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Scalable fabrication of nanostructured tin-oxide anodes for high-energy lithium-ion batteries Christian Heubner, Tobias Liebmann, Karsten Voigt, Mathias Weiser, Björn Matthey, Nils Junker, Christoph Laemmel, Michael Schneider, and Alexander Michaelis ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07981 • Publication Date (Web): 20 Jul 2018 Downloaded from http://pubs.acs.org on July 23, 2018

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ACS Applied Materials & Interfaces

Scalable Fabrication of Nanostructured Tin-Oxide Anodes for High-Energy Lithium-Ion Batteries Christian Heubner,*,† Tobias Liebmann,‡ Karsten Voigt,‡ Mathias Weiser,‡ Björn Matthey,‡ Nils Junker,† Christoph Lämmel,‡ Michael Schneider,‡ Alexander Michaelis †,‡ †

Institute of Materials Science, TU Dresden, 01062 Dresden, Germany Fraunhofer IKTS, Fraunhofer Institute for Ceramic Technologies and Systems, 01277 Dresden, Germany

‡

Abstract Although tin and tin-oxides have been considered very promising anode materials for future high-energy lithium-ion batteries due to high theoretical capacity and low cost, the development of commercial anodes falls short of expectations. This is due to several challenging issues related to a massive volume expansion during operation. Nanostructured electrodes can accommodate the volume expansion but typically suffer from cumbersome synthesis routes and associated problems regarding scalability and cost efficiency, preventing their commercialization. Herein, a facile, easily scalable and highly cost-efficient fabrication route is proposed based on electroplating and subsequent electrolytic oxidation of tin, resulting in additive-free tin-oxide anodes for lithium-ion batteries. The electrodes prepared accordingly exhibit excellent performance in terms of gravimetric and volumetric capacity as well as promising cycle life and rate capability, making them suitable for future high-energy lithium-ion batteries.

Keywords: lithium battery; high-energy; anode material; tin-oxide; electrode preparation ACS Paragon Plus Environment

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INTRODUCTION In the last two decades, lithium-ion batteries (LIBs) have turned into the leading energy storage devices for portable electronics, and are very promising candidates for large-scale applications such as all-electric or hybrid-electric vehicles 1–3. Current state-of-the-art anodes for LIBs are porous composites consisting of active material, binder and conductive additives coated on a metallic current collector. The anodes are commonly prepared via the following steps: 1) materials synthesis, 2) powder processing 3) slurry preparation, 4) coating, 5) drying, 6) calendaring. Afterwards, the anode foils are cut and then assembled with separator and cathode foils to form a LIB. Besides the rather limited specific capacities of state-of-the-art anodes, such as graphite with a theoretical capacity of 372 mAh g-1, significant improvement of the conventional electrode fabrication is needed regarding energy efficiency and costs as well as health and environmental aspects. Tin and tin-oxides have recently attracted intense attention as possible anode materials for next generation LIBs since these materials offer both a low electrode potential vs. Li/Li+ (0 - 1 V) and high gravimetric capacities (e.g. Sn ~1000 mAh g-1) 4–6. Furthermore, the materials are highly abundant, inexpensive and environmentally benign. However, a dramatic volume expansion during the lithiation of tin and tin-oxides causes several challenging issues related to mechanical stress and degradation, complicating the practical implementation and commercialization as anode materials in LIBs 7,8. Nevertheless, due to the very promising properties of tin-based active materials, tremendous efforts have been made to tackle these critical issues. Recent attempts include the nanostructuring of the electrodes and the application of buffer matrices to accommodate the volume expansion 9–15. Tin-based nanomaterials, such as 0-D nanoparticles 16,17, 1-D nanowires, nanorods and nanotubes 6,18,19, and 2-D nanosheets, nanobelts and nanofilms 20–22, have been proven to effectively accommodate the volume changes upon cycling and to improve the effective transport properties by shortening the lithium diffusion pathways 23–25. However, most of ACS Paragon Plus Environment

