Grid-scale Energy Storage Systems Research Articles

Aqueous organic redox flow batteries

Redox flow batteries (RFBs) are promising candidates to establish a grid-scale energy storage system for intermittent energy sources. While the current technology of vanadium RFBs has been widely exploited across the world, the rise in the price of vanadium and its limited volumetric energy density have necessitated the development of new kinds of redox active molecules. Organic molecules can be used as new and economical redox couples in RFBs to address these issues. In addition, the redox organic species also provide ample advantages to increase the voltage and solubility, provide multiple numbers of electron transfer, and ensure electrochemical/chemical stability by molecular engineering through simple synthetic methods. This review focuses on the recent developments in aqueous organic RFBs, including the molecular design and the corresponding cycling performance as these organic redox molecules are employed as either the negolyte or posolyte. Various strategies for tuning the electrochemical/chemical characteristics of organic molecules have improved their solubility, redox potential, cycling stability, and crossover issue across a separating membrane. We also put forward new strategies using nanotechnology and our perspective for the future development of this rapidly growing field.

Intercalation of Mg2+ into electrodeposited Prussian Blue Analogue thin films from aqueous electrolytes

So-called Prussian Blue Analogues (PBAs) attract significant research interest for a large variety of electrochemical applications. Recently, they have been recognized as promising electrode materials to implement in aqueous Na-ion batteries for grid-scale energy storage systems. Further types of aqueous batteries using, for instance, alkali metal cations such as Li+ and K+ or multivalent ions like Mg2+, Zn2+ and Al3+ as insertion species for PBA-based electrodes are being explored. In this study we systematically investigate intercalation of Mg2+ into four electrodeposited PBA thin films: Na2Ni[Fe(CN)6], Na2Co[Fe(CN)6], Na2VOx[Fe(CN)6], and NaIn[Fe(CN)6]. All four electrodes are capable of hosting Mg2+, while NaIn[Fe(CN)6] shows better stability upon repeated (dis)charging compared to the other three systems. Surprisingly, even though the hydration energies of Na+ and Mg2+ differ significantly, the mean half-charge potentials of (de)intercalation are comparable for both cations. A correlation between the empirical radii of transition metals present in the structure of PBAs and the mean half-charge potential of (de)intercalation, previously identified for Na+, is also found for Mg2+.

Modular Multilevel Converter With Partially Rated Integrated Energy Storage Suitable for Frequency Support and Ancillary Service Provision

Grid scale energy storage systems (ESSs) have received significant interest in recent years due to their ability to reduce/defer investment in transmission/distribution networks, as well their ability to act as primary reserve sources and provide emergency support to the transmission system. This paper investigates the dual purposing of an HVdc-Scale modular multilevel converter (MMC), allowing it to also act as an ESS. This has potential application in primary frequency response provision and other services such as de-coupled power oscillation damping. In the proposed topology, a certain percentage of sub-modules (SM) within the MMC have their capacitor interfaced through a dc-dc converter to an energy storage element (ESE), formed of a battery or ultracapacitor. By applying appropriate control, energy can be exchanged from the ESE to the main SM capacitor of each ESE-SM, and from there to either the ac or dc bus. It was found that for some operating points, an injection of circulating current was required to facilitate exchanging energy with the ESE-SMs. Analysis shows that, for instance, an additional power injection to ac or dc terminal of 0.1 per unit (10%) is possible with only 4% of the SMs replaced by full-bridge ESE-SMs, and no additional SMs added.

High-Performance Flexible Freestanding Anode with Hierarchical 3D Carbon-Networks/Fe7 S8 /Graphene for Applicable Sodium-Ion Batteries.

