Disentangling conduction pathways at the ionic–electronic interface in EMI-TFSI-covered graphene transistors

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).

Portable, reliable, and deployable devices for energy storage and conversion require fundamental changes in design of solid-state composites comprising ceramics and metals. These materials comprise the electrodes and electrolytes of next-generation solid-oxide fuel cells and all solid-state batteries, forming solid-state functional composites. The advent of solid-state batteries – which replace liquid electrolytes with solid electrolytes capable of lithium ion transport for reliable energy storage in portable batteries – and the increased demand space for all solid-state fuel cells capable of oxygen reduction at intermediate temperatures remain important challenges for improved material stability and decreased system cost. However, little is understood about three fundamental facets of materials that enable such solid-state energy applications. First, how do such materials deform, fracture, or delaminate under operando conditions? Second, how does such mechanical deformation limit or facilitate electronic and ionic transport within and across such material interfaces? Third, how we can predictably design interface-rich composites to engineer both structural and electrochemical stability? This COFFEI Group comprised expertise from Materials Science & Engineering and Nuclear Science & Engineering to integrate unique in situ experiments, simulations, and fabricated interfaces that address these fundamental questions in solid-state interfaces of nanoscale composites that will guide solid-state electrochemistry, transport kinetics, and mechanical deformation for nonstoichiometric materials that enable such applications. In particular, we built on COFFEI’s understanding of chemomechanical coupling among defect concentrations, ionic transport, electron transport, and stored elastic energy that is particularly acute in the far-from-equilibrium conditions typical of energy device applications. By tailoring our focus to solid-state interfaces, we addressed these important issues by (a) developing and applying advanced in situ and ex situ characterization tools to characterize model materials and interfaces synthesized with molecular-level control, under both laboratory-controlled and extreme environments representative of energy device operation; and (b) employing computational modeling and simulation frameworks to predict transport mechanisms, reactivity and stability of these model materials and interfaces under significant chemical strains typical of energy device operation. Recent progress provided insights to additional materials systems and electrochemomechanical fatigue and fracture that were not fully envisioned when the program was initiated. Specifically, in the final three years of COFFEI we pursued two integrated thrusts, with complementary focus. Thrust I focused on failure-resistant electrochemomechanical composites, while Thrust II focused on strain-modulated conductivity and reactivity across interfaces. In contrast to our initial COFFEI focus, these thrusts concentrated wholly on solid-state material interfacial interactions and included greater integration of multiscale visualization including in situ electron microscopy of strained structures/interactions and mesoscale simulations. Successful development of functionally superior and long-lived battery and fuel cell systems and stress adaptable oxides requires a deeper, fundamental understanding of the coupling among the historically important subfields of solid-state electrochemistry, transport kinetics, and mechanical deformation for nonstoichiometric metal oxide electrodes. In this program, the understanding and the application of chemomechanical coupling of defect concentrations, ionic transport, electro-catalytic activity and stored elastic energy, particularly acute in the far-from-equilibrium conditions typical of energy device applications, are being refined and implications for device operation clarified, including for miniaturized solid-state batteries and fuel cells.

Hydrogen fuel production from solar water splitting using an artificial photosynthetic device is the Holy Grail for sustainable energy generation. In the design of such a device, the development of new functional materials with high simultaneous transport of protons and electrons is an important quota of the work for an efficient energy system. In here, reduced graphene oxide (RGO)-Nafion nanocomposite membrane with both high proton and electron conductivity was successfully synthesized using a simple solution casting method followed by a hot press thermal reduction process at a temperature higher than the thermal stability of Nafion. Interestingly, in the hybrid material, the strong-coupling effects, exerted by the electron withdrawing groups of Nafion and the functional groups of RGO, have played an important role in the improvement of both electron and proton conductivity. The strong coupling interaction phenomena are proved by IR, Raman and X-ray photoelectron spectroscopy. The proton conductivity of the material increased by two order of magnitude (~30 times), from 7.30x10-3 for recast Nafion to 2.20x10-1 S cm-1 while the electron conductivity reached 2.20x10-2 S cm-1 with 5 wt.% of RGO/Nafion nanocomposite in wet condition. The electron conductivity of this same nanocomposite increased 5 times, from 4.40x10-3 for RGO to 2.15x10-2 S cm-1 at ambient condition. The presence of RGO in Nafion increases the water uptake by left oxygen-containing functional groups. The electron withdrawing groups of Nafion (-CF2 &-SO3H) are responsible for the increased electron conductivity of RGO. The proton conductivity of composite is further enhanced by the alignment or morphological change of the membrane’s proton channels during hot press thermal reduction. The proposed synthesis is a simple, environmentally benign and scalable process to produce Nafion/RGO nanocomposites without complex dispersion or chemical reduction processes.

