A negative plate of a phosphorus-carbon negative electrode system and a preparation method thereof

Abstract

The present application relates to a kind of negative pole piece of phosphorus-carbon negative electrode system and its preparation method, belong to the technical field of sodium-ion battery, for solving the one of existing phosphorus-carbon negative pole piece volume easy expansion, poor pole piece structure stability, poor cycle performance of battery problem.Phosphorus-carbon negative electrode system's negative pole piece includes current collector, first side surface and second side surface of current collector are sequentially symmetrical and stacked with first active layer and second active layer from inside to outside;Wherein, the preparation raw material of first active layer includes first active substance, phosphorus-carbon material, first binder, first conductive agent and first boron carbide;The preparation raw material of second active layer includes second active substance, phosphorus-carbon material, second binder, second conductive agent and second boron carbide;Wherein, the D50 of first boron carbide is b, the D50 of second boron carbide is c, c<b.The structure stability of the negative pole piece of phosphorus-carbon negative electrode system of the present application is good, and the expansion rate is low, not easy to drop material, and the cycle performance of battery is excellent.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, and in particular to a phosphorus-carbon anode system and its preparation method. Background Technology

[0002] As a crucial component of sodium-ion batteries, the performance of the anode material directly impacts the overall performance of the battery. Phosphorus-carbon anode materials have attracted widespread attention due to their high specific capacity and good cycle stability. Among various anode materials, the phosphorus-carbon composite system, with its ultra-high theoretical specific capacity of phosphorus and excellent electronic conductivity of carbon, has become a research hotspot for improving the energy density of sodium-ion batteries. However, the phosphorus-carbon composite system faces a severe volume expansion problem in practical applications. During cycling, the anode sheet may shed material, leading to instability in the electrode structure and poor cycle performance. Therefore, effectively suppressing volume expansion and enhancing the stability of the electrode structure are key technical challenges that urgently need to be overcome for this system. Summary of the Invention In view of the above analysis, the present invention aims to provide a phosphorus-carbon anode system and its preparation method to solve one of the following technical problems: existing phosphorus-carbon anodes are prone to volume expansion, have poor electrode structure stability, and have poor cell cycle performance.

[0003] On one hand, the present invention provides a negative electrode sheet of a phosphorus-carbon negative electrode system. The negative electrode sheet of the phosphorus-carbon negative electrode system includes a current collector, and a first active layer and a second active layer are symmetrically stacked from the inside to the outside on the first side surface and the second side surface of the current collector. The raw materials for preparing the first active layer include a first active material, a phosphorus-carbon material, a first binder, a first conductive agent, and a first boron carbide. The raw materials for preparing the second active layer include a second active material, a phosphorus-carbon material, a second binder, a second conductive agent, and a second boron carbide. The D50 of the first boron carbide is b, and the D50 of the second boron carbide is c, where c < b.

[0004] Furthermore, the first active material includes one or both of hard carbon or soft carbon, and the second active material is of the same type as the first active material.

[0005] Furthermore, the first adhesive includes styrene-butadiene rubber and sodium carboxymethyl cellulose, and the second adhesive is of the same type as the first adhesive.

[0006] Furthermore, the D50 values ​​'a' of the first and second active substances are the same, and 'a', 'b', and 'c' conform to the following relationship: c < b < ( -1)a.

[0007] Furthermore, the D50 values ​​a of the first and second active substances are 5~5.5 μm, the D50 value b of the first boron carbide is 1.64~2 μm, and the D50 value c of the second boron carbide is 1.1~1.64 μm.

[0008] Furthermore, the thickness of the first active layer is less than the thickness of the second active layer.

[0009] The present invention also provides a method for preparing the negative electrode sheet of the above-mentioned phosphorus-carbon negative electrode system, comprising the following steps: Step 1: Mix the first active material, phosphorus carbon material, first conductive agent, first binder, and first boron carbide powder evenly in N-methylpyrrolidone and water, and stir at room temperature to obtain the first active layer slurry; Step 2: Mix the second active material, phosphorus carbon material, second conductive agent, second binder, and second boron carbide powder evenly in N-methylpyrrolidone and water, and stir at room temperature to obtain the second active layer slurry; Step 3: Coat the first active layer slurry onto both sides of the current collector and dry it to form the first active layer; Step 4: Apply the second active layer slurry to the side of the first active layer away from the current collector, and dry it to form the second active layer.

