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Amorphous silicon honeycombs as a binder/ carbon-free, thin-film Li-ion battery anode† Yu Zhao, Lele Peng, Yu Ding and Guihua Yu*

Received 9th July 2014, Accepted 1st September 2014 DOI: 10.1039/c4cc05303f www.rsc.org/chemcomm

Amorphous silicon thin films with honeycombed structures have been prepared using a self-assembled monolayer of polystyrene spheres as the template. The as-prepared thin films may serve as a good anode candidate for thin film Li-ion batteries. This approach can be extended to a wide range of coating materials and substrates with controlled periodic structures.

Thin-film energy storage devices such as capacitors and batteries are attractive for ‘‘massless’’ energy storage, which shows their potential to make a considerable difference to how we store and deliver energy in the future.1 Such devices show the ability to save weight and volume: they may use materials with high galvanometric/volumetric energy density to reduce the weight/ volume, and they may exhibit ultrathin features and thus do not require a specified space for installation. Thin film Li-ion batteries, which are similar to conventional Li-ion batteries but composed of nanometre- or micrometre-thick components, may offer such an opportunity by combining two conventionally independent sub-systems, for example, shaping a battery as the skin for portable electronic devices. Materials with a higher capacity than the ones currently being employed are required in consideration of their ultrathin nature. Silicon may be promising owing to its high theoretical capacity that is ten times higher than that of graphite currently being used in commercial Li-ion batteries.2 However, the huge volume change of silicon upon alloying–dealloying with Li results in pulverization, which is one of the most critical issues leading to capacity fading.3 Many strategies have been proposed to alleviate the consequent issues by volume change, including nanostructuring of silicon,4 making silicon composites,5 and applying efficient binders.6 These strategies make a further step towards the practical application of silicon for conventional Li-ion batteries. Nevertheless, the use of a conductive

Materials Science and Engineering Program and Department of Mechanical Engineering, The University of Texas at Austin, Austin, Texas 78712, USA. E-mail: [email protected] † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4cc05303f

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additive and binder would lower the galvanometric/volumetric fraction of silicon. In addition, it is hard to control the electrode thickness in nanometres or several micrometres for thin-film Li-ion battery applications. A conventional silicon thin-film electrode prepared via magnetron sputtering, chemical vapour deposition or thermal evaporation undergoes repeated elastic deformation upon cycling,7 which would result in the formation of cracks and detachment of the thin film if the substrate constraint is not strong enough to balance the elastic strain. An alternative strategy is to create interior voids, which should possess a higher resistance to mechanical failure compared to its bulk counterpart, and facilitate strain/stress release within silicon anodes during repeated alloying–dealloying processes.8 Herein, we report a binder- and carbon-free Li-ion battery anode based on amorphous silicon (a-Si) thin film with a honeycomb structure. The honeycomb structure was achieved on a self-assembled monolayer (SAM) of a polystyrene sphere (PS) template, on which a-Si layer was deposited in a controlled manner. The interconnected interior voids created by the PS SAM template maintained the integrity of the film thus facilitating strain/stress release. In addition, the as-prepared a-Si honeycombs show good adhesivity to the applied substrates, and thus could be directly made into a thin film electrode as a Li-ion battery anode without additional binders and conductive additives. Moreover, due to the flexible honeycomb geometry and thin walls of individual cells, which facilitate in turn the strain/stress release and Li+ ion diffusion, the a-Si honeycomb electrode showed a high reversible capacity of 1700 mA h g 1 and good rate capability. The fabrication procedure uses a self-assembly technique9 and is briefly summarized in Fig. 1 (more details can be found in the ESI†). Firstly, a flat substrate was pre-positioned beneath the water surface and then a modified self-assembly technique was used to fabricate the PS SAM in the water/air interface (Fig. 1a). The water was slowly drained until the PS SAM was deposited onto the flat substrate prepositioned beneath the water (Fig. 1b). After it was completely covered with PS SAM, the substrate was allowed to dry under ambient conditions, which would result in an intact PS SAM (Fig. 1c). Then thermal annealing of the PS SAM-coated substrate was carried out,

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Fig. 1 Schematic illustration of the fabrication procedure of a-Si honeycombs. (a) PS SAM formed in the water/air interface. (b) PS SAM deposited on a flat substrate. (c) Intact PS SAM-coated substrate formed after drying. (d) Dehydrated PS SAM formed after thermal treatment. (e) a-Si deposited onto PS SAM to form a-Si microspheroid arrays. (f) a-Si film peeled off from the substrate by PS SAM dissolution in chloroform to form free-standing a-Si honeycombs.

