Secondary battery, battery module, battery pack, and electric device

Abstract

The present invention provides a secondary battery, a battery module, a battery pack, and an electric device. The secondary battery includes a positive electrode sheet and a non-aqueous electrolyte, the positive electrode sheet includes a positive electrode active material having a core-shell structure, the positive electrode active material includes a core and a shell covering the core, and the chemical formula of the core is Li 1+x Mn 1-y A y P 1-z R z O4, and the first coating layer is crystalline Li pyrophosphate a MP2O7 and / or M b (P2O7) c the second coating layer comprises crystalline pyrophosphate XPO4; the third coating layer is carbon; the non-aqueous electrolyte comprises a first lithium salt and a first additive; the first lithium salt is LiN(C m F 2m+1 SO2)(C n F 2n+1 The first additive is one or more selected from the group consisting of Li(FSO2), Li(FSO2)2N, where m and n are positive integers, and the first additive comprises one or more of the compounds shown in Formula 1. The cycle performance of the manganese phosphate lithium secondary battery is improved. JPEG2024515150000039.jpg9170

Description

Technical Field

[0001] This application relates to the technical field of lithium batteries, and particularly to secondary batteries, battery modules, battery packs, and electrical devices.

Background Art

[0002] In recent years, as the application range of lithium-ion batteries has become increasingly wide, lithium-ion batteries have been widely applied in energy storage power systems such as hydraulic, thermal, wind, and solar power plants, as well as in multiple fields such as power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, and aerospace. Due to the significant development of lithium-ion batteries, higher requirements are put forward for their energy density, cycle performance, safety performance, etc.

Summary of the Invention

Problems to be Solved by the Invention

[0003] Compared with other cathode active materials, lithium manganese phosphate cathode active material has high safety and cycle life, but the disadvantage of lithium manganese phosphate is its low rate performance. Currently, this problem is generally solved by means such as coating or doping. However, it is still desirable to further improve the rate performance, cycle performance, high-temperature stability, etc. of the lithium manganese phosphate cathode active material.

Means for Solving the Problems

[0004] This application is made in view of the above problems, and its purpose is to solve the problem of the low cycle performance of lithium manganese phosphate secondary batteries by providing secondary batteries, battery modules, battery packs, and electrical devices.

[0005] To achieve the above object, the first aspect of this application provides a secondary battery, including a positive electrode sheet and a non-aqueous electrolyte. Here, the positive electrode sheet includes a cathode active material having a core-shell structure, and the cathode active material includes a core and a shell covering the core. The chemical formula of the core is Li 1+x Mn1-y A y P 1-z R z is O4, where x is any numerical value within the range of -0.100 to 0.100, y is any numerical value within the range of 0.001 to 0.500, z is any numerical value within the range of 0.001 to 0.100, A is one or more elements selected from the group consisting of Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb, and Ge, one or more elements among Fe, Ti, V, Ni, Co, and Mg can be selected, R is one or more elements selected from the group consisting of B, Si, N, and S, preferably, R is one element selected from B, Si, N, and S, the values of x, y, and z satisfy the condition of maintaining electrical neutrality throughout the core, and the shell includes a first coating layer covering the core, a second coating layer covering the first coating layer, and a third coating layer covering the second coating layer. Here, the first coating layer is crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) c including, where 0 ≦ a ≦ 2, 1 ≦ b ≦ 4, 1 ≦ c ≦ 6, and the values of a, b, and c satisfy the condition of maintaining electrical neutrality of crystalline pyrophosphate Li a MP2O7 or M b (P2O7) c and M in crystalline pyrophosphate Li a MP2O7 and M b (P2O7) c are each independently one or more elements selected from the group consisting of Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, and Al. The second coating layer includes crystalline phosphate XPO4, where X is one or more elements selected from the group consisting of Li, Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, and Al. The third coating layer is carbon. The non-aqueous electrolyte includes a first lithium salt and a first additive. The first lithium salt is LiN(C m F 2m+1 SO2)(C n F 2n+1One or more selected from the group consisting of SO2) and Li(FSO2)2N, where m and n represent positive integers, The first additive contains one or more of the compounds represented by Formula 1,

Chemical formula

[0006] In this specification, crystalline means that the crystallinity is 50% or more, that is, 50% - 100%. Those with a crystallinity of less than 50% are called glassy states. The crystallinity of the crystalline pyrophosphate and crystalline phosphate described in this application is 50% - 100%. Pyrophosphates and phosphates with a certain crystallinity not only help to fully exert the functions of preventing the elution of manganese ions by the pyrophosphate coating layer, having excellent lithium ion conduction ability by the phosphate coating layer, and reducing interfacial side reactions, but also enable the pyrophosphate coating layer and the phosphate coating layer to achieve better lattice matching, thereby realizing a tight bond between the coating layers.

[0007] This application dopes element A into the manganese sites of lithium manganese phosphate and dopes element R into the phosphorus sites to obtain a core of doped lithium manganese phosphate, and by performing three-layer coatings on the surface of the core in sequence, a novel lithium manganese phosphate cathode active material having a core-shell structure is provided. Applying this cathode active material to a secondary battery can significantly improve the high-temperature cycle performance, cycle stability, and high-temperature storage performance of the secondary battery.

[0008] At the same time, the non-aqueous electrolyte uses a first lithium salt as the main lithium salt. Due to its excellent thermal stability and hydrolysis resistance, it can effectively reduce the acidity of the electrolyte, reduce the elution of manganese ions, and improve the high-temperature cycle and storage performance. An isocyanate group compound shown in Formula 1 is introduced into the non-aqueous electrolyte, which can react with trace amounts of water in the battery to generate -NHCOOH, preventing the generation of HF by the action of trace amounts of water and the non-aqueous electrolyte, further reducing the acidity of the electrolyte and the elution of manganese ions, and further improving the high-temperature cycle and storage performance. In addition, the isocyanate group compound shown in Formula 1 can further form a film on the negative electrode to generate a uniform SEI film, reducing the reduction of eluted Mn at the negative electrode, and further improving the high-temperature cycle and storage performance.

[0009] In some embodiments, the above R1 is a C2-C10 alkylene group, C2-C10 oxyalkylene group, C2-C10 azaalkylene group, phenylene group, naphthylene group, anthrylene group, cyclobutylene group, cyclopentylene group, cyclohexylene group, biphenylene group, methylenediphenylene group which may or may not be substituted by one or more R a Among them, preferably, R1 is a C2-C6 alkylene group, phenylene group, naphthylene group, anthrylene group, cyclobutylene group, cyclopentylene group, cyclohexylene group, biphenylene group, methylenediphenylene group which may or may not be substituted by one or more R a Among them, and / or R aIt contains one or more selected from halogen atoms, -CN, and C2-C3 alkyl groups, and / or R in the compound represented by Formula 1 a The number of which is 0, 1, 2, 3, or 4.

[0010] Preferably, the first additive contains one or more of the following compounds.

Chemical formula

Chemical formula

[0011] LiN(C m F 2m+1 SO2)(C n F 2n+1 SO2) in the above first lithium salt can be selected from any corresponding specific substances in the prior art. In some embodiments, m and n each independently represent 0, 1, 2, or 3, thereby providing excellent thermal stability.

[0012] In some embodiments of the present application, when the first lithium salt is any one selected from LiN(CF3SO2)(FSO2), LiN(CF3SO2)(C2F5SO2), and LiN(C3F7SO2)(CF3SO2), the first additive is any one selected from the following compounds,

Chemical formula

[0013] In some embodiments, based on the total weight of the non-aqueous electrolyte, the content of the first lithium salt is W1 wt%, where W1 is 0.1-48 (e.g., 0.1, 0.5, 1, 2, 3, 4, 5, 7, 10, 12, 14, 15, 16, 18, 20, 22, 25, 28, 30, 40, 45 or 48), preferably 5-20. This alleviates the corrosion problem of the aluminum foil caused by the first lithium salt at high operating voltages.

[0014] In some embodiments, the content of the first additive is W2 wt%, where W2 is 0.01-20 (e.g., 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 1, 2, 3, 4, 5, 8, 10, 12, 15, 18 or 20), preferably 0.1-10 or 0.3-5. When the mass ratio of the first additive in the electrolyte is within the above range, it not only reduces the acidity of the electrolyte and decreases the elution of manganese ions, but also avoids the deterioration of the negative electrode impedance, further improves the high-temperature cycle and storage performance of the lithium-ion battery, and at the same time does not affect the performance of the lithium-ion battery capacity and rate performance.

[0015] In some embodiments, the above W1 / W2 is defined as M, where M is 0.001-3 (e.g., 0.001, 0.002, 0.005, 0.007, 0.1, 0.2, 0.3, 0.4, 0.5, 1, 2 or 3), preferably 0.005-0.5. When W1 / W2 is within the above range, the two can exert a better synergistic effect, ensuring that the acidity of the system is low, the elution of manganese ions is reduced, and the lithium-ion high-temperature cycle and storage performance are excellent.

[0016] Based on controlling the content of the first lithium salt, in some embodiments, the lithium salt in the non-aqueous electrolyte can be replenished by adding a second lithium salt, that is, the non-aqueous electrolyte further contains a second lithium salt, and the second lithium salt includes one or more selected from lithium difluorophosphate, lithium difluorodioxalate phosphate, lithium difluoroxalate borate, lithium bisoxalate borate, LiPF6, LiBF4, LiAsF6. The above second lithium salt is added as a lithium salt type additive to the non-aqueous electrolyte, which preferentially decomposes on the surface of the aluminum foil, and the decomposition product combines with aluminum ions to form an insoluble or hardly soluble precipitate that adheres to the surface of the aluminum foil, thereby forming a further passivation film to prevent direct contact between the aluminum foil and the electrolyte, protect the aluminum foil, and further cooperate with the first lithium salt to improve the high-temperature cycle and storage performance.

[0017] In some embodiments, based on the total weight of the non-aqueous electrolyte, the content of the second lithium salt is W3% by weight, where W3 is 0.01 - 20 (for example, 0.01, 0.02, 0.05, 0.08, 0.1, 0.2, 0.3, 0.5, 0.8, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20), preferably 0.1 - 10 or 0.3 - 5. When the mass ratio of the second lithium salt in the electrolyte is within the above range, corrosion of the aluminum foil can be suppressed, the high-temperature cycle and storage performance of the lithium-ion battery can be improved, and the capacity performance and rate performance of the lithium-ion battery are not deteriorated.

[0018] In some embodiments, (W2 + W3) / W1 is defined as N, where N is 0.01 - 5 (e.g., 0.01, 0.02, 0.05, 0.08, 0.1, 0.5, 0.8, 1.0, 2, 3, 4, or 5), preferably 0.02 - 1. When W1 / W2 and (W2 + W3) / W1 are within the above ranges, the three components can exhibit a better synergistic effect, the acidity of the system is low, the elution of manganese ions is reduced, excellent lithium-ion high-temperature cycle and storage performance are guaranteed, the corrosion of the aluminum foil is effectively suppressed, the high-temperature cycle and storage performance of the lithium-ion battery are improved, and at the same time, the capacity and rate performance of the lithium-ion battery are not deteriorated.

[0019] In some embodiments, the non-aqueous electrolyte further contains a second additive, and the second additive includes one or more selected from the group consisting of a cyclic carbonate compound containing an unsaturated bond, a halogen-substituted saturated cyclic carbonate compound, a sulfate ester compound, a sulfite ester compound, a sultone compound, a disulfonic acid compound, a nitrile compound, an aromatic compound, a phosphazene compound, a cyclic acid anhydride compound, a phosphite ester compound, a phosphate ester compound, and a borate ester compound. Those skilled in the art can select the corresponding second additive from the above substances according to actual needs, and the usage amount of the second additive in the non-aqueous electrolyte can also refer to the prior art, and the description thereof is omitted in this application.

[0020] In some embodiments, the non-aqueous electrolyte further contains an organic solvent. The type of the organic solvent is not particularly limited and can be selected according to actual needs. Specifically, the organic solvent includes one or more of a cyclic carbonate compound, a chain carbonate compound, and a carboxylic acid ester compound. Preferably, the organic solvent includes one or more of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, ethylene carbonate, propylene carbonate, butylene carbonate, γ-butyrolactone, methyl formate, ethyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, butyl propionate, and tetrahydrofuran.

[0021] In some embodiments, calculated based on the weight of the core, the coating amount of the first coating layer is C1 wt%, where C1 is greater than 0 and not more than 6, preferably greater than 0 and not more than 5.5, and more preferably greater than 0 and not more than 2. And / or calculated based on the weight of the core, the coating amount of the second coating layer is C2 wt%, where C2 is greater than 0 and not more than 6, preferably greater than 0 and not more than 5.5, and more preferably 2 - 4. And / or calculated based on the weight of the core, the coating amount of the third coating layer is C3 wt%, where C3 is greater than 0 and not more than 6, preferably greater than 0 and not more than 5.5, and more preferably greater than 0 and not more than 2.

[0022] In the cathode active material having the core-shell structure of the above embodiment, the coating amounts of the three coating layers are preferably within the above ranges, so that the core can be sufficiently coated, and on the premise of not sacrificing the theoretical capacity of the cathode active material, the kinetic performance and safety performance of the secondary battery are further improved.

[0023] In some embodiments, when calculated based on the weight of the core, the coating amount of the first coating layer is C1 wt%, where C1 is greater than 0 and not more than 6, preferably greater than 0 and not more than 5.5, more preferably greater than 0 and not more than 2, and / or when calculated based on the weight of the core, the coating amount of the second coating layer is C2 wt%, where C2 is greater than 0 and not more than 6, preferably greater than 0 and not more than 5.5, more preferably 2 - 4, and / or when calculated based on the weight of the core, the coating amount of the third coating layer is C3 wt%, where C3 is greater than 0 and not more than 6, preferably greater than 0 and not more than 5.5, more preferably greater than 0 and not more than 2. Define the ratio of (W1 + W2) / (C1 + C2 + C3) as Q, where Q is 0.1 - 10, preferably 0.5 - 5. By controlling the Q value within the above range, when the Q value is smaller than the above range, there are not enough lithium salts and additives to reduce the acidity of the electrolyte, the dissolution of manganese ions cannot be sufficiently and thoroughly suppressed, and the improvement degree of high-temperature cycle and storage performance is insufficient. When the Q value is larger than the above range, the impedance due to film formation is too large, which affects the improvement of the battery capacity and rate performance of lithium ions.

[0024] In any embodiment, the crystal plane spacing range of the crystalline pyrophosphate in the first coating layer is 0.293 - 0.470 nm, and the included angle range of the crystal direction (111) is 18.00° - 32.00°. The crystal plane spacing range of the crystalline phosphate in the second coating layer is 0.244 - 0.425 nm, and the included angle range of the crystal direction (111) is 20.00° - 37.00°. Both the first coating layer and the second coating layer in the cathode active material of the above embodiment use crystalline substances, and the ranges of their crystal plane spacing and included angle are within the above ranges. Thereby, heterogeneous phases in the coating layer can be effectively avoided, thereby improving the theoretical capacity, cycle performance, and rate performance of the material.

