Manganese-based composite lithium supplement and preparation method thereof and positive plate

Abstract

The application provides a manganese-based composite lithium supplementing agent, a preparation method thereof and a positive plate, and belongs to the field of lithium batteries. The application coats a continuous non-porous MnO layer on the surface of Li6MnO4, avoids the contact between air or electrolyte and Li6MnO4, and realizes physical isolation. Lewis acidic sites (divalent manganese ion empty orbit) on the surface of manganese oxide can interact with LiOH or Li2O on the surface of Li6MnO4, reduce the adsorption activity of LiOH and Li2O on carbon dioxide in the air, inhibit the generation of lithium carbonate, thereby avoiding the blockage of ion channels by lithium carbonate and the aggregation of Li6MnO4. In addition, manganese oxide can form a chemically stable protective layer (such as Mn3O4) through surface oxidation, inhibit the dissolution of Li6MnO4 manganese ions and the loss of lattice oxygen, and stabilize the phase structure.

Description

Technical Field

[0001] This invention relates to the field of lithium battery technology, and in particular to a manganese-based composite lithium supplement agent, its preparation method, and a positive electrode sheet. Background Technology

[0002] During the first charge of a lithium-ion battery, the formation of a solid electrolyte interphase (SEI) film on the negative electrode surface irreversibly consumes the active lithium in the battery system, affecting the battery's energy density and cycle life. Replenishing the active lithium with a lithium replenisher is an effective way to solve this problem.

[0003] Lithium-added cathode materials are easier to scale up due to their strong compatibility with existing battery manufacturing processes, high safety, and controllable cost. Among them, manganese-based lithium-added materials have attracted widespread attention due to their high theoretical specific capacity; for example, Li6MnO4 with an anti-fluorite structure has a theoretical specific capacity of up to 1001 mAh / g. However, due to the poor air stability of Li6MnO4, free lithium ions and alkaline lithium species (such as Li2O and LiOH) on its surface readily react with water in the air to form LiOH, which then rapidly adsorbs CO2 from the air to form a Li2CO3 insulating layer, blocking the lithium ion transport channels and causing the lithium-added material particles to agglomerate, resulting in low specific capacity and capacity retention rate during the first charge of the lithium-ion battery. At the same time, the unstable valence state of manganese in Li6MnO4 is prone to Jahn-Teller distortion, which, under the synergistic effect of air and moisture, undergoes a disproportionation reaction to generate soluble manganese ions, destroying the bulk structure of the lithium-added material and further reducing the specific capacity and capacity retention rate during the first charge of the lithium-ion battery.

[0004] Coating Li6MnO4 with polymethyl methacrylate can solve the above-mentioned problems of manganese-based lithium supplements, but when used in lithium-ion batteries, the specific capacity of the first charge is increased by only 13.1%, and the specific capacity retention rate after 100 cycles is increased by only 14.5%, which still needs to be improved. Summary of the Invention

[0005] The purpose of this invention is to provide a manganese-based composite lithium supplement, its preparation method, and a positive electrode sheet. The manganese-based composite lithium supplement provided by this invention has good air stability, and the resulting positive electrode sheet has high initial charge specific capacity and high capacity retention.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution: A manganese-based composite lithium supplement includes Li6MnO4 and an MnO layer coated on the surface of the Li6MnO4; the mass ratio of MnO to Li6MnO4 is 1:(6~100).

[0007] Preferably, the thickness of the MnO layer is 10~300nm.

[0008] This invention also provides a method for preparing the manganese-based composite lithium supplement described in the above technical solution, comprising: After mixing nanoscale manganese source and lithium source, a first sintering and a second sintering are performed sequentially to obtain manganese-based composite lithium supplement; after the first sintering is completed, the obtained product is directly cooled from the temperature of the first sintering to the temperature of the second sintering for a second sintering; the cooling rate is 0.5~5℃ / min.

[0009] Preferably, the molar ratio of manganese in the manganese source to lithium in the lithium source is 1:(5~8).

[0010] Preferably, the particle size of the nanoscale manganese source is 300~800nm.

[0011] Preferably, the lithium source includes one or more of Li2O, LiOH·H2O, Li2CO3, LiNO3, Li2SO4 and CH3COOLi.

