Layered lithium batteries

Dr. Neil Canter, Contributing Editor | TLT Tech Beat November 2009

This emerging technology would enable a car to drive 40 miles powered only by a battery. 

KEY CONCEPTS
• Lithium-ion batteries exhibit superior energy-to-weight ratios and slower loss of discharge compared to other battery types. But they do not cycle at a fast enough rate, and there are concerns about their safety.
• A transitional lithium battery cathode has been developed that contains a high concentration of nickel ions in the core, which generate high energy but are thermodynamically unstable. In moving away from the core, nickel ions are gradually replaced by more stable manganese ions.
• This transitional cathode material improves the possibility of developing an automobile that can be driven for 40 miles just on a battery without recharging. In the U.S., 78% of the commuting is done within this range.

The high price of oil continues to spur interest in the development of alternative energy sources. One such option is lithium batteries for automotive applications that can provide high performance and better durability over a longer operating life.

An attractive strategy is to develop batteries for use in plug-in hybrid electric vehicles (PHEV). This car combines an electric battery with a small combustion engine to maximize performance, both in driving short and long distances. In the U. S., there is a goal to have more than a million PHEV on the road by 2015. The convenience aspect of this approach is that the consumer would recharge a depleted battery by plugging it into the readily accessible electric grid.

Difficulty has been encountered in developing the right type of battery for this application. Lithium-ion batteries exhibit superior energy-to-weight ratios and slower charge loss than other battery types. But they generally do not cycle (charge and discharge) at a fast enough rate, and there are concerns about their safe use. These issues have prompted the automotive industry to rely upon nickel-metal hydrides.

In a past column, work on preparing a well-ordered lithium nickel manganese oxide was described (1). This cathode has the potential to exhibit higher energy density and more rapid cycling.

Dr. Khalil Amine, senior scientist and manager of the Battery Technology Group at Argonne National Laboratories, in Argonne, Ill., says, “The two types of PHEV currently under development have all-battery ranges of 10 and 40 miles, respectively.” In the former case, battery technology can be designed right now, according to Amine, with the only outstanding issues being calendar life, safety and cost performance.

But the latter case is much more challenging. Amine adds, “The 40-mile target is very desirable because 78% of the commuting in the U.S. is done within this mileage range. Unfortunately, no chemistry is currently available that can enable a vehicle to travel for this distance using just the battery based on specifications prepared by a U.S. advanced battery consortium known as USABC.”

A big hurdle is meeting all the energy and power requirements within the weight and volume that is acceptable to the automotive industry. Vehicle weight, as we know from current internal combustion engine technology, is a critical factor in determining the fuel economy of a gasoline or diesel engine-powered vehicle. It is no different with batteries.

Amine says, “Current state-of-the art lithium batteries can generate between 160 and 180 watt-hours per kilogram at the cell level. However, for PHEV-40 miles, we need similar energy densities but at the pack level, which is very challenging.”

The specifications outlined by USABC require that a lithium-ion battery must exhibit the combination of superior performance, enhanced durability, safety and low cost. Such a battery has not been developed until now.

TRANSITIONAL COMPOSITION
A group of researchers has developed a battery cathode material that contains a lithium nickel cobalt manganese oxide cathode of variable composition in order to balance high energy and capacity with added safety and durability. Amine explains, “Nickelrich cathodes have proven to generate high energy but are inherently unstable. The high concentration of Ni⁺⁴ ions is thermodynamically unstable and can lead to a violent reaction with the electrolyte. Replacement of nickel ions with more stable manganese ions in the outer layer of the cathode material can lead to a more stable battery cathode material that still retains high energy characteristics.”

The core of the cathode battery material still contains high concentrations of nickel ions. But in moving away from the core towards the surface, the nickel ions are gradually replaced by more stable manganese ions so that the surface is predominantly the latter ion.

This transitional battery cathode material is prepared by a unique co-precipitation process. Amine says, “A nickel manganese hydroxide precursor is initially prepared in a continuously stirred reactor. Added to this precursor is a nickel manganese species that has a high concentration of nickel. As the process continues, the ratio of nickel to manganese is adjusted gradually to higher levels of manganese.”

An image of a cathode particle showing its transitional composition is shown in Figure 2. The key to the process is an interface in which the composition gradually changes from the nickel-rich interior to the manganese-rich surface.


Figure 2. A transitional cathodic material has been developed that contains a higher concentration of nickel in the core to generate high energy. In moving toward the surface, the nickel is replaced with more stable manganese. This cathode has the potential to be used in a battery, which can power an automobile for 40 miles. (Courtesy of Argonne National Laboratories)

Analysis of the cathode particles is conducted by electronprobe X-ray microanalysis and scanning electron microscopy. Amine says, “The size of the cathode particles is between 10-12 microns in diameter.”

Evaluation of the battery containing the transitional cathode particle was conducted in a small cell charged to 4.4 volts and cycled at 55 C. In one study, a cell based on the transitional material retained 96% of its capacity after 50 cycles, while a cell made from a nickel-rich salt retained only 67% of its capacity after the same number of cycles.

Amine believes the transitional battery cathode material has the potential to power an automobile close to the 40-mile targeted figure. He says, “The gradient particle approach improves the possibility that a car can exhibit a mileage range between 25 and 30 miles.”

Future work involves attempting to transfer the technology out of the lab. Amine says, “We are still doing optimization work and need to increase the concentration of the manganese component in the cathode.”

A 20-liter reactor is being set up for scale-up of material. Further information on the preparation and analysis of the battery cathode material can be found in a recent article (2) or by contacting Amine at amine@anl.gov. 

REFERENCES
1. Canter, N. (2006), “Higher Performing Lithium Batteries,” TLT, 62 (5), pp. 12–15.
2. Sun, Y., Myung, S., Park, B., Prakash, J., Belharouak, I. and Amine, K. (2009), “High-Energy Cathode Material for Long-Life and Safe Lithium Batteries,” Nature Materials, 8, pp. 320–324.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.