Batteries and Fuel Cells
I learned of battery research when I hired into Bell Telephone Laboratories (BTL)
You're surprised that "the telephone company" was interested in batteries?
You shouldn't be: Why do you think telephones still work when the power goes out?
Because telephone buildings are stuffed with batteries capable of driving
the low power transistors in telephone switches (BTL's crowning invention!)
In the basic research arm of BTL (~4%), to encourage breakthrough inventions:
Newly hired researchers (right out of school!) had the freedom
to choose their own problems, and their own collaborators
But then, in mid/late 70's, I was counseled to avoid battery research because it was:
An old almost fully researched field where, in the future,
major improvements (much less breakthroughs) were very unlikely
But that view has now changed, because of energy – Reasons?
Portable Electronics: We, the public, decided we HAD TO lug around our PC's
But early laptops were intolerably big and heavy - due to their batteries!
Grid Load Leveling: Long known that energy storage enhances "Grid" efficiency
But big conservative power companies just never
got around to setting up the necessary battery research projects
THEN renewable energy sources arrived with wildly variable power cycles
Transforming grid energy storage into full blown necessity!
Air pollution/global warming => Need to abandon fossil-fueled transportation
Which will require much better batteries
And could be enhanced by addition of fuel cells
Which also opens the door to non-polluting hydrogen fuel
These new motivations have re-energized battery research
But have they yet produced needed breakthroughs?
I'll try to answer that today by:
- Explaining the basic science behind traditional classes of batteries
Then critiquing them from the perspective of modern energy storage
- Describing present-day research on batteries targeted for Grid load leveling
- Followed by research on batteries intended for electrified transportation
- I'll then conclude by explaining fuel cells which have the promise of
enhancing electrical transport via hydrogen fueled vehicles
To begin: I still have a basic problem with batteries:
I was taught about batteries from elementary school onward
But what I was taught bears ZERO resemblance to the batteries I now see!
We made batteries by sticking pennies & nails in lemons:
We learned about a 2000 year old "Baghdad Battery"
Then, high school chemistry taught us "redox" reactions
<= Supposedly explaining modern batteries like this
But the lemon batteries didn't require salt-filled tubes
Nor has any battery I have ever owned!
And they never fully explained all the other chemicals
Or how they got there, or got ionized, in the first place!
Top figure (and excellent tutorial): http://www.edinformatics.com/math_science/how_does_a_battery_work.htm
Bottom figure: http://courses.washington.edu/bhchem/c456/ch11.pdf
So let's start by revisiting high school chemistry:
Which taught about "redox" reactions (i.e., "oxidation" and "reduction")
Which don't even require oxygen (as long as they mimic its effect):
"Oxidation" = When a chemical specie loses an electron (or electrons)
"Reduction" = When a chemical specie gains an electron (or electrons)
And, most often, redox reactions seemed to involve a metal, in contact with water
The redox reactions for two different metals can then be represented as:
M1 (solid)  M1+ + e- + DE1
M2 (solid)  M2+ + e- + DE2
Or, highlighting the dissolution of the metal, as this:
+ ++ -
+ ++ -
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
So it's mostly about atoms ionizing as they dissolve into water!
If energy has to be added to get that dissolution & ionization
then the energy yield in these two redox reactions would be negative:
M1  M1+ + e- + DE1
M2  M2+ + e- + DE2
But there is absolutely no reason that DE1 should equal DE2
So we can play one of these reactions off against the other!
By putting ionized atoms of the more difficult to ionize metal
in proximity to non-ionized atoms of the other metal:
Start:
Electron transfer:
After some rearrangement
+ +
+
+ +
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
But this isn't enough to make a battery!
