A method for quantifying lithium deposition in lithium-ion batteries based on pressure signals

Abstract

The present invention relates to the technical field of lithium-ion batteries and discloses a method for quantifying lithium deposition in lithium-ion batteries based on pressure signals. The method comprises the following steps: performing charge and discharge cycles at different rates on a test cell and a comparison cell at a temperature T until a specified number of cycles are reached, obtaining a discharge capacity set Q, a voltage set V, and an expansion force signal set P of the test cell and the comparison cell during the discharge phase; subtracting the initial expansion force from the expansion force signals of the test cell and the comparison cell during the discharge phase to obtain an expansion force variation curve; and performing differentiation on the expansion force variation curve to obtain a differential expansion force-discharge capacity curve. The method can realize non-destructive lithium deposition detection and simultaneously perform reversible lithium quantification analysis on the lithium deposition battery. Compared with a single relaxation voltage quantification method, the method introduces an expansion force signal. Experiments show that when the starting point of the differential expansion force-discharge capacity curve during the discharge phase is less than the first differential expansion force, it is determined that lithium deposition has occurred in the cell.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and in particular to a method for quantifying lithium deposition in lithium-ion batteries based on pressure signals. Background Art

[0002] Lithium-ion batteries have the characteristics of high energy density, long cycle life and high power capacity, and are the mainstream power source for electric vehicles. During the charging process, lithium-ion batteries will undergo significant volume expansion caused by lithiation, resulting in greater stress in the battery module or battery pack, which in turn affects the battery's cycle life and even safety performance.

[0003] Existing methods for detecting lithium deposition in lithium-ion batteries mainly include the relaxation voltage method and the disassembly method. The former requires the battery to be immediately left to rest (relax) or discharged at a low current after a series of charge and discharge cycles. By monitoring the changes in voltage during the relaxation process, it is possible to determine whether lithium deposition has occurred in the battery and to quantify the amount of lithium deposition. This has been proven to be an effective lithium deposition detection method by most researchers. The latter involves disassembling a fully charged lithium battery and observing the surface of the battery's negative electrode. If metallic lithium is present, it is determined that lithium deposition has occurred in the lithium battery.

[0004] However, the voltage platform in the relaxation stage is affected by the combined effects of lithium stripping and lithium intercalation. When the temperature decreases, the lithium ion intercalation rate decreases. Therefore, the lithium stripping time reflected by the same lithium deposition amount is prolonged, which in turn affects the accuracy of lithium deposition quantification. The disassembly method is a destructive test and its usage scenarios are limited. Therefore, a lithium deposition quantification method for lithium-ion batteries based on pressure signals is proposed to solve the above problems. Summary of the Invention

[0005] In response to the shortcomings of the existing technology, the present invention provides a lithium-ion battery lithium deposition quantification method based on pressure signals, which has the advantages of quantitatively detecting the reversible lithium capacity in the lithium deposition battery while avoiding damage to the lithium battery, and improving the accuracy of lithium deposition quantification, thereby solving the problems mentioned in the above background technology.

[0006] To achieve the above object, the present invention provides the following technical solution: a method for quantifying lithium deposition in lithium-ion batteries based on pressure signals, characterized in that it comprises the following steps:

[0007] S1: charging and discharging the test cell and the reference cell at different rates for a specified number of cycles at temperature T, and obtaining a discharge capacity set Q, a voltage set V, and an expansion force signal set P of the test cell and the reference cell during the discharge phase;

[0008] S2: Subtract the initial expansion force of discharge from the expansion force signal of the test cell and the reference cell during discharge to obtain an expansion force change curve, and perform differentiation on the expansion force change curve to obtain a differential expansion force-discharge capacity curve A;

[0009] S3: judging the lithium plating condition of the battery cell to be tested based on the differential expansion force-discharge capacity curve A of the battery cell to be tested and the comparison battery cell;

[0010] S4: Differentiating the differential expansion force-discharge capacity curves A of the test cell and the comparison cell again to obtain second-order differential expansion force-discharge capacity curves A' of the test cell and the comparison cell;

[0011] S5: judging the lithium plating condition of the battery cell to be tested based on the second-order differential expansion force-discharge capacity curve A' of the battery cell to be tested and the comparison battery cell;

[0012] S6: Quantify the lithium plating situation of the battery cell to be tested.