[0012] Preferably, the nanoscale manganese source includes one or more of Mn3O4, MnCO3, MnO2, and MnO.

[0013] Preferably, the temperature of the first sintering is 800~1200℃, and the time of the first sintering is 10~20h.

[0014] Preferably, the second sintering temperature is 300~700℃, and the second sintering time is 15~30h.

[0015] The present invention also provides a positive electrode sheet, comprising a current collector and a positive electrode material coated on the surface of the current collector, wherein the positive electrode material comprises a lithium active material, a manganese-based composite lithium supplement, a conductive agent and a binder, wherein the manganese-based composite lithium supplement is the manganese-based composite lithium supplement described in the above technical solution or the manganese-based composite lithium supplement prepared by the preparation method described in the above technical solution.

[0016] This invention provides a manganese-based composite lithium supplement, comprising Li6MnO4 and a MnO layer coated on the surface of the Li6MnO4; the mass ratio of MnO to Li6MnO4 is 1:(6~100). This invention achieves physical isolation by coating the surface of Li6MnO4 with a continuous, non-porous MnO layer, preventing contact between air or electrolyte and Li6MnO4; the presence of Lewis acidic sites (empty orbitals of divalent manganese ions) on the surface of manganese oxide allows for acid-base interactions with LiOH or Li2O on the Li6MnO4 surface, reducing the adsorption activity of LiOH and Li2O on carbon dioxide in the air and inhibiting the formation of lithium carbonate, thereby preventing lithium carbonate from blocking ion channels and causing Li6MnO4 aggregation; furthermore, manganese oxide itself can form a chemically stable protective layer (such as Mn3O4) through surface oxidation, inhibiting the dissolution of manganese ions from Li6MnO4 and the loss of lattice oxygen, thus stabilizing the phase structure. The results of the examples show that the positive electrode sheet prepared with the manganese-based composite lithium supplement provided by the present invention can achieve a first charge capacity of 741.4 mAh / g after 24 hours of exposure to air (relative humidity of 30%). Attached Figure Description

[0017] Figure 1 The XRD patterns of the manganese-based composite lithium supplement before and after air exposure in Example 3 of this invention are shown. Figure 2 This is a SEM image of the surface morphology of the manganese-based composite lithium supplement in Example 3 of the present invention; Figure 3 This is a TEM image of the cross-sectional morphology of the manganese-based composite lithium supplement in Example 3 of the present invention; Figure 4 The first charging curves are shown for the lithium-ion batteries prepared from the positive electrode sheets of Test Examples 3 and 6 of this invention. Detailed Implementation

[0018] The present invention provides a manganese-based composite lithium supplement comprising Li6MnO4 and a MnO layer coated on the surface of the Li6MnO4; the mass ratio of MnO to Li6MnO4 is 1:(6~100).

[0019] The manganese-based composite lithium replenisher provided by this invention includes Li6MnO4. This invention utilizes the high specific capacity of Li6MnO4, which can release a large number of lithium ions during the first charge cycle to compensate for irreversible lithium losses caused by SEI film formation and transition metal ion dissolution, significantly improving the coulombic efficiency and energy density of lithium-ion batteries during the first charge cycle.

[0020] The manganese-based composite lithium supplement provided by this invention further includes a MnO layer coated on the surface of Li6MnO4. This invention achieves physical isolation by limiting the MnO layer coated on the surface of Li6MnO4, preventing contact between air or electrolyte and Li6MnO4. The presence of Lewis acidic sites (empty orbitals of divalent manganese ions) on the surface of manganese oxide allows for acid-base interactions with LiOH or Li2O on the Li6MnO4 surface, reducing the adsorption activity of LiOH and Li2O on carbon dioxide in the air and inhibiting the formation of lithium carbonate. This prevents lithium carbonate from blocking ion channels and causing Li6MnO4 aggregation. Furthermore, manganese oxide itself can form a chemically stable protective layer (such as Mn3O4) through surface oxidation, inhibiting the dissolution of manganese ions and lattice oxygen loss from Li6MnO4, thus stabilizing the phase structure. In one embodiment of this invention, the thickness of the MnO layer can be 10~300 nm, 50~250 nm, or 100~200 nm. This invention limits the thickness of the MnO layer to ensure that the air stability of Li6MnO4 is improved without affecting the insertion / extraction of lithium ions in Li6MnO4.

