ES2397522T3 - Electrolyte for lithium-sulfur batteries and lithium-sulfur batteries that use it - Google Patents

Abstract

An electrolyte for a lithium-sulfur battery, the electrolyte comprising a solution of at least one electrolyte salt in at least two aprotic solvents, characterized in that the first aprotic solvent is sulfolane and the second aprotic solvent, is a sulfone, and in where the concentrations of the components of the solution are selected so that the solution is at its eutectic concentration or at most ± 30% of its eutectic concentration.

Description

Electrolyte for lithium-sulfur batteries and lithium-sulfur batteries that use it.

Technical field

The present invention generally pertains to the field of non-aqueous electrolyte compositions for use in chemical sources of electrical energy incorporating negative electrodes comprising highly reactive alkali metals. More particularly, the present invention pertains to non-aqueous electrolyte compositions comprising non-aqueous electrolyte solvents, salts and additives and which are suitable for use in secondary electricity producing cells. The present invention also pertains to chemical sources of electrical energy comprising said electrolyte compositions.

Basis

Throughout this application, reference is made to various publications, patents and patent applications published by means of an identification citation. The descriptions of published publications, patents and patent memories referenced in this application, more fully describe the state of the art to which the invention pertains.

Because the rapid evolution of batteries continues, and in particular, because secondary electric batteries such as lithium ion and lithium metal batteries have been more widely accepted for a variety of uses, the need for safe rechargeable cells , of long duration (more than 300 cycles), with a wide range of working temperature, is increasingly important. U.S. patents no. 5,460,905, 5,462,566,

5,582,623 and 5,587,253 describe the basic elements and operating needs of secondary lithium batteries and their components. A key feature in the development of high energy secondary batteries is the choice of electrolyte composition to improve cycle life, working temperature range and battery safety.

One of the many problems encountered in the process of producing electrolyte elements is that there is a difficulty in obtaining good cycling efficiency, cycle life, temperature stability and cell safety due to the reactivity of the electrolyte element with Electrode elements, particularly due to reactions with the anode. This is especially true with anodes comprising lithium, which is highly reactive. The reactions of lithium with the electrolyte are undesirable since they lead to self-discharge and early battery failure. The reaction of lithium with organic electrolyte solvents can also result in the formation of a surface film at the anode, which subsequently reduces the effectiveness of the anode, and can cause irregular coating that can lead to dendrite formation. These factors limit the number of potential electrolyte compositions that can be used to form the electrolyte element.

Desirable electrolyte elements provide high cycling efficiency, good ionic conductivity, good thermal stability and reasonable cost. The number of times a lithium battery can be recharged is dependent on the efficiency of each cell's charge and discharge cycle and provides a measure of cycling efficiency. Cycling efficiency means the percentage of lithium (or other anode material) that is coated or reduced at the anode at full charge compared to the amount of freshly pickled or oxidized lithium from the anode at the previous full discharge of the cell. Any deviation in this percentage of 100 percent represents the lithium that has been lost in terms of useful availability for carrying out the load / discharge of the cell. Cycling efficiency is primarily a function of the quality of the electrolyte composition.

Safety factors that affect the choice of electrolyte solvents include the safety margin against cell overload. The overload safety margin is determined by the difference in voltage between the end of the electrode recharge and the electrolyte breakdown. For example, in lithium-ion cells, the potential difference of the anode and cathode is approximately 4 V. Tarascon and Guyomard,

J. Electrochem. Soc., 1991, 138, 2864-2868, describe the upper voltage range of a potential sweep that is limited to 4.5 V versus Li / Li + by electrolyte breakdown to higher potentials (4.6 V versus Li / Li +) in a 1M 50:50 EC (ethylene carbonate) LiClO4 electrolyte: DME (dimethoxyethane). In addition, for example, Ein-Eli et al.,

J. Electrochem. Soc., 1997, 144, L205-L207, presents the beginning of electrolyte oxidation at 5.1 V for an electrolyte composition comprising LiPF6 1.2M ethylene carbonate: dimethyl carbonate (2: 3 by volume). The need for electrolyte compositions that do not decompose at high potentials is emphasized by the recent recommendation of Zhong et al., J. Electrochem. Soc., 1997, 144, 205-213, that certain lithium-ion cathode materials should be charged at approximately 5 V.

