Methods for TEM analysis of lithium battery materials

The atomic and electronic structures of lithium-ion battery materials directly determine the performance of the battery. Transmission electron microscopy, with its atomic-scale spatial resolution capability, can acquire structural distortions and electronic structure changes on the atomic scale, which plays a crucial role in the study of lithium-ion battery materials.
Table of Contents

TEM model characterization

TEM modes are mainly divided into two categories: image mode and diffraction mode. The image mode is usually used to observe the morphology of the sample. In addition, using high resolution transmission electron microscopy (HRTEM), structural images can be obtained with atomic scale resolution.

Diffraction mode usually uses the selected area electron diffraction (SEAD) method to obtain the electron diffraction results of a selected area, which can be analyzed for crystallinity and phase structure information at the selected location.

(a)TEM brightfield photo (b)Electron diffraction images and high-resolution photographs of selected areas

Fig. 1 Effect of TEM characterization of ternary materials

STEM model characterization

The STEM mode scans the surface of the sample with a converging electron beam and uses a ring detector to receive scattered electrons at different receiving angles for imaging. High angle annular dark field imaging (HAADF) and annular bright field imaging (ABF) are widely used in lithium ion batteries.

Among them, HAADF is sensitive to heavy elements, and ABF is sensitive to light elements, and can be used to directly image light elements such as Li and O, which is crucial for the study of Li-ion battery materials, as shown in Figure 2.

(a) ABF image of pristine LiFePO4;(b) ABF image of LiFePO4 in the fully delithiated state; (c) ABF image of LiFePO4 in the semi-delithiated state, where the order structure of Li and Li vacancies was found.

Fig. 2 Atomic scale structure of LiFePO4 cathode material at different amounts of delithiation

X-ray spectrum

X-rays are emitted when the excited electrons in the sample return to the ground state and are received to obtain an X-ray spectrum (EDS), which analyzes the characteristic X-rays emitted from the surface of the sample to obtain information about the elements contained in the sample.

In the TEM mode, the spectral information is averaged, reflecting the average elemental composition and proportions of the area irradiated by the electron beam. In STEM mode, the relationship between elemental species and elemental positions can be established to obtain the distribution map of elements, as shown in Figure 3.


Electronic holography

Electron holography can study the potential distribution of materials, which is important for lithium battery materials. The holography mentioned here usually refers to off-line holography, i.e., the incident electron beam passes half through the sample and half through the vacuum, thus forming the object wave and the reference wave.

As shown in Fig. 4, the object and reference waves are deflected by an electron prism and interfere with each other to form a holographic pattern. The pattern is then reconstructed by data processing to obtain the potential distribution. Through the electron holography method, the potential distribution of the battery material can be obtained during the cycling process.

(a) The object and reference waves interfere with each other to form a holographic pattern after the action of an electron prism; (b) Reconstruction of the object wave by Fourier transform of the holographic pattern. The phase of the reconstructed object wave is the potential distribution.

Fig. 4 Schematic diagram of electronic holography

Electron energy loss spectrum

In transmission electron microscopy the electron beam is scattered after passing through the sample, the energy of the electrons scattered elastically remains constant and the electrons scattered inelastically have a change in energy. The Electron Energy Loss Spectrum (EELS) analyzes the distribution of energy loss after inelastic scattering of incident electrons with a fixed energy from a sample.

Inelastic scattering involves Coulombic interactions between electrons and electrons outside the nucleus of the sample atom, where the electrons outside the nucleus undergo a selective jump by receiving the energy of the incident electrons, while the incident electrons lose a corresponding amount of energy.

Different elements in different states of the energy required for the selective jump is different, so according to the energy loss of incident electrons can be obtained from the sample’s elemental information and electronic structure information, which includes obtaining the thickness of the sample, distinguishing the type and content of the element, determine the valence state of the element and other structural information.

The differences between EELS and EDS are shown in Table 1:

Energy resoutions Energy resolution of about 100 eV Energy resolution better than 1eV
Signal interval Up to 2000 eV or more 0-1000eV, suitable for resolving fine electronic structures
Acquisition time Allows for longer periods of time compared to EELS acquisition Seriously affected by sample drift, not easy to collect too long
Professionalism requirements Easier to operate, visualization of results is good Complicated operation, poor visualization of the results obtained, more specialized data processing required
Electron-energy-loss-spectrumFig. 5 Analysis of carbon bonding environments in the vicinity of branches

Convergent beam electron diffractio

Convergent Beam Electron Diffraction (CBED) allows obtaining structural information at the electron orbital level. CBED measures the Fourier coefficients of the Coulomb potential (structure factor) of a crystal, which is converted into X-ray structure factor and the electron density is obtained by Fourier transform.

The structure factor measurement by electron diffraction has the advantages of being able to measure low-order structure factors, sensitive to electronic states, and capable of precise micro-region analysis, which ensures the accuracy of the electron density obtained.

