More powerful batteries are now possible thanks to MIT’s new electrode design.

The new lithium metal anode might create batteries that have more power per kilogram and last longer.

An MIT team has devised a lithium metal anode that can increase the lifespan and energy efficiency of the upcoming batteries.

New research carried out by engineers from MIT, and other institutions could result in batteries that pack greater power for each pound and last for longer in line with the long-sought purpose to use pure lithium as the batteries’ two electrodes the anode.

The concept for the new electrode comes from the lab at the lab of Ju Li, the Battelle Energy Alliance Professor of Nuclear Science and Engineering and professor of engineering and materials sciences. The concept is presented by the scientific journal Nature in a research article co-authored with Yuming Chen and Ziqiang Wang from MIT and 11 other researchers at MIT and in Hong Kong, Florida, and Texas.

The idea forms part of the plan for developing safe all-solid-state batteries that use the polymer gel, or liquid, that is usually employed as an electrolyte between the two electrodes in the battery. Electrolytes permit lithium ions to move through the charging and discharging phases of the battery. Furthermore, an all-solid model could be more secure than liquid electrolytes, which contain high levels of volatility and have been the cause of explosions in lithium batteries.

“There has been a lot of work on solid-state batteries, with lithium metal electrodes and solid electrolytes,” Li states. However, these efforts have run into many challenges.

One of the most significant issues is that atoms accumulate inside the lithium metal when the battery is being charged, leading to it expanding. The metal shrinks further in the course of discharge, depending on how the battery is utilized. The constant changes in metal’s dimensions, similar to the process of exhaling and inhaling, can make it difficult for the metals to stay at the same level of contact and can cause the electrolyte in the solid to split or break off.

Another issue lies in the fact that no of the suggested solid electrolytes is truly chemically stable even when they come into contact with lithium, a highly reactive metal. Furthermore, they tend to degrade with time.

The majority of attempts to solve these issues have been concentrated on creating solid electrolyte substances resistant to lithium metal, but this can be a challenge. However, Li and his team chose a unique design that incorporates two other types of solids, “mixed ionic-electronic conductors” (MIEC) and “electron and Li-ion insulators” (ELI), which are chemically stable and completely upon contact with lithium metal.

Researchers have developed a 3-D nanostructure in the shape of the honeycomb-like hexagonal MIEC tubes, partially filled with solid lithium metal that forms one electrode for the battery, with additional space in each tube. When lithium expands during the charging process, it flows out into the space within the tube’s interior and flows like a liquid but retains its solid crystal structure. The flow, which is completely contained within the honeycomb structure, reduces the pressure of the charge-induced expansion; however, it does not alter the electrode’s dimensions outside of the boundaries between the electrolyte and electrode. The other component, the ELI, is an important mechanical binder between the MIEC wall and electrolyte layer that is solid.

“We designed this structure that gives us three-dimensional electrodes, like a honeycomb,” Li states. The gaps in the tube of the structure permit lithium to “creep backward” into the tubes, “and that way, it doesn’t build up stress to crack the solid electrolyte.” The expanding and contracting lithium in the tubes flows through and out, kind of a car’s pistons in their pistons. Since these structures are constructed at nanoscales (the tubes measure between 100 to 300 nanometers diameter and 10 to 30 microns in height), it’s as “an engine with 10 billion pistons, with lithium metal as the working fluid,” Li claims.

Since chemically stable MIEC constructs the honeycomb-like walls of these structures, lithium does not lose its electrical connection with MIEC Li states. Therefore, the entire solid battery will remain chemically and mechanically stable throughout its usage cycle. The team has tested this concept in a lab setting by using a test device for hundred cycles of charge and discharging without breaking any solids.

Li states that although many other groups are researching solid batteries, most of them require a liquid electrolyte mixed with the solid electrolyte. “But in our case,” Li states, “it’s truly all solid. There’s no liquid or gel or of any kind.”

The new system may create safer anodes that weigh less than one-quarter of the weight of the traditional counterparts of lithium-ion batteries, with the same storage capacity. If combined with the development of new ideas for lighter models of another electrode called the cathode, this research could result in significant decreases in the battery’s weight. For instance, the team believes that it will lead to phones which can be changed every three days without making the devices any larger or heavier.

A new idea for a lighter cathode has been proposed by a team of researchers headed by Li in a paper published in Nature Energy, co-authored by MIT postdoc Zhi Zhu and graduate student Dawei Yu. The material could lower the amount of cobalt and nickel, which are toxic and expensive and are commonly used in cathodes today. The new cathode doesn’t depend on only the capacity of the transition metals in the cycle of the battery. Instead, it relies more heavily on oxygen’s redox capabilities and is lighter and abundant. However, during this process, the oxygen ions are more mobile, leading them to escape the cathode particle. Researchers used an extremely high-temperature surface treatment using melting salt to create an anti-corrosive layer on manganese and lithium-rich metal-oxide, which means that the amount lost oxygen is dramatically decreased.

While the surface layer is thin, ranging from 5-20 nanometers on a particle with a diameter of 400 nanometers, It provides adequate protection for the substrate material. “It’s almost like immunization,” Li claims, referring to the damaging negative effects of oxygen loss within batteries at temperatures of room temperatures. The latest versions offer at the very least a 50 percent increase in the quantity of energy stored at a given weight and provide more stable cycling.

The team has built small-scale devices in the lab so far; however, “I expect this can be scaled up very quickly,” Li claims. The raw materials required, most notable manganese, are considerably less than nickel and cobalt used in other systems. Therefore, the cathodes could cost less than one-fifth of the standard versions.

References:

“Li metal deposition and stripping in a solid-state battery via Coble creep” by Yuming Chen, Ziqiang Wang, Xiaoyan Li, Xiahui Yao, Chao Wang, Yutao Li, Weijiang Xue, Dawei Yu, So Yeon Kim, Fei Yang, Akihiro Kushima Guoge Zhang Haitao Huang Nan Wu Yiu-Wing Mai John B. Goodenough and Ju Li, 3 February 2020, Nature.

“Gradient Li-rich oxide cathode particles immunized against oxygen release by a molten salt treatment” by Zhi Zhu, Dawei Yu, Yang Yang, Cong Su, Yimeng Zhang, Yanhao Dong, Iradwikanari Waluyo Baoming Wang Adrian Hunt, Xiahui Yao Jinhyuk Lee Weijiang Zhang, and Ju Li, 12 December 2019 Nature Energy.

The research teams consisted of experts of MIT, Hong Kong Polytechnic University, and The University of Central Florida, the University of Texas at Austin, Brookhaven National Laboratories in Upton, New York.

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