Li-ion batteries have been deployed in a wide range of energy-storage applications, ranging from energy-type batteries of a few kilowatt-hours in residential systems with rooftop photovoltaic arrays to multi-megawatt containerized batteries for the provision of grid ancillary services.

How Lithium Ion Batteries Work

The term “lithium-ion” refers not to a single electrochemical couple but to a wide array of different chemistries, all of which are characterized by the transfer of lithium ions between the electrodes during the charge and discharge reactions. Li-ion cells do not contain metallic lithium; rather, the ions are inserted into the structure of other materials, such as lithiated metal oxides or phosphates in the positive electrode (cathode) and carbon (typically graphite) or lithium titanate in the negative (anode).

The term “lithium polymer” (or more correctly, lithium-ion polymer) refers to a Li-ion design in which the electrodes are bonded together by a porous polymer matrix. Liquid electrolyte is infused into the porous matrix and becomes immobilized, allowing the electrode stacks to be assembled into foil “pouches” that provide geometric flexibility and improved energy density compared to cylindrical cells. However, such advantages are less significant as the cells are scaled up to larger capacities.

Note that there are also “lithium metal polymer” technologies, in which metallic lithium negative is implemented with a conductive polymer to make a solid-state battery system. Such technologies do not fall under the Li-ion umbrella and have not yet been successfully deployed in energy-storage applications.

Technologies with lithiated metal oxide positives and carbon negatives have high cell voltages (typically 3.6 V to 3.7 V) and correspondingly high energy density. These technologies have widely differing life and safety characteristics. Cells with positive materials based on lithium iron phosphate are inherently safer than their metal oxide/carbon counterparts but the voltage is lower (around 3.2 V), as is the energy density. Designs with lithiated metal oxide positives and lithium titanate negatives have the lowest voltage (around 2.5 V) and low energy density but have much higher power capability and safety advantages.

Li-ion cells may be produced in cylindrical or prismatic (rectangular) format. These cells are then typically built into multi-cell modules in series and/or parallel arrays, and the modules are connected together to form a battery string at the required voltage, with each string being controlled by a battery management system. Electronic subsystems are an important feature for Li-ion batteries, which lack the capability of aqueous technologies (e.g. lead-acid batteries) to dissipate overcharge energy. Safety characteristics of Li-ion batteries are ultimately determined by the attributes of system design, including mechanical and thermal characteristics, electronics and communications, and control algorithms – regardless of electrochemistry.

During discharge, an electron is released via an oxidation reaction from a high chemical potential state on the negative or anode side of the battery. The electron moves through an external circuit to do useful work. Finally, the electron is accepted via a reduction reaction at a lower chemical potential state on the positive or cathode side of the battery. The direction of the current and the chemical reactions are reversed during charging.

The total difference in chemical potential between the chemical states of the active elements on the two sides of the battery determines the electromotive force (emf or voltage) generated in each cell of the battery. The voltage developed by the RFB is specific to the chemical species involved in the reactions and the number of cells that are connected in series. The current developed by the battery is determined by the number of atoms or molecules of the active chemical species that are reacted within the cells as a function of time. The power delivered by the RFB is the product of the total current and total voltage developed in the electrochemical cells. The amount of energy stored in the RFB is determined by the total amount of active chemical species available in the volume of electrolyte solution present in the system.

How Redox Flow Batteries Work

The separation of power and energy is a key distinction of RFBs, compared to other electrochemical storage systems. As described above, the system energy is stored in the volume of electrolyte, which can easily and economically be in the range of kilowatt-hours to tens of megawatt-hours, depending on the size of the storage tanks. The power capability of the system is determined by the size of the stack of electrochemical cells. The amount of electrolyte flowing in the electrochemical stack at any moment is rarely more than a few percent of the total amount of electrolyte present (for energy ratings corresponding to discharge at rated power for two to eight hours). Flow can easily be stopped during a fault condition. As a result, system vulnerability to uncontrolled energy release in the case of RFBs is limited by system architecture to a few percent of the total energy stored. This feature is in contrast with packaged, integrated cell storage architectures (lead-acid, NAS, Li Ion), where the full energy of the system is connected at all times and available for discharge.

