Positive pole piece of lithium ion battery, preparation method of positive pole piece and lithium ion battery

Abstract

The invention discloses a lithium ion battery positive pole piece, a preparation method thereof and a lithium ion battery, and belongs to the technical field of lithium ion batteries. The lithium ion battery positive pole piece provided by the invention comprises a current collector, and a composite positive active material, a binder and a conductive agent which are arranged on the current collector, the composite positive electrode active material comprises lithium iron phosphate, nickel cobalt lithium manganate and lithium manganese iron phosphate in a mass ratio of (80-90): (5-15): (3-8), and the particle size relationship meets the following conditions: D1 is more than D3 and less than D2; d1 is the D50 particle size of lithium iron phosphate, D2 is the D50 particle size of nickel cobalt lithium manganate, and D3 is the D50 particle size of lithium manganese iron phosphate. According to the battery assembled by the positive pole piece disclosed by the invention, good combination of high energy density, high rate capability and long cycle life is realized.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a positive electrode sheet for a lithium-ion battery and its preparation method, as well as a lithium-ion battery. Background Technology

[0002] The information disclosed in the background section of this invention is intended only to enhance the understanding of the overall background of the invention and is not necessarily to be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

[0003] The booming development of the new energy vehicle industry has placed increasingly stringent demands on the comprehensive performance of power batteries. Currently, lithium iron phosphate batteries and ternary lithium batteries constitute the mainstream choices in the market. The former is known for its excellent safety performance and cycle life, but its energy density is usually difficult to exceed 200 Wh / kg; the latter can achieve a higher energy density of over 230 Wh / kg, but faces inherent challenges in terms of thermal safety and cost. This situation has resulted in a technological gap in the key performance range of approximately 190 Wh / kg to 230 Wh / kg. The industry urgently needs a battery technology that can achieve a breakthrough in this range, that is, significantly improve energy density while ensuring core safety characteristics.

[0004] To improve the mass energy density of lithium iron phosphate (LFP) systems, the industry's common strategy is to combine them with other cathode materials. However, this multi-material composite system faces a series of complex challenges in practical applications. First, there is an inherent trade-off between safety and energy density; how to introduce high-capacity materials without compromising the inherent safety advantages of the battery system is a core challenge. Second, different types of active materials exhibit significant differences in their physical properties (such as particle morphology, particle size distribution, and surface properties) and electrochemical characteristics. If these differences are not effectively coordinated, they often restrict the optimization of electrode microstructure, affect the lithium-ion transport efficiency in the electrode, and may trigger a series of side reactions, ultimately making it difficult to simultaneously achieve ideal fast-charging performance, process adaptability, and long-term cycle stability. Therefore, developing a cathode technology that can systematically solve the above contradictions and achieve synergistic optimization of multiple performance indicators is currently the main technical obstacle facing this field. Summary of the Invention

[0005] In view of this, the present invention provides a positive electrode sheet for a lithium-ion battery and a method for preparing the same, as well as a lithium-ion battery. The present invention achieves high energy density, a smooth charging voltage platform and excellent safety performance through the synergistic effect of lithium iron phosphate, lithium nickel cobalt manganese oxide and lithium manganese iron phosphate.

[0006] In a first aspect, the present invention provides a positive electrode sheet for a lithium-ion battery, comprising a current collector and a composite positive electrode active material, a binder, and a conductive agent provided on the current collector; The composite positive electrode active material comprises lithium iron phosphate, lithium nickel cobalt manganese oxide, and lithium manganese iron phosphate with a mass ratio of (80~90) : (5~15) : (3~8), wherein the particle size relationship satisfies: D3 < D1 < D2; D1 is the D50 particle size of lithium iron phosphate, D2 is the D50 particle size of lithium nickel cobalt manganese oxide, and D3 is the D50 particle size of lithium manganese iron phosphate.

[0007] Preferably, in the composite positive electrode active material, the BET specific surface area satisfies: BET2 < BET1 < BET3; where BET1 is the BET specific surface area of lithium iron phosphate, BET2 is the BET specific surface area of lithium nickel cobalt manganese oxide, and BET3 is the BET specific surface area of lithium manganese iron phosphate.

