Patent Application
20200395583
The present invention relates to separators for use in electrochemical cells, such as rechargeable lithium ion batteries, and methods for manufacturing the separators and electrochemical cells incorporating the separators.
Continued improvements in electrical energy storage by using rechargeable lithium ion batteries are of particular interest in a wide range of applications—for example, personal electronic devices, electric automobiles, home energy storage, renewable power generation storage, and the like. For portable devices, there is particular interest in improved high temperature operational stability, miniaturization, ease of manufacture, etc. In particular, separators between anodes and cathodes are key elements in lithium ion batteries for use in electronic devices. Such separators have traditionally been made with a ceramic material being disposed on either or both sides of a sheet of a porous polymer substrate material, so-called ceramic-coated plastic separators (CCS), and placed between the electrodes. One of a number of considerations for the material used in a separator is tolerance for high temperatures while the battery is in use and plastic materials tend not to be suitable for very high temperatures. As lithium ion batteries have incorporated less safe, higher energy density electrode materials into more energy dense batteries of increasing sizes, such as 40 Ah to 100 Ah lithium ion cells in battery packs of many cells, ceramic-coated plastic separators (CCS) do not provide the enhanced safety needed, such as in the nail penetration test of a forced short circuit on the battery. The heat shrinkage of the plastic polymer support material of CCS at temperatures starting around 110° C. also limits the ability to vacuum dry the CCS and stacks of the CCS and the electrodes in order to remove any residual water and other volatiles prior to filling the dry lithium ion cell with electrolyte. Also, the plastic polymer support material is a thermal insulator and does not have the thermal conductivity to efficiently spread the heat to provide more safety when a sudden heating event occurs in the cell.
Techniques on forming free-standing all-ceramic separators (CSP) for use in electrochemical cells have been developed in light of the above-described issues related to polymer substrates, for example, as described in U.S. Patent Application Publication Nos. 2013/0171500 to Xu et al. and 2015/0030933 to Goetzen et al. One challenge with previous CSPs is obtaining sufficient mechanical strength for cell assembly without any breakage of the separator and for withstanding the mechanical stresses of the cell cycling, while providing the high % porosity of the separator needed for acceptable ionic conductivity and not showing any incompatibility and degradation with the battery cell chemistry. The requirement for mechanical strength of the separator has significantly increased in recent years due to the large increase in the cell assembly speeds in order to lower the cost of the batteries. Besides mechanical strength in the separator, it is also important that the separator have a high modulus up to 2% elongation to avoid stretching during cell assembly and a high level of flexibility to withstand the bending and any folding during cell assembly.
In view of the foregoing, it is an objective of the present invention to provide an improved free-standing, and substantially all-ceramic, composite separator (CSP), without a porous polymer substrate, that is suitable for use in an electrochemical cell, such as a lithium ion cell.
In an exemplary embodiment of the invention, an electrochemical cell incorporating one or more porous inorganic/organic composite separator layers reaches a peak temperature at or below 100° C. in a nail penetration test conducted with a 3 mm diameter nail penetrating at approximately 80 mm/s.
In an exemplary embodiment, an electrochemical cell incorporating one or more inorganic/organic composite separator layers maintains operation at or below 150° C. over a period of at least 3 hours, preferably at or below 200° C. over a period of at least 2 hours, and more preferably at or below 100° C. over a period of at least 30 hours (when heated to those temperatures over those time periods in an accelerating rate calorimeter (ARC) test).
In an exemplary embodiment, a composite separator comprises one or more porous inorganic/organic composite separator layers with an overall thickness at or below 20 microns, preferably at or below 15 microns, and more preferably at or below 12 microns.
In an exemplary embodiment, a composite separator comprises one or more porous inorganic/organic composite separator layers, which composite separator swells by or below 5% after soaking for 1 hour in a non-aqueous electrolyte or in propylene carbonate, preferably by or below 3%.
In an exemplary embodiment, a composite separator comprises one or more porous inorganic/organic composite separator layers, which composite separator has an overall resistance at or below 1.5 ohms, preferably at or below 1.4 ohms, and more preferably below 1.0 ohm.
In an exemplary embodiment, a composite separator comprises one or more porous inorganic/organic composite separator layers, which composite separator has a maximum tensile stress of, at least, approximately 1700 psi, a maximum tensile load of, at least, approximately 0.5 kg, and a percentage elongation at break of, at least, approximately fifteen (15) percent (%).
In an exemplary embodiment, a composite separator is made by coating one or more composite separator layers in a predetermined order on a release substrate, delaminating the layers from the release substrate, and vacuum drying the delaminated layers at approximately 130° C.-150° C. for approximately 1 hour-4 hours to provide a free-standing composite separator.
In an exemplary embodiment, a multi-layer composite separator comprises a porous layer comprising particles and a polymer that is non-swelling in a non-aqueous electrolyte, and a porous inorganic/organic composite separator layer on one or both sides of the porous layer, wherein the composite separator layer comprises a polymer that swells in the non-aqueous electrolyte.
In an exemplary embodiment, the particles comprise inorganic particles selected from the group consisting of inorganic oxides and inorganic nitrides.
In an exemplary embodiment, the inorganic particles comprise boehmite particles.
In an exemplary embodiment, the particles comprise polymer particles that are insoluble in water.
In an exemplary embodiment, the particles comprise polymer particles that are insoluble in propylene carbonate.
In an exemplary embodiment, the porous layer comprises a crosslinking agent that reacts with said polymer particles.
In an exemplary embodiment, the particles comprise inorganic particles selected from the group consisting of inorganic oxides and inorganic nitrides and further comprise polymer particles that are insoluble in water.
