Negative electrode, lithium battery with long service life and safety and preparation method of lithium battery

Abstract

The invention provides a negative electrode, a lithium battery with long service life and safety and a preparation method of the lithium battery, and relates to the technical field of lithium batteries. The invention aims to provide a lithium battery which comprises a negative electrode containing a binder II and inorganic particles, an electrolyte containing a binder III, a second substance layer formed by the binder II and the inorganic particles, and a positive electrode layer formed by the second substance layer and located on one side of an electrode substance layer far away from a current collector, the binder III is located in the electrode material layer and in pores between the binder II and the inorganic particles, and forms gel together with the electrolyte; through the synergistic effect of the binder II, the inorganic particles and the binder III, the contact between the negative electrode material layer and the positive electrode material layer and the volume change of the negative electrode active material layer are inhibited at the same time, so that the thermal safety of the battery cell in a fresh state is improved, and the volume change of the negative electrode in the use process is inhibited; and meanwhile, the impedance of the battery cell is reduced, and the coulombic efficiency, the thermal safety of the battery cell and the cycling stability are improved.

Description

Technical Field

[0001] This invention relates to the field of lithium battery technology, and in particular to a lithium battery with a negative electrode, a long lifespan, and a method for preparing the same. Background Technology

[0002] Increasing the energy density of lithium-ion battery cells can effectively reduce the overall size of the battery cells in a vehicle while maintaining the same driving range, thereby reducing physical safety risks in the event of a car accident. However, if high-energy-density cells are paired with electrode materials with poorer thermodynamic stability, the thermal safety risks increase.

[0003] Preparing a solid electrolyte coating with a high-temperature resistant oxide ceramic as the main phase on the electrode surface is an effective safety strategy to suppress positive and negative electrode contact above the membrane breakage temperature, thereby preventing thermal runaway. From the perspectives of both processability and safety, the negative electrode of a lithium battery is generally considered a more suitable coating carrier than the positive electrode, for the following reasons:

[0004] Processability: The energy density of traditional negative electrode active materials is higher than that of positive electrode active materials. Therefore, when the N / P ratio is close to 1, the thickness of the negative electrode sheet is significantly lower than that of the positive electrode. After constructing an additional solid electrolyte coating, it is less likely to cause common problems such as tab breakage, material loss, or even strip breakage during the rolling process due to excessively high areal density. In addition, the stability of traditional aqueous negative electrode slurry during the coating process is higher than that of oil-based positive electrode slurry. Therefore, the thickness uniformity of the electrode sheet is higher than that of the positive electrode, making it more suitable as a coating carrier.

[0005] Safety: Because the negative electrode area is larger than the positive electrode, a spatial relationship is usually formed in the battery cell where the negative electrode surrounds the positive electrode. If the coating is located on the surface of the positive electrode, above the film breaking temperature, even a small amount of missed coating or burrs at the edge of the positive electrode could potentially lead to an internal short circuit. Coating the negative electrode surface avoids this problem. Furthermore, because water-based binders have higher heat resistance than oil-based binders, they can better maintain the integrity of the coating at the film breaking temperature.

[0006] CN105529433A discloses a coated electrode and a lithium-ion battery including the coated electrode. The coating comprises "a first material layer adjacent to the electrode and a second material layer adjacent to the first material layer and away from the electrode. The first material layer comprises first inorganic particles and a first binder, and the second material layer comprises a second binder." The first inorganic particles, when generating significant heat in the battery, can better decompose and absorb the heat generated, thus enabling faster heat dissipation. The second binder "prevents the first material layer from deforming or cracking due to endothermic reactions of the inorganic particles, making the first material layer less prone to peeling off, thereby reducing the occurrence of short circuits between the negative and positive electrodes at high temperatures, improving battery safety, and making the battery less prone to combustion and explosion."

