Engineers at MIT and elsewhere have conducted a novel study that can potentially lead to the development of batteries that are more durable and have a larger amount of power per pound.
The study is built on the long-sought aim of utilizing pure lithium metal as the anode—one of the two electrodes used in batteries.
The novel electrode concept resulted from the laboratory of Ju Li, a professor of materials science and engineering and the Battelle Energy Alliance Professor of Nuclear Science and Engineering.
The electrode has been recently reported in the Nature journal, in a paper jointly written by Yuming Chen and Ziqiang Wang from MIT, together with 11 others from MIT and in Texas, Florida, and Hong Kong.
The design, which is a part of an idea for creating safe and all-solid-state batteries, eliminates the polymer gel or liquid that is often used as the electrolyte material between the two electrodes of the battery.
An electrolyte causes lithium ions to move to and fro during the battery’s charge-discharge cycles. An all-solid version is likely to be safer when compared to liquid electrolytes, which exhibit extreme volatility and have been known to cause explosions in lithium batteries.
There has been a lot of work on solid-state batteries, with lithium metal electrodes and solid electrolytes.
Ju Li, Professor, Department of Materials Science and Engineering, MIT
But, according to Li, these efforts have experienced many problems.
One of the most significant issues is that whenever the battery is charged up, atoms tend to build up within the lithium metal and cause it to expand. As the battery is used, the lithium metal again reduces in size during the discharge process.
Such frequent changes in the dimensions of the metal, similar to the inhaling and exhaling process to some extent, make it difficult for the solids to sustain continuous contact and are likely to cause the solid electrolyte to detach or breakdown.
Yet another issue is that none of the suggested solid electrolytes are actually chemically stable when they remain in contact with the extremely reactive lithium metal, and these are likely to breakdown eventually.
Most of the efforts made to resolve these issues have worked on developing solid electrolyte materials that are completely stable against lithium metal, which becomes too complicated.
Instead, Li and his research team implemented a unique design that uses two more groups of solids, “mixed ionic-electronic conductors” (MIEC) as well as “electron and Li-ion insulators” (ELI), which are fully chemically stable when they make contact with lithium metal.
The scientists created a three-dimensional (3D) nanoarchitecture in the shape of a honeycomb-like array of hexagonal MIEC tubes. This nanoarchitecture is partly infused with the solid-lithium metal to develop into a single battery electrode but with more space left within each tube.
When lithium expands during the charging process, it flows into the vacant space present in the tubes’ interior, and moves just like a liquid, although it maintains its solid crystalline structure.
Fully limited within the honeycomb structure, this flow eases the pressure from the expansion induced by charging but without altering the external dimensions of the electrode, or the boundary that exists between the electrolyte and electrode. The ELI—the other material—acts as an important mechanical binder between the solid electrolyte layer and the MIEC walls.
“We designed this structure that gives us three-dimensional electrodes, like a honeycomb,” added Li. The empty spaces present in each structural tube enable the lithium to “creep backward” inside the tubes, “and that way, it doesn’t build up stress to crack the solid electrolyte,” Li further added.
The contracting and expanding lithium within these tubes shifts in and out, just like the pistons of a car engine that move within their cylinders. Since these structures are developed at nanoscale dimensions (the tubes measure tens of microns in height and around 100 to 300 nm in diameter), the outcome is just like “an engine with 10 billion pistons, with lithium metal as the working fluid,” added Li.
Since the walls of these honeycomb-like structures are composed of MIEC, which are chemically stable, the lithium always establishes electrical contact with the material, added Li.
Hence, the entire solid battery can continue to be chemically and mechanically stable as it goes through its cycles of use. The researchers have demonstrated the theory at the experiment level, subjecting a test device through 100 charge-discharge cycles without breaking down the solids.
According to Li, while most of the other teams are working on the so-called solid batteries, a majority of those systems work more optimally with certain liquid electrolyte combined with the solid electrolyte material. “But in our case,” Li added, “it’s truly all solid. There is no liquid or gel in it of any kind.”
The novel system may help develop safe anodes that weigh just a quarter of their traditional counterparts in lithium-ion batteries for the same proportion of storage capacity. If integrated with other concepts relating to lightweight versions of cathodes—which happens to be the other electrode—the study may lead to considerable reductions in the total weight of lithium-ion batteries.
The researchers are hoping that the work may help develop cellular phones that can be charged only once in three days without rendering the phones any bulkier or heavier.
One novel theory for a lighter cathode was elucidated by another research team, headed by Li, in a paper published last month in the Nature Energy journal and co-authored by Zhi Zhu, an MIT post-doctoral researcher, and Daiwei Yu, a graduate student.
The material would cut down the use of cobalt and nickel used in existing cathodes, which are dangerous and costly. The new cathode does not solely depend on the capacity contribution from such transition-metals in battery cycling, but rather, it would depend more on oxygen’s redox capacity.
Oxygen is relatively lighter and exists in large amounts. However, in this procedure, the oxygen ions are more mobile, which can cause them to escape from the cathode particles.
The scientists utilized a high-temperature surface treatment with molten salt to form a protective surface layer on particles of lithium-and manganese-rich metal-oxide, and hence the amount of oxygen loss is significantly decreased.
Although the surface layer is extremely thin, measuring only 5 to 20 nm thick on a 400-nm wide particle, it offers excellent protection for the fundamental material.
“It’s almost like immunization,” Li added, against the adverse impacts of oxygen loss in batteries utilized at room temperature. The current versions offer a minimum of 50% enhancement in the proportion of energy that can be preserved for a specified weight, with relatively better cycling stability.
To date, the researchers have only developed small laboratory-scale devices but they "expect this can be scaled up very quickly,” added Li.
The required materials, mostly manganese, are considerably cheaper when compared to the cobalt or nickel utilized in systems of other types. Therefore, these cathodes are likely to cost only one-fifth as much as the traditional ones.
The research group comprised scientists from Hong Kong Polytechnic University, MIT, the University of Texas at Austin, the University of Central Florida, and Brookhaven National Laboratories located in Upton, New York.
The study was funded by the National Science Foundation.