+ +
+
+ +
An electron is transferred to the metal that holds on to its electrons more strongly
And thus, with that electron transfer, energy is released
But this all occurs on a local, atomic, one atom to another atom, scale
And the energy release does NOT drive electrons out through wires
And thus does NOT produce electricity I can use to do some sort of work
INSTEAD WE'VE GOT TO SOMEHOW:
- Separate the metal getting the electron from the metal giving the electron
- Compel them to transfer that electron ONLY via an external link (a wire)
- Set up the starting conditions (e.g., one metal ionized, the other not)
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
So with that new understanding, let's look at the 1st "modern" battery:
Which was invented by Allessandro Volta, in 1800
(From whose name we get our electrical unit of Volts)
Immediate problem: Its operation depends on only ONE metal, zinc
The second redox player = hydrogen (from sulfuric acid / salt brine)
Zn (solid) => Zn+2 + 2 e-
2 H+ + 2 e- => H2 (gas)
The other electrode (green) doesn't participate beyond just passing thru electrons:
Anode (Zn metal atoms ionizing/dissolving)
-
+
+
+
+
+
Cathode (hydrogen de-ionizing/forming H2 gas
H2
H+
+
+
+
-
H+
H+
http://www.edinformatics.com/math_science/how_does_a_battery_work.htm
Critique from a modern energy storage perspective:
1) We now want to store a LOT of energy!
In Volta-style battery:
Energy is partially from zinc = GOOD, we have a lot of zinc in the solid electrode
Energy is partially from hydrogen ions = BAD, there are limited number in the acid
And we can only the increase concentration of the acid so much
Further, more concentrated acid could eat away at the battery's container
Unless we use heavy, fragile, expensive glass containers
2) We now want to repeatedly store energy = A rechargeable battery:
In a discharging Volta-style battery, hydrogen ions combine to form hydrogen gas
Which forms bubbles
Which rise to the surface, pop, and are GONE!
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
So let's move on to a more modern Zinc / Copper style of battery:
As depicted on Wikipedia webpage:
Chemistry textbook's silly salt-filled glass tubes are gone (good riddance!)
Further, above does not oversimplify by suggesting that battery is just:
Two metal bars stuck in acid
Instead: The zinc electrode is immersed in a ZnSO4 solution (=> Zn+2 + SO4-2)
The copper electrode is immersed in a CuSO4 solution (=> Cu+2 + SO4-2)
http://en.wikipedia.org/wiki/Galvanic_cell
So we can now see how battery action gets started:
Initial concentration of metal ions doesn't have to first come off the electrodes
Metal ions are instead provided by the solutions added to the electrodes
Further, the metal giving electrons is now separated from the metal getting electrons
And electron transfer can only occur via the external wire
Problem: Negative charge seems to be building up at the right
Which will create a strengthening electric field pushing electrons backward!
Zn anode (metal atoms ionizing/dissolving)
+
-
+
+
+
+
+
+
+
-
Cu cathode (metal atoms de-ionizing/precipitating)
+
+
-
+
+
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
That's where the center "porous disk" and "anion flow" come in:
Negative electron flow to the right, via the wire, must be countered by either:
- Simultaneous movement of negative ions ("anions") back to left OR
- Simultaneous movement of positive ions ("cations") to right
EITHER prevents the build up of NET charge or electric fields anywhere
Here the anions must be the must be the SO4-2 ions from the CuSO4 solution
But, because lone electrons don't like to go into water solutions,
they'll still be forced through external circuit (=> electricity for our use)
But what would happen if the porous disk were removed?
Solutions would mix:
-
Zn Anode
-
+
+
+
+
Cu cathode
+
+
+
+
-
+
+
And eventually we'd revert to local electron transfer between Zn and Cu atoms/ions
Zn Anode
Cu cathode
+
-
+
+
+
+
+
+
+
+
-
Eliminating need for electrons
to flow out through the wire
+
Eliminating the "electricity!"
Critique from a modern energy storage perspective:
The need for porous disks (or clumsy glass salt bridges) is finally explained
Good News: Because things don't leave the cell, and reactions are reversible
Recharging now seems possible (at least for this general class of battery)
Bad News: Capacity is determined by concentration of certain ions (Zn+2 / Cu+2)
Which, in water, can't be all that numerous
BAD NEWS: Zn+2 and Cu+2 ions will eventually diffuse through the separator
And, indeed, I found certain sources alluding to this
This would occur, I'd guess, in days, weeks or (at most!) months
Doesn't that mean such batteries would die in days, weeks or months?
