Separator coated with a transition metal-phosphorus compound catalyst and a lithium-sulfur secondary battery containing this

Abstract

The present invention provides a separator coated with a transition metal-phosphorus compound catalyst and a lithium-sulfur secondary battery comprising this separator. By applying a separator comprising a catalyst layer containing a transition metal-phosphorus compound catalyst to a lithium-sulfur secondary battery, it is possible to prevent the migration of lithium polysulfide and damage to the active material, thereby improving the electrochemical performance and lifetime of the lithium-sulfur secondary battery.

Description

Background of the invention 1. Field of the invention

[0001] The present invention relates to a separator coated with a transition metal-phosphorus compound catalyst and a lithium-sulfur secondary battery comprising the same. More specifically, the present invention relates to a lithium-sulfur secondary battery comprising a sulfur cathode catalyst consisting of a transition metal-phosphorus compound and a separator coated with the catalyst. 2. Description of related technology

[0002] With technological advances, the demand for high-energy devices and energy storage systems (ESS) has increased significantly. In line with this trend, lithium-sulfur batteries, with higher energy density than conventional batteries, have attracted considerable attention as next-generation batteries. Lithium-sulfur batteries use sulfur as the cathode material and lithium metal as the anode material, offering approximately eight times the theoretical capacity of lithium-ion batteries. Furthermore, sulfur is abundant, inexpensive, and lightweight, further increasing its attractiveness.

[0003] Lithium-sulfur batteries generate electrical energy through the gradual conversion of sulfur present in the cathode. The S8 ring structure, consisting of eight sulfur atoms, is shortened through sequential reduction reactions and converted into lithium polysulfide with a linear structure, resulting in energy generation. Eventually, due to its ability to be reduced until two lithium ions are bonded to a single sulfur atom, a significant amount of lithium can be oxidized. During charging, the oxidation reactions occur in reverse, returning to the S8 ring structure. At this point, two phase transitions occur in the reduction process, leading to the appearance of liquid lithium polysulfide intermediates that penetrate through the electrolyte into the separator and migrate to the lithium metal anode.This causes the loss of active material, and the liquid lithium polysulfide penetrating the anode creates a layer composed mainly of Li2S and Li2S2, which acts as a resistor, reducing the stability and lifespan of the battery.

[0004] Therefore, there is a need for a technology that can prevent the migration of lithium polysulfide through the separator to the anode.

[0005] CN 1 08 336 308 A discloses a positive electrode protective material for a lithium-sulfur battery and a lithium-sulfur battery with the positive electrode protective material. The positive electrode protective material may comprise Cu and a metal phosphide and may be coated on a separator.

[0006] CN 1 11 477 873 A discloses a conductive agent for a lithium-sulfur battery based on a nano-transition metal phosphide / carbon composite material.

[0007] WO 2023 / 187823 A1 discloses a phosphide-coated separator as a barrier to restrict ion transport.

[0008] CN 1 05 140 449 A discloses a membrane with a protective coating for protecting the negative electrode of a lithium-sulfur battery. The additional metal of the coating can be a copper phosphide.

[0009] DE 10 2016 123 898 A1 discloses a lithium battery component comprising a lithium donor, which may be a layer on a polymer separator, e.g., Li2CuP2.

[0010] US 2019 / 0 214 640 A1 discloses an anode composition of a lithium-ion battery. Brief description of the invention

[0011] The present invention has been made in an effort to solve the above-described problems associated with the prior art, and a first object of the present invention is to provide a separator coated with a transition metal-phosphorus compound catalyst.

[0012] A second object of the present invention is to provide a lithium-sulfur secondary battery comprising the transition metal-phosphorus compound catalyst coated separator of the above-mentioned first object.

[0013] To achieve the above-mentioned first object, the present invention provides a separator coated with a transition metal-phosphorus compound catalyst. The catalyst-coated separator refers to a catalyst layer comprising a separator and a transition metal-phosphorus compound catalyst coated on the separator.

[0014] To achieve the above-mentioned second object, the present invention provides a lithium-sulfur secondary battery comprising a separator coated with a transition metal-phosphorus compound catalyst. The lithium-sulfur secondary battery consists of a sulfur cathode, a lithium metal anode located in a direction opposite to the sulfur cathode, and a catalyst-coated separator formed between the sulfur cathode and the lithium metal anode. The catalyst-coated separator includes a catalyst layer formed in a direction opposite to the lithium metal anode.

