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

Abstract

The application relates to a preparation method of a high-voltage lithium cobalt oxide positive electrode material, which comprises the following steps: step 1, dissolving a soluble salt of cobalt in a solvent and stirring uniformly; adding a lithium cobalt oxide matrix and stirring at a constant temperature; mixing with ammonia water, adjusting the pH value and continuously stirring; carrying out a co-precipitation reaction until a dry gel is formed; drying in a vacuum drying box to obtain a cobalt hydroxide coated LCO material, which is Co(OH)2 & lithium cobalt oxide; step 2, mixing and sintering the cobalt hydroxide coated LCO material with a sodium salt to prepare a P2 phase-sodium cobaltate coated LCO material, which is P2-NaXCoO2 & lithium cobalt oxide; step 3, mixing and grinding the P2 phase-sodium cobaltate coated LCO material with a lithium source and then carrying out low-temperature heat treatment, cooling to room temperature, and then carrying out water washing and drying to obtain a lithium cobalt oxide positive electrode material with an O2 phase lithium cobalt oxide coated layer, which is O2-LiCoO2 & lithium cobalt oxide. The method solves the problem that the existing scheme cannot effectively improve the phase transition process of the O3 phase and the mixed phase (H1-3) on the surface of the lithium cobalt oxide material, and simultaneously reduces the capacity performance and the kinetic performance.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of electrode materials, and particularly relates to a preparation method of high-voltage lithium cobalt oxide positive electrode material. BACKGROUND

[0002] O3-type lithium cobalt oxide (O3-LCO) positive electrode material has super-high compaction density and volume energy density, which is extremely competitive in the field of portable electronic products. However, the actual capacity (170 mAh / g, 4.45 V) of the currently commercialized O3-LiCoO2 is significantly lower than the theoretical capacity (274 mAh / g), thereby limiting the potential energy density improvement of LiCoO2 material. Increasing the working voltage to above 4.55 V to release more lithium ions is an effective strategy to promote LiCoO2 to obtain higher energy density. At high voltage, O3-LCO faces irreversible harmful phase transition and serious surface side reactions, which directly destroys the stability of its lithium storage structure.

[0003] At high cut-off voltage (such as 4.55 V), Co4 + -3d orbitals overlap with O 2- -2p orbitals, resulting in the oxidation of lattice oxygen to form active On-(n<2), which causes serious deterioration of the LCO crystal structure. When the charging voltage of LCO is higher than 4.55 V, the phase transition process of LixCoO2 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 bulk phase and surface of LCO, resulting in poor charge-discharge reversibility. This phase transition is accompanied by lattice slippage and partial collapse of the lattice structure. This leads to the accumulation of internal strain and subsequently leads to the formation of cracks and particle pulverization. Therefore, inhibiting harmful phase transition is crucial for stabilizing the LCO structure and breaking through the cut-off voltage above 4.55 V.

[0004] When LixCoO2 (0.5≤x≤1.0) is charged, electrons are extracted from the 3d orbitals 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 exhausted, and the Fermi level drops significantly as the top of the O-2p band overlaps with the broadened Co-3d state. This leads to the oxidation of oxygen and the formation of peroxide or O2, which develops more severely on the surface due to unstable structure and external stimuli. Therefore, at voltages higher than 4.55 V, oxygen evolution is considered to be the main challenge for LCO. At the same time, the continuous loss of oxygen during the cycle process will trigger irreversible phase transition (CoO2→Co3O4). The generated Co3O4 hinders the extraction of Li +The transport of the active species O2 and O- free radicals generated during the oxygen evolution process can react with the organic components in the electrolyte, leading to side reactions, initiating a large number of cracks, and thus exacerbating chemical effects such as etching, corrosion, and side reactions. Therefore, surface optimization is also considered an effective means to improve the stability of high-voltage LCO.

[0005] In the high delithiation state, the oxidized Co 4+ reacts with the carbonate electrolyte solvent, leading to a significant increase in the interfacial resistance and polarization of the battery. Charging above 4.55V triggers the phase transition of LCO from the O3 phase to the mixed-phase H1-3 hexagonal phase, making the surface lattice prone to structural collapse and crystal fragmentation.

