Method of making a lithium-bromine cell

A solid electrolyte primary cell comprising a lithium anode, a bromine cathode and a lithium bromide electrolyte. A solid lithium element operatively contacts the cathode material, and one form of cathode material is a charge transfer complex of an organic donor component material and bromine. The organic donor component material can be poly-2-vinyl pyridine. Another cathode material is liquid bromine. The surface of the lithium anode element which operatively contacts the cathode material can be provided with a coating of an organic electron donor component material. When the lithium anode operatively contacts the bromine cathode, a solid lithium bromide electrolyte begins to form at the interface and an electrical potential difference exists between conductors operatively connected to the anode and cathode.

BACKGROUND OF THE INVENTION 
This invention relates to the conversion of chemical energy to electrical 
energy, and more particularly to a solid electrolyte primary cell having a 
lithium anode, a bromine cathode and a lithium bromide electrolyte. 
In recent times a solid electrolyte primary battery has been developed to 
provide relatively high voltage and high energy density in a battery which 
is especially useful for long life, low current drain applications. 
Lithium is generally recognized as the most satisfactory material for the 
negative electrode, i.e. the anode on discharge, in a non-aqueous cell. In 
selecting a material for the positive electrode, i.e. cathode on 
discharge, it is necessary to consider, among other factors, relative 
chemical activity and energy density. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of this invention to provide a a new and 
improved solid electrolyte battery having relatively high voltage and high 
energy density, and being especially useful for long life, low current 
drain applications. 
It is a further object of this invention to provide a solid electrolyte 
primary cell having a lithium anode, a bromine cathode and a lithium 
bromide electrolyte. 
It is a further object of this invention to provide a lithium-bromine cell 
which is relatively convenient and economical to manufacture. 
The present invention provides a lithium-bromine cell comprising a lithium 
anode, a bromine cathode and a solid lithium bromine electrolyte. In one 
aspect of the invention the cathode material comprises a charge transfer 
complex of an organic donor component and bromine. In another aspect 
thereof, the cathode material comprises liquid bromine. The surface of the 
lithium anode which operatively contacts the cathode material can be 
provided with a coating of an organic electron donor component material. 
The foregoing and additional advantages and characterizing features of the 
present invention will become clearly apparent upon a reading of the 
ensuing detailed description together with the included drawing wherein:

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
In the development of solid electrolyte batteries, lithium is recognized as 
a very desirable material for the negative electrode, i.e. the anode on 
discharge, in a non-aqueous cell. The cell of the present invention 
includes a lithium anode and a bromine cathode to utilize the desirable 
characteristics of bromine, among which are a significant degree of 
chemical activity, a moderately low molecular weight, and a significant 
level of energy density. 
Referring now to the drawing, a lithium-bromine cell according to the 
present invention is generally designated 10 and includes a housing or 
casting element having a generally cup-shaped base portion 12 and a 
peripheral rim or flange portion 14. The base portion 12 can be of 
rectangular or circular configuration, and the casing is of a material 
which is non-reactive with bromine. One form of material found to perform 
satisfactorily is a fluoropolymer material commercially available under 
the name Halar, a trademark of the Allied Chemical Company. The cell of 
the present invention includes an anode in the form of a solid lithium 
element 20 and a current collector element 22 contacting a surface of 
lithium element 20. An anode lead 24 connected such as by welding at one 
end to current collector 22 extends out through an aperture in the housing 
base portion 12 making external electrical connection to a load circuit. 
