Negative plate, preparation method of negative plate and battery

Abstract

The invention provides a negative electrode plate, a preparation method of the negative electrode plate and a battery, the negative electrode plate comprises a negative electrode current collector and a negative electrode material layer, the negative electrode material layer comprises a silicon layer and a conductive layer, the silicon layer is arranged on the surface of the negative electrode current collector, a crack is formed in the silicon layer, and the conductive layer is arranged on the silicon layer. The conductive layer is arranged on the surface of the silicon layer and in the crack; the average width D of the cracks is greater than or equal to 1 mu m and less than or equal to 50 mu m. According to the negative plate provided by the invention, the silicon layer and the conductive layer are arranged on the negative current collector, and meanwhile, the conductive layer is used for filling and covering the silicon layer crack, so that the conductivity is ensured, and the situation that the battery energy density is reduced due to overhigh proportion of the conductive layer is avoided, thereby solving the problem of volume expansion of a silicon negative electrode; and meanwhile, when the average width D of the cracks is greater than or equal to 1 mu m and less than or equal to 50 mu m, on the basis of improving the expansion of the silicon negative electrode, the side reaction of the battery is further relieved, and the charging speed is improved.

Description

Technical Field

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

[0002] Silicon anodes, due to their high theoretical specific capacity, have become key candidate materials for improving battery energy density. They are often prepared using deposition methods such as PVD (physical vapor deposition) and CVD (chemical vapor deposition). However, 100% pure silicon anodes have certain drawbacks. On the one hand, pure silicon anodes have extremely poor conductivity, directly affecting the charge transport efficiency of the battery. On the other hand, pure silicon anodes undergo significant volume expansion during charge and discharge, leading to structural cracking of the silicon material and repeated rupture and recombination of the SEI film, resulting in poor battery cycle performance. To address these issues, existing technologies... While carbon deposition on the silicon layer can improve conductivity to some extent, the massive expansion of silicon can cause the conductive layer to break simultaneously, limiting the improvement effect. In addition, existing methods also use porous carbon to deposit silicon internally. Although the porous structure of porous carbon can provide a buffer space for silicon expansion, it reduces the silicon content (to only about 50%), resulting in a significant reduction in the material's specific capacity to half that of pure silicon. Furthermore, the hard carbon porous material used has a low compaction density, requiring the addition of additional binders. Even so, the interparticle adhesion is still far lower than that of pure silicon, making it difficult to fully suppress the negative effects of silicon expansion. Summary of the Invention

[0003] To address the problem of volume expansion of silicon anodes during charging and discharging in existing technologies, which affects the overall electrical performance of the battery, a negative electrode sheet, a method for preparing the negative electrode sheet, and a battery are provided.

[0004] The technical solution adopted by the present invention to solve the above-mentioned technical problems is as follows: On one hand, the present invention provides a negative electrode sheet, including a negative electrode current collector and a negative electrode material layer, the negative electrode material layer including a silicon layer and a conductive layer, the silicon layer being disposed on the surface of the negative electrode current collector, the silicon layer being expanded and cracked to form cracks, and the conductive layer being disposed on the surface of the silicon layer and in the cracks; The average width of the crack is D, where 1μm≤D≤50μm.

[0005] Optionally, 1 ≤ D ≤ 25 μm.

[0006] Optionally, the depth of the crack is 10 to 50% of the thickness of the silicon layer.

[0007] Optionally, there is a gap between the conductive layers located on opposite sidewalls of the crack.

[0008] Optionally, the thickness of the silicon layer is 5~20μm.

[0009] Optionally, the thickness of the conductive layer is 0.2~4μm.

[0010] Optionally, the silicon layer is composed of silicon, and the conductive layer is composed of a conductive material.

[0011] Optionally, the conductive material is carbon.

[0012] Optionally, the mass percentage A% of the conductive material in the negative electrode material layer is 1% to 20%.

[0013] Optionally, the mass percentage B% of silicon in the negative electrode material layer is 80%~99%.

[0014] Optionally, the method for preparing the negative electrode includes the following operations: Silicon is placed on the negative electrode current collector to form a silicon layer. The silicon layer expands and cracks to form a crack, thus obtaining a pre-fabricated negative electrode sheet. A conductive layer is deposited on the pre-fabricated negative electrode sheet to obtain the negative electrode sheet.

[0015] Optionally, the silicon layer is formed on the negative electrode current collector by physical vapor deposition or chemical vapor deposition.

[0016] Optionally, the method of "expanding the silicon layer to form cracks" includes the following operations: The negative electrode current collector with a silicon layer is placed in the electrolyte and charged and discharged in a cycle, so that the silicon layer expands and contracts and cracks.

