Analysis method of cathode used in lithium-sulfur battery

Abstract

The present invention provides a method for analyzing a positive electrode used in a lithium-sulfur secondary battery, comprising: (S1) a step of preparing a positive electrode sample by disassembling the battery and separating the positive electrode after charging and discharging a lithium-sulfur secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte in which a lithium salt is dissolved as an electrolyte in an organic solvent, wherein the positive electrode sample is used in a composite of a sulfur (S8)-carbon-based material; (S2) a step of performing EA (elemental analyzer), IC (ion chromatography), and ICP-OES (inductively coupled plasma optical emission spectrometer) analysis on the positive electrode sample, respectively; and (S3) a step of confirming the overall composition of the positive electrode sample and the content of by-products by combining the content of each component measured by each of the respective analyses, wherein in step (S2), the ICP-OES analysis is performed with a solution in which the positive electrode sample is dissolved in ultrapure water.

Description

Technology Field

[0001] The present invention relates to an analysis method for evaluating the degradation of a positive electrode used in a lithium-sulfur secondary battery. Background Technology

[0002] A lithium secondary battery basically has a structure in which an electrolyte, which is a transport medium for lithium ions, is impregnated into an electrode assembly composed of a positive electrode, a negative electrode, and a separator for separating the two electrodes.

[0003] Lithium-ion batteries using transition metal oxides as the cathode active material have been commercialized and are used in electronic devices such as mobile phones, laptops, and camcorders, but their theoretical capacity is only about 250 mAh / g or less, which limits their use in electric vehicles and large-capacity power storage systems that require high energy density. To overcome these limitations, lithium-sulfur batteries capable of achieving a high theoretical capacity density of 2,600 mAh / kg are attracting attention.

[0004] The above lithium-sulfur battery uses a sulfur-based compound having disulfide bonds as the positive electrode active material and a material containing lithium metal or lithium ions as the negative electrode active material. The sulfur used in the positive electrode active material is an inexpensive and environmentally friendly material with an electrical conductivity of 5×10 -30 Due to the disadvantage of having a low S / cm value, which makes it difficult for electrons generated by electrochemical reactions to move, it is used in combination with carbon-based conductive materials to form a composite.

[0005] Meanwhile, the sulfur exists in the structure of cyclic S8. During the discharge process, a reduction reaction occurs in which the oxidation number of S decreases as the SS bond is broken, and during the charging process, an oxidation reaction occurs in which the oxidation number of S increases as the SS bond is reformed. For example, as the discharge progresses, sulfur (S8) reacts continuously with lithium ions and is reduced through stepwise phase changes from S8→L2S8→(Li2S6)→Li2S4→Li2S2→Li2S, which is characterized by exhibiting stepwise discharge voltages.

[0006] In the above stepwise reduction process, the intermediate product lithium polysulfide (Li2Sx, 4≤x≤8) is easily dissolved in the organic electrolyte and exists in a liquid phase, while the lithium sulfides of Li2S2 and Li2S are insoluble and exist in a solid phase, and due to this difference in solubility, a region in which phase separation occurs is formed.

[0007] The above-mentioned insoluble lithium sulfides (Li2S2 and Li2S) are very stable phases that accumulate as byproducts within the anode and act as insulators, thereby degrading potential characteristics, and the accumulation of byproducts during battery degradation can affect performance degradation.

[0008] Therefore, it is necessary to evaluate the degree of degradation caused by battery operation by analyzing the content of byproducts accumulated in the cathode used in lithium-sulfur batteries, in addition to the initial materials of carbon and sulfur. The problem to be solved

[0009] The objective of the present invention is to provide a method for accurately analyzing the content of by-products accumulated in the cathode as the charge-discharge cycle of a lithium-sulfur secondary battery progresses. means of solving the problem

[0010] According to one aspect of the present invention, a method for analyzing a positive electrode used in a lithium-sulfur secondary battery,

[0011] (S1) A step of preparing a positive electrode sample by disassembling the battery and separating the positive electrode after charging and discharging a lithium-sulfur secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte using a composite of sulfur (S8)-carbon-based materials,

[0012] (S2) A step of performing EA (elemental analyzer), IC (ion chromatography), and ICP-OES (inductively coupled plasma optical emission spectrometer) analyses on the anode sample, respectively; and

[0013] (S3) A step of confirming the total composition of the anode sample and the content of by-products by combining the content of each component measured by each of the above analyses, and

[0014] In the above step (S2), the ICP-OES analysis is performed using a solution in which the anode sample is dissolved in ultrapure water. This provides an analysis method.

