JP2009216399A - High precision analytical method for cations by ion chromatograph method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high precision analytical method for cations, capable of performing continuous measurement without causing the clogging of the flow channel of a suppressor when the constituent element of the positive polar material of a lithium secondary cell is analyzed by an ion chromatograph method. <P>SOLUTION: A fraction containing no measuring target element is discharged to a drain 13 by changing over the flow channel switching valve 12, which is provided between a separation column 5 and the suppressor 6 corresponding to the holding time of the component element in a sample solution, using an ion chromatograph apparatus equipped with the separation column 5 filled with a cation exchange resin and the anion exchange suppressor 6 and only a fraction containing the measuring target element is selectively introduced into the suppressor 6 by the mobile phase sent by a mobile phase sending auxiliary pump 14. <P>COPYRIGHT: (C)2009,JPO&amp;INPIT

Description

  The present invention relates to a high-accuracy analysis method of cations by ion chromatography, and particularly relates to a method for analyzing lithium ions 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. However, it is difficult to apply a highly accurate titration method or gravimetric method, particularly in the measurement of lithium. 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.

  From such a current state, regarding the measurement of the lithium secondary battery positive electrode material, Japanese Patent Application Laid-Open No. 11-287793 and Japanese Patent Application Laid-Open No. 2002-174446 describe a method of applying an ion chromatography method. According to the ion chromatograph method, the lithium repeat measurement accuracy can be expected to be about 0.2% by RSD. However, according to a thorough investigation by the inventors, particularly in an ion chromatograph equipped with an anion exchange suppressor, stable continuous measurement is difficult due to the influence of specific coexisting elements such as nickel, cobalt and manganese. There was a case.

JP-A-11-287793 JP 2002-174446 A

  As described above, when analyzing the constituent elements of the positive electrode material of a lithium secondary battery by the conventional ion chromatograph method, if a specific element such as cobalt coexists, the liquid feeding pressure or the liquid feeding flow rate is unstable. Thus, the measurement accuracy is likely to be significantly reduced. When the flow path in the suppressor that caused the accuracy drop is washed with oxalic acid, coexisting elements such as nickel and cobalt are detected in the washing solution, and the measurement using the suppressor immediately after washing significantly improves the measurement accuracy. Is recognized. From these facts, it is presumed that the measurement accuracy has decreased for the following reasons.

  That is, in an ion chromatograph apparatus equipped with an electrical conductivity detector, a suppressor is used to reduce an increase in background electrical conductivity caused by the ion species of the mobile phase. For example, strong acids and organic acids are generally used for the mobile phase, but suppressors use ionic species such as nitrate ions and methanesulfonate ions from the mobile phase through the anion exchange membrane and hydroxide ions. It has a function of removing from the mobile phase by exchange.

  This action makes it possible to obtain a high signal-to-noise ratio by significantly reducing the background electrical conductivity. However, since hydroxide ions are introduced into the mobile phase by ion exchange, the pH of the flow path in the suppressor is Gradually it becomes alkaline. When the pH of the flow path in the suppressor becomes alkaline, nickel or cobalt coexisting in the sample solution generates hydroxide precipitates due to the hydrolysis reaction, causing the flow path to be clogged. It is considered that the flow rate becomes unstable and the measurement accuracy is significantly reduced.

  In view of such a current situation, the present invention provides a blockage of the flow path even when the pH of the flow path in the suppressor becomes alkaline, particularly when the constituent elements of the positive electrode material of the lithium secondary battery are analyzed by ion chromatography. An object of the present invention is to provide a highly accurate analysis method of cations that does not occur and can be continuously measured.

  In order to achieve the above object, the high-accuracy analysis method for cations provided by the present invention uses a separation column packed with a cation exchange resin and an ion chromatograph apparatus equipped with an anion exchange suppressor, and a mobile phase transfer. In the analysis method that detects the cation concentration in the sample solution sent together with the mobile phase with the liquid pump, a flow path switching valve is provided between the separation column and the suppressor, and the flow path is switched according to the retention time of the component elements in the sample solution. By switching the valve, the fraction not containing the element to be measured is discharged out of the system, and only the fraction containing the element to be measured is selectively introduced into the suppressor.

  In the cation high-accuracy analysis method according to the present invention, the fraction not containing the element to be measured is discharged out of the system by switching the flow path switching valve, and at the same time, passed through the separation column by the mobile phase liquid feeding auxiliary pump. The fraction containing the element to be measured is introduced into the suppressor by the mobile phase that is sent without any problem. In the method for analyzing cations with high accuracy according to the present invention, a further effect can be expected when the cations are detected by an electric conductivity detector.

