JP6265070B2 - Metal element concentration analysis method using inductively coupled plasma emission spectrometer - Google Patents

Description

  The present invention relates to a metal element concentration analysis method using an inductively coupled plasma emission spectrometer, and in particular, it is possible to analyze the concentration of a metal element such as lithium contained in a positive electrode material for a lithium secondary battery with a desired accuracy. The present invention relates to a concentration analysis method for metal elements using a novel inductively coupled plasma emission spectrometer.

  Lithium secondary batteries are mounted on portable electronic devices such as notebook computers, video cameras, and mobile phones because of their excellent light weight and charge / discharge cycle characteristics. Recently, lithium secondary batteries have been attracting attention in the automobile field against the background of global environmental problems and resource depletion problems, and their installation in fuel cell vehicles and hybrid vehicles has been studied earnestly.

  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 technology for managing the composition of elements constituting these compounds is capacity density, charge / discharge This is extremely important because it affects characteristics such as cycle life, safety, and economy.

  In addition to the concentration of the main component of the positive electrode material, the composition management targets include the molar concentration ratio of the total amount of other metal component elements to lithium (hereinafter simply referred to as lithium / metal ratio), sodium and chlorine. However, since the lithium / metal ratio is an extremely important management item, it is required that analysis can be performed with more stable accuracy.

  Conventionally, to measure the concentration of a metal element such as lithium constituting the positive electrode material of a lithium secondary battery, the sample is decomposed with acid, alkali, etc., and the element to be measured in the obtained sample solution is inductively coupled plasma emission A method of measuring by spectroscopic analysis (ICP-AES), flame atomic absorption method or flame light method is applied. These analysis methods are generally used as metal element detection means, but are easily affected by coexisting elements and plasma or flame fluctuations, and sufficient analysis accuracy may not be obtained.

  Therefore, depending on the element, a titration method or a gravimetric method that can be expected to have higher analytical accuracy may be applied. However, in nickel, for example, in order to prevent the influence of coexisting elements, it is necessary to perform precipitation separation using dimethylglyoxime, and such a pretreatment operation is a factor that reduces analytical accuracy. In addition, with alkali metals such as lithium, it is difficult to apply a titration method or a gravimetric method due to their properties.

  As a result, the repeated measurement accuracy of each element in the above analysis method is 1% or more in terms of relative standard deviation (hereinafter abbreviated as RSD), and in particular, the accuracy of the lithium / metal ratio is further reduced due to the error additivity. Had the problem. Such a large analysis error of constituent elements of the positive electrode material is not accepted in the current battery development or battery manufacturing field, and higher analysis accuracy is required.

  Under such circumstances, Patent Document 1 discloses that an internal standard element is prepared on a weight basis for measurement of lithium, nickel, cobalt, aluminum, or manganese in the positive electrode material of the lithium secondary battery, and inductively coupled plasma emission spectroscopic analysis is performed. Describes a method of applying an inductively coupled plasma emission spectroscopic analysis method in which the excitation temperature of the plasma used in the method is 4900K or higher. According to this inductively coupled plasma emission spectroscopy, it is described that the measurement accuracy per element can be expected to be 0.2% or less in RSD.

JP 2010-078381 A

  Although the above-mentioned technique of Patent Document 1 can improve the analysis accuracy of inductively coupled plasma optical emission spectrometry to some extent, in recent years, there has been a trend toward demand for high-performance, high-quality lithium secondary batteries. Accordingly, a method capable of stably analyzing with higher accuracy is desired. Under such circumstances, the present inventor has intensively studied a concentration analysis method for metal elements using an inductively coupled plasma emission spectrometer, and the analysis accuracy of the inductively coupled plasma emission spectrometry is inductively coupled plasma. The present inventors have found that the temperature control of the refrigerant that cools the detection unit of the emission spectroscopic analyzer is greatly affected.

