Maintenance-free open industrial type alkaline electrolyte storage battery

A maintenance-free open industrial storage battery includes an electrode assembly comprising at least one positive electrode, one negative electrode, one separator disposed between the negative electrode and the positive electrode, an alkaline electrolyte covering the top end of the assembly before electrical cycling and a valve the relative operating pressure of which is less than 1 bar. The total capacity of the negative electrodes is greater than the total capacity of the positive electrodes. The separator is permeable to oxygen and the storage battery contains an oxygen recombination device such that after at least one cycle of charging and discharging the storage battery operates without loss of electrolyte at a charging current at least equal to Ic/10 where Ic is the current discharging the capacity of the storage battery in one hour.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention concerns an industrial aqueous alkaline electrolyte
 open secondary storage battery, i-e. one having a high capacity in the
 range 10 Ah to 200 Ah and "open" in the sense that it operates at a low
 pressure (less than 1 bar relative). Storage batteries of this kind are in
 particular of the nickel-cadmium (Ni--Cd) or nickel-metal hydride (Ni--MH)
 type.
 2. Description of the Prior Art
 An open industrial storage battery includes an electrode assembly
 comprising a plurality of electrode pairs consisting of a positive
 electrode, a negative electrode and a separator that is only slightly
 permeable to gases disposed between the positive and negative electrodes,
 together with an alkaline electrolyte in which the assembly is immersed
 and the level of which is above the top edge of the electrodes. On
 overcharge, aqueous electrolyte storage batteries generate oxygen at the
 positive electrode and hydrogen at the negative electrode. An open storage
 battery operates at a relative pressure (pressure difference relative to
 atmospheric pressure) less than 1 bar and the gases generated on
 overcharge escape, consuming water from the electrolyte. The storage
 battery therefore requires regular maintenance, i.e. water must be added
 periodically. The maintenance frequency depends on the operating
 conditions of the storage battery in the application concerned, in
 particular on the charged capacity.
 To avoid topping up the electrolyte level after periods of operation sealed
 industrial type storage batteries have been derived from those previously
 described (U.S. Pat. No. 5,576,116). A sealed industrial (high-capacity)
 storage battery includes an electrode assembly comprising a plurality of
 electrode pairs consisting of a positive electrode, a negative electrode
 and a gas-permeable separator disposed between the negative and positive
 electrodes, a limited quantity of alkaline electrolyte and an oxygen
 recombination device. The oxygen formed at the positive electrode
 increases the pressure inside the storage battery which depends on the
 overcharge conditions employed. Permanent conditions are established
 thereafter in which the oxygen produced at the positive electrode is
 reduced, or recombined, at the negative electrode. A sealed industrial
 storage battery has a safety valve operating at a relative pressure higher
 than 1 bar. Although a sealed storage battery can solve the technical
 problem associated with maintenance, its energy per unit mass and per unit
 volume are lower than those of an open storage battery. On the one hand
 the sealed storage battery has a precharge and an excess of negative
 capacity designed to prevent the release of hydrogen at the end of
 charging. On the other hand such storage batteries contain a small
 quantity of electrolyte and consequently the yield of the active material
 is lower than in open storage batteries.
 It has therefore appeared desirable to work towards reducing the
 maintenance of open storage batteries. U.S. Pat. No. 5,128,217 proposes
 self-limitation of the charge of an Ni--Cd storage battery based on the
 sharp increase in the voltage at the end of charging. This open industrial
 storage battery operates at a relative pressure less than 1 bar and
 contains excess electrolyte.
 The aim of the present invention is to propose an open industrial storage
 battery requiring no maintenance with energy per unit volume and per unit
 mass higher than those of a sealed industrial storage battery.
