WO2016178186A1

WO2016178186A1 - Zinc-air cell with airlift pump

Info

Publication number: WO2016178186A1
Application number: PCT/IB2016/052594
Authority: WO
Inventors: Suren Martirosyan; Didier Guillonnet
Original assignee: Individual
Current assignee: Individual
Priority date: 2015-05-06
Filing date: 2016-05-06
Publication date: 2016-11-10
Anticipated expiration: 2017-11-06
Also published as: EP3292577A1; WO2016178185A1; CN107836052A; WO2016178187A1; WO2016178184A1

Abstract

The invention relates to a zinc-air secondary cell, for use in an alkaline electrolyte secondary battery, comprising: - a zinc-electrode; - an air electrode; - a charging electrode used for charging said zinc-electrode, said charging electrode facing at-least a face to said zinc-electrode, and said cell comprising further a Riser Tube, said Riser Tube being at least partially filled with electrolyte at start of charging, said Riser Tube defining an Airlift pump being only activated by the oxygen evolving from said charging electrode during the charge of said zinc-electrode.

Description

Zinc-Air cell with Airlift Pump

The present invention is concerned with electrically rechargeable batteries, metal-air batteries, zinc-electrodes for such batteries and especially electrically rechargeable Zinc-Air batteries.

Electrically rechargeable Zinc-Air batteries are famous for their energy density comparable to Li-ion batteries (at least 3 to 6 times more than Lead-Acid batteries) and their low cost per kWh (comparable or cheaper than Lead-Acid batteries and 5 to 10 times cheaper than Li-ion batteries).

These batteries would be very useful for many applications including Electric Vehicles and Stationary Electricity Storage if they could offer a sufficient service life. However so far nobody could offer this type of batteries with characteristics suitable for an application.

For example, considering the low cost of these batteries, we estimate that the minimum requirement for Electric Scooters would be something like: at least 70 Wh/kg Energy Density ; 15 W/kg Power Density and 6 months service life with 200 cycles.

THE SPECIFIC PROBLEM

It is well-known that if the electrolyte is not circulated in the Zinc-electrode or the charging electrode compartments, then the concentration of ions are not evenly distributed throughout the Zn electrode height - especially Zincate and OH^– ions - and many problems can happen: dendrites can form much faster, zinc can precipitate in the charging electrode compartment, carbonation can damage the air electrode, shape change damages the cell, etc, as well zincate ion aging could happen. In this case it is difficult to get more than 10 cycles.

The invention intends to obviate the prior art problems.

It is well known that electrolyte circulation is improving the cycling capacity of zinc-air cells. However, in a classical way, the electrolyte circulation is performed by the way of external pumps.

The inventor succeeded in associating the well-known technology of Airlift Pumps (see https://en.wikipedia.org/wiki/Airlift_pump) to zinc-air cells so that the said Zinc-Air cells can enjoy electrolyte circulation during charging without requiring external pumping equipment.

In Airlift Pumps, air is injected in the lower part of a Riser Tube that transports a liquid. By buoyancy the air, which has a lower density than the liquid, rises quickly. By fluid pressure, the liquid is taken in the ascendant air flow and moves in the same direction as the air. The calculation of the volume flow of the liquid is possible thanks to the physics of two-phase flow. This type of pump is very reliable.

A variant known by the man skilled in the art, called the "geyser pump", pumps with greater suction and less air.

The inventors observed that both Airlift Pumps and its variant Geyser Pumps can be used in zinc-air cells during charging to pump the alkaline electrolyte with the evolving oxygen. Compared to the operation of a classical Airlift pumps, in the present invention the air is substituted by oxygen evolving from the electrode under anodic polarization, called throughout this patent application the Charging Electrode, via the electrolytic oxidation of OH^-ions during charging, and the liquid transported by the Airlift Pump is the alkaline electrolyte of the cell.

