APPARATUS FOR CIRCULATING AN ELECTROLYTE IN AN ELECTROCHEMICAL CELL AND A BATTERY OF ELECTROCHEMICAL CELLS

Abstract: The invention provides an apparatus for circulating an electrolyte in an electrochemical cell whereby the cell is provided with a discharge (7) from the bottom to above the electrolyte level in the cell, causing that when a gas pressure is applied in the tank the electrolyte present therein flows into the space above the acid and that is further applied from a small opening at the bottom of the tank or of the discharge, which apparatus is characterized in that the tank (2) is provided with a float (3), coming on to the seating (4) when the tank is empty or substantially empty and thus limits the volume of the displaced liquid. Such an apparatus enables stirring the electrolyte by means of gas-supply (8) without the danger of bubbles. The apparatus according to the invention can excellently be used in electrochemical cells that are combined to batteries which enables a central gas-supply. This enables a simple and expedient circulation in the electrochemical cell.

Description: APPARATUS FOR CIRCULATING AN ELECTROLYTE IN AN ELECTROCHEMICAL CELL AND A BAT. -TERY OF ELECTROCHEMICAL CELLS The invention relates to an apparatus for the circulation of electrolyte in an electrochemical cell, whereby the cell is provided with a tank provided with a gas-supply, a discharge from the bottom to above the electrolyte level in the cell whereby the application of gas-pressure in the tank causes the electrolyte present therein to flow in the space above the acid and that is further provided with a small opening at the bottom of the tank or of the discharge, to an electrochemical cell provided with such an apparatus, to a battery of electrochemical cells and to a process for performing of a electrolysis in a such a cell or a battery. It is known from the literature (a.o. J. of Power Sources, Vol.22 (1988), p.115) that during the use of secondary batteries with free electrolyte, stratification occurs. Higher concentrated electrolyte amass because of its higher density, at the bottom of the battery-cells whereby the concentration of the electrolyte above in the cells is lower. This phenomenon occurs the stronger with the larger vertical sizes of a cell. It is necessary with such cells that can be sometime more 100 cm high, to circulate the electrolyte. Several disadvantages of this stratification are known from the lead-sulphur acid cell. Mentioned are : sulphating, increased corrosion on places with a increased electrolyte concentration (reducing the cyclic lifetime) and decrease of the discharge capacity by shortness of electrolyte on places with an decreased electrolyte concentration. Several methods of acid circulation are known, such as mechanical stirrers, hydraulic and pneumatic systems. Mechanical stirrers can on many occasions not be used in view of the limited space and are practical of no importance. Indeed some hydraulic circulation-systems are efficient, however require an additional installation-volume, have a considerable energy consumption and cause corrosion problems in the pumps used in the system. Pneumatic circulation-systems are less susceptible to these disadvantages. In this systems air bubbles are introduced under in the cell that send in a rising-tube and drag concentrated electrolyte to the space above the acid (air-lift pomp), causing a downward electrolyte stream within the cells. The air bubbles escape at the upside of the cell. However, this system has some disadvantages caused by the air cell. However, this system has some disadvantages caused by the air bubbles in the electrolyte. Said disadvantages relate to the contamination of the electrolyte by substances in the compressed air, the formation of droplets of electrolyte by the escape of air bubbles at the electrolyte-surface what requires additional provisions (droplet- scavengers), and the generation of air in a frequency range that is not desired for certain uses. US patent 4,221,847 describes a process and a apparatus that is not subject to the disadvantages of the above mentioned hydraulic and pneumatic circulation-systems and is based on the use of compressed air to circulate the electrolyte without that air is introduced into the electrolyte. In this process an electrolyte tank is used that is provided in the battery-cell and that empties by applying a pulse of compressed air into the above acid space. Between the pulses of air the air-pressure is decompressed and the tank fills again with electrolyte originating from the bottom of the cell where the electrolyte with a high acid concentration is. to prevent air bubbles in the electrolyte and the disadvantages connected therewith it is important that the expanded volume of the air pulse does not exceed the volume of the tank under the pressure occurring. In use for more battery cells the volume of the compressed air is adjusted accordingly where the cells are