ZA200200335B - High discharge electrolytic manganese dioxide and an electrode and alkaline cell incorporating the same. - Google Patents
High discharge electrolytic manganese dioxide and an electrode and alkaline cell incorporating the same. Download PDFInfo
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- ZA200200335B ZA200200335B ZA200200335A ZA200200335A ZA200200335B ZA 200200335 B ZA200200335 B ZA 200200335B ZA 200200335 A ZA200200335 A ZA 200200335A ZA 200200335 A ZA200200335 A ZA 200200335A ZA 200200335 B ZA200200335 B ZA 200200335B
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- potassium
- emd
- electrode
- manganese dioxide
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- NUJOXMJBOLGQSY-UHFFFAOYSA-N manganese dioxide Chemical compound O=[Mn]=O NUJOXMJBOLGQSY-UHFFFAOYSA-N 0.000 title claims description 50
- 229910052700 potassium Inorganic materials 0.000 claims description 126
- 239000011591 potassium Substances 0.000 claims description 126
- ZLMJMSJWJFRBEC-UHFFFAOYSA-N Potassium Chemical compound [K] ZLMJMSJWJFRBEC-UHFFFAOYSA-N 0.000 claims description 125
- 239000012535 impurity Substances 0.000 claims description 55
- 239000003792 electrolyte Substances 0.000 claims description 11
- 239000006258 conductive agent Substances 0.000 claims description 7
- 229910052725 zinc Inorganic materials 0.000 claims description 4
- 239000011701 zinc Substances 0.000 claims description 4
- HCHKCACWOHOZIP-UHFFFAOYSA-N Zinc Chemical compound [Zn] HCHKCACWOHOZIP-UHFFFAOYSA-N 0.000 claims description 3
- 239000007774 positive electrode material Substances 0.000 claims description 2
- 229910045601 alloy Inorganic materials 0.000 claims 1
- 239000000956 alloy Substances 0.000 claims 1
- 210000004027 cell Anatomy 0.000 description 64
- 238000000034 method Methods 0.000 description 62
- 239000000523 sample Substances 0.000 description 53
- 239000000243 solution Substances 0.000 description 51
- AMWRITDGCCNYAT-UHFFFAOYSA-L hydroxy(oxo)manganese;manganese Chemical compound [Mn].O[Mn]=O.O[Mn]=O AMWRITDGCCNYAT-UHFFFAOYSA-L 0.000 description 40
- 239000000203 mixture Substances 0.000 description 38
- 239000008367 deionised water Substances 0.000 description 37
- KWYUFKZDYYNOTN-UHFFFAOYSA-M Potassium hydroxide Chemical compound [OH-].[K+] KWYUFKZDYYNOTN-UHFFFAOYSA-M 0.000 description 36
- 230000008569 process Effects 0.000 description 27
- VEXZGXHMUGYJMC-UHFFFAOYSA-N Hydrochloric acid Chemical compound Cl VEXZGXHMUGYJMC-UHFFFAOYSA-N 0.000 description 19
- 239000011550 stock solution Substances 0.000 description 19
- 239000012475 sodium chloride buffer Substances 0.000 description 18
- 239000003153 chemical reaction reagent Substances 0.000 description 17
- 239000011572 manganese Substances 0.000 description 17
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical group [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 15
- PWHULOQIROXLJO-UHFFFAOYSA-N Manganese Chemical compound [Mn] PWHULOQIROXLJO-UHFFFAOYSA-N 0.000 description 15
- 229910052748 manganese Inorganic materials 0.000 description 15
- 238000007747 plating Methods 0.000 description 14
- 239000011521 glass Substances 0.000 description 11
- 239000002245 particle Substances 0.000 description 11
- FAPWRFPIFSIZLT-UHFFFAOYSA-M Sodium chloride Chemical compound [Na+].[Cl-] FAPWRFPIFSIZLT-UHFFFAOYSA-M 0.000 description 10
- QAOWNCQODCNURD-UHFFFAOYSA-N Sulfuric acid Chemical compound OS(O)(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-N 0.000 description 10
- 238000004458 analytical method Methods 0.000 description 9
- 229910052799 carbon Inorganic materials 0.000 description 9
- 239000000463 material Substances 0.000 description 9
- 239000011159 matrix material Substances 0.000 description 9
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 9
- 239000004744 fabric Substances 0.000 description 8
- VNWKTOKETHGBQD-UHFFFAOYSA-N methane Chemical compound C VNWKTOKETHGBQD-UHFFFAOYSA-N 0.000 description 8
- 238000012360 testing method Methods 0.000 description 8
- 238000006243 chemical reaction Methods 0.000 description 7
- 238000002386 leaching Methods 0.000 description 7
- 229940075397 calomel Drugs 0.000 description 6
- ZOMNIUBKTOKEHS-UHFFFAOYSA-L dimercury dichloride Chemical compound Cl[Hg][Hg]Cl ZOMNIUBKTOKEHS-UHFFFAOYSA-L 0.000 description 6
- 239000003517 fume Substances 0.000 description 6
- XEEYBQQBJWHFJM-UHFFFAOYSA-N iron Substances [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 description 6
- 229910002804 graphite Inorganic materials 0.000 description 5
- 239000010439 graphite Substances 0.000 description 5
- PXHVJJICTQNCMI-UHFFFAOYSA-N nickel Substances [Ni] PXHVJJICTQNCMI-UHFFFAOYSA-N 0.000 description 5
- 239000000843 powder Substances 0.000 description 5
- 239000011780 sodium chloride Substances 0.000 description 5
- 235000002639 sodium chloride Nutrition 0.000 description 5
- 238000012546 transfer Methods 0.000 description 5
- 229920002274 Nalgene Polymers 0.000 description 4
- WCUXLLCKKVVCTQ-UHFFFAOYSA-M Potassium chloride Chemical compound [Cl-].[K+] WCUXLLCKKVVCTQ-UHFFFAOYSA-M 0.000 description 4
- 238000002835 absorbance Methods 0.000 description 4
- 238000010521 absorption reaction Methods 0.000 description 4
- 239000002253 acid Substances 0.000 description 4
- 239000011262 electrochemically active material Substances 0.000 description 4
- 229910052935 jarosite Inorganic materials 0.000 description 4
- 239000012088 reference solution Substances 0.000 description 4
- 239000007787 solid Substances 0.000 description 4
- 238000005406 washing Methods 0.000 description 4
- HEMHJVSKTPXQMS-UHFFFAOYSA-M Sodium hydroxide Chemical compound [OH-].[Na+] HEMHJVSKTPXQMS-UHFFFAOYSA-M 0.000 description 3
- 239000011324 bead Substances 0.000 description 3
- 238000001354 calcination Methods 0.000 description 3
- 238000010276 construction Methods 0.000 description 3
- 238000002474 experimental method Methods 0.000 description 3
- 238000000705 flame atomic absorption spectrometry Methods 0.000 description 3
- 238000005259 measurement Methods 0.000 description 3
- 238000002360 preparation method Methods 0.000 description 3
- 241000270728 Alligator Species 0.000 description 2
- RTAQQCXQSZGOHL-UHFFFAOYSA-N Titanium Chemical compound [Ti] RTAQQCXQSZGOHL-UHFFFAOYSA-N 0.000 description 2
- 239000003513 alkali Substances 0.000 description 2
- 238000013459 approach Methods 0.000 description 2
- 238000003705 background correction Methods 0.000 description 2
- 239000011248 coating agent Substances 0.000 description 2