[0007] CN115312974A proposes a negative electrode sheet with an isolation layer, stating that "the cross-linked electronic isolation layer has a rupture temperature of over 180°C and an increased puncture strength of 10%." This electronic isolation layer is composed of polymers, and "batteries using negative electrode sheets with isolation layers can significantly improve the thermal stability and mechanical properties of the battery, delaying the battery failure temperature from 150°C to 180°C."

[0008] CN114927751A proposes a solid electrolyte composite membrane that can be used as an electrode protective layer for lithium metal batteries. The membrane contains ceramic powder with In element and polyacrylonitrile binder, which can "suppress the growth of lithium dendrites during battery charging and discharging and improve the safety performance of the battery".

[0009] However, the drawback of building a coating on the negative electrode side is that the volume change of the negative electrode is more severe than that of the positive electrode during charging and discharging. During repeated charging and discharging of the battery cell, the coating will be damaged due to the repeated expansion and contraction of the negative electrode, exposing the electrode material with electronic conductivity, thereby destroying the design effect of the negative electrode protective layer. Summary of the Invention

[0010] In order to simultaneously reduce cell impedance and improve coulombic efficiency, cell thermal safety and cycle stability, this application provides a negative electrode, a lithium battery with long life safety and a method for preparing the same.

[0011] Technical solution:

[0012] Firstly, this application provides a negative electrode, including...

[0013] current collector;

[0014] Electrode material layers located on one or both sides of the current collector;

[0015] A second material layer located on the side of the electrode material layer furthest from the current collector;

[0016] and adhesive III located within the electrode material layer, at the interface between the electrode material layer and the second material layer, and inside the second material layer;

[0017] The electrode material layer comprises a composition of binder I, conductive agent, silicon-carbon anode and graphite, wherein the binder I accounts for 1-8 wt% of the electrode material layer by mass.

[0018] The adhesive I includes any one or a combination of several of CMC, PAA and PMMA;

[0019] The second material layer comprises a composition of binder II, PAA, SBR, and inorganic particles; the mass ratio of binder II to inorganic particles is (0.5-1):(0.9-20);

[0020] The adhesive II is any one or a combination of several of CMC, SBR, PAA-Na, PAA-Li, PVDF, PMMA, and PVDF-HFP.

[0021] Furthermore, the electrode material layer comprises a composition of binder I, conductive agent, silicon-carbon anode and graphite in a mass ratio of 1-3:1-3:18-20:70-85.

[0022] Furthermore, the inorganic particles are any one or a combination of several of the following: alumina, boehmite, titanium dioxide, silicon dioxide, zirconium oxide, LATP, LLZO, and LLTO.

[0023] Preferably, the inorganic particles are LLZO.

[0024] Furthermore, the particle size of the inorganic particles is between 200 and 1000 nm.

[0025] Preferably, the inorganic particles have a particle size of 700-1000 nm.

[0026] Furthermore, the second material layer comprises a composition of binder II, PAA, SBR, and inorganic particles in a mass ratio of 1:1:1:9-19.

[0027] Furthermore, the thickness of the second material layer is 3-6 μm.

[0028] Secondly, this application provides a lithium battery with long life and safety, including a positive electrode, a negative electrode as described in this application, a separator, and an electrolyte filled between the positive electrode, the negative electrode, and the separator.

[0029] Thirdly, this application provides a method for preparing a lithium battery with long lifespan and safety.

[0030] The preparation steps include the following:

[0031] S1. Mix the raw materials of the electrode material layer to prepare a slurry, coat it onto the surface of the current collector, and evaporate the solvent;

[0032] S2. Prepare the second material layer raw material into a slurry and coat it on the surface of the electrode material layer away from the current collector, then evaporate the solvent;

[0033] S3. The negative electrode sheet containing the current collector, electrode material layer and second material layer is co-rolled to obtain the negative electrode sheet; the positive electrode sheet, negative electrode sheet and separator are stacked and packaged to prepare a dry cell.

