The cathode material is the most critical component of a lithium-ion battery. Improving the physical and chemical properties of lithium-ion battery cathode materials can significantly enhance the performance of lithium-ion batteries and drive their development.
Currently, the most widely used cathode materials include lithium cobalt oxide, lithium manganese oxide, lithium iron phosphate, and lithium nickel cobalt manganese oxide. The physicochemical and electrochemical properties of lithium-ion battery cathode materials, such as particle size, morphology, specific surface area, tap density, structure, and composition, have a significant impact on their application. Accurate analysis and measurement of these performance parameters are of great importance to both researchers and users of lithium-ion battery cathode materials. This article reviews the analytical methods for these physicochemical and electrochemical properties, providing a reference for researchers working on lithium-ion battery cathode materials.
I. Methods for analyzing the physicochemical properties of lithium-ion battery cathode materials
The physicochemical properties that need to be analyzed for lithium-ion battery cathode materials mainly include particle size, morphology, specific surface area, tap density, structure, and composition.
1. Particle size analysis of lithium-ion battery cathode materials
The particle size and particle size distribution of lithium-ion battery cathode materials have a significant impact on battery safety performance and electrode compaction density, as well as the electrical properties of the battery materials. Particle size analysis methods mainly include sedimentation, sieving, Coulter method, electron microscopy statistical observation, electro-ultrasonic particle size analysis, laser diffraction, and dynamic light scattering. Laser scattering is the most widely used method for particle size analysis of lithium-ion battery cathode materials, and the testing procedures mainly refer to the standard GB/T19077-2016.
The principle of laser scattering is as follows: a particle sample is dispersed at an appropriate concentration in a suitable liquid or gas, and then passed through a monochromatic light beam. When the light encounters the particles, it is scattered at different angles, and the scattered light is measured by a multi-element detector. Through appropriate optical models and mathematical processes, these quantified scattering data are transformed to obtain a series of discrete particle size ranges representing the percentage of particle volume relative to the total particle volume, thus yielding the particle size distribution. A schematic diagram of the principle is shown in Figure 1.
Figure 1. Schematic diagram of the principle of laser dispersive spectroscopy.
Laser scattering is a simple, fast, widely applicable, and highly repeatable method. However, when using laser scattering to measure the particle size and size distribution of lithium-ion battery cathode materials, it is crucial to accurately determine the refractive index and absorptivity of the cathode material; otherwise, measurement errors will occur. Zhu Guo et al. used a laser particle size analyzer based on the laser scattering principle to test the particle size of lithium cobalt oxide cathode materials. Their analysis showed that the test data had small errors and good reproducibility.
2. Morphology Analysis of Lithium-ion Battery Cathode Materials The most commonly used methods for morphology analysis of materials are scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Sometimes, depending on the specific requirements of the material morphology, scanning tunneling microscopy (STM) and atomic force microscopy (AFM) are also used.
The morphology analysis of lithium-ion battery cathode materials mainly utilizes scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM typically only provides micron- or submicron-level morphological information for lithium-ion battery cathode materials. TEM offers higher resolution, reaching 0.1 nm to 0.2 nm, and can analyze not only the morphology of samples but also their crystal structure. Su Zhi et al. used TEM to analyze the morphology of lithium manganese oxide, and the results are shown in Figure 2. The figure clearly shows the morphology of the sample, and also indicates good crystallinity.
Figure 2 TEM image of the sample
3. Specific Surface Area Analysis of Lithium-ion Battery Cathode Materials The specific surface area of lithium-ion battery cathode materials is related to their processing performance. Therefore, specific surface area analysis is an important aspect of the physicochemical property analysis of lithium-ion battery cathode materials. The main method for specific surface area analysis of lithium-ion battery cathode materials is the BET method. The principle of the BET method is as follows: the sample is placed in a sample container and then immersed in liquid nitrogen. Under these conditions, physical adsorption will occur on the sample surface. The equilibrium adsorption pressure at which adsorption reaches equilibrium is measured using a pressure gauge, and the amount of gas adsorbed by the sample is measured using volumetric or chromatographic methods. Finally, the equilibrium adsorption pressure and the volume of adsorbed gas are substituted into the BET equation to calculate the specific surface area of the sample. The BET test method and procedures refer to GB/T13390-2008.
4. Tap Density Analysis of Lithium-ion Battery Cathode Materials The tap density of lithium-ion battery cathode materials is related to the compaction density during battery manufacturing and is one of the key indicators of material energy density. The tap density analysis of lithium-ion battery cathode materials mainly employs the vibration method. The principle is as follows: a certain amount of sample is weighed and placed in a glass graduated cylinder. A vibration device is used to gradually compact the powder in the cylinder until the powder volume reading no longer changes, and the volume at this point is recorded. Dividing the previously weighed powder mass by the recorded volume yields the tap density of the sample. The tap density testing method and procedures refer to GB/T5162-2006.
5. Structural Analysis of Lithium-ion Battery Cathode Materials The structure of lithium-ion battery cathode materials determines the different lithium-ion insertion/extraction pathways, significantly affecting the electrochemical performance of lithium-ion batteries. Methods for structural analysis of lithium-ion battery cathode materials include X-ray diffraction (XRD), infrared spectroscopy, and Raman spectroscopy. XRD is the most commonly used structural analysis method. XRD can be used for qualitative and quantitative phase analysis of materials, as well as to determine the crystallinity of the materials. Zhang Lifeng et al. analyzed the structure of lithium iron phosphate prepared by different methods using XRD, and the analysis spectra are shown in Figure 3. The figure shows that the XRD patterns of lithium iron phosphate samples prepared by different methods have certain differences, indicating that XRD analysis can effectively reflect the structural changes of the samples.
