EVS24
Stavanger, Norway, May 13 - 16, 2009
Performance Characteristics of Lithium- ion Batteries of Various Chemistries for Plug- in Hybrid Vehicles
Andrew Burke, Marshall Miller
University of California- Davis, Institute of Transportation Studies, California 95616 afburke@ ucdavis. edu
Abstract
This paper is concerned with the testing and evaluation of various battery chemistries for use in PHEVs. Test data are presented for lithium- ion cells and modules utilizing nickel cobalt, iron phosphate, and lithium titanate oxide in the electrodes. The energy density of cells using NiCo ( nickelate) in the positive electrode have the highest energy density being in the range of 100- 170 Wh/ kg. Cells using iron phosphate in the positive have energy density between 80- 110 Wh/ kg and those using lithium titanate oxide in the negative electrode can have energy density between 60- 70 Wh/ kg. The situation regarding the power capability ( W/ kg) of the different chemistries is not as clear because of the energy density/ power capability trade- offs inherent in battery design.
Simulations of Prius plug- in hybrids were performed with Advisor utilizing lithium- ion batteries of the different chemistries. The UC Davis test data were used to prepare the battery input files needed in Advisor. Simulations were made for battery packs weighing 60 kg and 120 kg. The simulation results show that the selection of the battery chemistry for plug- in hybrids is closely linked to the details of the vehicle design and performance specifications and expected driving cycle. Economic factors such as cycle life and battery cost and battery management and safety issues must also be considered in selecting the most appropriate battery chemistry of plug- in hybrids.
Keywords: lithium- ion batteries, plug- in hybrid vehicles, energy density, pulse power
1 Introduction
It is well recognized that the key issue in the design of a plug- in hybrid- electric vehicle is the selection of the battery. The consensus view is the battery will be of the lithium- ion type, but which of the lithium- ion chemistries to use is still a major question. The selection will depend on a number of factors: useable energy density, useable power density, cycle and calendar life, safety ( thermal
stability), and cost. The most developed of the lithium- ion chemistries is that used in consumer electronics – that is carbon/ graphite in the negative electrode and nickel cobalt and other metal oxides in the positive electrode. That chemistry yields the best performance ( energy density and power density), but also has the greatest uncertainty concerning safety. The other chemistries ( iron phosphate in the positive and lithium titanate in the EVS24 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium 1
negative) being developed are known to have less favorable performance, but less concern regarding safety and longer cycle life. These latter chemistries have been evaluated in detail in the present study.
A number of companies world- wide are presently developing lithium- ion batteries utilizing the various electrode chemistries. Most of these companies are relatively small and are not well known in the battery business, but nevertheless their technologies are representative of the possibilities for the development of the emerging battery technologies. Hence a strong effort was made to obtain cells from a number of these companies for testing and evaluation. Reasonable success was achieved in obtaining lithium- ion cells from a number of sources for testing. This paper is concerned with analyzing the performance of the various cells/ chemistries based on testing of the cells. In addition, simulation results are presented for a plug- in Prius- type vehicle using different battery technologies and their suitability for use in plug- in hybrids assessed.
2 Lithium- ion battery chemistries
The lithium- ion battery technology used for consumer electronics applications is reasonably mature and in 2008 over one billion, small ( 18650) cells were manufactured and sold. These cells utilized graphite/ carbon in the negative and nickelate ( LiNiCoAlO) in the negative. This is the baseline chemistry with which the other emerging chemistries are compared. The graphite/ nickelate chemistry yields cells with the highest energy density and power capability of the chemistries being developed for vehicle applications primarily because the cell voltage and the specific charge ( mAh/ gm) of the positive electrode material are higher than for the other chemistries. The material and cell characteristics of the various chemistries are shown in Table 1. If performance of the cell was the only consideration, there would be little interest in developing cells/ batteries with the other chemistries. However, cycle life and safety ( thermal stability) as well as cost are important considerations in selecting batteries for vehicle applications. Unfortunately the graphite/ nickelate chemistry has shown in the consumer electronics applications to have safety and cycle life limitations, which can become even more serious for the large cells/ batteries needed for vehicle applications. Hence development is underway using lithium manganese spinel and iron phosphate for the positive electrodes and lithium titanate oxide for the negative electrode. As indicated in Table 1, these chemistries have significantly lower performance than the graphite/ nickelate chemistry, but longer cycle life and higher thermal stability. It is more difficult to compare the power capability of the different chemistries, because there is the inherent trade- off between energy density and power capability via the design of the electrodes and choice of material properties ( primarily particle size and surface area). Nevertheless, the cells with the higher cell voltage tend to have higher power capability. The goal of the developments of the other chemistries is to minimize the penalty in performance without significant sacrifice of the inherent advantages of the respective emerging chemistries.
