MIT Team New electrode lithium design may cause more powerful batteries

MIT Feb 5, 2020

An MIT team has devised a lithium metal anode that would improve the longevity and energy density of future batteries.

Credit: MIT News

New research by engineers at MIT et al. could lead on to batteries which will pack more power per pound and last longer, supported the long-sought goal of using pure lithium metal together of the battery’s two electrodes, the anode.

The new electrode concept comes from the laboratory of Ju Li, the Battelle Energy Alliance Professor of Nuclear Science and Engineering and professor of materials science and engineering. it's described today within the journal Nature, during a paper co-authored by Yuming Chen and Ziqiang Wang at MIT, along side 11 others at MIT and in Hong Kong , Florida, and Texas.

The design is a component of an idea for developing safe all-solid-state batteries, dispensing with the liquid or polymer gel usually used because the electrolyte material between the battery’s two electrodes. An electrolyte allows lithium ions to travel back and forth during the charging and discharging cycles of the battery, and an all-solid version might be safer than liquid electrolytes, which have high volatilility and are the source of explosions in lithium batteries.

“There has been tons of labor on solid-state batteries, with lithium metal electrodes and solid electrolytes,” Li says, but these efforts have faced variety of issues.

One of the most important problems is that when the battery is charged up, atoms accumulate inside the lithium metal, causing it to expand. The metal then shrinks again during discharge, because the battery is employed . These repeated changes within the metal’s dimensions, somewhat just like the process of inhaling and exhaling, make it difficult for the solids to take care of constant contact, and have a tendency to cause the solid electrolyte to fracture or detach.

Another problem is that none of the proposed solid electrolytes are truly chemically stable while in touch with the highly reactive lithium metal, and that they tend to degrade over time.

Most attempts to beat these problems have focused on designing solid electrolyte materials that are absolutely stable against lithium metal, which seems to be difficult. Instead, Li and his team adopted an unusual design that utilizes two additional classes of solids, “mixed ionic-electronic conductors” (MIEC) and “electron and Li-ion insulators” (ELI), which are absolutely chemically stable in touch with lithium metal.

The researchers developed a three-dimensional nanoarchitecture within the sort of a honeycomb-like array of hexagonal MIEC tubes, partially infused with the solid lithium metal to make one electrode of the battery, but with extra space left inside each tube. When the lithium expands within the charging process, it flows into the empty space within the interior of the tubes, moving sort of a liquid albeit it retains its solid crystalline structure. This flow, entirely confined inside the honeycomb structure, relieves the pressure from the expansion caused by charging, but without changing the electrode’s outer dimensions or the boundary between the electrode and electrolyte. the opposite material, the ELI, is an important mechanical binder between the MIEC walls and therefore the solid electrolyte layer.

“We designed this structure that provides us three-dimensional electrodes, sort of a honeycomb,” Li says. The void spaces in each tube of the structure allow the 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 inside these tubes moves in and out, kind of sort of a car engine’s pistons inside their cylinders. Because these structures are built at nanoscale dimensions (the tubes are about 100 to 300 nanometers in diameter, and tens of microns in height), the result's like “an engine with 10 billion pistons, with lithium metal because the working fluid,” Li says.

Because the walls of those honeycomb-like structures are made from chemically stable MIEC, the lithium never loses contact with the fabric , Li says. Thus, the entire solid battery can remain mechanically and chemically stable because it goes through its cycles of use. The team has proved the concept experimentally, putting a test device through 100 cycles of charging and discharging without producing any fracturing of the solids.

Reversible Li metal plating and stripping in a carbon tubule with an inner diameter of 100nm. Courtesy of the researchers.

Li says that though many other groups are performing on what they call solid batteries, most of these systems actually work better with some liquid electrolyte mixed with the solid electrolyte material. “But in our case,” he says, “it’s truly all solid. there's no liquid or gel in it of any kind.”

The new system could lead on to safe anodes that weigh only 1 / 4 the maximum amount as their conventional counterparts in lithium-ion batteries, for an equivalent amount of storage capacity. If combined with new concepts for lightweight versions of the opposite electrode, the cathode, this work could lead on to substantial reductions within the overall weight of lithium-ion batteries. for instance , the team hopes it could lead on to cellphones that would be charged just one occasion every three days, without making the phones any heavier or bulkier.

One new concept for a lighter cathode was described by another team led by Li, during a paper that appeared last month within the journal Nature Energy, co-authored by MIT postdoc Zhi Zhu and grad student Daiwei Yu. the fabric would scale back the utilization of nickel and cobalt, which are expensive and toxic and utilized in present-day cathodes. The new cathode doesn't rely only on the capacity contribution from these transition-metals in battery cycling. Instead, it might rely more on the redox capacity of oxygen, which is far lighter and more abundant. But during this process the oxygen ions become more mobile, which may cause them to flee from the cathode particles. The researchers used a high-temperature surface treatment with molten salt to supply a protective surface layer on particles of manganese- and lithium-rich metal-oxide, therefore the amount of oxygen loss is drastically reduced.

Even though the surface layer is extremely thin, just 5 to twenty nanometers thick on a 400 nanometer-wide particle, it provides good protection for the underlying material. “It’s almost like immunization,” Li says, against the destructive effects of oxygen loss in batteries used at temperature . this versions provide a minimum of a 50 percent improvement within the amount of energy which will be stored for a given weight, with far better cycling stability.

The team has only built small lab-scale devices thus far , but “I expect this will be scaled up very quickly,” Li says. The materials needed, mostly manganese, are significantly cheaper than the nickel or cobalt employed by other systems, so these cathodes could cost as little as a fifth the maximum amount because the conventional versions.

The research teams included researchers from MIT, Hong Kong Polytechnic University, the University of Central Florida, the University of Texas at Austin, and Brookhaven National Laboratories in Upton, New York. The work was supported by the National Science Foundation.