Electrochemical device for storing electrical energy in rectangular geometric cells, narrow stack geometry, comprising: a straight sturdy rectangular parallelepiped-shaped housing made of a material resistant to oxidizing conditions with good mechanical strength, and which is hermetically sealed, an electrode stack with rectangular narrow stack geometry housed inside the housing, comprising a stack of positive electrodes inserted on one metal rod following the sequence: a positive electrode a positive electrode separating element a stack of negative electrodes inserted on another metal rod following the sequence a negative electrode a negative electrode-separating element characterized in that said housing comprises a base provided with a metal substrate fitted with through holes, a cover, a front side (4.1), a rear side (4.2), wherein each of said front and rear sides (4.1, 4.2) is provided with an inner chamber, the metal rods are hollow rods that serve as current collectors and as means for cooling, which run over the metal substrate of the base and through its through holes, and wherein the electrochemical device further comprises: two metal plates fitted with central holes, wherein each metal plate is incorporated in each inner chamber of each side (4.1, 4.2), two metal-ion sheets, two grids, each one screwed to each of the metal plates holding one of the metal-ion sheets between them, and metal external connections that connect with the metal-ion sheets through the central holes of the metal plates. a straight sturdy rectangular parallelepiped-shaped housing made of a material resistant to oxidizing conditions with good mechanical strength, and which is hermetically sealed, an electrode stack with rectangular narrow stack geometry housed inside the housing, comprising a stack of positive electrodes inserted on one metal rod following the sequence: a positive electrode a positive electrode separating element a stack of negative electrodes inserted on another metal rod following the sequence a negative electrode a negative electrode-separating element a stack of positive electrodes inserted on one metal rod following the sequence: a positive electrode a positive electrode separating element a positive electrode a positive electrode separating element a stack of negative electrodes inserted on another metal rod following the sequence a negative electrode a negative electrode-separating element a negative electrode a negative electrode-separating element said housing comprises a base provided with a metal substrate fitted with through holes, a cover, a front side (4.1), a rear side (4.2), wherein each of said front and rear sides (4.1, 4.2) is provided with an inner chamber, the metal rods are hollow rods that serve as current collectors and as means for cooling, which run over the metal substrate of the base and through its through holes, two metal plates fitted with central holes, wherein each metal plate is incorporated in each inner chamber of each side (4.1, 4.2), two metal-ion sheets, two grids, each one screwed to each of the metal plates holding one of the metal-ion sheets between them, and metal external connections that connect with the metal-ion sheets through the central holes of the metal plates.