EP4169113A2 - Zn-air rechargeable battery - Google Patents

Abstract

The invention relates to a novel Zn-air rechargeable battery with charcoal/graphite cathode and dicarboxyl telechelic polyethylene glycol containing additive, e.g. PEG or POLOXAMER. The synthesized dicarboxyl telechelic PEGs were successfully applied to suppress the formation of dendritic and mossy deposits on the surface of the zinc electrode. The weight of the electrode did not change after the long-term test.

Description

Zn-Air Rechargeable Battery

FIELD OF THE INVENTION

The invention relates to a novel Zn-air rechargeable battery with charcoal/graphite cathode and dicarboxyl telechelic polyethylene glycol containing additive, e.g. PEG or POLOXAMER. The synthesized dicarboxyl telechelic PEGs were successfully applied to suppress the formation of dendritic and mossy deposits on the surface of the zinc electrode. The weight of the electrode did not change after the long term test.

BACKGROUND ART

The storage and utilization of the renewable energy sources are of paramount importance for building of our sustainable society. For this reason, for the appropriate energy storage effective and long-life batteries should be developed. To date, the most frequently used and the most popular energy storage device seems to be the Li-ion battery applied in electric vehicles and in other electronics [1] However, there are some limitations of the usage of these batteries such as their insufficient energy density, absence of high capacity due to safety problems and the very limited availability of Li-supplies [2,3] Recently, attempts to develop various metal-air batteries have been started owing to their high energy density nature [4-8] Among these batteries the rechargeable zinc-air batteries have been in the focus of research interest [9-11]. One of the main advantages of zinc-air batteries over those of based on Li-ions is that zinc is an abundant element in the Earth's crust as opposite to lithium, furthermore, oxygen is available directly from the air. The other advantages of the zinc-air batteries are their high energy densities (1350 Wh/kg) and in addition, the aqueous-based electrolytes provide safe operation condition for the high capacity rechargeable batteries. The chargeability of these batteries can be granted by developing bifunctional cathodes, which can promote the oxygen evolution reaction (OER, for charging) and oxygen-reduction reaction (ORR, for discharging) [9] Interestingly, in some cases different electrodes were applied for the charging and discharging processes (using different catalysts) [12-15], while others developed cathode with bifunctional activity providing both reactions using only one electrode [16-19] However, the development of catalyst with high OER activity seems to be a big challenge; and in addition, the control of the morphology of the zinc deposited during the charging process is also a frequently occurring problem [20-23] The morphology of the zinc deposition depends mainly on the operation conditions, for instance, at low current density mossy structure is formed., Moreover, upon increasing current densities layer-like deposition (at middle density) can occur, while at high current densities formation of zinc dendrites is expected to take place. Accordingly, the morphology of the surface of the zinc electrode can be controlled by appropriate charging protocols [24] On the other hand, several studies pointed out that such deleterious dendrite formation can be prevented by adding different additives to the electrolyte. For instance, trimethyloctadecylammonium chloride (STAC) [20], tetrabutylammonium bromide [25], or nickel, indium [26] dissolved in the electrolyte have been shown to suppress dendrite formation. It has also been demonstrated that mossy zinc deposition could be reduced using fluorinated surfactants [28]

Banik and Akolkar used low molecular weight poly(ethylene glycol-diol) (PEG200) for that purpose and report a model that predicts an order of magnitude reduction in the zinc dendrite growth rates in the presence of high concentration of PEG (10000 ppm), consistent with experimental findings [27]

Based on the literature, it can be concluded that there is still a need to construct a rechargeable Zn-air battery wherein both deposit formation can be reduced and ZnO precipitation on the surface of the Zn- electrode can be avoided.

BRIEF DESCRIPTION OF THE INVENTION

The invention relates to a rechargeable Zn-air alkaline battery with charcoal-based air cathode and zinc anode, said alkaline battery comprising a dicarboxyl telechelic polymer of C₂-C₃ alkylene glycol units (subunits or monomeric units), preferably said dicarboxyl telechelic polymer comprising ethylene-oxide units.

The invention also relates to a rechargeable Zn-air alkaline battery with charcoal-based air cathode and zinc anode, said alkaline battery comprising a dicarboxyl telechelic polymer comprising ethylene-oxide units.

Preferably said dicarboxyl telechelic polymer is used as additive in the electrolyte of the battery. Preferably said dicarboxyl telechelic polymer reduces the rate of charge transfer process.

Preferably said dicarboxyl telechelic polymer reduces preferably avoids the formation of dendritic and or other type deposits on the Zn electrode.

Preferably poly(alkylene-glycol) according to the invention has a molecular weight, preferably an average molecular weight of not higher than 3500 Da, preferably not higher than 2000 Da, more preferably not higher than 1000 Da.

The additives are preferably chelating the Zn(ll) ions. Preferably, the Zn-poly-alkylene-glycol(COO)₂ reduce the rate of charge transfer process and, preferably, adsorb onto the surface of the Zn-electrode.

In particular, the dicarboxyl telechelic polymer means a,w dicarboxyl polymer comprising C₂-C₃ alkylene glycol subunits, wherein the monomers of the polymer are selected from 1,2-ethanediol, 1,2- propanediol, 1,3-propanediol. Preferably, at least a part of the subunits are ethylene-glycol subunits. Preferably, at least 10% (w/w), at least 20% (w/w), more preferably at least 30% (w/w) or in particular at least 40% (w/w) of the dicarboxyl telechelic polymer consists of ethylene-glycol subunits.

In a preferred embodiment the dicarboxyl telechelic polymer is dicarboxyl telechelic PEG (a, co dicarboxyl-poly(ethylene glycol)).

Preferably the molecular weight of the dicarboxyl telechelic PEG is not higher than 3300 Da, preferably not higher than 1500 Da.

In a certain embodiment dicarboxyl telechelic PEG600 is disclaimed. In a certain embodiment dicarboxyl telechelic PEG, in particular bi-carboxy methyl ether (PEG BCME) is disclaimed. In a preferred embodiment the molecular weight of the dicarboxyl telechelic PEG is at least 100 or at least 200 Da.

In a preferred embodiment the molecular weight of the dicarboxyl telechelic PEG is 100 to 1400 Da, preferably 200 to 1000 Da, in particular 300 to 900 Da, highly preferably 400 to 800 Da or 200 to 600 Da, particularly preferably about 600 Da, e.g. 500 to 700 Da.

In another preferred embodiment dicarboxyl telechelic polymer is a a,w dicarboxyl-poly(ethylene- glycol)-poly(propylene glycol)-poly(ethylene glycol) copolymer (POLOXAMER). Preferably, at least 10% (w/w), at least 20% (w/w), more preferably at least 30% (w/w) or in particular at least 40% (w/w) of the dicarboxyl telechelic polymer consists of ethylene-glycol subunits.

