Large format battery anodes containing silicon particles

Abstract

Provided are large-scale anodes containing a high weight percentage of silicon suitable for use in lithium-ion energy storage devices and batteries, and methods for making the same. [Solution] The anode material described herein can include a thin film cast on a current collector substrate, the thin film including a plurality of active material particles and a conductive polymer film coated on the active material particles. In some embodiments, the conductive polymer film includes polyacrylonitrile (PAN). A method for manufacturing the anode material can include preparing a slurry including the active material particles and the conductive polymer material, casting the slurry on a current collector substrate, and subjecting the composite material to drying and heat treatment.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATION(S) This application claims priority to U.S. Provisional Patent Application No. 62 / 407,938, filed October 13, 2016, the entire contents of which are incorporated herein by reference.

[0002] STATEMENT REGARDING FEDERALLY FUNDED RESEARCH This invention was made with government support under Grant No. DE-SC0013852 awarded by the Department of Energy. The government has certain rights in this invention.

[0003] The present disclosure relates to energy storage devices such as lithium-ion electrochemical cells and batteries. More specifically, the present disclosure relates to the scalable production of silicon anode electrode sheets suitable for use in, for example, lithium-ion energy storage devices and batteries. [Background technology]

[0004] Batteries have become ubiquitous in today's society, powering everything from hearing aids to smartphones, forklifts, and even transportation equipment. Current battery technologies range from heavy, bulky, and inexpensive lead-acid batteries to lighter, smaller, and more expensive lithium-ion batteries (LIBs). However, rechargeable LIBs have dominated the portable electronics market for nearly a decade, and in recent years they have gained significant traction in specialty markets, including the electric transportation sector and military applications. Minor improvements in materials processing and device manufacturing have enabled energy density improvements of approximately 5 to 6% each year, a slow, incremental development. To date, improvements in Li-ion technology have primarily succeeded in modifying first-generation materials and fitting them into smaller, safer packages. Prior-art batteries remain heavy, bulky, expensive, and unsafe, creating obstacles to the power and cost requirements of next-generation applications. Reaching future energy storage goals will require breakthroughs in next-generation electrode materials, incorporating active materials with higher energy density.

[0005] In recent years, silicon has been identified as one of the most attractive high-energy anode materials for LIBs. Silicon's low operating voltage and high energy capacity (3579 mAhg), nearly 10 times higher than that of prior art graphite anodes, have led to the development of -1The high theoretical specific capacity of Si has spurred extensive research efforts aimed at developing practical Si-based electrodes. Despite the advantages of Si electrodes, several challenges, primarily linked to the material's severe volume expansion, have hindered their commercialization. While commercial graphite electrodes expand by roughly 10–13% during lithium intercalation, Si expansion approaches 300%, causing structural degradation and instability of the critical solid-electrolyte intercalation layer (SEI). Such instability ultimately shortens battery life to insufficient levels. The degradation of the active material can be mitigated by incorporating materials smaller than 150 nm or by nanostructuring the electrode architecture to reduce the expansion. Unfortunately, electrode architectures proposed in previous work lack sufficiently high Coulombic efficiency, primarily because the volume change during Si alloying and dealloying mechanically destabilizes the SEI at the Si-electrolyte interface.

[0006] Many efforts aimed at utilizing silicon in lithium-ion battery anodes have sought to combine silicon with conventional active materials. This provides higher capacity while minimizing the drawbacks of silicon materials (e.g., volume expansion, active material utilization, etc.). The incorporation of nanosilicon (nano-Si) particles in prior art graphite electrodes has been implemented in commercial practice to increase the capacity of current anodes. However, this process is limited to the inclusion of only about 5% (by mass) of nano-Si active material. Any amount above the 5% limit would destroy the conventional network of the electrode due to the severe volume expansion and contraction of Si during lithiation and delithiation.

[0007] Preliminary work conducted by the applicant has demonstrated the impressive long-term cycling stability of a nano-Si electrode / room-temperature ionic liquid (RTIL or IL) system and its combination with a commercially available "L333" cathode for Li-ion cells, capable of delivering 1.35 times the specific energy of today's prior art technology, normalized to the electroactive material mass. Nano-silicon-cyclized polyacrylonitrile (nSi-cPAN) electrodes, when combined with an imide-based RTIL electrolyte, maintain an average half-cell coulombic efficiency of over 99.97% due to the synergistic effects of a robust electrode architecture and the formation of a stable solid-electrolyte interlayer (SEI) layer. U.S. Patent No. 6,275,629, incorporated herein by reference in its entirety, describes the composition formed during the combination and use of an nSi-cPAN electrode with a specific RTIL electrolyte composition in Li-ion batteries. Specifically, U.S. Patent No. 6,275,629, discloses the composition of the SEI formed between the nSi-cPAN electrode and the RTIL electrolyte.