  As described in Japanese Patent Application Laid-Open No. 2007-3320, the detection unit includes a cooling mechanism such as a Peltier element that cools the detector in addition to the detector that detects the emission spectrum of the element to be analyzed. Is provided as necessary. Conventionally, the temperature control of the refrigerant for cooling the detection unit has not been regarded as important, but it has been found that it is effective to further reduce the temperature control range of the refrigerant in order to further improve the analysis accuracy. .

  On the other hand, in order to reduce the temperature control range of the refrigerant, it is necessary to improve the performance of the chiller that is the supply source, which increases the equipment cost. In some cases, the size of the chiller is increased or a type with high noise is adopted. There was a need. The present invention has been made in view of such a current situation, and measures the concentration of each metal element and the lithium / metal ratio in a sample such as a lithium secondary battery positive electrode material using an inductively coupled plasma emission spectrometer. At this time, it is an object to provide an analysis method capable of measuring with a desired accuracy stably and with a low environmental load without costing more than necessary.

In order to achieve the above object, the metal element concentration analysis method provided by the present invention applies an internal standard correction method using yttrium or copper as an internal standard element, and uses inductively coupled plasma emission using an excitation temperature of plasma of 4900K or higher. A method for analyzing the concentration of a metal element in a sample solution using a spectroscopic analyzer, wherein the measurement accuracy of the lithium / metal ratio and the temperature control of the refrigerant for the detector of the analyzer are measured by using the analyzer A relational expression with the width is obtained from a plurality of temperature control widths , and based on this relational expression, the temperature control width of the refrigerant necessary to satisfy the required analysis accuracy of the lithium / metal ratio is determined. The temperature of the refrigerant for the detection unit is controlled.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to perform analysis with a desired precision stably and with low environmental load, without costing more than necessary at the time of the concentration analysis of the metallic element using an inductively coupled plasma emission spectroscopic analyzer. In particular, it is extremely useful when measuring a lithium / metal ratio, which is extremely important as a management item for a lithium secondary battery positive electrode material.

1 is a configuration diagram of a specific example of an inductively coupled plasma optical emission spectrometer that is preferably used in a high-precision analysis method for metal elements according to the present invention. FIG. It is process drawing which shows the measurement order of the sample in the inductively coupled plasma emission spectroscopic analyzer of an Example. It is a graph which shows the relationship between the positive / negative temperature fluctuation width with respect to the preset temperature of the refrigerant | coolant in Example 1, and a measurement precision. It is a graph which shows the relationship between the positive / negative temperature fluctuation width with respect to the preset temperature of the refrigerant | coolant in Example 2, and a measurement precision.

  First, an embodiment of an inductively coupled plasma emission spectroscopic apparatus suitably used in a method for analyzing a metal element by inductively coupled plasma emission spectrometry according to the present invention will be described. As shown in FIG. 1, the inductively coupled plasma optical emission spectrometer of this embodiment includes a plasma torch unit 1 that forms plasma, a spectroscope 2 that splits light of a target wavelength from the light emission of the plasma, The control unit 5 mainly includes a detection unit 3 for detecting light, a high-frequency power source 4 for generating an electromagnetic field in the plasma torch unit 1, and a CPU for controlling these hardware and a CPU for data processing.

  More specifically, a work coil (not shown) is wound around the outside of the plasma torch unit 1, and a high-frequency alternating current produced by the high-frequency power source 4 flows therethrough. As a result, argon gas supplied from an argon gas supply system (not shown) is ionized to generate plasma composed of argon ions and electrons. In order to prevent the work coil from being damaged due to thermal expansion or the like, the work coil is cooled with a coolant such as cooling water cooled to a predetermined temperature.

  The sample solution is misted by a nebulizer of a sample supply system (not shown) and then introduced into the plasma together with argon gas as a carrier gas. Each element contained in the sample introduced into the plasma emits light at a specific wavelength after being thermally excited by the plasma. The emitted light of the plasma is collected by an incident lens system (not shown) and introduced into the spectrometer 2.

Since various elements are contained in the sample solution, and these elements emit light at different wavelengths, light having a target wavelength is separated by the spectroscope 2 including a prism, a diffraction grating, and the like. Note that light with a wavelength of less than 200 nm is easily absorbed by oxygen (O ₂ ). Therefore, when measurement of a wavelength of less than 200 nm is required, the spectroscope is evacuated or replaced with argon or nitrogen. preferable. FIG. 1 illustrates a case where the inside of the spectrometer 2 is evacuated by the vacuum pump 6.

