Wax and Soap Wash Away Barriers to Cheaper Batteries
Made with a one-step method, these flakes of lithium manganese phosphate can serve as electrodes for batteries.
CREDIT: Daiwon Choi, PNNL
A little wax and soap could smooth the way for less costly lithium ion batteries, according to a recent study.
Building electrodes using wax and soap will allow researchers to explore materials for batteries based on cheaper metals such as manganese or iron.
"Paraffin [wax] provides a medium in which to grow good electrode materials," said materials scientist Daiwon Choi of the Department of Energy's Pacific Northwest National Laboratory (PNNL) and lead author of the study in the August 11 issue of Nano Letters.
Lighter costs than lithium
Consumers use long-lasting rechargeable lithium ion batteries in everything from cell phones to the latest portable gadget, and many electronic vehicles also use lithium ion cells .
Most of the batteries available today are designed with an oxidized – that is, having oxygen atoms attached – version of metals such as cobalt, nickel, or manganese.
While cobalt oxide performs well in lithium batteries, cobalt and nickel are more expensive than manganese or iron. In addition, substituting phosphate for oxygen provides a durable structure for lithium.
Lithium iron phosphate batteries are commercially available in some power tools and solar products, but synthesis of the electrode material is complicated. Choi and colleagues wanted to develop a simple method to turn lithium metal phosphate into a quality electrode.
Lithium manganese phosphate (LMP) can theoretically store some of the highest amounts of energy in a rechargeable battery structure, weighing in at 171 milliamp-hours per gram of material. This high storage capacity allows the batteries to be light.
But other investigators working with LMP have not been able to eke out even 120 milliamp-hours per gram from the materials they have synthesized.
Choi reasoned that this 30 percent loss in capacity could be due to lithium and electrons having to battle their way through the metal oxide, a property called resistance.
The less distance lithium and electrons have to travel out of the cathode, he thought, the less resistance and the more electricity could be stored.
A smaller particle would decrease that distance. But growing smaller particles requires lower temperatures. Unfortunately, lower temperatures means the metal oxide molecules fail to line up properly and form crystals.
Randomness is unsuitable for cathode materials, so the researchers needed a framework in which the ingredients – lithium, manganese and phosphate – could arrange themselves into neat, useful crystals.
Wax on, wax off
Paraffin wax is made up of long straight molecules that do not react with many other materials, so the scientists suspected the chainlike molecules might help get things lined up. Soap – specifically a surfactant called oleic acid – could make the growing crystals disperse evenly as well.
So the scientists mixed the electrode ingredients with melted paraffin and oleic acid and let the crystals grow as they slowly raised the temperature.
By 752 degrees Fahrenheit (400 degrees, Celsius or four times the temperature of boiling water), crystals had formed and the wax and soap had boiled off.
Materials scientists generally strengthen metals by subjecting them to high heat, so the team raised the temperature even more to meld the crystals into a plate.
To measure the size of the miniscule plates, the team used a transmission electron microscope. Up close, tiny, thin rectangles poked every which way. The nanoplates measured about 50 nanometers thick – about a thousand times thinner than a human hair -- and up to 2,000 nanometers on a side. Other analyses showed the crystal growth was suitable for electrodes.
The best performance of this new battery knocked at the door of the theoretical maximum at 168 milliamp-hours per gram, when it was slowly charged and discharged over two days.
However, charging and discharging in an hour – a reasonable goal for use in consumer electronics – only allowed it to store a comparatively measly 54 milliamp-hours per gram.
Choi said the real advantage to this work is that the easy, one-step building method will let researchers explore a wide variety of cheap materials that have traditionally been difficult to work with in developing lithium ion rechargeable batteries.
"This method is a lot simpler than other ways of making lithium manganese phosphate cathodes," said Choi. "Other groups have a complicated, multi-step process. We mix all the components and heat it up."