Abstract text (incl. figure legends and references)

Introduction

Secondary ion mass spectroscopy (SIMS) in combination with scanning electron microscopy (SEM), focused ion beam (FIB) and energy-dispersive X-ray spectroscopy (EDS) is a powerful tool for chemical imaging of various technical devices. For the development and optimization of new Lithium batteries FIB-ToF (time of flight)-SIMS is essential as Lithium is not detectable with conventional EDS detectors and only hardly detectable with windowless EDS detectors. The solid-state electrolyte [1] family based on Lithium-Lanthanum-Zirconium-Oxide (LLZO) garnet is a promising material for next generation solid-state batteries. This electrolyte offers higher energy density and safer operation compared to standard Lithium ion batteries. To understand the internal composition and the interface of the Lithium cathode we analyzed the lateral and depth distribution of Lithium inside a LLZO pellet after processing and cycling with different parameters.

Materials & methods

LLZO pellets with a diameter of 12mm and thickness of 1mm are in development at DLR institute for technical thermodynamics. For FIB-SEM and ToF-SIMS investigation we used a ZEISS Crossbeam 540 FIB-SEM microscope equipped with a Tofwerk SIMS detector. The SIMS detector uses an orthogonal ToF system therefore no complex trigger timing for the primary gallium ion beam is necessary. A transfer optics collects the secondary ions from the sample surface and transfers the ions to the ToF detector for mass discrimination. A mass range from 1 to 500 is measured. SIMS is an optimal addition to EDS because it offers high surface sensitivity, detection of Lithium, and detection of isotopes. A lateral resolution of better than 100nm is possible. LLZO pellets were mounted on SEM sample holders. SEM, EDS and FIB- SIMS analysis was performed on the surface and on cross section of the LLZO pellets.

Results

The combination of SEM imaging with EDS and FIB-SIMS imaging gives important analytical data regarding the distribution of the elements and impurities inside the LLZO ceramic. With SIMS we could clearly resolve the distribution of Lithium inside the solid state electrolyte with high depth resolution and high lateral resolution. In the case of aluminium doped LLZO we find inhomogeneous distribution of aluminium oxide in the volume of the pellet.

Conclusion

ToF-SIMS in combination with FIB/SEM and EDS is a powerful tool in the field of Lithium ion battery research as Lithium is detectable with very high sensitivity with SIMS. It is also possible to detect the oxidation of Lithium with high depth and lateral resolution. In this way it is possible to optimize ­the production of LIBs and Lithium metal batteries as well as understanding degradation processes while operating the battery.

References

[1] Lei Cheng et al., J. Mater. Chem. A, 2014, 2, 172-181

[2] https://www.tofwerk.com/products/fibtof.