2015, Spring Semester

Energy Engineering (Class 458.624)

n Professor

¨ Dae-Hyeong Kim : 302-816, 880-1634,dkim 98@ snu.ac.kr

¨ Classroom :302-720

¨ Class tim e : M onday, W ednesday 11:00 ~ 12:15

n Textbook

(1) Introduction to Solid State Physics (Charles Kittel)

(2) Solid State Electronic Devices (Ben G Streetm an, Sanjay Kum ar Benerjee)

Energy vs. Power Density

S upercapacitors bridge between batteries and conventional capacitors“Energy – the capacity to do work” versus “Power – how fast the energy is delivered”

Battery has high energy but low power, Capacitor has low energy but high power

Capacitor is electrostatic capacitor, while supercapacitoris electrochem icalcapacitor.

Supercapacitors are able to attain greater energy densities while still m aintaining the high power density of conventional capacitors.

Supercapacitors are a potentially versatile solution to a variety of em erging energy applications based on their ability to achieve a wide range of energy and power density.

Ragone plot of energy storage system s*

ES R: Equivalent S eries Resistance

G oal

Batteries Vs. Supercapacitors

Supercapacitors:

-Higher power density

-M uch faster charge and discharge rate than the battery

-Environm entally friendly

-Extrem ely low internal resistance

-High efficiency (97-98%)

-Over a m illion charge-discharge cycles

Batteries:

-Have higher energy density

-Typically 200–1000 charge-discharge cycles

-Contain highly reactive and hazardous chem icals

-Negatively affected by low tem peratures

•Batteries are suitable for applications where we need an energy delivery profile. For exam ple, to feed a load during the night when the only source is PV m odules.•However, batteries are not suitable for applications with power delivery profiles. For exam ple, to assist a slow load-following fuel cell in delivering power to a constantly and fast changing load. For this application, supercapacitors seem to be m ore appropriate.•US DOE has designed supercapacitors as im portant as batteries for future energy storage system .•The m ajor problem of conventional capacitor is its low energy density, which can be solved by using electrochem ical supercapacitors.

TypeEnergy/ weight

[W h/kg]Power/S ize [W /kg]

Cell [V]Cycles

Durability [#]

Charge tim e [h]

Lead (Pb) 20 ~30 1~30 0 2 20 0 ~30 0 8 ~16

Ni-Cd 30 ~55 10 ~90 0 1.25 1,50 0 1

Ni-M H 50 ~8 0 20 ~1,0 0 0 1.25 30 ~50 0 2~4

Li-ion110 ~

16 01,8 0 0 3.7 50 0 ~1,0 0 0 2~4

S upercap 3.9~5.7 470 ~ 13,8 0 0 1~2.7 1,0 0 0 ,0 0 0 0 ~30 s

Batteries Vs. Supercapacitors

•Ni-M H: Hybrid Cars(Toyota), m ainly due to its safety•Li-ion: Electronics(Sm artphone), EV(Tesla M otors), Hybrid Cars(Hyundai-Kia, BM W , etc..), high capacity electricity storage•Supercapacitor:

n Back up for uninterruptable power supplies (UPS)n Light weight power supplies for sm all aircraftn Provide short duration power for various vehicle system s such as breaking or steeringn Used to absorb power during short periods of generation such as Regenerative Brakingn Extend range and battery life in Hybrid Electric Vehicles (HEV)

§ The purpose of use of supercapacitor is to allow higher accelerations and deceleration of the vehicle with m inim al loss of energy, and conservation of the m ain battery pack. i.e.)i) Use supercapacitors in parallel with the battery to provide enough power foracceleration and to recover energy during braking and storage in supercapacitorsii) Use supercapacitors for peak power requirem ents to increase the efficency and the life cycle of the ESU system (They are also used in electric vehicle and for load leveling to extend the life of batteries)

Hybrid Electric Vehicles

Ex) China is experim enting with a new form of electric bus that runs 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 term inus.

Electrostatic Capacitor

C: Capacitance εr : Dielectric Constant of M edium (Electrolyte) ε0: Dielectric Constant of Vacuum A : S urface A rea d: Electrode Distance

Store Energy as Electric Potential between Two Electrodes and a Dielectric LayerCapacitance lim ited by flat surface area and dielectric properties

Capacitive Storage System s

+++++

-----

ElectrodeElectrolyte

+

ElectrodeElectrolyte

M +n

M +n+1 -

Electrochem ical Capacitors (S upercapacitor)

EC Double Layer Capacitor

Pseudocapacitors

Non-Faradaic(no transfer of charge through the surface)

PseudocapacitanceCharge transfer through surface

Faradaic, redox reactions

•A supercapacitor (or ultracapacitor) is an electrochem icalcapacitor that has anunusually high energy density when com pared to com m on capacitors.