[0034] S4. Disperse the monomers of binder III and the initiator in the electrolyte, inject them into the dry cell, and heat to induce cross-linking of binder III monomers to form binder III, thereby preparing a lithium battery.

[0035] Furthermore, the liquid precursor of the adhesive III is any one or a combination of several of the following: ethylene glycol dimethacrylate, triethylene glycol dimethacrylate, (ethylene glycol) diacrylate, ethylene glycol methyl ether methacrylate, ethoxylated trimethylolpropane triacrylate, trimethylolpropane triacrylate, propoxylated trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, and dipentaerythritol hexaacrylate;

[0036] The mass ratio of binder III monomer to electrolyte is (5-10): (95-90); the mass ratio of binder III monomer to initiator is 100:0.5-1.5.

[0037] Preferably, binder III is a monomer containing 3-6 carbon-carbon double bond functional groups.

[0038] More preferably, binder III is any one or a combination of several of the following: trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, ethoxylated trimethylolpropane triacrylate, propoxylated trimethylolpropane triacrylate, and dipentaerythritol hexaacrylate containing three carbon-carbon double bond functional groups.

[0039] Using monomers containing 3-6 carbon-carbon double bond functional groups and having a higher degree of crosslinking can improve the mechanical strength of binder III after curing, thereby achieving more effective suppression of negative electrode expansion and reducing the lithium-ion transport path within the electrode, thus improving the capacity utilization of the cell. The high degree of crosslinking of the monomer binder III is combined with binder I and binder II and small-particle inorganic particles that effectively suppress negative electrode expansion. Binder II promotes the dispersion of inorganic particles and improves the flexibility of the final crosslinked network, forming a flexible coating layer on the surface of inorganic particles. Together with the high degree of crosslinking of binder III, it forms a high-strength, tough, and high lithium-ion transport network that suppresses negative electrode expansion.

[0040] Preferably, the inorganic particles are garnet-type LLZO, which has high ionic conductivity and stability to Li metal. It can be better compatible with the highly cross-linked monomer binder III, reducing the lithium ion transport path in the electrode and improving the stable lithium ion conduction speed. This makes the negative electrode material not only have low expansion rate and high stability, but also high coulombic efficiency.

[0041] Optionally, the initiator is AIBN.

[0042] Beneficial effects: 1. The purpose of this invention is to provide a lithium battery, including a negative electrode containing binder II and inorganic particles, and an electrolyte containing binder III. The binder II and inorganic particles form a second material layer located on the side away from the current collector electrode material layer. The binder III is located inside the electrode material layer, in the pores between the binder II and the inorganic particles, and forms a gel together with the electrolyte. Through the synergistic effect of binder II, inorganic particles, and binder III, the contact between the negative electrode material layer and the positive electrode material layer is suppressed, as well as the volume change of the negative electrode active material layer. This not only improves the thermal safety of the battery cell in its fresh state, but also suppresses the volume change of the negative electrode during use by increasing the internal bonding force of the negative electrode, thereby extending the lifespan of the thermal safety improvement effect. At the same time, it reduces the cell impedance and improves the coulombic efficiency, thermal safety, and cycle stability of the battery cell.

[0043] 3. Furthermore, the inorganic particles with a particle size of 700-1000 nm in the binder III with a high degree of cross-linking monomer and the second material layer can not only effectively suppress the expansion of the negative electrode, but also reduce the transport path of lithium ions in the electrode, improve the capacity of the cell, further reduce the cell impedance and improve the coulombic efficiency, while having high thermal safety and cycle stability of the cell.

[0044] 4. Furthermore, the preferred LLZO inorganic particles and their particle size in the second material layer can effectively improve the capacity utilization of the cell. The extremely high mechanical strength of LLZO can more effectively suppress negative electrode expansion and suppress internal side reactions of the cell, thereby achieving higher cycle stability. In addition, the combination of oil-based PMMA binder and hydrophilic LLZO can solve the compatibility problem between LLZO and oil-based binder PVDF or water-based slurry, further improving the overall performance of the cell.

