WO2018038957A1

WO2018038957A1 - Graphene oxide-based electrodes for secondary batteries

Info

Publication number: WO2018038957A1
Application number: PCT/US2017/046776
Authority: WO
Inventors: Nikhil Koratkar; Rahul Mukherjee; Eklavya Singh
Original assignee: Enermat Technolgies, Inc.
Priority date: 2016-08-22
Filing date: 2017-08-14
Publication date: 2018-03-01

Abstract

The disclosed technology generally relates to electrodes for secondary batteries, and more particularly to electrodes for secondary batteries having graphene oxide (GO) sheets. In one aspect, an electrode for a secondary battery includes a current collector having coated thereon an active material. The active material includes graphite microparticles intermixed with nanoparticles or microparticles, where the nanoparticles or microparticles comprises silicon, silicon monoxide, silicon dioxide, tin, tin oxide, germanium, aluminum or combinations thereof. The graphite microparticles have a mean cross-sectional dimension in a range of about 1 micron to about 200 microns. The electrode additionally includes a plurality of contiguous graphene oxide (GO) sheets formed over the active material, wherein the GO sheets have a mean carbon-to-oxygen ratio of about 2:1 to about 20:1, wherein each of the GO sheets contacts at least another of the GO sheets, such that the GO sheets generally encapsulate the active material to provide cohesion and mechanical stability thereto.

Description

GRAPHENE OXIDE-BASED ELECTRODES FOR SECONDARY BATTERIES

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application No. 62/377,898, filed on August 22, 2016, the content of which is incorporated by reference herein in its entirety.

BACKGROUND

Field

[0002] The disclosed technology generally relates to electrodes for secondary batteries, and more particularly to electrodes for secondary batteries having graphene oxide (GO) sheets.

Description of the Related Technology

[0003] Lithium ion batteries form the lifeblood of our economy today. From consumer electronics to electric cars, they bridge the gap from powering the smallest sensors and devices produced by the semiconductor industry, to electric bikes, cars and buses. As electrified transportation options grow, the battery production and consumption economics would need to drive the cost down. There are a few ways to do this at production scale for cell manufacturers, and one is by optimizing existing production lines and equipment that perform the tasks of slurry coating, drying, calendering, slitting and winding or stacking for cylindrical or prismatic/pouch cell form factors respectively. This has already been done to a great extent and manufacturing lines have been reported to cost approximately $10.5 to $11.6 million according to an NREL's Clean Energy Manufacturing Analysis Center report on the lithium-ion supply chain and manufacturing considerations. Another strategy would be to change the material that goes into the mixing step, and with an innovation in the formulation chemistry one could dramatically affect not only cell level performance, but also plant scale economics. SUMMARY

[0004] In one aspect, an electrode for a secondary battery includes a current collector having coated thereon an active material. The active material includes graphite microparticles intermixed with nanoparticles or microparticles, where the nanoparticles or microparticles comprises silicon, silicon monoxide, silicon dioxide, tin, tin oxide, germanium, aluminum or combinations thereof. The graphite microparticles have a mean cross-sectional dimension in a range of about 1 micron to about 200 microns. The nanoparticles or microparticles have a mean cross-sectional dimension in a range of about 0.01 μιη to about 40 μιη. The electrode additionally includes a plurality of contiguous graphene oxide (GO) sheets formed over the active material, wherein the GO sheets have a mean carbon-to-oxygen ratio of about 2: 1 to about 20: 1, wherein each of the GO sheets contacts at least another of the GO sheets, such that the GO sheets generally encapsulate the active material to provide cohesion and mechanical stability thereto. The electrode further includes a carbon-based polymer binder network interconnecting the current collector, the graphite microparticles, the nanoparticles or microparticles and the GO sheets.

[0005] In another aspect, a secondary battery includes a cathode, an anode and a separator interposed between the cathode and the anode, where the above-described electrode is configured serve as the anode.

[0006] In another aspect, an electrode for a secondary battery includes a current collector having coated thereon an active material. The active material includes lithium transition metal oxide particles having a mean cross-sectional dimension in a range of about 0.1 micron to 20 microns. The electrode additionally includes a plurality of contiguous graphene oxide (GO) sheets formed over the active material, wherein the GO sheets have a mean carbon-to-oxygen ratio of about 2: 1 to about 20: 1, wherein each of the GO sheets contacts at least another of the GO sheets, such that the GO sheets generally encapsulate the active material to provide cohesion and mechanical stability thereto. The electrode further includes a carbon-based polymer binder network interconnecting the current collector, the microparticles, the nanoparticles and the GO sheets.

[0007] In another aspect, a secondary battery includes a cathode, an anode and a separator interposed between the cathode and the anode, where the above-described electrode is configured serve as the cathode. [0008] In another aspect, a method of forming an electrode for a secondary battery, e.g., an anode, includes providing a current collector. The method additionally includes preparing a slurry mixture by a process including intermixing a carbon-based binder with water to form a first dispersion, and intermixing graphene oxide (GO) sheets and nanoparticles or microparticles and one or both of water and ethanol to form a second dispersion. The nanoparticles or microparticles include one of silicon, tin or aluminum and have a mean cross-sectional dimension in a range of about 0.01 μπι to about 40 μπι. Forming the first dispersion or the second dispersion further include dispersing graphite microparticles having a mean cross-sectional dimension in a range of about 1 micron to about 200 microns. Preparing the slurry mixture additionally includes mixing the first dispersion and the second dispersion to form the slurry mixture. The method additionally includes coating the current collector with the slurry mixture and drying the slurry mixture to form an active material layer on the current collector. In some embodiments, upon drying, the active material layer includes a plurality of contiguous GO sheets formed over the nanoparticles or microparticles and the graphite microparticles, wherein each of the GO sheets contacts at least another of the GO sheets, such that the GO sheets generally encapsulate the nanoparticles or microparticles and the graphite microparticles and provide cohesion and mechanical stability to the active material layer.

[0009] In another aspect, a method of forming an electrode for a secondary battery, e.g., a cathode, includes providing a current collector. The method additionally includes preparing a slurry mixture by a process including intermixing a carbon-based binder with water to form a first dispersion, and intermixing graphene oxide (GO) sheets and lithium transition metal oxide particles having a mean cross-sectional dimension in a range of about 0.1 micron to 20 microns to form a second dispersion, and mixing the first dispersion and the second dispersion to form the slurry mixture. The method additionally includes coating the current collector with the slurry mixture and drying the slurry mixture to form an active material layer on the current collector. In some embodiments, upon drying, the active material layer comprises a plurality of contiguous GO sheets formed over the transition metal oxide particles, wherein each of the GO sheets contacts at least another of the GO sheets, such that the GO sheets generally encapsulate the transition metal oxide particles and provide cohesion and mechanical stability to the active material layer. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] FIG. 1A illustrates a rechargeable or a secondary battery having an electrode according to embodiments.

[0011] FIG. IB illustrates an electrode for the secondary battery including graphene oxide, according to embodiments.

[0012] FIG. 2A illustrates an overall process flow for a battery electrode manufacturing.

[0013] FIG. 2B illustrates a method of forming an electrode for a secondary battery including graphene oxide sheets encapsulating an active material layer, according to embodiments.

[0014] FIG. 2C illustrates a method of preparing a slurry for forming an active material layer encapsulated by graphene oxide sheets for an anode, according to embodiments.

[0015] FIG. 2D illustrates a method of preparing a slurry for forming an active material layer encapsulated by graphene oxide sheets for a cathode, according to embodiments.

[0016] FIG. 2E is a flow chart illustrating a method of forming an electrode structure for a secondary battery, according to embodiments.

[0017] FIG. 3 is a schematic diagram illustrating a pre-lithiation process, according to embodiments.

[0018] FIG. 4 is an X-Ray diffraction spectra of a coating of an electrode structure for a secondary battery according to embodiments.

[0019] FIGS. 5(a)-5(c) are scanning electron micrographs of electrode structures according to embodiments.

[0020] FIG. 5(d) is a schematic depiction of the configurations of the electrode structures illustrated in FIGS. 5(a)-5(c).

[0021] FIG. 6(a) illustrates a pristine untreated anode, displaying first cycle capacity loss of -45%.

[0022] FIG. 6(b) illustrates an annealed anode, demonstrating a -25% loss in first cycle capacity. [0023] FIG. 6(c) illustrates a calendered and pre-lithiated anode, depicting a first cycle loss of only -15%, according to embodiments.

[0024] FIG. 7 illustrates parylene-coated active anode material, according to embodiments.

[0025] FIG. 8 illustrates voltage profiles of different compositions of active anode materials, synthesized through aqueous slurry dispersions, thereby enabling higher mass loading of the nanoparticle constituents as well as a better cycle life, according to embodiments.

[0026] FIG. 9(a) illustrates charge-discharge voltage profiles for electrodes containing 7.5% silicon, according to embodiments.

[0027] FIG. 9(b) illustrates charge-discharge voltage profiles for electrodes containing 20% silicon by weight, according to embodiments.

[0028] FIG. 10 illustrates cycle life and coulombic efficiency attained for silicon containing anodes having <10% by weight of Si, according to embodiments.

[0029] FIG. 11 is a graph illustrating coulombic efficiency of silicon-graphene or GO-graphite composite anodes according to embodiments, as a function of cycle index and depth of discharge.

[0030] FIG. 12 is a schematic representation of the slurry formulation and mixing process being integrated with a roll-to-roll electrode coating (slot-die in this case) machines for high speed deposition, drying and calendering, according to embodiments.

[0031] FIG. 13 illustrates variability in coating thickness as a function of particle size of silicon monoxide.

[0032] FIG. 14 illustrates GO-silicon monoxide-based anodes according to embodiments, tested in a half-cell configuration.

[0033] FIG. 15 illustrates half-cell testing results of silicon dioxide-based anodes according to embodiments, tested in a half-cell configuration.

DETAILED DESCRIPTION

[0034] The present disclosure seeks to address the multitude of problems associated with silicon-based anodes through a facile cost-competitive synthesis and post- synthesis strategy relying on prevalent industrial infrastructure, with the objective to promote ease of adoption and commercial viability of such high-performance silicon-based anodes.

Current Collector for a Secondary Battery Having Contiguous Graphene Oxide (GO) Sheets Formed Over the Active Material

[0035] FIG. 1A illustrates a rechargeable or a secondary battery 100 comprising an electrode according to embodiments. The secondary battery 100 includes an electrode assembly 102, electrolyte, and a can type or pouch type casing 104 accommodating the electrode assembly 102 and the electrolyte. The electrode assembly can be formed by stacking or winding a positive electrode or a cathode 110, a negative electrode or an anode 120, and a separator 130 disposed therebetween.

[0036] The positive electrode 1 10 includes a positive electrode current collector 111 formed of, e.g., aluminum, and positive electrode active material 1 12. The positive electrode active material 112 can be disposed on one both surfaces of the positive electrode current collector 111. The negative electrode 120 includes a negative electrode current collector 121 formed of, e.g., copper. The negative electrode active material 122 can be disposed on one or both surfaces of the negative electrode current collector 121. The separator 130 is disposed between the positive electrode 110 and the negative electrode 120 to insulate the positive electrode 110 and the negative electrode 120. The separator 130 is formed of a film including, e.g., one of polyethylene, polypropylene, and a combination thereof. The separator

[0037] FIG. IB illustrates an electrode comprising graphene oxide (GO) for the secondary battery 100, which can be a positive electrode 110 or a negative electrode 120, according to embodiments. The electrode 110, 120 includes a current collector 111, 122 and an electrode active material 112, 122 formed on the current collector 111, 122. The electrode active material 112, 122 comprises a plurality of contiguous GO sheets 140, wherein each GO sheet 140 contacts at least one other GO sheet 140 , such that the GO sheets generally encapsulate the electrode active material 112, 122 to provide cohesion and mechanical stability. In various embodiments, the GO sheets 140 have a mean carbomoxygen ratio between about 2: 1 and about 20: 1. In various embodiments, the GO sheets have a mean lateral dimension between about 0.1 μπι and about 200 μπι. In various embodiments, the electrode active material 112, 122 includes different particles 142, 144, depending on whether the electrode is configured as an positive electrode 110 or a negative electrode 120. The different particles 142, 144 may include, for example, graphite microparticles or nanoparticles or microparticles as described below when the electrode is configured as an anode, and may include, for example, lithium transistion metal oxide particles when the electrode is configured as a cathode.