[0015] Additionally, the present invention provides a positive electrode for a lithium-sulfur secondary battery analyzed by the above method, wherein after 100 charge-discharge cycles, the byproduct of Li2S or Li2S2 is present in an amount of less than 20% by weight based on the total content of the positive electrode. Effects of the invention

[0016] According to the present invention, by analyzing the cathode used in a lithium-sulfur secondary battery using EA, IC, and ICP-OES respectively and synthesizing the results according to mass balance, the overall composition of various phases derived from C, S, O, H, N, F, and Li components contained in the degraded cathode can be confirmed. In particular, when analyzing the cathode sample with ICP-OES, the cathode sample is pretreated with ultrapure water to extract Li2S and Li2S2, which are insoluble in the electrolyte but soluble in water, thereby allowing for the accurate measurement of the content of byproducts accumulated in the cathode degraded by charging and discharging. Specific details for implementing the invention

[0017] The present invention will now be described in detail. Terms and words used in this specification and claims should not be interpreted as being limited to their ordinary or dictionary meanings, but should be interpreted in a meaning and concept consistent with the technical spirit of the present invention, based on the principle that the inventor can appropriately define the concepts of the terms to best describe his invention.

[0018] In addition, the configurations exemplified in the embodiments described in this specification are merely one preferred embodiment of the present invention and do not represent all of the technical ideas of the present invention; therefore, it should be understood that various equivalents and modifications that can replace them may exist at the time of filing this application.

[0020] One embodiment of the present invention relates to a method for analyzing a positive electrode used in a lithium-sulfur secondary battery, comprising a step of preparing a positive electrode sample (S1), a step of analysis (S2), and a step of confirming the overall composition and byproduct content of the positive electrode sample (S3).

[0021] The analysis method of the present invention is described in detail step by step below.

[0022] Step (S1) of the analysis method according to the present invention is for preparing an anode sample, wherein a lithium-sulfur secondary battery in which an electrolyte is impregnated into an electrode assembly comprising an anode, a cathode, and a separator interposed between the two electrodes is first charged and discharged.

[0023] The above anode may be one in which an electrode layer comprising a composite of a sulfur (S8)-containing material and a carbon-based material is formed on an anode current collector such as aluminum.

[0024] The above sulfur (S8)-containing material is sulfur (S8), Li2S n (n≥1), may include organic sulfur compounds or mixtures thereof.

[0025] The above carbon-based material is intended to compensate for the low electrical conductivity of sulfur and can be selected from the group consisting of carbon black, graphite, graphene, activated carbon, carbon fibers, carbon nanotubes, and combinations thereof.

[0026] The mixing ratio of the sulfur (S8)-containing material and the carbon-based material can be appropriately selected according to the intended electrode performance, and, for example, can be applied in a ratio of 1:1 to 3:1, but is not limited thereto.

[0027] The above-mentioned cathode is formed by forming an electrode layer containing an active material capable of reversibly inserting and releasing lithium ions on a cathode current collector such as copper, and the cathode active material may be a lithium-containing compound, lithium metal, lithium alloy, or a mixture thereof.

[0028] The above separator enables the transport of lithium ions between the anode and the cathode while separating or insulating the anode and the cathode from each other. There are no specific limitations on the porous substrate commonly used in the field, and for example, a polyolefin-based porous membrane or a nonwoven fabric may be used.

[0029] The above electrolyte may be a non-aqueous electrolyte in which a lithium salt is dissolved in an organic solvent as the electrolyte. The lithium salts include LiCl, LiBr, LiI, LiClO4, LiBF4, and LiB 10 Cl 10One or more selected from the group consisting of LiB(Ph)4, LiC4BO8, LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, LiSO3CH3, LiSO3CF3, LiSCN, LiC(CF3SO2)3, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiN(SO2F)2, lithium chloroborane, lithium lower aliphatic carboxylate, lithium tetraphenylborate, and lithium imide may be used. One or more selected from the group consisting of dimethoxyethane, diglyme, triglyme, tetraglyme, 1,3-dioxolane, diethyl ether, N-methylpyrrolidone, 3-methyl-2-oxazolidone, dimethyl formamide, sulfolane, dimethyl acetamide or dimethyl sulfoxide, dimethyl sulfate, ethylene glycol diacetate, dimethyl sulfite, ethylene glycol sulfite, bis(2,2,2-trifluoroethyl) ether, and 2-methylfuran may be used as the organic solvent.

