EA040496B1 - LITHIUM SULFUR (LIS) BATTERY WITH LOW SOLVATIVE ELECTROLYTE - Google Patents

Description

Technical field

The invention relates to a lithium-sulfur (LiS) battery with a low solvating electrolyte.

State of the art

Liquid-containing lithium-sulfur (LiS) battery cells typically use electrolytes in which most of the electrochemically active sulfur, especially polysulfides, is dissolved in the electrolyte.

Examples are disclosed in US 7,354,680 assigned to Sion Power, specifically disclosed are electrolytes containing an acyclic ether such as 1,3-dioxolane (DOL, C ₃ H ₅ O ₂ ) and a cyclic ether such as dimethoxyethane (DME), and also lithium salts, for example LiN(CF ₃ SO2) 2 (lithium salt of bis(trifluoromethane)sulfonimide), which is also called LiTFSI. In addition, the electrolyte contains lithium nitrate LiNO3 as an additive, which is generally believed to prevent a rapid decrease in battery performance due to polysulfide migration. However, the formation of gases is one of the problems of this technology.

Alternative approaches include low solvating electrolytes in which electrochemically active sulfur is only sparingly dissolved. In the prior art for battery electrolytes, the term low solvating is used together with the term moderately solvating, i.e. these electrolytes dissolve only a small amount of available polysulfides.

Since there is no need for a large volume to dissolve the polysulfides, smaller amounts of electrolyte can be used, which in turn reduces the overall weight of the cell and hence the potential for a corresponding increase in energy density.

This topic is discussed in the article Sparingly Solvating Electrolytes for High Energy Density Lithium-Sulfur Batteries published by Cheng, Curtiss, Za-vadil, Gewirth, Shao and Gallagher in 2016 in ACS Energy Letters, which is available online at: http ://pubs.acs.org/journal/aelccp. This article reports that about 1 ml of electrolyte per gram of sulfur is needed to compete with lithium-ion technology in terms of energy density, but achieving such a low amount is considered a challenge.

Another discussion can be found in Directing the Lithium-Sulfur Reaction Pathway via Sparingly Solvating Electrolytes for High Energy Density Batteries published by Lee, Pang, Ha, Cheng, SangDon Han, Zavadil, Gallagher, Nazar, and Balasubramanian in 2017 in the journal ACS Central Science, available online at http://pubs.acs.org/journal/acscii.

Examples of moderately solvating electrolytes are disclosed in Chinese patent applications CN 107681197 A, CN 108054350 A, CN 108281633 A and CN 108091835 A. Other examples are disclosed in Korean WO 2018/004110, especially a mixture of a cyclic ether such as DOL and a glycol ether for such an electrolyte, which also contains a lithium salt.

Other examples of moderately solvating electrolytes are disclosed in the German patent application DE 102017209790.6 assigned to Fraunhofer and in the corresponding international patent application WO 2018/224374. In these publications, the preferred electrolyte contains hexyl methyl ether (HME) and 1,2-dimethoxyethane in a volume ratio of 80:20. Other examples of electrolytes with HME and additional ether are not specifically disclosed in WO 2018/224374.

As follows from the above, in general, electrolytes with moderate solvating power have been proposed and it has been recognized that the possibility of using them in small quantities is an advantage. However, no practical technical solution has yet been proposed. In particular, no satisfactory technical solution has yet been found for electrochemical cells with an electrolyte content of only 2 ml of electrolyte/1 g or even less.

Description/Summary of the Invention

Therefore, the purpose of the invention is to improve the art. In particular, the goal is to provide a LiS battery with a high energy density. Another object is to provide an electrochemical cell with a small amount of electrolyte with moderate solvating power, in particular with an electrolyte amount of less than 2 ml/g.

These goals are achieved with a lithium sulfur (LiS) battery cell, as will be explained in more detail below. In particular, it has been demonstrated that a lithium sulfur (LiS) battery design is possible with electrolyte volumes of less than 2 ml/g sulfur. Experimentally, electrochemical cells with high energy density over 400 Wh/kg have been obtained with electrolyte volumes as low as 1.6 ml/g. Here, the relative volumes of electrolyte are given in ml of electrolyte per gram of sulfur (ml/g), which is equivalent to the units of µl/mg.

