deposition, mass transport of precursors into the structure and surface reaction kinetics must be examined. First, we consider kinetic limitations related to a (ideal) TiCl4 half-cycle. Adsorption of the TiCl4 precursor onto a OH-terminated surface can be given by:


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(−OH)s + TiCl4 (g) ↔ (O − TiCl3 )s + HCl (g)

(2)

For this reaction, a rate constant of adsorption, kads, and desorption, kdes, can be formulated, which give an expression for the surface reaction rate 𝜈𝜈 (m−2 s−1): 𝜈𝜈 =

d𝑆𝑆ads d𝑡𝑡

d𝜃𝜃

= 𝑆𝑆0 d𝑡𝑡 = 𝑘𝑘ads 𝑃𝑃TiCl4(𝑥𝑥 = 0,𝑡𝑡) (1 − 𝜃𝜃) − 𝑘𝑘des 𝑃𝑃HCl(𝑥𝑥 = 0,𝑡𝑡) 𝜃𝜃

(3)

with Sads the number of adsorbed species and S0 the total number of reaction sites (m−2), t is time (s), 𝜃𝜃 the fraction of reacted sites (θ = Sads/S0), and 𝑃𝑃TiCl4 (𝑥𝑥=0,𝑡𝑡) and 𝑃𝑃HCl(𝑥𝑥 = 0,𝑡𝑡) the (time

dependent) TiCl4 and HCl partial pressures (in Pa) at the surface (x = 0), respectively. For simplicity, we assume that the desorption term can be neglected (kdes = 0), and that the reaction is described by first-order kinetics. Based on the kinetic theory of gases, kads can be written in an Arrheniustype relationship as:26

𝑘𝑘ads = �

1

�2π𝑚𝑚𝑘𝑘B 𝑇𝑇

𝐸𝐸 ads

(4)

� 𝜅𝜅 exp �− 𝑘𝑘a 𝑇𝑇 � B

with m the mass of the precursor molecule (kg), kB the Boltzmann constant (1.381 × 10−23 J K-1), T the temperature (K), 𝜅𝜅 the transmission coefficient, and 𝐸𝐸aads the activation energy of adsorption

(J). The pre-exponential term within the brackets is the collision frequency of ideal gas molecules hitting the surface. Note that eq. 4 is normalized so that the units of kads (Pa−1 m−2 s−1) are

compatible with eq. 3. By combining eq. 3 and 4, and neglecting the desorption step, the surface reaction rate becomes:

d𝑆𝑆ads d𝑡𝑡

= 𝑃𝑃TiCl4 (𝑥𝑥 = 0,𝑡𝑡)

(1−𝜃𝜃)


𝐸𝐸 ads

𝜅𝜅 exp �− 𝑘𝑘a 𝑇𝑇 � B

(5)

Note that 𝜃𝜃 increases over time (as more surface sites get occupied), and 𝑃𝑃TiCl4(𝑥𝑥=0,𝑡𝑡) decreases

if the consumption rate of precursor molecules is faster than their supply. To find a simple



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expression for the surface coverage as a function of time, all terms in eq. 5 are assumed independent on time, and a parameter, termed the reaction probability 𝛽𝛽, is introduced: d𝑆𝑆ads d𝑡𝑡

= 𝑃𝑃TiCl4

𝛽𝛽

(6)


In this case, 𝛽𝛽 includes all energetic effects related to the surface reaction kinetics, including the

surface coverage term (1 − 𝜃𝜃) and the transmission coefficient 𝜅𝜅 , and is valid for a nontemperature activated reaction ( 𝐸𝐸aads ≪ 𝑘𝑘B 𝑇𝑇) . Although in reality, 𝛽𝛽 varies as the reaction

proceeds, often a constant value for 𝛽𝛽 is used to describe the kinetics under certain experimental conditions. Integration of eq. 6 for time then leads to:

𝑃𝑃TiCl4 ∙ 𝑡𝑡 =

𝑆𝑆ads �2π𝑚𝑚𝑘𝑘B 𝑇𝑇 𝛽𝛽

=

𝜃𝜃𝑆𝑆0 �2π𝑚𝑚𝑘𝑘B 𝑇𝑇 𝛽𝛽

(7)

where "𝑃𝑃TiCl4 ∙ 𝑡𝑡", or generally P∙t, is known as the “exposure dose” (Pa∙s) needed to reach a certain surface coverage. Equation 7 shows that to ensure full saturation (𝜃𝜃 = 1), either the

precursor partial pressure or the exposure time can be increased. For a reaction probability (𝛽𝛽) of 1 and a total number of reaction sites (S0) for TiCl4 of 3.4 × 1014 cm−2,28 a minimum theoretical

exposure dose of 3.4 × 10-4 Pa∙s at 100 °C is required. A previous experimental study showed that the minimum exposure dose to reach full coverage of a OH-terminated Si surface by TiCl4 under UHV conditions at 100 °C was 0.027 Pa∙s.27 Using eq. 7, this suggests that for this system, the reaction probability, 𝛽𝛽, was about 0.01.

In a typical experiment, the (experimental) exposure dose is obtained from the product of TiCl4 partial pressure in the reactor and exposure time. In our experiments (see next section) TiCl4 is dosed using a bubbler at 20 °C. This equals a vapor pressure of 1300 Pa 29, which gives a partial pressure of 65 Pa over the substrate after 95% dilution with the N2 carrier gas. Therefore, when only considering kinetics, an exposure time between 5 × 10-6 s (𝛽𝛽 = 1) and 5 × 10-4 s (𝛽𝛽 = 0.01) is required in this case.


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Next, precursor diffusion into the micropillar structure is addressed. An analytical expression for the exposure dose of a diffusion limited deposition inside micropillar structures can be formulated under assumptions of:30–32 a constant partial pressure at the top of the microstructure, a linear concentration profile within the structure, and simplified reaction kinetics (e.g. by eq. 7). Previously, an analytical expression for the exposure dose was developed for coating of microholes at atmospheric pressures.30 Using a similar approach, we derived an expression for the exposure dose valid for micropillar structures, given by (see SI for derivations):

𝑃𝑃 ∙ 𝑡𝑡 = �

π𝑑𝑑ℎ

2 π𝑑𝑑2 �𝑑𝑑+𝑠𝑠p � − 4

��

�2π𝑚𝑚𝑘𝑘B 𝑇𝑇𝑆𝑆0 𝛽𝛽

+

ℎ𝑆𝑆0 𝑇𝑇𝑇𝑇

2𝐷𝐷g 𝑁𝑁A

�

(8)

with P the pressure (Pa), d, h and sp the pillar diameter, height and inter-pillar spacing (all in m), Dg the gas precursor diffusion coefficient (m2 s−1), NA Avogadro’s constant (6.022 × 1023 mol−1) and R the ideal gas constant (8.314 J mol−1 K−1). Equation 8 gives the exposure dose needed for the deposition front to reach the bottom of the pillar structure. To understand the conditions needed for conformal deposition of a thin film inside the micropillar array, we calculated the exposure dose needed to fully cover an optimized geometry. We considered here micropillar arrays that can reach 1.25 mAh cm-2. As detailed in the previous section, such a capacity can be achieved with a variety of pillar aspect ratios, while keeping the electrode film thickness below 1 µm (i.e. AR between 20 and 200). Since we are considering the optimized geometry, the selection of the aspect ratio also fixes the values of the inter-pillar spacing and electrode thicknesses. The theoretical exposure dose as a function of aspect ratio is shown in Figure 3. We plotted the exposure dose for three different values of 𝛽𝛽. As expected, the lower the value of 𝛽𝛽, the higher the required exposure dose. Also, increasing the aspect ratio also

increases the required exposure dose. A higher AR will have a longer diffusion path for the precursors deeper into the structure and will therefore need a higher exposure dose. For example, to saturate the complete surface of a micropillar array with an AR of 200 and with 𝛽𝛽 = 0.001, one would require about 0.6 s of exposure time (using a partial pressure for TiCl4 of

65 Pa). If the same amount of time is needed for H2O exposure and for the purging steps, a total time of 2.4 s for a full ALD cycle is obtained. The thickness of the electrode in this case is about