[0038] As described herein, graphite is a three-dimensional carbon based material made up of millions of layers of graphene, as understood in the relevant industry. Graphite oxide refers to a material formed by oxidation of graphite using strong oxidizing agents, thereby introducing oxygenated functionalities to graphite. For example, graphite oxide may expand the layer separation, or make the material hydrophilic. Graphite oxide can be exfoliated, e.g., in water using sonication, thereby producing a two-dimensional material comprising a single or a few layers of graphite oxide, which is referred to in the industry and herein a graphene oxide (GO). According to various embodiments described herein, GO refers to a structure having one or more layers but fewer than about 100, 50, 20, 10 or 5 sheets, any number of sheets within a range defined by any of these values.

[0039] It will be appreciated that the structure and properties of GO can depend the synthesis method and the degree of oxidation. GO generally preserves the layered structure of the parent graphite or graphene, but the layers can be buckled. GO can have an interlayer spacing that is as much as two times greater (-0.7 nm) compared to graphite. GO can have epoxide groups or bridging oxygen atoms, or other functional groups including one or more of carbonyl (C=0), hydroxyl (-OH), phenol and/or organosulfate groups, to name a few.

[0040] According to various embodiments described herein, GO can have a carbon-to-oxide ratio between about 1 to 100, 1 to 2, 2 to 5, 5 to 10, 10 to 15, 15 to 20, 20 to 25, 25 to 30, 30 to 35, 35 to 40, 40 to 45, 45 to 50, 50 to 55, 55 to 60, 60 to 65, 65 to 70, 70 to 75, 75 to 80, 80 to 85, 85 to 90, 90 to 95, 95 to 100, or a ratio within a range defined by any of these values. For instance, according to some embodiments, GO sheets have a mean carbon-to-oxygen ratio of about 2: 1 to about 20: 1. According to some other embodiments, GO sheets have a mean carbon-to-oxygen ratio of about 5: 1 to about 20: 1. [0041] According to various embodiments, the GO sheets have a surface area of

2 2 2 2 2 about 10 m /g to about 3000 m /g, about 10 m /g to about 100 m /g, about 100 m /g to about 1000 m²/g, about 1000 m²/g to about 3000 m²/g, or a surface area in a range defined by any of these values. For instance, GO sheets according to some embodiments have a surface area of about 100 m²/g to about 3000 m²/g.

[0042] When the electrode is an anode, the anode active material comprises graphite microparticles intermixed with nanoparticles. In various embodiments, the graphite microparticles have a mean cross-sectional dimension of about 1 micron to 200 microns. In various embodiments, the nanoparticles have a mean cross-sectional dimension in the range of about 0.01 μπι to about 40 μπι. In some embodiments, the anode active material further comprises lithium. In some embodiments, the nanoparticles are formed of silicon, tin, tin oxide and aluminum nanoparticles. Additionally, a carbon-based polymer binder network interconnects the current collector, the graphite microparticles, the nanoparticles and the GO sheets. In some embodiments, the carbon-based binder network comprises a water-soluble binder selected from the group consisting of carboxymethyl cellulose, poly-acrylic acid, styrene butadiene rubber or a combination thereof. The anode active material region also comprises a conductive carbon additive to provide an electrically conductive framework for the transport of electrons. The anode active material region further comprises one or more water-soluble polymers such that the water-soluble polymers form polymeric or co- polymeric blocks coating at least one of the graphite microparticles and the nanoparticles, thereby further providing cohesion and mechanical stability to the anode active material region. In some embodiments, the water-soluble polymers are one or more of polyvinylpyrrolidone (PVP), poly-ethylene oxide (PEO), poly-acrylic acid (PAA), polyaniline, polyethylene glycol (PEG) and polyvinyl alcohol (PVA).

[0043] When the electrode is a cathode, the cathode active material comprises lithium transition metal oxide particles having a mean cross-sectional dimension in the range of about 0.1-50 microns. The cathode active material region further comprises one or more of water-soluble binders consisting of carboxymethyl cellulose, poly-acrylic acid, styrene butadiene rubber or a combination thereof, thereby further providing cohesion and mechanical stability to the cathode active material region. In some embodiments, GO sheets are added to the cathode active material in order to promote dispersion and prevent agglomeration of the cathode active material in the solution. In addition, conductive carbon additives are incorporates to promote electrical conductivity. Such conductive carbon additives may include but are not limited to graphite, graphene, carbon nanotubes, activated carbon, Super-P or a combination thereof. In some embodiments, the water-soluble polymers are poly-ethylene oxide (PEO), poly-acrylic acid (PAA), polyaniline, polyethylene glycol (PEG) and polyvinyl alcohol (PVA).

[0044] FIG. 2A schematically illustrates an overall process flow for battery electrode manufacturing. The processing begins with mixing 202 a slurry of electroactive materials, polymeric binders and conductive carbon additives in an organic or aqueous dispersion. The slurries are coated 204 onto metallic current collector foils, dried 204, and are pressed or "calendered" 206 for achieving better packing and adhesion of the particles. The coated metallic current collector foils are subsequently slit 208 to form the individual electrodes. Some of the solvent may be recovered using a solvent recovery system 210 for recovering organic solvents such as N-Methyl pyrollidone are used.

[0045] The process shown in FIG. 2A includes mixing 202 the active components with binders and solvents to form a slurry, coating 204 the slurry on to a metallic sheet for forming metallic current collectors, and drying 204 the solvent from the slurry, and subsequently cutting the sheet into electrodes and assembling them into cells. Graphite is one of the most widely used anode active material. Silicon is another promising electrode candidate, with extensive research efforts focused on stabilizing its performance to develop a commercially viable product. Some approaches use graphene, or in fact carbon, to coat and stabilize high capacity materials such as silicon (theoretical capacity of -4200 mAh/g for silicon as compared to -372 mAh/g for graphite). Such approaches have been relatively widely used, albeit with limited success. Such approaches often lack manufacturing scalability and economic viability, attributed to extensive and expensive chemical synthesis steps associated. In other approaches of synthesizing silicon anodes, a graphene substrate is coated with a graphite film, wherein the coating process uses chemical vapor deposition and microwave or plasma-assisted polymerization techniques. It is to be noted that both these approaches techniques can be incompatible with current lithium ion electrode manufacturing lines and may utilize expensive and quality-controlled, complex infrastructure adjustments in order to establish manufacturing feasibility. Similarly, they may also utilize a chemical vapor deposition step, which can often employ toxic liquid silane pre-cursors to grow porous silicon nanowires and additional chemical or physical vapor deposition or high temperature polymerization-carbonization to coat the silicon structures with a protective carbon shell. Therefore, there is a need for a more benign, chemically less intensive and scalable manufacturing strategy, which is also compatible with current industry infrastructure for ease of adoption and cost-competitiveness.

[0046] Material processing for electrode materials is a complex supply chain. The graphite materials (synthetic graphite derived through petroleum derivatives or natural flake graphite that is mined) are heavily processed to ensure they are packed tightly, with high tapping densities (-0.9 kg/m³), and the particles are engineered to be semi -spherical and potato shaped on the size scale of 10-30 μιη in diameter. Smaller particles are preferred since they make it easier for the lithium-ions to diffuse through the graphite particles and pack better, leading to high volumetric energy density. Appropriately sized graphite flakes are the most widely used commercially available anode material.

[0047] Silicon, including silicon oxide and silicon monoxide, is an attractive anode material, but is inherently unstable in a lithium ion operating environment for three major reasons, the first being that its high lithium-ion intercalation capacity inherently leads to dramatic volume expansion (-200% for SiO and -300-400% for Si/SiO_x) and contraction with each charge cycle, leading to breakage or pulverization of the electrode material. The second reason is associated with the poor adhesion characteristics of silicon, that result in delamination of the silicon particles from the copper current collector, resulting in drastic loss of active material and electrical contact. The final reason is that the solid-electrolyte interphase (SEI) layer continuously forms, breaks and re-forms on the surface of silicon in an electrochemical environment, leading to capacity loss over extended cycling. Other high capacity active materials such as tin/tin oxides, aluminum/aluminum oxides, and germanium also face similar challenges when used as a lithium-ion host. Graphite on the other hand has a much more electrochemically stable SEI, and does not expand as much (just around -10%) volumetrically. It is therefore of little surprise that silicon-loaded anodes often incorporate a graphitic coating. The objective is to prevent the silicon structures from coming in direct contact with the lithium ion battery electrolyte that results in the formation of the SEI layer. Instead, the graphite coating is utilized to form a more stable SEI layer. However, such an approach may call for an additional, ex-situ processing step involving chemical vapor deposition or decomposition of carbon-containing compounds.

[0048] In yet another challenge confronting silicon anodes, the adhesion properties between active silicon and copper current collectors have to be addressed in order to prevent delamination and loss of electrical contact in the electrochemical environment. A common strategy adopted as a solution involves the introduction of a thin adhesive, conductive film on top of the copper current collector. Such films, typically metals such as chromium, titanium and sometimes, carbon black, provide a good interface for adhesion of the active silicon component to the substrate while simultaneously providing a pathway for the electrons. However, incorporation of such a thin film of adhesion promoter would call for additional steps such as physical or chemical vapor deposition or spray deposition. In addition, the metallic or carbonaceous films are not inherently elastic and would eventually result in delamination of the active material owing to a large mismatch between the volume expansion of silicon and the elastic strength of the adhesive. Therefore, all of the aforementioned strategies largely involve solving only one out of the three major challenges inherently associated with silicon-based anodes, specifically, (i) mechanical robustness; (ii) electrochemical stability; and (iii) adhesion. Thus, there is a further need for a cost-effective and scalable approach to systematically increasing the percentage of high-performance lithium-intercalating active materials, e.g., silicon and aluminum in battery electrodes, e.g., anodes, thereby improving the energy density of the cell, while maintaining good cycle life characteristics.

[0049] One such problem is the adoption of an in-situ polymerization of an electrochemically active polymer. The polymer forms a stable SEI layer in the electrochemical redox environment and therefore prevents the silicon from undergoing repeated SEI formation cycles. However, such graphite and polymer coatings are largely non-elastic compared to the deformation range of silicon (up to -400% by volume) and therefore do not mitigate the structural degradation of silicon or translate to a significant improvement in performance characteristics.

[0050] Another such problems is the use of elastic binders and incorporation of carbonaceous shells around a silicon core with sufficient room for silicon to expand without causing stress on adjacent components. However, such strategies still involve non-standard and expensive battery electrode materials and the use of manufacturing strategies that are largely incompatible with industrial norms. For instance, there is a description of a strategy to form a nano-scale coating of active silicon around an interconnected polymeric template, followed by in-situ carbonization and partial etching of the template to create adequate space to accommodate volumetric changes. In addition to the aforementioned drawbacks associated with incompatible and expensive manufacturing steps, such structures have critical limitations with respect to scalability as increasing the thickness of the silicon coating will have an adverse impact on the porosity and stability of the nano-structures, as well as its mechanical robustness during volumetric changes. Moreover, the presence of large cavities will dramatically lower the volumetric energy density of such anodic structures.

[0051] The present disclosure describes a stable silicon-based formulation and post-synthesis treatment strategies that promote adhesion with current collectors and mitigates the structural and electrochemical instabilities while delivering superior performance characteristics over extended cycling. The disclosure described herein includes a method to formulate a high-performance active material slurry, and use it to create a better lithium-ion rechargeable cell electrode. The method describes how to introduce nanomaterials including but not limited to carbon nano-sheets, graphene, GO, carbon nanotubes, fullerenes, aluminum, tin, tin oxide, germanium and silicon into an aqueous based slurry mixture, which can be used on a traditional battery manufacturing process line. Using water instead of MP (N-Methyl Pyrollidone, the standard organic solvent) eliminates the need for a solvent recovery system and saves nearly $1.1 million on a manufacturing capital investment. Water also dries faster, leading to a 1.2-1.3x increase in the throughput speed.

