Quantitative analysis method and system for chemical valence distribution of iron element in lithium iron phosphate material

Abstract

The invention provides a quantitative analysis method and system for chemical valence distribution of an iron element in a lithium iron phosphate material, and relates to the technical field of combined analysis. Aiming at the technical problems that ferrous ions in a lithium iron phosphate solid material are extremely easy to oxidize and iron ions are difficult to separate in a conventional chromatographic column, the method comprises the following steps of: dissolving a sample by using an acid solvent subjected to deoxygenization treatment under a strict inert gas protection atmosphere, and introducing a specific iron ion valence stabilizer; the valence variation in the dissolving process is effectively prevented. Furthermore, a specific complexing agent is added to form a stable complex with remarkable chromatographic behavior difference with iron ions in different valence states, so that baseline separation of Fe < 2 + > and Fe < 3 + > on a liquid chromatographic column is realized, and high-sensitivity quantitative detection is performed by using mass spectrum. The method can accurately determine the content of trace Fe < 3 + > impurities and the ratio of Fe < 2 + > to Fe < 3 + > in the lithium iron phosphate material, and provides a powerful tool for quality control of a high-performance battery material.

Description

Technical Field

[0001] This invention belongs to the technical field of lithium-ion battery material analysis and testing technology, and in particular relates to a method and system for quantitative analysis of the chemical valence distribution of iron in lithium iron phosphate materials. Background Technology

[0002] Lithium iron phosphate (LiFePO4, LFP) is a crucial cathode material for lithium-ion batteries, and its electrochemical performance largely depends on the distribution of the chemical valence states of iron within the material. Ideally, all iron in LFP should exist in the form of divalent iron (Fe(II)). However, during the synthesis and preparation of the material (e.g., incomplete sintering, raw material residues), storage, or use, small amounts of trivalent iron (Fe(III)) impurities or defect phases (such as FePO4, Fe2O3, etc.) are inevitably introduced or generated. The presence of these Fe(III) impurities significantly reduces the actual specific capacity, electronic conductivity, and ion diffusion rate of the material, thereby severely affecting the cycle life and rate performance of the battery. Therefore, establishing a method that can accurately and sensitively analyze the simultaneous qualitative and quantitative distribution of Fe(II) and Fe(III) content in lithium iron phosphate materials is of paramount importance for optimizing production processes, improving product quality, and gaining a deeper understanding of material failure mechanisms.

[0003] Currently, methods for analyzing the valence state of iron mainly include solid surface / bulk phase analysis techniques, represented by Mössbauer spectroscopy and X-ray photoelectron spectroscopy (XPS), and wet chemical analysis techniques, represented by titration and chromatography-spectroscopy. While Mössbauer spectroscopy and XPS can provide in-situ valence state information, these instruments are expensive, complex to operate, and require sophisticated sample preparation. They typically only provide semi-quantitative relative content ratios, making it difficult to meet the needs of rapid and accurate quantification in industrial production. Furthermore, they exhibit significant errors in analyzing trace phases with a content below 1%, making them unsuitable as routine quality control methods for detecting trace Fe(III) impurities.

[0004] Traditional wet chemical titration methods (such as the potassium dichromate method for determining total iron, combined with other methods for determining Fe(II)) are cumbersome and easily interfered with by other redox substances in the sample. More importantly, traditional wet analysis faces significant challenges in sample pretreatment (dissolution): LFP has a stable structure and usually requires strong acid and high temperature digestion. Under acidic conditions, especially in the presence of dissolved oxygen, the main component Fe(II) is easily oxidized to Fe(III), leading to severely overestimated Fe(III) results (false positives). Although existing techniques often use reducing agents such as ascorbic acid to protect Fe(II), this will also reduce trace Fe(III) impurities already present in the sample to Fe(II), resulting in underestimated or even undetectable Fe(III) results (false negatives), failing to accurately reflect the original state of the material.

[0005] In recent years, high-performance liquid chromatography (HPLC) or ion chromatography (IC) coupled with inductively coupled plasma mass spectrometry (ICP-MS) (LC-ICP-MS or IC-ICP-MS) has become a powerful tool for elemental speciation analysis due to its high separation capability and high sensitivity. However, there are still significant technical bottlenecks in applying this technology to LFP materials: (1) There is a lack of effective pretreatment methods to "lock" the valence state of iron while dissolving LFP, preventing Fe(II) oxidation or Fe(III) reduction; (2) The extremely high concentration of phosphate ions (PO4) in the LFP matrix 3- (2) Under conventional chromatographic conditions, it readily forms an insoluble ferric phosphate precipitate with Fe(III), leading to column blockage and loss of Fe(III) signal; (3) Direct injection of strongly acidic solutions can cause drastic fluctuations in the pH environment within the chromatographic column, damaging the stability of the complex, causing peak tailing, or even separation failure; (4) ICP-MS detection of iron (main isotopes) 56 Fe) is affected by polyatomic ions (such as Fe) 40 Ar 16 O + The interference is severe, affecting the accuracy of trace analysis.

[0006] Therefore, there is an urgent need to develop a new method that can overcome the above-mentioned defects and achieve accurate and sensitive determination of the forms and contents of Fe(II) and Fe(III) in lithium iron phosphate while preventing changes in valence state.

[0007] In view of this, the present invention is hereby proposed. Summary of the Invention

[0008] The purpose of this invention is to provide a method and system for quantitative analysis of the chemical valence state distribution of iron in lithium iron phosphate materials. This invention aims to solve problems existing in the prior art, such as the easy alteration of iron valence state during sample pretreatment (especially the easy oxidation of ferrous iron), Fe(III) precipitation due to a high-phosphorus matrix, and insufficient detection sensitivity.

[0009] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted: In a first aspect, the present invention provides a method for quantitative analysis of the chemical valence distribution of iron in lithium iron phosphate materials, comprising: include: The non-oxidizing acid solution used to dissolve the sample, as well as the volatile buffer salt, pH adjuster, blank diluent and volume-determining water added subsequently, were deoxygenated. Under an inert atmosphere with an oxygen content ≤10 ppm, the lithium iron phosphate sample was dissolved in a non-oxidizing acid solution containing a specific complexing agent that had been deoxygenated. Then, a volatile buffer salt that had been deoxygenated was added and the pH value was adjusted to match the pH value of the eluent used for subsequent chromatographic separation. The solution was then diluted to volume with deoxygenated water to obtain a test sample solution containing ferrous and ferric complexes. The sample solution to be tested was diluted with a deoxygenated blank diluent containing the same concentration of volatile buffer salt and specific complexing agent to obtain the final injection solution. The final injection solution was introduced into the chromatograph and separated using a strong anion exchange column to obtain column eluents of iron elements in different valence states. The eluent used for separation contained the volatile buffer salt and the specific complexing agent, and its pH value matched that of the final injection solution. The elution of iron in different valence states from the chromatographic column was introduced into an inductively coupled plasma mass spectrometer, and the polyatomic ion interference was eliminated by using collision or reaction modes before detection. Standard curves for ferrous iron (Fe2+) and ferric iron (Fe3+) were constructed separately. The quantitative analysis results were substituted into the standard curves, and the mass percentages of ferrous iron and ferric iron in the samples were calculated.

[0010] Furthermore, the inert atmosphere is selected from nitrogen and / or argon.

[0011] Furthermore, the oxygen content in the inert atmosphere is ≤1 ppm.

[0012] Furthermore, the specific complexing agent is selected from any one or a combination of at least two of pyridine-2,6-dicarboxylic acid, ethylenediaminetetraacetic acid, citric acid, diethylenetriaminepentaacetic acid, and hypozonyltriacetic acid.

[0013] Furthermore, in the non-oxidizing acid solution and the final diluted sample solution, the concentration of the specific complexing agent is 2.0~20.0 mmol / L, preferably 4.0~10.0 mmol / L.

[0014] Furthermore, the non-oxidizing acid is hydrochloric acid.

[0015] Furthermore, in the non-oxidizing acid solution used for dissolution, the concentration of the non-oxidizing acid is 0.5~4.0 mol / L.

[0016] Furthermore, the pH value is adjusted to a range that matches the eluent: the pH value of the solution is adjusted to 4.0~7.0, preferably 5.0~6.0.

[0017] Furthermore, the reagent used to adjust the pH value is selected from ammonia and / or acetic acid.

[0018] Furthermore, during the preparation of the test sample solution, it is necessary to record the mass m of the lithium iron phosphate sample, the volume V of the final volume, and the dilution factor D.

[0019] Furthermore, the deoxygenation treatment method includes inert gas bubbling and / or ultrasonic-assisted degassing.

[0020] Furthermore, the chromatograph includes an ion chromatograph or a liquid chromatograph.

[0021] Furthermore, the strong anion exchange column contains a hydrophilic polymer matrix packing material with quaternary ammonium salt functional groups, and the specifications of the strong anion exchange column are: column length range of 50~250 mm, inner diameter of 2.0~4.6 mm, and particle size of 3~10 μm.

[0022] Furthermore, the eluent used for the separation includes: a specific complexing agent, a volatile buffer salt, a pH adjuster, and a solvent.

[0023] Furthermore, the specific complexing agent in the eluent is selected from any one or a combination of at least two of pyridine-2,6-dicarboxylic acid, ethylenediaminetetraacetic acid, and citric acid, preferably pyridine-2,6-dicarboxylic acid.

