Article

Characterization of cathode-electrolyte interface in all-solid-state batteries using AES, TOF-SIMS, XPS, and UPS/LEIPS

Surface Analysis Spotlight: XPS

by Jenny Mann XPS Scientist

All-solid-state batteries (ASSBs) are attracting attention for use in electric vehicles, commercial electronics, energy storage systems, and numerous other applications. However, they are still under development and there are several challenges that must be overcome before they can be expanded to commercial use. One of these challenges is that the charge-discharge cycling performance is limited by the internal resistance at the interface. In a recent series of papers from Ulvac-PHI published in the Journal of Vacuum Science and Technology B and Surface and Interface Analysis, a comprehensive characterization of the interface between the solid electrolyte and cathode was performed on a thin film ASSB sample with a metal lithium anode, LiCoO₂ cathode, and LiPON electrolyte. Several surface analysis techniques were used to investigate the internal resistance issue in ASSBs: Auger electron spectroscopy (AES), time-of-flight secondary ion mass spectrometry (TOF-SIMS), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and low-energy inverse photoemission spectroscopy (LEIPS).

A focused ion beam (FIB) on the PHI nanoTOF 3 was used to create a cross-section, and the exposed interface between the LiPON electrolyte and LiCoO₂ cathode was visualized using AES. In general, solid electrolytes are often sensitive to electron beam irradiation, making them very difficult to handle and characterize. Here, the high sensitivity analyzer on the PHI VersaProbe 4 enabled collection of AES chemical maps with a lower beam current and in a shorter amount of time than typical analysis, which reduced beam damage and preserved chemistry for successful imaging of the electrolyte-cathode interface (Figure 1).

Figure 1: Schematic of the 2.2 µm thick layered ASSB sample (left) and AES chemical map of the electrolyte and cathode layers (right).

The interface was then characterized by TOF-SIMS to track chemical speciation as a function of depth. Multiple species were identified by depth profiling through the thin film layer structure using a PHI nanoTOF II. The major species of interest in the film stack were Li4PO4+, Li3O+, and Co+. Li4PO4+ is a secondary ion generated from LiPON, while Li3O+ and Co+ secondary ions are observed in the LiCoO2 layer. A sharp increase in intensity of the Co+ species in the TOF-SIMS mass spectrum was observed in the LiCoO2 layer, dividing it into low- and high-intensity Co+ regions. Li3O+ was also observed in the LiCoO2 layer. The relative intensities between Li3O+ and Co+ are negatively correlated. Li3O+  is localized in the LiCoO2 layer near the interface with LiPON in the 2.2 µm thick LiPON sample, but not in the 100 nm thick sample. The TOF-SIMS depth profile of the 2.2 µm sample is shown in Figure 2. Note the increase of Li3O+ at the interface between LiPON and LiCoO2.

Figure 2: TOF-SIMS depth profile of 2.2 µm thick ASSB sample.

The lack of Li₃O⁺ species at the interface of the thinner sample indicates chemical/physical differences between the two samples when the LiPON thickness is changed. To further investigate this, XPS analysis with a PHI VersaProbe III was performed on the 100 nm sample before and after in-situ heating in the XPS instrument, to simulate the heating process necessary to create a 2.2 µm thick LiPON layer. The concentration ratio of N/P stayed essentially the same before and after heating, while the O/P ratio increased slightly upon heating. After heating, P-O-P bonding decreased while P=O bonding increased in the O 1s spectra shown in Figure 3. The increase in the O/P ratio suggests incorporation of oxygen into LiPON upon heating. Additional evidence for Co reduction is observed in the Co 2p_3/2 spectra.  A satellite peak indicative of Co³⁺ is observed in the sample before heating and is no longer present after heating, indicating a change in chemical state to Co⁰⁺.

Figure 3: Co 2p_3/2, O 1s, N 1s, P 2p and Li 1s XPS spectra of the surface of 100 nm ASSB sample (a) before heating, (b) after heating and (c) Co 2p_3/2, O 1s and Li 1s spectra of LiCoO₂ provided for reference.

The combination of TOF-SIMS and XPS results showed that differences in the chemical properties of the LiPON/LiCoO₂ of two different thicknesses of LiPON, 100 nm and 2.2 µm, were due to temperature rise during the manufacturing process.

Analysis of the energy band diagrams using a combination of UPS and LEIPS (Figure 4) indicated that electron diffusion from LiPON to LiCoO₂ might have triggered the reduction of Co. Moreover, the temperature rise likely facilitated the interaction between LiPON and LiCoO₂, resulting in a measured band gap for LiPON that was slightly lower than the theoretical band gap. It was concluded that suppressing the reduction of Co would be a crucial factor in minimizing the internal resistance at the interface.

Figure 4: Band gap diagram for the LiPON solid electrolyte and LiCoO₂ cathode calculated by UPS and LEIPS data.

The use of AES, TOF-SIMS, XPS, and UPS/LEIPS in this study demonstrated their effectiveness in providing detailed information on the characteristics of ASSBs. The combination of techniques was able to provide a comprehensive analysis of the solid electrolyte-cathode interface that would not be achievable with one technique alone. A similar analysis is also possible on beam-sensitive samples, a major challenge with Li-containing battery materials, using the PHI vacuum transfer vessel  that can transport samples between PHI instruments for multi-technique analysis on the same sample without exposure to air. Comprehensive surface analysis is a critical method for evaluating materials and further understanding the interactions between the solid electrolyte and electrode. This knowledge can contribute to the improvement of commercial production methods and the development of better batteries.

For more information on how surface analysis techniques can be used to study battery materials, please attend the upcoming CQLMNS conference, where Sarah Zaccarine, Ph.D., Staff Scientist is giving a talk entitled "Advances in XPS Analysis of Battery Materials".