[0052] Graphene and other polymeric molecules are used to suspend and encapsulate nano-p articulate silicon and aluminum in a simple, but effective solution based process. The formulation and coating technique described herein enhances silicon's and aluminum's electrochemical stability in the cell and allows manufacturers to load much higher concentrations of such materials in their anodes, while managing the first cycle loss and capacity fade associated with such high capacity anode materials. The process can be scaled up with industrially available equipment and coating machines, and can be extended to other materials as well. Improved cycle-life performance with silicon-graphene and aluminum-graphene electrode composites has been demonstrated here, with the concept extending to crystalline silicon, amorphous silicon, silicon oxides, silicon oxy-nitrides, tin or aluminum containing materials, sulfur containing materials, and germanium containing materials.

[0053] Silicon nanoparticles that are on the size scale of 0.01 - 40 μιη are dispersed along with atomically thin carbon such as GO sheets in an aqueous dispersion. Ethanol or other organic solvents may be used to improve the dispersion stability, since the organic non-polar regions of the organic solvents molecule help silicon nanoparticles disperse better in a highly polar aqueous environment. Other components in the slurry include a water soluble polymeric binder as well as conductive carbon additives. In some embodiments, carboxymethyl cellulose (CMC) is used as a binder and thickening agent that improves the viscosity of electrode slurries prior to coating. In some embodiments, Super-P (activated carbon) is used as a carbon additive for enhanced electrical conductivity.

[0054] CMC has a structure with several oxygen moieties that are very compatible with GO in a dispersion. These large organic, cellulose molecules can form hydrogen bonds with GO, thus effectively creating a heavily interlinked network that can hold together constituents from various size scales. Large graphite particles (-10-30 microns) are held together by a cellulosic film that adheres strongly to the underlying current collector. The unique process step that incorporates highly flexible, micron scale GO sheets, with atomic thickness creates a better percolation network through the non-conducting cellulosic binder material. The silicon nanoparticles are attached to and embedded within these carbonaceous layers through solution based process steps. Rather than introducing reduced, pristine graphene sheets, a highly functionalized carbon such as GO dispersion is used. Post- deposition reduction strategies include thermal or flash based techniques that have been described by the inventors in previous filings. A strategy of calendering the electrodes with heated rollers in order to increase its mechanical robustness and volumetric density is also described in the present disclosure. The reduction is performed in order to improve the electrical conductivity of the electrode. The extent of reduction should be carefully controlled however to ensure the structural robustness of the resulting electrode. As the film is deoxygenated, conductivity increases, but the film may break or degrade as the oxygen based network is broken down. In addition, the present disclosure also details a process of pre- lithiation, through which lithium atoms are introduced into the silicon-graphene composite anodes prior to cell assembly. The pre-lithiation strategy aids in controlling the first cycle capacity loss and improves the cycling repeatability by inducing the formation of a stable and artificial SEI layer.

[0055] The GO sheets have hydroxyls, epoxides and carboxylic acid groups on the basal plane and edges, which can interact with the oxygen functional groups present in the cellulosic structure of CMC, as well as water, when in dispersion. However, large portions of the GO sheet are also highly non-polar, given that they have only sp² hybridized carbon atoms in the six membered rings that are characteristic of pristine graphene. These regions of the large GO macro-molecule attract silicon nanoparticles, and act as anchors during electrochemical lithiation and de-lithiation. This type of suspended, carbonaceous matrix allows for the expansion and contraction of silicon nanoparticles within the electrode, but more importantly the highly embedded nature of the active materials reduces the repeated SEI formation and breakage. Nano-particulate silicon is held by, and encapsulated by a graphene-CMC network, which also holds together and binds the larger graphite particles. Furthermore, upon casting onto the current collector, the CMC's cellulosic structure also forms powerful hydrogen bonds with the copper/copper oxide interface of the current collector foil, thus providing good adhesion during electrochemical reactions. Steps have been described to ensure proper mixing and coating of the particles on a large scale. Large scale mixers including planetary mixers, high speed mixers, homogenizers, universal type mixers and static mixers are generally used in industry.

[0056] The present disclosure is compatible with different equipment types, and only calls for a specific nanoscale material assembly approach. This approach focuses on understanding the relative forces of attraction and repulsion between nanoparticles, nanosheets and macroscopic constituents in a dispersion. Hydrophobic and hydrophilic interactions, electrostatic interactions, dispersion forces, hydrogen bonding, and other interactions are leveraged to assemble nanoparticle composite structures.

Methods of Forming an Electrode Having Coated Thereon Particles Encapsulated by Graphene Oxide Sheets

[0057] FIG. 2B illustrates a method of forming an electrode for a secondary battery including graphene oxide sheets encapsulating an active material layer, according to embodiments. Referring to FIG. 2B, a method 212 of forming an electrode for a secondary battery, e.g., an anode, includes providing 214 a current collector. The method 212 additionally includes preparing 216 a slurry mixture. The method 212 additionally includes coating the current collector with the slurry mixture comprising graphene oxide and drying 220 the slurry mixture to form an active material layer on the current collector.

[0058] FIG. 2C illustrates a method of preparing a slurry for forming an active material layer encapsulated by graphene oxide sheets for an anode, according to embodiments. Referring to FIG. 2C, when the electrode is an anode, preparing 216 the slurry mixture includes intermixing 222 a carbon-based binder with water to form a first dispersion, and intermixing 224 graphene oxide (GO) sheets and nanoparticles or microparticles and one or both of water and ethanol to form a second dispersion.

[0059] The nanoparticles or microparticles include one of silicon, tin or aluminum, e.g., oxides thereof, and have a mean cross-sectional dimension in a range of about 10 nm to about 100 μπι, 10 nm to about 100 nm, 100 nm to 500 nm, 500 nm to 1 μπι, 1 μπι to 10 μπι, 10 μπι to 40 μπι, 40 μπι to 100 μπι, or a range defined by any of these values. For instance, the nanoparticles or microparticles can have a mean cross-sectional dimension in a range of about 10 nm to 500 nm or about 0.01 μπι to 40 μπι.

[0060] Forming the first dispersion or the second dispersion further include dispersing graphite microparticles having a mean cross-sectional dimension in a range of about 0.1 μπι to 1000 μπι, 0.1 μπι to 10 μπι, 10 μπι to 20 μπι, 20 μπι to 50 μπι, 50 μπι to 100 μπι, 100 μπι to 200 μπι, 200 μπι to 500 μπι, 500 μπι to 1000 μπι, or a mean cross- sectional dimension in a range defined by any of these values. For instance, the graphite microparticles can have a mean cross- sectional dimension in a range of about 1 μπι to 200 μπι. Preparing the slurry mixture additionally includes mixing the first dispersion and the second dispersion to form the slurry mixture.

[0061] In some embodiments, upon drying, the active material layer includes a plurality of contiguous GO sheets formed over the nanoparticles or microparticles and the graphite microparticles, wherein each of the GO sheets contacts at least another of the GO sheets, such that the GO sheets generally encapsulate the nanoparticles or microparticles and the graphite microparticles and provide cohesion and mechanical stability to the active material layer. [0062] FIG. 2D illustrates a method of preparing 216 a slurry for forming an active material layer encapsulated by graphene oxide sheets for a cathode, according to embodiments. Referring to FIG. 2D, when the electrode is a cathode, preparing 216 the slurry mixture includes intermixing 228 a carbon-based binder with water to form a first dispersion, intermixing graphene oxide (GO) sheets and lithium transition metal oxide particles. According to various embodiments, the lithium transition metal oxide particles have a mean cross-sectional dimension in a range of about 0.1 μπι to 1000 μπι, 0.1 μπι to 10 μπι, 10 μπι to 20 μπι, 20 μπι to 50 μπι, 50 μπι to 100 μπι, 100 μπι to 200 μπι, 200 μπι to 500 μπι, 500 μπι to 1000 μπι, or a mean cross-sectional dimension in a range defined by any of these values. For instance, the lithium transition metal oxide particles can have a mean cross-sectional dimension in a range of about 0.1 μπι to 20 μπι. Preparing the slurry mixture additionally includes mixing 232 the first dispersion and the second dispersion to form the slurry mixture

[0063] In some embodiments, upon drying, the active material layer comprises a plurality of contiguous GO sheets formed over the transition metal oxide particles, wherein each of the GO sheets contacts at least another of the GO sheets, such that the GO sheets generally encapsulate the transition metal oxide particles and provide cohesion and mechanical stability to the active material layer

Incorporation of Materials into an Aqueous-Based Slurry Mixture

[0064] FIG. 2E is a flow chart illustrating further details of the method 216 of preparing a slurry described above with respect to FIGS. 2C and 2D, according to embodiments. The illustrated process flow for slurry formulation technique allows for the incorporation of high-capacity additives in an electrochemically stable electrode structure. Silicon can be added to graphite through encapsulation and suspension in a graphene- polymer structure, enhancing the energy density of lithium-ion cells, while maintaining good cycle life.

[0065] A first solution is created by mixing a polymeric binder and thickening agent premixed with water. In some embodiments, the polymeric binder and thickening agent is CMC with molecular weight (MW) in the range of -50,000-70,000 g/mol. In some embodiments, the solution has a polymeric binder and thickening agent concentration in the range of 5-50 mg/mL.

[0066] The polymeric binder and thickening agent are added to the water so that it is well dispersed and does not agglomerate. Adding the polymeric binder and thickening agent in sequential portions may be performed to avoid clumps from forming. In some embodiments, adding water to CMC dry powder produces clumps of solid CMC that are rather difficult to dissolve. In some embodiments, in addition to CMC, a small concentration of polyethylene glycol based surfactant such as polysorbate-80, or similar molecules may be added, between 0.01 weight% and 5 weight% to promote the dispersion of nanoparticles in the slurry. In some embodiments, a small volume of ethanol may be added to the aqueous slurry to further promote the uniform dispersion of silicon nanoparticles in the aqueous slurry.

[0067] Pre-wetting the polymeric binder and thickening agent with a spray and hopper setup may be useful, and ethanol or water may be used for pre-wetting. The polymeric binder and thickening agent solid may be added to the vortex of stirring water. Stirring should be gentle and the constituents may be shaken intermittently. The solution may be maintained at 30-60 °C. Constant stirring with a magnetic stir bar may be one way for mixing, although it is not as effective in terms of the time taken for dissolution. High heat may be applied in some instances although it is not needed and may actually slow down the solubilization process. Alternatively, a mixing device, such as an impeller-type rotor that produces a vortex may be used because it will allow the powder to be drawn into the solvent, but it may also cause shearing of the molecules.

[0068] A second solution is created with nanoparticles and GO dispersed in water, organic alcohol solvents, or a combination of both. In some embodiments, the nanoparticles are silicon nanoparticles, tin nanoparticles, and aluminum nanoparticles. In some embodiments, the silicon nanoparticles may be silicon oxide (SiO_x) nanoparticles made of silicon with a silicon oxide coating, or may be silicon monoxide (SiO) nanoparticles. In some additional embodiments, silicon comprises of microparticles of silicon oxide (SiO_x) or silicon monoxide. The silicon nanoparticles are heavy, inorganic and non-polar enough to not form a stable dispersion in water. Thus, the silicon nanoparticles agglomerate and settle down very quickly. Therefore, organic alcohol solvents may be used to create a more stable dispersion. In some embodiments, the organic alcohol solvents are ethanol, IPA, or methanol. When the nanoparticles or microparticles intermixed with graphite microparticles are formed of silicon oxide (SiO_x), experimental results establishing criticality of the size range have been obtained, as illustrated below with respect to FIGS. 13-15.

