NANOscientific

Professor Dr. Christine Kranz Institute of Analytical and Bioanalytical Chemistry, Ulm University, Germany

Dr. Christine Kranz is spearheading innovative research in scanning electrochemical probe microscopy (SEPM), particularly within the realms of energy storage and conversion. A leading expert in the intricate dance of electrochemistry and nanotechnology, Dr. Kranz has adeptly combined atomic force microscopy (AFM) with scanning electrochemical microscopy (SECM) that yield not only high-resolution topographical detail but also vital electrochemical information. Her group's pioneering efforts in miniaturized amperometric sensor technology and FIB-based microfabrication have laid the groundwork for significant advancements in mapping lightdriven photocatalysis and post-lithium battery research, promising to revolutionize energy storage systems.

The Kranz research team aims to unravel the complex electrochemical processes at the electrode electrolyte interface that will contribute knowledge to enable more efficient, sustainable, and cost-effective energy storage solutions. Additionally, Dr. Kranz and her team members explore scanning electrochemical cell microscopy for screening experiments of the dynamic behavior of formed interphases.

Dr. Kranz, could you share some insights into your current research at the Institute of Analytical and Bioanalytical Chemistry (IABC), Ulm University? What are the main goals and objectives of your research team?

IABC has a strong interest in instrumental analysis and instrumental development, ranging from trace element analysis to infrared spectroscopy (using QCL-based IR sensors), molecularly imprinted polymers, and in situ surface (bio) analytics. My research team focuses on developing miniaturized sensors that can be used in combination with, for example, scanning electrochemical microscopy (SECM) or hybrid AFM-SECM for applications ranging from energy-related topics to bio(medical) research. Additionally, we are interested in AFM force spectroscopy and scanning electrochemical cell microscopy (SECCM) for surface modification, for instance, for single cell studies that include bacterial cell adhesion on antimicrobial films. Regarding energy-related topics, we are engaged in multi-PI projects such as the POLiS Cluster of Excellence and the Transregio CataLight, where our research is dedicated to local in situ activity studies in light-driven molecular photocatalysis and post-Li battery research.

From a broader perspective, what do you believe will be the most significant impact of sodium-ion battery technology on the energy sector in the coming years?

Sodium-ion batteries (SIBs) are a promising alternative to traditional lithium-ion batteries (LIBs), offering future benefits such as lower cost, higher safety, and more abundant raw materials. For example, with the recent rollout of the Yiwei Sehol-E10X and JMEV EV3 in China, EVs using SIBs are already available on the market, boasting a range of about 250 km. In the context of stationary grid storage, where battery weight is less critical and low-temperature operation is advantageous, SIBs could revolutionize energy storage solutions by storing renewable energy for use during peak demand periods at low cost. Therefore, SIBs are poised to have a significant impact in the near future. However, there are still challenges, such as lower gravimetric energy density and a not yet well-established supply chain for the materials. Despite more than 40 years of LIB research, a knowledge gap still exists for SIBs.

Can you share your thoughts on the interdisciplinary nature of battery research and development and how various scientific disciplines contribute to innovations in this field?

In my opinion, battery research is inherently interdisciplinary, as it spans the entire chain from developing new electrode materials and electrolytes to gaining fundamental insights into interface processes, as well as designing and producing battery cells, and crucially, analyzing sustainability and circularity. This requires a distinctly interdisciplinary approach. Materials science, physics, and chemistry are focused on the development and characterization of new materials for cathodes, anodes, and electrolytes. In contrast, engineering is dedicated to optimizing battery design and scale-up, ensuring durability and safety. Computer science contributes through simulation and analysis. Collaborations across these disciplines, integral to the POLiS Cluster of Excellence, are vital for the sustainable advancement of battery technology and for addressing interdisciplinary challenges in developing innovative energy storage concepts.

Looking at the evolution of energy storage, how do you anticipate the role of researchers and scientists changing as new battery technologies move from the lab to everyday applications?

Researchers are inherently flexible and able to adapt to new challenges, ensuring their work is dynamic and responsive to changing needs. Their efforts are crucial in driving the development, advancement, and integration of solutions to meet the increasing demand for sustainable energy.

Regarding your research, what inspired you to focus on the solid electrolyte interphase (SEI) in sodium-ion batteries, particularly on hard carbon (HC) composite electrodes?