[0030] In a lithium-sulfur secondary battery comprising the above configuration, the sulfur (S8) contained in the positive electrode has a cyclic structure having disulfide bonds and, as discharge proceeds, reacts continuously with the lithium ions of the negative electrode to undergo a stepwise phase change from S8→L2S8→(Li2S6)→Li2S4→Li2S2→Li2S and is reduced. In this stepwise reduction process, the intermediate product, lithium polysulfide (Li2Sx, 4≤x≤8), is easily dissolved in the organic electrolyte and exists in a liquid state, while the lithium sulfides of Li2S2 and Li2S are insoluble and exist in a solid state, and a region in which phase separation occurs is formed due to this difference in solubility.

[0031] The above-mentioned insoluble lithium sulfides (Li2S2 and Li2S) are very stable phases that accumulate as byproducts within the anode and act as insulators, thereby degrading potential characteristics, and the accumulation of byproducts during battery degradation can affect performance degradation. Therefore, it is necessary to analyze the content of lithium sulfides accumulated in the anode after charging and discharging.

[0032] Meanwhile, the number of cycles for the above-mentioned charge and discharge can be adjusted according to the intended evaluation purpose, for example, and can be carried out with 100 cycles.

[0033] After charging and discharging, the lithium-sulfur secondary battery is disassembled, the positive electrode is separated, and then washed and dried to prepare a positive electrode sample.

[0034] The above washing is performed using an organic solvent, such as 1,2-dimethoxyethane (DME), and can remove the electrolyte remaining on the anode and components dissolved by the electrolyte.

[0035] The above drying can be performed at room temperature.

[0037] In step (S2) of the analysis method according to the present invention, EA (elemental analyzer), IC (ion chromatography), and ICP-OES (inductively coupled plasma optical emission spectrometer) analyses are performed on the anode sample prepared in the previous step, respectively.

[0038] The above EA analysis is intended to measure the components of C, S, N, H, and O contained in the anode sample and can be performed using conventional methods in the field. For example, a portion of the sample is combusted in the presence of oxygen at 900 to 1100°C, e.g. 990°C, to ionize the four elements C, S, N, and H, followed by continuous oxidation and reduction. Then, to measure the element O, another portion of the sample is thermally decomposed in the absence of oxygen, and finally, the generated gases are separated by a GC column and detected by a thermal conductivity detector (TCD).

[0039] In addition, during the above EA analysis, a calibration curve for each elemental component can be prepared using a standard substance whose elemental content is already known, and then the GC and TCD detection results for the anode sample can be substituted into the calibration curve to measure the content of the components containing each element. The standard substances may include BBOT (2,5-Bis(5-tert-butyl-benzoxazol-2-yl)thiophene), L-cystine, etc.

[0040] In the components of C, S, N, H, and O identified by EA analysis of this process, the S and C components correspond to sulfur (S8) and carbon-based materials used as cathode materials, and the N, H, and O components may correspond to by-products generated from the electrolyte during the charging and discharging process.

[0041] The above IC analysis is intended to measure the F content contained in the anode sample and can be performed using conventional methods in the field. For example, the sample is combusted in a furnace in an oxygen atmosphere to generate F gas, which is then absorbed into water, and the absorption solution is delivered to an IC system for detection. At this time, a combustion aid such as WO3 may be additionally added to increase the combustion efficiency of the sample, and WO3 may be used in an amount of 2 to 5 times the weight of the sample.

[0042] The F component identified by IC analysis of this process is attributed to the electrolyte components contained in the electrolyte, and may be a component accumulated due to the decomposition of the electrolyte as charge-discharge cycles progress.

[0043] The above ICP-OES analysis is a component analysis performed by applying high thermal energy to a sample with a high-temperature plasma induced by an inert gas, argon, thereby making atoms and ions in the sample excited, and then detecting the lines emitted as they return to a low energy level, which can measure the components of Li and S contained in the anode sample.

[0044] In the present invention, considering that the subject of analysis is a cathode used in a lithium-sulfur secondary battery, the cathode sample is pretreated with ultrapure water during ICP-OES analysis to perform component analysis.

[0045] As explained above, in a lithium-sulfur secondary battery, the sulfur (S8) contained in the positive electrode undergoes a stepwise phase change from S8→L2S8→(Li2S6)→Li2S4→Li2S2→Li2S during discharge, and among these, the lithium polysulfide (Li2Sx, 4≤x≤8) exists as an unstable liquid phase that is easily dissolved in the electrolyte, while the lithium sulfides of Li2S2 and Li2S exist as a stable solid phase that is insoluble in the electrolyte.

