Flow-through electrochemical cell electrode with permeable membrane

Abstract

Providing a flow-through electrochemical cell electrode with a permeable membrane. [Solution] This disclosure provides a porous electrode for a flow-through rechargeable electrochemical cell, comprising a high-porosity metal current collector, an active material surrounding the metal current collector, and a self-supporting synthetic film material surrounding the active material. This disclosure further includes a flow-through rechargeable battery comprising a plurality of electrochemical cells, a closed loop, and a pump. The metal current collector comprises a metal foam. The metal current collector comprises a woven wire mesh. The synthetic film completely coats the active material.

Description

Technical Field

[0001] (Cross - Reference to Related Applications) This application claims the benefit of U.S. Provisional Application No. 62 / 849,632, filed on March 12, 2020, titled "FLOW - THROUGH ELECTROCHEMICAL CELL ELECTRODE WITH PERMEABLE MEMBRANE", which is hereby incorporated by reference in its entirety.

[0002] (Technical Field) The present disclosure generally relates to rechargeable electrochemical cells, and more particularly to electrodes for flow - through electrochemical cells.

Background Art

[0003] (Background) An electrochemical cell has two electrodes (anode and cathode) and an electrolyte. The electrodes contain a material called an active material that is both electronically and chemically active. The active materials in the anode and cathode can acquire and lose the same ions, typically cations (positive ions) called the working ions of the electrochemical cell. The electrolyte conducts the working ions but is an electronic insulator. As a result, any movement of electrons between the anode active material and the cathode active material must occur through an external circuit that is in electronic contact with both the anode and the cathode. Typically, the anode active material, the cathode active material, or both contain the working ions prior to the operation of the electrochemical cell.

[0004] A rechargeable electrochemical cell or rechargeable battery is typically identified by the type based on its working ions, which leads to identifications such as "lithium - ion battery", "hydrogen - ion battery", etc.

[0005] During circulation, electrolytes conduct working ions inside the electrochemical cell, while electrons move through the external circuit. Electrons tend to flow without requiring external energy input during discharge, allowing the energy stored within the electrochemical cell to power, for example, a device. During charging, an external energy supply is typically used to reverse the flow of working ions and store energy from an energy supply within the electrochemical cell to cause electrons to flow in the opposite direction. [Overview of the project] [Means for solving the problem]

[0006] (summary) This disclosure provides an electrode for a flow-through rechargeable electrochemical cell, comprising a high-porosity metal current collector, an active material surrounding the metal current collector, and a self-supporting synthetic film material surrounding the active material.

[0007] The electrodes may further include the following features, which can be combined with each other and with other features disclosed herein in any combination, unless explicitly mutually exclusive:

[0008] i. A high-porosity metal current collector can be a metal foam.

[0009] ii. A high-porosity metal current collector can be a woven wire mesh.

[0010] iii. The synthetic film can completely coat the active material.

[0011] iv. The synthetic film may contain polymers.

[0012] v. The synthetic film may contain a polymer resin.

[0013] vi. The synthetic film may contain semicrystalline polyolefins.

[0014] vii. The synthetic film may contain polyoxymethylene.

[0015] viii. The synthetic membrane may contain isotactic poly(4-methyl-1-pentene).

[0016] ix. The synthetic membrane may contain ethylene propylene diene monomer rubber.

[0017] x. The synthetic membrane may contain polyethylene.

[0018] xi. The synthetic membrane may contain polypropylene.

[0019] xii. The synthetic membrane may contain a graft polymer.

[0020] xiii. The synthetic membrane may be made of polyethylene.

[0021] xiv. The synthetic membrane may be made of polypropylene.

[0022] xv. The synthetic membrane may contain polyvinylidene fluoride.

[0023] xvi. The synthetic membrane may contain a fluoropolymer elastomer.

[0024] xvii. The metal current collector may have a porosity of 80 - 90%.

[0025] xviii. The active material may be a cathode active material.

[0026] xix. The active material may be an anode active material.

[0027] The present disclosure provides a cathode for a flow-through rechargeable electrochemical cell, comprising a high-porosity metal current collector, a cathode active material surrounding the metal current collector, and a self-supporting synthetic membrane material surrounding the cathode active material.

[0028] The electrodes may further include the following features, which can be combined with each other and with other features disclosed herein in any combination, unless explicitly mutually exclusive:

[0029] i. A high-porosity metal current collector can be a metal foam.

[0030] ii. A high-porosity metal current collector can be a woven wire mesh.

[0031] iii. The synthetic film can completely coat the active material.

[0032] iv. The synthetic film may contain polymers.

