ADDITIVES FOR PRIMARY ELECTROCHEMICAL CELLS HAVING MANGANESE DIOXIDE CATHODES

Abstract: The invention relates to alkaline cells containing manganese dioxide cathode active material. A substance selected from the group of compounds Bi2O3, PbO2, SnO2, Co3O4, CoO, Bi2O33ZrO3 and K2Cr2O7 is added to the cathode of conventional alkaline cells typically having an anode comprising zinc and cathode comprising manganese dioxide and an alkaline electrolyte. The additive increases the specific capacity (amp-hr/g) of the manganese dioxide in the cathode.

Description: -- ADDITIVES FOR PRIMARY ELECTROCHEMICAL CELLS HAVING MANGANESE DIOXIDE CATHODES The invention relates to alkaline electrochemical cells with manganese dioxide cathode and compounds selected from the group Bi 2 0 3 , Pb0 2 , Sn0 2 , Co 3 0 4/ CoO, Bi 2 0 3 3Zr0 3 and K 2 Cr 2 0 7 added to the cathode material to improve the specific capacity of the manganese dioxide- Primary alkaline cells typically contain zinc anode active material, alkaline electrolyte, a manganese dioxide cathode active material, and an electrolyte permeable separator film, typically of cellulose. Conventional alkaline cells may contain zero-added mercury such that the total mercury content is less than about 50 parts mercury per million parts by weight of total cell. The anode active material comprises zinc particles admixed with conventional gelling agents, such as carboxymethylcellulose or acrylic acid copolymers, and electrolyte. The gelling agent holds the zinc particles in place and in contact with each other. A conductive metal nail, known as the anode current collector, is typically inserted into the anode active material. The alkaline electrolyte is typically an aqueous solution of potassium hydroxide, but other alkali solutions of sodium or lithium hydroxide may also be employed. The cathode material is typically of manganese dioxide and may include small amounts of carbon or graphite to increase conductivity. Conventional alkaline cells are encased in a steel container to retain the cell components and reduce the chance of leakage. Since commercial cell sizes are fixed it has been desirable to attempt to enhance performance and/or useful service life of the cell by increasing the surface area of the electrode acti material and by packing greater amounts of the active materi into the cell. This approach has practical limitations, sin if the active material is packed too densely into the cell th can reduce the rate of electrochemical reaction duri discharge, in turn reducing service life. Other deleterio effects such as polarization can occur, particularly at hi current drain rates. Polarization limits the mobility of io within the electrode active material and within the electrolyt which in turn retards performance and service life. Thus, it desirable to provide a way of retarding such deleterious effec which in turn can increase performance and/or useful servi life of the cell. One way of retarding such deleterious effects is increase the actual specific capacity of the Mn0 2 catho material, typically electrolytic manganese dioxide (EMD) . T EMD in conventional alkaline cells has a theoretical specif capacity of about 308 mAmp-hr per gram. The actual specif capacity of the EMD material realized in discharge of conventional alkaline cell is less than theoretical because the inefficiencies caused by polarization effects and oth phenomena which effect the cathode reactions and efficiency electron transfer. For example, the actual specific capacity EMD in a standard alkaline cell when discharged at a high ra (at 3.9 ohms load) to a potential of 0.8 volts may be about 1 mAmp-hr/g for a continuous discharge and 220 mAmp-hr f intermittant discharge. This would correspond to an efficency EMD of 63% and 71%, respectively. It has been discovered that the addition of small amoun of specific compounds to the cathode of conventional prima zinc/Mn0 2 alkaline cells increases the actual specific capaci (amp- hr/g) of the Mn0 2 cathode active material in the cell. The compounds which have been found to increase the actual specific capacity of the Mn0 2 material in such cells are: Bi 2 0 3 , Pb0 2 , Sn0 2 , Co 3 0 4 ,CoO, Bi 2 0 3 3Zr0 3 and K 2 Cr 2 0 7 and combinations thereof. The inclusion of a cathode additive selected from the foregoing group of compounds advantageously improves the specific capacity of the Mn0 2 cathode active material in such cells. These compounds may be added to the cathode so that their total weight comprises between about 0.1 to 10 percent by weight of the total cathode. The following examples illustrate the invention and advantages derived therefrom. (All compositions are by weight unless otherwise specified.) Example 1 (Comparative Example) : A conventional primary zinc/manganese dioxide alkaline (standard D cell) , is prepared with conventional cathode and anode active material, electrolyte and separator membrane. The anode material may be in the form of a gelled mixture containing mercury free (zero-added mercury) zinc alloy powder. The total mercury content of the cell is thus less than 50 parts mercury per million parts cell weight. The anode mixture may typically contain aqueous KOH solution, gelling agent, e.g. acrylic acid copolymer such as CARBOPOL C934 from B.F. Goodrich; and surfactants e.g. organic phosphate ester surfactant GAFAC RA600 from Rhone Poulenc. The separator membrane can be a conventional electrolyte permeable membrane of polyvinyl alcohol/rayon material. The electrolyte is an aqueous solution of KOH containing about 40 wt% KOH and 2 wt% ZnO, hereinaft referred to as "aqueous KOH solution" . The cathode active material in the standard cell has t following composition: Electrolytic manganese dioxide (84 wt%) , graphite (9 wt%) and a 7 Normal "aquesous KOH solution" (6.5 wt%) . Fresh standard cells are dicharged to a cut-off voltage 1.0 volt and 0.8 volt each on the basis of a continuo discharge and an intermittant discharge. The continuo discharge is carried out by discharging the cell at a contsta current drain rate of about 410 milliamp (equivalent to average load of about a 2.2 ohm) throughout. The intermitta discharge is carried out by discharging the cell at this sa constant current but applied for l hour followed by 5 ho recovery cycles. In each case the actual specific capaci (mAmp-hr/g) of Mn0 2 in the cathode is calculated by multiplyi the current drain by the time taken to reach the designat cut-off voltage divided by the weight of Mn0 2 in the cathode. Example 2 (Comparative Example) : The same standard cells as in Example 1 are prepare These cells are discharged to 1.0 and 0.8 volts under continuo and intermittant discharge conditions as in Example 1 exce that a constant current drain of 274 mAmp (equivalent to a 3. ohm load) is employed. In each case the actual specif capacity of the Mn0 2 in the cathode is calculated in the mann described in Example 1. Example 3 : Experimental zinc/Mn0 2 size D alkaline cells identical that referenced in Example 1 are prepared, except that in maki the experimental cell an amount (gms) of Pb0 2 is added so that the total cathode material comprises 5.0 percent by weight Pb0 2 . The amount of Mn0 2 in the cathode is reduced by an equal amount (gms) so that the total cathode weight in the experimental cell is the same as in the standard cell of Example 1. Thus the cathode composition of the experimental cell is: 79% electrolytic manganese dioxide (EMD), 9.5% graphite, 6.5% KOH solution and 5% Pb0 2 . Experimental cells are discharged to 1.0 volt and 0.8 volt at the same continuous and intermittant discharge conditions as in Example 1. During discharge the drain rates are kept constant at 410 millia ps (equivalent to an average load of about 2.2 ohms) as in Example 1. The specific capacity (mAmp-hr/g) of the Mn0 2 in the experimental cells is calculated in each case from the weight of Mn0 2 in the cells, the current drain, and discharge time required to reach the designated cut-off voltage, as described in Example 1. In each case the actual specific capacity of the Mn0 2 in the experimental cell increases over the specific capacity of the Mn0 2 in the standard cell, at the same discharge conditions and drain rates. At a constant drain rate of 410 mAmp the increase in specific capacity of the Mn0 2 in the experimental cell over that of the standard cell is 