Batteries and Energy Storage

Batteries and Energy Storage

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Description: In 1745, scientists (Musschenbroek and Cunaeus) noticed that one could “charge” up a glass filled with water and get a shock by touching a metal nail Shortly thereafter, this was simplified to just metal foil wrapped around the inside and outside of a jar with a chain connecting the inner layer. We know these devices as capacitors, but they work by storing charge ELECTROSTATICALLY. Although they still didn’t know all that much about electricity, they now had methods of storing and generating electricity, but it was still a research tool (and a parlor trick).

 
Author: Craig B. Arnold  | Visits: 177 | Page Views: 503
Domain:  Green Tech Category: Battery & Fuel Cell 
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Contents:
Batteries and Energy Storage
Craig B. Arnold
Department of Mechanical and
Aerospace Engineering

How do we make and store electricity ?
Today, we can convert energy
from many different forms into
usable electricity:

•Coal
•Wood
•Nuclear fission
•Oil
•Water
•Solar
•Wind

But the main problem for electricity
is how we store and transport it from
the generation source
•Transmission wires
•Batteries

But how did we get here ?
For today, we’ll focus on batteries for portable energy storage
In ancient times, the generation of electricity was purely
accidental.
By rubbing certain materials together, static charges
can be accumulated
•Drag feet on carpet
•Pet a cat
•Take off a sweater
Ancient Greeks rubbed amber on fur to generate electricity. In
fact, the word elektron comes from the greek word for amber
By the mid 1600’s, static electricity could be readily generated by
rubbing insulating materials together: fur/cloth, sulfur, amber, etc.
We still use this method today: Van de Graff generators

But in the 1600’s, scientists did not really know much about
electricity or how to use it. The spark generators were mostly
used by scientists to study the nature of the sparks
In 1745, scientists (Musschenbroek and
Cunaeus) noticed that one could “charge” up a
glass filled with water and get a shock by
touching a metal nail
Shortly thereafter, this was simplified to just
metal foil wrapped around the inside and
outside of a jar with a chain connecting the
inner layer.

Leyden Jar
We know these devices as capacitors, but they work
by storing charge ELECTROSTATICALLY

Although they still didn’t know all that much about electricity, they
now had methods of storing and generating electricity, but it was
still a research tool (and a parlor trick)
In fact, this enabled many important experiments of the time
1746: Nollet assembled a line of 200 monks each holding the
end of a wire to test if electricity can travel faster than human
communication. Without warning he connected a Leyden Jar to
the ends …
1747-1753 Cavendish used Leyden Jars to discover many of the
fundamental physics laws of electricity
Inverse square law for force, electric potential,
capacitance, resistance
But Cavendish did not publish all that much and these discoveries
were rediscovered years later by Faraday, Ohm, Coulomb, Maxwell

1752: Ben Franklin and his famous kite
experiment
Showed that lightening is the same as
electricity stored in Leyden Jar
Franklin’s other main contributions to the field include the
concept of current as the flow of positive charges, and the term
battery
We later found out he was very wrong, but unfortunately it was
too late. This is why current goes in the opposite direction of
electron flow
Two disadvantages of the Leyden Jar are that it doesn’t store
charge all that long (This is true in general for electrostatic
storage) and it doesn’t store all that much energy

1786: Galvani’s famous experiments on frog legs
He took two dissimilar metals (Zn, Cu) and touched them to the
ends of a dead frog’s leg
Surprisingly, the leg moved and Galvani attributed this to bioelectricity

But Volta did not believe that the electricity came from the frog.
He believed the electricity came from the metals
In 1799, he showed that by combining different metals that are
separated by a salt or acidic solution it was possible to generate
electricity
Side note: Galvani died one year
VOLTA PILE
First commercially
available battery

An enabling technology for the telegraph

earlier and never knowing the
answer

How does all of this work ?

It’s all electrochemistry!!

Chemical Energy is Converted to Electrical Energy Through
Oxidation and Reduction reactions
• Voltaic(Galvanic)
•Spontaneous
•Generates energy
• Electrolytic
•Non-spontaneous
•Needs energy to occur

But, if reactions are spontaneous …

… how do we harness them?

