CN112540297A - Method for researching overcharge safety redundancy boundary of lithium ion battery - Google Patents
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- 229910001416 lithium ion Inorganic materials 0.000 title claims abstract description 155
- HBBGRARXTFLTSG-UHFFFAOYSA-N Lithium ion Chemical compound [Li+] HBBGRARXTFLTSG-UHFFFAOYSA-N 0.000 title claims abstract description 149
- 238000000034 method Methods 0.000 title claims abstract description 29
- 230000008859 change Effects 0.000 claims abstract description 41
- 238000002474 experimental method Methods 0.000 claims abstract description 22
- 239000000463 material Substances 0.000 claims abstract description 20
- 238000011056 performance test Methods 0.000 claims abstract description 18
- 238000012512 characterization method Methods 0.000 claims abstract description 14
- 230000007246 mechanism Effects 0.000 claims abstract description 14
- 238000004458 analytical method Methods 0.000 claims abstract description 10
- 238000004519 manufacturing process Methods 0.000 claims abstract description 4
- 230000010287 polarization Effects 0.000 claims description 17
- 238000007600 charging Methods 0.000 claims description 15
- 238000002441 X-ray diffraction Methods 0.000 claims description 8
- WHXSMMKQMYFTQS-UHFFFAOYSA-N Lithium Chemical compound [Li] WHXSMMKQMYFTQS-UHFFFAOYSA-N 0.000 claims description 6
- 238000004364 calculation method Methods 0.000 claims description 6
- 229910052744 lithium Inorganic materials 0.000 claims description 6
- 229910052751 metal Inorganic materials 0.000 claims description 6
- 239000002184 metal Substances 0.000 claims description 6
- 239000013078 crystal Substances 0.000 claims description 5
- 238000010277 constant-current charging Methods 0.000 claims description 4
- 230000003213 activating effect Effects 0.000 claims description 3
- 238000007599 discharging Methods 0.000 claims description 3
- 230000014509 gene expression Effects 0.000 claims description 3
- 238000007781 pre-processing Methods 0.000 claims description 3
- 238000002203 pretreatment Methods 0.000 claims description 2
- 238000011076 safety test Methods 0.000 abstract 1
- 239000007773 negative electrode material Substances 0.000 description 7
- 238000012360 testing method Methods 0.000 description 6
- 238000010586 diagram Methods 0.000 description 5
- 230000008569 process Effects 0.000 description 5
- 238000011160 research Methods 0.000 description 4
- 239000010405 anode material Substances 0.000 description 3
- 206010000369 Accident Diseases 0.000 description 2
- 239000003792 electrolyte Substances 0.000 description 2
- 230000009286 beneficial effect Effects 0.000 description 1
- 230000007812 deficiency Effects 0.000 description 1
- 238000013461 design Methods 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 238000004146 energy storage Methods 0.000 description 1
- 238000012544 monitoring process Methods 0.000 description 1
- 238000006722 reduction reaction Methods 0.000 description 1
- 238000009781 safety test method Methods 0.000 description 1
- 238000004626 scanning electron microscopy Methods 0.000 description 1
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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
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N23/00—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00
- G01N23/20—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00 by using diffraction of the radiation by the materials, e.g. for investigating crystal structure; by using scattering of the radiation by the materials, e.g. for investigating non-crystalline materials; by using reflection of the radiation by the materials
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N23/00—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00
- G01N23/22—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00 by measuring secondary emission from the material
- G01N23/225—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00 by measuring secondary emission from the material using electron or ion
- G01N23/2251—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00 by measuring secondary emission from the material using electron or ion using incident electron beams, e.g. scanning electron microscopy [SEM]
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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]
- G01R31/385—Arrangements for measuring battery or accumulator variables
Abstract
The invention belongs to the technical field of lithium ion battery safety test, and relates to a method for researching overcharge safety redundancy boundary of a lithium ion battery, which comprises the following steps: step 1: pretreating the selected battery; step 2: carrying out an overcharge thermal runaway experiment on the selected first lithium ion battery, recording the temperature, the voltage, the capacity and the like of the first lithium ion battery at different moments, and determining the safety protection boundary of the battery; and step 3: disassembling the selected second lithium ion battery, and manufacturing half batteries respectively comprising the anode and the cathode of the second lithium ion battery, wherein a positive half battery and a negative half battery are grouped, and each group of half batteries are respectively overcharged to an overcharge SOC point and a thermal runaway SOC point which are more than 100% of SOC; and 4, step 4: disassembling the overcharged positive and negative half-cells in the step (3) and performing battery material characterization analysis; and 5: and (3) analyzing a battery change mechanism by combining the half-battery material characterization result and the first lithium ion battery basic performance test result, and determining the performance protection boundary of the battery together.