In forming the anode for the cell of the present invention, current 
collector 22 is moved into position adjacent the inner surface of the base 
portion 12 and lead 24 is inserted through the opening and used to draw or 
pull current collector 22 tightly against the surface of the housing. If 
desired, an element or button 26 of anode material, i.e. lithium, can be 
placed between collector 22 and the surface of casing 12 as shown in the 
drawing. The current collector 22 can comprise No. 12 zirconium mesh 
having a thickness of about 0.004 inch and lead 24 can be a relatively 
thin strip of zirconium. Then lithium element 20, initially in plate or 
sheet form, is placed in casing portion 12 adjacent collector 22. The 
entire assembly then is positioned in a suitable holding fixture and then 
force is applied to the exposed surface of lithium element 22 in a manner 
forcing or extruding it along the inner surface of casing portion 12 and 
along the inner surface of casing portion 14 [and along the inner surface 
of portion 14] so that it conforms to the inner surface of the casing with 
a resulting shape as shown in the drawing. A seal or patch 28 of suitable 
material, for example a fluoropolymer materially comercially available 
from the Dupont Company under the trademark Tefzel, can be placed over the 
outer surface of the housing around the aperture through which lead 24 
extends and sealed in place by a suitable cement such as the cyanoacrylate 
cement commercially available from Techni-Tool Inc. under the designation 
Permabond 100. In addition, the exposed surface of lithium element 20 
preferably is provided with a coating 30 of an organic electron donor 
component material, and the nature of coating 30 and its role in the cell 
of the present invention will be described in further detail presently. 
The cell of the present invention further comprises a bromine cathode 
including a region of cathode material 32 within the assembly and 
operatively contacting lithium element 20 and a cathode current collector 
34 operatively contacting the cathode material 32. According to a 
preferred mode of the present invention, the cathode material 32 comprises 
a charge transfer complex of an organic donor component and bromine. A 
preferred organic donor component is polyvinyl pyridine polymer and in 
particular two vinyl pyridine polymer. Cathode material 32 preferably 
comprises a mixture of bromine and poly-two-vinyl pyridine in a weight 
ratio of 6:1 bromine to polymer. The mixture is allowed to stand until it 
develops a rubbery consistency and is of a generally brown-red coloration. 
A quantity of the cathode material then is placed in the assembly in 
contact with the coated lithium element 20 and in an amount filling the 
open interior region. A cathode current collector and lead combination is 
positioned in the assembly and in contact with the cathode material. 
Cathode current collector 34, which can comprise No. 12 mesh platinum 
metal, is secured at the periphery as by welding to one end of a cathode 
lead 36 which can be a thin strip of platinum iridium alloy, which is 
enclosed by a sheet of insulating material 38, for example the 
aforementioned Halar material, which lead 36 extends out from the 
periphery of the casing for making external electrical connection thereto. 
Then a casing closure element 40 in the form of a sheet of suitable 
material is placed over the end of the assembly in contact with the 
peripheral rim or flange 14 and the components are then heat sealed 
together. The marginal or peripheral portion of sheet 40 and the rim or 
flange 14 therefore must be of a material which is heat sealable, and this 
requirement is satisfied by the aforementioned Halar material. Heat 
sealing is performed by placing the assembly in a suitable fixture and 
applying a heated platen to the peripheral end or flange portion at a 
temperature of about 495.degree. .+-. 5.degree. F and at a force of about 
60 pounds .+-. 10 pounds which have been found suitable to provide an 
adequate seal. While heat is being applied to the periphery of the 
assembly, the remainder of the cell assembly can be adjusted to low 
temperature refrigeration or gas to prevent expansion and leakage of the 
cathode material 32. 
The lithium-bromine cell according to the present invention operates in the 
following manner. As soon as the bromine-containing cathode material 32 
placed in the assembly operatively contacts lithium element 20, a solid 
lithium-bromine electrolyte begins to form at the interface, and an 
electrical potential difference will exist between the anode and cathode 
electrical leads 24 and 36, repectively, when the current collectors are 
in operative position. The mechanism by which the foregoing is 
accomplished is believed to include migration of lithium ions through the 
electrolyte whereby lithium is the ionic species in the cell. 
Table 1 presents electrical data obtained from a lithium-bromine half cell 
according to the present invention as a function of cell life in days. For 
example, the data entered in the first row of Table 1 was obtained 1 day 
after the half cell was placed in operation. The impedance quantities 
indicate impedance measured at 1000 hertz, and impedance measurements were 
made with a 100 kilohm resistance connected in parallel with the cell 
under test. 
