CA2389907A1 - Small format, high current density flat plate rechargeable electrochemical cell - Google Patents

Abstract

An improved flat plate rechargeable alkaline manganese dioxide-zinc cell comprising a paste-like cathode applied to a first metal based current collector; a mercury-free paste-like anode applied to a second metal based current collector, a separator, and an aqueous alkaline electrolyte. The paste-like cathode comprises active electrolytic manganese dioxide, active electrolytic manganese dioxide, 2-15% by wt of at least one of graphite and carbon black, 2-5%
by wt of a polymeric binding agent. 1% or less by wt of electrolyte absorbing agent, 1-15% by wt of a barium compound, and 0.01-10% by wt hydrogen recombination catalyst.
The paste like anode comprises a zinc active material, 25-50% by wt of an aqueous electrolyte, 0.5-5.0% by wt thickening agent, 0-5% by wt of electrolyte absorbing agent, and 0-20% by wt of solid zinc oxide. A method of forming paste-like cathodes and anodes for flat plate electrodes is also provided.

Description

SMALL FORMAT, HIGH CURRENT DENSITY FLAT PLATE
RECHARGEABLE ELECTROCHEMICAL CELL
FIELD OF THE INVENTION
This invention relates to a method of producing flat plate electrodes in a small format that exhibit high current densities, higher utilization of the active materials, and better rechargeability. The method of forming the electrodes requires the active materials, binders, thickening agents, additives, and an alkaline electrolyte to form a paste that is applied to a current collector.
BACKGROUND OF THE INVENTION
Flat plate or prismatic cells are currently found in many applications requiring high current densities such as portable computers and cellular phones. Typical rechargeable flat plate cells available in the market include nickel cadmium, nickel metal hydride, and lithium ion. Each of these battery design systems offers certain advantages depending on their related applications. It is possible to replace these systems with a rechargeable alkaline manganese flat plate cell offering various advantages that include no memory effect, longer shelf life, improved charge retention at higher operating temperatures, lower cost, and environmental superiority.
Rechargeable alkaline manganese flat plate cells have been described in the prior art in particular in U.S. Patent No. 3,945,847 to Kordesch et al. This patent describes a flat plate cell with a coherent manganese dioxide electrode. This electrode contains a conductive binder consisting of a non-conductive polymer such as epoxy and colloidal graphite having a surface area of less than about 100m2/g, such as Acheson DAG
No.
155. The colloidal conductive binder provides the manganese dioxide electrode with improved cohesion and efficiency of the electrode. The flat cells constructed by Kordesch with the conductive binder were based on a 10-cm2-area cell where the cathode was the capacity limiting electrode for the 1 electron conversion of Mn02 ~Mn203 on discharge.

U.S. Patent No. 5,660,953 to Conway et al. describes a flat plate manganese dioxide electrode that is enforced with metallic bismuth and graphite powder where the cathode capacity is increased towards the theoretical 2-electron capacity of the cathode for the discharge from MnO2~Mn(OH)Z. It is found that the cathode capacity becomes greater than 75% of the theoretical 2-electron capacity after 4-7 cycles. In addition, the graphite incorporated with a smaller particle size increased the initial cathode capacity by almost 30%; however, the active material loading of the cathode is only between 20 and 51 % Mn02 and the majority of the available capacity is delivered below 0.9 V
versus zinc electrode, which is considered too low for many practical applications.
In order to meet current demands placed on small format electronic applications, there is still a need to further improve the rechargeability of alkaline manganese dioxide-zinc cells with respect to effective material utilization, voltage level of discharge curve, and capacity fade.
SUMMARY OF THE INVENTION
This present invention provides a flat plate rechargeable alkaline manganese dioxide-zinc cell exhibiting higher utilization of the cathode and anode materials based on cell configuration and cell balance. In addition, the invention provides for an anode and cathode formulation and method to achieve a longer cycle life with reduced capacity fade.
In one embodiment, the invention provides a flat plate rechargeable electrochemical cell comprising a paste-like manganese dioxide cathode pressed onto a metal based current collector, a separator, and paste-like zinc anode applied to a metal based current collector, and an aqueous alkaline electrolyte wherein the minimal cell configuration equates to a 1.5V cell with a cell balance that is the ratio of theoretical zinc capacity (0.820 Ah/g) over the theoretical 1-electron Mn02 capacity (0.308 Ah/g),within the range between 50 and 180%, wherein capacity fade remains fairly constant after the 4'h cycle for up to at least 25 cycles. The embodiment also provides a rechargeable flat cell with a Utilization Factor (UF = initial cell capacity/ 1-electron MnOz capacity) of at least 60%
and a Multiples Utilization Factor (MUF = cumulative cell capacity/1-electron Mn02 capacity) over 25 cycles between 9 and 11. The term cumulative cell capacity defines the total delivered discharge capacity over 25 discharge/charge cycles by addition of the individual capacities from each cycle. The term Multiples Utilization Factor (MUF) defines the multiples of the theoretical 1-electron Mn02 capacity that are obtained over a number of discharge/charge cycles.
In another embodiment, the invention provides a flat plate rechargeable electrochemical cell comprising a paste-like manganese dioxide cathode, a separator, and a paste-like zinc anode, and an aqueous alkaline electrolyte wherein the cathode mixture allows for at least a 10% increase in the multiples utilization factor (MUF) over 25 cycles. The paste-like cathode comprising a binder, a electrolyte absorbing agent, manganese dioxide, and graphite wherein a small portion of the graphite is mixed in with the binder and wherein this portion of graphite has an average particle size between 2 and 6 microns based on the laser diffraction method at 90% minimum.
In another embodiment, the invention provides a flat plate rechargeable electrochemical cell comprising a paste-like manganese dioxide cathode, a separator, a paste-like zinc anode, and an aqueous alkaline electrolyte wherein the anode mixture incorporates a thickening agent, zinc or zinc alloys, nucleation agents, optionally an electrolyte absorbing agent, and zinc oxide wherein the portion of zinc oxide added replaces the zinc available; hence, lowering cell balance but allowing for a change in the physical characteristics of the paste that is easily spread onto the metal based current collector. Also, the addition of zinc oxide at least up to 15% has a minimal effect on the utilization factor.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other features of the invention will become more apparent in the following detailed description wherein reference is made to the appended Figures 1 and 1 (a) which are cross-sectional views of the minimal 1.5 V RAM flat plate cell configuration according to one embodiment of the invention.

