Keywords

1 Introduction

Over the past few decades, a wide variety of electrochemical storage systems have been developed and made commercially available. Today’s prominent electrochemical storage systems encompass technologies like lead-acid, nickel-cadmium, and nickel-metal hydride alkaline batteries, sodium-nickel chloride and sodium-sulfur high-temperature batteries, and lithium-ion technology.

Each of these unique electrochemical storage systems possesses specific features. Product variants, adjusted to meet the requirements of individual applications, are offered with tailored performance characteristics [20]. These tailored performance characteristics primarily encompass energy and power density, life cycle, design life, and the operating temperature range.

Consequently, certain applications are dominated by specifically adjusted electrochemical systems. Due to the complexity of application-specific requirements, selecting the most suitable electrochemical storage technology based solely on tabulated performance data can be challenging. Additional necessary system components, such as thermal management and battery management, can significantly impact the performance of the overall system and should not be overlooked.

A technology substitution within a particular application only occurs if a technology reaches its performance limits and can no longer meet the changed or increased requirements of the application. Other reasons for a technology change in specific applications are usually economical (e.g., total cost of ownership) or environmental (e.g., bans on heavy metals, safety concerns).

Sustainability aspects, such as the recycling and recovery of battery materials, will become increasingly important for end customers seeking to reduce the CO2 footprint of their applications.

The following subchapters will discuss the main performance criteria and costs associated with the different technologies, addressing each technology separately.

2 Lead-Acid Batteries

Lead-acid batteries accounted for the largest share of the global battery market, at around 90%, until 2010. From then on, the market share of lithium-ion batteries has been growing steadily, and in 2021, the market share of lithium-ion batteries was already slightly higher than the market share of lead-acid batteries, due to the upcoming demand for electromobility.

For the automotive and industrial market, the lead battery has been the predominant energy storage system for over 100 years.

The lead-acid batteries are on the market in two major different construction designs. On the one side, there is the flooded or “vented” construction design, requiring maintenance. On the other side, there is the maintenance-free valve-regulated (VRLA) batteries [1].

Today, the share of maintenance-free lead-acid batteries has increased to about 80% of the total lead market. There are two types of VRLA batteries, which differ in the way the electrolyte (sulfuric acid) is fixed. In one type, the electrolyte is fixed in a silica gel; in the other, it is in an AGM (absorbent glass mat) material.

Lead-acid batteries with AGM technology have become the most important technology in the field of automotive and industrial applications [3]. The share of AGM types, which are more efficient than gel types, is 85% of the VRLA market.

The lead-acid battery innovation has historically been market driven, primarily by the end-user applications. This explains the very wide range of specific battery products, sizes, and construction designs.

The positive electrodes in lead-acid batteries can have either a tubular, flat grid plate design or plante design, while the negative electrodes are always constructed as a flat grid plate design.

Table 8.1 shows the most important positive electrode designs and their main performance characteristics and cost. It is evident that there are significant differences between the various types of electrodes and their properties. For instance, the plante electrode exhibits high-power density but comes with a higher price tag and a lower life cycle. In contrast, the tabular electrode has a lower power density. The grid electrode, currently employed in VRLA batteries, offers lower costs and a higher power density compared to the tabular electrode. Additionally, ongoing development is focused on the bipolar electrode, as outlined in the “Further Development” section. This electrode aims to combine high performance, extended cycle-life, and a lower price.

Table 8.1 Main performance and cost parameters of the most important positive electrode designs used with the lead-acid technology
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The main application fields for lead-acid batteries are as follows:

  • Automotive mobility applications (grid electrode)

  • Material handling and logistics applications (tabular electrode)

  • Stationary energy storage applications (grid, tabular, and plante electrode)

Advantages

  • High-power densities

  • Very robust and abuse tolerant (safe without an additional battery management system (BMS)

  • Cost-effective with low maintenance cost

  • Application approved

  • High recyclability

Disadvantages

  • Low gravimetric energy density

  • Mandatory ventilation for flooded lead-acid batteries

  • Water loss that requires maintenance in flooded cells

  • Contains lead.

Further Development

In recent years, the lead-acid battery has undergone many relevant improvements in terms of lifetime and performance.

