NiMH Batteries for Energy Storage
Applications
John J.C. Kopera
Cobasys
Who Is Cobasys?
An independently managed 50-50 joint venture to further develop and advance the
commercialization of Ovonic Nickel Metal Hydride battery energy storage systems.
ChevronTexaco Technology
Ventures
50%
Ovonic Battery Company
50%
2
Batteries for Power and Energy
•
Nickel Metal Hydride (NiMH) batteries have been in use for many
years in consumer and automotive applications due to their superior
energy density and environmentally friendly attributes.
•
Cobasys is applying the maintenance free, long life attributes of
Ovonic NiMH to batteries for stationary energy storage applications.
¾ Spring-boarding off of the technology portfolio of Ovonic Battery
Company, the inventor of the practical NiMH battery
•
The NiMH battery chemistry has many attributes that make it ideally
suited to a variety of stationary energy storage applications.
¾ Energy density up to three times that of lead acid alternatives
¾ Higher energy, longer reserve times and more power in less volume
and weight
¾ Inherently long cycle life and little to no maintenance requirements
3
Secondary Battery Technology Comparison
Battery
System
Negative
Electrode
Positive
Electrode
Electrolyte
Lead-Acid
Pb
PbO2
H2SO4
Nickel Iron
Fe
NiOOH
Nickel
Cadmium
Cd
Nickel
Metal
Hydride
Nominal
Voltage
(V)
Theoretical
Specific
Energy
(Wh/kg)
Practical
Specific
Energy
(Wh/kg)
Practical
Energy
Density
(Wh/L)
Major Issues
2
252
35
70
Heavy, Low Cycle Life, Toxic
Materials
KOH
1.2
313
45
60
Heavy, High Maintenance
NiOOH
KOH
1.2
244
50
75
Toxic materials, maintenance,
cost
H
(as MH)
NiOOH
KOH
1.2
278 – 800
(depends on MH)
70
170
Cost
Nickel Zinc
Zn
NiOOH
KOH
1.6
372
60
120
Low cycle life
Silver Zinc
Zn
AgO
KOH
1.9
524
100
180
Very expensive, limited life
Zinc Air
Zn
O2
KOH
1.1
1320
110
80
Low Power, limited cycle life,
bulky
Zinc
Bromine
Zn
Bromine
Complex
ZnBr2
1.6
450
70
60
Low Power, hazardous
components, bulky
Lithium
Ion
Li
LixCoO2
PC or DMC w/
LiPF6
4
766
120
200
Sodium
Sulfur
Na
S
Beta Alumina
2
792
100
>150
High Temperature Battery,
Safety, Low Power Electrolyte
Sodium
Nickel
Chloride
Na
NiCl2
Beta Alumina
2.5
787
90
>150
High Temperature Battery,
Low Power Electrolyte
Safety Issues, Calendar Life,
Cost
4
NiMH Batteries
•
The NiMH battery is termed an alkaline storage battery due to the
use of potassium hydroxide (KOH) as the electrolyte.
•
Electrically, NiMH batteries are very similar to nickel cadmium
batteries.
•
Rechargeable alkaline storage batteries are a dominant factor in
the secondary battery market for several technically important
reasons:
¾
High electrolyte conductivity allows for high power
applications
¾
The battery system can be sealed, minimizing maintenance
and leakage issues
¾
Operation is possible over a wide temperature range
¾
Long life characteristics offset higher initial cost as
compared to some other technologies
¾
Higher energy density and lower cost per watt or watt-hour
(depending on design)
•
Cobasys NiMH batteries have been developed to respond to a
number of specific market requirements:
¾
Recyclability
¾
High power
¾
High energy density
¾
Long life
5
NiMH Batteries
•
Electrolyte
¾
An aqueous solution of potassium hydroxide
ƒ Very high conductivity and does not enter into the cell reaction to any significant extent
¾
¾
•
Active materials
¾
¾
¾
•
The electrolyte concentration (and therefore a major component of cell resistance) remains
fairly constant over the entire range of state of charge or discharge
These factors lead to a battery with high power performance and long cycle life
Composed of metal compounds or metallic oxides which (in a charged state) are relatively
good conductors
The nickel oxide – hydroxide electrode only exchanges a proton in the charge-discharge
reaction and the electron transfer is very rapid contributing to high power capacity
Very good mechanical electrode stability and thus longer cycle life
NiMH batteries can be fabricated in virtually any size from tens of milliampere hours
to hundreds of ampere hours or more
¾
¾
¾
Due to the compatibility of steel with the KOH electrolyte, the batteries can be
manufactured in steel cans which are very rugged and exhibit good thermal performance
Large energy storage systems for a variety of applications been successfully applied using
NiMH technology
This versatility is constantly leading to new applications for NiMH batteries where
performance and environmental factors are of utmost importance
6
Positive Electrode
•
The positive electrode of the NiMH battery is nickel hydroxide
¾
¾
¾
¾
Very well developed electrode material with decades of history and development
Nickel based alkaline batteries are attractive since the nickel electrode can be
fabricated with very large surface areas which lead to high capacities and high
current densities
The electrolyte does not enter into the electrode reaction so that conductivity stays at
a high level throughout the usable capacity of the battery
The nickel active material is insoluble in the Potassium Hydroxide (KOH) electrolyte
ƒ Leads to longer life and better abuse tolerance
¾
Only a proton is involved in the charge/discharge reaction
ƒ Means very small density changes and improved mechanical stability of the electrode
during cycling
¾
•
The gravimetric and volumetric energy densities are very good for the nickel
electrode
The simplified nickel electrode reaction in the cell is:
Ni(OH )
2
+ OH
−
Ch
arg

