CN116162974A

CN116162974A - Preparation method of nano porous zinc and application of nano porous zinc in zinc battery

Info

Publication number: CN116162974A
Application number: CN202111506478.8A
Authority: CN
Inventors: 陈擎; 李良昱; 曾雍泽
Original assignee: Hong Kong University of Science and Technology HKUST
Current assignee: Hong Kong University of Science and Technology HKUST
Priority date: 2021-11-24
Filing date: 2021-12-10
Publication date: 2023-05-26

Abstract

The invention belongs to the field of electrochemical cells, and particularly relates to a method for preparing integrated nano porous zinc in an electrochemical cell, the integrated nano porous zinc prepared by the method, and an electrode and a cell prepared by the integrated nano porous zinc. The method is simple and scalable, the nanoporous zinc thus obtained has bi-continuous pores and zinc fibers, and the respective sizes of the pores and zinc fibers are uniform. Batteries comprising electrodes made from the nanoporous zinc have longer rechargeability or cycle life.

Description

Preparation method of nano porous zinc and application of nano porous zinc in zinc battery

Technical Field

The invention belongs to the field of electrochemical cells, and particularly relates to a self-supporting porous zinc electrode prepared from a powder compact made of a zinc compound by an electrochemical method.

Background

Zinc metal has been of great interest because of its potential to be undermined in rechargeable batteries. One of the most abundant metals on earth, zinc has a theoretical capacity of 3694Ah/L. The slow hydrogen evolution reaction of the zinc surface enables it to work in aqueous solutions, especially alkaline electrolytes, eliminating any fire hazard and reducing costs. It can be used as a substitute for the hydride of expensive metals to fully utilize the mature alkaline nickel battery. It can also be paired with an air cathode to provide practical energy densities of up to 400 Wh/kg. However, under practical conditions, neither cell has achieved a sufficiently long cycle life, i.e., neither cell has a high area capacity, a high depth of discharge, and a low capacity electrolyte. Most cells fail within 150 cycles and are less competitive than ever-improving lithium ion cells.

The root of the failure is that the phase transition path between zinc and zinc oxide during the cell reaction is not ideal. In alkaline electrolytes, the most common zinc anode starts with a bed of predominantly ZnO powder, which is partially reduced to zinc metal in the first charging step. The reduction can excite the long-range transmission of zincate ions in the electrolyte to form a large number of zinc particles which grow cycle by cycle at the cost of sacrificing the uniformity of the electrode structure, the conductivity and other properties. The dense particles, which are reaction hot spots, will be covered with a thin layer of zinc oxide, which passivates the dense particles from complete discharge, while the zinc in the rest of the electrode is attracted to these hot spots, leaving even macroscopic depletion areas, a shape change that has plagued zinc anode rechargeability for decades. The solution to this problem must achieve two goals simultaneously: uniform distribution of zinc formation and sustainable transition between zinc and zinc oxide.

U.S. patent application 20170338479A1 discloses an electrochemical cell comprising: an anode current collector; an anode in electrical contact with the anode current collector; an electrolyte; a cathode current collector; a cathode comprising silver or silver oxide in electrical contact with the anode current collector; and a separator between the anode and the cathode. The anode is manufactured by the following method: providing a mixture comprising a metallic zinc powder and a liquid phase emulsion; drying the mixture to form a sponge; annealing and/or sintering the sponge in an inert atmosphere or under vacuum at a temperature below the melting point of zinc to form an annealed and/or sintered sponge having a metallic zinc surface; and heat annealing and/or sintering the sponge in an oxidizing atmosphere at a temperature above the melting point of zinc to form an oxidized sponge comprising a zinc oxide shell on the surface of the oxidized sponge. The anode includes: a continuous network comprising metallic zinc; a continuous network of interstices that extend through the zinc network; and a metallic zinc bridge connecting the metallic zinc particle cores. The electrolyte fills the voids.

U.S. patent application US20170025677A1 provides a superbranched nanoporous zinc foam that can be used in an electrode, preferably an anode. Zinc foam is a three-dimensional network of crystalline branches including nanometers that is electrochemically active and electrically conductive. Thus, in certain preferred embodiments, the zinc foam electrode is a nanoporous zinc structure comprising primary and secondary dendrites. The superbranched nanoporous zinc foam exhibits a specific surface area of the zinc foam that is at least 2 times that of the zinc mass. Such high relative surface to volume ratio zinc foam provides better and more uniform charge distribution, better charge capacity retention over multiple charge and discharge cycles, and longer useful life of the rechargeable battery.

Patent application WO2019183083A1 provides a composition comprising a nanoporous material comprising interconnected fibers defining pores therebetween, the pores being open to the environment outside the nanoporous material, the nanoporous material optionally comprising a metal having a standard reduction potential at 0V vs SHE that is less than that of a standard hydrogen electrode (standard hydrogen electrode, SHE), and the pores being characterized as having an average cross section in the range of about 3 to 100 nm. The invention also provides a power battery, which comprises: the composition according to the invention, optionally placed in a removable cartridge; an amount of water, the power cell configured to affect contact between the composition and water; and a collector configured to collect hydrogen evolved from contact between the water and the composition.

