Large-scale Energy Storage Research Articles

Physics-Based Continuum Modeling for an Aqueous Rechargeable Zn/MnO2 Battery

While Lithium-ion batteries dominate high-energy-density applications, their reliance on scarce and expensive materials poses a challenge for large-scale renewable energy storage. Aqueous Zinc/Manganese Oxide (Zn/MnO2) batteries, utilizing abundant and cost-effective materials, emerge as a viable alternative. However, the lack of understanding of their fundamental reaction mechanisms hampers their optimization and commercialization. In this study, we present a physics-based model to aid in elucidating the reaction mechanisms governing Zn/MnO2 batteries. When coupled with statistical parameter estimation, the model accurately replicates the discharge behavior across various C-rates and offers predictive insights into the underlying phenomena. Our findings inform the design of future targeted experiments and lay the groundwork for in-depth mechanistic studies. Moreover, the model serves as a tool for in-silico optimization, accelerating the path to commercial viability.

(Invited) Exploring Catalyst Characteristics through in-Situ Studies of Solid-Liquid Interfaces

Scientists are driven by the urgent need to combat global climate warming and environmental pollution, leading to the development of highly efficient and eco-friendly energy technologies. Hydrogen, a key player in achieving deep decarbonization for net-zero carbon dioxide emissions by 2050, is currently produced through the widely used steam-methane reforming process, which, unfortunately, relies on fossil fuels and generates significant CO2 as a byproduct. This process is environmentally unfriendly and unsustainable, emphasizing the necessity for green hydrogen production. The sun emerges as a clean, renewable, and abundant energy source, offering an ideal solution to the environmental challenges posed by fossil fuels. Hydrogen, an excellent fuel for fuel cell applications, must be generated from renewable and carbon-free resources, particularly utilizing natural energies like sunlight. Enter the photoelectrochemical cell, a promising method for hydrogen generation. This cell comprises semiconductor photoelectrodes capable of harnessing light energy to directly split water. Photocatalysis, leveraging light energy for chemical reactions, holds the key to large-scale solar energy storage, overcoming challenges such as cost and efficiency. Despite over four decades of dedicated research, photocatalysis still faces hurdles in terms of cost and efficiency, hindering progress in this crucial field. A major obstacle lies in the limited understanding of the mechanisms underlying its low performance. To address this challenge, I will overview several experimental studies that have successfully unveiled catalytic reactions at solid/liquid interfaces in real time, focusing on the electrochemical interface of photocatalysis and electrocatalysis. These studies utilize operando X-ray characterization, providing unique insights into the actual reaction mechanisms.

(Invited) Organosilicon Coatings Made By a Cold Plasma Processes at Atmospheric Pressure for Electrochemical Applications

Cell manufacturing is estimated to account for up to 40% of battery-industry and the related market will grow 14.1% in the next five years (108.4 B$ in 2019). In this context, high-power and stationary large-scale energy storage systems, such as lithium-ion batteries (LiBs), will be essential to make electricity quickly accessible to the power grid. This is particularly true when it comes to power plants located in remote areas or the use of renewable energy sources. Despite their advantages, commercial LiBs are today limited by their environmental and safety hazards. These limitations lead researchers around the world to discover and employ new technologies for safe recycling operations and a better control of the cycling processes. Currently, works are focused on the development of new advanced materials, composite design as well as advanced interfacial layers. In this context, surface modification by cold plasma is one of the most promising solutions to control and modify charge transport mechanisms at the interfaces. Although, the synthesis of nano-layers made by plasma at atmospheric pressure has already received a lot of interest, their use to modify electrochemical devices, such as electrochemical cells remain limited. In this work, the synthesis of organosilicon coatings on composite conductive layers by atmospheric pressure DBD is studied in detail. The influence of different organosilicon molecules has been analysed to highlight the effect of the growth mode as well as chemical functionalities on the operation of the final electrochemical device. The physical and chemical modifications of plasma on the conductive layer as well as the water stability of the coatings were investigated by ATR-FTIR, XRD, SEM and contact angle. Figure 1

Electrolyte and Cutoff Potential Effects on Cycle Life of Li4Ti5O12/LiNi0.9Mn0.1O2 Batteries for Behind-the-Meter Storage Applications