The term moderately solvating is used for electrolytes designed to dissolve only a small fraction of the available polysulfides during battery charging and discharging. For example, the share is less than 5%, possibly less than 2%.

The electrochemical cell contains: a negative lithium electrode; a positive sulfur electrode, wherein the sulfur electrode includes an electrically conductive porous carbon matrix, the pores of which contain sulfur; a current collector adjacent to the sulfuric electrode; a separator located between the lithium electrode and the sulfur electrode; an electrolyte located between the electrodes for transferring Li ions between the electrodes.

Good results have been obtained with an electrolyte which contains a non-polar acyclic non-fluorinated ether, in particular hexyl methyl ether, C7H ₁₆ O (HME), and a polar ether, in particular 1,3-dioxolane C ₃ H ₆ O ₂ (DOL).

Possibly this mixture can be represented in a HME:DOL volume ratio ranging from 2:1 to 1:2, possibly the range is an open range so endpoints may not be included in the range. For example, the range is 1.5:1 to 1:1.5, possibly around 1:1, such as 1:1.2 to 1.2:1.

It has been observed that the above HME:DOL ratio is far from the 80:20 HME:1,2dimethoxyethane ratio described in the aforementioned WO 2018/224374. It is also stated in WO 2018/224374 that the preferred concentration ratio of non-polar ether and polar aprotic organic solvents is more than 2:1, even more preferably more than 3:1, in particular in the range from 3:1 to 9:1. From WO 2018/224374 it follows that the amount of HME should be substantially greater, even several times greater than the volume of polar aprotic organic solvents. On the contrary, this is not necessary in the present invention, where equal amounts were used in the experiments, and it is also possible to use a smaller amount of HME than the amount of DOL.

Advantageously, the invention also contains a lithium salt, for example LiN(CF ₃ SO ₂ )2 (lithium salt of bis(trifluoromethane)sulfonimide), which is also referred to as LiTFSI, preferably in a molar concentration in the range of 1 to 4M. In the experiments, a concentration of 1.5 M was used.

Such an electrolyte is a low solvating electrolyte. For example, the electrolyte is designed to dissolve only a fraction of the available polysulfides during battery charging and discharging, and this proportion is less than 5% or even less than 2%.

It has been experimentally confirmed that with such an electrolyte, an electrochemical cell requires less than 2 ml of electrolyte per 1 g of sulfur to achieve a high specific energy expressed in Wh/kg.

In particular, it has been experimentally confirmed that the specific energy of this electrochemical cell exceeds 400 Wh/kg for at least 5 cycles and exceeds 350 Wh/kg for at least 20 cycles when it is charged and discharged at a rate of charge 0.1C.

The mass density of the positive sulfur electrode is preferably above 0.55 g/cm ³ , for example above 0.6 g/cm ³ . The high mass density means that the pores are relatively small. In some embodiments, the porous carbon matrix includes pores with a defined pore volume, wherein at least 50% of the pore volume is defined by pores having an average diameter of less than 0.1 microns. Small pores are advantageous since the electrolyte volume inside the cathode is minimized. In addition, the pressure fracture resistance of the cathode material is minimized. The latter circumstance is an important aspect if the electrochemical cell is subjected to pressure, for example, in a stack of cells.

With small pore volumes, a high mass density of the cathode is possible. In experiments, the mass density of the cathode was more than 0.6 g/cm ³ .

The pore volume of the cathode is advantageously in the range of 0.25 to 0.45 ml/g, for example in the range of 0.3 to 0.4 ml/g. In some embodiments, the pore volume of the cathode is 0.35 ml/g.

Possibly the battery is designed and intended to apply force to the electrochemical cell, possibly to the stack of cells, in a direction perpendicular to the active surfaces of the electrodes during battery charging, the force being in the range of 10 to 50 N/cm ² . In experiments, a force of 37 N/cm ² was successfully applied.

In some embodiments, the current collector is a perforated metal sheet with perforations, the total area of which is more than 50%, more than 70% or even more than 80% of the area of one side of the metal sheet. In the experiments, current collectors with a perforation area of about 80% of the sheet area were used.

