Method for detecting content of ferrous phosphide in lithium iron phosphate

Abstract

The invention provides a method for detecting the content of ferrous phosphide in lithium iron phosphate, and belongs to the technical field of chemical analysis. The detection method comprises the following steps: soaking a lithium iron phosphate sample in hydrochloric acid to dissolve lithium iron phosphate to obtain a mixed solution; then carrying out solid-liquid separation, and cleaning solid substances; the cleaned solid matter is digested with nitric acid, and a digestion solution is obtained; and detecting the contents of Li < + > and Fe < 3 + > in the digestion solution, and calculating to obtain the content of ferrous phosphide in the lithium iron phosphate. The detection method provided by the invention has the advantages of stable and accurate detection result, low consumption of samples and short detection period.

Description

Technical Field

[0001] The invention belongs to the technical field of chemical analysis, and in particular relates to a method for detecting the content of ferrous phosphide in lithium iron phosphate. Background Art

[0002] With the widespread application of lithium-ion batteries in electric vehicles and other fields, the market has higher and higher requirements for their safety and energy density. Correspondingly, the control of metal foreign matter in the positive electrode materials of lithium-ion batteries has become more and more stringent. Metal foreign matter will be oxidized into metal ions during the battery charging and discharging process, penetrate the diaphragm and deposit on the negative electrode. The metal element that is continuously deposited on the negative electrode will cause self-discharge, and even worse, cause the risk of short circuit. Therefore, the control of metal foreign matter is particularly important.

[0003] Lithium iron phosphate (LiFePO 4 , LFP) positive electrode material has the advantages of high safety, long cycle life, and good high temperature performance. It is one of the commonly used positive electrode materials in lithium-ion batteries. However, the specific capacity of lithium iron phosphate is relatively low. In order to improve the energy density of lithium-ion batteries, it can be achieved by increasing the compaction density of the positive electrode material. At present, the industry often increases the compaction density of lithium iron phosphate by increasing the sintering temperature, but the increase in temperature will cause the ferrous phosphide (Fe 2 As the content of ferrous phosphide increases, it brings risks to the control of metal foreign matter. Therefore, it is necessary to develop a detection method for the content of ferrous phosphide in lithium iron phosphate in order to monitor the quality of lithium iron phosphate positive electrode materials.

[0004] There are few methods for detecting iron phosphide at present. CN116148242A discloses a method for detecting magnetic metal foreign matter and iron phosphide content in lithium iron phosphate powder, and CN115356178A discloses a method for detecting magnetic foreign matter in lithium iron phosphate. Both methods are to adsorb iron phosphide from lithium iron phosphate powder by adsorption with a magnetic bar for detection. However, after actual measurement, the adsorbate was observed under a scanning electron microscope and found that the adsorbate was basically lithium iron phosphate, not iron phosphide. A considerable amount of iron phosphide still remained in the lithium iron phosphate powder, and the test results were inaccurate and fluctuated greatly. Moreover, the method of magnetic bar adsorption requires kilogram-level materials, consumes a large amount of samples and takes a long time for pre-treatment, and manual operation has a great influence on the results. Summary of the invention

[0005] In view of the shortcomings of the prior art, the present invention aims to provide a method for detecting the content of ferrous phosphide in lithium iron phosphate. The detection method provided by the present invention has the advantages of stable and accurate detection results, small sample consumption (only a few grams), and short detection cycle.

[0006] To achieve this object, the present invention adopts the following technical solutions:

[0007] In a first aspect, the present invention provides a method for detecting the content of ferrous phosphide in lithium iron phosphate, the detection method comprising the following steps:

[0008] (1) soaking the lithium iron phosphate sample in hydrochloric acid to dissolve the lithium iron phosphate and obtain a mixed solution;

[0009] (2) separating the mixed solution obtained in step (1) into solid and liquid, and washing the solid matter;

[0010] (3) digesting the cleaned solid matter obtained in step (2) with nitric acid to obtain a digestion solution;

[0011] (4) Detecting Li in the digestion solution + and Fe 3+ The content of ferrous phosphide in lithium iron phosphate is obtained according to the following formula;

[0012]

[0013] Among them, w Fe is the Fe 3+ The mass percentage, w Li is the Li in the digestion solution + The mass percentage, M Fe is the molar mass of Fe, M Li is the molar mass of Li, m 1 is the mass of the lithium iron phosphate sample, m 2 is the mass of the digestion solution, W Fe2P is the mass percentage of ferrous phosphide in the lithium iron phosphate sample.

