Positive electrode material, preparation method thereof and lithium ion battery

Abstract

The invention discloses a positive electrode material, a preparation method thereof and a lithium ion battery. The preparation method comprises the following steps: after a silane coupling agent is hydrolyzed in a solvent, the silane coupling agent and a positive electrode core with residual alkali on the surface are uniformly mixed and calcined to obtain the positive electrode material, the positive electrode material comprises the positive electrode core and a silicon-based coating layer, the silicon-based coating layer is bonded with the positive electrode core through a chemical bond, and the silicon-based coating layer is coated with the silicon-based coating layer. The silicon-based coating layer comprises a lithium element and an oxygen element. The method provided by the invention can improve the air / humidity storage stability of the positive electrode material, especially the layered nickel-rich positive electrode material, and ensures that the positive electrode material has good electrochemical performance.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, and relates to a positive electrode material, a preparation method thereof and a lithium ion battery. BACKGROUND

[0002] The rapid development of electric vehicles has increased the demand for high-performance lithium ion batteries. In order to meet the endurance requirements of electric vehicles, lithium ion batteries must achieve higher energy density, which mainly depends on the specific capacity and working voltage of the positive electrode. Compared with LiCoO2, LiFePO4 and LiMn2O4 positive electrode materials, layered nickel-rich positive electrode materials (LiNi x Co y Mn z O2, x≥0.8, y≥0, z≥0) have become a viable candidate material for the next generation of lithium ion batteries due to their high reversible specific capacity (>200 mAh / g) and good rate performance.

[0003] However, layered oxide positive electrode materials (such as LiCoO2, LiMn2O4, layered nickel-rich positive electrode materials, etc.) often have residual alkaline substances (mainly LiOH and / or Li2CO3) on the surface of the synthesized material due to the use of excess lithium salt in the preparation process to compensate for lithium volatilization at high temperature sintering. The surface of the material is often residual alkaline. The poly-anion positive electrode material such as LiFePO4, due to less lithium volatilization in the synthesis process, usually uses low excess lithium source, and the residual alkaline problem on the surface of the material is usually not significant. On the other hand, the positive electrode material also encounters major challenges in practical application. For example, the layered nickel-rich positive electrode material, because of the unique electronic structure of nickel ion (Ni 2+ , Ni 3+ or Ni 4+ ) and its sensitivity to humid air, leads to instability in the environment atmosphere, under the condition of environment atmosphere, the particles of the layered nickel-rich positive electrode material easily adsorb H2O and CO2, forming LiOH and Li2CO3 residual alkaline on the surface. These inactive lithium compounds increase the pH value of the electrode slurry, causing gelation during preparation, hindering uniform coating on the current collector. In addition, the active oxygen generated by the reduction reaction of the components in the air and the surface Ni 3+ reacts, leading to the formation of impurities, which induces harmful subsurface phase transformation, transforming the layered structure into a non-oxidation-reduction NiO-like rock salt structure. More seriously, this transformation hinders lithium diffusion channels, increases the energy barrier of (de)lithiation, and thus has a negative impact on rate performance and cycle stability. In addition, during the operation of the battery, the anisotropic volume expansion and contraction of the single battery produce microcracks (mainly intergranular cracks) inside the particles, which then propagate to the surface, further accelerating the capacity decay. Intergranular cracks promote the penetration of electrolyte along the newly generated channels in the particles, leading to active Ni 4+Reduction of the residual alkali leads to the accumulation of harmful NiO-like rock salt phases.

[0004] Therefore, the strategy to alleviate surface and bulk structural instability is of great importance for the practical application of cathode materials, especially layered nickel-rich cathode materials, in advanced lithium-ion batteries.

