Aqueous liquid electrolytes are pumped from storage tanks to flow-through electrodes, where chemical energy is converted to electrical energy discharge or vice versa charge. Between the anode and cathode compartments is a membrane that selectively allows cross-transport of non-active species e. Unlike traditional batteries that store energy in electrodes, FRB batteries are more like regenerative fuel cells in which the chemical energy in the incoming fuels is converted into electricity at the electrodes.
As such the power and energy capacity of an FRB system can be designed separately. The power kW of the system is determined by the size of the electrodes and the number of cells in a stack, whereas the energy storage capacity kWh is determined by the concentration and volume of the electrolyte. Both energy and power can be easily adjusted for storage from a few hours to days or even weeks, depending on the application, which is another important advantage for the renewable integration. Generally, FRB sustains no damage to the cells when completely discharged, although overcharging may need to be avoided.
There is only negligible self-discharge irreversible loss in optimized flow systems, and generally no problems associated with short circuiting. The liquid electrolyte and intimate interfaces with electrodes make high current densities and quick response in a matter of sub-seconds possible for utility applications. Simplicity in cell and stack structure allows for building large systems based on module design, which is another important advantage for electrical grid applications. A critical issue with the early redox system was the permeation of iron species into the chromium electrolyte and vice versa, causing quick performance degradation.
The energy conversions in the battery are realized via changes in vanadium valence states through the following electrode reactions:. The overall electrochemical reaction gives a standard cell voltage of 1. Since then systems up to MWh levels were developed and demonstrated.
In , Sumitomo Electric Industries successfully demonstrated a 4. Figure 3 depicts electrochemical reactions of these two RFBs. The PSB systems employ electrolytes of sodium bromides and sodium polysulfides. In ZBB, the electrolyte is an aqueous solution of zinc bromides plus agents. These two batteries have a higher voltage than VRB and potential higher energy densities. But their cycle-life, efficiency, and reliability may be inferior to VRB. In addition, the formation of zinc dendrites upon deposition and the high solubility of bromine in the aqueous zinc bromide electrolyte has hindered the ZBB development.
With all the stated advantages and the successful demonstration of systems up to MWh levels, none of the RFB technologies have seen broad market penetration. First and foremost, the current technologies are still expensive. This type of battery is known as a sodium-sulfur Na-S battery. Both Na-S and ZEBRA are traditionally built in tubular designs, as schematically shown in Figure 4, which also depicts their electrochemical reactions. The Na-S battery was initially developed by the Ford Motor Company in the late s and s for electrical vehicle applications, and halted in the mids with the emergence of battery technologies such as nickel-metal hydride and later lithium-ion.
By the late s, varied systems up to the MWh scale had been developed. A number of MWh systems have been demonstrated on the electrical grid. The Na-S battery demonstrates an energy density comparable to some lithium-ion chemistries. In addition, the molten electrodes in the battery ensure a high current density and a quick response to changing power conditions. There is also the need to improve safety, durability, reliability, etc. Molten sulfur is not a good conductor and corrosive to the container. A structural breakdown of the oxide electrolyte would lead to direct contact of molten sulfur and sodium, resulting in fire or potential catastrophic failure.
During off-times, the system must be maintained at elevated temperatures. Freezing cycles induce mechanical stress, potentially causing structural failure often after only a few cycles. The cost is still too high for broad market penetration, although technology advancement and scaled production have reduced it. The use of solid or semisolid cathodes makes Na-NiCl 2 batteries intrinsically safer and less corrosive than Na-S batteries. The high voltage of Na-NiCl 2 batteries helps energy density.
Nevertheless, there is need of further improvement in power, reliability, etc. Lithium-ion Batteries Lithium-ion batteries store electrical energy in electrodes made of lithium-intercalation or insertion compounds Figure 5. Among these is that the early lithium-ion chemistries are inherently unsafe. The lithiated-graphite electrode operates at a potential close to that of metallic lithium, leading to lithium-dendrite growth and potential electrical shorting. In the presence of flammable organic electrolyte solvents currently in use, there is a risk of heat generation, thermal runaway, and fire.
An additional challenge is the high cost that may not be critical to electronic applications, but is very important for scaled-up vehicle applications, which so far consider lithium-ion as the most promising technologies. In the past decade or so, substantial progress has been made in advancing the lithium-ion technologies, mainly driven by broad interests and extensive efforts for hybrid, plug-in hybrid, and electrical vehicles.
While promising progress has been made with the high-capacity alloy anodes, structural stability issues remain that are ascribed to large-volume expansion during alloying with lithium. While sacrificing energy density to some extent, the relatively high potential versus lithium makes titanate electrodes intrinsically safer than graphite. There are no, or few, side reactions with electrolytes directly related to the irreversible capacity and power loss. This allows for the use of nanostructures to improve rate capability, and thus power, without side reactions with electrolytes.
