Grid-scale Energy Storage Systems Research Articles

(Industrial Electrochemistry and Electrochemical Engineering Division New ElectrochemicalTechnology (NET) Award) Development and Commercialization of Rechargeable Zinc Manganese Alkaline Batteries to Enable a Sustainable Low-Carbon Energy Future

Urban Electric Power Inc. (UEP), a 2013 spinoff from the City University of New York Energy Institute (CUNY-EI), manufactures and markets rechargeable large-format zinc manganese dioxide (Zn-MnO2) alkaline battery cells in Pearl River, NY. The active materials in the UEP rechargeable cells are Zn and MnO2, both of which have high energy density, low cost, good safety characteristics, and are readily available with abundant supplies. These materials are key components of primary (non-rechargeable) alkaline batteries that presently dominate the disposable battery market, showing established commercial acceptability. This technology holds enormous potential to address a variety of energy storage applications, however the traditional chemistries used in primary alkaline batteries lead to irreversible degradation of the electroactive components which makes them unsuitable for the thousands of charge/discharge cycles desired for rechargeable grid-scale energy storage systems. UEP and the CUNY-EI have achieved several breakthroughs in addressing these challenges, utilizing chemical dopants and additives to stabilize both the Zn and MnO2 electrodes, which allow for more complete utilization of the battery capacity while maintaining hundreds to thousands of cycles.MnO2 and Zn deliver theoretical capacities, respectively, of 617mAh/g and 820mAh/g through a two-electron reaction in alkaline electrolyte (Figure 1). The theoretical energy density of this cell can be >400Wh/L, which is higher than many varieties of lithium-ion batteries. The Zn delivers its capacity through a dissolution-precipitation reaction, while MnO2 can exhibit solid-state proton insertion and dissolution-precipitation reactions depending on the type of MnO2 polymorph used. Electrolytic manganese dioxide (EMD, γ-MnO2), which is electrochemically synthesized in large quantities and used in primary batteries, exhibits proton insertion and dissolution-precipitation reactions. In batteries consisting of EMD, these two mechanisms each provide part of the two-electron capacity, where proton insertion delivers the first electron reaction (308mAh/g), and dissolution-precipitation delivers the second electron reaction. In contrast, birnessite(δ-MnO2), which has a layered crystal structure and is most commonly found in soils, exhibits only dissolution-precipitation reactions to deliver the two-electron capacity.The CUNY-EI made a breakthrough in 2014 and accessed the 2nd e- capacity of MnO2 reversibility. This was the first report of its complete reversibility for >6,000 cycles. EMD was used to make birnessite electrochemically in-situ, and also, because of ease of manufacturing, materials logistics and economic reasons. The additives, when mixed with EMD in the first discharge-charge formation process to make δ-MnO2, stabilized the layered cathode structure and prevented Mn3O4 formation. Bi2O3 prevented Mn3O4 formation electrochemically by complexing with the dissolved Mn(OH)6 3- and Mn(OH)4 2- ions. It also promoted δ-MnO2 formation in the charging process. The main discovery made at CUNY-EI involved recognizing that intercalating the MnO2 layers with Cu ions stabilized the cathode crystal structure and reduced the charge transfer resistance during the δ-MnO2’s electrochemical reactions. It was shown for the very first time in alkaline battery literature that energy densities approaching to 200Wh/L could be accessed from δ-MnO2 when paired with highly utilized Zn anodes.UEP’s current proton intercalation chemistry utilizes about 120-140 mAh/g-MnO2 and >160-200mAh/g-Zn. This creates an energy density of the cell of 150 Wh/L, which is comparable to existing lead-acid batteries, meaning UEP is competitive in many stationary battery applications including renewable integration, grid energy storage, backup power, including uninterruptible power supply (UPS), and residential backup applications. The company established a pilot scale manufacturing plant in Pearl River, NY with current production capacity of 100 MWh per year. Early production runs have gone to support battery systems for grid applications, backup power, and renewable integration projects. The batteries can be produced at cost lower than lead acid and lithium-ion batteries of comparable performance. Zn and MnO2 make up a small fraction of the cost per unit of storage capacity compared with costly or less environmentally friendly materials like cobalt, nickel, lithium, and lead that are used in other batteries. When manufactured in large volumes, UEP anticipates that its battery packs will cost less than $50’P3/kWh allowing building energy storage systems with a levelized cost of energy (LCOE) below the $0.05/kWh that ARPA-E has identified as the target for widespread adoption of energy storage on the electric grid. At $50/kWh, the technology becomes competitive for wide range grid applications including demand charge reductions, renewable load shifts and transmission and distribution upgrades. Moreover, these batteries offer a new level of inherent safety and environmental friendliness as they contain none of the flammable, toxic, or volatile electrolytes that have plagued other batteries considered for grid-scale storage. Figure 1

