Polysiloxane-derived silicon nanoparticles for Li-ion battery
Graphical abstract
Introduction
Academia and the industry have both shown great interest in next-generation Li-ion battery (LIB) anode materials with characteristics such as high capacity, a proper charging/discharging potential, and safe and low-cost manufacturing and usage [1]. Among all potential LIB anodes, silicon (Si) is one of the most promising candidates to replace graphite due to its high theoretical specific capacity: 4212 mAh g−1 for Li4.4Si (at high temperatures) and 3579 mAh g−1 for Li15Si4 (at room temperature) versus 372 mAh g−1 of graphite anode [2]. Si exhibits a low discharge potential plateau (~ 0.1 V vs Li+/Li) that finds a good balance between retaining a reasonable open circuit voltage and avoiding adverse lithium plating processes [3]. Unfortunately, ≈300% volume change of silicon during lithiation and delithiation leads to severe particle pulverization, unstable solid-electrolyte interphase (SEI) formation, and loss of electrical contact at the electrode level, resulting in fading capacity and limited cycle life [4].
Nano-sized silicon is regarded as an effective solution to reduce the volumetric expansion of Li-ion battery anodes. Currently, different methods have been developed regarding the fabrication of Si nanostructures. One of the main issues of Si NP synthesis methods is the choice of a low-cost and commercially available precursor for Si. This issue is a crucial factor that should be considered when developing environmentally friendly and large-scale production methods. As of this writing, a number of inorganic and organic silicon precursors have been tested for Si NPs synthesis. For instance, pulsed laser ablation [5], [6], [7], [8], [9] and electrochemical techniques [10], [11], [12], [13], [14] employ Si metal as a precursor. Heating degradation [15], [16], [17], [18], [19] and laser-induced degradation methods [20], [21], [22], [23], [24], [25] use silane (SiH4). The magnesiothermic reduction technique uses SiO2 NPs and 3D template mesoporous SiO2 nanostructures derived from TEOS (Si(OC2H5)4 [26], [27], [28], [29], [30], [31], [32]. Alkali metal naphthalenide [33], [34], [35], [36], [37] and Zintl salt reduction processes [38] use SiCl4. Moreover, several living plants such as rice husks [39], natural attapulgite clay [40], and reed plants [41] were tested as low-cost and non-standard precursors for Si NPs synthesis. Silica obtained from the thermal annealing of dried plants is reduced into silicon nanoparticles by magnesium reduction method. Despite the fact that the as-prepared Si displays a good reversible capacity (2650 mAh g−1 [39], ~954 mAh g−1 [40] after 200 cycles and 420 mAh g−1 after 4000 cycles [41]), natural plants cannot be considered as acceptable precursors for synthesizing Si nanostructures.
Even though precursor material guidelines for designing Si nanostructures provide possible solutions for improved gravimetric capacity and capacity retention for practical applications, there are still many other important figures of merit to improve upon, including material costs, fire and explosion safety, toxicity and commercial availability. Among the known precursors, SiH4, SiCl4, and (Si(OC2H5)4 are harmful and flammable gaseous or liquid species and are not suitable for large-scale production processes. 3D template SiO2 precursors require special techniques for its preparation. Although Si metal is a low-cost precursor, the evaporation of Si metal is an energy-consuming process.
In this context, silicon-organic polymers, particularly polysiloxanes (PSs) with the chemical formula (R2SiO)n (also known as silicones) [42,43] can be regarded as commercially attractive precursors for synthesizing Si NPs, as they are not toxic and low-cost ($1–10 per kg) with production volumes of approximately 400,000 t/year worldwide. In 1997, Xing et al. [44,45] conducted the pyrolizis of siloxane polymers in inert gas at 1000 °C to produce Si–O–C compounds for Li-ion battery anode tests. The electrochemical properties of these materials were measured using coin-type test cells. The study demonstrated that the Si–O–C compounds with the largest reversible specific capacity for lithium (about 900 mAh/g) were approximately 43% carbon, 32% oxygen, and 25% silicon (atomic percentage).
In this study, a combustion approach to convert PSs into Si NPs with an average size between 5 and 100 nm is reported. In addition, the electrochemical performance of Si NPs as anode material for Li-ion battery was demonstrated. To produce Si NPs, the PSs were first converted to SiO2 nanoparticles (SiO2 NPs) by burning the organic matter in air, which was followed by the reduction of SiO2 by Mg under an auto-combustion regime. Compared to previously reported methods of producing nanostructured Si, our method has several advantages: (i) PSs are commercially abundant and low-cost materials with a supply that far exceeds the demand for LIB anode materials; (ii) the process enables the production of Si nanoparticles with a wide size range (5–100 nm) and high BET specific areas (65–107 m2 g−1); (iii) intermediate SiO2 NPs could be highly promising for biological applications; (iv) even though the process has several stages, it is a simple process that is easy to scale-up; (v) further electrolysis of MgCl2 can be used to regenerate the Mg metal.
Section snippets
Materials
PS in the form of a clear silicone sealant adhesive was purchased from a store. Prior to the experimental procedure, the silicone sealant was partially cured under a ventilation system for 24 h to transform the sealant into a resin. The resin was cut into small pieces (1–3 cm) to ensure an easier burning process. Mg powder (98% purity, particle size 50–250 μm) and NaCl (99% purity, particle size <500 μm) were obtained from Samchun Chemicals (Korea). Before mixing, NaCl was ground into a fine
Burning of PS in air and TEM morphology of SiO2 nanoparticles
We applied the thermodynamic software “THERMO” [46] to estimate the adiabatic temperatures (Tad) and equilibrium reaction species (C) for the burning of PS in oxygen. For simplicity, the calculation was performed for polydimethylsiloxane - [Si(CH3)2O]n depending on the moles of oxygen (α) and the results are shown in Fig. 2a. The highest temperature (1425 °C) was predicted at α = =4.0, which corresponds to the complete burning of PS:
The burning product (α
Conclusions
In summary, whereas silicon nanostructures have potential applications in a number of areas, green, cost-effective, and scalable synthesis still remains a challenge. To overcome this barrier, we developed a combustion route to recover Si NPs directly from PS, which is a synthetic compound composed of repeating units of siloxane, which in turn is a chain of alternating silicon atoms and oxygen atoms. PS was initially burned in air and subsequently annealed at 800 °C to produce SiO2
Declaration of Competing Interest
I confirm that there is no conflict of interest.
Acknowledgments
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (NRF-2018R1A4A1024691 and NRF-2017M2B2B1072889).
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