Method of preparing a rechargeable modified manganese-containing material by electrolytic deposition and related material
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a method of electrolytically preparing a rechargeable modified manganese-containing material, as defined herein. 
     2. Background of the Invention and Description of the Prior Art 
     Materials for battery electrodes which have the capability of being recharged are of increasing usefulness in today&#39;s society due to the effort being made to develop energy alternatives. These efforts have resulted from increased awareness of environmental threats presented by use of conventional fossil fuels and depletion of resources of such fuels. In addition, certain fuels are impractical or impossible to use in applications such as space travel and satellites. Moreover, electric energy sources which are rechargeable such as rechargeable battery cells obviate the need for continuously replacing non-rechargeable cells which have been exhausted. Such energy sources are particularly useful as backups for solar energy systems. 
     A material which has dominated the primary &#34;throw away&#34; battery since the early 1950&#39;s is =modified form of manganese dioxide. This modified manganese dioxide is a high energy, low-cost material. As such, many processes have been tried to bring long life rechargeability to this material. For example, a method of making a birnessite compound which included one type modified manganese dioxide compound was disclosed in U.S. Pat. No. 4,520,005. The method disclosed is a batch method which is not suitable for use on a large scale in commercial applications. Additionally, U.S. Pat. No. 4,451,534 discloses a battery cell which includes a positive electrode composed of a modified manganese dioxide compound. The modified manganese dioxide compound disclosed in the patent is mixed with a percentage of bismuth, lead or mixtures thereof. The cell also includes a zinc negative electrode. Furthermore, the material disclosed can only be made in small batches over a 12 to 24 hour period. The amount of product developed during a relatively lengthy preparation time is generally unacceptable for any commercial application. In addition, the material disclosed in the two aforementioned patents has to cycle with a suitable anode. As noted, zinc is the preferred anode yet it has problems cycling because soluble zinc (zincate) in the electrolyte chemically reacts with the anode material yielding a non-rechargeable mixed anode. 
     Some of these types of methods have resulted in a rechargeable material. The materials show rechargeability on the order of about 250 cycles, but a continuous drop in capacity occurs as the number of cycles increases. 
     A method of producing traditional manganese dioxide, as opposed to a modified manganese dioxide material were disclosed in U.S. Pat. No. 3,535,217. The method disclosed includes using a chloride solution to produce electrolytic, anodic manganese dioxide. The method is suitable for treating naturally-occurring manganese ores. The resulting material is useful in dry cells, however, it is not rechargeable material. U.S. Pat. No. 4,048,027 discloses a method of producing electrolytic manganese dioxide from molten manganese nitrate hexahydrate. Higher current densities used in this method allow higher yields of depolarized material. Improved performance in dry cells is stated to be an advantage. Again, this involves traditional manganese dioxide. 
     Methods have been developed for ease of removal of electrolytically produced manganese dioxide powder. See U.S. Pat. Nos. 4,170,527 and 4,295,943. 
     Other uses of manganese dioxide have been known such as use of manganese dioxide as a catalytic coating for oxygen evolution electrodes. See U.S. Pat. Nos. 4,072,586 and 4,476,104. U.S. Pat. No. 4,863,817 discloses a method of making electrolytic gamma-MnO 2  involving stirring the solution with a gas. 
     Manganese dioxide prepared by the prior art electrolytic methods mentioned hereinbefore is used generally in primary alkaline manganese dioxide-zinc cells. The cells are typically made in a charged state and then discarded. The use of the manganese dioxide-zinc system is limited to primary cells because the manganese dioxide material produced is not fully rechargeable. Although some rechargeability can be achieved, the capacity significantly reduces with each successive cycle. This is particularly true if the material has been deep discharged, as would be understood by those skilled in the art to be greater than about fifty percent of the theoretical two-electron capacity. 
     It has been shown that electrolytic manganese dioxide may be recharged in the range of 100 times, if it is discharged to less than about thirty percent of the theoretical one electron capacity, or less than about fifteen percent of the theoretical two electron capacity. See K. Kordesch, et. al., Electrochemica Acta, 26, 1495 (1981). However, the restriction of shallow discharge does limit the amount of energy which can be generated by such cells, and thus reduces rechargeability of the cells. 
     There remains a need, therefore, for a method of making a highly rechargeable modified manganese dioxide material which can be produced on a commercial scale in a feasible amount of time. There remains a further need for a method of making rechargeable modified manganese dioxide using an electrolytic process. 