Additional factors that affect the choice of electrolyte compositions can be illustrated by reference to cells comprising intercalated carbon electrodes. Ein-Eli et al., J. Electrochem. Soc., 1996, 143, L273277, show that graphite electrodes, which are normally sensitive to the composition of the electrolyte solution, can be successfully cycled to high reversible capacities in electrodes comprising ethylmethyl carbonate. These results are interesting because lithium ions cannot be sandwiched in graphite in diethyl carbonate solutions and cycle poorly in dimethyl carbonate solutions.

A large number of non-aqueous organic solvents have been suggested and investigated as electrolytes in connection with various types of cells containing lithium electrodes. U.S. patents no. 3,185,590, 3,578,500, 3,778,310, 3,877,983, 4,163,829, 4,118,550, 4,252,876, 4,499,161, 4,740,436 and 5,079,109 describe many possible combinations of electrolyte elements and solvents of electrolyte, such as borates, substituted and unsubstituted ethers, cyclic ethers, polyethers, esters, sulfones, alkylene carbonates, organic sulphites, organic sulfates, organic nitrites and organic nitro compounds.

One class of organic electrolyte solvents that have received attention as a component of electrolyte elements for electrochemical cells and other devices are sulfones. The sulfones can be divided into two types: i) cyclic or aromatic sulfones, and ii) aliphatic sulfones. Sulfones form a potentially attractive group of organic solvents that have high chemical and thermal stability.

The use of cyclic sulfones, sulfolane (tetramethylene sulfone) together with its alkyl-substituted derivatives, 3-methylsulfolane and 2,4-dimethylsulfolane, as electrolyte solvents has been investigated.

U.S. Pat. no. 3,907,597 to Mellors describes a liquid organic electrolyte consisting essentially of sulfolane or its alkyl substituted liquid derivatives in combination with a co-solvent, preferably a low viscosity solvent such as 1,3-dioxolane, and an ionizable salt. Sulfolane and its alkyl substituted liquid derivatives, such as 3-methylsulfolane, are good non-aqueous solvents although they have the disadvantage that they have a relatively high viscosity. Thus, when the metal salts dissolve in these solvents in order to improve the ionic conductivity of the solvents, the viscosity of the solvent and the salt becomes too high for efficient use as an electrolyte for non-aqueous cell applications. For example, in .597, sulfolane is used in combination with a low viscosity co-solvent to overcome the viscosity problem.

Japanese patent publications numbers JP 08-298229, published on November 12, 1996 and JP 08298230, published on November 12, 1996, describe electrolytes for double layer electric capacitors comprising sulfolane as one of the electrolyte components.

U.S. Pat. no. 4,725,927 to Morimoto et al., Describes the use of sulfolane and its derivatives, 3-methylsulfolane and 2,4-dimethylsulfolane, for the use of double-layer electric capacitors. However, it is noted that the sulfolane solvent has a high viscosity and a relatively high solidification temperature. Therefore, when used for an electrolyte solution, the ionic conductivity tends to be low.

U.S. Pat. no. 5,079,109 by Takami et al., Describes a mixture of non-aqueous electrolyte solvents that may comprise sulfolane as one of the components for the use of secondary rechargeable lithium batteries. U.S. Pat. no. 5,219,684 to Wilkinson et al., Describes an electrolyte consisting essentially of sulfolane and a glyme for an electrochemical cell comprising a lithium-containing anode and a cathode that includes LixMnO2 active cathode material.

