Method for nondestructively determining lithium separation boundary of lithium battery

Abstract

The invention discloses a method for losslessly determining a lithium separation boundary of a lithium battery, which comprises the following steps: (1) carrying out constant-current charging on the lithium battery by using a current to be measured, standing for 4.8-5.2 seconds after the SOC of a target is increased in each charging process, then continuing to carry out constant-current charging, and carrying out such setting until the charging reaches an upper limit voltage; (2) calculating impedance according to the voltage at the end of charging, the voltage after shelving and the charging current under each SOC, and drawing according to the impedance and the corresponding SOC to obtain an impedance-SOC curve; if the impedance-SOC curve has an inflection point at the last phase, it is represented that lithium precipitation occurs in the lithium battery, and the SOC at the inflection point serves as the lithium precipitation boundary of the lithium battery under the current to be measured. According to the method, the lithium precipitation boundary can be rapidly and conveniently determined in a lossless manner, the accuracy is high, characterization can be carried out in the whole life cycle of the lithium battery, the safety of the battery is ensured, and accidents are reduced.

Description

Technical Field

[0001] The present invention relates to the field of lithium batteries, and in particular to a method for non-destructively determining the lithium plating boundary of a lithium battery. Background Art

[0002] The current market demands that lithium batteries possess both high energy density and fast charging capabilities. However, when batteries are charged at low temperatures, high rates, and high SOC, lithium plating often occurs, leading to battery performance degradation and, in severe cases, even safety incidents. Therefore, it is necessary to explore the boundaries of lithium plating in lithium batteries.

[0003] Currently, there are several main methods for determining the lithium plating boundary of batteries: (1) destructive disassembly to observe whether lithium plating occurs at the negative electrode interface. This method will damage the battery and cannot be quantified; (2) voltage relaxation method. This method cannot determine when lithium plating occurs through a single test; (3) three-electrode test. This method will damage the battery. Therefore, it is urgent to propose a non-destructive and convenient method to explore the lithium plating boundary of batteries. Summary of the Invention

[0004] The present invention is based on the inventor's discovery and understanding of the following facts and problems: the current method for determining the boundary of battery lithium deposition has many problems, such as causing battery damage, being unable to quantify, and being unable to determine when lithium deposition occurs through a single test.

[0005] The present invention aims to address, at least to some extent, one of the technical problems in the related art. To this end, embodiments of the present invention provide a method for non-destructively determining the lithium plating boundary of a lithium battery. This method can quickly and easily determine the lithium plating boundary with high accuracy and can characterize the lithium battery throughout its life cycle, thereby ensuring battery safety and reducing the risk of accidents.

[0006] An embodiment of the present invention provides a method for non-destructively determining the lithium deposition boundary of a lithium battery, comprising the following steps:

[0007] (1) Charge the lithium battery at a constant current with the current to be measured. After each charge to increase the target SOC, wait for 4.8-5.2 seconds, then continue charging at a constant current until the upper voltage limit is reached.

[0008] (2) Calculate the impedance by combining the voltage at the end of charging at each SOC with the voltage after the battery is idle and the charging current (current to be measured), and plot the impedance against the corresponding SOC to obtain an impedance-SOC curve; if the impedance-SOC curve has an inflection point at the end, it means that lithium plating has occurred in the lithium battery, and the SOC at the inflection point serves as the lithium plating boundary of the lithium battery at the current to be measured (the rate).

[0009] The advantages and technical effects of the method for non-destructively determining the lithium plating boundary of a lithium battery according to an embodiment of the present invention are as follows: the lithium plating boundary is determined by impedance analysis. Since the lithium plating layer on the surface of the negative electrode consumes part of the current when the battery is plating lithium, the lithium plating layer and the negative electrode sheet form a parallel circuit, which reduces the impedance. On the impedance-SOC curve, it can be intuitively seen that the curve has an inflection point at the end, so that the SOC point at the inflection point can be determined as the lithium plating point of the lithium battery at this rate. The present invention calculates the impedance by the voltage difference between the voltage at the end of charging at each SOC and the voltage after 4.8-5.2s of storage and the charging current. After each charge increases the target SOC, the battery is stored for 4.8-5.2s, and the internal polarization effect of the battery is alleviated accordingly. It takes a certain amount of time for the ions to diffuse internally, which is beneficial to improve the test accuracy. If the storage time is 1-4s, the storage time is too short, and the concentration difference polarization and electrochemical polarization on the electrode surface cannot be fully dissipated, which may result in a small pressure difference, no inflection point, and the inability to determine the lithium plating boundary of the lithium battery. When left alone for 6-10s, although the polarization is reduced, the ions and current are too stable, resulting in the inability to obtain the final boundary more accurately due to the diffusion of ions at the lithium deposition point.