The separation of power and energy also provides design flexibility in the application of RFBs. The power capability (stack size) can be directly tailored to the associated load or generating asset. The storage capability (size of storage tanks) can be independently tailored to the energy storage need of the specific application. In this way, RFBs can economically provide an optimized storage system for each application. In contrast, the ratio of power to energy is fixed for integrated cells at the time of design and manufacture of the cells. Economies of scale in cell production limit the practical number of different cell designs that are available. Hence, storage applications with integrated cells will usually have an excess of power or energy capability.

RFBs can be divided into two categories: 1) true redox flow batteries, where all of the chemical species active in storing energy are fully dissolved in solution at all times; and 2) hybrid redox flow batteries, where at least one chemical specie is plated as a solid in the electrochemical cells during charge. Examples of true RFBs include the vanadium-vanadium and iron-chromium systems. Examples of hybrid RFBs include zinc-bromine and zinc-chlorine systems.

Complete Separation of Power and Energy

True RFBs achieve the complete separation of power and energy, along with the full advantages. In hybrid RFBs, complete separation of power and energy is not achieved, because energy is stored in the metal which is plated in the electrochemical stack during charge. Larger energy storage capacity requires a larger stack, so the distinction of the hybrid RFB from integrated cell architectures is only partly achieved.

Finally, RFBs are well suited for applications with power requirements in the range of tens of kilowatts to tens of megawatts, and energy storage requirements in the range of 500 kilowatt-hours to hundreds of megawatt-hours. RFBs can be the most economical choice in this range because storage tanks and flow controls are easy and economical to scale, and electrochemical stacks can have repeat units with power ratings in the tens to hundreds of kilowatts.

Redox flow batteries have one main architectural disadvantage compared with integrated cell architectures of electrochemical storage. RFBs tend to have lower volumetric energy densities than integrated cell architectures, especially in the high power, short duration applications. This is due to the volume of electrolyte flow delivery and control components of the system, which is not used to store energy, so a system is not as compact as other technologies might be for a similar output. In spite of this, RFBs are available with system footprint below the EPRI substation target of <500 ft2 / MWh.

Redox flow batteries offer an economical, low vulnerability means to store electrical energy at grid scale. Redox flow batteries also offer greater flexibility to independently tailor power rating and energy rating for a given application than other electrochemical means for storing electrical energy. Redox flow batteries are suitable for energy storage applications with power ratings from tens of kW to tens of MW and storage durations of two to 10 hours.

How Vanadium Flow Redox Batteries Work

During the discharge cycle, V2+ is oxidized to V3+ in the negative half-cell and an electron is released to do work in the external circuit (either DC or, for AC systems, through an AC/DC converter).

In the positive half-cell, V5+ in the form of VO2+ accepts an electron from the external circuit and is reduced to V4+ in the form of VO2+. Hydrogen (H+) ions are exchanged between the two half-cells to maintain charge neutrality. The hydrogen ions diffuse through the anion or cation-ion permeable polymer membrane that separates the half cells. Charged vanadium species and water can also diffuse across the membrane. The cross-diffusion results in direct energy loss for that cycle. However, when vanadium is the only element present on both sides of the cell, this cross diffusion mechanism does not result in permanent capacity loss, as long as the total vanadium in the system remains constant (i.e., there is no loss due to precipitation).
In a V-only system there is no need to maintain balance between positive and negative sides of the system. In the positive half-cell, the vanadium is present in solution as oxy-cations. These oxy-cations are vulnerable to irreversible precipitation as V2O5 if the electrolyte temperature exceeds ~50-60oC. However, when precipitation occurs, it does so typically in the form of benign compounds, not V2O5.

