JP5120124B2 - High-precision analysis of lithium by ion chromatography - Google Patents

Description

  The present invention relates to a high-accuracy analysis method for lithium by ion chromatography, and more particularly to a method for analyzing lithium in a positive electrode material for a lithium secondary battery with high accuracy.

  Lithium secondary batteries are excellent in light weight and charge / discharge cycle characteristics, and are therefore mounted in portable electronic devices such as personal computers, video cameras, and mobile phones. Recently, attention has been paid to the automobile field against the background of global environmental problems and resource depletion problems, and installation in fuel cell vehicles and hybrid vehicles has been intensively studied.

  Generally, a lithium secondary battery includes a positive electrode made of a metal oxide, a negative electrode made of carbon, an electrolytic solution in which a lithium salt is dissolved in an organic solvent, and a separator. As the positive electrode material, lithium-containing transition metal oxides such as lithium cobaltate, lithium manganate and lithium nickelate are common, but the composition management and composition control technology of the elements constituting these compounds are capacity density, charge / discharge It is extremely important in characteristics such as cycle life, safety and economy.

  Conventionally, the concentration of metal elements such as lithium constituting the positive electrode material of a lithium secondary battery is measured by decomposing the sample with acid or alkali to form a solution, and the element to be measured in the resulting solution is inductively coupled A method of detecting and measuring by plasma emission spectroscopic analysis, flame atomic absorption method, flame light method or titration method is applied. These analytical methods are generally used as metal element detection means, but it is difficult to apply a relatively high titration method or gravimetric method, particularly in the measurement of lithium, and inductively coupled plasma emission spectroscopy or Flame atomic absorption is easily affected by coexisting elements, plasma, and flame fluctuations.

  Therefore, the repeated measurement accuracy in the above analysis method was 1% or more in terms of relative standard deviation (hereinafter abbreviated as RSD). Such an analysis error of a large positive electrode material constituent element is not accepted in the current battery development or manufacturing field, and further improvement in analysis accuracy is required. In other words, the specific measurement accuracy currently required for the composition management of the positive electrode material is extremely strict with an RSD of 0.2% or less per element.

  Under such circumstances, Japanese Patent Application Laid-Open No. 11-287793 and Japanese Patent Application Laid-Open No. 2002-174446 describe a method in which an ion chromatographic method is applied to the measurement of the lithium secondary battery positive electrode material. According to the ion chromatograph method, it is expected that the lithium repeat measurement accuracy is about 0.2% by RSD.

  However, according to a thorough investigation by the present inventors, particularly in an ion chromatograph equipped with an anion-exchange suppressor, stable continuous measurement is difficult due to the influence of coexisting elements such as nickel, cobalt, and manganese. There was a case. In order to solve this problem, the present inventors have proposed an ion chromatograph measurement method capable of stable continuation without being affected by coexisting elements (see Japanese Patent Application No. 2008-057162).

  Measurements using ion chromatography include weighing and disassembling actual samples, constant volume operation to adjust the volume of the sample solution in solution to a constant volume, dilution operation to dilute the solution and the standard solution for the calibration curve to a fixed magnification, etc. It has been clarified that when a series of pretreatment operations are continuously performed, the accuracy of lithium analysis is significantly lowered.

  As a result of investigating the details of this problem, the cause of the decrease in accuracy is due to the volume error caused by the constant volume operation and dilution operation, which greatly exceeds the tolerance of the full volume flask and the full volume pipette used. It was a thing. That is, this error is caused by an artificial ability error, and it has been difficult to completely prevent the occurrence particularly in an environment where a large number of samples are handled.

JP-A-11-287793 JP 2001-174446 A

  When the lithium concentration in the lithium secondary battery positive electrode material is measured by the ion chromatography method, it is desirable to dilute the sample 10,000 times or more in consideration of the influence of the coexisting element concentration and the like. The constant volume operation and the number of dilution operations during the pretreatment process depend on the amount of sample to be decomposed, but at least 2 to 3 constant volume operations and at least 1 to 2 dilution operations are performed. .

  The constant volume operation and the dilution operation are generally performed using a full volume flask or a full volume pipette volume whose tolerance is defined by JIS standards. This method is the most versatile and highly accurate. However, in an environment where a large number of samples are handled, the accuracy of the alignment of all the flasks and pipettes and the accuracy of the wake management of the whole pipette are reduced. It is easy to cause a significant decrease in analysis accuracy.

