Rice Husk Silica-Derived Nanomaterials for Battery Applications: A

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Rice Husk Silica Derived Nanomaterials for Battery Applications – A Literature Review Yafei Shen J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.6b04777 • Publication Date (Web): 04 Jan 2017 Downloaded from http://pubs.acs.org on January 5, 2017

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Journal of Agricultural and Food Chemistry

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Rice Husk Silica Derived Nanomaterials for Battery Applications – A

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Literature Review

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Yafei Shen 1,2 *

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Jiangsu Engineering and Technology Research Center of Environmental Cleaning Materials (ECM), Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control (AEMPC), Collaborative Innovation Center of Atmospheric Environment and Equipment Technology (AEET), School of Environmental Science and Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, China 2

Department of Environmental Science and Technology, Tokyo Institute of Technology, Yokohama 226-8502, Japan

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For Table of Content Only

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ACS Paragon Plus Environment


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ABSTRACT

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Silica-rich rice husk (RH) is an abundant and sustainable agricultural waste. The recovery of value

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added products from RH or its ash to explore an economic way for the valorization of agricultural

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wastes have attracted wide attention. For instance, RH can be converted to biofuels and biochars

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simultaneously via thermochemical processes. In general, the applications of RH biochars include

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soil remediation, pollutants removal, silicon battery materials, etc. This review article concludes

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recent progresses in the synthesis of RH-derived silicon materials for lithium-ion batteries (LIBs)

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applications. Silica nanomaterials produced from RH is initially discussed. RH amorphous silica

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can also be fabricated to crystal silicon used for battery materials via wide-used magnesiothermic

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reduction. However, the RH-derived Si nanoparticles suffer from a low coulombic efficiency in the

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initial charge/discharge and limited cycle life as anode materials due to high surface reactions and

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low thermodynamic stability. The synthesis of Si materials with nano/micro-hierarchical structure

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would be an ideal way to improve their electrochemical performances. Embedding nano-Si into

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3D conductive matrix is an effective way to improve the structure stability. Among the Si/carbon

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composite materials, CNTs is a promising matrix due to the wired morphology, high electronic

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conductivity, and robust structure. Additionally, CNTs can easily form 3D cross-linked conducting

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networks, ensuring effective electron transportation among active particles. Si nanomaterials

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with micro-hierarchical structures, in which CNTs are tightly intertwined between the RH-derived

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Si nanoparticles, have been proved to be ideal LIBs anode materials.

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KEYWORDS: rice husk (RH); biosilica; silicon; battery materials; magnesiothermic reduction

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1. INTRODUCTION

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1.1. Biogenic Silica

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Silicon (Si), which is a ubiquitous and quantitatively the second most prominent element in the

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earth crust after oxygen (O) [1], is released in soil by means of biological or chemical processes

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[2-5]. Plants contribute significantly to the biogeochemical cycle of silicon in chemosphere [4, 5].

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They take up silicon from soil water in the form of water-soluble silicic acid (H4SiO4) [6], which is

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polymerized and precipitated as amorphous silica (SiO2), frequently in close proximity to the

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transpiration conduit. After plant death, SiO2 returns back to the soil and then the plant-decay

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generates humic acid, which increases the weathering activity in soils. Bio-cycling of SiO2 in soil

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also occurs via microbial activities that involve fungi, bacteria, and actinomycetes [7]. Therefore,

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plants and microbes, through their intricate interplay with soil minerals, contribute appreciably

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to the global silicon cycle [3-8].

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Silicon (Si) is accumulated mainly in the form of phytoliths in plants [9], consisting primarily

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of the amorphous hydrated silica (SiO2 with 5-15% H2O). Phytoliths-containing plants include

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dicots (e.g., Myrtaceae, Casuarinaceae, Proteaceae, Xanthorrhoeaceae, Mimosaceae),

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monocots (e.g., Cyperaceae, Gramineae, Palmae), conifers (e.g., Pinaceae, Taxodiaceae), and

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sphenophytes/scouring rushes (e.g., Equisetaceae) [10]. The ash content of Equisetaceae

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and Gramineae/Poaceae may consist of 50-70% silica [11]. Some polyamines, carbohydrates,

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proteins, and glycoproteins from diatoms and sponges are capable of polymerizing silicic

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acid at neutral to acidic pH [12-14]. Some plant carbohydrates and proteins would play a key

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role in biogenic silica polymerization [15-18]. In the biosphere, silica is usually accumulated

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predominantly in the form of amorphous silica (opal or silica gel) [9]. 3



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1.2. Rice Husk (RH) Silica

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Rice (Oryza sativa), a member of the family Gramineae, typically constitutes 20-22% of its total