Sodium-ion batteries (SIBs) have gained tremendous interest for grid scale energy storage system and power energy batteries. However, the current researches of anode for SIBs still face the critical issues of low areal capacity, limited cycle life, and low initial coulombic efficiency for practical application perspective. To solve this issue, a kind of hierarchical 3D carbon-networks/Fe7 S8 /graphene (CFG) is designed and synthesized as freestanding anode, which is constructed with Fe7 S8 microparticles well-welded on 3D-crosslinked carbon-networks and embedded in highly conductive graphene film, via a facile and scalable synthetic method. The as-prepared freestanding electrode CFG represents high areal capacity (2.12 mAh cm-2 at 0.25 mA cm-2 ) and excellent cycle stability of 5000 cycles (0.0095% capacity decay per cycle). The assembled all-flexible sodium-ion battery delivers remarkable performance (high areal capacity of 1.42 mAh cm-2 at 0.3 mA cm-2 and superior energy density of 144 Wh kg-1 ), which are very close to the requirement of practical application. This work not only enlightens the material design and electrode engineering, but also provides a new kind of freestanding high energy density anode with great potential application prospective for SIBs.

Rechargeable Na/Ni batteries based on the Ni(OH)2/NiOOH redox couple with high energy density and good cycling performance

Rechargeable battery systems that use Na-based anodes as alternatives to Li-ion batteries are highly desirable for grid-scale energy storage systems owing to the high abundance and low cost of Na.

Organic Eutectic Electrolytes for Future Flow Batteries

In this issue of Chem, Yu and coworkers report phthalimide-based eutectic anolytes, which achieved a high concentration and enhanced redox reversibility. The organic-molecule-based eutectic electrolytes take advantages of both the superior tunability of the organic molecule and the high molar concentration of the redox-active molecules of the eutectic solvents. With combined computational and experimental analysis, this work demonstrates that forming eutectic electrolytes with self-containing redox-active organics is a promising strategy for the future development of high-energy-density redox-flow batteries.

Unraveling the effect of salt chemistry on long-durability high-phosphorus-concentration anode for potassium ion batteries

Phosphorus-based anode materials are of considerable interest for grid-scale energy storage systems due to their high theoretical capacity. Nevertheless, the low electrical conductivity of P, large volume changes during cycling, and highly-reactive phosphide surface are hindering their potential applications. Herein, outstanding long-term cycling stability with high retained potassium storage capacity (213.7 mA h g−1 over 2000 cycles) was achieved via the introduction of an alternative potassium bis(fluorosulfonyl)imide (KFSI) salt and by using a layered compound (GeP5) with a high phosphorus concentration as anode material. Fourier transform infrared spectroscopic mapping results suggest that KFSI salt helps to form an uniform solid electrolyte interphase (SEI) layer and reduces the side reactions at the electrode/electrolyte interface, thus enhancing the cycling performance. In-operando synchrotron X-ray diffraction analysis has revealed the synergistic reaction mechanisms of the K-P and K-Ge reactions. These findings indicate the enormous potential of phosphorus-based anodes for high-performance potassium ion batteries and can attract broad interest for regulating the SEI layer formation through manipulating the salt chemistry.

Highly Stable Sodium Metal Anodes for Sodium Batteries By Tactically Regulating the Solid Electrolyte Interphase

Sodium (Na) metal holds great promise as an attractive anode material for grid-scale energy storage systems owing to its high theoretical specific capacity, low electrochemical potential, low cost and high natural abundance. However, its highly reactive nature with organic electrolyte leads to the formation of unstable solid electrolyte interphase (SEI) and dendritic/mossy Na growth. To overcome these issues, we present highly stable and dendrite-free Na metal anodes over a wide current range and long-term cycling via versatile tactical tunings of the SEI. Stable Na metal cycling in carbonate electrolyte without any additives over 100 cycles can be achieved at a current density up to 2 mA/cm2 with a high capacity of 3 mAh/cm2. In ether-based electrolyte, a superior cycling performance of Na anode can be achieved at a high current density up to 10 mA/cm2 and a capacity up to 5 mAh/cm2 over 100 cycles. Moreover, we found that despite the many similarities in the chemical and physical properties of lithium (Li) and Na, the strategy that was able to stabilize Li metal could not be directly applied to Na metal, but requires more meticulous investigations, thus providing new insights into understanding the differences between Li and Na systems.

