среда, 13 сентября 2017 г.

Battery Technologies for Energy Storage



Reference Module in Earth Systems and Environmental Sciences
Encyclopedia of Sustainable Technologies
2017, Pages 469–486

Battery Technologies for Energy Storage

Current as of 5 April 2017
  
Available online 14 July 2017
https://doi.org/10.1016/B978-0-12-409548-9.10154-X

http://www.sciencedirect.com/science/article/pii/B978012409548910154X
Get rights and content

Abstract

The pressing need for green, renewable, alternative energy sources is due to the fast depletion of fossil fuels, increasing demands for energy, and the adverse effects of the use of fossil fuel on the environment. The world is embarking on ambitious solar and wind energy projects, as they can meet future energy requirements. However, supplied energy from these sources is intermittent and weather-dependent. Therefore, it is necessary to store the energy generated from these sources for continuous supply. Thus, electrical storage technologies are essential for both storing excess power and for meeting peak power demands (Tiwari and Mishra, 2011). The leading energy storage technologies include flywheels, superconducting magnetic energy storage, compressed air energy storage, water electrolysis and electrochemical energy storage devices such as batteries (Alotto et al., 2014). However, high installation costs and poor efficiency are the major limitations of their widespread use. Batteries and electrochemical capacitors can be cost-effective and allow for flexibility in deployment. Battery technologies are also ideal for transportation where instant power needs to be available for the vehicle for reasonable lengths of time. They are inexpensive and ensure high levels of safety, reliability, and durability. They come in various sizes and capacities, suitable for stationary and portable applications over a broad spectrum of human activity (Dunn et al., 2011).

Keywords

  • Battery technology; 
  • Conversion and alloying reaction; 
  • Deintercalation; 
  • Electrocatalysts; 
  • Electrochemical energy storage; 
  • Intercalation; 
  • Li–ion; 
  • Li–sulfur; 
  • Metal–air; 
  • Nanostructured electrode; 
  • Na–sulfur; 
  • Redox flow; 
  • Smart cathodes

Introduction

The pressing need for green, renewable, alternative energy sources is due to the fast depletion of fossil fuels, increasing demands for energy, and the adverse effects of the use of fossil fuel on the environment. The world is embarking on ambitious solar and wind energy projects, as they can meet future energy requirements. However, supplied energy from these sources is intermittent and weather-dependent. Therefore, it is necessary to store the energy generated from these sources for continuous supply. Thus, electrical storage technologies are essential for both storing excess power and for meeting peak power demands (Tiwari and Mishra, 2011). The leading energy storage technologies include flywheels, superconducting magnetic energy storage, compressed air energy storage, water electrolysis and electrochemical energy storage devices such as batteries (Alotto et al., 2014). However, high installation costs and poor efficiency are the major limitations of their widespread use. Batteries and electrochemical capacitors can be cost-effective and allow for flexibility in deployment. Battery technologies are also ideal for transportation where instant power needs to be available for the vehicle for reasonable lengths of time. They are inexpensive and ensure high levels of safety, reliability, and durability. They come in various sizes and capacities, suitable for stationary and portable applications over a broad spectrum of human activity (Dunn et al., 2011).

Thermodynamics of Electrochemical Devices

Batteries are electrochemical devices that generate electricity by converting chemical energy stored in the two electrode materials. Ions migrate from anode to cathode and electrons travel through an external circuit to produce electricity. The expected voltage of an electrochemical cell (Ecello) is determined by the difference in electrochemical potential of the positive and negative electrodes. The cell potential can be written as follows:
equation1
View the MathML source
Turn MathJaxon
The maximum energy that can be delivered by an electrochemical device depends on the change in the free energy (ΔG) of the redox couple. If all of this energy can be converted to useful electrical energy, it represents an ideal electrochemical cell. However, in practice, the passage of load current will lead to polarization losses. These losses are termed activation polarization (the electrochemical reaction at the electrode surface is driven by this) and concentration polarization (difference in the concentration of reactants and products will lead to this as a result of mass transfer). These polarizations consume some amount of available energy in the cell and convert it into heat. The amount of heat generated will vary depending on the load current (the rate at which energy is dissipated).
Another important factor that affects the rate of energy dissipation is the internal resistance of the cell. The internal resistance will lead to voltage drop during cell operation by consuming some amount of useful energy as heat, which is termed ohmic polarization or IR drop. The ohmic polarization or IR drop is the contribution of ionic resistance of the electrolyte (within the separator and the porous electrode), the electronic resistance of active mass, the current collectors on which electrode materials are coated and the resulting contact resistance and the electrical tabs. All of these resistances that contribute to internal impedance are ohmic in nature, with a linear relationship between current and voltage drop (follows Ohm’s law). When an external load (R) is applied to the cell, the cell voltage (E) can be expressed as
equation2
View the MathML source
where Eo is open circuit potential of the cell, (ηct)a and (ηct)c are activation polarization or charge-transfer overpotential at anode or cathode, respectively. Concentration polarization of anode and cathode represented by (ηc)a and (ηc)c are concentration polarization at anode and cathode, respectively, i is the operating current of a cell on load and Ri is the internal resistance of the cell ( Speight, 2005).
The polarization and internal IR drop will reduce the voltage of the cell from open circuit potential as shown in Fig. 1. The theoretically available energy of the cell can only be realized at very small operating currents, where polarization and IR drop are small and lead to minimum deviation of voltage from open circuit potential. From a thermodynamic standpoint, the basic thermodynamic equations for a reversible electrochemical reaction are given as follows:
equation3
View the MathML source
where ΔG represents Gibbs free energy or energy of a reaction available for useful work; ΔH is the enthalpy or energy released by the reaction; ΔS and T are entropy and absolute temperature, respectively, with TΔS being the heat associated with electrochemical redox reactions of materials.
Cell polarization curve.
Fig. 1.
Cell polarization curve.
Figure options
Since ΔG represents the net useful energy available from a given reaction, the net available electrical energy from an electrochemical reaction in a cell is given by
equation4
View the MathML source
where,n is the number of electrons transferred per mole of reactants, F is Faraday’s constant, and E is the voltage of the cell. The free energy relationship for bulk chemical reaction is defined as
equation5
View the MathML source
where R is the universal gas constant, T is absolute temperature, Ap is the activity product of the products, and Ar is the activity product of the reactants. The corresponding Nernst equation is given as
equation6
View the MathML source
The reversible heat effect can be defined from thermodynamic reversibility from Eqs. (3), (4) as follows:
View the MathML source
equation7

Various thermodynamic quantities for the materials can be experimentally determined by measuring cell voltage as a function of temperature. If dE/dT is positive, the cell will heat up upon the charge process (heat is generated in the cell) and cool upon the discharge process. In general, when a cell is in operation, the amount of entropic heat released is small compared to irreversible heat generated in the cell as given in
equation8
View the MathML source
The voltage upon irreversible heat loss will deviate from the thermodynamic or equilibrium voltage as given in Eq. (8), where Er is the practical cell voltage, EOCV is the open circuit voltage, and Q is the total heat released by the cell. The total heat released during discharge on the surface of the electrode is given as a sum of the thermodynamic entropy contribution and the irreversible contribution due to the shift in voltage. In practice, heat released at a high rate performance must be dissipated, as otherwise it will lead to thermal runaway and other catastrophic situations in the cell ( Speight, 2005).
The different kinds of battery technologies for electrical energy storage are described in the following sections.

