Long-life lithium secondary battery

Abstract

The present invention relates to a lithium-sulfur battery having long life characteristics, and provides new parameters for the characteristics of the lithium-sulfur battery having long life characteristics. A lithium-sulfur battery according to one embodiment of the present invention includes a positive electrode containing a sulfur-carbon composite, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte. The negative electrode includes a lithium metal layer and a solid electrolyte interlayer (SEI) formed on at least one surface of the lithium metal layer. The weight of elemental sulfur (S) present in the negative electrode at a state of charge (SOC) of 100% is 3 wt% or less based on the total weight of the negative electrode.

Description

[Technical Field]

[0001] The present invention relates to a lithium secondary battery, and in particular to a lithium-sulfur battery having a long life.

[0002] This application claims priority based on Korean Patent Application No. 10-2023-0058000 filed on May 3, 2023, and Korean Patent Application No. 10-2023-0133560 filed on October 6, 2023, the entire contents of which are incorporated herein by reference. [Background technology]

[0003] As the range of applications of lithium secondary batteries expands from portable electronic devices to electric vehicles (EVs) and electric storage systems (ESSs), there is an increasing demand for lithium secondary batteries with high capacity, high energy density, and long life.

[0004] Among various lithium secondary batteries, lithium-sulfur batteries are battery systems that use sulfur-based materials with sulfur-sulfur bonds as the positive electrode active material and lithium metal, carbon-based materials in which lithium ions are inserted / extracted, or silicon or tin that forms an alloy with lithium as the negative electrode active material.

[0005] Sulfur, the main positive electrode active material in lithium-sulfur batteries, has advantages such as being lightweight per atom, abundant in resources, easy to supply and demand, inexpensive, non-toxic, and environmentally friendly.

[0006] In addition, lithium-sulfur batteries utilize the conversion reaction between lithium ions and sulfur at the positive electrode (S8+16Li + +16e -The theoretical specific capacity of the lithium secondary battery (Li-FeS) is 1,675mAh / g, and when lithium metal is used as the anode, it exhibits a theoretical energy density of 2,600Wh / kg. This is significantly higher than the theoretical energy densities of other battery systems currently being researched (Ni-MH battery: 450Wh / kg, Li-FeS battery: 480Wh / kg, Li-MnO2 battery: 1,000Wh / kg, Na-S battery: 800Wh / kg) and lithium-ion batteries (250Wh / kg). Therefore, it is attracting attention as an environmentally friendly, high-capacity, and inexpensive lithium secondary battery among the secondary batteries currently being developed.

[0007] During discharge, sulfur accepts electrons at the positive electrode of a lithium-sulfur battery, causing a reduction reaction. At this time, lithium polysulfide (Li2S x , x=1-8) are generated, some of which are easily dissolved in the electrolyte and completely reduced to deposit on the anode in the solid form of lithium sulfide (LiS), or lithium polysulfide reacts with the lithium metal of the anode to contaminate the surface of the anode, causing sulfur (S) to become passivated at the anode. This shuttle phenomenon, in which the active material is irreversibly lost, limits the realization of long-life characteristics.

[0008] Therefore, there is a need to develop a lithium-sulfur battery that can suppress the passivation phenomenon of the negative electrode of the lithium-sulfur battery due to repeated charging and discharging and has long-life characteristics. Summary of the Invention [Problem to be solved by the invention]

[0009] The present invention aims to solve the above-mentioned problems and provide a lithium-sulfur battery with long life characteristics.

[0010] The present invention also seeks to provide a lithium-sulfur battery with high energy density. [Means for solving the problem]

[0011] In order to solve the above-mentioned problems, According to one aspect of the present invention, there is provided a lithium-sulfur battery according to the following embodiment:

[0012] The lithium-sulfur battery according to the first embodiment is A lithium-sulfur battery comprising a positive electrode and a negative electrode, each of which includes a sulfur-carbon composite, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, the negative electrode includes a lithium metal layer and a solid electrolyte interphase (SEI) formed on at least one surface of the lithium metal layer; The weight of elemental sulfur (S) present in the negative electrode at a SOC (State of Charge) of 100% is 3 wt % or less based on the total weight of the negative electrode.

[0013] According to the second embodiment, in the first embodiment, The SOC 100% may have a potential of 2.4V to 2.7V.

[0014] According to the third embodiment, in the first or second embodiment, The weight of elemental sulfur (S) in the negative electrode may be 3 wt % or less based on the total weight of the elemental sulfur (S) and elemental lithium (Li) in the negative electrode.

[0015] According to the fourth embodiment, in any one of the first to third embodiments, The weight of elemental sulfur (S) in the negative electrode may be 2 wt % or less based on the total weight of the elemental sulfur (S) and elemental lithium (Li) in the negative electrode.

[0016] According to the fifth embodiment, in any one of the first to fourth embodiments, The weight of elemental sulfur (S) in the negative electrode may be 1 wt % or less based on the total weight of the elemental sulfur (S) and elemental lithium (Li) in the negative electrode.

[0017] According to the sixth embodiment, in any one of the first to fifth embodiments, The negative electrode may have a thickness of 120 μm or less.

[0018] According to the seventh embodiment, in any one of the first to sixth embodiments, The negative electrode may have a thickness of 70 μm or less.

[0019] According to the eighth embodiment, in any one of the first to seventh embodiments, The sulfur-carbon composite may have a sulfur to carbon weight ratio (S / C weight ratio) of 2.5 g / g or less.

[0020] According to the ninth embodiment, in any one of the first to eighth embodiments, the positive electrode includes a current collector and a positive electrode active material layer including the sulfur-carbon composite; The weight of the sulfur-carbon composite may be 90 wt % or more based on the total weight of the positive electrode active material layer.

[0021] According to the tenth embodiment, in any one of the first to ninth embodiments, The weight ratio of the electrolyte to sulfur in the sulfur-carbon composite (El / S weight ratio) may be 3.5 g / g or less.

[0022] According to the eleventh embodiment, in any one of the first to tenth embodiments, The electrolyte may include a non-aqueous solvent, a lithium salt, and an additive.

[0023] According to the twelfth embodiment, in any one of the first to eleventh embodiments, The additive may include a nitrogen compound.

[0024] According to the thirteenth embodiment, in any one of the first to twelfth embodiments, The additive may include lithium nitrate (LiNO3).

[0025] According to the 14th embodiment, in any one of the 1st to 13th embodiments, The content of the additive may be 1 wt % to 5 wt % based on the total weight of the electrolyte solution.

[0026] According to the fifteenth embodiment, in any one of the first to fourteenth embodiments, The ratio of the molar concentration of the lithium salt to the molar concentration of the additive (molar concentration of lithium salt / molar concentration of additive) may be 0.5 to 5.

[0027] According to the 16th embodiment, in any one of the 1st to 15th embodiments, The lithium-sulfur battery can have a lifespan of 190 or more cycles when repeatedly charged and discharged at a rate of 0.3 to 0.5 C in the range of 1.8 V to 2.5 V at room temperature.

[0028] According to the seventeenth embodiment, in any one of the first to sixteenth embodiments, The normal temperature may be a temperature of 23°C to 25°C.

[0029] According to the 18th embodiment, in any one of the 1st to 17th embodiments, The lithium-sulfur battery may have an energy density of 300 Wh / kg or more.

[0030] According to another aspect of the present invention, there is provided a method for evaluating the lifespan of a lithium-sulfur battery, according to the following embodiment.

[0031] The method for evaluating the lifespan of a lithium-sulfur battery according to the 19th embodiment includes: The battery includes a positive electrode including a sulfur-carbon composite, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte solution, A method for evaluating the life of a lithium-sulfur battery, wherein the negative electrode includes a lithium metal layer and a solid electrolyte membrane (SEI) formed on at least one surface of the lithium metal layer, The method includes determining that the battery has a long life if the weight of elemental sulfur (S) present in the negative electrode at an SOC of 100% is 3 wt % or less based on the total weight of the negative electrode.

[0032] According to the 20th embodiment, in the 19th embodiment, The long-life lithium-sulfur battery can have a life of 190 or more cycles when repeatedly charged and discharged at a rate of 0.3 to 0.5 C in the range of 1.8 V to 2.5 V at room temperature. [Effects of the Invention]

[0033] A lithium-sulfur battery according to an embodiment of the present invention may have excellent capacity retention even after repeated charge and discharge.