12.8% for continuous discharge to 1.0 volt and 15.2% for continuous discharge 0.8 volt. At the same drain rate of 410 mAmp the increase in specific capacity of the Mn0 2 in the experimental cell over that of the standard cell is 9.5% for intermittent discharge to 1.0 volt and 10.2% for intermittent discharge to 0.8 volt. These increases are summarized in Table 1. Example 4 The same experimental cells as in Example 3 are prepare execpt that Bi 2 0 3 is added to the cathode material instead o Pb0 2 . Thus the cathode composition for the experimental cell is: 79% electrolytic manganese dioxide (EMD), 9.5% graphite 6.5% KOH solution and 5% Bi 2 0 3 . These experimental cells are discharged at a constant current of 274 mAmp (equivalent to an average load of 3.9 ohm) in the same manner as in discharg of the standard cell of Example 2. The specific capacit (mAmp-hr/g) of the Mn0 2 in the experimental cells is calculate in each case in the manner described in Example 1. In each case the specific capacity of the Mn0 2 in th experimental cell increases over the specific capacity of th Mn0 2 in the standard cell (Example 2) for the correspondin discharge condition and current drain. Thus, at a constan current drain of 274 mAmp (3.9 ohm load) the increase i specific capacity of the Mn0 2 in the experimental cell over tha of the standard cell is 9.0% for continuous discharge to 1. volt and 12.0% for continuous discharge to 0.8 volt. At th same drain rate of 274 mAmp the increase in specific capacity o the Mn0 2 in the experimental cell over that of the standard cel is 5.9% for intermittent discharge to 1.0 volt and 6.8% fo intermittent discharge to 0.8 volt. These increases ar summarized in Table 1. Example 5: The same experimental cells as in Example 3 are prepare execpt that Sn0 2 is added to the cathode material instead of Pb0 2 . Thus the cathode composition for the experimental cell is: 79% electrolytic manganese dioxide (EMD), 9.5% graphite 6.5% KOH solution and 5% Sn0 2 . These experimental cells are discharged at a constant current of 274 mAmp (equivalent to an average load of 3.9 ohm) in the same manner as in discharge of the standard cell of Example 2. The specific capacity (mAmp-hr/g) of the Mn0 2 in the experimental cells is calculated in each case in the manner described in Example 1. In each case the specific capacity of the Mn0 2 in the experimental cell increases over the specific capacity of the Mn0 2 in the standard cell (Example 2) for the corresponding discharge condition and current drain. Thus, at a constant current drain of 274 mAmp (3.9 ohm load) the increase in specific capacity of the Mn0 2 in the experimental cell over that of the standard cell is 9.3% for continuous discharge to 1.0 volt and 7.9% for continuous discharge to 0.8 volt. At the same drain rate of 274 mAmp the increase in specific capacity of the Mn0 2 in the experimental cell over that of the standard cell is 4.5% for intermittent discharge to 1.0 volt and 5.3% for intermittent discharge to 0.8 volt. These increases are summarized in Table 1. Example 6 The same experimental cells as in Example 3 are prepared execpt that Co 3 0 4 is added to the cathode material instead of Pb0 2 . Thus the cathode composition for the experimental cells is: 79% electrolytic manganese dioxide (EMD), 9.5% graphite, 6.5% KOH solution and 5% Co 3 0 4 . These experimental cells are discharged at a constant current of 274 mAmp (equivalent to a 3.9 ohm load) in the same manner as in discharge of the standard cell of Example 2. The specific capacity (mAmp-hr/g) of the Mn0 2 in the experimental cells is calculated in the manner described in Example 1. In each case the specific capacity of the Mn0 2 in the experimental cell increases over the specific capacity of the Mn0 2 in the standard cell (Example 2) for the corresponding discharge condition and current drain. Thus, at a constant current drain of 274 mAmp (3.9 ohm load) the increase in specific capacity of the Mn0 2 in the experimental cell over tha of the standard cell is 4.2% for continuous discharge to 1.0 volt and 5.6% for continuous discharge to 0.8 volt. These increases are summarized in Table 1. Example 7 The same experimental cells