Optionally, the positive electrode is blindly connected to the current collector, thus forming an electrode/current collector sheet block.

Optionally, a battery is constructed from electrochemical cells, containing a plurality of such electrochemical cells arranged in a stack, where two adjacent cells share one current collector, so that the current collector is located between the specified sulfuric electrode and another identical sulfuric electrode of the neighboring cell.

To obtain an efficient rechargeable battery, several of the above electrochemical cells are stacked, for example, 10, 20, 30 or 40 layers of the above multilayer combinations of anode, separator, current collector and cathode.

It has been experimentally found that it is an advantage to apply pressure on the stack

- 2 040496 in a direction perpendicular to the stack, for example in the range from 10 to 50 N/cm ² , possibly in the range from 20 to 50 N/cm ² . In experiments, a force of 37 N/cm ² was successfully applied.

Typically, stacks are provided as pouch cells.

A suitable material for the separator is polyethylene (PE) or polypropylene (PP) film, which has through perforations to allow electrolyte to pass through the perforations. These films are manufactured by Celgard®, see www.Celgard.com.

In some practical embodiments of stacks, the separator includes a coating of a porous sulfur-containing porous carbon matrix to provide a combination of cathode and separator, with each two of such combinations containing one current collector between them so that the cathode sides of these two combinations face the current collector and are attached to it. Optionally, the combinations are also attached to each other through the perforation of the current collector.

In alternative embodiments, the cathode carbon and the sulfur-containing material are electrically conductive, for example, due to the presence of integrated electrically conductive nanoparticles in the cathode material. One possibility is to produce a cathode by hot pressing carbon nanotubes (CNT) and sulfur particles to form a composite, optionally containing carbon black. In this case, the cathode conducts current to the electrical connector located on the edge of the sulfur cathode, and thus the metal current collector can be omitted. For example, the average CNT diameter is in the range of 5 to 10 nm.

In particular, as is known, there are no requirements for the use of lithium nitrate LiNO ₃ as an additive for an electrochemical cell, despite its high performance in its use.

The electrolyte is usually liquid. Alternatively, it may be provided as a gel or solid.

Aspects of.

In the following, some aspects of the invention will be explained in conjunction.

Aspect 1. A battery containing an electrochemical cell containing a negative lithium electrode; a positive sulfur electrode, the sulfur electrode comprising an electrically conductive porous carbon matrix, the pores of which contain sulfur; a current collector adjacent to the sulfuric electrode; a separator located between the lithium electrode and the sulfur electrode; an electrolyte located between these electrodes to transfer Li ions between the electrodes. For example, the electrolyte is low solvating. Optionally, it is designed to dissolve only a fraction of the available polysulfides during battery charging and discharging, with this proportion being less than 5%.

Aspect 2 The battery according to aspect 1 wherein the electrolyte between the electrodes is present in an amount of less than 2 μl per 1 mg of sulfur.

Aspect 3 A battery according to any of the preceding aspects wherein the electrolyte comprises a non-polar, acyclic non-fluorinated ether and a polar ether and a Li salt.

Aspect 4 The battery according to aspect 3, wherein the electrolyte contains a non-polar, acyclic, non-fluorinated ether and a polar ether in a ratio of 2:1 to 1:2, and a Li salt with a molar concentration of 1 to 4M.

Aspect 5 The battery according to aspect 3 or 4 wherein the polar ether is 1,3-dioxolane (DOL) and the non-polar ether is hexyl methyl ether (HME).

Aspect 6 The battery according to aspect 5 wherein the Li salt is the lithium salt of bis(trifluoromethane)sulfonimide (LiTFSI).

Aspect 7. A battery according to any of the preceding aspects wherein the specific energy of the electrochemical cell is above 400 Wh/kg for at least 5 cycles and above 350 Wh/kg for at least 20 cycles when charged and discharge with a charge rate of 0.1C.

Aspect 8. A battery according to any of the preceding aspects, wherein the mass density of the positive electrode is greater than 0.5 g/cm ³ .