[0033] v. The synthetic film may contain a polymer resin.

[0034] vi. The synthetic film may contain semicrystalline polyolefins.

[0035] vii. The synthetic film may contain polyoxymethylene.

[0036] viii. The synthetic film may contain isotactic poly(4-methyl-1-pentene).

[0037] ix. The synthetic film may contain ethylene propylene diene monomer rubber.

[0038] x. The synthetic film may contain polyethylene.

[0039] xi. The synthetic film may contain polypropylene.

[0040] xii. The synthetic film may contain graft polymers.

[0041] xiii. The synthetic membrane may be made of polyethylene. xiv. The synthetic film may be made of polypropylene. xv. The synthetic film may contain polyvinylidene fluoride.

[0042] xvi. The synthetic film may contain a fluoropolymer elastomer.

[0043] xvii. Metal current collectors may have a porosity of 80-90%.

[0044] This disclosure provides an anode for a flow-through rechargeable electrochemical cell, comprising a high-porosity metal current collector, an active material surrounding the metal current collector, and a self-supporting synthetic film material surrounding the anode active material.

[0045] The anode may further include the following features, which can be combined with each other and with other features disclosed herein in any combination, unless explicitly mutually exclusive:

[0046] i. A high-porosity metal current collector can be a metal foam.

[0047] ii. A high-porosity metal current collector can be a woven wire mesh.

[0048] iii. The synthetic film can completely coat the active material.

[0049] iv. The synthetic film may contain polymers.

[0050] v. The synthetic film may contain a polymer resin.

[0051] vi. The synthetic film may contain semicrystalline polyolefins.

[0052] vii. The synthetic film may contain polyoxymethylene.

[0053] viii. The synthetic film may contain isotactic poly(4-methyl-1-pentene).

[0054] ix. The synthetic film may contain ethylene propylene diene monomer rubber.

[0055] x. The synthetic film may contain polyethylene.

[0056] xi. The synthetic film may contain polypropylene.

[0057] xii. The synthetic film may contain graft polymers.

[0058] xiii. The synthetic membrane may be made of polyethylene. xiv. The synthetic film may be made of polypropylene. xv. The synthetic film may contain polyvinylidene fluoride.

[0059] xvi. The synthetic film may contain a fluoropolymer elastomer.

[0060] xvii. Metal current collectors may have a porosity of 80-90%. The present invention provides, for example, the following items: (Item 1) An electrode, A high porosity metal current collector, The active material used to coat the aforementioned metal current collector, A self-supporting synthetic film coating the aforementioned active material and An electrode equipped with (Item 2) The aforementioned metal current collector is the electrode according to item 1, comprising a metal foam. (Item 3) The aforementioned metal current collector is the electrode according to item 1, comprising a woven wire mesh. (Item 4) The electrode according to item 1, wherein the synthetic film completely coats the active material. (Item 5) The aforementioned synthetic film is the electrode according to item 1, comprising a polymer resin. (Item 6) The electrode according to item 5, wherein the polymer resin is selected from the group consisting of semicrystalline polyolefins, polyoxymethylene, isotactic poly(4-methyl-1-pentene), and combinations thereof. (Item 7) The electrode according to item 1, wherein the synthetic film comprises a polymer selected from the group consisting of polyethylene, polypropylene, and combinations thereof. (Item 8) The aforementioned synthetic film is the electrode according to item 1, comprising ethylene propylene diene monomer rubber. (Item 9) The aforementioned synthetic film is the electrode according to item 1, comprising a graft polymer. (Item 10) The aforementioned synthetic film is the electrode according to item 1, comprising polyvinylidene fluoride. (Item 11) The aforementioned synthetic film is the electrode according to item 1, comprising a fluoropolymer elastomer. (Item 12) The high porosity metal current collector is the electrode described in item 1, having a porosity of 80% to 90%. (Item 13) It is a cathode, A high porosity metal current collector, A cathode active material for coating the aforementioned metal current collector, A self-supporting synthetic film coating the cathode active material and A cathode equipped with this. (Item 14) The aforementioned metal current collector is a cathode according to item 13, comprising a metal foam. (Item 15) The aforementioned metal current collector is a cathode as described in item 13, comprising a woven wire mesh. (Item 16) The synthetic film completely coats the cathode active material, as described in item 13. (Item 17) The synthetic film comprises a polymer resin and is a cathode as described in item 13. (Item 18) The polymer resin is selected from the group consisting of semicrystalline polyolefins, polyoxymethylenes, isotactic poly(4-methyl-1-pentenes), and combinations thereof, as described in item 17. (Item 19) The synthetic film comprises a polymer selected from the group consisting of polyethylene, polypropylene, and combinations thereof, as described in item 13, the cathode. (Item 20) The synthetic film comprises an ethylene propylene diene monomer rubber, and is a cathode as described in item 13. (Item 21) The synthetic film comprises a graft polymer and is a cathode as described in item 13. (Item 22) The synthetic film is a cathode according to item 13, comprising polyvinylidene fluoride. (Item 23) The synthetic film comprises a fluoropolymer elastomer, and is the cathode described in item 13. (Item 24) The high porosity metal current collector is a cathode as described in item 13, having a porosity of 80% to 90%. (Item 25) It is an anode, A high porosity metal current collector, an anode active material for coating the metal current collector, A self-supporting synthetic film coating the anode active material and an anode equipped with (Item 26) The metal current collector is an anode as described in item 25, comprising a metal foam. (Item 27) The metal current collector comprises a woven wire mesh, as described in item 25. (Item 28) The synthetic film completely coats the anode active material, as described in item 25. (Item 29) The synthetic film is an anode according to item 25, comprising a polymer resin. (Item 30) The polymer resin is selected from the group consisting of semicrystalline polyolefins, polyoxymethylenes, isotactic poly(4-methyl-1-pentenes), and combinations thereof, as described in item 29. (Item 31) The synthetic film comprises a polymer selected from the group consisting of polyethylene, polypropylene, and combinations thereof, as described in item 25. (Item 32) The synthetic film is an anode according to item 25, comprising ethylene propylene diene monomer rubber. (Item 33) The synthetic film comprises a graft polymer, as described in item 25. (Item 34) The synthetic film is an anode according to item 25, comprising polyvinylidene fluoride. (Item 35) The synthetic film is an anode according to item 25, comprising a fluoropolymer elastomer. (Item 36) The high porosity metal current collector is an anode as described in item 25, having a porosity of 80% to 90%. [Brief explanation of the drawing]

