Nondestructive lithium precipitation detection method for lithium ion battery cell

Abstract

The invention discloses a lithium ion cell nondestructive lithium precipitation detection method, which specifically comprises the following steps: at a certain temperature, determining the charging open circuit voltage OCV of a normal lithium precipitation-free cell and a lithium ion battery to be detected, and generating an SOC-OCV table; the method comprises the following steps: carrying out charge-discharge circulation on a lithium ion battery at a certain charge rate and temperature to obtain a voltage-temperature rise curve, and roughly determining a voltage interval of stable heat production of a battery cell based on the voltage-temperature rise curve; a charging dQ / dV curve is obtained based on an incremental capacity method, the inflection point is accurately determined in the voltage interval of stable heat production of the battery cell in the dQ / dV curve, and the polarization heat production value corresponding to the interval from the inflection point to the charging end is calculated; and carrying out polarization heat proportion calculation on the batch of battery cells, and screening out the battery cells lower than a threshold value based on an outlier method as lithium precipitation battery cells. According to the method, lithium precipitation detection can be carried out under the condition that a sensor and a reference electrode are not additionally arranged, and risk assessment is carried out on early lithium precipitation of the high-capacity lithium ion battery cell.

Description

Technical Field

[0001] The invention belongs to the technical field of batteries, and in particular relates to a non-destructive lithium deposition detection method for lithium-ion batteries. Background Art

[0002] When a lithium-ion battery is charging, lithium ions are deintercalated from the positive electrode and embedded in the negative electrode; however, when some abnormal conditions occur, such as insufficient lithium embedding space in the negative electrode, too much resistance to lithium ion embedding in the negative electrode, lithium ions are deintercalated too quickly from the positive electrode but cannot be embedded in the negative electrode in equal amounts, the lithium ions that cannot be embedded in the negative electrode can only gain electrons on the surface of the negative electrode, thereby forming a silvery-white metallic lithium single substance, which is often referred to as "lithium precipitation". The precipitated metallic lithium often does not form a flat coating, but exists in the form of dendrites, which are called lithium dendrites. Lithium dendrites have very strong reduction activity and a large specific surface area, and their growth can bring many hazards. For example, excessively grown lithium dendrites may pierce the diaphragm, causing a short circuit between the positive and negative electrodes, and even cause fire and explosion of lithium-ion batteries.

[0003] The methods for judging the lithium deposition behavior of lithium-ion batteries are mainly divided into two categories: non-destructive analysis and lossy analysis. Among them, lossy analysis is mainly to confirm the interface by disassembling the battery after it is fully charged. There are many types of non-destructive analysis, such as calorimetry, relaxation voltage curve, AC impedance test method and three-electrode lithium potential measurement method. At present, for the completed module battery cells, the signals that can be collected are often only voltage, current, and temperature, and only non-destructive measurement methods can be used.

[0004] Among them, the calorimetric method, AC impedance test method and three-electrode method all require external instruments and can only measure a single battery cell. In actual applications, the battery cell is wrapped in iron sheets, and the above three methods are only used when the battery pack is disassembled. The relaxation voltage analysis technology is usually applied to small-capacity batteries (such as soft-pack batteries) or when the battery cell has a large amount of lithium deposition. For high-capacity batteries (such as aluminum shell 280Ah batteries), the voltage change caused by the amount of lithium deposition is not obvious. Summary of the invention

[0005] In view of the above problems, the present invention provides a non-destructive lithium deposition detection method for lithium-ion batteries, which is a non-destructive testing method. It can perform lithium deposition detection without adding sensors and reference electrodes, and conduct risk assessment on early lithium deposition of large-capacity lithium-ion batteries.

[0006] The technical solution is as follows:

[0007] A non-destructive lithium deposition detection method for a lithium-ion battery cell, comprising:

[0008] Step 1, based on two types of lithium-ion batteries A and B with the same chemical system, type A is a normal non-lithium-precipitated battery cell, and type B is a battery cell to be tested, at a certain temperature, the charging open circuit voltage OCV of the two types of lithium-ion batteries is determined respectively, and a SOC-OCV table is generated; at a certain charging rate and temperature, the two types of lithium-ion batteries are charged and discharged respectively, and the voltage, current, battery cell large surface temperature, ambient temperature, capacity and energy data are recorded during the whole process;

[0009] Step 2, based on the voltage, the large surface temperature of the battery cell and the ambient temperature change, a voltage-temperature rise curve is obtained, and based on the voltage-temperature rise curve, a voltage range in which the battery cell generates heat stably is determined; based on the incremental capacity method, a charging dQ / dV curve is obtained, an inflection point is determined within the voltage range in which the battery cell generates heat stably in the dQ / dV curve, and a polarization heat generation value corresponding to the inflection point to the charging end range is calculated;

[0010] Step 3, calculate the polarization heat ratio of the batch of battery cells, compare the polarization heat ratios of Class A and Class B battery cells horizontally, take the polarization heat ratio of Class A normal non-lithium-depositing battery cells as the benchmark, and screen out the battery cells below the threshold as lithium-depositing battery cells based on the outlier method.