These include material innovations such as the addition of novel carbon additives and expanders to the anode to avoid negative electrode sulfation and the use of innovative separator materials and the development of more resistant alloys for high-temperature environments. The outstanding feature in this process is that these improvements have been tailored to the specific application. These novel designs are referred to as “advanced lead-acid batteries,” which include bipolar, lead-carbon, and pure lead thin plate technologies [2].

Bipolar Lead-Acid Batteries

While bipolar and monopolar designs share the same lead-based chemistry, they differ in that in bipolar batteries, the cells are stacked in a sandwich construction so that the negative plate of one cell becomes the positive plate of the next cell. Stacking these cells next to one another allows the potential of the battery to be built up in 2 V increments. Since the cell wall becomes the connection element between cells, bipolar plates have a shorter current path and a larger surface area compared to connections in conventional cells. This construction reduces the power loss that is normally caused by the internal resistance of the cells. At each end of the stack, single plates act as the final anode and cathode. This construction leads to reduced weight since there are fewer plates and bus bars are not needed to connect cells together. The net result is a battery design with higher power than conventional monopolar lead-based batteries.

Until recently, the main problem limiting the commercialization of bipolar lead-acid batteries was the availability of a lightweight, inexpensive, and corrosion resistant material for the bipolar plate and the technology to properly seal each cell against electrolyte leakage.

Architectural advantages are as follows:

  • Lower inner resistance, i.e., higher power density

  • Increased energy density up to 63 Wh/kg

The lead-acid battery technology has a well-established circular economy. At the end of their life, lead-based batteries are collected for recycling. Within the EU, almost 100% of lead-based batteries are returned and recycled in a closed loop with a high efficiency of over 80% [20]. The market for lead-based batteries in the EU is mainly served by recycled material, and the demand for primary lead reserves is low.

3 Li-Ion Technology

As lithium shows the most negative normal potential of 3.05 V against hydrogen, Li-Ion batteries achieve higher gravimetric and volumetric energy densities compared to other widely used technologies like lead-acid or nickel-based systems.

In the past few decades, lithium-ion batteries have replaced the NiMH batteries in the field of portable and mobility applications due to their higher energy densities (80 Wh/kg vs. up to about 300 Wh/kg).

Currently, the expected market growth for Li-Ion batteries will be more than 3 TWh by 2030 and will largely serve with up to 85–90% the e-mobility and the energy storage markets [20].

The high market growth will most likely result in lower cost per kilowatt-hour due to standardization and mass production.

Due to the variety of possible combinations of cathode and anode materials, the resulting Li-ion batteries show specific and individual performance characteristics suitable for different kinds of applications. The development of Li-ion technologies suitable for industrial and automotive applications is still a challenge in terms of material research process, production, development, recycling, safety, and transportation [4].

Typical cathode active materials are as follows:

  • LCO—lithium cobalt oxide (LiCoO2).

  • LMO—lithium manganese oxide spinel (LiMn2O4).

  • LFP—lithium iron phosphate (LiFePO4).

  • NCA—lithium-nickel-cobalt-aluminum oxide (LiNiCoAlO2).

  • NMC—lithium-nickel-cobalt-manganese oxides (Li(NiCoMn)O2).

The most relevant cathode material for the e-mobility is the NMC material, which is currently further developed to meet the major requirement for higher energy densities to achieve an increased driving range. This results in modifications of nickel-cobalt-manganese oxide (NMC) materials, from NMC 111 to NMC 811, with increased nickel and reduced cobalt content. Typically, these NMC materials are combined with anode materials of high capacity (Fig. 8.1).

Fig. 8.1
A graph illustrates gravimetric versus volumetric energy density. The plots are deployed in an oval shape. They are Li S, Li-Air, S S B, Gen 2 a, Gen 2 b, Gen 3 a, Gen 3 b, and Na-Ion R T. It depicts N M C materials combined with anode materials of high capacity.

Gravimetric and volumetric energy densities of most relevant Li-Ion and post-lithium battery technologies

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Table 8.2 and Fig. 8.1 show the performance and cost parameters of the most relevant anode/cathode combinations.

