UINT IV
ELECTROCHEMICAL POWER SOURCES: Electrochemical cells – emf, electrode potential,
dependence of emf on electrolyte concentration – Nernst equation. Batteries–performance
characteristics. Materials, construction, reactions, characteristics of lechlanche cell, primary
lithium batteries, lead - acid battery and lithium-ion batteries. Supercapacitors – EDLC –
fundamentals, electrode materials, electrolytes, pseudocapacitors– materials.
Electrochemistry is the branch of chemistry which deals with the relationship between
chemistry and electricity. The industrial importance of electrochemistry can be understood from
the electrolytic cell and electrochemical cell.
Differences between Electrolytic cell and Electrochemical cell
Electrolytic cell
Electrochemical cell
Electrical energy is converted into chemical Chemical energy is converted into electrical
energy
energy
Electrical
reactions
energy
brings
about
a
redox Electrical energy is generated by redox
reactions
Anode is positive while cathode is negative
Anode is negative while cathode is positive
Oxidation reaction occurs at anode, reduction occurs at cathode
ELECTROCHEMICAL CELL
A metal is in contact with its own ions in solution constitutes an electrode. When Zn electrode is
immersed in ZnSO4 solution an equilibrium reaction is established within seconds as
Zn
Zn2+ + 2e-
Zinc has a higher tendency to undergo oxidation. Hence, the Zinc electrode gets a negative
charge and Zn2+ ions in the solution are arranged nearby to form an electrical double layer or
Helmholtz electrical double layer. There a potential difference exists between the metal surface
and the solution/electrolyte due to this double layer. This potential is called electrode potential or
single electrode potential (E).
When the concentration of the solution is 1M and the temperature is 25̊ C, the electrode
potential is referred as standard electrode potential (E0). Electrode potential is measured in
Volts.
1
2
Silver/Silver Chloride Electrodes
Another common reference electrode is the silver/silver chloride electrode, which is based on
the reduction of AgCl to Ag, commonly used in electrochemical measurements.
The electrode functions as a reversible redox electrode and the equilibrium is between the solid
(s) silver metal (Ag(s)) and its solid salt—silver chloride (AgCl(s), also called silver(I) chloride) in a
chloride solution of a given concentration.
AgCl(s) + e− ⇌ Ag(s) + Cl−(aq)
A typical Ag/AgCl electrode is shown in the below image, which consists of a silver wire, the end
of which is coated with a thin film of AgCl, immersed in a solution that contains the desired
concentration of KCl. A porous plug serves as the salt bridge. The electrode’s short hand
notation is written as Ag(s)|AgCl(s),KCl(aq,aCl−=x)∥
The standard electrode potential E0 against standard hydrogen electrode (SHE) is 0.230 V ±
10 mV.
3
4
5
6
7
8
9
10
11
12
13
14
ELECTROCHEMICAL POWER SOURCES
BATTERIES: It is a device where chemical energy is converted into electrical energy or in other
words a battery is an electrochemical cell or series of cells that produces an electric current.
Batteries consist of cells.
Cell - single unit consisting of two electrode , electrolyte and current collectors.
✓ A spontaneous redox reaction occurs producing electrical energy.
✓ The half cell reactions occur separately at the electrodes – oxidation at anode and
reduction at cathode.
✓ A voltage is generated because of the difference in the potentials between the two
electrodes.
Battery - Several cells connected together form a battery. The connection can be made in series
or parallel.
Connections in parallel: Cells are said to be connected in parallel when they are joined
positive to positive and negative to negative such that current is divided between the cells. The
final voltage of the battery remains unchanged while the capacity is increased.
Parallel connection of four 3.6 V Li-ion cells
The emf of the battery is the same as that of a single cell.The current in the external circuit is
divided equally among the cells.
Connections in series : Cells are connected in series when they are joined end to end so that
the same quantity of electricity must flow through each cell. The final voltage is the sum of all
battery voltages added together while the capacity remains unchanged.
Series connection of four 3.6 V Li-ion cells
Adding cells in a string increases the voltage; the capacity(A/Hr) remains the same.
The emf of the battery is the sum of the individual emfs. The current in each cell is the same
and is identical with the current in the entire arrangement.
Types: Primary and secondary batteries.
