Method for detecting mixing uniformity of first-injection electrolyte and second-injection electrolyte

Abstract

The invention relates to the technical field of lithium ion battery manufacturing, in particular to a method for detecting the mixing uniformity of a first-injection electrolyte and a second-injection electrolyte. According to the method, non-uniform electrolyte is frozen and fixed under a low-temperature condition, diffusion of a non-uniform solvent or solute is suspended, the lithium ion battery is charged under the low-temperature condition, and whether the first-injection electrolyte and the second-injection electrolyte are uniformly mixed or not is judged according to the electrochemical characteristic difference caused by non-uniform components in the charging process. According to the method, the diffusion of the electrolyte is slowed down at the low temperature, and the lithium ion battery is charged at the low temperature, so that the electrochemical characteristic difference caused by non-uniform mixing of the first-injection electrolyte and the second-injection electrolyte can be amplified, and the electrochemical characteristic difference is more obvious, so that the electrochemical characteristic difference is easy to find and detect; and the sensitivity of mixing uniformity detection of the first-injection electrolyte and the second-injection electrolyte is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion battery manufacturing, and in particular to a method for detecting mixing uniformity of one-injection electrolyte and two-injection electrolyte. BACKGROUND

[0002] In the manufacturing process of lithium ion batteries, one-injection and two-injection processes are often used, and there is a formation process between the one-injection and two-injection processes. After formation, the one-injection electrolyte and the two-injection electrolyte are prone to have inconsistent additive concentrations. Moreover, due to the differences in composition and function between the one-injection electrolyte and the two-injection electrolyte, if the mixing is not uniform, it may lead to unstable interface film formation, intensified local side reactions, battery performance degradation, and other problems. Therefore, the detection of the mixing uniformity of the one-injection electrolyte and the two-injection electrolyte is very important. Some commonly used detection methods have the defect of insufficient sensitivity. SUMMARY

[0003] The present application provides a method for detecting the mixing uniformity of one-injection electrolyte and two-injection electrolyte, which amplifies the electrochemical property differences caused by uneven components, thereby improving the sensitivity of the mixing uniformity detection of the one-injection electrolyte and the two-injection electrolyte.

[0004] The present application provides a method for detecting the mixing uniformity of one-injection electrolyte and two-injection electrolyte, which amplifies the electrochemical property differences caused by uneven components, thereby improving the sensitivity of the mixing uniformity detection of the one-injection electrolyte and the two-injection electrolyte.

[0005] J1) Obtain a lithium ion battery after two-injection electrolyte injection is completed.

[0006] J2) Freeze the lithium ion battery to inhibit the diffusion of the one-injection electrolyte and the two-injection electrolyte.

[0007] J3) Perform full charging on the lithium ion battery at low temperature.

[0008] J4) Judge the mixing uniformity of the one-injection electrolyte and the two-injection electrolyte based on the electrochemical property differences caused by uneven components during charging.

[0009] Based on the method for detecting the mixing uniformity of one-injection electrolyte and two-injection electrolyte provided by the present application, the method slows down the diffusion of the electrolyte at low temperature, and charges the lithium ion battery at low temperature. In this way, the electrochemical property differences caused by uneven mixing of the one-injection electrolyte and the two-injection electrolyte can be amplified, making the electrochemical property differences more significant, thereby facilitating detection and improving the sensitivity of the mixing uniformity detection of the one-injection electrolyte and the two-injection electrolyte.

[0010] In one possible design, the charging rate in step J3) is greater than the charging window of the lithium ion battery.

[0011] Step J4) includes:

[0012] J41) disassembling the full lithium ion battery.

[0013] J42) observing the lithium precipitation on the negative electrode surface. If the whole negative electrode surface is golden yellow, it is determined that the first electrolyte and the second electrolyte are mixed uniformly. If part of the negative electrode surface is white and part of the negative electrode surface is golden yellow, it is determined that the first electrolyte and the second electrolyte are not mixed uniformly.

[0014] Based on the detection method for the mixing uniformity of the first electrolyte and the second electrolyte provided by the embodiment, the lithium ion battery is charged at a charge rate exceeding the charge window at low temperature, which amplifies the defects on the negative electrode surface. By observing the lithium precipitation on the negative electrode surface, it is determined whether the first electrolyte and the second electrolyte are mixed uniformly. The method has the characteristics of being intuitive, easy to judge, time-saving and high efficiency. In addition, the method only needs conventional temperature control equipment and charging equipment, without additional equipment, and has the characteristics of easy operation and cost saving.

[0015] In a possible design, between step J1) and step J2), there is also step S1): storing the lithium ion battery at high temperature.