     SUMMARY OF THE INVENTION 
     These and other needs are satisfied by the method of the present invention in which a method of electrolytically producing a rechargeable modified manganese dioxide material compound and the resultant compound and a battery cell incorporating the compound are disclosed. The method includes the steps of placing manganese nitrate in an aqueous solution. A comparatively small, predetermined percentage of a doping material such as, for example, lead nitrate, bismuth nitrate, copper nitrate or mixtures thereof is then added to the solution. Two electrodes are immersed into the solution. The electrodes are preferably composed of graphite, graphite powder, nickel or other materials hereinafter discussed. Both electrodes may be composed of the same material, or they may be composed of different materials. An electric current is passed through the electrodes and a modified manganese-containing compound is electrolytically deposited onto both the anode and the cathode. The material deposited onto the anode includes manganese in the Mn 4+  oxidation state which is the charged state. The material which is deposited onto the cathode is in the Mn 2+  oxidation state which is the discharge state. In this way, two materials can be simultaneously produced, one in the charged state and the other in discharged state. The materials produced are highly rechargeable in that a given capacity is maintained throughout a number of successive cycles. This is as compared to prior art methods in which some level of rechargeability is achieved, yet, as the material is cycled, the capacity is substantially diminished from cycle to cycle. 
     The process of the present invention provides a compound which is highly rechargeable and has high energy density. The associated battery which is disclosed has a greatly increased cycling capacity over know rechargeable batteries. 
     It is an object of the present invention to provide a method of producing a modified manganese dioxide as well as other manganese-containing materials. The method can be used to produce a compound in either a charged state or a discharge state. Additionally, the method includes a process for simultaneously generating the material in both states. 
     It is another object of the invention to provide a method of making a modified manganese dioxide material in a manner which can be economically used for rechargeable batteries on a large scale. 
     It is a further object of the invention to provide a method of producing a modified manganese dioxide material compound which has high energy density. 
     It is a further object of the invention to provide a method of producing a modified manganese dioxide compound which is highly rechargeable and capable of being manufactured on a commercial scale. 
     These and other objects of the invention will be more fully understood from the following description of the invention with reference to the illustrations appended hereto. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic cross-sectional illustration depicting a system used in practicing the method of invention. 
     FIG. 2 is a discharge curve for the PbO 2  •MnO 2  material anodically deposited on graphite rod/powder in accordance with the present invention. 
     FIG. 3 is a graph depicting the cycling capacity in volts over time of the material of FIG. 2 made in accordance with the method of the present invention. 
     FIG. 4 is a discharge curve for the PbO 2  •Bi 2  O 3  •MnO 2  •graphite material made in accordance with the present invention. 
     FIG. 5 is a cycling curve for the material of FIG. 4. 
     FIG. 6 is a voltammogram curve for the eleventh cycle of cathodically prepared Mn(OH) 2  •Pb(OH) 2  in accordance with the method of the present invention. 
     FIG. 7 is a voltammogram curve for several cycles of anodically prepared MnO 2  •PbO 2  in accordance with the method of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     This invention is directed towards a method of making a rechargeable &#34;modified manganese-containing compound&#34; for use, primarily, in rechargeable battery cells. As used herein, the terms &#34;modified manganese-containing compound&#34; and/or &#34;modified manganese dioxide material&#34; include mixtures of manganese dioxide or manganese hydroxide with another material which may be bismuth, lead, copper, or mixtures of oxides thereof, or other metals whose oxides show stability in alkaline electrolytes and which possess redox behavior in the potential of modified manganese dioxide. Such a potential would be about -0.2 V to -0.6 V with reference to a Hg/HgO reference electrode. 
     The method of the present invention may be best understood with reference to FIG. 1. The method includes providing a container 5 which may be composed of glass or plastic which has an aqueous electrolyte solution 7 therein. Container 5 preferably has an anodic portion 8 and cathodic portion 10. The electrolyte solution 7 contains distilled water, manganese nitrate, Mn(NO 3 ) 2  and a second material which is a doping material selected from the group consisting of lead nitrate, Pb(NO 3 ) 2 , or bismuth nitrate, Bi(NO 3 ) 2 , mixtures of Pb(NO 3 ) 2  and Bi(NO 3 ) 2 , copper nitrate Cu(NO 3 ) 2 , copper sulfate or lead acetate and/or nitric acid or mixtures of any two or more of the aforementioned compounds. It is preferred to provide about 1 to 10M manganese nitrate and about 0.1 to 0.01M of the second material. The preferred range of the percentage of the second material in the mixture is preferably between about 0.1 to 5% on a molecular weight ratio basis. 
