Functional Electron Microscopy for Electrochemistry Research: From the Atomic to the Micro Scale

Objective: Despite recent global efforts to establish all-solid-state batteries (ASSB) as potentially safe and stable high-energy and high-speed storage technology, long-term performance, specific power, and economic viability remain challenging. However, efficient and short ionic and electronic transport pathways and optimized interfacial contacts between the cell materials and compounds are essential. This poses a significant challenge for the microstructure of cathodes and a careful design and tailoring of anodes and separators in ASSB, which rely solely on solid-state contacts. Here, we present our recent work on ASSB with thiophosphate electrolytes conducted within the German Competence Cluster for Solid-State Batteries (FestBatt). Our work aims to investigate relevant parameters in different cell components on material, electrode, and cell level, such as the importance of particle size distributions, microstructures, and kinetic factors to optimize ASSB cathodes, separators, and anodes. Methodology: The work will be presented in four sections, which showcase key challenges as well as optimization strategies mainly focusing on cathode composites, but also discussing selected examples for separators and anodes in ASSB with thiophosphate electrolytes. Further, an outlook for relevant production steps of ASSB electrodes and cells will be given. For these studies, we mainly conducted focused ion beam scanning electron microscopy tomography, electrochemical impedance spectroscopy, cycling experiments on ASSB cells with thiophosphate solid electrolytes. Results: Fast conducting solid electrolytes are necessary to ensure sufficient transport in composite cathodes but does not represent the only key factor for high-quality cathode composites in ASSB. In our studies, we aimed to optimize the microstructure of composite cathodes consisting of intercalation-type active material and a thiophosphate-based solid electrolyte. We varied the particle size distribution of the solid electrolyte to create various cathode microstructures and assessed the effectiveness of electronic and ionic conduction pathways using partial conductivities. We used both electronically and ionically blocking electrodes for electrochemical impedance spectroscopy measurements to quantify the respective partial conductivities. Our analysis revealed that the particle size of the solid electrolyte significantly influences the charge transport and electrochemical performance of the cathodes. Additionally, using FIB-SEM tomography, we created detailed 3D reconstructions of the cathode microstructure and correlated the obtained partial conductivities with microstructural descriptors such as tortuosity factors, identifying possible kinetic bottlenecks. We found that minimizing residual porosity, which blocks ion and electron transport, is necessary to optimize cathode microstructures and enhance ASSB performance. Moreover, high-performance anodes, along with protective concepts, are crucial for establishing dense, high-energy ASSB cells. Lithium metal anodes may not be the ultimate solution, and silicon anodes can pose advantages, e. g. in terms of electrochemical stability for thiophosphate separators. Discussion: Our studies demonstrate the importance of optimizing the microstructure of cathodes and carefully designing Si anodes for improved ASSB cells. By manipulating the particle size distribution of the solid electrolyte, we achieved more efficient electronic and ionic transport pathways, leading to improved battery performance. The presented work is primarily published in references 1-5, detailing key challenges and showcasing optimization strategies for these topics. Beyond that, diversity in materials, research teams, and approaches is crucial for long-term successful development of ASSB. Acknowledgement: Scientific and administrative coordination of the BMBF Competence Cluster for Solid-State Batteries (FestBatt), FB2-Koord (03XP0431), and cell platform Thiophosphates, FB2-Thio (03XP0430A).

Transport of electrons and ions at carbon surfaces immersed in electrolytes is instrumental for a wide variety of membrane processes as well as energy storage in batteries and supercapacitors. Ion transport in a nanoporous electrode strongly depends on its electronic conductance and on the interfacial capacitance with the electrolyte. In this study, we use in-plane impedance spectroscopy to disentangle in-plane ionic and electronic transport on a single crystal graphene transistor covered by an ionic liquid droplet (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, EMI-TFSI). droplet. Due to the atomic thickness of graphene combined to the strong affinity of EMI+ for carbon, this transistor maximizes ion-electron couplings. Using gate- and temperature-dependent in-plane spectroscopy, we extract both the electronic and ionic conductance of the transistor on a wide range of charge carrier density and over several decades of electrolyte conductivity. We show that despite an exceptionally high capacitive coupling at the carbon-EMI-TFSI interface, the ionic and electronic transport pathways are decoupled at the micrometric scale, in agreement with predicted lengthscales involved in the electronic-ionic interfacial transport.