[0025] In any embodiment, in the core, the ratio of y to 1 - y is 1:10 - 1:1, preferably 1:4 - 1:1. This further improves the cycle performance and rate performance of the secondary battery.

[0026] In any embodiment, in the core, the ratio of z to 1 - z is 1:999 - 1:9, preferably 1:499 - 1:249. This further improves the cycle performance and rate performance of the secondary battery.

[0027] In any embodiment, the carbon of the third coating layer is a mixture of SP2 - form carbon and SP3 - form carbon. Preferably, the molar ratio of SP2 - form carbon to SP3 - form carbon is any value within the range of 0.1 - 10, preferably any value within the range of 2.0 - 3.0. By restricting the molar ratio of SP2 - form carbon to SP3 - form carbon within the above - mentioned range in the above - mentioned embodiment, the overall performance of the secondary battery is improved.

[0028] In any embodiment, the thickness of the first coating layer is 1 - 10 nm, and / or the thickness of the second coating layer is 2 - 15 nm, and / or the thickness of the third coating layer is 2 - 25 nm.

[0029] In the above - mentioned embodiment, when the thickness range of the first coating layer is 1 - 10 nm, it is possible to avoid the adverse effect on the kinetic performance of the material that may occur when it is too thick, and to avoid the problem that the migration of transition metal ions cannot be effectively inhibited when it is too thin. When the thickness of the second coating layer is within the range of 2 - 15 nm, the surface structure of the second coating layer is stable and the side reaction with the electrolyte is small, so the interfacial side reaction can be effectively reduced, thereby improving the high - temperature performance of the secondary battery. When the thickness range of the third coating layer is 2 - 20 nm, the electrical conductivity of the material can be improved and the compaction density performance of the electrode sheet of the battery manufactured using the positive electrode active material can be improved.

[0030] In any embodiment, based on the weight of the positive electrode active material having a core-shell structure, the content of manganese element is in the range of 10 wt% - 35 wt%, preferably in the range of 15 wt% - 30 wt%, more preferably in the range of 17 wt% - 20 wt%, the content of phosphorus element is in the range of 12 wt% - 25 wt%, preferably in the range of 15 wt% - 20 wt%, and the range of the weight ratio of manganese element to phosphorus element is 0.90 - 1.25, preferably 0.95 - 1.20.

[0031] In the positive electrode active material having a core-shell structure of the above embodiment, the content of manganese element is within the above range, and problems such as poor stability of the material structure and decreased density due to too high content of manganese element can be effectively avoided, thereby improving the performance such as cycle, storage and tap density of the secondary battery, and problems such as low voltage plateau due to too low content of manganese element can be avoided, thereby improving the energy density of the secondary battery.

[0032] In the positive electrode active material having a core-shell structure of the above embodiment, the content of phosphorus element is within the above range, and the following situations can be effectively avoided. If the content of phosphorus element is too high, the covalent bond of P - O may be too strong and may affect the small polaron conduction, thereby affecting the conductivity of the material. If the content of phosphorus element is too low, it may reduce the stability of the pyrophosphate in the core and the first coating layer and / or the phosphate lattice structure in the second coating layer, thereby possibly affecting the stability of the whole material.

[0033] In the positive electrode active material having a core-shell structure of the above embodiment, the weight ratio of manganese element to phosphorus element is within the above range, and the following situations can be effectively avoided. If the weight ratio is too high, the elution of transition metal may increase, which may affect the stability of the material and the cycle and storage performance of the secondary battery. If the weight ratio is too low, the discharge voltage plateau of the material may decrease, thereby possibly reducing the energy density of the secondary battery.

[0034] In any embodiment, the lattice change rate before and after complete lithium desorption of the cathode active material having a core-shell structure is 4% or less, preferably 3.8% or less, and more preferably 2.0-3.8%. The cathode active material having a core-shell structure of the above embodiment can achieve a lattice change rate before and after lithium desorption of 4% or less. Therefore, by using the cathode active material, the theoretical capacity and rate characteristics of the secondary battery can be improved.

[0035] In any embodiment, the concentration of Li / Mn antisite defects in the cathode active material having a core-shell structure is 4% or less, preferably 2.2% or less, and more preferably 1.5-2.2%. When the concentration of Li / Mn antisite defects is within the above range, Mn 2+ prevents the transport of Li + and at the same time, the theoretical capacity and rate performance of the cathode active material can be further improved.

[0036] In any embodiment, the tap density of the cathode active material having a core-shell structure at 3T (ton) is 2.2 g / cm 3 or more, preferably 2.2 g / cm 3 or more and 2.8 g / cm 3 or less. Thereby, when the tap density is improved, the weight of the active material per unit volume increases, which helps to improve the volume energy density of the secondary battery.

[0037] In any embodiment, the surface oxygen valence of the cathode active material having a core-shell structure is -1.90 or less, preferably -1.90 to -1.98. Thereby, for example, by limiting the surface oxygen valence of the cathode active material within the above range, the interfacial side reaction between the cathode material and the electrolyte can be further reduced, thereby improving the performance such as the cycle of the cell and the gas generation during high-temperature storage.

[0038] The second aspect of the present application further provides a battery module, which includes a secondary battery, and the secondary battery is any one of the above secondary batteries of the present application.

[0039] The third aspect of the present application further provides a battery pack, which includes a battery module, and the battery module is the above battery module of the present application.

[0040] The fourth aspect of the present application further provides an electrical device, which includes at least one of a secondary battery, a battery module, or a battery pack, and the above secondary battery, battery module, and battery pack are all the secondary battery, battery module, and battery pack provided by the present application.

[0041] Thereby, the battery module and battery pack of the present application have high cycle performance and rate characteristics. In particular, the high-temperature stability is also significantly improved, and furthermore, high power cycle stability and high-temperature operation stability are provided to the electrical device having the secondary battery, battery module, or battery pack of the present application.

Brief Description of the Drawings

[0042]

Figure 1

Figure 2

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Figure 5

Figure 6

Embodiments for Carrying Out the Invention

[0043] Hereinafter, embodiments specifically disclosing the secondary battery, battery module, battery pack, and electric device of the present application will be described in detail with appropriate reference to the drawings. However, detailed descriptions that are more than necessary may be omitted. For example, detailed descriptions of well-known matters and duplicate descriptions of the same actual configuration may be omitted. This is to avoid making the following description unnecessarily redundant and to facilitate the understanding of those skilled in the art. Also, the drawings and the following description are provided for those skilled in the art to fully understand the present application and do not limit the subject matter described in the claims.

[0044] The "range" disclosed in the present application is limited in the form of a lower limit and an upper limit. A predetermined range is limited by selecting one lower limit and one upper limit, and the selected lower limit and upper limit define the boundary of a special range. The range limited in this way may or may not include the end values and can be arbitrarily combined, that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60 - 120 and 80 - 110 are enumerated for a specific parameter, ranges of 60 - 110 and 80 - 120 are also expected. Also, if the enumerated minimum range values are 1 and 2, and the enumerated maximum range values are 3, 4, and 5, then all of the following ranges can be expected: 1 - 3, 1 - 4, 1 - 5, 2 - 3, 2 - 4, and 2 - 5. In the present application, unless otherwise specified, the numerical range "a - b" represents an abbreviation of any real number combination between a and b, where both a and b are real numbers. For example, the numerical range "0 - 5" represents that all real numbers between "0 - 5" are enumerated in this specification, and "0 - 5" is an abbreviation of these numerical combinations. Also, when a certain parameter is denoted as an integer ≧2, it corresponds to disclosing that the parameter is, for example, the integers 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0045] Unless otherwise specifically described, all embodiments and selectable embodiments of the present application can be combined with each other to form new technical solutions.

[0046] Unless otherwise specified, all technical features of this application and selectable technical features can be combined with each other to form new technical solutions.

[0047] Unless otherwise specified, all steps of this application may be performed in sequence or randomly, preferably in sequence. For example, the method includes steps (a) and (b), and it can be shown that the method can include steps (a) and (b) performed in sequence, and further can include steps (b) and (a) performed in sequence. For example, the mentioned method can further include step (c), and it can be shown that step (c) can be added to the method in any order. For example, the method can include steps (a), (b) and (c), and further can include steps (a), (c) and (b), and further can include steps (c), (a) and (b).

[0048] Unless otherwise specified, the terms "comprising" and "including" mentioned in this application indicate an open form and may also be in a closed form. For example, the "comprising" and "including" can represent further comprising or including other components not listed, or can also comprise or include only the listed components.

[0049] Unless otherwise specified, in this application, the term "or" is inclusive. For example, the phrase "A or B" represents "A, B, or both A and B". More specifically, any of the following conditions satisfies the condition "A or B". A is true (or exists) and B is false (or does not exist), A is false (or does not exist) and B is true (or exists), or both A and B are true (or exist).

[0050] In addition, in this specification, the term "coating layer" refers to a layer of substance coated on a lithium manganese phosphate core, and the layer of substance can completely or partially coat the lithium manganese phosphate core. The use of the "coating layer" is only for the convenience of explanation and is not intended to limit the present invention. Similarly, the term "thickness of the coating layer" refers to the thickness in the radial direction of the lithium manganese phosphate core of the layer of substance coated on the lithium manganese phosphate core.

[0051] In actual work, the inventor of the present application has discovered that in the current conventional lithium manganese phosphate positive electrode active material, the elution of manganese ions is serious during the deep charge and discharge process. In the prior art, there has been an attempt to coat lithium iron phosphate on lithium manganese phosphate to reduce the interfacial side reaction, but such a coating cannot prevent the eluted manganese from continuously migrating into the electrolyte. After the eluted manganese migrates to the negative electrode, it is reduced to metallic manganese. The metallic manganese thus generated is equivalent to a "catalyst", which can catalyze the decomposition of the SEI film (solid electrolyte interphase) on the surface of the negative electrode to generate by-products. Since some of the by-products are gaseous, the secondary battery expands, affecting the safety performance of the secondary battery. In addition, some of the other by-products are deposited on the surface of the negative electrode, inhibiting the channels for lithium ions to enter and exit the negative electrode, increasing the impedance of the secondary battery, and thereby affecting the kinetic performance of the secondary battery. In addition, in order to supplement the lost SEI film, the electrolyte and the active lithium inside the battery are continuously consumed, irreversibly affecting the capacity retention rate of the secondary battery.

[0052] After a large amount of research, the inventor can obtain a positive electrode active material having a novel core-shell structure by modifying lithium manganese phosphate and performing multi-layer coating on lithium manganese phosphate. The positive electrode active material can significantly reduce the elution of manganese ions and reduce the lattice change rate, which is used in secondary batteries and can improve the cycle performance, rate performance, and safety performance of the batteries and improve the capacity of the batteries. [Secondary battery]

[0053] A secondary battery is also called a rechargeable battery or a storage battery, and refers to a battery that can continue to be used by activating the active material in a charging manner after the battery has been discharged.

[0054] Generally, a secondary battery includes a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte. In the charge and discharge process of the battery, active ions (such as lithium ions) are reciprocally inserted and escaped between the positive electrode sheet and the negative electrode sheet. The separator is installed between the positive electrode sheet and the negative electrode sheet, mainly playing a role in preventing the short circuit of the positive and negative electrodes, and at the same time allowing active ions to pass through. The electrolyte is between the positive electrode sheet and the negative electrode sheet, mainly playing a role in conducting active ions.

[0055] Some embodiments of the present application provide a secondary battery, including a positive electrode sheet and a non-aqueous electrolyte. Here, the positive electrode sheet includes a positive electrode active material having a core-shell structure. The positive electrode active material includes a core and a shell covering the core. The chemical formula of the core is Li 1+x Mn 1-y A y P 1-z R z O4, where x is any numerical value within the range of -0.100 to 0.100, y is any numerical value within the range of 0.001 to 0.500, z is any numerical value within the range of 0.001 to 0.100, A is one or more elements selected from the group consisting of Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb, and Ge, preferably one or more elements among Fe, Ti, V, Ni, Co, and Mg, R is one or more elements selected from the group consisting of B, Si, N, and S, preferably R is one element selected from B, Si, N, and S. The values of x, y, and z satisfy the condition of maintaining electrical neutrality throughout the core. The shell includes a first coating layer covering the core, a second coating layer covering the first coating layer, and a third coating layer covering the second coating layer. Here, the first coating layer is crystalline pyrophosphate Li a MP2O7 and / or M b (P2O7) cincluding, where 0 ≦ a ≦ 2, 1 ≦ b ≦ 4, 1 ≦ c ≦ 6, and the values of a, b, and c are such that the crystalline pyrophosphate Li a MP2O7 or M b (P2O7) c is electrically neutral, and the crystalline pyrophosphate Li a MP2O7 and M b (P2O7) c wherein M in each is independently one or more elements selected from the group consisting of Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, and Al. The second coating layer contains crystalline phosphate XPO4, where X is one or more elements selected from the group consisting of Li, Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, and Al. The third coating layer is carbon. The non-aqueous electrolyte contains a first lithium salt and a first additive. The first lithium salt is LiN(C m F 2m+1 SO2)(C n F 2n+1 SO2), Li(FSO2)2N, or one or more selected from the group consisting thereof, and m, n represent positive integers. The non-aqueous electrolyte contains a first lithium salt and a first additive. Preferably, the first lithium salt contains one or more selected from the group consisting of LiN(C m F 2m+1 SO2)(C n F 2n+1 SO2), Li(FSO2)2N, and m, n represent positive integers. The first additive contains one or more of the compounds shown in Formula 1. [Chemical formula] R1 is a C2-C10 alkylene group, a C2-C10 heteroalkylene group, a C6-C18 arylene group, a C2-C18 heteroarylene group, a C3-C18 alicyclic group, or a C3-C18 heteroalicyclic group, which may or may not be substituted with one or more R a . R a is a halogen atom, -CN, -NCO, -OH, -COOH, -SOOH, -COOR b, contains one or more of C2-C10 alkyl groups, C2-C10 alkenyl groups, C2-C10 alkynyl groups, and C2-C10 oxaalkyl groups, R b is any one selected from C1-C10 alkyl groups.

[0056] The positive electrode sheet generally includes a positive electrode current collector and a positive electrode film layer provided on at least one surface of the positive electrode current collector, and the positive electrode film layer contains a positive electrode active material.

[0057] As an example, the positive electrode current collector has two surfaces facing each other in its own thickness direction, and the positive electrode film layer is provided on either one or both of the two opposing surfaces of the positive electrode current collector.

