WO2021217684A1

WO2021217684A1 - High concentration solution, application thereof, and preparation method

Info

Publication number: WO2021217684A1
Application number: PCT/CN2020/088550
Authority: WO
Inventors: 高超; 蔡盛赢
Original assignee: 浙江大学
Priority date: 2020-05-01
Filing date: 2020-05-01
Publication date: 2021-11-04

Abstract

A high concentration solution, applicable to an electrolyte. The solution includes at least zinc ions, bromide ions, and chloride ions, and forms a novel mixture of dissolved salts by means of co-dissolution of multiple salts, thereby greatly increasing the concentration of the solution. Further provided is an ultra-high concentration solution further including zinc acetate. Capping functions of the zinc acetate prevent salts from continuing to aggregate and grow, thus forming high-molecular-weight inorganic oligomers that stably exist in the solution. The concentration thereof can reach 45 mol kg^-1. A high concentration electrolyte effectively inhibits the electrolysis of water, and reduces the oxidation potential of bromide ions, so as to further promote the intercalation of bromine in a positive electrode material for batteries, and greatly increase the specific capacity of positive electrodes (~638 mAh g^-1). In addition, an increase in the concentration of active ions in the electrolyte can reduce the amount of electrolyte used, thereby improving the energy density of non-rocking chair batteries.

Description

A high-concentration solution and its application and preparation method

Technical field

The present invention relates to a high-concentration solution and its application in an aqueous dual-ion battery. The concentration of the solution can reach ^{more than 30 mol kg -1} and can greatly promote the specific capacity of the battery.

Background technique

Due to the rapid development of renewable energy technologies in recent years, the demand for grid-level energy storage is increasing. Although lithium-ion batteries dominate the electrochemical energy storage market including applications such as portable electronic devices and electric vehicles due to their high energy density, they are used in consideration of the natural abundance and uneven distribution of natural lithium resources. Gigawatt-level grid energy storage is unreasonable, and it will also bring higher cost issues. Water-based electrochemical energy storage devices are not only cheap, but also extremely safe, providing a solution for large-scale storage of renewable energy.

However, the current energy density of lithium-free batteries is generally much lower than that of lithium-ion batteries, and this phenomenon is particularly obvious in water systems. Although the use of "water-in-salt" electrolytes can effectively increase the energy density of water-based electrochemical energy storage devices, most of these electrolytes are based on expensive organic lithium salts. Whether it is possible to use high natural abundance of non-lithium elements to prepare water-based electrolytes with ultra-high concentration, and to develop low-cost, high-energy-density lithium-free water-based rechargeable battery technology is an urgent problem to be solved, but it is also facing huge challenge.

Summary of the invention

An object of the present invention is to provide a high-concentration aqueous solution suitable for use in electrolyte. The solution contains at least zinc ions, bromide ions and chloride ions, wherein the concentration of zinc ions is above ^{30 mol kg -1.} Specifically, the solution includes at least two solutes of zinc chloride and zinc bromide dissolved in water. The solution composed of zinc bromide and zinc chloride through the "multi-salt co-dissolution" function, that is, the high-concentration salt solution serves as a new The solvent dissolves another unhydrated salt and forms a new "dissolved salt mixture", which greatly increases the concentration of the solution. The concentration of the zinc ion can reach ^{more than 30 mol kg -1} , and the concentration mol kg ^-1 refers to It is the amount of solute dissolved per kilogram of water.

In the above solution, zinc bromide and zinc chloride can be in any ratio. Usually, the concentration of bromide ion is in the range of 15-90 mol kg ^-1 , and the concentration of chloride ion is in the range of 15-90 mol kg ^-1 . In some embodiments, the concentration ratio of bromide ion to chloride ion is 1:3, which can effectively increase the zinc ion concentration in the dual system solution.

Another object of the present invention is to provide an ultra-high concentration solution, which also contains acetate ions, specifically, contains solute zinc acetate, and further through the "end-capping" effect of zinc acetate, the salt cannot continue to aggregate and grow, and then The inorganic oligomers with larger molecular weights stably exist in the solution. In order to maintain the liquid state, the concentration of zinc ions should be below 50 mol kg ^-1 and the concentration of acetic acid high ions should be below 10 mol kg ^-1 .

