METAL HYDRIDE CELLS FOR HIGH RATE/LOW TEMPERATURE PERFORMANCE

Abstract: A metal hydride rechargeable cell, such as a nickel hydride cell, is provided that discharges during a test discharge or initial operational discharge at a rate of 8C to 18C and a cell temperature of 0 DEG C wherein the voltage remains above 0.2 volts during the half of the discharge curve. The cell comprises a negative electrode of hydrogen storage alloy material having a particle size volume distribution whose d(50) value does not exceed 2(2Dht)<1/2>, where Dh is the hydrogen diffusion coefficient for the allow at 0 DEG C and t is a time period from about 200 to about 450 seconds. A process for manufacturing the cell according to the present invention involves an activation protocol comprising at least one formation cycle, a heat treating step, and a cycling step. The process avoids unnecessary processing of the metal alloy material prior to incorporation into the negative electrode.

Description: METAL HYDRIDE CELLS FOR HIGH RATE LOW TEMPERATURE PERFORMANCE This invention relates to metal hydride cells, particularly nickel metal hydride cells, for use in high rate applications in low temperature environments. Nickel cadmium cells have long been the standard for high discharge applications such as power tools. However, increasing emphasis is being placed on nickel metal hydride electrochemical cells as replacements for nickel cadmium cells. Nickel metal hydride cells avoid environmental concerns relating to cadmium and have improved energy density. Nickel metal hydride cells are mainly used with low rate discharge devices, such as cellular phones and portable computers, due to problems in nickel metal hydride power performance when used with high rate discharge devices. The shortfalls in power performance characteristics generally occur at high discharge rates and at low temperatures during initial use of existing nickel metal hydride cells. When discharged at a high pulsed rate, for example 18C (a current in amperes which is 18 times the value of the rated capacity of the cell) as in most power tools, nickel metal hydride batteries known in the art exhibit a voltage polarisation during the initial operational discharge. At low temperatures, for example at or below 0°C, this voltage polarisation is even more pronounced and severely limits the power capabilities of the battery. Cycling of the cell or battery that occurs during subsequent charging and discharging may reduce the effects of this polarisation, but can have no effect on the disabling voltage polarisation encountered initially. Consequently, it would be desirable to be able to enhance the performance of metal hydride electrochemical cells during the initial operational discharge of the cells. It would, furthermore, be desirable to provide a metal hydride cell that can be used for high drain applications in a low temperature environment and that exhibits a less pronounced initial voltage polarisation compared with known metal hydride cells. We have now found, surprisingly, that it is possible to achieve these and other advantages in a metal hydride cell. Accordingly, in a first aspect, the present invention provides a metal hydride cell wherein, when the cell is discharged at a discharge rate of 8C to 18C in a test discharge or initial operational discharge initiated at a cell temperature of 0°C, the voltage remains above 0,2 volts during the first half of the discharge curve. The metal hydride cell is comprised of a positive electrode, preferably utilising nickel hydroxide as the active material, a negative electrode utilising a hydrogen storage metal alloy material as the active material, and an alkaline electrolyte. These components are arranged in a sealed container in a manner effective to provide electrochemical energy. In another aspect of the present invention, the negative electrode of the cell according to the present invention comprises a metal alloy with a particle size volume distribution whose d(50) value prior to the initial operational discharge of the cell does not exceed the required maximum particle size. The required maximum particle size = 1/2 2(2Dι-t) , where D h is the hydrogen diffusion coefficient for the alloy at 0°C and t is a time from 200 to 450 seconds, corresponding to a discharge rate from 18C to 8C. In yet another aspect of the present invention, the metal hydride cell of the present invention is prepared by subjecting the cell to at least one formation cycle, heat treating the cell and cycling the cell, wherein the steps are performed prior to the initial operational discharge of the cell. In a preferred embodiment, this method comprises a formation cycle at about a C/8 to a C/6 charge for approximately 11 to 16 hours followed by a discharge at 2C to 0.9 volts. The heat treatment preferably comprises subjecting the cell to a temperature of 30°C to 120°C for approximately 20 to 24 hours followed by a hold period sufficient to enable the cell to return to room temperature. The cycling preferably comprises 1 to 10 charge/discharge cycles. The cells do not experience disabling voltage polarisation during the initial operational discharge of the cell, and can be manufactured without processing the hydrogen storage metal alloy material prior to incorporating the material into the negative electrode. The metal hydride cells of the present invention are particularly suited to meet the demands of power tools, household appliances, electric vehicles and other battery- powered devices that require power at high drain rates. Furthermore, the metal hydride cells can provide required power at high discharge rates and at low temperatures during the initial operational discharge, and are therefore particularly suitable for use in low temperature environments. It is a further advantage of the present invention that rechargeable power source for high drain applications at low temperatures can be provided with no added cadmium. Moreover, the present invention allows the manufacture of a metal hydride cell which can provide required power at high discharge rates and low temperatures during the initial operational discharge, by a process which, advantageously, can avoid