JP7748320B2 - How to estimate the lifespan of nickel-metal hydride batteries - Google Patents

Description

The present invention relates to a method for estimating the lifespan of a nickel-metal hydride battery.

For example, nickel-metal hydride batteries, which make up backup power sources for uninterruptible power supplies and other devices, require accurate lifespan determination to maintain the reliability of the power source. However, because battery capacity and internal resistance remain stable until the end of their lifespan, predicting their lifespan is difficult. Furthermore, backup power sources require periodic battery replacement to maintain functionality. Therefore, extending battery life is required to reduce the frequency of battery replacement, but the considerable time required to evaluate batteries to confirm their lifespan has been a development challenge.

Because batteries utilize electrochemical reactions between chemical substances, corrosion of the electrode plates progresses as the battery deteriorates. For example, as disclosed in Patent Document 1, batteries are evaluated at various environmental temperatures and charge/discharge conditions to measure the amount of corrosion of the negative electrode until the battery reaches the end of its life, and the battery life is estimated based on this amount of corrosion.

International Publication No. 2006/013881

However, when batteries are used as backup power sources in low-temperature environments, estimation methods based solely on the amount of corrosion of the negative electrode can result in actual lifespans that are shorter than the initial estimated lifespan. Therefore, to ensure the battery's reliability as a backup power source, batteries are replaced earlier than the predicted period of actual use, resulting in battery waste.

The object of the present invention is to provide a method for estimating the lifespan of a nickel-metal hydride battery that can accurately estimate the battery lifespan.

To achieve the above objective, the present invention provides a method for estimating the lifespan of a nickel-metal hydride battery, which is a method for estimating the lifespan of a nickel-metal hydride battery in which a positive electrode plate and a negative electrode plate are stacked together with a separator made of nonwoven fabric sandwiched between them and housed in an outer can together with an electrolyte. The lifespan is estimated from the amount of electrolyte held by the separator and the amount of corrosion of the negative electrode plate.

The nickel-metal hydride battery life estimation method of the present invention makes it possible to more accurately estimate the battery life even when continuously charging a nickel-metal hydride battery in a low-temperature environment.

1 is a partially cutaway view of a nickel-metal hydride battery according to an embodiment of the present invention; FIG. 10 is a diagram showing the correlation between the estimated value of the separator retention capacity estimated according to the present embodiment and the actual measured value of the separator retention capacity.

This embodiment describes a method for estimating the lifespan of a nickel-metal hydride battery.

Nickel-metal hydride battery (hereinafter referred to as "battery") 1 is a cylindrical battery of AA size, as shown in Figure 1, for example. Battery 1 contains an electrode group 6, which is made up of a positive electrode plate 3 and a negative electrode plate 4 stacked with a separator 5 between them and wound into a spiral shape, housed in a cylindrical outer can 2 with an open top and a closed bottom, together with alkaline electrolyte, and the top is sealed with a sealing body 7.

The bottom wall 8 of the outer can 2 is conductive and functions as the negative electrode terminal. The sealing body 7 includes a lid plate 9 and a positive electrode terminal 10. The lid plate 9 is conductive and has a gas vent hole 11 in the center. A rubber valve body 12 that closes the gas vent hole 11 is disposed on the outer surface of the lid plate 9. The lid plate 9 is disposed on the open end of the outer can 2 via a ring-shaped gasket 13, and the opening is closed by crimping the edge of the opening of the outer can 2. A positive electrode terminal 10 is attached to the lid plate 9.

The electrode group 6 is composed of strip-shaped positive and negative electrode plates 3, 4, and a separator 5. The separator 5 is sandwiched between the positive and negative electrode plates 3, 4, and wound in a spiral shape to form a roughly cylindrical shape. In other words, the positive and negative electrode plates 3, 4 face each other with the separator 5 interposed between them, and are stacked radially around the outer can 2.

Inside the outer can 2, a positive electrode lead 14 is disposed between one end of the electrode group 6 and the cover plate 9, and each end of the positive electrode lead 14 is electrically connected to the positive electrode plate 3 and the cover plate 9, respectively.

The positive electrode plate 3 consists of a conductive positive electrode substrate with a porous structure and a positive electrode mixture held within the pores of the positive electrode substrate and on its surface. The positive electrode substrate can be, for example, a nickel-plated mesh, sponge, or fibrous metal body, or nickel foam.

