Electrolyte for lithium sulfur battery

Abstract

The present invention relates to an electrolyte for use in a lithium-sulfur battery, the electrolyte comprising a non-aqueous solvent, a lithium salt, and a redox-mediating additive. The redox-mediating additive is a compound having a structure in which two or more rings are conjugated and the total number of ring atoms is 8 to 14.

Description

Electrolyte for lithium-sulfur batteries

[0001] The present invention relates to an electrolyte usable in a lithium-sulfur battery.

[0002] This application claims priority to Korean Patent Application No. 10-2024-0052295, filed April 18, 2024, and Korean Patent Application No. 10-2025-0050447, filed April 17, 2025, the entire contents of which are disclosed in the specification and drawings of the aforementioned applications are incorporated herein by reference.

[0003] As the scope of application of lithium secondary batteries expands beyond portable electronic devices to include electric vehicles (EVs) and electric storage systems (ESS), the demand for lithium secondary batteries with high capacity, high energy density, and long lifespan is increasing.

[0004] Among various lithium secondary batteries, the lithium-sulfur battery is a battery system that uses a sulfur-based material containing a sulfur-sulfur bond as a positive electrode active material, and lithium metal, a carbon-based material in which lithium ions can be inserted / deinserted, or silicon or tin that forms an alloy with lithium as a negative electrode active material.

[0005] Lithium-sulfur batteries are based on the conversion reaction of lithium ions and sulfur (S8+16Li) at the cathode. + +16e - -> The theoretical specific capacity from 8Li2S reaches 1,675 mAh / g, and when lithium metal is used as the negative electrode, it shows a theoretical energy density of 2,600 Wh / kg. This is a very high figure compared to the theoretical energy density of other battery systems currently being studied (Ni-MH battery: 450 Wh / kg, Li-FeS battery: 480 Wh / kg, Li-MnO2 battery: 1,000 Wh / kg, Na-S battery: 800 Wh / kg) and lithium ion batteries (250 Wh / kg), and therefore, it is attracting attention as a high-capacity, eco-friendly, and low-cost lithium secondary battery among the secondary batteries being developed so far.

[0006] However, lithium-sulfur batteries have problems such as a decrease in reversible capacity and poor cycle stability due to repeated charge and discharge cycles due to the high energy barrier of the lithium sulfide conversion reaction.

[0007] To address this, various forms of electrochemical catalysts are being developed. However, most battery performance improvements occur under conditions of excess electrolyte, which facilitates the formation of a three-phase interface (catalyst / conductive support / electrolyte). However, using excessive electrolyte within a battery has the disadvantage of lowering the battery's energy density, making it unsuitable for battery development requiring high energy density performance.

[0008] Therefore, although many previous studies have been conducted on electrochemical catalysts that are active under extremely low electrolyte conditions, their catalytic activity is limited due to the decrease in lithium ion conduction and low lithium polysulfide accessibility to the three-phase interface caused by the extremely high viscosity of the electrolyte under extremely low electrolyte conditions.

[0009] Therefore, the development of catalysts and additives that exhibit high catalytic activity even under extremely small electrolyte conditions when used in lithium-sulfur batteries is becoming a very important task.

[0010] The present invention is to solve the above-described problem,

[0011] The goal is to develop and provide an electrolyte additive that can promote redox reactions in lithium-sulfur batteries even under extremely low electrolyte conditions. Through this, the goal is to improve the reversible capacity decay of lithium-sulfur batteries even under extremely low electrolyte conditions and provide an electrolyte composition that significantly enhances the energy density of lithium-sulfur batteries.

[0012] Ultimately, the goal is to provide a lithium-sulfur battery with improved performance using the provided electrolyte.

[0013] To achieve the above purpose,

[0014] According to one aspect of the present invention, electrolytes for lithium-sulfur batteries of the following embodiments are provided.

[0015] The electrolyte according to the first embodiment is

[0016] Containing a non-aqueous solvent, a lithium salt and a redox mediator additive,

[0017] The above redox mediating additive is a compound having a structure in which two or more rings are conjugated and the total number of ring atoms is 8 to 14.

[0018] According to the second embodiment, in the first embodiment,

[0019] The above redox mediator may be a compound having a structure in which two or three rings are conjugated and the total number of ring atoms is 9 to 13.

[0020] According to the third embodiment, in the first embodiment or the second embodiment,

[0021] The above redox mediating additive may include 9-fluorenone, 2,1,3-benzothiadiazole, N-methyl phthalimide, or a mixture of two or more thereof.

[0022] According to the fourth embodiment, in any one of the first to third embodiments,

[0023] The molar concentration of the above redox mediating additive may be 1 mM to 100 mM.

[0024] According to the fifth embodiment, in any one of the first to fourth embodiments,

[0025] May contain additional inorganic nitrates.

[0026] According to the sixth embodiment, in any one of the first to fifth embodiments,

[0027] The above inorganic nitrate may include lithium nitrate (LiNO3).

[0028] According to the seventh embodiment, in any one of the first to sixth embodiments,

[0029] The above non-aqueous solvent may contain 80% or more of an ether solvent by volume.

