Battery

A lithium ion secondary battery, comprising an electrode assembly resulting from stacking a positive electrode, a separator and a negative electrode,

• wherein the battery comprises, as the separator, a first separator and a second separator having mutually different characteristics, the first separator and the second separator being disposed inside the electrode assembly in a state where the first separator and the second separator are not in contact with each other in a stacking direction of the electrode assembly,
• a given surface pressure is applied in the stacking direction of the electrode assembly, and
• the first separator and the second separator have the following characteristic:
• in a case where the battery is constructed comprising an electrode assembly that results from stacking the positive electrode, the first separator and the negative electrode, a resistance increase rate X=Delta X/Delta P satisfies X>0 (positive value), X being worked out from a change amount Delta P (MPa) of surface pressure applied in the stacking direction of the electrode assembly and a IV resistance increment Delta X (%) of the battery upon application of the surface pressure change amount Delta P, and
• in a case where the battery is constructed comprising an electrode assembly that results from stacking the positive electrode, the second separator and the negative electrode, a resistance increase rate Y=Delta Y/Delta P satisfies Y<0 (negative value), Y being worked out from a change amount Delta P (MPa) of surface pressure applied in the stacking direction of the electrode assembly and a resistance increment Delta Y (%) of the battery upon application of the surface pressure change amount Delta P,
• wherein the IV resistance is worked out on the basis of the difference between the voltage value (initial voltage value) before charge and discharge at a current value of 30 C in an environment at 25 degrees Celsius, and a voltage value at a point in time after 10 seconds had elapsed since the initial voltage value, and the resistance increase rate X is calculated from the slopes in the graph that denotes the correlation between surface pressure and IV resistance.