[0069] The second solution can alternatively be made by adding positively or negatively charged species and very small quantities of surfactants or emulsifiers to help make coating the surface of silicon with GO more energetically favorable, leading to an enhanced protective carbon coating. Surfactants or emulsifiers are used to create a very stable dispersion of silicon nanoparticles in water. In some embodiments, surfactants or emulsifiers represent -1-3% of the total solution. In some embodiments, the surfactant or emulsifier is polysorbate-80 (trade name Tween 80). Tween 80 has a polyethylene-glycol head that is extremely polar and hydrophilic, and a sorbitan monoleate chain of hydrocarbons that is non- polar and hydrophobic.

[0070] The first solution and the second solution are mixed together and energy is introduced into the mixture to separate the GO sheets in water. In some embodiments, the energy introduced is in the form of sonication. The organic alcohol solvent keeps the silicon stabilized in the mixture, but as the mixing process goes on, heat causes evaporation of the organic alcohol solvent. Removal of organic alcohol solvents from the mixture leads to non- polar regions of silicon nanoparticles to preferentially interact with non-polar regions of GO over water molecules. Polar regions of GO, however, keep the GO sheet stable since the oxygen moieties form hydrogen bonds with the highly polar water molecules. The GO sheets thus attract and attach themselves to the silicon nanoparticle surface in a self-assembly process.

[0071] The first and second solutions are mixed together in an industrially compatible mixing machine. A conductive carbon additive may also be pre-mixed with the GO and nanoparticle mixture, or with the binder mixture for more uniform dispersion depending on the availability of different types of mixers. The conductive carbon additive may be premixed with CMC and GO to provide the best percolation network. In some embodiments, the first and second solution mixture includes graphene oxide encapsulated silicon nanoparticles, dispersed in an aqueous slurry comprising graphite, super-P conductive carbon and CMC or other positively or negatively charged binders. In some embodiments, the concentrations of GO, CMC binder and super-P conductive carbon are typically less than 10 weight% each. In some embodiments, the concentration of graphite ranges from 20 weight% to 80 weight%. In some embodiments, the concentrations of silicon are 7.5 weight%, 15 weight%, 20 weight%, 30 weight% and 40 weight%. Polymers may be mixed in as well to serve as a solid electrolyte matrix.

In-Situ Polymerization

[0072] In-situ polymerization of a slurry component can be achieved through an initiator or other monomers to form a polymer or co-polymer coating around the nanoparticle. In some embodiments, the nanoparticles are silicon nanoparticles, tin nanoparticles, and aluminum nanoparticles. In some embodiments, the initiator is benzoyl peroxide, methyl acrylate, acrylonitrile, vinyl acetate, alginic acid, PANI, or PEDOT:PSS. In some embodiments, the monomer is methyl acrylate, acrylonitrile, vinyl acetate, alginic acid, PANI, or PEDOT:PSS. In some embodiments, the polymer or copolymer coating formed is a poly-ethylene oxide (PEO), poly-methyl methacrylate (PMMA), poly-vinyl acetate (PVA), poly-acrylic acid (PAA), or poly-l-lactic acid (PLLA) or their various permutations and combinations which may include polyanilines, PEDOT:PSS and other conductive polymers.

[0073] In-situ polymerization can be achieved within the slurry prior to coating of the slurry onto the current collector or while the slurry is being cast onto the current collector. If polymerization is performed to cross-link polymers or gel-polymers in the dispersion, care should be taken to cure the slurry directly on the current collector as the coating process is taking place, since cross-linking polymers would quickly form large agglomerates in the slurry. However, casting it directly on the current collector as it is curing would create a stronger, more robust electrode.

[0074] In some embodiments, application of heat to partially reduce the oxygen- containing groups in GO and the polymeric additives further improves the conductivity of the electrode active material region. The partially reduced GO sheets serve as an encapsulating structural backbone for electrode active material region. Electrode active material region is an electrically conductive network for the transport of electrons and an ionically conductive network for the transport of active metal ions. Solidified polymer acts as a solid electrolyte matrix encapsulating the active electrode material. Electrolytes such as lithium hexafluorophosphate salts in ethylene carbonate and diethyl carbonates with or without additives such as fluoroethylene carbonate (FEC) and propane sultone (PS) may be added to the anode to create a gel-polymer layer. The solidified polymer or gel polymer could serve as a solid electrolyte that is essentially holding active anode materials within it, while providing a percolation network.

[0075] The electrode active material region percolating through the solid electrolyte matrix would allow for the conduction of lithium ions, while protecting the silicon nanoparticles, tin nanoparticles and aluminum nanoparticles from repeated SEI formation and breakage, since the electrolyte would have even lesser direct interaction with the surface of silicon nanoparticles, tin nanoparticles and aluminum nanoparticles. In-situ polymerization step can yield electrochemically stable SEI. The present disclosure is unique in the incorporation of solid electrolyte materials within the electrode to protect the active materials from SEI formation and breakage. Thus the cycle life performance may be improved for silicon and other advanced anodes. This embodiment has in mind a different class of electrode material, produced by casting an aqueous suspension of active material, conductive additive, and polymeric binder, as well as a graphene based percolation network.

[0076] The overall electrode active material region percolating through the solid electrolyte matrix provides a percolating electrical path between the nanoscale silicon, tin and aluminum and micron scale graphite particles, while binding them strongly to the current collector. Thus the present disclosure provides not only structural robustness of the electrode, but also a flexible enough architecture that can accommodate the expansion and contraction of silicon, tin and aluminum. In some embodiments, the flexibility is coupled with a 2-5 mAh/cm² areal current density with current optimized mass loadings. In some embodiments, the areal loading density is in the range of 10-15 mg/cm², the capacity per unit area will increase by approximately five times.

[0077] This electrode-suspended in electrolyte strategy has been used for liquid electrolytes, but not in solid-electrolytes such as PEO, PMMA and others as used as electrolyte or separator layers in their gel-polymer form in Li-Po batteries. These electrolytes typically would have to be used in gel-polymer form to maintain high ionic conductivity.

Calendering of the Electrode

[0078] Calendering involves applying static or dynamic pressure to reduce the cross-sectional thickness of an electrode. In some embodiments, the electrode is passed through rollers for calendering to reduce the thickness and obtain desired porosity and packing characteristics for the finished electrode. The electrodes can, during the calendering stage, be heated to temperatures of 50-1200 °C for the purpose of reducing GO to graphene under an inert atmosphere, regular conditions, or reducing agents. In some embodiments, the inert atmosphere comprises nitrogen or argon. In some embodiments, the reducing agents are hydrogen. In some embodiments, the electrode is under a static or dynamic pressure of up to 10,000 psi. In some embodiments, the cross-sectional thickness of the electrodes is reduced by up to 50%.

[0079] Heating will also lead to a loss of oxygen functional groups, introduce vacancies and porosity in the final electrode structure, and improve overall conductivity. Loss of oxygen functional groups occurs through pyrolysis of the hydrocarbons. Hydrocarbons include any CMC/PEO/PAA/PVA/PMMA type polymers. The extent of this pyrolysis has to be controlled in order to influence different electrochemical behaviors such as the battery performance metrics, including controlling the first cycle capacity loss, capacity fade per cycle, total usable charge or discharge capacity, and rate capability. Moreover, calendering also improves the overall volumetric energy density of the electrodes, measured as a function of the net energy stored normalized by the volume of the electrode at the cell-level or volume of the entire battery package at the device-level.

[0080] In some embodiments, calendering of the silicon and aluminum-based anode composites is carried out in rollers by applying a simultaneous compressive and tensile force, static or dynamic, on the coated anodes. The pressure dies are pre-heated and can range from 50-1200 °C. The temperature is a function of the GO concentration and the desired level of reduction, as temperatures higher than 200 °C lead to the loss of oxygen- containing functional groups leading to a further reduction. In some embodiments, for GO concentrations ranging from 0-5 weight%, a temperature range of 50-250 °C maybe sufficient. In some embodiments, at GO concentrations greater than 10 weight %, temperatures can be as high as 800 °C. In some embodiments, for even higher GO concentrations, temperatures can go up to 1200 °C. In some embodiments, the calendering process is carried out for a time period not less than 0.01 minutes and not greater than 8 hours. The pressure applied is dependent on the desired final electrode thickness and can be as high as 10,000 psi. The temperature, pressure and calendering time are closely related to various factors including the GO concentration, silicon or aluminum loading and electrode thickness and should as such be carefully monitored to prevent degradation of the binders and other hydrocarbon chains that may adversely affect the electrode integrity.

Pre-lithiation of Anodes

[0081] Usually a large surface area on an electrode results in large first cycle losses. Pre-lithiation techniques essentially use some lithium, before the assembly, to create a SEI on the anode. Pre-lithiation saves the cathodic lithium from being used up for the same purpose and affords a much lower first cycle capacity loss. Lesser total cathode material is thus used, resulting in a thinner and lighter battery cell.

[0082] FIG. 3 is a schematic diagram illustrating a pre-lithiation process, according to embodiments. In particular, the illustrated prelithiation relates to an electrochemical pre-lithiation of silicon-based anode composites. In the present disclosure, pre-lithiation of the anode is used to neutralize the oxygen-containing groups in GO and polymeric additives and reduce the initial capacity loss. In some embodiments, pre-lithiation has been achieved through an in-situ electrochemical shorting process. In some embodiments, such a shorting process is as depicted in FIG. 3. FIG. 3 is a schematic depiction of electrochemical pre-lithiation of silicon-based anode composites. Essentially, a lithium metal foil and the active, coated anode material are pre-wetted with an electrolyte and brought in contact with each other. In some embodiments, the electrolyte is lithium hexafluorophosphate dissolved in ethylene carbonate and diethyl carbonate. This essentially induces a shorting between the electrodes and creates a path of least resistance for lithium ions to flow through the electrolyte and diffuse into the active anode. The pressure applied between the two electrodes ranges from < 1-100 psi and is intended to ensure the absence of air gaps, gas pockets and other sources of internal resistance build-ups. Internal resistance build-ups could result in a non-uniform pre-lithiation and would lead to mismatch in material response to the electrochemical environment, thereby compromising the performance. Pre- lithiation proceeds favorably at defect sites in graphene, GO, graphite or other active components in the anode, creating a localized charge build-up. The localized charge build-up further promotes and accelerates lithium insertion and subsequent reduction. Pre-lithiation also introduces an artificial pseudo-SEI layer, passivating the reactive constituents in the electrode material. [0083] Pre-lithiation of graphene and GO-containing electrodes also achieves a further passivation of the oxygen-containing functional groups, through the formation of irreversible oxides, hydroxides and carbonates of lithium. Any reactive oxygen molecule present within the electrode constituents can act as a host for lithium ions in an electrochemical environment and result in the formation of irreversible lithium oxides, hydroxides or carbonates. This in turn leads to a drop in capacity and a significant reduction in available active lithium ions for subsequent charge or discharge cycles. In addition, reducing graphene or GO constituents also improve both the ionic and electrical conductivity of the matrix. Finally, the presence of defect sites in such carbon nano-materials further promote and accelerate the pre-lithiation of graphene and GO based composites. Defective graphene has in fact been shown to accommodate a surplus of lithium ions, associated with the formation of intercalates as high as Li₃C₈, as opposed to LiC₆ seen in conventional graphitic anodes. This concentration of excess lithium ions at such defect sites therefore improve the rate of formation of the passivation layers and hence, the artificial SEI.

[0084] In some embodiments, pre-lithiation is achieved via an electrochemical lithiation technique whereby the anode is configured against a lithium alloy such as lithium manganese oxide or lithium cobalt oxide, separated by an ion-permeable insulating polymer film such as polypropylene, followed by the application of a steady voltage between 3 and 6 V. In some embodiments, pre-lithiation can be achieved via spray-coating of dispersed stabilized lithium metal powder (SLMP) directly on to the surface of the electrode material. In some embodiments, pre-lithiation can be achieved via sublimation and gas-phase coating of lithium oxide or lithium peroxide directly on to the surface of the electrode material.