(implying that long-lived batteries must be made differently!)
Further, we now need a LOT of energy storage per volume or mass:
So we're not going to tolerate spread out structures such as these:
+
-
+
+
H2
+
H+
+
+
-
H+
+
H+
+
Instead, we're going to move the electrodes as close to one another as possible
Introducing another potential problem:
Recharging will require metal ions to come out of solution, and back onto electrodes
This:
Will have to revert to this:
+
+
+
+
+
+
But that is NOT how crystals (such as metals) usually grow!
Didn't you every use sugar water to grow sugar crystals?
Pretty crystal spires grow because atoms condense more quickly
on only certain planes of crystal surfaces
=> dendrites / dendritic growth
So metal ions re-depositing on electrodes (during recharge) more likely produce:
This:
Leading to this in a new
more compact battery:
Thus in modern denser battery (w/ closely spaced electrodes) recharge can easily:
"Short out" (i.e., permanently connect) the electrodes
Indeed, this is what kills off most of my battery-powered tools!
Photo: http://geyserofawesome.com/post/102873046022/its-a-classic-case-of-science-vs-the-sweet
One way around this: Household alkaline batteries:
Which DO eliminate leaky separators, and DO exploit dense solid electrodes
But the electrodes never dissolve, they just rearrange into new solids!
Anode (Zn/ZnO)
Cathode (MnO2/Mn2O3)
O H
-
O
O
H
-
-
-
O H
O H
-
-
Left electrode is zinc being (literally!) oxidized:
Zn (solid) + 2 OH−  ZnO (solid) + H2O + 2e−
Right electrode is manganese oxide (with its oxygen content being reduced):
2 MnO2(solid) + H2O + 2e−  Mn2O3(solid) + 2 OH−
All facilitated by use of an alkaline (basic) potassium hydroxide electrolyte
With traditional batteries now (hopefully) making more sense
And having highlighted their strengths and weaknesses
Let's move onto:
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
The use of batteries for modern grid load leveling
This refers to when you can cheaply produce more power than you now need
It occurs, for instance, with nuclear plants . . . every single night!
Fuel cost of a nuclear plant is miniscule compared its building cost
And you're constantly paying off building cost in the form of loan interest
But plants are sized to power the Grid all day long
Then people turn things off as they go to sleep
Making nuclear plant's ~ zero-added-cost power, literally, useless!
A solar plant can similarly produce power (at noon) when we don't most need it
So there's a STRONG incentive to somehow save energy for later
For such grid load leveling, what will the battery priorities be?
In my subsequent lecture on Power Cycles and Energy Storage, I calculate that:
U.S. daily energy production is about 11,089 GW-h per day
For effective Grid load leveling, you must temporarily store a major fraction of this
So overwhelming priority will be AFFORDABLE TOTAL STORAGE CAPACITY
I insert the word "affordable" because the total cost is going to be huge
And cost will likely overshadow the battery's energy/volume or energy/weight
Because batteries will just be wired together in huge warehouses
Sitting on the floor or possibly stacked onto shelves
So it will probably be cheaper to buy a larger warehouse or stronger shelves
Than to pay a lot more for exceptionally energy dense batteries
Batteries used / researched for "Grid load leveling:"
As cited in a U.S. National Renewable Energy Lab report
1
- Lead-acid batteries
- Nickel-electrode batteries
- Molten sodium-sulfur modular batteries
- Zinc-bromine, batteries
- Vanadium redox batteries
- Polysulfide-bromide flow batteries
Let's look more closely at a few of these
To at least get a feel for issues, priorities, and the directions of research
1) Advanced Power Electronic Interfaces for Distributed Energy Systems
http://www.nrel.gov/docs/fy08osti/42672.pdf
Topping the list (and use): Old fashioned lead acid batteries
http://en.wikipedia.org/wiki/Lead–acid_battery
PbSO4
Weak
H2SO4
PbSO4
(Discharged)
In their fully discharged state, lead acid batteries are ridiculously simple
Consisting of TWO IDENTICAL lead sulfate plates separated by weak sulfuric acid
However, negative voltage applied to one plate (cathode) drives reaction:
PbSO4 (solid) + H+ + 2 e- => Pb (solid) + HSO4While positive voltage applied to other plate (anode) drives reaction:
PbSO4 (solid) + 2 H2O => PbO2 (solid) + HSO4- + 3 H+ + 2 eSo charging:
Converts one plate into pure lead
Converts other plate to pure lead oxide
Plus strong intervening sulfuric acid
Pb
Strong
H2SO4
(Charged)
PbO2
And discharging (battery use) just reverses both reactions!
