Secondary battery

Abstract

The invention provides a secondary battery, the secondary battery comprises a positive plate and an electrolyte, the positive plate comprises a positive current collector and a positive active layer arranged on at least one side of the positive current collector; in the thickness direction of the positive electrode current collector, the positive electrode active layer comprises a first active layer and a second active layer which are stacked, and the first active layer is close to the positive electrode current collector; the first active layer comprises a P2 phase layered oxide; the second active layer comprises an O3-phase layered oxide; in an X-ray diffraction pattern, the P2-phase layered oxide comprises a first diffraction peak at 15-17 degrees, the O3-phase layered oxide comprises a second diffraction peak at 16-17 degrees, and the angle theta 1 of the first diffraction peak is smaller than the angle theta 2 of the second diffraction peak; the electrolyte comprises a sulfur additive and a phosphorus additive. The secondary battery provided by the invention is high in cold start discharge end voltage, small in direct current impedance and excellent in cycle performance and high-temperature storage performance.

Description

Technical Field

[0001] This invention belongs to the field of battery technology, and particularly relates to a secondary battery. Background Technology

[0002] With the rapid development of new energy technologies, sodium-ion batteries, due to their abundant resources, low cost, and excellent rate performance, are gradually becoming a potential alternative to lithium-ion batteries. However, existing sodium-ion batteries still face performance bottlenecks in applications. Significant deficiencies exist in cold-start discharge voltage, DC impedance, cycle performance, and high-temperature storage performance, limiting their commercial application. Therefore, developing a battery with high cold-start discharge voltage, low DC impedance, and excellent cycle and high-temperature storage performance has become an urgent technical problem to be solved. Summary of the Invention

[0003] The main objective of this invention is to provide a secondary battery that has a high cold start discharge terminal voltage, low DC impedance, and excellent cycle performance and high temperature storage performance.

[0004] The present invention provides a secondary battery, including a positive electrode sheet and an electrolyte. The positive electrode sheet includes a positive current collector and a positive active layer disposed on at least one side of the positive current collector. In the thickness direction of the positive current collector, the positive active layer includes a first active layer and a second active layer stacked thereon, wherein the first active layer is close to the positive current collector.

[0005] The first active layer comprises a P2 phase layered oxide as shown in Formula 1; the second active layer comprises an O3 phase layered oxide as shown in Formula 2;

[0006] Na y1 Ni y2 Fe y3 Mn y4 Cu y5 Me1 y6 O2 Formula 1,

[0007] Na x1 Ni x2 Fe x3 Mn x4 Me2 x5 O2 Formula 2,

[0008] Among them, 0.6 < y1 < 0.85, 0.16 < y2 < 0.41, 0 ≤ y3 < 0.17, 0.39 < y4 < 0.79, 0 < y5 < 0.24, 0 < y6 < 0.16, and Me1 includes at least one of Li, Mg, Zn, Ti, Nb, B, and Al; 0.85 < x1 < 1, 0.21 < x2 < 0.49, 0.17 < x3 < 0.34, 0.14 < x4 < 0.58, 0 < x5 < 0.36, and Me2 includes at least one of Li, K, Cu, Mg, Zn, Ti, B, and Al;

[0009] In the X-ray diffraction pattern, the P2-phase layered oxide includes a first diffraction peak at 15° - 17°, and the O3-phase layered oxide includes a second diffraction peak at 16° - 17°. The angle θ1 of the first diffraction peak is less than the angle θ2 of the second diffraction peak;

[0010] The electrolyte includes a sulfur-based additive and a phosphorus-based additive;

[0011] The mass percentage of the sulfur-based additive in the electrolyte is 0.1% - 5%;

[0012] The mass percentage of the phosphorus-based additive in the electrolyte is 0.1% - 3%.

[0013] For the secondary battery as described above, the mass percentage of copper element in the positive electrode active layer is W1, the electrolyte further includes a fluorine-based additive, and the sum of the fluorine-based additive and the phosphorus-based additive accounts for the mass percentage W2 of the electrolyte, and 0.4 ≤ W1 / W2 ≤ 4;

[0014] Preferably, the mass percentage of copper element in the positive electrode active layer is 0.5% - 5%, and the sum of the fluorine-based additive and the phosphorus-based additive accounts for the mass percentage of 1% - 5% of the electrolyte.

[0015] For the secondary battery as described above, Me1 and Me2 include titanium element. In the thickness direction of the positive electrode current collector, the content of titanium element on the side close to the positive electrode current collector is less than the content of titanium element on the side far from the positive electrode current collector;

[0016] Preferably, the mass percentage of titanium element in the positive electrode active layer is 0% - 4%.

[0017] For the secondary battery as described above, the median particle size Dv1 of the P2-phase layered oxide is less than the median particle size Dv2 of the O3-phase layered oxide;

[0018] Preferably, 2μm < Dv1 ≤ 6μm, 6μm < Dv2 ≤ 10μm.

[0019] In the secondary battery described above, in the thickness direction of the positive electrode current collector, the content of the first element on the side closer to the positive electrode current collector is less than the content of the first element on the side farther from the positive electrode current collector; the first element includes at least one of Fe, Ni, and Na.

[0020] Preferably, the difference between the Fe content on the side away from the positive electrode current collector and the Fe content on the side closer to the positive electrode current collector is 17,500 ppm to 70,000 ppm.

[0021] In the secondary battery described above, in the thickness direction of the positive electrode current collector, the content of the second element on the side closer to the positive electrode current collector is greater than the content of the second element on the side farther away from the positive electrode current collector; the second element includes at least one of Mn and Cu.

[0022] In the secondary battery described above, the first active layer accounts for 5% to 55% of the mass percentage of the positive electrode active layer;

[0023] And / or, the second active layer accounts for 45% to 95% of the mass percentage of the positive electrode active layer.

[0024] The secondary battery described above further includes a negative electrode, wherein in the Raman spectrum of the negative electrode, the intensity ratio of the d peak to the g peak of the negative electrode is Id / Ig > 1.1.

[0025] And / or, in the discharge capacity curve of the negative electrode sheet during the coin cell test, the capacity of the ramp region accounts for 30% to 50% of the capacity of the negative electrode sheet;

[0026] Wherein, the capacity of the ramp zone is the capacity release in the coin cell test voltage range > 0.1V, and the capacity of the negative electrode is the capacity release in the coin cell test voltage range of 0.01V~0.1V.

[0027] As described above, in the secondary battery, the negative electrode sheet includes a first negative electrode active material and a second negative electrode active material; the Id / Ig ratio of the first negative electrode active material is greater than that of the second negative electrode active material, the specific surface area of ​​the first negative electrode active material is greater than that of the second negative electrode active material, and the median particle size of the first negative electrode active material is smaller than that of the second negative electrode active material.

[0028] Preferably, the Id / Ig ratio of the first negative electrode active material is 1.1~1.3;

[0029] The Id / Ig ratio of the second negative electrode active material is 0.85~1.06;

[0030] The specific surface area of ​​the first negative electrode active material is 3m².2 / g~8m 2 / g;

[0031] The specific surface area of ​​the second negative electrode active material is 1m². 2 / g~3m 2 / g;

[0032] The median particle size of the first negative electrode active material is 2 μm to 6 μm;

[0033] The median particle size of the second negative electrode active material is 6 μm to 10 μm.

[0034] In the secondary battery described above, the electrolyte further includes boron-based additives;

[0035] The boron-based additive accounts for 1% to 2% of the mass percentage of the electrolyte.