[0034] As an example, the lithium-sulfur battery of the present invention can retain more than 80% of its initial capacity after more than 190 charge / discharge cycles.

[0035] Furthermore, the lithium-sulfur battery according to an embodiment of the present invention can have a high energy density, particularly an energy density of 300 Wh / kg or more.

[0036] Furthermore, the lithium-sulfur battery according to an embodiment of the present invention can have a long lifespan while having a high energy density. DETAILED DESCRIPTION OF THE INVENTION

[0037] The present invention will be described in more detail below.

[0038] Throughout this specification, when a part "includes," "comprises," "has," or "has" a certain element, it does not exclude other elements and means that it may further include other elements, unless otherwise specified.

[0039] Also, as used throughout this specification, terms such as "about", "substantially", etc. are used to mean the numerical value or a value close thereto when manufacturing and material tolerances inherent to the recited meaning are presented, in order to prevent unscrupulous infringers from improperly using the disclosure where an exact or absolute numerical value is recited to aid in the understanding of the present application.

[0040] Throughout this specification, the description "A and / or B" means "A, B, or all of these".

[0041] As used in this specification, the term "composite" means a substance in which two or more materials are combined to form physically and chemically different phases while exhibiting a more effective function.

[0042] As used in this specification, the term "(poly)sulfide" means "(poly)sulfide ion (S x 2- , 1 ≦ x ≦ 8)" and "lithium (poly)sulfide (Li2S x or Li2S x - , 1 ≦ x ≦ 8)", and includes all such concepts.

[0043] In this specification, the term "polysulfide" means "polysulfide ion (S x 2- , 1 < x ≦ 8)" and "lithium polysulfide (Li2S x or Li2S x - , 1 < x ≦ 8)", and includes all such concepts.

[0044] The unit "mAh / g s " used in this specification is for indicating the capacity per weight of sulfur (S) unless otherwise specified, and can be used interchangeably with other expression methods such as mAh / g(s), mAh / gs, etc.

[0045] <00°0219>The unit "mg s / cm 2" is intended to indicate the weight of sulfur (S) per unit area, mg(s) / cm unless otherwise specified. 2 , or mAh / g as loading amount s It can be used in combination with other expression methods such as

[0046] Lithium-sulfur batteries have the problem that lithium (poly)sulfides formed by the reduction reaction of sulfur (S8) at the positive electrode during charging and discharging dissolve from the positive electrode into the electrolyte, causing the battery capacity to fall short of its theoretical capacity.

[0047] According to one embodiment of the present invention, a lithium-sulfur battery is provided that has a long life, specifically, an excellent capacity retention rate after repeated charge and discharge.

[0048] A lithium-sulfur battery according to an embodiment of the present invention includes a positive electrode including a sulfur-carbon composite, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte.

[0049] In one embodiment of the present invention, the lithium-sulfur battery specifically includes an electrode assembly including a positive electrode, a negative electrode, and a separator, and a case that accommodates the electrode assembly and an electrolyte.

[0050] In this case, the negative electrode includes a lithium metal layer and a solid electrolyte membrane (SEI) formed on at least one surface of the lithium metal layer, and the weight of sulfur (S) present in the negative electrode at an SOC of 100% is 3 wt % or less based on the total weight of the negative electrode.

[0051] A lithium-sulfur battery contains inorganic sulfur (S8) as a positive electrode active material. In the lithium-sulfur battery, lithium (poly) sulfide is formed by a reduction reaction at the positive electrode during discharge. The formed lithium (poly) sulfide is dissolved by the electrolyte and elutes from the positive electrode into the electrolyte, and the eluted lithium (poly) sulfide reacts with the lithium metal at the negative electrode, so that sulfur element (S) passivated by deposited lithium sulfide etc. remains on the surface of the negative electrode. As a result, there is a risk that the negative electrode is damaged and the life of the lithium-sulfur battery is reduced.

[0052] In consideration of such a mechanism, in this specification, substances containing sulfur element (S) derived from the positive electrode active material of the lithium-sulfur battery are collectively referred to as "sulfur-based compounds". The sulfur-based compounds may include all sulfur-containing compounds formed through, for example, a reduction reaction of inorganic sulfur (S8) or an oxidation reaction of lithium sulfide (Li2S), and more specifically, inorganic sulfur (S8), lithium sulfide (Li2S), lithium polysulfide (Li2S x 、1<x≦8), disulfide compounds, carbon-sulfur polymers ((C2S y ) n 、y = 2.5 to 50, n≧2), or may include two or more of these.

[0053] Specifically, the sulfur-based compound may be inorganic sulfur (S8), lithium sulfide (Li2S), lithium polysulfide (Li2S x 、1<x≦8), or a mixture of two or more of these.

[0054] In this specification, lithium (poly) sulfide may also be represented by the chemical formula of "Li2S x (1≦x≦8)", and in such a case, the lithium (poly) sulfide is used as a term including lithium sulfide (Li2S).

[0055] The inventors have found that, when the lifespan of a lithium-sulfur battery is reduced due to repeated charge-discharge cycles, by controlling the content of sulfur (S) formed on the anode to a certain level or less, it is possible to achieve a capacity of 80% or more of the initial capacity even after 190 or more charge-discharge cycles, or even 195 or more charge-discharge cycles, thereby completing the present invention.

[0056] According to one embodiment of the present invention, the lithium-sulfur battery may include a nitrogen compound, such as lithium nitrate (LiNO3), as an additive in the electrolyte to prevent deterioration of the lithium-sulfur battery.

[0057] As described below, if the content of the additive in the electrolyte is too high, the additive may precipitate rather than dissolve in the non-aqueous solvent, which may accelerate the deterioration of the lithium-sulfur battery. Therefore, taking into consideration the solubility of the additive in the non-aqueous solvent, the additive may be included in an amount of 5 wt % or less, for example, 1 wt % to 5 wt %, based on the total weight of the non-aqueous solvent, but the present invention is not limited thereto. The composition of the additive in the electrolyte will be described later.

[0058] According to one embodiment of the present invention, a new range of sulfur (S) content present in the negative electrode in a fully charged state, i.e., at an SOC of 100%, is proposed.

[0059] Specifically, one aspect of the present invention provides a lithium-sulfur battery in which the weight of elemental sulfur (S) present in the negative electrode at an SOC of 100% is 3% by weight or less based on the total weight of the negative electrode.

[0060] The anode includes a lithium metal layer and a solid electrolyte membrane (SEI) formed on at least one surface of the lithium metal layer. To achieve a long life of the lithium-sulfur battery, the sulfur (S) content is limited to 3 wt % or less based on the total weight of the lithium metal layer and the solid electrolyte membrane.

[0061] In one embodiment of the present invention, the weight of elemental sulfur (S) in the negative electrode may be, for example, 0 wt % to 3 wt %, 0.1 wt % to 3 wt %, 0.1 wt % to 3.0 wt %, 0.1 wt % to 2.5 wt %, 0.1 wt % to 2 wt %, 0.1 wt % to 1 wt %, or 0.1 wt % to 0.5 wt %, based on the total weight of the lithium metal layer and the solid electrolyte membrane.

[0062] In one embodiment of the present invention, the lithium metal layer is a lithium foil made of lithium metal, which is a negative electrode active material of a lithium-sulfur battery, or a lithium alloy foil made of a material capable of forming an alloy with lithium metal, and its thickness may be, for example, 140 μm or less.

[0063] In one embodiment of the present invention, the thickness of the lithium metal layer may be 140 μm or less, specifically 120 μm or less, 100 μm or less, more specifically 80 μm or less, or 70 μm or less. For example, the thickness of the lithium metal layer may be 20 μm to 140 μm, 20 μm to 120 μm, or 20 μm to 100 μm, specifically 30 μm to 80 μm, 40 μm to 70 μm, 50 μm to 70 μm, 50 μm to 65 μm, 55 μm to 65 μm, or 55 μm to 60 μm. In one embodiment of the present invention, when the thickness of the lithium metal layer is within the above range, it may be advantageous in terms of the weight and energy density of the battery.