[0005] To reduce the air sensitivity of layered nickel-rich cathode materials, effective modification strategies such as washing, secondary sintering and surface coating are proposed. The strategy of washing removes residual alkali (including LiOH and / or Li2CO3) by water washing, thereby helping to reduce the pH value of the electrode slurry and prevent gelation during the manufacturing process. However, it can have a negative impact on the underground structure, affecting Li + / H + exchange, increasing the instability during charging and discharging, and making the material more susceptible to air, thus requiring further protective measures. The strategy of secondary sintering effectively eliminates residual alkali through thermal decomposition and solid-phase reaction, without generating waste water, making it an environmentally friendly method. However, it cannot prevent surface degradation of the material caused by long-term exposure to air, and precise control of parameters during thermal decomposition is required, which increases the complexity of manufacturing. In contrast, the surface coating strategy is the most effective strategy to improve the interface stability of cathode materials, especially layered nickel-rich cathode materials.

[0006] In summary, washing, secondary sintering and surface coating are commonly used to remove residual alkali to improve the electrochemical performance of cathode materials, but it is difficult to avoid the generation of surface residual alkali during the storage of layered nickel-rich cathode materials. CN111370684A discloses a method for reducing the surface residual alkali content of lithium-ion battery nickel-rich cathode materials, which reduces residual alkali by adding a certain amount of acid or acid derivative in a non-aqueous inactive hydrogen organic solvent, improving the processing performance and cycle life of lithium-ion battery nickel-rich cathode materials, but cannot guarantee the air storage performance of the material. CN105336927B discloses a modified super-hydrophobic material coated lithium-ion battery nickel-rich cathode material, which bridges the material particles by modifying the super-hydrophobic material and surface modifying the super-hydrophobic material, improving the hydrophobicity and conductivity, and improving the storage performance of the nickel-rich cathode material. However, the super-hydrophobic material is essentially an electrochemically inert insulator, and surface modification of the super-hydrophobic material has limited effect on the improvement of the electrochemical performance of the nickel-rich cathode material, and the process is complex and difficult to industrialize.

[0007] Therefore, it is a technical problem to be solved to provide a strategy to improve the air / humidity storage stability of cathode materials, especially layered nickel-rich cathode materials, and ensure that the cathode material has good electrochemical performance. SUMMARY

[0008] In view of the above technical problems existing in the prior art, the present application aims to provide a positive electrode material, a preparation method thereof and a lithium ion battery. The method can improve the air / humidity storage stability of the positive electrode material, especially a layered nickel-rich positive electrode material, and ensure that the positive electrode material has good electrochemical performance.

[0009] To achieve the above-mentioned purpose, the present application adopts the following technical solutions:

[0010] In a first aspect, the present application provides a preparation method of a positive electrode material, which comprises the following steps:

[0011] After the silane coupling agent is hydrolyzed in a solvent, it is uniformly mixed with a positive electrode core having residual alkali, and then calcined to obtain a positive electrode material, which comprises a positive electrode core and a silicon-based coating layer, the silicon-based coating layer is chemically bonded to the positive electrode core, and the silicon-based coating layer comprises lithium and oxygen elements.

[0012] In the present application, the residual alkali is a residual lithium compound, for example, the residual alkali comprises LiOH and / or Li2CO3.

[0013] In the present application, the silicon-based coating layer is introduced on the surface of the positive electrode material particles (i.e. the positive electrode core) by means of chemical grafting of a hydrophobic silane coupling agent. Since the silane coupling agent is hydrolyzed before being mixed with the positive electrode material particles, the stable contact between the positive electrode core and the silicon-based coating layer can be achieved by chemical bonding. In addition, the silicon-based coating layer has hydrophobicity, which strongly reduces the hydrophilicity of the positive electrode material, inhibits the adverse chemical reaction with H2O and CO2, inhibits the formation of residual alkali, and prevents phase change when exposed to air, effectively solving the problem of air sensitivity of the positive electrode material. In addition, the silicon-based coating layer can reduce the c-lattice parameter shrinkage rate and improve the strain tolerance of the positive electrode material in a high charge state. The silicon-based coating layer can maintain the structural integrity of the material by strongly inhibiting the formation of surface impurities and reducing lithium migration, thereby significantly improving the air / water stability of the positive electrode material. The silicon-based coating layer also improves the mechanical elasticity of the material, reducing the problem of particle fracture and delamination cracking caused by isotropic strain during long-term operation of the battery. Finally, since the silicon-based coating layer comprises lithium and oxygen elements, the lithium element is derived from the residual alkali, which reduces the interfacial impedance and promotes uniform ion flux through the electrode / electrolyte interface, thereby improving the electrochemical performance of the material.

