Preparation method of high-voltage lithium cobalt oxide positive electrode material

Abstract

The invention relates to a preparation method of a high-voltage lithium cobalt oxide positive electrode material, which comprises the following steps of: 1, dissolving soluble salt of cobalt in a solvent, and uniformly stirring; adding a lithium cobalt oxide matrix, and stirring at constant temperature; adding ammonia water, mixing, adjusting the pH value, and continuously stirring; carrying out a co-precipitation reaction until xerogel is formed; drying in a vacuum drying box to obtain a cobalt hydroxide coated LCO material, namely Co (OH) 2amp; lithium cobalt oxide; step 2, mixing and sintering the cobalt hydroxide coated LCO material and a sodium salt to prepare a P2 phase-sodium cobaltate coated LCO material, namely P2-NaXCoO2amp; lithium cobalt oxide; 3, mixing and grinding the P2 phase-sodium cobaltate coated LCO material and a lithium source, performing low-temperature heat treatment, and cooling to room temperature; and washing and drying to obtain the lithium cobalt oxide positive electrode material coated with O2-phase lithium cobalt oxide, namely O2-LiCoO2amp, of which the coating layer is coated with O2-phase lithium cobalt oxide, and lithium cobalt oxide. The problems that in an existing scheme, the phase transformation process of an O3 phase and a mixed phase (H1-3) on the surface of the lithium cobalt oxide material cannot be effectively improved, meanwhile, the capacity exertion can be reduced, and the dynamic performance is reduced are solved.

Description

Technical Field

[0001] The present invention relates to the field of electrode materials, and in particular to a method for preparing a high-voltage lithium cobalt oxide positive electrode material. Background Art

[0002] O3-type lithium cobalt oxide (O3–LCO) cathode materials have ultra-high compaction density and volume energy density, making them extremely competitive in the field of portable electronic products. However, the actual capacity of commercially available O3–LiCoO2 (170mAh / g, 4.45V) is significantly lower than its theoretical capacity (274mAh / g), which limits the potential energy density improvement of LiCoO2 materials. Increasing the operating voltage to above 4.55V to extract more lithium ions is an effective strategy to promote LiCoO2 to achieve higher energy density. Under high voltage, O3–LCO faces irreversible harmful phase changes and severe surface side reactions, which directly destroy the stability of its lithium storage structure.

[0003] At high cut-off voltage (such as 4.55V), Co4 + -3d orbital and O 2- -2p orbital overlap, resulting in lattice oxygen oxidation to form active On-(n<2), leading to severe deterioration of the LCO crystal structure. When the charging voltage of LCO is higher than 4.55V, LixCoO2 undergoes a phase transition process from an octahedral three-phase (O3) to an octahedral single phase (O1) and an O3 mixed phase (H1-3). The transition between O3 and H1-3 causes degradation in the LCO bulk and surface, resulting in poor charge and discharge reversibility. This phase transition is accompanied by lattice sliding and partial collapse of the lattice structure. This leads to the accumulation of internal strain and subsequently to the formation of cracks and particle crushing. Therefore, suppressing harmful phase transitions is crucial to stabilizing the LCO structure and breaking through the cut-off voltage above 4.55V.

[0004] When LixCoO2 (0.5≤x≤1.0) is charged, electrons are extracted from the 3d orbital of Co, and the valence state of Co rises from +3 to +4. When x≤0.5, the electrons at the top of the Co-3d orbital are almost depleted, and the Fermi level drops significantly as the top of the O-2p band overlaps with the broadened Co-3d state. This leads to oxygen oxidation and the formation of peroxides or O2, which develop more violently on the surface due to the unstable structure and external stimuli. Therefore, oxygen evolution is considered to be a major challenge for LCO at voltages above 4.55V. At the same time, continued oxygen loss during cycling can trigger an irreversible phase transition (CoO2→Co3O4). The generated Co3O4 hinders Li +transport, resulting in a sharp increase in impedance and capacity attenuation. More importantly, the active substances O2 and O- radicals produced during the oxygen evolution process can react with organic components in the electrolyte, leading to side reactions and inducing a large number of cracks, thereby exacerbating chemical effects such as etching, corrosion and side reactions. Therefore, surface optimization is also considered to be an effective means to improve the stability of high-voltage LCO.

[0005] Under high delithiation conditions, the oxidized Co 4+ Reacting with carbonate electrolyte solvents, the interfacial resistance and polarization of the battery increased significantly. Charging above 4.55V triggers the transformation of LCO from the O3 phase to the mixed H1-3 hexagonal phase, making the surface lattice prone to structural collapse and crystal fragmentation.

[0006] The main bottleneck of the current high-voltage LCO is: harmful phase changes in the bulk phase lead to structural collapse and surface instability. The vast majority of existing patents are mainly about sintering doped metal / non-metallic elements, and then coating the surface to stabilize the bulk phase and surface structure. For example: Chinese invention patent: CN114050270A, discloses a preparation method for Al, Co, and Li co-coated high-voltage lithium cobalt oxide materials; Chinese invention patent: CN117525359A, discloses a fast ion conductor coated high-voltage high-specific capacity lithium cobalt oxide positive electrode and preparation method; Chinese invention patent: CN117727881A, proposes a fluorine-containing lithium-rich crystalline phase material coated high-voltage lithium cobalt oxide positive electrode material and its preparation method; CN109755530B, proposes a method for coating a high-voltage lithium cobalt oxide positive electrode material on the surface of a titanium-barium bimetallic oxide, etc. Most of these patents are achieved through direct solid-phase mixed sintering or liquid-phase coating followed by sintering. The surface coating layer is also concentrated on single / multiple metal oxides or fast ion conductors or core-shell structures. The effect is concentrated on using the coating layer to inhibit the contact between the positive electrode and the electrolyte and improve the cycle performance of high-voltage positive electrode materials. However, it cannot effectively improve the phase transformation process between the O3 phase and the mixed phase (H1-3) on the surface of the lithium cobalt oxide material. At the same time, there are side effects such as reduced capacity utilization and reduced kinetic performance.