[0064] In this specification, the thickness of the lithium metal layer may be measured by a known method for measuring the thickness of each component of a battery, for example, but not limited to, a thickness gauge manufactured by Mitutoyo Corporation. In one embodiment of the present invention, the thickness of the negative electrode may be measured in accordance with ASTM D374.

[0065] In one embodiment of the present invention, the solid electrolyte membrane (SEI) is a thin passivation film formed on the surface of the lithium metal layer during charging in the activation process of a lithium-sulfur battery, and serves to protect the lithium metal and inhibit decomposition of the electrolyte, but the present invention is not limited thereto.

[0066] In one embodiment of the present invention, the total thickness of the negative electrode including the lithium metal layer and the solid electrolyte membrane may be 150 μm or less, specifically 120 μm or less, 100 μm or less, more specifically 70 μm or less, For example, the total thickness of the negative electrode may be 40 μm to 120 μm, e.g., 40 μm to 70 μm or 55 μm to 65 μm.

[0067] In one embodiment of the present invention, the lithium-sulfur battery is advantageous for achieving long life characteristics when the content of elemental sulfur (S) in the negative electrode is 3 wt% or less at a state of charge (SOC) of 100%. Specifically, the negative electrode may have a sulfur content of 3 wt% or less based on the total weight of elemental sulfur (S) and elemental lithium (Li) in the negative electrode. For example, the sulfur content of the negative electrode may be 3 wt% or less, 2 wt% or less, 1 wt% or less, 0.5 wt% or less, or 0 wt% based on the total weight of elemental sulfur (S) and elemental lithium (Li) in the negative electrode.

[0068] More specifically, the negative electrode may contain elemental sulfur (S) in an amount of 0 wt % to 3 wt %, 0.1 wt % to 3 wt %, 0.1 wt % to 2.5 wt %, 0.1 wt % to 2 wt %, 0.1 wt % to 1 wt %, or 0.1 wt % to 0.5 wt % based on the total weight of sulfur (S) and lithium (Li) in the negative electrode at an SOC of 100%, i.e., in a fully charged state.

[0069] In this case, "0 wt%" includes not only the case where no sulfur (S) is contained at all, but also the case where a trace amount of sulfur (S) is contained but is measured as 0 wt% due to the limit of significant figures in the measurement or measurement error, etc. Therefore, the 0 wt% may also mean 0.1 wt% based on the total weight of the negative electrode.

[0070] In one embodiment of the present invention, the weight of lithium (Li) in the negative electrode may be, for example, 96 wt % or more, 99 wt % or more, 96 wt % to 100 wt %, 96 wt % to 99.9 wt %, or 99 wt % to 99.9 wt % based on the total weight of elemental sulfur (S) and lithium (Li) in the negative electrode. When the lithium content is within the above range based on the total weight of elemental sulfur (S) and lithium (Li) in the negative electrode of the lithium-sulfur battery at the fully charged state, i.e., SOC 100%, it is advantageous for the long life characteristics of the lithium-sulfur battery, but the present invention is not limited thereto.

[0071] In one embodiment of the present invention, the solid electrolyte membrane may have a composition that varies depending on the composition of the lithium salt and additives used in the electrolyte of the lithium-sulfur battery, and is not limited to a specific composition.

[0072] In one embodiment of the present invention, the solid electrolyte membrane may include, for example, Li-F functional groups.

[0073] In one embodiment of the present invention, in the lithium-sulfur battery with an SOC of 100%, the elements contained in the negative electrode including the lithium metal layer and the solid electrolyte membrane may include sulfur (S), lithium (Li), fluorine (F), etc. Specifically, the elements contained in the negative electrode may be sulfur (S), lithium (Li), and fluorine (F).

[0074] In one embodiment of the present invention, the lithium-sulfur battery may have a sulfur (S) content of 3 wt % or less, 2 wt % or less, 1 wt % or less, 0.5 wt % or less, 0.1 wt % or less, or 0 wt % or less, based on the total weight of sulfur (S), lithium (Li), and fluorine (F) in the negative electrode, when in a state of charge (SOC) of 100%. Here, "0 wt %" includes not only cases where no sulfur (S) is present, as described above, but also cases where a trace amount of sulfur (S) is measured as 0 wt % due to the significant digit limit or measurement error.

[0075] In one embodiment of the present invention, the solid electrolyte membrane may include a complex formed by a chemical reaction between an electrolyte additive (described below) and lithium metal. In this case, the electrolyte additive may be, for example, a material containing nitrogen (N), and thus the solid electrolyte membrane may include nitrogen (N).

[0076] Therefore, in one embodiment of the present invention, the lithium-sulfur battery may have a sulfur (S) content of 3 wt % or less, 2 wt % or less, 1 wt % or less, 0.5 wt % or less, 0.1 wt % or less, or 0 wt % or less, based on the total weight of sulfur (S), lithium (Li), and nitrogen (N) in the negative electrode, when in a state of charge (SOC) of 100%. Here, "0 wt %" includes not only cases where no sulfur (S) is present, as described above, but also cases where a trace amount of sulfur (S) is measured as 0 wt % due to the limit of significant figures or measurement error.

[0077] In another embodiment of the present invention, the lithium-sulfur battery may have a sulfur (S) content of 3 wt % or less, 2 wt % or less, 1 wt % or less, 0.5 wt % or less, 0.1 wt % or less, or 0 wt % or less, based on the total weight of sulfur (S), lithium (Li), fluorine (F), and nitrogen (N) in the negative electrode, when in a charged state, i.e., at an SOC of 100%.

[0078] Hereinafter, a method for measuring the content of sulfur (S) in the negative electrode will be described.

[0079] In one embodiment of the present invention, "SOC 100%" refers to a fully charged state of a battery. For example, the SOC 100% may be a state in which the battery is charged to have a potential of 2.4 V to 2.7 V. Specifically, the SOC 100% may be a state in which the battery is charged to have a potential of 2.5 V.

[0080] Specifically, the sulfur (S) content in the negative electrode at an SOC of 100% can be measured by disassembling the lithium-sulfur battery in a fully charged state, for example, in the range of 2.4 V to 2.7 V, specifically, 2.5 V, and then cleaning the electrolyte adhering to the negative electrode and analyzing the composition.

[0081] In one embodiment of the present invention, the component analysis may be performed using, for example, ion chromatography (IC) analysis, inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis, elemental analysis (EA), ONH analysis, etc., but the component analysis method is not limited to these. Therefore, the sulfur (S) content in the negative electrode based on the total weight of the negative electrode at an SOC of 100%, or the sulfur (S) content in the negative electrode based on the total weight of sulfur (S) and lithium (Li) in the negative electrode at an SOC of 100%, may be measured by the above-mentioned component analysis, for example, ICP-OES.

[0082] In an embodiment of the present invention, before measuring the sulfur (S) content, the lithium-sulfur battery may be activated by charging and discharging at least one cycle, for example, 5 or more cycles, or 10 or more cycles, at room temperature, for example, 23°C to 25°C, from 1.8V to 2.5V, where one cycle is discharge at 0.5C and charge at 0.3C.

[0083] As described above, in conventional lithium-sulfur batteries, passivated elemental sulfur (S) is formed in the negative electrode by sulfur-based compounds derived from the positive electrode during discharge, which can cause battery degradation due to repeated charge-discharge cycles. However, according to one embodiment of the present invention, the upper limit of the elemental sulfur (S) content in the negative electrode at a fully charged state, i.e., SOC 100%, is limited to 3 wt % or less, specifically 2 wt % or less, 1 wt % or less, or 0.5 wt % or less, thereby providing a lithium-sulfur battery with a long life.

[0084] Hereinafter, each component of the lithium-sulfur battery other than the negative electrode will be described in detail.

[0085] <Positive electrode> In one embodiment of the present invention, the positive electrode may include a positive electrode current collector and a positive electrode active material layer coated on one or both surfaces of the positive electrode current collector.

[0086] The positive electrode current collector is not particularly limited as long as it supports the positive electrode active material and has high conductivity without inducing chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, palladium, fired carbon, those with surface treatment such as carbon, nickel, silver, etc. on the surface of copper or stainless steel, aluminum-cadmium alloy, etc. can be used. [[ID=​​​​​​​​​​​​​​​​​​​​​

[0091] 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 to uniformly and stably fix the sulfur-based compound, and improves the conductivity of the positive electrode. Any porous carbon material may be used without any particular limitation as long as it is a porous carbon material.

