Multiscale Hyperporous Silicon Flake Anodes for High Initial Coulombic Efficiency and Cycle Stability Jaegeon Ryu, Dongki Hong, Myoungsoo Shin, and Soojin Park* Department of Energy Engineering, School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, South Korea S Supporting Information *
ABSTRACT: Three-dimensional (3D) hyperporous silicon flakes (HPSFs) are prepared via the chemical reduction of natural clay minerals bearing metal oxides. Natural clays generally have 2D flake-like structures with broad size distributions in the lateral dimension and varied thicknesses depending on the first processing condition from nature. They have repeating layers of silicate and metal oxides in various ratios. When the clay mineral is subjected to a reduction reaction, metal oxide layers can perform a negative catalyst for absorbing large amounts of exothermic heat from the reduction reaction of the silicate layers with metal reductant. Selectively etching out metal oxides shows a hyperporous nanoflake structure containing 100 nm macropores and meso-/micropores on its framework. The resultant HPSFs are demonstrated as anode materials for lithium-ion batteries. Compared to conventional micro-Si anodes, HPSFs exhibit exceptionally high initial Coulombic efficiency over 92%. Furthermore, HPSF anodes show outstanding cycling performance (reversible capacity of 1619 mAh g−1 at a rate of 0.5 C after 200 cycles, 95.2% retention) and rate performance (∼580 mAh g−1 at a rate of 10 C) owing to their distinctive structure. KEYWORDS: hyperporous silicon flakes, clay materials, lithium-ion battery anodes, high initial Coulombic efficiency nanowires,10,11 nanotubes,12,13 nanosheets,14−16 and 3D porous structures,17−20 have been extensively explored for advanced battery anodes. Along with distinctive features of nanostructured silicon materials, such as excellent mechanical durability, unusual electronic properties, and compatibility with existing technologies, their electrochemical performance (cycling stability and rate capability) must be significantly improved from past decades. Nevertheless, there was rare progress on the initial Coulombic efficiency, which is an essential part for practical applications, because nanostructured materials cannot exhibit high initial Coulombic efficiency due to its intrinsically larger surface resulting in irreversibility of initial capacity. The rational design of electrode materials involves retaining the whole structure on a micrometer scale but nanostructuring on its framework,5−7 which typically creates porosity as an effective strategy to overcome the disadvantages of conventional nanostructured materials.5−7 Porous structures generally provide large surface areas for specific reactions, promote
I
n order for lithium-ion batteries (LIBs) to meet the rigorous standards, development of new electrode materials superseding the graphite-based anodes has been addressed. Silicon delivers the highest gravimetric capacity (∼3579 mAh g−1 in a form of Li15Si4 at room temperature) among possible alloy-based anode materials and has multiple strengths over conventional anodes, such as abundance in nature and low working potential (300%) upon lithiation, which cause fracture, pulverization, contact loss with current collector, and eventually rapid capacity fading.5−7 Under the impact of severe swelling and shrinkage, continuous decomposition of organic electrolyte occurs, leading to irreversible formation of thicker solid-electrolyte interphase (SEI) layer. This incurred degradation of battery performance owing to electrically insulating nature of the thick SEI and longer diffusion length of lithium ion through it. Aforementioned issues, structural failure and surface instability of silicon anodes have been primarily mitigated by nanostructuring and surface engineering strategies, because nanostructures can endure the large mechanical stress from alloy reaction due to high surface-to-volume ratio. For example, diversely textured silicon materials, such as nanoparticles,8,9 © 2016 American Chemical Society
Received: October 10, 2016 Accepted: November 10, 2016 Published: November 10, 2016 10589
DOI: 10.1021/acsnano.6b06828 ACS Nano 2016, 10, 10589−10597
Article
www.acsnano.org
Article
ACS Nano
Figure 1. Schematic illustration of HPSF synthesized from layered talc clay minerals and morphology/phase characterization. (a) Schematic illustration of 3D hyperporous structure. Bowl-like cavity with 100 nm-sized uniform macropores and meso-/micropores are formed on the framework. SEM images of HPSF taken at (b) low and (c) high magnification. (d) XRD pattern of HPSF showing phase transition from natural clay to pure silicon.
reduction of silicates. The resultant 3D HPSF is demonstrated as anode materials for LIBs. Compared to reported micro-Si anodes, HPSF anodes exhibit remarkably high initial Coulombic efficiency (>92%), outstanding cycling performance (reversible capacity of 1619 mAh g−1 at a rate of 0.5 C after 200 cycles and 946 mAh g−1 at a rate of 1 C after 400 cycles), and rate performance (∼580 mAh g−1 at a rate of 10 C).