[0030] According to the eighth embodiment, in any one of the first to seventh embodiments,

[0031] The above lithium salt may include an organic lithium salt containing fluorine (F).

[0032] According to the ninth embodiment, in any one of the first to eighth embodiments,

[0033] The molar concentration of the above redox mediating additive may be lower than the molar concentration of the above lithium salt.

[0034] According to the tenth embodiment, in any one of the first to ninth embodiments,

[0035] The ratio of the molar concentrations of the lithium salt and the redox mediator additive may be 50:1 to 1000:1.

[0036]

[0037] According to another aspect of the present invention, lithium-sulfur batteries of the following embodiments are provided.

[0038] A lithium-sulfur battery according to the 11th embodiment,

[0039] A lithium-sulfur battery comprising an electrolyte, a positive electrode, an negative electrode, a separator interposed between the negative electrode and the positive electrode, and a battery case according to any one of the first to tenth embodiments, wherein the positive electrode comprises a sulfur series compound containing a sulfur (S)-sulfur (S) bond as an active material, and the negative electrode comprises a lithium metal layer.

[0040] According to the 12th embodiment, in the 11th embodiment,

[0041] The active material of the positive electrode may include a sulfur-carbon complex in which the sulfur-based compound is supported on at least one of the outer surface and the interior of the pores of the porous carbon material.

[0042] According to the 13th embodiment, in the 11th embodiment or the 12th embodiment,

[0043] The above lithium metal layer may include a lithium metal (Li) foil or a lithium alloy foil.

[0044] According to the 14th embodiment, in any one of the 11th to 13th embodiments,

[0045] The weight ratio (El / S) of sulfur (S) in the active material of the positive electrode to the weight of the electrolyte for the lithium-sulfur battery may be 10 g / g or less.

[0046] According to the 15th embodiment, in any one of the 11th to 14th embodiments,

[0047] The above lithium-sulfur battery may be a coin-type, pouch-type or cylindrical battery.

[0048] Conventional techniques for introducing catalysts to catalyze the conversion reaction of lithium sulfide (Li2S) into the anode of lithium-sulfur batteries have limited the formation of a three-phase interface under extremely low electrolyte conditions, as the catalyst is fixedly positioned on the anode. This has led to a problem where lithium-sulfur battery activity remains low even under extremely low electrolyte conditions.

[0049] In contrast, according to one aspect of the present invention, a redox mediator additive introduced into a lithium-sulfur battery electrolyte can move fluidly within the battery, thereby enhancing accessibility to the three-phase interface. This facilitates the conversion reaction of lithium sulfide, thereby providing a lithium-sulfur battery with enhanced reversible capacity and cycle stability even under extremely small electrolyte conditions.

[0050] Through this, by using the electrolyte of the present invention, a lithium-sulfur battery having improved energy density, improved overvoltage problem, and improved charge / discharge cycle stability can be provided.

[0051] Figure 1 shows a CV curve at the point of 10 repeated charge / discharge cycles of a lithium-sulfur battery using the electrolyte of Example 1 in the present specification.

[0052] Figure 2 shows the CV curve at the first charge / discharge cycle of a lithium-sulfur battery using the electrolyte of Example 1 and Comparative Example 1 in the present specification.

[0053] Figure 3 shows the resistance curve at the first charge / discharge cycle of a lithium-sulfur battery using the electrolyte of Example 1 and Comparative Example 1 in the present specification.

[0054] Figure 4 shows a discharge capacity curve according to repeated charge / discharge cycles of a lithium-sulfur battery using the electrolyte of Example 1 and Comparative Example 1 in the present specification.

[0055] Hereinafter, the present invention will be described in more detail.

[0056] The term "composite" used in this specification refers to a material in which two or more materials are combined to form physically and chemically different phases and exhibit more effective functions.

[0057] The term "(poly)sulfide" as used herein means "(poly)sulfide ion (S x 2- , 1≤x≤8)" and "lithium (poly)sulfide (Li2S x or Li2S x - It is a concept that includes all of "1≤x≤8".

[0058] The term "polysulfide" as used herein means "polysulfide ion (S x 2- , 1 <x≤8)" 및 "리튬폴리설파이드(Li2S x or Li2S x - 1 <x≤8)"를 모두 포함하는 개념이다.

[0059] The energy density of a lithium secondary battery can be measured using the following formula.

[0060]

[0061] Energy density = [(discharge capacity) X (driving voltage)] / (cell weight)

[0062] As shown in the equation above, even if a similar discharge capacity is achieved at the same operating voltage, a larger cell weight leads to a decrease in energy density. Consequently, even if the reversible capacity is increased indefinitely by increasing the amount of electrolyte, the increased cell weight due to the electrolyte itself will not be beneficial for improving energy density.

[0063] According to one aspect of the present invention, an electrolyte additive is provided that favors the formation of a three-phase interface of catalyst / conductive support / electrolyte in a lithium-sulfur battery by providing an additional reaction path for the conversion reaction of lithium (poly)sulfide even under lean electrolyte conditions.

