A thermal-runaway retardant (TRR) of lithium-ion batteries (LIBs), dibenzylamine (DBA), is investigated. In a TRR-modified LIB, DBA can be encapsulated in packages made of inert materials. When the LIB is subjected to mechanical abuse, the packages would be broken apart and the TRR is released. In nail penetration and impact tests, addition of 4 wt% DBA reduces the temperature increase of fully charged LIR-2450 cells by nearly 50%. The influence of TRR packages on the cycling performance of LIBs is negligible. The working mechanism of DBA is associated with the decrease in electrolyte conductivity, the increase in charge transfer resistance, and the reduction in lithium ion (Li+) transference numbers.