Method for cleaning positive electrode material, modified positive electrode material, secondary battery

Abstract

This application relates to the field of lithium-ion battery technology, and particularly to a cleaning method for cathode materials, modified cathode materials, and secondary batteries. A cleaning solution is prepared by mixing raw materials including a water-soluble electrolyte with water; the cathode material is then mixed with the cleaning solution to obtain the cleaned cathode material; wherein the water-soluble electrolyte contains at least two hydrophilic groups, and the pKa values ​​of the hydrophilic groups are either the same or different, ranging from 6 to 10. During the mixing process, the aqueous solvent dissolves residual alkali on the surface. The hydrophilic groups ionize in the aqueous solution to release at least two anions, which then form hydrated anions with a volume much larger than H+. + Multiple hydrated anions can be adsorbed onto the surface of the cathode material through electrostatic interactions. The significant steric hindrance effect of hydrated anions can significantly reduce H+. + The increased proximity to the cathode material surface suppresses lithium-hydrogen exchange and reduces capacity loss. Furthermore, it can form coordination bonds with the transition metal of the cathode material, improving the stability of the layered structure.

Description

Technical Field

[0001] This application relates to the field of lithium-ion battery technology, and in particular to cleaning methods for cathode materials, modified cathode materials, and secondary batteries. Background Technology

[0002] Cathode materials play a decisive role in the performance of lithium-ion batteries. Among existing cathode materials, ternary cathode materials, such as nickel-cobalt-manganese ternary cathode materials, are widely used. High-nickel ternary materials, in particular, possess outstanding high energy density advantages. However, their rapid capacity decay and poor thermal stability caused by their layered structure and composition have consistently hindered their development in the lithium-ion electric vehicle market. This is mainly due to the instability of their layered structure leading to the dissolution of transition metal ions and causing interfacial side reactions. Furthermore, the Ni content in high-nickel ternary materials... 3+ Due to the presence of unpaired single electrons, Ni is easily reduced. 2+ At the same time, the oxygen in the lattice (O 2- ) is oxidized to reactive oxygen species (O) - Or O 2- These reactive oxygen species react with H2O and CO2 in the air, as well as Li on the surface. + The reaction will generate residual alkalis such as LiOH and Li2CO3, which will adhere to the surface of the positive electrode material.

[0003] Residual alkali negatively impacts the structural stability, electrochemical performance, processing performance, and safety performance of cathode materials. 1. Residual alkali readily absorbs moisture in the environment, easily leading to gelation of the cathode slurry, uneven coating, affecting processing performance, and even corroding the current collector. 2. Residual alkali covers the surface of cathode material particles, hindering lithium-ion insertion / extraction, resulting in impaired lithium-ion transport and reduced effective capacity. 3. Residual alkali undergoes side reactions with the electrolyte, consuming active lithium and increasing interfacial impedance, further hindering lithium-ion transport, leading to capacity decay, reduced cycle performance, and decreased rate performance. 4. During cycling, the residual alkali side reactions continuously consume the electrolyte. This reaction is exothermic and easily leads to thermal runaway, while also generating a large amount of CO2, causing increased internal pressure and ultimately affecting battery safety and stability. Besides residual alkali, direct contact between the cathode material and the electrolyte also easily generates interfacial side reactions, resulting in poor battery cycle performance, rate performance, and high-temperature stability.

[0004] To address the structural and interfacial issues of high-nickel ternary cathode materials, the mainstream modification strategies currently include weak acid washing, elemental doping, and surface coating. 1. Industrially, solution washing is commonly used to remove residual alkali from the material surface. While water washing or acid washing is inexpensive, it can cause severe lithium-hydrogen exchange and transition metal ion dissolution. Furthermore, the washed cathode active material comes into direct contact with the electrolyte, often resulting in more severe surface reconstruction and poor battery performance. 2. Elemental doping enhances lattice stability by introducing highly stable elements to replace cations in the crystal lattice. However, single doping methods cannot improve interfacial side reactions caused by residual alkali. 3. Surface coating improves battery stability by forming a physical coating layer on the material surface to isolate it from the external environment and electrolyte influences. However, it still cannot remove residual alkali, and most coating methods are complex, failing to balance ease of processing with the effectiveness of performance improvement. Furthermore, methods 2 and 3 are limited to upstream cathode material manufacturers. For downstream battery manufacturers, if the ternary cathode material contains residual alkali, methods 2 and 3 cannot be used for modification to remove the residual alkali, nor can they be used to prevent the ternary cathode material itself from continuing to generate residual alkali. Therefore, a more effective treatment method is urgently needed for ternary cathode materials. Summary of the Invention

[0005] The purpose of this application is to provide a cleaning method for cathode materials, modified cathode materials, and secondary batteries, aiming to solve the problem of capacity loss caused by cleaning residual alkali from cathode materials in the prior art.

