GB2203280A - Recombination electric storage cells - Google Patents

Abstract

A recombination electric storage cell comprises a container which contains alternating positive and negative plates interleaved with separators and is full of electrolyte. The separators comprise sheets of polypropylene fibres whose pores are normally substantially filled with electrolyte. When gas is evolved at the plates on overcharge of the cell the electrolyte is expelled from a proportion of the pores whereby oxygen produced at the positive plates diffuses through the separators and is recombined at the negative plates.

Description

RECOMBINATION ELECTRIC STORAGE CELLS The present invention relates to recombination electric storage cells, in particular of lead acid type, and is particularly but not exclusively concerned with such cells for deep cycling duties such as for motive power.

Conventional lead acid cells are flooded with electrolyte. The electrolyte constitutes a hazard when the cells are transported or tilted and is progressively lost due to gassing caused by electrolysis of the water when the cell is overcharged. It has been proposed in, for instance, British Patent Specification No. 2062943 that lead acid batteries may be constructed to be of recombination type, that is to say that the gas which is evolved on overcharge is substantially only oxygen and is induced to recombine within the battery rather than being vented to the atmosphere, thereby curing the problem of the progressive loss of electrolyte. In such batteries, oxygen evolved at the positive plates diffuses through the separator material and the film of electrolyte on the negative plates and is recombined at the negative plates.The rate of oxygen diffusion through a liquid is extremely low and thus if a sufficient volume of oxygen is to be able to diffuse through the separator material it is essential that the latter has an appreciable void volume.

For this reason all commercially available recombination batteries have contained a reduced amount of electrolyte whereby substantially all the electrolyte is absorbed in the plates and separator material with the latter being unsaturated and therefore exhibiting a substantial empty volume.

The separator material in a reduced electrolyte recombination battery has to perform not only all the functions of the separator material in a conventional flooded electrolyte battery but also the further function of providing the active material on the plates with sufficient electrolyte for its electrochemical requirements. The separator material must thus not only have a high porosity so as to exhibit the necessary void volume but also a high electrolyte retentivity and capillarity. The only material which has been found to operate satisfactorily is a microfine glass fibre material, typically with a fibre diameter of 0.2 to and a surface area of 0.1 to 20 square metres per gram.

Whilst this glass fibre material functions effectively in lead acid recombination automotive batteries it is found not to be so effective in motive power cells. The reason for this is firstly that the plates in such cells tend to be thicker than those in automotive batteries so that they are able to withstand the mechanical forces to which they are subjected and secondly that such cells tend to be deep cycled, that is to say they are regularly deeply discharged and then fully recharged. Both these factors lead to the electrolyte requirement per unit area of theplates being substantially greater than in automotive batteries.Whilst that additional electrolyte could be readily provided by increasing the thickness of the separator material this has the disadvantages of increasing the separator cost for a battery and also increasing the diffusion path length between the plates which would reduce the rate of recombination.

Accordingly it is an object of the present invention to provide a recombination electric storage cell, in particular of motive power type, in which gas evolved on overcharge is induced substantially to recombine within the cell but in which the plates are provided with sufficient electrolyte for their electrochemical requirements whereby the performance of the cell is not electrolyte limited.

According to the present invention there is provided a recombination electric storage cell, e.g. of motive power type and preferably of lead acid type, which comprises a container containing alternating positive and negative plates interleaved with separators and containing free electrolyte, the separators being so constructed and/or treated that their void volume is normally substantially filled with electrolyte but on the evolution of gas at least a proportion of the electrolyte is expelled from at least a proportion of the voids whereby oxygen produced at the positive plates can diffuse through the separators to the negative plates.

The invention is based on the recognition that whilst effective recombination operation requires the presence of an unsaturated void volume in the separator through which oxygen may diffuse, this void volume need not be present all the time but need only be present when it is required, namely whilst gas is being evolved.

It is now appreciated that the evolution of gas at the positive plates is inherently associated with a local increase in pressure and that this pressure can itself displace electrolyte from the pores or voids in the separators to create a diffusion pah for oxygen. The cell of the present invention has free electrolyte and is preferably flooded with electrolyte, i.e. the active material on the plates is totally submerged in the electrolyte, which inherently results in the plates having sufficient electrolyte for their electrochemical requirements but the construction and/or treatment of the separators ensures that they exhibit a sufficient empty void volume whilst oxygen is being evolved for the oxygen to diffuse through them.A major advantage of the necessary empty void volume being created by the evolution of the gas rather than being permanently present is that the area of the diffusion path automatically varies as a function of the rate of gas evolution whereby the proportion of the area of the plates which is contacted by free electrolyte is always at the maximum value consistent with the rate at which oxygen is being evolved.