Primary batteries
Chemical reactions are irreversible
Reaction product cannot come back to original
form
Once use battery
Ex: Lechlanche cell or drycell, lithium primary
battery
Secondary batteries
Chemical reactions are reversible
Reaction product come back to original form
Recharged and reused
Ex: Lead acid, Ni-Cd, Ni-metal hydride
battery, lithium ion battery.
BATTERY CHARACTERISTICS
Batteries are designed and manufactured for a particular duty. For e.g., to power a torch, for
starting a car or bike, or for an uninterrupted power supply (UPS). For each application the
requirement is different.
Requirements of a car traction battery::
➢ Voltage : 90-400V
➢ Size : 0.1 to 0.5m3
➢ Rechargeable – should be able to withstand at least 1000 charge discharge cycles and
also overcharging.
Requirement for SLI (start – lighting- Ignition) automobile batteries
➢ Voltage 12V
➢ Usage may be intermittent of prolonged use with uneven demand
➢ Enough capacity to power head lights, AC, music system.
Requirement of battery for computer volatile memory:
➢ Voltage – 1-3V
➢ Size – few cm3
➢ Need not be rechargeable.
In order to judge whether a battery is suitable for a particular application or to design a
battery for a specific purpose, there are a set of defined characteristics which can be measured
and specified.
BATTERY PERFOMANCE CHARACTERISTICS
1. Voltage - The terminal voltage of a cell depends on the free energy change of the cell
reaction, and therefore on the electrodes used, kinetics of electrode reactions and cell
internal resistance.
Ecell = E0 C – E0A - ƞA - ƞC - iRcell
Where the ηA and ηc are over potentials for anode and cathode reactions. And iR cell is the
internal resistance of the cell. EOC and EOA are the standard electrode potentials of anode and
cathode.
The overpotentials for the cathode and anode reactions are caused by:
➢ Concentration overpotential – due to depletion of electroactive species at the
electrodes, this in turn is due slow diffusion of ions in the electrolyte. Current withdrawn
at a faster rate causes concentration over potential. This is cab be minimized by (i) by
using electrolytes containing ions with high transport number, (ii) by avoiding withdrawal
of higher currents than prescribed for the given battery system.
➢ Activation overpotential – due to slow electron transfer between the electrodes and
active species. By making right choice of electrode materials this can be minimised.
Internal resistance in the cell can be minimised by using highly conducting current
collectors, electrolytes and reducing the interelectrode gap.
Cut-off Voltage – The minimum allowable voltage. It is this voltage that generally
defines the “empty” state of the battery.
2. Capacity or Nominal Capacity (Ah for a specific C-rate) – It is the charge that can be
obtained from a battery. It depends upon size of the battery.
The coulometric capacity, the total Amp-hours available when the battery is discharged at a
certain discharge current (specified as a C-rate) from 100 percent state-of-charge to the cut-off
voltage. Capacity is calculated by multiplying the discharge current (in Amps) by the discharge
time (in hours) and decreases with increasing C-rate.
C- and E- rates – In describing batteries, discharge current is often expressed as a C-rate in
order to normalize against battery capacity, which is often very different between batteries.
➢ A C-rate is a measure of the rate at which a battery is discharged relative to its
maximum capacity.
➢ A 1C rate means that the discharge current will discharge the entire battery in 1 hour.
For a battery with a capacity of 100 Amp-hrs, this equates to a discharge current of 100
Amps.
➢ A 5C rate for this battery would be 500 Amps, and a C/2 rate would be 50 Amps.
Similarly, an E-rate describes the discharge power.
➢ A 1E rate is the discharge power to discharge the entire battery in 1 hour.
C = i x t Ampere hour.
If the capacity of a battery is 10 Ah it signifies that if a load connected to the battery withdraws
1A current, the battery will last for 10 hrs.
If the load withdraws 0.5A current, the battery will last for 20 hrs.
3. Specific Energy (Wh/kg) – The nominal battery energy per unit mass, sometimes referred
to as the gravimetric energy density. Specific energy is a characteristic of the battery chemistry
and packaging. Along with the energy consumption of the vehicle, it determines the battery
weight required to achieve a given electric range.
4. Specific Power (W/kg) – The maximum available power per unit mass. Specific power is a
characteristic of the battery chemistry and packaging. It determines the battery weight
required to achieve a given performance target.