[0036] And / or, the electrolyte further includes a first solvent and a compound containing a cyano group;

[0037] The first solvent includes at least one of diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, and 1,2-methoxyethane;

[0038] The first solvent accounts for 5% to 70% of the mass percentage of the electrolyte;

[0039] And / or, the cyano-containing compound includes at least one of malononitrile, succinic anionyl, glutaronitrile, ethylenedionitrile, 1,2,3-propanetricarbonyl, 1,3,5-pentanetricarbonyl, and 1,3,6-hexanetrionitrile;

[0040] The cyano-containing compound accounts for 4% to 10% of the mass percentage of the electrolyte.

[0041] The secondary battery provided by this invention has a positive electrode sheet with a double-layer coating structure. The first active layer is close to the positive electrode current collector, and the second active layer is far away from the positive electrode current collector. The composition of each active layer and electrolyte is defined, which can improve the cold start discharge end voltage, cycle performance and high temperature storage performance of the secondary battery, while reducing DC resistance. Attached Figure Description

[0042] To more clearly illustrate the technical solutions in the embodiments of the present invention or related technologies, the accompanying drawings used in the description of the embodiments of the present invention or related technologies are briefly introduced below. Obviously, the drawings described below are merely some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0043] Figure 1A cross-sectional SEM image of the positive electrode sheet of Embodiment 1 provided by the present invention;

[0044] Figure 2 The mapping diagram of Ni element in the positive electrode of Embodiment 1 provided by the present invention;

[0045] Figure 3 The mapping diagram of Fe element in the positive electrode of Embodiment 1 provided by the present invention;

[0046] Figure 4 Mapping diagram of Mn element in the positive electrode of Embodiment 1 provided by the present invention;

[0047] Figure 5 The mapping diagram of Ti element in the positive electrode of Embodiment 1 provided by the present invention;

[0048] Figure 6 Cross-sectional EDS diagram of the positive electrode sheet of Embodiment 1 provided by the present invention;

[0049] Figure 7 The mapping diagram of Ni element in the positive electrode of Comparative Example 1 provided by the present invention;

[0050] Figure 8 The mapping diagram of Fe element in the positive electrode of Comparative Example 1 provided by the present invention;

[0051] Figure 9 The mapping diagram of Mn element in the positive electrode of Comparative Example 1 provided by the present invention;

[0052] Figure 10 The mapping diagram of Ti element in the positive electrode of Comparative Example 1 provided by the present invention;

[0053] Figure 11 Raman spectrum of the negative electrode sheet of Example 1 provided by the present invention;

[0054] Figure 12 The XRD pattern of the positive electrode sheet of Embodiment 1 provided by the present invention. Detailed Implementation

[0055] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Obviously, the described embodiments are only some embodiments of this invention, not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0056] As the global energy structure shifts towards cleaner and lower-carbon energy, electrochemical energy storage technology has become central to supporting this transformation. Among various energy storage devices, sodium-ion batteries are widely regarded as a potential alternative to lithium-ion batteries in large-scale energy storage and low-speed electric vehicles due to their advantages such as abundant sodium resources, low raw material costs, environmental friendliness, and excellent rate performance. However, despite the theoretical feasibility of sodium-ion batteries, their overall performance, particularly their reliability in high-power, wide-temperature-range applications, still has significant shortcomings, severely hindering their large-scale commercial application.

[0057] Currently, commercially available sodium-ion batteries still suffer from the following problems: First, under low-temperature, high-rate discharge conditions, the battery's voltage polarization is extremely pronounced, leading to insufficient cold-start performance. This is mainly due to two factors: Firstly, in traditional single-layer coated positive electrode structures, the migration kinetics of sodium ions along the electrode thickness are limited, especially at low temperatures, where the inner active material cannot be effectively utilized, resulting in low actual capacity utilization and a sharp drop in discharge voltage. Secondly, the viscosity of conventional electrolyte systems increases significantly at low temperatures, while the impedance of the negative electrode interface increases, both contributing to the battery's inability to meet the stringent requirements of high-rate pulse discharge and its difficulty in meeting practical application needs such as vehicle cold starts. Secondly, the battery's DC resistance (DCR) increases too rapidly during long-term cycling and exhibits poor high-temperature storage stability. During cycling, the uneven distribution of the electric field within the positive electrode and changes in the material's structure cause a continuous accumulation of interfacial and bulk impedance, leading to a significant increase in DCR. Meanwhile, existing electrolyte systems lack compatibility with highly active cathode materials, easily triggering severe interfacial side reactions under high-temperature storage conditions. This leads to the loss of active sodium ions and electrode structure degradation, manifesting as accelerated capacity decay. Ultimately, these issues collectively result in battery cycle life failing to meet commercial requirements. The phase transition structural instability of the cathode active material during deep sodium insertion / extraction is one of the fundamental factors limiting cycle life. Under the combined effects of impedance growth and interfacial side reactions, the battery's capacity retention rate rapidly declines, failing to meet the expected long lifespan.

[0058] The inventors of this application have discovered through research that by designing the positive electrode in layers and limiting the composition of the electrolyte, the cold start discharge terminal voltage, cycle performance, and high-temperature storage performance of the secondary battery can be significantly improved, while reducing DC impedance.

[0059] Based on this, in a first aspect, the present invention provides a secondary battery, including a positive electrode sheet and an electrolyte, wherein the positive electrode sheet includes a positive current collector and a positive active layer disposed on at least one side of the positive current collector; in the thickness direction of the positive current collector, the positive active layer includes a first active layer and a second active layer stacked thereon, wherein the first active layer is close to the positive current collector.

[0060] The first active layer includes a P2-phase layered oxide represented by Formula 1; the second active layer includes an O3-phase layered oxide represented by Formula 2;

[0061] Na y1 Ni y2 Fe y3 Mn y4 Cu y5 Me1 y6 O2 Formula 1,

[0062] Na x1 Ni x2 Fe x3 Mn x4 Me2 x5 O2 Formula 2,

[0063] where 0.6 < y1 < 0.85, 0.16 < y2 < 0.41, 0 ≤ y3 < 0.17, 0.39 < y4 < 0.79, 0 < y5 < 0.24, 0 < y6 < 0.16, and Me1 includes at least one of Li, Mg, Zn, Ti, Nb, B, and Al; 0.85 < x1 < 1, 0.21 < x2 < 0.49, 0.17 < x3 < 0.34, 0.14 < x4 < 0.58, 0 < x5 < 0.36, and Me2 includes at least one of Li, K, Cu, Mg, Zn, Ti, B, and Al.

[0064] In the X-ray diffraction pattern, the P2-phase layered oxide includes a first diffraction peak at 15° to 17° (for example, it can be 15°, 15.5°, 15.5°, 15.7°, 16°, 16.2°, 16.5°, 16.7°, 17° or a range composed of any two of them), and the O3-phase layered oxide includes a second diffraction peak at 16° to 17° (for example, it can be 16°, 16.1°, 16.2°, 16.3°, 16.4°, 16.5°, 16.6°, 16.7°, 16.8°, 16.9°, 17° or a range composed of any two of them), and the angle θ1 of the first diffraction peak is less than the angle θ2 of the second diffraction peak.

[0065] The electrolyte includes a sulfur-based additive and a phosphorus-based additive.

[0066] The mass percentage of the sulfur-based additive in the electrolyte is 0.1% to 5%, for example, it can be 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5% or a range composed of any two of them.