Most of the cells for the consumer electronics applications are spiral wound packaged in a rigid container. Some cells are prismatic ( thin, flat) in shape, but they are also packaged in a rigid container. All these cells ( Figure 1) are small ( 1- 3 Ah) and can be used in vehicle applications only if larger cells/ modules are assembled by placing many of the small cells in parallel. This can be done, but it requires special attention to safety issues. For vehicle applications, larger cells ( up to 100 Ah) are being developed so it is not necessary to assemble parallel strings of the cells in the modules. In all cases, the modules consist of a number of cells in series to attain a reasonably high module voltage. In some cases, the larger cells ( Ah > 10 Ah) are packaged in a soft laminated pouch ( see Figure 2), which are then placed in a rigid container to form a high voltage module. Some of the larger cells are spiral wound ( see Figure 3), but the trend in cell development seems to be toward soft packaging. Whether this proves to be a wise trend remains to be seen as there are strong, well founded concerns about the robustness and reliability of the soft packaging for vehicle applications.
3 Battery testing
3.1 Test procedures and the batteries tested
EVS24 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium 2
Table 1: Characteristics of lithium- ion batteries using various chemistries
Chemistry
Anode/ cathode
Cell voltage
Max/ nom.
Ah/ gm
Anode/ cathode
Energy density Wh/ kg
Cycle life
( deep)
Thermal stability
Graphite/ NiCoMnO2
4.2/ 3.6
.36/. 18
100- 170
2000- 3000
fairly
stable
Graphite/
Mn spinel
4.0/ 3.6
.36/. 11
100- 120
1000
fairly
stable
Graphite/ NiCoAlO2
4.2/ 3.6
.36/. 18
100- 150
2000- 3000
least
stable
Graphite/
iron phosphate
3.65/ 3.25
.36/. 16
90- 115
> 3000
stable
Lithium titanate/
Mn spinel
2.8/ 2.4
.18/. 11
60- 75
> 5000
most
stable
Figure 1: Small, spiral wound cells
Figure 2: Pouch packaged cells
For each of the cells/ modules, the following tests were performed:
1)
Constant current tests starting at C/ 1 and up to currents at which the Ah capacity of the cell begins to show a significant decrease with rate.
2)
Constant power tests starting at about 100 W/ kg and up to powers ( W/ kg) at which the energy density ( Wh/ kg) begins to show a significant decrease with rate.
3)
5 sec pulse tests at high currents ( 5- 10C) at states of charge between 90% - 10% to determine the open- circuit voltage and
44 Ah cell
7.5 Ah
Figure 3: Spiral wound large cells
resistance from which the power capability of the cells can be calculated.
The power capability of the cells/ modules was determined in the present study by determining the open- circuit voltage and resistance as a function of state- of- charge and calculating the pulse power using the following equation:
P = Eff ( 1- Eff) Voc 2 / R
where Eff is the pulse efficiency, Eff= Vpulse / Voc
The power density is simply calculated as P/ battery weight or volume. This method is not too different from that given in the USABC test manual for PHEV batteries and can be applied for
EVS24 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium 3
cells/ modules independent of the vehicle in which they would be used.
The cells tested in the present study are listed in Table 2. As indicated in Table 3, modules were available for some of the batteries. Testing of the modules is still in progress. Photographs of a few of the cells and modules are shown in Figures 1- 4.
3.2 Test data for selected cells and modules
Detailed data were taken for all the cells listed in Table 2. Selected data for some of the cells are shown in Tables 3- 8 as illustrations of the performance of the iron phosphate and lithium titanate oxide cells.
