A lithium-ion battery, a preparation method thereof, and a power vehicle

Abstract

The present application provides a lithium ion battery and a preparation method thereof, wherein LiFe 1‑a Mn a PO4 material and ternary material are mixed as the positive electrode active material of the positive electrode sheet, and the positive electrode active material in the first positive electrode coating close to the positive electrode current collector is controlled to include LiFe 1‑ a Mn a The positive electrode active material in the second positive electrode coating layer, which is located away from the positive electrode current collector, comprises a ternary material. By controlling the ratio of these two materials in each layer, the defined parameter Y is in the range of -7 to 4.5, and the parameter Z is in the range of 0-6. This ensures that the battery has both high energy density and excellent safety performance. The present application also provides a power vehicle.

Description

Technical Field

[0001] The present application relates to the field of battery technology, and in particular to a lithium-ion battery, a preparation method thereof, and a power vehicle. Background Art

[0002] Lithium-ion batteries are a new generation of green high-energy batteries, widely used in electronic equipment, power vehicles and other fields. As an important component of lithium-ion batteries, the selection of positive electrode materials directly affects the performance of lithium-ion batteries. Among them, the positive electrode materials used in power batteries mainly include LiFe 1-a Mn a PO4 (lithium manganese iron phosphate, lithium iron phosphate), high energy density ternary materials (such as lithium nickel cobalt manganese oxide), but both types of materials have their own disadvantages, such as LiFe 1-a Mn a PO4 has a low specific capacity and the safety performance of ternary materials is poor. The industry believes that mixing these two materials can potentially take into account the advantages of both and reduce their disadvantages.

[0003] Existing LiFe 1-a Mn a The mixing of PO4 and ternary materials involves directly coating the mixed slurry of the two onto the current collector. However, this mixing solution does not effectively improve battery safety, especially when the mixing ratio of the ternary materials is high. Therefore, it is necessary to provide a battery and its preparation method that can truly achieve both high safety and high energy density. Summary of the Invention

[0004] In view of this, this application will LiFe 1-a Mn a PO4 (0≤a≤0.8) and ternary materials are used as active materials of the positive electrode sheet, and the positive electrode sheet is controlled to include LiFe 1-a Mn a The first positive electrode coating of PO4 (0≤a≤0.8) contains LiFe 1-a Mn a The second cathode coating of PO4 and ternary materials is achieved by regulating LiFe 1-a Mn a The ratio of PO4 in the two positive electrode coatings can fully utilize the capacity of the overall positive electrode active material while giving the battery excellent safety performance.

[0005] In the first aspect, the embodiment of the present application provides a lithium-ion battery comprising a positive electrode sheet, a negative electrode sheet, and a separator located between the positive electrode sheet and the negative electrode sheet, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector, wherein the positive electrode active material layer comprises a first positive electrode coating and a second positive electrode coating, wherein the first positive electrode coating is close to the positive electrode current collector, and the positive electrode active material in the first positive electrode coating comprises LiFe 1- a Mn a PO4, the positive electrode active material in the second positive electrode coating includes LiFe 1-a Mn a PO4 and ternary materials; of the total positive active materials of the positive electrode sheet, LiFe 1-a Mn a The mass proportions of PO4 and ternary materials are A1 and A2 respectively; the LiFe in the first positive electrode coating is 1-a Mn a The mass ratio of PO4 in the total positive electrode active material is A5, and the LiFe 1-a Mn a The mass proportion of PO4 in the total positive electrode active material is A3;

[0006] Define the following parameters:

[0007] Y=[(Q3×(1-η3)×M3-Q1×(1-η1)×(M1+M2)×A1-Q2×(1-η2)×(M1+M2)×A2)] / β

[0008] Z=α×(Q1×η1×A1+Q2×η2×A2) / β

[0009] and said Y is in the range of -7 to 4.5, and said Z is in the range of 0 to 6;

[0010] Wherein, 0≤a≤0.8, Q1, η1 are respectively the LiFe 1-a Mn aThe first charge specific capacity and first efficiency of PO4, Q2 and η2 are respectively the first charge specific capacity and first efficiency of the ternary material, M1 and M2 are respectively the amount of the positive electrode active material in the first positive electrode coating and the positive electrode active material in the second positive electrode coating on the positive electrode sheet; Q3 and η3 are respectively the first discharge specific capacity and first efficiency of the negative electrode active material in the negative electrode sheet, M3 is the amount of the negative electrode active material on the negative electrode sheet; α is the longitudinal thermal diffusion rate of the positive electrode sheet, in mm / s; β is the energy density of the lithium-ion battery, wherein the units of Q1, Q2, and Q3 are all mAh / g, the unit of β is mWh / g; the units of M1, M2, and M3 are all g / m 2 .

[0011] It should be noted that the above Q1 and η1, Q2 and η2, Q3 and η3 can be obtained by respectively 1-a Mn a The actual test results are obtained by testing button batteries made of PO4 (0≤a≤0.8), ternary materials, and negative electrode active materials (such as graphite), which will be described in detail below in this application. M1, M2, and M3 are parameters that are determined when designing the battery and can also be obtained through actual measurement. β is obtained by actual testing of the above-mentioned square shell lithium-ion battery, and α is obtained by actual testing of the longitudinal thermal diffusion rate of the positive electrode of the above-mentioned lithium-ion battery, which will be described in detail below in this application. The unit of Y is g / (m 2 *V), the unit of Z is mm / (s*V).

