Patent Application
20180294518
Traditional batteries and supercapcitors contain two electrodes, a liquid electrolyte, and a semi-permeable membrane separating the electrodes. A typical battery or supercapacitor cell is assembled by stacking two active material-containing metal foil electrodes and separating the two by an inert membrane soaked in a liquid electrolyte solution. In Li ion batteries, lithium salts dissolved in organic carbonate solvents such as ethylene carbonate, dimethyl carbonate, or diethyl carbonate, serve as the electrolyte. A cell designed in this manner has certain disadvantages. Its flexibility is limited by the metal foils. Also, the active materials may detach from the metallic collectors due to poor adhesion, and the electrolyte may leak, any of which leads to poor cycling. Solid-state electrolytes have been used to avoid these issues, to reduce weight, and to increase options with regard to volume and shape.
A solid-state electrolyte should possess high ionic conductivity, negligible electronic conductivity, a wide electrochemical window, and thermal and mechanical stability. One difference between liquid and solid-state electrolytes is the wetting of the electrode by the electrolyte. A liquid electrolyte can easily penetrate a porous active material and provide good contact with the material (
Gel polymer electrolytes exhibit superior conductivity compared to solid-state electrolytes based on conventional polymer-salt complexes. Studies on solid-state supercapacitors and lithium ion batteries have described the use of gel polymer electrolytes in a simple sandwich assembly where a polymer electrolyte film is placed between the electrodes. However, the poor electrode-electrolyte interface, characteristic of such assemblies, results in high internal resistance, limiting energy delivery rate and power density.2
Hydrogel electrolytes can improve the interface between solid-state electrolytes and electrodes.3-6 All solid-state energy devices have excellent mechanical and cycling stability. Further, due to the highly porous nature of the active material, they have highly improved accessibility of electrolytes to electrode surfaces. However, these devices are not suitable for industrial applications because their maximum operation voltage is under 1V, and because they have relatively low energy density due to the fact that hydrogel electrolytes contain water.7
Nanostructured active materials provide benefits in terms of capacity, power, and cost. These benefits are related to the small size of the materials which reduces the path of diffusion of ions and electrons and accommodates strains associated with lithium insertion and removal reactions. However, problems associated with poor packing, which leads to a significant proportion of the nanomaterial remaining inactive, limits the energy that can be stored per unit volume or mass.
There is a need for new solid electrolyte systems that allow better integration with electrodes for high capacity energy storage and rapid charge and discharge rates.
The present technology provides an integrated and flexible electrode/solid-state electrolyte structure for use in supercapacitors and batteries. The integrated electrode/electrolyte system includes an electrically conductive substrate (electrode surface), a nanostructured active material layer deposited on the substrate, and a porous membrane coating the nanostructured active material. The porous membrane includes a polymer network and a conductive additive, is flexible, and enhances access of ions and electrons to the nanostructured active material.
As used herein, “nanostructured active material layer” refers to a layer of positive or negative electrode active material that is nanostructured, i.e., the material itself is made up of nanosized structures such as nanoparticles, nanowires, nanorods, and micro/nano sized 3D porous particles, or is mixed with a nanostructured material. Examples of suitable positive electrode active materials include lithium-transition metal oxides, such as LiCoO2, LiNiO2, LiMnO2, LiMn2O4, and LiNi1-x-yCOxMyO2 (where, 0≤x≤1, 0≤y≤1, 0≤x+Ey≤1, M is Al, Sr, Mg or La). Examples of suitable negative electrode active materials include lithium alloying compounds such as Si, Al, Sn, Sb, and Ge, as well as 3D porous silicon, silicon nanotubes, silicon nanowires, SiOx/C coating on Si nanoparticles, Si/CNT composite film, Co3O4 nanoparticles, Co3O4 nanowires, mesoporous Co3O4, SnS2 nanoplates, SnS2 nanoflowers, and CuO particles (1 μm and 0.15 μm). Examples of a nanostructured material which can be mixed with a positive or negative electrode active material include 3D nanomaterials such as carbon nanotubes, carbon nanocups, and graphene.