[0064] In this specification, the additive is referred to as a 'redox mediator additive' in that it promotes the redox reaction of lithium (poly)sulfide.

[0065] In the present invention, the redox mediating additive is a compound having a structure in which two or more rings are conjugated and the total number of ring atoms is 8 to 14.

[0066] The above redox mediator additive has a conjugated structure with delocalized electrons within the structure to provide additional reaction paths for the redox reaction of lithium (poly)sulfide. In particular, it includes two or three ring structures so that all of the conjugated structures are connected, and includes a heteroatom, such as an oxygen atom, a nitrogen atom, a sulfur atom, a phosphorus atom, or two or more atoms thereof, within the structure to exhibit effects such as increasing affinity with a non-aqueous solvent in an electrolyte.

[0067] Specifically, taking into account the solubility of the redox mediator additive in the electrolyte, etc., the redox mediator additive may have two or three conjugated rings, and the total number of ring atoms may be 8 to 14, specifically the total number of ring atoms may be 9 to 14, and more specifically the total number of ring atoms may be 9 to 13, 9 to 12, 9 to 11, 9 to 10, or 8 to 9.

[0068] More specifically, the heteroatom included in the redox mediating additive may include an oxygen atom, a nitrogen atom, a sulfur atom, or two or more atoms thereof to form a stable conjugate system, but the present invention is not limited thereto.

[0069] In one embodiment of the present invention, the redox mediating additive may be a polycyclic aromatic compound having two or three conjugated aromatic rings. The redox mediating additive may include, but is not limited to, naphthalene, 1-naphthol, acenaphthylene, anthracene, benzophenone, 9-fluorenone, 2,1,3-benzothiadiazole, N-methyl phthalimide, or a mixture of two or more thereof.

[0070] In one embodiment of the present invention, in terms of high efficiency of the redox mediating additive, the two or three rings may be directly bonded and have a form in which they are in contact with each other.

[0071] In one embodiment of the present invention, the redox mediating additive may include one or more heteroatoms in its structure, and may have a form in which two or three rings are directly bonded and in contact with each other.

[0072] In one embodiment of the present invention, the redox mediating additive may include, for example, 9-fluorenone, 2,1,3-benzothiadiazole, N-methyl phthalimide, or a mixture of two or more thereof, but the present invention is not limited thereto.

[0073] [Chemical formula of 9-fluorenone]

[0074]

[0075]

[0076] [Chemical formula of 2,1,3-benzothiadiazole]

[0077]

[0078]

[0079] [Chemical formula of N-methyl phthalimide]

[0080]

[0081]

[0082] In one embodiment of the present invention, the redox mediating additive may include 9-fluorenone.

[0083] In one embodiment of the present invention, the redox mediating additive may include 2,1,3-benzothiadiazole.

[0084] In one embodiment of the present invention, the redox mediating additive may include N-methyl phthalimide.

[0085] In one embodiment of the present invention, the molar concentration of the redox mediator additive in the electrolyte may be, for example, preferably 1 mM to 100 mM. Specifically, the molar concentration of the redox mediator additive in the electrolyte may be 1 mM to 50 mM, 1 mM to 25 mM, 1 mM to 20 mM, 5 mM to 15 mM, or 5 mM to 10 mM. When the molar concentration of the redox mediator additive is within the above range, it may be preferable in terms of increasing the solubility in the electrolyte and the reversible capacity of a lithium-sulfur battery and improving the overvoltage, but the present invention is not limited thereto.

[0086] In one embodiment of the present invention, the redox mediating additive is dissolved in the electrolyte and is present in the positive electrode, such as inorganic sulfur (S8) or lithium sulfide (Li2S) and lithium polysulfide (Li2Sx, 1 <x≤8) 간의 산화환원 반응에 대한 추가적인 반응 경로를 제공하는 역할을 수행할 수 있는 양으로 포함되면 충분하고, 전해질 내에서 상기 리튬염보다 낮은 농도로 포함되는 것이 바람직할 수 있다.

[0087] In this respect, according to one embodiment of the present invention, it may be preferable that the molar concentration of the redox mediator additive be lower than the molar concentration of the lithium salt. Specifically, the ratio of the molar concentrations of the lithium salt and the redox mediator additive in the electrolyte may be, for example, 50:1 to 1000:1, specifically 50:1 to 500:1, 50:1 to 250:1, 50:1 to 200:1, 50:1 to 150:1, 100:1 to 200:1, or 100:1 to 150:1.

[0088] In the present invention, the lithium salt is a lithium ion (Li) as a cation. + ) is a general term for all salt compounds containing lithium salts. The lithium salts may be included as a medium for transferring lithium ions and electrons between the positive and negative electrodes.

[0089] In one embodiment of the present invention, the lithium salt may include an organic lithium salt, an inorganic lithium salt, or a mixture thereof.

[0090] Specifically, the lithium salt may include an organic lithium salt containing fluorine (F). For example, the lithium salt may include a fluorine-containing organic lithium salt such as LiCF3SO3, LiCF3CO2, LiSO3CF3, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiC(SO2CF3)3, or a mixture of two or more thereof, but the present invention is not limited thereto.

