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Saturday, November 5, 2011

The Lithium Air UltraBattery Makes Progress

The research team at Japan’s National Institute of Advanced Industrial Science and Technology (AIST) can show heat-treated metal-free graphene nanosheets (GNS) have much more stable cycling performance in a Lithium-air battery.

This might be important and does show a research lead, because of lithium air’s capacity of 5-10 times that of standard Lithium-ion batteries. That makes lithium air batteries the subject of intense, increased, and widespread research focus for use as high-energy batteries. What makes them different is lithium air batteries fundamentally use a catalytic air cathode that converts oxygen to lithium peroxide, which is an electrolyte, and a lithium anode.

AIST researchers demonstrated metal-free graphene nanosheets show good performance as a catalyst for reducing oxygen in the air in a lithium air battery with a hybrid electrolyte, although with poor cycling performance results. Then the team heat-treated the GNSs. These tests showed similar catalytic activity in reducing oxygen in the air, but also showed much more stable cycling performance in the test battery.

The problem in simple terms is in a lithium air battery the solid reaction product, lithium oxide (Li2O or Li2 O2), is insoluble in the organic electrolytes. That clogs up the air electrode on the cathode side in the discharge process. If the cathode air electrode is fully clogged up the O2 from the atmosphere cannot be reduced – clogging things to a stop.

In previous research the AIST team developed a new type of lithium air battery with a hybrid electrolyte designed to overcome that problem. In that new battery, an organic electrolyte is used on the anode (metallic lithium) side, and an aqueous electrolyte is used on the cathode (air) side. The two electrolytes are separated by a solid-state electrolyte called lithium super-ion-conductor glass film, so that they do not mix, to keep the Li2 O2 on one side.

With the glass film only lithium ions pass through the solid electrolyte, and the battery reactions proceed smoothly. Also the discharge reaction product is not a solid substance such as lithium oxide (Li2O), but lithium hydroxide (LiOH), which dissolves back into the aqueous electrolyte. So clogging of the pores does not occur at the carbon cathode.

The matters of building a long running very high cycle battery aren’t resolved just yet. In a paper published in the journal ACS Nano, AIST’s Eunjoo Yoo and Haoshen Zhou report preparing both GNSs and heat-treated GNSs by a chemical method, and studied their electrochemical properties as air-electrode catalysts for the new type of lithium air battery saying,
Corrosion of the air electrode catalysts is considered to result from oxidation of carbon to form Li2CO3. It is therefore important to choose suitable electrode materials to improve the performance of the battery.

In general, the air electrode for a lithium air battery is prepared from a combination of Pt-Au or a metal oxide such as Mn3O4 supported on a carbon material. In almost all cases, mesoporous carbon has been used as the support for the metal nanoparticles. Such mesoporous carbon-supported electrocatalysts have shown quite moderate performance in lithium air batteries, and several major obstacles arising from the carbonaceous air cathode, such as carbon’s oxidation in both charge and discharge processes, remain to be overcome if the cycling efficiency and cycle life of lithium air batteries are to be improved.

…GNSs were expected to be useful novel catalysts for lithium air batteries. However, until now, although GNSs have been widely investigated as support hosts for catalysts, metal-free GNSs have not been directly used as catalysts in air electrodes for either fuel cells or lithium air batteries.

A hybrid electrolyte lithium–air battery, in which a lithium-anode in a non-aqueous electrolyte and an air catalytic cathode in an aqueous electrolyte solution were separated by a ceramic LISICON film, has been investigated in our previous work. In the present work, a capacitor electrode was put in the non-aqueous electrolyte solution as an additional cathode in parallel with the air catalytic cathode.

The proposed lithium–air batteries with an additional capacitor cathode can successfully unite capacitor character and lithium–air battery character in one device which is now nominated as a “lithium–air capacitor–battery based on a hybrid electrolyte”. When high power is needed, the capacitor cathode would play the main role of peak power output; when high energy is demanded, the air catalytic cathode would display its high energy character. The adjustability of power output and energy output demonstrate that the proposed lithium–air capacitor–battery should be a promising power system for future electric vehicles.

The AIST team’s news is far from being market ready but the clue on how to take carbon further with heat treatment is an innovation that will catch a lot of researchers by surprise. Just how it is being done is a question of significance for the entire lithium air and likely the zinc air battery field as well.

Metal air batteries are at this date, the mountain top goal of energy density. At a low of 5 times the energy of an equivalent standard lithium battery, a point needs made. Many will surmise that the battery at market would be 1/5th the size. That would suggest the same number of cycles in a time period. But if costs are down for an air battery such that the capacity is 5 times then the cycles would be 1/5th as often – making the lifetime 5 times longer as well. The choices aren’t going to get easier, but they are getting simpler.

Depending on price, the life expectancy, battery replacement costs and ownership leading to trade in value, batteries are going to be a new realm of consumer considerations. But charging up once in five days or perhaps a week, or a full days drive before needing to stop to recharge has a very different meaning than charging every night or couple of hours of driving time.

Source: AIST

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