[0008] Following the demonstration of the nSi-cPAN system, applicant developed a "micron Si" (μSi) anode. The use of μSi is made possible by leveraging the mechanical strength of the cPAN coating. By encapsulating μSi particles in a durable, conductive coating matrix, the pulverization of larger Si particles is suppressed. This mechanism is called "self-restrained fragmentation." The fragmented silicon particles remain adhered to the cPAN coating matrix, allowing for long-term, full utilization of the material with minimal capacity loss. This mechanism is verified by the electrode's ability to retain its capacity over many cycles, demonstrating that the silicon particles maintain access to the electronically conductive cPAN matrix even after pulverization. This is demonstrated in Figure 1 and described in U.S. Patent No. 5,629,499, which describes a composition formed by electrochemically pulverizing large silicon particles in a cPAN matrix.

[0009] While the development of the nSi-RTIL system and μSi-cPAN electrode has resulted in world-record performance of Li-ion full cells containing silicon anodes (high silicon loading mass, no pre-treatment / pre-lithiated silicon anode, long cycle life, high energy), this performance has only been demonstrated on a laboratory scale. The anodes used to demonstrate those inventions contained more than 70% silicon relative to the total anode mass, but were thin and not suitable for commercial applications. These were "bench-top" demonstrations performed for proof-of-concept and feasibility. The slurries used to fabricate those anodes contained 12.5 to 25 wt.% solids (extremely low solids content, not suitable for commercial manufacturing). Laboratory-standard current collector substrates (thickness >25 to 30 microns), low electrode coating thickness (~2 mAh cm -2 The small electrode area and low current (in the microampere range, suitable for coin cell demonstrations) made such demonstrations possible. Translating these technologies from the benchtop to commercial manufacturing lines presents an entirely new set of challenges.

[0010] Commercial anodes have a capacity of at least 2 mAhcm so that they can be paired with cathodes for improved energy density and cost in large format Li-ion batteries. -2 This means that previously developed anodes must be scaled up (by 2x in most metrics, including payload and thickness, to achieve attractive energy densities) and processed by commercially practical means. Commercial anodes must have their capacity per area (mAhcm) scaled down to properly match the cathode capacity when stacked or wound into pouch or cylindrical cells, respectively. -2 ) must also be developed to remain consistent across the entire anode sheet. When anodes are scaled up to commercial levels, the adhesion between the coating and the current collector substrate, the physical properties of the coating, and even the anode electrochemistry change. -2It is well known that achieving realistic high performance silicon anodes with aerial loading capacities above 1000 kJ / cm2 is extremely difficult, and this is particularly true for anodes containing a high mass percentage (greater than 10 wt.%) of silicon material. This is due to adhesion (between the electrode and copper current collector substrate) and cohesion (maintaining the structural integrity of the electrode within itself) problems that arise due to the expansion and contraction of the silicon active material during lithiation and delithiation, respectively. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] International Publication No. 2016 / 123396 Summary of the Invention [Problem to be solved by the invention]

[0012] The present invention is directed to solving the problems of the background art. [Means for solving the problem]

[0013] This application describes various embodiments of processes and compositions used to facilitate consistent, high-quality, commercial-scale Si-cPAN anodes for Li-ion batteries. In some embodiments, the anodes include a thin film cast on a current collector substrate, the thin film including active material particles (e.g., silicon particles) and a conductive polymer film coating on the active material particles. In some embodiments, the conductive polymer film coating includes a thermoplastic processed to form a cyclized, non-plastic ladder compound. Such anodes can be incorporated into energy storage devices in conjunction with a cathode and an electrolyte. Methods for fabricating the disclosed anodes are also described. In some embodiments, the methods include preparing a slurry containing an active material, an additive powder, a polymer powder, and a solvent. The slurry is then mixed for a period of time, and the slurry is then cast onto a current collector substrate. A drying and heating step is then performed. [Brief explanation of the drawings]

[0014] [Figure 1A] ~ [Figure 1B] FIG. 1 is a pair of graphs illustrating cycling data for μSi-cPAN half-cells containing fluorinated electrolyte additives, showing the rapid CE stabilization achieved by use of Applicant's previously disclosed μSi-cPAN / mRTIL system. [Figure 2A] ~ [Figure 2D]

[0033] High-resolution transmission electron microscopy images (HR-TEM) of nanospherical silicon particles coated with polyacrylonitrile are provided. [Figure 3] 1 is a graph illustrating a three-electrode experiment of a full-cell pouch with a micron silicon (anode) and NMC (cathode) working electrode and a lithium counter electrode, with an N / P ratio of 0.9. [Figure 4] 1 is a pair of graphs illustrating pouch full cell cycling data comparing the impact of low N / P ratios to sufficient N / P ratios. [Figure 5A] ~ [Figure 5E] Figure 1 provides a graph showing the half-cell cycling performance of silicon / PAN anodes highlighting the performance of 1 to 3 micrometer silicon particles (D50 size) on various coppers with different surface roughness, and images of the surface topography (a)-(e) of various copper materials with varying surface roughness. [Figure 6]10 is a graph showing full cells (coins) containing silicon / carbon active material with a PAN conductive binder (30-35% silicon normalized to total anode coating mass) and an NMC

