Graphite charge cathode

Abstract

A process for fabricating an electrode involves forming a solid layer of an electrode material on a surface of a solid current collector by compression molding. The electrode material contains a mixture of an electrically conductive graphite and a binder, the binder being a moldable polymer. A layer of an electrically insulating material is formed, for example by molding, on a surface of the layer of the electrode material so that a portion of the surface of the layer of the electrode material remains uncovered by the insulating material. An electrode fabricated by the process is preferably a charge cathode for an electrochemical cell.

Description

[0001] GRAPHITE CHARGE CATHODE

[0002] Cross-reference to Related

[0003] This application claims the benefit of United States provisional application USSN 63 / 615,102 filed December 27, 2023, the entire contents of which is herein incorporated by reference.

[0004] Field

[0005] This application relates to electrochemical devices, in particular to electrodes useful in electrochemical devices.

[0006] During charging of zinc-based electrochemical cells, zincate reduction reaction to form zinc metal happens at the cathode. The zinc is electroplated and mechanically removed from the active sites at specific intervals to control the size and morphology of the zinc metal. This event happens multiple times until the charge is completed. Therefore, the charge electrode requires high electrical conductivity to decrease the charge transfer resistance, and active sites with low adhesion and robustness for easy removal of the electroplated zinc and to withstand the mechanical stress.

[0007] The charge cathode consists of one insulate area (epoxy), one conductive area (active sites / graphite), and the current collector (mild steel). An electrode that has multiple materials is prone to failure due to the difference in mechanical, electrical, thermal, and compatibility properties. Current technology uses graphite rods that are difficult to tailor and are expensive compared to untreated or powdered graphite. Also, the manufacturing process of current charge cathodes is complicated, labor-intensive, and costly, which makes it not feasible for mass production.

[0008] Graphite rods are porous due to the manufacturing process (i.e., extrusion), and the options to change its properties are limited. In current technology, the graphite rods are pressed into a steel current collector. During this process, the graphite gets scratched and creates paths for the electrolyte to the current collector, which gets corroded. The corrosion affects the electrical conductivity, resulting in high charge voltage and H2evolution levels. Also, the electrolyte diffusion through the graphite rods causes delamination of the epoxy that insulates the electrode. Another problem related to the graphite porosity is that, during charging, the Zn starts growing inside the graphite. That results in higher forces to remove the Zn, and therefore, degradation of the graphite. There remains a need for more robust graphite-based electrodes and processes for production of such electrodes, which are simpler and less costly to implement, especially for mass production.

[0009] Summary

[0010] A process for fabricating an electrode comprises: forming a solid layer of an electrode material on a surface of a solid current collector by compression molding, wherein the electrode material comprises a mixture of an electrically conductive graphite and a binder, the binder comprising a moldable polymer; and, forming a layer of an electrically insulating material on a surface of the layer of the electrode material so that a portion of the surface of the layer of the electrode material remains uncovered by the insulating material.

[0011] An electrode is fabricated by the process. In an embodiment, the electrode is a charge cathode. In other embodiments, the electrode is an anode or a different type of cathode.

[0012] In one embodiment of a charge cathode for an electrochemical cell, the charge cathode comprises: a current collector comprising a mesh of a solid electrically conductive material, the current collector having opposed first and second surfaces, the mesh comprising through-apertures between the first and second surfaces; a first layer of a solid cathode material compression molded on to the first surface of the current collector and a second layer of the solid cathode material compression molded on to the second surface of the current collector thereby encapsulating the current collector between the first layer and the second layer of the solid cathode material, the first layer and the second layer compression bonded to each other through the apertures in the mesh, the first and second layers having raised regions on outer surfaces thereof, the solid cathode material comprising electrically conductive graphite in a matrix of a moldable polymeric binder; a layer of an electrically insulating material covering the layers of the cathode material so that distal faces of the raised regions relative to the current collector are not covered by the insulating material; and, a bus bar electrically connected to the current collector.

[0013] The process provides one or more of the following advantages: permits mass production of graphite electrodes through compression molding; avoids press-fitting graphite rods into a current collector; provides a continuous layer of electrode material on the current collector rather than localized graphite nodes; permits easy modification of the electrode’s layout, surface area, and active sites shape and configuration; permits easy modification of the composition components (e.g., relative amounts of binder and graphite) to improve mechanical, electrical and electrochemical properties of the electrode; and lowers production costs compared to other processes.

[0014] Electrodes made using the process have one or more of the following favorable characteristics: enhanced material stability in an electrolyte environment; better compatibility (reduction in the tendency to delaminate) and stability during charging because the insulating material may comprise the same material as the moldable polymer used as the binder in the conductive layer; and reduction in adhesion with electroplated zinc without losing control over zinc growth and morphology. Low porosity of the electrode material results in low adhesion between electroplated zinc and the active sites of the electrode.

[0015] The solid electrode material comprising a mixture of an electrically conductive graphite and a binder may have one or more of the following favorable characteristics in an electrochemical cell. In some embodiments, conductivity of the electrode material is at least 2 times higher, or at least 3 times higher, or at least 4 times higher, or at least 5 times higher, than conductivity of a 99% purity commercial graphite electrode. In some embodiments, zinc metal removal force for zinc adhered to an active site of the electrode is at least 2 times less, or at least 3 times less, or at least 4 times less, or at least 5 times less than the zinc removal force at an active site of a 99% purity commercial graphite electrode. In some embodiments, hydrogen evolution potential of the electrode material is at least -1.75 V, or at least -1.8 V, as measured against a Hg / HgO reference electrode containing 20 wt% KOH. In some embodiments, corrosion rate of the electrode material in an electrolyte comprising 40 wt% KOH is at least 2 times lower, or at least 3 times lower, or at least 4 times lower, or at least 5 times lower, than the corrosion rate of a 99% purity commercial graphite electrode.

[0016] The electrode comprises a solid current collector, at least one solid layer of an electrode material on a surface of the solid current collector and an electrically insulating material on a surface of the electrode material so that a portion of the surface of the electrode material remains uncovered by the insulating material. The electrode may also comprise a bus bar electrically connected to the current collector.