From the above, it is known in the prior art that the preparation of porous zinc mainly comprises two types of processes, one using electroplating and one involving sintering (sintering) and calcining (sintering) steps. Both of these methods have a number of disadvantages. For example, both methods are relatively complex to operate, are not scalable, and cannot or are not easily tuned for the porosity, tortuosity, pore characteristics, etc. of the porous zinc produced. As another example, both of these methods are incapable of producing porous zinc that achieves structural uniformity at the nanoscale. Therefore, when the porous zinc obtained by these two methods is used for preparing a battery electrode, the battery thus obtained is poor in rechargeable properties (i.e., cycle life under deep discharge).

Thus, there is a need in the art to find a nanoporous zinc that can produce zinc fibers and pores of desired and consistent dimensions to further enhance the rechargeability of zinc cells.

Disclosure of Invention

Based on the above-mentioned shortcomings of the prior art, there is still a need for further research in the manufacture of raw materials for zinc electrodes and methods of preparing the same to improve the rechargeability of zinc batteries made from such electrodes, and to actually exert the advantages of zinc as an electrode.

Accordingly, in a first aspect, the present invention provides a method of preparing integrated nanoporous zinc in an electrochemical cell comprising a working electrode, a separator, an electrolyte, a reference electrode and a counter electrode, the working electrode comprising a conductive substrate and a zinc powder compact, wherein the zinc powder compact is obtained by densification of a zinc compound powder on the conductive substrate, the method comprising applying a reduction potential of-1.4 volts to-1.6 volts relative to an Ag/AgCl reference electrode to the working electrode to reduce the zinc compound, thereby forming integrated nanoporous zinc on the conductive substrate.

In a second aspect, the present invention provides an integrated nanoporous zinc having bi-continuous pores and zinc fibers, and the pores and zinc fibers each having a uniform size of 200nm to 1000 nm.

In a third aspect, the present invention provides an electrode comprising the integrated nanoporous zinc of the second aspect of the invention and a conductive substrate to which it is attached.

In a fourth aspect, the present invention provides a battery comprising an electrode according to the third aspect of the present invention as an anode.

The invention has one or more of the following technical effects:

(1) Compared with the method for preparing the porous zinc by an electroplating method or by sintering and calcining in the prior art, the invention provides a novel and extensible method for preparing the nano porous zinc, which is simple and easy to operate, and the zinc powder briquette can be prepared into the nano porous zinc by only applying a proper reduction potential to the zinc powder briquette;

(2) The method can prepare the integrated nano porous zinc with high load and high surface capacity by customizing the load of the zinc precursor;

(3) The method is implemented in an electrochemical cell, and the method can adjust the porosity, bending degree, pore characteristics and other material parameters of the obtained nano porous zinc by adjusting a plurality of parameters of the electrochemical cell such as the distance between a separator and a working electrode, the type of zinc compound, the reduction overpotential, the electrolyte formula, or a combination thereof.

Drawings

The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification. It will be apparent that the figures in the following description relate only to some embodiments of the invention and do not constitute a limitation of the invention. Other embodiments may be made by those of ordinary skill in the art without undue burden from the figures.

Fig. 1 shows a schematic of the structure of an electrochemical cell for carrying out the method of the invention, the cell comprising an anode (working electrode) 1, a separator 2 on the anode, a reference electrode 3 and a cathode (counter electrode) 4;

FIG. 2 shows a Scanning Electron Microscope (SEM) photograph of a nanoporous zinc prepared from zinc oxide as a precursor, which shows that the nanoporous zinc has a distinct bicontinuous structure of pores and fibers;

FIG. 3 shows an SEM image of a nanoporous zinc prepared from zinc carbonate as a precursor, which shows that the nanoporous zinc has particles that are significantly spherical and the particles are bicontinuous structures;

FIG. 4 shows the polarization curve of porous zinc;

FIG. 5 shows 25mA/cm ² The stability performance of the porous zinc in 40% DOD circulation under the charge and discharge conditions;

FIG. 6 shows 25mA/cm ² The stability performance of porous zinc at 60% DOD cycle under charge and discharge conditions.

Detailed Description

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. It will be apparent that the described embodiments are only some, but not all, embodiments of the invention. All other embodiments that can be obtained by a person skilled in the art based on the embodiments given in the present application are within the scope of protection of the present invention.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the subject matter described herein belongs. Before describing the present invention in detail, the following definitions are provided to better understand the present invention.

Where a range of values is provided, such as a concentration range, a percentage range, or a ratio range, it is to be understood that each intervening value, to the tenth of the unit of the lower limit, between the upper and lower limit unless the context clearly dictates otherwise, and any other stated or intervening value in that stated range, is encompassed within the subject matter. The upper and lower limits of these smaller ranges may independently be included in the smaller ranges, and such embodiments are also included in the subject matter, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the subject matter.

