Advancements in Anode-Free Solid-State Batteries Bring New Possibilities for Energy Storage

Anode-Free Solid-State Batteries: Fundamental Insights Bring Them Closer to Practical Use

From laptops to electric vehicles, lithium-ion batteries are integral to our daily lives. However, as the demand for longer-lasting devices continues to grow, researchers are seeking more advanced battery technologies. A team led by Kelsey Hatzell, an associate professor of mechanical and aerospace engineering at the Andlinger Center for Energy and the Environment, has made significant strides in developing a new type of battery known as an anode-free solid-state battery, which could surpass the limitations of lithium-ion technology.

By investigating how these advanced batteries function and fail under various conditions, Hatzell’s research is paving the way for improvements in their performance and manufacturability. This progress is crucial for transitioning from laboratory research to real-world applications that support clean energy initiatives. Hatzell states, “If we can successfully introduce these up-and-coming batteries, we can access energy densities that are impossible with conventional batteries. It would mean that your laptop and your phone would last longer on a charge. It could allow electric vehicles to achieve over 500 miles per charge and move us towards ambitious goals like electrified aviation.”

This work is part of Hatzell’s role as the manufacturing leader for the Mechano-Chemical Understanding of Solid Ion Conductors (MUSIC) Energy Research Frontier Center, which focuses on advancing electrochemical energy storage systems. MUSIC, led by the University of Michigan at Ann Arbor, includes 16 faculty members from nine institutions, including Princeton University. Jeff Sakamoto, the director of MUSIC and a professor at the University of California-Santa Barbara, notes, “Solid-state batteries can revolutionize energy storage technology, but a significant challenge is developing a process for manufacturing them at scale. Hatzell’s work is crucial in improving the solid-state manufacturing process, showcasing how integrated research approaches can help overcome complex, multidisciplinary challenges.”

Batteries: A Look Under the Hood

Traditionally, batteries consist of two electrodes – a positive electrode (cathode) and a negative electrode (anode), each paired with a thin metal foil known as a current collector. These electrodes are separated by an electrolyte. The movement of ions between the electrodes generates power. During charging, ions flow from the positive electrode through the electrolyte to the negative electrode. During discharge, the process reverses.

The batteries studied by Hatzell and her team differ from standard lithium-ion batteries in two fundamental ways. Firstly, while lithium-ion batteries use a liquid electrolyte, solid-state batteries use a solid electrolyte. This distinction is crucial; solid-state batteries can store more energy in less space, enabling longer driving ranges for electric vehicles. They also perform better across a wider temperature range and offer greater durability compared to lithium-ion batteries. Secondly, the anode-free design of these batteries eliminates the negative electrode, allowing ions to flow directly from the positive cathode to the current collector. As the battery charges, ions plate onto the current collector, forming a thin metal layer. This design not only reduces costs but also makes the battery more compact, while avoiding the manufacturing challenges associated with lithium metal foils used in traditional solid-state batteries. Hatzell emphasizes, “If you could assemble a battery without a lithium metal anode, you would significantly cut costs while leveraging existing manufacturing processes. Both of these advantages are key to making a significant impact in the battery market.”

Cracking Under Pressure

Despite their promising potential, anode-free solid-state batteries encounter various practical challenges. A primary concern is ensuring effective contact between the solid electrolyte and the current collector, as this contact is critical for uniform ion deposition during charging and discharging. In a recent study published in ACS Energy Letters, Hatzell and first-author Se Hwan Park, a postdoctoral researcher, examined how factors such as applied pressure affect this contact.

Park explains, “During charging and discharging, the battery undergoes an electrochemical reaction. By applying external pressure, we’re also introducing mechanical forces. It’s a very complex system, with many interacting forces.” Unlike the liquid electrolytes in traditional batteries, solid electrolytes are rigid. Consequently, any surface defects or irregularities can impede contact quality, leading to uneven ion plating and stripping on the current collector. The team discovered that low pressures did not sufficiently improve contact, resulting in uneven ion deposition and the formation of voids, which ultimately caused short-circuiting. Conversely, high pressures improved contact but also intensified imperfections, leading to fractures.

Hatzell remarks that both failure modes provide valuable insights into optimizing the manufacturing and operation of anode-free solid-state batteries, stating, “The Holy Grail in this area will be to figure out how to maintain solid contact at low pressures, since manufacturing a defect-free electrolyte is practically impossible. If we want to realize the potential of these batteries, we have to solve the contact issue.”

A Silver Lining

While their findings underscored the importance of uniform contact, a second paper published in Advanced Energy Materials explored methods to achieve this. The researchers demonstrated that introducing a thin coating between the current collector and the electrolyte facilitated better ion transport, leading to more uniform ion plating and stripping. They tested various coatings, known as interlayers, to assess how their structure and composition influenced ion deposition.

The team found that interlayers composed of carbon and silver nanoparticles were the most effective in achieving uniform metal deposition. The silver in these interlayers formed alloys with ions during battery operations, promoting even plating and stripping. However, they also noted that the size of the silver nanoparticles was critical; larger particles led to uneven metal structures that reduced battery durability. In contrast, smaller nanoparticles resulted in denser, more stable structures, enhancing power output.

Park states, “Only a few groups have investigated the actual processes occurring in these interlayers. We demonstrated that the stability of these systems is linked to the morphology of the metal as it plates and strips from the current collector.” This understanding is pivotal for informing strategies to fabricate effective interlayers, ensuring strong contact and uniform plating at low pressures.

Charging into the Future

In addition to experimental research, Hatzell and her collaborators reviewed the current state of anode-free solid-state batteries in a paper published in Nature Materials. They summarized recent advancements and identified critical research gaps, particularly the need to scale successful laboratory techniques for integration into existing manufacturing processes.

Hatzell notes that there is renewed optimism in the battery sector, with countries like Japan and South Korea planning to commercialize solid-state batteries soon. For instance, Samsung aims to mass-produce solid-state batteries by 2027, while Toyota targets 2030 for mass production. Hatzell concludes, “The challenge will be transitioning from research to real-world applications in just a few years. Hopefully, the work we’re doing now at MUSIC can support the development and large-scale deployment of these next-generation batteries.”