In-situ Investigation Of Mechanical Behavior Of Li-ion Battery Anode Material

In this dissertation, we in situ investigated the effect of coating layers, with different physical and chemical properties, on the mechanical behavior and the reaction mechanism of tin-based anode materials during electrochemical cycles inside transmission electron microscopy (TEM).In situ TEM investigated the effect of graphite structure carbon coating on mechanical behavior of Sn anode material during electrochemcial cycles. Carbon-coated Sn nanowires (CSNWs) were synthesized by chemical vapor deposition (CVD) method. The microstructure of the CSNW comprises a stacking of～10graphene layers roughly aligned in one direction. Shortened graphene sheets caused highly defective contents, which can serve as fast channels for lithium diffusion. In-situ transmission electron microscopy reveals a sequential phase transformation of Sn NW during Li insertion, which is in a reverse order during Li extraction. Both the bright field image and the electron diffraction show a reversible crystalline-crystalline phase transformation. The pulverization occurs during delithiation by agglomeration of regular-shape voids, showing a different mechanism from the cracking-dominated fracture in Si. It is noted that the crystalline tin has a more open lattice to readily accommodate Li ions up to Li2Sn5phase while retaining the crystallinity, which distinguishes Sn from its metalloid counterparts. The connected voids along [001] inside lattice form a helix pipe for fast Li diffusion, indicating the openness of the Sn lattice. The ab initio simulations reveal facile Li diffusion along [001] with a low migration barrier of0.014eV.Meanwhile chemical vapor deposited wrinkled carbon shell (WCS) shows high electrical conductivity, excellent thermal stability and remarkable mechanical robustness, which help in retaining structural integrity around the tin (Sn) anode core despite variation in volume of250%during repetitive lithiation. In situ transmission electron microscopy reveals no embrittlement in the lithiated WCS, which fully recovers its original shape after severe mechanical deformation and with no obvious structural change. Further analysis indicates that the capacity to accommodate large strains is closely related to the construction of the carbon shell, that is, the stacking of wrinkled few-layer graphenes. Both the pre-existing wrinkles and the few-layer thickness confer on the carbon shell a superior flexibility and good elasticity under conditions of bending or expansion of the interior volume.Lithiation induced fracture in individual carbon coating layer on SnO2NW electrode was studied in real time with in situ transmission electron microscopy (TEM). Amorphous carbon shell was coated on the surface of SnO2NWs via hydrothermal method.The carbon coating shell availably changed the volume deformation behavior compared to that noncoated SnO2NW due to the mechanical restraining effect, which restricted the axial elongation of SnO2NW when lithium inserted into nanowire. Moreover, a strong thickness dependence of fracture was discovered; that is to say, there exist a critical particle thickness of～10nm for SnO2nanowire diameter range of20～140nm, above which the carbon shell neither cracked nor fractured upon first lithiation, and below which the carbon shell initially formed surface cracks and then fractured due to lithiation-induced radial expansion. While even thicker carbon shells could not obviously decrease the radial strain found in this experiment, which because the binding force was not large enough to prevent the volumetric expansion during lithium ion insertion into SnO2NW core. In addition, the dislocation dynamic was studied when lithium insertein into SnO2NW. The results revealed that the propagation of dislocation inside SnO2NW with time could be fitting by the exponential function y=240×e4+240. The real form of dislocation, initiated and grew along axil of SnO2nanowire, is dislocation loop located in the {200}.Al2O3and TiO2thin films were deposited on the surface of SnO2nanowires (NW) and the composite electrode were tested inside nanobattery setup. In situ TEM study indicated that Al2O3thin film maintain intact undergone large volum change of SnO2NW during lithiation, however, the TiO2thin film was destroyed, with fature and cracking when the dimater of SnO2expanded to～140%. Coating layer can effectively improve capacitance and cyclability, which is attributed to the mechanical restraining effect of the coating shell. The cyclability of SnO2/Al2O3electrode was stable at～450 mAh g-1for50cycles, revealing in real battery system. However, the electrochemcial performance was deteriorated via coated TiO.2. Our results demonstrated that in situ TEM study held a light for designing advanced electrode materials used in real lithium ion battery.The capability of atomic layer deposition (ALD) to conformally coat3D nano-topography was examined by depositing amorphous (a-), polycrystalline (p-) and single-crystalline (s-) TiO2films over SnO2nanowires (NWs). Structural characterizations reveal a strong correlation between the surface morphology and the microstructures of ALD films. Conformal growth can only be rigorously achieved in amorphous phase with circular sectors developed at sharp asperities. Morphology evolution convincingly demonstrates the principle of ALD, i.e. sequential and self-limiting surface reactions result in smooth and conformal films. Orientation-dependent growth and surface reconstruction generally leads to non-conformal coating in polycrystalline and single-crystal films. The comparison study suggests that ALD process is codetermined by the interplay of both thermodynamic and kinetic factors. The observation challenged the traditional understanding for conformal coating via ALD.In summary, WCS has a bettery property compared amorphous carbon shell and oxide film. The electron conductivity of WCS is better than oxide thin film, and the mechanical strength is larger than amorphous carbon shell, which is more favorable for using as coating layer to improve the electrochemcial performance of advanced anode materials. With the help of advanced in situ TEM, the dynamic process during battery operation can be visualized in real time and with unprecedented high spatial resolution. The results provide deep understanding of the fundamental sciences of lithium ion battery and critical guidance in developing advanced lithium ion battery for electrical vehicle.