[0010] In the embodiment of the present invention, compared to the destructive disassembly that will cause damage to the battery, the method of the present invention is a non-destructive and convenient method for exploring the lithium plating boundary of the battery; at the same time, compared to the voltage relaxation method, the present invention can determine when lithium plating occurs through a single test; compared to the three-electrode test, it does not cause damage to the battery and the test accuracy is high. In addition, the method of the present invention can be widely used in the development process of battery cells to quickly determine the lithium plating boundary of the battery. At the same time, the method can also be used in battery packs (packs) and can be characterized throughout the entire life cycle, ensuring the safety of the battery and reducing the occurrence of accidents.

[0011] In some embodiments, before constant current charging the lithium battery with the current to be tested, the lithium battery is activated first; the activation of the lithium battery includes: activating the lithium battery just off the production line by circulating it three times at a current of 0.33C at room temperature.

[0012] In some embodiments, the discharge capacity of the third cycle is taken as the actual capacity of the lithium battery.

[0013] In some embodiments, in step (1), the charging is shelved after each 1% SOC increase;

[0014] And / or, wait for 5 seconds after each charge to increase the target SOC.

[0015] In some embodiments, in step (1), the constant current charging rate is 0.1-5C.

[0016] In some embodiments, step (1) is performed at room temperature.

[0017] In some embodiments, in step (2), the impedance is obtained by dividing the voltage difference between the voltage at the end of charging and the voltage after storage at each SOC by the charging current.

[0018] In some embodiments, in step (2), if the curve does not have an inflection point at the end, it means that lithium plating has not occurred in the lithium battery.

[0019] In some embodiments, in step (2), the error of the lithium deposition boundary is less than 1% SOC.

[0020] In some embodiments, the lithium battery includes a lithium iron phosphate battery or a lithium iron manganese phosphate battery;

[0021] and / or, used to determine the lithium plating boundary of a lithium battery cell or battery pack;

[0022] And / or, used to determine the lithium plating boundary of the entire life cycle of the lithium battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 1 and 2 are impedance-SOC curves of Example 1 and Example 2.

[0024] Figure 2 This is the three-electrode negative electrode potential and SOC curve of comparative example 1.

[0025] Figure 3 This is an impedance-SOC curve diagram for different shelf times in Comparative Example 2. DETAILED DESCRIPTION

[0026] The embodiments of the present invention are described in detail below, and examples of the embodiments are shown in the accompanying drawings. The embodiments described below with reference to the accompanying drawings are exemplary and intended to be used to explain the present invention, but should not be understood as limiting the present invention.

[0027] An embodiment of the present invention provides a method for non-destructively determining the lithium deposition boundary of a lithium battery, comprising the following steps:

[0028] (1) Charge the lithium battery at a constant current with the current to be measured. After each charge to increase the target SOC, wait for 4.8-5.2 seconds, then continue charging at a constant current until the upper voltage limit is reached.

[0029] (2) Calculate the impedance by taking the voltage at the end of charging at each SOC, the voltage after 4.8-5.2s of storage, and the charging current, and plot the impedance against the corresponding SOC to obtain an impedance-SOC curve; if the impedance-SOC curve has an inflection point at the end, it means that lithium deposition has occurred in the lithium battery, and the SOC at the inflection point is used as the lithium deposition boundary of the lithium battery under the current to be tested.

[0030] The method for non-destructively determining the lithium plating boundary of a lithium battery according to an embodiment of the present invention determines the lithium plating boundary by impedance analysis. Since the lithium plating layer on the surface of the negative electrode consumes a portion of the current when the battery is plating lithium, the lithium plating layer and the negative electrode sheet form a parallel circuit, which reduces the impedance. On the impedance-SOC curve, it can be intuitively seen that the curve has an inflection point at the end, so that the SOC point at the inflection point can be determined as the lithium plating point of the lithium battery at this rate. The present invention calculates the impedance by the voltage difference and charging current between the voltage at the end of charging at each SOC and the voltage after 4.8-5.2s of storage. After each charge increases the target SOC, the battery is stored for 4.8-5.2s, and the internal polarization effect of the battery is alleviated accordingly. It takes a certain amount of time for the ions to diffuse internally, which is beneficial to improve the test accuracy. If the storage time is 1-4s, the storage time is too short, and the concentration difference polarization and electrochemical polarization on the electrode surface cannot be fully dissipated, which may result in a small pressure difference, no inflection point, and the inability to determine the lithium plating boundary of the lithium battery. When left alone for 6-10s, although the polarization is reduced, the ions and current are too stable, resulting in the inability to obtain the final boundary more accurately due to the diffusion of ions at the lithium deposition point.

