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nanotimes Research
10-07/08 :: July/August 2010
Batteries // Paraffin and Surfactant Oleic Acid Improve Synthesis of Lithium Manganese Phosphate Electrodes
C
onsumers use long-lasting rechargeable lithium ion batteries in everything from cell phones
to the latest portable gadget. Some carmakers want to use them in vehicles. Most lithium ion batteries available today are designed with an oxide of metal such as cobalt, nickel, or manganese. Daiwon Choi and colleagues at PNNL and State University of New York at Binghamton, USA, wanted to explore both cheaper metals and the more stable phos- phate in place of oxide.
These rechargeable batteries work because lithium is selfish and wants its own electron. Positively charged lithium ions normally hang out in metal oxide, the stable, positive electrode in batteries. Metal oxide generously shares its electrons with the lithium ions.
Charging with electricity pumps electrons into the negative electrode, and when the lithium ions see the free-floating negative charges across the battery, they become attracted to life away from the metal oxide cage. So off the lithium ions go, abandoning the metal oxide and its shared electrons to spend time enjoying their own private ones.
But the affair doesn‘t last – using the battery in an electronic device creates a conduit through which the slippery electrons can flow. Losing their elec- trons, the lithium ions slink back to the ever-waiting metal oxide. Recharging starts the whole sordid
process over. While cobalt oxide performs well in lithium batteries, cobalt and nickel are more expen- sive than manganese or iron. In addition, substituting phosphate for oxide provides a more stable 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 good electrode.
Lithium manganese phosphate – LMP – can theoreti- cally store some of the highest amounts of energy of the rechargeable batteries, weighing in at 171 milli- Amp hours per gram of material. High storage capa- city allows the batteries to be light. But other investi- gators working with LMP have not even been able to eek out 120 milliAmp hours per gram so far from the material they‘ve synthesized.
Choi reasoned the 30% 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 tempera- tures. Unfortunately, lower temperatures means the
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