Imaging grain boundaries that impede lithium-ion migration in solid-state batteries

A NIMS research team has developed a new technique to image grain boundaries obstructing lithium-ion migration in solid-state batteries—a promising type of next-generation battery.

Solid-state batteries—next-generation rechargeable batteries—are intended to be safer and have higher energy densities than conventional lithium-ion batteries by replacing liquid organic electrolytes with solid electrolytes. A major issue in current solid-state battery R&D is the obstruction of lithium-ion migration at the interfaces between active materials and solid electrolytes and at the grain boundaries within solid electrolytes.

These obstructions lower charge/discharge rates and reduce energy density in batteries. A solid electrolyte is composed of crystalline grains and the boundaries between them. Existing ionic conductivity evaluation methods had only been able to measure average ionic conductivity across a solid electrolyte and were unable to quantify ionic conductivity at individual grain boundaries and identify boundaries restricting ionic migration.

This research team succeeded in imaging and quantifying ionic migration/diffusion at individual grain boundaries within a solid electrolyte using secondary ion mass spectrometry (SIMS). SIMS enables the imaging of chemical element distribution across a solid electrolyte specimen by sputtering the surface of the specimen with a focused primary ion beam and collecting and analyzing ejected secondary ions.

The team first replaced a portion of a stable lithium isotope, ⁷Li (mass number: 7, natural abundance: 92%), constituting an electrolyte specimen with another lithium isotope, ⁶Li (mass number: 6, natural abundance: 8%), at the edge of the specimen using an isotope exchange technique.

The team then observed the diffusion of ⁶Li within the specimen using SIMS. Because it was impossible to image and quantify the distribution of fast-diffusing ⁶Li using conventional SIMS, the team significantly slowed ⁶Li diffusion by cooling the specimen (i.e., cryo-SIMS), enabling the team to precisely measure the ⁶Li distribution and identify grain boundaries acting as bottlenecks to ionic migration.

The cryo-SIMS technique can be used to directly observe lithium-ion diffusion, identify interfaces/grain boundaries acting as bottlenecks among the many interfaces/boundaries existing in a solid-state battery, and determine the causes of these obstructions. This approach is expected to contribute to the development of higher-performance solid-state batteries.

The work is published in the Journal of Materials Chemistry A.