[0622] cathode, comparing anode current collector roughness. [Figure 7] An overview of the different copper types, their associated roughness parameters, and a profilometer spectrum of a representative surface of the first copper type ("OM10um (coarse)" copper) are provided. [Figure 8] 1 illustrates a polyacrylonitrile heating process. [Figure 9] We provide SEM and EDS (with mapping) of nanocomposites driven by polymers containing silicon and PAN after heat treatment under argon. [Figure 10A] ~ [Figure 10B] A pair of graphs illustrating the electrochemical performance of PAN / Si nanocomposite anodes, heat-treated at 300 °C under argon (top) and air (bottom) environments, followed by heat-treatment at 600 °C under argon. [Figure 11] 1 is a graph comparing cyclization in a vacuum oven and in a tube furnace under argon flow, showing high CE and high capacity when cyclized in either environment. [Figure 12A] ~ [Figure 12D] 1 is a series of graphs showing first cycle voltage profiles of full cells containing a nickel-rich NCM cathode and a Si-cPAN anode, in which the anode components were heat-treated according to various procedures. [Figure 13] 1 is a graph illustrating an exemplary half-cell containing 30-35% silicon (normalized to total anode mass) anode produced by the methods described herein (6 mAh / cm 2 loading). [Figure 14] 1 is a pair of graphs illustrating a full-cell single-stack pouch containing an exemplary anode described herein, mass-produced, suitable for commercial-level performance (commercial payload, commercially realistic auxiliary components). DETAILED DESCRIPTION OF THE INVENTION

[0015] The following description details various embodiments of methods associated with producing commercial-scale Si-cPAN electrodes, the electrochemical implications associated with some or all of those methods, and various embodiments of the resulting compositions. The description is divided into sections according to the steps used to fabricate the electrodes, and each step describes physical parameters that can be used to obtain improved battery performance.

[0016] Traditionally, Si-based electrodes are fabricated by mixing a polymer binder (e.g., polyvinylidene fluoride, polyacrylic acid, styrene-butadiene rubber, or carboxymethyl cellulose), a conductive additive (usually carbon black), and Si particles in an organic solvent such as N-methylpyrrolidone (NMP) to produce a viscous slurry. The slurry is then blade-coated onto a copper foil current collector and dried to form the anode electrode. The embodiments described herein relate to the fabrication of a polymer-driven composite Si anode, which differs from conventional methods in significant ways.

[0017] Remarkably, the method described herein is compatible with existing manufacturing infrastructure, enabling the first truly "drop-in" high-load silicon anodes available to the Li-ion market. Other silicon anode production methods are cost- and resource-intensive, providing significant value to the method discussed below. As described in more detail below, silicon active material is coated with a conductive polymer such as polyacrylonitrile (PAN), cast onto copper foil, and then thermally processed and coupled to the cathode in a specific manner to enable full-cell performance.

[0018] Although polyacrylonitrile is discussed herein as an exemplary conductive polymer for application by the disclosed methods, other polymers may be used. Other suitable polymers include, but are not limited to, poly(acrylic acid) (PAA), carboxymethyl cellulose (CMC), and alginic acid. PAN is a resinous, fibrous organic polymer made from a mixture of monomers, primarily acrylonitrile. PAN fibers, when appropriately modified, are chemical precursors to high-quality carbon fibers, which are found commercially in a variety of high-tech and common-day applications.

[0019] Numerous active material types can also be utilized with the methods described herein. While silicon is discussed as an exemplary anode active material for application with this method, any silicon form could be incorporated into the anode slurry and electrode sheet. Silicon forms include, but are not limited to, nanospheres, nanowires, nanorods, whiskers, "coral-shaped" silicon, microspherical silicon, and various nanofeatured large particle silicon materials. Silicon-graphite, silicon-graphene, silicon-hard carbon, and other silicon-carbon composite materials are also non-exhaustive exemplary anode active materials for application with the methods described herein. Mixtures of silicon and carbonaceous materials such as graphite or hard carbon are also non-exhaustive exemplary anode active materials.

[0020] Large-volume slurry mixing To fabricate Si-cPAN electrodes on a large scale, the methods described herein generally begin by preparing a slurry. Typically, the slurry is prepared by mixing the active material, polymer, auxiliary materials, and additives in a solvent. Preferably, the resulting slurry has specific rheological properties to provide the highest electrochemical cycling performance for Li-ion batteries. In some embodiments, the composition added to the solvent includes about 10 to about 50 wt.% PAN and about 50 to about 90 wt.% active material. In some embodiments, the slurry contains about 30 to about 60 wt.% solids in about 70 to about 40 wt.% solvent.

