WO2024018487A1

WO2024018487A1 - Methods for preparation of electroconductive slurry for an electrode in energy storage devices

Info

Publication number: WO2024018487A1
Application number: PCT/IN2023/050701
Authority: WO
Inventors: Chaitra K; Shankar THIYAGARAJAN; Jude JOHN; Robin George; Hemant CHARAYA
Original assignee: Log 9 Materials Scientific Private Limited
Priority date: 2022-07-21
Filing date: 2023-07-20
Publication date: 2024-01-25

Abstract

The present invention provides methods for preparing an electroconductive slurry for an electrode in energy storage devices. The method comprises providing a first mixture that includes a first binder and a conductive additive in a first solvent. An active material and a second solvent are added to the first mixture with constant stirring to form a final mixture. The active material is added sequentially to the first mixture in a plurality of batches. A second binder is mixed with the final mixture to form the electroconductive slurry. An acid solution can be added prior to the addition of the active material or after the addition of the active material to maintain a pH of the electroconductive slurry in a defined range. The slurry so formed is coated on a current collector to form an electrode of the energy storage device.

Description

METHODS FOR PREPARATION OF ELECTROCONDUCTIVE SLURRY FOR AN

ELECTRODE IN ENERGY STORAGE DEVICES

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to and the benefit of the Non-Provisional Patent application titled “METHODS FOR PREPARATION OF ELECTROCONDUCTIVE SLURRY FOR AN ELECTRODE IN ENERGY STORAGE DEVICES”, with application number 202241041857, filed in the Indian Patent Office on 21 st July 2022. The specification of the above referenced patent application is incorporated herein by reference in its entirety.

BACKGROUND

Technical field

[0002] The embodiments herein are related generally to energy storage devices. More specifically, the embodiments herein are related to methods for preparation of electroconductive slurry for an electrode in energy storage devices, and in particular lithium-based energy storage devices.

Description Of The Related Art

[0003] Lithium-ion batteries (LIBs) are the most widely used power storage and generation devices due to their comprehensive superiority in power density, energy density, cost, and safety. To meet the ever-growing demand for present and future applications, efforts are being made to further improve the characteristics of LIBs.

[0004] A typical LIB is composed of a cathode, an anode, and an electrolyte. The performance of the LIBs depends on the materials used and the battery design. The cathode and anode of LIBs are formed as sheets by coating a slurry, comprising electrode active material, on current collectors. As the electrodes determine the energy density of LIBs, electroconductive slurry plays a vital role in enhancing the performance of the resultant LIBs.

[0005] In a typical electroconductive slurry, electrode active materials are dispersed in a suitable solvent along with conductive additives and binders. The selection of slurry materials is based on the compatibility of the material with other slurry ingredients and with other components of the LIB. It is essential that the current collector has no adverse reaction with slurry materials and the solvent medium. Additionally, the viscosity of the slurry should remain constant over a period of time for uniform coating on the current collector. Hence, processability is also a key factor in enhancing slurry performance.

[0006] As the LIB market is expanding, there is a need for processes and materials that overcome some of the drawbacks of existing slurry technology. Further, there is a need for nontoxic, cheap, and eco-friendly slurry methods and materials.

[0007] Limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of described systems with some aspects of the present invention, as set forth in the remainder of the present application and with reference to the drawings.

SUMMARY

[0008] In an embodiment of the present invention, an energy storage device comprising a current collector coated with an electroconductive slurry is provided. The electroconductive slurry is prepared by a method comprising (i) providing a first mixture that includes a first binder and a conductive additive in a first solvent. The method further comprises (ii) adding an active material and a second solvent to the first mixture with constant stirring to form a final mixture. Adding the active material to the first mixture comprises sequentially adding a portion of the active material in a plurality of batches. The method further comprises (iii) admixing a second binder with the final mixture to form the electroconductive slurry. The method further comprises a step of providing an acid solution to maintain a pH of the electroconductive slurry in a defined range. The acid solution is added (a) to the first mixture after step (i) and before step (ii), or (b) to the final mixture after step (ii) and before step (iii).

[0009] In another embodiment of the present invention, a method of preparing an energy storage device comprising a current collector is provided. The method comprising providing a first mixture that includes a first binder and a conductive additive in a first solvent. The method further comprises (ii) adding an active material and a second solvent to the first mixture with constant stirring to form a final mixture. Adding the active material to the first mixture comprises sequentially adding a portion of the active material in a plurality of batches. The method further comprises (iii) admixing a second binder with the final mixture to form the electroconductive slurry. The method further comprises a step of providing an acid solution to maintain a pH of the electroconductive slurry in a defined range. The acid solution is added (a) to the first mixture after step (i) and before step (ii), or (b) to the final mixture after step (ii) and before step (iii). The method further comprises (iv) coating the electroconductive slurry on the current collector.

