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A neutron-based mapping technique has been used to track the movement of lithium ions in real time inside a functioning all-solid-state battery.
Researchers at the Institut Laue-Langevin (ILL) in France discovered unexpected structural complexities, which could inform the design of safer, more efficient solid-state batteries.
The operando neutron powder diffraction technique was used to peer inside the battery interface. Interestingly, the experiment revealed a chaotic structure within the electrode, showing that lithium doesn’t flow through the battery nearly as smoothly as previously thought.
Peering inside a solid-state battery
Rechargeable batteries use liquid electrolytes that pose safety risks and limit performance. All-solid-state batteries aim to replace this liquid with a solid material for safer operation, higher energy density, and better performance under extreme temperatures.
However, making these batteries reliable is difficult because lithium ions often move unevenly through solid structures, causing different areas to charge at mismatched speeds. To solve this mystery, researchers used neutron diffraction for the first time to look deep inside a thick, functioning solid-state battery. It helped track lithium movement in real time to understand and optimize their operation.
The typical laboratory X-rays scatter off heavy electrons and miss light elements, such as lithium. But neutrons interact directly with atomic nuclei. This makes them perfectly tuned to detect lithium. Furthermore, neutrons possess a unique superpower: extreme penetration. Neutrons offer a non-destructive way to probe the entire bulk of a thick battery pellet, rather than just its surface.
This makes operando neutron powder diffraction an invaluable tool for tracking structural changes as a battery actually charges and discharges.
Heat to the rescue
To get a clean signal, the ILL team had to build a battery cell about 2.5 millimeters thick, which required 140 milligrams of active cathode material. Thicker components typically mean massive internal resistance, which kills electrical current.
A newly synthesized, high-conductivity solid electrolyte (a mixed-halide argyrodite) helped overcome this issue. It has the ionic conductivity six times that of conventional materials. Even with unusual thickness, the battery’s electrolyte still allowed more than half of the lithium to be successfully extracted.
What they saw changed our understanding of solid-state mechanics.
Even when charging at a snail’s pace, the lithium did not flow smoothly. The electrode split into two competing structural phases, dubbed H1 and H2. Different regions of the battery effectively lived in different time zones, charging at completely different rates because the electrical current was an issue.
But then, the temperature was raised. When the team repeated the experiment at 100°C, the chaotic two-phase behavior completely vanished. The heat supercharged the material’s ionic conductivity, smoothing out the current and forcing the lithium to march in perfect, uniform lockstep. Meanwhile, the solid electrolyte framework held perfectly fine, showing no signs of degradation.
The solid electrolyte remained structurally stable throughout the entire test, an encouraging sign for the long-term viability of sulfide-based all-solid-state batteries.
This discovery provides battery architects with a precise dial for performance. It has laid out a roadmap for the future of EV design that internal traffic jams within the electrode can be ironed out through targeted thermal management and optimized conductivity.
The study was published in the journal Advanced Energy Materials.