24 V, 50 Ah modules from Altairnano 70V modules from EIG
Figure 4: Lithium- ion battery modules for testing
Table 2: Batteries tested - manufacturers, technology, and characteristics
Manufacturer
Technology type
Ah
Voltage range
Weight kg/
Volume L
K2
Iron phosphate
2.4
3.65- 2.0
.083/. 035
EIG
Iron phosphate
10.5
15.7
3.65- 2.0
.325/. 13
.424/--
A123
Iron phosphate
2.1
3.6- 2.5
.07/--
Lishen
Iron Phosphate
10.2
3.65- 2.0
----
EIG
Graphite/ Ni CoMnO2
18
4.2- 3.0
.45/--
GAIA
Graphite/
LiNiCoO2
42
4.1- 3.0
.32/--
Quallion
Graphite/
Mn spinel
1.8
2.3
4.2- 3.0
.043/. 017
.047/. 017
Altairnano
Lithium Titanate
11
52
2.8- 1.5
.34/--
1.6/--
EIG
Lithium Titanate
12.0
2.7- 1.5
Table 3: Lithium- ion battery modules available for testing
Chemistry
Anode/ cathode
Developer
Voltage
Ah
Resistance
mOhm
Weight
kg
pack. fact.
Volume
L
Pack. fact.
Nickel Cobalt
EIG
72
20
60
13.4
.67
11.3
.41
Iron Phosphate
EIG
74
14
55
13.6
.69
11.3
.34
Lithium titanate
Altairnano
16V
11
2
16.3
----
11.4
----
Lithium titanate
Altairnano
24V
50
10
21.4
.75
12.6
EVS24 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium 4
Table 3: Test data for the 15 Ah EIG iron phosphate cell
Iron Phosphate
FO 15A
Weight .424kg
3.65- 2.0V
Power ( W)
W/ kg
Time ( sec)
Wh
Wh/ kg
62
142
2854
49.5
117
102
240
1694
48.0
113
202
476
803
45.1
106
302
712
519
43.5
103
401
945
374
41.7
98
Current ( A)
Time ( sec)
Ah
Crate
Resistance
mOhm
15
3776
15.7
.95
30
1847
15.4
1.95
2.5
100
548
15.2
6.6
200
272
15.1
13.2
300
177
14.8
20.3
Table 4: Test data for the Altairnano 11Ah lithium titanate oxide cell
Constant current test data ( 2.8- 1.5V)
I( A)
nC
Time ( sec)
Ah
Resistance mOhm
10
.8
4244
11.8
--
20
1.7
2133
11.9
--
50
4.5
806
11.2
2.2
100
9.2
393
10.9
2.1
150
15.3
235
9.8
--
200
---
116
6.4
--
Resistance based on 5 sec pulse tests
Constant power test data ( 2.8- 1.5V)
Power W
W/ kg
Time
sec
nC
Wh
Wh/ kg
30
88
2904
1.2
24.2
71.2
50
147
1730
2.1
24.0
70.7
70
206
1243
2.9
24.2
71.0
100
294
853
4.2
23.7
69.7
150
441
521
6.9
21.7
63.8
170
500
457
7.9
21.6
63.5
260
764
255
14
18.4
54.2
340
1000
103
35.0
9.7
28.6
Mass: .34 kg
Table 5: Test data for the Altairnano 50Ah lithium titanate oxide cell
Constant current discharges ( 2.8- 1.5V)
Current A
nC
Time sec
Ah
Resistance mOhm
50
.96
3773
52.4
100
1.95
1847
51.3
1.0
200
4.0
904
50.2
.95
300
6.1
588
49.0
1.0
EVS24 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium 5
EVS24 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium 6
Constant power discharge ( 2.8- 1.5V)
Power W
W/ kg
Time
sec
nC
Wh
Wh/ kg
100
62
3977
.9
111
69
200
125
1943
1.85
108
67
300
188
1244
2.9
102
64
400
250
849
4.2
94
59
500
313
636
5.66
88
55
600
375
516
7.0
86
54
weight: 1.6 kg
Table 6: Pulse characteristics of the EIG 20Ah NiCo cell at various states- of- charge
Voc
DOD %
V2 sec
Effic. %
R mOhm
Power W
W/ kg
4.12/ 250A
0
3.33
80.8
3.16
833
1850
3.98/ 250A
10
3.24
81.4
2.96
810
1800
3.88/ 250A
20
3.14
80.9
2.96
785
1744
3.78/ 250A
30
3.06
81.0
2.88
765
1700
3.72/ 250A
40
2.98
80.1
2.96
745
1655
3.67/ 250A
50
2.90
79.0
3.08
725
1611
3.63/ 250A
60
2.84
78.2
3.16
710
1578
3.59/ 250A
70
2.74
76.3
3.4
685
1522
3.54/ 100A
80
3.18
89.8
3.6
318
706
3.48/ 100A
90
2.96
85.1
5.2
296
658
Table 7: Pulse characteristics of the EIG 14Ah Iron phosphate cell at various states- of- charge
Voc
DOD %
V2 sec
Effic. %
R mOhm
Power W
W/ kg
3.45/ 75A
0
3.08
89
4.9
231
711
3.3/ 75A
10
3.02
91.5
3.73
227
698
3.28/ 75A
20
3.0
91.5
3.73
225
692
3.26/ 75A
30
2.98
91.4
3.73
224
689
3.25/ 75A
40
2.96
91.0
3.87
222
683
3.25/ 75A
50
2.94
90.5
4.13
220
679
3.24/ 75A
60
2.91
89.8
4.4
218
672
3.21/ 75A
70
2.85
88.8
4.8
214
658
3.17/ 75A
80
2.74
86.4
5.7
206
632
2.58/ 75A
90
2.06
79.8
6.9
155
475
Table 8: Comparisons of the power characteristics of the EIG NiCo and iron phosphate cells
90% effic. 80% effic.
