S. B. Schougaard and D. Bélanger
8 Electrochemical energy storage systems
8.1 Introduction
The ability to efficiently store and retrieve electrical energy is at the heart of the mobile revolution which has swept through society since the late 1980s. The development of green energies such as solar and wind, which could ultimately be used for
transportation in hybrid and electric cars, began during the same period. Making this
“green” transition will require an efficient, low cost energy storage system. Two of
the most promising candidates to meet the high power-high energy density storage
requirements are lithium-ion batteries and advanced electrochemical capacitors. Yet
the current state-of-the art performance requires improvements to meet customer expectations. To this end, the active materials still need to be perfected.
Materials used in energy storage systems which convert chemical energy into electrical energy must have unique properties for this specific application. A wide variety
of materials such as metal, metal oxides, and even polymers have been used for this
purpose over the past two centuries. One of the first batteries developed which has
reached widespread commercial use is the lead-acid battery. This battery still occupies a major segment of the energy storage market today, due to its use in cars and
other automotives. Other common battery technologies include alkaline and nickelcadmium cells. Materials used in current and future energy storage systems must satisfy design criteria such as low cost, stability, low environmental impact, and toxicity,
and must be produced from widely available resources. This is obviously not the case
for all batteries thus far developed. Safety is also a major concern due to the quest for
higher energy and power density, which require more reactive materials.
The aim of this chapter is to present the fundamental concepts associated with
electrochemical energy storage systems in such a way that they will be accessible to a
broad audience working in the area of functional materials. Firstly, the metrics used to
evaluate the performance of batteries will be described. Secondly, the inner workings
of electrochemical energy storage will be explained using thermodynamic, kinetic,
structure, and mass transport properties of battery materials. The current state-of-the
art materials used in these electrochemical energy storage systems will also be presented.
8.2 Metrics and performance evaluation
The most fundamental characteristic of any energy storage system is how much energy per unit of mass can be stored (specific energy), and how fast this energy can be
retrieved and/or stored (power density). Alternatively, the storage capacity and power
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density may be expressed in terms of volume, when space constraints are more relevant. Electrochemical energy storage systems may be analyzed using several different protocols, one of the most common being the constant current. Here, the current
is sourced (controlled) and the potential (voltage) is measured as a function of time.
Importantly, several different units and nomenclatures are used to express the controlled current. Among the more common ones are A/g, A/cm2 and C-rate, where x C
represents full discharge in C/x hour, so that a current density of C/10 represents full
charge in 10 hours. Batteries and electrochemical capacitors differ in their fundamental charge storage mechanism; as such, it is not surprising that their electrochemical
responses are also different (Fig. 8.1).
10A/g
5A/g 1A/g
4.2
0.9
Potential (V)
Potential (V)
1.2
0.6
3.8
3.4
3.0
0.3
0.2A/g
0.05A/g
0
0
(a)
40
80
120
Capacity (mA.h/g)
160
2.6
0
0.005A/g
40
80
120
160
Capacity (mA.h/g)
(b)
Fig. 8.1. Potential vs. capacity curves. (a) electrochemical capacitor, and (b) battery. The stored
energy is proportional to the area under the curve when extended to 0 V.
From Fig. 8.1 it is evident that capacity is dependent on the current density used. To
put this into a meaningful metric, the power density can be defined as the product of
voltage and current density. As this product varies over the charge/discharge cycle,
an average is normally cited. This definition makes comparison of different devices
according to their power delivery ability and their total energy density in the Ragone
Plot possible (Fig. 8.2).
Figure 8.2 shows that electrochemical capacitors provide high power while batteries provide high energy density. It is therefore important to understand the required
specifications for a specific application before choosing a battery or an electrochemical capacitor. However, for an electric car, it is not a question of battery or electrochemical capacitor, since both serve distinct design parameters; i.e., electrochemical
capacitors are ideal for storing the power generated when the car decelerates rapidly
(e.g., regenerative braking), and for the high power needed when the car accelerates
from a standing position. However, the battery is essential to give the car a significant
range of autonomy due to its high energy density.
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8 Electrochemical energy storage systems | 173
107
Capacitors
Specific power (W/kg)
106
Combustion
engine
105
104
103
Electrochemical
capacitors
100
Batteries
Fuel
cells
10
1
0.01
0.1
100
1
10
Specific energy (Wh/kg)
1000
Fig. 8.2. The Ragone Plot comparing
different energy technologies.