unnecessary processing of the hydrogen storage metal alloy material prior to incorporating the material into the negative electrode. The present invention will be further understood from the following description and drawings, in which: Figure 1 is a graph of voltage versus time during discharge of a known nickel metal hydride cell at the indicated discharge rates and a cell temperature of 0°C; Figure 2 is a graph of voltage versus time for each electrode of a known nickel metal hydride cell as well as the overall cell voltage for the cell; Figure 3 is a graph of voltage versus time during discharge at 7.5C and a cell temperature of 0°C of a known nickel metal hydride cell according to comparative Example 1; Figure 4 is a graph of voltage versus time during discharge at 7.5C and a cell temperature of 0°C of a nickel metal hydride cell of the present invention according to Example 2; Figure 5 is a graph of voltage versus time during a pulsed discharge at 18C and a cell temperature of 0°C of a known nickel metal hydride cell according to comparative Example 3; Figure 6 is a graph of voltage versus time during a pulsed discharge at 18C and a cell temperature of 0°C of a nickel metal hydride cell of the present invention according to Example 4; Figure 7 is a graph of voltage versus time during discharge at 8C and a cell temperature of 0°C of a nickel metal hydride cell of the present invention according to Example 5; and Figure 8 is a graph of average minimum voltage over time of heat soak for various temperatures. The present invention provides metal hydride cells which are able to provide high power output even in cold temperatures during the initial operational discharge of the cell. The term "initial operational discharge" of the cell, as used herein, is the first cell discharge initiated by the end user of a battery powered device, such as a power tool, connected to the cell to provide current to the device. The cell according to the present invention does not experience disabling voltage polarisation during a test discharge and will therefore avoid disabling voltage polarisation during the initial operational discharge at the same cell temperature and discharge rate, since voltage polarisation tends to diminish with increasing cycle number. For purposes of the present invention, "disabling voltage polarisation" refers to a drop in cell voltage to 0.2 volts or less during the first half of a discharge curve. A "test discharge" of the cell, as used herein, refers to any discharge prior to the initial operational discharge, such as cell discharges occurring during the manufacture and testing of the cell or the device. The cell of the present invention can be substantially free of cadmium, meaning that there is no added cadmium to any of the cell components. The total amount of cadmium present within the cell is then only the residual amount, if any, naturally present in the cell components. When the cell is disposed of, no amount of cadmium is added to the environment that is greater than the amount that was present before the cell was assembled. The metal hydride cells of the present invention are otherwise of conventional construction. Thus, they may comprise an alkaline electrolyte, a positive electrode and a negative electrode, arranged in a sealed container in a manner effective to provide electrochemical energy. The cell container is preferably a cylindrical metallic can of steel, nickel or nickel plated steel. Alternatively, the cell configuration can be prismatic or button-shaped, or of any other configurations as are known in the art. The container is preferably hermetically sealed or equipped with a resealable safety valve, although the present invention can also be used with a semi-sealed or flooded cell. It will be appreciated that any suitable cell construction can be used in accordance with the present invention, for example a "jellyroll construction". In an embodiment of a cell according to the present invention, using a typical "jellyroll" construction, the electrodes are formed into sheets and sandwiched together in layers with a porous nylon separator or other suitable material located between the positive and negative electrodes to provide electrical isolation between the electrodes while allowing sufficient ionic diffusion between them. The electrode sheets and separator are spirally wound and placed within the can. The can has a positive terminal positioned on the sealed end of the can preferably formed from a positive electrode tab welded to the cover, and a negative terminal formed from contact between the negative electrode and the inner sides of the can. This contact can be a result of an interference fit, or due to spot welding or the like between the can and the negative electrode, or any combination thereof. The cell according to the present invention preferably employs a nickel based material as the electrochemically active material for the positive electrode. Preferably, the active material is nickel hydroxide, but can also be selected from other suitable materials known in the art, such as silver or silver oxide, or manganese oxides. Various additives, including calcium, cobalt, cobalt oxide, barium, magnesium, zinc, strontium, manganese and combinations thereof may be incorporated into the active material. The positive electrode is preferably a sintered positive electrode. For example, the electrode substrate is suitably formed by sintering nickel powder onto a perforated metal substrate. The porous sintered nickel substrate is then impregnated with nickel nitrate. The nickel nitrate is then chemically converted to nickel hydroxide by exposing the impregnated substrate to an alkaline solution, preferably sodium hydroxide. Pasted nickel hydroxide electrodes can also be used to practice the present invention and can, for example, be produced by depositing, drying and then pressing a nickel hydroxide paste into contact with an appropriate current collector substrate, such as foam, screen, felt, or expanded metal. Other suitable electrode constructions known in the art can also be used in accordance with the present invention. The cell according to the present invention employs a hydrogen