Application of multi-electron redox mediator in metal-air battery or lithium-sulfur battery

Abstract

The invention provides an application of a multi-electron redox mediator in a metal-air battery or a lithium-sulfur battery. The multi-electron redox mediator is selected from one or more of heteropoly acid, small organic molecule sulfide and small organic molecule nitride. According to the invention, a multi-electron transfer strategy is provided, and a redox mediator capable of transferring multiple electrons at the same time is utilized, so that the reaction rate of the charging and discharging process can be effectively improved, and the possibility of side reaction is reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of redox mediators, and particularly relates to the application of a class of multi-electron redox mediators in metal-air batteries or lithium-sulfur batteries. Background Art

[0002] Metal-air batteries have received extensive attention due to their ultra-high theoretical energy density. This battery technology can greatly extend the driving range of electric vehicles and accelerate the development of advanced power energy storage systems. However, so far, there are still many problems in the development of metal-air batteries. The charge-discharge reaction mechanism is still controversial, and many technical problems need to be solved urgently. Among them, how to improve the reaction kinetics during the charge and discharge process of metal-air batteries is a hot issue in current research. In response to this problem, this patent proposes a method of adding multi-electron redox mediators to enable multi-electron transfer to occur simultaneously during the reaction process.

[0003] Taking the aprotic lithium-oxygen battery as an example:

[0004] In a typical current aprotic lithium-oxygen battery, its charge-discharge process is the reversible generation and decomposition of Li 2 O 2 During battery discharge, the lithium negative electrode loses electrons to become Li + , and O 2 dissolved in the electrolyte gains electrons at the positive electrode to generate superoxide radicals (O 2 -), which further combines with Li + to generate lithium superoxide (LiO 2 ), and finally lithium peroxide (Li 2 O 2 ) is generated through an electrochemical reaction or a chemical disproportionation reaction. During battery charging, Li + gains electrons and is reduced to metallic lithium, and the discharge product Li 2 O 2 decomposes to release O 2 and Li + . The specific reaction equations are as follows:

[0005] During discharge: Anode Li - e - → Li +

[0006] Cathode O 2 + e - → O 2 - (ORR process)

[0007] O 2 - + Li + → LiO 2

[0008] 2LiO 2 →Li 2 O 2 +O 2 (Chemical disproportionation reaction)

[0009] LiO 2 +Li + +e - →Li 2 O 2 (Electrochemical reaction)

[0010] During charging: At the negative electrode, Li + +e - →Li

[0011] At the positive electrode, Li 2 O 2 →O 2 +2Li + +2e - (OER process)

[0012] In a lithium-oxygen battery, the ORR (oxygen reduction reaction) and OER (oxygen evolution reaction) processes are two-electron reactions, involving the generation of multiple reactive oxygen intermediates (such as O 2 - , LiO 2 and 1 O 2 etc.). The discharge product Li 2 O 2 generated during the ORR process is a non-conductive solid. Excessive deposition of Li 2 O 2 on the electrode surface will passivate the porous positive electrode, resulting in serious electrode clogging, increased overpotential, and difficulty in subsequent electrochemical cycling.

[0013] Currently, redox mediators such as lithium iodide (LiI), organic compounds such as tetrathiafulvalene (TTF), and some stable free radicals like TEMPO, which are applied to the charging process of lithium-oxygen batteries, can effectively catalyze the decomposition process of Li 2 O 2 . The RM is first oxidized to RM + , and electrons are transferred from Li 2 O 2 to RM + , and Li 2 O 2 gradually releases Li + , and finally releases O 2 . Similarly, RMs with relatively low potentials, such as 2,5-di-tert-butyl-1,4-benzoquinone (DBBQ), have also been proven to be effectively applied to the discharge process of lithium-oxygen batteries.

[0014] All the reported RMs mentioned above use redox mediators with single electron action. One electron is gained or lost in one reaction, that is, single electron transfer occurs, forming superoxide intermediates. SUMMARY OF THE INVENTION

[0015] In view of this, the object of the present invention is to provide the application of a class of multi-electron redox mediators in metal-air batteries or lithium-sulfur batteries. This class of redox mediators can perform multi-electron transfer simultaneously.