[0010] Furthermore, in step 1, the viscosity of the first active layer slurry is controlled to be 4000~5000 mPa·s.

[0011] Furthermore, in step 2, the viscosity of the second active layer slurry is controlled to be 2000~3000 mPa·s.

[0012] Furthermore, in step 3, the drying temperature is controlled at 60~65℃.

[0013] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects: a) The phosphorus-carbon anode system of the present invention effectively reduces the volume expansion of the anode sheet and reduces the shedding of the anode sheet by adding a certain amount of boron carbide to the first active layer and the second active layer and precisely controlling the particle size relationship of boron carbide in different layers.

[0014] (b) In the phosphorus-carbon anode system of the present invention, the boron carbide particle size in the first and second active layers of the anode sheet is smaller than that of the active material. -1) times, which can effectively fill the gaps between active substances and does not hinder the stacking of particles after the active substances are rolled.

[0015] c) In the phosphorus-carbon anode system of the present invention, the first active layer has a large proportion of binder and high viscosity, which greatly enhances the bonding force between the first active layer and the current collector, thereby enhancing the peeling force of the anode sheet; the second active layer has a low proportion of binder, which indirectly increases the proportion of main material, improves the capacity of the cell, and enhances the cycle performance of the cell.

[0016] d) In the preparation method of the phosphorus-carbon anode system of the present invention, by precisely controlling the process parameters of each step, the thickness of the active layer of the anode sheet is uniform and the surface quality is good, thereby ensuring that the anode sheet has good structural stability, low expansion rate and is not easy to lose material.

[0017] e) The phosphorus-carbon anode system of the present invention effectively reduces the volume expansion of the anode sheet by precisely controlling the types and amounts of each component in the first and second active layers, thereby reducing electrode shedding and ensuring good structural stability, low expansion rate, and minimal shedding. For example, the coating peel force is above 0.09 mN / 20 mm, such as 0.091~0.097 mN / 20 mm; the rolling peel force is above 0.15 mN / 20 mm, such as 0.158~0.165 mN / 20 mm; after 100 cycles, the electrode expansion rate is below 9%, such as 7.8%~8.7%, and the shedding area length is below 103 mm, such as 67~103 mm; the cell including the anode sheet of the present invention has excellent cycle performance, for example, the capacity retention rate after 500 cycles at 25°C and 1C is above 91%, such as 91.3%~92.5%.

[0018] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description and drawings. Attached Figure Description

[0019] The accompanying drawings are for illustrative purposes only and are not intended to limit the scope of the invention.

[0020] Figure 1 This is a schematic diagram of the structure of the negative electrode sheet in the phosphorus-carbon negative electrode system of the present invention.

[0021] Figure Labels 1-Current collector, 2-First active layer, 3-Second active layer. Detailed Implementation

[0022] The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which constitute a part of the present invention and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0023] This invention provides a negative electrode sheet for a phosphorus-carbon negative electrode system, as follows: Figure 1 As shown, the negative electrode sheet of the phosphorus-carbon negative electrode system includes a current collector 1. A first active layer 2 and a second active layer 3 are symmetrically stacked from the inside to the outside on the first and second side surfaces of the current collector 1. The raw materials for preparing the first active layer 2 include a first active material, a phosphorus-carbon material, a first binder, a first conductive agent, and a first boron carbide. The raw materials for preparing the second active layer 3 include a second active material, a phosphorus-carbon material, a second binder, a second conductive agent, and a second boron carbide. The D50 of the first boron carbide is b, and the D50 of the second boron carbide is c, where c < b.

[0024] The phosphorus-carbon anode system of the present invention effectively reduces the volume expansion of the anode sheet and reduces electrode shedding by adding a certain amount of boron carbide to the first and second active layers and precisely controlling the particle size relationship of boron carbide in different layers.

[0025] Specifically, current collector 1 is aluminum foil.

[0026] Specifically, the first active material includes one or both of hard carbon and soft carbon.

[0027] Specifically, the second active substance is the same type as the first active substance.