[0024] Furthermore, the concentration of the specific complexing agent in the eluent is 0.5~10.0 mmol / L, preferably 2.0~5.0 mmol / L.

[0025] Furthermore, the volatile buffer salt is selected from any one or a combination of at least two of ammonium acetate, ammonium hydroxide, or ammonium formate.

[0026] Furthermore, the concentration of the volatile buffer salt in the eluent is 20-200 mmol / L, preferably 50-100 mmol / L.

[0027] Furthermore, the pH adjuster in the eluent is selected from ammonia and / or acetic acid.

[0028] Furthermore, the pH of the eluent is 5.0 to 7.0.

[0029] Furthermore, the solvent in the eluent is ultrapure water; wherein the resistivity of the ultrapure water is ≥18.2 MΩ·cm.

[0030] Furthermore, during the separation process, the flow rate of the eluent is 0.2~1.2 mL / min.

[0031] Furthermore, during the separation process, the column temperature of the chromatographic column is 20~40℃.

[0032] Furthermore, the inductively coupled plasma mass spectrometer monitors the signal intensity of the main isotope m / z 56 in real time, and simultaneously monitors the signal intensity of the auxiliary confirmatory ions m / z 54 and / or m / z 57.

[0033] Furthermore, the parameter settings of the inductively coupled plasma mass spectrometer include: a semiconductor-cooled cyclone atomization chamber temperature of 2~5℃; a plasma radio frequency power of 1300~1600 W; and a sampling depth of 7~10 mm.

[0034] Furthermore, during the quantitative analysis using the aforementioned inductively coupled plasma mass spectrometer, helium collision mode or hydrogen reaction mode is employed to eliminate interference from polyatomic ions generated by the plasma.

[0035] Furthermore, in the helium collision mode, the helium flow rate is 4.0~6.0 mL / min.

[0036] Furthermore, in the hydrogen reaction mode, the hydrogen flow rate is 2.0~8.0 mL / min.

[0037] Furthermore, the method for constructing the standard curve specifically includes the following steps: A series of standard solutions of ferrous salts with different concentrations and a series of standard solutions of ferric salts with different concentrations were prepared respectively. The standard solutions of ferrous salts and ferric salts were injected and analyzed under the same chromatographic and mass spectrometric conditions as the test sample. Standard curves for ferrous iron and ferric iron were constructed by plotting the mass concentration of iron in the corresponding valence state on the x-axis and the chromatographic peak area in the corresponding valence state on the y-axis. Preferably, the method for preparing the standard solution of the divalent ferric salt is as follows: To prepare standard stock solutions of ferrous salts, dilute them with a diluent containing a specific complexing agent and a volatile buffer salt of similar type and concentration to the final injection solution and the eluent, and with a pH value consistent with the eluent, to obtain a series of standard solutions of ferrous salts with different concentrations. To prepare standard stock solutions of ferric salts, dilute them with a diluent containing a specific complexing agent and a volatile buffer salt of similar type and concentration to the final injection solution and the eluent, and with a pH value consistent with the eluent, to obtain a series of standard solutions of ferric salts with different concentrations.

[0038] Furthermore, the divalent ferric salt is selected from ferrous ammonium sulfate; the trivalent ferric salt is selected from ferric chloride or ferric nitrate.

[0039] Furthermore, the concentration gradient of the standard solution of the divalent ferric salt is set between 0.01 and 1000 mg / L; the concentration gradient of the standard solution of the trivalent ferric salt is set between 0.001 and 100 mg / L.

[0040] Furthermore, the calculation method includes: The peak areas of ferrous iron and ferric iron obtained from the above quantitative analysis are input into the standard curves of the corresponding valence states to obtain the actual concentration of iron in the corresponding valence states. Then, combined with the mass m of the sample, the volume V of the constant volume, and the dilution factor D, the mass percentage of ferrous iron and ferric iron in the sample are calculated. The formula for calculating the mass percentage of divalent iron in the sample is as follows:

[0041] in, This represents the mass percentage of ferrous iron. This represents the actual concentration of ferrous iron in the sample solution (in mg / L). Represents the volume at constant volume (in liters). The value represents the mass (in mg) of the lithium iron phosphate sample to be tested, and D represents the dilution factor. The formula for calculating the mass percentage of ferric iron in the sample is as follows:

[0042] in, This represents the mass percentage of ferric iron. This represents the actual concentration of ferric iron in the sample solution (in mg / L). Represents the volume at constant volume (in liters). The value represents the mass (in mg) of the lithium iron phosphate sample to be tested, and D represents the dilution factor.

[0043] Furthermore, the calculation also includes calculating the mass percentage of ferric iron in the total iron content, and the formula is as follows:

[0044] in, This represents the mass percentage of ferric iron (Fe3+) in the total iron content. This represents the mass percentage of ferric iron. This represents the mass percentage content of divalent iron.

[0045] Secondly, the present invention provides a quantitative analysis system for the chemical valence distribution of iron in lithium iron phosphate materials, comprising: A protective dissolution and complexation device is provided, which has an inert atmosphere with an oxygen content ≤10 ppm (preferably ≤1 ppm) for dissolving the lithium iron phosphate sample in a non-oxidizing acid solution containing a specific complexing agent, and then adjusting the pH value by adding a volatile buffer salt and bringing the volume to obtain a test sample solution containing ferrous and ferric complexes; the device may also include a unit for diluting the test sample solution. A separation chromatograph, wherein the chromatograph has a built-in strong anion exchange column and is equipped with an eluent containing a volatile buffer salt and the specific complexing agent, and the pH value of the eluent is matched with that of the sample solution to be tested, for processing the sample solution to separate the complexes of iron elements in different valence states; The inductively coupled plasma mass spectrometer is equipped with a collision / reaction cell to receive the effluent of iron in different valence states from the chromatographic column and to analyze the iron in different valence states in the sample to be tested. The data processing unit is used for data analysis and processing.

[0046] Furthermore, the separation chromatograph includes a liquid chromatograph or an ion chromatograph; Furthermore, the quantitative analysis system also includes a four-way valve injector; wherein, one passage of the four-way valve injector is connected to the protective dissolution and complexation device and the chromatographic column in the separation chromatogram, and the other passage is connected to the mobile phase storage device and the chromatographic column in the separation chromatogram; Furthermore, a high-pressure pump is installed between the mobile phase storage device and the chromatographic column in the separation chromatograph to extract the mobile phase and deliver it to the chromatographic column through a four-way valve injector.

[0047] Compared with the prior art, the present invention has the following beneficial effects: (1) This invention effectively ensures the stability of the valence state of iron throughout the entire analytical process through an extremely rigorous "double deoxygenation" strategy. This invention not only operates under a strictly inert atmosphere with an oxygen content ≤10 ppm (preferably ≤1 ppm), but more importantly, it thoroughly deoxygenates all liquid reagents used for dissolution and dilution. Combined with a specific complexing agent, in-situ encapsulation and dissolution under non-oxidizing acid conditions allows ferrous and ferric ions to form thermodynamically stable complexes upon dissolution, fundamentally blocking the Fe(II) oxidation pathway caused by dissolved oxygen. This solves the core problem of inflated Fe(III) values ​​due to oxidation in traditional methods, ensuring that the measured value is accurate and reflects the actual valence state distribution of the raw materials.

[0048] (2) This invention employs an innovative "stepwise matrix matching" pretreatment strategy to ensure high efficiency and stability of chromatographic separation. By first dissolving the sample with a non-oxidizing acid, then adding a volatile buffer salt and adjusting the pH to match the mobile phase, the non-oxidizing acidic sample matrix is ​​successfully converted into a buffer system compatible with the chromatographic system. This not only avoids pH shocks during injection that could lead to complex dissociation or Fe(III) precipitation, but also ensures the stable retention and separation of the target complex on the chromatographic column. Combined with the volatile salt mobile phase system, it avoids mass spectrometry interface blockage and guarantees the long-term stability of the method.

[0049] (3) This invention significantly improves the detection capability and quantitative accuracy of trace ferric iron impurities, while also taking into account the determination of the main components. Relying on the ultra-high sensitivity of inductively coupled plasma mass spectrometry with collision / reaction cell technology, mass spectrometry interference such as argon oxide is effectively eliminated. More importantly, this invention specifically constructs two sets of standard curves covering the low concentration range (for trace Fe(III)) and the high concentration range (for the main Fe(II)), and combined with necessary sample dilution steps, overcomes the matrix effect and broadens the linear range, achieving simultaneous and accurate calibration of ppb-level trace oxidation defects and high-content main elements.

[0050] (4) This invention achieves direct, simultaneous, and non-destructive quantification of the chemical valence state of iron. Stable ferrous (Fe2+) and ferric (Fe3+) complexes are physically separated by liquid chromatography, and the signals are independently responded to by mass spectrometry, completely avoiding the systematic errors caused by the traditional "total iron minus ferrous (Fe2+)" calculation method. Even with extremely low ferric (Fe3+) content, reliable direct quantitative results can be obtained, providing a powerful and high-quality analytical tool for material defect diagnosis, process optimization, and research on storage aging behavior. Attached Figure Description

[0051] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0052] Figure 1 This is a schematic diagram of the structure of the quantitative analysis system for the chemical valence distribution of iron in lithium iron phosphate materials provided by the present invention.