Description
Technical Field
The invention belongs to the technical field of safety testing of lithium ion batteries, and particularly relates to a method for researching an overcharge safety redundancy boundary of a lithium ion battery.
Background
The lithium ion battery has the advantages of high energy density, long service life and the like, and is widely applied to the fields of electric automobiles and energy storage power stations. But in recent years, the thermal runaway accidents of the electric automobiles emerge endlessly; according to incomplete statistics, the accidents of electric automobile ignition of dozens of electric automobiles continuously happen in China only in 2020; after the fire accident is analyzed, the proportion of the fire accident caused by overcharge is found to be large. In order to better prevent the occurrence of the accident of the overcharge thermal runaway of the lithium ion battery, it is necessary to perform overcharge protection on the battery. The research on the overcharge protection of the battery is mainly focused on the research on the risk tolerance of the battery under different grades of the battery so as to analyze the risk tolerance of the battery under a specific scene when faults occur in a Battery Management System (BMS), a charger and the like in the battery system, and most importantly, the determination of the overcharge safety redundancy boundary of the lithium ion battery can provide safety guidance for the use of the lithium ion battery and ensure the safe use of the battery.
Most of the existing lithium ion battery overcharge boundary research methods only consider the establishment of a safety protection boundary and a thermal model under the condition of battery thermal runaway, ignore performance protection boundaries and lack the analysis of the change mechanism of the battery under different overcharge degrees.
Disclosure of Invention
In view of the problems and deficiencies described in the background above, it is an object of the present invention to: while considering the safety protection boundary, the Performance protection boundary of the lithium ion battery is determined by disassembling half batteries with different overcharge degrees to perform X-ray diffraction (XRD) and Scanning Electron Microscope (SEM) analysis and analyzing the mechanism change of the lithium ion battery by combining the results of Reference Performance Test (RPT) (reference Performance test), and the specific technical scheme is briefly described as follows:
a method for researching the overcharge safety redundancy boundary of a lithium ion battery comprises the following steps:
step 1: firstly, randomly selecting two lithium ion batteries in the lithium ion batteries with the same model, and then preprocessing the two selected lithium ion batteries;
respectively determining the two selected lithium ion batteries as follows: a first lithium ion battery and a second lithium ion battery;
step 2: selecting a plurality of SOC (state of charge) points, selecting overcharge multiplying power, and carrying out an overcharge thermal runaway experiment on the first lithium ion battery;
determining the SOC, the voltage change rate and the temperature change rate in thermal runaway as the safety protection boundary of the lithium ion battery;
the number of SOC points includes: overcharge SOC point and thermal runaway SOC pointmax；
Wherein, the overcharge SOC points which are more than 100% SOC are x-1;
and step 3: discharging a second lithium ion battery to a discharge cut-off voltage, and then disassembling the second lithium ion battery to obtain a positive pole piece and a negative pole piece of the second lithium ion battery;
manufacturing x positive half batteries and x negative half batteries in a glove box by using the obtained positive and negative pole pieces of the second lithium ion battery;
the positive electrode of the positive electrode half cell is the positive electrode of the second lithium ion cell, the negative electrode of the positive electrode half cell is metal lithium, the negative electrode of the negative electrode half cell is the negative electrode of the second lithium ion cell, and the positive electrode of the negative electrode half cell is metal lithium;
grouping a positive half cell and a negative half cell;
respectively overcharging each group of half batteries to a plurality of overcharge SOC points and thermal runaway SOC points SOC selected in the step 2 and larger than 100% SOC according to the overcharge multiplying power in the step 2max；
And 4, step 4: disassembling each group of positive half batteries and negative half batteries which are overcharged to different SOC points, and performing material characterization analysis on the lithium ion battery;
and 5: analyzing the change mechanism of the lithium ion battery by combining the result of the characterization analysis of the lithium ion battery material and the result of the first overcharge thermal runaway experiment of the lithium ion battery, determining the performance protection boundary of the lithium ion battery together, and determining the overcharge safety redundancy boundary of the lithium ion battery by combining the safety protection boundary determined in the step 2;
the performance protection boundary comprises: overcharge onset boundary, voltage rate performance protection boundary, and voltage maximum boundary Vmax；
The overcharge starting boundary is a charge cut-off voltage;
the voltage change rate performance protection boundary is 0.001V/s;
the voltage maximum boundary VmaxThe maximum voltage value during the overcharge thermal runaway experiment.