- 238000000576 coating method Methods 0.000 description 2
- 239000013078 crystal Substances 0.000 description 2
- 230000003247 decreasing effect Effects 0.000 description 2
- 238000001514 detection method Methods 0.000 description 2
- 239000012153 distilled water Substances 0.000 description 2
- 230000000694 effects Effects 0.000 description 2
- 238000010894 electron beam technology Methods 0.000 description 2
- 238000005187 foaming Methods 0.000 description 2
- 238000000227 grinding Methods 0.000 description 2
- 229910052739 hydrogen Inorganic materials 0.000 description 2
- 239000001257 hydrogen Substances 0.000 description 2
- 238000003780 insertion Methods 0.000 description 2
- 230000037431 insertion Effects 0.000 description 2
- 238000004519 manufacturing process Methods 0.000 description 2
- 238000003801 milling Methods 0.000 description 2
- 238000006386 neutralization reaction Methods 0.000 description 2
- 230000003472 neutralizing effect Effects 0.000 description 2
- 229910052760 oxygen Inorganic materials 0.000 description 2
- 239000001103 potassium chloride Substances 0.000 description 2
- 235000011164 potassium chloride Nutrition 0.000 description 2
- 239000012488 sample solution Substances 0.000 description 2
- 229960002668 sodium chloride Drugs 0.000 description 2
- 239000002904 solvent Substances 0.000 description 2
- 238000003756 stirring Methods 0.000 description 2
- 150000004763 sulfides Chemical class 0.000 description 2
- 229910052717 sulfur Inorganic materials 0.000 description 2
- 230000002195 synergetic effect Effects 0.000 description 2
- 239000010936 titanium Substances 0.000 description 2
- 229910052719 titanium Inorganic materials 0.000 description 2
- 230000004580 weight loss Effects 0.000 description 2
- RYGMFSIKBFXOCR-UHFFFAOYSA-N Copper Chemical compound [Cu] RYGMFSIKBFXOCR-UHFFFAOYSA-N 0.000 description 1
- 229920000742 Cotton Polymers 0.000 description 1
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 description 1
- 239000005909 Kieselgur Substances 0.000 description 1
- 229910021380 Manganese Chloride Inorganic materials 0.000 description 1
- GLFNIEUTAYBVOC-UHFFFAOYSA-L Manganese chloride Chemical compound Cl[Mn]Cl GLFNIEUTAYBVOC-UHFFFAOYSA-L 0.000 description 1
- ZOKXTWBITQBERF-UHFFFAOYSA-N Molybdenum Chemical compound [Mo] ZOKXTWBITQBERF-UHFFFAOYSA-N 0.000 description 1
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N Silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 description 1
- 229920002125 Sokalan® Polymers 0.000 description 1
- 229910001297 Zn alloy Inorganic materials 0.000 description 1
- 239000006230 acetylene black Substances 0.000 description 1
- 239000012670 alkaline solution Substances 0.000 description 1
- 239000007853 buffer solution Substances 0.000 description 1
- 239000010406 cathode material Substances 0.000 description 1
- 210000003850 cellular structure Anatomy 0.000 description 1
- 239000010941 cobalt Substances 0.000 description 1
- 229910017052 cobalt Inorganic materials 0.000 description 1
- GUTLYIVDDKVIGB-UHFFFAOYSA-N cobalt atom Chemical compound [Co] GUTLYIVDDKVIGB-UHFFFAOYSA-N 0.000 description 1
- 150000001875 compounds Chemical class 0.000 description 1
- 229910052802 copper Inorganic materials 0.000 description 1
- 239000010949 copper Substances 0.000 description 1
- 230000001419 dependent effect Effects 0.000 description 1
- 238000009826 distribution Methods 0.000 description 1
- 238000005553 drilling Methods 0.000 description 1
- 239000011263 electroactive material Substances 0.000 description 1
- 239000003822 epoxy resin Substances 0.000 description 1
- 239000000706 filtrate Substances 0.000 description 1
- 238000001914 filtration Methods 0.000 description 1
- 229910001385 heavy metal Inorganic materials 0.000 description 1
- ZMFWDTJZHRDHNW-UHFFFAOYSA-N indium;trihydrate Chemical compound O.O.O.[In] ZMFWDTJZHRDHNW-UHFFFAOYSA-N 0.000 description 1
- 230000003993 interaction Effects 0.000 description 1
- 235000015110 jellies Nutrition 0.000 description 1
- 239000008274 jelly Substances 0.000 description 1
- 239000011565 manganese chloride Substances 0.000 description 1
- 235000002867 manganese chloride Nutrition 0.000 description 1
- 238000000691 measurement method Methods 0.000 description 1
- 238000002156 mixing Methods 0.000 description 1
- 229910052750 molybdenum Inorganic materials 0.000 description 1
- 239000011733 molybdenum Substances 0.000 description 1
- 229910052759 nickel Inorganic materials 0.000 description 1
- 229920000647 polyepoxide Polymers 0.000 description 1
- 229920000642 polymer Polymers 0.000 description 1
- 239000002244 precipitate Substances 0.000 description 1
- 239000000047 product Substances 0.000 description 1
- 238000000746 purification Methods 0.000 description 1
- 238000011002 quantification Methods 0.000 description 1
- 239000002994 raw material Substances 0.000 description 1
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- 230000004044 response Effects 0.000 description 1
- 230000035945 sensitivity Effects 0.000 description 1
- 238000004513 sizing Methods 0.000 description 1
- HYHCSLBZRBJJCH-UHFFFAOYSA-M sodium hydrosulfide Chemical compound [Na+].[SH-] HYHCSLBZRBJJCH-UHFFFAOYSA-M 0.000 description 1
- 238000004611 spectroscopical analysis Methods 0.000 description 1
- 239000010935 stainless steel Substances 0.000 description 1
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Description
HIGH DISCHARGE ELECTROLYTIC MANGANESE DIOXIDE AND
AN ELECTRODE AND ALKALINE CELL INCORPORATING THE SAME
The present invention generally relates to electrochemical cells. More specifically, the present invention relates to an improved electrolytic manganese dioxide (EMD) for an alkaline electrochemical cell.
Manufacturers of alkaline electrochemical cells are constantly attempting to increase the service life of the cells, and more particularly, the high-rate service life of their cells to meet the demands of current battery-operated devices, which draw increasingly larger current levels from the batteries. Because the outer dimensions of the battery are generally fixed by various standards, battery manufacturers cannot arbitrarily increase the outer dimensions of the battery in order to accommodate more of . the electrochemically active materials in their batteries. Thus, substantial effort has been made to make more efficient use of the space provided in the interior of the battery so as to enable more electrochemically active materials to be contained inside of the battery.
