Scientists discover thin interphase key to solid-state battery success

Scientists at the Oak Ridge National Laboratory have made a breakthrough in solid-state battery technology by using neutron reflectometry to study the inner workings of a working battery. The researchers discovered that the battery’s exceptional performance is due to an extremely thin layer that facilitates the rapid flow of charged lithium atoms between the anode and cathode, blending into a solid electrolyte.

The team, led by Andrew Westover and James Browning, aims to develop better batteries with improved energy density, lower cost, faster and safer charging, and longer lifespan. To address the flammability issues associated with liquid electrolytes used in current batteries, the researchers are investigating the use of solid electrolytes.

One promising solid electrolyte is lithium phosphorus oxynitride (LiPON), which was invented at the Oak Ridge National Laboratory almost 30 years ago. However, the exact mechanisms that make LiPON effective have not been fully understood. To scale up LiPON technology, it is crucial to gain a comprehensive understanding of its functioning.

Previous studies have shown that the solid electrolyte interphase (SEI), a protective layer that forms in the battery, plays a vital role in its charging and discharging capabilities. In the case of LiPON-based solid-state batteries, the SEI consists of a lithium-rich layer that gradually transitions into pure LiPON, forming a chemical gradient.

The quality of the SEI determines the performance of the battery. A well-formed SEI ensures proper battery functioning, while a poorly formed one leads to capacity degradation over time, as observed in liquid-based batteries commonly used in cell phones. In contrast, the SEI layer in LiPON-based batteries forms a stable passivating layer that prevents further reactions, maintaining its stability and performance.

The researchers employed neutron reflectometry combined with electrochemistry to measure and examine the stable interphase between LiPON and lithium in solid-state batteries. They discovered that the interphase layer was only about 7 nanometers thick, equivalent to approximately 70 atoms. This finding demonstrates that it is possible to create thin interfaces in solid-state batteries, resulting in excellent performance.

The researchers chose to use neutron reflectometry because of the small scale and solid-state nature of the battery materials. Neutron reflectometry allowed them to non-destructively probe the battery at the desired scale, providing insights into the interphase without the need for invasive techniques.

The combination of neutron reflectometry and electrochemistry has significantly accelerated the understanding of the interphase between lithium metal and solid electrolytes in solid-state batteries. This powerful approach enables researchers to investigate stability, long-term cyclability, and other crucial factors affecting battery performance.

The team is now focusing on studying different types of solid electrolytes to expand their knowledge and identify materials that can enable the development of fast-charging, high-energy batteries. The invention of new materials with improved stability will be instrumental in designing future high-performance batteries.

Source: Oak Ridge National Laboratory

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