One-Pot Bottom-Up Synthesis of SiO₂ Quantum Dots and Reduced Graphene Oxide (rGO) Nanocomposite as Anode Materials in Lithium-Ion Batteries

Abstract

Here, crystalline SiO₂ quantum dots (QDs) of 3–5 nm size were grown within the layers of reduced graphene oxide (rGO) by a solution mode chemical growth process at a relatively low temperature (100 °C). The composite was applied as a negative electrode in a Li-ion half-cell battery and the electrochemical investigation confirmed a distinct first-cycle discharge/charge capacity (~865 mAhg⁻¹/387 @ 51 mAg⁻¹). The battery could retain a capacity of 296 mAhg⁻¹ after 60 charge/discharge cycles with 99% coulombic efficiency. Furthermore, at a high current rate of 1.02 Ag⁻¹, the battery was able to display an apparent rate capability (214.47 mAhg⁻¹), indicating the high chemical and mechanical stability of the composite at a high current rate. A structural analysis revealed clear distinct diffraction peaks of SiO₂ and high-resolution transmission electron microscopy images showed discrete atomic planes, thereby confirming the growth of crystalline SiO₂ QDs within the layers of rGO.

1. Introduction

Recently, SiO₂ has attracted a lot of attention in lithium-ion battery research due to it high abundance, low cost, high theoretical capacity (~1965 mAhg⁻¹), better cycling stability, easy availability and good working potential [1,2,3]. The formation of Li₂O (lithium oxide) and Li silicates (Li_xSiO_y) during the lithiation reaction usually buffers the volume change, thereby improving the electrochemical property. However, the poor electrical conductivity and low initial coulombic efficiency (ICE) due to the irreversible consumption of active Li ions, leading to large volume expansion, are the bottlenecks for future applications such as electric hybrid vehicles. In order to enhance the electrochemical properties of SiO₂-based anodes, several approaches have been undertaken to engineer the size/morphology as hollow SiO₂ microspheres [4], nanobelts [5], submicron silica [6], nanospheres [7] and hollow and porous structures [8,9], thereby achieving good electrochemical performance (750 mAhg⁻¹ (@ 100 mAg⁻¹) until the 500th cycle [4], 534 mAhg⁻¹ (@ 0.1 C) after 70 cycles [5], 602 mAhg⁻¹ (@ 100 mAg⁻¹) at the 150th cycle [6], 876.7 mAhg⁻¹ (@ 1 C) over 500 cycles [7] and 919 mAhg⁻¹ over 30 cycles [8]).

Investigations were carried out to enhance the electronic and ionic conductivity of SiO₂ by preparing composites in the form of SiO₂ nanotubes/C nanofiber mat [10], SiO₂ nanotubes coated with nitrogen-doped carbon layers [11], SiO₂@C core-shell sub-microsphere [12], carbon-coated silica nanoparticles [13], SiO₂/C composite fibers [14], a silica nanonetwork confined to a nitrogen-doped ordered mesoporous carbon framework [15] and nanostructured silica/carbon composite spheres [16,17]. Zhang et al. [10] synthesized a SiO₂ nanotube–carbon nanofiber mat by electrospinning which delivered a good capacity of 1067 mAhg⁻¹ (562 mAhg⁻¹) at a current density of 100 mAg⁻¹ (1000 mAg⁻¹) after 300 (700) cycles. The composite had high mechanical flexibility and could be used as a free-standing electrode without any binder, carbon black or current collector. Zhang et al. [11] described the synthesis of a SiO₂ nanotube coated with N-doped carbon layers that achieved a distinct specific capacity (781 mAhg⁻¹ (100 mAg⁻¹) after the 200th cycle). The composite was synthesized by a combination of the sol–gel method and dopamine self polymerization and N-doped carbon layers enhanced the electronic conductivity and Li⁺ diffusion rate of SiO₂. Similarly, other SiO₂/C nanocomposites delivered relatively good electrochemical performances (349.2 mAhg⁻¹ @ 100 mAg⁻¹ after 200 cycles [12], 440.7 mAhg⁻¹ @ 0.5 Ag⁻¹ after 500 cycles [13] and 800 mAhg⁻¹ @ 200 mAg⁻¹ after 20 cycles [15]). All these interesting electrochemical performances of SiO₂, SiO₂/C composites inspired us to revisit and investigate the SiO₂-based composite with graphene [18,19] or reduced graphene oxide, which has hardly been studied.