[0015] According to the present invention described above, the catalyst coated on the separator is a catalyst consisting of a transition metal-phosphorus compound. When the transition metal-based compound forms chemical bonds with lithium polysulfides, it can adsorb lithium polysulfides and form Li-P and PS, thereby increasing the adsorption capacity for lithium polysulfide compared to the prior art. Specifically, the transition metal bonds to the sulfur of the polysulfide, while the surface of the phosphorus is oxidized to form a bond between phosphorus and oxygen, thus serving as an adsorption site different from that of the transition metal.In addition, the transition metal phosphorus compound exhibits higher conductivity than other transition metal compounds, which can help reduce the energy required for the phase transition that occurs during battery operation.

[0016] Therefore, when applied to a lithium-sulfur secondary battery, the separator coated with a transition metal-phosphorus compound catalyst not only acts as a catalyst, but also suppresses the migration of lithium polysulfide migrating from the cathode to the anode during battery operation and reduces the damage to the active material, thereby improving the battery life and performance. Brief description of the drawings

[0017] The above and other features and advantages of the present invention will be explained in more detail below using exemplary embodiments with reference to the accompanying drawings. Fig. 1 is a schematic diagram illustrating a lithium-sulfur secondary battery including a catalyst-coated separator according to a preferred embodiment of the present invention; Fig. 2 is a graph illustrating the results of UV-Vis spectroscopy evaluating the chemical performance of a catalyst according to a preferred embodiment of the present invention; Fig. 3 Scanning electron microscope (SEM) images and energy dispersive spectrometer (EDS) images illustrating the analysis of the catalyst surface according to a preferred embodiment of the present invention; Fig. 4 is a graph illustrating the results of cyclic voltammetry of lithium-sulfur secondary batteries according to a preferred embodiment of the present invention; Fig. 5 is a diagram illustrating the results of an experiment in which a constant current is applied to lithium-sulfur secondary batteries according to a preferred embodiment of the present invention; and Fig. 6 is a graph comparing overpotential versus catalyst ratio according to a preferred embodiment of the present invention. Detailed description of the invention

[0018] Unless otherwise defined herein, all terms used herein, including technical or scientific terms, have the same meanings as those commonly understood by those skilled in the art to which this disclosure relates. It is further understood that terms defined in dictionaries that are in common use should be interpreted to have a meaning consistent with their meaning in the context of the relevant art and should not be interpreted to have ideal or overly formal meanings unless clearly defined in the present application.

[0019] It is understood that when an element such as a layer, film, region or plate is referred to as being “on” or “over” another element, it may be directly on top of the other element or there may be intervening elements.

[0020] Hereinafter, various embodiments of the present invention will be described in more detail with reference to the accompanying drawings. Examples

[0021] Fig. 1 is a schematic diagram illustrating a lithium-sulfur secondary battery including a catalyst-coated separator according to a preferred embodiment of the present invention.

[0022] With reference to Fig. 1, the lithium-sulfur secondary battery includes a lithium metal anode 100, a sulfur cathode 110 made of sulfur, and a catalyst-coated separator 120 disposed between the anode 100 and the cathode 110. The catalyst-coated separator 120 is formed by coating a catalyst layer 121 on a separator 122 opposite the anode 100.

[0023] The catalyst layer 121 formed on the separator 122 can prevent lithium polysulfide from the sulfur cathode 110 from penetrating into the separator 120 and migrating to the anode 100 during battery operation, thereby improving the electrochemical performance of the battery.

[0024] The catalyst layer 121 preferably has a thickness of 15 μm to 30 μm, more preferably 20 μm to 25 μm. If the thickness of the catalyst layer 121 is less than 15 μm, it may not be able to sufficiently adsorb lithium polysulfide, making it difficult to suppress the migration of lithium polysulfide to the anode. If the thickness of the catalyst layer 121 exceeds 30 μm, it may be difficult for lithium ions to move during battery operation, and thus the oxidation / reduction reaction of sulfur may not be enabled. However, the thickness of the catalyst layer 121 is not limited to this, as long as the catalyst layer 121 can prevent the lithium polysulfide from migrating to the anode 100 and improve the performance of the battery.