  In the cation high-accuracy analysis method according to the present invention, the sample solution is preferably a solution obtained by decomposing a positive electrode material for a lithium secondary battery. In that case, the element to be measured is preferably at least one of lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, strontium, barium, and ammonium. Moreover, it is particularly effective when the sample solution contains at least one of aluminum, nickel, cobalt, manganese, and iron as an element other than the element to be measured.

  According to the present invention, when cations in a sample solution are measured by ion chromatography, even if the pH of the flow path in the suppressor becomes alkaline, the flow path is not clogged, and it is extremely highly accurate and continuous. Long-term stable measurement is possible. Therefore, the high-accuracy analysis method for cations of the present invention is particularly useful for the measurement of lithium in the positive electrode material for lithium secondary batteries, which requires extremely high-precision measurement.

  The high-accuracy analysis method of cations according to the present invention will be specifically described with reference to the drawings. FIG. 1 is a flow chart of a typical ion chromatograph apparatus used in the method of the present invention. This apparatus, like a general ion chromatograph apparatus, has a mobile phase tank 1 for storing a mobile phase, a mobile phase liquid feed pump 2 for pumping the mobile phase, an injector 3 for introducing a sample solution into the mobile phase, Pre-column 4 for removing foreign substances in the mobile phase, separation column 5 filled with a cation exchange resin, anion-exchange suppressor 6 for reducing background conductivity, detector for the element to be measured The electrical conductivity detector 7 is provided.

  Furthermore, in the ion chromatograph apparatus of the present invention, the 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.

  More specifically, in a normal state, the mobile phase stored in the mobile phase tank 1 is fed at a constant pressure and flow rate by the liquid feed pump 2 and guided to the injector 3. After the sample solution is introduced into the mobile phase by the injector 3, the mobile phase containing the sample solution is removed by the pre-column 4 and then enters the separation column 5 filled with the cation exchange resin. In this separation column 5, the component elements that interact with the ion exchange resin are retained, but each component element in the sample solution is separated for each component element because the retention time differs based on the difference in the selection coefficient. .

  The mobile phase containing the sample solution that has passed through the separation column 5 passes through the flow path switching valve 12 when the flow path switching valve 12 is in a position that forms a flow path at the connection point 1-4 shown by a solid line in FIG. Then, after being introduced to the anion-exchange suppressor 6, the anion contained in the mobile phase is exchanged and removed with hydroxide ions, and then introduced into the electrical conductivity detector 7 to measure the electrical conductivity of the element to be measured. And discharged to the drain 8. After performing this operation on the standard sample solution, creating a calibration curve with the horizontal axis as the concentration of the standard solution and the vertical axis as the electric conductivity, the concentration of the element to be measured can be calculated from the electric conductivity measured for the sample solution. it can. Incidentally, the regenerating liquid is continuously fed from the regenerating liquid tank 9 to the suppressor 6 by the regenerating liquid feed pump 10, and after the repressor 6 is regenerated, it is discharged to the drain 11.

  However, in the ion chromatographic method in the cation exchange mode, since an acidic mobile phase is generally used, the pH of the mobile phase in the front stage of the suppressor is maintained acidic. On the other hand, in the suppressor, the anion in the mobile phase is exchanged for hydroxide ions by the anion exchange reaction, so that the pH of the mobile phase rises. From the suppressor to the downstream mobile phase, the pH changes from neutral to weakly alkaline. Maintained. At this time, if the sample solution contains ionic species that form a precipitate such as hydroxide under neutrality or weak alkalinity by hydrolysis reaction, the deposit accumulates in the flow path in the suppressor. Since the flow path is closed, the liquid supply pressure and the liquid supply amount fluctuate remarkably, and stable measurement becomes difficult. Examples of ionic species that cause such a phenomenon include aluminum, nickel, cobalt, manganese, and iron. When lithium of a positive electrode material for lithium secondary batteries containing these elements is measured by an ion chromatography method, Stable measurement becomes difficult due to the blockage of the flow path.

  In order to eliminate this phenomenon, in the present invention, as described above and as shown in FIG. 1, a flow path switching valve 12 is provided between the separation column 5 and the suppressor 6 of the ion chromatograph apparatus. Then, before the fraction containing the element to be measured (including ionic species that cause a precipitation reaction under neutrality or alkalinity) is introduced from the separation column 5 to the suppressor 6, the connection point is changed by switching the flow path switching valve 12. The flow path illustrated by the solid line 1-4 is switched to the flow path illustrated by the dotted line at the connection point 1-2, and the fraction not including the measurement target element is discharged to the drain 13. At the same time, the flow path illustrated by the dotted line of the connection point 3-4 is formed, and the flow path is switched by feeding the mobile phase from the mobile phase tank 1 by driving the mobile phase liquid supply auxiliary pump 14 to this flow path. The same mobile phase is continuously sent to the suppressor 6 before and after the valve 12 is switched.