5. Energy Density (Wh/L) – The nominal battery energy per unit volume, sometimes referred
to as the volumetric energy density. Specific energy is a characteristic of the battery
chemistry and packaging. Along with the energy consumption of the vehicle, it determines
the battery size required to achieve a given electric range.
6. Power Density (W/L) – The maximum available power per unit volume. Specific power is a
characteristic of the battery chemistry and packaging. It determines the battery size required
to achieve a given performance target.
7. Cycle life:
A secondary battery is expected to be capable of repetitive charge and discharge cycles. The
cycle life is the number of charge and discharge cycles that are possible before failure
occurs.
In a charge discharge cycle it is essential that the active material is reformed in the correct
chemical composition, morphology and distribution on the electrodes for further discharge.
The cycle life strongly depends on the depth of discharge. Complete discharge of a battery
are damaging to the electrodes. The common causes of failure are (i) Corrosion of current
collectors and plates (ii) Shedding of active material from the plates (iii) Shorting due to
dendritic growth between the electrodes (iv) Changes in morphology.
8. Shelf life:
Applies to primary batteries. This denotes the period of storage before its usage. Shelf life of
a battery is reduced due to self discharge.
PRIMARY BATTERIES
Batteries that are designed for one use.
ZINC - CARBON BATTERIES
Have been in use for more than 100 years. Widely used for low power applications:
flash lights, clocks, remote control, toys, shavers etc. The batteries utiliseZn-MnO2
chemistry for generating electrical energy Their open circuit voltage is 1.5V.
The zinc carbon battery is marketed as general purpose (lechlanche cell) or also called
dry cell.Other variants of lechlanche cell that are based on the Zn-MnO2 chemistry are
the heavy duty or super heavy duty (Zinc Chloride cells) and the alkaline batteries
They are available in various sizes and hence with different capacities.
LECHLANCHE CELL
CONSTRUCTION
➢ The zinc serves as both the container and the anode.
➢ The manganese dioxide/carbon mixture is wetted with electrolyte and shaped
into a cylinder (bobbin shape) which is hollow at the centre.
The electrolyte is a solution with the following composition:
NH4Cl
26.0%
ZnCl2
H2O
Corrosion inhibitor
8.8 %
65.2 %
0.25 - 1.0 %
➢ A carbon rod is inserted into the centre of the bobbin, which serves as a current
collector. It is also porous to allow gases to escape, and provides structural
support.
➢ The separator is either cereal paste or treated absorbent kraft paper. The paste
is in a gel form which holds the electrolyte.
LECHLANCHE CELL COMPONENTS AND ROLES
Components
Roles
Zinc can
Anode
Carbon rod
Cathode current collector
Bobbin shaped MnO2
cathode material
Natural ore – 70-85%
Cathode material also called cathode
depolariser. Without MnO2, the cathode
reaction would be evolution of H2 gas
which is undesirable as it will get
adsorbed on the carbon cathode and
increase its resistance. It may also lead
to pressure buildup in the battery.
Hence MnO2 is called the cathode
depolariser.
Synthetic - 90-95%
Electrolytic MnO2
Electrolyte
Graphite powder
Added to improve conductivity of
MnO2(poor conductor)
NH4Cl - 26%
Increases the
electrolyte.
ZnCl2 - 8%
The product of the cell reaction is
alkaline. ZnCl2 being acidic helps in
maintaining the pH which is necessary
to maintain the gel structure of the
electrolyte.
conductivity
of
the
H2O - 65%
Dichromate,
salts 1%
Separator
chromate Zinc undergoes self corrosion and
hence reduces the shelf life of the
battery. The dichromate salts act as
corrosion inhibitors by forming a
passive oxide film on Zinc.
Kraft paper coated with Holds the electrolyte in a gel form and
cereal paste( wheat flour acts as an electronic insulator between
and starch)
the anode and cathode.
Gelled paste cereal paste
Electrical contacts
The terminals of the battery, are tin plated steel or brass. They aid in conductivity and
prevent external exposure of the zinc.
Seal
The battery is sealed using asphalt pitch, wax/resin mix, or plastic (usually
polyethylene or polypropylene) An airspace is usually left between the seal and the
cathode to allow for expansion. The function of the seal is to prevent evaporation of
the electrolyte, and to prevent oxygen entering the cell and corroding the zinc.