  In view of such a current situation, the present invention is not affected by a capacity error that occurs in a sample pretreatment step, particularly when lithium in a positive electrode material of a lithium secondary battery is analyzed by an ion chromatography method, and the ion An object of the present invention is to provide a highly accurate analysis method that suppresses errors caused by fluctuations in the chromatographic apparatus, fluctuations in sample introduction into the ion chromatographic apparatus, and the like, and can perform continuous and stable measurement.

  In order to achieve the above object, the present inventors have applied the measurement technique using an internal standard element to the ion chromatograph method and have found the optimal internal standard element for lithium to complete the present invention. Is. The measurement method using an internal standard element is generally called an internal standard correction method, and is a method generally used in measurement methods other than ion chromatography, such as coupled plasma emission spectrometry, and the like. Various fluctuation factors occurring in the process can be corrected.

  That is, the high-accuracy analysis method for lithium provided by the present invention uses an ion chromatograph apparatus equipped with a separation column packed with a cation exchange resin and an anion exchange suppressor, and uses a mobile phase feed pump together with the mobile phase. In the analytical method for detecting the lithium concentration in the sample solution to be sent, at least one of potassium, ammonium and an amine compound is measured as an internal standard element. The amine compound is preferably any one of methylamine, dimethylamine, ethylamine, diethylamine, ethanolamine and diethanolamine.

  In the above-described high-accuracy analysis method for lithium according to the present invention, a flow path switching valve is provided between the separation column and the suppressor of the ion chromatograph device, and the flow path switching valve is set according to the retention time of the component elements in the sample solution. By switching the above, the fraction not containing lithium and the internal standard element is discharged out of the system, and only the fraction containing the measurement target element can be selectively introduced into the suppressor.

  Furthermore, in the high-accuracy analysis method for lithium according to the present invention, a certain amount of a solid or liquid compound containing an internal standard element is added before the sample is decomposed, or immediately after the sample is decomposed and cooled to room temperature. It is characterized by adding. At that time, it is preferable to add a predetermined amount of the liquid of the compound containing the internal standard element and the fixed amount of the liquid of the standard sample used for the calibration curve by grasping by weight.

  In the high-accuracy analysis method for lithium according to the present invention, the sample solution is obtained by decomposing a positive electrode material for a lithium secondary battery into a solution, and in particular, elements other than lithium are aluminum, nickel, cobalt, manganese, and iron. When it contains at least one kind, a further effect can be expected.

  According to the present invention, a lithium concentration in a sample can be measured with high accuracy and continuously for a long period of time by an ion chromatograph method without being affected by a capacity error generated in a pretreatment process. It becomes. Therefore, the lithium high-accuracy analysis method of the present invention is particularly useful for the measurement of lithium in a positive electrode material for a lithium secondary battery that requires extremely high-accuracy measurement.

  In the method for measuring lithium by the ion chromatograph method of the present invention, at least one of potassium, ammonium and amine compounds is used as an internal standard element, and the area ratio of lithium peak to internal standard element peak in the ion chromatogram (lithium peak area / By measuring the internal standard element peak area) or peak height ratio (lithium peak height / internal standard element peak height), errors caused by fluctuations in detectors, fluctuations in the amount injected into the separation column, etc. are reduced. be able to.

  Hereinafter, the lithium highly accurate analysis method according to the present invention will be described in detail. In the analysis of lithium by ion chromatography, for example, when preparing a sample solution of a positive electrode material for a lithium secondary battery, a sample is weighed into a beaker, etc., and then nitric acid and hydrogen peroxide water are added to the hot plate or the like. The sample is decomposed by heating using a heating device. At that time, a certain amount of the solid or liquid of the compound containing the internal standard element is added before decomposition of the sample, or is added immediately after the sample is decomposed and cooled to room temperature.

  For example, when potassium is used as the internal standard element, there is no possibility of decomposition or loss even if an acid is used for the above decomposition treatment. Therefore, when a sample is weighed in a beaker, a compound containing potassium is also weighed at the same time. Can do. On the other hand, when ammonium or an amine compound is used as an internal standard element, these are decomposed by the above-described decomposition treatment using an acid to change the form, and cannot be stably detected by an ion chromatograph. Therefore, it is desirable to add a compound containing an ammonium or a solid or liquid of an amine compound after the sample is decomposed and allowed to cool.