  After the fraction not containing the element to be measured is discharged to the drain 13 and the discharge of the ionic species causing the precipitation reaction is finished, the flow path switching valve 12 is operated to connect the connection point 1- 4, the mobile phase that has passed through the separation column 5 is introduced into the suppressor 6. Through the above-described channel switching operation, it becomes possible to block the channel of the suppressor 6 and discharge ion species unnecessary for measurement out of the system. It should be noted that there is no problem if this flow path switching operation is performed either before or after the fraction containing the element to be measured is introduced into the suppressor 6.

  With the switching operation of the flow path switching valve 12, the mobile phase is disturbed for a very short time. As a result, a slight peak appears in the chromatogram, but the electrical conductivity recovers to a stable background position in a very short time. Further, since the holding time can be easily discriminated between the element to be measured and the other components, no practical adverse effect is recognized.

  In the description of the above specific example, a four-way valve is used as the flow path switching valve 12 as shown in FIG. 1, but there is no particular limitation on the form of the flow path switching valve. For example, a six-way valve is used. It is also possible to use it. For example, when using a 6-way valve, as shown in FIG. 2, the 6-way valve that is the flow path switching valve 15 may be provided with a bypass line illustrated by a dotted line at connection point 4-5. In addition, there is no fundamental difference between FIG. 1 and FIG. 2 other than the flow path switching valve, and the same parts are denoted by the same reference numerals.

  As shown in FIG. 2, a flow path switching valve 16 composed of a three-way valve can be provided between the mobile phase liquid supply auxiliary pump 14 and the flow path switching valve 15 for the purpose of venting the flow path. When the flow path switching valve 16 is switched, the mobile phase is discharged to the drain 17 by performing switching to the flow path illustrated by the dotted line of the connection point 2-3 to perform air venting. The mobile phase is returned to the flow path switching valve 15 by returning to the flow path shown by the solid line of the connection point 2-1.

  Next, other conditions in the method of the present invention will be described. The sample solution is not particularly limited, and may be a solution obtained by decomposing a solid sample using an acid or alkali in addition to a solution such as river water or drainage. 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.

  In the analysis of the positive electrode material for a lithium secondary battery, a solution obtained by decomposing and appropriately diluting using nitric acid and hydrogen peroxide water is used as a sample solution. In addition, when a high concentration organic solvent or oxidizing agent is contained, it may cause significant deterioration of the ion exchange resin. Therefore, it is desirable to reduce the oxidizing power in advance by separation or reduction treatment. 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 it with a membrane filter having a pore diameter of about 0.45 μm.

  The mobile phase is generally a strong acid or an organic acid, but is not particularly limited. Typical examples include nitric acid, hydrochloric acid, and sulfuric acid 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. However, when a corrosive compound is contained in the mobile phase and the sample solution, 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 polytetra Fluoroethylene (PTFE) or the like is generally used.

  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 necessary 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. The pre-column 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 described 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. However, any method that achieves a reduction in background may be selected and used.

  In addition, both membrane dialysis and column removal suppressors are discharged from the ion chromatograph as well as the chemical regeneration method in which regeneration is performed by passing tetramethylammonium hydroxide or sodium hydroxide through the regeneration method. There is no problem even if any of the electric regeneration methods using the alkali obtained by electrolyzing the mobile phase or water is employed. 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.

  Moreover, there are no particular restrictions on the conditions relating to the ion chromatograph, such as the mobile phase velocity and the sample injection amount, and it is only necessary to select measurement conditions that sufficiently separate other cations coexisting with the measurement target component.

[Example 1]
Weigh 1.0 g of the positive electrode material powder for lithium secondary battery containing lithium, nickel and cobalt, put it in a clean 300 ml glass beaker, add 10 ml of nitric acid and 2 ml of hydrogen peroxide, and use a hot plate at about 300 ° C. Decomposed by heating. 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 sample was completely decomposed by repeating this operation at least twice.

  After cooling the obtained solution to room temperature, the solution was transferred to a 100-ml volumetric flask and pure water was added to adjust the solution volume to 100 ml. Subsequently, 10 ml of the solution was accurately collected using a 10 ml volumetric pipette, transferred to a 200 ml volumetric flask, and pure water was added to adjust the volume of the solution to 200 ml. Furthermore, using a 10 ml total volume pipette, accurately dispense 10 ml of this solution, transfer it to a 100 ml volumetric flask, add pure water to adjust the volume to 100 ml in the same manner as above, and use this as the sample solution. did.