Recent Advances in Sodium-Ion Battery Materials

Grid-scale energy storage systems with low-cost and high-performance electrodes are needed to meet the requirements of sustainable energy systems. Due to the wide abundance and low cost of sodium resources and their similar electrochemistry to the established lithium-ion batteries, sodium-ion batteries (SIBs) have attracted considerable interest as ideal candidates for grid-scale energy storage systems. In the past decade, though tremendous efforts have been made to promote the development of SIBs, and significant advances have been achieved, further improvements are still required in terms of energy/power density and long cyclic stability for commercialization. In this review, the latest progress in electrode materials for SIBs, including a variety of promising cathodes and anodes, is briefly summarized. Besides, the sodium storage mechanisms, endeavors on electrochemical property enhancements, structural and compositional optimizations, challenges and perspectives of the electrode materials for SIBs are discussed. Though enormous challenges may lie ahead, we believe that through intensive research efforts, sodium-ion batteries with low operation cost and longevity will be commercialized for large-scale energy storage application in the near future.

The development in aqueous lithium-ion batteries

To meet the growing energy demands, it is urgent for us to construct grid-scale energy storage system than can connect sustainable energy resources. Aqueous Li-ion batteries (ALIBs) have been widely investigated to become the most promising stationary power sources for sustainable energy such as wind and solar power. It is believed that advantages of ALIBs will overcome the limitations of the traditional organic lithium battery in virtue of the safety and environmentally friendly aqueous electrolyte. In the past decades, plentiful works have been devoted to enhance the performance of different types of ALIBs. In this review, we discuss the development of cathode, anode and electrolyte for acquiring the desired electrochemical performance of ALIBs. Also, the main challenges and outlook in this field are briefly discussed.

Electrode Materials of Sodium-Ion Batteries toward Practical Application

Advances in developing affordable batteries are vital for integrating renewable and environmentally friendly energy sources into the power grid. Benefiting from the abundance of sodium resources, sodium-ion batteries (SIBs) have attracted great attention as one of the most promising energy storage and conversion devices for grid-scale energy storage systems. From this perspective, we present a succinct and critical survey of the emerging electrode materials, such as layered transition-metal oxides, polyanionic compounds, Prussian blue analogue cathode materials, and hard carbon anode materials, that have potential value for large-scale applications.

Suppression of Dendrites in Lithium Sulfur Batteries Using a Composite Polymer Electrolyte (CPE)