[0028] Specifically, the first binder includes styrene-butadiene rubber (SBR) and sodium carboxymethyl cellulose (CMC). The first binder using the above-mentioned composite binder can play the following roles: (1) Strong and tough bonding network: SBR is an elastomer that forms a soft network structure after drying and rolling, which firmly bonds the particles to the aluminum foil and buffers volume changes. CMC acts as an auxiliary binder, connecting the particles to the SBR network; thereby ensuring that the electrode has high peel strength and flexibility, and is not easy to shed powder or delaminate during charge and discharge cycles; (2) Reduce internal resistance and optimize interface: SBR itself is insulating, but when the amount is appropriate and the dispersion is good, it does not affect the electron / ion pathway; the good dispersibility of CMC avoids local binder enrichment; using the above-mentioned Composite binders can maintain low electrode polarization, which is conducive to the formation of a stable and dense SEI (solid electrolyte interface) film, reducing side reactions and improving the first coulombic efficiency; (3) Improve mechanical stability and cycle life: The rigid skeleton of CMC and the flexible elasticity of SBR combine to form a "rigid and flexible" bonding network. The electrode maintains structural integrity during repeated charge and discharge volume expansion and contraction, significantly improving the battery cycle life; (4) Both CMC and SBR are soluble in water and form an aqueous binder system, which greatly reduces the cost of organic solvent recycling, environmental pollution and safety hazards. At the same time, CMC and SBR have low costs.

[0029] Specifically, the first conductive agent includes conductive carbon black (SP) and single-walled carbon nanotubes (SWCNTs). The first conductive agent, using SP and SWCNT, can construct a multi-level conductive network. SP particles fill the spaces between active material particles in a "point contact" manner, forming basic short-range conductive pathways; while SWCNTs, acting as "long-range conductors," connect particles or regions that are far apart. Together, they construct a dense and efficient three-dimensional conductive network.

[0030] Specifically, the second adhesive is of the same type as the first adhesive, and the second conductive agent is of the same type as the first conductive agent.

[0031] Specifically, the phosphorus-carbon material was purchased from Tailing Technology Co., Ltd., model number HED-M1.

[0032] Specifically, the D50 of the first and second active substances is the same, both being a.

[0033] Considering the large gaps between spherical particles of a single size when stacked, adding smaller particles can fill these gaps, resulting in a denser packing structure that makes the electrode less prone to cracking under high compaction. While ensuring compaction density, maintaining sufficient ion transport channels and uniformly dispersing boron carbide particles among the larger active material particles effectively absorbs the stress generated by expansion. Taking all factors into account, controlling a, b, and c to meet the following relationship: c < b < ( -1)a.

[0034] Specifically, the D50 value a of the first and second active substances is 5~5.5μm, for example, 5μm, 5.1μm, 5.2μm, 5.3μm, 5.4μm, and 5.5μm.

[0035] Specifically, the D50 value b of the first boron carbide is 1.64~2μm, for example 1.64μm, 1.7μm, 1.8μm, 1.9μm, 2μm; and the D50 value c of the second boron carbide is 1.1~1.64μm, for example 1.1μm, 1.2μm, 1.3μm, 1.4μm, 1.5μm, 1.64μm.

[0036] Specifically, the mass ratio of the first binder to the total mass of the raw materials for preparing the first active layer is greater than the mass ratio of the second binder to the total mass of the raw materials for preparing the second active layer. This setting ensures that the binder proportion in the first active layer is larger and the viscosity is higher, which greatly enhances the adhesion between the first active layer and the current collector, thereby enhancing the peeling force of the negative electrode sheet. The binder proportion in the second active layer is lower, which indirectly increases the proportion of the main material, improves the capacity of the cell, and enhances the cycle performance of the cell.