[0061] Embodiments of the present disclosure will be described in further detail as examples with reference to the accompanying drawings, which are not necessarily to a consistent scale. [Figure 1A] Figure 1A is a schematic cross-sectional view of a flow-through rechargeable electrochemical cell during discharge.

[0062] [Figure 1B] Figure 1B is a schematic cross-sectional view of the flow-through rechargeable electrochemical cell shown in Figure 1A during charging.

[0063] [Figure 2A]Figure 2A is a schematic cross-sectional view of the wire of the woven wire mesh electrode.

[0064] [Figure 2B] Figure 2B is a schematic cross-sectional view of the wire of the woven mesh electrode after expansion. [Modes for carrying out the invention]

[0065] (Detailed explanation) This disclosure relates to an electrode with a permeable membrane for a rechargeable electrochemical cell.

[0066] A rechargeable electrochemical cell is a device capable of undergoing at least one charge / discharge cycle. The terms “battery” and “electrochemical cell” are sometimes used synonymously or given specific meanings in different contexts. The term “electrochemical cell” is used in this disclosure to describe a device comprising one anode, one cathode, and an electrolyte. The term “battery” is used in this disclosure to describe a device containing multiple electrochemical cells.

[0067] The electrochemical cell of this disclosure may have a porous cathode and anode, a separator to enable fluid flow, and a pump for circulating a fluid electrolyte through the electrochemical cell. The working ion is lithium ion (Li + ), sodium ions (Na + ), or potassium ions (K + ) such alkali metal ions, hydrogen ions (H + ), or hydroxyl ion (OH - ) is also acceptable.

[0068] Referring here to Figures 1A and 1B, the electrochemical cell 100 includes a cathode 102, an anode 104, an electrolyte 106, and an electronic isolation separator 108, all housed in a container 110. To facilitate the flow of the electrolyte 106 through the cathode 102 and anode 104, the container 110 is fluidically connected to a closed loop 114, which includes a pump 112.

[0069] As shown in Figures 1A and 1B, the electrochemical cell is configured such that the electrodes and separator 108 are arranged in a stack located between the anode 104 and the cathode 102, in order to help prevent electronic contact between the anode 104 and the cathode 102, other than through the external circuit 136. The closed loop 114 is connected to the container 110 so that the fluid electrolyte enters the container 110 from the closed loop 114, flows through the stack of anode 104, separator 108, and cathode 102, and then exits the container 110 and re-enters the closed loop 114. As shown in Figures 1A and 1B, the closed loop 114 is connected to the container 110 on the opposite side of the stack, but other configurations are also possible as long as the fluid flows through the stack.

