The growing demand for electric vehicles and renewable energy requires new methods and approaches to ensure fire safety of lithium-ion batteries (LIBs). At present, only passive protection methods are used to localize LIB fires at an early stage and active fire extinguishing is not implemented. Therefore, development of new methods allowing implementation of active fire extinguishing at the early stage of LIB fire, at the level of several lithium-ion cells or a module, without using additional engineering equipment is an urgent task. The study assesses the efficiency of using functional material containing microencapsulated gaseous fire extinguishing agent perfluoro(2-methyl-3-pentanone) to extinguish fires in lithium-ion cell assemblies. Fire extinguishing sheets containing 50% wt of perfluoro(2-methyl-3-pentanone) were used to protect LIB fire. Cell assemblies of NMC 30 Ah type numbering 2 and 6 pcs were selected as the test object. The cell thermal runaway and fire were initiated by external heating. Thermometric methods and visual inspection were used to evaluate the efficiency of fire extinguishing sheets. For the 2-cell assembly protected by fire extinguishing sheets, no fire or inflammation were recorded during the tests, even when using external ignition sources. For the 6-cell assembly protected by fire extinguishing sheets, no open combustion was observed either and thermal runaway was delayed from 15 to 180 s. Fire extinguishing sheets containing microencapsulated perfluoro(2-methyl-3-pentanone) effectively extinguish cell burning. They keep off ignition in confined spaces due to preventive release of fire extinguishing agent.