Cell
Wh/ kg at C/ 1
10% DOD
80% DOD
10% DOD
80% DOD
NiCo
20Ah
140
1056 W/ kg
696 W/ kg
1875 W/ kg
1238 W/ kg
Iron phosphate
14 Ah
90
808 W/ kg
488 W/ kg
1437 W/ kg
67 W/ kg
The resistance of the cells was determined from pulse tests performed at various states- of- charge. Pulse data for the EIG iron phosphate and NiCo cells are shown in Tables 6 and 7. A comparison of the power characteristics of the NiCo and iron phosphate cells is given in Table 8.
Test data for a 16V module of the Altairnano 11Ah cells are shown in Table 9. The characteristics of the module follow directly from the characteristics of the 11Ah cells. Table 9: Test data for the Altairnano 16V module )
Constant current discharge ( 8 cells in parallel, 6 in series)
I( A)
Time ( sec)
nC
Ah
Resistance mOhm
50
6908
.52
95.9
100
3419
1.05
95.0
200
1704
2.11
94.7
1.95
300
1113
3.23
92.8
2.0
400
833
4.32
92.6
2.0
Cell mass: 16.3 kg, resistance based on 5 sec pulses of the module
90% efficiency pulse: 11.5 kW, 706 W/ kg
Constant power discharges
Power ( W)
( W/ kg) cells
Time ( sec)
kWh
( Wh/ kg) cells
1000
61
4576
1.27
77.9
1500
92
2975
1.24
76.1
2000
122
2217
1.23
75.5
2500
250
1756
1.22
75.0
3000
184
1459
1.22
75.0
3500
215
1221
1.19
73.0
3600
221
1222
1.22
75.0
Charge at 88A to 16.3, discharge from 16.3 to 9V
Table 10: Summary of the performance characteristics of lithium- ion cells of different chemistries from various battery developers
Manufacturer
Technology type
Ah
Voltage range
Wh/ kg
at 300 W/ kg
( W/ kg) 90% eff.
50% SOC
K2
Iron phosphate
2.4
3.65- 2.0
86
667
EIG
Iron phosphate
10.5
15.7
3.65- 2.0
83
113
708
919
A123
Iron phosphate
2.1
3.6- 2.5
88
1146
Lishen
Iron Phosphate
10.2
3.65- 2.0
82
161
EIG
Graphite/ Ni CoMnO2
18
4.2- 3.0
140
895
GAIA
Graphite/
LiNiCoO2
42
4.1- 3.0
94
1742
at 70% SOC
Quallion
Graphite/
Mn spinel
1.8
4.2- 3.0
144
491
at 60% SOC
2.3
4.2- 3.0
170
379
at 60% SOC
Altairnano
Lithium Titanate
11
52
2.8- 1.5
70
57
684
340
EIG
Lithium Titanate
12.0
2.7- 1.5
43
584
4 Comparisons of the performance of lithium- ion cells of the different chemistries from various battery developers
A summary of the data for the different chemistries is shown in Table 10. It is clear from the table that both the energy density and power capability of the cells vary over a wide range and that there are significant trade- offs between energy and power with all the chemistries. Energy density and power
EVS24 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium 7
capability are discussed separately the following sections.
