Deciphering the Thermal Failure Mechanism of Anode-Free Lithium Metal Pouch Batteries

Anode-free lithium metal batteries (AFLMBs) are the subject of increasing attention due to their ultrahigh energy density, simplified structure, reduced cost, and relatively high safety, but their thermal runaway performance under abuse conditions has been rarely explored, and a clear understanding of whether the absence of a highly-reactive lithium metal anode is equal to thermal runaway free remains elusive. Herein, by systematically examining the thermal runaway characteristics of a 2.0 Ah AFLMB, it is revealed that under elevated temperatures, discharged anode-free pouch cell is safe while the fully-charged one indeed undergoes thermal runaway, but with a milder intensity than that of a lithium metal battery with the same capacity. Moreover, mechanistic investigations demonstrate that thermal runaway of an AFLMB employing a conventional electrolyte is triggered and dominated by anode-induced exothermic interactions and the broken separator induced electrodes interaction. Moreover, it is shown for the first time that adding fluoroethylene carbonate in an electrolyte leads to ring-opening repolymerization at 170 degrees C to form a thermal-stable solid layer between anode and cathode, which inhibits the direct contact of electrodes and effectively postpones violent self-heating. This comprehensive exploration of thermal runaway characteristics and mechanisms of large format AFLMBs sheds fresh light on developing high energy density and safety-enhanced lithium metal batteries.

Automated summary

Anode-free lithium metal batteries (AFLMBs) are gaining attention for their high energy density, simple structure, low cost, and relatively high safety. However, the thermal runaway performance of AFLMBs has not been well-studied, and the absence of a reactive lithium metal anode does not guarantee thermal runaway-free conditions. This study examines the thermal runaway characteristics of a 2.0 Ah AFLMB and finds that the discharged battery is safe under elevated temperatures, while the fully-charged battery does undergo thermal runaway, but with less intensity compared to a lithium metal battery. Mechanistic investigations reveal that an AFLMB with a conventional electrolyte experiences thermal runaway due to anode-induced exothermic interactions and broken separator induced electrodes interaction. Additionally, adding fluoroethylene carbonate in the electrolyte forms a thermal-stable solid layer between the anode and cathode, preventing direct contact of the electrodes and delaying violent self-heating. This comprehensive exploration of AFLMBs' thermal runaway characteristics and mechanisms contributes to the development of high energy density and safety-enhanced lithium metal batteries.