Positive electrode sheet, secondary battery, and electric device

Abstract

The application discloses a positive plate, a secondary battery and an electric device, and relates to the technical field of new energy sources.The positive plate comprises a current collector and a first active material layer and a second active material layer which are sequentially arranged on at least one side of the current collector, wherein: the first active material layer is located between the current collector and the second active material layer; the material of the first active material layer comprises a single-crystal structure ternary positive material as an inner core and a lithium manganese iron phosphate material which is at least coated on the surface of the inner core; the second active material layer is arranged away from the current collector, and the material of the second active material layer comprises a polycrystal structure ternary positive material and a solid-state electrolyte.The occurrence of thermal runaway reaction can be effectively inhibited, and the kinetic performance of lithium ion transmission is effectively improved.Meanwhile, compared with the single-crystal ternary material, the polycrystal ternary material is more conducive to improving the fast-charging performance of the positive plate, and the demand of high-power density application is met.

Description

Technical Field

[0001] This application relates to the field of new energy technology, and in particular to a positive electrode, a secondary battery, and an electrical device. Background Technology

[0002] With the rapid development of new energy technologies, lithium-ion batteries have been widely used in electric vehicles and portable electronic devices due to their advantages such as high energy density, long cycle life, and environmental friendliness. However, the safety performance of lithium-ion batteries, especially ternary lithium-ion batteries, remains a critical technical challenge. Due to their high energy density and chemical reactivity, ternary materials may experience safety issues such as thermal runaway, combustion, and even explosions during use. Furthermore, traditional ternary cathode materials exhibit poor stability at high temperatures and are prone to thermal runaway.

[0003] To improve the safety performance of lithium batteries, researchers have adopted various optimization strategies, such as improving battery structural design and optimizing battery material formulation. However, these methods often only improve the safety of batteries in one aspect, such as overcharging or short circuits, and cannot comprehensively improve battery safety. Currently, common solutions include adding heat dissipation devices to improve thermal stability and designing flame-retardant electrolytes to improve battery safety. However, these methods still have certain limitations and cannot simultaneously achieve both thermal safety and fast-charging performance. Summary of the Invention

[0004] This application provides a positive electrode sheet, a secondary battery, and an electrical device to simultaneously improve the thermal safety and fast-charging performance of ternary positive electrode materials.

[0005] In a first aspect, this application provides a positive electrode sheet, comprising a current collector and a first active material layer and a second active material layer sequentially stacked on at least one side of the current collector, wherein: The first active material layer is located between the current collector and the second active material layer; The material of the first active material layer includes a ternary cathode material with a single crystal structure as the core and lithium manganese iron phosphate material at least covering the surface of the core portion; The second active material layer is disposed away from the current collector, and the material of the second active material layer includes a polycrystalline ternary cathode material and a solid electrolyte.

[0006] This application significantly improves the thermal stability and safety of the cathode material by employing a composite coating layer design formed by blending a single-crystal ternary cathode with lithium manganese iron phosphate. The single-crystal ternary cathode has a continuous crystal lattice structure without grain boundaries, which, compared to polycrystalline materials, better suppresses physical failures caused by particle breakage and exhibits better thermal stability. Simultaneously, lithium manganese iron phosphate has a higher thermal decomposition temperature; coating the surface of the ternary material prevents direct contact between the cathode and the electrolyte, stabilizes the CEI film, and effectively suppresses thermal runaway reactions. By introducing a second active material layer of solid electrolyte and polycrystalline ternary material, the kinetic performance of lithium-ion transport is effectively improved. Solid electrolyte materials have excellent ion conductivity, which can significantly improve the transport efficiency of lithium ions within the cathode. Furthermore, compared to single-crystal ternary materials, polycrystalline ternary materials are more conducive to improving the fast-charging performance of the cathode, meeting the requirements of high power density applications.

[0007] In some embodiments, the Dv of the single-crystal ternary cathode material 50 The Dv of the single-crystal ternary cathode material is 2μm~5μm. 50 Within this range, single-crystal particles can possess high structural integrity and compaction density, while avoiding the problems of excessively long lithium-ion solid-phase diffusion paths due to excessively large particle size, and excessively large specific surface area and exacerbated side reactions due to excessively small particle size. This also facilitates the formation of a uniform and continuous coating layer on the surface of nano-lithium manganese iron phosphate; and / or, The mass ratio of the single-crystal ternary cathode material to lithium manganese iron phosphate material is (19~99):1. Within this range, a sufficiently covering isolation and protective layer can be formed on the surface of the single-crystal ternary particles, effectively isolating the electrolyte, stabilizing the CEI film, and suppressing thermal runaway. Simultaneously, it avoids a significant decrease in the overall specific capacity of the composite material and a significant increase in interfacial impedance due to excessively high lithium manganese iron phosphate content; and / or, The particle size of the lithium manganese iron phosphate material is 50nm~100nm. Within this particle size range, the lithium manganese iron phosphate material can achieve a tight and uniform coating on the surface of a single-crystal ternary cathode through methods such as ball milling, improving the density of the coating layer and simultaneously shortening the diffusion distance of lithium ions in the coating layer, thus reducing the interfacial charge transfer resistance.