[0012] Although the positive electrode sheet in this application contains LiFe 1-a Mn a PO4 and ternary materials, but they are distributed in different proportions, so that the positive electrode active material layer of the positive electrode sheet includes a first positive electrode coating and a second positive electrode coating with different compositions, wherein the positive electrode active material in the first positive electrode coating close to the positive electrode current collector contains LiFe 1-a Mn a PO4, the positive electrode active material in the second positive electrode coating away from the positive electrode current collector side contains a ternary material, and when A3 is not 0, it also contains LiFe 1- a Mn a PO4 and ternary materials. Based on LiFe 1-a Mn a PO4 has good thermal safety performance, which can ensure that when the battery has thermal runaway, the LiFe 1-a Mn a The PO4 first positive electrode coating can effectively hinder the heat conduction between the layers of the electrode. Of course, when both positive electrode coatings contain LiFe 1-aMn a PO4 (A3>0) can better hinder heat conduction; the ternary material with high specific capacity and high conductivity is mainly placed on the second positive electrode coating on the outermost side of the positive electrode, which has more complete contact with the electrolyte and smoother surface deintercalation / intercalation of Li+, which is conducive to showing its high capacity characteristics; and the LiFe 1- a Mn a After the ternary material is partially discharged, the PO4 material is + High concentration, internal LiFe 1-a Mn a Li in PO4 + Li generated at low concentrations + Under the concentration difference, Li + To the internal LiFe 1-a Mn a PO4 diffuses, making LiFe 1-a Mn a The potential balance and rate performance between PO4 materials and ternary materials are excellent, which can ensure that the overall battery cell can exert high capacity.

[0013] Therefore, LiFe 1-a Mn a When the ratio of PO4 and ternary materials is the same, compared with LiFe 1-a Mn a The positive electrode sheet is made by a one-time coating process in which PO4 and ternary materials are evenly mixed. The lithium-ion battery using the above-mentioned positive electrode sheet of the present application can have both good thermal safety performance and high battery cell capacity.

[0014] In the above definition of Y, Q3×(1-η3)×M3 represents the theoretical value of the irreversible capacity of the negative electrode active material, Q1×(1-η1)×(M1+M2)×A1 and Q2×(1-η2)×(M1+M2)×A2 represent the theoretical value of the irreversible capacity of all LiFe 1-a Mn a The theoretical value of irreversible capacity contributed by PO4 materials and ternary materials, [(Q3×(1-η3)×M3-Q1×(1-η1)×(M1+M2)×A1-Q2×(1-η2)×(M1+M2)×A2)] represents the theoretical amount of reversible active lithium in the battery system. Based on the first efficiency of ternary materials < the first efficiency of negative electrode active materials < LiFe 1-a Mn aThe first efficiency of PO4 material, when Y is negative, it means that a certain amount of lithium is pre-stored in the negative electrode of the battery (provided by the positive electrode itself, to be precise, the irreversible capacity of the ternary material can meet the lithium replenishment demand), and no additional lithium replenishment is required; when Y is positive, the battery needs external lithium replenishment, and the molecule of Y at this time specifically represents the theoretical value of the amount of external lithium replenishment required. Since the lithium replenished by the battery itself is not necessarily all reflected in the energy, part of the Li is pre-stored in the negative electrode, and the above molecule is divided by the measured value of energy density β, the resulting Y represents the actual amount of reversible active lithium after deducting the energy. Y reflects the limiting relationship between the amount of reversible active lithium and the energy density of the battery. Controlling Y in the range of -7 to 4.5 can ensure that the ternary material and LiFe 1-a Mn a The overall mixing ratio of PO4 materials is appropriate, which not only ensures that there is no risk of lithium plating at the negative electrode when a sufficient amount of lithium is pre-stored in the negative electrode of the battery, but also ensures that when the positive electrode of the battery is in an appropriate lithium-deficient state, the actual energy exerted by the positive electrode of the battery will not be too low.

[0015] The above Z represents the limiting relationship between the safety performance and energy density of the battery. When the energy density of the battery cell is higher, its relative retained energy is also higher, and the easier it is to lose control. (Q1×η1×A1+Q2×η2×A2) represents the theoretical specific energy of the positive electrode of the battery. This application divides it by β and multiplies it by the actual longitudinal thermal diffusion rate of the positive electrode to obtain the judgment of the safety of the battery cell after deducting the energy effect. Controlling Z in the range of 0-6 can ensure that the safety performance of the actual electrode after deducting the overall energy of the battery cell in the overall hybrid system is better, which not only meets the effect of introducing ternary materials to improve the energy density of the battery, but also ensures the safety characteristics of the overall electrode and battery cell.

[0016] From the above analysis, it can be seen that in the lithium-ion battery provided by the present application, by 1-a Mn a The positive electrode active materials of PO4 and ternary materials are distributed in different proportions to obtain positive electrode sheets with two layers of positive electrode coatings with different compositions. Based on the regulation of A3, A2, A5, etc., the above-mentioned custom parameters Y and Z can be within a suitable range, thereby ensuring that the battery has excellent safety performance, higher capacity, and longer cruising range.

[0017] In some embodiments of the present application, Y is in the range of -5 to 4.5, and may further be in the range of -3.5 to 4.5. Z is in the range of 0-4.

[0018] In this application, the lower the α, the better. A lower α means that the longitudinal heat diffusion of the positive electrode is slower, the safety performance of the electrode is higher, and accordingly, the value of Z is also lower. The above β is a measured value and can be adjusted according to the specific type of ternary material and its relationship with LiFe 1- a Mn aThe specific amount of the PO4 material varies. Generally, β is within the range of 430 - 600 mWh / g.

