Chapter 3: Batteries and Supercapacitors Flashcards
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
Battery Classifications
Primary:
- Disposable batteries
- Use once and discard
- Zinc carbon, Alkaline batteries, ….
Secondary:
- Rechargeable batteries
- Recharge and use multiple times
- Lead, NiCd, NiZn, NiMH , Li-ion batteries,…
Classification for labeling batteries
5 elements to designate a battery
Internation electrochemical commision
- On digit for number of cells connected in series
- One letter to denote the electrochemical system
- On letter to denote shape (R= round, P = not round)
- Two or three digits as a unique physical dimension designation
- One or two letters as designation modifiers
L - ‘Alkaline’
S - ‘Silver Oxide’
C - ‘Lithium/MnO2’
B - ‘Lithium/CF’
4a. Two digit code denoting the maxium diamter
4b. Two-digit code denoting the maximum height
“Primary” batteries lifetime
- between 8-20% of their original charge lost every year at room T ==> self-discharge
==> “side” chemical reactions which occur if no load is applied
==> possible solution: storing at low T but avoiding freezing
“secondary” batteries lifetime
- Lifetime depends on the chemical reactions occurring between the battery parts and the electrolyte
==> side reactions:
corrosion of internal parts
Conversion of the active materials into inactive forms
==>
Chemical composition may change during charge/discharge cycles causing variations in the volume. This may limit the cycle life of the battery
Example : 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
Zinc - Carbon
Alkaline
NiCd
NiMH
Li-ion
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)
Standard modern batteries (zinc carbon)
Used in all inexpensive AA, C and D drycell batteries. The electrodes are zinc and carbon, with an acidic paste between them that serves as the electrolyte (disposable)
Standard modern batteries (alkaline)
Used in common duracell and energizer batteries, the electrodes are zinc and manganese-oxide, with an alkaline electrolyte (disposable)
Standard modern batteries (NiCd)
Rechargeable but “memory effect”
Discharge:
Cd + 2OH^- ==> Cd(OH)2 + 2e^- (@Cd electrode)
2NiO(OH) + 2H2O + 2e^- ==> 2Ni(OH)2 + 2OH^- (@Ni electrode)
Charge:
2NiO(OH) + Cd + 2H2O ==> 2Ni(OH)2 + Cd(OH)2
Electrolyte: alkaline electrolyte (KOH)
Standard modern batteries (NiMH)
Rechargeable, “memory effect” (but much less than NiCd)
Charge:
(-) H2O + “M” +2e^- <==> OH^- +MH
(+) Ni(OH)2 + OH^- <==> NiO(OH) + H2O + e^-
“M” = intermetallic specie
“M” = AB_2, AB_5, i.e. A = Ti, V or rare earth mixture elements; B =Zr, Ni
Electrolyte: alkaline electrolyte (KOH)
Discharge
Standard modern batteries (Li-ion)
Rechargeable, no “memory effect”
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
State of charge (SOC) vs Depth of discharge (DOD)
The SOC is the percentage of energy remaining in a battery compared to the fully charge capacity
Dischargin a battery decreases the state of charge, while charging increases the state of charge
For example, a battery that has had three quaters of its capacity removed is at 25% state of change
Self-discharge rate
Self discharge is the gradual reduction in the state of charge while at the steady state condition
self discharge is also referred to as “stand-by” or “shelf” loss
self discharge is a result of internal electrochemical mechanisms and losses
The rate of self-discharge differs among battery types and increases with battery age
Self discharge rates are typically specified in percentage of rated capacity per month
Higher temperatures result in higher self-discharge rates
Charge rate
The electrons passing through in the opposite direction reverse the chemical reactions and restore the active materials and electrolyte to their original compositions
Charge rate is quantified in the same way as discharge rate
For example a charging rate of C/50 to a 100 Ah battery applies 2 A of current until the battery reaches a specific fully charged voltage
Battery selection criteria
Each battery type has design and performane features suited for particular applications
System designers must consider the advantages and disadvantages of different battery types with respect to the requirements of a particular system
Considerations include lifetime, deep cycle performance, tolerance to high temperatures and overcharge, and maintenance requirements
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
Lead - acid batteries
(equation)
The relation between current, discharge time, and capacity is approximated (over a certain range of current values) by Peukerts’s law
t = Q_p / I^k
t = amount of time ( in hrs) that the battery can sustain
Q_p = capacity when discharged at a rate of 1 amp
I = current drawn from the battery
k = constant =~ 1.3
Lithium (Li) - batteries
Lithium batteries are primary (disposable) batteries in which the anode is composded by lithium (Li) metal of lithium compounds
Depending on the design and chemical compounds used, Li batteries can produce voltages between 1.5 V - 3.7 V (ca. twice the voltage of an ordinary Zn - carbon battery or alkaline battery)
Lithium batteries are widely used in products such as portable consumer electronic devices
They refer to a family of batteries with different chemistries, comprising many types of cathodes and elctrolytes
- Li-Batteries :
Non-rechargeable
Use of Li metal , unstable - Li -ion batteries:
Rechargeable;
Use of Li compounds, more stable
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^+)
Some examples of (Li-metal batteries)
- Li - MnO2 (Li-Mn, “CR”)
ca. 80% of the Li battery market.