Which is about as simple as a battery can get
Further, starting lead sulfate plates are easily manufactured:
By just applying first reaction, in reverse, to single pure lead plates
Thus it's no surprise that lead acid batteries were invented early, in 1859
And now account for almost half of all batteries sold worldwide:
1
They are cheap
They are very dependable and relatively trouble free
But their energy storage per weight or volume is not particularly high:
Energy / mass = 33-42 W-h / kg
1
Energy / volume = 60-110 W-h / liter
And they do use (and require the mining and disposal of) toxic lead
1) http://en.wikipedia.org/wiki/Lead–acid_battery
Leading to research into alternative Grid load-leveling batteries
The previous Secretary of Energy singled out Ion Flow Batteries 1
Which have this strange and complex configuration:
With two different electrolytes circulated in (via pumps) from external tanks
To a central cell with an "ion selective membrane"
Plus simple metal electrodes to either side
1) http://www.nytimes.com/gwire/2010/10/15/15greenwire-doe-promotes-pumped-hydro-as-option-for-renewa-51805.html
Figure from: Electrochemical Energy Storage for Green Grid, Yang et al., Chemical Reviews 111, 3577–3613 (2011)
Zooming in on the center structure of this vanadium ion version:
LEFT SIDE:
VO2+1 ion is pumped in
It reacts with H+ ion
taking electron from electrode
Becoming VO2+2 ion
and releasing water
RIGHT SIDE:
V+2 ion is pumped in
Giving electron to electrode
It is converted ion to V+3
CENTER: H+ consumed on left is replaced by H+ selectively crossing membrane from right
That is, on left (cathode) side:
VO2+1 + 2 H+ + e- => VO+2 + H2O
And on the right (anode) side:
V+2 => V+3 + e-
Note that these electrodes are just acting as simple, dumb, inert, slabs of metal
Then zooming back out:
Battery is completely discharged only when:
VO2+1 originally filling left tank is completely replaced by VO+2 leaving cell
V+2 originally filling right tank is completely replaced by V+3 leaving cell
To recharge: Reverse reactions by forcing electrons FROM left electrode to right
Editorial comment a la James Clerk Maxwell:
Tanks MUST ALSO contain charge-balancing negative ions
or electrostatic forces (charge repulsion) would blow them apart!
Big advantages of such ion flow batteries:
1) Battery capacity is NOT determined by cell size
Capacity is instead determined by simple external storage tanks
Which could be gigantic => Gigantic capacity!
2) Electrodes are not being rebuilt during recharging
Electrodes are instead just static metal plates
Thus no problem with dendrite short circuits between them!
This eliminated almost all common electrode problems:
Including: Limited size, slow surface reactions or diffusion in/out, dendrites . . .
