JP2772054B2 - Nickel hydride rechargeable battery - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial application field) The present invention relates to a nickel-metal hydride secondary battery, in particular, a capacity ratio between a cathode and an anode mainly composed of a hydrogen storage alloy and injection of an electrolytic solution. The present invention relates to a nickel-hydrogen secondary battery with an appropriate amount.

(Prior Art) Nickel-metal hydride using a hydrogen storage alloy capable of reversibly absorbing and releasing hydrogen as a cathode and a nickel hydroxide used as a known nickel cadmium secondary battery as an anode Secondary batteries are attracting attention because they allow for higher capacities.

Examples of the hydrogen storage alloy include a titanium-based alloy, a manganese-based alloy, a rare earth-based alloy, and the like.As an alloy capable of electrochemically storing and releasing hydrogen electrochemically in an electrolytic solution, LaNi _5- based alloys are known. However, LaNi ₅ as it is does not provide satisfactory results in both electric capacity and charge / discharge cycle life, and takes into account changes in lattice spacing when hydrogen is occluded in the alloy, which is directly related to those characteristics. Therefore, it is necessary to design a hydrogen storage alloy having optimum characteristics when it is applied to the cathode of a secondary battery by adding another metal element.

Based on research on conventional hydrogen storage alloys, part of Ni was converted to Mn and A
l, substituted by Co, and represented by LmNi _a Mn _b Al _c Co _d (where Lm is a lanthanum-enriched misch metal),
It has been found that the hydrogen storage alloy defined as a> 3.6, d ≦ 1, 4.8 ≦ a + b + c + d ≦ 5.2 has almost satisfactory characteristics as the cathode of the above-mentioned secondary battery. However, when a secondary battery is configured by combining a cathode made of such a hydrogen storage alloy and an anode made of a nickel electrode, if the composition of the hydrogen storage alloy changes, the characteristics of the alloy change, and as a result, the cathode characteristics Changes. As a result, the optimum capacity of the cathode and anode changes, and the LaNi ₅
However, there is a problem that the optimum capacity ratio obtained in the above cannot be applied as it is.

In a nickel-metal hydride secondary battery using a hydrogen storage alloy, it is necessary to find an optimum cathode-to-anode capacity ratio every time the alloy composition is changed. The cycle life becomes longer as the capacity of the cathode increases, but the capacity of the battery is regulated by the nickel electrode serving as the anode. For this reason, if the volume of the cathode increases with the increase in the capacity of the cathode, the volume of the anode, which is the counter electrode, must be reduced because the power generation element group is housed in a metal can of a fixed volume. The capacity is reduced, and thus the battery capacity is reduced. Conversely, if the capacity of the cathode is reduced, the capacity of the battery can be increased.However, the rate of deterioration of the electrode capacity is higher in the hydrogen storage alloy as the cathode than in the nickel electrode as the anode. As a result, the capacity of the cathode becomes smaller than the capacity of the anode, and thereafter the capacity of the battery rapidly decreases, and at the time of charging, hydrogen is generated from the cathode to increase the internal pressure of the battery.

On the other hand, in a nickel-metal hydride secondary battery using a hydrogen storage alloy, a cathode, a separator, a power generation element group including an anode and an electrolytic solution are put in a limited space of a metal can. There is also a problem that the optimum injection amount of the electrolytic solution also changes, and the above-described optimum injection amount obtained for LaNi ₅ cannot be directly applied.

That is, in general, the larger the amount of the electrolyte, the more the space inside the electrode is filled with the electrolyte, so that the utilization rate of the electrode is increased and the battery capacity is increased. Discharge becomes possible. However, in a sealed secondary battery, it is necessary to reduce oxygen generated from the anode side during charging on the cathode surface, but the oxygen reduction rate and the amount of electrolyte are closely related, and the amount of electrolyte increases. As the rate of oxygen reduction on the cathode surface decreases, the internal pressure of the battery increases, and the state becomes more dangerous. This is for the following reason. When the cathode is made of a hydrogen storage alloy, oxygen generated at the anode is electrochemically reduced on the surface of the hydrogen storage alloy of the cathode, but the surface of the hydrogen storage alloy is covered with a thin liquid film of an electrolytic solution. Oxygen dissolves on the surface of the liquid and diffuses to the surface of the hydrogen storage alloy. If the electrolyte layer on the surface of the hydrogen storage alloy is thick, the diffusion rate of oxygen becomes slow, and the rate of oxygen reduction cannot keep up with the rate of oxygen generation.
As described above, the internal pressure of the voltage increases, and a dangerous state occurs.