[0021] The active material used to prepare the slurry can include combinations of materials in different compositions. For example, carbonaceous active materials (such as graphite, graphene, and hard carbon) can be mixed to form a 10:90 silicon:carbonaceous material weight ratio or a 90:10 silicon:carbonaceous material weight ratio. An exemplary commercial Si:cPAN anode can include a 30:55:15 Si:carbonaceous material:PAN weight ratio. Other exemplary weight ratios include 40 to 80 wt.% silicon, 5 to 50 wt.% carbonaceous material, and 10 to 20 wt.% PAN. The carbonaceous material can include a mixture of active material and conductive additives, including, but not limited to, carbon black or carbon nanotubes.

[0022] The mixture of active material and conductive binder powders is dispersed in a solvent to form a slurry. In some embodiments, the solvent is selected so that it can dissolve the conductive binder. For example, N,N-dimethylformamide (DMF, 99%) is an exemplary solvent for application by the methods described herein when utilizing PAN polymer. Other suitable solvents include, but are not limited to, dimethyl sulfone (DMSO), dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMAc), ethylene carbonate (EC), and propylene carbonate (PC).

[0023] Slurry viscosity determines mixing quality, coating quality, and the ability to generate large thin films on current collector substrates. Exemplary slurry viscosities can be determined using a Brookfield viscosity (spindle 64) (all measurements taken at room temperature, 23°C). Slurry viscosity is determined by the solvent / polymer mass ratio and polymer chain length, as well as by the mass of solvent relative to the total slurry mass. Exemplary slurries made using the materials and methods described herein exhibit significantly more Newtonian behavior than conventional Li-ion anode slurries. A Newtonian fluid is a fluid in which the viscous stress resulting from fluid flow is linearly proportional to the strain rate, or range of change in fluid deformation. This means that as shear forces are applied to the slurries described herein, the slurries exhibit less shear thinning relative to conventional Li-ion anode slurries. This has implications for the anode mixing and coating process, as the slurries can be mixed at significantly higher RPMs and successfully coated at much lower thicknesses. The low shear thinning properties of slurries can be studied using a Brookfield viscometer, a common instrument for rheological characterization. In some embodiments, the slurries have Brookfield viscosities ranging between 3000 and 6000 centipoise (cP) at room temperature and spindle rotations of 12 to 100 RPM (steel spindle #64), varying by less than 1000 cP for a given slurry mixture. The relatively low variation in viscosity over various shear forces (described by spindle rotations in RPM) suggests the Newtonian nature of the slurry. Another exemplary slurry has a viscosity ranging from 3500 to 5000 cP at room temperature and spindle rotations of 20 to 100 RPM.

[0024] Slurry mixing parameters are contributors to the resulting anode performance. Slurry mixing time and slurry volume, along with mass percentage, powder mix, and slurry viscosity, are important factors in determining the quality of the polymer coating on the active material. Slurry time is important to ensure a uniform polymer coating on the suspended active material particles. In some embodiments, a slurry mixing time of up to 12 hours is sufficient. In some embodiments, a lower mixing time, such as 2 hours, is sufficient depending on the appropriate equipment.

[0025] Slurry mixing can be accomplished by a variety of equipment: vacuum and non-vacuum planetary centrifugal mixers (e.g., "Thinky mixers" or "Ross" mixers), slurry planetary dispersing vacuum mixers, planetary double disperser mixers, homogenizers, and simple stirring in a beaker with a stir bar or stir plate can produce sufficient mixing conditions.

[0026] Slurry volume is important because a sufficient amount of material must be present to produce adequate mixing. Slurry volume also affects the electrochemical performance of the resulting anode. If the slurry volume is too low, a relatively large portion of the material will not receive agitation / mixing, and a uniform coating will not be applied. For example, a mixture consisting of 200 mg active material plus polymer powder and 1.6 g solvent (87.5% solvent by total slurry mass) will not mix properly. In this case, regardless of the mixing method, there is not enough slurry to produce adequate mixing. A slurry utilizing 1.2 g active material plus polymer powder in 8.4 g solvent (87.5% solvent by total slurry mass) mixes as well as a slurry utilizing 1.2 g active material plus polymer powder in 4-6 g solvent, producing a uniform polymer coating on the active material. However, this solids content (12.5 wt.% solids) is not suitable for coating using large-scale manufacturing equipment. These slurries have viscosities that are too low to be coated by roll-to-roll methods (e.g., comma bars, slot dies, etc.).

[0027] The quality of the PAN coating on the silicon material, resulting from the aforementioned parameters, becomes increasingly important as the anode payload increases. Achieving a sufficient coating can be confirmed by determining the coating's uniformity and thickness using microscopy, such as transmission electron microscopy. A PAN coating that provides strong shape retention throughout electrochemical cycling should be at least 3 to 5 nanometers thick, be found on the entire surface of the active material particles, and be present throughout the electrode. An example of an exemplary coating formed using the method described above is shown in Figure 2. Electron energy loss spectroscopy (EELS) highlights the silicon and PAN coating (d). A uniform 3 to 5 nm coating is present on the particles throughout the electrode matrix.