 SUMMARY OF THE INVENTION
 The present invention consists in a maintenance-free open industrial
 storage battery including an electrode assembly comprising at least one
 positive electrode, one negative electrode, one separator disposed between
 the negative electrode and the positive electrode, an alkaline electrolyte
 covering the top end of the assembly before electrical cycling and a valve
 the relative operating pressure of which is less than 1 bar, wherein the
 total capacity of the negative electrodes is greater than the total
 capacity of the positive electrodes, the separator is permeable to oxygen
 and the storage battery contains an oxygen recombination device such that
 after at least one cycle of charging and discharging the storage battery
 operates without loss of electrolyte at a charging current at least equal
 to Ic/10 where Ic is the current discharging the capacity of the storage
 battery in one hour.
 In the early cycles the storage battery of the invention operates like an
 open storage battery. The abundant quantity of electrolyte means that the
 high performance of open storage batteries can be achieved and conserved.
 As long as the electrolyte is in excess, the recombination device is
 relatively inaccessible and oxygen reduction is not encouraged. During
 charging the gas pressure rises and the valve allows the gas to escape,
 and water is therefore consumed. The water loss is estimated at
 approximately 0.3 cm.sup.3 per ampere-hour of overcharge. As the excess
 electrolyte is consumed the reduction of the oxygen occurs with a higher
 yield. After a few cycles the storage battery has achieved equilibrium
 between the release and the recombination of oxygen. The pressure inside
 the storage battery remains below the operating pressure of the valve,
 generally in the range 0.5 bar to 1 bar. The storage battery of the
 invention then operates like a sealed storage battery and requires no
 further maintenance.
 The negative electrodes have a low excess capacity relative to the capacity
 of the positive electrodes. At the end of charging the negative electrodes
 are completely charged. The total capacity of the negative electrodes is
 preferably in the range 100% to 150% of the total capacity of the positive
 electrodes.
 The gas-permeable separator allows access from the negative electrode to
 the oxygen generated at the positive electrode so that recombination
 occurs. The presence of a recombination device significantly increases the
 rate of recombination and enables equilibrium to achieved, even in deep
 cycling duties. A recombination system of this kind is described in U.S.
 Pat. No. 5,576,116.
 The distance between the negative electrodes and the positive electrodes is
 advantageously in the range 0.2 mm to 0.5 mm. The distance between the
 electrodes is made as small as possible whilst avoiding the risk of
 short-circuits. Depending on the application it can range from 0.2 mm for
 applications that are relatively undemanding, for example the aeronautical
 field, up to 0.5 mm if the cycling conditions lead to greater variations
 in the electrode dimensions, in particular in the case of use in an
 electric vehicle.
 In one variant the positive electrodes are of the sintered type and the
 negative electrodes are of the paste type on a conductive support selected
 from a two-dimensional support such as a solid or perforated strip,
 expanded metal, a grid or woven material and a three-dimensional support
 such as foam or felt.
 In another variant the positive electrodes are of the paste type on a
 three-dimensional conductive support and the negative electrodes are of
 the paste type on a two-dimensional or a three-dimensional conductive
 support.
 The maintenance-free storage battery of the invention is particularly
 suitable for use in the aeronautical or railroad field and for electric
 vehicle propulsion.
 The invention will be better understood and other advantages and features
 will appear from a reading of the following description of one embodiment
 given by way of non-limiting illustrative example and from the
 accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
 Referring to FIG. 1, an open storage battery of the invention comprises a
 plastics material parallelepiped-shape casing 1 and an electrode assembly
 2 comprising a plurality of pairs of electrodes consisting of a positive
 electrode 3 and a negative electrode 4 between which is a gas-permeable
 separator 5.
 At the end of the assembly 2 is a recombination electrode 6 associated with
 a spacer 7 that is preferably incompressible and hydrophobic. The
 recombination electrode 6 is electrically connected to the negative
 polarity. The electrode assembly usually includes a additional negative
 electrode so that it is bordered by two external negative electrodes 4',
 each being in contact with a recombination electrode 6 associated with a
 spacer 7.
