A method for detecting carbon-coated lithium iron phosphate based on water soaking iron steady state stage

Abstract

The present application relates to the technical field of lithium iron phosphate positive electrode material, and more particularly to a carbon-coated lithium iron phosphate detection method based on water soaking iron steady state stage, which comprises the following steps: after removing free moisture on the surface of the LFP@C to be detected, mixing with ultrapure water and soaking in a sealed environment; during the soaking process, sampling at preset time intervals, filtering and detecting the soluble iron and phosphorus content in the obtained filtrate; and drawing an iron content-time curve and a phosphorus content-time curve; extracting an iron content linear slow rising stage from the iron content-time curve; and based on the iron content and phosphorus content within the time range of the iron content linear slow rising stage, evaluating the performance of the carbon-coated lithium iron phosphate to be detected. The present application realizes rapid and accurate evaluation of the reduction property and interfacial bonding force of the carbon coating layer of the carbon-coated lithium iron phosphate to be detected.

Description

Technical Field

[0001] This invention relates to the field of lithium iron phosphate cathode material technology, and in particular to a method for detecting carbon-coated lithium iron phosphate based on the iron steady-state stage after water immersion. Background Technology

[0002] Lithium iron phosphate (LFP) has become the mainstream cathode material for energy storage and power lithium-ion batteries due to its advantages such as low cost, high safety, and long cycle life. However, LFP with an olivine structure has extremely low intrinsic conductivity, necessitating surface carbon coating to improve its electrochemical performance. Carbon coating is the core method for addressing the low intrinsic conductivity of LFP and improving its electrochemical performance. Besides the density and integrity of the carbon coating layer, its intrinsic chemical properties, including reducibility, surface functional group content, and interfacial bonding with LFP particles, are another key factor determining the rate performance, cycle stability, and hydrolysis resistance of LFP. Among these, the reducibility of the carbon layer directly determines the Fe content during LFP sintering. 3+ The reduction effect is achieved, avoiding the formation of impurity phases, and suppressing interfacial side reactions during cycling. The oxygen-containing functional groups on the carbon layer surface determine the dispersibility of LFP in aqueous slurry, directly affecting the electrode processing performance. The interfacial bonding force between the carbon layer and LFP determines whether the carbon layer will fall off during cycling, which is the core influencing factor for long-term cycling stability.

[0003] Common methods for detecting and evaluating carbon-coated lithium iron phosphate (LFP) materials include: X-ray photoelectron spectroscopy (XPS) / Raman spectroscopy. However, these methods can only detect the functional group ratio and graphitization degree on the carbon layer surface, and cannot quantitatively evaluate the overall reducibility of the carbon layer; the detection depth is only at the nanometer level, which cannot reflect the bonding characteristics of the carbon layer and LFP interface; the equipment cost is extremely high; the detection cycle is long; and the sample preparation is complex, making it unsuitable for batch quality inspection in production lines. Thermogravimetric analysis (TG) evaluates carbon content through oxidation weight loss in an air atmosphere, but cannot distinguish the reducibility and functional group characteristics of the carbon layer, and has poor correlation with the electrochemical performance of the material. Electrochemical impedance spectroscopy (EIS) evaluates interfacial impedance through coin cell testing, indirectly reflecting the characteristics of the carbon layer. However, the detection cycle is as long as several days, and it is greatly affected by the electrolyte, electrode preparation, and assembly process, and cannot quantitatively determine the intrinsic characteristics of the carbon layer alone. Existing water immersion detection methods only focus on the total amount of iron dissolved and the peak time to evaluate the physical barrier performance of the carbon layer, and cannot achieve quantitative detection of the reducibility and interfacial characteristics of the carbon layer. Summary of the Invention

[0004] This invention aims to solve the above-mentioned problems by providing a method for detecting carbon-coated lithium iron phosphate based on the steady-state stage of iron phosphate after water immersion. This method is fast, has small error, and can accurately evaluate the reducibility of the carbon coating layer and the interfacial bonding force between the carbon coating layer and lithium iron phosphate.