[0021] In this invention, the mass ratio of MnO to Li6MnO4 can be 1:(6~100). In embodiments of this invention, the mass ratio of MnO to Li6MnO4 can specifically be 1:10, 1:20, 1:40, 1:60, 1:80, or 1:100. This invention ensures that MnO can fully coat the surface of Li6MnO4 by limiting the mass ratio of MnO to Li6MnO4, thereby ensuring good air stability of Li6MnO4.

[0022] This invention achieves physical isolation by coating the surface of Li6MnO4 with a continuous, non-porous MnO layer, preventing air or electrolyte from contacting Li6MnO4. The presence of Lewis acidic sites (empty orbitals of divalent manganese ions) on the manganese oxide surface allows for acid-base interactions with LiOH or Li2O on the Li6MnO4 surface, reducing the adsorption activity of LiOH and Li2O on carbon dioxide from the air and inhibiting the formation of lithium carbonate. This prevents lithium carbonate from blocking ion channels and causing Li6MnO4 aggregation. Furthermore, manganese oxide itself can form a chemically stable protective layer (such as Mn3O4) through surface oxidation, inhibiting the dissolution of manganese ions and lattice oxygen loss from Li6MnO4, thus stabilizing the phase structure.

[0023] This invention also provides a method for preparing the manganese-based composite lithium supplement described in the above technical solution, comprising: After mixing nanoscale manganese source and lithium source, a first sintering and a second sintering are performed sequentially to obtain manganese-based composite lithium supplement; after the first sintering is completed, the obtained product is directly cooled from the temperature of the first sintering to the temperature of the second sintering for a second sintering; the cooling rate is 0.5~5℃ / min.

[0024] In one embodiment of the present invention, the nanoscale manganese source may include one or more of Mn3O4, MnCO3, MnO2, and MnO. In another embodiment, the particle size of the nanoscale manganese source may be 300-800 nm. In specific embodiments of the present invention, the particle size of the nanoscale manganese source may be 300 nm, 400 nm, 600 nm, or 800 nm. The present invention ensures more uniform dispersion of the nanoscale manganese source during the reaction process by limiting the type and particle size of the nanoscale manganese source, thereby forming a dense, continuous coating layer on the surface of Li6MnO4.

[0025] In one embodiment of the present invention, the nanoscale manganese source can be obtained by sequentially wet-milling and vacuum-drying a micron-sized manganese source. In another embodiment, the wet-milling additive can be water, and the mass ratio of the wet-milling additive to the nanoscale manganese source can be (5~15):1; the wet-milling rotation speed can be 1000~2500 rpm, and the wet-milling time can be 1~5 hours. In yet another embodiment, the vacuum drying temperature can be 60~140℃, and the vacuum drying time can be 5~20 hours, or 10~15 hours.

[0026] In one embodiment of the present invention, the lithium source may include one or more of Li₂O, LiOH·H₂O, Li₂CO₃, LiNO₃, Li₂SO₄, and CH₃COOLi. By limiting the type of lithium source, the present invention facilitates the preparation of high-purity manganese-based composite lithium supplements with manganese sources. The present invention does not impose any special limitation on the particle size of the lithium source; particle sizes commonly used in the art can be employed.

[0027] In one embodiment of the present invention, the molar ratio of manganese in the manganese source to lithium in the lithium source can be 1:(5~8). In embodiments of the present invention, the molar ratio of manganese in the manganese source to lithium in the lithium source can specifically be 1:5, 1:5.5, 1:6, 1:6.6, 1:7, or 1:8. The present invention ensures sufficient generation of Li6MnO4 and adjusts the thickness of manganese oxide by limiting the molar ratio of manganese in the manganese source to lithium in the lithium source.

[0028] In one embodiment of the present invention, the manganese source and the lithium source are mixed in a high-speed mixer. In another embodiment, the high-speed mixer can rotate at 4000-6000 rpm, the mixing cycle can be 4-8 times, and the mixing time for each cycle can be independently 90-150 seconds.