[0008] In some embodiments, the thickness of the coating layer formed by the lithium manganese iron phosphate material is 20 nm to 50 nm. Within this thickness range, the coating layer can form a continuous and dense physical barrier, effectively suppressing side reactions between the cathode and electrolyte and the dissolution of transition metals, stabilizing the CEI film. Simultaneously, this thickness range does not significantly increase interfacial impedance, ensuring rapid lithium-ion transport.

[0009] In some embodiments, the lithium manganese iron phosphate material is ball-milled onto at least a portion of the surface of the single-crystal ternary cathode material. Ball milling allows for the refinement of the lithium manganese iron phosphate nanoparticles using mechanical force, resulting in a uniform and robust physical coating on the single-crystal ternary surface. This process is simple and efficient, and avoids interdiffusion of elements and the formation of harmful interfacial phases that may occur during high-temperature sintering, thus maintaining the structural and performance stability of each component.

[0010] In some embodiments, the ball milling speed is 300 rpm to 600 rpm. Within this range, the ball milling speed provides sufficient mechanical energy to refine the particle size of the nano-lithium manganese iron phosphate particles, effectively coating them onto the surface of the single-crystal ternary cathode. Simultaneously, it avoids uneven and weak coating due to excessively low speeds, and particle breakage or the introduction of excessive impurities due to excessively high speeds; and / or, The ball milling time is 2 to 4 hours. Within this range, the ball milling time can ensure a uniform and complete coating layer, avoiding insufficient coating due to too short a milling time, and avoiding material structural damage, amorphization, or particle agglomeration due to too long a milling time, thus balancing the coating effect and the intrinsic properties of the material.

[0011] In some embodiments, the Dv of the solid electrolyte 50 The Dv of the solid electrolyte is 0.5μm~2μm. 50 Within this range, it can form a good particle size distribution with polycrystalline ternary cathode material particles, fully filling the voids between polycrystalline particles, constructing a continuous lithium-ion conduction network, reducing the tortuosity of ion transport inside the electrode, and improving the overall ionic conductivity of the composite electrode; and / or, The mass ratio of the solid electrolyte to the polycrystalline ternary cathode material is 1:(10~19). Within this range, the mass ratio provides sufficient interfacial ion transport channels for the polycrystalline ternary particles, significantly improving kinetic performance, while avoiding a decrease in electronic conductivity and a reduction in the energy density of the composite material due to excessive solid electrolyte addition; and / or The polycrystalline ternary cathode material Dv 50 The diameter is 5μm to 15μm. The Dv of the polycrystalline ternary cathode material... 50 Within this range, the abundant grain boundaries of polycrystalline particles can provide short lithium-ion diffusion paths, fully leveraging the fast charging capability at high rates. At the same time, this particle size range can ensure good electrode processability and high compaction density.

[0012] In some embodiments, the thickness ratio of the first active material layer to the second active material layer is 1:(2~6). This thickness ratio allows for a reasonable match between the first active material layer, which has high thermal stability, and the second active material layer, which provides high power characteristics. This effectively improves safety while fully utilizing the high ion conductivity and fast-charging capability of the second active material layer, thus balancing the battery's energy density and power density; and / or, The thickness of the first active material layer is 10 μm to 50 μm. Within this range, a reliable thermally stable insulating layer can be formed, effectively delaying or blocking the propagation of thermal runaway from the material layer. Simultaneously, this thickness does not cause excessive ion transport resistance, thus having a minimal impact on battery rate and energy density; and / or, The thickness of the second active material layer is 80μm~120μm. Within this range, the electrode can have a high areal capacity and high power output characteristics. At the same time, the ion-conducting network constructed by the polycrystalline ternary electrolyte and the solid electrolyte can still maintain good ion transport performance at this thickness, avoiding the aggravation of polarization and the decay of fast charging capability caused by excessive electrode thickness.

[0013] In some embodiments, the single-crystal ternary cathode material includes LiNi. 0.9 Co 0.05 Mn 0.05 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.5 Co 0.2 Mn 0.3 At least one of O2; and / or, The lithium manganese iron phosphate material includes LiMn 0.7 Fe 0.3 PO4, LiMn 0.65 Fe 0.35 PO4, LiMn 0.6 Fe 0.4 At least one of PO4; and / or, The polycrystalline ternary cathode material includes LiNi. 0.9 Co 0.05 Mn 0.05 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.6 Co 0.2 Mn 0.2O2, LiNi 0.5 Co 0.2 Mn 0.3 At least one of O2; and / or, The solid electrolyte includes at least one of lithium lanthanum zirconium oxide (LLZO), lithium titanium aluminum phosphate (LATP), lithium lanthanum titanium oxide (LLTO), and lithium lanthanum zirconium tantalum oxide (LLZTO).

[0014] Secondly, this application provides a secondary battery, including a positive electrode, a negative electrode, and a separator located between the positive electrode and the negative electrode, wherein the positive electrode is the positive electrode described in the first aspect.