[0091] In one embodiment of the present invention, the lithium salt may include an inorganic lithium salt containing fluorine (F). For example, the lithium salt may include LiBF4, LiPF6, LiAsF6, LiSbF6, LiN(SO2F)2, or a mixture of two or more thereof as a fluorine-containing inorganic lithium salt, but the present invention is not limited thereto.

[0092] In one embodiment of the present invention, the lithium salt may include a mixture of an organic lithium salt containing fluorine (F) and an inorganic lithium salt containing fluorine (F), but the present invention is not limited thereto.

[0093] In another embodiment of the present invention, the lithium salt may be composed of a fluorine-containing organic lithium salt. Specifically, the lithium salt may comprise LiN(SO2CF3)2(LiTFSI) alone. When the lithium salt comprises LiTFSI alone, it may be preferable in terms of improving the reactivity of lithium (poly)sulfide by a redox mediator additive in the electrolyte, but the present invention is not limited thereto.

[0094] In one embodiment of the present invention, the lithium salt is not limited to a lithium salt other than a lithium salt containing fluorine, as long as it does not hinder the purpose of the present invention, and can be used as a lithium salt in the electrolyte of a lithium-sulfur battery. For example, lithium salts that can be further included include, for example, LiCl, LiBr, LiI, LiClO4, LiB. 10 Cl 10 , LiC4BO8, LiAlCl4, LiSO3CH3, lithium chloroborane, lithium lower aliphatic carboxylic acid, lithium 4-phenylborate, lithium imide, or two or more thereof, but the present invention is not limited thereto.

[0095] In one embodiment of the present invention, the molar concentration of the lithium salt may be limited to, for example, 1.75 M or less, specifically, 1.5 M or less, or 1.0 M or less, based on the mixture of the non-aqueous solvent and the lithium salt. Alternatively, the molar concentration of the lithium salt may be, for example, 0.5 M to 1.75 M, 0.75 M to 1.5 M, or 0.75 M to 1.0 M, based on the mixture of the non-aqueous solvent and the lithium salt. When the concentration of the lithium salt is in the above-described range, it may exhibit a beneficial effect on the solubility of the lithium salt in the electrolyte and the improvement effect of the battery performance by the redox mediating additive, but the present invention is not limited thereto.

[0096] Meanwhile, lithium secondary batteries have a characteristic in that lithium ions are reduced on the surface of the negative electrode during charging and discharging, and lithium metal accumulates on the negative electrode through repeated charging and discharging. According to one aspect of the present invention, in order to improve the lifespan and coulombic efficiency of a lithium-sulfur battery using the same by suppressing and preventing the formation of lithium ion dendrites on the surface of the negative electrode including a lithium metal layer, the electrolyte may further include nitrate.

[0097] In the present invention, the nitrate is an anion of nitrate (-NO3 - ) is a general term for all salt compounds containing nitrate. At this time, lithium nitrate (LiNO3) may be included as a type of inorganic nitrate among the nitrates, and since the lithium nitrate contains lithium ion as a cation and nitrate ion as an anion, it may correspond to both lithium salt and nitrate. In the present invention, the lithium nitrate is not included solely as a lithium salt, but may be included as an additive additionally included in addition to the lithium salt as a nitrate.

[0098] The above nitrate may be used to prevent deterioration of the negative electrode and improve the life of the battery by inhibiting the dissolution of polysulfide from the positive electrode into the electrolyte phase, by preventing the decomposition and loss of lithium salt, or by forming a protective layer on the negative electrode surface, and may include, for example, inorganic nitrate, organic nitrate, or a mixture thereof.

[0099] In the present invention, -NO3 in the nitrate - Although the role of ions is not limited to this, they can play a role in improving the life of lithium-sulfur batteries by forming LiNxOy materials in the protective layer formed on the negative electrode (also called the solid electrolyte interface (SEI)), thereby stabilizing the negative electrode.

[0100] The above inorganic nitrate may include, for example, a monovalent nitrate, and may typically include lithium nitrate (LiNO3), but the present invention is not limited thereto.

[0101] In one embodiment of the present invention, the electrolyte may further include the lithium nitrate together with the redox mediating additive, thereby exhibiting advantageous effects in terms of improving the conversion reaction of lithium (poly)sulfide and stabilizing the negative electrode.

[0102] In one embodiment of the present invention, the inorganic nitrate may be included in an amount of 0.1 to 8 parts by weight, specifically 0.5 to 6 parts by weight, 1 to 5 parts by weight, 1 to 3 parts by weight, 2 to 3 parts by weight, or 1 to 2 parts by weight, based on 100 parts by weight of the sum of the non-aqueous solvent and the lithium salt, in terms of solubility in the electrolyte. When the inorganic nitrate is included in an amount within the above-described range, it may exhibit a beneficial effect in terms of improving the lifespan of the battery due to the inorganic nitrate, but the present invention is not limited thereto.