[0058] In some embodiments, the positive electrode current collector can employ a metal foil sheet or a composite current collector. For example, as the metal foil sheet, an aluminum foil can be employed. The composite current collector may have a base layer of a polymer material and a metal layer formed on at least one surface of the base layer of the polymer material. The composite current collector can be formed by forming a metal material (such as aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a polymer material substrate (such as substrates of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0059] Unless otherwise specified, in the chemical formula of the above core, when A is two or more elements, the limitation on the y numerical range is not only the limitation on the stoichiometric number of each element of A, but also the limitation on the sum of the stoichiometric numbers of each element of A. For example, when A is two or more elements A1, A2... An, the stoichiometric numbers y1, y2... yn of each of A1, A2... An must all be within the numerical range limited by y in the present application, and the sum of y1, y2... yn must also be within the numerical range. Similarly, for the case where R is two or more elements, the limitation on the numerical range of the R stoichiometric number in the present application also has the above meaning.

[0060] In a preferred embodiment, when A is one, two, three, or four elements selected from Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb, and Ge, A y is Q n1 D n2 E n3 K n4 where n1 + n2 + n3 + n4 = y, and n1, n2, n3, n4 are all positive numbers and not zero at the same time. Q, D, E, and K are each independently one element selected from Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb, and Ge. Preferably, at least one of Q, D, E, and K is Fe. Preferably, one of n1, n2, n3, n4 is zero and the rest are not zero. More preferably, two of n1, n2, n3, n4 are zero and the rest are not zero. Even more preferably, three of n1, n2, n3, n4 are zero and the rest are not zero. The core Li 1+x Mn 1-y A y P 1-z R z In O4, it is advantageous to dope the manganese site with one, two, three, or four of the above A elements. Preferably, one, two, or three of the above A elements are doped. Also, it is advantageous to dope the phosphorus site with one or two R elements, which helps to uniformly distribute the doping elements in this way.

[0061] The core Li 1+x Mn 1-y A y P 1-z R zIn O4, the magnitude of x is affected by the valences of A and R and the magnitudes of y and z, ensuring that the entire system exhibits electrical neutrality. If the value of x is too small, the lithium content of the entire core system will decrease, affecting the exertion of the theoretical capacity of the material. The y value limits the total amount of all doping elements. If y is too small, i.e., the doping amount is too small, the doping elements will not function. If y exceeds 0.5, the Mn content in the system will be low, affecting the voltage plateau of the material. The R element is doped at the position of P, stabilizing the P-O tetrahedron. Since a too large z value will affect the stability of the material, the z value is limited to 0.001 - 0.100.

[0062] The cathode active material of this application can improve the theoretical capacity, cycle performance, and safety performance of secondary batteries. Although the mechanism is not clear, the lithium manganese phosphate cathode active material of this application has a core-shell structure. By doping element A and element R into the manganese site and phosphorus site of the core of lithium manganese phosphate respectively, the elution of manganese ions can be effectively reduced, further reducing the manganese ions migrating to the anode, reducing the electrolyte consumed by the decomposition of the SEI film, not only improving the cycle performance and safety performance of secondary batteries, but also promoting the adjustment of the Mn-O bond, reducing the migration barrier of lithium ions, promoting the migration of lithium ions, and improving the rate performance of secondary batteries. By coating the core with a first coating layer containing crystalline pyrophosphate, the migration resistance of manganese can be further increased, its elution can be reduced, and the lithium ion content on the surface can be reduced, reducing the contact between the core and the electrolyte, thereby reducing the interfacial side reaction and gas generation, and improving the high-temperature storage performance, cycle performance, and safety performance of secondary batteries. By coating a crystalline phosphate coating layer with better lithium ion conduction ability, the interfacial side reaction on the surface of the cathode active material can be effectively reduced, and the high-temperature cycle and storage performance of secondary batteries can be further improved. By further coating a carbon layer as the third coating layer, the safety performance and kinetic performance of secondary batteries can be further improved.

[0063] In addition, in the core, element A doped into the manganese site of lithium manganese phosphate further reduces the lattice change rate of lithium manganese phosphate during the lithium desorption process of the material, improves the structural stability of the lithium manganese phosphate cathode material, significantly reduces the elution of manganese, and helps to lower the oxygen activity on the particle surface. Element R doped into the phosphorus site further helps to change the difficulty of the distance change of the Mn-O bond, thereby improving the electronic conductivity, reducing the lithium ion migration barrier, promoting lithium ion migration, and improving the rate performance of the secondary battery.

[0064] In addition, the entire core system can maintain electrical neutrality and ensure that there are as few defects and heterophases as possible in the cathode active material. If there is an excessive transition metal (such as manganese) in the cathode active material, since the structure of the material system itself is stable, the excess transition metal may precipitate in the form of a single substance or form a heterophase inside the crystal lattice. By maintaining electrical neutrality, such heterophases can be minimized. In addition, by ensuring the electrical neutrality of the system, lithium vacancies can be generated in the material in some situations, thereby making the kinetic performance of the material more excellent and further improving the kinetic performance of the secondary battery.

[0065] At the same time, the non-aqueous electrolyte uses the first lithium salt as the main lithium salt. Due to its excellent thermal stability and hydrolysis resistance, it can effectively reduce the acidity of the electrolyte, reduce the elution of manganese ions, and improve the high-temperature cycle and storage performance. Introducing the isocyanate group compound shown in Formula 1 into the non-aqueous electrolyte can react with trace amounts of water in the battery to generate -NHCOOH, prevent the generation of HF by the action of trace amounts of water and the non-aqueous electrolyte, further reduce the acidity of the electrolyte, reduce the elution of manganese ions, and further improve the high-temperature cycle and storage performance. In addition, the isocyanate group compound shown in Formula 1 can further form a film on the negative electrode to generate a uniform SEI film, reduce the reduction of the eluted Mn at the negative electrode, and further improve the high-temperature cycle and storage performance.

[0066] The positions of the main characteristic peaks in the XRD pattern of lithium manganese phosphate after doping with A element and R element are consistent with those of undoped LiMnPO4, indicating that no heterogeneous phase is introduced during the doping process. Therefore, the improvement of the core performance mainly comes from element doping rather than heterogeneous phase. After manufacturing the above-mentioned cathode active material, the inventor of the present application cut out the intermediate region of the cathode active material particles manufactured by focused ion beam (abbreviated as FIB), and tested it by transmission electron microscope (abbreviated as TEM) and X-ray spectroscopic analysis (abbreviated as EDS), and found that the distribution of each element is uniform and no aggregation occurs.

[0067] In the present application, the crystalline substance has a crystallinity of 50% or more, that is, it means 50%-100%. Those with a crystallinity of less than 50% are called glassy states. The crystallinity of the crystalline pyrophosphate and crystalline phosphate of the present application is 50% - 100%. Pyrophosphates and phosphates with a certain crystallinity not only help the pyrophosphate coating layer to fully exert its functions of inhibiting the elution of manganese ions, excellent lithium ion conduction ability, and reducing interfacial side reactions, but also enable the pyrophosphate coating layer and the phosphate coating layer to achieve better lattice matching, thereby realizing a tighter bonding of the coating layer.

[0068] In the present application, the crystallinity of the crystalline pyrophosphate of the first coating layer material and the crystalline phosphate of the second coating layer material of the cathode active material can be measured by general technical means in this field, such as density method, infrared spectroscopy, differential scanning calorimetry and nuclear magnetic resonance absorption method, and can also be measured by, for example, X-ray diffraction method.

[0069] The method for measuring the crystallinity of the crystalline pyrophosphate of the first coating layer and the crystalline phosphate of the second coating layer of the cathode active material by a specific X-ray diffraction method can include the following steps.

[0070] Take a certain amount of cathode active material powder and measure the total scattering intensity by X-rays. This is the sum of the scattering intensities of the entire spatial material and is only related to the intensity of the primary radiation, the chemical structure of the cathode active material powder, and the total number of electrons participating in diffraction, i.e., the amount of mass, and has nothing to do with the order of the sample. Next, separate the crystalline scattering and the amorphous scattering from the diffraction pattern, and the crystallinity is the ratio of the total scattering intensity of the crystalline part scattering.

[0071] In addition, in the present application, the crystallinity of the pyrophosphate and phosphate in the coating layer can be adjusted, for example, by adjusting the process conditions such as the sintering temperature and sintering time during the sintering process.

[0072] In the present application, since metal ions are difficult to migrate in pyrophosphate, the pyrophosphate can effectively separate the metal ions doped as the first coating layer from the electrolyte. The structure of the crystalline pyrophosphate is stable. Therefore, the crystalline pyrophosphate coating can effectively suppress the elution of transition metals and improve the cycle performance.

[0073] The junction between the first coating layer and the core is similar to a heterojunction, and the firmness of the junction is limited by the lattice matching degree. When the lattice mismatch is 5% or less, the lattice matching is good, and it is easy for the two to join tightly. The tight junction can ensure that the coating layer does not fall off during the subsequent cycle process and helps to ensure the long-term stability of the material. The determination of the degree of junction between the first coating layer and the core is mainly carried out by calculating the mismatch degree of each lattice constant. In the present application, after doping elements A and R into the core, compared with not doping elements, the matching degree between the core and the first coating layer is improved, and the core and the pyrophosphate coating layer can be joined more tightly. First coating with the In the present application, after doping elements A and R into the core, compared with not doping elements, the matching degree between the core and the first coating layer is improved, and the core and the pyrophosphate coating layer can be joined more tightly.

[0074] A crystalline phosphate is selected as the second coating layer. First, its lattice match with the crystalline pyrophosphate of the first coating layer is high (the mismatch is only 3%). Next, the stability of the phosphate itself is higher than that of the pyrophosphate, and coating the pyrophosphate helps to improve the stability of the material. The structure of the crystalline phosphate is very stable, and it has excellent lithium ion conduction ability. Therefore, coating with the crystalline phosphate can effectively reduce the interfacial side reactions on the surface of the cathode active material, thereby improving the high-temperature cycle and storage performance of the secondary battery. The lattice matching method between the second coating layer and the first coating layer is similar to the bonding situation between the first coating layer and the core. When the lattice mismatch is 5% or less, the lattice match is good, and the two are likely to be closely bonded.

[0075] The main reason for carbon to be the third coating layer is that the carbon layer has good electronic conductivity. What occurs during the application to a secondary battery is an electrochemical reaction, which requires the participation of electrons. Therefore, in order to increase the electron transport between particles and the electron transport at different positions on the particles, the cathode active material can be coated with carbon having excellent conductive performance. This carbon coating can effectively improve the conductive performance and desolvation treatment ability of the cathode active material.

[0076] Each coating layer of the present application may be completely coated or partially coated.

[0077] In some embodiments, the non-aqueous electrolyte contains a first lithium salt and a first additive, and the first lithium salt exists as the main lithium salt. In the above first lithium salt, LiN(C m F 2m+1 SO2)(C n F 2n+1 SO2) can be selected from any corresponding specific substances in the prior art. In some embodiments, m and n each independently represent 0, 1, 2, or 3, thereby providing excellent thermal stability. For example, LiN(C m F 2m+1 SO2)(C n F 2n+1SO2) is selected from substances such as lithium (trifluoromethanesulfonylimide)(fluorosulfonylimide) Li(CF3SO2)(FSO2)N, lithium bis(trifluoromethanesulfonylimide) (LiN(CF3SO2)2), lithium bis(pentafluoroethanesulfonylimide) (LiN(C2F5SO2)2), lithium (trifluoromethanesulfonylimide)(pentafluoroethanesulfonylimide) (LiN(CF3SO2)(C2F5SO2)), lithium (trifluoromethanesulfonylimide)(heptafluoropropanesulfonylimide) (LiN(CF3SO2)(C3F7SO2)), lithium (trifluoromethanesulfonylimide)(nonafluorobutanesulfonylimide) (LiN(CF3SO2)(C4F9SO2)), lithium (pentafluoroethanesulfonylimide)(heptafluoropropanesulfonylimide) LiN(C2F5SO2)(C3F7SO2), Li(FSO2)2N, etc.

[0078] The isocyanate compound shown in 1 used in the present application can be selected from isocyanate compounds generally used in electrolytic solutions in the prior art. In some embodiments, the above R1 is one or more Rs a which is an optionally substituted C2-C10 alkylene group, C2-C10 oxyalkylene group, C2-C10 azaalkylene group, phenylene group, naphthylene group, anthrylene group, cyclobutylene group, cyclopentylene group, cyclohexylene group, biphenylene group, methylenediphenylene group, preferably R1 is one or more Rs a which is an optionally substituted C2-C6 alkylene group, phenylene group, naphthylene group, anthrylene group, cyclobutylene group, cyclopentylene group, cyclohexylene group, biphenylene group, methylenediphenylene group, and / or R a contains one or more selected from halogen atoms, -CN, C2-C3 alkyl groups, and / or R in the compound shown in Formula 1 a The number of is 0, 1, 2, 3 or 4.

[0079] Preferably, the first additive contains one or more of the following compounds.

Chemical formula

Chemical formula

[0080] Each of the above compounds has good dispersion and dissolution performance in the solvent of the non-aqueous electrolyte, reacts sufficiently with trace amounts of water in the battery to produce -NHCOOH can be generated It can prevent the generation of HF due to the action of trace amounts of water and the non-aqueous electrolyte, further reduce the acidity of the electrolyte and the elution of manganese ions, and further improve the high-temperature cycle and storage performance. In addition, each of the above isocyanate group compounds is likely to form a uniform SEI film on the negative electrode, reduce the reduction of the eluted Mn on the negative electrode, and help further improve the high-temperature cycle and storage performance.

[0081] In some embodiments of the present application, when the first lithium salt is any one selected from LiN(CF3SO2)(FSO2), LiN(CF3SO2)(C2F5SO2), LiN(CF3SO2)(C3F7SO2), the first additive is any one selected from the following compounds.

Chemical formula

[0082] At this time, the advantages of both are fully exerted, not only reducing the acidity of the electrolyte and the elution of manganese ions, but also avoiding the deterioration of the negative electrode impedance, further improving the high-temperature cycle and storage performance of the lithium-ion battery, and at the same time not affecting the performance of the lithium-ion battery capacity and rate performance.

[0083] When the above-mentioned first lithium salt is adopted as the main lithium salt of the non-aqueous electrolyte, the high-temperature cycle and storage performance of the secondary battery can be effectively improved. However, under certain conditions, this substance corrodes the lithium salt. For example, when the operating voltage of a lithium-ion battery > 4.2V, there is a problem that the aluminum foil corrodes. The possible mechanism here is that after the original oxide film on the surface of the aluminum foil is destroyed in the electrolyte, the more active aluminum is exposed, and then the aluminum oxidizes to generate Al 3+ ions, and then FSI- or TFSI- in the electrolyte combines with Al 3+ to form soluble Al(FSI)3 or Al(TFSI)3, and after Al(FSI)3 or Al(TFSI)3 dissolves, it causes the corrosion of aluminum. Therefore, in order to alleviate the corrosion problem of the aluminum foil caused by the first lithium salt at a high operating voltage, in some embodiments, based on the total weight of the non-aqueous electrolyte, the content of the first lithium salt is W1 wt%, where W1 is 0.1 - 48, preferably 5 - 20.