100 nm, which corresponds to approximately 1000 ALD cycles. Therefore, the upper limit for the deposition time would be around 40 min. Although this calculation requires several assumptions (e.g. chemistry, reaction probability, etc.), it does show that even for a high AR of 200, the total deposition time for one layer is still below an hour. Therefore, it is safe to assume that in the extreme case of using an AR of 200 and low reaction probability, the time to coat the full 3D thin-film battery stack by ALD would be in the order of hours. Reactions with higher reaction probability, or precursors with higher vapor pressures, will significantly speed this up. 50

β = 0.001 β = 0.01 β=1

40 Exposure dose (Pa s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 30

30 20 10 0

50

100

150

200

Aspect ratio Figure 3. The theoretical exposure dose as a function of aspect ratio for an optimized micropillar array reaching 1.25 mAh cm−2. The calculated exposure doses for different values of the reaction probability (𝛽𝛽) are given. The exposure dose is considered for the diffusion of TiCl4 into micropillar arrays arranged in a square lattice, at a deposition temperature of 100 °C.

Coating of chlorine doped am-TiO2 on 3D structures As mentioned in the introduction, Cl-doped am-TiO2 is chosen as the high-performance anode for the 3D TFB. In our previous study, Cl-doped am-TiO2 fabricated by S-ALD was demonstrated, and its properties were analyzed in detail.14 Here, we mainly focus on the aspect of conformal coating using S-ALD for potential large-scale fabrication. As a reference, we also deposit Cl-doped amTiO2 using conventional (temporal) ALD. Silicon micropillar arrays were used as the model 3D substrate for ALD and S-ALD. The micropillar arrays were fabricated using standard photolithography and deep reactive ion etching of 300 mm Si wafers. A (nominal) height of 50 µm,


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with a diameter and inter-pillar spacing of 2 µm (AR = 25) was chosen for the micropillars. A 23 nm TiN coating is deposited on the pillars using conventional (temporal) ALD, and acts as a current collector and Li+ diffusion barrier towards the silicon.18 From SEM inspection of the pillars, an average pillar height of 57 µm was determined. For this experimental part, we did not yet focus on achieving the highest capacity possible. Rather, the aim was to determine the feasibility of SALD as a high throughput deposition technique and to characterize the conformality of the coatings. Therefore, we selected a thickness of 100 nm for the TiO2 electrode as this comes into the thickness range needed for a realistic stack and sufficient for SEM and EDX analysis. Using TiCl4 as the precursor together with our microfabricated pillar geometry, a theoretical exposure dose of 2.0 Pa∙s is calculated from eq. 8 (𝛽𝛽 = 0.01). Using our experimental conditions for the

TiCl4 partial pressure (i.e. 65 Pa, see above), a minimum exposure time of 30 ms is expected.

In our (custom built) S-ALD reactor, the gas precursor exposure (and purging) time is controlled by the substrate rotation frequency. In a typical deposition run, four 2 × 2 cm2 samples are loaded simultaneously into the S-ALD reactor. The micropillar array is defined in a 1 × 1 cm2 area, and is located at the center of the sample (see Figure 4a inset). After deposition, samples were manually cleaved parallel to the substrate rotation direction, so that both the trailing and leading edge (relative to the rotation direction) of the samples could be investigated by SEM. Note that the pillar array is recessed into the substrate, and therefore, the edge of the array consists of a Si/TiN “trench wall”. More schematics and information about our pillar array are provided Figure S5. Depositions were carried out using TiCl4 and H2O as precursors, for which an effective exposure dose of 70 (at 100 and 115°C), 90 and 140 (at 115 °C) ms were explored. This is about 2, 3 and 5 times the theoretical (diffusion limited) exposure dose of 30 ms for TiCl4 calculated. To compare the S-ALD deposited films with an ideally conformal process, TiO2 was also deposited using conventional (low-pressure) ALD from TiCl4 and H2O. To obtain a high degree of conformality with ALD, three consecutive TiCl4 pulses were applied with an exposure time of 100 ms each. Conformalities of the films were determined by measuring film thicknesses at different locations along the “trench wall” and in-between the pillars across the array. Figure 4a and b show crosssection SEM images of a micropillar array coated with nominal thickness of 98 nm Cl-doped amTiO2 by S-ALD, with a tilted top-view in Figure 4a and a side-view in Figure 4b. The side-view image



shows the edge of the pillar array with the trench wall. The right panels show a close-up of the coating along this trench wall near the top, middle and bottom parts.

