Method for evaluating cycle stability of lithium iron phosphate cathode material

Abstract

The present application relates to the technical field of battery, in particular to a kind of lithium iron phosphate positive material cycle stability evaluation method.The evaluation method includes the following steps: (a) the lithium iron phosphate positive material to be tested is prepared into battery;(b) the battery is simultaneously carried out charge-discharge test and XRD test;(c) obtain XRD data set and charge-discharge data set;According to XRD data set, XRD curve graph is drawn, and the characteristic peak of lithium iron phosphate and iron phosphate is marked;XRD data set and charge-discharge data set are corresponded by absolute time, and the SOC of the characteristic peak of iron phosphate in the charging process begins to appear LFP And the SOC of the characteristic peak of lithium iron phosphate disappears FP ;With L=SOC FP -SOC LFP The numerical size of the lithium iron phosphate cycle stability is evaluated, and the smaller the L value, the better the cycle stability of the lithium iron phosphate positive material.The method is short in evaluation time, low in cost and accurate in result.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and more specifically, to a method for evaluating the cycle stability of lithium iron phosphate cathode materials. Background Technology

[0002] Lithium iron phosphate (LFP) has an olivine structure. Due to the strong three-dimensional PO-Fe bonds, it is not prone to oxygen evolution, resulting in structural stability and good cycle and safety performance, making it widely used in the field of power batteries. When designing LFP cells or developing secondary power supplies, it is necessary to select suitable LFP materials, among which cycle performance is particularly important. Currently, there are few publicly available methods for evaluating the cycle stability of LFP batteries. Generally, they are fabricated into button cells or pouch cells, and the performance differences between materials are evaluated and compared by the capacity retention rate after long-term cycling; or their performance differences are evaluated by physicochemical properties such as iron dissolution.

[0003] In existing technologies, long-cycle evaluation is time-consuming, and because lithium iron phosphate materials have excellent cycle performance, it's often impossible to distinguish between superior and inferior materials even after 1000 cycles, meaning the cycle time can often last for nearly six months. Accelerated testing requires finding corresponding relationships, which is quite complex. Physicochemical performance tests are generally for factory-release powder and cannot fully represent their application status in battery cells, thus having limitations.

[0004] In view of this, the present invention is hereby proposed. Summary of the Invention

[0005] One objective of this invention is to provide a method for evaluating the cycle stability of lithium iron phosphate cathode materials. This method has a short evaluation time, low cost, and the evaluation process closely resembles the application scenario of lithium iron phosphate. It can intuitively show the changes of lithium iron phosphate during the charge and discharge process, and the results are accurate.

[0006] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted:

[0007] The evaluation method for the cycle stability of lithium iron phosphate cathode materials includes the following steps:

[0008] (a) The lithium iron phosphate cathode material to be tested is used to prepare a battery;

[0009] (b) Set the parameters of the in-situ XRD testing equipment and the charging / discharging equipment; perform charging / discharging tests and XRD tests on the battery simultaneously;

[0010] (c) Obtain the XRD data set from the XRD test and the charge / discharge data set from the charge / discharge test in step (b); plot the XRD curve based on the XRD data set, marking the characteristic peaks of lithium iron phosphate and iron phosphate; correlate the XRD data set with the charge / discharge data set using absolute time to obtain the SOC at which the characteristic peak of iron phosphate begins to appear during the charging process. LFP and the SOC when the characteristic peak of the lithium iron phosphate disappears FP ; with L=SOC FP -SOC LFP The value of L is used to evaluate the cycle stability of the lithium iron phosphate cathode material. The smaller the L value, the better the cycle stability of the lithium iron phosphate cathode material being tested.

[0011] In one embodiment, the parameters of the in-situ XRD testing equipment include: step size, scan speed, and scan angle;

[0012] The step size is 0.01° to 0.03°;

[0013] The scanning speed is 1–3° / min;

[0014] The scanning angle is 27° to 34°.

[0015] In one embodiment, the step size is 0.02° and the scan rate is 2° / min.

[0016] In one embodiment, the parameter settings of the charging and discharging device include discharging at 0.1-2C to 2-2.5V, resting for 10 minutes, and charging at 0.1C constant current to 3.65-4.5V.

[0017] In one embodiment, the data in the charge / discharge data set includes voltage, absolute time, current, and capacity.

[0018] In one implementation, the data in the XRD data set includes: the angles and corresponding intensities of all XRD curves, and the absolute time at which each XRD curve began testing.