[0084] Based on controlling the content of the first lithium salt, in some embodiments, by adding a second lithium salt, the lithium salt in the non-aqueous electrolyte can be replenished. That is, the non-aqueous electrolyte further contains a second lithium salt, and the second lithium salt includes one or more selected from lithium difluorophosphate, lithium difluorodioxalate phosphate, lithium difluorooxalate borate, lithium bisoxalate borate, LiPF6, LiBF4, LiAsF6. The above-mentioned second lithium salt is added to the non-aqueous electrolyte as a lithium salt type additive, which preferentially decomposes on the surface of the aluminum foil, and the decomposition product combines with aluminum ions to form an insoluble or hardly soluble precipitate that adheres to the surface of the aluminum foil, thereby forming a further passivation film to prevent direct contact between the aluminum foil and the electrolyte, protect the aluminum foil, and further cooperate with the first lithium salt to improve the high-temperature cycle and storage performance.

[0085] However, too much lithium salt type additive also increases the impedance of the positive and negative electrodes, resulting in abnormal lithium-ion battery capacity and poor rate performance. In some embodiments, based on the total weight of the non-aqueous electrolyte, the content of the second lithium salt is W3 wt%, where W3 is from 0.01 to 20, preferably from 0.1 to 10 or from 0.3 to 5. When the mass ratio of the second lithium salt in the electrolyte is within the above range, corrosion of the aluminum foil can be suppressed, the high-temperature cycle and storage performance of the lithium-ion battery can be improved, and the capacity and rate performance of the lithium-ion battery are not deteriorated.

[0086] The first additive can form a film on the negative electrode to generate a uniform SEI film, reduce the reduction of the eluted Mn on the negative electrode, and further improve the high-temperature cycle and storage performance. However, too much isocyanate group compound also increases the negative electrode impedance, resulting in abnormal lithium-ion battery capacity and poor rate performance. Preferably, the content of the first additive is W2 wt%, where W2 is from 0.01 to 20, preferably from 0.1 to 10 or from 0.3 to 5. When the mass ratio of the first additive in the electrolyte is within the above range, not only the acidity of the electrolyte is reduced and the elution of manganese ions is decreased, but also the deterioration of the negative electrode impedance is avoided, the high-temperature cycle and storage performance of the lithium-ion battery are further improved, and at the same time, the capacity and rate performance of the lithium-ion battery are not affected.

[0087] In some embodiments, the above W2 / W1 is defined as M, where M is from 0.001 to 3, preferably from 0.005 to 0.5. Define (W2 + W3) / W1 as N, where N is from 0.01 to 2, preferably from 0.02 to 1.

[0088] When W1 / W2 and (W2 + W3) / W1 are within the above ranges, the three can exhibit a better synergistic effect, the acidity of the system is low, the elution of manganese ions is reduced, the high-temperature cycle and storage performance of lithium ions are guaranteed to be excellent, the corrosion of the aluminum foil is effectively suppressed, the high-temperature cycle and storage performance of the lithium-ion battery are improved, and at the same time, the capacity and rate performance of the lithium-ion battery are not deteriorated.

[0089] In some embodiments, the non-aqueous electrolyte further includes a second additive, and the second additive includes one or more selected from the group consisting of a cyclic carbonate compound containing an unsaturated bond, a halogen-substituted saturated cyclic carbonate compound, a sulfate compound, a sulfite compound, a sultone compound, a disulfonic acid compound, a nitrile compound, an aromatic compound, a phosphazene compound, a cyclic acid anhydride compound, a phosphite compound, a phosphate compound, and a borate compound. Those skilled in the art can select the corresponding second additive from the above substances according to actual needs, and the usage amount of the second additive in the non-aqueous electrolyte can also refer to the prior art, and the description of which is omitted in this application.

[0090] In some embodiments, the non-aqueous electrolyte further includes an organic solvent. The type of the organic solvent is not particularly limited and can be selected according to actual needs. Specifically, the organic solvent includes one or more selected from the group consisting of a cyclic carbonate compound, a chain carbonate compound, and a carboxylic acid ester compound. Preferably, the organic solvent includes one or more selected from the group consisting of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, ethylene carbonate, propylene carbonate, butylene carbonate, γ-butyrolactone, methyl formate, ethyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, butyl propionate, and tetrahydrofuran.

[0091] In some embodiments, when calculated based on the weight of the core, the coating amount of the first coating layer is C1 wt%, where C1 is greater than 0 and not more than 6, preferably greater than 0 and not more than 5.5, and more preferably greater than 0 and not more than 2. And / or when calculated based on the weight of the core, the coating amount of the second coating layer is C2 wt%, where C2 is greater than 0 and not more than 6, preferably greater than 0 and not more than 5.5, and more preferably 2 - 4. And / or when calculated based on the weight of the core, the coating amount of the third coating layer is C3 wt%, where C3 is greater than 0 and not more than 6, preferably greater than 0 and not more than 5.5, and more preferably greater than 0 and not more than 2.

[0092] In the present application, the coating amount of each layer is not zero.

[0093] In the cathode active material having a core - shell structure in the above - mentioned embodiment, the coating amounts of the three coating layers can be selected within the above - mentioned ranges, so that the core can be sufficiently coated, and on the premise of not sacrificing the theoretical capacity of the cathode active material, the kinetic performance and safety performance of the secondary battery can be further improved.

[0094] For the first coating layer, when the coating amount is within the above - mentioned range, the following situations can be avoided. If the coating amount is too small, it means that the thickness of the coating layer is thin, and there is a possibility that the migration of transition metals cannot be effectively inhibited. If the coating amount is too large, it means that the coating layer is too thick, which affects the migration of Li+ and further affects the rate performance of the material.

[0095] For the second coating layer, when the coating amount is within the above - mentioned range, the following situations can be avoided. If the coating amount is too large, it may affect the plateau voltage of the whole material. If the coating amount is too small, there is a possibility that a sufficient coating effect cannot be achieved.

[0096] For the third coating layer, the carbon coating mainly serves to enhance the electron transport between particles. However, since the structure contains a large amount of amorphous carbon, the density of carbon is low. Therefore, if the coating amount is too large, it will affect the tap density of the electrode sheet.

[0097] In the positive electrode active material having the core-shell structure of the above embodiment, the coating amounts of the three coating layers are preferably within the above range, so that the core can be sufficiently coated, and on the premise of not sacrificing the theoretical capacity of the positive electrode active material, the kinetic performance and safety performance of the secondary battery are further improved.

[0098] In some embodiments, the ratio of (W1 + W2) / (C1 + C2 + C3) is defined as Q, and Q is 0.1 - 10, preferably 0.5 - 5. By controlling the Q value within the above range, when the Q value is smaller than the above range, there is not enough lithium salt and additive to reduce the acidity of the electrolyte, and the elution of manganese ions cannot be sufficiently and thoroughly suppressed, avoiding insufficient improvement in high-temperature cycle and storage performance. When the Q value is larger than the above range, the impedance due to film formation is too large, affecting the improvement of the battery capacity and rate performance of lithium ions.

[0099] In any embodiment, the range of the crystal plane spacing of the crystalline pyrophosphate in the first coating layer is 0.293 - 0.470 nm, and the range of the included angle of the crystal direction (111) is 18.00° - 32.00°; the range of the crystal plane spacing of the crystalline phosphate in the second coating layer is 0.244 - 0.425 nm, and the range of the included angle of the crystal direction (111) is 20.00° - 37.00°.

[0100] The crystalline pyrophosphate and crystalline phosphate in the coating layer may be characterized by general technical means in this field, for example, may be characterized by a transmission electron microscope (TEM). By testing the crystal plane spacing with TEM, the core and the coating layer can be distinguished.

[0101] The specific test method for the interplanar spacing and included angle of crystalline pyrophosphate and crystalline phosphate in the coating layer can include the following steps.

[0102] Put a certain amount of coated cathode active material sample powder into a test tube, inject a solvent such as alcohol into the test tube, then stir well to disperse, and then use a clean disposable plastic straw to take an appropriate amount of the above solution and drop it onto a 300-mesh copper mesh. At this time, some powder remains on the copper mesh. Transfer the copper mesh together with the sample to a TEM sample cavity for testing to obtain the original image of the TEM test and save the original image.

[0103] Open the original image obtained in the above TEM test with diffraction meter software, perform Fourier transform to obtain a diffraction pattern, and measure the distance from the diffraction spot in the diffraction pattern to the center position to obtain the interplanar spacing. The included angle can be obtained by calculation based on the Bragg equation.

[0104] The interplanar spacing range of crystalline pyrophosphate and the presence difference of crystalline phosphate can be directly judged by the numerical value of the interplanar spacing.

[0105] Both the first coating layer and the second coating layer in the cathode active material of the above embodiment use crystalline substances, and the range of their interplanar spacing and included angle is within the above range. Thereby, heterogeneous phases in the coating layer can be effectively avoided, thereby improving the theoretical capacity, cycle performance, and rate performance of the material. And the crystalline pyrophosphate and crystalline phosphate within the above range of interplanar spacing and included angle can more effectively suppress the lattice change rate of lithium manganese phosphate and the elution of manganese ions during the process of lithium desorption, thereby improving the high-temperature cycle performance, cycle stability, and high-temperature storage performance of the secondary battery.

[0106] In some embodiments, in the core, the ratio of y to 1 - y is 1:10 - 1:1, preferably 1:4 - 1:1. When the above conditions are met, the energy density and cycle performance of the secondary battery using the positive electrode active material can be further improved.

[0107] In some embodiments, in the core, the ratio of z to 1 - z is 1:9 - 1:999, preferably 1:499 - 1:249. Here, z represents the sum of the stoichiometric numbers of the P-site doping element R. When the above conditions are met, the energy density and cycle performance of the secondary battery using the positive electrode active material can be further improved.

[0108] In some embodiments, the carbon of the third coating layer is a mixture of SP2 - form carbon and SP3 - form carbon. Preferably, the molar ratio of SP2 - form carbon to SP3 - form carbon is any value within the range of 0.1 - 10, preferably any value within the range of 2.0 - 3.0.

[0109] In some embodiments, the molar ratio of SP2 - form carbon to SP3 - form carbon may be about 0.1, about 0.2, about 0.3 ..., about 0.4, about 0.5, about 0.6, about 0.7, about 0.8, about 0.9, about 1, about 2, about 3, about 4, about 5, about 6, about 7, about 8, about 9 or about 10, or within any range of any of the above values.

[0110] In the present application, a numerical value with "about" indicates a range, which means the range of ±10% of the numerical value.

[0111] By selecting the form of carbon in the carbon coating layer, the overall electrical performance of the secondary battery can be improved. Specifically, by using a mixed form of SP2-form carbon and SP3-form carbon and restricting the ratio of SP2-form carbon to SP3-form carbon within a certain range, the following situations can be avoided. If all the carbon in the coating layer is in the amorphous SP3 form, the conductivity is low. If all is in the graphitized SP2 form, the conductivity is good, but there are few lithium-ion channels, which is disadvantageous for the desorption of lithium. Also, restricting the molar ratio of SP2-form carbon to SP3-form carbon within the above range can not only achieve good conductivity but also guarantee the lithium-ion channels, thus contributing to the optimization of the secondary battery function and the improvement of its cycle performance.

[0112] The mixing ratio of the SP2 form and the SP3 form of the carbon in the third coating layer can be controlled by sintering conditions such as the sintering temperature and the sintering time. For example, when manufacturing the third coating layer using sucrose as the carbon source, after decomposing sucrose at a high temperature, it is deposited on the second coating layer, and at the same time, under the action of high temperature, a carbon coating layer having both the SP3 form and the SP2 form is generated. The ratio of the SP2-form carbon to the SP3-form carbon can be adjusted by selecting the high-temperature decomposition conditions and the sintering conditions.

[0113] The structure and characteristics of the carbon in the third coating layer can be measured by Raman spectrum, and the specific test method is as follows. By performing peak splitting on the energy spectrum of the Raman test, Id / Ig (where Id is the peak intensity of the SP3-form carbon and Ig is the peak intensity of the SP2-form carbon) is obtained, and thereby the molar ratio of the two is confirmed.

[0114] In some embodiments, the thickness of the first coating layer is 1 - 10 nm, and / or the thickness of the second coating layer is 2 - 15 nm, and / or the thickness of the third coating layer is 2 - 25 nm.

[0115] In some embodiments, the thickness of the first coating layer may be about 2 nm, about 3 nm, about 4 nm, about 5 nm, about 6 nm, about 7 nm, about 8 nm, about 9 nm, or about 10 nm, or within any range of any of the above values.

[0116] In some embodiments, the thickness of the second coating layer may be about 2 nm, about 3 nm, about 4 nm, about 5 nm, about 6 nm, about 7 nm, about 8 nm, about 9 nm, about 10 nm, about 11 nm, about 12 nm, about 13 nm, about 14 nm, or about 15 nm, or within any range of any of the above values.

[0117] In some embodiments, the thickness of the third coating layer may be about 2 nm, about 3 nm, about 4 nm, about 5 nm, about 6 nm, about 7 nm, about 8 nm, about 9 nm, about 10 nm, about 11 nm, about 12 nm, about 13 nm, about 14 nm, about 15 nm, about 16 nm, about 17 nm, about 18 nm, about 19 nm, about 20 nm, about 21 nm, about 22 nm, about 23 nm, about 24 nm, or about 25 nm, or within any range of any of the above values.

[0118] When the thickness range of the first coating layer is 1 - 10 nm, it is possible to avoid the adverse effect on the kinetic performance of the material that may occur if it is too thick, and it is also possible to avoid the problem that if it is too thin, the migration of transition metal ions may not be effectively inhibited.

[0119] When the thickness of the second coating layer is within the range of 2 - 15 nm, the surface structure of the second coating layer is stable and the side reaction with the electrolyte is small, so the interfacial side reaction can be effectively reduced, thereby further improving the high-temperature performance of the secondary battery.

[0120] When the thickness range of the third coating layer is 2 - 25 nm, the electrical conductivity performance of the material can be further improved and the consolidation performance of the battery electrode sheet manufactured using the cathode active material can be better improved.

