CN115327421A - Irreversible lithium analysis amount in-situ quantitative detection method based on differential voltage - Google Patents
Irreversible lithium analysis amount in-situ quantitative detection method based on differential voltage Download PDFInfo
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- 229910052744 lithium Inorganic materials 0.000 title claims abstract description 108
- WHXSMMKQMYFTQS-UHFFFAOYSA-N Lithium Chemical compound [Li] WHXSMMKQMYFTQS-UHFFFAOYSA-N 0.000 title claims abstract description 107
- 230000002427 irreversible effect Effects 0.000 title claims abstract description 77
- 238000004458 analytical method Methods 0.000 title claims abstract description 42
- 238000001514 detection method Methods 0.000 title claims abstract description 19
- 238000011065 in-situ storage Methods 0.000 title claims abstract description 17
- 230000032683 aging Effects 0.000 claims abstract description 57
- 238000000926 separation method Methods 0.000 claims abstract description 41
- 238000000034 method Methods 0.000 claims abstract description 32
- 230000002441 reversible effect Effects 0.000 claims abstract description 13
- 238000004364 calculation method Methods 0.000 claims abstract description 12
- HBBGRARXTFLTSG-UHFFFAOYSA-N Lithium ion Chemical compound [Li+] HBBGRARXTFLTSG-UHFFFAOYSA-N 0.000 claims description 23
- 238000002474 experimental method Methods 0.000 claims description 11
- 125000004122 cyclic group Chemical group 0.000 claims description 9
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 claims description 8
- 238000001914 filtration Methods 0.000 claims description 8
- 239000000178 monomer Substances 0.000 claims description 8
- 229910002804 graphite Inorganic materials 0.000 claims description 7
- 239000010439 graphite Substances 0.000 claims description 7
- 239000003792 electrolyte Substances 0.000 claims description 4
- 238000007747 plating Methods 0.000 claims description 4
- 239000002245 particle Substances 0.000 claims description 3
- 238000007599 discharging Methods 0.000 claims description 2
- 238000005498 polishing Methods 0.000 claims description 2
- 238000010008 shearing Methods 0.000 claims description 2
- 229910001416 lithium ion Inorganic materials 0.000 description 10
- 230000008021 deposition Effects 0.000 description 7
- 235000012431 wafers Nutrition 0.000 description 6
- 238000011066 ex-situ storage Methods 0.000 description 4
- 210000001787 dendrite Anatomy 0.000 description 3
- 238000005516 engineering process Methods 0.000 description 3
- 230000003247 decreasing effect Effects 0.000 description 2
- 230000015572 biosynthetic process Effects 0.000 description 1
- 230000015556 catabolic process Effects 0.000 description 1
- 210000004027 cell Anatomy 0.000 description 1
- 230000007547 defect Effects 0.000 description 1
- 238000006731 degradation reaction Methods 0.000 description 1
- 238000011161 development Methods 0.000 description 1
- 230000018109 developmental process Effects 0.000 description 1
- 238000010586 diagram Methods 0.000 description 1
- 238000003487 electrochemical reaction Methods 0.000 description 1
- 238000000691 measurement method Methods 0.000 description 1
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- 229910001220 stainless steel Inorganic materials 0.000 description 1
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- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
- G01R31/36—Arrangements for testing, measuring or monitoring the electrical condition of accumulators or electric batteries, e.g. capacity or state of charge [SoC]
- G01R31/392—Determining battery ageing or deterioration, e.g. state of health
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
- G01R31/36—Arrangements for testing, measuring or monitoring the electrical condition of accumulators or electric batteries, e.g. capacity or state of charge [SoC]
- G01R31/367—Software therefor, e.g. for battery testing using modelling or look-up tables
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
- G01R31/36—Arrangements for testing, measuring or monitoring the electrical condition of accumulators or electric batteries, e.g. capacity or state of charge [SoC]
- G01R31/378—Arrangements for testing, measuring or monitoring the electrical condition of accumulators or electric batteries, e.g. capacity or state of charge [SoC] specially adapted for the type of battery or accumulator
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
- G01R31/36—Arrangements for testing, measuring or monitoring the electrical condition of accumulators or electric batteries, e.g. capacity or state of charge [SoC]
- G01R31/389—Measuring internal impedance, internal conductance or related variables
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
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- Y02E60/10—Energy storage using batteries
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Abstract
The invention relates to an irreversible lithium-separating amount in-situ quantitative detection method based on differential voltage, which comprises the following steps: s1, obtaining a differential voltage curve and irreversible lithium separation quantity of a battery in different aging stages in an off-line manner; s2, obtaining the battery LLI in different aging stages through off-line calculation based on the differential voltage curve obtained in the step S1, and establishing a quantitative relation between the LLI in the different aging stages and the irreversible lithium analysis amount; s3, acquiring a differential voltage curve of the battery to be tested on line; s4, calculating the corresponding LLI of the battery to be tested according to the differential voltage curve obtained in the step S3; and S5, substituting the LLI of the battery to be tested, which is obtained by calculation in the step S4, into the quantitative relational expression established in the step S2, and outputting to obtain the corresponding irreversible lithium separation amount of the battery to be tested. Compared with the prior art, the method can effectively distinguish reversible lithium analysis from irreversible lithium analysis, can accurately obtain the quantitative detection result of the irreversible lithium analysis amount under the condition of not disassembling the battery, and improves the range of usable scenes.
