Lithium-Ion Cells with Ultracapacitor Performance
David Ofer, Celine Yang, Leah Nation, Christopher McCoy, Brian Barnett,
and Suresh Sriramulu
TIAX LLC
35 Hartwell Avenue
Lexington, MA 02421
Keywords: ultracapacitor; lithium-ion; usable energy;
CAM-7; lithium titanate.
Introduction
Existing battery technologies have difficulty meeting
electrical energy storage (EES) requirements for accepting
and delivering high power associated with load leveling
and power conditioning while also having significant
energy density storage, or meeting similar requirements for
certain emerging vehicular applications. For example, the
high rate charging capability (dynamic charge acceptance)
needed by micro-hybrid (start-stop) vehicle batteries is
proving challenging even for lead-acid batteries. Military
EES applications can be even more demanding, for
example, requiring capability to power pulsed energy
weapons. The bidirectional (charge and discharge) power
densities required in such applications are not available in
present batteries, but can be provided by electrochemical
double-layer capacitors, which are often referred to as
ultracapacitors.
Ultracapacitors function by charge and discharge of
electrochemical double layers at high surface area
electrodes. Their lack of discrete electron transfer and
associated chemical changes results in extremely fast
kinetics (i.e., high power) and very long cycle life.
However, ultracapacitors have much lower energy densities
than batteries, up to only about 10 Wh/l, for fundamental
reasons including that their ultimate source of charge is the
relatively low concentration ionic content of the electrolyte
solution.1 For example, a typical 1M organic electrolyte
solution of ~1 g/cc density contains ~27 Ah of charge
(cations or anions) per kg electrolyte. If half that charge
can be separated and capacitively stored by a 3 V charging
voltage (an aggressive assumption), upon discharge to 1.5
V the electrolyte alone will have delivered only ~15
Wh/kg, before the masses of electrodes, packaging, and
other cell components are even considered. In order to
achieve high energy density, electrochemical energy
storage devices must therefore employ at least one Faradaic
(i.e., battery-type as opposed to capacitive) electrode that
serves as the high capacity (high charge density) source of
the charge-compensating ions involved at both electrodes.2
Accordingly, in a Navy-sponsored program, TIAX is
developing a lithium-ion cell technology that provides the
energy density of batteries and the cell-level power density
of electrochemical double layer capacitors, targeting 70
Wh/l usable energy within which the cell can sustain
charge and discharge pulsing at 5 kW/l power density for 1
second duration. This novel Li-ion technology, based on
TIAX’s proprietary CAM-7 cathode material, a nanostructured Li4Ti5O12 (LTO) anode material and high powercapable electrolyte and separator, is directed towards
meeting NAVFAC EES targets for 200 Wh/l overall energy
density and 25,000 full depth of discharge cycle life.
Cell Chemistry and Design
CAM-7: TIAX’s CAM-7 is a stabilized LiNiO2-based
cathode material offering an outstanding combination of
high capacity and high rate capability as shown in Figure 1.
250
Discharge capacity (mAh/g)
Abstract: TIAX is developing Li-ion cells capable of very
high charge and discharge power acceptance/delivery
based on TIAX’s high energy, high power CAM-7 cathode
material and high rate capability lithium titanate (LTO)
anode material. This Li-ion technology can provide much
higher energy density than ultracapacitors for pulsed or
continuous charge or discharge at the 5 kW/l power density
level, while also meeting the life requirements of energy
storage applications.
200
150
100
50
0
0
20
40
60
80
100
C-Rate (C)
Figure 1. Discharge rate capability of CAM-7 in low loading
(~2 mg/cm2) Li metal half cells on basis of 200 mAh/g 1C
rate.
TIAX has installed a 50 metric ton per year CAM-7 pilot
plant in Rowley, Massachusetts. CAM-7 is being sampled
to many of the prominent domestic and foreign battery
manufacturers and developers for both vehicle and
consumer electronics applications.
1.9
200
160
120
s5
s0
s3
s6
s7
s4
s2
S1
80
40
1.8
1.7
Voltage (V vs. Li)
showing how strongly the separators impact the cells’ rate
capability. Commercially available, very high power
capability separators are being used in this high power Liion technology.
Specific Capacity (mAh/g)
LTO: Although graphitic carbon is the predominant Li-ion
anode active material, it can not be lithiated (as occurs in
Li-ion cell charging) at high rates without a high
probability of undesirable Li metal plating because its
potential is close to that of Li. LTO anode material was
first noted for its negligible volume change during
electrochemical cycling, earning it designation as a “zero
strain material,”3 and giving it exceptional cycling stability.