Embodiments of the integrated electrode/solid-state electrolyte system can include one or more of the following features. The conductive additive can be an acid, a salt, or an ionic liquid. In one embodiment, the conductive additive is phosphoric acid. Suitable ionic liquids include, for example, ionic liquids containing an imidazolium cation, a piperidinium cation, a pyrrolidinium cation, or an ammonium cation associated with an anion selected from a bis(trifluoromethansulfonyl)imide anion, a bis(fluorosulfonyl)imide anion, a tetrafluoroborate anion, and a hexafluorophosphate anion. An example is 1-butyl, 3-methylimidazolium chloride. Suitable ionic liquids are preferably liquid at ambient temperature, such as room temperature (15-30° C.) or less, such as ionic liquids including as cation 1-alkyl-3-methylimidazolium, 1-alkylpyridinium, N-methyl-N-alkylpyrrolidinium, ammonium, phosphonium, tetrafluoroborate or hexafluorophosphate combined with anions such as bistriflimide, triflate, tosylate, formate, alkylsulfate, or glycolate.
The nanostructured active material can include a carbon based 3D nanomaterial, an inorganic nanostructures material, or a hybrid of the two. In some embodiments, the carbon based 3D nanomaterial is selected from the group consisting of: assembled carbon nanotubes (CNT), vertically aligned carbon nanotubes, carbon nanocups, carbon nanofibers, graphene, doped graphene, a hybrid of CNT and graphene, a hybrid of CNT and carbon nanocups, and carbon black. The inorganic nanostructured material can be a nanoparticle, a nanowire, a nanosheet, each including a substance selected from the group consisting of: a metal/semiconductor, a metal oxide, a metal phosphide, a metal nitride, and a metal sulfide; or a nanocomposite comprising two or more of said substances. In some embodiments, the polymer of the polymer network is selected from the group consisting of: poly(vinyl alcohol), poly(vinylpyrrolidone), poly(acrylic acid), polyurethane, poly(ethylene glycol), poly(propylene glycol), poly(vinyl methyl ether), poly(N-isopropyl acrylamide), polymethacrylate, poly(vinyl methyl ether) and poly(N-isopropyl acrylamide), In other embodiments the polymer network contains a block copolymer containing two or more of any of the above polymers or other polymers.
By “nanocomposite” is meant a multiphase solid material in which one of the phases has one, two, or three dimensions of less than 100 nanometers, or structures having nanoscale repeat distances between the different phases that make up the material. Nanocomposites include porous media, colloids, gels, and copolymers, as well as the solid combination of a bulk matrix and nano-dimensional phase(s) differing in properties due to dissimilarities in structure and chemistry. The mechanical, electrical, thermal, optical, electrochemical, catalytic properties of the nanocomposite differ markedly from that of the component materials.
Another aspect of the present technology is a supercapacitor having two electrodes as described above. The electrolyte is a solid-phase electrolyte containing one or more conductive additives selected from the group consisting of: an acid, a salt solution, and an ionic liquid. For immiscible reagents and solvent combinations, a phase transfer catalyst such as a quaternary ammonium cation can be used.8
Yet another aspect of the present technology is a solid-state electrolyte containing a flexible porous membrane that encloses at least one conductive additive. The membrane contains a polymer network or matrix. The polymer of the polymer network can be selected from the group consisting of: poly(vinyl alcohol), poly(vinylpyrrolidone), poly(acrylic acid), polyurethane, poly(ethylene glycol), poly(propylene glycol), poly(vinyl methyl ether), poly(N-isopropyl acrylamide), polymethacrylate, poly(vinyl methyl ether) and poly(N-isopropyl acrylamide); and a block copolymer. The at least one conductive additive can be, for example, H3PO4, a salt, or an ionic liquid.
