Electrochemical energy storage systems convert chemical energy into electrical energy and vice versa through redox reactions. There are two main types: galvanic cells which convert chemical to electrical energy, and electrolytic cells which do the opposite. A basic electrochemical cell consists of two electrodes separated by an electrolyte. Primary cells cannot be recharged, while secondary cells are rechargeable through reversible chemical reactions. Lithium-ion batteries have become widely popular due to their high energy density and lack of memory effect.

1. By Brhane Amha
ELECTROCHEMICAL ENERGY
STORAGE SYSTEMS

2. ELECTROCHEMICAL ENERGY
• Electrochemistry is a branch of chemistry which deals with the study of interconvertion
of Electrical energy into chemical energy and vice varsa.
• Inter-conversion of energies takes place through a redox reaction.
• Electrochemical Cell is a device which converts chemical energy into electrical energy
and electrical energy into chemical energy.
 Types of Electrochemical Cell: There are two types of electrochemical cell.
i) Galvanic Cell: - A Galvanic cell is a device in which the chemical energy is
converted into electrical energy. Ex: Dry cell, Pb-Acid cell, Ni-Cd cell etc…
ii) Electrolytic Cell: - It is a device in which electrical energy is converted into
chemical energy. Ex: Nelson’s cell, Down’s cell etc….
 An electrochemical cell typically consists of:
- Two electronic conductors (also called electrodes)
- An ionic conductor (called an electrolyte)

3. ELECTROCHEMICAL ENERGY
Classification of Galvanic Cells:
I) Primary Galvanic cell: - A Primary Galvanic cell is one in which the chemical
changes takes place are
irreversible. Ex: Dry cell.
II) Secondary Galvanic cell: - A Secondary Galvanic cell is one in which the chemical
changes takes place are reversible… Ex: Pb-Acid Battery Etc…
 Many portable electrical and electronic devices are designed to be powered from
batteries – and in a lot of cases, from primary or non-rechargeable batteries.
 This is the familiar kind of battery which has a fixed amount of energy stored in it
during manufacture, and once that energy has been used up the battery is simply
thrown away and replaced.

4. ELECTROCHEMICAL ENERGY
Batteries:- devices that transform chemical energy into electricity
• Every battery has two terminals: the positive cathode (+) and the negative anode (-)
• Device switched on -> chemical reaction started - electrons produced - electrons travel from (-) to (+)
electrical work is produced.
An electrochemical cell comprises:
1. a negative electrode to which anions (negatively charged ions) migrate, i.e., the anode – donates
electrons to the external circuit as the cell discharges
2. a positive electrode to which cations (positively charged ions) migrate, i.e., the cathode
3. electrolyte solution containing dissociated salts, which enable ion transfer between the two
electrodes, providing a mechanism for charge to flow between positive and negative electrodes
4. a separator which electrically isolates the positive and negative electrodes.

5. ELECTROCHEMICAL ENERGY
Primary and Secondary Batteries
• Primary batteries are disposable because their electrochemical
reaction cannot be reversed.
• Secondary batteries are rechargeable, because their electrochemical
reaction can be reversed by applying a certain voltage to the battery in
the opposite direction of the discharge.

6. ELECTROCHEMICAL ENERGY
Primary cells are non-rechargeable cells in which the electrochemical reaction
is irreversible.
• They contain only a fixed amount of the reacting compounds and can be discharged
only once.
• The reacting compounds are consumed by discharging, and the cell cannot be used
again.
• A well-known example of a primary cell is the Daniell element, consisting of zinc
and copper as the electrode materials.

7. ELECTROCHEMICAL ENERGY
Secondary cells are rechargeable several times.
• Only reversible electrochemical reactions offer such a possibility.
• After the cell is discharged, an externally applied electrical energy forces a reversal
of the electrochemical process; as a consequence the reactants are restored to their
original form, and the stored electrochemical energy can be used once again by a
consumer.
• The process can be reversed hundreds or even thousands of times, so that the
lifetime of
the cell can be extended.
• This is a fundamental advantage, especially as the cost of a secondary cell is
normally much higher than that of a primary cell.
• Furthermore, the resulting environmental friendliness should be taken into account.

8. ELECTROCHEMICAL ENERGY
 Fuel cells : In contrast to the cells so far considered, fuel cells operate in
a continuous process.
 The reactants – often hydrogen and oxygen – are fed continuously to the cell from outside.
Fuel cells are not reversible systems.
 Typical fields of application for electrochemical energy storage systems are in portable
systems such as cellular phones, notebooks, cordless power tools, SLI (starter-light-ignition)
batteries for cars, and electrically powered vehicles.
• There are also a growing number of stationary applications such as devices for
emergency current and energy storage systems for renewable energy sources (wind, solar).
• Especially for portable applications the batteries should have a low weight and volume, a
large storage capacity, and a high specific energy density.
• Most of the applications mentioned could be covered by primary batteries, but economical
and ecological considerations lead to the use of secondary systems.