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

  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. The suppressor 6 is a membrane dialysis type anion exchange suppressor, and a chemical regeneration method using tetramethylammonium hydroxide was adopted for regeneration.

  A 40 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. 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 by a calibration curve method. The calibration curve was set to 4 points of 0 mg / l, 2 mg / l, 4 mg / l and 6 mg / l.

  Under the measurement conditions described above, the lithium retention time was 4.7 min. On the other hand, the retention times of nickel and cobalt coexisting in the sample solution were 12.0 min for nickel and 11.5 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 of 0 to 10 minutes 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 10 to 15 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 15 minutes, the flow path switching valve 15 was returned to the flow path shown by the solid line again.

  The chromatogram obtained in Example 1 is shown in FIG. Peak 1 is lithium, and peak 2 is a peak due to background fluctuation caused by the flow path change by the flow path switching valve. However, the peak 2 that appeared due to the change of the flow path was slight, and it returned to a stable background in an extremely short time, and no tailing was observed, and no problem was found in practice.

  Further, the same sample solution was continuously measured 200 times under the above measurement conditions. Meanwhile, as a result of monitoring the pressure of the flow path of the ion chromatograph apparatus including the suppressor and the like, the pressure was almost constant at 1350 ± 20 psi. Moreover, the measurement accuracy of the obtained lithium was 0.08% in RSD and extremely high accuracy.

[Example 2]
In the same manner as in Example 1, measurement was also performed on sodium, potassium, rubidium, cesium, magnesium, calcium, strontium, barium, and ammonium. However, 30 mM methanesulfonic acid was used for the mobile phase for potassium and magnesium, and 50 mM methanesulfonic acid was used for the mobile phase for rubidium and cesium.

  As a result, the measurement accuracy obtained was RSD, 0.10% for sodium, 0.10% for potassium, 0.09% for rubidium, 0.18% for cesium, 0.08% for magnesium and 0 for calcium. 0.09%, strontium 0.12%, barium 0.11%, and ammonium 0.10%.

[Comparative Example 1]
In the ion chromatograph apparatus shown in FIG. 2, a conventional apparatus not equipped with the flow path switching valves 15 and 16 and the mobile phase liquid feeding auxiliary pump 14 is used, and the separation column 5 and the membrane dialysis type anion exchange suppressor 6 are used. The lithium was measured in the same manner as in Example 1 except that was directly connected.

  When the same sample solution as in Example 1 was continuously measured, the flow path pressure increased from 1350 psi to 1400 psi after approximately 15 measurements. When the measurement was further continued, the pressure in the flow path increased to 1600 psi after about 20 measurements, and liquid leakage occurred from the suppressor body. After the pressure increased to 1600 psi, the suppressor was removed and the flow path was washed with 0.2 M oxalic acid for about 60 minutes and mounted again, and measurement was attempted, but the same pressure increase occurred and continuous measurement was difficult. there were.

  A measured value in which a pressure increase was recognized was rejected as an abnormal value, and a section where the pressure in the flow path was 1350 ± 50 psi was adopted as a normal value. As a result of obtaining the measurement accuracy from 50 measurement values obtained by repeated tests, the RSD was 0.28%. From the above results, it has been found that it is difficult to perform continuous and stable measurement, and the measurement accuracy is insufficient.

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 (6)

  In an analytical method for detecting the concentration of cations in a sample solution sent together with a mobile phase using a mobile phase pump by using an ion chromatograph equipped with a separation column packed with a cation exchange resin and an anion exchange suppressor In addition, by providing a flow path switching valve between the separation column and the suppressor and switching the flow path switching valve according to the retention time of the component elements in the sample solution, the fraction not containing the measurement target element is discharged out of the system, A highly accurate analysis method for cations, wherein only a fraction containing an element to be measured is selectively introduced into a suppressor.   By switching the flow path switching valve, the fraction not containing the element to be measured is discharged out of the system to the drain, and at the same time, the element to be measured is transferred by the mobile phase fed by the mobile phase feed auxiliary pump without passing through the separation column. The high-accuracy analysis method for cations according to claim 1, wherein a fraction containing them is introduced into a suppressor.   The method for high-accuracy analysis of cations according to claim 1 or 2, wherein detection of the cations is performed by an electric conductivity detector.   4. The method for highly accurate analysis of cations according to claim 1, wherein the sample solution is a solution obtained by decomposing a positive electrode material for a lithium secondary battery.   5. The cation according to claim 1, wherein the measurement target element is at least one of lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, strontium, barium, and ammonium. High-precision analysis method.   The high-accuracy analysis of a cation according to any one of claims 1 to 5, wherein the sample solution contains at least one of aluminum, nickel, cobalt, manganese, and iron as an element other than the element to be measured. Method.

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