Researchers at the Max Planck Institute for Polymer Research (MPI-P) in Germany, together with colleagues from Japanese universities, have characterized space charge effects in all-solid-state lithium batteries, a development that could boost future performance. Their findings, published in ACS Nano, focus on how ionic accumulation at electrode–electrolyte interfaces generates additional internal resistance during charge and discharge cycles.
Solid-state batteries replace flammable liquid electrolytes with solid materials, offering higher operating voltages, increased capacity, and enhanced safety. However, space charge layers—regions in which mobile ions are repelled or attracted near internal interfaces—have so far limited their efficiency. Until now, the spatial extent and contribution of these layers to overall battery resistance were poorly understood.
Using a thin-film model cell, the team applied operando Kelvin probe force microscopy (KPFM) and nuclear reaction analysis (NRA) to observe space charge formation in real time and quantify lithium enrichment at the positive electrode interface. KPFM scans revealed a charge layer less than 50 nanometers thick—comparable to the finest regions of a soap film—while NRA confirmed dynamic changes in lithium concentration as the battery cycled between states of charge.
Quantitative analysis showed that this nanoscale layer accounts for approximately 7% of total cell resistance, though its impact can vary with the specific solid electrolyte and electrode materials used. The researchers note that earlier studies reported widely differing estimates of space charge thickness, owing to methodological constraints. By combining two advanced microscopy techniques, the international team has provided a more consistent picture of this phenomenon.
To do this, they built a thin-film model battery and examined it using Kelvin probe force microscopy and nuclear reaction analysis. With the help of Kelvin probe force microscopy, they were able to scan the cross-section of the battery – a cut-open battery, so to speak – with a fine needle, learn more about the local influence of the voltage and observe electrical potentials in real time. Using Nuclear Reaction Analysis, they were able to detect the enrichment of lithium at the interface to the positive terminal of the battery.
“Both techniques are new in battery research and can also be used for other questions in the future,” explains Taro Hitosugi from the University of Tokyo. With further investigations, the researchers hope to find a way to reduce resistance and further increase the performance of solid-state batteries by modifying the material or structure of the electrode.
Source: Max Planck Institute for Polymer Research press release