The electron density can get the information of orbitals and topological states of the crystal by multipole fitting. The charge density and bonding in LiNiO2 material are shown in Fig. 6. Since the CBED method requires a long time and a large dose of the electron beam to act on the sample, the CBED method cannot be widely used in the study of lithium ion battery chemistry at present.

(a) CBED patterns of LiNiO2 material; (b) best fit of experimental data and theoretical calculations after refinement

Fig. 6 

Cryoelectron microscopy

Lithium battery materials are usually very sensitive to electron beam irradiation, such as lithium metal anode and solid state battery electrolyte materials, which limits electron microscopy to many electron beam sensitive materials.

Recently, Prof. Yi Cui’s team at Stanford University and Prof. Ying Meng’s team at the University of California, San Diego have recently performed HRTEM characterization of lithium metal using frozen sample rods at liquid nitrogen temperature, respectively.

The lithium ion battery electrolyte is an important component of the battery, but the fact that most of the electrolytes are liquids has led to a dearth of research on the structure and properties of liquid electrolytes.

Recently, thanks to the development of cryo-electron microscopy methods and cryo-FIB (cryo-FIB), it has become possible to study the state of the liquid electrolyte system during different charging and discharging processes in the electron microscope, as shown in Fig. 7.

(a) FIB images of type I woven crystals, SEI membrane and electrolyte; (b) FIB images of type II woven crystals and electrolyte; (c) HAADF Cryo-STEM images of type I wafers, SEI membrane and electrolyte; (d) HAADF Cryo-STEM image of type II woven crystal and electrolyte.

Fig. 7

In-situ electricity generation

The lifetime of a lithium battery is spent in the cycle of charging and discharging, so in-situ characterization during the charging and discharging of lithium ion battery is crucial.

In 2009, Allard et al. used a microelectromechanical system (MEMS) chip to carry the sample and designed a new in-situ sample rod, which realizes a rapid heating and cooling process up to a maximum temperature of more than 1,000 degrees Celsius, while at the same time the stability of the sample rod is sufficient to ensure that atomic scale images are obtained under the STEM;

Gong et al. applied the chip-based sample bars to the in-situ study of lithium-ion battery materials, successfully constructed a microscopic all-solid-state battery on an in-situ chip, and realized the in-situ observation of lithium-ion migration at the atomic scale (see Fig. 8), and further extended the characterization range to the three-dimensional atomic scale.

With the advantages of tiltability, high stability, maneuverability, and ease of further processing, the chip-on-chip sample rod has become the mainstream of in situ research.

(a) SEM image of an all-solid-state Li-ion battery built using FIB; (b) Schematic of the constructed all-solid-state battery; (c) Atomic-scale ABF image of the pristine LiCoO2 cathode material; (d) The corresponding atomic scale HAADF image of the pristine LiCoO2 cathode material.

Fig. 8 Initial structure of microscopic all-solid-state battery material

In-situ variable temperature

Temperature affects the performance of batteries in practical applications, and the performance of batteries at high or low temperatures is critical to the promotion of batteries in practical applications.

The heating and low temperature tests in in situ electron microscopy use different principles. Heating is controlled by the heat generated by an electric current, while cryogenic temperature is controlled by the balance of liquid nitrogen and electric heating to bring the sample in the interval from room temperature to liquid nitrogen temperature.

Figure 9 shows the structure of the electrode material at different temperatures, which is important for understanding the performance of Li-ion batteries in real-world operating environments. In the future, it is hoped that the combination of in-situ denaturation and electrification will be of more practical value and significance to the study of lithium battery materials.

(a) HRTEM image before heating; (b) HRTEM image after heating at 100°C; (c) HRTEM image after heating at 200 °C; (d) HRTEM image after heating at 300℃.

Fig. 9 [HRTEM images of overcharged Li0.15Ni0.8Co0.15Al0.05O2 particles

The inset vignette in each image shows the selected zone electron diffraction pattern of the sample at the respective temperature.

3D reconstruction

There are usually two methods to obtain the three-dimensional structure structural information in electron microscopy, one is to record the sample structural information by tilting the sample at different angles in the electron microscope, and then restore the three-dimensional structure of the sample; the other is to restore the three-dimensional structure of the sample by the outgoing wave reconstruction method.

The schematic diagrams of the two methods are shown in Fig. 10. At present, the atomic scale 3D reconstruction method is more demanding for the sample and has not been applied in lithium battery materials.

However, through the multi-orientation structural characterization of the samples, the three-dimensional structural information hidden behind the two-dimensional projection results has been discovered. It is believed that with the progress of science and technology, the 3D reconstruction method can achieve fruitful results in the research of lithium battery materials in the future.

(a) 3D reconstruction method for continuously tilted samples; (b) outgoing wave reconstruction method for obtaining 3D structure

Fig. 10 

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