A highly-stable layered Fe/Mn-based cathode with ultralow strain for advanced sodium-ion batteries

Sodium-ion batteries (SIBs) with iron- and manganese-based cathode electrodes have exhibited great promise in the grid-scale energy storage systems, on the basis of the satisfactory theoretical capacity, as well as huge abundance, low price and non-toxicity of raw materials. However, the inferior cycle life of cathode materials originating from their poor structural stability remains a formidable challenge towards practical applications. Here, an efficient strategy of improving the structure durability is demonstrated in iron- and manganese-based cathodes by dual heteroatom doping. The as-obtained P2-type Na0.65Li0.08Cu0.08Fe0.24Mn0.6O2 cathode delivers superior cyclability (88.2% capacity retention for 500 cycles at 2C), fabulous rate capability (76% capacity retention at 5C compared to 0.1C), and a useable reversible capacity of around 85 mAh g−1 at 0.1C. Through in-depth characterizations, the underlying structure-property relationship is established, revealing that the complete solid-solution reaction during cycling ensures the ultralow volume variation (as small as 0.7%) and excellent electrochemical performance. These results highlight the significance of fabricating a stable host for the design and development of advanced SIBs with long life.

Risk-Averse Storage Planning for Improving RES Hosting Capacity Under Uncertain Siting Choices

This paper presents a trilevel risk-averse strategy to configure the grid-scale energy storage systems (ESS) in active distribution network. Unlike the traditional planning framework, which presumes that ESS and renewable energy sources (RES) are cooperatively planned, we consider a practical situation that the siting choice of individual RES owners could be conflict with systems operation target of hosting capacity improvement. To address this challenge, a trilevel ESS planning formulation with “min-max” risk constraint is developed. Also, a scenario-based stochastic program model is incorporated in the lower level problem to handle the random fluctuation of RES outputs. As the resulting risk-constrained trilevel formulation is computationally difficult, we develop a customized column-and-constraint generation algorithm with finite convergence. Numerical studies on 33-bus test system and a real-world 56-bus distribution network indicate that the proposed methodology can effectively enhance the robustness of look-ahead ESS configuration against those non-cooperative and uncertain integration choices for renewable energy construction. Moreover, our customized algorithm demonstrates a superior solution capacity and scalability to support the efficient decisions for risk-averse storage planning.

A novel approach to increase the share of renewable purchase obligation for planning of distribution network including grid scale energy storage

Abstract In recent times, producing electricity with lower carbon emissions has resulted in strong clean energy incorporation into the distribution network. The technical development of weather-driven renewable distributed generation units, the global approach to reducing pollution emissions, and the potential for independent power producers to engage in distribution network planning (DNP) based on the participation in the increasing share of renewable purchasing obligation (RPO) are some of the essential reasons for including renewable-based distributed generation (RBDG) as an expansion investment. The Grid-Scale Energy Storage System (GSESS) is proposed as a promising solution in the literature to boost the energy storage accompanied by RBDG and also to increase power generation. In this respect, the technological, economic, and environmental evaluation of the expansion of RBDG concerning the RPO is formulated in the objective function. Therefore, a novel approach to modeling the composite DNP problem in the regulated power system is proposed in this paper. The goal is to increase the allocation of PVDG, WTDG, and GSESS in DNP to improve the quicker retirement of the fossil fuel-based power plant to increase total profits for the distribution network operator (DNO), and improve the voltage deviation, reduce carbon emissions over a defined planning period. The increment in RPO and decrement in the power purchase agreement will help DNO to fulfill round-the-clock supply for all classes of consumers. A recently developed new metaheuristic transient search optimization (TSO) based on electrical storage elements’ stimulation behavior is implemented to find the optimal solution for multi-objective function. The balance between the exploration and exploitation capability makes the TSO suitable for the proposed power flow problem with PVDG, WTDG, and GSESS. For this research, the IEEE-33 and IEEE-69 low and medium bus distribution networks are considered under a defined load growth for planning duration with the distinct load demand models’ aggregation. The findings of the results after comparing with well-known optimization techniques DE and PSO confirm the feasibility of the method suggested.