[0019] In the embodiments of the present application, A1 + A2 = 1, A3 + A5 = A1. In some embodiments of the present application, the first positive electrode active material includes LiFe 1-a Mn a PO4 and does not include ternary materials. At this time, A3 + A2 = A5 = 50%, A3 ≥ 0. That is, all ternary materials are placed in the second positive electrode active material, and the mass ratio of the ternary materials in the second positive electrode active material in the total positive electrode active material is also A2. The masses of the first positive electrode active material and the second positive electrode active material are equal. Among them, by controlling A3 + A2 = A5 = 50%, the first positive electrode coating can better hinder the heat conduction between the layers of the electrode sheet, and while ensuring excellent rate performance of the two stacked positive electrode coatings, it is also beneficial for the positive electrode sheet to better balance safety performance and high energy density.

[0020] In some embodiments of the present application, A2 is within the range of 5% - 50%. Among them, when A3 + A2 = A5 = 50% and A2 = 50%, A3 = 0. At this time, the second positive electrode active material is accurately only ternary materials, but it can still ensure that the above lithium-ion battery has good safety performance. Preferably, A2 is within the range of 5% - 45%.

[0021] In the embodiments of the present application, the ratio of M1 + M2 to M3 is within the range of 1.5 - 2.2. This can not only avoid lithium deposition on the negative electrode sheet during charging, but also facilitate the capacity utilization of the positive electrode active material, helping to improve the energy density of the battery. In some embodiments, the ratio of M1 + M2 to M3 is between 1.6 - 1.9. In some embodiments, the surface densities of the first positive electrode coating and the second positive electrode coating are controlled to be the same, that is, M1 = M2.

[0022] In the present application, the structural general formula of the ternary material is LiNi x Co y M z O2, M is at least one metal element from the third subgroup to the fifth main group, 0.33 ≤ x ≤ 0.98, 0 < y < 1, 0 < z < 1, and x + y + z = 1. For example, M is at least one of Mn, Al, Zr, Ti, Y, Sr, and W, etc., and it is more common that M is Mn and Al. When the value of x is relatively high, the specific capacity of this ternary material is relatively high and the rate performance is good. At this time, the ternary material can be called a "high-nickel ternary material". Preferably, the value range of x is: 0.70 ≤ x ≤ 0.98, further preferably 0.80 ≤ x ≤ 0.90, and more preferably 0.83 ≤ x ≤ 0.88. Optionally, y satisfies: 0.01 ≤ y ≤ 0.33, and z satisfies: 0.01 ≤ z ≤ 0.33.

[0023] In this application, when a=0, the general formula is LiFe 1-a Mn a The material of PO4 is specifically lithium iron phosphate material; when a>0, the material is specifically lithium manganese iron phosphate material, wherein a≤0.8, which is conducive to ensuring the stability of the structure of lithium manganese iron phosphate material. 1-a Mn a The particle size of the PO4 material is smaller than that of the ternary material. Optionally, the particle size D50 of the ternary material is 3-6 μm, for example, 4-5 μm. Preferably, the ternary material is a single crystal material, and its structural stability is higher than that of the ternary material in agglomerate state. Optionally, the LiFe 1-a Mn a The particle size D50 of the PO4 material is 0.8-1.8 μm, for example 0.8-1.5 μm. 1-a Mn a The surface of the PO4 material also has a carbon coating layer to improve its conductivity; LiFe 1-a Mn a PO4 can contain doping elements to improve its rate performance.

[0024] In the embodiment of the present application, the negative electrode active material includes graphite, which can be natural graphite or artificial graphite. The first efficiency of graphite is between that of ternary materials and LiFe 1-a Mn a Of course, the negative electrode active material can also be other materials besides graphite.

[0025] In this application, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector. The positive electrode active material layer includes a first positive electrode coating layer and a second positive electrode coating layer, with the first positive electrode coating layer being adjacent to the positive electrode current collector. In addition to containing the corresponding positive electrode active materials, the first and second positive electrode coating layers also contain a binder, a conductive agent, and the like. In other words, the primary and secondary coating positive electrode slurries used to form the above-mentioned positive electrode sheet, in addition to containing the corresponding positive electrode active materials, also include a binder, a conductive agent, and a solvent.

[0026] Among them, binders and conductive agents are conventional choices in the field of batteries. For example, the binder can be selected from one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl alcohol (PVA), styrene-butadiene rubber (SBR), polyacrylonitrile (PAN), polyimide (PI), polyacrylic acid (PAA), polyacrylate, polyolefin (such as polyethylene, polypropylene, polystyrene), sodium carboxymethyl cellulose (CMC), sodium alginate, etc. The conductive agent can be at least one of carbon black (such as acetylene black, Ketjen black), carbon nanotubes (CNT), graphene, carbon fiber, graphite, etc., but is not limited thereto. The solvent can be selected from one or more of N-methylpyrrolidone (NMP), dimethylformamide (DMF), diethylformamide (DEF), water and alcohols, with NMP being preferred. The content of the solvent is not particularly limited, as long as it can meet the fluidity and uniformity of the slurry coating. In addition, the positive current collector can include but is not limited to metal film materials, foam metal mesh, etc., and can specifically be aluminum foil, carbon-coated aluminum foil, etc.

[0027] The present application has no restrictions on the surface density and compaction of the above-mentioned positive electrode sheet, which can be designed according to the specific electrochemical system. In some embodiments, the positive electrode sheet of the present application has a double-sided surface density of 300-500g / m 2 The compacted density of the pole piece can be 2.40-3.3g / cm 3 Among them, when performing the above-mentioned primary coating and secondary coating, the positive electrode slurry supplied to the coated grinding head is different, specifically, the ratio of the positive electrode active material therein is different, which can be referred to the above description of this application.

