PARTITION MEMBER AND BATTERY ASSEMBLY

A partition member having a thickness direction and a plane direction perpendicular to the thickness direction, partitioning single cells, or a single cell and a member other than a single cell, in the thickness direction, including a liquid and a thermal insulating material capable of retaining the liquid, and an external package housing the thermal insulating material and the liquid, having an area (S1) of an internal space of the external package and an area (S2) of the thermal insulating material in plane view of the external package and the thermal insulating material in the thickness direction, a thickness (D1) of the thermal insulating material, and a volume (V1) of the liquid that satisfy the relationship of the following expression 1 and/or the following expression 2, and a battery assembly using the same. 0.25≤1/(1×1)≤0.70 expression 1: 0.35≤2/1 expression 2: A partition member that has a high opening temperature, and can properly switch the thermal resistance before and after opening, and a battery assembly can be provided.