[0017] Optionally, the number of charge-discharge cycles is 1 to 10.

[0018] On the other hand, the present invention provides a battery comprising a positive electrode, a separator, and a negative electrode, or a negative electrode prepared by a method for preparing the negative electrode.

[0019] The beneficial effects of this application are as follows: The negative electrode sheet provided in this application includes a negative electrode current collector, a silicon layer, and a conductive layer. In this application, the silicon layer is pre-expanded to generate the cracks, which provides reserved space for the volume expansion of silicon during subsequent charging and discharging processes. This effectively buffers the expansion stress of silicon and prevents the overall structure of the silicon layer from cracking and the conductive layer from breaking. At the same time, the conductive layer not only covers the surface of the silicon layer but also covers the inside of the cracks. This arrangement increases the contact area between the conductive layer and the silicon layer, improves the overall conductivity of the pure silicon negative electrode, and improves the rate performance of the battery. On the other hand, it can stabilize the silicon layer structure and reduce the cracking and recombination of the SEI film, thereby reducing the degradation of electrical performance during charging and discharging. Furthermore, preliminary verification revealed that when the average width of the crack is in the range of 1μm≤D≤50μm, it not only improves the expansion of the silicon anode but also helps to further alleviate battery side reactions and increase charging speed. At this average crack width, on the one hand, the conductive layer can fully cover the inside of the crack, which reduces the direct contact area between silicon and electrolyte to suppress side reactions and forms a continuous conductive path through the crack in the silicon layer. On the other hand, the average crack width of 1μm≤D≤50μm can balance the contact between the volume expansion space of the silicon layer and the conductive layer, avoiding both insufficient coverage of the conductive layer due to excessive crack width and failure to alleviate expansion stress due to excessive crack width. Thus, it reduces side reactions while increasing charging speed. In other words, the anode sheet provided in this application solves the adverse effects of silicon anode volume expansion on battery cycle performance and electrical performance through the synergistic effect of silicon layer, silicon layer crack, average crack width, and conductive layer. Attached Figure Description

[0020] Figure 1 This is a structural diagram showing the changes in the preparation process of the negative electrode sheet provided by the present invention; Figure 2 This is a partial structural schematic diagram of the negative electrode sheet provided by the present invention.

[0021] The reference numerals in the accompanying drawings are as follows: 1. Negative current collector; 2. Silicon layer; 3. Conductive layer; 4. Crack; 5. Average width of the crack. Detailed Implementation

[0022] To make the technical problems solved, the technical solutions, and the beneficial effects of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0023] Specifically, the width of the crack gradually decreases along the direction from the silicon layer surface to the negative electrode current collector. Therefore, the average width of the crack is equivalent to the width of a "crack of equal width". The calculation method is to first determine the area S of the cross-section of the crack, and then measure the depth L of the crack. The average width of the crack is the ratio of S to L. The crack depth can be measured using the method of CP slicing + SEM cross-section measurement.

[0024] Reference Figure 1-2 The present invention provides a negative electrode sheet, comprising a negative electrode current collector 1 and a negative electrode material layer, wherein the negative electrode material layer comprises a silicon layer 2 and a conductive layer 3, the silicon layer 2 is disposed on the surface of the negative electrode current collector 1, and a crack 4 is formed on the silicon layer 2, and the conductive layer 3 is disposed on the surface of the silicon layer 2 and in the crack 4; The average width (5) of the crack is D, where 1 μm ≤ D ≤ 50 μm.

[0025] Specifically, the negative electrode sheet provided in this application includes a negative electrode current collector 1, a silicon layer 2, and a conductive layer 3. In this application, the silicon layer 2 is pre-expanded to generate the crack 4, which provides reserved space for the silicon volume expansion during subsequent charging and discharging processes. This can effectively buffer the expansion stress of silicon and prevent the overall structure of the silicon layer 2 from cracking and the conductive layer 3 from breaking. At the same time, the conductive layer 3 not only covers the surface of the silicon layer 2 but also covers the interior of the crack 4. This arrangement increases the contact area between the conductive layer 3 and the silicon layer 2, improves the overall conductivity of the pure silicon negative electrode, and improves the rate performance of the battery. On the other hand, it can stabilize the structure of the silicon layer 2 and reduce the cracking and recombination of the SEI film, thereby reducing the degradation of electrical performance during charging and discharging. Furthermore, preliminary verification revealed that when the average width 5 of the crack is in the range of 1μm≤D≤50μm, it not only improves the expansion of the silicon anode but also helps to further alleviate battery side reactions and increase charging speed. With an average crack width of 5, on the one hand, the conductive layer 3 can fully cover the interior of the crack 4, which reduces the direct contact area between silicon and electrolyte to suppress side reactions and forms a continuous conductive path through the crack 4 of silicon layer 2. On the other hand, the average crack width 5 of 1μm≤D≤50μm can balance the contact between the volume expansion space of silicon layer 2 and conductive layer 3, avoiding both excessive crack width leading to insufficient coverage of conductive layer 3 and insufficient crack width failing to alleviate expansion stress. Thus, it reduces side reactions while increasing charging speed. In other words, the anode sheet provided in this application solves the adverse effects of silicon anode volume expansion on battery cycle performance and electrical performance through the synergistic effect of silicon layer 2, silicon layer 2 crack 4, average crack width 5, and conductive layer 3.