Therefore, it is necessary to inject an optimal amount of the electrolytic solution into the metal can, but the optimal amount of the electrolytic solution also changes due to a change in the electrode characteristics due to a difference in the composition of the hydrogen storage alloy. The optimum amount of the electrolytic solution also changes depending on the anode characteristics. When a paste-type nickel electrode having high water absorption is used as the anode, the amount of the electrolytic solution is larger than that of the sintered nickel electrode.

(Problems to be Solved by the Invention) The present invention has been made in order to solve the above-mentioned conventional problems, and it is an object of the present invention to optimize a capacity ratio of a cathode and an anode containing a hydrogen storage alloy as a main component and an amount of an electrolyte to be injected. To provide a nickel-metal hydride secondary battery that satisfies both electrode capacity and charge / discharge cycle life and prevents an increase in battery internal pressure.

[Structure of the Invention] (Means for Solving the Problems) The nickel-metal hydride secondary battery according to the present invention comprises LnNi _a Mn _b Al
_c Co _d (Ln is lanthanum alone or a mixture of rare earth elements containing lanthanum, a> 3.6, d ≦ 1, 4.8 ≦ a + b + c +
d ≦ 5.2), a power generating element group including a cathode mainly composed of a hydrogen storage alloy represented by the formula: an anode mainly composed of a nickel compound; and a separator interposed between the cathode and the anode. In a metal can, an alkaline electrolyte is injected, and the opening of the can is sealed with a lid.In an AA-size nickel-metal hydride secondary battery, the amount of hydrogen storage alloy contained per unit area of the cathode is determined. M (g / cm ² ), and when the capacity per unit area of the anode is C (mAh / cm ² ), the value of x _{2 represented} by the relationship of C = x ₂ · M is 100 or more, 300 When M is set so as to be as follows, and the injection amount of the electrolytic solution is L (cc), the value of y represented by the relationship of L = y · C becomes 0.035 or more and 0.1 or less. L is set as described above.

As Ln of the hydrogen storage alloy represented by the above composition formula, lanthanum (La) alone or a rare earth element containing La can be used, but in practice, the amount of cerium (Ce) among the rare earth elements is 15%. A misch metal represented by Lm, which is reduced as follows and whose La content is relatively increased (enriched), is preferable. The total amount of Ni, Mn, Al and Co (a + b + c + d)
The reason for setting the range of 4.8 to 5.2 is that if the composition is less than 4.8 and exceeds 5.2, the amount of hydrogen storage per 1 g of the hydrogen storage alloy is significantly reduced. The reason for limiting the amount a of Ni and the amount b of Co is that if a is 3.6 or less and b exceeds 1, the battery voltage is reduced. More preferable composition ratios represented by the above formula are Mn, Al, Co content of 0.1 to 0.1 respectively.
0.7. Hydrogen storage alloy of such composition,
Hydrogen storage capacity at normal pressure is converted to electric capacity, 360mAh per gram
The amount of hydrogen that can be stored and released reversibly electrochemically reaches 300 mAh in terms of electric capacity when charged at 1 C.

The anode mainly containing the nickel compound may be a sintered electrode or a paste electrode, but a paste electrode is preferable from the viewpoint of increasing the battery capacity. As such a paste electrode, for example, a three-dimensional mesh core such as sponge-like nickel and nickel short fiber sintered body, which is filled with nickel oxide and press-molded, can be used.

The reason for limiting the value of x in the above relational expression is that x is
If it exceeds 300, the charge / discharge cycle life decreases, while if it is less than 100, it becomes difficult to obtain a large capacity secondary battery.
More preferred values of x are in the range of 140-220.

The reason for limiting the value of y in the above relational expression is that y is
If it exceeds 0.1, the internal pressure of the battery will increase. On the other hand, if it is less than 0.035, the amount of the electrolyte will be insufficient and the capacity of the battery will decrease. A more preferable value of y is 0.04 or more and 0.08 or less.