A previously developed theory of particulate electron conduction enzymes was based on a model of an enzyme particle catalyzing the oxidation-reduction of two different substrates at two different enzymatic sites on the same particle with conduction of electrons between the two sites through the enzyme particle. Using the simplifying assumption that the percent reduction of the second substrate is held constant, there was previously shown to be a hyperbolic relationship between the first order rate constant (k′) and the sum (C x ) of oxidized plus reduced substrate, of the formk′=α/(C x +β), where α and β are positive constants. It is shown here that if this simplifying assumption is omitted, a positive constant is added to the right hand side of this equation, which describes exactly the experimental data of Smith and conrad on cytochrome oxidase. If electron transport is assumed to be coupled to ion transport, this equation becomesk′=(α/C x )−γ (where γ is a positive constant) which describes the experimental data of Eadie and Gale on pyruvic carboxylase of yeast. It seems probable that the same theory is applicable to coupled ion-ion transport and coupled electron-electron transport in both membranous systems, and in particulate preparations consisting of membrane fragments.

Rechargeable magnesium batteries (RMBs) have been considered a promising alternative to lithium‐ion batteries (LIBs) due to the high energy density and abundance of magnesium resources. However, the development of high‐performance cathode materials for magnesium batteries has been a significant challenge. Herein, through comprehensive and simple hydrothermal, selenization and impregnation methods, Cu‐doped CoSe2 nanoparticles encapsulated into carbon nanotubes (Cu‐CoSe2@NC) was fabricated. CoSe2 nanoparticles confined in carbon nanotubes (CNTs) growing in three dimensions on the surface of nanofibers have abundant active sites and high doping degree. Cu doping further improves the electron conductance of the Cu‐CoSe2@NC for RMBs. As cathode, Cu‐CoSe2@NC delivers a reversible capacity of 294 mAh g−1 at 20 mA g−1 and 104 mAg g−1 at 500 mA g−1, which exhibits a reasonable specific capacity and rate capability. The characterization of the Cu‐CoSe2@NC by ex‐situ transmission electron microscopy (TEM) after cycles shows that it can be well adapted to the (de)intercalation of magnesium ions. Density Functional Theory (DFT) calculation shows that CoSe2 band gap decreases obviously after Cu doping, favoring the electron and ion transport. This work provides a reference for the design of cathode materials based on transition metal selenide for RMBs.

The operation of batteries, fuel cells, and other energy storage and conversion devices is underpinned by the intricate synergy of ion transport, phase transformations, and electrochemical reactions. While electrochemical processes in well-defined molecular systems are amenable to classical chemical techniques including in situ optical spectroscopies and nuclear magnetic resonance (NMR), phase transformations and reactivity in solids and at solid interfaces remain largely an enigma. The presence of multiple types of structural defects and imperfections, competing electron and ion transport pathways, and multiple possible surface reconstruction types, combined with the directionality of ion flows and state of charge transformation fronts render these phenomena inaccessible to macroscopic interrogation techniques. At the same time, knowledge of the individual mechanisms involved in electrochemical reactivity of solids at the level of individual, atomic size structural elements is required for understanding the fundamental mechanisms of energy storage, factors controlling charge/discharge rates, degradation and failure processes, and ultimately knowledge-driven prediction and optimization of energy materials. 1-3

Exploring metal organic frameworks for energy storage in batteries and supercapacitors

BACKGROUND: The application potential of conducting polymers depends on their conductivity. It is generally assumed that the conductivity determined in the dry state is a parameter that unambiguously characterizes them.RESULTS: The conductivity of polyaniline (PANI) films immersed in aqueous solutions of sulfuric acid may be more than 1000 times higher compared with that obtained by measurement of dry films in air, and is estimated to reach a value exceeding 3300 S cm−1 in 1 mol L−1 sulfuric acid. This is explained by the reduction of conductivity barriers between conducting PANI islands.CONCLUSION: The organized polymer chains in the conducting islands of a PANI film are separated by disordered regions of low conductivity in the dry state. The penetration of sulfuric acid solution into the disordered areas increases the overall conductivity of the PANI film by improving the electrical contact between the islands through ionic charge transport. The electronic conductivity of the PANI film in the dry state thus converts to mixed electron–proton conduction in acidic aqueous solutions, electron conductivity being dominant in ordered regions and ionic conductivity in disordered regions separating them. Weakly bound protons are the most important ionic charge carriers hopping along the PANI chains. Copyright © 2009 Society of Chemical Industry

