RSC Advances View Article Online PAPER View Journal | View Issue Nanoporous silicon from low-cost natural clinoptilolite for lithium storage† Cite this: RSC Adv.,2015,5, 56772 Rongrong Miao,a Jun Yang,*a Yanan Wu,a Jiulin Wang,a Yanna Nulia and Wei Lub Despite the fact that silicon materials can be synthesized from various sources, deriving them from earth- abundant resources is of strategic significance for industrial processing. Here nanoporous silicon (pSi) was Received 9th May 2015 derived from earth-abundant natural clinoptilolite (NCLI) without complicated pretreatment. After surface Accepted 22nd June 2015 carbon coating, the pSi–C composite displayed a superior stable capacity of ca. 1257 mA h gÀ1 and good DOI: 10.1039/c5ra08622a cycling stability with 87.5% capacity retention on the 200th cycle versus the 3rd one, which benefit from www.rsc.org/advances its nanoporous structure, very small primary particle size of 10 nm and highly conductive carbon-matrix. Introduction materials have been proven to be effective in improving cycle life and/or rate properties, but the high-cost silica source (e.g. Lithium ion batteries, as the most dominant power source SBA-15,9 KIT-6 (ref. 10)) and complicated synthesis process may currently, have been extensively used in portable devices, hinder its large-scale production. As we all know, with the (hybrid) electric vehicles (HEV) and grid-scale stationary energy increase of transportation markets in hybrid electrical vehicles storage. However, the limitation in energy density and power (HEVs), plug-in hybrid electrical vehicles (PHEVs) and electrical density of existing lithium ion battery systems based on vehicles (EV), this broader and large-scale application raises conventional graphite anodes and lithium metal oxide or issues of price and environment, as well as scalability, thus new phosphate cathodes (LiCoO2, LiMn2O4, LiFePO4) is becoming designs of Si anodes with the desired combination of functional more and more prominent.1 During searching for new material features have to be supplemented by using cost-effective silica with much higher energy density, silicon (Si) attracts tremen- precursor and preparation method, which is becoming dous attention due to its ten times higher theoretical capacity extremely signicant for its practical application. À (4200 mA h g 1) than traditionally used graphite anodes.2 Recently, the developments on synthesizing porous or nano- Whereas the main impediment of silicon as anode material is scaled Si from low cost silica precursors look quite promising.11 its severe particle pulverization and loss of electronic conduc- Some synthetic silica precursors (e.g. modied stober silica or tivity of the electrode originated from huge volumetric change silica aerogels) are employed to synthesize porous silicon.12,13 during repeated lithiation/delithiation. Therefore, extensive Although excellent cyclability can be obtained, the preparation research has been devoted to address the aforementioned issue. process of silica precursor seems to be complicated. Thereby, For instance, downsizing the dimensions of silicon to nanoscale some natural silica precursors also attract attention. For could buffer the large (de)lithiation strains without fracture to instance, Wang et al.14 synthesized porous silicon by employing some extent; other strategies are to design some inner free diatomaceous earth as cheap silica source. Nevertheless, the space in silicon to accommodate the large volume expansions obtained porous silicon with carbon coating demonstrated and thereby well-designed silicon with various special struc- relatively poor performance with a reversible capacity of 633 À tures have been proposed, including pomegranate-like hierar- mA h g 1 aer 30 cycles and silicon without carbon coating was À chical structure, lotus-root-like mesoporous structure, etc.3–5 In even lower with only 376 mA h g 1. Other cheap silica sources addition, carbon materials have also been put inside to form such as sand or rice husk have also been selected and some diverse Si–C composite, which is used to stabilize the whole silicon materials keeping the original silica structures were structure and further enhance electronic conductivity of active prepared successfully by magnesiothermic reduction – material.6–8 Among these strategies, constructing porous silicon process.15 17 However, the tedious pre-treatments of sand, such as milling, calcining and washing with HCl, HF and NaOH, may be unfavorable for its potential industrial application.17 aShanghai Electrochemical Energy Devices Research Center, School of Chemistry & Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China. Although silicon derived from rice husk exhibits high reversible À1 E-mail: [email protected]; Fax: +86-21-54747667; Tel: +86-21-54747667 capacity of 2790 mA h g and long cycle life of 86% capacity bDepartment of Chemical Engineering, University of Michigan, Ann Arbor, MI 48109, retention