In an exemplary embodiment, the polymer in said porous layer is a polyvinyl alcohol.
In an exemplary embodiment, the polymer in said composite separator layer is a polyvinylidene difluoride (PVdF).
In an exemplary embodiment, the composite separator layer comprises inorganic particles selected from the group consisting of inorganic oxides and inorganic nitrides.
In an exemplary embodiment, the inorganic particles comprise boehmite particles.
In an exemplary embodiment, the weight percent of inorganic particles in said composite separator layer is 60% to 95%.
In an exemplary embodiment, the composite separator layer is a xerogel layer.
In an exemplary embodiment, the porous layer is a safety shutdown layer.
In an exemplary embodiment, the multi-layer composite separator swells by or below 0.5% after soaking in electrolyte for 1 hour.
In an exemplary embodiment, a multilayer separator is made by coating a porous layer and one or more composite separator layers in a predetermined order on a release substrate, and delaminating the layers from the release substrate to provide a free-standing multilayer composite separator.
In an exemplary embodiment, each layer is provided by a coating step.
In an exemplary embodiment, a lamination step is performed after one or more coating steps.
In an exemplary embodiment, the multi-layer separator is compressed in thickness in narrow lanes prior to slitting in the lanes to provide the desired width and to increase the mechanical strength along the edges of the multi-layer separator.
In an exemplary embodiment, a multi-layer composite separator comprises a porous layer comprising a polymer that is non-swelling in a non-aqueous electrolyte, the porous layer being formed by a phase inversion method, and a porous inorganic/organic composite separator layer on one or both sides of said porous layer, the composite separator layer comprising a polymer that swells in the non-aqueous electrolyte.
Other features and advantages of the present invention will become readily apparent from the following detailed description and the accompanying drawings.
The above and related objects, features and advantages of the present invention will be more fully understood by reference to the following, detailed description of the preferred, albeit illustrative, embodiments of the present invention when taken in conjunction with the accompanying figures, wherein:
Reference will now be made in detail to the example embodiment(s), as illustrated in the accompanying drawings. Whenever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts.
The term “substrate” refers to any supporting structure including, but not limited to, a thin sheet of porous polymeric material or polymer separator layer, such as a porous polyolefin substrate or layer as used to manufacture ceramic-coated separators (CCS) or a porous non-woven polymer substrate or layer as used to manufacture ceramic-coated and impregnated separators. The term “substrate” also refers to the thick but flexible sheet of a plastic release film used in the methods of manufacturing embodiments of the invention.
A free-standing composite separator without an attached substrate, or an all-ceramic separator (CSP), for use in an electrochemical cell is desirable for ease of manufacture and cell fabrication, and for high temperature performance in comparison to conventional CCS comprising polymer substrates. The present invention is directed to techniques for forming a separator comprised substantially of ceramic material that is applicable as a free-standing element positioned between electrodes in an electrochemical cell. The present invention is also directed to the CSP itself, including its chemical composition, formulation, physical properties and performance properties.
I. Coating Mix Process
A process for forming separator material according to an exemplary embodiment of the present invention is shown in
At step S120 in
At step S140, after the boehmite is added, the stirring speed of stir shaft 215 is increased—to 2650 RPM—and the mixture is stirred for approximately 60 minutes. According to a preferred embodiment, the mixture is stirred at an elevated temperature—for example, between approximately 40-60° C. to facilitate full dispersion. After the stirring, at step S145, the mixture is cooled to room temperature.
Referring now to
At step S160, after the binder solution is diluted, the stirring speed of stir shaft 215 is increased—to 1000 RPM—and the ceramic dispersion—for example, the boehmite dispersion formed by process 100b shown in
According to an exemplary embodiment of the invention, the coating mixture for manufacturing a CSP separator may be formed by the processes 100a-100c, as shown in
As examples, four particular types of CSP separators with specified approximate material proportions have been made with satisfactory testing results, shown in Table 1 below.
Properties of boehmite pigments from Sasol, Lake Charles, La., and Duren, Germany, are summarized below:
In this and following Examples on testing performed on CSP-A, CSP-B, CSP-B2, and CSP-B3 separators, the 10F4 Boehmite is surface-treated with formic acid and the 10SR Boehmite is surface-treated with p-toluene sulfonic acid (p-TSA). The surface treatment of boehmite particles with formic acid makes the boehmite more dispersible in water and in protic solvents, such as alcohols, but does not enable dispersion in NMP, which is one of several aprotic solvents that are capable of fully dissolving PVdF at a 3% by weight and higher solids content. As used herein, the terms “insoluble” and “not soluble” mean that less than 3% by weight of the material dissolved or was soluble at room temperature, or 25° C., in the particular solvent or water. The surface treatment of boehmite particles with p-toluene sulfonic acid enables dispersion in NMP. One of ordinary skill in the art would appreciate, based on the description herein, that material proportions, surface treatment of respective particles, and the like, may be altered without departing from the spirit and scope of the invention. Inorganic particles, such as boehmite, of different types, which may or may not be surface treated, may be blended at different proportions to form a CSP separator layer. For example, the ratio of Boehmite 1 to Boehmite 2 may be increased from 60%-40% by weight, as in CSP-B2 and CSP-B3, up to and including 75%-25% by weight. This increased ratio of Boehmite 1 to Boehmite 2 may be accompanied by a corresponding increase in P:B ratio. Other blends of inorganic particles may be similarly adjusted along with P:B ratio to maintain or increase mechanical strength and to lower swelling in organic carbonate solvents and in electrolytes while maintaining or increasing porosity and rate capability of the resulting separator.