[0028] When preparing the slurry, it may also be desirable to add electrode additives that can improve the performance of full-cell Li-ion batteries. Exemplary electrode additives include, but are not limited to, lithium metal powder (e.g., stabilized lithium metal powder (SLMP)) and lithium nitride (LiN), as well as other high-lithium powders and salts. These dry powders can be added directly to the slurry or mixed with other slurry components. Oxalic acid can also be added to the slurry to improve dispersion and adhesion properties.

[0029] Generally speaking, the polymer is dissolved in a solvent, and the slurry is then mixed so that the polymer material thoroughly coats the active material powder dispersed in the polymer / solvent solution.

[0030] Electrode Coating After mixing the slurry, it is cast onto the current collector substrate, for example, by a roll-to-roll coating method. Roll-to-roll coating machines can be used to produce hundreds of meters of electrodes in a single run. The roll-to-roll coating process is determined by the physical properties of the slurry (e.g., shear, viscosity, etc.). The current collector foil is pulled through the coater at a speed of 0.2 to 50 meters per minute, and the coated foil passes through a dryer set at a temperature of 30 to 70°C for water-based slurries and 110 to 160°C for solvent-based slurries. In the case of the technology described herein, the dryer temperature should be set at 30 to 70°C, even though it is a solvent-based system.

[0031] Important factors for electrode coatings are the area-specific loading of silicon per square centimeter, the capacity it provides, and how these numbers match the cathode used in the full cell. To pair with high-energy cathode materials, including "nickel-rich NMC" (nickel cobalt manganese oxide cathodes), the area-specific capacity of the anode should be 1.3 to 2.0 times the area-specific capacity of the cathode. This factor is known in the industry as the "N / P ratio" (negative electrode capacity / positive electrode capacity). In conventional Li-ion cells containing graphite anodes, the N / P ratio is typically 1.1 to 1.2 and is set to prevent lithium plating on the anode during cycling. The N / P ratios devised for the systems described herein are designed to cover the efficiency loss during the initial cycles and to increase the anode half-cell voltage to Li / Li. +The anode is designed to "pin" between 0.01 and 1.5 V relative to the total anode film mass. If the N / P ratio is too low, the anode half-cell voltage will drop below 0.01 V (due to complete and over-lithiation of silicon) and the anode will be destroyed. This is illustrated in Figure 3. Figure 4 shows a cell with a poor N / P ratio compared to a cell with a strong N / P ratio. A higher relative N / P ratio also prevents wrinkling and deformation of the anode film due to expansion and contraction of the silicon material present in the film. The desired N / P ratio depends on the silicon loading mass relative to the total anode film mass. If the anode film contains 20 to 50 wt.% silicon, the N / P ratio should be 1.2 to 1.6. If the anode film contains more than 50 wt.% silicon, the N / P ratio should be greater than 1.6. In some embodiments, to obtain the minimum N / P ratio for the system, the weight percent of silicon in the anode can be added to "1." In other words, if the anode contains 40 wt.% silicon, the resulting N / P ratio for the full cell system should be greater than 1.4.

[0032] Current collector substrate A current collector substrate, typically a metal foil, is used to transfer electrons from outside the cell to the electrode, and vice versa. The electrode slurry is cast onto the foil as a uniformly thick coating. For Li-ion batteries to function properly, the electrode coating must adhere sufficiently to the current collector foil and maintain this adhesion during cycling. This is particularly challenging when using large-format electrodes (which tend to be very thick) given the expansion characteristics of the active material when alloying electrodes such as silicon. A conductive binder is responsible for adhering the anode film to the current collector substrate. For large-format electrodes, sufficient binder must be present to enable adhesion. In the silicon-plus-PAN system described herein, a minimum polymer content of 10 to 25% PAN relative to the total anode coating mass is acceptable. This is typical for scaled, large-format silicon anodes with thicknesses greater than 5 micrometers.

[0033] Importantly, and surprisingly, the physical properties of the current collector substrate are highly critical to the performance of the resulting anode sheet. In conjunction with the anode composition, the physical properties of the current collector substrate enable long cell life. Copper foil is an exemplary current collector substrate and is the most commonly used in conventional Li-ion cells. Copper foil properties are discussed in this application.

[0034] An important physical property of copper in relation to anodic thin film adhesion is its surface roughness. One measure of roughness is the 10-point height or maximum height (R z ) which is the root mean square value. R z is defined as the average number of peaks to valleys in a given scan area, with at least five consecutive points measured (5 highest peaks + 5 highest values ​​= 10 points). For some embodiments of the anodes described herein to remain adhered during cycling, copper R z should be at least 1.5 micrometers. Other embodiments relying on larger active materials may require higher surface roughness, up to 6 to 7 micrometers. Other embodiments relying on nanoscale active materials may require R greater than 0.5 micrometers. z Copper foils used in conventional / previously commercialized Li-ion anodes typically have an R of 0.5 microns or less. z It has a value.

