CN105842266A - Fluorescence analysis method for determining element content in lithium iron phosphate - Google Patents

Abstract

The invention discloses a fluorescence analysis method for determining the content of phosphorus and iron elements in lithium iron phosphate, which comprises dividing the lithium iron phosphate sample into three equally; using hydrochloric acid to treat the first lithium iron phosphate sample, and after filtering, dichromate Potassium standard solution was titrated to measure the iron content; the second lithium iron phosphate sample was treated with hydrochloric acid, and after filtration, the phosphorus content was measured by the ammonium phosphomolybdate volumetric method; the third lithium iron phosphate sample was prepared into Six standard samples; then pressed into flakes respectively, and then measure the fluorescence intensity of iron and phosphorus elements in each flake standard sample by X-ray fluorescence spectrometry; draw the XRF standard curve; press the lithium iron phosphate sample to be measured into In flakes, the fluorescence intensity is measured; through the XRF standard curve, the content of iron and phosphorus elements can be obtained. The invention first utilizes the wet chemical method combined with the X fluorescence spectrometry method to be fast and high-speed, and has simple operation, small error, long time consumption, and is environmentally friendly to environmental pollution.

Description

A Fluorescent Analysis Method for Determination of Element Content in Lithium Iron Phosphate

technical field

The invention relates to the technical field of lithium ion batteries, in particular to a method for determining the content of iron and phosphorus in lithium iron phosphate, a cathode material of lithium batteries, by using an X-ray fluorescence spectroscopic analysis method.

Background technique

Lithium iron phosphate is a phosphate with an olivine structure, and its structure is stable. It is one of the cathode materials commonly used in lithium-ion batteries. At present, industrially synthesized lithium iron phosphate also contains a small amount of impurities. Therefore, as a lithium ion battery cathode material, the content of its main elements iron and phosphorus is the key indicator of its qualification standard.

At present, the method for testing the content of iron and phosphorus in the lithium-ion battery industry mostly adopts "GB/T30835-2014 Carbon-composite Lithium Iron Phosphate Cathode Material for Lithium-ion Batteries". This method is a wet chemical method, which is cumbersome to operate, has large operating errors, takes a long time, and has many kinds of reagents; and the required reagents, such as potassium dichromate, also cause serious environmental pollution. Therefore, it is of great significance to develop a fast and green method to detect the element content in lithium iron phosphate.

X-ray fluorescence spectrometry (XRF) is a method with fast analysis speed, simple sample preparation, no need to use toxic and polluting reagents, and no time-consuming pretreatment process. It can be used to test the iron and The content of phosphorus element, this method relies on the standard curve established by the fluorescence intensity and content of the element in the standard sample. At present, XRF is mostly used in the analysis of samples in the fields of building materials, ceramics, and metal materials. For now, there is no report on the rapid measurement of iron and phosphorus in lithium iron phosphate cathode materials in the lithium ion battery industry using XRF.

Contents of the invention

The object of the present invention is to provide a method for detecting the content of iron and phosphorus in industrially synthesized lithium iron phosphate by providing a method with convenient operation, simple sample preparation, and the detection process does not require toxic and polluting reagents.

The purpose of the present invention can be achieved through the following technical solutions:

A fluorescence analysis method for determining the content of phosphorus and iron elements in lithium iron phosphate, comprising the following steps:

(1) Take the same batch of lithium iron phosphate samples and divide them into three equal parts;

(2) Treat the first lithium iron phosphate sample with hydrochloric acid, filter the unreacted carbon, and form the sample solution to be tested; titrate it with potassium dichromate standard solution, and measure the content of iron in the sample solution to be tested;

(3) Treat the second lithium iron phosphate sample with hydrochloric acid, filter the unreacted carbon, and form the sample solution to be tested; use the ammonium phosphomolybdate volumetric method to measure the content of phosphorus in the sample solution to be tested;

(4) Divide the third lithium iron phosphate sample into at least six equal parts, add calcium carbonate reference substances with different contents respectively, and prepare standard samples with different contents; then use powder compression method to compress each standard sample into tablets respectively Shape, then measure the fluorescence intensity of iron and phosphorus in each flake standard sample by X-ray fluorescence spectrometry;

(5) Compare the fluorescence intensity of iron and phosphorus elements in the lithium iron phosphate sample measured by X-ray fluorescence spectrometry in step (4) with the iron and phosphorus elements in the lithium iron phosphate sample measured in steps (2) and (3). and phosphorus content to form an XRF standard curve with the fluorescence intensity as the ordinate and the iron and phosphorus content as the abscissa;

(6) Press the lithium iron phosphate sample to be tested into a sheet using the same powder compression method as in step (4) to obtain the sample to be tested, and measure the fluorescence of each element in the sample to be tested by X-ray fluorescence spectrometry Intensity; through the XRF standard curve obtained in step (5), the content of iron and phosphorus elements in the sample to be tested can be obtained.