(Operation) Inside the nickel-metal hydride secondary battery, a cathode and an anode are closely arranged via a separator. When the distance between the cathode and the anode is very small, an electrode reaction occurs between the opposed cathode and the anode. For this reason, designing a battery based on the ratio of the total capacity of the electrodes incorporated in the metal can, including the capacity of the electrode part where the cathode and anode are not opposed to each other, is not possible if the battery sizes are different. Alternatively, when the electrode size changes even with the same battery size, the optimum capacity ratio between the cathode and the anode must be obtained again, which is not a universal and theoretical design method.

Therefore, when determining the optimum capacitance ratio between the cathode and the anode, it is necessary to obtain the capacitance ratio between the surfaces facing the electrodes. Specifically, it is appropriate to determine the capacity per unit area of the cathode and the anode. It is assumed that the capacity reversibly obtained from the hydrogen storage alloy by the electrochemical method is equal to the capacity calculated from the maximum hydrogen storage capacity of the hydrogen storage alloy, and that the capacity of the hydrogen storage alloy electrode is not deteriorated in the alkaline electrolyte without any deterioration. When the decrease does not occur, the capacity of the hydrogen storage alloy electrode per unit area may be equal to the capacity per unit area of the nickel electrode as the counter electrode. That is, x in the above relational expression
The value of may be 360. However, the capacity actually obtained reversibly from a hydrogen storage alloy electrode by an electrochemical method is:
The capacity is smaller than the capacity calculated from the maximum storage alloy amount of the hydrogen storage alloy, and is gradually oxidized in the alkaline electrolyte, so that the capacity decreases.

In the present invention, in consideration of the above two points, the amount of the hydrogen storage alloy per unit area of the cathode mainly containing the hydrogen storage alloy having a specific composition is M (g / cm ² ), and the nickel compound is mainly used. The capacity per unit area of the anode as a component is C (mAh /
cm ² ), the upper limit value of x expressed by C = x / M...
By setting M so that the lower limit value is set to 300 and the lower limit value is set to 100 in relation to the capacity, the capacity ratio between the cathode and the anode mainly composed of the hydrogen storage alloy is optimized.

Further, the injection amount of the electrolyte is L (cc), and the capacity per unit area of the anode mainly composed of a nickel compound is C (mAh / c
m ² ), L is set so that the value of y expressed by the relationship L = y · C is not less than 0.035 and not more than 0.1. Is an optimized version of

Therefore, by setting the amount M of the hydrogen storage alloy and the amount L of the electrolytic solution contained per unit area of the cathode so that x and y in the above relational expressions become predetermined values, the electrode capacity and the charge / discharge are set. A high performance and high reliability nickel-metal hydride secondary battery that satisfies both cycle life and suppresses an increase in internal pressure can be obtained.

(Example) Hereinafter, an example of the present invention will be described in detail.

Example 1 A battery element group was wound by using a cathode, a nickel electrode as an anode, and a separator, which will be described later, in a state where the separator was interposed between the cathode and the anode. Then, the anode lead was connected to a lid, an alkaline electrolyte was injected into the can, and then the lid was attached to the upper opening of the can and hermetically closed to assemble an AA size secondary battery. The lid is provided with a valve that operates when the inside of the can becomes 15 kg / cm ² or more and allows gas to escape to the outside. The outermost periphery of the cathode side is connected to the inner surface of the can at the time of winding without interposing a lead.

The nickel electrode is a paste electrode manufactured by filling a nickel oxide as an active material into a three-dimensional mesh core made of nickel sintered fiber and press-molding the same.
Capacitor C 35 mAh / cm ² per unit area, 40mAh / cm ^2, 45mAh /
Three electrodes of cm ² were used.

The cathode uses a hydrogen storage alloy made of LmNi _4.2 Mn _0.3 Al _0.3 Co _0.3 (Lm; a lanthanum-enriched misch metal) whose value obtained by converting the amount of hydrogen storage at normal pressure into electric capacity is 360 mAh / g. Before absorbing and releasing gaseous hydrogen, it is pulverized,
This powder was mixed with carbon and polytetrafluoroethylene powder, kneaded to form a sheet, and the sheet was pressed against a nickel net to produce a sheet. In addition, as the cathode, the value of x obtained by calculating the amount M of the hydrogen storage alloy contained per unit area from the above-mentioned relational expression was determined for each nickel electrode.
Fifteen types set to be 80, 130, 220, 300, and 400 were used.

The separator used was a nonwoven fabric whose main fiber was made of polyolefin and had a thickness of 0.2 mm.

