The same process that drains the battery of your cell phone even when it’s turned off is even more of a problem for lithium-metal batteries, which are being developed for the next generation of smaller, lighter electronic devices, far-ranging electric vehicles and other uses.
Now scientists at Stanford University and the Department of Energy’s SLAC National Accelerator Laboratory have taken the first atomic-scale look at how this process, called “calendar aging,” attacks lithium-metal anodes, or negative electrodes. They discovered that the nature of the battery electrolyte, which carries charge between the electrodes, has a big impact on aging – a factor that needs to be taken into account when developing electrolytes that maximize a battery’s performance.
The study also revealed that calendar aging can drain 2-3% of a lithium-metal battery’s charge in just 24 hours – a loss that would take three years in a lithium-ion battery. Although this charge seepage slows over time, it quickly adds up and can reduce the battery’s lifetime by 25%.
“Our work suggests that the electrolyte can make a big difference in the stability of stored batteries,” said SLAC and Stanford Professor Yi Cui, who led the study with Stanford Professor Zhenan Bao. “This is something people haven’t really spent time looking at or using as a way to understand what’s going on.”
The research team described their results in Nature Energy today.
Lighter batteries for far-ranging cars
Like today’s lithium-ion batteries, lithium-metal batteries use lithium ions to ferry charge back and forth between the electrodes. But where lithium-ion batteries have anodes made of graphite, lithium-metal batteries have anodes made of lithium metal, which is much lighter and has the potential to store a lot more energy for a given volume and weight. This is especially important for electric vehicles, which spend a significant amount of energy lugging their heavy batteries around. Lightening their load could drop their cost and increase their driving range, making them more appealing to consumers.
The DOE’s Battery 500 Consortium, including SLAC and Stanford, has a goal of developing lithium-metal batteries for electric vehicles that can store almost three times as much charge per unit weight as today’s EV batteries. While they’ve made a lot of progress in increasing the energy density and lifetime of these batteries, they still have a ways to go. They’re also wrestling with the problem of dendrites, finger-like growths on the anode that can make a battery short out and catch fire.
Over the past few years, Bao and Cui, who are investigators with the Stanford Institute for Materials and Energy Sciences at SLAC, have teamed up to find solutions to these problems, including a new coating to prevent dendrite growth on lithium-metal anodes and a new electrolyte that also keeps dendrites from growing.
Most such studies have focused on minimizing damage caused by repeated charging and discharging, which strains and cracks electrodes and limits the battery’s working lifetime, said David Boyle, a PhD student in Cui’s lab.
But in this study, he said, the team wanted to test a variety of electrolytes with different chemical makeups to get a general picture of how lithium-metal anodes age.