9. ELECTROCHEMICAL ENERGY
 Chemical energy can be converted to other forms as well; for example, in combustion processes, the chemical energy of
combustion is converted to mechanical energy by an engine.
 In nuclear power plants, the chemical energy of nuclear fission is converted to heat and then to electricity.
 Electrochemical devices are unique in that they convert chemical energy directly to electrical energy.
 Because electrochemical processes do not involve the transfer of heat, Carnot limitations are avoided and processes can be very
efficient.
 For example, the round-trip energy efficiency (for example, the amount of energy you get out of battery compared to the amount
of energy used to charge it) for a battery is typically greater than 85%.
 Charge process: When the electrochemical energy system is connected to an external source (connect OB in Figure1), it is
charged by the source and a finite
charge Q is stored.
• So the system converts the electric energy into the stored
chemical energy in charging process.
 Discharge process: When the system is connected to an external resistive circuit it releases the stored charge Q and generates a
current through the external circuit.
• The system converts the stored chemical energy into electric energy in discharging process.

10. ELECTROCHEMICAL ENERGY
 A simple example of energy storage system is capacitor.
 Figure 2(a) shows the basic circuit for capacitor discharge. Here we talk about the integral
capacitance. The capacitance is defined as a constant,

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13. ELECTROCHEMICAL ENERGY
• Electrochemical power sources differ from others such as thermal power plants in
the fact that the energy conversion occurs without any intermediate steps; for
example, in the case of thermal power plants, fuel is first converted into thermal
energy (in furnaces or combustion chambers), then into mechanical energy, and
finally into electric power by means of generators.
• In the case of electrochemical power sources, this multistep process is replaced by
one step only.
• As a consequence, electrochemical systems show some advantages such as high
energy efficiency.
• The existing types of electrochemical storage systems vary according to the nature
of the chemical reaction, structural features, and design.
• This reflects the large number of possible applications.

14. ELECTROCHEMICAL ENERGY
• The simplest system consists of one electrochemical cell – the so-called
galvanic
element [1]. This supplies a comparatively low cell voltage of 0.5–5 V.
• To obtain a higher voltage the cell can be connected in series with others,
and for a higher capacity it is necessary to link them in parallel.
• In both cases the resulting ensemble is called a battery.
• Apart from the improvement and scaling up of known systems such as the
lead–acid battery, the nickel–cadmium, and the nickel–metal hydride
batteries, new types of cells have been developed, such as the lithium-ion
system. The latter seems to be the most promising system, as will be
apparent from the following sections.

15. ELECTROCHEMICAL ENERGY
• To judge which battery systems are likely to be suitable for a given potential
application, a good understanding of the principles of functioning and of the various
materials utilized is necessary (see Table 1.1).
• The development of high-performance primary and secondary batteries for different
applications has proved to be an extremely challenging task because of the need to
simultaneously meet multiple battery performance requirements such as high energy
(watt-hours per unit battery mass or volume), high power (watts per unit battery
mass or volume), long life (5–10 years and some hundreds of charge-discharge
cycles), low cost (measured per unit battery capacity), resistance to abuse and
operating temperature extremes, near-perfect safety, and minimal environmental
impact (see Table 1.2 and Table 1.3).
• Despite years of intensive worldwide R&D, no battery can meet all of these goals.

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18. ELECTROCHEMICAL ENERGY
• The capacitors only function is to store electric charge.
• A capacitor is simply two metallic plates separated by a gap, where the gap
between the plates is usually filled with a material called a dielectric.
• What we will find is that the amount of charge that a capacitor can store is a
geometric property of the capacitor.
• Each metallic plate stores charge of the same magnitude and of opposite
sign.
• These capacitor plates are separated by a distance d, and because there is a
separation of electric charge, a difference in electric potential exists across
the metal plates

19. ELECTROCHEMICAL ENERGY
• Experimentally we find that the potential difference that exists is
proportional to the
amount of charge that we have on each plate, meaning that if the
charge on the plates, say doubles, so too does the electric potential
difference across the plates.
• To quantify this result, we multiply by a constant so that ∆Q = C∆V ,
where this constant of proportionality is called the capacitance, C, in
units of Coulombs per Volt, also called a Farad.

20. ELECTROCHEMICAL ENERGY
• In the uncharged state, the charge on either one of the conductors in
the capacitor is zero.
• During the charging process, a charge Q is moved from one
conductor to the other one,
giving one conductor a charge +Q , and the other one a charge .
• A potential difference is created, with the positively charged
conductor at a higher potential than the negatively charged conductor.
• Note that whether charged or uncharged, the net charge on the
capacitor as a whole is zero.