[0014] The method of the present application stabilizes the surface structure of the positive electrode material, so that the prepared positive electrode material has good chemical stability to the ambient air / humidity and good storage stability. In addition, the battery assembled by using the positive electrode material has good electrochemical performance, especially high initial coulombic efficiency and good cycle performance. The present application provides an industrial solution for the large-scale application of the positive electrode material, especially the layered nickel-rich positive electrode material.

[0015] In one embodiment, the chemical composition of the silicon-based coating layer is Li x SiO y wherein x>0, y>0.

[0016] The following are preferred technical solutions of the present application, but not as a restriction on the technical solutions provided by the present application. Through the following preferred technical solutions, the technical purposes and beneficial effects of the present application can be better achieved and realized.

[0017] Preferably, the silane coupling agent includes at least one of KH550, KH560 and KH602. It should be noted that not all silane coupling agents are suitable for the present application. For example, high-temperature calcination of KH-570 can cause carbonization, destroying the uniformity of the coating; the sulfide in KH-580 can trigger side reactions and has poor thermal stability. Therefore, KH-570 and KH-580 cannot be used in the present application.

[0018] Preferably, the positive electrode core is a layered nickel-rich positive electrode material, and the chemical formula of the layered nickel-rich positive electrode material is LiNi x Co y Mn z O2, x≥0.8, y≥0, z≥0. The positive electrode material has high reversible specific capacity and good rate performance, and the air / humidity storage stability of the positive electrode material is poor. The method of the present application can effectively solve the shortcomings of poor air / humidity storage stability.

[0019] The present application does not make specific limitations on the preparation method of the layered nickel-rich positive electrode material. For example, a simple solid-phase method can be used to prepare the layered nickel-rich positive electrode material, which specifically includes the following steps:

[0020] After mixing the lithium salt and the hydroxide precursor Ni x Co y Mn z (OH)2(x≥0.8, y≥0, z≥0), calcining, the layered nickel-rich positive electrode material is obtained.

[0021] Preferably, in the preparation method of the layered nickel-rich positive electrode material, the lithium salt includes lithium hydroxide and / or lithium carbonate.

[0022] Preferably, in the preparation method of the layered nickel-rich positive electrode material, the molar ratio of the lithium salt to the hydroxide precursor Ni x Co y Mn z (OH)2 is 1.05-1.08, for example, it can be 1.05, 1.06, 1.07 or 1.08, etc.

[0023] Preferably, in the preparation method of the layered nickel-rich positive electrode material, the calcination is performed at 450-550°C (for example, 450°C, 460°C, 470°C, 480°C, 490°C, 500°C, 515°C, 525°C, 530°C, 540°C or 550°C, etc.) for 3-5h (for example, 3h, 3.2h, 3.5h, 3.7h, 3.8h, 4h, 4.5h or 5h, etc.), and then heated to 750-800°C (for example, 750°C, 760°C, 770°C, 780°C, 790°C or 800°C, etc.) and calcined for 10-14h (for example, 10h, 10.5h, 11h, 11.5h, 12h, 12.5h, 13h, 13.5h or 14h, etc.).

[0024] Preferably, the solvent comprises alcohol and water.

[0025] Preferably, the volume ratio of alcohol to water is (1-2):1, for example, 1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, 1.7:1, 1.8:1 or 2:1, etc.

[0026] Preferably, the mass fraction of the silane coupling agent is 0.5-1.5wt.% based on the mass of the positive electrode core, for example, 0.5wt.%, 0.6wt.%, 0.7wt.%, 0.8wt.%, 1wt.%, 1.1wt.%, 1.3wt.% or 1.5wt.%, etc.

[0027] Preferably, the hydrolysis temperature is 40-80°C, for example, 40°C, 45°C, 50°C, 55°C, 60°C, 65°C, 70°C, 75°C or 80°C, etc.

[0028] Preferably, the uniform mixing is performed by stirring at 60-90°C (for example, 60°C, 65°C, 70°C, 75°C, 80°C, 85°C or 90°C, etc.) for 5-7h (for example, 5h, 5.5h, 6h, 6.5h or 7h, etc.).

[0029] Preferably, before calcination, the obtained mixture is dried at a temperature of 80-120°C (for example, 80°C, 85°C, 90°C, 95°C, 100°C, 105°C, 110°C, 115°C or 120°C, etc.) for a time of 10-14h (for example, 10h, 10.5h, 11h, 11.5h, 12h, 12.5h, 13h, 13.5h or 14h, etc.).

[0030] Preferably, the temperature of the calcination is 500-700℃, for example, it can be 500℃, 525℃, 550℃, 570℃, 600℃, 625℃, 650℃, 680℃ or 700℃, etc.

[0031] Preferably, the time of the calcination is 4-8h, for example, it can be 4h, 4.5h, 5h, 5.5h, 6h, 6.5h, 7h, 7.5h or 8h, etc.

[0032] In the second aspect, the application provides a positive electrode material, which is prepared by the method of the first aspect.

[0033] In the third aspect, the application provides a lithium ion battery, which comprises the positive electrode material of the second aspect.

[0034] The numerical range of the application includes not only the point values listed above, but also any point values between the above numerical ranges that are not listed. Due to the limited space and the consideration of simplicity, the application does not list all the specific point values included in the range.

[0035] Compared with the prior art, the application has the following beneficial effects:

[0036] (1) The application introduces a silicon-based coating layer on the surface of the positive electrode material particles (i.e. the positive electrode core) by adopting the method of hydrophobic silane coupling agent chemical grafting, which can not only improve the combination of the coating layer and the positive electrode core, but also effectively improve the chemical stability of the positive electrode material to the environment air / humidity, and the storage stability. Moreover, the battery assembled by using the positive electrode material has good electrochemical performance, especially good cycle performance.

[0037] (2) The positive electrode material of the application has good hydrophobicity. The contact angle with water is tested in air with a humidity of 80% to characterize its hydrophobicity. After 0 days of storage, the contact angle with water is above 50.4%; after 14 days of storage, the contact angle with water is above 49.6%; and after 28 days of storage, the contact angle with water is above 47.2%.

[0038] (3) The battery assembled by using the positive electrode material of the application has an initial discharge specific capacity of 0.2C above 203.2mAh / g; a first coulombic efficiency of 0.2C above 86.4%; an initial discharge specific capacity of 1C above 184.7mAh / g; and a capacity retention rate of 1C / 1C cycle for 100 cycles above 79.6%.

[0039] (4) After the battery assembled by using the positive electrode material of the application is stored in air with a humidity of 80% for 28 days, the initial discharge specific capacity of 1C is above 177.6mAh / g, and the capacity retention rate of 1C / 1C cycle for 100 cycles is above 80.5%. DETAILED DESCRIPTION

[0040] The technical solutions of the present application are further illustrated by the specific embodiments below.

[0041] Preparation Example 1

[0042] Ni 0.85 Co 0.1 Mn 0.05 (OH)2and LiOH H2O are mixed in a molar ratio of 1:1.05, and are sequentially calcined at 500°C for 4h and at 780°C for 12h to obtain a layered nickel-rich positive electrode material.

[0043] Illustratively, the layered nickel-rich positive electrode material prepared in Preparation Example 1 is taken as the inner core, and is coated to form the desired positive electrode material.

[0044] However, it should be noted that the type of positive electrode material is not limited to the positive electrode material prepared in Preparation Example 1, nor is it limited to the layered nickel-rich positive electrode material. As long as the surface of the inner core has residual alkali, the positive electrode material commonly used in the art is suitable for the present application.