[0031] In the embodiment of the present invention, compared to destructive disassembly that will cause damage to the battery, the method of the present invention is a non-destructive and convenient method for exploring the boundaries of lithium deposition in the battery; at the same time, compared to the voltage relaxation method, the present invention can determine when lithium deposition occurs through a single test; compared to the three-electrode test, it will not cause damage to the battery and the test accuracy is high. In addition, the method of the present invention can be widely used in the development process of battery cells to quickly determine the boundaries of lithium deposition in the battery. At the same time, the method can also be used in battery packs and can be characterized throughout the entire life cycle, ensuring the safety of the battery and reducing the occurrence of accidents.

[0032] In some embodiments, before constant current charging the lithium battery with the current to be tested, the lithium battery is first activated; activating the lithium battery includes: activating the lithium battery that has just come off the production line by circulating it three times at a current of 0.33C at room temperature; and taking the discharge capacity of the third cycle as the actual capacity of the lithium battery.

[0033] In the present embodiment, a newly-released lithium battery is activated by cycling it three times at room temperature at a current of 0.33C to ensure the stability of the battery's internal systems, thereby guaranteeing the accuracy of the test results. Without activation, the capacity difference before and after cycling is significant. The lithium battery is cycled at room temperature at a current of 0.33C, where the temperature rise is minimal. These three cycles represent charge and discharge at 0.33C. Activation is performed at room temperature; using a high-temperature constant-capacity battery would increase the capacity.

[0034] In some embodiments, in step (1), the battery is left for 4.8-5.2 seconds after each 1% increase in SOC. In the embodiment of the present invention, the battery is left for 4.8-5.2 seconds after each 1% increase in SOC to reduce errors caused by thermal accumulation and polarization effects.

[0035] In some embodiments, in step (1), each charging step increases the target SOC and is paused for 4.8-5.2 seconds, specifically, for example, 4.8 seconds, 4.9 seconds, 5.0 seconds, 5.1 seconds, 5.2 seconds, preferably, 5.0 seconds.

[0036] In some embodiments, in step (1), there is no restriction on the current to be measured, and the lithium deposition boundary under any current can be measured. For example, the constant current charging rate can be 0.1-5C, specifically, for example, 0.1C, 0.5C, 1C, 2C, 3C, 4C, 5C. In the embodiment of the present invention, the constant current charging rate can be 0.1-5C, and the maximum critical point at each rate, that is, the lithium deposition boundary, can be measured. Fast charging is generally accumulated by charging to different SOCs at different rates, and the fast charging step of the final battery or battery pack can be designed by the SOC corresponding to the lithium deposition potential at each rate.

[0037] In some embodiments, step (1) is performed at room temperature. In the present invention, high temperature may lead to super performance due to the influence of kinetic thermodynamics, while low temperature severely limits kinetics. Therefore, step (1) is performed at room temperature.

[0038] In some embodiments, in step (1), the battery is charged to 100% SOC.

[0039] In some embodiments, in step (2), the impedance is obtained by dividing the voltage difference between the voltage at the end of charging and the voltage after the standby at each SOC by the charging current, that is, R=U / I, R is the impedance, I is the charging current, and U is equal to the voltage difference between the voltage at the end of charging and the voltage after the standby, that is, the voltage at the end of charging minus the voltage after the standby.

[0040] In some embodiments, in step (2), if the curve does not have an inflection point at the end, it means that lithium plating has not occurred in the lithium battery.

[0041] In some embodiments, in step (2), the error of the lithium deposition boundary is less than 1% SOC.

[0042] In some embodiments, the lithium battery includes a lithium iron phosphate battery or a lithium manganese iron phosphate battery; optionally, the upper limit voltage of the lithium battery is 3.65V.

[0043] In some embodiments, the non-destructive method for determining the lithium plating boundary of a lithium battery is used to determine the lithium plating boundary of a lithium battery cell or battery pack. In these embodiments, the method can be widely used during the development of battery cells to quickly determine the lithium plating boundary of a battery. The method can also be used in battery packs.