The strategy was to make the solid metal electrodes almost superfluous
Instead transferring almost all of the action to (re-circulating) redox liquids
Another way of doing this is to stick with active electrodes
But to make the electrodes, themselves, liquid
(Which can then easily mix and refresh their redox-able surfaces)
But you must then somehow keep the two electrode liquids from mixing
Because if they did, they'd just swap electrons locally (atom to atom)
And we'd again loose electron flow out though wires (= "electricity")
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
This is done in "molten sodium beta alumina" batteries
Overall structure:
- Central (anode) reservoir of molten sodium (green)
- Membrane capable of passing Na+ ions (gray)
Typically: Al2O3 "beta alumina" ceramic
- Surrounding (cathode) outer cylinder (orange)
Typically: Sulfur / Sodium Sulfide (Na2Sx)
In the central anode:
At the outer cathode:
2 Na => 2 Na+ + 2 e-
x S + 2 Na+ + 2 e- => Na2Sx
With Na+ ions formed in anode migrating through beta alumina toward cathode
These are promoted for long (50 year+) lifetimes / rapid (high power) discharge
Figure: Electrochemical Energy Storage for Green Grid, Yang et al., Chemical Reviews 111, 3577–3613 (2011)
1) http://en.wikipedia.org/wiki/Molten_salt_battery
1
But in recent research wholly liquid batteries have been built:
With three liquids chosen for redox properties AND their mass density
Because goal is to have them naturally segregate into the three layers of:
Anode / Separating Electrolyte / Cathode
Top: Puddle of lighter liquid lithium
(floating at stainless steel rod / in Fe-Ni foam)
Middle: Denser molten salt electrolyte
Bottom: Very dense molten antimony-lead
As reported in: Molten metal batteries aimed at the grid, BBC News – Science & the Environment, 21 September 2014
Based on: Lithium–antimony–lead liquid metal battery for grid-level energy storage, K. Wang et al., Nature 514, p. 348 (2014)
Operation:
In fully charged state, the battery is as described in figure:
But when load is connected, it discharges via:
- First, at top liquid to liquid interface (black/blue):
Li (liquid metal) => Li+ (in molten salt electrolyte) + eLithium ions then diffuse down through that (blue) electrolyte layer
Then, at bottom liquid to liquid interface (blue/red):
Li+ (in molten salt electrolyte) + e- => Li (dissolved in molten Sb–Pb)
With everything just reversing when the battery is recharged
Molten metals?!
Top: Li metal must be above 180°C
Middle: 20% LiCl / 50% LiF / 30% LiI must be above 430°C
Bottom: 18% Pb / 82% Sb must be above 253°C
Molten metal batteries aimed at the grid, BBC News – Science & the Environment, 21 September 2014
Lithium–antimony–lead liquid metal battery for grid-level energy storage, K. Wang et al., Nature 514, p. 348 (2014)
Comparison of Grid load leveling batteries?
Comprehensive comparative data were very hard to find!
Most data instead pertained to batteries targeting transportation
This was the most complete data set I found (from the University del Pais Vasco, Spain):
Ion Flow Batteries
Conventional Batteries
(Pumped Hydro,
Compressed Air)
Lead Acid
Molten sodium
Ion Flow
http://www.sc.ehu.es/sbweb/energias-renovables/temas/almacenamiento/almacenamiento.html
These (and other) batteries target Grid load leveling by:
Providing potentially huge energy storage capacities
Largely via extremely complete and effective use of their redox materials
And given that redox materials are automatically refreshed by mixing/circulation
Battery designs end up being (overall) rather simple
All of which should, at least eventually, make cost per energy-stored small
However:
It's very unlikely that 450°C molten-metal batteries will ever go into your car
And Grid batteries are optimized to charge/discharge on Grid timescales
Grid timescale = Charging over many hours (when energy is too available)
= Discharging over the many hours of peak evening load
So let's move on to:
Batteries for electrified transportation
What will the priorities be for batteries in this application?
And how will these drive us toward battery types different than above?
As consumers we've been rather inflexible
We pretty much demand that transportation batteries be:
Cheap, or at least not much more expensive than the gasoline engine alternative
Long lasting, in terms of both years and number of charge/discharge cycles
Fast, charging in the mere minutes we now spend at gas stations
High capacity, so that we can still drive hundreds of miles
Which translates as into energy storage per volume
Because we'll want that range without batteries filling our back seat/trunk
= ALL the advantages of gas engines, omitting ALL disadvantages!