CONSEQUENCE

Thus, in the present invention, the electrolyte is circulated by Airlift pumping assured by the oxygen evolution on the charging electrode whereby leading to a very simple, reliable and cheap solution for preventing shape change and growth of dendrites.

Thus the invention relates to a zinc-air secondary cell, for use in an alkaline electrolyte secondary battery, comprising

- a zinc-electrode ;

- an air electrode;

- a charging electrode used for charging the zinc-electrode, said charging electrode facing at-least a face to the zinc-electrode, and

said cell comprising a layer of alkaline liquid electrolyte in contact with at least one side the charging electrode,

said cell comprising further a Riser Tube, said Riser Tube being at least partially filled with electrolyte at start of charging,

said Riser Tube comprising at least one Air Inlet, eventually at its bottom, receiving the oxygen evolving from the charging, eventually mixed with electrolyte, and a Mixture Outlet placed higher than the Air Inlet, out of which a mixture of oxygen and electrolyte is flowing out during charging,

said Riser Tube defining an Airlift pump being only activated by the oxygen evolving from the charging electrode during the charge of said zinc-electrode,

said cell advantageously comprising a Gas Collector collecting and transmitting to said Air Inlet, eventually with the help of an Air Supply Line, the said oxygen evolving eventually mixed with electrolyte.

This Airlift system is assured in the charging electrode compartment by using the bubbling of oxygen evolution on the charging electrode (Auxiliary-electrode or Bifunctional Air-Electrode). The oxygen bubbles are directed through a Riser Tube having a limited cross section where the oxygen bubbles are mixed with the electrolyte liquid; and these oxygen bubbles are by Archimedes’s law pushing the electrolyte from the bottom of the Riser Tube to its output which should be at a higher level. As said above such pump principles are well-known as “Airlift pump” and its “Geyser pump” variant, and are described for example in (http://www.uwex.edu/uwmril/pdf/RuralEnergyIssues/aquaculture/90_Air_Lift_Theory.pdf). Some other variants can also be found for example in Jacob Riglin, "Performance Characteristics of Airlift Pumps with Vortex Induced by Tangential Fluid Injection" (2011). Honor’s Theses. However, from what the inventors know, the present invention is the first attempt and description of how to apply Airlift pump principles to electrochemical cells, especially in order to circulate electrolyte in compartments of a battery where there is no bubbling, such as the Zinc-electrode compartment (comprising the air electrode, a separator insuring electrical insulation from the zinc-electrode and the charging electrode, and the electrolyte in between). This solution is very simple to implement and cost effective because it does not require any external pump. From what the inventors know, it is also the first time that a zinc air battery is described having an individual pump assigned to each one of its cells.

Advantageously, the invention relates to the cell defined above, wherein the Mixture Output of the said Airlift pump is minimum 20 mm higher than the said Air Inlet whereby providing sufficient electrolyte transportation power or transportation efficiency.

Advantageously, the invention relates to the cell defined above, wherein the Riser Tube has a cross section less or equal to 15 mm² in surface, and advantageously a cross section equal to or higher than 2 mm².

It is to be noted that the cross section of the Riser Tube should not be too large otherwise no Airlift effect would take place. Preferably, the inside diameter of the said Riser Tube is from 1 mm and 4 mm, preferably a diameter of 2 mm, or advantageously the Riser Tube having a cross section between 1 to 14 mm².

Advantageously, the invention relates to the cell defined above, wherein the Riser Tube is essentially vertical during charging.

Advantageously, the invention relates to the cell defined above, wherein the Riser Tube is positioned higher than the top of the zinc-electrode.

Advantageously, the invention relates to the cell defined above, wherein the cell also includes an expansion reservoir, receiving during charging the output flow of the Airlift pump, the mixture of electrolyte and oxygen evolving during charging; said expansion reservoir comprising at least a gas exhaust (for oxygen exhaust).

Advantageously, the invention relates to the cell defined above, wherein the said expansion reservoir is communicating to the bottom of the zinc-air cell whereby allowing the electrolyte to return and circulate during charging.