[0025] The catalyst layer 121 comprises a transition metal-phosphorus compound catalyst. The transition metal is preferably copper, but is not limited thereto. The transition metal-phosphorus compound catalyst comprises CuP2.

[0026] The transition metal within the catalyst can bind with sulfur from lithium polysulfide and adsorb it to the separator, preventing migration of lithium polysulfide to the anode.

[0027] The surface of the phosphorus within the catalyst is oxidized to form a bond between phosphorus and oxygen, providing an adsorption site for lithium polysulfide that is different from that of the transition metal.

[0028] Therefore, when the lithium-sulfur battery is discharged, the liquid lithium polysulfide generated during the reduction reaction is adsorbed on the catalyst layer 121 coated on the separator 120, thereby suppressing the migration of lithium polysulfide to the anode. This makes it possible to prevent the loss of active material and the formation of a Li2S or Li2S2 layer on the anode by lithium polysulfide, thereby preventing a decrease in the capacity of the lithium-sulfur secondary battery and improving its stability.

[0029] Furthermore, by using the transition metal-phosphorus compound as a catalyst, the energy required for the phase transition occurring during operation of the lithium-sulfur secondary battery is reduced, thereby reducing the overpotential. Consequently, the electrochemical performance of the battery can be improved.

[0030] The catalyst layer 121 comprises a base solid consisting of a transition metal phosphorus compound catalyst, a conductive material, and a binder. However, it is not limited to these components, and additional substances may also be included for the purpose of improving performance.

[0031] The transition metal-phosphorus compound catalyst is added in an amount of 7.5 wt% to 13.5 wt%, more preferably 7.5 wt% to 12 wt%, based on the total weight of the base solid. If the amount of the transition metal-phosphorus compound catalyst is less than 7.5 wt%, there may not be enough catalyst on the catalyst layer 121 to adequately adsorb lithium polysulfide. As a result, it is not sufficient to suppress the migration of lithium polysulfide from the cathode to the anode and to increase the electrical conductivity, making it difficult to improve the battery performance. Furthermore, if the amount of the transition metal-phosphorus compound catalyst is above the range of 7.5 wt% to 13.5 wt%, it may lead to a higher overpotential, thereby deteriorating the battery performance.

[0032] The conductive material preferably contains carbon. Furthermore, the amount of the conductive material is preferably 75 wt% to 85 wt%, more preferably 75 wt% to 82.5 wt%, based on the total weight of the base solid. However, it is not limited to this, as long as it can improve the conductivity of the catalyst.

[0033] The electrolyte charged into the lithium-sulfur secondary battery preferably comprises at least one of a carbonate-based and ether-based solvent. Specifically, the solvent may include, but is not limited to, at least one selected from the group consisting of ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), dimethoxyethane, methyl tert-butyl ether, dioxane, tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-MeTHF), 1,3-dioxolane (DOL), dimethyl ether (DME), dibutyl ether, and methyl tert-butyl ether. Preparation Example 1: Preparation of a copper-phosphorus compound catalyst (CuP2 catalyst)

[0034] CuCl2 and red phosphorus were mixed in a 1:2 copper to phosphorus ratio, with oleic acid and oleylamine added in a 1:1 ratio, and the resulting mixture was stirred at 80°C for 2 hours to form a first solution. The first solution was subjected to a primary heat treatment by heating to 200°C at 5°C / min for 4 hours in a N2 atmosphere to obtain a product. Subsequently, ethanol was added to the resulting product to remove oleic acid, oleylamine, and impurities, followed by repeated centrifugation at 8000 rpm three times for 15 minutes each. Then, the resulting product was subjected to a secondary heat treatment by heating to 450°C at 5°C / min for 2 hours in a N2 atmosphere to obtain a copper-phosphorus compound (CuP2).At this time, the weight ratio of copper to phosphorus in the compound was about 49.47 wt% and about 50.53 wt%, respectively. Production Example 2: Production of a lithium-sulfur secondary battery containing a copper-phosphorus compound catalyst

[0035] A base solid containing 10 wt% of the copper-phosphorus compound catalyst prepared in Preparation Example 1, 55 wt% of Super P and 25 wt% of vapor glassy carbon type H (VGCF-H) as conductive materials, and 10 wt% of polyvinylidene fluoride (PVDF) as a binder was dispersed in a 1-methyl-2-pyrrolidone (NMP) solution to prepare a slurry. A Celgard 2320 product with a thickness of 20 μm was used as the separator, and the slurry was coated on one side of the separator and then dried in an oven at 60°C to prepare a catalyst-coated separator. At this time, the catalyst layer on the separator had a thickness of approximately 20 μm.