[0007] It should be noted that the information disclosed in the above background technology section is only used to enhance the understanding of the background of the present disclosure, and therefore may include information that does not constitute the prior art known to ordinary technicians in the field. Summary of the invention

[0008] In view of the shortcomings of the above-mentioned prior art, the purpose of the present invention is to provide a method for preparing a high-voltage lithium cobalt oxide positive electrode material to solve the problem that the prior art cannot effectively improve the phase transition process between the O3 phase and the mixed phase (H1-3) on the surface of the lithium cobalt oxide material, and at the same time will reduce the capacity and kinetic performance.

[0009] To achieve the above object, the technical solution of the present invention is as follows:

[0010] A method for preparing a high-voltage lithium cobalt oxide positive electrode material;

[0011] The steps include:

[0012] Step 1, dissolving a soluble salt of cobalt in a solvent and stirring evenly; adding a lithium cobalt oxide matrix and stirring at a constant temperature; then adding ammonia water and mixing, adjusting the pH value, and stirring continuously; performing a coprecipitation reaction to form a dry gel; drying in a vacuum drying oven to obtain a cobalt hydroxide-coated LCO material, which is Co(OH)2 & lithium cobalt oxide;

[0013] Step 2, the LCO material coated with cobalt hydroxide is mixed with sodium salt and sintered to prepare a P2 phase-sodium cobaltate-coated LCO material, which is P2-NaXCoO2&lithium cobaltate;

[0014] Step 3, the P2 phase-sodium cobalt oxide coated LCO material is mixed with a lithium source and ground, then subjected to low-temperature heat treatment and cooled to room temperature; then washed and dried to obtain a lithium cobalt oxide positive electrode material having a coating layer of O2 phase lithium cobalt oxide coated thereon, namely O2-LiCoO2&lithium cobalt oxide.

[0015] A further technical solution is that in step 1:

[0016] The soluble salt of cobalt is a cobalt source, which is one or more of cobalt chloride (CoCl2·6H2O), cobalt nitrate (Co(NO3)2·6H2O), cobalt sulfate (CoSO4·7H2O), cobalt oxalate (CoC2O4·2H2O), and cobalt acetate (C4H6CoO4·4H2O);

[0017] The solvent is at least one of water, ethanol, n-butanol and isopropanol.

[0018] A further technical solution is that in step 1:

[0019] The lithium cobalt oxide matrix has the following general formula: Li1+xCo 1-y M y O2, 0.01≤x≤0.07, 0.01≤y≤0.07, M is selected from one or more of B, Mg, Al, Mn, Ni, Zr, Ti, Cu, Zn, Y, Ce, Sm, La, Mo, Nb, Sn, V, W; particle size D50 ranges from 4 to 20 um;

[0020] The molar ratio of cobalt in the soluble salt of cobalt to the molar ratio of cobalt in the lithium cobaltate substrate, i.e. the molar ratio of soluble salt cobalt to substrate cobalt, ranges from 0.01 / 1 to 0.1 / 1; the concentration of the soluble salt solution of cobalt is 0.5 to 2 mol / L;

[0021] The constant temperature stirring temperature is 50-90°C, and the stirring speed is 100-400 rpm.

[0022] A further technical solution is that in step 1:

[0023] Adjust the pH value to 9-11;

[0024] Dry in a vacuum drying oven at 80-150°C for 4-12 hours.

[0025] A further technical solution is that in step 2:

[0026] The sodium salt is selected from at least one of sodium carbonate, sodium bicarbonate, sodium acetate, sodium oxalate, sodium citrate, sodium nitrate and sodium hydroxide;

[0027] The molar ratio of Na in the sodium salt to the molar ratio of Co in Co(OH)2, i.e. the molar ratio of sodium salt sodium: molten salt cobalt, ranges from 0.5 / 1 to 0.8 / 1.

[0028] A further technical solution is that in step 2:

[0029] The sintering temperature is 700-1000°C; the sintering time is 5-20h, the heating rate is 2-7°C / min; the sintering atmosphere is oxygen.

[0030] A further technical solution is that in step 3:

[0031] The lithium source is one or more of lithium nitrate, lithium chloride, lithium hydroxide, lithium bromide, lithium iodide, lithium sulfide, lithium fluoride, lithium carbonate, and lithium sulfate;

[0032] The molar ratio of Li in the lithium salt to the molar ratio of sodium in the sodium salt, i.e., the molar ratio of lithium salt lithium: sodium salt sodium, ranges from 10 / 1 to 5 / 1.

[0033] A further technical solution is that in step 3:

[0034] The low-temperature heat treatment temperature is 200-400°C; the low-temperature heat treatment time is 3-20h, and the heating rate is 2-7°C / min; the drying temperature is 80-150°C, the drying time is 4-10h, and the drying atmosphere is oxygen or air.

[0035] Compared with the prior art, the beneficial technical effects of the present invention are as follows: (1) The present application adopts a coprecipitation reaction to generate Co(OH)2 in situ on the surface of the lithium cobalt oxide substrate. The in-situ coprecipitation alleviates the core-shell two-phase interface problem between the materials existing in the prior art solution. Compared with the direct dry mixing of the lithium cobalt oxide substrate and Co(OH)2, the former has a more uniform surface Co(OH)2 coating layer, and then sodium salt is added for mixed sintering, which helps to form a uniform P2 phase sodium cobalt oxide on the surface of the substrate.

[0036] (2) The present application utilizes in-situ co-precipitation to achieve uniform coating, which is beneficial for the in-situ growth of lithium cobalt oxide during the subsequent sintering process; during the co-precipitation process, the thickness of the coating layer can be controlled by controlling the amount of metal salt added; thereby obtaining a material with optimal performance.

