New titanate system for HEVs. The manganese spinel system, Amine said, has excellent power capability—much better than any other existing lithium-ion system—allowing the possibility of designing small batteries, which could lead to significant cost reduction. Argonne modelling suggests that the material cost for a spinel cell is some 43% less than the materials cost for an NCA (LiNi_0.8Co₁₅Al_0.05O₂) cell.

A Mn-spinel cathode/carbon anode system has calendar life issues during cycling and again at high temperatures. A small amount of manganese dissolution in the electrolyte (5 ppm) can lead to significant capacity loss. While other researchers have looked at stabilizing the spinel, Amine said, Argonne looked at replacing the carbon with a lithium titanate material.

Lithium titanate offers low cost of production; outstanding safety (no lithium plating and limited reactivity with the electrolyte); long calendar life and excellent cycle life characteristics; and improved low-temperature performance. EnerDel uses such a Mn-Spinel/titanate system (based on earlier work at Argonne) for one of its battery solutions.

XRD of new Argonne nano-Li4Ti5O12 spinel.  Click to enlarge.

Argonne, in partnership with EnerDel, recently made uniform and small particle Li₄Ti₅O₁₂ with nano-size primary particles for application in high power batteries using a new proprietary process. The nanophase titanate allows easier manufacturing and provides a high packing density that can increase the battery’s energy density and provide the power needed for vehicle acceleration and charging.

Peak power of the new Mn-spinel/titanate system is almost 70 kW—three times more power, along with the ability to “knock down” the size and weight by almost three times.

Performance of NCA cathodes, coated and pristine. Click to enlarge.

AlF₃ coating. Argonne is investigating improving the performance of conventional layered oxide cathodes with a metal fluoride coating (AlF₃). The coating thickness is very uniform, and less than 8 nm.

The LiNi_0.8Co_0.15Al_0.05O₂ (NCA) cathode material has a capacity of 140 mAhg^-1 at 3.9V and 190 mAhg^-1 at 4.3V—but a 4.3V, the life of the battery is poor.

With the metal fluoride coating, NCA performance was enhanced at the higher voltage.

The AlF₃-coating on different cathodes shows lower impedance, better cycling characteristics at high temperature, improved safety and reduced metal ion dissolution when compared to non-coated cathodes.

LiFePO₄. Iron phosphate cathode materials for Li-ion batteries have excellent safety characteristics, but a poor rate performance. To address this, companies such as A123Systems have used a combination of dopant and nano-particle design.

Argonne, in collaboration with Hanyang University, developed a nano-porous, micron-size LiFePO₄ particle that has high packing density and high volumetric energy density.

The resulting material yields double the volumetric energy density of standard LiFePO₄ while keeping the rate capability and pulse power almost similar. “This pretty much doubles the capability of iron phosphate to enable a 20-30 mile PHEV,” said Amine.

New high energy composite cathode. Argonne has been working on a new flourinated composite layered cathode with very high capacity (270 mAhg^-1): Li[Li_0.2Ni_0.175Co_0.10Mn_0.525]O_1.95F_0.05 (ANLCC). Argonne also developed a new carbonate process to obtain dense ANLCC material with a high rate capability.

The carbonate synthesis process is able to increase the tab density of the materials from 0.8 g/cc resulting from earlier processes to 2.1 g/cc to take advantage of the material high capacity in the cell.

The material, said Amine, shows outstanding cycle life and improved rate capabilities, and thermal stability superior to current state-of-the-art Ni-based cathode materials.

Resources

• Low Cost Components: Advanced High Power & High Energy Battery Materials (Amine, DOE Merit Review, 2008)