[0036] A slurry containing 60 wt% sulfur, 30 wt% Super P and 10 wt% PVDF was tested with a sulfur loading of 1.0 to 1.5 mg / cm 2layered onto an aluminum foil and then dried in an oven at 60 °C to produce a cathode.

[0037] A lithium metal with a thickness of 350 μm was used as the anode. The cathode was placed in a direction opposite to the anode, and then a catalyst-coated separator was placed between the anode and the cathode to fabricate a lithium-sulfur secondary battery. At this time, the direction in which the catalyst layer is formed is toward the anode. An electrolyte containing 1 M LiTFSI and 0.5 M LiNO3 was added to a mixture of 1,3-dioxolane (DOL) and dimethyl ether (DME) (1:1 by volume), and the mixture was then filled into the lithium-sulfur secondary battery. Production Example 3: Production of a lithium-sulfur secondary battery containing a copper-phosphorus compound catalyst separator

[0038] A lithium-sulfur secondary battery was prepared under the same conditions as in Preparation Example 2, except that a base solid containing 5 wt% copper-phosphorus compound catalyst, 60 wt% Super P and 25 wt% VGCF-H as conductive materials, and 10 wt% PVDF as a binder was used. Production Example 4: Production of a lithium-sulfur secondary battery containing a copper-phosphorus compound catalyst separator

[0039] A lithium-sulfur secondary battery was prepared under the same conditions as in Preparation Example 2, except that a base solid containing 7.5 wt% copper-phosphorus compound catalyst, 57.5 wt% Super P and 25 wt% VGCF-H as conductive materials, and 10 wt% PVDF as a binder was used. Production Example 5: Production of a lithium-sulfur secondary battery containing a copper-phosphorus compound catalyst separator

[0040] A lithium-sulfur secondary battery was prepared under the same conditions as in Preparation Example 2, except that a base solid containing 12 wt% copper-phosphorus compound catalyst, 53 wt% Super P and 25 wt% VGCF-H as conductive materials, and 10 wt% PVDF as a binder was used. Production Example 6: Production of a lithium-sulfur secondary battery containing a copper-phosphorus compound catalyst separator

[0041] A lithium-sulfur secondary battery was prepared under the same conditions as in Preparation Example 2, except that a base solid containing 13.5 wt% copper-phosphorus compound catalyst, 51.5 wt% Super P, and 25 wt% VGCF-H as conductive materials, and 10 wt% PVDF as a binder was used. Production Example 7: Production of a lithium-sulfur secondary battery containing a copper-phosphorus compound catalyst separator

[0042] A lithium-sulfur secondary battery was prepared under the same conditions as in Preparation Example 2, except that a base solid containing 14 wt% copper-phosphorus compound catalyst, 50 wt% Super P and 25 wt% VGCF-H as conductive materials, and 10 wt% PVDF as a binder was used. Comparative Example 1: Production of a lithium-sulfur secondary battery separator without catalyst

[0043] A lithium-sulfur secondary battery was manufactured under the same conditions as in Preparation Example 2, except that a separator without a layered catalyst was used. Comparative Example 2: Production of a lithium-sulfur secondary battery containing a carbon layer separator

[0044] A lithium-sulfur secondary battery was prepared under the same conditions as in Preparation Example 2, except that a Super P coated separator was used instead of the copper-phosphorus compound catalyst. Experimental Example 1

[0045] Fig. Figure 2 is a graph illustrating the results of UV-Vis experiments after adding the product from Preparation Example 1 to a lithium polysulfide solution and a lithium polysulfide solution. 1 g of the product from Preparation Example 1 was added to the lithium polysulfide solution and then allowed to stand for 24 hours, and the supernatant was compared with the lithium polysulfide solution.