On the basis of the technical scheme, the pretreatment method in the step 1 comprises the following steps:
and (3) activating the two lithium ion batteries by current of 1/3C, and circulating for 3 times to ensure the stable state of the lithium ion batteries.
On the basis of the technical scheme, the method for the overcharge thermal runaway experiment in the step 2 comprises the following steps:
and selecting a plurality of SOC points to perform reference performance test, recording the temperature, the voltage, the battery capacity, the SOC, the direct current internal resistance, the polarization internal resistance and the polarization capacitance of the first lithium ion battery at different moments, and determining the safety protection boundary of the lithium ion battery.
On the basis of the technical scheme, the method for performing the reference performance test through the overcharge thermal runaway experiment in the step 2 comprises the following steps:
selecting overcharge multiplying power and determining charging current Icharge；
After the first lithium ion battery is discharged to the discharge cut-off voltage, the determined charging current I is usedchargeConstant-current charging, namely continuously charging the lithium ion battery at constant current until the lithium ion battery is thermally out of control, recording parameters such as the capacity, SOC, voltage, temperature, direct-current internal resistance, polarization capacitance and the like of the lithium ion battery from the beginning of an experiment to the thermal out of control, and determining the safety protection boundary of the lithium ion battery by combining the voltage change rate and the temperature change rate;
performing Reference Performance Test (RPT) on the lithium ion battery every n% of SOC (state of charge), wherein n represents the interval of SOC and is a natural number greater than 1;
among the reference performance tests, only the hybrid pulse power performance test (HPPC) at a specific SOC point is included.
On the basis of the technical scheme, the overcharge multiplying power is selected according to the actual application condition of the lithium ion battery, and the charging current is determined according to the selected overcharge multiplying power and the capacity of the lithium ion battery.
On the basis of the technical scheme, the calculation formula of the x in the step 2 and the step 3 is shown as the formula (1),
therein, SOCmaxIs a thermal runaway SOC point, n is the SOC interval selected in step 2,
is rounding up the symbol.
On the basis of the technical scheme, the step 4 of performing the characterization analysis on the lithium ion battery material comprises the following steps:
after disassembling each group of the anode half cell and the cathode half cell which are overcharged to different SOC points, performing material characterization such as X-ray diffraction (XRD) and Scanning Electron Microscope (SEM) on the internal material, and analyzing the crystal structure of the lithium ion battery by combining with fine adjustment calculation.