[0037] (3) The present application adopts a low-temperature molten salt ion exchange method to change the surface P2 phase sodium cobalt oxide into O2 phase lithium cobalt oxide, which is simpler to operate than the conventional aqueous phase ion exchange method.

[0038] (4) The present application uses a low-temperature molten salt ion exchange method to obtain a surface O2-phase lithium cobalt oxide coating layer. In addition to inhibiting the contact between the positive electrode material and the electrolyte as a coating layer, improving the material's tolerance to HF, and preventing the occurrence of side reactions, compared to other coating materials, while increasing the capacity, as an O2-phase lithium cobalt oxide coating layer, it can also effectively reduce the phase transition process of the O3 phase and the mixed phase (H1-3) on the surface of the lithium cobalt oxide material, inhibit the instability of the surface structure, and improve the high-voltage cycle performance of the positive electrode material. This effectively solves the problem that conventional coatings reduce capacity, while conventional lithium cobalt oxide surface coatings have serious O3 phase transition problems.

[0039] (5) The coating layer and the matrix material of the present application are both lithium cobalt oxide, which can improve the capacity relative to the electrochemically inactive coating layer, while effectively reducing the core-shell interface problem between the coating layer and the matrix material.

[0040] (6) The present application reduces HF corrosion to the material during the cycle process, reduces side reactions with the electrolyte and the generation of CEI, thereby significantly improving the high-voltage cycle life and rate performance of the positive electrode material. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 A scanning SEM comparison diagram of the sample in step 1 of embodiment 1 of the present invention and the sample in step 1 of comparative example 1 is shown.

[0042] Figure 2 The scan image of Example 1 after ion exchange in step 3 is shown.

[0043] Figure 3 The XRD comparison diagram of P2-Na0.7CoO2 & lithium cobalt oxide obtained in step 2 of Example 1 and the final product O2-LiCoO2 & lithium cobalt oxide obtained in step 3 of Example 1 is shown.

[0044] Figure 4 A dQ / dV comparison chart of Example 1 and Comparative Example 6 samples after 50 cycles of 1C buckling is shown. DETAILED DESCRIPTION

[0045] In order to make the purpose, technical scheme and advantages of the present invention clearer, the device proposed by the present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. According to the following description, the advantages and features of the present invention will be clearer. It should be noted that the accompanying drawings adopt a very simplified form and use non-precise proportions, which are only used to conveniently and clearly assist in explaining the purpose of the embodiments of the present invention. In order to make the purpose, features and advantages of the present invention more obvious and easy to understand, please refer to the accompanying drawings. It should be noted that the structure, proportion, size, etc. illustrated by the accompanying drawings of this specification are only used to match the content disclosed in the specification for people familiar with this technology to understand and read, and are not used to limit the limiting conditions for the implementation of the present invention, so they have no technical substantive significance. Any modification of the structure, change of the proportional relationship or adjustment of the size, without affecting the effect that the present invention can produce and the purpose that can be achieved, should still fall within the scope of the technical content disclosed by the present invention.

[0046] The preparation method of high-voltage lithium cobalt oxide positive electrode material comprises the following steps:

[0047] Step 1, dissolve the soluble salt of cobalt in a solvent and stir evenly. The soluble salt of cobalt is a cobalt source, which is one or more of cobalt chloride (CoCl2·6H2O), cobalt nitrate (Co(NO3)2·6H2O), cobalt sulfate (CoSO4·7H2O), cobalt oxalate (CoC2O4·2H2O), and cobalt acetate (C4H6CoO4·4H2O). The solvent is at least one of water, ethanol, n-butanol, and isopropanol.

[0048] Add lithium cobalt oxide matrix and stir at constant temperature. The lithium cobalt oxide matrix has the following general formula: Li1+xCo 1-y M y O2, 0.01≤x≤0.07, 0.01≤y≤0.07, M is selected from one or more of B, Mg, Al, Mn, Ni, Zr, Ti, Cu, Zn, Y, Ce, Sm, La, Mo, Nb, Sn, V, W. The particle size D50 ranges from 4 to 20 μm. The molar ratio of cobalt in the soluble salt of cobalt to the molar ratio of cobalt in the lithium cobaltate substrate, i.e., the molar ratio of soluble salt cobalt to substrate cobalt, ranges from 0.01 / 1 to 0.1 / 1; the concentration of the soluble salt solution of cobalt is 0.5 to 2 mol / L. The constant temperature stirring temperature is 50 to 90°C, and the stirring speed is 100 to 400 rpm.

[0049] Add ammonia water and mix, adjust the pH value, and stir continuously. Adjust the pH value to 9-11. Perform a coprecipitation reaction until a dry gel is formed. Dry in a vacuum drying oven at a temperature of 80-150°C for 4-12 hours. Obtain a cobalt hydroxide-coated LCO material, which is Co(OH)2 & lithium cobalt oxide.

[0050] By controlling the coprecipitation pH value and stirring speed, the in-situ growth of the Co(OH)2 coating layer on the lithium cobalt oxide substrate is achieved. The pH value is adjusted to the set range of 9 to 11 to avoid the coprecipitation reaction causing the cobalt hydroxide to grow too slowly when the pH value is low, which is not conducive to the complete reaction of the cobalt salt. When the pH value is high, the coprecipitation reaction causes the cobalt hydroxide to grow too fast, and it is easy to deposit, the coating layer is thick, and it is not conducive to uniform coating on the substrate surface.

[0051] During the constant temperature stirring process, the stirring speed also needs to be limited to a certain range. Avoiding a high stirring speed will make it difficult for cobalt hydroxide to grow on the substrate surface. At a low stirring speed, cobalt hydroxide is prone to flaky deposition and uneven deposition, which will affect the next step of sintering reaction with sodium salt.