Uses inexspensive materials
Suitable for low-drain, long life, low-cost applications
High energy density per both mass and volume
Can deliver high pulse currents
Wide T range
Maximum T limited to about 60 degree C
High self discharge at high T
Anode: Li metallic
Cathode: MnO2
Electrolyte: Li salt dissolved into organic solvent: LiCIO4 dissolved in propylene carbonate
Nominal voltage: 3V
OCV: 3.5V
Specific energy: 208 Wh/kg
- Li-SOCI2
Liquid cathode
Anode: Li metallic
Cathode: thionyl chloride, SOCl2
Electrolyte: LiAICl2 dissolved in SOCl2
Nominal voltage: 3.6 V
OCV: 3.65V
Specific energy: 500Wh/kg
Suitable for low T (upto -55 C), retaining over 50% of its rated capacity
toxic
electrolyte reacts with water
Low-current cells used used for portable electronics and memory backup
High-current cells used in military applications
Passivation layer on anode, which may lead to tempory voltage delay when put into service ( after long term storage)
High cost and safety concerns limit use in civilian applications
Can explode when shorted
- Li-FeS2
Anode: :Li metallic
Cathode: iron disulfide, FeS2
Electrolyte: propylene carbonate
Nominal voltage: 1.6 V - 1.4V
OCV: 1.8 V
Specific energy: ca. 300Wh/kg
Used in energizer lithium cells as a replacement for alkaline zinc manganese chemistry
2.5 times higher lifetime for high current discharge regime than alkaline batteries, better storage life due to lower self-discharge, 10 years storage time
FeS2 is cheap
Cathode often designed as a paste of iron sulfide powder mixed with powdered graphite. Variant is Li-CuFeS2
( Lithium metal may explode) - safety issued even from the electrolyte (organic solvent)
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 moleculoe (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
What is “graphite intercalation”
Where a molecule X is intercalated between the graphite layers. in this type of compound, the graphite layers remain largely intact and the guest mlecules or atoms (Li^+) are located in between them
Both the anode and cathode are materials into which, and from which, lithium can migrate. During intercalation Li^+ moves into the electrode
During the reverse process (extraction / de-intercalation) , Li^+ moves back out
When a Li-based cell is discharging, the lithium is extracted from the anode and inserted into the cathode
When the cell is charging , lithium is extracted from the cathode and inserted back into the anode
Graphite intercalation: (reactions)
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
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
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
Why lithium ion batteries are suitable for mobile products…
- high energy density
- High voltage (three times higher than Ni-Cd and Ni-MH)
- Light weight
- No memory effect
Li-ion battery : performances
- Power - High energy density mean s greater power in smaller package
160% greater than NiMH
220% greater than NiCs
- 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
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
Automotive application of Li-air batteries
Energy density :
Gasoline = 13 kWh/kg ==> 1.7 kWh/kg (when accounting losses)
Li-air = 12 kWh/kg (excluding oxygen mass) ==> 1.7 kWh/kg (theoretically predicted)
Overall:
5-15 times more energy density than Li-ion batteries
Storing 5-15 times more energy in the same size or the same energy stored in 1/5 th to 1/15th the size
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)
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.
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
Dielectric material
The dielectric material determines the type of capacitor
- common types of materials in capacitors are:
Mica
Ceramic
Plastic film
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
Two models of capacitance (?)
- when the electrodes are charged, there is an accumulation of ionic species of opposed charges at the interface
Two models:
- The helmhotz model
The helmotz model considers that the charges accumulated at the interface form a parallel plan with the interface, definition of helmhotz plan
- The Gouy-Chapman model
The Gouy-Chapman model: the double layer does not limit itself to the helmotz plan, but takes into account a diffuse layer of ionic species
Other models take into accoujt the interactions between charges, the polarizability of the molecules of the electrolyte and solvent
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
Maxwell supercapacitors
25 kW, fast response time
90% efficiency
Reduces lifetime maintenance costs
Durable and reliable with over a million cycles
Eliminates reliance on batteries or maintenance issues with hydraulic systems
High performance in all weather conditions (-40 degree C to + 65 degree C) especially for off shore installations
Suggested applications:
- Use as power back-up in products such as mobile phones, laptops, radio tuners, digital cameras, televisions, and computers
- Use for energy storage for solar panels, motor starters, photocopiers, to larger industrial drive systems
- There is a large interest from the autmotive industry for hybrid vehicles and as supplementary for battery electric vehicles
If the cost of super capacitors can be decreased, more applications may become available
Applications of super capacitors (real life examples)
Public transport:
- Shanghai (2006) : electric bus running without power lines using power stored in large onboard supercapacitors, which are quickly recharged whenever the electric bus stops at any bus stop, and get fully charged in the terminus)
- VAG (2002), the public transport operator in Nuremburg, Germany tested a bus which used hybrid-diesel drive system with supercapacitors
Energy:
- Siemens is developing a mobile energy storage based on double-layer capacitors called sibac energy storage. The company Celgelec is also developing a supercapacitor based energy storage system
- Boostcap (maxwell Technologies) to power emergency acuation systems for doors and evacuation slides in passenger aircraft such as the new airbus 380 jumbo jet
Graphene-based supercapacitors
Specific energy density of 85.6 Wh/kg at T and 136 Wh/kg a 80 degree C
These are the highest ever values for “electric double layer” supercapcitors based on carbon nanomaterials
Theoretically high specific surface area of single-layer graphene = 2.675 m^2/g
Supercapacitance of 550 F/g has not been reached in a real device because the graphene sheets tend to re-stack together
Graphene sheets stick to each other face to face
Curved graphene sheets to avoid stacking
Carbon aerogel supercapacitors
Usually made of non-woven paper made from carbon fibers and coated with organic aerogel, which then undergoes pyrolysis
Energy density equation
Energy density =
(V * I * t)/ m
V = voltage V
I = current ( A )
t = time (s)
m = mass (kg)
Power density equation
power density =
V * I / m
V = voltage V
I = current ( A )
m = mass (kg)