[0028] In this application, the negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector. In addition to containing the negative electrode active material, the negative electrode active material layer typically also includes a binder and, optionally, a conductive agent. Similarly, the scope of the binder and conductive agent in the negative electrode active material layer can be found in the description of the positive electrode sheet above. The negative electrode current collector may include, but is not limited to, metal film materials, metal foam mesh, and may specifically include copper foil, carbon-coated copper foil, and the like.

[0029] In the present application, in addition to the above-mentioned positive electrode sheet and negative electrode sheet, a diaphragm and an electrolyte are required in the assembly process of the above-mentioned lithium-ion battery. Among them, the diaphragm is used to separate the positive electrode sheet and the negative electrode sheet to maintain the insulation and liquid retention characteristics between the two; the diaphragm and the positive electrode sheet and the negative electrode sheet together constitute the battery cell, and the battery cell is accommodated in the battery shell, which can be an aluminum-plastic film. The diaphragm can be a polymer diaphragm, a non-woven fabric, or other commonly used battery diaphragms, including but not limited to single-layer PP (polypropylene) film, single-layer PE (polyethylene) film, double-layer PP / PE, double-layer PP / PP and three-layer PP / PE / PP diaphragms. The battery shell is injected with an electrolyte, which is a medium for the transmission of lithium ions between the positive and negative electrodes. The injection coefficient of the electrolyte is generally 2.0-4.5g / Ah. The specific composition of the electrolyte is a conventional choice in the battery field and is not limited to this.

[0030] This application has no limitation on the shape of the lithium-ion battery. The patent of this invention preferably uses an aluminum shell battery for testing to facilitate the arrangement of the temperature sensor.

[0031] The preparation method of the above-mentioned lithium-ion battery includes: stacking the positive electrode sheet, the separator and the negative electrode sheet in sequence to form a battery cell, accommodating the battery cell in a battery shell, injecting electrolyte, and then sealing the battery shell to obtain a lithium-ion battery.

[0032] In a second aspect, the present invention also provides a method for preparing a lithium-ion battery, comprising the following steps:

[0033] A first slurry containing a first positive electrode active material and a second slurry containing a second positive electrode active material are sequentially coated on the positive electrode current collector to form a first positive electrode coating and a second positive electrode coating stacked to obtain a positive electrode sheet; wherein the first positive electrode active material includes LiFe 1-a Mn a PO4, the second positive electrode active material includes LiFe 1-a Mn a PO4 and ternary materials; of the total positive active materials of the positive electrode sheet, LiFe 1-a Mn a The mass proportions of PO4 and ternary materials are A1 and A2 respectively, the mass proportion of the first positive electrode active material in the total positive electrode active material is A5, and the mass proportion of LiFe in the second positive electrode active material is A1. 1-a Mn a The mass proportion of PO4 in the total positive electrode active material is A3;

[0034] The positive electrode sheet and the negative electrode sheet are assembled into a lithium-ion battery, and the following parameters are defined:

[0035] Y=[(Q3×(1-η3)×M3-Q1×(1-η1)×(M1+M2)×A1-Q2×(1-η2)×(M1+M2)×A2)] / β

[0036] Z=α×(Q1×η1×A1+Q2×η2×A2) / β

[0037] and said Y is in the range of -7 to 4.5, and said Z is in the range of 0 to 6;

[0038] Wherein, 0≤a≤0.8; Q1, η1 are respectively the LiFe 1-a Mn a The first charge specific capacity and first efficiency of PO4, Q2 and η2 are the first charge specific capacity and first efficiency of the ternary material, respectively, M1 and M2 are the dressing amounts of the first positive electrode active material and the second positive electrode active material on the positive electrode sheet, Q3 and η3 are the first discharge specific capacity and first efficiency of the negative electrode active material in the negative electrode sheet, M3 is the dressing amount of the negative electrode active material on the negative electrode sheet, α is the longitudinal thermal diffusion rate of the positive electrode sheet, in mm / s, β is the energy density of the lithium ion battery, wherein the units of Q1, Q2, and Q3 are all mAh / g, the unit of β is mWh / g, and the units of M1, M2, and M3 are all g / m 2 .

[0039] The method for preparing the lithium-ion battery can be used to prepare the lithium-ion battery described in the first aspect of the present application. The parameters involved here are the same as those described above in the present application and will not be repeated here.

[0040] In the preparation method of the lithium-ion battery provided in the present application, when preparing the positive electrode sheet, the positive electrode slurry is coated twice, and the positive electrode active materials during the two coatings are different. Among them, when the slurry is coated once (i.e., when the first slurry is coated) to form the first positive electrode coating, the first positive electrode active material used includes LiFe 1-a Mn a PO4, when the second coating (ie, coating the second slurry) is performed to form the second positive electrode coating, the second positive electrode active material used includes LiFe 1-a Mn a PO4 and ternary materials. In this way, the positive active material in the first positive electrode coating close to the positive electrode current collector contains LiFe 1-a Mn a PO4, the positive electrode active material in the second positive electrode coating away from the positive electrode current collector side also contains LiFe 1-a Mn a PO4 and ternary materials, and by regulating the LiFe in the second cathode coating 1-a Mna The mass ratio of PO4 in the total positive electrode active material of the positive electrode sheet is A3 (A3 can be 0 or not), the mass ratio of LiFe in the total positive electrode active material of the positive electrode sheet is 1-a Mn a The mass proportion of PO4 is A1 and the mass proportion of ternary materials is A2, which can make the above-defined lithium replenishment parameter Y and safety parameter Z within an appropriate range, thereby ensuring that the lithium-ion battery has excellent safety performance, high capacity and long driving range.