[0053] Among them, 100 is a protective dissolution and complexation device, 101 is a sealed syringe, 200 is a separation chromatograph, 300 is an inductively coupled plasma mass spectrometer, 400 is a data processing unit, 500 is a four-way valve injector, 600 is a high-pressure pump, and 700 is a mobile phase storage device.

[0054] Figure 2 The LC-ICP-MS chromatogram is obtained from the quantitative device for the chemical valence distribution of iron in lithium iron phosphate materials using liquid chromatography-mass spectrometry provided in Example 1 of this invention. Detailed Implementation

[0055] Unless otherwise defined herein, the scientific and technical terms used in conjunction with this invention shall have the meanings commonly understood by one of ordinary skill in the art. The meaning and scope of terms shall be clear; however, in any case of potential ambiguity, the definitions provided herein shall prevail over any dictionary or foreign definitions. In this application, unless otherwise stated, the use of "or" means "and / or". Furthermore, the use of the term "comprising" and other forms is non-limiting.

[0056] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0057] All oxygen contents mentioned in the present invention are volume ratios.

[0058] In a first aspect, the present invention provides a method for quantitative analysis of the chemical valence distribution of iron in lithium iron phosphate materials, comprising: The non-oxidizing acid solution used to dissolve the sample, as well as the volatile buffer salt, pH adjuster, blank diluent and volume-determining water added subsequently, were deoxygenated. Under an inert atmosphere with an oxygen content ≤10 ppm, the lithium iron phosphate sample was dissolved in a non-oxidizing acid solution containing a specific complexing agent that had been deoxygenated. Then, a volatile buffer salt that had been deoxygenated was added and the pH value was adjusted to match the pH value of the eluent used for subsequent chromatographic separation. The solution was then diluted to volume with deoxygenated water to obtain a test sample solution containing ferrous and ferric complexes. The sample solution to be tested was diluted with a deoxygenated blank diluent containing the same concentration of volatile buffer salt and specific complexing agent to obtain the final injection solution. The final injection solution was introduced into the chromatograph and separated using a strong anion exchange column to obtain column eluents of iron elements in different valence states. The eluent used for separation contained the volatile buffer salt and the specific complexing agent, and its pH value matched that of the final injection solution. The elution of iron in different valence states from the chromatographic column was introduced into an inductively coupled plasma mass spectrometer, and the polyatomic ion interference was eliminated by using collision or reaction modes before detection. Standard curves for ferrous iron (Fe2+) and ferric iron (Fe3+) were constructed separately. The quantitative analysis results were substituted into the standard curves, and the mass percentages of ferrous iron and ferric iron in the samples were calculated.

[0059] It should be noted that this invention adopts a combined technical route of "inert complexation dissolution + chromatographic column separation + ICP-MS isotope detection"; it specifically addresses the problem that existing technologies cannot simultaneously, directly, and with high sensitivity detect the content of iron in different valence states in lithium iron phosphate, aiming to (1) achieve direct speciation analysis: without the need for differential calculation, the signals of Fe(II) and Fe(III) can be separated and detected simultaneously in one analysis process. (2) extremely high sensitivity: by utilizing the ultra-low detection limit (ppt level) of ICP-MS, the trace trivalent iron impurities in LFP materials can be accurately quantified. (3) in-situ stability: through a specific complexation protection mechanism, the valence state transformation of the sample is prevented during the separation process.

[0060] First, to effectively prevent the oxidation of Fe(II) and the hydrolysis of Fe(III) during the lattice destruction process of lithium iron phosphate (LFP), the pretreatment must be carried out in an inert gas protection device (such as a glove box) with strictly controlled oxygen content (≤10 ppm, preferably ≤1 ppm), and all liquid reagents used must be thoroughly deoxygenated beforehand. The specific operation is as follows: First, accurately weigh a certain mass (m) of lithium iron phosphate sample and place it in a sealed reaction vessel. Add a non-oxidizing acidic solution containing a specific complexing agent that has been deoxygenated to dissolve the sample. Under these acidic conditions, the complexing agent can quickly form stable complexes (such as Fe(II)-PDCA and Fe(III)-PDCA) with Fe(II) and Fe(III) in the system, effectively inhibiting the oxidation of Fe(II). Subsequently, add a volatile buffer salt that has been deoxygenated (and a pH adjuster if necessary) to adjust the pH of the solution to a range that matches the subsequent chromatographic flow (such as pH 5.0~7.0). This step is crucial, as it transforms the non-oxidizing acidic sample matrix into a buffer system. This avoids pH shocks during injection that could lead to complex dissociation or Fe(III) precipitation, while ensuring the stable retention of the target complex on the column. Finally, the volume is adjusted to the set volume (V) using deoxygenated water. Through this stepwise dissolution-pH adjustment process, the initial valence state distribution of iron can be "frozen" at the molecular level, laying the foundation for accurate analysis of the iron valence state composition in lithium iron phosphate materials.

[0061] Next, the pretreated sample solution (or the final injection solution after appropriate dilution) was filtered through a filter membrane and then precisely injected into the liquid chromatograph for analysis. A strong anion exchange column was used in the chromatographic separation stage. To ensure the thermodynamic stability of the iron-complex during separation and to prevent its dissociation on the column, the mobile phase system used a specific ratio of volatile buffer salt and a specific complexing agent, and the pH was adjusted to the optimal stability range. It is worth noting that an additional complexing agent of the same concentration as in the pretreatment step was added to the mobile phase to maintain the chemical equilibrium of the system and inhibit the reverse migration of the complexation reaction. Finally, the inherent differences in charge density and hydrophobicity between Fe(II) and Fe(III) complexes were utilized to achieve the separation of the target analytes, laying the foundation for subsequent quantitative analysis.

[0062] Furthermore, the column effluent is directly introduced into the inductively coupled plasma mass spectrometer (ICP-MS) via a concentric nebulizer. To effectively reduce oxide interference and improve the detection signal-to-noise ratio, the injection system should preferably be equipped with a semiconductor-cooled cyclone nebulizer and employ collision / reaction cell technology (such as helium collision or hydrogen reaction mode) to eliminate polyatomic ion interference (e.g., ... 40 Ar 16 O + right 56(Interference from Fe). Under the optimized experimental conditions described above, the mass spectrometer uses m / z 56 (the main isotope of iron) as the core monitoring ion for quantitative analysis. Simultaneously, m / z 54 or m / z 57 can be collected as auxiliary confirmatory ions according to actual needs. Matrix effects and mass spectrometry interferences are corrected through multi-isotope alignment. For high concentrations of ferrous iron, monitoring auxiliary confirmatory ions m / z 54 and / or m / z 57 can be selected to avoid detector saturation. This coupled technique, through the synergistic effect of chromatographic separation and mass spectrometry detection, can accurately distinguish and quantify iron elements of different valence states in lithium iron phosphate materials.

[0063] Finally, to ensure the accuracy of quantitative analysis, a standard curve was established using the mobile phase matrix matching method. Considering the high content of divalent iron and low content of trivalent iron in lithium iron phosphate materials, different concentration gradients of standard solutions were set. Fe(II) and Fe(III) standard stock solutions were prepared separately, and serially diluted with a deoxygenated diluent containing specific complexing agents and volatile buffers of similar types and concentrations to those in the final injection solution and eluent, with a pH value similar to those in the final injection solution and eluent, to prepare a series of mixed standard solutions. This process ensures that the iron ions in the standard solution form a complexation morphology consistent with the analyte, effectively eliminating matrix effect interference. Under the same chromatographic-mass spectrometry analysis conditions, the standard series solutions were injected and detected. Linear regression equations for Fe(II) and Fe(III) were constructed with iron element mass concentration (X, mg / L) as the x-axis and the corresponding peak area (Y, CPS·s) as the y-axis, and the correlation coefficient R0 was used. 2 All methods must meet the methodological requirements. For unknown samples, the "retention time-signal intensity" chromatogram is first obtained, and Fe(II) and Fe(III) are qualitatively identified based on the retention times determined by the standards. After integrating the target peak area using a chromatography workstation, the absolute iron ion content is calculated by substituting it into the corresponding standard curve equation. Finally, combining the sample weight (m, g), the final volume (V, L), and the dilution factor (D), the mass fractions of Fe(II) and Fe(III) in the lithium iron phosphate material are calculated, and the mass percentage of Fe(III) in the total iron content is further converted, providing a quantitative basis for the redox state and quality evaluation of the material. The entire analytical process strictly follows the instrument operating procedures, ensuring the reliability of the method and the precision and accuracy of the detection results.

[0064] As an optional implementation, the inert atmosphere is selected from nitrogen and / or argon.

[0065] It should be noted that operating under such a strictly low oxygen atmosphere blocks the oxidation of ferrous iron in the sample solution by atmospheric oxygen, ensuring that the Fe(II) released during the dissolution process is not converted into Fe(III), thus truly preserving the intrinsic valence distribution of the material; at the same time, it avoids distortion of valence ratios and subsequent chromatographic peak shape due to oxidation, significantly improving the accuracy and reproducibility of the detection results, and providing a reliable basis for the quality assessment and process traceability of lithium iron phosphate materials.

