(a) Schematic diagram of functional graphene sheet structure (top), with ideal bimodal porous structure (lower image), which is very beneficial for lithium-oxygen battery operation. (b) The discharge curve of the lithium-oxygen battery uses FGS (C / O = 14) as the air electrode (PO2 = 2 ATM). Source: Pacific Northwest National Laboratory

One such system is the metal/air battery, which has much higher specification energy than most existing primary and rechargeable batteries.

“Metal/air batteries are unique because positive active materials are not stored in batteries,” said Dr. Ji-Guang Zhang, a researcher at the Pacific Northwest National Laboratory's Transition Materials Science Program. "In contrast, the oxygen in the environment is reduced, and the catalytic surface in the air electrode forms an oxygenated or peroxide ion that further reacts with the cationic component of the electrolyte. Li/O2 Particularly attractive because it may have a specification energy of **, which is the case in all known electrochemical couples."

Among the various electrochemical energy storage systems discussed so far, the most promising technology for lithium-air batteries is that the theoretical energy density is nearly ten times that of conventional lithium-ion batteries. This is because lithium metal acts as an anode and has a capacity ten times that of a conventional graphite anode, while oxygen, which is the anode of a lithium-air battery, is freely absorbed from the environment, which significantly reduces battery weight and cost.

For use in utility equipment such as electric vehicles, researchers expect lithium-air batteries to achieve an energy density of about 800 watt-hours/kg, which is three times that of lithium-ion batteries. Therefore, lithium-air batteries have great potential for many applications, which requires energy storage systems to surpass lithium-ion batteries, such as remote electric vehicles, which can travel more than 500 kilometers per charge.

Their results were reported on October 10, 2011, in the online edition of Nano Letters, entitled "Layered Porous Graphene as a Lithium Battery Electrode" (Hierarchically Porous Graphene as a Lithium-Air Battery Electrode) Zhang Jiguang and his team at the Pacific Northwest National Laboratory have demonstrated that a new air electrode, containing an unusual layered array of functionalized graphene sheets (without catalyst), can deliver extremely high capacity for lithium-oxygen batteries. It is 15,000 mAh/g, which is the value reported in this field.

The performance of such lithium-air batteries is affected by many factors, such as electrolyte composition, macrostructured air electrodes, microscopically up to nanostructured carbonaceous materials. The reaction product (such as Li2O2) precipitates on the carbon-containing electrode and eventually blocks the oxygen channel, limiting the capacity of the lithium-air battery.

The recent work of the Pacific Northwest National Laboratory team has circumvented the problem of air electrode blockages to a significant extent, significantly increasing capacity.

“Previous work used mesoporous carbon or graphene sheets, or cast into a stacked two-dimensional structure, which limited its available capacity,” Zhang said. “Our work demonstrates for the first time a self-assembled, bimodal graphene structure with open pores of micron size that accelerates oxygen diffusion and a large number of nanopores (2-50 nm) that catalyze the Li-O2 reaction. At the same time, prevent the rapidly growing discharge products from blocking the chemical channels."

In addition, studies have shown that graphene defects and functional groups are beneficial for the formation of isolated nano-sized Li2O2 particles, helping to prevent air blockage in the air electrode. The layered ordered porous structure of graphene can be practically applied to promote most graphite flakes using this structure.

When the air electrode is prepared by using functionalized graphene sheets, the functionalized graphene sheets are first dispersed in the microemulsion solution, and the solution also contains the electrode binder material. After casting and drying, a very unusual form will be made.

“Amazingly, this functionalized graphene sheet polymerizes into a loosely packed “broken egg” structure, leaving a large interconnected tunnel that penetrates deep into the electrode,” says Zhang Jiguang. “These tunnels act like a large number of arteries, constantly supplying oxygen into the interior of the electrode, which is the case during the discharge. More importantly, this complex pore structure remains after the electrolyte has penetrated, unlike other tested ones. Porous carbon material."

They used a scanning electron microscope (SEM) to look at the "shell" of "broken eggs," and the researchers found that these "shells" contained many smaller continuous nanopores that connected large tunnels. Zhang Jiguang pointed out that this unique form is an ideal design for air electrodes. “In the process of discharge, a powerful large tunnel can be used as a “highway” to supply oxygen into the interior of the air electrode. At the same time, the small holes in the wall are outlets, providing a three-phase (solid-liquid-gaseous) zone for oxygen reduction. ."

Some obstacles still need to be overcome, and then this battery system can be applied on a large scale. The main obstacles include: low discharge rate; electrolyte stability and reversibility of lithium-air batteries; oxygen selective membranes are needed to mitigate capacity degradation, which is derived from moisture infiltration; need to prevent dendrite growth, Extend cycle life.