KAIST researchers identify 'false signals' that mimic ion movement in battery analysis

- Rough material surfaces found to generate measurement errors resembling ion movement

- Surface polishing eliminates errors, improving accuracy in solid-state and sodium battery analysis

KAIST researchers who conducted the study. From left: Professor Choi Nam-soon, doctoral candidate Jin Dong-yeon, and professors Hong Seung-bum and Yuk Jong-min. [KAIST]

Signals long interpreted as evidence of smooth ion movement inside batteries may in fact be optical illusions created by rough material surfaces, a new study has found.

KAIST announced Monday that a joint research team — led by professors Hong Seung-bum and Yuk Jong-min of the Department of Materials Science and Engineering and Professor Choi Nam-soon of the Department of Chemical and Biomolecular Engineering — had identified the source of "false signals" in nanoscale battery analysis that can be mistaken for actual ion movement, and developed a method to reduce them.

During charging and discharging, lithium or sodium ions travel through the interior of a battery. How quickly and smoothly those ions move is a key factor determining battery performance and lifespan.

One of the primary tools for observing this movement at the nanometer scale is electrochemical strain microscopy, or ESM. The technique uses the fine probe of an atomic force microscope to scan a material's surface, measuring tiny deformations that occur when ions move and thereby indirectly tracking ion migration.

The research team found, however, that when a battery material's surface is uneven, signals resembling ion movement can appear even when no ions are actually moving.

To pinpoint the cause, the team deliberately etched microscopic grooves into the surface of a single-crystal silicon sample — a material in which ions do not move. Experiments showed that variations in surface height alter the degree of contact between the microscope probe and the sample, and that this change alone can generate signals nearly identical to those produced by actual ion movement.

The same phenomenon was confirmed in real battery materials: graphite anodes and a sodium solid electrolyte (Na₂Zn₂TeO₆).

Particularly noteworthy are grain boundaries — the interfaces where small crystals within a battery material meet. In previous analyses, strong ESM signals at grain boundaries had led researchers to interpret them as fast-ion-transport pathways.

A schematic diagram showing trace-retrace discrepancies observed in DART-ESM measurements. [KAIST]

When the team smoothed the material surfaces using a cryogenic cross-section polisher with an argon ion beam, however, the strong signals at the grain boundaries disappeared. The finding suggests that some signals previously attributed to ion movement were in fact measurement errors produced by surface irregularities.

The research is expected to help clarify the operating principles of next-generation batteries — including all-solid-state and sodium-ion batteries as well as conventional lithium-ion cells. As accurate nanoscale data accumulates, the findings could also improve the reliability of AI and machine learning applications in new material design and performance prediction.

"We have clearly demonstrated how surface topography affects measurement results when analyzing battery materials at the nanoscale," Hong said. "This will contribute to a more accurate understanding of ion movement inside batteries and to the design of next-generation battery materials."

The findings were published in the international academic journal Small Methods.

nbgkoo@heraldcorp.com