[0019] In one embodiment, the XRD curves in the XRD curve graph are 85 to 110.

[0020] In one embodiment, under the same test conditions, n kinds of lithium iron phosphate cathode materials to be tested are tested respectively, n≥2, and their respective L values ​​are obtained. By comparing the L values ​​of the n kinds of lithium iron phosphate cathode materials to be tested, the cycle performance of the n kinds of lithium iron phosphate cathode materials to be tested is evaluated.

[0021] In one embodiment, the battery includes a positive electrode sheet; the positive electrode sheet includes a positive current collector and a positive active layer disposed on at least one side surface of the positive current collector; the positive active layer includes the lithium iron phosphate positive electrode material, a binder and a conductive agent; the mass ratio of the lithium iron phosphate positive electrode material, the binder and the conductive agent is (88-92):(4-6):(4-6).

[0022] In one embodiment, the method for preparing the battery includes: placing and assembling the in-situ tooling base, the positive electrode, the separator, the electrolyte, the lithium sheet, the gasket, the spring sheet, and the in-situ tooling cover in sequence under the conditions of oxygen content < 0.1 ppm and water content < 0.1 ppm.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0024] The method for evaluating the cycle stability of lithium iron phosphate cathode material of this invention is quick, low-cost, easy to assemble, and the evaluation process closely resembles the application scenario of lithium iron phosphate, allowing for a direct observation of the changes in lithium iron phosphate during charge and discharge, with accurate results. Attached Figure Description

[0025] 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.

[0026] Figure 1 This is a graph showing the voltage and capacity of group A batteries in Embodiment 1 of the present invention as a function of relative time.

[0027] Figure 2 This is a graph showing the change of the index of group A of the spectra in Embodiment 1 of the present invention over relative time.

[0028] Figure 3 The graphs show the changes in voltage and capacity of battery group B in Embodiment 1 of the present invention over relative time.

[0029] Figure 4 This is a graph showing the change of the sequence number of the B group of spectra in Embodiment 1 of the present invention over relative time.

[0030] Figure 5 The XRD pattern of lithium iron phosphate material A in Example 1 of this invention;

[0031] Figure 6 The XRD pattern of lithium iron phosphate material B in Example 1 of this invention;

[0032] Figure 7 This is a graph showing the capacity retention rate of lithium iron phosphate material A and lithium iron phosphate material B as a function of cycle number, tested using conventional methods according to the present invention. Detailed Implementation

[0033] The embodiments of the present invention will be described in detail below with reference to examples. However, those skilled in the art will understand that the following examples are for illustrative purposes only and should not be considered as limiting the scope of the invention. Unless otherwise specified in the examples, conventional conditions or conditions recommended by the manufacturer are followed. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0034] The evaluation method for the cycle stability of lithium iron phosphate cathode materials includes the following steps:

[0035] (a) The lithium iron phosphate cathode material to be tested is used to prepare a battery;

[0036] (b) Set the parameters of the in-situ XRD testing equipment and the charging / discharging equipment; perform charging / discharging tests and XRD tests on the battery simultaneously;

[0037] (c) Obtain the XRD data set from the XRD test and the charge / discharge data set from the charge / discharge test in step (b); plot the XRD curve based on the XRD data set, marking the characteristic peaks of lithium iron phosphate and iron phosphate; correlate the XRD data set with the charge / discharge data set using absolute time to obtain the SOC at which the characteristic peak of iron phosphate begins to appear during the charging process. LFP (State of charge) and SOC when the characteristic peak of lithium iron phosphate disappears FP (State of charge); with L = SOC FP -SOC LFP The value of L is used to evaluate the cycle stability of the lithium iron phosphate cathode material. The smaller the L value, the better the cycle stability of the lithium iron phosphate cathode material being tested.

[0038] Lithium iron phosphate (LFP) is a cathode material with an olivine structure, belonging to the orthorhombic crystal system. The lattice parameters of the orthorhombic system are a≠b≠c, α=β=γ=90°, and it belongs to the Pnmb space group. Before charging, LFP is a single phase. During charging, i.e., during delithiation, a new phase, iron phosphate, is formed, which also belongs to the orthorhombic crystal system. The charging and discharging mechanism of LFP is not yet fully understood. To explain the phenomena during the process, models such as the "core-shell model" and the "mosaic model" have been proposed, but they all have limitations. It is certain that during charging and discharging, two phases, LFP and iron phosphate, coexist. This can be identified by corresponding characteristic peaks in the XRD pattern of the cathode material. This phase transition generally results from the interaction leading to ordered and organized thermal motion, causing disorder and chaos. Diffusion coefficient tests also show that the diffusion coefficient is lowest in the LFP and iron phosphate coexisting region. The coexistence region is a dynamic resistance zone, which can cause volume changes and interfacial stresses, leading to material fracture, increased side reactions, and poor contact with the current collector, thus affecting the material's cycle stability. The shorter the coexistence region, the less conducive it is to cycle stability.