  In the detection unit 3, the intensity of light of a specific wavelength separated by the spectroscope 2 is measured. Since the light intensity is proportional to the concentration, the measured light intensity of the solution of unknown concentration is compared with the light intensity of the solution of unknown concentration to estimate the element concentration of the solution of unknown concentration. Can do. As the detector itself, which is a main component of the detection unit 3, a photomultiplier tube (photomultiplier), a CCD (or CID), or the like is generally used, but the analysis device of this embodiment uses a CCD detector. .

  Since the CCD detector outputs a signal derived from the temperature of the CCD detector itself even when no light is incident, this is an error in detecting the light intensity. In the analyzer according to the embodiment of the present invention, the refrigerant used for cooling the plasma torch unit 1 and the refrigerant used for cooling the detection unit 3 described above are different systems even if they are the same system as long as their respective cooling functions can be achieved. However, there is no particular limitation. However, by cooling the work coil when cooling the detection unit, the refrigerant supplied to the detection unit 3 is made a separate system, unlike the case where the refrigerant flows in series in the order of the work coil and the detection unit in one refrigerant circulation system. The cooling function is improved because it is possible to avoid the influence of the temperature fluctuation of the cooling water after the cooling.

  Next, a method for analyzing a metal element by inductively coupled plasma emission spectroscopy using the above inductively coupled plasma emission spectrometer will be described. For example, when a positive electrode material for a lithium secondary battery is targeted as a sample to be analyzed, first, a sample is weighed in a beaker or the like to prepare a sample solution, and then nitric acid and hydrogen peroxide water are added. The sample is dissolved by heating using a heating device such as a plate. At that time, a certain amount of the solid or solution of the compound containing the internal standard element is added before the sample is dissolved, or is added immediately after the sample is dissolved and cooled to room temperature.

  When the element to be measured is lithium, aluminum, nickel, cobalt, or manganese, it is preferable to use yttrium or copper as the internal standard element. Since these internal standard elements are not decomposed or lost by the acid used for the dissolution treatment of the sample, the compound containing the internal standard element can be simultaneously measured when the sample is measured in a beaker.

  The compound containing yttrium and copper used as the internal standard element is not particularly limited, but the content does not change due to moisture absorption or self-decomposition, and it is easily dissolved or decomposed during the sample dissolution process. Those that form copper are preferred, and generally a solid metal or a solution thereof is used.

  In the preparation of the sample solution, when a certain amount of the solid or solution of the compound containing the internal standard element is weighed, in any case, it is preferable to weigh and add a constant amount. The same applies to the preparation of the standard sample used for the calibration curve. As a result, errors due to capacitance errors can be eliminated. That is, by measuring by mass, it is possible to prevent the occurrence of an artificial error that may be a concern when a constant amount is collected with a pipette or the like.

  Next, an example is given and the preparation method of the standard solution for calibration curves and a sample solution is demonstrated concretely. First, a certain amount of a compound containing an internal standard element is decomposed or dissolved, and after cooling to room temperature, an internal standard solution expressed by mass concentration Ni (g / kg) is prepared by adding water to a constant mass. Next, a certain amount of the compound containing the element to be measured is decomposed or dissolved, cooled to room temperature, added with water to a constant mass, and a standard stock solution for a calibration curve expressed in mass concentration Ns (g / kg) is prepared. To do.

  The prepared standard stock solution for calibration curve is weighed into separate containers with a mass of two or more levels, and a certain amount of the above internal standard solution is weighed into these containers. Furthermore, a certain amount of water and acid are added to these containers and stirred well to obtain a standard solution for a calibration curve. Here, As (g) is obtained by multiplying the mass of the standard stock solution for calibration curve measured at the time of preparation of the standard solution for calibration curve by the mass concentration Ns, and the mass concentration of the internal standard solution is also represented by the mass concentration described above. The product of Ni is Ai (g), and the ratio As / Ai is obtained.

