image: Polymer-embedded molecular fuse mechanism for thermally responsive safety in Li batteries.
Credit: Ying Zhang, Yu-Guo Guo, Chunli Bai / Institute of Chemistry, Chinese Academy of Sciences
An electrical fuse protects a circuit by cutting off the current when something goes wrong. But could a similar safeguard work inside a lithium battery, intervening in dangerous chemical reactions before they spiral out of control?
Researchers at the Institute of Chemistry, Chinese Academy of Sciences, have developed a polymer "molecular fuse" designed to do just that. Integrated directly into high-nickel battery cathodes, it aims to suppress the heat-producing reactions that drive thermal runaway. The findings were published online in National Science Review.
Such protection is particularly important for high-nickel lithium batteries, which offer high energy density for applications such as electric vehicles but face significant thermal safety challenges. When these batteries overheat, oxygen released from the cathode can react with the electrolyte, generating heat and flammable gases. Like falling dominoes, these reactions can reinforce one another, eventually driving the battery into thermal runaway.
The molecular fuse works through temperature-sensitive phosphorus-carbon (P–C) bonds within the polymer. Under normal conditions, the polymer remains stable and supports lithium-ion transport. During abnormal heating, these bonds begin to break, generating phosphorus-centered radicals that can interfere with the reactive species responsible for heat-producing chain reactions.
Importantly, the temperature at which the polymer responds can be adjusted through molecular design. By modifying its phosphonate side chains, the researchers raised the onset of thermal decomposition from approximately 102°C to 112°C. This adjustment allows the polymer to better withstand typical cathode drying temperatures of around 100°C without premature activation, while retaining its ability to respond during the early stages of thermal runaway.
Laboratory tests demonstrated the protective effect. Fully charged high-nickel cathodes containing the molecular fuse released 76% less heat than unmodified cathodes, while gas generation was also reduced. Structural measurements further revealed less lattice expansion during early heating and delayed structural degradation, indicating improved thermal stability of the cathode.
The researchers also tested the approach in 0.6 Ah lithium-metal pouch cells. Under external heating, the modified cells exhibited a lower peak surface temperature near the cathode, while the temperature at which abrupt voltage collapse occurred increased from 144.7°C to 196.9°C. These results demonstrated that the molecular fuse could delay severe electrical failure and moderate thermal runaway at the cell level. Electrochemical tests also showed favorable lithium-ion transport and improved cycling performance.
Beyond developing a new protective material, the study introduces a broader design strategy: using tunable chemical bonds to actively intervene in hazardous reactions during the early stages of thermal runaway. Rather than relying solely on passive thermal barriers or flame suppression, this approach offers a new direction for improving the safety of high-energy lithium batteries.