A lithium-ion battery and its preparation method

Abstract

This invention provides a lithium-ion battery and its preparation method. The lithium-ion battery includes a positive electrode, a negative electrode, and a separator disposed between the positive and negative electrodes. The positive electrode includes a positive electrode active coating, a functional coating, and a current collector. The positive electrode active coating is connected to the current collector through the functional coating. The functional coating includes vinyl chloride-acrylate resin, aqueous acrylic resin, and metal oxides. The metal oxides include at least one selected from aluminum oxide, zirconium oxide, zinc oxide, magnesium oxide, and lithium oxide. The separator includes a base film and a surface coating. The surface coating includes a sodium fast ion conductor. This lithium-ion battery possesses both good structural stability and cycle performance, and exhibits excellent flame retardant properties, overcharge resistance, and safety even under overcharge conditions.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, specifically, it relates to a lithium-ion battery and its preparation method. Background Technology

[0002] With the increasing demands for energy density in lithium-ion batteries from pure electric vehicles, hybrid vehicles, and portable energy storage devices, people are looking forward to developing lithium-ion batteries with higher energy and power densities to achieve longer battery life and energy storage for electric devices.

[0003] However, overcharging lithium-ion batteries not only generates a large amount of heat and gas, leading to increased internal pressure, battery deformation, and leakage, but also easily triggers side reactions, degrading battery performance. On one hand, under overcharging conditions, excess lithium elements can easily embed into the negative electrode, causing lithium dendrites to grow on the surface, which can easily puncture the separator and trigger thermal runaway. On the other hand, under overcharging conditions, excess lithium elements can escape from the positive electrode, causing the positive electrode structure to collapse, releasing a large amount of heat and oxygen. This oxygen further accelerates the decomposition of the electrolyte, leading to an abnormal increase in internal pressure. This loss of internal pressure is a precursor to thermal runaway. Thermal runaway is a chain reaction; once triggered, heat accumulates rapidly inside the battery, the temperature rises sharply, and irreversible changes in the structure of the electrode active materials and the decomposition of the electrolyte occur, generating even more heat and creating a vicious cycle. Ultimately, this leads to safety hazards such as fire and explosion.

[0004] The main overcharge protection measures for lithium-ion batteries are as follows:

[0005] (1) By installing current interruption devices inside or outside the battery, such as installing a current interruption device (CID), an explosion-proof safety valve (Vent), and a PTC polymer switch inside the battery's safety cap. However, before the overcharge protection device cuts off the battery current, the internal temperature of the battery is already high. At this time, the battery separator usually shrinks, melts, or collapses, causing a short circuit between the positive and negative electrodes, which in turn leads to a sharp rise in battery temperature and triggers battery thermal runaway.

[0006] (2) By introducing anti-overcharge additives into the positive electrode and / or electrolyte, the anti-overcharge additives can preferentially decompose on the surface of the positive electrode to form a CEI film during battery cycling, which can protect the integrity of the electrode materials to a certain extent and reduce the side reactions caused by the contact between the high-voltage positive electrode and the electrolyte. However, anti-overcharge additives are not conducive to battery cycling and affect the battery life.

[0007] (3) By using thermal shutdown separators or ceramic-coated separators inside the battery, the heat resistance of the battery can be improved, but there are certain limitations and the effect on improving thermal runaway caused inside the battery is not obvious.

[0008] (4) The purpose of improving overcharge safety is achieved by using a positive temperature coefficient (PTC) coating. However, the PTC coating has a slow response and limited overcharge protection effect. Summary of the Invention

[0009] The purpose of this invention is to provide a lithium-ion battery and its preparation method. This lithium-ion battery improves the structural stability and cycle performance of the lithium-ion battery by using a positive electrode with a functional coating and a separator including a surface coating, as well as the flame retardant performance of the lithium-ion battery under overcharge conditions, thereby improving the overcharge resistance and safety of the lithium-ion battery.

[0010] According to one aspect of the present invention, a lithium-ion battery is provided, comprising a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode; the positive electrode comprises a positive electrode active coating, a functional coating, and a current collector, the positive electrode active coating being connected to the current collector through the functional coating, the functional coating comprising vinyl chloride-acrylate resin, aqueous acrylic resin, and metal oxide, the metal oxide comprising at least one of aluminum oxide, zirconium oxide, zinc oxide, magnesium oxide, and lithium oxide; the separator comprises a base film and a surface coating, the surface coating comprising a sodium fast ion conductor.