Experiments have shown that the concentration ratio of bromide ion to chloride ion is 1:3, which can effectively increase the zinc ion concentration in the three-system solution.

Another object of the present invention is the application of the above solution in the battery electrolyte. The high-concentration electrolyte not only effectively inhibits the electrolysis of water, but also reduces the oxidation potential of bromide ions, and promotes the insertion of bromine in the battery cathode material. Layer, which greatly improves the specific capacity of the positive electrode (~638mAh g ^-1 ). In addition, the increase in the concentration of active ions in the electrolyte can reduce the amount of electrolyte, which is beneficial to the improvement of the energy density of non-rocking chair batteries.

Another object of the present invention is the application of the above-mentioned solution in the battery electrolyte. The high-concentration electrolyte realizes the breakthrough of the I-stage intercalation of bromine from the existing single-layer graphene to the macroscopic material (in the prior art, The I-order intercalation of bromine is only observed on the single-layer graphene, which can only achieve the II-order intercalation in the macrographite materials, and the single-layer graphene cannot be used industrially), which gives the battery cathode a high specific capacity.

In some embodiments, the positive electrode of the battery uses carbon materials, especially carbon nanotubes, natural graphite, expanded graphite, graphene, graphene assemblies, and other carbon materials with a graphite lattice structure. The examples prove that no matter what kind of cathode material, the I-stage intercalation of bromide ions can be realized.

In some embodiments, the negative electrode of the battery adopts a zinc negative electrode, including metallic zinc, zinc-containing alloy, or other inert conductive substrates that can support zinc.

Another object of the present invention is the preparation method of the above-mentioned high-concentration solution, which is specifically as follows:

Add ZnCl ₂ , ZnBr ₂ and deionized water to the container and mix uniformly, control the concentration of zinc ions below 35 mol kg ^{-1 to} obtain two solute solutions of zinc chloride and zinc bromide. Among them, during the mixing process, appropriate heating (40-100°C, 2-24 hours) and stirring can be used to promote dissolution.

Add ZnCl ₂ , ZnBr ₂ and deionized water to the container, mix and add anhydrous Zn(OAc) ₂ , stir well and heat at 100～120℃ for more than 2 hours to obtain zinc chloride, zinc bromide and acetic acid A solution of three solutes of zinc. Among them, during _{the mixing process of ZnCl 2} and ZnBr ₂ , heating (40-100° C., 2-24 hours) and stirring may be appropriate to promote dissolution. Experiments have proved that in the three-system solution, the concentration of zinc ions is below 50mol kg ^-1 and the concentration of acetic acid high ions is below 10 mol kg ^-1 , which can maintain the liquid state and be used as a liquid electrolyte. Therefore, by adding Zn(OAc) ₂ , a solution with a zinc ion concentration of 35-50 mol kg ^-1 can be obtained.

The beneficial effect of the present invention is that the hybrid solute system of the present invention has properties that any single component does not possess, and its solubility in water is much higher than that of a single zinc chloride, zinc bromide or zinc acetate. The high concentration of the two-component solution composed of zinc bromide and zinc chloride is derived from the "multi-salt co-dissolution" effect, that is, the highly concentrated salt solution acts as a new solvent to dissolve another unhydrated salt and form a new "Mixed Salt Mixture". The formation of ultra-concentrated zinc chloride-zinc bromide-zinc acetate-water "copolymer" is based on another mechanism: when a large amount of electrolyte salt dissolved in water at high temperature is cooled to room temperature, due to the "end capping of acetate ion""It cannot continue to aggregate and grow, and then becomes inorganic oligomers with larger molecular weights that stably exist in the electrolyte solution instead of forming crystal grains. The formation mechanism of the latter is completely different from the known "salt-in-water" and "water-in-salt" electrolytes. Its discovery breaks through the electrolyte concentration limit that can be achieved by conventional dissolution methods and can be regarded as the "third type of aqueous electrolyte"". The above-mentioned various ultra-concentrated electrolytes not only effectively inhibit the electrolysis of water, but also reduce the oxidation potential of bromide ions, further promote the intercalation of bromine in the cathode material, and realize the first-order intercalation of bromine in the cathode material. The discharge specific capacity (~638mAh g ^-1 ) and coulomb efficiency of the positive electrode are greatly improved. Thanks to the extremely high ion concentration in the electrolyte, the assembled dual-ion battery has high specific energy density, simple preparation and low cost, and has important application prospects in the large-scale stationary grid-level electrochemical energy storage market .