[0011] Preferably, in step 1, among the two types of lithium-ion batteries, the NP ratio of the negative electrode coating weight to the positive electrode coating weight of the normal lithium-ion battery of type A is 1.17, and the NP ratio of the negative electrode coating weight to the positive electrode coating weight of the lithium-ion battery of type B is 1.10; the positive electrode material is lithium iron phosphate, the negative electrode material is single-particle graphite or single-particle mixed secondary particle graphite, the electrolyte is 1M LiPF6 (EC / DMC), and the diaphragm material is polyethylene or polypropylene.

[0012] Preferably, in step 1, the method for testing the charging open circuit voltage OCV of the battery cell is: first, measuring the initial capacity C0 of the battery cell; then, using 0.5C0 to discharge to the discharge cut-off voltage, and standing for 3 hours; using 0.5C0 to charge to the following SOC and then standing for 3 hours respectively, and recording when the SOC is 1%, 3%, 5%, 7%, 10%, 13%, 16%, 18%, 20%, 22%, 25%, 28%, 31%, 34%, 82%, 85%, 88%, 91%, 94%, 97%, 98%, 99%, and 100%, respectively, to obtain a SOC-OCV table.

[0013] Preferably, in step 1, only the test temperature is changed to obtain the open circuit voltage OCV of the battery cell at 5°C, 15°C, 25°C, 35°C, and 45°C, so as to perform lithium deposition detection on lithium-ion batteries at different temperatures.

[0014] Preferably, the charging rate in the battery cell cycle test includes: 0.2C, 0.33C, 0.5C or 0.2P, 0.33P, 0.5P; the discharge rate is consistent with the charging, which is 0.2C, 0.33C, 0.5C or 0.2P, 033P, 0.5P.

[0015] Preferably, step 2 comprises:

[0016] Step 2.1, based on the voltage-temperature rise curve, obtain the voltage range where the battery cell temperature tends to be flat;

[0017] Step 2.2, performing differentiation processing on the voltage and capacity data to obtain a charging dQ / dV curve, and finding an inflection point and a voltage V1 corresponding to the inflection point in the voltage range of the dQ / dV curve;

[0018] Step 2.3, sample data between V1 and the charge cut-off voltage Vmax, record the voltage at the i-th moment as Ui, the voltage as Ii, the temperature as Ti, and the SOC as SOCi. Use SOCi and Ti to look up the SOC-OCV table at different temperatures, and interpolate and calculate according to the temperature to obtain the open circuit voltage value OCVi, polarization heat generation Qi, total polarization heat generation Qp in the interval of V1 to Vmax, and the polarization heat generation proportion ω in this interval.

[0019] Preferably, the method for finding the inflection point is: find the characteristic peak of dQ / dV in the interval, calculate the slope k1 = (dQVmax-dQVp) / (Vmax-Vp) of the line between the characteristic peak (dQVmax, Vmax) and the voltage point at the end of charging (dQVp, Vp), and find the point in the dQ / dV curve where the tangent slope is closest to the inflection point.

[0020] Preferably, the polarization heat generation Qi at the i-th moment is expressed as: Qi=Ii*(Ui-OCVi); the total polarization heat generation Qp from V1 to Vmax is: The energy charged into the battery cell from V1 to Vmax is W, and the polarization heat accounts for ω=Qp / W*100%.

[0021] Preferably, in step 3, steps 1-2 are repeated to calculate the polarization heat generation ratio of each number of turns of class A and class B battery cells, and the polarization heat generation ratio of normal class A battery cells without lithium deposition is used as a benchmark, and the polarization heat generation ratio values ​​of the battery cells to be tested are compared horizontally for the same number of turns; for the values ​​of batch battery cells, the battery cells whose polarization heat ratio ω exceeds the threshold value are screened out based on the 3Sigma criterion, namely, the lithium deposition battery cells.

[0022] Preferably, the number of lithium-ion batteries of both types tested is greater than 3, and they are fully charged and disassembled after a certain number of charging cycles to verify the results of the lithium plating risk assessment.