Table 8.2 Cost and performance data of most relevant anode/cathode combinations for NMC batteries
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The electrolyte composition is usually LiPF6 in organic solvents with additional additives in order to improve certain properties. Field of developments is electrolytes that can withstand higher voltages and consequently higher energy densities.

Depending on the active material combinations, some advantages and disadvantages become less or more apparent.

Advantages

  • Very high energy densities.

  • High cell voltages: up to 3.7 V nominal.

  • Can be optimized for specific application performance requirements.

  • Tolerate to high discharge currents (discharged rate > 40 C).

  • Fast charging possible.

  • Batteries can be almost completely discharged without affecting cycle durability, lifetime, or high current output.

  • Very low self-discharge rate (3–5%/month).

Disadvantages

  • Sensitivity to deep discharge, overcharge, and excessive temperatures requires active battery management and monitoring.

  • Relatively high sensitivity to high or low temperatures.

4 Post-Li-Ion Battery Technologies

In response to the growing demand for energy-efficient and environmentally sustainable energy storage, researchers are seeking alternatives to conventional lithium-ion batteries. This exploration is motivated by the need to address not only energy efficiency but also the ecological and social impacts of current battery technologies. A key challenge is developing electrodes that are durable and stable while also offering high energy densities and quick charge-discharge rates.

To address these challenges, studies are concentrating on materials that are more abundant and environmentally friendly compared to traditional lithium sources.

The exploration of post-lithium battery technologies, based on alternative materials, presents both challenges and unique opportunities.

4.1 Lithium All-Solid-State Battery Technologies

Structure of Solid-State Batteries

The electrochemical system of lithium solid-state batteries is similar to lithium-ion batteries. The major difference is that, at the very least, the electrolyte is in a solid state.

All-solid-state batteries are generally considered to be a class of batteries that will enable higher energies in the future (see Table 8.3 and Fig. 8.2). The actual increase in energy comes only through the elimination of the graphite by the metallic Li anode.

Table 8.3 Cost and performance data of most relevant post-Li-Ion and sodium-ion battery technologies
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Fig. 8.2
A graph illustrates gravimetric and volumetric energy density. The plot is deployed in an oval shape. They are N M C, L F P, Ni-Cd, V R F B, Na S, Na Ni Cl, Ni M h, and lead. It depicts the anode or cathode combination's cost and performance characteristics.

Gravimetric and volumetric energy densities of different battery technologies

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Solid-state batteries use an electrolyte made of solid material instead of the usual liquid electrolyte. The electrodes are also made of solid material in terms of an all-solid-state battery [5].

Currently there are solid/liquid hybrid cells on the market which are also described as a certain kind of solid-state system.

The main components of the SSB cell are the anode and cathode active materials and the solid electrolytes. Various materials are suitable for use in SSB.

Anode Active Material

The most promising anode active materials in order to achieve high energy density are lithium metal and silicon. Lithium metal anodes are considered the most promising, as they enable the highest possible energy density on the anode side.

Cathode Active Material

Soon, Ni-rich layered oxides (NMC, NCA) and lithium iron phosphate (LFP) will become the most likely to be dominant cathode active materials. These materials are already commonly used in state-of-the-art LIB.

Electrolyte

With solid-state batteries, there is the possibility that part of the solid electrolyte can be incorporated into the electrodes. The main advantages of future solid-state batteries are that the energy density of the cells would increase significantly in the future and the risk of fire would also decrease due to the less pronounced flammability of the electrolyte.

Compared with Li-ion cells, high-power densities cannot be achieved with the solid-state technology. The reason for this is usually the high Li+ contact resistance at the phase boundary between the cathode and the solid electrolyte.

Solid-state cells are being developed with both polymeric and inorganic solid electrolytes. Inorganic electrolytes can be distinguished between sulfide and oxide solid electrolytes.