In a particular embodiment the propylene glycol content of the POLOXAMER is 30-70% w / w, preferably 40-60% w/ w, highly preferably about 50% w/ w.

Preferably the molecular weight of which is not higher than 5000 Da, preferably is about 1800 Da.

In a preferred embodiment in the rechargeable Zn-air alkaline battery the cathode consists (essentially) of graphite and charcoal.

Preferably the battery has an initial cell potential of 1.2 V to 1.6 V, preferably 1.25 V.

In a particularly preferred embodiment in the rechargeable Zn-air alkaline battery cellophane i.e. regenerated cellulose serves as the semi-permeable membrane, i.e., the separator (or diaphragm).

In a preferred embodiment the electrolyte of the battery contains alkaline hydroxide solution, preferably potassium-hydroxide solution and the dicarboxyl telechelic poly(alkylene-glycol) of the invention as additive.

In a preferred embodiment the electrolyte of the battery contains only KOH and [Zn(OH)₄]² and said non-toxic dicarboxyl telechelic PEG additives. In a preferred embodiment the battery has an initial cell potential of 1.2 V to 1.6 V, preferably 1.25 V.

Preferably in the rechargeable Zn-air alkaline battery polyethylene and cotton is the skeleton of the electrode. i) In a preferred embodiment in the rechargeable Zn-air alkaline battery the anode comprises a Zn plate, the air cathode comprises or consists of graphite and charcoal, the separator is a semi-permeable membrane made of regenerated cellulose (cellophane), and the electrolyte comprises KOH and [Zn(OH)₄]² and non-toxic dicarboxyl telechelic poly(alkylene- glycol) as an additive.

Preferably the concentration of the KOH is 2 to 10 M, preferably about 6 M, and the concentration of the [Zn(OH)₄]² is 0.1 to 0.5 M, preferably 0.25 M at the initial stage of the battery.

In a preferred embodiment the battery has an initial cell potential of 1.2 V to 1.6 V, preferably 1.25 V. The battery during normal operation of the battery no H₂ gas is developed. ii) In a preferred embodiment in the rechargeable Zn-air alkaline battery the anode comprises a Zn plate, the air cathode comprises or consists of graphite and charcoal, the separator is a semi-permeable membrane made of regenerated cellulose (cellophane), and the electrolyte comprises KOH and [Zn(OH)₄]² and non-toxic dicarboxyl telechelic PEG additives. Preferably in the electrolyte the concentration of the KOH and [Zn(OH)₄]² is as defined in paragraphe i).

Preferably the concentration of the non-toxic dicarboxyl telechelic PEG additives is 50 to 500 ppm, preferably 50 to 190 ppm, in particular 80 to 160 ppm.

Preferably the concentration of the non-toxic dicarboxyl telechelic PEG additives is 0.5 to 5 x 10⁴ M, preferably 1 to 3 x 10⁴ M, in particular 1.5. to 2.5 x 10⁴ M.

Preferably the dicarboxyl telechelic PEG is 300 to 900 Da, highly preferably 400 to 800 Da or particularly preferably 200 to 500 Da or 200 to 600 Da or about 600 Da, e.g. 500 to 700 Da. iii) In a preferred embodiment in the rechargeable Zn-air alkaline battery the anode comprises a Zn plate, the air cathode comprises or consists of graphite and charcoal, the separator is a semi-permeable membrane made of regenerated cellulose (cellophane), and the electrolyte comprises KOH and [Zn(OH)₄]² and non-toxic dicarboxyl telechelic a,w dicarboxyl- poly(ethylene-glycol)-poly(propylene glycol)-poly(ethylene glycol) copolymer as additive.

Preferably in the electrolyte the concentration of the KOH and [Zn(OH)₄]² is as defined in paragraph ')

Preferably the concentration of the non-toxic dicarboxyl telechelic poly(ethylene-glycol)- poly(propylene-glycol) copolymer (preferably dicarboxyl telechelic POLOXAMER) additives is 0.5 to 5 x 10⁴ M, preferably 1 to 3 x 10⁴ M, in particular 1.5 to 2.5 x 10⁴ M. Preferably the copolymer is 300 to 2000 Da, highly preferably 300 to 1500 Da or 500 to 1800 Da.

In a preferred embodiment the electrolyte of the battery contains only KOH and [Zn(OH)₄]² and non toxic dicarboxyl telechelic PEG additives, the cathode consists of graphite and charcoal, wherein cellophane serves as the semi-permeable membrane, i.e., the separator, and the cathode skeleton comprises preferably polyethylene and cottonn wherein the battery has an initial cell potential of 1.2 V to 1.6 V, preferably 1.25 V.

In a preferred embodiment the battery has rectangular cells.

In a preferred embodiment the rechargeable Zn-air alkaline battery the charcoal-based air cathode comprises a graphite body. In a particular embodiment in the 20 cathode graphite body is a 21 graphite rod whereas th charcoal is 22 activated carbon. In a particular embodiment the 10 anode comprises an 11 Zn plate. At least during operation of the battery both to the cathode and to the anode 60 electric wires are connected.

In a particular set of embodiments the invention relates to the following: 1. The invention relates to a rechargeable Zn-air alkaline battery with charcoal-based air cathode and zinc anode, said alkaline battery comprising a dicarboxyl telechelic polymer comprising ethylene-oxide units.

2. The rechargeable Zn-air alkaline battery of claim 1 wherein the dicarboxyl telechelic polymer is dicarboxyl telechelic PEG (a,w dicarboxyl-poly(ethylene glycol)) as additive in the electrolyte of the battery, wherein preferably the molecular weight of the dicarboxyl telechelic PEG is not higher than 3300 Da, preferably not higher than 1500 Da.

3. The rechargeable Zn-air alkaline battery of claim 2 wherein the molecular weight of the dicarboxyl telechelic PEG is about 3300 Da, about 1500 Da or about 600 Da, preferably about 600.

4. The rechargeable Zn-air alkaline battery of claim 1 wherein the dicarboxyl telechelic polymer is polyethylene glycol-polypropylene glycol-polyethylene glycol (POLOXAMER) preferably the molecular weight of which is not higher than 5000 Da, preferably is about 1800 Da.

5. The rechargeable Zn-air alkaline battery of claim 4, wherein the propylene glycol content of the POLOXAMER is 30-70% w / w, preferably 40-60% w / w, highly preferably about 50% w / w.

