href="#ulink_07404e48-d5b4-57e8-a209-f61d9b127428">Figure 2.4 High signal-to-noise EEL spectrum acquired by the accumulating 1 s exposures while scanning repeatedly. The insets present a 50 frame average in false color from the stacks of images created during the acquisitions, showing clear threefold coordination (a) or fourfold coordination (b) of the Si atom. Source: Reproduced with permission from Ramasse et al. [61]. Copyright 2013, American Chemical Society.
In summary, the EELS acquired by Cs-TEM can yield precise electronic structure information at the sub-nano level. Optimizing acquisition procedures would result in very high SNR data. The foreign species on a substrate and even bonding differences between dopant species would be distinguishable with electron energy loss spectroscopy. Another concept that should be emphasized in this part is that experimental configuration for atomic column imaging is not only to make a small incident probe but also to optimize the acquisition condition associated with the delocalization in elastic and inelastic scattering. The incoherent EELS imaging allows to interpret the core loss images, which are informative in material science. The observation of local inhomogeneity would endow the discussion of local crystal distortions and its unique material properties beyond the stoichiometric understanding of the average crystal structure. The development of Cs-TEM has proved its powerful function of elemental and chemical analyses of site occupancy in material microstructure characterization.
2.1.4 Applications in Amorphous Nanomaterial Characterization
The recent progress of Cs-TEM provides a solid foundation to investigate the structures and compositions at the atomic level or in a complex environment with gas or liquid. This can then be integrated with energy-dispersive X-ray spectroscopy (EDS) and EELS techniques to observe the structure evolution of the materials and to investigate the mechanism of the composition changes. Notably, one achievement in the last few decades is the application of in situ TEM that involves various stimuli to nanomaterials with high-resolution imaging and spectroscopy. These stimuli may include heat, stress, electrical biasing, and ultrashort photon pulses to the materials. However, the high vacuum of TEM within the column to protect the electron gun and to avoid the electron scattering by gases and liquids makes it not compatible with gaseous or liquid environments. The development of environmental transmission electron microscopy (ETEM) has offered great chances to study the dynamic changes in materials with ultrahigh resolution in complex gaseous or liquid environments.
Since the inception of in situ TEM techniques for battery research in 2010 [69], continuous efforts have been made to give a better understanding of material dynamics during electrochemical reactions. In the battery analysis by using TEM, the scientific challenges include how cathodes experience thermal degradation with compromised battery safety, what is the charge storage mechanism for electrodes with different elements, and how Li dendrites evolve during Li intercalation. As a stable electrode, intercalation should work without obvious structural degradation during ion insertion/extraction. To evaluate the structural changes with the high spatial and temporal resolution, the advantages of in situ TEM with aberration corrector are obvious. One typical example is the case of MnO2 cathode, which possesses a one-dimensional tunneled structure. An asynchronous lattice expansion was found to be driven by a sequential Jahn–Teller distortion of [MnO6] octahedral [70]. The dynamic observation of structure degradation demonstrates the powerful capability of in situ TEM in studying the localized reaction mechanisms. Inspired by these observations, the battery performance can be improved by structure modification, such as minimizing the particle size or tracing the dopant component to reduce the structural degradation. Interestingly, nanosized transitional metal oxides, sulfides, and fluorides (MX) showed that lithium storage through a conversion reaction between metal oxide and LiX is reversible, but the intermediate steps involve multiphase reactions, resulting in a low Coulombic efficiency (CE), a large overpotential, and a fast capacity fading upon cycling [71]. In addition, even for materials with the same metal cations, there is still a difference in conversion kinetics. Taking Fe as an example, an intermediate phase can be confirmed as LixFe3O4 before the conversion of nanosized Fe3O4 to Fe [72], while no similar phenomena can be observed with nanosized Fe2O3 [73]. In addition, for iron sulfide as a sodium battery electrode, the coexistence of Fe3S4, FeS2, and FeS can be observed by HRTEM, and those quantum-sized FeSx ensured a synergistic and highly reversible conversion reaction, leading to a superior cyclability and rate capability [74–76]. Recently, we also investigated the charge storage mechanism of bismuth as a promising anode material for the state of the art rechargeable batteries [77]. In our work, a 2D structure of few-layer bismuthene was designed (as shown in Figure 2.5), which undergoes a two-step mechanism of ion intercalation, followed by a reversible crystalline phase evolution. This structure can alleviate the stress accumulated along the critical z-axis and allow sodium ions to rapidly diffuse due to a shorter diffusion distance, which is very helpful to develop high-performance batteries. Meanwhile, ex-situ TEM can also be a powerful tool to study the structural evolution of an amorphous electrode transition. For example, we prepared a Prussian blue analog (PBA) Co3[Co(CN)6]2 as nonaqueous potassium-ion anode material [78]. The HRTEM image revealed that tiny crystallites of metallic Co of 5 nm in size are dispersed in the amorphous matrix. This observation is quite similar to the lithiation behavior of metal oxides, in which metal nanoparticles are found in the Li2O matrix [71]. The metallic Co formed during the lithiation process may enhance the electronic conductivity. The amorphous matrix might contribute to the good cycling performance because the isotropic nature of the amorphous materials can tolerate homogeneous volume changes and accommodate volume strain. Besides, we have also carried out research with amorphous FeVO4, which is a bimetallic element oxide for K-ion battery [79]. Local structural information of amorphous FeVO4 after potassiation can be obtained based on the HRTEM images, which displayed tiny crystallites of VO2, V2O3, and FeO with sizes below 5 nm. These tiny crystallites are surrounded by amorphous materials with solid electrode interface (SEI) or other potassiated products. The conversion was reversible because those crystalline phases partially recover to amorphous FeVO4 after subsequent depotassiation. The particle size for the crystalline counterpart is much larger, showing lower potassiation/depotassiation capacities. This observation suggested that particle size plays a role in determining the electrochemical performance of amorphous materials. Another important issue is the observation of SEI and Li (de)plating on the electrode/electrolyte interface with high spatial resolution. By using in-situ TEM, Li dendrite growth and SEI formation or decomposition in a LiPF6/ethylene carbonate (EC)/diethyl carbonate (DEC) electrolyte can be recorded at nanoscale resolution [80]. The SEI formation is not uniform but in the shape of dendrites. The growth kinetic of SEI can further be a valuable reference for understanding battery failure. Moreover, the beam irradiation with hundreds of keVs can cause side reactions in targeted materials and affect the imaging process, including atomic displacement, e-beam sputtering due to the elastic scattering, and heating or contamination damage due to inelastic scattering [81]. For a solid-state open cell, the main concern is the stability of Li2O under the electron beam and the subsequent effect on the battery electrochemical performance. An effective solution to reduce the electron dosage to a safe value (approximately 1 A cm−2) is to suppress the chemical lithiation [82]. Meanwhile, in situ liquid cell TEM is subject to more side reactions such as the electrolyte breakdown [83] and the nanoparticles’ precipitation/dissolution [84]. The LiPF6-based electrolyte has proven to be stable as the formation of fewer and smaller nanoparticles, and the SEI nucleation and growth can be captured on the Li deposit [85]. In addition, similar cyclic voltammetry (CV) curves were observed under and without electron beam irradiation, which demonstrates the suitability of applying liquid TEM in a real battery system [86].