The specific energy/energy density of solid-state cells can be increased by the following:

  • Lower electrolyte/separator thickness

  • Use of Li anodes

  • Use of nonporous active materials

  • Increase in cell voltage due to the higher electrochemical stability (electrochemical window) of the electrolyte

Advantages of solid-state batteries compared to liquid electrolyte Li-Ion batteries are as follows:

  • Higher energy density than Li-ion

  • Safety—instead of flammable organic liquid electrolyte, use of a solid-state electrolyte (ceramic, polymer)

  • No electrolyte leakage

  • Can fit easier casing shapes

  • Solid-polymer Li-ion cells can be made as thin as 0.1 mm or about one-tenth the thickness of the thinnest prismatic liquid Li-ion cells

  • Potentially lower manufacturing costs

Disadvantages of solid-state cells compared to liquid electrolyte cells are as follows:

  • Power limited by low ionic conductivity of electrolyte

  • High interfacial resistance

  • Poor interface contacts

In addition, due to the non-combustibility (at least of the ceramic electrolyte) and the higher temperature resistance of the electrolyte, safety-relevant components can be reduced in the module/pack, thus achieving a volume/mass reduction for the battery system.

4.2 Li-S Batteries

Lithium-sulfur batteries represent a new type of battery that promises high gravimetric energy densities at a moderate cost (see Table 8.3 and Fig. 8.2).

Design

Li-S batteries usually have a cathode consisting of sulfur and carbon and an anode of lithium metal to take advantage of the high specific capacity of the sulfur cathode [6].

Currently, there are also approaches of using a liquid polysulfide solution instead of a solid-state sulfur electrode. This idea is not new and was published as early as 1975.

Challenges and Opportunities

The cathode material of common lithium-ion cells is the most expensive component of a battery (more than 20% of the cell cost). It contains cobalt and nickel. Both are rare raw materials whose costs tend to rise rather than fall when batteries are mass-produced. In lithium-sulfur batteries, this cost item is eliminated, potentially saving more than 20% of the cost, because sulfur is very inexpensive and available in large quantities. Dry electrode manufacturing processes can also reduce production costs.

The theoretical gravimetric energy density of around 2500 Wh/kg is almost ten times that of conventional lithium-ion batteries.

However, the Li-S battery has disadvantages in terms of volumetric energy density. Future applications could be for situations in which the low weight of the battery is more important than its size, such as quadcopters, aircrafts, and ships. These appear to be more realistic applications for Li-S batteries.

The cycle stability, i.e., the lifetime of Li-S batteries, is currently still very low (limited to less than 200 cycles). Cells from pre-commercial production achieve only about 100 cycles at an energy density of 350 Wh/kg. However, cells with a significantly longer service life of several thousand cycles have already been achieved on a laboratory scale.

Advantages

  • Main advantage of Li-S batteries is the potentially high gravimetric energy density, estimated up to 600 Wh/kg in perfect systems.

  • Hope for low cost: Compared to LIB, it is hoped that the low cost of sulfur will result in significantly lower prices for energy storage. However, this is partially offset by the higher cost of metallic lithium (compared to graphite).

  • The usage of pure materials allows higher recyclability.

Disadvantages

  • Volumetric energy density lower than lithium-ion batteries (currently about 50%). Even in fully developed cells, at most the level of today’s lithium-ion batteries can be achieved.

  • Achievable performance lower than in commercial lithium-ion batteries.

  • Current prototypes can only be operated at currents of approx. C/2.

The low cost and high abundance of sulfur (i.e., the active cathode material) make LiSB more appealing than Li-ion batteries, given the fact that the latter use critical materials, such as cobalt and nickel, in the manufacturing of the cathodes. LiSB are promising because of the high energy density, low cost, and natural abundance of sulfur.

4.3 Lithium-Air Battery Technologies

Lithium-air batteries possess a great potential for efficient energy storage applications in order to resolve future energy and environmental issues. The extremely high theoretical energy density is attractive, but there are still various technical limitations to overcome. The performance of lithium-air batteries is governed mainly by electrochemical reactions that occur on the surface of the cathode [7]. Widespread interest in various carbons and their applicability as cathode materials in lithium-air batteries are a result of their highly specific surface area and porosity, their lightweight, and their low production cost.

Among the group of metal-air batteries, intensive research is being carried out in particular on the development of lithium-air batteries. Since lithium has the highest electrochemical potential of all metals, these batteries offer the highest energy density by far of all metal-air systems that can theoretically be achieved [8, 18]. Compared with the state-of-the-art, it is hoped that energy densities can be achieved that are about ten times higher in practice, in order to make the ranges of electric vehicles based on such batteries competitive with today’s gasoline-powered cars.