[0008] Further, the BET specific surface area of lithium iron phosphate is 10~16m 1-x ,

[0013] , , / g, the BET specific surface area of lithium nickel cobalt manganese oxide is 0.3~0.8m 2 / g, and the BET specific surface area of lithium manganese iron phosphate is 15~25m 2 / g.

[0009] Further, the composite positive electrode active material satisfies the following relational expression: 80 ≤ (D2 / D1) × (BET3 / BET1)× (W1 / (W2 + W3)) × (D1 / D3) ≤ 400, where W1 is the mass fraction of lithium iron phosphate, W2 is the mass fraction of lithium nickel cobalt manganese oxide, W3 is the mass fraction of lithium manganese iron phosphate, and W1, W2, and W3 are all expressed in decimal form.

[0010] Preferably, the D50 particle size of lithium iron phosphate is 0.5~3μm, the D50 particle size of lithium nickel cobalt manganese oxide is 8~20μm, and the D50 particle size of lithium manganese iron phosphate is 0.5~1.5μm.

[0011] Preferably, the chemical formula of lithium nickel cobalt manganese oxide is LiNi x Co y Mn 1-x-y O4, where x is 0.79~0.9 and y is 0.05~0.2.

[0012] Preferably, the chemical formula of lithium manganese iron phosphate is LiMn x Fe 1-x PO4, where x is 0.4~0.8.

[0013] Preferably, the mass ratio of the composite positive electrode active material, the binder, and the conductive agent is (92~98) : (1~4) : (1~4).

[0014] Secondly, the present invention provides a method for preparing the above-mentioned positive electrode sheet of a lithium-ion battery, comprising the following steps: A positive electrode slurry is prepared by mixing lithium iron phosphate, lithium nickel cobalt manganese oxide, lithium manganese iron phosphate, conductive agent, and binder, which is then coated onto a current collector, dried, and rolled to obtain the final product.

[0015] Thirdly, the present invention provides a lithium-ion battery, comprising the above-described lithium-ion battery positive electrode sheet or the lithium-ion battery positive electrode sheet prepared by the above-described preparation method.

[0016] Compared with the prior art, the present invention has achieved the following beneficial effects: (1) This invention combines lithium iron phosphate, lithium nickel cobalt manganese oxide and lithium manganese iron phosphate in a specific mass ratio of (80~90): (5~15): (3~8) and strictly controls the particle size of the three active materials to satisfy the gradient relationship of D3<D1<D2. It successfully improves the energy density of the battery while maintaining the inherent safety of the lithium iron phosphate system, effectively filling the technical gap of existing power batteries in the critical range of 190~230 Wh / kg. At the same time, the specific particle size gradient distribution optimizes the particle size distribution of the electrode, enabling the positive electrode sheet to achieve high compaction density, providing a structural basis for high volumetric energy density.

[0017] (2) The present invention utilizes lithium nickel cobalt manganese oxide with low specific surface area to reduce side reactions with electrolyte, improve the processability and high temperature storage stability of battery; while lithium iron phosphate and lithium manganese iron phosphate with high specific surface area provide abundant reactive sites, which together contribute to excellent rate performance, enabling battery to have fast charging and fast discharging capability; the synergistic effect of the three characteristics ultimately achieves a good combination of high energy density, high rate performance and long cycle life.

[0018] (3) By selecting specific cathode materials, optimizing their proportions, and designing physical properties (particle size, specific surface area) in a coordinated manner, the present invention provides cathode plates and batteries that overcome the problem of the inflection point of the charging voltage curve when lithium iron phosphate is mixed with ternary materials. This makes the charging and discharging process smoother, reduces the DC internal resistance (DCR) and the temperature rise during the charging and discharging process, and improves the rate performance and fast charging performance of the battery. In addition, the preparation method of this scheme is simple and easy to scale up, and has significant industrial application value. Attached Figure Description

[0019] The accompanying drawings, which form part of this specification, are used to provide a further understanding of the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention and do not constitute an undue limitation thereof. Obviously, those skilled in the art can obtain other drawings based on these drawings without any inventive effort.