Electrochem ical Double Layer Capacitors (EDLC)

EDLCs store charge electrostatically at electrode/electrolyte interface as charge separation.

C: 10-50 m F/cm 2

There is no charge transfer between electrode and electrolyte.

Intrinsically high power devices (short response tim e), lim ited energy storage, very high cycling stability (~10 6).

Different form s of high surface area carbon are used as an electrode m aterial:

activated carboncarbon aerogelscarbon nanotubes

Electrolyte: KOH, organic solutions, sulfuric acid.

+++++

-----

++

--

+++++

-----

++

--

Electrolyte

+ -

Electrode

• W hen the voltage is applied to positive plate, it attracts negative ions fromelectrolyte.W hen the voltage is applied to negative plate,it attracts positive ionsfrom electrolyte.•Therefore,there is a form ation ofa layer ofions on the both side ofplate.This iscalled ‘Double layer’form ation.For this reason,the ultracapacitor can also becalled Double layercapacitor.• The ions are then stored nearthe surface ofcarbon.• Ultracapacitor stores energy via electrostatic charges on opposite surfaces ofthe electric double layer.• They utilize the high surface area of carbon as the energy storage m edium ,resulting in an energy density m uch higherthan conventionalcapacitors.• The am ount ofenergy stored is very large as com pared to a standard capacitorbecause of the enorm ous surface area created by the (typically) porous carbonelectrodes and the sm all charge separation (10 angstrom s) created by thedielectric separator• The distance between the plates is in the orderofangstrom s.According to the form ula for the capacitance,

Dielectric constant of m edium X area of the plateCapacitance = -----------------------------------------------------------------

Distance between the plates

Pseudocapacitors

+ -e-

A

Electrolyte

H + or Li+

Pseudocapacitors store the energy by charge transfer between the electrode and the electrolyte.(no charge transfer in EDLC)

The charge is transferred at the surface or in the bulk near the surface through i) adsorption, ii) redox reaction and iii) intercalation of ions.

Pseudocapacitors can achieve higher specific capacitance and energy density than EDLCs.

ex. H ydrous RuO 2 (70 0 + F/g)*

Typical electrode m aterials:M etal oxidesConducting polym ers

•Charge transferthrough surface Faradaic RedoxReactions

•Sim ilarto EDLCs,but electrodes are m ade from m etal-oxides orconducting polym ers (veryoften on the carbon supports forthe rapid charge transferand large surface area)- Electrolyte ions diffuse into pores and undergo fast,reversible surface reactions- Can achieve very high capacitances & energies

•Advantages- High surface area and fast Faradaic reactions allow for higher energy densities thanEDLCs (Hydrous Ruthenium Oxide or M anganese Oxide can achieve extraordinarycapacitance)

•Disadvantages- G enerally,lowerpowerdensities than EDLCs (Reaction takes m ore tim e.)- Cycle life can be lim ited by m echanical stress caused during the reduction/oxidationreactions (in com parison with EDLC,but betterthan battery)- Negatively charged conducting polym erelectrodes are not very efficient- Best m etal-oxide electrodes are very expensive and require aqueous electrolytes (but

environm ent-friendly),im plying lowervoltage and capacitance.

Cyclic Voltam etry

W (energy) = ½ CU2

C = capacitance

U = voltage

Perform ance of supercapacitor= electrostatic (EDLC) + faradaic redox reaction (Pseudocap.)

I = current = C(dU/dt)

Cyclic Voltam etry

#1.Idealdouble layercapacitance-An ideal behavior is expressed in a rectangular shape of voltam m etry-The sign of current is reversed upon reversal of the potential sweep-Phenom enon is purely electrostatic-M aterials with pseudocapacitanceproperties show deviation from such a rectangular shape (redox peaks: 4)

#4.Pseudocapacitance-M aterials with pseudocapacitanceproperties show deviation from such a rectangular shape (redox peaks: 4)

#. Enhancem ent of specific capacitance -Oxidation of carbon for increasing the surface functionality-Carbon/conducting polym ers com posites by electropolym erization of m onom er (aniline, pyrrole) on the carbon surface

-Insertion of electroactive particles of transition m etals oxides, such as RuO2, TiO2, Cr2O3, M nO2, Co2O3 into the carbon m aterial.

