METHOD OF PREPARING ENERGY STORAGE ELECTRODES

Abstract

An energy storage electrode is formed by heat-pressing preformed electrode membranes into the pore structures of a metal mesh current collector without use of any solvents. The electrodes are utilized primarily for Li-ion batteries, as well as supercapacitors. This solvent-free method for electrode preparation is more cost-efficient and environmentally-friendly, in comparison with the method involving preparation and casting slurries or pastes onto metal foil current collectors, where a solvent is required.

Claims

1. A method of making an energy storage electrode comprising: a) forming an electrode membrane by heat-pressing a mixture of powdery materials consisting of an electrode active material, a conducting additive, and a polymer binder; b) forming the energy storage electrode by heat-pressing a pair of the electrode membranes sandwiching a metal mesh current collector.

2. The method according to claim 1, wherein said electrode membrane having a thickness ranging from 10 microns to 200 microns.

3. The method according to claim 1, wherein said metal mesh current collector comprising a material selecting from the group consisting of Al, Cu, Ni, Sb, Cr, Fe, and Si.

4. The method according to claim 1, wherein said metal mesh having a wire diameter ranging from 10 microns to 100 microns, and a mesh pore size ranging from 10 microns to 100 microns.

5. The method according to claim 1, wherein said energy storage electrode having a thickness ranging from 20 microns to 200 microns.

6. The method according to claim 1, wherein said energy storage electrode is a Li-ion cathode, and wherein said active material comprising a material selecting from the group consisting of S, LiCoO.sub.2, LiNiO.sub.2, LiMn.sub.2O.sub.4, LiFePO.sub.4, and LiNi.sub.xCo.sub.yMn.sub.z, said conductive additive comprising a material selecting from the group consisting of graphite and carbon black, and said polymer binder comprising a material selecting from the group consisting of PVDF, EPDM, CMC, PTFE and SBR.

7. The method according to claim 1, wherein said energy storage electrode is a Li-ion anode, and wherein said active material comprising a material selecting from the group consisting of graphite and silicon, said conductive additive comprising a material selecting from the group consisting of graphite and carbon black, and said polymer binder comprising a material selecting from the group consisting of PVDF, EPDM, CMC, PTFE and SBR.

8. The method according to claim 1, wherein said energy storage electrode is a supercapacitor electrode, and wherein said active material comprising a material selecting from the group consisting of activated carbons, carbon nanotubes, graphenes, RuO.sub.2, NiO, and IrO.sub.2, said conductive additive comprising a material selecting from the group consisting of graphite and carbon black, and said polymer binder comprising a material selecting from the group consisting of PVDF, EPDM, CMC, PTFE and SBR.

9. The method according to claim 1, wherein said heat-pressing temperature ranging from 25 C. to 400 C.

10. The method according to claim 1, wherein said heat-pressing pressure ranging from 0 to 65 psig.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The principle of the present invention may be understood with reference to the detailed description, in conjunction with the accompanying drawings, in which:

[0010] FIG. 1 shows a cross-sectional view of a mesh electrode.

[0011] FIG. 2 shows a cross-sectional view of a press mold where electrode materials mixture is loaded in a press mold for making electrode membranes.

[0012] FIG. 3 shows a cross-sectional view of the press mold where electrode membrane is made upon pressing.

[0013] FIG. 4 shows a cross-sectional side view of a pair of heat press plates where a pair of electrode membranes sandwiching a metal mesh is being pressed for making the electrode.

[0014] FIG. 5 shows a cross-sectional side view of an apparatus consisting of a pair of heat press rollers where a pair of electrode membranes sandwiching a metal mesh is delivered and pressed to make the electrode continuously.

[0015] FIG. 6 shows a cross-sectional side view of an apparatus for continuously making electrode membranes.

[0016] FIG. 7 shows a front cross-sectional view of the feeding part of the apparatus for continuously making electrode membranes.

