Common characterization techniques for 1.2V NiMH batteries materials

During the development of lithium-ion batteries, we hope to obtain a large amount of useful information to help us analyze the data of materials and devices to understand their various performances. At present, the research methods commonly used for lithium-ion battery materials and devices mainly include characterization methods and electrochemical measurements.

Electrochemical testing is mainly divided into three parts:

(1) Charge and discharge test, mainly to look at the battery charge and discharge performance and rate, etc.;

(2) Cyclic voltammetry, mainly to look at the battery charge and discharge reversibility, peak current, and peak position;

(3) EIS AC impedance, to look at the battery resistance and polarization, etc.

The following is a brief introduction to the characterization methods used in comprehensive 1.2V NiMH batteries research, which can be roughly divided into eight parts: composition characterization, morphology characterization, crystal structure characterization, material functional group characterization, material ion transport observation, material micromechanical properties, material surface work function and other experimental techniques.

I. Composition characterization

(1) Inductively coupled plasma (ICP)

It is used to analyze the constituent elements of a substance and the content of various elements. ICP-AES can well meet the needs of routine analysis of major, minor and trace elements in the laboratory; ICP-MS is a newly developed technology in recent years compared to ICP-AES. The instrument is more expensive and has a lower detection limit. It is mainly used for trace/ultra-trace analysis.

Aurbac et al. used ICP to study the solubility of LiC0O2 and LiFePO4 in the electrolyte when studying the interface problem between the positive electrode material and the electrolyte. By changing the temperature, the type of lithium salt in the electrolyte and other parameters, ICP was used to measure the changes in the Co and Fe content in the electrolyte when the parameters were changed, thereby finding the key to reducing the dissolution of the positive electrode material in the electrolyte. It is worth noting that if the element content is high (for example, higher than 20%), the error will be large when using ICP detection, and other methods should be used at this time.

(2) Secondary ion mass spectrometry (SIMS)

By emitting hot electrons to ionize argon or oxygen plasma to bombard the surface of the sample, the charged ions or ion clusters overflowing from the sample surface are detected to characterize the sample composition. The isotope distribution can be imaged to characterize the sample composition; the longitudinal distribution of the sample composition can be detected.

Ota et al. used TOF-SIMS technology to study the composition of the SEI film formed on the surface of graphite negative electrode and LiC0O2 positive electrode after adding ethylene sulfite as an additive to the standard electrolyte. Castle et al. used SIMS to detect the distribution of Li+ from the electrode surface to the inside of V2O5 after lithium insertion to study the diffusion process of Li+ in V2O5.

(3) X-ray photon spectroscopy (XPS)

It was gradually developed and improved by Professor Kai Siegbahn and his team at the Institute of Physics, Uppsala University, Sweden in the 1950s and 1960s. X-ray photoelectron spectroscopy can not only determine the constituent elements of the surface, but also provide information on the chemical state of each element. It has high energy resolution and a certain spatial resolution (currently micrometer scale) and time resolution (minute level).

It is used to determine the constituent elements of the surface and provide information on the chemical state of each element.

Hu Yongsheng et al. used XPS to study the composition of SEI generated by VEC on the graphite surface under high voltage. It is mainly composed of C, O and Li. Combined with FTIR, it was found that the main component is alkoxy lithium salt.

(4) Electron Energy Loss Spectroscopy (EELS)

The energy lost by incident electrons due to inelastic scattering such as electron excitation and ionization on the surface of the material can be used to obtain the elemental composition by analyzing the location of energy loss. Compared with EDX, EELS has a better resolution effect on light elements, with an energy resolution 1 to 2 orders of magnitude higher. Due to the transmission electron microscopy technology, the spatial resolution can also reach the order of 10?10 m. It can also be used to test the thickness of thin films and has a certain time resolution. By fitting the EELS spectrum with density functional theory (DFT), accurate information on the valence state of elements and even the electronic state can be further obtained.

AI.Sharab et al. used the STEM-EELS joint technology to study the chemical element distribution, structural distribution and iron valence state distribution of the nanocomposite of iron fluoride and carbon under different charge and discharge states when studying the electrode material of nanocomposite of iron fluoride and carbon.

(5) Scanning Transmission X-ray Microscopy (STXM)

A new spectroscopic microscopy technology based on the third-generation synchrotron radiation source, high-power laboratory X-ray source, and X-ray focusing technology. Using the principle of transmission X-ray absorption imaging, STXM can achieve three-dimensional imaging with high spatial resolution of tens of nanometers and provide certain chemical information. STXM can achieve non-destructive three-dimensional imaging, which can provide key information for understanding complex electrode materials, solid electrolyte materials, diaphragm materials, electrodes and batteries, and these technologies can realize the function of in-situ testing.