[0011] By applying functional coatings to both sides of the current collector on the positive electrode and a surface coating to the separator, the battery's overcharge resistance and flame retardancy can be improved while maintaining its cycle performance. On one hand, during normal battery operation, the introduction of vinyl chloride-acrylate resin and water-based acrylic resin as binders into the functional coating on the positive electrode not only enhances the internal structural stability of the functional coating, reducing issues like powdering and cracking, but also facilitates strong adhesion between the functional coating and the current collector and the positive electrode active coating, thereby improving the structural stability of the positive electrode and reducing the impedance of the lithium-ion battery. The metal oxides in the functional coating improve its conductivity and electrolyte permeability, further mitigating the impedance increase introduced into the positive electrode structure. Simultaneously, during battery cycling, the surface coating in the separator containing sodium fast ion conductors improves the ionic conductivity of the separator, effectively enhancing the battery's rate performance and cycle performance.

[0012] On the other hand, when the battery is overcharged, the functional coating on the positive electrode and the surface coating on the separator can improve the battery's flame retardant performance and reduce the probability of thermal runaway due to overcharging. By using binders and metal oxides composed of vinyl chloride-acrylate resin and water-based acrylic resin, the positive electrode can be endowed with self-extinguishing properties, which also helps improve its thermal stability. The functional coating absorbs the heat accumulated during overcharging, forming the first layer of overcharge protection and preventing thermal runaway. Simultaneously, the surface coating on the separator forms the second layer of overcharge protection. The sodium fast ion conductors in the separator's surface coating react with the electrolyte during cell formation to form a protective film. This protective film can block crosstalk of flammable gases such as oxygen between the positive and negative electrodes, preventing battery thermal runaway.

[0013] In summary, by introducing the aforementioned functional coatings into the positive electrode and surface coatings into the separator, the lithium-ion battery maintains good structural and cycle stability while possessing excellent overcharge resistance and flame retardant properties, thereby improving the safety performance of the battery device. This is reflected in the battery device's ability to withstand voltages exceeding its normal charging limit during charging, and its ability to tolerate overcharging to a certain extent, reducing safety risks caused by improper charging.

[0014] Preferably, the chlorine content in the functional coating is 2.2–13.6 wt%. By adjusting the content of vinyl chloride-acrylate resin and the content of vinyl chloride monomer in the vinyl chloride-acrylate resin in the functional coating, the chlorine content can be adjusted to the above range, thereby improving the flame retardant and self-extinguishing properties of the functional coating. When the battery is overcharged, a large number of lithium ions are released from the positive electrode, causing the positive electrode structure to collapse and releasing a large amount of heat and gas. The chlorine in the functional coating can be released into the electrolyte, reducing the flammability of the electrolyte and facilitating the formation of a stable interface film at the positive electrode, thus inhibiting the interface side reactions caused by battery overcharging. When the battery experiences thermal runaway due to overcharging, the internal temperature of the battery rises significantly. At this time, the chlorine in the functional coating can capture the free radicals required for the combustion process, interrupting the chain propagation of the combustion reaction and achieving a flame retardant effect.

[0015] Preferably, the mass ratio of vinyl chloride-acrylate resin to waterborne acrylic resin is 5-30:5-30.

[0016] Preferably, the proportion of vinyl chloride-acrylate resin in the raw materials used to prepare the functional coating is 5 to 30 wt%.

[0017] Preferably, the proportion of waterborne acrylate resin in the raw materials used to prepare the functional coating is 5 to 30 wt%.

[0018] Preferably, the proportion of metal oxide in the raw materials used to prepare the functional coating is 2 to 40 wt%.

[0019] Preferably, in the raw materials for preparing vinyl chloride-acrylate resin, the content of vinyl chloride is 70-90 wt%.

[0020] Preferably, in the functional coating, the metal oxide includes metal oxide A and metal oxide B. Metal oxide A includes at least one of aluminum oxide, zirconium oxide, zinc oxide, and magnesium oxide, while metal oxide B includes lithium oxide. The combination of metal oxide A and metal oxide B is beneficial for improving both the structural and cycle stability of the lithium-ion battery, as well as its safety performance. Metal oxide A can broaden the upper limit of heat absorption by the functional coating, while metal oxide B possesses good thermal and electrochemical stability. Simultaneous use of metal oxide A and metal oxide B helps to balance the thermal stability and conductivity of the cathode, thereby enabling the lithium-ion battery to exhibit excellent cycle capacity retention and overcharge resistance.