[0029] In one embodiment of the present invention, the temperature of the first sintering can be 800~1200℃, or 900~1000℃. In another embodiment, the rate of heating to the first sintering temperature can be 1~5℃ / min. In yet another embodiment, the time for the first sintering can be 10~20h, 12~18h, or 14~16h. The present invention ensures the formation of Li6MnO4 and the formation of a coating layer by limiting the temperature and time of the first sintering, and by controlling the presence of Lewis acidic sites (empty orbitals of divalent manganese ions) on the MnO surface to interact with LiOH or Li2O on the Li6MnO4 surface, thereby enhancing the interfacial bonding between the coating layer and Li6MnO4. In another embodiment, the atmosphere for the first sintering can be one or more of Ar, N2, CO, H2, Ar+5%H2, and Ar+10%H2.

[0030] As one embodiment of the present invention, after the first sintering is completed, the obtained product can be directly cooled from the temperature of the first sintering to the temperature of the second sintering for a second sintering; the cooling rate can be 0.5~5℃ / min.

[0031] In this invention, the cooling rate after the first sintering is 0.5~5℃ / min, or it can be 1~3℃ / min. In embodiments of this invention, the cooling rate can specifically be 0.5℃ / min, 1℃ / min, 3℃ / min, or 5℃ / min. This invention reduces thermal stress and prevents cracking of the MnO coating layer by limiting the cooling rate.

[0032] In one embodiment of the present invention, the second sintering temperature can be 300~700℃. In specific embodiments of the present invention, the second sintering temperature can be 300℃, 450℃, 500℃, or 600℃. In one embodiment of the present invention, the second sintering time can be 15~30h. In specific embodiments of the present invention, the second sintering time can be 15h, 20h, 25h, or 30h. The present invention, by limiting the temperature and time of the second sintering, enables the MnO coating layer to "spread and form a film" on the surface of Li6MnO4, forming a continuous MnO coating layer without obvious pores.

[0033] In one embodiment of the present invention, the atmosphere for cooling and the second sintering is the same as that for the first sintering, and will not be described again here. The present invention does not impose any particular limitation on the cooling method after the second sintering; any cooling method well known in the art can be used.

[0034] In one embodiment of the present invention, after the second sintering is completed, the product after the second sintering is sequentially subjected to natural cooling, ball milling, and sieving. In one embodiment of the present invention, the ball milling speed can be 300~1000 rpm. In specific embodiments of the present invention, the ball milling speed can be 300 rpm, 400 rpm, 500 rpm, 600 rpm, 700 rpm, 800 rpm, 900 rpm, or 1000 rpm. In one embodiment of the present invention, the ball milling time can be 1~15 hours. In specific embodiments of the present invention, the ball milling time can be 1 hour, 3 hours, 5 hours, 7 hours, 9 hours, or 15 hours. In one embodiment of the present invention, the sieve mesh size can be 300~500 mesh. In specific embodiments of the present invention, the sieve mesh size can be 300 mesh, 400 mesh, or 500 mesh. The present invention does not impose any special limitations on the sieving; any sieving method well known in the art can be used.

[0035] This invention ensures the formation of Li6MnO4 and the full diffusion of MnO to form a coating layer through a first sintering process. It also regulates the presence of Lewis acidic sites (empty orbitals of divalent manganese ions) on the MnO surface, which can interact with LiOH or Li2O on the Li6MnO4 surface to enhance the interfacial bonding between the MnO coating layer and Li6MnO4. Then, by limiting the cooling rate, it reduces the cracking of the MnO coating layer caused by thermal stress. Finally, a second sintering process allows the MnO coating layer to "spread out" on the Li6MnO4 surface to form a continuous MnO coating layer without obvious pores.

[0036] The present invention also provides a positive electrode sheet, comprising a current collector and a positive electrode material coated on the surface of the current collector. The positive electrode material comprises a lithium active material, a manganese-based composite lithium supplement, a conductive agent, and a binder. The manganese-based composite lithium supplement is the manganese-based composite lithium supplement described in the above technical solution or the manganese-based composite lithium supplement prepared by the above preparation method.

[0037] In one embodiment of the present invention, the current collector may include aluminum foil, carbon-coated aluminum foil, or titanium foil. In one embodiment of the present invention, the lithium active material may include one or more of lithium nickel cobalt manganese oxide, lithium iron phosphate, lithium manganese iron phosphate, lithium cobalt oxide, and lithium manganese oxide. In one embodiment of the present invention, the conductive agent may include one or more of graphene, acetylene black, carbon nanotubes, and carbon black. In one embodiment of the present invention, the adhesive may include one or more of polyvinylidene fluoride, polyethyl methacrylate, polyethylene glycol, nitrile rubber, polytetrafluoroethylene, polymethyl methacrylate, polyethyl methacrylate, and poly(3-methoxymethyl acrylate). In one embodiment of the present invention, the organic solvent may include one or more of N-methylpyrrolidone, dimethylformamide, diethylformamide, and tetrahydrofuran.

[0038] In one embodiment of the present invention, the mass ratio of the lithium active material to the manganese-based composite lithium supplement can be 100:(1~10). In embodiments of the present invention, the mass ratio of the lithium active material to the manganese-based composite lithium supplement can specifically be 100:1.26, 100:3.89, or 100:6.66.

[0039] In one embodiment of the present invention, the mass of the manganese-based composite lithium supplement can be 1-5% or 2-3% of the mass of the cathode material.

[0040] In one embodiment of the present invention, the method for preparing the positive electrode sheet may include: mixing a manganese-based composite lithium supplement, a lithium active material, a conductive agent, a binder, and an organic solvent, and then sequentially coating, drying, rolling, and die-cutting the mixture on the surface of a current collector to obtain the positive electrode sheet. In one embodiment of the present invention, the drying temperature may be 80~120℃. In one embodiment of the present invention, the number of rollers in the rolling process may be 4~5, the thickness of the roller gap may be 10μm, and the pressure applied during the rolling process may be 50t.

[0041] As one embodiment of the present invention, the mass ratio of the manganese-based composite lithium supplement, the lithium active material, the conductive agent and the binder can be (1.0~10):(70~89):(5.0~15):(5.0~15).

[0042] This invention does not impose any particular limitation on the mixing of the manganese-based composite lithium supplement, lithium active material, conductive agent, binder, and organic solvent. A mixing method well-known in the art can be used to mix the manganese-based composite lithium supplement, lithium active material, conductive agent, binder, and organic solvent. In one embodiment of this invention, the manganese-based composite lithium supplement, lithium active material, conductive agent, binder, and organic solvent are mixed to obtain a slurry; the solid content of the slurry can be 50-75%. In embodiments of this invention, the solid content of the slurry can specifically be 50%, 55%, 60%, 70%, or 75%.

[0043] In one embodiment of the present invention, the drying temperature can be 80~120℃, and the drying time can be 4~5h.

[0044] The present invention does not impose any special limitations on the coating, rolling and die-cutting methods, and any coating, rolling and die-cutting methods well known in the art can be used.

[0045] The positive electrode sheet prepared by this invention can achieve gradient lithium replenishment under high and low voltage conditions.

[0046] The technical solutions of this invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0047] Example 1 A manganese-based composite lithium supplement is composed of Li6MnO4 and a MnO layer coating the surface of the Li6MnO4; the mass fraction ratio of MnO to Li6MnO4 is 1:99; and the thickness of the MnO layer is 10 nm.

[0048] The preparation method of the above-mentioned manganese-based composite lithium supplement is as follows: Micron-sized MnO and water were placed in a sand mill and crushed at 1500 rpm for 3 hours. After centrifugation, filtration, and vacuum drying at 60℃ for 10 hours, MnO with an average particle size of 50 nm was obtained. The mass ratio of MnO to water was 5:1. The MnO with an average particle size of 50 nm and the Li2O with an average particle size of 20 μm were placed in a high-speed mixer and mixed at 4000 rpm for 4 times, with each mixing time being 90 s. Then, the mixture was placed in a tube furnace and heated to 1000 °C for 12 h under an argon atmosphere at a heating rate of 3 °C / min. Subsequently, it was cooled to 500 °C for 25 h at a furnace cooling rate of 5 °C / min. After natural cooling to room temperature, it was ball-milled at 300 rpm for 10 h to obtain a manganese-based composite lithium supplement, denoted as Li6MnO4@MnO-1. The molar ratio of manganese in MnO to lithium in Li2O is 1:5.5.

[0049] Example 2 The difference between this embodiment and Embodiment 1 is that the first sintering time is 20 hours, and the manganese-based composite lithium supplement is denoted as Li6MnO4@MnO-2.

