Electronic Design

Electric Cars Look To Li-Air Batteries For Exponentially Better Range

Chemistry research moves at a snail’s pace, and battery innovation is no exception. It needs a catalyst. Pardon the pun, but the world is clamoring for alternatives to gasoline-powered vehicles, and today’s batteries have a long way to go.

Batteries are expensive and heavy, and their costs and performance are the major hurdles to the widespread adoption of electric vehicles (EVs). The energy density of gasoline is 50 times that of the best batteries available. The 16-kilowatt-hour pack in the Chevrolet Volt reportedly costs $8000 and weighs about 400 pounds, and most EVs offer no more than 100 miles of range.

I haven’t done a formal study, but it seems to take at least 10 to 15 years before new battery technology is adopted. The new iron-phosphate batteries used in many power tools was first patented in the mid-1990s, the cobalt-oxide technology that powers most laptops and cell phones was developed in the mid-1980s, and lead-acid car batteries were a major innovation of the 19th century.

The electronics and other systems managing battery use have evolved much more quickly than the battery cells themselves, but I am hoping that the recent influx of funding for alternative energy and electric vehicles will move battery chemistry from the research stage to reality a little faster.

Argonne On The Cutting Edge
A battery contains two electrodes separated by an electrolyte and separator. The cathode and the anode electrodes determine key features such as voltage, capacity, and rate capability. Most lithium-ion (Li-ion) batteries use an anode made of carbon or graphite. There are various cathode chemistries, though, so the bulk of the research dedicated to improving Li-ion batteries over the past two decades has focused on the cathode.

General Motors is licensing cathode technology developed by Argonne National Laboratory that boosts the performance of Li-ion batteries by 50% to 100% (see www.anl.gov/Media_Center/News/2011/news110106.html). The patented cathode material has a mixture of manganese-spinel and cobalt-oxide materials. It is a step in the right direction, as electric cars could have a less expensive battery with better range. But is it enough?

Argonne is also licensing the technology to LG Chemical, which makes the battery for the Chevy Volt (see www.anl.gov/Media_Center/News/2011/news110106a.html). So, the Volt might be using a similar mixture of cobalt, manganese, and nickel in its battery already.

Mixed cathodes have become common in portable electronics applications. It’s a great feat for Li-ion batteries and electric vehicles to offer a 50% performance improvement, but this performance still pales in comparison to the energy density of gasoline and the range of a traditional or hybrid vehicle.

The Li-Air Battery
Li-ion technology has done amazing things for portable electronics, but a more radical innovation may be needed for electric vehicles to overtake traditional gas and diesel power. Probably in anticipation of the challenge, Argonne is now pursuing research into Li-air batteries, which use a catalytic air cathode that converts oxygen to lithium peroxide, an electrolyte, and a lithium anode. Li-air batteries promise to increase energy density by as much as five to 10 times compared to traditional Li-ion batteries.

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In Li-air batteries, the lithium anode is electrochemically coupled to atmospheric oxygen through an air cathode. During discharge, lithium cations flow from the anode through an electrolyte and combine with oxygen at the cathode (typically consisting of porous carbon) to form lithium oxide (Li2O) or lithium peroxide (Li2O2), which is inserted in the cathode. Electrons flow from the battery’s anode to the cathode through the external circuit.

Li-air batteries have higher energy density than Li-ion batteries because of the lighter cathode and abundant availability of atmospheric oxygen. Theoretically, with oxygen as an unlimited cathode reactant from the environment, the anode rather than the cathode limits the capacity of the battery.

Oxygen from the atmosphere enters the pores of the carbon cathode to serve as the cathode active material. In the discharge of the Li-air battery, this oxygen is reduced and stored in the pores of the carbon electrode. As a result, the cell capacity is expressed in capacity per weight of the carbon in the cathode alone. The open-circuit voltage is almost 3 V, and the theoretical energy density is 5200 Wh/kg. In practice, oxygen is not stored in the battery, so the theoretical energy density is more than 11,000 Wh/kg.

A 50-fold improvement, rather than 50%, represents the kind of change needed to move the masses. If this technology can be made commercially, then battery-powered vehicles can have the range of gasoline vehicles. There are significant technical challenges to any new technology, and the implementation of Li-air may be a long way off. But at least the goal to have an electric vehicle that competes with gasoline is the correct, formidable target.

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