Carbonaceous Coating of Anodes

[0085] In some embodiments, a thin, conformal chemical vapor deposition coating of the active anode by a carbonaceous coating can further improve the performance specifications. The carbonaceous coating can be active intercalation material such as soft carbon, graphite, mesoporous carbon or inactive intercalation material such as parylene. Parylene is a dimer that is known for its chemical inertness while being permeable to the diffusion of lithium ions, thereby making it a perfect material as a pseudo-SEI layer. In addition, parylene is mechanically robust and acts as a scaffolding to the underlying active anode constituents, preventing pulverization, delamination and subsequent loss of active material and conductive network. Parylene is also a dielectric and can promote lithium storage by creating a localized charge concentration and lithium ion reduction at the electrode-electrolyte interface. Furthermore, the dielectric can be used in conjunction with the underlying active material or as a standalone material for promoting power density as well as energy density.

Apparatus for Forming an Electrode

[0086] According to embodiments, a roll-to-roll manufacturing line for forming an electrode for a secondary battery comprises one or more of: mixing chambers, configured to agitate and disperse solutions and include a provision to control the flow of water and ethanol into the mixing chamber; rollers, configured to apply pressure to current collector substrates; a coating system, configured to coat the slurry coming from the mixing chambers onto the current collector substrates; a drying zone, configured to let dry the solvent in the slurry; a calendering zone, configured to apply heat and/or pressure; and a pre-lithiation zone, configured to decrease save the cathodic lithium from being used up and decrease first cycle capacity loss.

[0087] In some embodiments, the mixing chambers are used to agitate and disperse GO, graphite, silicon or other active nanoparticle, positively or negatively charged binders such as CMC, conductive additives such as super-P and other cross-linking polymers in a deionized water bath to form an electrode slurry.

[0088] In some embodiments, the mixing chambers are connected via flow tubes and pressure control valves to a slot-die coating system.

[0089] In some embodiments, the provision to control the flow of water and ethanol into the mixing chamber is used to maintain viscosity at < 50,000 cps.

[0090] In some embodiments, the current collector substrate is passed through the rollers and is coated by the slot-die system with the electrode slurry.

[0091] In some embodiments, the drying zone is set-up immediately after the slot- die coating system to allow the solvent in the electrode slurry to dry by a drying method.

[0092] In some embodiments, drying includes applying hot air through resistive convective heat or hot air guns. [0093] In some embodiments, drying includes controlling the humidity of the environment surrounding the drying zone with or without the application of hot air.

[0094] In some embodiments, drying includes applying heat through conduction, achieved by pre-heated rollers.

[0095] In some embodiments, the calendering zone follows the drying zone whereby pre-heated rollers press the electrodes.

[0096] In some embodiments, the calendering zone subjects electrodes to high temperatures, through conductive, convective or radiative heating.

[0097] In some embodiments, the pre-lithiation zone follows the calendering zone whereby a low-humidity section to is used to prevent moisture from reacting with lithium.

[0098] In some embodiments, the pre-lithiation zone is configured for an in-situ electrochemical shorting process.

[0099] In some embodiments, the pre-lithiation zone is configured for an electrochemical lithiation technique whereby the anode is configured against a lithium alloy such as lithium manganese oxide or lithium cobalt oxide, separated by an ion-permeable insulating polymer filum such as polypropylene, followed by the application of a steady voltage between 3 and 6 V.

[0100] In some embodiments, the pre-lithiation zone is configured for spray- coating of dispersed stabilized lithium metal powder (SLMP) directly onto the surface of the electrode material.

[0101] In some embodiments, the pre-lithiation zone is configured for sublimation and gas-phase coating of lithium oxide or lithium peroxide directly onto the surface of the electrode material.

[0102] A continuous roll-to-roll manufacturing line integrates the method of forming an electrode and the electrode final products. An example of such a continuous roll- to-roll manufacturing line is shown in FIG. 12. FIG. 12 is a schematic representing slurry formulation and mixing process being integrated with a slot die coating machine, drying, calendering and pre-lithiation. The manufacturing line comprises one or more mixing chambers to agitate two solutions. In some embodiments, the solutions are kept in a deionized water bath. The manufacturing line further comprises a provision to control the flow of water and ethanol into the mixing chamber in order to maintain viscosity at < 50,000 cps.

[0103] The first solution uses a polymeric binder and thickening agent premixed with water. In some embodiments, the polymeric binder and thickening agent is CMC with molecular weight (MW) in the range of -50,000-70,000 g/mol. In some embodiments, the solution has a polymeric binder and thickening agent concentration in the range of 10-25 mg/mL. In some embodiments, in addition to CMC, a small concentration of polyethylene glycol based surfactant such as polysorbate-80, or similar molecules may be added, between 0.01 weight% and 5 weight% to promote the dispersion of nanoparticles in the slurry. In some embodiments, a small volume of ethanol may be added to the aqueous slurry to further promote the uniform dispersion of silicon nanoparticles in the aqueous slurry. In some embodiments, a spray and hopper setup is used to pre-wet the polymeric binder and thickening agent. In some embodiments, ethanol or water may be used for pre-wetting. The polymeric binder and thickening agent solid are added to the vortex of stirring water.

[0104] The second solution is created with nanoparticles and GO dispersed in water, organic alcohol solvents, or a combination of both. In some embodiments, the nanoparticles are silicon nanoparticles, tin nanoparticles, and aluminum nanoparticles. In some embodiments, the silicon nanoparticles may be silicon oxide (SiOx) nanoparticles made of silicon with a silicon oxide coating, or may be silicon monoxide (SiO) nanoparticles. In some embodiments, the organic alcohol solvents are ethanol, IP A, or methanol. The second solution can alternatively be made by adding positively or negatively charged species and very small quantities of surfactants or emulsifiers to help make coating the surface of silicon with GO more energetically favorable, leading to an enhanced protective carbon coating. In some embodiments, the surfactant or emulsifier is polysorbate-80 (trade name Tween 80).

[0105] The mixing chambers allow for gentle stirring and intermittent shaking of the constituents. The mixing chambers additionally allow for the application of heat and maintaining of temperature between 30-60 °C. In some embodiments, a mixing device such as an impeller-type rotor that produces a vortex will allow powders to be drawn into the solvent, but it may also cause shearing of the molecules. The mixing chambers allow for the mixing of the first and second solutions and the introduction of energy to separate the GO sheets in water. In some embodiments, the energy is introduced in the form of sonication. In some embodiments, the first and second solutions are mixed together in an industrially compatible mixing machine.

[0106] In some embodiments, the mixing chambers contain a conductive carbon additive may also be pre-mixed with the GO and nanoparticle mixture, or with the binder mixture for more uniform dispersion. The conductive carbon additive may be premixed with CMC and GO to provide the best percolation network. In some embodiments, the mixing chamber also includes initiators or other monomers. In some embodiments, the initiator is benzoyl peroxide, methyl acrylate, acrylonitrile, vinyl acetate, alginic acid, PANI, or PEDOT:PSS.

[0107] In some embodiments, the mixing chambers are connected via flow tubes and pressure control valves to a slot-die coating system. The slurry in the mixing chambers can further be polymerized in-situ. The slurry coming out of the mixing chambers are coated onto current collector substrates. In some embodiments, the current collector substrates are passed through rollers. In some embodiments, the current collector is a battery grade copper. In some embodiments, in-situ polymerization can further occur while the slurry is being cast onto the current collector. Care should be taken to cure the slurry directly on the current collector as the coating process is taking place, since cross-linking polymers would quickly form large agglomerates in the slurry. However, casting it directly on the current collector as it is curing would create a stronger, more robust electrode.

[0108] In some embodiments, a drying zone is set-up immediately after coating the slurry to the current collectors to allow the solvent in the slurry to dry by a drying method. In some embodiments, the drying method is applying hot air through resistive convective heat or hot air guns. In some embodiments, the drying method is controlling the humidity of the environment surrounding the drying zone with or without the application of hot air. In some embodiments, the drying method is applying heat through conduction, achieved by pre-heated rollers.

[0109] In some embodiments, a calendering zone follows the drying zone whereby pre-heated rollers press the electrodes. In some embodiments, a separate region may be integrated to subject the electrodes to high temperatures, through conductive, convective or radiative heating. In some embodiments, the calendering zone can provide conditions such as an inert atmosphere comprising nitrogen or argon. In some embodiments, reducing agents such as hydrogen are added in the calendering zone. In some embodiments, the calendering zone can simultaneously apply a compressive and tensile force, static or dynamic pressures of up to 10,000 psi. In some embodiments, the calendering zone can reduce the cross- sectional thickness of the electrodes by up to 50%.

[0110] In some embodiments, the calendering zone is finally followed by a pre- lithiation zone. In some embodiments, the pre-lithiation zone is comprised of a low-humidity section to prevent moisture from reacting with lithium. In some embodiments, a lithium metal foil and the active, coated anode material are pre-wetted with an electrolyte and brought in contact with each other. In some embodiments, the electrolyte is lithium hexafluorophosphate dissolved in ethylene carbonate and diethyl carbonate. In some embodiments, pressure is applied between the two electrodes and ranges from <1-100 psi. In some embodiments, the pre-lithiation zone allows for an electrochemical lithiation technique whereby the anode is configured against a lithium alloy such as lithium manganese oxide or lithium cobalt oxide, separated by an ion-permeable insulating polymer film such as polypropylene, followed by the application of a steady voltage between 3 and6 V. In some embodiments, the pre-lithiation zone allows for spray-coating of dispersed stabilized lithium metal powder (SMLP) directly onto the surface of the electrode material. In some embodiments, the pre-lithiation zone allows for sublimation and gas-phase coating of lithium oxide or lithium peroxide directly onto the surface of the electrode material.

Exemplary Material and Electrochemical Characterization

X-ray Characterization

[0111] FIG. 4 is an X-Ray diffraction spectra of a coating of an electrode structure for a secondary battery according to embodiments. In order to gain a better understanding of the material, and ensure the presence of silicon and carbon within the final cast electrode film, X-ray diffraction (XRD) spectra, as shown in FIG. 4, were obtained. FIG. 4 shows a XRD of electrode samples containing 7.5%, 20%, and 40% silicon. The relative amplitude or intensity of the carbon peak decreases proportionally as silicon concentration is increased. The XRD spectra confirm the presence of silicon, while showing strong peaks for crystalline carbon and copper as large crystals for both these elements are present in a coated electrode sample. Silicon nanoparticles also result in clear peaks for the 111, 220 and 311 orientations, but have lower amplitudes as a result of their small particle size. However, it is observed, that as the silicon concentration is increased and graphite concentration reduced, the relative amplitude for the carbon peak is reduced proportionally.