Solar cells hold a function of photovoltaic conversion, while rechargeable metal batteries have an advantage of high energy storage. The conventional charge mode of batteries is made based on complete utilization of electric energy. The combination of solar cells and rechargeable metal batteries brings a new opportunity for the development of photo‐assisted rechargeable batteries, in which the solar energy can be utilized to partially achieve photo‐charging with or without external electrical bias. This review highlights the working mechanism and structure design of photo‐assisted rechargeable metal batteries according to the characteristics of rechargeable metal batteries and advantage of the photovoltaic technology. In particular, the recent advances are introduced for photo‐assisted rechargeable batteries based on light‐weight metal anodes, including metal lithium, metal sodium, and metal zinc. The working features of the integrated devices are also discussed for energy saving under photo‐assisted charging mode. Finally, a future outlook is provided for further improving the performance of photo‐assisted rechargeable metal batteries.

NiFe-based nanomaterials are extensively studied as one of the promising candidates for the oxygen evolution reaction (OER). However, their practical application is still largely impeded by the unsatisfied activity and poor durability caused by the severe leaching of active species. Herein, a rapid and facile combustion method is developed to synthesize the vertical graphene (VG) supported N-doped carbon modified (NixFe1-x)Se composites (NC@(NixFe1-x)Se/VG). The interconnected heterostructure of obtained materials plays a vital role in boosting the catalytic performance, offering rich active sites and convenient pathways for rapid electron and ion transport. The incorporation of Se into NiFe facilitates the formation of active species via in situ surface reconstruction. According to density functional theory (DFT) calculations, the in situ formation of a Ni0.75Fe0.25Se/Ni0.75Fe0.25OOH layer significantly enhances the catalytic activity of NC@(NixFe1-x)Se/VG. Furthermore, the surface-adsorbed selenoxide species contribute to the stabilization of the catalytic active phase and increase the overall stability. The obtained NC@(NixFe1-x)Se/VG exhibits a low overpotential of 220mV at 20mAcm-2 and long-term stability over 300h. This work offers a novel perspective on the design and fabrication of OER electrocatalysts with high activity and stability.

Our work’s objective is to improve the performance of advanced battery materials through the use of structurally engineered electrodes that are designed to exploit simultaneously the benefits of both the nano- and micro- length scales. Consequently, a three- dimensional fabrication approach has been developed to enable hierarchical control over electrode structures and surface areas. Potential advantages include improved pathways for electron and ion transport, enhanced mechanical stability, and additional control over interfacial areas and compositions for improved stability and performance. Silicon is an advanced material with promise for use in next generation batteries. Among anode materials, silicon stands out because of its potential for extremely high capacities – more than ten times the specific capacity of the commercial graphite anodes. Two issues that negatively impact the usable capacity and performance of Si anodes are i) large volume changes during charging/discharging and ii) the formation of an unstable solid-electrolyte interphase (SEI). The use of nanostructured electrodes has been effective in accommodating the large volume changes that occur during cycling [1, 2]. However, nanostructuring leads to a very high surface area, which exacerbates problems associated with SEI formation and stability. This study seeks to improve the performance of Si anodes through the use of hierarchically structured electrodes to provide the nanoscale framework needed to accommodate large volume changes while controlling the interfacial area – which affects SEI formation. To accomplish this, electrodes will be fabricated from vertically aligned carbon nanotubes (VACNT) infiltrated with silicon. On the nanoscale, these electrodes allow us to adjust the surface area, porosity, and silicon layer thickness. On the microscale, we have the ability to control the electrode thickness and the incorporation of micro-sized features. For instance, microscopic patterning of the CNT matrix to form vertical holes, as shown in Figure 1, has the potential to impact transport through the electrode – as well as its interfacial area and mechanical stability. Treatment of the surface between the nano- and micro-structured areas can be used to control the size and chemistry of the interface between the electrolyte and the electrode matrix. VACNT-Si composite electrodes were prepared by first synthesizing VACNTs on Si wafers using photolithography for catalyst patterning, followed by aligned CNT growth. Nano-layers of silicon were then deposited on the aligned carbon nanotubes via LPCVD at 200mTorr and 570°C. Figure 2 shows a cross-sectional SEM image of a synthesized VACNT-Si array. A thin copper film was used as the current collector. Electrochemical testing was performed on the electrodes assembled in a CR2025 coin cell with a metallic Li foil as the counter electrode. The impact of the electrode structure on the capacity at various C-rates was investigated. Experiment results demonstrate the influence of structurally engineered electrodes on the performance of silicon electrodes for next generation batteries. References C. K. Chan, H. Peng, G. Liu, K. McIlwrath, X. F. Zhang, R. A. Huggins, and Y. Cui, Nat Nanotechnol,3 (2008), 31-35.K. Evanoff, J. Khan, A. A. Balandin, A. Magasinski, W. J. Ready, T. F. Fuller, and G. Yushin, Adv Mater, 24 (2012), 533-540.