e →
←


Disch arg e
NiOOH
−
+ H 2O + e
7
Negative Electrode
•
The active material for the negative electrode in the NiMH
battery is hydrogen
¾
¾
¾
•
The hydrogen ions (protons) are stored in the metal hydride structure
which also serves as the electrode
The metal hydride can, depending on its composition, hold between 1%
and 7% hydrogen by weight
ƒ As a hydrogen storage material, the metal hydride is very efficient,
achieving better volumetric efficiency than liquid hydrogen
Today’s practical materials for NiMH batteries hold between 1% and 2%
by weight hydrogen
Many elemental metal hydride materials existed but were not
practical for battery applications
¾
High equilibrium pressure was exhibited by these materials at room
temperature
8
Negative Electrode
•
•
Multi-component alloys composed of disordered (amorphous) materials were
developed that combined strong and weak hydride forming materials
¾ Tailoring the metal hydrides for the desired equilibrium pressure and other
chemical properties was achieved by adjusting the ratio between these two
types of material components
¾ The compounds are divided into groups classified by AxBy based on their
composition and crystal structure
¾ The A and B components can each consist of a number of different elements in
varying ranges of stoichiometry
¾ The variation of the components of the metal hydride allows the design of
materials with the desired characteristics for use in battery applications such as
low equilibrium pressure, resistance to corrosion, mechanical stability,
reversibility, hydrogen storage ability, etc.
Cobasys NiMH batteries use a specifically developed metal hydride alloy for the
negative electrode. The reactions for the negative electrode can be written as:
M
+ H 2O + e
−
Ch
arg

e →
←


Disch arg e
MH
+ OH
−
M represents the metal hydride material.
9
The NiMH Cell
•
The complete cell is
represented schematically
as shown
•
The combined whole cell
reaction can be written as:
M + Ni(OH ) + H O
2
2
Ch
arg