[0015] Thirdly, this application provides an electrical device including the secondary battery described in the second aspect. Detailed Implementation

[0016] To make the objectives, technical solutions, and advantages of this application clearer, the technical solutions of this application will be clearly and completely described below in conjunction with the embodiments of this application. Obviously, the described embodiments are only some, not all, of the embodiments of this application. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0017] With the rapid development of new energy technologies, lithium-ion batteries have been widely used in electric vehicles and portable electronic devices due to their advantages such as high energy density, long cycle life, and environmental friendliness. However, the safety performance of lithium-ion batteries, especially ternary lithium-ion batteries, remains a critical technical challenge. Due to their high energy density and chemical reactivity, ternary materials may experience safety issues such as thermal runaway, combustion, and even explosions during use. Furthermore, traditional ternary cathode materials exhibit poor stability at high temperatures and are prone to thermal runaway.

[0018] To improve the safety performance of lithium batteries, researchers have adopted various optimization strategies, such as improving battery structural design and optimizing battery material formulation. However, these methods often only improve the safety of batteries in one aspect, such as overcharging or short circuits, and cannot comprehensively improve battery safety. Currently, common solutions include adding heat dissipation devices to improve thermal stability and designing flame-retardant electrolytes to improve battery safety. However, these methods still have certain limitations and cannot simultaneously achieve both thermal safety and fast-charging performance.

[0019] For example, a gradient cathode sheet for lithium-ion batteries and its dry preparation method are described. This method uses a dry process to prepare a double-layer gradient thick electrode sheet to improve energy density and ion / electron transport efficiency. The first cathode layer (closer to the current collector) contains the cathode active material, electrolyte material, conductive agent, and binder. The second cathode layer (away from the current collector) also contains the above components, but in different proportions. By adjusting the ratio of electrolyte and conductive agent in the two layers, a gradient channel for ion and electron transport is constructed, solving the problem of transport obstruction within the thick electrode, thereby improving the overall energy density. The main issues addressed are transport kinetics and energy density. If the active material in the first active layer undergoes thermal decomposition, the high decomposition temperature of LFMP cannot suppress the spread of thermal runaway. The first active material layer uses a common polycrystalline ternary electrode, which is prone to grain boundary breakage during cycling, leading to microcracks and side reactions.

[0020] For example, a three-layer protective positive electrode sheet and its preparation method are proposed, which uses an intermediate layer for physical isolation and protection. The structure is designed as a three-layer sandwich structure. Bottom layer: Positive electrode slurry (usually ternary material) coated on the current collector. Middle layer: Lithium iron phosphate (LFP) slurry layer. Top layer: Ceramic slurry layer. The middle LFP layer acts as an isolation wall, blocking direct contact between the ternary material and the electrolyte, inhibiting the corrosion of the positive electrode by HF acid, and the top ceramic layer prevents burrs from piercing the separator, thus improving safety. The middle layer is lithium iron phosphate (LFP), whose operating voltage platform (approximately 3.2V) is much lower than that of ternary materials (approximately 3.7V-3.8V). Introducing an LFP layer inside the positive electrode will directly lower the overall average voltage of the battery, leading to a decrease in energy density. LFP itself has relatively low electronic and ionic conductivity. Using it as an intermediate layer is equivalent to setting a speed bump in the lithium-ion transport path, which will significantly increase the interface impedance and deteriorate the battery's rate performance and fast charging capability.

[0021] In view of this, this application provides a positive electrode sheet, a secondary battery, and an electrical device to simultaneously improve the thermal safety and fast charging performance of ternary positive electrode materials.

[0022] In a first aspect, this application provides a positive electrode sheet, comprising a current collector and a first active material layer and a second active material layer sequentially stacked on at least one side of the current collector, wherein: The first active material layer is located between the current collector and the second active material layer; The material of the first active material layer includes a ternary cathode material with a single crystal structure as the core and lithium manganese iron phosphate material at least covering the surface of the core portion; The second active material layer is disposed away from the current collector, and the material of the second active material layer includes a polycrystalline ternary cathode material and a solid electrolyte.

[0023] This application significantly improves the thermal stability and safety of the cathode material by employing a composite coating layer design formed by blending a single-crystal ternary cathode with lithium manganese iron phosphate. The single-crystal ternary cathode has a continuous crystal lattice structure without grain boundaries, which, compared to polycrystalline materials, better suppresses physical failures caused by particle breakage and exhibits better thermal stability. Simultaneously, lithium manganese iron phosphate has a higher thermal decomposition temperature; coating the surface of the ternary material prevents direct contact between the cathode and the electrolyte, stabilizes the CEI film, and effectively suppresses thermal runaway reactions. By introducing a second active material layer of solid electrolyte and polycrystalline ternary material, the kinetic performance of lithium-ion transport is effectively improved. Solid electrolyte materials have excellent ion conductivity, which can significantly improve the transport efficiency of lithium ions within the cathode. Furthermore, compared to single-crystal ternary materials, polycrystalline ternary materials are more conducive to improving the fast-charging performance of the cathode, meeting the requirements of high power density applications.