storage metal alloy material as the electrochemically active material for the negative electrode. Typical alloys for the negative electrode include the so-called AB2-type as disclosed, for instance, in US-A- 4,716,088 (Reichman, et al). These AB2 a ^°y materials are based on TiN2 and typically encompass elements such as Ti, N, Cr, Zr, or other suitable selected elements. Preferably, the cell contains a negative electrode having hydrogen storage alloy materials of the so-called A-35-type, a common example of which is described by the formula MmΝiaCobMn j -Al-i (Νi, Co, Mn and Al together being the B- component) wherein Mm refers to a lanthanum-rich misch metal including various other rare earth metals, and wherein 2.5<a<5.0, 0<b<2.5, 0<c<0.5 and 0<d<0.5. The A-component can also include other misch metals of various composition. Hydrogen storage alloys of this class are disclosed, for instance, in US-A-4,216,274 (Bruning, et al) and US-A- 4,375,257 (Bruning, et al). Additional metallic constituent elements may be included in the particular AE^-type alloy, or added as substitutes for the listed elements. Typically, this alloy is finished to a powder having a particle size of about 150 Tyler mesh with a d(50) of about 40 μm. A 2 B alloys, such as disclosed in US-A- 4,431,561, and mixtures of alloys can also be used to practice the present invention. The negative electrode may be of the pasted or compressed powdered type. ® Typically a paste is prepared comprising the alloy powder, a binder such as Teflon or CMC (carboxymethylcellulose) or HPMC (hydroxypropyl methylcellulose) or the like, and solvent. Other additives such as nickel powders, carbon and graphite may be added to increase conductivity or facilitate some other desired performance attribute. This paste is coated onto a perforated nickel plated metal substrate, perforated nickel substrate, or a similar substrate material or imbedded into a foam metal structure or a screen. Preferably, the substrate is a nickel plated perforated steel. An alternative is to fabricate a dry powder electrode by pressing or rolling the alloy powder onto a nickel or nickel plated steel substrate or the like to produce a compacted powder electrode. This dry powder electrode may be treated at elevated temperatures and special atmospheres to "sinter" the material to impart additional attributes. The preferred separator material is a nylon 6/66 blend. Other suitable separator materials may include polypropylene, polyethylene, and surface grafted modifications thereof. The preferred electrolyte is a mixture of 35 weight % potassium hydroxide and 1.5 weight % lithium hydroxide solution, although other alkaline electrolytes may be employed to practice the present invention. Existing nickel metal hydride cells demonstrate disabling voltage polarisation during the initial operational discharge at high discharge rates and low temperatures. Disabling voltage polarisation at high discharge rates and low temperatures can be observed in the initial operational discharge of a drill, for example, where the drill bit slows and may cease to turn. Figure 1 illustrates the discharge curves for existing nickel metal hydride cells during a test discharge at discharge rates of 2 amperes, 5 amperes, 10 amperes and 15 amperes (corresponding to a 1C, 2.5C, 5C and 7.5C discharge rate for a cell having a nominal capacity of 2 ampere-hours) and a cell temperature of 0°C. "A cell temperature of 0°C" as used herein means that the reported cell was stored in an ambient temperature of 0°C for a period of at least three hours prior to the discharge. When the discharge rate reaches 15 amperes the cell voltage exhibits disabling voltage polarisation. In order to address the shortfalls in power performance of existing nickel metal hydride cells, investigations were carried out to isolate the causes for the disabling voltage polarisation at high discharge rates and low temperatures. A sub C size nickel metal hydride cell of 2 ampere hours nominal capacity was fitted with a reference electrode. The hydrogen storage alloy material of the negative electrode had a nominal formula MmN-3 6Cop , Mnn 3AI 0, 4. The lanthanum-rich misch metal was 33.34 wt. per cent of the total alloy weight and had the following composition: lanthanum (50-55% by wt.), cerium (30-35% by wt.), neodymium (9-13% by wt.) and praseodymium (3-5% by wt.). The nickel substituted metal was 66.64 wt. per cent of the total alloy weight and had the following composition: nickel (50.33% by wt.), cobalt (9.83% by wt.), manganese (3.93% by wt.) and aluminium (2.57 % by wt.). This alloy is commercially available. One source is Treibacher Auermet. The metal hydride powder was nominally 150 Tyler mesh with a mean particle diameter, d(50), of 39 μm. It was determined from the separate discharge profiles for each electrode and the overall cell discharge profile (Figure 2) that the polarisation was caused by the negative electrode. We have found that the rate of hydrogen diffusion through the hydrogen storage metal alloy of the negative electrode is a limiting factor in eliminating disabling voltage polarisation with metal hydride cells at high discharge rates and low temperatures. Solid state diffusion theory relates the maximum hydrogen diffusion path distance with the hydrogen diffusion coefficient, D h , and time. For a constant temperature, the maximum diffusion path distance for a time, t (seconds), is: 1/2 maximum diffusion path distance = (2D j . t) The time, t, is the time associated with a desired discharge rate nC, by the formula t=3600/n. For a C-rate discharge, the associated time is 3600 seconds. For an 8C discharge rate, the associated time is 3600/8 = 450 seconds. For an 18C discharge rate, the associated time is 3600/18 = 200 seconds. The hydrogen diffusion coefficient, Dj. is an intrinsic property of a given alloy and has a constant value at a given temperature. A preferred method to determine the hydrogen diffusion coefficient for an alloy is the potential step technique. For a single alloy particle, the discharge current is related to the hydrogen diffusion coefficient, D^, when sufficient time has elapsed so that secondary effects subside. This is