Throughout this application, many embodiments use the expression "comprising," including, "or" consisting essentially of … …. The expression "comprising," "including," or "consisting essentially of … …" is generally understood to mean an open-ended expression that includes not only the individual elements, components, assemblies, method steps, etc., specifically listed thereafter, but also other elements, components, assemblies, method steps. In addition, the expression "comprising," "including," or "consisting essentially of … …" is also to be understood in some instances as a closed-form expression, meaning that only the elements, components, assemblies, and method steps specifically listed thereafter are included, and no other elements, components, assemblies, and method steps are included. At this time, the expression is equivalent to the expression "consisting of … …".

For a better understanding of the present teachings and without limiting the scope of the present teachings, all numbers expressing quantities, percentages or proportions used in the specification and claims, and other numerical values, are to be understood as being modified in all instances by the term "about" unless otherwise indicated. Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained. Each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

As described above, there is a need in the art to find a nanoporous zinc that can produce zinc fibers and pores of a desired and consistent size to further enhance the rechargeability of the zinc cell.

As described above, the method of the invention is carried out in an electrochemical cell comprising a working electrode as anode, a separator, an electrolyte, a reference electrode and a counter electrode as cathode. In conventional electrochemical cells, a flexible separator is typically used to separate the anode and cathode, but in electrochemical cells in which the method of the invention is practiced, a separator is used in place of the separator, thereby effecting adjustments in the thickness and porosity of the nanoporous zinc to be formed.

The separator is also positioned between the working electrode and the counter electrode as the separator, and has the same function as the separator, namely, has the function of separating the positive and negative plates to avoid short circuit caused by direct contact of the positive and negative plates of the battery, and also has the functions of ion conduction and insulation. However, both the separator and the membrane also differ in terms of structure and function:

first, the separator is made of a rigid organic material. In the present invention, "rigidity" means a property that a material is not easily deformed by an external force. There are a wide variety of rigid organic materials that can be used for the separator, such as, but not limited to, acrylic materials, polypropylene (PP), polyethylene (PE), and/or Polyetherketone (PEEK).

Thus, in one embodiment, the separator is located between the working electrode and the counter electrode, and the separator is made of a rigid organic material. In a preferred embodiment, the rigid organic material comprises an acrylic material, polypropylene (PP), polyethylene (PE) and/or Polyetherketone (PEEK).

Second, the inventors have found that the distance between the separator and the working electrode affects the porosity of the final formed nanoporous zinc and the size of the zinc fibers. Therefore, by adjusting the distance between the separator and the working electrode, the thickness and porosity of the final porous zinc electrode and the size of the zinc fibers can be adjusted as shown in formula (1):

wherein epsilon is the porosity of the porous zinc electrode and epsilon ₀ For the initial porosity of the zinc precursor (i.e., zinc compound), α is the ratio of the distance between the separator and the working electrode to the initial thickness of the working electrode, V _{m, zinc compound} And V is equal to _m,Zn The molar volumes of zinc compound and zinc, respectively.

The distance between the separator and the working electrode is positively correlated to the porosity of the formed nanoporous zinc and the zinc fiber size. Specifically, if the distance between the separator and the working electrode is smaller, such as when α=1 (when the separator is in full contact with the working electrode), the porosity of the resulting nanoporous zinc is smaller, and the zinc fiber size is larger, i.e., the zinc fibers are thicker; conversely, if the distance between the separator and the working electrode is greater, such as when α=2, the porosity of the resulting nanoporous zinc is greater and the zinc fiber size is smaller, i.e., the zinc fibers are finer. Accordingly, it is possible to previously correlate the distance between the separator and the working electrode with the porosity and/or zinc fiber size of the nanoporous zinc to be formed and adjust the distance between the separator and the working electrode according to the desired porosity and/or zinc fiber size, thereby obtaining the desired nanoporous zinc.

Thus, in one embodiment, the method further comprises adjusting the porosity of the formed nanoporous zinc and the zinc fiber size by adjusting the distance between the separator and the working electrode. Preferably, the porosity of the formed nanoporous zinc is increased and/or the zinc fiber size is reduced by increasing the distance between the separator and the working electrode, or the porosity of the formed nanoporous zinc is reduced and/or the zinc fiber size is increased by decreasing the distance between the separator and the working electrode.

In addition to the distance between the separator and the working electrode affecting the internal structure of the nanoporous zinc, the concentration of hydroxide ions of the electrolyte and the type of electrolyte in the electrochemical cell may also affect the internal structure of the finally formed nanoporous zinc.