But there is one way in which we'll break with gasoline-powered vehicles:
When we brake, kinetic energy created by engine is now wasted as heat
Whereas, in electric car, when we brake, electric motors become electric generators
So most of that kinetic energy can be "regenerated" (put back into battery)
This includes the kinetic energy of our now possibly very heavy batteries
Thus energy storage per mass may be become relatively unimportant
Leading transportation battery candidates?
The early choice was once again lead acid batteries
Followed by nickel metal hydride batteries
Then lithium ion batteries (also our favorite for electronics)
And more recently by lithium air batteries
I’ve already covered the first two batteries on that transportation list:
Lead acid batteries were detailed in my section on battery basics
Nickel metal hydride batteries are an alkaline battery (described in same section)
Reactions in a common household alkaline battery (Zn/MnO2):
Anode being oxidized: Zn (solid) + 2 OH−  ZnO (solid) + H2O + 2e−
Cathode losing oxygen: 2 MnO2(solid) + H2O + 2e−  Mn2O3(solid) + 2 OH−
Versus analogous reactions in a nickel metal hydride battery (Ni(OH)2/Metal):
Anode being oxidized: Ni(OH)2 (solid) + OH−  NiO(OH) (solid) + H2O + e−
Cathode losing oxygen: M(solid) + H2O + e−  MH (solid) + OH−
With metal, M, a mixture of La, Ce, Nd, Pr, Co, Mn, Al, V, Zr or Ni 1
Both employing an alkaline (basic) potassium hydroxide electrolyte
1) http://en.wikipedia.org/wiki/Nickel–metal_hydride_battery
Which brings us to lithium ion batteries:
These, once again, use a Column I / Group I / alkali metal: Li, Na, K, Rb, Cs, Fr
Alkali metals hold onto their electrons VERY loosely (= least electronegative)
Highest voltage batteries pair alkali metals
with highly electronegative atoms or compounds
http://www.chemistry-reference.com/pdictable/
But that is a double edged sword:
At right of the periodic table, oxygen is one of the most electronegative elements
Alkali metal reacting with oxygen (or water) maximizes energy of electron transfer
RESULT:
Alkali metals can burn spontaneously in contact with air
Some even explode violently if dropped in water
Thus Li in lithium batteries is generally provided as a compound, e.g.: LiCoO2
Which is used as the battery's cathode:
Ionization/decomposition:
n LiCoO2 = (n-m) Li + n CoO2 + m Li+ + m e-
For anode, carbon or silicon are the first choices (for reasons I'll soon explain):
Ionization/decomposition:
LimC6 (solid) = 6 C (solid) + m Li+ + m e-
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
Yielding Li ion battery structure and behavior:
During CHARGING, Li is actually transferred from inside cathode to inside anode
-
Anode:
Li absorbing
and deionizing
+
+
Cathode:
Li dissolving
and ionizing
+
DISCHARGE reverses this: Li transferred from inside the anode to inside cathode:
-
Anode:
Li desorbing
and ionizing
+
+
+
Cathode:
Li absorbing
and
deionizing
Look more closely at action during charging of the Li ion battery:
Cathode starts out as a naturally layered chemical compound LiCoO2 :
During charging Li must diffuse out:
+
+
But anode starts out as pure solid carbon or silicon:
But then, during charging,
how the heck does Li get inside!
+
+
It's made possible by uniquely accommodating crystal structures of C / Si
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
Carbon and silicon are column IV neighbors in the periodic table:
They thus share the tetrahedrally bonded "diamond" crystal structure:
Diamond Carbon (bond length = 0.154 nm )
Silicon (bond length = 0.235 nm)
"Diamond" type of crystal structure has a lot of open space
Li is small enough to squeeze into spaces between between Si atoms
But Li is less likely to squeeze into spaces between closely spaced C atoms
And such true diamond electrodes would be hopelessly expensive anyway!