In a further scheme, in the step (2), the first lithium iron phosphate sample is added to the hydrochloric acid solution, heated to boil and kept for 18-25min, then cooled to room temperature, and unreacted carbon is filtered out, and the filtered solution is used The tin dichloride solution reduces the ferric iron in it, and then adds an indicator to form a solution to be tested.

In a further solution, the hydrochloric acid aqueous solution refers to the mixture of concentrated hydrochloric acid with a mass concentration of 36% and first-grade water according to a mass ratio of 1:1;

Described tin dichloride solution refers to that tin dichloride is dissolved in hydrochloric acid, then diluted with water to concentration is 50g/L;

The indicator is CuSO ₄ -isatin indicator, sodium diphenylamine sulfonate indicator; add CuSO ₄ -isatin indicator to turn the light yellow solution into green, then add TiCl ₃ solution dropwise until the green disappears, and the excess half drop, the solution turns blue after standing; add sulfuric acid/phosphoric acid mixed acid and sodium diphenylamine sulfonate indicator to form the solution to be tested.

In a preferred embodiment, the CuSO ₄ -isatin indicator refers to that isatin is dissolved in aqueous sulfuric acid solution, and then fixed to copper sulfate solution;

Described TiCl Solution refers to the titanium trichloride hydrochloric _acid solution that concentration is 20g/L;

The sulfuric acid/phosphoric acid mixed acid is mixed with 95~98wt% concentrated sulfuric acid and 85wt% phosphoric acid according to the volume ratio of 1:1, and then diluted with primary water to 1L to make 15wt%;

The sodium diphenylamine sulfonate indicator refers to a concentration of 0.05g/L sodium diphenylamine sulfonate aqueous solution.

In a further scheme, in the step (2), potassium dichromate standard solution is used for titration, when the solution changes from blue to purple, it is the end point of the reaction.

In a further scheme, in the step (3), the solution after filtering the unreacted carbon is neutralized with ammonia water until a hydroxide precipitate appears, and then neutralized with a nitric acid standard solution to make the hydroxide precipitate just disappear; then Add excess nitric acid standard solution, slowly add ammonium molybdate solution, shake for 2 to 3 minutes, precipitate, and place it for more than 4 hours to obtain the content of phosphorus element in it.

The present invention utilizes the wet chemistry method and the method that X fluorescence spectrometry combines first, draws the XRF standard curve with the fluorescence intensity as the ordinate, and the content of iron and phosphorus elements as the abscissa, and then uses this XRF standard curve as a standard, and The fluorescence intensity of each element in the sample to be tested is measured by X-ray fluorescence spectrometry to detect the content of phosphorus and iron elements in any lithium iron phosphate to be tested. The detection method of the invention is fast and high-speed, and has simple operation, small error, long time consumption, and is environmentally friendly to environmental pollution.

The beneficial effects of the present invention are as follows:

(1) The present invention adopts the combination of wet chemical method and X-ray fluorescence spectrometry, so as to quickly and accurately determine the content of iron and phosphorus in lithium iron phosphate;

(2) The present invention checks the content of various elements in the lithium iron phosphate sample by wet chemical method, and based on this, the XRF standard sample is made, and the corresponding standard curve is obtained by XRF, which effectively solves the problem of the lithium iron phosphate sample. Difficulties in making XRF standard curves;

(3) The present invention adopts the XRF standard curve, and measures the content of iron and phosphorus in lithium iron phosphate by X-ray fluorescence spectrometry, thereby reducing the use of toxic chemical reagents, reducing the cost, and improving the test time. It is a kind of Green test method.

detailed description

Below in conjunction with embodiment, the present invention is described in more detail.

In the following examples, hydrochloric acid, sulfuric acid, and phosphoric acid are commercially available undiluted reagents.

The selected hydrochloric acid aqueous solution refers to the mixture of concentrated hydrochloric acid with a mass concentration of 36% and primary water at a mass ratio of 1:1;

Described tin dichloride solution refers to that tin dichloride is dissolved in hydrochloric acid, then diluted with water to concentration is 50g/L;

The CuSO ₄ -isatin indicator means that isatin is dissolved in an aqueous sulfuric acid solution, and then fixed to a copper sulfate solution;

Described TiCl Solution refers to the titanium trichloride hydrochloric _acid solution that concentration is 20g/L;

The sulfuric acid/phosphoric acid mixed acid is mixed with 95~98wt% concentrated sulfuric acid and 85wt% phosphoric acid according to the volume ratio of 1:1, and then diluted with primary water to 1L to make 15wt%;

The sodium diphenylamine sulfonate indicator refers to a concentration of 0.05g/L sodium diphenylamine sulfonate aqueous solution.