[0016] The present invention provides the application of a class of multi-electron redox mediators in metal-air batteries or lithium-sulfur batteries.

[0017] In the present invention, a pair of redox peaks corresponds to one redox reaction, that is, it corresponds to the change from a certain oxidation state to a reduction state, or from an oxidation state to a reduction state. This class of multi-electron redox mediators is subdivided into the following two types: The first type is represented by heteropolyacids, which is characterized by having multiple pairs of redox peaks, and one pair of redox peaks corresponds to the gain or loss of one electron. Catalyzing multi-electron reactions requires the combined action of its multiple pairs of peaks. The second type is represented by organic small molecule sulfides or organic small molecule nitrides, which is characterized by having one or more pairs of redox peaks, but one pair of its redox peaks can correspond to the gain or loss of multiple electrons. Catalyzing multi-electron reactions may only require the action of a certain pair of its peaks.

[0018] The multi-electron redox mediators in the present invention are selected from one or more of heteropolyacids, organic small molecule sulfides and organic small molecule nitrides.

[0019] The above-mentioned multi-electron redox mediators are applied to the charge and discharge processes of metal-air batteries or lithium-sulfur batteries to carry out multi-electron reaction processes.

[0020] In the present invention,

[0021] The multi-electron redox mediators are specifically applied to the redox reactions of products that need to gain or lose multiple electrons during the charge and discharge processes of metal-air batteries or lithium-sulfur batteries;

[0022] The products refer to charging products, discharging products or intermediate products of metal-air batteries or lithium-sulfur batteries; the products include but are not limited to Li 2 O 2 , Na 2 O 2 , Li 2 CO 3 , Na 2 CO 3 , K 2 CO 3 , ZnO x , MgO2 and Mg 2 O 2 or one or more of them.

[0023] The advantages of the present invention include:

[0024] (1) If this type of multi - electron redox mediator has multiple pairs of redox peaks, the potential difference between the redox peaks is usually about 500 mV. If the potential is appropriate, multiple pairs of redox couples of this type of redox mediator can be utilized simultaneously, and multi - electron transfer can occur directly during the reaction;

[0025] (2) This type of multi - electron redox mediator can perform multi - electron transfer simultaneously, accelerating the reaction kinetics rate of the charge - discharge process of the metal - air battery;

[0026] (3) This type of multi - electron redox mediator can reduce or avoid the possibility of the existence of unstable intermediate products in the metal - air battery, improving the reaction stability;

[0027] (4) This type of multi - electron redox mediator can reduce the existence time of unstable intermediates in the metal - air battery, reducing the occurrence of side reactions;

[0028] (5) By applying a suitable multi - electron redox mediator, a rapid reaction at a low potential can be achieved.

[0029] The present invention specifically uses polyoxometalates as representative multi - electron redox mediators; the characteristics of this type of multi - electron redox mediator are as follows: 1) This type of multi - electron redox mediator has multiple pairs of redox peaks, and each pair of peaks corresponds to the transfer of a single electron; 2) This type of multi - electron redox mediator requires multiple pairs of peaks to act together, and the intermediate valence state cannot be oxidized alone. Thermodynamically, the equilibrium potential of the redox couple RM ox1 / RM re1 is lower than the decomposition potential of Li 2 O 2 or Na 2 O 2 and cannot be used alone to oxidize Li 2 O 2 or Na 2 O 2 ; the equilibrium potential of the redox couple RM ox2 / RM re2 is higher than the decomposition potential of Li 2 O 2 or Na 2 O 2 , and RM ox1 / RM re1 and RM ox2 / RM re2The average potential of these two electrode pairs is higher than that of Li 2 O 2 or Na 2 O 2 The decomposition potential of. The two electrode pairs can be used simultaneously to oxidize Li 2 O 2 or Na 2 O 2 .

[0030] The multi-electron redox mediators represented by organic small molecule sulfides or organic small molecule nitrides are specifically used in the present invention; the characteristics of this type of multi-electron redox mediators are as follows: 1) One redox reaction of this type of multi-electron redox mediator can gain and lose multiple electrons, that is, a pair of redox peaks can correspond to the transfer of multiple electrons; 2) When this type of multi-electron redox mediator conducts multi-electron catalysis, only a certain pair of peaks may be required to act alone. If a certain pair of peaks of the multi-electron redox mediator corresponds to a multi-electron redox reaction and meets the number of electrons to be transferred required for the catalytic reaction, then as long as the redox equilibrium potential of this pair of peaks is higher than the thermodynamic potential of the reaction, the requirement of simultaneous transfer of multiple electrons during catalysis can be achieved.

