Abstract
In this work, we study the lithiation behaviours of both porous silicon (Si) nanoparticles and porous Si nanowires by in situ and ex situ transmission electron microscopy (TEM) and compare them with solid Si nanoparticles and nanowires. The in situ TEM observation reveals that the critical fracture diameter of porous Si particles reaches up to 1.52 μm, which is much larger than the previously reported 150 nm for crystalline Si nanoparticles and 870 nm for amorphous Si nanoparticles. After full lithiation, solid Si nanoparticles and nanowires transform to crystalline Li15Si4 phase while porous Si nanoparticles and nanowires transform to amorphous LixSi phase, which is due to the effect of domain size on the stability of Li15Si4 as revealed by the first-principle molecular dynamic simulation. Ex situ TEM characterization is conducted to further investigate the structural evolution of porous and solid Si nanoparticles during the cycling process, which confirms that the porous Si nanoparticles exhibit better capability to suppress pore evolution than solid Si nanoparticles. The investigation of structural evolution and phase transition of porous Si nanoparticles and nanowires during the lithiation process reveal that they are more desirable as lithium-ion battery anode materials than solid Si nanoparticles and nanowires.
Highlights
To understand the lithiation/delithiation process of Si, it is of importance to directly observe the structural and chemical evolution during the process and correlate with the battery properties
The in situ transmission electron microscopy (TEM) observation of lithiation process of porous Si nanoparticles reveals that the lithiation proceeds in an end-to-end manner, which is different from the surface-to-center manner for solid Si nanoparticles under the same experimental condition
First-principle molecular dynamic simulation was conducted to investigate the effect of domain size on the phase stability of c-Li15Si4, which confirms the effect of nanostructure on phase transition
Summary
To understand the lithiation/delithiation process of Si, it is of importance to directly observe the structural and chemical evolution during the process and correlate with the battery properties. The lithiation reaction velocity of a-Si is approximately constant and does not slow as in c-Si, which suggests different stress evolution during lithiation and implies that a-Si may be a more desirable active material than c-Si27 These studies have led to fundamental understanding of the lithiation/delithiation process of Si nanoparticles and nanowires; these studies cannot provide direct explanation of better electrochemical performance achieved by newly reported nanostructured Si than solid Si nanoparticles and nanowires. Much larger critical fracture diameter is achieved in porous Si particle than previously reported for c-Si and a-Si particles Another interesting feature in the lithiation process of porous Si nanoparticles and nanowires is that a-LixSi does not transform to c-Li15Si4 even after full lithiation, which is distinct from that of solid Si nanoparticles and nanowires. Structural evolution of porous and solid Si nanoparticles under successive lithiation/delithation cycles are compared through ex situ TEM, which confirms that porous Si is a more desirable anode material for LIBs than solid Si
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