[0021] Preferably, the aluminum oxide includes at least one of boehmite (γ-Al2O3·H2O) and aluminum oxide.

[0022] Preferably, the zirconium oxide includes zirconium oxide.

[0023] Preferably, the zinc oxide includes zinc oxide.

[0024] Preferably, the magnesium oxide includes magnesium oxide.

[0025] Preferably, the lithium oxide includes at least one of lithium iron phosphate (LiFePO4), lithium manganese iron phosphate (LiMnFePO4), and lithium manganese oxide (LiMn2O4).

[0026] Preferably, in the functional coating, the mass ratio of metal oxide A to metal oxide B is 1-20:1-20.

[0027] Preferably, the proportion of metal oxide A in the raw materials used to prepare the functional coating is 1 to 20 wt%.

[0028] Preferably, the proportion of metal oxide B in the raw materials used to prepare the functional coating is 1 to 20 wt%.

[0029] Preferably, in the functional coating, the mass ratio of metal oxide A to metal oxide B is 3-10:2-9.

[0030] Preferably, in the functional coating, the mass ratio of metal oxide A to metal oxide B is 6.5:5.5.

[0031] Preferably, the thickness of the functional coating in the positive electrode is 1–5 μm. Adjusting the thickness of the functional coating to the above range can reduce the impedance of the lithium-ion battery, thereby extending the cycle life of the battery device, and can also improve the overcharge resistance and flame retardant characteristics of the lithium-ion battery.

[0032] Preferably, the functional coating further includes a conductive agent, which includes at least one of acetylene black, Ketjen black, carbon nanotubes, carbon fibers, and graphene.

[0033] Preferably, the proportion of conductive agent in the raw materials used to prepare the functional coating is 0.1 to 3 wt%.

[0034] Preferably, the functional coating further includes a dispersant, which includes at least one of sodium carboxymethyl cellulose (CMC), sodium sulfate, sodium sulfonate, and acrylate-type polymeric dispersants.

[0035] Preferably, the proportion of dispersant in the raw materials used to prepare the functional coating is 0.1 to 3 wt%.

[0036] Preferably, the positive electrode active coating includes polyvinylidene fluoride (PVDF).

[0037] Preferably, the positive electrode active coating includes lithium cobalt oxide.

[0038] Preferably, the surface coating in the separator further includes boron compounds and / or phosphorus compounds. On one hand, during normal battery operation, the inorganic materials such as sodium fast ion conductors, boron compounds, and phosphorus compounds in the surface coating provide support for the base film, effectively mitigating thermal shrinkage and improving the structural stability of the base film during battery cycling. This effectively prevents short circuits between the positive and negative electrodes caused by separator shrinkage during normal operation of the lithium-ion battery. On the other hand, when the lithium-ion battery is overcharged, the boron compounds and / or phosphorus compounds in the surface coating undergo thermal decomposition under the overcharge voltage, releasing water of crystallization. This provides heat absorption, cooling, and oxygen dilution, thereby reducing the probability of thermal runaway due to overcharging.

[0039] Preferably, the surface coating of the diaphragm also includes zinc borate.

[0040] Preferably, the total mass percentage of boron compounds and phosphorus compounds in the raw materials used to prepare the surface coating is 1–10 wt%. By controlling the amount of boron and phosphorus compounds added to the surface coating, it is possible to prevent thermal runaway caused by overcharging and reduce the impact of the thermal decomposition products of boron and phosphorus compounds on the battery cycle performance.

[0041] Preferably, the surface coating also includes polyvinylidene fluoride (PVDF).

[0042] Preferably, the proportion of polyvinylidene fluoride (PVDF) in the raw materials used to prepare the surface coating is 3 to 20 wt%.

[0043] Preferably, the sodium fast ion conductor (NASCION type) includes at least one of lithium aluminum titanium phosphate (LATP) and lithium aluminum germanium phosphate (LAGP).

[0044] Preferably, the proportion of sodium fast ion conductor (NASCION type) in the raw materials used to prepare the surface coating is 3 to 20 wt%.

