CAM-7/LTO Lithium-Ion Cells for a 6T Battery with Excellent LowTemperature Performance
David Ofer, Daniel Kaplan, Oksu Han, Shawn Montgomery, Yoo-Eup Hyung,
Brian Barnett, and Suresh Sriramulu
TIAX LLC
35 Hartwell Avenue
Lexington, MA 02421
Keywords: laminate packaging; lithium-ion;
temperature electrolyte, CAM-7; lithium titanate.
low-
Introduction
Military applications can require rechargeable batteries to
operate in extreme environments with temperatures as low as
-50 °C or as high as 70 °C, e.g., the Silent Watch
application, which is driving development of vehicle
batteries with increased run times.
However, these
temperature extremes extend beyond the operational
temperature range of current commercial Li-ion technology,,
hindering the replacement of low energy density lead-acid
technology used in current military vehicle 6T batteries with
more energy dense and deep-cycling-tolerant Li-ion cells.
Operational temperature limitations of Li-ion batteries are
related to their electrolytes and he active materials. At low
temperatures, performance is hampered by low electrolyte
conductivity and poor electrochemical kinetics associated
with ion-desolvation processes.1 Low-temperature charge
acceptance capability is particularly poor because the
graphitic carbon anodes used in Li-ion cells are prone to
plating lithium metal when charged rapidly or at low
temperature: a condition giving rise to rapid fade and
serious safety concerns. At high temperatures, Li-ion
battery lifetime is compromised by thermal instability of
the electrolytes and of electrode surface films they form,
and by the electrolytes’ reactivity with the charged
electrode materials; in particular by decomposition
reactions at the graphitic carbon anode.2 Furthermore,
decomposition reactions of Li-ion electrolytes at high
temperature can yield gaseous products that may limit the
use of laminated prismatic pouch cell designs, which are
otherwise very advantageous for their light weight and easy
form factor reconfiguration.
In an Army TARDEC-sponsored program, TIAX is
developing low-temperature-capable (to -50 °C) laminated
prismatic lithium-ion cells employing TIAX’s high-energy,
high-power CAM-7 cathode material, a high rate capability
lithium titanate (LTO) anode material, and a novel lowtemperature-capable electrolyte formulation. Although
LTO anode material yields lower energy density cells than
graphite anode materials, LTO offers performance, life and
cell design advantages that better facilitate meeting Army
performance targets, and the use of high energy content
CAM-7 cathode material enables LTO anode to be used
with the least possible cost to cell energy content. The
novel TIAX cells are targeting implementation in a 6Tformat battery, with cell-level energy content of about 90
Wh/kg and 250 Wh/l, and capability for charge and
discharge at temperatures approaching -50 °C.
Cell Chemistry
CAM-7: CAM-7 is a dopant-stabilized LiNiO2-class
cathode material with a unique combination of high energy
content and power capability as shown in Figure 1.
4.5
C/20
1C
5C
10C
30 C
50 C
100 C
4.3
4.1
Voltage (V)
Abstract: TIAX is developing laminated prismatic
lithium-ion (Li-ion) cells capable of charging at low
temperature (to -50 °C) to replace current lead-acid cells
in 6T vehicle batteries. The novel cells target cell-level
energy content of 90 Wh/kg and 250 Wh/l and are based on
TIAX’s high energy, high power CAM-7 cathode material,
high rate capability lithium titanate (LTO) anode material,
and a low-temperature-capable electrolyte formulation.
Tests of pouch cells ranging up to 3 Ah in size show
excellent performance at -46 °C and robust cycle life at
ambient temperature.
3.9
3.7
3.5
3.3
3.1
2.9
0
40
80
120
160
200
240
Specific Capacity (mAh/g)
Figure 1. Capacity and rate capability of CAM-7 measured
in a low loading (~2 mg/cm2) electrode in a coin cell vs. a Li
metal counter electrode. The cell was charged in CCCV
mode to 4.3 V, and discharged at rates shown on basis of
200 mAh/g 1C rate.
CAM-7 also delivers outstanding low-temperature
performance as shown by Figure 2, which compares the
discharge performance of CAM-7 relative to commercially
available NCA (LiNi0.8Co0.15Al0.05O2) and NCM
(LiNi1/3Co1/3 Mn1/3O2) cathode materials at -20 ºC. CAM-7
has higher discharge capacity at a higher average voltage.