[0016] Based on the detection method for the mixing uniformity of the first electrolyte and the second electrolyte provided by the embodiment, in some cases, the lithium ion battery can be stored at high temperature before low temperature charging, so that the uneven mixing of the first electrolyte and the second electrolyte is further deteriorated, so that the difference in electrochemical properties caused by uneven components is more obvious after low temperature charging.

[0017] In a possible design, between step J1) and step J2), there is also step F1): performing normal temperature cyclic charging and discharging on the lithium ion battery.

[0018] Based on the detection method for the mixing uniformity of the first electrolyte and the second electrolyte provided by the embodiment, before freezing the lithium ion battery, the lithium ion battery is first subjected to normal temperature cyclic charging and discharging to simulate the normal use state of the lithium ion battery. If the first electrolyte and the second electrolyte are not mixed uniformly, the influence of the uneven components will be accumulated during the cyclic charging and discharging. Then, the lithium ion battery is frozen and subjected to low temperature charging, so that the influence of the uneven components acts on the negative electrode interface, and then whether the first electrolyte and the second electrolyte are mixed uniformly can be determined according to the characteristics of the negative electrode interface.

[0019] In a possible design, between step J1) and step F1), there is also step F0): storing the lithium ion battery at high temperature.

[0020] The detection method for the mixing uniformity of the first electrolyte and the second electrolyte provided by the embodiment can increase the high-temperature storage step before the cycle charging and discharging of the lithium ion battery, so as to reduce the cycle test number and improve the detection efficiency.

[0021] In a possible design, in step J2), the freezing temperature is-20 to-40 DEG C, and the freezing time is 2-8 hours.

[0022] In a possible design, the temperature range in step J3) is 0 to-10 DEG C.

[0023] In a possible design, in step F0), the storage temperature of the lithium ion battery is 45 to 60 DEG C, and the storage time is 3-30 days.

[0024] In a possible design, the cycle number in step F1) is 5-10 cycles. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 A flow chart of the detection method for the mixing uniformity of the first electrolyte and the second electrolyte provided by the embodiment. DETAILED DESCRIPTION

[0026] In order to make the purpose, technical scheme and advantages of the embodiments of the present application clearer, the technical scheme of the embodiments of the present application will be described clearly and completely below in combination with the drawings of the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments of the present application.

[0027] The embodiments given in the present application can be combined with each other without contradiction. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative labor belong to the scope of protection of the present application.

[0028] All raw materials used in the present application are not particularly limited in source, and can be purchased on the market or prepared according to the conventional method well known to those skilled in the art.

[0029] If the parameters of all raw materials used in the present application are not particularly limited, they are all the raw materials generally used in the art, and those skilled in the art can purchase them on the market or prepare them by conventional methods according to the name of the raw material and the corresponding use.

[0030] The processes used in the present application are all conventional abbreviations in the art, and the specific steps and conventional parameters of each abbreviation are clear and explicit in its related field, and those skilled in the art can realize them by conventional methods according to the abbreviation.

[0031] Unless otherwise defined, all terms used herein have the meanings as commonly understood by one of ordinary skill in the art.

[0032] The present application will be described in detail from the background art.

[0033] In the process of manufacturing lithium ion batteries, one injection and two injection processes are often used, and there is a formation process between one injection and two injection. After formation, the additive concentration of the one injection electrolyte and the two injection electrolyte is not consistent. And because the composition and function of the one injection electrolyte and the two injection electrolyte are different, if mixed unevenly, it may cause problems such as unstable interface film, accelerated local side reaction, battery performance degradation, etc. Some commonly used detection methods have the defect of insufficient sensitivity.

[0034] In view of this, please refer to Figure 1 , Figure 1 The flow chart of a one injection electrolyte and two injection electrolyte mixing uniformity detection method provided by the present application, the one injection electrolyte and two injection electrolyte mixing uniformity detection method provided by the present application, comprising the following steps:

[0035] J1) Obtain a lithium ion battery after two injection electrolyte injection is completed.

[0036] J2) Freeze the lithium ion battery to inhibit the diffusion of the one injection electrolyte and the two injection electrolyte.

[0037] J3) Fully charge the lithium ion battery at low temperature.

[0038] J4) Based on the electrochemical property difference caused by the uneven composition during charging, judge the mixing uniformity of the one injection electrolyte and the two injection electrolyte.

[0039] Specifically, for step J2), the purpose of this step is to slow down or even stop the diffusion of the one injection electrolyte and the two injection electrolyte by using low temperature. That is, the flowability of the one injection electrolyte and the two injection electrolyte becomes poor and almost stops. Among them, the selection of freezing temperature should not only effectively inhibit the diffusion of the one injection electrolyte and the two injection electrolyte, but also avoid damaging the battery.