Such efforts have included minimising the volume occupied by the current collector and seal that are contained inside of the battery as well as increasing the density of the electrochemically active materials at the expense of other component materials, such as electrolyte or conductive agents. Other efforts have focused on increasing the high-rate discharge efficiency by utilising electrode constructions that optimise the interfacial surface area between the positive and negative electrodes. In addition, battery manufacturers have studied the electroactive materials themselves to increase their discharge efficiency.
As will become apparent to those skilled in the art, the present invention addresses the latter approach through a discovery that leads to an increased high-rate discharge efficiency for EMD, which is the electrochemically active material commonly used in the positive electrode of an alkaline electrochemical cell. To better understand the present invention, a description is provided below of the manner by which EMD is commonly produced.
EMD that is suitable for use in an alkaline electrochemical cell generally includes about 92 percent manganese dioxide (MnO,). A large percentage of the remainder of the EMD is Mn,O,. EMD additionally includes many different impurities at relatively low levels. Ideally, the EMD includes as high a percentage of MnO, as possible, to maximise cell service performance.
MnO, is a naturally occurring compound that is mined as an ore. The ore generally includes fairly high levels of impurities. The specific impurities and levels of
Impurities may vary considerably. Nevertheless, a typical analysis of a raw ore shows - that it contains the following:
MnO, - 75 percent )
Fe - 3-4 percent
K - 0.7-0.8 percent
Mo - 15-20 ppm
Co - 1200 ppm
Ni - 600 ppm
ALO, - 6 percent
Si0, - 3 percent
The raw ore is then processed through many different purification steps to arrive at a suitable form of EMD. The first step is a calcining process. The MnO, in the raw ore is insoluble in acid, which makes it difficult to further process the raw ore. Thus, the calcining process is used to convert the insoluble MnO, to manganese oxide (MnO), which is soluble in sulfuric acid. To produce the MnO (calcined ore), methane is used as a reagent in the presence of significant heat to cause the reduction of MnO, to MnO as shown in the formula below: 1000°C 4MnO, + CH, = 4MnO +2H,0 + CO,
A typical analysis of a calcined ore is:
MnO - 60 percent
MnO, - 1-2 percent
Fe - 3-4 percent
K - 0.7-0.8 percent
Mo - 15-20 ppm
Co - 1200 ppm - 15 Ni - 600 ppm } However, the levels of impurities can vary considerably, depending upon the raw ore.
The calcining process is typically carried out in brick-lined rotary kilns operated at about 1000°C. The calcined ore is then cooled and transferred to storage bins.
The second step in the process is known as the leaching process. There are several different leaching processes. One of the more common ones is known as the
Jarosite process. In the Jarosite leaching process, the stored calcined ore is dissolved in sulfuric acid in order to remove iron (Fe) and potassium (K) impurities. The following reactions may take place in the leaching process:
MnO + H,SO, = MnSO, + H,0
FeO {ore} + H,SO, = FeSO, +H,0 ' 2FeSO, + MnO, + 2H,SO, = Fe, (SO), + MnSO, + 2H,0
The leaching process generally takes place in one or more leach tanks. The initial pH in the leach tank is about 0.9. The calcined ore is added incrementally to slowly raise the pH to 4.2. As the pH rises, the mix undergoes the following reactions: pH=1.9
K,S0, + 3Fe,(SO,), + 12H,0 = 2KFe,(SO,),(OH), + 6H,SO,
Fe? +H,0,+2H" = 2Fe” +H,0 pH=3.6
Fe,(SO,), +6H,0 = 2Fe(OH), + 3H,SO,
The first of the three above reactions is known as the Jarosite reaction. At the end of the leach bath, polymer may be added to the tanks to help settle suspended solids.
These solids are then removed by filtering,
The clear solution having the solids removed is then processed by the third step } known as the sulfiding process. The sulfiding process is typically performed in a holding tank. The sulfiding process is used to precipitate heavy metal impurities (M), such as molybdenum (Mo), cobalt (Co), and nickel (Ni). The solution that overflows from the filter in the leaching process is mixed with sodium hydrosulfide (NaSH). The
NaSH is converted to H,S, which then precipitates the impurities as sulfides. Thus, the solution undergoes the following reactions: pH=3.8-4.2 2NaSH + H,SO, = 2H,S + Na SO,
M* + H,S = MS + 2H*
The solid sulfides are then filtered out through two rotary vacuum drum filters. The filter material is diatomaceous earth. The resultant filtrate constitutes what is known as purified cell feed.
The cell feed is fed into one or more plating cells. Each plating cell may include many negative and positive plating electrodes. Each plating cell includes at least one negative and one positive electrode. Titanium is often used for the negative electrodes, 5 and copper or lead can be used for the positive electrodes. Current flows through each cell to deposit the EMD on the negative electrode. Through this process, MnO, is plated onto the titanium negative electrode via the following reactions: negative electrode: Mn?* + 2H,0 = MnO, +2¢ +4H" (sulfuric acid) positive electrode: 2H, 0 +2 = H, + 20H
The cell bath is maintained at the desired temperature and acid concentration. . 15 The total process is a closed-loop system. The plating cells generate sulfuric acid and plate MnO, while the leach process consumes the sulfuric acid that is generated during ) the plating process and dissolves manganese.
After terminating the plating, the EMD is stripped off the negative electrode.
The material is then ready for the finishing operation, which may include milling, washing and/or neutralising. Washing and neutralising may be done before, during or after milling. For example, in one finishing operation chunks of EMD are crushed to about % inch (1.9 cm) average external diameter. This material is then sent to one or more neutralisation tanks. In these tanks, an alkaline solution such as NaOH or KOH is used to increase the pH of the material to a predetermined level to meet finished product specifications. After the material is neutralised, it is milled and screened to the desired particle size distribution. The EMD is then ready for use in cell manufacture. The EMD may be first mixed with a conductive agent and impact-moulded directly into the cylindrical can of the battery or may be mixed with a conductive agent and pre-moulded into rings that may subsequently be inserted into the cell.
It would be desirable to be able to provide an electrochemical cell, specifically an alkaline electrochemical cell, having improved high-rate discharge properties. We have found, surprisingly, that this may be achieved by using electrolytic manganese dioxide having a pH-voltage of at least about 0.860 volt. Furthermore, we have found that the high-rate discharge may further be improved by using electrolytic manganese dioxide having less than about 250 parts per million (ppm) of potassium impurities by weight.
Accordingly, in a first aspect, the present invention provides an electrode for an electrochemical cell comprising electrolytic manganese dioxide having a pH-voltage of at least 0.860 volt.
In a second aspect, the present invention provides an electrochemical cell comprising a negative electrode, an electrolyte, and a positive electrode, wherein the positive electrode comprises electrolytic manganese dioxide having a pH-voltage of at least 0.860 volt. .