The positive electrode mixture contains positive electrode active material particles, a conductive material, a positive electrode additive, and a binder. The positive electrode active material particles are nickel hydroxide (Ni(OH) ₂ ) particles or higher-order nickel hydroxide particles. It is preferable that at least one of zinc, magnesium, and cobalt is solid-dissolved in these nickel hydroxide particles.

The negative electrode plate 4 has a strip-shaped conductive negative electrode core, on which a negative electrode mixture is supported. The negative electrode core is made of a sheet-like metal material with distributed through-holes, such as a punched iron sheet with a nickel-plated surface. When held by the negative electrode core, the negative electrode mixture forms a negative electrode mixture layer.

The negative electrode mixture contains hydrogen storage alloy particles, a negative electrode additive, a conductive material, and a binder.

A hydrogen storage alloy is an alloy that can absorb and release hydrogen, which is the negative electrode active material. A typical hydrogen storage alloy can be used. In this disclosure, it is preferable to use a rare earth-Mg-Ni hydrogen storage alloy that contains rare earth elements, Mg, and Ni.

The separator 5 is made of, for example, a nonwoven fabric made of polypropylene fibers that have been treated with fluorine or sulfonation.

The electrode group 6 is housed in the outer can 2 so that the negative electrode side is in contact with the bottom wall 8 of the outer can 2.

After a predetermined amount of alkaline electrolyte is poured into the outer can 2, the opening of the outer can 2 is sealed. The alkaline electrolyte is impregnated into the positive electrode plate 3, negative electrode plate 4, and separator 5, and is involved in the electrochemical reaction between the positive electrode plate 3 and negative electrode plate 4, the so-called charge-discharge reaction. The alkaline electrolyte used contains NaOH as the main solute.

An initial activation process is performed on the battery 1 created as described above, making the battery 1 usable.

Next, a method for estimating the battery life will be described below.
When used as a backup power supply, the battery 1 is electrically connected between a load, such as electrical equipment, and a commercial power source. The backup power supply discharges and supplies power to the load when the commercial power source is interrupted or a power outage occurs. For this reason, the battery 1 is constantly charged, and after being fully charged, a small amount of current flows to compensate for self-discharge, thereby continuing charging, a so-called continuous charging (overcharge) state. In other words, "continuous charging" refers to a state in which, after the battery 1 is fully charged, a small amount of current flows to compensate for self-discharge, thereby continuing charging.

When the battery 1 is continuously charged in a low-temperature environment, such as 0°C, the positive electrode plate 3 swells (positive electrode swelling), causing the electrolyte to be absorbed into the positive electrode plate 3.

As a result of the positive electrode swelling, the positive electrode plate 3 increases in thickness in the radial direction of the electrode group 6. However, because the inner diameter of the outer can 2 remains unchanged, when the positive electrode swelling occurs, the separator 5 located between the positive electrode plates 3, which are expanding radially, is compressed in the thickness direction, i.e., radially, by the positive electrode plate 3 from both the center and the outside of the electrode group 6. This compression causes the electrolyte held in the separator 5 to be pushed out and absorbed into the swollen positive electrode plate 3. Therefore, the amount of electrolyte held in the separator 5 decreases. In this disclosure, the capacity of the electrolyte held in the separator 5 is referred to as the "separator electrolyte retention volume." A decrease in the separator 5 electrolyte retention volume increases the overall internal resistance of the battery 1, and therefore, the progression of positive electrode swelling leads to a shortened battery life. Positive electrode swelling tends to progress more rapidly when the temperature of the location where the battery 1 is installed, i.e., the ambient temperature, is low, such as 0°C, than when it is high.

This is because oxygen is generated on the positive electrode plate 3 as a side reaction after full charge during continuous charging. Nickel-metal hydride secondary batteries form sealed cells through a reaction in which oxygen generated on the positive electrode is converted back into water on the surface of the hydrogen storage alloy of the negative electrode. However, side reactions are more likely to occur on the positive electrode when the ambient temperature is high, and less likely to occur when the ambient temperature is low. Therefore, continuous charging of battery 1 in a low-temperature environment where side reactions are less likely to occur makes the positive electrode more likely to swell.