U.S. Pat. no. 4,550,064 by Yen et al., Describes electrolytes with sulfolane type solvents having relatively high dielectric constants and low vapor pressure. Sulfolane-containing electrolytes also show pickling / coating cycling efficiency enhanced by excellent reduction stability. However, the use of sulfolane solvents is inhibited by incompatibility of the polar sulfolane liquid with the hydrophobic separator and with the non-polar cathode binder. Methods for improving the wettability of the separator and the cathode electrode are described.

The use of aliphatic sulfones, dimethylsulfone and dipropylsulfone, has been investigated as electrolyte solvents. U.S. Pat. no. 4,690,877 to Gabano et al., Presents electrolyte compositions containing at least one linear aromatic or aliphatic sulfone for use in operable cells at temperatures between 100 ° C and 200 ° C. Dimethylsulfone is particularly preferred.

Sulfone-based electrolytes comprising dimethylsulfone, dipropylsulfone and sulfolane have been described by J. Pereira-Ramos et al., J. Power Sources, 1985, 16, 193-204 for use in lithium interleaving batteries. Melted dimethylsulfone at 150 ° C as an electrolyte for a rechargeable γ-MnO2 lithium battery is described by Bach et al., J. Power Sources, 1993, 43-44, 569-575.

U.S. patents no. 4,060,674 and 4,104,451 of Klemann and Newman describe electrolyte compositions for reversible alkali metal cells consisting essentially of a solvent and an electronically active alkali metal salt. The organic electrolyte solvents employed are generally those selected from the group consisting of ethers, esters, sulfones, organic sulphites, organic sulfates, organic nitrites or organic nitro compounds, inertly substituted and unsubstituted. Examples of organic solvents include propylene carbonate, tetrahydrofuran, dioxolane, furan, sulfolane, dimethyl sulphite, nitrobenzene, nitromethane and the like. Preferred solvents are ethers, and an electrolyte solvent containing dioxolane is preferred.

JP Patent Publication No. JP 09-147913, published June 6, 1997, describes electrolyte solvents containing sulfones of the formula R1-SO2-R2, where R1 and R2 are C1-4 alkyl groups, and R1 and R2 They are different. Preferably the anodes are carbonaceous anodes of interaction of Li.

Most of the electrolyte systems proposed for lithium-ion batteries are not useful in lithium-sulfur batteries. Low molecular weight sulfones are good solvents for Li-S battery electrolyte systems, although these sulfones have high melting temperatures, which means they cannot be used at low temperatures. U.S. Pat. no. 6,245,465 proposes (as solvents for Li-S batteries) non-cyclic sulfones or fluorinated non-symmetric non-cyclic sulfones, which have lower melting temperatures. This patent further describes the use of mixtures of the aforementioned sulfones with other solvents such as carbonates, glymes, siloxanes and others. However, the melting temperatures of the proposed sulfones are not low enough to produce electrolytes with desirable low temperature properties. On the other hand, the proposed sulfones are very expensive, and this restricts their wide use.

Despite the numerous electrolyte solvents proposed for use in rechargeable cells, there remains a need to improve non-aqueous electrolyte compositions that provide beneficial effects over the life of chemical sources of electrical energy, and which can be easily and completely incorporated in the cell without significant extra cost.

The embodiments of the present invention are sought to provide an improved non-aqueous electrolyte composition that is suitable for use in rechargeable cells.

The embodiments of the present invention can also be sought to provide non-aqueous electrolyte contents that have greater overload stability.

The embodiments of the present invention can be further sought to provide a non-aqueous electrolyte solvent with high ionic conductivity and low volatility of the solvent.

The embodiments of the present invention can also be sought to provide an improved non-aqueous electrolyte composition for chemical sources of electrical energy comprising negative alkali metal electrodes (including lithium).

The embodiments of the present invention can also be sought to provide non-aqueous electrolyte compositions that are useful with both lithium metal and lithium ion anodes for secondary battery cells.