[0020] Figure 1 These are the charging curves of lithium-ion batteries assembled with positive electrode sheets of Embodiment 1 and Comparative Example 2 of the present invention at 1C. Detailed Implementation

[0021] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0022] This invention provides a positive electrode sheet for a lithium-ion battery, comprising a current collector and a composite positive electrode active material, a binder, and a conductive agent disposed on the current collector; The composite positive electrode active material includes lithium iron phosphate, lithium nickel cobalt manganese oxide, and lithium manganese iron phosphate in a mass ratio of (80~90): (5~15): (3~8), wherein the particle size relationship satisfies: D3 < D1 < D2; D1 is the D50 particle size of lithium iron phosphate, D2 is the D50 particle size of lithium nickel cobalt manganese oxide, and D3 is the D50 particle size of lithium manganese iron phosphate.

[0023] In this invention, D50 particle size refers to the median diameter in the particle size distribution, specifically the particle size corresponding to 50% of the cumulative volume or mass in a particle group, which can be determined by laser diffraction.

[0024] The above-mentioned technical solution of the present invention, through the organic combination of a specific mass ratio and a strict particle size gradient relationship, works together to affect the microstructure of the electrode, lithium-ion transport kinetics and macroscopic electrochemical performance, thereby solving the technical contradictions that are difficult to overcome by single materials or conventional mixed systems.

[0025] First, from the perspective of the synergy between material function distribution and mass ratio, the ratio of (80~90) : (5~15) : (3~8) constitutes a stable system with lithium iron phosphate (LFP) with extremely high safety as the structural matrix and capacity basis, lithium nickel cobalt manganese oxide (NCM) with high capacity as the main energy density contributor, and lithium manganese iron phosphate (LMFP) with a high voltage platform as the "modulator" of electrochemical performance. Under this ratio, the absolute dominant position of LFP ensures that the battery system inherits the inherent thermal stability and cycle life of lithium iron phosphate, which is the safety cornerstone; while restricting the proportion of NCM within 15%, it can provide a significant capacity increase, and at the same time, due to its low content, it will not have an obvious impact on the safety of the entire system, avoiding the thermal runaway risk brought by high-nickel ternary materials. At the same time, the introduction of LMFP can not only provide additional capacity, but more importantly, use its voltage platform between LFP and NCM to play a smoothing role on the charge-discharge curve, effectively improving the voltage platform inflection point that may occur when LFP and NCM are mixed, making the charging process smoother, which is beneficial to reducing polarization and improving fast charging performance.

[0026] Second, the specific particle size gradient of D3<D1<D2 in the present invention is the key to achieving the above-mentioned functional synergy and further optimizing the electrode microstructure. Among them, the large-particle NCM particles (D2) constitute the structural skeleton of the electrode active layer, ensuring that the electrode has good pore structure and electrolyte wettability; the medium-particle LFP particles (D1) are filled in some of the voids between the large-particle NCM, increasing the filling amount of the active material; and the smaller-particle LMFP particles (D3) further fill the tiny voids between the LFP and NCM particles. This multi-level filling mode can significantly improve the compaction density of the electrode under the same process conditions, thus directly contributing to the improvement of the volume energy density. In addition, the small-particle LMFP and the LFP with a high specific surface area are distributed around the large-particle NCM, which can provide more lithium ion migration channels for it, being beneficial to improving the rate performance under high current, while the large-particle and low-specific surface area NCM itself also helps to reduce the contact area with the electrolyte, inhibit the occurrence of side reactions, and improve the storage and cycle stability.

[0027] In an optional embodiment of the present invention, in the composite cathode active material, the BET specific surface area satisfies: BET2 < BET1 < BET3; where BET1 is the BET specific surface area of lithium iron phosphate, BET2 is the BET specific surface area of lithium nickel cobalt manganese oxide, and BET3 is the BET specific surface area of lithium manganese iron phosphate. The low specific surface area of lithium nickel cobalt manganese oxide (NCM, BET2) means that it has fewer surface atomic active sites, which can effectively reduce its side reactions with the electrolyte at high voltages, especially inhibiting the dissolution of transition metal elements such as nickel and cobalt, thereby significantly improving the high-temperature storage performance and cycle life of the battery. At the same time, the high specific surface area of lithium manganese iron phosphate (LMFP, BET3) provides extremely rich active sites for lithium ion insertion / extraction, which greatly improves the reaction kinetics of the electrode and enables it to be competent for fast charging and discharging (high rate performance). The specific surface area of lithium iron phosphate (LFP, BET1) is between the two, which not only provides better rate performance than NCM but also avoids the increase in side reactions and the loss of first efficiency that may be brought by too high specific surface area. Further, the BET specific surface area of lithium iron phosphate is 10~16m 2 / g, the BET specific surface area of lithium nickel cobalt manganese oxide is 0.3~0.8m 2 / g, and the BET specific surface area of lithium manganese iron phosphate is 15~25m 2 / g.