A further aspect of the present technology is a rechargeable or non-rechargeable battery having an anode and a cathode, each having a structure as described above. The pair of electrodes contains two matched electrode materials, each coated with a solid-phase electrolye containing one or more conductive additives, which combined provide the required battery chemistry. The conductive additives can be, for example, one or more salts selected from an alkali salt, an alkaline earth salt, and a transitional metal salt, such as NaClO4, NaI, Mg(ClO4)2, LiClO4, LiI, LiN(CF3SO2)2, LiCF3SO3, LiBC4O8, AgNO3, LiPF6, LiBF4, LiSbF6, LiAsF6, LiN(C2F5SO2)2, LiAlO4, LiAlCl4, LiN(CxF2x+1SO2)(CyF2x+1SO2), where, x and y are positive integers.
Embodiments of the rechargeable battery with solid-state electrolyte support can include one or more of the following features. The anode and/or cathode can include a conductive active material selected from the group consisting of: a carbon based 3D nanomaterial, an inorganic nanostructured material, or a combination of the two. The carbon based 3D nanomaterial can be selected from the group consisting of: assembled carbon nanotubes (CNT), vertically aligned carbon nanotubes, carbon nanocups, carbon nanofibers, graphene, doped graphene, a hybrid of CNT and graphene, a hybrid of CNT and carbon nanocups, and carbon black. The inorganic nanostructured material can be a nanoparticle, a nanowire, or a nanosheet, each comprising a substance selected from the group consisting of a metal/semiconductor, a metal oxide, a metal phosphide, a metal nitride, and a metal sulfide; or a nanocomposite comprising two or more of these substances. Any known lithium battery chemistries can be employed.9
Yet another aspect of the present technology is a method of making an electrode. The method includes the steps of: (a) providing an electrode comprising a surface coated with a nanostructured active material, a polymer solution, and a conductive additive; (b) coating the nanostructured material with the polymer solution; (c) performing one or more freeze/thaw cycles on the product of step (b), whereby the polymer solution forms a hydrogel; (d) dehydrating the hydrogel, leaving a porous polymer membrane surrounding components of the nanostructured material; (e) soaking the porous polymer membrane in a solution comprising the conductive additive, whereby the conductive additive becomes incorporated into pores of the porous polymer membrane, and; (f) drying the porous polymer membrane to obtain the electrode. Embodiments of the method of making the electrode can include one or more of the following features. The conductive additive can be an acid, a salt, or an ionic liquid. The freezing and thawing can be repeated two to ten times. Drying can be performed at room temperature. Alternatively, drying can be performed at 80° C. under vacuum.
Still another aspect of the present technology is another method of making an electrode. The method includes the steps of: (a) providing an electrode comprising a surface coated with a nanostructured active material and a solution containing a polymer and a conductive additive; coating the nanostructured material with the solution; (c) performing one or more freeze/thaw cycles on the product of step (b), whereby the solution forms a hydrogel; and (d) dehydrating the hydrogel, leaving a porous polymer membrane and the conductive additive surrounding components of the nanostructured material, whereby the electrode is obtained.
The present technology is further summarized by the following list of embodiments.
1. A solid-state electrolyte comprising a porous polymer network containing a conductive additive selected from the group consisting of an acid, a salt dissolved in a non-aqueous solvent, and an ionic liquid.
2. The solid-state electrolyte of embodiment 1, wherein the polymer network comprises one or more polymers selected from the group consisting of poly(vinyl alcohol), poly(vinylpyrrolidone), poly(acrylic acid), polyurethane, poly(ethylene glycol), poly(propylene glycol), poly(vinyl methyl ether), poly(N-isopropyl acrylamide), polymethacrylate, poly(vinyl methyl ether) and poly(N-isopropyl acrylamide.
3. The solid-state electrolyte of embodiment 1 or embodiment 2, wherein the polymer network comprises a block co-polymer.
