Thermally regenerative battery with intercalatable electrodes and selective heating means

The battery contains at least one electrode such as graphite that intercalates a first species from the electrolyte disposed in a first compartment such as bromine to form a thermally decomposable complex during discharge. The other electrode can also be graphite which supplies another species such as lithium to the electrolyte in a second electrode compartment. The thermally decomposable complex is stable at room temperature but decomposes at elevated temperatures such as 50.degree. C. to 150.degree. C. The electrode compartments are separated by a selective ion permeable membrane that is impermeable to the first species. Charging is effected by selectively heating the first electrode.

TECHNICAL FIELD 
This invention relates to a regenerative battery having intercalatable 
electrodes and, more particularly, this invention relates to a thermally 
or electrically regenerative battery containing electrode compartments 
separated by an ion selective membrane, each of the compartments 
containing a graphite electrode. 
BACKGROUND OF THE INVENTION 
The premium on weight of a spacecraft limits the amount of fuel that can be 
carried into space. The success of any space mission depends on providing 
adequate power for the functioning of the spacecraft. Most spacecraft rely 
on solar panels to provide power. However, solar panels are idle during 
night periods. Fuel cells regenerated by solar cells can provide power 
during the night periods but are limited by the low efficiency of solar 
cells. 
Waste heat or solar generated heat is also available on a spacecraft. The 
heat can be converted to electricity. Conventionally heat from Radioactive 
Thermal Generators, (RTG) is fed to thermocouples or thermionic devices to 
produce electrical power. However, these conversion methods are very 
inefficient because devices only provide a single stage of electrical 
generation in the millivolt range and are limited in conversion 
efficiency. 
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List of References 
Patent No. Patentee 
______________________________________ 
H1076 SLANE, et al. 
4,294,898 HARSTEIN 
4,707,423 KALNIN, et al. 
4,861,690 OPE, et al. 
4,863,818 YOSHIMOTO, et al. 
5,028,500 FONG, et al. 
5,069,683 FONG, et al. 
5,344,724 OZAKI, et al. 
5,344,726 TANAKA, et al. 
5,385,777 HIGUCHI, et al. 
5,387,479 KOKSBANG 
5,426,006 DELNICK, et al. 
5,427,872 SHEN, et al. 
5,436,092 OHTSUKA, et al. 
5,436,093 HUANG, et al. 
5,443,601 DOEFF, et al. 
5,443,928 TAKEUCHI, et al. 
5,451,477 OMARU, et al. 
5,478,672 MITATE 
5,478,673 FUNATSU 
5,482,797 YAMADA, et al. 
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STATEMENT OF THE PRIOR ART 
Kalnin et al. disclose simultaneous intercalation at the cathode and the 
anode from an electrolyte with positive and negative ions, respectively. 
In the process of the invention, Br.sub.2 intercalates into the cathode 
from the electrolyte to yield Br.sup.- ions whereas Li.sup.+ comes out 
of the anode into the electrolyte. 
In the invention, use is made of an ion exchange membrane to separate the 
anode and cathode compartments. The membrane is impermeable to bromine and 
hence to Br.sub.3.sup.- ions. These differences make it clear that the 
battery of the invention works by a different mechanism than the battery 
disclosed by Kalnin et al. The non aqueous solvents disclosed by Kalnin et 
al. are different than those used in the invention. 
In the present invention, the battery is regenerable by heat, making use of 
a unique behavior of bromine intercalated graphite where most of the 
bromine intercalation is reversed by heating the bromine intercalated 
graphite to about 120.degree. C. Such heat may be obtained by solar means 
or by use of waste heat. This thermal regeneration of the battery greatly 
increases its potential uses and applications. 
STATEMENT OF THE INVENTION 
The present invention provides a more direct and more efficient way of 
utilizing heat to store electrical energy. The heat is directly converted 
at high efficiency into stored chemical energy according to the invention 
in a battery containing a thermally regenerable electrode. After 
discharge, the battery can be recharged by heating the thermally 
regenerable battery. 