[0041] In a third aspect, the present invention also provides a power vehicle equipped with the lithium-ion battery. Due to the use of the lithium-ion battery, the power vehicle has a long battery life and high safety.

[0042] Advantages of the embodiments of the present application will be partially explained in the following description, and some will be obvious from the description, or can be learned through the implementation of the embodiments of the present application. DETAILED DESCRIPTION

[0043] The embodiments of the present application are further described below with reference to a number of specific embodiments.

[0044] Example 1

[0045] Preparation of a lithium-ion battery:

[0046] (1) Preparation of positive electrode sheet:

[0047] Prepare a first slurry, which includes a first positive electrode active material (LFP), a conductive agent (carbon nanotubes CNT and carbon black SP), a binder-PVDF and a solvent NMP, and LFP:CNT:SP:PVDF:NMP=100:12:0.3:2.5:55;

[0048] Prepare a second slurry, which includes a second positive electrode active material (LFP and ternary material), a conductive agent (CNT and carbon black SP), PVDF and NMP, and the second positive electrode active material: CNT: SP: PVDF: NMP = 100:12:0.3:2.5:55;

[0049] In the total positive electrode active material consisting of the first positive electrode active material and the second positive electrode active material, the mass proportions of LFP and the ternary material are A1 and A2, respectively, A1 + A2 = 1, the mass proportion of the first positive electrode active material in the total positive electrode active material is A5, the mass proportions of LFP and the ternary material in the second positive electrode active material in the total positive electrode active material are A3 and A2, respectively, and A3 + A2 = A5 = 50%. The composition of the positive electrode active materials in the first slurry and the second slurry is adjusted according to the ratios provided in Table 1.

[0050] Take 12μm aluminum foil as the positive electrode current collector, and apply the coating amount M1 (M1=(M1+M2) / 2=200g / m 2 ) Apply the first slurry and then apply the dressing amount M2 (M2 = 200g / m 2 ) apply the second slurry, and after drying, form a positive electrode active material layer with a first positive electrode coating layer and a second positive electrode coating layer stacked, wherein the first positive electrode coating layer is formed by drying the first slurry, and the second positive electrode coating layer is formed by drying the second slurry, and the first positive electrode coating layer is close to the aluminum foil; then, in the same manner, the positive electrode active material layer with the first positive electrode coating layer and the second positive electrode coating layer stacked is sequentially formed on the other side surface of the aluminum foil to obtain a double-sided positive electrode sheet; and the double-sided positive electrode sheet is rolled, slit and die-cut to obtain a double-sided surface density of 400g / m 2 The positive electrode plate to be used.

[0051] (2) Preparation of negative electrode sheet:

[0052] Natural graphite, binder CMC, binder SBR, and water were mixed in a mass ratio of 100:1.5:2.5:3:130 to obtain a negative electrode slurry; the negative electrode slurry was applied in an amount M3 (unit: g / m 2 ) is coated on both sides of an 8μm thick copper foil, baked at 110°C to remove water, and then a negative electrode active material layer is formed on the copper foil, which is then rolled, slit, and die-cut to obtain a double-sided negative electrode sheet.

[0053] (3) Assembly of full battery:

[0054] The above-mentioned double-sided positive electrode sheets, negative electrode sheets and PP separators are stacked in a Z-shaped stacking manner to assemble into battery cells in one direction. The hot-pressed battery cells are placed in a battery casing, and the electrolyte is injected under vacuum according to the injection coefficient of 2.8g / Ah. The casing is sealed and then vacuum-sealed after high-temperature aging, formation, aging, and capacity separation to produce a long-cell battery with a length of 500mm and a width of 12mm.

[0055] The test method for Q1 and η1 of the LFP material mentioned above is as follows: LFP is used as the positive electrode active material, and a single-surface density of 1.5 g / dm is prepared according to the similar method described in Example 1. 2An LFP positive electrode sheet was prepared using a positive electrode slurry with a ratio of LFP:CNT:SP:PVDF:NMP = 100:12:0.3:2.5:55. This slurry was then coated on one side of an aluminum foil. The LFP positive electrode sheet, a lithium metal sheet, a separator, and a predetermined amount of electrolyte were assembled into a CR2025 button cell in a glove box. The button cell was left for approximately 4 hours to allow the electrode sheet to fully soak in the electrolyte. The button cell was charged at a constant current of 0.1C to a voltage of 3.8V, then charged at a constant voltage of 3.8V to a cutoff current of 0.02C, and then discharged at a constant current of 0.1C to a voltage of 2.0V. The ratio of the discharge capacity to the charge capacity during the first cycle was recorded as the initial efficiency η1, and the ratio of the charge capacity during the first cycle to the mass of LFP active material on the electrode sheet was recorded as the initial charge specific capacity Q1 of the LFP material.