[0010] In another embodiment of the present invention, a method for preparing an electroconductive slurry is provided. The method comprises providing a first mixture that includes a first binder and a conductive additive in a first solvent. The method further comprises providing an acid solution to the first mixture to form a pH controlled first mixture. Providing the acid solution to the first mixture maintains a pH of the electroconductive slurry in a defined range. The method further comprises adding an active material and a second solvent to the pH controlled first mixture with constant stirring to form a final mixture. Adding the active material to the pH controlled first mixture comprises adding sequentially a portion of the active material in a plurality of batches. The method further comprises admixing a second binder with the final mixture to form the electroconductive slurry.

[0011] In another embodiment of the present invention, a method for preparing an electroconductive slurry is provided. The method comprises providing a first mixture that includes a first binder and a conductive additive in a first solvent. The method further comprises adding an active material and a second solvent to the first mixture with constant stirring to form an intermediate mixture. Adding the active material to the first mixture comprises adding sequentially a portion of the active material in a plurality of batches. The method further comprises providing an acid solution to the intermediate mixture to form a final mixture. Providing the acid solution to the intermediate mixture maintains a pH of the electroconductive slurry in a defined range. The method further comprises admixing a second binder with the final mixture to form the electroconductive slurry.

[0012] These and other features and advantages of the present invention may be appreciated from a review of the following detailed description of the present invention, along with the accompanying figures in which like reference numerals refer to like parts throughout.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG. 1 is a schematic diagram of an energy storage device comprising current collectors coated with electroconductive slurry, in accordance with an exemplary embodiment of the present invention.

[0014] FIG. 2 is a flow chart that illustrates a method for preparing an electroconductive slurry, in accordance with an exemplary embodiment of the invention. [0015] FIG. 3 is a flow chart that illustrates another method for preparing an electroconductive slurry, in accordance with another exemplary embodiment of the invention.

[0016] FIG. 4 is a flow chart that illustrates a method of preparing an energy storage device using the electroconductive slurry prepared in FIG. 2 or FIG. 3, in accordance with another exemplary embodiment of the invention.

[0017] FIG. 5 is a diagram that illustrates a plot of pH at each step of methods of FIGS. 2 and 3, in accordance with an exemplary embodiment of the invention.

[0018] Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description of exemplary embodiments is intended for illustration purposes only and is, therefore, not intended to necessarily limit the scope of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0019] The following description illustrates some exemplary embodiments of the disclosed invention in detail. Those of skill in the art will recognize that there are numerous variations and modifications of this invention that are encompassed by its scope. Accordingly, the description of a certain exemplary embodiment should not be deemed to limit the scope of the present invention.

[0020] The term “comprising” as used herein is synonymous with “including,” or “containing,” and is inclusive or open-ended and does not exclude additional, unrecited elements, or method steps.

[0021] All numbers expressing quantities of ingredients, property measurements, and so forth used in the specification are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth herein are approximations that may vary depending upon the desired properties sought to be obtained.

[0022] As used herein, the term “lithium-ion based energy storage device” or “lithium-ion battery” (LIB) may refer to any conventional lithium-ion battery that has an anode, a cathode, a separator, electrolyte, and two current collectors. Example of LIBs may further include anode- free LIBs, lithium-ion polymer batteries, batteries with liquid electrolytes, and solid-state batteries.

[0023] As used herein, the terms “cathode” and “anode” may refer to the electrodes of a battery (interchangeably referred to as an energy storage device). During a charge cycle in a Li- ion battery, Li ions migrate from the cathode towards the anode through an electrolyte, while the electrons migrate from the cathode towards the anode via an external circuit. During a discharge cycle in the Li-ion battery, Li ions migrate from the anode towards the cathode through an electrolyte, while the electrons migrate from the anode towards the cathode via an external circuit.

[0024] As used herein, the term “electrolyte” may refer to a material that allows ions, for example, Li ions, to migrate therethrough, but does not allow electrons to conduct therethrough.

[0025] As used herein, the term “current collector” may refer to a bridging component that collects electrical current generated at the electrodes and connects with external circuits.

[0026] EIG. 1 is a schematic diagram that illustrates an energy storage device, in accordance with an embodiment of the invention. Referring to EIG. 1, an energy storage device 100 is shown. In an embodiment, the energy storage device 100 may be a lithium-based energy storage device, for example, a lithium-ion battery, a lithium-ion polymer battery, or the like. The energy storage device 100 comprises a cathode current collector 102 and an anode current collector 104. The cathode current collector 102 is coated with an electroconductive slurry 105a comprising cathode active material to form a cathode 106. An anode 108 is formed by coating an electroconductive slurry 105b comprising anode active material on the anode current collector 104. The energy storage device 100 further includes an electrolyte 110 that allows lithium ions to migrate therethrough and a separator 112 to separate the cathode 106 and the anode 108.

[0027] The electrolyte 110 used in the energy storage device 100 may be a solid electrolyte or a liquid electrolyte. Non-limiting examples of the electrolyte 110 may include LiPF6, LiAsF6, LiBOB, L1C1O4, L1BF4, and L1PF6.