Synthesis of Flower-Like, Hyperbranched Na3FePO4CO3 Nanocrystals and Their Electrochemical Performance as Cathodes in Aqueous Rechargeable Sodium-Ion Batteries

Aqueous rechargeable sodium-ion batteries (ARSBs) have received much attention due to their low cost, the vast abundance of sodium, and the possible application for a smart grid-scale energy storage system. Here, we synthesized Na3FePO4CO3, a cathode material for ARSBs by a low-temperature ionothermal method using deep eutectic solvent and investigated its electrochemical performance. The physical characterization of the synthesized cathode material was done using X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray analysis, thermogravimetry analysis, differential thermal analysis, and X-ray photoelectron spectroscopy to evaluate crystal size, structure, elemental composition, morphology, and oxidation state. The electrochemical behavior of Na3FePO4CO3 in 2 M Na2SO4 was studied using cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge-discharge techniques. The study revealed that at high current rates, the synthesized cathode material, Na3FePO4CO3 exhibited excellent electrochemical performance. The full cell NaTi2(PO4)3/2 M Na2SO4/Na3FePO4CO3 delivers a discharge capacity of 78.6 mAh g−1 at C/5 rate which is 82% of the theoretical capacity (96 mAh g−1 for one-electron reaction) and 41.62 mAh g−1 at 2 C rate Further, it retains a discharge capacity of 70 mAh g−1 over 100 cycles with good cyclability.

Anode Materials: Design Principles and Applications of Next‐Generation High‐Energy‐Density Batteries Based on Liquid Metals (Adv. Mater. 29/2021)

Liquid metals with superior mechanical properties and low reduction potentials are a class of promising metal anodes for high-energy-density batteries, that are intrinsically dendrite-free and conformal at various interfaces. Designs based on liquid metals promise intermittent renewable energy resources as power sources for flexible electronics, electric automobiles, and even grid-scale energy-storage systems, as discussed by Guihua Yu and co-workers in article number 2100052.

In Situ Construction of a Multifunctional Quasi-Gel Layer for Long-Life Aqueous Zinc Metal Anodes.

Aqueous zinc (Zn)-ion batteries are considered very promising in grid-scale energy storage systems. However, the dendrite, corrosion, and H2 evolution issues of Zn anode have restricted their further applications. Herein, to solve these issues, a hydrophilic layer, consisting of a covalent organic polymer (COP) and carboxylmethyl cellulose (CMC), is designed to in situ construct a multifunctional quasi-gel (COP-CMC/QG) interface between Zn metal and the electrolyte. The COP-CMC/QG interface can significantly improve the rechargeability of the Zn anode through enhancing Zn2+ transport kinetics, guiding uniform nucleation, and suppressing Zn corrosion and H2 evolution. As a result, the COP-CMC-Zn anode exhibits a reduced overpotential (12 mV at 0.25 mA cm-2), prolonged cycle life (over 4000 h at 0.25 mA cm-2 and 2000 h at 5 mA cm-2 in symmetrical cells), and elevated full-cell (Zn/MnO2) performance. This work provides an efficient approach to achieve long-life Zn metal anodes and paves the way toward high-performance Zn-based and other metal-ion batteries.

