JP2002134092A - Polymer meso porous separator element for laminated lithium ion battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a simple, economical method for manufacturing a battery separator element showing a high ionic conductivity and circulation stability under a long period of storage and usage of the battery. SOLUTION: A manufacturing method of a meso porous polymer film employed as an ionic conductive interelectrode separator for a battery cell, comprising the steps of: (a) preparing a polymer material, a volatile fluid solvent for the polymer material, and a compound containing a second fluid with the volatility lower than that of the solvent, the compound being able to be mixed with the solvent and be coated; (b) casting the compound so that layers are formed; (c) volatilizing the fluid from the layers under the condition that the solvent volatilizes actually faster than the above non-solvent; (d) keeping the solvent volatilized up to the completion of the gellation of the above polymer material parent; and (e) continuing the volatilization of the above non-solvent from the above droplet until the process is complete, so that meso porous voids are equally dispersed among the above film parent.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

FIELD OF THE INVENTION The present invention relates to a polymer film composition electrode and separator element, and a metal foil or mesh current collector, typically laminated under heat and pressure to form a single battery cell structure. The present invention relates to a rechargeable electrolytic battery cell. In particular, the present invention is extremely porous, and therefore can absorb and retain a significant amount of electrolytic solution even after high temperature lamination processing, thereby increasing the storage and use of batteries over a wide range of temperatures and for extended periods of time. Provided is a simple and economical method of producing a separator element film or membrane that can provide ionic conductivity and circulation stability.

[0002]

BACKGROUND OF THE INVENTION Electrolytic battery cells which are particularly preferably used in the present invention include Li-ion insertion cells of the type described in US Pat. No. 5,460,904, which is preferably disclosed in US Pat. No. 5,418,091. It includes a separator element as described and the disclosures of these patents are hereby incorporated by reference. Such cells are composed of positive and negative electrode elements, respectively, containing finely divided active materials such as a lithium-source LiMn ₂ O ₄ and carbon dispersed in a polymer matrix, and formed into a flexible layer or membrane. It is formed. These elements are laminated with an electrically insulating separator membrane between them, which usually contains a similar polymeric material, which is ultimately an organic material in which the lithium salt is uniformly dispersed. It contains a solution, acts as an ionic conductive bridge of electrolyte between the electrodes, and reversibly flows from the active material of these electrodes to the active material through the charge and discharge cycles of the battery cell.
Enables i-ion insertion. Ultimately, to facilitate the attendant flow of electrons in the battery cell circuit, each of the positive and negative electrode elements has a collector element, which is used for conducting wire fasteners that are directed to the application during use. Work as a terminal base.

In the fabrication and use of these conventional battery cells, a separator membrane element composition containing a polymer-compatible plasticizer compound is employed, the plasticizer being prepared for activation by the addition of an electrolyte solution. As part of the final cell separator component. Not only these polymer separator compositions but also similar polymer battery electrode compositions are generally prepared by extraction using a solvent inert to the polymer to remove the added compatible plasticizer. It was particularly unique in preconditioning the matrix for early absorption of the active electrolyte without imparting porosity therein.

[0004] On the other hand, conventional polymer battery cell separator elements that rely on the porosity of the matrix to promote electrolyte absorption are described in US Patent 3,351,495. However, the preparation of such separator elements relies on a similar solvent extraction procedure, in which a so-called "plasticizer" component, which is incompatible with the added inert filler particles, is used to obtain porosity. Physically removed from the solidified composition.

[0005]

In accordance with the present invention, there is provided a battery separator membrane element which has a degree of mesoporosity which can result in improved electrolyte solution absorption and which results in a high mesoporosity. Has ionic conductivity,
The preparation is more time-consuming and does not depend on expensive extraction operations. Also, the method of preparing the battery cell element of the present invention includes a simple element film coating method or a casting method, which forms the desired porosity by differential evaporation of the coating composition fluid vehicle component.