[0017] In some embodiments, the current collector comprises an electrically conductive sheet of material. The current collector may comprise any suitable electrically conductive material, for example a metal (e.g., copper, nickel, steel, gold, silver, and the like), a conductive non-metal material (e.g., conductive carbon, doped polymers, and the like) or any mixture thereof. In some embodiments, copper or nickel is preferred. In some embodiments, the current collector comprises copper metal. In some embodiments, the layer of electrode material is compression bonded to the surface of the current collector. In some embodiments, the current collector comprises through-apertures therein to permit bonding of electrode material layers to each other through the apertures when opposed first and second surfaces of the current collector have layers of the electrode material thereon. Bonding of the electrode material layers to each other through the apertures in the surface of the current collector provides for a more secure mechanical bonding of the electrode material layers to the current collector. In some embodiments, solid layers of the electrode material are compression bonded to each other through the apertures in the current collector. In some embodiments the current collector is a mesh. The entirety of or only a portion of the surface of the current collector may be covered by the layer of the electrode material. In some embodiments, the entirety of the current collector is covered by the layer of the electrode material. In some embodiments, the current collector is configured for connection to the bus bar. In one embodiment, a first layer of a solid electrode material is compression molded on to a first surface of the current collector and a second layer of the solid electrode material is compression molded on to a second surface of the current collector thereby encapsulating at least a portion of the current collector, for example the entirety of the current collector, between the first layer and the second layer of the solid electrode material, the first layer and the second layer compression bonded to each other through apertures in the current collector.

[0018] The solid electrode material comprises a mixture of an electrically conductive graphite and a binder. In some embodiments, the electrode material is a cathode material. In some embodiments, the electrode material comprises the electrically conductive graphite in a matrix of the binder. In some embodiments, the binder is electrically non-conductive. In some embodiments, the electrode material comprises the graphite and the binder in a ratio of graphite-to-binder in a range of 90: 10 w / w to 50:50 w / w, or 80:20 w / w to 60:40 w / w, or 80:20 w / w to 65:35 w / w, or 80:20 w / w to 70:30 w / w. Graphite-to-binder ratios in these ranges lead to improved electrical and mechanical properties of the electrode material. Electrode materials having such graphite-to-binder ratios provide low charge transfer resistance, especially when combined with a current collector (especially Cu or Ni mesh).

[0019] The binder has at least the following functions: to bind the graphite into a solid monolithic piece of electrode material; to reduce porosity of the electrode; and, to create insulated areas with high hydrophobicity on the active sites where zinc gets electroplated, the hydrophobic areas reducing the area of contact between the zinc and the graphite thereby lowering adhesion of zinc to the electrode and lowering force required to remove the zinc from the electrode. Further, the binder material can also be used as the electrically insulating material for the electrode resulting in less possibility that the insulating layer will delaminate from the electrode.

[0020] In some embodiments, the graphite is in powder form. In some embodiments, the graphite comprises natural graphite, synthetic graphite, or a hybrid of synthetic and natural graphite in a particle size range of 1 pm to 100 pm. In some embodiments, the binder comprises a moldable polymer, especially a compression malleable polymer. In some embodiments, the moldable polymer comprises a thermoplastic, an elastomer or a mixture thereof. In some embodiments, the moldable polymer has a melting temperature in a range of 0°C to 300°C, or 40°C to 300°C, or 50°C to 300°C. In some embodiments, the moldable polymer comprises a polyolefin. In some embodiments, the moldable polymer comprises low-density polyethylene (LDPE), high-density polyethylene (HDPE), ultra-high molecular weight polyethylene (UHMWPE), linear low-density polyethylene (LLDPE), polypropylene (PP), polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP), thermoplastic urethane (TPU), ethylene-propylene-diene-monomer (EPDM) elastomer, or any mixture thereof.

[0021] The electrode material covers and is in contact with at least one surface of the current collector. In an electrochemical cell (e.g., a battery and the like), the electrode material is in contact with an electrolyte so that electrical current can pass between the electrode material and the current collector. The current collector should be otherwise electrically insulated from the electrolyte. For this reason, and to increase electrical contact between the electrode material and the electrolyte, in some embodiments at least a portion of the current collector is encapsulated between two layers of the electrode material, a first layer of the electrode material on a first surface of the current collector and a second layer of the electrode material on a second, preferably opposed, surface of the current collector. Thus, the current collector may be sandwiched between the first and second layers of the electrode material. In some embodiments, the layer or layers of the electrode material have raised regions on an outer surface or outer surfaces thereof. An outer of the layer of electrode material is a surface in contact with the electrolyte, which is opposed to a surface of the electrode material that is in contact with a surface of the current collector.

[0022] In some embodiments, the electrode comprises a layer of an electrically insulating material covering a surface of the layer of the electrode material so that a portion of the surface of the layer of the electrode material remains uncovered by the insulating material. In this manner, only a portion of the electrode material is in contact with the electrolyte. The uncovered portion of the electrode material becomes the active site or sites for the electrode. Current flows between the electrolyte and the current collector only through the portion of the electrode material that is exposed. In some embodiments, when the layer of the electrode material has raised regions on an outer surface thereof, the portion of the surface of the layer of the electrode material which remains uncovered by the insulating material are distal faces of the raised regions relative to the current collector. Preferably, the outer layer of the electrode is entirely comprised of the electrically insulating material except where the electrode material is exposed. In some embodiments, the electrically insulating material comprises a polymeric material. The polymeric material may be the same as or different than the binder. Thus, in some embodiments, the polymeric material is low-density polyethylene (LDPE), high-density polyethylene (HDPE), ultra-high molecular weight polyethylene (UHMWPE), linear low-density polyethylene (LLDPE), polypropylene (PP), polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP), thermoplastic urethane (TPU), ethylene-propylene-diene-monomer (EPDM) elastomer, or any mixture thereof. Other materials include, for example, thermoset polymers derived from resins such as epoxy, another curable thermoset resin or mixtures thereof. Because the electrically insulating material can be the same as the binder, compatibility between the binder and the electrically insulating layer is enhanced reducing the possibility of delamination of the electrode over time.

[0023] The electrode may comprise a bus bar electrically connected to the current collector for connecting the electrode to an electrical circuit in the electrochemical cell. The bus bar comprises an electrical conductor, for example metals such as stainless steel, copper, nickel and the like.