[0028] The electroconductive slurry 105a and the electroconductive slurry 105b are prepared according to methods described later in conjunction with FIG. 2 or FIG. 3.

[0029] Although the energy storage device 100 is shown to include the anode 108, in other embodiments, the energy storage device 100 may be an anode-free battery without deviating from the scope of the invention.

[0030] FIG. 2 is a flow chart that illustrates a method for preparing an electroconductive slurry, in accordance with an embodiment of the present invention. Referring to FIG. 2, there is shown a flow chart 200 that illustrates exemplary operations 202 through 208 for preparing an electroconductive slurry, for example, the electroconductive slurry 105a and the electroconductive slurry 105b.

[0031] At 202, a first mixture is provided. The first mixture includes a first binder and a conductive additive in a first solvent. Examples of the first solvent may include water, deionized water, or distilled water.

[0032] In an embodiment, to provide the first mixture, the first binder is added to the first solvent (for example, deionized water) with constant stirring to form a binder solution, followed by addition of the conductive additive to the binder solution with constant stirring. Addition of the first binder and the conductive additive may be carried out at room temperature or at elevated temperatures that promote mixing and at the same time avoid degradation of the materials. In an embodiment, the addition of the first binder and the conductive additive is accomplished at room temperature. In another embodiment, the addition of the first binder and the conductive additive is accomplished at a temperature in a range of 25 degree Celsius (°C) to 100 °C.

[0033] Any suitable mixers or stirrers may be utilized, such as an ultrasonicator, magnetic stirrers, or commercial mixers such as planetary mixers to obtain the first mixture. Depending on the mixer and the mixing conditions, such as time of mixing and the temperature, the mixing speed (in terms of rpm) may be determined. For example, the stirring rate may vary from 200 rpm to 1000 rpm. The term, “constant stirring” may indicate stirring, mixing, and/or agitation to promote uniform mixing. The term “constant stirring”, as used herein, also includes the scenario where mixing is performed at one mixing rate (rpm) and is changed to a different mixing rate during the process.

[0034] At 204, an acid solution is added (or provided) to the first mixture to form a pH controlled first mixture. Addition of the acid solution controls and maintains a pH of the electroconductive slurry in a defined range, for example, in a range of 8 to 11. In one embodiment, the acid solution comprises only an acid. In another embodiment, the acid solution comprises an acid in an aqueous medium. Examples of a suitable acid used herein may include, but are not limited to, phosphoric acid, diphosphoric acid, triphosphoric acid, oxalic acid, formic acid, acetic acid, hydrochloric acid, or any combinations thereof. In another embodiment, the acid solution may comprise acid derivatives or salts thereof that may release hydrogen ions to lower a basicity of the first mixture. For example, phosphoric acid salts such as ammonium phosphate may be used to control the pH. [0035] The concentration of the acid, or its derivative in the acid solution may be determined based on the required range of the pH and liquid to solids ratio required to form subsequent mixtures. The term “liquid to solids ratio” refers to a ratio of liquid (for example, solvents) to solids (for example, binders, active materials, and the conductive additive). The liquid to solids ratio is critical in achieving a desirable viscosity in the electroconductive slurry. The concentration of the acid, its derivative, or salts may be in a range of 0.1 to 3 weight percent to a concentration of an active material added at a subsequent step of the method. In one embodiment, phosphoric acid is added in an amount of 0.1 weight percent to 0.9 weight percent of the active material that is added at a subsequent step. In a preferred embodiment, the phosphoric acid is added in an amount of 0.5 weight percent of the active material.

[0036] At 206, an active material and a second solvent are added to the pH controlled first mixture to form a final mixture. The active material may be added over a period with constant stirring. The period may range from 20 minutes to up to 1 hour. The term, “constant stirring” may indicate stirring, mixing, and/or agitation of a resultant mixture so as to promote uniform mixing. The addition of the active material comprises adding sequentially a portion of the active material in a plurality of batches to the pH controlled first mixture with constant stirring. In a sequential batch addition, a batch (for example, a portion) of the active material is added with constant stirring followed by addition of a subsequent batch. It is desired to have at least two portions or two sequential additions. The number of batches may vary from 2 to 5. In one embodiment, each of the batches comprises equal amounts of the active material. For example, for a batch addition having three portions, each portion will have 33.33% of the active material to the total amount of the active material. In yet another embodiment, an amount of a first batch is lower than a subsequent portion of the active material. In one embodiment, a first portion of the active material (for example, a first batch of the plurality of batches) is added to the pH controlled first mixture with constant stirring at variable rotations per minute (rpm). Variable rpm may refer to stirring at different rpm for different time durations. Addition of the first portion is followed by addition of the second portion of the active material (for example, a second batch of the plurality of batches) with constant stirring at variable rpm. A final portion of the active material (for example, a final batch of the plurality of batches) is added to the above mixture with constant stirring, followed by addition of the second solvent to the resultant mixture with constant stirring to form the final mixture. The second solvent may be added during the addition of the active material (for example, during the addition of the final portion of the active material) or after complete addition of the active material. The addition of the second solvent prevents agglomeration of the solids and supports in maintaining the required liquid to solids ratio.