High-performance rechargeable batteries are in great demand for portable electronic devices, electrical vehicles and grid-scale energy storage systems. Lithium sulfur (Li-S) batteries is considered as one of the most promising Li metal-based batteries due to their high theoretical energy density (2600 Wh kg-1)1, low cost and natural abundance2. However, the large volume expansion (~80%)3 of sulfur accompanying the electrochemical cycling, the low utilization of sulfur resulting from poor room – temperature electronic conductivity of sulfur (~10-15 S/cm) 4 , shuttle effect of highly soluble polysulfides in the organic ether-based electrolytes and Li dendrite growth results in fast capacity fading leading to safety concerns. As a result, commercialization of Li-S batteries thus far has been largely limited. To address these challenges facing the sulfur cathodes, significant efforts have been made to demonstrate advanced composite cathodes using various carbon materials5, polymers6 and metal-organic framework (MOF) materials7. In addition, solid electrolytes8 and electrolytes additives have also been identified to engineer a viable Li-S battery system. Despite these efforts to improve the performance of sulfur cathodes, suppression of dendrites growth on the lithium anode is the most crucial factor for practical application of Li-S batteries. Li metal, due to its high reactivity interacts instantly with most organic electrolyte solvents and Li salts to form a solid electrolyte interphase (SEI) layer that prevents further consumption of Li and electrolyte9. However, the SEI layer cannot withstand the volume change from repeated Li plating/stripping process due to its low mechanical strength. Hence, Li ions would aggregate toward the fresh Li exposed to the electrolyte due to the “tip effect”10 leading to uncontrolled Li dendrite growth. The dendrites protruding from the anode surface may pierce through the separator and cause internal short circuits and safety concerns. Due to the low utilization of Li metal and rapid consumption of electrolyte resulting from the Li dendrite growth, practical application of lithium metal as anode is highly restricted. In this work, we demonstrate the use of a composite polymer electrolyte (CPE) with fine-tuned physical and chemical properties to act as a separator-electrolyte complex in Li-S batteries. The superior mechanical properties of the CPEs coupled with excellent electrochemical conductivity and low electrolyte: sulfur (E:S) ratio will significantly facilitate prevention of both polysulfide dissolution and dendrite formation. Results of these studies will be presented and discussed. Acknowledgements: The authors acknowledge the financial support of DOE grant DE-EE 0006825 and DE-EE-0008199, Edward R. Weidlein Chair Professorship funds and the Center for Complex Engineered Multifunctional Materials (CCEMM). References V. S. Kolosnitsyn and E. V. Karaseva, Russian Journal of Electrochemistry, 2008, 44, 506-509.Y. Sun, G. Li, Y. Lai, D. Zeng and H. Cheng, Scientific Reports, 2016, 6, 22048.B. Dunn, H. Kamath and J.-M. Tarascon, Science, 2011, 334, 928-935.M. Edeling, R. W. Schmutzler and F. Hensel, Philosophical Magazine Part B, 1979, 39, 547-550.J. Jin, Z. Wen, G. Ma, Y. Lu and K. Rui, Solid State Ionics, 2014, 262, 170-173.P. J. Hanumantha, B. Gattu, O. Velikokhatnyi, M. K. Datta, S. S. Damle and P. N. Kumta, Journal of The Electrochemical Society, 2014, 161, A1173-A1180.P. M. Shanthi, P. J. Hanumantha, B. Gattu, M. Sweeney, M. K. Datta and P. N. Kumta, Electrochimica Acta, 2017, 229, 208-218.B. A. Boukamp, I. D. Raistrick, C. Ho, Y.-W. Hu and R. A. Huggins, in Superionic Conductors, eds. G. D. Mahan and W. L. Roth, Springer US, Boston, MA, 1976, DOI: 10.1007/978-1-4615-8789-7_65, pp. 417-417.E. Peled, C. Menachem, D. Bar‐Tow and A. Melman, Journal of The Electrochemical Society, 1996, 143, L4-L7.F. Ding, W. Xu, G. L. Graff, J. Zhang, M. L. Sushko, X. Chen, Y. Shao, M. H. Engelhard, Z. Nie, J. Xiao, X. Liu, P. V. Sushko, J. Liu and J.-G. Zhang, Journal of the American Chemical Society, 2013, 135, 4450-4456.

Engineering Pore Architecture into 2D Morphology: Synthesis of Multifunctional Nitrogen-Doped Porous Carbon Nanosheets Towards Long Calendar Life Lithium-Sulfur Battery