[0006] To achieve the above-mentioned objectives, the technical solution adopted in this application is as follows:

[0007] In a first aspect, this application provides a method for cleaning a positive electrode material, comprising the following steps:

[0008] Prepare a cleaning solution by mixing raw materials, including water-soluble electrolytes, with water;

[0009] The cathode material to be cleaned is mixed with the cleaning solution to obtain the cleaned cathode material.

[0010] Among them, the water-soluble electrolyte contains at least two hydrophilic groups, and the pKa values ​​of the hydrophilic groups are the same or different, ranging from 6 to 10.

[0011] In the cleaning method described in this application, when the cathode material to be cleaned is mixed with the cleaning solution, the aqueous solvent dissolves the residual alkali on the surface of the cathode material. Simultaneously, because the water-soluble electrolyte contains at least two hydrophilic groups with pKa values ​​within a certain range, it will ionize into at least two anions in the aqueous solution. These anions are surrounded by water molecules due to electrostatic interactions to form stable hydrated anions, and their volume is much larger than that of H+. +Hydrated anions contain a negative charge. Multiple hydrated anions preferentially adsorb onto positively charged sites (metal ion sites) on the surface of the cathode material via electrostatic interactions. This allows the water-soluble electrolyte to bind to the cathode material surface after ionization. The significant steric hindrance effect of hydrated anions can significantly reduce H+. + The probability of approaching the surface of the cathode material increases the concentration of H+ in the aqueous solution. + The energy barrier for cathode material intercalation inhibits lithium-hydrogen exchange between the cathode material and the aqueous solution, reducing lithium-ion loss, improving the stability of the layered structure of the cathode material, and minimizing capacity loss. In summary, this cleaning method not only removes residual alkali from the cathode material surface but also solves the problem of capacity loss caused by cleaning cathode materials in existing technologies.

[0012] Secondly, this application provides a modified cathode material, which is the cleaned cathode material obtained by the cleaning method described above.

[0013] Because the above cleaning method removes residual alkali from the surface of the cathode material, reduces capacity loss during the process, and binds water-soluble electrolytes to the surface of the cathode material, it further stabilizes the layered structure of the cathode material. This fundamentally solves the problem of continuous generation of residual alkali and dissolution of transition metals in the cathode material. Therefore, the cleaned cathode material can be used as a modified cathode material, which has good processing performance for battery manufacturing, and produces secondary batteries with high effective capacity, good cycle performance and rate performance, outstanding high-temperature stability, and good safety.

[0014] Thirdly, this application provides a secondary battery, wherein the positive electrode of the secondary battery contains the cleaned positive electrode material obtained by the cleaning method of the above application, or contains the modified positive electrode material of the above application.

[0015] Since the cleaning method described in the above application removes residual alkali from the surface of the cathode material, inhibits lithium-hydrogen exchange, and binds water-soluble electrolytes to the surface of the cathode material, thereby improving the stability of the layered structure of the cathode material, the modified cathode material has good processing performance in battery manufacturing, and the resulting secondary battery has high effective capacity, good cycle performance and rate performance, outstanding high-temperature stability, and good safety. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of this application, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 This is a surface morphology diagram of the positive electrode material being cleaned in Embodiment 1 of this application;

[0018] Figure 2 This is a surface morphology diagram of the cathode material in Comparative Example 1 of this application;

[0019] Figure 3 This is a surface morphology diagram of the cleaned cathode material in Comparative Example 2 of this application;

[0020] Figure 4 This is a graph showing the residual alkali content of the positive electrode material cleaned in Example 1 of this application, the positive electrode material in Comparative Example 1, and the positive electrode material cleaned in Comparative Example 2.

[0021] Figure 5 This is a comparison chart of the cycle performance of the secondary batteries prepared in Examples 1-7 and Comparative Examples 1 and 2 of this application at room temperature;

[0022] Figure 6 This is a comparison chart of the rate performance of the secondary batteries prepared in Example 1, Comparative Examples 1 and 2 of this application;

[0023] Figure 7 This is a comparison chart of the cycle performance of the secondary batteries prepared in Example 1, Comparative Examples 1 and 2 of this application at high temperature. Detailed Implementation

[0024] To make the technical problems, technical solutions, and beneficial effects of this application clearer, the following detailed description is provided in conjunction with embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0025] In this application, the term "and / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. A and B can be singular or plural. The character " / " generally indicates that the preceding and following related objects have an "or" relationship.

[0026] In this application, "at least one" means one or more, and "more than one" means two or more. "At least one of the following" or similar expressions refer to any combination of these items, including any combination of single or multiple items.

[0027] It should be understood that in the various embodiments of this application, the order of the above processes does not imply the order of execution. Some or all steps may be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of this application.

