Advanced Characterization Technologies for Secondary Batteries E-Book
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- Herausgeber: Bentham Science Publishers
- Kategorie: Fachliteratur
- Sprache: Englisch
This book focuses on crucial characterization methods adopted for materials, design, and performance of secondary batteries. The book is divided into eight chapters aiming to provide comprehensive and essential guidance on battery characterizations. Each chapter focuses on a specific technique: electron microscopy, focused ion beam methods, atomic force microscopy, X-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectra, neutron diffraction, synchrotron-radiation X-ray tomography, and ultrasonic nondestructive testing.
Key Features
· Comprehensive coverage of characterization techniques for secondary battery technology
· Explains the working principle, essential functions and data analysis for each technique
· In-depth review of recent applications of secondary batteries from both material and device perspectives
· Detailed reference list for advanced readers
This monograph is intended as a resource for the broad research community involved in materials and device testing for batteries at academic and industrial levels. It also serves as a reference for engineering students required to learn advanced characterization techniques for developing rechargeable battery technology.
Readership
Students (technology and engineering), researchers and professionals (battery technology).
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Veröffentlichungsjahr: 2024
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FOREWORD
The growing global demands for electric energy storage and renewable resources promote the development of various secondary batteries, including metal-ion batteries, metal-air batteries, etc. Despite significant progress obtained in recent decades, permanent goals exist in secondary batteries to continually improve their energy density and durability/safety. To achieve this, it becomes imperative to elucidate the relationship between micro-level structures/properties and device performance. However, conventional measurements may fail to meet such stringent requirements.
In this book, eight typical advanced characterization tools in battery research are introduced, including various microscopy, spectroscopy, and diffraction techniques. More importantly, it includes several attractive techniques that have been employed in battery studies yet have not been in-depth discussed, such as neutron diffraction and ultrasonic nondestructive testing. Furthermore, related applications and future prospects are reviewed and presented. I believe that this book will be an essential and important guide for the broad battery research community.
Congratulations to the editors and all authors for successfully accomplishing this book.
PREFACE
The development of high-energy density and stable secondary batteries fulfills the growing global demand for electric energy storage capacity. Although tremendous efforts have been devoted, further enhancement of battery performance will significantly rely on the precise unravelling of atomic-level structure and micro-properties in related materials/devices. Therefore, in this book, we would like to highlight several essential yet commonly employed characterization tools in secondary battery investigations.
In chapters 1 to 3, we focus on morphology measurements in secondary batteries, discussing basic structure, diverse accessories, sample preparation, recent research progresses, applications and current limitations of transmission electron microscopy (TEM), focused ion beam (FIB) methods, and atomic force microscopy (AFM) in related battery studies.
Chapters 4, and 5 present a discussion on X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS), respectively, aiming to unravel the chemical states of electrode materials and solid electrolyte interphase/cathode electrolyte interphase (SEI/CEI) film.
In chapters 6-7, we review diffraction-related technologies (e.g., neutron diffraction, synchrotron-radiation X-ray tomography) in secondary batteries, which are commonly adopted to acquire chemical and crystallographic details.
The last chapter 8 introduces an emerging method, ultrasonic nondestructive testing technology, to probe defects in battery materials/devices.
We expect the above contents could promote future developments of both advanced characterization methods and secondary battery studies. The potential audience of this book includes the broad research community of secondary batteries and characterization, such as research scientists, professors, engineers, and students.
List of Contributors
Transmission Electron Microscopy and its Applications in Secondary Batteries
Lixia Bao1,Ruiwen Shao2,Tinglu Song3,*,Yong Yang4,Fan Xu5,*Abstract
Transmission electron microscopy (TEM) has been widely employed as an important technique for detecting morphology in various secondary battery applications. When integrated with diverse accessories, TEM could attain sub-angstrom level detection, cryogenic TEM (cryo-TEM), and other functionalities, which enables the observation and comprehension of the microstructure of secondary battery materials, as well as structural variations during battery charging and discharging processes. In this chapter, essential structures, main functions, and sample preparation methods of TEM are introduced. Additionally, applications of TEM in characterizing materials used in secondary batteries are discussed and summarized. The aim is to provide essential guidance for employing TEM and to facilitate its future development.