[0070] The electrochemical cell 100 illustrated in Figures 1A and 1B includes a separator 108, but alternative electrochemical cells may lack a separator. For example, the cathode 102 and anode 104 may be held in place by attachment to a container 110 within the electrochemical cell 100 or to another non-separator structure so that they do not come into electronic contact. Such a configuration lacking a separator may not function in many conventional electrochemical cells due to the formation of dendritic crystals that are blocked by the separator, but the flow of the fluid electrolyte through the electrochemical cell 100 may prevent or substantially reduce dendritic crystal formation compared to similar electrochemical cells that do not require a separator, rather than being a flow-through cell.

[0071] The electrochemical cell 100 is illustrated with a cylindrical container 100, but other container shapes such as rectangular, cubic, or coin-shaped are also possible. The container 100 may contain any suitable material capable of resisting decomposition by the fluid electrolyte or other chemicals found within the electrochemical cell 100, including the chemicals formed during the circulation of the electrochemical cell 100. Suitable materials may also be able to maintain the shape of the electrochemical cell 100 during charging and discharging or over the expected lifespan of the electrochemical cell 100. Suitable materials include steel, glass, bituminous compounds, ceramic materials, and polymers. The container 110 may contain multiple materials, such as a metal outer layer with a decomposition-resistant inner layer or coating.

[0072] The closed loop 114 may contain the same material as the container 100 or a different material. The closed loop 114 may contain any suitable material capable of resisting decomposition by fluid electrolytes or other chemicals found within the electrochemical cell 100, including chemicals formed during the circulation of the electrochemical cell 100. Suitable materials may also be able to maintain their shape, such as cross-sectional shape, without collapsing over the expected lifespan of the electrochemical cell 100. Suitable materials include steel, glass, bituminous compounds, ceramic materials, and polymers. The closed loop 114 may contain multiple materials, such as a metal outer layer with a decomposition-resistant inner layer. The closed loop 114 may be flexible or rigid.

[0073] The pump 112 within the closed loop 114 may be any pump sufficient to cause the fluid electrolyte 106 to flow through the closed loop 114 and the electrodes in the container 110, and to make it reversible to cause flow in one direction during charging and in the opposite direction during discharging. The pump 112 may be a positive displacement pump, peristaltic pump, rotary vane pump, or progressive cavity pump, as illustrated. The pump 112 may be located within the fluid flow path of the closed loop 114, as illustrated, or it may be an external pump such as a peristaltic pump. The pump 112 may operate at a fixed flow rate or a variable flow rate.

[0074] The flow rate within the closed loop 114 may be measured as a linear flow rate through the electrodes or a volumetric flow rate through the electrodes. If the theoretical linear flow rates differ between the two electrodes, the lower linear flow rate will determine the actual linear flow rate through the electrodes. The linear flow rate can be determined by several factors, including the permeability of the electrodes and separator (if present) to the pump 112 and the fluid electrolyte 106. In general, peristaltic pumps or rotary vane pumps can provide low or moderate flow rates with moderate power requirements. Progressive cavity pumps can provide higher flow rates with higher power requirements. Due to their higher power requirements, progressive power pumps may be more suitable for use in flow-through electrochemical cells in large steady-state batteries, such as those in grid storage and other electric utility applications, or in marine batteries.

[0075] Pump 112 may be powered by an external power source during charging, discharging, or both of the electrochemical cell 100. In particular, pump 112 may be powered by the same external energy source, such as a charger 144, during charging. Pump 112 may be powered by the electrochemical cell 100 itself during discharging. Pump 112 may include electronic isolation components so that, in particular, if it is located in a closed loop 114, the electrochemical cell 100 cannot be discharged except through the external circuit 136. Pump 112 may also be a positive displacement pump, such as a peristaltic pump, which physically isolates a short section of the electrolyte and thereby physically disrupts any possible electronic circuits along the closed loop 114. If pump 112 is located in the fluid flow path of the closed loop 114, pump 112 may include, or be coated with, a material capable of resisting decomposition by the fluid electrolyte or other chemicals found in the electrochemical cell 100, including the chemicals formed during the circulation of the electrochemical cell 100.

[0076] Figures 1A and 1B illustrate a single pump 112, but multiple pumps may be present. For example, a battery containing multiple electrochemical cells may include pumps between the electrochemical cells or sets of electrochemical cells to maintain the flow of the fluid electrolyte, especially when there are many electrochemical cells or when the permeability of each cell is not high. Multiple pumps 112 may be driven by one motor or multiple motors to help them remain synchronized.