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produce in the form of RH, and a great deal of RH is presently disposed by rice mill industry as a

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waste. Interestingly, the highest SiO2 content in rice is observed in its husk, which varies from 8.7%

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to 12.1%, averaging close to 10.6% [19]. However, SiO2 present in RH is in a hydrated amorphous

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form similar to that present in most of the other entities in the biosphere [20]. The biological

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formation of inorganic materials (e.g., silica materials) with complex forms (i.e. biominerals) is a

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widespread phenomenon in nature [21]. In recent decades, the recovery of value added products

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from RH or its ash to explore an economic way for the valorization of agricultural wastes have

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attracted wide attention [22]. However, there is lack of a comprehensive article on the synthesis

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and application of value-added silicon materials derived from RH. Therefore, this review article

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presents recent progresses in the synthesis of RH-derived silicon materials for lithium-ion battery

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applications.

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2. RICE HUSK SILICA PRODUCTION

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Silica is recognized as an extremely important inorganic material and is extensively used for a

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wide range of commercial applications such as molecular sieves, catalysts, and in biomedical and

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electrical applications [23-29]. In general, porous inorganic microstructures are of interest as low

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density and thermally stable particles, and as mechanically resistant encapsulation structures.

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The chemical syntheses of silica materials are not only relatively expensive and eco-hazardous,

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but also require strict synthesis conditions. In contrast, biosilicification by living organisms such

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as cyanobacteria, diatoms, sponges, and plants proceeds under mild physiological conditions and

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results in a diversity of complex and hierarchical biogenic silica nanostructural frameworks [9]. 4


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Silica-rich rice husk ash (RHA) can be achieved by thermal treatment at elevated temperatures or

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it can be extracted from RH in the form of sodium silicate by a solvent extraction method. In most

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applications, RHA is more favorable compared to RH. RHA is a general term describing all forms of

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the ash produced from RH. In practice, the form of RHA obtained varies considerably according to

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the temperature. Silica in the RHA undergoes structural transformations based on the conditions

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(time, temperature, etc.) of combustion. Amorphous silica is formed at 550-800 oC and crystalline

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silica occurs at temperatures greater than this [30]. These types of silica have different properties

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and it is significant to produce RH silica of the correct specification for the particular end use [31].

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In general, the structure and properties of RH silica are sensitive to the methods. The forms of RH

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silica are different from amorphous to distinct crystal phases depending on the temperature or

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chemical treatments [32]. Fig. 1 summarizes the methods for producing silica from RH, including

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impurities removal before and after thermal processes. Of the different synthesis methods, the

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chemical method consisting of simple acid leaching and post annealing is one of the most simple

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and successful techniques to synthesize the ultrafine SiO2 nanoparticles from RHA.

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Figure 1 Different methods for producing different structural silica from RH

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Leaching of RH with different chemical solutions (e.g., HCl, H2SO4, H3PO4, HNO3, HF, NH4OH, and

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NaOH) before thermal processes under different conditions (e.g., temperature, time) can be so

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effective in accelerating the hydrolysis of cellulose and hemicelluloses in RH and removing most

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of the metallic impurities, which allows producing white-color silica completely, with high surface

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area [33-38]. Recently, Zeng and Bai [39] prepared the polyethyleneimine (PEI)-functionalized

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hierarchically porous silica nanoparticles from RH via a simple template-free method. Compared

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with traditional alkaline fusion and surfactant-templating methods for preparing waste-derived

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porous silica materials, this method had specific important advantages in being an inexpensive,

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and energy-saving process with faster production rate. In this work, the addition of hydrofluoric

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acid (HF) followed by ammonium hydroxide corresponding to the formation of (NH4)2SiF6 salt can

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serve as an effective porogen, so high-temperature is unnecessary for extraction SiO2 from RH.

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Apart from the synthesis conditions, the types of silica precursors can influence the properties of

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silica materials. Sankar et al. [40] synthesized the silica nanopowders from different RH sources

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(as shown in Fig. 2). The prepared silica nanoparticles had a uniform surface morphology with

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respect to the particle size distribution, and the sizes of silica nanoparticles decreased from 50

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nm (sticky RHA) to 10 nm (brown RHA). The mesoporous silica nanopowder synthesized from

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brown RHA had the lowest particle size distribution and the highest surface area. Besides, various

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sizes ranging from nanometer to micrometer of amorphous spherical silica particles could be

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controllably synthesized from RH [40].