[0037] Specifically, in the preparation of the first active layer, if the quality of the first active material is too high, the slurry becomes unstable, prone to sedimentation, and the electrode becomes brittle and sheds powder; if the quality of the first active material is too low, the energy density is low and the first-cycle coulombic efficiency (ICE) decreases. If the quality of the phosphorus-carbon material is too high, the volume expansion becomes uncontrolled, the electrode pulverizes and falls off, the conductive network fails, the internal resistance increases dramatically, and the SEI film repeatedly breaks down and regenerates; if the quality is too low, the energy density is not sufficiently improved, and the first-cycle coulombic efficiency decreases instead of increasing. If the quality of SP is too high, the energy density decreases, and the first-cycle coulombic efficiency and cycle life decrease; if the quality is too low, the electronic conductive network is incomplete, the internal resistance increases dramatically, the utilization rate of the active material is low, and the capacity is not fully utilized. If the quality of SWCNT is too high, the side reactions become uncontrolled and dispersion becomes extremely difficult; if the quality is too low, the long-range conductive network is missing, the active particles cannot be effectively "anchored," and the structure collapses. If the quality of SBR is too high, the energy density decreases and the electronic resistance increases; if the quality is too low, the electrode structure collapses and the cycle life is short. Excessive CMC quality leads to deterioration of slurry rheology, increased electrode brittleness, elevated internal resistance, and decreased energy density; excessively low CMC quality results in severe slurry sedimentation, poor dispersibility, insufficient electrode adhesion, and reduced electrode flexibility. Excessive boron carbide quality significantly reduces energy density, deteriorates conductivity, embrittles the electrode structure, and increases cost; excessively low boron carbide quality leads to failure in volume expansion suppression, oxidation resistance, and side reaction control. Therefore, the mass ratio of each component in the first active layer to the total mass of the raw materials in the first active layer is controlled as follows: First active material: 83%~85%, phosphorus-carbon material: 6.5%~7.5%, SP: 2.3%~2.8%, SWCNT: 0.15%~0.3%, SBR: 2%~3%, CMC: 1%~2%, and first boron carbide: 2%~3%.

[0038] Specifically, in the preparation of the second active layer, considering the synergistic matching of the effects of each component, the mass ratio of each component in the second active layer to the total mass of the raw materials of the second active layer is controlled as follows: second active substance: 85%~87%, phosphorus carbon material: 6.5%~7.5%, SP: 2.3%~2.8%, SWCNT: 0.1%~0.2%, SBR: 1.5%~2.5%, CMC: 0.5%~1.5%, second boron carbide: 1%~2%.

[0039] Specifically, the thickness of the first active layer is less than that of the second active layer. This configuration can reduce contact resistance and improve rate performance; create a porosity gradient to optimize ion transport; buffer volume expansion and enhance structural stability; balance energy density and power density; and improve safety and interface stability.

[0040] Specifically, the thickness of the first active layer is 30~35μm, for example 30μm, 31μm, 32μm, 33μm, 34μm, 35μm.

[0041] Specifically, the thickness of the second active layer is 35~40μm, for example 35μm, 36μm, 37μm, 38μm, 39μm, 40μm.

[0042] The present invention also provides a method for preparing the negative electrode sheet of the above-mentioned phosphorus-carbon negative electrode system, comprising the following steps: Step 1: Mix the first active material, phosphorus carbon material, first conductive agent, first binder, and first boron carbide powder evenly in N-methylpyrrolidone (NMP) and water, and stir at room temperature to obtain the first active layer slurry; Step 2: Mix the second active material, phosphorus carbon material, second conductive agent, second binder, and second boron carbide powder evenly in NMP and water, and stir at room temperature to obtain the second active layer slurry; Step 3: Coat the first active layer slurry onto both sides of the current collector and dry it to form the first active layer; Step 4: Apply the second active layer slurry to the side of the first active layer away from the current collector, and dry it to form the second active layer.

[0043] Specifically, in step 1 above, the room temperature is 20~25℃, for example, 20℃, 21℃, 22℃, 23℃, 24℃, or 25℃. Considering that excessive stirring time can damage the conductive network, increase the slurry temperature, and cause solvent evaporation, while insufficient stirring time results in high electrode resistance and poor batch consistency, the stirring time is controlled to 2.5~3 hours, for example, 2.5 hours, 2.6 hours, 2.7 hours, 2.8 hours, 2.9 hours, or 3 hours.

[0044] Specifically, in step 1 above, considering that if the viscosity of the first active layer slurry is too high, the internal resistance of the electrode will increase, making filtration and transportation difficult, and residual air bubbles will be difficult to eliminate; if it is too low, coating sagging and edge defects will occur, the electrode compaction density will decrease, and the binder distribution will be uneven. Therefore, the viscosity of the first active layer slurry at room temperature is controlled to be 4000~5000 mPa·s, for example, 4000 mPa·s, 4200 mPa·s, 4400 mPa·s, 4600 mPa·s, 4800 mPa·s, and 5000 mPa·s.

[0045] Specifically, in step 2 above, considering that if the stirring time is too long, the binder structure is easily damaged, the slurry temperature rises, and the solvent evaporates; if it is too short, the conductive network is incomplete and the slurry settling is poor. Therefore, the stirring time is controlled to be 2.5~3h, for example 2.5h, 2.6h, 2.7h, 2.8h, 2.9h, or 3h.

