Main industrial systems (mass produced) •  Lead acid •  NiMH •  Li-­‐ion 1 Lead acid •  Lead-­‐acid ba;eries are a convenient choice based on cost •  Lead-­‐acid ba;eries are worse than other technologies based on most performance characterisAcs. Disposal is another important issue because of the toxic materials •  In parAcular, lead-­‐acid ba;ery has low energy density and life is significantly shortened when they discharged very rapidly or with frequent deep cycles •  Lead-­‐acid ba;eries are very sensiAve to temperature effects •  Despite these drawbacks they are employed widely in cars (low cost and reliable, well understood) •  The nominal voltage produced is about 2 V/cell. Cells are usually connected in series to achieve higher voltages, usually 6V, 12 V, 24 V and 48V. Pb + PbO2 + H2SO4 2PbSO4 + 2H2O 2 rcentage of its capacity available when fully charged relative to its rated capacity. For example, a battery
ted at 30 Ah, but only capable of delivering 24 Ah when fully charged, will have a state-of-health of
Lead-­‐acid ba;eries takes
into account the loss of capacity as the battery ages.
/30 " 100 # 80%. Thus, the state-of-health
•  PosiAve eBatteries
lectrode: Lead dioxide (PbO2) 0.3 Lead-Acid
•  NegaAve electrode: Lead (Pb) Electrolyte: SoluAon of sulfuric acid (H2SO4) and water 0.3.1 • Theory
of Operation
he chemical
reactionsathat
in a lead-acid
battery
are represented
by theions following
equations:
•  Sulfuric cid occur
dissociates into protons and sulfate SO42− $
Positive electrode: PbO 2 $ H 2 SO 4 $ 2H $ 2e
Negative electrode: Pb $ H 2 SO 4
!
discharge
---------------------charge
Overall cell reaction: PbO 2 $ pb $ 2H 2 SO 4
discharge
---------------------charge
PbsO 4 $ 2H 2 O
$
PbSO 4 $ 2H $ 2e
discharge
---------------------charge
!
2PbSO 4 $ 2H 2 O
(1)
(2)
(3)
As the cell is charged, the sulfuric acid (H2SO4) concentration increases and becomes highest when the
l is fully charged. Likewise, when the cell is discharged, the acid concentration decreases and becomes
ost dilute when the cell is fully discharged. The acid concentration generally is expressed in terms of
3 ecific gravity, which is weight of the electrolyte compared to the weight of an equal volume of pure water.
finishing charge and overcharge reactions that electrolyze water
to
full
capacity.
e of hydrogen (H2) Lead-­‐acid and oxygenba;ery (O2) gases.
At the positive
plate, [8]
o
vercharge e finishing charge and overcharge reactions that electrolyze wate
ture•  of
hydrogen
(H2)aand
At ithe
positive plate, [8
During overcharge t the oxygen
posiAve e1(O
lectrode oxygen s formed 2) gases.
þ
"
H2 O ) 2H þ =2 O2 þ 2e
gative plate [9]
H2 O ) 2Hþ þ 1=2 O2 þ 2e"
•  During overcharge at the negaAve electrode hydrogen is formed negative plate [9]
•  The effect is that 2Hþ þ 2e" ) H2 :
2Hþ þ 2e" ) H2 :
otal overcharge reaction is the electrolysis of water to form oxyge
n [10] o  Bubbles form in the electrolyte e totalo overcharge
Build up of preaction
ressure is the electrolysis of water to form oxyg
gen [10]
o  Water is lost 1=2 O :
H
O
)
H
þ
2
2
2
o  Mixture is highly explosive o  Need to allow the gH
as tO
o e)
scape H þ 1=2
O2 :
modynamic measurements of these reactions at platinum electrodes
kes
less energymeasurements
to electrolyze of
water
in reactions
the electrolyte
than toelectrode
recharg
ermodynamic
these
at platinum
4 ftakes
this were
true
in
the
lead-acid
battery,
it
could
not
be
recharged.