For historical reasons, the energy storage capability of the electrochemical capacitor
is often expressed as capacitance (units: Farad (F)), using the idealized properties of
capacitors i.e., E = Q/C where Q is the stored charge, E is the potential, and C is the capacitance (see below). This relationship leads to W = 1/2 CE2 , so the stored energy (W)
is proportional to the capacitance.
Caution: Specific energy, specific power, and capacities are highly dependent on the
current density. As a rule of thumb, these values should not be extrapolated. Moreover,
care should be taken to identify the mass or volume on which the analysis is based (the
entire device including housing, the electrode, or the active material).
8.3 Models and theory of electrochemical charge storage
Electrochemical energy storage takes place by separating the connection between
two electrodes into two different charge transport paths, one for the ions (electrolyte)
which blocks the electrons, and one for the electrons (external circuit) which blocks
the ions (Fig. 8.3). The external circuit serves as the point where the electric energy is
injected or retrieved. It is this electrical energy that is converted into chemical energy
in the device.
The major difference between the electrochemical capacitor and the battery is the
processes which take place at the electrodes. In the battery it is the chemical process of
electron transfer, i.e., oxidation/reduction, while in the capacitor the energy is stored
electrostatically as “charges” on the electrode surfaces.
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External circuit
Positive electrode
Li+ + e–+ TiS2
LiTiS2
Li+ + e–
Li
Negative electrode
e–
Conducts
Ions
Electrons
Electrolyte
Yes
No
External
circuit
No
Yes
Li+
Electrolyte
Fig. 8.3. Schematic of an electrochemical cell.
8.3.1 Battery operation – a Faradaic process
The separation of the charge transport allows the separation of the redox reaction (cell
reaction) into two Faradaic reactions (half cell), which occur simultaneously in two different locations. A Faradaic process involves a gain or loss of at least one electron. For
example, the relevant reactions during charging are given below for the first lithium
metal battery developed by Whittingham [1]:
Half cell (positive electrode):
Half cell (negative electrode):
LiTiS2 → TiS2 + Li+ + e−
Li+ + e− → Li
Cell reaction:
LiTiS2 → Li + TiS2
(8.1)
The operational principle of charging relies on electrons and lithium ions released
from the oxidation of LiTiS2 to TiS2 in the positive electrode being transported to the
negative electrode by the external circuit (e− ) and electrolyte (Li+ ) where they are used
to form Li(0). Therefore, the overall process does not generate or remove electrons or
ions; they are only transported by the energy source in the external circuit to a location
where they have higher potential energy. Thermodynamics tells us that the work done
by the electrons in the external circuit during discharge cannot exceed the chemical
energy stored in the chemical reaction of the battery. Moreover, the maximum work
possible for an electron in the external circuit is its potential energy, measured as the
voltage between the positive and the negative electrode when no current is flowing (E).
The maximum work that can be done by the chemical reaction (at constant pressure
and temperature) is its Gibbs free energy (Δ r G). This relationship leads to:
Δ r G = −nFE,
(8.2)
where F is the Faraday constant 96 485 C/mol, and n is the number of electrons transferred (1 for the Li/TiS2 cell reaction). Δ r G can be determined from either thermodynamic tables, calorimetry, first principles, or empirical considerations, which are
especially useful when designing new battery materials.
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175
Armed with a method for approximating the potential of a battery (the y-axis of
Fig. 8.1(b)), another for predicting the maximum theoretical value for the x-axis would
be useful. The challenge is determining the charge transferred through the external
circuit by the reaction of one gram of reactant in (1). Thus:
theoretical capacity [in C/g] = nF/M,
(8.3)
where M is the molecular mass of the reactant (g/mol), F is the Faraday constant, n is
the number of electrons transferred per formula unit. (Note: 1 C/g = 1/3.6 mAh/g).
8.3.2 Electrochemical capacitor operation – a non-Faradaic process
The mechanism behind the electrochemical capacitor differs fundamentally from the
battery, as no redox reaction occurs (e.g. no electron transfer). The classical electrochemical double layer capacitor comprises two porous carbon electrodes in the presence of an appropriate electrolyte, separated by a porous and inert separator. At the
electrode/electrolyte interface, opposite charges accumulate on the electrode surface
(within the porous structure) and in the electrolyte. At equilibrium, the electronic
charges in the electrode are counterbalanced by ionic charges from the electrolyte.