during which the PS shares dehydrated in situ, and formed nanometre-scale gaps to facilitate a-Si deposition (Fig. 1d). Afterwards, the substrate was loaded into a radio-frequency magnetron sputtering system for a-Si deposition (Fig. 1e). By controlling the deposition time, microspheroid arrays with a well-controlled aspect ratio could be obtained. Finally, the substrate with microspheroid arrays was immersed in chloroform so that the microspheroid array would peel off from the substrate as a result of PS SAM dissolution to form a-Si honeycombs (Fig. 1f ), which could be further transferred onto Cu foil with good adhesivity. Fig. 2 represents the morphology evolution of PS SAM to a-Si honeycombs. Fig. 2a shows the scanning electron microscopy (SEM) image of a segment of PS SAM, in which a defect-free hexagonal close packing of PS can be observed. The PS dehydrated in situ to yield an B20% reduction of diameter from B980 nm to B780 nm upon thermal annealing (Fig. S1, ESI†). After magnetron sputtering a thin layer of a-Si, the microspheroid arrays were formed as shown in Fig. 2b and Fig. S2 (ESI†). The a-Si microspheroid arrays inherited the hexagonal close packing and integrity from the PS SAM. The magnified SEM image (inset of Fig. 2b) shows the structural details of the microspheroids revealing a rough surface and an aspect ratio of approximately 2 : 1. Each microspheroid contained a PS sphere inside its lower half part, and the thickness of the upper part could be controlled by sputtering time (Fig. S3, ESI†). Consequently, the microspheroid arrays possessed hollow interiors, which are clearly observable after removal of the host PS SAM and pattern transfer to form honeycombed structures as shown in Fig. 2c and d. The average outer and inner diameter corresponded well with that of PS before and after thermal annealing, respectively.

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Fig. 2 Morphological transformation from PS SAM to a-Si honeycombs. (a) SEM image of the as-prepared PS SAM. (b) SEM image (451 tilt view) of a-Si microspheroid array formed on dehydrated PS SAM. Inset is the corresponding cross-sectional view. (c) Low- and (d) high-magnification (451 tilt view) SEM images of the free-standing a-Si honeycombs after PS SAM removal.

The cell wall thickness was B200 nm, which would contribute to fast Li+ ion diffusion. A distinct feature of this method relies on the fact that the formation of PS SAM and subsequent targeted material deposition is applicable to wide ranging permutations of insulating and conducting flat surfaces regardless of a hydrophilic or hydrophobic surface of the supporting substrates, indicating the general compatibility, easy manipulation, and universality of this manufacturing process. Further composition analysis of the a-Si honeycombs is carried out using Raman and X-ray photoelectron spectroscopy (XPS). The Raman spectrum of the a-Si honeycombs is shown in Fig. 3a, in which a prominent band (400–550 cm 1) and a shoulder toward the low energy tail (350–400 cm 1) were observed. The 400–550 cm 1 band could be attributed to the first-order scattering of vibrational transverse optical phonon modes of a-Si.10 The characteristic Raman band of crystalline Si located around 540 cm 1, which arises from the first-order Raman scattering of the longitudinal optical and the transverse optical phonon modes degenerated at the Brillouin zone centre in crystalline Si, was not observed, suggesting that the as-prepared honeycombs were mainly composed of amorphous silicon. The surface composition of the a-Si honeycombs was revealed by XPS as shown in Fig. 3b. The binding energy of around 99.5 eV corresponds to the overlapped Si2p3/2 and Si2p1/2 of Si–Si bonds in silicon,11 while the peaks at 102.5 eV correspond to the surface SiOx component.12 According to the random-bonding model of the structure of Si–(Six–O4 x)x (0 o x o 4),13 the Si2p spectra could be described by a superposition of five-component (Si0, Si+, Si2+, Si3+ and Si4+) peaks, corresponding to Si atoms with zero, one, two, three, and four Si–Si bonds replaced by Si–O bonds, respectively. These results suggest that the surface of the a-Si honeycombs was covered by a thin layer of SiOx. It has been reported that the existence of an outer SiOx layer would offer a favourable mechanical behaviour,14 because the yield strength of lithiated SiOx (Li2Si2O5,

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Fig. 3 Composition analysis of the a-Si honeycombs. (a) Raman spectra revealing the amorphous nature of as-deposited silicon. (b) Core level XPS spectra of Si2p. The black and yellow curves stand for the as-obtained and simulated curves, respectively. The red and blue regions correspond to the simulated Si and SiOx contribution, respectively. The grey curve stands for the base line.