[0066] In a preferred embodiment, the oxygen content in the inert atmosphere is ≤1 ppm.

[0067] As an optional implementation, the liquid reagents (non-oxidizing acid solutions, buffer salt solutions, pH adjusters, water for volume adjustment, and blank diluents) need to be deoxygenated before use in an inert atmosphere. The deoxygenation method includes one or a combination of inert gas bubbling, freeze-drying, or ultrasonic-assisted degassing. This further eliminates the oxidation risk posed by dissolved oxygen.

[0068] As an optional implementation, the specific complexing agent is selected from any one or a combination of at least two of pyridine-2,6-dicarboxylic acid (PDCA), ethylenediaminetetraacetic acid (EDTA), citric acid, diethylenetriaminepentaacetic acid (DTPA), and nitrotriacetic acid (NTA).

[0069] In a preferred embodiment, the specific complexing agent is pyridine-2,6-dicarboxylic acid (PDCA).

[0070] As an optional implementation, the concentration of the specific complexing agent in the non-oxidizing acid solution and the final diluted sample solution is 2.0~20.0 mol / L, for example, it can be 2.0 mol / L, 3.0 mol / L, 4.0 mol / L, 5.0 mol / L, 6.0 mol / L, 7.0 mol / L, 8.0 mol / L, 9.0 mol / L, 10.0 mol / L, 15.0 mol / L, 20.0 mol / L, etc.

[0071] In a preferred embodiment, the concentration of the specific complexing agent in the non-oxidizing acid solution and the final volume-adjusted sample solution is 4.0~10.0 mol / L.

[0072] As an optional implementation, the non-oxidizing acid in the non-oxidizing acid solution and the final volume-adjusted sample solution is hydrochloric acid.

[0073] As an optional implementation, the concentration of the acid in the non-oxidizing acid solution used for dissolution is 0.5~4.0 mol / L, for example, it can be 0.5 mol / L, 0.6 mol / L, 0.8 mol / L, 1.0 mol / L, 2.0 mol / L, 3.0 mol / L, 4.0 mol / L, etc.

[0074] As an optional implementation, adjusting the pH value to a range that matches the eluent means adjusting the pH value of the solution to 4.0~7.0, for example, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, etc.

[0075] In a preferred embodiment, adjusting the pH value to a range that matches the eluent means adjusting the pH value of the solution to 5.0~6.0.

[0076] It should be noted that the pH value of the sample solution to be tested and the pH value of the eluent used are matched, meaning that the difference between their pH values ​​is within ±0.1.

[0077] As an optional implementation, the reagent used to adjust the pH value is selected from ammonia and / or acetic acid.

[0078] As an optional implementation, during the preparation of the test sample solution, the mass m of the lithium iron phosphate sample, the volume V of the final volume, and the dilution factor D should be recorded.

[0079] As an optional implementation, the deoxygenation treatment method includes inert gas bubbling and / or ultrasonic-assisted degassing.

[0080] As an optional implementation, the sample solution to be tested needs to be filtered before being injected into the chromatograph.

[0081] As an optional implementation, the filtration includes: filtering the treated sample solution through a 0.22 µm filter membrane and then injecting it into the chromatograph.

[0082] As an optional implementation, the chromatograph includes any one of ion chromatography (IC) and liquid chromatography (LC).

[0083] It should be noted that in this invention, both ion chromatography (IC) and liquid chromatography (LC) employ strong anion exchange columns as the core separation component. The principle is not to directly separate iron cations, but rather to utilize the positively charged functional groups (such as quaternary ammonium salts) on the surface of the column packing material and the negatively charged divalent iron-specific complexing agent complex (e.g., [Fe(II)-(PDCA)2]²) in the test solution. - ) and trivalent iron-specific complexing agents (e.g., [Fe(III)-(PDCA)2]- Differential ion exchange occurs, thereby achieving baseline separation of iron complexes with different valence states. Furthermore, regardless of the type of chromatograph used, a metal-free tubing system (such as PEEK) is recommended to minimize interference from background metal dissolution on trace iron detection.

[0084] In a preferred embodiment, the strong anion exchange column contains a hydrophilic polymer matrix packing material with quaternary ammonium salt functional groups.

[0085] It should be noted that this invention preferably employs a strong anion exchange column with a hydrophilic polymer matrix packing material incorporating quaternary ammonium salt functional groups. Compared to traditional silica gel-based columns, the polymer matrix has an extremely low background of metal impurities, which is crucial for trace detection in ICP-MS. Simultaneously, it exhibits excellent chemical stability over a wide pH range (especially under weakly acidic to neutral conditions), and is not prone to matrix hydrolysis. Combined with the strong anion exchange groups of quaternary ammonium salts, this type of column is particularly well-suited for the separation of negatively charged iron-complexes (such as Fe-PDCA anions). Its highly inert surface effectively prevents non-specific adsorption, dissociation, or catalytic redox reactions of iron complexes on the column, thereby ensuring symmetrical peak shapes and stable retention times, significantly improving the resolution and quantitative repeatability of separating ferrous and ferric iron.

[0086] As an optional implementation, the specifications of the strong anion exchange chromatographic column are: column length ranging from 50 to 250 mm, inner diameter from 2.0 to 4.6 mm, and particle size from 3 to 10 μm.

[0087] As an optional implementation, the strong anion exchange column includes, but is not limited to, ThermoScientific IonPac AS series columns, Agilent Bio SAX series columns, or anion exchange columns with equivalent performance.

[0088] As an optional implementation, the eluent used for separation includes: a specific complexing agent, a volatile buffer salt, a pH adjuster, and a solvent.

[0089] It should be noted that, in order to ensure the thermodynamic stability of the iron-complex during the separation process and prevent on-column dissociation, the present invention employs a specific combination of complexing agent and volatile buffer salt in the dynamic phase system; furthermore, a pH adjuster is used to precisely control the acid-base environment to match the optimal stability region of the complex, constructing a chemical microenvironment completely consistent with the sample pretreatment, effectively inhibiting the dissociation, hydrolysis, or valence state transformation of the iron complex during chromatography; the volatile components also prevent the deposition of non-volatile salts at the ICP-MS cone, ensuring long-term stable operation of the instrument and balancing separation selectivity and mass spectrometry compatibility.

[0090] As an optional implementation, the specific complexing agent in the eluent is selected from any one or a combination of at least two of pyridine-2,6-dicarboxylic acid (PDCA), ethylenediaminetetraacetic acid (EDTA), and citric acid.

[0091] In a preferred embodiment, the specific complexing agent in the eluent is pyridine-2,6-dicarboxylic acid.

[0092] As an optional implementation, the concentration of the specific complexing agent in the eluent is 0.5~10.0 mmol / L, for example, it can be 0.5 mmol / L, 1.0 mmol / L, 2.0 mmol / L, 3.0 mmol / L, 4.0 mmol / L, 5.0 mmol / L, 6.0 mmol / L, 7.0 mmol / L, 8.0 mmol / L, 9.0 mmol / L, 10.0 mmol / L, etc.

[0093] In a preferred embodiment, the concentration of the specific complexing agent in the eluent is 2.0~5.0 mmol / L.

[0094] It should be noted that, especially importantly, an additional complexing agent (such as PDCA) at a concentration of 0.5–10 mmol / L (preferably 2.0–5.0 mmol / L) should be added to the mobile phase, the same as that used in the pretreatment step, to inhibit the shift of chemical equilibrium.

[0095] As an optional implementation, the volatile buffer salt is selected from any one or a combination of at least two of ammonium acetate, ammonium hydroxide, or ammonium formate.

[0096] As an optional implementation, the concentration of the volatile buffer salt in the eluent is 20~200 mmol / L, for example, it can be 20 mmol / L, 25 mmol / L, 30 mmol / L, 35 mmol / L, 40 mmol / L, 45 mmol / L, 50 mmol / L, 55 mmol / L, 60 mmol / L, 65 mmol / L, 70 mmol / L, 75 mmol / L, 80 mmol / L, 85 mmol / L, 90 mmol / L, 95 mmol / L, 100 mmol / L, 110 mmol / L, 120 mmol / L, 130 mmol / L, 140 mmol / L, 150 mmol / L, 160 mmol / L, 170 mmol / L, 180 mmol / L, 190 mmol / L, 200 mmol / L, etc.

[0097] In a preferred embodiment, the concentration of the volatile buffer salt in the eluent is 50-100 mmol / L.

[0098] As an optional implementation, the pH adjuster in the eluent is selected from ammonia and / or acetic acid.

[0099] As an optional implementation, the pH of the eluent is 5.0 to 7.0, for example, it can be 5.0, 5.2, 5.4, 5.5, 5.6, 5.8, 6.0, 6.2, 6.4, 6.5, 6.6, 6.8, 7.0, etc.

[0100] As an optional implementation, the solvent in the eluent is ultrapure water.

[0101] As an optional implementation, the resistivity of the ultrapure water in the eluent is ≥18.2 MΩ·cm.

[0102] As an optional implementation, during the separation process, the elution procedure can be selected as isocratic elution or gradient elution based on the peak separation degree.