[0024] The process for fabricating the electrode comprises: forming a solid layer of an electrode material on a surface of a solid current collector by compression molding, wherein the electrode material comprises a mixture of an electrically conductive graphite and a binder, the binder comprising a moldable polymer; and, forming a layer of an electrically insulating material on a surface of the layer of the electrode material so that a portion of the surface of the layer of the electrode material remains uncovered by the insulating material. Forming layers of material on surfaces of other material may be accomplished by various methods including molding (e.g., compression molding), doctor blading, screen printing, spray coating and the like. Molding is preferred.

[0025] Forming the solid layer of the electrode material on the surface of the solid current collector is accomplished by compression molding, preferably in a compression mold. The electrode material may be a mixture of the graphite and the binder (e.g., a mixture of graphite and binder powders or a mixture of graphite in liquid binder), which is compression molded directly on the surface of the current collector. However, the compression molding preferably comprises molding a pre-form of the electrode material on to the surface of the current collector. In some embodiments, the graphite and the binder are mixed in a ratio of graphite-to-binder in a range of 90:10 w / w to 50:50 w / w, or 80:20 w / w to 60:40 w / w.

[0026] A pre-form comprises a monolithic solid piece of electrode material. The pre-form comprises graphite dispersed in a matrix of solid binder. The pre-form is preferably prepared by mixing graphite powder with the binder and then molding the mixture, for example in a mold, into the monolithic solid piece of electrode material. The mixing process may involve the use of liquid powdered binder, however binder in powder form is preferred. In some embodiments, the pre-form has a regular polygonal shape, for example a rectangular prism. Molding the mixture into the pre-form is accomplished at a temperature and pressure that depends on the melting point of the binder. In some embodiments, the molding temperature is in a range of 130°C to 350°C. In some embodiments, the molding temperature is in a range of 200°C to 300°C. In some embodiments, the molding temperature is in a range of 250°C to 280°C. In some embodiments, the molding pressure is in a range of 0.05 kN / cm2to 1.0 kN / cm2. In some embodiments, the molding pressure is in a range of 0.1 kN / cm2to 0.75 kN / cm2. In some embodiments, the molding pressure is in a range of 0.2 kN / cm2to 0.5 kN / cm2.

[0027] Compression molding of the electrode material on to the surface of the current collector may be accomplished in a compression mold. In the compression mold, the electrode material is situated on the portion of the current collector to be covered, and the electrode material and the current collector are pressed together at a temperature and pressure to form a composite with the electrode material compression bonded to the current collector. The temperature and pressure during compression molding depends on the melting point of the binder. In some embodiments, the compression molding temperature is in a range of 130°C to 350°C. In some embodiments, the compression molding temperature is in a range of 200°C to 300°C. In some embodiments, the compression molding temperature is in a range of 250°C to 280°C. In some embodiments, the compression molding pressure is in a range of 1.0 kN / cm2to 3.0 kN / cm2. In some embodiments, the compression molding pressure is in a range of 1.25 kN / cm2to 2.5 kN / cm2. In some embodiments, the compression molding pressure is in a range of 1.5 kN / cm2to 2.0 kN / cm2.

[0028] In some embodiments, the electrode material is situated on portions of opposed faces of the current collector so that compression molding results in a composite in which the current collector is encapsulated between two layers of the electrode material. Thus, in some embodiment, the process comprises forming a first solid layer of the electrode material on a first surface of the solid current collector and forming a second solid layer of the electrode material on a second surface of the solid current collector, the second surface opposed to the first surface so that the current collector is sandwiched between the first and second solid layers of the electrode material. In some embodiments, the current collector comprises through-apertures therein (e.g., the current collector is a mesh) and the two layers of the electrode material situated on opposed sided of the current collector are compression bonded to each other through the apertures in the current collector. In some embodiments where a pre-form of the electrode material is used, the pre-form may be formed larger than the compression mold and the pre-form cut to a size to fit in the compression mold. In some embodiments, the compression mold comprises inner surface features that impart corresponding features to an outer surface of the layer of electrode material. In some embodiments, the compression mold has depressions on an inner surface thereof, which result in forming raised regions on an outer surface of the layer of the electrode material during the compression molding. The raised regions may be of any desired shape and / or size. In some embodiments, one or more of the raised regions are cylindrical in shape.

[0029] To further protect the electrode from corrosive environments such as an electrolyte the process further comprises forming a layer of an electrically insulating material on a surface of the layer of the electrode material. Forming the layer of insulating material is performed so that a portion of the surface of the layer of the electrode material remains uncovered by the insulating material, thereby electrically insulating the composite while the uncovered portion of the layer of insulating material becomes the active site or sites of the electrode. The insulating material may be the same or different than the binder. Preferably, the insulating material is the same as or compatible with the binder so that a stronger bond is formed between the layer of insulating material and the layer of electrode material, which lessens the possibility of delamination of the electrode. In some embodiments, the layer of electrically insulating material is formed on the layer of electrode material by molding. Molding is performed at a temperature and pressure that depends on the melting point of the insulating material. In some embodiments, the molding temperature is in a range of 80°C to 200°C. In some embodiments, the molding temperature is in a range of 100°C to 180°C. In some embodiments, the molding temperature is in a range of 110°C to 150°C. In some embodiments, the molding pressure is in a range of 0.02 kN / cm2to 1.0 kN / cm2. In some embodiments, the molding pressure is in a range of 0.05 kN / cm2to 0.5 kN / cm2. In some embodiments, the molding pressure is in a range of 0.1 kN / cm2to 0.25 kN / cm2. To provide a portion of the surface of the layer of the electrode material that remains uncovered by the insulating material, one or more techniques may be applied. In some embodiments, the layer of the electrode material is first fully covered by the electrically insulating material and then a portion of the layer of the insulating material is removed to expose the portion of the electrode material. Removal of the insulating material may be accomplished by any suitable technique including milling, laser ablation, chemical treatment, and the like. Physical techniques, such as milling and laser ablation, are preferred. In some embodiments, the layer of the electrode material is only partially covered during molding of the insulating material so that the portion of the layer of electrode material to be left uncovered by the insulating material is not covered when the electrode comes out of the final molding step. Whatever technique is used, when the layer of the electrode material comprises raised regions, the uncovered portion of the layer of electrode material preferably comprises distal faces of the raised regions. The distal faces of the raised regions of the layer of electrode material are the faces that are directly opposed to the face of the layer of electrode material that is in contact with the current collector. Thus, the distal faces of the raised regions are distal relative to the current collector. In some embodiments, the distal faces of the raised regions are flush with an outer surface of the layer of the insulating material. When the distal faces of the raised regions are flush with an outer surface of the layer of the insulating material, the electrode has an outer surface that is smooth, which assists with removing materials (e.g., zinc metal) that collect on the electrode during operation of the electrode.