Capacitive Storage System s

Capacitive Storage System s

EDLC type Peudocapacitor type

M aterials for Supercapacitor(EDLC type)

n W hy Carbon for Electrodes?

-Carbon electrode is well polarizable and its conductivity is high. Its surface area can be increased a lot by introducing 3D nanostructures.

n Characteristics of Carbon?

-Chem ically stable in both acid and base. Environm entally friendly. Cheap.

n W hat kind of structures?

-Activated carbon, carbon aerogels, carbon nanotubes

n How is the capacitance increased?

-Addition of conducting polym ers or oxides of transition m etals (EDLC ®Pseudocapacitor)

Carbon nanotubes, activated carbons and carbon aerogels, are typical m aterials for EDLC and can be used for supercapacitors by adding redox m aterials (conducting polym ers, m etal oxides). Carbon nanotubes have excellent nanoporosity properties, allowing tiny spaces for the polym er to sit in the tube and act as a dielectric. Som e polym ers (eg. polyacenes) have a redox (reduction-oxidation) storage m echanism along with a high surface area.

•Supercapacitors technology: Surface Area ­

•Key principle: area is increased and distance is decreased.

•There are som e sim ilarities with batteries but there areno reactions here.

Traditional standard electrical double layer capacitor

Double layer capacitor with activated carbons

Ultracapacitorwith carbon nano-tubes electrodes

ACd

e=

M aterials for Supercapacitor(EDLC)

Activation of carbons to increase the surface area

M aterials for Supercapacitor(Pseudocapacitor)

n W hat is the difference of Pseudocapacitors from EDLC?

-W hen potential is applied, fast, reversible faradaic redox reactions take place.

-Perform ance = electrostatic (EDLC) + faradaic redox reaction (Pseudocap.)

n Electrochem ical Processes?

-Reversible adsorption

-Redox reactions of Transition m etal oxide

-Reversible electrochem ical doping/dedopingin conductive polym ers

n W here does the reaction take place?

-The electrochem ical processes happen both at the surface and in the bulk (near surface).

-M uch higher capacitance than EDLC (EDLC ~ electrostatic process at the surface only)

n W hy lower power density than EDLC?

-Faradaic processes are slower than electrostatic surface processes.

Conway, B. E., Birss, V. & W ojtowicz, J. Journal of Power S ources 6 6 , 1-14 (1997)

Doping-Dedoping of Polyaniline

Doping Dedoping

Redox reactions of RuO2

Electrolytes

n Are there safe organic electrolytes?

- W aterbase electrolytes can be used instead particularly forthe safety

n How can we exploit ionic liquids to im prove electrochem icalwindow?

-It has higheroperating voltages and leads to higherenergy density (W =1/2CV2 )

- But it has higher resistance than aqueous electrolytes (slow m ovem ent ofelectrolytes and thereby slow operation ofsupercapacitor)

n How does the resistance ofelectrolyte affect the perform ance?

- The electrolyte determ ines the series resistance of the supercapacitors (therealways are trade-offs)

W hat is the battery ?

1. Definition of Battery

- Batteries are electrochem icalcells,electrically interconnected each other,each ofwhich containstwo electrodes and an electrolyte.- Redox (reduction-oxidation) reactions occur at these electrodes,which convert electrochem icalenergy into electricalenergy and vice versa.-In 1800,Alessandro Volta invented the first battery (unit ofvoltage,V,com es from his nam e).

- Batteries are classifieds as two types, prim ary batteries (for one tim e use) and secondarybatteries (rechargeable battery).

Prim ary and S econdary Battery

1.Prim ary batteries

In prim ary batteries,the electrochem icalreaction is not reversible.

During discharging, the chem ical com pounds are perm anently changed andelectrical energy is released until the original com pounds are com pletelyexhausted. Thus the cells can be used only once.

Alkaline battery :Eveready,Duracell,Energizer

2.Secondary batteries

In secondary batteries,the electrochem icalreaction is reversible.

The originalchem icalcom pounds can be reconstituted by the application of anelectricalpotentialbetween the electrodes injecting energy into the cell. Suchcells can be discharged and recharged m any tim es.

Li-ion battery :Sam sung,LG ,SK,Panasonic,Sony

G alvanic / Electrolytic Cell

-Two different operation m odes of electrochem ical cells

1) G alvanic Cell

- An electrochem ical cell that derives electrical energy from spontaneous redox reactions taking place within the cell.