The invention provides a thermally regenerable battery which produces high 
energy efficiency. The battery contains at least one electrode that 
intercalates a first species from the electrolyte in the first electrode 
compartment to form a thermally decomposable complex during discharge 
while the other electrode supplies another species to the electrolyte in 
the second electrode compartment. The thermally decomposable complex is 
stable at room temperature but is decomposable at an elevated temperature 
of 50.degree. C. to 150.degree. C., preferably 50.degree. C. to 
100.degree. C., suitably 80.degree. C. The electrode compartments are 
separated by a selective ion permeable membrane that is impermeable to the 
first species. 
Charging can be effected either electrochemically or thermally. During 
thermal charging the first electrode is heated to a temperature sufficient 
to decompose the complex. The complex decomposes and the first electrode 
releases the first species into the electrolyte in the first compartment 
while the second species reacts with the second electrode to provide a 
recharged battery. 
The thermally rechargeable battery of the invention can be utilized to 
provide high efficiency conversion of solar energy, Radioactive Thermal 
Generators (RTG) or other waste heat directly to chemical energy which can 
be stored for night time use. The stored chemical energy is directly 
convertible to electrical energy during night or day periods. The 
invention provides direct conversion of solar to electric energy, not 
limited by the efficiency of the solar cell which has an upper limit of 
only about 20 percent. Also the thermally rechargeable battery of the 
invention can be utilized to efficiently convert RTG heat or other heat to 
provide several watts of electricity rather than milliwatts at low 
efficiency as produced by thermocouples or thermionic generators. 
A battery according to the invention can be used to power electrical 
apparatus in space or on earth as long as the power needs of the apparatus 
are consistent with the specific energy or energy density of the thermally 
rechargeable battery. 
These and many other features and attendant advantages of the invention 
will become apparent as the invention becomes better understood by 
reference to the following detailed description when considered in 
conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
The thermally rechargeable battery 8 of the invention as shown in FIG. 1 
includes a positive, thermally rechargeable cathode 10 and a negative 
anode 12 immersed in a common body of electrolyte 14 contained within a 
housing 16. The cathode 10 and anode 12 are separated by a membrane 18 
which is impermeable to the ion species 21 generated on heating the 
cathode by means of a heater 20. 
When a load 22 is placed in the circuit 24 connecting the electrodes 10, 12 
electric current will flow. The ion species 21 will be absorbed by the 
cathode 10. The battery 8 is regenerated by activating heater 20 to 
selectively heat the cathode 10 to a temperature to desorb the ion species 
21. 
Since only the cathode need be heated, the cell configuration illustrated 
in FIG. 2 thermally isolates the cathode 100 from the anode 102. The cell 
104 has cell compartments 106, 108 which are physically separated from 
each other but are joined by a bridge member 110 which contains a membrane 
separator 112. When a heat source 114 such as a resistance heating coil is 
selectively applied to the cathode compartment 106, while the electrodes 
100, 102 are connected by a circuit 116 containing a load 118, 
Br.sub.3.sup.- will be desorbed from the cathode 100 while Li.sup.+ is 
intercalated into the graphite anode 102. 
The cathode is preferably a solid with evenly spaced layers allowing the 
thermally regenerable species to intercalate between the layers forming a 
weak bond with the cathode material. Graphite is known to form 
intercalation complexes with halogens such as bromine and alkali metals 
such as potassium, cesium or lithium. The guest species enter the spacing 
between uniform layers of graphite atoms to form weak compounds at room 
temperature. The intercalation of halogens leads to a positive charge on 
graphite while the halogen ions are negatively charged. Similarly, the 
intercalation of alkali metals leads to a negative charge on graphite. 
The desorption of bromine from a graphite intercalated with about 35 
percent by weight of bromine is shown in FIG. 3. At a temperature of about 
120.degree. C., essentially all bromine is desorbed from the graphite. 
The electrolyte contains a solvent for dissolving the guest species and the 
ions emanating from the anode during discharge. The solvent can be aqueous 
or non-aqueous. 
