SiO2/C Composite as a High Capacity Anode Material of LiNi0.8Co0.15Al0.05O2 Battery Derived from Coal Combustion Fly Ash

1. Introduction

In recent decades, energy shortage and environmental pollution have become serious issues along with the world population growth. Extensive research on renewable energy sources has been rapidly performed. Current renewable energy cannot be separated from energy storage systems. Lithium-ion batteries (LIBs) have become the main candidates for a wide range of applications owing to their fast charging, long lifespan, low cost, high energy density, and ability to be used repeatedly with little degradation in performance [1,2,3,4,5]. Currently, graphite is widely used as a common anode material in the industry owing to its advantage of long lifespan and low cost [6,7,8]. In addition to graphite, Li4Ti5O12 or LTO has also become a promising candidate because it has fast charging and long shelf life [9,10]. However, both of them exhibit low storage capacity (graphite = 372 mAh g−1 and LTO = 160 mAh g−1), which has become a huge obstacle to further application, especially for electric vehicles and electronic devices [1,11]. Various different anode materials with higher capacities have been studied as a candidate of the next generation of anode material. Among the various anode materials studied, Si is considered as the most promising anode material owing to its high theoretical capacity (4200 mAh g−1), moderate operating voltage (0.1–0.5 V vs. Li/Li+), and low stable plateau potential [12,13]. Unfortunately, Si anode materials experience some challenges such as low electrical conductivity, large volume expansion (>300%) during lithiation and delithiation, and an unstable solid electrolyte interphase (SEI), which leads to poor cycling performance [1,9]. Therefore, silicon suboxide or SiOx (x < 2) has been suggested as an alternative anode material. Even though SiOx has a smaller theoretical specific capacity (1961 mAh g−1) than Si, it exhibits better cycling stability, smaller volume expansion, lower cost, and eco-friendliness. SiOx also has a drawback of poor conductivity and low Coulombic efficiency at the initial charge–discharge cycle. On the other hand, it is difficult to produce SiOx owing to the partial oxidation of silicon or semi-reduction of SiO2, which require complex and convoluted synthesis steps. It is advantageous to use SiO2 as an anode material owing to its high availability and because it can be easily extracted from biomass-based waste. Similar to SiOx, SiO2 has low conductivity, which needs to be improved before being applied in full batteries [14,15]. Many approaches have been tried to overcome these problems such as inserting Li into SiO2 and modifying the size of SiO2 [14]. However, one of the most effective ways is to use composite SiO2 with carbon. The composite improves electrical conductivity and also reduces volume expansion during the charge–discharge cycling to maintain a stable electrochemical performance [16]. For example, it has been reported that the SiO2/C composite showed an impressive reversible capacity of 635.7 mAh g−1 at the discharge current of 100 mA g−1 [17]. The 3D SiO2@graphene aerogel composite shows a reversible capacity of 300 mAh g−1 at the discharge current of 500 mA g−1 [18].However, the SiO2-carbon (SiO2/C) composite preparation for anode material usually requires many complex steps such as additional process to modify the morphology, expensive silicon precursor, e.g., Tetraethyl orthosilicate (TEOS), pretreatments [18] and multiple high temperature and long heating process [17], which greatly hinders further improvement. Thus, these additional processes will directly affect the overall production cost of SiO2/C anode material. The use of cheap raw materials, especially from waste, and a simple processing method reduce the production cost in a significant amount. Hence, it is desirable to design a facile method to fabricate SiO2/C composites to realize high-performance anode materials for LIBs. In this article, we reported a facile and inexpensive method for synthesizing the SiO2/C composite from coal combustion-derived fly ash for active anode material by mechanically milling SiO2/C and heat-treating it in an inert atmosphere. The use of silica from coal fly ash is a sustainable approach to recycle, reduce, and reuse industrial waste. SiO2/C was directly used as the anode material for the LiNi0.8Co0.15Al0.05O2 battery. This approach has never been studied before.

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