[0028] The weights of the relevant components mentioned in the embodiments of this application can refer not only to the specific content of each component, but also to the proportional relationship between the weights of the components. Therefore, any scaling up or down of the content of the relevant components according to the embodiments of this application is within the scope disclosed in the embodiments of this application. Specifically, the mass in the embodiments of this application can be a well-known unit of mass in the chemical industry, such as µg, mg, g, or kg.

[0029] The terms "first" and "second" are used for descriptive purposes only, to distinguish objects, such as substances, from one another, and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. For example, without departing from the scope of the embodiments of this application, a first feature may also be referred to as a second feature, and similarly, a second feature may also be referred to as a first feature. Thus, a feature defined with "first" and "second" may explicitly or implicitly include one or more of that feature.

[0030] In existing technologies, layered transition metal oxide cathode materials, such as ternary materials, especially high-nickel ternary materials, exhibit inherent characteristics that allow the transition metal ions (especially Ni) to be easily reduced. 3+ The layered structure of the cathode material is unstable, and lattice oxygen is easily oxidized into reactive oxygen species, readily generating residual alkalis such as LiOH and Li₂CO₃ that adhere to the surface. When battery manufacturers receive this cathode material, they often can only remove the surface alkalis by washing with water or a weak acid. However, water contains ionized hydrogen ions, and weak acids contain even more hydrogen ions. During the cleaning process, a lithium-hydrogen exchange occurs between the cathode material and the cleaning solution, i.e., hydrogen ions intercalate while lithium ions dissolve, leading to lithium ion loss and capacity loss, especially the initial capacity decay. Furthermore, this cleaning method only removes surface alkalis; due to the intrinsic characteristics of the cathode material, residual alkalis continue to form, failing to fundamentally solve the problem. Additionally, the cleaned cathode material still suffers from transition metal dissolution within the battery, where transition metal ions escape from the lattice and dissolve in the electrolyte, causing capacity decay. Moreover, during charge-discharge cycles, direct contact between the cathode material and the electrolyte leads to interfacial side reactions, especially under conditions of fast charging, high temperatures, and acidic electrolytes. Therefore, existing methods for cleaning residual alkali can cause a series of problems, such as battery capacity decay, cycle performance, rate performance and high temperature stability deterioration.

[0031] To address a series of problems such as capacity decay caused by residual alkali during cleaning, the first aspect of this application provides a cleaning method for positive electrode materials, comprising the following steps:

[0032] S10: Prepare a cleaning solution by mixing raw materials, including water-soluble electrolytes, with water;

[0033] S20: The cathode material to be cleaned is mixed with the cleaning solution to obtain the cleaned cathode material;

[0034] Among them, the water-soluble electrolyte contains at least two hydrophilic groups, and the pKa values ​​of the hydrophilic groups are the same or different, ranging from 6 to 10.

[0035] In the cleaning method of this application embodiment, when the positive electrode material to be cleaned is mixed with the cleaning solution, the aqueous solvent dissolves the residual alkali on the surface of the positive electrode material. Simultaneously, because the water-soluble electrolyte contains at least two hydrophilic groups with pKa values ​​within a certain range, it will ionize into at least two anions in the aqueous solution. These anions are surrounded by water molecules due to electrostatic interactions to form stable hydrated anions, and their volume is much larger than that of H+. + Hydrated anions contain a negative charge. Multiple hydrated anions preferentially adsorb onto positively charged sites (metal ion sites) on the surface of the cathode material via electrostatic interactions. This allows the water-soluble electrolyte to bind to the cathode material surface after ionization. The significant steric hindrance effect of hydrated anions can significantly reduce H+. + The probability of approaching the surface of the cathode material increases the concentration of H+ in the aqueous solution. + The energy barrier for cathode material intercalation inhibits lithium-hydrogen exchange between the cathode material and the aqueous solution, reducing lithium-ion loss, improving the stability of the layered structure of the cathode material, and reducing capacity loss of the cathode material itself. In summary, the cleaning method of this embodiment not only removes residual alkali from the surface of the cathode material but also solves the problem of capacity loss caused by cleaning cathode materials in existing technologies.

[0036] Furthermore, hydrophilic groups with pKa values ​​within the aforementioned range are all strongly polar groups. The vast majority of these hydrophilic groups, when ionized in water, form anions that can provide electron pairs, classifying them as Lewis bases. During the mixing of the cleaning solution with the cathode material, these Lewis bases coordinate with the transition metal ions in the cathode material, saturating empty orbitals and forming more robust coordination bonds. The formation of these coordination bonds significantly stabilizes the layered crystal structure of the cathode material. On one hand, this alleviates the dissolution of transition metals, slows down the structural degradation from the interface to the bulk phase, further reduces capacity loss, and improves cycle stability. On the other hand, it fundamentally alleviates the phenomenon of lattice oxygen transforming into reactive oxygen, thus fundamentally mitigating the problem of continuous residual alkali formation in the cathode material.