[0121] The test of the thickness of the coating layer is mainly carried out by FIB, and the specific method can include the following steps. Randomly select a single particle from the positive electrode active material powder to be tested, cut a thin slice with a thickness of about 100 nm from the middle position or near the middle position of the selected particle, then perform a TEM test on the thin slice to measure the thickness of the coating layer, measure 3-5 positions, and take the average value.

[0122] In some embodiments, based on the weight of the positive electrode active material, the content of manganese element is in the range of 10 wt%-35 wt%, preferably in the range of 15 wt%-30 wt%, more preferably in the range of 17 wt%-20 wt%, the content of phosphorus element is in the range of 12 wt%-25 wt%, preferably in the range of 15 wt%-20 wt%, and the range of the weight ratio of manganese element to phosphorus element is 0.90-1.25, preferably 0.95-1.20.

[0123] In the present application, when only the core of the positive electrode active material contains manganese, the content of manganese can correspond to the content of the core.

[0124] In the above embodiments, by restricting the content of manganese element within the above range, problems such as poor stability of the material structure and decreased density when the content of manganese element is too large can be effectively avoided, thereby effectively improving the performance of the secondary battery in terms of cycling, storage, and compaction, etc. And when the content of manganese element is too small, problems such as low voltage plateau can be avoided, thereby further improving the energy density of the secondary battery.

[0125] In the above embodiments, by restricting the content of phosphorus element within the above range, the following situations can be effectively avoided. When the content of phosphorus element is too large, the covalent bond property of P-O may be too strong, which may affect the small polaron conduction and thereby affect the conductivity of the material. When the phosphorus content is too small, it may reduce the stability of the core, the pyrophosphate in the first coating layer and / or the phosphate lattice structure in the second coating layer, thereby possibly affecting the stability of the whole material.

[0126] The magnitude of the weight ratio of the manganese and phosphorus contents has the following effects on the performance of the secondary battery. If the weight ratio is too large, it means that there is too much manganese element, resulting in an increase in the elution of manganese ions, which affects the stability of the positive electrode active material and the exertion of the theoretical capacity, and further affects the cycle performance and storage performance of the secondary battery. If the weight ratio is too small, it means that there is too much phosphorus element, which easily forms a heterogeneous phase, reduces the discharge voltage plateau of the material, and thereby reduces the energy density of the secondary battery.

[0127] The measurement of the manganese element and the phosphorus element can be carried out by using general technical means in this field. In particular, the contents of the manganese element and the phosphorus element are measured by the following method. The material is dissolved in dilute hydrochloric acid (concentration 10 - 30%), the content of each element in the solution is measured by ICP, and then the content of the manganese element is measured and converted to obtain its weight ratio.

[0128] In some embodiments, the core-shell structured positive electrode active material has a lattice change rate of 4% or less before and after complete lithium desorption, preferably 3.8% or less, and more preferably 2.0 - 3.8%.

[0129] The lithium desorption process of lithium manganese phosphate (LiMnPO4) is a two-phase reaction. The interfacial stress between the two phases is determined by the magnitude of the lattice change rate before and after lithium desorption. The smaller the lattice change rate, the smaller the interfacial stress, and the easier the Li + transport becomes. Therefore, reducing the lattice change rate of the core helps to improve the Li + transport ability, thereby improving the rate performance of the secondary battery. The core-shell structured positive electrode active material of the above embodiments can achieve a lattice change rate of 4% or less before and after lithium desorption, and therefore the rate performance of the secondary battery can be improved by using the positive electrode active material. The lattice change rate can be measured by a method known in this field, such as an X-ray diffraction pattern (XRD).

[0130] In some embodiments, the concentration of Li / Mn antisite defects in the cathode active material having a core-shell structure is 4% or less, preferably 2.2% or less, and more preferably 1.5 - 2.2%.

[0131] The Li / Mn antisite defects of the present application refer to the exchange of the positions of Li + and Mn 2+ in the LiMnPO4 crystal lattice. Accordingly, the Li / Mn antisite defect concentration refers to the percentage of Li 2+ exchanged with Mn + in the total amount of Li + . In the present application, the Li / Mn antisite defect concentration can be tested, for example, based on JIS K0131-1996.

[0132] The cathode active material having a core-shell structure of the above embodiment can achieve a low Li / Mn antisite defect concentration as described above. Although the mechanism is not yet fully clear, the inventors of the present application believe that in the LiMnPO4 crystal lattice, the positions of Li + and Mn 2+ are exchanged, and since the Li + transport channel is a one-dimensional channel, it is presumed that Mn 2+ transitions in the Li + channel and it is difficult to inhibit the transport of Li + . As a result, the cathode active material having a core-shell structure described in the present application has a low Li / Mn antisite defect concentration within the above range. Therefore, it is possible to avoid Mn 2+ from inhibiting the transport of Li + , and at the same time improve the theoretical capacity performance and rate performance of the cathode active material.

[0133] In some embodiments, the tap density of the cathode active material at 3T is 2.2 g / cm 3 or more, preferably 2.2 g / cm 3 or more and 2.8 g / cm 3The following holds. The higher the tap density, the greater the weight of the active material per unit volume. Therefore, improving the tap density helps improve the volumetric energy density of the cell. The tap density can be measured based on GB / T 24533-2009.

[0134] In some embodiments, the surface oxygen valence of the positive electrode active material is -1.90 or less, preferably -1.90 to -1.98.

[0135] The stable valence of oxygen is -2. The closer the valence is to -2, the stronger its electron-withdrawing ability, that is, the stronger its oxidizing property. Generally, its surface valence is -1.7 or less. By limiting the surface oxygen valence of the positive electrode active material within the above range as described in the above embodiments, the interfacial side reaction between the positive electrode material and the electrolyte can be reduced, thereby improving the performance of the cell such as cycling and high-temperature storage gas generation.

[0136] The surface oxygen valence can be measured by a method known in the art, for example, by electron energy loss spectroscopy (EELS).

[0137] The present application further provides a method for manufacturing a positive electrode active material, including the following steps.

[0138] Providing a core material, wherein the core chemical formula is Li 1+x Mn 1-y A y P 1-z R zIt is O4, where x is any numerical value within the range of -0.100 to 0.100, y is any numerical value within the range of 0.001 to 0.500, z is any numerical value within the range of 0.001 to 0.100, and the said A is one or more elements selected from Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb, and Ge, preferably one or more elements selected from Fe, Ti, V, Ni, Co, and Mg, the said R is one or more elements selected from B, Si, N, and S, preferably the said R is one element selected from B, Si, N, and S.

[0139] A coating step, each being Li a MP2O7 and / or Mb(P2O7) c and an XPO4 suspension are provided, the said core material is added to the above suspension and mixed, and then sintered to obtain a cathode active material, where 0 ≤ a ≤ 2, 1 ≤ b ≤ 4, 1 ≤ c ≤ 6, and the values of the said a, b, and c satisfy the condition that the crystalline pyrophosphate Li a MP2O7 or Mb(P2O7) c is electrically neutral, M is each independently one or more elements selected from Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, or Al, and X is one or more elements selected from Li, Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, or Al.

[0140] Here, the said cathode active material has a core-shell structure, which includes the said core and a shell covering the core, and the said shell includes a first coating layer covering the core, a second coating layer covering the first coating layer, and a third coating layer covering the second coating layer. The first coating layer includes the crystalline pyrophosphate Li a MP2O7 and / or Mb(P2O7) c The second coating layer includes the crystalline pyrophosphate XPO4, and the third coating layer is carbon.

[0141] In some embodiments, the step of providing the said core material includes the following steps.

[0142] Step (1): Mix and stir a manganese source, a dopant of element A, and an acid in a container to obtain manganese salt particles doped with element A.

[0143] Step (2): Mix the manganese salt particles doped with element A, a lithium source, a phosphorus source, and a dopant of element R in a solvent to obtain a slurry, and sinter under the protection of an inert gas atmosphere to obtain a core doped with element A and element R. Here, the core doped with element A and element R is Li 1+x Mn 1-y A y P 1-z R z O4, where x is any numerical value within the range of -0.100 to 0.100, y is any numerical value within the range of 0.001 to 0.500, z is any numerical value within the range of 0.001 to 0.100, the A is one or more elements selected from Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb, and Ge, preferably one or more elements selected from Fe, Ti, V, Ni, Co, and Mg, and the R is one or more elements selected from B, Si, N, and S, preferably one element selected from B, Si, N, and S.

[0144] The manufacturing method of the present application does not particularly limit the supply source of the material, and the supply source of a certain element can include one or more of the simple substance, sulfate, halide, nitrate, organic acid salt, oxide, or hydroxide of the element. the The supply source can achieve the purpose of the manufacturing method of the present application. on the premise that 。

[0145] In some embodiments, the dopant of element A is one or more selected from the simple substances, carbonates, sulfates, chlorides, nitrates, organic acid salts, oxides, and hydroxides of one or more elements selected from Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb, and Ge.

[0146] In some embodiments, the dopant of the element R is one or more inorganic acids, oxyacids, organic acids, sulfates, chlorides, nitrates, organic acid salts, oxides, hydroxides of each of one or more elements selected from B, Si, N, and S.

[0147] In the present application, the manganese source may be a manganese-containing substance used in the production of lithium manganese phosphate known in the art. By way of example, the manganese source may be one or more selected from elemental manganese, manganese dioxide, manganese phosphate, manganese oxalate, and manganese carbonate.

[0148] In the present application, the acid may be one or more selected from hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, silicic acid, organic acids such as tetraethoxy, and organic acids such as oxalic acid. In some embodiments, the acid is a dilute organic acid with a concentration of 60 wt% or less.

[0149] In the present application, the lithium source may be a lithium-containing substance used in the production of lithium manganese phosphate known in the art. By way of example, the lithium source is one or more selected from lithium carbonate, lithium hydroxide, lithium phosphate, and lithium dihydrogen phosphate.

[0150] In the present application, the phosphorus source may be a phosphorus-containing substance used in the production of lithium manganese phosphate known in the art. By way of example, the phosphorus source is one or more selected from diammonium hydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate, and phosphoric acid.

[0151] In some preferred embodiments, after reacting the manganese source, the dopant of the element A, and the acid in a solvent to obtain a manganese salt suspension doped with the element A, the suspension is filtered, dried, and sanded to obtain manganese salt particles doped with the element A having a particle size of 50 - 200 nm.

[0152] In some preferred embodiments, the slurry in step (2) is dried to obtain a powder material, and then the powder material is sintered to obtain a core doped with element A and element R.

[0153] In some embodiments, step (1) is mixed at a temperature of 20 - 120°C, preferably 40 - 120°C, and / or the stirring in step (1) is carried out at 400 - 700 rpm for 1 - 9 hours, preferably 3 - 7 hours.

[0154] Preferably, the reaction temperature in step (1) is carried out at about 30°C, about 50°C, about 60°C, about 70°C, about 80°C, about 90°C, about 100°C, about 110°C or about 120°C, and the stirring in step (1) is carried out for about 2 hours, about 3 hours, about 4 hours, about 5 hours, about 6 hours, about 7 hours, about 8 hours or about 9 hours. Preferably, the reaction temperature and stirring time in step (1) can be within any range of any of the above values.

[0155] In some embodiments, step (2) is mixed at a temperature of 20 - 120°C, preferably 40 - 120°C for 1 - 12 h. Preferably, the reaction temperature in step (2) is carried out at about 30°C, about 50°C, about 60°C, about 70°C, about 80°C, about 90°C, about 100°C, about 110°C or about 120°C, and the mixing in step (2) is carried out for about 2 hours, about 3 hours, about 4 hours, about 5 hours, about 6 hours, about 7 hours, about 8 hours, about 9 hours, about 10 hours, about 11 hours or about 12 hours. Preferably, the reaction temperature and mixing time in step (2) can be within any range of any of the above values.

[0156] When the temperature and time in the manufacturing process of the core particles are within the above ranges, the lattice defects of the obtained core and the cathode active material manufactured thereby are few, which helps to suppress the elution of manganese ions, reduce the interfacial side reaction between the cathode active material and the electrolyte, and thereby improve the cycle performance and safety performance of the secondary battery.

[0157] In some embodiments, preferably, in the process of manufacturing manganese hypophosphate particles doped with element A and element R, the pH of the solution is controlled to be 3.5 - 6, preferably, the pH of the solution is controlled to be 4 - 6, and more preferably, the pH of the solution is controlled to be 4 - 5. It should be noted that in the present application, the pH of the mixture obtained by a method generally used in this field can be adjusted, for example, it can be obtained by adding an acid or a base.

[0158] In some embodiments, preferably, in step (2), the molar ratio of the manganese salt particles, the lithium source, and the phosphorus source is 1:0.5 - 2.1:0.5 - 2.1, and more preferably, the molar ratio of the manganese salt particles doped with element A, the lithium source, and the phosphorus source is about 1:1:1.

[0159] In some embodiments, preferably, the sintering conditions in the process of manufacturing lithium manganese phosphate doped with element A and element R are as follows. Sinter at 600 - 950 °C for 4 - 10 hours in an inert gas or a mixed atmosphere of an inert gas and hydrogen gas. Preferably, the sintering can be carried out at about 650 °C, about 700 °C, about 750 °C, about 800 °C, about 850 °C, or about 900 °C for about 2 hours, about 3 hours, about 4 hours, about 5 hours, about 6 hours, about 7 hours, about 8 hours, about 9 hours, or about 10 hours. Preferably, the sintering temperature and sintering time can be within any range of any of the above numerical values. In the process of manufacturing lithium manganese phosphate doped with element A and element R, if the sintering temperature is too low and the sintering time is too short, the crystallinity of the material core is low, which affects the overall performance. If the sintering temperature is too high, heterogeneous phases are likely to occur in the material core, thereby affecting the overall performance. If the sintering time is too long, the length of the material core particles is large, thereby affecting the theoretical capacity, tap density, rate performance, etc.

[0160] In some preferred embodiments, preferably, the protective atmosphere is a mixed gas of 70 - 90 vol% nitrogen gas and 10 - 30 vol% hydrogen gas.

[0161] In some embodiments, the coating step includes the following.

[0162] A first coating step of dissolving a source of element M, a phosphorus source, an acid, and a selectable lithium source in a solvent to obtain a first coating layer suspension, mixing the core obtained in the core step and the first coating layer suspension obtained in the first coating step sufficiently, drying, and then sintering to obtain a material coated with the first coating layer.

[0163] A second coating step of dissolving a source of element X, a phosphorus source, and an acid in a solvent to obtain a second coating layer suspension, mixing the coating material of the first coating layer obtained in the first coating step and the second coating layer suspension obtained in the second coating step sufficiently, drying, and then sintering to obtain a material coated with two coating layers.

[0164] A third coating step of dissolving a carbon source in a solvent, dissolving it sufficiently to obtain a third coating layer solution, then adding the material coated with two coating layers obtained in the second coating step to the third coating layer solution, mixing uniformly, drying, and then sintering to obtain a material coated with three coating layers, that is, a positive electrode active material.