Behind-the-Meter Storage (BTMS) is a stationary battery energy storage system that is connected to the electrical distribution system on the customer’s side of the utility’s service meter. BTMS systems are used to store electrical energy from the grid as well as inconstant, renewable energy, such as local solar and wind generation. A successful BTMS system will allow the customer to pair their energy generation and storage to optimize electrical consumption from the grid, improving reliability and minimizing cost. For BTMS applications, batteries must be designed and optimized with different set of criteria from other leading segments of the Li-ion battery market, like transportation, due the system being stationary and proximal to the residential or commercial building it’s benefitting. BTMS applications prioritize safety, cost (low/no-critical materials), reliability (20-year calendar life), and durability (10,000 cycle life), while having the ability to (minimally) compromise energy density and rate capability.Lithium titanate (Li4Ti5O12, LTO) is a promising anode candidate for BTMS applications due to its high safety and capacity retention, while maintaining a reasonable 160 mAhg-1 reversable capacity and composition of relatively abundant materials. (1) Specifically, LTO has a high working voltage which helps to prevent Li dendrite formation, improving safety. Furthermore, LTO also has negligible lithiation-based volume change, leading to less mechanical pulverization, or loss of active material, upon cycling. For the cathode, materials with little or no Co are of high interest due to the high cost and low abundance of Co. LiMn2O4 (LMO) has been paired with LTO for BTMS applications in the past due to its safety, low cost (abundancy), and reasonably high operating voltage. (2-4) However, the low capacity of LMO limits energy density and specific energy. While not the highest priority for BTMS applications, increasing energy density will enable deployment in space constrained BTMS applications and decrease total cost. LiNi0.9Mn0.1O2 (LNMO) is a recently developed material with promise due to its high operating voltage and relatively low price. (5) However, Ni-rich layered oxides, including LNMO, tend to struggle with capacity retention during high-voltage cycling due to mechanical pulverization, irreversible phase transitions, and unstable solid-electrolyte interphase.The study presented here focuses on building an understanding of how electrolyte solvent and varied cutoff potentials will impact the cycle life of LTO/LNMO cells. Specifically, a comparison is provided between ethylene carbonate (EC), ethyl methyl carbonate (EMC), fluoroethylene carbonate (FEC), and Gen2 electrolyte solvents with 1M Lithium hexafluorophosphate (LiPF6) salt, cycling to two upper termination potentials, 2.6V and 2.7V. Electrochemical testing and diagnostics (e.g., differential capacity analysis, area specific impedance, constant voltage hold, and rate capability) and post-mortem characterization will be used to understand the aging behavior and failure mechanisms of the 8 cell combinations (four electrolytes and two voltage cutoffs). Cells with FEC electrolyte showed a lower initial capacity compared to cells with Gen2, EMC, and EC cycling at both voltages; however, the cells with FEC showed consistent trends in capacity retention with 2.6V and 2.7V termination potentials, while the cells with the other electrolytes showed much higher rates of capacity loss when cycling to the higher voltage. These results indicate that FEC may play a role in improving durability of high-voltage, Ni-rich electrode systems for use in high-cycle applications, such as BTMS.This work was authored in part by the National Renewable Energy Laboratory, operated by Alliance for Sustainable Energy, LLC, for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. Funding was provided by the U.S. Department of Energy's Vehicle Technologies Office under the Behind-the-Meter Storage (BTMS) Consortium directed by Samuel Gillard and managed by Anthony Burrell. The electrodes used in this manuscript are from Argonne's Cell Analysis, Modeling and Prototyping (CAMP) Facility, which is fully supported by the DOE Vehicle Technologies Office (VTO). The views expressed in the article do not necessarily represent the views of the DOE or the U.S. Government. The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes.

Long-Term Calendar Aging across Commercial Lithium-Ion Cell Chemistries - Part II: Modeling and Early Prediction

Lithium-ion (Li-ion) batteries are widely used in applications such as mobility, stationary grid systems, and various consumer and commercial systems. In electric vehicles (EVs), batteries often remain idle, with only about 10% utilization. Moreover, in stationary battery energy storage systems (BESS) designed for peak shaving, resting periods tend to cause more aging effects than the operational cycles. Consequently, quantifying the effect of calendar aging is crucial. However, current calendar aging models are inadequate for accurately modeling and predicting the diverse aging behaviors of commercial Li-ion batteries. In this study, we analyze a new, long-term Li-ion battery calendar aging dataset, which includes eight different commercial battery types. We compare and validate three calendar aging models—semi-empirical, symbolic regression, and Long Short-Term Memory Network (LSTM). We examine the transferability of these models across various commercial cell types, offering insights into their generalizability. Additionally, we introduce a novel trajectory-to-trajectory approach for early prediction of calendar aging lifetime. For the first time, we investigate the model's ability to interpolate and extrapolate aging behavior under different storage temperatures. This research provides practical insights for optimizing battery management strategies in conditions dominated by calendar aging.