4. The solid-state electrolyte of any of the previous embodiments, wherein the polymer network comprises a hydrophobic polymer or a hydrophilic polymer.
5. The solid-state electrolyte of any of the previous embodiments, wherein the conductive additive is H3PO4.
6. The solid-state electrolyte of any of embodiments 1-4, wherein the conductive additive is an ionic liquid.
7. An electrode comprising:
an electrically conductive substrate;
a nanostructured active material layer deposited on the substrate; and
the solid-state electrolyte of any of the previous embodiments configured as a porous membrane coating the nanostructured active material.
8. The electrode of embodiment 7, wherein the nanostructured active material comprises a carbon-based 3D nanomaterial, an inorganic nanostructured material, or a combination thereof.
9. The electrode of embodiment 8, wherein the carbon-based 3D nanomaterial is selected from the group consisting of assembled carbon nanotubes, vertically aligned carbon nanotubes, carbon nanocups, carbon nanofibers, graphene, doped graphene, a hybrid of carbon nanotubes and graphene, a hybrid of carbon nanotubes and carbon nanocups, and carbon black.
10. The electrode of embodiment 7 or embodiment 8, wherein the inorganic nanostructured material is in the form of nanoparticles, nanowires, nanosheets, and/or nanocrystals and comprises a metal, a semiconductor, a metal oxide, a metal phosphide, a metal nitride, a metal sulfide, or a combination thereof.
11. The electrode of any of embodiments 7-10 configured for use in a battery or supercapacitor.
12. A supercapacitor comprising a pair of electrodes of embodiment 11.
13. A battery comprising a first electrode of embodiment 11 configured as an anode and a second electrode of embodiment 11 configured as a cathode.
14. The battery of embodiment 13 that is rechargeable.
15. The battery of embodiment 13 or embodiment 14 that is a lithium ion battery.
16. The battery of any of embodiments 13-15, wherein the porous membrane of the solid-state electrolyte serves as separator.
17. A method of making an electrode, the method comprising the steps of:
The present technology provides a new method for fabricating a highly integrated, flexible, reconfigurable, and miniaturizable combined electrode/solid-state electrolyte system for use in batteries and supercapacitors. A key component of the electrode/electrolyte system is a porous, flexible, 3D polymer network (also referred to herein as a “membrane” or “polymer film”) made from a polymer-based hydrogel. The membrane coats a nanostructured active material deposited on the electrode's conductive surface. Additionally, the solid-state electrolyte contains one or more conductive additives and enhances the access of ions and electrons to the nanostructured active material. Such electrodes are suitable for the production of devices, such as lightweight and flexible, all solid-state, high-performance supercapacitors; batteries, and sensors for use in portable, wearable, and flexible electronic devices, electric and hybrid-electric vehicles, and energy-efficient cargo ships, locomotives, and aircraft.