However, it may result in the need for a lot of additional technology and electronics (e.g., to clean the air), so that the weight and space this takes up reduces the theoretical energy density to such an extent that the batteries hardly have any advantages over more advanced lithium-ion batteries. For example, the theoretical material energy density of a lithium-air system is up to 3450 Wh/kg, but if the entire periphery is taken into account, the possible energy density is reduced to about 1000 Wh/kg. Currently available primary cells achieve energy densities of around 800 Wh/kg.1.

Major challenges currently lie in achieving an acceptable number of charge cycles and reducing voltage losses during charging and discharging.

4.4 Sodium Ion Room-Temperature Technology

In comparison with the state-of-the-art high-temperature sodium batteries, the upcoming new sodium-ion battery technology operates at room temperature. The sodium-ion battery has a similar working principle to the Li-ion battery. Sodium ions also shuttle between the cathode and the anode to store and release energy [16].

The significantly higher global equal distribution of sodium and absence of critical raw materials like cobalt and nickel in the cathode lead to cost reductions and lower environmental impact of the sodium system compared to lithium-ion [15]. Due to the technological similarities with existing Li-ion batteries, the industrialization process of sodium-ion batteries can be accelerated. A significant advantage is that the sodium-ion battery can be manufactured with the same production facilities as lithium-ion cells.

For cathode materials, the most important part of sodium-ion batteries, Prussian blue analog, layered metal oxides, and NASICON (sodium (Na) super ionic conductor) have their own advantages in different aspects. The most critical indicators based on potential application scenarios are higher energy density, longer life cycle, and better low temperature performance. Overall, the cost and safety advantages of sodium batteries will gradually gain in prominence. Therefore, it is likely that sodium-ion batteries will be used in different automotive and industrial applications.

So far, the technology is not yet fully developed and still needs to be tested in practical applications. Safety concerns have also not been conclusively assessed, especially given the lower melting point of sodium in potential sodium plating scenarios. It is believed that its properties at low temperatures are superior to those of lithium-ion batteries.

5 Nickel-Based Batteries

Alkaline batteries that use nickel hydroxide as a cathodic material belong to the mainstream battery systems.

The most important representatives of this technology are the nickel-cadmium (NiCd), nickel-metal hydride (NiMh), as well as nickel-zinc (NiZn) and Ni-iron (NiFe) systems. In past decades, only NiCd and NiMH technology has played an important role in portable, automotive, and industrial applications. Therefore, only the cost and performance of the NiCd and NiMH systems will be discussed below.

It can already be pointed out that neither NiCd nor NiMh play a significant role in the current markets and are only used to a small extent exclusively in niche applications.

Due to the performance and the costs of these systems, it can be assumed that they will be substituted by other technologies, such as Li-Ion, by 2030.

5.1 Nickel-Cadmium Batteries

Nickel-cadmium batteries played a major role in the past, competing with lead-acid batteries in certain applications.

The general advantages of the very reliable NiCd battery system is the higher energy density, the robustness, and a high deep discharge cycle-life—even at low temperatures when compared with lead-acid batteries.

The electrodes for NiCd systems are classified into five electrode types: pocket electrodes, sintered electrodes, plastic-bonded electrodes, nickel foam electrodes, and fiber electrodes [9].

Table 8.4 presents the key properties and costs of these electrodes. The pocket plate electrode, characterized by its relatively thick electrode, is primarily used in applications that require low to medium performance. It has been the dominant choice for standby applications like uninterruptible power supplies (UPS) and emergency power supply systems for railway rolling stock. On the other hand, sintered electrodes are designed for high-power cells that can deliver significant power output. However, the sintered structure undergoes mechanical stress during charge and discharge cycles, which can potentially affect the overall durability and longevity of the system. Moreover, this technology is associated with higher costs compared to other alternatives. Despite these limitations, this energy storage system finds application in demanding sectors such as aircraft, military, railway, and vehicles where high performance and reliability are crucial requirements.

