PhD Thesis Presentation
Ever-growing global electric energy storage demand arising from the booming markets of portable electronics, electric vehicles (EVs) and unmanned aerial vehicles motivates the development of safe, sustainable, and cost-effective energy storage systems with high energy/power densities and long cyclic life. Apart from the conventional lithium ion batteries (LIBs), sodium ion batteries (SIBs) and lithium sulfur batteries (LSBs) exhibit great potentials as alternative candidates for next-generation batteries due to the much cheaper precursor materials, environmental benignity and electrochemical performance comparable to LIBs. In order to realize their successful applications to power EVs and smart grids in the near future, it is essential to develop energy storage materials with abundant resources, rationally designed functional and structural features and excellent structural stabilities. This thesis focuses mainly on exploring novel energy storage nanomaterials based on group-15 elements, e.g., red P and Sb2S3, in an effort to mitigate the critical challenges known to rechargeable batteries and accelerate their practical commercialization. Combing the cutting-edge experimental techniques, such as ex situ XRD and in situ TEM/SAED, with theoretical calculations, the underlying relationship between the microstructural features and electrochemical properties are well established.
Chemically stable red P possesses an attractive theoretical capacity and safe working potential as anodes for SIBs, but suffers from a very poor electrical conductivity and a large volume change during cycles. To promote widespread application of red P-based anodes, the mechanisms of P adsorption process are elucidated by combining molecular dynamics (MD) simulations and density functional theory (DFT) calculations. Inspired by the new discoveries, precisely controlled hollow microporous carbon nanosphere/red phosphorus composites are synthesized as anodes for SIBs, which illustrate exceptional mechanical stability upon sodiation/desodiation according to in situ TEM.
Apart from red P, Sb2S3 has also drawn significant attention as anodes in both LIBs and SIBs. First, the sodiation kinetics and phase evolution in carbon-coated 1D weak van der Waals force stacked Sb2S3 nanorod anodes are investigated using state-of-the-art tools including in situ TEM/SAED examination and DFT calculations/MD simulations. A unique two-step reaction mechanism, namely ultrafast Na+ ion intercalation and conversion/alloying reactions, and unexpectedly small volume expansion are revealed in the 1st sodiation process. Such evolution of an unusual phase arises from the synergy between the extremely low Na+ ion diffusion barrier and the sharply increased electrical conductivity upon the formation of amorphous NaxSb2S3 intermediate phases. Further, inspired by the weak van der Waals forces in the layered Sb2S3, few-layer 2D Sb2S3 nanosheets are prepared using commercial Sb2S3 powder based on a chemical exfoliation method, which present a well-defined layered structure with uniform lateral sizes of several tens of micrometers. Benefiting from the ultrathin thickness and large surface area, the 2D Sb2S3 nanosheet electrode exhibits a remarkable rate capability and steady cyclic performance in both LIBs and SIBs. Interestingly, the Sb2S3 nanosheet electrode presents comparable or an even better pseudocapacitive performance in SIBs than LIBs, which can be partially attributed to the lower ion diffusion barrier, i.e. 164 meV for Na vs. 189 meV for Li, and better structural integrity after Na adsorption according to the first-principles calculations.
In order to alleviate the shuttle effect of polysulfides in LSBs, 2D Sb2S3 nanosheets (SSNSs) with a high surface-to-mass ratio are prepared by a novel approach involving electrochemical Li intercalation and exfoliation, and their potential as an effective coupling material to entrap and recycle the soluble Li2Sx species is demonstrated by combining experiments with DFT calculations. The LSBs containing a separator uniformly coated with Sb2S3 nanosheet/carbon nanotube (SSNS/CNT) coupling layer deliver a much improved specific capacity with remarkable cyclic stability. The calculations further reveal the merits of 2D SSNSs, which possess appropriate binding strengths to entrap polysulfides while maintaining the internal Li-S bonds, along with a low energy barrier for efficient Li diffusion on the SSNS surface.
(Supervisor: Prof. Jang-Kyo Kim)