Researchers have resolved a long-standing challenge in high-Ni cathode materials, a key component of next-generation electric vehicle (EV) batteries. The team successfully reidentified the location of residual Li compounds, which have long been considered a chronic issue in high-Ni cathodes, and proposed a new material design strategy to significantly minimize residual lithium content.

The paper is published in the Journal of Materials Chemistry A. The research team includes Wooyoung Jin and Hyungyeon Cha from the Ulsan Advanced Energy Technology R&D Center at the Korea Institute of Energy Research.

High-Ni cathode materials are a core component of next-generation lithium-ion batteries used in electric vehicles (EVs) and other applications. As the Ni content in the cathode increases, so does the battery’s energy density, leading to longer driving ranges for EVs. With a Ni composition of up to 80%, high-Ni cathodes are emerging as a key technology in the future EV battery market.

However, as the Ni content increases, excessive formation of residual Li compounds tends to occur on the surface of the cathode material. This leads to a phenomenon known as gelation, where the electrode slurry hardens into a gel-like state.

As a result, the active material particles become unevenly distributed, and the adhesion between electrode components decreases by approximately 20%, ultimately compromising electrode integrity and performance. Notably, this issue has been observed even in commercially available cathode materials, highlighting the urgent need for solutions to ensure stable manufacturing and reliable battery performance

Previously, it was widely believed that residual Li existed on the surface of cathode particles. Accordingly, surface-washing processes using distilled water or external coating techniques were employed to remove it. However, these approaches failed to fully resolve the performance degradation issues in lithium-ion batteries.

In a breakthrough discovery, the research team was the first to confirm that residual Li is not only present on the surface, but also exists between the internal particles of high-Ni cathode materials challenging conventional assumptions. This finding revealed that the overlooked internal structure of the cathode plays a critical role in battery performance degradation and reduced lifespan. Based on this insight, the team proposed a new material design strategy aimed at fundamentally suppressing the formation of residual lithium.

Utilizing advanced analytical techniques including high-resolution electron microscopy, nitrogen adsorption analysis, and electron energy loss spectroscopy, the research team conducted a detailed investigation of the cathode material. They identified that residual Li compounds exist in crystalline form within the intergranular pores between particles, and confirmed that this is one of the primary causes of battery performance degradation.

Based on these findings, the researchers proposed the use of single-crystal structured high-Ni cathode materials to suppress the formation of residual Li within the cathode. Since single-crystal structures have minimal or no grain boundaries between primary particles, they prevent the formation of interparticle gaps, effectively eliminating the space where residual Li compounds could crystallize.

The research team reported that using single-crystal high-Ni cathode materials can reduce residual Li levels by up to 54% compared to conventional cathodes. This significant reduction brings the industry and academic community closer to achieving the target of maintaining residual Li compounds below 2,000 ppm.

Dr. Jin and Dr. Cha, who led the research team, stated, “This study marks the first in-depth analysis to move beyond surface-level approaches and examine residual Li issues within the internal structure of cathode particles. It represents a critical turning point in understanding the structural stability and performance degradation mechanisms of high-Ni cathodes.

“We believe these insights, when applied to cathode material design and processing, will play a significant role in advancing the development and commercialization of high-energy-density lithium-ion batteries.”