[0045] Preferably, the thickness of the surface coating is 0.5–3 μm. Adjusting the thickness of the surface coating to this range can, on the one hand, shorten the ion migration channel, reduce polarization, decrease the impedance of the lithium-ion battery, thereby extending the cycle life of the battery device and increasing the energy density of the lithium-ion battery; on the other hand, it can improve the mechanical strength and insulation effect of the separator, enabling the separator to withstand various stresses during battery assembly and use, preventing short circuits between the positive and negative electrodes, and also enhancing the thermal stability of the separator, improving the overcharge resistance and flame retardant characteristics of the lithium-ion battery.

[0046] Preferably, in the diaphragm, the base membrane is coated with a surface coating layer on each side.

[0047] Preferably, the lithium-ion battery further includes an electrolyte, wherein the lithium salt of the electrolyte includes lithium hexafluorophosphate (LiFP6).

[0048] According to another aspect of the present invention, a method for preparing the above-mentioned lithium-ion battery is provided, comprising the following operations: a. preparing a positive electrode: applying a functional coating slurry for preparing a functional coating to both sides of a current collector, curing it to obtain a composite layer containing a functional coating and a current collector, wherein the functional coating slurry includes vinyl chloride-acrylate resin, aqueous acrylic resin, metal oxide and solvent; then applying a positive electrode active slurry for preparing a positive electrode active coating to both sides of the composite layer, curing it to obtain a positive electrode; b. preparing a separator: applying a surface coating slurry for preparing a surface coating to both sides of a base film, curing it to obtain a separator.

[0049] Preferably, the solid content of the functional coating slurry is 10-15%.

[0050] Preferably, the method for preparing a lithium-ion battery further includes an assembly operation: stacking the positive electrode, separator, negative electrode, and separator in sequence, and then performing a winding or stacking operation to obtain a cell; the cell undergoes post-processing to obtain a lithium-ion battery.

[0051] Preferably, the curing process includes baking.

[0052] Preferably, the post-processing includes at least one of the following: encapsulation, baking, liquid injection, formation, secondary sealing, sorting, and testing the open-circuit voltage of the battery.

[0053] Optionally, during the stacking process, the surface coating of the diaphragm is oriented toward the positive electrode.

[0054] Optionally, during the stacking process, the surface coating of the diaphragm is oriented toward the negative electrode.

[0055] Preferably, during the stacking process, the positive electrode active coating of the positive electrode is oriented toward the separator. Attached Figure Description

[0056] Figure 1 This is a schematic diagram of the structure of the positive electrode in a lithium-ion battery provided by the present invention;

[0057] Figure 2 This is a schematic diagram of the structure of the separator in the lithium-ion battery provided by the present invention;

[0058] The correspondence of the reference numerals in the above figures is as follows: 1. First positive electrode active coating, 2. First functional coating, 3. Current collector, 4. Second functional coating, 5. Second positive electrode active coating, 6. First surface coating, 7. Base film, 8. Second surface coating. Detailed Implementation

[0059] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of this invention will be clearly and completely described below with reference to the embodiments and accompanying drawings. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0060] Example 1

[0061] This embodiment provides a lithium-ion battery, including a positive electrode, a negative electrode, and a separator placed between the positive electrode and the negative electrode.

[0062] (1) Positive electrode

[0063] The structure of the positive electrode is as follows Figure 1 As shown, the electrode includes a first positive electrode active coating 1, a first functional coating 2, a current collector 3, a second functional coating 4, and a second positive electrode active coating 5, arranged sequentially. Both the first functional coating 2 and the second functional coating 4 are functional coatings. The thickness of both the first functional coating 2 and the second functional coating 4 is 3 μm. The current collector 3 is aluminum foil.

[0064] The raw material composition used to prepare the functional coating for the positive electrode is shown in the table below:

[0065] Table 1. Raw material composition for preparing functional coatings for cathodes

[0066]

[0067] In the raw materials for preparing the aforementioned vinyl chloride-acrylate resin, the vinyl chloride monomer content is 80%. Since the solvent in the functional coating slurry evaporates during the curing process, the chlorine content in the functional coating is calculated to be 6.8 wt%.

[0068] The method for preparing the positive electrode includes the following steps:

[0069] S1. Prepare the raw materials according to the composition shown in Table 1, mix the components, and prepare a functional coating slurry with a solid content in the range of 10-15%. After grinding the functional coating slurry with a sand mill, apply the functional coating slurry to both sides of the current collector 3 with a gravure coating machine, and cure it to obtain a composite layer composed of the functional coating and the current collector 3.