4.1 Energy density
It is clear from Table 10 that the energy density of cells using NiCo ( nickelate) in the positive electrode have the highest energy density being in the range of 100- 170 Wh/ kg. Cells using iron phosphate in the positive have energy density between 80- 110 Wh/ kg and those using lithium titanate oxide in the negative electrode can have energy density between 60- 70 Wh/ kg. Hence in terms of energy density, the rankings of the different chemistries are clear and the differences are significant: 1. NiCo, 2. iron phosphate, 3. lithium titanate oxide. The question of what fraction of the energy density is useable in a specific vehicle application could decrease the relative advantage of the different chemistries.
4.2 Power capability
The situation regarding the power capability ( W/ kg) of the different chemistries is not as clear as was the case for energy density because of the energy density/ power capability trade- offs inherent in battery design. Further the question of the maximum useable power density is also application specific. In order to have a well- defined basis for comparing the different chemistries and cells, the power density ( W/ kg) for a 90% efficient pulse at 50% SOC is shown in Table 10 for most of the cells. The power densities can vary over a wide range even for a given chemistry. This is particularly true for the graphite/ NiCoMn chemistry. In general, it seems possible to design high power batteries ( 500- 1000 W/ kg at 90% efficiency) for all the chemistries if one is willing to sacrifice energy density and likely also cycle life. The data in Table 10 indicate that high power iron phosphate cells can be designed without a significant sacrifice in energy density. When power densities greater than 2000 W/ kg for lithium- ion batteries are claimed, it is for low efficiency pulses. For example, for an efficiency of 65%, the 15Ah EIG iron phosphate battery has a pulse power of 2330 W/ kg rather than the 919 value for a 90% efficient pulse.
5 Considerations for selecting batteries/ energy storage for Plug- in Hybrid vehicles
The selection of the battery for plug- in hybrid vehicle is complicated process and depends on many factors. In simplest terms, the battery must meet the energy storage ( kWh) and peak power ( kW) requirements of the vehicle and fit into the space available. In addition, the battery must satisfy the cycle life requirements both for deep discharge cycles in the charge depleting mode and shallow cycling in the charge sustaining mode of operation. Further the battery unit must be designed to meet the thermal management, cell- to- cell monitoring, and safety requirements. The final considerations are concerned with the initial and life cycle costs of the battery.
This paper has dealt in detail with the performance of the lithium- ion batteries using different chemistries. Even though electrode chemistry has a significant effect on the performance of the battery, these differences alone are far from sufficient for selecting a battery for a PHEV. The other factors – cycle life and the effect on cycle life of depth- of- discharge, safety and thermal issues, and cost can be critical in influencing battery selection.
As indicated earlier in the paper, a primary reason for the present development of lithium- ion batteries of various chemistries is related to safety issues with the batteries using NiCo and other metal oxides in the positive electrode. There have been some instances in which those cells/ batteries have experienced thermal runaway events and as a result, the NiCo based battery systems are treated with considerable caution. They incorporate extensive cell monitoring circuitry as protection against possible destructive thermal events.
Cells using iron phosphate in the positive electrode are thought to be much less prone to thermal runaway both because they are less energetic ( significantly lower energy density) and do not produce oxygen on overcharge which can react exothermically with the graphite in the negative electrode. Cells using lithium titanate oxide ( LTO) in the negative are even less energetic ( lower energy density) than cells using iron phosphate and in addition the LTO replaces the graphite in the negative electrode removing a combustible substance in the cell. Hence both the iron EVS24 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium 8
EVS24 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium 9
nom
s especially true of the lithium
titanate chemistry.
( CD) electric ranges of the various designs
phosphate and lithium titanate chemistries are inherently safer than the NiCo chemistry.
Another important issue in evaluating lithium- ion battery chemistries is cycle life and calendar life. In a plug- in hybrid vehicle, a battery life of at least ten years is thought to be necessary. This means that the battery must be able to sustain about 3000 deep discharge cycles in the charge depleting mode and several hundred thousand shallow cycles at low states- of- charge in the charge sustaining mode. Hence a PHEV battery must have the life cycle characteristics of an EV battery and a HEV battery. Whether any of the lithium battery chemistries can meet these life cycle requirements has not yet been determined.