[0040] Then, in step J3), the lithium ion battery is charged at low temperature. The purpose of this step is to amplify the electrochemical property difference caused by the uneven composition. Specifically:

[0041] Whether or not low temperature, if a note of electrolyte, two injection electrolyte mixed uneven phenomenon, then, in the charging process, the SEI film of different regions of the negative electrode surface will show different ion transport capacity due to the difference in composition and concentration of the first injection electrolyte and the second injection electrolyte. In the region where the first injection electrolyte and the second injection electrolyte are mixed unevenly, the lithium ion transport is blocked, and the polarization voltage increases during charging. In theory, the electrochemical property difference caused by uneven composition can be shown, but this electrochemical property difference is usually small and difficult to find and detect.

[0042] Therefore, in the scheme of the present application, the flow of the first injection electrolyte and the second injection electrolyte is inhibited at low temperature, and charging is carried out in a low temperature environment, so that the electrochemical property difference caused by the uneven mixing of the first injection electrolyte and the second injection electrolyte can be amplified. The theoretical basis is:

[0043] Low temperature deteriorates ion transport kinetics: According to the Arrhenius equation, the ion conductivity (σ) and the diffusion coefficient of lithium ions in graphite decrease exponentially with temperature, which greatly increases the resistance of lithium ions to embed in graphite.

[0044] It is easier to approach the lithium precipitation critical window: At low temperature, the kinetics of lithium ion embedding in graphite becomes extremely slow, while the reaction resistance of lithium precipitation changes relatively small. Therefore, low temperature greatly increases the risk of lithium precipitation. The region where the electrolyte is mixed unevenly (such as low lithium salt concentration, insufficient additives) will more easily reach the overpotential condition of lithium precipitation.

[0045] Among them, the selection of low temperature charging temperature is based on the principle of electrochemical kinetics, and the purpose is to create a harsh but controllable environment to significantly expose the uneven "symptoms".

[0046] In summary, the detection method for the uniformity of the first injection electrolyte and the second injection electrolyte provided by the present application can amplify the electrochemical property difference caused by the uneven mixing of the first injection electrolyte and the second injection electrolyte by inhibiting the diffusion of the electrolyte at low temperature and charging the lithium ion battery at low temperature, so that the electrochemical property difference is more significant, thereby being easy to find and detect, and the sensitivity of the detection of the uniformity of the first injection electrolyte and the second injection electrolyte is improved.

[0047] For step J4), the electrochemical property difference caused by uneven composition can be manifested as changes in electrolyte bulk resistance and charge transfer impedance during charging.

[0048] Among them, the electrolyte bulk resistance represents the sum of all ohmic resistances in the lithium ion battery, mainly including the ion resistance of the electrolyte itself, the electronic resistance of the electrode material and the current collector, and the contact resistance between each component, reflecting the resistance of ion migration in the electrolyte.

[0049] The charge transfer resistance represents the dynamic resistance of the electrochemical reaction itself occurring at the electrode / electrolyte interface. It reflects the ease of charge exchange. The smaller the charge transfer resistance, the faster the reaction and the better the kinetic performance.

[0050] Specifically, during the low-temperature charging process, a high-frequency (1 kHz-100 kHz) AC disturbance can be applied, and the change in the bulk resistance of the electrolyte can be monitored synchronously. The change in the bulk resistance of the electrolyte can indirectly reflect the distribution state of the electrolyte inside the battery. For example, an abnormal increase in the bulk resistance of the electrolyte can indicate that the additive is depleted in a local area or that a blocking layer with poor ionic conductivity is formed.

[0051] During the low-temperature charging process, a medium-frequency (10 Hz-1 kHz) AC disturbance can be applied, and the change in the charge transfer resistance can be monitored synchronously. The change in the charge transfer resistance can directly reflect the dynamic evolution of the electrochemical reaction kinetics at the electrode interface. For example, an increase in the charge transfer resistance usually means that the interface is deteriorated and the reaction activity is reduced (such as film thickening and surface passivation).

[0052] For step J4), the electrochemical property difference caused by the uneven composition during the charging process can also be analyzed by differential voltage analysis (DVA), which is a method of indirectly diagnosing the internal state of the battery (especially the active lithium distribution and the phase transition behavior of the electrode material) through the differential processing of the voltage-capacity curve. It is mainly used to analyze the distribution of lithium ions, but the uneven distribution of the electrolyte will interfere with the electrode reaction kinetics and the phase transition process by affecting the ion transport, and finally present characteristic signals on the DVA curve.