Lead-Acid Battery

The lead-acid battery has been profoundly acclaimed for its commercial value for over a century. The legacy of stationary electrical energy storage dates back to the turn of the 19th century, with the serious power shutdowns by the power stations overnight, with lead-acid accumulators supplying the residual loads on the direct current networks. Although there had been previous discussions about batteries containing sulfuric acid or lead components by many scientists, the modern lead-acid battery chemistry was first developed by French physicist Gaston Plante in the year 1859 (Kurzweil, 2010). Since then, rechargeable electrochemical devices have been widely used worldwide. In Plante’s fabrication method, two long strips of lead foil and intermediate layers of coarse cloth were spirally wound and immersed in a solution of about 10% sulfuric acid, as shown in Fig. 2.
(A) An illustration of original lead acid battery, called Plante cell (Usatoday, ...
Fig. 2.
(A) An illustration of original lead acid battery, called Plante cell (Usatoday, 2004). (B) Schematic representation of lead-acid battery.
Figure options
A typical lead acid battery consists of (in the charged state) lead dioxide (PbO2) as a positive electrode and metallic lead (Pb) with high surface area as a negative electrode in an electrolyte of about 37% (5.99 molar) sulfuric acid, and in the discharged state both electrodes turn into lead sulfate and the electrolyte loses its dissolved sulfuric acid and becomes primarily water. The chemical reactions during charging/discharging of a typical lead acid battery are as follows:
Negative electrode:
equation9
Pb(s)+SO42−(aq)→PbSO4(s)+2e−
Positive electrode:
equation10
PbO2(s)+SO42−(aq)+4H3O+(aq)+2e−→PbSO4(s)+6H2O(l)
Overall reaction:
equation11
Pb(s)+PbO2(s)+2SO42−(aq)+4H3O+(aq)→2PbSO4(s)+6H2O(l)
The overall reaction gives a standard voltage of 2.1 V and allowed end-voltage on a usual discharging is 1.75 V per cell. The electrolyte concentration may change the open circuit voltage. Thus the relative density of the sulfuric acid can help in understanding the state of charge in the lead-acid battery system. In the case of overcharging above 2.39 V, the aqueous electrolyte decomposes by an evolution of hydrogen at the negative and oxygen at the positive electrodes, respectively. This leads to loss of water in the electrolyte, requiring maintenance by adding water (Aifantis, 2010).
Overcharging reaction:
At negative electrode
equation12
2H++2e−→H2
At positive electrode
equation13
View the MathML source
The latest commercially available lead battery consists of six cells that are placed in series inside the casing such that the positive and negative active materials are applied to each side of the electrode plates and interspersed with insulating separators. Each cell provides a voltage of 2 V, which makes it a 12 V battery. The water loss due to overcharging is a serious inconvenience in lead acid batteries. To overcome this, various models have been adopted, including the flooded battery requiring regular topping up with distilled water, the sealed maintenance-free battery having a gelled/absorbed electrolyte, the valve-regulated battery and the ultrabattery with a split design for the negative electrode (Aifantis, 2010).
However, there are some unavoidable intrinsic limitations of lead-acid batteries that restrain it from global acceptance. These include corrosion on the surfaces of positive plates, which takes place even at 21°C and which severely damages the battery performance and decreases cycle life. Another contributing factor towards the capacity loss is irreversible sulfate formation on the negative plate.
The lower manufacturing cost and extreme customizability allow lead acid batteries to provide a wide range of outputs from 1 Ah to nearly 1000 Ah sized batteries with power ranges from 1 kW to 10 MW. State-of-the-art lead acid batteries almost singularly dictate the energy supply for engine starting, vehicle lighting, and engine ignition (SLI), that is, from starting engines in electric vehicles (EVs) to electrical energy storage in various renewable energy systems (Baker and Collinson, 1999). Also, this type of battery shows a very high cycle life of 5–12 years and additionally the cell components can be recycled at a high rate of 97% from the used batteries, which in turn makes the lead acid battery very popular in the market.

Li–Ion Battery

Rechargeable lithium batteries which operate at room temperature propose several advantages compared to the age-old aqueous technologies, including higher energy density, higher cell voltage (up to about 4 V per cell, and longer shelf life (up to 5–10 years).

Basic Components of Lithium–Ion Battery

Similar to other battery systems, a Li–ion battery (LIB) mainly comprises cells that employ lithiated intercalation compounds as the positive and negative materials. The components are mainly divided into three vital components: a cathode, an anode, and a liquid electrolyte between.
The anode is the negative electrode/terminal of a cell associated with oxidative chemical reactants that release electrons into the external circuit. In LIBs, the state-of-the-art anode material is carbonaceous material, especially graphite, and also sometimes carbon derivatives. Graphite was introduced in the 1980s by Bell Labs as an anode material for LIBs (Julien et al., 2015). Graphite is a layered structured material that facilitates Li+ to intercalate between its layers; where six carbon atoms accommodate one Li+ results in delivery of 372 mAh g− 1 specific capacity against Li/Li+. However, the basic demerit of carbon materials is associated with fast rate charging, solid electrolyte interphase (SEI) formation, and lithium plating. And even though in recent years several anode materials have been developed, namely Fe2O3, MoS2, Li4Ti5O12, TiO2, SnS2, SiO2, etc., (Goriparti et al., 2014) to overcome these demerits, still graphite remains as the ultimate choice in commercial batteries with very negligible replacements with Li4Ti5O12 and other carbon–Sn composites. Further, anode materials are classified according to the reaction step. The classifications of insertion reaction are
(a)
Intercalation, carbon, TiO2, Li4Ti5O12
(b)
Conversion-based material: Fe2O3, MoS2
(c)
Alloying-based material: Sn, SnO2 etc.
LiCoO2 is the most widely used cathode material in commercial LIBs. Lithium cobalt oxide offers a very good + electrical performance, is easily prepared, has good safety properties, and is relatively oblivious to process variation and moisture.
 Iitially, layered LiNiO2 was used as the cathode material. It has a layered structure which is used to give a very good capacity. But the problem with the material is the unstable delithiated structure, that is, Li1 −xNiO2, which was overcome by layered LiCoO2. It also has good thermal stability and structural stability through the intercalation–deintercalation processes. 
Even though LiCoO2 has the capability to give more lithium, that is, beyond 0.5Li, it is restricted to 4.2 V. So, research then focused on inert di- or trivalent metal (Al, Mg, Ti) doping to the Ni or Co metal sites. This leads to LiNi1 −XMgX/2TiX/2 phases, with a higher capacity of around 180 mAh g− 1 compared to 140 mAh g− 1 of LCO. 
At the same time, another line of focus was on a soft chemistry–based synthesis of layered LiFeO2and LiMnO2 to explore the redox couples of Fe3 +/Fe4 + and Mn3 +/Mn4 +, due to their stability and lower cost. 
The layered electrochemically active LiFeO2 was a failure, but LiMnO2 was fruitful. On the other hand, the olivine-type (LiMPO4) cathodes are very promising for 1D Li motion, having a voltage of 3.5 V versus Li and a theoretical capacity of 170 mAh g− 1 (Goodenough and Park, 2013; Tarascon and Armand, 2001; Etacheri et al., 2011 ;  Linden and Reddy, 2001).
Beside the electrodes, the electrolytes play an important role in the proper functioning of the battery. The electrolyte is a substance that produces an electrically conducting solution when it is dissolved in polar solvents.
 Ethylene carbonate (EC) is used in many electrolytes (LiPF6, LiBOB, and LiClO4) to make a protective layer on the graphite electrode to suppress the side reactions; it is thinned with other solvents owing to its high melting point. The research interest in polymer-based electrolytes is growing with the LIB, but at present, it is in the infancy stage at room temperature, so the battery manufacturers are mainly focusing on the liquid electrolyte which operates beyond 4.5 V. Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) is one of the most widely used cross-fertilizers to achieve high voltage goals, and also research on ionic liquid-based electrolytes is going on simultaneously (Tarascon and Armand, 2001; Etacheri et al., 2011 ;  Linden and Reddy, 2001).
Many materials have been studied to facilitate the purpose of the current collector in a LIB. Where the charges are collected and move on to the external circuit, commonly aluminum (Al) for cathodes and copper (Cu) for anodes are used. It is of central importance that these mentioned materials are low cost and have high electronic conductivity due to the high purity of (Al/Cu) in LIBs.
A separator is made of a chemically and electrochemically stable microporous polymeric permeable membrane to separate anodes and cathodes from the physical contacts, to prevent short circuits during the passage of ionic charges. Separators are also considered to be a key parameter for batteries to enhance the cyclability, performance, and energy and power densities, along with being a considerable safety factor.