3D MXene-holey graphene hydrogel for supercapacitor with superior energy storage

Long-range ion transport is composed of microscopic steps of ion migration, which is governed by local structures and interactions of the charge carriers with their surroundings. Therefore, understanding the local environments of conducting ions and their impact on ion dynamics and transport pathways are critical to design of high-performance solid electrolytes. We have synthesized and studied a variety of Li superionic conductors. Systematic solid-state nuclear magnetic resonance (NMR) investigations, complemented by pair distribution analysis (PDF) and DFT/MD simulations, have revealed that entropies in structures, compositions, and Li-coordination play a key role in determining ion hoping frequencies, jumping distance, transport pathways, and activation barriers. Via maximizing these entropies, “liquid-like” Li-sublattice is produced with a flattened energy landscape yielding extremely low activation barrier for ion transport. A few such examples will be discussed, including glass, ceramic, and glass-ceramic composites.

A vital aspect in energy storage planning and operation is to accurately model its operational cost, which mainly comes from the battery cell degradation. Battery degradation can be viewed as a complex material fatigue process that is based on stress cycles. Rainflow algorithm is a popular way for cycle identification in material fatigue process, and has been extensively used in battery degradation assessment. However, the rainflow algorithm does not have a closed form, which is the major difficulty to include it in optimization. In this paper, we prove the rainflow cycle-based cost is convex. Convexity enables the proposed degradation model to be incorporated in different battery optimization problems with a guarantee of the solution quality. We provide a subgradient algorithm to solve the problem. A case study on PJM regulation market demonstrates the effectiveness of the proposed degradation model in maximizing the battery operating profits as well as extending its lifetime.

Problem definition: Energy storage has become an indispensable part of power distribution systems, necessitating prudent investment decisions. We analyze an energy storage facility location problem and compare the benefits of centralized storage (adjacent to a central energy generation site) versus distributed storage (localized at demand sites). This problem encompasses optimizing storage capacities across all locations, with the objective of minimizing the total storage investment and energy generation costs. Methodology/results: We employ a stylized model that captures essential features of an energy distribution system, including convex costs, stochastic demand, storage efficiency, and line losses. Using dynamic programming, we optimize storage operations and derive value function properties that are key to analyzing the storage investment decisions. We discern fundamental differences between centralization/localization decisions at the capacity investment stage and the centralization/localization decisions at the storage operations level. Operationally, centrally stored energy offers more flexibility, which is consistent with the conventional understanding of inventory pooling. However, we find that localized storage often emerges as the preferred option at the investment stage under various circumstances. Managerial implications: Storage investment should first be made at the demand locations with positive minimum demand regardless of the level of demand variability. Subsequent storage investment should consider the tradeoffs between centralized versus localized investment. Operationally, the relative magnitudes of storage and line losses drive different optimal storage policies. Despite the differences, these policies are guided by common principles such as pooling inventory and balancing local storage levels. Supplemental Material: The online appendices are available at https://doi.org/10.1287/msom.2020.0652 .

Supercapacitors represent an important strategy for electrochemical energy storage, but are usually limited by relatively low energy density. Here we report a three-dimensional holey graphene framework with a hierarchical porous structure as a high-performance binder-free supercapacitor electrode. With large ion-accessible surface area, efficient electron and ion transport pathways as well as a high packing density, the holey graphene framework electrode can deliver a gravimetric capacitance of 298 F g(-1) and a volumetric capacitance of 212 F cm(-3) in organic electrolyte. Furthermore, we show that a fully packaged device stack can deliver gravimetric and volumetric energy densities of 35 Wh kg(-1) and 49 Wh l(-1), respectively, approaching those of lead acid batteries. The achievement of such high energy density bridges the gap between traditional supercapacitors and batteries, and can open up exciting opportunities for mobile power supply in diverse applications.