Table 8.4 Cost and performance data of most relevant NiCd electrode designs
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The fiber electrode enables the fabrication of both high-power and high-energy cells by adjusting the electrode thickness accordingly. These cells exhibit long life due to their ability to compensate for volume changes in the active material during cycling, thereby preventing shedding of the active material. They also offer fast rechargeability, facilitating quick energy replenishment. This versatile system finds applications in various domains, including standby and mobile applications such as railway rolling stock and automated guided vehicles (AGVs). Additionally, it is suitable for use in aviation and space flight applications where high performance, durability, and reliability are essential.

The foam technology emerged in the late 1980s to address the need for energy cells with higher volumetric and specific energy capacities. Unlike sintered technology, which allows only a maximum of 50% of the electrode volume for active material loading, the foam approach utilizes a metallic nickel substrate. The electrode structure is formed by electrolytic nickel plating of polyurethane foam, followed by the removal of the organic core material under high-temperature conditions in a reducing atmosphere. The resulting isotropic reticulated substrate exhibits exceptionally high porosity exceeding 95%. As a result, the available porosity for active material loading increases by approximately 30% compared to sintered substrates. Overall, this technique offers enhanced active material utilization and provides higher porosity for improved energy capacity, meeting the requirements for higher volumetric and specific energy cells.

Cell designs for NiCd batteries are prismatic or spiral wound, flooded, or maintenance-free valve regulated.

Nickel-cadmium cells have been manufactured as maintenance-free, sealed battery system since the 1950s.

The sale of new cadmium-containing batteries in the EU has only been permitted for a few applications since December 2009, and it must be ensured that batteries are returned for recycling at the end of life. The few exceptions are portable batteries for emergency or alarm systems, including emergency lighting and medical equipment, and certain industrial applications.

The NiCd battery system has become a subject of environmentally related discussions, due to the fact that cadmium is a hazardous heavy metal and has been banned from many applications. This has resulted in the NiCd being substituted in many applications by the NiMh technology.

Advantages

  • High-power capability

  • Fast rechargeability

  • Good low temperature performance (−40 °C)

  • Good energy density

Disadvantages

  • Poor environmental compatibility: Cadmium is a toxic heavy metal

  • High self-discharge rate

  • Low cell voltage of 1.2 V/cell

5.2 Nickel-Metal Hydride Batteries (NiMH)

Since the 1990s, NiCd batteries in the consumer markets for portable devices have been displaced by NiMH batteries. The reason for this was the higher energy storage capability achievable with NiMH and the general concern about cadmium as an environmentally hazardous material. Beneficial for an easy substitution of the NiCd technology by NiMh is the similar cell voltage of 1.2 V/cell [10].

The NiMH system was developed in the 1980s for electronic devices with high energy demand. During the 1990s, NiMH was the dominating electrochemical storage system for portable devices, which was replaced by Li-ion battery system in the middle of the 1990s. The Li-ion technology has shown even higher gravimetric and volumetric energy densities, which are essential for advanced portable devices. Another major application where NiMH played a very important role was the introduction of the first hybrid electrical vehicles at the beginning of the twenty-first century. In particular, the high-power performance of the NiMH battery systems was the greatest attraction for automotive applications.

The basic technologies for the electrodes as well as for other components used in NiMH cells are very similar to what has been developed for NiCd cells in a variety of applications.

The preferred electrode types of NiMH are the sinter-type and the foam-type electrode.

Two major designs for NiMH batteries are realized, the cylindrical and prismatic types.

Advantages

  • High energy and power density

  • Robust, but not as robust as NiCd for deep discharge and overcharge

  • Temperature operation range from −15 °C to 40 °C

  • Fast rechargeability

  • Environmentally friendlier compared to NiCd (no heavy metals)

  • Good recyclability

Disadvantages

  • Need for battery management system

  • High self-discharge rate.

  • Low cell voltage of 1.2 V/cell

6 Sodium-Based Batteries

6.1 High-Temperature Sodium Batteries

High-temperature sodium batteries consist of liquid electrodes and a solid electrolyte (usually an ion-conducting (e.g., Na+) ceramic). These batteries require relatively high operating temperatures of >300 °C to maintain the sodium-based electrode in the liquid state as well as to increase the conductivity of the solid electrolyte.

Mainstream technologies are the sodium-nickel-chloride (NaNiCl) and the sodium-sulfur battery (NaS).