[0053] Specifically, for a lithium iron phosphate / graphite battery: when the first electrolyte and the second electrolyte are mixed uniformly, the Stage II→Stage I of the negative electrode graphite produces a single phase transition peak (half-peak width ≈ 10 mV). When the first electrolyte and the second electrolyte are not mixed uniformly, the Stage II→Stage I of the negative electrode graphite produces a double peak (the lithium intercalation of the negative electrode near the tab side is not synchronized with the middle part of the negative electrode).

[0054] For step J4), the electrochemical property difference caused by the uneven composition during the charging process can also be manifested as the difference in lithium precipitation on the negative electrode surface.

[0055] In this embodiment, the charging rate in step J3) is greater than the charging window of the lithium ion battery.

[0056] Step J4) comprises:

[0057] J41) disassembling the fully charged lithium ion battery.

[0058] J42) Observe the lithium precipitation on the negative electrode surface. If the whole negative electrode surface is golden yellow, it is determined that the first electrolyte and the second electrolyte are mixed uniformly. If part of the negative electrode surface is white and part of the negative electrode surface is golden yellow, it is determined that the first electrolyte and the second electrolyte are not mixed uniformly.

[0059] The principle of this embodiment is explained as follows:

[0060] Firstly, the functions of the first electrolyte and the second electrolyte are different:

[0061] The first electrolyte: preferentially participates in the reaction in the formation stage, the additives are consumed in large quantities, and a relatively thick but possibly loose SEI film is formed.

[0062] The second electrolyte: contains functional additives, which mainly function to repair and strengthen the SEI film.

[0063] After the formation process, if the second electrolyte does not diffuse sufficiently, the first electrolyte is depleted of additives, the SEI film repair capability decreases, and the interface impedance increases.

[0064] Secondly, for the negative electrode sheet, the structure near the negative electrode tab is compact, the electrolyte wettability is poor, and the electrolyte penetration is difficult. Therefore, if the first electrolyte and the second electrolyte are mixed unevenly, the second electrolyte may be enriched in the middle of the electrode, and the side of the negative electrode near the tab still relies on the first residual electrolyte due to the limited electrolyte flow. Since the additives in the first electrolyte near the tab of the negative electrode have been consumed in large quantities during the formation stage, the SEI film near the tab of the negative electrode is prone to defects, which in turn triggers lithium precipitation near the tab of the negative electrode.

[0065] The lithium precipitation mechanism is as follows:

[0066] Current density concentration and interface impedance imbalance. The negative electrode tab is the current access channel, and the current density is naturally high. If the SEI film impedance on the side of the negative electrode near the tab is significantly higher than that in other regions, the lithium ion embedding resistance on the side of the negative electrode near the tab is greater, and is forced to be reduced to metallic lithium (lithium precipitation) on the surface.

[0067] Lithium ion transport kinetics imbalance. After the formation process, if the second electrolyte is difficult to supplement the first electrolyte on the side of the negative electrode near the tab, the impedance of the SEI film on the side of the negative electrode near the tab will increase, which in turn leads to low lithium ion migration rate, thereby causing lithium ion accumulation on the negative electrode surface. Under high current density, the ion embedding rate exceeds the diffusion capacity, and the lithium precipitation threshold is triggered.

[0068] The lithium precipitation near the tab side of the negative electrode will further block the electrode pores and exacerbate the uneven wetting of the electrolyte. In addition, the exothermic reaction of lithium precipitation may cause local high temperature, accelerate the decomposition of additives in the first electrolyte and the second electrolyte, and form a vicious cycle of "additive depletion→lithium precipitation→thermal runaway".

[0069] Therefore, in step J42), if the first electrolyte and the second electrolyte are not mixed uniformly, the negative electrode surface will exhibit different degrees of lithium precipitation in different areas. The prominent area of lithium precipitation is generally on the side of the negative electrode close to the tab. Conversely, whether the first electrolyte and the second electrolyte are mixed uniformly can be determined according to the lithium precipitation on the negative electrode surface.

[0070] Specifically, if the first electrolyte and the second electrolyte are mixed uniformly, lithium ions are normally inserted to form LiC6 with the graphite of the negative electrode. Since LiC6 is golden yellow, the entire negative electrode surface is golden yellow. If the first electrolyte and the second electrolyte are not mixed uniformly, the local area of the negative electrode surface (the side of the negative electrode close to the tab) exhibits white color due to the precipitation of lithium. Specific cases can be referred to Example 1 below.

[0071] It should be noted that in this embodiment, the charge rate is greater than the charge window of the lithium ion battery, and the charge rate needs to be determined in advance. The specific process can be as follows: in the case where it is known that the first electrolyte and the second electrolyte are not mixed uniformly, different charge rates are used for charging, and the charge rate that can make the side of the negative electrode close to the tab precipitate lithium and the other areas of the negative electrode preferably not precipitate lithium is selected as the detection rate of this type of lithium ion battery.