The inventors have found that the hydroxide ion concentration of the alkaline solution has a great influence on the internal structure of the finally formed nanoporous zinc. In the case of hydroxide ion concentration of 3M to 6M, nanoporous zinc of a desired microstructure can be obtained. When the hydroxide ion concentration is lower than 3M, the reduction speed is too slow, the reaction time is too long, and when the hydroxide ion concentration is higher than 6M, the dissolution speed of zinc oxide is too fast, which is unfavorable for the formation of nano porous zinc. Thus, in one embodiment, the electrolyte is an alkaline solution having a hydroxide ion concentration of 3M to 6M.

The inventors have also found that the desired nanoporous zinc can be obtained using alkali metal hydroxides such as sodium hydroxide (NaOH), potassium hydroxide (KOH), lithium hydroxide (LiOH), and the like. Thus, in one embodiment, the base is an alkali metal hydroxide. The alkali metal hydroxide may be sodium hydroxide, potassium hydroxide, lithium hydroxide, or the like.

In addition, other metal ions which can participate in the reduction reaction, such as bismuth, indium, calcium and the like, can be additionally contained in the electrolyte, and are simultaneously reduced and doped in the finally formed nano porous zinc structure in the process of reducing the zinc compound, so that the side reactions such as hydrogen evolution and the like in the running process of the battery are reduced.

In the present invention, a working electrode as an anode includes a conductive substrate and a zinc powder compact, wherein the zinc powder compact is obtained by densifying zinc compound powder on the conductive substrate.

First, the conductive matrix serves as a support and is in most cases made of metal due to its conductive properties. In addition, the form of the conductive substrate is not particularly limited as long as it can perform a sufficient supporting function. Thus, in a specific embodiment, the conductive substrate is a metal substrate such as a metal foam, a metal foil, or a metal mesh. In another specific embodiment, the metal may be copper, titanium, tin, or the like.

In addition, the metal substrate may be tin plated prior to densification of the zinc compound powder onto the conductive substrate. The tin plating treatment is to reduce side reactions of hydrogen evolution on the copper surface. Thus, in an optional embodiment, the metal substrate is tin plated.

Next, for zinc dust compacts, as described above, they are obtained by densifying zinc compound powders on the conductive matrix.

By zinc compound is meant any compound that contains zinc and is capable of being reduced to elemental zinc, such as zinc oxide, zinc carbonate, zinc chloride, and/or zinc acetate. The present inventors have found that nanoporous zinc having a non-identical microstructure can be obtained using different zinc compounds as reaction raw materials. Thus, in practice, a suitable zinc compound may be selected as a raw material depending on the desired microstructure of the nanoporous zinc.

In addition, the densification may be achieved in any suitable manner, but is typically achieved mechanically, such as by applying a force to zinc compound powder deposited on a conductive substrate to produce zinc compound in discrete powder form into a bulk or unitary form, and the conductive substrate and bulk zinc compound (i.e., zinc dust compact) thus obtained may be used as a working electrode in an electrochemical cell of the invention. It is understood that the pressure applied to the zinc dust compact will affect the density of the zinc dust compact and thus the microstructure of the final nanoporous zinc. Specifically, according to formula (1) If the pressure is too great, e.g. over 9 tons, ε ₀ The smaller the porosity of the obtained nano porous zinc is, the smaller the porosity of the obtained nano porous zinc is; if the pressure is too small, for example, less than 0.5 ton, the zinc compound powder cannot be completely bonded, resulting in powder fall apart. Thus, in one embodiment, the zinc dust compact is obtained by densification of the conductive substrate by applying a pressure of 0.5 to 9 tons to the zinc compound powder.

Electrode capacity can also be controlled by tailoring the loading of zinc compound powder on the conductive matrix. In this regard, specific choices may be made according to the needs of the application.

Other additives may also be incorporated into the zinc compound powder during the preparation of the zinc dust compact to impart additional properties or functions. For example, carbon black, carbon fiber, antimony oxide, and/or calcium hydroxide may be incorporated, thereby making the reduction reaction more uniform.

A reference electrode is also used in electrochemical cells, which is an electrode used as a reference when measuring various electrode potentials. In one embodiment, the reference electrode may be an Ag/AgCl electrode, a mercury/mercury oxide electrode, a standard hydrogen electrode. In a preferred embodiment, the reference electrode may be an Ag/AgCl electrode.

In the method of the invention, zinc compounds in zinc powder compacts forming part of the working electrode can be reduced to elemental zinc by applying a certain reduction potential to the working electrode, thereby forming integrated nanoporous zinc on the conductive substrate. The magnitude of the reduction potential has a significant effect on the microstructure of the finally formed nanoporous zinc: if the reduction potential is lower than-1.6V (V) vs Ag/AgCl reference electrode, the hydrogen evolution side reaction in the reduction process is serious, and uneven macropores exist in the obtained nano porous zinc; if the reduction potential is higher than-1.4 Vvs Ag/AgCl reference electrode, nano-porous zinc cannot be obtained because the zinc compound cannot be sufficiently reduced. Thus, in one embodiment, the reduction potential is any one of-1.4V to-1.6V (vs Ag/AgCl electrode), such as-1.4V, -1.45V, -1.5V, -1.55V, -1.6V (vs Ag/AgCl electrode). In addition, the specific reduction potential values given herein are relative to an Ag/AgCl reference electrode; where other reference electrodes than Ag/AgCl reference electrodes are employed, scaling may be performed on the basis of the specific reduction potential values given herein with respect to the Ag/AgCl reference electrodes to obtain specific reduction potential values with respect to the other reference electrodes, as is within the ability of those skilled in the art.