Quantum-Chemistry Enabled AI for Materials Discovery for Energy Storage

A priori and reliable simulations can enable timely and cost-efficient design and discovery of materials for energy. Therefore, ‘Let’s Start from Computing’ is an optimal approach to initialize modern day R&D processes. In energy storage, beyond lithium-ion (BLI) research has the potential to revolutionize consumer electronics including portable and stationary power, transportation, and grid energy storage sectors. Multi-valent (Mg, Ca, Zn) energy storage or economically viable monovalent (Na, K) batteries, high-density metal-air, metal-sulfur batteries, or grid-storage systems are considered in the beyond lithium-ion research and development. All these R&D efforts require significant fundamental knowledge via a priori computations for materials discovery, property prediction, and optimization. Atomistic modeling when coupled with reliable Artificial Intelligence (AI) approaches can provide accurate insights to accelerate discovery of optimalelectrolytes, electrodes, and membranes for BLI systems to reduce the cost. Thus, coupled with AI and multi-scale simulations techniques, atomistic modeling can address prediction of molecular level properties of materials (redox potentials, solvation, spectroscopic, and reactivity) to down-select optimal materials or material combinations. In this presentation, I will describe some of our recent efforts in active learning coupled with large scale first principles simulations to down select/optimize desired molecules for flow battery technology. This concept can be utilized for design of experiments using autonomous experimentation. Additionally, I will describe some of our quantum chemistry-informed molecular property predictions redoxmers and liquid organic hydrogen carriers. In addition to molecules, I will present. A data driven approach to study longer time scale diffusion of ions for multivalent battery concepts. Figure 1

Stable Cycling of Ultrahigh Mass Loaded MnO2 for Low-Cost Sodium-Ion Batteries

Achieving cost-effective and sustainable solutions for large-scale energy storage is critical for advancing the global clean energy transition. The intermittent nature of green energy underscores the inadequacy of current large scale energy storage technologies like pumped hydroelectricity which is susceptible to geographic and water resource constraints, making batteries a prime candidate for this role. Although lithium-ion batteries (LIBs) with high energy and power density are a viable solution, the high cost and uneven distribution of lithium reserves limits the widespread application of LIBs for grid energy storage. Considering these developments and the challenges posed by limited lithium reserves, sustainable and low-cost sodium-ion batteries (SIBs) emerge as a promising alternative.Among the various battery electrode materials, manganese dioxide (MnO2) stands out as a promising choice for large-scale energy storage applications due to its earth abundance, cost-effectiveness and non-toxic nature. Although MnO2 is known as a pseudocapacitive material with superior cycling stability in aqueous electrolytes, its dissolution in non-aqueous electrolytes has restricted its widespread use in long lifetime batteries. In this study, we demonstrate that an ether-based electrolyte solution (1M NaClO4 in diglyme) is able to achieve stable cycling of electrodeposited ε-MnO2 as a cathode for SIBs, reaching a over 75% capacity retention over 1000 cycles. This response surpasses conventional ester-based electrolytes (58% of 1M NaClO4 in EC/PC). A comparative cycling stability study, coupled with electrochemical quartz crystal microbalance characterization, validates the efficacy of the ether-based electrolyte in mitigating MnO2 dissolution. Furthermore, the integration of 3D printed graphene aerogel (3D GA) as a scaffold for electrodeposited MnO2 leads to enhanced electrochemical performance of the MnO2 electrode (3D MnO2/GA), resulting in a high areal capacity of 4.4 mAh cm-2 at a current density of 10 mA cm-2. The 3D MnO2/GA electrode possesses scalable electrochemical performance with mass loadings from 20 mg cm-2 to 80 mg cm-2. Using this electrode, a high mass loaded sodium-ion battery (SIB) device was fabricated by using utilizing 58 mg cm-2 3D MnO2/GA as the cathode and 16 mg cm-2 dip-coated TiO2@Cu foam as the anode. This MnO2 | TiO2 device delivered a notable areal capacity of 6.2 mWh cm-2 and a high areal power density of 70.7 mW cm-2. Moreover, the SIB device demonstrates 85% capacity retention after 500 cycles, underscoring the stable cycling of this high-energy and power-density SIB device.

Operando Visualization of Redox Flow Batteries Using Fluorescence Spectroscopy with Alizarin Red S Electrolyte