[0072] Based on the method for detecting the uniformity of the mixture of the first electrolyte and the second electrolyte provided in this embodiment, the lithium ion battery is charged at a low temperature using a charge rate exceeding the charge window, and the defects on the negative electrode surface are amplified. By observing the lithium precipitation on the negative electrode surface, whether the first electrolyte and the second electrolyte are mixed uniformly can be determined. This method is very intuitive, easy to judge, time-saving and efficient. In addition, this method only needs conventional temperature control equipment and charging equipment, without the need for additional equipment, and has the characteristics of easy operation and cost saving.

[0073] In practical applications, it is found that for some lithium ion batteries, the electrochemical property difference caused by the uneven components is not easy to appear when the above-mentioned method is used to detect the uniformity of the mixture of the first electrolyte and the second electrolyte.

[0074] In view of this situation, a step S1) of storing the lithium ion battery at a high temperature is further included between step J1) and step J2).

[0075] The detection method provided by the embodiment is more suitable for a first injection electrolyte and / or a second injection electrolyte containing additives or solvents that are difficult to diffuse.

[0076] In theory, the mixing of the first injection electrolyte and the second injection electrolyte after high-temperature storage should be more uniform, and more conducive to the stability of the SEI film. However, in some cases, it exacerbates the lithium precipitation on the side of the negative electrode close to the tab.

[0077] This seemingly contradictory phenomenon is caused by the coupling of the "diffusion-consumption competition mechanism" and the "interface self-catalytic effect".

[0078] For example, under the action of high temperature, the overall concentration of the additives (such as vinylene carbonate) in the electrolyte tends to be balanced, but the actual distribution is dominated by the interface reaction rate. On the side of the negative electrode close to the tab: the structural defects provide a large number of active sites, and the vinylene carbonate is consumed before it is diffused to the site. In the middle region of the negative electrode, the interface is relatively complete, the vinylene carbonate is slowly formed into a film, and there is still a reserve.

[0079] Therefore, although high temperature can promote the mixing of the first injection electrolyte and the second injection electrolyte, due to the competition mechanism of diffusion and consumption, if the diffusion rate of the additives or solvents in the second injection electrolyte is less than the consumption rate under high-temperature conditions, then high temperature not only cannot improve the uniformity of the mixing of the first injection electrolyte and the second injection electrolyte, but also exacerbates the difference between the two. Therefore, in some cases, before low-temperature charging, the lithium ion battery can be stored at high temperature, so that the uneven mixing of the first injection electrolyte and the second injection electrolyte is further deteriorated, so that the difference in electrochemical properties caused by uneven components is more obvious after low-temperature charging.

[0080] In the embodiment, the selection of the high-temperature storage temperature is a trade-off between chemical reaction kinetics and material safety boundaries, and the purpose is to achieve the preliminary homogenization of the electrolyte and the "pre-segregation" of the interface state in the shortest time under the premise of safety.

[0081] The selection of the high-temperature storage temperature is as follows:

[0082] Firstly, it should be lower than the flash point temperature of the solvents in the first injection electrolyte and the second injection electrolyte. The flash point of common solvents (such as EMC) is above 70℃, and the storage temperature is below 60℃, which leaves sufficient safety margin.

[0083] Secondly, the stability of the SEI film needs to be considered. If it is higher than 60-70℃ for a long time, the SEI film may begin to decompose, which may cause irreversible damage to the battery.

[0084] In some embodiments of the application, the storage temperature range is 45℃-60℃, and the storage time is 3-30 days.

[0085] It should be noted that the storage temperature and storage time at high temperature need to be explored in advance. The exploration method can be that, in the case that it is known that the first electrolyte and the second electrolyte are mixed unevenly, the storage time and the storage temperature are taken as variables, after storage at high temperature, low-temperature charging is performed, and the condition of the negative electrode surface is observed. The appropriate storage time and storage temperature are selected therefrom.

[0086] It is also found in practical application that some lithium ion batteries contain extremely difficult-to-diffuse additives in the first electrolyte and / or the second electrolyte. The influence caused by the unevenness of the additives can only be manifested after the lithium ion batteries are used normally for a period of time. For such cases, step F1) of performing normal-temperature cyclic charging and discharging on the lithium ion battery is further included between step J1) and step J2).

[0087] In this embodiment, before the lithium ion battery is frozen, normal-temperature cyclic charging and discharging is performed to simulate the normal use state of the lithium ion battery. If the first electrolyte and the second electrolyte are mixed unevenly, the influence caused by the unevenness will be accumulated during the cyclic charging and discharging. Then, the lithium ion battery is frozen and subjected to low-temperature charging, so that the influence caused by the unevenness acts on the negative electrode interface, and then whether the first electrolyte and the second electrolyte are mixed evenly can be judged according to the characteristics of the negative electrode interface.