[0056] The test method of Q2 and η2 of the above ternary material is as follows: using the ternary material as the positive electrode active material, a single-surface density of 1.5 g / dm 2 A ternary positive electrode sheet was prepared using a positive electrode slurry with the ternary materials: CNT:SP:PVDF:NMP = 100:12:0.3:2.5:55. This slurry was then coated on one side of an aluminum foil. This ternary positive electrode sheet, lithium sheet, separator, and a predetermined amount of electrolyte were assembled into a CR2025 button cell in a glove box. The button cell was left for approximately 4 hours to allow the electrode to fully soak in the electrolyte. The button cell was charged at a constant current of 0.1C to 4.3V, then charged at a constant voltage of 4.3V to a cutoff current of 0.01C, and then discharged at a constant current of 0.1C to 3.0V. The ratio of the discharge capacity to the charge capacity during the first cycle was recorded as the initial efficiency η2, and the ratio of the charge capacity during the first cycle to the mass of active material on the electrode sheet was recorded as the initial charge specific capacity Q2 of the high-nickel ternary material.

[0057] The test method of Q3 and η3 of the above graphite is as follows: prepare a graphite with a single surface density of 1.05 g / dm according to the method of Example 1. 2 The graphite electrode. The graphite electrode, lithium sheet, separator and a certain amount of electrolyte are assembled into a CR2025 button battery in a glove box. The button battery is placed for about 4 hours to allow the electrode to be fully immersed in the electrolyte. The button battery is (1) discharged at a constant current of 0.1C to 0.005V; (2) discharged at a constant current of 0.09C, 0.08C...0.02C to 0.001V; (3) left for 15 minutes; (4) charged at a constant current of 0.1C to 1.5V; (5) left for 15 minutes. The ratio of the charge capacity to the discharge capacity during the first cycle is recorded as the first efficiency η3, and the ratio of the discharge capacity in the first cycle to the mass of the active material on the electrode is recorded as the first discharge specific capacity Q3 of the graphite material.

[0058] The energy density β of the full battery and the longitudinal thermal diffusion rate α of the positive electrode sheet of Example 1 were tested, and the values of the parameters Y and Z defined above were calculated. The gram capacity of the overall positive electrode active material in Example 1 was also tested, and the results are summarized in Table 1 below.

[0059] The positive electrode gram capacity and β are tested as follows: at 25°C, the full battery of Example 1 is fully charged to 4.1V at a constant current and constant voltage of 0.2C, with a cutoff current of 0.05C. After standing, the battery is discharged at a constant current of 0.2C to 2.0V, and this is repeated three times. The capacity released the third time is recorded as C0, and the ratio of C0 to the mass of all the positive electrode active materials in Example 1 is the gram capacity of the positive electrode active material, expressed in mAh / g. In addition, the ratio of the energy released the third time (the product of C0 and the median discharge voltage) to the mass of all the positive electrode active materials in Example 1 is the energy density β of the positive electrode active material, expressed in mWh / g.

[0060] The test method for the longitudinal thermal diffusion rate α of the positive electrode plate is as follows: after the above-mentioned full battery is normally formed and divided into capacity, it is charged to 4.1V using a 1 / 3C constant current and constant voltage method with a cut-off current of 0.05C. Then, in the glove box, the battery is disassembled, and the fully charged positive electrode plate is taken out. After drying, the positive electrode plates are stacked together in sequence, and temperature sensors are arranged on the third layer and the third to last layer. Then, the electrode core height is recorded to obtain the longitudinal thermal diffusion detection electrode core. Take another normal battery, charge it to a full charge in the above manner, and then disassemble it. Place the normal electrode core on the above longitudinal heat diffusion detection electrode core, and use the stacked structure of the two as the runaway inducing electrode core. Wrap it with aluminum-plastic film, and inject an appropriate amount of electrolyte inside. Replenish the electrolyte according to the injection coefficient of 2.3g / Ah. The capacity is calculated according to the battery capacity before disassembly. Then, vacuum seal it to obtain a longitudinal heat diffusion detection battery. Use a 3-8mm diameter steel needle to penetrate the battery at a speed of 10mm / s. The penetration thickness is about 2 / 3 of the thickness of the runaway inducing electrode core. Subsequently, record the electrode temperature and absolute time obtained by the two temperature sensors. The heat transfer time can be calculated. The thickness between the electrodes where the two temperature sensors are located is divided by the heat transfer time to obtain the longitudinal heat diffusion rate α of the positive electrode.

[0061] In addition, the positive electrode sheets and batteries of the remaining examples were prepared according to the parameters listed in Table 1, and the relevant test results are also summarized in Table 1 below. Comparative Examples 1-5 are also provided as shown in Table 1 below. The positive electrode active material layers of Comparative Examples 1-5 are all composed of two identical coating layers. A6 represents the mass ratio of the ternary material in the first positive electrode coating layer (close to the positive electrode current collector) of the comparative example, and A4 represents the mass ratio of the ternary material in the second positive electrode coating layer (far away from the positive electrode current collector) of the comparative example. (In the embodiment of the present application, this ratio is A2.

[0062] The preparation and parameter determination of the lithium-ion batteries of Examples 2-7 and Comparative Examples 1-5 were carried out in the same manner as in Example 1, except that the first positive electrode active material of Examples 6-7 and Comparative Examples 4-5 differed from that of Example 1. Correspondingly, when testing Q1 and η1 in Examples 6-7 and Comparative Examples 4-5, the voltage range was 3.0-4.4 V. When testing the positive electrode specific capacity and β, and the longitudinal thermal diffusion rate α of the positive electrode sheet in Examples 6-7 and Comparative Examples 4-5, the voltage range was 2.8-4.3 V.