[0028] Further, the composite cathode active material satisfies the following relational expression: 80 ≤ (D2 / D1) × (BET3 / BET1)× (W1 / (W2 + W3)) × (D1 / D3) ≤ 400, where W1 is the mass fraction of lithium iron phosphate, W2 is the mass fraction of lithium nickel cobalt manganese oxide, and W3 is the mass fraction of lithium manganese iron phosphate, and W1, W2, and W3 are all expressed in decimal form. The above relational expression constructs a quantitative model for comprehensively evaluating the synergistic effect of the cathode material system by coupling the particle size gradient, surface activity, and mass ratio. Among them, (D2 / D1) reflects the basic structural relationship between the large-particle lithium nickel cobalt manganese oxide forming the electrode skeleton and the small-particle lithium iron phosphate for filling; (BET3 / BET1) characterizes the synergistic enhancement degree of the high-surface-activity lithium manganese iron phosphate and the substrate lithium iron phosphate in providing lithium ion migration channels; (W1 / (W2 + W3)) ensures the main position of the high-safety lithium iron phosphate in the composite system; the term (D1 / D3) strengthens the constraint on the particle size gradient relationship between lithium iron phosphate and lithium manganese iron phosphate to ensure the formation of an optimized particle size distribution structure. When the value of this relational expression is within the range of 80 - 400, it indicates that each material has achieved the best balance in structural construction, reaction kinetics, and safety performance, and can simultaneously achieve the synergistic improvement effect of high tap density, excellent rate performance, and long cycle life.

[0029] In this invention, the BET specific surface area is determined by the BET (Brunauer-Emmett-Teller) nitrogen adsorption method, which involves allowing nitrogen gas to adsorb onto the sample surface at liquid nitrogen temperature and calculating the surface area based on the BET adsorption isotherm model.

[0030] In optional embodiments of the present invention, the D50 particle size of lithium iron phosphate is 0.5~3μm, for example, it can be 0.5μm, 1μm, 2μm, 3μm, etc.; the D50 particle size of lithium nickel cobalt manganese oxide is 8~20μm, for example, it can be 8μm, 10μm, 14μm, 16μm, 18μm, 20μm, etc.; the D50 particle size of lithium manganese iron phosphate is 0.5~1.5μm, for example, it can be 0.5μm, 0.8μm, 1.0μm, 1.2μm, 1.5μm, etc.

[0031] In this invention, the D50 particle size of lithium iron phosphate, lithium nickel cobalt manganese oxide, and lithium manganese iron phosphate can be controlled by synthesis methods known in the art (such as solid-state methods, co-precipitation methods, and hydrothermal methods), and further refined through airflow milling and airflow classification to obtain the particle size distribution required by this invention. The BET specific surface area of ​​lithium iron phosphate, lithium nickel cobalt manganese oxide, and lithium manganese iron phosphate can be achieved by adjusting sintering process parameters (such as temperature, time, and atmosphere) and controlling the morphology and pore structure of the particles during the precursor stage.

[0032] In an optional embodiment of the present invention, the chemical formula of lithium nickel cobalt manganese oxide is LiNi. x Co y Mn 1-x-y O4, where x is 0.79~0.9 and y is 0.05~0.2. In one or more embodiments of the present invention, the chemical formula of lithium nickel cobalt manganese oxide is LiNi. 0.8 Co 0.1 Mn 0.1 O4. In an optional embodiment of the present invention, the chemical formula of lithium manganese iron phosphate is LiMn. x Fe 1-x PO4, where x is 0.4~0.8. In one or more embodiments of the present invention, the chemical formula of lithium manganese iron phosphate is LiMn. 0.6 Fe 0.4 PO4.