Redox flow batteries (RFBs) charge and discharge by transferring electrons between two pairs of redox species separated by a proton exchange membrane. Unlike traditional batteries that store energy in redox-active materials within an enclosed cell, RFBs store energy in liquid electrolytes housed in external reservoirs1. These electrolytes are pumped through the cell, enabling redox reactions on the surface of electrodes. The energy storage capacity of RFBs is determined by the concentration of the active species and the size of the reservoirs, making them a promising technology for large-scale energy storage applications2. While RFBs possess advantages such as long lifetime, flexible design for battery capacity and power, and rapid response, they suffer from drawbacks like low energy density, power density, and energy efficiency3. Efforts to address these issues have primarily focused on modifications to the electrode materials or the flow field4 , 5. However, evidence supporting performance enhancements often relies on global parameters such as energy efficiency, energy density, and voltage drop, lacking detailed mechanistic and microscopic studies. Real-time observation of the electrode-electrolyte interface during redox reactions and the transport of redox species in the flow field holds the potential to significantly contribute to understanding the electrode structure and mass transport phenomena in RFBs6. This understanding, in turn, facilitates prediction and purposeful enhancement of battery performance by developing desirable structures for both electrode and flow fields. Microscopy, particularly confocal microscopy, serves as a powerful tool for observing microscopic structures. Confocal microscopes yield sharper images with higher resolution than widefield microscopy, enabling rapid acquisition of high-resolution images.In this study, we employed a confocal laser scanning microscope for operando visualization of RFBs. We designed a transparent framework for the battery, allowing light to pass through, and covered its surface with a glass cover to visualize the porous carbon electrode. We selected a fluorescent redox-active species, Alizarin Red S to enable visualization using fluorescence spectroscopy. Leveraging its unique fluorescence properties, we established a correlation between fluorescence intensity and the state of charge. This innovative approach enables us to observe the distribution of electrolytes, the dynamics of redox reactions on the electrode surface (with fast speed of imaging ＜0.2s), the mass transfer behavior of the electrolyte on the electrode surface, and the uniformity of electrolyte flow in the flow field, including areas of stagnation or recirculation. Zhang, J. et al. An all-aqueous redox flow battery with unprecedented energy density. Energy Environ. Sci. 11, 2010–2015 (2018).Rubio-Garcia, J., Kucernak, A. & Charleson, A. Direct visualization of reactant transport in forced convection electrochemical cells and its application to redox flow batteries. Electrochem. commun. 93, 128–132 (2018).Tolmachev, Y. V. Flow Batteries From 1879 To 2022 And Beyond. Qeios 1–79 (2023).Wang, R. et al. Gradient-distributed NiCo2O4 nanorod electrode for redox flow batteries: Establishing the ordered reaction interface to meet the anisotropic mass transport. Appl. Catal. B Environ. 332, 122773 (2023).Kumar, S. & Jayanti, S. Effect of flow field on the performance of an all-vanadium redox flow battery. J. Power Sources 307, 782–787 (2016).Wong, A. A., Rubinstein, S. M. & Aziz, M. J. Direct visualization of electrochemical reactions and heterogeneous transport within porous electrodes in operando by fluorescence microscopy. Cell Reports Phys. Sci. 2, 100388 (2021).

Optimizing Mid-Temperature Sodium Iodine Batteries for Sustainable Large-Scale Energy Storage: A Comprehensive Modeling Approach and Experimental Validation

The need for safe, eco-friendly, low-cost batteries for large scale implementation rises in the presence of the expansions of renewable energy sources. High-temperature liquid sodium batteries (e.g. sodium-sulfur) are a well-established technology for large-scale grid storage. Combining the molten sodium anode with an aqueous iodine cathode overcomes the problems of thermal loses and sealing by reducing the operating temperature to about 100°C. This leads to higher cost efficiency, energy density and a simplified cell design.Here we present a novel modelling approach for complex carbon-based cathode structures and its limits and abilities on enhancing the utilizable capacity. Due to their sustainability and high efficiency, which is comparable to Li-Ion –batteries, mid-temperature sodium-iodine batteries are a promising candidate for stationary energy storage applications. With a high specific energy of around 200 Wh kg-1, sodium-iodine batteries have the ability to meet the growing demand of short-term energy storage caused by the integration of renewable energy sources into the power grid. Up to this day, the detailed processes inside the battery are not well-understood. Quantities like initial species concentrations, C-rate, cell design and cathode geometry have large impacts on the overall battery performance. A deeper understanding of these processes is inevitable for enhancing the battery performance by choosing the optimal abovementioned quantities. We set up a fully resolved 3D Simulation model to gain insight into microstructural processes inside the battery, as experimental studies are limited to macroscopic quantities like electronic current and potential. The methodology is based on continuum hypothesis by using an open-source CFD software, which is rewritten accordingly. The proposed spatially resolved three-dimensional simulation model takes charge and mass transport, heterogeneous and homogeneous reactions as well as the electrochemical processes into account. Results show that transport limitations confine the utilizable capacity of the battery [1].This contribution focuses on enhancing the battery energy density and efficiency by including open-pored glassy carbon foam structures in the cathode half-cell [2, 3]. We state simulation predicted, optimal morphology properties of such a structure. Furthermore, we describe the evaluation of improved thermodynamic data of the aqueous iodine solution by refitting simulation results to polarization curves. We utilize a novel impedance spectroscopy simulation method for sodium iodine batteries to improve parameterization of the underlying electrochemical model by comparing the results with experimental data. References Gerbig, F., Cernak, S., Nirschl, H. (2021). 3D Simulation of Cell Design Influences on Sodium–Iodine Battery Performance. Energy Technology, vol. 9, no. 6, p. 2000857Gerbig, F., Cernak, S. and Nirschl H. (2021), Towards a Novel Sodium-Iodine Battery with an Aqueous Catholyte: Numerical Investigations of Complex Cathode Structures, ECS Trans., vol. 104, no. 1, pp. 123–130Gerbig, M. Holzapfel, and H. Nirschl, Simulating the Impact of Glassy Carbon Foam Electrodes on the Performance of Sodium Iodine Batteries, J. Electrochem. Soc., vol. 170, no. 4, p. 40517, 2023, doi: 10.1149/1945-7111/accab7. Figure 1