interfacial transport, and shorten the diffusion length or reduce the diffusion effect, making them efficient host materials to be integrated with and to stabilize other active materials.21,22 By extension, hierarchical porous materials have rendered specific solutions for energy conversion and storage systems.22,22 However, conventional approaches for synthesizing porous structures, such as the top-down method (electrochemical or metal-assisted etching),23,24 template-assisted method,25−27 infiltration,28,29 and magnesiothermic reduction, 30,31 are not practical. Meanwhile, more realistic production methods have recently been reported. For example, Liu et al. fabricated 3D porous hierarchical silicon architectures using reed leaves via purification and the chemical reduction method.19 Even if their structures are well retained as a finalized form, oversized particles (>10 μm) require further downsizing processes for wide applications, and they exhibit extremely low yield (∼4%). Furthermore, Ge et al. reported a scalable way of developing 3D porous silicon materials in high yield from inexpensive metallurgical silicon via the ball-milling and stainetching methods.20 However, the materials show partly uniform size distributions and still have metal impurities that can hinder further applications. Herein, we have successfully prepared 3D hyperporous silicon flakes (HPSFs) from natural clay via selective magnesiothermic reduction. This clay mineral has a single layer of magnesium oxides and hydroxide sandwiched by two silicate layers with a perfectly pure composition of Si/Mg/O. Above the melting point of magnesium (>650 °C), magnesium vapors can only react with silicate layers to convert them into pure silicon covered with magnesium oxide byproducts. After interjacent and covered magnesium oxides are etched out, there are many void spaces both inside and outside of the silicon materials. A large portion of magnesium oxide generates macropores over the whole flake structure along with meso-/ micropores on the framework attributed to the chemical
RESULTS AND DISCUSSION A crystal structure of talc clay minerals is depicted in Figure S1a (see Supporting Information, (SI)), which consist of 2:1 ratio of two different layers, silicate tetrahedral and magnesium oxide layers. It belongs to the so-called smectite group among the various classes of clay minerals.32 Talc clay has tightly stacked flake structure with 1 nm-thick single layer of phyllosilicates, where those regular laminating structures are sustained by interfacial repulsive forces between adjacent layers (see SI, Figure S1b−d). Because of it is natural sources, broad distribution in their size (0.5−5 μm) and thickness (10−50 nm) are unavoidable. One of the scalable production methods for silicon is the magnesiothermic reduction process, generally following reaction equation: SiO2 (s) + 2Mg (g) → Si (s) + 2MgO (s) (weight ratio of SiO2:Mg = 1:0.8). Likewise, the clay minerals are subjected to magnesiothermic reduction after considering stoichiometric ratio of reactants. The mixture of talc and Mg was loaded to a stainless steel reactor filled with inert Ar gas (see SI, Figure S2) and gradually heated up to 650 °C for 3 h. Above the melting point of Mg, it vaporizes and is trapped inside the reactor on the purpose of preventing escape of Mg vapors, delivering high pressure on overall system. As a result, Mg vapors can easily react with clay minerals beginning with the surface dual phase of silicates and silica. Subsequently, all of the byproducts are completely etched out after reduction for sufficient time. As schematically shown in Figure 1a, the 10590
DOI: 10.1021/acsnano.6b06828 ACS Nano 2016, 10, 10589−10597
Article
ACS Nano
Figure 2. Structural analyses of HPSF. (a) TEM images showing the entire structure of HPSF and (b) STEM dark-field image (inset is the corresponding FFT diffraction pattern). (c) TEM image showing macropore regions of HPSF and (d−f) magnified TEM image showing edge region of HPSF. (g) Nitrogen adsorption−desorption isotherm curve, (h) BJH pore size distribution curve, and (i) pore volumes distribution of HPSF.
its ability to absorb the heat from the reduction reaction by serving as a negative catalyst.37 Because negative catalysts (also known as inhibitors of catalysis) have high specific heat capacity as much as typical salts and accordingly regulate the reaction rate, structural collapse can be greatly prevented. Apart from MgO layers, any kind of clay which contains inactive metal oxide layers toward metal reductant can reproduce the same phenomena. Structural superiority of developed 3D HPSF on the strength of negative catalyst layers was examined by transmission electron microscopy (TEM). As presented in Figure 2, HPSF has multistacked hierarchical porous flake structures with highly pure silicon phase developed (see SI, Figure S5). First, about 100 nm of macropores are regularly developed over the few micrometer-sized flake frame through selective chemical reduction process which is analogous to inverse-opal morphology (Figure 2a). By observing the degree of overlapped macropores, we can surmise that more than two flakes are irregularly stacked to each other, of which whose thickness can still be called nanoflakes (
