WO2002021626A1

WO2002021626A1 - Prezzurized metal hydride batteries

Info

Publication number: WO2002021626A1
Application number: PCT/NO2001/000368
Authority: WO
Inventors: Lars Ole VALØEN
Original assignee: Norsk Hydro ASA
Current assignee: Norsk Hydro ASA
Priority date: 2000-09-07
Filing date: 2001-09-07
Publication date: 2002-03-14
Anticipated expiration: 2003-03-07
Also published as: AU2001290362A1; NO20004463D0; NO20004463L

Abstract

The invention concerns a metal hydride batteries where the battery cell is pressurized. One or more battery cells could be arranged in a pressurized container. The invention also concerns a method of improving the properties of metal hydride batteries by applying an external pressure in the battery cell. An external gas pressure could be applied directly to the battery cell or to a container with one or more battery cells. It is used an inert gas or a gas that is chemically inert to the materials used in the battery. Preferred gases are nitrogen, argon, helium or neon. It is preferred to apply pressures above 1 bar. Commonly used metal hydride alloys are applicable in the metal hydride battery according to the invention. In addition, the invention makes it possible to utilise hydrogen absorption alloys with high equilibrium pressures. The inert gas pressure prevents hydrogen evolution during charging, and as such improving the charge efficiency.

Description

Pressurized metal hydride batteries

The invention relates to improved metal hydride batteries and a method for improving the properties of a metal hydride battery by applying an external gas pressure.

Batteries are crucial for a lot of different applications. Recently, cars and buses powered in part or in full by electrical energy have gained increased popularity around the world. To maximize the mileage of such vehicles, the braking energy should preferably be regenerated and used to charge the batteries. To ensure that as much as possible of the braking energy is recovered and also to ensure satisfactory acceleration, a battery with a high power density, i.e. capable of being charged at a high rate is crucial.

Rechargeable batteries all have some common components. A negative electrode where a reactant is oxidized during discharging and a species is reduced during charging. A positive electrode where a reactant is reduced during discharging and a species is oxidized during charging. An electrolyte where the electrolyte is a charge transport medium with preferably a pure ionic conductivity. It is desirable to have an electrolyte with an as high as possible ionic conductivity. A separator which prevents a direct electrical contact.

US 4,636,445 is directed to a metal hydride battery which has an increased internal pressure to improve the capacity of the battery. The pressure is regulated by raising and lowering the temperature. It is not used inert gas to pressurize the battery.

US 5,264,301 describes battery cells placed in a vessel where the hydrogen pressure in the vessel is maintained within a given range by sequential venting of hydrogen during cycling of the battery. This is a NiH battery and does not contain metal hydrides. Inert gas is not used.

US 4,216,274, US 4,605,603 and US 3,874,928 describe a metal hydride battery system. Unlike several other battery systems, the battery system described in said patents does not contain substances that are enriched through the food chain. A problem with this type of batteries has been, however, a notably lower power density than for instance lead acid batteries.

The metal hydride alloy in this type of batteries often consist of, but are not limited to alloys with an AB5 composition. AB5 type alloys, with the A-component being misch metal (a commercial mixture of rare earth elements: lanthanum, cerium, neodymium and praseodymium) and the B-component being nickel (with a variety of other metal additions, such as aluminium, manganese, cobalt etc.) are widely used as negative electrodes in rechargeable nickel metal hydride (NiMH) batteries because of their beneficial kinetic properties, reasonable cost, and long cycle life. Cobolt is often added to enhance the life time of the batteries. The expansion by hydrogen absorption is reduced, the alloys containing cobolt are more ductile and do not crack during cycling. Cobolt is also present in the oxide layer on the surface of the metal hydrode particles. However, cobolt is expensive and makes about 50 % of the cost of the alloy. In addition, cobolt causes reduced power density and poorer kinetics.

Also other alloys could be used, as alloys with an AB composition, as TiFe, TiFeH or A2B alloys as Mg2Ni, MgaNE *., Ti^i or Ti₂NiH. Alloys of type AB₂ could be ZrMn₂, TiMn₂, ZrCr₂ or ZrV₂.

A bottleneck which has prevented a higher power density in the metal hydride batteries has up till now been the negative metal hydride electrode. Another problem has been a higher cost of this type of batteries compared to for instance NiCd batteries mainly as a result of additions of cobalt to the metal hydride electrode.