This leads to the formation of an electrochemical double layer, whose thickness is in
the nanometer range. The energy of an electrochemical double layer capacitor is determined by the potential being developed and the capacitance of the electrodes, as
discussed previously in the metrics section.
The first description of a model for the double layer was proposed by Helmholtz,
and is illustrated in Fig. 8.4(a) [2]. In this simple model, which is similar to that of
a two-plate capacitor, two layers of opposite charge are present at the electrode/
electrolyte interface and are separated by a distance of molecular order. The Gouy–
Chapman model considers the fact that in contrast to the electrode, for which the
excess of charge is confined to the surface, the excess of charge required in the electrolyte for charge compensation is spread over a thickness which depends on the concentration of the electrolyte and is referred to as a diffuse layer (Fig. 8.4(b)). A third
model, known as the Gouy–Chapman–Stern model, combines the first two models
and includes a first compact layer next to the electrode surface, followed by a diffusion
layer. In this model, the inner Helmholtz plane (IHP) refers to the plane that crosses
the species of the first layer, and the outer Helmholtz plane (OHP) passes through
the centres of solvated ions which are close to the electrode surface (Fig. 8.4(c)). The
effect of specific adsorption and the predominance of water molecules (in aqueous
electrolytes) for which the dipoles are oriented according to the electrode charge
was later proposed in a model by Grahame, Bockris, Devananthan, and Muller as
illustrated in Fig. 8.4(d) [3].
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Diffusion
layer
Stern
layer
Stern
layer
Diffusion
layer
D
ψ
Positively charged surface
ψ
Positively charged surface
Positively charged surface
Anion
Diffusion layer
Solvated
cation
ψ0
Positively charged surface
ψ0
ψ0
Solvent
Inner
plane Outer plane
(a)
(b)
(c)
Primary
solvent
layer
Secondary
solvent
layer
(d)
Fig. 8.4. Schematic representation of the: (a) Helmholtz, (b) Gouy–Chapman, (c) Gouy–Chapman–
Stern, and (d) Bockris et al. models for the electrochemical double layer [2, 3].
The capacitance (C) of such an interface can be roughly estimated by analogy with a
double plate capacitor using the equation:
C = εr ε0 A/d,
(8.4)
where εr is the dielectric constant of the electrolyte, ε0 is the dielectric constant of
vacuum, A is the area of the electrode, and d is the thickness of the double layer. From
this equation, a typical double layer capacitance of 10–15 μF cm−2 can be estimated
for metals in pure water by taking a value of 5 for the dielectric constant of a first layer
of water molecules and the radius of a water molecule [3].
Since the capacitance of an electrode material is related to its surface area, an
obvious approach to improvement has been to develop porous materials with a
large surface area. The pores of these materials are classified into four categories:
ultramicropores (< 1 nm), micropores (< 2 nm), mesopores (2–50 nm); and macropores (> 50 nm). However, it is now clear that parts of the surface area are not electrochemically accessible when the pore size is smaller than the diameter of the electrolyte
ions. Nonetheless, recent studies have demonstrated a significant increase of capacitance for electrodes presenting micropores (< 2 nm), which are smaller than the size
of hydrated ions [4].
Another important parameter of an electrochemical capacitor is the cell voltage.
Figure 8.5 shows a schematic representation of an ideal electrochemical double layer
capacitor, in which both electrode/electrolyte interfaces behave similarly. Initially, the
two electrodes have the same potential and therefore the potential of the cell is null.
When the electrochemical capacitor is charged, electrons are forced to flow through
an external circuit from one electrode to the other. The electrode which accumulates
electrons is termed the negative electrode, and cations from the electrolyte move to the
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8 Electrochemical energy storage systems | 177
electrode surface under the influence of the electric field. Simultaneously, the second
electrode, from which electrons have been removed, becomes the positive electrode,
which is accompanied by displacement of anions of the electrolyte to the electrode
surface. Note that the terms negative and positive are used instead of anode and cathode. This is because in contrast to the latter, which refers to the electrode where electron transfer occurs and involves a change in the oxidation state of the electrode or
solution species (see above), no electron transfer occurs between the electrodes and
the electrolyte in a classical electrochemical double layer. In these instances, the two
electrodes are said to be blocking electrodes.
