| dc.description.abstract | Li-ion batteries have a predominant market share as mobile energy storage devices, especially in consumer electronics. New concepts for electrode material designs are, however, necessary to boost their energy and power densities, and most importantly, the long term cycle stability. This will allow for these devices to gain widespread acceptance in electric vehicles, an area with immense market potential and environmental benefits. From a practical perspective, new electrode materials must be developed by simplistic, environmentally friendly and low cost processes.
As a new class of electrode materials, mesoporous Sn/SnO2/Carbon composites with uniformly distributed Sn/SnO2 embedded within the carbon pore walls have been rationally designed and synthesized. These nanocomposites have been characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), x-ray photoelectron spectroscopy (XPS), and tested as negative electrodes in a cell using lithium foil as the counter electrode. The inclusion of metallic Sn in SnO2/CMK3 resulted in a unique, ordered structure and provided a synergistic effect which resulted in an impressive initial reversible capacity of 799 mAh g-1. In addition, at a high current of 800 mAg-1, the heterogeneous structure was able to provide a stable capacity of 350 mAhg-1 and a retention capacity of ~ 670 mAh g-1 after 60 cycles.
While Sn/SnO2 composites have been deemed very promising, Si materials boast improved energy storage capacities, inspiring us to investigate these materials as new anode structure. A novel one-pot synthesis for the sub-eutectic growth of (111) oriented Si nanowires on an in-situ formed nickel nanoparticle catalyst prepared from an inexpensive nickel nitrate precursor is developed. Anchoring the nickel nanoparticles to a simultaneously reduced graphene oxide support created synergy between the individual components of the c-SiNW-G composite, which greatly improved the reversible charge capacity and its retention at high current density when applied as an anode for a lithium-ion battery. The c-SiNW-G electrodes in a Li-ion battery achieved excellent high-rate performance, producing a stable reversible capacity of 550 mAh g-1 after 100 cycles at 6.8 A g-1 (78% of that at 0.1 A g-1). Thus, this process creates an important building block for a new wave of low cost silicon nanowire materials and a promising avenue for high rate Li-ion batteries.
While excellent rate capability was obtained by using SiNW/graphene based material, simplifying the process may drive Si based materials to commercialization. A novel, economical flash heat treatment to fabricate silicon based electrodes is introduced to boost the performance and cycle capability of Li-ion batteries. The treatment results in a high mass fraction of Si, improved interfacial contact, synergistic SiO2/C coating and a conductive cellular network for improved electronic conductivity, as well as flexibility for stress compensation. The developed electrodes achieve first cycle efficiency of ~84% and a maximum charge capacity of 3525 mA h g-1, which is almost 84% of silicon’s theoretical maximum. Furthermore, a stable reversible charge capacity of 1150 mA h g-1 at 1.2 A g-1 can be achieved over 500 cycles. Thus, the flash heat treatment method introduces a promising avenue for the production of industrially viable, next-generation Li-ion batteries.
Even though we obtained a dramatic improvement to a treated electrode based on commercial silicon, we still need to boast the cycle stability and high areal capacity achieved by higher electrode loading. Thus, we report a scalable approach that relies on covalent binding commercially available Si nanoparticles (SiNP) to sulfur-doped graphene (SG) followed by shielding them with cyclized polyacrylonitrile. The covalent synergy led to improved material property that can deliver stable reversible capacity of 1033 mAh g-1 for more than 2000 cycles at a rate of 1 A g-1. The areal capacity was 3.5 mAh cm-2 at 0.1 A g-1, approaching the commercial demand. The spatial arrangement of Si after cycling reveals that it was confined in nanowires morphology. This reveals that the solid electrolyte interphase remains stable leading to superior cyclability. Our DFT calculations revealed covalent hybrid interaction between Si, S, and C leading to stable material configuration. Furthermore, the structure synergy facilitated lithium diffusion, which strongly supports our results. This simple, low cost, feasible, and safe approach provide new avenues for engineering electrode structure for enhanced performance. | en |
