NCM811 materials have been considered as the main cathode materials for high energy density Li-ion batteries. However, cracks of poly-crystalline NCM materials caused by stress accelerate the loss of active materials, limiting the life cycle. Hence, monitoring and understanding the chemo-mechanical evolution of NCM materials is of importance.

To address this challenge, the research team led by Prof. Yunhui Huang and Prof. Zhen Li introduced an optical fiber to in operando detect the stress evolution of the NCM811 cathode. Through designing the integration of optical fiber and battery, the stress evolution at material level has been successfully monitored. Meanwhile, due to the small size and chemical stability, the implantation of fiber also has little impact on the performance of battery and transmission of sensing signals.

They found that the stress evolution of poly-crystalline NCM811 mainly consists of chemical stress and structural stress. Chemical stress is caused by (de)lithiation of materials, which is normal and unavoidable. Of importance is the fact that structural stress is found to induce cracks, which is harmful to performance. Hence, the elimination of cracks, which means the remission of structural damage, is the key to improving performance.

Combined with crystallographic results, structural stress is proved to be induced by the nonmonotonic variation of the c-axis of crystal during (de)lithiation and the anisotropy of primary particles in poly-crystalline. Hence, improving the anisotropy of primary particles to construct an ordered arrangement structure is proposed to obtain the chemo-mechanical stable poly-crystalline materials.

It has been proved that the structural stress of poly-crystalline with ordered arrangement structure is relieved. Meanwhile, this material delivers a high capacity retention of 82% after 500 cycles at 0.5C.

"With many advantages, optical sensing has received widespread attention in the field of battery monitoring. We implanted the fiber into the electrode to obtain the stress information at material level, decoding the chemo-mechanical evolution of materials and helping guide the construction of chemo-mechanical stable materials. In the future, optical sensing will play an important role in the battery, helping to build better, safer and smarter batteries," Prof. Huang said.

The research paper, titled "Operando chemo-mechanical evolution in LiNi_0.8Co_0.1Mn_0.1O₂ cathode," was recently published in National Science Review.

More information: Yi Zhang et al, Operando chemo-mechanical evolution in LiNi_0.8Co_0.1Mn_0.1O₂ cathodes, National Science Review (2024). DOI: 10.1093/nsr/nwae254