On the basis of the technical scheme, the specific steps of the step 5 are as follows:
combining the representation result of the lithium ion battery material at the same SOC point with the direct current internal resistance, polarization internal resistance and polarization capacitance result of the first lithium ion battery, analyzing the change mechanism of the lithium ion battery, defining different stages according to the characteristic expressions of voltage, voltage change rate, temperature change rate and the like in the overcharge thermal runaway experiment of the first lithium ion battery, summarizing the mechanism changes of the different stages, and determining the performance protection boundary of the lithium ion battery together;
and finally, combining the performance protection boundary with the result of the safety protection boundary determined in the step 2 (namely the SOC value, the voltage change rate and the temperature change rate of the first lithium ion battery at the stage before thermal runaway) to determine the overcharge safety redundancy boundary of the lithium ion battery.
The invention has the following beneficial technical effects:
the invention can determine the overcharge safety redundancy boundary of the lithium ion battery. According to the method for testing, parameters such as voltage change rate, temperature change rate and SOC in the previous stage of thermal runaway in a full battery test can be used as the overcharge safety protection boundary of the lithium ion battery, the positive half battery and the negative half battery which are overcharged to different SOC points are disassembled, the mechanism evolution rule of the lithium ion battery in the overcharge process is analyzed by combining the test result of the overcharge thermal runaway test RPT through X-ray diffraction XRD and scanning electron microscope SEM analysis, and the parameters such as the battery voltage and the voltage change rate are selected as the performance protection boundary of the lithium ion battery, so that important basis is provided for state monitoring and thermal runaway protection design of the battery in the overcharge process in practical application.
Drawings
The invention has the following drawings:
FIG. 1 is a schematic flow diagram of the process of the present invention.
Fig. 2 is a curve diagram in a lithium ion battery overcharge thermal runaway experiment according to an embodiment of the invention.
Fig. 3 is a schematic view of scanning the negative electrode material of the negative electrode half cell under the scanning electron microscope according to the embodiment of the invention.
Fig. 4 is a schematic diagram of the X-ray diffraction results of the negative electrode material of the negative electrode half cell of the embodiment of the invention.
Fig. 5 is a schematic diagram of a crystal structure of an anode material of the anode half cell according to the embodiment of the invention.
Detailed Description
The steps of the present invention will be described in further detail below with reference to the accompanying drawings and test examples.
As shown in fig. 1, to show a flow of a method for researching an overcharge safety redundancy boundary of a lithium ion battery, the method comprises the following steps:
step 1: and preprocessing the selected lithium ion battery.
Firstly, randomly selecting two lithium ion batteries in the lithium ion batteries with the same model, then activating the two lithium ion batteries by 1/3C current, and circulating for 3 times to ensure the stable state of the lithium ion batteries.
Respectively determining the two selected lithium ion batteries as follows: a first lithium ion battery and a second lithium ion battery.
Step 2: carrying out an overcharge thermal runaway experiment on the first lithium ion battery;
in the experiment process of the over-charging thermal runaway, a plurality of SOC points are selected for reference performance test, the temperature, the voltage, the battery capacity (capacity for short), the SOC, the direct current internal resistance, the polarization internal resistance and the polarization capacitance of the first lithium ion battery at different moments are recorded, and the safety protection boundary of the lithium ion battery (battery for short) is determined, and the specific steps are briefly described as follows:
according toSelecting overcharge rate according to practical application condition of the lithium ion battery, and determining charging current I by combining with capacity of the lithium ion batterychargeAfter the first lithium ion battery is discharged to the discharge cut-off voltage, the selected charging current I is usedchargeAnd constant-current charging is carried out, the constant-current charging is continued until the lithium ion battery is out of control thermally, parameters such as battery capacity, SOC, voltage, temperature, direct-current internal resistance, polarization internal resistance and polarization capacitance from the beginning of an experiment to the out of control thermally are recorded, and the safety protection boundary of the lithium ion battery is determined by combining the voltage change rate and the temperature change rate.
Performing Reference Performance Test (RPT) on the lithium ion battery every n% of SOC (state of charge);
where the RPT test only comprises a mixed pulse power performance test (HPPC) at a certain SOC point.