Batteries, Fuel Cells, corrosion
Electrodeposition, electrowinning, polishing

Voltaic or Galvanic Cells
Separate reactions into half-cells

Oxidation occurs at anode

Reduction occurs
at cathode

Material gives up
electrons

Material takes in
electrons

i.e. Zn

Zn+2 + 2 e-

i.e. Cu+2 + 2e-

Salt bridge: allows ions to move between cells

Cu

Electrochemistry: chemical reactions require charge transfer
This occurs through redox reactions
Oxidation

M

Reduction

Mn+ + e-

Mn+ + neM(n-1)+

Oxidation is loss of electrons
Ions dissolve into solution
Reduction gains electrons
Ions deposited from solution

Oxidation occurs at the anode anodic reaction
Reduction occurs at the cathode cathodic reaction
These are called Half-Cell reactions
Both need to happen… electron is generated at the anode and
must be consumed at the cathode so net charge is conserved in
overall process
We need a salt bridge to complete the circuit so that the
charge remains balanced otherwise, the charge would build
up and the battery would stop working

In the process of oxidation and reduction, energy is
converted from chemical into electrical
i.e. Electrons are free to run through the circuit and do work
But they must go through the external wires

In previous example,
Zn can lower its energy by giving up electrons and dissolving into
solution
Cu+ can lower its energy by capturing electron and ‘plating’ out on
electrode

Voltage of the cell is determined by the oxidized and reduced
species and related to the change in free energy
Zn Zn+2 + 2 e- +0.763 V
Cu+2 + 2e- Cu +0.337 V
1.10 V

Can think of this as analogous to
water flowing downhill where the
voltage is the height of the hill

The important thing here is that every material has a
slightly different potential

Electrochemical Energy Storage
Batteries are a compact method of producing a voltaic cell
Other methods, fuel cell, photovoltaic cell, electrochemical capacitors etc.
e-

Primary: Non-rechargeable
Secondary: rechargeable
Voltage
Potential difference
between anode and cathode.
Related to energy of reactions
Capacity amount of charge stored
(usually given per unit mass or
volume)
Fuel Cell PEC Cell
TiO2
Anode H2
Cathode O2
Metal

e

Current
Collectors

e-

-

Anode
Electrolyte/Separator
Cathode
eee-

Anode (Oxidation):
Zn + 2 OHZn(OH)2 + 2e-

E = 1.25 V

Cathode (Reduction):
Ag2O + H2O + 2e2 Ag + 2 OH-

E = 0.34 V

So this raises a potential problem with the Volta Pile
If Zn is the anode and H is the cathode, there is an evolution of
hydrogen gas that could passivate the electrode or generate a
large resistance to flow
Solution is to use a wet cell that has the proper ions around

(1836) Daniel Cell: Cu/Zn in sulfate
solutions
(1859) Plante: Pb/PbSO4 cell

Wet cells have an obvious disadvantage
for transport, but they do work well

But actually, a wet cell battery may have existed well before Volta
Archeologists found this clay pot in the Baghdad area in the 1930’s
Carbon dating places it ~250 BC

How does this work?

Leclanche in 1866 developed another kind of wet cell that had a
better shelf life and was less reactive with the environment
This battery was constructed with Zn
as the anode and MnO2 + C as the
cathode
The cathode was mixed into a paste
and placed in a porous pot
The Zn anode was immersed in a
chloride electrolyte

Very popular with the new Telegraph !!

Other variations came about as well
Different materials: Ni/Cd (Junger) 1899
Different electrolyte: Alkaline (Edison) 1911, “dry” cell
(Gassner) 1888
They all pretty much looked about the same, but some were
better than others
•Lifetime
•Capacity
•Marketing
•Convenience
•Rechargeability

Edison Cell
Ni/Fe in
KOH
electrolyte

Edison was actually trying to make a battery
for the Automobile, but gas engines were
too good

Modern Batteries

Modern Button Cell

Battery Types
Battery chemistry has not changed much since 1800’s
Leclanché
Alkaline
Silver-Oxide
Rechargeable Alkaline

1866, 1888
1949
1950
1978

Zn-MnO2
Zn-MnO2
Zn-Ag2O
Zn-MnO2

Lead-acid

1859

Pb-PbSO4

Ni- based
NiCad
Metal Hydride

1899
1990

Cd-NiOOH
MH-NiOOH

Li- based
Lithium-Iodine
Lithium ion
Plastic/Polymer

1968
1991
1995

Li-I2
Li-LiCoO2
Li-LiCoO2

How does a Li-ion battery work?
Li ions intercalate into the crystal structure of the electrode materials
But Li is very reactive (high V) which causes side reactions and
passivation layers (solid electrolyte interphase)
Critical to the proper functioning of these cells
More advanced topic

So what makes a battery rechargeable ?
In reality, as long as the electrochemical reaction is
reversible, the battery should be rechargeable
However, other effects are important
•Decay of structural properties as ions move in and out of electrodes
•Growth of metal on electrodes
•Decay/contamination of electrolyte
•Cost
•Power signature

Lead-acid works well

Impedance Matching
So faced with all these choices, how do we choose an
appropriate power source?
The optimal power transfer to the load will occur when the
impedance (resistance) is equal to the internal resistance
Can derive this from Ohm’s laws
However, we would not typically run a battery at this
current as it would heat up too much

Ragone relation
Energy density
Specific Energy

Energy per unit area/volume
Energy per unit mass

In general, electrochemical systems always
show the characteristically downward
curved plots
High power
High energy

