Patent Application
20230027323
The present disclosure relates to battery cell manufacturing, and particularly to an electrode coating having a spatially varied porosity and a method of forming the same by using a porous current collector.
Electrodes are widely used in a range of devices that store electrical energy, including primary (non-rechargeable) battery cells, secondary (rechargeable) battery cells, fuel cells, and capacitors. An ideal electrode needs to balance various electrical energy storage characteristics, such as, for example, energy density, power density, maximum charging rate, internal leakage current, equivalent series resistance (ESR), charge-discharge cycle durability, high electrical conductivity, and low tortuosity. Electrodes often incorporate current collectors to supplement or otherwise improve upon these electrical energy storage characteristics. Current collectors, for example, can be added to provide a higher specific conductance and can increase the available contact area to minimize the interfacial contact resistance between the electrode and its terminal.
A current collector is typically a sheet of conductive material to which the active electrode material is attached. Aluminum foil is commonly used as the current collector of an electrode. In some electrode fabrication processes, for example, a film that includes activated carbon powder (i.e., the active electrode material) is attached to a thin aluminum foil using an adhesive layer. To improve the quality of the interfacial bond between the film of active electrode material and the current collector, the combination of the film and the current collector is processed in a pressure laminator, for example, a calender. This process is generally known as calendering. Thus, the fabrication of an electrode typically involves the production of an active electrode material film and the lamination of that film onto a current collector.
Technical methods described herein include the manufacture and design of an electrode coating having a spatially varied porosity. In one exemplary embodiment, an electrode film includes a porous current collector having a bulk material and a plurality of voids. The electrode film can further include an electrode coating having an active electrode material. The porous current collector and the electrode coating can be compressed together in a calendering process to define the electrode film. In some embodiments, the electrode film includes a spatially varied porosity (e.g., regions of lower porosity and regions of higher porosity).
In some embodiments, the electrode coating fills the plurality of voids prior to calendering. In one exemplary embodiment, a distribution of the plurality of voids in the porous current collector introduces regions of different calendering pressures during the calendering process. In some embodiments, higher-pressure regions during the calendering process correspond to the lower porosity regions in the electrode film and lower-pressure regions during the calendering process correspond to the higher porosity regions in the electrode film.
In another exemplary embodiment, the porous current collector includes a mesh structure having equally sized and distributed voids. In yet other exemplary embodiments, the porous current collector includes a foam structure having a three-dimensional network of struts and pores. In still other embodiments, the plurality of voids in the porous current collector further comprise laser-patterned cut-outs. In some embodiments, the laser-patterned cut-outs have a same shape, while in other embodiments a first laser-patterned cut-out is made of a first shape and a second laser-patterned cut-out is made of a second shape different from the first shape.
Aspects of the disclosure include a method for forming an electrode coating having a spatially varied porosity. An exemplary method can include forming a porous current collector having a bulk material and a plurality of voids. The porous current collector can be coated, infused, or otherwise saturated with an electrode coating having an active electrode material. The porous current collector and the electrode coating can be compressed in a calendering process to define the electrode film. The distribution of the plurality of voids in the porous current collector provides for regions of different calendering pressures during the calendering process. The regions of different calendering pressures leads to regions of higher and lower porosity in the resultant electrode film. In other words, an electrode film having a spatially varied porosity.
The above features and advantages, and other features and advantages of the disclosure are readily apparent from the following detailed description when taken in connection with the accompanying drawings.
Other features, advantages and details appear, by way of example only, in the following detailed description, the detailed description referring to the drawings.
The following description is merely exemplary in nature and is not intended to limit the present disclosure, its application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features. Additionally, the term “exemplary” is used herein to mean “serving as an example, instance or illustration.” Any embodiment or design described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments or designs. The terms “at least one” and “one or more” are understood to include any integer number greater than or equal to one, i.e. one, two, three, four, etc. The term “a plurality” is understood to include any integer number greater than or equal to two, i.e. two, three, four, five, etc. The term “connection” can include an indirect “connection” and a direct “connection”.
As shown and described herein, various features of the disclosure will be presented. Although similar reference numbers may be used in a generic sense, various embodiments will be described and various features may include changes, alterations, modifications, etc. as will be appreciated by those of skill in the art, whether explicitly described or otherwise would be appreciated by those of skill in the art.
Electrodes often incorporate current collectors to supplement or otherwise improve upon the electrical energy storage characteristics of the final integrated device (e.g., a battery). A current collector typically includes a sheet of conductive material (e.g., aluminum foil) to which an active electrode material is attached. To improve the quality of the interfacial bond between the film of the active electrode material and the current collector, the combination of the film and the current collector is processed in a pressure laminator. Thus, the fabrication of an electrode typically involves the production of an active electrode material film and the lamination of that film onto a current collector (the so-called calendering process).