Sun et al. studied that carbon-coated Li4Ti5O12 has better rate performance and cycle performance than before coating. The authors used STXM-XANES and high-resolution TEM to determine that the amorphous carbon layer is uniformly coated on the surface of LTO particles with a coating thickness of about 5 nm. Among them, the authors obtained the distribution of C, Ti, and O of a single LTO particle through STXM, in which C is coated on the surface of the particle.

(6) X-ray absorption near-edge spectroscopy (XANES)

It is a technology for calibrating elements and their valence states. The same element with the same valence state in different compounds has high absorption of X-rays of a specific energy, which we call near-edge absorption spectrum. In the field of lithium batteries, XAS is mainly used for charge transfer research, such as the valence change of transition metals in positive electrode materials.

Kobayashi et al. used XANES to study the positive electrode material LiNi0.80Co0.15Al0.05O2. XANES detected that the particle surface contained Li2Co3 and other additional cubic phase impurities.

(7) X-ray fluorescence spectrometry (XRF)

A method of using primary X-ray photons or other microscopic ions to excite atoms in the substance to be tested, causing them to produce fluorescence (secondary X-rays) to analyze the composition of the material and study its chemical state. According to the different excitation, dispersion and detection methods, it is divided into X-ray spectroscopy (wavelength dispersion) and X-ray energy spectroscopy (energy dispersion). According to the different dispersion methods, X-ray fluorescence analyzers are divided into X-ray fluorescence spectrometers (wavelength dispersion) and X-ray fluorescence energy spectrometers (energy dispersion). XRF is widely used in the industry for the analysis of the main components and impurity elements of lithium-ion battery materials. The detection limit for some elements can reach the order of 10-9.

2. Morphological characterization

(1) Scanning electron microscopy (SEM)

Collect secondary electron information on the sample surface to reflect the surface morphology and roughness of the sample. SEM with EDS accessories can further analyze the element types, distribution and semi-quantitative analysis of element content. Although the resolution of SEM is much smaller than that of TEM, it is still the most basic tool for characterizing the particle size and surface morphology of battery materials.

Li Wenjun et al. redesigned the sample holder of the scanning electron microscope for metal lithium electrodes based on the use of sealed transfer boxes to transfer samples, and studied the formation process of surface holes and dendrites on the metal lithium electrode during the insertion and extraction of Li.

(2) Transmission electron microscopy (TEM)

The morphology and characteristics of the surface and interface of the material are often introduced in the literature on surface coating and the description of surface SEI. TEM can also be equipped with energy spectrum accessories to analyze the types and distribution of elements. Compared with SEM, TEM can observe smaller particles, and high-resolution transmission electron microscopy can observe the lattice. In-situ TEM is more powerful. In-situ batteries are assembled in the TEM electron microscope cavity. At the same time, with the help of TEM's high-resolution characteristics, the morphology and structural evolution of battery materials during the cycle process are measured and analyzed in real time.

Huang Jianyu et al. used an in-situ sample rod to in-situ characterize the morphology and structural evolution of SnO2 during the lithium insertion and extraction process in ionic liquid. Subsequently, they improved the device of TEM in-situ battery experiment, using lithium oxide naturally produced on metal Li as electrolyte to replace the ionic liquid used originally, which improved the stability of the experiment and better protected the electron microscope cavity.

(3) Atomic force microscopy (AFM)

Observation of nanoscale flat surfaces is widely used in the characterization of carbon materials.

3. Crystal structure characterization

(1) X-ray diffraction technology (XRD)

Through XRD, we can obtain information such as the crystal structure, crystallinity, stress, crystal orientation, superstructure, etc. of the material. It can also reflect the average crystal structure properties of the bulk material and the average unit cell structure parameter changes. After fitting, we can obtain atomic occupancy information.

Thurston et al. first applied in-situ XRD technology to lithium-ion batteries. By using the hard X-rays of the synchrotron radiation source to detect the bulk electrode material in the in-situ battery device, we can intuitively observe the results of lattice expansion and contraction, phase change, and multiphase formation.

(2) Extended X-ray absorption fine spectrum (EXAFS)

Through the interaction between X-rays and the electrons of the sample, the absorption of some incident photons of specific energy is used to reflect the local structural differences and changes of the material. It has a certain energy and time resolution ability and mainly obtains information such as radial distribution, bond length, order, coordination number, etc. in the crystal structure. Usually, a strong light source of the synchrotron radiation source is required to realize the EXAFS experiment.

Jung et al. studied the electrochemical properties of SnOx/CuOx embedded carbon nanofiber negative electrode materials by using EXAFS analysis, indicating that the SnOx/CuOx embedded carbon nanofibers have a disordered structure, forming a special distribution of SnOx particles, which leads to improved electrochemical performance.