Lithium–Ion Battery Working Principle

During the charge/discharge process, lithium ions from the cathode are extracted or inserted from interstitial space between the atomic layers within the active materials. Fig. 3 shows the schematic representation of a LIB and the following mechanism shows the electrode and net cell reaction of a LIB in the charged state.
1.
During charging, the lithium ions flow from the positive electrode LiMO2 (where M = Co, Ni, Fe…) to the negative electrode through the electrolyte, whereas electrons flow through the outer circuit from the cathode to anode.
2.
During the discharge process, the ions flow back to the positive electrode from the negative electrode through the electrolyte, whereas the electrons flow in the outer circuit to combine the ions at the cathode. When all the ions have moved back, the battery is termed as fully discharged and requires recharging.
At positive
equation14
LiMO2↔Li1−xMO2+xLi++xe−
At negative
equation15
C+xLi++xe−↔LixC
Overall
equation16
LiMO2+C↔LixC+Li1−xMO2
Fig. 3. Schematic illustration of lithium–ion battery.
Schematic illustration of lithium–ion battery.
Figure options
Different types of LIBs are classified in Fig. 4. The advantages and disadvantages of LIBs are summarized in Table 1. The high specific energy (150 Wh kg–1) and energy density (400 Wh/L) make them a suitable candidate for lightweight and other applications. LIBs offer a very low self-discharge rate of 2%–8% per month, a long cycle life greater than 3000 cycles and a broad temperature range of operation (charge at 20–60°C, discharge at 40–65°C), empowering a wide variety of applications with different sizes and shapes available from a variety of manufacturers. A single cell typically operates in the range of 2.5–4.2 V, approximately three times higher than that of NiCd or NiMH cells, and thus requires a set of cells to attain the required voltage battery. LIBs can offer a high rate capability: around 5C–25C without compromising the cycle efficiency has been demonstrated. The combination of these qualities in a cost-effective package has enabled these batteries to be used for various technological applications.
Fig. 4. Typical examples of Li–ion batteries (Linden and Reddy, 2001).
Typical examples of Li–ion batteries (Linden and Reddy, 2001).
Figure options
Table 1.
Advantages and disadvantages of Li–ion batteries
AdvantagesDisadvantages
No maintenance requiredInitial cost
Sealed cellsDegradation at high temperature
Wide range of operational temperatureNeed for protective circuits
Long shelf/calendar lifeCapacity loss or thermal runaway when overcharged
Rapid charge–discharge capabilityPossibilities of thermal runaway when it is crushed
High rate and power discharge capabilityNaturally, the cylindrical designs offer lower power density than NiCd or NiMH
High coulombic and energy efficiency
High specific energy and energy density
Zero memory effect
Table options
Table 1 describes the benefits and drawbacks of LIBs relative to other types of energy storage devices.

Sodium–Sulfur Battery

The sodium–sulfur (Na–S) battery system was first developed in the 1970s as a result of intensified research on various alternative sustainable energy technologies. The main characteristic feature of this system is that the electrodes are maintained in a molten state during the charge and discharge process, and it usually operates at a high temperature of 300–350°C. Traditionally a sodium–sulfur system is constructed in a tubular vessel and consists of a Na anode, S cathode, and β-alumina, as a solid electrolyte and a separator (Fig. 5).
Schematic of high-temperature Na–S battery.
Fig. 5.
Schematic of high-temperature Na–S battery.
Figure options
The elevated operational temperature strictly marginalizes the application of liquid electrolytes and additives. However, the enhanced ionic conductivity of the Na+ ion at ∼ 300°C insists on a very robust electrode-electrolyte interface. The thermal and chemical stability alongside the high ionic conductivity of 0.2–0.4 S/cm of β-alumina positively contributes to this factor. The introduction of a small number of Li+ or Mg2 + ions has been observed to drastically improve the electrochemical performance and stability (Lu et al., 2014; Wen et al., 2013 ;  Oshima et al., 2004). However, for the sustenance of battery operation, proper sealing of the alumina material is necessary. The temperature intolerance of most organosilane and polyacrylonitrile compounds creates remarkable inconvenience in their application. Glass-based ceramic systems are highly temperature resilient and, therefore, a whole species of glass ceramic composites are instrumental in working with the Na–S battery since they act as a primary boundary between the two electrodes. These glass-ceramic sealants with their high thermal expansion coefficient at high temperature (300°C) further benefits the situation. Among them, borosilicates are highly efficient and produce attractive results, although the Group I oxides [Na2O, Cs2O, etc.] also have been observed to successfully facilitate mechanical strength and lead to healthier interaction between the glass and solid electrolytes (Wen et al., 2013; Oshima et al., 2004 ;  Sudworth and Tilley, 1985).
Looking at the electrochemical reaction of a Na–S system, at the negative electrode Na is oxidized to Na+ ions while discharging and disodium pentasulfide (Na2S5) appears at the positive electrode from the reaction of the Na+ ion with S, which further leads to the formation of a polysulfide moiety in the presence of sulfur.
Negative electrode:
equation17
2Na↔2Na++2e−
Positive electrode:
equation18
nS+2Na++2e−↔Na2Sn
Overall reaction:
equation19
2Na+nS↔Na2Sn
The prime benefit of a Na–S system is its low redox potential of nearly 2 V and high theoretical specific energy of 750 Wh kg–1. The elemental abundance of sulfur at surface volcanoes and Na minerals in the earth’s crust, together with the relatively low-cost extraction processes of both Na and S, rank the Na–S battery as one of the most promising, industrially applicable, large-scale energy storage devices, which can achieve a long cycle life of 2500 years. This system has five times the enhanced energy density of the leading lead acid battery.
The major issue with this technology lies in the discharged product, sodium polysulfide. These are a series of Na–S compounds with varying degrees of stoichiometry. These insulator compounds create a substantial traction in the electron transport process, leading to rapid deceleration of capacity at the current collector. The presence of carbon particles has been observed to alleviate the conductance issue to a significant extent. Additionally, improper sealing of molten cathodes leads to severe corrosion. Thus, an anticorrosive current collector is required for Na–S batteries (Tischer, 1983). In September 2011, a 2000 kW Na–S battery plant at Japan erupted into violent flames. The investigation committee reported that the molten electrodes from a single cell flooded over the sealing materials and came into direct contact, resulting in a short circuit and fiery explosion (Ryu et al., 2011). Thus, safety precautions remain a big challenge for Na–S systems.
The charge/discharge process of a Na–S cell has been proposed to be a complicated one, since it has the transition processes of sodium and sulfur via a series of reactions, which result in the formation of many polysulfides (Na2Sn, 1 ≤ n ≤ 8) in the compartment (Manthiram and Yu, 2015). The whole reaction is divided into four regions, where the transformation of sodium and sulfur takes place into different forms of polysulfide. Finally, the Na2S2 and Na2S form in the fourth region, which are highly insulative in behavior, resulting in the slower kinetics and high polarization of room-temperature (RT) Na–S batteries.
In the initial trials of RT Na–S cells, Li–S cathodes were used but did not perform well. So, different approaches have been taken to improve cathode performance. One approach is the development of a sulfur composite which is favorable to charge-discharge with good cyclability. Hollow carbon sphere–sulfur (HCS–S) cathodes, and polyacrylonitrile (PAN)-derived carbon—sulfur composites with a 1D fibrous structure and small sulfur molecule cathodes with carbon nanotube (CNT) scaffold have been used. The common binder material polyvinylidene difluoride (PVDF) has generally been used in sulfur–carbon cathodes for RT Na–S batteries. Also, great attention is being paid to sodium–metal anodes. In one study, a Na–Sn–C alloy anode improved the output voltage of Na–S batteries (Sudworth and Tilley, 1985 ;  Manthiram and Yu, 2015) (Fig. 6).
Schematic illustration of RT Na–S battery.
Fig. 6.
Schematic illustration of RT Na–S battery.
Figure options
Polymer-based gel electrolytes also have been studied, but show a relatively poor performance. The significant advance in the electrolyte is the use of liquid-phase solvents, namely dimethoxyethane and/or tetraethylene glycol dimethyl ether (TEGDME). Also, the use of modified porous separators showed a better performance (Manthiram and Yu, 2015). In addition, there have been significant improvements in the electrolyte additive. To conclude, the improvements in the cathode, modified anode and electrolytes should demonstrate the feasibility of RT Na–S battery systems as near-future energy storage devices.