Tri-Electrode Structured Zinc-Air Batteries: A Leap in Energy Storage Efficiency and Stability

Rechargeable zinc-air batteries (ZABs) are gaining attention as a potent solution for large-scale energy storage, offering high capacity and an environmentally friendly profile. Utilizing zinc, an abundant and cost-effective alternative to lithium, ZABs capitalize on the plentiful oxygen in ambient air, presenting a more sustainable option than traditional lithium-ion batteries. In this work, we address the operational challenges of ZABs, primarily focusing on the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) that occur during the discharge and charge cycles. ORR, a three-phase reaction, necessitates efficient water management and high-performance electrocatalysts, while OER is a two-phase reaction prone to creating oxygen bubbles that can damage the electrode structure. A critical issue in ZABs is the instability of ORR catalysts during the OER phase, and vice versa. To counter this, we propose a tri-electrode cell structure that separates the ORR and OER electrodes, allowing each to be optimized independently. This design enhances the stability and overall efficiency of the batteries. In our study, MnO2/C deposited on nickel foam was employed as the ORR electrode, and layered double hydroxides (LDHs) deposited directly on stainless steel mesh served as the OER electrode. A graphite electrode was used as the common negative electrode. Our proposed system, tested at 100 mAh/cm² and 25 mA/cm², demonstrated remarkable durability, sustaining at least 100 cycles with a Coulombic efficiency of 98% and a 60% roundtrip efficiency. These findings offer valuable insights into the future design of stable, high-efficiency rechargeable zinc-air batteries, marking a significant step forward in sustainable energy storage technology.

Dual-Ion-Doped Hydrated Vanadium Oxides Nanowires As Cathode Materials for High-Rate Aqueous Zinc Ion Batteries

Due to environmental-friendliness, high safety and stability, high theoretical capacity as well as low cost, aqueous zinc ion batteries (AZIBs) are budding as attractive alternatives for large-scale energy storage applications. However, the lack of suitable positive electrode materials becomes the main issue for AZIBs to pursue high energy density and excellent rate capability. Herein, the dual-ion-doped hydrated vanadium oxide nanowires (VOH) are synthesized through hydrothermal method, which is suitable to be cathode materials for AZIBs. Through metal ions and crystal water pre-intercalation, the interlayer spacing of (001) becomes larger, which is proposed to have better capability to accommodate Zn2+. Moreover, the pre-intercalated metal ions can serve as structural pillars between layers, providing excellent stability during discharging/charging process. In this study, the novel dual-ion-doped VOH exhibits more than 420 mA h g−1 at 0.1 A g−1 as well as more than 260 mA h g−1 at 5 A g−1. In addition, it can retain more than 80% capacity after 500 cycles at 5 A g−1. The energy storage mechanism of dual-ion-doped VOH is revealed via operando x-ray absorption near edge spectroscopy. This study implies that this dual-ion-doped VOH nanowires can serve as suitable positive electrode materials for high-performance aqueous zinc ion batteries.

Design and Synthesis of K-Ion Solid Electrolytes Based on Antiperovskites and Chalcogenides

Potassium batteries with organic liquid electrolytes, including K-ion, K–O2, and K–S batteries, have emerged as promising candidates for large-scale energy storage due to the high earth abundance and low redox potential of potassium (−2.93 V versus SHE). However, the issues brought by using liquid electrolyte still hinder the development of K-batteries. The utilization of solid electrolytes will not only have the commonly recognized benefits in suppressing dendritic metal plating and enhancing battery safety, but also in blocking oxygen or sulfur crossover from the cathode. In this presentation, I will present our recent progress in developing K-ion solid electrolytes based on antiperovskites, K3OX (X=Cl, Br and I) and chalcogenides (K3SbS4 and K3SbSe4). Among them, K2.92Sb0.92W0.08S4 exhibits high conductivity of 1.4×10−4 S cm−1 at 40 °C, which is among the best reported potassium-ion conductors at ambient temperature.Their applications in room-temperature K-S and K-O2 batteries have been successfully demonstrated.