[0006] The main bottleneck of current high-voltage LCO is the bulk phase transition leading to structural collapse and surface instability. Most existing patents mainly involve doping metal / non-metal elements and then coating a surface layer to stabilize the bulk and surface structure. For example, Chinese invention patent CN114050270A discloses a preparation method for Al, Co, and Li co-coated lithium cobalt oxide material for high voltage; Chinese invention patent CN117525359A discloses a fast ion conductor coated high-voltage high-specific-capacity lithium cobalt oxide positive electrode and a preparation method; Chinese invention patent CN117727881A proposes a fluorine-containing lithium-rich crystal phase material coated high-voltage lithium cobalt oxide positive electrode material and a preparation method; CN109755530B proposes a method for titanium-barium bimetallic oxide surface-coated high-voltage lithium cobalt oxide positive electrode material. These patents mostly use direct solid-phase mixing sintering or liquid-phase coating followed by sintering. The surface coating layer is mainly composed of single / multiple metal oxides or fast ion conductors or a core-shell structure. The effect is focused on using the coating layer to inhibit the contact between the positive electrode and the electrolyte, improving the cycle performance of the high-voltage positive electrode material, but it cannot effectively improve the phase transition process of the O3 phase and the mixed phase (H1-3) on the surface of the lithium cobalt oxide material, and it also has the side effects of reducing capacity and reducing kinetics.

[0007] It should be noted that the information disclosed in the above background section is only used to strengthen the understanding of the background of the present disclosure, and therefore can include information that does not constitute prior art known to those of ordinary skill in the art. SUMMARY

[0008] In view of the shortcomings of the prior art described above, the purpose of the present application is to provide a preparation method for a high-voltage lithium cobalt oxide positive electrode material to solve the problem that the prior art cannot effectively improve the phase transition process of the O3 phase and the mixed phase (H1-3) on the surface of the lithium cobalt oxide material, while reducing capacity and reducing kinetics.

[0009] To achieve the above-mentioned purpose, the technical solution of the present application is as follows:

[0010] A method for preparing high-voltage lithium cobalt oxide cathode material;

[0011] Includes the following steps:

[0012] Step 1: Dissolve the soluble salt of cobalt in a solvent and stir until homogeneous; add the lithium cobalt oxide matrix and stir at a constant temperature; then add ammonia water and mix, adjust the pH value and stir continuously; carry out the co-precipitation reaction until a dry gel is formed; dry in a vacuum drying oven to obtain 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 the P2 phase-sodium cobaltate coated LCO material, which is P2-NaXCoO2 & lithium cobalt oxide;

[0014] Step 3: The P2 phase-coated sodium cobalt oxide LCO material is mixed and ground with the lithium source, then subjected to low-temperature heat treatment and cooled to room temperature; then washed and dried with water to obtain a lithium cobalt oxide cathode material with an O2 phase-coated lithium cobalt oxide coating, namely O2-LiCoO2 & lithium cobalt oxide.

[0015] A further technical solution is as follows, in step 1:

[0016] The soluble salts of cobalt are cobalt sources that are one or more of the following: 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 as follows, 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 μm;

[0020] The molar ratio of cobalt in a soluble salt to cobalt in the lithium cobalt oxide substrate is called the soluble salt cobalt: matrix cobalt molar ratio, which 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℃, and the stirring speed is 100-400 rpm.

[0022] Further technical solutions are that in step 1:

[0023] The pH value is adjusted to 9-11;

[0024] Drying in a vacuum drying oven, temperature 80-150 DEG C, time 4-12h.

[0025] Further technical solutions are 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, sodium hydroxide;

[0027] The ratio of the moles of Na in the sodium salt to the moles of Co in Co(OH)2, i.e. the sodium salt sodium: salt-dissolved cobalt molar ratio, is in the range of 0.5 / 1-0.8 / 1.

[0028] Further technical solutions are that in step 2:

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

[0030] Further technical solutions are 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, lithium sulfate;

[0032] The ratio of the moles of Li in the lithium salt to the moles of Na in the sodium salt, i.e. the lithium salt lithium: sodium salt sodium molar ratio, is in the range of 10 / 1-5 / 1.

[0033] Further technical solutions are that in step 3:

[0034] The low-temperature heat treatment temperature is 200-400 DEG C; the low-temperature heat treatment time is 3-20h, the heating rate is 2-7 DEG C / min; the drying temperature is 80-150 DEG 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 application are as follows: (1) the present application adopts co-precipitation reaction to generate Co(OH)2 in situ on the surface of the lithium cobaltate matrix, which slows down the problem of the core-shell two-phase interface between materials existing in the prior art. Compared with direct dry mixing of the lithium cobaltate matrix and Co(OH)2, the former has a more uniform Co(OH)2 coating layer, and then adding sodium salt and mixing and sintering helps to form a uniform P2 phase sodium cobaltate on the surface of the substrate.