[0077] The separator 108 may be a permeable membrane that allows the passage of working ions and the flow of the fluid electrolyte 106 through the membrane. The separator 108 may include woven fibers, nonwoven fibers, polymer films, ceramics, and naturally occurring materials. Nonwoven fibers may include cotton, nylon, polyester fibers, paper, and glass fibers. Polyester films may include polyethylene, polypropylene, poly(tetrafluoroethylene), poly(ethylene terephthalate), and polyvinyl chloride films. Naturally occurring materials may include rubber, asbestos, and wood. The separator 108 may have a thickness of 10 μm to 5,000 μm, 10 μm to 1,000 μm, 10 μm to 500 μm, 10 μm to 100 μm, or 20 μm to 70 μm, with thinner separators being most useful in many electrochemical cells 100.

[0078] The cathode 102 may be a porous cathode. The cathode 102 may include a cathode high-porosity metal current collector, such as a metal foam, which is permeable to the fluid electrolyte. The cathode high-porosity metal current collector is an electron conductor and provides a path for electrons to flow between the cathode active material and the external electronic circuit. The cathode high-porosity metal current collector also generally provides mechanical support for the cathode active material. Suitable high-porosity metal foams include Ni, Fe, Cu, and Al foams. The porosity of the cathode high-porosity metal current collector can be greater than 40%, such as 40% to 90% or 40% to 99%. In particular, the porosity may be 80% to 90%. The porous nature also allows the fluid electrolyte 106 to flow through the cathode 102.

[0079] A cathode high-porosity metal current collector may be coated with a cathode active material. In particular, the pore surface of the cathode high-porosity metal current collector may be coated with a cathode active material. After coating with the cathode active material, the cathode 102 may still maintain a porosity of at least 5%, at least 10%, or at least 20%, such as 5%-50%, 10%-50%, and 20%-50%, or a porosity sufficient to allow the fluid electrolyte 106 to flow at a set linear or volumetric flow rate.

[0080] The cathode active material may be coated with a synthetic film material to form a penetrable synthetic film around the cathode active material. The penetrable synthetic film can provide structural support to the cathode active material. The penetrable synthetic film can prevent the decomposition of the cathode active material by holding the cathode active material in place on the cathode high porosity metal current collector. Exemplary film materials, methods for forming the film, and film structures are discussed in more detail below.

[0081] The anode 104 may be a porous anode. For example, the anode 104 may include a highly porous metal current collector, such as a highly porous metal foam, which is permeable to the fluid electrolyte. The highly porous metal current collector is an electron conductor and provides a path for electrons to flow between the anode active material and the external electronic circuit. The highly porous metal current collector also generally provides mechanical support for the anode active material. Suitable highly porous metal foams include Ni, Fe, Cu, and Al foams. The porosity of the highly porous metal current collector can be greater than 40%, such as 40% to 90% or 40% to 99%. In particular, the porosity may be 80% to 90%. The porous nature also allows the fluid electrolyte 106 to flow through the anode 104.

[0082] The anode high-porosity metal current collector may be coated with an anode active material. In particular, the pore surface of the anode high-porosity metal current collector may be coated with an anode active material. After coating with the anode active material, the anode 104 may still maintain a porosity of at least 5%, at least 10%, or at least 20%, such as 5%-50%, 10%-50%, and 20%-50%, or a porosity sufficient to allow the fluid electrolyte 106 to flow at a set linear or volumetric flow rate.

[0083] The anode active material may be coated with a synthetic film material to form a penetrable synthetic film around the anode active material. The penetrable synthetic film can provide structural support to the anode active material. The penetrable synthetic film can prevent the decomposition of the anode active material by holding the anode active material in place on the anode high porosity metal current collector. Exemplary film materials, methods for forming the film, and film structures are discussed in more detail below.

[0084] In conventional electrochemical cells and batteries, expansion and physical distortion of the cathode and anode active materials, particularly during electrochemical cell cycles, can cause physical damage to the electrochemical cell, such as a disconnection between the active material and other electronically conductive materials present in the electrochemical cell. Volume changes can also result in distortion of the electrodes themselves, as well as the decomposition of the active material into loose powder that can fall off the electrodes. This causes performance degradation over multiple cycles. For example, an electrochemical cell may suffer a decrease in capacity over multiple cycles.

[0085] Generally, it is also desirable to design materials so as to maximize the energy density of the cell by maximizing the volume of the active material, thereby absorbing or releasing the maximum amount of mobile ions per unit volume of the electrode. The expansion and contraction of the volume of the active material can be significant, ranging from a minimum of 5% to a maximum of 50%. In current prior art, silicon anode materials, such as those used in some lithium-ion cells, have, for example, very high volume changes and suffer performance degradation over multiple cycles.