In a third aspect, the present invention provides electrolytic manganese dioxide for use in an electrochemical cell having a pH-voltage of at least 0.860 volt. : 20
In a fourth aspect, the present invention provides the use of electrolytic manganese dioxide having a pH-voltage of at least 0.860 volt as positive electrode active material of an alkaline electrochemical cell.
The EMD has a pH-voltage of at least about 0.860 volt, and more preferably at least about 0.870 volt. We have found that by maintaining the pH-voltage of the EMD to at least about 0.860 volt, the EMD, and hence the electrode and electrochemical cell, will provide improved high-rate service.
i WO 01/11703 | PCT/US00/21025
We have also found that by ensuring that the EMD has less than about 250 ppm of potassium impurities, the EMD and hence the electrode and electrochemical cell will provide improved high-rate service.
Accordingly, the EMD preferably has less than about 250 ppm of potassium impurities, more preferably less than about 200 ppm, still more preferably less than about 150 ppm, even more preferably less than about 75 ppm, and most preferably less than about 30 ppm of potassium impurities by weight. In a further aspect, the present invention provides an electrode for an electrochemical cell comprising electrolytic manganese dioxide having less than 30 ppm of potassium impurities by weight.
We have further found that an EMD having a pH-voltage of at least about 0.860 volt and less than about 250 ppm of potassium impurities, will exhibit surprising synergistic improvements in high-rate service. } The electrochemical cell constructed in accordance with the present invention comprises a negative electrode, a positive electrode, and an electrolyte. The cell may have essentially any construction. For example, the electrodes may have a bobbin-type, spiral-wound (i.e., jelly roll), stacked or any other construction.
The negative electrode preferably includes zinc or an alloy of zinc and the electrolyte preferably includes potassium hydroxide.
The posttive electrode comprises EMD and, in view of the poor conductivity of manganese dioxide itself, preferably also a conductive agent. By ‘conductive agent’ is meant any material that provides electronic conductivity to the cathode, as is known by those skilled in the art. A suitable conductive agent is carbon, preferably in the form of graphite, or acetylene black.
By ‘potassium impurities’ in the EMD, as used and defined herein, is meant that potassium that is incorporated into the EMD crystalline structure, entrapped in voids in the EMD crystals or adsorbed onto the surface of the EMD crystals during plating, but does not include potassium added to the EMD after plating (e.g., from the neutralisation process or from contact with other cathode or cell components).
The present invention will be further understood and appreciated by those skilled in the art by reference to the drawings, in which:
Figure 1 is a plot of both the pH-voltage and potassium impurity levels of various samples of prior art EMD provided from five different suppliers;
Figure 2 is a bar graph illustrating the percent increase of overall service for cells constructed in accordance with the present invention as compared to the service of a conventional cell; and
Figure 3 is a graph of temperature vs. volts for determining temperature corrected potential values for converting potential readings from a calomel electrode to standard hydrogen electrode potential values.
Figure 1 is a plot illustrating the measured pH-voltage (in volts) and potassium } content (in parts per million by weight) of various EMD samples provided from five different suppliers. These values represent prior art EMD samples. As apparent from
Figure 1, current EMD:s utilised in electrochemical cells have pH-voltages less than 0.860 volt. One commercially available EMD has exhibited a potassium impurity level as low as 35 ppm, however this EMD has a pH-voltage below 0.860 volt.
While the above-noted process for producing EMD includes steps for removing potassium impurities and that can increase pH-voltage levels, those processes have never been utilised to further reduce the impurity levels of potassium or to further increase the pH-voltage of the EMD to the levels of the current invention for commercial use in electrochemical cell cathodes, especially for alkaline cells.
For instance, the potassium impurity levels may be decreased by either starting with raw materials that do not have high starting levels of potassium impurities, by incorporating an effective potassium removal step of the plating bath cell feed in the
EMD production process, or by refining or retreating the ore in the leaching process where potassium is removed during the Jarosite reaction.
To increase the operative pH-voltage of the EMD, three approaches are known tobe possible. The first optional process would be to alter the EMD plating conditions $0 as to maximise the resultant EMD plating voltage (i.e., raise the acid level of the plating bath). The second process would be to chemically treat the EMD after the plating process (i.e., acid wash the EMD). The third would be to preferentially select the material from the whole population of the EMD. While such processes are known for further decreasing potassium impurities and for further increasing the operating pH- voltage, EMD has not been produced meeting both of these criteria, because neither battery manufacturers nor producers of EMD had previously recognised the need for further reducing potassium impurities and increasing pH-voltage operating levels of the
EMD.
C15 } As used and defined herein, the pH-voltage is the voltage measured with the
EMD at a pH level of 6.0. The technique for measuring the pH-voltage is described : below. This same technique is that which was utilised for measuring the pH-voltage of the cells constructed in the example that is also provided below. Also described below are the techniques for measuring the impurity levels of potassium in the EMD. It is noted that different test techniques may produce different results. The measurement techniques are therefore described below to provide the tests and basis for which one skilled in the art may determine whether a particular EMD falls within the scope of the claimed invention.
The invention will be further illustrated by reference to the following non- limiting example: :
To illustrate the advantages of the present invention, eighty AA (LR6) size cells were made as follows. A cathode mix containing 85.31 weight % EMD, 5.69 wt%
expanded graphite, 7.4 wt% (45%-conc) KOH solution, and 1.6% water was blended in a the following sequence. EMD and graphite were mixed, followed by the addition of the electrolyte and water. Mixing was continued. The mix was densified, and then broken up and screened through a 70 mesh screen. The densified and screened mix was placed into a AA size can with a graphite coating on its inside surface and impact moulded to “ form a compacted cathode along the inside surface of the can. The resulting cathode ” had a height of 1.674 inches (4.3 cm), a thickness of 0.158 inch (4 mm), and a weight of 11.1 grams. Once moulded, two pieces of separator having a length of 3.75 inches 9.5 cm), a width of 0.680 inch (1.7 cm), and a thickness of 0.004 inch (0.1 mm) were inserted into the cathode 90° of each other.
An anode mix comprising 70 weight % Zn, 0.02 wt% In(OH)3, 0.42 wt%
Carbopol, 1.17 wt% 0.1 N KOH, and 28.39 wt% (40% conc) KOH solution is made. A : total of 6.05 grams of the resulting anode mix was placed inside of the separator along : 15 with 1.08 grams of 37% conc KOH. An anode collector/seal assembly was then inserted and the cells were crimped. The cells were aged for one week and then : discharged at either 1000 mA or 1500 mA. Service results were determined by the discharge time required for the cell to be discharged to a voltage of 1.0 V.
Twenty of the cells made in accordance with the above procedure contained an
EMD having high potassium impurity levels (i.e., average impurity levels of 400 ppm) and low pH-V (i.e., average pH-V of 0.848 volt, resulting in a cell open circuit voltage of 1.603 volts).
