Synthesis and characterization of carbon-based electrodes derived from biomass

Nearly 30 years ago, following the commercialization of the world's first lithium-ion battery by Sony Energetic, the persistent demand for high-performance rechargeable batteries led to continuous academic and commercial developments in this technology. Today, rechargeable batteries are used widely in numerous applications, ranging from laptops to automobiles and even to the power supply of entire cities. Lithium-ion batteries are particularly attractive due to their relatively low cost, non-toxicity, long service life, and high energy density. To meet the requirements of lithium-ion batteries, many research groups have extensively investigated potential cathode materials such as LiCoO2, LiNi1−x−yCoxMnyO2, LiMn2O4, and LiFePO4. Among these, LiMn2O4 (LMO) stands out as a promising cathode material owing to its low cost, abundant availability, environmental friendliness, and low safety risk. In addition to its capacity of 148 mAh/g, LMO attracts significant attention because its face-centered cubic spinel structure in the Fd-3m space group allows three-dimensional (3D) ion transport, thereby greatly enhancing power capacity. LMO (approximately 4.0 V) also offers advantages such as higher nominal voltage compared to LiCoO2 (approximately 3.6 V), high thermal stability, and ease of implementation. However, LMO has several disadvantages, including the Jahn-Teller distortion (particularly above 55°C), which leads to a transition from the cubic spinel phase to the tetragonal spinel phase and causes a 1 V drop. Additionally, the dissolution of manganese (Mn) into the electrolyte and the micro-cracks that form on the electrode surface during electrochemical cycling adversely affect cycling behavior and reduce electrical conductivity (10−6/cm). Various strategies such as synthesizing different morphologies, doping, and coating have been proposed in the literature to address these issues. Among these, coating is one of the simplest methods for mitigating the aforementioned problems of LMO. Carbon coating, in particular, has garnered significant attention because it provides a continuous electronic pathway throughout the coated layer, ensuring that particles remain electrically conductive. Moreover, coating LMO with carbon or carbon-like materials greatly enhances its electrochemical properties. Recent studies have also highlighted the importance of graphene (a honeycomb structure derivative of carbon) due to its two-dimensional architecture, which provides exceptional electronic conductivity, a large surface area (2600 m2/g), and good mechanical flexibility. One study reported that a LiMn2O4/Graphene hybrid composite electrode exhibits very little capacity loss even after 200 cycles. The electrochemical applications of silicon have evolved through various discoveries since the 1950s, especially gaining momentum with the production of porous silicon (P-Si). Early studies on electrochemical shaping in 1956 and the subsequent 1990 discovery of porous silicon exhibiting visible photoluminescence at room temperature opened up significant research avenues in sensors, lasers, biomedical applications, and optoelectronics. Silicon nanoparticles produced via electrochemical methods have been widely used in areas ranging from drug delivery to improving the efficiency of photovoltaic systems. Porous silicon structures with different surface properties have enabled new possibilities in biomedical imaging and solar fuel production. In the 2000s, the electrochemical reduction of silicon in molten salt environments was developed, offering an eco-friendly and safe approach for producing high-purity silicon. Meanwhile, the electrochemical doping of silicon and the fabrication of P–N junctions have shown promise for reducing costs and improving energy efficiency in solar cells and semiconductor devices. Current research on silicon electrodes for lithium-ion batteries focuses on developing materials that can compensate for volume changes, self-heal, and deliver high performance. Strategies such as graphene coating, silicon–carbon nanocomposites, and porous designs seek to enhance the cycle life and minimize capacity losses. Silicon is considered one of the most promising anode materials for high-energy lithium-ion batteries due to its abundance in nature, eco- friendliness, approximately 10 times higher capacity (around 4200 mAh/g for Li4.4Si) compared to graphite, and a reasonably low electrochemical potential (~0.4 V vs. Li+ /Li). Unlike graphite, silicon does not have a layered structure; it generally crystallizes in a diamond- like structure with a face-centered cubic lattice and stores lithium by forming an alloy with lithium. During the initial discharge, lithium ions diffuse into the silicon surface to form a Li–Si alloy, which involves significant volumetric expansion. Among various Li–Si phases, Li22Si5 offers the highest theoretical capacity (4200 mAh/g) and the greatest volume expansion (~420%). Another attractive material for lithium-ion battery applications today is carbon aerogel. Compared with other aerogel materials, carbon aerogels possess many remarkable properties, including ultra-low density, large surface area, high electrical conductivity, thermal and chemical durability, and good mechanical characteristics. Carbon aerogels (CA) can be categorized into three groups, depending on the carbon precursor: phenolic resin-derived carbon aerogels, aerogels derived from other carbon allotropes (carbon nanotube and graphene), and biomass-derived carbon aerogels. Carbon aerogels with a three-dimensional hierarchical network are considered ideal for lithium-ion batteries because of their high specific surface area, tunable porosity, large pore volume, high electrical conductivity, and hydrophobic nature. In the master's thesis, carbon aerogels synthesized from corn starch biomass via the sol-gel method were utilized to produce carbon aerogel/LiMn2O4 (CA/LMO) composite cathodes and carbon aerogel/silicon (CA/Si) composite anodes. These carbon aerogels exhibit unique properties, such as ultra-low density, large surface area, high electrical conductivity, and exceptional thermal and chemical stability. The goal of utilizing these properties was to improve the performance and stability of the electrodes. Specifically, the carbon aerogel aims to suppress Mn dissolution in LMO cathodes and act as a buffer against volume changes in silicon anodes, thereby enhancing electrode stability, electrical conductivity, cycle life, and electrochemical properties. Composite electrodes were fabricated by combining pure LMO and silicon nanopowders with the carbon aerogels derived from corn starch biomass. This novel approach seeks to leverage a low-cost, environmentally friendly precursor to create sustainable, long-cycle-life LMO cathodes and Si anodes. The morphological, chemical, and electrochemical properties of the carbon aerogels synthesized from corn starch biomass and the resulting CA/LMO and CA/Si composite electrode materials were thoroughly analyzed. Electrochemical tests demonstrated that the carbon aerogels significantly improved the performance of both the Si anodes and LMO cathodes. The results confirmed that the carbon aerogels contributed positively to the electrochemical properties of the Si anodes and LMO cathodes, enhancing their stability and suitability for lithium-ion battery electrodes.