[0103] In one embodiment of the present invention, the non-aqueous solvent may be used without particular limitation as long as it can be used as a solvent that can dissolve the lithium salt, the redox mediator additive, and, if the electrolyte of a lithium-sulfur battery further includes an inorganic nitrate, dissolve the inorganic nitrate. For example, the non-aqueous solvent may be an ether solvent, an ester solvent, an amide solvent, a carbonate solvent, or a mixture of two or more thereof. However, if the electrolyte further includes an inorganic nitrate, it may be preferable that the non-aqueous solvent not include a carbonate solvent in terms of the solubility of the inorganic nitrate.

[0104] The above ether solvent may include an acyclic ether, a cyclic ether, or a mixture thereof.

[0105] The above acyclic ether may include, for example, dimethyl ether, diethyl ether, dipropyl ether, dibutyl ether, diisobutyl ether, ethyl methyl ether, ethylpropyl ether, ethyl tertbutyl ether, dimethoxymethane, trimethoxymethane, dimethoxyethane, diethoxyethane, dimethoxypropane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, ethylene glycol divinyl ether, diethylene glycol divinyl ether, triethylene glycol divinyl ether, dipropylene glycol dimethylene ether, butylene glycol ether, diethylene glycol ethyl methyl ether, diethylene glycol isopropyl methyl ether, diethylene glycol butyl methyl ether, diethylene glycol tertbutyl ethyl ether, ethylene glycol ethyl methyl ether, or a mixture of two or more thereof. Preferably, it may include dimethoxyethane, diethoxyethane, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, or a mixture of two or more thereof. More preferably, it may include dimethoxyethane.

[0106] The above cyclic ethers include, for example, 1,3-dioxolane, 4,5-dimethyl-dioxolane, 4,5-diethyl-dioxolane, 4-methyl-1,3-dioxolane, 4-ethyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, 2,5-dimethoxytetrahydrofuran, 2-ethoxytetrahydrofuran, 2-methyl-1,3-dioxolane, 2-vinyl-1,3-dioxolane, 2,2-dimethyl-1,3-dioxolane, 2-methoxy-1,3-dioxolane, 2-ethyl-2-methyl-1,3-dioxolane, tetrahydropyran, 1,4-dioxane, 1,2-dimethoxybenzene, 1,3-Dimethoxybenzene, 1,4-dimethoxybenzene and isosorbide dimethyl ether, furan, 2-methylfuran, 3-methylfuran, 2-ethylfuran, 2-propylfuran, 2-butylfuran, 2,3-dimethylfuran, 2,4-dimethylfuran, 2,5-dimethylfuran, 2H-pyran, 4H-pyran, 2-methylpyran, 3-methylpyran, 4-methylpyran, benzofuran, 2-(2-Nitrovinyl)furan or a mixture of two or more selected from these, but is not limited thereto.

[0107] In one embodiment of the present invention, the non-aqueous solvent may preferably contain at least 80% by volume of an ether solvent. More specifically, the non-aqueous solvent may be composed solely of an ether solvent.

[0108] In one embodiment of the present invention, the non-aqueous solvent may include a mixture of an acyclic ether and a cyclic ether, for example, a mixture of dimethoxyether (DME) and 1,3-dioxolane (DOL).

[0109] In one embodiment of the present invention, when the non-aqueous solvent includes a mixture of an acyclic ether and a cyclic ether, the mixing volume ratio of the acyclic ether and the cyclic ether may be specifically 9:1 to 1:9, for example, 5:5, but the present invention is not limited thereto.

[0110] The ester solvent may be, for example, one selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone, or a mixture of two or more thereof, but is not limited thereto.

[0111] The above carbonate solvent may be, for example, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethylmethyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, vinylene carbonate, vinylethylene carbonate, halides thereof (e.g., fluoroethylene carbonate, etc.), or mixtures of two or more thereof, but is not limited thereto.

[0112] As described above, in one embodiment of the present invention, since the carbonate-based solvent does not dissolve the nitrate, specifically, the inorganic nitrate, or exhibits low solubility, the non-aqueous solvent may not contain the carbonate-based solvent. In one embodiment of the present invention, the non-aqueous solvent may contain a very small amount of the carbonate-based solvent such that the carbonate-based solvent does not affect the solubility of the nitrate, and for example, when the non-aqueous solvent contains the carbonate-based solvent, the content of the carbonate-based solvent may be 3 wt% or less, 2 wt% or less, 1 wt% or less, 0.5 wt% or less, or 0 wt% (i.e., not contained at all) based on the total weight of the electrolyte for the lithium-sulfur battery.

[0113] As described above, according to one aspect of the present invention, an electrolyte having a novel composition that can be used in a lithium-sulfur battery to exhibit high reversible capacity and improved cycle stability is provided.

[0114]

[0115] According to another aspect of the present invention, a lithium-sulfur battery comprising an electrolyte for a lithium-sulfur battery having the composition described above is provided.

[0116] The above lithium-sulfur battery includes the above-described electrolyte, positive electrode, negative electrode, a separator interposed between the negative electrode and positive electrode, and a battery case.

