Power/Performance Bits: Jan. 14

Contrary to conventional wisdom, MIT researchers found cathodes made of disordered lithium compounds can perform better than perfectly ordered ones; researchers at Purdue have developed synthetic crystals with an inverse opal structure to reflect, diffract and bend incoming sunlight in an effort to improve solar cells.

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Disorderly conduct
With a significant ability to store power per a given weight, lithium batteries have been a major focus of research to enable use in everything from portable electronics to electric cars and now researchers at MIT and Brookhaven National Laboratory have found the use of disordered materials – generally considered unsuitable for batteries – can be used in a new avenue for such research.

In a rechargeable lithium-based battery, lithium ions — atoms that have given up an electron, and thus carry a net charge — are pulled out of the battery’s cathode during the charging process, and returned to the cathode as power is drained. But these repeated round-trips can cause the electrode material to shrink and expand, leading to cracks and degrading performance over time.

Today’s lithium batteries contain cathodes that are usually made of an orderly crystalline material, sometimes in a layered structure. When slight deviations from that perfect order are introduced, the battery’s efficiency generally goes down — so disordered materials have mostly been ignored in the search for improved battery materials.

However, it turns out this correlation is far from universal and certain kinds of disorder can provide a significant boost in cathode performance, the researchers found through a combination of computer modeling and laboratory experiments.

The materials that can release and then reabsorb the lithium ions act as a kind of reversible sponge. In today’s batteries, the cathodes are striated materials, made up of lithium layers alternating with oxides of transition metals. Scientists had thought the layering was necessary to provide a pathway for lithium to pass in and out of the cathodes without bumping into the transition metal oxide layer — a channel with nothing in the way.

Further, while disorder usually significantly reduces the lithium ion mobility, which is essential for an efficient rechargeable battery, it turns out that a significant excess of lithium in the material changes things dramatically. In the traditional ordered structure, there is an exact balance between the number of lithium and metal atoms. But if there is enough lithium excess, there are new channels created that can take over from the channels that have been closed off.

In essence, the researchers’ analysis shows a new direction that can be taken in searching for even better materials, opening a whole new category of possibilities that had previously been ignored.

Inverse opal structure for improved thin-film solar cells
Purdue University researchers have shown how to increase the efficiency of thin-film solar cells with a technology that could facilitate low-cost solar energy by using 3D photonic crystals to absorb more sunlight than conventional thin-film cells.

The synthetic crystals have what the researchers call an inverse opal structure to make use of and enhance properties found in the gemstones to reflect, diffract and bend incoming sunlight. Typically in thin-film silicon solar cells, much of the sunlight comes right back out, but using this approach the light comes in and it is diffracted, causing it to propagate in a parallel path within the film, they explained.

And compared to solar cells made of silicon wafers, cost is reduced 100 times for the thin films, but the downside is lower efficiency. The researchers wonder if they can make up that lower efficiency by introducing new approaches to light trapping for thin film solar cells thereby combining low cost and high performance.

The researchers are the first to demonstrate incorporation of the 3D photonic crystals to increase light trapping in crystalline silicon solar cells. Experimental findings of solar cells that contain the 3D photonic crystals indicate roughly a 10% increase in efficiency over conventional silicon thin films, with further potential for improvement.

 

 The illustrations at the top (steps A through D) depict the creation of 3-D "photonic crystals" to improve the efficiency of thin-film solar cells, a technology that could bring low-cost solar energy. The "inverse opal" structure is formed by treating the crystals with hydrofluoric acid (d). Scanning electron microscope images show its small-scale crystal structure from three different angles (e and g), and a photograph (h) shows how the same membrane can be wrapped around a glass pipette. (Source: Purdue University)

The illustrations at the top (steps A through D) depict the creation of 3-D “photonic crystals” to improve the efficiency of thin-film solar cells, a technology that could bring low-cost solar energy. The “inverse opal” structure is formed by treating the crystals with hydrofluoric acid (d). Scanning electron microscope images show its small-scale crystal structure from three different angles (e and g), and a photograph (h) shows how the same membrane can be wrapped around a glass pipette. (Source: Purdue University)