[0013] Preferably, the number of cycles in step S1 is ten times, and the charging rate is 0.2C-1C.

[0014] Preferably, the specific steps in step S2 are:

[0015] S2.1: Interpolate the data points of the expansion force signal set P and the discharge capacity set Q to extrapolate the correlation function of the expansion force-discharge capacity curve, and perform difference calculation on the extrapolated curve. The expression is:

[0016]

[0017]

[0018] Among them, P i is an expansion force value in the expansion force signal set P, △P is the expansion force change value, Q i is a capacity value in the discharge capacity set Q, and △Q is the change value of the discharge capacity;

[0019] S2.2: The discharge capacity change value △Q after difference calculation is used as the horizontal axis, and the expansion force change value △P is used as the vertical axis to obtain the differential expansion force-discharge capacity curve A.

[0020] Preferably, the specific steps in step S3 are:

[0021] S3.1: Determine the starting point of the differential expansion force-discharge capacity curve A of the comparison cell as the first differential discharge capacity P r The starting point of the differential expansion force-discharge capacity curve A of the battery cell to be tested is the first differential discharge capacity P r ';

[0022] S3.2: Determine the first differential discharge capacity P r '<First differential discharge capacity P rIf yes, lithium deposition occurs in the battery under test; otherwise, no lithium deposition occurs in the battery under test.

[0023] Preferably, the specific steps in step S5 are:

[0024] S5.1: Determine the target capacity of the maximum peak value of the second-order differential expansion force-discharge capacity curve A' of the comparison cell as Q r-plating The target capacity of the maximum peak value of the second-order differential expansion force-discharge capacity curve A' of the cell to be tested is Q r-plating ';

[0025] S5.2: Determine the target capacity Q of the maximum peak r-plating '>Maximum peak target capacity Q r-plating If yes, the battery to be tested has lithium deposition phenomenon, and the target capacity of the maximum peak value is Q r-plating ' is the reversible lithium content C of the battery to be tested rev-plating If not, it is determined that the battery cell under test has no lithium plating.

[0026] Preferably, the specific steps of determining the maximum peak value of the second-order differential expansion force-discharge capacity curve A' in step S5 are:

[0027] Step 1: Remove invalid values ​​in the second-order differential expansion force-discharge capacity curve A' to ensure the validity and integrity of the data;

[0028] Step 2: Traverse the second-order differential expansion force-discharge capacity curve A', the experimental data is (x i, f i ), directly find the point with the largest amplitude and record its index position. The expression is:

[0029] ;

[0030] Among them, x i is the discharge capacity value, f i is the second-order differential expansion force-discharge capacity signal amplitude, N is the number of data points, is the index position with the largest amplitude, is the maximum second-order differential expansion force-discharge capacity signal maximum amplitude, is the discharge capacity value corresponding to the maximum second-order differential expansion force-discharge capacity signal amplitude;

[0031] Step 3: Obtain the corresponding discharge capacity value according to the index of the maximum amplitude point;

[0032] Step 4: Output the maximum second-order differential expansion force-discharge capacity signal maximum amplitude And the corresponding discharge capacity value , which is the reversible lithium content C of the battery to be tested rev-plating, and visually mark the peak position in the signal.

[0033] Preferably, the specific steps of step S6 are:

[0034] ;

[0035] ;

[0036] ;

[0037] ;

[0038] Among them, C ch is the charging capacity, C dc is the discharge capacity, C plating is the total capacity generated by lithium plating, C rev.plating is the reversible coating capacity, C irrev.plating is the irreversible coating capacity, n plating is the amount of lithium in the coating, F is the Faraday constant, m plating is the mass of the lithium plating layer, M Li is the molar mass of lithium.