Figure 4. SEM images of 3D micropillar arrays coated with am-TiO2 by S-ALD (a-b) and SEM thickness measurements for different deposition conditions (c-e). (a) Tilted top-view image of a typical TiO2 coated micropillar array, with an inset showing the substrate together with the pillar array. (b) Cross-section view of the pillars near the array trench wall, with insets showing the thickness near the top, at 12 µm from the top and at the bottom. Thickness as a function of normalized distance from the top of the trench wall, measured at the (c) leading and (d) trailing edge wall. (e) The thickness at the bottom in-between the pillars is measured from the leading to the trailing edge. The leading edge is the part of the substrate that first reaches the reaction zone. The legend denotes the S-ALD and ALD deposition parameters of temperature and exposure time. Error bars denote sample standard deviation obtained for a measurement of minimum 3 and maximum 10 measurements.


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At close inspection, the TiN current collector and am-TiO2 film can be discerned. The complete coating of the trench wall demonstrates the ability of the S-ALD technique to coat high-aspect ratio structures with TiO2. Figure 4c shows the thicknesses determined by SEM of am-TiO2 films deposited at 100 °C and 115 °C along the trench wall at the leading edge of the substrate. As a benchmark, Cl-doped am-TiO2 deposited at 100 °C by conventional ALD is added. For ALD, the film thickness is nearly constant from the top to bottom with a thickness of ~90 nm; as expected from the self-limiting growth mechanism and long exposure times. On the other hand, a thickness change across the trench wall is observed for S-ALD. The different deposition conditions give a film thickness of 88–102 nm at the top, which decreases to 65–84 at the bottom, and corresponds to a conformality between 74% and 83%. There is no significant influence of exposure time and temperature on the conformality. Figure 4d gives the thicknesses measured at the trailing edge wall for S-ALD. In this case, at the top of the trench wall, a thickness of ~130 nm was measured for all conditions, except for the sample deposited at 115 °C with 140 ms exposure time (i.e. 175 nm). A thickness larger than the GPC suggests that some TiCl4 and H2O precursors accumulated at this side and were improperly removed during the purging step. The increased thickness at the trailing edge likely explains the color gradient observed visually next to the pillar array (see inset Figure 4c). In this case, a top-to-bottom conformality of ~50% was determined. Note that the thickness at the bottom of the pillars is the same whether it is measured at the leading or trailing edge. Figure 4e shows the film thickness at the bottom in-between the pillars as a function of distance away from the leading to the trailing trench wall (“array x-coordinate”). Conventional ALD shows a uniform thickness across the array. For S-ALD however, issues with uniformity are present. Most strikingly, for layers deposited at 115 °C with 70 ms exposure time, the thickness decreases to about 8 nm (i.e. a 10% uniformity) at 7.5 mm away from the leading trench wall. The uniformity increases for longer exposure time; for 90 and 140 ms exposure time, a uniformity of 65% and 72% is found, respectively. For deposition at 100 °C with 70 ms exposure time, the uniformity is 43%. Hence, cross-array uniformity is improved by a longer exposure time or lower deposition temperature, or both.