[0092] The porous carbon material can generally be prepared by carbonizing various carbon precursors. The porous carbon material contains irregular pores, with an average pore diameter of 1 nm to 200 nm and a porosity of 10 to 90% by volume of the total volume of the porous carbon material. If the average pore diameter is below the above range, the pore size is merely at the molecular level and cannot be impregnated with sulfur. If the average pore diameter exceeds the above range, the mechanical strength of the porous carbon material is weakened, making it difficult to apply to electrode manufacturing processes.

[0093] In one embodiment of the present invention, the average pore diameter can be measured by any known method for measuring the pore diameter of porous materials in the art, and the measurement method is not particularly limited. For example, the pore diameter can be measured by a scanning electron microscope (SEM), a field emission electron microscope, a laser diffraction method, or a Brunauer-Emmett-Teller (BET) method. Measurement using the laser diffraction method can be performed using, for example, a commercially available laser diffraction particle size analyzer (e.g., Microtrac MT3000). Measurement using the BET method can be performed using, for example, a BELSORP series analyzer manufactured by BEL Japan, but is not limited thereto.

[0094] In one embodiment of the present invention, the "porosity" refers to the ratio of the volume occupied by pores to the total volume of a structure, and is expressed in units of "%." The term may be used interchangeably with terms such as void ratio, porosity, etc. In one embodiment of the present invention, the measurement of the porosity is not particularly limited, and may be measured by, for example, the BET method using nitrogen gas, mercury intrusion porosimetry (Hg porosimeter), or ASTM D2873.

[0095] The shape of the porous carbon material may be any shape commonly used in lithium-sulfur batteries, such as spheres, rods, needles, plates, tubes, or bulks, without limitation.

[0096] The porous carbon material may be any material commonly used in the art, having a porous structure or a large specific surface area. For example, the porous carbon material may be at least one selected from the group consisting of graphite, graphene, carbon black (e.g., denka black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black), carbon nanotubes (CNTs) (e.g., single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs)), carbon fibers (e.g., graphite nanofibers (GNFs), carbon nanofibers (CNFs), and activated carbon fibers (ACFs), graphite (e.g., natural graphite, artificial graphite, and expanded graphite), and activated carbon. Preferably, the porous carbon material is carbon nanotubes.

[0097] In one embodiment of the present invention, the porous carbon material may include, for example, carbon nanotubes (CNTs).

[0098] In one embodiment of the present invention, the sulfur-to-carbon composite may have a sulfur-to-carbon weight ratio of, for example, 5 g / g or less, e.g., 4 g / g or less, more specifically, 2.5 g / g or less, e.g., 2.3 g / g or less. For example, the sulfur-to-carbon composite may have a sulfur-to-carbon weight ratio of 2.3 g / g. The S / C ratio of the sulfur-carbon composite within the above range is preferable in terms of ensuring the electron transfer ability (conductivity) and electrochemical specific surface area of ​​the sulfur-carbon composite. For example, the usable surface of the sulfur-carbon composite is increased, which makes it easier to suppress sulfur elution from the positive electrode. However, the present invention is not limited thereto.

[0099] In one embodiment of the present invention, the S / C ratio can be calculated from the weight (g) of sulfur and the weight (g) of carbon present in the sulfur-carbon composite. Alternatively, it can be calculated from the weight (wt%) of sulfur and the weight (wt%) of carbon based on the total weight of the sulfur-carbon composite. Thus, the S / C ratio can be a unitless value.

[0100] In one embodiment of the present invention, the S / C weight ratio may be, for example, 0.5 to 5.0, 0.5 to 4.0, 1.0 to 3.0, 1.5 to 2.5, 2.0 to 2.45, 2.25 to 2.35, or 2.3 to 3. For example, the S / C ratio may be calculated from the ratio of the weight of the inorganic sulfur (S8) to the weight of the porous carbon material. For example, the S / C ratio may be calculated from the weight ratio of S8 to CNT.

[0101] The method for preparing the sulfur-carbon composite is not particularly limited, and may be a method commonly used in the art. For example, the sulfur and the porous carbon material may be simply mixed and then heat-treated to form a composite.

[0102] In addition to the above-mentioned composition, the positive electrode active material may further include one or more selected from a transition metal element, a Group 13 element, a Group 14 element, a sulfur compound of these elements, and an alloy of these elements with sulfur.

[0103] The transition metal elements may include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Os, Ir, Pt, Au, or Hg, and the Group 13 elements may include Al, Ga, In, or Ti, and the Group 14 elements may include Ge, Sn, or Pb.

[0104] In one embodiment of the present invention, the sulfur-carbon composite may be included in an amount of 50 wt % or more, based on the total weight of the positive electrode. Specifically, the sulfur-carbon composite may be included in an amount of, for example, 80 wt % or more, 90 wt % or more, or 95 wt % or more, based on the total weight of the positive electrode active material layer. Specifically, the sulfur-carbon composite may be included in an amount of 80 wt % to 100 wt %, more specifically, 85 wt % to 99 wt %, 90 wt % to 99 wt %, 95 wt % to 98 wt %, 95 wt % to 97 wt %, or 96 wt % based on the total weight of the positive electrode active material layer. In one embodiment of the present invention, the above-mentioned content of the sulfur-carbon composite may refer to the content of the sulfur-carbon composite during the manufacturing stage of the lithium-sulfur battery. In another embodiment of the present invention, the content of the sulfur-carbon composite may be measured by disassembling the lithium-sulfur battery after charging it to 100% SOC and then analyzing the cathode. If the content of the sulfur-carbon composite is less than the above range, the relative contents of auxiliary materials such as a conductive material and a binder increase, while the content of the sulfur-carbon composite decreases, making it difficult to realize a battery with a high capacity and a high energy density. If the content of the sulfur-carbon composite exceeds the above range, the content of the conductive material or binder, which will be described later, becomes relatively insufficient, resulting in a problem of degraded physical properties of the electrode.

[0105] The conductive material electrically connects the electrolyte and the positive electrode active material and serves as a path for electrons to move from the current collector to the positive electrode active material. The conductive material is a component of the electrode that is physically distinct from the carbon contained in the sulfur-carbon composite, and may be any conductive material.

[0106] In one embodiment of the present invention, the conductive material may be, for example, carbon black such as Super P, Denka Black, acetylene black, Ketjen Black, channel black, furnace black, lamp black, thermal black, or carbon black; carbon derivatives such as carbon nanotubes or fullerenes; conductive fibers such as carbon fibers or metal fibers; metal powders such as carbon fluoride, aluminum, or nickel powder; or conductive polymers such as polyaniline, polythiophene, polyacetylene, or polypyrrole, which may be used alone or in combination.

[0107] In one embodiment of the present invention, the content of the conductive material may be 0 to 40 wt%, 1 to 40 wt%, 15 to 40 wt%, 20 to 40 wt%, 25 to 35 wt%, or 0 to 10 wt%, for example, 1 to 10 wt%, based on the total weight of the positive electrode active material. If the content of the conductive material is below the above range, electron transfer between the positive electrode active material and the current collector may be difficult, resulting in a decrease in voltage and capacity. If the content of the conductive material is above the above range, the proportion of the positive electrode active material may be relatively reduced, resulting in a decrease in the total energy (charge) of the battery. Therefore, it is preferable to determine the appropriate content within the above range.

[0108] In an embodiment of the present invention, the binder supports the positive electrode active material on the positive electrode current collector and organically connects the positive electrode active materials to further enhance the binding strength therebetween. Any binder known in the art may be used.

[0109] For example, the binder may be one or a mixture of two or more selected from the group consisting of polyvinylidene fluoride (PVdF), polyvinylidene fluoride-based polymers containing at least one vinylidene fluoride repeating unit, polytetrafluoroethylene (PTFE), or a fluororesin-based binder including a mixture of two or more of these; rubber-based binders including styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber, and styrene-isoprene rubber; acrylic binders; cellulose-based binders including carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, and regenerated cellulose; polyalcohol-based binders; polyolefin-based binders including polyethylene and polypropylene; polyimide-based binders; polyester-based binders; silane-based binders; polyacrylic acid-based binders; and polyacrylonitrile-based binders. In one embodiment of the present invention, the binder may include, for example, polyacrylate (PAA).