- Energy is released from a spontaneous redox reaction- ∆G < 0 (∆G = -nFE)

- ex) Corrosions, Batteries(discharge), Fuel Cells

2) Electrolytic Cell

- An electrochem ical cell that undergoes a redox reaction when electrical energy is applied.- Energy is absorbed to drive a nonspontaneous redox reaction- ∆G > 0 (∆G = -nFE)

- ex) Recharging Rechargeable Batteries, Electrolysis, Electrorefining, Electroplating (silver and gold)

n Oxidation: rem oval of electrons from a species, X ® X+ + e-

n Reduction: addition of electrons to a species, X+ + e-® X

n Redox (Reduction and Oxidation) reaction: a reaction accom panying an electron transfer

n Reducing agent (reductant): an electron donor (which m akes others reduced, i.e. m erge with electrons) in a redox reaction

n Oxidizing agent (oxidant): an electron acceptor (which m akes others oxidized, i.e. lose electrons) in a redox reaction

n H alf-reaction: a conceptual reaction showing the gain of electrons, any redox reaction can be expressed as the difference of two reduction half-reactions

n Redox couple: the reduced and oxidized species in a half-reaction for redox couple, i.e. Ox + ne-® Red

n A node: the electrode at which oxidation occurs

n Cathode: the electrode at which reduction occurs

reduction

Term inology

H istory of Batteries

Establishm ent of electrochem istry study 1600 England

Discovery of ‘anim al electricity’ 1791 Italy

Voltaic battery, Zn-Cu with NaClelectrolyte 1800 Italy

Daniellcell, Zn-Cu with H 2SO 4 and CuSO 4 electrolyte 18 36 England

Fuel cell, H 2/O2 1839 England

Lead A cid battery, Pb-PbO 2 with H 2SO 4 electrolyte

(First rechargeable battery, Pb toxicity issue)

18 59 France

Carbon zinc dry cell (Com pletion of dry cell) 1896 G erm any

NikelCadm ium (Ni-Cd) battery with KOH electrolyte

(Cd toxicity issue, 1.2V)

1910 Sweden

NiOOH-Fe rechargeable battery with potassium hydroxide electrolyte 1914 US

Zn-M nO 2 with alkaline electrolyte

(alkaline prim ary battery, Eveready battery)

1955 US

Ni-H 2 long life rechargeable batteries put in satellites 1970s US

Nickel–M etal hydride (NiM H) battery

(environm ent-friendly, high safety, 1.2V, Toyota Prius)

198 9 US

Com m ercialization of Li -ion batteries by Sony

(high energy density, 3.7V, Sam sung/LG /SK)

1991 Japan

LiFePO4 invented 1997 US

Introduction to Batteries

4. Types of Batteries

Battery(Chemical cell)

Primary Battery

Alkaline

Lithium

Zinc-air

Zinc Carbon

Secondary Battery

Lead-Acid

Ni-Cd

Ni-MH

Lithium ion

Fuel Cell

PEMFC

SOFC

MCFC

PAFC

DaniellCell

1) oxidation (anode): loss of electronsZn(s) → Zn2+(aq) + 2e-

2) Reduction (cathode): gain of electronsCu2+(aq) + 2e-→ Cu (s)

3) Overall reactionZn(s) + Cu2+ (aq) → Zn2+(aq) + Cu(s)

Daniellcell

S tandard electrode potential: Potential of given half-reaction at standard state

Cell potential (EoCell): potential difference between

two electrodes of a cell m easured in voltsEo

Cell= Eoreduction – Eo

oxidation = EoCu/cathode – Eo

Zn/anode

= 0.34 V – ( -0.76 V) = 1.1 V∆G (reaction G ibbs energy) = -nFEo

= -2 (m ol) x 96,485 (C/m ol) x 1.1 (V)= -212,267 J < 0 (S pontaneous reaction)

F is Faraday s̀ constant, F=eN A, E is the cell potential, ν is the stoichiom etric coefficient of the electron (ν=2 in above case) in the half-reactions

-Electrodes in different electrolytes

-Cu is a cathode and Zn is an anode.

-Electrons leave the cell from anode and enter the cell again through cathode.