The embodiments of the present invention can be further sought to provide non-aqueous electrolyte compositions that provide reversible interleaving of lithium to graphite.

The embodiments of the present invention can also be sought to provide non-aqueous electrolyte compositions that increase cycle life and safety of secondary cells.

The embodiments of the present invention can be further sought to provide non-aqueous electrolyte compositions that provide better temperature stability.

The embodiments of the present invention can be further sought to provide secondary lithium cells that employ the electrolytes of the present invention.

According to a first aspect of the present invention, an electrolyte has been provided as defined in claim 1.

Preferably, the concentrations of the components of the solution are selected such that the solution is at most ± 20% of its eutectic concentration, and even more preferably at most ± 10% of its eutectic concentration.

The use of eutectic or almost eutectic compositions dramatically improves the low temperature operating properties of the electrolyte. A significant reduction of the limit of the capacity of lithium-sulfur batteries at reduced temperatures is obtained, since it is an improvement in the characteristics of capacity and power at low temperature. In addition, the battery cycle life can be extended at low temperatures.

The sulfone that is present in addition to the sulfolane preferably has a molecular weight of 94 to 150.

The electrolyte salt may be at least one salt or a mixture of salts selected from a group comprising: lithium hexafluorophosphate (LiPF6), lithium hexafluoroarseniate (LiAsF4), lithium perchlorate (LiClO4), lithium sulfonylimide trifluoromethane (LiN ( CF3SO2) 2) and lithium trifluorosulfonate (CF3SO3Li).

The concentration of the at least one electrolyte salt is preferably between 0.1M and a saturation concentration.

The electrolyte may further include one or more organic or non-organic additives for the contribution to a solid electrolyte interface (IES) phase formation on a surface of a negative electrode (anode) and to improve cycling of the negative electrode. The additive may be at least one substance or a mixture of substances selected from a group comprising: S, Li2Sn, CO2, SO2, N2O, Cl2, Br, I and amines.

The amount of additive is preferably between 0.2% and 10% of the total mass of the electrolyte.

According to a second aspect of the present invention, a lithium-sulfur battery is provided comprising a negative electrode that includes a lithium-containing material, a positive electrode that includes a sulfur-containing material and an electrolyte of the first aspect.

The negative electrode may include a negative electrochemically active material selected from a list that

10 comprises: metallic lithium, lithium-containing alloy, lithium combined with non-active sulfur, a compound that can reversibly interleave lithium ions (such as metal powders, intercalated lithium-carbon metal and lithium-graphite metal and mixtures thereof). same), and compounds that can reversibly undergo a reduction-oxidation process with lithium ions.

The positive electrode may include a positive electrochemically active material comprising at least one

Sulfur-based material selected from a list comprising: elemental sulfur, Li2Sn compounds (with n ≥ 1), organic sulfur compounds and sulfur-containing polymers.

For a better understanding of the present invention and to show how it can be done, reference will now be made by way of the example to the accompanying drawing, in which:

FIGURE 1 is a constitution diagram of a sulfolane-methylpropylsulfone system.

20 Example 1

Low molecular weight sulfones were synthesized. Its properties were studied and the results are presented in the table.

Name

Molecular mass Density 103 * kg / m3 Viscosity N * s / m2, 103 Molar Volume, m3 / mol * 106 Freezing Temperature ºC Refractive index nD Permission ε

Methylene Sulfone *

108.2 1.1638 * 4.75 * 93.0 * 34.5 1,4453 57.5

Methylpropylsulfone

122.2 1,1081 5.22 110.3 32.5 1,4472 40.2

Methylbutyl Sulfone

136.2 1,0686 6.58 127.5 30.3 1,4485 35.1

Sulfolane

120.2 1,2594 9.04 95.4 28.4 1.4820 42.9

2,4-methylsulfolane

148.2 1,1263 6.74 131.6 18.0 1,4708 30.0

* t = 40 ° C

Example 2

A composition was prepared from 0.8 ml of methylpropylsulfone (melting temperature 32.5 ° C) and 0.2 ml of sulfolane (melting temperature 28.4 ° C). The melting temperature of the mixture was + 21 ° C.

