Question 3 - Batteries and super capacitors

Question 1

Describe electrolysis and potential industry applications

Answer 1

An electrochemical process that uses an electric current to drive a non-spontaneous chemical reaction. It involves the decomposition of a compound into its constituent elements or ions through the passage of electric current.

The process of electrolysis typically takes place in an electrolytic cell, which consists of two electrodes—an anode (positive electrode) and a cathode (negative electrode)—submerged in an electrolyte solution. When an electric current is passed through the electrolyte, chemical reactions occur at the electrodes, resulting in the desired products.

• Hydrogen production
• Metal extraction
• Energy storage
• Water treatment

Question 2

Descibe Li-ion batteries and the electrochemical reactions involved

Answer 2

Rechargeable energy storage devices commonly used in portable electronics, electric vehicles, and renewable energy systems. The basic principle of a Li-ion battery involves the movement of lithium ions between the battery’s electrodes during charge and discharge cycles.

Cathode (positive): Typically lithium

Anode (negative) : Graphite (can intercalate lithium ions)

Electrolyte: non aqueous solution containing lithium salts. Transfers ions whilst preventing direct contact

Anode: LiC6 ↔ Li+ + C6 + e-
Cathode: Li1-xMn2O4 + xLi+ + xe- ↔ Li1-xMn2O4Li
Overall: LiC6 + Li1-xMn2O4 ↔ C6 + Li1-xMn2O4Li

repeated movement of li ions between the anode and cathode allow the battery to release and store energy

No memory effect

Question 3

Primary vs Secondary batteries

Answer 3

Primary : Single use, limited lifespan, high energy density,
Secondary : Rechargeable, longer lifespan, lower energy density,

Environmental impact

Question 4

Li-ion vs Li-metal batteries

Answer 4

Both rechargeable
Li-ion is safer (dendrite) although have slightly lower energy density
metal lithium has a higher specific capacity for storage
-li-metal are an active area of research

Question 5

Explain the memory effect

Answer 5

The memory effect occurs due to the formation of crystal-like deposits on the electrodes of the battery. When a NiCd battery is repeatedly discharged and recharged without being fully drained, these deposits can build up and create a localized reduction in the available active material surface area. As a result, the battery effectively “remembers” the lower capacity and exhibits a diminished performance.

Question 6

Battery (definition)

Answer 6

A battery is a an electrochemical cell (or a collection of multiple electrochemical cells) that produces electricity from a chemical energy

Chemical ==> Electrical Energy

Question 7

Battery design

Answer 7

• A battery cell consists of one or more sets of (+) and (-) plates immersed in an electrolyte solution

A plate is an electrode consisting of active material supported on a grid framework

Active material is chemically reactive compound

The amount of active material is a proportional to the energy storage capacity of a battery

The grid is a metal framework that supports the active material of a battery cell and conducts electricity

Question 8

NiCd batteries

Answer 8

NiCd: 10% of charge lost during first 24hr, then at 10% every month

Some NiCd batteries already partially charged when purchased

NiCd batteries have to be fully discharged before recharge. without full discharge, crystals may build up on the electrodes, thus decreasing the active surface area and increasing internal resistance

“memory effect” (decrease battery capacity)

Question 9

Batteries (4 steps)

Answer 9

• Every battery has two terminals, the positive cathode (+) and the negative anode (-)

(1) device switched on
(2) chemical reaction starts
(3) electrons travel from (-) to (+)
(4) electrical work is produced

Question 10

Standard modern batteries: Lead - Acid

Answer 10

Used in cars, the electrodes are lead and lead oxide, with an acidic electrolyte. (rechargeable)

Discharge:

(-) Anode
Pb (s) + HSO4^- (aq) ==> PbSO4 (s) + H^+ (aq) + 2e^-

(+) Cathode
PbO2 (s) + HSO4^- (aq) + 3H^+ (aq) + 2e^- ==> PbSO4 (s) + 2H2O

Charge : Overall

PbO2 (s) + Pb (s) + 2H2SO4 ==> 2PbSO4 (s) + 2H2O (l)

Question 11

Battery Capacity
(Lead -acid batteries)

Answer 11

Temperature and discharge rate may affect capacity

Warmer batteries are capable of storing and delivering more charge than colder batteries

However, high temperatures decrease the useful life of a battery. Manufacturers generally rate lead-acid batteries performance and life cycle at 25 degrees C

Question 12

Battery Capacity
(Lead -acid batteries)

Answer 12

Temperature and discharge rate may affect capacity

Warmer batteries are capable of storing and delivering more charge than colder batteries

However, high temperatures decrease the useful life of a battery. Manufacturers generally rate lead-acid batteries performance and life cycle at 25 degrees C

Question 13

Rate of discharge

Answer 13

Discharge rate is expressed as a ratio of the nominal battery capacity to the discharge time in hours

For example, a 5 A discharge for a
nominal 100 Ah battery would be a
C/20 discharge rate

The designation C/20 indicates that 1/20th of the rated capacity is discharged per hour, or that the battery will be completely discharged after 20 hrs