Twenty of the above cells contained an EMD having high potassium impurity levels and high pH-V (i.e., an average pH-V of 0.885 volt, resulting in a cell open circuit voltage of 1.622 volts).
Twenty of the cells contained an EMD having low potassium impurity levels (i.e, average potassium impurity levels of 220 ppm) and low pH-V.
oo WO 01/11703 | PCT/US00/21025 CE
The remainder of these cells contained an EMD having low potassium impurity levels and high pH-V. All the cells were then discharged to 1.0 V and the relative service (discharge time) of the cells was measured.
Figure 2 is a graph illustrating the percentage increase of overall service based on 1500 mA and 1000 mA continuous service. As illustrated, the conventional cells, which had an EMD with high average potassium impurity levels and high average pH-
V, served as the reference point from which the other three sets of cells were measured.
Thus, for example, the cells having an EMD with high levels of potassium impurities and high pH-V exhibited approximately a 3% increase of overall service.
Similarly, the cells with EMD having low pH-V and low potassium impurities exhibited about an 8% increase of overall service. Given a 3% increase of overall service for raising the pH-V level of the EMD and an 8% increase as a result of only lowering the © 15 potassium impurity levels in the EMD, one would expect to achieve about an 11% increase of overall service by forming a cell having both high pH-V and low potassium impurity levels. However, as illustrated in Figure 2, the observed service data for such a cell exhibited an increase of overall service of approximately 15%. It is therefore apparent that the results of raising the pH-V of the EMD and lowering the potassium impurity levels in the EMD provides unexpected synergistic results.
The pH-V level of the EMD was measured using the technique described below.
Further, the potassium impurity levels of the EMD was determined using the first two techniques described below for measuring potassium impurity levels. A third test for measuring potassium impurity levels is also provided, which enables the potassium impurity levels of the EMD to be measured for a cell that has already been manufactured. 1. Technique for Measuring pH-Voltage
To measure the pH-voltage of EMD, an electrically conductive mixture is first prepared by manually grinding 3 grams of fully neutralised EMD with 1.0 gram of graphite until the mixture will pass through a 200 mesh screen. (Note: mechanical on grinding should not be used since it may generate enough heat to reduce the EMD.) A . thin layer of electrically conductive mixture is spread on each of two rectangular pieces
Bh of cloth for each EMD sample to be tested. The pieces of cloth may be dense cotton, o 5 Pellon®, or another synthetic material that is free of sizing material, or oxidising reducing substances. The rectangular cloths are approximately % inch (1.9 cm) by 3 inch (7.6 cm) rectangles. The electrically conductive mixture is pressed onto the cloth by firm horizontal strokes using a stainless steel spatula.
One end of a 6 inch (15.2 cm) electrode is placed along one of the narrow ends of each coated cloth. The 6 inch (15.2 cm) electrode is a cylindrical carbon electrode
R (unimpregnated) with a 0.17-0.18 inch (0.43-0.48 cm) diameter. Each coated cloth is
N rolled onto one end of a carbon electrode as tightly as possible with the coated surface ) facing the carbon electrode. Each cloth is secured to a carbon electrode with a single i i 15 size 8 rubber band, tightly stretched in three places across the cloth surface. Two #14 rubber stoppers are prepared by drilling through each of their centres a hole of a : diameter sufficient for insertion of a calomel reference electrode and one 0.185 inch to 0.19 inch (0.47 to 0.48 cm) diameter hole for each EMD sample to be tested (up to 12 holes spaced evenly about the perimeter of the stopper) to allow insertion of the carbon electrodes constructed in the manner discussed above. A #0 stopper is placed in the centre hole of each #14 stopper until the reference electrode is to be inserted. One of each pair of carbon electrodes prepared as described above is inserted into each #14 stopper, with the wrapped end extending from the bottom of the stopper and a sufficient length of carbon electrode extending from the top for connecting a potential measuring cable.
Two solutions are prepared as follows and each is poured into a wide-mouth jar that holds approximately 473 ml, and is 92 mm high and 93 mm in diameter. Enough solution is poured into the jar to cover the cloth end of the carbon electrodes once they are inserted into the jars. Each jar is marked to identify the solution therein.
The first solution is made with 10.04 g of ZnCl, 24.50 g of NH, Cl, 3.5 ml of
NH,OH, and 55.60 ml of de-ionised H,O. This solution has an approximate pH of 5.8.
The second solution is made with 10.00 g of ZnCl, 24.50 g of NH,Cl, 11.00 ml of
NH ,OH, and 55.60 ml of de-ionised HO. The second solution has an approximate pH of6.9.
Once the two jars are filled with these two respective solutions, the stoppers containing the carbon electrodes are carefully placed into each of the two solutions. The assembly is allowed to stand 18 to 24 hours before reading potential and pH.
The potentials of the samples are first measured. The #0 stopper is removed from the centre hole in the first jar, and a calomel reference electrode, attached to a multimeter, is inserted in this hole, using care to avoid disturbing the solution and the - EMD samples. A 30 inch (76 cm) wire cable is also attached to the multimeter, and an alligator clip on the other end of the measuring cable is attached to one of the carbon ) electrodes. The potential in volts is then read from the multimeter. The alligator clip is then attached to other carbon electrodes in the stopper and the potentials read in the ) - same manner. The multimeter must be one which will not discharge the sample. The multimeter used in the experiments described below was a Keithley 177 multimeter available from Keithley Instruments, Inc. of Cleveland, Ohio. Next, the calomel reference electrode is removed from the first jar, washed, dried, and carefully inserted into the stopper in the second jar, and the potentials of the electrodes in the second jar are read. After the potentials have been read, the #14 stoppers, with carbon electrodes, should be removed from both jars. The calomel reference electrode and a universal glass pH electrode are connected to a pH meter having a scale of 0-14 and then inserted into a 50 ml beaker containing pH 7 buffer solution to standardise the meter. The electrodes are then removed and rinsed with de-ionised water. After measuring the temperature of the solution in the first jar, these two electrodes are inserted into the first jar and the pH of the solution therein is read, adjusting the meter to compensate for oo temperature. Electrodes are rinsed, dried and inserted into the second jar to read the pH
Ca of the solution therein, adjusting the meter to compensate for temperature.
To calculate the pH-voltage of the EMD sample, the potential values read versus 5 the calomel electrode (SCE) are converted to potential values versus a standard - hydrogen electrode (SHE) by adding the temperature corrected potential value from
Figure 3 to the potential values read from the pH meter. Next, the temperature corrected - pH versus SHE potential is plotted on linear graph paper for both the first and second
Jars. The two plotted data points are then connected by a straight line and the potential ce 10 at pH at 6.0 is read from the graph as the pH-voltage of the EMD sample. 2. Technique for Measuring Potassium Impurity Levels Greater than 150 ppm > © i The principle behind this test technique is that the EMD sample is dissolved in - 15 hydrochloric acid and the potassium level is determined by flame atomic absorption spectroscopy (FAAS) at 766.5 nm. In this test, there are three potential interferences i that may skew the results obtained using this technique. The first of these interferences ~ is that potassium is partially ionised in an air-acetylene flame. The effects of ionisation may be substantially overcome by adding another alkali (1000-2000 pg/ml) to the samples and standards. A second interference is that manganese concentrations above 500 ppm suppress the potassium signal. Hydrochloric acid concentrations above 0.25% also suppress the signal.
