NOVELTY - A polyimide binder is polyimide obtained by subjecting an aqueous solution of a polyamic acid comprising repeating unit (I) to imidization reaction. The tensile elastic modulus of polyimide is 1.5-2.7 GPa.    USE - Polyimide binder is used for preparing negative electrode sheet and positive electrode sheet for preparing electrical storage device, and improving electrical property of electrical storage device (all claimed). Uses include but are not limited to electrical double layer capacitor, lithium-ion capacitor, lithium primary battery, lithium secondary battery, mobile telephone, notebook computer, electric vehicle and electric power storage.    ADVANTAGE - The polyimide binder having excellent stability is manufactured by efficient and easy process, and provides lithium secondary battery having excellent cycle characteristics and low-temperature discharge characteristic, high energy density and physical property and suppressed gas-evolution.    DETAILED DESCRIPTION - A polyimide binder is polyimide obtained by subjecting an aqueous solution of a polyamic acid comprising repeating unit of formula (I) to imidization reaction. The tensile elastic modulus of polyimide is 1.5-2.7 GPa.    A=tetravalent group obtained by removing carboxyl group from aromatic tetracarboxylic acid which does not contain fluorine group, tetravalent group obtained by removing carboxyl group from aliphatic tetracarboxylic acid, or tetravalent group obtained by removing carboxyl group from aromatic tetracarboxylic acid containing fluorine group;and    B'=divalent group obtained by removing amine group from aromatic diamine having solubility of 0.1 g/L or more at 2 degrees C in water and does not contain fluorine group, divalent group obtained by removing amino group from aliphatic diamine having molecular weight of 500 or less, or divalent group obtained by removing amino group from aromatic diamine containing fluorine group.    55 Mol% or more B' within the total content of repeating unit is divalent group in which amine group is removed from aliphatic diamine having molecular weight of 500 or less. INDEPENDENT CLAIMS are included for the following:    (1) negative electrode sheet, which is obtained by mixing polyimide binder, silicon-containing negative electrode active material and carbon material, coating the resultant product on the surface of copper electrical power collector, and forming negative electrode active material layer;    (2) positive electrode sheet, which is obtained by coating positive mix containing polyimide binder, carbon material and lithium transition metal oxide on the surface of aluminum electrical power collector, and forming positive electrode active material layer;    (3) electrical storage device, which is provided with the positive electrode, negative electrode and non-aqueous electrolyte which is electrolytic salt dissolved in nonaqueous solvent. The negative electrode is negative electrode sheet;    (4) usage of polyimide binder in electrical storage device; and    (5) method of improving electrical property of electrical storage device using polyimide binder.