1. INTRODUCTION
The growing demands for renewable energy and the rising concern over environmental pollution have created significant opportunities for the develop-
ment of secondary battery technology [1], including investigations for materials related to other optoelectronic devices [2-4]. The macroscopic performance of batteries, including capacity, density of energy, circulation life, safety performance, operating voltage, and operating temperature range, is closely related to the microstructures of assembled battery materials. These microstructures, such as size, surface morphology, surface reconstruction, crystal structure, and grain boundaries, exhibit a pivotal role in determining the overall performance of the battery. Therefore, it becomes imperative to gain a comprehensive understanding of the microscopic structural and chemical changes in electroactive materials during battery operation, which may be achieved via advanced techniques for in-depth characterization of electroactive materials [5]. Such an understanding not only enhances the accuracy of examining the correlation between microstructure and performance but also facilitates the fundamental understanding of high-performance batteries. Furthermore, it may function as a roadmap for battery design, thus advancing the overall development of secondary battery technology.
Various techniques, such as microscopes, X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), secondary ion mass spectroscopy (SIMS), X-ray photoelectron spectroscopy (XPS), have been utilized to investigate the structure of secondary battery materials. However, there is still a lack of powerful tools for atomic-level detection. TEM emerges as an indispensable tool to probe local atomic structures with high resolution, which also possesses multiple functions, providing valuable insights into real space, reciprocal space, or energy space, thus enabling an in-depth understanding of the mechanisms connecting microstructure changes to battery performance [6]. TEM has been used to characterize the microstructure of perovskite solar cells to better understand the macroscopic performance of batteries [7]. Moreover, TEM and high-resolution TEM (HRTEM) further allow for the observation of the surface and interface morphology of battery materials, as well as detailed information on the lattice structure. Spherical aberration-corrected TEM and scanning TEM (STEM) have been developed to reveal atom arrangements, atomic-level real-space imaging, and atomic occupancy. Furthermore, when configured with X-ray energy dispersive spectroscopy (EDS), TEM becomes a powerful tool for analyzing the types and distribution of elements. Electron energy loss spectroscopy (EELS) is particularly effective for light elements, even surpassing the resolution of EDS, making it highly relevant in lithium-ion batteries (LIBs) research. By fitting the EELS spectra with density functional theory (DFT), accurate information on the valence and even electronic states of elements could be obtained. The functionality of in-situ TEM is significantly more potent in battery investigations. By integrating in-situ batteries within the TEM column, it could achieve real-time measurement and analysis of the morphology and structural evolution of battery materials during cycling, which unveils critical information about the position, concentration, and chemical environment of lithium ions in electrode materials, offering insights into both static and dynamic conditions [8, 9]. With the development of electron microscopy technology, new TEM techniques have also rapidly evolved, including annular bright-field (ABF), integrated differential phase contrast (iDPC), and ptychography, which provide the possibility of directly imaging lithium ions, surpassing traditional annual dark field (ADF) and high-angle annular dark field (HAADF) methods [10, 11].
This chapter provides an in-depth review of three essential aspects of TEM, including the structure, functionality, and sample preparation procedures. Additionally, it delves into the practical application of TEM in characterizing battery materials, with a particular emphasis on secondary battery materials. The primary objective is to elevate the comprehension of the intricate interplay between microstructure and the performance of battery materials by presenting a detailed analysis of the microstructural characteristics inherent in these materials. This enhanced understanding is anticipated to advance the comprehension of battery failure mechanisms, as well as provide valuable insights that will guide the future development of high-performance batteries.
2. Introduction of TEM
2.1. The Structure of TEM
TEM is an electron-optical instrument renowned for its high resolution and magnification, which utilizes an extremely short-wavelength electron beam as the illumination source and an electromagnetic lens to focus and image the transmitted electron beam. The TEM can be divided into four major components: the electronic optical system, the vacuum system, the power supply control system, and the additional instrument system.
(1) The electronic optical system comprises three essential components: the illumination system, the imaging system, and the observation and recording system. Functioning as the core of the TEM, the optical path diagram and cross-section diagram are shown in Fig. (1). The illumination system mainly consists of the electron gun, the condenser, the translation centering and tilt adjustment device, working as the primary light source for the imaging system. The electron gun, which is the source of the electron beam, may be categorized into thermionic and field-emission guns, respectively. These electron guns emit electron sources with distinct brightness, temporal coherence, special coherence, energy emissivity, and stability. Different electron sources are employed in TEM based on their performance characteristics. The performance comparison of various electronic guns is shown in Table 1.