CELL CHEMISTRY
The probable half-cell reactions are:
Anode: Zn → Zn2+ + 2e–
Cathode: MnO2 +H2O + 2e– → MnO(OH) + OHThe OH- ions generated at the cathode migrate to the anode and combine with the Zn2+
ions and form Zn(OH)2 which increases the pH. The acidic ZnCl2 combine with the
hydroxide to form a neutral complex.
Zn(OH)2 + ZnCl2 → neutral complex
MnO(OH) + Zn(OH)2 → ZnO + Mn2O3 + H2O
Sometimes the ammonium ions react with the hydroxide to give free ammonia. This
happens especially when high current is withdrawn.,
NH4+ + OH- → NH3 + H2O
The ammonia combines with the ZnCl2 to form a complex.
ZnCl2 + 2NH3 → Zn(NH3)2Cl2
The overall reaction in the cell is:
Zn + 2MnO2 → ZnO + Mn2O3
BATTERY CHARACTERISTICS
➢ Open circuit voltage – 1.5V.
Electrode potential of zinc is – 0.76 V and that of manganese dioxide is 1.23V.
Therefore, theoretical voltage of each cell Ecell= 1.23-(-0.76) = 1.99 V but due to
polarisation the actual voltage of a standard zinc carbon battery is not more than 1.5
V.
➢
➢
➢
➢
Low energy density
Voltage falls steadily with discharge.
Low efficiency under high current drain.
Service Life: 110 min (if used continuously). But, it is designed for intermittent
use. For best capacity it should be used intermittently and removed from the
device when not required.
➢ Comparitively poor shelf life: ~ 1 – 2 years (at room temperature). Shelf life
can be improved it refrigerated.
OTHER VARIANTS
✓ Heavy duty lechlanche cell in which electrolytic MnO2 is used.
✓ Heavy duty and super heavy duty Zinc chloride cell in which very little ammonium
chloride is used and electrolytic MnO2 is used.
✓ Alkaline batteries in which the electrolyte is potassium hydroxide, which is highly
conducting, resulting in low internal resistance for the cell. The zinc anode does not
form the container. But it is in the form of a powder, giving a large surface area. The
zinc anode forms the central part of the cell and is surrounded by the MnO2 cathode
material. These batteries have longer service life.
LITHIUM PRIMARY BATTERIES
The term “lithium battery” refers to all the battery systems utilizing lithium as the anode but
differing in cathode material, electrolyte, and construction.
Characteristics of lithium that make it an exceptional electrode material:
➢ Lithium is the lightest metal and has an intrinsic negative potential, which exceeds that
of all other metals. Hence lithium offers the highest specific energy (energy per unit
weight) and energy density (energy per unit volume) of all available battery chemistries
and have normal open-circuit voltages (OCVs) between 2.7 and 3.6 V.
➢ It is soft and malleable and can be extruded into thin foils.
➢ Lithium reacts with water and for this reason all Lithium cells use a non-aqueous
electrolyte.
CHARACTERISTICS OF LITHIUM BATTERIES
➢ High terminal voltage.
➢ High specific energy and energy density.
➢ High rated discharge capability under intermittent or continuous drain (high power
construction).
➢ Immediate start-up capability and flat discharge.
➢ Superior pulse capability.
➢ Wide operating temperature range.
➢ Lowest self discharge and hence long shelf life almost 10years
➢ Highly reliable and longer service life than all other primary batteries.
LITHIUM BATTERY SYSTEMS
Lithium batteries are classified by the cathode material used as listed below:
SVO – Silver Vanadium oxide; (CF)n – Fluorinated Carbon.
Battery constructions:
➢ Coin cells with capacities ranging from 75 to 550 mAh;
➢ spiral-wound cells are available in 160 and 1,300 mAh capacities.
➢ bobbin cells ranging from 650 to 1,900 mAh capacities.
Lithium/thionyl chloride battery:
Anode : Lithium
Cathode : carbon ( It does not involve in reaction. Only acts as a site at which the reduction
takes place).
Electrolyte: SOCl2 (thionyl chloride) containing a lithium salt like LiAlCl4. The thionyl
chloride is also the active cathode material that undergoes reduction.