[0046] Referring to Fig. 2, Preparation Example 1 showed a lower absorption than Li2S6, a liquid lithium polysulfide solution. This suggests that the product from Preparation Example 1 reacted with Li2S6 and precipitated, resulting in a relatively lower concentration of lithium polysulfide in the supernatant of the solution containing the product from Preparation Example 1. It can therefore be concluded that the CuP2 catalyst layer from Preparation Example 1 adsorbed the lithium polysulfide eluted from the cathode.

[0047] Fig. Figure 3 shows scanning electron microscope (SEM) images and energy dispersive spectrometer (EDS) images illustrating the analysis of the catalyst surface after the product from Preparation Example 1 adsorbed the liquid lithium polysulfide.

[0048] With reference to Fig. Figure 3 (a) is an SEM image of the CuP2 catalyst adsorbing the liquid lithium polysulfide. Based on this, the EDS analysis results for each element are presented in (b) to (d). 23.24 wt% sulfur was contained in (b), 52.90 wt% copper was contained in (c), and 23.85 wt% phosphorus was contained in (d). It was observed that sulfur was evenly distributed on the surface of the catalyst. Therefore, it can be concluded that the product of Preparation Example 1 containing the transition metal-phosphorus compound catalyst layer adsorbs the sulfur from the liquid lithium polysulfide onto the catalyst surface.

[0049] Therefore, the Fig. 2 and Fig. 3, it was found that the product from Preparation Example 1 containing the CuP2 catalyst layer adsorbs lithium polysulfide. Consequently, the transition metal-phosphorus compound catalyst, CuP2, can prevent the migration of lithium polysulfide from the cathode to the anode during operation of the lithium-sulfur secondary battery and reduce the loss of active material. Experimental Example 2

[0050] Fig. Figure 4 is a graph illustrating the cyclic voltammetry results of lithium-sulfur secondary batteries prepared in Comparative Example 1 and Preparation Example 2. Referring to Fig. 4, when the voltage range was 1.5 V to 3.0 V and the scan rate was 0.1 mV / s, the product of Preparation Example 2 showed a reduced overpotential during the oxidation process and a large increase in current during the reduction process compared to Comparative Example 1. Therefore, it can be seen that the catalyst-coated separator of Preparation Example 2 adsorbs lithium polysulfide to reduce the loss of active material, thereby increasing the current, and the CuP2 catalyst on the catalyst-coated separator improves the electrochemical reaction, thereby reducing the overpotential, resulting in an increased reaction rate and higher activity in the lithium-sulfur secondary battery.

[0051] Fig. Figure 5 is a graph illustrating the results of constant current charge / discharge tests to measure the cycle life of a lithium-sulfur secondary battery. The tests were conducted with four cycles each at 0.1 C and 0.2 C, followed by a repeat cycle at 0.5 C. The voltage range was between 1.5 V and 3.0 V.

[0052] Referring to Fig. 5, Comparative Example 1 showed an initial specific capacity of about 900 mAh / g, while Preparation Example 2 showed a higher initial specific capacity of about 1050 mAh / g. In addition, when the cycle was repeated, the specific capacity of both Comparative Example 1 and Preparation Example 2 decreased. However, Preparation Example 2 showed a higher discharge capacity compared to Comparative Example 1 until the cycle was repeated 100 times, and the rate of capacity deterioration from the initial value to 100 cycles was lower in Preparation Example 2. Therefore, the use of the lithium-sulfur secondary battery containing a catalyst-coated separator can reduce the permeability of lithium polysulfide to the anode, thereby improving stability, and reduce the loss of active material, thereby achieving a higher specific capacity.

[0053] Consequently, Fig. 4 and Fig. 5 shows that the lithium-sulfur secondary battery containing a catalyst-coated separator has superior battery performance and a longer cycle life. Experimental Example 3

[0054] Fig. Figure 6 is a graph comparing the overpotential versus catalyst ratio within the catalyst-coated separator.