[0026] Furthermore, the average width 5 of the crack can be 1μm, 5μm, 10μm, 15μm, 20μm, 25μm, 30μm, 35μm, 40μm, 45μm, or 50μm, that is, the average width 5 of the crack can be within the range of 1μm≤D≤50μm.

[0027] In some embodiments, the average width 5 of the crack is 1≤D≤25μm.

[0028] Extensive experimental verification in the early stage shows that when the average width 5 of the crack is 1≤D≤25μm, it is beneficial to better exert the synergistic effect of silicon layer 2, crack 4 and conductive layer 3, and solve the adverse effects of silicon anode volume expansion on battery cycle performance and electrical performance.

[0029] In some embodiments, the depth of the crack 4 is 10 to 50% of the thickness of the silicon layer 2.

[0030] Specifically, when the depth of the crack 4 is 10% to 50% of the thickness of the silicon layer 2, the crack 4 does not completely penetrate the silicon layer 2, which can prevent the connection structure between the silicon layer 2 and the negative electrode current collector 1 from breaking, ensuring the overall structural stability of the electrode and preventing the blocky silicon structure from falling off during charging and discharging. On the other hand, the depth of the crack 4 can fully match the thickness range of the silicon layer 2, providing sufficient reserved space for the volume expansion of silicon, effectively buffering the expansion stress, and at the same time, it can allow the conductive layer 3 to smoothly penetrate into the crack 4, maximizing the contact area between the conductive layer 3 and the silicon layer 2. This not only improves the overall conductivity of the pure silicon negative electrode to improve the battery rate performance, but also reduces the direct contact between silicon and electrolyte through the coverage of the crack 4 by the conductive layer 3, inhibiting the rupture and recombination of the SEI film, reducing the degradation of electrical performance, and ultimately synergistically improving the battery cycle life.

[0031] In some embodiments, there is a gap between the conductive layers 3 on opposite sidewalls of the crack.

[0032] Specifically, the gap formed between the conductive layers 3 on opposite sidewalls of the crack 4 can further accommodate the volume expansion deformation of silicon during charging and discharging, thus preventing the conductive layers from breaking due to the expansion and compression of silicon.

[0033] In some embodiments, the thickness of the silicon layer 2 is 5~20 μm.

[0034] Specifically, a silicon layer 2 thickness in the range of 5~20μm can retain the high specific capacity of pure silicon material, avoiding the problems of insufficient silicon content and low battery energy density caused by excessively thin thickness (below 5μm). At the same time, by reasonably controlling the volume of silicon layer 2, the degree of overall volume expansion during charging and discharging can be reduced, thereby reducing the risk of damage to the silicon layer 2 structure by expansion stress. Meanwhile, 5~20μm can ensure that cracks 4 can be generated uniformly and at a reasonable depth, providing sufficient space for subsequent expansion and facilitating the conductive layer 3 to fully fill the cracks 4, ensuring effective contact between the conductive layer 3 and silicon layer 2 to improve conductivity. In addition, this thickness can also balance the overall weight and structural stability of the electrode, avoiding structural breakage caused by decreased electrode flexibility or excessive expansion stress exceeding the buffering capacity of conductive layer 3 due to excessively thick silicon layer 2 (above 20μm), thus working together with conductive layer 3 to achieve stable improvement in battery cycle performance and electrical performance.

[0035] Furthermore, the thickness of the silicon layer 2 can be 5μm, 6μm, 8μm, 10μm, 12μm, 14μm, 15μm, 16μm, 18μm, or 20μm, that is, the thickness of the silicon layer 2 can be in the range of 5 to 20μm.

[0036] In some embodiments, the thickness of the conductive layer 3 is 0.2~4μm.

