A team of UC Riverside engineers has discovered why a key solid-state battery material stays remarkably cool during operation — a breakthrough that could help make the next generation of lithium batteries safer and more powerful.
The study, published in PRX Energy, focused on a ceramic material known as LLZTO — short for lithium lanthanum zirconium tantalum oxide. The substance is a promising solid electrolyte for solid-state rechargeable batteries, which could deliver higher energy density than today’s lithium-ion batteries while reducing overheating and fire risks.
Until now, scientists did not fully understand why LLZTO’s thermal conductivity — its ability to transfer heat — remains exceptionally low.
“It’s a material that stays thermally quiet, even as ions zip through it,” said Xi Chen, the study’s corresponding author and an associate professor of electrical and computer engineering at UCR’s Marlan and Rosemary Bourns College of Engineering.
“We reviewed the thermal properties of this material and explained why — at the atomic level — its thermal conductivity is low. This insight can help us predict temperature profiles inside batteries and improve thermal management, which means we can design safer batteries with higher energy density.”
When a battery charges or discharges, heat builds up. If that heat isn’t dissipated quickly, it can degrade performance, shorten lifespan, or, in extreme cases, cause thermal runaway — a dangerous chain reaction leading to fire or explosion. That’s why the federal Transportation Security Administration controls what kinds of batteries passengers may take onto commercial airplanes.
Understanding how LLZTO naturally impedes heat flow could be vital to picturing the temperature distribution and preventing safety problems, Chen said.
“For solid-state batteries, the electrolyte sits between the cathode and anode. Knowing how heat flows through that layer is essential,” he said. “We need batteries that can store more energy without getting dangerously hot. Our study gives insights into how to design materials that make that possible.”
To understand LLZTO’s unusual behavior, UCR graduate student Yitian Wang — first author of the paper — grew single crystals of the material using a floating-zone method. Unlike polycrystalline samples, which contain many tiny grains that scatter heat, single crystals are structurally pristine — revealing the material’s intrinsic properties.
The results surprised the team. Even without defects, LLZTO’s thermal conductivity was as low as 1.59 watts per meter-kelvin, which is nearly 250 times lower than that of copper.
“This tells us that the low thermal conductivity is built into the material itself,” Chen said.
By combining neutron scattering experiments at Oak Ridge National Laboratory with advanced simulations, the researchers traced the cause to the way atoms vibrate within the crystal lattice.
In solids like LLZTO, heat is carried by phonons — quantized vibrations of atoms. The team discovered two key factors that disrupt phonon movement and limit heat transport.
First, LLZTO contains many optical phonon modes — vibrations where atoms move out of sync with their neighbors. These optical vibrations interact with the main heat-carrying acoustic phonons, scattering them and impeding heat flow.
“When phonons scatter more, they don’t carry heat efficiently,” Wang said. “That’s why we see such low thermal conductivity.”
Second, LLZTO has a large anharmonicity, which quantifies how much the vibrations deviate from the ideal case. This property, which is linked to the motion of mobile ions within the material, suggests that traditional models of thermal transport may not fully apply to LLZTO.
“How thermal conductivity changes with temperature does not fit the phonon model.” Wang said. “New mechanisms might emerge in this case.”
The discovery gives researchers new tools to engineer materials that regulate heat at the atomic level, helping prevent failures in powerful, compact batteries.
“By linking lattice vibrations and ionic movement to thermal behavior, it is possible to design materials that not only conduct ions efficiently but also manage heat safely,” Chen said. “We’re looking at the big picture — how atomic-scale dynamics influence macroscopic behavior in energy systems.
“That’s the future of battery innovation.”
The study’s title is “Origin of Intrinsically Low Thermal Conductivity in a Garnet-Type Solid Electrolyte: Linking Lattice and Ionic Dynamics with Thermal Transport.” In addition to Chen and Wang, UCR co-authors are Yaokun Su, Youming Xu, Shucheng Guo, Qingan Cai, and Prof. Chen Li. Collaborators include Jesús Carrete and Georg K.H. Madsen from Vienna Technical University; Nan Wu, Yutao Li, Hongze Li, and Jiaming He from University of Texas, Austin; Huanyu Zhang, Kostiantyn V. Kravchyk, and Maksym V. Kovalenko from Swiss Federal Institute of Technology in Zurich; and Douglas L. Abernathy and Dr. Travis Williams from Oak Ridge National Laboratory.