Zn(s) | ZnS O 4(aq)║CuS O 4(aq) | Cu(s)anode cathodem em brane

S tandard Hydrogen Electrode (SHE): A special electrode, whose potential is zero, and used as a reference of single electrode

A lkaline Battery

A dvantages over carbon zinc type batteries-Higher energy density-Lower internal resistance-Longer shelf life-G reater resistance to leakage-Better dim ension stability

-Cathode: M ixture of electrolytic m anganese dioxide and carbon conductor-A node: G elled m ixture of zinc powder and electrolyte-Electrolyte: Potassium hydroxide solution in water-S eparator: Non-woven, fibrous fabric that prevents m igration of any solid particles in the battery-S teel: cathode collector-Brass collector: anode collector

-Overall reaction: Zn + 2M nO2 + H 2O → ZnO + 2M n2O3 (e0 = 1.43 V)

-Cathode: 2M nO2 + H 2O+ 2 e-→ 2M n2O3 + 2OH

- (e0 = +0.15 V)-A node: Zn + 2OH -→ ZnO + H 2O + 2e

-(e0 = -1.28 V)

Prototype (1957)

First com m ercial battery (1955) by Lew Urry

Energy Density Com parison of Rechargeable Batteries

- Negative electrode : Pb- Positive electrode : PdO2

- Electrolyte : Sulfuric acid (H 2SO4)

Lead - A cid Battery

- Chem ical reaction (discharge)- Positive electrodePbO2 (s) + HSO4

- (aq) + 3H + (aq) + 2e- PbSO4 (s)+ 2H 2O (l)

- Negative electrodePb (s)+ HSO4

- (aq) PbSO4 (s) + H+ (aq) 2e-

- Overall reactionPb (s) + PbO2 (s) + 2H 2SO4 (aq) 2PbSO4 (s) + 2H 2O (l)

A dvantages

- Inexpensive and sim ple to m anufacture — in term s of cost per watt hours, the sm all sealed lead acid battery (SLA) is the least expensive.

- M ature, reliable and well-understood technology — when used correctly, the SLA is durable and provides dependable service.

- Low self-discharge — the self-discharge rate is am ong the lowest in rechargeable battery system s.

- Low m aintenance requirem ents & Capable of high discharge rates.

Lim itations

- Low energy density — poor weight-to-energy density lim its use to stationary and wheeled applications.

- A llows only a lim ited num ber of full discharge cycles — well suited for standby applications that require only occasional deep discharges.

- Environm entally unfriendly — the lead content causes environm ental dam age. Electrolyte (sulfuric acid) is also harsh.

- Transportation restrictions on flooded lead acid — there are environm ental concerns regarding spillage in case of an accident.

Lead - A cid Battery

- Negative electrode : Cadm ium (Cd) - Positive electrode : Nickel oxyhydroxide (NiO(OH))- Electrolyte : Potassium hydroxide (KOH) solution

Ni-Cd Battery

- Chem ical reaction (discharge)- Positive electrode2NiO(OH) + 2H 2O + 2e- 2Ni(OH)2 + 2OH -

- Negative electrodeCd + 2OH - Cd(OH)2 + 2e-

- Overall reaction2NiO(OH) + Cd + 2H 2O 2Ni(OH)2 + Cd(OH)2

- The electrolyte is not affected because it does not participate in the reaction.

A dvantages

- Fast and sim ple charge — even after prolonged storage.- H igh num ber of charge/discharge cycles — if properly m aintained, the

Ni-Cd provides over 1000 charge/discharge cycles.- Long shelf life – in any state-of-charge.- S im ple storage and transportation — m ost airfreight com panies

accept the Ni-Cd without special conditions.- G ood low tem perature perform ance.- Forgiving if abused — the Ni-Cd is one of the m ost rugged rechargeable

batteries.- Econom ically priced — the Ni-Cd is the lowest cost battery in term s of

cost per cycle.- Available in a wide range of sizes and perform ance options — m ost

Ni-Cd cells are cylindrical.

Lim itations

- Relatively low energy density — com pared with newer system s (Li-ion).- Environm entally unfriendly — the Ni-Cd contains toxic m etals. Som e

countries are lim iting the use of the Ni-Cd battery.- H as relatively high self-discharge — needs recharging after storage.

Ni-Cd Battery

- Negative electrode : M etal Hydride such as AB 2 (A=titanium and/or vanadium , B= zirconium or nickel, m odified with chrom ium , cobalt, iron, and/or m anganese) or AB 5 (A=rare earth m ixture of lanthanum , cerium , neodym ium , praseodym ium , B=nickel, cobalt, m anganese, and/or alum inum )

- Positive electrode : Nickel oxyhydroxide (NiO(OH))- Electrolyte : Potassium hydroxide (KOH)

Ni-M H Battery

- Chem ical reaction (discharge)- Positive electrode- Negative electrode- Overall reaction

- The electrolyte is not affected because it does not participate in the reaction.