 A maintenance-free nickel-cadmium storage battery A was made in accordance
 with the invention. The plasticized type negative electrode comprised a
 perforated nickel-plated steel tape onto which had been deposited a paste
 comprising a polymer binder, cadmium in metallic form (Cd) and in oxide
 form (CdO) and the usual shaping additives. The sintered type positive
 electrode was formed on a sintered porous nickel support incorporating an
 active material based on nickel hydroxide. The gas-permeable separator
 comprised two layers of polypropylene felt impregnated with electrolyte in
 the form of an aqueous solution of potassium hydroxide KOH and lithium
 hydroxide LiOH at a concentration of 7.8 N.,
 The quantity of electrolyte introduced into the storage battery
 corresponded to the volume of electrolyte absorbed by the electrode
 assembly and the recombination device (reference quantity) augmented by an
 excess quantity corresponding to complete immersion of the electrode
 assembly and the recombination device to the point where the top end of
 the electrode assembly was covered. This excess quantity represented at
 least 32% of the reference quantity. The valve opened towards the exterior
 when the pressure difference between the interior of the storage battery
 and atmospheric pressure was equal to at least 0.5 bar.
 A sealed nickel-cadmium industrial storage battery B was made for
 comparison. This storage battery differed from the storage battery A in
 that it did not contain any excess electrolyte, had an uncharged excess
 negative capacity to prevent the release of hydrogen at the end of
 charging of the storage battery, and was equipped with a safety valve
 operating at a relative pressure above 1 bar.
 An open nickel-cadmium industrial storage battery C was made for
 comparison. This storage battery differed from the storage battery A in
 that it contained a gas-impermeable separator consisting of two layers of
 polypropylene felt and a microporous membrane. The electrolyte was an
 aqueous solution of potassium hydroxide KOH and lithium hydroxide LiOH at
 a concentration of 6 N.
 The quantity of electrolyte introduced into the storage battery
 corresponded to the volume of electrolyte absorbed by the electrode
 assembly (reference quantity) augmented by an excess quantity
 corresponding to complete immersion of the electrode assembly to the point
 where the top end of the electrode assembly was covered to a depth of 20
 mm. The excess quantity represented 60% of the reference quantity.
 The storage batteries A, B and C were evaluated by the following
 comparative electromechanical tests.
 (I)--Capacity test.
 A capacity test was conducted to determine the real capacity of the storage
 battery:
 charge at Ic5 where Ic is the current to discharge the capacity of said
 storage battery in one hour, then
 overcharge at Ic/10 with an overcharge coefficient of +50%,
 discharge at Ic/2 to a cut-off voltage of 0.9 volts.
 The initial characteristics of the storage batteries A, B and C including
 the capacities actually measured are set out in table I below. The gain
 corresponds to the difference between the real and nominal capacities
 divided by the nominal capacity.
 TABLE I
 Initial characteristics:
 A B C
 Distance between electrodes (mm) 0.5 0.25 0.5
 Excess electrolyte (%) +32 0 +60
 Nominal capacity (Ah/electrode) 3.1 3.1 3.1
 Real capacity (Ah/electrode) 3.5 3.1 3.5
 Gain (%) +13 0 +13
 At the start life the capacity recoverable from storage battery B conformed
 to the theoretical capacity; that for the storage batteries A and C was
 higher than predicted and indicates the high activation of the positive
 electrode associated with the excess electrolyte.
 (II) Cycling test.
 A cycling test of 270 cycles was carried out to observe the electrochemical
 behavior of the storage battery:
 charge at Ic/5,
 overcharge at Ic/10 with an overcharge coefficient of +20%,
 rest for 1 h,
 discharge at Ic/2 to a depth of discharge corresponding to 70% of the
 nominal capacity,
 rest for 4 h.
 Table II summarizes the comparative results for operation of storage
 batteries A, B and C during the first 44 cycles. Table II gives the
 internal pressure, the mass variation and the rate of recombination
 observed after an overcharge at Ic/10. The internal pressure was limited
 to 0.5 bar by the valve in the case of storage battery A; the internal
 pressure reached by the storage battery B in this case was very much lower
 than the limit imposed by the valve.
