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Mechanically compressing the solid electrolyte in solid-state batteries can prevent lithium dendrites from growing vertically and causing short circuits, allowing the batteries to operate for thousands of charge cycles, a new study claims.
The study by a research team from the SLAC National Accelerator Laboratory and Stanford University provides direct evidence that lithium dendrites can originate from defects inside the solid electrolyte rather than only at its surface, resolving a long-standing debate in solid-state battery research.
Solid-state batteries are being developed as a potential alternative to conventional lithium-ion batteries because they could ultimately provide about twice the energy density of current batteries while improving reliability. However, their commercial development has been constrained by dendrites, narrow lithium metal intrusions that form in defects in the solid electrolyte during charging and can eventually reach the electrodes and cause a short circuit.
When a solid-state battery charges, lithium ions move from the positive electrode through a solid ceramic electrolyte towards the negative electrode. When the ions enter nanoscopic defects in the electrolyte, they can form lithium metal and develop into dendrites.
Researchers have struggled to determine where dendrites originate because they are thinner than tens of nanometres and can propagate within seconds.
The team found that applying mechanical compression to the solid electrolyte changed the direction in which dendrites propagated. A metal ring placed around the electrolyte prevented vertical dendrites, which can reach the electrodes and cause short circuits, from forming during charging.
Instead, dendrites propagated horizontally inside the electrolyte. Because these dendrites did not reach the electrodes, they did not immediately short-circuit the battery.
The batteries continued working for thousands of charge cycles despite the formation of many internal dendrites.
“We want to make reliable, energy-dense batteries that are fast charging,” said Teng Cui, who conducted the research as a postdoctoral researcher at Stanford University and was part of the SLAC Stanford Battery Centre team and is an assistant professor at the University of Waterloo in Canada. “This research shows us several of the steps that need to happen to make that possible.”
The researchers found that dendrites form on two timescales. Surface dendrites emerge quickly, while internal dendrites form later when surface dendrite growth is suppressed.
X-rays from SLAC’s Stanford Synchrotron Radiation Lightsource showed that internal dendrites formed at defects such as pores and grain boundary junctions without changing the bulk crystal structure of the electrolyte.
The finding indicates that dendrite initiation is linked to defects within the electrolyte, rather than being limited to its surface.
To test whether mechanical stress could control dendrite propagation, Cui and his colleagues used a shape memory alloy ring around the solid electrolyte. The ring changes shape when heated and was heated to 170 degrees Celsius, causing it to shrink and compress the battery.
Under compression, dendrites continued to form but propagated horizontally rather than vertically. The researchers said the unexpected durability of the compressed batteries could make mechanical compression a potential design feature for future solid-state batteries.
“We generated an unprecedented number of dendrites,” said Cui. “But the dendrites did not short the battery. It shows that there is this intimate relationship between mechanics and electrochemistry that could lead to new design strategies for batteries.”
The researchers identified two potential approaches to preventing internal dendrite formation. One is to develop solid electrolytes with extremely low electronic leakage, smooth surfaces and highly defect-free internal structures. The other is to design batteries in which the electrolyte remains under constant mechanical compression.
“These batteries could ultimately offer higher energy density and improved reliability,” said Wendy Gu, associate professor of mechanical engineering at Stanford University, who co-led the study with William Chueh, director of the SLAC Stanford Battery Centre and the Stanford Precourt Institute for Energy, professor of photon science at SLAC, and professor of materials science and engineering and energy science and engineering at Stanford University.
The next stage of the research will examine the interfaces between the cathode, anode and solid electrolyte, to improve contact between the materials to levels comparable with those achieved using liquid electrolytes.
“Many different areas of expertise came together- mechanics, electrochemistry and characterisation- to make this work possible,” said Chueh. “Insights synthesised in this work provide an actionable pathway to advance solutions for the energy storage grand challenge.”
The research team included scientists from Kyungpook National University in South Korea and Arizona State University. The work was supported by the US Department of Energy’s Office of Technology Transportation, Office of Critical Minerals and Energy Innovation, and Basic Energy Sciences, Office of Science. The Stanford Synchrotron Radiation Lightsource is an Office of Science user facility.