[0037] Specifically, this thickness range ensures that the conductive layer 3 fully covers the surface of the silicon layer 2 and fills the crack 4. This avoids situations where the thickness is less than 0.2 μm, resulting in incomplete coverage and insufficient filling, which would prevent effective improvement of conductivity and stability of the silicon layer 2 structure. It also prevents excessive conductive material due to a thickness greater than 4 μm. Furthermore, a thickness of 0.2~4 μm can balance conductivity and battery energy density. By constructing a continuous conductive path through the conductive layer 3 to improve battery rate performance and buffer the expansion stress of the silicon layer 2 to prevent structural cracking, it further enhances the protective effect and conductivity stability of the silicon layer 2, thereby solving the problem of silicon anode volume expansion and achieving simultaneous optimization of battery cycle performance and electrical performance.

[0038] Furthermore, the thickness of the conductive layer 3 can be 0.2μm, 0.5μm, 1μm, 2μm, 3μm, or 4μm, that is, the thickness of the conductive layer 3 can be in the range of 0.2 to 4μm.

[0039] In some embodiments, the silicon layer 2 is composed of silicon, and the conductive layer 3 is composed of a conductive material, wherein the conductive material is carbon.

[0040] Specifically, the silicon layer 2 is composed of silicon, which can effectively improve the theoretical specific capacity of the negative electrode, thereby ensuring the high energy density of the battery; the conductive layer 3 is made of carbon material, which not only has excellent electronic conductivity, but also can form a stable three-dimensional conductive network on the surface of the silicon layer 2 and in the mesh cracks 4, enhancing the electron transport efficiency of the electrode to improve rate performance. At the same time, it has good flexibility and interface compatibility, which can adapt to the volume expansion deformation during silicon charging and discharging, alleviate the damage of expansion stress to the electrode structure, and thus achieve a synergistic improvement in high energy density and excellent cycle performance.

[0041] In some embodiments, the mass percentage A% of the conductive material in the negative electrode material layer is 1% to 20%.

[0042] Specifically, the 1% to 20% proportion of conductive material ensures that the conductive material fully covers the surface of silicon layer 2 and fills the crack 4. This not only improves the overall conductivity of the pure silicon anode by increasing the contact area with silicon layer 2 and improving the rate performance of the battery, but also stabilizes the structure of silicon layer 2, reduces the rupture and recombination of the solid electrolyte interface film, and reduces the degradation of electrical performance during charging and discharging. If A% is less than 1%, the conductivity is poor. On the other hand, this proportion range can control the amount of conductive material used to prevent excessive conductive material from affecting the energy density. While ensuring that the conductive layer 3 plays a role in buffering silicon expansion and improving conductivity, it balances conductivity and energy density.

[0043] In some embodiments, the mass percentage B% of silicon in the negative electrode material layer is 80% to 99%.

[0044] Specifically, the mass percentage B% of silicon in the negative electrode material layer is limited to 80%~99%, so that the content is matched with the proportion of the conductive layer 3 material. This allows the conductive layer 3 to fully cover the silicon layer 2 and the surface of the crack 4, achieving the effects of buffering silicon expansion, improving conductivity, and reducing the rupture and recombination of the solid electrolyte interface film. At the same time, it does not cause the effective capacity of silicon to be squeezed or affect the lithium-ion insertion / extraction efficiency due to an excessively high proportion of conductive material. Ultimately, while ensuring the high energy density of the silicon negative electrode, the battery cycle performance and rate performance are further optimized, effectively mitigating the impact of the volume expansion of the silicon negative electrode during charging and discharging on the overall electrical performance of the battery.

[0045] In some embodiments, the method for preparing the negative electrode sheet includes the following operations: Silicon is placed on the negative electrode current collector 1 to form a silicon layer 2. The silicon layer 2 is expanded to form cracks 4, thus obtaining a pre-fabricated negative electrode sheet. A conductive layer 3 is deposited on the pre-fabricated negative electrode sheet to obtain the negative electrode sheet.

[0046] Specifically, by first forming a silicon layer 2 on the negative electrode current collector 1 and expanding the silicon layer 2 to create cracks 4 to obtain a pre-fabricated negative electrode sheet, and then depositing a conductive layer 3 on the pre-fabricated negative electrode sheet, the silicon layer 2 is expanded in advance to form cracks 4, reserving sufficient space for the volume expansion of silicon during subsequent battery charging and discharging, buffering the expansion stress from the source, and preventing the silicon layer 2 from structurally breaking due to secondary expansion in actual use. At the same time, it provides a channel for the subsequent coverage and filling of the conductive layer 3. Then, the conductive layer 3 is deposited on the pre-fabricated negative electrode sheet with cracks 4, which ensures that the conductive layer 3 not only covers the surface of the silicon layer 2, but also fully penetrates into the interior of the cracks 4. This increases the contact area between the conductive layer 3 and the silicon layer 2 to improve the overall conductivity of the pure silicon negative electrode and improve the battery rate performance. It also stabilizes the structure of the silicon layer 2 by covering the cracks 4 with the conductive layer 3, thereby improving the battery cycle performance and electrical performance.