During charging, hydrogen may be evolved as a side reaction. This may increase the internal cell pressure or cause leaks of the electrolyte in batteries not intended to handle high internal pressure. This is often seen as a problem in batteries. The object of the invention is to provide improved metal hydride batteries. Another object is to obtain a method for improving the properties of a metal hydride battery. This being properties as charge efficiency, power density, initial activation etc. A third object is to be able to produce batteries at reduced material costs.

These and other objects of the invention are obtained by the apparatus and method as described below. The invention is further defined and characterised by the enclosed patent claims.

The invention thus concerns a metal hydride battery where the battery cell is pressurized. One ore more battery cells could be arranged in a pressurized container. The invention also concerns a method of improving the properties of metal hydride batteries by elevating the internal pressure in the battery cell by applying an external gas pressure directly to the battery cell or to a container with one or more semi-open battery cells. It is used an inert gas or a gas that is chemically inert to the materials used in the battery. Preferred gases are nitrogen, argon, helium or neon. It is preferred to apply pressures above 1 bar. Pressures above 10 bar are even more preferred.Commonly used metal hydride alloys are applicable in the metal hydride battery accoring to the invention. In addition, the invention makes it possible to utilise hydrogen absorption alloys with high equilibrium pressures. The inert gas pressure prevents hydrogen evolution during charging, and as such improving the charge efficiency.

The invention also concerns the use of pressurized metal hydride battery cells or one or more pressurized containers with one or more hydride battery cells for hybrid electric drive trains, where the batteries are charged during braking or via a generator or from another source of electricity such as fuel cell or solar panel, and discharged to give power to the electric motor.

The invention will be further described with reference to the accompanying drawings, figures 1-5, where Fig. 1 shows a commercial NiMH battery.

Fig. 2 shows a schematic view of a pressurized battery cell.

Fig. 3 shows a schematic view of a pressurized battery container.

Fig. 4 is a graph showing discharge capacity versus cycle index for different applied pressures.

Fig. 5 is a graph showing relative capacity versus discharge current density for different alloy systems at different external pressures after 30 charge/discharge cycles. Various pressurized systems and a commercially available alloy is shown for reference.

Fig. 1 shows a commercial NiMH battery, spiral wound cylindrical type. The negative electrode 1 is the metal hydride electrode and the positive electrode 2 is made of NiOOH/NiOH₂. The battery further comprises a separator 3, an insulator 4, a case (-) 5, a positive electrode collector 6, a cap (+) 7, a safety vent 8, a sealing plate 9 and an insulation ring 10.

Fig. 2 is a schematic view of a pressurized battery cell 11. The pressure Pi inside the cell 1 is higher than atmospheric pressure.

Fig. 3 is a schematic view of a pressurized battery container 12. The individual cells 13 are semi-open, having the same pressure as the outer reinforced pressurized container 12. The container could be made of for example steel or carbon fibre.

The advantages of the invention will be illustrated by the following examples. Example 1

Electrochemical cycling is a very important way of characterizing the performance of rechargeable batteries. During electrochemical cycling, the battery is repeatedly charged and discharged and various parameters are monitored. One of the most important parameters is the discharge capacity. An experiment was carried out where a metal hydride electrode was pressurized with different pressures during charge/discharge cycling. The applied pressures were 10 Bar, 15 Bar and 25 Bar. The gases used were argon or nitrogen. The results are shown in a graph in Fig. 4.

Fig. 4 shows discharge capacity versus cycle index for different applied pressures. It is illustrated how the initial activation properties are improved when a metal hydride electrode is pressurized during charge/discharge cycling. From the figure it is also seen that each charge/discharge cycle has a higher discharge capacity as the external pressure is increased. This will in practice enhance the practical life of a battery. The reason for this is assumed to be that the hydrogen evolution parasitic reaction during charge of the electrode is suppressed, resulting in a better charge efficiency.

Example 2 In another experiment, different external pressures were applied using argon or nitrogen. After 30 charge/discharge cycles the relative capacity discharged versus current density for different alloy systems were measured. The alloys and pressures were: LaNis with applied pressures 3,5 Bar, 7,5 Bar, 15 Bar and 25 Bar, MrnNis

at pressure 25 Bar and a commercial alloy composition for reference (MmNi₃.₇Mno.₃Alo. Co₀. ) and applied pressure 1 Bar. The results are shown in Fig. 5.