II. Mixture Filtration
Given the high filler loading, optimization of boehmite particle size distribution, and blend ratios of surface-treated/“un-treated” (e.g., 10SR/10F4 or 10SR/D60 described above) boehmites, this inevitably yields small quantities of large (>10 micron) aggregates that must be removed for optimal separator performance. According to an exemplary embodiment, suitable filtration of the coating mixture is conducted prior to a die coating process, with a filter pore size of approximately 10 to 25 microns.
Samples of a coating mix by processes 100a-100c described above, as shown in
Correspondingly, as reflected in
III. Ceramic Die Coating Process
A process for making a thin film ceramic precursor to a CSP separator according to an exemplary embodiment of the present invention is shown in
According to an exemplary embodiment of the invention, the films 415a and 415b are removed from liner 405 and slit to width for assembly into a battery cell. Vacuum drying, as further described below, may be conducted before and/or after the films 415a and 415b are assembled into cells prior to filling the cells with electrolyte.
Given the high filler loading, high viscosity, specific preferred solvent system (NMP), and particular rheological characteristics, pumping of the coating mix fluid to (and through) a coating die—for example, dispensers 410a and 410b—may present unique challenges. According to an exemplary embodiment of the invention, progressive cavity pumps with appropriate NMP-resistant fittings may be used for feeding dispensers 410a and 410b for continuous use without degradation of the pump or the coating fluid.
IV. Cell Fabrication
Following die coating, in accordance with exemplary embodiments of the invention and as described in further detail below, the film 415a (and/or) 415b may undergo one or more processes for characteristic improvements, including, but not limited to, one or more of: vacuum drying (VD); crosslinking (or cross-linker coating); and edge reinforcement (ER). As detailed below, VD may be conducted before and/or after cell stacking or inserting the cell stacks into a casing (and before electrolyte filling). As also described in further detail below, the die coat film (415a and/or 415b), and separators made by alternative coating methods such as gravure coatings, may contain multiple ceramic coating layers and may be incorporated with one or more non-swelling layers and/or safety shutdown layers before cell fabrication in accordance with exemplary embodiments of the invention.
When ready for cell fabrication, the separator film (415a and/or 415b) after slitting is rolled into reel form for cell stacking. Cell stacking can be performed by a number of known processes and the characteristic improvements to the separator film (415a and/or 415b), described above and detailed below, address issues presented by such cell stacking and associated cell fabrication processes—for example, edge reinforcement processing on the separator film (415a and/or 415b) improves, among other factors, edge tear resistance when the film is handled by cell stacking and fabrication machinery.
In accordance with an exemplary embodiment of the invention, the separator film (415a and/or 415b), in reel form, is pulled through a z-fold stacking machine in a central “stacking” location. Cell stacking is performed, preferably, in a clean room environment. The separator is then folded back and forth as anodes and cathodes are placed on the stack. Once the stack is built up with enough electrode layers (for example, 17 cathodes and 18 anodes in a stack), the separator is wound around the stack a predetermined number of times and cut/taped to form a “dry cell stack.” The “dry cell stack” is then moved to a tabbing/welding station—for example, ultrasonic welding—for connecting the anode and cathode tabs to create the two respective leads (plus and minus). The tabbed cells are then placed in a HiPot (defect) tester, where a high voltage is applied for a short amount of time. If there is a hole in the separator or other defect that causes the cell to short-circuit or the current to jump, the cell would be rejected. Accepted stacks are then placed into a casing, such as a pouch, and sealed, leaving an opening for adding electrolyte (electrolyte filling) via the opening in the pouch. Once the electrolyte filling step is completed, the pouch is completely sealed. The cells then undergo final cell formation processing, which may include heating the cells to form gas that is then removed by opening the pouch and resealing it. The cells may also be cycled (charged and discharged) a predetermined number of times in a cell formation process before being released as a finished cell product for end use.
V. Shutdown Layer
In accordance with an exemplary embodiment of the invention, one or more shutdown layers of a porous thermally fusible particle-filled coating may be disposed on either or both sides of a ceramic-based nanocomposite to form a free-standing CSP separator. The shutdown layer may also be intermediate between two ceramic-based nanocomposite layers.
Testing was conducted on the thermal stability of lithium-ion cells comprising CSP films produced in the manner described above. In particular, Accelerating Rate Calorimetry (ARC) was performed on such lithium-ion cells against cells with a variety of different separators, all in pouch form with approximately 5 Ah in rated discharge capacity, namely:
For the “CSP” ARC testing in this Example, pouch cells with the CSP-A version separator were used. The cells were fully-charged using a CC/CV charge protocol with 0.5 C charge rate, 4.20 V charge cut-off voltage and a C/20 rate taper charge cut-off. The cells utilized a lithium nickel manganese cobalt oxide (NMC) positive electrode chemistry and carbon (graphite) negative electrode chemistry. Excess pouch material was folded and taped down. Insulated wires were connected to the cell tabs and then connected to an external data-logger for voltage measurements during testing. The cells were placed in an Accelerating Rate Calorimeter (ARC) chamber and a thermocouple was affixed to the side of the pouch using high-temperature tape.
The ARC testing was performed in a “Heat-Wait-Search” Mode where an ARC heats the chamber to the starting temperature, 50° C., and waits until the sample temperature matches the chamber temperature. The system then waits a prescribed amount of time while searching for an exotherm from the sample. If an exotherm is not detected, the system heats the chamber and sample to the next step (heat step=5° C.). If an exotherm above the threshold is detected, the ARC continues to match the chamber temperature to the sample temperature and no longer undergoes heating steps (adiabatic region). The results are shown in Table 2.