typically greater than about 400 sec. This relationship is as follows: ln(i) = ln(-2FAD h Δc/1)-D h 2 t/4r 2 wherein F, A, D j ., 1, and t denote the Faraday constant, area of the electrode, hydrogen diffusion coefficient, diffusion layer thickness, and time respectively, and Δc represents the difference in the equilibrium hydrogen concentrations before and after the potential step load and r is the particle radius. The hydrogen diffusion coefficient, D j ., can be obtained from the slope of the ln(i) vs. time plot. To determine the hydrogen diffusion coefficient, D n , in accordance with the present invention, microelectrodes are fabricated with a single spherical particle of metal hydride forming alloy, where the typical diameter of the particle ranges from about 100 to 400 μm. The counter electrode is a gold film, and a Hg/HgO electrode is used as the reference. The working metal hydride forming alloy electrode particle is immersed in 7N potassium hydroxide electrolyte and charged with hydrogen for sufficient amount of time to ensure that the particle is fully loaded with hydrogen. The electrode is then stepped to a potential of -0.5 volts vs. Hg HgO electrode for the particle to discharge. The current is measured as a function of time during this discharge process which is carried out at a constant temperature. After a sufficient amount of time (400-500 sec), the log (current) is plotted as a function of time. The slope of this plot provides the hydrogen diffusion coefficient value, Dj., for the alloy at the discharge temperature. In accordance with the present invention, Table 1 below shows the relationship between discharge rate, hydrogen diffusion coefficient and the required maximum particle size of the alloy powder for a range of hydrogen diffusion coefficients typical for hydrogen storage metal alloys. The required maximum particle size is equal to twice the maximum diffusion path distance as calculated above. We have found, as illustrated in the examples contained herein, that disabling voltage polarisation during the initial operational discharge of a cell of the present invention can be eliminated if the alloy particles do not exceed the required maximum particle size. The required maximum particle size is therefore the maximum metal alloy particle size required to eliminate disabling voltage polarisation during the initial operational discharge of a cell of the present invention. From Table 1, it can be noted that as the discharge rate increases at a cell temperature of 0°C (e.g., from IC to 8C to 18C), the required maximum particle size for a given hydrogen diffusion coefficient should decrease. Specifically, as the discharge rate increases from 1 C to 8C (a factor of 8), the required maximum particle 1/2 size should be reduced by a factor of (8) = 2.83. As the discharge rate increases from 8C to 18C (a factor of 2.25), the required maximum particle size should be 1/2 reduced by a factor of (2.25) = 1.50. It can also be noted that for a particular discharge rate, as the hydrogen diffusion coefficient decreases, the required maximum particle size should be reduced. Specifically, for discharge rates of IC, 8C, and 18C as the hydrogen diffusion coefficient decreases by a power of 10, the required maximum particle size should be reduced by a factor of 3.16. D j - values for two known AB5 alloys have been measured using the potential step technique at both 25°C and 0°C and the results tabulated in Table 2. MmNi3 gCoø 7MJI0 3AI0 4 and LaNiø -jSnn 3 are both alloys available from Treibacher Auermet. The required maximum particle sizes for the two alloys at a desired discharge rate were calculated from the D^ data and reported in Table 2. Based on these results, it can be concluded that at the 18C rate at a cell temperature of 0°C, an alloy having a hydrogen diffusion coefficient on the order of -8 2 2x10 cm /sec has a required maximum particle size in the range of less than 56 μm. Under the same conditions, the misch metal alloy, having a hydrogen diffusion -9 2 coefficient on the order of 2x10 cm /sec, has a corresponding required maximum particle size of approximately 18 μm. Another aspect of the present invention relates the required maximum particle size with the particle size distribution of the metal alloy powder. This relationship advantageously minimises the amount of alloy processing required to avoid disabling voltage polarisation during the initial operational discharge of a cell and allows for efficient alloy powder characterisation. Existing crushing, grinding, sieving, screening and size classification equipment in use in the metal powder industry is capable of making a wide range of hydrogen storage metal alloy particle size distributions. making a wide range of hydrogen storage metal alloy particle size distributions. However, the larger the required maximum particle size for the desired high rate low temperature performance, the better, since small particle size requirements may be difficult to meet. This is due to the fact that smaller particles are often difficult to transport, process and handle and present safety issues due to their increased pyrophoricity. It is also advantageous to minimise the volume of alloy that must conform to the required maximum particle size, so as to avoid unnecessary processing. It has been discovered, as illustrated in the examples contained herein, that disabling voltage polarisation during the initial operational discharge of a cell at high discharge rates and low temperatures can be avoided so long as the hydrogen storage metal alloy powder has a particle size volume distribution whose d(50) value does not exceed the required maximum particle size. Particle size distributions for a metal alloy are referred to in the art in terms of characteristic particle diameters reported as, for example, d(10), d(50) and d(80) values based on a volume distribution. For example, a d(50) value of 27 μm means that 50% of the sample volume is occupied by particles having a characteristic particle diameter of not more than 27 μm. Particle size distributions for purposes of the present invention are determined using a Microtrac XI 00 equipped to perform a three laser analysis, as