[0103] As an optional implementation, during the separation process, the flow rate of the eluent is 0.2~1.2 mL / min, for example, it can be 0.2 mL / min, 0.3 mL / min, 0.4 mL / min, 0.5 mL / min, 0.6 mL / min, 0.7 mL / min, 0.8 mL / min, 0.9 mL / min, 1.0 mL / min, 1.1 mL / min, 1.2 mL / min, etc.

[0104] As an optional implementation, during the separation process, the column temperature of the chromatographic column is 20~40℃, for example, it can be 20℃, 22℃, 24℃, 26℃, 28℃, 30℃, 32℃, 34℃, 36℃, 38℃, 40℃, etc.

[0105] As an optional implementation, an nebulizer interface is provided between the quantitative analysis inductively coupled plasma mass spectrometer (ICP-MS) and the aforementioned chromatograph to connect the two.

[0106] As an optional implementation, the column effluent is directly introduced into the concentric nebulizer of the ICP-MS.

[0107] As an optional implementation, the inductively coupled plasma mass spectrometer monitors the signal intensity of the main isotope m / z 56 in real time, and simultaneously monitors the signal intensity of the confirmatory ions m / z 54 and / or m / z 57. In cases where high concentrations of ferrous iron may cause detector saturation, quantification can be performed by monitoring the signal intensity of the confirmatory ions m / z 54 and / or m / z 57.

[0108] As an optional implementation method, quantification is performed using isotope dilution method (ID-ICP-MS): enriched isotopes (such as...) are added to the sample. 57 Fe-labeled Fe(II) can be absolutely quantified using isotope dilution mass spectrometry, which can further eliminate errors caused by matrix effects.

[0109] As an optional implementation, in order to reduce oxide interference and improve the signal-to-noise ratio, the sample introduction system is preferably equipped with a semiconductor-cooled swirling atomization chamber.

[0110] As an optional implementation, to ensure sufficient ionization of high-matrix samples, the parameters of the inductively coupled plasma mass spectrometer are set as follows: the temperature of the semiconductor-cooled cyclone atomization chamber is 2~5℃ (e.g., 2℃, 2.5℃, 3℃, 3.5℃, 4℃, 4.5℃, 5℃, etc.); the plasma radio frequency power is 1300~1600 W (e.g., 1300 W, 1350 W, 1400 W, 1450 W, 1500 W, 1550 W, 1600 W, etc.); and the sampling depth is 7~10 mm (e.g., 7 mm, 7.5 mm, 8 mm, 8.5 mm, 9 mm, 9.5 mm, 10 mm, etc.).

[0111] As an optional implementation, during the quantitative analysis using the inductively coupled plasma mass spectrometer, helium collision mode or hydrogen reaction mode is used to eliminate interference from polyatomic ions generated by the plasma.

[0112] It should be noted that the polyatomic ions include: interference 56 Fe polyatomic ions (e.g.) 40 Ar 16 O + , 40 Ca 16 O + ),interference 54 Fe polyatomic ions (e.g.) 40 Ar 14 N + , 38 Ar 16 O + , 37 Cl 16 O 1 H + ),interference 57 Fe polyatomic ions (e.g.) 40 Ar 16 OH + , 40 Ca 16 OH +Any one or at least two of the following.

[0113] As an optional implementation, in the helium collision mode, the helium flow rate is 4.0~6.0 mL / min, for example, it can be 4.0 mL / min, 4.5 mL / min, 5.0 mL / min, 5.5 mL / min, 6.0 mL / min, etc.

[0114] As an optional implementation, in the hydrogen reaction mode, the hydrogen flow rate is 2.0~8.0 mL / min, for example, it can be 2.0 mL / min, 2.5 mL / min, 3.0 mL / min, 3.5 mL / min, 4.0 mL / min, 4.5 mL / min, 5.0 mL / min, 5.5 mL / min, 6.0 mL / min, 6.5 mL / min, 7.0 mL / min, 7.5 mL / min, 8.0 mL / min, etc.

[0115] It should be noted that, in response to the interference of polyatomic ions generated by argon plasma, the present invention uses collision / reaction pool technology to eliminate it, specifically including any of the following modes: (1) Helium collision mode, where the helium flow rate is controlled at 4.0~6.0 mL / min, and the kinetic energy discrimination effect is used to preferentially attenuate the interference of polyatomic molecules; or, (2) Hydrogen reaction mode, where the hydrogen flow rate is controlled at 2.0~8.0 mL / min, and the interference of argon oxides is completely eliminated through charge transfer reaction.

[0116] As an optional implementation, under the above conditions, the mass spectrometer monitors the signal intensity of m / z 56 (the main isotope) in real time, and can simultaneously monitor m / z 54 or m / z 57 as auxiliary confirmatory ions as needed, thereby achieving accurate quantification of iron elements in different valence states in lithium iron phosphate materials.

[0117] As an optional implementation, the method for constructing the standard curve specifically includes the following steps: A series of standard solutions of ferrous salts with different concentrations and a series of standard solutions of ferric salts with different concentrations were prepared respectively. The standard solutions of ferrous salts and ferric salts were injected and analyzed under the same chromatographic and mass spectrometric conditions as the test sample. Standard curves for ferrous iron and ferric iron were constructed by plotting the mass concentration of iron in the corresponding valence state on the x-axis and the chromatographic peak area in the corresponding valence state on the y-axis.

[0118] It should be noted that, in the final data processing, to ensure the accuracy of quantification, a standard curve was established using a method matching the mobile phase matrix. Standard stock solutions of Fe(II) and Fe(III) were prepared separately and serially diluted to a series of mixed standard solutions (e.g., 0.1, 0.5, 1.0, 5.0, 10.0 mg / L) using diluents containing the same concentration of specific complexing agents and buffer salts (such as PDCA and ammonium acetate). At this point, the iron ions in the standard solutions had been converted to a complex form consistent with the analyte. The standard series was analyzed under the same chromatographic and mass spectrometric conditions. Linear regression equations for Fe(II) and Fe(III) were constructed, with iron concentration (X, mg / L) as the x-axis and the corresponding peak area (Y, CPS s) as the y-axis. When analyzing unknown samples, Fe(II) and Fe(III) were qualitatively identified based on the retention time determined by the standards using the obtained time-signal intensity chromatogram. The peak areas were integrated and substituted into the standard curve to calculate their absolute contents. Finally, by combining the sample weight (m), the fixed volume (V), and the dilution factor D, the mass fraction (w) of Fe(II) and Fe(III) in the lithium iron phosphate material is calculated, and the mass percentage of Fe(III) in the total iron content is further calculated, thereby completing the quantitative evaluation of the material quality.

[0119] As an optional implementation, the method for preparing the standard solution of the divalent ferric salt is as follows: To prepare standard stock solutions of ferrous salts, dilute them with a deoxygenated diluent containing specific complexing agents and volatile buffers of similar types and concentrations as those in the final injection solution and eluent, and with a pH value consistent with the eluent, to obtain a series of standard solutions of ferrous salts of different concentrations. To prepare standard stock solutions of ferric salts, dilute them with a deoxygenated diluent containing specific complexing agents and volatile buffers of similar types and concentrations as those in the final injection solution and eluent, and with a pH value consistent with the eluent, to obtain a series of standard solutions of ferric salts with different concentrations.

[0120] As an optional implementation, the divalent ferric salt is selected from ferrous ammonium sulfate; the trivalent ferric salt is selected from ferric chloride or ferric nitrate.

[0121] As a preferred embodiment, according to the national standard requirements for the content of iron elements in different valence states in lithium iron phosphate materials, the concentration gradient ranges of divalent iron and trivalent iron standard solutions are respectively set: the divalent iron salt standard solution covers a higher concentration range, wherein the concentration gradient of divalent iron ions is set between 5.0 mg / L and 50.0 mg / L to match the concentration of the main element after appropriate dilution for quantification; the trivalent iron salt standard solution covers a lower concentration range, wherein the concentration gradient of trivalent iron ions is set between 0.05 mg / L and 10.0 mg / L to meet the detection requirements for trace impurities or defects. As an optional embodiment, the calculation method includes: The peak areas of ferrous iron and ferric iron obtained from the above quantitative analysis are input into the standard curves of the corresponding valence states to obtain the actual concentration of iron in the corresponding valence states. Then, combined with the mass m of the sample, the volume V of the fixed volume and the dilution factor D, the mass percentage of ferrous iron and ferric iron in the sample are calculated. The formula for calculating the mass percentage of divalent iron in the sample is as follows:

[0122] in, This represents the mass percentage of ferrous iron. This represents the actual concentration of ferrous iron in the sample solution (in mg / L). Represents the volume at constant volume (in liters). The value represents the mass (in mg) of the lithium iron phosphate sample to be tested, and D represents the dilution factor. The formula for calculating the mass percentage of ferric iron in the sample is as follows:

[0123] in, This represents the mass percentage of ferric iron. This represents the actual concentration of ferric iron in the sample solution (in mg / L). Represents the volume at constant volume (in liters). The value represents the mass (in mg) of the lithium iron phosphate sample to be tested, and D represents the dilution factor.

[0124] As an optional implementation, the calculation also includes calculating the mass percentage of ferric iron in the total iron content, and the calculation formula is as follows:

[0125] in, This represents the mass percentage of ferric iron (Fe3+) in the total iron content. This represents the mass percentage of ferric iron. This represents the mass percentage content of divalent iron.

