MIT engineers have developed electrodes to avoid the stresses that cause component fracturing in solid-state batteries.
Solid-state batteries need a mechanism that allows for expansion and contraction during charge/discharge cycles, without changing the electrode’s outer dimensions.
A new electrode concept from the laboratory of Ju Li, the Battelle Energy Alliance Professor of Nuclear Science and Engineering and Professor of Materials Science and Engineering could meet that need.
Li and his team adopted a design that uses two additional classes of solids, “mixed ionic-electronic conductors” (MIEC) and “electron and Li-ion insulators” (ELI), which are chemically stable in contact with lithium metal.
The researchers developed a three-dimensional nano-architecture in the form of a honeycomb-like array of hexagonal MIEC tubes, partially infused with the solid lithium metal to form one electrode of the battery, but with extra space left inside each tube.
When the lithium expands in the charging process, it flows into the empty space in the interior of the tubes, moving like a liquid even though 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 other material, the ELI, serves as a crucial mechanical binder between the MIEC walls and the solid electrolyte layer.
Because the walls of these honeycomb-like structures are made of chemically stable MIEC, the lithium never loses electrical contact with the material, Li says.
Thus, the whole solid battery can remain mechanically and chemically stable as it goes through its cycles of use.
Li says that though many other groups are working on what they call solid batteries, most of those 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 is no liquid or gel in it of any kind.”
The new system could lead to safe anodes that weigh only a quarter as much as their conventional counterparts in lithium-ion batteries, for the same amount of storage capacity.
Another team led by Li described a new concept for a lighter cathode.
The material would reduce the use of nickel and cobalt, which are expensive and toxic and used in present-day cathodes. The new cathode would rely on the redox capacity of oxygen, which is much lighter and more abundant.
The researchers used a high-temperature surface treatment with molten salt to produce a protective surface layer on particles of manganese- and lithium-rich metal-oxide, so the amount of oxygen loss is drastically reduced.
Even though the surface layer is very thin, just 5-20 nanometers thick on a 400 nanometer-wide particle, it provides good protection for the underlying material. The present versions provide at least a 50% improvement in the amount of energy that can be stored for a given weight, with much better cycling stability
The materials needed, mostly manganese, are significantly cheaper than the nickel or cobalt used by other systems, so these cathodes could cost as little as 20% of conventional versions.