[0024] In conjunction with the first aspect, in some embodiments provided in this application, the Dv of the single-crystal ternary cathode material... 50 The Dv of the single-crystal ternary cathode material is 2μm~5μm. 50 Within this range, it is possible to ensure that the single crystal particles have high structural integrity and compaction density, while avoiding the problems of excessively long lithium-ion solid-phase diffusion paths due to excessively large particle size, and excessively large specific surface area and aggravated side reactions due to excessively small particle size. It is also conducive to the formation of a uniform and continuous coating layer on the surface of nano-manganese iron lithium phosphate.

[0025] In conjunction with the first aspect, in some embodiments provided in this application, the mass ratio of the single-crystal ternary cathode material to the lithium manganese iron phosphate material is (19~99):1. Within this range, a sufficiently covering isolation and protective layer can be formed on the surface of the single-crystal ternary particles, effectively isolating the electrolyte, stabilizing the CEI film and suppressing thermal runaway, while avoiding a significant decrease in the overall specific capacity of the composite material and a significant increase in the interfacial impedance due to excessive lithium manganese iron phosphate content.

[0026] In conjunction with the first aspect, in some embodiments provided in this application, the particle size of the lithium manganese iron phosphate material is 50 nm to 100 nm. With a particle size within this range, the lithium manganese iron phosphate material can achieve a tight and uniform coating on the surface of a single-crystal ternary cathode through methods such as ball milling, thereby improving the density of the coating layer and shortening the diffusion distance of lithium ions in the coating layer, thus reducing the interfacial charge transfer resistance.

[0027] In conjunction with the first aspect, in some embodiments provided in this application, the thickness of the coating layer formed by the lithium manganese iron phosphate material is 20 nm to 50 nm. Within this thickness range, the coating layer can form a continuous and dense physical barrier, effectively suppressing side reactions between the cathode and electrolyte and the dissolution of transition metals, stabilizing the CEI film. Simultaneously, this thickness range does not significantly increase interfacial impedance, ensuring rapid lithium-ion transport.

[0028] In conjunction with the first aspect, in some embodiments provided in this application, the lithium manganese iron phosphate material is ball-milled onto at least a portion of the surface of the single-crystal ternary cathode material. The advantage of ball-milling is that it allows for the refinement of the lithium manganese iron phosphate nanoparticles using mechanical force, resulting in a uniform and robust physical coating on the single-crystal ternary surface. The process is simple and efficient, and it avoids elemental interdiffusion and the formation of harmful interfacial phases that may occur during high-temperature sintering, thus maintaining the structural and performance stability of each component.

[0029] In conjunction with the first aspect, in some embodiments provided in this application, the ball milling speed is 300 rpm to 600 rpm. Within this range, the ball milling speed can provide sufficient mechanical energy to refine the particle size of nano-lithium manganese iron phosphate particles, effectively coating them onto the surface of the single-crystal ternary cathode, while avoiding uneven or weak coating due to excessively low speed, and avoiding breakage of single-crystal particles or introduction of too many impurities due to excessively high speed.

[0030] In conjunction with the first aspect, in some embodiments provided in this application, the ball milling time is 2 to 4 hours. Within this range, the ball milling time ensures a uniform and complete coating layer, avoiding insufficient coating due to excessively short milling times, and also avoiding material structural damage, amorphization, or particle agglomeration caused by excessively long milling times, thus balancing the coating effect with the intrinsic properties of the material.

[0031] In conjunction with the first aspect, in some embodiments provided in this application, the Dv of the solid electrolyte... 50 The Dv of the solid electrolyte is 0.5μm~2μm. 50 Within this range, it can form a good particle size distribution with polycrystalline ternary cathode material particles, fully fill the gaps between polycrystalline particles, construct a continuous lithium-ion conduction network, reduce the tortuosity of ion transport inside the electrode, and improve the overall ionic conductivity of the composite electrode.

[0032] In conjunction with the first aspect, in some embodiments provided in this application, the mass ratio of the solid electrolyte to the polycrystalline ternary cathode material is 1:(10~19). Within this range, the mass ratio of the solid electrolyte to the polycrystalline ternary cathode material can provide sufficient interfacial ion transport channels for the polycrystalline ternary particles and significantly improve the kinetic performance, while avoiding the decrease in electronic conductivity and the reduction in the energy density of the composite material due to excessive addition of solid electrolyte.

[0033] In conjunction with the first aspect, in some embodiments provided in this application, the Dv of the polycrystalline ternary cathode material 50 The diameter is 5μm to 15μm. The Dv of the polycrystalline ternary cathode material... 50 Within this range, the abundant grain boundaries of polycrystalline particles can provide short lithium-ion diffusion paths, fully leveraging the fast charging capability at high rates. At the same time, this particle size range can ensure good electrode processability and high compaction density.