[0045] 5. The lithium batteries prepared using the method of this application have FCC 9000-9200 mAh, FDC 7000-7700 mAh, ICE 81-85%, Retention 80-92%, and δ24-35%. At the same time, the cell pass rate at 180℃ after formation is 75-100%, and the cell pass rate at 180℃ after 100 cycles is 50-100%. Attached Figure Description

[0046] Figure 1 This is a schematic diagram of the negative electrode sheet obtained using the method of Embodiment 1 of the present invention. Detailed Implementation

[0047] To make the technical solution of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0048] Example 1: A method for preparing a lithium battery with long lifespan and safety, comprising the following preparation steps:

[0049] S1: Preparation of positive electrode sheet

[0050] The binder (PVDF) was prepared as a 7 wt% NMP adhesive solution, and then a conductive agent (using carbon nanotubes) and a positive electrode active material (LiNi) were added sequentially. 0.8 Co 0.1 Mn 0.1 O2) is prepared into a uniformly dispersed adhesive solution, with the mass ratio of binder, conductive agent, and positive electrode active material being 2:2:96. The adhesive solution is coated onto the surface of the current collector and the solvent is dried to form an electrode material layer. The positive electrode layer and the current collector are then rolled together to obtain the positive electrode sheet.

[0051] S2 Preparation of negative electrode sheet

[0052] Adhesive I uses adhesive 1 (CMC) and adhesive 2 (PAA).

[0053] Preparation of the electrode material layer: A 1.3 wt% aqueous adhesive solution of binder 1 (CMC) was prepared, followed by the sequential addition of conductive agent (carbon fiber), binder 2 (PAA), silicon-carbon anode material, graphite, and SBR emulsion to form a uniformly dispersed adhesive solution. The mass ratio of binder 1, binder 2, conductive agent (carbon nanotubes), silicon-carbon anode material, and graphite was 1:1:1:19.4:77.6. The adhesive solution was coated onto the surface of the current collector, and the solvent was dried to form the electrode material layer.

[0054] Preparation of the second material layer: The binder II (CMC) was prepared as an aqueous adhesive solution of 1.3 wt%, and inorganic particles (alumina with D50=700nm), binder PAA, and SBR emulsion were added sequentially to form a uniformly dispersed adhesive solution. The mass ratio of binder II to inorganic particles was 1:9; the ratio of binder II, PAA, and SBR was 1:1:1; the adhesive solution was coated on the surface of the electrode material layer and the solvent was dried to form the second material layer.

[0055] A negative electrode sheet is prepared by co-rolling a negative electrode sheet comprising a current collector, an electrode material layer, and a second material layer. The thickness of the second material layer is 6 μm. A schematic diagram of the obtained negative electrode sheet is shown below. Figure 1 It includes a current collector 101, an electrode material layer 102, a second material layer 103 (including binder II 103A and inorganic particles 103B), and binder III 104.

[0056] S3 Preparation of Dry Cells

[0057] The positive electrode, negative electrode, and separator are vacuum dried at 85℃ for 12 hours, and then the dry cell is prepared by stacking and aluminum-plastic film packaging.

[0058] In a protective atmosphere with a moisture and oxygen content of no more than 10 ppm, binder III monomer (ethylene glycol dimethacrylate, with 2 carbon-carbon double bond functional groups), electrolyte (1 mol / L lithium hexafluorophosphate dissolved in a mixed solution of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, and dimethyl carbonate in a mass ratio of 30 / 5 / 45 / 20), and initiator are mixed in a ratio of 10:90:0.05 and stirred to form a homogeneous and stable precursor. The precursor is injected into the dry cell at an injection coefficient of 2.3 g / Ah and then sealed by vacuum extraction.