6. The rechargeable Zn-air alkaline battery of any of claims 1 to 5 wherein the cathode consists of graphite and charcoal.

7. The rechargeable Zn-air alkaline battery of any of claims 1 to 6 wherein cellophane serves as the semi- permeable membrane, i.e., the separator.

8. The rechargeable Zn-air alkaline battery of any of claims 1 to 7 wherein the electrolyte of the battery contains only KOH and [Zn(OH)₄]² and said non-toxic dicarboxyl telechelic PEG additives.

9. The rechargeable Zn-air alkaline battery of any of claims 1 to 8 wherein the battery has rectangular cells.

10. The rechargeable Zn-air alkaline battery of any of claims 1 to 8 wherein the battery has an initial cell potential of 1.2 V to 1.6 V, preferably 1.25 V.

Preferably in the rechargeable Zn-air alkaline battery polyethylene and cotton is the skeleton of the electrode.

DEFINITIONS

A "dicarboxyl telechelic polymer" means herein an a,w dicarboxyl polymer. According to the invention the dicarboxyl telechelic polymer comprises C₂-C₃ alkylene glycol subunits.

A poly(alkylene-glycol) polymer means here a polymer of alkylene-glycol monomers, in particular a polymer of 1,2-ethanediol, 1,2-propanediol and/or 1,3-propanediol derivable by polycondensation with the elimination of water. Poly(alkylene-glycol), poly(alkylene-oxide), polyalkylene-glycol or poly(alkylene- oxide are used interchangeably herein and the same type of alternative names may applied to, mutatis mutandis, poly(ethylene-glycols) and poly(propylene-glycols) as well.

A poly(alkylene-glycol) (or polyalkylene-glycol, used interchangeably) polymer may be a homopolymer of a single type of alkylene glycol subunits or copolymer of more than one type of alkylene glycol subunits. In particular, the poly(alkylene-glycol) polymer comprises ethylene glycol and/or propylene glycol subunits, preferably comprises at least ethylene glycol subunits. Suitable PAGs include polyethylene glycol (PEG) and copolymers of ethylene glycol with propylene glycol (e.g. poloxamers, meroxapols; e.g., PLURONIC^® surfactants.)

In a particular embodiment poly(alkylene-glycol), preferably "PEG" or "poly(ethylene glycol)," or a copolymer of ethylene glycol with propylene glycol as used herein, is meant to encompass any water- soluble poly(alkylene-glycol).

The terminal groups and architecture of the poly(alkylene-glycol) polymers, preferably polyethylene glycol) comprising polymers of the invention are carboxylate groups.

The numbers that are often included in case of poly(alkylene-glycol), e.g. in the names of PEGs indicate their average molecular weights (e.g. a PEG with n = 9 would have an average molecular weight of approximately 400 Da (Daltons), and would be labeled PEG 400). Most PEGs include molecules with a distribution of molecular weights (i.e. they are polydisperse).

"Molecular weight" as used herein refers to the average molecular mass (or "molecular weight") of a polymer, typically determined by any appropriate method, e.g. size exclusion chromatography or light scattering techniques or can be measured by mass spectrometry or any other appropriate means. The size distribution can be characterized statistically by weight average molecular weight (Mw) or by number average molecular weight (Mn), the ratio of which is called the polydispersity index.

"Electrolyte" as used herein refers to a substance that is capable of conducting electric current as a result of comprising positively and negatively charged ions, which are able to migrate in the electrolyte, wherein preferably the electrolyte is a solution wherein migration of ions takes place negative and positive terminals (cathode and anode) of an electric circuit, respectively.

A "separator" is used herein as a permeable membrane placed between a battery's anode and cathode. As used in this specification, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise.

The terms "comprises" or "comprising" (replaceable by the alternative "including") are to be construed here as having a non-exhaustive meaning and allow the addition or involvement of further features or method steps or components to anything which comprises the listed features or method steps or components. The term "consisting of" relates herein a list of features or method steps or components which is exhaustive and does not allow further members to be included. However, the skilled person may understand in a technical description that a device ore method may comprise further features which may be necessary for operation under certain conditions, however, do not belong to the very merit of the inventive idea and therefore, as a teaching, "consisting of" can be amended to "consisting essentially of" without addition of new matter herein.

The expression "consisting essentially of" or "comprising substantially" is to be understood as consisting of mandatory features or method steps or components listed in a list e.g. in a claim whereas allowing to contain additionally other features or method steps or components which do not materially affect the essential characteristics of the use, method, composition or other subject matter. It is to be understood that "comprises" or "comprising" or "including" can be replaced herein by "consisting essentially of" or "comprising substantially" if so required without addition of new matter.

BRIEF DESCIPTION OF THE DRAWINGS

Figure 1. The MALDI-TOF mass spectrum of the dicarboxyl telechelic PEG1500. The main series corresponds to the potassiated dicarboxyl telechelic polyethylene glycol. The inset reveals the two additional series appearing in the mass spectrum. The numbers on the top of these peaks show the measured and the theoretical m/z values (for the main series, these are shown in brackets). The potassium salt of dicarboxyl telechelic PEG ionized by potassium ion is denoted by series A, while series B corresponds to the potassiated monocarboxyl PEG.

Figure 2. Arrangement and drawing of the rectangular Zn-air rechargeable battery: side-view (Figure 2a), top-view (Figure 2b)

Figure 3a. Long-term discharging of cylindrical primary battery. The red solid line represents the fitted curve using a stretched exponential function (equation 1).

Figure 3a. Photo of the rectangular Zn-air rechargeable battery

Figure 4. Charging and discharging of the cylindrical rechargeable battery. The charging steps operated at a current density of 7.5 mAcm ², while the discharging current density was 0.625 mAcm ². Prior to the second charging step dicarboxyl telechelic PEG (M_n: 1500 Da, 2xl0⁴ M) was added. (Insets show the real time images of the zinc electrode)

Figure 5. Cyclic voltammograms of the first cycle in the absence of dicarboxyl telechelic PEG additive (Figure 5a) and in the presence of dicarboxyl telechelic PEG additive (M_n: 600 Da) (Figure 5b), the presence of dicarboxyl telechelic PEG additive (M_n: 600 Da) (Figure 5c), the presence of dicarboxyl telechelic PEG additive (M_n: 600 Da) (Figure 5d). The concentration of the additives were 2 mM in each case, while the solution contains 6 M KOH and 0.25 M ZnO. The scan rate was 20 mVs ¹.

Figure 6. Cycling tests of the rectangular Zn-Air secondary battery. The current density was kept to be constant, i.e., 2 mAcm ², while the time limit if charging and discharging was set to 5 min.