The good chemical compatibility, along with the relative high potential vs. Other alternatives include LiMn 2 O 4 spinel and its derivatives that have a voltage of over 4. However, the material exhibits low lithium-ion and electronic conductivity. Introducing nanostructuring, carbon coatings, and doping shortens the lithium-diffusion distance and enhances electron conduction, substantially improving the performance of the olivine structured chemistry as cathodes. There has been discussion on the use of the lithium-ion battery stacks after their life service on hybrid or electrical vehicles.
This would extend the values of the batteries that are initially developed for the transportation applications. It remains questionable, however, if the after-use batteries can meet the performance and economic matrix for stationary applications. Alternatively, there are increasing incentives and the need to develop lithium-ion batteries specifically for stationary applications.
This becomes particularly important, given the difference in requirements between stationary and transportation applications and the fact that, currently, no lithium-ion chemistry meets the performance and economic matrixes for both applications. The lithium-ion technologies for the stationary applications should focus on cost-effective chemistries or materials that provide long calendar and cycle life. Altairnano developed a lithium-ion battery based on nano-titanite anodes and demonstrated up to 2. Operating with a relatively lower voltage 2. However, it could only last up to 15 minutes at the name-tag power.
A developed a lithium-ion battery based on nanostructured LiFePO 4 cathode and demonstrated up to 2. Similarly, the A system lasted only 15 minutes at the name power. Heat management appears among major challenges for better technologies. Overall, lithium-ion technologies have not yet been fully demonstrated to meet the performance and economic matrix for the utility sector. Further investment and efforts are needed to develop suitable lithium-ion technologies that can support increasing penetration of renewable energy and stabilizing of the electrical grid.
Significant advancements are needed in materials, processing, design, and system integration for the technologies to achieve broad market penetration.
The current trend toward reducing greenhouse gas emission and increasing penetration of renewable energy, along with increasing demands of high-quality power, calls for urgent development and implementation of EES. Without suitable EES, the current electrical grid could allow for only a limited level of penetration of renewable energy generated from intermittent sources such as wind and solar. Over-penetration would destabilize the grid, potentially causing shutdowns and even blackouts. Further challenging is that intermittent renewable sources are typically rich in certain areas, which are often far away from load centers.
Installing EES into the grid would not only facilitate increasing penetration of renewables, but ensure quality power for a society becoming increasingly digitized. Implementing EES would help reduce greenhouse gas emissions by replacing fossil-burning turbines currently employed to stabilize the grid. Energy storage can be a key enabler for a future grid that integrates extensive renewable generation and provides power for plug-in vehicles. Detailed studies on effective and economically viable use of EES in the future grid are needed. A number of potential technologies for EES exist, and some of these have been demonstrated for utility applications.
However, these technologies are facing either challenges in meeting the performance and economic matrix for the stationary applications, or limits in environment, site selection, etc. This calls for both basic and applied research to further develop current technologies and to discover new technologies that can address the needs for renewable and utility applications. This is particularly true compared to storage technology research and development for vehicle applications. Unfortunately, there are only a few government-funded programs worldwide for developing electricity storage technologies for stationary applications.
There is a general public and political lack of awareness of the need for new technology for these applications. Even renewable energy industries are reluctant to lend support due to concerns about adding extra cost to renewable power systems as they struggle to reduce system cost. Lately, however, there appear signs that the current trend is reversing.
Along with the electrical storage for vehicle applications, development and demonstrations of largescale storage technologies have been proposed in the American Recovery and Reinvestment Act of A number of other countries have also shown increasing interest in stationary storage research and development, suggesting a bright outlook for development of stationary energy storage technology for the future electric grid.
Rising to the Challenge: U.S. Innovation Policy for the Global Economy.
The authors acknowledge financial support from the U. Imhoff, and Jamie D. Yang can be reached at zgary. Overview: Energy Storage Technologies. Daily profiles of wind power projected by 7x output in April for the year in Tehachapi, California. Courtesy of ISO California. A schematic of applications of electricity storage for generation, transmission, distribution and end uses, and a future smart grid that integrates with intermittent renewables and plug-in hybrid vehicles through two-way digital communications between loads and generation or distribution grids. A schematic of a redox flow battery and selected redox chemistries.
Single-cell and tubular design of a sodium-beta battery and electrode reactions. A comparison of varied electrical storage technologies: a discharge time hours vs. With increasing use of renewable power generated from intermittent sources such as solar and wind, interest has grown in research and development of stationary electrical storage. To help the materials community gain insight into the emerging area, this paper offers an overview on the needs, requirements, and potential technologies. A number of existing and emerging technologies are potential candidates for energy storage applications.