The growth of crystalline SiO₂ quantum dots over reduced graphene oxide (rGO) might be one of the preferable choices for enhancing the active surface area, electrical conductivity and flexibility to accommodate the volume change and achieve a high capacity. Furthermore, the synthesis of SiO₂ quantum dots directly from the Si precursor, with a direct Si-O bond using a low temperature and bottom-up approach, might be an economic and energy-efficient method compared to high-temperature processes and is worth investigating. For example, Si starting materials, such as APTMS (3-(aminopropyl)-trimethoxysilane), have a direct Si-O bond in the complex, making them very suitable starting precursor materials for the production of SiO₂. Therefore, we developed a solution mode low-temperature (100 °C) growth process for the production of SiO₂-rGO nanocomposites where the crystallization of SiO₂ from a Si precursor and the chemical reduction of GO to rGO take place at the same time.

2. Materials and Methods

First, 10 mL of the Si starting material, (3-Aminopropyl) trimethoxysilane (APTMS, H₂N(CH₂)₃Si(OCH₃)₃), was mixed with 40 mL deionized (DI) water in a round-bottomed (RB) flask; an ascorbic acid (AA, 0.66075 g, 0.3 M) solution in DI water (12.5 mL) was added slowly under continuous stirring for 20 min. Then, the solution mixture in the RB flask was placed on a heating mantle at 100 °C for 5 h under continuous stirring, which was then cooled down naturally to room temperature. Then, the modified APTMS solution was gradually injected into GO (1 g) solution in deionized water (100 mL) under stirring conditions for 30 min. A further chemical reaction was carried out under a reducing condition with the drop by drop addition of an aqueous solution of 1.5 g NaBH₄ (sodium borohydride) in 20 mL of water and the chemical reaction was continued for 60 min. Black color precipitates produced in the solution were repeatedly washed and dried by a standard protocol; the dried powder was then annealed at 650 °C for 4 h in Ar atmosphere for further investigation.

X-ray diffraction was used to confirm the crystallographic structure of the nanocomposite. The morphology and concentration of the different elements were characterized by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectrometer (EDS) attached with TEM. The different bond vibrations were identified by Fourier transform infrared spectroscopy. Anodes in the form of CR2032-type coin cells were fabricated for electrochemical studies.

The slurry was prepared by combining the solid components as 65 wt. % of active materials, 20 wt. % Super P (MMM Carbon, Belgium), 9 wt. % poly (acrylic acid) (PAA, Sigma-Aldrich Co.), 3.5 wt. % styrene butadiene rubber (SBR, Zeon Co.), and 2.5 wt. % carboxymethyl cellulose (CMC, Sigma-Aldrich Co.). Mixing was performed in a planetary mixer (G-Mixer 400S, Gold Max Applied Materials Co.). The electrodes were prepared by casting the slurry onto a sheet of copper foil (Nippon Foil Co.) followed by drying in an oven at 90 °C for 1 h. The composite anode was stored in a glove box (with oxygen and humidity maintained below 10 ppm) for more than 24 h before electrochemical testing. For the cycle test, the cells underwent galvanostatic charge/discharge tests between 0.002 and 1.5 V versus Li⁺/Li using a multichannel battery testing system (AcuTech Systems BAT-750B).