Figures 2 and 2 (a) are cross-sectional views of the electrodes embodied in the RAM flat plate cell configuration.
Figures 3 and 4 are graphs of discharge capacity versus the discharge/charge cycle for a 1.5 V RAM flat plate single sided and sandwich cell where capacity fade is limited after the 4th cycle.
Figure 5 is a graph of discharge capacity versus the discharge/charge cycle for a RAM flat plate cell with a cathode containing a small portion of a fine graphite in the binder that is compared to a colloidal binder system as disclosed in prior art (US
3,945,847 to Kordesch et al.) DESCRIPTION OF THE PREFERRED EMBODIMENTS
Figures 1 and 1(a) show two configurations of a 1.5V minimum flat plate rechargeable alkaline manganese dioxide-zinc cell,10 and 20. The flat plate RAM cell comprises the following main units: positive, 21 (a), snf negative, 21(b), end terminals or current collectors, the positive terminal being optionally coated with a conductive graphite coating; a cathode assembly 23 as shown in Figures 2 and 2 (a), comprising a paste-like manganese dioxide cathode 26 formed and pressed one side or dual side onto a positive current collector 27 such as nickel-, stainless steel-, silver-, nickel -plated steel collectors, in the form of foam, mesh, or screen, optionally coated with a conductive graphite coating; an anode assembly 25 as shown in Figure 2 (b), comprising a paste-like zinc anode 28 made of an anode gel and pasted onto a negative current collector 29 such as copper-, brass-, bronze-, nickel-, silver collectors, in the form of foam, mesh, or screen, optionally plated with indium or tin; and a separator 24 separating the anode 28 from the cathode 26. The ionic conductivity between the anode and the cathode is provided by the presence of potassium hydroxide, KOH, electrolyte added into the cell in a predetermined quantity. Optionally the potassium hydroxide electrolyte can have zinc oxide dissolved in the range of 2 to 6% by weight.
The 1.5V minimum cell configurations, sandwich format,10 and single sided format, 20, have outer terminals or current collectors for the cathode 21 (a) and anode 21 (b). Middle sections may vary, but produce the same minimum required voltage of 1.5V.

