RICHLAND (Washington) - Researchers have increased the life expectancy of a promising battery for electric vehicles to a record high. This is a significant step towards the goal of making future electric cars lighter, more affordable, and longer-lasting. Nature Energy published the results on June 28th.These batteries, which are the goal of research groups around the globe, are seen as an important part in the solution to climate change. Scientists are currently exploring a wide range of options.A lithium-metal electric vehicle battery is one solution. These batteries are twice as powerful as their lithium-ion counterparts and lighter. This combination makes it possible to create an electric vehicle that is lighter and can go further on one charge. However, lithium-metal batteries used in laboratory experiments have had a short life span and are susceptible to premature death.A team of scientists from the U.S. Department of Energy's Pacific Northwest National Laboratory have created a lithium-metal batteries that can last 600 cycles. This is far more than any other results. It can be fully charged 600 times before it goes dead.Although it's a significant step forward in a promising technology, lithium-metal technology has not yet been ready for primetime. Although the current lithium-ion batteries in electric cars hold less energy, they can last for more than 1,000 cycles. However, vehicles won't get as far with a single charge of lithium-ion batteries as they will with a more powerful lithium-metal battery.This new research was conducted by DOE's Innovation Center for Battery500 Consortium. It is a multi-institutional effort led PNNL to create lighter, more efficient, and cheaper electric vehicle batteries. PNNL is the leader of the consortium. It is responsible for integrating new developments from partner institutions into high-energy pouch cell devices and demonstrating improved performance in real world conditions.Lithium metal: Thin lithium strips mean a longer life expectancyThe PNNL team discovered a surprising way to extend the battery's life span. The team instead of using more lithium anodes, they used extremely thin lithium strips, 20 microns in width, which is far less than a human hair's width.Jie Xiao is the corresponding author. He was also Jun Liu's director at Battery500 Consortium. But this is not always the case. Depending on the cell energy and its design, each lithium-metal batteries has an optimal thickness.The Battery500 team created a lithium-metal battery with a 350 watt-hours of energy per kilogram (Wh/kg). This is not an unusual value, but it is still quite high. The battery's life expectancy is what makes the new findings more valuable. The battery retained 76% of its initial capacity after 600 cycles.Four years ago, a prototype lithium-metal battery was capable of operating for 50 cycles. This has rapidly increased; the PNNL team was able to achieve 200 cycles two years ago, and now they can do 600. Moreover, the PNNL's battery is a pouch-cell, which closely mimics real-world conditions better than a coin cell. This makes it a more realistic device for many battery research projects.Thickness is important in lithium metalThe team decided to test thinner lithium strips based on their detailed understanding of the anode's molecular dynamics as described in the Nature Energy paper.Scientists discovered that thicker strips directly contribute to battery failure. Complex reactions surrounding a film on anode called the solid electrolyteinterphase (SEI) are responsible for this. Side reactions between lithium and electrolyte result in the SEI. It is an important gatekeeper, allowing certain molecules to move from the anode into the electrolyte while keeping others at bay.It is a crucial job. A SEI that works effectively allows certain lithiumions to pass through, but it limits unwanted chemical reactions that can reduce battery performance or accelerate cell failure. Researchers have set out to minimize unwanted side reactions between lithium metal and electrolyte. This is in order to encourage important chemical reactions, while also limiting undesirable ones.Researchers found that thin lithium strips can create what one might consider good SEI. Thicker strips, on the other hand, have a greater chance of creating harmful SEI. The researchers used the terms "wet" and "dry" SEI in their paper. The wet version maintains contact between the anode and liquid electrolyte, making it possible to perform important electrochemical reactions.The liquid electrolyte does not reach all the lithium in the dry version. Because the lithium strips are thicker the electrolyte must flow into deeper pockets. As it does, other parts of the lithium become dry. This prevents important electrochemical reactions from taking place, which effectively stops them from happening. It also contributes to the battery's early death.This is a critical issue in real batteries such as pouch cells where the electrolyte amount available is 20-30 times lower than in experimental coin cells.If the frying pan is not thoroughly cleaned after each use, it will slowly build up grease. The layer becomes a barrier and reduces the flow of energy, making the surface less efficient. A dry SEI layer can also prevent the efficient transfer of energy inside a battery.Battery500 is a great resource for progressThanks to the Battery500 Consortium, significant progress has been made in lithium-metal battery development. The Battery500 Consortium aims to increase the energy density of long-lasting, safe and affordable batteries. A lighter vehicle can travel farther with one charge because it uses less energy per pound. The current electric vehicle battery levels are around 200-250 Wh/kg. Battery500 is striving for 500 Wh/kg."The Battery500 Consortium has made great strides in increasing the energy density, and extending the life of the cycle," stated Distinguished Professor M. Stanley Whittingham from Binghamton University. He is the 2019 Nobel Prize laureate for chemistry and a coauthor. There is still much to do. There are safety concerns with lithium-metal battery that need to be addressed. The Battery500 team is working hard on this issue.###DOE's Office of Energy Efficiency and Renewable Energy’s Vehicle Technologies Office funded the work. The majority of microscopy used to evaluate the battery was performed at EMSL (the Environmental Molecular Sciences Laboratory), a DOE Office of Science user facility at PNNL.Whittingham, Liu, Xiao, and Whittingham are also authors. They include PNNL scientists Joshua Lochala and Joshua Niu. Xiao (Jason) Zhang and Liu have also been appointed at the University of Washington.