[0014] In some embodiments of the present invention, the mass concentration of the hydrochloric acid is 5-20%.

[0015] In some embodiments of the present invention, the soaking time is 15-60 minutes.

[0016] In some embodiments of the present invention, step (2) comprises:

[0017] Filtering the mixed solution obtained in step (1), and adding water to wash it to obtain a first filter cake;

[0018] ultrasonically cleaning the first filter cake and the filter membrane in water;

[0019] The mixed liquid obtained after the ultrasonic cleaning is filtered, and water is added for cleaning to obtain a second filter cake, which is the cleaned solid matter.

[0020] In some embodiments of the present invention, the ultrasonic cleaning time is 5-20 minutes.

[0021] In some embodiments of the present invention, the mass concentration of the nitric acid is 30-35%.

[0022] In some embodiments of the present invention, the temperature during digestion is 200-350°C;

[0023] In some embodiments of the present invention, the digestion time is 20-50 min.

[0024] In some embodiments of the present invention, in step (3), the cleaned solid matter obtained in step (2) is digested together with the filter membrane using nitric acid.

[0025] In some embodiments of the present invention, the detection of Li in the digestion solution + and Fe 3+ The method for determining the content of the compound is inductively coupled plasma optical emission spectrometry.

[0026] In some embodiments of the present invention, the mass of the lithium iron phosphate sample is 1-5g.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] The detection method provided by the present invention extracts and detects ferrous phosphide by a chemical method, thereby reducing the influence of human operation factors on the detection results, and has the advantages of stable and accurate detection results, small sample consumption (only a few grams), and short detection cycle. DETAILED DESCRIPTION

[0029] In order to more clearly understand the above-mentioned objectives, features and advantages of the present invention, the scheme of the present invention will be further described below. It should be noted that the embodiments of the present invention and the features in the embodiments can be combined with each other without conflict.

[0030] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein; it is obvious that the embodiments in the specification are only part of the embodiments of the present invention, rather than all of the embodiments.

[0031] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by technicians in the technical field to which this application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit this application.

[0032] In a first aspect, an embodiment of the present invention provides a method for detecting the content of ferrous phosphide in lithium iron phosphate, the detection method comprising the following steps:

[0033] (1) soaking the lithium iron phosphate sample in hydrochloric acid to dissolve the lithium iron phosphate and obtain a mixed solution;

[0034] (2) separating the mixed solution obtained in step (1) into solid and liquid, and washing the solid matter;

[0035] (3) digesting the cleaned solid matter obtained in step (2) with nitric acid to obtain a digestion solution;

[0036] (4) Detecting Li in the digestion solution + and Fe 3+ The content of ferrous phosphide in lithium iron phosphate is obtained according to the following formula;

[0037]

[0038] Among them, w Fe is the Fe 3+ The mass percentage, w Li is the Li in the digestion solution + The mass percentage, M Fe is the molar mass of Fe, M Li is the molar mass of Li, m 1 is the mass of the lithium iron phosphate sample, m 2 is the mass of the digestion solution, W Fe2P is the mass percentage of ferrous phosphide in the lithium iron phosphate sample.

[0039] It is easy for a person skilled in the art to understand that the above calculation formula is only a kind of operation logic, which does not represent the calculation steps. In addition, the actual calculation process can be performed by a detection instrument, or by a person, or by a detection instrument and a person together. Therefore, the data output by the detection instrument may not be the Li content in the digestion solution. + and Fe 3+ The content is not the intermediate result after a certain operation. For example, after the initial parameters are input, the inductively coupled plasma optical emission spectrometer (ICP-OES) can directly output the Fe content in the lithium iron phosphate sample (corresponding to the Fe content in the digestion solution). 3+ ) and Li (corresponding to Li + ), but the overall calculation logic is still the same as Formula I and Formula II.

[0040] The detection method provided by the present invention first dissolves lithium iron phosphate and other soluble impurities with hydrochloric acid to separate ferrous phosphide solid, then digests the ferrous phosphide solid with nitric acid (in this process, divalent iron will be oxidized to trivalent iron) and detects Li in the digestion solution. + and Fe 3+ Among them, due to the ferrous phosphide (Fe 2P) The solid may contain a small amount of undissolved lithium iron phosphate (LiFePO 4 ), which will lead to Fe 3+ Content Fe Higher than Fe from ferrous phosphide 3+ The content of w Fe Correction is made by subtracting Fe from lithium iron phosphate 3+ The content of Fe (Formula I). 3+ Content w′ Fe That is, Fe from ferrous phosphide 3+ The content in the digestion solution, combined with formula II, can be used to calculate the content of ferrous phosphide in the lithium iron phosphate sample.