less energy to electrolyze water in the electrolyte than to rechar
2
2
Lead-­‐acid ba;eries: SulfaAon •  When the ba;ery is discharged both plates are coated with lead sulfate (PbSO4) – a poor conductor •  During charge the sulfate coaAng returns into soluAon •  When the sulfate coaAng hardens on the plates, it can no longer be converted •  Ba;ery output becomes limited and eventually leads to complete ba;ery failure •  A failed ba;ery is said to be sulfated •  Electrode sulfaAon is one of the primary effects that affects ba;ery life 5 Lead-­‐acid ba;eries: Types •  Flooded (Wet Cells) –  Electrolyte is a liquid. –  Ba;ery is usually not sealed and not spill-­‐proof. –  Usually vent hydrogen gas during charging. •  Valve-­‐Regulated Lead-­‐Acid (VRLA) –  Gelled Electrolyte (if led at rest the electrolyte gels) •  Electrolyte is immobilized by a thickening agent. –  Absorbed Glass Mat (AGM) •  Electrolyte is absorbed in separators of ma;ed glass fibers. –  VRLA ba;eries are hermeAcally sealed and designed to produce li;le or no hydrogen gas during charging. •  All use the same chemistry but construcAon varies 6 Lead-­‐acid ba;eries: Types •  VRLA also called maintenance free or sealed •  A VRLA ba;ery uAlizes a one-­‐way, pressure-­‐relief valve system •  The oxygen normally produced on the posiAve plate is absorbed by the negaAve plate •  This suppresses the producAon of hydrogen at the negaAve plate •  Water (H2O) is produced instead, retaining the moisture within the ba;ery •  SAll, gas evoluAon can happen, in which case a valve opens to release excess gas •  Flooded designs uAlize a vent to allow the gas to escape and water needs topping up regularly •  The liquid electrolyte can spill and cause corrosion if Apped or punctured 7 Lithium ba;eries •  Power – High power density means greater power in a smaller package. o  160% greater than NiMH o  220% greater than NiCd •  Higher voltage –allows it to power complex mechanical devices. •  Long shelf-­‐life – only 5% discharge loss per month. o  10% for NiMH, 20% for NiCd •  For portable electronics, a reliable choice •  Expensive -­‐ 40% more than NiCd. •  Delicate -­‐ ba;ery temp must be monitored from within (which raises the price), and sealed parAcularly well. •  Regula;ons -­‐ when shipping Li-­‐Ion ba;eries in bulk (which also raises the price (hazardous material) 8 Lithium-­‐ion •  A Li-­‐ion ba;ery has a four-­‐layer structure •  A posiAve electrode (cathode) made with Lithium Cobalt Oxide (current collector made of thin aluminum foil) •  A negaAve electrode made with specialty carbon (current collector of thin copper foil) •  A separator -­‐ fine porous polymer film. •  An electrolyte made with lithium salt in an organic solvent •  The electrolytes are selected in such a way that there should be an effecAve transport of Li-­‐ion to the cathode during discharge •  Li-­‐ion ba;eries depends on an "intercalaAon" mechanism •  This involves the inserAon of lithium ions into the crystalline lajce of the host electrode without changing its crystal structure •  These electrodes have two key properAes. One is the open crystal structure, which allow the inserAon or extracAon of lithium ions and the second is the ability to accept compensaAng electrons at the same Ame 9 Lithium-­‐ion •  A separator is necessary to electrically isolate the two electrodes •  It may consist of solvent, salt, separator, addiAve, and/or a solid ion-­‐
conducAng membrane or a combinaAon thereof •  The electrolyte should be durable, highly ionic-­‐conducAve, and facilitate electrochemical reacAons •  If a liquid electrolyte is used, an addiAonal membrane is required to electronically separate the two electrodes •  This membrane must be porous and allow the liquid electrolyte to flow through •  A soaked porous plasAc film can be used as separator •  If the membrane itself is a Li ion conductor, the liquid electrolyte is not a necessity •  Can also incorporate the liquid electrolyte into the polymer matrix to form a polymer gel electrolyte. These are the most common types of membranes used in a Li-­‐ion ba;eries 10 Lithium-­‐ion •  During charging, lithium in the +ve
electrode material is ionized and
migrates across the electrolyte to be
inserted into the anode material
•  During discharge Li ions are
dissociated from the –ve electrode and
migrate across the electrolyte to be
inserted into the crystal structure of the
host compound of the +ve electrode
•  At the same time the compensating
electrons travel in the external circuit
and are accepted by the host to
balance the reaction
Lithium Cobalt-­‐oxide (LiCoO2) Li containing speciality carbon •  The process is completely reversible.