[0035] Another measure of surface roughness is the arithmetic mean height (Sa), which expresses the magnitude of the difference in height at each point compared to the arithmetic mean of the surface. Yet another measure of surface roughness is the interfacial developed area ratio (Sdr), which is the percentage of the surface area contributed by texture compared to the plane-defined area (i.e., a perfectly flat surface has Sdr=0). Each of these parameters, along with their relative magnitudes, are important to the performance of the systems described herein.

[0036] As previously mentioned, the required copper surface roughness for realistic performance is highly dependent on the active material size. Current data suggest that a surface roughness R of more than 0.5 micrometers is required when the electrode film expansion is below 50% in the z-axis (perpendicular to the electrode substrate) and the active material particle size is above 500 nanometers. z It has been suggested that copper with a surface roughness R of more than 2 micrometers provides the best performance when the electrode film expansion is more than 50% in the z-axis (perpendicular to the electrode substrate) and the active material particle size is more than 500 nanometers. z Copper having a .gtoreq. ...

[0037] Unfortunately, higher surface roughness leads to a consequential higher thickness, which is detrimental to battery energy density because thicker current collectors (auxiliary materials that do not contribute to cell capacity) take up space and provide no energy. The improved adhesion on rougher copper surfaces is explained by the increased surface area available for adhesion between the conductive polymer and copper. Referring to Figure 5, various copper surfaces were imaged by optical microscopy for comparison.

[0038] The performance of half cells containing silicon plus PAN anodes is also shown in Figure 5, demonstrating the clear need for adequate copper surface roughness to maintain adhesion to the anode film during many charge-discharge cycles, based on the active material morphology and anode film structure. In half cells, the first cycle coulombic efficiency (CE) is higher in cells containing rougher copper foils, and the CE stabilizes much more quickly. This is attributed to maintained electronic contact and faster electron transport within the cell. The CE behavior and combined electron transport / adhesion properties are manifested in higher performance full cells with active material particles sized greater than 500 nanometers, as shown in Figure 6. R at or below 1 micron is significantly higher. z In full cells containing an anode current collector thin film having

[0039] An overview of various copper types, their associated roughness parameters, and a profilometer spectrum of a representative surface of the first copper type ("OM10 um (coarse)" copper) is provided in Figure 7. The copper types indicated by the shaded font ("OM10 um (coarse)" and "VL10|23 um" copper) provide the best performance with various silicon material types and anode thin film microstructures. In general, the materials shown in Figure 7 work best for the large-scale electrode systems described herein. Therefore, a new parameter, Sa / Sdr, is used to describe the best-performing copper foils based on roughness parameters. Sa / Sdr describes the ratio of the average height of the peaks and valleys on the foil surface to the magnitude of the percentage of the surface caused by roughness. In other words, a high Sa / Sdr means that the copper surface has very high peaks and low frequency relative to the total roughness. Here, an Sa / Sdr closer to 1 suggests that the copper surface peak / valley heights are more evenly distributed. Sa / Sdr closer to 1 is found to be advantageous, and Sa / Sdr below 3 is found to be sufficient for high performance for a variety of silicon material types and anode film compositions and microstructures.

[0040] Electrode calendaring In some embodiments, the electrode is dried after coating using a roll-to-roll coating machine, with the dryer temperature set at 30 to 70°C under airflow. After slurry casting onto the current collector foil and solvent drying / evaporation, conventional graphite anodes are calendered to about 70% of their original thin-film thickness. This calendering results in a porosity of about 40 to 50%. Such a process provides a higher degree of particle contact while still allowing sufficient electrolyte penetration. Above 50% porosity, conventional anodes lack sufficient mechanical strength to withstand battery production and operation. The system described in this application is different. The electrode requires a higher porosity to accommodate the volume expansion of the silicon material, and a higher inter-film surface area promotes the formation of a robust SEI layer and faster Li+ / Li ... +This is advantageous for ion transport. The anodes described herein, including silicon-plus-PAN anodes, are calendered to 40 to 70% porosity. Exemplary porosities for Si-cPAN composites would be 50 to 60%. This is comparable to conventional graphite anodes, which have a porosity of about 30 to 40%. Today, conventional electrodes (with binders such as PAA, CMC, and SBR) containing silicon ranging up to 15 wt.% Si have porosities ranging from 40 to 50%. Active materials that exhibit higher expansion require electrodes with higher porosity.

[0041] Electrode heat treatment A key aspect of the anodes described herein is the ability of the conductive polymer binder to act both as a binder material and as an electronically conductive matrix capable of providing efficient charge transport within the composite. Producing high-performance anode thin films of this type on a large scale is quite challenging and requires understanding many of the complexities described above, but the need to process many conductive polymers to achieve electronic conductivity adds another layer of complexity. Exemplary conductive polymers discussed herein (e.g., PAN) can be heated in a reducing atmosphere or under vacuum to exhibit electronic conductivity. At the same time, the polymer can be processed so that the polymer matrix does not become too brittle for the mechanical effects of battery cycling (as can be caused by heat treatment at very high temperatures or in an insufficient atmosphere). Furthermore, the processing must be carried out so that the supporting components of the electrode (i.e., copper) remain unaffected and in the correct condition for battery operation.