[0047] In some embodiments, the silicon layer 2 is formed on the negative electrode current collector 1 by physical vapor deposition or chemical vapor deposition.

[0048] Specifically, in the preparation of the negative electrode sheet in this application, the silicon layer 2 is formed on the negative electrode current collector 1 by physical vapor deposition (PVD) or chemical vapor deposition (CVD). Both deposition processes can achieve uniform and dense growth of the silicon layer 2 on the surface of the current collector, avoiding the concentration of expansion stress during charging and discharging due to uneven local thickness of the silicon layer 2. At the same time, this deposition method can enhance the bonding force between the silicon layer 2 and the current collector, preventing the silicon layer 2 from falling off during subsequent charging and discharging.

[0049] In some embodiments, the method of "expanding silicon layer 2 to form crack 4" includes the following operations: The negative electrode current collector 1 with silicon layer 2 is placed in the electrolyte for charge-discharge cycle, so that silicon layer 2 expands and contracts to form crack 4, and an electrolyte interface film is formed on the surface of silicon layer and the surface of crack.

[0050] After the battery is charged and discharged, an electrolyte interface film can be further formed on the surface of the conductive layer 3.

[0051] The negative electrode current collector 1, on which the silicon layer 2 is formed, is placed in the electrolyte for charge-discharge cycles, so that the silicon layer 2 expands and contracts to form a network of cracks 4.

[0052] Specifically, the charge-discharge cycle simulates the expansion and contraction of silicon during actual battery use, allowing the silicon layer 2 to naturally generate cracks 4. This avoids structural damage or uneven distribution of cracks 4 that may be caused by other operations. Furthermore, the size and density of the generated cracks 4 match the actual expansion requirements of the silicon layer 2, which can precisely reserve buffer space for the volume expansion of silicon during subsequent charge and discharge, effectively alleviating expansion stress and reducing the risk of overall cracking of the silicon layer 2. On the other hand, the charge-discharge cycle in the electrolyte environment can simultaneously allow the silicon layer 2 to undergo the lithium-ion insertion / extraction process, thereby reducing the magnitude of structural changes in the silicon layer 2 when the battery is put into use. This reduces the possibility of the conductive layer 3 breaking due to secondary expansion of the silicon layer 2, and also reduces the frequency of repeated cracking and recombination of the solid electrolyte interface film, thus reducing the degradation of electrical performance.

[0053] In some embodiments, the number of charge-discharge cycles is 1 to 10.

[0054] Specifically, the number of charge-discharge cycles, from 1 to 10, can control the size and density of the formed cracks 4. If the number of cycles is too few, the cracks 4 will be insufficient and shallow, with limited expansion space and poor buffering effect. If the number of cycles is too many, the silicon layer 2 may fragment due to repeated expansion and contraction, destroying the overall structural stability and affecting the subsequent coverage of the conductive layer 3 and the battery performance. This application has verified through a large number of previous experiments that when the number of charge-discharge cycles is in the range of 1 to 10, the cracks 4 can be generated uniformly and appropriately, which not only meets the expansion buffering requirements of subsequent charge and discharge, but also prevents the conductivity of the silicon layer 2 from decreasing due to too many or too large cracks 4.

[0055] Another embodiment of the present invention provides a battery comprising a positive electrode, a separator, and the aforementioned negative electrode, or a negative electrode prepared by the method for preparing the aforementioned negative electrode.