is known in the art. Such an analyser is commercially available from Honeywell. The sample is dispersed in water, a 60 mL/second flow rate is used, and the measurement period is 60 seconds. The analyser is operated in the reflective mode. The cells of the present invention have unique and highly advantageous properties which make these cells particularly well adapted for use in high drain rate applications. Surprisingly, these cells can provide high power output at very low temperatures, even during the initial operational discharge. In a specific embodiment, these cells can be used in power tools, such as drills, or electric vehicles, which are used at temperatures of 0°C or less. The cells of the present invention can be characterised by their performance during certain test discharges. The test discharge is initiated after the cell has been fully charged and when the cell temperature is 0°C at the initiation of the discharge. The cell is then pulsed at 18C for 2 seconds on and 8 seconds off to simulate actual power tool field use. Alternatively, the test discharge is at an 8C constant discharge rate. The cells of the present invention maintain a voltage during the test discharge of more than 0.2 volts during the first half of the discharge curve. In a preferred embodiment, the voltage during the test discharge remains above 0.3 volts for this period. In a most preferred embodiment, the voltage during the test discharge remains above 0.4 volts for this period. Another aspect of the present invention provides novel methods for obtaining the required maximum particle size in the metal alloy of the negative electrode without additional sieving, crushing or grinding of the metal alloy powder prior to incorporation of the powder into the electrode. In accordance with the present invention, it is possible to activate a cell in an extended activation protocol comprised of initial formation, heat treating, and cycling. As a result of this unique activation process, disabling voltage polarisation even during the initial operational discharge at high rates and low temperatures can be avoided. The activation protocol of the present invention can be varied as described herein to produce the desired results. In a preferred embodiment, the basic steps for activation include (1) formation, (2) heat treating and (3) cycling of the cell (or battery). As described herein, each of these steps may be performed in a number of ways utilising variable parameters to obtain specific desired results. The formation step generally involves applying a current to a cell for a predetermined period of time followed by a discharge, typically draining the cell of usable capacity. The heat treating step (also called heat soak) involves subjecting the cell to a predetermined temperature or temperature range over a predetermined time period. Finally, the cycling step involves charging and discharging the cell for a predetermined number of times of which the depth of discharge may be varied as well as the temperature, amperage and time. In a preferred embodiment, the steps are performed in the above-noted order. However, changing the order of heat treating and cycling does not affect the results and, therefore, provides an alternative method of this invention, as well as any other ordering of steps which produce the desired results. In a preferred embodiment, the formation step involves cycling the cell at least once until it reaches a desired performance level, but not to the point of bulging, leaking or venting. A preferred formation cycle involves charging the cell at a charge of about C/10 to about C/5, preferably about C/8 to about C/6, for at least 10 hours, preferably for 10 to 18 hours, more preferably for 11 to 16 hours, and still more preferably for 12 to 15 hours, followed by a discharge at, preferably, 2C to 0.9 volts. In a preferred embodiment, the formation cycle consists of a cell charge at 0.175C for 11.5 hours at room temperature, followed by a cell discharge at 2C to 0.9 volts. In an alternative embodiment, the formation cycle is performed at an elevated ambient temperature, preferably above 35 °C, more preferably about 40 °C, in which case a separate heat soak step can be omitted. In another alternative embodiment, formation includes an extended formation charging period, preferably of at least 100 hours, more preferably about 120 hours at a C/6 rate, in which case a separate heat soak step can also be omitted. Other variations of the formation step are illustrated in the examples section to follow. In a preferred embodiment, the heat treating step comprises exposing the cell to an ambient temperature between about 30° C and about 120°C, where the maximum temperature may be limited by degradation of cell components. In a more preferred embodiment, the temperature is from about 65 ° C to about 105 ° C. In a preferred embodiment, for example when the formation step is charged at 0.175C for 11.5 hours, the heat treating step is at a temperature of approximately 98 ° C for a period of approximately 24 hours. In an alternative preferred embodiment, for example when the formation step is charged at C/8 for 16 hours, the heat soak is at a temperature of about 90°C for approximately 22 hours. The duration of the heat treatment can be manipulated as described herein to achieve the desired results. As can be seen from Figure 8, generally the higher the temperature, the shorter the time period necessary to heat treat the cell to obtain the desired minimum discharge voltage. Likewise, for a given heat treatment temperature, the longer the exposure time period, the higher the minimum discharge voltage. Other variations of this step are illustrated in the examples section to follow. Since the heat treating step results in a cell having the positive electrode in a highly discharged state, in certain instances, a charge following heat treating is utilised. Such charge can take the form of a "boost" or a complete charge/discharge cycle. In an alternative embodiment, the formation step and the heat treating step can be combined. Preferably the combined formation and heat treating step comprises charging at C/6 at room temperature for 11.5 hours and