Proton Activity and Pathway in Aqueous Organic Redox Flow Battery Electrolyte

Aqueous soluble organic (ASO) redox-active materials have recently shown great promise as alternatives to transition metal ions employed as energy-bearing active materials in redox flow batteries for large-scale energy storage because of their structural tunability, cost-effectiveness, availability, and safety features. However, development so far has been limited to a small palette of organics that are aqueous soluble. This presentation will use fluorenone as an example to showcase how a natively redox-inactive molecule can be tuned to possess two-electron redox reversibility through hydrogenation and dehydrogenation. The modified fluorenone molecules demonstrated high energy density and recorded stable cycling. Furthermore, research has shown the unique chemical-electrochemical coupled redox reactions mechanism; thus, the system rate capabilities can be improved by incorporating suitable hydrogen acceptors that regulate the proton pathway for faster kinetics.Reference:Feng et al., Science 372, 836–840, 2021Feng et al., Joule 7, 1–14, 2023

Concentrated Chloride Electrolyte Enabling SEI for Fe Metal Anode

Iron is one of the most abundant metal elements in the Earth’s crust and has been widely used worldwide since ancient times. Due to this natural abundance and well-established mass production methods, iron is a promising candidate as a battery electrode for cost-effective large-scale energy storage systems. Edison’s Ni-Fe battery, invented in 1900, used an alkaline electrolyte without ferrous or ferric ions that eventually limited its anode design using iron oxide. More recent Fe redox flow batteries and the newly suggested Fe-ion batteries that use an acidic electrolyte containing ferrous ions (Fe2+) can utilize iron foil as an anode. Combining iron foil with Fe2+ electrolyte enables a simple design and faster kinetics on the anode side. However, introducing acidic electrolytes accelerates hydrogen evolution reaction (HER) on the iron surface. Severe HER affects cell performance in various ways, including lower Coulombic efficiency (CE), drying out of the electrolyte by water consumption, and precipitation of Fe2+ ions due to increased pH of the electrolyte. In this work, a concentrated chloride-based electrolyte was introduced to Fe-ion batteries to suppress HER and achieve a high-CE Fe2+/Fe electrode. With the help of nonpolar [ZnCl4]2- which enables low-polarity organic additives to be soluble in the aqueous solution, an organic solid electrolyte interface (SEI) can be formed on top of the electrode while plating iron. Fe anode protected by the SEI shows a noticeable decrease in HER during cycling, which leads to higher CE of more than 98% and twice longer cycle life.

Recent Advancements in Materials for Proton-Conducting Reversible Solid Oxide Cells

The escalating threats posed by climate change and environmental issues underscore the need for urgent exploration of clean energy technologies. In this context, proton-conducting reversible solid oxide cells (P-RSOCs) have emerged as promising contenders for large-scale energy storage and conversion. Their efficacy at intermediate temperatures (400 - 600 °C) is a distinctive advantage, attributed to the remarkable ionic conductivity of proton conductors within this temperature range. This feature positions P-RSOCs favorably for the efficient, cost-effective, and enduring conversion between clean electricity and green hydrogen.However, the successful commercialization of P-RSOCs hinges on the rational design of novel materials and meticulous surface/interface modifications to significantly enhance performance and durability. Achieving prolonged durability during cycling between fuel cell and electrolysis modes while maintaining high Faraday efficiency and roundtrip energy efficiency is of paramount importance. A central challenge involves the development of electrolyte materials exhibiting exceptional stability and a high ionic transference number under operating conditions. Additionally, bifunctional electrodes/catalysts, particularly for the oxygen electrode, must demonstrate elevated activity for efficient dual-mode operation.To effectively tackle these challenges, it is imperative to devise new protocols for controlling the structure, composition, and morphology of materials across multiple length scales. Equally crucial is gaining a fundamental understanding of degradation mechanisms and electrode processes. This presentation will illuminate the critical scientific challenges inherent in developing a new generation of P-RSOCs, providing insights into the latest advancements in electrolyte materials and bifunctional electrodes/catalysts. Furthermore, it will highlight strategies for enhancing durability and electrode activity through surface/interface modification, along with the latest developments in modeling, simulation, and in situ characterization techniques to deepen our understanding of the underlying mechanisms. The presentation will conclude prospects for achieving low-cost, highly efficient, and durable P-RSOCs through innovative material-based solutions.

Regulated Interfacial Proton and Water Activity Enhances Mn2+/MnO2 Platform Voltage and Energy Efficiency