The thickness conformality was also determined directly on the pillars by performing EDX linescans, as shown in Figure 5. Since the pillars are also coated with TiN, the Ti signal originates from both TiN and am-TiO2, and therefore, the O:Si ratio is taken as a measure of thickness change along the pillar height. The O:Si ratio was chosen since the pillar diameter varies slightly across the length of the pillar. Besides Ti, O, Si and N, a signal originating from Cl was clearly detectable, for the films deposited by ALD and S-ALD (see Figure S6). The O:Si ratio measured by EDX was analyzed for am-TiO2 films deposited by ALD and S-ALD at 100 °C (see Figure 5). For ALD, the conformality is 95% on a pillar at the edge, and 90% on a pillar in the center of the array. For SALD, a conformality of 85% at the edge, and 50% at the center of the array is measured. 1.0 0.8 0.6 0.4 0.2 0.0 10 µm

ALD - array trench ALD - array center S-ALD - array trench S-ALD - center array 0 10 20 30 40 50 Distance from top of pillar (µm)

Figure 5. EDX line-scans of pillars situated near the trench wall and near the center of the pillar array, comparing ALD with S-ALD. The O:Si signal is chosen as representative to the am-TiO2 thickness distribution across the array. The O:Si signal was smoothed by an adjacent-average method using 15 points, and then normalized for the first point (distance from top of pillar of 0 µm). The S-ALD and ALD TiO2 films were deposited at 100 °C with an exposure time of 70 ms and 300 ms, respectively.

The conformality measured by EDX at the edge is similar to what was determined by SEM along the trench wall for S-ALD. As such, the conformality along the trench wall correlates well with the conformality on nearby pillars. For the pillars at the center of the array, a 50% conformality suggests that the top of the pillar has an am-TiO2 thickness of 80 to 100 nm, as 40 to 50 nm was measured by SEM at the bottom for this deposition condition. EDX line scans of the Cl:O signal (Figure S6) were also analyzed, and showed that the Cl content is constant across the length of the pillars for S-ALD (100 °C / 70 ms) and ALD (100 °C / 300 ms). An average Cl:O2 atomic ratio of 0.041 ± 0.011 for S-ALD, and 0.043 ± 0.006 for ALD was determined (where the error denotes the


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95% confidence interval). Hence, the Cl:O2 ratios and composition for ALD and S-ALD are equal within experimental error. For the fastest exposure time applied (i.e. 70 ms), an experimental exposure dose of 4.6 Pa∙s is obtained, which is above the dose required to fully saturate the surface calculated theoretically (i.e. 2.0 Pa∙s). A possible explanation for the non-conformality of the coatings can be found when considering the total number of molecules needed for saturation versus the actual number supplied in the reactor. Taking a reaction site density for TiCl4 of 3.4 × 1014 cm−2,28 and an effective surface area of 24 cm2 for the 2 × 2 cm2 sample comprising the micropillars, leads to 1.4 × 10−8 mol reaction sites. Our flow rate of 1 standard liters per minute (slm) and TiCl4 partial pressure of 65 Pa corresponds to a molar flow rate of 3 × 10−7 mol s−1 at 115 °C. During each cycle, in total 1 × 10−8 mol were supplied for the shortest exposure time. Hence, two-thirds of the supplied TiCl4 are consumed with complete adsorption (𝜃𝜃 = 1), and therefore, the effective partial pressure drops during deposition and amounts to only one-third of its initial value at the end of each TiCl4

cycle. If we take one-third of our experimental exposure dose, the value drops below the limit of 2.0 Pa∙s, and conformal deposition cannot be ensured. Therefore, rather than being limited by precursor diffusion into the micropillar structure alone, the deposition is limited by the supply of precursor into the reactor (which in turn slows down diffusion into the structure). In this case, eq. 8 does not adequately describe our system, and mass-transport fluxes in the complete reactor must be solved. From our experiments, it follows that increasing the exposure time from 70 to 140 ms improves the cross-array uniformity. This can directly be linked to an increased amount of precursor supplied, since a longer exposure time will increase the effective molar flow rate into the reactor. Alternatively, increased supply could be achieved by increasing the precursor partial pressure (temperature of the TiCl4 bubbler), or by varying the mixing ratio of the N2 carrier gas and TiCl4 vapor.