Referring to Table 2, when tested in an adiabatic environment (no heat loss to surroundings) the cells began to self-heat with the following properties. The CSP-A cells (or “CSP 1” and “CSP 2”) (2 tested) demonstrated self-heat onset when the cell temperature was >150° C. Other cells tested exhibited onset temperatures between 87° C. and 129° C., except for one anomalous “CCS on Anode” sample in which onset occurred at 145° C. Cell voltage behavior (e.g. shut-down or separator failure) depended on the material(s) of the separator. The cells with polymeric separators (full or partial), except for the one anomalous “CCS on Anode” sample, exhibited voltage drops around 130° C., consistent with melting temperatures of common polymeric polyethylene (PE) separator materials. The two “CSP” cells maintained the cell voltage until the temperature of the cell was >190° C. Sudden heating events were apparent in most cells before full cell failure (defined as when d2T/dt2>100° C./min2). Heating events were observed in non-“CSP′” cells between 120° C. and 170° C. except for the one anomalous “CCS on Anode” sample in which onset of the exotherm was above 200° C. (the other polymer separator cells showed onset of the exotherm at about 130° C.). Heating events occurred in both “CSP” cells above 200° C. and appear to have been induced by the opening of the seal of the pouch cell and the release of the volatile electrolyte solvents. As reflected in these results, the tested CSP-A cells with their lack of shrinkage at temperatures to 220° C. and above maintained operation through higher temperatures against the comparative cells, with both higher self-heating onset and voltage drop temperatures. The improved results against, at least, the “CCS” and “Control” samples are attributable to the absence of a porous polymer substrate that shrinks and melts at about 130° C.
As shown in
For comparison with the “Shutdown on Anode” cell, testing was performed on a “CCS on Anode 1” cell, results of which are shown in
Achieving shutdown at 105° C. has long been a desired result for lithium ion cells because lithium ion cells typically experience a rapid thermal runaway starting at about 110° C., so it is important to terminate the operation of the cell before it reaches the temperature of thermal runaway. Further, for safety and cell performance reasons, lithium ion cells should not be used after they have reached about 100° C. and experienced some degradation of the cell, so shutting the cell down at 105° C. automatically prevents any further use or operation of the lithium ion cell. As can be seen in the below examples, CCS with a PE substrate and PE separators do not shut down until a much higher temperature, such as at 125° C. to 130° C. or higher, and also show high exothermic heat buildup and cell fires at those temperatures and thus are not safety shutdown layers that consistently prevent fires and explosions in lithium ion electrochemical cells containing volatile and flammable organic electrolyte solvents.
For further comparison with the “Shutdown on Anode” cell, testing was performed on a “CCS on Anode 2” cell, results of which are shown in
Finally, for comparison with the “CSP” cells, tests were conducted on PE separator cells “Control 1” and “Control 2,” the results of which are shown in
For “Control 2,”
In summary, the “CSP 1” and “CSP 2” cells showed notable improvement in thermal stability, particularly in comparison to conventional CCS and PE “control 1” and “control 2” cells.
Testing was also conducted on the thermal conductivity of CSP films for use as separators produced in the manner described above, with comparisons to CCS and polymer sheets having corresponding dimensions. For the “CSP” thermal conductivity testing in this Example, a CSP version separator with two different treated boehmites, similar to the CSP-A version was used. The advantages of excluding any polymer substrate, as in a traditional CCS separator, is confirmed by laser flash (transient) tests conforming to ASTM E1461 on the 20 μm CSP separator (“CSP 20”) and the PE polymer sheet. The ASTM E1461 method is accurate in providing quantitative and reproducible thermal conductivity measurements for thin separator samples. As reflected in
Nail penetration safety testing was conducted on lithium-ion pouch cells incorporating CSP films produced in the manner described above as separators, with comparisons to pouch cells incorporating PE (polyethylene) polymer separators. Separate tests were conducted on 3.5 Ah capacity cells and 75 Ah cells, respectively. For the “CSP” nail penetration safety testing in this Example, pouch cells with the CSP-A version separator were used.
The high voltage 3.5 Ah pouch cells were charged to 4.4 V prior to testing and a 3 mm diameter nail was driven to penetrate the approximate center of the cell at approximately 8.13 cm/s. The results of the tests are shown in Table 3 below.
The 75 Ah pouch cells were charged to 4.1 V prior to testing and a 3 mm diameter nail was driven to penetrate the approximate center of the cell at approximately 8 cm/s. The results of the tests are shown in Table 4 below.
As shown in both Tables 3 and 4, the CSP cells provided substantially improved mechanical and attendant fire safety in the nail penetration test, with both sets of tests showing no ignition in the CSP cells and peak temperatures below 100° C. As cells become larger, such as 40 Ah cells or larger, and as cells use less safe electrode materials, such as 80:10:10 nickel manganese cobalt (811 NMC) and high nickel cobalt aluminum (NCA) cathode materials and silicon anode materials, it becomes increasingly difficult to pass the nail penetration tests with CCS, and particularly with PE separators. Safety is the most important property in lithium ion batteries and is an absolute requirement, so it is very beneficial if the separator is very heat stable and thermally conductive and can by itself enhance the battery safety compared to CCS and plastic separators. There are other cell designs that are known to improve the safety of lithium ion cells but they are expensive and sometimes not consistently effective, so it is very helpful to be able to use a separator with excellent safety properties. Accordingly, the testing reflects improved safety of cells incorporating CSP separators formed in accordance with the invention in the manner described herein, in preventing ignition events and in maintaining below ignition peak temperatures through induced mechanical failures, such as penetration of the separator and cell by a conductive metal, while in operation.