(3) Neutron diffraction (ND)

When there are larger atoms in lithium-ion battery materials, X-rays will find it difficult to accurately detect the lithium ion occupancy. Neutrons are more sensitive to lithium in lithium-ion battery materials, so neutron diffraction plays an important role in the research of lithium-ion battery materials.

Arbi et al. determined the Li+ occupancy in the lithium-ion battery solid electrolyte material LATP by neutron diffraction.

(4) Nuclear magnetic resonance (NMR)

NMR has high energy resolution and spatial resolution capabilities, and can detect chemical information in materials and image them, detect dendrite reactions, determine lithium ion self-diffusion coefficients, and study phase transition reactions inside particles.

Grey et al. have conducted extensive research on the application of NMR in lithium-ion battery positive electrode materials. It shows that the NMR spectrum of the positive electrode material can obtain rich chemical information and information such as local charge order and disorder, and can detect paramagnetic or metallic materials. It can also detect the weak changes in the electronic structure caused by doping to reflect the information of the elemental chemical state. In addition, combined with isotope tracing, it can also study the side reactions in the battery.

(5) Spherical aberration corrected scanning transmission electron microscopy (STEM)

Purpose: used to observe the arrangement of atoms and atomic-level real-space imaging, and can clearly see the lattice and atomic occupancy; high requirements for samples; can realize in-situ experiments.

Oshima et al. used annular bright field imaging spherical aberration corrected scanning transmission microscopy (ABF-STEM) to observe the atomic arrangement of Li, V, and O in Li2VO4 in real space.

(6) Raman

Raman spectroscopy was used to study the crystal structure of LiC0O2 in the early days. There are two Raman active modes in LiC0O2, the peak of Co-O stretching vibration Alg and the peak of O-Co-O bending vibration Eg. It is also often used to characterize the degree of graphitization of carbon materials in lithium-ion batteries.

4. Characterization of functional groups

Functional groups, also known as functional groups, are atoms and atomic groups that determine the chemical properties of organic compounds. Common functional groups include hydrocarbon groups, halogen-containing substituents, oxygen-containing functional groups, nitrogen-containing functional groups, and phosphorus- and sulfur-containing functional groups.

(1) Raman spectroscopy (RS)

It was discovered by Indian physicist Raman in an experiment in which monochromatic light irradiated liquid benzene and scattered spectral lines with different frequencies from the incident light. Information about molecular vibration and rotation can be obtained from Raman spectroscopy. Raman spectroscopy is suitable for molecules with symmetrical structures with small polarity. For example, for molecules with fully symmetrical vibration modes, under the action of excitation photons, molecular polarization will occur, resulting in Raman activity, and the activity is very strong.

When characterizing lithium-ion battery electrode materials, since the disassembly and transfer process will inevitably cause interference to the electrode materials due to human or atmospheric reasons, in-situ technology is used together with Raman spectroscopy in the characterization of electrode materials. Raman spectroscopy is very sensitive to the symmetry, coordination and oxidation state of material structures, and can be used to measure transition metal oxides.

In the case where the sensitivity of Raman spectroscopy is not enough, some metals such as Au and Ag can be used to treat the sample surface. The enhanced electromagnetic field on the conductor surface or in the sol of these special metals close to the sample surface leads to enhanced Raman spectral signals of adsorbed molecules, which is called surface enhanced Raman scattering (SERS).

Peng et al. used SERS to confirm that there is indeed an intermediate product LiO2 in the charging and discharging process of lithium-air batteries, but LiO2 was not observed during the charging process, indicating that the discharge process of lithium-air batteries is a two-step reaction process with LiO2 as the intermediate product, while the charging process is an asymmetric one-step reaction. The direct decomposition of Li2O2 is difficult due to its poor conductivity, which is also the reason why the charging polarization is greater than the discharge polarization.

(2) Fourier transform infrared spectroscopy (FT-IS)

The band used by infrared spectroscopy is similar to that of Raman. Many molecules with weak Raman activity can be characterized by infrared spectroscopy. Infrared spectroscopy can also be used as a supplement to Raman spectroscopy. Infrared spectroscopy is also called molecular vibration spectroscopy and belongs to molecular absorption spectroscopy.

According to the different wavelengths of infrared light, infrared light can be divided into three regions:

① Near infrared region, i.e., over-band region, refers to the region with wave number above 4000 cm?1, mainly measuring the double frequency absorption of O-H, C-H, and N-H bonds;

② Mid-infrared region, i.e., fundamental vibration region, with wave number range of 400-4000 cm?1, is also the most studied and applied region, mainly measuring molecular vibration and accompanying vibration;

③ Far infrared region, i.e., molecular vibration region, refers to the region with wave number below 400 cm?1, mainly measuring the rotation information of molecules.

Since water is a very polar molecule, its infrared absorption is very strong, so the infrared spectrum cannot be directly measured by aqueous solution. Usually, the sample of infrared spectrum needs to be ground into KBr tablets.