In the present invention, unless otherwise specified, the concentration is expressed by the ratio of solute to solvent. 60-100 mol kg ^-1 means that the amount of solute dissolved per kilogram of water is 60-100 mol.

Description of the drawings

Figure 1 is the mass spectrum of the salt-water "copolymer" of Example 1. It can be seen from the figure that there are a large number of molecular ion peaks in the solution with a relative molecular mass greater than any component in the system, which means that the salt-water "copolymer" The presence of oligomers.

Figure 2 is a photograph of the salt-water "copolymer" electrolyte and its precursors of Example 1. It can be seen from the figure that the molar ratio of zinc chloride, zinc bromide and zinc acetate is 33.75:11.25:1 with a small amount of Water is mixed and heated in a specific stoichiometric ratio to form a colorless and transparent salt-water "copolymer" that is stable at room temperature.

Figure 3 is a constant current charge and discharge diagram of the battery of Example 1. It can be seen from the figure that its specific charge and discharge capacity is greater than 650 mAh ^{g -1} ^{at a current density of 1A g -1} , and the coulombic efficiency is greater than 95%.

Figure 4 is a cycle diagram of the battery of Example 1. It can be seen from the figure that its capacity retention rate is greater than 98% after being cycled for 200 cycles at a current density of ^{1A g -1.}

Figure 5 is a photo of the electrolyte and its precursor of Example 2. It can be seen from the figure that the molar ratio of zinc chloride, zinc bromide and zinc acetate is 28:14:1 mixed with a small amount of water in a specific stoichiometric ratio After heating, a colorless and transparent salt-water "copolymer" that is stable at room temperature can be formed.

Figure 6 is a photograph of the water-phase eutectic salt electrolyte and its precursors of Example 3. It can be seen from the figure that the molar ratio of zinc chloride, zinc bromide and a small amount of water are mixed in a specific stoichiometric ratio of 3:1 After heating, a colorless, transparent, water-phase eutectic salt that is stable at room temperature can be formed.

Figure 7 is a photograph of the salt-water "copolymer" electrolyte of Example 4 and its precursor. It can be seen from the figure that the molar ratio of zinc chloride, zinc bromide and zinc acetate is 22.5:22.5:1 with a small amount of zinc chloride, zinc bromide and zinc acetate. Water is mixed and heated in a specific stoichiometric ratio to form a colorless and transparent salt-water "copolymer" that is stable at room temperature.

FIG 8 is a positive electrode material Example 1 Macro - positive graphene film in the salt - the electrochemical water "copolymer" in situ Raman spectra, it will be seen, the process of charging the graphene G band from 1579 cm ^{- 1} becomes 1625cm ^-1 , which represents the formation of I-order Br intercalated graphite compound; the G peak changes back to 1579cm ^-1 during the discharge process, which proves that the charge-discharge process is completely reversible.

Figure 9 is the electrochemical in-situ XRD spectra of the macroscopic cathode material-graphene film cathode in the salt-water "copolymer" of Example 1. It can be seen that the (002) surface peak position of graphene varies from The original 26.5° changed to 25.3°, representing the formation of I-order Br intercalated graphite compound; during the discharge process, it changed back to 26.3°, which proved that the charge and discharge process is completely reversible.

Detailed ways

The present invention will be described in detail below through examples. This example is only used to further illustrate the present invention and cannot be construed as a limitation on the protection scope of the present invention. Those skilled in the art make some non-essential changes based on the content of the above-mentioned invention. And adjustments belong to the protection scope of the present invention.

