PARTICLES IN ELECTROSPUN POLYMER FIBERS WITH THERMAL RESPONSE PROPERTIES

Abstract

The preset invention provides an electrode structure for a lithium ion battery comprising an electrode selected from a cathode including a lithium-based material or an anode including a conductive material, and a melt-convertible encapsulation layer covering at least one surface layer of the electrode. The melt-convertible encapsulation layer comprises a network of nanofibers having the diameter ranging approximately from 100 to 300 nm and polymer microspheres embedded in and coated on the nanofibrous network, wherein the ratio of the diameter of the polymer microspheres to the diameter of the nanofiber is over 30. The polymer microspheres melt to form a dielectric coating of the electrode so as to prevent fire or thermal runaway at a temperature approximately from 100 to 200° C.

Claims

1. An electrode structure for a lithium ion battery comprising: an electrode selected from a cathode including a lithium-based material selected from the group consisting of Lithium Manganese Oxide (LMO), Lithium Cobalt Oxide (LCO), Lithium Nickel Manganese Cobalt Oxide (NMC), and Lithium Iron Phosphate (LFP), or an anode including a conductive material selected from the group consisting of carbon black, carbon nanotubes, graphene, and graphite; a three-dimensional structure with nanofiber bonded microspheres forming a melt-convertible encapsulation layer on at least one surface of the electrode, the melt-convertible encapsulation layer comprising: a network of nanofibers, wherein the diameter of the nanofibers is approximately from 100 to 300 nm for carrying polymer microspheres on the electrode surface via stacking of layers of polymer microspheres-coated nanofiber interconnecting network; the polymer microspheres being embedded in and coated on at least one surface of the network of nanofibers, wherein a ratio of the diameter of the polymer microspheres to the diameter of the nanofibers is over 30 and a mass ratio of the polymer microspheres to the polymer nanofibers is at least 3:1, wherein the polymer microspheres melt to form a dielectric coating, thereby covering the surface of the electrode and nanofibers to provide a medium on spreading the molten polymer microspheres so as to prevent short circuit, fire or thermal runaway at a temperature approximately from 100 to 200° C.

2. The electrode structure for a lithium ion battery of claim 1, wherein a polydispersity of the polymer microspheres in the network of nanofibers ranges approximately from 0.6 to 1.0.

3. The electrode structure for a lithium ion battery of claim 1, wherein the nanofibers comprise one or more polymers of polyvinylidene fluoride (PVDF), poly(vinylidene fluoride-co-hexafluoropropylene)(PVDF-HFP), polyimide (PI), and polyethylene (PE), polypropylene (PP).

4. The electrode structure for a lithium ion battery of claim 1, wherein the polymer microspheres comprise one or more polymers of polyvinylidene fluoride (PVDF), poly(vinylidene fluoride-co-hexafluoropropylene)(PVDF-HFP), polyethylene (PE), and polypropylene (PP).

5. The electrode structure for a lithium ion battery of claim 1, wherein the melting point of the nanofibers ranges approximately from 180 to 200° C. and the decomposition temperature of the nanofibers ranges approximately from 300 to 500° C.

6. The electrode structure for a lithium ion battery of claim 1, wherein the melting point of the polymer microspheres ranges approximately from 80 to 200° C. and the decomposition temperature of the polymer microspheres approximately from 300 to 500° C.

7. The electrode structure for a lithium ion battery of claim 1, wherein the porosity of the network of the nanofiber is approximately from 50 to 90%.

8. The electrode structure for a lithium ion battery of claim 1, wherein the coverage rate of melt-convertible the encapsulation layer covering at least one surface layer of the electrode is approximately from 50 to 80%.

9. The electrode structure for a lithium ion battery of claim 1, wherein the melt-convertible encapsulation layer is fabricated by one or both of electrospinning and blowspinning.

10. The electrode structure for a lithium ion battery of claim 1, wherein the thickness of the melt-convertible encapsulation layer is approximately from 10-50 μm.