  The potassium-containing compound used as the internal standard element is not particularly limited, but the potassium content does not change due to moisture absorption or self-decomposition, and it easily dissolves or decomposes in the sample decomposition process to form potassium ions. In general, a solid of potassium carbonate or a solution thereof is used.

  Moreover, there is no restriction | limiting in particular as a compound containing ammonium and an amine compound, However, The solid and liquid of a stable compound with which the density | concentration of ammonium or an amine compound does not change for a long period of time are preferable. For example, the compound containing ammonium includes ammonium chloride. As the amine compound, methylamine, dimethylamine, ethylamine, diethylamine, ethanolamine, diethanolamine and the like are used.

  In the preparation of the sample solution, when a certain amount of liquid of the compound containing the internal standard element is weighed, it is preferable to grasp and add a certain amount of the liquid by weight. The same applies to the adjustment of the standard sample used for the calibration curve. This is because, when a certain amount of liquid is collected with a pipette or the like, there is a concern about the occurrence of artificial errors as described above. By this method, the error due to the capacity error can be greatly reduced.

  Since the sample solution prepared as described above contains a certain amount of the internal standard element, it is not particularly necessary to adjust it to a certain volume. However, in order to further reduce the cause of error and prepare a more uniform solution, it is preferable to transfer the sample solution to a full-volume flask, make a constant volume, stir well, and store with a tight stopper. In addition, the instrument used for dilution operation should just be a clean instrument, and generally the whole quantity pipette is used. However, in consideration of operability and quickness, a well-maintained push button type liquid microvolume meter is practical.

  The obtained sample solution is supplied to an ion chromatograph equipped with a separation column packed with a cation exchange resin and an anion exchange suppressor, and sent along with the mobile phase by a mobile phase feed pump to detect the lithium concentration. Is done. If the lithium ion and the internal standard element ion are uniformly dispersed in the sample solution by the stirring operation, the volume error that occurs in the above constant volume operation or dilution operation is as described above. Can be corrected by measuring the peak area ratio or the peak height ratio.

  Specifically, it is as follows. A standard curve standard solution containing a constant concentration of the internal standard substance and varying the lithium concentration at at least 3 to 4 levels is prepared, and the height or area of the lithium peak and the internal standard element peak is measured. Calibration curve with the ratio Ms / Mi of the lithium amount Ms and the internal standard element amount Mi on the horizontal axis and the ratio Ps / Pi of the peak area or height Ps of lithium to the peak area or height Pi of the internal standard element on the vertical axis Create Next, a measurement solution is prepared by adding substantially the same amount of the same internal standard element as described above to the sample solution, and the ratio Ps / Pi between the area and height of lithium and the internal standard element peak is obtained from the measured value. The amount of lithium in the sample solution is obtained from the relational expression of Ms / Mi and Ps / Pi obtained from the line. Thus, since the ratio of lithium to the internal standard element is measured at the time of measurement, for example, the influence of the volume error generated in the pre-analysis process is so small that it can be ignored.

  If the sample solution contains a high-concentration organic solvent or oxidizing agent, it may cause significant deterioration of the separation column packed with the ion exchange resin. It is desirable. In addition, if particles such as undecomposed matter are present, the flow path of the ion chromatograph apparatus is blocked, and therefore, it is preferable to remove in advance with a membrane filter having a pore diameter of about 0.45 μm, for example.

  However, in an ion chromatograph apparatus, stable continuous measurement may be difficult due to the influence of a specific coexisting element. In other words, an acidic mobile phase is generally used in the ion chromatographic method in the cation exchange mode, but in the suppressor, the anion in the mobile phase is exchanged with hydroxide ions, so that the pH rises. Becomes neutral to weakly alkaline. Therefore, if the sample solution contains ionic species that form a precipitate such as hydroxide under neutral or weak alkalinity, the deposit accumulates in the suppressor, and the flow path is blocked and stable measurement is performed. It becomes difficult. Examples of ionic species that cause such a phenomenon include aluminum, nickel, cobalt, manganese, iron, and the like, and these elements are often contained in a sample solution prepared from a positive electrode material for a lithium secondary battery.

