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Thin-film rechargeable lithium batteries were developed
by Dr. John Bates and his team of scientists and engineers
from more than a decade of research at the Oak Ridge
National Laboratory (ORNL). Unlike conventional batteries,
thin film batteries can be deposited directly onto chips
or chip packages in any shape or size, and when fabricated
on thin plastics, the batteries are quite flexible.
Some of the unique properties of thin-film batteries
that distinguish them from conventional batteries include:
- All solid state construction
- Can be operated at high and low temperatures (tests
have been conducted between -20°C and 140°C)
- Can be made in any shape or size
- Cost does not increase with reduction in size (constant
$/cm2)
- Completely safe under all operating conditions.
 |
| Fig. 1. Miniature
thin film lithium battery on a ceramic substrate
for use in an implantable medical device. |
Thin-film lithium-ion batteries have the additional
advantage of being unaffected by heating to 280°C.
Many integrated circuits are assembled by the solder
reflow or surface mount process in which all of the
electronic components are soldered on the board at the
same time by heating to temperatures as high as 280°C
for a few minutes. Conventional batteries, such as coin
or button cells, contain organic liquid electrolytes
that cannot survive such temperatures and therefore
must be added to the circuits as a separate component,
often manually.
Because of their unique features, thin-film batteries
have a wide range of uses as power sources for consumer
and industrial products such as non-volatile memory,
semiconductor diagnostic wafers, smart cards, sensors,
radio frequency identification tags, and medical products
such as implantable defibrillators and neural stimulators.
The small size of this new battery technology will improve
existing consumer and medical products and enable the
development of many new products.
The purpose of this section is to introduce thin film
battery technology and to present test results on ORME
prototype batteries. A brief discussion of
batteries and terminology is given in Battery
Basics.
 |
| Fig. 2. Schematic layout of a
thin film battery. |
The construction of a thin film solid state battery
is illustrated by the schematic drawing in Fig. 2. The
different layers are deposited by sputtering or evaporation,
methods which are commonly used in the semiconductor
and optical coating industries. The deposited battery
stack from current collector to anode is less than 5
micrometers (μm) thick. Depending on the choice of substrate
and packaging method, the total battery thickness ranges
from 0.35 mm to 0.62 mm. The battery can be fabricated
in any shape and size provided that the electrolyte
film completely isolates the cathode and anode films.
Descriptions of the methods of fabrication have been
published in the scientific and patent literature
(see references). Because the batteries
can have any shape and size, they can be tailored to
meet the specific energy, power, and space requirements
for each application.
 |
| Fig. 3. Discharge of a thin-film
lithium battery at current densities of 0.02, 0.1,
0.2, 0.5, 1.0, 2.0, 5.0, and 10.0 mA/cm2. |
One of the unique features of thin film batteries
is their capability to deliver high current densities
with good cathode utilization as illustrated in Fig.
3. The current density and discharge capacity are based
on the area of the cathode. The curves in Fig. 3 were
obtained by charging the battery to 4.2 V, holding the
potential fixed until the current density decreased
to 1μA/cm2, and then discharging the cell to 3.0 V.
The charge and discharge steps are repeated for each
current density. At a current density of 5 mA/cm2,
this battery delivered 50% of its maximum capacity
at 37°C (body temperature).
 |
| Fig. 4. Discharge of a thin-film
lithium-ion battery at current densities of 0.02,
0.1, 0.2, 0.5, 1.0, 2.0, 5.0, and 10.0 mA/cm2. |
A set of discharge curves for a thin-film lithium-ion
battery are shown in Fig. 4. For the particular combination
of anode and cathode of this cell, the upper cutoff
potential of the charge step is 4.0 V while the lower
cutoff potential of the discharge step is 2.0 V. It
is typical of lithium-ion cells that the operating voltages
are lower than those of batteries with lithium anodes
for comparable current densities. For example, for the
lithium-ion cell of Fig. 4, the potential at the midpoint
of the 0.02 mA/cm2 discharge curve is 3.54 V whereas
the midpoint potential of the lithium cell of Fig. 3
is 3.93 V. A discussion of the differences between lithium
and lithium-ion batteries is given in the section, Battery
Basics.