[0037] Therefore, the cleaning method of this application embodiment is completely different from existing water washing and weak acid cleaning methods. Water washing utilizes the properties of lithium hydroxide being soluble in water and lithium carbonate being slightly soluble in water to remove residual alkali. Weak acid cleaning further utilizes acid-base neutralization reactions to remove residual alkali. Weak acids include citric acid, acetic acid, oxalic acid, phosphoric acid, etc., with a pH range of 2.5 to 4.5 at a 1 wt% concentration. Both of these methods lead to severe lithium-hydrogen exchange, resulting in capacity loss. Moreover, these two methods are difficult to fundamentally improve the stability of the layered structure of the cathode material. The cleaning method of this application embodiment can solve the above problems.

[0038] Furthermore, the formation of the aforementioned coordination bonds also facilitates the binding of water-soluble electrolyte electrolysis products to the surface of the cathode material, synergistically enhancing the adsorption binding strength by electrostatically adsorbing the aforementioned hydrated anions onto the cathode material surface. When the amount of water-soluble electrolyte adhering to the cathode material surface reaches a certain level, a uniform and tightly coated layer can spontaneously form. On one hand, this coating layer acts as a physical barrier, directly isolating H+. + This process reduces lithium-hydrogen exchange and capacity loss. Furthermore, it minimizes direct contact between the cathode material and the electrolyte, reducing interfacial side reactions and mitigating electrolyte penetration into the crystal lattice, thus improving the battery's cycle performance, rate performance, and high-temperature stability. This coating layer forms spontaneously during the mixing process, eliminating the need for additional surfactants and the high-temperature sintering required for carbon coatings as in existing technologies. The cleaning method described in this application is simpler, more environmentally friendly, and energy-efficient.

[0039] Step S10 is the preparation of the cleaning solution. The water-soluble electrolyte contains at least two hydrophilic groups. In the example, this can include any value or a range between any two of 2, 3, 4, 5, or 6 hydrophilic groups. The pKa value is 6 to 10. In the example, the pKa value can include, but is not limited to, any value or a range between any two of 6, 7, 8, or 10. A suitable number of hydrophilic groups can ionize into an appropriate amount of anions in the aqueous solution and form hydrated anions, which is beneficial for inhibiting lithium-hydrogen exchange.

[0040] In some embodiments, the water-soluble electrolyte contains Lewis base active sites, which are atoms or groups that can provide electron pairs. These sites can form coordination bonds with the cathode material, reducing transition metal dissolution, improving cycle stability, enhancing the stability of the layered structure of the cathode material, and mitigating the conversion of lattice oxygen into reactive oxygen species. This fundamentally solves the technical problem of continuous residual alkali formation in the cathode material. These Lewis base active sites can be sites inherent in the ionized hydrophilic groups or sites contained in other groups besides the hydrophilic groups in the water-soluble electrolyte. In some embodiments, the water-soluble electrolyte may also contain amine groups and / or hydroxyl groups. The nitrogen in the amine group contains lone pairs of electrons that can provide electron pairs, as does the oxygen in the hydroxyl group. The amine group has a relatively stronger electron-donating ability, but it is also prone to catalyzing side reactions during battery cycling. Therefore, selection needs to be based on the specific properties exhibited by the water-soluble electrolyte. Thus, both amine and hydroxyl groups can form Lewis base active sites, which is beneficial for the water-soluble electrolyte to form coordination bonds with transition metals in the cathode material, inhibiting transition metal dissolution and further suppressing residual alkali formation.

[0041] In some embodiments, the hydrophilic groups include at least one of phosphate groups and sulfonic acid groups. On the one hand, these two types of groups are hydrophilic and can form corresponding hydrated anions after ionization and depletion. After being electrostatically attracted and bound to the surface of the cathode material, they can suppress lithium-hydrogen exchange through steric hindrance. On the other hand, after depletion, these two types of groups are highly polar, and the oxygen atoms can provide lone pairs of electrons, which are themselves Lewis base active sites. They can form coordination bonds with transition metals in the cathode material, such as POM and SOM bonds (M is Ni, Co, Mn, etc.), fixing the transition metal ions, alleviating transition metal dissolution, and further improving the stability of the layered structure of the cathode material. This stabilizes the lattice oxygen and fundamentally solves the technical problem of continuous residual alkali generation in the cathode material. Furthermore, the first two points enable the water-soluble electrolyte to bind to the surface of the cathode material after electrolysis. When the binding amount reaches a certain level, a coating layer or even a full coating layer can be formed. This coating layer, in addition to the physical barrier mentioned above, prevents H from being absorbed. + In addition to penetrating and reducing direct contact between the cathode material and the electrolyte, it is also rich in phosphoric acid and sulfonic acid groups to conduct lithium ions, forming a lithium ion-conducting network structure. Therefore, the coating layer formed by the water-soluble electrolyte containing phosphoric acid and sulfonic acid groups is also a lithium ion conductor layer. The cleaned cathode material is beneficial to the surface diffusion and insertion / extraction kinetics of lithium ions in the battery, improving the battery's discharge specific capacity and reversible capacity at high rates.

