Question 3 - Batteries and super capacitors Flashcards
Describe electrolysis and potential industry applications
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
Descibe Li-ion batteries and the electrochemical reactions involved
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
Primary vs Secondary batteries
Primary : Single use, limited lifespan, high energy density,
Secondary : Rechargeable, longer lifespan, lower energy density,
Environmental impact
Li-ion vs Li-metal batteries
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
Explain the memory effect
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.
Battery (definition)
A battery is a an electrochemical cell (or a collection of multiple electrochemical cells) that produces electricity from a chemical energy
Chemical ==> Electrical Energy
Battery design
- 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
NiCd batteries
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)
Batteries (4 steps)
- 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
Standard modern batteries: Lead - Acid
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)
Battery Capacity
(Lead -acid batteries)
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
Battery Capacity
(Lead -acid batteries)
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
Rate of discharge
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
Batteries in series
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
Batteries in parallel
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
Li - metal batteries
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^+)
Li-ion batteries
(pros and cons)
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)
Chemical intercalation
Intercalation is the reversible inclusion of a molecule (or group) between two other molecule (or groups)
Ex: include graphite intercalation compounds
Graphite intercalation compounds
- 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
Discharging
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
Charging
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
Li-air ( or Li-oxygen) battery
advantages and disadvantages
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
Li - air battery (anode and cathode)
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,)
Challenges of Li-air battery
- 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
Drawbacks of Na-ion batteries
- 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.
Na-air (Na-oxygen) battery
Na-ion battery
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)
Capacitor
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
Components of a capacitor:
- negative charge connection
- positive charge connection
- dielectric
- metal plate
- aluminum
- plastic insulation
Explain capacitors:
+
Charge stored between plates
- 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
Basic equations for capacitors
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
What is a double layer? (capacitance)
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
What is a ‘supercapacitor’
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
Supercapacitor vs capacitor
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)
The features of a “supercapacitor”
(advantages relative to batteries)
- 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
Disadvantages of a “super capacitor”
(relative to batteries)
- 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
The uses of supercapacitors
- 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