4.3
Solid curve: C/20 discharge
Dashed curves: 1 C discharge
4.1
Voltage (V)
3.9
CAM-7
3.7
CAM-7
3.5
NCA
3.3
NCA
NCM
3.1
NCM
2.9
2.7
0
40
80
120
160
200
Specific Capacity (mAh/g)
Figure 2. Discharge voltage profiles of CAM-7 and
competing cathode materials at -20 ºC, measured for 10-11
mg/cm2 electrodes vs. Li metal in coin cells. Cells were
charged to 4.2 V at RT, equilibrated at -20 ºC, and
discharged at C/20 and 1C on basis of 200 mAh/g 1C rate.
CAM-7 is produced by a readily scalable solid-state
synthetic process. A separate company, CAMX Power, has
been established to commercialize CAM-7, and has
installed and is operating a 50 t/y plant for producing
CAM-7 in Rowley, Massachusetts.
LTO: Graphitic carbon, the predominant Li-ion anode
active material, can not be lithiated (charged) at low
temperatures without a high probability of undesirable Li
metal plating. Nano-structured Li4Ti5O12 or LTO anode
material has exceptionally high rate capability and can be
lithiated very rapidly as shown in Figure 3, because its
potential is far positive of Li metal (~1.55 V vs. Li), which
also prevents high impedance SEI films from forming on
its surface.
Voltage (V vs. Li)
1.7
1.6
C/20
1.5
C/5
1C
1.4
2C
1.3
5C
1.2
10C
1.1
15C
20C
1.0
0.9
0
20
40
60
80
100
120
140
160
180
Specific Capacity (mAh/g)
Figure 3. Ambient temperature half cell lithiation profiles as
function of C-rate (1C = 170 mA/g) for LTO electrodes with
2
0.7 mAh/cm active material loading.
Low-temperature electrolyte: Current commercial Li-ion
electrolytes consist almost exclusively of LiPF6 salt in
mixtures of cyclic ethylene carbonate (EC) with various
linear carbonates, these having emerged as providing the
best overall balance of properties for Li-ion batteries in
consumer electronics.3
However, cells with these
electrolytes generally do not have acceptable performance
below -20 °C because of the high freezing point and
viscosity of EC as well as its high activation energy for
lithium ion desolvation.
Reducing the electrolyte EC content, and replacing linear
carbonates with alternative higher dielectric constant, low
viscosity, low freezing point cosolvents has been shown to
greatly improve Li-ion low-temperature performance.4 A
comparison of the properties of alternative cosolvents to
those of standard carbonate solvents is shown in Table 1.
Nitriles or esters can offer a better combination of high
polarity with low viscosity and extended liquid temperature
range than linear carbonates do.
Solvent
Abb.
Molecular
Formula
Ethylene
Carbonate
EC
C3H4 O3
Melting Dielectric Viscosity
Point constant (cp @
25 °C)
(o C) (20 oC)
90
1.86
38
(40 °C) (40 °C)
C5H 10 O3
-43
2.8
0.75
DMC
C3H6 O3
4
3.1
0.59
EMC
C4H8 O3
-55
2.9
0.65
Butyronitrile
BN
C3H 7CN
-112
20.7
0.52
Methyl Acetate
MA
CH3 CO2 CH3
-99
6.7
0.37
Methyl Butyrate
MB
C3 H7CO2 CH3
-85
5.6
0.58
Diethyl Carbonate DEC
Dimethyl
Carbonate
Ethyl Methyl
Carbonate
Table 1. Comparison of electrolyte solvent physical
properties.
Initial development of this CAM-7/LTO/low-temperature
electrolyte cell chemistry has taken place in coin cells, and
it is now being tested in stacked-electrode laminate pouch
cells ranging from 1 Ah to 3 Ah in size.