Additional nail penetration testing was further conducted on 5 Ah CSP-A pouch cells under varying conditions. For this testing, the electrodes were NMC/graphite, the 5 Ah CSP-A cells were LiPF6 constructions with an organic carbonate solvent system, and the tests were conducted at 100% state of charge (SOC) (4.2 V). As shown by the results in Table 5 below, all of the CSP cells maintained maximum temperature below 70° C.
For another nail penetration test, 5 Ah pouch cells with 16 micron and 20 micron CSP-A separators were tested at 100% SOC against control polyolefin (PE) separator cells with a 3 mm diameter nail at 2.5 cm/sec penetration speed. As shown in Table 6 below, the CSP-separator cells showed safety improvement.
In Table 6, an immediate hard short with the PE separator cells was shown by a drop in the voltage to 0 volts and an explosion of the cell in about 3 seconds. This resulted in much of the energy of the cell being discharged immediately and rapidly heating the cell to the very high peak temperatures measured. The significant short with the CSP-A separator was shown by a short that took about 5 minutes to an hour to fully discharge the cell to 0 volts. This much more gradual energy discharge kept the peak temperature in the cell below 100° C. These results are reflected in
Life Cycle testing was conducted on 5 Ah pouch cells incorporating CSP separators with the CSP-A version separator, which showed comparable performance to control polymer (polyolefin)(PE) separator cells. For the “CSP” Life Cycle testing in this Example, pouch cells with the CSP-A version separator were used.
As shown in
5 Ah pouch cells incorporating CSP-A separators of various thicknesses were evaluated for rate capability. For the “CSP” Rate Capability testing in this Example, pouch cells with the CSP-A version separator were used. As reflected in
Various storage characteristics of cells incorporating CSP separators were evaluated. For a 28 day storage test, cells with 20 micron CSP separators (CSP-A version) were compared to cells incorporating 16 micron PE separators (“Control”). As shown in
The CSP-separator (CSP-A version) cells were also tested for discharge capacity and voltage changes over storage at ambient conditions for over one (1) year, the results of which are shown in
5 Ah pouch cells incorporating 16 micron CSP separators (CSP-A version) were evaluated for discharge resistance against 16 micron PE separator cells (“Control”) at 50% SOC using a 10 second pulse resistance method. For the discharge resistance testing in this Example, cells with the CSP-A version separator were used. As shown in
Thermomechanical analysis (TMA) was conducted on CCS, High Density Polyethylene (HDPE), and CSP separators (CSP-A version), respectively. The CCS samples were both one-side ceramic coated PE (CCS 1) and two-side ceramic coated PE (CCS 2). For the TMA conducted in this Example, a CSP-A version separator was used. As shown in
VI. Vacuum Drying (CSP-A)
As described above, in accordance with an exemplary embodiment of the invention, CSP-separator cells may be vacuum dried at an elevated temperature—in, for example, an unsealed pouch—before electrolyte filling for improved life cycle performance. In a preferred embodiment, the cells may be vacuum dried for approximately one (1) to four (4) hours at approximately 130° C. to 150° C., or at a temperature above 130° C. as long as the temperature and the time of the heating do not damage or degrade the anode, the cathode, the pouch casing material, or any other component of the dry cell before the filling with electrolyte.
Life Cycle testing was conducted on CSP-separator cells that were undried, vacuum dried, and stack dried before electrolyte filling For the Life Cycle testing conducted in this Example, cells incorporating CSP-A version separators were used. As shown in
Additionally, as shown in
As described above, including in Table 1, all-ceramic free-standing separators may be formed by varying proportions of respective boehmite grades in the coating mixture. Separators from coating mixtures having the above-described CSP-B, CSP-B2, and CSP-B3 proportions have been shown to exhibit qualities suitable for use in lithium-ion cells and comparable to the results described herein for lithium-ion cells with the CSP-A separator.
Cycling rate Capability testing was conducted on CSP-separator cells with CSP-A and CSP-B (as described above) version separators, respectively.
Correspondingly, cold crank amp (CCA) measurement testing was conducted on CSP-separator cells with CSP-A and CSP-B (as described above) version separators, respectively.
VII. Vacuum Drying (CSP-B2)
As described above, in accordance with an exemplary embodiment of the invention, CSP-separator cells may be vacuum dried at an elevated temperature—in, for example, an unsealed pouch—before electrolyte filling for improved life cycle performance. In an exemplary embodiment, a free-standing CSP separator, or its precursor in roll form, may also be vacuum dried before cell fabrication for improved mechanical properties. Vacuum drying at elevated temperature affects the degree of crystallinity and, accordingly, the resultant mechanical properties. The vacuum state is important for moisture removal—and perhaps other volatiles, such as residual NMP and p-TSA. Additionally, an initial coating and drying process may provide for annealing a PVdF binder in the presence of the boehmite or other ceramic particles, which would result in enhanced mechanical properties during cell fabrication.
In a preferred embodiment, the separator (e.g., films 415a and 415b described above) may be vacuum dried for approximately one (1) to four (4) hours at approximately 130° C. to 150° C.
20 micron CSP-B2 samples were tested for mechanical properties before and after vacuum drying in roll form.
As reflected in
Additional mechanical testing was conducted on CSP-B2, CSP-B3, and other films with the same blend ratio of boehmite particles, various pigment:binder proportions, and separator thicknesses, as summarized in Table 8 below, before and after vacuum drying, the results of which are shown in
As shown in
Additional testing was conducted on CSP-B2 and CSP-B3 samples before and after vacuum drying under various parameters.
VIII. Calendering
In accordance with an exemplary embodiment of the invention, films 415a and 415b described above with reference to
Samples of CSP-A, CSP-B2, and CSP-B3 films were produced as described above and cut before delamination to provide about 18 to 24 inch length sheets up to approximately 12 inches wide with the CSP layers still on the PET release substrate and were subjected to calendering while heating the bottom roll.