The equipment used in this technique includes an atomic absorption spectrophotometer equipped with background correction; volumetric flasks of 1000 ml, 500 ml, 250 ml, 200 ml, and 100 mi; pipettes of 20.00 ml, 10.00 ml, and 2.00 ml; burettes of 50 ml and 25 ml; Carboy-Nalgene, 5/2 gallon (21 L) with spigot from the 1998 Fisher Scientific Catalog #02 963 BB; a fume hood; beakers of 150 ml; and watch glasses to fit the 150 ml beakers.
Additionally, the following reagents are used in this technique: 1. De-ionised water: Fill a 5%2 gallon (21 L) Nalgene carboy with de- ionised water and let it adjust overnight to room temperature. Use this water to dilute all samples and standards. Also allow other solutions for sample preparation/measurement to adjust to the same temperature by placing them in proximity to this source of de- 1onised water, preferably hours before use. 2. Hydrochloric acid (concentrated, reagent grade). 3. Sodium chloride buffer solution (15,000 pg/ml): Allot 38.13 grams of an ACS or finer grade of sodium chloride to a 1000 ml volumetric flask. Allow mixture of NaCl and water to adjust to room temperature, then dilute to the mark with de-ionised water and mix well. Reserve for dispensing with a 50 ml burette. 4. 1,000 pg/ml Potassium reference solution: Fisher Scientific Catalog #PLK2-2X. Solute: potassium chloride. Solvent: distilled water. : 5. 10.00 pg/ml Potassium stock solution: Pipette 10 ml from the 1000 pg/ml potassium reference solution into a 1000 ml volumetric flask. Dilute to the mark oo with de-ionised water and mix well. Reserve for dispensing with a 50 ml burette. 6. 2.00 pg/ml Potassium stock solution: Dispense 50 ml from the : 10.00 pg/ml potassium stock solution into a 250 ml volumetric flask. Dilute to the mark with de-ionised water and mix well.
The following range of working standards are prepared with the following concentrations, as needed: 1. 2.00 pg/ml Potassium: Dispense 40 ml of the 10.00 pg/ml potassium stock solution into a 200 mi volumetric flask. Add 20 ml of the sodium chloride buffer solution. Dilute to the mark with de-ionised water and mix well. 2. 1.50 pg/ml Potassium: Dispense 30 ml of the 10.00 pg/ml potassium stock solution into a 200 ml volumetric flask. Add 20 ml of the sodium chloride buffer solution. Dilute to the mark with de-ionised water and mix well.
3. 1.00 pg/ml Potassium: Dispense 20 ml of the 10.00 pg/ml potassium stock solution into a 200 ml volumetric flask. Add 20 ml of the sodium chloride buffer solution. Dilute to the mark with de-ionised water and mix well. 4, 0.50 pg/ml Potassium: Dispense 10 ml from the 10.00 pg/ml potassium stock solution into a 200 ml volumetric flask. Add 20 ml of the sodium chloride buffer solution. Dilute to the mark with de-ionised water and mix well. : 3. 0.25 pg/ml Potassium: Dispense 25 ml of the 2.00 pg/ml - potassium stock solution into a 200 ml volumetric flask. Add 20 ml of the sodium - chloride buffer solution. Dilute to the mark with de-ionised water and mix well. : 10 6. 0.10 pg/ml Potassium: Dispense 10 ml of the 2.00 ug/ml . potassium stock solution into a 200 ml volumetric flask. Add 20 ml of the sodium chloride buffer solution. Dilute to the mark with de-ionised water and mix well.
X 7. 0.05 pg/ml Potassium: Dispense 5 ml of the 2.00 pg/ml potassium stock solution into a 200 ml volumetric flask. Add 20 ml of the sodium chloride buffer . 15 solution. Dilute to the mark with de-ionised water and mix well. : The procedure includes the following steps: 1. Determine (by weight loss after 4 hours at 120°C) the moisture on approximately 10 grams of the EMD sample and use this value in calculating the potassium concentration. 2. Weigh 3.0000 grams of EMD, place into a 150 ml beaker, and cover with a watch glass. Transfer to a fume hood and add 15 ml of concentrated hydrochloric acid slowly down the side walls of the beaker. Allow it to stand until foaming subsides; swirl gently from time to time until mixture reacts only mildly. 3. Transfer a second 150 ml beaker to the fume hood, add 15 ml of hydrochloric acid, and cover with a watch glass. This is a reagent blank and should be carried through the entire procedure. 4, Place the EMD sample and reagent blank on a hot plate and heat slowly until the EMD sample solution clears and the MnO, has dissolved completely. Heat the reagent blank until the HCI volume has been reduced to less than 4 ml. Remove both from the hot plate and allow them to cool for at least 5 minutes. Then rinse watch glasses and side walls of the beakers with de-ionised water from a wash bottle. Replace the watch glasses and swirl contents gently. Adjust the volume of solution in the beakers to about 75 ml and let cool to room temperature. 5. Filter the sample and reagent blank, with the aid of a clean stirring rod, through 540-Whatman filter paper into separate 500 ml volumetric flasks. Wash the residues with de-ionised water from a wash bottle at least ten times, allowing the filters to drain after each washing. Dilute to the mark with de-ionised water. Mix well. 6. Pipette 10.00 ml of the EMD sample and the blank into separate 100 ml volumetric flasks. Add 10 ml of the sodium chloride buffer solution to each and dilute to the mark with de-ionised water and mix well. 7. Peak the wavelength on the AA spectrophotometer near 766.5 nm, with : the slit set at an opening of 1.4 nm. Aspirate standards 1 through 7 into an oxidising © 15 (lean, blue) flame, followed by reagent blank and EMD samples from step 6. ] 8. Construct a calibration graph of absorbance versus concentration of standards in (g/ml K) on linear graph paper. Read each sample concentration from the graph in (ug/ml) for each corresponding absorbance value. Insert the concentration value (ng/ml) in the equation below for calculations of potassium impurity levels of the
EMD sample.
It should be noted that if any EMD sample reading is above the highest standard (i.e., the 2.00 mg/ml potassium working standard), dilute it to bring it in the proper range. If any sample reading is below the lowest standard (i.e., 0.05 mg/ml potassium working standard), this method cannot be used to analyse that sample.
