UC San Diego

Lithium-ion batteries are prone to severe, mechanically-induced short circuit events that can lead to thermal runaway. Risk mitigating components commonly include primary protection structures and thermally-triggered failsafe mechanisms active at high temperatures, but features taking effect in the early stages of the joule heating regime are uncommon. To bridge this gap, the nature of the rate-limiting resistance dynamics is examined experimentally to clarify the progression of discharge events, and how to effectively address them upon short circuit initiation. Direct current internal resistance, external shorting, and nail penetration experiments are performed on LIR2450 format 120 mAh LiCoO2 / graphite coin cells to probe resistance and consequent heat generation dynamics over resolute temperature and time scales. The study reveals a low-resistance, electrically-controlled capacitive discharge event occurs immediately upon shorting which is subsequently throttled by increasing ionic resistances. An electrolyte resistance model is postulated and validated experimentally in terms of concentration, temperature, permittivity, and viscosity, to show how charge carriers polarize and reallocate within a cell when operated under stress.

The information attained identifies the need to exacerbate electrical resistance via mechanical response to suppress the powerful capacitive discharge feature immediately upon impact and suppress ion transport through electrolyte to halt continued discharge thereafter. Forming thick interpenetrating phase composite electrodes within brittle porous metal current collectors are shown to curb discharge power by preventing the formation of electrically conductive pathways between current collectors, as well as increasing the distance charge carriers must travel to liberate joule heat. Poisons capable of hindering ion transport to halt continued discharge are identified consulting the postulated electrolyte resistance model and tested via simultaneous injection and nail penetration testing of LIR2450 coin cells. Multifunctional design strategies are discussed for imparting greater degrees of integration within electronics by the lithium-ion batteries to reduce overall weight and volume upon further development of safe-cell technologies.