Nano-structured LTO is capable of exceptionally high rate
capability and can be lithiated (charged) very rapidly
without danger of Li plating because of its high potential
(~1.55 V vs. Li). LTO’s high potential also means that
high impedance films of electrolyte reduction products
(SEI) do not form on its surface, enabling high surface area
material to be used without incurring high 1st cycle
irreversible capacity loss. LTO’s ability to sustain high rate
charging is illustrated by half cell data shown in Figure 2.
1C
1.6
5C
1.5
20C
1.4
51C
101C
1.3
0
0
5
151C
1.2
10
15
20
25
30
35
Discharge C-Rate
201C
2
Figure 3. Discharge rate capability of ~7.5 mg/cm CAM-7
electrodes in Li metal half cells dependence on separator
type. Rates on basis of 200 mAh/g 1C rate.
1.1
1.0
0.9
0
20
40
60
80
100 120 140 160 180 200
Specific Capacity (mAh/g)
Figure 2. Half cell lithiation profiles as function of C-rate (1C
= 170 mA/g) for LTO electrodes with 0.07 mAh/cm2 active
material loading. Commercially available LTO was
fabricated into electrodes at TIAX.
Although LTO has very attractive attributes with respect to
stability and charging rate capability, its low capacity and
high potential impose significant sacrifice of Li-ion cell
energy. However, LTO remains the only commercially
available Li-ion anode alternative to carbon materials and is
the only material capable of being charged at very high rates
and low temperatures, and thus the use of a cathode material
having the highest possible energy content (CAM-7) along
with suitable high rate capability helps offset the loss of celllevel energy content that comes with use of LTO.
Separator: Separator selection can be a very important
factor determining the power capability of Li-ion cells.
Separator properties vary greatly depending on the
applications for which they are intended, with high powercapable separators tending to be thin with high porosity and
low tortuosity.4 TIAX has sourced and evaluated various
commercial and experimental separators from the leading
separator manufacturers. Figure 3 summarizes capacity
data for discharge of Li metal half cells employing identical
CAM-7 electrodes (~7.5mg/cm2 active material loading;
C/5 capacity of ~ 1.5 mAh/cm2) and electrolyte (1M LiPF6
in 1:1:1 EC:DMC:EMC) but different separator materials,
The high power CAM-7/LTO cell chemistry is being
implemented and tested in 2 cm2 coin cells using electrode
design parameters (formulations, loadings and balance; i.e.,
anode to cathode ratio) that specifically target high power
density and long life. An 18650 cell (1.8 cm diameter, 6.5
cm height cylindrical format) engineering model based on
these electrode designs is used to derive test conditions for
the coin cells that scale to 5 kW/l in 18650 cells made with
identically designed electrodes, and then uses the same
model to project 18650 cell performance from the coin cell
results. State-of-the-art commercial 3F ultracapacitors are
tested in parallel under similarly-scaled test conditions that
have been derived by the same approach of measuring the
ultracapacitor’s electrode design parameters and inputting
them in the 18650 engineering model.
Cell Performance
Continuous 5 kW/l cycling energy acceptance & delivery:
Direct comparison of the continuous 5 kW/l power
acceptance or delivery capabilities of a CAM-7/LTO cell to
those of an ultracapacitor demonstrates the enormous energy
advantage of the Li-ion system, as shown in Figure 4. The 5
kW/l charge to 2.85 V and discharge to 1.0 V data were
obtained at ambient temperature following low rate
discharge of the cells to 1.4 V and low rate charge of the
cells to 2.75 V, respectively. The Li-ion cell’s electrodes
employ a low-loading (1.7 mAh/cm2), high power design
that projects to ~200 Wh/l (low rate) in an 18650 cell. The
actual power densities used to generate the Figure 4 data
(i.e., scaling to 5 kW/l in an 18650 cell design) were 90
mW/cm2 of face-to-face electrode area for the Li-ion cell and
140 mW/cm2 for the ultracapacitor, reflecting the different
design parameters of the relatively thinner Li-ion and thicker
ultracapacitor electrodes. Even under these very high power
charge and discharge conditions, the Li-ion cell accepts or
delivers more than 5 times the energy density of the
ultrapacitor (i.e., operates for over 5 times as long).