Figure 6.1. The cycling test of the rectangular Zn-Air secondary battery with no additive.

Figure 6.2. The cycling test of the rectangular Zn-Air secondary battery for PEG-(COOH)2 with Mn=600 Da molecular weight

Figure 6.3. The cycling test of the rectangular Zn-Air secondary battery for PEG-(COOH)2 with Mn=1500 Da molecular weight

Figure 6.4. The cycling test of the rectangular Zn-Air secondary battery for PEG-(COOH)2 with Mn=3300 Da molecular weight

Figure 7. Discharging of the rectangular Zn-Air rechargeable battery in the absence and presence of dicarboxyl telechelic PEG additives during the cycling test: in the absence of dicarboxyl telechelic PEG additives (a), in the presence of dicarboxyl telechelic PEG600 (b), PEG1500 (c) and PEG3300 (d). The black solid lines represent the fitted curves by eq. 1. Experimental conditions: see Fig. 6. caption.

Figure 8. Long-term cyclic performance (E_batterv< capacity and the Coulombic efficiency) of the rectangular Zn-air rechargeable battery with dicarboxyl telechelic PEG600 additive. The current density was 2 mAcm ², while the time limit if charging and discharging was set to be 5 min, respectively.

Figure 9. The SEM images of the zinc electrode after 68-hour cyclic performance test with the current density of 2 mAcm ². with no additive, and with carboxyl telechelic PEG600, PEG1500 and PEG3300, respectively.

Figure 10. EDS spectrum of the zinc-electrode after 68-hour cyclic performance test with dicarboxyl telechelic PEG600 additive.

Figure 11. Photo of experimental battery - essentially the same as the one used with dicarboxyl telechelic PEG600 additive.

Figure 12. Charge-discharge test of a battery containing additive dicarboxylic telechelic POLOXAMER (Figure 12a). Magnified diagram of charge-discharge cycles (Figure 12b). Battery coulomb efficiency over 270 cycles used (Figure 12c).

Figure 13. The Zn anode surface in the cell after the charge-discharge cycle 270 (Figure 13a) removed from the cell (Figure 13b).

DETAILED DESCRIPTION OF THE INVENTION

We can distinguish three fundamental processes taking place on the surfaces of Zn electrodes and its vicinity, i.e., in the electric double-layer of the Zn-electrode during recharging: (i) dissociation of Zn- complexes to yield Zn ²⁺ ions, (ii) charge transfer process to neutralize Zn²⁺ ions to form metallic Zn, (iii) incorporation of metallic zinc in the crystalline lattice of the Zn-electrode.

Deposit formation on the surface of Zn-electrode is expected if the charge transfer process is much faster than that of the incorporation of the metallic Zn formed into the crystalline lattice of the Zn electrode. Accordingly, if deposit formation is to be avoided one should fundamentally define conditions to slow down the charge transfer process, i.e., the charge transfer process should be the rate determining step in the overall recharging process.

The dicarboxylic telechelic poly(alkylene-glycols) used in the batteries of the invention can be prepared from e.g. poly(alkylene-glycols) which are well-known and commercially available and can be obtained from commercial sources.

The terminal carboxyl groups can be synthesized by methods known in the art, e.g. as disclosed in US8067505B2 or by a method described by Fishman, A et al [29] and references cited therein.

The present inventors focused on the construction of a Zn-air rechargeable battery with heavy metal- free, charcoal-based air cathode applying a,w dicarboxyl-poly(ethylene glycol) electrolyte additives. First, in the presence of chromium (Vl)-oxide catalyst a,w dicarboxyl-PEG derivatives with different molecular weights were synthesized. Then cylindrical and rectangular cells were made to test the electrochemical performance of the system. Furthermore, cyclic voltammetry measurements were carried out to understand the effect of dicarboxyl telechelic additives.

The invention relates to a novel rechargeable Zn-air alkaline battery developed with charcoal/graphite cathode and zinc anode. Using of charcoal/graphite cathode, continuous oxygen supply was achieved and it was demonstrated by long-term performance tests that it is an appropriate electrode in the rechargeable Zn-air alkaline battery. To avoid the formation of dendritic and or other type deposits, dicarboxyl telechelic PEGs of different molecular weights (600 Da, 1500 Da, and 3300 Da) were synthesized and applied as additives in the electrolyte. A possible mechanism for the effect of these additives is proposed.

In a particular embodiment the rechargeable Zn-air alkaline battery of the invention comprises charcoal- based air cathode comprising a graphite body or member, e.g. a 20 graphite rod and a Zn-anode preferably a 11 Zn-plate anode arranged in a cell wherein the anode space and the cathode space are separated by a separator (diaphragm) made of a cellulose membrane e.g. a regenerated cellulose, e.g. cellophane. In a preferred embodiment the cell has a rectangular arrangement. In a particular arrangement the 20 cathode is fixed within the cell, e.g. is placed in a frame, e.g. in a plastic frame, like an 50 frame (PE or polyethylene frame). The telechelic dicarboxyl poly(alkylene-glycol) is present in the electrolyte, preferably is placed to the same place as the anode, preferably the 11 Zn plate anode, i.e. the anode side of the separator. The electrode comprises electrolyte ions for which the separator is permeable.

In an embodiment the electrode has a flat arrangement. In this arrangement the graphite/charcoal air 20 cathode is present in a flat 50 frame to provide mechanical strenth, the 30 separator to separate the cathode space from the anode space is cellulose and the anode comprises a flat 11 Zn plate surrounded by a 70 reservoir.

In a preferred embodiment the rechargeable Zn-air alkaline battery is a flexible rechargeable Zn-air alkaline battery, wherein the rechargeable Zn-air alkaline battery has significantly improved mechanical flexibility under repeated bending deformation compared to the conventional Zn-air battery technologies. In a preferred embodiment the cell of the flexible rechargeable Zn-air alkaline battery has a planar arrangement with a bendable structure characteristics, e.g. the cells are placed in a frame or separated by a separator or cross-linked each other within the planar arrangement. Preferably the flexible rechargeable Zn-air alkaline battery may be used for wearable electronics applications.