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these nanomaterials are limited in terms of scalability due to harsh experimental conditions with respect to toxic chemicals, high temperatures, impurities, etc. 5,6,26. Moreover, the gravimetric capacity related to the mass of these nanomaterials might be excellent but the energy density at the cell level (including current collectors, separator, electrolyte, and additives) is often insufficient since the mass ratio between active and inactive materials is quite unfavorable (e.g. area specific capacities < 1 mAh cm-2). Furthermore, despite the superior electrochemical properties of the nanostructures, their fabrication is not cost-effective and thus not applicable at an industrial level. For example, battery-grade graphite, which is the most commonly used anode material sells for 10 - 15 $/kg (3 – 4 cent/Ah at the material level) 27,28. This benchmark should be considered when claims on direct application of new materials are made 27. Many of the reported nanostructured materials will fail this benchmark and possible advantages in performance might be insufficient to compensate for this. Therefore, in order to move from an experimental proof-of-concept device towards practical anodes for LIBs, research groups are seeking for alternative fabrication routes 5,29–31. Herein, we demonstrate a facile, easily scalable and highly cost-efficient route to fabricate nanostructured tin-oxide anodes for high-energy LIBs based on electrodeposition and subsequent electrolytic oxidation of tin. Figure 1 schematically shows the proposed fabrication route. First, tin is electrochemically deposited on a suitable substrate, e.g. a copper foil as typical current collector material. Afterwards, the tin-layer is converted into tin-oxide by electrolytic oxidation in an acidic electrolyte. As prepared electrodes can be used directly as anodes in LIBs. Obviously, such a straightforward process is much faster, more costefficient and better scalable in comparison to the conventional coating technologies currently used to fabricate graphite-based anodes for LIBs, while providing the advantages of nanostructured high-energy materials.

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Figure 1. Schematic illustration of the fabrication of tin-oxide anodes for lithium-ion batteries by electrodeposition and subsequent electrolytic oxidation of tin.

MATERIALS AND METHODS Electrodeposition: Copper substrates (99.9 wt%, 0.25 mm thickness) were cleaned as follows: 5 min alkaline bath (20 g L-1 NaOH, 1 g L-1 sodium dodecyl sulfate, 60°C), 2 min acidic bath (7.5 wt% H2SO4, 25 °C), rinsed in deionized water and dried under compressed air. Potentiostatic electrodeposition (-0.8 V vs Ag/AgCl) was carried out in a tin electrolyte (0.1 M tin(II) methanesulfonate, 1 M methanesulfonic acid, 10 g L-1 hydroquinon, 10 g L-1 Triton™ X-100, 25 °C). The current efficiency of the tin deposition was determined by comparing the mass change of the sample with the theoretical mass change computed from the consumed electric charge by using Faraday’s law. The growth rate of the tin-layer was estimated from the average thickness, based on Faraday’s law, and the deposition time. Electrolytic oxidation: Electrodeposited tin-electrodes were immersed in a 0.3 M oxalic acid electrolyte (25 °C). Electrolytic oxidation was carried out by applying a constant voltage of 7.0 V vs Ag/AgCl. The process was stopped when the current starts to decrease, which indicates that the reaction zone reaches the copper substrate, forming a passive layer by the precipitation of copper oxalate 32. Afterwards, the electrodes were rinsed with deionized water and dried at 80 °C under vacuum. Electrochemical Characterization: As prepared electrodes were punched into circular sheets with 12 mm in diameter. T-type Swagelok® cells were assembled in an argon filled glove box ACS Paragon Plus Environment

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(< 5 ppm H2O, 10 ppm O2). The counter and the reference electrodes were metallic lithium (99.9% trace metal basis ALDRICH Chemistry). A borosilicate glass-microfiber filter (CAT No. 5401-090E, Whatman) was used as the separator. The electrolyte was high purity 1 M LiPF6 solved in a 1:1 wt% ratio mixture of ethylene carbonate and dimethyl carbonate (ALDRICH Chemistry). Galvanostatic charge-discharge measurements and electrochemical impedance spectroscopy (f-range: 200 kHz – 100 mHz, P2P-amplitude: 10 mV around OCP) were carried out using a multi-channel potentiostat /galvanostat (VMP3, Biologic). Materials diagnostics: The electrodes were carefully examined by scanning electron microscopy (CrossBeam NVision 40®, Carl Zeiss SMT AG). The analysis of the elemental composition was carried out using an XMax detector in combination with the Aztec software (Ltd. Oxford Instruments). The calculation of the EDS phase maps is based on the convolution of the elemental distribution maps of the single elements using the Aztec software. XRD analysis was performed on a 3003TT (Ltd., GE Sensing & Inspection technologies) using CuKα radiation and a Meteor1D position sensitive detector.