The normal operating temperature of a VRB is approximately between 10-40oC. Active cooling sub-systems are employed if ambient temperatures exceed 40-45oC. Being able to cool the system actively is an advantage since the system can remain operating without risking any damage to it. By contrast, if integrated cell architectures overheat, the best option is to stop using them until they cool down.

The cell voltage is 1.4-1.6 volts and cell power densities are hundreds mW/cm2 (although Prudent Energy reports their power densities are higher). The DC-DC efficiency of this battery has been reported in the range of 60-80%. According to EPRI, the vanadium redox battery is suitable for power systems in the range of 100 kW to 10 MW, with storage durations in the 2-8 hour range.

The vanadium redox battery offers a relatively high cell voltage, which is favorable for higher power and energy density compared with other true RFBs, like the iron-chromium system. However, the higher voltage and highly oxidative V5+ electrolyte puts more chemical stress on the materials used in the cell electrodes, membranes, and fluid handling components. Cross-transport of vanadium ions across the membrane is also reported as a challenge, and fairly expensive ion-exchange membranes must be used to minimize losses due to cross-membrane transport. These membranes can be vulnerable to fouling, wherein vanadium ions become irreversibly trapped in the membrane and increase resistive losses in the cell. On the other hand, lower cost membranes are under development.

Vanadium is a readily available material, used in steel manufacturing and as a chemical catalyst, which is found naturally and can also be recovered from various waste streams. The market price of vanadium as V2O5 has, however, been fairly volatile since 2017 after enjoying several years of low prices..

Notes
¹ VRB®, VRB-ESS®, and VRB ENERGY STORAGE SYSTEM® are registered trademarks of JD Holding, Inc., the parent company of Prudent Energy Corporation, a Delaware corporation. JD Holding, Inc. is the owner of U.S. Patent Nos. 6,143,443, 6,468,688, 6,562,514, 7,078,123, 7,181,183, 7,184,903, 7,227,275, 7,265,456, 7,353,083, 7,389,189, 7,517,608 and corresponding foreign patents. Additional patent rights are pending.

How Nickel-Cadmium Batteries Work

Early Ni-Cd cells used pocket-plate technology, a design that is still in production today. Sintered plates entered production in the mid-20th century, to be followed later by fiber plates, plastic-bonded electrodes and foam plates. Cells with pocket and fiber plates generally use the same electrode design for both the nickel positive and cadmium negative, while sintered and foam positives are now more commonly used with plastic-bonded negatives.

All industrial Ni-Cd designs are vented types, allowing gases formed on overcharge to be dissipated but requiring some degree of water replenishment to compensate. This has led to the implementation of separator designs that allow varying levels of recombination, with some products designed for telecom or off-grid renewable energy applications achieving near maintenance-free operation with respect to the electrolyte.

Ni-Cd batteries found use in some earlier energy-storage applications, most notably the Golden Valley Electric Association BESS, sized for 27 megawatts for 15 minutes and commissioned in 2003. Ni-Cd has also been used for stabilizing wind-energy systems, with a 3 megawatt system on the island of Bonaire commissioned in 2010 as part of a project for the island to become the first community with 100% of its power derived from sustainable sources.

How Sodium Sulfur Batteries Work

The active materials in a NaS battery are molten sulfur as the positive electrode and molten sodium as the negative. The electrodes are separated by a solid ceramic, sodium alumina, which also serves as the electrolyte. This ceramic allows only positively charged sodium-ions to pass through. During discharge electrons are stripped off the sodium metal (one negatively charged electron for every sodium atom) leading to formation of the sodium-ions that then move through the electrolyte to the positive electrode compartment. The electrons that are stripped off the sodium metal move through the circuit and then back into the battery at the positive electrode, where they are taken up by the molten sulfur to form polysulfide. The positively charged sodium-ions moving into the positive electrode compartment balance the electron charge flow. During charge this process is reversed. The battery must be kept hot (typically > 300 ºC) to facilitate the process (i.e., independent heaters are part of the battery system). In general Na/S cells are highly efficient (typically 89%).