3. Results and Discussion

The un-annealed and annealed (650 °C) SiO₂ QD-rGO nanocomposite revealed several distinct X-ray diffraction peaks, as observed in the XRD spectra (Figure 1a), which confirmed the polycrystalline nature of the SiO₂ QDs. The main XRD peak at 26.56° for un-annealed sample shifted toward a lower Bragg angle of 26.33° after annealing at 650 °C. All the other peaks were indexed, matching well with the standard JCPDS database (PDF 46-1045) of SiO₂. The nanocomposite showed relatively good absorption at 992 cm⁻¹ in the FTIR spectra (Figure 1b), which is attributed to Si-O-Si bond. Several other weak peaks at 1563 cm⁻¹, 1443 cm⁻¹, 783 cm⁻¹ and 687 cm⁻¹ were also observed, corresponding to C-C and Si-C bonds, respectively. The Si 2p XPS spectrum (Figure 1c) revealed an asymmetric peak indicating contributions from both Si³⁺ (102.30 eV for SiO₃C) and Si⁴⁺ (103.5 eV for SiO₂) oxidation states [20]. The corresponding Raman spectra (Figure 1d) showed peaks at ~1343 cm⁻¹ and 1583 cm⁻¹ corresponding to the D band (disordered or amorphous carbon) and the G band (graphitic carbon), respectively. The D band is k point phonon in A1g symmetry, whereas the G band is E_2g phonon of sp2 bonded carbon atoms in the honey-comb lattice. The intensity ratio (I_D/I_G) increased from 1.01 to 1.04 after annealing, indicating a decrease in the average size of the sp2 domains due to the annealing-induced reduction of the rGO [21,22].

The formation of highly crystalline SiO₂ quantum dots of 3–5 nm size over the rGO sheets [23] was confirmed by a high-resolution TEM image with clear distinct atomic planes (Figure 1e) and the lattice spacing was evaluated as 0.277 nm and 0.312 nm, corresponding to the (110) and (101) planes. The EDS analysis in Figure 1f shows all the elements, including Si (~20 at. %), and area mapping revealed complete coverage of Si and O over rGO.

The nanocomposite was used to fabricate the battery half-cell that delivered an obvious high discharge/charge capacity of 865 mAhg⁻¹ (388.6 mAhg⁻¹), 417 mAhg⁻¹ (370.6 mAhg⁻¹), and 398 mAhg⁻¹ (363.4 mAhg⁻¹) in the first, second and third cycles at a current rate of 51 mAg⁻¹ (Figure 2a)_. After 60 charge/discharge cycles, the composite could retain a noticeable capacity of ~286 mAhg⁻¹ and the coulombic efficiency could reach as high as 96%, as shown in Figure 2b. The composite delivered good capacities (Figure 2c) of 350 mAhg⁻¹ (51 mAg⁻¹), 280.78 mAhg⁻¹ (200 mAg⁻¹) and ~213 mAhg⁻¹ @ 1 Ag⁻¹ with increasing current rate, indicating the high physical and chemical stability of the SiO₂ QD-rGO nanocomposite at a high current rate. The capacity further returned to 308 mAhg⁻¹ with the reversal of the current rate to 51 mAg⁻¹ at 60 cycles, which indicated the high physical stability of the composite after 50 charge/discharge cycles at a high current rate, thereby increasing its applicability as future anode material in LIBs. Unlike in earlier studies [23,24,25], where the specific capacity of milled SiO dropped drastically with cycle number, the capacity of the SiOx/rGO nanocomposite is very stable after several charge/discharge cycles.

4. Conclusions

A strategic solution-based low-temperature synthesis methodology was developed to grow crystalline SiO₂ QDs within rGO layers. The nanocomposite was subsequently fabricated as an anode electrode in a Li-ion battery half-cell, delivering a good electrochemical performance in terms of cyclic stability and rate capability (~213 mAhg⁻¹ @ 1 Ag⁻¹, after 50 cycles) and confirming the high physical and chemical stability of the composite.