Aqueous zinc battery (AZB) is one of the most promising energy storage systems for large-scale applications due to its high safety, low cost, and environmental friendliness. To match the high capacity of zinc metal (~820 mAh g-1), various cathode materials, such as oxides, open structure analogues, chalcogens, and halogens, have been explored for AZBs. Among them, manganese dioxide (MnO2) has been considered to be one of the most attractive candidates due to its high capacity (~308 mAh g-1 for one-electron transfer) and adequate redox potential (~0.70 V vs. SHE). Recently, the MnO2 dissolution/deposition charge storage mechanism has been demonstrated to further increase the voltage and capacity of Mn-based AZBs. The dissolution/deposition (Mn2+/MnO2) is a two-electron transfer process, which doubles the theoretical capacity of MnO2 cathode to ~616 mAh g-1 and considerably increases the redox potential (~1.229 V vs. SHE). The charge storage via the dissolution/deposition mechanism pushes the aqueous-based energy density even higher, to over 1000 Wh kg-1.However, this charge storage mechanism involves liquid-solid transition, as well as the thermodynamics and kinetics of this reaction are highly dependent on the acidity and interfacial environment. Yet, the opposite effect of interfacial H+ on the dissolution/deposition processes and the role of interfacial H2O are rarely discussed. Here we introduce tetrafluoroborate anion (BF4 -) into the sulfate-based electrolyte to regulate the interfacial H+ and H2O activity. First, BF4 - hydrolysis increases the electrolyte’s acidity, promoting MnO2 dissolution. Second, BF4 - forms an H-bond network with interfacial H2O that assists H+ diffusion while retaining sufficient H2O supply to facilitate MnO2 deposition. As a result, the cathode-free Zn//MnO2 electrolytic cell achieves a high plateau of ~1.92 V and energy efficiency of ~84.23 % in the BF4 - containing electrolyte. Significantly, the cell delivers 1000 cycles at 1 C with ~100 % Coulombic efficiency and a high energy efficiency retention of 93.65 %. Our findings disclose a new strategy to promote Mn2+/MnO2 platform voltage and energy efficiency for future large-scale energy storage systems.

Cu/F Dual-Doping on the Ni–Mn Based Layered Oxide As Cathode Material for Sodium-Ion Batteries

As demands for energy storage are ever-increasing, sodium ion batteries (SIB) have been widely conducted for superior electrochemical performance and reduction in cost compared to lithium ion batteries (LIBs), one of the most commonly used energy storage devices nowadays. Among all the parts in batteries, cathode materials play a crucial role with regard to performance. Here, layered transition metal oxides (LTMOs) (NaxTMO2, TM = Cu, Mn, Fe, Ti, Co, Cr, Ni, etc.) have kept catching researchers’ eyes due to their mature developments in commercial LIBs over decades.P2-type Ni/Mn-based LTMOs (such as Na0.67Ni0.33Mn0.67O2, NNMO) exhibit high theoretical specific capacity (~173 mAh g− 1) with environmentally friendly and cheap elements. However, they show limited stability upon charging over 4.2 V, directly restricting their energy density and lowering operating voltage. Besides, disadvantages such as poor rate performance and air/moisture sensitivity are remaining improvements for commercialization. Hence, our study mainly focuses on improvements for shortages of NNMO by applying a facile cation-and-anion co-doping strategy. Via transition metal Cu ion doping, the cycling stability of NNMO can be improved without sacrificing too much capacity, and the air stability can also be improved. In addition, F doping to replace oxygen can further enhance the structural stability and increase the rate performance by modifying interlayer spacing of the layered structure. A facile sol-gel method with heat treatment is used to synthesize cation-and-anion co-doped NNMO. The material characterization shows successful synthesis after cation and anion doping. In addition, the long cycle test shows that capacity retention increases from ~50% to ~90% over 100 cycles after Cu doping. The specific discharge capacity also increases from ~110 mAh/g to ~130 mAh/g at 0.1C with a slight increase in cycle retention from ~75% to ~77% after F doping. Also, the capacity at 20C could be maintained at ~50 mAh/g, around 45% of the initial capacity at 0.1C. The detailed charge storage mechanism will be studied by applying operando X-ray absorption spectroscopy and ex-situ X-ray photoelectron spectroscopy. As for the moisture sensitivity part, the XRD spectra indicate that no noticeable structural changes or impurity formation occurred after air exposure for one month, showing good air stability. Furthermore, operando XRD is conducted to confirm structural reversibility during charging and discharging.In summary, by applying a simple Cu/F co-doping strategy, Na0.67Ni0.13Cu0.20Mn0.67O2-xFx exhibits sufficient cycle stability for over 100 cycles as well as excellent rate capability even at a high current density of 20C, which proves itself a promising candidate as high-performance cathode material in SIBs. In addition, the cheap and environmentally friendly elemental usage (Ni, Mn and Cu) and air-stability enhancement also increase possibility of this material for large-scale energy storage application in industry.

(Invited) Research and Development Trends in Sodium-Ion Batteries

Lithium-ion batteries (LIBs) are the most used energy storage technology for electronic devices and electric vehicles. However, the increasing demand for lithium may make it difficult to support the widespread use of LIBs in electric vehicles, utility grids, and other applications. Therefore, developing alternative technologies to LIBs is critical for large-scale energy storage. Studies suggest that using sodium-ion batteries with renewable energies can significantly reduce the cost of electricity. Despite the lower energy density, sodium-ion batteries are appealing for several reasons: (1) sodium is abundant, (2) less critical components are used, and (3) the manufacturing knowledge of LIBs can be used. Herein, we report for the first time a hydrothermal synthesis of a new class of sodium-rich cathode material with the formula Na3+xNixV2-x(PO4)2F2O that could enable the compensation of the irreversible sodium loss in the first charge in a full cell with hard carbon. Trends in sodium-ion batteries will be discussed, considering the new developments in layered oxides, oxy-fluoro-phosphates, and Prussian blue cathodes.