Laminate Pouch Cell Performance
Ambient temperature: The CAM-7/LTO cell chemistry
cycles very stably. This cycling stability was demonstrated
by initial ~1 Ah pouch cells built with electrode loading of
1.3 mAh/cm2 in an anode-limited design (cathode
utilization limited to ~85%) and conventional allcarbonate-solvent electrolyte. Cells of this type cycled over
their full depth of discharge (DOD) 1,000 times with no
decrease in capacity or increase in area specific impedance
(ASI), as shown by Figure 4. Cycling was conducted at
ambient laboratory temperature conditions at 1C/1C (1A)
charge/discharge rate between limits of 1.4V and 2.75V,
with interruptions at 100 cycle intervals for a C/5 cycle and
a 1C pulsed current characterization of impedance at 1
second and 10 seconds pulse duration as a function of state
of charge (SOC).
The low loading electrodes used in these initial 1 Ah cells
projected to yield scaled-up pouch cells with energy
content of ~80 Wh/kg and ~220 Wh/l, which are below the
targets. Therefore, subsequent larger pouch cells employed
electrodes having 1.7 mAh/cm2 loading and projecting to
meet the 90 Wh/kg and 250 Wh/l targets in 10 Ah cells.
1.0
0.9
Discharge Capacity (Ah)
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0
100
200
300
400
500
600
700
800
900
1000
Cycle Number
25
38% SOC
2.8
15
2.6
2.4
10
2.2
1s
10 s
5
Voltage
discharge ASI (  x cm 2)
20
Figure 6 shows data for ambient laboratory temperature
characterization of a 3 Ah pouch cell built with 1.7
mAh/cm2 loading electrodes and an ester-based lowtemperature-capable electrolyte formulation.
This
unoptimized cell weighs 81g and delivers about 7 Wh
energy at C/20, i.e., yielding 86 Wh/kg. The Figure 6 data
show that the cell has high rate capability, with only ~100
mV polarization measured for charge or discharge at the 3C
rate relative to the C/20 rate, and also demonstrate the cell’s
suitability for rapid charging. More extended cycling tests
are in progress.
1.8
0
0
100
200
300
400
500
600
700
800
900
1000
1.6
Number of cycles
Figure 4. Top: 1C discharge capacity of 1 Ah CAM-7/LTO
pouch cell vs. cycle number, and C/5 discharge capacity
measured at 100-cycle intervals. Bottom: 1 second and 10
second ASI for the 1 Ah pouch cell at 38% SOC measured
at 100-cycle intervals during 1C/1C cycling.
1 Ah cells filled with ester-based low-temperature
electrolyte formulation had good power capability at
-46 °C. They also had good ambient temperature
performance, as shown by Figure 5 which compares ASI
measured as a function of SOC for cells filled with
conventional electrolyte and low-temperature electrolyte.
55
0
3.2
1 s, low-temperature electrolyte
3.0
10 s, low-temperature electrolyte
35
2.8
30
2.6
25
2.4
1.8
1.6
0
1.4
30
40
50
60
70
80
90
100
State of Charge (%)
Figure 5. 1 second and 10 second ASI dependence on
SOC for 1 Ah pouch cells filled with conventional allcarbonate-solvent
electrolyte
and
low-temperature
electrolyte.
2.5
3
3.5
2.0
5
20
2
2.2
10
10
1.5
Low temperature (-46 °C): CAM-7/LTO pouch cells filled
with low-temperature-capable electrolyte demonstrate
outstanding performance at -46 °C, as illustrated by Figure
7 which shows -46 °C data for a 2.5Ah pouch cell built
with 1.7 mAh/cm2 loading electrodes.
10 s, conventional electrolyte
0
1
Figure 6. Ambient temperature charge and discharge
voltage characteristics for a 3 Ah CAM-7/LTO pouch cell
built with low-temperature electrolyte.
1 s, conventional electrolyte
15
0.5
Capacity, Ah
45
20
3A (3C)
1.2
50
40
0.15A (C/20)
1.4
Voltage
discharge ASI (  x cm 2)
2.0
0.1A (C/20)
0.5A (C/5)
2.5A (1C)
5A (2C)
1.2
1.0
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Capacity, Ah
Figure 7. Voltage characteristics at -46 °C for a 2.5 Ah
CAM-7/LTO/ low-temperature electrolyte pouch cell.