The samples were calendered in different configurations, “Up” means that the CSP side was against the upper, unheated roll, and the PET film side was against the heated bottom roll, and conversely, for “Down”.
The first experiments were run at a pressure of approximately 1500 psi (or about 1250 pounds per linear inch), with the bottom roll warmed to about 110° F. to about 140° F.
Sample (1) was a 20 micron CSP-B2, 3.75 pigment:binder (P:B). Samples (1), (1)A, and (1)B were calendered one pass in the “Up” orientation. The 20 micron starting CSP became more transparent with calendering with a thickness around 13 to 15 microns, which is about a 30% thickness reduction. Sample (1)C was two passes through the calender with the second pass being done from the opposite direction. The thickness appeared to be not significantly changed by the second calendering pass. Due to some non-uniformity in the calendering pressure across the width of the sheets, the transparentization of these samples was not uniform with one side being more transparent and there being partially transparent, partially less opaque areas in the samples.
Samples (1)D and (1)E were the 20 micron CSP-B2, and were run in the “Down” orientation. This gave less reduction in thickness to about 16 to 17 micron compared to the “Up” orientation. Sample (1)F was two passes but didn't appear to further reduce the CSP-B2 thickness.
Sample (2) was the 20 micron CSP-B3, 4.25:1 P:B. Samples (2) and (2)A were run in the “Up” orientation and gave a CSP thickness of about 16 to 17 microns. There was no significant change in thickness with two passes with Sample (2)B. Samples (2)C and (2)D were calendered in the “Down” position and gave a CSP thickness of about 17 to 18 microns. There was no significant change in thickness with two passes with Sample 2(E).
Sample (3) was the 12 micron (labelled 10 micron in Table 9) CSP-B2, 3.75:1 P:B. Samples (3) and (3)A were run in the “Up” orientation and gave a CSP thickness of about 10 microns. There was no significant change in thickness with two passes with Sample (3)B. Unlike the 20 micron CSP, there was some “pickoff” of the 12 micron thick CSP coating in places from the calendering operation. It appeared to be due more to an abrasion and tear of the thinner CSP, rather than to any adhesion of the CSP layer to the calender roll. There was no significant change in thickness with two passes with Sample 3(B). The thickness changes were the same on Samples (3)C and (3)D with one pass and on Sample (3)E with two passes.
Sample (4) was a CSP-A, 3.75 P:B of about 26 to 27 microns in thickness. Sample (4) was calendered in the “Up” orientation and gave a thickness of about 20 to 21 microns.
Sample (4)A was calendered in the “Down” orientation and also gave a thickness of about 20 to 21 microns.
The next calendering experiments were done at 750 psi, which is about 700 pli (pounds per lineal inch). This gave much less transparentization. Sample (1)G was calendered in the “Up” orientation and gave a CSP thickness of 16 microns. Sample (1)H was calendered in the “Down” position. Samples (2)F and (2)G were calendered in the “Up” and “Down” orientations, respectively. Samples (3)F and (3)G were calendered in the “Up” and “Down” orientations, respectively. Samples (4)B and (4)C were calendered in the “Up” and “Down” orientations, respectively.
The calender was then set to 250° F. (its maximum setting) for “hot” calendering. The temperature read-out was about 280° F. This temperature was too hot for calendering without distorting the base PET film. The following samples were run. Samples (1)I and (1)J were calendered one pass in the “Up” orientation and gave a CSP thickness of about 14 to 15 microns. There was no significant change in thickness with two passes with Sample (1)K. Other samples were run, but all had some distortion of the PET substrate. This indicates that the temperature of the calender rolls had been set too high for this set of experiments.
The above-described calendering conditions are summarized below in Table 9.
RT/750
Samples (1, 1A, 1B, 1C (2×), and 1G) and (2G) in Table 9 were tested for mechanical properties and air permeability (Gurley), the results of which are reflected in
IX. Cross-Linking
In accordance with an exemplary embodiment of the invention, a cross-linking coating material may be applied to a CSP film—for example, films 415a and 415b described above—prior to cell fabrication for enhancing mechanical properties of the resulting separator. The cross-linking coating may be a polyaziridine cross-linker, an isocyanate cross-linker, and the like.
Samples of CSP films (CSP-A and CSP-B2) applied with di- and tri-functional isocyanate cross-linkers incorporated into the CSP-A and CSP-B2 coating mixes were tested for mechanical properties.
Using CSP at 3.5:1 with Solef® 5130 as a baseline formulation (CSP-A version), Teracure® N33 (Desmodurg N3300)(tri-functional) and Desmodur® W (di-functional) at loadings of 0.375 phr (per hundred parts resin) and 0.75 phr, respectively, were utilized.
The samples were coated on a release coated SR2/SKC Skyrol® SH-400 PET liner substrate at a target dry coating thickness of 20 microns. Drying conditions were 400° F. for about 2 minutes and a linespeed of 5.5 fpm (feet per minute). A portion of each section was given a second pass through the coater under the same drying conditions.
As shown in
The results imply that mechanical properties of standard CSP separators—i.e., 3.5:1 pigment:binder (P:B) ratio with no cross-linker—may be matched using 4.0:1 pigment:binder ratio with a di-functional cross-linker, while also attaining much lower Gurley values and, perhaps, higher porosity.