All these technologies are, however, facing challenges to meet economic and performance targets for wide market penetration, which requires substantial advances in materials, design, system engineering, etc. Batteries are widely used to store electrical energy for electronics and now hybrid vehicles. Can these batteries be used to store renewable energy? The answer may be yes.
But they have to be capable of storing it at large scales and being costeffective. Substantial advancement is required for the current battery technologies, along with nonelectrochemical means, to meet the economic and performance targets. Wang et al. The ex situ XRD results Figure 11 d of electrodes obtained at different stages of discharging indicated progressive amorphization of V 2 O 5.
When the electrolyte was modified to eliminate all Al ions in solution, however, Wang et al. With discharge, graphite sheets also exhibited an increase in interlayer distances by 0. This gave strong evidence of the crystallographic insertion of foreign species into the graphitic layers. By using multiple characterization techniques, Nacimiento et al. First, semiquantitative microanalysis was used to determine the composition of raw, first charged, and discharged samples, and agreed well with the corresponding theoretical composition calculations obtained from electrochemical data Table 6.
Third, XPS results indicated high Al peak intensity in discharged samples, whereas vanadium peaks reversibly shifted to higher and lower energy values following battery charge and discharge, respectively. Based on all these characterizations, Nacimiento et al. At a fundamental level, batteries store energy via the redox coupling of the active materials within the negative electrode and the active materials within the positive electrode. However, for energy conversion to occur, auxiliary battery components such as the electrolyte and the current collector must be in perpetual interaction with the active materials.
A poor control of deleterious side reactions generated within ZIABs and AIABs would cripple any prospects for their mainstream adoption in electric grids. This prediction was later verified experimentally by Knight et al. Besides the stable coordination environments of the intercalating cation within the lattice, the transitory coordination environments involved in the cation migration path can also dictate the level of cation mobility.
Rong et al. During ion hopping, the coordination configuration changes between octahedra and square pyramids in layered V 2 O 5 , whereas the coordination configuration changes in spinel Mn 2 O 4 Figure 13 b are between octahedra and tetrahedra. Based on these energetics arguments, Rong et al. However, Rong et al. On the other hand, replacing the oxide sublattice with a sulfide sublattice typically induces better ion mobilities.
This information is summarized in what is known as an E —pH or Pourbaix diagram, for example, Figure 14 a,b for manganese and vanadium. During operation, when MnO 2 cathodes cycle between 0 and 1. Only at applied potentials higher than 1. Here, it is important to realize that the dynamic transformations between solid and aqueous phases of a transition metal are not necessarily reversible. The presence of dissolved oxygen in aqueous battery systems has been shown to be damaging to electrochemical cycling performances by Luo et al.
Aqueous electrolytes are typically less stable than nonaqueous electrolytes from an electrochemical standpoint. This is because water has a thermodynamic oxidation potential and a thermodynamic reduction potential which differ by a narrow 1. This limited voltage window often means that water electrolysis may occur when water is placed in direct contact with materials containing metallic elements, either readily at rest or with an externally applied bias. With more cations intercalated, the host material undergoes electrochemical reduction.
Forcing current beyond this anodic potential limit may induce redox coupling directly between the intercalated host material and H 2 O. This equilibrium exists at a potential V M. In theory, an electrode potential higher than V M would drive this reaction backward. That is, under an applied bias potentiodynamically shifting toward the negative voltage direction, the host material undergoes reduction until reaching V M. Therefore, only above V M would cations exist in a stable state within the host material. Based on the argument above, equilibrium at a lower V M is favorable for stabilizing intercalation host materials, while equilibrium at a higher V M renders electrodes to be progressively unstable in an aqueous battery system.
Hence, lower values of V M increase the choices for intercalation host materials for electrodes in aqueous battery systems. In this section, we derive V Li , V Zn , and V Al parameters in the presence and in the absence of dissolved O 2 , using an approach similar to Li et al. Therefore in equilibrium:. With increasing salt concentration, V M decreases marginally by at most 0. Interestingly, the concentration dependence of V M is stronger for the case of Li as compared to Zn and Al.
V M shows a stronger dependence on pH, varying by a difference as large as 0. We also note that since kinetic effects and additional side reactions have been neglected during this derivation, the applicability of these equations is limited and some anomaly is expected. For example, 1 He et al. The galvanostatic intermittent titration technique profile during discharging also showed two separate regions Figure 16 d with much higher charge transfer resistance and diffusion resistance in region II.
While Sun et al. H 2 O was found to be negligible. It leads to the formation of a stable, ionically conductive, and electronically insulating layer which stabilizes the electrode—electrolyte interfaces EEIs. Unfortunately, the direct decomposition products of water in an aqueous electrolyte system are either volatile gases or soluble ions. Zhi et al. Based on this, Zhi et al.