[0031] The multi-electron redox mediator described in the present invention is selected from H 3 PW 12 O 40 , Li 3 PW 12 O 40 , Na 3 PW 12 O 40 , H 4 SiW 12 O 40 , Li 4 SiW 12 O 40 , Na 4 SiW 12 O 40 , H 6 P 2 W 18 O 62 , H 3 BW 12 O 40 , Na 3 BW 12 O 40 , H 5 PMo 12 O 41 , Na 3 PMo 12 O 40 , H 6 SiMo 12 O41 , Na 4 SiMo 12 O 40 , H 6 ZnW 12 O 40 , H 3 PW 11 MoO 40 , one or more of 1,5 - Dithiocane, hexahydropyrazolo[1,2 - a]pyrazole, and 2 - mercaptobenzoxazole.

[0032] In the present invention, the metal - air battery described is selected from Li - O 2 / CO 2 battery, Na - O 2 / CO 2 battery, K - O 2 / CO 2 battery, Mg - O 2 / CO 2 battery, or Zn - O 2 / CO 2 battery.

[0033] In the present invention, for the multi - electron redox mediator, if a pair of redox peaks corresponds to the gain and loss of one electron and there are multiple pairs of reversible redox peaks, multiple pairs of peaks need to act together during catalysis; the potential of the RM ox2 / RM re2 electrode pair is lower than the catalytic potential, the potential of the RM ox1 / RM re1 electrode pair is higher than the catalytic potential, the difference in peak potentials between the RM ox1 / RM re1 and RM ox2 / RM ox2 is greater than 300 mV, and the average potential of the two electrode pairs of RM ox1 / RM re1 and RM ox2 / RM ox2 is higher than the catalytic potential, and the RM ox1 / RM re1 and RM ox2 / RM ox2 two electrode pairs can be used for simultaneous catalysis.

[0034] In the present invention, for the multi - electron redox mediator, if a pair of redox peaks corresponds to the gain and loss of multiple electrons and there is one or more pairs of reversible redox peaks, only a certain pair of its peaks is required to act for catalyzing the multi - electron reaction;

[0035] If a pair of peaks of a multi-electron redox mediator corresponds to a multi-electron redox reaction and satisfies the number of electrons to be transferred required for the catalytic reaction, then as long as the redox equilibrium potential of this pair of peaks is higher than the thermodynamic potential of the reaction, the requirement of simultaneous transfer of multiple electrons during catalysis can be achieved.

[0036] In the present invention, the multi-electron redox mediator accelerates the kinetic rate of the reaction process by means of simultaneous transfer or transmission of multiple electrons.

[0037] The present invention provides an application of a multi-electron redox mediator in a metal-air battery or a lithium-sulfur battery; the multi-electron redox mediator is selected from one or more of heteropolyacids, organic small molecule sulfides, and organic small molecule nitrides. The present invention proposes a strategy for multi-electron transfer. By using a class of redox mediators capable of simultaneously transferring multiple electrons, the reaction rate of this process can be effectively increased, and the possibility of side reactions can be reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 For Example 1 of the present invention, the redox potential diagram of Na 3 PW 12 O 40 ;

[0039] Figure 2 For Example 1 of the present invention, the stability of the reduced state RM 2- ;

[0040] Figure 3 For Example 1 of the present invention, the gas production experiment of the reaction between Na 2 O 2 and RM 0 ;

[0041] Figure 4 For Example 1 of the present invention, the apparent rate constant of the multi-electron redox mediator oxidizing Na 2 O 2 ;

[0042] Figure 5 For Example 1 of the present invention, the charge-discharge cycle stability of the sodium-oxygen battery;

[0043] Figure 6 For Example 2 of the present invention, the redox potential of 2-mercaptobenzoxazole;

[0044] Figure 7 For Example 2 of the present invention, the apparent rate constant of the multi-electron redox mediator oxidizing Li 2 O 2 ;

[0045] Figure 8 For Example 2 of the present invention, the charge-discharge cycle stability of the lithium-oxygen battery. Detailed implementation mode

[0046] To further illustrate the present invention, the following describes in detail the application of the multi-electron redox mediator provided by the present invention in a metal-air battery or a lithium-sulfur battery in conjunction with embodiments, but they should not be construed as limiting the protection scope of the present invention.