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Figure 2 Digital and SEM images of rice plant, rice, RH, RHA and synthesized silica nanoparticles. Copyright with the permission from ref. [40]

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RH can be directly converted into bio-oil and syngas by the pyrolysis process. Combined with the

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chemical extraction, the high-yield amorphous silica can be recovered from pyrolysis char. More

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significantly, the activated carbon with high specific surface area can also be produced by using

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physical or chemical activation (as illustrated in Fig. 3) [41]. Co-production of silica with other

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valuable products mainly including activated carbon, biofuels and lignocellulosic components

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would become the most promising way for RH valorization [41-45].

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Figure 3 Production of both amorphous silica and activated carbon from RH. Copyright with the permission from ref. [41]

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Rice grows by taking in silicic acid as well as CO2, water and various minerals [45]. As illustrated in

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Fig. 4, rice deposits amorphous silica on their cell walls, forming silica-cuticle and silica-cellulose

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double layers on the surface of leaves, stems and husks [46, 47]. Therefore, amorphous silica on

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the cell walls of RH can play as a role of the natural template for the formation of porous carbon

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(PC), as well as silica recycling. Tabata et al. [48] developed an environmentally friendly process

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for synthesis of value-added hierarchical porous carbon (HPC) material, comprising micro-, meso-

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and macro-pores by using unique structural cell assemblies of RH. Noteworthy, steam activation

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of PC contributed to the deepening of the meso- and micro-pores with respect to the formation

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of HPC with low-toxicity and superior in vivo properties. In addition, it can be applied to many

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significant applications not in the electronic and industrial fields but in the medical field for use

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as adsorbents, drug delivery and bio-interface materials, and scaffolds for cell incubation [48].

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Figure 4 Schematic illustration of the porous carbon (PC) synthesis from RH and SEM images of

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silica-carbon (SiO2-C) composites (a-c) and PC (d-f). Copyright with the permission from ref. [48]

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In addition, RH silica materials with specific morphologies and structures can be synthesized for

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several particular applications [49-52]. Wang et al. [49] explored the intrinsic morphology and

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microstructure of the biosilica in RH by controlling the pretreatment and reaction conditions to

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maintain their original structure. Here, porous silica nanoparticles with narrow size distribution

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and high surface area were initially prepared from the HCl-treated RH by means of controlled

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pyrolysis. Subsequently, the semi-crystalline porous silica frameworks with tunable pore sizes

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were formed by doping the silica nanoparticles with K+ cations and tailoring pyrolysis conditions

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(as illustrated in Fig. 5). Compared with amorphous porous silica, the synthesized porous silica

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frameworks have wide applications and exhibit superior performance because of the increased

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crystallinity and structural integrity [49]. In general, RHA has wide range of applications, including

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the fabrication of silica gels, silicon chip, silica-carbon composite, construction materials, catalysts,

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zeolites, battery materials, graphene, energy storage/capacitor, carbon capture, as well as in drug

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delivery vehicles [51].

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Figure 5 Synthesis of silica nanoparticles from RH and formation of porous silica framework. Copyright with the permission from ref. [49]

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3. BATTERY MATERIALS

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Silicon with high-purity and crystalline structures is often required for the preparation of battery

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materials. The conventional process of producing silicon is relatively expensive [53]. Thus, many

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efforts are being directed to develop a low-cost, high-volume and commercially feasible process

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for producing high purity silicon in electrical applications [54-56]. One such process involves RHA

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reduction to silicon. Several approaches have been developed to produce silicon from RH. For

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instance, amorphous silica can be reduced to silicon via carbothermal reduction at relatively high

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temperatures [57, 58]. Marchal et al. [58] synthesized the solar grade silicon (≈99.9999% pure)

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from RHA as a biogenic silica source. RHA is initially purified using acid milling/boiling water wash

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purification steps and pelletization followed by carbothermal reduction using an experimental 50

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kW electric arc furnace (EAF) operated at 1700-2100 oC in batch mode. In addition, reduction of

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amorphous silica to silicon by metallic metals has been studied extensively. The metallic elements

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of Mg, Ca, Al and Ti can reduce SiO2 at comparatively lower temperature and forms mixtures of

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condensed phase products [59-62]. The overall reactions with corresponding free energy change

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and adiabatic temperature rise per mole of silicon are shown in Table 1.