  The sample solution is prepared as follows. After weighing a certain amount of the sample into the decomposition container, a certain amount of the internal standard solution is added to the decomposition container. At this time, the weight of the weighed sample is Ws (g), and the weight of the added internal standard solution is multiplied by the aforementioned mass concentration Ni is Wi (g). In addition, it is preferable to adjust the addition amount of an internal standard solution suitably considering the dilution rate in the dilution operation mentioned later. Specifically, it is preferable to adjust so that the amount of the internal standard element in the standard solution for the calibration curve and the sample solution are substantially equal. After decomposing or dissolving the sample in this state, add a certain amount of water and stir well. When dilution is necessary, a dilution operation is appropriately performed, and this is used as a sample solution.

  For the calibration curve standard solution and the sample solution obtained by the above method, the emission intensity of the measurement target element and the internal standard element is sequentially measured by inductively coupled plasma emission spectrometry. Here, the emission intensity of the measurement target element in the standard solution for the calibration curve is Bs, the emission intensity of the internal standard element is Bi, the emission intensity of the measurement target element in the sample solution is Cs, and the emission intensity of the internal standard element is Ci. To do. The luminescence intensity ratio Bs / Bi obtained from the luminescence intensity measured for the standard curve standard solution is taken as the vertical axis, and the mass ratio As / Ai of the measurement target element and the internal standard element in the corresponding standard curve standard solution is taken as the horizontal axis. Create a calibration curve by plotting. Next, Cs / Ci is obtained from the emission intensity ratio of the measurement target element and the internal standard element obtained for the sample solution, and the measurement target element and internal standard element in the sample solution are determined from the calibration curve of As / Ai and Bs / Bi. The mass ratio Ds / Di is obtained. Using the data obtained by the above operation, the content C (%) of the element to be measured in the sample is calculated from the following equation.

[Equation 1]
C = (Ds / Di × Wi) / Ws × 100

  As described above, the concentration of the element to be measured is determined by adding the internal standard element before the analysis pretreatment process including the sample decomposition operation and measuring the emission intensity ratio between the measurement element and the internal standard element after the pretreatment process. Therefore, even if an error occurs due to dilution in the pre-analysis process, for example, the calculation formula does not include the dilution factor, so it is not affected by the dilution error. It should be noted that the concentration of the element to be measured in the sample solution is preferably adjusted so as to be within the concentration range of the calibration curve.

  In order to perform measurement with an inductively coupled plasma emission spectrometer, it is necessary to adjust the plasma excitation temperature to a certain level or more. The simplest method for measuring the excitation temperature is the two-wire emission method. In this method, a solution containing iron, titanium, magnesium, or the like is sprayed, and the excitation temperature is calculated from the following equation by simultaneously measuring the emission intensity of two neutral atomic beams with different excitation energies from this one element. can do. Note that two neutral atomic beams of argon may be used instead of iron, titanium, magnesium, etc. In this case, since the plasma is formed of argon, the emission intensity can be increased by spraying pure water. Can be obtained.

[Equation 2]
T = 5041 × ΔE / [log (g ₁ A ₁ / g ₂ A ₂ ) −log (λ ₁ / λ ₂ ) −log (I ₁ / I ₂ )]

[Equation 3]
ΔE = E ₁ −E ₂

Here, E ₁ and E ₂ are excitation energy (eV) of two wavelengths, λ ₁ and λ ₂ are two wavelengths (nm), and I ₁ and I ₂ are emission intensity (arbitrary unit) of each wavelength. Show. gA is obtained by multiplying the statistical weight of each excited state by the transition probability, and is generally disclosed as a gA value.

  The wavelength to be selected is preferably one that can obtain a sufficient emission intensity, has an accurate transition probability disclosed, has a large excitation energy range, and has wavelengths close to each other. For example, when an iron standard solution having a concentration of 1 g / L is used, the excitation temperature can be calculated by a combination of wavelengths 302.403 nm and 303.015 nm, or a combination of 370.557 nm and 370.925 nm.