[0088] In practical application, for the lithium ion battery containing extremely difficult-to-diffuse additives in the first electrolyte and / or the second electrolyte, and the influence caused by the unevenness of the additives can only be manifested after the lithium ion battery is used normally for a period of time, in order to reduce the cycle number and improve the test efficiency, in some embodiments of the present application, step F0) of storing the lithium ion battery at high temperature is further included between step J1) and step F1). In this embodiment, the role of high-temperature storage is the same as described above, which will not be described here.

[0089] In some embodiments of the present application, after the high-temperature storage is added, the cycle number of the lithium ion battery is reduced from 100-200 cycles to 5-10 cycles, and the test efficiency is greatly improved.

[0090] Among the above four methods, the first method only uses low-temperature freezing, and then low-temperature full charging, so that the uniformity of the first electrolyte and the second electrolyte can be judged by the unevenness of lithium precipitation on the negative electrode surface.

[0091] The second method adds the step of high-temperature storage before low-temperature freezing on the basis of the first method. The third method adds the step of cyclic charging and discharging before low-temperature freezing. The fourth method adds the step of high-temperature storage before cyclic charging and discharging.

[0092] In practice, the components, concentrations, and viscosities of the solvents and additives in the first electrolyte and the second electrolyte are various, and it is difficult to directly determine which of the four methods should be used to judge the mixing uniformity of the first electrolyte and the second electrolyte.

[0093] Generally, when detecting the first electrolyte and the second electrolyte, the first method is used first. If the first method is not feasible, the second method is used. If the second method does not work well, the third method is used. If the third method does not work well, the fourth method is used.

[0094] As a tendency guide, for electrolytes with low viscosity and easy diffusion, the first method can be used for testing first. For electrolytes with high viscosity and difficult diffusion, such as electrolytes containing vinylene carbonate, the second method can be used for testing first. For electrolytes with extremely difficult diffusion, such as electrolytes containing high molecular weight film-forming agents and high concentration lithium salts, the third method or the fourth method can be used for testing.

[0095] In some embodiments of the present application, the freezing temperature in step J2) is -20°C to -40°C, and the freezing time is 2-8 hours.

[0096] Specifically, the purpose of step J2) is to achieve the glass transition of the first electrolyte and the second electrolyte, prevent their continuous diffusion at room temperature, and provide a "frozen" sample for subsequent detection.

[0097] The selection basis and principle are as follows:

[0098] First, the freezing temperature is lower than the freezing point of the electrolyte: The freezing point of the electrolyte of a conventional lithium ion battery is usually between -20°C and -30°C. A temperature lower than this temperature is selected to ensure that the electrolyte as a whole enters a solid state or a supercooled state, and the molecular diffusion movement tends to stop.

[0099] Second, the glass transition temperature is reached: rapid cooling (rate > 5°C / min) to the range of -30°C to -40°C can make the electrolyte avoid forming crystals (crystallization will destroy the microstructure), and directly form a kind of amorphous "glassy" solid. In this state, molecular motion is greatly inhibited, and the diffusion time scale is extended from seconds / minutes to hours / days, thus perfectly "locking" the instantaneous state of non-uniformity.

[0100] Finally, avoid battery damage: temperature is not the lower the better. Too low a temperature (such as lower than -50°C) can cause damage to battery materials (such as adhesives and separators) due to low-temperature brittleness, excessive electrolyte volume shrinkage resulting in internal stress, and unnecessary energy consumption. -30°C is a balance point that takes into account "effective freezing" and "safety and reliability".

[0101] In some embodiments of the present application, the charging temperature in step J3) ranges from 0°C to -10°C. This charging temperature, combined with the charging rate, can cause the lithium deposition on the surface of the negative electrode to exhibit differences in the case of uneven mixing of the first electrolyte and the second electrolyte.

[0102] Specifically, at 0°C, the ionic conductivity is about 40% of that at 25°C. At -10°C, the ionic conductivity is only 20% of that at 25°C or even lower. This greatly increases the resistance of lithium ions to intercalate into graphite.

[0103] The lower limit of the low temperature is chosen to be -10°C instead of -20°C to strike a balance between "effective amplification of signals" and "avoiding general failure in all regions", so that the unevenness is more clearly contrasted.