[0033] In an optional embodiment of the present invention, the mass ratio of the composite positive electrode active material, binder, and conductive agent is (92~98):(1~4):(1~4), more preferably (96~97.5):(1.5~2.5):(1~1.5). The present invention does not impose special restrictions on the selection of binder, conductive agent, and current collector; commonly used binders, conductive agents, and current collectors in the art can be used. Binders include, but are not limited to, at least one of: polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polymethyl methacrylate (PMMA), polypropylene (PP), polyacrylate, polyvinylidene fluoride hexafluoropropylene (PVDF-HFP), sodium carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), or nitrile rubber. Conductive agents include, but are not limited to, at least one of: conductive carbon black, Ketjen black, carbon nanotubes, graphene, conductive graphite, or conductive carbon fiber.

[0034] In an optional embodiment of the present invention, the compaction density of the lithium-ion battery positive electrode sheet is 2.7~2.9 g / cm³. 3 High compaction density means that more active material can be packed into the same volume, thus directly and significantly improving the volumetric energy density of the battery.

[0035] The present invention also provides a method for preparing the above-mentioned positive electrode sheet of lithium-ion battery, comprising the following steps: A positive electrode slurry is prepared by mixing lithium iron phosphate, lithium nickel cobalt manganese oxide, lithium manganese iron phosphate, conductive agent, and binder, which is then coated onto a current collector, dried, and rolled to obtain the final product.

[0036] This invention does not impose special restrictions on the specific process conditions in the preparation process; commonly used process conditions in the field can be used.

[0037] The present invention provides a lithium-ion battery, comprising the above-described lithium-ion battery positive electrode sheet or the lithium-ion battery positive electrode sheet prepared by the above-described preparation method.

[0038] This invention does not impose any special restrictions on the specific preparation method of lithium-ion batteries; any commonly used lithium-ion battery preparation method in this field can be used.

[0039] The technical solution of the present invention will be further described below with reference to specific embodiments. The present invention does not impose any special restrictions on the source of reagents used in the following embodiments; commercially available products well known to those skilled in the art can be used.

[0040] Example 1 This embodiment provides a positive electrode sheet for a lithium-ion battery and its preparation method.

[0041] A composite positive electrode active material was prepared using lithium iron phosphate, lithium nickel cobalt manganese oxide, and lithium manganese iron phosphate in a mass ratio of 80:10:10. The lithium iron phosphate had a particle size D50 of 1 μm and a BET specific surface area of ​​13 m². 2 / g; Lithium nickel cobalt manganese oxide (LiNi 0.8 Co 0.1 Mn 0.1 The particle size D50 of O4 is 12 μm, and the BET specific surface area is 0.6 m². 2 / g; Lithium manganese iron phosphate (LiMn) 0.6 Fe 0.4 The particle size D50 of PO4 is 0.6 μm, and the BET specific surface area is 20 m². 2 / g.

[0042] A composite positive electrode active material, conductive carbon, carbon nanotubes, and PVDF were mixed in a mass ratio of 96.8:1.5:0.5:1.2, and N-methylpyrrolidone was added to prepare a positive electrode slurry with a solid content of 50 wt%. The positive electrode slurry was coated onto the surface of a 14 μm thick carbon-coated aluminum foil and dried, ensuring that the weight of the coating after drying was 200 g / m². 2 With a compaction density of 2.8 g / cm³ 3 Rolling is performed to obtain the rolled lithium-ion battery positive electrode sheet.

[0043] Example 2 This embodiment provides a positive electrode sheet for a lithium-ion battery and its preparation method.

[0044] A composite positive electrode active material was selected using lithium iron phosphate, lithium nickel cobalt manganese oxide, and lithium manganese iron phosphate in a mass ratio of 80:15:5. The lithium iron phosphate had a particle size D50 of 1.2 μm and a BET specific surface area of ​​12 m². 2 / g; Lithium nickel cobalt manganese oxide (LiNi 0.8 Co 0.1 Mn 0.1 The particle size D50 of O4 is 12 μm, and the BET specific surface area is 0.6 m². 2 / g; Lithium manganese iron phosphate (LiMn) 0.6 Fe 0.4 The particle size D50 of PO4 is 0.8 μm, and the BET specific surface area is 18 m². 2 / g.

[0045] A composite positive electrode active material, conductive carbon, carbon nanotubes, and PVDF were mixed in a mass ratio of 96.8:1.5:0.5:1.2, and N-methylpyrrolidone was added to prepare a positive electrode slurry with a solid content of 50 wt%. The positive electrode slurry was coated onto the surface of a 14 μm thick carbon-coated aluminum foil and dried, ensuring that the weight of the coating after drying was 200 g / m². 2With a compaction density of 2.8 g / cm³ 3 Rolling is performed to obtain the rolled lithium-ion battery positive electrode sheet.