[0052] Step 2: The cobalt hydroxide-coated LCO material is mixed with a sodium salt and sintered to prepare a P2 phase-sodium cobaltate-coated LCO material, namely, P2-NaXCoO2&lithium cobaltate.

[0053] The sodium salt is selected from at least one of sodium carbonate, sodium bicarbonate, sodium acetate, sodium oxalate, sodium citrate, sodium nitrate and sodium hydroxide. The molar ratio of Na in the sodium salt to Co in Co(OH)2, i.e. the molar ratio of sodium salt sodium: soluble salt cobalt, is between 0.5 / 1 and 0.8 / 1.

[0054] The sintering temperature is 700-1000° C., the sintering time is 5-20 hours, the heating rate is 2-7° C. / min, and the sintering atmosphere is oxygen.

[0055] By controlling the sodium salt type and sodium / cobalt ratio, as well as the sintering temperature, the surface Co(OH)2 can be converted into P2-NaXCoO2 without changing the lithium cobalt oxide matrix material.

[0056] By setting the molar ratio of sodium salt sodium: molten salt cobalt and setting the sintering temperature in the range of 700-1000°C, it is possible to avoid incomplete reaction of too low sodium salt with cobalt hydroxide and to avoid too high sodium salt easily entering the lithium cobalt oxide matrix and easily changing the matrix properties.

[0057] Step 3, the P2 phase-sodium cobaltate coated LCO material is mixed and ground with a lithium source and then subjected to low temperature heat treatment, the low temperature heat treatment temperature is 200-400°C, the low temperature heat treatment time is 3-20h, the heating rate is 2-7°C / min, and then cooled to room temperature. The lithium source is one or more of lithium nitrate, lithium chloride, lithium hydroxide, lithium bromide, lithium iodide, lithium sulfide, lithium fluoride, lithium carbonate, and lithium sulfate. The molar ratio of Li in the lithium salt to the molar ratio of sodium in the sodium salt, i.e., the molar ratio of lithium salt lithium: sodium salt sodium, is in the range of 10 / 1 to 5 / 1.

[0058] After washing and drying, the drying temperature is 80-150° C., the drying time is 4-10 hours, and the drying atmosphere is oxygen or air, to obtain a lithium cobalt oxide positive electrode material with a coating layer of O2-phase lithium cobalt oxide, which is O2-LiCoO2&lithium cobalt oxide.

[0059] Low-temperature heat treatment realizes molten salt ion exchange, controls the low-temperature heat treatment temperature, realizes the conversion of surface P2-NaXCoO2 into O2-LiCoO2, and avoids the formation of O3-LiCoO2 on the high-temperature surface. The low-temperature heat treatment temperature has a certain range. If the low-temperature heat treatment temperature is too low, the moisture content of the positive electrode material will be high. At the same time, if the low-temperature heat treatment temperature is too high, the Li / Na exchange will become intense, which will lead to a decrease in the stability of the generated O2 phase structure. And the O2 phase structure itself is a thermodynamically unstable metastable structure. At about 350°C, the O2 phase structure will undergo a phase transition to a more thermodynamically stable O3 phase structure, and at 450°C, the O2 phase structure will complete the transformation to the O3 phase structure, so the heat treatment temperature range needs to be controlled at 200-400°C.

[0060] The molar ratio of lithium salt lithium: sodium salt sodium needs to be controlled within a certain range. First, the Li / Na molar ratio of the lithium source and the P2 sodium precursor should not be too high. If the lithium source content is too high, a large amount of impurity anions will remain in the prepared O2-type lithium-rich manganese-based material, which will seriously hinder the transmission of lithium ions and penetrate into the matrix, affecting its rate performance and coulombic efficiency. The Li / Na ratio should not be too low either. If the lithium source content is too low, the Li / Na exchange of the prepared O2-type lithium-rich manganese-based material will be incomplete, and P2-NaXCoO2 will still exist on the surface, resulting in a decrease in its capacity.

[0061] The ion exchange in this application is a molten salt ion exchange. In this application, the P2 phase-sodium cobaltate coated LCO material is evenly mixed with lithium nitrate, wherein the molar ratio of lithium salt lithium: sodium salt sodium is 8 / 1, mainly the P2 phase-sodium cobaltate and lithium nitrate undergo molten salt reaction, specifically the sodium ions in the sodium cobaltate and the lithium ions in the lithium nitrate undergo ion exchange. Among them, the lithium ion concentration is 8 times the sodium ion concentration. Under a large concentration difference and a suitable reaction temperature, sodium and lithium are exchanged, and finally the P2 phase-sodium cobaltate is converted into O2 phase lithium cobaltate.

[0062] The present application is explained below through a number of embodiments and comparative examples:

[0063] Embodiment 1:

[0064] The preparation method of high-voltage lithium cobalt oxide positive electrode material comprises the following steps:

[0065] Step 1, weigh cobalt sulfate (CoSO4·7H2O) and dissolve it in 500 mL of water and stir evenly to prepare a solution with a cobalt metal content of 1 mol / L;

[0066] Then, Lithium ions with a particle size of 17 μm were added to the solution at a molar ratio of 0.04 / 1 for the dissolved cobalt and the matrix cobalt. 1.02 Co 0.95 Al 0.05 O2 lithium cobalt oxide matrix.

[0067] While the mixture was stirred at a constant temperature of 80°C and 200 rpm, ammonia water was slowly added dropwise to adjust the pH value of the solution to 10.5, and the mixture was stirred continuously until the mixture completely turned into a dry gel.

[0068] The obtained dry gel was placed in a vacuum drying oven at 100° C. and dried for 8 h to obtain a cobalt hydroxide-coated LCO material, which is Co(OH)2&lithium cobaltate.