Low energy
Low power

One might think that it is best to just
push the limits of energy or power
http://www.powerstream.comz/ragone.gif

Other issues such as total weight, size, voltage, environmental
concerns can limit our selection process

Rate effects
Another major factor in optimizing energy storage is the rate at which the energy is needed

Duracell ‘D’ cell

As with all batteries, there is a decrease in capacity (efficiency) as the power is increased
Main limiting factor is rate of ion movement across electrolyte

Energy demands differ from application to application
But many application for batteries do not require extended periods of high power draw

Rate effects
Although standard specs can be reported, there are a variety of important issues that
ultimately affect the lifetime and performance of a battery system for a desired application
Current Drain: Different batteries respond differently to current
In general, as current is increased, the available voltage and capacity decrease

Peukert’s equation:
In x t = C

I is current in A
t is time in hr
C is rated capacity

But since C depends on rate, one must correct for this

Modified
Peukert’s law

t=

H
⎛ IH ⎞


⎝ C ⎠

n

Where H is the
hours for the
rated capacity
Handbook of Batteries 3e, Eds Linden and Reddy

This is a significant issue for small batteries being used to power
transmitters and it is non-linear

Discharge Characteristics
Different chemistries will have different voltages and different characteristic discharge curves
Appropriate choice will depend on application limitations

Cut-off voltage of the device is an
important parameter as it will
determine the actual capacity that can
be used

Handbook of Batteries 3e, Eds Linden and Reddy

For instance, Silver Oxide batteries have a very flat discharge at 1.5 volts
compared to Li-ion batteries with a sloping curve around 3 V

Pulsed Discharges
Batteries operated in pulsed applications will last longer than constant discharge at same current
During rest time, battery recovers voltage
more of the theoretical capacity can be used
Pulsing between high and low current (e.g.
transmit/receive operations) will have a similar
effect
voltage will oscillate
Handbook of Batteries 3e, Eds Linden and Reddy

Voltage response to pulse
will vary with chemistry
High current pulsed can
lead to catastrophic failure

Alkaline

Zn-C

Factors such as maximum current and duty cycle will have
a profound effect on this issue

Other Considerations
We mentioned

•Current drain
•Shape of discharge curve
•Pulsed operation

Other considerations include:
•Effects of temperature
•Discharge mode (constant power, current, load)
•Self discharge
•Depth of discharge (for secondary)

Handbook of Batteries 3e, Eds Linden and Reddy

Supercapacitors
Electronically conductive
electrodes

Capacitors

Electrode
Electrolyte
Electrode

Ionic conduction in liquid
or solid-state electrolyte

Electrochemical capacitors
Supercapacitors
pseudocapacitors

Batteries

ultracapacitors
An ultracapacitor has properties of both battery and capacitor
It has a high power density and can be cycled like a capacitor
But it also has a significant energy density like a battery
First ultracapacitor was patented in 1957 using porous carbon electrodes

Different Flavors: carbon, transition metal oxides
Double Layer: Charge stored in double layer at interface between electrolyte and electrode
Pseudocapacitance: Faradaic charge storage at electrode surface

Require high surface area electrodes to achieve large capacitance

Supercapacitors
Supercapacitors have certain advantages over traditional batteries for tracking applications
Pros

Cons

•Greater cycle life
•Better reversibility low capacity fade
•Higher power
•Rapid charging discharging

•Lower energy density
•Shorter shelf life
•Higher self-discharge
•Cost

Red line shows
results from our work

One could envision a supercapacitor for transmitting information, but one must take care
not to have too large a duty cycle or it will not sufficiently recharge

Limitations
Why can’t we make a super battery that solves all the problems?
Limited energy density: There are only so many coulombs of charge we can store in the system
For instance, Li Li+ + eThis means for every mole of Li, I get 1 mole of electrons

3.86 Ah/g

Limited conductivities: ions must move from one place to another
Material

Specific Energy (Wh/kg)

Hydrogen

> 30,000

Gasoline

12,200

Methanol

6400

Primary Li battery

2190 (600)

However, that is not the whole story

Secondary Li battery

635 (180)

Zn-air

1300 (370)

•Mass/volume of entire system
•Economics of production

Alkaline battery

360 (150)

Pb-acid

250 (35)

Compressed air

35

Based on energy density of ‘fuel’
Combustion fuels win out

The “best” energy device
depends on the applications

Materials Challenges
Reactions occur at the electrode surfaces
Need to have ions and electrons together for reactions to occur

We want to get as high a surface area as possible
However

Electrons must still be able to get outside the cell
Electron resistance cannot be too high
Energy of the reactions also depend on the surface properties
e.g. Nanoparticles behave differently than bulk materials
Separator must be robust and allow rapid transfer of ions

Fundamental materials properties need to be understood

Basic materials properties
Transport in porous materials
Interfacial properties