[0042] Many conductive polymers can undergo chemical changes that result in electronic conductivity. However, most require the addition of a cross-linking agent to catalyze their chemical reaction. PAN and PAN copolymers are unique in that they are self-catalytic via a thermal treatment process. PAN has the molecular formula (C3H3N) nPAN is a unique linear, semi-crystalline organic polymer with a molecular structure consisting of carbon chains with coordinated nitrile groups. PAN chemistry is particularly interesting due to its unique autocatalytic cyclization and crosslinking due to thermal stabilization. PAN chains decompose before they reach a molten state, and the decomposition process, commonly referred to as "cyclization," converts the linear PAN chains into a thermally stable, conjugated ladder-like structure that does not flow or melt. This is illustrated in Figure 8. This thermal stabilization allows the fibers to withstand carbonization and graphitization temperatures (approximately 1000 to 3000°C) without excessive weight loss or chain scission, yielding high-performance carbon fibers.

[0043] Heat stabilization of PAN refers to the low-temperature (typically 200 to 300°C) conversion of the polymer fiber into a high-temperature-resistant fiber. This conversion is necessary for the fiber to survive carbonization (800 to 1300°C) and graphitization (1300 to 3000°C) and have the highest possible carbon yield and superior properties. The primary chemical reactions involved in this process are known as cyclization, dehydrogenation, oxidation, and crosslinking, which result in the formation of a thermally stable conjugated ladder structure.

[0044] Cyclization is a key reaction during PAN stabilization and is the primary focus of the anodic process described herein. Cyclization occurs when nitrile bonds (C≡N) react to develop crosslinks between PAN molecules, forming stable conjugated ladder polymers of double bonds (C═N) and fused pyridine rings. The thermal stability of stabilized fibers is attributed to the formation of ladder structures through cyclization of the nitrile groups, allowing stabilized PAN to operate at high temperatures with minimal volatilization of carbonaceous materials. Cyclization is the reason stabilized fibers change color from white to yellow, brown, or black. Cyclization is exothermic and can damage the fibers if performed too quickly. The fibers may shrink excessively, lose significant mass, and even melt and condense together. Conversely, if the stabilization procedure is too conservative (both in terms of time and heat), the fibers will only be partially stabilized. Unlike dehydrogenation, cyclization does not require the presence of oxygen to occur, so it can occur in an inert atmosphere. The reaction atmosphere is important in the process described herein because the anode copper foil current collector is involved in the heat treatment of PAN, and any exposure to oxygen at temperatures above 100°C will oxidize the foil and cause defect problems (electronic resistance and electrochemical side reactions).

[0045] In the embodiments described herein, PAN is processed only up to its stabilization (specifically, "cyclization") stage, and the resulting pyridine-based conjugated polymer is then applied as an electrode binder / coating with robust mechanical and unique electronic properties. The reason for deviating from the conventional stabilization procedure (oxidation, dehydrogenation, and cyclization to achieve carbonization and graphitization) for high carbon yield and high performance carbons is to prevent the formation of a highly oriented (basal plane-to-basal plane alignment), rigid, and brittle coating around the active material, which is prone to high expansion and contraction.

[0046] Stabilization of PAN from linear molecules to ladder polymer compounds via cyclization can be achieved by heating in an inert environment from 100 to 500°C. An exemplary temperature for the PAN cyclization heat treatment is 300°C, at a rate of 5°C / min, with a hold time of 2 to 12 hours at the peak temperature. Running at various temperatures allows for the best peak electrochemical performance of the PAN / silicon configuration to be determined. Figure 9 shows scanning electron microscopy (SEM) images and energy dispersive spectroscopy (EDS) analysis of some of these polymer-driven nanocomposite samples. PAN / silicon samples were tested using the methods described above. Others were then processed for carbonization of the PAN and held for 1 hour at peak temperatures of 500°C, 600°C, 700°C, 800°C, 900°C, and 1000°C under the same argon atmosphere. A heating rate of 5°C / min was again maintained for this second heating stage. Figures 10 and 11 illustrate the electrochemical data for the various heat treatment tests.