[0044] In some embodiments, the non-destructive method for determining the lithium plating boundary of a lithium battery is used to determine the lithium plating boundary throughout the battery's life cycle. In this embodiment of the present invention, characterization can be performed throughout the battery's life cycle, ensuring battery safety and reducing the risk of accidents.

[0045] The present invention is described below with reference to specific examples. It should be noted that these examples are merely illustrative and do not limit the present invention in any way.

[0046] Example 1

[0047] A method for non-destructively determining the lithium plating boundary of a lithium battery comprises the following steps:

[0048] (1) A 166Ah square lithium iron phosphate battery just off the production line was activated by cycling it three times at room temperature with a current of 0.33C to ensure the stability of the battery. The discharge capacity of the third cycle (167.4Ah) was taken as the actual capacity C0 of the battery.

[0049] (2) Charge the lithium battery at a constant current of 0.1C0 (0.1C rate) at room temperature. The steps are as follows: after charging 1% SOC (0.01*C0), wait for 5 seconds, and then continue constant current charging until the battery reaches 3.65V (upper limit voltage).

[0050] (3) The impedance is calculated by the voltage at the end of charging at each SOC, the voltage after 5s of storage, and the charging current according to R = U / I, and the impedance is plotted against SOC to obtain the impedance-SOC curve, as shown in Figure 1 As shown in curve a.

[0051] Curve a does not have an inflection point at the end, indicating that when the battery is charged at a constant current of 0.1C0, lithium plating does not occur in the lithium battery. This is because the low current density matches the ion embedding rate.

[0052] Example 2

[0053] A method for non-destructively determining the lithium plating boundary of a lithium battery comprises the following steps:

[0054] (1) A 166Ah square lithium iron phosphate battery (from the same batch as Example 1) was activated by cycling it three times at room temperature with a current of 0.33C to ensure the stability of the battery. The discharge capacity of the third cycle (167.4Ah) was taken as the actual capacity C0 of the battery.

[0055] (2) Charge the lithium battery at a constant current of 2C0 at room temperature. The steps are as follows: after charging 1% SOC (0.01*C0), wait for 5 seconds, then continue charging at a constant current until the battery reaches 3.65V.

[0056] (3) The impedance is calculated by using the voltage at the end of charging at each SOC, the voltage after 5 seconds of storage, and the charging current according to R = U / I, and the impedance is plotted against SOC to obtain the impedance-SOC curve, as shown in Figure 1 As shown in curve b.

[0057] Curve b has an inflection point at the end, indicating that when the battery is charged at a constant current of 2C0, lithium plating occurs in the lithium battery. The SOC of the inflection point is 87% SOC, which is the lithium plating boundary of the lithium battery at this rate. When lithium plating occurs, the lithium plating layer on the surface of the negative electrode consumes part of the current, so that the lithium plating layer and the negative electrode form a parallel circuit, thereby reducing the impedance. This can be seen intuitively on the curve, so that the SOC point can be determined as the lithium plating point of the battery at this rate.

[0058] Comparative Example 1

[0059] A method for determining the lithium deposition boundary of a lithium battery using three electrodes comprises the following steps:

[0060] (1) One battery was selected from the same batch of batteries as in Example 1 to make three electrodes. After it was off the production line, it was activated by cycling for three cycles at room temperature with a current of 0.33 C. The discharge capacity of the third cycle (167.5 Ah) was taken as the actual capacity C1 of the battery.

[0061] (2) Charge the lithium battery at a constant current of 2C1 at room temperature, and monitor the potential change of the negative electrode with a multi-channel instrument;

[0062] (3) Plot the negative electrode potential change and SOC obtained by the multi-channel monitoring above to obtain Figure 2 , Figure 2 The SOC value when the negative electrode potential is 0 is 87.1% SOC, that is, lithium deposition occurs at 87.1% SOC.

[0063] However, the testing method in Comparative Example 1 requires implanting a copper wire during the battery manufacturing process, which may damage the battery and cannot achieve non-destructive and convenient determination of the battery lithium plating boundary.

[0064] Comparative Example 2

[0065] The method is the same as that of Example 2, except that the holding time is 3s, 5s, and 8s respectively.

[0066] contrast Figure 1 and Figure 2 It can be seen that for the method of the present invention, Figure 1 Curve b in FIG1 has an inflection point at the end of charging, which is 87% SOC. The SOC value of the method in Comparative Example 1 when the negative electrode potential is 0 is 87.1% SOC. Figure 1 The 87% SOC point corresponding to the inflection point of lithium deposition determined by the method of the present invention is Figure 2 The 87.1% SOC when the negative electrode potential of lithium deposition is 0 is basically consistent. Therefore, it can be explained that the lithium deposition SOC point determined by the method of the present invention is accurate, and the method of the present invention can be used as a method for exploring the lithium deposition boundary of lithium batteries.

