A team at Korea’s Ulsan National Institute of Science and Technology (UNIST), led by Dr. Jaephil Cho, has developed a new high-power NCA (nickel-cobalt-aluminum) Li-ion cathode material: LiNi0.81Co0.1Al0.09O2. Variations of NCA systems are currently used in some very high profile battery systems: the Tesla-Panasonic cell used in the Tesla Model S and the AESC cell used in the Nissan LEAF, for example.
The new UNIST NCA material exhibits an excellent rate capability of 155 mAh g−1 at 10 C with a cut-off voltage range between 3 and 4.5 V, corresponding to 562 Wh kg−1 at 24 °C. It additionally provides significantly improved thermal stability and electrochemical performance at the high temperature of 60 °C, with a discharge capacity of 122 mAh g−1 after 200 cycles with capacity retention of 59%. A paper on the work is published in the journal Advanced Energy Materials.
The new material overcomes limitations of other nickel-cobalt-aluminum (NCA) systems—such as those arising when they are used use at high discharge/charge rates (>5C) and in high temperature (60 °C) environments. As a result, the team suggests, their new NCA material holds great promise for commercial use in batteries within EV and HEV systems.
Nickel-based layered oxides with an isostructure of LiCoO2 have been extensively investigated because of their higher specific energy, lower cost, and low toxicity compared to conventional lithium cobalt oxide materials.
However, pure LiNiO2 is unstable at elevated temperatures; additionally, nickel ions migrating into the Li layer interfere with the Li ion pathway during Li intercalation/deintercalation, resulting in poor electrochemical performance. Additionally, pure LiNiO2 has poor thermal stability.
Much work has gone into determining how partial substitutions (doping) could address these limitations. While adding cobalt significantly improved the structural stability and reversibility of the material, thermal and chemical instabilities remained.
The addition of aluminum, which is electrochemically inactive, further stabilized the layered structure, the researchers explained, as well as enhanced the capacity/power retention of the Ni-based layered oxides.
Among the numerous possible compositions of the LiNi1-y-zCoyAlzO2 system, the LiNi0.8Co0.15Al0.05O2 where y = 0.1-0.15 and z = 0.05 has been carefully selected to meet safety criteria without sacrificing the energy, power or cost advantages of Ni-based cathodes. However, it still has limited applications within EV systems because the capacity can fade both at high discharge/charge rates and at high temperature environment of 60 ˚C, the resistance can increase during cycling at elevated temperatures, and thermal instability can occur at temperatures above 200 ˚C.
In order to develop better cathode materials for EV systems, there have been extensive investigations into the partial substitution of Co and Al into the LiNi1-y-zCoyAlzO2 system to achieve desirable electrochemical performances and thermal stabilities with minimizing loss of reversible capacity.
After all of our research efforts, herein we determined that LiNi0.81Co0.1Al0.09O2 was the most suitable cathode material and shows great potential for commercial use in batteries for EV and HEV systems.—Jo et al.
Jo, M., Noh, M., Oh, P., Kim, Y., Cho, J. (2014) “A New High Power LiNi0.81Co0.1Al0.09O2 Cathode Material for Lithium-Ion Batteries” Adv. Energy Mater., 4: 1301583. doi: 10.1002/aenm.201301583