Lithium–Sulfur Batteries

The lithium–sulfur battery appears as one of the promising candidates for next-generation energy storage technologies. Realization of a Li–S battery is of great interest because the theoretical cell capacity could be up to 1165 mAh g− 1 with an average voltage of 2.1 V, as estimated based on the Gibbs formation energy. The theoretical specific energy is about 2447 Wh kg− 1 (Yang et al., 2013) (which is three to five times that of conventional LiCoO2/graphite batteries) for Li–S battery chemistry, making it a suitable candidate for large-scale energy storage applications.
A Li–S battery typically consists of a lithium anode, a sulfur cathode, and an electrolyte containing lithium salts (Fig. 7). The anode and cathode are separated from each other by a porous insulating separator. During discharge of a Li–S battery, the lithium metal anode is oxidized to produce Li+ ions leaving behind the electrons at the anode. Then, Li+ions flow freely from anode to cathode through a porous separator, while electrons move from anode to cathode through an external wire. When both Li+ ion and electron reach the cathode, sulfur starts to be reduced to form lithium polysulfides (Li2S8). However, the reduction of sulfur (S8) mainly takes place via a stepwise reduction, with polysulfides of different chain length as intermediates (Zhang and Zhang, 2015). In contrast to conventional lithium ion batteries, lithium dissolution takes place from the anode during discharge and reverse lithium plating to the anode occurs while charging. The overall electrochemical reaction of lithium and sulfur can be described as
equation20
16Li+S8→8Li2S
The reduction of sulfur mainly takes place via a stepwise reduction with polysulfides of different chain length as intermediates (Yang et al., 2013). The polysulfides are reduced at the cathode in the discharging process by the following steps to finally produce lithium sulfide:
equation21
S8+2Li++2e−→Li2S8
equation22
3Li2S8+2Li++2e−→4Li2S6
equation23
2Li2S6+2Li++2e−→3Li2S4
equation24
3Li2S4+2Li++2e−→4Li2S3
equation25
2Li2S3+2Li++2e−→3Li2S2
equation26
Li2S2+2Li++2e−→2Li2S
These polysulfides with longer chain length are very soluble in organic electrolytes. They also diffuse through the electrolyte to the anode surface and produce shorter chain length polysulfides, which reduce to Li2S. The resulting polysulfides recombine with other dissolved polysulfides and subsequently transport back to the cathode, where they oxidize again. However, short chain length polysulfides of Li2S and Li2S2are insoluble in the organic electrolyte. These polysulfides are found both on the anode surface and the cathode surface, forming a fine insulator layer and causing further hindrance to the charge and discharge process. This phenomenon is called the polysulfide shuttle effect, which leads to rapid capacity fading and poor coulombic efficiency due to the increase in internal resistance of the cell. On the other hand, this shuttle effect provides an advantage of intrinsic overcharge protection for lithium–sulfur batteries (Yang et al., 2013 ;  Zhang and Zhang, 2015).
Schematic illustration of Li–S battery.
Fig. 7.
Schematic illustration of Li–S battery.
Figure options

Smart Cathodes

Technoeconomic studies suggest that the cathodes with areal sulfur loadings of > 7 mg cm− 2 are essential for Li–S batteries to find some practical applications in transportation (Eroglu et al., 2015). Significant efforts have been devoted to solving the lithium polysulfide dissolution problem within the cathode and also to making the electrode reasonably stable. As a result, a better degree of capacity retention throughout the battery cycling has been attained. Building high sulfur loading Li–S cells poses great challenges, since the increase of thickness above a certain limit hinders the specific capacity and conductivity of the material even though the specific capacity increases with thickness. The deprived use of sulfur in the thick electrodes is significantly attributed to the mass transport and the three-dimensional distribution of polysulfides throughout the electrodes. It leads to electrolyte inability at high current rates and slow diffusion kinetics. The main concern for thick electrodes arises from the design aspects, since the proper architecture is important to accommodate a large volume of sulfur, the electronic conductivity and also to correspond to its volume changes over cycling. To achieve high performance, not only high areal sulfur loading but also,a low fraction of the electrochemical inactive materials with the properly maintained porous material are necessary. In this direction, the increase of carbon and binder only help to achieve the advantageous high energy density. Free-standing 3D electrodes and crosslinked nanostructured sulfur hosts are the two main types of smart cathodes.

Free-Standing 3D Electrodes

From the engineering point of view of carbon-based electrodes, it is suggested to directly impregnate the sulfur into the void structures in the electrode. This ensures charge transfer and mechanical stability for the electrode over cycling: for example, the incorporation of porous carbon and porous, spongy graphene structures containing sulfur or polysulfide catholyte results in higher specific energy as well as high stability. A free-standing activated carbon cloth with up to 6.5 mg cm− 2 sulfur loading gives a stable performance up to 80 cycles. Layer by layer carbon-based materials and a 3D scaffold fabricated with the help of carbon nanofiber and CNTs offer high sulfur loading and result in high performance and structural tailoring that is promising in Li–S batteries (Yang et al., 2013; Zhang and Zhang, 2015 ;  Eroglu et al., 2015).

Cross-Linked Sulfur Hosts

The 3D structure provides a superior mechanical, ionic and electronic conductivity along with a high areal sulfur loading. The main disadvantage of this is the lower volumetric energy due to the high areal occupancy of carbon and also the improper wetting of the electrolyte due to the void space availability in the 3D framework (Eroglu et al., 2015 ;  Pope and Aksay, 2015). Consequently, modified conventional crack-free electrodes cast on a current collector with sulfur composites that can easily facilitate the smooth transferring of the electrons are becoming more interesting. Here, the cross-linked sulfur–carbon nanostructure comes from the secondary particle growth (Yang et al., 2013). Fine-tuning of the conductive pathway, elasticity and void space volume are necessary. Issues related to the anode are neglected due to the thin coating of it in the cell. However, the anode issue will be more dangerous in thick electrodes.