In the example, the charging current IchargeThe reference performance test was performed every 5% SOC, with the interval n of SOC being 5, from 0% SOC to 167% SOC (thermal runaway SOC point), determined as 1C. As shown in fig. 2, the lithium ion battery reaches the maximum SOC point, i.e., the thermal runaway SOC point SOCmaxWhen the voltage begins to decrease rapidly, the temperature of the lithium ion battery begins to rise rapidly, and finally the SOC is obtained>167% voltage change rate dV/dt<-1V/s, temperature rate of change dT/dT>And 1 ℃/s is taken as the safety protection boundary of the lithium ion battery.
And step 3: discharging a second lithium ion battery to a discharge cut-off voltage, and then disassembling the second lithium ion battery to obtain positive and negative electrode plates (namely, half batteries respectively containing the positive and negative electrodes of the selected second lithium ion battery);
manufacturing x positive half batteries and x negative half batteries in a glove box by using the obtained positive and negative pole pieces of the second lithium ion battery;
the positive electrode of the positive electrode half cell is the positive electrode of the second lithium ion cell, the negative electrode of the positive electrode half cell is metal lithium, the negative electrode of the negative electrode half cell is the negative electrode of the second lithium ion cell, and the positive electrode of the negative electrode half cell is metal lithium;
the number x of half cells and the calculation formula thereof are shown in formula (1),
therein, SOCmaxAnd n is the SOC interval selected in step 2, i.e. n is 5,
is a rounded up symbol;
and (3) grouping a positive half battery and a negative half battery into a group, and respectively overcharging each group of half batteries to a plurality of overcharge SOC points and thermal runaway SOC points which are larger than 100% SOC and selected in the step (2) according to the overcharge multiplying power in the step (2).
Example x is 14, thermal runaway SOC Point SOCmaxAt 167%, the 14 half-cells were overcharged every 5% SOC to 105%, 110%, …, 165% and 167% SOC, respectively.
And 4, step 4: disassembling each group of positive half batteries and negative half batteries (namely positive half batteries and negative half batteries) with different overcharge SOC points, and performing characterization analysis on lithium ion battery materials, wherein the method comprises the following specific steps:
after disassembling each group of the anode half cell and the cathode half cell which are overcharged to different SOC points, performing material characterization such as X-ray diffraction (XRD) and Scanning Electron Microscope (SEM) on the internal material, and analyzing the crystal structure of the lithium ion battery by combining with fine adjustment calculation.
In the example, the negative electrode half-cell at each SOC point was disassembled, and the negative electrode material of the negative electrode half-cell was subjected to material characterization with a scanning electron microscope, to obtain the results shown in fig. 3;
then, the negative electrode material was subjected to X-ray diffraction to obtain the results shown in fig. 4;
subtracting the observed value from the calculated value to obtain a difference value, and judging the collapse condition of the negative electrode material according to the size and the position of the difference value;
finally, the crystal structure diagram of the anode material is derived according to the results shown in fig. 3 and fig. 4, as shown in fig. 5.
And 5: and analyzing the change mechanism of the lithium ion battery by combining the characterization and analysis result of the lithium ion battery (namely, a half battery) material and the result of the first lithium ion battery overcharge thermal runaway experiment (namely, a basic performance test result), and jointly determining the performance protection boundary of the lithium ion battery. The method comprises the following specific steps:
the method comprises the steps of combining a representation result of a lithium ion battery material at the same SOC point with a direct current internal resistance, polarization internal resistance and polarization capacitance result of a first lithium ion battery, analyzing a change mechanism of the lithium ion battery, defining different stages according to characteristic expressions such as voltage, voltage change rate, temperature change rate and the like in a first lithium ion battery overcharge thermal runaway experiment, summarizing mechanism changes of the different stages, and determining a performance protection boundary of the lithium ion battery together.
The performance protection boundary comprises: overcharge onset boundary, voltage rate performance protection boundary, and voltage maximum boundary Vmax；
The overcharge-starting boundary is a charge-cutoff voltage, which in the example is 4.2V;
the voltage rate performance protection boundary is 0.001V/s, which in the example is the boundary for phase 1 (described later): the rate of change of voltage dV/dt is > 0.001V/s;
the voltage maximum boundary VmaxThe maximum voltage during the overcharge thermal runaway experiment, in the example, is the boundary of phase 2 (described later): the voltage reaches a maximum value VmaxThe value was 5.1V.