pneumatically combined. When the process according to patent US 4,221,847 is used for battery-systems of a large surface (a great number of batteries in series- and/or parallel) then it is impossible when one air-compressor and one air-accumulator is used, to provide each cell in a short time with a sufficient volume of air. This is caused by the fact that during the pulse of air pressure drops in the supplies occur causing an insufficient air supply to the cells at a larger distance of the accumulator and that for this reason in said cells the circulation of the electrolyte is lower and thus the mixing is poor. It is important from the point of view of the voluminous required, however especially from the point of view from the energy consumption of the air compressor, to make the supplies of such a large diameter that the pressure-drops are small. This would require a great volume of air to be pressured where only a small part of it would be used for the electrolyte circulation. Now an apparatus has been found of the type as described in the preamble characterising that the tank is provided with a float that lands on a valve-seat with a empty or substantially empty tank and thus limits the volume of the displaced liquid. It is obvious that in said apparatus air is preferably used as the gas. With the apparatus according to the invention it is possible to provide many cells from a central air- accumulator or compressor with a suited amount of air whereby the volume of supplies can be very small so that every cell is closed of from the air supply when the desired electrolyte level in the tank is reached. The float prevents that the air comes into the electrolyte. According to a preferred embodiment the apparatus is characterized in that the gas supply to press the electrolyte from the tank is connected with an air compressor. It is especially preferred that the gas supply is also connected with an accumulator with an adapted volume. The pressing of the electrolyte from the tank is expediently performed by means of a cylinder provided with a piston. The apparatus is then characterized in that it is provided with a cylinder and a piston to provide a gas to press the electrolyte from the tank. According to a other embodiment this cylinder is provided with an opening what makes that with a complete expansion the gas in the cylinder is contacted with the atmosphere on the outside. A further improved embodiment encompasses a one-way valve positioned in the small opening, serving as an electrolyte supply. The apparatus can be used by a periodic supply of gas to the electrolyte tank. This causes an hesitation of the electrolyte. The invention is elucidated by means of the following drawings. Drawing 1 shows a cross-section of the circulation system 1 with tank 2, float 3, seat 4, opening 5 (which can be provided with a one-way valve), and a second tank (a riser) 6 with outlet opening 7 and a central air provision 8. In a different embodiment elucidated in drawing 2 the central air supply (8 in figure 1) is not provided with a combination compressor/accumulator however by a cylinder 9 provided with piston 10 that can transport air which is distributed over the cells of a battery by supply 11. The float valves take care that every cell is supplied with a correct amount of air. The piston velocity in the compressor is determined by the flow resistance of the electrolyte in the discharge 6 (see drawing 1), and the piston velocity in the expansion apparatus depends on the velocity with which the electrolyte flows in from the lower space by supplies 5 into tanks 2 (see drawing 1 ) . This embodiment does not require pneumatic falls and not a separate air compressor. The piston can be driven by electro-motor 12, which can be provided with different rotations for both directions of rotation. It is further possible to control the piston velocities in the compressor and expansion phase pure mechanical for instance with liquid pressure. It is beneficial to contact the air in the cylinder with the outside air in complete expansion to compensate for air losses if any. According to a practical embodiment the wall of the cylinder near to the point of complete expansion provided with an opening 13 what is passed by the piston so that the valve is superfluous. The cells according to the invention are suited to compose battery cells wherein the air (or if desired an other gas is used) can be supplied from a central point. Example The new system has been constructed and tested in a lead- sulphur-acid cell of 2 v with a capacity of 35 Ah and a height of 27 cm. The pump-system has been constructed from PVC. to visualise the working one side has been provided with transparent PVC. The cross-section of supply 5 (drawing 1 ) down in the system is 1 mm, that of the discharge 6 (fig.1) 2 mm (the surface being 80% of the total discharge surface). It requires about 7 seconds to fill the system with liquid for the 35 Ah labcel via the supply