The main cause of failure in a state-of-the-art lithium metal battery, considered a prospect for use in long-range electric vehicles, has been identified.

Researchers from Stony Brook University, U.S. Pacific Northwest Laboratory, U.S. Idaho National Laboratory and U.S. Brookhaven National Laboratory pinpointed the main failure mechanism with high-energy X-rays that allowed them to follow the cycling-induced changes at points all across the battery, and to map the performance variations in thousands of these points.

A pouch cell battery is readied for testing. Source: Andrea Starr, U.S. Pacific Northwest National LaboratoryThe amount of cathode material and its local state of charge was calculated at each point. By combining these findings with complementary electrochemical measurements, the loss of battery capacity after multiple charge-discharge cycles was connected to depletion of the liquid electrolyte.

A group of universities and national labs known as the Battery500 Consortium has been working on next-generation lithium-metal anodes with three times the energy density of current automotive batteries. The challenge comes from lithium metal’s ability to work as an anode in a continually cycling rechargeable battery with high density because it is highly reactive and degrades with each battery cycle. The degradation reactions consume the liquid electrolyte, as well as other battery parts, over time.

The consortium researchers are working to increase the lifetime of high-energy-density lithium-metal anodes to 1,000 cycles or more to meet the needs of electric vehicles. To date, they have improved the number of cycles from 10 or less to 200 for the battery cell in the current study and to 400 cycles in 2020.

The researchers fabricated and tested lithium-metal batteries in a prototype pouch cell geometry with multiple layers, observing that the cell lost only 15% capacity in the first 170 cycles, but lost 75% over the next 25 cycles. Electrolyte depletion was cited as the failure mechanism most consistent with the synchrotron X-ray and electrochemistry data.

The synchrotron X-ray diffraction studies provided the amount of NMC (lithium nickel manganese cobalt oxide) present at each position on the cathode, revealing why failure occurred there. Regions with smaller amounts of NMC had the worst failure compared to the rest of the cell. The charge-discharge cycle occurs more frequently and completely when less of the NMC cathode is present, consuming electrolyte more rapidly and accelerating failure. Even small reductions in the cathode amount (5% or less) can accelerate failure. Improving manufacturing processes to produce more uniform cathodes should lead to longer-lasting batteries.