DE102017007858A1 - Method for direct application of glass-based separators to battery electrodes - Google Patents

Abstract

Separator for an electrochemical cell, preferably a lithium-ion battery, comprising a porous layer containing the following components:
(A) 5 to 99.9% by weight of at least one glass flake having an average thickness of 0.05 μm to 30 μm and an average edge length of 0.1 μm to 150 μm;
(B) 0.1 to 95% by weight of at least one binder; wherein the components (A) and (B) add up to 100 wt .-% and wherein the separator is applied by means of a coating method directly on a battery electrode and a porosity of 5 to 80% and a total thickness of 0.5 .mu.m to 200 .mu.m having.

Description

Description of the technical field of the invention

The described invention is in the field of electrochemical storage separators, e.g. Lithium-ion batteries. The novelty is that on the one hand by the use of the material glass in the form of glass flakes (glass flakes) a much better temperature stability, improved mechanical stability and an optimized wetting with an electrolyte liquid of the separator is achieved. On the other hand results from the direct application of the separator to an electrode procedural simplification for cell production, since this process via a roll-to-roll method is feasible.

• • Elektrisch isolierend
• • Hochgradig permeabel für Ionen aus dem Elektrolyten
• • Hochtemperaturstabil bis mindestens 500°C ohne signifikanten Schrumpf
• • Chemisch stabil im Elektrolyten
• • Dicke vorzugsweise kleiner 200µm
• • Mechanisch stabil und flexibel
• • Gute Benetzbarkeit für organische Elektrolyten

The invention relates to the composite formation of micrometer glass flakes as separators and battery electrodes via a doctor blade, calendering or spraying process. The separator layer has the following properties:

• • Electrically insulating
• • Highly permeable to ions from the electrolyte
• • High temperature stable up to at least 500 ° C without significant shrinkage
• • Chemically stable in the electrolyte
• • Thickness preferably less than 200μm
• • Mechanically stable and flexible
• • Good wettability for organic electrolytes

According to the current state of the art, polymer-based, microporous films or fabrics are usually used as separators for lithium-ion batteries, which can consist of several layers of different polymers with different melting points [1]. However, these are mechanically stable only up to temperatures far below 200 ° C and show a significant shrinkage already from 100 ° C, so that the function of the physical separation of the electrodes is no longer guaranteed. A further development of this is the coating of these polymer-based membranes or network structures with inorganic, ceramic particles [1,2,3] to increase the temperature stability and thus safety. Here, a temperature stability of up to 250 ° C and also an improved wetting with electrolyte liquids is achieved.

However, no separator of the prior art is known which satisfies the above functions and properties in the desired manner.

A component of modern separators according to the prior art is usually a polymer. This results in a correspondingly low high-temperature strength and poor wetting. By contrast, glass-based separators are known only as fiber mats, which are very thick or as porous glass plates, which are mechanically inflexible.

By using glass in the form of glass flakes in the arrangement of a brick wall structure (1), printed on an electrode active material with current conductor (2,3) in the function as a separator is a distinction from the prior art (see Figure 1/2) , This ensures, on the one hand, good wetting with electrolytes and permeability to ions from the electrolyte, high stability at high temperatures, high mechanical stability and at the same time mechanical flexibility and stability through the use of micrometer glass flakes. In addition, with the developed method very thin layers of less than 200μm can be realized.

Technical task

Separators for electrochemical cells, especially batteries and accumulators, provide for the physical separation of the two electrodes, anode and cathode and thus prevent a short circuit of these. In addition, separators must be permeable to ions to allow cell reactions such as loading and / or unloading. Furthermore, separators must be chemically stable in the electrolyte and also mechanically stable. In addition, separators must reliably fulfill their function of physically separating the electrodes, even at high temperatures. In terms of process technology, a separator must be able to absorb a liquid electrolyte as quickly as possible in order to reduce cell construction time. Furthermore, a separator should be made thin in order to fill the available space in a battery cell or accumulator with as much active material and vice versa to achieve a high specific capacity of the cell.