  In order to eliminate this phenomenon, in the ion chromatograph apparatus used in the present invention, a flow path switching valve is provided between the separation column and the suppressor, and the flow path switching valve is set according to the retention time of the component elements in the sample solution. By switching, fractions that do not contain lithium and internal standard elements (including ionic species that cause a precipitation reaction under neutral or alkaline conditions) are discharged out of the system before the suppressor, and the measurement target elements lithium and internal standard elements are removed. Those having the function of selectively introducing only the contained fraction into the suppressor are preferred. By using this apparatus, it is possible to suppress hydrolysis and precipitation of the coexisting elements in the suppressor in an alkaline atmosphere, so that more stable measurement is possible.

  For example, as shown in FIG. 1, a specific ion chromatograph apparatus includes a mobile phase tank 1, a mobile phase liquid feed pump 2, an injector 3 for introducing a sample solution into the mobile phase, A pre-column 4 for removing foreign substances, a separation column 5 filled with a cation exchange resin, an anion-exchange suppressor 6, an electric conductivity detector 7 as a detector for the element to be measured, a sample solution after measurement, A drain 8 for receiving the mobile phase is provided. In FIG. 1, 9 is a regenerating liquid tank of the suppressor 6, 10 is a regenerating liquid feed pump, and 11 is a regenerating liquid drain.

  Furthermore, in this ion chromatograph apparatus, a flow path switching valve 12 for switching the mobile phase flow path is provided between the separation column 5 and the suppressor 6. By switching this flow path switching valve 12, the fraction not containing the element to be measured is discharged to the drain 13 outside the system, and at the same time, depending on the mobile phase fed from the mobile phase tank 1 by the mobile phase feed auxiliary pump 14, A fraction containing the element to be measured is introduced into the suppressor 6.

  In the apparatus of FIG. 1, a four-way valve is used as the flow path switching valve 12. However, the form of the flow path switching valve is not particularly limited, and for example, a six-way valve can be used. In addition, a flow path switching valve composed of a three-way valve may be provided between the mobile phase liquid supply auxiliary pump 14 and the flow path switching valve 12 for the purpose of venting the flow path.

  The mobile phase is generally a strong acid or an organic acid, but is not particularly limited. As typical mobile phases, there are nitric acid, hydrochloric acid, sulfuric acid and the like as inorganic acids, and methanesulfonic acid and oxalic acid as organic acids, which can be used alone or in combination. In order to improve the degree of separation of the components to be measured, a complexing agent such as ethylenediaminetetraacetic acid (EDTA) can be added.

  When the mobile phase or the sample solution contains a corrosive compound, it is preferable to use a highly corrosion-resistant material for the flow path of the ion chromatograph apparatus. For example, polyether ethyl ketone (PEEK) or polytetrafluoroethylene (PTFE) is generally used as a highly corrosion-resistant material used for the flow path.

  As the cation exchange resin used for the separation column, either a strong acid ion exchange resin or a weak acid ion exchange resin can be used. Depending on the usage history, there is a possibility that the degree of separation of the component to be measured may decrease, so it is desirable to periodically inspect the degree of separation with a standard solution or the like. A precolumn is generally used for protecting the separation column, and the use of the precolumn is not limited in the present invention. In addition, the precolumn and the separation column are usually installed in a thermostat in order to maintain a predetermined temperature.

  As the suppressor, in addition to the membrane dialysis suppressor shown in the above specific example, a column removal type using a column filled with an ion exchange resin, a suspension resin adsorption type using ion exchange resin particles suspended in a mobile phase, etc. Therefore, any method that can achieve background reduction can be arbitrarily selected and used.

  For both membrane dialysis type and column removal type suppressors, as a regeneration method, in addition to a chemical regeneration method in which tetramethylammonium hydroxide, sodium hydroxide, or the like is passed through and regenerated, it is discharged from the ion chromatograph. There is no problem in adopting any of the electric regeneration methods using the alkali obtained by electrolyzing the generated mobile phase or water. However, since the chemical regeneration method is generally excellent in the stability of the detected electrical conductivity, it is preferable to apply the chemical regeneration method when higher accuracy analysis is required.

  In addition, there are no particular restrictions on the conditions related to ion chromatography such as mobile phase velocity and sample injection amount. Select measurement conditions that sufficiently separate lithium ions from internal standard element ions and other cations that coexist. That's fine.

  The present invention can be applied to other than the lithium measurement of a positive electrode material for a lithium secondary battery. By performing the same operation as described above using potassium, ammonium, or an amine compound as an internal standard element, the target element can be obtained. It is possible to greatly improve the analysis accuracy of lithium. However, if the contaminant component coexists at a high concentration, it may be difficult to measure the measurement target component. Therefore, it is preferable to separate the contaminant component by using dilution with pure water, a pretreatment column, or the like.

