As electricity flows through a battery, the materials inside the battery gradually wear away. Physical forces such as stress and strain also play a role in this process, but their exact impact on battery performance and life is not fully understood. A research team led by researchers at the U.S. Department of Energy's Oak Ridge National Laboratory has developed a framework for considering mechanics when designing solid-state batteries (SSBs). Their paper, published in the journal Science, reviews how these factors alter the cycling process of solid-state batteries.
"Our goal is to emphasize the importance of mechanics in battery performance," said Sergiy Kalnaus, a staff scientist in ORNL's Multiphysics Modeling and Flow Group. "Many studies focus on chemical or electrical properties but neglect to reveal the underlying mechanical properties."
The team spans multiple research areas at ORNL, including computing, chemistry and materials science. They conducted a comprehensive study of the various conditions affecting SSB from different scientific perspectives to paint a more cohesive picture. "We're trying to bridge the gap between disciplines," Kalnaus said.
Solid electrolytes: a safer, more robust alternative
In a battery, charged particles flow through a material called an electrolyte. Most electrolytes are liquids, as in lithium-ion batteries found in electric cars, but solid electrolytes are also being developed. These conductors are usually made of glass or ceramic, which offer advantages such as greater safety and strength.
"True solid-state batteries have no flammable liquids inside them," Kalnaus said. "This means they are less hazardous than batteries commonly used today."
However, solid-state electrolytes are still in the early stages of development due to the challenges faced by these new materials. Solid-state battery components expand and contract during charging and mass transfer, changing the system. The electrodes continue to deform during battery operation, creating delamination and voids at the interface with the solid electrolyte. "In today's system, the best solution is to apply a lot of pressure to keep everything together.
These dimensional changes can damage solid electrolytes because solid electrolytes are made of brittle materials. They often break under strain and pressure. If these materials could be made more ductile, they could withstand stress by flowing rather than cracking. This behavior is achieved through a number of techniques that introduce small crystalline defects in ceramic electrolytes.
Engineered Anodes and Solid Electrolytes
The electrons leave the system through the anode. In solid-state batteries, the anode can be made from pure lithium metal, which has the highest energy density. While this material has advantages in terms of battery power, it can also create stress that can damage the electrolyte.
"During the charging process, uneven plating and the absence of a stress relief mechanism can cause stress concentrations. These stress concentrations can create significant stress, causing lithium metal to flow," said Erik Herbert, leader of ORNL's Mechanical Properties and Mechanics Group. "To optimize the performance and lifetime of solid electrolyte separators, we need to design next-generation anodes and solid electrolytes that can maintain the mechanical stability of the interface without the solid electrolyte separator breaking."
The team's work is part of ORNL's long history of studying SSB materials. In the early 1990s, the lab developed a glass electrolyte called lithium phosphorus oxide, or LiPON. Lithium phosphorus oxide has been widely used as an electrolyte in thin-film batteries with metallic lithium anodes. This component can withstand multiple charge-discharge cycles without failure, largely due to LiPON's ductility. When exposed to mechanical stress, it will flow rather than crack.
"In recent years, we have learned that LiPON has strong mechanical properties that complement its chemical and electrochemical durability," said Nancy Dudney, an ORNL scientist who led the team that developed the material.
The team's efforts highlight an under-researched aspect of SSBs - understanding the factors that influence SSB longevity and efficacy. "The scientific community needs a road map," Kalnaus said. In our paper, we provide an overview of the material mechanics of solid-state electrolytes and encourage scientists to consider these factors when designing new batteries. "
Reference "Solid-state batteries: The key role of mechanics," by Sergiy Kalnaus, Nancy J. Dudney, Andrew S. Westover, Erik Herbert, and Steve Hackney, September 22, 2023, Science.
DOI:10.1126/science.abg5998
Compiled source: ScitechDaily