• • Physische Trennung der eingesetzten Elektroden
• • Permeabel für Ionen aus dem Elektrolyten
• • Temperaturstabil über weiten Temperaturbereich
• • Mechanisch stabil
• • Chemisch stabil im Elektrolyten
• • Geringe Dicke
• • Gute Benetzbarkeit für Elektrolyten

An electrochemical storage separator, such as a primary or secondary lithium-ion battery, must meet the following requirements:

• • Physical separation of the electrodes used
• • Permeable to ions from the electrolyte
• • Temperature stable over a wide temperature range
• • Mechanically stable
• • Chemically stable in the electrolyte
• • Small thickness
• • Good wettability for electrolytes

Technical solution of the task

Glass can be processed by special manufacturing processes to thin glass slides, for example by a Rotationszerstäubungsprozess. With the respective adaptation of the process parameters glass flakes of different thicknesses can be produced. Typically, glass slides with average thicknesses in the range of 0.1-30 μm and average edge lengths of 5-200 μm can be produced here. This usually results in a specific aspect ratio (edge length / thickness ratio) in the range of 1: 5 to 1: 100.

For the production of glass flakes, the glass is first melted in a glass melting plant or a crucible. Here, depending on the glass compositions, temperatures in the range of 900-1600 ° C are usually required. For the glass compositions given, temperatures ranging from 1350 ° C (Type 1/2) to 1600 ° C (Type 3) are suitable.

Glasses are differently stable in organic media, connected with conductive salts, for example an electrolyte, for example consisting of ethylene carbonate and dimethyl carbonate as the solvent and lithium hexafluorophosphate (LiPF ₆ ) as conductive salt.

The following glass composition was found to be chemically stable in the electrolyte for the preparation of glass flakes: Table 1: Glass composition for separator glasses Type SiO ₂ B ₂ O ₃ Na ₂ O Li ₂ O Al ₂ O ₃ ZrO ₂ BaO literature 1 49.27 41.36 7.87 1:00 0.5 house development 2 49.27 41.36 7.87 1:00 0.5 house development 3 80.11 19.89 house development 4 70 23 7 [4] 5 55 10 10 25 [8th] Indication in wt .-%

In general, the glasses (type 1 to type 3) may contain the following constituents (in mole% based on oxide): SiO ₂ (0.1-75), B ₂ O ₃ (0-70), Na ₂ O (0-70) 16), Li ₂ O (0-16), Al ₂ O ₃ (0-5), ZrO ₂ (0-5), SnO ₂ (0-1), TiO ₂ (0-1), K ₂ O ( 0-1), MoO ₃ (0-1), RO (<15%), R ₂ O ₃ (<15%), refining agents in conventional amounts of up to 2%, where RO is the sum of ZnO, MgO, CaO , BaO and R ₂ O _{3 is} the sum amount of Fe ₂ O ₃ and Sb ₂ O ₃ .

For the development described glass flakes are produced as small as possible thickness. With the addition of a binder / solvent in a specific ratio results in a processable with a doctor blade, calender, - spray or dip coating process mass of defined viscosity. Binder may be polymer-based binders or organic water-based binders or organic solvents a ratio of binder to solvent of 1:10 to 1: 100 can be used. These include, for example, PVDF, celluloses, starches, alginates, guar, styrene-butadiene rubber (SBR), polyacrylic acid (PAA), as well as modifications or mixtures thereof.

The glass flake and binder suspension is applied directly to an electrode with a defined layer thickness so that it is completely covered. In this case, an alignment of the glass flakes is achieved in the horizontal plane by adjusting the processing parameters, for example in a doctor blade process, with several glass flakes overlap each. The layer thickness is limited, assuming at least 3-5 layers of glass flakes applied to an electrode in a horizontal plane on top of each other, by the average thickness of the glass flakes produced. Accordingly, there are layer thicknesses in the range of 20-200μm. The binder content in the suspension can be adjusted and is 0.5-30% by weight. The applied glass flake binder suspension is then dried, preferably under vacuum or in air, and the electrode-separator composite can be installed in an electrochemical cell.

Innovation advantage, industrial property rights

Publications on glass-based separators are available, but are clearly different from the described development.

The publication [4] describes a battery separator comprising a porous, ion-permeable glass sheet or glass-ceramic, which are produced, for example, by means of known drawing methods, in particular the down-draw method. However, no glass flakes are described here.

The publications [5, 6] describe glass separators based on glass webs, which are used for VLRA AGM batteries.

The publications [7] printed batteries describes a printed battery in which the current conductors, electrode materials and separators are processed by means of dispersions. Here, however, not explicitly specified that glass flakes are used as separators, but only electrically non-conductive fibers and / or particles are used.

Furthermore, there are publications which, however, are limited to the description of polymer-based separators or ceramic-particle-reinforced polymer-based separators [1-3,9].