[0039] Compared with the prior art, the present invention provides a method for quantifying lithium deposition in lithium-ion batteries based on pressure signals, which has the following beneficial effects:

[0040] 1. This pressure signal-based lithium deposition quantification method for lithium-ion batteries can achieve non-destructive lithium deposition detection and simultaneously perform reversible lithium quantification analysis on lithium-deposited batteries. Compared with the single relaxation voltage quantification method, this method introduces an expansion force signal. Experiments show that when the starting point of the differential expansion force-discharge capacity curve during the discharge stage is less than the first differential expansion force, it is determined that lithium deposition has occurred in the battery cell.

[0041] 2. Compared with existing lithium deposition detection technologies, this pressure signal-based lithium deposition quantification method for lithium-ion batteries does not require additional processing methods. By comparing the first differential expansion force, the occurrence of lithium deposition can be intuitively judged, which reduces the detection time, reduces the limitations of the detection model usage scenarios, and has high detection accuracy. On the other hand, in the second-order differential expansion force-discharge capacity curve, the capacity corresponding to the maximum peak intensity is the reversible lithium capacity of the lithium deposition battery. This discovery fills the gap in the current research on the quantification of lithium deposition by expansion force signals. It can not only avoid damaging lithium-ion batteries, but also has higher accuracy than the existing commonly used relaxation voltage method, thereby improving the accuracy and adaptability of lithium deposition quantification. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 This is a flow chart of a method for quantifying lithium deposition in lithium-ion batteries based on pressure signals proposed by the present invention;

[0043] Figure 2 This is a diagram of the expansion force change of a lithium-ion battery under low current discharge at 0°C, based on a pressure signal-based lithium deposition quantification method proposed by the present invention;

[0044] Figure 3 This is a graph of the differential expansion force change during low-current discharge at 0°C versus discharge capacity in a pressure signal-based lithium deposition quantification method for lithium-ion batteries proposed by the present invention.

[0045] Figure 4 This is a graph of the change in the second-order differential expansion force of a small current discharge at 0°C versus the discharge capacity in a lithium-ion battery lithium deposition quantification method based on a pressure signal proposed by the present invention;

[0046] Figure 5 This is a differential voltage diagram of a small current discharge at 0°C in a lithium-ion battery lithium deposition quantification method based on a pressure signal proposed by the present invention;

[0047] Figure 6 for Figure 5 Middle partial detail drawing;

[0048] Figure 7 This is a relaxation voltage diagram of a small current discharge at 0°C in a lithium-ion battery lithium deposition quantification method based on a pressure signal proposed by the present invention;

[0049] Figure 8 for Figure 7 Partial detail drawing. DETAILED DESCRIPTION

[0050] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0051] See also Figure 1-8 A method for quantifying lithium deposition in lithium-ion batteries based on pressure signals comprises the following steps:

[0052] S1: charging and discharging the test cell and the reference cell at different rates for a specified number of cycles at temperature T, and obtaining a discharge capacity set Q, a voltage set V, and an expansion force signal set P of the test cell and the reference cell during the discharge phase;

[0053] The number of cycles is ten times, and the charging rate is any value between 0.2C and 1C;

[0054] In a constant temperature incubator at 0°C, four experimental soft-pack batteries of the same specifications were charged and discharged at different charge and discharge rates (0.2C, 0.5C, 0.75C, and 1C). After 10 charge and discharge cycles, each experimental battery was charged with constant current and constant voltage at the same charge rate as the last charge rate, and then immediately discharged at 1A to obtain the expansion force signal, voltage signal, and discharge capacity signal of the lithium-ion battery during the discharge process.

[0055] It should be noted that in the currently widely used battery material systems, whether it is cathode materials such as lithium iron phosphate, lithium iron manganese phosphate, and ternary materials, or anode materials such as graphite and silicon, the lattice spacing within the cathode and anode materials changes during the lithium ion extraction (embedding) process, resulting in a change in the electrode volume. This volume change is ultimately converted into a change in expansion force detected by the sensor.

[0056] The lithium-ion battery used in this example uses lithium iron phosphate as the positive electrode material and graphite as the negative electrode material. Its rated voltage is 2.0V-3.65V and its nominal capacity is 10Ah. The experimental equipment used includes a three-ply constant gap device, an Arbin charge and discharge tester (SN: 215671), a Galaxy high and low temperature chamber (SDJ405F), and expansion force acquisition using a Smowo LCS-C3 spoke-type sensor (accuracy of 0.03mV / V) connected to an Arbin pressure acquisition instrument.