[0023] Beneficial effects:

[0024] (1) The non-destructive lithium deposition detection method for lithium-ion batteries disclosed in the present invention has high detection sensitivity and can detect early lithium deposition: according to the data of each cycle, real-time monitoring can be performed to detect early local lithium deposition in large-capacity batteries, thereby advancing the warning time;

[0025] (2) The non-destructive lithium deposition detection method for lithium-ion batteries disclosed in the present invention is simple in testing method and does not require additional testing equipment; the testing method is based on conventional constant power charging data analysis, and relative to the AC impedance testing method, no additional electrochemical workstation is required; relative to the three-electrode lithium deposition potential method, no built-in reference electrode is required;

[0026] (3) This method uses charging time period data, which is neither a relaxation method nor a discharge voltage method. The heat generation is calculated by collecting conventional data (voltage, current, time). The voltage and current have smaller errors than temperature collection. At the same time, no external instruments are required, which is more conducive to practical implementation. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below.

[0028] Figure 1 2 is a schematic diagram of a battery core winding structure according to an embodiment of the present invention;

[0029] Figure 2 is a schematic diagram of disassembling a battery cell according to an embodiment of the present invention;

[0030] Figure 3 25°C charging SOC-OCV curves of A and B cells in the embodiment of the present invention;

[0031] Figure 4 The B-type battery cell of the embodiment of the present invention undergoes lithium deposition after 100 cycles;

[0032] Figure 5 2 are voltage-temperature rise curves of different cycle numbers of an embodiment of the present invention;

[0033] Figure 6 : The charging voltage-capacity curves of different cycle numbers of the embodiment of the present invention;

[0034] Figure 7 dQ / dV curves of different charging cycles of the embodiment of the present invention;

[0035] Figure 8 This is the dQ / dV curve of the first cycle of charging of a 25°C A battery cell according to an embodiment of the present invention;

[0036] Fig. 9 This is a comparison chart of the polarization heat generation ratio of A and B battery cells with different numbers of turns in an embodiment of the present invention. DETAILED DESCRIPTION

[0037] The technical scheme in the embodiment of the present invention will be described in more detail below in conjunction with the drawings in the embodiment of the present invention. Obviously, the described embodiment is only a part of the embodiment of the present invention, not all of the embodiments. Based on the embodiment of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0038] Example

[0039] like Figures 1 to 9 As shown, an embodiment of the present invention provides a non-destructive lithium deposition detection method for lithium-ion batteries, based on the lithium deposition test principle: during the charging process of lithium-ion batteries, once lithium deposition occurs, lithium ions begin to transform into metallic lithium on the surface of the graphite negative electrode, and the charging current is divided into lithium insertion current and lithium deposition current, generating a new equivalent circuit branch. When lithium deposition begins, the impedance of the electrolyte and the transfer impedance of the SEI remain unchanged, while the impedance of the charge transfer process decreases due to the addition of a parallel branch. At this time, the polarization internal resistance of the lithium-ion battery cell decreases. Under the same SOC state, the voltage of the lithium-ion battery cell for lithium deposition is lower than that of the normal battery cell. According to the Bernadi formula, polarization heat generation Q = I*(U-UOCV), and the lithium deposition situation is judged by comparing the reduction value of polarization heat generation of abnormal batteries and normal batteries.

[0040] A non-destructive lithium deposition detection method for a lithium-ion battery cell comprises the following steps:

[0041] Step 1: Based on Class A and Class B lithium-ion batteries with the same chemical system, at a certain temperature, the open circuit voltage (OCV) of the two types of lithium-ion batteries is determined respectively, and a SOC-OCV table is generated; the two types of lithium-ion batteries are charged and discharged at a certain charging rate and temperature, and the voltage, current, battery cell surface temperature, ambient temperature, capacity and energy data are recorded during the whole process.