[0165] In some embodiments, the source of the element M is one or more of the respective simple substances, carbonates, sulfates, chlorides, nitrates, organic acid salts, oxides, hydroxides of one or more elements selected from Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, or Al.

[0166] In some embodiments, the source of the element X is one or more of the respective simple substances, carbonates, sulfates, chlorides, nitrates, organic acid salts, oxides, hydroxides of one or more elements selected from Li, Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, or Al.

[0167] The addition amount of each source of the elements A, R, M, and X depends on the target doping amount, and the ratio of the usage amounts of the lithium source, manganese source, and phosphorus source matches the stoichiometric ratio.

[0168] As an example, the carbon source is one or more selected from starch, sucrose, glucose, polyvinyl alcohol, polyethylene glycol, and citric acid.

[0169] In some embodiments, in the first coating step, the pH of the solution of the source in which element M is dissolved, the phosphorus source, the acid, and the selectable lithium source is controlled to 3.5 - 6.5, then stirred and reacted for 1 - 5 hours, then the temperature of the solution is raised to 50 - 120°C, and this temperature is maintained for 2 - 10 hours, and / or the sintering is carried out at 650 - 800°C for 2 - 6 hours.

[0170] Preferably, in the first coating step, the reaction is carried out sufficiently. Preferably, in the first coating step, the reaction is carried out for about 1.5 hours, about 2 hours, about 3 hours, about 4 hours, about 4.5 hours, or about 5 hours. Preferably, in the first coating step, the reaction time of the reaction can be within any range of any of the above numerical values.

[0171] Preferably, in the first coating step, the pH of the solution is controlled to 4 - 6. Preferably, in the first coating step, the temperature of the solution is raised to about 55°C, about 60°C, about 70°C, about 80°C, about 90°C, about 100°C, about 110°C, or about 120°C, and maintained at this temperature for about 2 hours, about 3 hours, about 4 hours, about 5 hours, about 6 hours, about 7 hours, about 8 hours, about 9 hours, or about 10 hours. Preferably, in the first coating step, the temperature of the temperature rise and the holding time can be within any range of any of the above numerical values.

[0172] Preferably, in the first coating step, the sintering can be carried out at about 650°C, about 700°C, about 750°C, or about 800°C for about 2 hours, about 3 hours, about 4 hours, about 5 hours, or about 6 hours. Preferably, the sintering temperature and sintering time can be within any range of any of the above numerical values.

[0173] In the first coating step, by controlling the sintering temperature and time within the above ranges, the following situations can be avoided. When the sintering temperature in the first coating step is too low and the sintering time is too short, the crystallinity of the first coating layer is low, there are many amorphous substances, which results in a decrease in the effect of suppressing the elution of metal, thereby affecting the cycle performance and high-temperature storage performance of the secondary battery. When the sintering temperature is too high, it leads to the occurrence of heterogeneous phases in the first coating layer, thereby affecting the effect of suppressing the elution of metal, and thereby affecting the cycle and high-temperature storage performance, etc. of the secondary battery. When the sintering time is too long, it increases the thickness of the first coating layer, affects the migration of Li+, and thereby affects the theoretical capacity performance and rate performance, etc. of the material.

[0174] In some embodiments, in the second coating step, after dissolving the source of element X, the phosphorus source and the acid in a solvent, stirring and reacting for 1 - 10 hours, then raising the temperature of the solution to 60 - 150 °C and holding the temperature for 2 - 10 hours, and / or sintering at 500 - 700 °C for 6 - 10 hours.

[0175] Preferably, in the second coating step, the reaction is carried out sufficiently. Preferably, in the second coating step, the reaction is carried out for about 1.5 hours, about 2 hours, about 3 hours, about 4 hours, about 4.5 hours, about 5 hours, about 6 hours, about 7 hours, about 8 hours, about 9 hours or about 10 hours. Preferably, in the second coating step, the reaction time of the reaction can be within any range of any of the above numerical values.

[0176] Preferably, in the second coating step, the temperature of the solution is raised to about 65 °C, about 70 °C, about 80 °C, about 90 °C, about 100 °C, about 110 °C, about 120 °C, about 130 °C, about 140 °C or about 150 °C and held at that temperature for about 2 hours, about 3 hours, about 4 hours, about 5 hours, about 6 hours, about 7 hours, about 8 hours, about 9 hours or about 10 hours. Preferably, in the second coating step, the temperature of the temperature rise and the holding time can be within any range of any of the above numerical values.

[0177] In the steps of providing the core material, the first coating step, and the second coating step, before sintering, that is, in the production of the core material where a chemical reaction occurs (steps (1)-(2)) and in the production of the first coating layer suspension and the second coating layer suspension, by selecting appropriate reaction temperatures and reaction times as described above, the following situations can be avoided. If the reaction temperature is too low, the reaction may not occur or the reaction rate may be slow. If the temperature is too high, the product may decompose or form a heterogeneous phase. If the reaction time is too long, the particle size of the product may be large, which may increase the time and difficulty of subsequent processes. If the reaction time is too short, the reaction may be incomplete and the resulting product may be less.

[0178] Preferably, in the second coating step, the sintering can be carried out at about 550 °C, about 600 °C, or about 700 °C for about 6 hours, about 7 hours, about 8 hours, about 9 hours, or about 10 hours. Preferably, the sintering temperature and sintering time can be within any range of any of the above numerical values.

[0179] In the second coating step, by controlling the sintering temperature and time within the above range, the following situations can be avoided. If the sintering temperature in the second coating step is too low and the sintering time is too short, the crystallinity of the second coating layer is low, there is a lot of amorphous material, and the performance of reducing the surface reaction activity of the material decreases, thereby affecting the cycle and high-temperature storage performance of the secondary battery, etc. If the sintering temperature is too high, a heterogeneous phase occurs in the second coating layer, thereby affecting the effect of reducing the surface reaction activity of the material, and thereby affecting the cycle and high-temperature storage performance of the secondary battery, etc. If the sintering time is too long, the thickness of the second coating layer increases, affecting the voltage plateau of the material, thereby reducing the energy density of the material.

[0180] In some embodiments, the sintering in the third coating step is carried out at 700 - 800 °C for 6 - 10 hours. Preferably, in the third coating step, the sintering can be carried out at about 700 °C, about 750 °C or about 800 °C for about 6 hours, about 7 hours, about 8 hours, about 9 hours or about 10 hours. Preferably, the temperature and time of the sintering can be within any range of any of the above numerical values.

[0181] In the third coating step, by controlling the sintering temperature and time within the above range, the following situations can be avoided. If the sintering temperature in the third coating step is too low, the graphitization degree of the third coating layer decreases, affecting its conductivity, thereby affecting the exertion of the theoretical capacity of the material. If the sintering temperature is too high, the graphitization degree of the third coating layer is too high, affecting the transport of Li + and thereby affecting the exertion of the theoretical capacity of the material. If the sintering time is too short, the coating layer is too thin, affecting its conductivity, thereby affecting the exertion of the theoretical capacity of the material. If the sintering time is too long, the coating layer is too thick, affecting the tap density of the material, etc.

[0182] In the above first coating step, second coating step, and third coating step, the drying is all carried out at a drying temperature of 100 °C to 200 °C, preferably 110 °C to 190 °C, more preferably 120 °C to 180 °C, even more preferably 120 °C to 170 °C, and most preferably 120 °C to 160 °C. The drying time is 3 - 9 hours, preferably 4 - 8 hours, more preferably 5 - 7 hours, and most preferably about 6 hours.

[0183] A positive electrode active material manufactured by the method for manufacturing a positive electrode active material described in the present application, wherein the manufactured secondary battery has a reduced elution amount of Mn and Mn site doping elements after cycling, and improved high-temperature stability, high-temperature cycle performance, and rate performance. In addition, the raw material has a wide supply source, low cost, and a simple process, which is useful for realizing industrialization. In any embodiment, the surface oxygen valence of the positive electrode active material having a core-shell structure is -1.90 or less, preferably -1.90 to -1.98. Thereby, by limiting the surface oxygen valence of the positive electrode active material within the above range as described above, the interfacial side reaction between the positive electrode material and the electrolyte can be further reduced, thereby improving the performance such as cell cycling and high-temperature storage gas generation. In some embodiments, the positive electrode film layer may further include other positive electrode active materials for secondary batteries known in the art. As an example, the positive electrode active material may include at least one of olivine-structured lithium-containing phosphates, lithium transition metal oxides, and modified compounds thereof. However, the present application is not limited to these materials, and conventional materials that can be used as other secondary battery positive electrode active materials can be further used. These positive electrode active materials may be used alone or in combination of two or more. Here, examples of the lithium transition metal oxide include at least one of lithium nickel oxide (for example, LiNiO2), lithium manganese oxide (for example, LiMnO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, and modified compounds thereof, but are not limited thereto. Examples of the olivine-structured lithium-containing phosphate include at least one of lithium iron phosphate (for example, LiFePO4 (which may be abbreviated as LFP)), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate (for example, LiMnPO4), a composite material of lithium manganese phosphate and carbon, lithium manganese iron phosphate, and a composite material of lithium manganese iron phosphate and carbon, but are not limited thereto.

[0184] In some embodiments, the positive electrode film layer can further selectively include an adhesive. For example, the adhesive can include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride - tetrafluoroethylene - propylene terpolymer, vinylidene fluoride - hexafluoropropylene - tetrafluoroethylene terpolymer, tetrafluoroethylene - hexafluoropropylene copolymer, and fluorine - containing acrylate resin.

[0185] In some embodiments, the positive electrode film layer can further selectively include a conductive agent. For example, the conductive agent can include at least one of superconducting carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0186] In some embodiments, the positive electrode sheet can be manufactured in the following manner. Components for manufacturing the positive electrode sheet, such as a positive electrode active material, a conductive agent, an adhesive, and any other components, are dispersed in a solvent (such as N - methylpyrrolidone) to form a positive electrode slurry. The positive electrode slurry is applied to a positive electrode current collector, and after processes such as drying and cold pressing, a positive electrode sheet can be obtained. [Negative electrode sheet]

[0187] The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer disposed on at least one surface of the negative electrode current collector, and the negative electrode film layer includes a negative electrode active material.

[0188] For example, the negative electrode current collector has two surfaces facing each other in its own thickness direction, and the negative electrode film layer is disposed on either one or both of the two opposing surfaces of the negative electrode current collector.

[0189] In some embodiments, the negative electrode current collector can employ a metal foil sheet or a composite current collector. For example, a copper foil can be employed as the metal foil sheet. The composite current collector may have a base layer of a polymer material and a metal layer formed on at least one surface of the base material of the polymer material. The composite current collector can be formed by forming a metal material (such as copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a polymer material base (such as a base of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0190] In some embodiments, the negative electrode active material can employ a negative electrode active material for batteries known in the art. As an example, the negative electrode active material can include at least one of artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate. The silicon-based material can be selected from at least one of elemental silicon, silicon oxide, silicon-carbon composite, silicon-nitrogen composite, and silicon alloy. The tin-based material can be selected from at least one of elemental tin, tin oxygen compound, and tin alloy. However, the present application is not limited to these materials, and other conventional materials that can be used as battery negative electrode active materials can be used. These negative electrode active materials can be used alone or in combination of two or more kinds.

[0191] In some embodiments, the negative electrode film layer can further selectively include an adhesive. As an example, the adhesive is selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0192] In some embodiments, the negative electrode film layer can further selectively include a conductive agent. For example, the conductive agent can be selected from at least one of superconducting carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0193] In some embodiments, the negative electrode film layer can further selectively include other auxiliaries, such as a thickening agent (e.g., sodium carboxymethyl cellulose (CMC-Na)), etc.

[0194] In some embodiments, the negative electrode sheet can be manufactured in the following manner. Components for manufacturing the negative electrode sheet, such as a negative electrode active material, a conductive agent, an adhesive, and any other components, are dispersed in a solvent (e.g., deionized water) to form a negative electrode slurry. The negative electrode slurry is applied to a negative electrode current collector, and after processes such as drying and cold pressing, a negative electrode sheet can be obtained. [Separator]

[0195] In some embodiments, the secondary battery further includes a separator. The present application does not particularly limit the type of the separator, and any known porous structure separator having good chemical stability and mechanical stability can be selected.

[0196] In some embodiments, the material of the separator can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, and is not particularly limited. When the separator is a multi-layer composite film, the materials of each layer may be the same or different, and are not particularly limited.

[0197] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be used to manufacture an electrode assembly by a winding process or a lamination process.

[0198] In some embodiments, the secondary battery can include an exterior package. This exterior package is used to package the above electrode body and electrolyte.

[0199] In some embodiments, the exterior package of the secondary battery may be a rigid case, such as a rigid plastic case, an aluminum case, a steel case, etc. The exterior package of the secondary battery may also be a soft pack, such as a bag-type soft pack. The material of the soft bag may be plastic, and examples of the plastic include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0200] The present application does not particularly limit the shape of the secondary battery, and it may be cylindrical, square, or any other arbitrary shape. For example, FIG. 1 shows a secondary battery 5 having a square structure as an example.

[0201] In some embodiments, as shown in FIG. 2, the exterior package can include a case 51 and a cover plate 53. Here, the case 51 can include a bottom plate and side plates connected to the bottom plate, and the bottom plate and the side plates enclose to form an accommodation cavity. The case 51 has an opening communicating with the accommodation chamber, and the cover plate 53 can cover the opening, thereby sealing the accommodation chamber. The positive electrode sheet, the negative electrode sheet, and the separator can form an electrode assembly 52 by a winding process or a lamination process. The electrode assembly 52 is enclosed in the accommodation chamber. The electrolyte is impregnated in the electrode assembly 52. The number of electrode assemblies 52 included in the secondary battery 5 may be one or more, and those skilled in the art can select according to specific actual needs.

[0202] In some embodiments, the secondary battery can be assembled into a battery module, and the number of secondary batteries included in the battery module may be one or more. Those skilled in the art can select the specific number according to the application and capacity of the battery module.

[0203] Figure 3 shows a battery module 4 as an example. As shown in Figure 3, in the battery module 4, a plurality of secondary batteries 5 may be arranged and installed in sequence along the longitudinal direction of the battery module 4. Naturally, they can be arranged in any other optional manner. Furthermore, the plurality of secondary batteries 5 may be fixed with fasteners.

[0204] Preferably, the battery module 4 can further include a casing having an accommodation space, and the plurality of secondary batteries 5 are accommodated in the accommodation space.