[0057] S2: Subtract the initial expansion force of discharge from the expansion force signal of the test cell and the reference cell during discharge to obtain an expansion force change curve, and perform differentiation on the expansion force change curve to obtain a differential expansion force-discharge capacity curve A;

[0058] S3: judging the lithium plating condition of the battery cell to be tested according to the differential expansion force-discharge capacity curve A of the battery cell to be tested and the comparison battery cell;

[0059] S4: Differentiating the differential expansion force-discharge capacity curves A of the test cell and the comparison cell again to obtain second-order differential expansion force-discharge capacity curves A' of the test cell and the comparison cell;

[0060] S5: judging the lithium plating condition of the battery cell to be tested based on the second-order differential expansion force-discharge capacity curve A' of the battery cell to be tested and the comparison battery cell;

[0061] At the end of charging and the transition to low-current discharge, the negative electrode undergoes multiple processes, including lithium stripping and delithiation. Lithium stripping requires relatively low energy, so in the initial stages of discharge, the battery is primarily driven by lithium stripping. Because lithium is deposited on the negative electrode surface, unlike normal batteries where lithium ions are embedded in the graphite lattice, changes in expansion force exhibit differences from those of normal batteries. Therefore, the reversible lithium quantification of lithium in lithium-ion batteries can be performed by detecting changes in expansion force per unit capacity.

[0062] S6: Quantify the lithium plating situation of the battery cell to be tested.

[0063] Specific steps in step S2:

[0064] S2.1: Interpolate the data points of the expansion force signal set P and the discharge capacity set Q to extrapolate the correlation function of the expansion force-discharge capacity curve, and perform difference calculation on the extrapolated curve. The expression is:

[0065] ;

[0066] ;

[0067] Among them, P i is an expansion force value in the expansion force signal set P, △P is the expansion force change value, Q i is a capacity value in the discharge capacity set Q, and △Q is the change value of the discharge capacity;

[0068] S2.2: The discharge capacity change value △Q after difference calculation is used as the horizontal axis, and the expansion force change value △P is used as the vertical axis to obtain the differential expansion force-discharge capacity curve.

[0069] The specific steps in step S3 are:

[0070] S3.1: Determine the starting point of the differential expansion force-discharge capacity curve A of the comparison cell as the first differential discharge capacity P r The starting point of the differential expansion force-discharge capacity curve A of the battery cell to be tested is the first differential discharge capacity P r ';

[0071] S3.2: Determine the first differential discharge capacity P r '<First differential discharge capacity P r If yes, lithium deposition occurs in the battery under test; otherwise, no lithium deposition occurs in the battery under test.

[0072] The specific steps in step S5 are:

[0073] S5.1: Determine the target capacity of the maximum peak value of the second-order differential expansion force-discharge capacity curve A' of the comparison cell as Q r-plating The target capacity of the maximum peak value of the second-order differential expansion force-discharge capacity curve A' of the cell to be tested is Q r-plating ';

[0074] S5.2: Determine the target capacity Q of the maximum peak r-plating '>Maximum peak target capacity Q r-plating If yes, the battery to be tested has lithium deposition phenomenon, and the target capacity of the maximum peak value is Q r-plating ' is the reversible lithium content C of the battery to be testedrev-plating If not, it is determined that the battery cell under test has no lithium plating.

[0075] The specific steps for determining the maximum peak value of the second-order differential expansion force-discharge capacity curve A' in step S5 are:

[0076] Step 1: Remove invalid values ​​in the second-order differential expansion force-discharge capacity curve A' to ensure the validity and integrity of the data;

[0077] Step 2: Traverse the second-order differential expansion force-discharge capacity curve A', the experimental data is (x i, f i ), directly find the point with the largest amplitude and record its index position. The expression is:

[0078] ;

[0079] Among them, x i is the discharge capacity value, f i is the second-order differential expansion force-discharge capacity signal amplitude, N is the number of data points, is the index position with the largest amplitude, is the maximum second-order differential expansion force-discharge capacity signal maximum amplitude, is the discharge capacity value corresponding to the maximum second-order differential expansion force-discharge capacity signal amplitude;

[0080] Step 3: Obtain the corresponding discharge capacity value according to the index of the maximum amplitude point;

[0081] Step 4: Output the maximum second-order differential expansion force-discharge capacity signal maximum amplitude And the corresponding discharge capacity value , which is the reversible lithium content C of the battery to be tested rev-plating , and visually mark the peak position in the signal.