[0006]

SUMMARY OF THE INVENTION The present invention relates to a laminated Li
Provided is a method for preparing a mesoporous polymer matrix layer or membrane that works uniformly and well as a separator and an electrode element of an ion insertion battery cell. Such membranes are prepared by a simple coating or film forming method utilizing a polymer composition comprising a combination of a polymer solvent component and a non-solvent coated vehicle component, the method comprising the steps of: The non-volatile non-solvent is finely dispersed in the polymer matrix which solidifies through the initial solvent evaporation. After the polymer matrix gels or solidifies as a result of the evaporation of the solvent component, the non-solvent component eventually evaporates and diffuses from the polymer matrix to produce a mesoporous matrix structure, It readily absorbs the electrolyte solution to provide a high level of ionic conductivity in the battery cells.

[0007] In addition to the mixture of the matrix polymer and the coating vehicle, the composition preferably contains a finely divided inert filler such as silica, the particles of which are initially uniformly dispersed in the coating layer. Partially migrate and aggregate as lumps in the dispersed non-solvent vehicle droplets through gelation of the matrix film. Thus, the inert filler not only provides structural strength to the final layer due to its uniform dispersion, but also provides a support that maintains the open structure of the mesopores after diffusion of the non-solvent vehicle, and further later provides the separator with the separator. And an electrode element to provide another support through compression of a thermal lamination operation to form a single battery cell structure.

The complementary electrode coating composition comprises, in addition to a matrix polymer and a mixture of a solvent and a non-solvent, powdered carbon and an intercalating compound;
For example, each contains an electrolytically active component such as LiMn ₂ O ₄ spinel. These active ingredients are dispersed in the coating composition in finely divided form, but they significantly impede the formation of mesopores, since the size of these particles is usually much larger than the mesopores. do not do.

After the formation of the mesoporous separator and the electrode elements is completed, a metal grid or foil collector is laminated on each electrode element by heat, and then an intervening separator element is laminated on these electrode subassemblies. This is described in the above-mentioned patent specification. The fabrication of the battery cell is completed by activation, absorption of the electrolyte solution into the mesoporous membrane, and final packaging, as is customary.

[0010]

DETAILED DESCRIPTION OF THE INVENTION The present invention essentially comprises a method of making a mesoporous polymer separator and electrode element of a rechargeable electrolytic battery cell without resorting to costly and time consuming solvent extraction steps. The invention further comprises the separator and the electrode element obtained, in addition to a battery cell assembled by a thermally laminated assembly of these separator and electrode element and the conductive collector element. The methods used to make such assemblies and battery cells are described in large part in U.S. Patents 5,470,357 and 5,540,741 and related patents.

A typical battery cell assembly 10 is shown in FIG. 1, which includes a positive membrane element 13 and a negative membrane element 17,
Between these is located a separator element 15 prepared according to the invention, into which the lithium salt electrolyte solution is finally absorbed. The electrodes 13 and 17 are each made of a lithiated insertion compound such as LiM.
n ₂ O ₄ and supplements that can reversibly insert lithium ions, such as carbon in the form of petroleum coke or graphite. The conductive collectors 11, 19, which are preferably made of aluminum and copper or nickel, have respective electrode elements 1
3, 17, preferably pre-laminated to form sub-assemblies, and these sub-assemblies are laminated by heat and adhere to an intermediate separator element 15, A single battery cell 10 is formed. At least one, and preferably two, current collecting elements, such as in the form of a perforated foil or expanded metal grid, to facilitate subsequent processing of the cell, particularly to incorporate the electrolyte solution, It is.

The polymeric materials utilized in making the separators and similar electrode elements of the present invention include, for example, polymers and copolymers of vinyl chloride, acrylonitrile, vinyl acetate, vinyl fluoride, vinylidene chloride, and vinylidene fluoride. Polymers and copolymers of acrylonitrile and vinyl chloride or vinylidene chloride; polymers and copolymers of vinylidene fluoride and tetrafluoroethylene; and the weight of trifluoroethylene, chlorotrifluoroethylene, or hexafluoropropylene. About 75-97% by weight of vinylidene fluoride (V
Copolymers of dF) and 3-25% by weight of hexafluoropropylene (HFP) are particularly preferred, the proportion of VdF comonomer generally varying with its molecular weight.