[0046] Example 3 This embodiment provides a positive electrode sheet for a lithium-ion battery and its preparation method.

[0047] A composite positive electrode active material was prepared using lithium iron phosphate, lithium nickel cobalt manganese oxide, and lithium manganese iron phosphate in a mass ratio of 90:5:5. The lithium iron phosphate had a particle size D50 of 1 μm and a BET specific surface area of ​​13 m². 2 / g; Lithium nickel cobalt manganese oxide (LiNi 0.8 Co 0.1 Mn 0.1 The particle size D50 of O4 is 12 μm, and the BET specific surface area is 0.6 m². 2 / g; Lithium manganese iron phosphate (LiMn) 0.6 Fe 0.4 The particle size D50 of PO4 is 0.5 μm, and the BET specific surface area is 22 m². 2 / g.

[0048] A composite positive electrode active material, conductive carbon, carbon nanotubes, and PVDF were mixed in a mass ratio of 96.8:1.5:0.5:1.2, and N-methylpyrrolidone was added to prepare a positive electrode slurry with a solid content of 50 wt%. The positive electrode slurry was coated onto the surface of a 14 μm thick carbon-coated aluminum foil and dried, ensuring that the weight of the coating after drying was 200 g / m². 2 With a compaction density of 2.8 g / cm³ 3 Rolling is performed to obtain the rolled lithium-ion battery positive electrode sheet.

[0049] Comparative Example 1 The difference between this comparative example and Example 1 is that the mass ratio of lithium iron phosphate, lithium nickel cobalt manganese oxide, and lithium manganese iron phosphate in this comparative example is 70:15:15.

[0050] Comparative Example 2 The difference between this comparative example and Example 1 is that the composite positive electrode active material in this comparative example does not contain lithium iron manganese phosphate, and the mass ratio of lithium iron phosphate to lithium nickel cobalt manganese oxide is 75:25.

[0051] Comparative Example 3 The difference between this comparative example and Example 1 is that the lithium iron phosphate in this comparative example has a particle size D50 of 0.5 μm and a BET specific surface area of ​​18 m². 2 / g; Lithium manganese iron phosphate (LiMn) 0.6 Fe 0.4 The particle size D50 of PO4 is 1 μm, and the BET specific surface area is 14 m². 2 / g.

[0052] Comparative Example 4 The difference between this comparative example and Example 1 is that the mass ratio of lithium iron phosphate, lithium nickel cobalt manganese oxide, and lithium manganese iron phosphate in this comparative example is 10:10:80.

[0053] The parameters and calculation results of formula (I) for Examples 1-3 and Comparative Examples 1-4 are shown in Table 1.

[0054] Equation (I) = (D2 / D1) × (BET3 / BET1) × (W1 / (W2+ W3)) × (D1 / D3). The final value of Equation (I) is dimensionless. In the calculation, D1 is the D50 particle size of lithium iron phosphate in "μm"; D2 is the D50 particle size of lithium nickel cobalt manganese oxide in "μm"; D3 is the D50 particle size of lithium manganese iron phosphate in "μm"; BET1 is in "m 2 The BET specific surface area of ​​lithium iron phosphate is expressed in units of " / g", while BET3 is expressed in "m²". 2 The BET specific surface area of ​​lithium manganese iron phosphate is expressed in units of / g, W1 is the mass fraction of lithium iron phosphate in decimal form, W2 is the mass fraction of lithium nickel cobalt manganese oxide in decimal form, and W3 is the mass fraction of lithium manganese iron phosphate in decimal form.

[0055] Table 1. Parameters and calculation results of Equation (I) for Examples 1-3 and Comparative Examples 1-4

[0056] Test case Preparation of lithium-ion pouch batteries: Artificial graphite, conductive carbon, carbon nanotubes, CMC, and SBR were mixed in a mass ratio of 93:2:2:1.5:1.5. Deionized water was then added in a high-speed mixer and the mixture was uniformly mixed to form a negative electrode slurry with a solid content of 48 wt%. This negative electrode slurry was coated onto one side of an 8-micrometer-thick copper foil and dried, maintaining a dry coating weight of 43.75 g / m². 2 Then, the same process is applied to the other side of the copper foil, followed by coating, drying, and rolling to obtain the negative electrode sheet. The negative electrode sheet is then processed and welded to the exposed metal foil portion of the positive electrode sheet of the lithium-ion battery in Examples 1-3 and Comparative Examples 1-4 to form an electrode tab, which is then wound with a separator to form a core. After wrapping the core with aluminum-plastic film to form a semi-finished cell, an electrolyte (1 M LiPF6 dissolved in a mixed solvent of EC and DEC at a volume ratio of 1:1) is injected, and the finished lithium-ion pouch battery is obtained through formation and capacity testing.