11. The electrode structure for a lithium ion battery of claim 1, wherein the polymer microspheres have an average size of 1 to 10 μm.

12. The electrode structure for a lithium ion battery of claim 11, wherein the polymer microspheres have an average size of 1 to 3 μm.

13. The electrode structure for a lithium ion battery of claim 1, wherein a diameter of the nanofibers is from 100 to 300 nm.

14. The electrode structure for a lithium ion battery of claim 1, wherein a ratio of the diameter of the polymer microspheres to the diameter of the nanofibers is 30.

15. A lithium ion battery comprising the electrode structure of claim 1 comprising at least an anode, a cathode, a separator and at least one three-dimensional nanofiber-microspheres-incorporated, melt-convertible encapsulation layer being applied on at least one surface of each of the anode and cathode in the absence of an order that the anode must be followed by an encapsulation layer, the separator, and then another encapsulation layer followed by the cathode, or vice versa.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Embodiments of the present invention are described in more detail hereinafter with reference to the drawings.

[0024] FIG. 1A and FIG. 1B illustrate a melt-convertible encapsulation layer including nanofibers and polymer microspheres coating on at least one surface of the electrode.

[0025] FIGS. 2A to 2D shows the polymer microspheres embedding in the network of the nanofibers.

[0026] FIG. 3 illustrates the coating of the melt-convertible encapsulation layer on both surfaces of the electrode according to one embodiment of the present invention.

[0027] FIG. 4 shows the formation of the dielectric coating on the electrode after a thermal treatment at 100° C.

[0028] FIG. 5 shows the result of short circuit test (curve of Temperature-Time). The thickness of the melt-convertible encapsulation layer is approximately 35.8 μm for B79A and 35.4 μm for B79B, respectively.

[0029] FIG. 6 are SEM images showing the formation of the dielectric coating on the electrode after short circuit test.

[0030] FIG. 7 shows the result of nail penetration test (change of Temperature against Time). The thickness of the melt-convertible encapsulation layer for B79C is approximately 36.9 μm while that for B79D is 36.5 μm.

[0031] FIG. 8A is an SEM image showing the formation of the dielectric coating on the electrode close to the nail penetration area.

[0032] FIG. 8B is an SEM image showing no dielectric coating formation on the electrode away from the penetration area after the test.

DEFINITION

[0033] References in the specification to “one embodiment”, “an embodiment”, “an example embodiment”, etc., indicate that the embodiment described can include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

[0034] The terms “a” or “an” are used to include one or more than one and the term “or” is used to refer to a nonexclusive “or” unless otherwise indicated. In addition, it is to be understood that the phraseology or terminology employed herein, and not otherwise defined, is for the purpose of description only and not of limitation. Furthermore, all publications, patents, and patent documents referred to in this document are incorporated by reference herein in their entirety, as though individually incorporated by reference. In the event of inconsistent usages between this document and those documents so incorporated by reference, the usage in the incorporated reference should be considered supplementary to that of this document; for irreconcilable inconsistencies, the usage in this document controls.

[0035] In the methods of preparation described herein, the steps can be carried out in any order without departing from the principles of the invention, except when a temporal or operational sequence is explicitly recited. Recitation in a claim to the effect that first a step is performed, and then several other steps are subsequently performed, shall be taken to mean that the first step is performed before any of the other steps, but the other steps can be performed in any suitable sequence, unless a sequence is further recited within the other steps. For example, claim elements that recite “Step A, Step B, Step C, Step D, and Step E” shall be construed to mean step A is carried out first, step E is carried out last, and steps B, C, and D can be carried out in any sequence between steps A and E, and that the sequence still falls within the literal scope of the claimed process. A given step or sub-set of steps can also be repeated. Furthermore, specified steps can be carried out concurrently unless explicit claim language recites that they be carried out separately. For example, a claimed step of doing X and a claimed step of doing Y can be conducted simultaneously within a single operation, and the resulting process will fall within the literal scope of the claimed process.