[0039] This invention utilizes in-situ XRD testing technology to measure the length of the coexistence region of characteristic peaks of lithium iron phosphate and iron phosphate during the charging process of lithium iron phosphate, thereby evaluating the cycle stability of lithium iron phosphate. This evaluation method is short in time, low in cost, easy to assemble, and the evaluation process closely resembles the application scenario of lithium iron phosphate, allowing for a direct observation of the changes of lithium iron phosphate during the charging and discharging process, with accurate results.

[0040] In one embodiment, the parameters of the in-situ XRD testing equipment include: step size, scan rate, and scan angle; the step size is 0.01° to 0.03°; the scan rate is 1 to 3° / min; and the scan angle is 27° to 34°. In one embodiment, the target material for XRD testing is a copper target. In one embodiment, the step size is 0.02°, and the scan rate is 2° / min.

[0041] First, select the angular range of the characteristic peaks and scan a specific range. This avoids scanning the entire spectrum, saving time and obtaining a set of XRD data with good continuity.

[0042] In one embodiment, the parameter settings of the charging and discharging device include: discharging at 0.1-2C to 2-2.5V, resting for 10 minutes, and charging at 0.1C constant current to 3.65-4.5V. In another embodiment, the parameter settings of the charging and discharging device include: discharging at 0.1C to 2V, resting for 10 minutes, and charging at 0.1C constant current to 3.75V.

[0043] In one implementation, the charge / discharge data set includes voltage, absolute time, current, and capacity. The charge / discharge status is determined by the current.

[0044] In one implementation, voltage versus relative time and capacity versus relative time curves are plotted based on the data from the charge / discharge data set.

[0045] In one implementation, the data in the XRD data set includes: the angles and corresponding intensities of all XRD curves, and the absolute time at which each XRD curve began testing.

[0046] In one implementation, the XRD pattern plotted based on the data from the XRD data set includes multiple XRD curves, and the XRD curves in the XRD pattern plot are numbered according to the absolute time of the test, and a curve of the pattern number changing with relative time is plotted based on the numbered XRD curves.

[0047] For the XRD data set, continuity must be maintained throughout the charging process. In one embodiment, the SOC interval for each XRD curve is less than 1.5%. Each XRD curve corresponds to the SOC of charge and discharge over an absolute test time.

[0048] In one implementation, a complete set of XRD data includes 85 to 110 XRD curves, such as 85, 88, 90, 95, 100, etc.

[0049] In one embodiment, under the same test conditions, n kinds of lithium iron phosphate cathode materials to be tested are tested respectively, n≥2, and their respective L values ​​are obtained. By comparing the L values ​​of the n kinds of lithium iron phosphate cathode materials to be tested, the cycle performance of the n kinds of lithium iron phosphate cathode materials to be tested is evaluated.

[0050] The obtained XRD data sets need to be correlated with charge / discharge data using absolute time to obtain the battery state corresponding to each XRD set. The XRD data sets are then sequentially unfolded using the battery's SOC (State of Charge) to obtain the desired data sets. Since charging data is typically recorded every 30 seconds, and a full charge usually takes 10 hours, this set of charging data contains over 1200 sets. Therefore, approximately 90 of these 1200 sets need to be extracted using absolute time and correlated with approximately 90 sets of XRD data.

[0051] In one embodiment, the battery includes a positive electrode sheet; the positive electrode sheet includes a positive current collector and a positive active layer disposed on at least one side surface of the positive current collector; the positive active layer includes the lithium iron phosphate positive electrode material, a binder, and a conductive agent; the mass ratio of the lithium iron phosphate positive electrode material, the binder, and the conductive agent is (88-92):(4-6):(4-6), for example, 90:5:5, etc.

[0052] In one embodiment, the binder comprises polyvinylidene fluoride. The conductive agent comprises Super Li.