In considering the possible application of different
batteries, it is important to know how much energy can
be delivered at a specified discharge rate or discharge
power. Graphs of the energy/cm2 delivered at a given
power density for the lithium and lithium-ion batteries
discussed above are given in Fig. 5.
 |
| Fig. 5. Graph of energy vs. power
per unit area of the cathode from the discharge
data for the lithium and lithium-ion batteries in
Figs. 3 and 4. |
These types of graphs are known as Ragone plots,
and they show how the energy delivered by a battery
depends on power, i.e on the discharge rate. The values
of energy E were obtained by integrating the potential
ε(q) as a function of the capacity per unit area,
q, over the discharge curves,
E = ∫ε(q)dq
The average power was calculated from
P = E•i/q
where i is the current density. All batteries exhibit
the same qualitative trend shown in Fig. 5: As more
power or current is required, less energy is delivered
due to internal resistive losses.
In order to obtain high cell capacities or energies
at high discharge rates as in Fig. 3 and 4 , it is necessary
to heat treat the cathode at temperatures of 700°C or
above. This means that the substrate upon which the
battery is fabricated must be able to withstand these
high temperatures. Thin film batteries also can be fabricated
on flexible polymer substrates as well, but the temperatures
at which the cathode films can be heat treated are limited
by the substrate.
 |
| Fig 6. Discharge curves for a
thin film battery on a polyimide substrate. |
Some examples of discharge curves measured for a lithium
battery fabricated on a 0.005" thick polyimide
sheet are shown in Fig. 6. Because the cathode anneal
temperature was limited to less than 400°C, the internal
resistance of the battery was about 60 times higher
than a lithium battery made on a rigid ceramic substrate
with a cathode of comparable thickness annealed above
700°C. The advantage of the battery fabricated on thin
polyimide is that is can be bent or twisted or rolled
into a cylinder. This feature can be important in applications
that require a battery conform to a curved surface or
for devices in which it is desirable to fabricate the
battery on the same substrate as a flexible circuit.
 |
| Fig 7. Discharge curves for a
thin film lithium battery at 125°C. |
Current R&D at ORME is focused on the design, fabrication,
and testing of prototype thin film batteries for potential
medical, commercial, and government customers, on the development of new thin film battery chemistries
to achieve better performance in harsh environments,
and on achieving better discharge rate capability of
batteries fabricated on flexible substrates. In addition
to the need for thin film batteries that can survive
high temperature assembly processes, there are significant
markets for batteries that can operate continuously
at temperatures of 125°C or higher. The results obtained
recently for a lithium battery cycled at 125°C (Fig.
7) encourage the view that a lithium-ion cell using
the same type of cathode will be capable of continuous
operation at even higher temperatures.
Battery Basics
The principal components of a battery are the anode,
the cathode, and the electrolyte. During discharge,
a battery converts the heat that would be released in
the chemical reaction between the anode and the cathode
into electrical energy. This is accomplished by imposing
the electrolyte between the cathode and anode. When
no current is flowing, the "open circuit voltage"
(OCV) of a battery is determined solely by the chemical
composition of the anode and cathode and is not related
to the size of the battery. The capacity and energy,
on the other hand, are determined by the size of the
battery. When current is flowing, the voltage can decrease
below the OCV value due to internal resistance in the
battery. These points will be discussed later in this
section.
Consider a battery with a metallic lithium
(Li) anode (negative terminal) and a lithium cobalt
oxide (LiCoO2) cathode (positive terminal).