Usually, the data of infrared spectrum needs to be processed by Fourier transform, so the infrared spectrometer and Fourier transform processor are used together, which is called Fourier transform infrared spectroscopy (FITR). In the research of lithium-ion battery electrolyte, there are many works using infrared spectroscopy.

Mozhzhukhina et al. used infrared spectroscopy to study the stability of dimethyl sulfoxide (DMSO), a commonly used solvent in lithium-air battery electrolytes, and found that DMSO was not stable in lithium-air batteries mainly due to the attack of superoxide ions (O2-). The presence of SO2 signals was observed in the infrared spectrum. This reaction is difficult to avoid. Even at a potential as low as 3.5 V, DMSO is unable to stabilize.The method is stable.

(3) Deep ultraviolet spectroscopy (UV)

Mainly used for the analysis of characteristic functional groups in solutions.

V. Phenomena of material ion transport

(1) Neutron diffraction (ND)

Combined with the maximum entropy simulation analysis method, information on the Li+ diffusion channel in the electrode material can be obtained.

(2) Nuclear magnetic resonance (NMR)

The NMR spectra of some elements were measured as a function of heat treatment temperature, and the self-diffusion coefficient of Li+ was measured. Gobet et al. used pulse gradient field NMR technology to characterize the changes of 1H, 6.7Li, and 31P NMR spectra in β-Li3PS4 solids as a function of heat treatment temperature, and measured the self-diffusion coefficient of Li+, which is consistent with the order of magnitude of Li+ conductivity reported previously.

(3) Atomic force microscopy series technology (AFM)

The van der Waals force between the atoms at the tip and the atoms on the sample surface is used to feedback the surface morphology information of the sample. AFM has high spatial resolution (about 0.1?) and time resolution. Since it does not detect energy and does not have energy resolution, it was first used in lithium-ion battery research in 1996.

Zhu et al. prepared a full battery using solid electrolyte by magnetron sputtering, and then used in situ AFM to detect the changes in the surface morphology of the TiO2 negative electrode with the loaded triangular waveform voltage.

VI. Micromechanical properties of materials

Battery materials are generally polycrystalline, and there is stress inside the particles. During the charging and discharging process, the insertion and extraction of lithium will cause lattice expansion and contraction, resulting in changes in local stress, which will further cause volume changes of particles and electrodes, stress release, lattice stacking changes, and cracks in particles and electrode layers.

(1) Atomic force microscopy series technology (AFM) and nanoimprint technology, as well as combined testing with nanoprobes and STM probes in TEM

Observe morphological characteristics, and in situ mechanical properties and stress measurements can be performed when using solid-state batteries.

Jeong et al. used AFM to in situ observe the thickness of the surface film formed on the HOPG basal surface during cyclic voltammetry.

(2) SPM probe

Purpose: Study the mechanical properties of SEI film

In contact mode, the probe is inserted into the film with a constant force, and the response curve of the penetration depth with force can be obtained, and then the Young's modulus and other information can be obtained.

VII. Material surface work function

(1) Kelvin probe force microscopy (KPFM)

The potential distribution on the sample surface is obtained by detecting the force exerted by the surface potential on the probe.

agpure et al. used Kelvin probe microscopy (KPFM) to measure the surface potential of aged lithium-ion batteries. The aged battery has a lower surface potential, which can be attributed to the influence of particle size, phase change of the surface layer and the physical and chemical properties of the new deposits.

(2) Electron holography

Measure the change of potential during the charge and discharge process of all-solid-state lithium-ion batteries, and obtain the distribution of potential at the interface under different systems.

The Yamamoto group directly observed the change of potential during the charge and discharge process of all-solid-state lithium-ion batteries by electron holography, and successfully obtained the distribution of potential at the interface under different systems, verifying the conclusion that the potential is mainly distributed at the positive electrode/electrolyte interface.

(3) Photoemission electron microscopy (PEEM)

Used to obtain the distribution of surface potential

In addition to the above characterization methods, some other characterization techniques are also used in actual experiments, such as:

(1) Angle-resolved photoelectron spectroscopy (ARPES), purpose: directly measure the material band structure;

(2) DFT calculation, purpose: obtain the electronic structure of the material;

(3) Electron flooding technique (PAT), purpose: measure defect structure and electronic structure;

(4) Rutherford backscattering (RBS), purpose: can measure the composition of thin films;

(5) Resonant inelastic X-ray scattering (RIXS), purpose: study the magnetic interaction between atoms;

(6) Auger electron imaging technology (AES), purpose: directly detect the spatial distribution of lithium elements on the surface of particles and electrodes, and can also perform element depth analysis through Ar ion stripping. Of course, electrochemical characterization is also very important when studying lithium batteries.

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