Electrochemical Properties of Amorphous Silica As an Anode Material for Li-Ion Batteries

Abstract

After conquering the scene of portable electronics (i.e. mobile phones and laptop computers) since their introduction to the marketplace in 1991, lithium ion batteries (LIBs) are set to further enable wide-scale deployment of electric vehicles (EVs) and hybrid electric vehicles (HEVs). However, a series of challenges still remain with respect to a number of important factors, such as the operational safety of LIBs, raw material supply, cost, recycling and environmental aspects. In order to fulfill future demands for large-scale applications, significant improvements and deeper understanding are required in these areas. As of today, graphite is the anode (negative electrode) material primarily used in commercial LIBs, mainly due to its charge-discharge capacity retention capability and an attractively low, flat operational voltage as well as a manageable voltage hysteresis upon (de)intercalation which altogether maximizes both the energy density and energy efficiency of the cell in which it is employed. However, recent projections regarding the future cost and availability of graphite have prompted researchers to explore alternatives in the context of cheap and sustainable materials for electrochemical energy storage. Silica (SiO2) is currently being explored as a possible alternative candidate material in this context. SiO2 was long disregarded as a potential electrode material due to its apparent electrochemical inactivity. However, studies in recent years have sparked renewed interest in SiO2 after it was demonstrated that modified nano-SiO2 may indeed be electrochemically active. SiO2 is a conversion-type anode material which irreverisbly produces various silicates (e.g. Li4SiO4 and Li2Si2O5) as well as elemental Si during electrochemical lithiation. Although the precise reaction mechanism is still a topic for debate, it is believed that the by-products formed in-situ during electrochemical cycling can mechanically buffer the stress and strain associated with continued (de)lithiation of Si and thereby extending the cycle life of the electrode material. The present study systematically investigates the electrochemical properties of amorphous SiO2 as a negative electrode material for Li-ion batteries. The SiO2 materials in question include spherical nano-particulate SiO2 recovered as an industrial by-product or naturally nano-sized 3D-architectured SiO2 extracted from a renewable resource of biological origin – the main motivation here clearly being the study of dirt-cheap, highly abundant materials and the possible prospect of their implementation in electrochemical storage devices. By virtue of its electronically insulating properties, SiO2 needs to be carefully engineered in order to activate the material. Important factors in this regard include control of particle morphology, surface area and porosity as well as enhancement of electronic conductivity. Consequently, various carbon sources (i.e. glucose, sucrose, starch) have been utilized in varying amounts in order to achieve a conductive carbon coating on the SiO2 particles via high-temperature pyrolysis (i.e. carbonization), ultimately yielding SiO2/C composites with improved electrochemical properties in relation to bare SiO2. Here, these variations in carbon precursor, carbon content and coating thickness etc. are directly related to the electrochemical activity of the different SiO2 materials. These aspects are additionally highlighted by means of a series of advanced characterization techniques. The influence of particle size and morphology is further made possible by the direct comparison of spherical SiO2 nanoparticles to the intricate 3D-architecture of the SiO2 derived from harvested biomass. Preliminary results indicate it appears possible to electrochemically activate (i.e. displace Si followed by its electrochemical lithiation at low voltages vs. Li+/Li) the latter via careful considerations in terms of electrode design and electrochemical testing procedures which results in gravimetric capacities of >1000 mAh g-1 at an average (de)lithiation voltage in the range of 0.2-0.4 V vs. Li+/Li. By comparison, the spherical nanoparticles exhibit significantly lower electrochemical activity, i.e. a reversible gravimetric capacity of 300-400 mAhg-1 with highly sloping voltage profiles in an electrochemical cycling windowof ~0-2 V vs. Li+/Li.