[0046] When the degraded cathode of a lithium-sulfur secondary battery that has undergone such a stepwise reduction process is pretreated with a strong acid such as nitric acid, as in general ICP-OES analysis, the Li component present in the cathode can be dissolved.

[0047] Meanwhile, sulfur (S8) used as the positive electrode active material is known to be soluble in organic solvents but insoluble in both acid and water, and Li2S2 and Li2S among the lithium sulfide components are known not to be soluble in organic solvents (e.g., DME) used in the electrolyte. Accordingly, if the positive electrode is separated from the lithium-sulfur battery and washed with an organic solvent such as DME, components other than Li2S2 and Li2S are removed, and the remaining Li2S2 and Li2S can be extracted by dissolving them in ultrapure water and then analyzed.

[0048] In this way, the present invention allows for the accurate measurement of the content of accumulated byproducts, such as Li2S2 and Li2S, along with the overall composition of the anode that has undergone charging and discharging, by performing sample pretreatment using the difference in solubility due to the phase difference of the components present in the anode and then conducting ICP-OES analysis.

[0049] Therefore, the Li component identified by ICP-OES analysis in the present invention is derived from a byproduct generated from the negative electrode active material inserted in the negative electrode or the electrolyte in the electrolyte during the charging and discharging process, and the S component identified by ICP-OES may be derived from a byproduct of Li2S or Li2S2 formed by the stepwise reduction of sulfur (S8) used in the positive electrode during charging and discharging.

[0050] In one embodiment of the present invention, the ultrapure water treatment of the anode sample may be performed by adding ultrapure water in a range of 1 to 10 ml based on 10 mg of a fractionated sample and then heating at a high temperature (e.g., 130 to 150°C).

[0052] In step (S3) of the analysis method according to the present invention, the content of each component measured by each of the above analyses is combined to determine the total composition of the anode sample and the content of by-products.

[0053] Specifically, the total content of the degraded anode after charging and discharging can be determined by summing the contents of C, S, O, H, N, F, and Li components measured by the analysis of the above EA, IC, and ICP-OES, and the confirmed total content may show an error of 10 wt%, for example, 5 wt% or less or 2 wt% or less based on 100 wt%.

[0054] At this time, the content of Li2S2 and Li2S measured by analyzing the solution of the anode sample pretreated with ultrapure water using ICP-OES may be less than 20% by weight, for example, less than 10% by weight, based on the total content of the anode.

[0055] Since the above Li2S2 and Li2S have a high boiling point of about 1372°C, they are components that are not analyzed by EA, which analyzes the gas phase generated by combustion at 900 to 1100°C, but are analyzed only by ICP-OES, making them suitable for use as factors to balance the mass of the entire anode composition.

[0057] According to the analysis method of the present invention as described above, the composition of the anode degraded by charging and discharging and the content of accumulated by-products can be accurately measured, and the results can be used for quality control of the anode material and battery design.

[0058] Accordingly, the present invention further provides a positive electrode of a lithium-sulfur secondary battery analyzed by the above method.

[0059] The above anode may have a byproduct of Li2S or Li2S2 present in an amount of less than 20% by weight, for example, less than 10% by weight, based on the total content of the anode after 100 charge-discharge cycles.

[0061] Hereinafter, the present invention will be described in detail with reference to examples to aid in understanding. However, the embodiments according to the present invention may be modified in various different forms, and the scope of the present invention should not be interpreted as being limited to the following embodiments. The embodiments of the present invention are provided to more completely explain the present invention to those with average knowledge in the art.

[0062] Example:

[0063] (Step 1) Preparation of the anode sample

[0064] A lithium-sulfur secondary battery was prepared by using an electrolyte in which an active material layer containing sulfur (S8) and CNT in a weight ratio of 75:25 was formed on an aluminum current collector, a lithium metal negative electrode, a porous polyolefin separator was interposed between the positive and negative electrodes, and an electrolyte of LiN(SO2F)2 (LiFSI) dissolved at a concentration of 0.5 M in a solvent mixed with bis(2,2,2-trifluoroethyl) ether (BTFE), 2-methyl furan, and dimethoxyethane (DME) in a volume ratio of 10:20:70, and LiNO3 3 wt% was added, and charging and discharging were performed for 100 cycles under conditions of 0.2 C charging and 0.3 C discharging. After the charging and discharging, the battery was disassembled to separate the positive electrode, washed with 1,2-dimethoxyethane (DME), and dried at room temperature to prepare a positive electrode sample.