[0110] In one embodiment of the present invention, the content of the binder may be 1 to 10 wt % based on the total weight of the positive electrode active material layer. If the content of the binder is less than the above range, the physical properties of the positive electrode may be deteriorated, causing the positive electrode active material and the conductive material to fall off. If the content of the binder exceeds the above range, the ratio of the positive electrode active material to the conductive material in the positive electrode may be relatively reduced, causing a decrease in battery capacity. Therefore, it is preferable to determine the appropriate content within the above range.

[0111] In one embodiment of the present invention, the method for manufacturing the positive electrode for a lithium secondary battery is not particularly limited, and any method known to those skilled in the art or various methods modified therefrom may be used.

[0112] For example, the positive electrode for the lithium secondary battery may be manufactured by preparing a positive electrode slurry composition including the above-described composition and then applying the same to at least one surface of the positive electrode current collector to form the positive electrode active material layer.

[0113] The positive electrode slurry composition may include the above-described positive electrode active material, and may further include a binder, a conductive material, and a solvent.

[0114] The solvent used is one that allows the positive electrode active material to be uniformly dispersed. The most preferred solvent is water, which may be distilled water or deionized water. However, the solvent is not limited thereto. If necessary, a lower alcohol that is easily miscible with water may be used. Examples of the lower alcohol include methanol, ethanol, propanol, isopropanol, and butanol. These may be preferably mixed with water.

[0115] The solvent may be contained in an amount that allows easy coating, and the specific amount may vary depending on the application method and device.

[0116] The positive electrode slurry composition may further contain, as needed, substances commonly used in the art for the purpose of improving performance, etc. For example, a viscosity modifier, a fluidizing agent, a filler, etc.

[0117] The method for applying the positive electrode slurry composition is not particularly limited, and examples thereof include a doctor blade, die casting, comma coating, screen printing, etc. Alternatively, the positive electrode slurry may be formed on a separate substrate and then applied to the positive electrode current collector by pressing or laminating.

[0118] After coating, a drying process may be performed to remove the solvent. The drying process is performed at a temperature and time sufficient to sufficiently remove the solvent, and the conditions are not particularly limited as they may vary depending on the type of solvent. Examples include drying with warm air, hot air, or low-humidity air, vacuum drying, and drying by irradiation with (far) infrared rays or electron beams. The drying rate is usually adjusted to remove the solvent as quickly as possible without causing cracks in the positive electrode active material layer due to stress concentration or causing the positive electrode active material layer to peel off from the positive electrode current collector.

[0119] Furthermore, after drying, the current collector may be pressed to increase the density of the positive electrode active material in the positive electrode. Examples of pressing methods include die pressing and roll pressing.

[0120] The positive electrode manufactured using the above-described composition and manufacturing method, specifically the porosity of the positive electrode active material layer, may be 50% to 80% by volume, specifically 60% to 75% by volume. If the positive electrode porosity is less than 50% by volume, the filling rate of the positive electrode slurry composition containing the positive electrode active material, conductive material, and binder becomes too high, preventing sufficient electrolyte for ionic and / or electrical conduction from being maintained between the positive electrode active materials. This leads to reduced battery output and cycle characteristics, and serious problems such as overvoltage and reduced discharge capacity. On the other hand, if the positive electrode porosity is excessively high, exceeding 80% by volume, the physical and electrical connection with the current collector may be weakened, reducing adhesion and reaction difficulty. Furthermore, excessive pores may be filled with electrolyte, resulting in reduced battery energy density. Therefore, the porosity should be appropriately adjusted within the above range.

[0121] In one embodiment of the present invention, the sulfur loading in the positive electrode is, for example, 2.5 mg(s) / cm 2 For example, 0 to 2.5 mg(s) / cm 2 , 0.1 to 2.4 mg(s) / cm 2 , or 1.7 to 2.1 mg(s) / cm 2 The sulfur loading amount can be calculated from the total weight of sulfur (S) contained as a positive electrode active material in the positive electrode. In this case, the weight of sulfur in the positive electrode can be measured from the positive electrode active material added during manufacturing, or can be measured after manufacturing by thermogravimetric analysis (TGA) of the positive electrode. Meanwhile, when disassembling a lithium-sulfur battery after manufacturing, a charged lithium-sulfur battery can be disassembled in a non-active atmosphere to obtain a positive electrode, and the positive electrode can be washed and dried using an appropriate washing solvent. The positive electrode active material layer can then be scraped off and the resulting product can be subjected to thermogravimetric analysis (TGA) to measure and calculate the sulfur (S) content derived from the active material, but the measurement method is not limited thereto.

[0122] In another embodiment of the present invention, the sulfur loading in the positive electrode is, for example, 3 mAh / cm 2 Specifically, 0mAh / cm 2 ~3mAh / cm2 For example, the sulfur loading in the positive electrode may be 0.1 to 2.9 mAh / cm 2 , 0.5~2.8mAh / cm 2 , 1~2.7mAh / cm 2 , 1.5~2.6mAh / cm 2 , or 2.0~2.5mAh / cm 2 The sulfur loading amount may be a value calculated from the total weight of sulfur (S) contained as a positive electrode active material in the positive electrode and the resulting capacitance value of the electrode.

[0123] <Negative electrode> The negative electrode may include a negative electrode current collector and a negative electrode active material layer formed on at least one surface of the negative electrode current collector, or the negative electrode may be a free-standing lithium metal plate or lithium metal foil without a current collector.

[0124] In one embodiment of the present invention, the negative electrode current collector supports the negative electrode active material and can be any of those described above for the positive electrode current collector.

[0125] The negative electrode active material is lithium (Li + The lithium ion-containing compound may include a material capable of reversible intercalation or deintercalation of lithium ions, a material capable of reversibly reacting with lithium ions to form a lithium-containing compound, lithium metal, or a lithium alloy.

[0126] The lithium ion (Li + The material capable of reversibly inserting or desorbing lithium ions (Li) can be, for example, crystalline carbon, amorphous carbon, or a mixture thereof. +The material capable of reacting with lithium (Li) to reversibly form a lithium-containing compound may be, for example, tin oxide, titanium nitride, or silicon. The lithium alloy may be, for example, an alloy of lithium (Li) and a metal selected from the group consisting of sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), francium (Fr), beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), radium (Ra), aluminum (Al), and tin (Sn).

[0127] <Separation membrane> The separator separates or insulates the positive electrode and the negative electrode and transports lithium ions between them. The separator may include a porous, non-conductive or insulating material. Any material commonly used as a separator for lithium secondary batteries may be used without any particular limitation. The separator may be an independent member such as a film, or a coating layer applied to the positive electrode and / or the negative electrode.

[0128] The separation membrane preferably has low resistance to ion migration of the electrolyte and excellent wettability with respect to the electrolyte.

[0129] In one embodiment of the present invention, the separator may include a porous substrate, which may be any porous substrate commonly used in secondary batteries, such as a porous polymer film alone or in combination with other porous polymer films, including, but not limited to, a nonwoven fabric made of high-melting-point glass fiber, polyethylene terephthalate fiber, or a polyolefin-based porous membrane.

[0130] The material of the porous substrate is not particularly limited, and any porous substrate commonly used in electrochemical devices can be used. For example, the porous substrate may include at least one selected from the group consisting of polyolefins such as polyethylene and polypropylene, polyesters such as polyethylene terephthalate and polybutylene terephthalate, polyamides, polyacetals, polycarbonates, polyimides, polyether ether ketones, polyether sulfones, polyphenylene oxides, polyphenylene sulfides, polyethylene naphthalate, polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl chloride, polyacrylonitrile, cellulose, nylon, polyparaphenylene benzobisoxazole, and polyarylates.

[0131] In one embodiment of the present invention, the thickness of the porous substrate is not particularly limited, but may be 1 μm to 100 μm, preferably 5 μm to 50 μm. The thickness of the porous substrate is not limited to the above range, but if it is thinner than the above lower limit, the mechanical properties may be reduced, and the separator may be easily damaged during use of the battery.

[0132] In one embodiment of the present invention, the average diameter and porosity of the pores present in the porous substrate are not particularly limited, but may be 0.001 μm to 50 μm and 10 vol % to 95 vol %, respectively.