[0036] (2) The application realizes uniform coating by in-situ co-precipitation, which is conducive to the in-situ growth of lithium cobalt oxide in 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 the best performance.

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

[0038] (4) The application obtains a surface O2 phase lithium cobaltate coating layer by a low-temperature molten salt ion exchange method. In addition to serving as a coating layer to inhibit the contact between the positive electrode material and the electrolyte, improve the material's resistance to HF, and prevent the occurrence of side reactions, compared to other coating materials, as an O2 phase lithium cobaltate coating layer, it can also effectively reduce the phase transition process of O3 phase and mixed phase (H1-3) on the surface of lithium cobaltate material, inhibit the instability of the surface structure, and improve the high-pressure cycle performance of the positive electrode material. It effectively solves the problem of capacity reduction caused by conventional coating materials and the serious O3 phase change of conventional lithium cobaltate surface coating.

[0039] (5) The coating layer and the base material of the application are both lithium cobaltate, which can improve the capacity compared to non-electrochemically active coating layers, and effectively reduce the problem of the core-shell interface between the coating layer and the base material.

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

[0041] Figure 1 The scanning SEM comparison chart of the sample of step 1 of Example 1 and the sample of step 1 of Comparative Example 1 is shown.

[0042] Figure 2 The scanning chart of Example 1 after step 3 ion exchange is shown.

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

[0044] Figure 4 The dQ / dV comparison chart of Example 1 and Comparative Example 6 sample 1C discharge cycle after 50 cycles is shown. DETAILED DESCRIPTION

[0045] In order to make the purpose, technical solutions and advantages of the present application more clear, the device proposed by the present application is further described in detail below in combination with the drawings and specific embodiments. The advantages and features of the present application will be more clear according to the following description. It should be noted that the drawings are greatly simplified and all use non-precise proportions, only to facilitate, clear and assist the purpose of describing the embodiments of the present application. In order to make the purpose, features and advantages of the present application more obvious and easy to understand, please refer to the drawings. It should be noted that the structure, proportion, size and the like shown in the drawings attached to the present specification are only used to cooperate with the content disclosed in the specification, so that those skilled in the art can understand and read, and are not used to limit the conditions for implementing the present application, so they do not have technical significance. Any modification of structure, change of proportion relationship or adjustment of size, without affecting the effect and purpose that can be achieved by the present application, should still fall within the scope of the technical content disclosed by the present application.

[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 the solvent and stir uniformly. The soluble salt of cobalt 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 cobaltate matrix and constant temperature stirring. The lithium cobaltate 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, and W. The particle size D50 ranges from 4 to 20 um. The molar ratio of cobalt in the soluble salt of cobalt to the molar of cobalt in the lithium cobaltate matrix, i.e. the molar ratio of soluble salt cobalt to matrix cobalt, ranges from 0.01 / 1 to 0.1 / 1. The concentration of the soluble salt of cobalt solution 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 revolutions per minute.

[0049] Add ammonia water and mix, adjust the pH value, and continue stirring. The adjusted pH value is 9 to 11. Perform co-precipitation reaction until dry gel is formed. Dry in a vacuum drying oven, dry in a vacuum drying oven, temperature 80 to 150°C, time 4 to 12h. Obtain cobalt hydroxide coated LCO material, Co(OH)2& lithium cobaltate.

[0050] By controlling the co-precipitation pH value and stirring speed, the Co(OH)2coating layer is grown in-situ on the lithium cobalt oxide substrate. The pH value is adjusted to the set range of 9-11 to avoid the co-precipitation reaction from growing too slowly when the pH value is too low, and the complete reaction of the cobalt salt is not conducive. When the pH value is too high, the co-precipitation reaction grows too fast, and 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 higher stirring speed, the growth of cobalt hydroxide to the substrate surface is not conducive. At a lower stirring speed, cobalt hydroxide is prone to flaky deposition and is not uniform, affecting the next sintering reaction with sodium salt.

[0052] Step 2, the Co(OH)2coated LCO material is mixed with sodium salt to sinter to prepare P2 phase-sodium cobaltate coated LCO material, which is 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 in the sodium salt to cobalt in the dissolved salt, is in the range of 0.5 / 1-0.8 / 1.

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

[0055] By controlling the type of sodium salt and the sodium / cobalt ratio, as well as the sintering temperature, the surface Co(OH)2is converted to P2-NaXCoO2without changing the lithium cobaltate substrate material.