[0126] Secondly, this invention provides a quantitative analysis system for the chemical valence distribution of iron in lithium iron phosphate materials, such as... Figure 1 As shown, it includes: The protective dissolution and complexation device 100 has an inert atmosphere with an oxygen content ≤10 ppm (preferably ≤1 ppm) for dissolving the lithium iron phosphate sample in a non-oxidizing acid solution containing a specific complexing agent, and adjusting the pH value by adding a volatile buffer salt and then bringing the volume to obtain a test sample solution containing ferrous and ferric complexes; the device may also include a unit for diluting the test sample solution. The separation chromatograph 200 has a built-in strong anion exchange column and is equipped with an eluent containing a volatile buffer salt and the specific complexing agent. The pH value of the eluent is matched with that of the sample solution to be tested. It is used to process the sample solution to separate the complexes of iron elements in different valence states. The Inductively Coupled Plasma Mass Spectrometer 300 is equipped with a collision / reaction cell to receive the effluent from the column of iron with different valence states and to analyze the iron elements with different valence states in the sample to be tested. The data processing unit 400 is used for data analysis and processing.

[0127] As an optional implementation, the separation chromatograph 200 includes a liquid chromatograph or an ion chromatograph.

[0128] As an optional implementation, the quantitative analysis system further includes a four-way valve injector 500; The four-way valve injector 500 has one channel connected to the protective dissolution and complexation device 100 and the chromatographic column in the separation chromatogram 200, and the other channel connected to the mobile phase storage device 700 and the chromatographic column in the separation chromatogram 200.

[0129] As an optional implementation, a high-pressure pump 600 is provided between the mobile phase storage device 700 and the chromatographic column in the separation chromatogram 200, for extracting the mobile phase and then delivering it to the chromatographic column through a four-way valve injector.

[0130] The present invention will be further illustrated below through examples. Unless otherwise specified, the materials in the examples were prepared according to existing methods or purchased directly from the market. All chemical reagents used were of analytical grade or higher purity, and the experimental water was ultrapure water (resistivity ≥18.2 MΩ·cm).

[0131] Example 1 This embodiment provides a method for quantitative analysis of the chemical valence state distribution of iron in lithium iron phosphate materials. The method includes: (1) Protective dissolution and complexation: First, all liquid reagents required for the experiment (including dilute hydrochloric acid solution, ammonium acetate solution, ammonia, water for volume adjustment, and blank diluent) are thoroughly deoxygenated (e.g., bubbled with high-purity nitrogen for at least 30 minutes) to ensure extremely low dissolved oxygen content. The dissolution operation is performed in a glove box filled with high-purity nitrogen, with the oxygen content strictly controlled below 1 ppm. Accurately weigh the lithium iron phosphate powder sample (m = 0.1025 g) and place it in a sealed reaction flask. Add 20 mL of deoxygenated dilute hydrochloric acid solution containing pyridine-2,6-dicarboxylic acid (PDCA) (hydrochloric acid concentration of 4.0 mol / L, PDCA concentration of 10.0 mmol / L) for dissolution. After the sample is completely dissolved, add an appropriate amount of deoxygenated ammonium acetate solid (or a high-concentration solution) as a volatile buffer salt matrix, and add deoxygenated ammonia dropwise while continuously stirring, finely adjusting the pH of the solution to approximately 5.5. Subsequently, the solution was accurately brought to 100 mL (V = 100 mL) using deoxygenated water. Under these conditions, the Fe(II) and Fe(III) ions released from the sample were rapidly captured and locked in their structures by PDCA, forming stable complexes (Fe(II)-PDCA and Fe(III)-PDCA) at the current pH, thus obtaining the sample stock solution.

[0132] To match the instrument's detection range and reduce matrix effects, before injection, the sample stock solution was diluted 100-fold (D=100) with a deoxygenated blank diluent (whose matrix composition was consistent with that of the sample stock solution, i.e., containing the same concentration of PDCA and ammonium acetate, and with a pH of 5.5) to obtain the final injection solution.

[0133] (2) Chromatographic separation (IC unit): The final injection solution was filtered through a 0.22 µm filter membrane and then injected into the ion chromatograph (IC). The chromatographic separation step was performed using a strong anion exchange column, such as the Thermo Scientific IonPac AS7 column (the packing material is a hydrophilic polymer matrix with quaternary ammonium salt functional groups; specifications: 250 × 4.0 mm, particle size 9 μm). The mobile phase uses ultrapure water as the solvent and contains ammonium acetate at a concentration of 60 mmol / L as a volatile buffer salt. The pH of the mobile phase is strictly adjusted to 5.8 using ammonia water to match the optimal stability region of the complex. In addition, PDCA at a concentration of 3.0 mmol / L is added to the mobile phase to suppress the shift of the complexation equilibrium on the column and ensure sharp peaks. The elution program employed isocratic elution, with a flow rate controlled at 0.8 mL / min and a column temperature maintained at a constant 30°C. Under these conditions, baseline separation was achieved by utilizing the differences in charge density and hydrophobicity between the Fe(II)-PDCA and Fe(III)-PDCA complexes.

[0134] (3) ICP-MS detection: The elution of iron in different valence states from the chromatographic column was introduced into the nebulizer of an inductively coupled plasma mass spectrometer (ICP-MS) to perform quantitative analysis of iron in different valence states in the sample to be tested. To reduce oxide interference and improve the signal-to-noise ratio, the sample introduction system is preferably equipped with a semiconductor-cooled cyclone atomization chamber with a temperature set at 2°C; the plasma RF power is set to 1550 W and the sampling depth is 8.0 mm to ensure sufficient ionization of high matrix samples; for the polyatomic ion interference generated by argon plasma, this embodiment uses a helium collision mode to eliminate it, with the helium flow rate controlled at 4.5 mL / min, utilizing the kinetic energy discrimination effect to preferentially attenuate the interference of larger polyatomic molecules; Under the aforementioned conditions, the mass spectrometer monitors the signal intensity of m / z 56 (the main isotope) in real time, and can simultaneously monitor m / z 54 or m / z 57 as auxiliary confirmatory ions as needed, thereby achieving precise quantification of iron elements in different valence states in lithium iron phosphate materials.

[0135] (4) Data Processing and Quantitative Calculation: To ensure the accuracy of quantification, a strict matrix-matched external standard method was used to establish a standard curve. First, high-concentration standard stock solutions of Fe(II) and Fe(III) were prepared. Then, the stock solutions were serially diluted using a "blank diluent" (i.e., a solution containing the same concentration of PDCA and ammonium acetate as the final injection solution and with the same pH) to prepare a series of mixed standard solutions. The concentration gradient of the Fe(II) standard series was designed to cover the main content range: 0.2, 1.0, 5.0, 10.0, 25.0, 50.0 mg / L; the concentration gradient of the Fe(III) standard series was designed to cover the trace impurity range: 0.02, 0.05, 0.1, 0.5, 1.0, 5.0 mg / L. During this process, the iron ions in the standard solution were converted into a complex form completely consistent with the sample to be tested. The standard series solutions were injected and analyzed under the same chromatographic and mass spectrometric conditions as the samples. Linear regression equations for Fe(II) and Fe(III) were constructed with iron mass concentration (X, mg / L) as the abscissa and the chromatographic peak area (Y, CPS s) of the corresponding valence state as the ordinate. Y Fe(II) =18500X Fe(II) + 240; R Fe(II) 2 = 0.9998; Y Fe(III) = 19200X Fe(III) + 120; R Fe(III) 2 = 0.9996.

[0136] When analyzing unknown samples, a time-signal intensity chromatogram is obtained (e.g., Figure 2 (As shown). Based on the retention time determined by the standard, the chromatographic peaks of Fe(II) and Fe(III) were qualitatively identified. The peak areas of each chromatographic peak were integrated, and the peak areas were substituted into the corresponding standard curve equation to calculate the absolute mass concentration (c, mg / L) of Fe(II) and Fe(III) in the final injection solution. Finally, combining the sample weight (m), the final volume (V), and the dilution factor (D), the mass fraction (w) of Fe(II) and Fe(III) in the lithium iron phosphate material was calculated, and the mass percentage of Fe(III) in the total iron was further calculated, thus completing the quantitative evaluation of the material quality; whereby the absolute mass concentration c of ferrous iron in the final injection solution was... Fe(II) = 3.600 mg / L; Among them, the absolute mass concentration c of ferric iron in the final injected solution Fe(III) = 0.039 mg / L; The mass percentage of divalent iron in the sample. = 35.12%; The mass percentage of ferric iron in the sample. = 0.38%; Of which, trivalent iron accounts for the mass percentage of total iron. = 1.07%.

[0137] The average recovery rate of divalent iron after three spikes was 99.8%, and the average recovery rate of trivalent iron after three spikes was 98.5%.

[0138] Example 2 This embodiment provides a method for quantitative analysis of the chemical valence distribution of iron in lithium iron phosphate materials. The only difference between this method and Example 1 is the specific complexing agent and buffer system used.