Fig. (1)) The cross-section (a) and the electron optical schematic (b) of TEM.The imaging system is primarily composed of the objective lens, intermediate lens, projection lens, objective lens aperture, and selected area diffraction aperture, as shown in Fig. (1b). The objective lens determines the final resolution of TEM, making it the most critical component. The imaging process in an electron microscope is referred as the tertiary magnification image of the sample. Initially, the objective lens forms a primary magnification image on the image plane, which is then magnified progressively through the intermediate lens and projection lens, ultimately being projected onto the fluorescent screen. The imaging modes of TEM include diffraction and image modes, whose difference is the position of the intermediate lens image plane.
To observe the sample, it is necessary to detect the electron intensity generated by the specimen, which must be converted into visible light. The fluorescent screening of traditional TEM is created by coating a layer of ZnS fluorescent powder on an aluminum plate, thereby providing direct images to the operator. However, in several latest TEM models, the traditional operator's viewing screen is absent. Instead, all information is displayed on flat-panel computer monitors at a separate console that is separated from the column. Additionally, image recording systems based on film, CCD, or CMOS are integrated into these TEMs, allowing operators to remove or insert these devices into the electron beam path as required.
(2) The vacuum system comprises the mechanical pump, the diffusion pump, and the ion pump. The vacuum system is responsible for evacuating residual gases from the microscope column, which ensures the vacuum integrity of the column, and operates as a crucial safeguard for the electron as the light source.
(3) The power supply control system retains the high-voltage power supply, lens power supply, vacuum system power supply, and circuit control system power supply. This system provides the requisite energy for each component of the TEM to ensure its operation.
(4) The additional instrument system comprises the X-ray energy dispersive spectrometer and electron energy loss spectrometer. The additional instrument system functions as an elemental analysis device integrated based on structural analysis.
2.2. The Primary Functions of TEM
2.2.1. Morphological and diffraction imaging
In TEM, variations in the intensity of the electron beam passing through the sample lead to uneven intensity displayed on the fluorescent screen, which is known as amplitude contrasts, mainly including mass-thickness contrast and diffraction contrast. Mass-thickness contrast arises from differences in the thickness or average atomic number of different regions in the material, leading to variations in transmitted beam intensity. On the other hand, diffraction contrast results from discrepancies in the diffraction intensity due to variations in Bragg reflection conditions and structural amplitudes within the crystal sample.
The theoretical basis for morphological imaging in TEM refers to the mass-thickness contrast. TEM shows the capability to achieve magnification ranging from tens to over a million times, allowing for a wide range of sample examination, localization, and micro-area amplification.
Bright-field imaging, dark-field imaging, and electron diffraction imaging are all based on the diffraction contrast, which shows extreme sensitivity to crystal structure and orientation, and thus are primarily adopted for microstructure analysis (e.g., grain shape, grain size, grain boundaries, precipitates, etc.) and crystal defect analysis (e.g., dislocations, twins, stacking faults, etc.).
2.2.2. High-resolution TEM (HRTEM)
HRTEM is a phase-contrast imaging technique that captures interference patterns formed by the phase difference between all diffracted beams participating in the imaging process, which enables the imaging of atomic columns in most crystalline materials. There are two main types of HRTEM images: one-dimensional lattice images and two-dimensional lattice images. One-dimensional lattice images are formed by reflecting the electron beam from specific crystal planes, which enables the direct measurement of interplanar spacing and the observation of twin boundaries, as well as the structures of long-range periodic layered crystals. The latter two-dimensional lattice images are obtained by reflecting the electron beam from a crystal zone axis, which permits the study of complex and distorted structures such as dislocations and grain boundaries.
2.2.3. Scanning TEM (STEM)
STEM concentrates the electron beam into a small probe and scans it point by point over the atomic surface. The electrons scattered after interacting with the atoms are detected by various electron detectors to generate images. The optical path diagram for STEM is illustrated in Fig. (2a). STEM offers several imaging modes, including bright-field (BF) for coherent phase-contrast imaging, incoherent annular dark-field (ADF), and high-angle annular dark-field (HAADF) for Z-contrast imaging. Each mode collects only a portion of the scattered electrons, as illustrated in Fig. (2b). Notably, HAADF imaging could attain sub-angstrom resolution, especially when equipped with a spherical aberration corrector, enabling the identification of the precise positions of atoms and atomic columns.