Example 3

A composition was prepared from 0.6 ml of methylpropylsulfone (melting temperature 32.5 ° C) and 0.4 ml of sulfolane (melting temperature 28.4 ° C). The melting temperature of the mixture was + 6 ° C.

Example 4

A composition was prepared from 0.4 ml of methylpropylsulfone (melting temperature 32.5 ° C) and 0.6 ml of sulfolane (melting temperature 28.4 ° C). The melting temperature of the mixture was -8.5 ° C.

Example 5

A composition was prepared from 0.2 ml of methylpropylsulfone (melting temperature 32.5 ° C) and 0.8 ml of sulfolane (melting temperature 28.4 ° C). The melting temperature of the mixture was + 0.5 ° C.

The constitution diagram of Figure 1 shows the melting temperatures of pure sulfolane and pure methylpropylsulfone, and also the melting temperatures of the mixtures thereof. Extrapolation provides the composition of the eutectic mixture and its melting temperature. The data obtained indicate that the melting temperature of the eutectic mixture is approximately -17 ° C, which is approximately 47 ° C less than that of the initial sulfone components.

Example 6 (Comparison)

A lithium-sulfur battery was produced, the battery comprising an anode made of metallic lithium foil, a Celgard® separator and a sulfur cathode containing elemental sulfur (70% by weight) as a depolarizer, carbon conductive additive (Ketjenblack EC-600JD, 10% by weight) and a binder (polyethylene oxide with molecular mass 4000000, 20% by weight). The specific energy of the cathode was measured to be 2 mAh / cm2. The mounted battery was filled with an electrolyte in the form of a 1M solution of LiClO4 in sulfolane. The battery was cycled with a current density of 0.3 mA / cm2 at 25 ° C. The discharge capacity in the first cycle was 1.45 mAh / cm2. The degree of sulfur utilization was 72.5%.

Example 7 (Comparison)

A lithium-sulfur battery was produced as described in Example 6. The battery was cycled with a current density of 0.3 mA / cm2 at 0 ° C. The discharge capacity in the first cycle was 0.42 mAh / cm2. The degree of sulfur utilization was 21%.

Example 8 (Comparison)

A lithium-sulfur battery was produced as described in Example 6. The battery was cycled with a current density of 0.3 mA / cm2 at -10 ° C. The discharge capacity in the first cycle was 0.02 mAh / cm2. The degree of sulfur utilization was 1%.

Example 9

A lithium-sulfur battery was produced as described in Example 6, although using, instead, as the electrolyte, a 1M solution of LiClO4 in a eutectic mixture of sulfolane (2M) and ethylbutyl sulfone (dM). The battery was cycled with a current density of 0.3 mA / cm2 at 25 ° C. The discharge capacity in the first cycle was 1.53 mAh / cm2. The degree of sulfur utilization was 76.5%.

Example 10

A lithium-sulfur battery was produced as described in Example 6, although using, instead, as the electrolyte, a solution of LiClO4 in a eutectic mixture of sulfolane (2M) and ethylbutyl sulfone (1M). The battery was cycled with a current density of 0.3 mA / cm2 at -10 ° C. The discharge capacity in the first cycle was 1.01 mAh / cm2. The degree of sulfur utilization was 50.5%.

Example 11 (Comparison)

A lithium-sulfur battery was produced as described in Example 6, although using, instead, as the electrolyte, a 1M solution of LiClO4 in 2,4-dimethylsulfolene. The battery was cycled with a current density of 0.3 mA / cm2 at -10 ° C. The discharge capacity in the first cycle was 0.13 mAh / cm2. The degree of sulfur utilization was 6.5%.