DETAILED DESCRIPTION OF THE INVENTION

[0017] Referring now to FIG. 1 of the drawings, an energy storage electrode 16, comprises a metal mesh 14 and a pair of electrode materials membranes 12, with the membranes inherently bonded together and locked into the pore structures of the mesh by heat-pressing a pair of the membranes sandwiching the metal mesh. The mesh electrode 16 is essentially an electrode of metal mesh/wire reinforced ceramic/polymer composite, showing improved mechanical strength and increased electrode materials loading over conventional electrode using metal foil current collector. The mesh wire diameter and mesh pore/opening size may range, for example, from 10 to 100 microns, whereas the thickness of the finished electrode from 10 to 300 microns. In a particularly preferred embodiment of the invention, mesh wire diameter and mesh pore size are 25 microns and finished electrode thickness 75 microns.

[0018] Referring to FIG. 2 and FIG. 3 of the drawings, an electrode membrane 12 is prepared by loading into mold 20A a well-mixed sample 10 consisting of powdery active material, conductive additive, and polymer binder, followed by pressing using mold 20B with or without heat. The thickness of 12 may range from 10 to 200 microns. Electrode active materials may include: a) high surface area carbon materials for supercapacitors, b) sulfur (S) for LiS batteries, 3) Li-ion cathode materials such as LiCoO.sub.2, LiNiO.sub.2, LiMn.sub.2O.sub.4, LiFePO.sub.4, and LiNi.sub.xCo.sub.yMn.sub.z, and 4) Li-ion anode materials such as graphite, and silicon. Conductive additives may include carbon or non-carbon conducting additives and polymer binders may include polyvinylidene fluoride (PVDF), ethylene-propylene diene (EPDM), carboxy methyl cellulose (CMC), polytetrafluoroethylene (PTFE) and sodium carboxymethyl cellulose (SBR). The temperature for the heat-pressing may range from room temperature to 400 C., and the pressure from 0 to 65 psig.

[0019] Referring to FIG. 4 of the drawings, an energy storage electrode 16 is made by heat-pressing a pair of electrode membranes 12 sandwiching a metal mesh 14 using a pair of hot-plates 22 until the pair of the membranes are bonded inherently within the framework of the mesh with desired thickness. The temperature for the heat-pressing may range from room temperature to 400 C., and the pressure from 0 to 65 psig.

[0020] Referring to FIG. 5 of the drawings, a pair of membranes 12 sandwiching a metal mesh sheet 14 are fed continuously into a heat press apparatus consisting of a pair of rollers 24, to form mesh electrode 16, which may be collected by winding it into a roll. The pair of rollers may be replaced with a series of pairs of rollers for better preparation of the mesh-based electrodes, where the spacing between the rollers of the same pair decreases sequentially. The temperature of the rolling process may range from room temperature to 400 C.

[0021] Referring to FIG. 6 and FIG. 7 of the drawings, a mixture of powder 10 is spread continuously onto a release film 28 that is supported by bottom board 26A and guided with side guard 26B. The powder mixture along with the release film is fed into a pair of rollers 24 and pressed to form membrane 12, which may be collected by winding it into a roll. The pair of rollers may be replaced with a series of pairs of rollers for better preparation of the electrode membranes, where the spacing between the rollers of the same pair decreases sequentially. The temperature of the rolling process may range from room temperature to 400 C.

[0022] The present invention discloses a method of preparing an energy storage electrode on metal mesh current collector without use of organic solvents, whereas for preparation of electrodes on metal foil, solvent is required to prepare slurries or pastes that are subsequently cast onto metal foil. These mesh-based electrodes are primarily used in Li-ion batteries and supercapacitors. In accordance with embodiments of the present invention, a metal mesh, having dimensions (for example, mesh wire diameter and pore size) ranging from several microns to several hundred microns, is utilized as a current collector, and a pair of premade electrode materials membranes, sandwiching the mesh, are heat-pressed partially into the pore structures of the metal mesh, forming a mesh electrode.

[0023] Supercapacitors, also called ultracapacitors or electrochemical double layer capacitors, as energy storage devices, use high surface area carbon as electrode materials. These include activated carbons, carbon nanotubes, graphenes, as well as pseudo-capacitance metal oxides such as RuO.sub.2, NiO, and IrO.sub.2. Such materials in powder form may be mixed with a polymer binder without presence of a solvent and pressed with or without heat, forming supercapacitor electrode membranes, a pair of which, sandwiching a mesh, may be pressed partially into the pores of the metal mesh substrate under pressure and heat, forming mesh-based supercapacitor electrodes without using any solvents. The relative proportions of active materials and polymer binder may range, for example, from 50 wt % to 95 wt % of active material, the balance being polymer binder.