[0056] Specifically, the battery includes the negative electrode sheet provided in this application, wherein the silicon layer 2 is pre-expanded to generate the crack 4, providing reserved space for the volume expansion of silicon during subsequent charging and discharging, which can effectively buffer the expansion stress of silicon and prevent the overall structure of silicon layer 2 from cracking and the conductive layer 3 from breaking. At the same time, the conductive layer 3 not only covers the surface of silicon layer 2, but also covers the inside of the crack 4. This setting increases the contact area between the conductive layer 3 and silicon layer 2, improves the overall conductivity of the pure silicon negative electrode, and improves the rate performance of the battery. On the other hand, it can stabilize the structure of silicon layer 2 and reduce the cracking and recombination of SEI film, thereby reducing the degradation of electrical performance during charging and discharging. Furthermore, preliminary verification revealed that when the average width 5 of the crack is in the range of 1μm≤D≤50μm, it not only improves the expansion of the silicon anode but also helps to further alleviate battery side reactions and increase charging speed. With an average crack width of 5, on the one hand, the conductive layer 3 can fully cover the interior of the crack 4, which reduces the direct contact area between silicon and electrolyte to suppress side reactions and forms a continuous conductive path through the crack 4 of silicon layer 2. On the other hand, the average crack width 5 of 1μm≤D≤50μm can balance the contact between the volume expansion space of silicon layer 2 and conductive layer 3, avoiding both excessive crack width leading to insufficient coverage of conductive layer 3 and insufficient crack width failing to alleviate expansion stress. Thus, it reduces side reactions while increasing charging speed. In other words, the anode sheet provided in this application solves the adverse effects of silicon anode volume expansion on battery cycle performance and electrical performance through the synergistic effect of silicon layer 2, silicon layer 2 crack 4, average crack width 5, and conductive layer 3.

[0057] In this invention, there are no particular limitations on the negative electrode current collector 1, as long as it can achieve the purpose of this application. For example, it can be copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, or composite current collector, etc.

[0058] In some preferred embodiments, the negative current collector 1 comprises copper foil.

[0059] In some embodiments, the positive electrode includes a positive electrode active material, which includes one or more of transition metal lithium oxide, lithium iron phosphate, lithium manganese oxide, lithium manganese iron phosphate, and lithium vanadium phosphate. Specifically, the positive electrode active material can be one or more of the above-mentioned materials.

[0060] In some embodiments, the positive electrode includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector, the positive active material layer including a positive active substance.

[0061] In this application, there is no particular limitation on the type of positive electrode current collector; it can be any known material suitable for use as a positive electrode current collector. In one embodiment, the positive electrode current collector includes metallic materials such as aluminum, stainless steel, titanium, and tantalum, as well as carbon materials such as carbon cloth and carbon paper.

[0062] In one embodiment, the positive current collector is a metallic material.

[0063] In some embodiments, the positive electrode active material layer further includes a positive electrode conductive agent, a positive electrode binder, and a solvent.

[0064] In some embodiments, the type of positive conductive agent mentioned in this invention is not limited, and any known conductive agent can be used.

[0065] In some embodiments, the positive electrode conductive agent mentioned in this invention includes at least one of carbon materials such as natural graphite, artificial graphite, acetylene black, needle coke, carbon nanotubes, and graphene.

[0066] In one embodiment, the type of positive electrode binder mentioned in this invention is not limited, and any known positive electrode binder can be used.

[0067] In some embodiments, the positive electrode binder includes at least one of polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, and nitrocellulose.

[0068] In the batteries mentioned in this application, a separator is usually provided between the positive and negative electrodes to prevent short circuits. There are no particular restrictions on the material and shape of the separator.

[0069] In some embodiments, the diaphragm includes a porous sheet-like or non-woven material with excellent liquid retention properties. The diaphragm includes resin or glass fiber diaphragm materials, which include, but are not limited to, polyolefins, aromatic polyamides, polytetrafluoroethylene, polyethersulfone, etc.

[0070] In some embodiments, the battery further includes an electrolyte, which includes an organic solvent, an electrolyte salt, and additives. It should be noted that this application does not impose any particular limitation on the preparation method of the electrolyte. Those skilled in the art can prepare the electrolyte using conventional techniques, such as mixing the components (organic solvent, electrolyte salt, and additives) evenly according to the specified ratio.

[0071] Table 1 The present invention will be further illustrated by the following examples.

[0072] Example 1 This embodiment illustrates the preparation method of the negative electrode sheet and the battery disclosed in this invention, and includes the following steps: Preparation of positive electrode sheet 97.8% of the positive electrode active material lithium cobalt oxide, 1% of the positive electrode conductive agent conductive carbon black, and 1.2% of the binder PVDF are added to N-methylpyrrolidone and stirred to prepare a positive electrode slurry. The positive electrode slurry is then coated on both surfaces of the positive electrode current collector Al foil. After drying, cold pressing, and slitting, the positive electrode sheet is obtained.