[0047] Example 1

[0048] 1), Sodium phosphotungstate (Na 3 PW 12 O 40 ) as a redox mediator for catalyzing the decomposition of Na 2 O 2 has a suitable redox potential

[0049] Sodium-ion exchanged phosphotungstic acid retains the characteristic of having multiple pairs of redox peaks of polyacid. Figure 1 For the redox potential of Na 3 PW 12 O 40 in the solvent dimethyl sulfoxide (DMSO). The test uses a three-electrode system. The working electrode is a glassy carbon electrode with a diameter of 3 mm, the reference electrode is an Ag / AgCl electrode (the internal filling electrolyte is 1M NaOTf DMSO), and the counter electrode is a carbon rod. Cyclic voltammetry tests are carried out in a 1M NaOTf 3mM Na 3 PW 12 O 40 DMSO solution. By changing the electrolyte system and electrolyte components, the potential of phosphotungstic acid is adjusted to obtain a suitable redox potential as shown in Figure 1 . In the figure, sodium phosphotungstate has multiple pairs of redox peaks. The equilibrium potential (the average of the oxidation peak potential and the reduction peak potential) of the electrode pair (A) is higher than the decomposition potential of Na 2 O 2 . The equilibrium potential of the electrode pair (B) is lower than the decomposition potential of Na 2 O 2 . Therefore, the electrode pair (B) cannot be used alone to oxidize Na 2 O 2 . However, the average potential of the electrode pair (A) and the electrode pair (B) is higher than the decomposition potential of Na 2 O 2 . Therefore, we plan to use the electrode pair (A) and the electrode pair (B) together to oxidize Na 2 O 2 is thermodynamically feasible.

[0050] Since Na 3 PW 12 O 40The highest oxidation state is 0, denoted as RM 0 , the valence state changes corresponding to redox couple (A) and redox couple (B) are as Figure 1 shown, and the reduced states are respectively denoted as RM - , RM 2- , RM 3- .

[0051] 2), The reduced state RM 2- of sodium phosphotungstate has high stability

[0052] The test adopted a three-electrode system. The working electrode was a 3-mm diameter glassy carbon electrode, the reference electrode was an Ag / AgCl electrode (with 1M NaOTf DMSO as the internal electrolyte), and the counter electrode was a carbon rod. Cyclic voltammetry tests were carried out in a 1M NaOTf 10mMNa 3 PW 12 O 40 DMSO solution. As Figure 2 shown, the black line is the cyclic voltammogram of freshly prepared RM 2- , and the red line is the cyclic voltammogram of RM 2- after standing for two weeks. After standing, the state of RM 2- in the solution did not change, and no extra peaks appeared on the cyclic voltammogram, proving that the reduced state RM 2- itself has high stability, avoiding the disproportionation and decomposition side reactions of RM 2 -itself during the reaction.

[0053] 3), The gas production of the reaction between RM 0 and Na 2 O 2 conforms to a two-electron process

[0054] Take 1 ml of electrolyte (1M NaOTf 3mM RM 0 DMSO) and react it with 50 mg of Na 2 O 2 . Analyze the produced gas using a mass spectrometer. As Figure 3 shown, Figure 3 a in it is the amount of gas produced per minute during the reaction, Figure 3 b in it is the total amount of gas obtained by integrating the curve in a. Among them, the reactant Na 2 O 2 is in excess, the amount of the reactant RM 0 is about 2.1*10 -6 mol, and about 1.02*10 -6 mol of oxygen is produced. The amount of the produced oxygen is about twice that of the reactant RM 0 , and the gas production conforms to RM0 and Na 2 O 2 The assertion that the reaction is a two - electron process.