[0042] It should be noted that the same chemical system includes the same positive electrode material, the same negative electrode material, the same diaphragm material and the same electrolyte. The positive electrode coating uses a positive electrode active material, while the negative electrode coating uses a negative electrode active material. In lithium-ion batteries, the positive electrode active material is usually a transition metal oxide (such as lithium manganese oxide, lithium cobalt oxide, lithium nickel oxide, etc.) or a compound such as a phosphate. The negative electrode active material is usually a material such as graphite or silicon. During the charge and discharge process, the positive electrode material undergoes lithium ion insertion / extraction reactions, and the negative electrode material undergoes lithium ion release / absorption reactions. These reactions enable the battery to store and release electrical energy. The positive electrode material usually has a higher specific capacity and can store more lithium ions, thereby achieving a higher energy density. The energy density of the negative electrode material is relatively low. In this embodiment, two types of battery cells, A and B, are set for comparison, one type is a normal non-lithium deposition battery cell, and the other type is a lithium deposition battery cell. The negative electrode coating weight of the A-type lithium-ion battery is to the positive electrode coating weight, that is, the NP ratio is 1.17 (normal battery cell), and the negative electrode coating weight of the B-type lithium-ion battery is to the positive electrode coating weight, that is, the NP ratio is 1.10 (artificial easy lithium precipitate battery cell). In this embodiment, the B-type battery cell is used to prove the effectiveness and feasibility of the method. In the normal lithium precipitation risk determination, the B-type battery cell is a batch of battery cells to be tested. The positive electrode material is lithium iron phosphate, the negative electrode material is single-particle graphite or single-particle mixed secondary particle graphite, the electrolyte is 1M LiPF6 (EC / DMC), and the diaphragm material is polyethylene and polypropylene.

[0043] The method for testing the charging open circuit voltage OCV of the battery cell is as follows: first, measure the initial capacity C0 of the battery cell according to the national standard GB / T 36276-20236.4.1.1.1 battery monomer test method; then, use 0.5C0 to discharge to the discharge cut-off voltage and let it stand for 3 hours; use 0.5C0 to charge to the following SOC and let it stand for 3 hours respectively, and record when the SOC is 1%, 3%, 5%, 7%, 10%, 13%, 16%, 18%, 20%, 22%, 25%, 28%, 31%, 34%, 82%, 85%, 88%, 91%, 94%, 97%, 98%, 99%, 100%, and obtain the SOC-OCV table or SOC-OCV curve. The state of charge (SOC) of a battery indicates the ratio of the remaining power of the battery to its rated capacity, usually expressed as a percentage.

[0044] The open circuit voltage (OCV) test of the battery cell is performed by only changing the test temperature: 5℃, 15℃, 25℃, 35℃, 45℃ to obtain the open circuit voltage (OCV) of the battery cell at different temperatures.

[0045] The charging rates in the battery cycle test include 0.2C, 0.33C, 0.5C or 0.2P, 0.33P, 0.5P; the discharge rate is the same as the charging rate, including 0.2C, 0.33C, 0.5C or 0.2P, 0.33P, 0.5P.

[0046] In this embodiment, a constant capacity test is performed on a battery cell A with a NP ratio of 1.17 and a battery cell B with a NP ratio of 1.10 at 25°C to measure the actual total capacity of each battery cell, and then the charging SOC-OCV curves are measured at 25°C, as shown in FIG. Figure 3 As shown,

[0047] Two cells of each type, A and B, were cycled at 25°C at 0.5P for 100 cycles and disassembled after 20 and 100 cycles, respectively. Due to the relatively low NP, lithium deposition occurred in the B cell after 100 cycles. Figure 7 shown.

[0048] Taking the first cycle of cell A as an example, the SOC-OCV curve of charging at 25°C is shown in Table 1 below:

[0049] Table 1A battery cell charging SOC-OCV curve at 25℃

[0050] Real-time voltage (V) OCV(V) SOC(%) 3.3873 3.3048 33.51％ 3.4333 3.3380 80.82％ 3.4432 3.3386 83.77％ 3.4529 3.3394 86.73％ 3.4569 3.3410 89.69％ 3.4847 3.3427 96.59％ 3.4948 3.3447 97.57％ 3.5123 3.3468 98.56％ 3.6500 3.3520 100.00％ .

[0051] Step 2, based on the voltage and the temperature change of the large surface of the battery cell, a voltage-temperature rise curve is obtained, and based on the voltage-temperature rise curve, a voltage range in which the battery cell generates heat stably is determined; based on the incremental capacity method, a charging dQ / dV curve is obtained, an inflection point is determined within the voltage range in which the battery cell generates heat stably in the dQ / dV curve, and a polarization heat generation value corresponding to the inflection point is calculated.

[0052] Specifically,

[0053] Step 2.1, based on the voltage-temperature rise curve, obtain the voltage range where the battery cell temperature tends to be flat.

[0054] Since lithium deposition occurs when the battery is about to be fully charged, it is necessary to find the starting point of a stable voltage. The temperature rise value of the voltage-temperature rise curve in this embodiment is the difference between the measured temperature of the large surface of the battery and the ambient temperature. Figure 5 As shown in the figure, the voltage-temperature rise curve has a peak near 3.4V, and then the temperature rise slowly decreases and tends to be flat. It is preliminarily determined that the voltage range in which the battery cell temperature tends to be flat is 3.4~3.5V.