[0086] Figure 1A illustrates the electrochemical cell 100 during discharge when the active material is an alkali metal ion active material. The fluid electrolyte 106 is pumped in direction 118 by a positive displacement pump 112 rotating in direction 120. Electrons are conducted in direction 122 along an external circuit 136 through an electrical load 138, such as a device powered by the electrochemical cell 100. Working ions, lithium ions in this embodiment, are conducted in direction 124 between the anode 104 and the cathode 102. Electrochemical reactions within the anode 104 release lithium ions from the anode active material, while electrochemical reactions within the cathode 102 capture lithium ions within the cathode active material.

[0087] Figure 1B illustrates the electrochemical cell 100 during charging when the active material is an alkali metal ion active material. The fluid electrolyte 106 is pumped in the opposite direction 118 by a positive displacement pump 112 rotating in direction 126. Electrons are conducted in direction 130 along an external circuit 136 through a charger 144 which may be connected to an external power source. Working ions, lithium ions in this embodiment, are conducted in the opposite direction 124 between the anode 104 and the cathode 102 in direction 134. Electrochemical reactions in the cathode 102 release lithium ions from the cathode active material, while electrochemical reactions in the anode 104 capture lithium ions within the anode active material.

[0088] The flow of the fluid electrolyte 106 through the anode 104 and cathode 102 in direction 124, or through the cathode 102 and anode 104 in direction 134, increases the effective ion mobility of working ions in the anode 104, cathode 102, or typically both, and the higher rate of flow has a further effect. Higher working ion mobility in one or both electrodes can reduce resistive losses in the electrolyte during charging, discharging, or typically both.

[0089] (penetrable synthetic membrane) The active material may be coated by a permeable synthetic film. The film may be made from a film material. The film can be coated over the entire surface of the active material. The film is mechanically strong but can allow free conduction of ions between the electrode and the electrolyte. This type of film may also be described as "semi-permeable," meaning that only certain defined ion species can traverse the film material. For example, some films may allow hydrogen ions (i.e., protons) to pass through, but hydroxyl ions cannot. The film may be made from the same type and thickness as those known in the art for use in separators.

[0090] The membrane may be "self-supporting." A self-supporting membrane is a membrane that provides support for an active material. The self-supporting membrane maintains contact with the active material even when the active material expands or contracts during charging or discharging. The self-supporting membrane may expand and contract together with the active material.

[0091] The film may be formed on the active material by dissolving the film material in a liquid solvent, resulting in a low-viscosity liquid that can impregnate the electrode. The solvent can then be removed by drying and heating, depositing the film material onto the active material and forming a film. The concentration of the film material in the solution can be used to determine the final thickness of the film. The thickness may be in the range of 0.01 to 10 microns.

[0092] The membrane material may be a polymer resin. Polymer resins are used because they maintain flexibility and elasticity after polymerization. Exemplary polymer resins may include, but are not limited to, semicrystalline polyolefins, polyoxymethylenes, and isotactic poly(4-methyl-1-pentene). Hybrids of incompatible polymers, such as polyethylene-polypropylene, polystyrene-polypropylene, poly(ethylene terephthalate)-polypropylene hybrids, and ultra-high molecular weight polyethylene, in which at least one polymer has a crystalline structure, may also be used.

[0093] The polymer resin may be deposited on the active material using a wet process. The wet process includes the steps of mixing, heating, impregnation, and additive removal. The polymer resin may first be mixed with other additives such as paraffin oil, antioxidants, and substitute powders. The mixture may then be heated to produce a homogeneous solution. The heated solution may be sprayed into the pores of the electrode to create a gel-like film that coats the active material, which may then be dried and cured. Vacuum impregnation may also be used to eliminate the possibility of air pockets in the pores of the electrode, which may prevent complete coating. The substitute additives may then be removed using a volatile solvent to form a microporous film coating.

[0094] The wet process can also be suitable for both crystalline and amorphous polymers. Some polymers can impart desirable mechanical properties to the film, such as stopping cell operation when they become too hot. Polymers suitable for use as film materials may include polyolefin-based materials with a semi-crystalline structure. Polyolefin materials with a semi-crystalline structure may include, but are not limited to, polyethylene, polypropylene, and its hybrids such as polyethylene-polypropylene.

[0095] The membrane material may also contain graft polymers. For example, microporous poly(methyl methacrylate)-grafted and siloxane-grafted polyethylene may be the membrane material. These graft polymers may exhibit favorable surface morphological structures and electrochemical properties compared to conventional polyethylene. In another embodiment, polytriphenylamine (PTPAn)-modified materials are electroactive, which may provide reversible overcharge protection. In yet another embodiment, polyvinylidene fluoride (PVDF) nanofiber webs may be used as the membrane material to improve conductivity and dimensional stability.