By adopting the method, example research is carried out, the mechanism change of the lithium ion battery in different stages is summarized, and the overcharge process is divided into the following 3 stages:
stage 1: from the time when the charging voltage of the first lithium ion battery exceeds the charging cut-off voltage of the first lithium ion battery to the time when the change rate of the charging voltage of the battery is more than 0.001V/s, the lithium ions of the positive electrode of the battery are remained at the stage, the lithium ions are removed from the positive electrode and are embedded into a negative electrode material through a diaphragm, and the temperature change is small;
and (2) stage: from the moment that the voltage change rate is larger than 0.001V/s to the moment that the voltage reaches the maximum value, the voltage and the temperature of the positive electrode are continuously increased at this stage, the electrolyte starts to decompose, the side reaction in the battery is intensified, a large amount of gas and heat are generated, meanwhile, the SEI film of the negative electrode starts to decompose at high temperature, the negative electrode material and the electrolyte generate a reduction reaction, the gas and the heat are generated, the temperature rise rate is increased, and the temperature rise is faster;
and (3) stage: from the moment the charging voltage reaches the maximum value to the moment the battery is out of control by touching heat, structural collapse of the battery anode material occurs at the stage, the voltage is reduced, the temperature is rapidly increased until the battery diaphragm shrinks, large-scale internal short circuit is caused, a large amount of heat is released, and thermal out of control occurs;
and finally, combining the performance protection boundary with the result of the safety protection boundary determined in the step 2 (namely the SOC value, the voltage change rate and the temperature change rate of the first lithium ion battery at the stage before thermal runaway) to determine the overcharge safety redundancy boundary of the lithium ion battery.
The above embodiments describe the technical solutions of the present invention in detail. It will be clear that the invention is not limited to the described embodiments. Based on the embodiments of the present invention, those skilled in the art can make various changes, but any changes equivalent or similar to the present invention are within the protection scope of the present invention.
Those not described in detail in this specification are within the knowledge of those skilled in the art.
Claims (7)
1. A method for researching the overcharge safety redundancy boundary of a lithium ion battery is characterized by comprising the following steps:
step 1: firstly, randomly selecting two lithium ion batteries in the lithium ion batteries with the same model, and then preprocessing the two selected lithium ion batteries;
respectively determining the two selected lithium ion batteries as follows: a first lithium ion battery and a second lithium ion battery;
step 2: selecting a plurality of SOC points, selecting overcharge multiplying power, and carrying out an overcharge thermal runaway experiment on the first lithium ion battery;
determining the SOC, the voltage change rate and the temperature change rate in thermal runaway as the safety protection boundary of the lithium ion battery;
the number of SOC points includes: overcharge SOC point and thermal runaway SOC pointmax；
Wherein, the overcharge SOC points which are more than 100% SOC are x-1;
and step 3: discharging a second lithium ion battery to a discharge cut-off voltage, and then disassembling the second lithium ion battery to obtain a positive pole piece and a negative pole piece of the second lithium ion battery;
manufacturing x positive half batteries and x negative half batteries in a glove box by using the obtained positive and negative pole pieces of the second lithium ion battery;
the positive electrode of the positive electrode half cell is the positive electrode of the second lithium ion cell, the negative electrode of the positive electrode half cell is metal lithium, the negative electrode of the negative electrode half cell is the negative electrode of the second lithium ion cell, and the positive electrode of the negative electrode half cell is metal lithium;
grouping a positive half cell and a negative half cell;
respectively overcharging each group of half batteries to a plurality of overcharge SOC points and thermal runaway SOC points SOC selected in the step 2 and larger than 100% SOC according to the overcharge multiplying power in the step 2max；
And 4, step 4: disassembling each group of positive half batteries and negative half batteries which are overcharged to different SOC points, and performing material characterization analysis on the lithium ion battery;
and 5: analyzing the change mechanism of the lithium ion battery by combining the result of the characterization analysis of the lithium ion battery material and the result of the first overcharge thermal runaway experiment of the lithium ion battery, determining the performance protection boundary of the lithium ion battery together, and determining the overcharge safety redundancy boundary of the lithium ion battery by combining the safety protection boundary determined in the step 2;
the performance protection boundary comprises: overcharge onset boundary, voltage rate performance protection boundary, and voltage maximum boundary Vmax；
The overcharge starting boundary is a charge cut-off voltage;
the voltage change rate performance protection boundary is 0.001V/s;
the voltage maximum boundary VmaxThe maximum voltage value during the overcharge thermal runaway experiment.