High-performance hybrid supercapacitor with 3D hierarchical porous flower-like layered double hydroxide grown on nickel foam as binder-free electrode

Energy devices are having an increasing technological impact on our society and have become an essential part of our daily lives. Printing technology has been used to manufacture a wide range of energy storage and conversion devices, including solar cells, supercapacitors, lithium-ion batteries, zinc-ion batteries, nanogenerators, etc. High-resolution screen-printing technology is widely used in the fabrication of commercial solar cells. The development of flexible energy devices is a key part of the realization of wearable electronics, and printing technology has unique advantages. In this chapter, the structure and mechanisms of conventional printed energy storage and conversion devices and manufacturing strategy are illustrated.

To meet the growing demand for energy storage, lithium-ion batteries (LIBs) with fast charging capabilities has emerged as a critical technology. The electrode materials affect the rate performance significantly. Organic electrodes with structural flexibility support fast lithium-ion transport and are considered promising candidates for fast-charging LIBs. However, it is a challenge to create organic electrodes that can cycle steadily and reach high energy density in a few minutes. To solve this issue, accelerating the transport of electrons and lithium ions in the electrode is the key. Here, it is demonstrated that a ferrocene-based polymer electrode (Fc-SO3Li) can be used as a fast-charging organic electrode for LIBs. Thanks to its molecular architecture, LIBs with Fc-SO3Li show exceptional cycling stability (99.99% capacity retention after 10 000 cycles) and reach an energy density of 183Wh kg-1 in 72 seconds. Moreover, the composite material through insitu polymerization with Fc-SO3Li and 50 wt % carbon nanotube (denoted as Fc-SO3Li-CNT50) achieved optimized electron and ion transport pathways. After 10000 cycles at a high current density of 50C, it delivered a high energy density of 304Wh kg-1. This study provides valuable insights into designing cathode materials for LIBs that combine high power and ultralong cycle life.

Over the last few decades several innovative ideas have been explored in the energy storage areas, ranging in size, capacity, design complexity, and targeted applications. Some of them are designed for large scale power system applications, others for smallor medium-scale renewable energy or hybrid power systems, while the others are designed to perform short-term energy storage ride through for critical infrastructure (communication systems, hospitals, military facilities, etc.). Energy storage has become an enabling technology for renewable energy applications, grid integration and enhancing power quality and stability in the power transmission and distribution, having a great potential to improve power grid quality and stability and to provide an alternative to fossil fuel-based energy generation. The major constraints for renewable energy penetration are the availability, intermittency, and variability, which can be addressed through energy storage. The energy storage choice depends on specific usage requirements, often incorporating several energy storage systems in order to increase system reliability, capacity, and supply security. In the electric power system, the renewable energy promise lies in its potential to increase grid efficiency, reliability, or in optimizing power flows and supporting variable power supplies. The parameters used in comparisons of various energy storage technologies include efficiency, energy capacity and density, run time, costs, system's response time, lifetime in years and cycles, self-discharge, and maturity of each energy storage technology. The most common energy storage technologies include compressed air, pumped hydro, batteries, fuel cells, flywheels, and super-capacitors. The last four are suitable for the medium scale applications. The chapter discussed energy storage technologies and gives an up to date comparative summary of their performances. After completing this chapter, the readers are able to understand the role, importance, configurations and topologies of energy storage systems, operation principles, characteristics, performances, and operation of major energy storage systems used in power systems, buildings, and industrial facilities. Another benefit is that readers are able to understand the critical role and necessity of energy storage systems in power and renewable energy systems, the differences between large-, mediumand small-scale energy storage systems, and how a system is selected on specific applications based on system characteristics and performances. Major energy storage technologies discussed in this chapter are compressed air energy storage, pumped hydropower storage systems, batteries, flywheels, hydrogen energy storage, fuel cells, supercapacitors, and superconducting energy storage systems. Thermal energy storage systems are covered in detail in the next chapter. This chapter provides comprehensive reviews of the energy storage technologies and gives an up to date comparative summary of their performances, characteristics, and applications.