[0037] Active material may be a cathode active material or an anode active material. The cathode active material is used when the electroconductive slurry 105a is to be prepared for coating on the cathode current collector 102 and the anode active material is used when the electroconductive slurry 105b is to be prepared for coating on the anode current collector 104.

[0038] At 208, a second binder is mixed with the final mixture to form the electroconductive slurry. The second binder may hold the active material and the conductive additive on to a substrate or a current collector (for example, the cathode current collector 102 or the anode current collector 104), thereby maintaining the mechanical and electrical integrity of an electrode (for example, the cathode 106 and the anode 108) thus formed.

[0039] In an embodiment, the method of FIG. 2 may be carried out at room temperature without applying any additional heat. In another embodiment, the method of FIG. 2 may be carried out at elevated temperatures that promote mixing and at the same time avoid degradation of the materials.

[0040] FIG. 3 is a flow chart that illustrates another method for preparing an electroconductive slurry, in accordance with another embodiment of the invention. Referring to FIG. 3, there is shown a flow chart 300 that illustrates exemplary operations 302 through 308 for preparing the electroconductive slurry (for example, the electroconductive slurry 105a or the electroconductive slurry 105b).

[0041] At 302, a first mixture is provided. The first mixture comprises a first binder and a conductive additive in a first solvent. Examples of the first solvent may include water, deionized water, or distilled water.

[0042] In an embodiment, to provide the first mixture, the first binder is added to the first solvent (for example, deionized water) with constant stirring to form a binder solution, followed by addition of the conductive additive to the binder solution with constant stirring. The stirring rate is determined based on the mixer employed and may vary from 200 rpm to 1000 rpm. Addition of the first binder and the conductive additive may be carried out at room temperature or at elevated temperatures that promote mixing and avoid degradation of the materials. In an embodiment, the addition of the first binder and the conductive additive is accomplished at room temperature. In another embodiment, the addition of the first binder and the conductive additive is accomplished at a temperature in a range of 25 degree Celsius (°C) to 100 °C. Any suitable mixers or stirrers may be utilized to perform stirring.

[0043] At 304, an active material and a second solvent are added (or provided) to the first mixture to form an intermediate mixture. The active material may be added over a period with constant stirring. The period may range from 20 minutes to up to 1 hour. The addition of the active material comprises adding sequentially a portion of the active material in a plurality of batches to the first mixture with constant stirring. In a batch addition, a batch (for example, a portion) of the active material is added with constant stirring followed by addition of a subsequent batch. It is desired to have at least two portions or two sequential additions. The number of batches may vary from 2 to 5. In one embodiment, each of the batches comprises equal amounts of the active material. In yet another embodiment, an amount of a first batch is lower than a subsequent batch of the active material. In one embodiment, a first portion of the active material (for example, a first batch of the plurality of batches) is added to the first mixture with constant stirring at variable rotations per minute (rpm). Addition of the first portion is followed by addition of a second portion of the active material (for example, a second batch of the plurality of batches) with constant stirring at variable rpm. A final portion of the active material (for example, a final batch of the plurality of batches) is added to the above mixture with constant stirring, followed by addition of the second solvent to the resultant mixture with constant stirring to form the intermediate mixture. The second solvent may be added during the addition of the active material (for example, during the addition of the final portion of the active material) or after complete addition of the active material. The addition of the second solvent prevents agglomeration of the solids and supports in maintaining the required liquid to solids ratio.

[0044] Active material may be a cathode active material or an anode active material. The cathode active material is used when the electroconductive slurry 105a is to be prepared for coating on the cathode current collector 102 and the anode active material is used when the electroconductive slurry 105b is to be prepared for coating on the anode current collector 104.

[0045] At 306, an acid solution is provided to the intermediate mixture form a final mixture. Addition of the acid solution controls and maintains a pH of the electroconductive slurry in a defined range, for example, a range of 8 to 11. In one embodiment, the acid solution comprises only an acid. In another embodiment, the acid solution comprises an acid in an aqueous medium. Examples of a suitable acid used herein may include, but are not limited to, phosphoric acid, diphosphoric acid, triphosphoric acid, oxalic acid, formic acid, acetic acid, hydrochloric acid, or any combinations thereof. In another embodiment, the acid solution may comprise acid derivatives or salts thereof that may release hydrogen ions to lower a basicity of the first mixture. For example, phosphoric acid salts such as ammonium phosphate may be used to control the pH.

[0046] A concentration of the acid, or its derivative in the acid solution may be determined based on the required range of the pH and liquid to solids ratio required to form subsequent mixtures. The concentration of the acid, its derivative, or salts may be in a range of 0.1 to 3 weight percent to a concentration of the active material added at 304. In one embodiment, phosphoric acid is added in an amount of 0.1 weight percent to 0.9 weight percent of the active material that is added at 304. In a preferred embodiment, the phosphoric acid is added in an amount of 0.5 weight percent of the active material added at 304.