Stabilizing zinc deposition with sodium lignosulfonate as an electrolyte additive to improve the life span of aqueous zinc-ion batteries

Thanks to high safety and low cost, rechargeable zinc-ion batteries (RZIBs) have become a promising candidate for grid-scale energy storage systems. However, zinc anodes suffer from severe dendrite growth and irreversible side reactions, leading to poor cyclability of RZIBs. In this work, low-cost sodium lignosulfonate (SL) is utilized as the electrolyte additive to solve this problem. The added amount of SL is optimized to be 0.02%, which enables the Zn//α-MnO2 battery to deliver a large capacity of 146 mAh g−1 after 1000 cycles at 1 A g−1, corresponding to a high capacity retention of 83.5%. The Zn//Zn symmetric cell with the modified electrolyte also shows excellent cyclability even under a commercial level of areal specific capacity (4 mAh cm−2). Overall, the results of this study confirm that the SL additive can improve the ionic conductivity of electrolyte, restrict the two-dimensional planar diffusion of Zn2+ ions at the electrode/electrolyte interface, lower the nucleation overpotential of Zn2+ ions, prevent side reactions, and inhibit the corrosion of Zn metal. Therefore, the dendrite growth and byproduct formation can be effectively suppressed. This study provides new insights into protecting metal electrodes of electrochemical energy storage devices.

POWER management and control of A PHOTOVOLTAIC system with hybrid battery-supercapacitor energy storage BASED ON HEURISTICS METHODS

Grid-scale electrical energy storage (EES) systems are enabling technologies to enhance the flexibility and reliability of electricity grids with high penetration of intermittent renewable energy sources such as solar and wind. They allow excess of generation to be stored for later use and can respond quickly to power fluctuations. Unfortunately, there is no single type of EES technology that can effectively fulfill all the desired requirements. Hybrid EES (HEES), combining two or more EES, are an emerging, viable solution. While batteries have a higher energy density, supercapacitors (SC) have a higher power density and are characterized by a fast discharge rate. The combination of both technologies results in a HESS solution which can address the challenges associated with the large-scale deployment of distributed renewable energy sources and enhance the grid reliability. The goal is to design a power management strategy to enhance the performance of the HEES. This paper proposes various robust design methods for the control of the power electronics converters and enhance the performance of the power management of the HEES. These robust design strategies are based on pole placement, linear matrix inequalities (LMI), particle swarm optimization (PSO) and genetic algorithm (GA). The performance of these control schemes is compared in terms of the transient response time and robustness. The results obtained demonstrate the effectiveness of the power management strategy (PMS) for the photovoltaic (PV) system with HEES and the enhanced robustness of the controllers using GA and PSO-based tuning techniques.

Inspired by “quenching-cracking” strategy: Structure-based design of sulfur-doped graphite felts for ultrahigh-rate vanadium redox flow batteries

Vanadium redox flow batteries (VRFBs) are perceived as promising candidates for grid-scale energy storage systems. However, limited improvements in electrode structures restrict the operation of VRFBs at high current densities. Herein, finite element simulations are used to guide the construction direction of the electrode structure. Afterwards, a quenching-cracking strategy is ingeniously employed to successfully construct parallel-aligned micron flow channels on electrode fibers in high agreement with the model, and the consistency of the flow channel structure is verified via deep learning technique. The well-constructed flow channels achieve high specific surface areas of electrodes while enabling the smooth flow of electrolyte over the fiber surfaces. Subsequent graphitization and sulfur-doping processes yield hierarchical fibers with highly conductive cores and well-active surfaces. Benefiting from fine structural modulation, the battery equipped with the as-prepared electrodes delivers an energy efficiency of 80.44 % at an ultra-high current density of 500 mA cm−2 and achieves a peak power density of 1.68 W cm−2. Additionally, the battery is consistently cycled for 1000 cycles at 500 mA cm−2 and the average energy efficiency decay is only 0.01032 % per cycle. Notably, finite element simulations are applied to investigate the velocity distribution of electrolyte in the flow channels, and first-principle calculations are employed to reveal the cause for energy efficiency decay of the battery after long-term cycling. Most importantly, the establishment of structure-activity relationships highlights the significance of comprehensive modulation of electrode fiber structures towards enhancing the performance of VRFBs.