Porous carbon
+
–
–
Current collector
+
+–
+
–
+
–
–
+
–
+
Electric
charge
+–
Electric
double layer
capacitor
Liquid
(electrolyte)
+
–
+
–
+
–
+
–
+
+
–
–
+
+
–
–
+
–
+
–
+
–
+
–
Liquid
(electrolyte)
ψ0+ψ1
ψ0
ψ0
Discharged state
ψ0–ψ1
Charged state
Fig. 8.5. Schematic representation of the charges of each component (electrodes and electrolyte)
of an electrochemical capacitor at the electrode/electrolyte interface (top) and the potential profile
(bottom) for both its discharged and charged states.
Upon charging by application of a constant current, the double layer will become
charged to a level that will depend on the applied potential (Fig. 8.6). In the case of an
ideal electrochemical capacitor, the potential of each electrode varies linearly but in
opposite directions, with the charge passed during the charging process of the device.
The potential of each electrode can vary up to a limit, determined by the potential at
which oxidation or reduction of electrolytes commences.
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Potential/Reference
Voltage
178 | Part II Development of new materials for energy applications
0
+
–
Charge
Fig. 8.6. Variation of the potential of the positive (- - -) and
negative (– –) electrodes as a function during constant current
charge/discharge together with the evolution of the voltage
(—) of the electrochemical capacitor.
8.4 Electrolytes
From the operational principles of the battery and electrochemical capacitor, it is clear
that the energy required for transportation of charge through the electrolyte and the
potential limits outside which the electrolyte is oxidized or reduce, are key physicochemical design parameters.
Transport of species in solution (mass transport) is related to three phenomena:
diffusion, migration, and convection. Convection, which is forced movement of the liquid including all its components, is only of importance in flow-batteries [5]. Diffusion,
which is important in batteries, is the process by which high concentration solutions
flow towards low concentration at a speed proportional to the concentration difference until equilibrium has been reached. Migration is the mechanism by which an
electric field moves charged species through a solution, and is crucial in both batteries and electrochemical capacitors. Importantly, only migration can carry the current
required to close the circuit in Fig. 8.3. The drag force caused by moving ions through
the solution has two effects. First, there is a significant resistance to charge transport.
This leads to Ohm’s law behavior, where the current (I) is proportional to the potential difference between the electrodes (E), and conductivity (κ) is proportional to the
concentration of free ions (C+ , C− ) and their mobility (u+ , u− ), i.e.,
Iionic = E ∗ κ and κ ∝ u+ C+ + u− C−
(8.5)
The energy lost to resistive heating in the electrolyte should be kept to a minimum,
which suggests that C and u should be maximized. Electrolytes should therefore be
highly concentrated, low viscosity solutions of free ions. This is a challenge, since as
the concentration of electrolyte increases, so does the tendency for ion-pairing. The
result is a viscous solution and a significant fraction of ions that are no longer free to
engage in migration. The second effect of the drag force is that it is different for ions
of different sizes. This leads to strong concentration differences during high current
operation, which the diffusion process is slow to counteract. In particular, the lithium
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179
ion in the Li-battery electrolyte typically moves at only about 65 % of the speed of the
counter-ion. This leads to major problems during high power operation, since only
lithium ions can take part in the charge transfer at the electrodes.
The electrochemical potential range of stability of the electrolyte limits the maximum quantity of energy that can be stored per electron. For example, in aqueous
electrolytes used for electrochemical capacitors, this value is set at 1.23 V by the thermodynamic standard potentials of the H2 O/O2 and H2 O/H2 redox systems. However,
in practice a much lower value of about 1 V can be achieved. A typical nonaqueousbased device using acetonitrile or propylene carbonate as solvent can, however, reach
close to 3 V. Even higher cell voltage could be obtained using materials with a wider
potential range of stability, such as room temperature ionic liquids, which are salts
that are liquid at room temperature. A list of common electrolyte systems is given in
Table 8.1.
Table 8.1. Various types of electrolytes and relevant properties.
Electrolyte type
Example
Practical
conductivity
(mS/cm)
Potential
window
Comment
Aqueous solutions
KCl (aq)
∼ 100
∼ 1.23 V
Cheap, nontoxic,
nonflammable,
limited potential
window.
Nonaqueous solutions
LiPF6 in organic carbonates
∼ 10
∼ 4.3 V
Flammable, large
potential window,
compatible with
low potential anodes.