Furthermore, it is known that corrosion of the hydrogen storage alloy (negative electrode corrosion) progresses in the negative electrode plate 4. Corrosion of the hydrogen storage alloy consumes the electrolyte in the negative electrode plate 4, and like positive electrode swelling, it draws electrolyte out of the separator 5, leading to a reduction in the amount of electrolyte held by the separator 5.

As mentioned above, when the amount of electrolyte held in the separator 5 decreases due to positive electrode swelling and negative electrode corrosion, the internal resistance of the battery 1 increases, shortening the battery's life.

The inventors therefore developed the following formula, which takes into account not only negative electrode corrosion but also the amount of electrolyte absorbed into both electrodes 3 and 4 due to positive electrode swelling, allowing for a more realistic estimation of the battery 1 lifespan.

The characteristics of Battery 1 are evaluated at each environmental temperature and under each charge/discharge condition, and the lifespan of Battery 1 is estimated based on these evaluations as follows:

The liquid retention capacity of separator 5 can be calculated using the following formula (1):

Each parameter in the above formula (1) will now be explained.
Regarding the initial liquid volume,
C: Amount of liquid retained in separator during continuous charging [g]
L: Amount of electrolyte injected during battery production [g]
Sr: distribution ratio of injected electrolyte to separator, 0<Sr<1
Ce: Amount of electrolyte at which the separator begins to be impregnated with the electrolyte during injection [g]
The initial amount of electrolyte held in the separator is the amount of electrolyte held in the separator at a ratio Sr to the amount of electrolyte exceeding Ce from the injected amount.

Regarding the amount of positive electrode uptake during continuous charging,
Wγ: Electrolyte consumption per amount of positive electrode swelling [g]
Dγ: Positive electrode swelling progress rate [0<Dγ<1] Dγ=0 indicates that 100% of the positive electrode active material is in the β phase, and Dγ=1 indicates that 100% of the positive electrode active material is in the γ phase.

Regarding negative electrode corrosion,
W _M : Electrolyte consumption per amount of corrosion progress of negative electrode [g^2/emu]
M: Amount of corrosion progress of negative electrode [emu/g] M is an index of the amount of magnetization per weight of the negative electrode, the amount of corrosion progress.

When using formula (1), the unknown coefficient can be determined by preliminarily evaluating the battery at each ambient temperature and under each charge/discharge condition, and measuring the amount of electrolyte retained in the separator each time.

Next, the formula for determining the positive electrode expansion progress rate Dγ will be explained below.
More specifically, the amount of positive electrode uptake is calculated from the following formulas (2) to (4).

In the formulas (2) to (4), the parameters are as follows:
i _rate : Battery current rate during continuous charging [C]
i _side : Current rate used for side reaction of the positive electrode [C]
i _γ : Current rate used in the positive electrode swelling reaction [C]
The values of i _side and i _γ are calculated by solving simultaneous equations (2) and (3) using the interface overvoltage Δφ _pos of the positive electrode surface as an unknown.
I: Exchange current density I [C] at the reference temperature T _ref in each of the side reaction and the positive electrode swelling reaction in the Butler-Volmer equation
α: Transfer coefficient in each of the side reaction and the positive electrode swelling reaction in the Butler-Volmer equation Δφ: Interfacial overvoltage in each of the side reaction and the positive electrode swelling reaction in the Butler-Volmer equation [V]
Q: Activation energy [J/mol] for each of the side reaction and the positive electrode swelling reaction in the Butler-Volmer reaction
T: Temperature during continuous charging [K]
T _ref : Reference temperature [K]
R: gas constant [J/mol k]
F: Faraday constant [A sec/mol]
The reference temperature T _ref is the reference temperature used to determine each coefficient. For example, when the temperature T during charging is equal to T _ref , the exponent terms in equations (2) and (3) become 1, and calculations can be performed without temperature-related correction. In other words, I _side and Iγ represent the exchange current density coefficients when the charging temperature is T _ref . In this way, T _ref can be set arbitrarily, and I _side and Iγ corresponding to the set T _ref can be used. However, when performing life prediction calculations or comparing and analyzing each coefficient, it is preferable to set T _ref to 25°C.