Upon operations of the cell 60 electric wires are connected to the electrodes while electrode reactions take place as explained herein and potential difference is generated between the electrodes which is measureable through e.g. the electric wires and may result in electric current if the electric circuit is closed between the electrodes preferably e.g. 60 electric wires are connected. Theoretical aspects and working of the novel Zn-air rechargeable battery

In order to avoid the formation of deposits, the rate of the charge transfer process should be reduced. Variations of the reaction rates of the electrode processes are possible using additives that affect the concentrations of the free Zn²⁺ ions. Based on the generally accepted electrochemical processes of such systems we take into account the following reactions taking place on the Zn electrode (Scheme 1): Zn(OH)₂ — ZnO + H₂0 R3

Scheme 1. Reactions taking place on the zinc electrode If Zn(OH)₂ is present in the electrolyte in the boundary of the Zn electrode, an irreversible dehydration reaction may occur resulting in the formation of a solid ZnO layer on the surface of the electrode. The formed ZnO layer decreases the active surface area and thus highly reduces the capacity and Coulomb efficiency of the battery, which, ultimately led to the corrosion of the zinc electrode.

Charcoal/graphite air electrode

A suitable amount of charcoal is necessary to maintain stable, stationary state of the air electrode by adsorption/desorption of oxygen (scheme 2, R4). To achieve our fundamental goals, i.e., to design a high-performance Zn -air rechargeable battery we elaborated and used the following approaches:

To reduce the rate of charge transfer process we used strong complexing and/or chelating agents, i.e., PEG-(COOH)₂ of different number-average molecular weights. This way, the concentration (activity) of Zn²⁺ ions can be reduced in the vicinity of Zn electrode. It is important to note that according to the invention the macromolecular chelating agent should be of bifunctional in order to be able to exclude the OH ions from the inner coordination sphere of the mixed complex. The presence of macromolecular PEG-(COOH)₂ in the mixed complexes also increases the hydrodynamic volumes of the complexes, and thus reduces the ion mobility. This effect brings about that the mixed complex remains basically in the vicinity of the Zn electrode excluding the [Zn(OH)₄]² ions. The process can be visualized as follows on Scheme 2.:

Zn²⁺ + PEG-(COO-)₂ — Zn(PEG-(COO)₂) R6

Zn(PEG-(COO)₂) + 2 OH ^ [Zn(PEG-(COO)₂)(OH)₂]²- R7

Scheme 2. Cathode (R4) and anode (R5) reactions furthermore, the suggested complex formation processes (R6, R7) taking place on the zinc electrode In a particular embodiment, according to Scheme 2, in the presence of PEG-(COO )₂ no Zn(OH)₂ formation takes place, thus excluding the precipitation of ZnO. The results of the rechargeable battery test verifying the suppressing effect of dicarboxyl telechelic PEGs on the deposit formation are discussed below.

In the invention dicarboxyl telechelic poly(alkylene-glycol) is used in the electrolyte as additive to reduce In the Zn electrode preferably a Zn plate is used.

In the arrangement according to the invention the battery has an initial cell potential of 1.2 V to 1.6 V, preferably 1.25 V.

The battery during normal operation no H₂ gas is developed if the dicarboxyl telechelic poly(alkylene- glycol) as defined herein is added to the electrolyte.

The suppression of dendritic deposition was demonstrated by cyclic charging and discharging the battery with high current density (7.5 mAcm ²) in the absence and presence of additive. Intensive deposit formation was observed in the first cycle. On the contrary, with the addition of the dicarboxyl telechelic PEG, the dendritic and mossy depositions were prevented. Cyclic voltammetry measurements were carried out, and it was found that both the anodic and cathodic peaks were shifted to higher and lower potential, resulting in increasing peak separation.

The stability of the battery, the charging/discharging cycles were tested in the presence of each additive. Broadening potential range was observed in the additive-free case, while the synthesized dicarboxyl telechelic PEG additives stabilized the cyclic performance up to four hours. Furthermore, nearly 100% Coulomb efficiency and constant capacity were achieved in each case. The 20^th discharging steps without and with additives were also investigated. Stretched exponential fitting proved to be appropriate to describe the change of battery potential in the absence of additives. On the contrary, the presence of dicarboxyl telechelic PEG additives yielded a more complex decay of E_batterv with time. For the description of these E_batterv versus time curves an equation consisting a fast simple exponential decay and a slow starched exponential decay term was proposed. The dicarboxyl telechelic PEG600 was subjected to a 68- hour test where almost constant capacity and Coulomb efficiency was obtained (100%). The surfaces of the zinc-electrodes were characterized by SEM. SEM-images showed the formation of mossy like deposition without the use of the additive. Flowever, in the presence of dicarboxyl telechelic PEG600 the surface of the zinc-electrode remained smooth after a long-term performance test in the battery of the invention.

In an alternative embodiment production of a prototype of a stable Zn-air battery. Synthesis of telechelic polyethylene glycol-polypropylene glycol-polyethylene glycol (POLOXAMER) (Mn, starting: 1800 g / mol, propylene glycol content 50% w / w). From the commercially available starting material, which is a dihydroxy-telechel derivative, the dicarboxyl derivative was prepared by chromic acid oxidation using the experimental method described previously. The structure of the obtained oxidized product was identified by MALDI-TOF-MS. The electrode reactions P6 and P7 can be described mutatis mutandis.

Surprisingly, stable battery potentials were obtained with 100% Coulombic efficiency during the entire measurement. The internal resistance of the cell decreased and thus allowed charging / discharging in a narrow voltage range. This range was significantly smaller than the values observed with the carboxyl telechelic PEG polymers.

Below the invention is further illustrated by Examples to explain certain preferred embodiments. EXAMPLES

Materials and Methods

Materials

Zinc foil (100 x 100 x 0.25mm, 99.9%), Poly(ethylene glycol) samples with different molecular weights (PEG600, PEG1500 and PEG3300 with Mn = 600 Da, 1500 Da and 3300 Da), POLOXAMER (Mn, starting: 1800 g / mol, propylene glycol content 50% w / w)., activated carbon, i.e., charcoal, (analytical grade), Chromium(VI) oxide (analytical grade), Sulfuric acid (analytical grade), Sodium chloride (analytical grade), Magnesium sulfate (analytical grade) were received from Merck (Darmstadt, Germany). The Zn electrodes were cleaned by rinsing them with n-hexane and dried before use. Graphite rod with spectroscopic grade was used as graphite rod (Ceramics Praha, Czech Republic, Prague). Dichloromethane (HPLC grade), n-Hexane (HPLC grade), Diethyl ether (reagent grade), Zinc oxide (analytical grade), Potassium hydroxide (analytical grade) from VWR (Debrecen, Hungary). All chemicals were used as received.

Potentiostate

The test of the home-made Zinc-air rechargeable battery and the cyclic voltammetry (CV) measurements were performed by BioLogic SP-150 potentiostate (Seyssinet-Pariset, France) equipped with EC-Lab software package.