[0139] The specific differences are as follows: In step (1), the specific complexing agent in the dilute hydrochloric acid solution used to dissolve the sample is replaced by PDCA with an equal molar concentration of ethylenediaminetetraacetic acid (EDTA, 10.0 mmol / L). When adjusting the pH value, ammonium acetate is not added; instead, deoxygenated ammonia water is added directly to adjust the pH value of the solution to about 5.5 (at this time, the buffer system formed in the solution mainly consists of EDTA and its ammonium salt, as well as ammonium chloride produced by neutralization). The matrix of the blank diluent and standard solution is also changed accordingly. In step (2), the specific complexing agent in the mobile phase is replaced with 3.0 mmol / L EDTA. The buffer salt system of the mobile phase is replaced with: using ultrapure water as a solvent, ammonium hydroxide is added to make its concentration reach about 60 mmol / L (using NH4) + The pH of the mobile phase was adjusted to 5.8 using deoxygenated acetic acid.

[0140] Other steps, including deoxygenation operation requirements, instrument conditions, standard curve construction planning and calculation methods, are consistent with those in Example 1.

[0141] The specific test results (for samples identical to those in Example 1) are as follows: Among them, the absolute mass concentration c of ferrous iron in the final injected solution Fe(II) = 3.593 mg / L; Among them, the absolute mass concentration c of ferric iron in the final injected solution Fe(III) = 0.043 mg / L; The mass percentage of divalent iron in the sample. = 35.05%; The mass percentage of ferric iron in the sample. = 0.42%; Of which, trivalent iron accounts for the mass percentage of total iron. = 1.18%.

[0142] The average recovery rate of divalent iron after three spikes was 99.5%, and the average recovery rate of trivalent iron after three spikes was 97.9%.

[0143] Example 3 This embodiment provides a method for quantitative analysis of the chemical valence distribution of iron in lithium iron phosphate materials. The only difference between this embodiment and Example 1 is the type of specific complexing agent and buffer salt used.

[0144] The specific differences are as follows: In step (1), the specific complexing agent in the dilute hydrochloric acid solution used to dissolve the sample is replaced by diethylenetriaminepentaacetic acid (DTPA, concentration 10.0 mmol / L) at an equal molar concentration. When adjusting the pH value, the added volatile buffer salt is replaced by ammonium acetate with deoxygenated ammonium formate, and the pH value is adjusted to about 5.5 with deoxygenated ammonia or formic acid. The matrix of the blank diluent and standard solution is also changed accordingly. In step (2), the specific complexing agent in the mobile phase is replaced by 3.0 mmol / L DTPA. The volatile buffer salt in the mobile phase is replaced by ammonium acetate with ammonium formate at a concentration of 60 mmol / L, and the pH value of the mobile phase is adjusted to 5.8 with formic acid or ammonia.

[0145] Other steps, including deoxygenation operation requirements, instrument conditions, standard curve construction planning and calculation methods, are consistent with those in Example 1.

[0146] The specific test results (for samples identical to those in Example 1) are as follows: Among them, the absolute mass concentration c of ferrous iron in the final injected solution Fe(II) = 3.606 mg / L; Among them, the absolute mass concentration c of ferric iron in the final injected solution Fe(III) = 0.036 mg / L; The mass percentage of divalent iron in the sample. = 35.18%; The mass percentage of ferric iron in the sample. = 0.35%; Of which, trivalent iron accounts for the mass percentage of total iron. = 0.99%.

[0147] The average recovery rate of divalent iron after three spikes was 99.3%, and the average recovery rate of trivalent iron after three spikes was 98.2%.

[0148] Example 4 This embodiment provides a method for quantitative analysis of the chemical valence distribution of iron in lithium iron phosphate materials. The only difference from Embodiment 1 is that the type of chromatograph and the model of chromatographic column used in step (2) are slightly different. The specific differences are as follows: In step (2), a high-performance liquid chromatograph (HPLC) is used as the separation unit (its core separation principle is still ion exchange). Chromatographic separation uses an Agilent Bio SAX series strong anion exchange column (the packing material is a hydrophilic polymer matrix with quaternary ammonium salt functional groups; specifications: 250 × 4.6 mm, particle size 5 μm). The mobile phase composition is the same as in Example 1 (containing 60 mmol / L ammonium acetate and 3.0 mmol / L PDCA, pH 5.8), the elution program is isocratic elution is selected, the flow rate is controlled at 0.8 mL / min, and the column temperature is maintained at 35 ℃.

[0149] Other steps, including the preprocessing in step (1), the ICP-MS detection parameters (He mode) in step (3), and the data processing in step (4), are consistent with those in Example 1.

[0150] The specific test results (for samples identical to those in Example 1) are as follows: Among them, the absolute mass concentration c of ferrous iron in the final injected solution Fe(II) = 3.596 mg / L; Among them, the absolute mass concentration c of ferric iron in the final injected solution Fe(III) = 0.041 mg / L; The mass percentage of divalent iron in the sample. = 35.08%; The mass percentage of ferric iron in the sample. = 0.40%; Of which, trivalent iron accounts for the mass percentage of total iron. = 1.13%.

[0151] The average recovery rate of divalent iron after three spikes was 99.1%, and the average recovery rate of trivalent iron after three spikes was 97.3%.

[0152] Example 5 This embodiment provides a quantitative analysis method for the chemical valence distribution of iron in lithium iron phosphate materials. The only difference between this embodiment and Example 1 is that the ICP-MS interference elimination mode in step (3) is different.

[0153] The specific differences are as follows: In step (3), the interference of polyatomic ions generated by argon plasma (such as...)40 Ar 16 O + Elimination is achieved using a hydrogen (H2) reaction mode. Specifically, high-purity hydrogen is introduced into the collision / reaction tank at a flow rate controlled at 5.0 mL / min. This utilizes the efficient charge transfer or atom transfer reaction between hydrogen and interfering ions (e.g., 40 Ar 16 O + +H2→ 40 Ar + H2O+ or other neutral products), thereby completely eliminating its interference with the iron signal at m / z 56, while the iron ions to be measured hardly react with hydrogen.

[0154] Other steps, including pretreatment, chromatographic separation conditions, and data processing methods, are consistent with those in Example 1.

[0155] The specific test results (for samples identical to those in Example 1) are as follows: Among them, the absolute mass concentration c of ferrous iron in the final injected solution Fe(II) = 3.603 mg / L; Among them, the absolute mass concentration c of ferric iron in the final injected solution Fe(III) = 0.037 mg / L; The mass percentage of divalent iron in the sample. = 35.15%; The mass percentage of ferric iron in the sample. = 0.36%; Of which, trivalent iron accounts for the mass percentage of total iron. = 1.01%.

[0156] The average recovery rate of divalent iron after three spikes was 99.8%, and the average recovery rate of trivalent iron after three spikes was 98.0%.

[0157] Comparative Example 1 This comparative example provides a method for quantitative analysis of the chemical valence distribution of iron in lithium iron phosphate materials. The only difference from Example 1 is that PDCA is no longer added in step (1), while the other steps are the same as in Example 1.

[0158] Due to the lack of a complexing agent during the dissolution process, Fe(II) underwent significant oxidation, and some uncomplexed Fe(III) was lost as precipitate formed during pH adjustment, resulting in severely distorted quantitative results. Specific detection results (for samples identical to those in Example 1): Among them, the absolute mass concentration c of ferrous iron in the final injected solution Fe(II) = 2.921 mg / L; Among them, the absolute mass concentration c of ferric iron in the final injected solution Fe(III) = 0.702 mg / L; The mass percentage of divalent iron in the sample. =28.50%; The mass percentage of ferric iron in the sample. =6.85%; Of which, trivalent iron accounts for the mass percentage of total iron. =19.38%.

[0159] The average recovery rate of divalent iron after three spikes was 75.2%, and the average recovery rate of trivalent iron after three spikes was 60.5%.

[0160] Comparative Example 2 This comparative example provides a method for quantitative analysis of the chemical valence distribution of iron in lithium iron phosphate materials. The only difference from Example 1 is that in step (1), PDCA is replaced with ascorbic acid of equal concentration. The other steps are the same as in Example 1.

[0161] Because of the addition of the strong reducing agent ascorbic acid, the trace amounts of Fe(III) originally present in the sample were reduced to Fe(II), making oxidation defects in the material undetectable. Specific detection results (for samples identical to those in Example 1): Among them, the absolute mass concentration c of ferrous iron in the final injected solution Fe(II) = 3.637 mg / L; Among them, the absolute mass concentration c of ferric iron in the final injected solution Fe(III) = 0 mg / L; The mass percentage of divalent iron in the sample. =35.48%; The mass percentage of ferric iron in the sample. =ND (Not Detected); Of which, trivalent iron accounts for the mass percentage of total iron. =0%.

[0162] The average recovery rate of divalent iron after three spikes was 100.5%, and the average recovery rate of trivalent iron after three spikes was 2.1%.

[0163] In summary, the methods provided in each embodiment of this invention can effectively achieve the separation and direct quantification of divalent and trivalent iron in lithium iron phosphate materials. Through strict oxygen removal protection and specific complexation, errors in valence state conversion are avoided. Using chromatography-mass spectrometry (GC-MS), independent chromatographic peaks of divalent and trivalent iron can be directly observed, eliminating the logical defects of traditional calculation methods. This method possesses extremely high sensitivity, capable of detecting trace amounts of Fe(III) impurities, helping researchers discover extremely small oxidation defects. The chromatograms not only distinguish valence states but also identify the presence of other forms of iron complexes, providing more structural information.

