First close-ups of how a lithium metal electrode ages

The same process that depletes your cell phone battery even when it is turned off is even more problematic for lithium metal batteries, which are being developed for the next generation of smaller and lighter electronic devices, long-range electric vehicles and others use it.

Now, scientists at Stanford University and the Department of Energy’s SLAC National Accelerator Laboratory have taken a first atomic-scale look at how this process, called “calendar aging,” attacks lithium metal anodes, or negative electrodes. They found that the nature of the battery’s electrolyte, which carries the charge between the electrodes, has a major impact on aging – a factor that needs to be taken into account when developing electrolytes that maximize battery performance.

The study also revealed that aging the calendar can drain 2 to 3 percent of the charge from a lithium metal battery in just 24 hours – a loss that would take three years on a lithium-ion battery. Although this charge infiltration decreases over time, it increases rapidly and can reduce battery life 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 that people haven’t really spent a lot of 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 long-range cars

Like today’s lithium ion batteries, lithium metal batteries use lithium ions to carry 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 much more energy for a given volume and weight. This is especially important for electric vehicles, which spend a significant amount of energy charging their heavy batteries. Lightening your load can reduce your costs and increase your autonomy, making them more attractive to consumers.

DOE’s Battery 500 Consortium, including SLAC and Stanford, aims to develop lithium metal batteries for electric vehicles that can store almost three times as much charge per unit weight as today’s EV batteries. While much progress has been made in increasing the energy density and life of these batteries, there is still a lot to do. They are also grappling with the problem of dendrites, finger-like protuberances on the anode that can short-circuit the battery and catch fire.

In recent years, Bao and Cui, who are researchers at the Stanford Institute for Materials and Energy Sciences at SLAC, have come together to find solutions to these problems, including a new coating to prevent the growth of dendrites on lithium metal anodes and a new electrolyte that also prevents the growth of dendrites.

Most of these studies have focused on minimizing damage from repeated charges and discharges, which deform and crack the electrodes and limit battery life, said David Boyle, a Ph.D. student in the Cui lab.

But in this study, he said, the team wanted to test a variety of electrolytes with different chemical compositions to get an overall picture of how lithium metal anodes age.

Aggressive corrosion

First, Boyle measured the charging efficiency of lithium metal batteries containing various types of electrolytes. Then he and his Ph.D. student William Huang carefully disassembled the batteries that had been fully charged and left there for a day, removed the anode and froze it in liquid nitrogen to preserve its structure and chemistry at a specific point of the calendar aging process.

Next, Huang examined the anodes with a cryogenic electron microscope, or cryo-EM, on the Stanford campus to see how the various electrolytes affected the anode on an almost atomic scale. It is a pioneering approach by Cui’s group a few years ago to examine the internal life of battery components.

In today’s lithium-ion batteries, the electrolyte corrodes the surface of the anode, creating a layer called the solid electrolyte interphase or SEI. This layer is both Jekyll and Hyde: it consumes a small amount of battery capacity, but it also protects the anode from future corrosion. Therefore, in balance, a smooth and stable SEI layer is good for battery operation.

But in lithium metal batteries, a thin layer of lithium metal is deposited on the surface of the anode each time the battery is charged, and this layer offers a new surface for corrosion during the aging of the calendar. In addition, “we found a much more aggressive growth of the SEI layer on these anodes due to more aggressive chemical reactions with the electrolyte,” said Huang.

Each electrolyte tested gave rise to a distinct SEI growth pattern, with some clumps forming, films or both, and these irregular growth patterns were associated with faster corrosion and a loss of charging efficiency.

Finding a balance

Contrary to expectations, electrolytes that would otherwise withstand highly efficient charging were just as likely to drop in efficiency due to the aging of the calendar as low-performance electrolytes, said Cui. There was no electrolytic chemistry that would do both.

Therefore, to minimize aging of the calendar, the challenge will be to minimize the corrosive nature of the electrolyte and the extent of the lithium metal on the anode surface that it can attack.

“What is really important is that it gives us a new way to investigate which electrolytes are most promising,” said Bao. “It points to a new electrolyte design criterion to achieve the parameters we need for the next generation of battery technology.”