  In the method for measuring a metal element by inductively coupled plasma optical emission spectrometry according to the present invention, the excitation temperature is 4900K or higher. This is because satisfactory measurement accuracy cannot be obtained at less than 4900K. That is, in order to perform measurement with high accuracy, it is necessary to suppress or correct various fluctuation factors generated in the inductively coupled plasma emission spectrometer. For this reason, in the internal standard correction method, it is preferable that the behaviors of the measurement target element and the internal standard element coincide with all the apparatus variation factors. Specifically, the correlation coefficient R between the measurement target element and the internal standard element is It is desirable to be closer to 1.0. However, when the excitation temperature is less than 4900K, the correlation coefficient between the element to be measured and the internal standard element is remarkably lowered, so that the measurement accuracy is lowered.

  Two types of inductively coupled plasma emission spectroscopic analyzers are commercially available: an apparatus with a high frequency power supply frequency of 40.68 MHz for generating plasma and an apparatus with a 27.12 MHz specification. Although there is no restriction on the frequency to be used, a device with a 27.12 MHz specification can easily obtain a higher excitation temperature. When measurement is performed with a 40.68 MHz specification apparatus, the high frequency output and the carrier gas flow rate or photometric height are adjusted to form plasma of 4900K or more and measurement is performed. Specifically, when the high frequency output is increased and the carrier gas flow rate is decreased, the excitation temperature tends to increase.

  Although the decrease in the carrier gas flow rate has the effect of increasing the residence time of the measurement element in the plasma, the same effect can also be realized by increasing the center diameter of the plasma lighting torch having a triple tube structure. This is because the linear velocity of the gas decreases due to the thicker carrier gas channel. It should be noted that, by lowering the photometric height, measurement in a higher temperature region is possible. However, since it becomes easy to be affected by various coexisting elements, a sufficient investigation is required when changing the photometric position. The emission intensity measurement method includes an axial photometry method and a synchrotron photometry method, but there is no particular limitation in the present invention. However, it should be noted that the axial photometry method may be more sensitive to coexisting elements.

  In addition to yttrium and copper, ytterbium, cobalt, scandium, beryllium, thallium and the like may be used as the internal standard element. In addition, when the element to be measured contains at least two of lithium, aluminum, nickel, cobalt, and manganese, the internal standard element is optimized in order to further improve the analysis accuracy by optimizing the internal standard element. As for, it is preferable to use yttrium for lithium, aluminum or manganese and to use copper for nickel or cobalt. Thus, by selecting the optimal internal standard element for each measurement target element, the correlation coefficient R between the measurement target element and the internal standard element can be set to 0.95 or more.

  In selecting the optimum internal standard element, it is preferable to consider the excitation energy of each element or the sum of the excitation energy and ionization energy. That is, the combination of the measurement wavelength of each element to be measured and the internal standard element is between neutral atomic beams or ion beams, and when selecting a neutral atomic beam, the upper level of the measurement wavelength of the internal standard element is selected. The excitation energy at the position is within a range of 1.0 eV above and below the excitation energy at the upper level of the measurement wavelength of the element to be measured, and when the ion beam is selected, the measurement wavelength of the internal standard element is It is desirable that the sum of the excitation energy and ionization energy of the upper level to be within a range of 1.0 eV above and below the sum of the excitation energy and ionization energy of the upper level of the measurement wavelength of the measurement target element.

  Furthermore, when measuring the lithium / metal ratio of a lithium battery positive electrode material made of, for example, lithium, nickel, cobalt, and aluminum, the positive electrode material for a lithium secondary battery contained in the sample solution is configured in order to improve the analysis accuracy. It is preferred to measure all major metal elements simultaneously. This shows that when each element is measured alone, the accuracy of analysis tends to deteriorate due to the additive nature of the error. However, if these elements are measured simultaneously under a condition where the plasma excitation temperature is 4900 K or higher, the lithium generated due to the fluctuation of the apparatus This is because the behaviors of nickel, cobalt, and aluminum corresponding to the metal are consistent, and the lithium / metal ratio can be measured more accurately. The means for performing simultaneous measurement is not particularly limited, but it is preferable to use a multi-element simultaneous detector such as a CCD detector, a CID detector, or a polychromator equipped with a plurality of photomultiplier tubes.