The publication [8] describes a glass which can be used in Li-ion batteries to improve high rate capability. Although methods such as printing the glasses on electrodes are protected, it is specified that the glass is used in the form of particles (no flakes) and is at best processed on a polymer or together with a polymer to form a self-supporting separator. Furthermore, it is described that the glass should be as far as possible Na ₂ O-free. The main application of the glass should be an electrode additive.

Publication [10] describes a separator applied to an electrode, which is made of special Li-ion conductive ceramics or glasses. A specification of the shape, in the case of the glasses mentioned, is not given. Furthermore, the indicated glasses differ significantly from the glass composition described herein.

The publication [11] describes a glass-based separator consisting of glass fibers and glass plates. However, this is a self-supporting separator, which is not printed directly on an electrode. Furthermore, the given composition of the glass shows significant differences from the invention described here. In addition, this publication results in significant differences in the specified porosity and the degree of filling of glass flakes in the separator.

embodiment

Glass flake production

A glass of defined composition (type 1, see Table 1) is melted in a glass melting plant or in a crucible at temperatures of 1350 ° C. In this case, a heating rate of 10 ° C / min and a hold time of 2h at 1350 ° C was selected. The molten glass of defined viscosity is poured into a rotating at 10,000 rpm and pre-heated at 800 ° C atomizer unit of a rotary atomizer. Due to the centrifugal force, the glass melt is ejected over the edge of the atomizer unit in the form of a film (lamella), which is further stretched by the prevailing forces until the glass breaks by cooling. About a suction with intermediate cyclone, the glass semi-finished products are discharged from the rotary atomizer. The product is glass flakes (glass flakes).

Suspension production

Size-classified glass flakes are blended together with a binder solution, the mean edge length of the glass flakes used being less than 32 μm and the mean thickness being less than 7 μm. In this case, an already prepared binder (modified potato starch) in an aqueous solvent in a ratio of 1:10 is used. The mixing ratios are adjusted so that a suitable processability via a doctor blade, calender, spray or dip coating method results. The components are mixed via a rotary mixer for 10 minutes at a rotational speed of 1500 rpm.

Production of the electrode-separator composite

The glassflake binder suspension is added, e.g. via a doctor blade method, applied directly to the electrode with a defined layer thickness. An automatic feed rate of 2.5mm / s was selected and the height of the squeegee adjusted to the desired layer thickness plus the thickness of the electrode. In the present example the thickness of the electrode (active material incl. Current conductor) is 120μm. Thus, the squeegee was set to a height of 170μm to obtain a separator layer thickness of 50μm. After the doctoring process, the electrode printed with a separator layer is preferably dried under vacuum.

Electrochemical measurements

Electrochemical measurements were carried out using a measuring cell constructed from swagelok parts. Cell construction takes place under an inert gas atmosphere in a glove box. Here, the printed with the separator layer electrode is first placed in the empty measuring cell and wetted with electrolyte. The cell assembly is completed with a corresponding counter electrode and optionally a metal plate and a metal spring, before the cell is sealed gas-tight. The active materials used were graphite as the anode material and lithium iron phosphate (LFP) as the cathode material.

Impedance analyzes as well as loading and unloading were carried out and it turned out that cells equipped with the produced separators have excellent electrochemical properties.

The internal resistance of a test cell equipped with a glass-based separator printed electrode shows values that are 20-40% lower compared to test cells with polymer-based separators according to the prior art. This is due to pore diameters measured in the range of 0.1-5μm with the majority of the pores having an average pore diameter of 3μm and the concomitant optimized diffusion of ions through the separator. The porosity values and the values for the average pore diameters were determined by measurements by means of mercury porosimetry.

In cyclic load tests of test cells with glass-based printed separators, it was found that, on the one hand, high cycle stability can be achieved (Coulomb efficiency> 99.9% at 1 C current load, discharge depth 100%) and, on the other hand, the test cells are also exposed to very high currents can be, without the separator is a significant limiting factor. At a current load of 10C (complete charging and discharging of the battery cell within 6 minutes at 100% discharge depth), 40% of the nominal capacity could still be charged and discharged. This shows that the development described can also be used for batteries in fast charging mode.

Thermal stability / wetting behavior

To investigate the thermal stability, self-supporting glass-based separators were produced in a glass mat in the function of a support matrix and heated to 600 ° C. (hot-oven test). No significant shrinkage can be detected (<2% at 600 ° C). In addition, the mechanical integrity is largely retained. Furthermore, the glass-based separator described in the so-called hot-punch test do not pierce with a hot mandrel (2mm diameter, 200 ° C) at a pressure of up to 1.5kg / mm ² (hot-punch test).