over 300 cycles, the relatively low silica content (23% USA in mass) in rice husks largely reduces the single conversion † Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra08622a 56772 | RSC Adv.,2015,5, 56772–56779 This journal is © The Royal Society of Chemistry 2015 View Article Online Paper RSC Advances yield of silicon.16 And the sophisticated pretreatments are also low-cost, energy-efficient and simple to be realized in large-scale disadvantageous to large-scale production. production. Herein, nanoporous silicon with primary particle size in 10 nm was directly synthesized from low-cost natural cli- noptilolite (NCLI) through magnesiothermic reduction method Experimental with simple and facile pretreatment. Natural clinoptilolite is an Synthesis of porous silicon earth-abundant resource of aluminosilicate available all over Scheme 1 gives an overview of the synthesis procedure of the À1 the world with extremely low-cost (RMB 2.0 kg ). It is the most porous Si material from NCLI. Natural clinoptilolite (NCLI) common natural zeolite and commercially used in wastewater containing ca. 68.6 wt% SiO2 from Zhejiang province, was rstly processing, with cage-like structure containing AlO4 and SiO4 ball-milled with a Planetary Mono Mill P-6 (Fritsch, Germany) at 18,19 tetrahedra linked through the common oxygen atoms. It a rotation speed of 500 rpm for 10 h to decrease the particle size belongs to heulandite family with the molar Si/Al ratio above and open the transfer channels (mass ratio of balls/material/ 20 4, and its speci c crystal building is characterized by two water ¼ 10 : 2 : 1). channels running parallel to c-axis: a channel consisting of a The mixture of obtained NCLI powder and magnesium – 10-member ring with the size of 0.44 0.72 nm and a channel powder (Sinopharm Chemical Reagent Co. Ltd, 100–200 mesh) consisting of 8-member ring with the size of 0.41–0.47 nm, and ¼ were loaded in an alundum boat with a molar ratio of Mg/SiO2 a channel running parallel to a-axis consisting of an 8-member 2.1. And then heated in a tube furnace at 650 C for 4 h under Ar tetrahedral ring with the size of 0.40–0.55 nm (Scheme 1).21 (95 vol%)/H2 (5 vol%) mixed atmosphere. The heating speed À These channels in NCLI framework structure form the primary was kept at 2 C min 1. The obtained brown powder was rstly micro-pores. Notably, the presence of secondary porosity is immersed in 2 M HCl solution for 12 h to remove MgO and another interesting feature of NCLI, which is connected with other impurities in NCLI. To further remove small amount of cleavage of NCLI grains and other minerals in the NCLI rocks.22 unreacted and surface-grown SiO2, 5 wt% HF/EtOH(10 vol%) This unique polymodal pore size distribution is favorable for solution was used and stirred for 15 min, then washed with the homogeneity of magnesiothermic reduction reaction and distilled water and ethanol by ltration, nally vacuum-dried at nanoporous silicon was successfully obtained even in the 65 C for 2 h. presence of alumina and other mineral species. Carbon was coated on the as-prepared porous silicon (pSi) by Compared with the reported silica templates for producing CVD method using toluene as the carbon source. The obtained porous silicon, the silica source of natural clinoptilolite (NCLI) pSi powder was loaded in an alundum boat and placed at the in this paper has several advantages: (i) the intricate cage and center of a quartz tube furnace. Next, the precursor gas (argon À channel architectures in NCLI are bene cial to obtain porous and toluene with the gas ow of 200 ml min 1) was introduced structured silicon with nano-scaled primary particles in size of into the furnace for 30 min to ush away any oxygen in the 10 nm, which accommodate the large volume expansion and reactor. Then, the furnace temperature was increased from À ensure the facile strain relaxation; (ii) as-synthesized porous room temperature to 800 C at a rate of 10 C min 1 and kept at silicon a er carbon modi cation exhibits excellent capacity 800 C for 25 min. The furnace was cooled slowly to room retention and rate performance; (iii) NCLI inherits relatively temperature. At high temperature, the toluene decomposed – high SiO2 content of 50 70 wt%, extremely low cost and abun- quickly and carbon deposited onto the surface of pSi particles. dant reserves in nature; (iv) there is no any complicated pre- The carbon content in the composite was controlled at treatment for the NCLI and the overall synthesis process is 23.6 wt%. Structure and morphology characterization X-ray diffraction (XRD) patterns were recorded using Cu-Ka radiation at 40 kV with an X-ray Diffractometer (D/max-2200/PC, Rigaku). The morphologies of synthesized silicon and related composite materials were observed by a eld emission scanning electron microscope (FESEM, JEOL JSM-7401F). Transmission electron microscope (TEM, JOEL JEM-100CX) was employed to characterize the microstructures of as-synthesized materials.