In addition, a sufficient time for applying the reduction potential is required, because the reduction reaction may be incomplete if the application time is insufficient. The reduction potential application time is generally determined in such a way that the reduction current no longer changes or in accordance with the theoretical capacity of the zinc precursor.

In summary, the method for preparing the nano-porous zinc of the present invention can change the zinc fiber size and/or the pore size of the prepared integrated nano-porous zinc by one or more means of adjusting the reduction potential, changing the composition or concentration of the electrolyte, adjusting the distance from the separator to the working electrode, etc. Therefore, the method of the invention is easy to manufacture the nano porous zinc with the required microstructure by adjusting factors such as reaction conditions, parameters and the like, and has extremely high adjustability and pertinence.

In addition, the method of the present invention does not require the addition of additional additives to control the morphological uniformity of zinc deposition, nor does it require the addition of polymeric binders to maintain the integrity of the zinc particles, nor does it require the addition of additives to inhibit dendrite formation, nor does it require additives such as emulsifiers or other metals to aid in the formation of porous zinc, which is simpler and easier to implement than prior art methods.

Also, the nanoporous zinc prepared by the inventive method has unique combination of characteristics not possessed by the prior art, namely, uniform pores and zinc fibers having a size range of about 200-1000nm, a pore structure of almost all open pores, a continuous zinc phase, and an integrated structure. Such combination properties are well suited for application of such nanoporous zinc to batteries and may enable better rechargeability.

In a second aspect of the invention, there is provided an integrated nanoporous zinc having bi-continuous pores and zinc fibers, and the pores and zinc fibers each having a uniform size of 200nm to 1000 nm.

It will be appreciated that the integrated nanoporous zinc of this aspect of the invention may be prepared by the method of the first aspect of the invention.

In the present invention, by "bicontinuous" is meant that, on the one hand, zinc fibers and zinc fibers in the nanoporous zinc are interconnected and, on the other hand, a substantial portion (e.g., at least 70%, at least 75%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or even 100%) of the pores in the nanoporous zinc are in communication with the pores, i.e., the pores are communicating pores, and the amount of closed pores contained therein is insufficient to affect the functioning of the nanoporous zinc as an electrode, and thus a substantial portion of the pores are in direct or indirect communication with the outside. In this way, when such nanoporous zinc is used to prepare an electrode and the electrode is used in a battery, the electrolyte in the battery can flood the entire nanoporous zinc structure, achieving a more uniform current distribution, reducing non-uniform concentration gradients and anodic polarization losses.

In addition, the pores and zinc fibers of the integrated nanoporous zinc are each uniform in size and comparable to the osmotic dissolution mechanism. By "the respective sizes of the pores and the zinc fibers are uniform" is meant herein that the pore sizes as well as the zinc fiber sizes are each distributed over a range of values, for example over a range of values from 200nm to 1000 nm. Within this range of values, the nanoporous zinc is on the one hand able to effectively maintain its structural stability and on the other hand also able to ensure a uniform current distribution when the nanoporous zinc is used as an electrode.

As zinc powder precursors for the synthesis of nanoporous zinc, as described in the first aspect of the invention, there are a wide variety of possible uses, such as zinc oxide, zinc carbonate, zinc chloride and/or zinc acetate. It will be appreciated that the microstructure of the nanoporous zinc obtained in the case of using different zinc powder precursors is also not the same. For example, in the case of using zinc oxide as a raw material, the obtained nanoporous zinc as a whole exhibits a relatively single primary structure composed of bicontinuous pores and zinc fibers, and the pores and the zinc fibers each have a uniform size of 200nm to 1000 nm. For another example, in the case of using zinc carbonate as a raw material, the obtained integrated nanoporous zinc exhibits different hierarchical structural morphologies as a whole; specifically, the integrated nanoporous zinc body exhibits a primary structure and a secondary structure, wherein the primary structure is in the form of spherical particles, each spherical particle comprising bi-continuous pores and fibers as the secondary structure. Thus, by selecting different zinc compound precursors as starting materials, the integrated nanoporous zinc finally formed can be made to have a microstructure specific for the precursor, i.e. to have a pore size and zinc fiber size specific for the precursor compound.

In one embodiment, the integrated nanoporous zinc is prepared from a working electrode comprising a zinc oxide powder compact, the integrated nanoporous zinc having only a primary structure comprised of pores and zinc fibers.