[0067] The mass percentage of the phosphorus-based additive in the electrolyte is 0.1% to 3%, for example, it can be 0.1%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3% or a range composed of any two of them.

[0068] For example, y1 can be a range consisting of 0.61, 0.65, 0.7, 0.75, 0.8, 0.84, or any two of these; y2 can be a range consisting of 0.17, 0.20, 0.25, 0.30, 0.35, 0.40, or any two of these; y3 can be a range consisting of 0, 0.05, 0.10, 0.12, 0.15, 0.16, or any two of these; y4 can be a range consisting of 0.40, 0.45, 0.50, 0.55, 0.60, 0.65, 0.70, 0.75, 0.78, or any two of these; y5 can be a range consisting of 0.01, 0.05, 0.10, 0.15, 0.20, 0.23, or any two of these; y6 can be a range consisting of 0.01, 0.02, 0.05, 0.10, 0.12, 0.15, or any two of these; x1 can be a range consisting of any two of the following: 0.86, 0.87, 0.88, 0.89, 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.97, 0.99, or any two of these. x2 can be a range consisting of any two of the following: 0.22, 0.25, 0.30, 0.35, 0.40, 0.45, 0.48, or these. x3 can be a range consisting of any two of the following: 0.18, 0.20, 0.22, 0.25, 0.30, 0.33, or these. x4 can be a range consisting of any two of the following: 0.15, 0.20, 0.25, 0.30, 0.40, 0.50, 0.57, or these. x5 can be a range consisting of any two of the following: 0.01, 0.05, 0.10, 0.20, 0.30, 0.35, or these.

[0069] Sulfur additives include at least one of vinyl sulfate and 1,3-propanesulfonyl lactone. Phosphorus additives include at least one of trimethyl phosphate, sodium difluorophosphate, trionitrile phosphate, and sodium difluorodioxane phosphate.

[0070] The secondary battery provided by the present invention has a double-coated structure for the positive electrode sheet. The first active layer is close to the positive electrode current collector, and the second active layer is far from the positive electrode current collector. By limiting the composition of each active layer and the electrolyte, the cold-start discharge terminal voltage of the secondary battery can be increased, as well as the cycle performance and high-temperature storage performance, while reducing the DC impedance. This is because by setting the P2-phase layered oxide as the first active layer on the side of the positive electrode current collector and the O3-phase layered oxide as the second active layer, the XRD diffraction peak angle θ1 of the P2-phase layered oxide is less than the diffraction peak angle θ2 of the O3-phase layered oxide. According to the Bragg equation nλ = 2dsinθ, the lower diffraction peak angle of θ1 indicates that the P2-phase layered oxide has a larger lattice spacing d, thereby enhancing the solid-phase sodium ion transport ability in the first active layer. This gradient design effectively alleviates the voltage polarization phenomenon caused by uneven internal electric field distribution in the positive electrode sheet. Especially under low-temperature and high-rate operating conditions, the utilization rate of the active material in the first active layer is significantly improved, thereby improving the DC impedance growth and cold-start performance of the secondary battery. By limiting the sodium content range of the P2-phase layered oxide to 0.6 < y1 < 0.85 and combining the doping of the Me1 element, the structural stability of the material can be optimized. At the same time, the x1 parameter of the O3-phase layered oxide is controlled within the range of 0.85 < x1 < 1 to ensure that it provides a high specific capacity as a capacity-type material.

[0071] The inorganic-organic composite CEI film formed by the phosphorus-based additive on the surface of the positive electrode sheet has certain toughness; the decomposition product of the sulfur-based additive forms an organic-inorganic composite film with good flexibility, which can better adapt to the volume change of the P2-phase or O3-phase positive electrode active material during cycling. When both the phosphorus-based additive and the sulfur-based additive are added to the electrolyte, a composite interface film with both good mechanical strength and adaptability can be formed on the surface of the positive electrode active material. The phosphorus-based additive forms a film to inhibit the dissolution of transition metals, and the sulfur-based additive forms a dense physical barrier to isolate the electrolyte. The two cooperate from both chemical and physical aspects to inhibit the side reactions at the interface between the positive electrode sheet and the electrolyte, improving the high-temperature storage and long-cycle performance of the battery. The solid electrolyte film formed by the sulfur-based additive can improve the ion conduction performance, which can compensate for the increase in interface impedance brought by the phosphorus-based additive, thereby taking into account both ion conductivity and interface stability. The structure of the positive electrode sheet combined with the electrolyte system can fully exert the low-temperature discharge capacity of the positive electrode active material and enable the secondary battery to have both high-temperature storage and long cycle life.

[0072] Therefore, the secondary battery provided by the present invention has a double-coated structure for the positive electrode sheet. The first active layer is close to the positive electrode current collector, and the second active layer is far from the positive electrode current collector. By limiting the composition of each active layer and the electrolyte, the cold-start discharge terminal voltage of the secondary battery can be increased, as well as the cycle performance and high-temperature storage performance, while reducing the DC impedance.

[0073] In some embodiments of the present invention, the mass percentage of copper in the positive electrode active layer is W1, and the electrolyte further includes fluorine additives, the total mass percentage of fluorine and phosphorus additives in the electrolyte is W2, and 0.4 ≤ W1 / W2 ≤ 4. For example, it can be a range of 0.4, 0.6, 1, 1.2, 1.5, 2, 2.2, 2.4, 3, 3.5, 4, or any two of these. Preferably, the mass percentage of copper in the positive electrode active layer is 0.5% to 5%, for example, it can be a range of 0.5%, 0.8%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, or any two of these. The total mass percentage of fluorine and phosphorus additives in the electrolyte is 1% to 5%, for example, it can be a range of 1%, 1.5%, 1.7%, 2%, 2.2%, 2.5%, 2.8%, 3%, 3.5%, 4%, 4.5%, 5%, or any two of these.

[0074] Fluorine additives include at least one of sodium difluorooxalate borate, sodium tetrafluoroborate, sodium difluorodioxalate phosphate, sodium difluorophosphate, and sodium bis(oxalate borate).

[0075] This invention effectively suppresses phase distortion and ion dissolution caused by copper doping under high voltage by controlling the copper content within the range of 0.5% to 5% and combining it with fluorine and phosphorus additives at a content of 1% to 5%. Specifically, copper doping in P2 phase materials can expand the interlayer spacing and promote sodium ion diffusion, but excessive doping can lead to the occupation of active sites and a reduction in specific capacity. Fluorine and phosphorus additives enhance interfacial stability by forming a CEI film, and their synergistic effect with copper ensures the structural integrity of the positive electrode active material under high voltage.

[0076] In some embodiments of the present invention, Me1 and Me2 include titanium, and the content of titanium on the side closer to the positive current collector is less than the content of titanium on the side farther from the positive current collector in the thickness direction of the positive current collector; preferably, the mass percentage of titanium in the positive active layer is 0% to 4%, for example, it can be 0%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4% or any two of them.

[0077] The gradient distribution design of titanium is based on its electron delocalization properties. Doping with titanium further away from the current collector expands the sodium ion transport channels, improving rate performance, and simultaneously forms a Na₂CO₃ / NaF-rich CEI film, suppressing side reactions in the electrolyte at high temperatures and enhancing storage performance. Conversely, the titanium content decreases closer to the current collector, avoiding active site blockage caused by excessive doping. Furthermore, this gradient distribution design improves the structural stability of the positive electrode.