[0034] In conjunction with the first aspect, in some embodiments provided in this application, the thickness ratio of the first active material layer to the second active material layer is 1:(2~6). Within this range, the thickness ratio of the first active material layer to the second active material layer can achieve a reasonable match between the first active material layer with high thermal stability and the second active material layer with high power characteristics. Under the premise of effectively improving safety, the high ion conduction and fast charging capability of the second active material layer are fully utilized, taking into account both the energy density and power density of the battery.

[0035] In conjunction with the first aspect, in some embodiments provided in this application, the thickness of the first active material layer is 10μm to 50μm. Within this range, the thickness of the first active material layer can form a reliable thermally stable isolation layer, effectively delaying or blocking the spread of thermal runaway from the material level. At the same time, this thickness does not cause excessive ion transport resistance, and has little impact on the battery rate and energy density.

[0036] In conjunction with the first aspect, in some embodiments provided in this application, the thickness of the second active material layer is 80μm~120μm. Within this range, the electrode can be guaranteed to have high areal capacity and high power output characteristics. At the same time, the ion-conducting network constructed by the polycrystalline ternary electrolyte and the solid electrolyte can still maintain good ion transport performance at this thickness, avoiding the aggravation of polarization and the decay of fast charging capability caused by excessive electrode thickness.

[0037] In conjunction with the first aspect, in some embodiments provided in this application, the single-crystal ternary cathode material includes LiNi. 0.9 Co 0.05 Mn 0.05 O2, LiNi 0.8 Co0.1 Mn 0.1 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.5 Co 0.2 Mn 0.3 At least one of O2.

[0038] In conjunction with the first aspect, in some embodiments provided in this application, the lithium manganese iron phosphate material includes LiMn 0.7 Fe 0.3 PO4, LiMn 0.65 Fe 0.35 PO4, LiMn 0.6 Fe 0.4 At least one of PO4.

[0039] In conjunction with the first aspect, in some embodiments provided in this application, the polycrystalline ternary cathode material includes LiNi. 0.9 Co 0.05 Mn 0.05 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.5 Co 0.2 Mn 0.3 At least one of O2.

[0040] In conjunction with the first aspect, in some embodiments provided in this application, the solid electrolyte includes at least one of lithium lanthanum zirconium oxide (LLZO), lithium titanium aluminum phosphate (LATP), lithium lanthanum titanium oxide (LLTO), and lithium lanthanum zirconium tantalum oxide (LLZTO).

[0041] Secondly, this application provides a secondary battery, including a positive electrode, a negative electrode, and a separator located between the positive electrode and the negative electrode, wherein the positive electrode is the positive electrode described in the first aspect.

[0042] Thirdly, this application provides an electrical device including the secondary battery described in the second aspect.

[0043] The present application will be described below with reference to specific embodiments.

[0044] Example 1 Preparation of the first active material layer The active material is a single-crystal ternary material LiNi. 0.8 Co 0.1 Mn 0.1O2 (D) 50 4μm) and nano-lithium manganese iron phosphate (LiMn) 0.65 Fe 0.35 PO4 (particle size 70 nm) was ball-milled at 500 rpm for 3 hours at a mass ratio of 97:3 to coat the surface of the single-crystal ternary material with LFMP (lithium manganese iron phosphate), forming a coating layer of approximately 40 nm thick, thus obtaining the first composite. The first composite was mixed with conductive agent CNT, binder PVDF, and dispersant PVP at a mass ratio of 96.5:1.5:1.8:0.2, and NMP was used as a solvent to prepare the first slurry. This slurry was coated onto a 12 μm carbon-coated aluminum foil and vacuum dried at 90 °C for 6 hours to obtain the first active material layer.

[0045] Preparation of the second active material layer Polycrystalline ternary material LiNi 0.6 Co 0.2 Mn 0.2 O2 (D) 50 The first active material layer (8 μm particle size) was mixed with solid electrolyte LLZO (1 μm particle size) at a mass ratio of 95:5, and then mixed with CNT, PVDF, and PVP at a mass ratio of 96.2:1.5:2.0:0.3. A second slurry was prepared using NMP as a solvent. This slurry was then coated onto the dried surface of the first active material layer and vacuum-dried at 90°C for 6 hours to form the second active material layer.

[0046] The double-coated electrode sheet is compacted with a roller pressing pressure of 1.5t / cm and a speed of 8m / min to obtain a positive electrode sheet with a total active material layer of about 130μm, wherein the thickness of the first active material layer is 30μm and the thickness of the second active material layer is 100μm.

[0047] Example 2 Preparation of the first active material layer The active material is a single-crystal ternary LiNi 0.9 Co 0.05 Mn 0.05 O2 (D) 50 5μm) and LFMP (LiMn) 0.6 Fe 0.4 PO4 (particle size 100nm) was ball-milled at 300rpm for 4 hours at a mass ratio of 99:1 to coat the surface of the single-crystal ternary material with LFMP, achieving a coating layer of approximately 50nm, thus obtaining the first composite. Slurry preparation: The first composite was mixed with conductive agent SP, CNT, binder PVDF, and dispersant PEG at a mass ratio of 96.7:0.5:0.5:2.0:0.3, using NMP as a solvent to prepare the first slurry. This slurry was coated onto a 10μm aluminum foil and vacuum-dried at 100℃ for 4 hours to obtain the first active material layer.