[0059] The battery cell was placed at 10°C and tilted at 30°C for 48 hours. Then, 10 PSI pressure was applied to the battery cell using clamps, and it was placed at 38°C for 72 hours to obtain a lithium battery.

[0060] Example 2, a method for preparing a lithium battery with long life and safety, differs from Example 1 in that the thickness of the second material layer is 3 μm.

[0061] Example 3, a method for preparing a lithium battery with long lifespan and safety, differs from Example 1 in that: the type of binder III monomer is different, and the amount of initiator is different; details are as follows:

[0062] The precursor is composed of binder III monomer (ethoxylated trimethylolpropane triacrylate with 3 carbon-carbon double bond functional groups), electrolyte, and initiator in a mass ratio of 10:90:0.1.

[0063] Example 4, a method for preparing a lithium battery with long life and safety, differs from Example 1 in that: the precursor is composed of binder III monomer (dipentaerythritol hexaacrylate, with 6 carbon-carbon double bond functional groups)), electrolyte, and initiator in a mass ratio of 5:95:0.05.

[0064] Example 5, a method for preparing a lithium battery with long life and safety, differs from Example 4 in that the particle size of alumina is D50=1000nm.

[0065] Example 6, a method for preparing a lithium battery with long life and safety, differs from Example 4 in that: the preparation method of the slurry of the second material layer is as follows: PMMA is prepared as 7wt% NMP adhesive solution, inorganic particles (alumina with D50=700nm) are added, and a uniformly dispersed adhesive solution is prepared. The mass ratio of binder II to inorganic particles is 1:9.

[0066] Example 7, a method for preparing a lithium battery with long life and safety, differs from Example 6 in that the inorganic particles are LLZO with D50=700nm.

[0067] Example 8, a method for preparing a lithium battery with long life and safety, differs from Example 7 in that the mass ratio of binder II to inorganic particles is 1:19.

[0068] Comparative Example 1, a method for preparing a lithium battery with long life and safety, differs from Example 4 in that: the negative electrode does not contain a second material layer, and the precursor is only an electrolyte (without binder III).

[0069] Comparative Example 2, a method for preparing a lithium battery with long life and safety, differs from Example 4 in that the precursor is only an electrolyte (containing a second material layer).

[0070] Comparative Example 3, a method for preparing a lithium battery with long lifespan and safety, differs from Example 4 in that:

[0071] The negative electrode does not contain a second material layer (but contains binder III).

[0072] Comparative Example 4, a method for preparing a lithium battery with long lifespan and safety, differs from Example 1 in that:

[0073] The thickness of the second material layer is 10 μm.

[0074] Comparative Example 5, a method for preparing a lithium battery with long life and safety, differs from Example 1 in that the particle size of alumina is D50=100nm.

[0075] The following examples and comparative data illustrate the electrochemical and safety performance of the lithium battery of the present invention.

[0076] (1) Electrochemical performance testing

[0077] The lithium batteries comprising the embodiments and comparative examples of this patent were manufactured and data collected using the following steps:

[0078] Formation performance: At 25℃, the lithium battery was subjected to a 10PSI pressure, constant current charging at 0.1C to 4.25V, and constant voltage charging at 4.25V to 0.05C to obtain the initial charge capacity QFCC (Ah); constant current discharge at 0.1C to 2.5V to obtain the 0.1C constant capacity QFDC (Ah); coulombic efficiency ICE = QFDC / QFCC*100%.

[0079] Cyclic stability: At 25°C, the circuit was charged at a constant current of 1.0C to 4.25V, charged at a constant voltage of 4.25V to 0.05C, and discharged at a constant current of 1.0C to 2.5V. This process was repeated 100 times, and the discharge capacity Qn (Ah) of the nth cycle was recorded.

[0080] Battery capacity retention rate after 100 cycles: Retention (%) = (Q100 / Q1)*100%.