2.2.4. X-ray Energy Dispersive Spectroscopy (EDS)
EDS is a widely utilized method for micro-area composition analysis, whose working principle is based on distinct characteristic X-rays that are emitted from each element. The process initiates when incident electrons excite or ionize the inner-shell electrons of sample atoms, which creates a vacancy in the inner shell and places the atom in an excited state with higher energy. Subsequently, outer-shell electrons transition to fill the vacancy in the inner shell, releasing characteristic X-rays with specific energy. The wavelength of characteristic X-rays corresponds to the energy released during the outer-shell electron transitions. EDS analyzes the composition based on the distinct characteristic X-ray energies emitted by different elements. When STEM attachment and EDS are simultaneously integrated into the TEM, EDS analysis could achieve both micro-area elemental composition analysis and micro-area elemental distribution analysis.
Fig. (2)) Optical path diagram of STEM (a) and the various electron detectors in a STEM (b).2.2.5. Electron Energy Loss Spectroscopy (EELS)
Another powerful function, EELS, has also been extensively integrated into the TEM, which can convey material information by capturing the electrons via an EELS detector, as illustrated in Fig. (3a). During the transmission of incident electrons through the specimen, interactions with atomic nuclei or electrons may occur. These interactions can be categorized as either elastic or inelastic scattering, depending on the energy changes involved. The outgoing electrons, which include transmitting electrons, elastically scattered electrons, and inelastically scattered electrons, are focused by the in-column or post-column magnetic prism, which is then separated and directed to different positions, and further recorded by distinguishing their energy differences via electron detectors/cameras. Therefore, the achievement of EELS relies on the comparison of energies between the incident and outgoing electrons. A typical EELS spectrum could be divided into three parts: the zero-loss peak, low-loss peak or plasmon peak, and high-loss peak or core-loss peak, as depicted in Fig. (3b). The zero-loss peak comprises transmitted and elastic scattered electrons, which exhibit no energy loss due to interaction with atomic nuclei. The plasmon peak (<50 eV) primarily arises from collective oscillations of electrons relative to ions in the crystal lattice when the incident electrons penetrate the sample and the plasmon excitation peak increases with the sample thickness. Conversely, the high-loss peak (>50eV) mainly originates from the inelastic scattering of incident electrons with inner-shell electrons of the sample atoms, including absorption edges, energy loss near-edge structure (ELNES), and extended energy loss fine structure (EXELFS). In practice, the combined analysis of the plasmon peak and the zero-loss peak is effective in calculating sample thickness. The high-loss peak is commonly employed to identify elemental types, band structures, and chemical and crystallographic states [12].
Fig. (3)) (a) Schematic diagram illustrating the structure of EELS. (b) Typical EELS spectra and the corresponding meaning of different peaks.EELS offers a wealth of information, including (a) Analysis of chemical composition. EELS is capable of analyzing elements with atomic numbers ranging from 1 to 92. It exhibits particular sensitivity to light elements, while EDS is more sensitive to heavy elements. In addition, the energy resolution of the EELS spectrum is much higher than that of EDS. (b) Providing information on chemical bond states. The ELNES in the EELS spectrum is highly sensitive to crystal structure, which serves as a “fingerprint” for confirming compounds, providing valuable insights into chemical bond states. (c) Analysis of valence state. EELS can be employed to analyze the valence state of elements, contributing to a comprehensive understanding of their electronic configurations. (d) Providing the electronic structure of elements. EELS offers insights into the electronic structure of elements, including details such as band structure and orbital occupancy of electronic states. (e) Extended fine structure (EXELFS). Through EXELFS, EELS can provide information about the number of coordination atoms and coordination distance.
3. TEM Sample Preparation for Secondary Battery Materials
TEM is a versatile tool applicable to a wide range of materials, including metals, non-metals, nanoparticles with sizes in the range of a few tens of nanometers in size, thin films with thicknesses ranging from a few nanometers to several tens of nanometers, as well as organic materials such as biological specimens. However, TEM samples must meet specific requirements to ensure optimal imaging conditions: (1) Suitable electron beam transparency, smooth surface, stability, ease of placement, resistance to electron beam bombardment, non-volatility, distortion-free, and non-radioactivity are crucial parameters for preparing effective TEM samples. (2) The sample size is generally around φ3mm, with a thickness of about 0.1 mm. Importantly, the thickness of the area intended for actual observation should be less than 100 nm. For high-resolution imaging, such as those prepared for spherical aberration-corrected TEM, even thinner samples are required. The choice of sample preparation method depends heavily on the type of material. This section focuses on the methodologies and considerations essential for preparing TEM samples of secondary battery materials.