[0078] It should be noted that in the cross-sectional SEM image of the positive electrode sheet, the thickness of the positive electrode active layer on one side is denoted as h. The region at a distance of 0.2h from the surface of the positive electrode current collector is denoted as the side close to the positive electrode current collector, and the region at a distance of 0.2h from the surface of the positive electrode active layer is denoted as the side far from the positive electrode current collector.

[0079] The doping of Ti element inhibits the Jahn-Teller distortion of Ni 2+ / Ni 3+ by occupying the octahedral sites of the transition metal layer, thereby reducing the structural phase change of the material during cycling and improving the cycle life. When the content of Ti element is 0% - 4%, it can effectively balance the expansion of the ion transport channel and the retention of active sites.

[0080] In some embodiments of the present invention, the median particle size Dv1 of the P2-phase layered oxide is smaller than the median particle size Dv2 of the O3-phase layered oxide; preferably, 2μm < Dv1 ≤ 6μm, for example, it can be 2.1μm, 2.5μm, 3μm, 3.5μm, 4μm, 4.5μm, 5μm, 5.5μm, 6μm or the range composed of any two of them; 6μm < Dv2 ≤ 10μm, for example, it can be 6.1μm, 6.5μm, 7μm, 7.5μm, 8μm, 8.5μm, 9μm, 9.5μm, 10μm or the range composed of any two of them.

[0081] The second active layer is formed of a material with a larger median particle size Dv2 to form a dense structure, which can reduce the side reactions occurring at the interface of the electrolyte and improve the high-temperature storage performance; the first active layer is constructed with a material with a smaller median particle size Dv1 to form a large-pore structure, which can promote the infiltration of the electrolyte and the rapid deintercalation and intercalation of sodium ions, thereby increasing the terminal voltage of cold start discharge.

[0082] In some embodiments of the present invention, in the thickness direction of the positive electrode current collector, the content of the first element on the side close to the positive electrode current collector is less than the content of the first element on the side far from the positive electrode current collector; the first element includes at least one of Fe, Ni, and Na; preferably, the difference between the content of Fe element on the side far from the positive electrode current collector and the content of Fe element on the side close to the positive electrode current collector is 17500ppm - 70000ppm, for example, it can be 17500ppm, 20000ppm, 30000ppm, 40000ppm, 50000ppm, 60000ppm, 70000ppm or the range composed of any two of them.

[0083] In the present invention, the gradient distribution of Fe, Ni, and Na elements can reduce the voltage polarization by enhancing the solid-phase sodium ion transport ability.

[0084] In some embodiments of the present invention, in the thickness direction of the positive electrode current collector, the content of the second element on the side closer to the positive electrode current collector is greater than the content of the second element on the side farther away from the positive electrode current collector; the second element includes at least one of Mn and Cu.

[0085] The gradient distribution of Mn and Cu elements can enhance electron delocalization, improve conductivity, and reduce sheet resistance. Furthermore, the higher content of Mn and Cu elements on the side closer to the positive electrode current collector facilitates the formation of a P2 phase structure with a larger interlayer spacing, which is beneficial for the diffusion of sodium ions.

[0086] In some embodiments of the present invention, the first active layer accounts for 5% to 55% of the mass percentage of the positive electrode active layer, for example, it can be a range of 5%, 10%, 20%, 30%, 40%, 50%, 55% or any two of these.

[0087] In some embodiments, the second active layer accounts for 45% to 95% of the mass percentage of the positive electrode active layer, for example, it can be a range of 45%, 50%, 60%, 70%, 80%, 90%, 95% or any two of these.

[0088] In this invention, the mass ratio of the first active layer to the second active layer can further improve the cold start discharge end voltage, DC impedance, cycle performance and high temperature storage performance of the secondary battery.

[0089] In some embodiments of the present invention, the secondary battery further includes a negative electrode sheet. In the Raman spectrum of the negative electrode sheet, the intensity ratio of the d peak to the g peak, Id / Ig, is greater than 1.1. For example, it can be a range of 1.11, 1.15, 1.2, 1.3, 1.4, 1.5, or any combination thereof. This provides sufficient defects to offer a high specific surface area and a fast ion transport channel, improves cold-start performance, and also ensures high-temperature storage performance.

[0090] In some embodiments, in the discharge capacity curve of the negative electrode sheet during coin cell testing, the capacity of the ramp region accounts for 30% to 50% of the total capacity of the negative electrode sheet. For example, it can be a range of 30%, 35%, 40%, 45%, 50%, or any two of these. The capacity of the ramp region is the capacity release within the coin cell testing voltage range of 0.1V to 1V (e.g., 0.1V, 0.2V, 0.3V, 0.4V, 0.5V, 0.6V, 0.7V, 0.8V, 0.9V, 1V, or any two of these), and the capacity of the negative electrode sheet is the capacity release within the coin cell testing voltage range of 0V to 2V (e.g., 0V, 0.5V, 1V, 1.5V, 1.8V, 2V, or any two of these). Selecting an appropriate ramp region capacity ratio can effectively balance the side reactions at the negative electrode sheet interface and the low-temperature ion transport performance, as well as the high-temperature storage performance and the low-temperature charge-discharge performance.

[0091] In some embodiments of the present invention, the negative electrode sheet includes a first negative electrode active material and a second negative electrode active material; the Id / Ig ratio of the first negative electrode active material is greater than that of the second negative electrode active material, the specific surface area of ​​the first negative electrode active material is greater than that of the second negative electrode active material, and the median particle size of the first negative electrode active material is smaller than that of the second negative electrode active material; preferably, the Id / Ig ratio of the first negative electrode active material is 1.1~1.3, for example, it can be a range of 1.1, 1.15, 1.2, 1.25, 1.3 or any two of these. The Id / Ig ratio of the second negative electrode active material is 0.85~1.06, for example, it can be a range of 0.85, 0.9, 0.95, 1.0, 1.06 or any two of these. The specific surface area of ​​the first negative electrode active material is 3m³. 2 / g~8m 2 / g, for example, can be 3m 2 / g、4m 2 / g、5m 2 / g、6m 2 / g、7m 2 / g、8m 2 The range is / g or any combination thereof. The specific surface area of ​​the second negative electrode active material is 1m². 2 / g~3m 2 / g, for example, can be 1m 2 / g, 1.5m 2 / g、2m 2 / g, 2.5m 2 / g、3m 2The median particle size of the first negative electrode active material is 2 μm to 6 μm, for example, it can be a range of 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, or any two of these. The median particle size of the second negative electrode active material is 6 μm to 10 μm, for example, it can be a range of 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, or any two of these.

[0092] The present invention can provide a fast ion transport channel through the first negative electrode active material (power type material) to improve the charge and discharge capability; while the second negative electrode active material (capacity type material) can ensure the high specific capacity of the secondary battery.

[0093] In some embodiments of the present invention, the electrolyte further includes boron-based additives; the boron-based additives include at least one selected from sodium bis(oxalato)borate, tris(trimethylsilane)borate, sodium tetrafluoroborate, and sodium difluorooxalatoborate. The boron-based additives in the electrolyte can inhibit the dissolution of transition metals through complexation and enhance the ionic conductivity of the first active layer.

[0094] The boron additive accounts for 1% to 2% of the electrolyte by mass, for example, it can be 1%, 1.2%, 1.4%, 1.5%, 1.7%, 1.9%, 2% or any combination thereof.

