AFM Evaluation of Different-Sized Active Materials and Interface of All-Solid-State Lithium-Ion Batteries

Significance of the Topic

All-solid-state lithium-ion batteries (ASSLiB) offer enhanced safety, longer cycle life and higher energy density compared with conventional liquid-electrolyte systems. Reducing interfacial resistance between active materials and solid electrolytes is critical to unlocking their full potential in electric vehicles and grid storage.

Objectives and Study Overview

This study investigates the impact of TiO₂ negative-electrode particle size (150 nm vs. 1 µm) on interface quality and electrochemical performance. Employing high-resolution scanning probe microscopy (SPM/AFM) and Kelvin probe force microscopy (KPFM), the work aims to visualize interface morphology, conductive pathways and degradation after charge/discharge cycling.

Methodology and Used Instrumentation

Sample Preparation and Cell Assembly

• Negative electrode: TiO₂ (150 nm or 1 µm), lithium aluminum germanium phosphate (LAGP) solid electrolyte, acetylene black conductive additive
• Positive electrode: LiCoPO₄, LAGP, acetylene black
• Hot pressing step to improve electrode–electrolyte contact

Instrumentation

• Shimadzu scanning probe microscope (SPM/AFM) inside a flow-type glove box (dew point –80 °C, O₂ < 1 ppm)
• Kelvin probe force microscopy (KPFM) for surface potential mapping
• Cross-section preparation by epoxy embedding and ion milling

Main Results and Discussion

Particle Size and Morphology

• AFM measurements confirmed average TiO₂ sizes of ~160 nm and ~1 µm, matching nominal values

Interface Evaluation

• Without hot pressing: voids at negative-electrode/solid-electrolyte interface, increasing interfacial resistance
• With hot pressing: elimination of voids and formation of dense, well-bonded interface

Conductive Path and Degradation

• KPFM on 150 nm TiO₂ positive electrode showed uneven distribution of conductive additive and incomplete current paths
• Charge/discharge tests delivered only ~50% of theoretical capacity (150 mAh/g), indicating ion-transport limitations
• Post-cycle KPFM revealed residual charge at ~2.98 V rather than full discharge to 0 V, suggesting impaired ion conduction

Benefits and Practical Applications of the Method

• High-resolution SPM/AFM imaging allows direct observation of particle morphology and interface bonding quality
• KPFM mapping identifies active regions and potential degradation hotspots without destructive sectioning
• Hot pressing combined with microscopic evaluation informs electrode fabrication strategies to minimize interfacial voids

Future Trends and Potential Applications

Advances in nanoscale microscopy will enable:
• Real-time monitoring of solid-state interfaces during electrochemical cycling
• Automated quantitative analysis of conductive network homogeneity
• Integration of multi-modal imaging (e.g., AFM coupled with Raman or X-ray microscopy) for comprehensive interface characterization

Conclusion

• Particle size control and hot pressing significantly improve electrode–electrolyte interface density
• KPFM reveals uneven conductive additive distribution and highlights areas for dispersion optimization
• The combined AFM/KPFM approach captures degradation mechanisms, guiding design of high-performance ASSLiBs

References

1) E. Iida et al. SPM/AFM Evaluation of Interface of All-Solid-State Lithium-Ion Batteries, IVC-22, Sapporo, Japan (September 13, 2022)