Description
Technical Field
The invention relates to the technical field of lithium battery detection, in particular to an irreversible lithium analysis amount in-situ quantitative detection method based on differential voltage.
Background
With the rapid development of electric automobiles, the service life and safety of a lithium ion battery as a power source of the electric automobile are influenced by various factors, wherein the factor with larger influence is lithium separation, and under the condition of overcharge of the lithium ion battery, a layer of lithium metal can be separated out on the surface of a negative electrode due to insufficient negative electrode allowance, so that on one hand, the lithium separation can cause a large amount of lithium ion loss, and internal resistance increase and capacity attenuation are caused; on the other hand, if the precipitated lithium continues to grow, lithium dendrites can be generated, and the lithium dendrites can pierce through the diaphragm to cause internal short circuit to trigger thermal runaway, so that the safety of the battery is greatly threatened. That is, lithium separation is classified into reversible lithium separation and irreversible lithium separation, and as the battery ages, the irreversible lithium separation accumulates, which not only accelerates the capacity degradation of the lithium ion battery, but also causes safety problems due to internal short circuits caused by the formation of lithium dendrites. Therefore, detecting lithium evolution is critical for lithium ion batteries.
At present, a plurality of methods for detecting and analyzing lithium are available, and the methods are divided into an in-situ technology and an ex-situ technology according to whether a battery is damaged or not. The in-situ technology mainly comprises the following two types: 1) Discharge curve method. And judging whether lithium analysis occurs or not according to whether a high-voltage platform appears on a battery discharge curve or not, and quantifying the lithium analysis amount through the length of the voltage platform. The method is only based on theory for analysis, and the accuracy is not proved by actually measuring the lithium analysis amount in experiments. Also, this method cannot detect the irreversible lithium evolution amount. 2) And (4) thickness detection. The volume change of the battery caused by lithium separation is far larger than that caused by the change of the graphite structure, and the thickness of the battery with the lithium separation is larger, so that whether the lithium separation occurs can be judged by measuring the thickness of the battery. However, the accuracy of the method is affected by the complicated electrochemical reaction and gas generation in the battery. In addition, this method is difficult to quantitatively detect the amount of lithium evolution, and cannot distinguish between reversible lithium evolution and irreversible lithium evolution. The ex-situ techniques mainly comprise SEM, ICP, NMR and the like, but ex-situ means that a battery is damaged, so that the available scenes are few, and many ex-situ techniques cannot distinguish reversible lithium analysis from irreversible lithium analysis and cannot quantitatively detect the amount of the irreversible lithium analysis.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provide an in-situ quantitative detection method for the irreversible lithium separation amount based on differential voltage, which can accurately detect and obtain the quantitative irreversible lithium separation amount under the condition of not disassembling and damaging a battery.