as in Example 3 are prepa execpt that CoO is added to the cathode material instead of Pb0 2 . Thus the cathode composition for the experimental cel is: 79% electrolytic manganese dioxide (EMD), 9.5% graphit 6.5% KOH solution and 5% CoO. These experimental cells are discharged at a constant current of 274 mAmp (equivalent to average load of 3.9 ohm) in the same manner as in discharge the standard cell of Example 2. The specific capaci (mAmp-hr/g) of the Mn0 2 in the experimental cells is calculat in each case in the manner described in Example 1. The specific capacity of the Mn0 2 in the experimental ce increases over the specific capacity of the Mn0 2 in the standa cell (Example 2) for the corresponding discharge condition a current drain in all cases except intermittent discharge to 1 volt. At a constant current drain of 274 mAmp (equivalent to average load of 3.9 ohm) the increase in specific capacity the Mn0 2 in the experimental cell over that of the standard ce is 7.0% for continuous discharge to 1.0 volt and 4.7% f continuous discharge to 0.8 volt. At the same drain rate of 2 mAmp there is a decrease in specific capacity of the Mn0 2 in t experimental cell over that of the standard cell of 0.1% f intermittent discharge to 1.0 volt and an increase of 2.5% f intermittent discharge to 0.8 volt. These changes are summarized in Table 1. Example 8 The same experimental cells as in Example 3 are prepared execpt that Bi 2 0 3 3Zr0 2 is added to the cathode material instead of Pb0 2 . Thus, the cathode composition for the experimental cells is: 79% electrolytic manganese dioxide (EMD), 9.5% graphite, 6.5% KOH solution and 5% Bi 2 0 3 3Zr0 2 . These experimental cells are discharged at a constant current of 274 mAmp (equivalent to a 3.9 ohm load) in the same manner as in discharge of the standard cell of Example 2. The specific capacity (mAmp-hr/g) of the Mn0 2 in the experimental cells is calculated in each case in the manner described in Example 1. For discharge at a constant current drain of 274 mAmp (3.9 ohm load) there is a decrease of 0.6% in the specific capacity of the Mn0 2 in the experimental cell compared to that of the standard cell (Example 2) for continuous discharge to 1.0 volt and an increase of 6.2% for continuous discharge to 0.8 volt. At the same drain rate of 274 mAmp the specific capacity of the Mn0 2 in the experimental cell compared to the specific capacity of the standard cell does not change for intermittent discharge to 1.0 volt but increases 5.5.% for intermittent discharge to 0.8 volt. These changes are summarized in Table 1. Example 9 The same experimental cells as in Example 3 are prepared execpt that K 2 Cr 2 0 7 is added to the cathode material instead of Pb0 2 . Thus the cathode composition for the experimental cells is: 79% electrolytic manganese dioxide (EMD), 9.5% graphite, 6.5% KOH solution and 5% K 2 Cr 2 0 7 . These experimental cells are discharged at a constant current of 274 mAmp (equivalent to average load of 3.9 ohm) in the same manner as in discharge the standard cell of Example 2. The specific capac (mAmp-hr/g) of the Mn0 2 in the experimental cells is calcula in the manner described in Example 1. In each case the specific capacity of the Mn0 2 in experimental cell increases over the specific capacity of Mn0 2 in the standard cell for the corresponding discha condition and current drain. Thus, at a constant current dr of 274 mAmp (3.9 ohm load) the increase in specific capacity the Mn0 2 in the experimental cell over that of the standard c is 9.3% for continuous discharge to 1.0 volt and 18.1% continuous discharge to 0.8 volt. At the same drain rate of 2 mAmp there is no change in specific capacity of the Mn0 2 in experimental cell over that of the standard cell f intermittent discharge to 1.0 volt and there is an increase 5.5% in the specific capacity of the Mn0 2 in the experiment cell over that of the standard cell for intermittent discha to 0.8 volt. These changes are summarized in Table 1. As seen from Table 1 the addition of any of the listed additives to the alkaline cell cathode results in an increase the Mn0 2 specific capacity at both continuous and intermitte discharge to 0.8 volts. If