[Example 1]
1.0 g of a powder sample of a positive electrode material for a lithium secondary battery containing lithium, nickel, and cobalt was weighed and placed in a clean 300 ml glass beaker. Next, potassium carbonate was dissolved in pure water, the potassium concentration of the internal standard element was adjusted to 40 g / kg, and 10 g of the obtained potassium carbonate aqueous solution was weighed and put into the beaker. Further, 10 ml of nitric acid and 2 ml of hydrogen peroxide were gradually added to the beaker and heated on a hot plate at about 300 ° C. to decompose the powder sample. After allowing to cool, 2 ml of hydrogen peroxide was further added, and the mixture was heated and decomposed in the same manner as described above. The powder sample was completely decomposed by repeating this operation at least twice.

  The resulting solution was cooled to room temperature and then transferred to a 100-ml volumetric flask and pure water was added to adjust the solution volume to 100 ml. Next, 1 ml of the solution was taken using a 1 ml push button type liquid microvolume meter, transferred to a 200-ml volumetric flask, and pure water was added to make the solution volume constant to 200 ml. A sample solution was obtained.

  About the obtained sample solution, lithium was measured using the ion chromatograph apparatus shown in FIG. The ion chromatograph apparatus actually used was ICS-1000 manufactured by Dionex, and the apparatus shown in FIG. The precolumn is IonPacCG16, the separation column is IonPacCS16, and the suppressor is CMMSIII-4 mm, both of which are manufactured by Dionex. The suppressor is a membrane dialysis type anion exchange suppressor.

  That is, in the ion chromatograph apparatus of FIG. 2, a 6-way valve was used as the flow path switching valve 15, and a PEEK tube was connected between the connection points 4-5 to form a bypass line. Further, between the mobile phase liquid supply auxiliary pump 14 and the flow path switching valve 15, a flow path switching valve 16 comprising a three-way valve was installed for the purpose of venting the flow path. During the air venting operation, the flow path is switched to the flow path indicated by the dotted line at the connection point 2-3, and at the time of measurement, the flow path illustrated by the solid line at the connection point 2-1 was used. Except for the above, the apparatus is the same as the apparatus of FIG.

  A 30 mM methanesulfonic acid solution was used as the mobile phase, and the mobile phase flow rate was 1.0 ml / min. An 80 mM tetramethylammonium hydroxide solution was used as the regeneration solution for the suppressor, and the control pressure of the regeneration solution was 5 psi. The peak area ratio (lithium peak area / potassium peak area) of lithium as the internal standard element and lithium was measured under the conditions of a sample solution injection amount of 50 μl, a column temperature of 35 ° C., and a measurement time of 15 min. The lithium concentration was calculated from a calibration curve created by the above-mentioned area ratio of lithium peak area / potassium peak area. The lithium concentration used for the calibration curve was about 0 mg / kg, 2 mg / kg, 4 mg / kg, and 6 mg / kg, and the internal standard element potassium ion was 20 mg / kg.

  Under the measurement conditions described above, the retention times of lithium and potassium were 5.2 min for lithium and 13.3 min for potassium. On the other hand, the retention times of nickel and cobalt present in the sample solution were 19.4 min for nickel and 18.6 min for cobalt. The pH of the mobile phase before the suppressor is about 1.5 and there is no possibility of precipitation of nickel and cobalt, but the pH of the mobile phase flowing out of the suppressor is as high as about 8.5 and is the pH at which nickel and cobalt are precipitated. Therefore, the channel was switched for the purpose of preventing the blockage of the channel in the suppressor.

  That is, during the holding time 0 to 16 min until the fraction containing lithium is introduced into the suppressor 6, the flow path switching valve 15 is indicated by the solid lines of the connection point 1-6, the connection point 2-3, and the connection point 4-5. Measurements were made as indicated channels. Thereafter, during the holding time of 16 to 22 minutes, the flow path switching valve 15 is switched to form the flow path illustrated by the dotted lines of the connection point 1-2, the connection point 3-4, and the connection point 5-6. At the same time that the contained fraction was discharged to the drain 13, the mobile phase fed through the flow path at the connection point 2-1 of the flow path switching valve 16 was introduced into the suppressor 6. After the holding time of 22 minutes, the flow path switching valve 15 was returned to the flow path shown by the solid line again.