The electrolyte used was adjusted so that KOH was 7N and LiOH was 1N. The injection amount L of each battery into the metal can was such that y obtained from the above relational expression was 0.05. Set.

After the above-mentioned secondary battery was allowed to stand at room temperature for one day, charging and discharging were performed at 0.3 C for 5 hours and discharging at 1 C to 1 V. However, a one-hour rest period was provided between charging and discharging, and between discharging and charging. The cycle life of each battery was determined from the number of cycles until the battery capacity reached 90% of the capacity of the initial cycle (third cycle). In addition, the valve operated during the charge / discharge cycle was checked, and for the valve operated, the charge / discharge cycle was stopped at that time, and the number of times was determined as the cycle life. FIG. 1 shows the relationship between x of the hydrogen storage alloy constituting the cathode and the cycle life. FIG. 2 shows the relationship between x of the hydrogen storage alloy constituting the cathode and the capacity ratio of the initial cycle (third cycle) of each battery. The capacity ratio was expressed as a relative value when the capacity where x was 300 was taken as 100. In FIG. 1 and FIG. 2, a circle indicates a nickel electrode having a capacity per unit area of 35 mAh / cm ^2, a square indicates a nickel electrode having a capacity of 40 mAh / cm ^{2, and} a triangle indicates a capacity of 45 mAh / cm ^2. c
It uses a nickel electrode of m ² . Further, in FIG. 2, the case where the value of x is 400 is not described because the valve was activated in the third cycle or more and the capacity could not be measured.

As is clear from FIG. 1, when the value of x is 220 or less, the cycle life is all 300 cycles. However, the cycle life does not mean 300 cycles, but a maximum of 300
This is because only cycles were performed. When the value of x is 300, the cycle life is about 280 cycles, and when the value of x is 400, it is only two cycles. Two cycles when the value of x is 400 is
This is because the internal pressure of the battery rose as expected and the valve was operated. Therefore, it is necessary to set the content of the hydrogen storage alloy having a specific composition of the cathode such that the value of x is 300 or less from the results of the cycle life shown in FIG.

Also, as is clear from FIG. 2, the battery capacity decreases as the value of x decreases, but it can be seen that the smaller the capacity per unit area of the nickel electrode, the smaller the battery capacity decrease. This is for the following reason. Decreasing the value of x is equivalent to increasing the content of the hydrogen storage alloy. In order to store the power generation element group in a metal can with a fixed volume, it is necessary to reduce the volume of the nickel electrode by increasing the content of the hydrogen storage alloy, but the capacity of the nickel electrode is originally small and the volume is small In this case, the volume of the nickel electrode to be reduced can be reduced. When the capacity per unit area of the nickel electrode is 45 mAh / cm ² , when the value of x is 80, the relative capacity ratio decreases to about 60. When the relative capacity ratio is 60, it is insufficient in terms of increasing the capacity, and it is necessary that the relative capacity ratio be 70 or more. For this purpose, the content of the hydrogen storage alloy having a specific composition of the cathode is determined by the value of x.
It must be set to be 100 or more.

Example 2 A battery element group was wound by using a cathode, a nickel electrode as an anode, and a separator, which will be described later, in a state where the separator was interposed between the cathode and the anode. And the alkaline electrolyte was injected into the can. The outermost periphery of the cathode was connected to the inner surface of the can at the time of winding without a lead, and a nickel lead was attached to the outside of the can. The anode was directly attached with a lead made of nickel so that it could be taken out of the can.
The can was housed in an acrylic resin container, and the upper opening of the container was sealed with an acrylic resin lid to which a pressure sensor was attached, to assemble a test cell.

The nickel electrode is a paste electrode manufactured by filling a three-dimensional mesh core made of nickel sintered fiber with nickel oxide as an active material and press-molding, and has a capacity C per unit area of 35 mAh. / cm ² , 40mAh / cm ² , 45
Three electrodes of mAh / cm ² were used.

The cathode uses a hydrogen storage alloy made of LmNi _4.2 Mn _0.3 Al _0.3 Co _0.3 (Lm; a lanthanum-enriched misch metal) whose value obtained by converting the amount of hydrogen storage at normal pressure into electric capacity is 360 mAh / g. Before absorbing and releasing gaseous hydrogen, it is pulverized,
This powder was mixed with carbon and polytetrafluoroethylene powder, kneaded to form a sheet, and the sheet was pressed against a nickel net to produce a sheet. In addition, as the cathode, the value of x obtained by calculating the amount M of the hydrogen storage alloy contained per unit area from the above-mentioned relational expression was determined for each nickel electrode.
The one set to be 220 was used.