In another embodiment, the integrated nanoporous zinc is prepared from a working electrode comprising a pressed block of zinc carbonate powder, the integrated nanoporous zinc being in the form of spherical particles as a primary structure and the spherical particles comprising bi-continuous pores and zinc fibers as a secondary structure. In addition, it is to be understood that such integrated nanoporous zinc has pores of the order of microns in addition to the pores of the order of nanometers.

It is noted that, as described in the first aspect above, the nanoporous zinc is obtained by applying a reduction potential to a zinc powder compact obtained by densification of a zinc compound powder on a conductive substrate. Therefore, the integrated nano porous zinc obtained by the method is self-supporting, has certain mechanical strength, and can be cut at will without changing the structure. Therefore, the integrated nano porous zinc is easier to popularize and apply in industry, and can be easily applied to preparing batteries with different sizes and specifications.

In a third aspect, the present invention provides an electrode comprising the integrated nanoporous zinc of the second aspect and a conductive substrate to which it is attached.

As described above, the integrated nanoporous zinc of the invention has a higher relative surface area, which results in better and more uniform charge distribution of the electrode fabricated from the porous zinc, better charge capacity retention over multiple charge-discharge cycles, and longer useful life of the rechargeable battery.

The anode of the secondary alkaline battery has better multiplying power performance and longer cycle stability and structure retention rate while having large charge and discharge depth, and can be directly used as the anode in the rechargeable alkaline battery (such as nickel zinc, silver zinc and zinc air) so as to realize good multiplying power performance, longer cycle life and structural stability in deep discharge.

Unlike the cells existing in the prior art, the cells employing the electrode made of the nanoporous zinc of the invention as anode have very excellent rechargeability or cycle life. As an example, the inventive battery has a lifetime of 200 cycles at 40% depth of discharge (DOD). As another example, there is a lifetime of 100 cycles at 60% depth of discharge (DOD). The battery of the invention has very excellent rechargeability, on the one hand thanks to the uniform distribution of the pores and fibres of the nanoporous zinc formed, and on the other hand thanks to the sustainable transition between zinc and zinc oxide, which is manifested by a high depth of discharge, a greater number of cycles, a high coulombic efficiency, etc.

In one embodiment, the battery is a secondary battery. In a specific embodiment, the secondary battery is a nickel zinc battery, a silver zinc battery, or a zinc air battery.

For nickel zinc batteries, the primary market will be the backup power source in view of their high safety, low cost and medium energy density. For zinc-air batteries, in addition to backup power sources, unmanned aerial vehicles and electric vehicles can also be potential fields of application due to their high energy density and low cost.

Examples

In the following description, for purposes of explanation and not limitation, specific details are set forth in order to provide a thorough understanding of the present disclosure.

EXAMPLE 1 preparation of Zinc oxide-based nanoporous Zinc

The typical bicontinuous porous zinc is prepared by electrochemically reducing zinc oxide (ZnO) to Zn. First, in the presence of 0.1M SnCl ₂ And 0.4. 0.4M K ₄ P ₂ O ₇ A tin (Sn) base film was deposited on a copper foam (thickness 1.6mm, MTI Co.) by constant potential plating at-1V (vs Ag/AgCl in saturated KCl). The final tin-deposited copper foam was rinsed with deionized water, ethanol, and dried as a matrix. Then, at a hydraulic pressure of 1 ton, a certain amount of design capacity was set to 50mAh/cm ² Is a commercially available zinc oxide powder (J)&K) Directly and uniformly pressed on the tinned copper foam. Using an electrochemical cell (Ni foam as counter electrode) in house, a bias of 1.55 to 1.6 volts (vs Ag/AgCl in saturated KCl) was maintained in 3M KOH until only a weak and flat hydrogen evolution current remained, thereby converting zinc oxide to zinc. After reduction, the sample is sequentially placed in acetone and methanol for at least 1H to remove residual H ₂ O and KOH, and then dried in vacuo at room temperature. The nanoporous zinc thus prepared was cut and used directly as an electrode. The resulting morphology was characterized by Scanning Electron Microscopy (SEM).

Fig. 2 shows a scanning electron micrograph of nanoporous zinc prepared using zinc oxide as a starting material. As can be seen from the figure, the thus obtained nanoporous zinc overall exhibits a single-level structure in which zinc fibers are connected to each other with pores uniformly distributed therebetween, and almost all the pores are open pores, which means that even the pores located inside the nanoporous zinc structure are capable of communicating with the outside, and thus the electrolyte can be sufficiently filled in the nanoporous zinc structure when an electrode comprising the structure is immersed in the electrolyte. In addition, the zinc fibers and pores have a size of 200-1000nm, which indicates that the zinc fibers and pores have a uniform size, which has a very excellent effect on the performance of the battery.

