A prismatic lithium secondary battery includes: a prismatic battery can
having a bottom, a side wall, and an open top; an electrode assembly; a
non-aqueous electrolyte; and a sealing plate covering the open top of the
battery can that accommodates the electrode assembly and the non-aqueous
electrolyte. The electrode assembly includes: a positive electrode; a
negative electrode; and a porous heat-resistant layer and a separator
that are interposed between the positive and negative electrodes. The
side wall of the battery can has two rectangular main flat portions that
are opposed to each other, and the thickness A of the porous
heat-resistant layer and the thickness B of each of the main flat
portions of the side wall satisfy the relation:
0.003.ltoreq.A/B.ltoreq.0.05.