Although not required for the preparation and processing of the separator and electrode elements of the present invention, the polymer composition is preferably dibutyl phthalate, such as dibutyl phthalate, to optimize flexibility and thermal aggregation through lamination. It may further contain a suitable amount of a compatible organic plasticizer. The positive and negative electrode compositions, usually in finely divided dispersion form, further comprise an electrolytically active component, which provides lithium ions substantially throughout the final battery discharge / charge cycle, and It is a substance that can be inserted.

The method of the present invention is a unique technique that differs from conventional battery cell manufacturing techniques for preparing coating compositions used to cast separator and electrode membranes. While conventional methods require the preparation of a uniform coating solution containing a host polymer and a coating vehicle solvent and, if necessary, a plasticizer, the coating composition employed in the present invention comprises a primary coating solution such as acetone or tetrahydrofuran. In addition to the polymer solvent of the present invention, a liquid coating vehicle component such as a lower alcohol, such as a lower alcohol, which is substantially non-solvent for the polymer, but is miscible with the first solvent to a considerable extent. Such a mixed soluble coating vehicle can be cast first with a clear, homogeneous polymer solution layer, and the non-solvent in the coating layer is concentrated by evaporating most of the first solvent. Then, gelling or coalescence of the polymer occurs locally in the layer, surrounding many small droplets of non-solvent components. Due to the low volatility of the non-solvent, uniform droplet dispersion is maintained until the subsequent evaporation of the non-solvent component causes the polymer film to sufficiently gel and maintain a mesoporous structure.

In a preferred embodiment of the present invention, the coating composition contains a significant amount of inert particulate filler, such as silica or alumina, which is initially homogeneously dispersed in the cast layer. This filler component substantially contributes to the physical strength of the final film, as it did in prior art compositions. However, when utilized in the present invention, the particulate filler adds a unique function. That is, through evaporation of the first solvent vehicle and thus coalescence of the polymer film matrix, the filler particles tend to accumulate in larger non-solvent droplets as the evaporation of the vehicle proceeds, and Larger dimensions eventually agglomerate within the initially occupied gaps, so that the final mesoporosity of the separator membrane is maintained.

This superior function of the particulate filler combined with the specific dissolution imbalance of the coating composition will be illustrated in FIGS. In FIG. 2, portion 15 of the separator membrane coating layer includes a continuous matrix 22 of polymer solution gelled by evaporating the first solvent to an extent sufficient to separate non-solvent droplets 26. Filler particles 24,
25 is the same as when the homogeneous composition was first cast and formed over the entire layer, ie, the polymer matrix 22 and the non-solvent droplets 2.
6 are dispersed in substantially the same homogeneous dispersion state.
When the gel is further formed by the evaporation of the first solvent, the viscosity of the polymer matrix 22 increases to fix the dispersion of the filler particles 24 as shown in FIG. Particles 25 within 36 are free to clump as the non-solvent evaporates. Further agglomeration may occur until the polymer matrix solidifies and the dimensions of the gap 36 can be sufficiently maintained.

It should be noted that the depictions of FIGS. 2 and 3 are shown in a very uncoordinated state for ease of understanding. The relative distribution of the particulate filler, such as fumed silica, inside the gap caused by the dispersion of the non-solvent in the polymer matrix is such that the actual size of the silica particles is about 10 nm, and the micrograph shows the diameter of the gap. May be drawn more closely, taking into account that is about 1 μm.

Another depiction of the gaps and particulate filler seen in FIG. 4 is that the battery cells are made by lamination using heat and pressure that easily collapses the gaps in the membrane and greatly limits the critical absorption of the electrolyte. In order to achieve the greatest role of supporting the interstitial structure of the separator membrane in an extreme state, it is intended to arrange a mass of filler particles. As indicated by the maximum compression, the agglomerated particles maintain an open network that provides the sustained mesoporosity of the separator 15. on the other hand,
Intact mesopores in the electrode films 13, 17 are largely maintained by the large sized particles in these compositions.