The sandwich format cell,10, middle section comprises the following: a cathode assembly, 23, in contact with the cathode end terminals or current collector, 21 (a), an anode assembly, 25, centered in the middle in contact with anode terminal or current collector 21 (b), and separators, 24, located between cathode and anode assemblies to prevent shorting. The single format cell, 20, has one cathode, 23, and one anode assembly, 25, with a single separator, 24, there in between. These configurations produce the minimal voltage requirements for a flat plate cell. To produce higher voltages, multiple layers can be constructed with varying configurations to give a stacked flat plate cell or a bipolar cell.
In accordance with the present invention, the paste-like cathode 26, comprises the following materials, active electrolytic manganese dioxide; 2% to 15% by weight of graphite and/or carbon black to provide conductivity; 2% to 5% by weight of a polymeric binding agent such as but not limited to polyisobutylene, polytetrafluoroethylene, polyamide, polyethylene or a metal stearate to bind and provide lubricity during processing and facilitate gas penetration into the electrode; minor amounts below 1.0%
by weight of an electrolyte absorbing agent such as a cross-linked sodium polyacrylate;
the addition of compounds such as barium oxide, hydroxide or sulfate, in the range of 1 to 15% by weight to improve performance during discharge/discharge cycling;
and 0.01 % to 10% by weight finely divided hydrogen recombination catalyst such as silver or its oxides or hydrogen absorbing alloys such as LaNix or NiTiy, to prevent pressure build-up from gassing resulting from corrosion of the zinc.
One method of producing the manganese dioxide electrodes of the invention can be by a process that comprises:
1. Mixing electrolytic manganese dioxide, graphite, carbon black, silver oxide, and barium sulfate to produce a homogeneous dry mixture;

2. Adding polymeric binder agent to said dry mixture, wherein the said polymeric binder such as polyisobutylene of a viscosity between 1.0 X105 Pa's and 1.0 x Pa s, is dispersed in a solvent such as toluene at typically 5.0% by weight;