[0117] According to one aspect of the present invention, the lithium-sulfur battery may mean a battery including a sulfur series compound containing a sulfur (S)-sulfur (S) bond as a positive electrode active material and including a lithium metal layer as a negative electrode.

[0118] In one embodiment of the present invention, the lithium metal layer may be a thin film layer composed only of lithium metal (Li).

[0119] In another embodiment of the present invention, the lithium metal layer may be a thin film layer made of lithium and a material that forms an alloy with lithium, such as silicon, tin, sodium, potassium, rubidium, cesium, francium, beryllium, magnesium, calcium, strontium, barium, radium, aluminum, or a lithium alloy alloyed with two or more of these materials.

[0120] In one embodiment of the present invention, the negative electrode may be provided as a free-standing film comprising a lithium metal layer and a protective layer without a separate support. In this case, the support may be, but is not limited to, a current collector used in a conventional lithium secondary battery electrode or a polyolefin porous support used as a separator.

[0121] In another embodiment of the present invention, the negative electrode may be provided in a form including a lithium metal layer and a protective layer (also called a solid electrolyte interface (SEI)) on a support.

[0122] In one embodiment of the present invention, the negative electrode may include the lithium metal layer and a solid electrolyte interface (SEI) formed on the surface of the lithium metal layer. At this time, the solid electrolyte interface (SEI) may contain, as a constituent, at least one element selected from fluorine atoms (F) and nitrogen atoms (N). More specifically, the solid electrolyte interface may contain at least one element from among fluorine atoms (F) derived from the fluorine-containing lithium salt and nitrogen atoms (N) derived from the inorganic nitrate.

[0123] In addition, the negative electrode according to one embodiment of the present invention may further include a conventional configuration that can be used for the negative electrode of a lithium secondary battery, particularly a lithium-sulfur battery, and is not particularly limited to the configuration further included as long as it does not impede the purpose of the present invention.

[0124] In one embodiment of the present invention, the positive electrode can be used without limitation as long as it includes a sulfur series compound containing a sulfur (S)-sulfur (S) bond as an active material.

[0125] In one embodiment of the present invention, the positive electrode active material may include a sulfur-carbon complex.

[0126] In one embodiment of the present invention, the sulfur-carbon composite may include a porous carbon material; and a sulfur-based compound supported on at least one of the outer surface and the interior of the pores of the porous carbon material. In the case of sulfur acting as the positive electrode active material, since sulfur alone does not have electrical conductivity, it is used in combination with a conductive material such as a carbon material, and a porous carbon material may be used to support sulfur. In addition, the sulfur-based compound may include, for example, inorganic sulfur (S8), lithium sulfide (Li2S), lithium polysulfide (Li2Sx, 1 <x≤8), 디설파이드 화합물, 탄소-황 폴리머((C2S y ) n , y = 2.5 to 50, n≥2), or two or more of these. Preferably, the sulfur compound may be inorganic sulfur (S8).

[0127] In one embodiment of the present invention, the porous carbon material supports a sulfur-based compound as a positive electrode active material, provides a framework in which the sulfur-based compound can be uniformly and stably fixed, and improves the conductivity of the positive electrode. Any porous carbon material can be used without particular limitation. The shape of the porous carbon material can be spherical, rod-shaped, needle-shaped, plate-shaped, tubular, or bulk-shaped, and can be used without limitation as long as it is commonly used in lithium-sulfur batteries. The porous carbon material may be any material that has a porous structure or a high specific surface area and is commonly used in the art.

[0128] For example, the porous carbon material may be at least one selected from the group consisting of graphite; graphene; carbon black such as Denka black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and summer black; carbon nanotubes (CNTs) such as single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs); carbon fibers such as graphite nanofibers (GNFs), carbon nanofibers (CNFs), and activated carbon fibers (ACFs); and graphite and activated carbon such as natural graphite, artificial graphite, and expanded graphite, but is not limited thereto. Preferably, the porous carbon material may be a carbon nanotube.

[0129] In one embodiment of the present invention, the porous carbon material may include, for example, carbon nanotubes (CNTs).

[0130] In one embodiment of the present invention, the sulfur-carbon composite may contain the sulfur-based compound in an amount of 65 wt% or more, for example, 65 wt% to 90 wt%, 65 wt% to 85 wt%, 70 wt% to 80 wt%, or 70 wt% to 75 wt%, based on the total weight of the sulfur-based compound and the porous carbon material. When the content of the sulfur-based compound in the sulfur-carbon composite is within the above-described range, it may be preferable in terms of the electron transfer area of ​​the sulfur-carbon composite and the wettability of the positive electrode with the electrolyte, and for example, it may be preferable in suppressing the elution of sulfur from the positive electrode because the available surface of the sulfur-carbon composite increases, but the present invention is not limited thereto.

[0131] The method for manufacturing the above sulfur-carbon composite is not particularly limited in the present invention, and any method commonly used in the art may be used. For example, a method of simply mixing the sulfur and the porous carbon material and then heat-treating to form a composite may be used. Specifically, a sulfur-carbon composite formed by a melt-diffusion method may be used, and for example, a sulfur-carbon composite formed by mixing the porous carbon material and the sulfur-based compound and then heat-treating at 150°C to 160°C for 10 to 24 hours, for example, at 155°C for 12 hours, may be used.