Electrochemical performance of 3D electrodes The electrochemical Li-ion insertion properties of planar and 3D thin-film electrodes deposited by ALD and S-ALD were tested in a three-electrode cell setup (see Figure S2 for a schematic of the setup). For this test, samples deposited at 100 °C were compared, as this yields the highest Cl content and thus best electrochemical performance. A cyclic voltammogram of TiO2 deposited by



S-ALD on micropillars is shown in Figure 6a. The shape of the voltammogram is typical for Cldoped amorphous TiO2,14 with broad peaks centered around 1 V for lithiation and around 2 V for delithiation. No other features are seen, which shows that there is no electrochemical contribution from the underlying TiN/Si substrate. Figure 6b shows the galvanostatic charge/discharge results of 3D TiO2 electrodes fabricated by S-ALD and ALD, measured at a rate of 0.5 C. The lithiation and delithiation curves for both ALD and S-ALD are nearly identical, with a sloped response across the full potential range. The footprint capacity for ALD and S-ALD films is almost the same, with an average capacity of 225 µAh cm−2 for S-ALD and 235 µAh cm−2 for ALD. The (delithiation) footprint capacity obtained after galvanostatic charge/discharge at different Crates is shown in Figure 6c. Planar TiO2 films reach a maximum capacity (measured at 0.25 C) of 10.3 µAh cm−2 for S-ALD, and 9.2 µAh cm−2 for ALD. The highest rate capabilities of these films are similar: at a rate of 25 C, still about 40% of the maximum capacity can be accessed for both planar films. For the 3D structured electrodes, a significant increase in maximum footprint capacity is obtained; 242 µAh cm−2 for S-ALD and 249 µAh cm−2 for ALD. On average, this corresponds to a capacity enhancement of 25× for S-ALD and 27× for ALD. The slightly higher capacity enhancement for ALD likely reflects the better conformality. More importantly, even though the capacity was increased to such an extent, 3D thin-film electrodes mostly preserved the high rateperformance of the planar films, with 35% of the maximum capacity accessible at 25 C. After each rate-performance test, the measurement at 0.5 C was repeated to check for reproducibility. The capacities in all cases did not decrease more than 8%. Figure 6d shows the result of cycling a 3D TiO2 electrode fabricated by S-ALD for 50 cycles at a rate of 2.5 C. During 50 cycles the capacity drops from 177 to 169 µAh cm−2 (i.e. a 4.5% drop), with an average coulombic efficiency of 99.83%. This shows that the integrity of the pillared electrodes is maintained during electrochemical cycling, and that a high capacity retention can be achieved.


Page 22 of 30

1 0 -1 -2 -3

S-ALD TiO2 (3D) 0

3D

100 /

S-ALD TiO2

/

ALD TiO2

10

Planar 1 C-rate

10

2.5

(b)

S-ALD delith. ALD delith.

2.0 1.5 1.0 S-ALD lith. ALD lith.

0.5 0.0

1 2 3 + Potential (V vs Li /Li)

(c)

0.1

Potential (V vs. Li+/Li)

2

3.0

(a)

Footprint capacity (µAh cm-2)

Footprint capacity (µAh cm-2)

3

0

50 100 150 200 250 -2 Footprint capacity (µAh cm )

(d)

100

200

75 100

0

S-ALD (lith.) S-ALD (delith.) Coulombic efficiency 0

10

20

30 40 Cycle

50 25

Coulombic efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Current density (mA cm-2)

Page 23 of 30

50

Figure 6. Electrochemical characterization of Cl-doped am-TiO2 films deposited on 3D and planar substrates. (a) Cyclic voltammogram of a 3D TiO2 electrode fabricated by S-ALD, measured at a scan rate of 10 mV s−1 between 3.2 V and 0.1 V. (b) Charge-discharge curves of 3D TiO2 electrodes fabricated by ALD and S-ALD, measured at 0.5 C. (c) Delithiation capacity vs. C-rate for planar and 3D electrodes deposited by ALD and S-ALD. (d) A 3D TiO2 electrode fabricated by S-ALD cycled for 50 cycles at a rate of 2.5 C. A rate of 1 C corresponds to a current of 10.3 µA cm-2 for planar and 0.2 mA cm-2 for 3D electrodes. A three-electrode setup using a Li reference and counter electrode in 1 M LiClO4 were used. Cut-off voltages of 3.0 and 0.1 V vs Li+/Li were used for delithiation and lithiation, respectively. The S-ALD and ALD TiO2 films were deposited at 100 °C with an exposure time of 70 ms and 200 ms, respectively.