[0005] The technical solution to the problem solved by this invention is to provide a method for detecting carbon-coated lithium iron phosphate based on the steady-state stage of iron immersion in water, comprising the following steps:

[0006] S1. Remove free moisture from the surface of the carbon-coated lithium iron phosphate sample;

[0007] S2. Mix the carbon-coated lithium iron phosphate to be tested with ultrapure water and immerse it in a sealed environment;

[0008] S3. During the soaking process, samples are taken at preset time intervals, filtered, and the soluble iron content and soluble phosphorus content in the filtrate are detected; an iron content-time curve is plotted with soaking time as the abscissa and soluble iron content as the ordinate.

[0009] S4. The iron content-time curve includes, in sequence, a stage of rapid increase in iron content, a stage of rapid decrease in iron content, and a stage of linear and slow increase in iron content;

[0010] The linear and slow increase in iron content during this phase satisfies the following conditions: iron content increases monotonically with soaking time, and the coefficient of determination R for linear fitting of content-time data at multiple consecutive time points is [missing information]. 2 ≥0.95; preferably, R 2 ≥0.98;

[0011] S5. Evaluate the performance of the carbon-coated lithium iron phosphate to be tested based on the iron content and phosphorus content within the time range of the linearly and slowly increasing iron content stage.

[0012] The inventors discovered that carbon-coated lithium iron phosphate exhibits a three-stage kinetic characteristic in a pure water immersion system: the soluble iron content first increases sharply, then decreases sharply, and finally increases linearly and slowly.

[0013] Phase 1: The iron content increases dramatically. Water molecules penetrate through the micropores, defects, and cracks in the carbon coating layer, contacting the LFP matrix. Soluble impurities on the surface and the surface lattice dissolve rapidly, releasing a large amount of soluble Fe. 2+ Fe at this stage 2+ The dissolution rate is much greater than the oxidation rate, Fe 2+ It accumulates rapidly in the solution, forming a peak.

[0014] Second stage: Iron content decreases sharply: Solution pH decreases with Li + / H + The exchange rate rises to a weakly alkaline state, and dissolved oxygen will cause Fe to... 2+ Rapid oxidation to Fe 3+ Fe 3+ solubility product ratio of Fe 2+ The iron content drops 23 orders of magnitude, instantly precipitating out as ferric hydroxide and ferric phosphate, resulting in a precipitous decrease in soluble iron content.

[0015] Phase 3: Iron content increases linearly and slowly: the system enters a weakly acidic steady state, the LFP bulk lattice dissolves uniformly, and Fe... 3+ / Fe 2+ The catalytic cycle and the carbon layer-mediated galvanic cell effect reach a dynamic equilibrium, and the soluble iron content increases linearly and slowly with bulk dissolution.

[0016] In the carbon-coated lithium iron phosphate pure water immersion system, the soluble phosphorus content shows a monotonically increasing characteristic; in addition, the molar ratio of total Fe (soluble + precipitate) to total P (soluble + precipitate) in the system is always 1:1.

[0017] In the third stage, the accumulation rates of soluble iron and soluble phosphorus may differ, due to:

[0018] Dual-source accumulation of Fe: Fe generated from reduction 2+ Part of it forms stable, soluble Fe with the oxygen-containing functional groups (carboxyl groups, phenolic hydroxyl groups) on the surface of the carbon layer. 2+ Carbon complexes are not oxidized or precipitated, and directly enter the solution; the other part is free Fe. 2+ Although it will be oxidized and precipitated again, the reduction reaction continues (more than 90% of the FePO4 and Fe(OH)3 precipitates generated in the second stage are adsorbed on the surface and in the micropores of the carbon coating layer, forming a tight solid-solid contact with the reduced carbon coating layer, providing a prerequisite for the interfacial reaction; while the amorphous carbon coating layer has strong reducing properties and excellent electronic conductivity, which allows the adsorbed Fe to be released at the solid-liquid interface). 3+ Precipitate reduced to Fe 2+ The solution is continuously replenished with soluble iron. Ultimately, the source of soluble iron equals the Fe dissolved from the bulk solution. 2 + +Fe precipitated and reduced 2+ The cumulative rate has been significantly improved.

[0019] Single-source confined accumulation of P: PO4 released by FePO4 reduction 3- It will immediately react with the free Fe in the solution. 3+ FePO4 precipitate is regenerated and remains in the solid phase, unable to enter the solution; only a very small amount of unprecipitated PO4 dissolves from the bulk. 3- It can remain in solution. Ultimately, the source of soluble phosphorus is only the unprecipitated PO4 dissolved from the bulk. 3- Without additional supplementation, the accumulation rate is much lower than that of Fe.