storing the cell at 101°C for 24 hours followed by a discharge at 2C to 0.9 volts. In a preferred embodiment, the process according to the present invention comprises a hold period following heat treatment. A hold time of 24 hours or more is preferred. This allows the cells to cool down to a typical room temperature (approximately 22 to 27 °C) prior to initiating the next step in the process, because cells at an elevated temperature (e.g., greater than about 35 °C) do not charge effectively or reliably. A maximum hold time in a preferred process is 72 hours. This maximum ensures that a cell having a highly discharged nickel positive electrode is not stored for long and variable periods of time, since the highly discharged positive electrode is subject to composition changes which may result in permanent degradation in the capacity that the cell is able to provide at usable voltages. In a preferred embodiment, the cycling step involves either cycling between about 1-20 cycles, more preferably 1-10 cycles, or "boosting" the cell. For cycling, the maximum number of cycles may be limited due to its potential effect on the total cycle life of the cell. In a most preferred embodiment the number of cycles is 10 cycles. In addition, if at least 10 cycles are used, for example charged at C/2 for 4 hours and discharged at 2C to 0.9 volts, then it may be possible to omit a separate heat soak step. Variations of this step are illustrated in the examples section to follow. "Boosting" may be utilised instead of or in addition to cycling. It is noted that "boosting" may not provide as superior results as variations on the cycling step, but it can provide excellent results for many high discharge rate applications and, in most cases, consumes less overall time in processing as well as less equipment time. In the present application, a "boost" is defined as a limited charge input to revive a cell (or battery) in a deeply discharged condition. The boost inputs charge so that functionality of the cells and/or battery can be determined (i.e., the cell and/or battery assumes a positive voltage above a threshold value indicating proper function). The "boost" also allows a charged cell and/or battery to better survive storage over periods of time because the cell or battery can be revived to a higher percentage of its original capacity. Boosting instead of cycling is preferably performed after the heat treating step. It will be appreciated that, once activated by the activation protocol in accordance with present invention, the cell is ready for use in an initial operational discharge by a consumer or other user. Therefore, the activated cell is preferably packaged, or is incorporated directly into a power tool or other discharge device which is then itself packaged, for presentation or display to consumers, before the initial operational discharge by the user. The present invention may be further understood by reference to the following examples: Example 1 (comparative) A nickel metal hydride negative electrode is fabricated by applying a paste consisting of metal hydride-forming alloy particles and a binder onto a perforated nickel plated steel substrate as described herein. The composition of the paste is as follows: binder, 1 wt. per cent; carbon, 0.5 wt. per cent, alloy, 98.5 wt. per cent. The metal alloy used is MmNi3 ^COQ . Mnø JAIQ 4 as described above. The hydrogen -9 2 diffusion coefficient for the alloy is 2 x 10 cm /sec at 0°C as determined using the potential step technique described herein. The particle size distribution for the alloy powder is determined prior to electrode fabrication using the Microtrac XI 00 as described above. Three samples of the powder are run, and a mathematical average for the distribution is reported as follows: d(10) = 11.4 μm; d(50) = 37.32 μm; d(80) = 60.5 μm. A conventional sintered nickel electrode as described herein is the companion electrode. Both of these electrodes are wound to form a compact jellyroll with the electrodes of opposite polarity isolated from each other by a porous separator of nylon 6/66. This jellyroll is inserted into a metal can, and the cell is filled with the desired electrolyte solution of potassium hydroxide. The negative electrode is contacted with the nickel plated steel case, and the positive is connected via a nickel tab to the cover. The cover is held within the container by means of a mechanical radial compression seal. The sub C cell as fabricated is charged at 350 milliamperes for 11.5 hours and subsequently discharged at 4 amperes to 0.9 volts. After this formation cycle, the cell is recharged at 2.4 amperes until fully recharged at 25°C and stored for three hours at 0°C. The cell has at least 80% of its nominal capacity remaining. The cell is discharged in a test discharge at a high rate of 7.5 C and a cell temperature of 0°C. The cell discharge curve is shown in Figure 3. The cell of this comparative example displays a significant disabling voltage polarisation 6 to 10% into the test discharge. The cell voltage drops to about -0.4 volts and the cell cannot deliver adequate power. The d(50) for the metal alloy in this comparative example exceeds the required maximum particle size of 27 μm for this alloy at this discharge rate, and therefore disabling voltage polarisation occurs. Figure 3 further illustrates that cell voltage recovery occurs after the minimum is reached and is attributed to the temperature increase due to resistive heating during discharge, heating which increases the hydrogen diffusion coefficient for the alloy. Example 2 The cell of this example is assembled as described in Example 1, using the same alloy. The alloy particles in this example are sieved prior to incorporation into the electrode. The particle size distribution for the sieved alloy powder is determined prior to electrode fabrication using the Microtrac XI 00 as described above. Three samples of the powder are run, and a mathematical average for the distribution is reported as follows: d(10) = 6.2 μm; d(50) = 15.5 μm; d(80) = 26.6 μm. The cell is charged at 350 milliamperes for 11.5 hours and subsequently discharged at 4 amperes to 0.9 volts as in Example 1. After this formation