TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred Component: A coolant passage (300A, 310A, 320A, 330A, 340A) is provided from the inlet to the outlet inside the battery through which the introduced coolant flows and comes into direct contact with one of the unit cells. The battery further comprises a heat sink (300, 340) provided on top of the stacked unit-cell layers on a bottom of the stacked unit-cell layers, or between the stacked unit-cell layers; and a network of layered sub-networks of coolant flow paths. Each sub-network is defined between the frame and the unit cells of the respective unit-cell layers. The inlet is formed to be larger in cross-sectional area than the outlet. Size of the inlet and/or the outlet varies between the unit-cell layers. The coolant passage is formed so that a flow of the coolant through increases its velocity as the coolant flows downstream. Each frame comprises an outer frame portion and cross member portions extended between the outer frame portion. Each unit cell is held at its periphery by the outer frame portion and/or the cross member portions. The inlet comprises an opening formed on the outer frame portion o the frame on one side of the battery. The outlet comprises an opening formed on the outer frame portion of the frame on the other side of the battery. The coolant passage includes openings each formed on the cross member portions of the frame, and a groove formed on a top surface or a bottom surface of the frame where the periphery of the unit cells contacts. The more downstream the openings of the coolant passage are provided, the smaller in size the openings become. The coolant passage further includes an interstice between a part of one of the cross member portions of the frame forming one of the openings and a projection formed on the heat sink at a position corresponding to the one of the openings of the cross member portion. The more downstream the interstice of the coolant passage, the smaller in size the interstice becomes. The unit cell also comprises a flat lithium-ion cell covered with a laminate film.