[0055] Step 2.2, performing differentiation processing on the voltage and capacity data to obtain a charging dQ / dV curve, and finding an inflection point and a voltage V1 corresponding to the inflection point in the voltage range of the dQ / dV curve.

[0056] Step 2.21, first determine the characteristic peak in the corresponding voltage range.

[0057] As shown in Figure 6, it is the charging voltage-capacity curve for different cycle numbers. The dQ / dV curve is obtained by calculating the change in battery capacity within a constant voltage interval. A small fluctuation in the lithium battery voltage within the platform range corresponds to a larger capacity, which is shown as a characteristic peak on the dQ / dV curve. Each peak represents an electrochemical reaction, and the peak point represents the phase change point of the material. Figure 5 As shown, the voltage point where a large amount of heat generation ends is found in the voltage range (3.4-3.5V) corresponding to the abscissa of the charging dQ / dV curve, that is, the voltage starting point of stable heat generation is found, that is, the characteristic peak in the range of 3.4-3.5V is found.

[0058] Step 2.22, then find the inflection point of this voltage range.

[0059] The inflection point of the dQ / dV curve after the high voltage characteristic peak in the dQ / dV curve decreases (i.e., from the left starting point of the voltage interval to the voltage end point at the end of charging, which is approximately 3.4-3.65V in this embodiment) is calculated, and the phase change of the corresponding material ends and the heat generation rate decreases.

[0060] According to the Cauchy middle value theorem, the method for finding the inflection point is: find the characteristic peak of dQ / dV in the interval, calculate the slope of the line between the characteristic peak (dQVmax, Vmax) and the voltage point at the end of charging (dQVp, Vp) k1 = (dQVmax-dQVp) / (Vmax-Vp), and find the point in the dQ / dV curve where the tangent slope is closest to the inflection point. The voltage corresponding to the inflection point is recorded as V1, and the charging cut-off voltage is recorded as Vmax, such as Figure 7 shown.

[0061] Step 2.3, data sampling is performed between V1 and the charge cut-off voltage Vmax. The voltage at the i-th moment is recorded as Ui, the voltage is recorded as Ii, the temperature is recorded as Ti, and the SOC is recorded as SOCi. The SOC-OCV table at different temperatures is checked through SOCi and Ti, and the open circuit voltage value OCVi, polarization heat generation Qi at the i-th moment is interpolated and calculated according to the temperature: Qi = Ii*(Ui-OCVi) and the polarization heat ratio Qp of V1~Vmax: The energy charged into the battery cell from V1 to Vmax is W, and the polarization heat accounts for ω=Qp / W*100%.

[0062] In this embodiment, Figure 8 As shown, taking the first cycle charging of a type A battery cell as an example, the characteristic peak of the battery cell high voltage (about 3.4-3.65V) corresponds to a voltage Vp=3.4131.

[0063] Vp and discharge end slope k1 = (dQVmax-dQVp) / (Vmax-Vp) = (15.3134-4480.785) / (3.65-3.4131) = -18849.6.

[0064] dQ / dV slope = (dQVi-dQVi-1) / (Vi-Vi-1). The tangent slope results corresponding to each point in the voltage range are shown in Table 2 below. Table 2 dQ / dV table for the first cycle of charging of A battery cell

[0065] dV dQ dQ / dV slope 3.4075 2924.054 370158 3.4103 3988.087 380012 3.4131 4480.785 175963 3.4159 4112.807 -131421 3.4187 3759.804 -126072 3.4215 3552.877 -73902 3.4243 3055.259 -177721 3.4271 2427.651 -224146 3.4299 2018.986 -145952 3.4327 1698.082 -114609 3.4355 1450.147 -88548 3.4383 1246.115 -72869 3.4411 1085.82 -57248 3.4439 925.8311 -57139 3.4467 809.4714 -41557 3.4495 722.158 -31183 3.4523 634.9646 -31141 3.4551 562.3542 -25932 3.4579 518.6812 -15598 3.4607 439.0954 -28424 3.4635 395.5531 -15551 3.4663 352.1199 -15512 3.4691 315.9186 -12929 3.4719 279.8474 -12883 …… …… …… 3.6500 15.3134

[0066] From Table 2, find the voltage V1=3.4579V corresponding to the inflection point with a slope closest to k1.