[0069] Step 2: The obtained cobalt hydroxide-coated LCO material is uniformly mixed with sodium carbonate in proportion, wherein the molar ratio of sodium salt sodium: molten salt cobalt is 0.7 / 1, and then the temperature is raised to 875°C at 3°C / min in an oxygen atmosphere and sintered for 10 hours to obtain a P2 phase-sodium cobaltate-coated LCO material, which is P2-Na 0.7 CoO2 & LiCoO 4.

[0070] Step 3: Evenly mix the P2 phase-sodium cobaltate-coated LCO material with lithium nitrate, wherein the molar ratio of lithium salt lithium: sodium salt sodium is 8 / 1. Then, heat to 280°C at 3°C / min in an air atmosphere, sinter for 8h, cool to room temperature, wash with water, and dry at 120°C in a drying oven for 6h to obtain the final product O2-LiCoO2&lithium cobaltate.

[0071] Embodiment 2:

[0072] The difference between Example 2 and Example 1 is that:

[0073] Step 1, weigh cobalt sulfate (CoSO4·7H2O) and dissolve it in 500 mL of water and stir evenly to prepare a solution with a cobalt metal content of 1 mol / L;

[0074] Then, Lithium ions with a particle size of 17 μm were added to the solution at a molar ratio of 0.08 / 1 for the dissolved cobalt and the matrix cobalt. 1.02 Co 0.95 Al 0.05 O2 lithium cobalt oxide matrix.

[0075] While the mixture was stirred at a constant temperature of 80°C and 200 rpm, ammonia water was slowly added dropwise to adjust the pH value of the solution to 10.5, and the mixture was stirred continuously until the mixture completely turned into a dry gel.

[0076] The obtained dry gel was placed in a vacuum drying oven at 100° C. and dried for 8 h to obtain a cobalt hydroxide-coated LCO material, which is Co(OH)2&lithium cobaltate.

[0077] Embodiment 3:

[0078] The difference between Example 3 and Example 1 is that:

[0079] Step 1, weigh cobalt sulfate (CoSO4·7H2O) and dissolve it in 500 mL of water and stir evenly to prepare a solution with a cobalt metal content of 1 mol / L;

[0080] Then, Lithium ions with a particle size of 17 μm were added to the solution at a molar ratio of 0.02 / 1 for the dissolved cobalt and the matrix cobalt. 1.02 Co 0.95 Al 0.05 O2 lithium cobalt oxide matrix.

[0081] While the mixture was stirred at a constant temperature of 80°C and 200 rpm, ammonia water was slowly added dropwise to adjust the pH value of the solution to 10.5, and the mixture was stirred continuously until the mixture completely turned into a dry gel.

[0082] The obtained dry gel was placed in a vacuum drying oven at 100° C. and dried for 8 h to obtain a cobalt hydroxide-coated LCO material, which is Co(OH)2&lithium cobaltate.

[0083] Embodiment 4:

[0084] The difference between Example 4 and Example 1 is that:

[0085] Step 2: The obtained cobalt hydroxide-coated LCO material is uniformly mixed with sodium carbonate in proportion, wherein the molar ratio of sodium salt sodium: molten salt cobalt is 0.75 / 1, and then the temperature is raised to 875°C at 3°C / min in an oxygen atmosphere and sintered for 10 hours to obtain a P2 phase-sodium cobaltate-coated LCO material, which is P2-Na 0.7 CoO2 & LiCoO 4.

[0086] Embodiment 5:

[0087] The difference between Example 5 and Example 1 is that:

[0088] Step 2: The obtained cobalt hydroxide-coated LCO material is uniformly mixed with sodium carbonate in proportion, wherein the molar ratio of sodium salt sodium: molten salt cobalt is 0.6 / 1, and then the temperature is raised to 875°C at 3°C / min in an oxygen atmosphere and sintered for 10 hours to obtain a P2 phase-sodium cobaltate-coated LCO material, which is P2-Na 0.7 CoO2 & LiCoO 4.

[0089] Embodiment 6:

[0090] The difference between Example 6 and Example 1 is that:

[0091] Step 3: Evenly mix the P2 phase-sodium cobaltate-coated LCO material with lithium nitrate, wherein the molar ratio of lithium salt lithium: sodium salt sodium is 9 / 1. Then, heat to 280°C at 3°C / min in an air atmosphere, sinter for 8h, cool to room temperature, wash with water, and dry at 120°C in a drying oven for 6h to obtain the final product O2-LiCoO2&lithium cobaltate.

[0092] Embodiment 7:

[0093] The difference between Example 7 and Example 1 is that:

[0094] Step 3: Evenly mix the P2 phase-sodium cobaltate-coated LCO material with lithium nitrate, wherein the molar ratio of lithium salt lithium: sodium salt sodium is 7 / 1. Then, heat to 280°C at 3°C / min in an air atmosphere, sinter for 8 hours, cool to room temperature, wash with water, and dry at 120°C in a drying oven for 6 hours to obtain the final product O2-LiCoO2 & lithium cobaltate.

[0095] Embodiment 8:

[0096] The difference between Example 8 and Example 1 is that:

[0097] Step 1, weigh cobalt chloride (CoCl2·6H2O) and dissolve it in 500 mL of water and stir evenly to prepare a solution with a cobalt metal content of 1 mol / L;

[0098] Then, Lithium ions with a particle size of 17 μm were added to the solution at a molar ratio of 0.04 / 1 for the dissolved cobalt and the matrix cobalt. 1.02 Co 0.95 Al 0.05 O2 lithium cobalt oxide matrix.

[0099] While the mixture was stirred at a constant temperature of 80°C and 200 rpm, ammonia water was slowly added dropwise to adjust the pH value of the solution to 10.5, and the mixture was stirred continuously until the mixture completely turned into a dry gel.

[0100] The obtained dry gel was placed in a vacuum drying oven at 100° C. and dried for 8 h to obtain a cobalt hydroxide-coated LCO material, which is Co(OH)2&lithium cobaltate.