[0047] At 308, the second binder is mixed with the final mixture to form the electroconductive slurry.

[0048] In an embodiment, the method of FIG. 3 may be carried out at room temperature without applying any additional heat. In another embodiment, the method of FIG. 3 may be carried out at elevated temperatures that promote mixing and at the same time avoid degradation of the materials.

[0049] The method described in conjunction with flow chart 200 differs from the method described in conjunction with flow chart 300 in that the acid solution is added after the addition of active material. The mixing sequences (for example, adding the active material in the plurality of batches) and the presence of acid solution of the methods 200 and 300 play an important role in forming a stable dispersion of the active material, the binders, and the conductive additive thereby avoiding phase separation and maintaining the integrity of the resulting slurry.

[0050] FIG. 4 is a flow chart that illustrates a method of preparing an energy storage device using the electroconductive slurry prepared in FIG. 2 or FIG. 3, in accordance with another exemplary embodiment of the invention. Referring to FIG. 4, there is shown a flow chart 400 that illustrates exemplary operations 402 through 404 for preparing the energy storage device 100 using the electroconductive slurry prepared by methods described in conjunction with FIG. 2 or FIG. 3.

[0051] At 402, coating the electroconductive slurry on a substrate or the current collector (for example, the cathode current collector 102 or the anode current collector 104). The electroconductive slurry is prepared by providing a first mixture, at step (i). The first mixture includes a first binder and a conductive additive in a first solvent. Examples of the first solvent may include water, deionized water, or distilled water. An active material and a second solvent are added to the first mixture with constant stirring to form a final mixture, at step (ii). The active material may be added over an extended period of time with constant stirring to facilitate uniform mixing. The period may range from 20 minutes to up to 1 hour. The active material is added sequentially in a plurality of batches. In a batch addition, a portion of the active material is added with constant stirring followed by a subsequent portion. It is desired to have at least two portions or two sequential additions. The number of batches may vary from 2 to 5. In one embodiment, each of the batches comprises equal amounts of the active material. In yet another embodiment, an amount of a first batch is lower than a subsequent portion of the active material. The second solvent may be added during the addition of the active material (for example, during the addition of the final portion of the active material) or after complete addition of the active material. At step (iii), a second binder is admixed with the final mixture to form the electroconductive slurry. According to embodiments of the present invention, a pH of the electroconductive slurry is maintained in a defined range of 8 to 11. In one embodiment (also described in the foregoing description of FIG. 2) this is achieved by adding an acid solution to the first mixture after step (i) and before step (ii). In another embodiment (also described in the foregoing description of FIG. 3), the acid solution is added to the final mixture after step (ii) and before step (iii).

[0052] The terms “substrates’ and “current collector’ are interchangeably used herein, the substrate in the context of the energy storage device 100 may correspond to a current collector (for example, the cathode current collector 102 or the anode current collector 104). Examples of substrates or current collectors may include aluminum, nickel, titanium, stainless steel, carbonaceous material, or copper. The substrate can be in the form of foil, mesh, or foam. It is preferred, for an anode to use a carbon-based current collector and for a cathode, to use an aluminum current collector. The substrate may have a thickness in a range of few microns (pm) to 20 microns.

[0053] In certain embodiments, the substrate may be a surface treated prior to coating with the slurry. The surface treatment may include subjecting the surface of the substrate to at least one selected from the group consisting of a plasma treatment, laser treatment, wet chemical treatment, ion beam treatment, electron beam treatment, and thermal etching treatment.

[0054] The coating of the substrate may be accomplished by means of spray coating, spin coating, dip coating, or similar such methods. In one embodiment, the slurry is spray coated on a surface of the current collector (for example, the cathode current collector 102 or the anode current collector 104) at room temperature. A second surface or opposite surface is coated subsequently. The coating may have a thickness in a range of 10 to 600 microns without deviating from the scope of the invention.

[0055] The coated substrate may be dried in any suitable dryer at room temperature or at elevated temperature to hasten the process. During drying, the first solvent and the second solvent volatize. In an embodiment, the weight ratio of the active material, the conductive additive, and the binders (e.g., the first binder and the second binder) in the electroconductive slurry after drying may be 92 to 99: 0.5 to 5.0: 0.5 to 3.0, specifically 92: 5: 3. The coated substrate may be calendared to enhance the bonding, density, and porosity of the electrodes. The calendaring, in one embodiment, is achieved by compacting the dried substrate through a roll compactor. The calendaring process can be a batch-process or a continuous process. The electroconductive slurry 105a and 105b that may be coated on current collectors 102 and 104 are suitable for both processes. The calendared substrate may be stamped and slit to the required dimension to fit the cell design of the energy storage device 100.

[0056] At 404, assemble various components including the coated substrate or the coated current collector to form the energy storage device 100. The components may be assembled using any known battery assembly technique, description of which is omitted for the sake of brevity.