[0070] S2. Preparation of the positive electrode:

[0071] A positive electrode active slurry for preparing a positive electrode active coating is prepared. The positive electrode active slurry includes polyvinylidene fluoride (PVDF), lithium cobalt oxide, and conductive agents, including conductive carbon black (SP) and carbon nanotubes (CNT). The positive electrode active slurry is coated on both sides of the composite layer, and after baking and rolling, a positive electrode is obtained.

[0072] In the positive electrode active coating, the content of lithium cobalt oxide is 97.6 wt%, the content of polyvinylidene fluoride (PVDF) is 1 wt%, the content of conductive agent is 1.4 wt%, and the mass ratio of conductive carbon black (SP) to carbon nanotubes (CNT) is 1:1.

[0073] (2) Negative electrode

[0074] The method for preparing the negative electrode includes the following operations:

[0075] A negative electrode active slurry for preparing the negative electrode active coating is prepared. The negative electrode active slurry includes graphite, conductive carbon black (SP), styrene-butadiene rubber (SBR), and dispersant sodium carboxymethyl cellulose (CMC). The negative electrode active slurry is coated on the surface of the negative electrode current collector (carbon-coated copper foil), and after baking and rolling, the negative electrode is obtained.

[0076] In the negative electrode active coating, the content of graphite is 96 wt%, the content of conductive carbon black (SP) is 1.3 wt%, the content of styrene-butadiene rubber (SBR) is 1.5 wt%, and the content of dispersant sodium carboxymethyl cellulose (CMC) is 1.2 wt%.

[0077] (3) Diaphragm

[0078] The structure of the diaphragm is as follows Figure 2 As shown, it includes a first surface coating 6, a base film 7, and a second surface coating 8 arranged sequentially. Both the first surface coating 6 and the second surface coating 8 are surface coatings. The base film 7 is a commercially available heat-resistant ceramic diaphragm. The thickness of the first surface coating 6 is 1 μm, and the thickness of the second surface coating 8 is 1 μm.

[0079] The raw material composition used to prepare the surface coating of the diaphragm is shown in the table below:

[0080] Table 2. Raw material composition for preparing the surface coating of the diaphragm

[0081]

[0082] The method for preparing the diaphragm includes the following steps:

[0083] Prepare the raw materials according to the composition shown in Table 2, mix the components and prepare the surface coating slurry. After grinding the surface coating slurry with a sand mill, apply the surface coating slurry to both sides of the base film 7 with a gravure coating machine and cure to obtain the diaphragm.

[0084] (4) Lithium-ion batteries

[0085] Assemble the above-prepared positive electrode, negative electrode, and separator according to the following steps:

[0086] After the positive and negative electrodes are slit and sheeted, the positive electrode, separator, negative electrode, and separator are stacked sequentially and wound to obtain a battery cell. The battery cell undergoes packaging, baking, electrolyte injection, formation, secondary sealing, sorting, and open-circuit voltage testing to obtain a lithium-ion battery. The electrolyte used is a commercially available conventional electrolyte containing lithium hexafluorophosphate (LiFP6).

[0087] In other embodiments, the composition and thickness of the positive electrode active coating and the negative electrode active coating can be adjusted according to actual application requirements, or the type and thickness of the current collector of the positive electrode, the current collector of the negative electrode, the base film 7 of the separator can be adjusted, or the type of electrolyte can be adjusted, provided that it does not affect the normal use of the battery device.

[0088] Test Example 1

[0089] (1) 1C6V test:

[0090] The lithium-ion battery was charged at a constant current and constant voltage of 0.2C until the battery voltage reached 4.45V. Charging was stopped when the charging current decreased to 0.02C, and the battery was allowed to rest for 10 minutes. Then, the lithium-ion battery was charged at a 1C current until the battery voltage reached 6V, and this charging was continued for 3 hours before stopping. The battery condition was evaluated; if the battery did not catch fire or explode during the test, it was considered to have passed the 1C6V test. To ensure data accuracy, ten of each type of lithium-ion battery were prepared and tested independently, and the 1C6V overcharge pass rate was recorded.

[0091] 1C6V overcharge pass rate = number of batteries that pass the 1C6V test / total number of test samples × 100%.