Suitable redox couples for anodes in an aqueous electrolyte are 
Zn/Zn.sup.++, Zn/Zno, Cd/Cd(OH).sub.2 and Fe/FeOOH. The redox couples are 
shown below: 
EQU Zn.sup.++ +2e.sup.- .revreaction.Zn 
EQU Cd(OH).sub.2 +2e.sup.- .revreaction.Cd+2OH 
EQU FeOOH+H.sub.2 O+3e.sup.- .revreaction.Fe+3OH.sup.- 
EQU ZnO+H.sub.2 O+2e.sup.- .revreaction.Zn+2OH.sup.- 
The preferred anode is graphite which is intercalated with lithium. This 
requires use of a polar non-aqueous solvent such as dimethyl carbonate, 
diethyl carbonate, dioxane, propylene carbonate, ethylene carbonate and 
mixtures thereof. 
Intercalation of graphite with lithium in non-aqueous media is illustrated 
below: 
Li.sup.+ +C+e.sup.- .revreaction.C--Li 
A salt of lithium is present in the electrolyte in an amount from 1% by 
weight up to saturation. Usually the salt is a salt of the intercalated 
species such as LiBr in the case of bromine. The ion exchange membrane is 
impermeable to bromine and Br.sub.3.sup.- ions. Membrane material should 
be capable of rejecting Br.sub.3.sup.- (anion--rejecting membrane). A 
class of such anion-rejecting membranes are sulfonic acid cation--exchange 
membranes such as: 
I. NAFION.sup.R (DuPont product) 
Perfluoro tetraethyl perfluoro alkoxysulfonic acid 
##STR1## 
II. Polyhydrocarbon sulfonic acid 
Dais corporation membranes 
##STR2## 
III. Poly aryloxy Ketone sulfonic acid Victrex.RTM. Sulfonated polymer PEEK 
(poly ether ether ketone) (ICI Product) 
##STR3## 
IV. Polyaryl ether sulfone sulfonic acid 
##STR4## 
sulfonated--Victrex.RTM. or UDEL.RTM. (Union-Carbide) 
A test cell was designed as follows: 
An aqueous solution of approximately 0.50 molar potassium sulfate (K.sub.2 
SO.sub.4) was prepared by dissolving 21.2 g of K.sub.2 SO.sub.4 in 300 ml 
of distilled water. This solution served as the electrolyte in the 
battery. 
Bromine intercalated graphite paper was utilized as the cathode. Bromine 
intercalated graphite is a highly reactive material. Pieces of graphite 
paper were intercalated with bromine by contacting them with saturated 
bromine vapor at room temperature inside a glass flask for a period in 
excess of 3 days. The bromine intercalated paper was then taken out and 
its top end was wrapped in a platinum foil. The platinum foil was 
pre-attached to an electrical cable. Another platinum wire was used to 
serve as the anode in the cell. The reference electrode used was a 
saturated Calomel electrode with a potential of 0.24 volts vs. NHE (Normal 
Hydrogen Electrode). 
Tests were carried out for comparison purposes on Br.sub.2 intercalated 
graphite electrode and a blank graphite electrode. In the first 
configuration, the cathode was the Br.sub.2 -intercalated graphite paper 
and the anode was a platinum wire. The electrolyte was the 0.50 molar 
K.sub.2 SO.sub.4 solution. In the second configuration, the cathode was a 
blank graphite paper and the anode was the same platinum wire. The 
electrolyte was also the same. 
In the test with the first configuration with the Br.sub.2 -intercalated 
graphite, the open circuit voltage was first measured and was found to be 
0.834 volts. This value is close to the Br.sub.2 /Br.sup.- standard 
reduction potential. The two electrodes were then connected and the 
Br.sub.2 electrode was cathodically polarized with a current of about 10.5 
micro-amps using an external power supply. About 20 minutes lapsed in 
connecting the electrodes and adjusting for the proper current. The 
electrode potential of the Br.sub.2 -graphite electrode was monitored as a 
function of time and the current was maintained at 10.5 micro-amps. The 
electrode potential vs. time profile obtained is shown in FIG. 4. The 
total test duration was about an hour. However, the test was interrupted 
three times to measure the open circuit voltage, which was found to be 
close to 0.80 volts. 