Table I 
______________________________________ 
Open Circuit Voltage 
Cell Impedance 
Cell Life in Days 
In Volts In Ohms 
______________________________________ 
1 3.456 79 
5 3.457 120 
7 3.457 128 
14 3.459 163 
19 3.458 190 
26 3.457 217 
33 3.458 250 
41 3.452 318 
51 3.451 349 
______________________________________ 
The cathode material 32 comprising a charge transfer complex of an organic 
donor component and bromine is prepared in the following manner. A 
preferred organic donor component material is poly-two-vinyl pyridine. The 
mixture is prepared in a pressure-tight container having a pressure-tight 
closure. The polymer material is placed in the container and then the 
liquid bromine is added thereto, the preferred ratio by weight of bromine 
to polymer being 6:1. The container is closed so as to be pressure-tight 
and is allowed to stand for about one-half day at room temperature. The 
result is a rubber-like, semi-solid plastic mass with no liquid bromine 
present. The material is removed from the container, this generally 
requiring some tool or implement, and is placed into the cell assembly in 
a manner as described above. It has been found that mixing the bromine and 
polymer in a somewhat greater weight ratio of bromine to polymer, for 
example 7.5:1, is not satisfactory. With such a weight ratio the liquid 
bromine was observed not to combine readily with the polymer but to remain 
in liquid form. Upon standing, when the mixture was solidified, it was 
observed to be a very sticky plastic mass which would adhere strongly to 
glass containers and would release copious amounts of bromine vapor and 
then upon heating would release liquid bromine. When the mixture is 
prepared with a bromine to polymer weight ratio considerably less than 
6:1, it was observed that not all of the polymer would react with the 
liquid bromine. 
The material of coating 30 on lithium element 20 is an organic electron 
donor material of the group of organic compounds known as charge transfer 
complex donors. The material of the coating can be the organic donor used 
in preparing the charge transfer complex of the cathode material 32, but 
other materials can be employed. A preferred material for the coating is 
polyvinyl pyridine and it is applied to the exposed surface of lithium 
element 20 in the following manner. A solution of poly-2-vinyl pyridine 
polymer in anhydrous benzene or other suitable solvent is prepared. The 
poly-2-vinyl pyridine is readily commercially available. The solution is 
prepared with 2-vinyl-pyridine present in the range from about 10% to 
about 20% by weight with a strength of about 14% by weight of 
2-vinyl-pyridine being preferred. While 2-vinyl pyridine, 4-vinyl pyridine 
and 3-ethyl 2-vinyl pyridine can be used, 2-vinyl pyridine is preferred 
because of its more fluid characteristics in solution. When the solution 
is prepared at a strength below about 10% the resulting coating can be 
undesirably too thin and when the solution is prepared at a strength 
greater than about 20% the material becomes difficult to apply. The 
solution is applied to the exposed surface of each lithium plate in a 
suitable manner, for example simply by application with a brush. The 
presence of the anhydrous benzene serves to exclude moisture thereby 
preventing any adverse reaction with the lithium plate. The coated anode 
then is exposed to a desiccant in a manner sufficient to remove the 
benzene from the coating. In particular the coated anode is placed in a 
chamber with barium oxide solid material for a time sufficient to remove 
the benzene, which can be in the neighborhood of 24 hours. 
The following illustrative examples further describe the cells of the 
present invention. 