[0101] Embodiment 9:

[0102] The difference between Example 9 and Example 1 is that:

[0103] Step 2: The obtained cobalt hydroxide coated LCO material is evenly mixed with sodium oxalate in proportion, wherein the molar ratio of sodium salt sodium: molten salt cobalt is 0.7 / 1, and then the temperature is raised to 875°C at 3°C / min in an oxygen atmosphere and sintered for 10 hours to obtain a P2 phase-sodium cobaltate coated LCO material, which is P2-Na 0.7 CoO2 & LiCoO 4.

[0104] Embodiment 10:

[0105] The difference between Example 10 and Example 1 is that:

[0106] Step 3: Evenly mix the P2 phase-sodium cobaltate-coated LCO material with lithium chloride, wherein the molar ratio of lithium salt lithium: sodium salt sodium is 8 / 1. Then, heat to 280°C at 3°C / min in an air atmosphere, sinter for 8h, cool to room temperature, wash with water, and dry at 120°C in a drying oven for 6h to obtain the final product O2-LiCoO2&lithium cobaltate.

[0107] Comparative Example 1:

[0108] The difference between Comparative Example 1 and Example 1 is that:

[0109] Step 1, weigh cobalt sulfate (CoSO4·7H2O) and dissolve it in 500 mL of water and stir evenly to prepare a solution with a cobalt metal content of 1 mol / L;

[0110] Then, Lithium ions with a particle size of 17 μm were added to the solution at a molar ratio of 0.04 / 1 for the dissolved cobalt and the matrix cobalt. 1.02 Co 0.95 Al 0.05 O2 lithium cobalt oxide matrix.

[0111] While the mixture was stirred at a constant temperature of 80°C and 200 rpm, ammonia water was slowly added dropwise to adjust the pH value of the solution to 11.5, and the mixture was stirred continuously until the mixture completely turned into a dry gel.

[0112] The obtained dry gel was placed in a vacuum drying oven at 100° C. and dried for 8 h to obtain a cobalt hydroxide-coated LCO material, which is Co(OH)2&lithium cobaltate.

[0113] Comparative Example 2:

[0114] The difference between Comparative Example 2 and Example 1 is that:

[0115] Step 2: The obtained cobalt hydroxide-coated LCO material is uniformly mixed with sodium carbonate in proportion, wherein the molar ratio of sodium salt sodium: molten salt cobalt is 0.9 / 1, and then the temperature is raised to 875°C at 3°C / min in an oxygen atmosphere and sintered for 10 hours to obtain a P2 phase-sodium cobaltate-coated LCO material, which is P2-Na 0.7 CoO2 & LiCoO 4.

[0116] Comparative Example 3:

[0117] The difference between Comparative Example 3 and Example 1 is that:

[0118] Step 2: The obtained cobalt hydroxide-coated LCO material is uniformly mixed with sodium carbonate in proportion, wherein the molar ratio of sodium salt sodium: molten salt cobalt is 0.7 / 1, and then the temperature is raised to 1050°C at 3°C / min in an oxygen atmosphere and sintered for 10 hours to obtain a P2 phase-sodium cobaltate-coated LCO material, which is P2-Na 0.7 CoO2 & LiCoO 4.

[0119] Comparative Example 4:

[0120] The difference between Comparative Example 4 and Example 1 is that:

[0121] Step 3: Evenly mix the P2 phase-sodium cobaltate-coated LCO material with lithium nitrate, wherein the molar ratio of lithium salt lithium: sodium salt sodium is 3 / 1. Then, heat to 280°C at 3°C / min in an air atmosphere, sinter for 8 hours, cool to room temperature, wash with water, and dry at 120°C in a drying oven for 6 hours to obtain the final product O2-LiCoO2 & lithium cobaltate.

[0122] Comparative Example 5:

[0123] The difference between Comparative Example 5 and Example 1 is that:

[0124] Step 3: Evenly mix the P2 phase-sodium cobaltate-coated LCO material with lithium nitrate, wherein the molar ratio of lithium salt lithium: sodium salt sodium is 12 / 1. Then, heat to 280°C at 3°C / min in an air atmosphere, sinter for 8h, cool to room temperature, wash with water, and dry at 120°C in a drying oven for 6h to obtain the final product O2-LiCoO2&lithium cobaltate.

[0125] Comparative Example 6:

[0126] The difference between Comparative Example 6 and Example 1 is that:

[0127] Step 3: Evenly mix the P2 phase-sodium cobalt oxide coated LCO material with lithium nitrate, wherein the molar ratio of lithium salt lithium: sodium salt sodium is 8 / 1. Then, heat to 500°C at 3°C / min in an air atmosphere, sinter for 8h, cool to room temperature, wash with water, and dry at 120°C in a drying oven for 6h to obtain the final product O2-LiCoO2 & lithium cobalt oxide.

[0128] This application uses O2-LiCoO2 & lithium cobalt oxide as the positive electrode to assemble button cells for electrochemical performance testing. The specific steps are as follows:

[0129] The positive electrode active material O2-LiCoO2 & lithium cobalt oxide, carbon black SuperP, and polyvinylidene fluoride (Solvey5130) were mixed in a mass ratio of 90:5:5 to form a slurry, which was evenly coated on the surface of the aluminum foil to obtain a positive electrode. The lithium sheet was used as the negative electrode, and 1 mol / L lithium hexafluorophosphate ethylene carbonate (EC) and dimethyl carbonate (DMC) solution was used as the electrolyte, where the mass ratio of EC to DMC was 1:1, and assembled in a glove box to obtain a lithium-ion battery.

[0130] The cycle performance of lithium-ion batteries was tested using an electrochemical tester at a temperature of 25°C, with the capacity retention rate measured at 3-4.6V, 0.1C current density and 50 cycles after the first charge and discharge at 1C.

