Battery research plays a crucial role in enabling the successful transition to the use of renewable energy sources by enhancing energy storage capabilities. To optimize battery performance, it is necessary to explore different concepts, including the integration of solid electrolytes in lithium-ion batteries and the utilization of lithium metal anodes. The performance of a battery is fundamentally influenced by the processes occurring at interfaces, particularly the formation of interphases when different components come into contact, which significantly impacts the kinetics of lithium transport. Understanding and characterizing these interphases and interfaces are pivotal in unraveling the mechanisms underlying impedance growth and related phenomena. Thorough analysis and characterization of electrodes, electrolytes, and their interfaces are essential for gaining insights into interface behavior, potential degradation processes, and for improving cell performance. This challenging task requires techniques with high lateral resolution, sensitivity, and the ability to provide chemical information.

In our research, we employ secondary ion mass spectrometry (SIMS) and complementary X-ray photoelectron spectroscopy (XPS) to comprehensively characterize both the raw materials and various components of a battery cell. Our analysis encompasses surface analysis, conventional depth profiling, and surface imaging, enabling us to perform analytical 3D tomography. Additionally, we utilize focused ion beam (FIB) crater preparations to assess interfacial phenomena over larger areas.

By leveraging the capabilities of time-of-flight secondary ion mass spectrometry (ToF-SIMS), we have acquired valuable insights into chemical and structural information at the nanometer scale, surpassing the capabilities of other techniques. This progress brings us closer to our goal of improving our understanding of the fundamental mechanisms governing cell performance, degradation, and transport phenomena in lithium (ion) batteries. Such knowledge is crucial for further advancements in battery technology and the realization of efficient and sustainable energy storage solutions

Titanium dioxide (TiO₂) surfaces are extensively utilized in various applications, including dye-sensitized solar cells (DSSCs),¹ biomaterial,² and photocatalysis³ due to their cost-effectiveness, stability, and suitable electronic properties. More recently, in solar cell applications, the use of functionalized anatase has been reported.⁴ Among the numerous functional groups, carboxylic acid (-COOH) is one of the most common surface modification candidates widely used in DSSCs. However, one of the drawbacks of anatase TiO₂ is its relatively low reactivity towards carboxylic acid anchor groups compared to other metal oxides⁵. Since Ni, Co, and Cu have reportedly shown a higher affinity towards –COOH groups,^5,6 we exploit the intrinsic affinity of these metals in the form of bilayers to enhance titania's reactivity. Herein, we develop metal-phosphate bilayers containing nickel (Ni), cobalt (Co), copper (Cu) and manganese (Mn) synthesized on anatase TiO₂ compact oxide as these metals facilitate attachment to carboxylic acid-based compounds. The successful formation of these bilayers was confirmed through comprehensive characterization techniques, including Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and Cyclic Voltammetry/Electrochemical Impedance Spectroscopy (CV/EIS), demonstrating the effectiveness of the phosphate layer as an intermediate between TiO₂ and the other metal species.

Furthermore, we specifically investigate copper metal-phosphate bilayers for biomaterial applications, as copper has well-known antibacterial properties that can contribute to the prevention of bacterial growth and infection. Herein, we validate the successful reactivity of copper by investigating the TiO₂-P-Cu sample’s influence on enzymatic activity.

The findings provide valuable insights into developing reactive surfaces with potential applications in surface modification for application both in photocatalysis and the biomedical field, aiding in the prevention of potential pathogenic interactions.

Keywords: Anatase TiO₂, copper, ToF-SIMS, bi-phosphate bilayer, reactivity

References:

[1] Gong, J., Sumathy, K., Qiao, Q., & Zhou, Z. (2017). Review on dye-sensitized solar cells (DSSCs): Advanced techniques and research trends. Renewable and Sustainable Energy Reviews, 68, 234–246. https://doi.org/10.1016/J.RSER.2016.09.097

[2] Killian, M. S., & Schmuki, P. (2014). Influence of bioactive linker molecules on protein adsorption. Surface and Interface Analysis, 46(S1), 193–197. https://doi.org/10.1002/sia.5497