[0053] In one embodiment, the preparation of the positive electrode sheet includes: taking lithium iron phosphate material, binder, conductive agent, and solvent in a certain proportion, mixing them evenly, coating them onto a positive electrode current collector (e.g., aluminum foil), vacuum drying, rolling, and cutting into sheets. In one embodiment, the areal density of the positive electrode coating is 8–9 mg / cm³. 2 In one embodiment, the drying temperature is 80–95°C, and the drying time is 1–3 hours. In another embodiment, the material is compacted by roller pressing until the density is 3–3.5 g / cm³. 3 .

[0054] In one embodiment, the battery preparation method includes: in a glove box environment with oxygen content <0.1ppm and water content <0.1ppm, sequentially placing an in-situ tooling base, a positive electrode plate, a separator, an electrolyte, a lithium plate, a gasket, a spring plate, and an in-situ tooling cover, and locking them in place to complete the installation.

[0055] The following explanation, in conjunction with specific embodiments, further clarifies the situation.

[0056] Example 1

[0057] The evaluation method for the cycle stability of lithium iron phosphate cathode materials includes the following steps:

[0058] (a) Obtaining the positive electrode sheet: Take lithium iron phosphate material A and lithium iron phosphate material B, and prepare them into positive electrode sheets according to the following methods: Take lithium iron phosphate material in a 90:5:5 ratio, binder and conductive agent, the binder is polyvinylidene fluoride, the conductive agent is SuperLi, take the solvent N-methylpyrrolidone, mix them evenly, and coat them on aluminum foil with an areal density of 8.75 mg / cm³. 2 Vacuum dried at 90℃ for 2 hours, then dried completely and compacted by roller pressing to a density of 3.3 g / cm³. 3 The electrode is cut into 12mm diameter plates using a cutting machine; lithium iron phosphate material A corresponds to positive electrode plate A, and lithium iron phosphate material B corresponds to positive electrode plate B.

[0059] (b) Prepare in-situ batteries for positive electrode A and positive electrode B respectively: In a glove box environment with oxygen content <0.1ppm and water content <0.1ppm, place the in-situ tooling base, positive electrode, separator, electrolyte, lithium sheet, gasket, spring sheet, and in-situ tooling cover in sequence, and lock them in place. Keep the above preparation conditions for positive electrode A and positive electrode B the same to obtain in-situ battery A (Group A) and in-situ battery B (Group B).

[0060] (c) Test the two in-situ batteries separately: Place the in-situ batteries on the in-situ XRD testing equipment. The equipment uses a copper target. Set the step size to 0.01°, the scan speed to 2° / min, and the scan angle to 27°-34°. At the same time, set the charging and discharging parameters of the charging and discharging equipment: discharge to 2V at 0.1C, stand for 10min, and charge to 3.75V at 0.1C constant current. After setting, start the charging and discharging test and XRD test simultaneously, keeping the test conditions of the two groups the same.

[0061] (d) Data Processing: Extract charge-discharge data for groups A and B, including voltage, absolute time, current, and capacity. Based on the charge-discharge data, plot the voltage versus relative time curve and the capacity versus relative time curve for group A, respectively. (See...) Figure 1 For the voltage and capacity variation curves of group B over relative time, please refer to [link / reference]. Figure 3 XRD data from groups A and B were extracted separately, including the angles and corresponding intensities of all XRD curves, and the absolute time of start of the test for each XRD curve. The XRD data during the charging process were then plotted into XRD spectra, which included multiple XRD curves. For the XRD spectra of lithium iron phosphate material A, please refer to [reference needed]. Figure 5 As shown, the XRD pattern of lithium iron phosphate material B is shown in [reference needed]. Figure 6 As shown; based on the curves of the XRD patterns in group A, plot the curves of the changes in the XRD pattern number of group A versus relative time, see [reference]. Figure 2 As shown, plot the curves of the XRD pattern sequence number versus relative time for group B based on the XRD pattern curves. (See [reference]) Figure 4 As shown; using charge / discharge data and absolute times from XRD tests, the SOC is mapped to each group of spectra; the characteristic peaks of lithium iron phosphate and iron phosphate in the XRD patterns are marked respectively, with the lithium iron phosphate characteristic peak around 29.1° and the iron phosphate characteristic peak around 30.2°. The SOC at which the iron phosphate characteristic peak in group A first appears during charging is also marked. LFP =40.2% and the SOC when the characteristic peak of lithium iron phosphate disappears FP =76.2%, L A =SOC FP -SOC LFP =76.2% - 40.2% = 36%. The SOC (Sodium Oxygen phosphate) characteristic peak of group B begins to appear. LFP =34.1% and the SOC when the characteristic peak of lithium iron phosphate disappears FP =90.9%, L B =SOC FP -SOC LFP =90.9%-34.1%=56.8%, L A <L BIt can be seen that the cycle stability of lithium iron phosphate material A is better than that of lithium iron phosphate material B.