  An example of the chromatogram obtained in Example 1 is shown in FIG. Peak 1 is a lithium peak, and peak 2 is a potassium peak. Under the above measurement conditions, the same sample was repeatedly decomposed and measured 80 times over 2 months, and the calibration was made in advance based on the peak area ratio between the peak 1 of lithium and the peak 2 of potassium, which is the internal standard element. As a result of analysis using a wire, the analysis accuracy of the obtained lithium was 0.16% as extremely high as RSD.

[Example 2]
As an internal standard element, lithium was measured in the same manner as in Example 1 except that ammonium, methylamine, dimethylamine, ethylamine, dimethylamine, ethanolamine, and diethanolamine were used instead of potassium in Example 1 above. went. A 40 mM methanesulfonic acid solution was used as the mobile phase. For this reason, the retention time of each element was slightly earlier than that of Example 1, and the flow path switching valves 15 and 16 were switched accordingly.

  In addition, ammonium chloride, methylamine hydrochloride, dimethylamine hydrochloride, ethylamine hydrochloride, dimethylamine hydrochloride, ethanolamine, and diethanolamine were used as the compounds of the respective internal standard elements. Each internal standard element concentration was 20 g / kg solution, each 10 g was weighed and added and mixed in a beaker containing a sample solution.

  As a result, the analytical accuracy of the obtained lithium was RSD for each internal standard element, 0.15% for ammonium, 0.16% for methylamine, 0.15% for dimethylamine, 0.14% for ethylamine, It was 0.18% for diethylamine, 0.17% for ethanolamine, and 0.15% for diethanolamine.

[Comparative Example 1]
Lithium ions were measured in the same manner as in Example 1 without applying the internal standard correction method. That is, the same sample solution as in Example 1 except that the internal standard element was not contained was repeatedly decomposed and measured 80 times over a period of 2 months. The analytical accuracy of lithium was very poor at 0.55% by RSD.

  When a solution that caused a significant decrease in accuracy was measured by performing only the dilution operation again, it was restored to a normal measurement value. From this result, it was estimated that the cause of the decrease in accuracy was the volume error caused by the dilution operation. However, it is difficult to always perform high-precision analysis when the internal standard correction is not performed. I understand.

It is a channel diagram of the ion chromatograph apparatus used for implementation of the method of the present invention. It is a flow-path figure of another ion chromatograph apparatus used for implementation of the method of this invention. It is an ion chromatogram obtained in Example 1 of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Mobile phase tank 2 Mobile phase liquid feed pump 3 Injector 4 Precolumn 5 Separation column 6 Suppressor 7 Electrical conductivity detector 9 Regeneration liquid tank 10 Regeneration liquid liquid feed pump 12, 15, 16 Flow path switching valve 14 Mobile phase liquid supply assistance pump

Claims (7)

  In an analysis method for detecting a lithium concentration in a sample solution sent together with a mobile phase by a mobile phase liquid feeding pump using an ion chromatograph apparatus equipped with a separation column packed with a cation exchange resin and an anion exchange suppressor, A method for highly accurate analysis of lithium, comprising measuring at least one of potassium, ammonium and an amine compound as an internal standard element.   The method for highly accurate analysis of lithium according to claim 1, wherein the amine compound is any one of methylamine, dimethylamine, ethylamine, diethylamine, ethanolamine and diethanolamine.   Fraction not containing lithium and internal standard elements by providing a flow path switching valve between the separation column and suppressor of the ion chromatograph device and switching the flow path switching valve according to the retention time of the component elements in the sample solution The method for highly accurate analysis of lithium according to claim 1, wherein only the fraction containing the element to be measured is selectively introduced into the suppressor.   The solid or liquid fixed amount of the compound containing the internal standard element is added before the decomposition of the sample, or is added immediately after the sample is decomposed and cooled to room temperature. The method for highly accurate analysis of lithium according to any one of the above.   The fixed amount of the compound containing the internal standard element and the fixed amount of the standard sample liquid used for the calibration curve are grasped by weight and added. High-accuracy analysis method for lithium.   6. The method for highly accurate analysis of lithium according to claim 1, wherein the sample solution is a solution obtained by decomposing a positive electrode material for a lithium secondary battery. The high-precision analysis method for lithium according to claim 1, wherein the sample solution contains at least one of aluminum, nickel, cobalt, manganese, and iron as an element other than lithium .

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