Electrolyte Developments

Other than conventional organic electrolytes, two new classes of electrolytes are deployed for Li–S battery systems:
I.
Polymer electrolyte
a.
Solid polymer electrolyte
b.
Gel polymer electrolyte
II.
Ionic liquid

Polymer electrolyte

This is usually defined as a membrane that possesses transport properties comparable to that of common liquid ionic solutions. The study of polymer electrolytes was started in 1973 by Fenton et al. and since then it has garnered nearly worldwide acclaim due to its many advantages over liquid electrolytes. These electrolytes consist of lithium salts (LiClO4, LiCF3SO3, LiBF4) dissolved in a solid polymeric host medium. The entire polymer electrolyte system can be classified into two broad categories: solid polymer electrolyte and gel polymer electrolyte systems.
In solid polymer electrolytes, the polymer matrix uses itself as a solid solvent along with lithium salt which does not contain any organic liquid. The main advantage of this solid electrolyte in a Li–S cell is that it can act as a barrier between two electrodes and also can control the dissolution of polysulfide anions. Polyethylene oxide (PEO) is the most investigated host polymer due to its nontoxic nature and highly flexible structure. Its complex with lithium salt can exhibit a higher ionic conductivity of 10− 4 S/cm. The polymer chains are arranged in helical or nonhelical pairs forming a tunnel through which the Li–ion can migrate easily, leaving its counter anion in the interchain space. The Li–ion transportation occurs through the segmental motion of the amorphous region of the polymer chain (Scheers et al., 2014). The ratio of PEO and Li-salt lies in the range from 3:1 to 20:1 according to the application. The main drawback of the PEO based solid polymer electrolyte has poor ionic conductivity (10− 7 to 10− 6 S/cm) at ambient temperature (30°C) and a high tendency to react with the lithium anode at elevated temperature (60°C).
To eliminate the obstacle of low ionic conductivity, gel polymer electrolytes (GPEs) were developed in which liquid components are entrapped in the polymer matrix. A plasticizer of low molecular weight aprotic solvents is used to induce a disorder in the crystalline structure by increasing the ionic conductivity. EC or propylene carbonate is widely used as a plasticizer in PEO-based polymer host systems. Due to high chemical resistance and thermal stability, PVDF, polymethyl methacrylate (PMMA), a copolymer of hexafluoropropylene with PVDF, that is, PVDF-HFP, are also chosen as promising candidates for GPEs. This class of electrolyte can attain ionic conductivity of 10− 4 S/cm at room temperature but it exhibits very poor mechanical properties when compared to solid polymer electrolytes (Scheers et al., 2014).
Further improvement of ionic conductivity can be attributed to the adding of ceramic filler such as Al2O3, SiO2, TiO2, or ZrO2 in the host polymer matrix. This class of electrolyte is called a composite polymer electrolyte. The ceramic filler increases ionic conductivity by decreasing the crystallinity of the polymer host. It decreases the interface resistance between the polymer electrolyte and the lithium electrode also, by lowering the contact area between lithium and the electrolyte. A composite polymer electrolyte can exhibit a maximum ionic conductivity of 10− 3 S/cm and 10− 2 S/cm at ambient temperature and 60°C, respectively (Scheers et al., 2014).

Ionic liquid

A new series of electrolytes that have successfully decreased the polysulfide dissolution consists of room temperature molten salts, that is, ionic liquid. Nonflammable, nonvolatile ionic liquids are well known for their low environmental impacts with high ionic conductivity. The main advantage of the ionic liquids is their electrochemical stability towards a wide range of the potential window. A mixture of N-butyl-N-methyl pyrrolidinium bis(trifluoromethanesulfonyl)-imide (PYR-TFSI), LITFSI and TEGDME is widely used as an electrolyte for Li–S batteries. This class of electrolyte obtains high discharge capacity and stability as it significantly increases the sulfur utilization by decreasing the polysulfide intermediates ( Scheers et al., 2014).
Other than the solubility of intermediately formed polysulfides in the electrolyte, lithium–sulfur batteries are facing some major failures in their commercialization, due to the low conductivity of elemental sulfur and lithium dendrite growth for the use of lithium metal as the anode. Table 2 summarizes the cathode and anode parts of the Li–S battery.

 Comparative study for cathode and binder for Li–S battery


Component
Characteristics
Advantage
Cathode
Porous carbon with sulfur
Encapsulation of sulfur in highly ordered porous carbonaceous material reducing the loss of active materials
Ionic conductivity increases. Cost is low
Graphene with sulfur
Ultrathin high surface structure
Excellent electronic conductivity and high mechanical strength
Porous silica with sulfur
Porous silica used as polysulfide reservoir It can reversibly adsorb and desorb soluble polysulfide resulting in reduced sulfur dissolution into the electrolyte
Enhanced coulombic efficiency and cycling stability
Organosulfide
Energy storage is provided by cleavage of a disulfide bond. Tetraethiuram disulfide (TEDT) and PAN-S oligomer composite are widely used as organic sulfide. Follows two-step reaction pathway: Charge transfer step followed by rate determining step
High specific capacity and excellent cycling performance
Binder
PEO
Gives uniform homogeneous porous surface when mixed with sulfur. PEO films coat the sulfur particle suppressing the loss of active materials
Stable cycle performance but sometimes highly entangled PEO chains form agglomeration with sulfur, leading to swelling of the binder. It will cause rapid increase in cell resistance and poor cell performance
PVDF
Most popular binder for Li–S system. Exhibits swelling at elevated temperature
Oxidatively stable in high potential applications. Shows good adhesion between sulfur and the current collector
Gelatin
Biological macromolecule soluble in water. An ionizable group such as COOH and NH2 helps to withstand polysulfide
Cost efficient, environment-friendly, good mechanical properties. Suppress active material agglomeration, enhanced initial capacity
CMC + SBR
Possesses higher elasticity, heat resistance, and binding force and holds huge volume change of sulfur during cycling
Improved capacity and high mechanical stability
The latest development in the lithium–sulfur battery is due to its potential energy content. Although a few agencies are attempting to commercialize a functional Li–S battery, their full potential is far from certain. There is much room for improvement in Li–S technologies, especially in decreasing polysulfide dissolution and increasing the ionic conductivity.

Metal–Air Batteries

Metal–air batteries have been considered as very promising energy storage devices for portable and stationary applications. Metal–air batteries are comprised of a metal anode and air breathing cathode which constantly draw out oxygen from ambient air. The air-breathing carbon cathode makes it different from the other batteries. The specification of the battery is determined by the particular anode. Though several types of metal–air batteries are theoretically possible, only a handful of metal-based air batteries could be able to overcome the technological hurdles to be economical. Nowadays, a number of metal–air batteries are under investigation and, hopefully, in the near future will be commercialized. Metal–air cells based on Li, Zn, Al, etc. are leading because of their rapid technological progress. The concept of metal–air batteries was first understood from Lechlance’s cell (Zn anode, MnO2 cathode), where the effect of oxygen/air was realized in 1878. The first primary Zn–air battery was commercialized by National Carbon Company mainly for railway signaling on the basis of the principle of an “air-depolarized primary battery” proposed by Heise and Schumacher in 1932. However, the miniaturization and development of the Zn–air cell in the late 1970s make it smarter than the other existing battery technologies, as it could deliver the highest energy densities with a low operational cost (Linden and Reddy, 2001) (Fig. 8).
Cross-sectioned view of Zn–air battery (Courtesy Duracell) (Linden and Reddy, ...
Fig. 8.
Cross-sectioned view of Zn–air battery (Courtesy Duracell) (Linden and Reddy, 2001).
Figure options

