PhD Thesis Presentation
There is increasing demand for energy storage devices along with the rapid development of renewable energy, electric vehicles and portable electronics. Advanced Li-ion battery (LIB) has been widely recognized as the most promising candidate for the abovementioned applications due to its high energy density, low self-discharge rate and high energy efficiency. Na-ion battery (NIB), on the other hand, is also widely studied recently because of its low cost compared to the LIB counterpart. However, the convectional LIB and NIB using a traditional graphite anode fail to deliver a satisfactory energy and power density due to the low capacity of carbon. Although many emerging anode materials have been studied recently, their electrochemical performances are still limited by several critical issues including the volume change of electrode materials, the poor electrochemical kinetics and the complicated synthesis routes.
In pursuit of the superior electrochemical performance of advanced LIB and NIB, this thesis is dedicated to addressing the aforementioned issues by systemically elucidating the lithiation/sodiation kinetics of emerging anode materials including SnO2, SnS2 and P. Besides, nanocarbons such as graphene and carbon nanotube (CNT) are utilized as conductive agent and mechanical buffer to fabricate novel anodes for advanced LIB and NIB applications. To be specific, the kinetically-controlled, reversible ‘conversion reaction’ between Na ions and SnO2 is proposed to responsible for Na ion storage in SnO2 anodes where the ion diffusion rate is the limiting factor. This revelation is contrary to the current understanding of ‘alloying reaction’ as the major reaction process. Aiming to fully utilize the theoretical capacity from the conversion reaction, a composite electrode consisting of carbon nanotubes coated with a mainly amorphous SnO2 phase together with crystalline nanoparticles is synthesized. The SnO2/CNT anodes deliver a superior specific capacity of 630.4 mAh g-1 at 0.1 A g-1 and 324.1 mAh g-1 at a high rate of 1.6 A g-1 due to the enhanced kinetics. The volume expansion of composite is accommodated by the CNT substrate, giving rise to an excellent 69% capacity retention after 300 cycles.
To design SnO2 anode for high power LIB, Sb is doped into SnO2 and the Sb-doped SnO2 nanoparticles are grown on the graphene-CNT aerogel conductive matrix based on a facile hydrothermal self-assembly route for the first time. The ATO/N-GCA composite presents a highly porous structure along with an electrical conductivity enhanced by over two orders of magnitude through the in situ Sb doping. The composite electrode delivers an outstanding rate capability of 659 mAh g-1 at 10 A g-1 with capacity retention of 73% after 1000 cycles at 1 A g-1 in LIBs.
A SnS2/graphene-CNT aerogel is developed as anode for NIB and Na hybrid capacitor (NHC) applications that fulfill the requirement of high power density. The SnS2 nanoplatelets are grown directly on SnO2/C composites to synthesize SnS2/graphene-CNT aerogel by pressurized sulfidation where the original morphology of carbon framework is preserved. The composite electrode possessing a large surface area delivers a remarkable specific capacity of 600.3 mAh g-1 at 0.2 A g-1 and 304.8 mAh g-1 at an ultrahigh current density of 10 A g-1 in NIBs. NHCs comprising a SnS2/GCA composite anode and an activated carbon cathode present exceptional energy densities of 108.3 and 26.9 Wh kg-1 at power densities of 130 and 6053 W kg-1, respectively. The in situ transmission electron microscopy (TEM) and the density functional theory (DFT) calculations reveal that the excellent pseudocapacitance originates from the combination of Na adsorption on the surface/Sn edge of SnS2 nanoplatelets and ultrafast Na+ ion intercalation into the SnS2 layers.
Finally, a freestanding few-layer BP film is developed, which delivers a high energy density when using as anode for flexible LIB application. The ab initio molecular dynamics (AIMD) simulations reveal fast kinetics of Li diffusion through the interface of the black phosphorus/graphene heterostructure. The oxygenated functional groups on the mildly reduced GO surface form P-O-C chemical bonds with BP, according to the DFT calculations, to produce a robust electrode capable of sustaining 500 steady cycles at an average Coulombic efficiency of 99.6 %. A flexible battery comprising a BP/rGO anode, a V2O5/carbon nanotube cathode and gel polymer electrolyte is assembled to deliver both ultra-high gravimetric and volumetric energy densities of 389 Wh kg-1 and 498 Wh L-1, respectively, with a high retention rate of 92.3 % after 100 cycles.
(Supervisor: Prof. Jang-Kyo Kim)