Ionic liquids
1-ethyl3-methylimidazolium
dicyanamide
(EMI-DCA)
∼ 0.1–30
> 5V
Nonflammable,
large potential
window.
Polymer
Polyethylene
oxide combined with
LiPF6 .
∼ 0.1
> 4V
One of the two
ions may be linked
to the polymer to
improve selective
ion transport.
Solid
Lix POy Nz
< 0.1 at room
temperature.
Highly dependent on
system chosen.
Only high temperature or thin film
application due to
poor conductivity.
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8.5 Electrode materials
8.5.1 Electrochemical capacitors
Carbon is the material of choice for the fabrication of electrode materials of electrochemical capacitor. This is primarily because its low cost, its good electrochemical
stability, high conductivity, and high specific surface area. A large variety of other
types of materials such as conducting polymers and metal nitrides have been used
for this application, but arguably the most promising materials are metal oxides such
as ruthenium dioxide and manganese oxide.
Carbon can be produced in various morphologies and shapes. Initial studies
mainly dealt with high specific surface area powders and high area fibers. Activated
carbons are the most widely investigated, but other forms such as templated carbons, aerogels, nanotubes, nano-onions, and graphene have also been examined for
electrochemical capacitor applications [6].
Carbon materials commonly used in electrochemical capacitors are characterized by
a high specific surface area which ranges from 1 000 to 2 500 m2 /g. Depending on the
production method, the porous texture of the carbon materials will display a variable
pore size distribution. As mentioned previously, it is now well-established that not
all of the porous surface area of the carbon material is active in the charge storage
process. Indeed, by taking a low value of 10 μF cm−2 for the double layer capacitance,
the expected specific capacitance of 250 F/g cannot be reached for the highest surface
area materials. Instead, the typical specific capacitance of standard activated carbons
in organic electrolyte is about 100 F/g [6]. It is possible to attain a higher value (150 F/g)
in aqueous electrolytes (e.g. aqueous KOH) due to the pseudocapacitive contribution
of oxygenated surface groups such as quinone. These surface functional groups will
also contribute to the wettability of carbon, thereby improving the capacitance of the
electrode.
Several single metal and mixed metal oxides have been investigated for their application as active electrode materials for electrochemical capacitors. One is ruthenium dioxide (RuO2 ), which is characterized by pseudocapacitive behavior that, in
contrast to capacitive materials such as carbon, is Faradaic in origin. Thus, pseudocapacitive electrode materials undergo reversible redox reaction involving change of
oxidation states of the metallic species [7]. These redox reactions are distributed over
a large potential range, and are consequently characterized by a rectangular cyclic
voltammogram shape and a linear charge/discharge profile such as the one shown by
carbon electrodes (Fig. 8.7). On the other hand, most other metal oxides are characterized by Faradaic redox processes (in fact they are battery material) which usually
encompass a relatively small potential window. Therefore, a symmetric electrochemical capacitor based on such material for both the positive and negative electrodes will
be characterized by a relatively small cell voltage, and would be almost useless unless
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181
the Faradaic metal oxide is used with a complementary electrode material in a hybrid
electrochemical capacitor, as discussed below.
Another pseudocapacitive material that has attracted attention in the past decade
is MnO2 . It is obtained in various crystalline forms, including many tunnel and layer
structures which allow cation exchange. Amorphous or poorly crystallized MnO2 was
also widely investigated as an electrochemical capacitor material. These poorly crystallized compounds consist of a random arrangement of MnO6 octahedra, which leads
to an intergrowth of different structures, alternating tunnels of different sizes alongside water molecules and alkaline cations (e.g. Na+ , K+ ). The charge storage processes
for the MnO2 electrode can occur by two mechanisms depending on the nature of
the oxide, and involves cations (C+ = H+ , Li+ ) from the electrolyte in both cases, the
Mn3+ / Mn4+ redox interconversion (equation (8.6)) and the following electrochemical
reactions:
MnOOC → MnO2 + C+ + e−
+
−
(MnO2 )surface + C + e →
(MnO−2 C+ )surface
(8.6)
(8.7)
Spectroscopy investigations, typically using thin-film MnO2 electrodes, have confirmed that the Mn3+ / Mn4+ redox couple is primarily responsible for the observed
pseudocapacitance in mild aqueous electrolytes, as demonstrated for poorly crystallized manganese dioxides using in-situ Mn K-Edge x-ray absorption and x-ray photoelectron spectroscopy [8].