RESULTS Figure 2 depicts the results of the electrochemical tin deposition on a copper foil, carried out from a methane sulfonic acid electrolyte. Tin deposition starts at approximately -0.45 V, at about -1.0 V, the hydrogen evolution begins to superimpose the tin deposition (Figure 2a). Moreover, the stationary potential-current density plot shows the typical kinetic regions including charge transfer control at low overpotentials (-0.4 to -0.5 V) and diffusion controlled electrodeposition at large overpotentials (-0.8 to -1.1 V) with a mixed controlled region in between (-0.5 to -0.8 V). In a potential range of -0.6 to -0.9 V, the deposition process exhibits excellent efficiencies close to 100 % and large growth rates of the tin-layer close to 2 µm min-1 (Figure 2b). Figure 2c shows a typical current density course during the



potentiostatic tin deposition at -0.8 V. After a short transient response, the current density becomes constant, indicating a continuous growth of the tin-layer.

Figure 2. Results of the electrochemical tin deposition on a copper substrate, carried out from a methane sulfonic acid electrolyte at room temperature. a) Stationary currentpotential relationship and b) potential dependence of current efficiency and growth rate. c) Current density course during potentiostatic electrodeposition at -0.8 V and schematic illustration of the layer growth.

Cross sectional SEM-micrographs of tin-layers deposited at -0.8 V show large grains in a columnar arrangement and an intermetallic compound layer separating the Cu substrate and the Sn layer (Figure 3). According to the literature, the magnitude of the deposition potential and the different crystallographic structures of Cu and Sn suggest a Volmer-Weber nucleation and growth mechanism 33–35. This commonly results in the formation of tin dendrites, which is, however, suppressed here by using a commercial surfactant (Triton™ X-100)34,36. From ACS Paragon Plus Environment

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Figure 3 can be seen that the morphology of the deposited tin-layer corresponds to the field orientated growing type 37, which is expected under diffusion controlled deposition conditions. The tin surface is not smooth but exhibits a texture related to the large grains grown during the deposition process.

Figure 3. a-b) Top view and c-d) cross-sectional SEM micrographs of the deposited tinlayer. (Please note the different magnifications, a), b) and c) SE-image, d) BSE-image).

Figure 4 illustrates the results of the electrolytic oxidation of the electrodeposited Sn-layers. The course of the current density during electrolytic oxidation (Figure 4a) is in good agreement to reports from the literature 38–41. At first, a distinct increase of the current density is observed, indicating rapid dissolution of tin. Afterwards, the current density decreases, culminating in a local minimum. According to Zaraska et al., this behavior is attributed to the formation of a passive tin-oxide layer on the metal surface 38. The subsequent increase of the current density is related to the local dissolution of the oxide layer and afresh conversion of tin into tin-oxide. After passing a local maximum, the current density tends to zero. This



indicates that the reaction zone reaches the copper substrate, forming a passive layer by the precipitation of copper oxalate 32. It should be noted that the well-pronounced features of current vs time behavior shown in Figure 4a are well suited for a targeted process control.

Figure 4. Results of the electrolytic oxidation process of electrodeposited tin-layers. a) Typical course of the current density during electrolytic oxidation and schematic illustration of the conversion into a nanostructured tin-oxide layer. SEM micrographs of b), d) the deposited tin and c), e) the nanostructured tin-oxide in different magnifications. f) Photographs of samples at different stages of the process: left) copper substrate, centered) after electrodeposition of tin, and right) after electrolytic oxidation. ACS Paragon Plus Environment

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Figure 5. a) – c) Cross-sectional SEM micrographs of the tin-oxide layer in different magnifications and d) statistical analysis of the pore diameter and the wall thickness.

After electrolytic oxidation, the electrodes exhibit a dark grey color (Figure 4f). SEMmicrographs of the initial tin-layer and the resulting tin-oxide layer are shown in the Figs. 4b – 4e. The large tin crystals obtained from electrodeposition are converted into nanostructured tin-oxide by the electrolytic oxidation process. Irregular pore structures are observed with pore diameters ranging from 30 - 70 nm and wall thicknesses < 10 nm (Figure 5). The porosity of the tin-oxide is estimated from a computer aided image analysis of the SEM micrographs to approximately 55 %. This relatively large porosity benefits the electrode performance since it provides sufficient void space for the volume changes of Sn/Sn-oxide during lithiation and delithiation. Cross-sectional EDS mapping confirms the conversion of tin into tin-oxide (Figure 6a). Furthermore, residual tin and a thin copper- and tin-rich layer are observed at the copper/tinoxide interface. These observations are supported by XRD phase analysis of the sample ACS Paragon Plus Environment


(Figure 6b). Clear reflections related to the copper substrate and the residual tin are found. Furthermore, the measurements indicate the existence of minor phases of copper oxalate and an intermetallic compound layer, Cu6Sn5 34. Obviously, the electrolytic oxidation of the tinlayer results in an amorphous tin-oxide, since no crystalline tin-oxide is observed except the occurrence of an elevated background between 25 and 37 °2ߠ.