Figures from my "UVA Virtual Lab" website: www.virlab.virginia.edu/VL/Semiconductor_crystals.htm
Fortunately, carbon has a second possible crystal structure:
Carbon in its alternate (also cheaper) "graphite" structure, with stacked planes:
Li CAN slide between these "graphitic" carbon planes
But in some ways, Si anodes are still more attractive
Because silicon crystal growth was perfected by the microelectronics industry
And crystals are now available in huge sizes (30 cm dia. x meters long)
With precut and fully polished wafers costing only ten's of dollars
Figures from my "UVA Virtual Lab" website: www.virlab.virginia.edu/VL/Nanocarbon.htm
But there is still a problem (or challenge) for Si anodes:
To increase Li battery capacity we cram as much Li into the anode as possible
But when a LOT of Li slithers into the spaces, the Si crystal actually expands
With enough added Li, silicon expands by 2-3 times, actually changing its structure:
Top from my "UVA Virtual Lab" website: www.virlab.virginia.edu/VL/Semiconductor_crystals.htm
Bottom: http://www.greencarcongress.com/2014/02/20140204-nmr.html
But when Li ion battery discharges, silicon anode shrinks and reorders:
Or at least it may do that the first few (or few hundred) times
But during charging it's likely that Li is not added uniformly to the Si
And during discharging it's likely that Li is not removed uniformly
Resulting in non-uniform expansion and contraction of the silicon =>
Huge non-uniform stress across the crystal =>
Eventual development of cracks and fractures
With these cracks/fractures, as silicon shrinks upon battery discharge:
Si pieces separate =>
Electrical contact between pieces is lost =>
Shrinking effective anode size & capacity
A solution is provided by nanoscale self-assembly:
On Si wafer, lay down nanopattern of metal, heat to melt, then expose to SiH4 vapor:
Si
<= SiH4 vapor approaching one of a vast array
of now molten metal dots
SiH4 decomposes, releasing Si to dissolve into the molten metal dot
Si
Si diffuses down to wafer where it solidifies creating a growing column of new Si:
Si
For details see my Nano class lecture note set: The Need for Self-Assembly
Resulting in tight arrays of Silicon nanowires:
Lorelle Mansfield -NIST:
http://www.nist.gov/public_affairs/techbeat/tb2006_0525.htm
U. Helsinki:
www.micronova.fi/units/ntq/research/nanowires.php
Small size / accessibility => Even Li absorption, even stress, minimal Si cracking:
New nano-structured
Li ion battery anodes
Designing nanostructured Si anodes for high energy lithium batteries, Wu & Cui, Nano Today 7, pp 414-29 (2012)
But what about well publicized Li ion battery fires?
Are Li+ batteries particularly prone to fires, and if so, why?
According to the American Chemical Society (ACS), their fires are extremely rare:
"Failure rates for rechargeable Li-ion batteries are on the order of one in 10 million cells
That’s not a reliability problem. It’s an exception" (1)
Further, during charge/discharge, metal ions slither inside the electrodes:
-
+
+
+
+
Which should eliminate dendrite short-circuits seen in other types of batteries
1) http://cen.acs.org/articles/91/i6/Assessing-Safety-Lithium-Ion-Batteries.html
However, if a (rare) short-circuit DOES occur:
Li+ batteries pack an exceptionally large amount of energy per mass:
(1)
Li ion batteries
More conventional batteries
AND, according the the American Chemical Society:
(2)
"Unlike other common types of batteries, in which the electrolytes consist of aqueous
solutions of acid or base, the electrolyte in Li-ion cells typically consists of lithium salts in
flammable organic solvents such as ethylene carbonate and ethyl methyl carbonate."
So if there IS a short-circuit, the battery itself is unusually likely to burn
1) http://www.sc.ehu.es/sbweb/energias-renovables/temas/almacenamiento/almacenamiento.html
2) http://cen.acs.org/articles/91/i6/Assessing-Safety-Lithium-Ion-Batteries.html
Bringing us to a related possibility: Lithium Air Batteries
Which actually do breathe, using oxygen as one of the redox partners:
Very complex triple layer electrolyte (organic => solid => liquid)
Extending from anode at left, rightward halfway through gaps in cathode
Anode reaction (left): Li (metal) => Li+ (in three layered electrolyte) + eCathode reaction (right): Li+ + e- + O2 => LiO2
Via O2 drawn from air (into OH-)!