[0050] Example 3 The difference between this embodiment and Embodiment 2 is that the molar ratio of manganese in MnO to lithium in Li2O is 1:6, and the manganese-based composite lithium supplement prepared is denoted as Li6MnO4@MnO-3.

[0051] Comparative Example 1 A manganese-based lithium supplement is Li6MnO4 with an antifluorite structure.

[0052] The preparation method of the above-mentioned manganese-based composite lithium supplement is as follows: Micron-sized MnO and micron-sized Li2O were mixed evenly using a mortar and pestle, and then placed in a tube furnace under an argon atmosphere. The mixture was heated to 1000℃ for 12 hours at a heating rate of 3℃ / min, followed by a second sintering at 500℃ for 25 hours at a furnace cooling rate of 5℃ / min. After natural cooling to room temperature, the mixture was ball-milled at 300 rpm for 10 hours to obtain a manganese-based lithium supplement, denoted as Li6MnO4. The molar ratio of manganese in MnO to lithium in Li2O was 1:5.5.

[0053] Application Example 1 A positive electrode sheet: comprising an aluminum foil current collector and a positive electrode material coated on the surface of the aluminum foil current collector, wherein the positive electrode material comprises a manganese-based composite lithium supplement agent prepared in Example 1, LiFePO4, acetylene black conductive agent, and polyvinylidene fluoride (PVDF) binder; the mass ratio of manganese-based composite lithium supplement agent, LiFePO4, acetylene black, and PVDF is 1:79:10:10; the mass ratio of LiFePO4 to manganese-based composite lithium supplement agent is 100:1.26; and the mass of the manganese-based composite lithium supplement agent is 1% of the mass of the positive electrode material.

[0054] Method for preparing positive electrode: The manganese-based composite lithium supplementer, LiFePO4, acetylene black conductive agent, and polyvinylidene fluoride (PVDF) binder prepared in Example 1 were mixed in N-methylpyrrolidone (NMP) solvent to obtain a positive electrode slurry. The positive electrode slurry was coated on one side of an aluminum foil current collector, then dried, rolled, and die-cut. The drying conditions were 100°C in a vacuum oven, 4-roller pressing, 10μm roll gap thickness, and 50t pressure, followed by die-cutting to obtain a positive electrode sheet. The solid content of the positive electrode slurry was 60%.

[0055] Application Example 2 The difference between this application example and application example 1 is that the mass ratio of manganese-based composite lithium supplement, LiFePO4, acetylene black and PVDF is 3:77:10:10; the mass ratio of LiFePO4 to manganese-based composite lithium supplement is 100:3.89; and the mass of the manganese-based composite lithium supplement is 3% of the mass of the cathode material.

[0056] Application Example 3 The difference between this application example and application example 1 is that the mass ratio of manganese-based composite lithium supplement, LiFePO4, acetylene black and PVDF is 5:75:10:10; the mass ratio of LiFePO4 to manganese-based composite lithium supplement is 100:6.66; and the mass of the manganese-based composite lithium supplement is 5% of the mass of the cathode material.

[0057] Application Example 4 The difference between this application example and application example 1 is that the manganese-based composite lithium supplement described in application example 1 is placed in air with a relative humidity of 40% for 24 hours, while the rest remains unchanged.

[0058] Application Example 5 The difference between this application example and application example 2 is that the manganese-based composite lithium supplement described in application example 1 is placed in air with a relative humidity of 40% for 24 hours, while the rest remains unchanged.

[0059] Application Example 6 The difference between this application example and application example 3 is that the manganese-based composite lithium supplement described in application example 1 is placed in air with a relative humidity of 40% for 24 hours, while the rest remains unchanged.

[0060] Comparative Application Example 1 The difference between this comparative application example and application example 1 is that the use of manganese-based composite lithium supplement is omitted, and the mass ratio of LiFePO4, acetylene black and PVDF is 80:10:10.

[0061] Comparative Application Example 2 The difference between this comparative application example and comparative application example 1 is that the manganese-based composite lithium supplement described in comparative application example 1 was placed in air with a relative humidity of 40% for 24 hours, while the rest remained unchanged.

[0062] Test Example 1 The difference between this test case and application example 1 is that LiFePO4 is not added; the mass ratio of the manganese-based composite lithium supplement, acetylene black and PVDF is 80:10:10.

[0063] Test Example 2 The difference between this test example and application example 1 is that: no LiFePO4 is added; the manganese-based composite lithium supplement prepared in example 2 is used; and the mass ratio of the manganese-based composite lithium supplement, acetylene black and PVDF is 80:10:10.

[0064] Test Example 3 The difference between this test example and application example 1 is that: no LiFePO4 is added; the manganese-based composite lithium supplement prepared in example 3 is used; and the mass ratio of the manganese-based composite lithium supplement, acetylene black and PVDF is 80:10:10.

[0065] Test Example 4 The difference between this test example and application example 1 is that the manganese-based composite lithium supplement agent of example 1 was placed in air with a relative humidity of 40% for 24 hours to prepare a positive electrode sheet; the mass ratio of the manganese-based composite lithium supplement agent, acetylene black and PVDF was 80:10:10.

[0066] Test Example 5 The difference between this test example and application example 1 is that the manganese-based composite lithium supplement agent of example 2 was placed in air with a relative humidity of 40% for 24 hours to prepare a positive electrode sheet; the mass ratio of the manganese-based composite lithium supplement agent, acetylene black and PVDF is 80:10:10.

[0067] Test Example 6 The difference between this test example and application example 1 is that: LiFePO4 is not added; the positive electrode sheet is prepared after the manganese-based composite lithium supplement agent of Example 3 is placed in air with a relative humidity of 40% for 24 hours; the mass ratio of the manganese-based composite lithium supplement agent, acetylene black and PVDF is 80:10:10.

[0068] Comparative Test Case 1 The difference between this comparative test example and application example 1 is that: no LiFePO4 is added; the manganese-based composite lithium supplement prepared in comparative example 1 is used; and the mass ratio of the manganese-based composite lithium supplement, acetylene black and PVDF is 80:10:10.

[0069] Comparative Test Example 2 The difference between this comparative test example and application example 1 is that: no LiFePO4 is added; the positive electrode sheet is prepared after the manganese-based composite lithium supplement of comparative example 1 is placed in air with a relative humidity of 40% for 24 hours; the mass ratio of the manganese-based composite lithium supplement, acetylene black and PVDF is 80:10:10.

[0070] The positive electrode sheets prepared in Application Examples 1-3, Comparative Application Example 1, Test Examples 1-6, and Comparative Test Examples 1-2 were assembled and pressed with the negative electrode sheet, electrolyte, and 16mm polypropylene separator to obtain a 2032 coin cell. The electrolyte consisted of 1 mol / L lithium hexafluorophosphate and a mixed solvent, wherein the mixed solvent was a mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 1:1:1.

[0071] The 2032 coin cells prepared with the corresponding test cases 1-3 and comparative application example 1 positive electrode at 4.4V voltage and 0.5C / 0.5C current were subjected to the first charge-discharge test on the 2032 coin cells prepared with the corresponding test cases 1-6 and comparative test cases 1-2 positive electrode at 4.4V voltage and 0.5C / 0.5C current. The 200 charge-discharge cycle test was carried out on the corresponding test cases 1-3 and comparative application example 1 at 4.4V voltage and 1C / 1C. The results are shown in Table 1.

[0072] Table 1. Test Results of Lithium-ion Batteries

[0073] Table 1 shows that, based on the first charge-discharge specific capacity and cycle performance data of the 2032 coin cells prepared by the cathode sheets of Application Examples 1-3 and Comparative Application Examples 1-2, it can be seen that the lithium-ion batteries assembled with cathode sheets prepared by Application Examples 1-3 at 1%, 3%, and 5% of the cathode material mass have higher first discharge specific capacity and specific capacity retention rates before and after air exposure than the lithium-ion batteries assembled with cathode sheets prepared by Comparative Application Examples 1-2 without adding manganese-based composite lithium supplementers. This is because the addition of manganese-based composite lithium supplementers can play a good role in lithium supplementation, compensating for the irreversible lithium loss during the first charge-discharge process of lithium-ion batteries, and at the same time stabilizing the SEI interface and improving the long-term cycle stability of lithium-ion batteries.