[0095] The above-mentioned boron additives can further inhibit the dissolution of transition metals, improve the stability of CEI film, and reduce the DCR growth rate.

[0096] In some embodiments, the electrolyte further includes a first solvent and a cyano-containing compound; the cyano-containing compound includes at least one selected from malononitrile, succinic anionyl nitrile, glutaronitrile, oxonium nitrile, 1,2,3-propanetricarbonate, 1,3,5-pentanetricarbonate, and 1,3,6-hexanetrionitrile; the first solvent includes at least one selected from diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, and 1,2-methoxyethane; the first solvent accounts for 5% to 70% of the electrolyte by mass, for example, it can be in the range of 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, or any two of these. The cyano-containing compound accounts for 4% to 10% of the electrolyte by mass, for example, it can be in the range of 4%, 5%, 6%, 7%, 8%, 9%, 10%, or any two of these.

[0097] In this invention, the first solvent is a low-viscosity solvent.

[0098] It can reduce the viscosity of the electrolyte at low temperatures and improve the ion transport efficiency at low temperatures, thereby increasing the cold start discharge terminal voltage of the secondary battery. Compounds containing cyano groups can increase the dielectric constant of the electrolyte and reduce the overall viscosity, thereby further improving the low-temperature discharge capability of the secondary battery.

[0099] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0100] Example 1

[0101] The method for preparing the secondary battery in this embodiment includes:

[0102] 1) Preparation of the positive electrode:

[0103] 95kg Na 0.62 Ni 0.23 Mn 0.67 Cu 0.05 Mg 0.04 Ti 0.01 O2 (P2 phase layered oxide), 3 kg of conductive carbon black, 2 kg of polyvinylidene fluoride and 100 kg of N-methylpyrrolidone were mixed and homogenized at high speed to obtain the first positive electrode slurry.

[0104] 95kg Na 0.96 Ni 0.34 Fe 0.25 Mn 0.33 Ti 0.05 Zn 0.03 O2 (O3 phase layered oxide), 3 kg of conductive carbon black, 2 kg of polyvinylidene fluoride and 100 kg of N-methylpyrrolidone were mixed and homogenized at high speed to obtain the second positive electrode slurry.

[0105] The first and second positive electrode slurries are extruded onto the upper and lower sides of an aluminum foil using a double-layer die, respectively, to form the first and second positive electrode active layers on the upper and lower sides of the aluminum foil. The areal density of the first positive electrode active layer is 0.01 g / cm³. 2 The areal density of the second positive electrode active layer is 0.005 g / cm³. 2 Then, the positive electrode sheet is obtained through oven drying, rolling, and die-cutting processes.

[0106] The positive electrode includes a positive current collector and a positive active layer disposed on at least one side of the positive current collector. In the thickness direction of the positive current collector, the positive active layer includes a first active layer and a second active layer stacked together, wherein the first active layer is close to the positive current collector. The first active layer includes Na. 0.62 Ni 0.23 Mn 0.67 Cu 0.05 Mg 0.04 Ti 0.01 O2 (P2 phase layered oxide), the second active layer includes Na 0.96 Ni 0.34 Fe 0.25 Mn 0.33 Ti0.05 Zn 0.03 The first diffraction peak angle θ1 of the O2 (O3 phase layered oxide) is 15.9°, and the second diffraction peak angle θ2 of the O3 phase layered oxide is 16.3°. In the thickness direction of the positive electrode current collector, the titanium content on the side closer to the positive electrode current collector is less than that on the side farther from the positive electrode current collector; the median particle size Dv1 of the P2 phase layered oxide is less than that of the O3 phase layered oxide; in the thickness direction of the positive electrode current collector, the content of the first element on the side closer to the positive electrode current collector is less than that on the side farther from the positive electrode current collector; the first element includes at least one of Fe, Ni, and Na; the difference in Fe content between the side farther from the positive electrode current collector and the side closer to the positive electrode current collector is 32870 ppm; in the thickness direction of the positive electrode current collector, the content of the second element on the side closer to the positive electrode current collector is greater than that on the side farther from the positive electrode current collector; the second element includes at least one of Mn and Cu.

[0107] 2) Preparation of the negative electrode:

[0108] 8 kg of power-type hard carbon (first negative electrode active material), 36 kg of capacity-type hard carbon (second negative electrode active material), 1.6 kg of conductive carbon black, 0.8 kg of styrene-butadiene rubber, 0.4 kg of sodium carboxymethyl cellulose and 18.6 kg of deionized water were mixed and homogenized at high speed to obtain the negative electrode slurry.

[0109] The negative electrode paste was coated on both the top and bottom sides of the aluminum foil, with an areal density of 0.005 g / cm³. 2 After drying, it is rolled until the compacted density is 1.0 g / cm³. 3 Then, it is die-cut to obtain the negative electrode sheet.

[0110] In the Raman spectrum of the negative electrode, the intensity ratio of the d peak to the g peak, Id / Ig, is 1.16. In the discharge capacity curve of the negative electrode under coin cell testing, the capacity in the ramp region accounts for 43% of the capacity of the negative electrode. The capacity in the ramp region is the capacity release in the coin cell testing voltage range >0.1V, and the capacity of the negative electrode is the capacity release in the coin cell testing voltage range of 0.01V~2V.

[0111] 3) Preparation of electrolyte:

[0112] The electrolyte contains dimethyl carbonate (DMC), propylene carbonate (PC), and malononitrile as solvents. The DMC content is 50 wt%, and the malononitrile content is 7.2 wt%. The electrolyte also contains additives 1,3-propanesulfonyl lactone (PS), trimethyl phosphate, sodium tetrafluoroborate, and sodium bis(oxalato)borate. The PS content is 3 wt%, the trimethyl phosphate content is 2 wt%, the sodium tetrafluoroborate content is 1 wt%, and the sodium bis(oxalato)borate content is 0.5 wt%. The electrolyte also contains 14 wt% sodium salt NaPF6. The remaining components after removing the above substances are all propylene carbonate.

[0113] Battery manufacturing:

[0114] The battery cell is manufactured by stacking the positive electrode, separator, and negative electrode in that order. The separator is made of polyethylene (PE) with a thickness of 8 μm, and the surface coating consists of ceramic particles and polyvinylidene fluoride (PVDF) with a thickness of 3 μm. The cells are then encapsulated, dried, and injected with the prepared electrolyte for 24 hours of aging. Finally, after chemical separation and sorting, a sodium-ion battery is obtained.

[0115] Examples 2-21, 26-35

[0116] The preparation methods of the secondary batteries in Examples 2-21 and 26-35 are basically the same as those in Example 1, except that one or more steps in the preparation method are changed, as shown in Table 1.

[0117] Example 22

[0118] The preparation method of the secondary battery in Example 22 is basically the same as that in Example 1, except that in step 2), the negative electrode sheet is prepared by mixing 10 kg of power-type hard carbon (first negative electrode active material), 34 kg of capacity-type hard carbon (second negative electrode active material), 1.6 kg of conductive carbon black, 0.8 kg of styrene-butadiene rubber, 0.4 kg of sodium carboxymethyl cellulose and 18.6 kg of deionized water, and then homogenizing at high speed to obtain the negative electrode slurry.