Owing to its high theoretical capacity, lithium-sulfur battery (Li-S) is considered as one of the leading alternatives to conventional lithium-ion batteries (LIBs) for electrical vehicles and grid-scale energy storage systems. The sulfur cathode has a theoretical capacity of 1675 mAh/g, accounting for an energy density as high as 2600 Wh/kg when coupled with lithium metal anode.1 Nevertheless, Li-S still need to overcome several challenges before being implemented in practical applications. The major issues include the low conductivity of sulfur, large volume change and severe polysulfide shuttling.2 In the recent reports, carbon material have been considered as an efficient sulfur host material to promote Li-S’s performance. However, it is difficult to solve all the problems simultaneously by a simple carbon material design. For instance, carbon nanotube has the advantages of its excellent conductivity, large surface area to volume ratio and prominent mechanical strength. However, it lacks porous sulfur reservoir to store the active sulfur, which makes their dissolution into electrolyte inevitable.3 Therefore, it is of great importance to develop multi-functional carbon materials for high-performance Li-S. Porous carbon materials have been intensively studied as the sulfur host due to its tunable porosity, large pore volume and controllable morphologies.4 The micro/meso pores can not only provide enough void space for sulfur volume expansion but also squeeze the cyclic sulfur molecules into short chain sulfur species and hence physically immobilize them from detrimentally shuttling to the anode side. To these aspects, we applied a dual-template strategy to synthesize nitrogen-doped two-dimensional porous carbon nanosheets (N-PCS). During the synthesis, we successfully integrating carbon pores into rational-designed secondary structure. The 2D carbon skeleton can effectively form conductive pathway for rapid electron/ion transfer. Moreover, when further doped with nitrogen, the carbon matrix enables a significantly improved conductivity and stronger interaction with polysulfides species due to the chemical bonding. With the advantages, when using N-PCS and sulfur composite as active material, an initial discharge capacity of 1360 mAh/g can be achieved at 0.1 C, showing a sulfur utilization as high as 81%. After 1000 cycles at 0.5 C, the cell can still retain ~50% of its initial capacity. At 1 C rate, N-PCS/S cathode is capable of delivering a high discharge capacity of 996 mAh/g, indicating its excellent electron conductivity and rate capability. Reference M. K. Song, E. J. Cairns and Y. Zhang, Nanoscale, 2013, 5, 2186-2204.Y. Yang, G. Zheng and Y. Cui, Chem. Soc. Rev., 2013, 42, 3018-3032.X. Liang, Z. Wen, Y. Liu, H. Zhang, J. Jin, M. Wu and X. Wu, J. Power Sources, 2012, 206, 409-413.S. Niu, W. Lv, G. Zhou, Y. He, B. Li, Q. H. Yang and F. Kang, Chem Commun (Camb), 2015, 51, 17720-17723. Figure 1

A Cost-Effective Lithium-Ion Battery Direct Recycling Process

Lithium-ion batteries have dominated the power source market for consumer electronics (W⋅h scale) and are becoming the dominant battery technology for electrified vehicles (EV, kW⋅h scale) and grid-scale electrochemical energy storage systems (MW⋅h scale) [1,2]. The growing ubiquity of lithium-ion batteries also brings a rapid growth in hazardous battery waste from manufacturing processes and end-of-life (EOL) disposal [3,4]. In the EV sector alone, approximately 50-100 million lithium-ion cells in the US and China will reach their EOL by 2020, thus generating large amounts of hazardous waste [5]. However, conventional pyrometallurgical and hydrometallurgical battery recycling processes require high temperature, strong acid leaching and extensive gas treatment, which lead to prohibitive cost, high energy consumption, water contamination and recycled materials with low resale value [6,7]. Since the cathode materials account for 30-40% of the overall battery cost, a cost-effective and eco-friendly direct recycling process that yields reusable battery-grade cathode materials can lower the energy consumption and battery materials cost. In this talk, we will demonstrate a scalable and cost-effective direct recycling process for EOL lithium-ion batteries to yield battery-grade cathodes. The recycled cathodes retain the microstructure and show equivalent electrochemical performance compared to the pristine cathode materials. We will also present life cycle analysis for several cathode materials widely used in current state-of-the-art electrified vehicles and compare the energy consumptions of different recycling approaches. Reference [1] M.S. Whittingham, Lithium Batteries and Cathode Materials, Chem. Rev. 104 (2004) 4271–4302. doi:10.1021/cr020731c. [2] M.S. Whittingham, Ultimate Limits to Intercalation Reactions for Lithium Batteries, Chem. Rev. 114 (2014) 11414–11443. doi:10.1021/cr5003003. [3] E. Gies, Recycling: Lazarus batteries, Nature. 526 (2015) 526S100a. doi:10.1038/526S100a. [4] L. Gaines, To recycle, or not to recycle, that is the question: Insights from life-cycle analysis, MRS Bull. 37 (2012) 333–338. doi:10.1557/mrs.2012.40. [5] Estimated based on the EV sales volume of US and China from 2011-2015, n.d. [6] J.B. Dunn, L. Gaines, J.C. Kelly, K.G. Gallagher, Life Cycle Analysis Summary for Automotive Lithiumion Battery Production and Recycling, in: Rewas 2016 Mater. Resour. Sustain., John Wiley & Sons, Inc., 2016: pp. 73–79. doi:10.1002/9781119275039.ch11. [7] L. Gaines, The future of automotive lithium-ion battery recycling: Charting a sustainable course, Sustain. Mater. Technol. 1–2 (2014) 2–7. doi:10.1016/j.susmat.2014.10.001.