[0067] Return to the recorded data of the charge and discharge cycle in step 1 to obtain the voltage, energy, capacity and other data corresponding to V1 = 3.4579V. For easy reading, the charging parameters of the first cycle of cell A are recorded in Table 3 below:

[0068] Table 3A battery cell first cycle charging parameter record (sampling: 1s)

[0069] Current / A Voltage / V Capacity / Ah Energy / Wh Polarization heat / Ws …… …… …… …… …… 145.2928 3.4579 305.417 1031.424 16.94589 145.2956 3.4579 305.4574 1031.564 16.9705 145.2804 3.458 305.4977 1031.703 16.98802 145.2882 3.4581 305.5381 1031.843 17.00258 145.2734 3.4582 305.5785 1031.982 17.01449 145.2713 3.4583 305.6188 1032.122 17.02789 145.267 3.4584 305.6591 1032.262 17.04100 145.2667 3.4584 305.6995 1032.401 17.04097 145.2629 3.4585 305.7399 1032.541 17.05416 145.2612 3.4585 305.7802 1032.68 17.05397 145.2563 3.4586 305.8206 1032.82 17.06704 145.2495 3.4588 305.8609 1032.959 17.09352 145.2503 3.4588 305.9012 1033.099 17.09362 145.2427 3.4589 305.9416 1033.238 17.10633 145.2409 3.459 305.982 1033.378 17.11977 145.2307 3.4591 306.0223 1033.518 17.13221 145.2354 3.4591 306.0627 1033.657 17.13276 145.2336 3.4592 306.103 1033.797 17.14616 …… …… …… …… …… 137.6391 3.65 323.0354 1093.112 41.01646

[0070] Substitute it into the calculation and we get:

[0071] 1. V1~Vmax battery cell charging energy W=1093.112-1031.564=61.548 (Wh)

[0072] 2. OCV1 interpolation from Table 1 shows: V1 = 3.4579 (V) corresponds to OCV1 = 3.3411 (V)

[0073] 3. V1 corresponds to polarization heat generation Qp1 = 145.2956*(3.4579-3.3411) = 16.9705 (Ws)

[0074] 4. Total polarization heat generation of the first cycle Qp = (Qp1 + Qp2 + ... + Qpmax) / 3600 = 2.3769 (Wh)

[0075] 5. Polarization heat generation ratio ω = Qp / W*100% = 2.3769 / 61.548*100% = 4.447%.

[0076] Because the voltage and current changes are small in the interval of being fully charged, the heat generation is large and the polarization heat generation is relatively small, so the heat generation is divided by the total energy to calculate the polarization heat generation ratio. At the same time, the polarization heat generation value of each point in the interval is accumulated to amplify the change rate of the polarization heat generation ratio in the interval, so that the calculation is more accurate.

[0077] Step 3: Calculate the polarization heat ratio of the batch of cells and select the lithium-deposited cells based on the 3Sigma criterion.

[0078] Repeat steps 1-2 to calculate the polarization heat generation ratio of other set turns of Class A cells and the polarization heat generation ratio of each turn of Class B cells. Based on the 3Sigma rule, cells with a lower polarization heat generation ratio ω are selected as lithium-ion cells. The 3Sigma rule is a statistical tool based on normal distribution. It determines the degree of abnormality or need for improvement by judging the deviation and range of data.

[0079] The method of cycle steps 1-2 is similar to calculating the polarization heat generation ratio of the 5th, 10th, 15th, 20th, 50th, 75th, and 100th cycles of normal A-type cells without lithium deposition, and the polarization heat generation ratio of each cycle of B-type cells is calculated in the same way. The following Table 4 is obtained, and the comparison chart is generated as follows Fig. 9 shown.

[0080] Table 4 Inflection points of Class A and B cells at the 1st, 5th, 10th, 15th, 20th, 50th, 75th and 100th cycles, i.e., the proportion of polarization heat generation

[0081]

[0082] According to the material of the specific battery cell, the capacity of lithium iron phosphate increases and the internal resistance decreases in the first 1-20 cycles or 1-50 cycles, and the corresponding heat generation decreases. The battery cell in this embodiment increases in capacity and reduces heat in 1-20 cycles, which is the activation process of the lithium ion battery. The internal resistance of a normal battery cell increases slowly with the cycle, and the internal resistance of a lithium-ion battery cell increases more slowly.

[0083] At the beginning of lithium deposition, the cell impedance decreases due to the addition of parallel branches of lithium dendrites. At this time, the heat generated by lithium ions decreases. The internal resistance of a normal cell increases slowly with the cycle, but the rate of increase of the internal resistance of a lithium deposition cell will slow down. Therefore, it can be seen from the table that the lithium iron phosphate cell is in the activation process in the first 20 cycles (the capacity increases and the internal resistance decreases in the first 1-20 cycles or 1-50 cycles). As the DC internal resistance decreases, the proportion of polarization heat generation also decreases. As the cycle progresses, the SEI becomes thicker and the proportion of polarization heat generation also increases.

