Compared with preexisting batteries, lithium ion secondary batteries and other secondary batteries are small, lightweight and have a high energy density, and also have an excellent power density. For these reasons, in recent years, they have been favored for use as so-called portable power sources for personal computers and handheld devices, and as on-board batteries in vehicles (e.g., power sources for vehicle propulsion).
However, in nonaqueous electrolyte secondary batteries intended for on-board use in vehicles and other applications, there exists a desire for even higher energy density and higher power density in order to increase performance. Such increased performance can be achieved by working innovatively with, for example, the conductive material. Existing art to this effect is described in, for example, Patent Documents 1 and 2. Patent Document 1 discloses that, by using carbon fibers having an average fiber diameter of from 1 to 200 nm as the conductive material, the proportion of active material within the active material layer can be increased and a high energy density can be achieved. Patent Document 2 discloses that, by using a carbon black composite of carbon black coupled with fibrous carbon as the conductive material, contact between the active material and the conductive material can be maintained and improved, enabling resistance within the active material layer to be reduced.