NaNiCl and NaS batteries have a service life of around 4500 cycles and an efficiency of 75–86%. Thermal losses due to the heating required to maintain the cell temperature must be considered if there are longer periods of time between charging and discharging. This can be influenced to a certain extent through a corresponding effort in thermal insulation.

6.1.1 Sodium-Nickel-Chloride Batteries

Sodium-nickel-chloride ZEBRA batteries were developed in 1985 in South Africa. The name ZEBRA stands for Zeolite Applied to Battery Research Africa. The cathode mainly consists of a porous nickel matrix which serves as a current conductor with nickel chloride (NiCl2), which is impregnated with sodium aluminum chloride (NaAlCl4). The anode is made of sodium [11].

Ceramic β-aluminum oxide is used as the separator and electrolyte, but the sodium ions do not allow electrons to pass between the anode and cathode. The operating temperature of this type of battery is between 270 °C and 350 °C, so that the electrodes (active material) are in the liquid state (melted) and the ceramic separator achieves high conductivity for sodium ions [12]. The specific energy of the cells is approximately 120 Wh/kg at a nominal voltage of 2.3 V to 2.6 V. Advantages over the sodium-sulfur battery are the inverse structure with liquid sodium on the outside, which allows the use of inexpensive rectangular steel housings instead of cylindrical nickel containers. The assembly is simplified in that the battery materials can be used in the uncharged state as sodium chloride and nickel, and the charged active materials are only generated in the first charging cycle. Sodium-nickel-chloride batteries have been used in the past for small series of electric vehicles in fleets and for stationary storage applications.

6.1.2 Sodium-Sulfur Batteries

The cells consist of an anode made of molten sodium and a cathode made of graphite fabric soaked with liquid sulfur to achieve electrical conductivity, as sulfur is an insulator. As in the case of the NaNiCl battery, the solid electrolyte β-aluminum oxide is used as the electrolyte, which becomes conductive for Na + ions above a temperature of approx. 300 °C. The optimum temperature range is between 300 °C and 340 °C. During the discharge process, positively charged sodium ions enter the solid electrolyte from the liquid sodium, releasing electrons. The sodium ions migrate through the electrolyte to the positive electrode, where they form sodium polysulfides. The cell voltage is 2 V. This process is reversed during charging.

A major advantage of the sodium-sulfur battery is that the internal resistance of the cell is almost independent of the state-of-charge. It only rises sharply toward the end of the charge because there is a decrease in sodium ions in the electrolyte. The required operating temperature is maintained in normal operation by the power dissipation of the cells themselves; in standby operation, it is achieved by an additional electric heater, which increases the battery’s own consumption.

One advantage of this battery is a long calendar life of over 15 years [13]. The technology has been commercialized since 2002, mainly for large-scale storage with more than 1 MWh of energy [14].

7 Redox Flow Batteries

Redox flow batteries have been under development since the 1970s. The vanadium redox flow battery, developed in the 1980s, is considered the best-studied redox flow battery system.

Research efforts are currently focused primarily on reducing equipment and maintenance costs and searching for new electrolyte systems for higher energy densities, electrode optimization for higher performance, membrane development for lower maintenance costs, and electrical system development.

In redox flow batteries, the electrolytes are stored in two circuits in external tanks, while the electrochemical reaction takes place in a “power stack” (reversed fuel cell). Unlike other battery technologies, redox flow batteries thus allow independent scaling of power and energy capacity, making them suitable for a wide range of stationary applications [8, 18].

RF batteries are suitable as stationary energy storage mainly for industrial applications (backup power, load management), at distribution grid level (MW and MWh range, grid management), and for off-grid applications and minigrids (kW and kWh range, long-term storage).

A characteristic of this type of battery is that the power (size of the reactor) can be scaled independently of the capacity (electrolyte volume), because the electro-active materials (electrolytes) can be stored in external tanks [19].

Lifetime.

A lifetime of 20 years is generally expected for VRFB (vanadium redox flow batteries), where temperature control (against precipitation) and regular internal re-initialization of the electrolyte (which is state-of-the-art) are established.

The electrolyte, which represents a significant part of the capital cost of the vanadium redox flow battery, has an unlimited lifetime due to the possibility of reprocessing.