The purpose of the invention can be realized by the following technical scheme: an irreversible lithium analysis amount in-situ quantitative detection method based on differential voltage comprises the following steps:
s1, obtaining a differential voltage curve and irreversible lithium separation quantity of a battery in different aging stages in an off-line manner;
s2, obtaining the LLI (loss of lithium ion) of the battery at different aging stages through off-line calculation based on the differential voltage curve obtained in the step S1, and establishing a quantitative relation between the LLI at different aging stages and the irreversible lithium analysis amount;
s3, acquiring a differential voltage curve of the battery to be tested on line;
s4, calculating the corresponding LLI of the battery to be detected according to the differential voltage curve obtained in the step S3;
and S5, substituting the LLI of the battery to be tested, which is calculated in the step S4, into the quantitative relational expression established in the step S2, and outputting to obtain the irreversible lithium separation amount corresponding to the battery to be tested.
Further, the specific process of acquiring the differential voltage curve of the battery in step S1 and step S3 is as follows:
acquiring a discharge voltage curve of the battery under the discharge working condition of 25 ℃ and 0.1C;
and obtaining an initial differential voltage curve according to the discharge voltage curve, and filtering the initial differential voltage curve to obtain a corresponding differential voltage curve.
Further, the initial differential voltage curve is a relationship curve between a battery differential voltage DV and a battery capacity Q, and the battery differential voltage is specifically:
wherein Δ V is electricityIncrease in cell voltage, Δ Q, is the increase in capacity, V t2 And V t1 The battery voltage, Q, at times t2 and t1, respectively t2 And Q t1 The battery capacities at time t2 and time t1, respectively.
Further, the filtering of the initial differential voltage curve is specifically to perform filtering by using a smooth function of Matlab.
Further, the specific process of obtaining the irreversible lithium deposition amount offline in step S1 is as follows:
a) Emptying a new battery with the same specification as the battery to be measured, disassembling the new battery, and measuring the SEI film thickness delta of the negative electrode of the new battery SEI,0 Measuring the content Li of lithium element in the negative electrode by ICP element analyzer 0 ；
b) Emptying the aged battery with the same specification as the battery to be measured, disassembling, and measuring the SEI film thickness delta of the negative electrode SEI,i The lithium element content Li of the negative electrode thereof was measured by ICP i ；
c) Calculating the irreversible lithium separation amount Li of the battery to be tested by the following formula plating,i ：
Wherein r is the radius of the graphite particles of the negative electrode.
Further, the specific process of emptying the battery in the steps a) and b) is as follows:
and discharging the battery in a constant-current and constant-voltage mode until the battery voltage is reduced to a lower cut-off voltage and the current is less than 0.02C, so that the reversible lithium separation is ensured to be re-inserted into the graphite or dissolved in the electrolyte, and the irreversible lithium separation is remained at the negative electrode, thereby realizing the aim of distinguishing the reversible lithium separation from the irreversible lithium separation.
Further, the method for measuring the thickness of the SEI film in the steps a) and b) specifically comprises the following steps:
after disassembling the battery, shearing a negative pole piece with a set size to prepare a sample piece;
vertically cutting a sample wafer to form a cross section, polishing the cross section by using a cross section polisher, observing the cross section of the polished sample wafer by using an electron microscope, searching an SEI film, and measuring the thickness of the initial SEI film by using electronic microscope matched software;
the above process is repeated to obtain initial SEI film thicknesses of a plurality of samples respectively, and the average value is obtained to be used as the SEI film thickness of the battery negative electrode.
Further, the step S1 specifically includes the following steps:
s11, selecting a plurality of battery monomers with the same specification as that of a battery to be tested, and taking one battery monomer as a reference battery, wherein the reference battery does not perform a cyclic aging experiment;
carrying out an off-line cyclic aging experiment on the rest batteries at the same time, and calibrating the capacity of all the batteries at set time intervals to determine a corresponding aging stage;
s12, firstly carrying out capacity calibration on the reference battery, obtaining a differential voltage curve, then emptying and disassembling, and measuring the thickness of an SEI film of a negative electrode and the irreversible lithium separation amount of the reference battery;
s13, carrying out capacity calibration on the residual batteries in different aging stages, obtaining a differential voltage curve, then selecting a single battery to be emptied for disassembly, measuring the thickness of an SEI film of the negative electrode and the content of a lithium element of the negative electrode, continuing a cyclic aging experiment on the residual batteries, and repeating the step until the number of the residual batteries is 0;
and S14, calculating to obtain the irreversible lithium analysis amount of the battery at different aging stages by combining the measured SEI film thickness and the negative lithium element content data.