[0074] Based on the initial charge-discharge specific capacity data of Test Examples 1-6 and Comparative Test Examples 1-2, it can be seen that the initial charge specific capacity of the lithium batteries prepared by the lithium replenishing agent in Test Examples 1-3 and Comparative Test Example 1 at 4.4V is 786.1, 780.4, 777.1 and 797.9 mAh / g, respectively; and the initial charge specific capacity after 24h of air exposure (relative humidity of 40%) is 703.2, 720.8, 741.4 and 671 mAh / g, respectively. This indicates that the manganese-based composite lithium replenishing agent prepared in this invention still has a high initial charge specific capacity after 24h of air exposure (relative humidity of 40%).

[0075] X-ray diffraction was used to analyze the phase composition of the manganese-based composite lithium supplement in Example 3 and the phase composition of the manganese-based composite lithium supplement in Example 3 after exposure to air (relative humidity of 40%) for 24 hours. The results are as follows. Figure 1 As shown in the figure, the phase structure of the manganese-based composite lithium supplement in Example 3 did not change before and after 24 hours of air exposure.

[0076] The surface morphology of the manganese-based composite lithium supplement prepared in Example 3 was analyzed using scanning electron microscopy, and the results are as follows: Figure 2 As shown in the figure, the material has a particle size of 500 nm to 3 µm and a relatively smooth surface morphology.

[0077] The coating state of the manganese-based composite lithium supplement prepared in Example 3 was analyzed using high-resolution transmission electron microscopy, and the results are as follows: Figure 3 As shown in the figure, in the manganese-based composite lithium supplement prepared in Example 3 of this invention, Li6MnO4 is coated with MnO.

[0078] The first charge-discharge tests of lithium-ion batteries prepared from the positive electrode sheets of Test Examples 3 and 6 at different voltages were conducted using the Blue Electric test system. The results are as follows: Figure 4 As shown in the figure, it can be seen that with the increase of voltage, the specific capacity of the lithium-ion battery made from the positive electrode obtained before and after air exposure of the manganese-based composite lithium supplement prepared in Example 3 is not significantly different.

[0079] In summary, the manganese-based composite lithium replenishing agent provided by this invention has good air stability, and the resulting positive electrode sheet has high initial charge specific capacity and high capacity retention.

[0080] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A manganese-based composite lithium supplement, comprising Li6MnO4 and a MnO layer coated on the surface of the Li6MnO4; wherein the mass ratio of MnO to Li6MnO4 is 1:(6~100).

2. The preparation method according to claim 1, characterized in that, The thickness of the MnO layer is 10~300nm.

3. A method for preparing the manganese-based composite lithium supplement according to claim 1 or 2, comprising: After mixing nanoscale manganese source and lithium source, the first sintering and the second sintering are carried out sequentially to obtain manganese-based composite lithium supplement; After the first sintering is completed, the resulting product is directly cooled from the temperature of the first sintering to the temperature of the second sintering for a second sintering; the cooling rate is 0.5~5℃ / min.

4. The preparation method according to claim 3, characterized in that, The molar ratio of manganese in the manganese source to lithium in the lithium source is 1:(5~8).

5. The preparation method according to claim 3 or 4, characterized in that, The particle size of the nanoscale manganese source is 300~800nm.

6. The preparation method according to claim 3 or 4, characterized in that, The lithium source includes one or more of LiOH·H2O, Li2CO3, LiNO3, Li2SO4, and CH3COOLi.

7. The preparation method according to claim 3 or 4, characterized in that, The nanoscale manganese source includes one or more of Mn3O4, MnCO3, MnO2, and MnO.

8. The preparation method according to claim 3, characterized in that, The first sintering temperature is 800~1200℃, and the first sintering time is 10~20h.

9. The preparation method according to claim 3, characterized in that, The second sintering temperature is 300~700℃, and the second sintering time is 15~30h.

10. A positive electrode sheet, comprising a current collector and a positive electrode material coated on the surface of the current collector, characterized in that, The positive electrode material includes a lithium active material, a manganese-based composite lithium supplement, a conductive agent, and a binder. The manganese-based composite lithium supplement is the manganese-based composite lithium supplement according to any one of claims 1 to 2 or the manganese-based composite lithium supplement prepared by the preparation method according to any one of claims 3 to 9.

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