Thus the lithium ions pass back and
forth between the electrodes during
charging and discharging.
11 Lithium-­‐ion chemistry •  Chemical reacAon (charge) •  PosiAve electrode •  NegaAve electrode •  Overall LiCoO2 CoO2 + Li+ + e-­‐ Through electrolyte Li+ + e-­‐ + 6C LiC6 External circuit LiCoO2 + C6 CoO2 + C6Li •  With discharge the reacAons are reversed 12 Lithium-­‐ion: charge discharge/ stack 13 Lithium-­‐ion: Safety •  Fire and/or explosion (rare but widely publicized) can occur for a number of reasons including: •  Poor cell design (electrochemical or mechanical) •  Cell manufacturing flaws •  External abuse of cells (thermal, mechanical, or electrical) •  Poor ba;ery pack design or applicaAon •  Poor protecAon electronics design or manufacture •  Poor charger/system design or manufacture resulAng in overcharging of ba;ery •  Another failure is thermal runaway, which is a rapid self-­‐heaAng of a cell due to the exothermic chemical reacAon involving the electrolyte and/or negaAve electrode Possible solutions
•  Replacement of the oxygen releasing +ve
electrode material (LiCoO2) with
structurally stable alternative compounds,
e.g. LiFePO4
•  Replacement of the flammable liquid
organic electrolyte with more stable
materials
14 Nickel Metal Hydride Ni-­‐MH •  Commonly found in devices such as portable music players, phones, cameras, electric toothbrushes, razors, torches etc. •  NegaAve electrode: The acAve material is actually hydrogen, but in a metal hydride such as AB2 (A=Atanium and/or vanadium, B= zirconium or nickel, modified with chromium, cobalt, iron, and/or manganese) or AB5 (A=rare earth mixture of lanthanum, cerium, neodymium, praseodymium, B=nickel, cobalt, manganese, and/or aluminum), e.g., lanthanum nickel LaNi5 •  PosiAve electrode: nickel oxyhydroxide (NiO(OH)) •  Electrolyte: Potassium hydroxide (KOH)
•  Advantages: •  Less sensiAve to high temperatures than Li-­‐ion and Lead-­‐acid •  Handle abuse (overcharge or over-­‐discharge be;er than Li-­‐ion) •  Disadvantages: •  Cell voltage around 1.2 so more cells needed compared to Li-­‐ion •  Cost (same as Li-­‐ion) 15 Nickel Metal Hydride Ni-­‐MH •  Cost and availability issues : o  NiMH negaAve electrode use rare earth materials (La, Ce, Nd, Pr) as negaAve oxide materials o  PosiAve electrode material can use addiAves such as Y or Yb or Nb o  Availability decrease & price increase will contribute to faster move to Li-­‐ion 16 Ni-­‐MH Chemistry •  Chemical reacAon (discharge) •  PosiAve electrode 6NiO(OH) + 6H2O + 6e-­‐ 6Ni(OH)2 + 6OH-­‐ Through electrolyte •  NegaAve electrode •  Overall external LaNi5H6 + 6OH-­‐ LaNi5 + 6 H2O + 6e-­‐ LaNi5H6 + 6NiO(OH) LaNi5 + 6Ni(OH)2 •  The electrolyte is aqueous KOH, which provides the water to the discharge and OH-­‐ for the charge 17 Nickel Metal Hydride Ni-­‐MH •  Its precursor was the nickel-­‐cadmium ba;ery •  Both ba;ery types use the same +ve electrode