[0046] Specifically, in step 2 above, considering that excessive viscosity of the second active layer slurry leads to difficulties in coating, poor coating uniformity, and increased internal resistance of the electrode, while insufficient viscosity results in slurry sedimentation, poor dispersion stability, and uneven binder distribution, the viscosity of the second active layer slurry at room temperature is controlled to be 2000~3000 mPa·s, for example, 2000 mPa·s, 2200 mPa·s, 2400 mPa·s, 2600 mPa·s, 2800 mPa·s, and 3000 mPa·s.

[0047] Specifically, in steps 3 and 4 above, considering that excessive coating speed can lead to insufficient drying, orange peel / cracks, uneven coating thickness, coating wrinkling, edge defects, and slurry overflow; while insufficient coating speed can easily cause overheating leading to binder failure, a significant decrease in production efficiency, and the risk of coating sagging, uneven thickness, and surface skinning, the coating speed is controlled at 9~10 m / min, for example, 9 m / min, 9.5 m / min, or 10 m / min.

[0048] Specifically, in step 3 above, considering that excessively high oven temperatures can easily cause the electrode sheets to crack and curl, leading to uneven adhesive distribution, while excessively low temperatures can easily result in coating appearance defects and reduced production efficiency, the oven temperature is controlled at 60~65℃, for example, 60℃, 61℃, 62℃, 63℃, 64℃, or 65℃; and the coating thickness of the first active layer is controlled at 30~35μm.

[0049] Specifically, in step 4 above, considering that excessively high oven temperatures can easily lead to adhesive migration and surface enrichment, as well as damage to the adhesive structure; while excessively low temperatures can result in incomplete drying of the electrode and ineffective curing of the adhesive, the oven temperature is controlled at 75~80℃, for example, 75℃, 76℃, 77℃, 78℃, 79℃, or 80℃; and the thickness of the second active layer is controlled at 35~40μm.

[0050] Specifically, the present invention also includes performance testing of the negative electrode and the battery cell: Step 5: Perform a peel force test on the negative electrode sheet after the first and second active layers have been coated; Step 6: Roll the negative electrode sheet under a preset pressure and test the peeling force of the negative electrode sheet after rolling. Step 7: In an environment controlled by a dew point of ≤-30℃, assemble the negative electrode sheet into a battery cell. Before filling with electrolyte, bake the battery cell at 80~90℃ to control the moisture content of the battery cell. Step 8: After the battery cell is manufactured, conduct battery cell performance testing.

[0051] Specifically, in step 5 above, a portion of the negative electrode sheet is cut off for a routine peel force test.

[0052] Specifically, in step 7 above, the battery cell baking time is 12~24 hours.

[0053] Specifically, in step 8 above, the prepared battery cell is cycled and then disassembled for verification.

[0054] Specifically, the phosphorus-carbon anode system of the present invention exhibits good structural stability, low expansion rate, and is less prone to material loss. For example, the coating peel force is above 0.09 mN / 20 mm, such as 0.091~0.097 mN / 20 mm; the rolling peel force is above 0.15 mN / 20 mm, such as 0.158~0.165 mN / 20 mm; after 100 cycles, the electrode expansion rate is below 9%, such as 7.8%~8.7%, and the length of the material loss area is below 103 mm, such as 67~103 mm; the battery cell including the anode of the present invention has good cycle performance, for example, the capacity retention rate after 500 cycles at 25°C and 1C is above 91%, such as 91.3%~92.5%.

[0055] The advantages of the present invention will be demonstrated below with specific embodiments and comparative examples.