[0084] During the lithium deposition stage, lithium ions begin to transform into metallic lithium on the surface of the graphite negative electrode, and the charging current is divided into lithium insertion current and lithium deposition current, generating a new equivalent circuit branch. When lithium deposition begins, the impedance of the electrolyte and the transfer impedance of the SEI remain unchanged, while the impedance of the charge transfer process decreases due to the addition of parallel branches. At this time, the polarization internal resistance of the lithium-ion battery cell decreases, and the heat generation ratio increases slowly, while the internal resistance of the normal battery cell increases slowly with the cycle. It is impossible to determine whether there is a risk of lithium deposition by only looking at the data of the lithium deposition battery cell, and a horizontal comparison with the normal battery cell is required. The horizontal comparison method is to use the polarization heat generation ratio of the normal battery cell as a benchmark at a certain number of laps (such as the 50th lap, the 75th lap), and if the polarization heat generation ratio of the battery cell to be tested is lower than the threshold value (baseline value - error value), it is judged to have a lithium deposition risk. In this embodiment, to avoid lengthy text, the data of Class B cells are relatively small. In actual application, the cell packs are generally tested in batches, with 50 to 100 cells in a cell pack. The polarization heat percentage of all cells is calculated, and the outlier data with low data is selected as the lithium-ion cells. There are many outlier methods. The 3Sigma criterion is used in this implementation. Specifically, the mean and standard deviation are calculated based on the distribution of the polarization heat percentage, with three standard deviations as the lower limit, and the threshold = mean - 3 * standard deviation. If it is lower than the threshold, it is judged as an outlier. It should be noted that this application does not limit the outlier method to 3Sigma. Other outlier methods such as k-means classification or other statistical methods are within the scope of protection of this application as long as they can achieve outlier judgment.

[0085] The lithium plating detection method disclosed in the present invention calculates the polarization heat generation ratio through the measured voltage, current, capacity and other data. Compared with the prior art, it is more accurate to directly judge through images based on the relaxation voltage interval; secondly, the selection of the starting point in the present application is different from the prior art. According to the principle that the heat generation becomes stable after the phase change of the material is completed, the voltage change is amplified by differentiating the voltage curve, and the starting point of the stable voltage is determined according to the peak point of the dq / dv curve. The peak point corresponds to the end point of the phase change of the material. In the voltage interval from the end of the phase change to the completion of charging, the polarization heat generation value of each point and the polarization heat generation ratio of the interval are calculated to further determine the risk of lithium plating. The detection sensitivity of the present invention is high and early lithium plating can be detected. In this embodiment, in order to verify the reliability of the method, the number of tests in each group of A and B battery cells is greater than 3, and they are fully charged and disassembled after 20 and 100 cycles, respectively. Figure 2 , Figure 4 As shown. Remove the battery case and expose the positive and negative electrodes at the same time. Since lithium deposition only occurs at the negative electrode, observe whether there is lithium deposition at the negative electrode after 20 / 100 cycles. When the number of lithium deposition folds exceeds 30% and the lithium deposition area of ​​lithium deposition folds is greater than 2%, it can be detected. (For wound cells, the winding R angle is used to distinguish them, and each circle is recorded as consisting of 2 folds, such as Figure 1The testing method of the present invention is simple, does not require additional testing equipment, and has high promotion value.

[0086] The embodiments described above are only examples of specific applications of the present invention and are not intended to limit the present invention. Those skilled in the art may modify, supplement or replace some of the technologies in the technical solutions of the specific embodiments. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention shall be included in the protection scope of the present invention.

Claims

1. A non-destructive lithium deposition detection method for lithium-ion batteries. It is characterized in that include: Step 1, based on two types of lithium-ion batteries A and B with the same chemical system, type A is a normal non-lithium-precipitated battery cell, and type B is a battery cell to be tested, at a certain temperature, the charging open circuit voltage OCV of the two types of lithium-ion batteries is determined respectively, and a SOC-OCV table is generated; at a certain charging rate and temperature, the two types of lithium-ion batteries are charged and discharged respectively, and the voltage, current, battery cell large surface temperature, ambient temperature, capacity and energy data are recorded during the whole process; Step 2, based on the voltage, the large surface temperature of the battery cell and the ambient temperature change, a voltage-temperature rise curve is obtained, and based on the voltage-temperature rise curve, a voltage range in which the battery cell generates heat stably is determined; based on the incremental capacity method, a charging dQ / dV curve is obtained, an inflection point is determined within the voltage range in which the battery cell generates heat stably in the dQ / dV curve, and a polarization heat generation value corresponding to the inflection point to the charging end range is calculated; Step 3, calculate the polarization heat ratio of the batch of battery cells, compare the polarization heat ratios of Class A and Class B battery cells horizontally, take the polarization heat ratio of Class A normal non-lithium-depositing battery cells as the benchmark, and screen out the battery cells below the threshold as lithium-depositing battery cells based on the outlier method.

