Abstract:SnO2 nanoparticles have been synthesized by a novel route of a sol-gel method assisted with biomimetic assembly using L-leucine as a biotemplate. The microstructure of as-prepared SnO2 nanoparticles was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectra (FT-IR), and Brunner−Emmet−Teller (BET) measurements. The results demonstrated that the growth of SnO2 could be regulated by L-leucine at a high calcination temperature. The electrochemical performance of SnO2 was also measured as anodes for lithium-ion battery. It is a guidance for the growth regulation of SnO2 at high temperature to obtain SnO2/C with nanosized SnO2 coated by a graphitic carbon.
1. Introduction
       Lithium-ion batteries (LIBs) are the dominant power supply for portable electronics and also show promising applications for electric vehicles and power storage systems, due to their high specific energy, good cycling performance, high coulombic, and energy efficiency [1, 2]. In accordance with the increasing energy requirement for these industries, LIBs develop toward higher capacity, higher energy, and higher power. Therefore, it is of great significance to explore electrode materials with high capacity for the next generation LIBs. Graphite is the principal commercialized anode for LIBs since invented in 1991, but its explored capacity has been reached the theoretical limit (372 mAh/g) [3–10]. Metal oxides (Co3O4, Fe3O4, and SnO2) have been studied as anodes to enhance energy density of LIBs, for they can deliver higher capacity than graphite [11–13]. In this regard, SnO2 has been considered as an outstanding alternative to graphite because of its high theoretical capacity of 782 mAh/g and moderate lithiation potential (∼0.6 V vs. Li+/Li) [8, 14–17]. It is commonly recognized that SnO2 experiences a two-step lithiation process, namely,
       The reaction shown in (1) is irreversible, which would induce low initial coulombic efficiency (CE) of ~50%. Additionally, SnO2 electrode would suffer a large volume variation (~260%) resulted by the reaction of (2) as well as Sn, which would cause crack and collapse. And then, the capacity fading of SnO2 is dramatical during cycling. To improve cycling performance for SnO2, tremendous investigations have indicated that SnO2/C composite anode with nanosized SnO2-coated carbon is the most effective strategy [15, 18–20]. In SnO2/C electrode, nanosized SnO2 could sustain large volume change of Sn during lithiation and delithiation and carbon can buffer the volume change and also maintain the conductivity network for the whole electrode. Therefore, the cycling life of SnO2/C would greatly be extended. However, most previous studies presented that SnO2 was usually coated by amorphous carbon via the pyrolysis of carbonaceous organic material at 400~500°C, such as glucose. Unfortunately, amorphous carbon with a higher specific surface area might bring large amounts of side reactions with electrolyte, leading to a lower initial CE of SnO2. Besides, amorphous carbon usually shows higher average lithiation/delithiation voltage and larger voltage hysteresis, which would contribute little improvement in terms of energy density for LIBs in fact. Now, even since it has been reported that amorphous carbon can be catalytically graphitized at a lower temperature of about 600~700°C [21–23], SnO2 particles would grow greatly large at this temperature and experience a rapid capacity fading as lithium-ion anodes. Accordingly, it is of importance to obtain nanosized SnO2 and suppress its growth at 600~700°C for the application implementation of SnO2/C.
       In this work, nanosized SnO2 were synthesized by a sol-gel method assisted with biomimetic assembly. Biomimetic synthesis is a novel route to fabricate nanosized inorganic particles with organic templates. Investigations have identified that specific molecular interactions at inorganic-organic interfaces could result in the controlled nucleation and growth of inorganic crystals [24–26]. During the biomimetic assembly process, the organic template could promote self-assembly, recognize the reactant substrate, guide the nucleation, and limit the growth of inorganic particles by utilizing biological adsorption, hydrogen bond, van der Waals force, and so on. Considering that L-leucine could regulate the synthesis and the growth of organic particles and even enzymes, while no researches related to the regulation of inorganic materials by L-leucine could be found [27], we would like to control SnO2 nucleation and growth by biomimetic assembly using L-leucine as a biomimetic template here. The regulated mechanism of SnO2 synthesis and growth is studied for the first time in this work. Moreover, the electrochemical performance of as-prepared SnO2 was also measured as lithium-ion anodes.