[0056] By setting the molar ratio of sodium in the sodium salt to cobalt in the dissolved salt in a range, and setting the sintering temperature in the range of 700-1000°C, the reaction of the sodium salt with cobalt hydroxide is not complete at too low a temperature, and the sodium salt is easily introduced into the lithium cobaltate substrate at too high a temperature, which easily changes the properties of the substrate.

[0057] Step 3, the P2 phase-sodium cobaltate coated LCO material is mixed with a lithium source after grinding, 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 the cooling is 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 Na in the sodium salt, i.e. the molar ratio of lithium in the lithium salt to sodium in the sodium salt, is in the range of 10 / 1-5 / 1.

[0058] After washing with water and drying, the drying temperature is 80-150℃, the drying time is 4-10h, and the drying atmosphere is oxygen or air, to obtain the lithium cobalt oxide anode material coated with O2 phase lithium cobalt oxide as the coating layer, which is O2-LiCoO2& lithium cobalt oxide.

[0059] The low-temperature heat treatment realizes the molten salt ion exchange. By controlling the low-temperature heat treatment temperature, the surface layer P2-NaXCoO2 is converted into O2-LiCoO2, and the formation of O3-LiCoO2 at high temperature is avoided. 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 anode material is relatively high. At the same time, if the low-temperature heat treatment temperature is too high, the Li / Na exchange will become violent, which will lead to a decrease in the stability of the generated O2 phase structure. Moreover, the O2 phase structure itself is a metastable structure that is thermodynamically unstable. At about 350℃, the O2 phase structure will undergo a phase transition to the O3 phase structure which is more thermodynamically stable. At 450℃, the O2 phase structure will complete the transition to the O3 phase structure. Therefore, the heat treatment temperature range needs to be controlled in the range of 200-400℃.

[0060] The molar ratio of lithium salt lithium to sodium salt sodium needs to be controlled within a certain range. First, the molar ratio of lithium source to Li / Na in P2 sodium precursor should not be too high. If the lithium source content is too high, there will be a large amount of residual impurity anions in the prepared O2 type lithium-rich manganese-based material, which will seriously hinder the lithium ion transmission, and at the same time, it will penetrate into the interior of the substrate, affecting its rate performance and coulomb efficiency. The Li / Na ratio should not be too low. If the lithium source content is too low, the Li / Na exchange of the prepared O2 type lithium-rich manganese-based material will not be complete, and P2-NaXCoO2 will still exist on the surface, resulting in a decrease in capacity.

[0061] In this application, the ion exchange is molten salt ion exchange. In this application, the P2 phase-sodium cobaltate coated LCO material is mixed with lithium nitrate uniformly, and the molar ratio of lithium salt lithium to sodium salt sodium is 8 / 1. The main purpose is to carry out molten salt reaction between P2 phase-sodium cobaltate and lithium nitrate, specifically, sodium ions in sodium cobaltate and lithium ions in lithium nitrate carry out ion exchange. Among them, the concentration of lithium ions is 8 times that of sodium ions. Under the condition of large concentration difference and suitable reaction temperature, sodium and lithium exchange, and finally P2 phase-sodium cobaltate changes into O2 phase lithium cobalt oxide.

[0062] The following explains the present application through multiple examples and comparative examples:

[0063] Example 1:

[0064] The preparation method of high-voltage lithium cobalt oxide anode material includes the following steps:

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

[0066] After that, according to the molar ratio of 0.04 / 1 of the dissolved salt cobalt: matrix cobalt, LiCoO2 matrix with a particle size D50 of 17um was added to the solution 1.02 Co 0.95 Al 0.05 O2 lithium cobaltate matrix.

[0067] At a constant temperature of 80℃, while stirring at 200rpm, ammonia was slowly added dropwise to adjust the pH value of the solution to 10.5, and the mixture was continuously stirred until it completely became a dry gel.

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

[0069] Step 2: The obtained cobalt hydroxide coated LCO material was mixed with sodium carbonate in proportion, wherein the molar ratio of sodium salt sodium: dissolved salt cobalt was 0.7 / 1, and then it was heated to 875℃ at a rate of 3℃ / min in an oxygen atmosphere and sintered for 10h, and then a P2 phase-sodium cobaltate coated LCO material was obtained, which was P2-Na 0.7 CoO2& lithium cobaltate.

[0070] Step 3: The P2 phase-sodium cobaltate coated LCO material was mixed with lithium nitrate in proportion, wherein the molar ratio of lithium salt lithium: sodium salt sodium was 8 / 1. After that, it was heated to 280℃ at a rate of 3℃ / min in an air atmosphere and sintered for 8h, and after cooling to room temperature, it was washed with water and dried in a drying oven at 120℃ for 6h to obtain the final product O2-LiCoO2& lithium cobaltate.