Scanning electron microscopy

To visualize the morphology of the Zn electrode surface Scanning Electron Microscopy (SEM) was used - Hitachi S-4300 scanning electron microscope (Tokyo, Japan)

Mass spectrometry

The Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry (MALDI-TOF MS) measurements were carried out with a Bruker Autoflex Speed mass spectrometer. In all cases 19 kV acceleration voltage was used in the positive ion mode. Ions were detected in the reflectron mode and 21 kV and 9.55 kV were applied as reflectron voltage 1 and voltage 2, respectively. A solid phase laser (355 nm, >100 pJ/pulse) operating at 500 Hz was applied to produce laser desorption and 2000 shots were summed. The MALDI-TOF MS spectra were externally calibrated with poly(ethylene glycol) standards (M_n= 600 Da, 1500 Da and 3300 Da).

Samples were dissolved in a mixture of water and methanol (80/20 V/V) at a concentration of 10 mg/mL. Samples for MALDI-TOF MS were prepared with 2,5-dihydroxy benzoic acid (DHB) matrix. The matrix at a concentration of 20 mg/mL was dissolved in the same water/methanol mixture as the sample. The matrix solution, the sample solution and potassiumtrifluoroacetate solution (5 mg/mL in water/methanol (80/20 V/V)), used as the cationization agent, were mixed in a 10:2:1 (v/v/v) ratio (matrix/analyte/cationization agent). A volume of 0.5 pL of the solution was deposited onto a metal sample plate and allowed to air-dry.

Synthesis and characterization of dicarboxyl telechelic PEGs

The oxidations of PEGs (PEG600, PEG1500 and PEG3300), were carried out according to ref. [29] In the following, the typical modified procedure for the synthesis of dicarboxyl telechelic PEGs are outlined through the example for PEG 1500 g/mol. A 100 mL round-bottom, three-necked flask equipped with magnetic stirring, thermometer, dropping funnel, reflux condenser was used. The reaction flask was charged with a mixture of 25 mL of water and 8 mL of sulfuric acid at room temperature and 5 g of PEG (1500 g/mol, 3.33mmol) was dissolved in it. After complete dissolution of PEG, the dropping funnel was charged with 2,62 g (26.2 mmol) of Cr0₃ dissolved in 5 mL of water. The oxidizing agent was introduced into the reactor for 20 minutes - the highest temperature was 55 °C. The reaction mixture was stirred for additional three hours. The color of the mixture changed from orange to green-blue showing the completion of the reaction. Then 25 mL of water was added to the reaction mixture, transferred into a separation funnel and extracted with 3 x 100 mL of CH₂CI₂. The organic layers were then combined and washed with water (2 x 25 mL) and saturated NaCI solution (2 x 25 mL) - (To increase the yield in the case of PEG600 only saturated NaCI solution was used for washing.) - The organic phase was dried with MgS0₄ for overnight. Solvent was evaporated at room temperature under vacuum. The honey-like product was mixed with 150 mL of cold diethyl ether and the precipitate was filtered and dried. Yield: 3.94 g (79 %).

The formations of the target reaction products, i.e., PEG-(COOH)₂-s were confirmed by MALDI-TOF MS as seen for dicarboxyl telechelic PEG1500 shown in Fig. la. The spectra for the other two dicarboxyl telechelic PEGs with M_n = 3300 Da and 600 Da are presented in Fig. lb. and Fig. lc., respectively. Synthesis and characterization of dicarboxyl telechelic poly(ethylene-glycol) poly(propylene-glycol) copolymers POLOXAMER

Synthesis of dicarboxyl telechelic poly(ethylene-glycol) - poly(propylene-glycol) copolymers POLOXAMER was carried out by the same method as the dicarboxyl telechelic PEGs, by oxidation of the starting material.

Construction ofZn-air rechargeable batteries

The Zinc-air rechargeable batteries were constructed using charcoal/graphite cathode and zinc anode with two different geometries. The 20 cathode compartment was fabricated from polyethylene (PE) skeleton with cylinder (with 4 cm diameter) or rectangular prism (with dimensions: 3 x 3 x 9.5 cm (length x width x height)) shape. 40 Cotton finger or lining (cotton textile) was connected to the 50 frame (PE skeleton) forming cotton textile walls inside the cylinder or the rectangular prism. Then cellophane, used as a semipermeable membrane, i.e. the 30 separator, was inserted inside the cotton cavity. Finally, the interior space was filled with charcoal (approx. 22 g) and the 21 graphite rod was placed into the middle of the inner part filled with activated carbon to form the 20 cathode. The 10 anode had a 11 zinc plate (thickness 0.25 mm, length 10 cm, width 11 mm) fixed to the PE skeleton and its distance from the cotton wall was kept at a constant value (approx. 2 mm). The prepared cell was inserted into a 70 reservoir (beaker) with 200 mL volume and filled out through the charcoal bed with 6 M aqueous potassium hydroxide solution containing ZnO at a concentration of 0.25 M. The level of the solution in the 70 reservoir (beaker) was kept constant (the height of the lye was 8 cm). To test the effect of the dicarboxyl telechelic PEG additives, these PEG derivatives were dissolved and/or emulsified in the KOH/ZnO solution at a concentration of 2 x 10⁴ M.

Results and discussion

Preliminary experiments and cell construction

Besides the appropriate performance of a battery, the applied materials are playing a very important role in the construction of the Zn-air rechargeable battery. The electrolyte of the battery contains only KOH and [Zn(OH)₄]² and non-toxic dicarboxyl telechelic PEG additives. The cathode consists of graphite and charcoal. In such configuration, polyethylene and cotton is the skeleton of the electrode, while cellophane serves as the semi-permeable membrane, i.e., the separator. All of the materials used for the construction of Zn-air rechargeable battery are easily available, non-toxic and are met the main criteria of green and sustainable chemistry. Arrangement and a picture from the novel Zn-air rechargeable battery are shown in Fig. 2.

In order to understand our simple Zn-charcoal/graphite air battery first a Zn-primary battery was created. The characteristics of the cell were determined by the continuous discharge of the cell with 5 mA discharging current.

The results of discharge of the cylindrical primary battery is shown in Fig. 3. The initial cell potential was 1.25 V. The discharge was forced-terminated after 60 hour running times. The approximately constant potential suggests that the lifetime of the cell may be much longer. The long lifetime is due to the continuous oxygen supplement from the air. During the discharge process, more than 300 mAh capacity was obtained. Fig. 3. shows that the discharge process is stable and the operation of the Zn-air cell is sustainable with the application of charcoal/graphite cathode. Interestingly, the discharge curve can well be described using stretched exponential function as shown in Fig. 3. where AE_S is the potential change. The k_s is the rate coefficient of the decay and m is the stretched exponential factor, while the £ is the terminal potential. The fitted parameters for £ , AE_S, k_s and m are 1.22 V, 0.029 V, 0.058 h ¹ and 0.606, respectively.