Example 1:

(1) 0.3375 mol of zinc chloride and 0.1125 mol of zinc bromide were sequentially added to 10 g of deionized water, and then heated at 60° C. for 2 hours to obtain a white turbid liquid composed of _{ZnCl 2} , ZnBr _{2 and water.} Add 0.01 mol zinc acetate, continue heating in an oven at 120°C for 2 hours, and cool naturally to obtain a zinc chloride-zinc bromide-zinc acetate-water "copolymer" solution ^{with a total concentration of 46 mol kg -1, in which bromide and chloride ions} The molar ratio of is 1:3, see Figure 2.

(2) The zinc negative electrode, the glass fiber separator impregnated with the electrolyte in (1), and the graphene film positive electrode are sequentially stacked in a battery mold for pressure and heat sealing.

The battery cell obtained through the above steps has a specific discharge capacity of 607.5mAh g ^-1 (calculated based on the positive active material, see Figure 3) ^{at a current density of 1A g -1, and has excellent cycle stability.} After 200 charge-discharge cycles, the capacity retention rate is greater than 98% (see Figure 4).

The battery charge-discharge process situ Raman and X- ray diffraction pattern of the graphene films are a positive electrode 8 and 9, it will be seen, the process of charging the graphene G peak position changes from 1579cm ^-1 1625cm ^{- 1} , represents the formation of I-order Br intercalated graphite compound; the G peak changes back to 1579 cm ^-1 during the discharge process, which proves that the charge and discharge process is completely reversible. According to the electrochemical in-situ XRD spectrogram, it can be seen that during the charging process, the position of the (002) plane peak of graphene has changed from 26.5° to 25.3°, which represents the formation of I-order Br intercalated graphite compound; during the discharge process It changed back to 26.3° again, proving that the charging and discharging process is completely reversible.

According to the Nernst equation:

E=E ^o -RT/zF·ln(α[R]/α[O])

The redox potential of anions is:

E _anion = E ^o -RT/zF·ln(c[anion]/γ[anion])

The increase of the total ion concentration in the electrolyte reduces the oxidation potential of bromide ions, increases the reduction potential of zinc ions, further avoids the side reaction of water molecules from oxygen evolution and hydrogen evolution, and helps to inhibit the "shuttle effect" of halogens and promote Its intercalation in macroscopic cathode materials.

Combining electrochemical in-situ Raman and X-ray diffraction techniques (Figures 8-9), it is proved that bromine has achieved I-stage intercalation in the graphite anode during charging, thus giving the battery cathode a high specific capacity.

Example 2:

(1) 0.28 mol of zinc chloride and 0.14 mol of zinc bromide were sequentially added to 10 g of deionized water, and then heated at 40° C. for 24 hours to obtain a white turbid liquid composed of _{ZnCl 2} , ZnBr _{2 and water.} Add 0.01mol zinc acetate, continue heating in an oven at 100°C for 10 hours, and cool naturally to obtain a zinc chloride-zinc bromide-zinc acetate-water "copolymer" solution ^{with a total concentration of 43mol kg -1, in which bromide and chloride ions} The molar ratio of is 1:2, see Figure 5.

(2) The graphene fiber non-woven fabric negative electrode substrate, the glass fiber separator impregnated with the electrolyte in (1), and the expanded graphite positive electrode are sequentially stacked in a battery mold for pressure and heat sealing.

The battery cell obtained through the above steps has a specific discharge capacity of 632mAh g ^-1 (calculated based on the active material of the positive electrode) ^{at a current density of 1A g -1, and has good cycle stability.} The capacity retention rate after discharge cycles is greater than 90%.

In-situ Raman test and XRD test were performed on the expanded graphite anode during the charging and discharging process of the battery, and the analysis was carried out according to Example 1. The specific capacity of the positive electrode of the battery is high.

Example 3:

(1) 0.245 mol of zinc chloride and 0.245 mol of zinc bromide were sequentially added to 10 g of deionized water, and then heated at 100° C. for 2 hours to obtain a white turbid liquid composed of _{ZnCl 2} , ZnBr _{2 and water.} Add 0.05mol zinc acetate, continue heating in an oven at 120℃ for 72h, and cool naturally to obtain a total concentration of 50mol kg ^-1 zinc chloride-zinc bromide-zinc acetate-water "copolymer" solution, in which bromide and chloride ions The molar ratio of is 1:1, see Figure 6.