NiO(OH) + H 2O + e- Ni(OH)2 + OH -

M H + OH - M + H 2O + e-

NiO(OH) + M H Ni(OH)2 + M

A dvantages

- 30 – 40 percent higher capacity over a standard Ni-Cd — The Ni-M H has potential for higher energy densities.

- S im ple storage and transportation — transportation conditions are not subject to regulatory control.

- Environm entally friendly — contains only m ild toxins; profitable for recycling.

Lim itations

- Lim ited service life — if repeatedly deep cycled, especially at high load currents, the perform ance starts to deteriorate after 200 to 300 cycles.

- Lim ited discharge current — although a Ni-M H battery is capable of delivering high discharge currents, repeated discharges with high load currents reduces the battery’s cycle life.

- H igh self-discharge — the Ni-M H has about 50 percent higher self-discharge com pared to the Ni-Cd.

- Perform ance degrades if stored at elevated tem peratures — the Ni-M H should be stored in a cool place.

- A bout 20 percent m ore expensive than Ni-Cd — Ni-M H batteries designed for high current are m ore expensive than the regular version.

Ni-M H Battery

Lithium Ion Battery

- Chem ical reactions

- Cathode : LiCoO2 Li1-xCoO2 + xLi+ + xe-

- Anode : xLi+ + xe- + 6C LixC6

- Overall reaction : LiCoO2 + C6 Li1-xCoO2 + LixC6

- W hen the battery is discharged, the Co is oxidized from Co3+ to Co4+. The reverse process (reduction) occurs when the battery is being charged.

- The traditionalbatteries are based on galvanic redox reactions but Lithium ion secondary batterydepends on an "intercalation"m echanism ,i.e.the lithium ion is inserted into and exerted from thelattice structure ofanode and cathode during charging and discharging.

- During charging,lithium in cathode is ionized and m oves from layer to layer and inserted into theanode.This involves the insertion of lithium ions into the crystalline lattice of the host electrodewithout changing its crystalstructure.

- During discharging,Liions are dissociated from the anode and m igrate across the electrolyte andare inserted into the crystalstructure of the host com pound of cathode.At the sam e tim e,thecom pensating electrons travelin the externalcircuit and are accepted by the host to balance theredox reaction.

- The process is com pletely reversible. Thus the lithium ions pass back and forth between theelectrodes during charging and discharging.

Lithium Ion Battery

A dvantages

- Relatively high voltage (lowest standard reduction potential).- Relatively low self-discharge — self-discharge is less than half

ofthat of Ni-Cd and Ni-M H.- Low m aintenance requirem ent.- H igh capacity — lithium is lightest/sm allest m etalion and thus

can serve the highest energy density (per weight or volum e).

Lim itations

- Requires organic electrolyte rather than aqueous one due to high operation voltage (> 3V), higher than the water dissociation voltage.

- Requires protection circuit — protection circuit lim its voltage and current.

- S ubject to aging, even if not in use — storing the battery in a cool place. The aging effect is m inim ized at 40 percent state-of-charge.

- Expensive to m anufacture — about 40 percent higher in cost than Ni-Cd.

Com ponents of Lithium Ion Battery

- Cathode : Lithiated form of a transition m etal oxide (e.g. lithium cobalt oxide (LiCoO2))

- A node : Carbon (usually graphite (C6)), M etal oxide m aterials

- Binder : PVDF, SBR/CM C- Current collector : Cu (anode), Al (cathode)

- Electrolyte : Solid lithium -salt liquid electrolytes (LiPF6, LiBF4, or LiClO4) and organic solvents (ether); if the electrolyte is polym er, this battery is called as lithium -ion polym er battery

- S eparator : Polyolefin based resin (PP, PE)- Encapsulation : Stainless steel, Al pouch

Structure of Cylindrical Type Lithium Ion BatteryStructure of Coin Cell Type Lithium Ion Battery

Structure of Pouch Type Lithium Ion Battery

Cathode M aterials

1. LiCoO 2 (LCO)• M ost com m only used cathode m aterial• Li intercalates into octahedral sites

between the edge sharing CoO2 layers• High electrical conductivity• Capacity ~ 145 m Ah/g• Voltage ~ 3.6 V• Energy density ~ 150 -250 W h/kg• Only 50% of the Li content can be taken

out before the structure collapses • Relatively low capacity• Less therm ally stable because of

oxygen loss at elevated tem peratures• Expensive and toxic• Not environm entally friendly

2. LiNiO 2

• Voltage 3.5 V• High capacity (~ 200 m Ah/g)• Difficult to m ass produce• Low therm al stability