Cell Chemistry
Anode (Oxidation)
: 4Li →4Li + +4 eCathode (Reduction): 2SOCl2 + 4e-→ S + SO2+ 4ClOverall reaction
: 4Li + 2SOCl2 → S + SO2 + 4LiCl
The open circuit cell voltage of Li/SOCl2 cell is 3.65 V.
Thionylchloride is the most widely used of all the liquid cathode electrolytes.
In a liquid cathode cell, the active cathode material is always in contact with the lithium anode
as a result of which Li reacts with the material to form a passive layer of LiCl.
This LiCl layer “seals” the lithium surface, protecting the lithium from further reaction with the
cathode. Without the LiCl layer, this type of cell could not exist.
This passivation may lead to a“voltage delay” when the load is connected but as the discharge
continues, the passivation layer breaks down and the voltage returns to normal.
Lithium /Manganese dioxide cell:
Anode : Lithium
Cathode : Specially prepared heat treated form of manganese dioxide.
Electrolyte : Mixture of organic solvents like propylene carbonate and 1,2- dimethoxy ethane
containing lithium salts.
Chemistry:
At anode: x Li → xLi+ + xe
At cathode : Mn4+O2 + xLi+ +xe → LiMn4+1-x Mn3+x O2
The Mn4+ is reduced to Mn3+ by the interstitially occupied lithium ions MnO2 lattice.
The open circuit voltage of the cell is 3.5 volts.
APPLICATIONS OF LITHIUM BATTERIES
➢ AMR ( Automatic meter reading) utilility metering (Electricity, gas, water meters)
➢ Military radio communication.
➢ Alarms and security wireless devices like door lockers, burglar alarms, smoke
detectors.
➢ GPS tracking systems.
➢ Digital cameras, High end electronic toys.
➢ Portable medical equipment.
➢ Motherboard, RTC, CMOS power.
➢ Hazardous gas sensors, temperature and humidity sensors.
➢ Electronic Toll collection.
➢ Bar code readers and scanners.
➢ Cardiac pace makers.
SECONDARY BATTERIES
Secondary batteries are those which are rechargeable.
LEAD ACID BATTERY
The lead acid battery is the most used battery in the world. The most common is the SLI
battery used for motor vehicles for engine Starting, vehicle Lighting and engine Ignition,
however it has many other applications (such as communications devices, emergency lighting
systems and power tools) due to its cheapness and good performance.
It was first developed in 1860 by Raymond Gaston Planté. Strips of lead foil with coarse
cloth in between were rolled into a spiral and immersed in a 10% solution of sulphuric acid. The
cell was further developed by initially coating the lead with oxides, then by forming plates of lead
oxide by coating an oxide paste onto grids. The electrodes were also changed to a tubular
design.
Fabrication:
Lead grids (plates) are coated with a paste of lead monoxide (PbO) and dilute sulphuric acid
and dried. Alternate plates are attached to two common terminals. Microporous PVC sheets are
placed between the plates to separate them from each other. (The separator are usually
cellulose, PVC, rubber, microporous polyethylene or non-woven polypropylene). This assembly
is housed in a PVC container and the top is closed with a PVC lid and sealed using pitch.
Suitable vents are provided for filling up of the electrolyte. The vents are closed with plastic caps
having small holes. Sulphuric acid of specific gravity 1.2 – 1.4 is filled up in the container.
Several such assemblies can be connected in series to increase the voltage to suit the
requirement. In practice, most cells contain upto 30 plates with separators between.
First Charging:
The common terminal of one set of plates is attached to positive and the other set to the
negative terminal of a DC power source. The PbO coating on the plates, connected to positive
terminal is oxidised to spongy porous electroactive lead dioxide (PbO2) while the PbO on the
other set of plates, connected to negative terminal, is reduced to spongy electroactive lead.
PbO2
+
(O)
Pb
PbO
(R)
Pb
The physical state of these coatings is entirely different from that of lead or lead dioxide which
are not electroactive. The conversions occur only during the first charge of the battery. Actually
the battery plates are formed only during first charge.
The cell notation is
–
+
Pb/H2SO4/PbO2
. These plates are now ready for discharge.