8.5.2 Hybrid electrochemical capacitors
A conventional electrochemical capacitor is usually comprised of two identical capacitive electrodes in a symmetrical configuration. On the other hand, in a hybrid electrochemical capacitor, one of the capacitive electrodes is replaced by a Faradaic electrode. Several types of hybrid electrochemical capacitors have been developed; the
most common entails the positive electrode of a symmetric carbon/carbon electrochemical capacitor being replaced with an electrode characterized by a much higher
charge-storage capability. These include pseudocapacitive and Faradaic electrode materials, such as conducting polymers and metal oxides such as MnO2 , NiOOH and
PbO2 [8]. This type of electrochemical device, also called asymmetric capacitor, takes
advantage of the best performance characteristics of electrochemical capacitors and
batteries. This can be achieved by the fast charge storage process of the capacitive
negative electrode and by the increase of the specific capacity of the cell due to the
Faradaic charge storage mechanism of the positive electrode. In addition, by selecting
two electrodes characterized by different operating ranges it is possible to extend the
voltage of an aqueous hybrid electrochemical capacitor beyond the thermodynamic
limit of 1.23 V, as illustrated in Figs. 8.7(b) and (c).
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8
Negative electrode 4
(porous carbon)
–1.0 –0.5
E (V) vs NHE
0
I (mA cm–2)
Positive electrode
(porous carbon)
0.5
1.0
1.5
2.0
–4
–8
(a)
8
Negative electrode 4
(porous carbon)
–1.0
–0.5
0
I (mA cm–2)
Positive electrode
(MnO2)
0.5
1.0
1.5
2.0
E (V) vs NHE –4
(b)
–8
8
Negative electrode
(porous carbon)
–1.0 –0.5
E (V) vs NHE
Positive electrode
(PbO2)
I (mA cm–2)
4
0
0.5
1.0
1.5
2.0
–4
(c)
–8
Fig. 8.7. Schematic representation of cyclic voltammograms for three different configurations of
aqueous-based electrochemical capacitors (ECs), in which areas shaded in black and white represent the potential window of the positive and negative electrode, respectively for: (a) symmetric carbon//carbon device in 1 M H2 SO4 , (b) asymmetric activated carbon//MnO2 device in 0.5 M K2 SO4 ,
and (c) asymmetric activated carbon//PbO2 device in 1 M H2 SO4 . NHE, normal hydrogen electrode,
I, measured current, and E, electrode potential.
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The operating range of an electrode material in a given electrolyte can be assessed
by cyclic voltammetry (Fig. 8.7). Figure 8.7(c) shows that the electrochemical potential
window of an activated carbon electrode ranges from −0.4 to 0.7 V in H2 SO4 (aqueous). Therefore, it can be used as a negative electrode of a hybrid device comprising
of a PbO2 positive electrode, with a range of electroactivity between 1.2 and 1.8 V, over
which the interconversion of PbO2 to PbSO4 is:
−
PbO2 + 4H+ + SO2−
4 + 2e → PbSO4 + 2H2 O
(8.8)
Thus, Fig. 8.7(c) indicates that a carbon/PbO2 hybrid electrochemical capacitor can
deliver a cell voltage of about 2.3 V, in contrast to a carbon/carbon device, which will be
limited to 1.1 V at the most (Fig. 8.7(a)). In a symmetrical device, each carbon electrode
operates in a limited electrochemical window of about 0.55 V, which is only 50 % of
the full electrochemical potential window of carbon (Fig. 8.7(a), white or black areas).
This means that the resulting capacitance (F/g) of the symmetrical device is only 25 %
that of a single carbon electrode.
1/CEC = 1/C+ + 1/C−
(8.9)
The use of a positive electrode with a larger capacitance (capacity) and a complementary electrochemical window will result in full utilization of the carbon material and will increase the energy density due to the capacitive and pseudocapacitive/Faradaic behavior of each electrode. Similarly, a carbon/MnO2 hybrid electrochemical capacitor can deliver a slightly larger cell voltage than a carbon/carbon device, since the electrochemical potential window of an MnO2 electrode extends to
slightly more positive value than a carbon electrode (Fig. 8.7(b)). The former is arguably the most thoroughly investigated hybrid electrochemical capacitor due to the
fact that MnO2 is characterized by low cost, low toxicity, natural abundance, and environmentally friendly use in mild aqueous electrolytes.