[0057] 1. Mass energy density test: The prepared pouch cell was charged at 25°C with a constant current of 0.2C to the upper limit voltage (4.25V), then switched to constant voltage charging until the current dropped to 0.05C. After resting for 5 minutes, it was discharged at a constant current of 0.2C to the cutoff voltage (2.5V). The initial discharge capacity (Ah) and the average discharge voltage (V) were recorded. The calculation formula is: Mass energy density (Wh / kg) = Discharge capacity (Ah) × Average discharge voltage (V) / Battery mass (kg).

[0058] 2. Rate performance and charging curve: At 25℃, the battery was charged to 4.25V at different rates (0.2C, 0.5C, 1C, 2C) using a constant current, and the 1C constant current charging curve was recorded. Then, it was discharged to 2.5V using a constant current of 0.2C. The capacity retention rate at 2C / 0.2C rates = (2C constant current charging capacity / 0.2C constant current charging capacity) × 100%.

[0059] 3. DC internal resistance (DCR) test: The battery was charged to 50% state of charge (SOC) at 25°C. The DC internal resistance (DCR) at 50% SOC was recorded before cycling and after 100 cycles. The test method was as follows: a 1C discharge pulse lasting 10 seconds was applied, and the voltage transient at the start of the pulse was recorded. The DCR was calculated using ΔV / ΔI. The calculation formula is as follows: DCR change rate = [(DCR after cycling - DCR before cycling) / DCR before cycling] × 100%.

[0060] 4. Cycle life test: The battery was placed in a constant temperature environment of 25°C and charged at a constant current and constant voltage of 1C to 4.25V (cutoff current 0.05C), and then discharged at a constant current of 1C to 2.5V. This process is counted as one cycle. The test was continued until the battery's discharge capacity decayed to 80% of its initial capacity (C0). The total number of cycles the battery underwent at this point was recorded, which directly reflects the battery's long-term durability.

[0061] 5. Differential Scanning Calorimetry (DSC) Test: The positive electrode plate, fully charged (100% SOC), was removed from the battery. A small sample of the positive electrode active material layer was carefully scraped off within an argon-protected glove box. The sample was sealed in a high-pressure crucible to prevent contact with air during testing. Testing was conducted under a nitrogen atmosphere at a temperature range of 25°C to 400°C, with a heating rate of 5°C / min. The initial peak temperature at which the material underwent a violent exothermic reaction was recorded (denoted as the "thermal runaway initiation temperature"). A higher temperature indicates better thermal stability of the material and superior battery safety performance.

[0062] The test results are summarized in Table 2.

[0063] Table 2 Electrochemical performance test results of the batteries assembled in the examples and comparative examples

[0064] As can be seen from Table 2, Examples 1-3 all exhibit excellent comprehensive performance: not only achieving a high energy density of 205-219 Wh / kg, but also having a capacity retention rate of over 89.5% under 2C fast charging, a cycle life exceeding 2800 cycles, a DCR growth controlled within 10%, and a thermal runaway onset temperature higher than 228°C, reflecting a perfect balance of high energy density, excellent fast charging performance, long cycle life and high safety.

