A new electrode material could help make lightweight, powerful rechargeable sodium batteries to replace lithium-ion batteries used in electronics and some electric vehicles. The material contains widely available iron, instead of the nickel and cobalt commonly used in these electrodes, and enables a similar energy density to electrodes in lithium batteries.

Sodium is an attractive candidate to replace lithium in batteries because it’s cheaper and widely available around the world. But building a sodium battery requires redesigning battery technology to accommodate the chemical reactivity and larger size of sodium atoms.

A rechargeable battery, whether lithium or sodium, contains two electrodes, the anode and the cathode. When a battery with an anode made from sodium metal discharges, electrons flow from that electrode to the other. The anode sloughs off positively charged sodium ions, which travel over to the cathode and wiggle inside the material to balance the extra negative charges coming in through the circuit.

When the battery is charged, this process is reversed: electrons flow out of the cathode, releasing the sodium ions inside. These ions float over to the other electrode, gain an electron, and replace the atoms lost during discharging.

The energy density of a battery depends on the amount of charged ions that the electrode can hold as well as the amount of energy released by each electron. The greater the energy density, the smaller a powerful battery can be.

Lithium batteries pack more of an electrical punch than sodium batteries because lithium atoms naturally release more energy when losing an electron than sodium does. So, for sodium batteries to reach energy densities similar to lithium ones, the positive electrode in the sodium battery has to hold more ions. And ideally, the ion-sized spaces in the electrode do not change size during the electron exchange, so that sodium ions can easily squeeze in and out as the battery charges and discharges.

To make this new electrode material, Shinichi Komaba, of Tokyo University of Science and his colleagues ground together iron oxide, sodium oxide, and manganese oxide. They squished the powder into a pellet and heated it to 900°C for 12 hours. This created a material with the formula Na 2/3 [Fe 1/2 Mn 1/2 ]O 2 .

Then the researchers made a battery using this new material as the positive electrode and sodium metal as the negative electrode. The capacity of the new material, which reflects how much charge one gram of electrode material can store, was 190 milliAmp-hours/gram, with an average voltage of 2.75 V. The capacity decreases over 30 charging cycles, meaning that the battery held less energy each time it was recharged.

The energy density of this material was estimated to be about 520 mWhr/g, similar to the energy density of LiFePO 4 and about 80 mWhr/g higher than LiMn 2 O 4 cathodes. Using carbon or titanium dioxide for negative electrode, instead of sodium, might allow the researchers to build a rechargeable battery that puts out three volts of power (the amount in two AA batteries).

Another recently published material using layered vanadium pentoxide as the positive electrode in a rechargeable sodium battery has a higher capacity and energy density than this material. It also maintains its capacity for 200 charging cycles.

The new material is unique because it contains cheap iron, says chemist Christopher Johnson, of Argonne National Laboratory in Illinois, who helped develop the vanadium-containing material. Since sodium battery technology is still new, it’s encouraging to see materials with energy densities similar to current lithium batteries, he says. “Who knows where we’ll be in 10 years?”

But using a cheaper material for the positive electrode does not necessarily reduce the overall cost of a battery, says Jay Whitacre, of Carnegie Mellon University. Dropping the total cost requires considering the other components, like electrolyte and anode, in the battery too. And a battery made with this new material does not use anything different than a standard lithium-ion battery, he says.

Nature Materials, 2012. DOI: 10.1038/NMAT3309 (About DOIs).