cycle, the cell is fully recharged and stored for three hours at 0°C as in Example 1. The cell has at least 80% of its nominal capacity remaining. The cell undergoes a test discharge at an 8C rate and at a cell temperature of 0°C at the initiation of the test discharge in the manner described in Example 1. A discharge curve for this cell is shown in Figure 4. The discharge curve shows no disabling voltage polarisation and exhibits significant improvement in terms of discharge voltage characteristics over Example 1. In accordance with the present invention, the d(50) for the metal alloy in this example does not exceed the required maximum particle size of 27 μm for this alloy at this discharge rate, thereby avoiding disabling voltage polarisation. Example 3 (comparative The cell of this example is assembled as described in Example 1, using the same alloy powder with the same particle size distribution as Example 1. The sub C cells as fabricated is charged at 350 milliamperes for 11.5 hours and subsequently discharged at 4 amperes to 0.9 volts as in Example 1. After this formation cycle, the cell is fully recharged and stored for three hours at 0°C as in Example 1. The cell has at least 80% of its nominal capacity remaining. The cell undergoes a test discharge at a pulsed discharge rate of 18C with 2 seconds on and 8 seconds off. The cell discharge curve for this comparative example is shown in Figure 5. During the early pulses, the voltage drops to -0.15 volts and cannot deliver power. The d(50) for the metal alloy in this comparative example exceeds the required maximum particle size of 18 μm for this alloy at this discharge rate, and therefore disabling voltage polarisation occurs. Example 4 The cell of this example is assembled as described in Example 1, using the same alloy. The alloy particles in this example are sieved prior to incorporation into the electrode. The particle size distribution for the sieved alloy used in this example is the same as in Example 2, and is determined prior to electrode assembly using the Microtrac X100 as described above. The cell is charged at 350 milliamperes for 11.5 hours and subsequently discharged at 4 amperes to 0.9 volts as in Example 1. After this formation cycle, the cell is recharged and stored for three hours at 0°C as in Example 1. The cell has at least 80% of its nominal capacity remaining. The cell undergoes a test discharge at a high pulsed rate of 18C, with 2 seconds on and 8 seconds off. The cell discharge curve is shown in Figure 6. The discharge curve shows no disabling voltage polarisation and exhibits significant improvement in terms of discharge voltage characteristics over Example 3. In accordance with the present invention, the d(50) for the metal alloy in this example does not exceed 18 μm, the required maximum particle size for this alloy at this discharge rate, and therefore disabling voltage polarisation does not occur. Example 5 A cell using a LaNi-j TΪ Q 3 hydrogen storage metal alloy in the negative electrode is assembled as described in Example 1. The particle size distribution for the alloy used in this example was determined prior to electrode assembly using the Microtrac XI 00 as described above. Three samples of the alloy powder are run, and a mathematical average for the distribution is reported as follows: d(10) = 9.6 μm; d(50) = 44.3 μm; d(80) = 89.7 μm. The hydrogen diffusion coefficient of this alloy is determined according to the potential step technique as described herein to be about 2 x -8 2 10 cm /sec at 0°C, about an order of magnitude higher than the alloy used in Example 1. The cell is formed, charged and discharged in a test discharge as in Example 1. A discharge curve for this cell is shown in Figure 7. The discharge curve shows no disabling voltage polarisation at an 8C discharge rate and a cell temperature of 0°C and exhibits significant improvement in terms of discharge voltage characteristics over Example 1. In accordance with the present invention, the d(50) for the metal alloy in this example does not exceed 85 μm, the required maximum particle size for this alloy at this discharge rate, and therefore disabling voltage polarisation does not occur. In Examples 6 through 19 that follow, sub C size cells are assembled as in Example 1 using the metal alloy of Example 1, and having a nominal capacity of 2 ampere-hours. However, the alloy powder has a d(50) of approximately 39 μm prior to incorporation into the negative electrode, as determined using the Microtrac XI 00 as described above. Examples 6 through 19 illustrate the process claimed herein for activating a cell fabricated as above, by forming, heat treating and cycling the cell. Examples 8, 10, and 12-19 have preferred results, with Example 19 having the most preferred result based on a combination of factors, including overall time. The discharge minimum voltage in Table 3 is a typical minimum voltage of a cell during a test discharge. The test discharge occurs after the indicated activation protocol is completed and after the cell is fully charged at a IC rate for one hour at 25°C and stored for three hours at 0°C. The test discharge is initiated at a cell temperature of 0°C and is a pulsed discharge at an 18C rate with two seconds on and eight seconds off. a: Discharge at 2C to 0.9 volts b: Charge at C/2 for 4 hours; Discharge at 2C to 0.9 volts Example 17 1) Formation: Charge at 0.175C for 11.5 hours Discharge at 2C to 0.9 volts 2) Heat Soak: Heat Treat at 98 °C (+/- 3°C) for 24 hours (+/- 2 hours) 3) Hold: 24-72 hours 4) Cycling: 10 cycles The above Example 17 results in a discharge minimum voltage of 0.60 volts. Example 18 1) Formation: Charge at 0.175C for 11.5 hours Discharge at 2C to 0.9 volts 2) Heat Soak: Heat Treat at 96°C for 24 hours 3) Hold: 24-72 hours 4) Boost: Charge at C/10 for 3 hours The cell is subsequently fully charged by the end user. The above Example 18 results in a discharge minimum voltage of 0.50 volts. Example 19 1) Formation: Charge at C/8 for 16 hours Discharge at 2C to 0.9 volts 2) Heat Soak: Heat Treat at 90 °C for 22 hours 3) Hold: 24-72 hours 4) Cycling: 10 cycles The above Example 19 results in a discharge minimum voltage of 0.50 volts.

