Transmission Electron Microscopy Investigation On Nanoscale Energy Materials

Energy conversion and storage is the most important part on developing and utilizing clean energy to relieve the global energy pressure.Energy material is the core and foundation of energy conversion and storage.With the miniaturization of equipment and the development of nanotechnology,nanomaterials have been widely studied due to their unique structures and properties.In this paper,the state-of-art techniques-in-situ transmission electron microscopy?TEM?and chemical sensitive electron tomography are used to explore the key issues involved in the preparation and working process of nanoscale energy materials,including the structural regulation,working principle,degradation mechanism and failure mechanism.The main results are summarized as follows:?1?In-situ TEM study on the failure mechanism of black phosphorus anode for lithium-ion batteries.Black phosphorus?BP?is one of the promising anode materials because of its high theoretical capacity,which however some limitations like the extremely low coulombic efficiency and rapid capacity fading after the first charge/discharge cycle.To explore the failure mechanism that plagues the BP LIBs,in-situ TEM technique was introduced to visual the nanostructures evolution of BP anode during electrochemical charging/discharging processes.It is found that BP anode sufferes a huge anisotropic size expansion during lithiation and cracking/fracturing during delithiation,leading to the loss of electrical contact and irreversible phase transformation,which should be responsible for the battery failure.This work provides important evidences for optimizing the properities of BP anode lithium-ion batteries.?2?In-situ TEM study on the electrochemical behaviors of MoO₃ nanobelts.The MoO₃nanobelts anode suffers serious capacity fading after first cycle,and the reaction mechanism during delithiation remains unclear.Aiming at these issues,in-situ TEM technique was used to explore the electrochemical behaviors of the MoO₃ nanobelt during lithium insertion/extraction process.It is found that MoO₃ nanobelts are transformed into amorphous Li₂MoO₃,that is,MoO₃ anode sufferes irreversible phase conversion during first cycle.The subsequent cycles are reversible change between Mo and Li₂MoO₃.On this basis,the energy storage mechanism and multistep phase transformation behaviors of MoO₃ nanobelts during sodiation/desodiation were explored,with the assistance of in-situ electron diffraction technique.This work provides important theoretical guidances for the application of MoO₃ nanobelts in sodium-ion batteries.?3?Study of PdFe@Pt nanocatalysts degradation mechanism with chemical sensitive electron tomography.Coating Pt monolayer on non-Pt metal-core is an important technology that can significantly reduce the use of Pt in proton exchange membrane fuel cells.Although the activity of this class of catalysts is tunable through optimizing the composition of the transition metal core,their insufficient durability during voltage cycling is still a remaining issue.Here,the investigation of a Pt coated Pd-Fe nanocatalyst was reported using statistically significant chemical sensitive transmission electron microscopy imaging.With the improved imaging statistics that procures sub-nanometer precision,our work reveals that the corrugated surface in the voltage cycled particle is due to the incomplete coverage of Pt which is found to undergo coarsening and thickening effect during cycles,a phenomenon to date has remained an untested theory at the nanoscale.More interestingly,we find although Pt layer can slow down the dissolution of Pd,it however instead induces a delocalized dealloying of the 3d transition metal,Fe,which contradicts the prevailing understanding.Our theoretical calculation suggests that the lower Fe vacancy formation and transition barrier in Pt-covered area and lower Fe segregation energy in metal Pd is responsible for the delocalized dealloying effect.This work reveals that the surface composition can play a very subtle and even delocalized role in tuning dissolution and dealloying of metal catalysts for the ORR reaction.?4?Bimetallic oxides growth mechanism in three dimensions by chemical sensitive electron tomography and in-situ TEM.Oxidation induced hollow-nanostructured bimetallic oxide is of particular importance because the synthetic method is low-cost and scalable.However,the degree of complexity of the oxidation process multiplies in the bimetallic system because of the incorporation of more than one element.Here,we provide an in-situ and chemical sensitive investigation of the growth mechanism of different morphologies NiFe oxides.We uncover three types of reaction route that results in three different morphologies-i.e.,porous,dual-cavity and hollow structures.The results demonstrate that the final morphology of the oxidized products is determined by the formation of pinholes in the oxide shell and voids at the core/shell interface.Statistical analysis shows that the resulting oxide morphology is strongly correlated with particle size and elemental composition:hollow products dominates the smaller particles while the porous ones dominates the larger particles,meanwhile the probability of forming porous structure decreases with increasing Fe concentration.The stress and composition evolutions inside the particles appear to have strong impact on the stability of the oxide shell.Our theoretical modeling verifies this hypothesis and shows that this size and composition-dependent oxidation behaviors are very likely a result of the particle stress caused by heterogeneous phase distribution and concentration gradient.By combining all these techniques and theoretical analysis,we have developed a framework that can predict the complex behavior in bimetallic and multi-metallic systems,which can guide the rational synthesis of samples with different hollow morphologies.