EXAMPLE I 
A housing for an experimental cell was provided by a length of Pyrex glass 
tubing having an overall length of about 3 inches and a diameter of about 
2 centimeters. A pair of closure elements in the form of end plugs of 
Delrin material fit into opposite ends of the tubing and extend axially 
outwardly therefrom. The end plugs are held in place by an assembly 
comprising a pair of plates abutting corresponding ones of the end plugs 
and connected together by axially extending bolts located radially 
outwardly of the tubing and which are threaded at each end and connected 
to the plates by means of wing nuts. The anode includes two lithium disks, 
one having a small hole or aperture in the center thereof, which disks are 
sandwiched together against a zirconium screen serving as the current 
collector and having a lead spot-welded thereto and protruding through the 
center hole in the one lithium disk. The cathode current collector 
comprises a disk of zirconium having a thickness of about 0.030 inches and 
having a lead spot-welded thereto. Both the anode and cathode leads extend 
through central bores or channels provided in the end plugs. A mixture of 
bromine and poly-2-vinyl pyridine was prepared in a weight ratio of 6:1 
bromine to polymer, in particular 30 grams bromine and 5 grams polymer, 
and allowed to stand overnight. The mixture became very rubbery and of a 
brown-red coloration. A plug of this material was placed inside the glass 
tube body of the test cell, the anode and cathode assemblies then were 
placed against opposite end surfaces of the cathode material, and the end 
plugs were inserted into opposite ends of the glass tube with the leads 
extending through the bores thereof. Then force was applied to the end 
plugs by tightening the wing nuts evenly until the cathode material was 
observed to fill the remaining space in the cell completely. The material 
occupied a region of the tube having an axial length of about one-half 
inch. All spaces where leaks possibly might occur were sealed with a halo 
carbon type grease. The pertinent electrical data which were immediately 
determined were an open circuit cell voltage of 2.9 volts, a voltage of 
1.7 volts when connected to a 50 kilohm load, a voltage of 0.1 volts when 
connected to a 1 kilohm load, and a cell impedance of greater than 300 
ohms. The foregoing measurements were obtained at room temperature or 
approximately 20.degree. C. The test cell was placed in a warm room for 
approximately 3 days and electrical data determined again at a temperature 
of 98.6.degree. F. Under these conditions, the open circuit voltage was 
2.36 volts, the voltage was 2.5 volts with a 50 kilohm load, the voltage 
was 0.038 volts with a 1 kilohm load, and the cell impedance was greater 
than 1000. The cell was removed from the warm room and placed on shelf, 
and after about 6 weeks a measurement indicated an open circuit voltage of 
greater than 2 volts and a cell impedance of greater than 10,000 ohms. 
EXAMPLE II 
A cell wherein the cathode material was in the form of liquid bromine was 
built using a generally hollow rectangular casing of epoxy material and 
open at one end. A cathode current collector in the form of a zirconium 
screen was placed in the casing so as to lie along the bottom or closed 
end and up along both opposite sidewalls thereof. The cathode lead then 
extended out and over an edge of the casing. The anode was prepared by 
pressing a pair of lithium plates together against a zirconium screen 
having a lead extending therefrom, and the opposite exposed faces of the 
lithium plates were coated with a charge transfer complex in the same 
manner as described above in connection with the cell 10 shown in the 
drawing. The anode assembly was suspended in the center of the case and 
the lead was passed through an opening provided in the casing lid. The lid 
was provided with another opening to which was cemented a small tube 
extending upwardly therefrom. The lid then was cemented in place in the 
open end of the casing, by suitable sealant such as liquid epoxy resin 
material. Liquid bromine was then introduced to the interior of the casing 
through the tube whereupon a small plug was cemented in place in the open 
end of the tube to seal the bromine within the casing. The liquid epoxy 
resin material was used for this seal also. The initial open circuit 
voltage of the cell was measured at about 3.4 volts. 
EXAMPLE III 
A cell casing and anode assembly were provided similar to that shown in the 
drawing and the bromine and poly-2-vinyl pyridine were introduced in a 
stepwise manner. In particular, a small amount of polymer was first 
sprinkled into the half cell on the lithium element, then bromine was 
added, then a little more polymer followed by bromine, etc. This stepwise 
procedure was continued until the half cell was completely filled with the 
mixture and having a fairly smooth surface adjacent the top. A closure 
element was sealed in place against the top in a suitable manner, for 
example with a fast-setting cement, and electrical measurements of the 
cell performance were made. On one side of the battery the open circuit 
voltage was 3.1 volts, the voltage under 50 kilohm load was 1.8 volts, and 
voltage under 1 kilohm load was 0.3 volts. On the other side of the 
battery the open circuit voltage was 2.9 volts, the voltage under 50 
kilohm load was 1.4 volts, and the voltage with one kilohm load was 0.2 
volts. 
It is therefore apparent that the present invention accomplishes its 
intended objects. While several embodiments of the present invention have 
been described in detail, this is done for the purpose of illustration, 
not limitation.