Although the present invention can utilize the battery electrodes and electrolyte components described in the above-mentioned patent specification and related publications,
For simplicity, the present invention will be described with reference to some representative compositions included in the following examples.

[0020]

EXAMPLE 1 A coating composition for a preferred embodiment of a separator membrane 15 was prepared according to the present invention at about 380 × 10 ³ MW (Atochem Kynar).
FLEX 2801) with 3.0 grams of 88:12 VdF:
It was prepared by dissolving the HFP copolymer in about 20 milliliters of acetone. About 10 milliliters of ethanol and 2.0 grams of fumed silica (SiO ₂ ), in which the copolymer was substantially insoluble, were stirred into this solution to form a clear, smooth, viscous composition. .
The composition was formed into a wet thickness of about 250 μm on the surface of an anti-adhesive polyethylene terephthalate film and dried by evaporating the vehicle components of the coating in moderately circulating air at about 30 ° C. The obtained separator membrane having a thickness of about 50 μm was self-supported when peeled from the molding substrate, and showed a homogeneous state. As a result of inspection of the micrograph, as shown in FIG. 2, circular voids and silica particles were evenly distributed through the polymer matrix in the film, and the density of the particles within the voids was reduced by the weight. Greater than the density of the particles in the combined matrix.

Example 2 A comparative example of the above general embodiment of the present invention was implemented. That is, the mesoporous membrane in which the inert filler is not dispersed,
The coating composition was prepared using the method of Example 1 except that the silica component was not added. The coating composition was molded to a thickness of about 350 μm to compensate for the increased flow of the composition and to obtain the same film thickness. The obtained separator membrane showed a limit physical strength, and as a result of microscopic inspection, showed the same mesoporous structure as in Example 1.
No inert dispersed particles were present.

Example 3 Another membrane sample was prepared by the method of Example 1 except that the non-solvent isopropanol was used instead of ethanol. The obtained film was visually indistinguishable from the film of Example 1.

Example 4 Another membrane sample was prepared according to the method of Example 1, except that the non-solvent methanol was used instead of ethanol. The obtained film was visually indistinguishable from the film of Example 1.

Example 5 A comparative membrane sample was prepared from the components of Example 1 except that about 8 milliliters of acetone was used in place of ethanol, thereby providing a coating vehicle consisting solely of a polymer solvent. I got This composition was molded in the same manner and dried to form a film having a macro appearance similar to that of Example 1. Examination of the micrographs revealed that the membrane consisted solely of silica particles evenly dispersed throughout the polymer matrix without the presence of a mesoporous structure.

Example 6 To test the effectiveness of the present invention, a comparative prior art separator membrane sample was prepared substantially as described above for 5,418,0.
Prepared by the method of Example 13 of the '91 patent by molding a FLEX 2801 VdF: HFP copolymer composition containing about 20% SiO ₂ and 25% dibutyl phthalate plasticizer dispersed in a solvent. Following the prior art methods described above, the resulting flexible separator membrane was extracted with diethyl ether to remove the plasticizer component, thereby conditioning the membrane for electrolyte absorption. Testing of the extracted membranes revealed that the silica particles were dispersed in the polymer matrix, but voids were not recognizable, as described in the referenced patent.

Example 7 Four sets of samples of the separator membranes of the present invention and the conventional example prepared according to Examples 1 and 6, respectively, were compared with each other in terms of electrolyte absorption capacity. To this end, the samples were weighed and then immersed for a few minutes in propylene carbonate (PC), a lithium salt solvent typically used as a vehicle for battery cell activating electrolyte solutions. After immersion, the PC accumulated on the surface of these samples was cleaned and reweighed to determine PC uptake. A graph comparing the absorption by these sets of samples is shown in FIG.
Which indicates that the simulated electrolyte absorption of the mesoporous membranes of the present invention is generally large.