Recent Progress in Iron-Based Electrode Materials for Grid-Scale Sodium-Ion Batteries.

Grid-scale energy storage batteries with electrode materials made from low-cost, earth-abundant elements are needed to meet the requirements of sustainable energy systems. Sodium-ion batteries (SIBs) with iron-based electrodes offer an attractive combination of low cost, plentiful structural diversity and high stability, making them ideal candidates for grid-scale energy storage systems. Although various iron-based cathode and anode materials have been synthesized and evaluated for sodium storage, further improvements are still required in terms of energy/power density and long cyclic stability for commercialization. In this Review, progress in iron-based electrode materials for SIBs, including oxides, polyanions, ferrocyanides, and sulfides, is briefly summarized. In addition, the reaction mechanisms, electrochemical performance enhancements, structure-composition-performance relationships, merits and drawbacks of iron-based electrode materials for SIBs are discussed. Such iron-based electrode materials will be competitive and attractive electrodes for next-generation energy storage devices.

Energy Management and Optimization Methods for Grid Energy Storage Systems

Today, the stability of the electric power grid is maintained through real time balancing of generation and demand. Grid scale energy storage systems are increasingly being deployed to provide grid operators the flexibility needed to maintain this balance. Energy storage also imparts resiliency and robustness to the grid infrastructure. Over the last few years, there has been a significant increase in the deployment of large scale energy storage systems. This growth has been driven by improvements in the cost and performance of energy storage technologies and the need to accommodate distributed generation, as well as incentives and government mandates. Energy management systems (EMSs) and optimization methods are required to effectively and safely utilize energy storage as a flexible grid asset that can provide multiple grid services. The EMS needs to be able to accommodate a variety of use cases and regulatory environments. In this paper, we provide a brief history of grid-scale energy storage, an overview of EMS architectures, and a summary of the leading applications for storage. These serve as a foundation for a discussion of EMS optimization methods and design.

Critical Review—Experimental Diagnostics and Material Characterization Techniques Used on Redox Flow Batteries

Grid-scale energy storage systems are of interest as the world increases reliance on renewable energy sources. Redox flow batteries are a type of grid-scale energy storage technology that shows exceptional promise for accommodating the dynamic output of wind and solar power sources. The research community employs dozens of diagnostic techniques to investigate nearly all facets of these devices. Material properties, operational losses, transport, and integrated system properties are studied through the lens of electrochemical, physical, and chemical phenomena that ultimately dictate cost by influencing efficiency, durability, power, and capacity. These diagnostic techniques can, if applied correctly, elucidate not only the types of losses in redox flow batteries, but also tie those losses to fundamental driving forces in such systems so that next generation systems and models can be designed. This review details various diagnostic techniques used in flow battery analysis. The benefits, unique insights, and limitations of these techniques are discussed. Recommendations are also made to assist researchers in identifying the diagnostics that can advance their particular investigations. The review concludes with a summary of opportunities for new diagnostics that are needed to enable solution of persistent issues in redox flow battery research and development.