[0063] Specifically, the particle size D50 of the LiFePO4 (LFP) material used in Examples 1-5 and Comparative Examples 1-3 of the present application is 1.0 μm, the initial charge specific capacity Q1 is 157 mAh / g, and the initial efficiency η1 is 98%. The ternary material used is a high nickel ternary single crystal material, and its general structural formula is LiNi 083 Co 0.12 Mn 0.05 The particle size D50 is 4.3μm, its first charge specific capacity Q2 is 238mAh / g, and its first efficiency η2 is 87%. The negative electrode active material used is natural graphite, its first discharge specific capacity Q3 is 355mAh / g, and its first efficiency η3 is 94%.

[0064] The LiFe used in Example 6 and Comparative Example 4 of this application 0.25 Mn 0.75 The particle size D50 of the PO4 material is 1.5 μm, the initial charge specific capacity Q1 is 152 mAh / g, and the initial efficiency η1 is 96%. The ternary material and negative electrode active material used are the same as those in Example 1.

[0065] The LiFe used in Example 7 and Comparative Example 5 of the present application 0.56 Mn 0.44 The particle size D50 of the PO4 material is 1.4, the initial charge specific capacity Q1 is 155 mAh / g, and the initial efficiency η1 is 96%. The ternary material and negative electrode active material used are the same as those in Example 1.

[0066] Table 1 Partial parameters and test results of various embodiments and comparative examples

[0067]

[0068]

[0069] It can be seen from Table 1 that the ratio of LFP and ternary material in the overall positive electrode active material in Comparative Examples 1-3 is the same as that in Examples 1, 4, and 5, respectively, but the ratio of each layer is different. The positive electrode active material layer of Comparative Examples 1-3 is composed of two layers of exactly the same coating. It can be seen that when the proportion of ternary material in the overall positive electrode active material is low, there is no significant change in the longitudinal thermal diffusion rate α of the pole piece between Example 1 and Comparative Example 1. However, when the proportion of ternary material in the overall positive electrode active material is high, the longitudinal thermal diffusion rate α of the pole piece of the comparative example is about twice that of the embodiment (Comparative Example 2 and Example 4, Comparative Example 3 and Example 5), which shows that the structure of the positive electrode pole piece provided in the embodiment of the present application can greatly reduce its longitudinal thermal diffusion, especially when the overall proportion of ternary material is high. The battery parameter Z of Comparative Example 3 is also more than twice that of Example 5, and the battery parameter Z of Comparative Example 2 is also more than five times that of Example 4. This is mainly because when the proportion of ternary materials increases, the total energy of the electrode will also increase, but based on the higher stability of LFP than the ternary materials, the positive electrode active materials coated once in this application are all LFP. After acupuncture, it can block the conduction of heat, reduce the heat conduction rate, maintain the overall safety performance of the battery (Z is lower), and meet the battery cell's demand for high energy density.

[0070] In addition, in Comparative Example 1, the overall proportion of the ternary material is relatively small, and its layering ratio is slightly different from that of Example 1. Although its Z value can meet the requirements of the 0-6 range, its Y value cannot meet the requirements of -7 to 4.5 defined in this application. This is mainly because the composition of the two-layer positive active material layer in Comparative Example 1 is exactly the same, which is not conducive to the LFP material to exert its full capacity, especially the LFP material close to the positive current collector. Therefore, the positive electrode gram capacity of Comparative Example 1 is relatively low. In the two stacked positive electrode coatings of the present application, the coating close to the positive current collector contains only LFP, and the coating away from the positive current collector contains ternary materials. This structure is beneficial for the outer ternary material and the inner LFP in the positive electrode system to fully exert their capacity. Accordingly, the positive electrode gram capacity and energy density β of the battery of the embodiment of the present application are better than those of the comparative example.

[0071] The material mixed with the ternary material in Examples 6-7 and Comparative Examples 4-5 is lithium iron manganese phosphate, not LFP. Similarly, the ratio of lithium iron manganese phosphate and ternary material in the overall positive electrode active material in Comparative Examples 4 and 5 is the same as that in Examples 6 and 7, respectively, but the ratio of each layer is different, wherein the positive electrode active material layer of Comparative Examples 4 and 5 is composed of two completely identical coating layers. It can be seen that the longitudinal thermal diffusion rate α of the pole piece of Comparative Example 4 is much higher than that of Example 6, and the longitudinal thermal diffusion rate α of the pole piece of Comparative Example 5 is much higher than that of Example 7, and the battery parameter Z of the comparative example is large and is not within the range defined in this application. This shows that the structure of the positive electrode pole piece provided in the embodiments of the present application can greatly reduce its longitudinal thermal diffusion, and the battery safety performance is high.

[0072] It can be seen from Table 1 that when LFP material and ternary material are mixed as the positive electrode active material of the positive electrode sheet, when the coating ratio of the upper and lower layers of the two makes the parameter Y in the range of -7 to 4.5 and the parameter Z between 0-6, it can be ensured that the assembled full battery can have both high energy density and excellent safety performance.

[0073] The above-described embodiments merely represent several implementation methods of the present application. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that a person of ordinary skill in the art may make various modifications and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be determined by the appended claims.