[0132] In one embodiment of the present invention, the positive electrode active material layer may further include a conductive agent, a binder, and additives in addition to the positive electrode active material described above. The conductive agent, binder, and other components that may be used in the positive electrode active material layer may utilize conventional techniques and are not particularly limited to the present invention.

[0133] In one embodiment of the present invention, the positive electrode includes a current collector having the positive electrode active material layer on one or both sides. The current collector supports the positive electrode active material and is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, palladium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, silver, etc., aluminum-cadmium alloy, etc. can be used.

[0134] The above positive electrode current collector can form fine irregularities on its surface to strengthen the bonding strength with the positive electrode active material, and can be used in various forms such as a film, sheet, foil, mesh, net, porous body, foam, and non-woven fabric.

[0135] In one embodiment of the present invention, the separator separates or insulates the positive and negative electrodes from each other and enables lithium ion transport between the positive and negative electrodes. The separator may be made of a porous non-conductive or insulating material, and any material commonly used as a separator in a lithium secondary battery may be used without particular limitation. The separator may be an independent member such as a film, or may be a coating layer added to the positive and / or negative electrodes.

[0136] In one embodiment of the present invention, the lithium-sulfur battery may have various shapes, for example, a coin shape, a pouch shape, or a cylindrical shape, but is not limited thereto.

[0137] In one embodiment of the present invention, the electrolyte for a lithium-sulfur battery described above exhibits the effect of implementing a lithium-sulfur battery having excellent performance even under lean electrolyte conditions.

[0138] At this time, the above lean electrolyte condition can be explained through the weight ratio of sulfur element (S) included as a positive electrode active material of a lithium-sulfur battery and the electrolyte.

[0139] For example, in the lithium-sulfur battery of the present invention, it may be preferable that the ratio (El / S) of the total weight of the electrolyte to the total weight of the sulfur element (S) in the positive electrode is, for example, 10 g / g or less, for example, 5 g / g or less. For example, the ratio (El / S) of the total weight of the electrolyte to the total weight of the sulfur element (S) in the positive electrode may be 2.0 g / g to 10 g / g, 2.0 g / g to 8.0 g / g, specifically 3.0 g / g to 7.0 g / g, and more specifically 4.5 g / g to 5.5 g / g. For example, the El / S may be 5.0 g / g.

[0140] When using an electrolyte according to one aspect of the present invention, a lithium-sulfur battery having an El / S ratio within the above-described range can be realized, thereby exhibiting an effect of improving energy density. However, a lithium-sulfur battery using the electrolyte having an El / S ratio higher than the above-described range can also be realized, and the present invention is not limited thereto.

[0141]

[0142] Hereinafter, preferred examples are presented to help understand the present invention, but the following examples are only illustrative of the present invention, and it is obvious to those skilled in the art that various changes and modifications are possible within the scope and technical idea of ​​the present invention, and it is also natural that such changes and modifications fall within the scope of the appended patent claims.

[0143] [Electrolyte production]

[0144] Comparative Example 1

[0145] An electrolyte was prepared by dissolving 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, LiN(SO2CF3)2) and 2 wt% lithium nitrate (LiNO3) as an inorganic nitrate in a mixed solvent (1:1 v / v) of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME).

[0146]

[0147] Example 1

[0148] An electrolyte was prepared by additionally adding 10 mM of N-methyl phthalimide as a redox mediator additive to the electrolyte of Comparative Example 1 prepared above and stirring for 24 hours.

[0149]

[0150] [Manufacturing of lithium-sulfur batteries]

[0151] A lithium-sulfur battery of a CR2032 coin cell was manufactured using each of the electrolytes of Comparative Example 1 and Example 1 manufactured above, using the following method.

[0152] As a cathode, a 200 μm thick lithium thin film was prepared, and as a separator, Celgard 2400 was used.

[0153] As a cathode, inorganic sulfur (S8) and multi-walled carbon nanotubes (MWCNT) as cathode active materials were mixed in a mass ratio of 7:3, and then heat-treated at 155°C for 12 hours to prepare a sulfur-carbon composite, and 90 wt% of the sulfur-carbon composite prepared above and 10 wt% of PVDF as a binder were dispersed in a solvent NMP to prepare a cathode slurry composition. The cathode slurry composition was applied to both sides of an aluminum current collector, dried at 80°C, and then rolled with a roll press to obtain 4 mg s / cm 2 An electrode having a loading amount of was prepared.

[0154] After placing the above-mentioned positive electrode / separator / negative electrode assembly into a coin-shaped case, each of the electrolytes manufactured above was injected, and the lid was covered and sealed to manufacture a lithium-sulfur battery. At this time, in order to confirm the performance of the redox mediator additive under lean electrolyte conditions, the electrolyte was injected so that the ratio of the amount of electrolyte to the amount of sulfur (S) in the positive electrode (El / S) was 5.0 g / g / .