Vanadium redox flow batteries are considered to have an unlimited lifetime, since there are no classical aging mechanisms as in typical battery systems.

Up until now, more than 50 different RFB systems have been described in the literature, of which only a small number have been commercialized or are in the commercialization phase.

The most relevant systems currently are as follows:

  • Iron/chromium (Fe2+/Fe3+; Cr2+/Cr3+)

  • Bromine/polysulfide (Br-/Br3-; S22-/S42-)

  • All-vanadium (V2+/V3+; V4+/V5+)

  • Vanadium/bromine.

Advantages

  • Good energy efficiency: 60–85%

  • Very long service life (> 20 years)

  • Electrolyte recyclable and reusable

  • Cycle stability (> 10,000 cycles)

  • Good response time (some micro- to milliseconds)

  • Scalable, modular design

  • Independent scaling of power and capacity

  • Due to separation of energy storage and converter

  • Overcharge and deep discharge tolerance

  • Low maintenance

  • Almost no self-discharge

Disadvantages

  • Low energy densities

  • Investment costs

8 Conclusion

The selection of an appropriate electrochemical storage system involves consideration of various performance factors such as energy and power density, cycle-life, design life, efficiency, and self-discharge. However, the compatibility of a storage system with specific application requirements is the most crucial factor in its selection. Additionally, factors like investment costs, total cost of ownership, system safety, reliability, and sustainability have gained importance in recent years, particularly with the introduction of the new European Batteries Regulation [21].

Traditional technologies, such as lead- and nickel-based systems, have undergone continuous application-specific development over the years. These developments have focused on adapting the design (including external dimensions, shape, and internal electrochemical design) to meet specific electrochemical requirements, such as high cycle-life for traction applications or an optimized design life for charge retention in UPS applications. The performance data ranges for these systems, listed in the overview in Table 8.5, are a result of appropriate internal designs aimed at achieving high performance in various areas or meeting high energy demands.

Table 8.5 Main performance and cost parameters of the most important battery technologies
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Usually, power density and energy density, as shown in Fig. 8.2, or cost per unit of energy are commonly compared. However, this approach can be misleading. For instance, a common mistake is often made when comparing systems based on cost per unit of energy content (€/kWh), as there are many applications where the cost per required power (€/kW) is more relevant. High-performance battery systems are typically more expensive in terms of energy content (€/kWh) due to their internal design.

Moreover, considering the cost based on energy throughput (cycle-life) for many applications is less meaningful, especially when such systems are used as backup systems for emergency power.

The progress of lithium-ion batteries for electric vehicle (EV) applications serves as a prime example. Lithium-ion batteries have undergone significant development to meet the specific demands of the EV industry. Initially utilized in consumer electronics, this technology has been tailored and tested in fleet trials to address the requirements of electric vehicles. These trials have emphasized the importance of maximizing energy density while accommodating reduced cycle lives, typically ranging from 1000 to 1500 energy throughputs, for EVs.

In contrast, stationary large-scale storage applications necessitate life cycles surpassing 6000 energy throughputs and extended design lives of up to 20 years. These requirements are significantly higher compared to EVs, highlighting the divergent needs of different applications in terms of battery longevity and reliability.

The development of lithium-ion batteries for electromobility applications illustrates the need for application-specific development to cater to the unique requirements of different applications. This example highlights the importance of tailoring battery technologies to meet specific demands, as seen in the progression of lithium-ion batteries for electric vehicles. Similarly, emerging technologies such as sodium-ion batteries (RT, room temperature) are expected to undergo a similar development process, where their characteristics and performance will be optimized to address the specific needs of various applications. This approach ensures that future energy storage solutions are customized and efficient, aligning with the diverse requirements of different industries and sectors.

In conclusion, the demand for electrochemical storage systems is increasing due to the electrification of the mobility sector and the integration of renewable energy sources. There is no universal battery system that fits all applications. The market and applications indicate that there will be a need for different mainstream and upcoming storage systems in the future.

Therefore, when selecting the optimal energy storage system for a specific application, it is crucial to carefully consider the application-specific requirements in order to choose the most optimal and sustainable system for the given application.