[0055] With reference to Fig.6, Comparative Example 1 using a separator without a coated catalyst showed the highest overpotential at 463 mV. Furthermore, Comparative Example 2, in which only the conductive material, Super P, was coated on the separator, showed a high overpotential of 393 mV. Compared to Comparative Example 1 and Comparative Example 2, Preparation Example 2 and Preparation Examples 4 to 6 using the catalyst-coated separator showed very low overpotential. However, Preparation Example 3 with a catalyst ratio of 5 wt% and Preparation Example 7 with a catalyst ratio of 15 wt% showed similar or slightly reduced overpotential compared to Comparative Examples 1 and 2. Therefore, it can be concluded that the catalyst ratio on the catalyst-coated separator is most effective when it falls within the range of 7.5 wt% to 13.5 wt%.

[0056] Therefore, according to the present invention, as described above, the formation of a catalyst layer containing a transition metal-phosphorus compound catalyst can improve the performance and cycle life of the lithium-sulfur secondary battery. When the transition metal-based compound in the catalyst layer forms chemical bonds with lithium polysulfides, it can adsorb lithium polysulfides and form Li-P and PS, thereby increasing the adsorption capacity for lithium polysulfide compared to the prior art. Specifically, the transition metal bonds to the sulfur of the polysulfide, while the surface of the phosphorus is oxidized to form a bond between phosphorus and oxygen, thereby serving as an adsorption site different from that of the transition metal.In addition, the transition metal phosphorus compound exhibits higher conductivity than other transition metal compounds, which can help reduce the energy required for the phase transition that occurs during battery operation.

[0057] Therefore, by applying the separator coated with a transition metal phosphorus compound catalyst to a lithium-sulfur secondary battery, it not only acts as a catalyst but also suppresses the migration of lithium polysulfide that migrates from the cathode to the anode during battery operation and reduces the damage of active material, thereby improving the battery life and performance. [Description of reference symbols] 100 lithium metal anode 110 Sulfur cathode 120 catalyst coated separator 121 Catalyst layer 122 Separator

[0058] While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. Therefore, the scope of the invention is defined by the appended claims rather than the detailed description of the invention, and all differences within the scope are understood to be included within the present invention.

Claims

[1] A transition metal phosphorus compound catalyst coated separator for a lithium-sulfur secondary battery, comprising: a separator and a catalyst layer comprising a transition metal phosphorus compound catalyst coated on one side of the separator, characterized by , that the transition metal phosphorus compound catalyst is a compound containing CuP2, and that the catalyst layer comprises a base solid consisting of CuP2, a conductive material and a binder, wherein the transition metal phosphorus compound catalyst is contained in an amount of 7.5 wt% to 13.5 wt% based on the total weight of the base solid. [2] A separator coated with a transition metal phosphorus compound catalyst according to claim 1, characterized bythat the conductive material is present in an amount of 75 wt% to 85 wt% based on the total weight of the base solid and contains carbon. [3] A separator coated with a transition metal phosphorus compound catalyst according to claim 1, characterized by that the catalyst layer has a thickness of 15 µm to 30 µm. [4] Lithium-sulfur secondary battery, comprising: a cathode; an anode; an electrolyte and a separator located between the cathode and the anode, characterized bythat: the separator forms a transition metal-phosphorus compound catalyst-coated separator according to any one of claims 1 to 3, wherein a catalyst layer comprising a transition metal-phosphorus compound catalyst is coated on one side of the separator, wherein the transition metal-phosphorus compound catalyst comprises CuP2, wherein the transition metal-phosphorus compound catalyst is contained in an amount of 7.5 wt% to 13.5 wt% based on the total weight of the base solid of the catalyst layer. [5] Lithium-sulfur secondary battery according to claim 4, characterized by that the lithium-sulfur secondary battery, consisting of the anode, the cathode and the separator coated with a transition metal-phosphorus compound catalyst, has a thickness of 11 µm to 400 µm. [6] Lithium-sulfur secondary battery according to claim 4, characterized bythat the electrolyte comprises at least one of a carbonate and ether-based solvent. [7] Lithium-sulfur secondary battery according to claim 6, characterized by that the solvent of the electrolyte comprises at least one selected from the group consisting of ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), dimethoxyethane, methyl tert-butyl ether, dioxane, tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-MeTHF), 1,3-dioxolane (DOL), dimethyl ether (DME), dibutyl ether and methyl t-butyl ether.

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