Lithium-Ion Battery Systems and Technology | SpringerLink
Similarly, Son et al. The coated Mg anode showed a slight decrease in capacity over 40 cycles, whereas the capacity for a cell with uncoated Mg metal anode dropped to zero within 8 cycles. This study might open avenues for Al metal to be used as the anode in aqueous electrolytes. Suo et al.
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A preferential reduction of salt anions over water electrolysis was speculated to be responsible for the formation of a favorable battery SEI. It has been hypothesized that unlike the case of superconcentrated electrolyte, in diluted aqueous electrolytes, the reduction products can quickly hydrolyze and dissolve, and thus be unable to form stable SEI. In another work of their group, Suo et al. The importance of a protective coating is illustrated in Figure 18 a, wherein the cycling stability of the electrode is improved with carbon coating and by the introduction of LiOTf in WIS. On similar lines, Wang et al.
Hydrogen evolution is strongly dependent on the operational voltage range for cycling, the electrolyte formulation, and the current collector used. Using a Pourbaix diagram, we can design electrolytes with appropriate pH values to minimize H 2 and O 2 evolution.
Recently, Lahan and Das demonstrated the importance of choosing an appropriate current collector for aqueous systems in order to suppress H 2 evolution. To the left of this maximum, the propensity for H 2 evolution remains low as H bonds weakly with the surface. Currently, Ti and stainless steel are popular current collectors for ZIABs, although better substitutes are being explored.
On the other hand, for AIABs Table 5 , whereas stainless steel corrodes strongly in highly concentrated electrolytes of Al salts, Pt, Ti, and Ni are expected to show adequate corrosion resistance, but are expensive. Another way of avoiding corrosion is to completely cover all the sides of a current collector such that current collector—electrolyte contact is minimized. This can be done by 1 completely coating the current collector by active material on all sides and 2 covering the bare current collector area with unreactive coatings.
For example, Liu et al. Overall, the choice of current collector material seems trivial, but is crucial to avoid unwanted hydrogen evolution and corrosion effects. Nonetheless, advancements in widening the electrochemical stability window of aqueous electrolytes, , , and the usage of protective interphases, might allow the use of Al metal as an anode in the near future.
Among these, manganese oxide full cell ZIABs have realized competitive energy densities: However, most reports on ZIAB and AIAB electrode materials show ordinary electrochemical performance, owing to various challenges highlighted in this review. Solving these problems requires intricate battery engineering, controlling design parameters involving the active material, electrolyte, and current collector together Figure Here, it is important to note that adjusting one parameter may cause untoward effects on other parameters, ultimately causing inefficiencies in the overall battery performance.
Therefore, at the cell level, BESS battery components must be optimized in unison, and not individually. One of the most essential nuances in electrochemistry between aqueous batteries and nonaqueous batteries is the role of water during cation insertion in electrode materials. Water comes into play in three cases: 1 within the crystal structure of the electrode active material or crystal water ; 2 as part of the hydration sphere of the intercalating cation; 3 at the electrode surface as chemisorbed H 2 O molecules. It is possible for crystal water introduced into intercalation host materials during synthesis or cell operation to be beneficial in three ways, by: 1 enlarging ion tunnels or layers in the crystallographic lattice to allow more facile cation insertion Figure 22 a , 51 , 2 providing structural support to the lattice framework for stable repeated de intercalation, 39 , 51 , and 3 providing electrostatic shielding for intercalating cations to weaken cation—anion bonds along the migration pathway.
However, water is often tightly bound to aqueous MV ions in the form of a hydration sphere. Further, water may drive specific reactions at the electrolyte—electrode interface. However, most of these ideas are not well established with direct experimental proofs.
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This represents excess energy, and a high risk of overgeneration during afternoons whereas the opposite scenario is expected during late evenings. Data sourced from ref. Data sourced from refs. Reproduced with permission. MnO 6 octahedra and water molecules are represented by brown polyhedral and white spheres, respectively. This led them to propose the following reaction: Figure 3 Open in figure viewer PowerPoint.
Figure 4 Open in figure viewer PowerPoint. Figure 5 Open in figure viewer PowerPoint. Figure 7 Open in figure viewer PowerPoint. Figure 8 Open in figure viewer PowerPoint. Figure 9 Open in figure viewer PowerPoint. Figure 10 Open in figure viewer PowerPoint. Figure 11 Open in figure viewer PowerPoint. The peak at Adapted with permission. Pourbaix diagram of a manganese and b vanadium in water. Figure 15 Open in figure viewer PowerPoint. V b Charge—discharge curve for VO 1. Figure 18 Open in figure viewer PowerPoint. Figure 19 Open in figure viewer PowerPoint.
Figure 20 Open in figure viewer PowerPoint.