[0133] In an embodiment of the present invention, the separator may further include a porous coating layer formed on at least one surface of the porous substrate and including inorganic particles and a binder.

[0134] In one embodiment of the present invention, the inorganic particles and binder contained in the porous coating layer may be any inorganic particles and binder commonly used in porous coating layers of separators, and the manufacturing method thereof is not particularly limited.

[0135] <Electrolyte> The electrolyte is a medium in which ions involved in the electrochemical reaction of the lithium-sulfur battery can move, and includes a non-aqueous solvent and a lithium salt as an electrolyte.

[0136] The electrolyte solution is not particularly limited as long as it has a composition that can be used in lithium secondary batteries, specifically lithium-sulfur batteries.

[0137] In one embodiment of the present invention, the electrolyte may include a non-aqueous solvent, a lithium salt, and an additive.

[0138] In an embodiment of the present invention, the non-aqueous solvent may be any solvent that can be used in a lithium-sulfur battery, without any particular limitation. For example, an ether solvent, an ester, an amide, a chain carbonate, a cyclic carbonate, etc. may be used.

[0139] In one embodiment of the present invention, the ester may be, for example, any 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.

[0140] In one embodiment of the present invention, the chain carbonate may be, for example, any one selected from the group consisting of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, and ethyl propyl carbonate, or a mixture of two or more thereof, but is not limited thereto.

[0141] In one embodiment of the present invention, the cyclic carbonate may be any one or a mixture of two or more selected from the group consisting of ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, vinylene carbonate, vinylethylene carbonate, and halides thereof, and examples of halides thereof include, but are not limited to, fluoroethylene carbonate.

[0142] In one embodiment of the present invention, the non-aqueous solvent may include an ether-based solvent.

[0143] In one embodiment of the present invention, the ether-based solvent may be present in an amount of 60% by volume or more, for example, 60% to 100% by volume, 70% to 100% by volume, 80% to 100% by volume, 85% to 100% by volume, 90% to 100% by volume, 95% to 100% by volume, 98% to 100% by volume, 90% to 98% by volume, or 90% to 95% by volume, based on the total volume of the non-aqueous solvent. The above-described range of the amount of the ether-based solvent, based on the total volume of the non-aqueous solvent, is advantageous in terms of the solubility of electrolyte components such as lithium salt, but the present invention is not limited thereto.

[0144] In one embodiment of the present invention, the ether-based solvent may include an acyclic ether, a cyclic ether, or a mixture thereof. The non-aqueous solvent is preferably an ether-based solvent in view of the solubility of the lithium salt and / or additives described below.

[0145] In one embodiment of the present invention, the acyclic ether may include one or more selected from the group consisting of dimethyl ether, diethyl ether, dipropyl ether, dibutyl ether, diisobutyl ether, ethyl methyl ether, ethyl propyl ether, ethyl tert-butyl 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 tert-butyl ethyl ether, and ethylene glycol ethyl methyl ether. Preferably, the acyclic ether may include one or more selected from the group consisting of dimethyl ether, dimethoxyethane, diethoxyethane, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether, and more preferably dimethoxyethane.

[0146] In one embodiment of the present invention, the cyclic ether may include, for example, one or more selected from the group consisting of 2-methylfuran, 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, tetrahydrofuran, 1,4-dioxane, 1,2-dimethoxybenzene, 1,3-dimethoxybenzene, 1,4-dimethoxybenzene, and isosorbide dimethyl ether. Preferably, it may contain one or more selected from the group consisting of 2-methylfuran, 1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, and 2,5-dimethyltetrahydrofuran, and more preferably it may contain 2-methylfuran.

[0147] In one embodiment of the present invention, the non-aqueous solvent may comprise a mixture of an acyclic ether and a cyclic ether.

[0148] In one embodiment of the present invention, the non-aqueous solvent may include dimethoxyethane (DME) and 2-methylfuran (2-MeF).

[0149] In one embodiment of the present invention, the volume ratio of the acyclic ether to the cyclic ether may be 5:95 to 95:5 (v / v), specifically 95:5 to 50:50, more specifically 90:10 to 70:30, 85:15 to 75:25, or 80:20 (v / v). In the present invention, the volume ratio corresponds to the ratio of "volume % of acyclic ether" to "volume % of cyclic ether" in the ether-based solvent.

[0150] In one embodiment of the present invention, the non-aqueous solvent may not contain a carbonate-based solvent in terms of electrolyte solubility, or may contain a small amount of carbonate-based solvent that does not affect the solubility of the lithium salt. For example, when the non-aqueous solvent contains a carbonate-based solvent, the content of the carbonate 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., no content) based on the total weight of the lithium secondary battery electrolyte.

[0151] In one embodiment of the present invention, the lithium salt may be any suitable lithium salt that can be used as an electrolyte for a lithium secondary battery. Examples of the lithium salt include LiCl, LiBr, LiI, LiClO4, LiBF4, and LiB 10 Cl 10 , LiPF6, LiCF3SO3, LiCF3CO2, LiC4BO8, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, CF3SO3Li, (CF3SO2)2NLi, (C2F5SO2)2NLi, (SO2F)2NLi, (CF3SO2)3CLi, lithium chloroborane, lithium lower aliphatic carboxylate, lithium tetraphenylborate, lithium imide, or two or more thereof.

[0152] In one embodiment of the present invention, the concentration of the lithium salt may be appropriately determined in consideration of ionic conductivity, solubility, etc., and may be, for example, 0.1 M to 4 M, preferably 0.25 M to 2 M, 0.5 M to 2 M, or 0.5 M to 1.5 M. When the concentration of the lithium salt is within the above range, it is easy to ensure ionic conductivity suitable for driving the battery, or the electrolyte has an appropriate viscosity, which is advantageous in terms of improving the mobility of lithium ions and suppressing the decomposition reaction of the lithium salt itself, but the present invention is not limited thereto.

[0153] In one embodiment of the present invention, the additive may further include a nitrogen compound in addition to the lithium salt to improve the electrical conductivity of the electrolyte and extend the life of the lithium-sulfur battery.

[0154] Specifically, the nitrogen compound has, but is not limited to, an effect of suppressing the reduction reaction of polysulfide that occurs during the charge / discharge process of a lithium-sulfur battery, thereby preventing irreversible consumption of polysulfide, and thereby improving the performance of the lithium-sulfur battery.

[0155] In one embodiment of the present invention, the additive, e.g., a nitrogen compound, is not particularly limited as long as it stably forms a solid electrolyte membrane (SEI) of the negative electrode and improves charge / discharge efficiency, and may be, for example, a nitrate compound, a nitrite compound, or a mixture thereof.

[0156] In one embodiment of the present invention, the nitrogen compound may be selected from the group consisting of inorganic nitric acid or nitrite compounds such as lithium nitrate (LiNO), potassium nitrate (KNO), cesium nitrate (CsNO), barium nitrate (Ba(NO)), ammonium nitrate (NHNO), lithium nitrite (LiNO), potassium nitrite (KNO), cesium nitrite (CsNO), and ammonium nitrite (NHNO); organic nitric acid or nitrite compounds such as methyl nitrate, dialkylimidazolium nitrate, guanidine nitrate, imidazolium nitrate, pyridinium nitrate, ethyl nitrite, propyl nitrite, butyl nitrite, pentyl nitrite, and octyl nitrite; organic nitro compounds such as nitromethane, nitropropane, nitrobutane, nitrobenzene, dinitrobenzene, nitropyridine, dinitropyridine, nitrotoluene, and dinitrotoluene, and combinations thereof, and preferably includes lithium nitrate.

[0157] In one embodiment of the present invention, the electrolyte solution may include a non-aqueous solvent and two lithium salts. The two lithium salts may be different, with one lithium salt being a nitrogen-containing lithium salt and the other being a nitrogen compound such as lithium nitrate (LiNO). For example, the two lithium salts may include LiTFSI as the electrolyte and LiNO as the additive.

[0158] In one embodiment of the present invention, the nitrogen compound may be present in an amount of, for example, 1 wt % to 10 wt %, 1 wt % to 8 wt %, 1 wt % to 6 wt %, 1 wt % to 5 wt %, 1.0 wt % to 5.0 wt %, 2.0 wt % to 5.0 wt %, 2.0 wt % to 4.9 wt %, 2.1 wt % to 3.5 wt %, or 3 wt % to 5 wt %, based on the total weight of the electrolyte. The nitrogen compound present in the above amounts is advantageous in terms of improving the electrical conductivity of the electrolyte and suppressing the reduction of polysulfides when applied to lithium-sulfur batteries, but the present invention is not limited thereto.