Zn–Air Battery

In a primary Zn–air cell, commonly a Zn amalgam is employed as a Zn anode. However, several Zn nanostructures have also been investigated to improve the performance of the cell. The metallic zinc is oxidized and consumed with the processes, hence a higher amount of zinc increases the durability of the cell. A 30% aqueous KOH solution is utilized as the electrolyte. The air cathode extracts O2 from the air and diffuses it into the electrolyte. The cathode provides the reaction site for oxygen reduction and could undergo a charge–discharge process for an infinite number of cycles, theoretically. The electrode reactions inside a Zn–air cell are as follows:
At positive electrode
equation27
View the MathML source
At negative electrode
equation28
View the MathML source
Overall cell reaction
equation29
View the MathML source
In alkaline electrolytes, the Zn anode is prone to corrode and produce hydrogen gas. This challenge could be met by the addition of gelling agents (carboxymethyl cellulose, graft polymers, etc.). Several polynuclear species such as Zn(OH)42 −, Zn(OH)3(H2O)−, and Zn(OH)2(H2O)2 are also generated during the discharge process, and it depends on the concentration of OH− and H2O (Lee et al., 2011). A separator for the Zn–air battery plays a crucial role, as it should have suitable pore size to allow the OH− ion to pass through it. The other requirements of the separator are to be stable in alkaline electrolyte and have high ionic and electronic conductivity. Polyethylene, polyvinyl alcohol, polyolefin–based separators are usually employed for the Zn–air battery (Lee et al., 2011). The oxygen reduction reaction (ORR) at the cathode is complex and crucial to determine the performance of the cell. The ORR can be undergone by two different processes:(i) direct reduction from O2 to water, and (ii) indirect reduction of O2 where a peroxide intermediate is generated:
Direct reduction
Indirect reduction
equation30
View the MathML source
equation31
View the MathML source
A direct pathway for the ORR is desirable for an energy-efficient metal–air battery. The reduction of the peroxide to hydroxide is the rate-determining step of the reaction (Kinoshita, 1992). The cathode catalyst facilitates the reduction process and typically four types of catalysts are present, that is, (i) noble metals and bimetallic alloy catalysts (Pt, PtAu, PtPd etc.), (ii) carbonaceous materials (activated carbon, graphene-based materials and porous carbon etc.), (iii) transition metal oxides and perovskites (MnO2, Co3O4, and LaMO3 where M is Ni, Mn, etc.), (iv) organic–inorganic hybrid (metal macrocycles, etc.) (Lee et al., 2011; Kinoshita, 1992; Cheng and Chen, 2012 ;  Shao et al., 2016). The cathode of the metal–air batteries has been developed simultaneously with the progress of cathode catalysts of low-temperature fuel cells (FCs). Different types of ORR catalysts are widely explored, and the kinetics of the ORR have been extensively studied for the last few decades (Lee et al., 2011; Kinoshita, 1992 ;  Cheng and Chen, 2012). The catalysts and respective ORR kinetics are more complex in nonaqueous-based electrolytes and will be discussed in the case of the Li–air battery.
The electrocatalytic reduction of oxygen is crucial as it is undergone at the interphases of the solid electrode, electrolyte and gaseous oxygen. A suitable high surface area cathode catalyst layer should be fabricated in which O2 can diffuse throughout the electrolyte and adsorb on the electrode surface, but it must not flood it to the exclusion of gas. A mixture of porous activated carbon and a small amount of MnO2 are widely used as cathode catalysts for the primary Zn–air cell. The catalyst layer is carefully coated with a gas diffusion layer or Ni mesh by a dual-layer approach. The electrolyte side is kept hydrophilic, and the gaseous side is made hydrophobic by coating with wax or a Teflon layer. The balance of the hydrophilic and hydrophobic nature in corresponding parts is very important to construct a proper cathode. An air-excess hole is generated on the positive terminal of the cell for oxygen flow. The flow rate of air is controlled by the area of the hole and the porosity of electrode materials. A schematic illustration of the Zn–air cell from Duracell is shown in Fig. 9. Commercialized primary Zn–air batteries from Duracell and Panasonic in different sizes, such as DA5, DA10, DA675, etc. for single cell and DA164, DA146 as multicells, are available. A cylindrical cell of a Zn–air primary battery with AA, AAA, C, 9V, etc., sizes has also been launched by Rayovac. Zn–air batteries have a wide range of energy density, from 300 to 450 Wh kg− 1. The primary Zn–air cell usually delivers an average capacity of 40–600 mAh depending on cell size. The cells exhibit an operating voltage from 0.9 to 1.25 V, and are being widely used in hearing aids, pagers, patient monitors, etc. (Linden and Reddy, 2001 ;  Girishkumar et al., 2010). However, the performance of the Zn–air battery largely varies with humidity and temperature. The best results are obtained in the temperature range from 10°C to 40°C and when the humidity is within 30%–80%. The deviation of performance of the cell with temperature and humidity is due to changes in the KOH concentration.
Different types of Li–air battery.
Fig. 9.
Different types of Li–air battery.
Figure options
In the case of rechargeable Zn–air batteries, bifunctional oxygen catalysts are employed as electrodes which facilitate the oxygen evolution reaction (OER) to charge the cell. The charging process is associated with the deposition of Zn2 + to metallic Zn and the OER at the air electrode. The chemistry concerned with charging of a rechargeable zinc air electrode is:
At positive electrode:
equation32
View the MathML source
At negative electrode:
equation33
View the MathML source
The cathode catalysts, employed in this cell, mostly composed of perovskite material based on La, Mn and Ni, etc., are highly efficient bifunctional oxygen catalysts with exceptional durability. Rechargeable Zn–air cells have been widely used in computers and cellular phones, etc. An important factor in the design and fabrication of rechargeable Zn–air batteries is to control the flow of the air that inserts into and leaves from the cell. Excessive air could make the cell dry, whereas too little an amount of air may impair performance. Therefore, an air-manager is employed to control the amount of air (Linden and Reddy, 2001; Kinoshita, 1992 ;  Cheng and Chen, 2012).