21. ELECTROCHEMICAL ENERGY
• Earlier we mentioned that the plates are usually separated by a material, called a
dielectric that is meant to keep the plates from touching and neutralizing.
• Capacitors have many important applications in electronics.
• Some examples include storing electric potential energy, delaying voltage changes
when coupled with resistors, filtering out unwanted frequency signals, forming
resonant circuits and making frequency-dependent and independent voltage
dividers when combined with resistors.
• If the plates have an area A and are separated by a distance d, the electric field
generated across the plates is

22. ELECTROCHEMICAL ENERGY
• The constant of proportionality C is referred to as the capacitance of the capacitor.
It is a function of the geometric characteristics of the capacitor - plate separation (d)
and plate area (A) - and by the permittivity (ε) of the dielectric material between the
plates.
• Capacitance represents the efficiency of charge storage.

23. ELECTROCHEMICAL -ENERGY
Standard Modern Batteries:-
• Zinc-Carbon: used in all inexpensive AA, C and D dry cell batteries.
• The electrodes are zinc and carbon, with an acidic paste between them that serves as the electrolyte.
(disposable);
• Alkaline: used in common Duracell and Energizer batteries, the electrodes are zinc and manganese-
oxide, with an alkaline electrolyte. (Disposable);
• Lead-Acid: used in cars, the electrodes are lead and lead oxide, with an acidic electrolyte.
(rechargeable).
• ‰Nickel-cadmium: (Ni-Cd); rechargeable, “memory effect”
• Nickel-metal hydride: (NiMH); rechargeable, “memory effect” (less susceptible than NiCd)
• Lithium-Ion: (Li-Ion); rechargeable, no “memory effect”, high energy density, power rate, cycle life,
costly

24. ELECTROCHEMICAL ENERGY
LithiumPioneering work for the lithium battery began in 1912 by G. N.
Lewis but it BatteryDevelopment
• was not until the early 1970’s when the first non-rechargeable lithium
batteries became commercially available.
• In the 1970’s, Lithium metal was used but its instability rendered it
unsafe.

25. ELECTROCHEMICAL ENERGY
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26. ELECTROCHEMICAL ENERGY
• Attempts to develop rechargeable lithium batteries
followed in the eighties, but failed due to safety
problems.
• The Lithium-Ion battery has a slightly lower energy
density than Lithium metal, but is much safer.

27. ELECTROCHEMICAL ENERGY
• 1972 Define the concept of chemical intercalation:- In chemistry,
intercalation is the reversible inclusion of a molecule between two other
molecules. Ex: graphite intercalation compounds.
• Graphite intercalation compounds are complex materials where an atom,
ion, or molecule is inserted (intercalated) between the graphite layers. In this
type of compound the graphite layers remain largely intact and the guest
species are located in between.
• A Li-ion battery is a electrochemical device which converts stored
chemical energy directly into electricity.
• During charging an external voltage source pulls electrons from the
cathode through an external circuit to the anode and causes Li-ions to move
from the cathode to the anode by
transport through an liquid electrolyte.
• During discharge the processes are reversed. Li-ions move from the anode
to the cathode through the electrolyte while electrons flow through the
external circuit from the
anode to the cathode and produce power.

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30. ELECTROCHEMICAL ENERGY
Key Battery Attributes
• Energy Density: Total amount of energy that can be stored per unit mass or volume.
How long will your laptop run before it must be recharged?
• Power Density: Maximum rate of energy discharge per unit mass or volume. Low
power: laptop, i-pod. High power: power tools.
• Safety: At high temperatures, certain battery components will breakdown and can
undergo exothermic reactions.
• Life: Stability of energy density and power density with repeated cycling is needed
for the long life required in many applications.
• Cost: Must compete with other energy storage technologies

31. ELECTROCHEMICAL ENERGY
The ideal battery:-
• High energy density [kWh/kg], [kWh/m3] light, small, with high capacity to
store energy
• High specific power [W/kg ]  it can get and store a big amount of energy all
together
• Long cycle life  charged and discharge several times without decrease of capacity
and performance
• Fast recharge
• Wide temperature range
• Safe
• Recyclable
• Low cost

32. ELECTROCHEMICAL ENERGY
Advantages of Using Li-Ion Batteries:-
• POWER – High energy density means greater power in a smaller package.
• 160% greater than NiMH
• 220% greater than NiCd
• HIGHER VOLTAGE – a strong current allows it to power complex
mechanical devices.
• LONG SELF-LIFE – only 5% discharge loss per month.
• 10% for NiMH, 20% for NiCd
• Due to high safety

33. ELECTROCHEMICAL ENERGY
Why Lithium?
• Small:– Intercalation
• Light :– 0.534 g/cm3
• Highly reactive:– Lowest Standard Electrode Potential:– E = 3.04 VSHE
• High Capacity:– 3.86 Ah/g

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