Comparative Analysis of Single- and Poly-Crystalline Layered Cathode Materials: Determining the Superior Choice
Authors: Seo Jae kwon , Koo Jinkyo , Kim Woo-Seong , Kim Young-Jun 
Published: August 2024

In the field of cathode material candidates of lithium-ion batteries, there has been ongoing debate regarding which material is superior: poly-crystalline (PC) or single-crystalline (SC) cathode active materials. The PC materials are characterized by primary particles that agglomerate into secondary particles, which PCs are prone to micro-crack due to significant c-axis volumetric expansion/contraction in their lattice structure.

As an alternative, the SC materials have been received attention for their physical resistance from the expansion/contraction and external pressure. However, in terms of lithium-diffusion pathway, it has been reported that SC’s structures possess certain disadvantages in lithium-ion diffusion compared to PC materials. This is attributed to the scarcity of grain boundaries and the extensive pathways of bigger primary particles in SC, which impede the efficient movement of lithium ions. Even though the comparative analysis has been needed between PC and SC above superiority, achieving an exact comparison under identical or similar conditions is challenging due to differences in particle size and surface environment, including binders and conducting materials.

In this study, we standardized the size of both PC and SC particles to 10 μm to evaluate their electrochemical performance under identical environmental conditions and using a common commercial electrode composition (96:2:2, active material, carbon black, and PVDF binder) in a wet process with a 60% nickel cathode (NCM622). Consequently, in comparing PC and SC materials, the focus has been on analyzing key factors that contribute to electrode performance, specifically concerning issues of lithium-ion diffusion and the composition of materials at the electrode level that may arise. Moreover, we fabricated SC electrodes using a solvent-free dry electrode process as an alternative of better process, aimed at creating higher energy density electrodes without the addition of carbon black, but rather by employing coated carbon nanotubes (CNT) on the SC surface with the composition (99.6:0:0.4, active material, carbon black, PTFE binder) to clarify the comparison between two main cathodes’ design.

Finally, we found that the dry-processed SC electrode demonstrated superior performance in full-cell tests and show excellent cycle performance at high temperature (45 °C) and high voltage (2.8 to 4.5V). The enhanced performance observed in dry-processed electrodes using CNT was found to be primarily due to the reduced ion resistance in environments devoid of internal carbon black, as confirmed through comparison with wet-processed electrodes. Although electron conductivity was ensured through the conductive material, a significant decrease in performance was noted if the environment was not conducive to the movement of internal lithium-ion. And it is attributable to enhanced surface stability under high-voltage storage conditions with a reduced specific surface area compared to PC cathode and PC wet process condition.

Figure 1