Li–Air Battery

The Li–air or Li–O2 battery has drawn major attention for being the most promising for EV applications. Though the aqueous Li–air concept was proposed in 1970, it still has not been widely commercialized due to the electrochemical instability of both the electrolyte and electrode. The nonaqueous, rechargeable Li–air battery was first reported by K. M. Abraham in 1996. Recently, IBM and their partners started Battery 500 research projects on Li–air batteries for automotive applications due to their having a higher theoretical specific energy density (5200 Wh kg− 1, including oxygen) than all other existing battery chemistries. The “500” signifies setting of a target of 500 miles/800 km per every single charge for a standard family car. In recent years the nonaqueous rechargeable Li–air battery has drawn attention because the predicted practical energy density (1700 Wh kg–1) can be similar to gasoline. Lithium can also deliver the highest theoretical voltage and electrochemical equivalence (3860 Ah kg− 1Li). Currently, four types of Li–air cell have been categorized (Fig. 9) on the basis of the employed electrolyte. These are:(i) aqueous, (ii) nonaqueous/aprotic, (iii) solid-state electrolyte, and (iv) hybrid, composed of aprotic and aqueous types (Cheng and Chen, 2012 ;  Shao et al., 2016).
The associated chemical reaction for discharge in an aqueous Li–air battery is
equation34
View the MathML source
where LiOH is generated as a product. Though the theoretical potential of the cell is 3.35 V, in practice open-circuit voltage of 2.85 V is obtained. A protective layer should be introduced at the metal surface to prevent it from corrosion. Degraded performance with consecutive cycles due to side reactions and Li corrosion are the major limitations of this type of cell. Another major challenge for the aqueous Li–air system is to develop a suitable Li+ conducting separator to prevent the Li anode from contamination with H2O. A protective glass-ceramic layer on Li (LiSICON, LiM2(PO4)3) was introduced by Polyplus Co. which could make the Li stable in water (Linden and Reddy, 2001; Lee et al., 2011 ;  Kinoshita, 1992).
Thanks to the high theoretical energy density of the nonaqueous Li–air battery, hope remains for next-generation automotive battery technology. Present research on the fundamental science of novel electrode materials and electrolytes is conferring progress on Li–air technology. The basic chemistry of the nonaqueous Li–air cell suggested for charge–discharge is
For charging
equation35
View the MathML source
For discharging
equation36
Li2O2=2Li+O2
During the discharge of the cell, an oxidation reaction occurs at the Li anode and consequently electrons flow through an external circuit to produce electricity. As produced, Li+ from this reaction reduces O2 to Li2O2 in the cathode. There is also a possibility of formation of Li2O at the cathode.
equation37
View the MathML source
A stable solid electrolyte interface (SEI) is also formed at the surface of the Li anode as in a conventional LIB and it protects from Li corrosion.
An Al–air battery having a theoretical energy density of 4300 Wh kg− 1 also has potential for use in EVs. The practical energy density obtained from the Al–air cell is 1300 Wh kg− 1. The energy and power produced from the Al–air system is also equivalent to gasoline. The chemical reaction associated with the full cell is the same as with other metal air batteries:
equation38
View the MathML source
Generally, an aqueous KOH solution is used as the electrolyte but solid polymer-based electrolytes have also been explored. The air cathode is made of cobalt catalyst in carbon and is cast over a Ni mesh. A PTFE film is coated over the air side to prevent leakage (Linden and Reddy, 2001).

Redox Flow Battery

Redox flow batteries (RFBs) were first developed by NASA in the 20th century, during the major energy crisis in the United States (Skyllas-Kazacos et al., 2011). Similar to other rechargeable batteries, RFBs utilize redox couples, which are stored in independent tanks and brought together to energy conversion cells through pumps when there is a need of energy, as shown in Fig. 10.
Schematic illustration of vanadium-based redox-flow battery.
Fig. 10.
Schematic illustration of vanadium-based redox-flow battery.
Figure options
As compared to other available electrochemical storage technologies, in RFBs the power conversion is separated from the energy storage, so they can be independently sized to achieve the desirable power output. This feature virtually leads to a limitless capacity simply by using a larger storage tank. In the current scenario, a capacity from 1 kW to 100 MW can be built by using RFBs, and these are similar to FCs in energy generation flexibility. They have more advantages compared to other electrochemical energy storage technologies when a longer duration of more than 4–6 h of storage is needed. Like FCs the membrane electrode assembly, which is sandwiched between the electrodes, is the heart of the RFBs. The electrode is made of a porous structure, since it has to allow the electrolyte to flow to the active sites. The porous structure is obtained by the utilization of carbon-based materials, namely CNTs, carbon fiber, carbon paper, etc., which allow a high degree of porosity up to 0.8, thereby achieving a good compromise between the electrode surface area and electrode permeability (Weber et al., 2011). In RFBs the electrochemical reactions are completely reversible, enabling the same cell to work as a generator as well as a converter of electrical energy into chemical energy. The principle of operation of the RFB is a change in the valence of the metal ion without its consumption, thereby giving long life. The lower power and energy density compared to the other available technologies have made these cells unsuitable for mobile applications. Also, due to the larger surface area, large dimensions of the electrode and chamber lead to high transverse of the solution, which apparently leads to a lowering of the average current density and nominal current with respect to the theoretical value achievable with the maximum constant current density.
Since the 1970s, several types of RFBs have been investigated; the list includes iron/chromium, vanadium/bromide, bromine/polysulfide, zinc–cerium, zinc/bromine (Zn/Br), and all-vanadium. Among them, all-vanadium (1.26 V) and Zn/Br (1.85 V) systems are noted as the most advanced and they have come up to the demonstration level for grid storage applications. The interesting thing in all-vanadium systems is that they have a single cationic element (here, the vanadium ion). The ions cross over through the membrane upon cycling, which is less harmful than other competing chemistries in RFBs.
RFBs possess a number of excellent advantages over conventional batteries: the simplicity of the electrode material during the reactions in terms of the phase transformations, electrode–electrolyte degradation, and change in electrode morphology, the stationary energy storage still perhaps uncertain. For this, one of the limitations will be its field trials. On the other hand, the other available battery technologies have benefited from their extensive flexibility in developing a range of applications for the electronics and automotive fields. A relative disadvantage of flow batteries is that the system requires pumps, sensors, reservoirs and flow management systems for their smooth operation (Weber et al., 2011).
Different types of RFBs have been developed. Some of the main popular technologies are discussed in the further sections.

All-Liquid Aqueous Flow batteries

Among all available flow batteries, the all-liquid aqueous flow battery gets more attention because of its design flexibility and the possibility of scaling up the process. They can be further divided into
•
All-vanadium RFB
•
Iron-chromium RFB
•
Polysulfide-bromine RFB
•
Soluble metal–bromine RFB
•
Other chemistries

All-vanadium flow batteries

Of aqueous flow batteries, the all-vanadium flow battery has been the most studied. It was first invented by Skyllas-Kazacos et al. in the 1980s at the University of New South Wales (Australia). The battery chemistry is solely based on highly reversible and fast redox couples of V2 +/V3 + in the anolyte (Eq. 39) and the V4 +/V5 + redox couple in the catholyte (Eq. 40), which is noticeably more complicated and therefore slower. Sulfuric acid is commonly used as the supporting electrolyte; it ensures the pH and increases the stability of electroactive spices in the solution. A high acidic concentration (up to 4M) is maintained to provide the higher conductivity. The solubility of the vanadium decides the theoretical energy density of the RFBs and it was assumed that the catholyte reaction is controlled by both charge transfer and diffusion steps. The reactions at the respective electrodes are as shown here:
Negative electrode:
equation39
V2+(II)↔V3+(III)+e−
Positive Electrode:
equation40
View the MathML source
Overall reaction:
equation41
VO2+(V)+2H++V2+(II)↔VO2+(IV)+H2O+V3+(III)
A typical all-vanadium system is as shown in Fig. 10; it is comprised of a set of external liquid electrolyte tanks having a redox couple of vanadium. V4 +/V5 + acts as a positive electrode and V2 +/V3 + acts as a negative electrode redox system, respectively with two stacked electrodes. These two electrodes are separated by an ion exchange membrane and each electrode material is replenished through a pumped circulation system connected to the respective anolyte and catholyte tank and circulated through the stack. The electrochemical reactions occur at the core, termed as the cell of the redox flow battery. These cell compositions follow a sandwiched assembly where the membrane is overlapped by a pair of electrodes and carbon felt (which acts as a gasket) and which is subsequently encased between a pair of current collectors followed by bipolar or end plates, as shown in Fig. 11. The power of the system is determined by the number of cells in the stacks, whereas the energy of the system is determined by the volume and concentration of the electrolyte in the tank. In a vanadium redox-flow battery, a V2 +/V3 +redox couple circulates in the negative (anolyte) compartment and a V4 +/V5 + redox couple circulates in the positive (catholyte) compartment when the cell has unit activity. The overall electrochemical reaction gives a cell voltage of 1.26V at 25°C.
Components of the redox-flow battery.
Fig. 11.
Components of the redox-flow battery.
Figure options

