Aug 152010
 

Manganese Phosphate

Rechargeable lithium ion batteries are used in everything from mobile phones to cars.

Most of the batteries available today are designed with an oxide of metal such as cobalt, nickel, or manganese, which adds to their cost. Researchers looking for lower-priced alternatives to existing lithium ion-metal oxide batteries have discovered that a little wax and soap can help build electrodes and will allow battery developers to explore lower-priced alternatives to the lithium ion-metal oxide batteries currently on the market.

Recharging process

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 and 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 electrons, the lithium ions slink back to the ever-waiting metal oxide. Recharging starts the whole sordid process over.

Cheaper, stabler

While cobalt oxide performs well in lithium batteries, cobalt and nickel are more expensive 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.

Material scientist Daiwon Choi and his colleagues at the Department of Energy’s Pacific Northwest National Laboratory wanted to develop a simple method to turn lithium metal phosphate into a good electrode.

Lithium manganese phosphate (LMP) can theoretically store some of the highest amounts of energy of the rechargeable batteries, weighing in at 171 milliAmp hours per gram of material. High storage capacity allows the batteries to be light. But other investigators 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 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 well in the 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 crystals.

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