The separator used was a nonwoven fabric whose main fiber was made of polyolefin and had a thickness of 0.2 mm.

The electrolyte used was adjusted so that KOH was 7N and LiOH was 1N, and the injection amount L into each battery was 0.025, 0.035, 0.05, 0.075, y obtained from the above relational expression.
It was set to be 0.10 and 0.12.

After the above-mentioned test cell is allowed to stand at room temperature for one day, it is charged at 0.3 C for 5 hours, and charged and discharged at 1 C to 1 V to change the capacity of the test cell according to the charge and discharge cycle (change obtained as a capacity ratio) And the change of the internal pressure of the test cell was measured. However, a one-hour rest period was provided between charging and discharging, and between discharging and charging. FIG. 3 shows the relationship between the injection amount y of the electrolytic solution and the capacity ratio of the test cell. The capacity ratio of each test cell is
The capacity at the 10th cycle was adopted, and the value was expressed as a relative value when the capacity at which the value of y was 0.05 was taken as 100. FIG. 4 shows the relationship between the injection amount y of the electrolytic solution and the maximum value of the internal pressure at the 100th cycle of the test cell. In FIGS. 3 and 4, a circle indicates a nickel electrode having a capacity per unit area of 35 mAh / cm ^2, a square indicates a nickel electrode having a capacity of 40 mAh / cm ² , and a triangle indicates a capacity of 45 m
A nickel electrode of Ah / cm ² is used.

As is clear from FIG. 3, when the value of y becomes 0.025, the capacity ratio decreases to about 60%. When the capacity ratio is 60%, it is insufficient from the viewpoint of increasing the capacity, and it is necessary that the capacity ratio be 70% or more. For that purpose, it is necessary to set the injection amount of the electrolytic solution so that the value of y is 0.035 (the capacity ratio is 80%) or more.

As is clear from FIG. 4, when the value of y is 0.1, the pressure is 12 kg / cm ^2, which is lower than the operating pressure of a safety valve attached to a normal battery. The internal pressure rises sharply and reaches 30 kg / cm ² . Therefore, based on the result of the internal pressure of the battery shown in FIG.
It must be set to be 0.1 or less.

[Effects of the Invention] As described in detail above, according to the present invention, the capacity ratio between the cathode and the anode mainly composed of a hydrogen storage alloy is optimized, the amount of the injected electrolyte is optimized, and the electrode capacity and the charge / discharge cycle life are obtained. Can be provided, and a high-performance and highly reliable nickel-metal hydride secondary battery in which an increase in battery internal pressure is prevented can be provided.

[Brief description of the drawings]

FIG. 1 is a characteristic diagram showing the relationship between the value of x and the cycle life of the secondary battery, FIG. 2 is a characteristic diagram showing the relationship between the value of x and the capacity ratio of the secondary battery, and FIG. FIG. 4 is a characteristic diagram showing the relationship between the value and the capacity ratio of the test cell, and FIG. 4 is a characteristic diagram showing the relationship between the value of y and the internal pressure of the test cell.

Claims (1)

(57) [Claims] 1. A _{LnNi a Mn b Al c Co d} ( provided that a mixture of rare earth elements Ln may include lanthanum alone or lanthanum, a> 3.6, d
.Ltoreq.1, 4.8.ltoreq.a + b + c + d.ltoreq.5.2), a cathode mainly composed of a hydrogen storage alloy, an anode mainly composed of a nickel compound, and a separator interposed between the cathode and the anode. Power generation element group in a metal can,
In an AA-size nickel-metal hydride secondary battery having a structure in which an alkaline electrolyte is injected and the opening of the can is sealed with a lid, the amount of hydrogen storage alloy contained per unit area of the cathode is M
(G / cm ² ), and the capacity per unit area of the anode is C (mAh / c
m ² ), M is set so that the value of x expressed by the relationship C = x · M is not less than 100 and not more than 300, and the injection amount of the electrolytic solution is L (cc). Wherein L is set so that the value of y represented by the following relationship: L = y · C is not less than 0.035 and not more than 0.1.