[0073] Preparation of negative electrode sheet A negative electrode current collector with a silicon layer is placed in an electrolyte and subjected to charge-discharge cycles to cause the silicon layer to expand and contract, forming cracks, thus obtaining a pre-fabricated negative electrode sheet. The process includes the following steps: Electrolyte preparation: EC (ethylene carbonate) / DEC (diethyl carbonate) / DMC (dimethyl carbonate) are mixed in a ratio of 1:1:1 as a solvent, and 1M LiPF6 is added as a lithium salt. After being fully dissolved, it is used as the electrolyte. Charge-discharge cycle: The fabricated silicon anode is used as the anode sheet and lithium cobalt oxide (conventional formula) is used as the cathode sheet. It is charged to 4.4V with a constant current of 16A and then discharged to 3V with a constant current of 1A. This cycle is repeated 10 times. After the cycle is completed, it is soaked in DMC solvent for 5 minutes, rinsed, and the residual electrolyte is removed to obtain a prefabricated anode sheet with cracks. After the pre-fabricated negative electrode sheet is vacuum dried at 100°C for 1 hour, a conductive material (5%) is deposited on the silicon layer by chemical vapor deposition. The conductive material simultaneously covers the surface of the silicon layer and fills the cracks inside, thus obtaining the negative electrode sheet. The average width / μm, crack depth / μm, silicon layer thickness / μm, and conductive layer thickness / μm of the prepared negative electrode sheet are recorded in Table 1.

[0074] The above-mentioned negative electrode, positive electrode and separator are assembled into a cell, placed in an aluminum-plastic film package and injected with electrolyte. It is charged to 4.2V at 3.2C at 45℃ and discharged to 3V at 0.5C according to the clamping pressure of 5MPa. After reaching the discharge state, the gas in the cell is extracted, and it is formed again. After the gas is extracted and packaged, the battery is obtained.

[0075] Examples 2-10 Examples 2-10 illustrate the preparation method of the negative electrode sheet and the battery disclosed in this invention, and include most of the operations in Example 1, except that: The mass percentages of conductive materials (A%) and silicon (B%) in Examples 2-10 are all based on Table 1. The average width / μm, crack depth / μm, silicon layer thickness / μm, and conductive layer thickness / μm of the negative electrode sheets prepared in Examples 2 to 10 are recorded in Table 1.

[0076] Comparative Example 1 This comparative example is used to illustrate the preparation method of the negative electrode sheet and the battery disclosed in this invention, including most of the operations in Example 1, the difference being: Preparation of negative electrode sheet A conductive material (5%) is deposited onto the silicon layer by chemical vapor deposition of a negative electrode current collector with a silicon layer to obtain a negative electrode sheet. The relevant parameters of the prepared negative electrode are recorded in Table 1.

[0077] Comparative Examples 2-3 Comparative Examples 2 and 3 are used to compare and illustrate the preparation method of the negative electrode sheet and the battery disclosed in this invention, including most of the operations in Example 1, the difference being: The mass percentages of conductive materials (A%) and silicon (B%) in Comparative Examples 2 and 3 are based on Table 1. The structural characteristics of the negative electrode sheets prepared in Comparative Examples 2 and 3 are recorded in Table 1.

[0078] Performance testing The following performance tests were performed on Examples 1-10 and Comparative Examples 1-3 prepared above: Cyclic test: At 25℃, charge at 0.5C to 4.2V, cut off at 0.05C, and then discharge at 0.5C to 3.0V; Rate discharge test: At 25℃, charge at 0.5C to 4.2V, cut off at 0.05C, and then discharge at 0.2C to 3.0V as the initial capacity; then charge at 0.5C to 4.2V, cut off at 0.05C, and then discharge at 2C to 3.0V as the rate discharge capacity; Capacity percentage = 2C discharge capacity / initial capacity.

[0079] The test results are entered into Table 2.

[0080] Table 2 As can be seen from the test results in Table 2, among Examples 1-3, Example 2 (average crack width 25μm) has the best overall performance, with 352 cycles at 25℃ where the capacity decays to 80% and the 2C discharge capacity accounts for 81%. Example 1 (crack 1μm) has poor cycle performance (203 cycles) due to the small average crack width, which leads to increased silicon layer breakage during cycling. Example 3 (crack 50μm) has a large average crack width, which increases the side reactions caused by silicon contact with the electrolyte, thus limiting the cycle performance (223 cycles). Since the increased average crack width leads to increased porosity in the crack gaps, which is beneficial for lithium ion transport, the rate performance of Example 3 is slightly higher (85%).

[0081] In Examples 4-7, when the crack width is the same as in Example 2 (25 μm), a low content of conductive material (such as 1% in Example 4) will result in insufficient conductivity and poor rate performance (77%). A high content of conductive material (such as 25% in Example 6) will overly cover the crack and compress the silicon expansion space, resulting in more silicon particle breakage and affecting both cycle performance and rate performance.