3. Evaporating the majority of the solvent (~ 90%) from said mixture above;

4. Forming said mixture above into a shape that can be pressed for example a sphere or ball, much similar to forming a "dough";
S. Placing shaped ball or sphere or cathode mass between two sheets of water repellant material, such as Teflon~ sheet or waxed paper;
6. Rolling flat the cathode mass to the desired thickness;
7. Removing one sheet of material and replace with current collector;
8. Roll press cathode mass into current collector until desired thickness is reached;
9. Dry the finished cathodes until most of the solvent is evaporated (~98%) and re-press the cathode electrode assembly in a final step between 100 to 3000 kp/cm2.
Other conventional coating, spreading, or pressing techniques can be employed.
A variety of anode active materials can be used with the paste-like cathode 26 of the present invention. The anode active materials may comprise one of mercury-free or mercury-free and lead-free zinc or zinc alloy. The mercury-free or mercury-free and lead-free zinc or zinc-alloy may be metallic, a powder, granular, particulate, fibers, or in the form of flakes but are not limited to these forms. The zinc alloy may comprise of mercury-free and lead-free zinc-bismuth alloy, mercury-free and lead-free zinc-bismuth alloy of finer particle size, zinc-lead alloy, zinc-aluminum-bismuth-indium alloy, zinc-calcium-bismuth-indium alloy, zinc-magnesium-bismuth alloy, and any combination thereof. The zinc may contain up to 800 ppm lead, up to 800 ppm indium, up to 500 ppm calcium, up to 500 ppm magnesium, up to 200 ppm bismuth and up to 200 ppm aluminum.
In the present invention, the paste-like anode 28, in addition to the zinc active materials disclosed above, further comprises, 25% to 50% by weight of an aqueous electrolyte such as potassium hydroxide from 4N to 12N, 0.5% to 5% by weight of a suitable thickening agent such as carboxymethyl cellulose, polyacrylic acid, starches, and their derivatives; 0% to 5% by weight of an electrolyte absorbing agent such as a cross-linked sodium polyacrylate; and 0% to 20% by weight of solid zinc oxide which may function to reduce the gassing of the active zinc, to permit overcharge of the cell, to act as an reserve active mass, and to assist in providing a paste that is easily applied to at least one side of a current collector, 29, such as copper, brass, bronze, nickel, or silver that may be optionally plated with indium or tin. The method of producing this type of anode mixture is similar to that of cylindrical RAM cell anode gel mixtures except that the flat plate mixture is applied to at Ieast one side of a current collector by a coating process such as rolling and optionally may be followed by a pressing process.
In another embodiment the paste-like anode 28, comprises a zinc active material as disclosed above, 0% to 40% by weight of an aqueous electrolyte, 4N to 12N
potassium hydroxide, up to about 20% by weight of solid zinc oxide, 4% to 10% by weight of a polymeric binder such as but not limited to polyisobutylene, polytetrafluoroethylene, polyamide, polyethylene or a metal stearate to bind and allow for the balance between a proper paste consistency and suspension of the zinc network; and may optionally include a minor amount below 1.0% by weight of an electrolyte absorbing agent such as a cross-linked sodium polyacrylate. In this embodiment, the paste-like anode, 28, is processed by kneading the zinc/zinc oxide powder mixture with the colloidal binder and electrolyte and optionally absorbing agent. This paste is subsequently applied to at least one side of the current collector, 29, such as copper, brass, bronze, nickel, or silver, that may be optionally plated with indium or tin, by the anode processing means disclosed above.
Material of separator 24 is generally a flexible structure which is impermeable to zinc dendrites, but which is permeable to ions and may be permeable to the passage of gases such as hydrogen or oxygen that are produced within the cell on overcharge, standby, or over-discharge conditions. The separator may comprise an absorber layer made from cellulose, RayonT"", polyamide, polypropylene or polyvinylalcohol fibers, and an ion permeable membrane layer made of cellulose, CellophaneT"", radiation grafted polyethylene, polypropylene, or the like.
The 1.5V minimum cell configurations, sandwich format,10 and single format, 20, flat plate RAM cell are constructed and composed of materials to provide a high current density cell with improved cycling performance and reduced capacity fade that is commercially viable. The constructed cells have an active electrochemical cell balance that is the ratio of the theoretical zinc capacity (0.820 Ah/g), zinc oxide is not taken into consideration as it is not active at that point, over the theoretical 1-electron Mn02 capacity (0.308 Ah/g), within the range between 50 and 180%, wherein the multiples utilization factor (MUF) over 25 cycles is between 9 and 11.
In a further aspect of the invention, the paste-like cathode, 26, comprise a small portion of conductive graphite in the polymeric binder wherein this graphite has an average particle size between 2 and 6 microns based on the laser diffraction method measured at 90% minimum. The selected particle size range allows for at least a 10%
increase in the multiples utilization factor (MUF) over 25 cycles. The graphite combined with the manganese dioxide may be of the same or different grade or particle size range as the graphite added to the polymeric binder. Further investigation is warranted for the appropriate graphite combinations for optimum cycling performance.
Furthermore, the addition of fine powder graphite in the binder has shown at least 5% increase in the MUF
over 25 cycles over the addition of colloidal graphite as disclosed in US
3,945,847 to Kordesch.
A further aspect of the invention, the paste-like anode 28 comprise a portion of zinc oxide wherein the zinc oxide replaces the available zinc, lowers cell balance, but provides for a even paste and better suspension of the zinc network as to easily spread the mixture onto the current collector, 29. Also, the addition of zinc oxide at least up to 15% has a minimal effect on the utilization factor on first discharge.
The following examples will assist those skilled in the art to better understand the invention and its principles and advantages. It is intended that these examples be illustrative of the invention and not limit the scope thereof.
Example 1 Flat plate rechargeable alkaline manganese dioxide 9 cm2 cells, AS-F76 and AS-F76(a), were prepared with cathodes of the following composition: 81%
electrolytic manganese dioxide, 10% graphite, 3% carbon black, 3% barium sulfate, and 3%
polyisobutylene. The cathodes were processed according to the above description. The pastes were pressed one sided onto a nickel based current collector, Exmet, of 0.3mm thickness. Cell AS-F76 was pressed at 300 kp/cm2 while cell AS-F76(a) was pressed at 850 kp/cmz onto the collector. It was previously determined that pressing forces of up to 150 kp/cm2 are insufficient for good cycle performance.
The paste-like anode was prepared for both cells of the following composition:
65% zinc, 2% zinc oxide, 1% magnesium dioxide, 1.5% CarbopolT"" 940, and 30.5%
electrolyte (9M KOH with 5% zinc oxide dissolved in it). This method of anode preparation being similar to cylindrical RAM cells. The anodes were pasted onto a brass current collector. Cells were constructed according to Figure 1 (a) with a cell balance of 150%, a laminated separator of a non-woven fiber material to cellophane, and electrolyte of 9M KOH added to thoroughly wet the separator and the cathode.
The cells were discharged through a constant resistor load of 7 ohms to a cut-off voltage of 0.9Volts, followed by a 260-minute recharge to 1.72V. This discharge/charge cycle was repeated 25 times. The 7-ohm resistor load in this example represented an approximate current density of 17 mA/cm2 or 200 mA/g Mn02.
Cycle life of the cells is shown in Figure 3 and measured utilization factor (UF=
initial cell capacity/MnOz capacity), multiples utilization factor (MUF=
cumulative cell capacity/Mn02 capacity) over 25 cycles, and first cycle charge factor (CF=
charge/discharge capacity) appear in Table 1. As disclosed above, the term cumulative cell capacity defines the total delivered discharge capacity over 25 discharge/charge cycles by addition of the individual capacities from each cycle. In addition, the term Multiples Utilization Factor (MUF) defines the multiples of the theoretical 1-electron Mn02 capacity that are obtained over a number of discharge/charge cycles.