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Thermic reduction of SiO2 can be accomplished via the above-mentioned mechanisms including

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carbothermal, magnesiothermic, aluminothermic, and calciothermic reduction. Carbothermal

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reduction often utilizes EAF operating at 2000 oC and is the primary mode for metallurgical silicon

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production [63]. However, this process is generally very energy intensive and liquefies the silicon,

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thus destroying any original morphology of SiO2. Recently, magnesiothermic reduction has gained

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much attention due to its lower operating temperatures (650 oC). Typically, Mg powder is placed

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adjacent to SiO2 powder and the furnace is heated until Mg vaporizes. However, this reduction 10


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scheme produces zonal variations in composition with Mg2Si forming near the Mg powder, Si in

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the middle, and unreacted SiO2 furthest from the Mg [64]. Luo et al. [65] proved the addition of

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NaCl to the reduction process aids in scavenging the large amount of heat generated during this

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highly exothermic reaction. NaCl effectively halts the reaction temperature rise at 801 oC during

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fusion, preventing the reaction from surpassing the melting point of silicon and thus aiding in

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preserving the original SiO2 morphology. Favors et al. [66] synthesized the porous nano-silicon via

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a scalable heat scavenger-assisted magnesiothermic reduction of sand. The addition of NaCl, as

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an effective heat scavenger for the highly exothermic magnesiothermic reduction, promotes the

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formation of an interconnected 3D network of nano-silicon with a thickness of 8-10 nm. Herein,

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carbon coated nano-silicon electrodes can achieve remarkable electrochemical performance with

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a capacity of 1024 mAhg

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and low cost SiO2 source allows for the production of nano-silicon with excellent electrochemical

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performance as an anode material for Li-ion batteries [66].

-1

at 2 Ag

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after 1000 cycles. This environmentally benign, abundant,

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Mishra et al. [56] investigated the purification of silicon through calcium reduction of RHA. They

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mixed RHA obtained at 500 oC and calcium thoroughly in stoichiometric proportions. Then the

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mixture was reduced at 720 oC. The reduced product leached with HNO3 and HF to achieve high

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purity of 99.9% silicon. Obtaining silicon of (99.9999%) purity by reducing RHA with magnesium

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at temperature of 800 oC was followed by several successive acid (mixtures of HF, H2SO4 and HCl)

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leaching treatments. With acid treatment, it is possible to remove MgO, Mg2Si and unreacted Mg

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as by-products. Larbi [59] reported that by utilization of pellet of RHA with Mg content 5 wt.% in

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excess of stoichiometry requirement and heating at 900 oC under the flowing argon, a maximum

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silicon yield can be achieved. The fine silicon nanocrystals was also formed via magnesiothermic 11



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reaction. The reduction of SiO2 to form Si has obtained popularity since the three-dimensionally

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structured Si replicas could be produced from parent SiO2 diatom by magnesiothermic reduction

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[60]. Magnesiothermic reduction of various types of silica/carbon (SiO2/C) composites has been

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frequently used to synthesize silicon/carbon (Si/C) composites and silicon carbide (SiC) materials,

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which are of great interest in the applications of lithium-ion batteries (LIBs) and nonmetal oxide

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ceramics, respectively [61]. Mg2Si has been considered as intermediates of Si in magnesiothermic

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reduction of SiO2 [67, 68]. On the basis of these facts, the kinetic product of Si can be formed if

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conversion from SiO2 to Si is completed before the silicon intermediates reach carbon. In other

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words, Si can be synthesized if insufficient diffusion of Si intermediates to carbon is prompted.

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Based on the methods discussed above, unique nanostructures of amorphous silica derived from

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abundant biogenic silica sources (e.g., RH) can be reduced to silicon or silicon-carbon composites

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in high-value LIBs applications [69-75]. Table 2 shows the reported works on RH-derived silicon

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materials in LIBs applications. Generally, silicon materials have relatively high potential to be used

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as the LIBs anodes compared with silica composites. Jung et al. [69] synthesized nanostructured

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Si from RH via magnesiothermic reduction process. After that, 3D porous Si was obtained after an

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additional two-stage acid etching process (as illustrated in Fig. 6). This work proved that RH can

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be employed to produce Si with an ideal porous nanostructure for high-capacity LIB anodes. Its

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interconnected nanoporous structure, developed via natural evolution for efficient cultivation of

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rice, can resolve important issues in the Si anode operation, enabling excellent cycling and power

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performance.

