NOVELTY - The power storage device has power storage elements (100) which include a positive electrode (101), a primary separator over the positive electrode, a negative electrode (103) over the primary separator and a secondary separator over the negative electrode. The power storage elements are stacked. The positive electrode, negative electrode, primary separator and secondary separator are provided with sheet-like shapes and flexibility. One end portion of positive and negative electrodes is fixed to a positive electrode tab (104) and a negative electrode tab (105) respectively.    USE - Power storage device such as lithium secondary battery used in consumer electrical device e.g. mobile phone. Uses include but are not limited to desktop personal computer, game machine, DVD, wall clock, electric rice cooker, hair dryer, escalator, motorcycle, rocket, smart phone etc.    ADVANTAGE - The force applied to the positive electrode tab can be prevented from being directly applied to the power storage element. The charge and discharge capacity of power storage device can be improved. The safety against liquid leakage is improved. The use of the negative electrode allows improvement in durability of the power storage element. The silicon whiskers and the negative electrode mix layer can be prevented from being pulverized and collapsed. The cycle characteristics of power storage device can be improved. The conductivity of the negative electrode can be improved, so as to enable charge and discharge to be performed at much higher speed. The usage rate of electric power in daytime can be reduced. The tripping of breaker of commercial power supply in use of electrical device can be prevented by using the power storage device. The transfer of electrons in the positive electrode active materials is facilitated, so as to increase in conductivity of the positive electrode mix layer.    DESCRIPTION OF DRAWING(S) - The drawing shows a sectional view of the power storage element.    Power storage element (100)    Positive electrode (101)    Separator (102)    Negative electrode (103)    Positive electrode tab (104)    Negative electrode tab (105)