Further, the calculation method of LLI in step S2 specifically is:
calculating the maximum capacity value Q of the differential voltage curve of the reference battery off line 0 And calculating the maximum capacity value Q of the battery differential voltage curve at a certain aging stage i Then LLI of the battery i Calculated from the following formula:
wherein, LLI i The corresponding LLI value of the ith battery.
Further, the specific process of step S2 is:
calculating LLIs of different aging stages according to the battery differential voltage curves of different aging stages obtained in the step S1;
and drawing the LLI and the irreversible lithium analysis amount in different aging stages on the same graph by taking the LLI as a horizontal coordinate and the irreversible lithium analysis amount as a vertical coordinate, and obtaining a relational expression through fitting calculation, wherein the relational expression is a quantitative relation between the LLI and the irreversible lithium analysis amount in different aging stages.
Compared with the prior art, the invention has the following advantages:
1. according to the invention, the differential voltage curves and irreversible lithium analysis quantities of the battery at different aging stages are obtained through earlier stage experiments, then the battery LLI at different aging stages is obtained through offline calculation, so that the quantitative relation between the LLI at different aging stages and the irreversible lithium analysis quantities is established, in practical application, the quantitative irreversible lithium analysis quantities corresponding to the battery to be detected can be accurately obtained only by substituting the online measured parameters of the battery to be detected into the quantitative relation expression, the battery to be detected does not need to be disassembled, the comprehensiveness and the fineness of a battery management system can be improved, and meanwhile, the application scene of detection is expanded.
2. When the irreversible lithium separation amount of the battery is obtained, the battery is discharged in a constant-current and constant-voltage mode until the voltage is reduced to a lower cut-off voltage and the current is less than 0.02C, so that the reversible lithium separation is ensured to be re-inserted into graphite or dissolved in electrolyte, and the irreversible lithium separation is left at the negative electrode, so that the aim of distinguishing the reversible lithium separation from the irreversible lithium separation is fulfilled, and the accuracy of battery service life estimation and safety early warning is improved.
Drawings
FIG. 1 is a schematic flow diagram of the process of the present invention;
FIG. 2 is a differential voltage curve of a battery disassembled last in the example;
FIGS. 3a to 3d are SEI films under an electron microscope in examples;
FIG. 4 shows the SEI film thickness at different aging stages of the battery in the example;
FIG. 5 shows irreversible lithium deposition at various stages of aging of the battery of the example;
FIG. 6 is the LLI of the battery in different aging stages in the example;
FIG. 7 is a graph of the fit of LLI to the amount of irreversible lithium deposition in examples.
Detailed Description
The invention is described in detail below with reference to the figures and specific embodiments.
Examples
As shown in fig. 1, an irreversible lithium deposition in-situ quantitative detection method based on differential voltage includes the following steps:
s1, obtaining a differential voltage curve and an irreversible lithium separation amount of a battery in different aging stages in an off-line manner;
s2, obtaining the battery LLI in different aging stages through off-line calculation based on the differential voltage curve obtained in the step S1, and establishing a quantitative relation between the LLI in the different aging stages and the irreversible lithium analysis amount;
s3, acquiring a differential voltage curve of the battery to be tested on line;
s4, calculating the corresponding LLI of the battery to be tested according to the differential voltage curve obtained in the step S3;
and S5, substituting the LLI of the battery to be tested, which is obtained by calculation in the step S4, into the quantitative relational expression established in the step S2, and outputting to obtain the corresponding irreversible lithium separation amount of the battery to be tested.
In this embodiment, the battery to be tested uses an NCM811 lithium ion battery, and the specific process of this embodiment using the above technical scheme is as follows:
in steps S1, S3, the differential voltage curve is obtained by:
and acquiring a discharge voltage curve of the corresponding battery under the discharge working condition of 25 ℃ and 0.1C, acquiring an initial differential voltage curve according to the discharge voltage curve, and filtering the initial differential voltage curve to acquire a corresponding differential voltage curve. The initial differential voltage curve refers to a relation curve of a battery differential voltage DV and a battery capacity Q, and the differential voltage DV is calculated by the following formula:
where Δ V is the battery voltage increment, Δ Q is the capacity increment, V t2 、V t1 Is the battery voltage at time t2 and time t1, Q t2 、Q t1 The battery capacity at time t2 and time t 1.