[0071] Example 2:

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

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

[0074] After that, according to the molar ratio of 0.08 / 1 of the dissolved salt cobalt: matrix cobalt, LiCoO2 matrix with a particle size D50 of 17um was added to the solution 1.02 Co 0.95 Al 0.05 O2 lithium cobaltate matrix.

[0075] At a constant temperature of 80℃, while stirring at 200rpm, ammonia was slowly added dropwise to adjust the pH value of the solution to 10.5, and the mixture was continuously stirred until it completely became a dry gel.

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

[0077] Example 3:

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

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

[0080] Then, according to the molar ratio of dissolved salt cobalt to matrix cobalt of 0.02 / 1, add Li 1.02 Co 0.95 Al 0.05 O2 lithium cobaltate matrix with a particle size D50 of 17 um to the solution.

[0081] Under constant temperature stirring at 80°C, slowly add ammonia water to adjust the pH of the solution to 10.5 while stirring at 200 rpm, and continue stirring until the mixture becomes a dry gel.

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

[0083] Example 4:

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

[0085] Step 2: The obtained cobalt hydroxide coated LCO material is mixed with sodium carbonate in proportion, and the molar ratio of sodium salt sodium to dissolved salt cobalt is 0.75 / 1, then heated to 875°C at a rate of 3°C / min in an oxygen atmosphere and sintered for 10h, then a P2 phase-sodium cobaltate coated LCO material is obtained, which is P2-Na 0.7 CoO2& lithium cobaltate.

[0086] Example 5:

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

[0088] Step 2: The obtained cobalt hydroxide coated LCO material is mixed with sodium carbonate in proportion, and the molar ratio of sodium salt sodium to dissolved salt cobalt is 0.6 / 1, then heated to 875°C at a rate of 3°C / min in an oxygen atmosphere and sintered for 10h, then a P2 phase-sodium cobaltate coated LCO material is obtained, which is P2-Na 0.7 CoO2& lithium cobaltate.

[0089] Example 6:

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

[0091] Step 3: The P2 phase-sodium cobaltate coated LCO material is mixed with lithium nitrate uniformly, where the lithium salt lithium: sodium salt sodium molar ratio is 9 / 1. After that, it is sintered at 280°C for 8h in air atmosphere, and after cooling to room temperature, it is washed with water, and dried in a drying oven at 120°C for 6h to obtain the final product O2-LiCoO2 & lithium cobaltate.

[0092] Example 7:

[0093] Example 7 is different from Example 1 in that:

[0094] Step 3: The P2 phase-sodium cobaltate coated LCO material is mixed with lithium nitrate uniformly, where the lithium salt lithium: sodium salt sodium molar ratio is 9 / 1. After that, it is sintered at 280°C for 8h in air atmosphere, and after cooling to room temperature, it is washed with water, and dried in a drying oven at 120°C for 6h to obtain the final product O2-LiCoO2 & lithium cobaltate.

[0095] Example 8:

[0096] Example 8 is different from Example 1 in that:

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

[0098] Then, according to the molar ratio of dissolved cobalt to matrix cobalt of 0.04 / 1, add Li 1.02 Co 0.95 Al 0.05 O2 lithium cobaltate matrix with a particle size D50 of 17um to the solution.

[0099] Under constant temperature stirring at 80°C, slowly add ammonia water to adjust the pH of the solution to 10.5, and continue to stir until the mixed solution becomes a dry gel.

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

[0101] Example 9:

[0102] Example 9 is different from Example 1 in that:

[0103] Step 2: The obtained cobalt hydroxide coated LCO material is mixed with sodium oxalate uniformly according to the proportion, where the sodium salt sodium: dissolved cobalt molar ratio is 0.7 / 1, and then sintered at 875°C for 10h in an oxygen atmosphere, to obtain a P2 phase-sodium cobaltate coated LCO material, which is P2-Na 0.7 CoO2 & lithium cobaltate.

[0104] Example 10:

[0105] Example 10 is different from Example 1 in that:

[0106] Step 3: The P2 phase-sodium cobaltate coated LCO material is mixed with lithium chloride uniformly, wherein the molar ratio of lithium salt lithium: sodium salt sodium is 8 / 1. Then it is heated to 280℃ at 3℃ / min in an air atmosphere, and sintered for 8h, after cooling to room temperature, it is washed with water, and dried in a drying oven at 120℃ for 6h to obtain the final product O2-LiCoO2& lithium cobaltate.