(2) The carbon cloth negative electrode substrate, the glass fiber separator impregnated with the electrolyte in (1), and the intermediate phase microsphere graphite positive electrode are sequentially stacked in a battery mold for pressure and heat sealing.

The battery cell obtained through the above steps has a specific discharge capacity of 637 mAh g ^-1 (calculated based on the positive electrode active material) ^{at a current density of 1A g -1, and has good cycle stability.} The capacity retention rate after discharge cycles is greater than 85%.

The in-situ Raman test and XRD test were performed on the middle phase microsphere graphite anode during the charging and discharging process of the battery, and the analysis was carried out according to Example 1. -Stage intercalation, which gives the battery a high specific capacity of the positive electrode.

Example 4:

(1) Add 0.2625mol of zinc chloride and 0.0875mol of zinc bromide to 10g of deionized water in turn, heat for 2h at 60°C, and cool naturally to obtain zinc chloride-bromide ^{with a total concentration of 35mol kg -1} The zinc-water phase co-dissolving salt solution, in which the molar ratio of bromide ion to chloride ion is 1:3, as shown in Figure 7.

(2) The zinc negative electrode, the glass fiber separator impregnated with the electrolyte in (1), and the artificial graphite positive electrode are sequentially stacked in a battery mold for pressure and heat sealing.

The battery cell obtained through the above steps has a specific discharge capacity of 382 mAh g ^-1 (calculated based on the positive active material) ^{at a current density of 1A g -1, and has good cycle stability.} The capacity retention rate after the discharge cycle is greater than 80%.

Example 5:

(1) Add 0.225mol of zinc chloride and 0.075mol of zinc bromide to 10g of deionized water, and heat for 2h at 60℃ to obtain a zinc chloride-zinc bromide water phase with a ^{total concentration of 30mol kg -1} Co-dissolving salt solution, wherein the molar ratio of bromide ion to chloride ion is 1:3.

(2) The zinc negative electrode, the glass fiber separator impregnated with the electrolyte in (1), and the pitch-based graphite positive electrode are sequentially stacked in a battery mold for pressure and heat sealing.

The battery cell obtained through the above steps has a specific discharge capacity of about 350mAh g ^-1 ^{(calculated based on the positive electrode active material) at a current density of 1A g -1} , and the capacity retention rate is greater than 70 after 200 charge-discharge cycles %.

Claims

A high-concentration solution, characterized in that the solution is an aqueous solution with at least zinc ions, bromide ions and chloride ions, wherein the concentration of zinc ions is above 30 mol kg -1 , and the concentration refers to the dissolved per kilogram of water The amount of solute substance.
The solution according to claim 1, wherein the concentration of bromide ions is in the range of 15 to 90 mol kg -1 , and the concentration of chloride ions is in the range of 15 to 90 mol kg -1 .
The solution according to claim 1, wherein the concentration ratio of bromide ion to chloride ion is 1:3.
The solution according to claim 1, further comprising acetate ions, and the concentration of zinc ions is below 50 mol kg -1 and the concentration of acetate ions is below 10 mol kg -1 .
The solution according to claim 4, wherein the concentration ratio of bromide ion to chloride ion is 1:3.
The use of the solution according to any one of claims 1 to 5 as a battery electrolyte.
The application according to claim 6, wherein the concentration of zinc ions in the electrolyte is 46 mol kg -1 , and the concentration ratio of bromide ions, chloride ions, and acetate ions is 11.25:33.75:1.
The method for preparing a solution according to claim 4, characterized in that the method at least comprises: adding ZnCl 2 , ZnBr 2 and deionized water into the container, after mixing, adding anhydrous Zn(OAc) 2 and stirring uniformly Afterwards, it is heated at 100～120℃ for more than 2h.
The production method according to claim 8, characterized in that the bromide ion concentration in the range of 15 ~ 90mol kg -1, chloride ion concentration range is 15 ~ 90mol kg -1, the concentration of zinc ions in 50mol kg - Below 1 , the concentration of acetate ion is below 10mol kg -1 .
The preparation method according to claim 8, wherein the concentration ratio of bromide ion to chloride ion is 1:3.