[0045] Example 1

[0046] The present embodiment provides a preparation method of a positive electrode material, comprising the following steps:

[0047] A silane coupling agent KH550 (purity 98wt.%) is weighed and dissolved in a mixture of anhydrous ethanol and water, the volume ratio of anhydrous ethanol and water being 2:1. After hydrolysis at 60°C, layered nickel-rich positive electrode material powder (Preparation Example 1) is added, the proportion of the silane coupling agent in the layered nickel-rich positive electrode material powder being 0.5wt.%, and stirring is carried out at 80°C for 6h. After solid-liquid separation, the material is dried at 100°C for 12h, and then calcined at 600°C for 6h to obtain a positive electrode material.

[0048] The positive electrode material prepared in the present embodiment comprises a positive electrode inner core and a silicon-based coating layer, the silicon-based coating layer being chemically bonded to the positive electrode inner core, and the silicon-based coating layer comprising lithium and oxygen elements.

[0049] Example 2

[0050] The difference between the present embodiment and Example 1 is that the proportion of the silane coupling agent in the layered nickel-rich positive electrode material powder is 0.7wt.%.

[0051] Example 3

[0052] The difference between the present embodiment and Example 1 is that the proportion of the silane coupling agent in the layered nickel-rich positive electrode material powder is 0.9wt.%.

[0053] Example 4

[0054] The difference between this embodiment and embodiment 1 is that the proportion of the silane coupling agent in the layered nickel-rich positive electrode material powder is 1.1wt.%.

[0055] Embodiment 5

[0056] The difference between this embodiment and embodiment 1 is that the proportion of the silane coupling agent in the layered nickel-rich positive electrode material powder is 1.3wt.%.

[0057] Embodiment 6

[0058] The difference between this embodiment and embodiment 1 is that the proportion of the silane coupling agent in the layered nickel-rich positive electrode material powder is 1.5wt.%.

[0059] Embodiment 7

[0060] The embodiment provides a preparation method of a positive electrode material, including the following steps:

[0061] The silane coupling agent KH550 (purity 98wt.%) is weighed and dissolved in a mixed solution of anhydrous ethanol and water, the volume ratio of anhydrous ethanol and water is 1:1, after hydrolysis at 45℃, the layered nickel-rich positive electrode material powder (preparation example 1) is added, the proportion of the silane coupling agent in the layered nickel-rich positive electrode material powder is 0.5wt.% and stirring is carried out at 90℃ for 5h. After solid-liquid separation, the material is dried at 80℃ for 14h, and then calcined at 700℃ for 4h to obtain the positive electrode material.

[0062] The positive electrode material prepared in the embodiment includes a positive electrode core and a silicon-based coating layer, the silicon-based coating layer is bonded to the positive electrode core through a chemical bond, and the silicon-based coating layer includes lithium and oxygen elements.

[0063] Embodiment 8

[0064] The embodiment provides a preparation method of a positive electrode material, including the following steps:

[0065] The silane coupling agent KH550 (purity 98wt.%) is weighed and dissolved in a mixed solution of anhydrous ethanol and water, the volume ratio of anhydrous ethanol and water is 2:1, after hydrolysis at 80℃, the layered nickel-rich positive electrode material powder (preparation example 1) is added, the proportion of the silane coupling agent in the layered nickel-rich positive electrode material powder is 0.5wt.% and stirring is carried out at 60℃ for 7h. After solid-liquid separation, the material is dried at 120℃ for 10h, and then calcined at 500℃ for 8h to obtain the positive electrode material.

[0066] The positive electrode material prepared in the embodiment comprises a positive electrode core and a silicon-based coating layer, the silicon-based coating layer is bonded to the positive electrode core through a chemical bond, and the silicon-based coating layer comprises lithium and oxygen elements.

[0067] Comparative Example 1

[0068] In the present comparative example, the layered nickel-rich positive electrode material prepared in Preparation Example 1 is directly used without coating.

[0069] Comparative Example 2

[0070] The difference between the present comparative example and Example 1 is that the silane coupling agent is not hydrolyzed, but is directly mixed with the layered nickel-rich positive electrode material powder, and then calcined at 600°C for 6h to obtain the positive electrode material.