Claims

1. A lithium-ion battery comprising a positive electrode sheet, a negative electrode sheet, and a separator located between the positive electrode sheet and the negative electrode sheet, wherein the positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector, characterized in that: The positive electrode active material layer includes a first positive electrode coating and a second positive electrode coating, and the first positive electrode coating is close to the positive electrode current collector, and the positive electrode active material in the first positive electrode coating includes LiFe 1-a Mn a PO4, the positive electrode active material in the second positive electrode coating includes LiFe 1-a Mn a PO4 and ternary materials; of the total positive active materials of the positive electrode sheet, LiFe 1-a Mn a The mass proportions of PO4 and ternary materials are A1 and A2 respectively. The LiFe 1- a Mn a The mass ratio of PO4 in the total positive electrode active material is A5, and the LiFe 1-a Mn a The mass proportion of PO4 in the total positive electrode active material is A3; definition The following parameters: Y=[(Q3×(1-η3)×M3-Q1×(1-η1)×(M1+M2)×A1-Q2×(1-η2)×(M1+M2)×A2)] / β Z=α×(Q1×η1×A1+Q2×η2×A2) / β and said Y is in the range of -7 to 4.5, and said Z is in the range of 0 to 6; Wherein, 0≤a≤0.8, Q1, η1 are respectively the LiFe 1-a Mn a The first charge specific capacity and first efficiency of PO4, Q2 and η2 are respectively the first charge specific capacity and first efficiency of the ternary material, M1 and M2 are respectively the amount of the positive electrode active material in the first positive electrode coating and the positive electrode active material in the second positive electrode coating on the positive electrode sheet, Q3 and η3 are respectively the first discharge specific capacity and first efficiency of the negative electrode active material in the negative electrode sheet, M3 is the amount of the negative electrode active material on the negative electrode sheet, α is the longitudinal thermal diffusion rate of the positive electrode sheet, the unit is mm / s, β is the energy density of the lithium-ion battery, wherein the units of Q1, Q2, and Q3 are all mAh / g, the unit of β is mWh / g, and the units of M1, M2, and M3 are all g.

2. The lithium-ion battery according to claim 1, wherein The Y is in the range of -5 to 4.

5.

3. The lithium-ion battery according to claim 2, wherein The Y is in the range of -3.5 to 4.

5.

4. The lithium-ion battery according to claim 1, wherein The Z is in the range of 0-4.

5. The lithium-ion battery according to claim 1, wherein A1+A2=1; A3+A5=A1.

6. The lithium-ion battery according to claim 5, wherein A3+A2=A5=50%.

7. The lithium-ion battery according to claim 1, wherein The A2 is in the range of 5%-50%, and A3≥0.

8. The lithium-ion battery according to claim 7, wherein The A2 is in the range of 5%-45%, and A3>0.

9. The lithium-ion battery according to claim 1, wherein The ratio of the M1+M2 to the M3 is in the range of 1.5-2.

2.

10. The lithium-ion battery according to claim 9, wherein The ratio of the M1+M2 to the M3 is in the range of 1.6-1.

9.

11. The lithium-ion battery according to any one of claims 1 to 10, wherein: The structural general formula of the ternary material is LiNi x Co y M z O2, where M is at least one metal element in Group IIIB to Group VA, 0.33 ≤ x ≤ 0.98, 0 < y < 1, 0 < z < 1, and x + y + z = 1.

12. The lithium-ion battery according to claim 11, wherein The M is at least one of Mn, Al, Zr, Ti, Y, Sr and W.

13. The lithium-ion battery according to claim 1, wherein The LiFe 1-a Mn a The particle size D50 of PO4 is 0.8-1.8 μm; the particle size D50 of the ternary material is 4-6 μm.

14. The lithium-ion battery according to claim 13, wherein The LiFe 1-a Mn a The particle size D50 of PO4 is 0.8-1.5 μm, and the particle size D50 of the ternary material is 4-5 μm.

15. A method for preparing a lithium ion battery, characterized in that: The following steps are involved: A first slurry containing a first positive electrode active material and a second slurry containing a second positive electrode active material are sequentially coated on the positive electrode current collector to form a first positive electrode coating and a second positive electrode coating stacked to obtain a positive electrode sheet; wherein the first positive electrode active material includes LiFe 1-a Mn a PO4, the second positive electrode active material includes LiFe 1-a Mn a PO4 and ternary materials; of the total positive active materials of the positive electrode sheet, LiFe 1-a Mn a The mass proportions of PO4 and ternary materials are A1 and A2 respectively, the mass proportion of the first positive electrode active material in the total positive electrode active material is A5, and the mass proportion of LiFe in the second positive electrode active material is A1. 1-a Mn a The mass proportion of PO4 in the total positive electrode active material is A3; The positive electrode sheet and the negative electrode sheet are assembled into a lithium-ion battery, and the following parameters are defined: Y=[(Q3×(1-η3)×M3-Q1×(1-η1)×(M1+M2)×A1-Q2×(1-η2)×(M1+M2)×A2)] / β Z=α×(Q1×η1×A1+Q2×η2×A2) / β and said Y is in the range of -7 to 4.5, and said Z is in the range of 0 to 6; Wherein, 0≤a≤0.8, Q1, η1 are respectively the LiFe 1-a Mn a The first charge specific capacity and first efficiency of PO4, Q2 and η2 are the first charge specific capacity and first efficiency of the ternary material, respectively, M1 and M2 are the dressing amounts of the first positive electrode active material and the second positive electrode active material on the positive electrode sheet, Q3 and η3 are the first discharge specific capacity and first efficiency of the negative electrode active material in the negative electrode sheet, M3 is the dressing amount of the negative electrode active material on the negative electrode sheet, α is the longitudinal thermal diffusion rate of the positive electrode sheet, in mm / s, β is the energy density of the lithium ion battery, wherein the units of Q1, Q2, and Q3 are all mAh / g, the unit of β is mWh / g, and the units of M1, M2, and M3 are all g / m 2 .

16. A powered vehicle, characterized in that: The power vehicle is equipped with a lithium-ion battery as described in any one of claims 1 to 14, or a lithium-ion battery prepared by the preparation method as described in claim 15.

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