[0082] In Examples 8-10, Example 9 (5% conductive material, 95% silicon content) achieved the optimal balance between cycle performance (390 cycles) and rate performance (82%). Compared with Comparative Example 1 (without cracks), Example 9 illustrates a scheme that pre-fabricates cracks and fills them with a conductive layer. This not only buffers silicon expansion and reduces SEI film rupture and recombination through cracks, but also improves conductivity through the conductive layer, resulting in a significant improvement in cycle and rate performance. Compared with Comparative Example 2 (crack width 0.5μm < 1μm lower limit), Example 9 shows that the insufficient crack width of Comparative Example 1 cannot effectively relieve expansion stress, resulting in a deterioration in performance. Compared with Comparative Example 3 (crack 60μm > 50μm upper limit), Example 9 showed that although the excessively wide crack did not significantly affect the rate performance, the increased side reactions led to a significant decrease in cycling performance.

[0083] Based on the analysis of the specific test data above, it can be seen that the negative electrode sheet provided by the present invention, by setting a silicon layer and a conductive layer on the negative electrode current collector and filling and covering the cracks in the silicon layer with a conductive layer, not only ensures the conductivity performance, but also avoids the decrease in battery energy density due to the excessive proportion of conductive layer, thereby solving the problem of silicon negative electrode volume expansion and achieving simultaneous improvement in battery cycle performance and electrical performance. At the same time, when the average width of the crack is in the range of 1μm≤D≤50μm, it is beneficial to further alleviate battery side reactions and improve charging speed on the basis of improving silicon negative electrode expansion.

[0084] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A negative electrode sheet, characterized in that, It includes a negative electrode current collector (1) and a negative electrode material layer, the negative electrode material layer including a silicon layer (2) and a conductive layer (3), the silicon layer (2) is disposed on the surface of the negative electrode current collector (1), the silicon layer (2) is expanded and cracked to form a crack (4), and the conductive layer (3) is disposed on the surface of the silicon layer (2) and in the crack (4); The average width (5) of the crack is D, where 1 μm ≤ D ≤ 50 μm.

2. The negative electrode sheet according to claim 1, characterized in that, 1μm≤D≤25μm.

3. The negative electrode sheet according to claim 1, characterized in that, The depth of the crack (4) is 10% to 50% of the thickness of the silicon layer (2).

4. The negative electrode sheet according to claim 1, characterized in that, There is a gap between the conductive layers (3) located on the opposite sidewalls of the crack (4).

5. The negative electrode sheet according to claim 1, characterized in that, The thickness of the silicon layer (2) is 5~20μm.

6. The negative electrode sheet according to claim 1, characterized in that, The thickness of the conductive layer (3) is 0.2~4μm.

7. The negative electrode sheet according to claim 1, characterized in that, The silicon layer (2) is composed of silicon, and the conductive layer (3) is composed of conductive material.

8. The negative electrode sheet according to claim 7, characterized in that, The conductive material is carbon.

9. The negative electrode sheet according to claim 7, characterized in that, The mass percentage A% of the conductive material in the negative electrode material layer is 1%~20%; the mass percentage B% of the silicon in the negative electrode material layer is 80%~99%.

10. The method for preparing the negative electrode sheet according to any one of claims 1 to 9, characterized in that, Includes the following operations: Silicon is placed on the negative electrode current collector (1) to form a silicon layer (2). The silicon layer (2) expands and cracks to form a crack (4) to obtain a pre-fabricated negative electrode sheet. A conductive layer (3) is deposited on the pre-made negative electrode sheet to obtain the negative electrode sheet.

11. The method for preparing the negative electrode sheet according to claim 10, characterized in that, The silicon layer (2) is formed on the negative electrode current collector (1) by physical vapor deposition or chemical vapor deposition.

12. The method for preparing the negative electrode sheet according to claim 10, characterized in that, The method of "expanding and cracking the silicon layer (2) to form cracks (4)" includes the following operations: The negative electrode current collector (1) with silicon layer (2) is placed in the electrolyte for charge and discharge cycle, so that the silicon layer (2) expands and contracts and cracks to form crack (4).

13. The method for preparing the negative electrode sheet according to claim 12, characterized in that, The charge-discharge cycle count is 1 to 10 times.

14. A battery, characterized in that, It includes a positive electrode sheet, a separator, and a negative electrode sheet as described in any one of claims 1 to 9, or a negative electrode sheet prepared by the method described in claims 10 to 13.

Images