[0042] Specifically, water-soluble electrolytes may include at least one of linear polyphosphoric acid, carbon-chain polyphosphoric acid, and carbon-ring polyphosphoric acid, and further contain at least two hydrophilic groups, and may also contain groups such as amine groups and hydroxyl groups containing Lewis base active sites. Among them, the number of phosphate units in linear polyphosphoric acid is 2 to 9, and the optional chemical formula is H... n+2 P n O 3n+1 Polyphosphoric acid, with n ranging from 2 to 9, is rich in phosphate groups. The main chain of the carbon-chain type includes at least one of a carbon chain and a heteroatom-containing carbon chain, with ≤4 units in the main chain. In an example, it may include at least one of aminotrimethylenephosphonic acid and hydroxyethylidene diphosphonic acid. The ring in the carbon-ring type includes at least one of an alicyclic ring and a benzene ring. In an example, it may include at least one of 4,5-dihydroxy-1,3-benzenedisulfonic acid, 1,3,5-benzenetrisulfonic acid, and cyclohexanehexaphosphate.

[0043] The aforementioned water-soluble electrolytes, such as polyphosphoric acid, 4,5-dihydroxy-1,3-benzenedisulfonic acid, 1,3,5-benzenetrisulfonic acid, cyclohexanehexaphosphate, aminotrimethylenephosphonic acid, and hydroxyethylidene diphosphonic acid, are rich in hydrophilic groups. These groups can ionize and form hydrated anions, which then bind to the surface of the cathode material via electrostatic interactions. Through steric hindrance, they can effectively mitigate lithium-hydrogen exchange. Furthermore, they are rich in groups containing Lewis base active sites, such as phosphate groups, sulfonic acid groups, nitrogen atoms, amino groups, and hydroxyl groups. These groups form coordination bonds with transition metal ions in the cathode material, inhibiting transition metal dissolution and further stabilizing the layered structure of the cathode material, fundamentally suppressing residual alkali. Furthermore, these water-soluble electrolytes contain a large number of Lewis base active sites, which can form multidentate chelates with the cathode material. Combined with the electrostatic effects and the formation of coordination bonds, this greatly improves the bonding strength between the water-soluble electrolyte and the cathode material. Additionally, when the amount of water-soluble electrolyte bound reaches a certain level, a coating layer is formed. Besides providing physical barriers, reducing interfacial side reactions, and forming an ion conductor layer, these coating layers can further reduce the surface energy of the cathode material. Unwashed cathode material surfaces contain a large number of high-valence metal ions (such as Ni). 3+ The surface energy of the residual alkali is relatively high, which can easily exacerbate lithium-hydrogen exchange. Reducing the surface energy can alleviate this phenomenon and reduce capacity loss when cleaning residual alkali. On the other hand, these water-soluble electrolytes exhibit excellent chemical stability under high temperature conditions. The coating layer formed can effectively suppress the release of lattice oxygen in the cathode material, reduce the reaction tendency of active oxygen species with active materials and electrolytes, reduce the bulk structure degradation caused by gas generation and interface deterioration during high-temperature cycling, and significantly improve the long-term stability of cathode materials, especially high-nickel cathode materials, under high-temperature conditions.

[0044] In some embodiments, the mass fraction of the water-soluble electrolyte in the cleaning solution is 0.25% to 5%, and may include, but is not limited to, any value or any two of 0.25%, 0.5%, 1%, 3%, and 5%. This further enhances the effects of cleaning residual alkali, inhibiting lithium-hydrogen exchange, and stabilizing the layered structure, and is more conducive to increasing the amount of water-soluble electrolyte bound to the surface of the cathode material, which is beneficial to the formation of the coating layer and improves the coating modification effect.

[0045] Furthermore, the cleaning method in this application does not require a low-pH solution, nor does it rely on acid-base neutralization reactions. In some embodiments, the pH value of the cleaning solution is 7-9, optionally 7, 8, or 9, to further improve the aforementioned effects of cleaning residual alkali, inhibiting lithium-hydrogen exchange, stabilizing the layered structure, and coating modification. If the pH is too low, similar to existing technologies, it may lead to problems such as lithium-hydrogen exchange, corrosion of the cathode material surface, and dissolution of metal ions. If the pH is too high, it is not conducive to the dissolution and cleaning of residual alkali on the cathode material surface. Therefore, in some embodiments, a pH adjuster may be added to the cleaning solution. The pH adjuster may include hydroxides and / or ammonia to adjust the pH of the cleaning solution.