Advantageously, the invention relates to the cell defined above, wherein the expansion reservoir is connected to a return tubing connected to an electrolyte return port at the bottom of the cell.

Advantageously, the invention relates to the cell defined above, wherein the level of the electrolyte in the expansion reservoir is at start of charging at least a minimum of 20 mm higher than the said Air-Input whereby the pressure head is sufficient to start and sustain the electrolyte circulation.

Advantageously, the invention relates to the cell defined above, wherein the Airlift pump, expansion reservoir and return tubing are all sharing a same casing whereby simplifying the design and number of interconnections.

Throughout the present invention, by “Zinc-electrode compartment” it is meant the space comprising the zinc-electrode and the electrolyte between the said zinc-electrode and a separator insuring electrical insulation from the said zinc-electrode and the charging electrode.

Similarly, throughout the present invention, by “charging-electrode compartment” it is meant the space comprising the charging-electrode and the electrolyte between the said charging -electrode and a separator insuring electrical insulation from the said zinc-electrode and the said charging electrode.

At the start of charging, many bubbles of oxygen are rapidly evolving on the surface of the charging electrode and get mixed with the electrolyte of the charging-electrode-compartment. The oxygen bubbles are taking the place of some volume of electrolyte which is pushed outside of the compartment. Thus, conveniently, the output of the Airlift pump of the cell is connected to an expansion reservoir and a gas exhaust allowing oxygen to escape the cell, the said expansion reservoir being large enough to withstand the increase of the volume required to keep not only the electrolyte but also the volume of the oxygen gas inside the cell.

Eventually, the expansion reservoir is advantageously connected to the bottom of the cell so that the electrolyte can circulate and be returned to the cell.

Moreover, in the present invention, considering that the oxygen evolution rate is directly linked to the charging current, the said cross section of the Riser Tube is advantageously adjusted to control the circulation rate of the electrolyte.

The invention also relates to a zinc-air battery system comprising at least one cell according to the above definition.

Advantageously, the invention relates to the above zinc-air battery, wherein a plurality of cells shares a common electrolyte expansion reservoir.

The invention relates to a vehicle comprising a zinc-air battery as defined above.

[Blending]

It is well-known that during charging, the pH of the electrolyte is changing at the vicinities of the anode and cathode while flowing through the anode and cathode compartments while some current is flowing in the cell. In case of charging a Zinc-Air cell, the pH would increase while flowing through the Zinc-electrode compartment and decrease through the charging counter-electrode compartment. In the case of battery cells including a Zinc electrode, it is also well-known that the dissolution of ZnO in a strong alkaline electrolyte is dependent on the KOH concentration at 1 (of ZnO) to 10 (KOH) ratio, however in the stronger KOH solutions dissolution rate is increasing. During charging this could lead to ZnO precipitation in the vicinity of the Charging Electrode, eventually leading to zincate ion aging when the active mass of Zn electrode becomes depleted during further cycling. Thus, according to a preferred way of the present invention, in order to avoid Zn precipitation in the charging-electrode compartment while the pH of the electrolyte is decreased, the cell is such that the electrolyte circulation is organized so that a common electrolyte flow is split to supply the Zinc & the charging compartments, and the electrolyte outputs from the Zinc-electrode and charging-electrode compartments are grouped and blended before being reintroduced at the bottom of the cell as a common electrolyte flow whereby averaging the pH of the electrolyte in the Zinc and Charging compartments.

It was observed also that no supersaturated zincate electrolytes could be produced with the here proposed electrolyte circulating method. The supersaturated zincate electrolyte, which is unstable, over time becomes saturated with resultant ZnO precipitation anywhere in the cell causing again zincate ion ageing.