Example 8 Simulated electrolyte absorption after exposure to various temperature conditions was tested on each sample of the inventive and comparative separator membranes of Examples 1-5. The above temperature condition is easily encountered through thermal lamination of the separator element and the electrode element in the production of the battery cell. It is expected that porosity and electrolyte absorption capacity will decrease as such lamination temperature increases. After exposure to lamination pressure and lamination temperatures of 100 ° C. and 170 ° C., the samples were tested for PC absorption by the method of Example 7. The results of comparative absorption of the films of each example are shown in the graph of FIG. Of particular note in these test results is the high level of electrolyte absorption initially exhibited by the mesoporous membranes of the present invention, as well as those of Examples 1, 3 and 4, which include particulate fillers that support the structure. The absorption of the preferred embodiment is kept fairly high. The susceptibility of unfilled membranes to mesopore collapse as shown in Example 2 is apparent from these results at temperatures above the melting point of the parent polymer. These films with very good initial absorption would, of course, be useful for low temperature or non-laminated battery cell applications. As further evident,
The non-porous structure of the comparative membrane of Example 5 is hardly affected by temperature, but shows minimal electrolyte absorption under any conditions.

Example 9 The highest effectiveness of mesoporous separators and electrode membranes prepared according to the present invention was determined using the above patents of 5,460,90.
The test was performed using the comparative laminated battery cell manufactured by the method of FIG. A prior art cell from which the plasticizer was extracted was prepared as in Example 15 of the patent, and a cell of the present invention was prepared as follows. As shown in FIG.
5 grams of FLEX2801VdF: HFP copolymer,
1 gram of SP conductive carbon, 5 grams of LiMn ₂ O ₄ ,
A positive electrode film 13 of about 375 μm was prepared from a composition consisting of 40 ml of acetone and 15 ml of ethanol.
It was cast to a thickness of m. The resulting mesoporous membrane was laminated to an expanded aluminum foil collector element 11 at about 150 ° C. in a heated roller device. 2.
5 grams of FLEX2801VdF: HFP copolymer,
Anode film 17 was prepared from a composition comprising 1 gram of SP conductive carbon, 5 grams of microbead carbon, 40 milliliters of acetone, and 15 milliliters of ethanol by about 4
It was cast to a thickness of 50 μm and molded. The resulting mesoporous membrane was laminated to an expanded copper foil collector element 19 at about 150 ° C. in a heated roller device. Mesoporous separator element 1 made according to Example 1 above
5 are placed between and in contact with the electrode elements 13, 17 of the subassembly, and the assembly is laminated in an apparatus at about 120 ° C. to form a single battery cell 10 and then The battery cell is E as described in the above referenced patent.
C: activated by immersion in PC: LiPF ₆ electrolyte solution.

The prior art battery cells and battery cells assembled with elements made in accordance with the present invention were prepared at a C / 5 cycle rate by the method described in Example 13 of 5,460,904 of the referenced patent. Circulation was carried out with a constant 10 mA between 0 V and 4.5 V cutoff voltage. Both cells operated similarly in the manner shown in FIG. 2 of the referenced patent, and exhibited discharge performance in the range of about 20-25 mAh.

The above embodiments are intended to be illustrative only and not limiting of the present invention. Other polymers and solvent / non-solvent combinations that produce the same result will be understood by those skilled in the art in light of the foregoing description based on the performance of certain experiments without departing from the scope of the invention as set forth in the appended claims. Will be achieved by

After the formation of the mesoporous separator and the electrode element is completed, a metal grid or foil collector is laminated on each electrode element by heat, and then an intervening separator element is laminated on these electrode subassemblies. This is described in the above-mentioned patent specification. The fabrication of the battery cell is completed by activation, absorption of the electrolyte solution into the mesoporous membrane, and final packaging, as is customary.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a composite Li-ion battery cell including a separator element and an electrode element of the present invention.

FIG. 2 is a cross-sectional view of a portion of a separator membrane of the present invention showing the relative dispersion of non-solvent vehicle droplets and inert filler particles during early stage gelation of the membrane polymer.

