The research is based on the goal of using pure lithium metal as one of a battery's two electrodes. The design is part of a concept for developing safe all-solid-state batteries by avoiding the liquid or polymer gel usually used as the electrolyte material - which allows lithium ions to travel back and forth during the charging and discharging cycles of the battery - between a battery's two electrodes.
While much work has been done pursuing solid-state batteries with lithium metal electrodes and solid electrolytes, say the researchers, such efforts have faced a number of issues. For example, when the battery is charged up, atoms accumulate inside the lithium metal, causing it to expand; the metal then shrinks again during discharge, as the battery is used. These repeated changes in the metal's dimensions make it difficult for the solids to maintain constant contact, and tend to cause the solid electrolyte to fracture or detach.
Another problem is that none of the proposed solid electrolytes are truly chemically stable when in contact with the highly reactive lithium metal, and tend to degrade over time. Most attempts to solve these issues, say the researchers, have focused on designing solid electrolyte materials that are absolutely stable against lithium metal, which turns out to be difficult.
The researchers therefore focused instead on a design approach that uses two additional classes of solids - "mixed ionic-electronic conductors" (MIECs) and "electron and Li-ion insulators" (ELIs) - that are absolutely chemically stable in contact with lithium metal. The result was a three-dimensional nanoarchitecture 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