3. LiM n2O 4

• High voltage 3.7 ~ 3.8V• Energy density 100 ~ 150 W h/kg• High therm al stability• Low cost• Low capacity (~120 m Ah/g)• Low chem ical stability

4. LiNi1/3Co1/3M n1/3O 2

• Voltage 3.6 ~ 3.7 V• Energy density 150 ~ 220 W h/kg• Relatively high capacity (~170 m Ah/g)• High power & therm al stability

5. LiFePO 4

• Voltage 3.2 ~ 3.45 V• Energy density 90 ~ 120 W h/kg • Low cost and plentiful elem ents• Environm entally benign• Relatively low capacity (~150 m Ah/g)• Low electrical conductivity (~10 -9 S/cm )

A node M aterials1. G raphite• M ost com m only used anode m aterial• Stacked graphene layers• Low voltage (~0.05 V)• Sm all volum e expansion• Low cost• Relatively low capacity ~372 m Ah/g

2. Li4Ti5O 12 (LTO)• High voltage ~1.5 V• Less than 0.2% volum etric change• Long cycle life• G ood rate capability• One of the stable m aterials for lithium ion battery• Low capacity ~175 m Ah/g• Low energy density 70 ~ 80 W h/kg

3. S ilicon• Very high theoretical capacity ~4200m Ah/g• Voltage 0.4 V• Abundant elem ent• Environm ental friendly• Large volum e expansion (~300 %)• Poor cyclability

Silicon nanowire anodeNature nanotechnology, Vol 3 (2008), 31-35

Double-walled silicon nanotube structureReference : H ui W u et al., Nature nanotechnology, Vol 7 (2012), 310-315

Cathode and A node M aterials for LIB

~ 3.7 V

Electrolyte

- Liquid electrolytes in lithium ion battery are com posed of lithium salts, organic solvents, and additives (e.g. VC, Biphenyl, Boranes, HM TP).

- Characteristics of electrolytes for lithium ion batteries• Large electrolyte potential window so does not decom pose across potential range.• M aintain good electrode/electrolyte interface during cycling even when the electrode

particles are changing their volum e during lithiation/delithiation.• Li -ion conductivity σLi>10

-4 S/cm over the tem perature range of battery operation.• Chem ical stability over tem perature ranges of battery operation.• Safe m aterials, i.e., preferably nonflam m able and nonexplosive even when short-

circuited.• Low toxicity and low cost.

Organic Carbonates and Esters as Electrolyte Solvents

Lithium Salts as Electrolyte Solutes

Reference : Kang Xu, Chemical Reviews, 2004, Vol. 104, No. 10

S eparator

― The separator m ust physically keep anode and cathode from contacting with eachother,while enable free ionic transport.Based on the m orphology ofthe separator,thereare generally two kinds ofseparators,m icroporous m em branes and nonwoven film s.

― Although separators are effective in preventing electricalshorts between anode andcathode, their presence in between the two electrodes decreases the effectiveconductivity ofthe electrolyte,raising cellim pedance.This would be expected since thepresence of the separator decreases the total cross sectional area of lithium ionconducting pathway,and the tortuosity ofthe open pores in the separatorprolongs theionic transport pathway.Forthis reason,the thinnerthe separator,the higherthe ionicconductivity.

― The m aterials used for the m icroporous polym er m em branes are sem i-crystallinepolyolefin,such as polyethylene (PE),polypropylene (PP),and theirblends (PE-PP).

Separator requirements for liquid lithium ion batteries (LIBs).

Reference : Min Yang et al., Membranes 2012, 2, 367-383

Li Polym er Battery

Solid polym er electrolyte (PEO-LiX): -Role of polym er: m edium for ionic transport + separatorG el type polym er electrolyte (PVDF-HFP, PM M A, PAN):-Polym er electrolyte + inorganic particles (TiO2, Al2O3, γ-LiAlO2, M gO etc)-Role of inorganic particles: prom oting ion transport & im proving m echanical stabilityLithium ion gel polym er-Liquid electrolyte (high ionic conductivity) + solid polym er electrolytes (elim ination of leakage problem s)G el coated and/or gel-filled separators-Im proved m echanical properties com pared to lithium ion gel polym er

Lithium Ion Battery: Liquid electrolyte

Li Polym er Battery: S olid-like electrolyte

Li ion

electrolyte

Polym er network

Inorganic particle

Cylindrical/Prism atic Cells Polym er Cells

-Lower m echanical strength due to thin encapsulation-Low capacity/energy density