[0048] Preparation of the second active material layer Polycrystalline ternary material LiNi0.5 Co 0.2 Mn 0.3 O2 (D) 50 The first active material layer was mixed with solid electrolyte LATP (0.5 μm particle size) at a mass ratio of 92:8, and then mixed with SP, CNT, PVDF and PEG at a mass ratio of 96.2:0.8:0.5:2.2:0.3. The mixture was prepared with NMP as solvent to form a second slurry. The slurry was coated on the surface of the dried first active material layer and dried at 100°C for 4 hours to form the second active material layer.

[0049] The double-coated electrode sheet was compacted by rolling at a pressure of 2.0 t / cm and a speed of 5 m / min to obtain a positive electrode sheet with a total active material layer of about 130 μm, wherein the thickness of the first active material layer is 20 μm and the thickness of the second active material layer is 110 μm.

[0050] Example 3 Preparation of the first active material layer The active material is a single-crystal ternary material LiNi. 0.6 Co 0.2 Mn 0.2 O2 (D) 50 3μm) and nano-lithium manganese iron phosphate (LiMn) 0.7 Fe 0.3 PO4 (particle size 60 nm) was ball-milled at 400 rpm for 2.5 h at a mass ratio of 95:5 to coat the surface of the single-crystal ternary material with LFMP, forming a coating layer of about 30 nm thick, thus obtaining the first composite. The first composite was mixed with conductive agent CNT, binder PVDF, and dispersant PVP at a mass ratio of 96.5:1.5:1.8:0.2, and NMP was used as a solvent to prepare the first slurry. The slurry was coated on a 12 μm carbon-coated aluminum foil and vacuum dried at 80 °C for 12 h to obtain the first active material layer.

[0051] Preparation of the second active material layer Polycrystalline ternary material LiNi 0.8 Co 0.1 Mn 0.1 O2 (10 μm crystals) and solid electrolyte LLTO (1.5 μm particle size) were mixed at a mass ratio of 90:10, and then mixed with CNT, PVDF, and PVP at a mass ratio of 96.2:1.5:2.0:0.3. A second slurry was prepared using NMP as a solvent. The slurry was coated onto the surface of the dried first active material layer and vacuum dried at 80 °C for 12 h to form the second active material layer.

[0052] The double-coated electrode sheet is compacted with a roller pressing pressure of 2.5t / cm and a speed of 10m / min to obtain a positive electrode sheet with a total active material layer of about 130μm, wherein the thickness of the first active material layer is 10μm and the thickness of the second active material layer is 120μm.

[0053] Example 4 Preparation of the first active material layer The active material is a single-crystal ternary material LiNi. 0.5 Co 0.2 Mn 0.3 O2 (D) 50 2μm) and nano-lithium manganese iron phosphate (LiMn) 0.7 Fe 0.3 PO4 (particle size 50 nm) was ball-milled at 600 rpm for 2 hours at a mass ratio of 97:3 to coat the surface of the single-crystal ternary material with LFMP, forming a coating layer of about 20 nm thick, thus obtaining the first composite. The first composite was mixed with conductive agent CNT, binder PVDF, and dispersant PVP at a mass ratio of 96.5:1.5:1.8:0.2, and NMP was used as a solvent to prepare the first slurry. This slurry was coated on a 12 μm carbon-coated aluminum foil and vacuum dried at 90 °C for 6 hours to obtain the first active material layer.

[0054] Preparation of the second active material layer Polycrystalline ternary material LiNi 0.9 Co 0.05 Mn 0.05 O2 (Dv) 50 A first active material layer (15 μm particle size) was mixed with solid electrolyte LLZTO (2 μm particle size) at a mass ratio of 95:5, and then mixed with CNT, PVDF, and PVP at a mass ratio of 96.2:1.5:2.0:0.3. A second slurry was prepared using NMP as a solvent. This slurry was then coated onto the dried surface of the first active material layer and vacuum-dried at 90°C for 6 hours to form the second active material layer.

[0055] The double-coated electrode sheet is compacted with a roller pressing pressure of 1.5t / cm and a speed of 8m / min to obtain a positive electrode sheet with a total active material layer of about 130μm, wherein the thickness of the first active material layer is 30μm and the thickness of the second active material layer is 100μm.

[0056] Comparative Example 1 Similar to Example 3, except that polycrystalline LiNi is used. 0.8 Co 0.1 Mn 0.1 O2 (Dv) 50 A 10μm positive electrode was used as the active material and mixed with SP, CNT, PVDF, and PVP in a ratio of 97:0.8:0.5:1.5:0.2 to form a homogenate. The homogenate was then coated onto an 8μm carbon-coated aluminum foil in a single pass, dried at 100℃ for 6 hours, and rolled at 1.5t / cm. This yielded a conventional positive electrode sheet with a thickness of approximately 130μm.