[0082] The specific steps of step S6 are:

[0083] ;

[0084] ;

[0085] ;

[0086] ;

[0087] Among them, C ch is the charging capacity, C dc is the discharge capacity, C plating is the total capacity generated by lithium plating, C rev.plating is the reversible coating capacity, Cirrev.plating is the irreversible coating capacity, n plating is the amount of lithium in the coating, F is the Faraday constant, F=96485.3399Cmol -1 , m plating is the mass of the lithium plating layer, M Li is the molar mass of lithium, M Li =6.94 gmol -1 .

[0088] During use, under the same charge and discharge conditions, the currently recognized lithium deposition quantification methods, the relaxation voltage method (VRP) and the differential voltage method (DVA), are used to verify the lithium deposition quantification method of lithium-ion batteries based on expansion force signals described in this article.

[0089] Based on existing research, it is generally believed that the differential voltage method (DVA) has a high accuracy in quantifying lithium deposition. Therefore, this study uses the DVA detection method as a benchmark;

[0090] Table 1

[0091]

[0092] Table 2

[0093]

[0094] In summary, this pressure signal-based lithium deposition quantification method for lithium-ion batteries can achieve non-destructive lithium deposition detection and simultaneously perform reversible lithium quantification analysis on lithium-deposited batteries. Compared with the single relaxation voltage quantification method, this method introduces an expansion force signal. Experiments show that when the starting point of the differential expansion force-discharge capacity curve during the discharge stage is less than the first differential expansion force, it is determined that lithium deposition has occurred in the battery cell.

[0095] Compared with existing lithium plating detection technology, this method does not require additional processing means. By comparing the first differential expansion force, the occurrence of lithium plating can be intuitively judged, which reduces the detection time, reduces the limitations of the detection model usage scenarios, and has higher detection accuracy. On the other hand, in the second-order differential expansion force-discharge capacity curve, the capacity corresponding to the maximum peak intensity is the reversible lithium capacity of the lithium plating battery. This discovery fills the gap in the current research on the quantification of lithium plating by expansion force signals. It can not only avoid damaging lithium-ion batteries, but also has higher accuracy than the existing commonly used relaxation voltage method, thereby improving the accuracy and adaptability of lithium plating quantification.

[0096] It should be noted that the terms "comprises," "includes," or any other variations thereof are intended to encompass non-exclusive inclusion, such that a process, method, article, or apparatus that includes a series of elements includes not only those elements but also other elements not explicitly listed, or elements inherent to such process, method, article, or apparatus. In the absence of further limitations, an element defined by the phrase "comprises a ..." does not preclude the presence of other identical elements in the process, method, article, or apparatus that includes the element.