2. The method of studying an overcharge safety redundancy margin for a lithium-ion battery of claim 1, wherein: the pretreatment method in the step 1 comprises the following steps:
and (3) activating the two lithium ion batteries by current of 1/3C, and circulating for 3 times to ensure the stable state of the lithium ion batteries.
3. The method of studying an overcharge safety redundancy margin for a lithium-ion battery of claim 1, wherein: the method for the overcharge thermal runaway experiment in the step 2 comprises the following steps:
and selecting a plurality of SOC points to perform reference performance test, recording the temperature, the voltage, the battery capacity, the SOC, the direct current internal resistance, the polarization internal resistance and the polarization capacitance of the first lithium ion battery at different moments, and determining the safety protection boundary of the lithium ion battery.
4. The method of studying an overcharge safety redundancy margin for a lithium-ion battery of claim 3, wherein: the method for carrying out the reference performance test through the overcharge thermal runaway experiment in the step 2 comprises the following steps:
selecting overcharge multiplying power and determining charging current Icharge；
After the first lithium ion battery is discharged to the discharge cut-off voltage, the determined charging current I is usedchargeConstant-current charging, namely continuously charging the lithium ion battery at constant current until the lithium ion battery is thermally out of control, recording the lithium ion battery capacity, SOC, voltage, temperature, direct-current internal resistance, polarization internal resistance and polarization capacitance parameters from the beginning of an experiment to the thermal out of control, and determining the safety protection boundary of the lithium ion battery by combining the voltage change rate and the temperature change rate;
performing Reference Performance Test (RPT) on the lithium ion battery every n% of SOC (state of charge), wherein n represents the interval of SOC and is a natural number greater than 1;
the reference performance test only includes the mixed pulse power performance test at a specific SOC point.
5. The method of studying an overcharge safety redundancy margin for a lithium-ion battery of claim 4, wherein: the calculation formula of the x in the step 2 and the step 3 is shown as the formula (1),
therein, SOCmaxIs a thermal runaway SOC point, n is the SOC interval selected in step 2,
is rounding up the symbol.
6. The method of studying an overcharge safety redundancy margin for a lithium-ion battery of claim 1, wherein: and 4, the steps for characterizing and analyzing the lithium ion battery material are as follows:
and (3) disassembling each group of positive half batteries and negative half batteries which are overcharged to different SOC points, carrying out X-ray diffraction and scanning electron microscope material characterization on the internal material, and analyzing the crystal structure of the lithium ion battery by combining with fine adjustment calculation.
7. The method of studying an overcharge safety redundancy margin for a lithium-ion battery of claim 1, wherein: the specific steps of step 5 are as follows:
combining the characterization result of the lithium ion battery material at the same SOC point with the direct current internal resistance, polarization internal resistance and polarization capacitance result of the first lithium ion battery, analyzing the change mechanism of the lithium ion battery, defining different stages according to the characteristic expressions of voltage, voltage change rate, temperature and temperature change rate in the overcharge thermal runaway experiment of the first lithium ion battery, summarizing the mechanism changes of the different stages, and determining the performance protection boundary of the lithium ion battery together;
and finally, combining the performance protection boundary with the result of the safety protection boundary determined in the step 2 to determine the overcharge safety redundant boundary of the lithium ion battery.
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