[0119] Example 23

[0120] The preparation method of the secondary battery in Example 23 is basically the same as that in Example 1, except that in step 2), the negative electrode sheet is prepared by mixing 4 kg of power-type hard carbon (first negative electrode active material), 40 kg of capacity-type hard carbon (second negative electrode active material), 1.6 kg of conductive carbon black, 0.8 kg of styrene-butadiene rubber, 0.4 kg of sodium carboxymethyl cellulose and 18.6 kg of deionized water, and then homogenizing at high speed to obtain the negative electrode slurry.

[0121] Example 24

[0122] The preparation method of the secondary battery in Example 24 is basically the same as that in Example 1, except that in step 2), the negative electrode sheet is prepared by mixing 2 kg of power-type hard carbon (first negative electrode active material), 42 kg of capacity-type hard carbon (second negative electrode active material), 1.6 kg of conductive carbon black, 0.8 kg of styrene-butadiene rubber, 0.4 kg of sodium carboxymethyl cellulose and 18.6 kg of deionized water, and then homogenizing at high speed to obtain the negative electrode slurry.

[0123] Example 25

[0124] The preparation method of the secondary battery in Example 25 is basically the same as that in Example 1, except that in step 2), the negative electrode sheet is prepared by mixing 16 kg of power-type hard carbon (first negative electrode active material), 28 kg of capacity-type hard carbon (second negative electrode active material), 1.6 kg of conductive carbon black, 0.8 kg of styrene-butadiene rubber, 0.4 kg of sodium carboxymethyl cellulose and 18.6 kg of deionized water, and then homogenizing at high speed to obtain the negative electrode slurry.

[0125] Comparative Example 1

[0126] The preparation method of the secondary battery in Comparative Example 1 is basically the same as that in Example 1, except that both the first active layer and the second active layer include Na. 0.96 Ni 0.34 Fe 0.25 Mn 0.33 Ti 0.05 Zn 0.03 O2 (O3 phase layered oxide).

[0127] Comparative Example 2

[0128] The preparation method of the secondary battery in Comparative Example 2 is basically the same as that in Example 1, except that both the first active layer and the second active layer include Na. 0.62 Ni 0.23 Mn 0.67 Cu 0.05 Mg 0.04 Ti 0.01 O2 (P2 phase layered oxide).

[0129] Comparative Example 3

[0130] The preparation method of the secondary battery in Comparative Example 3 is basically the same as that in Example 1, except that the first active layer includes Na. 0.96 Ni 0.34 Fe 0.25 Mn 0.33 Ti 0.05 Zn 0.03 O2 (O3 phase layered oxide), the second active layer includes Na 0.62 Ni 0.23 Mn 0.67 Cu0.05 Mg 0.04 Ti 0.01 O2 (P2 phase layered oxide).

[0131] Comparative Example 4

[0132] The preparation method of the secondary battery in Comparative Example 4 is basically the same as that in Example 1, except that the content of PS in the electrolyte is 6 wt%.

[0133] Comparative Example 5

[0134] The preparation method of the secondary battery in Comparative Example 5 is basically the same as that in Example 1, except that the content of PS in the electrolyte is 0.05 wt%.

[0135] Comparative Example 6

[0136] The preparation method of the secondary battery in Comparative Example 6 is basically the same as that in Example 1, except that the content of sodium difluorophosphate in the electrolyte is 4 wt%.

[0137] Comparative Example 7

[0138] The preparation method of the secondary battery in Comparative Example 7 is basically the same as that in Example 1, except that the content of sodium difluorophosphate in the electrolyte is 0.05 wt%.

[0139] Experimental example:

[0140] 1. XRD Testing: X-ray diffractometer (XRD, Bruker D8) was used to acquire electrode or powder diffraction data; the X-ray source was CuKα, with an operating voltage of 40kV and a current of 40mA. The sample scanning angle range was 10-80 degrees, and the step size was 0.02 degrees.

[0141] 2. Element content testing method: The content of each element is tested by ICP. The element content test of the first active layer requires scraping powder from the surface of the positive electrode to test, and the element content test of the second active layer requires scraping powder from the vicinity of the current collector to test.

[0142] ICP testing for elemental content: First, place the positive active electrode material in a polytetrafluoroethylene digestion vessel, slowly add the prepared hydrochloric acid and concentrated nitric acid mixed solution, cover and heat on a hot plate to 120℃ for 30 minutes until the solution is clear and transparent; after cooling, dilute to 30 mL with ultrapure water, and use this solution as the impurity analysis solution. After dilution, dilute the sample solution by 1 part with ultrapure water, and use this solution as the major element analysis solution. Two parallel test groups and two spiking groups are set up for each sample, and two preparative blank groups are also set up. The processed sample solutions are tested on the instrument, and the intensity of the elemental characteristic spectral lines (ICP-OES) is recorded. Finally, the concentration of each element in the sample is calculated according to the standard curve, and the original content is converted by combining the dilution factor and sample weight. Test method for characteristic element content of the first and second active layers: Randomly select the prepared positive electrode sheet and place it on the sample stage, and simultaneously obtain a smooth and flat cross-section by argon ion beam grinding. After locating the sample in low-magnification SEM mode, the mode is switched to high-magnification. Multiple line scans are performed on locations near the foil and the surface of the positive electrode to obtain reliable values ​​for each element, which represent the content of characteristic elements in the first and second active layers.

[0143] 3. Content of each additive in the electrolyte: tested by GC-MS.

[0144] 4. First solvent content: tested by GC-MS.

[0145] 5. Median particle size: can be determined using a laser particle size analyzer (such as MalvernMaster Size 300) in accordance with standard GB / T 19077.1-2016.

[0146] 6. Content of the first or second active layer: The content ratio of the first and second active layers is defined as the ratio of their thicknesses in the SEM. Since the positive electrode active materials of the first and second active layers are both layered oxygen materials with similar compaction densities, according to the formula: areal density = compaction density × thickness, and areal density × area = mass, taking the first and second active layers with the same area, the content ratio of the first and second active layers is obtained, which is the ratio of their thicknesses in the SEM. Since the sum of the contents of the first and second active layers equals 100%, the mass percentage of each active layer is obtained.

[0147] 7. Id / Ig Test: A Renishaw inVia Qontor Raman spectrometer was used, employing a 532nm laser as the excitation source with a power of 10mW. The Raman shift measurement range was set to 100cm. -1 ~3500cm -1Specific method: Place the sample to be tested on the sample stage of the spectrometer, ensuring the sample surface is flat and perpendicular to the excitation light, then collect the Raman spectrum. Locate the positions of the D peak and G peak in the Raman spectrum (the D peak is usually located at 1300 cm⁻¹). -1 The G peak is located at approximately 1580 cm. -1 (Left and right) Read the maximum value of the peak as the peak intensity, substitute the measured D peak intensity and G peak intensity into the formula Id / Ig=I(D) / I(G) to calculate the ratio.

[0148] 8. Capacity Ratio of the Slope Region: A coin cell is fabricated using a negative electrode sheet. The working electrode is the treated negative electrode sheet, and the reference electrode is a sodium metal sheet. Negative electrode sheet treatment: A relatively intact negative electrode sheet is taken, and a 1.21 cm² area is used... 2 The die is used to punch a round shape, and then the single-sided coating is wiped with an alcohol-soaked cotton cloth dipped in DMC solvent to obtain the processed negative electrode sheet. The coin cell capacitance is within the voltage range of 2V-0.01V. It is calibrated by cycling with a 1mA current for 3 cycles. The charging data from the third cycle is taken. The ramp capacitance, denoted as Q, is the capacitance occupied by the voltage greater than 0.1V. 斜坡 A capacitance of 0.01V-2V is denoted as Q. 总 Slope area capacity ratio = Q 斜坡 / Q 总 .