[0104] In summary, the detection method provided by the present application realizes rapid, low-cost, and engineering-friendly evaluation of the uniformity of electrolyte mixing without relying on precision instruments, perfectly making up for the shortcomings of existing methods in timeliness and practicality. Compared with traditional methods, the detection period is shortened to 3-10 days, and the detection cost is reduced by 90%, providing an efficient and reliable solution for electrolyte quality control in the manufacturing process of lithium ion batteries.

[0105] The method of the present application is further illustrated and verified by several specific examples below.

[0106] Example One:

[0107] A lithium iron phosphate square cell with completed second electrolyte injection was taken, and the additives of the first electrolyte and the second electrolyte included lithium difluorophosphate.

[0108] The lithium ion battery was maintained in a -30°C incubator for 4 hours to achieve glassy freezing of the first electrolyte and the second electrolyte.

[0109] Full charging at 0.2C in a -10°C environment, and disassembling the battery, it can be observed that lithium is deposited on the side of the negative electrode close to the tab, appearing white. The other areas of the negative electrode appear golden yellow, showing obvious differences.

[0110] Comparative Example One:

[0111] A lithium iron phosphate square cell with completed second electrolyte injection was taken, and the additives of the first electrolyte and the second electrolyte included lithium difluorophosphate.

[0112] After that, the lithium ion battery was maintained in a -30°C incubator for 4 hours to achieve glassy freezing of the first electrolyte and the second electrolyte.

[0113] After that, full charging at 0.2C in a -10°C environment, and disassembling the battery, it can be observed that the surface of the negative electrode as a whole appears golden yellow, with no obvious interface differences.

[0114] Example Two:

[0115] Take the lithium iron phosphate square shell battery with two injections of electrolyte, one of which contains vinylene carbonate.

[0116] The lithium ion battery was stored at 55°C for 7 days.

[0117] After that, maintain in a -30°C oven for 4 hours to achieve glassy freezing of the first and second injections of electrolyte.

[0118] After that, fully charge at 0.2C in a -10°C environment, and disassemble the battery. It can be observed that lithium is deposited on the side of the negative electrode close to the tab, appearing white. The rest of the negative electrode is golden yellow, showing a clear difference.

[0119] Comparative Example Two: Take the lithium iron phosphate square shell battery with two injections of electrolyte from the same batch as Example Two, and directly maintain in a -30°C oven for 4 hours to achieve glassy freezing of the first and second injections of electrolyte.

[0120] After that, fully charge at 0.2C in a -10°C environment, and disassemble the battery. No obvious difference can be observed on the surface of the negative electrode.

[0121] Example Three:

[0122] Take the lithium iron phosphate square shell battery with two injections of electrolyte, one of which contains polyvinylene carbonate.

[0123] The lithium ion battery was stored at 55°C for 7 days, and then subjected to 3 times of normal temperature cycling.

[0124] After that, maintain in a -30°C oven for 4 hours to achieve glassy freezing of the first and second injections of electrolyte.

[0125] After that, fully charge at 0.2C in a -10°C environment, and disassemble the battery. It can be observed that lithium is deposited on the side of the negative electrode close to the tab, appearing white. The rest of the negative electrode is golden yellow, showing a clear difference.

[0126] Comparative Example Three: Take the lithium iron phosphate square shell battery with two injections of electrolyte from the same batch as Example Three, and store it at 55°C for 7 days.

[0127] After that, maintain in a -30°C oven for 4 hours to achieve glassy freezing of the first and second injections of electrolyte.

[0128] After that, fully charge at 0.2C in a -10°C environment, and disassemble the battery. No obvious difference can be observed on the surface of the negative electrode.

[0129] Example 4: Take the same batch of lithium iron phosphate square shell battery as in Example 3, directly perform 100 times of normal temperature cycle.

[0130] After that, maintain in a-30℃ oven for 4 hours to realize the glassy freezing of the first electrolyte and the second electrolyte.

[0131] After that, perform full charging at 0.2C in a-10℃ environment, and disassemble the battery. It can be observed that lithium is deposited on the side of the negative electrode close to the tab, showing white color. The other areas of the negative electrode are golden yellow, showing obvious difference.

[0132] Through the comparison between Example 1 and Comparative Example 1, it can be known that for the same batch of lithium iron phosphate square shell battery, Example 1 does not perform high temperature storage before low temperature freezing, and the first electrolyte and the second electrolyte are locked in the state of uneven mixing. Therefore, after low temperature full charging, the surface of the negative electrode shows obvious difference. In Comparative Example 1, high temperature storage is performed before low temperature freezing, which promotes the diffusion of the first electrolyte and the second electrolyte, so that the first electrolyte and the second electrolyte are mixed uniformly. Therefore, after low temperature freezing and low temperature full charging, the surface of the negative electrode has no obvious difference.