[0071] Comparative Example 3

[0072] The present comparative example provides a method for preparing a positive electrode material, comprising the following steps:

[0073] The silane coupling agent KH550 (purity 98wt.%) is dissolved in a mixture of anhydrous ethanol and water, the volume ratio of anhydrous ethanol to water is 2:1, and after hydrolysis at 60°C, the layered nickel-rich positive electrode material powder (Preparation Example 1) is added, the proportion of the silane coupling agent in the layered nickel-rich positive electrode material powder is 0.5wt.%, and stirring is carried out at 80°C for 6h. After solid-liquid separation, the material is dried at 100°C for 12h to obtain the positive electrode material.

[0074] The difference between the present comparative example and Example 1 is that the calcination process is not carried out after the hydrolysis and coating of the silane coupling agent.

[0075] (I) Contact angle test:

[0076] The contact angle test is carried out on a contact angle measuring instrument, and the contact angle formed by a water droplet on a small disc of the positive electrode material is used to measure the hydrophobicity of the positive electrode material. The contact angles of the positive electrode materials after being placed in air (the humidity of the air is 80% tested by a dew point instrument) for 0 days, 14 days and 28 days are tested respectively. The specific method is as follows:

[0077] An appropriate amount of positive electrode material particles of Examples 1-8 and Comparative Examples 1-2 is taken respectively, and a tablet machine is used to press into a disc with a diameter of 10mm and a thickness of 2mm, the pressure is 10MPa, and the pressing time is 60 seconds. The time point for water droplet contact angle measurement is 1s after the water droplet contacts the surface of the small disc. In order to eliminate errors as much as possible, at least 5 different positions are taken to obtain the average value, and the influence of surface roughness is reduced. The results are shown in Table 1.

[0078] Table 1

[0079]

[0080]

[0081] The results show that, compared with Comparative Example 1, the contact angles between the positive electrode materials of Examples 1-8 and water droplets are reduced to varying degrees, indicating that the hydrophobicity of the silicon-based coated nickel-rich positive electrode material is significantly improved. Especially in Examples 3 and 4, when the proportion of silane coupling agent in the layered nickel-rich positive electrode material powder is 0.9wt.%-1.1wt.%, the contact angle with the water droplet is the largest, indicating that the hydrophobicity of the material is better.

[0082] In Comparative Example 2, when KH550 is not hydrolyzed, its ethoxy group (-OCH2CH3) cannot be converted into silanol (Si-OH), and cannot form a covalent bond (Si-O-Li) with the Li-O bond on the surface of the positive electrode material. It only covers the particles through physical adsorption. The physical adsorption layer is loose and easy to fall off, and cannot form a dense hydrophobic coating, resulting in a decrease in hydrophobicity.

[0083] Application Examples 1-8 and Application Comparative Examples 1-3

[0084] The positive electrode materials, polyvinylidene fluoride (PVDF) and acetylene black of Examples 1-8 and Comparative Examples 1-3 were added into solvent 1-methyl-2-pyrrolidone (NMP) in a mass ratio of 8:1:1 to form a slurry. The slurry was coated on an aluminum foil and dried at 90°C for 8h, and the electrode was punched into a circular sheet with a diameter of 13mm (the areal density of the active material was 2.5mg cm -2 ), to obtain a positive electrode.

[0085] A lithium metal foil was used as the negative electrode, a polypropylene separator (Celgard 2400) was used as the separator, and a solution of 1M lithium hexafluorophosphate (LiPF6) dissolved in ethylene carbonate (EC), diethyl carbonate (DEC) and methyl ethyl carbonate (EMC) (mass ratio 1:1:1) was used as the electrolyte.

[0086] The above positive electrode, negative electrode and separator were assembled into a CR2032 type button cell in a glove box (LS800D type, oxygen and water content ≤0.1ppm), injected with electrolyte and sealed, and the assembled battery was obtained.