     Two electrodes, namely cathode 9 and anode 11, are submerged into the aqueous solution 7 in container 5. The electrodes may be composed of a material selected from the group consisting of carbon, graphite, nickel, sintered rod nickel, titanium, lead or other materials. It is presently preferred to use graphite rods for both the cathode 9 and anode 11. An insulator 19 may also be provided on anode 9. The two electrodes 9 and 11 are placed in the aqueous bath and are connected by wires 13 and 15, respectively, to a suitable power supply 17. Power supply 17 may be a potentiostat, a galvanostat or a DC power supply. Current is passed through cathode 9 and anode 11 as discussed in greater detail hereinafter. In accordance with yet another preferred aspect of the invention, graphite powder 20 may be provided in anodic portion 8 of container 5. Graphite powder 20 is used as a current collector and a substrate upon which the material discussed hereinafter may be deposited. It may also be preferred in this application or in others to provide a separator 18 to block any graphite powder from passing into cathode portion 10 of container 5. 
     As noted hereinbefore, during electrolysis, current is passed through the electrodes 9 and 11. The current passed through the electrodes is preferably between about 0.1 ma/cm 2  and 200 ma/cm 2  and would preferably be between about 1 ma/cm 2  and 100 ma/cm 2 . This current is applied for between about 1 and 170 hours and preferably for about 10 to 33.3 hours. The current level is chosen based upon the application in which the method is to be utilized. A higher current results in a greater amount of material being produced per unit time. Conversely, at a given current, an extension of time during which the electrodes are exposed to the current results in a greater yield of material produced. 
     As the aqueous solution is electrolyzed a coating is obtained on both cathode 9 and anode 11. At anode 11 the material formed is in the charged state. Specifically, during anodic deposition MnO 2  •PbO 2  is formed, if for example, lead nitrate is used as the doping material. In that case, the reaction at the anode 11 is as follows: 
     
         Mn(NO.sub.3).sub.2 +Pb(NO.sub.3).sub.2 +H.sub.2 O→MnO.sub.2 •PbO.sub.2 +HNO.sub.3 
    
     If bismuth is used as the second material, then MnO 2  •Bi 2  O 3  is formed at anode 11. The manganese in this material is in the +4 oxidation state which is the charged state. 
     At cathode 9, the material obtained is in the discharge state. More specifically, during cathodic deposition, Mn(OH) 2  •Pb(OH) 2  is formed if, for example, lead oxide was used as the second material. In such a case, the reaction at cathode 9 is as follows: 
     
         Mn.sup.2+ +Pb.sup.2+ +4H.sub.2 O+4e→Mn(OH).sub.2 •Pb(OH).sub.2 +2H.sub.2 
    
     Alternatively, Mn(OH) 2  •Bi(OH) 3  is formed if bismuth is used as the doping material referred to hereinbefore. If combinations of lead or bismuth oxides are used, Mn(OH) 2  •Pb(OH) 2  •Bi(OH) 3  is formed. If another material is used this would, of course, affect and determine the material generated during cathodic and anodic deposition. For example, if copper nitrate is added to the electrolyte, the copper oxides will be formed at the anode with the MnO 2  and corresponding hydroxides will form at the cathode. This cathodically produced compound may be used as a modified manganese-containing material for a positive electrode in the discharge state in a battery cell. The oxidation state of the manganese in this discharge state is +2 . That is, during cathodic deposition Mn 2+  is generated. 
     The electrolysis is preferably carried out at room temperature which is between about 65 to 75 degrees F. The electrolysis is carried out at normal atmospheric pressure. It is significant to note that the electrolysis of the present invention can be employed at room temperature because conventional methods require the electrolysis to be carried out at temperatures of at least 180 degrees F. The method of the present invention represents an energy savings and simplification over conventional electrolysis methods. 
     After the electrolysis is carried out if it is performed on graphite rods, the materials at the cathode and the anode are scraped off the rods and are then washed to remove salts and dried. If, on the other hand, the material is deposited in graphite powder, then it may be used as is. The resulting material can be used in industrial applications in which rechargeable materials are required. 
     If desired, a reference electrode (not shown) can be used in order to study the performance of either cathode 9 or anode 11. Specifically, a mercury/mercury-oxide electrode may be used. Alternatively, Ag/AgCl or other reference electrodes suitable in the application may be used. With a reference electrode it is possible to determine what is happening at the cathode 9, for example, by measuring the voltage of cathode 9 versus the reference electrode. Similar such information can be determined about anode 11 by measuring the voltage with respect to the reference electrode. 
     It should be understood that the method of the present invention can be employed on a very large scale using materials which are already used in the industry in the preparation of conventional manganese dioxide material for use in battery cells. As stated hereinbefore, the conventional method generally provides for starting with manganese sulphate and operates at 180 degrees F. However, the conventional manganese dioxide is essentially not rechargeable to a reliable extent as significant degradation in capacity occurs from cycle to cycle. The present invention provides a method of generating rechargeable modified manganese dioxide in a manner which can easily be employed using materials and devices which are known in the industry. 
     It should be understood that this method is simple and requires only a comparatively short time to produce the material. In addition, both cathodic and anodic depositions can be carried out simultaneously to generate a higher yield of rechargeable material. For use in industry, the material which is created in the charged state can be converted to the discharge state before being supplied on a commercial basis, if desired. 
     In order to refine the disclosure of the invention several illustrative examples will be considered. 
     EXAMPLE 1 
     About 2M of manganese nitrate was mixed with about 0.1M of lead nitrate. This mixture was placed in a container 5 composed of stainless steel, plastics, glass or any chemically inert material containing 100 ml of distilled water. Two graphite rod electrodes 9 and 11 of dimensions of about 15 cm (length)×0.5 cm (diameter) were immersed into the aqueous bath. The electrodes 9, 11 were connected by a suitable conductive wire supplied with a current from a suitable power supply such as a DC power supply or a galvanostat. A current of about 2 ma/cm 2  to 20 ma/cm 2  was passed through the electrodes 9 and 11 for at least about 16.67 hours. After this time period, the graphite electrodes were removed from the solution and rinsed by soaking in H 2  O to remove excess Mn(NO 3 ) 2  •Mn x  (NO 3 ) y  which may remain after electrolysis. The materials deposited on the electrodes were then removed, dried and stored. 
     EXAMPLE 2 
     The conditions for depositing the active material in this example were as follows: an electrolyte 7 composed of about 20 ml of the following was used: about 5M Mn(NO 3 ) 2 , and about 0.05M Pb(NO 3 ) 2 , and about 0.2M HNO 3 . The anode 11 was composed of a graphite rod in about 4 grams of graphite powder 20. Cathode 9 was a graphite rod. The current applied was at least about 20 ma for at least about 60 hours. 
     This produced about 1.5 g of PbO 2  /MnO 2  •graphite powder which was then mixed with 0.2 g TEFLON and made into a positive electrode for use in a plexiglass test cell. The cycling results are shown in FIG. 2 and FIG. 3. Referring to FIG. 2, a plot of discharge in milli-ampere-hours against cycle number is shown. The high voltage charge/discharge region is the area designated by reference character 40 in which the potential was between about -0.35 v to 0.35 v. In this region 40 which was between 0 and 17 cycles, the capacity was low, but constant. At a lower voltage region designated by reference character 42, the potential was between -1.0 v to 0.35 v. In this region, the charge/discharge character is relatively high, which means the ratio of MnO 2  versus C is high, which is important for a high energy density battery. FIG. 3 shows curve 48 which is a plot of voltage over time in seconds and indicates the voltage drop during discharge and the recharging of the material. 
     An important feature of this invention is the use of graphite powder as the substrate for deposition. Conventionally, MnO 2  is deposited on to lead, carbon, or other inert substrate, then scraped or vibrated off from the substrate and made into powder. The MnO 2  powder is then mixed with graphite powder before being made into an electrode. The direct deposition of active material to the graphite powder in accordance with the present invention has several significant advantages over the prior art. It exhibits good electrical contact between the active material and electrode substrate/current collector, which can reduce the voltage drop through internal resistance. The energy density could be increased by increasing the MnO 2  /graphite ratio while keeping internal resistance within a desired limitation. There is an even thickness and distribution of the active material along with graphite powder, which will produce even current distribution and hence improve electrode behavior. This aspect of the method also involves simplification of manufacturing procedure. In addition, the extra step of grinding the electrode material and mixing it with graphite powder is avoided when using this aspect of the method of the present invention. 
     EXAMPLE 3 
     The conditions for depositing the active material in this example were as follows: an electrolyte 7 composed of about 20 ml of the following solution was used: 3M Mn(NO 3 ) 2 , 0.05M Pb(NO 3 ) 2 , 0.05M Bi(NO 3 ) 2  and 0.2M HNO 3 . The anode was a graphite rod and the cathode was also a thick graphite rod. The current applied was at least about 30 ma for at least about 80 hours. 
     This produced about 1.2 g of PbO 2  •Bi 2  O 3  •MnO 2  •graphite. This material was mixed with about 0.2 g of TEFLON and a positive electrode for a plexiglass test cell was made from the material. The cycling results are shown in FIGS. 4 and 5. Referring to FIG. 4, a discharge curve 50 is shown for the material made in accordance with this example. The discharge in milli-ampere-hours is plotted along the ordinate and the cycle number is plotted along the abscissa. As shown in FIG. 4, 350 cycles were performed with little loss in capacity. Referring to FIG. 5, a cycling curve 65 is shown for the first and second cycles of the electrode. Voltage is plotted along the ordinate and time in minutes is plotted on the abscissa. The first cycle is represented by portion 60 of curve 65 and the second cycle is represented by portion 67 of curve 65. It should be noted that a substantially constant voltage applies over a long period of time throughout the first cycle. In the second cycle the same results are found in that a substantially constant voltage occurs throughout the cycle. 
     EXAMPLE 4 
     A mixture of Mn(NO 3 ) 2  and Pb(NO 3 ) 2  was used to deposit a mixture of Mn and Pb oxides onto sintered nickel or nickel wire substrates. A cathodic deposition was carried out using a constant direct current. During the cathodic deposition Mn(OH) 2  •Pb(OH) 2  is formed. The electrodes were then cycled in a 9M KOH solution with either sintered nickel or nickel wire as the counter electrode and Hg/HgO as the reference electrode. FIG. 6 shows a cyclic voltammogram curve for cathodically prepared Mn(OH) 2  •Pb(OH) 2  in 9M KOH. The cathodically-deposited material showed good cycling capabilities over more than 300 cycles and still continuing. Good rechargeability and regeneration of capacity was shown by taking the electrodes to cathodic potentials. No initial activation cycling was required and the material was obtained with full rechargeability. The materials had a higher discharge potential which should lead to higher battery potentials during discharge. 
     EXAMPLE 5 
     A mixture of Mn(NO 3 ) 2  and Pb(NO 3 ) 2  was used to deposit a mixture of Mn and Pb oxides onto sintered nickel or nickel wire substrates. An anodic deposition was carried out using a constant direct current. During the anodic deposition MnO 2  •PbO 2  formed. The electrode was then cycled in 9M KOH solution with either sintered nickel or nickel wire as the counter electrode and Hg/HgO as the reference electrode. FIG. 2 shows curve 90 which is a cyclic voltammogram of anodically prepared MnO 2  •PbO 2 . The anodically deposited material showed good cycling capabilities. These cycling capabilities were obtained over more than 125 cycles and still recharging. More specifically, in this graph the cycles are shown separately. For example, reference character 119 designates to the curve for the 119th cycle. Reference character 120 refers to the 120th cycle. Reference character 121 refers to the 121st cycle. Reference characters 124 and 125 refer to cycle number 124 and 125. It should be noted that no initial activation cycle was required and the material was obtained with full rechargeability. The materials have a high discharge potential which leads to higher battery potentials during discharge. The deposited material had a peak couple at 0.0/-0.2 v, as shown in the set of curves 90 in FIG. 7. 
     One advantageous use of this material made in accordance with the method of the present invention is in a battery cell. A battery container would be provided with a positive electrode and a corresponding spaced apart negative electrode both placed in a suitable electrolyte. The battery housing would then be sealed in a manner such that the positive and negative electrodes are electrically connected together. 
     It should be understood that the material prepared by the method of the present invention has the advantage of being highly rechargeable and of low cost. Furthermore, good rechargeability and regeneration of capacity is provided by both the cathodically and the anodically prepared materials. This is particularly shown in the anodically prepared material. The method is simple and can be performed in a very short period of time. Therefore, it should be understood that the method and material so prepared provide a solution to the long-standing problem of achieving highly rechargeable modified manganese dioxide and other types of manganese-containing materials. In addition, the method provides for simultaneous generation of materials in both the discharge and the charged state. The materials can be readily used as positive electrode materials for a primary battery cell. 
     Whereas particular embodiments of the invention have been described for purposes of illustration, it will be appreciated by those skilled in the art that numerous variations of the details may be made without departing from the invention as described in the appended claims.