[0020] Therefore, the stronger the reducing power of the carbon coating, the faster the interfacial reduction reaction rate, and the greater the additional Fe replenishment; the tighter the bond between the carbon coating and the LFP interface, the more sufficient the contact between the precipitate and the carbon layer, the easier the reduction reaction occurs, and the greater the additional Fe replenishment; the more abundant the oxygen-containing functional groups on the carbon layer surface, the greater the amount of soluble Fe generated. 2+ The more carbon complexes there are, the faster the accumulation rate of soluble Fe in the solution. Therefore, by comparing the difference in the growth rate of soluble iron content and soluble phosphorus content within the third stage time range, we can reflect the reducing power of the carbon coating layer and its binding force with lithium iron phosphate.

[0021] As a preferred embodiment of the present invention, the difference between the accumulation rate of soluble iron and the accumulation rate of soluble phosphorus within the third stage time range reflects the reducibility of the carbon coating layer and its binding force with lithium iron phosphate. The greater the difference, the better the reducibility of the carbon coating layer and the greater its binding force with lithium iron phosphate.

[0022] As a preferred embodiment of the present invention, step S5 includes the following steps:

[0023] A linear fit was performed on the data of the iron content during the period of linear and slow increase in iron content to obtain the first slope k. Fe ;

[0024] A second slope k was obtained by linearly fitting the data of phosphorus content within the same time period as the linearly slow increase phase of iron content. P ;

[0025] According to the k Fe and k P The ratio R is used to evaluate the performance of the carbon-coated lithium iron phosphate (LiFePO4) under test. The performance of the LiFePO4 under test includes at least one of the following: the reducibility of the carbon coating layer and the interfacial bonding strength between the carbon coating layer and LiFePO4. The value of R is directly proportional to the performance of the LiFePO4 under test. The larger the R value, the better the reducibility of the carbon coating layer and the stronger the interfacial bonding strength between the carbon coating layer and LiFePO4.

[0026] As a preferred embodiment of the present invention, the iron content data and phosphorus content data are converted into molar concentrations before the fitting is performed. This eliminates atomic weight differences and ensures that the comparison is based on the same standard.

[0027] As a preferred embodiment of the present invention, the performance evaluation of the carbon-coated lithium iron phosphate to be tested specifically includes:

[0028] When R < 0.9, the reducibility of the carbon coating is rated as poor, and / or the interfacial bonding force between the carbon coating and lithium iron phosphate is rated as poor.

[0029] When 0.9≤R≤1.1, the reducibility of the carbon coating is rated as medium, and / or the interfacial bonding strength between the carbon coating and lithium iron phosphate is rated as medium.

[0030] When R > 1.1, the reducibility of the carbon coating is considered good, and / or the interfacial bonding between the carbon coating and lithium iron phosphate is considered good.

[0031] As a preferred embodiment of the present invention, it further includes a step of predicting the process compatibility of the carbon-coated lithium iron phosphate to be tested based on the evaluation results:

[0032] When the carbon coating layer has poor reducibility and / or the interfacial bonding force between the carbon coating layer and lithium iron phosphate is poor, it is predicted that the carbon-coated lithium iron phosphate under test will have poor rate performance, poor cycle performance, and carbon layer detachment.

[0033] When the carbon coating layer has medium reducibility and / or the interfacial bonding force between the carbon coating layer and lithium iron phosphate is medium, it is predicted that the carbon-coated lithium iron phosphate to be tested is compatible with conventional products.

[0034] When the carbon coating has good reducibility and / or the interfacial bonding between the carbon coating and lithium iron phosphate is good, it is predicted that the carbon-coated lithium iron phosphate to be tested is suitable for high-rate, long-cycle, and high-humidity environments.

[0035] In some embodiments, as a preferred embodiment of the present invention, the following step is also included: S6. Detecting the electrochemical performance of the carbon-coated lithium iron phosphate to be tested, and establishing a correlation equation between R and electrochemical performance.

[0036] Electrochemical performance can include rate performance (10C / 1C capacity ratio), long-cycle performance (capacity retention after 2000 cycles at 1C), etc. After establishing a correlation equation between the R value and electrochemical performance of several standard samples, the electrochemical indicators of the carbon-coated lithium iron phosphate to be tested can be predicted by R.