Figure 3. XRD pattern of lithium iron phosphate
Infrared spectroscopy belongs to molecular vibrational spectroscopy, and its band spectra are formed by transitions between molecular vibrational and rotational energy levels. Infrared spectroscopy is a method for qualitative and quantitative analysis of the molecular structure of substances using infrared spectra. In the structural analysis of lithium-ion battery cathode materials, a combination of XRD and infrared spectroscopy is widely used. By examining changes in the infrared absorption peaks of the cathode material, changes in the crystal structure of the cathode material can be analyzed.
Like infrared spectroscopy, Raman spectroscopy originates from vibrational and rotational energy level transitions in molecules. It provides direct information about the molecular structure of a substance. Raman spectroscopy boasts high resolution and good reproducibility. In the research of lithium-ion battery cathode materials, Raman spectroscopy can be used to study the structural changes during the charging and discharging process. In non-in-situ Raman spectroscopy, the lithium-ion battery cathode material is removed from its charging or discharging state, and its Raman spectrum is measured to obtain its current structure. This structure is then compared with its state before charging or discharging to reveal the structural changes before and after charging and discharging. In-situ Raman spectroscopy, on the other hand, allows for real-time acquisition of information on the structural changes of the lithium-ion battery cathode material during charging and discharging.
6. Composition Analysis of Lithium-ion Battery Cathode Materials The composition analysis of lithium-ion battery cathode materials mainly consists of bulk element composition analysis and dopant element composition analysis. Dopant element composition analysis is relatively simple because the dopant content is low. Depending on the content and type of dopant element, methods such as spectrophotometry, inductively coupled plasma optical emission spectrometry (ICP-OES), inductively coupled plasma mass spectrometry (ICP-MS), and atomic absorption spectrometry (AAS) can be used.
Because of the high content of elements, conventional analytical methods for low-content impurity elements are prone to significant errors in the analysis of main elemental composition. Therefore, titration is usually employed for main elemental analysis. For example, Chen et al. studied a method for determining the nickel content in cathode materials using dimethylglyoxime precipitation-EDTA titration in the presence of large amounts of interfering elements such as manganese and cobalt. The results showed that the standard deviation of this method was less than 0.054, the coefficient of variation was less than 0.20%, and the recovery rate was between 99.63% and 100.5%.
GB/T30835-2014 specifies that the analysis of iron content in lithium iron phosphate is performed using the potassium dichromate standard solution titration method. The analysis of phosphorus content is performed using the ammonium phosphomolybdate volumetric method. If the content of individual elements among the main elements is relatively low, ICP-OES, AAS, etc., can also be used. For example, Liu Ying et al. established an analytical method for Li in LiCoO2, a lithium-ion battery electrode material, and determined that AAS and ICP-AES are used. This method is accurate, simple, and rapid.
II. Electrochemical Performance Analysis Methods for Lithium-ion Battery Cathode Materials
The electrochemical performance of lithium-ion battery cathode materials mainly includes initial discharge specific capacity, initial charge-discharge efficiency, discharge plateau capacity ratio, rate performance, and cycle life.
For lithium cobalt oxide cathode materials, GB/T23665-2009 specifies the test methods for the initial discharge specific capacity and initial charge-discharge efficiency. These two parameters are tested using a half-cell, and the battery manufacturing method and testing process are detailed in the standard. GB/T23366-2009 specifies the test methods for the discharge plateau capacity ratio and cycle life of lithium cobalt oxide. The test procedures are also detailed in the standard. For lithium nickel cobalt manganese oxide cathode materials, YS/T798-2012 specifies the test methods for the initial discharge specific capacity, initial charge-discharge efficiency, plateau capacity ratio, and cycle life of lithium nickel cobalt manganese oxide. The standard stipulates that the determination of these parameters should be performed in accordance with the test methods for lithium cobalt oxide. For lithium iron phosphate cathode materials, GB/T30835-2014 specifies the test methods for the initial coulombic efficiency, initial reversible specific capacity, and rate performance of lithium iron phosphate. The test procedures are also detailed in the standard.
In addition to the methods mentioned above for analyzing the electrochemical performance of lithium-ion battery cathode materials, cyclic voltammetry and electrochemical impedance spectroscopy can also be used to characterize the kinetic processes of cathode materials. Wu Jianbo et al. used cyclic voltammetry and electrochemical impedance spectroscopy to analyze the kinetic processes of lithium manganese oxide powder and thin-film electrodes, identifying the main kinetic factors affecting their electrochemical performance. Figure 4 shows their cyclic voltammetry spectra. The redox potential difference in the spectra can reflect the reversibility of the electrode material.
Figure 4 Cyclic voltammetry curves
Figure 5 shows the membrane resistance and membrane capacitance of lithium manganese oxide powder at different electrode potentials. The membrane resistance and membrane capacitance in the figure can reflect the magnitude of the resistance to the lithium-ion intercalation/deintercalation reaction.
Figure 5. Changes in membrane resistance and membrane capacitance of lithium manganese oxide powder with electrode potential.
III. Summary
There are many methods for analyzing the physical and chemical properties and application performance of materials. Selecting and determining the appropriate analytical methods for lithium-ion battery cathode materials helps lithium-ion battery cathode material researchers accurately analyze the performance of their materials and facilitates data comparison among different lithium-ion battery cathode material researchers. This is of great significance for promoting the development of lithium-ion battery cathode materials.
This article provides analytical methods for each physicochemical property of lithium-ion battery cathode materials. Some of these methods are specified in lithium-ion battery cathode material standards, while others are the most widely used methods in lithium-ion battery cathode material analysis. These methods can serve as a reference for researchers working on lithium-ion battery cathode materials when selecting analytical methods for their physicochemical properties.