A possible reason for the occurrence of the stretched exponential function will be discussed later. However, the application of the Zn-air system as a rechargeable battery runs into difficulties due to the formation of Zn deposits and unstable operations as it is discussed in the introduction. In order to get deeper insight into operation of our cell, alternating charging and discharging events were performed. In addition, a new rectangular cell was prepared in order to obtain uniform geometry and to test the performance of our cell with the novel dicarboxyl telechelic PEG additives. Fig. 4. shows the cycles of charging and discharging steps in the absence of additive, while the second cycle was recorded in the presence of dicarboxyl telechelic PEG1500 additive.

The charging current density in this case was 7.5 mAcm ². During the first charging step, intensive deposit formation was observed as it is demonstrated in Fig. 4. The deposit is more expanded near the side of the electrode due to the higher local current density. The structure of the deposit is mainly mossy type, however, dendrite-like crystals have also appeared. The intensive charging event was followed by discharging with a current density of 0.625 mAcm ². This step removed most of the deposition from the surface of the Zn-electrode, confirming the presence of mossy-type deposit, indicating that deposition and dissolution are overwhelmingly reversible. Prior to starting the next cycle, dicarboxyl telechelic PEG1500 was added to the electrolyte. Applying the same charging conditions as in the case of the first step, the surface of the electrode after the charging remained almost the same as before. Importantly, significant amount of new deposits was not observed in the presence of dicarboxyl telechelic PEG additive, only the size of some deposit increased, which formed during the first cycle. Thus, the dicarboxyl telechelic PEG shows high efficiency in the suppression of the deposit formation.

Cyclic voltammetry with telechelic PEG

In order to investigate the zinc deposition and dissolution, cyclic voltammetry measurements were carried out. Fig. 5a and 5b. shows the voltammogram in the absence and presence of dicarboxyl telechelic PEG600 (M_n=600 Da), respectively, whereas Fig. 5b for dicarboxyl telechelic PEG 1500 and Fig. 5c for PEG 3300 (M_n= 1500 Da and 3300 Da, respectively). The working, the counter and the reference electrode were zinc wire, graphite and zinc wire, respectively.

The voltammograms were recorded in the range of -0.5-0.55 V vs. Zn/Zn²⁺. The cathodic peaks are detected at a lower potential than -0.1 V, while anodic peaks are around 0.1 V. In the presence of additives, the anodic/cathodic peak separations are higher than in the additive-free system. A similar shift was reported by Shimizu et. al. [20] for the anodic peak applying anionic additives in the electrolyte. The difference between the position of anodic and cathodic peaks reveals higher overpotential, however, the difference is low compared to other types of additives applied by Trudgeon et. al. [29] In voltammograms measured in the presence of PEG-(COOH)₂ additives (Fig. 2.) represent clear reduction and oxidation peaks. The results of the cyclic voltammetry measurement are shown in Table 1. Table 1. The results of cyclic voltammetry including the positions of anodic/cathodic peaks, the current densities and the peaks separations.

The CV figures reveal the shifts for both the anodic and cathodic peak positions. The cathodic shifts are 25 mV, 24 mV and 52 mV, while the shifts of the anodic peaks are 50 mV, 7 mV, 65 mV compared to the additive-free system for the dicarboxyl telechelic PEGs with the molecular weights of 600, 1500 and 3300 Da, respectively. The highest anodic/cathodic peak separation was observed for the dicarboxyl telechelic PEG3300, which might be the result of the incomplete solution of the polymer in the concentrated KOH solution.

Rectangular rechargeable Zn-air battery with telechelic PEG The cyclic performance of the novel Zn-air rechargeable battery with different additives was tested with the current density of 2 mAcm ² for charging and discharging. The time limit of the steps was 5 min, while the potential limits were 1.0 and 1.8 V. The battery potential was around 1.27 V. It is clearly visible in Fig. 6., that the additive-free system has an increasing battery potential range with time (efficiency and capacity curves are shown in the supporting information). The change of potential suggests the formation of irreversible deposit on the surface of the zinc electrode. Moreover, the dicarboxyl telechelic PEGs of different molecular weights have a favorable effect on the stability of the potentials. In every case, the potential falls into the range of 1.2 V to 1.6 V and the cycling performances show only little differences. The dicarboxyl telechelic PEG600 gives the most stable charging-discharging curves (Fig. 6), very small changes can be observed in the case of dicarboxyl telechelic PEG1500, while in the presence of dicarboxyl telechelic PEG3300 distorted cycles were obtained. The observed trend in the performance of dicarboxyl telechelic PEG additives may be due to the fact that the chelating ability of the carboxylate groups at the chain-end of PEGs towards the Zn(ll) ions decreases with the increasing chain-length. Furthermore, it can be assumed that Zn-PEG(COO)₂ complexes may adsorb onto the surface of the Zn- electrode and thus reduce the rate of charge transfer process. The Coulomb efficiency of the tested batteries were near 100 % indicating high reproducibility of the charging/discharging cycles. The battery potential change was investigated for the 20^th discharge cycles of each measurement (with and without additives) to visualize the effect of the dicarboxyl telechelic PEG additives (Fig. 7.).

The curve obtained without the addition of additives can be approached by a stretched exponential function (see eq. 1, Fig. 7a). Such behavior can be originated from a broad distribution of activation energies for the electrochemical reactions, caused by the diverse surface of the Zn electrode. On the contrary, the presence of PEG additives yields a more complex decay of E_batterv with time (Fig.7b-7d.). We found that the E_batterv versus time curves can adequately be approximated by eq. 2.

Ebattery ( where AE_f and AE_S are the potential change corresponding to fast and slow decay, respectively. The k_f and k_s are the rate coefficients of the fast and slow decay, respectively, and m is the stretched exponential factor, while the E„ is the terminal potential.

Eq. 2 consist of two exponential terms in which the first term represents a fast decay of E_battery indicating a fast rearrangement of the structure of electric double layer thus reducing the overpotential of the zinc electrode. The second term involves the stretched exponential function. Interestingly, the value of the stretched exponential factor (m) decreases as the chain length of dicarboxyl telechelic PEGs increases, while the corresponding k_s values does not change significantly in the case of PEG1500 and PEG3300.ln order to prove the long term stability of the novel Zn-air rechargeable battery, the cyclic performance measurements were extended up to more than 450 cycles (65 hours) in the presence of dicarboxyl telechelic PEG600 (Fig. 8.). The maximum potential of the cycles is slightly increasing, however, stable cycles were observed. The stability of the Zn-electrode is also confirmed by the capacity curve. It was found that the capacity remained constant during the investigated period and high Coulomb efficiency was obtained (near 100%). The reversibility of charging/discharging of the battery was also confirmed by measurement of the mass of the zinc-electrode before and after the long-term cycling performance measurements and no significant changes in the mass of the electrode was observed.

