A team of MIT engineers designed a new electrode with a pure lithium metal anode that could lead to more powerful batteries with improved longevity and energy density. The new electrode concept also involves swapping out the liquid or polymer gel used as the electrolyte material between the battery’s two electrodes, and replacing it with solid material – hence why these new experimental devices are called an all-solid-state battery.
The electrolyte is what enables lithium ions to move back and forth between the anode and cathode during the charging and discharging. Liquid electrolytes are prone to explosion and fire because they have high volatility. An all-solid version would be much safer, but they face several performance issues.
For one, atoms accumulate inside the lithium metal when the battery is charging up, which causes it to expand. Then, during discharge, the metal shrinks again. The constant fluctuation of dimensions leads to the metal having a difficult time maintaining constant contact – which causes the solid electrolyte to detach or fracture. A second issue is that solid electrolytes tend to degrade over time because they aren’t chemically stable during contact with the highly reactive lithium metal.
To overcome these problems, the team adopted an unusual design that employs two additional classes of solids, “electron and Li-ion insulators” (ELI) and “mixed ionic-electronic conductors” (MIEC), which are chemically stable in contact with lithium metal. These were built into a three-dimensional nanoarchitecture in the form of a honeycomb with an array of hexagonal tubes made from MIEC.
The tubes in the anode side are partially infused with the solid lithium metal and have enough extra space left so that when the lithium expands during charging it flows into the area inside the tubes. The lithium, therefore, flows like a liquid while retaining its solid crystalline structure, and the movement being wholly confined inside the honeycomb structure relieves the pressure from expansion caused by charging. Meanwhile, the electrode’s outer dimensions and the boundary between the electrode and electrolyte remains unchanged. The second solid material, the ELI, coats the walls of the tubes and acts as a binder between them and the solid electrolyte.
Leader of the team Ju Li, the Battelle Energy Alliance Professor of Nuclear Science and Engineering and MIT professor of materials science and engineering, said:
We designed this structure that gives us three-dimensional electrodes, like a honeycomb. The void spaces in each tube of the fabric allow the lithium to “creep backward” into the tunnels, 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, sort of like 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 is like an engine with 10 billion pistons, with lithium metal as the working fluid.
The solid-state battery remains chemically and mechanically stable as it cycles through the charging and discharging process thanks to the honeycomb-like structure being made of MIEC, which never loses electrical contact with lithium. The team even proved this by putting their device through a test of 100 charges. There wasn’t a single fracture in the solids. The research has been published in the journal Nature Energy.
Li expects the device can be scaled up very quickly. The materials need manganese, which is significantly cheaper than the cobalt or nickel used in conventional batteries. This means that the cathodes for the MIT solid-state battery would cost as little as a fifth of the price of current versions. The design also could lead to cellphones that weigh around a quarter of existing models and only need to be charged around twice a week.