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Figure 6 Generation of 3D nanoporous Si from RH. (A) Photographs of rice plant. (B) Photograph of RH after milling. (Upper Inset) Optical micrograph showing the morphological characteristic of outer/inner surfaces of RH. (Lower Inset) Circular chart indicating the main composition of RH. (C) Optical micrograph of a RH shell magnified from the black box in B. (Inset) Si-mapped SEM-EDS image suggesting that silica exists mostly along the outer rugged surface of RH. (D) The overall procedure of synthesizing the nanostructured Si from RH. (Left to Right) Pretreated RH by an acid-leaching process, RH silica by a thermal decomposition process, a Si/MgO mixture formed after a magnesiothermic reduction process, and the 3D porous Si obtained after an additional two-stage acid etching process. Copyright with the permission from ref. [69]

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Liu et al. [72] proposed an integrated magnesiothermic reduction process for producing nano-Si

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from RH. As shown in Fig. 7c-g, RH were first converted to nano-SiO2 by thermal decomposition,

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followed by magnesiothermic reduction to produce nano-Si (Fig. 7a). This innovative method has

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several advantages: (i) the recovered silicon inherits the intrinsic and unique nanostructure of the

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RH silica, allowing for excellent battery performance by mitigating pulverization; (ii) RH is an

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abundant and sustainable silica source; (iii) the whole process is facile, energy-efficient, and easy

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to scale-up; and (iv) the overall process does not use expensive silica precursors or reagents. And

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Mg is produced by electrolytic process with a relatively low cost. Mg can be regenerated from the

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electrolysis of MgCl2 (Fig. 7c). Therefore, this green process only consumes HCl and converts it to

273

Cl2 after the electrolysis. 13



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Figure 7 A large scale production of nano-Si from RH. (a) Comparison of carbothermal reduction and magnesiothermic reduction for producing nano-Si. Si(OR)4 denotes silicon alkoxide. (b) A panicle of ripe rice at a rice farm. (c) Flow chart of the process for recovering Si nanoparticles from RH. (d-g) Optical images of the intermediate substances. The inset of (f) shows an optical microscopy image (0.9 mm 3 0.6 mm) of one piece of thermal treated RH. Copyright with the permission from ref. [72]

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In addition, Wong et al. [74] synthesized the silicon nanoparticles from RH-derived silica via the

283

magnesiothermic reduction process. Subsequently, silicon nanoparticles were used to prepare a

284

binder-free composite of Si-graphene as an anode material for LIBs (as illustrated in Fig. 8). The

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Si-graphene composite yielded an initial capacity of 1000 mA h/g at high applied current density

286

of 1000 mA/g. In summary, all these studies open up the use of waste materials especially for

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agricultural wastes (e.g., RH) as a sustainable source in advanced technology applications.

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Figure 8 Synthesis of a binder-free composite of Si-graphene as an anode material for LIBs. Copyright with the permission from ref. [74]

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Praneetha et al. [75] proposed a rapid microwave assisted approach to extract SiO2 from various

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agricultural residues such as bamboo culm (BC), RH, and sugarcane bagasse (SB) followed by a

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magnesiothermic reduction of SiO2 into Si within 30 min at temperatures below 650 oC without

295

using any reducing gas atmosphere. This reduction reactor inside the microwave muffle furnace

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can be given by the SiC passive heating element (PHE) plates by absorbing microwave radiation

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and subsequent transfer of heat to the reactants with increase in reaction kinetics during raw

298

material (SB, BC, and RH) ashing and magnesiothermic reduction processes (Fig. 9A). Si-based

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nanohybrids were produced by a microwave assisted solvothermal (MW-ST) process that offer a

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uniform heating of polar solvent by absorbing microwave energy followed by heating (Fig. 9B).

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The pristine Si and their nanohybrids with carbon, multiwall carbon nanotubes (MWCNT), and

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graphene had been proved as a high capacity anode for LIBs for energy storage applications [75].

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Figure 9 (A) A rapid microwave assisted method to extract SiO2 from agricultural residues such as BC, RH, and SB followed by the magnesiothermic reduction of SiO2 to Si; (B) Si nanohybrids production by using a MW-ST process. Copyright with the permission from ref. [75].

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The fabrication of Si anode materials with biomass enables the effective utilization of agricultural

309

residues in battery industries, despite the electrochemical performances of these as-synthesized

310

Si materials still require improvements. The RH-derived Si nanoparticles always suffer from a low

311

coulombic efficiency in the initial charge/discharge and limited cycle life as anode materials due

312

to high surface reactions and low thermodynamic stability. Based on the reported works, the

313

fabrication of Si nanomaterials with nano/micro-hierarchical structure would be an ideal way to

314

improve their electrochemical performances, since the materials on the micrometer size possess

315

a good thermodynamic stability upon cycling. Embedding nano-Si into three-dimensional (3D)

316

conductive matrix is an effective way to improve the structure stability and form stable solid

317

electrolyte interphase (SEI) films on surfaces. Among various Si/carbon composite materials

318

[76-80], carbon nanotube (CNT) has been considered as a promising matrix because of the wired

319

morphology, high electronic conductivity, and robust structure [81]. Additionally, it is easy for CNT

320

to form 3D cross-linked conducting networks, ensuring effective electron transportation among

321

active particles [76]. Consequently, Si anode materials with nano/micro-hierarchical structure, in

322

which CNTs are tightly intertwined between the RH-derived Si nanoparticles, are supposed to be

323

ideal anode materials for LIBs.

