At EVS-23, EnerDel had highlighted its prototype Li-ion pack—which it had unveiled in October—in a Prius. The company expects to release third-party testing results on the battery’s performance in the Prius during the first quarter of 2008.

EnerDel currently is developing two different types of automotive lithium-ion batteries: high power batteries for HEV applications; and high energy batteries for full electric vehicle (EV) and plug-in hybrid electric (PHEV) applications. (Earlier post.)

EnerDel is designing its HEV Lithium Power System as a safe, small, light and high-power replacement for NiMH packs. EnerDel’s choice of active materials for the HEV cell is LiMn₂O₄ - spinel (LMO) for the cathode and Li₄Ti₅O₁₂ (LTO) for the anode.

Power capability. Although the operational voltage of an LMO/LTO system is lower than most conventional lithium-ion chemistry systems, the choice of LTO as the anode material provides the LMO/LTO system with high power capability.

Furthermore, the LMO/LTO cell can achieve the high power capability over a wide range of depths of discharge (DOD), therefore providing a wide range of usable energy, according to Taison Tan, EnerDel’s R&D manager, in a paper presented at EVS-23.

For example, the EnerDel battery pack in the Prius—although designed with essentially the same power specs as the OEM pack (1.1 kWh for the EnerDel Li-ion vs. 1.2 kWh for the OEM NiMH pack)—operates in a DOD window from 90% to 10%, effectively doubling the usable energy available from the battery. The OEM NiMH pack operates within a much narrower charge window—approximately 80% to 40%.) Although EnerDel has not yet taken the system to dynamometer testing, it expects that making approximately twice the electric energy available will substantially benefit the fuel economy of the vehicle.

Low-temperature operation. The LMO/LTO combination does not suffer from the degradation in power or capacity at low temperature as do some other Li-ion chemistries. The capacity of the EnerDel cell achieved at -30°C is more than 90% of the capacity achieved at 30°C.

In addition, discharging at lower temperatures brings the risk of dendrite formation with other chemistries, which can lead to shorts. According to Tan, this is not risk with the LMO/LTO chemistry, since the nominal voltage of the LTO anode is approximately 1.5V—providing a potential that does not allow for the formation of lithium dendrites.

Long life. Both active materials are of spinel structures, providing high stability under charge and discharge. During the intercalation and de-intercalation of lithium ions, the LTO material experiences a volume change of approximately 0.2%, compared to graphite which experiences about a 9% volume change. Capacity in the cell remain flat for almost 3,000 cycles.

Design. EnerDel is using a prismatic design. The large surface area proves better heat dissipation, and allows for a variety of cooling options.

EV Cells. In October, Think Global announced it had selected EnerDel as the supplier of choice for prismatic large format Li-ion batteries that will be used to power its Th!nk City electric vehicle.

On the Fox interview, Gassenheimer said that EnerDel batteries will be in the Think cars by the end of 2008. Under the supply agreement, EnerDel must deliver production prototypes in March 2008 and pre-production parts in July 2008. (Earlier post.)

Resources

• Taison Tan, Hiroyuki Yumoto, Derrick Buck, Bob Fattig, Chad Hartzog (2007) “Development of Safe and High Power Batteries for HEV” (EVS-23)