NaS battery technology has been demonstrated at over 190 sites in Japan. More than 270 MW of stored energy suitable for 6 hours of daily peak shaving have been installed. In Abu Dhabi, fifteen NaS systems acting in coordination provide 108 MW / 648 MWh to defer fossil generation investment and provide frequency response and voltage control services.

How Electrochemical Capacitors Work

When the two electrodes of an EC are connected in an external current path, current flows until complete charge balance is achieved. The capacitor can then be returned to its charged state by applying voltage. Because the charge is stored physically, with no chemical or phase changes taking place, the process is fast and highly reversible and the discharge-charge cycle can be repeated over and over again, virtually without limit. Because of the high surface area and the small thickness of the double layer, these devices can have very high specific and volumetric capacitances. This enables them to combine a previously unattainable capacitance density with an essentially unlimited charge-discharge cycle life. The operational voltage of one cell is limited only by the breakdown potential of the electrolyte and is usually less than 3 V. Thus, cells are connected in series for higher voltage operation, exactly like battery cells.

There are two types of ECs: those with 1) symmetric designs, where both positive and negative electrodes are made of the same high-surface-area carbon and 2) asymmetric designs with different materials for the two electrodes, one high-surface-area carbon and the other a higher capacity battery-like electrode. Symmetric ECs have specific energy values up to ~6 Wh/kg and higher power performance than asymmetric capacitors where designs having specific energy values approach 20 Wh/kg. There are other differences in the characteristics and performance of these two types leading to use in different applications.

Because of their high power, long cycle life, good reliability, and other characteristics, the market and applications for ECs have been steadily increasing. There are dozens of manufacturers and more are entering the market because of market growth. Applications range from portable electronics and medical devices to heavy hybrid and other transportation uses. ECs are better suited than batteries for applications requiring high cycle life and charge or discharge times of 1 second or less. The largest barrier to market growth has been the lack of understanding of the technology and the applications for which it is best suited. Aqueous electrolyte asymmetric EC technology offers opportunities to achieve exceptionally low-cost bulk energy storage.

There are difference requirements for energy storage in different electricity grid-related applications from voltage support and load following to integration of wind generation and time-shifting. Symmetric ECs have response times on the order of 1 second and are well-suited for short duration high-power applications related to both grid regulation and frequency regulation. Asymmetric ECs are better suited for grid energy storage applications that have long duration, for instance, charge-at-night/use-during-the-day storage (i.e., bulk energy storage). Some asymmetric EC products have been optimized for ~5 hour charge with ~5 hour discharge. Advantages of ECs in these applications include long cycle life, good efficiency, low life-cycle costs, and adequate energy density.

How Iron-Chromium Flow Batteries Work

During the discharge cycle, Cr2+ is oxidized to Cr3+ in the negative half-cell and an electron is released to do work in the external circuit through the negative and positive terminals of the AC/DC converter. In the positive half-cell during discharge, Fe3+ accepts an electron from the external circuit and is reduced to Fe2+. These reactions are reversed during charge, when current is supplied from the external circuit through the AC/DC converter. Hydrogen (H+) ions are exchanged between the two half-cells to maintain charge neutrality as electrons leave one side of the cell and return to the other side. The hydrogen ions diffuse through the separator, which electronically separates the half cells. In early implementations of the iron-chromium RFB, diffusion of the iron and chrome ions across the separator created an imbalance between the positive and negative electrolytes, resulting in an irreversible system capacity loss.

Modern electrolyte formulations using mixed iron and chromium on both sides of the separator have eliminated the irreversible loss and enabled the use of low cost, porous separator materials. These porous separators have also eliminated the “membrane fouling” failure mode that occurs with ion exchange membranes used in early iron-chrome and some other current RFB technologies.