With more Li+ then added via: LiO2 + Li+ + e- => Li2O2
http://www.aist.go.jp/aist_e/latest_research/2009/20090727/20090727.html
Passage of air & interaction with porous carbon is actually more like this:
Publicity figure from IBM battery project targeting electric vehicle with 500 mile range:
http://www.extremetech.com/computing/126745-ibm-creates-breathing-high-density-light-weight-lithium-air-battery
Energy/power densities for these new types of transportation batteries?
According to company that effectively pioneered this field with their Prius:
Standard Li ion (solid + liquid)
<= Higher Power for Hybrid Gas/Electrics
Higher Capacity for Plug in Electrics =>
Li ion air (solid + liquid + air)
http://www.toyota-global.com/innovation/environmental_technology/next_generation_secondary_batteries.html
Despite the fact that
Neither of the above discussions were anywhere near complete
Let's move on to the topic of fuel cells
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
Or, more specifically to hydrogen / water fuel cells:
As depicted by the U.S. National Renewable Energy Research Lab:
NREL: Advanced Power electronic Interfaces for Distributed Energy Systems, p. 35
http://www.nrel.gov/docs/fy08osti/42672.pdf
Showing this more schematically:
Incoming H2 gas
H
O
H
H+
H
H+
Outgoing water
Incoming O2 gas (in air)
O
H
O
H+
H
H
H
H
O
H+
High surface area
(or nano-porous) catalyst
H+
O
O
O
High surface area
(or nano-porous) catalyst
Electrolyte capable of passing H+ ions:
Aqueous OR Solid Solution OR Proton permeable membrane
This is VERY SIMILAR to earlier ion flow batteries with:
Inert metal electrodes + redox species pumped in from external tanks
DIFFERENCE: These gasses don't naturally disassociate / associate and thus:
Electrodes must act as catalysts promoting disassociation / association
Platinum, the chemists' favorite catalyst = classic fuel cell electrode
But platinum is a very expensive noble metal:
36,667 $/kg (27 March 2015)
However, catalysis occurs only on surfaces:
And surface area increases as something is ground into a powder
Thus early fuel cells used electrodes of slightly compacted platinum powder
Hence my reference to use of "porous" metal in the above schematic
But for same quantity of Pt, smaller powder particles => more total surface area
So use tiny minimally compacted particles (so don't squash back together)
OR nano Pt particles on surface of some other (cheaper) porous nano material
Fuel cells can also be charged and thus run backwards
Charging:
H2O => H2 + O2
H2O
Discharging: H2 + O2 (in air) =>
Further, as in the flow batteries, this occurs without the need to:
Deposit atoms back on surfaces of electrodes (worrying about dendrites)
OR diffusing atoms into or out of the insides of an electrode
So this can be done very quickly and with ~ no degradation of fuel cell
Finally: Air is readily available / Output of water vapor is no problem
Making H2 fuel cells great potential battery supplement / replacement
At least if we can (radically?) improve both cost and energy conversion efficiency:
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
Current hydrogen fuel cell efficiencies:
But these 40-60% energy conversion efficiencies are still discouraging!
Energy recovery efficiencies for other technologies:
FC = Hydrogen Fuel Cell
NaS = Molten Sodium Battery
VR = Vanadium Redox Flow Battery
CAES = Compressed Air Energy Storage
U.S. National Renewable Energy Lab: "Hydrogen Energy Storage Overview"
http://www.nrel.gov/hydrogen/pdfs/48360.pdf
Credits / Acknowledgements
Some materials used in this class were developed under a National Science Foundation "Research
Initiation Grant in Engineering Education" (RIGEE).
Other materials, including the "UVA Virtual Lab" science education website, were developed under even
earlier NSF "Course, Curriculum and Laboratory Improvement" (CCLI) and "Nanoscience Undergraduate
Education" (NUE) awards.
This set of notes was authored by John C. Bean who also created all figures not explicitly credited above.
Copyright John C. Bean (2015)
(However, permission is granted for use by individual instructors in non-profit academic institutions)
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm