Positive electrode sheet and preparation method therefor, and battery

Abstract

Provided in the present application are a positive electrode sheet and a preparation method therefor, and a battery. The positive electrode sheet comprises a current collector, a first active material layer and a second active material layer, wherein the first active material layer comprises lithium manganese iron phosphate, a first electrically conductive agent and a first lithium-supplementing agent, and the second active material layer comprises lithium manganese iron phosphate, a second electrically conductive agent and a second lithium-supplementing agent. The gram capacity K1 of the first lithium-supplementing agent is greater than the gram capacity K2 of the second lithium-supplementing agent; and the content D1 of the first electrically conductive agent is greater than the content D2 of the second electrically conductive agent.

Description

Positive electrode sheet and its preparation method, and battery

[0001] This application claims priority to Chinese Patent Application No. 202411720673.4, filed with the Chinese Patent Office on November 27, 2024, the entire contents of which are incorporated herein by reference. Technical Field

[0002] This application relates to the field of battery technology, specifically to a positive electrode sheet, a method for preparing a positive electrode sheet, and a battery. Background Technology

[0003] Lithium manganese iron phosphate (LMFP) is a positive electrode active material made by adding a certain proportion of manganese to lithium iron phosphate (LFP). LMFP and LFP both belong to the olivine structure and have the same theoretical specific capacity. However, LMFP has a higher voltage plateau, typically reaching around 4.1V, higher than LFP's 3.3V–3.5V. Therefore, under the same conditions, the theoretical energy density of LMFP is 15%–20% higher than that of LFP. Technical issues

[0004] However, compared to LFP, LMFP has poorer electronic conductivity and lithium-ion diffusion rate, preventing it from fully utilizing its capacity. Furthermore, LMFP has a poor cycle life, especially at high temperatures compared to LFP. To address these shortcomings, lithium replenishment in the cathode is necessary. Current technologies for lithium replenishment in LMFP systems are relatively simple, making it difficult for the replenishing agents to function effectively. Consequently, the capacity of the LMFP is not fully utilized, and the cycle life of the battery is difficult to improve. Technical solutions

[0005] This application provides a positive electrode sheet. The positive electrode sheet includes: a current collector and a first active material layer and a second active material layer disposed on the current collector, wherein the first active material layer and the second active material layer are sequentially distributed in a direction away from the current collector; both the first active material layer and the second active material layer contain lithium manganese iron phosphate; the first active material layer further contains a first conductive agent and a first lithium replenishing agent, and the second active material layer further contains a second conductive agent and a second lithium replenishing agent; the specific capacity K1 of the first lithium replenishing agent is greater than the specific capacity K2 of the second lithium replenishing agent; the mass percentage D1 of the first conductive agent in the first active material layer is greater than the mass percentage D2 of the second conductive agent in the second active material layer.

[0006] This application also provides a method for preparing a positive electrode sheet. The method for preparing the positive electrode sheet includes:

[0007] A first slurry is provided, comprising lithium manganese iron phosphate, a first conductive agent, and a first lithium replenishing agent;

[0008] A second slurry is provided, comprising lithium manganese iron phosphate, a second conductive agent, and a second lithium supplementing agent;

[0009] The first slurry is subjected to film formation treatment on the current collector to obtain the first active material layer;

[0010] The second slurry is formed into a film on the side of the first active material layer away from the current collector to obtain the second active material layer, and then the positive electrode sheet is obtained.

[0011] This application also provides a battery. The battery includes the above-described positive electrode sheet; or, the positive electrode sheet prepared by the above method. Beneficial effects

[0012] The positive electrode sheet provided in this application features a layered design of active material layers. In the first active material layer near the current collector, the specific capacity of the first lithium replenishing agent is relatively large, and the mass percentage of the first conductive agent is relatively high. Conversely, in the second active material layer further away from the current collector, the specific capacity of the second lithium replenishing agent is relatively small, and the mass percentage of the second conductive agent is relatively low. Thus, for the same mass, the first lithium replenishing agent can provide more lithium ions than the second lithium replenishing agent. Furthermore, compared to the second active material layer, the first active material layer has a denser conductive network. This results in lower lithium ion diffusion resistance during battery formation (i.e., the first charge-discharge process) when the first lithium replenishing agent releases a large amount of lithium ions to the negative electrode, optimizing lithium ion transport and promoting the utilization of the lithium manganese iron phosphate capacity in the positive electrode sheet, especially the portion of the positive electrode sheet near the current collector, thereby improving the cycle life of the lithium-ion battery. Attached Figure Description

[0013] Figure 1 is a schematic cross-sectional view of the positive electrode sheet provided in an embodiment of this application.

[0014] Explanation of reference numerals in the attached figures: 10, positive electrode sheet; 11, current collector; 12, first active material layer; 13, second active material layer. Embodiments of the present invention

[0015] In the description of this application, unless otherwise expressly specified and limited, the terms "connected," "linked," and "fixed" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this application according to the specific circumstances.

[0016] In this application, unless otherwise expressly specified and limited, "above" or "below" the second feature can include direct contact between the first and second features, or contact between the first and second features through another feature between them. Furthermore, "above," "over," and "on top" of the second feature includes the first feature being directly above or diagonally above the second feature, where the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature includes the first feature being directly below or diagonally below the second feature, where the first feature is at a lower horizontal level than the second feature.

[0017] In the description of this embodiment, the terms "upper," "lower," "left," "right," "front," and "rear," etc., refer to the orientation or positional relationship shown in the accompanying drawings. They are used for ease of description and simplification of operation, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application. Furthermore, the terms "first" and "second" are used for distinction in description and have no special meaning.

[0018] Firstly, please refer to Figure 1. This application provides a positive electrode 10, which is used in a battery, such as a lithium-ion battery.

[0019] Specifically, the positive electrode 10 includes a current collector 11, a first active material layer 12, and a second active material layer 13. The first active material layer 12 and the second active material layer 13 are disposed on the current collector 11 and are sequentially distributed along a direction away from the current collector 11. Both the first active material layer 12 and the second active material layer 13 contain the positive electrode active material lithium manganese iron phosphate (LMFP). The first active material layer 12 also contains a first conductive agent and a first lithium replenishing agent, and the second active material layer 13 also contains a second conductive agent and a second lithium replenishing agent. The specific capacity K1 of the first lithium replenishing agent is greater than the specific capacity K2 of the second lithium replenishing agent; the mass percentage D1 of the first conductive agent in the first active material layer 12 is greater than the mass percentage D2 of the second conductive agent in the second active material layer 13.

[0020] The chemical formula of lithium manganese iron phosphate is LiFe a Mn b PO4, 0 < a < 1, 0 < b < 1. As an example, a is 0.001, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, or 0.999; b is 0.001, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, or 0.999.

[0021] The lithium supplement can compensate for the irreversible lithium loss during the first charge-discharge process. The specific capacity of the lithium supplement refers to the capacity provided by each gram of the lithium supplement in the battery, usually expressed in mAh / g. The specific capacity of the lithium supplement varies depending on the specific composition.

[0022] As an example, lithium ferrite (LFO) is a lithium metal oxide with an antifluorite structure. The chemical formula of lithium ferrite is Li5FeO4, and its theoretical specific capacity can reach 700 mAh / g.

[0023] As an example, lithium nickelate (LNO) has a crystal structure belonging to layered lithium metal oxides. The chemical formula of lithium nickelate is LiNiO2, and its theoretical specific capacity can reach 400 mAh / g.

[0024] As an example, the chemical formula of the lithium-rich manganese-based material (LMR) is xLi2MnO3·(1-x)LiMO2, where 0 < x < 1 and M includes at least one transition metal. The theoretical specific capacity of the lithium-rich manganese-based material can reach 250 mAh / g.

[0025] The specific capacity K1 of the first lithium supplement is greater than the specific capacity K2 of the second lithium supplement. It can be understood that the first lithium supplement and the second lithium supplement are usually different substances.

[0026] The mass percentage content of the first conductive agent in the first active material layer 12 is different from the mass percentage content of the second conductive agent in the second active material layer 13, but the first conductive agent and the second conductive agent can be the same substance or different substances.

[0027] As an example, the first conductive agent and the second conductive agent can each independently include at least one of conductive carbon black, carbon nanotubes (CNT), mesoporous carbon materials, and vapor-grown carbon fibers (VGCF). As an example, conductive carbon black includes at least one of acetylene black, super p (abbreviated as SP), and Ketjen black. As an example, carbon nanotubes include at least one of single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT). As an example, the mesoporous carbon material includes the ordered mesoporous carbon material CMK-3.

[0028] In the embodiment of the present application, the positive electrode sheet 10 is provided by designing the active material layer in layers. Among them, the lithium supplement agent in the first active material layer 12 close to the current collector 11 has a relatively large specific capacity, and the mass percentage content of the first conductive agent is relatively high. While in the second active material layer 13 far from the current collector 11, the second lithium supplement agent has a relatively small specific capacity, and the mass percentage content of the second conductive agent is relatively low. In this way, under the same mass, the first lithium supplement agent can provide more lithium ions than the second lithium supplement agent, and compared with the second active material layer 13, the first active material layer 12 has a denser conductive network. Therefore, during the battery formation process (i.e., the first charge-discharge process), when the first lithium supplement agent releases a large amount of lithium ions to the negative electrode, it has a lower lithium ion diffusion resistance, optimizes the transmission of lithium ions, and further promotes the utilization of the capacity of lithium iron phosphate manganese in the positive electrode sheet 10, especially the utilization of the capacity of lithium iron phosphate manganese in the first active material layer 12, improving the capacity and cycle life of the battery.

[0029] In some embodiments, the first lithium supplement agent includes at least one of LiNiO2 and Li5FeO4. LiNiO2 and Li5FeO4 have a relatively high specific capacity. In a battery with a lithium iron phosphate manganese system, adding a small amount of LiNiO2 and / or Li5FeO4 can effectively improve the energy density of the battery. However, LiNiO2 and Li5FeO4 are unstable, not only having the characteristic of strong water absorption, but also being prone to decomposition when exposed to air for a long time. By adding LiNiO2 and / or Li5FeO4 to the first active material layer 12, and using the current collector 11 and the second active material layer 13 to shield the first active material layer 12, water vapor can be effectively isolated, reducing the risk of inactivation of LiNiO2 and / or Li5FeO4. Optionally, the first lithium supplement agent includes LNO. Compared with LFO, LNO has more stable properties and can more effectively promote the utilization of the capacity of lithium iron phosphate manganese.

[0030] In some embodiments, the second lithium supplement agent includes a lithium-rich manganese-based material, and the chemical formula of the lithium-rich manganese-based material is xLi2MnO3·(1 - x)LiMO2, where 0 < x < 1, and M includes at least one transition metal. Although the specific capacity of the lithium-rich manganese-based material is not as high as that of LiNiO2 and Li5FeO4, the lithium-rich manganese-based material is relatively stable, and the price of the lithium-rich manganese-based material is relatively low, which is beneficial to controlling the production cost of the battery. As an example, x is 0.001, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 or 0.999. As an example, M includes at least one of Ni, Co, and Mn.

[0031] In some embodiments, the ratio P1 of the mass percentage D2 of the second conductive agent in the second active material layer 13 to the mass percentage D1 of the first conductive agent in the first active material layer 12 is less than 0.78. This allows for further optimization of the conductive system while maintaining the total amount of conductive agent, promoting the effective functioning of the lithium supplement. Optionally, P1 is less than 0.67. As examples, P1 is 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, or 0.67.

[0032] In some embodiments, the mass percentage D1 of the first conductive agent in the first active material layer 12 is 0.3 wt% to 1.4 wt%. As examples, D1 is 0.3 wt%, 0.5 wt%, 0.6 wt%, 0.8 wt%, 1.0 wt%, 1.2 wt%, 1.3 wt%, or 1.4 wt%.

[0033] In some embodiments, the mass percentage D2 of the second conductive agent in the second active material layer 13 is 0.15 wt% to 0.7 wt%. As examples, D2 is 0.15 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt%, or 0.7 wt%.

[0034] In some embodiments, the mass percentage B2 of the second lithium replenishing agent in the second active material layer 13 is greater than the mass percentage B1 of the first lithium replenishing agent in the first active material layer 12. Compared to the first active material layer 12, the second active material layer 13 is closer to the surface of the positive electrode 10, thus the lithium ion transport path released by the second lithium replenishing agent is shorter and the diffusion resistance is lower. By increasing the mass percentage B2 of the second lithium replenishing agent in the second active material layer 13, the overall lithium ion transport in the positive electrode 10 can be further optimized.

[0035] In some embodiments, the ratio P2 of the mass percentage of the second lithium replenishing agent B2 in the second active material layer 13 to the mass percentage of the first lithium replenishing agent B1 in the first active material layer 12 is 1.25 to 6. Within this range, the capacity of lithium manganese iron phosphate in both the first active material layer 12 and the second active material layer 13 can be effectively utilized, thereby improving the battery capacity and cycle life. As examples, P2 is 1.25, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, or 6.

[0036] In some embodiments, the mass percentage B1 of the first lithium supplement agent in the first active material layer 12 is 2.0 wt% to 4.0 wt%. As examples, B1 is 2.0 wt%, 2.2 wt%, 2.4 wt%, 2.6 wt%, 2.8 wt%, 3.0 wt%, 3.2 wt%, 3.4 wt%, 3.6 wt%, 3.8 wt%, or 4.0 wt%.

[0037] In some embodiments, the mass percentage of the second lithium supplementer B2 in the second active material layer 13 is 5.0 wt% to 15.0 wt%. As examples, B2 is 5.0 wt%, 5.5 wt%, 6.0 wt%, 6.5 wt%, 7.0 wt%, 7.5 wt%, 8.0 wt%, 8.5 wt%, 9.0 wt%, 9.5 wt%, 10.0 wt%, 10.5 wt%, 11.0 wt%, 11.5 wt%, 12.0 wt%, 12.5 wt%, 13.0 wt%, 13.5 wt%, 14.0 wt%, 14.5 wt%, or 15.0 wt%.

[0038] In some implementations, the first lithium replenisher has an initial charge calibrated capacity greater than 300 mAh / g, and the second lithium replenisher has an initial charge calibrated capacity greater than 100 mAh / g. The initial charge calibrated capacity, also known as the first charge capacity, refers to the ratio of the capacity released by the lithium replenisher during the first charge of the lithium-ion battery to the mass of the lithium replenisher. The initial charge calibrated capacity of the lithium replenisher is only related to the type of lithium replenisher material; that is, once the type of lithium replenisher material is determined, the initial charge calibrated capacity is a fixed value, meaning it is an inherent property of the lithium replenisher itself. For example, the first lithium replenisher includes at least one of LFO and LNO, with LFO having an initial charge calibrated capacity of 590 mAh / g and LNO having an initial charge calibrated capacity of 350 mAh / g. For example, the second lithium replenisher includes LMR, with LMR having an initial charge calibrated capacity of 140 mAh / g.

[0039] In some embodiments, the mass percentage H1 of lithium manganese iron phosphate in the first active material layer 12 is 91.0 wt% to 95.3 wt%. Within this range, the energy density of the battery can be effectively guaranteed. As examples, H1 is 91.0 wt%, 92.0 wt%, 93.0 wt%, 94.0 wt%, or 95.3 wt%.

[0040] In some embodiments, the mass percentage of lithium manganese iron phosphate (H2) in the second active material layer 13 is 80.0 wt% to 92.55 wt%. Within this range, the energy density of the battery can be effectively guaranteed. As examples, H2 is 80.0 wt%, 82.0 wt%, 84.0 wt%, 86.0 wt%, 88.0 wt%, 90.0 wt%, or 92.55 wt%.

[0041] In some embodiments, both the first active material layer 12 and the second active material layer 13 further comprise an adhesive. Optionally, the adhesive comprises at least one of polyacrylic acid (PAA), polyvinylidene fluoride (PVDF), and polyvinyl alcohol (PVA).

[0042] In some embodiments, the binder content N1 in the first active material layer 12 is 1.5 wt% to 2.0 wt%, and the binder content N2 in the second active material layer 13 is 1.5 wt% to 2.0 wt%. As examples, N1 is 1.5 wt%, 1.6 wt%, 1.7 wt%, 1.8 wt%, 1.9 wt%, or 2.0 wt%; and N2 is 1.5 wt%, 1.6 wt%, 1.7 wt%, 1.8 wt%, 1.9 wt%, or 2.0 wt%.

[0043] In some embodiments, both the first active material layer 12 and the second active material layer 13 further comprise a dispersant. Optionally, the dispersant includes at least one selected from acid anhydrides, polyvinylpyrrolidone, sulfate salts, poly(N-vinylacetamide), polyvinyl alcohol, sulfonates, and polyethylene glycol.

[0044] In some embodiments, the content of dispersant F1 in the first active material layer 12 is 0.1 wt% to 0.2 wt%, and the content of dispersant F2 in the second active material layer 13 is 0.1 wt% to 0.2 wt%. As examples, F1 is 0.1 wt%, 0.12 wt%, 0.14 wt%, 0.16 wt%, 0.18 wt%, or 0.2 wt%; and F2 is 0.1 wt%, 0.12 wt%, 0.14 wt%, 0.16 wt%, 0.18 wt%, or 0.2 wt%.

[0045] In some embodiments, the current collector 11 can be an aluminum foil, a carbon-coated aluminum foil, a safety-coated aluminum foil, an etched aluminum foil, or an aluminum mesh. The thickness of the current collector 11 can be 5μm to 20μm, for example, 5μm, 10μm, 15μm, or 20μm.

[0046] Secondly, embodiments of this application also provide a method for preparing a positive electrode sheet, used to prepare the above-mentioned positive electrode sheet 10, comprising:

[0047] S1. Provide a first slurry, the first slurry comprising lithium manganese iron phosphate, a first conductive agent and a first lithium replenishing agent;

[0048] S2. Provide a second slurry, the second slurry comprising lithium manganese iron phosphate, a second conductive agent, and a second lithium replenishing agent;

[0049] S3. The first slurry is subjected to film formation treatment on the current collector to obtain the first active material layer;

[0050] S4. The second slurry is subjected to film formation treatment on the side of the first active material layer away from the current collector to obtain the second active material layer, thereby obtaining the positive electrode sheet.

[0051] In some embodiments, the first slurry further includes a first solvent, and S3 includes:

[0052] S31. The first slurry is coated on the current collector to obtain a first wet film layer;

[0053] S32. The first wet film layer is dried to obtain the first active material layer.

[0054] In some embodiments, the second slurry further includes a second solvent, and S4 includes:

[0055] S41. The second slurry is coated on the first wet film layer to obtain the second wet film layer;

[0056] S42. The second wet film layer is dried to obtain the second active material layer.

[0057] Optionally, the coating process includes, but is not limited to, at least one of gravure coating, microgravure coating, spray coating, and electrospinning techniques. The first solvent and the second solvent include N-methylpyrrolidone (NMP).

[0058] Thirdly, embodiments of this application also provide a battery, including the above-described positive electrode 10; or, a positive electrode prepared by the above method.

[0059] In some implementations, the battery is a lithium-ion battery.

[0060] The following description is based on specific embodiments.

[0061] Example 1

[0062] S1. Preparation of positive electrode: The positive electrode slurry is coated onto the carbon-coated aluminum foil current collector by a double-layer coating method. The upper and lower layers have the same density. After drying, it is cold-pressed into sheets.

[0063] The positive electrode slurry formulation is as follows:

[0064] 1. Upper layer slurry: LMFP (LiFe 0.5 Mn 0.5 The ingredients are PO4 (90.55 wt%), LMR (7 wt%), SP (0.3 wt%), CNT (0.25 wt%), PVDF (1.7 wt%), and dispersant (0.2 wt%). The chemical formula of LMR is 0.6Li2MnO3·0.4LiNiO2. LMR and LMFP are added simultaneously during stirring. LMR is the lithium replenishing agent in the upper layer.

[0065] 2. Lower layer slurry: LMFP (94.7wt%), LFO (2.5wt%), SP (0.6wt%), CNT (0.3wt%), PVDF (1.7wt%), dispersant (0.2wt%), wherein LFO is added simultaneously with LMFP during the stirring process, and LFO is the lithium supplement for the lower layer.

[0066] S2. Preparation of negative electrode: The negative electrode slurry is coated onto copper foil in a single layer, and after drying, it is cold-pressed into sheets.

[0067] The negative electrode slurry formulation is: graphite:SP:CMC:SBR = 96.7:0.6:1.2:1.5 (mass ratio).

[0068] S3. Battery preparation: The positive and negative electrode sheets are stacked to form a core pack. After assembly, the core pack is vacuum baked. After baking, the moisture content is qualified, and then liquid is injected (1M LiPF6 (EC:DEC:DMC=1:1:1, volume ratio)). After liquid injection, the core pack is left to stand for 24 hours to improve its capacity. After the capacity is improved, the core pack is vacuum sealed and the soft-pack cell is off the production line.

[0069] Example 2

[0070] The difference from Example 1 lies in the positive electrode slurry formulation:

[0071] 1. Upper layer slurry: LMFP (90.55wt%), LMR (7wt%), SP (0.3wt%), CNT (0.25wt%), PVDF (1.7wt%), dispersant (0.2wt%), wherein LMR and LMFP are added simultaneously during stirring, and LMR is the lithium supplement for the upper layer;

[0072] 2. Lower layer slurry: LMFP (94.7wt%), LNO (2.5wt%), SP (0.6wt%), CNT (0.3wt%), PVDF (1.7wt%), dispersant (0.2wt%), wherein LNO is added simultaneously with LMFP during the stirring process, and LNO is the lithium supplement for the lower layer.

[0073] Everything else is the same as in Example 1.

[0074] Example 3

[0075] The difference from Example 1 lies in the positive electrode slurry formulation:

[0076] 1. Upper layer slurry: LMFP (90.95wt%), LMR (7wt%), SP (0.1wt%), CNT (0.05wt%), PVDF (1.7wt%), dispersant (0.2wt%), wherein LMR and LMFP are added simultaneously during stirring, and LMR is the lithium supplement for the upper layer.

[0077] 2. Lower layer slurry: LMFP (94.7wt%), LFO (2.5wt%), SP (0.6wt%), CNT (0.3wt%), PVDF (1.7wt%), dispersant (0.2wt%), wherein LFO is added simultaneously with LMFP during the stirring process, and LFO is the lithium supplement for the lower layer.

[0078] Everything else is the same as in Example 1.

[0079] Example 4

[0080] The difference from Example 1 lies in the positive electrode slurry formulation:

[0081] 1. Upper layer slurry: LMFP (90.5wt%), LMR (7wt%), SP (0.4wt%), CNT (0.2wt%), PVDF (1.7wt%), dispersant (0.2wt%), where LMR and LMFP are added simultaneously during stirring, and LMR is the lithium supplement for the upper layer.

[0082] 2. Lower layer slurry: LMFP (94.7wt%), LFO (2.5wt%), SP (0.6wt%), CNT (0.3wt%), PVDF (1.7wt%), dispersant (0.2wt%), wherein LFO is added simultaneously with LMFP during the stirring process, and LFO is the lithium supplement for the lower layer.

[0083] Everything else is the same as in Example 1.

[0084] Example 5

[0085] The difference from Example 1 lies in the positive electrode slurry formulation:

[0086] 1. Upper layer slurry: LMFP (90.4wt%), LMR (7wt%), SP (0.45wt%), CNT (0.25wt%), PVDF (1.7wt%), dispersant (0.2wt%), wherein LMR and LMFP are added simultaneously during stirring, and LMR is the lithium supplement for the upper layer.

[0087] 2. Lower layer slurry: LMFP (94.7wt%), LFO (2.5wt%), SP (0.6wt%), CNT (0.3wt%), PVDF (1.7wt%), dispersant (0.2wt%), wherein LFO is added simultaneously with LMFP during the stirring process, and LFO is the lithium supplement for the lower layer.

[0088] Everything else is the same as in Example 1.

[0089] Example 6

[0090] The difference from Example 1 lies in the positive electrode slurry formulation:

[0091] 1. Upper layer slurry: LMFP (90.55wt%), LMR (7wt%), SP (0.3wt%), CNT (0.25wt%), PVDF (1.7wt%), dispersant (0.2wt%), wherein LMR and LMFP are added simultaneously during stirring, and LMR is the lithium supplement for the upper layer.

[0092] 2. Lower layer slurry: LMFP (94.89wt%), LFO (2.5wt%), SP (0.5wt%), CNT (0.21wt%), PVDF (1.7wt%), dispersant (0.2wt%), wherein LFO is added simultaneously with LMFP during the stirring process, and LFO is the lithium supplement for the lower layer.

[0093] Everything else is the same as in Example 1.

[0094] Example 7

[0095] The difference from Example 1 lies in the positive electrode slurry formulation:

[0096] 1. Upper layer slurry: LMFP (90.55wt%), LMR (7wt%), SP (0.3wt%), CNT (0.25wt%), PVDF (1.7wt%), dispersant (0.2wt%), wherein LMR and LMFP are added simultaneously during stirring, and LMR is the lithium supplement for the upper layer.

[0097] 2. Lower layer slurry: LMFP (94.5wt%), LFO (2.5wt%), SP (0.8wt%), CNT (0.3wt%), PVDF (1.7wt%), dispersant (0.2wt%), wherein LFO is added simultaneously with LMFP during the stirring process, and LFO is the lithium supplement for the lower layer.

[0098] Everything else is the same as in Example 1.

[0099] Example 8

[0100] The difference from Example 1 lies in the positive electrode slurry formulation:

[0101] 1. Upper layer slurry: LMFP (90.55wt%), LMR (7wt%), SP (0.3wt%), CNT (0.25wt%), PVDF (1.7wt%), dispersant (0.2wt%), wherein LMR and LMFP are added simultaneously during stirring, and LMR is the lithium supplement for the upper layer.

[0102] 2. Lower layer slurry: LMFP (94.2wt%), LFO (2.5wt%), SP (0.9wt%), CNT (0.5wt%), PVDF (1.7wt%), dispersant (0.2wt%), wherein LFO is added simultaneously with LMFP during the stirring process, and LFO is the lithium supplement for the lower layer.

[0103] Everything else is the same as in Example 1.

[0104] Example 9

[0105] The difference from Example 1 lies in the positive electrode slurry formulation:

[0106] 1. Upper layer slurry: LMFP (90.95wt%), LMR (7wt%), SP (0.1wt%), CNT (0.05wt%), PVDF (1.7wt%), dispersant (0.2wt%), wherein LMR and LMFP are added simultaneously during stirring, and LMR is the lithium supplement for the upper layer.

[0107] 2. Lower layer slurry: LMFP (95.3wt%), LFO (2.5wt%), SP (0.2wt%), CNT (0.1wt%), PVDF (1.7wt%), dispersant (0.2wt%), wherein LFO is added simultaneously with LMFP during the stirring process, and LFO is the lithium supplement for the lower layer.

[0108] Everything else is the same as in Example 1.

[0109] Example 10

[0110] The difference from Example 1 lies in the positive electrode slurry formulation:

[0111] 1. Upper layer slurry: LMFP (90.4wt%), LMR (7wt%), SP (0.5wt%), CNT (0.2wt%), PVDF (1.7wt%), dispersant (0.2wt%), where LMR and LMFP are added simultaneously during stirring, and LMR is the lithium supplement for the upper layer.

[0112] 2. Lower layer slurry: LMFP (94.4wt%), LFO (2.5wt%), SP (0.9wt%), CNT (0.5wt%), PVDF (1.7wt%), dispersant (0.2wt%), wherein LFO is added simultaneously with LMFP during the stirring process, and LFO is the lithium supplement for the lower layer.

[0113] Everything else is the same as in Example 1.

[0114] Example 11

[0115] The difference from Example 1 lies in the positive electrode slurry formulation:

[0116] 1. Upper layer slurry: LMFP (90.55wt%), LMR (7wt%), SP (0.3wt%), CNT (0.25wt%), PVDF (1.7wt%), dispersant (0.2wt%), wherein LMR and LMFP are added simultaneously during stirring, and LMR is the lithium supplement for the upper layer.

[0117] 2. Lower layer slurry: LMFP (93.2wt%), LFO (4.0wt%), SP (0.6wt%), CNT (0.3wt%), PVDF (1.7wt%), dispersant (0.2wt%), wherein LFO is added simultaneously with LMFP during the stirring process, and LFO is the lithium supplement for the lower layer.

[0118] Everything else is the same as in Example 1.

[0119] Example 12

[0120] The difference from Example 1 lies in the positive electrode slurry formulation:

[0121] 1. Upper layer slurry: LMFP (90.55wt%), LMR (7wt%), SP (0.3wt%), CNT (0.25wt%), PVDF (1.7wt%), dispersant (0.2wt%), wherein LMR and LMFP are added simultaneously during stirring, and LMR is the lithium supplement for the upper layer.

[0122] 2. Lower layer slurry: LMFP (95.2wt%), LFO (2.0wt%), SP (0.6wt%), CNT (0.3wt%), PVDF (1.7wt%), dispersant (0.2wt%), wherein LFO is added simultaneously with LMFP during the stirring process, and LFO is the lithium supplement for the lower layer.

[0123] Everything else is the same as in Example 1.

[0124] Example 13

[0125] The difference from Example 1 lies in the positive electrode slurry formulation:

[0126] 1. Upper layer slurry: LMFP (92.55wt%), LMR (5wt%), SP (0.3wt%), CNT (0.25wt%), PVDF (1.7wt%), dispersant (0.2wt%), wherein LMR and LMFP are added simultaneously during stirring, and LMR is the lithium supplement for the upper layer.

[0127] 2. Lower layer slurry: LMFP (94.7wt%), LFO (2.5wt%), SP (0.6wt%), CNT (0.3wt%), PVDF (1.7wt%), dispersant (0.2wt%), wherein LFO is added simultaneously with LMFP during the stirring process, and LFO is the lithium supplement for the lower layer.

[0128] Everything else is the same as in Example 1.

[0129] Example 14

[0130] The difference from Example 1 lies in the positive electrode slurry formulation:

[0131] 1. Upper layer slurry: LMFP (85.05wt%), LMR (12.5wt%), SP (0.3wt%), CNT (0.25wt%), PVDF (1.7wt%), dispersant (0.2wt%), wherein LMR and LMFP are added simultaneously during stirring, and LMR is the lithium supplement for the upper layer.

[0132] 2. Lower layer slurry: LMFP (94.7wt%), LFO (2.5wt%), SP (0.6wt%), CNT (0.3wt%), PVDF (1.7wt%), dispersant (0.2wt%), wherein LFO is added simultaneously with LMFP during the stirring process, and LFO is the lithium supplement for the lower layer.

[0133] Everything else is the same as in Example 1.

[0134] Example 15

[0135] The difference from Example 1 lies in the positive electrode slurry formulation:

[0136] 1. Upper layer slurry: LMFP (82.55wt%), LMR (15wt%), SP (0.3wt%), CNT (0.25wt%), PVDF (1.7wt%), dispersant (0.2wt%), where LMR and LMFP are added simultaneously during stirring, and LMR is the lithium supplement for the upper layer.

[0137] 2. Lower layer slurry: LMFP (94.7wt%), LFO (2.5wt%), SP (0.6wt%), CNT (0.3wt%), PVDF (1.7wt%), dispersant (0.2wt%), wherein LFO is added simultaneously with LMFP during the stirring process, and LFO is the lithium supplement for the lower layer.

[0138] Everything else is the same as in Example 1.

[0139] Example 16

[0140] The difference from Example 1 lies in the positive electrode slurry formulation:

[0141] 1. Upper layer slurry: LMFP (92.55wt%), LMR (5wt%), SP (0.3wt%), CNT (0.25wt%), PVDF (1.7wt%), dispersant (0.2wt%), wherein LMR and LMFP are added simultaneously during stirring, and LMR is the lithium supplement for the upper layer.

[0142] 2. Lower layer slurry: LMFP (93.9wt%), LFO (3.3wt%), SP (0.6wt%), CNT (0.3wt%), PVDF (1.7wt%), dispersant (0.2wt%), wherein LFO is added simultaneously with LMFP during the stirring process, and LFO is the lithium supplement for the lower layer.

[0143] Everything else is the same as in Example 1.

[0144] Example 17

[0145] The difference from Example 1 lies in the positive electrode slurry formulation:

[0146] 1. Upper layer slurry: LMFP (92.55wt%), LMR (5wt%), SP (0.3wt%), CNT (0.25wt%), PVDF (1.7wt%), dispersant (0.2wt%), wherein LMR and LMFP are added simultaneously during stirring, and LMR is the lithium supplement for the upper layer.

[0147] 2. Lower layer slurry: LMFP (93.2wt%), LFO (4wt%), SP (0.6wt%), CNT (0.3wt%), PVDF (1.7wt%), dispersant (0.2wt%), where LFO is added simultaneously with LMFP during stirring and serves as the lithium supplement for the lower layer.

[0148] Everything else is the same as in Example 1.