reacAon, but the -­‐ve electrode reacAon is different •  In the nickel-­‐cadmium ba;ery, cadmium is used •  Cadmium and its compounds are very toxic and as a result, ba;eries containing cadmium pose a threat to the environment, if they are not properly disposed of •  In the NiMH ba;ery, hydrogen is used instead of cadmium as a reducing agent, which makes this ba;ery environmentally safe •  Hydrogen can’t be used directly so a metal hydride is used •  Ni-­‐Cad is being replaced by Ni-­‐MH 18 Ni-­‐MH for Electric Vehicles Ni-­‐MH have been successfully field tested in many electric and hybrid vehicle applicaAons Toyota Prius v hybrid Honda Insight •  Hybrid mostly based on Ni-­‐MH (Honda, Ford, Toyota) •  It is expected that Li-­‐ion will replace Ni-­‐MH for full EV (other manufacturers developing Li-­‐ion systems) 19 Part 3: IntroducAon to flow ba;eries 20 Large-­‐Scale Storage ApplicaAons •  Renewable energy integraAon –  StabilizaAon of wind turbine output. Power generation
–  Hybrid with photovoltaic cells. •  Grid systems. •  UninterupAble power supply (UPS). •  Emergency standby. Distribution
Grid
Micro-grid
domestic
•  Remote area power systems. •  TelecommunicaAons. •  DomesAc/community storage. Storage
End user
Power
Load leveling
Peak shaving
Energy arbitrage
Energy
Frequency/voltage control
Power quality regulation
50 kW to 50 MW
100 kW to >500 MW
Systems
50 kW to 50 MW
100 kWh to >250 MWh
21 Which technology?
•  ConvenAonal ba;eries –  Lead acid is ubiquitous but has lifeAme and environmental concerns –  Most not readily scalable (surface area) –  Li-­‐ion cost does not come down significantly with scale •  Huge potenAal for large scale electrochemical, e.g. flow ba;eries –  Vanadium, Zn-­‐Br, others in future •  Sodium (Molten salt) –  NAS •  No single winning technology System Efficiency / % Cycle life Cost, $ kW-­‐1 Lead acid 80 1000 200-­‐800 NiCd 70 2500 500-­‐1500 NAS 75 2500 1000-­‐3000 Li 80 5000 200-­‐4000 Zn-­‐Br 60 2000 700-­‐2500 10000 600-­‐2500 Vanadium 70 22 Criteria for large-­‐scale ba;ery •  Low costs –  Capital: low cost components, readily scalable manufacturing processes –  Annual: high cycle life with extended maintenance/service intervals –  Must compete with lead acid benchmark of 200 – 800 $ kWh-­‐1 •  Convenience and suitability for applicaAon (criteria from E.On, Sandia) –  OperaAonal periods between <15 min to >10 hour –  10 to 20 year lifeAme –  kW to 400 MW Frequency regula;on UPS 50 – 1000 kW –  Cycles per year: 50 to >8000 1 – 100 MW 15 minute duraAon 4 – 10 hour duraAon >8000 cycles per year <50 cycles per year 15 year lifeAme 10 year lifeAme 23 A simple flow ba;ery Electrolyte Flow
Positive
Electrolyte
Reservoir
Electrode
Stack
Negative
Electrolyte
Reservoir
Electrolyte Pumps
24 Flow ba;ery storage capacity •  Volume of external electrolyte tanks.
•  Concentration of active species.
•  Electrons transferred per mole of active species.
