Hitachi Ltd and Hitachi Vehicle Energy Ltd. have developed Li-ion batteries for plug-in hybrid electric vehicles, a first for the Hitachi Group. These new batteries were developed in response to the growing demand for environment-friendly vehicles resulting from the increasingly strict regulations on automobile exhaust emissions around the world.

Plug-in hybrid electric vehicles (PHEVs) have two running modes: an EV (electric vehicle) mode, in which the vehicle runs on a motor alone, and an HEV (hybrid electric vehicle) mode, in which the vehicle is driven by both the engine and the motor. PHEVs offer improved gas mileage and reduced exhaust emissions, and so are considered a strong contender among environment-friendly vehicles of the future. The new Li-ion batteries offer the high performance and reliability required for use in PHEVs, achieving both high energy (durability; contribute to gaining cruising distance) and high output (instantaneous force) performance.

The Li-ion batteries feature electric capacity of 25Ah, allowing it to run for about 20 km in EV mode, using the motor alone. This represents 4-5x the capacity of the Li-ion batteries for HEVs that Hitachi has been developing up to now.

Hitachi has also developed new electrode that offer balance of high energy (EV running) and high power (EV running) performances. Normally, high energy and high power are reciprocal factors in battery design, but these new electrodes achieve both factors simultaneously by optimizing the electrodes' thickness and the composition of the active materials, and by enhancing the composition of the conductive materials to maintain high output.

In addition, the batteries also incorporate a heat-resistant separator that prevents internal short-circuits and dramatically improves safety. The separator is a key component of Li-ion batteries that separates the cathode and the anode, and at the same time maintains ionic conductivity. Because the electric capacity of PHEV batteries is considerably higher than that of HEV batteries, consideration for safety becomes even more important. The heat-resistant separator adopted in these new batteries was specially designed and developed for automotive applications, based on ceramic separator technologies.

To accommodate larger battery sizes, a variety of simulation technologies, including structural analysis and vibration analysis, have been utilized to increase battery strength and enhance the collector structure. In collaboration with Hitachi Research Laboratory and the Mechanical Engineering Research Laboratory, Hitachi has achieved a battery structure with an even higher level of reliability than previous units.

The development activities used some of the results of research contracted by the New Energy and Industrial Technology Development Organization (NEDO). Hitachi has been participating in a national project related to large-scale Li-ion batteries being promoted by NEDO and the Japanese Ministry of Economy, Trade and Industry since 1992, and will continue its efforts to incorporate the results of such contracted research into practical applications as quickly as possible.