FIG. 3 is a cross-sectional view of a portion of a separator membrane of the present invention showing the relative dispersion of non-solvent vehicle droplets and inert filler particles during the final gelation of the membrane polymer.

FIG. 4 is a cross-sectional view of a portion of a separator membrane of the present invention showing the relative dispersion of non-solvent-replaced mesoporous voids and inert filler particles after exposure to the temperature and pressure at which the membrane polymer is laminated.

FIG. 5 is a graph comparing electrolyte absorption in separator membrane samples of similar polymers made according to the present invention and the prior art.

FIG. 6 is a graph comparing electrolyte absorption in mesoporous separator membranes prepared from various compositions and exposed to moderate and high lamination temperatures.

 ──────────────────────────────────────────────────続 き Continuing on the front page F term (reference) 4F074 AA38A CB31 CB32 CB37 CB47 CC07Y CC22X DA24 DA49 4J002 BD031 BD101 BD121 BD131 BD141 BD161 BF021 BG101 DE146 DJ016 FD016 GQ00 5H021 BB04 BB12 CC04 EE10 EE15H01 A03 EE15H01 AL07 AM03 AM05 AM07 BJ04 BJ12 CJ06 HJ01 HJ08

Claims (10)

[Claims] 1. A method for producing a mesoporous polymer membrane used as a separator between ionic conductive electrodes of a storage battery cell, comprising: a) a polymer material, a volatile fluid solvent for the polymer material, and Preparing a coatable composition that includes a second fluid that is miscible with the solvent and that is less volatile than the solvent, wherein the second fluid exhibits significant solvency for the polymeric material. B) casting the composition to form a layer; and c) volatilizing the fluid from the layer under conditions in which the solvent volatilizes at a substantially faster rate than the non-solvent. Thereby causing gelling of the polymeric material in the non-solvent predominant regions of the layer, and isolating the non-solvent as substantially evenly dispersed droplets in the matrix of the polymeric material; d) The gelation of the matrix of the polymer substance is completed. And e) allowing the non-solvent to evaporate from the droplets until the next completion to equalize the mesoporous voids in the film matrix. The above method, comprising dispersing. 2. A uniform distribution of filler particles substantially inert to the remaining components of the composition in the coatable composition, such that the particles are in the gaps and in the film matrix. 2. The method of claim 1, further comprising dispensing therein. 3. The mesoporous separator membrane is contacted with a fluid electrolyte for a time sufficient to distribute the electrolyte into the gap by absorption into at least a portion of the gap, thereby causing the membrane to ionize. 3. The method of claim 2, further comprising making it conductive. 4. A mesoporous polymer membrane used as a separator between ion-conductive electrodes of a storage battery cell, wherein the membrane has a plurality of gaps distributed therein and a number of inert filler particles. The polymer film according to claim 1, further comprising a united matrix, wherein at least a part of the particles is located inside the gap. 5. The membrane according to claim 4, wherein said particles are distributed inside said gap with a higher spatial density than inside said matrix. 6. The membrane of claim 4, wherein said polymer matrix comprises a copolymer of vinylidene fluoride having 3 to 25% by weight of hexafluoropropylene. 7. A battery cell structure including a positive electrode element, a negative electrode element, and a separator membrane element provided therebetween, wherein the separator membrane has a number of gaps distributed therein and a number of inert fillers. The battery cell structure according to claim 1, further comprising a polymer matrix having agent particles, wherein at least a part of the particles is located inside the gap. 8. The storage battery structure according to claim 7, wherein the particles are distributed inside the gap with a higher spatial density than inside the matrix. 9. The storage battery structure according to claim 7, wherein said polymer matrix comprises a copolymer of vinylidene fluoride having 3 to 25% by weight of hexafluoropropylene. 10. Each of said electrode elements includes a polymer matrix, and each of said electrode elements and separator elements is bonded to an adjacent element at a respective interface to form a single flexible laminated structure. The storage battery structure according to claim 7.