Scanning Electron Microscopy (SEM)

[0112] SEM imaging was carried out to achieve a qualitative understanding of the effectiveness of the aqueous slurry and the role of GO in encapsulating silicon and aluminum nanoparticles. The encapsulation is critical and ensures that the silicon and aluminum nanoparticles do not come in direct contact with the electrolyte. Silicon and similar advanced energy storage materials such as germanium, tin, tin oxide and aluminum form a very unstable electrochemical interface with the electrolyte that results in a rapid loss of active lithium for intercalation kinetics. Therefore, a GO encapsulated silicon composite anode essentially improves the cycle life and performance characteristics by enhancing the electrochemical stability of the anode. While GO wrapping of metal oxide nanoparticles have been demonstrated before, the metal oxide surfaces were modified to carry a positive surface charge that would make the negatively charged GO to encapsulate the nanoparticles. On the other hand, silicon nanoparticles used in this disclosure were not subjected any treatment to alter the surface charge. As such, silicon nanoparticles tend to form an oxide layer in ambient or aqueous solutions, resulting in the formation of negative surface charge that should not interact favorably with GO. Presumably, successful encapsulation is possible owing to the structural morphology and charge distribution of GO sheets in an aqueous dispersion that causes the sheets to fold on to themselves, thereby trapping silicon nanoparticles. Moreover, it is to be noted that in order to prevent agglomeration of high surface area nanoparticles such as silicon, it is critical to incorporate a similar high surface area material as an encapsulating structural scaffold. In that respect, GO or partially reduced GO were found to be an ideal additive. The GO sheets provided a high surface area platform for silicon nanoparticles to uniformly distribute itself, thereby preventing agglomeration of the nanoparticles that would otherwise lead to a loss in charge storage capabilities and more importantly, structural degradation. Further, the use of such GO sheets can be extended to encapsulate miscellaneous nanoparticles including but not limited to aluminum, tin, tin oxide and germanium. [0113] FIGS. 5(a)-5(c) are scanning electron micrographs of graphene / graphene oxide-encapsulated silicon nano-particles composite anode structures, according to embodiments. FIG. 5(d) is a schematic depiction of the configurations of the electrode structures illustrated in FIGS. 5(a)-5(c). FIG. 5(a) is a top view showing the presence of pores and cracks for efficient electrolyte wettability and hence, improved lithium ion mobility. FIG. 5(b) is a magnified section of the anode surface showing uniformly distributed silicon nano-particles. The slurry preparation, through aqueous dispersions, ensures prevention of agglomerated nano-particles and therefore provides sufficient accessible area to lithium ions for intercalation kinetics. Moreover, the presence of silicon in the nano- particulate form minimizes volumetric instabilities of silicon. FIG. 5(c) is a further magnified section of the silicon-graphene / graphene oxide composite anode showing the presence of an inter-connected network of graphene / graphene oxide sheets wrapping around silicon nano- particles. The network maintains the integrity of the silicon-graphene composite and at the same time, prevents silicon from coming in direct contact with the electrolyte, resulting in a longer cycle life. FIG. 5(d) is a schematic representation of the morphology of the graphite- silicon-graphene / graphene oxide composite anode using aqueous CMC binders, cast on a current collector. Super-P conductive carbon additives are not depicted in the illustration for sake of simplicity.

Electrochemical Characterization

[0114] Electrochemical measurements include analyzing the voltage profiles, charge / discharge capacities, cycling and coulombic efficiencies and cycle life, with the objective to determine the entire range of operational conditions as well as performance specifications.

Calendering & Pre-lithiation

[0115] Prior to assembly, the silicon-graphene based composite anodes were subjected to calendering and pre-lithiation, with the objective to maximize volumetric energy densities and minimize first cycle capacity loss. As described previously, first cycle capacity loss is associated with the high surface area of nano-materials and the reactions at the electrolyte-electrode interface, and may be accompanied by higher loading of lithium content in order to compensate for the loss in capacity. Industrial standards dictate the first cycle capacity loss to be controlled to less than 20%. However, nano-materials based anodes typically demonstrate much higher losses, as depicted in FIG. 6 (a). FIG. 6(a) shows that the first cycle charge voltage profile of a pristine, untreated anode displaying a first cycle capacity loss of -45%. One approach to improve the first cycle loss is to eliminate the excess oxygen-containing functional groups in graphene and GO constituents. This can be achieved through ex situ annealing of the as-coated anode in an inert atmosphere at temperatures ranging from 200-1500 °C and over a time period ranging between 15 minutes and 6 hours. Alternatively, calendering of the electrodes at the aforementioned temperature range and following the process described earlier in the disclosure also achieves a similar effect. Such a strategy was shown to reduce the first cycle capacity loss to -25%, as shown in FIG. 6 (b). However, in order to bring the first cycle capacity loss down to within industrial standards, pre-lithiation may be utilized. Pre-lithiation serves as a passivating process, whereby a pseudo-SEI layer is created at the electrode-electrolyte interface through the interaction of lithium ions and reactive constituents on the electrode surface such as functional groups, un- terminated bonds, etc. Since the first cycle capacity loss involves loss of active lithium to SEI formation, pre-lithiation and prior passivation of the oxygen-containing functional groups and other reactive sites within the electrode matrix was found to lower the first cycle capacity loss to -15%), as shown in FIG. 6(c).

[0116] Finally, it is to be noted here that the calendering and pre-lithiation steps, along with the aqueous slurry processing to disperse and encapsulate the silicon nanoparticles, are critical to improving the cycle life and performance characteristics of the silicon-graphene based anode material. However, in addition to the aforementioned approaches, a thin, conformal chemical vapor deposition coating of the active anode by a carbonaceous coating can further improve the performance specifications. The carbonaceous coating can be active intercalation material such as soft carbon, graphite, mesoporous carbon or inactive intercalation material such as parylene. Parylene is a dimer that is known for its chemical inertness while being permeable to the diffusion of lithium ions, thereby making it a perfect material as a pseudo-SEI layer. In addition, parylene is mechanically robust and acts as a scaffolding to the underlying active anode constituents, preventing pulverization, delamination and subsequent loss of active material and conductive network. Parylene is also a dielectric and can promote lithium storage by creating a localized charge concentration and lithium ion reduction at the electrode-electrolyte interface, as depicted schematically in FIG. 7. Further, the dielectric can be used in conjunction with the underlying active material or as a standalone material for promoting power density as well as energy density.

[0117] FIG. 7 illustrates parylene-coated active anode material, according to embodiments. Parylene, a dimer, is capable of demonstrating excellent passivating properties and mechanical strength. This serves a two-fold purpose - first, the chemical inertness introduces a stable pseudo-SEI layer, preventing electrochemical decomposition of the underlying anode and second, the mechanical rigidity prevents delaminating and pulverization of silicon nano-particles. In addition, parylene as a dielectric also promotes localized charge build-up that assists in charge-induced benign plating of lithium metal to increase the energy density and rapid transport of ions for high power density applications.

Voltage Profile

[0118] The formulation process outlined in this disclosure achieves successfully the synthesis of a high-performance anode with uniformly dispersed nanoparticles, enabling a much higher mass loading of the nanoparticle constituent. Following the formulation, the post-synthesis treatment, including calendering and pre-lithiation, further improve the performance metrics of the materials. The entire process can as such be integrated with a host of high-performance active nano-material including but not limited to silicon, silicon oxide, aluminum, aluminum oxide, tin, tin oxide and germanium.

[0119] FIG. 8 illustrates voltage profiles of different compositions of active anode materials, synthesized through aqueous slurry dispersions, thereby enabling higher mass loading of the nanoparticle constituents as well as a better cycle life. As illustrated, the discharge and charge profiles of silicon-graphite anodes with and without graphene / graphene oxide additives are very similar, except for the fact that with the additive, the charge hysteresis is lowered. Aluminum nano-particles of course show a different voltage profile, attributed to a different intercalation kinetic than silicon-lithium interaction. However, the presence of graphene / graphene oxide additives are once again found to lower the charge voltage hysteresis, thereby limiting the loss in energy consumed during charging cycles. All the tests were conducted in half cell configurations in 2032 coin cell form factors.

[0120] For purpose of brevity, the voltage profiles of silicon-graphite, silicon- graphene/GO-graphite and aluminum -graphene/GO-graphite have been included in FIG. 8. FIG. 8 depicts voltage profiles of different compositions of active anode materials, synthesized through aqueous slurry dispersions, thereby enabling higher mass loading of the nano-particle constituents as well as a better cycle life. As can be seen in FIG. 8, the discharge and charge profiles of silicon-graphite anodes with and without graphene / graphene oxide additives are very similar, except for the fact that with the additive, the charge hysteresis is lowered. Aluminum nano-particles show a different intercalation kinetic than silicon-lithium interaction. However, the presence of graphene / graphene oxide additives are once again found to lower the charge voltage hysteresis, thereby limiting the loss in energy consumed during charging cycles. All the tests were conducted in half cell configurations in 2032 coin cell form factors.

[0121] FIG. 9 is a diagram of charge and discharge voltage profiles of the prepared anode material. At lower concentrations the voltage profile is smooth, however at higher concentrations of silicon, two distinct plateaus begin to appear, indicating the step voltage difference in intercalation potentials for silicon and carbon. This can provide batteries with an unexpected supply of battery power after their normal use of up to -350 mAh/g is used up, all the way to -794 mAh/g at 20% silicon loading. FIG. 9(a) depicts a charge-discharge voltage profile for electrodes containing 7.5% silicon. FIG. 9(b) depicts a charge-discharge profile for electrodes containing 20% silicon by weight. A noticeable difference in the discharge profiles indicate that higher concentration of silicon provide reversible capacities beyond the typical discharge plateau for carbon based anodes.

[0122] Manufacturers define the lifetime for a battery as the number of cycles it takes to get to a certain percentage (-80% usually, but may differ upon OEM specifications) and the number can range from a few hundred, to a few thousand depending upon the operational parameters chosen, such as depth of discharge (100% means using the full 3.0- 4.2 V voltage window, but is usually kept lower in actual devices to improve lifetime), operating temperature (higher temperatures increase capacity fade, and result in lower lifetime) as well as the charge-discharge current draw characteristics. Higher current rates may result in lower lifetime as well.

[0123] FIG. 10 illustrates cycle life and coulombic efficiency attained for silicon containing anodes having <10% by weight of Si, according to embodiments. In particular, FIG. 10 shows a life-cycle comparison between the silicon-based anode and commercial graphitic control. Silicon-graphene / GO-graphite composite has a slightly higher capacity loss in each cycle, measuring -1.64 mAh/g/cycle while graphitic control has a much lower capacity loss, -0.41 mAh/g/cycle. However, standard and pristine silicon-based anodes have a much higher capacity fade rate, often as much as 50-100 mAh/g/cycle and therefore, a capacity fade rate of 1.64 mAh/g/cycle is a significant improvement. In addition, silicon is capable of storing 2-5 times the charge compared to commercial graphite and hence, the performance of such silicon-based anodes over the entire operational lifetime is significantly better than graphite. FIG. 10 depicts the cycle life and coulombic efficiency attained for silicon containing anodes <10% by weight of silicon. Current optimizations are running at 20% and 40% silicon loading. Cells were assembled against standard Lithium NCA (Nickel- Cobalt- Aluminum) cathodes, and cycled at a constant current rate of 157.5 uA/cm² in this full cell configuration between 3.0-4.2 V. A control sample with graphite anode - NCA cathode was also assembled for comparison. The electrode delivers better gravimetric capacity for over -275 cycles of operation, making it suitable for various applications where high energy density is desired.

[0124] The silicon-graphene / GO-graphite composite tested here provided a net energy density of -482,728 Wh/kg (or 1749 Wh/kg/cycle at 3.7 V and normalized by anodic mass) over 276 cycles while graphitic control could deliver less than 65% of the energy, at -308,825 Wh/kg (or 1 1 19 Wh/kg/cycle at 3.7 V and normalized by anodic mass).

[0125] FIG. 1 1 is a graph illustrating coulombic efficiency of silicon-graphene or GO-graphite composite anodes according to embodiments, as a function of cycle index and depth of discharge. The cycle life of such anodes can be further improved by carefully controlling the operational characteristics. A 100% Depth of Discharge (DoD) of such anodes in the initial cycling stage results in a relatively lower coulombic efficiency of -98.5%, while a DoD of -80% improves the coulombic efficiency to -99.8%. Interestingly, following extended cycling (100-200 cycles) at 80% DoD, the coulombic efficiency in subsequent cycling at 100% DoD was higher at almost -100%. It will be appreciated that the cycle index is not indicative of the true cycling number and the results are plotted for the performance of the anode following the completion of the formation cycles.

[0126] The aforementioned observation suggests a possible strategy during the formation cycle that would control the DoD to less than 100% in order to ensure longevity. The formation cycle of such anodes would therefore involve a low current density cycling (0.01-0.5C) at DoDs between 80-100%. The effectiveness of the formation cycle can be further improved by cycling the cells in a temperature controlled environment, typically between 20 °C and 40 °C. In addition, controlling the DoD to less than 100% (ideally 70- 80%) or occasionally at -90%) over the normal operational lifetime would have a significant benefit on the performance and cycle life. The loss in capacity at 70-90%> DoDs is negligible, ranging from 1-10%.