[0037] Before testing, as a preferred embodiment of the present invention, the carbon-coated lithium iron phosphate to be tested is in powder form, which has a large contact area with ultrapure water, a high leaching rate, and reduces testing errors.

[0038] In step S1, before soaking, the free water on the surface of the carbon-coated lithium iron phosphate to be tested is removed. On the one hand, the presence of free water on the surface will cause some water molecules to pre-contact the material surface during the soaking moment, which is equivalent to starting the water permeation process in advance. This will lead to inconsistent starting points for iron dissolution in the first stage, thus affecting the accurate extraction of Tmax. On the other hand, the free water adsorbed in the environment may contain dissolved CO2, trace ions, etc., which will introduce additional pH disturbances or side reactions in the early stage of soaking, interfering with the three-stage kinetic process of iron dissolution-oxidation-precipitation.

[0039] As a preferred embodiment of the present invention, free moisture on the surface of the carbon-coated lithium iron phosphate to be tested is removed by vacuum drying at 105–120 °C for 2–4 h. For example, the temperature can be 105 °C, 107 °C, 110 °C, 112 °C, 115 °C, 117 °C, or 120 °C, and the time can be 2 h, 2.5 h, 3 h, 3.5 h, or 4 h.

[0040] As a preferred embodiment of the present invention, after vacuum drying, the sample is cooled to room temperature and sealed to ensure consistency of parallel samples.

[0041] In step S2, ultrapure water refers to pure water with a resistivity ≥18.2 MΩ·cm during immersion. The immersion process is completely sealed to avoid fluctuations in dissolved oxygen and to ensure that the experimental conditions for all samples are completely consistent.

[0042] The solid-liquid ratio of the carbon-coated lithium iron phosphate to be tested and ultrapure water is not limited, as long as the solid-liquid ratio of all samples to be tested is consistent. Preferably, the solid-liquid ratio of the carbon-coated lithium iron phosphate to ultrapure water is 0.001 to 1 (mass ratio). For example, it can be 0.001, 0.005, 0.01, 0.05, 0.1, 0.5, or 1.

[0043] The soaking process can be static or accompanied by stirring to accelerate iron dissolution, as long as the stirring speed of all samples to be tested is consistent. Preferably, stirring is performed during the soaking process at a speed of 100–500 rpm. Examples include 100 rpm, 150 rpm, 200 rpm, 250 rpm, 300 rpm, 350 rpm, 400 rpm, 450 rpm, and 500 rpm.

[0044] Immersion temperature affects the intensity of thermal motion of water molecules and their permeation rate through the micropores and defects of the carbon coating layer. Preferably, the sealed immersion system of the carbon-coated lithium iron phosphate to be tested and ultrapure water is at a temperature of 22–28 °C. Examples include 22 °C, 23 °C, 24 °C, 25 °C, 26 °C, 27 °C, and 28 °C. Using room temperature provides a sufficient reaction rate to ensure timely detection while avoiding the interference of accelerated side reactions caused by high temperatures.

[0045] Preferably, the temperature fluctuation does not exceed 0.5–1°C. The soaking temperature should be kept as constant as possible to reduce the impact of temperature fluctuations on dissolution.

[0046] Dissolved oxygen content affects Fe 2+ The oxidation rate is high, and excessive oxygen content leads to Fe 2+Immediate precipitation after dissolution affects the measurement of iron content. Preferably, the dissolved oxygen content in the sealed immersion system of the carbon-coated lithium iron phosphate and ultrapure water is 5–10 mg / L. Examples include 5 mg / L, 6 mg / L, 7 mg / L, 8 mg / L, 9 mg / L, and 10 mg / L; a saturation level of 8 mg / L is preferred.

[0047] Preferably, the dissolved oxygen content fluctuates by no more than 0.5–1 mg / L. The dissolved oxygen concentration is kept as constant as possible to ensure that Fe… 2+ Accumulation and Fe 3+ The competitive dynamics between precipitates remain constant.

[0048] In step S3, samples are taken at preset time intervals during the soaking process. The time intervals should not be too long, as excessively long intervals will smooth out the dramatic changes at the end of the first stage and the beginning of the second stage, making the curve flat and monotonous, and causing the extraction in the third stage, where the iron content increases linearly and slowly, to lose accuracy and reliability. Preferably, the preset time interval is 1 to 5 minutes. For example, it can be 1 minute, 2 minutes, 3 minutes, 4 minutes, or 5 minutes.