[0057] Comparative Example 2 Similar to Example 3, the difference is that the active material of the first active material layer is only single-crystal LiNi. 0.6 Co 0.2 Mn 0.2 O2 (Dv503μm), without LFMP, the slurry ratio and coating parameters are the same as those in Example 1 for the first active material layer, with a thickness of approximately 30μm. The second active material layer (polycrystalline NCM811 + 5% LLTO), with a thickness of 100μm, yields a gradient composite electrode without LFMP.

[0058] Comparative Example 3 Similar to Example 3, except that it does not contain a solid electrolyte.

[0059] Comparative Example 4 Similar to Comparative Example 1, the difference is that single-crystal LiNi is used. 0.8 Co 0.1 Mn 0.1 O2.

[0060] Performance testing The positive electrode sheets of Examples 1 to 4 and Comparative Examples 1 to 4 were subjected to performance tests. The test methods were as follows: Ion conductivity on / off ratio: Each positive electrode was punched into a 12mm diameter disc, and symmetrical cells were assembled using a blocking electrode (stainless steel). The AC impedance (frequency 0.1Hz~1MHz, perturbation 5mV) at 25℃ and 150℃ was measured on an electrochemical workstation, and the lithium-ion conductivity σ was obtained by fitting the data. 25 and σ 150 Calculate the ionic conductivity on / off ratio K = σ 25 / σ 150 .

[0061] Cycle stability: CR2032 coin cells were assembled using lithium metal sheets as the negative electrode, 1 mol / L LiPF6EC / EMC / DMC (volume ratio 1:1:1) as the electrolyte, and Celgard 2500 as the separator. The cells were cycled 200 times at 25℃ and within a voltage range of 2.8V to 4.3V, with constant current and voltage charging at 1C to 4.3V and constant current discharging at 1C to 2.8V. The capacity retention rate (%) was recorded.

[0062] Thermal runaway protection simulation response: A positive electrode plate charged to 4.3V was scraped off the current collector. Approximately 3mg of positive electrode powder was taken and added to an equal proportion of electrolyte. The mixture was placed in a high-pressure sealed crucible and heated from room temperature to 350℃ at a rate of 5℃ / min. Differential scanning calorimetry (DSC) was performed, and the onset temperature (T0) of the first major exothermic peak was recorded. onset (℃).

[0063] The test results are shown in Table 1.

[0064] Table 1. Positive electrode performance of Examples 1 to 4 and Comparative Examples 1 to 4

[0065] As shown in Table 1, Examples 1 to 3 have significantly improved ionic conductivity on / off ratio, excellent cycling stability, and higher thermal runaway initiation temperature, demonstrating the outstanding advantages of the composite coating layer design in constructing thermally induced ion transport shutdown function and stabilizing the interface. In Example 4, due to the smaller single-crystal ternary particle size in the first active material layer and the thinnest lithium manganese iron phosphate coating layer, the ionic conductivity on / off ratio and thermal runaway initiation temperature are slightly lower, but still better than Comparative Examples 1 and 2.

[0066] Comparative Example 1 uses polycrystalline LiNi 0.8 Co 0.1 Mn 0.1 As an active material, O2 has defects such as easy particle breakage along grain boundaries, lack of any coating protection leading to violent side reactions with electrolyte, and poor thermal stability, resulting in rapid cycle decay and an exothermic onset temperature of only 203℃.

[0067] Comparative Example 2: Because the active material of the first active material layer is only single-crystal LiNi 0.8 Co 0.1 Mn 0.1 O2, without the addition of LFMP, although the surface layer is a single crystal with good thermal stability, lacks the effective physical isolation formed by LFMP and the stabilizing effect of the CEI film, resulting in a low ionic conductivity on / off ratio and a significantly lower thermal runaway initiation temperature compared to the example.

[0068] Comparative Example 3 lacks a solid electrolyte, resulting in insufficient ion transport channels within the second active material layer. This leads to a decrease in the overall lithium-ion conductivity of the electrode, increased polarization, and reduced cycle stability. Furthermore, the absence of a solid electrolyte and a highly efficient ion network constructed from polycrystalline ternary electrolytes also affects the rate performance.

[0069] Comparative Example 4 uses single-crystal LiNi 0.8 Co 0.1 Mn 0.1 Although the intrinsic thermal stability of single-crystal O2 is worse than that of polycrystalline O2, it lacks the lithium manganese iron phosphate coating layer and solid electrolyte composite layer, and cannot effectively suppress the positive electrode-electrolyte reaction in the early stage of thermal runaway. The thermal runaway initiation temperature is only 208°C, and the cycle stability is not as good as the embodiment with a double-layer composite structure.