Aspect 9 A battery according to any of the preceding aspects, wherein the porous carbon matrix contains pores, wherein at least 50% of the pore volume is defined by pores having an average diameter of less than 0.1 microns.

Aspect 10. A battery according to any of the preceding aspects, designed and configured to apply a force to the electrochemical cell in a direction perpendicular to the active electrode surfaces during battery charging, the force being in the range of 10 to 50 N/cm ² .

Aspect 11. A battery according to any of the preceding aspects, wherein the current collector is a perforated metal sheet with perforations whose total area is more than 50% of the area of one side of the metal sheet; in this case, the positive electrode is blindly connected to the current collector, forming an electrode/current collector sheet block.

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Aspect 12. A battery according to any of the preceding aspects, comprising a plurality of electrochemical cells arranged in a stack where two adjacent cells share said current collector such that said current collector is between said sulfur electrode and another identical sulfur electrode of an adjacent cell.

Aspect 13 The battery of aspect 12 wherein the separator includes a coating of porous sulfur-containing porous carbon matrix to provide a cathode and separator combination, each two of such combinations containing one current collector between them such that the cathode sides of the two combinations face towards current collector and attached to it, and the combinations are attached to each other through the perforation of the current collector.

Aspect 14 A battery according to any of the preceding aspects, wherein the stack is under pressure with a force in the range of 20 to 50 N/cm ² .

Brief description of the drawings

The invention will be explained in more detail with reference to the drawings, where in FIG. 1 shows a schematic diagram of an electrochemical cell;

in fig. 2 is a diagram of electrochemical cells in stacks used for experiments;

in fig. 3 - pantograph, which is perforated to form a grid;

in fig. 4 - experimental values of specific energy at a ratio of electrolyte to sulfur, ranging from 2 to 2.5 ml of electrolyte per 1 g of sulfur;

in fig. 5 - experimental values of specific energy at 1.6 ml/g.

Detailed description of the invention

The electrochemical cell contains a negative lithium electrode; a positive sulfur electrode, the sulfur electrode comprising an electrically conductive porous carbon matrix, the pores of which contain sulfur; a current collector adjacent to the sulfuric electrode; a separator located between the lithium electrode and the sulfur electrode; an electrolyte located between the separator and each of the electrodes for transferring Li ions between the electrodes.

The components that were used in the experiments will be described in more detail below, while the amount of electrolyte between the electrodes is less than 2 μl per 1 mg of sulfur.

The electrochemical battery cell is free of lithium nitrate LiNO ₃ .

An anode made of lithium (Li) metal foil 50 µm thick was used in the experiments. The specific cell size was 71 mm x 46 mm.

Copper foil 10 µm thick and 7x20 mm in size was used as a contact for electrical connection to the lithium anode. For fastening, copper was pressed against Li foil. However, as an alternative, electrical contact may be made directly to the Li metal surface, for example, with a nickel contact welded or otherwise attached to the Li foil.

The sulfur cathode, separator and current collector were made in the form of a layered structure, which is shown in Fig. 1 in schematic form. The current collector forms the central layer of a multilayer structure in a stack of electrochemical cells.

The cathode sulfur material is supported by a separator film. A double cathode/separator layer is provided on opposite sides of the current collector and is located between the lithium anode layers.

In experiments, a double separator design was used for better separation, as shown in FIG. 2.

The current collector was a thick perforated aluminum foil 12 µm thick. The weight of the foil has been reduced by using a mesh pantograph, see FIG. 3, with through holes distributed throughout the aluminum foil. The holes made up about 80% of the pantograph area, and only 20% of the area was aluminum material.

The current collector contained a carbon primer coating, the thickness of which was on the order of a micrometer.

A porous polyethylene separator film with a thickness of 5 µm was provided. The pore size was in the range of 20–200 nm.

The sulfur electrode contains an electrically conductive porous carbon matrix, the pores of which contain sulfur. Porous carbon black particles (Printex™) were impregnated with sulfur in a ratio by weight of 1:2. Impregnation was carried out by obtaining a mixture of carbon black particles and micrometer-sized sulfur particles and heating the composite to 155° C. for 30 min under dry conditions, followed by cooling. As a result, sulfur was evenly distributed inside the pores. The composite was then ground to form carbon/sulfur composite particles having a size in the range of several micrometers.