 The recombination rate R was calculated using the equation:
 R=100.times.[1-.DELTA.m/(E.times.S)]
 where .DELTA.m is the variation in the mass of the battery, i.e. the loss
 of water, E=0.33 corresponds to the quantity of water consumed per 1
 ampere-hour overcharge and S is overcharge expressed as a number of
 ampere-hours, so that (E.times.S) represents the theoretical water loss.
 TABLE II
 A B C
 Internal pressure (bars) 0.5 0.6 0
 Loss of mass per cycle (mg) 393 0 1300
 Recombination rate (%) 75 100 15
 The theoretical consumption was 66 g of water for storage battery A of the
 invention during the first 44 cycles with a 200 Ah overcharge. The total
 loss of mass of storage battery A during this cycling was found to be 17.3
 g, much lower than the theoretical quantity. The internal pressure was set
 at 0.5 bar by the valve.
 During cycling of storage battery B the internal pressure stabilized at 0.6
 bar and the mass did not vary. storage battery C, the internal pressure of
 which was not limited by the valve, had to be topped up periodically
 during the cycling test.
 The results set out in table III below show the loss of mass and the
 recombination rate during cycling of storage battery A of the invention up
 to 270 cycles under the same conditions as previously.
 TABLE III
 Cycles Cycles Cycles
 45-147 149-249 252-270
 Loss of mass per cycle (mg) 168 7 0
 Recombination rate (%) 88 99.5 100
 Internal pressure (bar) 0.5 .apprxeq.0.5 &lt;0.5
 Three phases of operation of storage battery A of the invention could be
 distinguished during the cycling test. During the early cycles the
 internal pressure was higher than the pressure at which the valve opened,
 leading to high consumption of electrolyte on each cycle because the valve
 allowed the gases to escape. This was followed by an intermediate phase in
 which the pressure and the mass stabilized.
 Finally, after approximately 250 charge/discharge cycles, the internal
 pressure was below the pressure at which the valve opened. The
 recombination rate was close to 100% reflecting operation close to that of
 a sealed storage battery.
 Curve 20 in FIG. 2 is the charging curve for an open industrial storage
 battery requiring maintenance. When the storage battery is charged (charge
 rate.gtoreq.100%) hydrogen is generated (the rising part 21 of the curve)
 at the negative electrode in amounts corresponding to the overcharge.
 Curve 22 in FIG. 2 represents the charging of the maintenance-free open
 industrial storage battery of the invention after 250 cycles. The
 recombination of the gases 23 reduces the pressure rise. No hydrogen is
 generated.
 Curve 30 in FIG. 3 shows that the excess electrolyte initially introduced
 (+32%) decreased visibly up to cycle 149 and then tended to stabilize at a
 value in the order of +10%.
 Table IV below sets out the capacities observed after 272 cycles.
 TABLE IV
 A B C
 Nominal capacity (Ah/electrode) 3.1 3.1 3.1
 Initial real capacity (Ah/electrode) 3.5 3.1 3.5
 Capacity after 272 cycles (Ah/electrode) 3.8 3.3 3.8
 For the sealed storage battery B the capacity as measured after cycling
 conformed to the capacity expected from the design and the manufacture of
 the battery. For the open storage battery C and the maintenance-free
 storage battery A of the invention the capacity obtained was more than 20%
 (22.6%) higher than the theoretical value.
 The above electrical tests show up the following features of the storage
 battery A of the invention:
 a capacity higher than predicted for the design,
 very low electrolyte consumption after 150 cycles and virtually no
 consumption after 250 cycles,
 stable capacity during cycling,
 behavior very similar to a sealed industrial storage battery with an
 internal, pressure less than 0.5 bar after 250 cycles.
 The maintenance-free open industrial storage battery of the invention
 achieves gains in the order of 15% to 30% in terms of energy per unit mass
 and 20% to 40% in terms of energy per unit volume compared to a prior art
 sealed industrial storage battery.