Capacity is directly affected by the rate of discharge. Lower discharge rates are able to remove more energy from a battery before it reaches the cut off voltage. Higher discharge rates remove less energy before the battery reaches the same voltage

Question 14

Batteries in series

Answer 14

Batteries are first connected in series by connecting the (-) terminal of one battery to the (+) terminal of the next battery

There is only one path for the current flow, so the circuit current remains the same as individual battery current

For batteries of similar capacity and voltage connected in series, the circuit voltage is the sum of the individual battery voltages, and the circuit capacity is the same as the capacity of the individual batteries

If batteries with different capacities are connected in series, the capacity of the string is limited by the lowest capacity battery

Question 15

Batteries in parallel

Answer 15

Batteries are connected in parallel by connecting all the (+) terminals together and all the (-) together

Batteries connected in parallel provide more than one path for current to flow, so currents add together at the common connections

The current of the parallel circuit is the sum of the currents from the individual batteries

The voltage across the circuit is the same as the voltage across the individual batteries, and the overall capacity is the sum of the capacities of each battery

Series strings of batteries can also be connected in parallel in the same way

Question 16

Li - metal batteries

Answer 16

Positive electrode reaction:

Mn^(iv)O2 + Li^+ + e^- ==(discharge)==> Mn^(iii)O2 (Li^+)

Negative electrode reaction:

Li ===(discharge)==> Li^+ + e^-

Overall reaction:

Mn^(iv)O2 + Li^+ + e^- ==(discharge)==> Mn^(iii)O2 (Li^+)

Question 17

Li-ion batteries
(pros and cons)

Answer 17

Pros:

Higher energy density - i.e. they can store more charge at same weight or size

They operate at higher voltage - (3.5V (Li) vs 1.2 (NiMH or NiCd) (i.e. a single Li-ion battery can be used rather than multiple NiMH

Lower self-discharge rate (i.e. they retain charge for longer times)

Can be smaller and lighter

Cons:

More expensive than NiMH or NiCd (i.e more complex to manufacture and special circuitry to protect from over and under charging)

Not avalible in standard cells size (i.e AA, C, D like NiMH and NiCd)

Li-ion battery chargers are more sophisticated and thus expensive (wrong charger can lead to ignition)

Question 18

Chemical intercalation

Answer 18

Intercalation is the reversible inclusion of a molecule (or group) between two other molecule (or groups)

Ex: include graphite intercalation compounds

Question 19

Graphite intercalation compounds

Answer 19

• Materials having formula XC_y where element or molecule X is intercalated between the graphite layers

The graphite layers remain largely intact and the guest molecules or atoms are located inbetween

When graphite and the guest X interact by charge transfer the in-plane electrical conductivity generally increases

Instead, when the guest forms covalent bonds with the graphite layers as in fluorides or oxides the conductivity decreases as the conjugated sp^2 system collapses

Positive electrode reaction:

LiCoO2 <===> Li_1-x CoO2 + xLi^+ + xe^-

Negative electrode reaction:

C_y + xLi^+ + xe^- <===> C_yLi_x

Overall reaction:

LiCoO2 + C_y <===> Li_1-x CoO2 + C_yLi_x

Layered honeycomb lattice

Question 20

Discharging

Answer 20

As the battery is discharged, the Li-ion in the carbon material that form the anode migrate via a separator to the cathode material: a discharging current flows

Lithium polymer battery does not contain any metal lithium

Only Li-ions move between the positive and negative poles, leaving the cathode and anode materials unchanged

Question 21

Charging

Answer 21

As the battery is charged, the Li-ions in the cathode material migrate via a separator to the layers of carbon material that form the anode: a charging current flows

Question 22

Li-air ( or Li-oxygen) battery

advantages and disadvantages

Answer 22

Advantages:
-They rely on air as the cathode;

-do not require heavy casing;

• theoretically they can achieve high energy density

Disadvantages:

• Deposition of electrically resistive discharge products;
• poor oxygen redox kinetics;
• require thick membranes (resistive) in aqueous cells

Question 23

Li - air battery (anode and cathode)

Answer 23

Anode:
Li (s) <==> Li^+ + e^-

Cathode:

(1) aprotic: Li^+ + e^- + O2 + * ===> LiO2*

Li^+ + e^- + LiO2* ===> Li2O2

• = surface sites on Li2O2

(2) Aqueous

Acidic electrolyte:
2Li + 0.5O2 + 2H^+ ==> 2Li^+ + H2O

Alkaline electrolyte:
2Li + 0.5O2 + H2O ===> 2LiOH

Cathode material: carbon + catalyst (Mn, Co, Ru, Pt,)

Question 24

Challenges of Li-air battery

Answer 24

• cell voltage drop at the cathode: O2 is OK but humidity degrades the cathode as well as the discharge products
• Anode: Li is extremely reactive and theres formation of Li dendrite that lead to short-circuits

Question 25

Drawbacks of Na-ion batteries

Answer 25

• Larger ionic size of Na^+ requires more power to keep energy flowing
• Long time to charge (even several days) and discharge
• Lower operating voltage, lower energy density, higher T for optimal performances
• Slow discharge rate does not supply enough power density for high-power applications
• trade-off between the charge/discharge rate and capacity, so attempts to increase charge/discharge rates results in severely reduced capacity.