[0081] Negative electrode expansion rate: After the cycle test, the battery was charged at a constant current of 0.1C to 4.25V and then charged at a constant voltage of 4.25V to 0.05C. The cell was then disassembled in a water-controlled environment, and the average thickness L of all negative electrode sheets except the outermost two sheets was measured using a micrometer.

[0082] L electrode material layer = L negative electrode - L current collector - L second material layer (μm)

[0083] Expansion rate δ(%) = (L electrode material layer - electrode material layer density / electrode material layer compaction) / (electrode material layer density / electrode material layer compaction) * 100%.

[0084] The relevant test results are shown in Table 1.

[0085] Table 1 Electrochemical performance of each example and comparative example

[0086] experimental group FCC (mAh) FDC (mAh) ICE (%) Retention (%) δ (%) Example 1 9011.63 7389.76 82.00 81.40 30.40 Example 2 9058.23 7448.99 82.23 81.70 32.30 Example 3 9065.01 7456.13 82.25 82.90 29.10 Example 4 9104.89 7545.26 82.87 85.10 25.50 Example 5 9008.05 7402.25 82.17 84.40 25.90 Example 6 9099.78 7534.32 82.80 85.30 32.10 Example 7 9104.89 7654.26 84.07 88.10 28.40 Example 8 9150.39 7698.41 84.13 90.20 28.30 Comparative Example 1 8826.91 7606.73 86.18 76.3 43.70 Comparative Example 2 8841.27 7522.45 85.08 78.50 40.40 Comparative Example 3 9148.80 7566.61 82.71 80.60 33.90 Comparative Example 4 8964.70 7285.11 81.26 65.40 47.20 Comparative Example 5 9020.29 7325.39 81.21 71.30 45.50

[0087] The prepared lithium batteries have FCC 9000-9200 mAh, FDC 7000-7700 mAh, ICE 81-85%, Retention 80-92%, and δ24-35%.

[0088] Observing Comparative Examples 1, 2, and 3, it can be seen that the presence of the inert coating has a certain impact on the cell capacity. This is due to the increase in internal resistance caused by the coating (Comparative Example 2). The incomplete curing of binder III will cause some residual cells to participate in the film formation reaction during the first charging stage, resulting in a significant reduction in the cell ICE (Comparative Example 3). However, both the coating and binder III have the effect of inhibiting the expansion of the negative electrode, thus improving the cycle stability of the battery.

[0089] Observing Comparative Example 4, Example 1, and Example 2, it can be seen that the combined use of the second material layer and binder III can further suppress negative electrode expansion and improve the cycle stability of the battery cell, but it will also further increase the battery cell impedance and affect the battery cell capacity (Example 1); Regarding the thickness of the second material layer: coatings that are too thick or too thin will lead to an increase in the negative electrode expansion rate (Comparative Example 4, Example 2); Regarding the type of binder III: using monomers with a higher degree of crosslinking can improve the mechanical strength of binder III after curing, thereby achieving more effective suppression of negative electrode expansion, and thus reducing the lithium ion transport path within the electrode, which in turn improves the battery cell capacity (Examples 3, Example 4); Regarding the particle size of inorganic particles: using inorganic particles with a particle size that is too small (100 nm) will cause the coating to be too dense and the impedance to increase, and will also affect the shelf life of the coating slurry, 700-1000 Inorganic particles with a diameter of nm can effectively suppress the expansion of the negative electrode (Comparative Example 5, Example 5); if the particle size of inorganic particles exceeds 1 μm, the second material layer, which is already at the micrometer level in thickness, will have uneven thickness due to its high roughness, resulting in different impedances and uneven current density at different parts of the electrode.

[0090] Regarding the selection of materials for the second material layer: replacing inert inorganic particles with LLZO oxide solid electrolyte powder with a certain ionic conductivity is an effective strategy to reduce coating impedance. Among them, LLZO is the most resistant to reduction, but it has high alkalinity. Using oil-based PMMA binder in combination with hydrophilic LLZO can solve the compatibility problem between LLZO and oil-based binder PVDF or water-based slurry.