Figure 6. a) Results of cross-sectional EDS mapping and b) XRD pattern of the tin-oxide electrode. Figure 7 shows the results of the electrochemical characterization of as prepared tin-oxide electrodes in lithium half-cells. A sloping potential curve with several plateaus is observed in the first cycle, suggesting the following main reactions: 1) At potentials lower than 2 V vs Li/Li+, the electrolyte starts to decompose, forming a solid-electrolyte-interphase (SEI) layer


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42,43

. 2) Lithium insertion in tin-oxide results in the reduction to metallic tin via the conversion

reaction 44: 2xLi+ + 2xe- + SnOx ↔ Sn + xLi2O

(1)

3) Afterwards, further lithium insertion takes place via the alloying reaction 44: yLi+ + ye- + Sn ↔ LiySn

(2)

Figure 7. Results of the electrochemical characterization of tin-oxide electrodes in Li-half-



cells. a) Voltage profiles and b) Impedance spectra recorded at different cycles including a simplified interpretational approach (Rel – electrolyte resistance, Rct – charge transfer resistance, CPEct – constant phase element representing the double layer capacitance, WD – Warburg element representing solid-state diffusion), c) cycle life test at the 1.0 C rate and d), e) rate capability test ranging from 0.1 C to 10 C.

The relatively large irreversible capacity in the first cycle is attributed to both, the SEI formation and the partial irreversibility of the conversion reaction. Cycling in a range of 0 – 3 V yields a large initial capacity of approximately 1150 mAh g-1. This indicates that the conversion reaction is partially reversible in the nanostructured electrodes, since the theoretical capacity of the alloying reaction only amounts to 873 (786) mAh g-1, if the mass of SnO (SnO2) is taken into account. The partial reversibility of the conversion reaction (Eq. 1) becomes also notable in the shape of the charge/discharge curves. During the first lithiation, a distinct plateau is observed at approximately 1.2 V, related to the conversion of Sn-oxide and Li into Sn and Li2O 45. During delithiation, a ‘blurred’ plateau in a narrow stoichiometric range emerges at potentials > 1.2 V, indicating the partial reverse reaction of Sn and Li2O to Sn-oxide and Li. In the subsequent cycles, these ‘blurred’ plateaus at potentials > 1.2 V continuously shrink and a rapid capacity fade is observed when cycling the electrodes between 0 – 3 V (Figure 7c). Cycling in a range of 0 – 1 V yields an initial capacity of 815 mAh g-1, which corresponds to 93% (104%) of the theoretical capacity of the alloying reaction, if the mass of SnO (SnO2) is taken into account. In this case, an excellent cycling stability is achieved (Figure 7c), yielding a stable capacity of approximately 600 mAh g-1 after 200 cycles and coulombic efficiencies close to 100%. Electrochemical impedance spectroscopy is performed to analyze the electrode polarization behavior. The results shown in Figure 7b indicate a depressed semicircle in the mid frequency region, typically attributed to the charge transfer at the solid/electrolyte interface, followed by a ‘diffusion branch’, ACS Paragon Plus Environment

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typically attributed to Li-diffusion in the solid 46,47. It has to be noted that the EIS spectra shown in Figure 7b are measured in a limited frequency range of 200 kHz – 100 mHz. Therefore, possible slower processes are not captured and the Li-storage behavior of the fabricated Sn-oxide electrodes might be more complex than indicated by the simplified equivalent circuit model shown in Figure 7b 48,49. The impedance of the electrodes just slightly increases with the number of cycles, indicating both (electro)chemical and mechanical integrity of the nanostructured tin-oxide. It should be underlined that both the charge transfer as well as the diffusion limitations of the tin-oxide electrodes are extraordinarily low, compared to state-of-the-art graphite electrodes, while achieving similar area specific capacity and much higher gravimetric and volumetric capacity (Figure 8). We assume that these beneficial properties are related to the morphology of the tin-oxide electrodes, providing a large surface area and ultra-short diffusion pathways (cf. Figure 5). These features might also be responsible for the excellent rate performance (Figure 7d and e), allowing charging and discharging of 50 % of the nominal capacity in only 12 min (5 C).