Question 26

Na-air (Na-oxygen) battery

Na-ion battery

Answer 26

Na-ions as charge carrier; NaO2 more stable than Li_2O_2

Na-ion battery:

They store energy as Li-ion batteries (charge and discharge comparable to Li-ion)

During charg, Na ions moving from cathode to anode through electrolyte, the reverse during discharge

Na is abundant (6th most abundant element on earth)

Promising for large scale storage of renewable energy (cheaper per stored per unit of energy, scalable as Li-ion batteries)

Question 27

Capacitor

Answer 27

An electric circuit element used to store charge temporarily, consisting in two metallic plates separated and insulated from each other by dielectric material

The dielectric material is an insulator therefore no current flows through the capacitor

Question 28

Components of a capacitor:

Answer 28

• negative charge connection
• positive charge connection
• dielectric
• metal plate
• aluminum
• plastic insulation

Question 29

Explain capacitors:
+
Charge stored between plates

Answer 29

• A capacitor has two parallel plates separated by an insulating material
• A capacitor stores electrical charge between the two plates
• The unit of capacitance is Farad (F) ]
• Capacitance values are normally smaller, such as micro Farad, nano Farad, and pico Farad

Charge stored between plates:

• Electrons on the left plate are attracted toward the positive terminal of the voltage source
• This leaves an excess of positively charged holes
• The electrons are pushed toward the right plate
• Excess electrons leave a negative charge

Question 30

Basic equations for capacitors

Answer 30

Capacitance measures the ability of a capacitor to store charge in an electric field

C = q/V (Farad, F)

Capacitance is also a measure of the amount of electric potential energy stored (or separated) for a given electric potential

The energy stored in a capacitor is equal to the work done to charge it

dW = q/C dq ==> W_charging = 1/2 CV^2 = W_stored

C =ε_r * ε_0 * (A/d)

ε_r = permittivity
ε_0 = electric constant
d = separation between plates

A = area of overlap of two plates

C = ε_r * (A/ 4* pi * d)
W_charging = 1/2 * C * V^2

W_stored = 1/2 CV^2 = 1/2 ε_r * ε_0 * (A/d) * V^2

Question 31

What is a double layer? (capacitance)

Answer 31

A double layer (DL, also called an electrical double layer, EDL) is a structure that appears on the surface of an object when it is placed into a liquid

Question 32

What is a ‘supercapacitor’

Answer 32

Supercapacitor = electric double-layer capacitor (EDLC)

A supercapacitor is an electrochemical capacitor that has a very high energy density compared to common capacitors

It is similar to a battery, but stores energy in an electrostatic field, rather than as a chemical state.

No chemical reactions are involved

The application of voltage to the supercapacitor creates an electric field between the charged electrodes

This electric field cause the electrically charged ions in the electrolyte to migrate towards the electrodes of opposite polarity during charging.

Highly reversible

Question 33

Supercapacitor vs capacitor

Answer 33

Supercapacitors store electrical charge in a similar way to the convential capacitors, but the charges do not accumulate on two conductors

Instead the charges accumulate at the interface between the surface of the conductor and the electrolyte solution

The accumulated charges hence form an electric double-layer, the separation of each layer being of the order of a few angstroms

==>

Since no chemical action is involved, the effect is easily reversible and the typical cycle life is hundreds of thousands of cycles (vs. Hundreds of conventional batteries)

Question 34

The features of a “supercapacitor”

(advantages relative to batteries)

Answer 34

• Very high rates of charge and discharge
• Little degradation over hundreds of thousands of cycles
• good reversibility
• low toxicity of materials used
• high cycle efficiency (95% or more); low internal resistance
• narrow gap between electrodes (ultra-thin layers): large amount of charge stored in tiny volumes
• No danger overcharging
• Reducing battery cycling duty, and extending battery life

Question 35

Disadvantages of a “super capacitor”

(relative to batteries)

Answer 35

• The amount of energy stored per unit weight is considerably lower than that of an electrochemical battery (3-5 Wh/kg for an ultracapacitor compared to 30-40 Wh/kg for a battery)
• It is also only about 1/1000th the volumetric energy density of gasoline
• The voltage varies with the energy stored. To effectively store and recover energy, supercapacitor requires sophisticated electronic control and switching equipment
• The voltage across any capacitor drops significantly as it discharges

Question 36

The uses of supercapacitors

Answer 36

• Supercapacitors are generally employed in conjunction with a battery string to provide for uninterrupted power supply
• In this configuration, supercapacitor provides power during short duration interruptions and voltage sags
• by combining a supercapacitor with a battery power supply system, the life of the batteries can be extended
• The supercapacitors provide power only during the longer interruptions, reducing the cycling duty on the battery
• Small supercapaitors are commercially avaliable to extend battery life in electronic equipment, but large supercapacitors are still in development, and may soon become a viable component of the energy storage field