[0092] (2) Capacity retention rate: The lithium-ion battery was charged at a constant current and constant voltage of 0.2C until the battery voltage reached 4.45V. When the charging current decreased to 0.02C, the charging was stopped and the battery was left to stand for 10 minutes. Then it was discharged at 0.2C to 3.0V. The capacity discharged was recorded as the initial capacity. The lithium-ion battery was then charged at 0.7C at a temperature of 25±3℃. When the charging current decreased to 0.02C, the charging was stopped and the battery was discharged at 0.5C to 3.0V. This was used as a cycle. The discharge capacity of the test subject in the 400th cycle was recorded, and the capacity retention rate was recorded.

[0093] Capacity retention rate = Discharge capacity at 400th cycle / Initial capacity × 100%.

[0094] Example 2

[0095] To further investigate the influence of component content in the functional coating of the positive electrode, corresponding positive electrodes, separators, negative electrodes, and lithium-ion batteries were prepared according to the specific materials and amounts listed in Table 3 and following the methods and steps described in Example 1. The remaining raw material ratios and preparation methods were strictly consistent with those in Example 1. Then, tests were conducted according to the methods specified in Test Example 1. The final test results were averaged and are shown in Table 4. For ease of comparison, relevant information about the lithium-ion battery of Example 1 is also included in the table below.

[0096] Table 3. Composition of Functional Coatings

[0097]

[0098] Table 4. Performance test results of lithium-ion batteries

[0099] Group 1C6V overcharge pass rate (%) Capacity retention rate after 400 laps (%) Example 1 100％ 96.8％ Experimental group 2-1 80％ 96.5％ Experimental group 2-2 80％ 96.1％ Experimental group 2-3 100％ 96.5％ Experimental groups 2-4 100％ 96.0％ Experimental groups 2-5 100％ 96.4％ Experimental groups 2-6 100％ 96.1％ Control group 2-1 50％ 96.2％ Control group 2-1 40％ 96.4％

[0100] Results analysis:

[0101] Comparing the performance indicators of the experimental group and the control group, it can be found that the lithium-ion batteries provided by Example 1 and Experimental Groups 2-1 to 2-6 all have a 1C6V overcharge pass rate of more than 50%. Compared with the lithium-ion batteries provided by Control Groups 2-1 to 2-2, the lithium-ion batteries provided by Example 1 and Experimental Groups 2-1 to 2-6 have better overcharge resistance and flame retardant performance.

[0102] Comparing the lithium-ion batteries provided in Example 1 with those in Experimental Groups 2-1 to 2-2, it can be found that, compared to the lithium-ion batteries in Experimental Group 2-1 which only use metal oxide A for the functional coating and the lithium-ion batteries in Experimental Group 2-2 which only use metal oxide B for the functional coating, the lithium-ion battery provided in Example 1 uses both metal oxide A and metal oxide B for its functional coating. Furthermore, the lithium-ion battery provided in Example 1 has a 1C6V overcharge pass rate greater than 80% and a capacity retention rate greater than 96.5% after 400 cycles. This indicates that the combination of metal oxide A and metal oxide B is beneficial for improving both the structural stability and cycle stability of lithium-ion batteries, as well as their safety performance.

[0103] Example 3

[0104] To further investigate the influence of component content in the surface coating of the separator, corresponding positive electrode, separator, negative electrode, and lithium-ion battery were prepared according to the specific materials and amounts listed in Table 5 and following the methods and steps described in Example 1. The remaining raw material ratios and preparation methods were strictly consistent with those in Example 1. Then, tests were conducted according to the methods specified in Test Example 1. The final test results were averaged and are shown in Table 6. For ease of comparison, relevant information about the lithium-ion battery of Example 1 is also included in the table below.

[0105] Table 5. Composition of the surface coating

[0106]

[0107] Table 6. Performance test results of lithium-ion batteries

[0108]

[0109]

[0110] Results analysis:

[0111] Comparing the performance indicators of the experimental group and the control group, it can be found that the lithium-ion batteries provided by Example 1 and Experimental Groups 3-1 to 3-3 all have a 1C6V overcharge pass rate of more than 50%. Compared with the lithium-ion batteries provided by Control Groups 3-1 to 3-3, whose separator surface coating does not contain sodium fast ion conductors, the lithium-ion batteries provided by Example 1 and Experimental Groups 3-1 to 3-3 have better overcharge resistance and flame retardant performance.

[0112] Comparing the lithium-ion batteries provided in Example 1 with those in Experimental Group 3-1, it can be found that when the surface coating simultaneously employs sodium fast ion conductors, boron and / or phosphorus compounds, the overcharge resistance and cycle stability of the lithium-ion battery can be improved simultaneously.