2. The method according to claim 1, It is characterized in that In step 1, among the two types of lithium-ion batteries, the NP ratio of the negative electrode coating weight to the positive electrode coating weight of the normal lithium-ion battery of type A is 1.17, and the NP ratio of the negative electrode coating weight to the positive electrode coating weight of the lithium-ion battery of type B is 1.10; the positive electrode material is lithium iron phosphate, the negative electrode material is single-particle graphite or single-particle mixed secondary particle graphite, the electrolyte is 1M LiPF6 (EC / DMC), and the diaphragm material is polyethylene and polypropylene.

3. The method according to claim 1, It is characterized in that In step 1, the method for testing the charging open circuit voltage OCV of the battery cell is: first, measure the initial capacity C0 of the battery cell; then, use 0.5C0 to discharge to the discharge cut-off voltage and let it stand for 3 hours; use 0.5C0 to charge to the following SOC and let it stand for 3 hours respectively, and record when the SOC is 1%, 3%, 5%, 7%, 10%, 13%, 16%, 18%, 20%, 22%, 25%, 28%, 31%, 34%, 82%, 85%, 88%, 91%, 94%, 97%, 98%, 99%, and 100%, respectively, to obtain a SOC-OCV table.

4. The method according to claim 3, It is characterized in that In step 1, only the test temperature is changed to obtain the open circuit voltage OCV of the battery cell at 5°C, 15°C, 25°C, 35°C, and 45°C, so as to perform lithium deposition detection on the lithium-ion battery at different temperatures.

5. The method according to claim 3, It is characterized in that The charging rates in the battery cycle test include: 0.2C, 0.33C, 0.5C or 0.2P, 0.33P, 0.5P; the discharge rate is the same as the charging rate, which is 0.2C, 0.33C, 0.5C or 0.2P, 033P, 0.5P.

6. The method according to any one of claims 1 to 5, It is characterized in that Step 2 includes: Step 2.1, based on the voltage-temperature rise curve, obtain the voltage range where the battery cell temperature tends to be flat; Step 2.2, differentiate the voltage and capacity data to obtain the charging dQ / dV curve, find the inflection point and the voltage V corresponding to the inflection point in the voltage range of the dQ / dV curve 1 ; Step 2.3, sample data between V1 and the charge cut-off voltage Vmax, record the voltage at the i-th moment as Ui, the voltage as Ii, the temperature as Ti, and the SOC as SOCi. Use SOCi and Ti to look up the SOC-OCV table at different temperatures, and interpolate and calculate according to the temperature to obtain the open circuit voltage value OCVi, polarization heat generation Qi, total polarization heat generation Qp in the interval of V1 to Vmax, and the polarization heat generation proportion ω in this interval.

7. The method according to claim 6, It is characterized in that The method for finding the inflection point is: find the characteristic peak of dQ / dV in the interval, calculate the slope k1 = (dQVmax-dQVp) / (Vmax-Vp) of the line between the characteristic peak (dQVmax, Vmax) and the voltage point at the end of charging (dQVp, Vp), and find the point in the dQ / dV curve where the tangent slope is closest to the inflection point.

8. The method according to claim 7, It is characterized in that The polarization heat generation Qi at the i-th moment is expressed as: Qi = Ii*(Ui-OCVi); the total polarization heat generation Qp from V1 to Vmax is: The energy charged into the battery cell from V1 to Vmax is W, and the polarization heat accounts for ω=Qp / W*100%.

9. The method according to claim 7 or 8, It is characterized in that In step 3, repeat steps 1-2 to calculate class A, The polarization heat generation ratio of each cycle of Class B battery cells is based on the polarization heat generation ratio of normal Class A battery cells without lithium deposition, and is compared horizontally with the polarization heat generation ratio of the battery cells to be tested when the same number of cycles is performed; for the values ​​of batch batteries, the batteries whose polarization heat ratio ω exceeds the threshold are screened out based on the 3Sigma criterion, namely, the lithium deposition batteries.

10. The method according to claim 1, It is characterized in that The number of lithium-ion batteries of both types tested was greater than 3, and they were fully charged and disassembled after a certain number of charging cycles to verify the results of the lithium plating risk assessment.

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