The potassium in ppm is then calculated as follows: 1. Adjusted Sample Wt. = Actual Sample Wt. (Step 2) x [(100% - % moisture) + 100] 2. Corrected pg/ml K = pg/ml K for sample - pg/ml K for reagent blank 3. Potassium, ppm = Corrected pg/ml K x 100 ml x (500 ml +10 ml)
Adjusted Sample Wt., grams so 3. Technique for Measuring Potassium Impurity Levels Less than 150 ppm
The principle behind this test technique is that the EMD sample is dissolved in hydrochloric acid and the potassium level is determined by flame atomic absorption : - spectroscopy (FAAS) at 766.5 nm. Because the atomic absorption response to the: : potassium concentration is affected by the manganese concentration, manganese is added to the standards used in calibrating the instrument. In this test, there are three potential interferences that may skew the results obtained using this technique. The first of these interferences is that potassium is partially ionised in an air-acetylene flame.
The effects of ionisation may be substantially overcome by adding another alkali (1000 : to 2000 pg/ml) to samples and standards. A second interference is that manganese : concentrations above 500 ppm suppress the potassium signal. Hydrochloric acid - . concentrations above 0.25% also suppress the signal. In this test, an impact bead should : 15 be used in the AAS burner assembly for increased sensitivity.
The equipment used in this technique includes an atomic absorption spectrophotometer (AAS) equipped with background correction and burner assembly with impact bead; volumetric flasks of 1000 ml, 500 ml, 250 ml, 200 ml, and 100 ml; pipettes of 20.00 ml, 10.00 ml, and 2.00 ml; burettes of 50 ml and 25 ml; Carboy-
Nalgene, 5%2 gallon (21 L), with spigot - 1998 Fisher Scientific Catalog #02 963 BB; fume hood; beakers of 150 ml; and watch glasses to fit the 150 ml beakers.
Additionally, the following reagents are used in this technique: 1. De-ionised water: Fill a 52 gallon (21 L) Nalgene carboy with de- ionised water and let it adjust overnight to room temperature. Use this water to dilute all samples and standards. Also, allow other solutions for sample preparation/measurement to adjust to the same temperature by placing them in proximity to this source of de-ionised water, preferably hours before use. 2. Hydrochloric acid (concentrated, reagent grade).
3. Sodium chloride buffer solution (15,000 ug/ml): ~~ Allot 38.13 grams of an ACS or finer grade of sodium chloride (NaCl) to a 1000 ml volumetric flask. Allow mixture of NaCl and water to adjust to room temperature, then dilute to the mark with de-ionised water and mix well. Reserve for dispensing with a 50 ml burette. 4. Manganese matrix match solution: Dissolve 34.1499 grams of manganese (II) chloride (MnCl; - 4 H,0), Puratronic grade from Alfa, Stock #10804 in de-ionised water and quantitatively transfer the solution to a 100 ml volumetric flask.
Dilute to the mark with de-ionised water. 5. 1000 pg/ml Potassium reference solution: ~~ Fisher Scientific Catalog #PLK2-2X. Solute: potassium chloride. Solvent: distilled water. 6. 10.00 pg/ml Potassium stock solution: Pipette 10 ml from the 1000 pg/ml potassium reference solution into a 1000 ml volumetric flask. Dilute to the mark with de-ionised water and mix well. Reserve for dispensing with a 50 ml burette. 7. 2.00 pg/ml Potassium stock solution: Dispense 50 ml from the 10.00 pg/ml potassium stock solution into a 250 ml volumetric flask. Dilute to the mark with de-ionised water and mix well.
The following range of working standards are prepared with the following concentrations, as needed: 1. 2.00 pg/ml Potassium: Dispense 40 ml of the 10.00 pg/ml stock potassium solution into a 200 ml volumetric flask. Add 20 ml of the sodium chloride buffer solution and 2 ml of the manganese matrix match solution. Dilute to the mark with de-ionised water and mix well. 2. 1.50 pg/ml Potassium: Dispense 30 ml of the 10.00 pg/ml potassium stock solution into a 200 ml volumetric flask. Add 20 ml of the sodium chloride buffer solution and 2 ml of the manganese matrix match solution. Dilute to the mark with de-ionised water and mix well. : 3. 1.00 pg/ml Potassium: Dispense 20 ml from the 10.00 ug/ml potassium stock solution into a 200 ml volumetric flask. Add 20 ml of the sodium chloride buffer solution and 2 ml of the manganese matrix match solution. Dilute to the mark with de-ionised water and mix well. : 4. 0.50 pg/ml Potassium: Dispense 10 ml from the 10.00 pg/ml potassium stock solution into a 200 ml volumetric flask. Add 20 ml of the sodium chloride buffer solution and 2 ml of the manganese matrix match solution. Dilute the mark with de-ionised water and mix well. 5. 0.25 pg/ml Potassium: Dispense 25 ml of the 2.00 pg/ml _potassium stock solution into a 200 ml volumetric flask. Add 20 ml of the sodium chloride buffer solution and 2 ml of the manganese matrix match solution. Dilute to the mark with de-ionised water and mix well. 6. 0.10 pg/ml Potassium: Dispense 10 ml of the 2.00 pg/ml potassium stock solution into a 200 ml volumetric flask. Add 20 ml of the sodium chloride buffer solution and 2 ml of the manganese matrix match solution. Dilute to the . mark with de-ionised water and mix well. 7. 0.05 pg/ml Potassium: Dispense 5 ml of the 2.00 pg/ml potassium stock solution into a 200 ml volumetric flask. Add 20 ml of the sodium chloride buffer solution and 2 ml of the manganese matrix match solution. Dilute to the mark with de- ionised water and mix well.
The procedure includes the following steps:
I. Determine (by weight loss after 4 hours at 120°C) the moisture on approximately 10 grams of the EMD sample and use this value in calculating the potassium concentration. 2. Weigh 3.0000 grams of the EMD sample, place into a 150 ml beaker, and cover with a watch glass. Transfer to a fume hood and add 15 ml of concentrated hydrochloric acid slowly down the side walls of the beaker. Allow it to stand until foaming subsides; swirl gently from time to time until the mixture reacts only mildly. 3. Transfer a second 150 ml beaker to the fume hood and add 15 ml of hydrochloric acid and 20 ml of the manganese matrix match solution. Cover the beaker with a watch glass. This is a reagent blank and should be carried through the entire procedure. 4. Place the EMD sample and reagent blank on a hot plate and heat slowly until the EMD sample solution clears and MnO; has dissolved completely. Heat the reagent blank until the HCI volume is reduced to less than 4 ml. Remove both from the hot plate and allow them to cool for at least 5 minutes. Then rinse watch glasses and side walls of the beakers with de-ionised water from a wash bottle. Replace the watch glasses and swirl contents gently. Adjust the volume of solution in the beakers to about 75 ml with de-ionised water and let cool to room temperature. 5. Filter the sample and reagent blank, with the aid of a clean stirring rod, through 540-Whatman filter paper into separate 200 ml volumetric flasks. Wash the filters/residues with de-ionised water from a wash bottle at least ten times, allowing the filters to drain after each washing. Dilute to the mark with de-ionised water. Mix well. 6. Pipette 10.00 ml of the EMD sample and the blank into separate 100 ml © 15 volumetric flasks. Add 10 ml of the sodium chloride buffer solution to each and dilute to the mark with de-ionised water and mix well. 7. Peak the wavelength on the AA spectrophotometer near 766.5 nm, with the slit set at an opening of 1.4 nm. Install an impact bead in burner assembly. Aspirate standards 1 through 7 into an oxidising (lean, blue) flame, followed by the reagent blank and the EMD samples from step 6. 8. Construct a calibration graph of absorbance versus concentration of standards in (pg/ml K) on linear graph paper. Read each sample concentration from the graph in (ug/ml) for each corresponding absorbance value. Insert the concentration value (g/ml) in the equation below for calculations of the potassium impurity levels of the EMD samples.