Example 1

A fluorescence analysis method for determining the content of phosphorus and iron elements in lithium iron phosphate, comprising the following steps:

(1) Take the same batch of lithium iron phosphate samples and divide them into three equal parts;

(2) Add the first lithium iron phosphate sample to hydrochloric acid aqueous solution, heat to boil and keep for 18-25min, then cool to room temperature, and filter out unreacted carbon, and reduce the filtered solution with tin dichloride solution Ferric iron, then add CuSO ₄ -isatin indicator to turn the light yellow solution into green, then add TiCl ₃ solution dropwise until the green color disappears, then add half a drop in excess, and the solution turns blue after standing; then add sulfuric acid/phosphoric acid mixed acid and sodium diphenylamine sulfonate indicator to form the solution to be tested; titrate it with potassium dichromate standard solution, when the solution changes from blue to purple, it is the end of the reaction, and the iron element in the sample solution to be tested is measured content;

(3) Treat the second lithium iron phosphate sample with hydrochloric acid, filter the unreacted carbon, neutralize with ammonia water until the hydroxide precipitate appears, then neutralize the hydroxide precipitate with nitric acid standard solution, and make it just disappear Then add excess nitric acid standard solution, slowly add ammonium molybdate solution, precipitate after shaking for 2 ~ 3min, and place for more than 4h to form the sample solution to be tested; promptly record the content of phosphorus element in the sample solution to be tested;

(4) Divide the third lithium iron phosphate sample into at least six equal parts, add calcium carbonate reference substances with different contents respectively, and prepare standard samples with different contents; then use powder compression method to compress each standard sample into tablets respectively Shape, then measure the fluorescence intensity of iron and phosphorus in each flake standard sample by X-ray fluorescence spectrometry;

(5) The fluorescent intensity of iron, phosphorus and metal impurity elements in the lithium iron phosphate sample measured by X-ray fluorescence spectrometry in step (4) is compared with the phosphoric acid measured in steps (2) and (3). The content of iron and phosphorus elements in the iron-lithium sample is correlated to form an XRF standard curve with the fluorescence intensity as the ordinate and the content of iron and phosphorus as the abscissa;

(6) Press the lithium iron phosphate sample to be tested into a sheet using the same powder compression method as in step (4) to obtain the sample to be tested, and measure the fluorescence of each element in the sample to be tested by X-ray fluorescence spectrometry Intensity; through the XRF standard curve obtained in step (5), the content of iron and phosphorus elements in the sample to be tested can be obtained.

Example 2

1. Potassium dichromate standard solution titration method to determine the content of iron in lithium iron phosphate samples:

Weigh 1.000g±0.005g (accurate to 0.0001g) lithium iron phosphate (sample 1), a positive electrode material in the lithium-ion battery industry, into a 100mL beaker, add (1+1) hydrochloric acid, and heat and boil on an electric heating furnace for 20 minutes Left and right, turn off the electric heating furnace, cool to room temperature, filter and set the volume to a 100mL volumetric flask for later use. Take 20mL of the above solution, add 30mL of water, 5mL (1+1) hydrochloric acid, put it on an electric heating furnace and heat to a slight boil, add 50g/L tin dichloride solution dropwise while it is hot until light yellow, add two drops of CuSO ₄ -isatin indicator turns green, add 20g/L TiCl ₃ solution dropwise until the green color disappears, half a drop in excess, and the solution turns blue when left to stand. Add 20mL of 15% sulfuric acid/phosphoric acid mixed acid, a few drops of sodium diphenylamine sulfonate indicator, titrate with potassium dichromate standard solution, the solution turns from green to purple as the end point, and the consumed volume is V; Ferric ammonium 5mL, using the same reagent and dosage as the above steps, according to the same analysis steps, parallel operation, the volume of potassium dichromate consumed is recorded as V ₁ and V ₂ , and then according to the calculation formula, finally get the phosphoric acid The content of iron element in iron lithium. And parallel measurement three times, take the average value, range <3‰.