DISCUSSION As mentioned above, the area specific capacity is one of the key factors to push LIBs towards large-scale applications such as hybrid- and all-electric vehicles. The area specific capacity of the tin-oxide electrodes can be adjusted easily by the proposed fabrication route via the deposited amount of tin. For instance, tin-oxide layers of 4 and 9 µm yield approximately 1.5 and 2.8 mAh cm-2, which is clearly superior to the optimal area specific capacity that can be obtained from graphite based electrodes of similar thickness (cf. Figure 8).



Figure 8. Comparison of tin-oxide electrodes prepared by electrolytic oxidation and commercial graphite anodes with similar area specific capacity: a) impedance and b) thickness dependence of the areal capacity.

It should be noted that comparable approaches of fabricating nanostructured tin-oxide by electrolytic oxidation were previously reported in literature. Passerini and co-workers prepared sponge-like tin-oxide films with promising electrochemical properties by electrolytic oxidation of tin-layers, followed by carbon coating and thermal annealing 50. Ortiz et al. reported the synthesis of tin-oxide powder using electrolytic oxidation of bulk tin-foils in an electrolyte bath based on polyethylenglycol and oxalic acid 51. Electrodes prepared by a


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conventional slurry coating process exhibit excellent specific capacities and good cycling stability. Compared to these attempts of fabricating nanostructured tin-oxide electrodes by electrolytic oxidation 50,51, the presented approach is much more straightforward und potentially more cost efficient while achieving significantly improved electrode properties in terms of gravimetric and areal capacity, cycle life and rate performance. The main differences between previous attempts 50,51 and the presented results are:

•

The morphology and the microstructure of the deposited tin-layer.

•

The existence of an intermetallic compound layer between the copper substrate and the nanostructured tin-oxide.

•

The experimental conditions (electrolyte compositions, current-voltage regimes) during electrodeposition and electrolytic oxidation.

The morphology and the microstructure of the deposited tin-layer can be adjusted by the deposition parameters (current, voltage, pulse-regime, etc.) 37,52,53. The tin-layers deposited under diffusion controlled conditions (-0.8 V) exhibit large grains in a columnar arrangement according to the field orientated growing type (cf. Figure 3). The intermetallic compound layer (ICL) enhances the adhesion of the nanostructured tin-oxide to the copper substrate, resulting in significantly improved mechanical stability and low contact resistance. In general, it is assumed that bulk ICLs, consisting of the deposited metal and the substrate metal, are formed in the under potential deposition range instead of solely a monatomic layer, if the components can form solid solutions 33. The formation and growth of deposited metal– substrate metal alloys is also expected in systems forming intermetallic phases between substrate and depositing metals 54, as observed for several combinations 54–57. According to the existing models, the formation and growth of the ICL occurs via the following steps 58,59: 1) tin atoms are deposited at the copper surface. 2) The growth of the ICL is initiated by a site



exchanges between copper atoms and vacancy sites within a relatively mobile tin-layer. 3) The ICL grows, controlled by the mutual diffusion of tin and copper through the vacancy-rich ICL. 4) The successive deposition of tin occurs at the surface of the ICL. Surprisingly, the thickness of the ICL varies between 100 and 500 nm (cf. Figs. 3 and 6). Thereby, the ICL is thickest close to the grain boundaries of the deposited tin-layer, indicating fast grain boundary diffusion of Cu. Consequently, the microstructure of the deposited tin-layer is crucial for the formation and growth of the ICL. The formation of the tin-oxide is accompanied with a vigorous oxygen evolution during electrolytic oxidation, especially enhanced under high voltages, which disturbs a stable growth of the tin-oxide 60,61. Lee et al. suggested that the current efficiency ratio of oxygen evolution to tin oxide formation is the key factor responsible for internal crack development 61