e →
←


Disch arg e
MH + ( NiOOH • H O )
2
10
Charge and Discharge Characteristics
NiMH Charge Discharge Characteristic
1.7
1.5
Cell Voltage
1.3
1.1
0.9
0.7
0.5
0
10
20
30
40
50
60
70
80
90
100
State of Charge
11
Over-Charge and Over-Discharge
•
The NiMH battery has inherent electrochemical recombination processes which enable the battery
to tolerate the abuse of Over-Charge and Over-Discharge.
¾
•
The Oxygen Cycle and the Hydrogen Cycle
The oxygen cycle functions as follows on overcharge:
4 ⋅ OH − ⇒ 2 ⋅ H 2O + O2 + 4 ⋅ e −
¾
For the positive electrode:
¾
−
−
For the negative electrode: 2 ⋅ H 2O + O2 + 4 ⋅ e ⇒ 4 ⋅ OH
Net is no reaction, only heat generation equal to the energy input and an increase in cell pressure.
•
The hydrogen cycle functions as follows on over-discharge:
2 ⋅ H 2O + 2 ⋅ e − ⇒ H 2 + 2 ⋅ OH −
¾
For the positive electrode:
¾
−
−
For the negative electrode: H 2 + 2 ⋅ OH ⇒ 2 ⋅ H 2O + 2 ⋅ e
Net result is no reaction but heat and pressure are generated in the cell.
•
In an extreme case of overcharge the cell will become pressurized enough to cause the safety
vent to open and release the excess pressure, thus avoiding the danger of cell rupture.
12
•
•
The NiMH batteries, like all batteries, have a
certain level of self discharge that occurs
when the battery is at rest.
¾
The self discharge characteristic
typical of an available battery module
is illustrated to the right
Several factors contribute to self discharge:
¾
The contribution to self discharge
from the oxygen cycle is negligible
below about 70% state of charge
¾
Longer term contributions to self
discharge are caused by chemical ion
shuttles which continuously discharge
the cell over longer periods of time
¾
The rate of the self discharge is highly
dependent on the temperature of the
cell
ƒ
•
PERCENT CAPACITY
RETAINED
Self Discharge
110%
100%
90%
80%
70%
60%
50%
40%
0
48
96
144
192
240
STAND TIME, HOURS
-1.1°C (30°F)
21.1°C (70°F)
37.8°C (100°F)
Higher temperatures yield higher self
discharge rates
Reduction of self discharge is constantly
being improved by continuing development
of the technology and materials.
13
Characteristics
•
Advantages
¾
¾
¾
¾
¾
¾
¾
¾
•
•
Applications
Energy density (3+ times PbA)
High rate capability
Good thermal performance
Non toxic materials
Battery capacity is largely
independent of discharge rate
No hydrogen evolution on overcharge
Long life
Recyclable
Disadvantages
¾
¾
¾
¾
Starting Lighting Ignition (SLI)
ƒ
ƒ
ƒ
¾
¾
¾
¾
¾
¾
¾
Automotive
Aerospace
Military
Telecom
UPS
Switchgear
Motive Power
Electric and Hybrid-Electric Vehicles
Energy Storage
Portable Applications
Higher first cost than PbA
Higher self discharge than PbA and
some other technologies
New technology in industrial markets
A
Pb
MH
i
N
Size Comparison of PbA and NiMH batteries of comparable energy
14
Standby Applications
•
Cobasys develops a variety of
stationary energy storage
solutions:
¾ Photovoltaic energy storage
installations
¾ Telecom
¾ UPS
¾ Switchgear
¾ Distributed Generation
Example of UPS Battery System
15
Battery Module Building Blocks
SERIES 1000
•
•
•
•
8.8 Ah 12 V High Power Module Design
Liquid/Air* Cooled Package
Configurable as cassettes up to 48 V
Designed for Light duty vehicle to large SUV needs
*Air cooling for lower power applications.
SERIES 4500
•
•
•
•
43 Ah High Power Cell Design
Liquid Cooled Package
12 V Monoblock Module
HEV and EV applications, such as transit buses / military
SERIES 9500
•
•
•
•
•
85 Ah 12 V Cell Design
Air Cooled Package
Ultra long full depth cycle life
Configurable Module Capacity by Paralleling Cells
Heavy Duty HEV, EV and all stationary applications
16
Stationary System Products
Telecommunications
Distributed Generation
Uninterruptible Power Supply (UPS)
17
Transportation System Products
NiMHax 42 Volt System
NiMHax EV Solution
for Municipal Busses
NiMHax HEV Solutions for Small & Heavy Applications
18