[3] Linsebigler, A. L., Lu, G., & Yates, J. T. (1995). Photocatalysis on TiO₂ Surfaces: Principles, Mechanisms, and Selected Results. Chemical Reviews, 95(3), 735–758. https://doi.org/10.1021/CR00035A013/ASSET/CR00035A013.FP.PNG_V03

[4] Monteiro, M. C. O., Schmuki, P., & Killian, M. S. (2017). Tuning Anatase Surface Reactivity toward Carboxylic Acid Anchor Groups. Langmuir, 33(49), 13913–13922. https://doi.org/10.1021/ACS.LANGMUIR.7B03044/SUPPL_FILE/LA7B03044_SI_001.PDF

[5] Monteiro, M. C. O., Cha, G., Schmuki, P., & Killian, M. S. (2018). Metal-Phosphate Bilayers for Anatase Surface Modification. ACS Applied Materials and Interfaces, 10(7), 6661–6672. https://doi.org/10.1021/acsami.7b16069

[6] Meychik, N., Nikolaeva, Y., Kushunina, M., & Yermakov, I. (n.d.). Are the carboxyl groups of pectin polymers the only metal-binding sites in plant cell walls? https://doi.org/10.1007/s11104-014-2111-z

Composite solid electrolytes (CSEs), consisting of polymer and ceramic electrolytes, have garnered significant attention as promising materials for advanced energy storage systems, owing to their improved mechanical stability and enhanced ionic conductivity. Understanding the diffusion pathways of lithium ions within CSEs is crucial for optimizing their performance and designing efficient energy storage devices. The in-situ investigation of lithium ion diffusion pathways in CSEs using Secondary Ion Mass Spectrometry (SIMS) imaging is a novel approach to understand the lithium ion transport process in composite electrolyte systems. In our case we studied the diffusion behavior of lithium ions in the composite electrolyte system of polyethylene oxide (PEO) with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as polymer electrolyte matrix, and lithium lanthanum zirconate (LLZO) as ceramic electrolyte filler.

To achieve this, we perform in-situ SIMS analysis utilizing a custom-designed sample holder that enables electrical contact to a potentiostat. This innovative setup allows us to directly observe and quantify lithium ion diffusion within the CSEs during electrochemical cycling. The symmetric electrochemical cell used in our experiments consists of ⁶Lithium metal as the reference electrode, the PEO:LiTFSI and LLZO composite solid electrolyte, and lithium metal as the counter electrode. By cycling the cell within the SIMS chamber, we can monitor the real-time lithium ion diffusion processes, providing valuable insights into their dynamic behavior within the CSE.

Cross sections of the investigated cell setup were created beforehand by an ion polishing system, establishing optimal conditions for the following in-situ ToF-SIMS imaging. Using the ⁶Lithium isotope, we map the distribution and quantify the diffusion of lithium ions within the CSEs. The obtained SIMS data reveals the evolution of lithium ion isotope concentration gradients, diffusion pathways, and interfacial reactions between the polymer and ceramic components during cycling. Our analysis highlights the significance of these findings in elucidating the underlying mechanisms governing lithium ion transport in CSEs.

Within the specific CSE system of PEO:LiTFSI and LLZO, we conduct a detailed analysis of the observed lithium ion diffusion behavior based on various factors. We study the influence of polymer-to-ceramic ratio, temperature and current density on lithium ion diffusion pathways within the composite electrolyte system. Our in-situ SIMS analysis provides valuable insights into the role of interfacial dynamics in the CSEs, aiding in the development of strategies to achieve enhanced ionic conductivity and stability.

Safety, cycle life and performance of next-generation batteries need a stable solid electrolyte interphase (SEI). Understanding the nature, properties, composition, and structure of the SEI is critical to stabilising the next-generation batteries. The SEI layer is vital for lithium during battery cycles, as lithium ions need to pass through the SEI layer before reaching the bulk electrode. Formation of the SEI layer has an advantage of inhibiting further electron transmission from the electrode toward electrolyte, so avoiding further decomposition of electrolyte and enhancing battery performance. A change in the composition and/or shape of the SEI layer dramatically alters cell performance. Despite its great importance, the SEI layer is poorly understood due to its complexity, non-uniformity, and very thin thickness.