[0030] In some embodiments, two or all three of the molding steps are accomplished in a single common mold holder block using different mold inserts having appropriately designed cavities. In some embodiments, each of forming the pre-form, forming the composite, and insulating the composite are performed in the same mold holder block using three different inserts.

[0031] The process for fabricating the electrode produces an electrode, for example a charge cathode, which provides beneficial characteristics to the electrochemical cell in which the electrode is used. For example, in a zinc-based electrochemical cell, lower zinc adhesion to the electrode permits charging at higher molarities of zinc ion in the electrolyte, and higher hydrogen gas evolution overpotential permits charging at lower molarities of zinc ion in the electrolyte with lower current densities. Thus, the operable molarity range for zinc ion is wider rather than needing to keep the molarity around 1.6 M. In some embodiments, the zinc ion molarity is in a range of 0.1 M to 3.5 M or even broader. In some embodiments, the zinc ion molarity is in a range of 0.2 M to 3.0 M. In some embodiments, the zinc ion molarity is in a range of 0.25 M to 2.5 M. Further, using compression molding with different mold inserts and a common holder block provides greater flexibility of changing the electrode configurations, for example, the size and layout of active site, thereby providing a quick and easy way of adjusting charge current densities in the cell. In some embodiments, the electrode is operable at a current density in a range of 0.1 mA / cm2to 280 mA / cm2, preferably 11 mA / cm2to 50 mA / cm2. Furthermore, the robustness, compatibility and stability of the electrode enables a greater range of electrode wiping parameters (e.g., frequency, speed, force) and simplifies the wiper mechanism because zinc removal requires less force. In some embodiments, zinc metal adheres to active sites of the electrode with an adhesion force in a range of 0.5 N to 3 N per active site, preferably 1 N to 2 N per active site.

[0032] Further features will be described or will become apparent in the course of the following detailed description. It should be understood that each feature described herein may be utilized in any combination with any one or more of the other described features, and that each feature does not necessarily rely on the presence of another feature except where evident to one of skill in the art.

[0033] Brief Description of the Drawings

[0034] For clearer understanding, preferred embodiments will now be described in detail by way of example, with reference to the accompanying drawings, in which:

[0035] Fig. 1 depicts a flow chart of steps in an embodiment of a process for fabricating an electrode.

[0036] Fig. 2A depicts a side view of a pre-form comprising a monolithic solid piece of electrode material used in the process of Fig. 1.

[0037] Fig. 2B depicts a front perspective view of a current collector connected to a bus bar for use in the process of Fig. 1 .

[0038] Fig. 2C depicts a front perspective view of an electrode produced during the process of Fig. 1 but without a layer of insulating material.

[0039] Fig. 2D depicts an exploded view of an electrode produced by the process of Fig. 1.

[0040] Fig. 3 depicts optical images of surfaces of graphite / thermoplastic composites with different ratios of binders. Fig. 4 depicts optical images of surfaces of charge cathode active sites next to thermoplastic insulation where: a) active site A1 comprises graphite, LDPE and FEP and thermoplastic insulation T1 comprises LDPE.

[0041] Fig. 5 depicts a graph of load (N) at five columns of active sites on a pressed graphite cathode composed of a cathode material having 70 wt% graphite, 10 wt% FEP and 20 wt% LDPE on a Cu current collector illustrating zinc removal force in a wiper-based zinc removal system.

[0042] Fig. 6 depicts a graph of load (N) at three columns of active sites on a charge cathode comprising commercial graphite rods inserted into a mild steel current collector illustrating zinc removal force in a wiper-based zinc removal system.

[0043] Fig. 7 depicts a graph of load (N) at columns of active sites on a pressed graphite cathode composed of a cathode material having 70 wt% graphite, 10 wt% FEP and 20 wt% LDPE on a Cu current collector illustrating zinc removal force in a wiper-based zinc removal system when the current densities were 11 mA / cm2.

[0044] Fig. 8 depicts a graph of current (A) vs. potential (V) showing a typical result for a linear sweep voltammetry (LSV) experiment used to determine the hydrogen evolution overpotential of an electrochemical cell with a pressed graphite charge cathode material comprising 70 wt% graphite, 10 wt% FEP and 20 wt% LDPE.

[0045] Fig. 9 depicts a Tafel plot of potential (V) vs. current (A) extracted from linear sweep voltammetry (LSV) for determining corrosion rate of a pressed graphite charge cathode material comprising 70 wt% graphite, 10 wt% FEP and 20 wt% LDPE in an electrochemical cell containing an electrolyte comprising 40% KOH.

[0046] Detailed Description

[0047] With reference to Fig. 1 and Fig. 2Ato Fig. 2D, a process for producing an electrode 1 comprises the following Steps.

[0048] Step 1 comprises mixing electrode material components (e.g., powdered graphite, together with any additives, with powdered binder) to form a substantially homogeneous mixture of dry components. Mixing is performed in any suitable mixer, for example a mechanical mixer, a centrifugal mixer, a sonic mixer or the like. A substantially homogeneous mixture is blended sufficiently thoroughly that every sample of the mixture has about the same amounts of each substance. Step 2 comprises transferring the dry electrode material mixture to a first mold insert with a cavity designed to produce a solid pre-form 5 (see Fig. 2A), which is a flat plaque (i.e., a rectangular prism), when the mixture is heated to at least the melting point of the binder. While cooling the heated mixture at ambient temperature after melting the binder, application of low pressure, for example in a range of 0.2 kN / cm2to 0.5 kN / cm2, in the first mold insert can assist with molding the pre-form 5 while the mixture solidifies during pressing. The pre-form 5 comprises the graphite dispersed in a matrix of the binder, the melted and pressed binder having fused the mixture into a solid monolithic piece of electrode material.