Chemical vapor deposition (CVD) is a widely used technique for growing high‐quality carbon on metal surfaces such as Cu, Ni, and Si, particularly for energy storage applications. This work employs CVD to grow carbon on Ni foam, using iron chloride as a catalyst and acetylene as the carbon precursor at a temperature of 700 °C. The resulting carbon is thoroughly characterized using Raman spectroscopy, X‐ray diffraction, and transmission electron microscopy, which reveals the morphology as multiwall carbon nanotubes (MWCNT) anchored with iron nanoparticles. Due to their high specific surface area and conductivity, carbon‐based materials serve as excellent catalyst supports in energy conversion and storage devices, including fuel cells, supercapacitors, and batteries. The present work investigates Fe/MWCNT nanostructures as cathode materials for zinc‐air batteries. The Fe/MWCNT structures achieve a cycle life of 330 cycles (20 min each) at discharge and charge current densities of 5.2 mA cm−2, which is three times higher than bare carbon black and five times greater than reduced graphene oxide (rGO). This highlights the electrocatalytic role of Fe nanoparticles in enhancing the oxygen reduction and oxygen evolution reactions during Zn‐air battery operation. These findings emphasize the significant potential of CVD‐grown carbon materials in advancing next‐generation energy storage technologies.

Among the possible solutions for large-scale renewable energy storage, Power-to-Gas (P2G) and Compressed Air Energy Storage (CAES) appear very promising. In this work, P2G and an innovative type of CAES based on underwater storage volumes (UW-CAES) are compared from a techno-economic point of view, when applied in combination with a 48 MWe offshore wind power plant, selecting an appropriate location for both high productivity and favorable seabed depth. An optimization model is employed to study the system design and operation, maximizing the lifetime plant profitability, while considering differential installation and operation costs, market values of the products (i.e., hydrogen and electricity), and technological constraints. In the current economic and technological scenario, the resulting P2G system has a nominal power equivalent to about 10% of the wind park capacity, with a small hydrogen storage buffer. On the other hand, the compressor and the turbine of the UWCAES have a nominal power close to the full wind farm capacity, and large underwater compressed air tanks are required. Both options significantly impact the wind plant management but the most beneficial applications of the two systems are different: the P2G results in a compact and flexible unit, whereas the UW-CAES is able to exploit a higher average conversion efficiency (about 80% round-trip) against a much higher installed power and investment cost. Anyway, considering the current framework, the resulting economics are still inadequate, but their competitiveness can improve compatible with the expected evolution of energy markets in the next future.

Cost optimisation and life cycle analysis of SOEC based Power to Gas systems used for seasonal energy storage in decentral systems

The morphological structures, hierarchical porosity, heteroatom doping, and catalytically active centers of MOF-derived porous carbon as Se cathodes play essential roles in the electrochemical performances of Li–Se batteries. It is highly desirable to establish the structure–property relationship between the tunable architectures of MOF-derived porous carbons and their electrochemical performances in Li–Se batteries. In this study, nitrogen-doped porous carbon materials were derived from ZIF-8 precursors with precisely tuned morphologies by using particle-size adjustment and controlled tannic acid etching, which served as Se hosts for electrochemical analysis. Compared with the medium- and large-sized ZIF-8-derived porous carbon, the small-sized nitrogen-doped porous carbon (NPC) can effectively shorten ion and electron transport pathways and increase the fraction of surface-accessible active sites, which feasibly accelerates selenium conversion kinetics and enhances both capacity and cycling stability. Small-sized Se@NPC-S electrode possessed high reversible capacities of 670 mAh g–1 at 0.1 C and 446 mAh g–1 at 5 C. For the acid etched nitrogen-doped porous carbon (ENPC), the Se@ENPC-R2 cathode with moderate shell thickness exhibits a superior balance between capacity, rate capability, and long-term cycling stability which exceeded that of insufficiently etched and excessively etched counterparts. Se@ENPC-R2 delivers a reversible capacity of 480 mAh g–1 after 200 cycles at 0.1 C, and it sustains about 400 mAh g–1 after 800 cycles at 1 C. Electrochemical analyses reveal the improved Li+ diffusivity in Se@ENPC-R2, in which its moderate shell thickness can preserve continuous electronic pathways and provide sufficient internal space to accommodate volume variation and facilitate ion transport. These findings establish a clear correlation between the structural regulation and electrochemical behavior and suggest a practical route toward promising Li–Se batteries with a high discharge capacity, a superior rate capability, and excellent cycling stability built from MOF-derived porous carbons.