Preferably the capacity of the negative plate is greater, e.g. in excess of 10,20 or 30% greater than the positive plates to ensure that substantially only oxygen is evolved on overcharge. The cell may be provided with a safety vent, e.g. of Bunsen type, which opens at a pressure of e.g. 0.07 bar or in excess of 0.3 or even 0.6 bar to vent gas to atmosphere if the rate of gas evolution should exceed the rate of recombination thereby producing a pressure rise within the cell. However, it is preferred that a permanently open vent is provided of the type normally provided on batteries flooded with electrolyte whereby the interior of the cell permanently communicates -with the atmosphere.This produces a considerable financial economy as regards the cost of the vent and the cost of the container because the latter may be thinner and/or the ribs conventional in a sealed battery container may be eliminated since the container does not have to withstand any internal pressure. A non-return valve is not necessary as in reduced electrolyte recombination cells because the fact that the cell is flooded with electrolyte means that atmospheric oxygen is unable to gain access to these plates and thus react with them. Since the necessary unsaturated void volume is created by the expulsion of electrolyte from the voids the material and void size of the separators must be such that the pressure which is generated is adequate to expel the electrolyte.For this purpose the contact angle of the electrolyte with the material of the separators is ideally 900 and preferably in excess of 60 or 800. Fibrous material of a polyolefin, in particular polypropylene or polyethylene, is found to be particularly suitable.

Microfine borosilicate glass fibre material as is commonly used in reduced electrolyte recombination cells is not inherently suitable because its contact angle with electrolyte is substantially 0o but if treated with a suitable hydrophobic material it may be suitable.

In order that electrolyte is displaced from the pores of the separators at as low a pressure differential as possible it is desirable that the pores be as large as possible. However, the separators also have the function of preventing "leading through", i.e.

the growth of lead dendrites between adjacent plates and for this function the pores should be as small as possible. It is therefore necessary to reach a compromise with the pore size or to use a composite separator of two outer layers of large pored material and an inner layer of small pored material, such as microfine glass fibre material which provides good protection against leading through and exhibits hydrophobicity in some areas giving a permanently unsaturated pore volume over at least a proportion of its area through which gas can readily diffuse.

It is preferred that the plates and separators are in intimate contact and are more preferably under a mutual compressive force. This is believed to be of importance, because the intimate contact minimises the thickness of the layer of electrolyte on the positive plates through which gas might otherwise rise upwardly and escape from the cell rather than recombining at the negative plates. It is also desirable that the positive plates are under compression so as to inhibit the shedding of active material to which these plates are inherently subject.

The grids of the plates may be substantially pure lead or may include a small proportion of other metals.

In particular the grids, especially those of the positive plates, may contain antimony of an amount up to 3,6,9 or 12% by weight.

One separator material which has been found to be particularly effective is a polypropylene fibre material whose thickness is between 1 and 4mm, preferably between 2 and 3mm (measured at a contact pressure of 10 KPa). The material preferably has a pore size of 60 to 120 jim, e.g. 80 to 90 jim, and a diameter of 0.5 to 40 pm, e.g. 1.5 to 20 pm. Such a material is sold by the 3M Company and is known as LSM (liquid sorption medium). This particular material has a basis weight of 240gum'2, a tensile strength of 0.722 Nmm'l and a pressure of 14 to 16 cm H20 (measured with an H cell) was needed to expel sulphuric acid with a specific gravity of 1.3 from its pores.

In a comparative test four cells were made up comprising three positive plates and four negative plates interleaved with microfine glass fibre material of the type which is commonly used in reduced electrolyte recombination batteries. The cells were flooded with sulphuric acid electrolyte and then allowed to drain and their recombination efficiency at varying overcharge currents was measured. Four similar cells were made up using fibrous polypropylene separator material of LSM (as described above). The cells were filled with electrolyte and their recombination efficiency measured. The recombination efficiency of the conventional cells was substantially 100% at low overcharge rates but dropped to less than 40% at an overcharge of about 0.3 amps and thereafter steadily decreased. The recombination efficiency of the cells in accordance with the invention was only about 90% at low overcharge rates but at overcharge currents in excess of about 0.3 amps was of the order of 30 to 40% greater than that of the conventional cells.

The comparative test indicated that the addition of antimony to the grids of the cells of the present invention had a positively beneficial effect on the recombination efficiency. The addition of 2.5 by weight antimony improved the recombination efficiency by about 5% whilst the addition of 4% antimony improved the recombination efficiency by about a further 10%.

Claims (4)

1. A recombination electric storage cell comprising a container containing alternating positive and negative plates interleaved with separators and containing free electrolyte, the separators being so constructed and/or treated that their void volume is normally substantially filled with electrolyte but on the evolution of gas at least a proportion of the electrolyte is expelled from at least a proportion of the voids whereby oxygen produced at the positive plates can diffuse through the separators to the negative plates.

2. A cell as claimed in claim 1 in which the separators have a thickness of between 2 and 3 mm.

3. A cell as claimed in claim 1 or claim 2 in which the separators are of fibrous material with a fibre diameter of 1.5 to 20 pm.

4. A cell as claimed in any one of the preceding claims in which the separators are of polypropylene.