  Next, based on the relational expression between the measurement accuracy of the lithium / metal ratio obtained in advance by the measurement using the inductively coupled plasma optical emission spectrometer and the temperature control width of the refrigerant for the detector of the analyzer, a request is made. A method for controlling the temperature of the refrigerant for the detection unit with the temperature control range of the refrigerant necessary for satisfying the analysis accuracy of the lithium / metal ratio will be specifically described. First, using a chiller unit with high temperature control capability with respect to the set temperature of the refrigerant, the lithium / metal ratio is measured by the inductively coupled plasma emission spectrometry described above, and at the same time, the temperature fluctuation range of the refrigerant is recorded using a temperature recorder or the like. Keep it. At this time, the measurement by the inductively coupled plasma emission spectroscopy is performed using the calibration curve method described in Patent Document 1, and the same sample is measured a plurality of times with one calibration curve to obtain the RSD.

  Next, using a chiller unit that has a lower temperature control capability with respect to the set temperature of the refrigerant than the chiller unit used above, the lithium / metal ratio is measured by inductively coupled plasma emission spectroscopy, and the temperature fluctuation range of the refrigerant is simultaneously measured. Record using a temperature recorder. At this time, the measurement by the inductively coupled plasma emission spectroscopy is performed using the calibration curve method described in Patent Document 1, and the same sample is measured a plurality of times with one calibration curve to obtain the RSD.

  From the RSD of the lithium / metal ratio obtained by the measurement of the above two cases and the respective temperature fluctuation ranges, a relational expression between the RSD of the lithium / metal ratio and the temperature fluctuation range is calculated, and using this relational expression, It is possible to calculate the temperature control ability of the chiller refrigerant required to satisfy the required analysis accuracy of the lithium / metal ratio.

[Example 1]
About 1.0 g of a powder sample of a positive electrode material for a lithium secondary battery containing lithium, nickel, aluminum, and cobalt was accurately weighed and placed in a clean 300 mL glass beaker. Next, about 10 g each of the yttrium internal standard solution and the copper internal standard solution having a concentration of 6 g / kg were accurately 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 dissolve the powder sample. After standing to cool, 2 mL of hydrogen peroxide solution was further added and heated to dissolve as described above. This operation was repeated at least twice to completely dissolve the powder sample.

  After the obtained solution was cooled to room temperature, it was transferred to a 100-mL volumetric flask and pure water was added to adjust the solution volume to 100 mL. Next, 10 mL of the solution was collected using a 10 mL total volume pipette, transferred to a 200 mL volumetric flask, and further added with pure water to make the solution volume constant to 200 mL, which was used as a sample solution.

  The internal standard solution was prepared as follows. The standard solution for yttrium was dissolved by weighing 6.0 g of yttrium metal in a clean 300 mL glass beaker, adding 10 mL of water, 10 mL of nitric acid and 30 mL of hydrochloric acid, and then heating on a hot plate. After cooling the obtained solution to room temperature, it was transferred to a 1000 mL capacity container, and pure water was added to make 1 kg. The copper internal standard solution was prepared in the same manner as the yttrium internal standard solution except that 6.0 g of copper metal was dissolved by adding 20 mL of water and 40 mL of nitric acid.

  About the obtained sample solution, all the elements simultaneous measurement of lithium, nickel, aluminum, and cobalt was performed using the inductively coupled plasma emission spectroscopic analyzer. For the inductively coupled plasma emission spectroscopic analyzer, ICPE 9000 manufactured by Shimadzu Corp., using a synchrotron radiation photometry method, having a plasma frequency of 27.12 MHz and equipped with a CCD detector in the detection part was used. Cooling water was used as a coolant for cooling the detection unit having the CCD detector, and the temperature was controlled so that the temperature fluctuation range was within ± 0.1 ° C. The plasma output was 1.2 kW, the carrier gas flow rate was 0.6 L / min, and the photometric height was low. At this time, a 1 g / L iron standard solution was used, and the plasma excitation temperature determined from the emission intensity ratio between wavelengths 302.403 nm and 303.015 nm was 6220K.