8.5.3 Lithium battery¹ electrode materials
Charge storage requires that the charge/discharge cycle can be repeated thousands of
times. In batteries, this is a severe limitation for the type of chemical reactions which
can be employed since high yield is required (for example, a 99.99 % yield per cycle for
1 000 cycles leads to a 10 % total capacity loss). In fact, only three types of reactions
are currently being used in lithium batteries: insertion, alloying, and conversion.
1 Batteries where an intercalation electrode is used, both positive and negative electrodes, are known
as lithium-ion batteries.
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8.5.4 Negative (anode) electrode materials
The simplest negative electrode is arguably metallic lithium. However, difficulties in
plating lithium uniformly lead to dendrite formation in liquid electrolytes. In turn,
this can lead to short-circuiting the electrodes, entailing thermal runaway and even
explosions. Graphitic carbon where Li(0) atoms are nominally intercalated between
graphene sheets is used to circumvent this problem (Fig. 8.8).
Electrolyte
Fig. 8.8. Lithium (large circles) intercalation into graphite.
The reaction leads to a slight increase of the unit cell without
destroying the layered structure.
Current Graphite
collector
The limited capacity of the graphite lithium insertion reaction (339 mAh/g for LiC6 )
has led to the search for new materials for its replacement. A promising candidate is
silicon, which forms an alloy that can store 4.4 Li per Si atom (theoretical capacity
4 200 mAh/g). The accompanying 400 % volume change is however a major design
challenge, since it causes not only stress cracking of the silicon particle, but also in
the entire composite electrode. Another important property of all low potential negative electrodes is their lack of thermodynamic stability relative to the reduction of the
electrolyte. Fortunately, in organic carbonate electrolytes this reaction leads to the formation of a thin layer at the solid electrolyte interface (SEI), which allows Li-ions to
pass and limits further reactions between electrode and electrolyte; see Fig. 8.9.
Electrolyte
Solvent
reduction
Li2CO3, alkoxides
Li2O, RCO2Li
Li+
Li+
Current Graphite SEI
collector
Electrolyte
Fig. 8.9. The solid electrolyte interface (SEI) layer
formation (top) and ionic transport (bottom) at the
anode.
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8 Electrochemical energy storage systems | 185
8.5.5 The positive (cathode) electrode
Most practical positive electrode materials are based on redox cycling of 4th row transition metals. These are combined with highly electronegative elements like oxygen and
fluorine to form solid-state structures, where elevated oxidation states of the metal are
stable. Unique to the structures selected for battery electrodes is that they are electron conductors and allow insertion and extraction of lithium ions to compensate the
change in metal ion oxidation state associated with the redox process.
c
b
b
a
a
b
c
a
c
(a)
(b)
(c)
Fig. 8.10. Materials with (a) 3D, (b) 2D, and (c) one 1D lithium transport path, exemplified by a
spinel, layered α-NaFeO2 , and an olivine structure, respectively.
These intercalation compounds can be classified according to the dimensionality of
the lithium ions transport path, i.e. 3D, 2D, and 1D (Fig. 8.10). Arguably, the most well
known 3D material is the spinel LiMnTMO4 (TM: transition metal ion), which offers
“5V” vs. Li+ /Li. However, problems with Mn dissolution and electrolyte stability have
strongly limited its practical application. The 2D materials with α-NaFeO2 structure
have not suffered a similar fate. In fact, LiCoO2 , which was used in the first Li-ion battery launched by Sony in 1993, is still used in most cellphones today. More recently
Li(Ni0.8 Co0.15 Al0.05 )O2 , which has the same layered structure, has been targeted for
electric car batteries due to its vastly improved safety characteristics and lower cost.
Surprisingly, even a material with a 1D lithium transportation path, olivine LiFePO4 ,
has been commercialized. This material, once carbon coated, offers elevated storage
capacity, high power, and remarkable safety characteristics. However, its lithium insertion/extraction mechanism is still a subject of intense debate. Yet it is clear that the
phase boundary between the lithium-poor heterosite FePO4 phase and the lithiumrich olivine LiFePO4 phase formed during cycling plays a major role in reaction kinetics. While insertion materials currently dominate the market, some transformation
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186 | Part II Development of new materials for energy applications
materials have also been proposed. One notable example is nanosized FeF3 , which
is believed to convert reversibly into LiF and Fe, entailing a remarkable theoretical
capacity of 712 mAh/g.