[0049] Step 3 comprises cutting two pre-forms produced in accordance with Step 2 to a size that fits into a second mold insert. This Step can be omitted if the pre-forms are of suitable size directly from Step 2.

[0050] Step 4 comprises attaching a bus bar 15 to a metallic current collector 10 (see Fig. 2B). While the bus bar 15 may be attached to the current collector 10 before Step 5, it is also possible to attach the bus bar 15 after Step 5, after Step 6 or after Step 7. Attaching the bus bar 15 to the current collector 10 is accomplished by any suitable method, for example welding, conductive adhesive, riveting, bolting, clipping or the like. A robust, minimally intrusive method that maintains conductivity between the current collector and the bus bar, for example welding, is most desirable.

[0051] Step 5 comprises assembling the two pre-forms from Step 3 together with the current collector 10 in the second mold insert such that the current collector 10 is sandwiched between the two pre-forms and then heating the assembly to at least the melting point of the binder so that the binder flows through through-apertures 11 (only one labeled and only illustrated in Fig. 2D) in the current collector 10. The heated assembly is then compression molded in the second insert, for example at a pressure in a range of 1 .5 kN / cm2to 2.0 kN / cm2, while the heated assembly cools at ambient temperature to form a non-insulated electrode 2 (see Fig. 2C) having the current collector 10 encapsulated between two layers 5a, 5b of the electrode material. Inner surfaces of the cavity of the second insert are provided with depressions (dimples), which are responsible for forming raised regions 6 (only one labeled, see Fig. 2C) on outer surfaces of the two layers 5a, 5b of the electrode material. The raised regions 6 will be active sites of the fully produced electrode 1.

[0052] Step 6 comprises electrically insulating the non-insulated electrode 2 by placing the non-insulated electrode in a third mold insert along with two sheets of electrically insulating material, for example two sheets of the same material as the binder used in the pre-forms 5, such that the non-insulated electrode 2 is situated between the two sheets of insulating material. The third insert is heated to a temperature sufficient to soften the sheets of insulating material while not too high as to deform the non-insulated electrode 2. Pressure, for example in a range of 0.1 kN / cm2to 0.25 kN / cm2, is applied so that the softened sheets of insulating material in the third insert adhere to the binder of the two layers 5a, 5b of electrode material. The sheets of insulating material may be sufficiently large to ensure coverage of the two layers 5a, 5b of the electrode material as well as the current collector 10 and some of the bus bar 15. The insulated electrode thereby comprises an electrically insulating envelope 20.

[0053] Step 7 comprises machining apertures 21 into the insulating envelope 20 to expose outer faces of the active sites 6 thereby producing the fully formed electrode 1 (see Fig. 2D). The active sites 6 are the conductive areas in contact with the electrolyte during use of the electrode 1.

[0054] Overall, the process utilizes one compression mold and three mold inserts to produce an electrode by compression molding. Having one mold and different inserts provides flexibility to modify, at any time, dimensions and / or active site layout of the electrode. Any step in the process can be readily adjusted to change the properties and functionality of the electrode. The electrical conductivity has been improved compared to commercial graphite rod electrode resulting in a lower charge voltage by approximately 112 and 299 mV at 110 mA / cm2and 280 mA / cm2respectively. The porosity has been considerably reduced with the addition of a thermoplastic binder and additives in combination with the temperature and pressure used in the compression molding step. Optical images of various electrode materials produced from different ratios of graphite-to- binder are shown in Fig. 3. Also, using the same thermoplastic as the binder and as the insulation material results in better stability and reliability compared to other electrodes with different materials (see Fig. 4).

[0055] EXAMPLES

[0056] Example 1: Electrical Conductivity

[0057] A four-point probe technique is a commonly used method for measuring the resistivity of materials with high precision. The four-point probe technique is particularly useful for characterizing materials with low resistivity values, where traditional two-point measurements could be significantly affected by contact and lead resistance. The four- point probe technique relies on the fact that when a current is passed through a sample material using two outer probes, the voltage measured across two inner probes (positioned between the outer probes) is directly proportional to the resistivity of the material and independent of the probe resistances and contact resistances.

[0058] The main objective of the electrical conductivity test is to evaluate the effect of binders (low density polyethylene (LDPE) and fluorinated ethylene propylene (FEP)) and current collectors on the electrical conductivity of the pressed graphite composite electrode material.

[0059] Material

[0060] To prepare samples, graphite, LDPE, and FEP powders were mixed in different ratios using mechanical stirring. The mixed powder was heated up to 250°C and compressed at 1 .9 kN / cm2to form flat plaques with 3.5 mm in thickness. For the samples with a current collector, two flat plaques (pre-forms) were made by compression molding. The powder material was mixed, heated up to 250 °C, and pressed at 0.25 kN / cm2. Then, the two low-density pre-forms were pressed with the current collector mesh therebetween at 1 .9 kN / cm2. To compare the results, a medium extruded graphite plate from a third party (GraphiteStore™) was also tested.

[0061] Test Procedure

[0062] The electrical conductivity test was performed with a Siglent™ SDM3045X digital multimeter by applying direct current through the flat graphite plaques and measuring the voltage and resistance between equally spaced probes. The resistance was recorded, and the electrical conductivity was calculated with Error! Reference source not found.: where: o = electrical conductivity in Siemens.

[0063] R = resistance in ohms (ft).

[0064] A = cross section area in cm2

[0065] I = length of between probes in cm. Results

[0066] Table 1 shows the electrical conductivity of the different ratios of graphite, LDPE, and FEP. The results exhibited a maximum conductivity of 7.8e+3S / cm for the composition with 80 wt% graphite / 10 wt% LDPE / 10 wt% FEP, and almost a linear decrement of the conductivity as the ratio of graphite was decreased. The material with 80 wt% graphite showed 6 times higher conductivity compared to the commercial graphite plate.