Example 2: preparation of nano porous zinc based on zinc carbonate

Electrochemical method of zinc carbonate (ZnCO ₃ ) Reduced to Zn, a typical bicontinuous porous zinc is prepared. First, in the presence of 0.1M SnCl ₂ And 0.4. 0.4M K ₄ P ₂ O ₇ A tin (Sn) base film was deposited on a copper foam (thickness 1.6mm, MTI Co.) by constant potential plating at-1V (vs Ag/AgCl in saturated KCl). The final tin-deposited copper foam was rinsed with deionized water, ethanol, and dried as a matrix. Then, a certain amount of ZnCO is added under the hydraulic pressure of 1 ton ₃ The powder was pressed directly onto the tin-plated copper foam. Then, using an electrochemical cell (Ni foam as counter electrode) in house, a bias of 1.55 to 1.6 volts (vs Ag/AgCl in saturated KCl) was maintained in 3M KOH until only a weak and flat hydrogen evolution current remained, thereby reducing the zinc carbonate in the powder compact to zinc. Immersing the reduced sample in acetone and methanol in sequence for at least 1H to remove H ₂ O and KOH residues, and then dried under vacuum at room temperature. The porous zinc prepared can be cut and used directly as a battery anode. The resulting morphology was characterized by Scanning Electron Microscopy (SEM), as shown in figure 3.

Fig. 3 shows a scanning electron micrograph of nanoporous zinc prepared using zinc carbonate as a starting material. As can be seen from the figure, the overall nanoporous zinc exhibits a different hierarchical structure morphology. Specifically, the whole nano porous zinc is in the form of spherical particles, and the spherical particles are uniformly distributed in the whole nano porous zinc structure as a primary structure; in addition, each spherical particle also contains continuous pores and zinc fibers as a secondary structure. As with the nanoporous zinc structure obtained from the oxidizing property, zinc fibers in the nanoporous zinc are also connected to each other, and pores uniformly distributed therebetween are almost all open pores, so that the electrolyte can be sufficiently filled in the nanoporous zinc structure when an electrode including the structure is immersed in the electrolyte. In addition, the zinc fibers and pores in the nanoporous zinc also have a size of 200-1000nm, which indicates that the zinc fibers and pores have a uniform size, which has a very excellent effect on the performance of the battery.

EXAMPLE 3 application of nanoporous Zinc as anode for Nickel Zinc batteries

The electrochemical performance of nanoporous zinc in terms of rate (C-rate) and stability was studied in nickel zinc batteries. The configuration and assembly of nickel zinc cells is a simple button cell design. Square (0.25 cm) was assembled in 2032 button type cells ² ) Is a nano-porous zinc-based anode. From a fully charged commercial NiMH AAA battery (about 55mAh/cm ² 2600mAh,GP Batteries) to a size of 1cm ² Is provided to mate with the anode. In the cells reported herein, the total capacity ratio of cathode to anode is 3:1 to 4:1, as the performance of zinc-based anodes is a focus of the study. The anode and cathode were separated by a non-woven cellulose membrane and a Celgard 3501 separator.

For a cell that was cycled at 40% dod (depth of discharge), a solution consisting of 6M KOH/1M LiOH was used as electrolyte. Before assembly, 10wt% Ca (OH) was suspended therein ₂ The anode was permeated with a 6M KOH/1M LiOH solution. For a cell cycled at 60% dod, the aqueous electrolyte was 9M KOH saturated with ZnO. The total volume of the electrolyte was controlled to be about 150. Mu.L.

The battery was at 25mA/cm ² The charge/discharge ratio (C ratio) of (0.5C) was subjected to constant current circulation, and the cut-off voltage was 1.35V to 1.9V. Before the final condition of fixed capacity is met, the battery will be charged at a constant voltage of 1.9V until the desired capacity is reached or the current drops to 0.01C. While upon discharge, the battery stops operating by reaching a fixed capacity or lower cutoff voltage. DOD and C rates were calculated from the theoretical capacity of the anode. Polarization or LSV testing was performed at a scan rate of 1 mV/s. In the frequency range of 200kHz to 0.5Hz, a three-electrode split test cell (EQ-3 ESTC, MTI Corp) having the same structure as a coin cell was used, and Ag/Ag was used ₂ The O-line acts as a quasi-reference electrode between the cathode and anode and is subjected to Electrochemical Impedance Spectroscopy (EIS) testing at a perturbation of 10 mV.

FIG. 4 shows rate performance evaluated by polarization testing, which shows a relatively high dischargeThe electrical current and power density, which is mainly due to the high and stable conductivity and ionic conductivity of nanoporous zinc and its high specific surface area, which provides more sites for charge transfer. Owing to the unique bicontinuous structure of the nano porous zinc, the nickel zinc battery based on the nano porous zinc is 196.4mA/cm ² Can provide 264mW/cm at a current density of (C) ² Is a maximum power density of (c).