[0066] (Step 2) Analysis of the anode sample

[0067] <EA 분석>

[0068] Approximately 1 to 2 mg of the above anode sample was taken and placed into Sn capsules (for C, S, N, H analysis) and Ag capsules (for O analysis), respectively. After folding the samples to isolate them from the outside, they were fed into an EA device (Flash 2100, Thermo) and combusted at 990°C to generate gas. The generated gas was separated using a GC column to measure the components of C, S, N, H, and O.

[0069] Meanwhile, BBOT(2,5- as a standard substance for quantitative analysis Bis (5-tert-butyl-benzoxazol-2-yl)thiophene) was used.

[0070] Table 1 below shows the analysis results using EA.

[0071]

[0072] In Table 1 above, the anode containing sulfur (S8) and CNT in a weight ratio of 75:25 was analyzed by EA after 100 charge-discharge cycles, and the C content was found to be 24%, corresponding to the initial CNT content, and the S content was found to be 44.6% as a result of a stepwise reduction process. In addition, the content of N, H, and O can be determined to have originated from by-products from the electrolyte during the charge-discharge process.

[0073] To cross-verify the results of the EA analysis, TGA analysis was performed on the same anode sample, and the S content was confirmed to be 45%, so it can be determined that the S content measured by the EA analysis is a reasonable result.

[0075] <IC 분석>

[0076] About 1 to 10 mg of the above anode sample was taken and placed in a ceramic sample boat, and about 20 to 50 mg of WO3 was added as a combustion aid, and then burned in an automatic quick furnace under an oxygen atmosphere. After absorbing the F gas generated during the combustion process into water, the absorption solution was introduced into an IC instrument (Thermo Dionex ICS-6000) to measure the F component.

[0077] <IC 분석 조건>

[0078] Column: IonPac AS18 (4 X 250 mm)

[0079] Eluent type: KOH (30.5 mM, isocratic)

[0080] Eluent flow rate: 1 ml / min

[0081] Detector: suppressed conductivity detector

[0082] SRS current: 76mA

[0083] Injection volume: 25 µl

[0085] Table 2 below shows the analysis results using EA.

[0086]

[0087] In Table 2 above, the F component was found to be 2.2 wt% in the anode after 100 charge-discharge cycles, which is attributed to electrolyte components contained in the electrolyte, such as LiN(CF3SO2)2 and LiN(SO2F)2, and can be judged to be a component accumulated by the decomposition of the electrolyte during the charge-discharge process.

[0089] <ICP-OES 분석>

[0090] About 10 mg of the above anode sample was taken and placed in a container, about 1 ml of ultrapure water was added, and the sample was dissolved by heating to 150°C.

[0091] After about 2 to 3 hours, the sample solution was diluted with 50 g of ultrapure water, and undissolved components were removed using a 0.45 µm PTFE filter. The remaining filtrate was then fed into an ICP-OES instrument (AVIO 500, Perkin Elmer) to perform component analysis.

[0092] <ICP-OES 분석조건>

[0093] RF Power: 1300W

[0094] Torch Height: 15 mm

[0095] Plasma gas flow rate: 12 L / min

[0096] Sample gas flow rate: 0.8 L / min

[0097] Auxiliary gas flow rate: 0.20 L / min

[0098] Pump speed: 1.5 ml / min

[0100] The analysis results using ICP-OES are shown in Table 3 below.

[0101]

[0102] In Table 3 above, it can be confirmed that the S component was measured at 8.8 wt% along with the Li component by ICP-OES analysis of the ultrapure water-treated anode sample. Considering that sulfur (S8) itself is not soluble in both acid and water, the S component analyzed after pre-treating the anode sample with ultrapure water is determined to have originated from Li2S2 and Li2S. That is, Li2S2 and Li2S are insoluble in the electrolyte but were measured as being soluble in water.

[0104] (Step 3) Confirmation of total composition and byproduct content of the anode sample

[0105] The total composition and byproduct content of the anode sample were determined by combining the component contents measured by the above EA, IC, and ICP-OES analyses, and the results are shown in Table 4 below.

[0106]

[0107] From Table 4 above, the total content was calculated to be 98.1 wt% as a result of summing the contents of the components performed in all analyses, which indicates that it has an error of 2 wt% or less based on 100 wt%.