[0067] Table 1

[0068] After evaluating the electrical conductivity of different compositions of graphite, the effect of the Cu mesh current collector was tested (Table 2). The results showed an increased increment in each of the compositions of almost an order of magnitude, with the highest conductivity being 9.42e+4S / cm. The effect of the Ni mesh current collector was also tested in the composition 70 wt% graphite / 20 wt% LDPE / 10 wt% FEP. In this case, the conductivity increased from 2.4e+3to 2.61 e+3.

[0069] Table 2

[0070] The material with 80 wt% graphite showed 8.5 times higher electrical conductivity compared to the one with 60 wt% graphite and 6 times higher compared to the commercial graphite plate. After adding the Cu current collector, the conductivity increased almost 12 times in each composition. The highest conductivity of 9.42E+04was from the 80 wt% graphite / 10 wt% LDPE / 10 wt% FEP. In the case of the Ni current collector, the material showed a small increase in conductivity compared to the sample without a current collector, but significantly smaller compared to the one with Cu.

[0071] The voltage in a battery represents the electrochemical potential between two electrodes that drives a chemical reaction. Upon charging, an external power source is used to reverse the reactions involved in the system. In zinc air batteries, those two reactions are the oxygen evolution reaction (OER) at the anode (Error! Reference source not found.) and the zincate reduction at the cathode (Error! Reference source not found.). Overall, a low charging voltage is desired to increase the charge efficiency and reduce the H2evolution reaction rate (Error! Reference source not found.), which represents a safety condition and negatively impacts zinc morphology.

[0072] The charge voltage in zinc batteries is dynamic. During the electrodeposition of zinc metal on the active site, the active surface area increases and the cathodic resistance decreases, improving the overall voltage.

[0073] The main objective of the charge voltage test is to investigate the electrochemical performance and stability of the pressed graphite cathode versus the commercial graphite used in previous zinc electrochemical cells.

[0074] Eguipment

[0075] The tests were performed in a benchtop scale cell and an Arbin battery tester. Two electrically powered pumps were used to recirculate the electrolyte. One of the pumps was connected to the body of the cell, and the other one with the discharge to the headspace of the cell. Both pumps were powered with an external power supply. For the test at high temperatures, a stainless-steel reservoir was used with a heating element. The electrolyte was heated while being recirculated with a pump. The reservoir had a thermocouple that controlled the temperature. Material

[0076] The tests were performed with an electrolyte comprising 34% KOH, 1 .6 M Zn and 2 g / L silica. For the anode, a 10.5 cm x 30 cm corrugated Ni-Fe mesh was used. For the cathode, two pressed graphite configurations were used with a common composition of graphite, and binder materials one with a Cu current collector (sample GCu), and another one with a Ni current collector (sample GNi). Both electrodes had a common pattern of active sites with - so that the active area was common to both cathode specimens -. The graphite electrodes were fabricated as described above.

[0077] For comparison, the charge cathode configuration currently used in zinc electrochemical cells was also tested, which has a mild steel current collector with the same pattern of active sites. (GSS).

[0078] Test Procedure

[0079] The anode and cathode were connected to one of the channels in the Arbin battery tester. After every charge cycle, the zinc was scraped off the cathode. The charging cycles were conducted at cathodic current densities of 280 mA / cm2, 195 mA / cm2, and 110 mA / cm2relative to the exposed active sites on the charge cathode. The electrolyte was replaced with a new one after running each of the currents twice.

[0080] Results

[0081] The GCu sample exhibited between 69 mV and 81 mV lower voltage compared to the sample GNi, which suggests that Cu provides lower electronic resistance in the new graphite charge cathode. Also, the cathode with mild steel current collector and commercial graphite exhibited significantly higher voltage overthe new pressed graphite cathodes, with GSS having a 230 mV and 299 mV increase over GNi and GCu respectively. The main reason for the high voltage in the old cathode configuration can be attributed to the lower conductivity of mild steel and the high electronic resistance caused by the electrode’s design, where the graphite rods are pressed onto the orifices in the mild steel plate. During this process, the graphite gets scratched and the contact between both parts is poor. At lower current densities, the voltage difference started to decrease. At 195 mA / cm2, the sample GSS a 223 mV higher voltage compared to the sample GCu and 158 mV higher voltage than the GNi. At 110 mA / cm2, the voltage for the sample GSS was 112 mV higher voltage than the one in the sample GCu and 31 mV higher than the one in the sample GNi. The voltage difference between all the samples decreased at lower current densities due to the lower effect of the electronic resistance. A second test was performed on a pressed graphite cathode with Cu current collector to assess the charging voltage at 11 mA / cm2in a 2.5 M electrolyte at 25°C and 40°C. At 25°C, the average voltage over two hours was between 205 mV lower than the same test conducted at 110 mA / cm2. At 40°C, the voltage was 225 mV lower than the test conducted at 110 mA / cm2at 40°C. At this current density, the temperature had a lower effect since the ohmic resistance is minimal. Table 3 summarizes the relative voltage profile from the three samples, where the lowest voltage is assigned a baseline value of 0 and the other voltages are given as increases over the baseline.

[0082] Table 3

[0083] The pressed graphite charge cathode showed lower voltage compared to the one with commercial graphite rods and a mild steel current collector. The pressed graphite with Cu current collector showed better performance than the one with Ni current collector due to the overall better electronic conductivity of Cu. The test at 40°C exhibited higher performance in both pressed graphite samples likely due to the increase of ionic conductivity and more efficient reaction kinetics, especially at high current densities.

[0084] Example 3: Zinc Removal Force

[0085] The charge cycle in the zinc batteries works through the electroplating of zinc metal onto the charging cathode active site. The zinc metal is then removed periodically using a mechanical system (e.g., wipers) that scrapes the zinc metal from the active site. Zinc metal morphology and adhesion depend on the current density and zincate concentration in the electrolyte. High zincate concentration with low current density produces denser zinc metal and therefore, higher adhesion to the active site. However, in many applications, the charging cathode requires a surface with low zinc metal adhesion to operate at current densities between 11 mA / cm2and 280 mA / cm2and zincate molarities in a range of 0.2 M to 3.0 M or broader. However, current electrodes with commercial graphite can only charge below 1.6 M and current densities above 110 mA / cm2due to the high adhesion of zinc metal produced by the porosity of the graphite. The main objective of the zinc removal force test is to measure the force required to remove zinc metal from the active sites. The difference between commercial graphite rods and a pressed graphite cathode comprising FEP and LDPE binders was evaluated.