As described above, the formula for the amount of electrolyte taken up into the positive electrode is expressed by two equations (2) that represent the rate of progress of the side reaction during continuous charging of the positive electrode and the rate of progress of the positive electrode swelling reaction (current rate):
(3). The side reaction and the positive electrode swelling reaction each occur based on the charging current flowing through the battery during continuous charging. As shown in the following equation (4), the sum of the two reaction currents is equal to the charging current i _rate during continuous charging. Therefore, the unknown interfacial overvoltage Δφ _pos on the positive electrode surface can be derived, and the respective reaction currents i _side and i _γ can be calculated.

Next, the expansion rate Dγ of the positive electrode is calculated from iγ using the following procedure:

(1) Solve equations (2) and (3) at predetermined intervals, for example, every hour, to calculate iγ.

(2) If the above time interval is Δt = 1 hour, the reaction progress rate per calculation interval is iγ × Δt [A × (hours)], and the total amount of positive electrode swelling during continuous charging is Σ(iγ × Δt).

The amount of oxygen generated as a result of the side reaction at the positive electrode is measured, and the reaction amount of the side reaction, Σ(i _side × Δt), is calculated. Σ indicates the integration of the current during the period of continuous charging. The calculated reaction amount of the side reaction, Σ(i _side × Δt), is subtracted from the total charge amount, Σ(i _rate × Δt), when all swelling of the positive electrode active material is complete, to determine the charge capacity, Cγ, of the battery at which swelling is completed. At this time, the positive electrode swelling progress rate, Dγ, can be calculated as Dγ = Σ(iγ × Δt) / Cγ.

As shown in equation (4), the sum of the two reaction currents is equal to the charging current during continuous charging. Therefore, by deriving the unknown interfacial overvoltage Δφ _pos on the positive electrode surface, the respective reaction currents i _side and i _γ can be calculated using equations (2) and (3). By sequentially calculating and integrating the reaction current i _γ due to swelling of the positive electrode active material, the amount of positive electrode swelling Dγ of the positive electrode plate 3 in an overdischarged state can be calculated. It can be seen that these calculation formulas and calculation methods can explain the increase in the rate of positive electrode swelling in a low-temperature environment, as described above.

Also, negative electrode corrosion can be calculated using the following equation (5):

In addition, the parameters in equation (5) are as follows:
Mo: Initial magnetic susceptibility [emu/g] The starting value of magnetic susceptibility at the start of use (t=0). t is the time elapsed since the battery was used (the total of the period during which normal charging and discharging was performed, the period during which continuous charging was performed, and the unused period during which charging and discharging was not performed). When predicting the lifespan under continuous charging, it is the period during which continuous charging continues, and is expressed in months.
At: corrosion progression coefficient per month of storage [emu/g month]
Ac: corrosion coefficient per charge/discharge cycle [emu/g cyc]
Ncyc: Number of charge/discharge cycles [cyc]
Qt, Qc: Activation energy [J/mol] of corrosion progression over time and corrosion progression due to charge/discharge
T _ref : Reference temperature [K] The reference temperature for At and Ac

The amount of magnetization progress M of the negative electrode is determined by two factors: the total elapsed time of the battery (the sum of the period during which normal charging and discharging is performed, the period during which continuous charging is performed, and the unused period during which no charging or discharging is performed) and the number of charge and discharge cycles. The amount of corrosion progress for each is calculated and added together. Since the amount of electrolyte absorbed is known to be proportional to the progress of positive electrode swelling and negative electrode corrosion, if an evaluation is performed in advance and the coefficients (Wγ, W _M ) are calculated, it becomes possible to estimate the amount of electrolyte in the separator. Therefore, the internal resistance at that time can be used to more accurately estimate the battery life.

From the above, the lifespan can be estimated by using equations (1) to (5) as follows.
(I) Preliminary experiments are conducted in advance by varying the storage condition parameters such as the battery storage temperature, charging current value, and storage period in the estimation formulas (2) to (5), and the amount of electrolyte retained by the separator at each time is measured. The storage period may be determined using an accelerated test.

(II) The coefficient parameters in estimation equations (2) to (5) are determined using the least squares method or the generalized reduced gradient method (GRG) so that the separator's liquid retention capacity can be correctly calculated under each storage condition.