324 325

More recently, Zhang et al. [82] fabricated Si/nitrogen-doped carbon/carbon nanotube (SNCC)

326

nano/micro-structured spheres by using a polyacrylonitrile (PAN) assisted electrospray method.

327

RH-derived Si nanoparticles on the sizes of 50 nm were homogeneously dispersed and embedded

328

into the N-doped carbon matrix intertwined and threaded by CNT cross-linking networks, forming

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micro-spheres with the diameters of 3.270.8 μm. Given the high electronic conductivity, robust

330

structure, and improved surface/interface stability, SNCC spheres can exhibit high cycling stability 16


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(1031 mA h g-1 at 0.5 A g-1 after 100 cycles) and excellent rate capability as anode materials for

332

LIBs (Fig. 10). The synthesis of SNCC nano/micro-structured spheres is schematically depicted in

333

Fig. 10A. Firstly, RH were pretreated to obtain high purity nano-SiO2, which was then reduced to

334

RH-Si nanoparticles by magnesiothermic reduction. Subsequently, RH-Si nanoparticles and CNTs

335

were dispersed in N,N-dimethylformamide (DMF) solution containing PAN through adding a small

336

amount of sodium citrate to improve the dispersity of CNTs in DMF solution [83]. Finally, uniform

337

SNCC nano/micro-structured spheres were obtained through electrospray method accompanying

338

further carbonization process.

339 340

Cyclic voltammetry (CV) curves of electrode with the SNCC spheres at a scan rate of 0.5 mV s-1 are

341

shown in Fig. 10B. A broad cathodic peak appeared at 0.68 V in the first cycle and disappeared in

342

latter cycles, which results from the decomposition of electrolyte and the formation of SEI films,

343

leading to initial irreversible capacity [83]. In the next CV curves, there is a main cathodic peak at

344

0.18 V (the lithiation reaction of Si to form LixSi alloy), while the other two peaks at 0.36 and 0.52

345

V at the anodic sweep is assigned to delithiation process of LixSi [84]. The SNCC spheres have a

346

long flat discharge plateau at 0.09 V in the first cycle (Fig. 10C, the characteristic plateau of the

347

lithiation of crystalline Si) [85]. The SNCC spheres show promising stability and reversibility even

348

though 10 mV is used as the cutoff voltage. In the latter cycles, the crystalline Si transformed to

349

amorphous Si and exhibited the representative charge/discharge profiles of amorphous Si [84].

350

Nevertheless, a low initial coulombic efficiency (CE) of 46% for RH-Si electrode was obtained with

351

a high initial discharge specific capacity of 3006 mA h g-1. However, the initial discharge specific

352

capacity of SNCC spheres reached 2062 mA h g-1 with a high CE of 72%, demonstrating that 17



353

incorporation of Si nanoparticles into micro-sized hierarchical carbon matrix could significantly

354

reduce the undesired surface reaction. When cycling at the current density of 0.5 A g-1, the SNCC

355

spheres could maintain 92% of the initial charge specific capacity after 50 cycles, and deliver a

356

specific capacity as high as 1031 mA h g-1 even after 100 cycles with a high CE around 100% (Fig.

357

10D), nearly twice higher than the theoretical capacity of graphite (372 mA h g-1). However, the

358

specific capacity of RH-Si counterpart sharply decreased to below 200 mA h g-1 after 17 cycles.

359

And the SNCC spheres can keep the microsphere morphology well after 100 full charge/discharge

360

cycles, indicating their electrochemically stable structure toward Li+ intercalation/extraction. The

361

mass loading of the SNCC electrode entirely located between 0.8 and 1.0 mg cm-2, much higher

362

than that of the previous reported RH-derived Si-based electrodes (0.1-0.3 mg cm-2) [69, 72, 74].

363

Due to the 3D conductive networks built by N-doped carbon/CNT, the SNCC spheres electrode

364

also exhibits an excellent rate performance (Fig. 10E). At the current density of 0.1 and 1 A g-1,

365

the SNCC spheres electrode can achieve a specific capacity of 1460 and 978 mA h g-1, respectively,

366

corresponding to a capacity retention of 67% at high rate. Moreover, a specific capacity of 1236

367

mA h g-1 is recovered when the current density is lowered to 0.2 A g-1, showing high reversibility.