[0062] This invention employs conventional long-cycle testing methods to test the cycle stability of the aforementioned Group A and Group B batteries, specifically including: 30 minutes of rest, 1C discharge to 2.5V, 30 minutes of rest, 1C, 3.65V constant current / constant voltage charging to 3.65V, stopping when the capacity decays to 80% of the initial capacity. Figure 7 As shown, the cycle stability of lithium iron phosphate material B is poor. The capacity retention rate of group B in long-term cycling data is less than that of group A. That is, the cycle stability of lithium iron phosphate material A is better than that of lithium iron phosphate material B, which verifies the accuracy of the evaluation method of the present invention.

[0063] 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 therein. Such 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 evaluating the cycle stability of lithium iron phosphate cathode materials, characterized in that, Includes the following steps: (a) The lithium iron phosphate cathode material to be tested is used to prepare a battery; (b) Set the parameters of the in-situ XRD testing equipment and the charging / discharging equipment; perform charging / discharging tests and XRD tests on the battery simultaneously; (c) Obtain the XRD data set of the XRD test and the charge-discharge data set of the charge-discharge test in step (b); plot the XRD curve according to the XRD data set and mark the characteristic peaks of lithium iron phosphate and iron phosphate. By mapping the XRD data set to the charge / discharge data set using absolute time, the state of charge (SOC) at which the characteristic peak of the iron phosphate begins to appear during the charging process is obtained. LFP and the SOC when the characteristic peak of the lithium iron phosphate disappears FP ; with L=SOC FP -SOC LFP The value of L is used to evaluate the cycle stability of the lithium iron phosphate cathode material. The smaller the L value, the better the cycle stability of the lithium iron phosphate cathode material being tested.

2. The method for evaluating the cycle stability of lithium iron phosphate cathode material according to claim 1, characterized in that, The parameters of the in-situ XRD testing equipment include: step size, scan speed, and scan angle; The step size is 0.01° to 0.03°; The scanning speed is 1–3° / min; The scanning angle is 27° to 34°.

3. The method for evaluating the cycle stability of lithium iron phosphate cathode material according to claim 2, characterized in that, The step size is 0.02° and the scanning speed is 2° / min.

4. The method for evaluating the cycle stability of lithium iron phosphate cathode material according to claim 1, characterized in that, The parameter settings of the charging and discharging device include: discharging to 2-2.5V at 0.1-2C, resting for 10 minutes, and charging to 3.65-4.5V at 0.1C constant current.

5. The method for evaluating the cycle stability of lithium iron phosphate cathode material according to claim 1, characterized in that, The charge / discharge data set includes voltage, absolute time, current, and capacity.

6. The method for evaluating the cycle stability of lithium iron phosphate cathode material according to claim 1, characterized in that, The XRD data set includes: the angles and corresponding intensities of all XRD curves, and the absolute time at which each XRD curve began testing.

7. The method for evaluating the cycle stability of lithium iron phosphate cathode material according to claim 1, characterized in that, The XRD curves in the XRD curve diagram consist of 85 to 110 lines.

8. The method for evaluating the cycle stability of lithium iron phosphate cathode material according to claim 1, characterized in that, Under the same test conditions, n kinds of lithium iron phosphate cathode materials to be tested are tested respectively, n≥2, and their respective L values ​​are obtained. By comparing the L values ​​of the n kinds of lithium iron phosphate cathode materials to be tested, the cycle performance of the n kinds of lithium iron phosphate cathode materials to be tested is evaluated.

9. The method for evaluating the cycle stability of lithium iron phosphate cathode material according to claim 1, characterized in that, The battery includes a positive electrode sheet; the positive electrode sheet includes a positive current collector and a positive active layer disposed on at least one side surface of the positive current collector; The positive electrode active layer includes the lithium iron phosphate positive electrode material, binder and conductive agent; the mass ratio of the lithium iron phosphate positive electrode material, binder and conductive agent is (88-92):(4-6):(4-6).

10. The method for evaluating the cycle stability of lithium iron phosphate cathode material according to claim 9, characterized in that, The battery preparation method includes: under the conditions of oxygen content <0.1ppm and water content <0.1ppm, sequentially placing and assembling the in-situ tooling base, the positive electrode sheet, the separator, the electrolyte, the lithium sheet, the gasket, the spring sheet and the in-situ tooling cover.

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