(III) After obtaining the estimation formulas (2) to (5) with determined coefficient parameters, the storage conditions such as temperature and current value can be input into the estimation formulas (2) to (5) to calculate the liquid retention capacity of the separator from formula (1).

(IV) The amount of electrolyte held in the separator determines the internal resistance of the battery, allowing for the lifespan to be determined. In this way, the battery lifespan can be estimated from the history of the amount of electrolyte held in the separator 5 and the amount of swelling of the positive electrode plate, measured at various ambient temperatures and under various charge and discharge conditions.

(Example)
A continuous charge test was conducted using an AA-size battery with a capacity of 1000 mAh. The continuous charge test conditions were: (1) ambient temperature: −20 to 60°C; (2) charge rate: 0.05 It to 0.2 It; and (3) discharge frequency: none, once per month, once every two months, or once per three months. After a certain period of time, the battery was removed and the positive electrode electrolyte uptake, negative electrode electrolyte consumption, and separator retention volume were measured after continuous charging. These measurement results were used to formulate the equations. Figure 2 shows the correlation between the actual separator retention volume and the predicted separator retention volume calculated using the above equations (1) to (5). Figure 2 confirms the correlation between the actual separator retention volume and the predicted separator retention volume.

Specifically, the table below shows the relationship between the actual battery life and the life calculated using the life estimation formula when an AA-size, 1000mAh battery is continuously charged (without discharging) in a low-temperature environment, such as an ambient temperature of 0°C, at a current rate of 0.05C. In the table, the comparative example is a battery of the same type and continuously charged under the same conditions as the example in which the life was calculated based solely on conventional anode corrosion.

As can be seen from Table 1, the battery's life was 6.2 years when continuously charged in a low-temperature environment of 0°C at a current rate of 0.05C. On the other hand, the battery of the comparative example, whose life was estimated from the amount of corrosion of the negative electrode, was estimated to have a life of 12.8 years, while the battery of the example was estimated to have a life of 5.8 years. As such, it can be seen that the battery life estimation according to the example can calculate a number of years closer to the actual measured value than the comparative example. Therefore, it is clear that the accuracy of the life estimation according to this embodiment can be improved.

The estimated lifespan in the comparative example is thought to be higher than the actual measured value due to the influence of the environmental temperature in which continuous charging was performed. When continuous charging is performed in a low-temperature environment, such as 0°C, conventional estimation methods based on the amount of anode corrosion do not cause anode corrosion to progress as much as continuous charging in a temperature environment, such as 25°C, and therefore the estimated lifespan is longer than the actual measured value. When estimating lifespan based solely on anode corrosion, if the battery is continuously charged in a low-temperature environment, the estimated lifespan is longer than the actual measured value. As a result, batteries incorporated into backup power sources will deteriorate and become unusable sooner than originally estimated based on anode corrosion, and will not be able to function as backups in the event of a voltage drop or power outage in the commercial power supply.

Therefore, by estimating battery life from the amount of liquid held in the separator, as in this embodiment, a backup power supply incorporating such a battery can function appropriately in unexpected situations such as a voltage drop or power outage in the commercial power supply, and can make effective use of the battery life without waste.

1 Battery 2 Outer can 3 Positive electrode plate 4 Negative electrode plate 5 Separator

Claims (3)

A method for estimating the service life of a nickel-metal hydride battery in which a positive electrode plate and a negative electrode plate, which are stacked with a separator made of nonwoven fabric sandwiched between them, are housed in an outer can together with an electrolyte, comprising:
A method for estimating the life of a nickel-metal hydride battery, comprising estimating the life from the amount of electrolyte held by the separator and the amount of corrosion of the negative electrode plate. The method of claim 1, wherein the retained electrolyte volume is calculated from the initial amount of electrolyte held in the separator before the nickel-metal hydride battery is first used and the amount of electrolyte absorbed into the swollen positive electrode plate when the nickel-metal hydride battery is maintained at full charge. 2. The method according to claim 1, wherein the life is estimated by calculating the amount of retained electrolyte in the separator and the amount of swelling of the positive electrode plate from a history of the amount of retained electrolyte held in the separator and the amount of swelling of the positive electrode plate, the history being measured in a storage environment in which the nickel-metal hydride battery is stored and under predetermined charge and discharge conditions.