[0041] The invention extracts and detects ferrous phosphide by chemical methods, reduces the influence of human operation factors on the detection results, has the advantages of stable and accurate detection results, small consumption of samples (only a few grams), and short detection cycle.

[0042] It should be noted that, since the environment in the preparation process of lithium iron phosphate is a reducing environment (such as carbon), the impurities generated are mainly ferrous phosphide (ferrous iron), and the content of ferric phosphide (ferric iron) is very small and can be ignored. In addition, since the accuracy of inductively coupled plasma optical emission spectroscopy (ICP-OES) in detecting P content is relatively low, in the embodiment of the present invention, according to Fe 3+ Calculate the content of ferrous phosphide.

[0043] In some embodiments of the present invention, the mass concentration of the hydrochloric acid is 5-20%, for example, 5%, 8%, 10%, 12%, 15%, 18% or 20%, etc. However, the present invention is not limited to the listed values, and other values ​​not listed within the range are also applicable.

[0044] In the embodiment of the present invention, the amount of hydrochloric acid used can be conventionally selected by those skilled in the art to ensure that the lithium iron phosphate can be fully dissolved. As a non-limiting example, based on the mass of the lithium iron phosphate sample of 1-5g, the amount of hydrochloric acid used can be 50-200mL.

[0045] In the embodiment of the present invention, the soaking time can be conventionally selected by those skilled in the art to ensure that the lithium iron phosphate is fully dissolved. As a non-limiting example, the soaking time can be 15-60 minutes; for example, it can be 15 minutes, 20 minutes, 25 minutes, 30 minutes, 35 minutes, 40 minutes, 45 minutes, 50 minutes, 55 minutes or 60 minutes. However, the present invention is not limited to the listed values, and other values ​​not listed in the range are also applicable.

[0046] In some embodiments of the present invention, step (2) comprises:

[0047] S1: filtering the mixed solution obtained in step (1), and adding water to wash it to obtain a first filter cake;

[0048] S2: ultrasonically cleaning the first filter cake and the filter membrane in water;

[0049] S3: filtering the mixed solution obtained after the ultrasonic cleaning, and adding water for cleaning to obtain a second filter cake, which is the cleaned solid matter.

[0050] In the embodiment of the present invention, the purpose of step (2) is to clean the separated ferrous phosphide solid and reduce the residual amount of lithium iron phosphate. The embodiment of the present invention does not impose any special restrictions on the conditions in step (2), and those skilled in the art can make conventional selections. As a non-limiting example, the filtration can be carried out in a sand core filter; the pore size of the filter membrane can be 0.45 μm; based on the mass of the lithium iron phosphate sample of 1-5g, the amount of water in step S1 can be 200-1000mL, the amount of water in step S2 can be 100-500mL, and the amount of water in step S3 can be 200-1000mL.

[0051] In some embodiments of the present invention, the ultrasonic cleaning time is 5-20 minutes, for example, 5 minutes, 8 minutes, 10 minutes, 12 minutes, 15 minutes, 18 minutes or 20 minutes, etc. However, the present invention is not limited to the listed values, and other values ​​not listed in the range are also applicable.

[0052] In some embodiments of the present invention, the mass concentration of the nitric acid is 30-35%, for example, 30%, 32%, 33%, 34% or 35%, etc. However, the present invention is not limited to the listed values, and other values ​​not listed within the range are also applicable.

[0053] In the embodiment of the present invention, there is no particular restriction on the amount of nitric acid used, which ensures that the solid material obtained in step (2) is fully digested and the ion concentration in the digestion solution is within a suitable detection range. As a non-limiting example, based on the mass of the lithium iron phosphate sample of 1-5g, the amount of nitric acid used can be 20-100mL.

[0054] In some embodiments of the present invention, the temperature during digestion is 200-350°C; for example, it can be 200°C, 220°C, 230°C, 250°C, 260°C, 280°C, 300°C, 320°C, 330°C or 350°C, etc. However, the present invention is not limited to the listed values, and other values ​​not listed in the range are also applicable.

