NOVELTY - Lithium ion secondary battery (100) has positive electrode collector (221), positive mix layer (223) containing positive electrode active material and electroconductive material coated on positive electrode collector, negative electrode collector (241), negative mix layer (243) containing negative electrode collector and negative electrode active material coated on negative electrode collector and electrolyte solution (280) containing lithium ion in predetermined concentration. The positive mix layer has void through which the electrolyte solution infiltrates.    USE - Lithium ion secondary battery e.g. assembled battery is used for power source system for vehicle drive (all claimed).    ADVANTAGE - The lithium ion secondary battery has excellent charging/discharging characteristics.    DETAILED DESCRIPTION - Lithium ion secondary battery has positive electrode collector, positive mix layer containing positive electrode active material and electroconductive material coated on positive electrode collector, negative electrode collector, negative mix layer containing negative electrode collector and negative electrode active material coated on negative electrode collector and electrolyte solution containing lithium ion in predetermined concentration. The positive mix layer has void through which the electrolyte solution infiltrates. The negative mix layer has void through which the electrolyte solution infiltrates. The number of lithium ions (X) in the liquid electrolyte permeating per 1 cm3 of the positive electrode mixture layer, and the number of lithium ions (Y) in the liquid electrolyte permeating per 1 cm3 of negative electrode mixture layer, are both 3.75x 1019 or more. The number of lithium ion (X) is calculated by, X=(volume Vb of the void of the positive mix layer in per cm3 of the positive mix layer)x (lithium-ion-concentration of the electrolyte solution (mol/L))x Avogadro's number (NA). The number of lithium ion (Y) is calculated by, Y=(volume Vd of the void of the negative mix layer in per cm3 of the negative mix layer)x (lithium-ion-concentration of the electrolyte solution (mol/L))x Avogadro's number (NA).    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of lithium ion secondary battery during charging.    Lithium ion secondary battery (100)    Positive electrode sheet (220)    Positive electrode collector (221)    Positive mix layer (223)    Negative electrode sheet (240)    Negative electrode collector (241)    Negative mix layer (243)    Electrolyte solution (280)