In this embodiment, a smooth function of Matlab is used to perform filtering processing on the initial differential voltage curve.
In step S1, the irreversible lithium deposition amount is obtained as follows:
a) The new battery with the same specification as the battery to be measured is disassembled after being emptied, and the SEI film thickness delta of the negative electrode of the new battery is measured SEI,0 The lithium element content Li of the negative electrode thereof was measured by ICP 0 ；
b) The battery to be measured is disassembled after being emptied, and the SEI film thickness delta of the negative electrode of the battery is measured SEI,i The lithium element content Li of the negative electrode thereof was measured by ICP i ；
c) Calculating the irreversible lithium-separating amount Li of the battery to be tested by the following formula plating,i ：
Wherein r is the radius of the graphite particles of the negative electrode.
In steps a) and b), the battery is emptied as follows:
the battery is discharged in a constant-current constant-voltage mode until the voltage is reduced to 3.0V and the current is less than 0.02C, so that reversible lithium separation is ensured to be re-inserted into graphite or dissolved in electrolyte, and therefore the lithium remaining on the negative electrode is irreversible lithium separation, and the aim of distinguishing reversible lithium separation from irreversible lithium separation is fulfilled.
The SEI film thickness measurement method in step a) is as follows:
after the battery is disassembled, a negative pole piece with the size of a nail cover is cut off to prepare a sample wafer, a stainless steel blade is used for vertically cutting the sample wafer to form a cross section, the cross section is polished by a cross section polisher, the cross section of the polished sample wafer is observed by an electron microscope, an SEI film is searched, and the thickness of an initial SEI film is measured by adopting electronic microscope matched software. Repeating the method for four times, obtaining the initial SEI film thicknesses of the four sample wafers, and taking the average value as the SEI film thickness of the battery cathode.
The specific process of step S1 includes:
s11, selecting 9 battery monomers with the same specification as the battery to be tested, taking one battery monomer as a reference battery without carrying out a cyclic aging experiment, simultaneously carrying out an offline cyclic aging experiment on the rest 8 batteries, carrying out capacity calibration on all the batteries at regular intervals, and determining an aging stage of the batteries;
s12, firstly carrying out capacity calibration on the reference battery, obtaining a differential voltage curve, then emptying and disassembling, and measuring the thickness of an SEI film of a negative electrode and the irreversible lithium analysis amount of the reference battery;
s13, carrying out capacity calibration on the remaining 8 batteries in different aging stages and obtaining a differential voltage curve, then selecting one battery monomer to be emptied and disassembled, measuring the thickness of an SEI film of a negative electrode and the content of a lithium element of the negative electrode, continuing a cyclic aging experiment on the remaining 7 batteries, and repeating the step until the number of the remaining batteries is 0;
and S14, calculating the irreversible lithium analysis amount of the battery at different aging stages by combining the thickness of the SEI film and the lithium element content of the negative electrode.
In this embodiment, the differential voltage curves of the NCM811 lithium ion battery disassembled at last at different aging stages are as shown in fig. 2, and the differential curves of the other 8 batteries are similar to each other, and are not described again. In fig. 2, as the battery ages, the maximum capacity value of the differential voltage curve shifts to the left, and this change characteristic represents LLI, i.e., lithium ion loss.
In this embodiment, the SEI films observed under an electron microscope are shown in fig. 3a to 3d, and fig. 3a, 3b, 3c and 3d are respectively SEI film electron microscope photographs of the battery in cycle 60, cycle 270, cycle 480 and cycle 540, and it can be seen that the SEI film thickness increases from tens of nanometers to hundreds of nanometers as the battery ages. SEI film thickness at different aging stages of the battery as shown in fig. 4, the SEI film thickness increases linearly with decreasing SOH of the battery. SOH represents a certain aging stage of a battery, and is defined as a ratio of the current capacity of the battery to the capacity when the battery is not aged.
