Why solid-state lithium batteries fail

Researchers from the University of Oxford have figured out how lithium solid-state batteries fail, an advance that could lead to the development of more efficient batteries for electric vehicles and aviation.

Scientists at the Faraday Institution in Oxford's Departments of Materials, Chemistry, and Engineering Science used advanced imaging techniques to determine the causes of failure in Li-SSBs, according to a study described in the June 7 edition of Nature.

Unlike conventional batteries, Li-SSBs replace the flammable liquid electrolyte with a solid one and use lithium metal as the anode. This enables better safety and more energy storage, innovations that could revolutionize industry.

"Progressing solid-state batteries with lithium metal anodes is one of the most important challenges facing the advancement of battery technologies," said Dominic Melvin, one of the co-lead authors of the study.

The team reported that the formation and growth of "dendrites" caused the batteries to short-circuit, insights that could help address technological hurdles in solid-state battery development. Dendrites are filaments of lithium metal that crack through the ceramic electrolyte in the batteries.

The researchers used a high-resolution imaging method to visualize batteries in unprecedented detail during charging, which revealed the mechanisms that cause lithium metal solid-state batteries to fail.

If these can be overcome, they said solid-state batteries using lithium metal anodes could deliver a step-change improvement in EV battery range, safety, and performance, and help advance electrically powered aviation.

"While lithium-ion batteries of today will continue to improve, research into solid-state batteries has the potential to be high-reward and a game-changer technology," said Melvin, a Ph.D. student in Oxford's Department of Materials.

Short-circuiting research

Li-SSBs are distinct from other batteries because they replace the flammable liquid electrolyte in conventional batteries with a solid electrolyte and use lithium metal as the anode (negative electrode). The use of the solid electrolyte improves the safety and use of lithium metal, which enables it to store more energy.

A critical challenge with Li-SSBs, however, is that they are prone to short circuit when charging due to the growth of dendrites, stalagmite-like structures that grow from the lithium metal anode as lithium ions flow from the cathode (positive electrode) during charging.

Current efforts to commercialize solid-state lithium batteries worldwide are failing and will continue to fail until the fundamental processes taking place in the devices are understood.

As part of the Faraday Institution's five-year SOLBAT project, the Oxford researchers have led a series of in-depth investigations to understand more about how this short-circuiting happens.

Considerable progress has been made during the first five years, and a recent round of funding to continue the research until 2025 will support a focus on developing a deep understanding of the materials' properties and mechanisms behind the premature short-circuiting and failure of solid-state batteries, a crucial step towards avoiding such events and realizing the commercial potential of this technology.

The new insights could help overcome the technical issues with solid-state batteries and unlock game-changing battery technology.

Two separate processes

In this latest study, the group used an advanced imaging technique called X-ray computed tomography at Diamond Light Source to visualize dendrite failure in unprecedented detail during the charging process. The new imaging revealed that the initiation and propagation of the dendrite cracks are separate processes, driven by distinct underlying mechanisms.

"Previous models of dendrite penetration have generally focused on a single process for dendrite initiation and propagation, with lithium driving the crack at its tip," wrote Ziyang Ning, lead co-author of the scientific paper titled, "Dendrite initiation and propagation in lithium metal solid-state batteries."

"Here we show that initiation and propagation are separate processes. Initiation arises from Li deposition into subsurface pores, by means of microcracks that connect the pores to the surface. Once filled, further charging builds pressure in the pores owing to the slow extrusion of (lithium) (viscoplastic flow) back to the surface, leading to cracking. By contrast, dendrite propagation occurs by wedge opening, with Li driving the dry crack from the rear, not the tip. Whereas initiation is determined by the local (microscopic) fracture strength at the grain boundaries, the pore size, pore population density and current density, propagation depends on the (macroscopic) fracture toughness of the ceramic, the length of the Li dendrite (filament) that partially occupies the dry crack, current density, stack pressure and the charge capacity accessed during each cycle. Lower stack pressures suppress propagation, markedly extending the number of cycles before short circuit in cells in which dendrites have initiated," the scientists wrote in the article's abstract.

Dendrite cracks initiate when lithium accumulates in sub-surface pores. When the pores become full, further charging of the battery increases the pressure, leading to cracking. In contrast, propagation occurs with lithium only partially filling the crack through a wedge-opening mechanism which drives the crack open from the rear.

Understanding points to solution

This new understanding points the way forward to overcoming the technological challenges of Li-SSBs, according to the scientists.

"For instance, while pressure at the lithium anode can be good to avoid gaps developing at the interface with the solid electrolyte on discharge, our results demonstrate that too much pressure can be detrimental, making dendrite propagation and short-circuit on charging more likely," Melvin explained.

Sir Peter Bruce, Wolfson Chair, Professor of Materials at Oxford and chief scientist of the Faraday Institution, said, "The process by which a soft metal such as lithium can penetrate a highly dense hard ceramic electrolyte has proved challenging to understand with many important contributions by excellent scientists around the world.

"We hope the additional insights we have gained will help the progress of solid-state battery research towards a practical device," added Wolfson, who is also a corresponding author of the study.

According to a recent report by the Faraday Institution, solid-state batteries may satisfy 50% of the global demand for batteries in consumer electronics, 30% in transportation, and over 10% in aircraft by 2040.

"SOLBAT researchers continue to develop a mechanistic understanding of solid-state battery failure, one hurdle that needs to be overcome before high-power batteries with commercially relevant performance could be realized for automotive applications," said Professor Pam Thomas, CEO of the Faraday Institution. "The project is informing strategies that cell manufacturers might use to avoid cell failure for this technology.

"This application-inspired research is a prime example of the type of scientific advances that the Faraday Institution was set up to drive," she added.