[0055] In some embodiments of the present invention, the digestion time is 20-50 min; for example, it can be 20 min, 22 min, 25 min, 28 min, 30 min, 32 min, 35 min, 38 min, 40 min, 42 min, 45 min, 48 min or 50 min, etc. However, the present invention is not limited to the listed values, and other values ​​not listed within the range are also applicable.

[0056] In some embodiments of the present invention, in step (3), the cleaned solid matter obtained in step (2) is digested with nitric acid together with the filter membrane. This ensures that the solid matter is completely digested, and avoids residual solid matter on the filter membrane that affects the accuracy of the test results.

[0057] In some embodiments of the present invention, the detection of Li in the digestion solution + and Fe 3+ The method for detecting the content of Li in the digestion solution is inductively coupled plasma optical emission spectroscopy (ICP-OES). ICP-OES is a conventional detection method in the art, and the specific detection steps can be conventionally selected by those skilled in the art. + and Fe 3+ The method for detecting the content of Li is not limited to ICP-OES. + and Fe 3+ The method of determining the content of the compound can also be applied to the present invention.

[0058] The detection method provided in the embodiment of the present invention consumes little sample, and only a few grams are needed to complete the detection of the ferrous phosphide content. As a non-limiting example, the mass of the lithium iron phosphate sample can be 1-5g; for example, it can be 1g, 2g, 3g, 4g or 5g, etc. However, the present invention is not limited to the listed values, and other values ​​not listed within the range are also applicable.

[0059] In order to make the technical problems, technical solutions and beneficial effects solved by the present invention clearer, the present invention will be further described in detail in conjunction with the embodiments below. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is by no means intended to limit the present invention and its applications. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work belong to the scope of protection of the present invention.

[0060] If no specific techniques or conditions are specified in the examples, the conventional techniques or conditions in the art, or the techniques or conditions described in the literature, or the product instructions are used. If no manufacturer is specified for the reagents or instruments used, they are all conventional products that can be purchased commercially.

[0061] Examples 1-9 and Comparative Examples 1-4

[0062] Example 1-9 and Comparative Example 1-4 respectively provide a method for detecting the content of ferrous phosphide in lithium iron phosphate, and the steps are as follows:

[0063] (1) To lithium iron phosphate raw material (mass m 10 ) is added with ferrous phosphide (mass m 11 ) as the lithium iron phosphate sample (mass m 1 ), and then immersed in hydrochloric acid to dissolve the lithium iron phosphate to obtain a mixed solution;

[0064] (2) The mixed solution obtained in step (1) was filtered using a sand core filter with a pore size of 0.45 μm, and then deionized water (volume v 1 ) to wash to obtain a first filter cake;

[0065] The first filter cake and the filter membrane are placed in a container containing deionized water (volume v 2 ) in a beaker and ultrasonically cleaned;

[0066] The mixed solution obtained after ultrasonic cleaning was filtered with a sand core filter with a pore size of 0.45 μm, and then deionized water (volume v 3 ) to wash to obtain a washed solid substance;

[0067] (3) digesting the cleaned solid matter obtained in step (2) with nitric acid to obtain a digestion solution;

[0068] (4) Prepare a series of Li-containing solutions with a concentration gradient + and Fe 3+ The standard solution was used to detect Li in the standard solution by ICP-OES. + , Fe 3+ The strength of Li + , Fe 3+ The concentration and corresponding intensity of the standard curve were established; ICP-OES was used to detect the Li in the digestion solution. + and Fe 3+ The intensity of Li in the digestion solution was calculated by combining the standard curve. + and Fe 3+ Content;

[0069] (5) The content of ferrous phosphide in the lithium iron phosphate sample is obtained according to the following formula;

[0070]

[0071] Among them, w Fe is the Fe3+ The mass percentage, w Li is the Li in the digestion solution + The mass percentage, M Fe is the molar mass of Fe, M Li is the molar mass of Li, m 1 is the mass of the lithium iron phosphate sample, m 2 is the mass of the digestion solution, W Fe2P is the mass percentage of ferrous phosphide in the lithium iron phosphate sample.

[0072] The operating conditions and test results in each embodiment and comparative example are shown in Table 1 and Table 2.