[0131] Table 1 shows the electrochemical performance test results of each embodiment and comparative example:

[0132] (1) From Example 1 to Comparative Example 5, there is an O2 phase lithium cobalt oxide surface coating layer on the surface. Comparative Example 6 has only one conventional O3 phase lithium cobalt oxide surface coating layer. Therefore, the cycle stability of Comparative Example 6 is the worst at 93.1%, which proves that the presence of the O2 phase lithium cobalt oxide surface coating layer is beneficial to the improvement of the cycle stability of the lithium cobalt oxide material.

[0133] (2) Compared with Example 1, Example 1, Example 2 and Example 3 have a pH value of 11.5 during the coprecipitation process, which is relatively high. During the subsequent coprecipitation, a thicker coating layer appears, and the coating is uneven on the substrate surface, resulting in a decrease in relative capacity and stability.

[0134] (3) Compared with Comparative Example 2, in Example 2, the molar ratio of sodium salt sodium: soluble cobalt salt is 0.9 / 1. In the process of converting cobalt hydroxide into sodium cobaltate, sodium salt is excessive, which leads to an increase in lithium salt in the next step of lithium-sodium ion exchange and an increase in residual alkali on the surface of the final product, which ultimately leads to a decrease in product capacity, initial effect and stability.

[0135] (4) Compared with Comparative Example 4, in Example 1, Example 6 and Example 7, during the low-temperature molten salt ion exchange in Comparative Example 4, the molar ratio of lithium salt lithium: sodium salt sodium was 3 / 1, and the amount of lithium salt added was too small, resulting in the inability to completely convert the surface sodium cobaltate into O2 phase lithium cobaltate, and the presence of some P2 phase sodium cobaltate, resulting in a decrease in capacity and stability.

[0136] (5) Compared with Comparative Example 5, in Example 5, during the low-temperature molten salt ion exchange, the molar ratio of lithium salt lithium: sodium salt sodium was 12 / 1, and the amount of lithium salt added was too much. Although the surface sodium cobaltate could be completely converted into O2 phase lithium cobaltate, it resulted in excessive lithium salt enrichment on the surface of the material, high residual alkali, and poor final capacity, first effect and stability.

[0137] (6) Compared with Example 1 and Comparative Example 3, during the sintering process of surface cobalt hydroxide and sodium salt in Comparative Example 3, the sintering temperature was too high, resulting in some sodium ions entering the interior of the lithium cobalt oxide matrix in the form of doping, resulting in a relative excess of lithium salt during subsequent ion exchange, and at the same time, the final O2 phase lithium cobalt oxide coating layer was reduced, ultimately resulting in poor capacity, initial efficiency and stability.

[0138] (7) In Examples 8, 9 and 10, the cobalt salt, sodium salt and lithium salt are changed relative to those in Example 1, but the performance is better than that of the comparative example, which proves that the process has material universality.

[0139]

[0140] Table 1

[0141] Result analysis:

[0142] Figure 1 A scanning SEM comparison diagram of the sample in step 1 of embodiment 1 of the present invention and the sample in step 1 of comparative example 1 is shown. Figure 2 The scan image of Example 1 after ion exchange in step 3 is shown. Figure 3 The P2-Na obtained in step 2 of Example 1 is shown. 0.7 XRD comparison diagram of CoO2 & lithium cobalt oxide and the final product O2-LiCoO2 & lithium cobalt oxide obtained in step 3 of Example 1. Figure 4 The dQ / dV comparison diagram of Example 1 and Comparative Example 6 after 50 cycles of 1C buckling power is shown. Figure 1-Figure 4 As shown,

[0143] Figure 1 The left side is the SEM image of the sample in step 1 of Example 1. Figure 1 The right side of the figure is a scanning SEM image of the sample of step 1 of comparative example 1. Compared with comparative example 1, the coprecipitation pH of embodiment 1 is 10.5, while the coprecipitation pH of comparative example 1 is 11.5. Figure 1 It can be seen from the two scanning images that when the pH is high, the coprecipitation reaction causes cobalt hydroxide to grow too fast and deposition occurs at the same time. The coating layer is thick and the coating on the substrate surface is uneven, which confirms that the coprecipitation pH needs to be controlled in a suitable range.

[0144] from Figure 2 It can be seen that Example 1 has a uniform coating layer on the surface after ion exchange in step 3.

[0145] from Figure 3 From the XRD comparison diagram, it can be seen that in Example 1, after step 2, the characteristic peak of sodium electric layer oxygen P2 phase (002) appears on the XRD, confirming that sodium cobaltate is formed on the surface of the lithium cobaltate matrix. After step 3, the surface P2 phase sodium cobaltate disappears after low-temperature molten salt ion exchange, and sodium and lithium are completely exchanged to form a lithium cobaltate phase.

[0146] The low-temperature sintering temperature in step 3 has a certain range. If the temperature is too high, the Li / Na exchange will become intense, which will reduce the stability of the O2 phase lithium cobalt oxide generated on the surface. The O2 phase structure itself is a thermodynamically unstable metastable structure. At about 350°C, the O2 phase structure will undergo a phase transition to a more thermodynamically stable O3 phase structure, and at 450°C, the O2 structure will complete the transformation to the O3 structure.

[0147] Therefore, when the comparative example 6 is subjected to low-temperature molten salt ion exchange at 500°C, it has been completely transformed into a conventional O3 phase lithium cobalt oxide surface coating layer. Compared with the O2 phase lithium cobalt oxide surface coating layer of Example 1, this coating layer still has a phase transformation process between the O3 phase and the mixed phase (H1-3) during the high voltage electrochemical cycle.