FIG. 5 shows the flow rate at 25mA/cm _Zn ² Stability performance of nickel zinc cells with nanoporous zinc anodes (corresponding to 0.5C, based on theoretical capacity of the anode) when cycled at 40% dod in lean electrolyte under charge and discharge conditions. As can be seen from the figure, in the nickel zinc cell, the chargeable area capacity of the nano porous zinc can reach 20mAh/cm in the previous 100 cycles _Zn ² The coulomb efficiency is close to 100 percent, and the specific capacity is 328mAh/g _Zn The method comprises the steps of carrying out a first treatment on the surface of the After 100 cycles, the capacity of the battery is observed to be gradually attenuated, but the attenuation rate is quite low, and the battery still has 90% coulombic efficiency after 200 cycles; at 300 cycles, the battery is shut down, at which time only 50% of the initial capacity is available for delivery.

To achieve higher energy densities, greater than 40% dod may be desirable. Therefore, the present inventors have also tested that the current density at charge and discharge is 25mA/cm _Zn ² The cycling stability of the nickel zinc cell cycling at 60% dod is shown in figure 6. As shown in fig. 6, while the coulombic efficiency is slightly lower than that at 40% dod, a nickel zinc cell based on nanoporous zinc can be cycled steadily about 80 times before significant capacity decay occurs. The nickel zinc cell of the present invention may eventually cycle about 160 times when the capacity drops to half of the original capacity.

Claims

1. A method of preparing integrated nanoporous zinc in an electrochemical cell comprising a working electrode, a separator, an electrolyte, a reference electrode, and a counter electrode, the working electrode comprising a conductive substrate and a zinc powder compact, wherein the zinc powder compact is obtained by densifying a zinc compound powder on the conductive substrate, the method comprising applying a reduction potential of-1.4 volts to-1.6 volts relative to an Ag/AgCl reference electrode to the working electrode to reduce the zinc compound, thereby forming integrated nanoporous zinc on the conductive substrate.

2. The method of claim 1, wherein the zinc compound is zinc oxide, zinc carbonate, zinc chloride, and/or zinc acetate.

3. The process of any one of claims 1-2, wherein the zinc dust compact further comprises additives that promote uniform progress of the reduction reaction, such as carbon black, carbon fibers, antimony oxide, and/or calcium hydroxide.

4. The method according to any one of claims 1-3, wherein the reference electrode is selected from Ag/AgCl electrodes, mercury/mercury oxide electrodes, or standard hydrogen electrodes, etc.; preferably, the reference electrode is an Ag/AgCl electrode.

5. The method of any one of claims 1-4, wherein the separator is located between the working electrode and the counter electrode, and the separator is made of a rigid organic material; preferably, the rigid organic material comprises an acrylic material, polypropylene (PP), polyethylene (PE) and/or Polyetherketone (PEEK).

6. The method of any one of claims 1-5, wherein the electrolyte is an alkaline solution having a hydroxide concentration of 3M to 6M, the alkali being, for example, an alkali metal hydroxide, such as sodium hydroxide, potassium hydroxide, and/or lithium hydroxide; preferably, the electrolyte additionally contains other metal ions which can participate in the reduction reaction, such as bismuth ions, indium ions, calcium ions.

7. The method of any one of claims 1-6, wherein the method further comprises adjusting the porosity of the formed nanoporous zinc and zinc fiber size by adjusting the distance between the separator and the working electrode.

8. The method according to any one of claims 1-7, wherein the electrically conductive substrate is a metal substrate such as a metal foam, a metal foil or a metal mesh; the metal is, for example, copper, titanium, tin; optionally, the metal matrix is tin plated.

9. The process of any one of claims 1-8, wherein the zinc dust compact is obtained by densification onto the electrically conductive substrate by applying a pressure of 0.5-9 tons to the zinc compound powder.

10. An integrated nanoporous zinc having bi-continuous pores and zinc fibers, and each of the pores and zinc fibers having a uniform size of 200nm to 1000 nm; preferably, the integrated nanoporous zinc is prepared according to the method of any one of claims 1-8.

11. The integrated nanoporous zinc according to claim 10, wherein the integrated nanoporous zinc is prepared from a working electrode comprising a zinc oxide powder compact, the integrated nanoporous zinc having only a primary structure consisting of pores and zinc fibers.

12. The integrated nanoporous zinc according to claim 10, wherein the integrated nanoporous zinc is prepared from a working electrode comprising a pressed block of zinc carbonate powder, the integrated nanoporous zinc being in the form of spherical particles as primary structure and the spherical particles comprising bi-continuous pores and zinc fibers as secondary structure.

13. An electrode comprising the integrated nanoporous zinc of any one of claims 10-12 and a conductive substrate to which it is attached.

14. A battery comprising the electrode of claim 13 as an anode.

15. The battery of claim 14, wherein the battery has a lifetime of 200 cycles at 40% depth of discharge or the battery has a lifetime of 100 cycles at 60% depth of discharge.

16. The battery according to any one of claims 14-15, wherein the battery is a secondary battery such as a zinc battery, for example a nickel zinc battery, a silver zinc battery, or a zinc air battery.