-Excellent stability at high tem perature-Light due to thin encapsulation

Types

Pros & Cons

Necessity of S olid Electrolyte

Lithium Ion Battery

SCIENTIFIC REPORTS | 4 : 3815 | DOI: 10.1038/srep03815

- H igh energy density battery :Li-A ir,Li-S battery

- Low cost battery :S odium -ion battery

- Safe battery :A ll-solid-state battery

Lithium -A ir Battery

Theoretically, the capacity of cathode (or air electrode)can be rem arkable large because cathode active (O2) is not included in battery package. Li-Air: 4Li+O2 => 2Li2O2, 11,140W h/kg

2 Li + O2 → 2 Li2O2 (3.1 V, 3623 W h/kg)4 Li + O2 → 2 Li2O (2.9 V, 5024 W h/kg)4 Li + O2 → 2 Li2O (2.9 V, 11,140 W h/kg)

- M ost of the current lim itations in Li-air battery developm ent are at the cathode. Incom plete discharge due to blockage of the porous carbon cathode with discharge product such as lithium peroxide (in aprotic designs) is the m ost serious problem . Also atm ospheric oxygen m ust be present at the cathode, but contam inants such as water vapor can dam age it.

- Dendritic lithium deposits in anode can decrease capacity or trigger a short circuit.- In current cell designs, the charge overpotentialis m uch higher than the discharge

overpotential. Significant charge overpotentialindicates the presence of secondary reactions. As a result, electrical efficiency is only around 65%.

- Long term battery operation requires chem ical stability of all the com ponents of the cell. Current cell designs show poor resistance to oxidation by reaction products and interm ediates.

Lithium -S ulfur Battery

- Chem ical processes in the Li–S cell include lithium dissolution from the anode surface (and incorporation into alkali m etalpolysulfide salts) during discharge, and reverse lithium plating to the anode while charging.This contrasts with conventional lithium -ion cells, where the lithium ions are intercalated in the anode and cathodes.

- Each sulfur atom can host two lithium ions. Typically, lithium -ion batteries accom m odate only 0.5–0.7 lithium ions per host atom .Consequently Li-S allows for a m uch higher lithium storage density.

- Polysulfides are reduced on the cathode surface in sequence while the cell is discharging :

S 8 → Li2S 8 → Li2S 6 → Li2S 4 → Li2S 3

- Across a porous diffusion separator, sulfur polym ers form at the cathode as the cell charges :

Li2S → Li2S 2 → Li2S 3 → Li2S 4 → Li2S 6 → Li2S 8 → S 8

- Low electronic conductivity of S, Li2S and interm ediate Li-S products lim its battery’s rate capability.

- Reactivity of the lithium m etal anode can cause dendrite deposition, cell shorting, and safety issues.

S odium -Ion Battery

- There are significant drawbacks for usinglithium ion batteries: they are ratherexpensive and in addition the extraction oflithium m etal is problem atic from anenvironm entalpoint of view due to its highreactivity and relatively low abundance in theEarth’s crust (only 20 ppm ).

- One option to avoid these drawbacks m ightbe to replace lithium (Li)with sodium (Na)inelectrode m aterials. Sodium is sim ilar tolithium in term s of its chem icalproperties,but approxim ately 1,000 tim es m oreabundant in the ground (26,000 ppm ) andin the form ofsalt (NaCl)in norm alseawater(15,000 ppm ).

- This m akes sodium -based batteriespotentially m ore environm entally friendly andeasier to recycle,as wellas up to five tim esless expensive.

Safety Issues and A ll-solid-state LIB

Cons• Electrolyte leakage• Narrow working tem perature range (salts precipitation & electrolyte evaporation)• Sensitive to environm ental changes (high possibility of deform ation, expansion, & explosion)

Pros• High stability (non-explosive, flam e retardant, non-volatile)• Higher voltage stability• Ease of processing (thin film process, printing processing, etc.)• Realization of high voltage & high energy density battery by

m ulti-stacking process.

Conventional Lithium Ion Battery A ll-Solid-State Lithium Ion Battery

Explosion of LIB

iPhone 4 Dell com puter (Sony LIBs) Tesla M otorsiPod nano

A ll-S olid-S tate Battery by Toyota

5μm

LCO cathode

Solid electrolyte

LTO anode

20 20

20 30

- The world’s first operation of all-solid-state battery (release on Novem ber 5, 2010)

- Fine interfaces between the electrolyte layer & the electrodes (New Electrolytes)

- High density solid electrolyte layer (higher ionic conductivity)

- Electrode layers without high tem perature processing

- Toyota’s target: all-solid-state battery for EV in 2020, Li-air battery for EV in 2030