When formed, the battery is in the discharged state,
so the first step is to charge the battery to 4.2 V
which results in the extraction of 50% of the lithium
from the LiCoO2 cathode. The battery can
then be discharged to 3.0 V. The chemical reactions
of the charge and discharge steps are represented by:
| charge: |
LiCoO2
= 0.5 Li + Li0.5CoO2 |
| discharge: |
0.5
Li + Li0.5CoO2
= LiCoO2 |
To better understand how the chemical energy of the
discharge reaction is converted to electrical energy,
it is useful to separate the discharge process into
the steps that occur on the anode side and on the cathode
side:
| anode: |
0.5
Li = 0.5 Li+ + 0.5e- |
| cathode: |
0.5
Li+ + 0.5e- + Li0.5CoO2
= LiCoO2 |
where Li+ represents lithium ions with a single positive
charge, and e- represents electrons with a single negative
charge. The electrolyte which separates the anode and
cathode conducts lithium ions but not electrons, while
the metal contacts to the battery terminals at the anode
and cathode conduct electrons but not lithium ions.
When the anode and cathode are connected by electrical
leads through a load, the lithium ions travel from the
anode to the cathode through the electrolyte while the
electrons are forced by the electrolyte to travel in
the external circuit before they can recombine with
the lithium ions in the cathode.
 |
| Fig.9. Discharge of a lithium
battery. |
The Li+ ions move into the vacant spaces
that were created when lithium ions were extracted from
the cathode during the charge step. The cartoon in Fig.
9 illustrates this process. As the electrons travel
through the external circuit during discharge, electrical
work is performed on the load. Other materials that
can be used as cathodes in a lithium battery include
oxides such as LiNiO2, LiMn2O4,
and V2O5 and sulfides such as
MoS2 and TiS2.
In a lithium-ion battery, the metallic lithium anode
is replaced with another material that can accept large
concentrations of lithium ions. In "bulk"
lithium-ion batteries with liquid electrolytes such
as those used in cellular telephones and camcorders,
the anode is graphite and the cathode is LiCoO2.
The charge-discharge reaction of this type of battery
is represented by:
3C + LiCoO2
= Li0.5C3
+ Li0.5CoO2
 |
| Fig. 10. Discharge of a lithium-ion
battery. |
The concentration limit of lithium in graphite is about
1 Li per 6 carbon atoms, i.e. LiC6. The cartoon
in Fig. 10 illustrates the discharge of a lithium-ion
battery. Because the anode initially contains no lithium,
the choice of cathode is restricted to those compounds
that contain lithium such as LiCoO2, LiNiO2,
or LiMn2O4.
The capacity of a battery or the amount of charge
that can be supplied usually is expressed in as the
amount of current in amperes (A) flowing for a given
time in hours (h) with units of Ah, mAh, or μAh, where
1 Ah = 1000 mAh and 1 mAh = 1000 μAh. Since 1 A = 1
C/s where C is the charge in Coulombs, 1 Ah = 3600 C,
1 mAh = 3.6 C, and 1 μAh = 3.6 mC.
The energy of a battery is given by the operating
voltage x charge supplied and usually is expressed in
units of Wh, mWh, or μWh. So, for example, if a battery
delivers a charge of 1 mAh at a potential of 4 V, the
energy supplied is 4 mWh. In some instances, the energy
required will be expressed in Joules, J, where 1 Wh
= 3600 J, 1 mWh = 3.6 J, and 1μ Wh = 3.6 mJ. Unlike
the battery potential, the energy depends on the size
of the battery since the capacity or amount of charge
that can be supplied is proportional to the mass of
the cathode. The power delivered by a battery
is the energy supplied per unit time usually expressed
in units of W, mW, or μW, where 1 W = 1 J/s.
Important quantities that often are used to compare
different batteries are the energy and power per unit
volume and mass. The energy density and specific energy
of a battery are given by dividing the energy supplied
at a specified discharge rate by the total volume (in
liters) and total mass of the battery (in kg), respectively,
with units of Wh/l and Wh/kg or equivalently mWh/cm³
and mWh/g. The power density and specific power are
given by the dividing the power delivered by the volume
and mass of the battery, respectively, and usually expressed
in units of W/l and W/kg or mW/cm³ and mW/g. When comparing
batteries, it is important that the energy and power
or discharge current be considered together. Because
of the internal resistance of a battery, the higher
the power or discharge current delivered to a load,
the smaller the energy supplied. The relationship between
energy and power is illustrated in Fig. 5.
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