[0159] In one embodiment of the present invention, the concentration of the nitrogen compound in the electrolyte may be, for example, 0.15 to 1.5 M (mol / L), specifically 0.20 to 1.0 M, 0.25 to 0.80 M, or 0.30 to 0.60 M. When the concentration of the nitrogen compound is within the above range, the nitrogen compound improves the electrical conductivity in the electrolyte and suppresses an increase in the amount of sulfur in the negative electrode due to a reduction reaction of (poly)sulfide during use of a lithium-sulfur battery, thereby improving battery degradation.

[0160] In another embodiment of the present invention, the ratio of the molar concentration of the lithium salt to the molar concentration of the additive, e.g., the nitrogen compound, may be, for example, 0.5 to 6.0 (molar concentration of lithium salt / molar concentration of additive), 0.5 to 5.5 (molar concentration of lithium salt / molar concentration of additive), 0.5 to 5.0, 0.90 to 5.0, 1.0 to 4.0, 1.0 to 3.0, or 1.5 to 2.5. When the ratio of the molar concentration of the lithium salt to the molar concentration of the additive is within the above-mentioned range, the ionic conductivity and viscosity of the electrolyte are appropriate, which is advantageous in terms of improving the mobility of lithium ions and suppressing the decomposition reaction of the lithium salt itself, but the present invention is not limited thereto.

[0161] In still another embodiment of the present invention, the sum of the concentration of the lithium salt and the concentration of the additive in the electrolyte solution, i.e., the total concentration, may be, for example, 0.5 M or more, specifically 0.9 M or more, for example, 0.9 M to 2 M, 0.9 M to 1.5 M, or 1.0 M to 1.5 M. If the total concentration of the lithium salt and the additive in the electrolyte solution is within the above-mentioned range, it is preferable in terms of the electrical conductivity and viscosity of the electrolyte solution, and is thereby advantageous in terms of improving the mobility of lithium ions and suppressing the decomposition reaction of the lithium salt itself, but the present invention is not limited thereto.

[0162] In one embodiment of the present invention, the lithium-sulfur battery may have various energy densities depending on the ratio of the electrolyte to the cathode active material. However, in terms of energy density, a smaller ratio of the electrolyte to the cathode active material is advantageous. For example, the weight ratio of the electrolyte to sulfur in the sulfur-carbon composite (El / S weight ratio) may be 3.5 g / g or less.

[0163] In one embodiment of the present invention, the lithium-sulfur battery can be manufactured to have an El / S ratio of, for example, 3.0 g / g or less, or 2.9 g / g or less.

[0164] As described above, according to one embodiment of the present invention, the sulfur (S) content in the negative electrode at SOC 100% is limited to 3 wt % or less, specifically 3.0 wt % or less, and particularly 2.0 wt % or less, thereby achieving long life characteristics.

[0165] In this specification, the lithium-sulfur battery having a long life characteristic means that the battery has an excellent capacity retention rate even after repeated charge-discharge cycles.

[0166] In one embodiment of the present invention, the lithium-sulfur battery maintains 80% or more of its initial capacity up to 190 charge / discharge cycles. Specifically, when the lithium-sulfur battery is repeatedly charged / discharged at room temperature in the range of 1.8 V to 2.5 V at a rate of 0.3 to 0.5 C, it can maintain 80% or more of the capacity measured at the first charge / discharge cycle up to at least 190 cycles.

[0167] In one embodiment of the present invention, the room temperature may be, for example, a temperature of 23°C to 25°C, and specifically, 23°C.

[0168] In one embodiment of the present invention, the charge / discharge rate may be a 0.3C rate or a 0.5C rate, for example, charging at a 0.3C rate and discharging at a 0.5C rate, but is not particularly limited as long as charge / discharge is performed within the above-mentioned rate range.

[0169] In one embodiment of the present invention, the energy density of the lithium-sulfur battery may be, for example, 300 Wh / kg or more.

[0170] In one embodiment of the present invention, the energy density of the lithium-sulfur battery may be measured by a known method, and is not particularly limited by the measurement method. For example, the energy density of the lithium-sulfur battery may be measured at room temperature (e.g., 23°C), in the range of 1.8 V to 2.5 V, by discharging at a 0.5 C rate and charging at a 0.3 C rate.

[0171] In one embodiment of the present invention, the lithium-sulfur battery may have various shapes, such as, but not limited to, a coin type, a pouch type, or a cylindrical type.

[0172] According to another embodiment of the present invention, a method for assessing the lifetime of a lithium-sulfur battery is provided.

[0173] In one embodiment of the present invention, a lithium-sulfur battery to be evaluated for lifespan includes a positive electrode containing a sulfur-carbon composite, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte.

[0174] Specifically, the lithium-sulfur battery may be an electrode assembly including the positive electrode, the negative electrode, and a separator, housed in a case together with an electrolyte.

[0175] In this case, the negative electrode includes a lithium metal layer and a solid electrolyte membrane (SEI) formed on at least one surface of the lithium metal layer.

[0176] The evaluation method includes determining that a lithium-sulfur battery has a long life when the weight of elemental sulfur (S) is 3 wt % or less based on the total weight of the negative electrode at an SOC of 100%.

[0177] In one embodiment of the present invention, the long-life lithium-sulfur battery may have a life of 190 cycles or more, particularly 200 cycles or more, when repeatedly charged and discharged at a rate of 0.3 to 0.5 C in a range of 1.8 V to 2.5 V at room temperature.

[0178] In one embodiment of the present invention, the lithium-sulfur battery to be evaluated is preferably a battery activated by, for example, initial discharge and repeated charge / discharge cycles, such as 1 to 20 cycles, 5 to 15 cycles, or at least 10 cycles, such as 10 cycles, but the present invention is not limited thereto.

[0179] In one embodiment of the present invention, the lithium-sulfur battery to be evaluated may be activated by 0.5C discharge and 0.3C discharge.

[0180] In one embodiment of the present invention, the lithium-sulfur battery to be evaluated may be charged and discharged in the range of 1.8V to 2.5V.

[0181] Furthermore, activation of the lithium-sulfur battery to be evaluated can be performed at room temperature, for example, 23°C to 25°C, for example, 23°C.

[0182] In one embodiment of the present invention, the lithium-sulfur battery may be discharged at 0.5 C and charged at 0.3 C in the range of 1.8 V to 2.5 V at room temperature, 23° C. to 25° C.

[0183] In order to facilitate understanding of the present invention, preferred examples are presented below. However, the following examples are merely illustrative of the present invention, and it will be apparent to those skilled in the art that various changes and modifications can be made within the scope of the present invention and the technical concept thereof. It goes without saying that such changes and modifications also fall within the scope of the claims.

[0184] [Lithium-sulfur battery manufacturing] Example 1 A sulfur-carbon composite (S / C weight ratio = 2.3) was prepared as a positive electrode active material by mixing inorganic sulfur (S8) and carbon nanotubes (CNTs). 96 wt% of the prepared sulfur-carbon composite was mixed with 4 wt% of polyacrylate (PAA) as a binder to prepare a positive electrode slurry composition. The positive electrode slurry composition was applied to an aluminum current collector and dried to prepare a positive electrode. The loading of the prepared positive electrode was 2.08 mg(s) / cm. 2 (2.5mAh / cm 2 ) was.

[0185] A 60 μm thick lithium metal was used as the negative electrode.

[0186] The positive electrode and the negative electrode were positioned facing each other, and a polyethylene separator having a thickness of 16 μm and a porosity of 46 vol% was interposed therebetween to prepare an electrode assembly.

[0187] The prepared electrode assembly was placed in a pouch case, and an electrolyte solution consisting of 0.75M lithium salt (LiTFSI) and 2.1 wt% lithium nitrate (LiNO3) dissolved in a solvent of a 8:2 volume ratio of dimethoxyethane (DME) and 2-methylfuran (2-MeF) was added to produce a lithium-sulfur battery, with an El / S ratio of 2.9 g / g. This resulted in an electrolyte solution with a 0.33M concentration of LiNO3 as a nitrogen compound.