The -46 °C cycling shown in Figure 7 was performed
between 1.2V to 3V voltage limits that are expanded
relative to the 1.4V to 2.75V limits used for ambient
temperature cycling shown in Figure 6. These expanded
voltage limits are enabled by the use of LTO anode which
avoids Li metal plating concerns, and which therefore also
enables anode-limited cell design, preventing cathode
excursion to excessively high overcharge potentials. As
Figure 7 shows, the expanded voltage limits facilitate these
cells’ remarkable ability to be effectively charged at very
low temperature at reasonable rates.
Figure 7 also suggests that the cell’s rate capability benefits
from some amount of self-heating at rates above 1C. The
2C charge and discharge curves both show large and rapid
initial polarization, but then flatten. Most significantly, the
150-200 mV polarization between the 2C and 1C curves is
much less than the 300-500 mV polarization between the
1C and C/20 curves. Higher rate tests are in progress.
Cold cranking capability is an important attribute for
vehicle batteries. Specification MIL-PRF-32143B, which
governs testing and qualification of the 6T battery, calls for
measurement of the battery’s ability to sustain a 3.33C
pulse (400 A for a 120 Ah battery) for 30 seconds at -40 °C
before its voltage reaches 50% of the charge voltage.
CAM-7/LTO pouch cells perform well when subjected to a
scaled version of this test at -46 °C. Cells were first
charged to 2.75 V at the C/50 rate at -46 °C, and then
discharged by repeating the sequence of a 30 second 3.5C
pulse followed by a 30 minute rest at open circuit, until the
cell voltage reached 1.35 V. Figure 8 shows results of the
scaled MIL-PRF-32143B cold cranking test performed on
2.5 Ah and 3 Ah CAM-7/LTO pouch cells built with 1.7
mAh/cm2 loading electrodes.
2.3
2.5 Ah, 6.2 mA/cm2
2.2
3 Ah, 5.7 mA/cm2
2.1
2.0
Voltage
1.9
1.8
1.7
Figure 8 shows that at -46 °C, the CAM-7/LTO pouch cells
can support the 30 second pulsing at ~3.5C discharge rate
over about 40% of their capacity range, substantially
exceeding the requirements of MIL-PRF-32143B.
Testing of cycle and storage life of CAM-7/LTO/lowtemperature-capable electrolyte pouch cells at ambient and
elevated temperatures is being initiated. Cell gassing will
be studied as an important element of these efforts.
Conclusions
Li-ion pouch cells based on CAM-7 cathodes, LTO anodes
and low-temperature performance-enhancing electrolyte
offer great promise for improving the specific energy and
the low-temperature performance of the 6T battery.
Ambient temperature tests show that this chemistry has
potential to be very stable and long-lived, particularly with
respect to high depth-of-discharge cycling.
Further
assessment of this technology’s potential awaits results of
more aggressive testing at elevated temperatures.
Acknowledgements: This material is based upon work
supported by the United States Army under Contract No.
W56HZV-11-C-0065.
References
1. Kang Xu, “Charge-Transfer” Process at
Graphite/Electrolyte Interface and the Solvation
Sheath Structure of Li+ in Nonaqueous Electrolytes,”
Journal of the Electrochemical Society, Vol. 154, no.
3, pp. A162-A167, 2007.
2. Michel Broussely in “Advances in Lithium-Ion
Batteries,” W. van Schalwijk and B. Scrosati, eds.,
Chapter 13, Kluwer Academic/Plenum, 2002.
3. Kang Xu, “Nonaqueous Liquid Electrolytes for
Lithium-Based Rechargeable Batteries,” Chemical
Reviews, Vol. 104, pp. 4303-4417, 2004.
4. M.C. Smart, B.V. Ratnakumar, K.B. Chin and L.D.
Whitcanack, “Lithium-Ion Electrolytes Containing
Ester Cosolvents for Improved Low Temperature
Performance,” Journal of the Electrochemical Society,
Vol. 157, pp. A1361-A1374, 2010.
1.6
1.5
1.4
1.3
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
Capacity, Ah
Figure 8. Scaled MIL-PRF-32143B-based cold cranking
tests at -46 °C for 2.5 Ah and 3 Ah CAM-7/LTO pouch cells
built with ester-based electrolyte.
Disclaimer: Any opinions, findings and conclusions or
recommendations expressed in this material are those of the
author(s) and do not necessarily reflect the views of the
United States Army.