Reactions during discharge:
At anode: This is the negative plate and hence lead undergoes oxidation as:
Pb
Pb2+ + 2e-
Pb2+ + SO42-
PbSO4
Pb + SO42-
PbSO4 + 2e-
At cathode: This is the positive plate and lead dioxide undergoes reduction as:
PbO2 + 4H+ + 2e-
Pb2+ + 2H2O
Pb2+ + SO42-
PbSO4
PbO2 + 4H+ + SO42- + 2e-
PbSO4 + 2H2O
The total cell reaction is:
Pb + SO42-
PbSO4 + 2e-
PbO2 + 4H+ + SO42- + 2e-
PbSO4 + 2H2O
Pb + PbO2 + 4H+ + 2SO42-
2PbSO4 + 2H2O
(Pb + PbO2 + 2H2SO4
2 PbSO4 + 2H2O)
All these reactions are exactly reversed during charging
2PbSO4 + 2H2O
Pb + PbO2 + 2H2SO4
Thus the overall process can be represented as:
+
PbO2
+
(R)
(O)
Pb
PbO
PbSO4
(R)
Pb
Charging
(O)
Discharging
Functioning :
1. Quantity of materials required: According to the overall cell reaction, one mole of lead,
one mole of PbO2 and two moles of sulphuric acid are required to generate two faradays of
electricity. But if theoretical quantities are taken, the specified Ampere hour cannot be
withdrawn because (i) The products PbSO4 and water are poor conductors of electricity (ii) The
acid gets progressively diluted. Therefore the internal resistance often cell increases
enormously. Similarly, when the entire quantity of lead and PbO2 are converted to PbSO4,
recharging becomes almost impossible. Hence when 60-65% of the current is withdrawn from
the battery, the recharging should be done or excess quantity of the material over the theoretical
quantity should be taken. In practice, 1.6-1.7 times higher than the theoretical amount is taken
in the battery. Virtually, an 80 Ah battery has materials theoretically required for a 130 Ah
battery.
2. Discharge: Lead acid battery is designed for heavy current output. Two kinds of
coatings are given in these batteries depending on the end use. A thin coating over a large area
is given for a very high current output in unit time. These become possible because the acid can
percolate easily through a thin coating and react with a large surface area underneath producing
a high current. A battery of this type is necessary for starting automobile engines, aviation
engines and railway engines. On the other hand, a thick coating over a smaller area gives
sustained low or medium current for a long duration. A battery of this type is mainly used in
laboratory work. Batteries which are used in UPS back-up power supply employ moderately
thick coatings because the power demand and the frequency of demand are quite uncertain.
Accordingly, the battery manufacturers specify the optimal discharge rate on the battery
depending upon the type of coating employed. For example, an 80 Ah battery may have an
efficiency between 85 – 90% when its discharge does not exceed 8A. If a higher current is
withdrawn, the efficiency decreases exponentially. If too low a current is drawn, it also results in
a loss of efficiency because the self discharge rate becomes a significant part of the low ampere
output.
Open circuit voltage is 2V whenever a load is attached, irrespective of its magnitude. Voltage
decreases to 1.98 and then gradually decreases to 1.7V and thereafter the decreases to steep.
Hence there is no useful current can be taken from the battery below 1.7V. The battery has to
be then recharged. Therefore the working voltage range of the battery is 1.7 – 1.98V.
3. Performance
➢ The charging rate should be preferably 25-30% of the discharge rate specified or the
charging voltage should be between 2.1-2.3V.
➢ Overcharging amounts to charging at a higher voltage or charging for a higher period
than specified. This leads to scouring i.e., continuous evolution of hydrogen gas at
cathode damaging the porous coating.
➢ Undercharging leads to accumulation of PbSO4 on the plates, increasing the internal
resistance and makes it difficult to recharge.
4. Wrong polarity connection : During charging, the PbSO4 must be oxidised back to PbO2 on
the cathode and plates and PbSO4 must be reduced to Pb on the anode grids. During
discharge only 60-65% of active coating gets converted to PbSO4. The remaining 35% PbO2
will be present on the cathode plates and 35% Pb on the anode plates. If a wrong polarity
connection is given during charging, the lead sulphate will be reduced to Pb at the cathode
(instead of being oxidised to PbO2) gets mixed up with PbO2 present. Similarly, PbSO4 will be
oxidised to PbO2 at the anode gets mixed up with Pb already present.ie mix up of PbO 2 and
Pb occurs in both the electrodes. This gives rise to innumerous local cell formation and
heavy short circuiting results. Self discharge is heavy and the battery becomes dead in no
time.