R2
è
O2 + ne-
O1 + ne- è R1
Electrolyte storage capacity = nFC/3600 (Ah dm-3)
•  n = number of electrons transferred per active molecule
•  F = Faraday constant (96485 C mol-1)
•  C = concentration of active species (mol dm-3)
25 Flow BaLery Stacks 26 Power •  Current density •  Size and number of electrodes in stack •  Number and configuraAon of bipolar stacks EXAMPLE: 200 cell stack. Electrode area = 0.5 m2. Current density = 50 mA cm-­‐2. Average cell voltage = 1.5 V. Current = 50 x 10-­‐3 x 103 x 0.5 = 25 A Voltage = 200 x 1.5 = 300 V Power = 300 x 25 = 7.5 kW 27 Strategies for Choosing Redox Flow Cell Electrochemistry Look at the electrochemical series Find a pair of couples having high cell voltage One couple can be highly oxidised The other couple can be highly reduced But both redox couples must be sustainable: –  stable themselves and, preferably, in combinaAon –  kineAcally ‘redox-­‐able’ at pracAcal electrodes –  reasonable in cost, easily sourced, transported, –  Stored… •  AcAve material must be soluble in suitable electrolyte, e.g. H2SO4 •
•
•
•
•
28 Different Types of Flow Batteries
Number of solid phases and use of a membrane
Type 1 classical, membrane divided redox flow ba;ery uAlizing redox couples which remain solvated during charge and discharge cycling. Example: all-­‐vanadium V3+ + VO2+ + H2O ⇄ V2+ + VO2+ + 2H+ Type 2 half redox flow ba;ery and half metal flow ba;ery. Membrane divided. A phase change is required at one electrode. Example: zinc-­‐cerium Zn2+ + 2Ce3+ ⇄ Zn + 2Ce4+ Type 3 metal, metal oxide flow ba;ery where both electrode reacAons involve deposiAng solid phases during charging. Example: soluble lead 2Pb2+ + 2H2O ⇄ Pb + PbO2 + 4H+ 29 Vanadium redox flow ba;ery (VRFB) •  Based on the ability of vanadium to exist in four different oxidaAon states (V(II), V(III), V(IV), and V(V), each of which holds a different electrical charge •  The liquid electrolyte in the negaAve half-­‐cell has V3+ (V(III)) and V2+ (V(II)) ions, while the liquid electrolyte in the posiAve half-­‐cell contains VO2+ (V(V)) and VO2+ (V(IV)) ions •  The typical open-­‐circuit voltage created during discharge is 1.26 V •  Membrane divided cell •  Most widely studied and most popular RFB •  The simplicity of the design and the fact that all species remain in soluAon is the reason for its popularity 30 VRFB Charge and discharge VO2+ is also wriLen V5+ VO2+ is also wriLen V4+ During discharge Posi;ve Electrode: V5+ + 2H+ + e-­‐ → V4+ + H2O (E0 = 0.99 V vs. SHE)
Nega;ve Electrode: V2+ → V3+ + e-­‐ (E0 = -­‐0.26 V vs. SHE) 31 VRFB performance 32 Soluble lead Positive electrode
charge
Pb2+ + 2H2O - 2e-
PbO2 + 4H+
discharge
charge
Negative electrode
Pb2+
Overall cell reaction
2Pb2+ + 2H2O - 2e-
+ 2e
-
discharge
Pb
charge
discharge
Pb + PbO2 + 4H+
•  Undivided •  Based on staAc lead-­‐acid (same reacAons) •  A;racAve because no membrane is required (no membrane ohmic loss and cheaper) and because only one electrolyte is needed •  Complex lead oxide deposits (parAcularly on the posiAve electrode) lead to deterioraAng performance and low efficiency for repeated cycles •  Can be ameliorated using addiAves (bismuth) but no addAve works for both electrodes 33 Soluble lead performance Charge Discharge cycles with open circuit measurements
34 Commercial Redox Flow Battery Development
Chemistries Vanadium •  Ashlawn Energy •  Cellenium •  Cellstrom GmbH •  Lynergies •  Panggang Group •  Prudent Energy CorporaAon •  Renewable Energy Dynamics Technology Ltd. (RedT) •  Rongke Power •  Sumitomo Electric Industries •  Ubiq Tech •  V-­‐Power Iron-­‐chromium •  Deeya Energy Zinc-­‐halide •  Net-­‐Power •  Premium Power CorporaAon •  RedFlow Limited •  ZBB Energy CorporaAon •  Xin-­‐Long Installed Systems Power •  30 kW to 4 MW Capacity •  100 kWh to 8 MWh 35 Cell Voltage Components During charge: Ecell = EOCV − η+ ve + η−ve + ∑ k ΔVohm,k
During discharge: Ecell = EOCV − η+ ve + η−ve −
∑
k
ΔVohm,k
Minimise energy losses by reducing: overpotenSals -­‐ good electrocatalysis, good mass transport R, resistances -­‐ of electrolytes and membrane 36