It should be noted that if any sample reading is above the highest standard (i.e., the 2.00 mg/ml potassium working standard), dilute it to bring it in the proper range.
However, a new set of standards must be prepared which contains the proper amount of manganese. If any sample reading is below the lowest standard (i.e., the 0.05 potassium working standard), this method cannot be used to analyse the sample.
E Calculations: :1. Adjusted Sample Wt. = Actual Sample Wt. (Step 2) x [(100% - %moisture) +100] 2. Corrected pg/ml K for sample = pg/ml K for sample - mg/ml K for reagent blank 3. Potassium, ppm = Corrected ug/ml K x 100 ml x (200 ml +10 ml) 5 Adjusted Sample Wt., grams -4. Technique for Measuring Potassium Impurity Levels of EMD Retrieved from a - Completed Cell
In measuring the potassium impurity levels in EMD removed from a completed cell, the most significant obstacle is to develop a technique that avoids interference from potassium present as KOH in the electrolyte. In general, the KOH electrolyte is absorbed on the surface of the EMD particles. The potassium in the KOH electrolyte, g 15 however, does not penetrate to the centre of the EMD particles. Thus, this technique © .exposes the centres of the EMD particles so as to allow the potassium impurity levels of } the EMD to be determined as it existed at the time the cell was initially constructed. . In order to eliminate the interference of potassium from the KOH electrolyte absorbed on the surface of the EMD particles, electron probe microchemical analysis (EPMA) is used to measure the potassium concentration in the centre of individual
EMD particles. This is accomplished by focusing an electron beam onto the polished cross section of EMD powder particles and measuring the intensity of potassium X-rays emitted from the sample. Since the size of the analytical volume for EPMA is extremely small (~1 to 10 pm®), only the potassium in the centre of the particle is measured. By avoiding the surface of the EMD particles during the analysis, KOH absorbed on the surface of the particles is not measured.
To prepare the sample for use in the EPMA analysis, the cathode material is removed from an alkaline cell and rinsed in de-ionised to remove the majority of the
KOH electrolyte. The rinsed cathode is then dried and mounted in an epoxy resin. The epoxy-mounted cathode is then polished by conventional metallographic techniques through 0.05 pum grit. This procedure results in flat, polished cross sections of the EMD powder particles. The polished surface is then coated with a 10 nm layer of carbon to provide a conductive path for the electron beam used in the EPMA measurements.
To measure the potassium level using EPMA, the sample is bombarded with an electron probe with a diameter of approximately 1 um. The interaction between the electron probe and the sample results in the emission of X-rays that have energies that are dependent upon the elements present in the sample. The intensity of the X-rays are then measured and related to the elemental concentration of the sample. Due to the small size of the electron probe, the analytical volume is ~1 to 10 um’ using this technique. This extremely small analytical volume allows for the quantification of elemental concentration on a micron scale.
Prior to measuring unknown concentrations of potassium in EMD samples, the = } system is first calibrated using EMD with a known concentration. Specifically, a flat polished piece of EMD plate with a known potassium concentration of 319 ppm was analysed in the EPMA for calibration purposes. The intensity of potassium X-rays emitted from the sample was measured and stored. The analytical conditions used to calibrate the instrument with this standard are listed below:
Count time 60s
Probe current 50 nA
Electron energy 20 keV
Carbon coating 10 nm
Potassium concentration in standard | 319 ppm
Using this standard in these analytical conditions, a theoretical limit of detection for potassium in EMD of 4 ppm was obtained. This limit of detection can be reduced by increasing the counting time of the analysis.
To determine the concentration of potassium in an unknown EMD sample, the sample is analysed using the analytical conditions listed above. The intensity of potassium X-rays emitted from the unknown sample is then compared to the intensity of
X-rays emitted from the calibration sample, and the concentration of potassium in the unknown EMD is calculated. Utilising the analytical conditions in the sample preparation described above, any laboratory with EPMA capabilities should be able to duplicate this analysis. - To validate this technique, a control experiment was performed where potassium was measured in EMD powder removed from an alkaline cell. The measured concentration was compared to the known potassium concentration in the EMD powder used in the cell. In this control experiment, the known potassium concentration present in the EMD powder was 260 ppm. The concentration of potassium in the EMD removed from the cell was found to be 250 ppm +5 ppm by the EPMA technique. The
Be 15 measured value and known value agreed within the experimental error of this technique, which indicates that the EPMA technique is a valid method for measuring the inherent . potassium concentration in EMD removed from alkaline cells.
Claims (15)
1. An electrode for an electrochemical cell comprising electrolytic manganese dioxide having a pH-voltage of at least 0.860 volt.
2. An electrode according to claim 1, wherein said electrolytic manganese dioxide has a pH-voltage of at least 0.870 volt.
3. An electrode according to claim 1 or claim 2, wherein said electrolytic manganese dioxide has less than 250 ppm of potassium impurities by weight.
4. An electrode according to claim 3, wherein said electrolytic manganese dioxide has less than 200 ppm of potassium impurities by weight. © 15 Ss.
An electrode according to claim 4, wherein said electrolytic manganese dioxide has less than 150 ppm of potassium impurities by weight.
6. An electrode according to claim 5, wherein said electrolytic manganese dioxide has less than 75 ppm of potassium impurities by weight.
7. An electrode according to claim 6, wherein said electrolytic manganese dioxide has less than 30 ppm of potassium impurities by weight.
8. An electrode according to any preceding claim, further including a conductive agent.
9. An electrode for an electrochemical cell comprising electrolytic manganese dioxide having less than 30 ppm of potassium impurities by weight.
10. An electrochemical cell comprising a negative electrode, an electrolyte, and a positive electrode, wherein the positive electrode is an electrode as defined in any of claims 1to0 9. :
11. Anelectrochemical cell according to claim 10, wherein said negative electrode comprises zinc or an alloy including zinc.
12. Electrolytic manganese dioxide for use in an electrochemical cell having a pH- voltage of at least 0.860 volt.
13. Electrolytic manganese dioxide according to claim 12, wherein the electrolytic manganese dioxide has less than 250 ppm of potassium impurities by weight.
14. Use of electrolytic manganese dioxide having a pH-voltage of at least 0.860 volt as positive electrode active material of an alkaline electrochemical cell.
15. Use according to claim 14, wherein the electrolytic manganese dioxide has less than 250 ppm of potassium impurities by weight.
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