[0046] In some embodiments, the cleaning solution may also contain lithium ions, with a concentration of 0.0004–0.002 mol / mL. The purpose of adding lithium ions is to utilize the common ion effect to shift the chemical equilibrium and inhibit lithium-hydrogen exchange, which can work synergistically with the aforementioned water-soluble electrolyte to inhibit lithium-hydrogen exchange. Based on Le Chatelier's principle (the principle of equilibrium shift), refer to the following chemical reaction equation for the reversible reaction:

[0047] Li + + H2O → H + + LiOH;

[0048] If Li in the cleaning solution + An increase in concentration will shift the equilibrium to the left, decreasing H₂. + The concentration of Li generated is reduced, while the surface Li of the cathode material is decreased. + The dissolution of H+ reduces lithium-hydrogen exchange and capacity loss. This common ion effect further weakens H+ by modulating the solution chemical environment. + The intercalation driving force effectively alleviates the initial capacity decay problem caused by residual alkali during cleaning in existing technologies. Optionally, at least one of lithium hydroxide, lithium sulfate, lithium chloride, lithium nitrate, and lithium phosphate can be added to the cleaning solution to provide lithium ions, preferably lithium hydroxide. Lithium hydroxide not only provides lithium ions but also acts as a pH adjuster, providing a dual effect and reducing the introduction of impurity ions.

[0049] Step S20 is a step of mixing the cathode material. The cathode material can be a transition metal layered lithium oxide material, which has a prominent residual alkali problem. Optionally, the cathode material is a high nickel content cathode material, where the transition metal dissolution and residual alkali problems are most significant. It can be a ternary cathode material. In some embodiments, the general chemical formula of the cathode material is LiNi. x Co y M z O2, wherein 0.6≤x<1, 0<y<0.4, z=1-xy, and M includes at least one of Al, Mn, and Mg, and can be selected as a high-nickel ternary cathode material such as 811, 721, or 712. Optionally, the Dv50 particle size of the cathode material can be 2–4 μm.

[0050] During the mixing process, the amount of cleaning solution can be higher than the mass of the cathode material. This is beneficial for fully dissolving and washing away residual alkali and increasing the amount of water-soluble electrolyte binding on the surface of the cathode material. In some embodiments, the mass ratio of cathode material to cleaning solution can be 1:(5-25). In exemplary cases, it can be any ratio or any two of 1:5, 1:10, 1:15, 1:20, and 1:25. This is beneficial for further achieving the above-mentioned effects of cleaning residual alkali, inhibiting lithium-hydrogen exchange, stabilizing the layered structure, and coating modification. The cleaning effect of residual alkali is good and the capacity loss is small. The formed coating layer is also of uniform thickness and has a good modification effect.

[0051] When cleaning the cathode material with a cleaning solution, methods such as rinsing, soaking, and stirring can be used. The mixing process in step S20 can be selected as stirring, which facilitates sufficient contact between the cathode material and the water-soluble electrolyte in the cleaning solution, further reducing capacity loss when washing away residual alkali, and also benefits the coating modification of the cathode material. Optionally, the mixing process can be carried out at room temperature, with a selectable temperature of 5–28°C. In some embodiments, the mixing process may include stirring at a speed of 300–1000 rpm for 2–15 minutes. If the mixing time is too long, the possibility of lithium-hydrogen exchange during the process increases, potentially reducing the reversible capacity of the battery; if the mixing time is too short, residual alkali on the surface may not be completely removed, or the coating layer may not form in time, resulting in a high surface resistance of the cathode material, ultimately leading to a reduction in reversible capacity and rate performance at high rates.

[0052] After the above mixing process, the cleaning solution can remove residual alkali from the surface of the cathode material, reduce capacity loss during the process, and stabilize the layered structure of the cathode material. This fundamentally solves the problems of residual alkali formation and transition metal dissolution in the cathode material, and also forms a coating layer on the surface of the cathode material for modification. The slurry obtained after the mixing process needs further solid-liquid separation to obtain the cleaned modified cathode material. Solid-liquid separation can be performed by methods such as filtration, heating and drying, or freeze-drying. Preferably, the drying method is carried out in a protective atmosphere. Optionally, after the mixing process in step S20, the following steps are also included:

[0053] S30: The slurry obtained from the mixing process is filtered, and the filter residue is dried to obtain the cleaned cathode material.