[0149] Example 18

[0150] The difference from Example 1 lies in the positive electrode slurry formulation:

[0151] 1. Upper layer slurry: LMFP (90.55wt%), LMR (7wt%), SP (0.3wt%), CNT (0.25wt%), PVDF (1.7wt%), dispersant (0.2wt%), wherein LMR and LMFP are added simultaneously during stirring, and LMR is the lithium supplement for the upper layer;

[0152] 2. Lower layer slurry: LMFP (94.7wt%), LFO (1.25wt%), LNO (1.25wt%), SP (0.6wt%), CNT (0.3wt%), PVDF (1.7wt%), dispersant (0.2wt%), wherein LNO and LFO are added simultaneously with LMFP during the stirring process, and LNO and LFO are lithium supplementers for the lower layer.

[0153] Everything else is the same as in Example 1.

[0154] Comparative Example 1

[0155] The difference from Example 6 lies in step S1 (preparation of the positive electrode):

[0156] The positive electrode slurry is coated onto the carbon-coated aluminum foil current collector using a single-layer coating method, and after drying, it is cold-pressed into sheets.

[0157] The positive electrode slurry formulation is as follows: LMFP (92.72wt%), LMR (4.75wt%), SP (0.35wt%), CNT (0.28wt%), PVDF (1.7wt%), and dispersant (0.2wt%). LMR and LMFP are added simultaneously during the stirring process, and LMR is a lithium supplement.

[0158] Everything else is the same as in Example 1.

[0159] Comparative Example 2

[0160] The difference from Example 6 lies in step S1 (preparation of the positive electrode):

[0161] The positive electrode slurry is coated onto the carbon-coated aluminum foil current collector using a single-layer coating method, and after drying, it is cold-pressed into sheets.

[0162] The positive electrode slurry formulation is as follows: LMFP (92.72wt%), LFO (4.75wt%), SP (0.35wt%), CNT (0.28wt%), PVDF (1.7wt%), and dispersant (0.2wt%). LFO is added simultaneously with LMFP during the stirring process and serves as a lithium supplement.

[0163] Everything else is the same as in Example 1.

[0164] Comparative Example 3

[0165] The difference from Example 6 lies in the positive electrode slurry formulation:

[0166] 1. Upper layer slurry: LMFP (90.47wt%), LMR (7wt%), SP (0.35wt%), CNT (0.28wt%), PVDF (1.7wt%), dispersant (0.2wt%), wherein LMR and LMFP are added simultaneously during stirring, and LMR is the lithium supplement for the upper layer;

[0167] 2. Lower layer slurry: LMFP (94.97wt%), LFO (2.5wt%), SP (0.35wt%), CNT (0.28wt%), PVDF (1.7wt%), dispersant (0.2wt%), wherein LFO is added simultaneously with LMFP during the stirring process, and LFO is the lithium supplement for the lower layer.

[0168] Everything else is the same as in Example 1.

[0169] Comparative Example 4

[0170] The difference from Example 6 lies in the positive electrode slurry formulation:

[0171] 1. Upper layer slurry: LMFP (90.39wt%), LMR (7wt%), SP (0.5wt%), CNT (0.21wt%), PVDF (1.7wt%), dispersant (0.2wt%), wherein LMR and LMFP are added simultaneously during stirring, and LMR is the lithium supplement for the upper layer.

[0172] 2. Lower layer slurry: LMFP (95.05wt%), LFO (2.5wt%), SP (0.3wt%), CNT (0.25wt%), PVDF (1.7wt%), dispersant (0.2wt%), wherein LFO is added simultaneously with LMFP during the stirring process, and LFO is the lithium supplement for the lower layer.

[0173] Everything else is the same as in Example 1.

[0174] Comparative Example 5

[0175] The difference from Example 6 lies in the positive electrode slurry formulation:

[0176] 1. Upper layer slurry: LMFP (94.89wt%), LFO (2.5wt%), SP (0.5wt%), CNT (0.21wt%), PVDF (1.7wt%), dispersant (0.2wt%), wherein LFO is added simultaneously with LMFP during the stirring process, and LFO is the lithium supplement for the upper layer.

[0177] 2. Lower layer slurry: LMFP (90.55wt%), LMR (7wt%), SP (0.3wt%), CNT (0.25wt%), PVDF (1.7wt%), dispersant (0.2wt%), wherein LMR and LMFP are added simultaneously during stirring, and LMR is the lithium supplement for the lower layer.

[0178] Everything else is the same as in Example 1.

[0179] The parameters of each embodiment and comparative example are recorded in Table 1.

[0180] Table 1

[0181] The content of each material in Table 1 is in wt%.

[0182] The batteries prepared in the examples and comparative examples were tested as follows:

[0183] 1. Specific capacity test: At 25℃, the battery is charged at a rate of 0.33C, and then discharged at 0.33C after being fully charged. The discharge capacity is obtained, and the specific capacity is calibrated. The results are recorded in Table 2.

[0184] 2. Capacity retention test: The battery was subjected to a 1C / 1C cycle test at 25℃. The capacity retention rate was calculated by dividing the capacity after 500 cycles by the initial capacity. The test results are recorded in Table 2.

[0185] 3. DC internal resistance (DCR) growth rate test: DCR test was performed at 1C@10s 70% SOC DCR; at 25℃, the battery was subjected to 1C / 1C cycle test. The DCR growth rate was calculated by dividing the difference between the DCR after 500 cycles and the initial DCR by the initial DCR. The test results are recorded in Table 2.

[0186] Table 2

[0187] The results in Table 2 show that:

[0188] The batteries provided in Examples 1 to 18 maintain a specific capacity of 139.6 mAh / g or higher, a capacity retention of 95.3% or higher, and a DCR growth rate of 8.9% or lower.