[0067] Figure 3 This is a graph showing the impedance and SOC curves for different shelf times in Comparative Example 2. Figure 3 It can be seen that the biggest difference between the methods of Comparative Example 2 and Example 2 is the shelving time, and the length of the shelving time will affect the final test results. The lithium deposition SOC point determined when shelving for 5s in Example 2 is accurate. Shelving for 1-3s, for example, when shelving for 3s in Comparative Example 2, the shelving time is too short, and the concentration difference polarization and electrochemical polarization on the electrode surface cannot be fully dissipated, which may lead to a small pressure difference, no inflection point, and the inability to determine the lithium deposition boundary of the lithium battery, which will lead to the use of the wrong rate during subsequent battery use and accelerate the lithium deposition of the battery. Shelving for 8-10s, for example, when shelving for 8s in Comparative Example 2, the measured lithium deposition boundary is 90% SOC. Although the polarization is greatly reduced, the ions and current are too stable, resulting in the inability to obtain the final boundary more accurately due to the diffusion of ions at the lithium deposition point, which is not conducive to the lithium battery to perform its fast charging performance.

[0068] In the present invention, the terms "one embodiment", "some embodiments", "examples", "specific examples", or "some examples" mean that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art can combine and combine different embodiments or examples described in this specification and the features of different embodiments or examples without contradiction.

[0069] Although the above embodiments have been shown and described, it is understood that the above embodiments are illustrative and are not to be construed as limitations on the present invention. Changes, modifications, substitutions and variations of the above embodiments by those skilled in the art are all within the scope of protection of the present invention.

Claims

1. A method for non-destructively determining the lithium deposition boundary of a lithium battery, characterized in that: The following steps are involved: (1) Charge the lithium battery at a constant current with the current to be measured. After each charge to increase the target SOC, wait for 4.8-5.2 seconds, then continue charging at a constant current until the upper voltage limit is reached. (2) Calculate the impedance by taking the voltage at the end of charging at each SOC, the voltage after being left idle, and the charging current, and plot the impedance against the corresponding SOC to obtain an impedance-SOC curve; if the impedance-SOC curve has an inflection point at the end, it means that lithium deposition has occurred in the lithium battery, and the SOC at the inflection point is used as the lithium deposition boundary of the lithium battery under the current to be tested.

2. The method for non-destructively determining the lithium deposition boundary of a lithium battery according to claim 1, wherein: Before charging the lithium battery with a constant current at the current to be tested, activate the lithium battery first; Activating lithium batteries includes: circulating the newly-produced lithium batteries at room temperature for three cycles with a current of 0.33C.

3. The method for non-destructively determining the lithium deposition boundary of a lithium battery according to claim 2, wherein: The discharge capacity of the third cycle is taken as the actual capacity of the lithium battery.

4. The method for non-destructively determining the lithium deposition boundary of a lithium battery according to claim 1, wherein: In the step (1), the battery is left on hold after each charging step increases the SOC by 1%; And / or, wait for 5 seconds after each charge to increase the target SOC.

5. The method for non-destructively determining the lithium deposition boundary of a lithium battery according to claim 1, wherein: In the step (1), the constant current charging rate is 0.1-5C.

6. The method for non-destructively determining the lithium deposition boundary of a lithium battery according to claim 1, wherein: The step (1) is carried out at room temperature.

7. The method for non-destructively determining the lithium deposition boundary of a lithium battery according to claim 1, wherein: In the step (2), the impedance is obtained by dividing the voltage difference between the voltage at the end of charging and the voltage after the battery is left idle at each SOC by the charging current.

8. The method for non-destructively determining the lithium deposition boundary of a lithium battery according to claim 1, wherein: In the step (2), if the curve does not have an inflection point at the end, it means that lithium deposition has not occurred in the lithium battery.

9. The method for non-destructively determining the lithium deposition boundary of a lithium battery according to claim 1, wherein: In the step (2), the error of the lithium deposition boundary is less than 1% SOC.

10. The method for non-destructively determining the lithium deposition boundary of a lithium battery according to claim 1, wherein: The lithium battery includes a lithium iron phosphate battery and a lithium manganese iron phosphate battery; and / or, used to determine the lithium plating boundary of a lithium battery cell or battery pack; And / or, used to determine the lithium plating boundary of the entire life cycle of the lithium battery.

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