LiIiI I i GI

Figure 3 Discharge capacity of cells AS-F76 and F76a Flat cells AS-F76 & 76a 9 cm2, 7 Ohm discharge (17mA/cm2) ___ - -_ 200 _ _ --___.-- -f- F76 -300kg/cm' 175 f F76a - 850kglcm 125 _ .....-;, a I

__ p cycles Table 1: PERFORMANCE SUMMARY OF FLAT RAM CELLS
AS-F76 and F76a Cell Approx. Rate OF [%] MUF (t~ 25 1$' Cycle # of CF

Discharge cycles [%]

(mAIgMnOZ) F76 200 58 9.8 85.6 F76a 200 60 10.4 87.4 io This example demonstrates that the discharge capacity after the first 4 to 5 cycles remains fairly constant over the remaining cycles at least up to the 25th cycle. In addition, in comparison to US 3,945,847 to Kordesch, which in Figure 5 illustrates at 200 mA/gMn02 the Mn02 utilization factor is about 30%, in this invention, it is shown in Table 1 that the Mn02 utilization factor is about 60%, a 100% improvement over prior art. In addition, the cells of this embodiment are capable of repeated deep discharge cycling to predetermined cut-off voltage (i.e. 0.9V), whereas the Kordesch prior art cells were cycled at a maximum of 40% depth of discharge.
The example also demonstrates that the two pressing forces applied are both effective and show little difference in performance with the higher pressing forces. It was subsequently determined that the cycle performance remains about the same for pressing forces of up to 3000 kp/cm2. However, internal resistance of the cathode was reduced with higher pressing forces resulting in higher short circuit current capabilities, which is of advantage if an application requires fast, high pulse currents.
Example 2 Flat plate rechargeable alkaline manganese dioxide 22 cm2 sandwich cells, AS-F115 and AS-F115 (a), were prepared with cathodes of the following composition: 79%
electrolytic manganese dioxide, 9.75% graphite, 2.75% carbon black, 5% barium sulfate, 0.35% electrolyte absorbing agent, SANFRESH DK300T"", and 3% polyisobutylene.
The cathodes were processed according to the above description. The pastes were pressed one sided onto a nickel based current collector, Exmet, of 0.3mm thickness.
The paste-like anode was prepared for both cells of the following composition:
50% zinc, 2% zinc oxide, 1% magnesium dioxide, 2% CarbopolT"" 940, and 45%
electrolyte (9M KOH with 5% zinc oxide dissolved in it). This method of anode preparation being similar to cylindrical RAM cells. The anodes were pasted onto a brass current collector. Cells were constructed according to Figure 1, sandwich construction, i i~ i ~ ~i with a cell balance of 150%, two layers of a membrane separator, ScimatT""
SC31/08, and electrolyte of 9M KOH added to thoroughly wet the separator and the cathode.
The cells were discharged through different constant resistor loads. Cell AS-Fl 15 was discharged through 3.9 ohms and cell AS-F115 (a) was discharged through 10 ohms both to a cut-off voltage of 0.9Volts followed by a 5-hour recharge to 1.75 V.
In this example, the 3.9-ohm load resistor represented an approximate current density of 12.5 mA/cm2 or 190 mA/gMn02 and the 10-ohm load resistor 5.5 mA/cm2 or 80 mA/gMn02, respectfully.
Cycle life of the cells is shown in Figure 4 and measured utilization factor (UF=
initial capacity/Mn02 capacity), multiples utilization factor (MUF= cumulative capacity /Mn02 capacity) over 25 cycles, and first cycle charge factor (CF=
charge/discharge capacity) appear in Table 2.
Figure 4 Discharge capacity of cells AS-F115 and AS-F115a Flat cells AS-F115 & 115a 22 cm2, 3.9/10 Ohm discharge (12.5 / 5.5 mA/cm~) 250 __ __-______ - ___._. __. ._ 175 f 3.9 Ohm 150 ~ 10 Ohm v~ 125 ., - - - - --~.-~-- _-. -~---E

Cycles Table 2: PERFORMANCE SUMMARY OF FLAT RAM CELLS
AS-F115 & AS-F115a Cell Approx. Rate OF MUF ~ 25 1g' cycle CF [%]
# of [%]

