New findings on space charge effects could improve efficiency
Researchers have conducted a detailed investigation of the space charge layer that forms inside a lithium battery. The experiments show, that a thin layer of positive and negative charges (green line) is primarily formed at plus pole. This impedes further charge transport. The finding was amongst others achieved by nuclear reacion analysis. This method uses nuclei like protons (red balls) which are shot into the space charge layer. The resulting products are analyzed.
© MPI for Polymer Research
- Solid-state batteries could store electricity more efficiently and safely in the future than today's batteries with liquid electrolytes.
- Space charges that form in solid-state batteries have so far impaired their performance.
- Space charges form primarily at the positive pole of the batteries. This provides a starting point for preventing their formation by modifying the structure or material of the electrode.
Whether in e-mobility or stationary storage, solid-state batteries promise greater storage capacity and safety. This is because they no longer use liquid electrolytes, but solid ones instead. This means they cannot leak, and the fire risk that is very low, but often discussed with today's batteries is virtually non-existent.
Resistance in the charge pump
Researchers at the Max Planck Institute for Polymer Research and universities in Japan have now opened up the possibility of making solid-state batteries more powerful with a study published in the journal ACS Nano. "A battery is a kind of pump," explains Rüdiger Berger, group leader at the Max Planck Institute for Polymer research. "Ions, or charged atoms, move inside the battery, which must be balanced on the outside by a flow of electrons and thus a flow of current." When the ions migrate in the battery, so called space charge layers can form at the internal interfaces in the battery. This repels the other migrating ions. This charge layer creates additional resistance and thus losses within the battery - it hinders both the charging and discharging processes. As the Mainz team has now discovered, the effect occurs mainly at the positive electrode, where a charge layer less than 50 nanometers thick forms - as thin as the thinnest part of a soap bubble. Furthermore, they quantitatively found the space charge layer is dynamic, that means it depends on the charging state of the battery. This space charge layer accounts for approximately 7 percent of the battery's total resistance, but could - depending on the materials used for the electrolyte - be much larger.
Until now, little was known about the size of this charge layer and the extent to which it impedes the flow of current. Various research teams around the globe have already investigated this effect in previous studies, but depending on the method used, they arrived at completely different estimates of the thickness of the charge layer.
Examination in quasi-operating mode
The international team around Berger therefore used two microscopic methods for the first time to investigate where and how the charge layer forms. The challenge was to examine the boundary layer of a model battery using microscopic methods in quasi-operating mode and at different states of charge.
To do this, they built a thin-film model battery and examined it using Kelvin probe force microscopy and nuclear reaction analysis. Using Kelvin probe force microscopy, they were able to scan the battery's cross-section - a cut-open battery, so to speak - with a fine needle and learn more about the local influence of voltage and observe electrical potentials in real time. With nuclear reaction analysis, they were able to detect the accumulation of lithium at the interface with the positive pole 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 that by modifying the material or structure of the electrode, they will find a way to reduce resistance and further increase the performance of solid-state batteries.