[0056] Example 1 This embodiment provides a phosphorus-carbon anode system and its preparation method. The preparation method includes the following steps: Step 1: Hard carbon, phosphorus carbon material, SP, SWCNT, SBR, CMC, and first boron carbide powder are mixed evenly in NMP and water at a mass ratio of 83.8:7:2.5:0.2:2.5:1.5:2.5, and stirred at room temperature for 3 hours to obtain the first active layer slurry of the negative electrode. The viscosity of the first active layer slurry at room temperature is 4000 mPa·s; the D50 value a of hard carbon is 5.1 μm, and the D50 value b of the first boron carbide powder is 1.84 μm. Step 2: Hard carbon, phosphorus carbon materials, SP, SWCNT, SBR, CMC, and second boron carbide powder are mixed evenly in NMP and water at a mass ratio of 85.85:7:2.5:0.15:2:1:1.5 and stirred at room temperature for 3 hours to obtain the second active layer slurry of the negative electrode. The viscosity of the second active layer slurry at room temperature is 2000 mPa·s; the D50 value c of the second boron carbide powder is 1.12 μm. In steps 1 and 2, c < b < ( -1)a; Step 3: Apply the first active layer slurry to the blank aluminum foil current collector at a speed of 10 m / min, set the oven temperature to 65℃, and form the first active layer, wherein the coating thickness of the first active layer is 30 μm; Step 4: Apply the second active layer slurry to the side of the first active layer away from the current collector at a speed of 10 m / min, set the oven temperature to 80°C, and form the second active layer, wherein the coating thickness of the second active layer is 40 μm. Step 5: Perform a peel force test on the negative electrode sheet after the first and second active layers have been coated; Step 6: Under a preset pressure of 50 tons, roll the negative electrode sheet after the first and second active layers have been coated, and test the peeling force of the negative electrode sheet after roll rolling. Step 7: In an environment controlled by a dew point of ≤-30℃, assemble the electrode sheets into a battery cell. Before electrolyte injection, bake the battery cell at 90℃ for 24 hours. Step 8: After the battery cell is manufactured, conduct battery cell performance testing.

[0057] The performance characteristics of the negative electrode and the battery cell in this embodiment are shown in Table 1 below.

[0058] Example 2 This embodiment provides a phosphorus-carbon anode system and its preparation method. The preparation method includes the following steps: Step 1: Hard carbon, phosphorus carbon materials, SP, SWCNT, SBR, CMC, and first boron carbide powder are mixed evenly in NMP and water at a mass ratio of 84.4:7:2.3:0.3:2:2:2 and stirred at room temperature for 3 hours to obtain the first active layer slurry of the negative electrode. The viscosity of the first active layer slurry at room temperature is 4200 mPa·s; the D50 value a of the hard carbon material is 5.22 μm, and the D50 value b of the first boron carbide powder is 1.96 μm. Step 2: Hard carbon, phosphorus carbon materials, SP, SWCNT, SBR, CMC, and second boron carbide powder are mixed evenly in NMP and water at a mass ratio of 86.5:7:2.3:0.2:1.5:1.5:1, and stirred at room temperature for 3 hours to obtain the second active layer slurry of the negative electrode. The viscosity of the second active layer slurry at room temperature is 2300 mPa·s; the D50 value c of the second boron carbide powder is 1.64 μm. In steps 1 and 2, c < b < ( -1)a; Step 3: Apply the first active layer slurry to the blank aluminum foil current collector at a speed of 10 m / min, set the oven temperature to 63℃, and form the first active layer, wherein the coating thickness of the first active layer is 33 μm; Step 4: Apply the second active layer slurry to the side of the first active layer away from the current collector at a speed of 10 m / min, and set the oven temperature to 78°C, wherein the coating thickness of the second active layer is 40 μm. Step 5: Perform a peel force test on the negative electrode sheet after the first and second active layers have been coated; Step 6: Under a preset pressure of 50 tons, roll the negative electrode sheet after the first and second active layers have been coated, and test the peeling force of the negative electrode sheet after roll rolling. Step 7: In an environment controlled by a dew point of ≤-30℃, assemble the electrode sheets into a battery cell. Before electrolyte injection, bake the battery cell at 90℃ for 24 hours. Step 8: After the battery cell is manufactured, conduct battery cell performance testing.

[0059] The performance characteristics of the negative electrode and the battery cell in this embodiment are shown in Table 1 below.

[0060] Example 3 This embodiment provides a phosphorus-carbon anode system and its preparation method. The preparation method includes the following steps: Step 1: Hard carbon, phosphorus carbon materials, SP, SWCNT, SBR, CMC, and first boron carbide powder are mixed evenly in NMP and water at a mass ratio of 83.05:7:2.8:0.15:3:1:3 and stirred at room temperature for 3 hours to obtain the first active layer slurry of the negative electrode. The viscosity of the first active layer slurry at room temperature is 4500 mPa·s; the D50 value a of the hard carbon material is 5.32 μm, and the D50 value b of the first boron carbide powder is 1.64 μm. Step 2: Hard carbon, phosphorus carbon materials, SP, SWCNT, SBR, CMC, and second boron carbide powder are mixed evenly in NMP and water at a mass ratio of 85.1:7:2.8:0.1:2.5:0.5:2, and stirred at room temperature for 3 hours to obtain the second active layer slurry of the negative electrode. The viscosity of the second active layer slurry at room temperature is 2500 mPa·s; the D50 value c of the second boron carbide powder is 1.12 μm. In steps 1 and 2, c < b < ( -1)a.