[0097] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A method for quantifying lithium deposition in lithium-ion batteries based on pressure signals, characterized in that: The following steps are involved: S1: charging and discharging the test cell and the reference cell at different rates for a specified number of cycles at temperature T, and obtaining a discharge capacity set Q, a voltage set V, and an expansion force signal set P of the test cell and the reference cell during the discharge phase; S2: Subtract the initial expansion force of discharge from the expansion force signal of the test cell and the reference cell during discharge to obtain an expansion force change curve, and perform differentiation on the expansion force change curve to obtain a differential expansion force-discharge capacity curve A; S3: judging the lithium plating condition of the battery cell to be tested based on the differential expansion force-discharge capacity curve A of the battery cell to be tested and the comparison battery cell; S4: Differentiating the differential expansion force-discharge capacity curves A of the test cell and the comparison cell again to obtain second-order differential expansion force-discharge capacity curves A' of the test cell and the comparison cell; S5: judging the lithium plating condition of the battery cell to be tested based on the second-order differential expansion force-discharge capacity curve A' of the battery cell to be tested and the comparison battery cell; S6: Quantify the lithium plating situation of the battery cell to be tested; The specific steps in step S5 are: S5.1: Determine the target capacity of the maximum peak value of the second-order differential expansion force-discharge capacity curve A' of the comparison cell as Q r-plating The target capacity of the maximum peak value of the second-order differential expansion force-discharge capacity curve A' of the cell to be tested is Q r-plating '; S5.2: Determine the target capacity Q of the maximum peak r-plating '>Maximum peak target capacity Q r-plating If yes, the battery to be tested has lithium deposition phenomenon, and the target capacity of the maximum peak value is Q r-plating ' is the reversible lithium content C of the battery to be tested rev-plating If not, it is determined that the battery cell under test has no lithium plating.

2. The method for quantifying lithium deposition in lithium-ion batteries based on pressure signals according to claim 1, wherein: The number of cycles in step S1 is ten, and the charging rate is 0.2C-1C.

3. The method for quantifying lithium deposition in lithium-ion batteries based on pressure signals according to claim 1, wherein: The specific steps in step S2 are: S2.1: Interpolate the data points of the expansion force signal set P and the discharge capacity set Q to extrapolate the correlation function of the expansion force-discharge capacity curve, and perform difference calculation on the extrapolated curve. The expression is: ； ; Among them, P i is an expansion force value in the expansion force signal set P, △P is the expansion force change value, Q i is a capacity value in the discharge capacity set Q, and △Q is the change value of the discharge capacity; S2.2: The discharge capacity change value △Q after difference calculation is used as the horizontal axis, and the expansion force change value △P is used as the vertical axis to obtain the differential expansion force-discharge capacity curve A.

4. The method for quantifying lithium deposition in lithium-ion batteries based on pressure signals according to claim 1, wherein: The specific steps in step S3 are: S3.1: Determine the starting point of the differential expansion force-discharge capacity curve A of the comparison cell as the first differential discharge capacity P r The starting point of the differential expansion force-discharge capacity curve A of the battery cell to be tested is the first differential discharge capacity P r '; S3.2: Determine the first differential discharge capacity P r '<First differential discharge capacity P r If yes, lithium deposition occurs in the battery under test; otherwise, no lithium deposition occurs in the battery under test.

5. The method for quantifying lithium deposition in lithium-ion batteries based on pressure signals according to claim 4, wherein: The specific steps for determining the maximum peak value of the second-order differential expansion force-discharge capacity curve A' in step S5 are: Step 1: Remove invalid values ​​in the second-order differential expansion force-discharge capacity curve A' to ensure the validity and integrity of the data; Step 2: Traverse the second-order differential expansion force-discharge capacity curve A', the experimental data is (x i, f i ), directly find the point with the largest amplitude and record its index position. The expression is: ; Among them, x i is the discharge capacity value, f i is the second-order differential expansion force-discharge capacity signal amplitude, N is the number of data points, is the index position with the largest amplitude, is the maximum second-order differential expansion force-discharge capacity signal maximum amplitude, is the discharge capacity value corresponding to the maximum second-order differential expansion force-discharge capacity signal amplitude; Step 3: Obtain the corresponding discharge capacity value according to the index of the maximum amplitude point; Step 4: Output the maximum second-order differential expansion force-discharge capacity signal maximum amplitude And the corresponding discharge capacity value , which is the reversible lithium content C of the battery to be tested rev-plating , and visually mark the peak position in the signal.

6. The method for quantifying lithium deposition in lithium-ion batteries based on pressure signals according to claim 1, wherein: The specific steps of step S6 are: ; ; ; ; Among them, C ch is the charging capacity, C dc is the discharge capacity, C plating is the total capacity generated by lithium plating, C rev.plating is the reversible coating capacity, C irrev.plating is the irreversible coating capacity, n plating is the amount of lithium in the coating, F is the Faraday constant, m plating is the mass of the lithium plating layer, M Li is the molar mass of lithium.

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