[0205] In some embodiments, the above battery module can be further assembled into a battery pack. The number of battery modules included in the battery pack may be one or more, and a specific number can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0206] Figures 4 and 5 show a battery pack 1 as an example. As shown in Figures 4 and 5, the battery pack 1 can include a battery box and a plurality of battery modules 4 installed in the battery box. The battery box includes an upper housing 2 and a lower housing 3. The upper housing 2 can cover the lower housing 3 and form a sealed space for accommodating the battery module 4. The plurality of battery modules 4 may be arranged in the battery box in any manner.

[0207] In addition, the present application further provides an electrical device, and the electrical device includes at least one of the secondary battery, battery module, or battery pack provided by the present application. The secondary battery, battery module, or battery pack may be used as a power source of the electrical device, or may be used as an energy storage unit of the electrical device. The electrical device can include, but is not limited to, mobile equipment (such as mobile phones, notebook computers, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0208] As the electric device, a secondary battery, a battery module, or a battery pack can be selected according to the usage demand.

[0209] FIG. 6 shows an example of an electric device. The electric device is a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, or the like. In order to meet the demand for high power and high energy density for the secondary battery of the electric device, a battery pack or a battery module can be adopted.

Example

[0210] Hereinafter, examples of the present application will be described. The examples described below are exemplary and are only for explaining the present application, and should not be understood as limiting the present application. If specific techniques or conditions are not shown in the examples, they are carried out according to the techniques or conditions described in the literature in this field or according to the product specifications. The reagents or equipment used are not specified by the manufacturer, and all are conventional products that can be obtained commercially.

[0211] Among them, the first additive is selected from the following compounds.

Chemical formula

[0212] Step 1: Manufacture of the positive electrode active material

[0213] Step S1: Manufacture of manganese oxalate co-doped with Fe, Co, V, and S ​​Put 689.6 g of manganese carbonate, 455.27 g of ferrous carbonate, 4.65 g of cobalt sulfate, and 4.87 g of vanadium dichloride into a mixer and mix them thoroughly for 6 hours. Next, transfer the obtained mixture to a reaction kettle, add 5 L of deionized water and 1260.6 g of oxalic acid dihydrate, heat to 80 °C, stir thoroughly at a rotation speed of 500 rpm for 6 hours, mix uniformly, and stop until the reaction is completed and no bubbles are generated to obtain a manganese oxalate suspension co-doped with Fe, Co, and V. Next, filter the suspension, dry it at 120 °C, and further polish it to obtain manganese oxalate particles with a particle size of 100 nm.

[0214] Step S2: Core Li 0.997 Mn 0.60 Fe 0.393 V 0.004 Co 0.003 P 0.997 S 0.003 Production of O4 (1) Take 1793.1 g of manganese oxalate produced, 368.3 g of lithium carbonate, 1166.6 g of ammonium dihydrogen phosphate, and 4.9 g of dilute sulfuric acid, add them to 20 L of deionized water, stir thoroughly, mix uniformly and react at 80 °C for 10 hours to obtain a slurry. Transfer the slurry to a spray drying device for spray drying granulation, and dry at a temperature of 250 °C to obtain a powder material. Sinter the powder material at 700 °C for 4 hours in a protective atmosphere (90% nitrogen gas and 10% hydrogen gas) using a roller hearth kiln to obtain the above core material.

[0215] Step S3: Production of the first coating layer suspension Prepare a Li2FeP2O7 solution by dissolving 7.4 g of lithium carbonate, 11.6 g of ferrous carbonate, 23.0 g of ammonium dihydrogen phosphate, and 12.6 g of oxalic acid dihydrate in 500 mL of deionized water, control the pH to 5, then stir and react at room temperature for 2 hours to obtain a solution, and then heat the solution to 80 °C and maintain this temperature for 4 hours to obtain the first coating layer suspension.

[0216] Step S4: Coating of the first coating layer The 1571.9 g of the doped lithium manganese phosphate core material obtained in step S2 is added to the first coating layer suspension obtained in step S3 (the content of the coating substance is 15.7 g), and stirred and mixed thoroughly for 6 hours. After uniform mixing, it is transferred to an oven at 120 °C and dried for 6 hours. Then, it is sintered at 650 °C for 6 hours to obtain a material coated with pyrophosphate.

[0217] Step S5: Preparation of the second coating layer suspension 3.7 g of lithium carbonate, 11.6 g of ferrous carbonate, 11.5 g of ammonium dihydrogen phosphate and 12.6 g of oxalic acid dihydrate are dissolved in 1500 mL of deionized water, and then stirred and reacted for 6 hours to obtain a solution. Then, the solution is heated to 120 °C and this temperature is maintained for 6 hours to obtain the second coating layer suspension.

[0218] Step S6: Coating of the second coating layer The 1586.8 g of the material coated with pyrophosphate obtained in step S4 is added to the second coating layer suspension obtained in step S5 (the content of the coating substance is 47.1 g), and stirred and mixed thoroughly for 6 hours. After uniform mixing, it is transferred to an oven at 120 °C and dried for 6 hours. Then, it is sintered at 700 °C for 8 hours to obtain a two-layer coated material.

[0219] Step S7: Preparation of the third coating layer aqueous solution 37.3 g of sucrose is dissolved in 500 g of deionized water, and then stirred and dissolved thoroughly to obtain a sucrose aqueous solution.

[0220] Step S8: Coating of the third coating layer The 1633.9 g of the two-layer coated material obtained in step S6 is added to the sucrose solution obtained in step S7, stirred and mixed for 6 hours. After uniform mixing, it is transferred to an oven at 150 °C and dried for 6 hours. Then, it is sintered at 700 °C for 10 hours to obtain a three-layer coated material.

[0221] Step 2: Manufacture of the positive electrode sheet The manufactured cathode active material after the three-layer coating, conductive agent acetylene black, and adhesive polyvinylidene fluoride (PVDF) are added to N-methylpyrrolidone (NMP) according to a weight ratio of 97.0:1.2:1.8, and uniformly stirred and mixed to obtain a cathode slurry. Next, the cathode slurry is uniformly coated on an aluminum foil according to 0.280 g / 1540.25 mm 2 and a cathode sheet is obtained by drying, cold pressing, and cutting.

[0222] Step 3: Manufacture of the anode sheet The anode active material artificial graphite, hard carbon, conductive agent acetylene black, adhesive styrene-butadiene rubber (SBR), and thickener sodium carboxymethyl cellulose (CMC) are dissolved in deionized water as a solvent according to a weight ratio of 90:5:2:2:1, and uniformly stirred and mixed, and then an anode slurry is manufactured. The anode slurry is uniformly coated on an anode current collector copper foil according to 0.117 g / 1540.25 mm 2 and an anode sheet is obtained by drying, cold pressing, and cutting.

[0223] Step 4: Manufacture of the electrolyte In an argon atmosphere glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), the organic solvents ethylene carbonate (EC) / ethyl methyl carbonate (EMC) are uniformly mixed at a volume ratio of 3 / 7, and Compound 1 (the first additive, the mass content in the electrolyte is 1%), LiN(CF3SO2)(FSO2) (the first lithium salt, the mass content in the electrolyte is 10%), and LiPF6 (the second lithium salt, the mass content in the electrolyte is 1%) are added thereto, and uniformly stirred to obtain an electrolyte.

[0224] Step 5: Manufacture of the separator A commercially available PP-PE copolymer microporous film with a thickness of 20 μm and an average pore diameter of 80 nm (from Zhuogao Technologies Co., Ltd., model number 20) is used.

[0225] Step 6: Manufacture of the full battery Stack the above-mentioned positive electrode sheet, separator, and negative electrode sheet in sequence, position the separator in the middle of the positive and negative electrodes to play a role in isolation, and wind them up to obtain a bare cell. Place the bare cell in a container, inject the above-mentioned electrolyte, and package it to obtain a full battery (hereinafter also referred to as "full cell"). [Fabrication of Button Battery]

[0226] Add the above-prepared positive electrode active material, polyvinylidene fluoride (PVDF), and acetylene black to N-methylpyrrolidone (NMP) at a weight ratio of 90:5:5, and stir in a drying chamber to produce a slurry. Coat the slurry on an aluminum foil, dry it, and cold press it to form a positive electrode sheet. The coating amount is 0.2 g / cm 2 and the pressure density is 2.0 g / cm 3 is achieved.

[0227] Adopt a lithium sheet as the negative electrode, and use a solution of ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC) with a volume ratio of 1:1:1 for Compound 1 (the first additive with a mass content of 1% in the electrolyte), Li(FSO2)2N (the first lithium salt with a mass content of 15% in the electrolyte), and LiPF6 (the second lithium salt with a mass content of 3% in the electrolyte) as the electrolyte, and assemble it with the above-prepared positive electrode sheet in a button battery box to form a button battery (hereinafter also referred to as "button cell"). Examples 2 - 29, Comparative Examples 1 - 3

[0228] Manufacture the positive electrode active materials and batteries in Examples 2-29 and Comparative Examples 1-3 in a manner similar to Example 1. For the differences in the manufacture of the positive electrode active materials, refer to Table 1-6. Here, in Comparative Examples 2 and 3, since the first layer is not coated, Steps S3 and S4 are absent. In Comparative Example 1, since the second layer is not coated, Steps S5-S6 are absent. Note: In all examples and comparative examples of this application, unless otherwise specified, by default, all the first coating layer materials and / or second coating layer materials used are crystalline.

[0229] ​​

Table 1-1

Table 1-2

Table 1-3

Table 1-4

[0230]

Table 2

[0231]

Table 3

[0232]

Table 4

[0233]

Table 5-1

Table 5-2

[0234]

Table 6-1

Table 6-2

[0235] 1. Test method for grid change rate: ​​Under a constant temperature environment of 25°C, place the positive electrode active material sample in an XRD (model number Bruker D8 Discover), test the sample at 1° / min, organize and analyze the test data, refer to the standard PDF card, and calculate the lattice constants a0, b0, c0, and v0 at this time (a0, b0, and c0 indicate the lengths of various aspects of the unit cell, v0 indicates the volume of the unit cell, and can be directly obtained from the XRD finishing result).

[0236] Adopt the button battery manufacturing method in the above example, manufacture the above positive electrode active material sample into a button battery, and charge the button battery at a low rate of 0.05C until the current decreases to 0.01C. Next, take out the positive electrode sheet in the button battery and immerse it in dimethyl carbonate (DMC) for 8 hours. Then dry it, scrape off the powder, and select the particles with a particle size smaller than 500nm among them. Sample and calculate its unit cell volume v1 in the same way as the test of the above fresh sample, and express (v0 - v1) / v0×100% as the lattice change rate (unit cell volume change rate) before and after completely desorbing the lithium in a table.

[0237] 2. Li / Mn Antisite Defect Concentration Compare the XRD results measured by the "Lattice Change Rate Measurement Method" with the PDF (Powder Diffraction File) card of the standard crystal to obtain the Li / Mn antisite defect concentration. Specifically, introduce the XRD results measured by the "Lattice Change Rate Measurement Method" into the general-purpose structure analysis system (GSAS) software, automatically obtain the finishing result, which contains the occupancy status of different atoms, and obtain the Li / Mn antisite defect concentration by reading the finishing result.

[0238] 3. Tap Density Put 5g of the above-produced positive electrode active material powder into a special mold for compaction (CARVER mold from the United States, model number 13mm), and then place the mold in a density compaction device. Apply a pressure of 3T, read the thickness of the powder under pressure (the thickness after releasing the pressure) with the device, and calculate the tap density according to ρ = m / v. The area value used here is the standard small image area of 1540.25mm2 It is.

[0239] 4.3C charging constant current ratio Under a constant temperature environment of 25°C, the fresh full batteries manufactured in the above respective examples and comparative examples are left standing for 5 minutes, and discharged to 2.5V according to 1 / 3C. They are left standing for 5 minutes, charged to 4.3V at 1 / 3C, and then charged at a constant voltage until the current becomes 0.05 mA or less at 4.3V. They are left standing for 5 minutes, and the charging capacity at this time is recorded as C0. They are discharged to 2.5V according to 1 / 3C, left standing for 5 minutes, further charged to 4.3V according to 3C, left standing for 5 minutes, and the charging capacity at this time is recorded as C1. The 3C charging constant current ratio is namely C1 / C0×100%.

[0240] The higher the 3C charging constant current ratio, the higher the rate performance of the secondary battery is indicated.

[0241] 5. Transition metal Mn (and Fe doped with Mn sites) elution test After the capacity of the full batteries manufactured in the above respective examples and comparative examples decayed to 80% by cycles at 45°C, they are discharged to a cut-off voltage of 2.0V at a 0.1C rate. Next, the battery is disassembled, the negative electrode sheet is taken out, 30 wafers with a unit area of 1540.25 mm 2 ) are randomly taken from the negative electrode sheet, and inductively coupled plasma optical emission spectrometry (ICP) is measured with an Agilent ICP-OES730. Based on the ICP results, the amounts of Fe (if Fe is doped in the Mn sites of the positive electrode active material) and Mn are calculated, and thereby the elution amount of Mn (and Fe doped in the Mn sites) after cycling is calculated. The test standard conforms to EPA-6010D-2014.

[0242] 6. Surface oxygen valence Take the above-prepared 5 g of the positive electrode active material sample and fabricate a button cell according to the button cell fabrication method described in the above examples. Charge the button cell at a low rate of 0.05 C until the current decreases to 0.01 C. Then, take out the positive electrode sheet in the button cell and immerse it in DMC for 8 hours. Next, dry it, scrape off the powder, and select particles with a particle size smaller than 500 nm. Measure the obtained particles using electron energy loss spectroscopy (EELS, the model number of the equipment used is Talos F200S) to obtain the fine structure of the energy loss absorption edge (ELNES), which reflects the density of states and the energy level distribution of elements. Based on the density of states and the energy level distribution, calculate the number of occupied electrons by integrating the valence band density of states data, thereby estimating the valence of surface oxygen after charging.

[0243] 7. Measurement of Manganese and Phosphorus Elements in the Positive Electrode Active Material Dissolve 5 g of the above-prepared positive electrode active material in 100 mL of reverse aqua regia (concentrated hydrochloric acid: concentrated nitric acid = 1:3) (concentrated hydrochloric acid concentration ~ 37%, concentrated nitric acid concentration ~ 65%), measure the content of each element in the solution by ICP, and then measure and convert the content of manganese or phosphorus element (amount of manganese or phosphorus element / amount of positive electrode active material * 100%) to obtain its weight ratio.

[0244] 8. Measurement Method for the Initial Theoretical Capacity of the Button Battery Charge the button batteries fabricated in the above examples and comparative examples at 0.1 C to 4.3 V at 2.5 - 4.3 V, then perform constant voltage charging at 4.3 V until the current becomes 0.05 mA or less, let it stand for 5 minutes, and then discharge it at 0.1 C to 2.0 V. The discharge capacity at this time is the initial theoretical capacity, denoted as D0.

