10-07/08 :: July/August 2010
nanotimes Research
metal oxide molecules fail to line up well in the cry- stals. Randomness is unsuitable for cathode materi- als, so the researchers needed a framework in which the ingredients – lithium, manganese and phosphate – could arrange themselves into neat crystals.
So, Choi and colleagues mixed the electrode ingre- dients with melted paraffin and oleic acid and let the crystals grow as they slowly raised the temperature. By 400° Celsius / 752° Fahrenheit (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 tempera- ture even more to meld the crystals into a plate.
“This method is a lot simpler than other ways of ma- king lithium manganese phosphate cathodes,” said Choi. “Other groups have a complicated, multi-step process. We mix all the components and heat it up.”
To measure the size of the miniscule plates, the team used a transmission electron microscope in EMSL, DOE‘s Environmental Molecular Sciences Laboratory on the PNNL campus. Up close, tiny, thin rectangles poked every which way. The nanoplates measured about 50nm thick and up to 2000nm on a side. Other analyses showed the crystal growth was suita- ble for electrodes.
To test LMP, the team shook the nanoplates free from one another and added a conductive carbon backing, which serves as the positive electrode. The team tested how much electricity the material could store after charging and discharging fast or slowly. When the researchers charged the nanoplates slowly over a day and then discharged them just as slowly,
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the LMP mini battery held a little more than 150 milliAmp hours per gram of material, higher than other researchers had been able to attain. But when the battery was discharged fast -- say, within an hour, that dropped to about 117, comparable to other material.
Its best performance knocked at the theoretical maximum at 168 milliAmp hours per gram, when it was slowly charged and discharged over two days. Charging and discharging in an hour – a reasonable goal for use in consumer electronics – allowed it to store a measly 54 milliAmp hours per gram.
Although this version of an LMP battery charges slower than other cathode materials, Choi said the real advantage to this work is that the easy, one-step method will let them explore a wide variety of cheap materials that have traditionally been difficult to work with in developing lithium ion rechargeable batteries.
In the future, the team will change how they incor- porate the carbon coating on the LMP nanoplates, which might improve their charge and discharge rates. Made with a one-step method, these flakes of lithium manganese phosphate can serve as electrodes for batteries.
Daiwon Choi, Donghai Wang, In-Tae Bae, Jie Xiao, Zimin Nie, Wei Wang, Vilayanur V. Viswanathan, Yun Jung Lee, Ji-Guang Zhang, Gordon L. Graff, Zhenguo Yang, and Jun Liu: LiMnPO4 nanoplate grown via solid-state reaction in molten hydrocarbon for li-ion battery cathode, In: Nano Letters ASAP, July 19, 2010, DOI 10.1021/nl1007085:
http://pubs.acs.org/doi/abs/10.1021/nl1007085
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