[0047] In addition to the processing parameters of heating time and temperature, the equipment utilized and the resulting atmospheric conditions are important. As previously discussed, anodes should be processed under vacuum or inert gas flow. Argon and nitrogen are exemplary inert gas atmospheres, with pressures of 20 to 80 PSI providing the most consistent heating conditions. Heat treatment under vacuum also allows for adequate cyclization conditions and high-performance anodes. These atmospheres can be provided by various equipment types, including tube furnaces, glove boxes, vacuum ovens, or other atmosphere-controlled ovens. Gas flow during heat treatment allows for improved heating and electrochemical performance because byproducts of the polymer chemical reaction (including hydrogen outgassing during PAN cyclization) are flushed from the system and therefore cannot react with the electrode or current collector substrate. For best performance, gas flow in a tube furnace should be set at a rate of 100 to 1,000 liters per hour. An entire roll of anodes or multiple rolls of anodes can be processed through a single oven. Controlled atmosphere furnaces can also be added to industry-standard roll-to-roll coating systems used for electrode fabrication. Typically, electrodes are conveyed through a drying oven immediately after coating. Following initial drying, with conditions set to induce chemical changes in the polymer (i.e., PAN cyclization) before being wound onto a spool, the electrodes can also be passed through a controlled atmosphere furnace. Alternatively, tubular furnaces provide a commercially viable means of processing large rolls of anodes containing silicon / polyacrylonitrile composites.

[0048] Especially as it relates to commercial-scale manufacturing, it is particularly important to note that the heating time and heating ramp rate should be tuned according to the anode microstructure and size. Specifically, anode thickness, PAN weight percentage, and the amount of anode material undergoing processing influence these processing parameters. As shown in Figure 12, a heating time of just 2 hours can be sufficient for adequate electrochemical performance.

[0049] Electrochemical performance Figure 13 presents exemplary data for a micron silicon:polyacrylonitrile (Si:PAN) anode half-cell fabricated by the methods described herein. The payload, processing, and resulting composition are sufficient for commercial use. Figure 14 presents exemplary data for a full cell containing an exemplary anode described herein and a "nickel-rich" high-energy cathode ("NMC

[0622] ").

[0050] From the foregoing, it will be understood that specific embodiments of the invention have been described herein for purposes of illustration and that various modifications may be made without departing from the scope of the invention. Accordingly, the invention is not limited except as by the appended claims.

Claims

1. a cathode; Electrolytes, Surface roughness R of over 1.5 microns ｚ an anode comprising a thin film having a thickness of 10 to 80 micrometers cast on a current collector substrate having Including, The thin film, A) a plurality of active material particles (the active material particles include at least one of silicon, hard carbon, graphite, graphene, germanium, titanium oxide, tin, magnesium, antimony, and lead); B) a conductive polymer film coating on the active material particles, the conductive polymer film coating comprising a thermoplastic polymer that has been treated to form a cyclized non-plastic ladder compound; Including, Energy storage devices.

2. 10. The energy storage device of claim 1, wherein the anode comprises 30 to 60 wt. % silicon particles and the gravimetric capacity per area of ​​the anode is 1.3 to 1.6 times that of the gravimetric capacity per area of ​​the cathode.

3. 10. The energy storage device of claim 1, wherein the anode comprises greater than or equal to 60 wt. % silicon particles and the gravimetric capacity per area of ​​the anode is 1.6 to 2.0 times that of the gravimetric capacity per area of ​​the cathode.

4. 10. The energy storage device of claim 1, wherein the thermoplastic polymer processed to form the cyclized non-plastic ladder compound comprises polyacrylonitrile.

5. 10. The energy storage device of claim 1, wherein the electrolyte comprises an imide-based room temperature ionic liquid.

6. 10. The energy storage device of claim 1, wherein the porosity of the anode thin film is between 50 and 70%.

7. 2. The energy storage device of claim 1, wherein the magnitude of the arithmetic mean height Sa is less than three times the expansion ratio Sdr of the interface.

8. the thin film includes a plurality of active materials and a thermoplastic polymer that has been processed to form a cyclized non-plastic ladder compound; The method is A) preparing a slurry having a Brookfield viscosity of 2000-6000 cP at room temperature using a #64 spindle at 20-100 RPM by placing a mixture of active material, additive powder, and polymer powder in a solvent capable of dissolving the polymer powder; B) mixing the slurry for a period of 1 to 4 hours; C) casting the slurry onto a current collector substrate; D) drying the cast film; and E) applying heat to the cast film at a temperature of 200 to 400°C for a time period of 1 to 12 hours; Including, A method for making an anode comprising a thin film thickness of 10 to 80 micrometers cast on a current collector substrate.

9. 9. The method of claim 8, wherein the active material comprises at least one of silicon, hard carbon, graphite, germanium, titanium oxide, tin, magnesium, antimony, and lead.

10. 9. The method of claim 8, wherein the thermoplastic polymer treated to form the cyclized non-plastic ladder compound comprises polyacrylonitrile.

11. The method of claim 8 , wherein the additive powder comprises lithium metal powder.

12. The method of claim 8 wherein the additive powder comprises lithium nitride.

13. The method of claim 8 wherein the additive powder comprises oxalic acid.

14. 9. The method of claim 8, wherein the application of heat at 200 to 400°C is completed under vacuum or inert gas flow.

15. 10. An energy storage device comprising an anode, a cathode, and an electrolyte produced according to the method of claim 8, wherein the weight capacity per area of ​​the anode is 1.3 to 2.0 times that of the weight capacity per area of ​​the cathode.

Images