[0245] 10. Store the full cell at 60 °C for 30 days to conduct a battery swelling test: Store all the full batteries manufactured in the above-mentioned examples and comparative examples at 60 °C in a fully charged state (SOC). Monitor the SOC by measuring the open circuit voltage (OCV) and the alternating current internal resistance (IMP) of the cell before, during, and after storage, and measure the volume of the cell. Here, after storing for 48 hours, take out all the full batteries, let them stand for 1 hour, then measure the open circuit voltage (OCV) and the internal resistance (IMP), and after cooling to room temperature, measure the volume of the battery cell by the drainage method. The drainage method first measures the gravity F1 of the cell alone with a balance that automatically converts the dial data to the unit, and then places the cell completely in deionized water (the density of which is known to be 1 g / cm 3 ), and measure the gravity F2 of the cell at this time. The buoyancy F 浮き received by the cell is namely F1 - F2. Then, based on Archimedes' principle

Number

Number

[0246] Judging from the OCV and IMP test results, until the end of storage in this experimental process, the batteries of all examples always maintain an SOC of 99% or more.

[0247] After storing for 30 days, measure the cell volume, and calculate the percentage increase in the cell volume after storage relative to the cell volume before storage.

[0248] 11. Cycle performance test of the full battery at 45 °C In a constant temperature environment of 45 °C, charge at 2.5 - 4.3 V up to 4.3 V according to 1C, then charge at a constant voltage of 4.3 V until the current ≤ 0.05 mA, let it stand for 5 minutes, and then discharge to 2.5 V according to 1C. Denote the capacity as D n (n = 0, 1, 2,...). Repeat the above process until the capacity fade reaches 80%, and record the number of repetitions at this time, which is the number of cycles corresponding to the 80% capacity retention rate at 45 °C.

[0249] 12. Crystal Plane Spacing and Included Angle Test Take 1 g of each of the above-prepared cathode active material powders into a 50 mL test tube, inject 10 mL of alcohol with a mass fraction of 75% into the test tube, then stir well and disperse for 30 minutes. Next, take an appropriate amount of the above solution with a clean disposable plastic straw and drop it onto a 300-mesh copper mesh. At this time, some powders remain on the copper mesh. Transfer the sample on the copper mesh to the TEM (Talos F200s G2) sample cavity for testing to obtain the original TEM test image and save the original image format (xx.dm 3).

[0250] Open the original image obtained from the above TEM test in Digital Micrograph software and perform Fourier transform (automatically completed by the software after clicking the operation) to obtain a diffraction pattern. By measuring the distance from the diffraction spot to the center position in the diffraction pattern, the crystal plane spacing can be obtained, and the included angle can be calculated based on the Bragg equation.

[0251] By comparing the obtained crystal plane spacing and the corresponding included angle data with their standard values, different substances in the coating layer can be identified.

[0252] 13. <Coating Layer Thickness Test> The coating layer thickness test mainly cuts a thin slice with a thickness of about 100 nm from the middle of a single particle of the above-prepared cathode active material by FIB, then performs a TEM test on the thin slice to obtain the original TEM test image and save the original image format (xx.dm 3).

[0253] Open the original image obtained from the above TEM test in Digital Micrograph software, identify the coating layer based on the lattice spacing and included angle information, and measure the thickness of the coating layer.

[0254] Measure the thickness at three positions for the selected particles and take the average value.

[0255] 14. Measurement of the molar ratio of the SP2 and SP3 forms in the third-layer coating carbon This test is performed by Raman spectrum. By performing peak splitting on the energy spectrum of the Raman test, Id / Ig is obtained, where Id is the peak intensity of the SP3 form carbon and Ig is the peak intensity of the SP2 form carbon, thereby confirming the molar ratio of the two.

[0256] Refer to the following table for the performance test results of all examples and comparative examples.

[0257]

Table 7-1

Table 7-2

[0258] As can be seen from Table 7, compared with the comparative examples, the examples achieve a smaller lattice change rate, a smaller Li / Mn antisite defect concentration, a larger tap density, a surface oxygen valence number closer to -2, less elution of Mn and Fe after cycling, and better battery performance, such as better high-temperature storage performance and high-temperature cycle performance.

[0259]

Table 8-1

Table 8-2

Table 8-3

[0260] As can be seen from Table 8, by doping and triple coating the manganese site and phosphorus site of lithium iron manganese phosphate (manganese content is 35% and phosphorus content is about 20%), the content of manganese element and the weight content ratio of manganese element to phosphorus element in the positive electrode active material are significantly reduced. Also, as can be seen by referring to Table 7 when comparing Examples 1 - 14 with Comparative Example 1, Comparative Example 2, and Comparative Example 3, the reduction of manganese element and phosphorus element in the positive electrode active material results in a reduction in the elution amount of manganese iron and the battery performance of the manufactured secondary battery is improved.

[0261] Examples 30 - 65 Manufacture the positive electrode active material, button cell and full cell in the same manner as in Example 6, but change the composition of the electrolyte, specifically as shown in Table 9 below. And, measure the performance data according to the above performance test method for the button cells or full cells of Examples 30 - 65, as shown in Table 10.

[0262]

Table 9 - 1

Table 9 - 2

Table 9 - 3

[0263]

Table 10 - 1

Table 10 - 2

[0264] The electrolytes in all the full batteries of the above Examples 1-29 are replaced with an electrolyte obtained by uniformly mixing the organic solvents ethylene carbonate (EC) / ethyl methyl carbonate (EMC) at a volume ratio of 3 / 7, adding 12.5% by weight of LiPF6 (calculated based on the weight of the ethylene carbonate / ethyl methyl carbonate solvent), dissolving it in the above organic solvents, and uniformly stirring. The electrolyte of the button battery is replaced with a solution of 1 mol / L LiPF6 in ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC) at a volume ratio of 1:1:1. The amounts of Mn and Fe eluted (ppm) after cycling of the formed button batteries or full batteries of Comparative Examples 4-32, the 0.1C button battery capacity (mAh / g), the 3C charging constant current ratio (%), the number of cycles with a 45°C capacity retention rate of 80%, and the 60°C storage cell expansion rate (%) are detected according to the above method, and the detection results are recorded in Table 11.

[0265]

Table 11

[0266] As can be seen from the comparison between Table 7 and Table 11, the electrolyte composition of the present application can further improve the energy density and cycle performance of the secondary battery.

[0267] Note that the present application is not limited to the above embodiments. The above embodiments are merely illustrative, and any embodiments having a configuration that is substantially the same as the technical idea within the scope of the technical solution of the present application and exhibiting the same effects are included in the technical scope of the present application. Also, within the scope not departing from the gist of the present application, various modifications conceivable by those skilled in the art to the embodiments, and other forms constructed by combining some components in the embodiments are also included in the scope of the present application.

Explanation of Reference Numerals

[0268] 1: Battery pack, 2: Upper housing, 3: Lower housing, 4: Battery module, 5: Secondary battery, 51: Case, 52: Electrode assembly, 53: Cover Plate 。

Claims

1. A secondary battery including a positive electrode sheet and a non-aqueous electrolyte, wherein the positive electrode sheet includes a positive electrode active material having a core-shell structure, and the positive electrode active material includes a core and a shell covering the core, the chemical formula of the core is Li 1+x Mn 1-y A y P 1-z R z O 4 where x is any numerical value within the range of -0.100 to 0.100, y is any numerical value within the range of 0.001 to 0.500, z is any numerical value within the range of 0.001 to 0.100, and A is one or more elements selected from the group consisting of Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb, and Ge, and R is one or more elements selected from the group consisting of B, Si, N, and S, and the values of x, y, and z satisfy the condition of maintaining electrical neutrality throughout the core, the shell includes a first coating layer covering the core, a second coating layer covering the first coating layer, and a third coating layer covering the second coating layer, where the first coating layer includes crystalline pyrophosphate Li a MP 2 O 7 and / or M b (P 2 O 7 ) c where 0≦a≦2, 1≦b≦4, 1≦c≦6, and the values of a, b, and c satisfy the condition of maintaining electrical neutrality of the crystalline pyrophosphate Li a MP 2 O 7 or M b (P 2 O 7 ) c and the crystalline pyrophosphate Li a MP 2 O 7 and M b (P 2 O 7 ) cM in each case is one or more elements independently selected from the group consisting of Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, and Al, the second coating layer contains crystalline phosphate XPO 4 where X is one or more elements selected from the group consisting of Li, Fe, Ni, Mg, Co, Cu, Zn, Ti, Ag, Zr, Nb, and Al, the third coating layer is carbon, the non-aqueous electrolyte contains a first lithium salt and a first additive, and the first lithium salt contains one or more selected from LiN(C m F 2m+1 SO 2 )(C n F 2n+1 SO 2 ), Li(FSO 2 ), 2 N, m and n represent positive integers, the first additive contains one or more of the compounds represented by Formula 1, [Chemical Formula 1] R 1 is either a C2-C10 alkylene group, a C2-C10 heteroalkylene group, a C6-C18 arylene group, a C2-C18 heteroarylene group, a C3-C18 divalent alicyclic group, or a C3-C18 hetero divalent alicyclic group, which may or may not be substituted by one or more R a ; R a is a halogen atom, -CN, -NCO, -OH, -COOH, -SOOH, -COOR b , one or more selected from a C2-C10 alkyl group, a C2-C10 alkenyl group, a C2-C10 alkynyl group, and a C2-C10 oxaalkyl group, and R b is any one selected from a C1-C10 alkyl group, the surface oxygen valence of the cathode active material having the core-shell structure is -1.90 or less Secondary battery.

2. R 1 is one or more R a A C2-C10 alkylene group, a C2-C10 oxyalkylene group, a C2-C10 azaalkylene group, a phenylene group, a naphthylene group, an anthrylene group, a cyclobutylene group, a cyclopentylene group, a cyclohexylene group, a biphenylene group, or a methylenediphenylene group, which may or may not be substituted, and / or, said R a includes one or more selected from a halogen atom, -CN, and a C2-C3 alkyl group, and / or, the number of said R in the compound represented by Formula 1 a is 0, 1, 2, 3, or 4 The secondary battery according to claim 1.

3. The first additive includes at least one of the following compounds The secondary battery according to claim 1. 【Chemical Formula 2-1】 【Chemical Formula 2-2】

4. m and n each independently represent 0, 1, 2, or 3, The first additive is any one selected from the following compounds The secondary battery according to claim 1. 【Chemical Formula 3】

5. Based on the total weight of the non-aqueous electrolyte, the content of the first lithium salt is W1 wt%, W1 is 0.1 to 48, and / or the content of the first additive is W2 wt%, W2 is 0.01 to 20 The secondary battery according to claim 1.

6. The non-aqueous electrolyte further includes a second lithium salt, and the second lithium salt is lithium difluorophosphate, lithium difluorodioxalate phosphate, lithium difluorooxalate borate, bisoxalate borate lithium, LiPF 6 LiBF 4 LiAsF 6comprising one or more selected from the group consisting of The secondary battery according to claim 5.

7. Based on the total weight of the non-aqueous electrolyte, the content of the second lithium salt is W3% by weight, and W3 is 0.01 to 20 The secondary battery according to claim 6.

8. (W2 + W3) / W1 is defined as N, and N is 0.01 to 2 The secondary battery according to claim 7.

9. The non-aqueous electrolyte further contains a second additive, and the second additive contains one or more selected from the group consisting of a cyclic carbonate compound containing an unsaturated bond, a halogen-substituted saturated cyclic carbonate compound, a sulfuric acid ester compound, a sulfurous acid ester compound, a sultone compound, a disulfonic acid compound, a nitrile compound, an aromatic compound, a phosphazene compound, a cyclic acid anhydride compound, a phosphorous acid ester compound, a phosphoric acid ester compound, and a boric acid ester compound The secondary battery according to claim 1.

10. The non-aqueous electrolyte further contains an organic solvent The organic solvent contains one or more selected from the group consisting of a cyclic carbonate compound, a chain carbonate compound, and a carboxylic acid ester compound The secondary battery according to claim 1.

11. Calculated based on the weight of the core, the coating amount of the first coating layer is C1% by weight, C1 is greater than 0 and 6 or less, and / or Calculated based on the weight of the core, the coating amount of the second coating layer is C2% by weight, C2 is greater than 0 and 6 or less, and / or Calculated based on the weight of the core, the coating amount of the third coating layer is C3% by weight, C3 is greater than 0 and 6 or less The secondary battery according to claim 1.

12. When calculated based on the weight of the core, the coating amount of the first coating layer is C1% by weight, where C1 is greater than 0 and not more than 6, and / or, When calculated based on the weight of the core, the coating amount of the second coating layer is C2% by weight, where C2 is greater than 0 and not more than 6, and / or, When calculated based on the weight of the core, the coating amount of the third coating layer is C3% by weight, where C3 is greater than 0 and not more than 6, Define the ratio of (W1 + W2) / (C1 + C2 + C3) as Q, where Q is 0.1 to 10 The secondary battery according to claim 5.

13. In the first coating layer, the interplanar spacing range of the crystalline pyrophosphate is 0.293 to 0.470 nm, and the included angle range in the crystal direction (111) is 18.00° to 32.00°, In the second coating layer, the interplanar spacing range of the crystalline phosphate is 0.244 to 0.425 nm, and the included angle range in the crystal direction (111) is 20.00° to 37.00° The secondary battery according to claim 1.

14. In the core, the ratio of y to 1 - y is 1:10 to 1:1 The secondary battery according to claim 1.

15. In the core, the ratio of z to 1 - z is 1:9 to 1:999 The secondary battery according to claim 1.

16. The carbon in the third coating layer is a mixture of SP2 - form carbon and SP3 - form carbon The secondary battery according to claim 1.

17. The thickness of the first coating layer is 1 to 10 nm, and / or, The thickness of the second coating layer is 2 to 15 nm, and / or, The thickness of the third coating layer is 2 to 25 nm The secondary battery according to claim 1.

18. Calculated based on the weight of the positive electrode active material, the content of manganese element is in the range of 10% to 35% by weight, and the content of phosphorus element is in the range of 12% to 25% by weight. The secondary battery according to claim 1.

19. For the positive electrode active material having the core-shell structure, the lattice change rate before and after complete desorption of lithium is 4% or less. The secondary battery according to claim 1.

20. The Li / Mn antisite defect concentration of the positive electrode active material having the core-shell structure is 4% or less. The secondary battery according to claim 1.

21. For the positive electrode active material having the core-shell structure, the pressure density at 3T (ton) with respect to an area of 1540.25 mm2 is 2.2 g / cm 3 or more. The secondary battery according to claim 1.

22. A battery module including the secondary battery according to any one of claims 1 to 21.

23. A battery pack including the battery module according to claim 22.

24. Including at least one selected from the secondary batteries according to any one of claims 1 to 21. An electrical device.

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