[0087] (II) Electrochemical performance test:

[0088] The batteries of Examples 1-8 and Application Comparative Examples 1-3 were left to stand at room temperature for 10h, and then electrochemical tests were performed using a Wuhan Lanbatt CT2001A battery test system, with the test voltage range set to 2.8-4.3V (vs. Li + / Li).

[0089] (1) Test the initial specific discharge capacity and the first coulombic efficiency under 0.2C condition, wherein the first coulombic efficiency = the first discharge capacity / the first charge capacity x 100%.

[0090] (2) Test the initial specific discharge capacity and the 100th cycle capacity retention rate under 1C condition, wherein the 100th cycle capacity retention rate = the 100th cycle discharge capacity / the initial specific discharge capacity x 100%.

[0091] The test results are shown in Table 2.

[0092] Table 2

[0093]

[0094] The results of Table 2 show that the positive electrode material with the silicon-based coating layer modification has a slight sacrifice of specific capacity compared with the positive electrode material without modification (Comparative Example 1). However, the first coulombic efficiency and the capacity retention rate are significantly improved.

[0095] It can be known from the comparison of Example 1 and Comparative Example 2 that when the KH550 is not hydrolyzed, the ethoxy group (-OCH2CH3) thereof cannot be converted into silanol (Si-OH), and cannot form a covalent bond (Si-O-Li) with the Li-O bond on the surface of the positive electrode material. Only the particles are covered by physical adsorption. The physical adsorption layer is loose and easy to fall off, and cannot form a dense hydrophobic coating. The hydrophobicity decreases, and the protection effect of the hydrophobic layer decreases. The surface residual lithium and side reactions may increase, for example, the residual organic groups (such as amino groups and ethoxy groups) of the unhydrolyzed KH550 react with lithium salts (such as LiPF6) in the electrolyte to generate HF, which corrodes the surface of the positive electrode and accelerates the dissolution of transition metals (Ni and Mn), resulting in a decrease in the first coulombic efficiency and the cycle capacity retention rate.

[0096] It can be known from the comparison of Example 1 and Comparative Example 3 that calcination is a key step for removing the organic components (such as amino groups and ethoxy groups) in the KH550 and forming a stable Li x SiO y coating. When not calcined, the coating contains a large amount of organic residues, and the exposure of the hydrophilic groups (such as -NH2) thereof leads to a decrease in hydrophobicity. The residual organic matter of the KH550 is oxidized and decomposed during high-pressure charging and discharging, generating gas (such as CO2 and NH3) and carbon residues, which causes the expansion of the electrode and the increase in impedance, resulting in a decrease in the initial discharge capacity and the first coulombic efficiency. At the same time, the uncalcined coating is in an organic-inorganic mixed state, and has low mechanical strength. During the cycle, the coating is easy to crack and fall off, loses the protection effect, and causes a decrease in the cycle capacity retention rate.

[0097] (Three), test of the electrochemical performance after storage:

[0098] The positive electrode materials of Examples 1-8 and Comparative Examples 1-3 were used to assemble batteries after being stored in air for 28 days. The method of assembling the batteries and the parameters were the same as those of Examples 1-8 and Comparative Examples 1-3. The assembled batteries were tested for electrochemical performance, with a charge rate and a discharge rate of 1C. The initial discharge capacity and the capacity retention rate after 100 cycles were tested, where the capacity retention rate after 100 cycles = the discharge capacity after 100 cycles / initial discharge capacity x 100%. The results are shown in Table 3.

[0099] Table 3

[0100]

[0101] Table 3 evaluates the cycle stability of the positive electrode materials after being exposed to air (humidity 80%) for 28 days, and studies their chemical stability to H2O and CO2. Compared with the unmodified positive electrode material (Comparative Example 1), the initial discharge specific capacity and cycle stability of the positive electrode materials of Examples 1-8 are improved to varying degrees after being exposed to air for 28 days. Compared with Example 1, the initial specific capacity of Comparative Example 1 decreases sharply from 179.5 mAh / g to 170 mAh / g, and the capacity retention rate after 100 cycles at 1C decreases from 80.8% to 63.3%. This is due to the loss of active lithium and surface phase transition that hinders ion diffusion.