3
6.1 seconds
2.8
57 seconds
2.6
2.4
Ultracap 5 kW/l discharge
Voltage, V
2.2
Ultracap 5 kW/l charge
2
Li-ion 5 kW/l discharge
Li-ion 5 kW/l charge
1.8
1.6
1.4
1.2
1
4.5 seconds
30 seconds
0.8
0
10
20
30
40
50
60
70
80
90
Wh/l accepted / delivered
Figure 4. Voltage curves for a CAM-7/LTO Li-ion coin cell
and a commercial 3F ultracapacitor charged and discharged
at powers scaling to 5 kW/l in 18650 cell designs.
Although they fade more rapidly than ultracapacitors, high
power CAM-7/LTO cells nevertheless maintain superior
energy acceptance and delivery for continuous 5 kW/l
charge and discharge over many thousands of cycles, as
shown in Figure 5 below.
Figure 5. Charge and discharge energies for continuous 5
kW/l constant power cycling of a CAM-7/LTO Li-ion coin cell
between 1.0 V and 2.65 V and a 3F ultracapacitor between
1.0 V and 2.85 V scaled to 18650 cell-level energy density.
Continuous cycling shown in Figure 5 is interrupted at
1000 cycle intervals in order to measure 5 kW/l pulse
power capability across the cells’ full state of charge (SOC)
range. During continuous 5 kW/l cycling, the Li-ion cell is
cycled across only ~10% of its SOC range (although the
energy density accepted or delivered is still about 3 times
that of the ultracapacitor). However, the Li-ion cell’s SOC
cycling range and energy accepted/delivered are higher at
1,000 cycle intervals immediately following the pulse
power characterization because the pulse power
characterization ends with the cells at 100% SOC (fully
charged), and thus in subsequent 5 kW/l continuous
cycling, the cell’s SOC cycling range gradually narrows as
the upper limit adjusts back to an intermediate value. Other
fluctuations in the cell’s energy acceptance and delivery are
due to variations in laboratory temperature. Importantly,
the Li-ion cell cycles with approximately the same energy
efficiency as the ultracapacitor, that being ~70%. The
narrow SOC cycling range also results in low cyclinginduced degradation of the Li-ion cell; it maintains
capability for high energy density acceptance and delivery
of high power (~20% decline as compared to ~5% decline
for the ultracapacitor). The Li-ion cell’s high power
capability retention is also demonstrated by measurements
of 5 kW/l pulse power capability, discussed below.
Usable energy for pulsed 5 kW/l acceptance & delivery:
For the 5 kW/l pulse power capability evaluation
(conducted at 1,000 cycle intervals), 5 kW/l charges or
discharges are repeated across the cell’s full SOC range.
Li-ion cells are charged to 3 V cutoff and discharged to 1 V
cutoff at the scaled 5 kW/l power level beginning at 100%
state of charge (SOC) (for discharge power) or 0% SOC
(for charge power), and when the cutoff voltage is reached,
the cell is allowed to rest, and the SOC is decremented
(discharge) or advanced (charge) by 5% at the C/5 rate (i.e.,
for 36 seconds). The 5 kW/l, rest, C/5 sequence is repeated
until the C/5 discharge or charge steps reach 1.4 V or 2.75
V, respectively, these being the respective 0% and 100%
SOC voltages. An analogous procedure is applied to
ultracapacitors with the difference being that the 5 kW/l
cyling voltage limits are 2.85 V and 1.0 V.
Figure 6 shows results for the full 5 kW/l pulse charge and
discharge capability testing of a 3F ultracapacitor at
different 1,000 cycle intervals during the continuous 5 kW/l
cycling.
The triangular region formed under the
intersection of the charge and discharge energy vs. SOC
curves defines the SOC range over which an electrode/cell
design can both accept and deliver a given amount of
energy for charge and discharge at 5 kW/l constant power.
In practical applications requiring the ability to either
source or sink power at any given time, such as in hybrid
electric vehicle batteries and load leveling batteries, the
ability to accept and deliver a given level of power and
energy can define the allowable operational SOC range,
and thus the usable energy of an electrical energy storage
device.
10
5 kW/l discharge fresh
5 kW/l charge fresh
5 kW/l discharge 3000 cycles
5 kW/l charge 3000 cycles
5 kW/l discharge 6000 cycles
5 kW/l charge 6000 cycles
5 kW/l discharge 10000 cycles
5 kW/l charge 10000 cycles
Wh/l accepted / delivered
9
8
7
6
5
4
3
2
1
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
state of charge
Figure 6. 5 kW/l pulse capability as a function of SOC for a
3F ultracapacitor.