2. Determination of phosphorus content in lithium iron phosphate by ammonium phosphomolybdate volumetric method:

Weigh 1.000g±0.005g (accurate to 0.0001g) lithium iron phosphate (sample 1) into a 100mL beaker, add (1+1) hydrochloric acid, heat and boil on the electric heating furnace for about 20min, turn off the electric heating furnace, Cool to room temperature, filter and dilute to 100mL volumetric flask for use. Pipette 10mL of the above solution into an Erlenmeyer flask, neutralize with ammonia water until there is a precipitate, then neutralize the precipitate with nitric acid standard solution until it just disappears, add excess nitric acid standard solution, slowly add 75mL ammonium molybdate solution, shake for 2~3min, Precipitation placed more than 4h. Filter, wash the Erlenmeyer flask and the precipitate with nitric acid standard solution for 2 to 3 times, and then wash the Erlenmeyer flask and the precipitate with potassium nitrate solution until neutral. Add standard sodium hydroxide solution dropwise to the Erlenmeyer flask to dissolve the precipitate completely, then add 5mL~10mL filtered sodium hydroxide standard solution, record the volume V of sodium hydroxide consumed, add 2 drops of phenolphthalein indicator after a while, and use nitric acid standard The solution was dripped back until the solution became colorless, and the volume was recorded as V ₁ . Then, according to the calculation formula, the content of phosphorus element in the obtained lithium iron phosphate is finally obtained. And parallel measurement three times, take the average value, range <3‰.

Among them, the 70g/L ammonium molybdate solution is prepared by dissolving A solution (70g ammonium molybdate in 53mL ammonia water and 267mL water) slowly into B solution (mixed with 267mL nitric acid and 400mL water), and dilute to 1L .

The concentration of nitric acid standard solution is 0.1mol/L, the concentration of sodium hydroxide standard solution is 0.1mol/L; potassium hydrogen phthalate is the reference reagent, and the phenolphthalein indicator is 100mL of 95% dehydrated alcohol dissolved in 100mL of phenolphthalein indicator middle.

3. Preparation of XRF standard samples:

The lithium iron phosphate (sample 1) has been tested three times by wet chemical method to get the average value to obtain the content of iron element and phosphorus element. Use 150-mesh and 200-mesh standard sieves to screen 150-200-mesh powder samples, divide the screened lithium iron phosphate (sample 1) into six parts, mix in calcium carbonate reference materials with different contents, and configure them into different iron elements. Standard samples with different phosphorus content and content of iron element are: 33.50%, 32.38%, 31.27%, 30.14%, 29.03%, 27.92%, phosphorus content are: 18.85%, 18.22%, 17.59%, 16.96%, 16.34%, 15.71%. Then add a certain amount of glucose as a binder, mix well and set aside.

4. Powder compression method to compress XRF standard samples or samples to be tested:

Put the prepared six gradient lithium iron phosphate standard samples into molds such as aluminum rings with a diameter of 30mm, spread them evenly and compact them with a little pressure, then fill the entire mold with boric acid, and apply 20 tons on the tablet press. After pressing the pressure for 5 seconds, press it into a sheet-shaped standard sample.

5. Preparation of XRF standard curve:

Use X-ray fluorescence spectrometry to measure the fluorescence intensity of iron element and phosphorus element in each standard sample that is pressed into sheet shape, and take the fluorescence intensity of the element measured as the ordinate, and take the content of iron element and phosphorus element that has been measured before as The abscissa plots the XRF standard curve.

6. Detection of iron and phosphorus content in any batch of lithium iron phosphate samples:

Select any batch of lithium iron phosphate samples (sample 2), which is the positive electrode material of lithium-ion batteries, and place them in a mold such as an aluminum ring with a diameter of 30 mm. After applying 20 tons of pressure on the tablet machine and pressing for 5 seconds, it was pressed into a sheet shape, and the fluorescence intensity of iron and phosphorus elements was measured by X-ray fluorescence spectrometry, and the contents of iron and phosphorus elements were obtained through the XRF standard curve.

Example 3-5

Select any batch of lithium iron phosphate samples, which are the positive electrode material of lithium-ion batteries, as samples 3-5, place them in molds such as aluminum rings with a diameter of 30mm, spread them evenly, press them with a little pressure, and then fill the entire mold with boric acid , after applying 20 tons of pressure on the tablet press for 5 seconds, it was pressed into a sheet, and the fluorescence intensity of iron and phosphorus in the sample to be tested was measured by X-ray fluorescence spectrometry, and the XRF standard curve was drawn using Example 2 and Determination of iron and phosphorus content.

For Example 3, Example 4, and Example 5, 5 independent sample preparations and tests were carried out respectively, and the measurement results of the 5 times were counted, and the accuracy of the present invention was evaluated according to the relative standard deviation (RSD). The results are shown in Table 1.

