Positive electrode active material, electrochemical device, and electronic device

Abstract

The application provides a positive electrode active material, an electrochemical device and an electronic device. The positive electrode active material comprises a substrate and a coating on at least part of the surface of the substrate, wherein the coating comprises a first compound having a P42 / mnm crystal structure. The positive electrode active material can have excellent cycle stability at a high voltage.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of batteries, in particular to a positive electrode active material, an electrochemical device comprising the same and an electronic device. BACKGROUND

[0002] With the commercialization of 5G technology, there is a growing demand for high-capacity lithium ion batteries (LIBs). In the portable electronics market, lithium cobaltate still dominates the positive electrode material of LIBs due to its high volumetric energy density.

[0003] The commonly commercialized lithium cobaltate belongs to the R-3m space group structure, and its theoretical gravimetric capacity is 274 mAh / g. However, at present, when charged to 4.45 V (vs. Li / Li + ), the gravimetric capacity of lithium cobaltate is only 175 mAh / g, which is much lower than the theoretical value. In order to further improve the gravimetric capacity of lithium cobaltate, it is usually necessary to increase the charging voltage to extract more lithium ions. For example, the charging cutoff voltage of lithium cobaltate is increased to 4.6 V (vs. Li / Li + ), and its gravimetric capacity can be increased to 220 mAh / g, but inevitably brings about structural and surface instability problems, and the cycle performance deteriorates. In addition, another lithium cobaltate belonging to the P63mc space group has a higher reversible capacity than the traditional R-3m structure lithium cobaltate at high voltage, and is expected to achieve higher battery energy density. However, the P63mc structure lithium cobaltate has serious interface side reactions during the cycle process, which leads to continuous dissolution of transition metal elements (such as cobalt), and further causes rapid drop in cycle. Therefore, how to improve the cycle stability of lithium cobaltate at high charging voltage is still a major challenge for high-performance lithium cobaltate. SUMMARY

[0004] Therefore, it is necessary to provide a positive electrode active material with excellent cycle stability at high voltage.

[0005] In a first aspect, the present application provides a positive electrode active material, comprising: a substrate, and a coating on at least part of the surface of the substrate; the coating comprises a first compound having a P42 / mnm crystal structure.

[0006] In some embodiments, in the XRD diffraction spectrum of the positive electrode active material, the peak intensity of the strongest diffraction peak in the range of 18°-19° of 2θ diffraction angle is I1, the peak intensity of the strongest diffraction peak in the range of 26°-27° of 2θ diffraction angle is I2, and 0<I2 / I1≤10% is satisfied.

[0007] In some embodiments, the mass fraction of the coating based on the mass of the positive electrode active material is w, and 0

[0008] In some embodiments, the first compound comprises an Fe element, an O element, an F element, and optionally an M element, M being a transition metal element different from Fe, the molar amount of the M element being n M , the sum of the molar amounts of the M element and the Fe element being n M+Fe , satisfying 0 M / n M+Fe < 1.

[0009] In some embodiments, the first compound comprises an M a Fe 1-a O c F d , satisfying 0

[0010] In some embodiments, the M element comprises at least one of Ni, Mn, Co, Ti, Y, or La.

[0011] In some embodiments, the matrix is of R-3m crystal structure, the matrix comprising a Co element and optionally a T element, the T element comprising at least one of Ni, Mn, Ti, Zn, Y, La, Zr, Mg, Fe, Cu, Nb, Cr, or Al, in the matrix, the molar amount of the T element being n T , the sum of the molar amounts of the Co element and the T element being n Co+T , the ratio y1 of the n T to the n Co+T satisfying 0

[0012] In some embodiments, the matrix is of P63mc crystal structure, the matrix comprising a Co element and optionally a Q element, the Q element comprising at least one of Ni, Mn, Ti, Zn, Y, La, Zr, Mg, Fe, Cu, Nb, Cr, or Al, in the matrix, the molar amount of the Q element being n Q , the sum of the molar amounts of the Co element and the Q element being n Co+Q , the ratio y2 of the n Q to the n Co+Q satisfying 0

[0013] In some embodiments, the matrix is of P63mc crystal structure, the matrix further comprising a Na element, in the matrix, the molar amount of the Na element being n Na , the ratio z2 of the n Na to the n Co+Q satisfying 0

[0014] The second aspect of the present application further provides an electrochemical device comprising a positive electrode sheet, wherein the positive electrode sheet comprises the positive electrode active material according to any one of the preceding aspects.

[0015] The third aspect of the present application further provides an electronic device comprising the electrochemical device described above.

[0016] The present application coats the first compound having a P42 / mnm crystal structure on the surface of the substrate. Firstly, the first compound has good electrical conductivity and does not increase the impedance of the positive electrode active material. In addition, the coating on the surface of the positive electrode active material can effectively prevent the direct contact between the substrate and the electrolyte during the cycle process, inhibit the occurrence of the interface side reaction between the substrate and the electrolyte, and further realize the excellent cycle stability of the positive electrode active material at high voltage. BRIEF DESCRIPTION OF DRAWINGS

[0017] In order to better describe and illustrate the embodiments of the present application, reference can be made to the following drawings. Obviously, the following drawings should not be considered as limiting the scope of the present application.

[0018] Figure 1 Process flow chart for the preparation method of the positive electrode active material of an embodiment;

[0019] Figure 2 XRD spectrum of the lithium cobalt oxide prepared in Comparative Example 1;

[0020] Figure 3 XRD spectrum of the lithium cobalt oxide prepared in Comparative Example 2;

[0021] Figure 4 XRD spectrum of the FeOF material prepared in Example 1;

[0022] Figure 5 XRD spectrum of the positive electrode active material prepared in Example 1;

[0023] Figure 6 XRD spectrum of the positive electrode active material prepared in Example 15. DETAILED DESCRIPTION

[0024] In order to facilitate the understanding of the present application, the present application will be described more fully below with reference to specific embodiments. In the specific embodiments, preferred embodiments of the present application are given. However, the present application can be realized in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the disclosure of the present application more thorough and comprehensive.

[0025] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the description herein is for describing particular embodiments only and is not intended to be limiting of the application.

[0026] The conventional lithium cobalt oxide with R-3m space group structure will not only bring about structural and surface instability problems, cycle performance deterioration, but also cause harmful phase transition from O3 hexagonal phase to H1-3 mixed phase when the charging voltage exceeds 4.55 V (relative to Li / Li + ). This structural transition is accompanied by significant changes in lattice constants (a-axis expansion and c-axis contraction), resulting in internal stress and cracks of lithium cobalt oxide particles, further accelerating capacity decay. While the lithium cobalt oxide with P63mc space group cannot form an effective inert protective layer to isolate the positive electrode material from direct contact with the electrolyte due to the lack of spinel phase transition on the surface during the cycle process, the interface side reaction is very serious, leading to continuous dissolution of transition metal elements, and further causing rapid cycle diving.

[0027] To solve the above problems, the first aspect of the present application provides an embodiment of a positive electrode active material, comprising: a substrate and a coating on at least part of the surface of the substrate. The coating comprises a first compound having a P42 / mnm crystal structure.

[0028] By coating the first compound having a P42 / mnm crystal structure on the surface of the substrate, firstly, the first compound has good electrical conductivity and will not cause an increase in the impedance of the positive electrode active material; in addition, the coating on the surface of the positive electrode active material can effectively prevent direct contact between the substrate and the electrolyte during the cycle process, inhibit the occurrence of interface side reactions between the substrate and the electrolyte, and further realize excellent cycle stability of the positive electrode active material at high voltage.

[0029] In some embodiments, the first compound comprises Fe elements, O elements, F elements and optional M elements, M being a transition metal element different from Fe.

[0030] In some embodiments, in the first compound, the molar amount of M elements is n M , the sum of the molar amounts of M elements and Fe elements is n M+Fe , and satisfies 0≤n M / n M+Fe <1.

[0031] In some embodiments, the first compound comprises M a Fe 1-a O c F d , 0≤a<1, 0.95≤c≤1.05, 0.95≤d≤1.05.

[0032] In some embodiments, the matrix is ​​an R-3m crystal structure.

[0033] In some embodiments, the matrix includes Co and optionally T, wherein the T includes at least one of Ni, Mn, Ti, Zn, Y, La, Zr, Mg, Fe, Cu, Nb, Cr, or Al.

[0034] In some embodiments, the molar amount of element T in the matrix is ​​n. T The sum of the molar amounts of Co and T is n. Co+T The n T With the n Co+T The ratio y1 satisfies 0 ≤ y1 ≤ 0.5.

[0035] In some embodiments, the matrix further comprises Li element, wherein the molar amount of Li element in the matrix is ​​n. Li The n Li With the n Co+T The ratio x1 satisfies 0.6≤x1≤1.2.

[0036] In some embodiments, the matrix includes Li x1 Co 1-y1 T y1 O 2±b1 Wherein, 0.6≤x1≤1.2, 0≤y1≤0.5, 0≤b1≤0.2, and T includes at least one of Ni, Mn, Ti, Zn, Y, La, Zr, Mg, Fe, Cu, Nb, Cr, or Al.

[0037] In other embodiments, the substrate is a P63mc crystal structure.

[0038] In other embodiments, the matrix includes Co and optionally Q, wherein the Q element includes at least one of Ni, Mn, Ti, Zn, Y, La, Zr, Mg, Fe, Cu, Nb, Cr, or Al.

[0039] In other embodiments, the molar amount of Q element in the matrix is ​​n. Q The sum of the molar amounts of Co and Q is n. Co+Q The n Q With the n Co+Q The ratio y2 satisfies 0 ≤ y2 ≤ 0.15.

[0040] In other embodiments, the matrix further comprises Li element, wherein the molar amount of Li element in the matrix is ​​m. Li The m Li With the nCo+T The ratio x2 satisfies 0.6 ≤ x2 ≤ 0.95.

[0041] In some other embodiments, the matrix further contains Na element, and the molar amount of Na element in the matrix is n Na , and the n Na and the ratio z2 of the n Co+Q satisfies 0 < z2 ≤ 0.03.

[0042] In some other embodiments, the matrix includes Li x2 Na z2 Co 1-y2 Q y2 O 2±b2 , where 0.6 ≤ x2 ≤ 0.95, 0 ≤ y2 ≤ 0.15, 0 < z2 ≤ 0.03, 0 ≤ b2 ≤ 0.2, and Q includes at least one of Ni, Mn, Ti, Zn, Y, La, Zr, Mg, Fe, Cu, Nb, Cr or Al.

[0043] Specifically, in some embodiments, based on the mass of the positive electrode active material, the mass fraction of the coating is w, satisfying 0 < w ≤ 3%. For example, based on the mass of the positive electrode active material, the mass fraction of the coating is 0.5%, 1%, 1.5%, 2%, 2.5%, 3% or the range composed of any two of these values.

[0044] Specifically, in some embodiments, in the XRD diffraction spectrum of the positive electrode active material, the peak intensity of the strongest diffraction peak within the range of 2θ diffraction angle from 18° to 19° (i.e., the strongest diffraction peak of the matrix) is I1, and the peak intensity of the strongest diffraction peak within the range of 2θ diffraction angle from 26° to 27° (i.e., the strongest diffraction peak of the coating) is I2, satisfying 0 < I2 / I1 ≤ 10%. Further, 0 < I2 / I1 ≤ 6%, and even further, 3% ≤ I2 / I1 ≤ 5.5%. When I2 / I1 is within the above range, the protection effect of the coating on the surface of the matrix can be improved, and further, the cycle stability of the positive electrode active material at high voltage can be improved.

[0045] In some embodiments, M includes at least one of Ni, Mn, Co, Ti, Y and La. Experiments prove that compared with the coated FeOF material, after making a battery with the positive electrode active material coated with M a Fe 1-a OF material (a > 0), the cycle stability of the battery is further improved, and the specific capacity is not significantly reduced. This may be because the conductivity of the coating after compounding with other metal elements is higher, and the binding force with the surface of the matrix is stronger, further improving the interface stability. Further, M includes Co. Experiments prove that the coating effect after Co is compounded with FeOF material is better, with both excellent cycle stability and relatively high specific capacity.

[0046] Further, 0 < a ≤ 0.2. For example, a is 0.03, 0.05, 0.08, 0.1, 0.15, 0.15, 0.18, 0.2, or a range consisting of any two of these values. Compared with a > 0.2, when a ≤ 0.2, the cycle stability of the battery is better.

[0047] In the prior art, the poor cycle stability of lithium cobalt oxide in the R-3m space group at a high charging voltage is mainly improved by element doping and surface modification. Element doping includes single element doping (such as Mg, Al, Ti, Mn, Ca, and B) and multiple element doping (such as Ti-Mg-Al co-doping and Ni-Mn co-doping), which can inhibit the unintended phase transition of lithium cobalt oxide at a high voltage. Surface modification includes Al2O3, ZnO2, AlF3, and AlPO4 surface modification, which can inhibit surface side reactions, thereby improving the cycle performance of lithium cobalt oxide.

[0048] However, conventional element doping and surface modification can only improve the material structure stability to a certain extent, and often involve high-temperature treatment processes of 600°C and above. High-temperature heat treatment can cause unnecessary side reactions, generate harmful impurities, and the process is complex and costly, which limits their industrial applications. In addition, lithium cobalt oxide in the P63mc space group belongs to a low-temperature metastable phase, and its synthesis temperature is generally not more than 300°C. In the above high-temperature treatment process, the material is prone to R-3m phase transition, which cannot meet the surface coating requirements of lithium cobalt oxide in the P63mc space group. Therefore, there is an urgent need to develop a simple low-temperature coating treatment method to improve the high-voltage cycle stability of lithium cobalt oxide.

[0049] Based on this, the second aspect of the present application provides a preparation method of the positive electrode active material of the embodiment, please refer to Figure 1 , which comprises the following steps:

[0050] Step S110: mixing Fe powder, optional metal powder of M, and fluorosilicic acid solution, stirring at 40°C to 50°C for 12h to 24h, to prepare M a Fe 1-a SiF6 solution.

[0051] Wherein, M is a transition metal element different from Fe, and 0 ≤ a < 1. In some embodiments, M includes at least one of Ni, Mn, Co, Ti, Y, or La. Further, M includes Co.

[0052] Specifically, the number of moles of the metal powder of M is n M , the total number of moles of Fe powder and the metal powder of M is n M+Fe , and 0 ≤ n M / nM+Fe ≤0.2. For example, n M / n M+Fe is 0.03, 0.05, 0.08, 0.1, 0.15, 0.15, 0.18 or 0.2.

[0053] Step S120: mixing M a Fe 1-a SiF6 solution with lithium cobalt oxide, removing solvent, and heat treating at 220-300°C for 6-10h to prepare a positive electrode active material.

[0054] Specifically, M a Fe 1-a SiF6 solution is mixed with lithium cobalt oxide, and then solvent is removed, and M a Fe 1-a SiF6 is precipitated from the solution and uniformly adsorbed on the surface of the lithium cobalt oxide. Further, M a Fe 1-a SiF6 solution is an aqueous solution. In one embodiment, the mixture of M a Fe 1-a SiF6 solution and lithium cobalt oxide is dried at 110°C to remove solvent.

[0055] Specifically, M a Fe 1-a SiF6 is slowly decomposed during heat treatment to form M a Fe 1-a OF and uniformly coated on the surface of the lithium cobalt oxide, to obtain a lithium cobalt oxide material coated with M a Fe 1-a OF on the surface. Optionally, heat treatment is performed at 250°C for 8h.

[0056] Specifically, M a Fe 1-a SiF6 is slowly decomposed during heat treatment, and the product is oxygen-free M a Fe 1-a F2 and uniformly coated on the surface of the lithium cobalt oxide, to obtain a lithium cobalt oxide material coated with M a Fe 1-a F2 on the surface.

[0057] Specifically, M a Fe 1-a SiF6 in the solution is ≤0.07. a Fe 1-a SiF6 and lithium cobalt oxide.

[0058] In one embodiment, the lithium cobalt oxide has an R-3m crystal structure and a general formula of Li x1 Co 1-y1 T y1 O 2±b1 wherein 0.6≤x1≤1.2, 0≤y1≤0.5, 0≤b1≤0.2, and T includes at least one of Ni, Mn, Ti, Zn, Y, La, Zr, Mg, Fe, Cu, Nb, Cr, or Al.

[0059] In another embodiment, the lithium cobalt oxide has a P63mc crystal structure and a general formula of Li x2 Na z2 Co 1-y2 Q y2 O 2±b2 wherein 0.6≤x2≤0.95, 0≤y2≤0.15, 0<z2≤0.03, 0≤b2≤0.2, and Q includes at least one of Ni, Mn, Ti, Zn, Y, La, Zr, Mg, Fe, Cu, Nb, Cr, or Al.

[0060] The preparation method of the positive electrode active material of the present embodiment has at least the following advantages:

[0061] (1) The heat treatment temperature of the preparation method of the positive electrode active material of the present embodiment is lower than 300°C, which does not cause the R-3m phase transition of the P63mc space group lithium cobalt oxide, and effectively avoids the side reactions and harmful impurities that may be caused by high temperature heat treatment.

[0062] (2) The coating formed on the surface of the lithium cobalt oxide by the preparation method of the positive electrode active material of the present embodiment is a metal oxyfluoride, which has good electrical conductivity and does not cause an increase in material impedance. In addition, the coating can effectively prevent direct contact between the positive electrode active material and the electrolyte during the cycle process, inhibit the occurrence of side reactions at the interface between the positive electrode active material and the electrolyte, and thus achieve high cycle stability at high voltage.

[0063] (3) The preparation process of the positive electrode active material of the present embodiment is simple, low in cost, and easy to realize industrial production.

[0064] The third aspect of the present application provides a positive electrode sheet of an embodiment, which comprises a current collector and a positive electrode active layer arranged on at least one side surface of the current collector, and the material of the positive electrode active layer comprises the positive electrode active material of the above-mentioned embodiment.

[0065] It can be understood that the material of the positive electrode active layer can also include a conductive agent, a binder, etc. For example, the conductive agent is conductive carbon, and the binder is polyvinylidene fluoride. The above only lists one commonly used conductive agent and binder, but is not limited thereto, and can also be other conductive agents and binders commonly used in the art.

[0066] The fourth aspect of the present application provides an electrochemical device of an embodiment, which comprises a positive electrode sheet, and the positive electrode sheet is the positive electrode sheet of the above-mentioned embodiment.

[0067] It can be understood that in some embodiments, the electrochemical device further comprises a negative electrode sheet, an electrolyte and a separator. The specific negative electrode sheet, electrolyte and separator can be commonly used in the art, which will not be described here. For example, in one of the embodiments, the electrochemical device is a secondary battery or the like. Specifically, the secondary battery can be a lithium ion battery.

[0068] When the electrochemical device is a secondary battery, the secondary battery has high specific capacity and cycle stability at a voltage of 4.55 V and above (relative to Li / Li + ), and has great application potential.

[0069] The fifth aspect of the present application provides an electronic device of an embodiment, which comprises the electrochemical device of the above-mentioned embodiment. The electrochemical device provides power for the electronic device. The specific electronic device is not particularly limited and can be commonly used in the art, for example, the electronic device can include but is not limited to a notebook computer, a mobile phone, a tablet computer and the like.

[0070] In order to make the purpose and advantages of the present application more clear, the positive electrode active material of the present application and its effects will be further described in detail below in combination with specific embodiments, and it should be understood that the specific embodiments described herein are only used to explain the present application and should not be used to limit the present application:

[0071] Comparative Example 1

[0072] (1) Preparation of lithium cobalt oxide

[0073] 0.5 mol of cobalt sulfate (CoSO4) was weighed, deionized water was added and dissolved quickly under stirring, then ammonium carbonate ((NH4)2CO3) was added to adjust the pH to 8, until the reaction was complete and a homogeneous cobalt carbonate (CoCO3) precipitate was formed. The cobalt carbonate precipitate was sintered at 650°C for 12 h, and then crushed and sieved to obtain Co3O4 metal oxide.

[0074] Lithium carbonate (Li2CO3) and the above-mentioned Co3O4 metal oxide were weighed and mixed uniformly in a molar ratio of 1.05:1, and then heat-treated at 900°C for 12 h. After crushing and sieving through a 200-mesh sieve, lithium cobalt oxide was obtained. The XRD test results of the obtained lithium cobalt oxide are shown in Figure 2 As can be seen from Figure 2 , the obtained lithium cobalt oxide has an R-3m crystal structure.

[0075] (2) Preparation of a coin cell

[0076] The lithium cobalt oxide obtained above was mixed with conductive carbon (SP) as a conductive agent and polyvinylidene fluoride (PVDF) as a binder at a mass ratio of 90:5:5, and then a solvent N-methyl-2-pyrrolidone (NMP) was added to prepare a slurry. The slurry was coated on an aluminum foil with a thickness of 12 μm. Then, the coated aluminum foil was dried in a 90°C air-drying oven for 4 h and then baked in a 110°C vacuum drying oven for 24 h to obtain a pole piece. The fully dried pole piece was subjected to cold pressing, punching, weighing and other processes to obtain a positive electrode pole piece.

[0077] A coin-type half battery was assembled using the positive electrode pole piece, a separator and a negative electrode and an electrolyte under an inert atmosphere. The negative electrode was a lithium metal, and the electrolyte contained ethylene carbonate (EC) and dimethyl carbonate (DMC) at a volume ratio of 1:1 as solvents and lithium hexafluorophosphate (LiPF6) at a concentration of 1 mol / L as a lithium salt.

[0078] The first cycle discharge specific capacity of the coin-type half battery of Comparative Example 1 was 203.4 mAh / g, and the capacity retention rate after 50 cycles was 61.6%.

[0079] Comparative Example 2

[0080] (1) Preparation of sodium cobalt oxide

[0081] 0.5 mol of cobalt sulfate was weighed, and then deionized water was added thereto and stirred rapidly to dissolve, followed by the addition of ammonium carbonate to adjust the pH to 8 until the reaction was complete to form a homogeneous cobalt carbonate precipitate. The cobalt carbonate precipitate was sintered at 650°C for 12 h, and then crushed and sieved to obtain a Co3O4 metal oxide.

[0082] Sodium carbonate (Na2CO3) and the Co3O4 metal oxide obtained above were weighed and mixed uniformly at a molar ratio of 0.35:1, and then heat-treated at 800°C for 48 h. The heat-treated product was crushed and sieved through a 200-mesh sieve to obtain a sodium cobalt oxide having a crystal structure of P63 / mmc.

[0083] (2) Preparation of lithium cobalt oxide

[0084] The sodium cobalt oxide obtained above, lithium nitrate and lithium acetate were mixed uniformly at a molar ratio of 1:2:3, and then loaded into a corundum crucible and subjected to a solid-phase reaction at 250°C for 6 h to obtain a mixture containing lithium cobalt oxide.

[0085] The mixture containing lithium cobalt oxide obtained above was crushed, and then washed with deionized water several times to remove soluble sodium and lithium salts until the conductivity of the supernatant was less than 200 μS / cm. The powder was then dried and sieved to obtain a lithium cobalt oxide.

[0086] The XRD test results of the obtained lithium cobalt oxide are shown in Figure 3 As can be seen from Figure 3 It can be seen that the obtained lithium cobalt oxide has a P63mc crystal structure.

[0087] (3) Making button cells

[0088] The steps of making button cells are similar to those of Comparative Example 1, except that the positive active material is different. The positive active material used in Comparative Example 2 is the obtained lithium cobalt oxide having a P63mc crystal structure.

[0089] Comparative Example 3

[0090] (1) Preparation of positive active material

[0091] According to the molar ratio of iron powder: fluosilicic acid = 1.2:1, the iron powder was slowly added to the fluosilicic acid, followed by stirring at 45°C for 18h, and then filtering out the excess iron powder to obtain a FeSiF6 solution.

[0092] A certain amount of the above FeSiF6 solution was diluted, and a certain amount of the lithium cobalt oxide obtained in Comparative Example 1 was slowly added according to the designed ratio (see Table 1). After rapid stirring, the water was evaporated using a rotary evaporator, and FeSiF6 was precipitated from the solution and uniformly adsorbed on the surface of the lithium cobalt oxide.

[0093] The above obtained material was heat treated at 250°C for 8h under a nitrogen atmosphere. FeSiF6 slowly decomposed during the heat treatment process to form FeF2 and uniformly adsorb on the surface of the lithium cobalt oxide, obtaining a lithium cobalt oxide material coated with FeF2 on the surface. The above obtained material was crushed and sieved through a 200 mesh sieve to obtain the positive active material of Comparative Example 3.

[0094] (2) Making button cells

[0095] The steps of making button cells are similar to those of Comparative Example 1, except that the positive active material is different. The positive active material used in Comparative Example 3 is the obtained lithium cobalt oxide coated with FeF2.

[0096] Comparative Example 4

[0097] (1) Preparation of positive active material

[0098] According to the molar ratio of iron powder: fluosilicic acid = 1.2:1, the iron powder was slowly added to the fluosilicic acid, followed by stirring at 45°C for 18h, and then filtering out the excess iron powder to obtain a FeSiF6 solution.

[0099] A certain amount of the above FeSiF6 solution was diluted and a certain amount of the lithium cobalt oxide obtained in Comparative Example 2 was slowly added in a designed ratio (see Table 3). After rapid stirring, water was evaporated using a rotary evaporator, FeSiF6 was precipitated from the solution and uniformly adsorbed on the surface of the lithium cobalt oxide.

[0100] The above obtained material was kept at 250°C for 8h under a nitrogen atmosphere. FeSiF6 slowly decomposed during the heat treatment process to form FeF2, which was uniformly adsorbed on the surface of the lithium cobalt oxide, obtaining a lithium cobalt oxide material coated with FeF2 on the surface. The above obtained material was crushed and sieved through a 200 mesh screen, obtaining the positive electrode active material of Comparative Example 4.

[0101] (2) Preparation of button cells

[0102] The steps for preparing button cells were similar to those of Comparative Example 1, except that the positive electrode active material was different. The positive electrode active material used in Comparative Example 4 was the lithium cobalt oxide coated with FeF2 obtained above.

[0103] Example 1

[0104] (1) Preparation of positive electrode active material

[0105] Iron powder was slowly added to fluosilicic acid in a molar ratio of iron powder: fluosilicic acid = 1.2:1, followed by stirring at 45°C for 18h. The excess iron powder was then filtered off, obtaining a FeSiF6 solution.

[0106] A certain amount of the above FeSiF6 solution was vacuum dried, obtaining FeSiF6 powder, which was then kept at 250°C for 8h, obtaining the decomposition product of FeSiF6. The XRD test results of the decomposition product of FeSiF6 are shown in Figure 4 As can be seen from Figure 4 , the decomposition product of FeSiF6 obtained is a FeOF material with a P42 / mnm crystal structure.

[0107] A certain amount of the above FeSiF6 solution was diluted and a certain amount of the lithium cobalt oxide obtained in Comparative Example 1 was slowly added in a designed ratio (see Table 1). After rapid stirring, water was evaporated using a rotary evaporator, FeSiF6 was precipitated from the solution and uniformly adsorbed on the surface of the lithium cobalt oxide.

[0108] The above obtained material was kept at 250°C for 8h under an air atmosphere. FeSiF6 slowly decomposed during the heat treatment process to form FeOF, which was uniformly adsorbed on the surface of the lithium cobalt oxide, obtaining a lithium cobalt oxide material coated with FeOF on the surface. The above obtained material was crushed and sieved through a 200 mesh screen, obtaining the positive electrode active material of Example 1.

[0109] The XRD test results of the positive electrode active material in Example 1 are as follows: Figure 5 As shown, the obtained positive electrode active material has a composite structure of R-3m + P42 / mnm, where the ratio of the strongest (110) crystal plane diffraction peak intensity I2 of the P42 / mnm structure FeOF material to the strongest (003) crystal plane diffraction peak I1 of the R-3m structure lithium cobalt oxide is approximately 3.12%. After acid dissolution of the surface coating, the actual measured mass fraction of FeOF in the obtained positive electrode active material is 0.44%.

[0110] (2) Making a button battery

[0111] The steps for making the button cell are similar to those in Comparative Example 1, except that the positive electrode active material is different. The positive electrode active material used in Example 1 is the lithium cobalt oxide with FeOF coated on the surface obtained above.

[0112] Examples 2 to 5

[0113] Except for the different amount of FeSiF6 solution added during the preparation of the positive electrode active material, the other material preparation steps and coin cell fabrication process are the same as in Example 1. The elemental composition of the positive electrode active materials prepared in Examples 2 to 5 and the electrical performance test results of the coin cells are shown in Table 1.

[0114] Examples 6 to 11

[0115] Except for adjustments to the elemental composition of the lithium cobalt oxide used in the preparation of the positive electrode active material, the other material preparation steps and coin cell fabrication process were the same as in Example 1. Aluminum, nickel, manganese, titanium, and yttrium were added to deionized water along with cobalt sulfate in the forms of aluminum nitrate, nickel sulfate, manganese sulfate, titanium nitrate, and yttrium nitrate, respectively. The elemental composition of the positive electrode active materials prepared in Examples 6 to 11 and the electrical performance test results of the coin cells are shown in Table 2.

[0116] Examples 12 to 16

[0117] Except for the difference in the amount of lithium cobalt oxide coated (as in Comparative Example 2) and the amount of FeOF coating during the preparation of the positive electrode active material, the other material preparation steps and coin cell fabrication process are the same as in Example 1. The elemental composition of the positive electrode active materials prepared in Examples 12 to 16 and the electrical performance test results of the coin cells are shown in Table 3.

[0118] The XRD test results of the positive electrode active material obtained in Example 15 are as follows: Figure 6 As shown, by Figure 6It can be seen that the obtained positive active material is a composite structure of P63mc+P42 / mnm, wherein the ratio of the strongest (110) crystal face diffraction peak intensity I2 of the P42 / mnm structure FeOF material to the strongest (002) crystal face diffraction peak intensity I1 of the P63mc structure lithium cobalt oxide material is about 4.66%. After the surface coating is dissolved by acid, it is actually measured that the mass fraction of FeOF in the obtained positive active material is 1.97%.

[0119] Examples 17 to 21

[0120] Except that the element components containing sodium cobalt oxide used in the process of preparing sodium cobalt oxide are adjusted, the remaining material preparation steps and the process of making the button cell are the same as those of Example 13. Among them, nickel, manganese, aluminum and titanium elements are added in the form of nickel sulfate, manganese sulfate, aluminum nitrate, titanium nitrate together with cobalt sulfate in deionized water. The element composition of the positive active material prepared in Examples 17 to 21 and the test results of the electrical performance of the button cell are shown in Table 4.

[0121] Example 22

[0122] (1) Preparation of positive active material

[0123] According to the molar ratio of iron powder: nickel powder: fluorosilicic acid = 1.14:0.06:1, the iron powder and nickel powder are slowly added to the fluorosilicic acid solution, then stirred at 45°C for 18h, and the excess metal powder is filtered out to obtain Fe 0.95 Ni 0.05 SiF6solution.

[0124] A certain amount of Fe 0.95 Ni 0.05 SiF6solution is diluted, and a certain amount of lithium cobalt oxide obtained in Comparative Example 2 is slowly added. After rapid stirring, the water is evaporated by a rotary evaporator, and Fe 0.95 Ni 0.05 SiF6is precipitated from the solution and attached to the surface of the lithium cobalt oxide. Among them, the addition amount of Fe 0.95 Ni 0.05 SiF6solution and lithium cobalt oxide is calculated according to Table 5.

[0125] The above obtained material is kept at 250°C for 8h, and Fe 0.95 Ni 0.05 SiF6slowly decomposes during the heat treatment to generate Fe 0.95 Ni 0.05 OF and attach to the surface of the lithium cobalt oxide. The above obtained material is sieved to obtain the positive active material of Example 22. The test shows that the surface of the obtained positive active material is coated with Fe 0.95 Ni0.05 The mass fraction of OF is 0.98%.

[0126] (2) Preparation of button cell

[0127] The steps of preparing the button cell are similar to those of Comparative Example 2, except that the positive active material is different. The positive active material used in Example 22 is the positive active material obtained above.

[0128] The elemental composition of the positive active material prepared in Example 22 and the test results of the electrical performance of the button cell are shown in Table 5.

[0129] Examples 23 to 27

[0130] Except that the nickel powder is replaced by manganese powder, cobalt powder, titanium powder, yttrium powder and lanthanum powder in turn when preparing the metal fluorosilicate solution in the process of preparing the positive active material, the remaining material preparation steps and the button cell preparation process are the same as those of Example 22. The elemental composition of the positive active material prepared in Examples 23 to 27 and the test results of the electrical performance of the button cell are shown in Table 5.

[0131] Examples 28 to 31

[0132] Except that the proportions of iron powder and cobalt powder added when preparing the metal fluorosilicate solution in the process of preparing the positive active material are different, the remaining material preparation steps and the button cell preparation process are the same as those of Example 24. The elemental composition of the positive active material prepared in Examples 28 to 31 and the test results of the electrical performance of the button cell are shown in Table 6.

[0133] The following are specific test methods:

[0134] (1) XRD test

[0135] The positive active material is tested by an X-ray powder diffractometer (XRD, instrument model: Bruker D8 ADVANCE), the target material is Cu Kα, the voltage and current are 40KV / 35mA, the scanning angle range is 10° to 60°, and the scanning rate is 5° / min.

[0136] (2) Test of elemental composition and coating content of lithium cobalt oxide surface coating

[0137] The test principle is that lithium cobalt oxide itself is not soluble in acid, while the surface coating is easily soluble in acid.

[0138] The test method is to dissolve the surface coating of lithium cobalt oxide with a certain amount of dilute nitric acid solution, separate the residual powder, and the mass difference of the material before and after acid dissolution is the coating amount. The obtained filtrate contains F - , Fe 3+ and M 3+The solution can be tested for content. Among them, the content of metal elements such as Fe is measured by an Optima 7000DV inductively coupled plasma spectrometer (ICP) from PE Company, USA, and the content of F element is measured by a Thermo ion chromatograph.

[0139] (3) Discharge capacity and cycle capacity retention rate test

[0140] After the button cell is aged at a constant temperature (25°C) for 24 h, 50 cycles of charge-discharge test are repeatedly carried out in a voltage range of 3 V to 4.6 V (relative to Li / Li + ). Among them, the first 2 cycles are activated by low current charge-discharge at a rate of 0.2C, and from the 3rd cycle, charge-discharge is carried out at a current rate of 1C. Among them, 1C = 273 mA / g.

[0141] Among them, the 3rd cycle discharge capacity is taken as the reference benchmark for cycle capacity decay, that is, the nth capacity retention rate = nth discharge capacity / 3rd discharge capacity x 100%.

[0142] Table 1 gives the performance comparison of FeOF coated lithium cobalt oxide material belonging to R-3m space group. By comparing the first cycle discharge capacity of Comparative Example 1 to Example 5 and Comparative Example 1, it can be seen that when the coating amount is low, the discharge capacity decreases slightly with the increase of the FeOF coating amount; when the coating amount exceeds 2%, the discharge capacity decreases from 203.4 mAh / g of Comparative Example 1 to 198.4 mAh / g of Example 5. The reason for the decrease in discharge capacity is mainly that FeOF coating does not contain lithium and cannot provide active lithium, so with the increase of its relative content, the gram capacity of the positive active material gradually decreases. Comparative Example 3 uses FeF2 without oxygen for coating, and Example 2 is completely the same except that it uses the same amount of FeOF coating. By comparing the electrical performance, the capacity retention rate of Comparative Example 3 material after 50 cycles is similar to that of Comparative Example 1 material without any coating treatment, only 63.7%, which is much lower than that of Example 2 material 81.8%, indicating that oxygen replacing part of fluorine element and then coating is very critical for improving the electrical performance of the material.

[0143] By comparing the discharge capacity retention rate after 50 cycles, it can be seen that the capacity retention rate of Comparative Example 1 is 61.6%, while the capacity retention rates of Examples 1-5 are all greatly improved to more than 75%, indicating that the FeOF coating on the surface of lithium cobalt oxide can effectively block the interfacial reaction between the lithium cobalt oxide active material and the electrolyte, improve the interfacial stability, and improve the cycle performance. In addition, with the increase of the FeOF coating amount, the ratio I2 / I1 of the strongest (110) crystal face diffraction peak intensity I2 of the P42 / mnm structure FeOF material to the strongest (003) crystal face diffraction peak intensity I1 of the R-3m structure LiCoO2 material also gradually increases.

[0144] Table 2 shows the performance comparison of lithium cobalt oxide materials doped with different metal elements coated with FeOF belonging to the R-3m space group. By comparing Example 6 to Example 11 and Comparative Example 1, it can be seen that after the lithium cobalt oxide is doped with metal elements such as Al, Ni, Mn, Ti, Y, etc., the capacity retention rate of the positive electrode active material after 50 cycles is greatly improved without obvious deterioration of the first cycle discharge capacity. Compared with Comparative Example 1, the average cycle retention rate is increased by more than 20%, and the capacity retention rate after 50 cycles of Example 6 and Example 9 is increased by nearly 90%. It is obvious that FeOF coating modification can significantly improve the electrochemical performance of lithium cobalt oxide containing R-3m space group.

[0145] Table 3 shows the performance comparison of lithium cobalt oxide materials coated with FeOF belonging to the P63mc space group. From the table, it can be seen that the lithium cobalt oxide of Comparative Example 2 is not coated, and its first cycle discharge capacity is 223.5 mAh / g, and the capacity retention rate after 50 cycles is only 73.1%. After the lithium cobalt oxide materials of Example 12 to Example 16 are coated with FeOF, the discharge capacity is slightly reduced, but the cycle retention rate is greatly improved. When the coating amount of FeOF is about 1%, the capacity retention rate after 50 cycles is more than 93%, which is increased by 20% compared with Comparative Example 2. Comparative Example 4 uses FeF2 without oxygen for coating, and Example 13 is the same except that it uses the same amount of FeOF for coating. However, the materials of Comparative Example 4 and Example 13 show obvious differences in electrical performance. The capacity retention rate of Comparative Example 4 after 50 cycles is only 75.7%, which is much lower than the 93.1% of Example 13, which again shows that the coating method of the oxygen-containing metal fluoride material provided in the application can greatly improve the cycle performance of the material.

[0146] Table 4 shows the performance comparison of lithium cobalt oxide materials doped with different metal elements coated with FeOF belonging to the P63mc space group. The coating amount of FeOF in the positive electrode active materials of Example 17 to Example 21 is 1%, and it can be seen that after the different metal elements are doped and coated with FeOF, the first cycle discharge capacity of the materials of Example 17 to Example 21 is not significantly deteriorated, but the capacity retention rate after 50 cycles is greatly improved. Compared with Comparative Example 2, the cycle retention rate of the materials of the remaining examples after 50 cycles is more than 90% except for Example 18 and Example 19. Therefore, FeOF coating modification can significantly improve the electrochemical performance of lithium cobalt oxide containing P63mc structure.

[0147] Table 5 shows the performance comparison of lithium cobalt oxide material belonging to P63mc space group after coating composite FeOF material. First, by comparing Example 22 to Example 27 with Comparative Example 2, it can be seen that after coating the lithium cobalt oxide material belonging to P63mc space group with FeOF material composite with Ni, Mn, Co, Ti, Y, La on the surface, the first cycle discharge specific capacity is not significantly deteriorated, and the capacity retention rate after 50 cycles is further improved, and the average capacity retention rate is more than 94%. In particular, Example 24, the surface coated material is FeOF composite with 5mol% Co, the capacity retention rate after 50 cycles is as high as 95.2%. By coating with FeOF composite, the cycle retention rate of P63mc space group lithium cobalt oxide material is significantly improved, and the possible reason is that the conductivity of FeOF composite with other metal elements is higher, and the binding force with the surface of lithium cobalt oxide containing material is stronger, further improving the interface stability. 0.95 Co 0.05 OF, the capacity retention rate after 50 cycles is as high as 95.2%. By coating with FeOF composite, the cycle retention rate of P63mc space group lithium cobalt oxide material is significantly improved, and the possible reason is that the conductivity of FeOF composite with other metal elements is higher, and the binding force with the surface of lithium cobalt oxide containing material is stronger, further improving the interface stability.

[0148] Table 6 shows the performance comparison of P63mc structure lithium cobalt oxide material coated with different Co composite amount of FeOF. Based on the previous test results, the coating effect of Co composite FeOF material is the best, and different Co composite amount of FeOF coated material is further prepared in order to obtain the optimal composite amount. By comparing Example 24, Example 28 to Example 31, it can be seen that the optimal Co composite amount in FeOF is about 5mol%, and the cycle retention rate is slightly reduced when the composite amount is lower than this amount; In particular, when the composite amount of Co reaches 20mol%, the 50 cycle retention rate is reduced to 93.2%. Considering that the price of cobalt is much higher than that of iron, the composite amount of cobalt should not be too high, so it is considered that the maximum composite amount of Co in FeOF should be controlled within 20mol%.

[0149] Table 1

[0150]

[0151] Table 2

[0152]

[0153] Table 3

[0154]

[0155]

[0156] Table 4

[0157]

[0158] Table 5

[0159]

[0160] Table 6

[0161]

[0162]

[0163] Each technical feature of the above-described embodiments can be combined arbitrarily, and for the sake of brevity, each technical feature of the above-described embodiments is not described in all possible combinations, but it should be understood that the combinations of the technical features are within the scope of the present disclosure as long as the combinations do not contradict each other.

[0164] The above-described embodiments only express several implementation manners of the present application, facilitate specific and detailed understanding of the technical solutions of the present application, but should not be understood as a limitation on the protection scope of the present patent. It should be noted that, for those skilled in the art, some modifications and improvements can be made without departing from the concept of the present application, and these are within the protection scope of the present application. It should be understood that, the technical solutions obtained by those skilled in the art through logical analysis, reasoning or limited experiments on the basis of the technical solutions provided by the present application are within the protection scope of the appended claims of the present application. Therefore, the protection scope of the present patent should be subject to the contents of the appended claims, and the description and drawings can be used to explain the contents of the claims.

Claims

1. A positive electrode active material, comprising: a substrate, and a coating on at least part of a surface of the substrate; The coating includes a first compound having a P42 / mnm crystal structure; the first compound includes M a Fe 1-a O c F d , 0 < a < 1, 0.95 < c < 1.05, 0.95 < d < 1.05, the M element includes at least one of Ni, Mn, Co, Ti, Y, or La.

2. The positive electrode active material according to claim 1, characterized by in an XRD diffraction spectrum of the positive electrode active material, a peak intensity of a strongest diffraction peak in a range of 18°-19° of a 2θ diffraction angle is I1, a peak intensity of a strongest diffraction peak in a range of 26°-27° of the 2θ diffraction angle is I2, and 0 3. The positive electrode active material according to claim 1, characterized by a mass fraction of the coating based on a mass of the positive electrode active material is w, and 0 4. The positive electrode active material according to claim 1, characterized by The base body is of an R-3m crystal structure, the base body comprises a Co element and optionally a T element, the T element comprising at least one of Ni, Mn, Ti, Zn, Y, La, Zr, Mg, Fe, Cu, Nb, Cr or Al, a molar amount of the T element in the base body being n T , a sum of molar amounts of the Co element and the T element being n Co+T , a ratio y1 of the n T to the n Co+T satisfying 0≤y1≤0.

5.

5. The positive electrode active material according to claim 1, characterized by The base body is a P63mc crystal structure, the base body comprises a Co element and optionally a Q element, the Q element comprises at least one of Ni, Mn, Ti, Zn, Y, La, Zr, Mg, Fe, Cu, Nb, Cr or Al, a molar amount of the Q element in the base body is n Q , a sum of molar amounts of the Co element and the Q element is n Co+Q , and a ratio y2 of the n Q to the n Co+Q satisfies 0≤y2≤0.

15.

6. The positive electrode active material according to claim 5, characterized by The base further contains Na element, and a molar amount of the Na element in the base is n Na , and a ratio z2 of the n Na to the n Co+Q satisfies 0 < z2 ≤ 0.

03. 7.An electrochemical device, comprising a positive electrode tab, wherein the positive electrode tab comprises the positive electrode active material according to any one of claims 1 to 6. 8.An electronic device, comprising the electrochemical device according to claim 7.

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