  The measurement wavelength of each measurement target element was 670.153 nm for lithium, 221.647 nm for nickel, 396.153 nm for aluminum, and 238.892 nm for cobalt. The measurement wavelength of the internal standard element was 371.030 nm for yttrium and 324.754 nm for copper. Further, for lithium and aluminum, yttrium was used as an internal standard element, and for nickel and cobalt, copper was used as an internal standard element, and the emission intensity ratios of lithium / yttrium, aluminum / yttrium, nickel / copper, and cobalt / copper were measured. .

  Ten types of sample solutions of Samples 1 to 10 were prepared by the above-described method for preparing a powder sample of a positive electrode material for a lithium secondary battery, and analyzed using the above-described inductively coupled plasma emission spectrometer. As shown in FIG. 2, each analyzer first measures L (low concentration) and H (high concentration) of a standard solution for a calibration curve with a known concentration, and prepares a calibration curve (proportional expression of emission intensity and concentration). Thereafter, the above-described samples 1 to 10 were measured in the order of this number. This operation was repeated 8 times, and RSD% (n = 8), which is a relative standard deviation, was calculated from 8 analytical values obtained for each sample.

  Next, samples 1 to 10 were prepared in the same manner as described above except that the temperature control was performed so that the fluctuation range of the cooling water temperature for cooling the detection unit was within ± 3 ° C. The operation of measuring in order was repeated 8 times, and RSD% (n = 8), which is a relative standard deviation, was calculated from 8 analytical values obtained for each sample. The results of the RSD% (n = 8) of the lithium / metal ratio of samples 1 to 10 when the temperature fluctuation range of the cooling water thus obtained is ± 0.1 ° C. and the case of the fluctuation range ± 3 ° C. are shown below. Table 1 shows.

[Table 1]

  As can be seen from the results in Table 1, when the temperature of the detection unit was controlled with a temperature fluctuation range of ± 0.1 ° C., any element of any sample had RSD% ≦ 0.2%. On the other hand, when temperature control was performed with a temperature fluctuation range of ± 3 ° C., the RSD% increased for any element of any sample. In general, when the measurement conditions are stable and the analysis is performed with high accuracy, the RSD% <0.3%. Therefore, in both cases, the metal element concentration and the lithium / metal ratio are stable with high accuracy. It can be measured automatically.

  As described above, when the temperature of the refrigerant supplied to the detection unit fluctuates, the light intensity of the quantitative element and the internal standard element varies at a different rate. As a result, the quantitative element / internal standard element is not constant. It can be seen that the measurement accuracy of the concentration and the lithium / metal ratio is strongly influenced by the temperature control width of the refrigerant in the detection unit of the inductively coupled plasma emission spectrometer. That is, the smaller the variation from the set temperature of the refrigerant that cools the detection unit, the smaller the variation in the obtained lithium / metal ratio.

  Next, the largest RSD% was selected from the results in Table 1, and these were plotted in a graph in which the horizontal axis represents the temperature control width for cooling the detection unit as shown in FIG. Then, as shown in the graph, a relational expression between RSD% and the temperature control width for cooling the detection unit was obtained from the two plotted points. From this relational expression, the temperature control range that satisfies the required analysis accuracy is determined, and by selecting a chiller unit that can be controlled within that temperature control range, the analysis with the desired accuracy can be performed stably without costing more than necessary. And low environmental load.

[Example 2]
All these elements were measured using an inductively coupled plasma emission spectrometer in the same manner as in Example 1 except that a powder sample of a positive electrode material for a lithium secondary battery containing lithium, nickel, manganese, and cobalt was used as an analysis target. Simultaneous measurement was performed. At that time, similarly to the first embodiment, when the temperature control is performed so that the fluctuation range of the temperature of the cooling water for cooling of the detection unit is within ± 0.1 ° C., the fluctuation range is within ± 3 ° C. Thus, the measurement was performed twice when the temperature control was performed.