[0073] Table 1

[0074]

[0075]

[0076] Table 2

[0077]

[0078]

[0079] Examples 1-6 and Comparative Examples 1-2 add ferrous phosphide particles to lithium iron phosphate powder that does not contain ferrous phosphide (the added ferrous phosphide particles are samples extracted from other lithium iron phosphate powders containing ferrous phosphide) to verify the feasibility of the method. From the test results of Examples 1-6, it can be seen that when the amount of ferrous phosphide added is within the range of 1000-10000ppm, the recovery rate of ferrous phosphide measured by the method provided by the present invention is stable at about 95%, indicating that the method provided by the present invention has a wide range of application (ferrous phosphide content range) and the test results are relatively accurate.

[0080] Comparing Examples 1-4 with Comparative Example 1, it can be seen that the recovery rate of ferrous phosphide measured in Comparative Example 1 is higher, exceeding 100%. This is because the hydrochloric acid soaking time in Comparative Example 1 is shorter, resulting in undissolved lithium iron phosphate and other impurities in the first filter cake, which remain in the second filter cake after being washed with deionized water. After being digested with concentrated nitric acid, the undissolved lithium iron phosphate continues to dissolve, causing the Fe in the digestion solution to 3+ The content is high, even if Li + Even after the content was corrected, the recovery rate of ferrous phosphide was still high.

[0081] Compared with Examples 1-4, no ultrasonic cleaning was performed in Comparative Example 2, resulting in more residual lithium iron phosphate in the second filter cake, and thus also resulting in a higher recovery rate of ferrous phosphide, exceeding 100%.

[0082] Examples 7-9 and Comparative Examples 3-4 all use lithium iron phosphate powder containing ferrous phosphide for testing. Comparing Example 8 with Comparative Example 3, and Example 9 with Comparative Example 4, it can be seen that when the hydrochloric acid immersion time is short (Comparative Example 3) or ultrasonic cleaning is not performed (Comparative Example 4), the measured ferrous phosphide content is high, which further verifies the experimental results of Comparative Examples 1-2.

[0083] The above description is only a specific embodiment of the present disclosure, so that those skilled in the art can understand or implement the present disclosure. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present disclosure. Therefore, the present disclosure will not be limited to the embodiments described herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for detecting the content of ferrous phosphide in lithium iron phosphate, characterized in that: The detection method comprises the following steps: (1) soaking the lithium iron phosphate sample in hydrochloric acid to dissolve the lithium iron phosphate and obtain a mixed solution; (2) separating the mixed solution obtained in step (1) into solid and liquid, and washing the solid matter; (3) digesting the cleaned solid matter obtained in step (2) with nitric acid to obtain a digestion solution; (4) Detecting Li in the digestion solution + and Fe 3+ The content of ferrous phosphide in lithium iron phosphate is obtained according to the following formula; Among them, w Fe is the Fe 3+ The mass percentage, w Li is the Li in the digestion solution + The mass percentage, M Fe is the molar mass of Fe, M Li is the molar mass of Li, m1 is the mass of the lithium iron phosphate sample, m2 is the mass of the digestion solution, W Fe2P is the mass percentage of ferrous phosphide in the lithium iron phosphate sample.

2. The detection method according to claim 1, characterized in that: The mass concentration of the hydrochloric acid is 5-20%.

3. The detection method according to claim 1 or 2, characterized in that: The soaking time is 15-60 minutes.

4. The detection method according to any one of claims 1 to 3, characterized in that: Step (2) comprises: Filtering the mixed solution obtained in step (1), and adding water to wash it to obtain a first filter cake; ultrasonically cleaning the first filter cake and the filter membrane in water; The mixed liquid obtained after the ultrasonic cleaning is filtered, and water is added for cleaning to obtain a second filter cake, which is the cleaned solid matter.

5. The detection method according to claim 4, characterized in that: The ultrasonic cleaning time is 5-20 minutes.

6. The detection method according to any one of claims 1 to 5, characterized in that: The mass concentration of the nitric acid is 30-35%.

7. The detection method according to any one of claims 1 to 6, characterized in that: The temperature during the digestion is 200-350°C; And / or, the digestion time is 20-50 min.

8. The detection method according to any one of claims 1 to 7, characterized in that: In step (3), the washed solid matter obtained in step (2) is digested together with the filter membrane using nitric acid.

9. The detection method according to any one of claims 1 to 8, characterized in that: The method of detecting Li in the digestion solution + and Fe 3+ The method for determining the content of the compound is inductively coupled plasma optical emission spectrometry.

10. The detection method according to any one of claims 1 to 9, characterized in that: The mass of the lithium iron phosphate sample is 1-5g.

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