A typical battery comprises anode, cathode, electrolyte, separators, and battery shell. TEM is employed to characterize the morphology, structure, and composition of the electrode materials, the interface between electrode and electrolyte, as well as dendrites during charge and discharge processes. Due to the sensitivity of battery materials to electron beam irradiation, it is crucial to select appropriate sample preparation methods to ensure optimal results in TEM analysis.
3.1. Ultrasonic Dispersion Method
For most solid particles and solid-solid interfaces in battery materials, such as cathodes and interfaces between cathodes and solid electrolytes, the conventional approaches involve directly dispersing particles on TEM grids. However, it is crucial to exercise caution when applying ultrasound treatment for improved dispersion, as high-energy waves may damage fragile samples. In the case of a magnetic cathode, it is essential to undergo a demagnetization treatment before preparation. Subsequently, a double-stacked carbon film should be employed for TEM sample preparation.
3.2. In-situ Preparation and Cryo-transfer
It remains a challenge to mount air-sensitive nanomaterials from battery materials to a TEM grid without causing damage or contamination. in-situ preparation offers a possible solution by incorporating a TEM grid as part of the battery current collector in the battery to alleviate sample damage or contamination, e.g., Li/Na metal may be directly plated on TEM grids [13]. The grid will then be removed from the cell and sealed in the glovebox before transferring it to the TEM transfer holder, which includes cryogenic and cooling holders. When employing a cryogenic transfer holder, the TEM grid should be installed in a liquid nitrogen environment. When using another type of cooling holder, the sample installation and transfer processes should be carried out in a glovebox or argon-filled environment. After inserting the holder into the TEM chamber, the sample could be immediately cooled, which mitigates potential damage to lithium metal and other sensitive materials.
3.3. Cryo-FIB and Cryo-transfer
For large particles and buried solid-solid interfaces in battery materials, cryo-FIB is recommended for TEM sample preparation. Cryo-FIB could effectively reduce large bulk samples to the desired size without causing beam damage. Afterward, the obtained TEM sample is transferred to a holder and then transferred to a low-temperature TEM for further characterization.
In case of solid-liquid interface materials, during the immersion/freezing process, they will be transformed into a glass phase, converting the previous solid-liquid interface into a solid-solid interface, while retaining their contact with each other. Then, the fixed region can be sliced using the cryo-FIB method to prepare TEM samples for analysis, followed by cryo-transfer to the TEM chamber.
4. Applications of TEM in Characterization of Secondary Batteries
4.1. Applications of Conventional TEM in Characterization of Secondary Battery Materials
TEM modes could be categorized into imaging modes and diffraction modes. Imaging modes are typically employed to observe the morphology of the sample. In contrast, diffraction patterns are obtained from specific regions using selected area electron diffraction (SAED), providing information about the crystallinity and phase structure of the selected areas. Combining diffraction imaging, the crystal structure of the materials could be obtained. Therefore, TEM can be
adopted as the most valuable tool for investigating the morphology of battery materials, surface coatings, and other features at interfaces.
During the battery cycling process, the cathode material is prone to fracture and accompanied by side reactions, which seriously hampers the cycle life and rate performance of the battery. Surface coating shows great potential to reduce the side reaction and optimize the cathode. Lee and co-worker synthesized layered nickel-rich Li-Ni0.6Co0.2Mn0.2O2 (LNCM) cathode material and applied a double-layer coating of alumina nanoparticles and poly(3,4-ethylenedioxythiophene)-co-poly (ethylene glycol) (PEDOT-co-PEG) [14]. To verify the uniformity of the Al2O3 nanoparticles and PEDOT-co-PEG coating on the surface of LNCM particles, imaging analysis was conducted using TEM on both pristine LNCM and surface-modified LNCM particles shown in Fig. (4a). The pristine LNCM particles were uncoated. In contrast, the LNCM particles were uniformly coated with a polymer layer for cp-LNCM. In the TEM image of al-LNCM, the cathode particles were covered with Al2O3 nanoparticles, with coating thickness ranging from 18-38 nm. For dl-LNCM particles, the LNCM particles were covered with a dual-layer of Al2O3 and PEDOT-co-PEG. The results demonstrate that the dual-layer coating on cathode active materials can effectively enhance the cycling performance and thermal stability of Li-ion batteries. In order to enhance the lithium-ion diffusion rate of nickel-rich LNCM, some researchers have proposed a novel strategy of spontaneous grafting of benzene-diazonium tetrafluoroborate (C6H5N2+BF4−) onto the surface of LiNi0.6Co0.2Mn0.2O2 (LNCM-3) [15]. Such surface functionalization strategy can enhance the high performance of LIBs cathode materials. The morphology of LNCM-3 particles after surface functionalization with a thin conductive polymer layer was characterized using TEM, as shown in Fig. (4b). The TEM images, complemented by selected electron diffraction patterns, confirm the uniform coating of LNCM-3 particles with the thin conductive polymer layer.