[0024] Li-ion batteries, as energy storage devices, commonly use a metal oxide as cathode and a carbon material as anode. Any suitable Li-ion battery cathode materials may be used with LiCoO.sub.2, LiNiO.sub.2, LiMn.sub.2O.sub.4, LiFePO.sub.4, or LiNi.sub.xCo.sub.yMn.sub.z, as an example. Such materials in powder form may be mixed with a conductive additive and a polymer binder without presence of a solvent and pressed with or without heat, forming Li-ion cathode membranes, a pair of which, sandwiching a mesh, may be pressed partially into the pores of the metal mesh substrate under pressure and heat, forming mesh-based Li-ion cathodes without using any solvents. The relative proportions of active material, conductive additive, and polymer binder may range, for example, from 50 wt % to 90 wt % of active material, and 0 wt % to 15 wt % of conductive additive, the balance being polymer binder.

[0025] Any suitable Li-ion anode material may be used in the invention, with graphite, or Si as an example. Such materials in powder form may be mixed with a conductive additive and a polymer binder without presence of a solvent and pressed with or without heat, forming electrode membranes, a pair of which, sandwiching a mesh, may be pressed partially into the pores of the metal mesh under pressure and heat, forming mesh-based Li-ion anodes without using any solvents. The relative proportions of active material, conductive additive, and polymer binder may range, for example, from 50 wt % to 90 wt % of active material, and 0 wt % to 15 wt % of conductive additive, the balance being polymer binder.

[0026] One embodiment of the present invention involves preparation of mesh-based sulfur (S) electrodes to be used in LiS batteries, where elemental S powder is mixed with a conducting additive and a polymer binder without presence of a solvent and pressed with or without heat, forming electrode membranes, a pair of which, sandwiching a mesh, may be pressed partially into the pores of the metal mesh substrate under pressure and heat, forming mesh-based S electrodes without using any solvents. The relative proportions of sulfur, conductive additive, and polymer binder may range, for example, from 40 wt % to 90 wt % of sulfur, and 10 wt % to 50 wt % of conductive additive, the balance being polymer binder.

[0027] It is important to note that there are many factors having impacts on the solvent-free preparation of mesh-based electrodes, including material particle size, heat-press temperature, pressure, and time, and materials feeding rate into the pair of rollers. While it is difficult to provide a general guidance to determine optimal values of these factors, the followings are worth noting.

[0028] It is noted that the size of the particles of the ceramic materials including lithium metal oxides, graphite, and conductive carbons, must be significantly smaller than the pore size of the metal mesh, permitting these particles to squeeze into the pores along with the polymer binder, in an effort to form an integral, fine mesh reinforced composite of energy storage electrodes. It is also noted that the heat-pressing temperature should be sufficiently high to allow polymer binders to infiltrate into the voids of the mixture or to form a coated layer on the particles when heat-pressing. Therefore the heat-pressing temperature must be higher than the glass transition temperature (T.sub.g), or preferably higher than the melting point (T.sub.m) of the polymer. These temperatures, along with sufficiently high values of pressures, are critical for inherently bonding between the particles and between the two membranes within the mesh pore structures.

[0029] It is also noted that a single membrane may be used in preparation of the mesh-based electrode by heat-pressing with the mesh; however, a pair of membranes sandwiching a mesh is preferred as this ensures formation of a uniform, balanced electrode having electrode materials distributed evenly on both sides of the mesh.

[0030] It is further noted that when the pressing temperature is above the melting point of the polymer binder, the processes described in FIGS. 3 to 7 are essentially plastic thermoforming. In addition, the electrode materials mixture, along with the mesh substrate, may be directly fed into the heat-press apparatus, forming mesh-based electrodes without the step of making the electrode membranes.

[0031] Having generally described the invention, the following examples serve to illustrate the preferred embodiments of the present invention and should not be construed as limiting the scope of the invention:

EXAMPLE 1

Preparation of Supercapacitor Electrodes

[0032] A sample of well mixed powder consisting of PVDF (20 wt %) and Activated Carbon (80 wt %) is loaded evenly into mold 20A. (FIG. 2) The sample of the mixture is pressed with mold 20B being lowered into mold 20A at 185 C. and 5 psig for an hr. (FIG. 3) The process continues at 200 C. without pressure for another 5 hrs. The molds are cooled to room temperature and disassembled. The electrode membrane (605 cm10 cm) is collected from the mold.