Discharge cycles (mAIgMn02) F 115 190 66 10.8 88.7 F 115a 80 74 11.5 84.2 This example further demonstrates that the discharge capacity at varying rates continues to remain fairly constant after the first 4 to 5 cycles to at least the 25"' cycle. In addition, for these sandwich-constructed cells at a cell balance of 150%, it is shown in Table 2 that the Mn02 utilization factor continues to be an improvement over prior art, also at the lower current densities of approximately 80 mA/g Mn02, 74% OF is achieved with this invention vs. 40% OF with prior art. Once again, it is shown that cells of this embodiment are capable of repeated deep discharge cycling to predetermined cut-off voltage (i.e. 0.9~, whereas the Kordesch prior art cells were cycled at a maximum of 40% depth of discharge.
Example 3 Flat plate rechargeable alkaline manganese dioxide 11 cm2 cells, AS-F113 and AS-F 114, were prepared with cathodes of the following composition; AS-F 113:
78.4%
electrolytic manganese dioxide, 9.7% graphite, 2.9% carbon black, 4.95% barium sulfate, and 3% polyisobutylene containing 1.1% of a colloidal graphite DAGT"" No. 109B
by Acheson Colloids Inc; AS-F114: 78.4% electrolytic manganese dioxide, 9.7%
graphite, 2.9% carbon black, 4.95% barium sulfate, and 3% polyisobutylene containing 1.1 % of a graphite powder KS 2.5 by Timcal Inc. The cathodes were processed according to the above description, the only differential being the mixture of the binding agent. The pastes were pressed one sided onto a nickel based current collector. Both cells were pressed at 300 kp/cmz The paste-like anode was prepared for both cells of the following composition:
50% zinc, 2% zinc oxide, 1% magnesium dioxide, 2% CarbopolT"" 940, and 45%
electrolyte (9M KOH with 5% zinc oxide dissolved in it). This method of anode preparation being similar to cylindrical RAM cells. The anodes were pasted onto a brass current collector. Cells were constructed according to Figure 1 (a) with a cell balance of 150%, a laminated separator of a non-woven fiber material and cellophane, and electrolyte of 9M KOH added to thoroughly wet the separator and the cathode.
The cells were discharged thmugh a constant resistor load of 3.9 ohms to a cut-off voltage of 0.9Volts followed by a 5- hour recharge to 1.81 V. This discharge/charge cycle was repeated 25 times. In this example, the 3.9- ohm load resistor represents an approximate current density of 200mA/gMn02, Cycle life of the cells is shown in Figure 5 and measured utilization factor (UF=
initial capacity/Mn02 capacity), multiples utilization factor (MUF= cumulative capacity/Mn02 capacity) over 25 cycles, and first cycle charge factor (CF=
charge/discharge capacity) appear in Table 3.

I. i I', i1 . ~i Figure 5 Flat cells AS-F113-114 11 cm2, 3.9 Ohm discharge (25mA/cm2) 175 -1- KS6/KS2.5 a~ ---100 -_ _ 75 _____ _._-_.____ .__._ _ _ 50 ----.___ -_ _-_____..____ ___. __. -Cycles Table 3: PERFORMANCE SUMMARY OF FLAT RAM CELLS
AS-F113 & AS-F114 Cell Approx. Rate OF MUF ac 25 cycles15' cycle CF
of [%]

# Discharge [%]

(mAIgMn02) F 200 55 9.5 89.3 F 200 57 10.4 95.2 This example demonstrates that the addition of fine graphite to the binder versus the addition of colloidal graphite to the binder results in an improvement of 9.5% for the fine graphite as measured by the MUF over 25 cycles. This improvement is attributed to the better charge efficiency as measured by the 1St cycle CF. In regard to charging, it was noticed that the increase in charge voltage from 1.72V in example 1, to 1.75V
in example 2, to 1.81 V in example 3, slightly increased the 1 St cycle charge factor of about equal current densities. However, in none of the experiments overcharge from these charge voltages was observed and no damage from higher charge voltages was experienced for these flat cells. On the other hand, the charge voltage maximum is a very critical parameter for cylindrical RAM cells, which use fairly thick electrodes and higher charge voltage will more likely cause over charge and cell failure.
Example 4 Flat plate rechargeable alkaline manganese dioxide sandwich 22 cm2 and single sided, 13.5 cm2 cells, with zinc oxide content varied between 0% and 15% were prepared with the same cathodes of the following composition; 81 % electrolytic manganese dioxide, 10% graphite, 3% carbon black, 3% barium sulfate, and 3%
polyisobutylene.
The cathodes were processed according to the above description. The pastes were pressed one sided onto a nickel based current collector.
The paste-like anode cells with varying zinc oxide content were prepared of the following compositions as outlined in Table 4 below. The cells were constructed according to Figures 1 sandwich and 1 (a) single sided, with varying cell balance that is also presented in Table 4. The method of anode mix preparation being similar to cylindrical RAM cells. The anode mix was pasted onto a brass current collector and pressed to a uniform thickness.

