New research on solid-state batteries could lead to longer-lasting batteries

The researchers hope to develop strategies to stop or at least restrict growth at the negative pole.
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Battery production line stock image.
Battery production line stock image.

John D/iStock 

Researchers at the Max Planck Institute for Polymer Research have focused on the life cycle of solid-state batteries, and their research could lead to longer-lasting batteries.

The so-called "solid-state batteries" are considered the "Holy Grail" of battery development. They no longer have a liquid core, like modern batteries, but rather a solid substance. This has various benefits, including the fact that these batteries may be produced on a smaller scale and are trickier to ignite. Rechargeable batteries are used in many commonplace gadgets, including cordless screwdrivers, cell phones, and electric cars.

The movement does, however, have certain drawbacks. For instance, some cell phones were prohibited from being brought on board airplanes, and e-cars have caught fire. Commercial lithium-ion batteries of the present day are delicate to mechanical stress.

Fully made of solid material and simple to miniaturize

So-called "solid-state batteries might offer a solution." They are fully made of solid material, such as ceramic ionic conductors, and no longer have a liquid core or what is known as the electrolyte. As a result, they are temperature-insensitive, non-flammable, and mechanically strong. They are also simple to miniaturize.

Nevertheless, solid-state batteries begin to experience issues after numerous cycles of charging and discharging: The internal battery processes of "Lithium dendrites" progressively growing in the battery cause the positive and negative poles to become electrically joined after initially being electrically isolated from one another. These lithium dendrites expand incrementally throughout each charging cycle until the two poles are joined. The battery shorts out and "dies" as a result. However, further research is still needed to understand the precise physical processes involved in this process better.

The issue has now been taken up by a team from Hans-Jürgen Butt's department led by Rüdiger Berger, who used a unique microscope technique to examine the processes in greater depth. They looked into the issue of where lithium dendrites begin to form. Is this similar to a flowstone cave, where stalagmites and stalactites eventually unite in the middle to form a so-called "stalagnate"? A battery has no top or bottom, but do dendrites grow from the negative to the positive pole or the other way around? Or do they sprout from either pole equally? Or are there specific locations within the battery that promote nucleation and subsequent dendritic growth?

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Paying attention to "grain limits"

The team led by Rüdiger Berger paid close attention to what is known as "grain limits" in the ceramic solid electrolyte. These boundaries are created during the solid layer's production: The atoms are consistently organized in ceramic crystals. Yet, if the atoms are organized erratically, line-like structures termed "grain boundaries" are generated due to tiny, random fluctuations in crystal development.

Their "Kelvin Probe Force Microscopy" technique, which scans the surface with a fine point, makes these grain boundaries visible. As the solid-state battery is charged, according to Chao Zhu, a Ph.D. student working with Rüdiger Berger, "the Kelvin Probe Force Microscopy detects that electrons collect along the grain boundaries - especially near the negative pole." The latter shows that the grain boundary alters the electrical structure and the arrangement of the ceramics' atoms.

Positively charged lithium ions moving through the solid electrolyte may be reduced into metallic lithium due to the buildup of electrons or negative particles. As a result, lithium deposits and lithium dendrites develop. The dendrite will keep growing if the charging procedure is continued until the battery's poles are finally joined. Only at the negative pole - and only at this pole - were these earliest phases of dendritic growth shown to form. At the positive pole opposite, no growth was seen.

The researchers hope that by clearly understanding the growth processes, they will be able to create efficient strategies to stop or at least restrict growth at the negative pole, paving the way for safer lithium solid-state batteries in broadband applications in the future.

The study was published in Nature Communications on March 9.

Study abstract:

The growth of lithium dendrites in inorganic solid electrolytes is an essential drawback that hinders the development of reliable all-solid-state lithium metal batteries. Generally, ex situ post mortem measurements of battery components show the presence of lithium dendrites at the grain boundaries of the solid electrolyte. However, the role of grain boundaries in the nucleation and dendritic growth of metallic lithium is not yet fully understood. Here, to shed light on these crucial aspects, we report the use of operando Kelvin probe force microscopy measurements to map locally time-dependent electric potential changes in the Li6.25Al0.25La3Zr2O12 garnet-type solid electrolyte. We find that the Galvani potential drops at grain boundaries near the lithium metal electrode during plating as a response to the preferential accumulation of electrons. Time-resolved electrostatic force microscopy measurements and quantitative analyses of lithium metal formed at the grain boundaries under electron beam irradiation support this finding. Based on these results, we propose a mechanistic model to explain the preferential growth of lithium dendrites at grain boundaries and their penetration in inorganic solid electrolytes.

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