[0164] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for quantitative analysis of the chemical valence state distribution of iron in lithium iron phosphate materials, characterized in that, include: The non-oxidizing acid solution used to dissolve the sample, as well as the volatile buffer salt, pH adjuster, blank diluent and volume-determining water added subsequently, were deoxygenated. Under an inert atmosphere with an oxygen content ≤10 ppm, the lithium iron phosphate sample was dissolved in a non-oxidizing acid solution containing a specific complexing agent that had been deoxygenated. Then, a volatile buffer salt that had been deoxygenated was added and the pH value was adjusted to match the pH value of the eluent used for subsequent chromatographic separation. The solution was then diluted to volume with deoxygenated water to obtain a test sample solution containing ferrous and ferric complexes. The sample solution to be tested was diluted with a deoxygenated blank diluent containing the same concentration of volatile buffer salt and specific complexing agent to obtain the final injection solution. The final injection solution was introduced into the chromatograph and separated using a strong anion exchange column to obtain column eluents of iron elements in different valence states. The eluent used for separation contained the volatile buffer salt and the specific complexing agent, and its pH value matched that of the final injection solution. The elution of iron in different valence states from the chromatographic column was introduced into an inductively coupled plasma mass spectrometer, and the polyatomic ion interference was eliminated by using collision or reaction modes before detection. Standard curves for ferrous iron (Fe2+) and ferric iron (Fe3+) were constructed separately. The quantitative analysis results were substituted into the standard curves, and the mass percentages of ferrous iron and ferric iron in the samples were calculated.

2. The method for quantitative analysis of the chemical valence state distribution of iron in lithium iron phosphate materials according to claim 1, characterized in that, The inert atmosphere is selected from nitrogen and / or argon; Preferably, the oxygen content in the inert atmosphere is ≤1 ppm; Preferably, the specific complexing agent is selected from any one or a combination of at least two of pyridine-2,6-dicarboxylic acid, ethylenediaminetetraacetic acid, citric acid, diethylenetriaminepentaacetic acid, and hypozonotriacetic acid; Preferably, in the non-oxidizing acid solution and the final diluted sample solution, the concentration of the specific complexing agent is 2.0~20.0 mmol / L, more preferably 4.0~10.0 mmol / L; Preferably, the non-oxidizing acid is hydrochloric acid; Preferably, in the non-oxidizing acid solution used for dissolution, the concentration of the non-oxidizing acid is 0.5~4.0 mol / L; Preferably, the pH value is adjusted to a range that matches the eluent: the pH value of the solution is adjusted to 4.0~7.0, preferably 5.0~6.0; Preferably, the reagent used to adjust the pH value is selected from ammonia and / or acetic acid; Preferably, during the preparation of the test sample solution, the mass m of the lithium iron phosphate sample, the volume V of the final volume, and the dilution factor D should be recorded. Preferably, the deoxygenation treatment method includes inert gas bubbling and / or ultrasonic-assisted degassing.

3. The method for quantitative analysis of the chemical valence state distribution of iron in lithium iron phosphate materials according to claim 1, characterized in that, The chromatograph includes an ion chromatograph or a liquid chromatograph; Preferably, the strong anion exchange column contains a hydrophilic polymer matrix packing material with quaternary ammonium salt functional groups, and the specifications of the strong anion exchange column are: column length range of 50~250 mm, inner diameter of 2.0~4.6 mm, and particle size of 3~10 μm.

4. The method for quantitative analysis of the chemical valence state distribution of iron in lithium iron phosphate materials according to claim 1 or 3, characterized in that, The eluent used for the separation includes: a specific complexing agent, a volatile buffer salt, a pH adjuster, and a solvent; Preferably, the specific complexing agent in the eluent is selected from any one or a combination of at least two of pyridine-2,6-dicarboxylic acid, ethylenediaminetetraacetic acid, and citric acid, and is preferably pyridine-2,6-dicarboxylic acid; Preferably, the concentration of the specific complexing agent in the eluent is 0.5~10.0 mmol / L, more preferably 2.0~5.0 mmol / L; Preferably, the volatile buffer salt is selected from any one or a combination of at least two of ammonium acetate, ammonium hydroxide, or ammonium formate; Preferably, the concentration of the volatile buffer salt in the eluent is 20-200 mmol / L, more preferably 50-100 mmol / L; Preferably, the pH adjuster in the eluent is selected from ammonia and / or acetic acid; Preferably, the pH of the eluent is 5.0 to 7.0; Preferably, the solvent in the eluent is ultrapure water; wherein the resistivity of the ultrapure water is ≥18.2 MΩ·cm; Preferably, during the separation process, the flow rate of the eluent is 0.2~1.2 mL / min; Preferably, during the separation process, the column temperature of the chromatographic column is 20~40℃.

5. The method for quantitative analysis of the chemical valence state distribution of iron in lithium iron phosphate materials according to claim 1, characterized in that, The inductively coupled plasma mass spectrometer monitors the signal intensity of the main isotope m / z 56 in real time, and simultaneously monitors the signal intensity of the auxiliary confirmatory ions m / z 54 and / or m / z 57. Preferably, the parameter settings of the inductively coupled plasma mass spectrometer include: a semiconductor-cooled cyclone atomization chamber temperature of 2~5℃; a plasma radio frequency power of 1300~1600 W; and a sampling depth of 7~10 mm.

6. The method for quantitative analysis of the chemical valence state distribution of iron in lithium iron phosphate materials according to claim 1 or 5, characterized in that, During quantitative analysis using the aforementioned inductively coupled plasma mass spectrometer, helium collision mode or hydrogen reaction mode is used to eliminate interference from polyatomic ions generated by the plasma. Preferably, in the helium collision mode, the helium flow rate is 4.0~6.0 mL / min; Preferably, in the hydrogen reaction mode, the hydrogen flow rate is 2.0~8.0 mL / min.

7. The method for quantitative analysis of the chemical valence state distribution of iron in lithium iron phosphate materials according to claim 1, characterized in that, The method for constructing the standard curve specifically includes the following steps: A series of standard solutions of ferrous salts with different concentrations and a series of standard solutions of ferric salts with different concentrations were prepared respectively. The standard solutions of ferrous salts and ferric salts were injected and analyzed under the same chromatographic and mass spectrometric conditions as the test sample. Standard curves for ferrous iron and ferric iron were constructed by plotting the mass concentration of iron in the corresponding valence state on the x-axis and the chromatographic peak area in the corresponding valence state on the y-axis.

8. The method for quantitative analysis of the chemical valence state distribution of iron in lithium iron phosphate materials according to claim 7, characterized in that... The method for preparing the standard solutions of the ferrous salts and ferric salts is as follows: To prepare standard stock solutions of ferrous salts, dilute them with a diluent containing a specific complexing agent and a volatile buffer salt of similar type and concentration to the final injection solution and the eluent, and with a pH value consistent with the eluent, to obtain a series of standard solutions of ferrous salts with different concentrations. To prepare standard stock solutions of ferric salts, dilute them with a diluent containing a specific complexing agent and a volatile buffer salt of similar type and concentration to the final injection solution and the eluent, and with a pH value consistent with the eluent, to obtain a series of standard solutions of ferric salts with different concentrations. Preferably, the divalent ferric salt is selected from ferrous ammonium sulfate; the trivalent ferric salt is selected from ferric chloride or ferric nitrate. Preferably, the concentration gradient of the standard solution of the divalent ferric salt is set in the range of 0.01~1000 mg / L; the concentration gradient of the standard solution of the trivalent ferric salt is set in the range of 0.001~100 mg / L.

9. A quantitative analysis system for the chemical valence state distribution of iron in lithium iron phosphate materials, characterized in that, include: A protective dissolution and complexation device is provided, which has an inert atmosphere with an oxygen content ≤10 ppm (preferably ≤1 ppm) for dissolving the lithium iron phosphate sample in a non-oxidizing acid solution containing a specific complexing agent, and then adjusting the pH value by adding a volatile buffer salt and bringing the volume to obtain a test sample solution containing ferrous and ferric complexes; the device may also include a unit for diluting the test sample solution. A separation chromatograph, wherein the chromatograph has a built-in strong anion exchange column and is equipped with an eluent containing a volatile buffer salt and the specific complexing agent, and the pH value of the eluent is matched with that of the sample solution to be tested, for processing to separate the complexes of iron elements in different valence states in the sample solution to be tested; The inductively coupled plasma mass spectrometer is equipped with a collision / reaction cell to receive the effluent of iron in different valence states from the chromatographic column and to analyze the iron in different valence states in the sample to be tested. The data processing unit is used for data analysis and processing.

10. The quantitative analysis system for the chemical valence state distribution of iron in lithium iron phosphate materials according to claim 9, characterized in that, The separation chromatograph includes a liquid chromatograph or an ion chromatograph; And / or, the quantitative analysis system further includes a four-way valve injector; wherein, one passage of the four-way valve injector is connected to the protective dissolution and complexation device and the chromatographic column in the separation chromatogram, and the other passage is connected to the mobile phase storage device and the chromatographic column in the separation chromatogram; And / or, a high-pressure pump is provided between the mobile phase storage device and the chromatographic column in the separation chromatograph, for extracting the mobile phase and then delivering it to the chromatographic column through a four-way valve injector.

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