Efficiency analysis for a grid-connected battery energy storage system

Efficiency is one of the key characteristics of grid-scale battery energy storage system (BESS) and it determines how much useful energy lost during operation. The University of Manchester has been commissioned with 240 kVA, 180 kWh lithium-ion BESS. This paper investigates round-trip efficiencies, comparing energy extracted from and returned to the grid, of the BESS for different operation modes. This is done by developing simulation switch-level model of power components of the BESS. The simulation is validated against hardware test results and used to identify optimum operation modes, which vary depending on SOC and charging and discharging power rates of the BESS.

Passive Deposition of Carbon Nanoparticles for Robust Kinetic Enhancement in Vanadium Redox Flow Batteries

Vanadium redox flow batteries (VRFBs) are a competitive technology for grid-scale energy storage due to numerous advantages over other electrical energy storage systems. Cost remains a limiting barrier, however, as VRFBs are currently slightly more expensive than Li-ion systems at grid scale. Much of the current research focused on VRFBs is intended to find electrode and membrane materials that are less expensive than those in current industrial use while yielding acceptable performance. An alternative avenue to reduced component cost for economic viability is system improvement to achieve greater power density while retaining 75 – 80% voltage efficiency. The added benefit of achieving efficient, high power is that such a system is better able to accommodate fluctuations in demand. As losses in electrochemical systems can be broadly separated into kinetic, ohmic, and mass transport overpotentials, system improvement is accomplished by minimizing these overpotentials. This work demonstrates the mitigation of kinetic losses in conventional carbon felt electrodes or carbon paper electrodes by in-situ addition of high specific surface area carbon nanoparticles. This is in contrast with commonly-reported electrode pretreatments intended to alter the chemical composition and electrochemical activity of highly graphitized carbon electrodes, an approach that frequently suffers from poor electrochemical durability. It will be shown that some efforts to perform passive electrode modification on carbon felt led to negative impacts on mass transport in the porous carbon felt, thus there is opportunity to balance kinetic enhancement with mass transport resistance in VRFBs. Additionally, similar kinetic enhancement with detrimental mass transport impacts was observed when a thick film of reduced graphene oxide (rGO) was deposited on carbon paper electrodes. Kinetic enhancement for both carbon electrode materials is shown via alternating charging and discharging polarization curves to characterize VRFB performance at a representative state of charge, electrochemical impedance spectroscopy, and is supported by materials characterization of pre- and post-treatment electrodes. The addition of carbon nanoparticles was accomplished in-situ, thus this method is not limited to pretreatment of electrodes, but can be a low-cost retrofit to existing systems. This work illustrates a favorable avenue to improve the performance of a VRFB with little disruption to operation. These insights and approach can be adapted to other efforts to reduce the cost of these grid-scale energy storage systems. In the figure, scanning electron micrographs show (a) smooth carbon felt before addition of reduced graphene oxide (rGO) flakes and (b) rougher fibers after passive deposition of rGO flakes. Alternating polarization curves in (c) were accomplished at 50% state of charge throughout, indicating a three-fold improvement in current density at 80% voltage efficiency. Figure 1

Overview of Lithium-Ion Grid-Scale Energy Storage Systems

This paper provides a reader who has little to none technical chemistry background with an overview of the working principles of lithium-ion batteries specifically for grid-scale applications. It also provides a comparison of the electrode chemistries that show better performance for each grid application. Two of the main causes driving the growth of stationary energy storage technologies are the increasing environmental regulations that promote a high penetration of non-dispatchable generation and policy changes in the electricity markets that benefit the profit of fast response energy resources such as a battery. The combination of these two factors is drawing the attention of investors toward lithium-ion grid-scale energy storage systems. We review the relevant metrics of a battery for grid-scale energy storage. A simple yet detailed explanation of the functions and the necessary characteristics of each component in a lithium-ion battery is provided. We also discuss the chemistries currently used for cathode and anode materials. Discussed will be the trade-off of several materials with respect to cost, thermal stability, cyclability, environmental friendliness, and other important characteristics to be considered for grid applications. This paper also discusses the commercial availability of lithium-ion batteries for grid-scale storage and presents some of the containerized battery storage solutions available in the market.