[0057] The methods described in conjunction with FIGS. 2, 3, and 4 can be performed as a continuous, semi-continuous, or as a batch process. Various mixers can be utilized. For example, for large scale manufacturing commercial mixers such as planetary mixers could be used while for a lab-scale batch, an ultrasonicator, or magnetic stirrers could be utilized. Suitable other mixers may include, but are not limited to, hydrodynamic shear mixing, ball mill, or the like.

[0058] Examples of the first binder, used in the methods described in conjunction with FIGS. 2, 3, and 4, may include, but are not limited to, water soluble binders such as poly vinyl alcohols (PVA), polyvinyl pyrrolidone, polyethylene oxides (PEO), polyethylene glycols (PEG), polyacrylamide (PAAm), poly-N-isopropylearylamide, poly-N, poly (vinylidene difluoridehexafluoropropylene) (PVDF-HFP) copolymer, polyvinylidene fluoride (PVDF), N- dimethylacrylamide, polyethyleneimine, polytetrafluoroethylene (PTFE), polyoxyethylene, polyvinylsulfonic acid, poly(2-methoxyethoxyethoxyethylene), styrene butadiene rubber (SBR), fluorinated SBR, butadiene-acrylonitrile, rubber (NBR), hydrogenated NBR (HNBR), epichlorhydrin rubber (CHR) and acrylate rubber (ACM), polylactic acid (PLA), polyacrylic acid (PAA), polysuccinic acid, poly maleic acid and anhydride, poly furoic (pyromucic acid), poly fumaric acid, poly sorbic acid, poly linoleic acid, poly linolenic acid, poly glutamic acid, poly methacrylic acid, poly glycolic acid, poly aspartic acid, poly formic acid, poly acetic acid, poly propionic acid, poly butyric acid, poly sebacic acid, acrylic acid-type water-soluble polymers, maleic anhydride-type water-soluble polymers, poly(N-vinyl amides), polyacrylamides, for example N-methylacrylamide, N-ethyl acrylamide, N,N-dimethyl acrylamide, and N,N-diethyl acrylamide, poly (hydroxy- ethyl methacrylate), polyesters, poly(ethyl oxazolines), poly(oxymethylene), poly(vinyl methyl ether), poly(styrene sulfonic acid), poly(ethylene sulfonic acid), poly(vinyl phosphoric) acid, poly(maleic acid), starch, cellulose, carboxymethyl cellulose (CMC), protein, polysaccharide, dextrans, tannin, lignin, a polyethylene-polypropylene copolymer, or mixtures, or co-polymers thereof. The polymer binder may also comprise physically-modified and/or chemically-modified versions of any of the polymer binders listed above. In a preferred embodiment, the water-soluble first binder may comprise CMC.

[0059] Examples of the second binder, used in the methods described in conjunction with FIGS. 2, 3, and 4, may include, but are not limited to, binders described with reference to the first binder. In one embodiment, the second binder and the first binder are in a ratio of 1 :3. However, it is preferred, in certain embodiments, that the first binder be different from the second binder. In one embodiment, the first binder may be CMC and the second binder may be SBR. This is because a binder such as CMC may additionally act as a thickener in the first solvent which improves dispersion of the conductive additive and/or the active material. Hence, a blend of CMC and SBR respectively as the first binder and the second binder may reduce sedimentation and prevent phase separation of the resulting slurry over a period of time. In one embodiment, the first binder and the second binder independent of each other comprises CMC, SBR, fluorinated SBR, PVA, PTFE, PLA, PEG, PVDF, or any combinations thereof.

[0060] The first binder and the second binder support binding between the active material and the conductive additive, and also support in binding the active material and the conductive additive to the substrate. Thus, the first binder and the second binder increase the electrode stability and cycle life of the electrode thus formed by using the electroconductive slurry.

[0061] Examples of the conductive additive, used in the methods described in conjunction with FIGS. 2, 3, and 4, may include carbon and its derivative available in natural form or prepared synthetically, but are not limited to, carbon black, activated carbon, graphite, graphene, carbon nanotubes, carbon fibers, vapor grown carbon fibers (VGCF). In one embodiment, the conductive additive is activated carbon. The conductive additive functions to improve the transportation of the electrons in the electrodes 106 and 108, thereby enhancing the specific capacity and rate capability of the electrodes 106 and 108.

[0062] Suitable active material may comprise a cathode active material or an anode active material. The active materials determine the energy density of an electrode (for example the cathode 106 and the anode 108). Examples of the anode active material may comprise, but are not limited to, graphite, SiC nanocomposites, lithium titanium oxides (LTO) such as LiTiO2, Li4Ti5O12, Sn particulate, and Si particulates. Examples of the cathode active material may include, but are not limited to, lithium metal oxides such as LMO (lithium manganese oxide), Li- NCA (lithium nickel cobalt aluminum oxide), Li-NMC (lithium nickel manganese cobalt oxide), LCO (lithium cobalt oxide), LNO (lithium nickel oxide), LNMO (lithium nickel manganese oxide), LNCM (lithium nickel cobalt manganese oxide), LFP (lithium iron phosphate), LMP (lithium manganese phosphate), LCP (lithium cobalt phosphate), and similar such lithium-based metal oxides.