[0061] Step 3: Apply the first active layer slurry to the blank aluminum foil current collector at a speed of 9 m / min, and set the oven temperature to 62℃, wherein the coating thickness of the first active layer is 32 μm; Step 4: Apply the second active layer slurry to the side of the first active layer away from the current collector at a speed of 9 m / min, and set the oven temperature to 76℃, wherein the coating thickness of the second active layer is 38 μm. Step 5: Perform a peel force test on the negative electrode sheet after the first and second active layers have been coated; Step 6: Under a preset pressure of 50 tons, roll the negative electrode sheet after the first and second active layers have been coated, and test the peeling force of the negative electrode sheet after roll rolling. Step 7: In an environment controlled by a dew point of ≤-30℃, assemble the electrode sheets into a battery cell. Before electrolyte injection, bake the battery cell at 90℃ for 24 hours to control the moisture content of the battery cell. Step 8: After the battery cell is manufactured, conduct battery cell performance testing.

[0062] The performance characteristics of the negative electrode and the battery cell in this embodiment are shown in Table 1 below.

[0063] Extensive research was conducted during the study process, and some suboptimal solutions are presented here as comparative examples.

[0064] Comparative Example 1 This comparative example provides a phosphorus-carbon anode system and its preparation method. The preparation method includes the following steps: Step 1: Hard carbon, phosphorus carbon materials, SP, SWCNT, SBR, CMC, and boron carbide powder are mixed evenly in NMP and water at a mass ratio of 80.1:7:2.5:0.4:4.5:2.5:3 and stirred at room temperature for 3 hours to obtain the first active layer slurry of the negative electrode. The D50 value a of the hard carbon material is 5.1 μm, and the D50 value b of the boron carbide powder is 3.76 μm. Step 2: Hard carbon, phosphorus carbon materials, SP, SWCNT, SBR, CMC, and boron carbide powder are mixed evenly in NMP and water at a mass ratio of 81.9:7:2.5:0.1:3:2:2.5 and stirred at room temperature for 3 hours to obtain the second active layer slurry of the negative electrode, wherein the D50 value c of the boron carbide powder is 4.45μm; In steps 1 and 2, the condition c < b < ( -1)a.

[0065] Step 3: Apply the first active layer slurry to the blank aluminum foil current collector at a speed of 10 m / min, and set the oven temperature to 65℃, wherein the coating thickness of the first active layer is 30 μm; Step 4: Apply the second active layer slurry to the side of the first active layer away from the current collector at a speed of 10 m / min, and set the oven temperature to 80°C, wherein the coating thickness of the second active layer is 40 μm. Step 5: Perform a peel force test on the negative electrode sheet after the first and second active layers have been coated; Step 6: Under a preset pressure, roll the negative electrode sheet after the first and second active layers have been coated, and test the peel force of the negative electrode sheet after rolling. Step 7: In an environment controlled by a dew point of ≤-30℃, assemble the electrode sheets into a battery cell. Before electrolyte injection, bake the battery cell at 90℃ for 24 hours to control the moisture content of the battery cell. Step 8: After the battery cell is manufactured, conduct battery cell performance testing.

[0066] The performance characteristics of the negative electrode and the battery cell in this comparative example are shown in Table 1 below.

[0067] Comparative Example 2 This comparative example provides a phosphorus-carbon anode system and its preparation method. The preparation method includes the following steps: Step 1: Hard carbon, phosphorus carbon materials, SP, SWCNT, SBR, CMC, and boron carbide powder are mixed evenly in NMP and water at a mass ratio of 83.8:7:2.5:0.2:2.5:1.5:2.5 and stirred at room temperature for 3 hours to obtain a negative electrode slurry. The D50 value a of the hard carbon material is 5.1 μm, and the D50 value b of the boron carbide powder is 1.84 μm. Step 2: Apply the slurry to the blank aluminum foil current collector at a speed of 10 m / min, set the oven temperature to 65℃, and the coating thickness to 75 μm; Step 3: Perform a peel force test on the coated negative electrode sheet; Step 4: Roll the negative electrode sheet under a preset pressure and test the peeling force of the negative electrode sheet after rolling. Step 5: In an environment controlled by a dew point of ≤-30℃, the electrode sheets are assembled into a battery cell. Before electrolyte injection, the battery cell is baked at 90℃ for 24 hours to control the moisture content of the battery cell. Step 6: After the battery cell is manufactured, conduct battery cell performance tests.