[0049] The soluble iron content in the filtrate after filtration is detected by any method, for example, by inductively coupled plasma optical emission spectrometry (ICP-OES).

[0050] In some embodiments, as a preferred embodiment of the present invention, the lithium content in the filtrate is simultaneously detected when detecting the soluble iron and phosphorus content. Correspondingly, a lithium content-time curve is plotted simultaneously.

[0051] The perfect linear growth of Li is a prerequisite for the steady state of the system, which can prove that in the steady state stage, the LFP bulk lattice dissolves uniformly, releasing Fe. 2+ With PO4 3- The molar ratio is strictly 1:1, ensuring that the molar ratio of total Fe (soluble + precipitate) to total P (soluble + precipitate) in the system is always 1:1, thus guaranteeing the reliability of the evaluation conclusions.

[0052] The beneficial effects of this invention are:

[0053] 1. This invention utilizes the three-stage change mechanism of iron content in a carbon-coated lithium iron phosphate pure water immersion system. By comparing the iron and phosphorus contents within the steady-state stage of the linearly and slowly increasing iron content, the performance of the carbon-coated lithium iron phosphate under test is evaluated. This eliminates the interference of surface impurities and the physical barrier properties of the carbon layer, reflecting only the intrinsic chemical characteristics of the carbon layer. It achieves rapid quantitative detection of the reducibility and interfacial bonding characteristics of the LFP carbon coating layer, and the coefficient of variation of parallel samples can be controlled within 3%.

[0054] 2. This invention uses the ratio R of the accumulation rate of soluble iron to the accumulation rate of soluble phosphorus within the steady-state phase of linearly and slowly increasing iron content to evaluate the performance of carbon-coated lithium iron phosphate. The R value is also strongly correlated with the rate performance and long-cycle stability of the material. The test results can directly predict the electrochemical performance of the material, and the matching degree with the actual battery performance is >95%.

[0055] 3. The detection method of the present invention can obtain core evaluation results within 1 hour, and does not require large-scale high-end equipment such as XPS and Raman. It is fast, low-cost, and easy to standardize experimental conditions, making it suitable for batch quality inspection needs of industrial production lines.

[0056] 4. The detection method of the present invention can be extended to the detection of carbon layer characteristics of other carbon-coated cathode materials such as carbon-coated lithium manganese iron phosphate and carbon-coated ternary materials, and has a wide range of applications. Attached Figure Description

[0057] Figure 1 This is a graph showing the iron, lithium, and phosphorus content versus time obtained in Example 1 (the left vertical axis represents the units of iron and phosphorus, and the right vertical axis represents the units of lithium).

[0058] Figure 2 This is a graph of iron, lithium, and phosphorus content versus time obtained in Example 1 (the left vertical axis represents the units of lithium and phosphorus, and the right vertical axis represents the units of iron). Detailed Implementation

[0059] The following are specific embodiments of the present invention, and the technical solution of the present invention will be further described in conjunction with the accompanying drawings. However, the present invention is not limited to these embodiments.

[0060] Example 1

[0061] A method for detecting carbon-coated lithium iron phosphate based on the steady-state stage of iron immersion in water includes the following steps:

[0062] S1. Place the carbon-coated lithium iron phosphate powder to be tested in a vacuum drying oven and dry it at 110 ℃ for 3 h to remove free moisture from the surface of the powder. After cooling to room temperature, seal and store it.

[0063] S2. The pretreated carbon-coated lithium iron phosphate to be tested was added to ultrapure water (resistivity ≥18.2 MΩ·cm) at a mass ratio of 0.1:1 into a sealed reaction vessel to obtain the immersion system. The system temperature was controlled at 25±0.5 ℃, the dissolved oxygen content at 8±0.5 mg / L, and the stirring rate at 300 rpm. The entire process was sealed to avoid fluctuations in dissolved oxygen and to ensure that the experimental conditions for all samples were completely consistent.

[0064] S3. During the soaking process, samples were taken from the reaction vessel every 1 minute and filtered using a syringe and a water filter. The filtrate was then analyzed using an inductively coupled plasma optical emission spectrometer (ICP-OES) to detect the soluble iron content in the clarified solution; the lithium and phosphorus contents were also detected simultaneously.

