Nanostructured Si-C hybrids for Anode Application in Lithium Ion Batteries

Abstract

Rechargeable lithium-ion batteries (LIB) have achieved tremendous attention as one of the most efficient energy storage options. However, significant advancement in electronic devices demands for higher energy and power density, requiring that alternative electrode materials to be explored. Silicon is well recognized as a highly promising anode material due to its remarkable lithium storage capacity of 3590 mAh g-1. However, during the process of alloying or de-alloying with lithium, Si undergoes a significant expansion in volume, surpassing 300%. The repeated expansion and contraction during cycling of the cells leads to the disintegration of particles, accelerated formation of a solid electrolyte interphase (SEI), damage in the electrode calendars, all of which result in capacity fade and failure of the electrode. To mitigate the limitations linked to Si, nanostructuring was found to be an effective method. The purpose of nanostructuring is to reduce the strain on the electrode, mostly caused by the volume dilation of Si. However, the implementation of nanostructured Si individually as anode material causes rapid capacity fading and failure of the electrode. A promising strategy involves dispersing active silicon materials on a carbon-based matrix, which significantly enhances the electrical connectivity between the active material and the current collector and acts as a buffer to accommodate the volume dilation that occurs during continuous charge and discharge cycles. In a similar vein, in this thesis we have adopted an innovative approach using a carbon matrix synthesized from a precursor polymer-derived silicon oxycarbide (SiCO). Silicon oxycarbide, prepared from the inert pyrolysis of Si-containing preceramic polymers, is a versatile material with excellent thermodynamic stability and flexible stoichiometry. Interestingly, it serves as an excellent material for the integration of nanoparticles (in-situ and ex-situ) into a matrix. The microstructure of SiCO ceramic may be suitably designed by the meticulous selection of initial polymer precursors and by exercising control over the synthesis and processing conditions. One of the standout characteristics of these non- crystalline ceramics is their distinctive nanostructure, consisting of nanodomains composed of SiO2, carbon layers resembling graphene, and an intermediate region with a blend of tetrahedral bonds made up of Si, O, and C. In order to obtain carbon derived from SiCO ceramics, its microstructure can be altered by selectively removing the SiO2 nanodomains. This process results in the development of a highly porous, graphene-like carbon material. The resulting oxycarbide derived carbon (ODC) material, which is both conductive and flexible, can be used in combination with Si nanoparticulates for anode applications. The ODC serves as a conduit for interfacial adhesion and mechanical support and prevents capacity degradation by ensuring continuous and effective electrical contact between the silicon nanoparticles. The current work investigates a process to fabricate Si-C hybrid material with tailorable microstructure. The fabrication process involves integration of nanostructured Si into a carbon rich preceramic polymer precursor matrix to accommodate the volume change of Si during lithium alloying process. The initial step in this process involves preparing nanostructured silicon through high-energy mechanical milling, which form the source of nanoparticulate Si in every fabrication process described for the Si-C hybrids. The first kind of Si-C composites is prepared from high energy mechanical milling of nanocrystalline Si and oxycarbide derived carbon. Carbon-rich polysilsesquioxane polymer precursor was pyrolyzed at 1000 and 1200 °C to synthesize a silicon oxycarbide, which were further etched with HF to prepare the ODC. The Si-ODC composite produced from the mixing of Si and the ODC from 1200 ºC pyrolysis exhibited improved performance compared to the ODC obtained from 1000°C pyrolysis. The material showed reversible specific capacity of 1000 mAh g-1 after 200 cycles at a current density of 0.1 A g-1, and 420 mAh g-1 at a high current density of 2 A g-1. In the second synthesis route of Si-C hybrids, additional pore forming agent in the form of amorphous SiO2 fillers were added into the hybrid nanostructure which were subsequently removed by HF etching. The hybrid anode structures were fabricated from materials pyrolyzed at two different temperatures of 1000 and 1200 ºC to study the effect of pyrolysis temperature on the structure and the electrochemical behavior of the anodes. The final composites prepared at 1000 ºC delivered better electrochemical properties and displayed stable cycle life of 622 mAh g-1 for the 400th cycle at 0.1 A g-1 current density. The composite also exhibited better power capability of 229 mAh g-1 at 2 A g-1 rate. In an effort to further enhance the electrochemical performance, another method for fabricating Si-C hybrids was pursued. This approach places emphasis on improving the adhesion between nanostructured Si and the surrounding carbon matrix by using a non-ionic surfactant, Triton-X-100, which plays a crucial role in achieving a homogeneous dispersion of crystalline Si particles within amorphous SiCO matrix. In this Si-C systems in addition to variation of pyrolysis temperature, composition ratios (Si: C) were also varied to analyze how their phase and microstructure varies according to change in processing parameters. Additionally, their effect on the resulting electrochemical properties was also studied. The composite fabricated with 1:2 ratio of silicon nanoparticles and preceramic polymer pyrolyzed at 1200 ºC followed by etching exhibited the maximum stable capacity of 1048 mAh g-1 after 200th cycle at a current density of 0.1 A g-1. The sample also demonstrated excellent rate capabilities with a capacity of 840 mAh g-1 at a high current density of 2 A g-1. All Si-C hybrids discussed in this thesis demonstrated exceptional electrochemical behavior. The significant performance of the electrodes is attributed to the synergistic combination of high-capacity Si with structurally stable SiCO-derived carbon which provides a harmony between capacity and mechanical stability. The present study provides a significant advancement from prior research by introducing etching induced porosity into the polymer derived carbon phase. This porosity provides additional space to accommodate the volume expansion of Si particles. Further, the combination of porous microstructure and interconnected network of the preceramic polymer-derived carbon absorb the mechanical strain resulting from lithiation induced volume dilation of Si as well as enhances the conductivity of the anode structure.