CSP-B2 (or “G2 r2”) samples coated with a Desmodur® RE Series cross-linker of the following formula were also prepared for testing:
The reduction in ductility may be associated with increased fragility of the system (common upon cross-linking of polymers) or introduction of additional “point defects”—e.g., gels or boehmite aggregates—with higher loadings of the RE cross-linker. At the same time, as shown in
CSP-A samples coated with a polyaziridine cross-linker (PZ-33-pentaerythritol tris (3-(1-aziridinyl) propionate or PZ-28-trimethylolpropane tris(2-methyl-1-aziridine propionate)) were prepared and tested for mechanical properties, the results of which are reflected in
X. Edge Reinforcement
In accordance with an exemplary embodiment of the invention, an edge reinforcement coating may be applied to a CSP film—for example, films 415a and 415b described above—in narrow lanes where there will be subsequent slitting to provide the CSP rolls at the desired width, or after the CSP is slit to size and prior to cell fabrication, for reinforcing the edges of the film and improving tear resistance and mechanical strength properties. The edge coating may be a UV-cured material, a chemically cross-linked material, a strong and flexible polymer material, and the like. For additional tear resistance and mechanical strength, this edge coating may be combined with compression of the CSP in the edge areas either prior to slitting or after slitting.
A polyethylene glycol diacrylate/pentaerythritol tetraacrylate (72:25) blend was applied to a “standard” CSP-A sample at various coating deposition weights using a #3 Mayer rod with MEK (methyl ethyl ketone) solutions of varying solids. For testing purposes, the samples were coated at full coverage, instead of coating in a narrow lane for providing an edge reinforcement lane after slitting. Although an oligomer/monomer blend was applied as a solution from MEK, the coating may be applied at 100% NV. MEK solutions were used for this experiment to reliably control coating deposition weight and wettability of substrate. Coatings were dried in an oven at 100° C. for 60 seconds to remove the MEK. Resultant coated separators were then exposed to one pass through a Fusion UV system (H bulb, 20 ipm). From each coating sample, a single 100 cm2 sample was taken; and basis weight (mass/area) and thickness (4×/sample) were measured.
Additional testing on edge reinforcement coating was conducted by coating a radiation-curable formulation (PEGDA/ethoxylated-TMPTA, 75:25, at various % NV [20%-40%] in MEK) on “standard” CSP-A (P:B=3.5:1, 20 micron) samples at full coverage. The samples were, thereafter, submitted to thermal drying (120 seconds @ 110 C) and UV-curing (single pass through Fusion unit, 20 ipm)
As shown in
It is very important to note that the edge reinforcement layers of the CSP need to be only about 2 to 4 mm wide and, as such, these edges are outside of the electrode areas, as the separators are wider than the electrodes in lithium ion batteries to protect against short circuits when the highly insulating separator is not present. Because of this, there is no or very little effect on the energy capacity and cycling of the cells if these edge areas of CSP are partially filled or completely filled with a toughening material for more mechanical strength. It is still important that these edges be not thicker that the rest of the CSP since this would cause problems in winding slit rolls of CSP that are often up to 2,000 meters in length. The above results show that the edge reinforcement materials can be imbibed into the high porosity CSP layer without adding to the thickness on the edges.
XI. Non-Swelling Layer
As described above, PVdF may be used as a binder material for forming free-standing CSP separators. One desirable feature of PVdF is its stability up to 5V in lithium ion and lithium metal cells. However, very high molecular weight PVdF, such as Sole®5140, swells by approximately 20% to 50% in electrolyte solvents, which may be advantageous for electrolyte wetting and for lower impedance but can result in too much swelling for a CSP separator. The inorganic materials in the CSP reduce to some extent the swelling of the PVdF. To avoid excessive swelling of inorganic/organic composite separators in electrochemical cells, where the organic polymer and the separators may swell a few percentages in the presence of the organic electrolyte, one or more non-swelling porous layers may be added to the composite separator to reduce the swelling to an acceptable level. This multi-layer approach also has the benefits of: (1) reducing the likelihood of pinholes by going from a single separator layer to multiple separator layers, where it is less likely to have a pinhole through the multiple layers; and (2) providing a non-swelling layer that may have other useful properties, such as safety shutdown, inhibition of the migration of transition metal ions, and increased mechanical strength. The one or more non-swelling layers may be formed by inorganic oxides, inorganic nitrides, and the like, and may comprise a polymer, which may be insoluble in water or other solvents, such as propylene carbonate and the like, such as, for example, a polyvinyl alcohol (PVA) that is insoluble in propylene carbonate and also does not swell in propylene carbonate. As another example, the polymer may have approximately 150 nm diameter water-insoluble polymer latex particles, such as products under the tradenames of Lumiflon® and Joncryl®. As described above, one or more layers of the CSP separator may comprise a crosslinking agent that reacts with the polymer particles.
In an exemplary embodiment, the one or more non-swelling layers are intermediate between an inorganic/organic composite layer on both sides, i.e., in a “sandwich” configuration. This has the advantage of protecting the intermediate, non-swelling layer from direct interaction with the surface of one of the electrodes to provide better consistency and long term performance in the cell. Other configurations may also be used to incorporate one or more non-swelling layers, for example:
In an exemplary embodiment of the invention, a CSP separator incorporating one or more non-swelling layers, as described above, may have reduced swelling of approximately 3% to 4%, or preferably less than 1% to 2%, or more preferably less than 0.5%, when soaked in propylene carbonate or in an organic electrolyte for 1 hour. Swelling tolerance may be increased for cylindrical or metal prismatic cells, where the separator is very constrained in a metal case when the electrolyte is added, whereas the benefits of reduced swelling may be more significant for pouch cells, where the case is a metalized plastic.
Samples of a non-swelling microporous layer were formed by non-solvent induced phase inversion of polyamide (Elvamide® 8063)/polyvinylbutyral (Butvar® B-98) blends. A seven (7) micron thick non-swelling layer of various blends was laminated between two 3.5 micron thick CSP-B2 layers.