[0096] Methods for forming the film may also include solution-based treatment and aqueous emulsion polymerization. In solution-based treatment, typical organic solvents may include, but are not limited to, dimethylformamide and butanone. In aqueous emulsion polymerization, the fluorosurfactant perfluorononanoic acid may be used as a processing aid in the form of a negative ion by solubilizing the monomer. This process can be used to polymerize the PVDF and simultaneously deposit it on the surface of the active material to form a film.

[0097] PVDF films may also be used to mechanically stabilize active materials, which may be in the form of fine powders. A solution of 1-2% PVDF by mass in N-methyl-2-pyrrolidone (NMP) may be mixed with active material powder to form a slurry that can be used to impregnate a metal foam current collector, or it may be mixed with conductive additives such as carbon black, carbon nanofibers, or metal powders. The NMP may then be evaporated to form a composite electrode. PVDF may be used because it is chemically inert above the range of electrode potentials used and does not chemically react with common electrolytes.

[0098] PVDF may also be used because it is a thermoplastic material with high resistance to chemical modification. It is compatible with strong acids, weak acids, ionic salines, halogen compounds, hydrocarbons, aromatic solvents, aliphatic solvents, oxidizing agents, and weak bases. However, it exhibits chemical sensitivity to strong bases, esters, and ketones.

[0099] Synthetic rubbers such as Viton and fluoroelastomers may also be used as membrane materials. Synthetic rubbers are elastic and may be used as membranes for active materials that exhibit large volume expansion. Viton fluoroelastomers belong to a family consisting of copolymers of hexafluoropropylene (HFP) and vinylidene fluoride (VDF or VF2), terpolymers of tetrafluoroethylene (TFE), vinylidene fluoride (VDF), and hexafluoropropylene (HFP), and perfluoromethyl vinyl ether (PMVE)-containing copolymers. The fluorine content of the most common grades varies between 66 and 70%.

[0100] Viton is generally compatible with concentrated inorganic acids and bases. It is resistant to most hydrocarbons. Since it is soluble in acetone, methyl ethyl ketone, and ethyl acetate, these solvents can be used to deposit Viton onto active materials, and films can be formed using wet methods as previously described.

[0101] Fluorovinyl methylsiloxane rubber (FVMQ) may be used as a membrane material for NiMH cells using KOH electrolytes. FVMQ generally exhibits very high resistance to strongly alkaline solutions. Since FVMQ is soluble in ketones, ketones can be used as solvents for depositing FVMQ onto active materials, and membranes can be formed using wet methods as previously described.

[0102] (Design of electrodes with permeable membranes) The membrane is self-supporting and unlikely to be removed from the electrode under repeated charging and discharging cycles. For example, if the metal current collector is a metal foam, the active material may surround each web of the foam. Then, the membrane may also surround each web of the foam, forming a closed loop. This closed loop can prevent the membrane from being removed from the metal current collector.

[0103] Woven wire mesh electrodes are also suitable for coating with films that retain mechanical integrity throughout many charging and discharging cycles.

[0104] Figure 2A is a cross-sectional view of the wire 200 of the woven wire electrode. The metal current collector 210 forms the core of the wire 200, and the active material 220 surrounds the metal current collector 210. The film 230 surrounds the active material 220 and forms a closed loop that prevents the removal of the film 230.

[0105] Figure 2B is a cross-sectional view of the wire 200 of the woven wire electrode after the deposition of additional active material resulting from charging or discharging. The amount of active material 220 surrounding the metal current collector 210 is greater than that in Figure 2A, and the film 230 is stretched to accommodate the expansion of the active material 220. The stretched portion of the film 230 holds the active material 220 in place on the metal current collector 210.

[0106] Expansion occurs, for example, within the active material used in the anode of a lithium-ion cell during the charging process, when lithium ions are interposed within the active material. The membrane may expand without being removed, increasing the volume of the active material and improving the integrity of the electrode.

[0107] Figures 2A and 2B show a porous electrode including a woven wire mesh current collector, but the porous electrode may have a different form of current collector. For example, the porous electrode may include a metal foam current collector, which may have a porosity of 80-90%. Similarly, the porous electrode may include a 3D printed metal current collector.

[0108] The combination of the cathode active material, the anode active material, and the electrolyte may be based on known electrochemical cell principles, such that working ions can participate in electrochemical reactions that enable the electrochemical cell to function. For example, the anode and cathode may be capable of producing a given theoretical voltage, and the electrolyte may be an electrolyte that is expected to be stable at a given voltage.