Table 4: FLAT RAM CELL COMPOSITIONS AND CELL BALANCES
Cell # Zn Zn0 Carbopol ElectrolyteCell Balance Cell Balance [%] [%] j%] [%J for for Single Sided Sandwich Cell (a) [%~
Cell [%]

130/130 65 0 2 33 125 62.5 (a) 131/131 63 2 2 33 121 60.5 (a) 132/132 60 5 1 34 115 57.5 (a) 133/133 55 10 0.75 34.25 106 53 (a) 134/134 50 15 0.6 34.4 96 48 (a) The cells were discharged through a constant resistor load of 10 ohms to a cut-off voltage of 0.9Volts followed by a 5-hour recharge to 1.75V.
Measured utilization factors (UF= initial capacity/Mn02 capacity) appear in Table

5. Cell (a)'s are single sided format.
Table 5: FLAT RAM CELL PERFORMANCE FOR CELLS

Cell Cell Type Zn0 [%) OF [%J
#

130 Sandwich 0 70 130 (a) Single Sided0 63 131 Sandwich 2 71 131 (a) Single Sided2 65 132 Sandwich 5 67 132 (a) Single Sided5 62 133 Sandwich 10 65 133 (a) Single Sided10 60 134 Sandwich 15 60 134 (a) Single Sided15 63 m This example illustrates that the addition of zinc oxide can be used to modify pasting properties and at the same time still maintain the level of performance as measured by the utilization factor on initial discharge. In addition, the zinc oxide content and cell formats can be also used to adjust the balance of the cell. The example illustrates that the higher zinc oxide content will result in lower balances without diminishing the physical amount of anode and at the same time continues to maintain the level of performance.

Claims

Claims:

1. A flat plate rechargeable alkaline manganese dioxide-zinc cell comprising a paste-like cathode applied to a first metal based current collector, a mercury-free paste-like anode applied to a second metal based current collector, a separator, and an aqueous alkaline electrolyte, wherein the paste-like cathode comprises;
electrolytic manganese dioxide;
2-15% by wt of at least one of graphite and carbon black;
2-5% by wt of a polymeric binding agent;
1% or less by wt of electrolyte absorbing agent;
1-15% by wt of a barium compound; and 0.01-10% by wt hydrogen recombination catalyst; and wherein the paste-like anode comprises;
a zinc active material;
25-50% by wt of an aqueous electrolyte;
0.5-5.0% by wt thickening agent;
0-5% by wt of electrolyte absorbing agent; and 0-20% by wt of solid zinc oxide.

2. A flat plate rechargeable alkaline manganese dioxide-zinc cell comprising a paste-like cathode applied to a first metal based current collector, a mercury-free paste-like anode applied to a second metal based current collector, a separator, and an aqueous alkaline electrolyte, wherein the paste-like cathode comprises;
electrolytic manganese dioxide;
2-15% by wt of at least one of graphite and carbon black;
2-5% by wt of a polymeric binding agent;
1 % or less by wt of electrolyte absorbing agent;
1-15% by wt of a barium compound; and 0.01-10% by wt hydrogen recombination catalyst; and wherein the paste-like anode comprises;
a zinc active material;

0-40% by wt of an aqueous electrolyte;
4-10% by wt polymeric binder; and 0-20% by wt of solid zinc oxide.

3. The cell according to any one of claims 1 or 2 wherein the polymeric binding agent is selected from the group consisting of polyisobutylene, polytetrafluoroethylene, polyamide, polyethylene, and a metal stearate.

4. The cell according to any one of claims 1 to 3 wherein the electrolyte absorbing agent is a cross-linked sodium polyacrylate.

5. The cell according to any one of claims 1 to 4 wherein the barium compound is selected from the group consisting of barium oxide, barium hydroxide, and barium sulfate.

6. The cell according to any one of claims 1 to 5 wherein the hydrogen recombination catalyst is selected from the group consisting of silver, silver oxides, and hydrogen absorbing alloys.

7. The cell according to any one of claims 1 to 6 wherein the zinc active material is a lead-free zinc, or zinc alloy.

8. The cell according to claim 7 wherein the lead-free zinc, or zinc alloy is in metallic, powder, granular, particulate, fibrous, or flake form.

9. The cell according to claim 1 wherein the thickening agent is selected from the group consisting of carboxymethyl cellulose, polyacrylic acid, starch, and starch derivatives.