[0054] The filtration process can be vacuum filtration. Here, the drying process simply evaporates the solvent to remove moisture, resulting in the cleaned positive electrode material. Therefore, the coating material remains a water-soluble electrolyte and interacts with the positive electrode material, unlike existing technologies that use carbon precursors to coat the material and form a carbon coating layer through high-temperature sintering. In this embodiment, the drying temperature can be 80–120°C, including but not limited to any value or any two of 80°C, 90°C, 100°C, and 120°C. This removes the solvent and also promotes the stability of the water-soluble electrolyte material and its bonding to the surface of the positive electrode material. If the drying temperature is too high, it can easily lead to an increase in the surface impedance of the material.

[0055] The second aspect of this application provides a modified cathode material, which is the cleaned cathode material obtained by the cleaning method of the above-described application embodiments.

[0056] Because the above cleaning method removes residual alkali from the surface of the cathode material, reduces capacity loss during the process, and binds water-soluble electrolytes to the surface of the cathode material, it further stabilizes the layered structure of the cathode material. This fundamentally solves the problem of continuous generation of residual alkali and dissolution of transition metals in the cathode material. Therefore, the cleaned cathode material can be used as a modified cathode material, which has good processing performance for battery manufacturing, and produces secondary batteries with high effective capacity, good cycle performance and rate performance, outstanding high-temperature stability, and good safety.

[0057] In some embodiments, when the amount of water-soluble electrolyte bound reaches a certain level, the modified cathode material includes a cathode core and a coating layer. The coating layer covers the cathode material core, and the thickness of the coating layer can be 3–10 nm. The material of the core can refer to the types of cathode materials described above, and the material of the coating layer can refer to the substances obtained after the ionization of the water-soluble electrolyte. These materials are combined with the cathode material through electrostatic interactions, coordination bonds, or further through multidentate chelation.

[0058] A third aspect of this application provides a secondary battery, wherein the positive electrode of the secondary battery contains the cleaned positive electrode material obtained by the cleaning method of the above application embodiment, or contains the modified positive electrode material of the above application embodiment.

[0059] Since the cleaning method in the above-mentioned embodiment removes residual alkali from the surface of the cathode material, inhibits lithium-hydrogen exchange, and binds water-soluble electrolyte to the surface of the cathode material, thereby improving the stability of the layered structure of the cathode material, the modified cathode material has good processing performance in battery manufacturing, and the resulting secondary battery has high effective capacity, good cycle performance and rate performance, outstanding high-temperature stability, and good safety.

[0060] The manufacturing of secondary batteries involves first preparing a positive electrode slurry from the cleaned positive electrode material, followed by coating, drying, electrode fabrication, and battery assembly. The positive electrode slurry includes the cleaned positive electrode material, a conductive agent, a binder, and a solvent. The conductive agent can be a commonly used conductive agent, such as at least one from graphite, carbon black, acetylene black, graphene, carbon fiber, C60, and carbon nanotubes, with the conductive agent comprising 0.5 wt% to 8 wt% of the positive electrode slurry. The binder can be a commonly used electrode binder, such as at least one from polyvinylidene chloride, soluble polytetrafluoroethylene, styrene-butadiene rubber, hydroxypropyl methylcellulose, methylcellulose, carboxymethyl cellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan and its derivatives, polyacrylic acid and its salts, with the binder comprising 0.5 to 8 wt% of the positive electrode slurry. The solvent can be at least one from dimethyl sulfoxide, N-methylpyrrolidone, and N,N-dimethylformamide, with the solvent comprising 60 wt% to 80 wt% of the positive electrode slurry. Of course, the positive electrode slurry layer may also contain other positive electrode active materials, such as at least one of lithium cobalt oxide, lithium manganese oxide, lithium iron phosphate, lithium manganese iron phosphate, lithium vanadium phosphate, lithium vanadium oxide phosphate, lithium fluorinated vanadium phosphate, lithium titanate, lithium nickel cobalt manganese oxide, and lithium nickel cobalt aluminum oxide.

[0061] The positive electrode slurry is coated onto the current collector and then dried. The drying temperature can be 80–150℃, preferably 80–120℃. Excessive temperature can lead to increased surface resistance of the material and electrode. After drying, the positive electrode is prepared through rolling, die-cutting, and other steps. Finally, a secondary battery is assembled. Its structure includes a positive electrode, a separator, a negative electrode, an electrolyte, and a battery casing. There are no particular limitations on the separator, negative electrode, electrolyte, and battery casing.

[0062] The following description is based on specific embodiments.

[0063] Example 1

[0064] This embodiment provides a method for cleaning the positive electrode material and a secondary battery. The cleaning method comprises the following steps:

[0065] S1: Provide a 50 wt% cyclohexanehexyl phosphate solution. Add the cyclohexanehexyl phosphate solution, lithium hydroxide, and deionized water to a beaker at a mass ratio of 0.52%, 0.16%, and 99.32% respectively, and stir rapidly at 600 rpm for 10 min at 25°C to obtain a cleaning solution. Lithium hydroxide can be used as a pH adjuster and also provides lithium ions. The pH of the cleaning solution at 25°C is 7.75.