[0109] (Examples) The following embodiments are provided to further illustrate the principles and specific aspects of the present invention. They are not intended to, and should not be construed to, encompass the entire scope of all aspects of the present invention.

[0110] (Example 1: Nickel metal hydride (NiMH) cell electrode) Porous electrodes with a permeable membrane may be constructed as shown in Figures 2A and 2B.

[0111] First, the metal current collector is fabricated from a sheet of nickel foam with a pore density of 110 pores per inch (PPI). Typical pore sizes within the nickel foam sheet range from 100 μm at the 10th percentile to 500 μm at the 90th percentile, with the most common size being 230 μm at the 50th percentile. The nickel metal forms a fine, random mesh with wire diameters ranging from 50 μm to 150 μm.

[0112] The metal current collector is then uniformly coated with the active material to a thickness of 22 μm to 74 μm in order to maximize the volume of the active material in a given volume of the electrode, while maintaining an acceptable open volume to allow the electrolyte to pass through.

[0113] Next, the active material is produced by grinding a solid casting material into a powder. The resulting powder has a range of particle sizes. The powder then passes through a cascade of progressively finer sieves, classifying the powder into size intervals. The powder is classified according to the mesh size of the sieve from which it is collected. For example, powder collected by a sieve with a 400 wire / inch mesh (i.e., 400 mesh) is classified as "+400". Powder graded as "-400+600" has a particle size range of 22 μm to 38 μm, while coarser powder graded as "-200+400" has a range of 38 μm to 74 μm. In this embodiment, both -400+600 and -200+400 may be used. Using a given size range can result in a fixed and predictable final thickness of the active material deposited on the metal foam.

[0114] The powder is then mixed with a solution of polyvinyl alcohol (PVA) in distilled water at a weight ratio of 2% to 20% PVA / water. The PVA is dissolved by adding it to 90°C water while continuously stirring the mixture with a magnetic stirring rod, and then allowing the PVA to dissolve in the solution over a period of 1 hour. To deposit the active material onto a metal current collector, the metal current collector is lowered into the mixture, slowly withdrawn, and then air-dried. Very fine particle sizes in the range of 0.005 to 5 μm, graphite powder and / or pure nickel powder, may be added to the PVA / aqueous solution to facilitate good electrical contact between the active material and the nickel foam.

[0115] After the metal current collector is coated with the active material, the active material is coated with a film. A solution of uncured or partially cured ethylene propylene diene monomer (EPDM) rubber is dissolved in perchloroethylene (PERC) in a sealed flask over 1-2 hours, while heating to 60°C, using a condenser fitted to recover evaporated PERC. The solution is EPDM / PERC in a weight ratio of 1-10%, which can be adjusted to control the final film thickness. The EPDM rubber is obtained in a non-polymerized form and may be mixed with a suitable vulcanizing or curing agent.

[0116] The electrodes are immersed in an EPDM solution and air-dried before being placed in a forced-air convection oven at a temperature of 150-200°C for 1-5 hours to cure the rubber. Depending on the concentration of the rubber solution, a film thickness between 0.05 μm and 5 μm may be formed, which is mechanically stable while allowing active ions in the electrolyte solution to pass through. The EPDM rubber is not affected by the highly alkaline aqueous solutions typically used in NiMH electrolytes.

[0117] (Example 2: Measurement of permeable film) The quality of the film may be tested by immersing a 2.5 cm × 5 cm sample electrode in a 1 N KOH aqueous solution with an uncoated nickel foam counter electrode. To measure the complex impedance of the electrode with the active material, contact is made between both electrodes using a multi-frequency LCR meter. In one embodiment, 6.25 cm 2 A low-Q capacitive impedance within the range of 100 μF is measured for the active electrode area. This capacitance is relatively constant over the frequency range of 100 Hz to 10 kHz, demonstrating that the rubber membrane is the dominant component of the equivalent electronic circuit, while the measured parallel resistance impedance indicates that ions in the solution can easily traverse the membrane.

[0118] The details provided herein relate to the design and construction of NiMH type cells, but variations in materials and methods may be used without departing from the claimed invention to adapt the invention to different chemical phenomena.

[0119] The subject matter disclosed above is intended to be illustrative and not restrictive, and the appended claims are intended to cover all such modifications, improvements, and other embodiments that fall within the true spirit and scope of this disclosure. Therefore, to the maximum extent permitted by law, the scope of this disclosure shall be determined by the broadest permissible interpretation of the following claims and their equivalents, and shall not be limited or restricted by the foregoing detailed description.

Claims

[Claim 1] The invention described herein.

Images