[0107] Comparative Example 1:

[0108] Comparative Example 1 is different from Example 1 in that:

[0109] Step 1: Cobalt sulfate (CoSO4·7H2O) is weighed and dissolved in 500mL water, and stirred uniformly to prepare a solution with a cobalt metal content of 1mol / L;

[0110] Then, according to the molar ratio of dissolved cobalt: matrix cobalt of 0.04 / 1, Li 1.02 Co 0.95 Al 0.05 O2 lithium cobaltate matrix with a particle size D50 of 17um is added to the solution.

[0111] Under constant temperature stirring at 80℃, the solution is slowly dripped with ammonia water to adjust the pH value to 11.5, and stirred constantly until the mixed solution becomes a xerogel.

[0112] The obtained xerogel is placed in a vacuum drying oven at 100℃ for 8h to obtain a cobalt hydroxide coated LCO material, which is Co(OH)2& lithium cobaltate.

[0113] Comparative Example 2:

[0114] Comparative Example 2 is different from Example 1 in that:

[0115] Step 2: The obtained cobalt hydroxide coated LCO material is mixed with sodium carbonate uniformly according to the ratio, wherein the molar ratio of sodium salt sodium: dissolved cobalt is 0.9 / 1, and then it is heated to 875℃ at 3℃ / min in an oxygen atmosphere, and sintered for 10h, and then P2 phase-sodium cobaltate coated LCO material is obtained, which is P2-Na 0.7 CoO2& lithium cobaltate.

[0116] Comparative Example 3:

[0117] Comparative Example 3 is different from Example 1 in that:

[0118] Step 2: The obtained cobalt hydroxide coated LCO material was mixed with sodium carbonate in proportion, with a sodium salt sodium: salt-dissolved cobalt molar ratio of 0.7 / 1, and then heated to 1050°C at 3°C / min in an oxygen atmosphere, and sintered for 10h, and then a P2 phase-sodium cobaltate coated LCO material was obtained, which was P2-Na 0.7 CoO2& lithium cobaltate.

[0119] Comparative Example 4:

[0120] Comparative Example 4 is different from Example 1 in that:

[0121] Step 3: The P2 phase-sodium cobaltate coated LCO material was mixed with lithium nitrate in proportion, with a lithium salt lithium: sodium salt sodium molar ratio of 3 / 1. Then heated to 280°C at 3°C / min in an air atmosphere, and sintered for 8h, and after cooling to room temperature, washed with water, and dried in a drying oven at 120°C for 6h, to obtain the final product O2-LiCoO2& lithium cobaltate.

[0122] Comparative Example 5:

[0123] Comparative Example 5 is different from Example 1 in that:

[0124] Step 3: The P2 phase-sodium cobaltate coated LCO material was mixed with lithium nitrate in proportion, with a lithium salt lithium: sodium salt sodium molar ratio of 12 / 1. Then heated to 280°C at 3°C / min in an air atmosphere, and sintered for 8h, and after cooling to room temperature, washed with water, and dried in a drying oven at 120°C for 6h, to obtain the final product O2-LiCoO2& lithium cobaltate.

[0125] Comparative Example 6:

[0126] Comparative Example 6 is different from Example 1 in that:

[0127] Step 3: The P2 phase-sodium cobaltate coated LCO material was mixed with lithium nitrate in proportion, with a lithium salt lithium: sodium salt sodium molar ratio of 8 / 1. Then heated to 500°C at 3°C / min in an air atmosphere, and sintered for 8h, and after cooling to room temperature, washed with water, and dried in a drying oven at 120°C for 6h, to obtain the final product O2-LiCoO2& lithium cobaltate.

[0128] The application uses O2-LiCoO2& lithium cobaltate as the positive electrode to assemble a coin cell battery for electrochemical performance testing. The specific steps are as follows:

[0129] The positive active material O2-LiCoO2 & lithium cobaltate, carbon black Super P, and polyvinylidene fluoride (Solvey 5130) were mixed in a mass ratio of 90:5:5 to form a slurry, which was uniformly coated on the surface of an aluminum foil to obtain a positive electrode sheet. A lithium sheet was used as a negative electrode sheet, and a 1 mol / L lithium hexafluorophosphate solution in ethylene carbonate (EC) and dimethyl carbonate (DMC) was used as an electrolyte, wherein the mass ratio of EC to DMC was 1:1. The lithium ion battery was assembled in a glove box.