DETAILED DESCRIPTION

[0036] The present invention provides an electrode structure for a lithium ion battery. The electrode structure includes an electrode or/and an anode, and a melt-convertible encapsulation layer. The encapsulation layer includes a network of nanofibers and polymer microspheres. Advantageously, the polymer microspheres are embedded in the nanofibrous network such that the polymer microspheres melt to form a dielectric coating of the electrode so as to prevent fire or thermal runaway at a temperature approximately from 80 to 300° C.

[0037] As shown in FIGS. 1A and 1B, the melt-convertible encapsulation layer includes a network of nanofibers and polymer microspheres coated uniformly on at least one side of the electrode. Furthermore, the polymer microsphere is embedded in the nanofibrous network shown in FIG. 2A to 2D. The diameter of the nanofibers in the network is approximately from 100 to 500 nm, and the size of the polymer microspheres is approximately from 1 to 10 μm. Preferably, the nanofibers in the network may have the diameter in the range of about 100 to 300 nm and the polymer microspheres may have the size in the range of about 1 to 3 μm. With respect to the diameter of the nanofibers, the ratio of the diameter of the polymer microspheres to the diameter of the nanofiber is over 30. Preferably, the ratio of the diameter of the polymer microspheres to the diameter of the nanofiber is 50. Furthermore, the thickness of the melt-convertible encapsulation layer is approximately from 10 to 50 μm. Preferably, the melt-convertible encapsulation layer may have the thickness in the range of about 10 to 30 μm.

[0038] The melt-convertible encapsulation layer is prepared by dispersing the polymer microspheres in the polymer solution. Then, the polymer solution with the polymer microsphere dispersion is electrically charged and forming a so-called “Taylor cone” such that the polymer solution begins to be drawn out from the tip of the needle to the collector where the electrode is positioned as shown in FIG. 3. Methods for fabricating this encapsulation layer include, but not limited to, electrospinning, electro spraying and blow spinning, or any combination thereof, but preferably one or both of electrospinning and blowspinning. The solvents to prepare the polymer solution include, for example, but not limited to Dimethylformamide (DMF), Dimethylacetamide (DMAc) and Acetone, etc. The polymer microspheres may be dispersed at a concentration of approximately 5 to 30% of the polymer solution. To achieve a more evenly dispersed polymer microsphere-containing, melt-convertible encapsulation layer, one or both of the electrospinning and blowspinning solutions contain polymer microspheres such that they are both chemically and physically associated with the polymer nanofibrous network of the melt-convertible encapsulation layer in order to have the polymer microspheres embedded into and coated on the polymer nanofibers.

[0039] The electrode of the present invention is either or both of a cathode and an anode, wherein the cathode includes a lithium-based material selected from the group consisting of lithium manganese oxide (LMO), lithium cobalt oxide (LCO), lithium nickel Manganese cobalt oxide (NMC), and lithium iron phosphate (LFP); the anode includes a material selected from the group consisting of carbon black, carbon nanotubes, graphene, and graphite. The nanofibers in the network is made of one or more polymers of polyvinylidene fluoride (PVDF), poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), polyimide (PI), polyethylene (PE), and polypropylene (PP). In addition, the polymer microspheres is made of one or more polymers of polyvinylidene fluoride (PVDF), Poly(vinylidene fluoride-co-hexafluoropropylene)(PVDF-HFP), polyimide (PI), polyethylene (PE), and polypropylene (PP).

[0040] As shown in the present invention, the nanofibrous network in the melt-convertible encapsulation layer has high porosity in a range of approximately 50% to 90% such that the pores formed by the nanofibrous network will assist the absorption of the ion conductive electrolyte, resulting in high ionic conductivity. Meanwhile, the melt-convertible encapsulation layer also has a high surface area-to-volume ratio, such as those mentioned in U.S. Pat. No. 9,711,774 which is incorporated herein by reference in its entirety, so as to enhance the loading and improve the impedance of the battery.