[0189] Further comparing Example 6 and Comparative Examples 1 to 5, under the condition that the total content of each material in the electrode is the same, compared with Comparative Examples 1 and 2 (the positive electrode has only a single layer of active material, and the lithium supplement is simply mixed in the active material layer), Comparative Example 3 (the active material layer is set in layers, but the conductive agent content in the upper and lower active material layers is the same), Comparative Example 4 (the active material layer is set in layers, but the conductive agent content in the lower active material layer is less than that in the upper layer), and Comparative Example 5 (the active material layer is set in layers, but the conductive agent content in the lower active material layer is small), when the total content of each material in the electrode is the same, the comparison results are as follows: In Example 6, both the specific capacity and capacity retention were improved, while the DCR growth rate decreased. This indicates that by layering the active material layers, with the specific capacity of the lithium replenishing agent in the inner layer (i.e., the lower layer) being greater than that in the outer layer (i.e., the upper layer), and the content of the conductive agent in the inner layer being greater than that in the outer layer, the capacity of the LMFP in the active material layer can be effectively promoted, the cycle stability of the battery can be improved, and the DC internal resistance growth rate of the battery can be reduced, thereby improving the performance of the battery.

Claims

1. A positive electrode sheet comprising: A current collector, a first active material layer and a second active material layer disposed on the current collector, along the direction away from the current collector, the first active material layer and the second active material layer are sequentially distributed; both the first active material layer and the second active material layer contain lithium iron phosphate manganese; the first active material layer further contains a first conductive agent and a first lithium supplementing agent, and the second active material layer further contains a second conductive agent and a second lithium supplementing agent; the gram capacity K1 of the first lithium supplementing agent is greater than the gram capacity K2 of the second lithium supplementing agent; the mass percentage content D1 of the first conductive agent in the first active material layer is greater than the mass percentage content D2 of the second conductive agent in the second active material layer.

2. The cathode sheet of claim 1, wherein, The ratio P1 of the mass percentage content D2 of the second conductive agent in the second active material layer to the mass percentage content D1 of the first conductive agent in the first active material layer is less than 0.

78.

3. The cathode sheet of claim 1, wherein, The mass percentage content B2 of the second lithium supplementing agent in the second active material layer is greater than the mass percentage content B1 of the first lithium supplementing agent in the first active material layer.

4. The cathode sheet of claim 3, wherein, The ratio P2 of the mass percentage content B2 of the second lithium supplementing agent in the second active material layer to the mass percentage content B1 of the first lithium supplementing agent in the first active material layer is 1.25 to 6.

5. The cathode sheet of any one of claims 1 to 4, wherein, The mass percentage content B1 of the first lithium supplementing agent in the first active material layer is 2.0 wt% to 4.0 wt%; and / or, The mass percentage content B2 of the second lithium supplementing agent in the second active material layer is 5.0 wt% to 15.0 wt%.

6. The cathode sheet of any one of claims 1 to 4, wherein, The mass percentage content D1 of the first conductive agent in the first active material layer is 0.3 wt% to 1.4 wt%; and / or, The mass percentage content D2 of the second conductive agent in the second active material layer is 0.15 wt% to 0.7 wt%.

7. The cathode sheet of any one of claims 1 to 4, wherein, The first lithium supplementing agent includes at least one of LiNiO2 and Li5FeO4; and / or, the second lithium supplementing agent includes a lithium-rich manganese-based material, and the chemical formula of the lithium-rich manganese-based material is xLi2MnO3·(1-x)LiMO2, where 0 < x < 1, and M includes at least one transition metal.

8. The cathode sheet of any one of claims 1 to 4, wherein, The first charge calibration gram capacity of the first lithium supplementing agent is greater than 300 mAh / g; and / or, the first charge calibration gram capacity of the second lithium supplementing agent is greater than 100 mAh / g.

9. The cathode sheet of any one of claims 1 to 4, wherein, The mass percentage content H1 of the lithium iron phosphate manganese in the first active material layer is 91.0 wt% to 95.3 wt%; and / or, The mass percentage content H2 of the lithium iron phosphate manganese in the second active material layer is 80.0 wt% to 92.55.0 wt%.

10. The cathode sheet of any one of claims 1 to 4, wherein, Both the first active material layer and the second active material layer further contain a binder, the content N1 of the binder in the first active material layer is 1.5 wt% to 2.0 wt%, and / or, the content N2 of the binder in the second active material layer is 1.5 wt% to 2.0 wt%.

11. The cathode sheet of any one of claims 1 to 4, wherein, Both the first active material layer and the second active material layer further contain a dispersant, wherein the content of the dispersant F1 in the first active material layer is 0.1wt% to 0.2wt%, and / or the content of the dispersant F2 in the second active material layer is 0.1wt% to 0.2wt%.

12. A method for preparing a positive electrode sheet, used to prepare a positive electrode sheet as described in any one of claims 1 to 11, wherein the method for preparing the positive electrode sheet comprises: A first slurry is provided, the first slurry comprising lithium manganese iron phosphate, a first conductive agent and a first lithium replenishing agent; A second slurry is provided, the second slurry comprising lithium manganese iron phosphate, a second conductive agent, and a second lithium supplementing agent; The first slurry is subjected to film formation treatment on the current collector to obtain the first active material layer; The second slurry is deposited on the side of the first active material layer away from the current collector to form a second active material layer, thereby obtaining a positive electrode sheet.

13. A battery comprising a positive electrode as described in any one of claims 1 to 11; or a positive electrode prepared by the method described in claim 12.

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