[0149] 9. Specific Surface Area: The specific surface area of ​​the negative electrode active material is tested as follows: Weigh the total weight of the empty small test tube and the stopper. Soak the powder sample of the negative electrode active material to be tested in anhydrous ethanol for 4 hours. Then, take out the powder sample and dry it in an oven at 105℃ for half an hour. Next, put the powder sample into the sample tube and weigh the total weight of the powder sample, the small test tube and the stopper to calculate the sample mass. Turn on the degassing station and put the small test tube containing the powder sample into the degassing station at 105℃. Purge with nitrogen (pure nitrogen) for 30 minutes, cool for 15 minutes, and test on the instrument at 25℃ and 60% humidity. P / P0 with points in the range of 0.05~0.25 as the x-axis and P / V(P0-P) as the y-axis. Plot a graph using the BET equation and perform linear fitting to obtain the slope and intercept of the straight line, thereby calculating the BET specific surface area of ​​the powder sample.

[0150] 10. DC Impedance Test at Room Temperature (DCR Test): The secondary batteries in the above embodiments and comparative examples were subjected to DCR testing according to the following steps: The secondary battery was placed in a constant temperature chamber at 25°C for 5 minutes, charged at a constant current and constant voltage of 1C to the upper limit voltage, with a cutoff current of 0.05C, and then discharged at a current of 1C to the lower limit voltage to obtain the rated capacity C0. Then, at the rated capacity C0, with a cutoff current of 0.05C, the battery was allowed to stand at 0.33C until it reached 60% SOC for DCR testing. The test parameters were: 60% SOC, 5C, 10s (voltage sampling frequency: 0.1s per point). The DC impedance (DCR) was calculated using the following formula: R = (U1 - U2) / 5C × 1000, where U1 is the static voltage at 60% SOC, and U2 is the voltage at the end of the 5C pulse. The obtained DCR is the DC impedance of this application, which is the value at 60% SOC (i.e., state of charge), in milliohms.

[0151] 11. Cold Start Test: The secondary batteries in the above embodiments and comparative examples were subjected to a cold start test according to the following steps: 1) The sodium-ion battery was placed in a constant temperature chamber at 25°C for 5 minutes, charged at 1C constant current and constant voltage to the upper limit voltage, with a cutoff current of 0.05C, and then discharged at 1C current to the lower limit voltage to obtain the calibrated capacity C0. 2) The battery was charged at 1C standard constant current and constant voltage to the upper limit voltage, with a cutoff current of 0.05C, and then the SOC was adjusted to 75% at 1C. 3) Cell packaging test: After standing in a -20°C chamber for 4 hours, the battery was discharged sequentially at 5.5C for 10 seconds, 10.5C for 0.5 seconds, and 5.5C for 150 seconds, and the voltage at the end of the discharge was recorded with a sampling accuracy of 100ms.

[0152] 12. Cyclic Performance: Under 25℃ conditions, the capacitor is charged at a constant current rate of 3C to 3.9V, then charged at a constant voltage rate of 0.5C to 3.9V, and then discharged at a discharge rate of 3C to 2V. This charge-discharge cycle is repeated 300 times. The discharge capacity Q1 at the first cycle and the discharge capacity Q at the 300th cycle are measured. 300 The capacity retention rate after 300 cycles is Q = Q 300 / Q1×100%.

[0153] 13. High-Temperature Storage Performance: At 25℃, the battery is charged at 1C constant current and constant voltage to the cutoff voltage, then cut off at 0.05C. After standing for 30 minutes, it is discharged at 1C constant current to the cutoff voltage, and the initial discharge capacity C0 is recorded. Then, it is charged at 1C constant current and constant voltage to the cutoff voltage, and cut off at 0.05C. The battery is stored at 60℃ for 42 days, and the capacity is tested every 7 days to recover. The recovered capacity after 42 days is C1. The capacity recovery rate is calculated as C1 / C0 × 100%, and the high-temperature storage performance is evaluated based on the capacity recovery rate.

[0154] Recovery capacity test: At 25℃, discharge at 1C constant current to the cutoff voltage, charge at 1C constant current and constant voltage to the cutoff voltage, cut off at 0.05C, let stand for 30 minutes, and then discharge at 1C constant current to the cutoff voltage. This is the recovery capacity C1.

[0155] Figure 1 A cross-sectional SEM image of the positive electrode sheet of Embodiment 1 provided by the present invention.

[0156] from Figure 1 As can be seen, the positive electrode sheet of Example 1 includes a positive current collector and a positive active layer disposed on at least one side of the positive current collector. The positive active layer includes a first active layer disposed on at least one side of the positive current collector and a second active layer disposed on the side of the first active layer opposite to the positive current collector.

[0157] Figure 2 The mapping diagram of Ni element in the positive electrode of Embodiment 1 provided by the present invention.

[0158] from Figure 2 It can be seen that in the positive electrode sheet of Example 1, the content of Ni element on the side closer to the positive current collector is less than the content of Ni element on the side farther away from the positive current collector.

[0159] Figure 3 The mapping diagram of Fe element in the positive electrode of Embodiment 1 provided by the present invention;

[0160] from Figure 3 It can be seen that in the positive electrode sheet of Example 1, the Fe content on the side closer to the positive current collector is less than the Fe content on the side farther away from the positive current collector.

[0161] Figure 4 The mapping diagram of Mn element in the positive electrode of Embodiment 1 provided by the present invention.

[0162] from Figure 4 It can be seen that in the positive electrode sheet of Example 1, the content of Mn element on the side closer to the positive electrode current collector is greater than the content of Mn element on the side farther away from the positive electrode current collector.

[0163] Figure 5 The mapping diagram of Ti element in the positive electrode of Embodiment 1 provided by the present invention.

[0164] from Figure 5 It can be seen that in the positive electrode sheet of Example 1, the content of Ti element on the side closer to the positive electrode current collector is less than the content of Ti element on the side farther away from the positive electrode current collector.

[0165] Figure 6 EDS cross-sectional view of the positive electrode sheet of Embodiment 1 provided by the present invention.

[0166] from Figure 6 As can be seen, in the positive electrode sheet of Example 1, the Ti element accounts for 0.6% of the mass percentage of the positive electrode active layer, and the Cu element accounts for 2.8% of the mass percentage of the positive electrode active layer.

[0167] Figure 7 The mapping diagram of Ni element in the positive electrode of Comparative Example 1 provided by the present invention.

[0168] Figure 8 The mapping diagram of Fe element in the positive electrode of Comparative Example 1 provided by the present invention.

[0169] Figure 9 The mapping diagram of Mn element in the positive electrode of Comparative Example 1 provided by the present invention.

[0170] Figure 10 The mapping diagram of Ti element in the positive electrode of Comparative Example 1 provided by the present invention.

[0171] from Figures 7-10 It can be seen that Ni, Fe, Mn and Ti elements are uniformly distributed in the positive electrode of Comparative Example 1.

[0172] Figure 11 The Raman spectrum of the negative electrode sheet of Example 1 provided by the present invention.

[0173] from Figure 11 It can be seen that in the Raman spectrum of the negative electrode of Example 1, the intensity ratio of the d peak to the g peak is Id / Ig = 1.16.

[0174] Figure 12 The XRD pattern of the positive electrode sheet of Embodiment 1 provided by the present invention.