[0108] The S component identified by the above ICP-OES analysis is derived from Li2S2 and Li2S. Since Li2S2 and Li2S have a high boiling point of approximately 1372°C, they are components that are not analyzed by EA, which analyzes the gas phase generated by combustion at 900 to 1100°C, but are analyzed only by ICP-OES. Therefore, since no additional components are identified even when searching for the total elemental composition of the anode, the content of Li2S2 and Li2S analyzed by ICP-OES was used as a factor to balance the mass of the anode's overall composition.

[0109] Through this process, the total composition of the degraded cathode and the content of byproducts can be measured for each charge-discharge cycle of the lithium-sulfur secondary battery, thereby enabling efficient battery evaluation.

[0111] Reference Example:

[0112] After performing nitric acid pretreatment on the anode sample obtained in Step 1 of the example, ICP-OES analysis was performed under the conditions shown in Step 2, and the results are shown in Table 5 below.

[0113]

[0114] In Table 5 above, it can be seen that the content of the Li component confirmed by ICP-OES analysis after acid treatment of the anode sample is similar to the content of the Li component confirmed after ultrapure water pretreatment in Table 4.

[0115] These results demonstrate that the S component identified by ICP-OES analysis after ultrapure water treatment of the anode sample originated from Li2S2 and Li2S, indirectly confirming that although S8 is insoluble in water, it was analyzed as having water solubility after being reduced to Li2S2 and Li2S.

Claims

Claim 1 A method for analyzing a positive electrode used in a lithium-sulfur secondary battery, comprising: (S1) a step of preparing a positive electrode sample by disassembling the battery and separating the positive electrode after charging and discharging a lithium-sulfur secondary battery comprising a positive electrode using a composite of sulfur (S8)-carbon-based material, a negative electrode containing lithium metal as an active material, a separator, and an electrolyte in which a lithium salt is dissolved as an electrolyte in an organic solvent; (S2) a step of performing EA (elemental analyzer), IC (ion chromatography), and ICP-OES (inductively coupled plasma optical emission spectrometer) analysis on the positive electrode sample, respectively; and (S3) a step of confirming the total composition of the anode sample and the content of by-products by synthesizing the content of C, S, O, H, N, F, and Li components measured by each of the above analyses, wherein in step (S2), the ICP-OES analysis is an analytical method performed with a solution in which the anode sample is dissolved in ultrapure water, wherein the carbonaceous material is selected from the group consisting of carbon black, graphite, graphene, activated carbon, carbon fiber, carbon nanotube, and combinations thereof, and the by-products of the anode sample are L2S8, Li2S6, Li2S4, Li2S2, Li2S; and components accumulated from the decomposition of the electrolyte, wherein the electrolyte is LiCl, LiBr, LiI, LiClO4, LiBF4, LiB 10 Cl 10 An analytical method selected from the group consisting of LiB(Ph)4, LiC4BO8, LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, LiSO3CH3, LiSO3CF3, LiSCN, LiC(CF3SO2)3, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiN(SO2F)2, lithium chloroborane, lithium lower aliphatic carboxylate, lithium tetraphenylborate, and lithium imide. Claim 2 delete Claim 3 An analysis method according to claim 1, wherein components containing C, S, N, H, and O are identified by the EA analysis, components containing F are identified by the IC analysis, and components containing Li and S are identified by the ICP-OES analysis. Claim 4 In paragraph 3, the S and C components identified as EA are derived from sulfur (S8) and carbon-based materials used in the anode, and the N, H and O components identified as EA are derived from by-products generated from the electrolyte during the charging and discharging process. Claim 5 In paragraph 3, the analysis method wherein the F component identified by the IC is derived from a byproduct generated from the electrolyte in the electrolyte during the charging and discharging process. Claim 6 In paragraph 3, the Li component identified by ICP-OES is derived from a byproduct generated from a negative electrode active material inserted in the negative electrode or an electrolyte in the electrolyte during the charging and discharging process, and the S component identified by ICP-OES is derived from a byproduct of Li2S or Li2S2 formed by stepwise reduction of sulfur (S8) used in the positive electrode during charging and discharging. Claim 7 An analysis method according to paragraph 3 in which the S component identified by the ICP-OES is used as a factor for adjusting the mass balance of the entire anode composition. Claim 8 An analysis method according to claim 1, wherein the total content of the anode sample confirmed by combining the measurement values ​​of the EA, IC, and ICP-OES shows an error of 2% or less based on 100% by weight. Claim 9 A cathode of a lithium-sulfur secondary battery analyzed by the method according to claim 1, wherein after 100 charge-discharge cycles, the byproduct of Li2S or Li2S2 is present in an amount of less than 20% by weight based on the total content of the cathode.

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