The standard cell voltage is 1.18 volts and cell power densities are typically 70-100 mW/cm2. The comparatively low cell voltage results in a low energy density, and thus larger equipment than would be the case with other technologies, but developers can still meet the EPRI footprint target of 500 ft2 per MWh of storage. The DC/DC efficiency of this battery has been reported in the range of 70-80%. Efficiency of this system is enhanced at higher operating temperatures in the range of 40-60 oC (105-140 oF), making this RFB very suitable for warm climates and practical in all climates where electrochemical energy storage is feasible. The iron and chromium chemistry is environmentally benign compared to other electrochemical systems, in that the iron and chromium species present have very low toxicity and the dilute, water-based electrolyte has a very low vapor pressure. These factors combine to make the iron-chromium RFB one of the safest systems for energy storage in personnel and environmental terms.

The standard potential of the Cr2+ – Cr3+ couple is near the hydrogen evolution potential. Care must be taken in the design of iron-chrome RFBs to minimize parasitic side reactions and then to reverse the associated capacity loss and electrolyte imbalance. Current developers of iron-chromium RFBs appear to have mitigated this side reaction and implemented effective re-balancing subsystems with minimal system efficiency loss.

Iron-chromium flow batteries are available for telecom back-up at the 5 kW – 3 hour scale and have been demonstrated at utility scale. Current developers are working on reducing cost and enhancing reliability. These systems have the potential to be very cost effective at the MW – MWh scale.

How Zinc-Bromine Batteries Work

In each cell of a zinc-bromine battery, two different electrolytes flow past carbon-plastic composite electrodes in two compartments, separated by a micro-porous polyolefin membrane. The electrolyte on the anode (negative) side is purely water-based, while the electrolyte on the positive side also contains an organic amine compound to hold bromine in solution.

During charge, metallic zinc is plated (reduced) as a thick film on the anode side of the carbon-plastic composite electrode. Meanwhile, bromide ions are oxidized to bromine and evolved on the other side of the membrane. Bromine has limited solubility in water, but the organic amine in the catholyte reacts with the bromine to form a dense, viscous bromine-adduct oil that sinks to the bottom of the catholyte tank. The bromine oil must later be re-mixed with the rest of the catholyte solution to enable discharge.

During discharge, the zinc metal, plated on the anode during charge, is oxidized to Zn2+ ion and dissolved into the aqueous anolyte. Two electrons are released at the anode to do work in the external circuit. The electrons return to the cathode and reduce bromine molecules (Br2) to bromide ions, which are soluble in the aqueous catholyte solution. The bromine in the catholyte is decomplexed from the amine and converted into two bromide (Br-) ions at the cathode, balancing the Zn2+ cation and forming a zinc bromide solution. The chemical process used to generate the electric current increases the zinc-ion and bromide-ion concentration in both electrolyte tanks. The net DC-DC efficiency of this battery is reported to be in the range of 65-75%.

Practical Challenges

The zinc-bromine redox battery offers one of the highest cell voltages and releases two electrons per atom of zinc. These attributes combine to offer the highest energy density among flow batteries. However, the high cell voltage and highly oxidative element, bromine, demand cell electrodes, membranes, and fluid handling components that can withstand the chemical conditions. These materials are expensive. Bromine is a highly toxic material through inhalation and absorption. Maintaining a stable amine complex with the bromine is key to system safety. Active cooling systems are provided by system manufacturers to maintain stability of the bromine-amine complex when ambient temperatures may exceed 95°F. In addition, repeated plating of metals in general is difficult due to the formation of “rough” surfaces (dendrite formation) that can puncture the separator. Special cell design and operating modes (pulsed discharge during charge) are required to achieve uniform plating and reliable operation.

Integrated Zn/Br energy storage systems have been tested on transportable trailers (up to 1 MW/3 MWh) for utility-scale applications. Multiple systems of this size could be connected in parallel for use in much larger applications. Zn/Br systems are also being supplied at the 5-kW/20-kWh Community Energy Storage (CES) scale, and now being tested by utilities, mostly in Australia.