[0133] Through the comparison between Example 1 and Example 2, it can be known that for some lithium ion batteries, only through low temperature freezing and low temperature full charging, it can be judged whether the first electrolyte and the second electrolyte are mixed uniformly according to the lithium precipitation condition of the negative electrode interface.

[0134] Through the comparison between Example 2 and Comparative Example 2, it can be known that for other lithium ion batteries, only the method of low temperature freezing and low temperature full charging cannot judge whether the first electrolyte and the second electrolyte are mixed uniformly, and the step of high temperature storage before low temperature freezing is needed to judge.

[0135] By comparing Example 2 and Comparative Example 1, it can be seen that in Comparative Example 1, high temperature storage promotes the mixing uniformity of the first electrolyte and the second electrolyte. In Example 2, high temperature storage deteriorates the mixing uniformity of the first electrolyte and the second electrolyte.

[0136] The difference between the two depends on the "diffusion-consumption competition mechanism" mentioned earlier. In Comparative Example 1, the diffusion speed of lithium difluorophosphate is fast. When the diffusion speed is greater than the consumption speed, lithium difluorophosphate is distributed more and more uniformly under the action of high temperature.

[0137] In Example Two, the diffusion speed of vinylene carbonate is slower than the consumption speed. Then, on the side of the negative electrode close to the tab, vinylene carbonate is consumed before it diffuses to the site due to the large number of active sites provided by structural defects. The interface in the middle region of the negative electrode is complete, and vinylene carbonate slowly forms a film and is still reserved. Therefore, the distribution of vinylene carbonate inside the battery is more uneven.

[0138] Through the comparison of Example Three and Comparative Example Three, it can be seen that for some lithium ion batteries, it is still difficult to determine whether the first electrolyte and the second electrolyte are mixed uniformly by only using the high-temperature storage, low-temperature freezing, and low-temperature full charging method, and it is still necessary to add the step of cyclic charging and discharging between high-temperature storage and low-temperature freezing.

[0139] Through the comparison of Example Three and Example Four, it can be seen that for some lithium ion batteries, the number of room temperature cycles can be reduced by adding the step of high-temperature storage before low-temperature freezing.

Claims

1. A method for detecting the uniformity of mixing of a first-injection electrolyte and a second-injection electrolyte, characterized in that, Includes the following steps: J1) Obtain a lithium-ion battery after the second electrolyte injection is completed; J2) Freeze the lithium-ion battery to suppress the diffusion of the first and second electrolytes; J3) The lithium-ion battery is fully charged at low temperature; J4) Based on the differences in electrochemical characteristics caused by the non-uniform components during the charging process, the mixing uniformity of the first and second electrolyte injections is judged.

2. The method for detecting the uniformity of mixing of a first electrolyte injection and a second electrolyte injection according to claim 1, characterized in that, The charging rate in step J3) is greater than the charging window of the lithium-ion battery; Step J4) includes: J41) Disassembles a fully charged lithium-ion battery; J42) Observe the lithium deposition on the negative electrode surface. If the negative electrode surface is golden yellow, it is judged that the first and second electrolytes are mixed evenly. If some areas of the negative electrode surface are white and some areas are golden yellow, it is judged that the first and second electrolytes are not mixed evenly.

3. The method for detecting the uniformity of mixing of a single electrolyte injection and a second electrolyte injection according to claim 2, characterized in that, Between step J1) and step J2), there is also step S1): storing the lithium-ion battery at a high temperature.

4. The method for detecting the mixing uniformity of a single electrolyte injection and a second electrolyte injection according to claim 2, characterized in that, Between step J1) and step J2), there is also step F1): performing room temperature cyclic charging and discharging on the lithium-ion battery.

5. The method for detecting the mixing uniformity of a single electrolyte injection and a second electrolyte injection according to claim 4, characterized in that, Between step J1) and step F1), there is also step F0): storing the lithium-ion battery at high temperature.

6. The method for detecting the mixing uniformity of a single electrolyte injection and a second electrolyte injection according to claim 1, characterized in that, In step J2), the freezing temperature is -20℃ to -40℃, and the freezing time is 2-8 hours.

7. The method for detecting the mixing uniformity of a single electrolyte injection and a second electrolyte injection according to claim 1, characterized in that, The temperature range in step J3) is from 0°C to -10°C.

8. The method for detecting the mixing uniformity of a single electrolyte injection and a second electrolyte injection according to claim 5, characterized in that, The storage temperature range of the lithium-ion battery described in step F0) is 45℃-60℃, and the storage time is 3-30 days.

9. The method for detecting the mixing uniformity of a single electrolyte injection and a second electrolyte injection according to claim 8, characterized in that, The number of cycles in step F1) is 5-10.

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