It is expected that both the iron phosphate and lithium titanate chemistries will have significantly longer cycle life than the NiCo chemistry. This is especially true of the lithium titanate chemistry. Life cycle testing of cells done by Altairnano as part of their development program have indicated a very long cycle life of greater than 5000 cycles even for fast charge and discharge rates ( Reference 1).
Little information is available on the relative cost ($/ kWh) of lithium- ion batteries of the different chemistries. Further it is difficult to get good information on the costs of the various materials used in the batteries. If such information were available, it is relatively simple to estimate the differences in the electrode material costs for the different chemistries. This could be done using the following equation to estimate the $/ Wh for each chemistry:
$/ Wh={[($/ gm)/ Ah/ gm] anode + [$/ gm)/ Ah/ gm] cathode}/
V
Values for the Ah/ gm and Voc are given in Table 1. Calculated values for the electrode material costs ($/ kWh) are shown in Table 11 for the assumed unit costs of the various materials. The material unit costs used in the calculations are based on inquiries made of several sources involved with the manufacture of lithium batteries ( References 2 and 3). The results shown in Table 11 indicate the relative electrode material costs of the various chemistries and also that electrode material costs should not dominate the total battery cost. Note that in general the higher cost lithium battery chemistries have the potential for longer cycle life which on a life cycle cost basis can compensate for the higher initial cost of those chemistries. This i
6 Plug- in hybrid vehicle simulations using various battery chemistries
Simulations of Prius plug- in hybrids have been performed with Advisor utilizing lithium- ion batteries of the different chemistries ( References 4 and 5). The UC Davis test data were used to prepare the battery input files needed in Advisor. Simulations were made for battery packs weighing 60 kg and 120 kg. The results of the simulations are given in Table 12. Note from Table 12 that plug- in hybrids can be designed using the various lithium- ion batteries as well as a nickel metal hydride battery. However, the charge depleted
Table 11: Relative electrode material costs for various lithium battery chemistries
Chemistry
Anode/ cathode
Cell voltage
Max/ nom.
Electrode
material $/ kg
Anode/ cathode
Electrode material cost $/ kWh
Cycle life
( deep)
Graphite/ NiCoMnO2
4.2/ 3.6
12/ 25
48
2000- 3000
Graphite/
Mn spinel
4.0/ 3.6
12/ 8
30
1000
Graphite/ NiCoAlO2
4.2/ 3.6
12/ 25
48
2000- 3000
Graphite/
iron phosphate
3.65/ 3.25
12/ 20
49
> 3000
Lithium titanate/
Mn spinel
2.8/ 2.4
25/ 8
88
> 5000 Table 12: Simulation results for Prius PHEVs using various lithium- ion batteries
and their fuel economy in the CD mode are much different and the differences are highly dependent on the driving cycle. The CD ranges are larger for the batteries with the higher energy densities and the fuel economies in the CD mode are highest for the batteries that are capable of high peak power. High battery power capability permits the vehicle to operate in the all- electric mode ( engine off) until the energy in the battery is depleted. The fuel economy in the charge sustaining ( CS) mode is dependent on the driving cycle, but not significantly on the battery energy density and weight of the battery pack. The weight of the battery and its energy density has a large effect on CD operation as would be expected. The simulation results show that the selection of the battery chemistry for plug- in hybrids is closely linked to the details of the vehicle design and
EVS24 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium 10
performance specifications and expected driving cycle. Economic factors such as cycle life and battery cost and battery management and safety issues must also be considered in selecting the most appropriate battery chemistry of plug- in hybrids.