In the test with the second configuration with the blank graphite, the open 
circuit voltage was first measured and was found to be 0.132 volts. A 
cathodic current of about 10.5 micro-amps was then made to flow through 
the cell using the external power supply. The electrode potential as a 
function of time was monitored again as before. The measurements are shown 
in FIG. 4. It is seen that the electrode potential remains at -0.400 
volts, and is relatively independent of time. This voltage is basically 
determined by the H.sub.2 evolution reaction in the distilled water 
solution at a pH of 5.0. This is the only reaction that can take place in 
the absence of bromine. 
In test configuration 1, the expected reaction is: 
EQU CBr+e.sup.- Br.sup.- +C 
From FIG. 4, the observed open circuit voltage and the electrode potential 
decay with time are in agreement with the progress of above reaction. 
These results show that the above reaction can be sustained 
electrochemically. Thus, chemically intercalated Br.sub.2 can be 
de-intercalated electrochemically. The capacity of the Br.sub.2 electrode 
could be dependent on the type of graphite used and can be higher, 
depending on operating conditions. 
In the absence of Br.sub.2 in graphite (test configuration II), the 
reduction reaction sustained is H.sub.2 evolution. The observed electrode 
potential, which is relatively constant, is in agreement with this 
reaction. 
The remaining reaction to complete the cycle: [Br.sup.- 1/2Br.sub.2 
+e.sup.- ] is a well studied electrochemical process and its demonstration 
is not repeated here. Note: For regeneration, only the cathode is heated. 
The important parameters to evaluate the practical aspects of such a 
battery are the voltage obtained and the energy density. 
For the bromine cathode, the voltage E.sub.c is likely to be close to the 
standard reduction potential of bromine/Br-couple which is 1.08 volts. For 
the lithium anode, the voltage E.sub.a is close to -3.0 volts. Therefore, 
the cell voltage, E.sub.c -E.sub.a is approaching 
EQU E.sub.c -E.sub.a =1.08-(-3.0)=4.08 volts. 
Specific Energy Calculation: 
For carbon-lithium anode, the stoichiometry is C.sub.6 Li and the specific 
capacity obtained using Faraday's Law is 372 mAh/g. 
For carbon-bromine cathode, preliminary intercalation measurements made 
with bromine vapor at room temperature indicate a stoichiometry of 
C.sub.32 Br and a resultant specific capacity of 69 mAh/g of carbon. As 
this is the smaller number, the cathode energy density will be controlling 
and, hence, the battery specific capacity will be limited to 69 mAh/g of 
carbon. Even limited by the bromine electrode, the specific energy 
(amp-hours.times.volts) comes to be 280 mWh/g of carbon. It should be 
noted that when bromine intercalation is carried out at a higher vapor 
pressure, the cathode specific capacity (and hence specific energy) can be 
increased by a factor of 2 or more. 
Potential Uses: 
This invention has the potential to provide a direct high efficiency 
conversion of solar energy, RTG heat, or other waste heat to chemical 
energy storage which is directly convertible to electrical energy during 
night (or day) periods. Thus, the conversion of solar to electrical energy 
storage is not limited by the efficiency of the solar cell which has an 
upper limit of only about 20 percent. Also, the conventional approach of 
converting the RTG heat into electricity using thermocouples or 
thermoionic devices is rather inefficient because such devices have a 
single stage electrical generation in the millivolt range and are limited 
in conversion efficiency. The present concept, on the other hand, can 
provide about 4 volts at high efficiencies. 
A battery of this type may find several applications for use in space. The 
applications must be consistent with the specific energy or energy 
density. 
It is to be realized that only preferred embodiments of the invention have 
been described and that numerous substitutions, modifications and 
alterations are permissible without departing from the spirit and scope of 
the invention as defined in the following claims.