Table 1 Precision experiment (n=5) (unit: %)

It can be seen from Table 1 that the RSD (relative standard deviation) of iron and phosphorus elements measured by the method of the present invention is less than 3.5%, indicating that the method has good precision and the element measurement results have good reproducibility.

The above content is only an example and description of the structure of the present invention. Those skilled in the art make various modifications or supplements to the described specific embodiments or replace them in similar ways, as long as they do not deviate from the structure of the invention or Anything beyond the scope defined in the claims shall belong to the protection scope of the present invention.

Claims (6)

1. A fluorescence analysis method for measuring phosphorus and iron element content in lithium iron phosphate, is characterized in that: comprise the following steps: (1) Take the same batch of lithium iron phosphate samples and divide them into three equal parts; (2) Treat the first lithium iron phosphate sample with hydrochloric acid, filter the unreacted carbon, and form the sample solution to be tested; titrate it with potassium dichromate standard solution, and measure the content of iron in the sample solution to be tested; (3) Treat the second lithium iron phosphate sample with hydrochloric acid, filter the unreacted carbon, and form the sample solution to be tested; use the ammonium phosphomolybdate volumetric method to measure the content of phosphorus in the sample solution to be tested; (4) Divide the third lithium iron phosphate sample into at least six equal parts, add calcium carbonate reference substances with different contents respectively, and prepare standard samples with different contents; then use powder compression method to compress each standard sample into tablets respectively Shape, then measure the fluorescence intensity of iron and phosphorus in each flake standard sample by X-ray fluorescence spectrometry; (5) Compare the fluorescence intensity of iron and phosphorus elements in the lithium iron phosphate sample measured by X-ray fluorescence spectrometry in step (4) with the iron and phosphorus elements in the lithium iron phosphate sample measured in steps (2) and (3). and phosphorus content to form an XRF standard curve with the fluorescence intensity as the ordinate and the iron and phosphorus content as the abscissa; (6) Press the lithium iron phosphate sample to be tested into a sheet using the same powder compression method as in step (4) to obtain the sample to be tested, and measure the fluorescence of each element in the sample to be tested by X-ray fluorescence spectrometry Intensity; through the XRF standard curve obtained in step (5), the content of iron and phosphorus elements in the sample to be tested can be obtained. 2. The fluorescence analysis method according to claim 1, characterized in that: in the step (2), the first lithium iron phosphate sample is added to the hydrochloric acid solution, heated and boiled for 18-25 minutes, and then cooled to Room temperature, and unreacted carbon was filtered out, and the filtered solution was used to reduce the ferric iron in it with tin dichloride solution, and then an indicator was added to form a solution to be tested. 3. The fluorescence analysis method according to claim 2, wherein: the hydrochloric acid aqueous solution refers to a mixture of concentrated hydrochloric acid with a mass concentration of 36% and primary water at a mass ratio of 1:1; Described tin dichloride solution refers to that tin dichloride is dissolved in hydrochloric acid, then diluted with water to concentration is 50g/L; The indicator is CuSO ₄ -isatin indicator, sodium diphenylamine sulfonate indicator; add CuSO ₄ -isatin indicator to turn the light yellow solution into green, then add TiCl ₃ solution dropwise until the green disappears, and the excess half drop, the solution turns blue after standing; add sulfuric acid/phosphoric acid mixed acid and sodium diphenylamine sulfonate indicator to form the solution to be tested. 4. according to the fluorescence analysis method described in claim 3, it is characterized in that: said CuSO ₄ -isatin indicator refers to that isatin is dissolved in sulfuric acid aqueous solution, and then settled in copper sulfate solution; Described TiCl Solution refers to the titanium trichloride hydrochloric _acid solution that concentration is 20g/L; The sulfuric acid/phosphoric acid mixed acid is mixed with 95~98wt% concentrated sulfuric acid and 85wt% phosphoric acid according to the volume ratio of 1:1, and then diluted with primary water to 1L to make 15wt%; The sodium diphenylamine sulfonate indicator refers to a concentration of 0.05g/L sodium diphenylamine sulfonate aqueous solution. 5. The fluorescence analysis method according to claim 1, characterized in that: in the step (2), titrate with potassium dichromate standard solution, when the solution changes from blue to purple, it is the end point of the reaction. 6. The fluorescence analysis method according to claim 1, characterized in that: in the step (3), the solution after filtering the unreacted carbon is neutralized with ammonia water until hydroxide precipitates appear, and then the nitric acid standard Neutralize the hydroxide precipitate in the solution and make it just disappear; then add excess nitric acid standard solution, slowly add ammonium molybdate solution, shake for 2~3min, precipitate after 2~3min, and place it for more than 4h to obtain the content of phosphorus element.