[0086] Equipment

[0087] The tests were performed in a bench scale cell and an Arbin battery tester. Two electrically powered pumps were used to recirculate the electrolyte. One of the pumps was connected to the body of the cell, and the other one with the discharge to the headspace of the cell. Both pumps were powered with an external power supply.

[0088] Material

[0089] For the anode, a corrugated Ni-Fe mesh was used. For the cathode, a pressed graphite configuration was used with a composition of 70 wt% graphite, 20 wt% LDPE and 10 wt% FEP on a Cu mesh current collector (the electrode fabrication is detailed above). The tests were performed with an electrolyte comprising 40% KOH, 2.5 M zincate and 5 g / L silica.

[0090] Test procedure

[0091] The mini cell was charged at cathodic current densities of 11 mA / cm2, 110 mA / cm2and 195 mA / cm2. The zinc removal force was measured with a mechanical wiper system that scrapes the zinc metal from the active site. The wiper is attached to a force gauge that records the force values. The wiper and force gauged are moved by an electric motor coupled to a lead screw. The electrolyte was replaced with a new one after every run.

[0092] For comparison, a charge cathode was also tested, which has a mild steel current collector with an active area pattern with half the active area of the previous example that were pressed to create the active sites.

[0093] Results

[0094] At a cathodic current density of 110 mA / cm2, most of the zinc is fully removed from the active site of the pressed graphite cathode in contrast to the graphite rod cathode where a thick layer of zinc metal remained on the active sites. The big difference in adhesion on the pressed graphite cathode is likely at least in part due to the LDPE and FEP binders that fill most of the porosity in the graphite and provide hydrophobicity. These results match with the removal force tests illustrated in Fig. 5 showing the removal force of the pressed graphite charge cathode at current densities of 110 mA / cm2and 195 mA / cm2after a second run of charging and wiping. At 195 mA / cm2, the highest zinc removal force was around 3 N. Considering that each column has 3 active sites per side, 3 N divided by 6 active sites results in 0.5 N per active site. At 110 mA / cm2, the maximum zinc removal force was around 14 N, which is equivalent to 2.3 N per active site.

[0095] For the charge cathode based on graphite rods, the maximum zinc removal force at a charge density of 195 mA / cm2was around 24.5 N. Considering that there are 4 active sites per column, the removal force is around 6 N per site. However, considering that during wiping not all the zinc metal was removed due to the high adhesion, and the wipers instead were passing on top of the zinc metal, the measured force is unreliable. At 110 mA / cm2, the maximum removal force was 20 N which translates to 4 N per site. This value is lower than the one at 195 mA / cm2because there was less zinc removed from the active site. Fig. 6 shows that the third peak is lower compared to the others, which is likely due to the wiper jumping the zinc metal nugget. Thus, the force results for the charge cathode based on graphite rods are likely underestimating the force required to remove the zinc metal.

[0096] The pressed graphite was also tested at 11 mA / cm2to assess the wipe capabilities. Fig. 7 shows the Zn removal force over three cycles of charge and wipe. The average removal force over the first wipe was 0.65 kN per active site. For the second run, the removal force increased to 1.6 N per active site. Finally, for the third cycle, the removal force increased to 2 N per active site. The test demonstrated that after the second cycle, the Zn adhesion doesn't increase significantly. Another point to notice is that at 11 mA / cm2the Zn removal force was lower than at 110 mA / cm2. This is likely due to the change in morphology of the Zn.

[0097] Table 4 shows the zinc removal force for both electrodes at 110 mA / cm2and 195 mA / cm2.

[0098] Table 4

[0099] The force required to remove zinc metal from charge cathodes were measured and compared between charge cathodes with commercial graphite actives and charge cathodes with pressed graphite cathode material. Overall, the charge cathode with pressed graphite required 2 times less removal force at 110 mA / cm2and 12 times less removal force at 195 mA / cm2compared to the commercial graphite rods. The differences are likely much larger since the wipers could not entirely remove the zinc metal from the active sites on the commercial graphite rods. Instead, the wipers passed on top of the active site producing lower forces. Thus, the pressed graphite charge cathodes provide much lower zinc metal adhesion compared to the current configuration with commercial graphite rods. The tests also demonstrated the Zn removal capabilities at current densities as low as 11 mA / cm2. The adhesion was similar to the one found at 110 mA / cm2, which confirms the effectiveness of the polymers in graphite composite.

[0100] Example 4: Hydrogen Evolution Overpotential (HER)

[0101] Linear sweep voltammetry (LSV) was used to determine the overpotential for hydrogen gas evolution in an electrochemical cell with a pressed graphite charge cathode material comprising 70 wt% graphite, 20 wt% LDPE, and 10 wt% FEP. The charge cathode had a 1 cm2active surface area. The electrolyte contained 40 wt% KOH. Graphite was used as the counter electrode. The reference electrode was a Hg / HgO (20% KOH) reference electrode.

[0102] The test was performed by connecting the electrodes in a 3-electrodes setup and performing LSV from 0 V to -3 V at a scan rate of 0.2 mV / s. The onset potential was estimated from the LSV. Fig. 8 is a graph of current (A) vs. potential (V) showing a typical result for the LSV experiment. In Fig. 8, the HER was estimated to be -1.87 V vs. Hg / HgO.

[0103] Example 5 - Corrosion Rate Testing

[0104] Linear sweep voltammetry (LSV), Tafel plots and linear polarization resistance (LPR) techniques were used to calculate corrosion rate of a pressed graphite charge cathode material comprising 70 wt% graphite, 20 wt% LDPE, and 10 wt% in an electrochemical cell containing an electrolyte comprising 40% KOH. For comparison, corrosion parameters of commercial graphite plates were also determined. The charge cathode had a 1 cm2active surface area. Graphite was used as the counter electrode. The reference electrode was a Hg / HgO (20% KOH) reference electrode.

[0105] The test was performed by connecting the electrodes in a 3-electrodes setup and performing LSV in both reduction and oxidation potential range. The LSV plot was transformed to a Tafel representation. The Tafel plot was extrapolated to get the Tafel slopes (anodic Beta and cathodic Beta) to be used for corrosion measurements. Corrosion rate was calculated through linear polarization resistance (LPR). A typical Tafel plot extracted from LSV is shown in Fig. 9. The corrosion rate results are summarized in Table 5.