[0148] When the charging voltage of LCO is higher than 4.55V, LixCoO2 undergoes a phase transition process from octahedral three-phase (O3) to octahedral single phase (O1) and O3 mixed phase (H1-3). The transition between O3 and H1-3 causes degradation in the LCO bulk and surface, resulting in poor charge and discharge reversibility. On the conventional lithium cobalt oxide dQ / dV graph, it can be seen that there is a typical O3 phase between 3.8V and 3.9V. Then, as the depth of discharge increases, an O3 mixed phase (H1-3) will appear at around 4.5V. This mixed phase is unstable and has poor electrochemical reversibility. After that, an irreversible phase change CoO2→Co3O4 occurs during repeated cycles, resulting in a sharp increase in impedance and the overall dQ / dV will shift toward the high voltage direction.

[0149] like Figure 4 As shown, after 50 cycles of Example 1, the typical O3 phase between 3.8V and 3.9V and the mixed phase (H1-3) at about 4.5V can still be clearly seen, while after 50 cycles of Comparative Example 6, the O3 phase and the mixed phase (H1-3) become less obvious and shift toward the high voltage direction at the same time, confirming that the surface-coated O2 phase lithium cobalt oxide layer of Example 1 can effectively inhibit surface phase change, stabilize the structure, and improve the reversibility of electrochemical cycling compared to the conventional O3 phase lithium cobalt oxide layer, ultimately improving the stability of the material in high-voltage cycling.

[0150] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0151] The above-mentioned embodiments only express several implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A method for preparing a high-voltage lithium cobalt oxide positive electrode material, characterized in that: The steps include: Step 1, dissolving a soluble salt of cobalt in a solvent and stirring evenly; adding a lithium cobalt oxide matrix and stirring at a constant temperature; then adding ammonia water and mixing, adjusting the pH value, and stirring continuously; performing a coprecipitation reaction to form a dry gel; drying in a vacuum drying oven to obtain a cobalt hydroxide-coated LCO material, which is Co(OH)2 & lithium cobalt oxide; Step 2, the LCO material coated with cobalt hydroxide is mixed with sodium salt and sintered to prepare a P2 phase-sodium cobaltate-coated LCO material, which is P2-NaXCoO2&lithium cobaltate; Step 3, the P2 phase-sodium cobalt oxide coated LCO material is mixed with a lithium source and ground, then subjected to low-temperature heat treatment and cooled to room temperature; then washed and dried to obtain a lithium cobalt oxide positive electrode material having a coating layer of O2 phase lithium cobalt oxide coated thereon, namely O2-LiCoO2&lithium cobalt oxide.

2. The method for preparing a high-voltage lithium cobalt oxide positive electrode material according to claim 1, characterized in that: In step 1: The soluble salt of cobalt is a cobalt source, which is one or more of cobalt chloride (CoCl2·6H2O), cobalt nitrate (Co(NO3)2·6H2O), cobalt sulfate (CoSO4·7H2O), cobalt oxalate (CoC2O4·2H2O), and cobalt acetate (C4H6CoO4·4H2O); The solvent is at least one of water, ethanol, n-butanol and isopropanol.

3. The method for preparing a high-voltage lithium cobalt oxide positive electrode material according to claim 2, characterized in that: In step 1: The lithium cobalt oxide matrix has the following general formula: Li1+xCo 1-y M y O2, 0.01≤x≤0.07, 0.01≤y≤0.07, M is selected from one or more of B, Mg, Al, Mn, Ni, Zr, Ti, Cu, Zn, Y, Ce, Sm, La, Mo, Nb, Sn, V, W; particle size D50 ranges from 4 to 20 um; The molar ratio of cobalt in the soluble salt of cobalt to the molar ratio of cobalt in the lithium cobaltate substrate, i.e. the molar ratio of soluble salt cobalt to substrate cobalt, ranges from 0.01 / 1 to 0.1 / 1; the concentration of the soluble salt solution of cobalt is 0.5 to 2 mol / L; The constant temperature stirring temperature is 50-90°C, and the stirring speed is 100-400 rpm.

4. The method for preparing a high-voltage lithium cobalt oxide positive electrode material according to claim 3, characterized in that: In step 1: Adjust the pH value to 9-11; Dry in a vacuum drying oven at 80-150°C for 4-12 hours.

5. The method for preparing a high-voltage lithium cobalt oxide positive electrode material according to claim 1, characterized in that: In step 2: The sodium salt is selected from at least one of sodium carbonate, sodium bicarbonate, sodium acetate, sodium oxalate, sodium citrate, sodium nitrate and sodium hydroxide; The molar ratio of Na in the sodium salt to the molar ratio of Co in Co(OH)2, i.e. the molar ratio of sodium salt sodium: molten salt cobalt, ranges from 0.5 / 1 to 0.8 / 1.

6. The method for preparing a high-voltage lithium cobalt oxide positive electrode material according to claim 5, characterized in that: In step 2: The sintering temperature is 700-1000°C; the sintering time is 5-20h, and the heating rate is 2-7°C / min; The sintering atmosphere is oxygen.

7. The method for preparing a high-voltage lithium cobalt oxide positive electrode material according to claim 1, characterized in that: In step 3: The lithium source is one or more of lithium nitrate, lithium chloride, lithium hydroxide, lithium bromide, lithium iodide, lithium sulfide, lithium fluoride, lithium carbonate, and lithium sulfate; The molar ratio of Li in the lithium salt to the molar ratio of sodium in the sodium salt, i.e., the molar ratio of lithium salt lithium: sodium salt sodium, ranges from 10 / 1 to 5 / 1.

8. The method for preparing a high-voltage lithium cobalt oxide positive electrode material according to claim 7, characterized in that: In step 3: The low-temperature heat treatment temperature is 200-400°C; the low-temperature heat treatment time is 3-20h, and the heating rate is 2-7°C / min; the drying temperature is 80-150°C, the drying time is 4-10h, and the drying atmosphere is oxygen or air.

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