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First close-ups of how a lithium metal electrode ages

First close-ups of how a lithium metal electrode ages

Researchers at SLAC National Accelerator Laboratory and Stanford University have documented a process called calendar aging that can cause lithium metal batteries to lose 2-3% of their charge in just 24 hours, even when a device is turned off, cutting life by 25%. The study suggests that choosing the right electrolyte can minimize lumpy corrosion that causes aging. Credit: Greg Stewart / SLAC National Accelerator Laboratory

The same process that drains the battery of your mobile phone even when it is 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-reaching electric cars and other users.

Now, researchers at Stanford University and the Department of Energy̵

7;s SLAC National Accelerator Laboratory have taken the first atomic scale of 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 major impact on aging – a factor that must be taken into account when developing electrolytes that maximize battery performance.

The study also revealed that aging in the calendar can drain 2-3% of a lithium metal battery’s charge in just 24 hours – a loss that will take three years in a lithium ion battery. Although this charge is slow over time, it builds up quickly 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 people have not spent time looking at or using as a way to understand what is happening.”

The research group described the results in Natural energy Today.

Lighter batteries for far-reaching cars

Like today’s lithium-ion batteries, lithium-metal batteries use lithium ions to charge back and forth between the electrodes. However, where lithium-ion batteries have anodes made of graphite, lithium metal batteries have anodes made of lithium metal, which are much lighter and have the potential to store much more energy for a given volume and weight. This is especially important for electric cars, which use a significant amount of energy to lug their heavy batteries around. Lightening the load can reduce costs and increase the driving range, making them more appealing to consumers.

First close-ups of how a lithium metal electrode ages

The next generation of lithium metal batteries undergoes rapid aging in the calendar, which drains the battery even when not in use, and reduces the capacity to store energy. This occurs when the battery electrolyte corrodes lithium metal anode particles (top left) or lithium metal filaments that form on the anode during charging (top right). The corrosion creates irregular lumps of a layer called SEI (bottom left and right). A study by the SLAC National Accelerator Laboratory and Stanford University found that some electrolytes cause more of this harmful corrosion pattern than others. Credit: Hongxia Wang / Stanford University

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 of weight as current EV batteries. Although they have made great strides in increasing the energy density and life of these batteries, they still have a way to go. They also break with the problem of dendrites, finger-like growths on the anode that can cause a battery to short circuit and catch fire.

In recent years, Bao and Cui, researchers at 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 dendritic growth on lithium metal anodes and a new electrolyte. which also keeps dendrites from growing.

Most such studies have focused on minimizing damage caused by repeated charging and discharging, which charges and cracks electrodes and limits battery life, said David Boyle, Ph.D. student in Cuis lab.

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

Aggressive corrosion

First, Boyle measured the charging efficiency of lithium metal batteries that contain different types of electrolytes. So he and the doctoral student. Student William Huang carefully disassembled batteries that were fully charged and allowed to sit for a day, removed the anode and flared it frozen in liquid nitrogen to preserve the structure and chemistry at a specific point in the aging process in the calendar.

Huang then examined the anodes with a cryogenic electron microscope, or cryo-EM, on the Stanford campus to see how the different electrolytes affected the anode at near atomic scale. It’s an approach Cuis’s group pioneered a few years ago to look at the internal lives of battery components.

First close-ups of how a lithium metal electrode ages

Each time a lithium metal battery is charged, a new layer of lithium metal is deposited on the anode. The battery electrolyte corrodes this fresh metal, (left) and forms a layer called SEI on the anode surface. Corrosion continues (right) even after the battery is turned off, and the SEI layer becomes irregular and lumpy in a process called calendar aging that can drain 2-3% of the battery’s charge in just 24 hours, according to a study at Stanford University and SLAC National Accelerator Laboratory. These images were made with a cryogenic electron microscope, or cryo-EM. Credit: William Huang / Stanford University

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

However, 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 provides a fresh surface for corrosion during calendar aging. In addition, “We found much more aggressive growth of the SEI layer on these anodes due to more aggressive chemical reactions with the electrolyte,” Huang said.

Each electrolyte they tested gave rise to a distinctive pattern of SEI growth, with some forming lumps, films or both, and the irregular growth patterns were associated with faster corrosion and loss of charge efficiency.

Finding a balance

Contrary to expectations, electrolytes that would otherwise support high-efficiency charging were just as prone to declines in efficiency due to aging in the calendar as low-performance electrolytes, Cui said. There was no electrolyte chemistry that did both things well.

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

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

How short circuits in lithium metal batteries can be prevented

More information:
David T. Boyle et al. Corrosion of lithium metal anodes during aging of the calendar and its microscopic origin, Natural energy (2021). DOI: 10.1038 / s41560-021-00787-9

Provided by SLAC National Accelerator Laboratory

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