  For manganese, the measurement wavelength was 279.827 nm, and manganese / yttrium was measured using yttrium as an internal standard element of manganese. In measurement, the operation of measuring samples 1 to 10 prepared in the same manner as in Example 1 in the order of this number was repeated 8 times, and RSD% (n = 8) which is a relative standard deviation from the 8 analytical values obtained in each sample. ) Was calculated. The results of the RSD% (n = 8) of the lithium / metal ratio of samples 1 to 10 when the temperature fluctuation range of the cooling water thus obtained is ± 0.1 ° C. and the case of the fluctuation range ± 3 ° C. are shown below. It shows in Table 2.

[Table 2]

  As can be seen from the results of Table 2 above, when the temperature of the detection unit was controlled with a temperature fluctuation range of ± 0.1 ° C., any element of any sample had RSD% ≦ 0.2%. On the other hand, when temperature control was performed with a temperature fluctuation range of ± 3 ° C., the RSD% increased for any element of any sample. In general, when the measurement conditions are stable and analysis is performed with high accuracy, RSD% <0.3%, so that the temperature fluctuation range is within ± 0.1 ° C. When the control is performed, the metal element concentration and the lithium / metal ratio can be stably measured with high accuracy.

  Next, from the results in Table 2, the largest RSD% was selected, and these were plotted in a graph in which the horizontal axis represents the temperature control width for cooling the detector as shown in FIG. Then, as shown in the graph, a relational expression between RSD% and the temperature control width for cooling the detection unit was obtained from the two plotted points. From this relational expression, the temperature control range that satisfies the required analysis accuracy is determined, and by selecting a chiller unit that can be controlled within that temperature control range, the analysis with the desired accuracy can be performed stably without costing more than necessary. And low environmental load.

DESCRIPTION OF SYMBOLS 1 Plasma torch part 2 Spectrometer 3 Detection part 4 High frequency power supply 5 Control part 6 Vacuum pump

Claims (6)

An internal standard correction method using yttrium or copper as an internal standard element is applied, and the concentration of a metal element in a sample solution is analyzed using an inductively coupled plasma emission spectrometer using a plasma excitation temperature of 4900K or higher. Then, a relational expression between the measurement accuracy of the lithium / metal ratio and the temperature control width of the refrigerant for the detection unit of the analytical apparatus is obtained from a plurality of temperature control widths by measurement using the analytical apparatus, and based on the relational expression A method for analyzing the concentration of a metal element, characterized in that a temperature control range of a refrigerant necessary to satisfy the required analysis accuracy of a lithium / metal ratio is determined, and the temperature of the refrigerant for the detection unit is controlled by the temperature control range . The metal element concentration analysis method according to claim 1, wherein the plurality of temperature control ranges are within a range of ± 0.1 ° C. and ± 3.0 ° C. 2. The metal element concentration analysis method according to claim 1 or 2 , wherein the sample solution is a solution obtained by decomposing a positive electrode material for a lithium secondary battery. The sample solution contains at least two of lithium, aluminum, nickel, cobalt, and manganese as elements to be measured. For lithium, aluminum, or manganese, yttrium is used as an internal standard element, and nickel or cobalt is used. 4. The metal element concentration analysis method according to claim 1, wherein copper is used as an internal standard element. 5. The inductively coupled plasma optical emission spectrometer is equipped with a multi-element simultaneous detector, and the multi-element simultaneous detector is used as a main metal element constituting the positive electrode material for a lithium secondary battery contained in the sample solution. The method for analyzing the concentration of a metal element according to claim 3 , wherein all of the above are measured simultaneously, whereby the molar concentration ratio of the constituent metal elements is measured. When the internal standard solution containing the internal standard element is added to the sample solution and the calibration curve standard solution in the internal standard correction method, an internal standard solution whose concentration is adjusted on a mass basis is used. The method for analyzing the concentration of a metal element according to any one of claims 1 to 5 , wherein the metal element is added by weighing.

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