A customer story from Hanyang University in Korea

Our lab. focuses on the research and development of higher capacity, longer cycling, and safer battery materials with a specialization in cathode materials for advanced Li-ion batteries and next-generation battery systems. Based on the fundamental understanding of the physical and electrochemical properties of the materials, we develop and evaluate innovative cathode materials to improve the energy density, cycle life, and safety of lithium-ion batteries. Together with domestic and foreign chemical companies, secondary battery companies, and automobile companies, we conduct R&D of core technology with an understanding and experience of commercially available technologies. We also explore futuristic battery systems to turn them into reality. We seek to capitalize on these innovative battery materials and build a greener world for the future.

As the Ni content in the Li[Ni_xCo_yMn_1-x-y]O2 (NCM) cathode increases above 60%, the extent of microcracks rapidly increases owing to the build-up of anisotropic strain induced by abrupt lattice volume changes during the H2–H3 phase transition. The resultant microcracks in Ni-rich cathode particles can create channels through which the electrolyte infiltrates into the particle interior, thereby increasing the surface area exposed to the electrolyte attack. This increased surface area further accelerates the rate of capacity fading for Ni-rich cathodes. To suppress the degradation in Ni-rich cathode materials, our group focuses on the modification in microstructure which can dissipate the internal strain induced by lattice volume change.

The microstructure of cathode materials is largely determined by their hydroxide precursor and calcination process. When the mixture of hydroxide precursor and lithium hydroxide is calcined at a high temperature (700~800 ^oC), the layered crystal structure which can (de)intercalate Li+ ions is formed. However, coarsening of primary particles during calcination can destroy the microstructure, which undermines the mechanical stability of the cathode against microcrack formation. On the other hand, limiting the calcination temperature or soaking time to tune the primary particle morphology prevents the full crystallization of the cathodes; subsequent cation mixing can deteriorate the cycling behavior. Therefore, achieving full crystallization without coarsening of the cathode material is one of the most important challenges to be solved.

In general, during calcination, an optimal temperature exists at which the hydroxide precursor is fully crystallized. A high temperature can anneal structural defects such as anti-site defects, whereas an excessively high temperature induces Li deficiencies and cation mixing. The degree of cation mixing, which is a consequence of Li/Ni site exchange because of the similar radii of Li⁺ (0.076 nm) and Ni²⁺ (0.069 nm), can be used to judge the crystallinity of a layered structure. Combining the structural information with the microstructure of cathode particles from scanning electron microscopy (SEM), and the respective electrochemical performances, the optimal calcination temperature of the cathode material is determined.

Before X-ray diffractometers XRD was used, XRD analysis using a particle accelerator in Pohang was performed. Furthermore, transition electron microscopy (TEM) analysis which can determine atomic-scale crystal structure was performed.

As XRD is a basic analysis that provides a lot of crystal structural information, compact and powerful XRD analysis equipment which can be installed in the lab was needed. Malvern Panalytical XRD system can analyze not only the powder samples but also the pouch-type cells without disassembling them. The in-situ XRD analysis of cells can provide detailed structural changes upon charging/discharging the cells. As the capacity fading mechanism of Ni-rich cathode material is largely determined by the H2-H3 phase transition where abrupt structural change occurs, analyzing the structural changes of the cathodes without disassembling the cells is important in developing high-energy Ni-rich cathode materials.

Our lab owns ‘Empyrean’ XRD analysis equipment which can analyze in reflection and transmission mode. Reflection mode is used for analyzing cathode powder samples to determine their lattice parameters and layeredness. Transmission mode is used for analyzing the pouch-type cells which are composed of many components (electrodes, separator, Al pouches…). Upon charging and discharging the cells, the shift of peaks corresponding to the structural changes in cathode material is analyzed. For example, the changes in lattice parameter of cathode materials depending on their chemical composition can be analyzed by rietveld refinement. Furthermore, by deconvoluting (003) reflection in H2 and H3 peaks during H2-H3 phase transition, the structural reversibility of cathode materials can be compared. The structural information of cathode materials obtained by XRD analysis meets our expectations well.

Accurate analysis is possible with an intuitive interface. In addition, various accessories can be utilized according to the purpose.

Our lab plans to perform TR(Time-resolved)-XRD analysis using a high-temperature reactor chamber. By performing TR-XRD analysis, real-time analysis of phase changes and phase evolution during heat treatment similar to the actual calcination process is possible.

It will be a great help for analyzing the crystal structure not only in the cathode material analysis where the crystal structure is largely related to the electrochemical performance but also in the next-generation battery materials (All-solid-state batteries, Li-sulfur batteries, etc.).