NOVELTY - The heat sink comprises a flat graphite-containing material provided for bearing against battery cells (5, 5', 5'') and containing solidified graphite expandate. The flat material has a density of 0.9-1.1 g/cm3 as 1.0 g/cm3, has a thermal conductivity of 180-190 W/mK, a rear suspension of 4-10% in a thickness direction related to its output thickness, and a compressibility of 10-20% in thickness direction related to its output thickness. The flat material consists of a mixture of uniformly mixed graphite expandate and plastic particles formed before the solidification.    USE - Heat sink useful in an electrical energy storage unit (claimed).    ADVANTAGE - The heat sink improves the life and reliability of the battery cells, is easy to use, allows uniform heat distribution across the battery cells, and has good thermal conductivity and good adaptability.    DETAILED DESCRIPTION - The heat sink comprises a flat graphite-containing material provided for bearing against battery cells (5, 5', 5'') and containing solidified graphite expandate. The flat material has a density of 0.9-1.1 g/cm3 as 1.0 g/cm3, has a thermal conductivity of 180-190 W/mK, a rear suspension of 4-10% in a thickness direction related to its output thickness, and a compressibility of 10-20% in thickness direction related to its output thickness. The flat material consists of a mixture of uniformly mixed graphite expandate and plastic particles formed before the solidification. The flat material is impregnated with plastic introduced after the solidification superficially or up to in the core region of the flat material. The flat material has a metal coating at a front side provided for connecting with a cooling module (4) and/or a partial area of a main surface of the flat material is provided with a metal coating. The flat material is formed in a trough-like-, wave-like-, meander-like- or honeycomb-like manner. An INDEPENDENT CLAIM is included for an electrical energy storage unit.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic three-dimensional view of an electrical energy storage unit.    Electrical energy storage unit (1)    Housing (2)    Housing bottom (3)    Cooling module (4)    Battery cells. (5, 5', 5'')