Amorphous Cobalt Tin Oxide/Reduced Graphene Oxide Decorated on Graphite Felt as Positive Electrode for Vanadium Redox Flow Battery

The Vanadium Redox Flow Battery (VRFB) stands out as one of the most promising large-scale energy storage systems. Nevertheless, the graphite electrode materials commonly used in VRFBs often exhibit drawbacks such as limited electrochemical activity and insufficient conductivity, ultimately resulting in suboptimal performance for the VRFB. In this study, we utilized highly conductive reduced graphene oxide (rGO) as a carrier for cobalt tin oxide (CoSnO3), aiming to enhance their catalytic capability towards vanadium ions. CoSnO3/rGO composite was confirmed using XRD and TEM, revealing nanoboxes morphology and amorphous structure. XPS and EPR analysis showed an increase in oxygen vacancies after composite formation. In order to further prove the effect of CoSnO3/rGO composite catalyst modified graphite felt, the modified graphite felt electrode was applied to the positive electrodes of VRFB, and the charge and discharge tests at different current densities were carried out. At a current density of 160 mA/cm², the composite demonstrated a voltage efficiency of 74.22%, surpassing the heat treated graphite felt by 2.69%. Noteworthy enhancements in capacitance were also evident at this specific current density. Furthermore, stability testing over 50 cycles revealed no significant degradation, underscoring the superior performance and outstanding durability of the CoSnO3-rGO modified graphite felt throughout charge and discharge cycling. The result of this study highlight the promising potential of CoSnO3-rGO composites as efficient catalysts for vanadium redox flow batteries, providing improvements in electrochemical performance and long-term stability.

Aqueous Polysulfide-Based Redox Flow Battery with Soluble Molecular Catalysts

Aqueous polysulfide-based redox flow batteries (RFBs) are promising for large-scale energy storage applications due to their low cost and high safety1. However, polysulfide negolyte suffers from poor kinetics, resulting in low operating current density and low energy efficiency2-5. Herein, we proposed a molecular catalyst strategy to transfer the sluggish electrochemical polysulfide reduction reaction to a fast chemical reaction via homogeneous catalysis. Inspired by the electron transport chain in the respiratory process, we selected riboflavin sodium phosphate (FMN-Na) as the molecular catalyst to transfer electrons from electrode to polysulfide with the disulfide bond cleavage6. The reaction rate (or turnover frequency) kobs is determined to be 33 s–1 at 1 M K2S4 solution, and this spontaneous chemical electron transfer process is verified by ultraviolet-visible light spectroscopy. In the polysulfide-ferrocyanide (S-Fe) RFB, the overpotential is dramatically decreased from over 800 mV to 241 mV at 30 mA cm–2 with the assistance of FMN-Na as the molecular catalysts. The catalyzed S-Fe RFB and polysulfide-iodide RFB showed stable operation for over 1,000 cycles at 40 mA cm–2. This work offers a simple but effective method to resolve the bottleneck for aqueous polysulfide-based RFBs, and such a homogeneous catalysis strategy may shed light on other flow battery systems with poor kinetics. We will combine operando spectroscopic techniques and theoretical calculation to discuss the possible reaction mechanism and intermediate.AcknowledgmentThis work is supported by two grants from the Research Grants Council (RGC) T23-713/22-R and RFS2223-4S03. Reference Y. Yao, J. Lei, Y. Shi, F. Ai and Y.-C. Lu, Nat. Energy, 2021, 6, 582-588. Z. Li and Y.-C. Lu, Nat. Energy, 2021, 6, 517-528. X. Wei, G.-G. Xia, B. Kirby, E. Thomsen, B. Li, Z. Nie, G. G. Graff, J. Liu, V. Sprenkle and W. Wang, J. Electrochem. Soc., 2015, 163, A5150. Z. Li, M. S. Pan, L. Su, P.-C. Tsai, A. F. Badel, J. M. Valle, S. L. Eiler, K. Xiang, F. R. Brushett and Y.-M. Chiang, Joule, 2017, 1, 306-327. J. Lei, Y. Yao, Y. Huang and Y.-C. Lu, ACS Energy Lett., 2023, 8, 429-435. J. Lei, Y. Zhang, Y. Yao, Y. Shi, K. L. Leung, J. Fan and Y.-C. Lu, Nat. Energy, 2023, DOI: 10.1038/s41560-023-01370-0.