[0102] Furthermore, Examples 3 and 4 maintain a capacity retention rate of 87.6% and 87.4% after 100 cycles, indicating that the silicon-based coating layer effectively prevents the surface from reacting with moisture, thereby improving the storage stability and cycle performance under environmental conditions.

[0103] At the same time, by comparing Example 1 with Comparative Example 2, it can be seen that when KH550 is not hydrolyzed, its ethoxy group (-OCH2CH3) cannot be converted into silanol (Si-OH), and cannot form a covalent bond (Si-O-Li) with the Li-O bond on the surface of the positive electrode material. Only the particles are covered by physical adsorption. The physical adsorption layer is loose and easy to fall off, and cannot form a dense hydrophobic coating. The hydrophobicity decreases, the protection effect of the hydrophobic layer decreases, and the cycle stability of the battery decreases after being stored for 28 days.

[0104] By comparing Example 1 with Comparative Example 3, it can be seen that when not calcined, the coating contains a large amount of organic residues, and the exposure of the hydrophilic groups (such as -NH2) causes the hydrophobicity to decrease. The residual KH550 organic matter is oxidized and decomposed during high-pressure charging and discharging, producing gas (such as CO2, NH3) and carbon residues, causing the electrode to swell and the impedance to increase, resulting in a decrease in the initial discharge specific capacity. At the same time, the uncalcined coating is in an organic-inorganic mixed state, and has low mechanical strength. It is easy to crack and fall off during cycling, loses the protection effect, and causes the cycle capacity retention rate to decrease.

[0105] The applicant states that the present invention is intended to illustrate the detailed methods of the present invention through the above-described embodiments, but the present invention is not limited to the above-described detailed methods, that is, it does not mean that the present invention must rely on the above-described detailed methods in order to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions for various raw materials in the products of the present invention, addition of auxiliary ingredients, and selection of specific methods, etc., are all within the scope of protection and disclosure of the present invention.

Claims

1. A method for preparing a positive electrode material, characterized in that: The preparation method comprises the following steps: After the silane coupling agent is hydrolyzed in a solvent, it is evenly mixed with the positive electrode core with residual alkali on the surface and calcined to obtain a positive electrode material. The positive electrode material includes a positive electrode core and a silicon-based coating layer. The silicon-based coating layer is chemically bonded to the positive electrode core, and the silicon-based coating layer includes lithium and oxygen elements.

2. The preparation method according to claim 1, characterized in that The silane coupling agent includes at least one of KH550, KH560 and KH602; Preferably, the positive electrode core is a layered nickel-rich positive electrode material, and the chemical formula of the layered nickel-rich positive electrode material is LiNi x Co y Mn z O2, x≥0.8, y≥0, z≥0.

3. The preparation method according to claim 1 or 2, characterized in that The solvent includes alcohol and water; Preferably, the volume ratio of alcohol to water is (1-2):

1.

4. The preparation method according to any one of claims 1 to 3, characterized in that Based on the mass of the positive electrode core, the mass fraction of the silane coupling agent is 0.5wt.% to 1.5wt.%.

5. The preparation method according to any one of claims 1 to 4, characterized in that The hydrolysis temperature is 40°C to 80°C.

6. The preparation method according to any one of claims 1 to 5, characterized in that The uniform mixing method is: stirring at 60° C. to 90° C. for 5 h to 7 h.

7. The preparation method according to any one of claims 1 to 6, characterized in that Before the calcination, the mixture is dried at a temperature of 80° C. to 120° C. and for a time of 10 hours to 14 hours.

8. The preparation method according to any one of claims 1 to 7, characterized in that The calcination temperature is 500°C to 700°C; Preferably, the calcination time is 4 hours to 8 hours.

9. A positive electrode material, characterized in that The positive electrode material is prepared by the method according to any one of claims 1 to 8.

10. A lithium ion battery, characterized in that: The lithium-ion battery comprises the positive electrode material according to claim 9.

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