Fig.1

is a schematic representation of an Airlift pump disclosed in the application US 2007/0166171. The Airlift pump is powered by compressed air, raises fluid by entraining gas to reduce its density. 1. Air supply. 2. Liquid supply. 3. Air inlet port. 4. Air supply line. 5. Air port. 6. Air outlet. 7. Fluid intake. 8. Riser tube. 9. Air liquid mixture. 10. Pump outlet. L: Liquid, usually wastewater. LL: Liquid level. V: Vessel G: Gravel or solids.

Fig.2

is a schematic representation of an electrode according to the invention during charging, when the airlift pump is active. 1 1. Galvanic cell (including an air-electrode, a charging electrode and a zinc-electrode, installed vertically, parallel one to each other) of 15 Ah, height 150 mm, width 120 mm, thickness 10 mm. The electrolyte is 6M KOH. 2. Oxygen bubbles mixed with electrolyte. 3. Gas Collector, 10 mm height. 4. Air inlet. 5. Riser tube, a PVC tube of Dia 2 mm inside cross section. 6. Air liquid mixture output flow. 7. Gas Exhaust, a hole Dia 2 mm. 8. Expansion Reservoir, H=70 mm, W=40 mm, Thk=12 mm, the bottom of the reservoir placed 50 mm above top of galvanic cell. The reservoir is filed up to a level of approximately 10 mm at start of charging. 9. Return tubing, a PVC tube of Dia 4 mm inside cross section. 10. Electrolyte return port.

Claims

A zinc-air secondary cell, for use in an alkaline electrolyte secondary battery, comprising :
- a zinc-electrode ;
- an air electrode;
- a charging electrode used for charging the zinc-electrode, said charging electrode facing at-least a face to the zinc-electrode, and
said cell comprising a layer of alkaline liquid electrolyte in contact with at least one side the charging electrode,
said cell comprising further a Riser Tube, said Riser Tube being at least partially filled with electrolyte at start of charging,
said Riser Tube comprising at least one Air Inlet receiving the oxygen evolving from the charging, eventually mixed with electrolyte, and a Mixture Output placed higher than the Air Inlet, out of which a mixture of oxygen and electrolyte is flowing out during charging,
said Riser Tube defining an Airlift pump being only activated by the oxygen evolving from said charging electrode during the charge of said zinc-electrode.
The cell according to claim 1, wherein the Mixture Output of the said Airlift pump is minimum 20 mm higher than the said Air Inlet whereby providing sufficient electrolyte transportation efficiency.
The cell according to claim 1 or claim 2, wherein the Riser Tube has a cross section less or equal to 15 mm² in surface, and advantageously a cross section equal to or higher than 2 mm².
The cell according to anyone of claims 1 to 3, wherein the Riser Tube is essentially vertical during charging.
The cell according to anyone of claims 1 to 4, wherein the Riser Tube is positioned higher than the top of the zinc-electrode.
The cell according to anyone of claims 1 to 5, wherein the cell also includes an expansion reservoir, receiving during charging the output flow of the Airlift pump, the mixture of electrolyte and oxygen evolving during charging; said expansion reservoir comprising at least a gas exhaust (for oxygen exhaust).
The cell according to claim 6, wherein the said expansion reservoir is communicating to the bottom of the zinc-air cell whereby allowing the electrolyte to return and circulate during charging.
The cell according to claim 7, wherein the expansion reservoir is connected to a return tubing connected to the bottom of the cell.
The cell according to claim 7, wherein the level of the electrolyte in the expansion reservoir is at start of charging at least a minimum of 20 mm higher than the said Air-Input whereby the pressure head is sufficient to start and sustain the electrolyte circulation.
The cell according to claim 9, wherein the Airlift pump, expansion reservoir and return tubing are all sharing a same casing whereby simplifying the design and number of interconnections.
A zinc-air battery system comprising at least one cell according to anyone of claims 1 to 10.
The zinc-air battery according to claim 11, wherein a plurality of cells shares a common electrolyte expansion reservoir.
A vehicle comprising a zinc-air battery according to claim 11 or 12.