Table 12 ( continued): Simulation results for Prius PHEVs using various lithium- ion batteries EVS24 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium 11
7 Summary and conclusions
It is well recognized that the key issue in the design of a plug- in hybrid- electric vehicle is the selection of the battery. The consensus view is the battery will be of the lithium- ion type, but which of the lithium- ion chemistries to use is still a major question. The selection will depend on a number of factors: useable energy density, useable power density, cycle and calendar life, safety ( thermal stability), and cost. This paper is concerned with the testing and evaluation of various battery chemistries for use in PHEVs. Test data are presented for lithium- ion cells and modules utilizing nickel cobalt, iron phosphate, and lithium titanate oxide in the electrodes. The energy density of cells using NiCo ( nickelate) in the positive electrode have the highest energy density being in the range of 100- 170 Wh/ kg. Cells using iron phosphate in the positive have energy density between 80- 110 Wh/ kg and those using lithium titanate oxide in the negative electrode can have energy density between 60- 70 Wh/ kg. The situation regarding the power capability ( W/ kg) of the different chemistries is not as clear because of the energy density/ power capability trade- offs inherent in battery design. The power densities can vary over a wide range even for a given chemistry. This is particularly true for the graphite/ NiCoMn chemistry. In general, it seems possible to design high power batteries ( 500- 1000 W/ kg at 90% efficiency) for all the chemistries if one is willing to sacrifice energy density and likely also cycle life. The data indicate that high power iron phosphate cells can be designed without a significant sacrifice in energy density. When power densities greater than 2000 W/ kg for lithium- ion batteries are claimed, it is for low efficiency pulses. For example, for an efficiency of 65%, the 15Ah EIG iron phosphate battery has a pulse power of 2330 W/ kg rather than the 919 value for a 90% efficient pulse.
Simulations of Prius plug- in hybrids have been performed with Advisor utilizing lithium- ion batteries of the different chemistries. Simulations were made for battery packs weighing 60 kg and 120 kg. The simulation results show that the selection of the battery chemistry for plug- in hybrids is closely linked to the details of the vehicle design and performance specifications and expected driving cycle. Economic factors such as cycle life and battery cost and battery management and safety issues must also be considered in selecting the most appropriate battery chemistry of plug- in hybrids.
References
[ 1]
Manev, V, etals, Nano- Li4Ti5O12 based HEV Batteries, Advanced Automotive Battery and Ultracapacitor Conference, Fourth International Symposium on Large Lithium- ion Battery Technology and Applications, Tampa, Florida, May 2008
[ 2]
Private communications from South Korea and China on battery material costs, December 2008
[ 3]
Anderman, M., Performance of Large Lithium- ion batteries in key applications and gap analysis against requirements- Tutorial C, Advanced Automotive Battery Conference, Baltimore, Maryland, May 2006
[ 4]
Axsen, J., Burke, A. F., and Kurani, K., Batteries for Plug- in Hybrid Electric Vehicles ( PHEVs): Goals and State of the Technology ( 2008), Report UCD- ITS- RR- 08- 14, May 2008
[ 5]
Burke, A. F. and Van Gelder, E., Plug- in Hybrid- Electric Vehicle Powertrain Design and Control Strategy Options and Simulation Results using Lithium- ion Batteries, paper presented at EET- 2008 European Ele- Drive Conference, Geneva, Switzerland, March 12, 2008 ( paper on the CD of the proceedings of the conference)
Authors
Andrew Burke, Research faculty, ITS- Davis. Ph. D., 1967, Princeton University. Since 1974, Dr. Burke’s research has involved many aspects of electric and hybrid vehicle design, analysis, and testing. He was a key contributor on the US Department of Energy Hybrid Test Vehicles ( HTV) project while working at the General Electric Research and Development Center. He continued his work on electric vehicle technology, while Professor of Mechanical Engineering at Union College and later as a research manager with the Idaho National Engineering Laboratory ( INEL). Dr. Burke joined the research faculty of the ITS- Davis in 1994. He directs the EV Power Systems Laboratory and performs research and teaches graduate courses on advanced electric driveline technologies, specializing in batteries, ultracapacitors, fuel cells and hybrid vehicle design. Dr. Burke has authored over 80 publications on electric and EVS24 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium 12
hybrid vehicle technology and applications of batteries and ultracapacitors for electric vehicles.
Dr. Marshall Miller is a Senior Development Engineer at the Institute of Transportation Studies at the University of California, Davis. He is the Director of the Hydrogen Bus Technology Validation Program which studies fuel cell and hydrogen enriched natural gas buses. He also supervises testing in the Hybrid Vehicle Propulsion Systems Laboratory where he does research on fuel cells, advanced batteries, and ultracapacitor technology. His overall research has focused on advanced environmental vehicles and fueling infrastructure to reduce emissions, greenhouse gases, and oil usage. He received his B. S. in Engineering Science and his M. S. in Nuclear Engineering from the University of Michigan. He received his Ph. D. in Physics from the University of Pennsylvania in 1988.
EVS24 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium 13