368

Such a favorable electrochemical performance is probably attributed to the elegant structural

369

design. On one hand, the carbon matrix built by CNTs and N-doped carbon layers can provide

370

sufficient conductive pathways for electron diffusion and fast lithium ion transportation, enabling

371

a high specific capacity at large charge/discharge current; on the other hand, the flexible CNTs

372

and the robust N-doped carbon framework function as structural reinforcement can maintain the

373

whole integrity to overcome mechanical breaking during the huge volume changes of Si [82].

374 18


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Figure 10 (A) Schematic illustration of the preparation process for SNCC nano/micro-structured spheres; (B) CV curves of SNCC spheres electrode at a rate of 0.5 mV s-1; (C) Charge/discharge profiles of SNCC spheres electrode; (D) Cycling performance and CE of SNCC spheres electrode and RH-Si electrode at current densities of 0.1 and 0.2 A g-1 for the 1st and 2nd cycle, then 0.5 A g-1 for latter cycles; (E) Rate performance of SNCC spheres electrode at various current densities. The voltage range is 0.01-1.5 V vs. Li+/Li. Copyright with the permission from ref. [82].

19



383

4. CONCLUSIONS

384

Waste to energy or material is becoming increasingly important. In this context, waste biomass of

385

RH is being considered as a sustainable and renewable energy resource and has high potential as

386

low cost precursors for the production of value added materials. Harvesting silica or silicon from

387

RH can not only take full advantage of its highest potential value, but also minimize the related

388

environmental issues from the valorization of RH. Thermochemical processes such as pyrolysis,

389

gasification is considered as the most promising approach that can converted RH to biofuels and

390

biochars simultaneously. RH-derived silicon materials could become standard ingredients for LIBs.

391

RH silica may be utilized in future applications such as surface functionalized mesoporous silica,

392

development of hybrid mesoporous silica, synthesis of fluorescent silica particles, antireflective

393

optical coatings, addition of CNTs to mesoporous silica, and nanoparticles supported on silica.

394 395

RH amorphous silica could be synthesized to crystal silicon for battery materials (e.g., LIBs) via

396

carbothermal or magnesiothermic reduction performed at a relatively lower temperature. The

397

fabrication of Si anode materials by using biomass resources enables the effective utilization of

398

agricultural residues in battery industries. The RH-derived Si nanoparticles always suffer from a

399

low coulombic efficiency in the initial charge/discharge and limited cycle life as anode materials

400

due to high surface reactions and low thermodynamic stability. Based on the reported works, the

401

fabrication of Si nanomaterials with nano/micro-hierarchical structure would be an ideal way to

402

improve their electrochemical performances, since the materials on the micrometer size possess

403

a good thermodynamic stability upon cycling. Embedding nano-Si into 3D conductive matrix is an

404

effective way to improve the structure stability and form stable SEI films on surfaces. Among

405

various Si/carbon composite materials, CNT will become a promising matrix because of the wired 20


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406

morphology, high electronic conductivity, and robust structure. Additionally, CNTs can easily form

407

3D cross-linked conducting networks that ensure effective electron transportation among active

408

particles. Consequently, Si anode materials with nano/micro-hierarchical structure, in which CNTs

409

are tightly intertwined between the RH-derived Si nanoparticles, are supposed to be ideal anode

410

materials for LIBs. In summary, the amorphous silica-rich RH can become a potential resource of

411

low cost precursors for the fabrication of high value-added silicon nanomaterials in battery anode

412

applications.

413 414

AUTHOR INFORMATION

415

Corresponding Author

416

*E-mail: [email protected]

417

Notes

418

The authors declare no competing financial interest.

419 420

ACKNOWLEDGEMENTS

421

This work is financially supported by the Startup Fund for Introducing Talent at NUIST (grants no.

422

2243141501046), and the National Science Foundation of China (grant no. 91544220, 21607079).

423

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Figure Captions

652

Figure 2 Digital and SEM images of rice plant, rice, RH, RHA and synthesized silica nanoparticles.

653

Figure 3 Production of both amorphous silica and activated carbon from RH.

654 655

Figure 4 Schematic illustration of the porous carbon (PC) synthesis from RH and SEM images of silica-carbon (SiO2-C) composites (a-c) and PC (d-f).

656

Figure 5 Synthesis of silica nanoparticles from RH and formation of porous silica framework.