[0188] Example 2 A lithium-sulfur battery was fabricated in the same manner as in Example 1, except that lithium nitrate (LiNO3) was dissolved in the electrolyte at 3.5 wt %.

[0189] Example 3 A lithium-sulfur battery was fabricated in the same manner as in Example 1, except that lithium nitrate (LiNO3) was dissolved in the electrolyte at 5.0 wt%.

[0190] Comparative Example 1 A lithium-sulfur battery was fabricated in the same manner as in Example 1, except that lithium nitrate (LiNO3) was dissolved in the electrolyte at 0.8 wt%.

[0191] Comparative Example 2 A lithium-sulfur battery was fabricated in the same manner as in Example 1, except that lithium nitrate (LiNO3) was dissolved in the electrolyte at 0.4 wt%.

[0192] [Table 1]

[0193] [Evaluation of the energy density of lithium-sulfur batteries] The fabricated lithium-sulfur battery was discharged to 1.8 V at a rate of 0.5 C at room temperature (23 °C), and then charged once to 2.5 V at the same rate. The battery's energy density (Wh / kg) was then measured while discharging once to 1.8 V at a rate of 0.5 C.

[0194] It was confirmed that the energy density of the lithium-sulfur batteries according to the examples and comparative examples met 320 Wh / kg.

[0195] [Evaluation of sulfur (S) content in the negative electrode at SOC 100%] The lithium-sulfur batteries of Examples 1 to 3, Comparative Examples 1 and 2 were activated by charging and discharging them for 10 cycles, each cycle consisting of discharging at a rate of 0.5 C and charging at a rate of 0.3 C at room temperature (23°C) in the range of 1.8 V to 2.5 V. The batteries were then disassembled at SOC 100% to obtain the negative electrodes.

[0196] At this time, the obtained negative electrode includes the lithium metal layer used in the production of the lithium-sulfur battery and a solid electrolyte membrane (SEI) formed by discharging and charging.

[0197] To obtain the negative electrode, the lithium-sulfur battery to be measured was disassembled to separate the positive electrode, separator, and negative electrode. The separated negative electrode was then placed in a container filled with a predetermined mass of solvent and washed for at least 5 minutes. The solvent used was the same non-aqueous solvent as in the electrolyte.

[0198] The contents of lithium (Li), sulfur (S), nitrogen (N) and fluorine (F) in the obtained negative electrode were analyzed, and the analysis results are shown in Table 2 below.

[0199] The contents of lithium (Li), sulfur (S), nitrogen (N) and fluorine (F) elements were analyzed by the following method.

[0200] First, for the analysis of lithium, sulfur, and fluorine, a certain amount of sample was weighed in a dry room where moisture can be controlled, and then a primary oxidation reaction was induced with water. After the primary oxidation reaction, nitric acid was added to dissolve the components in the sample in an ionic state. The treated solution was quantitatively analyzed using ICP-OES (or ICP-MS). The fluorine content was analyzed by immediately quantifying the solution after the primary oxidation using IC.

[0201] To analyze the nitrogen content, a certain amount of the sample was encapsulated in paraffin film in a dry room, and the encapsulated sample was then pyrolyzed in an oxygen-filled atmosphere using an oxygen combustion chamber (bomb). The vaporized nitrogen was then saturated and dissolved in the internal solvent. The nitrogen-absorbed solution was quantitatively analyzed using IC.

[0202] The mass of each element analyzed as described above was calculated as a weight percent based on the total weight of the negative electrode being analyzed, and is shown in the following Table 2. The sum of the masses of each analyzed element was taken as the total weight of the negative electrode.

[0203] [Lifespan evaluation] The lithium-sulfur batteries of Examples 1 to 3 and Comparative Examples 1 and 2 were activated by discharging at a 0.5C rate in the range of 1.8V to 2.5V at room temperature (23°C) and charging at a 0.3C rate. Then, the batteries were repeatedly charged and discharged at a 0.5C rate, and the number of cycles required to maintain 80% of the battery capacity measured at the first cycle after activation was measured. The results are shown in Table 2 below.

[0204] [Table 2]

[0205] From Table 2, it was confirmed that a lithium-sulfur battery with long life characteristics can be realized when the sulfur (S) content in the negative electrode at SOC 100% is controlled to 3 wt% or less.

Claims

1. A lithium-sulfur battery comprising a positive electrode and a negative electrode each including a sulfur-carbon composite, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, The negative electrode includes a lithium metal layer and a solid electrolyte membrane (SEI) formed on at least one surface of the lithium metal layer, A lithium-sulfur battery, wherein the weight of elemental sulfur (S) present in the negative electrode at an SOC of 100% is 3% by weight or less based on the total weight of the negative electrode.

2. 2. The lithium-sulfur battery of claim 1, wherein the 100% SOC has a potential of 2.4V to 2.7V.

3. 2. The lithium-sulfur battery of claim 1, wherein the weight of elemental sulfur (S) in the negative electrode is 3 wt % or less based on the total weight of the elemental sulfur (S) and elemental lithium (Li) in the negative electrode.

4. 4. The lithium-sulfur battery of claim 3, wherein the weight of elemental sulfur (S) in the negative electrode is 2 wt % or less based on the total weight of the elemental sulfur (S) and elemental lithium (Li) in the negative electrode.

5. 4. The lithium-sulfur battery of claim 3, wherein the weight of elemental sulfur (S) in the negative electrode is 1 wt % or less based on the total weight of the elemental sulfur (S) and elemental lithium (Li) in the negative electrode.

6. 2. The lithium-sulfur battery of claim 1, wherein the thickness of the negative electrode is 120 μm or less.

7. 2. The lithium-sulfur battery of claim 1, wherein the thickness of the negative electrode is 70 μm or less.

8. 2. The lithium-sulfur battery according to claim 1, wherein the sulfur-carbon composite has a weight ratio of sulfur to carbon of 2.5 g / g or less.

9. the positive electrode includes a current collector and a positive electrode active material layer including the sulfur-carbon composite; 2. The lithium-sulfur battery according to claim 1, wherein the weight of the sulfur-carbon composite is 90 wt % or more based on the total weight of the positive electrode active material layer.

10. 2. The lithium-sulfur battery according to claim 1, wherein a weight ratio of the electrolyte to the sulfur in the sulfur-carbon composite is 3.5 g / g or less.

11. 2. The lithium-sulfur battery of claim 1, wherein the electrolyte solution comprises a non-aqueous solvent, a lithium salt, and an additive.

12. 12. The lithium-sulfur battery of claim 11, wherein the additive comprises a nitrogen compound.

13. The additive is lithium nitrate (LiNO ３ 12. The lithium-sulfur battery of claim 11, comprising:

14. 14. The lithium-sulfur battery according to claim 11, wherein the content of the additive is 1% by weight to 5% by weight based on the total weight of the electrolyte.

15. 12. The lithium-sulfur battery of claim 11, wherein the ratio of the molar concentration of the lithium salt to the molar concentration of the additive (lithium salt molar concentration / additive molar concentration) is 0.5 to 5.

16. 2. The lithium-sulfur battery of claim 1, wherein the lithium-sulfur battery has a lifespan of 190 cycles or more when repeatedly charged and discharged at a rate of 0.3 to 0.5 C in a range of 1.8 V to 2.5 V at room temperature.

17. 17. The lithium-sulfur battery of claim 16, wherein the room temperature is a temperature of 23°C to 25°C.

18. 2. The lithium-sulfur battery of claim 1, wherein the energy density of the lithium-sulfur battery is 300 Wh / kg or more.

19. a positive electrode including a sulfur-carbon composite, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte; A method for evaluating the life of a lithium-sulfur battery, wherein the negative electrode includes a lithium metal layer and a solid electrolyte membrane (SEI) formed on at least one surface of the lithium metal layer, and determining that the negative electrode has a long lifespan when the weight of elemental sulfur (S) present in the negative electrode at an SOC of 100% is 3 wt % or less based on the total weight of the negative electrode.

20. The evaluation method according to claim 19, wherein the long-life lithium-sulfur battery has a life of 190 cycles or more when charge-discharge cycles are repeated at room temperature in the range of 1.8 V to 2.5 V at a rate of 0.3 to 0.5 C.

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