2.52 GPa) was over 40 times higher than that of lithiated a-Si (Li22Si5, 60 MPa),15 and the a-Si inside might be prevented from outward expansion upon the alloying process. To investigate the electrochemical performance of a-Si honeycombs, three-electrode H-type cells were prepared with a-Si honeycombs as the working electrode and two pieces of Li metal foil as the counter and reference electrodes. Fig. 4a shows the galvanostatic charge–discharge characteristics of the a-Si honeycomb anode at 0.42 A g 1 in the potential range of 0.05–1 V vs. Li+/Li. In the first cycle, the discharge and charge capacities were 3056 and 2507 mA h g 1, respectively. The formation of an irreversible solid-electrolyte-interphase layer together with incomplete delithiation led to a relatively low Coulombic efficiency of B82% for the initial cycle. Flat plateaus, characteristic of the reaction of Li with a-Si,16 were observed at B0.2 V vs. Li+/Li during discharge and B0.4 V vs. Li+/Li during charge, consistent with the cyclic voltammetry profiles (Fig. S4, ESI†). The SEI was stabilized after the first a few charge–discharge cycles, i.e. in the 5th cycle, as suggested by the identical capacities of charge and discharge. The capacity retention and the corresponding Coulombic efficiency of the a-Si honeycombs are plotted as a function of cycle number as shown in Fig. 4b. Compared with the planar a-Si film, which was directly sputtered onto Cu foil with a thickness of B1.5 mm, the a-Si honeycomb electrode showed better performance with a reversible capacity of B1730 mA h g 1 after 200 cycles. The corresponding Coulombic efficiency was B99% of the whole range except for the first few cycles. To further evaluate the rate capability of the a-Si honeycomb electrode, charge–discharge cycling tests were performed under various current densities of up to 8.4 A g 1 (2 C) as summarized in Fig. 4c. When the current density increased 20-fold from 0.42 A g 1 to 8.4 A g 1, the discharge capacity retention was B60%, about 1480 mA h g 1, which is B4 times greater than the

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Fig. 4 Electrochemical performance of the a-Si honeycombs. (a) Charge– discharge profiles of a-Si honeycombs/Li half-cell cycled at a current density of 0.42 A g 1 (0.1 C). The cut-off voltage is set to 5 mV during discharge and 1 V during charge. (b) Cycling performance and corresponding Coulombic efficiency (CE) at a current density of 0.42 A g 1. Electrode made from a planar a-Si film (B1.5 mm thick) directly sputtered on Cu foil was tested for comparison. (c) Rate performance in a broad range of current density. The cell was firstly galvanostatic charge–discharged for 20 cycles before the rate capability test.

theoretical capacity of a graphite anode (372 mA h g 1). The demonstrated electrochemical performance of a-Si honeycombs is superior to those of a-Si thin films prepared by radio-frequency magnetron sputtering,17a silicon–carbon thin films with a macroporous inverse-opal structure,17b an Si–Cu thin film electrode with a Kirkendall voids structure,17c Si1 xGex thin films with a nanocolumnar-like structure,17d and a-Si thin films on a three dimensional nanopillar Cu electrode.17e The method of using a colloidal template for structural control has proven very successful for producing inorganic solids with ordered structures at different length scales.18 Such structures are important both for a fundamental study of structure–property relationships and for technological applications. In this study, a-Si honeycombed structure could be synthesized, in principle, on PS SAM with a controlled cell size, cell wall thickness and aspect ratio. As a demonstration, the a-Si honeycombs could serve as a binder- and carbon-free Li-ion battery anode aiming at becoming thin-film energy materials with a potential for use in future ‘‘massless’’ energy storage devices. The a-Si honeycombs delivered areal energy density in excess of 4000 mA h m 2 (estimated with a specific capacity of 1700 mA h g 1 and an areal mass loading of B250 mg cm 2). The honeycombed structure was effective in terms of strain/stress release of silicon during repeated

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alloying–dealloying processes, thus beneficial for maintaining the integrity of the electrode as is evident from the fact that the cycled a-Si honeycombs were not exfoliated from the current collector (Fig. S5a, ESI†), nor did they pulverize into small particles (Fig. S5b, ESI†). In contrast, planar a-Si film directly deposited on Cu foil showed serious pulverization upon cycling (Fig. S6, ESI†). It is necessary to mention that using this method we should be able to prepare a variety of functional materials with a similar structure that may find their corresponding applications.19 At present, this approach yields thin-films in the centimetre scale (Fig. S7, ESI†). Future challenges besides scale-up preparation remain in composition/crystallography optimization, interface engineering, device fabrication/integration, and manufacturing cost reduction. In conclusion, a free-standing a-Si thin film with a honeycomb structure has been synthesized using PS SAM as the template. Such highly ordered micro/nanostructures could have broad applications. As an example, the as-synthesized a-Si honeycombs show good electrochemical performance in that a reversible capacity of over 1700 mA h g 1 can be achieved as an anode for Li-ion batteries. This method may be compatible with other electrode materials with a similar structure that would potentially benefit the design and fabrication of thin-film Li-ion battery electrodes with structural integrity and flexibility. This work is supported by the faculty start-up grant from the University of Texas at Austin and the Welch Foundation grant (F-1861).

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carbon-free, thin-film Li-ion battery anode.

Amorphous silicon thin films with honeycombed structures have been prepared using a self-assembled monolayer of polystyrene spheres as the template. T...
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