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Figure 6 Generation of 3D nanoporous Si from RH. (A) Photographs of rice plant. (B) Photograph of RH after milling. (Upper Inset) Optical micrograph showing the morphological characteristic of outer/inner surfaces of RH. (Lower Inset) Circular chart indicating the main composition of RH. (C) Optical micrograph of a RH shell magnified from the black box in B. (Inset) Si-mapped SEM-EDS image suggesting that silica exists mostly along the outer rugged surface of RH. (D) The overall procedure of synthesizing the nanostructured Si from RH. (Left to Right) Pretreated RH by an acid-leaching process, RH silica by a thermal decomposition process, a Si/MgO mixture formed after a magnesiothermic reduction process, and the 3D porous Si obtained after an additional two-stage acid etching process.

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Figure 7 A large scale production of nano-Si from RH. (a) Comparison of carbothermal reduction and magnesiothermic reduction for producing nano-Si. Si(OR)4 denotes silicon alkoxide. (b) A panicle of ripe rice at a rice farm. (c) Flow chart of the process for recovering Si nanoparticles from RH. (d-g) Optical images of the intermediate substances. The inset of (f) shows an optical microscopy image (0.9 mm 3 0.6 mm) of one piece of thermal treated RH.

671

Figure 8 Synthesis of a binder-free composite of Si-graphene as an anode material for LIBs.

672 673 674

Figure 9 (A) A rapid microwave assisted method to extract SiO2 from agricultural residues such as BC, RH, and SB followed by the magnesiothermic reduction of SiO2 to Si; (B) Si nanohybrids production by using a MW-ST process.


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Figure 2 Digital and SEM images of rice plant, rice, RH, RHA and synthesized silica nanoparticles

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Figure 3 Production of both amorphous silica and activated carbon from RH

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Figure 4 Schematic illustration of the porous carbon (PC) synthesis from RH and SEM images of silica-carbon (SiO2-C) composites (a-c) and PC (d-f).

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Figure 5 Synthesis of silica nanoparticles from RH and formation of porous silica framework.

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Figure 6 Generation of 3D nanoporous Si from RH. (A) Photographs of rice plant. (B) Photograph of RH after milling. (Upper Inset) Optical micrograph showing the morphological characteristic of outer/inner surfaces of RH. (Lower Inset) Circular chart indicating the main composition of RH. (C) Optical micrograph of a RH shell magnified from the black box in B. (Inset) Si-mapped SEM-EDS image suggesting that silica exists mostly along the outer rugged surface of RH. (D) The overall procedure of synthesizing the nanostructured Si from RH. (Left to Right) Pretreated RH by an acid-leaching process, RH silica by a thermal decomposition process, a Si/MgO mixture formed after a magnesiothermic reduction process, and the 3D porous Si obtained after an additional two-stage acid etching process.

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Figure 7 A large scale production of nano-Si from RH. (a) Comparison of carbothermal reduction and magnesiothermic reduction for producing nano-Si. Si(OR)4 denotes silicon alkoxide. (b) A panicle of ripe rice at a rice farm. (c) Flow chart of the process for recovering Si nanoparticles from RH. (d-g) Optical images of the intermediate substances. The inset of (f) shows an optical microscopy image (0.9 mm 3 0.6 mm) of one piece of thermal treated RH.

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35



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Figure 8 Synthesis of a binder-free composite of Si-graphene as an anode material for LIBs.

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Figure 9 (A) A rapid microwave assisted method to extract SiO2 from agricultural residues such as BC, RH, and SB followed by the magnesiothermic reduction of SiO2 to Si; (B) Si nanohybrids production by using a MW-ST process.

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812

Table Captions

813

Table 1 Silica-metal reaction thermodynamic data

814

Table 2. Comparison of RH-derived Si materials in LIBs applications

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Table 1 Silica-metal reaction thermodynamic data [94] Reaction

∆H (kJ/mol)

Temperature

∆Go (kJ/mol)

(oC)

Adiabatic Temperature (oC)

SiO2 + 2Ca = Si + 2CaO

650

-363

-333

2348

SiO2 + 2Mg = Si + 2MgO

650

-312

-261

1906

SiO2 + 4/3Al = Si + 2/3Al2O3

650

-210

-180

1477

SiO2 + Ti = Si + TiO2

650

-34

-33

407

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Table 2. Comparison of RH-derived Si materials in LIBs applications Si Materials

Properties

Capacity (mAh/g)

Life Cycle

Ref.

Si nanoparticles

10-40 nm

2790

86% (300 cycles)

[107]

Nanoporous Si

40-60 nm

1554

100% (200 cycles)

[104]

Mesoporous Si

150.1 m2/g

1220.2

1000 mAh/g (100 cycles)

[105]

Carbon-SiO2 composite

500

100% (40 cycles)

[106]

Si-graphene composite

1000

100% (300 cycles)

[109]

820

38


Rice Husk Silica-Derived Nanomaterials for Battery Applications: A

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