In this embodiment, the calculated irreversible lithium deposition amounts at different aging stages of the battery are shown in fig. 5 by combining the SEI film thickness and the lithium element content of the negative electrode, and the irreversible lithium deposition amount linearly increases with the aging of the battery.
In step S2, the LLI calculation method specifically includes:
calculating the maximum capacity value Q of the differential voltage curve of the reference battery off line 0 And the maximum capacity value Q of the battery differential voltage curve in a certain aging stage is calculated i Calculated LLI of the battery i Then it is calculated by:
the specific process of the step S2 is as follows:
calculating LLIs in different aging stages according to the differential voltage curves of the batteries in different aging stages obtained in the step S1, drawing the LLIs in different aging stages and the irreversible lithium analysis amount on a graph by taking the LLIs as horizontal coordinates and the irreversible lithium analysis amount as vertical coordinates, and fitting a certain relational expression, wherein the relational expression is a quantitative relational expression for calculating the irreversible lithium analysis amount.
In this example, the LLI of the 9-section NCM811 lithium ion battery at different aging stages is shown in fig. 6. As can be seen in FIG. 6, the LLI increases linearly with decreasing SOH.
This example plots LLI and irreversible lithium evolution at different stages of aging, as shown in FIG. 7. Fitting the data by using a fitting tool in Origin, wherein the fitting result shows that the LLI and the irreversible lithium analysis amount are in a linear relation, and a quantitative relation is obtained as follows:
Li plating ＝73.31·LLI-57.90
and then measuring the discharge curve of the battery to be measured on line, drawing a differential voltage curve, calculating LLI, and substituting the differential voltage curve into the determined quantitative relational expression to obtain the irreversible lithium analysis amount of the battery to be measured.
In conclusion, the technical scheme can distinguish reversible lithium analysis from irreversible lithium analysis, and improve the accuracy of battery life estimation and safety early warning;
the irreversible lithium analysis amount can be calculated by acquiring the discharge curve of the battery to be tested on line under the condition of not disassembling the battery, and the application scenes are many;
the quantitative irreversible lithium separation amount can be accurately obtained, and the comprehensiveness and the fineness of the battery management system are improved.
Claims (10)
1. An irreversible lithium-separating amount in-situ quantitative detection method based on differential voltage is characterized by comprising the following steps:
s1, obtaining a differential voltage curve and irreversible lithium separation quantity of a battery in different aging stages in an off-line manner;
s2, obtaining the battery LLI in different aging stages through off-line calculation based on the differential voltage curve obtained in the step S1, and establishing a quantitative relation between the LLI in the different aging stages and the irreversible lithium analysis amount;
s3, acquiring a differential voltage curve of the battery to be tested on line;
s4, calculating the corresponding LLI of the battery to be detected according to the differential voltage curve obtained in the step S3;
and S5, substituting the LLI of the battery to be tested, which is obtained by calculation in the step S4, into the quantitative relational expression established in the step S2, and outputting to obtain the corresponding irreversible lithium separation amount of the battery to be tested.
2. The method for in-situ quantitative detection of the amount of irreversible lithium evolution based on differential voltage according to claim 1, wherein the specific process of obtaining the differential voltage curve of the battery in the steps S1 and S3 is as follows:
acquiring a discharge voltage curve of the battery under the discharge working condition of 25 ℃ and 0.1C;
and obtaining an initial differential voltage curve according to the discharge voltage curve, and filtering the initial differential voltage curve to obtain a corresponding differential voltage curve.
3. The method according to claim 2, wherein the initial differential voltage curve is a relationship curve between a battery differential voltage DV and a battery capacity Q, and the battery differential voltage is specifically:
where Δ V is the battery voltage increment, Δ Q is the capacity increment, V t2 And V t1 The battery voltage, Q, at times t2 and t1, respectively t2 And Q t1 The battery capacities at time t2 and time t1, respectively.
4. The method for in-situ quantitative lithium analysis amount detection based on the differential voltage as claimed in claim 2, wherein the filtering of the initial differential voltage curve is specifically filtering by using a smooth function of Matlab.