Calendering can be generally defined as the compression of a dried electrode (the latter typically resulting from the coating and drying of an electrode slurry) to reduce its porosity, improve the particles contacts, and enhance its energy or power density. Conventional calendering processes have been used to improve various aspects of battery technology by offering, for example, a higher specific conductance, greater contact areas, and lower contact resistance in the electrode. There are several challenges, however, in optimizing the calendering process. One such challenge is balancing the inherent tradeoff between providing both a high electrical conductivity (requiring a low film porosity) and a low tortuosity (requiring a high film porosity) as needed for efficient ion transport.
The current calendering process is not well-suited to address this fundamental tradeoff, as notably, there is no spatial variation of the porosity of the final electrode film when using conventional calendering. The typical process takes a coating that has relatively high porosity and compresses it to a coating with reduced porosity. This is usually done with high pressure rollers that can vary pressure, roller separation, and roller temperature. The resulting coating is denser, smoother, and thinner. Moreover, the porosity is markedly uniform post-calendering. Unfortunately, while this type of electrode compression can improve several electrode properties, such as energy or power density, this same process is detrimental for other critical electrode properties, such as the effective conductivity of the electrolyte phase.
Thick electrodes enable high energy density battery designs and are a requirement for some next-generation technologies, such as, for example, high range electric vehicles. Unfortunately, conventional calendering places practical limits on the electrode film thickness. For example, thicker electrode films lead to inefficient use of active material due to the inherit tradeoff between achieving a suitably high electrical conductivity and a low tortuosity. Also, the bonding strength represents a bottleneck for achieving thick electrodes. With a high area loading of materials, the limited adhesion strength between the current collector and a thick electrode coating layer can result in an insufficient electrical contact or a poor electrode integrity. These will eventually cause a degradation of electrode performance, especially when suffering any microstructure change during cycling (charging/discharging).
One or more embodiments address one or more of the above-described shortcomings by leveraging a porous current collector to produce an electrode coating or film having a spatially varied porosity. One novel aspect of this approach is that a porous current collector introduces zones with different calendering pressures. Without wishing to be bound by theory, these calendering pressure differentials are responsible for imparting porosity variations in the final calendered coating. Electrode coatings having spatially varied porosities can directly address the inherent calendering tradeoff between providing both a high electrical conductivity and a low tortuosity in an electrode film by offering films having both regions of high porosity and regions of low porosity. Moreover, these regions can be arbitrarily allocated in the film by modifying the structure (voids/pores) of the porous current collector against which the film is created. In other words, various embodiments can employ porous current collectors having a range of constructions and geometries, such as, for example, mesh, foam, and laser-patterned current collectors, to achieve arbitrarily spatially varied porosities in the final electrode coating.
Technical solutions described herein facilitate a range of improvements to battery technology. Electrode coatings having nonuniform porosity distributions formed according to one or more embodiments can reduce battery degradation, extend battery lifetime, and improve specific capacity. In addition, arbitrarily thick electrodes are possible (the porosity of such a film can be arbitrarily manipulated as needed). This can enable, for example, the efficient construction of higher loadings in lithium ion batteries.
Other advantages are possible. From the manufacturing perspective, current calendaring presses can be easily adapted to accommodate porous current collectors, as no new rollers are needed. Instead, only a slight modification to the coating line is required, since the current collector is now porous. Another advantage is an improved bond strength between the current collector and the coating layer. For example, current calendering processes use a “sheet” style current collector, offering only a one-dimensional (1D) bonding relationship between the current collector and the coating layer. In contrast, one or more embodiments provide a three-dimensional (3D) relationship between the current collector and the coating layer due to, for example, the inner pores of the current collector.
Yet another advantage is a reduction in surface cracks and coating delamination. In a conventional calendering process heat flux can only be transferred to the bottom of the coating layer, which can cause an inhomogeneous migration of binder material at that area. Under calendering stress, the result is a 2D solid materials spreading behavior of the coating composite, which at high stress may result in surface cracks and coating delamination. Embodiments of the present disclosure alleviate these concerns, as heat flux can be efficiently transferred through the porous structure to arbitrary depths of the coating layer. Under calendering stress, the result is a 3D solid materials spreading behavior of the coating composite, and the stress can be primarily relieved through the pores, which in the end prevents or mitigates cracks and delamination. Cycling is also improved, as the 3D structure formed according to one or more embodiments significantly increases the contact area between the current collector and the coating composite, thus leading to better bonding strength and preventing delamination or detachment during cycling. This is a marked improvement over the 2D structure of the conventional calendering process, which suffers from a relatively higher potential to cause coating degradation and delamination or detachment during cycling.
