In this paper, natural pyrite (FeS2) was used as the cathode material of a reserve small-sized cylindrical battery Li - FeS2. Li-FeS2 cylindrical reserve battery size: h = 55 mm; d = 6.3 mm.The theoretical specific discharge capacity of FeS2 is 890 mA·h/g. and the theoretical bulk energy is 6675 W·h/cm3 For comparison for sulfur electrode the theoretical bulk energy is 6400 W·h/cm3 As the anode, for this Li - FeS2 battery a lithium foil was used, the surface of which was modified with a LiAl alloy. An innovative method of electrochemical activation for increasing the specific energy of battery was developed. After battery activation the discharge voltage of the reserve battery rises, and as a result, the battery energy rises by about 30 %. The surface modification of the lithium anode with the LiAl alloy allows to decreased in 3 times complex impedance of the system at a temperature of + 25 °C, in 5-8 times the complex impedance of the system during long-term storage at temperature of + 60 °C and avoid surface deformation of the lithium electrode during long-term storage. After battery activation the battery has very low self - discharge. The developed reserve battery can be operated at the temperature range from plus 60 oC to minus 25 oC.