[0127] Finally, a schematic of the roll-to-roll process line is shown in FIG. 12 that incorporates nanomaterials such as silicon and graphene with a traditional, industrially scalable deposition technique. FIG. 12 depicts a schematic representing the slurry formulation and mixing process being integrated with a roll-to-roll electrode coating (slot-die in this case) machine for high speed deposition, drying and calendering. A unique roller based pre-lithiation technique can also be incorporated for reducing first cycle capacity loss if desired.

Electrodes Having Graphite Microparticles Intermixed with Si-based Particles

[0128] In addition to silicon nanoparticles, microparticles of silicon monoxide (SiO) and microparticles of silicon oxide (SiO_x) other than SiO, e.g., Si0₂, have been tested as anodes in lithium ion batteries. In the following, experimental results establishing criticality of the physical parameters of the microparticles are illustrated.

[0129] FIG. 13 illustrates variability in coating thickness as a function of particle size of silicon monoxide. Tested silicon monoxide microparticles measured between 1 μπι and 44 μπι in particle size. Any larger particle size was found to agglomerate in the anode slurry resulting in the formation of clumps, thus indicating criticality of the particle size. This phenomenon is likely attributed to the large particle size and the inefficient encapsulation of such silicon monoxide by graphene oxide. Agglomeration of particles in slurry result in non-uniform electrode coating with varying localized electrode thicknesses, making them unusable in batteries. Such anodes typically demonstrated a poorer cycle life, reaching 80%> of capacity in less than 50 cycles. On the contrary, silicon monoxide particles with a size less than or equal to 44 μπι demonstrated a cycle life of more than 200 cycles. This is illustrated in FIG. 14, which illustrates GO-silicon monoxide (20%) based anodes tested in a half-cell configuration at a C/8 charge and C/8 discharge rate. As illustrated, the capacity retention is -81% after 210 cycles.

[0130] Similarly, SiO_x was also tested as a potential anode material. SiO_x precursor material can be chosen from a list including but not limited to the following options: (i) sand (particle size ranging between 40 μπι and 100 μπι); (ii) zeolite (particle size ranging between 0.1 μπι and 50 μπι); (iii) MCM-41 (particle size ranging from 0.01 μπι and 50 μπι). SiO_x was partially reduced through hydrothermal treatment, in the presence of one or more of carbon, silicon, manganese, magnesium, aluminum and tin. The hydrothermal process involved mixing SiO_x particles, ranging from 0.01 μπι to 100 μπι in particle size, with one or more of the aforementioned components in an aqueous solution comprising of deionized water with or without either one of potassium hydroxide (0.01 M to 5 M), sulfuric acid (0.01 M to 1 M), nitric acid (0.01 M to 1 M), hydrochloric acid (0.01 M to 1 M) or acetic acid (0.01 M to 1 M). The mixture is poured in a hydrothermal pressure chamber, sealed and heated between 160 °C and 300 °C for a total time ranging from 1 hour to 24 hours. Following this step, the residue is washed with deionized water several times and filtered. The resulting partially-reduced SiO_x has a silicon-to-oxygen atomic ratio ranging from 100: 1 to 1 :2, or x in SiO_x is 0.01 to 2.

[0131] The as-obtained SiO_x was tested as a potential anode material by encapsulating up to 20 weight% of SiO_x within graphene oxide and mixing it with graphite microparticles, binders and thickening agent, as discussed elsewhere in the specification. The cells provided a capacity retention of -80% over 165 cycles. This is illustrated in FIG. 15, which shows half-cell testing results of SiO_x-based anodes, tested in a half-cell configuration at a C/10 charge and C/10 discharge rate. As illustrated, the capacity retention is -82% after 165 cycles. Such an observation is interesting, especially because the presence of oxygen in silicon results in introducing insulating properties in silicon and increasing the overall internal resistance of the anode. Moreover, unreduced and partially-reduced oxides are potential sites for irreversible lithium-oxygen reactions. However, none of the possible drawbacks were observed as the cell provided comparable capacities and cycle life to silicon nanoparticles. It is to be noted here that sourcing silicon from ultra-cheap precursor materials such as sand or zeolite is a highly effective and desired way to reduce the cost of the material. It is also to be noted here that the use of microparticles further reduces the cost of materials as compared to nanoparticles. It is surprising that silicon-based microparticles could provide such high capacities and impressive cycle life, especially because it is well known in literature that larger particle sizes are more susceptible to delamination and pulverization. It is therefore believed that the graphene oxide encapsulation plays a critical role in the electrode composition.

Example Electrodes

EXAMPLE 1

[0132] One example method of forming an electrode of the present disclosure includes the incorporating active nanomaterials including but not restricted to graphene, carbon nanotubes, fullerenes, aluminum, silicon, germanium, tin and tin oxide into an aqueous-based slurry mixture. The slurry mixture includes GO encapsulated silicon nanoparticles, dispersed in an aqueous solution comprising graphite, super-P conductive carbon and CMC or other positively or negatively charged binders. The concentration of silicon tested includes 7.5 weight%, 15 weight%, 20 weight%, 30 weight% and 40 weight%. The concentrations of GO, CMC binder and super-P conductive carbon tested are typically less than 10 weight% each. The concentration of graphite tested ranges from 20 weight% to 80 weight%. In addition to the CMC, a small concentration of polyethylene glycol based surfactant such as polysorbate-80, or similar molecules may be added, between 0.01 weight% and 5 weight% to promote the dispersion of silicon nanoparticles in the slurry. A small volume of ethanol may be added to the aqueous slurry to further promote the uniform dispersion of silicon nanoparticles in the aqueous slurry. A method to achieve in-situ polymerization of polymeric molecule used as a surfactant or slurry component uses an initiator, such as benzoyl peroxide, and other monomers (methyl acrylate, acrylonitrile, vinyl acetate, alginic acid, PANI, PEDOT:PSS etc.) to form a poly-ethylene oxide or PEO, and similarly PMMA, PVA or PAA -type polymer or co-polymer coating around the nanoparticles.

[0133] The example method further includes polymerization. Polymerization can be achieved within the slurry prior to coating of the electrode on to the current collector or while the slurry is being cast on to the current collector to let a robust cross-linking chain of polymers form. Application of heat to partially reduce the oxygen-containing groups in GO and the polymeric additives to further improve the conductivity of the electrode matrix. The partially reduced GO sheets serve as an encapsulating structural backbone for the active and inactive materials in the said composite electrode. The partially reduced GO sheets serve as an electrically conductive network for the transport of electrons. The partially reduced GO sheets serve as an ionically conductive network for the transport of active metal ions. The solidified polymer constituent acts as a solid electrolyte matrix encapsulating the active electrode material.

[0134] Electrolytes such as lithium hexafluorophosphate salts in ethylene carbonate and diethyl carbonates with or without additives such as fluoroethylene carbonate (FEC) and propane sultone (PS) may be added to the anode to create a gel-polymer layer. A process of calendering the electrodes under a static or dynamic pressure of up to 10,000 psi is used to reduce the cross-sectional thickness by up to 50%. A process to achieve simultaneous reduction of GO and structural integrity through the use of pre-heated pressure systems (such as rollers) is provided. Graphene oxide with concentrations less than 5 weight% may be simultaneously reduced by pre-heating the pressure systems up to 550 °C. Graphene oxide with concentrations up to 10 weight% may be simultaneously reduced by pre-heating the pressure systems up to 800 °C. Graphene oxide with concentrations greater than 10 weight% are subjected to much higher temperatures of up to 1200 °C. The calendering process is carried out for a time period not less than 0.01 minutes and not greater than 8 hours.

[0135] The example method further includes a method to pre-lithiate the anode material in order to neutralize the oxygen-containing groups in GO as well as the polymeric additives and reduce the initial capacity loss is provided. In one example, pre-lithiation is achieved via bringing a lithium metal foil in direct contact with the anode material, where both the lithium metal foil and electrode are pre-wetted with a standard lithium ion battery electrolyte. A pressure is applied between the electrode and lithium metal foil to eliminate air gaps, gas pockets and other sources of internal resistance build-ups. Pre-lithiation proceeds favorably at defect sites in graphene, GO, graphite or other active components in the anode, creating a localized charge build-up. The localized charge build-up further promotes and accelerates lithium insertion and subsequent reduction. Pre-lithiation also introduces an artificial pseudo-SEI layer, passivating the reactive constituents in the electrode material. [0136] In another approach, pre-lithiation is achieved via an electrochemical lithiation technique whereby the anode is configured against a lithium alloy such as lithium manganese oxide or lithium cobalt oxide, separated by an ion-permeable insulating polymer film such as polypropylene, followed by the application of a steady voltage between 3 and 5 V. In another approach, pre-lithiation can be achieved via spray-coating of dispersed stabilized lithium metal powder (SLMP) directly on to the surface of the electrode material.

[0137] In another approach, pre-lithiation can be achieved via sublimation and gas-phase coating of lithium oxide and / or lithium peroxide directly on to the surface of the electrode material.

EXAMPLE 2

[0138] One example electrode of the present disclosure is a silicon-GO / graphene-graphite (Si(20wt%)GO/G-G) anode, with 20 weight% silicon, prepared using the aqueous slurry formulation. The Si(20wt%)GO/G-G anode was assembled against a lithium cobalt oxide (LCO) or lithium nickel cobalt aluminum oxide (NCA) cathode in a full cell configuration. The electrolyte was composed of a lithium hexafluorophosphate salt in ethylene carbonate and diethyl carbonate with FEC and PS additives. A standard polypropylene separator was used to separate the anode and cathode.

[0139] The cell was cycled between 3 and 4.2 V. The Si(20wt%)GO/G-G anode delivered a reversible capacity greater than 600 mAh/g at a rate of 0.1 C.

EXAMPLE 3

[0140] One example electrode of the present disclosure is a silicon-GO / graphene-graphite (Si(7.5wt%)GO/G-G) anode, with 7.5 weight% silicon, prepared using an aqueous slurry formulation. The Si(7.5wt%)GO/G-G was assembled and cycled. The Si(7.5wt%)GO/G-G anode delivered a net energy density of 482,728 Wh/kg over 274 cycles while a commercial graphite anode control delivered a net energy density of only 308,825 Wh/kg over the same cycle life. In another example, the Si(20wt%)GO/G-G anode was subjected to varying depths of discharge, ranging from 80% -100%. At 100% depth of discharge, a coulombic efficiency of approximately 98.5% was observed during early cycling. [0141] At 80% depth of discharge, the coulombic efficiency was found to increase to approximately 99.8%. Following extended cycling (100-200 cycles) at 80%> depth of discharge, the coulombic efficiency was found to increase to approximately 100%> even at a 100%) depth of discharge.

EXAMPLE 4

[0142] One example method of forming an electrode of the present disclosure includes synthesizing anodes by subjecting them to a low current density cycling, typically between 0.01 C and 0.1 C, at temperatures ranging between ambient (25 °C) to 50 °C, in order to promote electrolyte redox reaction and electrolyte-electrode interface equilibration. This method of forming an electrode can achieve the formation cycle in order to ensure repeatable and reliable electrochemical stability. The low current density cycling can be carried out at a depth of discharge of approximately 80%>-90%> for 1-20 cycles, followed by a 100%) depth of discharge for the next 1-80 cycles, corresponding to total number of cycles of 2-100 during the formation.

[0143] The ability to cycle cells at lower cut-off voltages in real applications, provides increased capacity, but reduces voltages and prevents electrolytic oxidation at the cathode. A method to coat the anode material with a uniform thin layer of parylene or its derivatives (Parylene-D, Parylene-C, Parylene-F) is by chemical vapor deposition. The thickness of the parylene coating can range from 0.5-50 nm. The thickness of the parylene is controlled by either one, two or all of the following methods - (a) varying the parylene dimer loading in the sublimation chamber; (b) by controlling the monomer flux rate by adjusting the deposition pressure; (c) by controlling the deposition time.

[0144] In one such example, 0.5 nm of parylene is coated on to the silicon-GO / graphene-graphite anode (SiGO/G-G/PPX). The SiGO/G-G/PPX anode was assembled against an LCO cathode and tested as described in. The anode delivered a reversible capacity greater than 400 mAh/g at 0.1 C.