To understand the SEI layer, it is important to correlate the standard materials data from techniques including secondary ion mass spectrometry (SIMS) and hard X-ray photoelectron spectroscopy (HAXPES).HAXPES provides quantitative analysis of standards with a depth range of up to 100 nm. SIMS is generally considered a semi-quantitative technique, however, in contrast to HAXPES, SIMS is very sensitive to lithium compounds and can obtain high-resolution 2D or 3D images of the sample. The use of massive gas cluster ion beams (GCIBs) enables few-nm depth resolution deeper than 10 μm. Therefore, HAXPES and SIMS complement each other in the study of SEI layer.

Here a study of standard metal oxides with SIMS and HAXPES and investigation of the SEI layer of lithium-ion batteries is presented. Analysis of standard materials are important in solid-state physics and the development of next-generation energy storage devices. Here we investigated the analysis of various metal oxides related to LiNi_xMn_yCo_zO₂ (NMC) used as cathode in lithium-ion batteries. In SIMS, these samples were analysed using 70 keV GCIB. The obtained spectrum depends on the chemical nature of the primary ion and its velocity [1]. The primary ions of (CO₂)_n and (H₂O)_n were found to increase the relative yields of metal oxides and metal hydroxides compared to Ar_n ions. HAXPES is a powerful technique to investigate the electronic behaviour of the transition metals (TMs) oxides. HAXPES with a gallium x-ray source can be accessible to the 1s orbitals of the first-row transition metals. Due to the absence of the multiplet splitting of the core level and the spin-orbit splitting of the 1s core electrons, its XPS spectra is sensitive to the chemical environment and to the non-local charge transfer screening. Therefore, XPS 1s spectra of TMs can be used to recognize the charge-transfer satellite feature and to discriminate local and non-local screening characters. This can be used to accurately determine the oxidation states of the transition metals.

[1] A.H. Alsaedi, A.S. Walton and N.P. Lockyer, Secondary Ion Mass Spectrometry analysis of metal oxides using 70 keV argon, carbon dioxide and water gas cluster ion beams, J. Vac. Sci. Technol. B (2023) [in press]

Research into lithium ion batteries (LIBs) for electric vehicles has been accelerated in the last decades. For the Ni-rich layered materials functioning as the cathode in LIBs, the high capacity and low cost make it a promising candidate for high-energy batteries while its poor thermal and chemical stability, unsatisfying cycling behavior, and sensitivity to ambient moisture hinder its further applications. Considerable attempts have been performed on improving the performance of the Ni-rich cathode. However, to enhance the performance and safety of LIBs, it is essential to understand the degradation processes occurring both within the electrode materials, as well as at the electrodes/electrolyte interfaces. Preliminary degradation studies have already been carried out on intragranular cracks [1], phase transition with oxygen evolution [2] etc. analysis but the local chemical degradation processes on the utmost surface, at interfaces and in the bulk materials are yet to be fully unfolded.

To obtain more chemical information on the interface of the electrolyte and electrode, burgeoning investigations with Time-of-flight Secondary Ion Mass Spectrometry (ToF-SIMS) have been reported on lithium ion batteries, due to its surface sensitivity and the capability of depth profiling. However, the complexity of the chemical environment of battery electrodes and the instrumentation fundamentals increase the barrier for understanding the authentic behavior. In this work, studies on the chemical degradation process at the interface of the high Ni content positive electrode material LiNi_0.8Co_0.1Mn_0.1O_2-d (NCM811) has been performed by ToF-SIMS together with different surface-sensitive techniques. Furthermore, a unique plasma focused-ion-beam secondary ion mass spectrometry (FIB-SIMS), Hi-5, with unprecedent sensitivity to low mass elements coupled to a lateral resolution of 30 nm and simultaneous detection of both positive and negative ion is applied for the first time. To further deconvolute the solid-electrolyte interface (SEI) chemistry low energy ion scattering (LEIS) with utmost surface sensitivity is employed. The chemical environment and roles of different elements in the degradation processes of the cathode materials will be investigated and discussed.

Reference

[1] Z. Xu et al., J. Mater. Chem. A, 6 (2018) 21859

[2] R. Jung et al., J. Electrochem. Soc., 164(7) (2017) A1361