LITHIUM BATTERIES
Lithium batteries are becoming important for all type of electronic devices. For many
years the Ni- Cd battery available for portable devices such as wireless communication. In
1990, the nickel metal hydride and Li batteries emerged. Battery research is mainly focused on
lithium based batteries because it fulfils many of the demands made within the areas of portable
electronics and electrical vehicles offering high capacities .
Three pioneers of lithium-ion battery technology—John Goodenough, M. Stanley Whittingham,
and Akira Yoshino—shared the 2019 Nobel Prize in Chemistry for their groundbreaking work.
Lithium has greatest promise as a battery material,
•
•
•
•
Low weight (Atomic weight 6.94)
High voltage (greater than 3v compared to Ni-Cd and Lead- Acid batteries (1.5-2.0)
Favourable thermodynamic potential (-3.045V)
No passivation in non-aqueous solvent like propylene carbonate, ethylene carbonate.
Lithium – ion secondary batteries
In 1997, the Sony Corporation commercialized the first lithium-ion battery. Today more
than ten companies based in Japan are producing these batteries.
In this type of batteries, anode is made by using safer material such as graphite.
Cathode is made of lithium transition metal oxides (LiMO2). Electrolyte contains lithium ions with
organic carbonates such as ethylene and propylene carbonates.
During discharging and charging these electrodes can be able to intercalate and
deintercalate the lithium ions reversibly. The lithium in its ion state (Li+) is very stable and
unreactive. When intercalated in the negative electrode its potential is much lower than when
intercalated in the positive electrode. The battery works by shuttling lithium ions between anode
and cathode through an electrolyte.
During charging
The battery can be charged by supplying an electric current during which the lithium are
forced out at the positive electrodes into negative electrode. This charging process resets the
anode and cathode. So that the battery can once again power the devices.
Positive electrode (anode):
LiMO2
Li1-xMO2 + xLi+ + xeNegative electrode (cathode):
C + xLi+ + xeLixC
Net reaction:
LiMO2 + C
Li1-xMO2 +LixC
Upon charging lithium ions are extracted from the positive electrode material and
intercalated into the negative electrode material, upon discharging the reverse process takes
place.
During discharging
Lithium ions spontaneously shuttle from the negative electrode to positive electrode
through the electrolyte .This flow of electrons from anode into cathode produce electric current
which is used to power the devices.
Positive electrode (cathode)
Li1-xMO2 + xLi+ + xeLiMO2
Negative electrode (anode):
LixC
C + xLi+ + xeNet reaction
Li1-xMO2 + LixC
LiMO2 + C
Advantages
1. High energy density- potential for higher capacities.
2. Relatively low self discharge –self discharge is less than half that of Ni-Cd battery.
3. Low maintenance
4. No periodic discharge is required .
5. There is no memory effect (memory effect refers to the phenomenon where the
discharge capacity of the battery is reduced when it is repetitively incompletely
discharged and then recharged)
6. It is eco-friendly.
SUPER CAPACITORS
Capacitors use static electricity to store energy. Inside a capacitor, there are two
conducting metal plates with an insulating material called a dielectric in between them. Positive
and negative electrical charges build up on the plates and the separation between them, which
prevents them coming into contact, is what stores the energy.
Capacitors have many advantages over batteries: they weigh less, generally don't contain
harmful chemicals or toxic metals, and they can be charged and discharged many number of
times. But they cannot store the same amount of electrical energy as batteries.
A supercapacitor (or ultracapacitor) differs from an ordinary capacitor in two important
ways: its plates effectively have a much bigger area and the distance between them is much
smaller, because the separator between them works in a different way to a conventional
dielectric.
Structure of Supercapacitor
Schematic of a electric double-layer capacitors or EDLC
It does not consist of a dielectric material like ceramic capacitors or electrolyte capacitors do
have. As shown in above figure, supercapacitors consist of two porous electrodes, electrolyte, a
separator and current collectors
Current collector:
Current collectors are made up of metal foil generally of aluminum as it is cheaper than titanium,
platinum etc. They are coated with the electrode material.