8.5.6 Electrode production
The common positive electrode materials are hard ceramics with limited electronic
and ionic conductivity. They are therefore produced as small particles which are
mixed with conducting carbon particles and polymer binders and cast as thin films
(10–100 μm) onto metallic current collectors. This open composite structure allows
the electrolyte to penetrate the electrode. As such, it provides improved ionic and
electronic conduction compared to the active material itself. Unsurprisingly, the detailed structure and composition of the electrode has a profound effect on the power
performance of the battery [9], and must consequently be optimized for the specific
application.
8.6 Summary
Batteries and electrochemical capacitors are currently the technologies which allow
efficient electrical energy recovery. There are further great hopes both in industry
and in society as a whole that batteries may become the energy vector which makes
the use of solar, wind, and hydro energy in cars and trucks widely possible. Current
technology, however, does not fully reach the expectations of the average consumer.
Specifically, autonomy, i.e., driving distance per charge, appears to be a concern. The
challenge over the coming years will therefore be to improve the energy density of the
batteries or the charging times, to make them comparable to refuelling with gasoline
or diesel. Chemistries which have been identified as capable of yielding increased
energy density include lithium air and Li-sulphur, which pose unique challenges in
their own right. The alternative is to design batteries which allow for unprecedented
high charging rate, such as 70 % of a complete charge within five minutes. This rapid
charge will then be used for emergency charging in the few instances where the
average driver needs to travel beyond the range of a single charge within one day.
Obtaining such extreme redox kinetics will require that the charge compensating ions
involved in the redox process travel over very short distances inside the solid material.
As such, the distinction between the hybrid electrochemical capacitor and battery becomes less clear, since both rely on redox process in a thin layer of the solid. It will
therefore be fascinating to see if joint research in these fields will be able to combine
the high power of the electrochemical capacitor with the high energy density of the
battery in a single device.
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8 Electrochemical energy storage systems |
187
Further reading
Readers interested in methods used for the production of various carbon materials will find valuable
information in the monographs by Kinoshita [10] and Conway [11]. Similarly, those interested in information about the inner workings of batteries may consult references [12] and [13].
References
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
Whittingham, M.S. Electrical Energy Storage and Intercalation Chemistry (1976) Science, 192,
1126–1127.
Bard, A.J., Faulkner, L.R. Electrochemical methods Fundamentals and applications. 2nd edition.
John Wiley & Sons, New York, 2001.
Bockris, J.O’M., Khan, S.U.M. Surface electrochemistry A molecular level approach, Plenum
Press, New York, 1993.
Simon, P., Gogotsi, Y. Materials for electrochemical capacitors (2008) Nature Materials, 7,
845–854.
Shin, S.-H., Yun, S.-H., Moon, S.-H. A review of current developments in non-aqueous redox flow batteries: characterization of their membranes for design perspective. (2013)
RSC Advances. 3, 9095–9116.
Frackowiak, E. Carbon materials for supercapacitor application (2007) Physical Chemistry
Chemical Physics, 9, 1774–1785.
Conway, B.E. Transition from ’supercapacitor’ to ’battery’ behavior in electrochemical energy
storage (1991) Journal of the Electrochemical Society, 138, 1539–1548.
Long, J.W., Bélanger, D., Brousse, T., Sugimoto, W., Sassin, M.B., Crosnier, O. Asymmetric
electrochemical capacitors-Stretching the limits of aqueous electrolytes (2011) MRS Bulletin,
36, 513–522.
Yu, D.Y.W, Donoue, K., Inoue, T., Fujimoto. M., Fujitani, S. Effect of electrode parameters on
LiFePO4 cathodes. Journal of the Electrochemical Society (2006), 153, A835–A9.
Kinoshita, K. Carbon: Electrochemical and physiochemical properties. John Wiley & Sons,
New York, 1988.
Conway, B.E. Electrochemical Supercapacitors: Scientific Fundamentals and Technological
Applications, Kluwer Academic/Plenum Publishers, New York, 1999.
Newman, J., Thomas-Alyea, K.E., Electrochemical Systems. 3rd edition. John Wiley & Sons,
New York, 2004.
Reddy, T.B., Linden’s Handbook of Batteries. McGraw-Hill Companies. Inc., New York. 2011.
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