opening. The displacement of the liquid with compressed air requires about 3 seconds. Thus each pump-cycle requires 10 seconds wherein about 40 cm 3 of liquid is displaced (of which 30 cm 3 (75%) via discharge 6, drawing 1). When this operation is performed without pauses about 14 dm 3 /h can be pumped. The system used in the lead-sulphur acid cell would be able by simple scaling up (increase in length to a maximum of 1,2 m) could circulate 60 dm 3 /h in a 2 V, 10 kAh cell of a height of 1,2 m. this is usual for this type of cells. When the system according to drawing 2 is used with 400 cells of 10 kAh a cylinder contents of about 100 dm 3 is required. The cell with the new acid-circulation system has been subjected to standard charge/discharge-cycles. The discharge is performed at 11,5 A. The charging starts with constant current (10 A) followed by constant voltage (2,4 V) until 0,6 A has been reached and finally a constant current (0,6 A) during 3 hours and 47 minutes. Every tenth cycle an additional charge is performed. In that case the phase of constant voltage is extended by charging to 0,3 A followed by a constant current (0,3 A) during 3 hours and 47 minutes. The compressed air is supplied to the acid circulation system by means of a valve controlled by time. The pumping-cycle was initially adjusted at 20 seconds with a supply time of 3 seconds (circulation yield: 7 dm 3 /h) . During a number of periods the air-supply has been stopped or has different pumping-cycle times been used to find out the influence on the discharge capacity. Main purpose was to find whether the system would be working correctly during many hundreds of cycles. The experiments have been performed during 570 charge/discharge cycles. During about 470 cycles the pumping system has been working. The system worked during this 470 cycles well and well such that continuously sufficient electrolyte was recirculated. This appears from among others capacity-measurements. Even after that it appeared that starting from about cycle 200 the float did not reach the upper level with each pumping cycle, the float closed the system always on the correct moment for the air supply. The possible cause that the float did not move fully upwards is a not fully correct range of weight in the float so that it did not fully vertically moved upwards and than "stuck" on a certain level. During a number of cycles the new acid-circulation system has been stopped by not supplying air. During a number of these periods a decrease of the discharge capacity of about 5-10% and has been found what can be removed by starting the acid circulation again. The value of the decrease/increase can be called high since in generally it is assumed that only by higher cells great differences in acid density will occur causing a decrease of the capacity. The number of cycles that the pump worked that can now amounts about 1,3 millions (the charge/discharge cycle including the pause-times is about 15 hours), whereof to cycle 200 a half million of pump-cycle has been performed without sticking of the float. The corresponding operating time of the system was 7500 h (more than 300 days).

Claims: C L A I M S 1. Apparatus for the circulating of the electrolyte in a electrochemical cell wherein a tank is provided in the cell which tank is provided with a gas-supply and a discharge from the bottom to above the electrolyte level in the cell, such that when gas-pressure is applied in the tank the electrolyte present therein flows into the space above the acid and that is further provided with a small opening at the bottom of the tank or discharge characterized in that the tank is provided with a float, that comes onto a valve seat when the tank is discharged and thus limits the volume of the liquid displaced. 2. Apparatus according to claim 1 , characterised in that the gas supply to press out the electrolyte from the tank is connected with an air compressor. 3. Apparatus according to claim 2, characterised in that the gas supply is connected with an accumulator. 4. Apparatus according to claim 1 , characterised in that the apparatus is provided with a cylinder with a piston to supply gas to press out the electrolyte from the tank. 5. Apparatus according to claim 4, characterised in that the cylinder is provided with an opening that contacts the gas in the cylinder by complete expansion with the outside air. 6. Apparatus according to claims 1 to 3, characterised in that the small opening is provided with a one way valve. 7. Electrochemical cell characterised in that this is provided with an apparatus according to claims 1-6. 8. Battery of electrochemical cells according to claim 7.

Inventors: SCHILLEMANS RONALDUS ADRIANUS, SCHMAL DIRK, VAN DUIN PIETER JOHANNES, SCHILLEMANS, RONALDUS, ADRIANUS, ALBERT, SCHMAL, DIRK, VAN DUIN, PIETER, JOHANNES
Applicants: TNO, NEDERLANDSE ORGANISATIE VOOR TOEGEPAST-NATUURWETEN
Application year: 1992
Main classification: H01M2
IPC codes: H01M2