[0065] Plot an iron content-time curve with soaking time on the x-axis and soluble iron content on the y-axis; simultaneously plot lithium and phosphorus content-time curves, as shown below. Figure 1 and Figure 2 As shown.

[0066] S4. For example Figure 2 As can be seen from the iron content-time curve, the iron content increases sharply from 0 to 8 minutes, decreases sharply from 8 to 15 minutes, and then increases linearly and slowly after 15 minutes.

[0067] The iron content data after 15 minutes was converted to molar concentration and used as the ordinate, with the soaking time as the abscissa for linear fitting: y = 0.0003x + 0.0126, R0 2 = 0.9932, thus obtaining the first slope k Fe = 0.0003.

[0068] The phosphorus content data after 15 minutes was converted to molar concentration and used as the ordinate, with soaking time as the abscissa for linear fitting: y = 0.0042x + 0.4042, R0 2 = 0.9926, thus obtaining the second slope k P = 0.0042.

[0069] Therefore, R = k Fe / k p = 0.07.

[0070] S5. In this embodiment, R < 0.9, the carbon coating layer is rated as poor reducibility, the carbon coating layer and lithium iron phosphate interface bonding force is poor, the carbon-coated lithium iron phosphate material has poor rate performance, poor cycle life, and carbon layer detachment.

[0071] Example 2

[0072] The following are examples of carbon-coated lithium iron phosphate with different carbon coating layers and varying degrees of reducibility and bonding strength with lithium iron phosphate:

[0073] Sample A-1: ​​The carbothermic reduction method was used, with FePO4 and Li2CO3 as raw materials, and the molar ratio of Li:Fe:P = 1.05:1:1; glucose was used as the carbon source, and the carbon source mass accounted for 6 wt% of the total mass of the raw materials; after wet ball milling and spray drying, the sample was sintered at 700 ℃ for 10 h under a nitrogen atmosphere with a heating rate of 5℃ / min.

[0074] Sample B: The carbothermic reduction method was used, with FePO4 and Li2CO3 as raw materials in a molar ratio of Li:Fe:P = 1.05:1:1; glucose was used as the carbon source, accounting for 6 wt% of the total raw material mass. After wet ball milling and spray drying, the sample was sintered at 700 °C for 10 h under a nitrogen atmosphere with a heating rate of 5 °C / min. It was then dispersed in a 10% hydrogen peroxide solution, stirred at room temperature for 45 min, washed with deionized water, and vacuum dried at 60 °C. The introduction of oxygen-containing functional groups through hydrogen peroxide changed the hybridization state of the carbon atoms from sp... 2 To sp 3 change.

[0075] Sample C: The carbothermic reduction method was used with FePO4 and Li2CO3 as raw materials, with a molar ratio of Li:Fe:P = 1.05:1:1; the carbon source was 6 wt% glucose and the nitrogen source was 2 wt% urea; after wet ball milling and spray drying, the sample was sintered at 700 ℃ for 10 h under a nitrogen atmosphere with a heating rate of 5 ℃ / min.

[0076] Sample A-2: The carbothermic reduction method was used, with FePO4 and Li2CO3 as raw materials, and the molar ratio of Li:Fe:P = 1.05:1:1; polystyrene was used as the carbon source, accounting for 12 wt% of the total mass of the raw materials; after wet ball milling and spray drying, the sample was sintered at 700 ℃ for 10 h under a nitrogen atmosphere with a heating rate of 5 ℃ / min.

[0077] The above samples were tested according to the steps in Example 1, and the test results are shown in Table 1 below.

[0078] Table 1.

[0079] Sample A-1 1.00 middle Sample B 0.09 Difference Sample C 1.22 good Sample A-2 0.88 Difference

[0080] As shown in Table 1, R is positively correlated with the reducibility of the carbon coating and its binding force with lithium iron phosphate.

[0081] The specific embodiments described herein are merely illustrative of the spirit of the invention. Those skilled in the art to which this invention pertains may make various modifications or additions to the described specific embodiments or use similar methods to substitute them, without departing from the spirit of the invention or exceeding the scope defined by the appended claims.