[0055] 4), Sodium phosphotungstate as a redox mediator for catalyzing the decomposition of Na 2 O 2 can achieve fast reaction kinetics

[0056] The apparent rate constant kapp between the redox mediator Na 3 PW 12 O 40 and the reactant Na 2 O 2 was obtained by electrochemical scanning probe microscopy (SECM). -Na2O2 Na 3 PW 12 O 40 has a relatively high apparent rate constant at a lower redox potential (see Figure 4 ), which can achieve the rapid decomposition of Na 2 O 2 at a lower potential, reduce the charging overpotential, reduce or avoid the existence of unstable intermediate states, and reduce the occurrence of side reactions.

[0057] 5), Sodium phosphotungstate as a redox mediator can improve the cycle stability of sodium - oxygen batteries

[0058] The decomposition kinetics of the discharge product Na 2 O 2 is slow, resulting in a low Coulombic efficiency and a large overpotential during the cycling of sodium - oxygen batteries, and thus showing poor cycle stability. As Figure 5 shown, applying Na 3 PW 12 O 40 on the positive electrode side of a lithium - oxygen battery, the charge - discharge cycle test of the sodium - oxygen battery was carried out at a current density of 0.1 mA cm -2 in the voltage range of 1.8 V - 3 V. Without adding the redox mediator Na 3 PW 12 O 40 on the positive electrode side, the charging capacity of the sodium - oxygen battery decays severely during the charge - discharge process. After 25 cycles, the charging capacity is only 0.035 mAh cm -2 , and the voltage polarization increases significantly (see Figure 5 a); after adding Na 3 PW 12 O 40 , the charging capacity of the sodium - oxygen battery is still 0.28 mAh cm after 75 stable cycles -2, the capacity retention rate is 87%, and there is almost no change in voltage polarization during charge and discharge, and the battery exhibits improved cycle stability (see Figure 5 b) in

[0059] Example 2: Taking a lithium-oxygen battery as an example, if the discharge product Li 2 O 2 decomposition requires the loss of two electrons:

[0060] 1), 2-Mercaptobenzoxazole (hereinafter referred to as MBO) as a redox mediator for catalyzing Li 2 O 2 decomposition has a suitable redox potential

[0061] As Figure 6 shown, in the solvent diethylene glycol dimethyl ether (DEGDME), the equilibrium potential of MBO is 3.6 V (vs. Li + / Li), and this potential reaches the thermodynamic theoretical decomposition potential of Li 2 O 2 . The test was carried out using a three-electrode system, and cyclic voltammetry tests were performed in a DEGDME 1 M LiTFSI 10 mM MBO solution at a scanning rate of 50 mV / s. By coulometric titration, the number of electrons transferred corresponding to the redox peak of MBO in this solution was found to be two electrons, which meets the number of electrons required to be transferred during the catalysis of Li 2 O 2 decomposition.

[0062] 2), 2-Mercaptobenzoxazole as a redox mediator for catalyzing Li 2 O 2 decomposition can achieve fast reaction kinetics

[0063] By calculating the apparent rate constant of the multi-electron redox mediator oxidizing Li 2 O 2 through SECM experiments, the apparent rate constant of MBO oxidizing Li 2 O 2 is significantly higher than that of the common single-electron redox mediator oxidizing Li 2 O 2 (see Figure 7 ), and the redox potential of MBO is moderate, lower than that of single-electron redox mediators such as TEMPO. MBO has a high apparent rate constant kapp-Li 2 O 2 at a redox potential of about 3.6 V, and can achieve Li 2 O 2The rapid decomposition can theoretically reduce the charging overpotential, shorten the existence time of unstable intermediate states, and reduce the occurrence of side reactions.

[0064] 3) 2-Mercaptobenzoxazole, as a redox mediator on the positive electrode side, can improve the cycle stability of lithium-oxygen batteries

[0065] Applying MBO on the positive electrode side of a lithium-oxygen battery, within the voltage range of 2.5 V to 4 V, charge-discharge cycling tests were carried out at a current density of 0.1 mA cm -2 The positive and negative electrodes were separated by a solid electrolyte. The lithium-oxygen battery without MBO had poor cycle stability, especially during the charging process. The discharge product Li 2 O 2 was difficult to decompose, the charging capacity decayed severely, and the charging capacity was only 0.013 mAh cm -2 after 30 charge-discharge cycles; in contrast, the lithium-oxygen battery with MBO showed improved cycle stability, with a small charge-discharge voltage gap of only 0.78 V. After 95 stable charge-discharge cycles, the lithium-oxygen battery with MBO still had a charging capacity of 0.4 mAh cm -2 . Figure 8 It shows that adding MBO as a redox mediator for oxidizing Li 2 O 2 can help Li 2 O 2 decompose rapidly, reduce the decay of the capacity, and improve the cycle stability.