The examples listed above demonstrate the advantages of electrolyte batteries made from solutions of electrolyte salts in eutectic sulfone mixtures. At reduced temperatures (0 ° C to -10 ° C) the discharge capacity and the degree of sulfur utilization are respectively 2.5 and 6 times higher than for non-eutectic electrolyte solutions.

5 Throughout the description and claims herein, the words "understand" and "contain" and variations of the words, for example "that comprise" and "understand", mean "that they are included but not limited to", and it is not intended to exclude (or not do) other components, integers, residues, additives or stages.

Throughout the description and claims in this report, the singular encompasses the plural unless the context needs otherwise. In particular, where the indefinite article is used, it will be understood that the memory contemplates the

10 plurality in addition to the singularity, unless the context needs something else.

Claims (13)

one.

 An electrolyte for a lithium-sulfur battery, the electrolyte comprising a solution of at least one electrolyte salt in at least two aprotic solvents, characterized in that the first aprotic solvent is sulfolane and the second aprotic solvent, is a sulfone, and in where the concentrations of the components of the solution are selected so that the solution is at its eutectic concentration or at most ± 30% of its eutectic concentration.

2.

 An electrolyte as claimed in claim 1, wherein the concentrations of the components of the solution are selected so that the solution is at most ± 20% of its eutectic concentration.

3.

 An electrolyte as claimed in claim 1, wherein the concentrations of the components of the solution are selected such that the solution is at most ± 10% of its eutectic concentration.

Four.

 An electrolyte as claimed in any of the preceding claims, wherein the second aprotic solvent is a sulfone having a molecular weight of 94 to 150.

5.

 An electrolyte as claimed in any preceding claim, wherein the electrolyte salt is at least one salt or a mixture of salts selected from a group comprising: lithium hexafluorophosphate (LiPF6), lithium hexafluoroarsenate (LiAsF6), lithium perchlorate (LiClO4), lithium sulfonylimide trifluoromethane (LiN (CF3SO2) 2) and lithium trifluorosulfonate (CF3SO3Li).

6.

 An electrolyte as claimed in any preceding claim, wherein the concentration of the at least one electrolyte salt is between 0.1M and a saturation concentration.

7.

 An electrolyte as claimed in any one of the preceding claims, which further includes as an additive one or more organic or non-organic additives to contribute to a solid electrolyte interface (IES) phase formation on a surface of a negative electrode and for improve a negative electrode cycling.

8.

An electrolyte as claimed in claim 7, wherein said additive is at least one substance or a mixture of substances selected from a group comprising: S, Li2Sn, CO2, SO2, N2O, Cl2, Br, I and amines.

9.

 An electrolyte as claimed in claim 8 or 9, wherein the amount of additive is between 0.2% and 10% of the total mass of the electrolyte.

10.

 A lithium-sulfur battery comprising a negative electrode that includes a lithium-containing material, a positive electrode that includes a sulfur-containing material and an electrolyte as claimed in any one of claims 1 to 9.

eleven.

 A battery as claimed in claim 10, wherein the negative electrode includes a negative electrochemically active material selected from a list comprising: metallic lithium, lithium-containing alloy, lithium combined with non-active sulfur, a compound that can intercalate reversible lithium ions (which includes but is not limited to metallic powders, sandwiched with lithium-carbon metal and lithium-graphite metal and mixtures thereof, and compounds that can reversibly undergo an oxidation reduction process with lithium ion .

12.

 A battery as claimed in any one of claims 10 or 11, wherein the positive electrode includes a positive electrochemically active material comprising at least one sulfur-based material selected from a list comprising: elemental sulfur, Li2Sn compounds (with n ≥ 1), organic sulfur compounds and sulfur-containing polymers.

13.

 The use of an electrolyte as claimed in any one of claims 1 to 9 as an electrolyte in a sulfur lithium battery.