Claims

1. A method for detecting carbon-coated lithium iron phosphate based on the steady-state stage of iron immersion in water, characterized in that: Includes the following steps: S1. Remove free moisture from the surface of the carbon-coated lithium iron phosphate sample; S2. Mix the carbon-coated lithium iron phosphate to be tested with ultrapure water and immerse it in a sealed environment; S3. During the soaking process, samples are taken at preset time intervals, filtered, and the soluble iron content and soluble phosphorus content in the filtrate are detected; an iron content-time curve is plotted with soaking time as the abscissa and soluble iron content as the ordinate. S4. The iron content-time curve successively includes a stage of rapid increase in iron content, a stage of rapid decrease in iron content, and a stage of linear and slow increase in iron content; The linear and slow increase in iron content during this phase satisfies the following conditions: iron content increases monotonically with soaking time, and the coefficient of determination R for linear fitting of content-time data at multiple consecutive time points is [missing information]. 2 ≥0.95; S5. Evaluate the performance of the carbon-coated lithium iron phosphate to be tested based on the iron content and phosphorus content within the time range of the linearly and slowly increasing iron content stage.

2. The method for detecting carbon-coated lithium iron phosphate based on the steady-state stage of iron immersion according to claim 1, characterized in that: Step S5 includes the following steps: A linear fit was performed on the data of the iron content during the period of linear and slow increase in iron content to obtain the first slope k. Fe ; A second slope k was obtained by linearly fitting the data of phosphorus content within the same time period as the linearly slow increase phase of iron content. P ; According to the k Fe and k P The ratio R evaluates the performance of the carbon-coated lithium iron phosphate under test. The performance of the carbon-coated lithium iron phosphate to be tested includes at least one of the following: the reducibility of the carbon coating layer and the interfacial bonding force between the carbon coating layer and lithium iron phosphate. The value of R is directly proportional to the performance of the carbon-coated lithium iron phosphate to be tested.

3. The method for detecting carbon-coated lithium iron phosphate based on the steady-state stage of iron immersion according to claim 2, characterized in that: The iron and phosphorus content data were converted to molar concentrations before the linear fitting was performed.

4. The method for detecting carbon-coated lithium iron phosphate based on the steady-state stage of iron immersion according to claim 3, characterized in that: The specific evaluation of the performance of the carbon-coated lithium iron phosphate to be tested is as follows: When R < 0.9, the reducibility of the carbon coating is rated as poor, and / or the interfacial bonding force between the carbon coating and lithium iron phosphate is rated as poor. When 0.9≤R≤1.1, the reducibility of the carbon coating is rated as medium, and / or the interfacial bonding strength between the carbon coating and lithium iron phosphate is rated as medium. When R > 1.1, the reducibility of the carbon coating is considered good, and / or the interfacial bonding between the carbon coating and lithium iron phosphate is considered good.

5. The method for detecting carbon-coated lithium iron phosphate based on the steady-state stage of iron immersion according to claim 2, characterized in that: It also includes the following steps: S6. Detect the electrochemical performance of the carbon-coated lithium iron phosphate to be tested, and establish the correlation equation between R and electrochemical performance.

6. The method for detecting carbon-coated lithium iron phosphate based on the steady-state stage of iron immersion according to claim 1, characterized in that: In step S1, the free moisture on the surface of the carbon-coated lithium iron phosphate to be tested is removed by vacuum drying at 105-120 °C for 2-4 h.

7. The method for detecting carbon-coated lithium iron phosphate based on the steady-state stage of iron immersion according to claim 1, characterized in that: In step S2, the solid-liquid ratio of the carbon-coated lithium iron phosphate to be tested and ultrapure water is 0.001 to 1.

8. The method for detecting carbon-coated lithium iron phosphate based on the steady-state stage of iron immersion according to claim 1, characterized in that: In step S2, stirring is performed during the soaking process at a speed of 100–500 rpm.

9. A method for detecting carbon-coated lithium iron phosphate based on the steady-state stage of iron immersion according to claim 1 or 8, characterized in that: In step S2, the temperature fluctuation of the sealed immersion system of carbon-coated lithium iron phosphate and ultrapure water to be tested does not exceed 0.5 to 1 ℃, and the dissolved oxygen content fluctuation does not exceed 0.5 to 1 mg / L.

10. The method for detecting carbon-coated lithium iron phosphate based on the steady-state stage of iron immersion according to claim 9, characterized in that: In step S2, the temperature of the sealed immersion system of carbon-coated lithium iron phosphate and ultrapure water is 22-28°C, and the dissolved oxygen content is 5-10 mg / L.

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