[0068] The performance characteristics of the negative electrode and the battery cell in this comparative example are shown in Table 1 below.

[0069] The partial performance of the negative electrode sheet and battery cell of the embodiments and comparative examples of the present invention is shown in Table 1 below.

[0070] Table 1. Partial performance results of negative electrode and battery cell

[0071] As can be seen from Table 1 above, compared with the comparative example, the embodiments of the present invention show significant improvements in coating peel force and roll peel force, significant reductions in electrode expansion rate and material loss, and improved capacity retention.

[0072] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A negative electrode sheet for a phosphorus-carbon negative electrode system, characterized in that, The negative electrode sheet of the phosphorus-carbon negative electrode system includes a current collector, and a first active layer and a second active layer are symmetrically stacked from the inside to the outside on the first and second side surfaces of the current collector; wherein, the raw materials for preparing the first active layer include a first active material, a phosphorus-carbon material, a first binder, a first conductive agent, and a first boron carbide; the raw materials for preparing the second active layer include a second active material, a phosphorus-carbon material, a second binder, a second conductive agent, and a second boron carbide; wherein, the D50 of the first boron carbide is b, the D50 of the second boron carbide is c, and c < b.

2. The negative electrode sheet of the phosphorus-carbon negative electrode system according to claim 1, characterized in that, The first active material includes one or two of hard carbon or soft carbon, and the second active material is of the same type as the first active material.

3. The negative electrode sheet of the phosphorus-carbon negative electrode system according to claim 1, characterized in that, The first adhesive comprises styrene-butadiene rubber and sodium carboxymethyl cellulose, and the second adhesive is of the same type as the first adhesive.

4. The negative electrode sheet of the phosphorus-carbon negative electrode system according to claim 1, characterized in that, The first active substance and the second active substance have the same D50 value 'a', and a, b, and c satisfy the following relationship: c < b < ( -1)a.

5. The negative electrode sheet of the phosphorus-carbon negative electrode system according to claim 4, characterized in that, The D50 values ​​a of the first and second active substances are 5~5.5 μm, the D50 value b of the first boron carbide is 1.64~2 μm, and the D50 value c of the second boron carbide is 1.1~1.64 μm.

6. The negative electrode sheet of the phosphorus-carbon negative electrode system according to claim 1, characterized in that, The thickness of the first active layer is less than the thickness of the second active layer.

7. A method for preparing a negative electrode sheet of the phosphorus-carbon negative electrode system according to any one of claims 1 to 6, characterized in that, Includes the following steps: Step 1: Mix the first active material, phosphorus carbon material, first conductive agent, first binder, and first boron carbide powder evenly in N-methylpyrrolidone and water, and stir at room temperature to obtain the first active layer slurry; Step 2: Mix the second active material, phosphorus carbon material, second conductive agent, second binder, and second boron carbide powder evenly in N-methylpyrrolidone and water, and stir at room temperature to obtain the second active layer slurry; Step 3: Coat the first active layer slurry onto both sides of the current collector and dry it to form the first active layer; Step 4: Apply the second active layer slurry to the side of the first active layer away from the current collector, and dry it to form the second active layer.

8. The method for preparing the negative electrode sheet of the phosphorus-carbon negative electrode system according to claim 7, characterized in that, In step 1, the viscosity of the first active layer slurry is controlled to be 4000~5000 mPa·s.

9. The method for preparing the negative electrode sheet of the phosphorus-carbon negative electrode system according to claim 7, characterized in that, In step 2, the viscosity of the second active layer slurry is controlled to be 2000~3000 mPa·s.

10. A method for preparing the negative electrode sheet of the phosphorus-carbon negative electrode system according to any one of claims 7 to 9, characterized in that, In step 3, the drying temperature is controlled at 60~65℃.

Images