The wetting behavior was analyzed by means of a contact angle measurement. Here, a contact angle of less than 15 ° was found. It is additionally noted that one drop (10 .mu.l) of the electrolyte used for the measurement is absorbed instantaneously (<50 ms) completely into the separator layer.

literature sources

• [1] property rights WO 2014047126A1 , Thin battery separators and methods (2014-03-27), Celgard, LLC
• [2] property rights DE 102011003186 A1 , Thin, macroporous polymer films (2012-06-26). Evonik Degussa GmbH
• [3] property rights DE 10347566 A1 , Ceramic separator for electrochemical cells with improved conductivity (2005-05-12). Degussa
• [4] property rights WO 2010149265 A1 , Battery separator (2010-12-29). Schott AG
• [5] Intellectual Property US 5468572 A , Pre-compressed glass fiber separators for batteries (1995-11-21). Hollingsworth & Vose Company
• [6] Property right WO 2016210225 A1 , Improved absorbent glass mat separators, vrla batteries, and related methods of manufacture and use (2016-12-29). Daramic, LLC
• [7] Property right WO 2013076125 A1 , Printed Batteries (2013-05-30). Varta Microbattery GmbH
• [8] Intellectual Property WO 2013045350 A1 , Rechargeable lithium-ion accumulator and use of glass-based material therefor (2013-04-04). Schott AG
• [9] property rights WO 2009103537 A1 , Nonwoven fabric with cross-linking material (2009-08-27). Carl Freudenberg KG
• [10] Property right WO 2005076388 A1 , Organic / inorganic composite porous layer-coated electrode and electro-chemical device comprising the same (2005-02-05). LG CHEM LTD.
• [11] Intellectual Property EP 2605311 A1 , Separator (2013-06-19). Nippon Sheet Glass Company LTD.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• WO 2014047126 A1 [0032]
• DE 102011003186 A1 [0032]
• DE 10347566 A1 [0032]
• WO 2010149265 A1 [0032]
• US 5468572A [0032]
• WO 2016210225 A1 [0032]
• WO 2013076125 A1 [0032]
• WO 2013045350 A1 [0032]
• WO 2009103537 A1 [0032]
• WO 2005076388 A1 [0032]
• EP 2605311 A1 [0032]

Claims (6)

Separator for an electrochemical cell, preferably a lithium-ion battery, comprising a porous layer containing the following components: (A) 5 to 99.9% by weight of at least one glass flake having an average thickness of 0.05 μm to 30 μm and an average edge length of 0.1 μm to 150 μm; (B) 0.1 to 95% by weight of at least one binder; wherein the components (A) and (B) add up to 100 wt .-% and wherein the separator is applied by means of a coating method directly on a battery electrode and a porosity of 5 to 80% and a total thickness of 0.5 .mu.m to 200 .mu.m having. Separator after Claim 1 , characterized in that: the proportion of at least one glass flake (A) in the dry composite is 10 to 99.8 wt .-%, preferably 30 to 99.5 wt .-% and particularly preferably 50 to 99 wt .-%; and / or the proportion of the binder (B) in the dry composite is 0.2 to 20% by weight, preferably 0.5 to 10% by weight. Method according to Claim 1 or 2 comprising the steps of: (a) providing a battery electrode (C) consisting of current conductors and electrode material; (b) providing a binder solid (B) and solvent / dispersant; (c) mixing a suspension consisting of glass flakes (A), binder (B) and solvent / dispersant; (d) applying the suspension of step (c) on the battery electrode (C) by a coating method, after which a battery separator is obtained on the battery electrode (C); (e) withdrawing the solvent and / or drying the battery separator from step (d) Separator according to one of the preceding claims, characterized in that: The binder from the group of celluloses, starches, alginates, glycols, polyvinylidene fluorides, acrylic acids, rubbers and modifications and mixtures thereof and for mixing a liquid binder suspension, an aqueous and / or organic solvent is used. Separator according to one of the preceding claims, characterized in that: the glass plates (A) are preferably aligned and layered in a horizontal orientation with respect to the battery electrode surface by the coating method. Method according to one of the preceding claims, characterized in that: the glass flakes (A) - binder (B) - suspension is applied to a battery electrode via a coating method, preferably from the group of doctoring or spraying.

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