[0175] from Figure 12 It can be seen that the positive electrode in Example 1 includes a first diffraction peak at 15°~17° and a second diffraction peak at 16°~17°, and the angle θ1 of the first diffraction peak is smaller than the angle θ2 of the second diffraction peak.

[0176] Table 1

[0177]

[0178] Table 2

[0179]

[0180] Table 3

[0181]

[0182] Table 4

[0183]

[0184] Table 5

[0185]

[0186] Table 6

[0187]

[0188] Table 7

[0189]

[0190] Table 8

[0191]

[0192] As shown in Tables 1-8, compared with the comparative examples, the secondary battery provided by the present invention has a double-layer coating structure for the positive electrode sheet. The first active layer is close to the positive electrode current collector, and the second active layer is far away from the positive electrode current collector. The composition of each active layer and electrolyte is defined, which can take into account the cold start discharge end voltage, cycle performance and high temperature storage performance of the secondary battery, while reducing DC resistance.

[0193] Finally, it should be noted that other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and practice of the invention disclosed herein. This invention is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein, and is not limited to the precise structures described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of the invention is limited only by the appended claims.

Claims

1. A secondary battery, characterized in that, It includes a positive electrode sheet and an electrolyte. The positive electrode sheet includes a positive electrode current collector and a positive electrode active layer provided on at least one side of the positive electrode current collector; in the thickness direction of the positive electrode current collector, the positive electrode active layer includes a first active layer and a second active layer arranged in a stacked manner, wherein the first active layer is close to the positive electrode current collector. The first active layer includes a P2-phase layered oxide represented by Formula 1; the second active layer includes an O3-phase layered oxide represented by Formula 2. Na y1 Ni y2 Fe y3 Mn y4 Cu y5 Me1 y6 O2 formula 1 Na x1 Ni x2 Fe x3 Mn x4 Me2 x5 O2 formula 2, Where, 0.6 < y1 < 0.85, 0.16 < y2 < 0.41, 0 ≤ y3 < 0.17, 0.39 < y4 < 0.79, 0 < y5 < 0.24, 0 < y6 < 0.16, Me1 includes at least one of Li, Mg, Zn, Ti, Nb, B, Al; 0.85 < x1 < 1, 0.21 < x2 < 0.49, 0.17 < x3 < 0.34, 0.14 < x4 < 0.58, 0 < x5 < 0.36, Me2 includes at least one of Li, K, Cu, Mg, Zn, Ti, B, Al. In the X-ray diffraction pattern, the P2-phase layered oxide includes a first diffraction peak at 15° - 17°, and the O3-phase layered oxide includes a second diffraction peak at 16° - 17°, and the angle θ1 of the first diffraction peak is less than the angle θ2 of the second diffraction peak. The electrolyte includes a sulfur-based additive and a phosphorus-based additive. The mass percentage content of the sulfur-based additive in the electrolyte is 0.1% - 5%. The mass percentage content of the phosphorus-based additive in the electrolyte is 0.1% - 3%.

2. The secondary battery according to claim 1, characterized in that, The mass percentage content of copper element in the positive electrode active layer is W1. The electrolyte further includes a fluorine-based additive, and the total mass percentage content of the fluorine-based additive and the phosphorus-based additive in the electrolyte is W2, 0.4 ≤ W1 / W2 ≤ 4. Preferably, the mass percentage content of copper element in the positive electrode active layer is 0.5% - 5%, and the total mass percentage content of the fluorine-based additive and the phosphorus-based additive in the electrolyte is 1% - 5%.

3. The secondary battery according to claim 1 or 2, characterized in that, Me1 and Me2 include titanium element. In the thickness direction of the positive electrode current collector, the content of titanium element on the side close to the positive electrode current collector is less than the content of titanium element on the side far from the positive electrode current collector. Preferably, the mass percentage content of titanium element in the positive electrode active layer is 0% - 4%.

4. The secondary battery according to any one of claims 1-3, characterized in that, The median particle size Dv1 of the P2-phase layered oxide is less than the median particle size Dv2 of the O3-phase layered oxide. Preferably, 2μm < Dv1 ≤ 6μm, 6μm < Dv2 ≤ 10μm.

5. The secondary battery according to any one of claims 1-4, characterized in that, In the thickness direction of the positive electrode current collector, the content of the first element on the side close to the positive electrode current collector is less than the content of the first element on the side far from the positive electrode current collector; the first element includes at least one of Fe, Ni, Na. Preferably, the difference between the content of Fe element on the side far from the positive electrode current collector and the content of Fe element on the side close to the positive electrode current collector is 17500ppm - 70000ppm.

6. The secondary battery according to any one of claims 1-5, characterized in that, In the thickness direction of the positive electrode current collector, the content of the second element on the side closer to the positive electrode current collector is greater than the content of the second element on the side farther away from the positive electrode current collector; the second element includes at least one of Mn and Cu.

7. The secondary battery according to any one of claims 1-6, characterized in that, The first active layer accounts for 5% to 55% of the mass percentage of the positive electrode active layer; And / or, the second active layer accounts for 45% to 95% of the mass percentage of the positive electrode active layer.

8. The secondary battery according to any one of claims 1-7, characterized in that, The secondary battery also includes a negative electrode, and in the Raman spectrum of the negative electrode, the intensity ratio of the d peak to the g peak of the negative electrode is Id / Ig > 1.

1. And / or, in the discharge capacity curve of the negative electrode sheet during the coin cell test, the capacity of the ramp region accounts for 30% to 50% of the capacity of the negative electrode sheet; Wherein, the capacity of the ramp zone is the capacity release in the coin cell test voltage range > 0.1V, and the capacity of the negative electrode is the capacity release in the coin cell test voltage range of 0.01V~2V.

9. The secondary battery according to claim 8, characterized in that, The negative electrode sheet includes a first negative electrode active material and a second negative electrode active material; the Id / Ig of the first negative electrode active material is greater than the Id / Ig of the second negative electrode active material, the specific surface area of ​​the first negative electrode active material is greater than the specific surface area of ​​the second negative electrode active material, and the median particle size of the first negative electrode active material is smaller than the median particle size of the second negative electrode active material. Preferably, the Id / Ig ratio of the first negative electrode active material is 1.1 to 1.3; The Id / Ig ratio of the second negative electrode active material is 0.85~1.06; The specific surface area of ​​the first negative electrode active material is 3m². 2 / g~8m 2 / g; The specific surface area of ​​the second negative electrode active material is 1m². 2 / g~3m 2 / g; The median particle size of the first negative electrode active material is 2 μm to 6 μm; The median particle size of the second negative electrode active material is 6 μm to 10 μm.

10. The secondary battery according to any one of claims 1-9, characterized in that, The electrolyte also includes boron-based additives; The boron-based additive accounts for 1% to 2% of the mass percentage of the electrolyte. And / or, the electrolyte further includes a first solvent and a compound containing a cyano group; The first solvent includes at least one of diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, and 1,2-methoxyethane; The first solvent accounts for 5% to 70% of the mass percentage of the electrolyte; And / or, the cyano-containing compound includes at least one of malononitrile, succinic anionyl, glutaronitrile, ethylenedionitrile, 1,2,3-propanetricarbonyl, 1,3,5-pentanetricarbonyl, and 1,3,6-hexanetrionitrile; The cyano-containing compound accounts for 4% to 10% of the mass percentage of the electrolyte.

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