Fig. 5 is a graph showing relative capacity discharged versus discharge current density for different alloy systems at different external pressures after 30 charge/discharge cycles. It is seen from the figure that the high rate dischargeability increases considera- bly as the external pressure is increased and the differences in the high rate discharge- ability for the different alloys at different applied external pressures are notable. From the figure it is seen that the power density of the pressurized metal hydrode electrodes is between two and three times the power density of today's excisting system shown for reference.

An elevated battery pressure will also enable the use of metal hydride alloys with a high equivalent hydrogen pressure. These alloys could not be used in normal batteries. We have demonstrated that such alloys also could be used, see MmNis in Figure 5. These alloys will be less expensive since the cobalt content can be reduced substantially as it seems as the Mischmetal has much of the same effect as Co. This has as such the potential of lowering the total cost of the battery. The use of alloys with a plateau pressure above 1 bar has been difficult in normal batteries until now. We are now able to use pressures above 10, 20 , 30 bar .

In addition, alloys with a high equivalent hydrogen pressure has a corresponding high open circuit voltage. A high open circuit voltage is desirable for most applications. In a car propulsion battery, a higher open circuit voltage will give better acceleration. The case is similar for electrical power tools.

By making the batteries handle high pressure and applying an inert gas pressure, several beneficial properties can be obtained. The benefit of using an inert gas is that in contrary to hydrogen or oxygen evolved from the electrodes, it will not react with neither of electrode material nor the electrolyte. Applying an inert gas pressure will also ensure a stable, high pressure in the battery cell by preventing gas evolution from the electrodes. In case of still some hydrogen or oxygen evolution, the normal overcharge protection mechanisms will still be in effect.

The present invention provides a method and a device for providing a pressurized metal hydride battery cell by using an inert gas such as argon or nitrogen. The metal hydride cells can be of a known type, for instance as described in US 5,856,047, US 5,747,186 and US 4,605,603. The invention can be applied to all types of nickel-metal hydride batteries. The invention described here opens for a substantial improvement of the power density of metal hydride batteries. This increased pressure also eliminates at least the need for the addition of some of the cobalt, and by that reducing at least some of the cost related to the electrode materials in such batteries.

It was also found that the self discharge of the battery was eliminated by applying an external pressure to the battery. Self discharge is a big problem for NiMH batteries that is used for example in hybrid cars, electrical cars, offshore pipeline inspection vehicles, emergency backup systems etc.

The method and apparatus according to the invention thus provide faster initial action, has better practical cycle life, enable better high current discharge capacity, higher open circuit battery voltage, and give materials cost reductions for metal hydride batteries.

Claims

Patent claims

1. Metal hydride battery , characterised in that the battery cell is pressurized.

2. Metal hydride hydride battery according to claim 1, characterised in that one or more semi-open battery cells are arranged in a pressurized container.

3. Metal hydride battery according to claim 1 or 2, characterised in that there is applied a pressure of an inert gas or a gas that is chemically inert to the materials used in the battery.

4. Metal hydride battery according to claim 3, characterised in that the gas is nitrogen, argon, helium or neon.

5. Metal hydride battery according to any of the preceding claims, characterised in that the pressure is above 1 bar.

6. Metal hydride battery according to claim 5, characterised in that the pressure is above 10 bar.

7. Metal hydride battery according to claims 1 or 2, characterised in that hydrogen absorption alloys with high equilli- brium hydrogen pressure is used as the negative metal hydride electrode in the battery.

8. Container, wherein one or more semi-open metal hydride battery cells are arranged therein and where there is applied a pressure of an inert gas or a gas that is chemically inert to the materials used in the battery.

9. Method of improving the properties of rechargable metal hydride batteries, characterised in that an external gas pressure is applied.

10. Method according to claim 9, characterised in that the battery cell is pressurized or a container with one or more semi-open battery cells is pressurized.

11. Method according to claim 9 or 10, characterised in that there is used an inert gas or a gas that is chemically inert to the materials in the battery.

12. Method according to claim 11, characterised in that nitrogen, argon, helium or neon is used.

13. Method according to any of the claims 9 to 12, characterised in that the gas pressure is above 10 bar.

14. Use of pressurized metal hydride battery cells or one or more pressurized containers with one or more hydride battery cells for hybrid electric drive trains, where the batteries are charged during braking or via a generator or from another source of electricity such as a fuel cell or a solar panel, and discharged to give power to the electric motor.