. However, a certain macroporosity might be desirable to avoid significant electrolyte

depletion during fast discharging 62, since it can ensure improved electrolyte transport through the nanostructured oxide film. Consequently, the adjustment of the electrolytic oxidation conditions (electrolyte composition, current-voltage regime) is a key factor to achieve optimal electrochemical properties of the resulting tin-oxide electrode. In comparison with the previous reports 50,51, the lower voltage (7.0 V vs 10.0 V) used in this study during the electrolytic oxidation should result in reduced oxygen evolution. This might lead to a more favorable electrode structure (e.g. ratio of macro- to nanoporosity) with respect to the electrochemical properties and mechanical stability. Finally, the experimental conditions of electrodeposition and electrolytic oxidation lead to significantly improved rate and cycle life performance, when compared to previous attempts 50,51, even without additional process steps, such as carbon coating and thermal annealing. However, it has to be noted that we obtained the best results when cycling the electrodes in a potential range of 0 – 1 V vs Li/Li+, which excludes the conversion reaction (cf. Eq. 1). Thus, in this case, any limitations arising from the low conductivity of Sn-oxide do not affect the electrode performance. For cycling a in a ACS Paragon Plus Environment

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range of 0 – 3 V vs Li/Li+ the rate capability could be significantly reduced due to the formation of tin-oxide at high potentials. In this case, the usage of conductive additives might be inevitable. We want to underline, that both, the electrodeposition as well as the electrolytic oxidation are very fast, robust and easy to handle processes. For instance, the electrodeposition of a 20 µm tin-layer and subsequent electrolytic oxidation last approximately 10 and 5 min, respectively. Thereby, the process time is independent from the surface area of the substrate. Thus, in this example, the preparation of arbitrarily large electrode sheets with an area specific capacity > 3 mAh cm-2 lasts approximately 15 min. In our laboratory scale, the costs of such a tin-oxide anode are roughly estimated to 5 $/m2 (15 cent/Ah), including costs for materials, consumables and energy consumption. Any other costs, such as for equipment and personnel expenses are not considered in this calculation. By comparison, the laboratory scale fabrication of a comparable graphite based anode, with ready-to-use active material and without drying, lasts approximately 4 h, including the mixing of components, ball milling, vacuum mixing, coating, and calendaring. The costs of graphite based anodes for LIBs, fabricated at an industrial level, amount to approximately 4 $/m2 (13 cent/Ah) 63. Thus, in this rough estimate, the costs for the laboratory scale tin-oxide electrodes are almost identical to conventional graphite electrodes while providing significantly higher gravimetric and volumetric energy density and improved rate capability due to the nanostructuring. Another advantage of the proposed fabrication route is that both, electrodeposition 64,65 and electrolytic oxidation 66–68 can be carried out continuously wet-onwet using roll-to-roll processes. Furthermore, the presented fabrication approach is also applicable to 3D structured substrates (e.g. foam-like 69–71 or textile based 72 current collectors), which are not suitable for conventional tape casting processes. Due to the very good electrical conductivity, which is inherent to electrodeposited films, and the mechanical



integrity/flexibility of the nanostructured tin-oxide, a calendaring of the electrodes is not necessary. Therefore, the proposed fabrication route includes the benefits of nanostructured electrodes (fast kinetics, accommodation of volume changes, mechanical flexibility, etc.) while being potentially more cost-effective (materials and production costs) and environmentally benign (no toxic or environmentally harmful components) than conventional coating processes. However, the most important feature is that, in contrast to the most other synthesis routes for high capacity nanomaterials, electrodeposition and electrolytic oxidation are easily scalable to high-volume manufacturing. Since the presented fabrication route allows targeted nano- and microstructuring of the tinoxide 41, our current work includes the optimization of the plating conditions and the electrolytic oxidation procedure to further improve the cycle life performance. A further important task is to reduce the initial capacity loss, e.g. by prelithiation 73 or the formation of an artificial SEI-film after the electrolytic oxidation process 74. Another interesting attempt is to explore suitable nanostructures that enable a high reversibility of the conversion reaction to expand the usable potential window and to obtain even higher specific capacity.

CONCLUSIONS Nanostructured tin-oxide electrodes prepared by electrodeposition and subsequent electrolytic oxidation of tin clearly show superior performance in comparison with state-of-the-art anodes for LIBs. The excellent performance results from the large surface area, the short lithium diffusion pathways and the mechanical integrity (flexibility) as well as from the redundancy of binder and conductive additives in the tin-oxide nanostructures formed by the electrolytic oxidation process. Moreover, the proposed fabrication route is potentially very cost-effective (materials and production costs), environmentally benign (no toxic or environmentally harmful components), and easily scalable to high-volume manufacturing.


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ACKNOWLEDGMENTS The authors are grateful to Maria Striegler and Mathias Herrmann (Fraunhofer IKTS Dresden) for the EDX measurements and fruitful discussions.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; Phone: +49 351 2553-7324

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