[0106] Table 5 mpy = milli-inch per year

[0107] The pressed graphite electrode material has a corrosion rate in an electrolyte comprising 40 wt% KOH that is 8.2 times lower than that of commercial graphite plates.

[0108] The novel features will become apparent to those of skill in the art upon examination of the description. It should be understood, however, that the scope of the claims should not be limited by the embodiments but should be given the broadest interpretation consistent with the wording of the claims and the specification as a whole.

Claims

Claims:

1. A process for fabricating an electrode comprising: forming a solid layer of an electrode material on a surface of a solid current collector by compression molding, wherein the electrode material comprises a mixture of an electrically conductive graphite and a binder, the binder comprising a moldable polymer; and, forming a layer of an electrically insulating material on a surface of the layer of the electrode material so that a portion of the surface of the layer of the electrode material remains uncovered by the insulating material.

2. The process of claim 1 , wherein the graphite is in powder form and the process further comprises forming the mixture into a monolithic solid pre-form of the graphite in a matrix of the binder, and wherein forming the layer of the electrode material on the surface of the current collector comprises compression molding the pre-form onto the surface of the current collector in a mold.

3. The process of claim 2, the binder is in powder form in the mixture with the graphite.

4. The process of claim 2 or claim 3, wherein the pre-form is formed larger than the mold and the process further comprises cutting the pre-form to a size to fit in the mold.

5. The process of any one of claims 2 to 4, wherein the mold has depressions on an inner surface thereof, which result in forming raised regions on an outer surface of the layer of the electrode material during the compression molding.

6. The process of claim 5, further comprising: removing a portion of the layer of the insulating material from the layer of the electrode material to expose distal faces of the raised regions relative to the current collector; or, molding the layer of the insulating material on to the layer of the electrode material so that distal faces of the raised regions relative to the current collector remain uncovered by the insulating material during the molding of the layer of the insulating material on to the layer of the electrode material.

7. The process of claim 6, wherein the distal faces of the raised regions are flush with an outer surface of the layer of the insulating material.

8. The process of any one of claims 2 to 7, wherein: forming the mixture into a monolithic solid pre-form of the graphite in a matrix of the binder is performed at a temperature in a range of 150°C to 350°C and at a pressure in a range of 0.05 kN / cm2to 1 .0 kN / cm2; compression molding is performed at a temperature in a range of 150°C to 350°C and at a pressure in a range of 1 .0 kN / cm2to 3.0 kN / cm2; forming the layer of the electrically insulating material on the surface of the layer of the electrode material is performed at a temperature in a range of 80°C to 200°C and at a pressure in a range of 0.01 kN / cm2to 1.0 kN / cm2; or, any combination thereof.

9. The process of any one of claims 2 to 7, wherein: forming the mixture into a monolithic solid pre-form of the graphite in a matrix of the binder is performed at a temperature in a range of 250°C to 280°C and at a pressure in a range of 0.2 kN / cm2to 0.5 kN / cm2; compression molding is performed at a temperature in a range of 250°C to 280°C and at a pressure in a range of 1 .5 kN / cm2to 2.0 kN / cm2; forming the layer of the electrically insulating material on the surface of the layer of the electrode material is performed at a temperature in a range of 110°C to 150°C and at a pressure in a range of 0.1 kN / cm2to 0.25 kN / cm2; or, any combination thereof.

10. The process of any one of claims 1 to 9, wherein forming the solid layer of the electrode material on the surface of the solid current collector comprises forming a first solid layer of the electrode material on a first surface of the solid current collector and forming a second solid layer of the electrode material on a second surface of the solid current collector, the second surface opposed to the first surface so that the current collector is sandwiched between the first and second solid layers of the electrode material.11 . The process of claim 10, wherein the current collector is a mesh and the two solid layers of the electrode material are compression bonded to each other through apertures in the mesh.

12. The process of any one of claims 1 to 11 , wherein: a) the current collector comprises copper or nickel metal; b) the binder comprises a polyolefin in a graphite-to-binder amount in a range of 90: 10 w / w to 50:50 w / w; c) the insulating layer comprises a polymeric material that is the same as or different than the binder; or, d) a combination of any two or more of a) to c).

13. An electrode fabricated by the process of any one of claims 1 to 12, the electrode being a charge cathode.

14. A charge cathode for an electrochemical cell, the charge cathode comprising: a current collector comprising a mesh of a solid electrically conductive material, the current collector having opposed first and second surfaces, the mesh comprising through- apertures between the first and second surfaces; a first layer of a solid cathode material compression molded on to the first surface of the current collector and a second layer of the solid cathode material compression molded on to the second surface of the current collector thereby encapsulating the current collector between the first layer and the second layer of the solid cathode material, the first layer and the second layer compression bonded to each other through the apertures in the mesh, the first and second layers having raised regions on outer surfaces thereof, the solid cathode material comprising electrically conductive graphite in a matrix of a moldable polymeric binder; a layer of an electrically insulating material covering the layers of the cathode material so that distal faces of the raised regions relative to the current collector are not covered by the insulating material; and, a bus bar electrically connected to the current collector.

15. The charge cathode of claim 14, wherein:a) the current collector comprises copper or nickel metal; b) the binder comprises a polyolefin in a graphite-to-binder amount in a range of 90:10 w / w to 50:50 w / w; c) the insulating layer comprises a polymeric material that is the same as or different than the binder; or, d) a combination of any two or more of a) to c).

16. The charge cathode of claim 14 or claim 15, wherein: i) the charge cathode is operable at a current density in a range of 0.1 mA / cm2to 280 mA / cm2; ii) the charge cathode is operable in an electrolyte having a molarity of zinc ion in a range of 0.1 M to 3.5 M; iii) zinc metal adheres to active sites of the charge cathode with an adhesion force in a range of 0.5 N to 3 N per active site during operation of the charge cathode at a current density in a range of 0.1 mA / cm2to 280 mA / cm2and a molarity of zinc ion in electrolyte in a range of 0.1 M to 3.5 M; or, iv) any combination of i), ii) and iii).

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