Claims: CLAIMS: 1. A metal hydride electrochemical cell comprising a positive electrode, a negative electrode comprising a hydrogen storage metal alloy material, and an electrolyte wherein, when the cell is discharged at a discharge rate of from about 8C to about 18C in a test discharge or initial operational discharge initiated at a cell temperature of 0°C, the voltage remains above a minimum voltage of 0.2 volts, preferably above 0.3 volts, and more preferably above 0.4 volts, during the first half of the discharge curve. 2. A cell according to claim 1 wherein the cell is substantially free of cadmium. 3. A cell according to claim 1 or claim 2 wherein the voltage remains above the minimum voltage when the discharge rate is 8C and the discharge is constant. 4. A cell according to claim 1 or claim 2 wherein the voltage remains above the minimum voltage when the discharge rate is 18C and the discharge is a pulsed discharge of 2 seconds on and 8 seconds off. 5. A cell according to any preceding claim wherein the positive electrode comprises nickel hydroxide. 6. A cell according to any preceding claim wherein the positive electrode is a sintered electrode. 7. A cell according to any preceding claim wherein the alloy has the formula MmNi g Cot-Mn g Ald and a is between 2.5 and 5, b is between 0 and 2.5, c is between 0 and 0.5 and d is between 0 and 0.5. 8. A cell according to any preceding claim wherein the metal alloy has a particle size volume distribution whose d(50) value does not exceed a required maximum particle size defined as 2(2D n t) , wherein Dj- is the hydrogen diffusion coefficient for the alloy at 0°C and t is the time associated with the discharge rate nC by the formula r=3600/n. 9. A cell according to claim 8 wherein the hydrogen diffusion coefficient is -9 -7 2 between 10 and 10 cm /second. 10. A cell according to claim 9 wherein the required maximum particle size is from about 13 μm to about 190 μm. 11. A metal hydride electrochemical cell comprising a positive electrode, a negative electrode, and an electrolyte, the negative electrode comprising metal alloy particles, the metal alloy particles having a particle size volume distribution whose d(50) value prior to the initiation of an initial operational discharge does not exceed a required maximum particle size defined as 2(2Dht) , wherein D n is the hydrogen diffusion coefficient for the alloy at 0°C and t is a time period, associated with the discharge rate, of from about 200 seconds to about 450 seconds. 12. A cell according to any preceding claim, wherein the cell is presented for initial operational discharge. 13. A cell according to claim 12, wherein the cell is packaged, or is incorporated into a discharge device and the device is packaged. 14. A process for activating a metal hydride cell, wherein the process comprises the steps of: subjecting the cell to at least one formation cycle; heat treating the cell; and cycling the cell, wherein the steps are performed prior to the initial operational discharge of the cell. 15. A process according to claim 14, wherein the cycling step is performed prior to the heat treating step. 16. A process according to claim 14, wherein the cycling step is performed simultaneously with the heat treating step. 17. A process according to any of claims 14 to 16, wherein the formation cycle comprises charging the cell at a current ranging from about C/8 to about C/6, for a period of time ranging from about 11 hours to about 16 hours at a temperature ranging from about 20°C to about 40°C, and discharging the cell. 18. A process according to any of claims 14 to 16, wherein the formation cycle comprises charging the cell for a period of time of approximately 120 hours and discharging the cell. 19. A process according to any of claims 14 to 16, wherein the formation cycle comprises charging the cell, and discharging the cell at 2C to 0.9 volts. 20. A process according to any of claims 14 to 19, wherein the heat treating step comprises subjecting the cell to a temperature of between about 30°C and about 120°C over a time period of between about 12 hours and about 96 hours. 21. A process according to claim 20, wherein the temperature is about 90°C and the time period is about 22 hours or the temperature is about 98°C and the time period is about 24 hours. 22. A process according to any of claims 14 to 21, wherein the cycling step comprises a number of charge/discharge cycles of between about 2 and about 20. 23 . A process according to claim 22, wherein the number of cycles is 10 and the cell IS c harged at a C/2 rate for about 1 hour during the charge portion of each cycle. 24. A process according to claim 23, wherein the cell is discharged at a 1.5C rate during the discharge portion of each cycle and the time of discharge during the discharge portion of each cycle ranges from about 8 minutes to about 24 minutes. 25 . A process according to claim 23, wherein the cell is discharged at a C/2 rate during the discharge portion of cycles one through nine and the time of discharge during the discharge portion of cycles one through nine ranges from about 24 to about 48 minutes. 26. A process according to any of claims 14 to 16, wherein the formation step comprises a charge at 0.175C for 11.5 hours followed by a discharge at 2C to 0.9 volts and the heat treating step comprises subjecting the cell to a temperature of about 98°C for approximately 24 hours. 27. A process according to any of claims 14 to 26 further comprising a hold period after the heat treating step. 28. A process according to claim 27 wherein the hold period is between about 24 hours and about 72 hours. 29. A process according to any of claims 14 to 28 wherein the cycling step is replaced by a boost step. 30. A process according to claim 29 wherein the boost step comprises charging the cell at about 200 milliamperes for about 3 hours. 31. A process according to claim 14, wherein the formation step and the heat treating step are combined. 32. A process according to claim 30, wherein the cell is charged at C/6 at room temperature for about 11.5 hours, then stored at 101°C for about 24 hours and then discharged at 2C to 0.9 volts. 33. A process according to any of claims 14 to 32, wherein after activation and prior to initial operational discharge of the cell, the cell is packaged or is incorporated into a discharge device and the device is packaged. 34. An activated cell formed by a process according to any of claims 14 to 33.

Inventors: DEMOULLY THOMAS R, KLEIN MICHAEL R, PRAKASH JAI, PUGLISI VINCE, SINGH DEEPIKA B, ZHANG LIANYING, DEMOULLY, THOMAS, R., KLEIN, MICHAEL, R., PRAKASH, JAI, PUGLISI, VINCE, SINGH, DEEPIKA, B., ZHANG, LIANYING
Applicants: EVEREADY BATTERY INC, EVEREADY BATTERY COMPANY, INC.
Application year: 1998
Main classification: H01M
IPC codes: H01M2, H01M4, H01M10