Researchers propose a mechanics framework for solid-state batteries

A team led by researchers at the Department of Energy’s Oak Ridge National Laboratory (ORNL) has developed a framework for designing solid-state batteries (SSBs) with mechanics in mind. Their paper is published in Science.

One of the critical aspects of SSBs is the stress response of their microstructure to dimensional changes (strains) driven by mass transport. The compositional strains in cathode particles occur in liquid electrolyte batteries too, but in SSBs these strains lead to contact mechanics problems between expanding or contracting electrode particles and solid electrolyte.

On the anode side, plating of lithium metal creates its own complex stress state at the interface with the solid electrolyte. A critical feature of SSBs is that such plating can occur not only at the electrode–electrolyte interface but within the solid electrolyte itself, inside its pores or along the grain boundaries. Such confined lithium deposition creates areas with high hydrostatic stress capable of initiating fractures in the electrolyte.

Although the majority of failures in SSBs are driven by mechanics, most of the research has been dedicated to improving ion transport and electrochemical stability of electrolytes. As an attempt to bridge this gap, in this review we present a mechanics framework for SSBs and examine leading research in the field, focusing on the mechanisms by which stress is generated, prevented, and relieved.

—Kalnaus et al.

The team spans several ORNL research areas including computation, chemistry and materials science. Together, their review painted a more cohesive picture of the conditions that affect SSBs by using perspectives from across the scientific spectrum.

Solid electrolytes are typically made from glass or ceramic and could offer advantages such as enhanced safety and strength. However, solid electrolytes are still in the early stages of development due to the challenges associated with these novel materials. SSB components swell and shrink during charge and mass transport, which alters the system.

Electrodes constantly deform during the battery operation, creating delamination and voids at the interfaces with the solid electrolyte. In today’s systems, the best solution is applying a large amount of pressure to keep everything together.

—Sergiy Kalnaus, lead author

These dimensional changes damage solid electrolytes, which are made from brittle materials. They often break in response to strain and pressure. Making these materials more ductile would allow them to withstand stress by flowing instead of cracking. This behavior can be achieved with some techniques that introduce small crystal defects into ceramic electrolytes.

Electrons leave a system through anodes. In SSBs, this component can be made from pure lithium, which is the most energy dense metal. Although this material offers advantages for a battery’s power, it also creates pressure that can damage electrolytes.

During charging, nonuniform plating and an absence of stress-relief mechanisms can create stress concentrations. These can support large amounts of pressure, enabling the flow of lithium metal. In order to optimize the performance and longevity of SSBs, we need to engineer the next generation of anodes and solid electrolytes that can maintain mechanically stable interfaces without fracturing the solid electrolyte separator.

—Erik Herbert, the leader of ORNL’s Mechanical Properties and Mechanics group and co-author

SSBs offer a variety of multifunctional and safe solutions if important breakthroughs are made in engineering cell components and eliminating the need for tremendous external pressure to keep interfaces intact. Kalnaus et al.

The team’s work is part of ORNL’s long history of researching materials for SSBs. In the early 1990s, a glassy electrolyte known as lithium phosphorous oxynitride, or LiPON, was developed at the lab. LiPON has become widely used as an electrolyte in thin-film batteries that have a metallic lithium anode. This component can withstand many charge-discharge cycles without failure, largely due to the ductility of LiPON. When met with mechanical stressors, it flows instead of cracking.

In recent years we have learned that LiPON has robust mechanical properties to complement its chemical and electrochemical durability.

—Nancy Dudney, an ORNL scientist who led the team that developed the material

The team’s effort highlights an under-studied aspect of SSBs—understanding the factors that shape their lifespan and efficacy.

Several key challenges must be addressed, including (i) nonuniform lithium plating on a solid electrolyte surface and deposition of lithium metal within the solid electrolyte; (ii) loss of interfacial contact within the cell as a result of the volume changes associated with the electrochemical cycling that occurs at electrode contacts and also at grain boundaries; and (iii) manufacturing processes to form SSBs with a very thin solid electrolyte and a minimum of inactive components, including binders and structural supports. Mechanics is a common denominator connecting these problems.

Deposition of metallic lithium into the surface and volume defects of a ceramic solid electrolyte results in local high stresses that can lead to electrolyte fracture with further propagation of metallic lithium into the cracks. In manufacturing, as a minimum requirement, the cathode–electrolyte stacks should possess enough strength to withstand the forces applied by the equipment. A better understanding of the mechanics of SSB materials will transfer to the development of solid electrolytes, cathodes, anodes, and cell architectures, as well as battery packs designed to manage the stresses of battery manufacturing and operation.

—Kalnaus et al.

Resources

• Sergiy Kalnaus et al. (2023) “Solid-state batteries: The critical role of mechanics.” Science doi: 10.1126/science.abg5998