Nanomaterials used as active materials in batteries and supercapacitors have a high surface-to-volume ratio which increases the area of contact between the electrolyte and electrode leading to improved power density and energy efficiency. The nanosize dimensions of the components of such materials effectively reduces the distance that ions and electrons must travel during cycling in the solid state through electrode materials. However, nanomaterials have low tapped density (packing density) which leads to reduced volumetric energy density. Without being bound to any theory or mechanism of action, it is believed that the porous flexible membrane described herein, which swells when soaked in an electrolyte solution, fills the voids between the particles, thereby enhancing volumetric energy density. Many nanostructures are known which can be used as active materials on electrodes of the present technology.10-11
A method for making the above-described electrode/electrolyte system (sometimes referred to herein as an “electrode”) requires formation of a hydrogel on an electrode surface, and drying of the hydrogel to form a porous polymer film or membrane containing one or more conductive additives. The hydrogel is formed by depositing on the active elements of the electrode surface a solution containing one or more polymers or polymer precursors (e.g., monomeric units) and containing the conductive additive(s) in a suitable solvent. During cycles of freezing and thawing, some of the polymer material forms crystallites (see
Once the hydrogel is formed on the electrode, the solvent is removed to form the electrode/solid-phase electrolyte system. A preferred method of removing the solvent is by solvent exchange, during which the initial solvent is removed and gradually replaced with a volatile solvent that can be readily and completely removed to form a porous mechanically stable film. Optionally, the polymer film can be soaked in further solutions to add additional components, such as conductive additives, cross-linking agents, preservatives, and the like (see
The porosity of the polymer network of the solid-state electrolyte of the present technology is necessary in order to maintain a pathway for diffusion of ions to and from the active material on the electrode, and to maintain a conductive ion pathway between the electrodes of a device such as a supercapacitor or battery that contains more than one such electrode. The characteristics of the polymer network can be adjusted in order to control the electrical properties of a supercapacitor or battery in which the electrode/solid-state electrolyte system is used. Thus, properties such as pore size, pore density, porosity, thickness, and tortuosity can be adjusted by selecting the starting conditions, materials, and fabrication method of the polymer network together with the conductive additives (type and concentration) contained within the polymer network. In general, larger pore size leads to larger capacitance. The fabrication conditions (such as freeze/thaw process and number of cycles) and materials can be adjusted while using the capacitance, series resistance, energy density, and/or discharge rate values as feedback for optimizing the structure, performance, and fabrication of the final integrated electrode/solid-phase electrolyte system. The porosity of the solid-state electrolyte material of the present technology is also sensitive to drying conditions used during fabrication, both after dehydration or other solvent exchange and after final impregnation with electrolyte. If drying is performed to quickly or under harsh conditions (e.g., excessive temperature, time, or positive or negative pressure), the polymer network can lose sufficient porosity as to degrade the electrical properties of the integrated electrode/electrolyte system or a device in which it is used.
While it is preferable to use the polymer film of the present technology as a replacement for a conventional separator in a supercapacitor or battery device, a separator optionally can be added in such a device if required to achieve desired properties of the device.
In another embodiment of a method for making an electrode/solid-phase electrolyte system, the polymer solution is initially devoid of conductive additive, which are added after the porous polymer film is formed by soaking the film in a solution containing the desired ions or in an ionic liquid that serves as the conductive additive.
The present technology affords many advantages. The method for making the electrode provides a simple and efficient process for integrating electrodes with a conductive matrix, avoiding the need for adding a separate solid or liquid electrolyte. Impregnation of the electrode can be performed at the same time as the formation of the gelled matrix. The fabrication method ensures a good interface between the gel matrix and the electrode active material which is important for minimizing contact resistance in the final device. A mechanically stable porous membrane is formed by a simple to perform solvent exchange step followed by drying. The presence of the membrane covering the electrode surface avoids the need for adding a separator when combining two electrodes in a battery or supercapacitor, as the membrane serves the role of a separator, preventing shorting between the electrodes and regulating the ionic environment at the electrode surface.
The dry porous membrane coating the electrode active material serves as a versatile template that can be used to apply desired ions to the electrode. Such ions can be introduced by soaking the membrane in a liquid containing the ions required for a specific device. Soaking leads to swelling of the membrane which promotes access of ions and electrons present in the bulk electrolyte solution to the nanostructured active material on the electrode. The dimensions of the pores of the membrane can be controlled by different means, including by selecting an amount of solid content in the polymer or polymer-additive solution (by selecting a suitable concentration of initially dissolved polymer or polymer precursor), by the temperature during the polymerization or gelation process, or by applying positive or negative pressure, optionally with heat, during the dehydration and drying process. Uniform pores, high mechanical strength, and enhanced accessibility of ions to the electrode all serve to protect the electrode and the solvent inside the battery or supercapacitor cell, thereby ensuring higher and more stable cycling ability than with earlier technologies.