In some embodiments, the porous current collector 102 is configured in a mesh-like structure where the voids 104 are equally sized and evenly distributed in a symmetrical arrangement (as shown in
As used herein, the term “active electrode material” refers to materials that enhance the function of an electrode beyond simply providing a contact point or increased reactive area. In some embodiments, for example, a film of active electrode material includes particles with high porosity, so that the surface area of the electrode exposed to an electrolyte in which the electrode is immersed is increased well beyond the area of the visible external surface. In effect, the surface area exposed to the electrolyte becomes a function of the volume of the film made from the active electrode material. A variety of suitable active electrode materials are known, such as, for example, activated carbon, conductive carbon, and graphite. Similarly, a variety of suitable carrier fluids (bulk slurry fluid) are known, such as, for example, N-methyl-2-pyrrolidone.
The size of the active electrode material particles within the slurry is not meant to be particularly limited. In some embodiments, the particle size ranges from about 0.1 to 10 microns, for example 3 microns, although other particle sizes are within the contemplated scope of the disclosure. The concentration of the active electrode material can be varied as desired for the particular application. In some embodiments, the active electrode material constitutes 20 to 80 percent by weight of the slurry (i.e., 20-80% solids), although other solids contents are within the contemplated scope of the disclosure.
In dry electrode coating embodiments, for example, the active electrode material is applied to the porous current collector 102 in the form of dry particles. In some embodiments, the dry particles are applied to the bare surface of the porous current collector 102. In some embodiments, the surface of the porous current collector 102 is pre-treated prior to application of the dry particles. Suitable activated carbon materials for dry electrode coating are available from a variety of sources known to those skilled in the art. In some embodiments, the active electrode material comprises activated carbon, conductive carbon, or graphite.
In some embodiments, a dry blend of particles (e.g., carbon) and a binder are dry mixed (fibrillized; dry-blended) to form a dry powder material. In a dry process this is typically done without the addition of liquids, solvents, processing aids, or the like to the mixture. The binder can include, for example, vinylidene polyfluoride, polyvinyl alcohol, polyimide, polyamideimide, thermoset or thermoplastic particles, and/or Polytetrafluoroethylene (PTFE), although other binders are within the contemplated scope of the disclosure.
The actual mixing process used is not meant to be particularly limited. Dry-blending may be carried out, for example, for 1 to 10 minutes in a V-blender equipped with a high intensity mixing bar, until a uniform dry mixture of dry particles and dry binder is formed. The blending time can vary based on batch size, materials, particle size, densities, as well as other properties, and yet remain within the scope of the present disclosure.
After dry-blending, the mixed dry powder material can be dry fibrillized (fibrillated) using non-lubricated high-shear force techniques. In some embodiments, high-shear forces are provided by a jet-mill. The dry powder material is introduced into the jet-mill, wherein high-velocity air jets are directed at the dry powder material to effectuate application of high shear to the fibrillizable binder within the dry powder material. The shear forces that arise during the dry fibrillization process physically stretch the fibrillizable binder, causing the binder to form a network of fibers that bind the binder to other particles in the active electrode material.
In some embodiments, application of the dry fibrillized particles can occur prior to, or be incorporated within, the calendering step. For example, the dry fibrillized particles can be applied between the rollers of a calender and the surface of the porous current collector 102. In some embodiments, one or both of the rollers is heated to improve adhesion. In embodiments having thermoset or thermoplastic particles, for example, heating one or more of the rollers can also serve to soften or liquefy the particles such that they better effectuate adhesion of the active electrode material to the porous current collector 102.
As further shown in
In some embodiments, the foam may include about 4 to about 100 pores per centimeter at an average pore size of at about 1 to 50 although the particular pore size and distribution is not meant to be particularly limited. In some embodiments, for example, the average pore size can be bigger, or smaller. The average pore size and density can be modified as needed for a particular application. Reducing the average pore size will increase the effective surface area of the material but can impede or otherwise limit penetration of the active electrode material. Regardless of the average pore size, a total porosity value for the foam may be at least 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, or 95%. In other words, at least 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, or 95% of the foam structure may be included within the cavities 304.
Referring now to
At block 502, a porous current collector is formed. In some embodiments, the porous current collector includes a bulk material and a plurality of voids. In some embodiments, the porous current collector includes a mesh structure having equally sized and distributed voids (as shown in
At block 504, the porous current collector is coated with an electrode coating having an active electrode material. In some embodiments, the electrode coating fills the plurality of voids. At block 506, the porous current collector and the electrode coating are compressed in a calendering process to define the electrode film. In some embodiments, resultant the electrode film includes a spatially varied porosity. In some embodiments, the spatially varied porosity includes lower porosity regions and higher porosity regions.
In some embodiments, a distribution of the plurality of voids in the porous current collector introduces regions of different calendering pressures during the calendering process. In some embodiments, higher-pressure regions during the calendering process correspond to the lower porosity regions in the electrode film and lower-pressure regions during the calendering process correspond to the higher porosity regions in the electrode film.
While the above disclosure has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made, and equivalents may be substituted for elements thereof without departing from its scope. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the disclosure without departing from the essential scope thereof. Therefore, it is intended that the present disclosure not be limited to the particular embodiments disclosed, but will include all embodiments falling within the scope of the application.
