Wax Helps Scientists Reach Theoretical Storage Limit for Lithium Manganese Phosphate Batteries

In the war for efficiency, not only energy harvesters (like solar cells, wind turbines and others) have to win, but they will also have to be accompanied by storage devices, coming from behind. Daiwon Choi, working at DOE’s Pacific Northwest National Laboratory has discovered how paraffin (wax) can improve the battery manufacturing technology by enhancing their electrodes.

“Paraffin provides a medium in which to grow good electrode materials,” said Choi. “This method will help researchers investigate cathode materials based on cheaper transition metals such as manganese or iron.”

His targets are lithium manganese phosphate (LMP) batteries, which have the highest energy densities on the market, but which are expensive to produce because the synthesis of the electrode material is a multi-step, complicated process. Choi wants to develop a single-step method that should make lithium metal phosphate become a good electrode.

Theoretically, LMP has a 171 mAh/gram energy density, but up to now nobody has been able to exploit more than 120 mAh/g from it. The idea that Choi has had was that the loss happened due to the electrical resistance inside the battery (specifically, the metal oxide). Then, he theorized that if the distance between the electrodes was smaller, the resistance would drop and the capacity will rise. By using smaller particles they could lower the distance.

The next thing Choi and his colleagues did was mixing the electrode ingredients (lithium, manganese and phosphate) with melted paraffin and oleic acid (used in soap), and let the crystals grow while they raised the temperature up to 400 degrees Celsius. By that point, the was and soap had boiled off and only the formed crystals remained. Then, they raised the temperature even more to meld the formed crystals into 50-nanometers-thick plates.

Choi makes this process sound simpler than ever: “This method is a lot simpler than other ways of making lithium manganese phosphate cathodes. Other groups have a complicated, multi-step process. We mix all the components and heat it up.”

By adding a carbon backing that served as electrode, the team managed to prove the new material’s storing capacity going to a maximum of 168 mAh/g, but only in slow-discharging conditions (over a few days). Discharging it over a single day proved to be less efficient, but still over what others had obtained, 150 mAh/g. The problem was that discharging the energy stored in the battery over an hour proved unsatisfying: only 54 mAh/g.

Still, the team is not discouraged, but they will rather seek out new materials and will change how they incorporate the carbon coating on the LMP nanoplates, saying that this will enhance the high-discharge capabilities of the battery.

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