The above-described electrode can be used in the construction of a supercapacitor. Accordingly, the present technology provides a supercapacitor having two such electrodes. Since each electrode is actually an integrated electrode/electrolyte system, and includes a membrane (3D polymer network that serves as a semipermeable membrane and contains an ionic solution or an ionic liquid), no additional separator or electrolyte is required. The supercapacitor can be made by simply layering the two electrodes over one another. If desired, the PVA-CNT structures can be covered by a metal layer such as gold, silver or chromium to provide support and electrical contact.
Detailed characterization of supercapacitors is provided in Example 5. Standard electrochemical methods, such as impedance spectroscopy, cyclic voltammetry, and galvanostatic charge/discharge measurements can be used for characterization. Results obtained for supercapacitors using H3PO4 and an ionic liquid as the electrolyte are shown
The integrated electrode/solid-state electrolyte system of the present technology can be used in the construction of non-rechargeable or rechargeable batteries. In one embodiment, such rechargeable batteries have at least one integrated electrode/solid-phase electrolyte of the present technology, and preferably have two such electrodes. Because the integrated electrode/electrolyte includes the electrolyte, no additional electrolyte is needed (although a solid or liquid electrolyte optionally can be added), and no separator is required because of the presence of a porous membrane enclosing the electrolyte. A battery can be made simply by combining suitable electrodes which serve as the anode and cathode for the battery. A case and positive and negative contact structures can be added to enclose and/or provide contact with the electrodes. The materials chosen for the anode and cathode, as well as their electrolyte materials, particularly the conductive additives, provide the required chemistry for the battery.
The fabrication of a porous polyvinyl alcohol (PVA) film solid-state electrolyte is schematically shown in
The fabrication of a reconfigurable electrode/solid-electrolyte system is shown schematically in
Supercapacitors having highly integrated, porous, and flexible electrode/solid-state-electrolyte films were prepared using a vertically aligned carbon nanotube array, poly(vinyl alcohol), and H3PO4 solution or an ionic liquid as electrolyte. The electrode/solid-state electrolyte structures were prepared as described in Example 2 and then soaked in a 1.5M H3PO4 solution or in a hydrophilic ionic liquid to obtain the final electrode/solid-state electrolyte system.
A battery having two integrated electrode/solid-state electrolyte structures together with a lithium salt as conductive additive are prepared by the process depicted in
Supercapacitors made according to the process described in Example 3 were characterized by impedance spectroscopy, cyclic voltammetry, and galvanostatic charge/discharge measurements. Two different electrolyte systems were used in the design of the suprcapacitors: (1) H3PO4 (results shown in
The structure obtained by the integration procedure as described in Example 2 makes it possible to have a high cycle life with lower capacitance loss after 10,000 cycles. These characteristics are a result of high integration between the supercapacitor components which ensures proper electrode/electrolyte interface, guaranteeing extensive wetting of the electrode and access of ions to the high surface area porous polymer film (with narrow pore size effective for double-layer accumulation), and also a result of film porosity, which ensures access of electrolyte from the bulk of the film to the surface of the nanotubes. Results of cyclic voltammetry experiments performed with different electrochemical windows from 0.8 to 2.0V at 50 mV/s are shown in
As used herein, “consisting essentially of” allows the inclusion of materials or steps that do not materially affect the basic and novel characteristics of the claim. Any recitation herein of the term “comprising”, particularly in a description of components of a composition or in a description of elements of a device, can be exchanged with “consisting essentially of” or “consisting of”.
While the present technology has been described in conjunction with certain preferred embodiments, one of ordinary skill, after reading the foregoing specification, will be able to effect various changes, substitutions of equivalents, and other alterations to the compositions and methods set forth herein.
All publications referenced herein are incorporated by reference in their entirety.
This application claims the priority of U.S. Provisional Appl. No. 62/478,988, filed Mar. 30, 2017 and entitled “Highly Integrated, Porous, Flexible, and Miniaturizable Electrode/Electrolyte System for Energy Storage Applications”, which is hereby incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 62478988 | Mar 2017 | US |