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

[0131] The electrochemical performance test results of each example and the comparative example are shown in Table 1.

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

[0133] (2) Compared with Comparative Example 1, the pH value in the coprecipitation process of Example 1, Example 2, and Example 3 is 11.5, which is relatively large. During the subsequent coprecipitation, the coating layer is relatively thick, and the coating on the surface of the substrate is not uniform. The relative capacity and stability are decreased.

[0134] (3) Compared with Comparative Example 2, the molar ratio of sodium salt sodium to dissolved salt cobalt in Comparative Example 2 is 0.9 / 1. During the conversion of cobalt hydroxide to sodium cobaltate, the sodium salt is excessive, which leads to an increase in lithium salt during the next lithium-sodium ion exchange, and finally increases the residual alkali on the surface of the finished product, resulting in a decrease in the capacity, initial efficiency, and stability of the product.

[0135] (4) Compared with Comparative Example 4, the molar ratio of lithium salt lithium to sodium salt sodium is 3 / 1 during the low-temperature molten salt ion exchange of Comparative Example 4. The amount of lithium salt added is too small, which leads to incomplete conversion of the surface sodium cobaltate to O2 phase lithium cobaltate, and the existence of part of P2 phase sodium cobaltate, resulting in a decrease in capacity and stability.

[0136] (5) Compared with Comparative Example 5, the molar ratio of lithium salt lithium to sodium salt sodium is 12 / 1 during the low-temperature molten salt ion exchange of Comparative Example 5. The amount of lithium salt added is too large, which leads to complete conversion of the surface sodium cobaltate to O2 phase lithium cobaltate, but causes excessive enrichment of lithium salt on the surface of the material, resulting in high residual alkali, and finally poor capacity, initial efficiency, and stability.

[0137] (6) Compared with Comparative Example 3, Comparative Example 3 had an excessively high sintering temperature during the sintering process of cobalt hydroxide and sodium salt on the surface, which caused some sodium ions to enter the lithium cobalt oxide matrix in the form of doping. This resulted in a relative excess of lithium salt during subsequent ion exchange, and the final O2 phase lithium cobalt oxide coating layer was reduced, ultimately leading to poor capacity, first-time efficiency and stability.

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

[0139]

[0140] Table 1

[0141] Results analysis:

[0142] Figure 1 The image shows a comparison of the SEM scans of the sample from step 1 of Example 1 and the sample from step 1 of Comparative Example 1. 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 chart of CoO2 & lithium cobalt oxide and the final product O2-LiCoO2 & lithium cobalt oxide obtained in step 3 of Example 1. Figure 4 The diagram shows a comparison of dQ / dV after 50 coin cell cycles for Sample 1C of Example 1 and Comparative Example 6. Combined with... Figures 1-4 As shown,

[0143] Figure 1 The image on the left in the middle is a SEM image of the sample from step 1 of Example 1. Figure 1 The image on the right is a SEM image of the sample from step 1 of Comparative Example 1. Compared to Comparative Example 1, the coprecipitation pH in Example 1 was 10.5, while the coprecipitation pH in Comparative Example 1 was higher at 11.5. Figure 1 The two scan images show that at higher pH, the coprecipitation reaction causes cobalt hydroxide to grow too quickly, resulting in a thicker coating layer that is unevenly distributed on the substrate surface. This confirms that the pH for coprecipitation needs to be controlled within a suitable range.

[0144] from Figure 2 As can be seen from the example, after step 3 ion exchange, the surface of Example 1 has a uniform coating layer.

[0145] from Figure 3 The XRD comparison images show that after step 2, the characteristic peak of the sodium-ion layer oxygen P2 phase (002) appeared on the XRD of Example 1, confirming that sodium cobalt oxide was formed on the surface of the lithium cobalt oxide matrix. After step 3, the surface P2 phase sodium cobalt oxide disappeared, and sodium and lithium were completely exchanged to form the lithium cobalt oxide phase.

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

[0147] Therefore, in Comparative Example 6, the lithium cobalt oxide surface coating was completely transformed into a conventional O3 phase during low-temperature molten salt ion exchange at 500°C. Compared to the O2 phase lithium cobalt oxide surface coating in Example 1, this coating still exhibits a phase transition process between the O3 phase and the mixed phase (H1-3) during high-voltage electrochemical cycling.