Conclusions Three-dimensional thin-film solid-state batteries hold promise for high capacity and fast-charging microstorage. A major bottleneck in their realization and commercialization is the conformal coating of full battery stack, and scalable manufacturing tools. A design based on micropillar arrays is thought to reduce the technological challenge of conformal coating, since the open and regular structure facilitates deposition of consecutive layers. Our calculations show that theoretically, capacities can be increased multifold compared to planar thin-film batteries and



can even surpass that of coin cells. There is no maximum to the capacity that can be achieved, but the technological complexity will largely determine the limit. For example, a capacity ten times that of planar thin-film batteries (i.e. 1 mAh cm−2) is possible using an AR of 200 with an electrode thickness of 100 nm, or with an AR of 50 with electrode thickness of 370 nm. Additionally, since the film thickness affects the rate-performance, there is a clear trade-off between technological complexity (i.e. higher AR) and fast-charging capabilities (i.e. thinner films). At present, increasing the AR beyond 50 is challenging, but, for example, work on nanowire-based 3D batteries could enable this.33 For our simulations, assumptions were made about the volumetric capacities of the electrode films. The next step is to combine our models with experimental values of volumetric capacities as a function of electrode thickness and charging rate. A benefit of the 3D thin-film technology is that a clear scaling roadmap can be formulated, where AR can be increased for higher energy density, while film thickness can be decreased for higher rate-performance. The aspect of high-throughput manufacturing of 3D thin-film batteries was investigated for Cldoped am-TiO2 deposited by S-ALD. An analytical model developed for diffusion of precursors into micropillar arrays was used to calculate the minimal exposure dose needed for conformal coating. We calculated the minimal exposure dose needed when considering optimized pillar geometry. In the extreme case of a structure with an AR of 200 and a reaction with low reaction probability (𝛽𝛽 = 0.001), depositing a 100 nm film would require about 40 min. Although this cannot necessarily be considered fast, it does show that it is in a feasible range. The predicted exposure dose was tested experimentally, and it was found that our model predicted quite accurately the required dose (i.e. a 5 times higher exposure dose was needed than theoretically calculated). Non-conformal coating for exposure doses closer to the theoretical value were explained based on insufficient supply of precursor into the reactor itself. Although it is typically assumed that a perfect conformality is required for 3D thin-film batteries, it was shown that, as long as a constant film composition can be maintained, the conformality has negligible effect on the electrode performance. It is likely that the thickness dependent lithiationdelithiation kinetics somewhat balance the non-conformal and non-uniformity of the films.


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The next step is to test and validate optimized geometries, and determine what capacities can be practically achieved, especially under relevant C-rates. Depositing TiO2 on pillars with higher aspect ratios and smaller inter-pillar spacing will further validate S-ALD as a technique for highthroughput manufacturing. Future work will be needed for the deposition of solid-electrolytes and cathode films by S-ALD. These will have additional challenges, since lithium containing precursors are needed which often require more stringent deposition conditions (e.g. humidity control). Also, in the case of the solidelectrolyte layer, a high degree of conformality is required, as a small thickness decrease can lead to internal shorts and breakdown of the battery. Although many issues still must be solved to fabricate 3D thin-film batteries on a large-scale, the fast deposition rate and excellent electrode performance achieved with S-ALD shows it to be a powerful tool in the fabrication of electrodes for 3D thin-film batteries.

Associated Content Supporting information available: Comparison of planar thin-film (TFB) vs coin cell (CB) rechargeable batteries (Figure S1). Schematic of the three-electrode custom-made electrochemical cell used (Figure S2). Schematics of different pillar geometries (Figure S3) and analytical derivation of the theoretical capacity. Examples of the theoretical capacity for different pillar geometries (Figure S4). Additional SEM and EDX measurements (Figure S5 and S6). Analytical derivations to approximate the exposure dose for conformal deposition.

Acknowledgement S.M. gratefully acknowledges the support of a Ph.D. stipend from the Agency for Innovation by Science and Technology in Flanders (IWT).

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