5. The in-situ quantitative detection method for the amount of irreversible lithium evolution based on differential voltage according to claim 1, wherein the specific process of obtaining the amount of irreversible lithium evolution in step S1 off-line is as follows:
a) A new battery with the same specification as the battery to be measured is disassembled after being emptied, and the SEI film thickness delta of the negative electrode of the new battery is measured SEI,0 Measuring the content Li of lithium element in the negative electrode by ICP element analyzer 0 ；
b) Emptying the aged battery with the same specification as the battery to be measured, disassembling the battery, and measuring the SEI film thickness delta of the negative electrode of the battery SEI,i The lithium element content Li of the negative electrode thereof was measured by ICP i ；
c) Calculating the irreversible lithium separation amount Li of the battery to be tested by the following formula plating,i ：
Wherein r is the radius of the graphite particles of the negative electrode.
6. The method for in-situ quantitative detection of the irreversible lithium evolution quantity based on the differential voltage according to claim 5, wherein the specific process of emptying the battery in the steps a) and b) is as follows:
and discharging the battery in a constant-current and constant-voltage mode until the battery voltage is reduced to a lower cut-off voltage and the current is less than 0.02C, so that the reversible lithium separation is ensured to be re-inserted into the graphite or dissolved in the electrolyte, and the irreversible lithium separation is remained at the negative electrode, thereby realizing the aim of distinguishing the reversible lithium separation from the irreversible lithium separation.
7. The in-situ lithium-analysis quantitative detection method based on the differential voltage as claimed in claim 5, wherein the method for measuring the SEI film thickness in the steps a) and b) is specifically:
after disassembling the battery, shearing a negative pole piece with a set size to prepare a sample piece;
vertically cutting a sample wafer to form a cross section, polishing the cross section by using a cross section polisher, observing the cross section of the polished sample wafer by using an electron microscope, searching an SEI film, and measuring the thickness of an initial SEI film by using electron microscope matched software;
the above process is repeated to obtain initial SEI film thicknesses of a plurality of samples respectively, and the average value is obtained to be used as the SEI film thickness of the battery negative electrode.
8. The method for in-situ quantitative detection of the irreversible lithium evolution quantity based on the differential voltage according to claim 1, wherein the step S1 specifically comprises the following steps:
s11, selecting a plurality of battery monomers with the same specification as that of a battery to be tested, and taking one of the battery monomers as a reference battery, wherein the reference battery does not carry out a cyclic aging experiment;
carrying out an off-line cyclic aging experiment on the rest batteries at the same time, and calibrating the capacity of all the batteries at set time intervals to determine a corresponding aging stage;
s12, firstly carrying out capacity calibration on the reference battery, obtaining a differential voltage curve, then emptying and disassembling, and measuring the thickness of an SEI film of a negative electrode and the irreversible lithium separation amount of the reference battery;
s13, calibrating the capacity of the residual batteries in different aging stages, acquiring a differential voltage curve, then selecting a battery monomer to be emptied for disassembly, measuring the thickness of an SEI film of a negative electrode and the content of a lithium element of the negative electrode, continuing a cyclic aging experiment on the residual batteries, and repeating the step until the number of the residual batteries is 0;
and S14, calculating to obtain the irreversible lithium analysis amount of the battery at different aging stages by combining the measured SEI film thickness and the negative lithium element content data.
9. The in-situ lithium quantity quantitative detection method based on the irreversible lithium analysis of the differential voltage as claimed in claim 8, wherein the LLI in the step S2 is calculated in a specific manner as follows:
calculating the maximum capacity value Q of the differential voltage curve of the reference battery off line 0 And calculating the maximum capacity value Q of the battery differential voltage curve at a certain aging stage i Then LLI of the battery i Calculated from the following formula:
wherein, LLI i The corresponding LLI value of the ith battery.
10. The method according to claim 9, wherein the step S2 comprises the following steps:
calculating LLIs at different aging stages according to the battery differential voltage curves at different aging stages obtained in the step S1;
and drawing the LLI and the irreversible lithium analysis amount in different aging stages on the same graph by taking the LLI as a horizontal coordinate and the irreversible lithium analysis amount as a vertical coordinate, and obtaining a relational expression through fitting calculation, wherein the relational expression is a quantitative relation between the LLI and the irreversible lithium analysis amount in different aging stages.
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