[0091] Regarding the selection of materials for the second material layer: Compared to the aqueous binder formulation of CMC+PAA+SBR, PMMA as binder II has weaker mechanical strength and slightly poorer suppression effect on negative electrode expansion (Example 6). However, when combined with LLZO inorganic particles (Examples 7 and 8), it can effectively improve the capacity of the cell. The extremely high mechanical strength of LLZO can more effectively suppress negative electrode expansion, thereby achieving higher cycle stability. In addition, LLZO can adsorb anions and suppress side reactions inside the cell, thereby improving ICE (Example 7). Further increasing the proportion of LLZO in the second material layer can further improve the overall performance of the cell (Example 8).

[0092] (2) Safety performance test

[0093] The lithium batteries in the embodiments and comparative examples containing this patent underwent safety performance testing using the following steps:

[0094] Hot box safety: At 25℃, apply 10PSI pressure to the battery cell, charge it to 4.25V at a constant current of 0.1C, charge it to 0.05C at a constant voltage of 4.25V, and then place the battery cell in a 60℃ hot box for 150 minutes.

[0095] Increase the temperature to 130℃ at a rate of 5℃ / min and hold for 30 minutes;

[0096] Increase the temperature to 150℃ at a rate of 2℃ / min and hold for 30 minutes;

[0097] Increase the temperature to 170℃ at a rate of 2℃ / min and hold for 30 minutes;

[0098] Increase the temperature to 180℃ at a rate of 2℃ / min and hold for 30 minutes;

[0099] Cool down the battery cell until the temperature drops below 50°C, then end the test.

[0100] Each embodiment or comparative example selected 8 cells for verification, 4 of which were verified after formation and 4 after 100 cycles.

[0101] The relevant test results are shown in Table 2.

[0102] Table 2. Pass rate of the 180°C hot box for each embodiment and comparative example.

[0103] After the experimental group was formed, the battery cells were cycled.

[0104] experimental group After formation of the battery cell After cycling, the battery cell Example 1 100% 50% Example 2 75% 0% Example 3 100% 75% Example 4 100% 100% Example 5 100% 100% Example 6 100% 100% Example 7 100% 100% Example 8 100% 100% Comparative Example 1 0% 0% Comparative Example 2 50% 0% Comparative Example 3 0% 0% Comparative Example 4 100% 25% Comparative Example 5 75% 50%

[0105] The obtained lithium battery cells have a 180°C hot box pass rate of 75-100% after formation, and a 180°C hot box pass rate of 50-100% after 100 cycles.

[0106] Observing Comparative Examples 1, 2, and 3, it can be seen that without any protective measures, the battery cell cannot pass the 180°C hot box test (Comparative Example 1). The adhesive III alone cannot effectively improve safety. This is determined by the thermal runaway mechanism of the diaphragm melting and triggering an internal short circuit (Comparative Example 3). Applying the second material layer alone can improve the safety of the battery cell in the fresh stage after formation, but this effect cannot be maintained after 100 charge-discharge cycles (Comparative Example 2). Regarding the thickness of the second material layer: it can be seen that a second material layer with a thickness of 6 μm or more combined with adhesive III can effectively improve the safety of the battery cell in the fresh state. However, an excessively thick second material layer will also lead to excessively high battery cell impedance, uneven current density amplification during cycling, causing changes in the negative electrode volume and damage to the coating structure, thereby reducing the protection effect after 100 cycles (Comparative Example 4). When the thickness of the second material layer is too low, it will affect the initial protection effect (Example 2). A coating of about 6 μm has the highest all-time safety effect (Example 1).