[0033] A pair of the electrode membranes (605 cm10 cm) prepared above, sandwiching an aluminum mesh (6.5 cm11 cm) with mesh wire thickness of 50 and pore diameter 50, are placed between a pair of hot plates. (FIG. 4) The length of the membranes is aligned along one of the lengths of the mesh. The membrane/mesh assembly is pressed at 215 C. and 5 psig for 5 hrs, followed by cooling to room temperature. The top press plate is lifted, and the resulting mesh-based supercapacitor electrode is removed from the press plate, followed by cleaning off excess materials from the edges of the rectangle mesh substrate using a razor blade.

EXAMPLE 2

Preparation of Li-ion Cathodes

[0034] A sample of well mixed powder consisting of PVDF (15 wt %), LiFePO.sub.4 (80 wt %) and carbon black (5 wt %) is loaded evenly into mold 20A. (FIG. 2) The sample of the mixture is pressed with mold 20B being lowered into mold 20A at 185 C. and 5 psig for an hr. (FIG. 3) The process continues at 200 C. without pressure for additional 5 hrs. The molds are cooled to room temperature and disassembled. The electrode membrane (605 cm10 cm) is collected from the mold.

[0035] A pair of the electrode membranes (605 cm10 cm) made above, sandwiching an aluminum mesh (6.5 cm11 cm) with mesh wire thickness of 50, and pore diameter 50, are placed between a pair of hot plates (FIG. 4). The length of the membranes is aligned along one of the lengths of the mesh. The membrane/mesh assembly is pressed at 215 C. and 8 psig for 5 hrs, followed by cooling to room temperature. The top press plate is lifted, and the resulting mesh-based Li-ion cathode electrode is removed from the press plate, followed by cleaning off excess materials from the edges of the rectangle mesh substrate using a razor blade.

EXAMPLE 3

Preparation of Li-ion Anodes

[0036] A sample of well mixed powder consisting of PVDF (15 wt %), and graphite (85 wt %) is lowered evenly into mold 20A. (FIG. 2) The sample of the mixture is pressed with mold 20B being loaded into mold 20A at 185 C. and 5 psig for an hr. (FIG. 3) The process continues at 200 C. without pressure for additional 5 hrs. The molds are cooled to room temperature and disassembled. The electrode membrane (605 cm10 cm) is collected from the mold.

[0037] A pair of the electrode membranes (605 cm10 cm) made above, sandwiching a copper mesh (6.5 cm11 cm) with mesh wire thickness of 50 and pore diameter 50, are placed between a pair of hot plates (FIG. 4). The length of the membranes is aligned along one of the lengths of the mesh. The membrane/mesh assembly is pressed at 215 C. and 8 psig for 5 hrs, followed by cooling to room temperature. The top press plate is lifted, and the resulting mesh-based Li-ion anode electrode is removed from the press plate, followed by cleaning off excess materials from the edges of the rectangle mesh substrate using a razor blade.

EXAMPLE 4

Preparation of S Electrodes

[0038] A sample of well mixed powder consisting of PVDF (15 wt %), carbon black (35 wt %), and sulfur (50 wt %) is loaded evenly into mold 20A. (FIG. 2) The sample of the mixture is pressed with mold 20B being lowered into mold 20A at 185 C. and 5 psig for an hr. (FIG. 3) The process continues at 200 C. without pressure for additional 5 hrs. The molds are cooled to room temperature and disassembled. The electrode membrane (605 cm10 cm) is collected from the mold.

[0039] A pair of the electrode membranes (605 cm10 cm) made above, sandwiching a A1 mesh (6.5 cm11 cm) with mesh wire thickness of 50 and pore diameter 50, are placed between a pair of hot plates (FIG. 4). The length of the membranes is aligned along one of the lengths of the mesh. The membrane/mesh assembly is pressed at 215 C. and 8 psig for 5 hrs, followed by cooling to room temperature. The top press plate is lifted, and the resulting mesh-based S electrode is removed from the press plate, followed by cleaning off excess materials from the edges of the rectangle mesh substrate using a razor blade.