4.2. Applications of HRTEM in Characterization of Secondary Battery Materials
HRTEM has been adopted to acquire atomic-level images of structural information, which is of great significance in investigating cathode-electrolyte compatibility in battery studies. Among them, surface coating [16, 17], doping [18] to cathode materials, and cathode-electrolyte interphase are considered to effectively reduce the occurrence rate of side reactions between the electrolyte and the cathode [19]. Li et al. used HRTEM to demonstrate the existence of LiF induced by the surface fluorinated reconstruction on the surface layer of modified cathode materials, while it was absent in the bulk phase, which contributes to enhancing interface stability [20].
Fig. (4)) (a) TEM images of (1) pristine LNCM, (2) conductive polymer-coated LNCM (cp-LNCM), (3) alumina-coated LNCM (al-LNCM), and (4) double-layer-coated LNCM (dl-LNCM); Reused with authorization from Ref [14]. Copyright 2015, American Chemical Society. (b) Interface characterization of polyphenylene/LNCM-3 particles and SAED insets. Reused with authorization from Ref [15]. Copyright 2022, Institute of Physics Publishing, IOP.Yu et al. designed and prepared the LiNi0.9Co0.1O2 (NCAl-LAO) cathode using an oxalate-assisted deposition followed by a heat-driven diffusion method, incorporating simultaneous gradient Al-doped and LiAlO2 coating [21]. HRTEM was employed to reveal the microstructural characteristics of the NCAl-LAO, as depicted in Fig. (5a). The outermost layer of secondary NCAl-LAO particles consists of a uniform coating with a thickness of 2-4 nm. The lattice spacings of 2.2 Å and 4.7 Å were indexed to the (211) plane of the LiAlO2 and the (003) planes of the Ni-based layered oxides, respectively. These results suggest that simultaneous gradient Al doping within the primary particles and uniform LiAlO2 coating on the secondary particle surface can stabilize the crystal structure while preventing side reactions at the interface. Such a dual-modification approach could address two critical issues of crystal degradation and interface instability of the nickel-rich cathode, providing a promising strategy for high-energy cathodes. In another work, Ni and a co-worker devised a strategy combining atomic/microstructural reconstruction with interfacial shielding to enhance the LiNi0.94Co0.04Al0.02O2 (NCA) cathode by incorporating Sb5+ doping and Li7SbO6 coating [22]. HRTEM revealed that after Sb modification, the NCA cathode exhibits a typical layered structure throughout the particles, while a unique coating is formed at the grain boundaries of the 1Sb NCA cathode. This coating is identified as the (303) plane of the Li7SbO6 phase, which offers advantages in enhancing interface stability and Li+ diffusion capability (Fig. 5b).
Fig. (5)) (a) HRTEM images of surface for NCAl-LAO. Reprinted with permission from Ref [21]. Copyright 2021, Springer Nature; (b) HRTEM images of NCA and 1Sb-NCA. Reused with authorization from Ref [22]. Copyright 2023, American Chemical Society.4.3. Applications of HAADF-STEM in Characterization of Secondary Battery Materials
HAADF-STEM and ABF-STEM have been widely employed in the characterization of LIBs. Since HAADF and ABF are particularly sensitive to heavy and light elements, respectively, HAADF-STEM allows for the direct imaging of light elements such as lithium and oxygen, which is crucial for the study of LIB materials.