[0063] The second solvent comprises an alcohol. Examples of the second solvent may include, but are not limited to, isopropyl alcohol, ethanol, methanol, octanol, butoxyethanol, or any combinations thereof. The second solvent may be added in the range of 0.1% to 3% to an amount of the first solvent. In one embodiment, the second solvent is isopropyl alcohol (IP A). In one embodiment, IPA is added in an amount of 2% to an amount of the first solvent to maintain a required liquid to solids ratio.

[0064] FIG. 5 is a diagram that illustrates a plot 500 of pH at each step of the methods of FIGS. 2 and 3, in accordance with an exemplary embodiment of the invention. The plot 500 comprises two curves, the curve with starred pH values is hereinafter referred to as curve 1 and the curve with polygonned pH values is hereinafter referred to as curve 2. The curve 1 corresponds to the method as illustrated in FIG. 3 and the curve 2 corresponds to the method as illustrated in FIG. 2. The step 1 of the plot 500, corresponds to operation 202 and operation 302 of FIGS. 2 and 3, respectively, where the first mixture comprising the first binder and the conductive additive is provided in the first solvent.

[0065] For the curve 1, the steps 2 to 4 correspond to operation 304 of FIG. 3 where the active material is added in equal amounts, sequentially in 3 batches. At step 5 of the curve 1, an acid solution is added to the above mixture, corresponding to the operation 306 of FIG. 3. At step 6 of the curve 1, a second binder is added to the above mixture to form the electroconductive slurry corresponding to the operation 308 of FIG. 3.

[0066] As shown by the plot 500, step 2 of the curve 2 corresponds to addition of acid solution in the first mixture and corresponds to operation 204 of FIG. 2. The steps 3 to 5 of the curve 2 corresponds to sequential addition of the active material in equal batches of 3 and correspond to operation 206 of FIG. 2. At step 6 of the curve 2, a second binder is added to the above mixture to form the electroconductive slurry corresponding to the operation 208 of FIG. 2.

[0067] At steps 7 and 8, the electroconductive slurry prepared according to the methods of FIG. 2 and FIG. 3 are coated on an etched aluminum foil. The coating of the aluminum foil is in accordance with the method illustrated in FIG. 4 and corresponds to operation 402 of FIG. 4. At step 7, the electroconductive slurry is coated on one surface of the aluminum foil. At step 8, the uncoated surface of the aluminum foil is coated with the electroconductive slurry. The pH of the slurry of step 6, and the coated foils of steps 7 to 8 remains within the defined pH range and remains constant over a prolonged period. In one embodiment, the slurry remains stable for 5 days.

[0068] It is known that the electrochemical performance of the electrodes is related to the mixing conditions as well as the sequence of mixing. The slurry prepared according to embodiments of the present invention has a liquid to solids ratio in which a percentage of liquid is in a range of 42% to 48% and a percentage of solids is in a range of 52% to 58%. As a result, the electroconductive slurry does not undergo phase separation even when stored over a period of time (for example, 5 days). Further, the viscosity of the electroconductive slurry remains constant when stored over a period of time. In one embodiment, the electroconductive slurry may be stored for up to 5 days. In another embodiment, the slurry may remain stable for up to 3 days. [0069] The defined pH range of the electroconductive slurry is between 8 and 11. The defined pH helps maintaining the liquid to solids ratio. Further, the defined pH range minimizes corrosion of the current collector on which the slurry is to be coated.

[0070] The disclosed methods of FIGS. 2, 3, and 4 encompass numerous advantages. For example, the electroconductive slurry of the present invention has the pH maintained in the range of 8 to 11. As a result, corrosion phenomenon at the current collector is mitigated when the electroconductive slurry is coated on to the current collector. Addition of the active material in multiple batches prevents formation of agglomerates in the electroconductive slurry and supports in maintaining the required liquid to solids ratio. The electroconductive slurry of the present invention has a liquid to solids ratio in which a percentage of liquid is in the range of 42% to 48% and a percentage of solids is in the range of 52% to 58%. The achieved liquid to solids ratio increases the stability of the electroconductive slurry. As a result, the electroconductive slurry of the present invention is able to withstand phase separation even when stored over a period of time (for example, 5 days). Further, the methods of preparation of the electroconductive slurry as disclosed herein do not require any additional heat treatment.

[0071] It is to be understood that the above description is intended to be illustrative, and not restrictive. Furthermore, many other embodiments will be apparent to those of skill in the art upon reading and understanding the above description. Although the present invention has been described with reference to specific exemplary embodiments, it will be recognized that the invention is not limited to the embodiments described, but can be practiced with modification and alteration within the scope of the appended claims.