[0107] Regarding the type of binder III: it is known that replacing the binder III monomer with one containing a higher number of unsaturated double bonds results in better maintenance of coating integrity, with the combination of polydipentaerythritol hexaacrylate showing the best effect (Example 4); Regarding the particle size of inorganic particles: it is known that when the particle size of inorganic particles is too small, agglomeration is likely to occur during the film formation stage, inducing uneven current density during the cell cycling process, thereby compromising the safety effect of the coating (Comparative Example 5). Using inorganic particles with a particle size of 1000 nm can achieve the same safety effect as 700 nm particles (Example 5); Regarding the selection of the material type for the second material layer: oil-based PMMA as binder II and LLZO as inorganic particles also have effective thermal stability (Examples 6, 7, and 8).

[0108] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A negative electrode, characterized in that, include current collector; Electrode material layers located on one or both sides of the current collector; A second material layer located on the side of the electrode material layer furthest from the current collector; and adhesive III located within the electrode material layer, at the interface between the electrode material layer and the second material layer, and inside the second material layer; The electrode material layer comprises a composition of binder I, conductive agent, silicon-carbon anode and graphite, wherein the binder I accounts for 1-8 wt% of the electrode material layer by mass. The adhesive I includes any one or a combination of several of CMC, PAA and PMMA; The second material layer comprises a composition of binder II, PAA, SBR, and inorganic particles; the mass ratio of binder II to inorganic particles is (0.5-1):(0.9-20); The adhesive II is one or more of CMC, SBR, PAA-Na, PAA-Li, PVDF, PMMA, and PVDF-HFP.

2. The negative electrode according to claim 1, characterized in that, The electrode material layer comprises a composition of binder I, conductive agent, silicon-carbon anode and graphite in a mass ratio of 1-3:1-3:18-20:70-85.

3. A negative electrode according to claim 1 or 2, characterized in that, The inorganic particles are any one or a combination of several of the following: alumina, boehmite, titanium dioxide, silicon dioxide, zirconium oxide, LATP, LLZO, and LLTO.

4. A negative electrode according to claim 3, characterized in that, The inorganic particles are LLZO.

5. A negative electrode according to claim 3, characterized in that, The inorganic particles have a particle size of 200-1000 nm.

6. A negative electrode according to claim 4 or 5, characterized in that, The second material layer comprises a composition of binder II, PAA, SBR and inorganic particles in a mass ratio of 1:1:1:9-19.

7. A negative electrode according to claim 6, characterized in that, The thickness of the second material layer is 3-6 μm.

8. A lithium battery with long lifespan and safety, characterized in that, It includes a positive electrode, a negative electrode as described in any one of claims 1-7, a separator, and an electrolyte filled between the positive electrode, the negative electrode, and the separator.

9. A method for preparing a lithium battery with long lifespan and safety as described in claim 8, characterized in that, The preparation steps include the following: S1. Mix the raw materials of the electrode material layer to prepare a slurry, coat it onto the surface of the current collector, and evaporate the solvent; S2. Prepare the second material layer raw material into a slurry and coat it on the surface of the electrode material layer away from the current collector, then evaporate the solvent; S3. The negative electrode sheet containing the current collector, electrode material layer and second material layer is co-rolled to obtain the negative electrode sheet; the positive electrode sheet, negative electrode sheet and separator are stacked and packaged to prepare a dry cell. S4. Disperse the monomers of binder III and the initiator in the electrolyte, inject them into the dry cell, and heat to induce cross-linking of binder III monomers to form binder III, thereby preparing a lithium battery.

10. A lithium battery with long lifespan and safety according to claim 9, characterized in that, The liquid precursor of the adhesive III is any one or a combination of several of the following: ethylene glycol dimethacrylate, triethylene glycol dimethacrylate, (ethylene glycol) diacrylate, ethylene glycol methyl ether methacrylate, ethoxylated trimethylolpropane triacrylate, trimethylolpropane triacrylate, propoxylated trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, and dipentaerythritol hexaacrylate. The mass ratio of binder III monomer to electrolyte is (5-10):(95-90); the mass ratio of binder III monomer to initiator is 100:0.5-1.5.

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