[0066] As can be seen from the above embodiments, the present invention provides the application of multi-electron redox mediators in metal-air batteries or lithium-sulfur batteries; the multi-electron redox mediators are selected from one or more of heteropolyacids, organic small molecule sulfides, and organic small molecule nitrides. The strategy of multi-electron transfer proposed in the present invention uses a class of redox mediators that can transfer multiple electrons simultaneously, which can effectively increase the reaction rate of this process and reduce the possibility of side reactions occurring.

[0067] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. Application of a class of multi-electron redox mediators in metal-air batteries or lithium-sulfur batteries; The multi-electron redox mediator is selected from one or more of heteropolyacids, organic small molecule sulfides and organic small molecule nitrides.

2. The use according to claim 1, characterized in that: The multi-electron redox mediator is selected from H3PW 12 O 40 、Li3PW 12 O 40 、Na3PW 12 O 40 、H4SiW 12 O 40 、Li4SiW 12 O 40 、Na4SiW 12 O 40 、H6P2W 18 O 62 、H3BW 12 O 40 、Na3BW 12 O 40 、H5PMo 12 O 41 、Na3PMo 12 O 40 、H6SiMo 12 O 41 、Na4SiMo 12 O 40 、H6ZnW 12 O 40 、H3PW 11 MoO 40 , 1,5-Dithiocane, hexahydropyrazolo[1,2-a]pyrazole and 2-mercaptobenzoxazole.

3. The use according to claim 1, characterized in that: The multi-electron redox mediator is specifically used in the redox reaction of products that need to gain or lose multiple electrons during the charge and discharge process of metal-air batteries or lithium-sulfur batteries; The product refers to the charging product, discharging product or intermediate product of metal-air battery or lithium-sulfur battery, and the products include but are not limited to Li2O2, Na2O2, Li2CO3, Na2CO3, K2CO3, ZnO x , MgO2 and Mg2O2 or more.

4. The use according to claim 1, characterized in that: The metal-air battery is selected from Li-O2 / CO2 battery, Na-O2 / CO2 battery, K-O2 / CO2 battery, Mg-O2 / CO2 battery or Zn-O2 / CO2 battery.

5. The use according to claim 1, characterized in that: If a pair of redox peaks of the multi-electron redox mediator corresponds to the gain or loss of one electron, it has multiple pairs of reversible redox peaks, and multiple pairs of peaks are required to work together during catalysis; ox2 / RM re2 The potential of the electrode pair is lower than the catalytic potential, RM ox1 / RM re1 The potential of the electrode pair is higher than the catalytic potential, RM ox1 / RM re1 and RM ox2 / RM ox2 The difference in peak potential between them is greater than 300mV, and RM ox1 / RM re1 and RM ox2 / RM ox2 The average potential of the two electrodes is higher than the catalytic potential, RM ox1 / RM re1 and RM ox2 / RM ox2 Two electron pairs can be used for simultaneous catalysis.

6. The use according to claim 1, characterized in that: If a pair of redox peaks of the multi-electron redox mediator corresponds to the gain or loss of multiple electrons, it has one or more pairs of reversible redox peaks, and only one pair of peaks is needed to catalyze the multi-electron reaction; If a pair of peaks of a multi-electron redox mediator corresponds to a redox reaction of multiple electrons and meets the number of electrons required to be transferred during the catalytic reaction, the requirement of simultaneous transfer of multiple electrons during catalysis can be achieved as long as the redox equilibrium potential of the pair of peaks is higher than the thermodynamic potential of the reaction.

7. The use according to claim 1, characterized in that: The multi-electron redox mediator accelerates the kinetic rate of the reaction process by transferring or transmitting multiple electrons simultaneously.

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