Characterization of the zinc electrode surface by scanning electron microscopy (SEM) with telechelic PEG

The effect of additives on the formation of deposit was investigated by the surface analysis of the Zn- electrodes by SEM. The electrodes after the tests were washed with water and acetone then stored under cyclohexane prior to analysis.

Fig. 9. shows the recorded images of electrodes after long-term cyclic performance tests (68-hour) at different magnifications in the presence and in the absence of dicarboxyl telechelic PEG600 additives. The applied current density was 2 mAcm ². Mossy-like and porous deposition is formed on the surface of the electrode in the absence of dicarboxyl telechelic PEG additive. The deposit occupies large area on the electrode surface. On the contrary, the application of dicarboxyl telechelic PEG600 yielded highly improved morphology. The electrode surface is relatively smooth, needle-like dendritic, or mossy protrusions were not recognized. The elemental composition of deposits was proved to be metallic zinc by energy dispersive spectroscopy (EDS) measurements. A typical EDS spectrum is shown in Fig. 10.

The SEM images for the dicarboxyl telechelic PEG1500 and 3300 are shown in the Supporting Information (Fig. S9.). In the presence of these additives, some protrusions were formed during the cyclic performance test, however, their sizes remained very small.

Novel Zn-air rechargeable battery with telechelic polyethyleneglycol-propyleneglycol- polyethyleneglycol (POLOXAMER)

The design of the experimental equipment was identical to that described for the dicarboxyl telechelical PEG 600 experiments (Fig. 11). The composition of the electrolyte used was also the same as that of the electrolyte used in the previous experiments, except that the dicarboxyl telecelical additive PEG 600 was replaced by a dicarboxyl telechelic POLOXAMER derivative. When mixed with 6 M potassium hydroxide solution, the dicarboxylic POLOXAMER derivative was not completely dissolved but formed an opaque emulsion. Table 2 - Parameters for charging/discharging assay of the battery Potentiostatic measurements confirmed that the duration of the instationary (non-stationary) operation previously observed during the operation of the battery cell has been significantly shortened, and the charge/discharge range was reduced to 0.13 V (Figure 12) and stable battery potentials were obtained with 100% Coulombic efficiency during the entire measurement. The internal resistance of the cell decreased and thus allowed charging / discharging in a narrow voltage range. This range was significantly smaller (1.20 - 1.37 V) than the values observed with the carboxyl telechelic PEG polymers (1.12-1.60 V). The running of the charge/discharge cycles were also significantly different from those of the batteries with dicarboxyl telechelic PEGs (M_n: 600 Da, 1500 Da, 3300 Da).

INDUSTRIAL APPLICABILITY

The Zn-air batteries are promising energy storage devices because of their high storage capacity and inexpensive constructions. Herein the development of a novel Zn-air rechargeable battery with charcoal/graphite cathode and dicarboxyl telechelic polyethylene glycol (PEG) additives with different molecular weights (M_n: 600 Da, 1500 Da, 3300 Da) is reported. The battery is free of heavy metals and all of the used materials are easily available and non-toxic. The synthesized dicarboxyl telechelic PEGs were successfully applied to suppress the formation of dendritic and mossy deposits on the surface of the zinc electrode at both 7.5 and 2 mAcm ² charging current density. The stability of the rechargeable battery was tested, where stable operation was achieved with all the synthesized dicarboxyl telechelic PEG additives. Cyclic voltammetry measurements were also carried out to understand the effect of dicarboxyl telechelic additives. Furthermore, the long-term cyclic performance was investigated with charging/discharging cycles up to 68 hours, where stable battery potentials were obtained with near 100% Coulombic efficiency during the entire measurement. In the presence of additives, the discharging steps were also mathematically modeled. The electrode surface after the tests were analyzed by Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). The additive-free systems resulted in mossy-like deposition, while in the presence of dicarboxyl telechelic PEG additives the surface of the zinc electrode remained smooth and the weight of the electrode did not change after the long-term test.

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Claims

I. A rechargeable Zn-air alkaline battery with charcoal-based air cathode and zinc anode, said alkaline battery comprising a dicarboxyl telechelic polymer of C₂-C₃ alkylene glycol units (subunits or monomeric units), preferably comprising a dicarboxyl telechelic polymer comprising ethylene-oxide units.

2. The rechargeable Zn-air alkaline battery of claim 1 wherein the anode comprises a Zn-plate and the cathode consists of graphite and charcoal.

3. The rechargeable Zn-air alkaline battery of any of claims 1 to 2 wherein the electrolyte of the battery contains only KOH and [Zn(OH)₄]² and said non-toxic dicarboxyl telechelic PEG additives.

4. The rechargeable Zn-air alkaline battery of any of claims 1 to 3 wherein cellophane serves as the semi- permeable membrane, i.e., the separator.

5. The rechargeable Zn-air alkaline battery of any of claims 1 to 4 wherein the battery has an initial cell potential of 1.2 V to 1.6 V, preferably 1.25 V.

6. The rechargeable Zn-air alkaline battery of any of claims 1 to 5 wherein the dicarboxyl telechelic polymer is dicarboxyl telechelic PEG (a,w dicarboxyl-poly(ethylene glycol)) as additive in the electrolyte of the battery, wherein preferably the molecular weight of the dicarboxyl telechelic PEG is not higher than 3300 Da, preferably not higher than 1500 Da, preferably about 600 Da.

7. The rechargeable Zn-air alkaline battery of any of claims 1 to 6 wherein the dicarboxyl telechelic polymer is polyethylene glycol-polypropylene glycol-polyethylene glycol (POLOXAMER) preferably the molecular weight of which is not higher than 5000 Da, preferably is about 1800 Da.

8. The rechargeable Zn-air alkaline battery of claim 7, wherein the propylene glycol content of the

POLOXAMER is 30-70% w / w, preferably 40-60% w / w, highly preferably about 50% w / w.

9. The rechargeable Zn-air alkaline battery of any of claims 1 to 8 wherein the battery has rectangular cells.

10. The rechargeable Zn-air alkaline battery of any of claims 1 to 9 wherein no H₂ is produce during battery operation.

II. The rechargeable Zn-air alkaline battery of any of claims 1 to 9 wherein no dicarboxyl telechelic PEG600 is present in the electrolyte.