Claims

We claim,

1. An energy storage device comprising a current collector coated with an electroconductive slurry, wherein the electroconductive slurry is prepared by a method comprising:

(i) providing a first mixture comprising a first binder and a conductive additive in a first solvent;

(ii) adding an active material and a second solvent to the first mixture with constant stirring to form a final mixture, wherein adding the active material to the first mixture comprises adding sequentially a portion of the active material in a plurality of batches;

(iii) admixing a second binder with the final mixture to form the electroconductive slurry; and a step of providing an acid solution to maintain a pH of the electroconductive slurry in a defined range, wherein the acid solution is added:

(a) to the first mixture after step (i) and before step (ii), or

(b) to the final mixture after step (ii) and before step (iii).

2. A method of preparing an energy storage device comprising a current collector, the method comprising:

(iii) admixing a second binder with the final mixture to form the electroconductive slurry; a step of providing an acid solution to maintain a pH of the electroconductive slurry in a defined range, wherein the acid solution is added:

(a) to the first mixture after step (i) and before step (ii), or

(b) to the final mixture after step (ii) and before step (iii); and

(iv) coating the electroconductive slurry on the current collector. A method for preparing an electroconductive slurry, the method comprising:

(ii) providing an acid solution to the first mixture to form a pH controlled first mixture, wherein providing the acid solution to the first mixture maintains a pH of the electroconductive slurry in a defined range;

(iii) adding an active material and a second solvent to the pH controlled first mixture with constant stirring to form a final mixture, wherein adding the active material to the pH controlled first mixture comprises adding sequentially a portion of the active material in a plurality of batches; and

(iv) admixing a second binder with the final mixture to form the electroconductive slurry. The method as claimed in claim 3, wherein the first binder and the second binder independent of each other comprises carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR), fluorinated SBR, poly vinyl alcohols (PVA), polytetrafluoroethylene (PTFE), polylactic acid (PLA), polyethylene glycols (PEG), polyvinylidene fluoride (PVDF), or any combinations thereof.

5. The method as claimed in claim 3, wherein the conductive additive comprises carbon black, activated carbon, graphite, graphene, carbon nanotubes, carbon fibers, vapor grown carbon fibers, or any combinations thereof.

6. The method as claimed in claim 3, wherein the active material is a lithium metal oxide selected from the group comprising of: lithium titanium oxides, lithium manganese oxide, lithium nickel cobalt aluminum oxide, lithium nickel manganese cobalt oxide, lithium cobalt oxide, lithium nickel oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium iron phosphate, lithium manganese phosphate, lithium cobalt phosphate, or any combinations thereof.

7. The method as claimed in claim 3, wherein the second solvent comprises isopropyl alcohol, ethanol, methanol, octanol, butoxyethanol, or any combinations thereof.

8. The method as claimed in claim 3, wherein the first solvent comprises water, deionized water, distilled water, or any combinations thereof.

9. The method as claimed in claim 3, wherein the acid solution comprises an acid comprising phosphoric acid, diphosphoric acid, triphosphoric acid, oxalic acid, formic acid, acetic acid, hydrochloric acid, or any combinations thereof.

10. The method as claimed in claim 3, wherein the acid solution comprises an acid having a concentration in a range of 0.1 weight percent to 3 weight percent to a concentration of the active material.

11. The method as claimed in claim 3, wherein the pH of the electroconductive slurry is maintained in the defined range of 8 to 11.

12. The method as claimed in claim 3, wherein the second binder and the first binder are added in a ratio of 1 :3, where the second binder is SBR and the first binder is CMC.

13. The method as claimed in claim 3, wherein an amount of the second solvent is in a range of 0.1% to 3% to an amount of the first solvent.

14. The method as claimed in claim 3, wherein the electroconductive slurry has a liquid to solids ratio, wherein a percentage of liquid is in a range of 42% to 48% and a percentage of solids is in a range of 52% to 58% in the ratio.

15. A method for preparing an electroconductive slurry, the method comprising:

(ii) adding an active material and a second solvent to the first mixture with constant stirring to form an intermediate mixture, wherein adding the active material to the first mixture comprises adding sequentially a portion of the active material in a plurality of batches;

(iii) providing an acid solution to the intermediate mixture to form a final mixture, wherein providing the acid solution to the intermediate mixture maintains a pH of the electroconductive slurry in a defined range;

(iv) admixing a second binder with the final mixture to form the electroconductive slurry.

16. The method as claimed in claim 15, wherein an amount of the second solvent is in a range of 0.1% to 3% to an amount of the first solvent.

17. The method as claimed in claim 15, wherein the acid solution comprises an acid comprising phosphoric acid, diphosphoric acid, triphosphoric acid, oxalic acid, formic acid, acetic acid, hydrochloric acid, or any combinations thereof. The method as claimed in claim 15, wherein the acid solution comprises an acid having a concentration in a range of 0.1 weight percent to 3 weight percent to a concentration of the active material. The method as claimed in claim 15, wherein the pH of the electroconductive slurry is maintained in the defined range of 8 to 11. The method as claimed in claim 15, wherein the electroconductive slurry has a liquid to solids ratio, wherein a percentage of liquid is in a range of 42% to 48% and a percentage of solids is in a range of 52% to 58% in the ratio.