Figure 7 compares the dependence of usable energy on
cycle number for the ultracapacitor and the CAM-7/LTO
Li-ion cell. The figure illustrates how the boundaries of the
usable energy range are set by the length of time for which
a given level of bidirectional (charge or discharge) power
must be sustained.
10
9
Wh/l accepted / delivered
8
7
5
6
5 kW/l
5 kW/l
5 kW/l
5 kW/l
5 kW/l
5 kW/l
5 kW/l
5 kW/l
5
4
3
2
1
0
3.5 0
0.15 kW/l0.2
0.3 fresh0.4
discharge
5 kW/l charge fresh
3
0.5
discharge 1 fresh
charge 1 fresh
discharge 3000 cycles
charge 3000 cycles
discharge 6000 cycles
charge 6000 cycles
discharge 11000 cycles
charge 11000 cycles
0.6
0.7
0.8
0.9
time,
sec.
1
1
state of charge
5 kW/l discharge 3000
2
Wh/l accepted / delivered
5 kW/l charge 3000
5 kW/l discharge 6000
2.5
5 kW/l charge 6000
Conclusions
Suitably designed CAM-7 cathode, LTO anode Li-ion
technology can sustain cell-level power density comparable
to that of ultracapacitors for both discharge and charge.
The approximately 150 Wh/l usable energy range within
which bidirectional 5kW/l power can be sustained by this
Li-ion technology for 1 second is more than 20 times
higher than that of ultracapacitors. The novel cell
technology is robust, and shows promise to retain usable
energy density for high power acceptance/delivery superior
to that of ultracapacitors for over 10,000 cycles.
Acknowledgements: TIAX thanks the US Navy
NAVFAC for funding under Phase I SBIR contract #
N39430-13-P-1246.
References
1. J.P. Zheng, J. Huang, and T.R. Jow, “The Limitations
of Energy Density for Electrochemical Capacitors”
Journal of the Electrochemical Society, Vol. 144, no.
6, pp. 2026-2031, 1997.
2.
Jim P. Zheng, “High Energy Density Electrochemical
Capacitors without Consumption of Electrolyte”
Journal of the Electrochemical Society, Vol. 156, no.
7, pp. A500-A505, 2009.
3.
Tsutomu Ohzuku, Atsushi Ueda, and Norihiro
Yamamoto, “Zero-Strain Insertion Material of
Li[Lil/3Ti5/3]O4 for Rechargeable Lithium Cells,”
Journal of the Electrochemical Society, Vol. 142, no.
5, pp. 1431-1435, 1995.
K.M. Abraham, “Directions in Secondary Lithium
Battery Research and Development,” Electrochimica
Acta, Vol. 38, no. 9, pp. 1233-1248, 1993.
time,
sec.
5 kW/l discharge 10000
2
Like Figures 4 and 5, Figure 7 shows the energy advantage
that Li-ion cells can provide over ultracapacitors, even in
high power applications. Li-ion cells have the potential to
sustain 5 second long 5 kW/l charge or discharge pulses
over a broad usable energy range (~50% of the SOC range,
or ~100 Wh/l), whereas ultracapacitors can not do so at all.
The total projected energy density of an ultracapacitorbased 18650 cell is ~8 Wh/l, and thus the projected usable
energy density for 1 second duration bidirectional 5 kW/l
pulse power capability shown in Figure 6 does not exceed
5 Wh/l, whereas that for a Li-ion cell is ~150 Wh/l. The
usable energy of the ultracapacitor is unchanged after
10,000 continuous 5 kW/l constant power cycles, while the
5 sec. duration and 1 sec. duration usable energies of the
Li-ion cell decline ~38% and ~12%, respectively, after
11,000 cycles (but with almost all this loss occurring in the
first 6,000 cycles). Even with these losses, the Li-ion
technology’s usable energy density still remains far greater
than the ultracapacitor usable energy density.
5 kW/l charge 10000
1.5
1
1
0.5
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
state of charge
Figure 7. 5 kW/l pulse capability as a function of SOC at
intervals during 5 kW/l constant power cycling. Top: CAM-7
LTO Li-ion cell. Bottom: 3F ultracapacitor.
4.