[0113] Furthermore, comparing the lithium-ion batteries provided in Example 1, Experimental Groups 3-2 to 3-3 with Comparative Groups 3-1 to 3-3, it can be found that when the surface coating does not contain sodium fast ion conductors, the lithium-ion batteries in Comparative Groups 3-2 and 3-3 with surface coatings containing ammonium borate or ammonium polyphosphate have higher overcharge resistance than the lithium-ion battery in Comparative Group 3-1 with surface coatings containing zinc borate. However, the cycle stability of the battery in Comparative Group 3-1 is slightly better than that of the batteries in Comparative Groups 3-2 and 3-3. But when the surface coating contains sodium fast ion conductors, the lithium-ion battery in Example 1 with surface coatings containing zinc borate has a higher 1C6V overcharge pass rate and a higher 400-cycle capacity retention rate than the lithium-ion batteries in Experimental Groups 3-2 to 3-3 with surface coatings containing ammonium borate or ammonium polyphosphate. That is, under the premise that the surface coating contains sodium fast ion conductors, the lithium-ion battery with surface coatings containing zinc borate has better overcharge resistance and cycle stability.

[0114] Example 4

[0115] To further investigate the influence of the structure of the positive electrode and separator, as well as the thickness of the functional coating and surface coating, corresponding positive electrodes, separators, negative electrodes, and lithium-ion batteries were prepared according to the specific materials and amounts listed in Table 7 and following the methods and steps described in Example 1. The remaining raw material ratios and preparation methods were strictly consistent with those in Example 1. Tests were then conducted according to the methods specified in Test Example 1. The final test results were averaged and are shown in Table 8. For ease of comparison, relevant information about the lithium-ion battery from Example 1 is also included in the table below.

[0116] Table 7. Structure and functional coatings of the positive electrode and separator, and the thickness of the surface coating.

[0117]

[0118] Note: In the column for "Positive Electrode Structure," "Contains 2 functional coatings" indicates that the positive electrode in this experimental / control group comprises, in sequence, a first positive electrode active coating 1, a first functional coating 2, a positive electrode current collector 3, a second functional coating 4, and a second positive electrode active coating 5. In the column for "Positive Electrode Structure," "Contains 1 functional coating" indicates that the positive electrode in this experimental / control group consists of, in sequence, a first positive electrode active coating 1, a first functional coating 2, a positive electrode current collector 3, and a second positive electrode active coating 5. In the column for "Positive Electrode Structure," "Does not contain a functional coating" indicates that the positive electrode in this experimental / control group consists of, in sequence, a first positive electrode active coating 1, a positive electrode current collector 3, and a second positive electrode active coating 5.

[0119] If the column for "Diaphragm Structure" states "Contains Surface Coating," it means that in this experimental / control group, the diaphragm includes a first surface coating 6, a base film 7, and a second surface coating 8 arranged sequentially. If the column for "Diaphragm Structure" states "Does Not Contain Surface Coating," it means that in this control group, the diaphragm consists of the base film 7.

[0120] Table 8. Performance test results of lithium-ion batteries

[0121] Group 1C6V overcharge pass rate (%) Capacity retention rate after 400 cycles (%) Experimental group 4-1 60％ 97.7％ Experimental group 4-2 80％ 97.5％ Example 1 100％ 96.8％ Experimental group 4-3 100％ 95.2％ Experimental group 4-4 80％ 97.1％ Experimental group 4-5 100％ 95.7％ Experimental group 4-6 100％ 95.1％ Experimental groups 4-7 60％ 96.9％ Control group 4-1 0％ 97.9％ Control group 4-2 50％ 97.0％ Control group 4-3 20％ 97.5％

[0122] Results analysis:

[0123] Comparing the performance indicators of the experimental and control groups, it can be found that the lithium-ion batteries provided by Example 1 and Experimental Groups 4-1 to 4-7 all have a 1C6V overcharge pass rate of more than 50%. Compared with the lithium-ion batteries provided by Control Groups 4-1 to 4-3, which do not contain sodium fast ion conductors in the surface coating of the separator, the lithium-ion batteries provided by Example 1 and Experimental Groups 4-1 to 4-7 have better overcharge resistance and flame retardant performance.