[0145] In the foregoing, it will be appreciated that any feature of any one of the embodiments can be combined or substituted with any other feature of any other one of the embodiments. [0146] Aspects of this disclosure can be implemented in various electronic devices. Examples of the electronic devices can include, but are not limited to, consumer electronic products, parts of the consumer electronic products, electronic test equipment, cellular communications infrastructure such as a base station, etc. Examples of the electronic devices can include, but are not limited to, a mobile phone such as a smart phone, a wearable computing device such as a smart watch or an ear piece, a telephone, a television, a computer monitor, a computer, a modem, a hand-held computer, a laptop computer, a tablet computer, a personal digital assistant (PDA), a microwave, a refrigerator, a vehicular electronics system such as an automotive electronics system, a stereo system, a DVD player, a CD player, a digital music player such as an MP3 player, a radio, a camcorder, a camera such as a digital camera, a portable memory chip, a washer, a dryer, a washer/dryer, peripheral device, a clock, etc. Further, the electronic devices can include unfinished products.

[0147] Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise," "comprising," "include," "including" and the like are to be construed in an inclusive sense, as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to." The word "coupled", as generally used herein, refers to two or more elements that may be either directly connected, or connected by way of one or more intermediate elements. Likewise, the word "connected", as generally used herein, refers to two or more elements that may be either directly connected, or connected by way of one or more intermediate elements. Additionally, the words "herein," "above," "below," and words of similar import, when used in this application, shall refer to this application as a whole and not to any particular portions of this application. Where the context permits, words in the above Detailed Description using the singular or plural number may also include the plural or singular number, respectively. The word "or" in reference to a list of two or more items, that word covers all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list.

[0148] Moreover, conditional language used herein, such as, among others, "can," "could," "might," "may," "e.g.," "for example," "such as" and the like, unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include, while other embodiments do not include, certain features, elements and/or states. Thus, such conditional language is not generally intended to imply that features, elements and/or states are in any way required for one or more embodiments or whether these features, elements and/or states are included or are to be performed in any particular embodiment.

[0149] While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the disclosure. Indeed, the novel apparatus, methods, and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the disclosure. For example, while blocks are presented in a given arrangement, alternative embodiments may perform similar functionalities with different components and/or circuit topologies, and some blocks may be deleted, moved, added, subdivided, combined, and/or modified. Each of these blocks may be implemented in a variety of different ways. Any suitable combination of the elements and acts of the various embodiments described above can be combined to provide further embodiments. The various features and processes described above may be implemented independently of one another, or may be combined in various ways. All possible combinations and subcombinations of features of this disclosure are intended to fall within the scope of this disclosure.

Claims

WHAT IS CLAIMED IS:

1. An electrode for a secondary battery, comprising:

a current collector having coated thereon an active material, the active material comprising:

graphite microparticles intermixed with nanoparticles or microparticles, the nanoparticles and/or microparticles comprising silicon, silicon monoxide, silicon dioxide, tin, tin oxide, germanium, aluminum or combinations thereof, wherein the graphite microparticles have a mean cross-sectional dimension in a range of about 1 micron to about 200 microns, and wherein the nanoparticles and/or microparticles have a mean cross- sectional dimension in a range of about 0.01 μιη to about 40 μιη; and a plurality of contiguous graphene oxide (GO) sheets formed over the active material, wherein the GO sheets have a mean carbon-to-oxygen ratio of about 2: 1 to about 20: 1, wherein each of the GO sheets contacts at least another of the GO sheets, such that the GO sheets generally encapsulate the active material to provide cohesion and mechanical stability thereto; and

a carbon-based polymer binder network interconnecting the current collector, the graphite microparticles, the nanoparticles and/or microparticles and the GO sheets.

2. The electrode of Claim 1, wherein the electrode is configured to serve as an anode of the secondary battery.

3. The electrode of Claim 1, wherein the active material further comprises a conductive carbon additive configured to provide an electrically conductive network for transporting electrons.

4. The electrode of Claim 1, wherein the active material further comprises a polymer selected from the group consisting of poly-ethylene oxide (PEO), poly-acrylic acid (PAA), polyaniline, polyethylene glycol (PEG) and poly-vinyl acetate (PVA), wherein the polymer coats the graphite microparticles or the nanoparticles and/or the microparticles, thereby providing further cohesion and mechanical stability to the anode active material.

5. The electrode of Claim 1, wherein the the mean carbon-to-oxygen ratio is about 5: 1 to about 20: 1.

6. The electrode of Claim 1, wherein the GO sheets have a surface area of about 100 m²/g to about 3000 m²/g.

7. The electrode of Claim 1, wherein the GO sheets have a mean lateral dimension of about 0.1 μπι to about 200 μιη.

8. The electrode of Claim 1, wherein the carbon-based binder network comprises a binder selected from the group consisting of carboxymethyl cellulose, poly-acrylic acid and styrene butadiene rubber.

9. The electrode of Claim 1, wherein the electrode is configured as an anode prior to being assembled into the secondary battery, wherein the active material further comprises lithium.

10. An electrode for a secondary battery, comprising:

lithium transition metal oxide particles having a mean cross-sectional dimension in a range of about 0.1 micron to 20 microns;

a plurality of contiguous graphene oxide (GO) sheets formed over the active material, wherein the GO sheets have a mean carbon-to-oxygen ratio of about 2: 1 to about 20: 1, wherein each of the GO sheets contacts at least another of the GO sheets, such that the GO sheets generally encapsulate the active material to provide cohesion and mechanical stability thereto; and

a carbon-based polymer binder network interconnecting the current collector, the lithium transition metal oxide particles and the GO sheets.

11. The electrode of Claim 10, wherein the electrode is configured to serve as an cathode of the secondary battery.

12. The electrode of Claim 10, wherein the electrode is configured to serve as an anode of the secondary battery.

13. The electrode of Claim 10, wherein the active material further comprises a conductive carbon additive configured to provide an electrically conductive network for transporting electrons.

14. The electrode of Claim 10, wherein the active material further comprises a polymer selected from the group consisting of polyvinylpyrrolidone (PVP), poly-ethylene oxide (PEO), poly-acrylic acid (PAA), polyaniline, polyethylene glycol (PEG) and polyvinyl acetate (PVA), wherein the polymer coats the graphite microparticles or the nanoparticles and/or the microparticles, thereby providing further cohesion and mechanical stability to the anode active material.

15. The electrode of Claim 10, wherein the mean carbon-to-oxygen ratio is about 5: 1 to about 20: 1.

16. The electrode of Claim 10, wherein the GO sheets have a surface area of about 100 m²/g to about 3000 m²/g.

17. The electrode of Claim 10, wherein the GO sheets have a mean lateral dimension of about 0.1 μπι to about 200 μπι.

18. The electrode of Claim 10, wherein the carbon-based binder network comprises a binder selected from the group consisting of carboxymethyl cellulose, poly-acrylic acid and styrene butadiene rubber.

19. A secondary battery comprising a cathode, an anode and a separator interposed between the cathode and the anode, wherein the electrode of Claim 1 is configured serve as the anode.

20. A secondary battery of Claim 19, wherein the electrode of Claim 10 is configured to serve as the cathode.

21. A secondary battery comprising a cathode, an anode and a separator interposed between the cathode and the anode, wherein the electrode of Claim 10 is configured to serve as the cathode.

22. A method of forming an electrode for a secondary battery, comprising:

providing a current collector;

preparing a slurry mixture by a process comprising:

intermixing a carbon-based binder with water to form a first dispersion,

intermixing graphene oxide (GO) sheets and nanoparticles and/or microparticles and one or both of water and ethanol to form a second dispersion, wherein the nanoparticles and/or microparticles comprise one of silicon, tin or aluminum and have a mean cross-sectional dimension in a range of about 0.01 μπι to about 40 μπι,

wherein forming the first dispersion or the second dispersion further comprises dispersing graphite microparticles having a mean cross-sectional dimension in a range of about 1 micron to about 200 microns, and mixing the first dispersion and the second dispersion to form the slurry mixture;

coating the current collector with the slurry mixture; and

drying the slurry mixture to form an active material layer on the current collector.

23. The method of Claim 22, wherein upon drying, the active material layer comprises a plurality of contiguous GO sheets formed over the nanoparticles and/or microparticles and the graphite microparticles, wherein each of the GO sheets contacts at least another of the GO sheets, such that the GO sheets generally encapsulate the nanoparticles and/or microparticles and the graphite microparticles and provide cohesion and mechanical stability to the active material layer.

24. The method of Claim 23, wherein as-dried, the GO sheets have a mean carbomoxygen ratio between about 2: 1 and about 6: 1.

25. The method of Claim 24, further comprising pressure-rolling the active material using a heated roller to increase the mean carbomoxygen ratio to between about 5: 1 and about 20: 1.

26. The method of Claim 25, wherein the heated roller is heated to a temperature between about 50°C and about 900°C, and wherein pressure-rolling the active material increases the carbomoxygen ratio to between about 10: 1 and about 20: 1.

27. The method of Claim 23, wherein the electrode is an anode, the method further comprising pre-lithiating by incorporating lithium into the active material layer after drying the slurry mixture and prior to the electrode being assembled into the secondary battery.

28. The method of Claim 23, further comprising, after drying, calendering the electrode active material layer on the current collector, wherein calendering includes applying a static pressure or a dynamic pressure.

29. The method of Claim 27, wherein pre-lithiating includes forming a solid electrolyte interphase (SEI) layer comprising lithium.

30. The method of Claim 27, wherein pre-lithiating includes electrochemically pre- lithiating, comprising:

disposing the electrode active material layer on a lithium-containing material, wherein the electrode active material and the lithium-containing material contact and are separated by an ion-permeable insulating polymer; and

applying a steady-state voltage.

31. The method of Claim 30, wherein the lithium-containing material comprises lithium manganese oxide or lithium cobalt oxide.

32. The method of Claim 30, wherein the ion-permeable insulating polymer comprises polypropylene.

33. The method of Claim 30, wherein the steady-state voltage is between about 3V and 6 V.

34. The method of Claim 30, wherein pre-lithiating includes subliming and gas-phase coating of lithium oxide or lithium peroxide directly onto the electrode active material.

35. The method of Claim 22, further comprising forming a carbonaceous coating comprising an active intercalation material selected from the group consisting of soft carbon, graphite, mesoporous carbon and parylene.

36. The method of Claim 22, further comprising in-situ polymerizing monomers to form a polymer coating around the nanoparticles and/or microparticles prior to coating the slurry mixture on the current collector.

37. A method of forming an electrode for a secondary battery, comprising:

providing a current collector;

preparing a slurry mixture by a process comprising:

intermixing a carbon-based binder with water to form a first dispersion,

intermixing graphene oxide (GO) sheets and lithium transition metal oxide particles having a mean cross-sectional dimension in a range of about 0.1 micron to 20 microns to form a second dispersion; and

mixing the first dispersion and the second dispersion to form the slurry mixture;

coating the current collector with the slurry mixture; and drying the slurry mixture to form an active material layer on the current collector.

38. The method of Claim 37, wherein upon drying, the active material layer comprises a plurality of contiguous GO sheets formed over the transition metal oxide particles, wherein each of the GO sheets contacts at least another of the GO sheets, such that the GO sheets generally encapsulate the transition metal oxide particles and provide cohesion and mechanical stability to the active material layer.

39. The method of Claim 38, wherein as-dried, the GO sheets have a mean carbomoxygen ratio between about 2: 1 and about 6: 1.

40. The method of Claim 39, further comprising pressure-rolling the active material using a heated roller to increase the mean carbomoxygen ratio to between about 5 : 1 and 20: 1.

41. The method of Claim 40, wherein the heated roller is heated to a temperature between about 50°C and about 900°C, and wherein pressure-rolling the active material increases the carbomoxygen ratio to between about 10: 1 and about 20: 1.

42. The method of Claim 38, further comprising forming a carbonaceous coating comprising an active intercalation material selected from the group consisting of soft carbon, graphite, mesoporous carbon and parylene.