High-resolution STEM-HAADF imaging is usually utilized to elucidate the degradation mechanisms in batteries. For example, when exposed solely to the electrolyte without undergoing the process of charge and discharge, the surface reconstruction layer (SRL) is already formed on the surface of the cathode particles, shown by the pink line in Fig. (6a). Understanding the formation and characteristics of the SRL is crucial for improving both stability and performance of cathode materials. Zhang et al. revealed that the formation of SRL in the pristine LiNi0.80Co0.15Al0.05O2 sample is related to crystal facets [23]. Two different rock-salt structures with (002) and (111) surfaces were formed along the pristine (014_) and (003) planes of the layered phase, as shown in Fig. (6a). Compared to the rough (111) configuration, the (002) lattice plane was relatively smooth and exhibited a series of single-atom steps. Such structural variations suggested that the distribution of surface-formed rock-salt phase may be non-uniform. During the lithium deintercalation process, numerous defects are dynamically formed to release and adapt to the strain generated in electrochemical processes, which increases charge transfer resistance and decreases battery performance. Gong and co-workers observed the atomic-level structural changes of LiCoO2 (LCO) cathode after high-voltage delithiation by STEM-HAADF and STEM-ABF (Fig. 6b) [24]. The single crystal LCO decomposed into polycrystalline structures, with coherent twin boundaries (TB) and anti-phase boundaries (APB) forming along the [010] zone axis. After delithiation, the angle of the coherent TB changes from 109.5° to 112°. Additionally, a distinct contrast was observed between the two boundaries. The existence of these defects may lead to the instability of the high-voltage LCO cathode structure. To prevent the structural degradation of LCO at high voltage, Xia et al. synthesized the Lanthanide-doped LCO by a Li-deintercalation/doping strategy [25]. To confirm the doped Ln cation sites, the atomic structural differences between pristine LCO (PLCO) and Lu-doped LCO (Lu-LCO) were compared using STEM-HAADF. The results showed that Lu atoms occupied the 3a positions of Li atoms, indicating that Ln cations were introduced into the Li layer, thereby preventing structural collapse during high lithiation states.
Fig. (6)) Two types of surface reconstruction layer (SRL) (a); HAADF and ABF-STEM images of two boundaries including TB, Reused with authorization from Ref [23]. Copyright 2018, American Chemical Society, and APB for the delithiated LiCoO2 cathode (b), Reused with authorization from Ref [24]. Copyright 2017, American Chemical Society.4.4. Applications of EDS-Mapping in Characterization of Secondary Battery Materials
In the STEM mode, EDS-Mapping enables the acquisition of elemental distribution information, which can be used to analyze the element distribution at the electrode-electrolyte interface to determine the SEI composition, as well as to investigate the cathode surface coating before and after battery cycling [26]. The structural degradation and side reactions at the cathode interface are critical challenges that hinder the development of high-voltage sulfide-based all-solid-state lithium batteries (ASSLBs) (≥4.5 V). He et al. designed an LCO cathode coating with a nanometric Li1.175Nb0.645Ti0.4O3 (LNTO), which was characterized by EDS mapping to observe the coating layer before and after cycling (Fig. 7) [27]. The HAADF images and corresponding EDS mapping indicated that coating prior to cycling was approximately uniform, with Nb and Ti distributed on the surface of LCO (Fig. 7a). However, after 300 cycles, the combination of EDS mapping, STEM-HAADF images, and the line profiles of Ti and Nb demonstrated element segregation at the microscale interface after battery cycling (Fig. 7b).
4.5. Applications of EELS in Characterization of Secondary Battery Materials
In recent years, EELS has been extensively integrated into the TEM to reveal valuable information regarding the type of elements, their oxidation states, and concentration distributions. In fact, EELS has been extensively utilized in battery material research [12]. For instance, EELS are commonly employed in analyzing the heterogeneity and chemical structural evolution of battery materials including electrodes, electrolytes, and the interface between electrodes and electrolytes.
EELS has significantly promoted researchers’ understanding of batteries by offering chemical composition analyses with high spatial resolution of element and oxidation state concentrations, as well as radial distributions. It can be applied in five main aspects: analyses of battery material composition, characterization of intermediate states, dynamic Li+ behaviors, behaviors related to dynamic Li+ and interface, and thermal stability.
EELS could explicitly identify the composition of battery materials, providing accurate atomic-resolution information on element mapping, electronic states, and faults. Silicon (Si) is a high theoretical specific capacity anode material for rechargeable LIBs. However, the mechanism by which the chemical state of Si leads to capacity fading during battery cycling remains ambiguous. To investigate such evolution, Quinn et al.