[0124] Comparing the lithium-ion batteries provided in Example 1, Experimental Groups 4-7, and Control Group 4-3, it can be observed that as the number of functional coating layers in the positive electrode structure increases, the 1C6V overcharge pass rate of the lithium-ion battery increases, while the capacity retention rate after 400 cycles decreases. In Example 1, the positive electrode structure contains two functional coating layers, achieving a 1C6V overcharge pass rate of 100% and a capacity retention rate of 96.8% after 400 cycles.

[0125] Comparing the lithium-ion batteries provided in Example 1 and Comparative Groups 4-2 to 4-3, it can be found that the 1C6V overcharge pass rate measured in Example 1 is greater than the sum of the 1C6V overcharge pass rates of Comparative Groups 4-2 and 4-3. In other words, by setting a functional coating on the positive electrode of the lithium-ion battery and setting a surface coating on the separator, the flame retardant performance of the battery can be improved and the probability of thermal runaway due to overcharging can be reduced.

[0126] Comparing Example 1 with the lithium-ion batteries provided in Experimental Groups 4-1 to 4-3, it can be found that as the single-sided thickness of the functional coating in the positive electrode structure increases, the 1C6V overcharge pass rate of the lithium-ion battery increases, while the 400-cycle capacity retention rate decreases. When the single-sided thickness of the functional coating reaches 3 μm, the 1C6V overcharge pass rate reaches 100%. Comparing Example 1 with the lithium-ion batteries provided in Experimental Groups 4-4 to 4-6, it can be found that as the single-sided thickness of the surface coating in the separator structure increases, the 1C6V overcharge pass rate of the lithium-ion battery increases, while the 400-cycle capacity retention rate decreases. When the single-sided thickness of the surface coating reaches 1 μm, the 1C6V overcharge pass rate reaches 100%.

[0127] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A lithium-ion battery, characterized in that, It includes a positive electrode, a negative electrode, and a membrane placed between the positive electrode and the negative electrode; The positive electrode includes a positive electrode active coating, a functional coating, and a current collector. The positive electrode active coating is connected to the current collector through the functional coating. The functional coating includes vinyl chloride-acrylate resin, waterborne acrylic resin, and metal oxide. The metal oxide includes at least one of aluminum oxide, zirconium oxide, zinc oxide, magnesium oxide, and lithium oxide. The diaphragm includes a base membrane and a surface coating, wherein the surface coating includes a sodium fast ion conductor.

2. The lithium-ion battery as described in claim 1, characterized in that, The chlorine content in the functional coating is 2.2~13.6 wt%.

3. The lithium-ion battery as described in claim 1, characterized in that, In the functional coating, the metal oxide includes metal oxide A and metal oxide B, wherein metal oxide A includes at least one of aluminum oxide, zirconium oxide, zinc oxide, and magnesium oxide, and metal oxide B includes lithium oxide.

4. The lithium-ion battery as described in claim 3, characterized in that, In the functional coating, the mass ratio of metal oxide A to metal oxide B is 1~20:1~20.

5. The lithium-ion battery according to any one of claims 1 to 4, characterized in that, In the positive electrode, the thickness of the functional coating is 1~5μm.

6. The lithium-ion battery as described in claim 1, characterized in that, In the diaphragm, the surface coating further includes boron compounds and / or phosphorus compounds.

7. The lithium-ion battery as described in claim 6, characterized in that, The boron compound in the membrane also includes zinc borate.

8. The lithium-ion battery as described in claim 6, characterized in that, In the raw materials used to prepare the surface coating, the sum of the mass of the boron compound and the phosphorus compound accounts for 1 to 10 wt%.

9. The lithium-ion battery according to any one of claims 1-4 and 6-8, characterized in that, The thickness of the surface coating is 0.5~3μm.

10. The lithium-ion battery as described in claim 1, characterized in that, The positive electrode active coating of the positive electrode faces the separator.

11. A method for preparing a lithium-ion battery as described in any one of claims 1 to 10, characterized in that, Includes the following operations: a. Preparation of the positive electrode: A functional coating slurry for preparing the functional coating is coated on both sides of the current collector and cured to obtain a composite layer containing the functional coating and the current collector. The functional coating slurry includes vinyl chloride-acrylate resin, waterborne acrylic resin, metal oxide and solvent. Then, a positive electrode active slurry for preparing the positive electrode active coating is coated on both sides of the composite layer and cured to obtain the positive electrode. b. Preparation of the diaphragm: The surface coating slurry used to prepare the surface coating is applied to at least one side of the base membrane and cured to obtain the diaphragm.

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