Optimal Cell Utilization for Improved Power Rating and Reliability in a Grid-Scale Three-Phase Battery Energy Storage System Using Hybrid Modular Multilevel Converter Topology Without Redundant Cells

Redundant cells within a battery energy storage system (BESS) are an important aspect to be considered in a BESS planning, design, and operation in order to achieve high reliability. The conventional topology does not have the ability to take advantage of its idle cells, at least one-third of its cells are constantly idling in a typical three-phase BESS. This article presents a new topology for a three-phase grid-scale BESS, where it offers three ways to deal with its idle cells. First, the idle cells were eliminated from the BESS plant while still achieving the desired output power with reduced cost. Second, the idle cells were utilized where no redundant cell is required. Finally, they were utilized to meet the output power required during peak demand. Consequently, the overall construction cost of BESS plant, losses, size, and control complexity are reduced while the reliability is improved. The simulation results indicate that at least 140 out of 333 modules (3996 cells) are in an idle state during the BESS operation, with the three-phase output voltage achieved in both topologies. Using 333 modules, the three-phase output power has increased by 58.8% in the proposed topology (306 kW) compared with the conventional topology (180 kW).

An Artificial Polyacrylonitrile Coating Layer Confining Zinc Dendrite Growth for Highly Reversible Aqueous Zinc-Based Batteries.

Aqueous rechargeable zinc‐metal‐based batteries are an attractive alternative to lithium‐ion batteries for grid‐scale energy‐storage systems because of their high specific capacity, low cost, eco‐friendliness, and nonflammability. However, uncontrollable zinc dendrite growth limits the cycle life by piercing the separator, resulting in low zinc utilization in both alkaline and mild/neutral electrolytes. Herein, a polyacrylonitrile coating layer on a zinc anode produced by a simple drop coating approach to address the dendrite issue is reported. The coating layer not only improves the hydrophilicity of the zinc anode but also regulates zinc‐ion transport, consequently facilitating the uniform deposition of zinc ions to avoid dendrite formation. A symmetrical cell with the polymer‐coating‐layer‐modified Zn anode displays dendrite‐free plating/stripping with a long cycle lifespan (>1100 h), much better than that of the bare Zn anode. The modified zinc anode coupled with a Mn‐doped V2O5 cathode forms a stable rechargeable full battery. This method is a facile and feasible way to solve the zinc dendrite problem for rechargeable aqueous zinc‐metal batteries, providing a solid basis for application of aqueous rechargeable Zn batteries.

Assessing and mitigating potential hazards of emerging grid-scale electrical energy storage systems

Electrical energy storage (EES) systems consisting of multiple process components and containing intensive amounts of energy present inherent hazards coupled with high operational risks. Although the thermal hazards of batteries have aroused widespread attention, the safety issues of emerging large scale EES technologies persist. This study aims to begin to fill this gap by examining the hazards of typical 100 MWh or more EES systems which are used for grid applications. These systems include compressed and liquid air energy storage, CO2 energy storage, thermal storage in concentrating solar power plants, and Power-to-Gas. Hazard assessments are performed using a hybrid method to consider and evaluate the EES systems’ potential hazards from three novel aspects: storage, operability, and connectivity. Results reveal that for a similar energy storage capacity, cryogenic liquid systems have the least severe accident consequences while thermal energy storage using synthetic oil exhibits the largest. Concerning the operations in many cases, extreme operating conditions, complicated heat exchanger networks involving multiple flammable working fluids, and system operation intermittency present the major challenges to the safer operation of EES systems. Lastly, these systems themselves form one of many components of the power supply, each of which needs to accommodate fluctuations in supply and demand but also should be prevented from transmitting hazards to each other. Considering both engineering and administrative controls, this paper concludes with a discussion on the four grouped strategies from inherent to procedural for the elimination and mitigation of the identified hazards. Representative solutions and research perspectives including inherently safer design, operation uncertainty management, resilience analysis, energy barriers design, and life cycle safety assessment are suggested for the overall safety enhancement of industrial EES systems.