Iron-chromium RFB

The modern flow battery concept was first developed as an iron/chromium (Fr/Cr) system which was first invented by Thaller in the year 1970 at NASA, demonstrating a 1 kW/13 kWh energy, which was later improved by various groups (Rosseinsky, 1965). Two one-electron redox couples FeII/FeIII and CrII/CrIII in hydrochloric acid are used as the positive and negative reactants, respectively. The charge transfer reactions at each electrode are
equation42
View the MathML source
equation43
View the MathML source
The system showed better performance with the low-cost carbon-felt electrode with IEM/separator. The ion-selective membrane passes only protons and chloride anions but, in reality, cannot prevent cross-contaminations. Both the charge transfer reactions require only one single electron movement to accomplish the process; indeed they simplify the reaction. The slow chromium redox kinetics called for the development of redox catalysts, like bismuth or bismuth-lead on carbon materials. The addition of ethylenediaminetetraacetic acid to chromium electrolyte allows for stabilization of Cr5 +, and makes the all-chromium battery a possibility in the future (Grigorii, 2015). But the anodic reaction is slow. The design engineering of the system will greatly desires the improvement of crossovers of iron to the chromium stream and vice-versa.

Bromine/polysulfide

This system, the bromine/polysulfide RFB, was first patented by Grigorii, 2015 ;  Remick and Ang, 1984 and later was extensively studied. To date, three series of bromine/polysulfide RFBs have been developed as various power-rated systems. A system capable of 15 MW, which is comparable with commercial size, has been demonstrated, where the system comprises 120 modules and 200 bipolar electrodes to produce a storage capacity of about 12 MWh with two 1800 m3 electrolyte storage tanks (Price et al., 1999). In the bromine/polysulfide system, the state-of-the-art system consists of sodium bromide as the positive electrolyte and sodium polysulfide as the negative electrolyte, although the counter-ion could be replaced with another cation.
equation44
View the MathML source
equation45
View the MathML source
The key attributes of this system are the abundant and reasonable availability of the species, which are present in the electrolytes in large quantities. Also, their solubility in the aqueous electrolyte significantly minimizes the volume of electrolyte required for the efficient storage of the required quantity of energy. At the positive electrode, three bromide ions combine to form the tribromide ion (Eq. 44) and at the negative electrode, the sulfur in solution is shuttled between polysulfide and sulfide (Eq. 45). Since, the majority of the electroactive species in the system are anions, a cation-exchange membrane is desired to prevent the mixing of both the streams.

Hybrid Redox-Flow Batteries

Along with all-aqueous flow batteries, there are other hybrid types, with improved performance and some common issues with what we would classify as RFBs. The active material can be introduced to or removed from the cell without disassembling its initial structure. Here, the active material is not stored in liquid or gaseous form; as such, it is a semiflow cell with more complicated reactions than those of simple shuttling between the oxidation state of a single species.

Zinc/bromine

The prototypical hybrid or semiflow RFB is the zinc/bromine system where, the reactive species are stored in the external tank containing the electrolyte solutions. The electrolyte is passed through each cell in the stack, but the zinc reaction does not only involve dissolved species in the aqueous phase.
At positive electrode,
equation46
Br2+Br−↔Br3−
From this reaction, it is seen that the bromide ions can combine with bromine molecules to generate the tribromide ions, which usually occur in the liquid phase.
At the negative electrode,
equation47
View the MathML source
The zinc metal dissolves and deposits. To prevent self-discharge of the system with the combination of zinc and bromine, separate flowing streams of aqueous zinc bromide and bromine are made, separated by an ion exchange membrane (IEM) or a microporous film separator. The metal negative electrode allows the compactness of the electrode and thus increases the energy density. In addition, the zinc/bromine system has a high cell voltage, decent reversibility, and also the expectation of low material costs. However, the demonstration of zinc/bromine has been restricted due to the material inabilities: corrosive atmospheres, dendrite formation, electrical shorting, high self-discharge rates, low energy efficiencies, and short cycle life (Grigorii, 2015).

Hydrogen-Based Systems

Usually, a FC takes hydrogen and oxygen as the fuel to produce electricity and water, with hydrogen oxidizing at the anode and oxygen reducing at the cathode. The FC can also form an RFB system if it works in both the charge and discharge direction (regenerative FC) (Grigorii, 2015). The construction of such a system typically differs from the earlier discussed RFBs, since it comprises a gas reactant instead of a liquid one. This greatly improves the mass transfer at the cost of storage tank volume, and thus there is a need for suitable hydrogen compression or novel hydrogen-storage materials. Although the typical mass transfer is rapid, the oxygen reactions are known to be very slow and result in very large overpotentials, thereby making the overall efficiency of the system relatively low. To overcome these drawbacks and to cut down the costs, strategies like using an alkaline medium, high temperature, and closed systems which can contain oxygen, not air, have been looked at. Also, to overcome the oxygen reduction problem, examinations of different types of oxidants have been done (Livshits et al., 2006).

Conclusions

Different types of major electrochemical energy storage systems have been discussed. Although large-scale deployment is an inevitable part of future energy storage systems, today’s systems are not very economical for large-scale applications and result in large capital investments. To reduce the cost and make them economically viable in the near future, several efforts are being considered and have been described. At present, the LIBs show excellent performance in portable electronics and light motor vehicles, with their high energy density. However, their suitability in large-scale applications is hindered due to their safety issues. In this view, the sulfur, metal–air and redox flow-based chemistries show a prominent potential to expand the grid storage market. However, their practical application is questioned today.

References

    • Aifantis, 2010
    • K.E. Aifantis
    • High energy density lithium batteries: Materials, engineering, applications, Wiley, Hoboken, NJ (2010)
    • Grigorii, 2015
    • L. Grigorii
    • Soloveichik, flow batteries: Current status and trends
    • Chemica Reviews, 115 (2015), pp. 11533–11558
    • Julien et al., 2015
    • C. Julien, A. Mauger, A. Vijh, K. Zaghib
    • Lithium batteries: Science and technology
    • Springer, Berlin (2015)
    • Linden and Reddy, 2001
    • D. Linden, T.B. Reddy
    • Handbook of batteries, McGraw-Hill Professional, New York City (2001)
    • Lu et al., 2014
    • X. Lu, G. Li, J.Y. Kim, D. Mei, J.P. Lemmon, V.L. Sprenkle, J. Liu
    • Liquid-metal electrode to enable ultra-low temperature sodium–beta alumina batteries for renewable energy storage
    • Nature Communications, 5 (2014), p. 4578

    • Remick and Ang, 1984
    • R.J. Remick, P.G.P. Ang
    • Electrically rechargeable anionically active reduction-oxidation electrical storage-supply system
    • USA Patent 4485154 (1984) November 27, 1984
    • Speight, 2005
    • J.G. Speight
    • Lange’s handbook of chemistry
    • (16th edn.)McGraw-Hill, New York (2005)
    • Tischer, 1983
    • R.P. Tischer
    • The sulfur electrode
    • Academic Press, New York (1983)
    • Tiwari and Mishra, 2011
    • G.N. Tiwari, R.K. Mishra
    • Advanced renewable energy sources, Royal Society of Chemistry, London (2011)
    • Zhang and Zhang, 2015
    • Z. Zhang, S.S. Zhang
    • Rechargeable batteries: Materials, technologies and new trends, Springer, Berlin (2015)
Copyright © 2017 Elsevier Inc. All rights reserved.
Автор: на

Комментариев нет:

Отправить комментарий

Примечание. Отправлять комментарии могут только участники этого блога.

Подписаться на: Комментарии к сообщению (Atom)