Highly ordered TiO2@α-Fe2O3 core/shell arrays on carbon textiles (TFAs) have been fabricated by a stepwise, seed-assisted, hydrothermal approach and further investigated as the anode materials for Li-ion batteries (LIBs). This composite TFA anode exhibits superior high-rate capability and outstanding cycling performance. The specific capacity of the TFAs is much higher than that of pristine carbon textiles (CTs) and TiO2 nanorod arrays on carbon textiles (TRAs), indicating a positive synergistic effect of the material and structural hybridization on the enhancement of the electrochemical properties. This composite nanostructure not only provides large interfacial area for lithium insertion/extraction but should also be beneficial in reducing the diffusion pathways for electronic and ionic transport, leading to the improved capacity retention on cycling even at high discharge–charge rates. It is worth emphasizing that the CT substrates also present many potential virtues for LIBs as flexible electronic devices owing to the stretchable, lightweight and biodegradable properties. The fabrication strategy presented here is facile, cost-effective, and scalable, which opens new avenues for the design of optimal composite electrode materials for high performance LIBs.

Mixed ionic-electronic conductors have great potential as materials for energy storage applications. However, despite their promising properties, only a handful of metal-organic frameworks (MOFs) provide efficient pathways for both ion and electron transport. This work reports a proton-electron dual-conductive MOF based on tetrathiafulvalene(TTF)-phosphonate linkers and lanthanum ions. The formation of regular, partially oxidized TTF stacks with short S···S interactions facilitates electron transport via a hopping mechanism, reporting a room-temperature conductivity of 7.2 × 10-6 S cm-1. Additionally, the material exhibits a proton conductivity of 4.9 × 10-5 S cm-1 at 95% relative humidity conditions due to the presence of free -POH groups, enabling efficient proton transport pathways. These results demonstrate the potential of integrating electroactive building blocks along with phosphonate groups toward the development of mixed ionic-electronic conductors.

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.

Due to unreliability of power supply from national grid, high cost of diesel and maintenance of generators being an alternate for national grid, there is need to adopt a renewable energy source for efficient and reliable power supply. Savings associated with conversion of a stand-alone diesel generator powered system to a reliable PV/Diesel hybrid power system with energy storage in batteries for Bayero University Kano Library using HOMER software was presented in this study. The economic parameter of merit used in selecting an optimum energy system from set of configurations was total Net Present Cost (NPC) in billions of naira. Generator + PV + Battery system has a total NPC of (₦ 1.17B) and saves (₦ 0.9B), Generator + PV system has a total NPC of (₦ 1.68B) and saves (₦0.4B), Generator + Battery system has a total NPC of (₦ 2.05B) and saves (₦ 0.03B), PV + Battery system has a total NPC of (₦ 2.28B) and saves (₦ - 0.2B) when all compared with Generator-only system (₦ 2.08B). It can be seen that the best case is the PV + Gen + Battery system. Even though it has the highest capital cost, but it resulted to the least net present cost (NPC); as such, PV + Gen + Battery system is the optimized model to fulfill the load demands. Therefore, this research work shows that the integration of PV with battery storage into the existing diesel stand-alone system in the University library is more reliable at the lowest lifecycle cost.