[0065] In contrast, each comparative example has obvious defects due to deviating from the specific parameter combinations of the present invention: Although Comparative Example 1 has the highest energy density (225.61 Wh / kg), its thermal runaway onset temperature is the lowest (218°C) and the cycle life is significantly attenuated to 1654 cycles due to the excessive NCM content, proving that excessive NCM will seriously sacrifice safety and cycle stability. In Comparative Example 2, due to the lack of lithium iron manganese phosphate component, the capacity retention rate under 2C fast charging drops sharply to 68.9%. Figure 1 Figure 12 shows the charging curves of the batteries of Example 1 and Comparative Example 2 at 1C. It can be seen that the charging curve of Comparative Example 2 without LMFP has an obvious voltage plateau inflection point, while the charging curve of Example 1 is smoother, fully verifying the irreplaceable role of LMFP in smoothing the voltage plateau and improving fast charging performance. In Comparative Example 3, due to the destruction of the key particle size relationship (D3<D1), its energy density (192.38 Wh / kg) and rate performance (83.6%) are both significantly deteriorated, indicating that a specific particle size gradient is a prerequisite for achieving high tap density and optimizing the ion transport path. Although Comparative Example 4 has the best thermal safety, its energy density is the lowest (182.46 Wh / kg) because it is mainly composed of low-capacity LMFP, proving that using LFP as the matrix is the core of balancing energy density and safety.

[0066] The above are only the preferred embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, the present invention can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A positive electrode sheet for a lithium-ion battery, characterized in that, Includes a current collector and a composite positive electrode active material disposed on the current collector, a binder and a conductive agent; The composite positive electrode active material includes lithium iron phosphate, lithium nickel cobalt manganese oxide, and lithium manganese iron phosphate in a mass ratio of (80~90): (5~15): (3~8), wherein the particle size relationship satisfies: D3 < D1 < D2; D1 is the D50 particle size of lithium iron phosphate, D2 is the D50 particle size of lithium nickel cobalt manganese oxide, and D3 is the D50 particle size of lithium manganese iron phosphate.

2. The lithium-ion battery positive electrode sheet as described in claim 1, characterized in that, In the composite positive electrode active material, the BET specific surface area satisfies: BET2 < BET1 < BET3; Wherein, BET1 is the BET specific surface area of ​​lithium iron phosphate, BET2 is the BET specific surface area of ​​lithium nickel cobalt manganese oxide, and BET3 is the BET specific surface area of ​​lithium manganese iron phosphate.

3. The lithium-ion battery positive electrode sheet as described in claim 2, characterized in that, The BET specific surface area of ​​lithium iron phosphate is 10~16m². 2 The BET specific surface area of ​​lithium nickel cobalt manganese oxide is 0.3~0.8 m² / g. 2 / g, the BET specific surface area of ​​lithium manganese iron phosphate is 15~25m². 2 / g.

4. The lithium-ion battery positive electrode sheet as described in claim 2, characterized in that, The composite positive electrode active material satisfies the following relationship: 80≤(D2 / D1) × (BET3 / BET1) × (W1 / (W2 + W3)) × (D1 / D3)≤400, where W1 is the mass fraction of lithium iron phosphate, W2 is the mass fraction of lithium nickel cobalt manganese oxide, and W3 is the mass fraction of lithium manganese iron phosphate. W1, W2 and W3 are all expressed in decimal form.

5. The positive electrode of a lithium-ion battery as described in claim 1, characterized in that, The D50 particle size of lithium iron phosphate is 0.5~3μm, that of lithium nickel cobalt manganese oxide is 8~20μm, and that of lithium manganese iron phosphate is 0.5~1.5μm.

6. The positive electrode of a lithium-ion battery as described in claim 1, characterized in that, The chemical formula of lithium nickel cobalt manganese oxide is LiNi x Co y Mn 1-x-y O4, where x is 0.79~0.9 and y is 0.05~0.

2.

7. The positive electrode of a lithium-ion battery as described in claim 1, characterized in that, The chemical formula of lithium manganese iron phosphate is LiMn x Fe 1-x PO4, where x is 0.4 to 0.

8.

8. The positive electrode of a lithium-ion battery as described in claim 1, characterized in that, The mass ratio of the composite positive electrode active material, binder and conductive agent is (92~98): (1~4): (1~4).

9. The method for preparing the positive electrode sheet of a lithium-ion battery according to any one of claims 1 to 8, characterized in that, Includes the following steps: A positive electrode slurry is prepared by mixing lithium iron phosphate, lithium nickel cobalt manganese oxide, lithium manganese iron phosphate, conductive agent, and binder, which is then coated onto a current collector, dried, and rolled to obtain the final product.

10. A lithium-ion battery, characterized in that, This includes the lithium-ion battery positive electrode sheet according to any one of claims 1 to 8 or the lithium-ion battery positive electrode sheet prepared by the preparation method according to claim 9.

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