[0070] In summary, the composite coating design formed by blending monocrystalline ternary cathode and lithium manganese iron phosphate significantly improves the thermal stability and safety of the cathode material. The monocrystalline ternary cathode has a continuous lattice structure without grain boundaries, which, compared to polycrystalline materials, better suppresses physical failures caused by particle breakage and exhibits better thermal stability. Simultaneously, lithium manganese iron phosphate has a higher thermal decomposition temperature; coating the surface of the ternary material prevents direct contact between the cathode and the electrolyte, stabilizes the CEI film, and effectively suppresses thermal runaway reactions. Introducing a second active material layer of solid electrolyte and polycrystalline ternary material effectively improves the kinetic performance of lithium-ion transport. Solid electrolyte materials have excellent ion conductivity, which can significantly improve the transport efficiency of lithium ions within the cathode. Furthermore, compared to monocrystalline ternary materials, polycrystalline ternary materials are more conducive to improving the fast-charging performance of the cathode, meeting the requirements of high power density applications.

[0071] The terms "comprising" and "having," and any variations thereof, in the specification and claims of this application are intended to cover non-exclusive inclusion. For example, a process, method, apparatus, product, or device that includes a series of steps or units is not limited to the listed steps or units, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to such process, method, product, or device. The terms "first," "second," and "third," etc., are used to distinguish different objects, etc., and do not indicate a sequence, nor do they limit "first," "second," and "third" to different types.

[0072] In the description of the embodiments of this application, terms such as "exemplary," "for example," or "for instance" are used to indicate examples, illustrations, or explanations. Any embodiment or design described as "exemplary," "for example," or "for instance" in the embodiments of this application should not be construed as being more preferred or advantageous than other embodiments or designs. Specifically, the use of terms such as "exemplary," "for example," or "for instance" is intended to present the relevant concepts in a concrete manner.

[0073] In the description of the embodiments of this application, unless otherwise stated, " / " means "or". For example, A / B can mean A or B. The "and / or" in the text is merely a description of the relationship between related objects, indicating that there can be three relationships. For example, A and / or B can mean: A exists alone, A and B exist simultaneously, and B exists alone. In addition, in the description of the embodiments of this application, "multiple" means two or more.

[0074] The above are merely preferred embodiments of this application and do not limit the patent scope of this application. Any equivalent structural or procedural changes made using the content of this application's specification, or any direct or indirect application in other related technical fields, are similarly included within the patent protection scope of this application.

Claims

1. A positive electrode plate, characterized in that, It includes a current collector and a first active material layer and a second active material layer sequentially stacked on at least one side of the current collector, wherein: The first active material layer is located between the current collector and the second active material layer; The material of the first active material layer includes a ternary cathode material with a single crystal structure as the core and lithium manganese iron phosphate material at least covering the surface of the core portion; The second active material layer is disposed away from the current collector, and the material of the second active material layer includes a polycrystalline ternary cathode material and a solid electrolyte.

2. The positive electrode sheet as described in claim 1, characterized in that: The single-crystal structure ternary cathode material Dv 50 2μm~5μm; and / or, The mass ratio of the single-crystal ternary cathode material to lithium manganese iron phosphate material is (19~99):1; and / or, The particle size of the lithium manganese iron phosphate material is 50nm~100nm.

3. The positive electrode sheet as described in claim 1, characterized in that, The thickness of the coating layer formed by the lithium manganese iron phosphate material is 20nm~50nm.

4. The positive electrode sheet as described in claim 1, characterized in that, The lithium manganese iron phosphate material is ball-milled onto at least a portion of the surface of the single-crystal ternary cathode material.

5. The positive electrode sheet as described in claim 4, characterized in that: The ball mill rotates at 300 rpm to 600 rpm; and / or, The ball milling time is 2-4 hours.

6. The positive electrode sheet as described in claim 1, characterized in that: The Dv of the solid electrolyte 50 The range is 0.5μm to 2μm; and / or, The mass ratio of the solid electrolyte to the polycrystalline ternary cathode material is 1:(10~19). The polycrystalline ternary cathode material Dv 50 The size ranges from 5μm to 15μm.

7. The positive electrode sheet as described in claim 1, characterized in that: The thickness ratio of the first active material layer to the second active material layer is 1:(2~6); and / or, The thickness of the first active material layer is 10 μm to 50 μm; and / or, The thickness of the second active material layer is 80μm~120μm.

8. The positive electrode sheet as described in claim 1, characterized in that: The single-crystal ternary cathode material includes LiNi. 0.9 Co 0.05 Mn 0.05 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.5 Co 0.2 Mn 0.3 At least one of O2; and / or, The lithium manganese iron phosphate material includes LiMn 0.7 Fe 0.3 PO4, LiMn 0.65 Fe 0.35 PO4, LiMn 0.6 Fe 0.4 At least one of PO4; and / or, The polycrystalline ternary cathode material includes LiNi. 0.9 Co 0.05 Mn 0.05 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.5 Co 0.2 Mn 0.3 At least one of O2; and / or, The solid electrolyte includes at least one of lithium lanthanum zirconium oxide, lithium titanium aluminum phosphate, lithium lanthanum titanium oxide, and lithium lanthanum zirconium tantalum oxide.

9. A secondary battery, characterized in that, It includes a positive electrode, a negative electrode, and a separator located between the positive electrode and the negative electrode, wherein the positive electrode is the positive electrode as described in any one of claims 1 to 8.

10. An electrical appliance, characterized in that, Includes the secondary battery as described in claim 9.

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