Nanowelding to Improve the Chemomechanical Stability of the Ni-Rich Layered Cathode Materials

To satisfy the increasing energy density requirements for electric vehicles and grid-scale energy storage systems, Ni-rich layered oxide cathode materials are often fabricated as micron-sized secondary spherical particles consisting of nanosized single crystals. Unfortunately, the hierarchical structure inevitably induces intergranular cracks and parasitic reactions at the cathode-electrolyte interphase, aggravating chemomechanical instability and seriously hindering their practical application. Here, we propose a nanowelding strategy to build consolidation points at the grain boundary of the primary particles, which dramatically enhances the capacity retention and chemomechanical stability. Meanwhile, the oxygen vacancies in the ceria-based solid electrolyte possessing oxygen adsorbing and storage capability can restrain the active oxygenates in the surficial lattice to avoid oxygen evolution. Experimental characterization further confirms that this unique architecture can effectively prevent the liquid electrolyte from penetrating into the active material along the grain boundary and consequently eliminate the adverse effects, including intergranular cracks, cathode electrolyte interface formation and growth, and the layered structure-rock salt phase irreversible transition. This finding provides a promising approach to realize the rapid commercialization of highly stabilized nickel-rich cathode materials for high-performance lithium-ion batteries.

Pseudocapacitive MoOx anode material with super-high rate and ultra-long cycle properties for aqueous zinc ion batteries

Aqueous zinc-ion batteries (AZIBs) are widely concerned and considered as promising candidates for grid-scale energy storage systems due to the simple and feasible preparation process together with the demonstrated electrochemical properties, but it is still a tremendous challenge to develop new-type anode materials which can effectively overcome the zinc dendrites formation and low utilization efficiency of the typical zinc metal anodes, not to mention the ones with outstanding rate and cycle properties. Herein, for the first time, monoclinic MoOx is verified to be a novel and promising pseudocapacitive anode material for aqueous zinc-ion batteries and the MoOx shows excellent zinc ion storage abilities-especially at 10 A g−1, including ideal average potential of 0.53 V (vs. Zn2+/Zn), high discharge/charge capacities of 82.5/82.0 mAh g−1 and cycling life of 30,000 cycles. Further, zinc ion storage mechanism of the as proposed monoclinic MoOx is also studied and deduced to be: MoOx + yZn2++2ye−⇌ ZnyMoOx.

Investigation of a Biomass Hydrogel Electrolyte Naturally Stabilizing Cathodes for Zinc-Ion Batteries.

Aqueous zinc-ion batteries (AZIBs) have the potential to be utilized in a grid-scale energy storage system owing to their high energy density and cost-effective properties. However, the dissolution of cathode materials and the irreversible extraction of preintercalated metal ions in the electrode materials restrict the stability of AZIBs. Herein, a cathode-stabilized ZIB strategy is reported based on a natural biomass polymer sodium alginate as the electrolyte coupling with a Na+ preintercalated δ-Na0.65Mn2O4·1.31H2O cathode. The dissociated Na+ in alginate after gelation directly stabilizes the cathodes by preventing the collapse of layered structures during charge processes. The as-fabricated ZIBs deliver a high capacity of 305 mA h g-1 at 0.1 A g-1, 10% higher than the ZIBs with an aqueous electrolyte. Further, the hybrid polymer electrolyte possessed an excellent Coulombic efficiency above 99% and a capacity retention of 96% within 1000 cycles at 2 A g-1. A detailed investigation combining ex situ experiments uncovers the charge storage mechanism and the stability of assembled batteries, confirming the reversible diffusions of both Zn2+ and preintercalated Na+. A flexible device of ZIBs fabricated based on vacuum-assisted resin transfer molding possesses an outstanding performance of 160 mA h g-1 at 1 A g-1, which illustrates their potential for wearable electronics in mass production.

The Renaissance of Liquid Metal Batteries

Next-generation batteries with long life, high-energy capacity, and high round-trip energy efficiency are essential for future smart grid operation. Recently, Cui et al. demonstrated a battery design meeting all these requirements—a solid electrolyte-based liquid lithium-brass/zinc chloride (SELL-brass/ZnCl2) battery. Such a battery design overcomes some inherent problems of conventional ZEBRA batteries and lithium ion batteries.