[0148] When the charging voltage of LCO exceeds 4.55V, LixCoO2 undergoes a phase transformation from an octahedral three-phase (O3) to an octahedral single-phase (O1) and a mixed O3 phase (H1-3). The transformation between O3 and H1-3 causes degradation in both the bulk and surface of the LCO, resulting in poor charge-discharge reversibility. On a typical lithium cobalt oxide dQ / dV diagram, a typical O3 phase is observed between 3.8V and 3.9V. Subsequently, with increasing discharge depth, a mixed O3 phase (H1-3) appears around 4.5V. This mixed phase is unstable and exhibits poor electrochemical reversibility. During repeated cycling, an irreversible phase transition occurs: CoO2 → Co3O4, leading to a sharp increase in impedance and a shift in the overall dQ / dV towards higher voltage levels.

[0149] like Figure 4 As shown, after 50 cycles in Example 1, the typical O3 phase between 3.8V and 3.9V and the mixed phase (H1-3) around 4.5V are still clearly visible. However, after 50 cycles in Comparative Example 6, both the O3 phase and the mixed phase (H1-3) become less obvious and shift towards the high voltage direction. This confirms that the surface-coated O2 phase lithium cobalt oxide layer in Example 1 can effectively suppress surface phase transitions, 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 embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above 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 embodiments only express several implementation manners of the present application, and the description is more specific and detailed, but it should not be understood as a limitation on the scope of the patent. It should be noted that for ordinary skilled persons in the art, without departing from the concept of the present application, several modifications and improvements can be made, which are within the scope of protection of the present application. Therefore, the protection scope of the patent of the present application should be subject to the appended claims.

Claims

1. A method for preparing a high-voltage lithium cobalt oxide cathode material, characterized in that, Includes the following steps: Step 1: Dissolve the soluble salt of cobalt in a solvent and stir until homogeneous; add the lithium cobalt oxide matrix and stir at a constant temperature; then add ammonia water and mix, adjust the pH value and stir continuously; carry out the co-precipitation reaction until a dry gel is formed; dry in a vacuum drying oven to obtain cobalt hydroxide-coated LCO material, which is Co(OH)2 & lithium cobalt oxide; Step 2: The cobalt hydroxide-coated LCO material is mixed with sodium salt and sintered to prepare a P2 phase-sodium cobaltate-coated LCO material, which is P2-Na. X CoO2 & lithium cobalt oxide; 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 ratio of the molar amount of Na in sodium salt to the molar amount of Co in Co(OH)2, i.e., the sodium salt:cobalt molar ratio, ranges from 0.5 / 1 to 0.8 / 1. In step 2: the sintering temperature is 700–1000℃; in step 3: the low-temperature heat treatment temperature is 200–400℃. Step 3: The P2 phase-coated sodium cobalt oxide LCO material is mixed and ground with the lithium source, then subjected to low-temperature heat treatment and cooled to room temperature; then washed and dried with water to obtain a lithium cobalt oxide cathode material with an O2 phase-coated lithium cobalt oxide coating, namely O2-LiCoO2 & lithium cobalt oxide.

2. The method for preparing the high-voltage lithium cobalt oxide cathode material as described in claim 1, characterized in that, In step 1: The soluble salts of cobalt are cobalt sources that are one or more of the following: 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 the high-voltage lithium cobalt oxide cathode material as described in claim 2, characterized in that, In step 1: Lithium cobalt oxide matrix has the following general formula: Li 1+x Co 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 μm; The molar ratio of cobalt in a soluble salt to cobalt in the lithium cobalt oxide substrate is called the soluble salt cobalt: matrix cobalt molar ratio, which 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℃, and the stirring speed is 100-400 rpm.

4. The method for preparing the high-voltage lithium cobalt oxide cathode material as described in claim 3, characterized in that, In step 1: Adjust the pH value to 9-11; Dry in a vacuum drying oven at a temperature of 80–150℃ for 4–12 hours.

5. The method for preparing the high-voltage lithium cobalt oxide cathode material as described in claim 1, characterized in that, In step 2: Sintering time: 5–20 h; heating rate: 2–7 °C / min. The sintering atmosphere is oxygen.

6. The method for preparing the high-voltage lithium cobalt oxide cathode material as described in 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 ratio of the molar amount of Li in a lithium salt to the molar amount of sodium in a sodium salt, i.e., the lithium:sodium molar ratio, ranges from 10 / 1 to 5 / 1.

7. The method for preparing the high-voltage lithium cobalt oxide cathode material as described in claim 6, characterized in that, In step 3: the low-temperature heat treatment time is 3 to 20 hours, the heating rate is 2 to 7℃ / min; the drying temperature is 80 to 150℃, the drying time is 4 to 10 hours, and the drying atmosphere is oxygen or air.

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