Electrodes:
The capacitance value is proportional to the surface area of the electrode. Generally, as an
electrode material, highly porous powdered coated active carbon material or carbon nanotubes
are used. The porous nature of the material allows many more charge carriers (ions or radicals
from electrolyte) to be stored in given volume. This increases the capacitance value of
supercapacitors. The electrodes are coated on a current collector and immersed in an
electrolyte.
Electrolyte:
The electrolyte is the key factor in determining the internal resistance (ESR). The electrolyte
solution shall be either aqueous or non-aqueous in nature. The non-aqueous electrolytes are
mostly preferred as they provide high terminal voltage V. Non-aqueous solution consists of
conductive salts dissolved in solvents. Acetonitrile or propylene carbonate as solvents preferred
mostly. Tetraalkylammonium or lithium ions can be used as solutes.
Separator:
The separator is between electrodes and it is made up of material that is transparent to ions but
is an insulator for direct contact between porous electrodes to avoid short circuit.
Energy storage in Supercapacitors
On applying voltage, each collector attracts ions of opposite charge.Ions from electrolyte
get collected on the surface of the two current collectors.A charge is built on each current
collector.As we can see in figure , two separate layers of charge have been formed, hence
supercapacitor is also called as an Electrical Double Layer Capacitor (EDLC).Like conventional
capacitors, EDLCs store charge electrostatically, or non-Faradaically, and there is no transfer of
charge between electrode and electrolyte.
The structure of supercapacitor is unique and hence it differs from conventional batteries
and capacitors. Use of activated carbon increases surface area and hence increase in
capacitance value. Electrolyte with low internal resistance increases power density. These both
together brings ability in supercapacitors to store and release energy rapidly. The power [W] of
supercapacitor is given by,
P= V2/4R
where V [Volts] is the operating voltage and R [Ω] is internal resistance.
Batteries have a higher energy density (they store more energy per unit mass) but
supercapacitors have a higher power density (they can release energy more quickly). That
makes supercapacitors particularly suitable for storing and releasing large amounts of power
relatively quickly, but batteries are still king for storing large amounts of energy over long
periods of time.Although supercapacitors work at relatively low voltages (maybe 2–3 volts), they
can be connected in series (like batteries) to produce bigger voltages for use in more powerful
equipment. Since supercapacitors work electrostatically, rather than through reversible chemical
reactions, they can theoretically be charged and discharged any number of times.
Types of supercapacitors
Schematic diagram of Energy storage of SCs types: (a) (EDLCs) ;( b) pseudo-capacitors;(c) Hybrid
capacitors
Electrochemical double-layer capacitors (EDLCs)
EDLCs includes an electrolyte, two carbon based materials utilized an electrode as well
as a separator. EDLC are either able to electro-statically store the charges or via a non-Faradic
process that doesn’t need charge transfers between the electrolyte and electrode. The
electrochemical double layer is the concept of energy storage used by EDLCs. There was no
accumulation of charges on the surface of electrode when voltage is applied, since the
difference in the potential there is opposite charge attraction, resulting in the diffuse of
electrolyte ions over the separator as well as on the opposite charged electrode pores as shown
in figure (a).
Pseudocapacitors
In contrast to EDLCs, which store charge electrostatically, pseudocapacitors store
charge Faradaically through the transfer of charge between electrode and electrolyte. This is
accomplished through electrosorption, reduction-oxidation reactions, and intercalation
processes as shown in figure b. These Faradaic processes may allow pseudocapacitors to
achieve greater capacitances and energy densities than EDLCs. There are two electrode
materials that are used to store charge in pseudocapacitors, conducting polymers and metal
oxides.
Hybrid Capacitors
Hybrid capacitors attempt to exploit the relative advantages and mitigate the relative
disadvantages of EDLCs and pseudocapacitors to realize better performance characteristics.
Utilizing both Faradaic and non-Faradaic processes to store charge as shown in figure c. hybrid
capacitors have achieved energy and power densities greater than EDLCs.
Application of Supercapacitors
✓ One common application is in wind turbines, where very large supercapacitors help to
smooth out the intermittent power supplied by the wind.
✓ In electric and hybrid vehicles, supercapacitors are increasingly being used as
temporary energy stores for regenerative braking (the energy a vehicle would normally
waste when it comes to a stop is briefly stored and then reused when it starts moving
again).