The SEI layer is crucial in battery stability and performance. HC composite electrodes are mainly used as anode materials for SIBs due to their high capacity and stability. There is still a knowledge gap regarding the chemical composition and dynamics of the SEI in SIBs compared to LIBs. Therefore, it is an important aspect that sparks the interest of many research groups to understand the relevant parameters affecting the SEI and its stability during charging and discharging.

Can you discuss the benefits of ether-based electrolytes over carbonate-based ones for HC anodes in sodium-ion batteries?

Ether-based electrolytes are beneficial for their, lower viscosity, relatively low vapor pressure, and better chemical and thermal stability compared to carbonate-based electrolytes. The reduced viscosity enhances ion transport in the electrolyte, thereby boosting the rate capability and overall efficiency of SIBs. Improved electrochemical performance, such as increased coulombic efficiency and improved rate capability, has been demonstrated for HC anodes in ether-based electrolytes.

Your research includes spray-coating for HC composite electrodes. How does this technique impact the SEI compared to traditional methods?

The spray-coating technique in fabricating HC composite electrodes offers multiple advantages for laboratory-based fundamental studies compared to techniques like doctor blade coating. This method allows control over the material's thickness and the active material's mass loading on the current collector by adjusting the number of spray passes. Furthermore, it enables the creation of electrodes or coatings in various shapes needed for specific electrochemical cell applications, such as microcalorimetry or cantilever-based stress testing.

How does conductive atomic force microscopy (C-AFM) enhance the understanding of the SEI's conductivity?

C-AFM offers direct measurements of electrical conductivity at nanometer resolution, shedding light on the heterogeneity of the electrical conductivity of the SEI formed after cycling. Our C-AFM studies have examined various cycling regimes and electrolyte compositions, including additives. Comparing data from pristine and cycled samples gives important insights into the dynamic electrochemical phenomena within the SEI and their impact on conductivity. This knowledge could enable researchers to refine electrolyte formulations and create new materials to enhance battery performance and lifetime.

What unique insights have you gained from employing scanning electrochemical microscopy (SECM) alongside C-AFM?

Utilizing the Park NX10 allows for switching between C-AFM, EC-AFM, and AFM-SECM, offering the flexibility to conduct various experiments on the same sample and at the same sample positions. The SEI's primary function is to facilitate Na+ ion transport while blocking electron transport, thereby preventing further electrolyte decomposition and maintaining electrochemical reaction continuity. SECM can provide quantitative data on local electron transfer kinetics, revealing observed differences in the heterogeneous rate constant of electron transfer for pristine and cycled particles. Additionally, AFM-SECM experiments enable direct correlation of this data with the sample surface's morphological features. C-AFM is advantageous for visualizing electronic conductivity and assessing the homogeneity and electronic insulating properties of the formed interphases. In contrast, SECM yields quantitative data on electron transfer kinetics and insights into the heterogeneity and electrochemical properties of the formed layers. Analyzing the same sample both in situ (with AFM-SECM) and ex situ (using C-AFM and AFM force spectroscopy) offers a comprehensive view of the nanomechanical and electrochemical properties of the SEI.

You mention the impact of specific additives on the SEI's properties. How does this influence the battery's overall performance and longevity?

Electrolyte additives, particularly fluoroethylene carbonate (FEC), are widely utilized in lithium-ion and sodium-ion battery electrolytes to enhance performance and longevity. FEC aids in forming a stable and uniform SEI layer on the electrode surface, which helps suppress undesirable side reactions between the electrode materials and the electrolyte, thereby reducing electrolyte decomposition. This reduction in solvent decomposition enhances electrolyte stability, diminishes gas evolution, and prolongs battery life. Furthermore, FEC enhances ion transport through the SEI layer, contributing to the battery's stability and efficiency. However, it is important to note that for SIBs, findings from half-cell experiments using FEC might not directly apply to full-cell studies.

How do you foresee the role of scanning probe microscopy techniques in future research and the optimization of battery interfaces?

The field of battery research is rich with established microscopic and spectroscopic analytical techniques. However, there is growing interest in utilizing in situ scanning probe microscopy (SPM) techniques to examine interfacial processes, as traditional methods often lack spatial resolution. The high spatial resolution provided by SEPM and the ability to manipulate interfaces on the nano- to microscale, for instance, through SECCM, suggest that SPM studies may contribute to our understanding of battery interfaces. Such studies can elucidate key factors influencing battery performance, including charge transfer kinetics, ion diffusion pathways, and interface stability. It's important to note that studies on "real electrodes" might encounter artifacts, hence the potential need for model interfaces in fundamental research.

Thank you, Professor Dr. Kranz, for offering your insights and sharing your research with us.