The obtained particles were suspended in water and a binder was added. The binder consisted of equal mass amounts of carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR). In addition to water, the aqueous slurry contained 60% sulfur, 30% carbon black to which 60% sulfur was added, and 10% CMC/SBR binder. Relative amounts are expressed by weight.

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Claims (1)

The pores are predominantly small in size. For example, at least 50% of the pore volume is defined by pores having an average diameter of less than 0.1 microns. With such a small pore volume, a cathode mass density of about 0.65 g/cm ³ was achieved. In the experiment, the pore volume of the cathode was 0.35 ml/g. This means that only a small amount of electrolyte is contained inside the cathode cavities. The remaining electrolyte is located between the electrodes. Alternatively, to increase the electrical conductivity of the cathode, carbon nanotubes (CNTs), such as MWCNTs, are added to the cathode. The weight percentage is preferably in the range of 5-20%, eg 10%. The percentage of carbon black or both carbon black and part of the binder is reduced by an appropriate amount. For example, the mass ratio of sulfur:soot:carbon nanotubes:binder is 60:25:10:5. An aqueous suspension with a binder was applied with a doctor blade to a separator, the separator serving as a support for the cathode material. It has been experimentally found that a suitable layer thickness is in the range of 2-10 µm. In the experiment, which, in particular, is described here, the thickness was 5 μm. The coated separator was attached to both sides of the current collector as shown in FIG. 2. Experimentally, a suitable practical method has been found to fold the coated separator around the current collector and press it against the current collector for connection. It was experimentally found that the carbon primer coating of the current collector improves the adhesion of the cathode to the current collector. The electrolyte partially fills the pores of the cathode matrix and fills the space between the electrodes for the transfer of Li ions between the electrodes. With regard to sulfur, the electrolyte refers to the type of electrolyte with moderate or no solvating power. For the experiment, the electrolyte was a 1.5 M solution of LiTFSI in a mixture of hexyl methyl ether (HME) and 1,3-dioxolane (DOL) in a 9:1 volume ratio. Initially, experiments were carried out at various temperatures with relative electrolyte amounts of 2 μl/mg (= 2 ml/g) as well as 2.2, 2.5 and 2 μl/mg, as shown in FIG. 4. The inventors, having obtained unexpectedly stable performance in the range of 2-2.5 ml/g, carried out further experiments at a ratio of 1.6 ml/g. In FIG. 5 shows the experimental values of specific energy obtained for a single electrochemical cell at 30°C in a stack of cells, the cell having the characteristics described above, without the mass of packaging material included in the mass of the individual cell. The specific energy appears to be above 400 Wh/kg 7 cycles after the initial cycle. For 20 cycles, the specific energy was above 350 Wh/kg and remained above 300 Wh/kg for more than 30 cycles when charged and discharged at a charge rate of 0.1C, which is 1/10 of the discharge capacity per hour, so that a complete discharge takes 10 hours. The discharge was carried out until the discharge voltage criterion of 1.5 V was reached. The table below shows the mass composition for such a cell, with an electrolyte:sulfur ratio of 1.6 ml/g. Component Weight [g] % wt. Sulfur 0.54 18.9 Soot + binder 0.36 12.6 Electrolyte 0.93 32.5 Aluminum 0.13 4.5 Copper 0.08 2.8 Lithium 0.52 18.2 Separator 0.30 10.5 Total 2.86 100% To obtain an efficient rechargeable battery, several of the aforementioned electrochemical cells must be stacked, for example, in the form of 10, 20, 30 or 40 layers of multilayer combinations of the above type of anode, separator, current collector and cathode. Experimentally, it has been found that the predominant pressure on the stack is, for example, from 20 to 50 N/cm ² . A pressure of 37 N/cm ² was used in the experiments. CLAIM 1. A battery containing an electrochemical cell containing a negative lithium electrode; a positive sulfur electrode, the sulfur electrode comprising an electrically conductive porous carbon matrix, the pores of which contain sulfur; a current collector adjacent to the sulfuric electrode; -