|Rate capability of the Ener1 LMO/LTO HEV cells. Click to enlarge.|
In preparation for a series of investor meetings during November, Ener1 has published a presentation outlining its approach to the two different types of automotive lithium-ion batteries it currently has under development: high power batteries for hybrid electric vehicle (HEV) applications (earlier post), and high energy batteries for plug-in hybrid electric (PHEV) and full electric vehicle (EV) applications.
Ener1 is using two different sets of electrode materials for each type. In the HEV batteries, the company is using a lithium manganese spinel (LiMn2O4-spinel, LMO) for its cathode material, and a lithium titanate (Li4Ti5O12, LTO) for the anode material. The energy batteries (PHEV and EV) are based on a layered lithium metal oxide cathode—LiNiCoMnO2—and a hard-carbon anode.
|Overcharge and nail penetration test results comparing a commercial 26650 cell and a Gen1 Ener1 HEV cell. Click to enlarge.|
HEV batteries. EnerDel developed its LTO anode material in collaboration with Argonne National Laboratory (ANL). The LMO/LTO combination for HEV cells offers a combination of high power and safety, according to Ener1. LTO has high rate capabilities, and supports a fast charge even at low temperatures.
Use of LTO in the anode rather than graphite reduces the possibility of runaway thermal events and extends battery life. Whereas graphite may experience up to a 9% volume change over the battery life, LTO will experience an approximate 0.2% volume change.
The Ener1 HEV battery runs very cool, and does not require a liquid cooling system. It can discharge to a very low state of charge, and also has strong low temperature performance, which is critical for automotive applications.
The downside of the LTO anode, however, is a lower energy capacity. LTO as an anode material has a capacity of about 150 mAh/g—about half that of graphite. LTO also has a lower voltage.
PHEV and EV batteries. For the high energy capacity batteries required in PHEV and EV applications, EnerDel is turning to a layered lithium manganese oxide cathode, and a non-graphite carbon (hard carbon) anode.
In general, lithium manganese oxides are of interest as cathode materials due to the safety, low cost, and low toxicity of manganese-based materials as well as to the high power enabled.
However, the material can suffer poor rate performance and capacity fading during cycling. To address those and other structural issues with the material, researchers are exploring the use of layered materials, with LiNixCoyMnzO2 (called NMC) gaining momentum. One of the more popular formulations for NMC is 1/3-1/3-1/3—i.e., LiNi1/3Co1/3Mn1/3O2
|Rate capability testing for the NMC/HC 7 Ah cell.|
A hard carbon anode is capacity tunable, and supports better rate capabilities on charge and discharge than graphite.
EnerDel has begun cycle life and rate capability testing of a 7 Ah cell with the NMC/HC chemistry. (By comparison, the HEV cells come in 1.8 Ah and 5 Ah units.) Initial results indicate a good rate capability from 1C to 5C, and also a good cycle life. EnerDel also expects to be able to produce further improvements in the performance of the cells.