the amount of additive is adjuste e.g. between 0.1% and 10%, preferably between about 0.1 and by weight of the cathode, the service life of the cell c increase compared to the standard cell having an additon (equal) amount of Mn0 2 in place of the additive. In fact with t 5 wt% amount of additive employed in each of the above example service life of the cell when discharged to 0.8 volts increas over that of the standard cell when the additves Bi 2 0 3 , Pb0 2 , S and K 2 Cr 2 0 7 are employed. The percent increase in service life such cases are summarized in Table 1. The running voltage (i.e. the voltage vs. discharge time profile) of the cells containing the additives Pb0 2 is measured to be higher than the running voltage of the standard cell. In general the increase in specific capacity of Mn0 2 resulting from inclusion of any of the above described additives will tend to increase the running voltage of the experimental cell containing the respective additive. TABLE 1 EFFECT OF CATHODE ADDITIVES Discharge to 1.0 Volts Discharge to 0.8 Volts CATHODE % Increase % Increase % Increase % Increase ADDITIVE 1 DISCHARGE Sp.Capacity Service Life Sp. Capacity Service Life (5 wt%) TEST of MnO, of Cell 2 of MnO, of Cell 2 Bi 2 0 3 Continuous +9. .0 +2.5 +12.0 +5.3 Bi 2 0 3 Intermittent 3 +5, .9 -0.5 +6.8 +0.4 Pb0 2 Continuous +12. .8 +6.0 +15.2 +8.3 Pb0 2 Intermittent 3 +9. .5 +2.9 +10.2 +3.6 Sn0 2 Continuous +9. .3 +2.7 +7.9 +1.4 y-> t- Sn0 2 Intermittent 3 +4 .5 -1.8 +5.3 -1.0 Co 3 0 4 Continuous +4 .2 -2.1 +5.6 -0.7 Co 3 0 4 Intermittent 3 +4 .3 -2.0 +0.1 -5.9 CoO Continuous +7 .0 +0.6 +4.7 -1.6 CoO Intermittent 3 -0 .1 -6.1 +2.5 -3.7 Bi 2 033Zr0 3 Continuous -0 .6 -6.6 +6.2 -0.2 Bi 2 033Zr0 3 Intermittent 3 0 .0 -6.0 +5.5 -0.8 K 2 Cr 2 0 7 Continuous +9 .3 +2.7 +18.1 +11.0 Notes : 1. Additive comprises 5 wt% of cathode. 2. Service life comparison to same cell with more Mn0 2 in place of the additive. 3. Repetitive discharge cylces of 1 hour followed by 5 hour recovery. Although the present invention was described with respect to specific embodiments, it should be recognized that variations, for example, in cell size are possible without departing from the concept of the invention.

Claims: WHAT IS CLAIMED IS: 1. A primary electrochemical cell comprising an anode, an aqueous alkaline electrolyte solution, a separator and a cathod comprising manganese dioxide, characterized in that said cathod further includes Bi 2 0 3 , Pb0 2 , Sn0 2 , Co 3 0 4 , CoO, Bi 2 0 3 3Zr0 3 or K 2 Cr 2 or any combination thereof whereby the capacity of the cell is increased when discharged to 0.8 volts. 2. A primary electrochemical cell comprising an anode, a aqueous alkaline electrolyte solution, a separator and a cathod comprising manganese dioxide, characterized in that said cathode further comprises Sn0 2 , Co 3 0 4 , CoO, or K 2 Cr 2 0 7 or any combination thereof added in admixture with the manganese dioxide, and wherein said additive has the effect of increasing the capacity of the cell when the cell is discharged to 0.8 volts. 3. A primary electrochemical cell in accordance with Claim l wherein the cathode comprises manganese dioxide and Bi 2 0 3 or Pb0 2 or a mixture thereof. 4. The cell of claims 1,2 or 3 wherein the anode comprises zinc and the aqeuous electrolyte solution comprises potassium hydroxide. 5. The cell of claims 1,2 or 3 wherein said additive comprises between about 0.1 and 10 percent by weight of the cathode. 6. The cell of claims 1,2 or 3 wherein the total mercury content in the cell is less than 50 parts per million of total cell weight. 7. A primary electrochemical cell comprising an anode comprising zinc, an aqueous alkaline electrolyte solution comprising potassium hydroxide, a separator and a cathode comprising manganese dioxide, characterized in that said cathode further comprises Pb0 2 in admixture with the manganese dioxide, and wherein said additive has the effect of increasing the capacity of the cell when discharged to 0.8 volts.

Inventors: HOWARD MARTIN W, MIECZKOWSKA JOLA A, HOWARD, MARTIN, W., MIECZKOWSKA, JOLA, A.
Applicants: DURACELL INC, DURACELL INC.
Application year: 1996
Main classification: H01M4
IPC codes: H01M4, H01M6