Scientists at the Oak Ridge National Laboratory of the Department of Energy became pioneers in the utilization of neutron reflectometry for studying the electrochemistry of a functional solid-state battery. Their research revealed that the exceptional performance capacity of these batteries can be traced back to the incredibly thin layer that allows charged lithium atoms to move briskly from the anode to the cathode and integrate into a solid electrolyte.
The main aim of this research is to develop superior batteries characterized by higher energy density, lower costs, increased safety and longevity, and quicker charging times. Rechargeable batteries typically depend on lithium, a small metal atom that fits snugly into the negatively charged anode to maximize energy density. However, its general instability with most electrolytes raises concerns about flammability, a common problem observed in smartphone, laptop, and electric vehicle batteries that use liquid electrolytes.
The proposed solution to this flammability issue is the transition towards solid electrolytes. One such solid electrolyte, known as lithium phosphorus oxynitride or LiPON, was created at ORNL nearly three decades ago. However, questions remain on why exactly it functions efficiently. A recent study uncovered that the performance of a solid-state battery relies heavily on the formation of a protective layer known as a solid electrolyte interphase (SEI). This component safeguards and stabilizes the battery, allowing it to charge and discharge repeatedly.
As a battery undergoes charge cycles, changes occur in the composition and thickness of the SEI, affecting the performance of the device. A well-established SEI layer greatly benefits the normal functioning of a battery, while a poor one can undermine its performance. For instance, the decreasing battery life of smartphones observed over the years is traceable to an expanding SEI layer that consumes the battery’s liquid electrolyte.
In a LiPON-based solid-state battery, however, an efficient and thin SEI layer forms to neutralize lithium, making it non-reactive. This characteristic feature of a solid-state battery prevents SEI growth, observed in traditional batteries. Advanced techniques combining neutron reflectometry with electrochemistry enabled the precise measurement of this interphase for the first time, with an observed thickness of around 7 nanometers.
The small scale of the interphase and the solid-state design prompted scholars to use neutron reflectometry to non-invasively probe the workings inside the battery. Neutrons’ weak interactions allow them to probe a point inside the battery without causing interference, providing valuable insight into the area of interest – the interphase, in this case.
The application of neutron reflectometry has propelled the understanding of the interphase between lithium metal and solid electrolytes in solid-state batteries. This pairing of techniques paves the way for advances in assessing a broad spectrum of solid-state electrolyte materials, discovering those that can spearhead the development of high-energy, fast-charge batteries. Future explorations of diverse types of solid electrolytes are already underway, with a focus on the invention of new materials with this much-needed stability.