[0155]

[0156] [Performance Evaluation of Lithium-Sulfur Battery]

[0157] The lithium-sulfur battery manufactured as described above was activated by performing 0.1C discharge three times at 30°C, and then charge and discharge (CC mode, 1.8 V to 2.5 V) was repeated with 0.1C charge and 0.1C discharge as one cycle, and the results of evaluating the performance of the battery during the charge and discharge process are shown in FIGS. 1 to 4.

[0158] Figure 1 shows a CV curve of a lithium-sulfur battery using the electrolyte of Example 1 after 10 charge-discharge cycles. According to Figure 1, it was confirmed that the redox mediator additive according to one embodiment of the present invention can function reversibly.

[0159] Figure 2 shows CV curves for one charge / discharge cycle of a lithium-sulfur battery using the electrolytes of Comparative Example 1 and Example 1, respectively. According to Figure 2, it was confirmed that when an electrolyte with a redox mediator additive according to one embodiment of the present invention was used, the sulfur electrochemical reaction was promoted, thereby improving the overvoltage.

[0160] Figure 3 shows the results of measuring the resistance of a lithium-sulfur battery after one charge / discharge cycle using the electrolytes of Comparative Example 1 and Example 1, respectively. According to Figure 3, it was confirmed that when an electrolyte with a redox mediator additive according to one embodiment of the present invention was used, the resistance within the battery was lowered, and thus the sulfur conversion reaction was promoted, thereby improving the reversible capacity of the battery.

[0161] Figure 4 shows the results of measuring the discharge capacity according to repeated charge / discharge cycles of a lithium-sulfur battery using the electrolytes of Comparative Example 1 and Example 1, respectively. According to Figure 4, it was confirmed that when an electrolyte with a redox mediator additive according to one embodiment of the present invention was used, cycle stability was improved even under lean electrolyte conditions.

[0162] Through the above experiments, it was confirmed that a lithium secondary battery, particularly a lithium-sulfur battery, using an electrolyte according to one embodiment of the present invention can exhibit a significant lifespan improvement effect and a Coulomb improvement effect.

Claims

1. Contains a non-aqueous solvent, lithium salt and redox mediator additives, An electrolyte for a lithium-sulfur battery, wherein the above redox mediator additive is a compound having a structure in which two or more rings are conjugated and the total number of ring atoms is 8 to 14.

2. In claim 1, An electrolyte for a lithium-sulfur battery, wherein the above redox mediator additive is a compound having a structure in which two or three rings are conjugated and the total number of ring atoms is 9 to 13.

3. In claim 1, An electrolyte for a lithium-sulfur battery, wherein the above redox mediator additive comprises two or three aromatic rings directly bonded and in contact with each other and one or more heteroatoms.

4. In claim 1, An electrolyte for a lithium-sulfur battery, wherein the redox mediating additive comprises 9-fluorenone, 2,1,3-benzothiadiazole, N-methyl phthalimide, naphthalene, 1-naphthol, acenaphthylene, anthracene, benzophenone, or a mixture of two or more thereof.

5. In claim 1, An electrolyte for a lithium-sulfur battery, wherein the molar concentration of the redox mediating additive is 1 mM to 100 mM.

6. In claim 1, An electrolyte for a lithium-sulfur battery further comprising inorganic nitrate.

7. In claim 6, The above inorganic nitrate is an electrolyte for a lithium-sulfur battery, comprising lithium nitrate (LiNO3).

8. In claim 1, An electrolyte for a lithium-sulfur battery, wherein the non-aqueous solvent comprises an ether solvent of 80% or more by volume.

9. In claim 1, An electrolyte for a lithium-sulfur battery, wherein the lithium salt comprises an organic lithium salt containing fluorine (F).

10. In claim 1, An electrolyte for a lithium-sulfur battery, wherein the molar concentration of the redox mediating additive is lower than the molar concentration of the lithium salt.

11. In claim 10, An electrolyte for a lithium-sulfur battery, wherein the molar concentration ratio of the lithium salt and the redox mediator additive is 50:1 to 1000:

1.

12. A lithium-sulfur battery comprising an electrolyte, a cathode, an anode, a separator interposed between the cathode and the anode, and a battery case according to any one of claims 1 to 11, The above positive electrode includes a sulfur series compound containing a sulfur (S)-sulfur (S) bond as an active material, A lithium-sulfur battery, wherein the negative electrode comprises a lithium metal layer.

13. In claim 12, A lithium-sulfur battery, wherein the active material of the positive electrode comprises a sulfur-carbon complex in which the sulfur-based compound is supported on at least one of the outer surface and the interior of the pores of the porous carbon material.

14. In claim 12, A lithium-sulfur battery, wherein the lithium metal layer comprises a lithium metal (Li) foil or a lithium alloy foil.

15. In claim 12, A lithium-sulfur battery, wherein the ratio (El / S) of the total weight of the electrolyte for the lithium-sulfur battery to the total weight of sulfur (S) in the active material of the positive electrode is 10 g / g or less.

16. In claim 11, The above lithium-sulfur battery is a lithium-sulfur battery that is a coin-shaped, pouch-shaped or cylindrical battery.

Images