A techno-economic assessment of battery business models in the UK electricity market

The UK's transition to a zero-carbon economy is inevitably leading to an electricity grid with a high penetration of intermittent renewable energy generation, resulting in the need to install grid-scale energy storage systems at an accelerating rate. This study has taken the perspective of a business owner to assess the profitability of deploying battery technologies in the UK electricity market by analysing arbitrage in electricity markets, capacity market auctions, and ancillary services (frequency response and reserve). Additional work was conducted for integrating battery arbitrage cycling with solar and wind generation, comparing the utilisation of a linear programming model against a simplistic threshold cycling model. An overall economic assessment of value streams found that the lowest payback periods for premium ancillary services were 3.3 years (advanced lead-acid battery providing fast reserve) and 6.6 years (lithium-ion battery providing frequency response). Moreover, this research has demonstrated promising returns on investment for stacking short-term operating reserve with arbitrage in the balancing market. In the near future, battery systems will likely attain accelerated deployment due to reforms in ancillary service structures, cost reductions in battery technologies, greater price volatility in the balancing market, and effective policy incentives introduced by the UK government.

Exploring Redox Active and Electrochemically Stable Organic Molecules for > 3 V Non-Aqueous Redox Flow Batteries

Non-Aqueous Redox flow batteries (NARFBs) are emerging grid-scale energy storage systems that have the potential to overcome issues associated with aqueous RFBs, such as a limited voltage window due to water electrolysis and low energy densities. However, most non-aqueous electrolytes consist of metal coordination complexes that have significantly lower solubility due to their large molecular size compared to their aqueous counterparts and their cell voltages are typically limited to < 3 V.1 Besides, widespread acceptance of metal coordination complexes as the non-aqueous electrolyte is also restricted by the higher cost of the metal. Many of these metal coordination complexes undergo disproportionation; therefore, resulting flow batteries do not require selective membranes. However, cost targets outlined by Darling et al. and the U.S. Department of Energy require redox active materials to have > 3 V cell voltage and 4-5 M solubility and ~$5/Kg material cost.2 Therefore, there have been substantial efforts made to develop organic molecules either as anolyte or catholyte that meet many of these criteria for application in NARFBs. Some reported redox active organic molecules have solubility up to 5 M in common organic solvents.3-4 The lower molecular weights of organic molecules provide more room and freedom for molecular engineering to improve solubility, stability, and increase the potential window. However, deployment of these molecules in asymmetrical NARFBs raises the inherent issue of RFBs, i.e., irreversible capacity fade due to the absence of selective membranes.4 In this presentation, we will report a new class of sulfur and oxygen-containing redox active organic molecules for high voltage (> 3 V) NARFBs. Specifically, we will highlight the screening procedure and electrochemical properties of various anolytes and catholytes with higher redox potentials. We will also discuss strategies to minimize the capacity fade due to crossover. As proof of concept, anolyte and catholyte will be combined in a full cell, and cycling performance will also be discussed. Acknowledgements: The authors would like to thank Dr. Imre Gyuk and the U.S. Department of Energy, Office of Electricity for supporting this work.

Triggering the Reversible Reaction of V3+/V4+/V5+ in Na3V2(PO4)3 by Cr3+ Substitution.

Sodium-ion batteries (SIBs) have grabbed worldwide attention as an alternative to lithium-ion batteries on account of the abundance and accessibility of the sodium element in nature. For the sake of meeting the requirements for various applications containing grid-scale energy storage system, electric vehicles, and so forth, a stable and high-voltage cathode is decisive to enhance the energy and power density of SIBs. In this research, sodium super ionic conductor structured Na3V1.5-xCr0.5+x(PO4)3 with different V/Cr ratios to balance the V3+/V4+ and V4+/V5+ redox couples was investigated as the potential cathode for SIBs. Among these candidates, Na3V1.3Cr0.7(PO4)3 manifested high energy density together with good cycling performance and rate capability. Combining the structural analysis and density functional theory calculation, the underlying mechanism of V3+ substitution by Cr3+ was uncovered, accounting for the improvement of electrochemical performance.