Silicon/Mesoporous Carbon/Crystalline TiO2 Nanoparticles for Highly Stable Lithium Storage Wei Luo,† Yunxiao Wang,‡ Lianjun Wang,† Wan Jiang,† Shu-Lei Chou,‡ Shi Xue Dou,‡ Hua Kun Liu,‡ and Jianping Yang*,† †
State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, People’s Republic of China ‡ Institute for Superconducting & Electronic Materials, Australian Institute of Innovative Materials, University of Wollongong, Innovation Campus, Squires Way, North Wollongong, NSW 2500, Australia S Supporting Information *
ABSTRACT: A core−shell−shell heterostructure of Si nanoparticles as the core with mesoporous carbon and crystalline TiO2 as the double shells (Si@C@TiO2) is utilized as an anode material for lithium-ion batteries, which could successfully tackle the vital setbacks of Si anode materials, in terms of intrinsic low conductivity, unstable solid−electrolyte interphase (SEI) films, and serious volume variations. Combined with the high theoretical capacity of the Si core (4200 mA h g−1), the double shells can perfectly avoid direct contact of Si with electrolyte, leading to stable SEI films and enhanced Coulombic efficiency. On the other hand, the carbon inner shell is effective at improving the overall conductivity of the Si-based electrode; the TiO2 outer shell is expected to serve as a rigid layer to achieve high structural stability and integrity of the core−shell−shell structure. As a result, the elaborate Si@C@TiO2 core−shell−shell nanoparticles are proven to show excellent Li storage properties. It delivers high reversible capacity of 1726 mA h g−1 over 100 cycles, with outstanding cyclability of 1010 mA h g−1 even after 710 cycles. KEYWORDS: core−shell nanostructure, silicon anode, lithium-ion battery, sol−gel process, coating
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insertion and extraction processes, which leads to the loss of electrical contact between the active electrode material and the current collector, resulting in the continuous formation of an unstable solid−electrolyte interphase (SEI) layer and short battery lifetime.14−17 To circumvent these challenges, numerous approaches have been proposed, such as the use of nanostructured Si materials, adopting Si active/inactive nanocomposites, and exploring different binders.18 Reducing the particle size of the Si materials from the bulk to the nanoscale, in forms including nanoparticles, 19,20 nanotubes, 21 nanowires, 22 and porous spheres,23,24 can offer high interfacial area, fast electronic and ionic diffusion, and alleviation of the strain, thus demonstrating superior performance.25 The fabrication of Si active/inactive
lectrochemical energy storage devices are regarded as suitable advanced power sources for portable electronic products and electric vehicles. In this respect, lithiumion batteries (LIBs) have become the most promising energy storage devices and received increasing research interest due to their impressive energy density and rate capability.1,2 The rapid development of automotive energy storage applications requires rechargeable LIBs with a much high energy density and power density and long cycle life.3−6 For this purpose, silicon (Si) has been proposed as an appealing candidate as a high-performance anode material because of its abundant natural resources and ultrahigh theoretical capacity of ∼4200 mA h g−1, which is 10 times that of commercial graphite anode (372 mA h g−1).7−12 Two essential problems, however, restrict the practical application of Si-based anodes.13 The low intrinsic electrical conductivity limits fast Li+ transport at high current density. Another problem is the structural pulverization caused by the large volume changes (∼400%) during the lithium © 2016 American Chemical Society
Received: September 27, 2016 Accepted: October 24, 2016 Published: October 24, 2016 10524
DOI: 10.1021/acsnano.6b06517 ACS Nano 2016, 10, 10524−10532
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Figure 1. (A) Schematic illustration of the fabrication of Si@C@TiO2 core−shell−shell nanoparticles via the two-step sol−gel coating process. Scanning electron microscopy images of (B) commercially available silicon nanoparticles with the predominant size of ∼80 nm; (C) resorcinol−formaldehyde resin polymer-coated silicon core−shell nanoparticle (Si@RF); (D) Si@RF@TiO2 nanoparticles; Si@C@TiO2 core−shell−shell nanoparticles prepared by carbonization of Si@RF@TiO2 at (E) 600 °C (Si@C@TiO2-600), (F) 800 °C (Si@C@TiO2-800), and (G) 900 °C (Si@C@TiO2-900) under nitrogen atmosphere.
inner layers, confined between the Si core and TiO2 shell, can offer high conductivity for electron and ion transport and effectively limit the volume expansion during the lithiation and delithiation process. Second, the TiO2 shell works as a mechanically strong outer shell to ensure structural stability and integrity and promotes the electrical conductivity during the lithiation process. Third, these porous structures with sizeselective permeability provide a double barrier to prevent the accessible electrolyte from reaching the surface of the Si core and thus protect the nanoparticle electrodes from subsequent irreversible reaction with the electrolyte. As a consequence, these well-designed Si@C@TiO2 core−shell−shell nanoparticles exhibit outstanding cyclability of 1010 mA h g−1 after 710 cycles at a current density of 420 mA g−1 with high Coulombic efficiency of >98%.
nanocomposites is a straightforward and effective approach to avoid direct contact between the surface of the Si and the electrolyte, and therefore, the nanocomposites maintain a relatively stable SEI film and improved cycling performance.26−32 For example, amorphous carbon-coating strategies are attractive because of the outstanding electronic conductivity, and more importantly, they provide a barrier layer to isolate Si active materials from the electrolytes.33−38 The compact carbon layers, unfortunately, are prone to fracture due to the rigid structure and large volume expansion of Si.39 An addition of void spaces around Si particles is necessary to ensure the structural integrity during the cycling process, such as by the formation of hollow core−shell,40,41 yolk−shell,42 and porous structures.43,44 Such constructions are usually accompanied by the use of hazardous hydrofluoric acid and, as well, give rise to the inferior mechanical and thermal stability.42 As an alternative, titanium oxide (TiO2) has received significant attention because it yields a fast lithium insertion/extraction rate with negligible volume expansion (