Samples having a corresponding CSP-non-swelling layer-CSP structure were separately tested for swelling. A polyvinyl alcohol (PVA)-boehmite layer of approximately 16 micron thickness, made according to the fourth paragraph of Example 1 of U.S. Pat. No. 8,883,354 to Carlson, et al., with 11 parts of ethylene carbonate, 6 parts of the divinyl ether of triethylene glycol (DVE-3), and 3 parts of polyethylene oxide (molecular weight of 200), served as the non-swelling layer that was laminated between two CSP-B2 layers of approximately 11 microns in thickness.
The resulting properties of the laminate are summarized in Table 10 below.
As shown in Table 10 above, the laminate showed less than 0.5% swelling in propylene carbonate after soaking for 1 hour, thus confirming the effectiveness of laminating solvent-swellable, but strong and flexible and very stable and safe in lithium ion batteries at high temperature, versions of CSP to a non-swellable, but low in elongation, layer to yield low net swelling. The samples exhibited a net reduction in tensile strength relative to CSP-B2 separators and a reduction in elongation/ductility, but this can be improved by adjustments for better elongation/ductility in the composition of non-swellable layers to better match the higher elongation before break of the solvent-swellable and high safety versions of CSP, such as those described herein for CSP-A, CSP—B, CSP-B2, and CSP-B3 with their flame retardant PVdF polymer binder. The net effect on the Gurley permeability number was negligible (once normalized to 20 microns).
Samples of separators formed in conformance with the CSP-B and CSP-B2 blend proportions were evaluated against samples that were formed with the CSP-A coating mixture blend proportions.
While maintaining very similar mechanical strength to the CSP-B separator, the CSP-B2 separators showed a significant reduction in the Gurley air permeability number to about 550 seconds/100 cc compared to 1050 seconds/100 cc for CSP-A and about 900 seconds/100 cc for CSP-B. While the CSP-B separators showed substantial improvements in mechanical strength compared to CSP-A, as reflected in
As shown above, the CSP-B2 blend yielded separators with improved mechanical strength at approximately 2,000 psi, while significantly lowering the Gurley values, separator impedance, and swelling properties over the CSP-A separators. A summary of the results is further provided in Table 11 below.
XII. Extraction
As described above, one or more ceramic components in a coating mix blend may be surface treated—with, for example, p-toluene sulfonic acid—to facilitate dispersion in organic solvents instead of using dispersants, which may present low molecular weight species and potentially migratory components that may degrade the cell performance. Furthermore, as described above, applying a cross-linker is one of a number of options for further enhancement of CSP mechanical properties and/or electrochemical performance—in other words, adding low molecular weight additives that may not be fully “bound” via chemical bonding to the primary separator components (such as boehmite and PVdF binder).
As further described above, Desmodur® RE is an example of a cross-linker that showed, in testing, improved mechanical properties at loading levels of ˜1%-6% w/w of PVdF polymer binder (i.e., very low overall loadings) in CSP separators. For this example, the resultant separator coatings have a distinct pink-purple color (depending on loading of the cross-linker, and exposure to light). One cause for concern is the fact that this distinct color is readily leached from the separator when exposed to electrolyte solvents. Accordingly, extraction testing was performed to confirm a preservation of positive mechanical properties through solvent exposure.
Extraction of modified CSP-A dry separators (D10SR/D10F4 50:50, S5140, 2% RE) using an aqueous alcohol solution was performed in a water/IPA (isopropyl alcohol) (50:50) bath over 60 minutes. The extracted and control samples were subsequently dried at 100° C. for 60 minutes.
As shown in
In summary, provided are free-standing CSP separators with improved safety, heat stability, and thermal conductivity properties compared to CCS separators. The added heat stability of the CSP of this invention enables high temperature vacuum drying, such as at 130° C. to 150° C., of CSP, of cell stacks of electrodes with CSP, and of dry cells with CSP, that are not possible with CCS. This high temperature vacuum drying results in improved cell cycle life, rate capability, and cell storage stability. Very importantly, CSP shows superior safety results compared to CCS when tested in cells by ARC and nail penetration testing methods.
The free-standing CSP separators of this invention have improved performance over prior CSP separators, particularly in higher mechanical strength while providing excellent elongation/ductility and higher ionic conductivity and rate capability. A number of process and materials options to obtain the improved performance for the CSP separators of this invention are described herein, including, but not limited to, the use of particular blends of ceramic particles and the mix and filtration processes for the CSP coating with these blends, the use of functionalized PVdF, such as Solef® 5130 and Solef® 5140, with preferred functionality and molecular weight, the use of different non-swelling porous layers to reduce the swelling of the CSP in the electrolyte, vacuum drying at high temperatures for both CSP and for dry cells with CSP, the use of edge reinforcement materials on the slit edges, the use of cross-linkers in the CSP layer, the use of a calendering process on the CSP, the use of solvent/water extraction to remove soluble materials, the addition of a shutdown layer, and combinations of the foregoing.
Now that embodiments of the present invention have been shown and described in detail, various modifications and improvements thereon will become readily apparent to those skilled in the art. Accordingly, the spirit and scope of the present invention is to be construed or broadly and not limited by the foregoing specification
This application claims the benefit of and priority to U.S. Provisional Patent Application No. 62/569,964, filed on Oct. 9, 2017, the entire contents of which are incorporated by reference herein.
This invention was made with government support under W56HZV-13-C-0063 awarded by the Department of Defense. The government has certain rights in the invention.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2018/055014 | 10/9/2018 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62569964 | Oct 2017 | US |