10. The cell according to claim 2 wherein the paste-like anode comprises 0 to 1% by wt of electrolyte absorbing agent 11. The cell according to any one of claims 1 to 10 wherein the cathode further comprises conductive graphite in the polymeric binder, the conductive graphite having an average particle size between 2 and 6 microns.

12. The cell according to any one of claims 1 to 11 wherein the utilisation capacity of the cell is at least 60%.

15. A method of forming a cathode electrode and an anode electrode for a flat plate rechargeable alkaline manganese dioxide-zinc cell, the method consisting of;
a) preparing the cathode electrode, the cathode comprising electrolytic manganese dioxide; 2-15% by wt of at least one of graphite and carbon black; 2-5% by wt of a polymeric binding agent; 1% or less by wt of electrolyte absorbing agent; 1-15% by wt of a barium compound; and 0.01-10% by wt hydrogen recombination catalyst, wherein the cathode is prepared by;
-mixing the electrolytic manganese dioxide, at least one of graphite and carbon black, the hydrogen recombination catalyst, the barium compound, and the electrolyte absorbing agent to produce a homogenous dry mixture;
-adding the polymeric binding agent to the dry mixture in a solvent to form a first paste;
-evaporating the solvent;
-applying the paste to a first metal based current collector; and b) preparing the anode electrode, the anode comprising a zinc active material;
25-50% by wt of an aqueous electrolyte; 0.5-5.0% by wt thickening agent; 0-5% by wt of electrolyte absorbing agent; and 0-20% by wt of solid zinc oxide wherein the anode is prepared by;
-mixing the zinc active material, the aqueous electrolyte, and the zinc oxide, with the thickening agent to produce a second paste;
-coating at least one side of a second current collector with the second paste.

16. A method of forming a cathode electrode and an anode electrode for a flat plate rechargeable alkaline manganese dioxide-zinc cell, the method consisting of;
a) preparing the cathode electrode, the cathode comprising electrolytic manganese dioxide; 2-15% by wt of at least one of graphite and carbon black; 2-5% by wt of a polymeric binding agent; 1% or less by wt of electrolyte absorbing agent; 1-15% by wt of a barium compound; and 0.01-10% by wt hydrogen recombination catalyst, wherein the cathode is prepared by;
-mixing the electrolytic manganese dioxide, at least one of graphite and carbon black, the hydrogen recombination catalyst, the barium compound to produce a homogenous dry mixture;
-adding the polymeric binding agent to the dry mixture in a solvent to form a paste;
-evaporating the solvent;
-applying the first paste to a first metal based current collector; and b) preparing the anode electrode, the anode comprising a zinc active material;
0-40% by wt of an aqueous electrolyte; 4-10% by wt polymeric binder; and 0-20% by wt of solid zinc oxide, wherein the anode is prepared by;
-mixing the zinc active material, the aqueous electrolyte, and the zinc oxide with the polymeric binder to form a second paste;
- applying the second paste to at least one side of a second metal based current collector.

17. The method according to any one of claims 15 or 16 wherein the polymeric binding agent is selected from the group consisting of polyisobutylene, polytetrafluoroethylene, polyamide, polyethylene, and a metal stearate.

18. The method according to any one of claims 15 to 17 wherein the electrolyte absorbing agent is a cross-linked sodium polyacrylate.

19. The method according to any one of claims 15 to 18 wherein the barium compound is selected from the group consisting of barium oxide, barium hydroxide, and barium sulfate.

20. The method according to any one of claims 15 to 19 wherein the hydrogen recombination catalyst is selected from the group consisting of silver, silver oxides, and hydrogen absorbing alloys.

21. The method according to any one of claims 15 to 20 wherein the zinc active material is a lead-free zinc, or zinc alloy.

22. The method according to claim 21 wherein the lead-free zinc, or zinc alloy is in metallic, powder, granular, particulate, fibrous, or flake form.

23. The method according to claim 15 wherein the thickening agent is selected from the group consisting of carboxymethyl cellulose, polyacrylic acid, starch, and starch derivatives.

23. The method according to claim 16 wherein the paste-like anode comprises 0 to 1% by wt of electrolyte absorbing agent 24. The method according to any one of claims 15 to 23 wherein the cathode further comprises conductive graphite in the polymeric binder, the conductive graphite having an average particle size between 2 and 6 microns.

25. The method according to any one of claims 15 to 24 wherein the first metal current collector and the second metal current collector are made of a material selected from the group consisting of copper, brass, bronze, nickel, and silver.

26. The method according to claim 25 wherein at least one of the first metal current collector and the second metal current collector are plated with a material selected from the group consisting of indium and tin.