Uneven charging of electrode particles found to be key driver of lithium battery degradation

Scientists have identified a new cause behind the degradation of lithium-ion batteries used in electric vehicles and large-scale energy storage systems.

The National Research Foundation of Korea said Friday that a research team led by Kim Min-gyu, a professor at Inha University, has found that a charge-state imbalance among electrode particles triggers spontaneous redistribution of lithium ions between particles. The internal current generated during this process concentrates on specific particles, accelerating battery degradation.

High-capacity batteries used in electric vehicles and ESS suffer sharp performance declines as they are repeatedly charged and discharged — a degradation problem that has long been a major obstacle to market expansion and long-term use.

Previous research had focused primarily on the structural stability of individual materials and single particles, leaving the degradation mechanisms arising from interactions among multiple particles at the electrode level poorly understood.

Overcoming the limitations of high-performance battery design requires clearly identifying the microscopic interactions and degradation mechanisms that occur at the electrode level, researchers say.

The team fabricated composite electrodes by mixing two types of cathode particles of different sizes in various ratios, then combined a self-developed reaction-decomposition analysis platform with synchrotron-based micro X-ray diffraction mapping and transmission X-ray microscopy to track the distribution of reactions across the entire electrode and within individual particles.

The analysis confirmed that even after external charging stopped, lithium ions spontaneously redistributed between the two particle groups to equalize their states of charge.

While this internal lithium migration serves to synchronize the electrode's state of charge, the team found that in compositions where one particle group is present in low proportions, the internal current generated during redistribution concentrates on the minority particles. This creates additional localized electrochemical stress that does not appear in external current measurements.

Particles subjected to concentrated internal current developed thick organic surface films through side reactions with the electrolyte. Their normal layered crystal structure — through which lithium ions travel — also transformed into an electrochemically inactive rock-salt structure, ultimately accelerating degradation across the entire electrode.

"This research can be applied to optimizing cathode particle size, mixing ratios, and electron and ion transport pathways within the electrode," Kim said. "Designing all particles to react at similar rates can reduce the concentration of internal current and degradation in specific particles."

He added that "going forward, designing all particles within an electrode to react in a uniform and synchronized manner will become the core standard for developing high-capacity, long-life batteries."

The research, supported by the Ministry of Science and ICT and the National Research Foundation of Korea through the Nano and Materials Technology Development project, was published in the international journal Advanced Energy Materials on July 30.