[0041] As shown in FIG. 4, the polymer microspheres of the melt-convertible encapsulation layer are not involved in the charge-discharge process of the lithium ion battery, and will melt to form a dielectric coating of the electrode so as to prevent fire or thermal runaway at a temperature approximately from 80 to 300° C. Unlike the commercial shutdown separator which is damaged or shrunk when electrode is shortened, the ionic conduction is completely blocked by the in situ formation of the dielectric coating formed by the molten polymer microspheres in the present invention. The dielectric coating is able to prevent lithium ion transportation between electrodes, resulting in shutdown of the battery.

[0042] The temperature of the dielectric coating formation in the present invention mainly depends on the properties such as the melting point and size of the polymer microspheres. With different combinations of the polymer microspheres, it is able to form different dielectric coatings at different temperatures corresponding to different specific lithium ion battery designs. In some embodiments of the present invention where the polymer microsphere is made of polyethylene (PE), the temperature for dielectric coating formation is approximately at 90° C.

EXAMPLES

[0043] A polymer solution with polymer microspheres was prepared by dissolving the polymer PVDF materials (for fiber formation) into the organic solvent, heat at 90° C. until all the polymer had been dissolved to form a clear yellow polymer solution. The solution was allowed to cool down to room temperature and the PE microspheres (for polymer microspheres) was added and dispersed into the as prepared polymer solution. The resulting solution was allowed to sonicate at 40-50° C. for 3-5 hours until a uniform suspension of polymer microspheres in polymer solution was formed.

[0044] The as prepared polymer solution-particles suspension will be transferred into electrospinning machine for fiber formation which the polymer solution will be transformed into fiber under high voltage condition while polymer particles will be suspended around the fibers network. Lithium ion battery electrodes (either anode or cathode) will be used as the fiber collector which fibers and particles will be formed both side of electrode surface (FIG. 1). FIG. 3 depicts the schematic diagram on electro spinning.

[0045] Referring to FIG. 5, the electrodes are coated with the melt-convertible encapsulation layer having the thickness about 35.8 μm (B79A) and 35.4 μm (B79B), respectively. The Tmax for B79A was about 117.3° C. at approximately 0 to 50 seconds and the Tmax for B79B was about 127.6° C. at approximately 50 to 80 seconds after the short circuit test. However, the batteries without the melt-convertible encapsulation layer coating experienced dramatic temperature rise after short circuit test, with the Tmax of C1 and C2 up to 533.4° C. and 731.4° C., respectively. The SEM images in FIG. 6 show a dielectric coating formed from the molten polymer microspheres encapsulating the electrode after short circuit test. These results suggest that the internal short-cutting is efficiently inhibited due to the melt-convertible encapsulation layer, thereby achieving a significant improvement in safety performance of the batteries.

[0046] Referring to FIG. 7, the electrodes were coated with the melt-convertible encapsulation layer having a thickness of about 36.9 μm (B79C) and 36.5 μm (B79D), respectively. The Tmax for B79C was about 77.9° C. at approximately 100 to 200 seconds and the Tmax for B79D was about 106.5° C. at approximately 60 to 80 seconds after the nail penetration test. However, the batteries without the melt-convertible encapsulation layer coating experience dramatical temperature rise after penetrations, with the Tmax going up to 664.4° C. The SEM images in FIG. 8A show a dielectric coating formed from the molten polymer microspheres encapsulating the electrode close to the nail penetration area after the test. On the contrary, the structure is intact in the area away from nail penetration test shown in FIG. 8B.

[0047] It will be appreciated by those skilled in the art, in view of these teachings, that alternative embodiments may be implemented without deviating from the spirit or scope of the invention, as set forth in the appended claims. This invention is to be limited only by the following claims, which include all such embodiments and modifications when viewed in conjunction with the above specification and accompanying drawings.