Chapter 3: Batteries and Supercapacitors

Question 1

Battery (definition)

Answer 1

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 2

Battery design

Answer 2

• 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 3

Battery Classifications

Answer 3

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,…

Question 4

Classification for labeling batteries

5 elements to designate a battery

Answer 4

Internation electrochemical commision

1. On digit for number of cells connected in series
2. One letter to denote the electrochemical system
3. On letter to denote shape (R= round, P = not round)
4. Two or three digits as a unique physical dimension designation
5. 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

Question 5

“Primary” batteries lifetime

Answer 5

• 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

Question 6

“secondary” batteries lifetime

Answer 6

• 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

Question 7

Example : NiCd batteries

Answer 7

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 8

Batteries (4 steps)

Answer 8

• 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 9

Standard Modern Batteries

Answer 9

Lead - acid

Zinc - Carbon

Alkaline

NiCd

NiMH

Li-ion

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

Standard modern batteries (zinc carbon)

Answer 11

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)

Question 12

Standard modern batteries (alkaline)

Answer 12

Used in common duracell and energizer batteries, the electrodes are zinc and manganese-oxide, with an alkaline electrolyte (disposable)

Question 13

Standard modern batteries (NiCd)

Answer 13

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)

Question 14

Standard modern batteries (NiMH)

Answer 14

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

Question 15

Standard modern batteries (Li-ion)

Answer 15

Rechargeable, no “memory effect”

Question 16

Battery Capacity
(Lead -acid batteries)

Answer 16

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 17

Rate of discharge

Answer 17

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 18

State of charge (SOC) vs Depth of discharge (DOD)

Answer 18

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

Question 19

Self-discharge rate

Answer 19

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

Question 20

Charge rate

Answer 20

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

Question 21

Battery selection criteria

Answer 21

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

Question 22

Batteries in series

Answer 22

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 23

Batteries in parallel

Answer 23

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 24

Lead - acid batteries
(equation)

Answer 24

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

Question 25

Lithium (Li) - batteries

Answer 25

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

Question 26

Li - metal batteries

Answer 26

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 27

Some examples of (Li-metal batteries)

Answer 27

- 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)

Question 28

Li-ion batteries (pros and cons)

Answer 28

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 29

Chemical intercalation

Answer 29

Intercalation is the reversible inclusion of a moleculoe (or group) between two other molecule (or groups) Ex: include graphite intercalation compounds

Question 30

Graphite intercalation compounds

Answer 30

- 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

Question 31

What is "graphite intercalation"

Answer 31

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

Question 32

Graphite intercalation: (reactions)

Answer 32

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

Question 33

Discharging

Answer 33

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 34

Charging

Answer 34

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 35

Charging

Answer 35

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 36

Why lithium ion batteries are suitable for mobile products...

Answer 36

- high energy density - High voltage (three times higher than Ni-Cd and Ni-MH) - Light weight - No memory effect

Question 37

Li-ion battery : performances

Answer 37

- 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

Question 38

Li-air ( or Li-oxygen) battery advantages and disadvantages

Answer 38

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 39

Li - air battery (anode and cathode)

Answer 39

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 40

Challenges of Li-air battery

Answer 40

- 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 41

Automotive application of Li-air batteries

Answer 41

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

Question 42

Na-air (Na-oxygen) battery Na-ion battery

Answer 42

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 43

Drawbacks of Na-ion batteries

Answer 43

- 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 44

Capacitor

Answer 44

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 45

Components of a capacitor:

Answer 45

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

Question 46

Explain capacitors: + Charge stored between plates

Answer 46

- 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 47

Dielectric material

Answer 47

The dielectric material determines the type of capacitor - common types of materials in capacitors are: Mica Ceramic Plastic film

Question 48

Basic equations for capacitors

Answer 48

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 49

What is a double layer? (capacitance)

Answer 49

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 50

Two models of capacitance (?)

Answer 50

- 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

Question 51

What is a 'supercapacitor'

Answer 51

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 52

Supercapacitor vs capacitor

Answer 52

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 53

The features of a "supercapacitor" (advantages relative to batteries)

Answer 53

- 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 54

Disadvantages of a "super capacitor" (relative to batteries)

Answer 54

- 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 55

The uses of supercapacitors

Answer 55

- 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

Question 56

Maxwell supercapacitors

Answer 56

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

Question 57

Suggested applications:

Answer 57

- 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

Question 58

Applications of super capacitors (real life examples)

Answer 58

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

Question 59

Graphene-based supercapacitors

Answer 59

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

Question 60

Carbon aerogel supercapacitors

Answer 60

Usually made of non-woven paper made from carbon fibers and coated with organic aerogel, which then undergoes pyrolysis

Question 61

Energy density equation

Answer 61

Energy density = (V * I * t)/ m V = voltage V I = current ( A ) t = time (s) m = mass (kg)

Question 62

Power density equation

Answer 62

power density = V * I / m V = voltage V I = current ( A ) m = mass (kg)