Institute of Chemistry CAS: polymer molecular fuse cuts heat release by 76% in high-nickel cathodes

Researchers at the Institute of Chemistry, Chinese Academy of Sciences, have developed a polymer "molecular fuse" built into high-nickel lithium battery cathodes. According to the paper in National Science Review, the additive reduced heat release by 76% in LiNi0.8Co0.1Mn0.1O2 cathodes and delayed thermal runaway in 0.6 Ah lithium-metal pouch cells doi.orgNational Science Reviewdoi.orgOpen source ↗. The work was done in the laboratory on cathodes and small pouch cells.

The problem addressed is thermal runaway in high-energy lithium batteries. It is driven by autocatalytic radical reactions and the accumulation of flammable gases. In high-nickel cells, oxygen released from the cathode on overheating can react with the electrolyte, producing heat and gas, and these reactions reinforce one another doi.orgNational Science Reviewdoi.orgOpen source ↗.

How the molecular fuse works

The polymer contains sacrificial phosphorus–carbon (P–C) bonds. Under normal conditions the polymer is stable and supports lithium-ion transport. During abnormal heating the bonds break and form phosphorus-centered radicals, which suppress the exothermic chain reactions doi.orgNational Science Reviewdoi.orgOpen source ↗.

The activation temperature is set by molecular design. By tuning the inductive and hyperconjugative effects of the phosphonate side chains, the authors changed the dissociation energy of the P–C bonds. This shifted the onset of thermal decomposition by about 10°C, from roughly 102°C to 112°C, as supported by density functional theory and in situ variable-temperature Raman spectroscopy doi.orgNational Science Reviewdoi.orgOpen source ↗. The higher threshold lies above typical cathode drying temperatures of about 100°C, so radicals are not generated prematurely during fabrication.

Test results

Fully charged high-nickel cathodes with the fuse released 76% less heat than unmodified ones, and gas generation was also reduced. Structural measurements showed less lattice expansion during early heating and delayed structural degradation doi.orgNational Science Reviewdoi.orgOpen source ↗.

In 0.6 Ah lithium-metal pouch cells under external heating, the modified cells showed a lower peak surface temperature near the cathode. The temperature of abrupt voltage collapse rose from 144.7°C to 196.9°C doi.orgNational Science Reviewdoi.orgOpen source ↗. Electrochemical tests also showed favorable lithium-ion transport and improved cycling performance, according to the authors.

The authors describe the approach as a design strategy: using tunable chemical bonds to intervene in hazardous reactions at the early stages of thermal runaway, rather than relying only on passive thermal barriers or flame suppression doi.orgNational Science Reviewdoi.orgOpen source ↗.