[0066] S2: According to a mass ratio of 1:25, LiNi 0.8 Co 0.1 Mn 0.1 O2 (811 high-nickel ternary) was added to the cleaning solution, and the mixture was stirred at 600 rpm for 7 minutes at room temperature to obtain a slurry.

[0067] S3: The slurry is filtered, and the filter residue is placed in an 80℃ vacuum drying oven for 4 hours to dry the solvent, thus obtaining the cleaned cathode material.

[0068] Then, prepare the secondary battery according to the following steps:

[0069] S4: Add the modified cathode material, carbon black conductive agent (Super P), polyvinylidene fluoride (PVDF), and N-methylpyrrolidone into the mixing tube at a mass ratio of 24:3:2:71 and stir at high speed for 3 minutes each time to disperse evenly, to obtain the cathode slurry.

[0070] S5: Coat the aluminum foil with a uniform positive electrode slurry, then dry it in a vacuum oven at 110°C for 7 hours. After rolling, cut it into a positive electrode sheet that fits the battery case.

[0071] S6: Assembly of button cell: Assemble the above positive electrode sheet and polypropylene separator into the positive electrode shell, add an appropriate amount of high voltage electrolyte, put in the lithium sheet, gasket and negative electrode shell, and then press the battery to obtain a secondary battery.

[0072] Example 2

[0073] This embodiment provides a cleaning method for positive electrode materials and a secondary battery. The only difference from Embodiment 1 is that in step S1, the mass ratio of cyclohexanehexyl phosphate solution to lithium hydroxide is changed to 3.5:1, and the pH value of the cleaning solution is 7; all other aspects are the same.

[0074] Example 3

[0075] This embodiment provides a cleaning method for positive electrode materials and a secondary battery. The only difference from Embodiment 1 is that in step S1, the mass ratio of cyclohexanehexyl phosphate solution to lithium hydroxide is changed to 2.5:1, and the pH value of the cleaning solution is 10; all other aspects are the same.

[0076] Example 4

[0077] This embodiment provides a cleaning method for positive electrode materials and a secondary battery. The only difference from Embodiment 1 is that step S3 is replaced with freeze-drying to remove the solvent; all other steps are the same.

[0078] Example 5

[0079] This embodiment provides a method for cleaning positive electrode materials and a secondary battery. The only difference from Embodiment 1 is that in step S5, the drying temperature of the positive electrode sheet is 150°C; all other aspects are the same.

[0080] Example 6

[0081] This embodiment provides a cleaning method for positive electrode materials and a secondary battery. The only difference from Embodiment 1 is that in step S1, the 50 wt% cyclohexanehexyl phosphate solution is replaced with aminotrimethylenephosphonic acid, and the pH value of the cleaning solution is 7.5; all other aspects are the same.

[0082] Example 7

[0083] This embodiment provides a cleaning method for positive electrode materials and a secondary battery. The only difference from Example 1 is that in step S1, the 50 wt% cyclohexanehexyl phosphate solution is replaced with 20 wt% 4,5-dihydroxy-1,3-benzenedisulfonic acid, and the pH value of the cleaning solution is 9; all other aspects are the same.

[0084] Example 8

[0085] This embodiment provides a method for cleaning positive electrode materials and a secondary battery. The only difference from Embodiment 1 is that in step S1, the mass ratio of positive electrode material to cleaning solution is changed from 1:25 to 1:5; all other aspects are the same.

[0086] Example 9

[0087] This embodiment provides a method for cleaning the positive electrode material and a secondary battery. The only difference from Embodiment 1 is that in step S1, the 50 wt% cyclohexanehexyl phosphate solution is replaced with 50 wt% tripolyphosphate H5P3O. 10 The pH of the cleaning solution was 9, and everything else was the same.

[0088] Example 10

[0089] This embodiment provides a cleaning method for positive electrode materials and a secondary battery. The only difference from Embodiment 1 is that in step S1, instead of adding cyclohexanehexyl phosphate solution, lithium hydroxide, and deionized water to the beaker in a mass ratio of 0.52%, 0.16%, and 99.32%, the method is changed to adding cyclohexanehexyl phosphate solution, 25 wt% ammonia water, lithium sulfate solid, and deionized water to the beaker in a mass ratio of 1%, 0.87%, 0.13%, and 98%. The pH value of the cleaning solution is 8.0, and all other aspects remain the same.

[0090] Example 11

[0091] This embodiment provides a cleaning method for positive electrode materials and a secondary battery. The only difference from Embodiment 1 is that in step S1, instead of adding cyclohexanehexyl phosphate solution, lithium hydroxide, and deionized water to the beaker in a mass ratio of 0.52%, 0.16%, and 99.32%, the cyclohexanehexyl phosphate solution, 25 wt% ammonia solution, and deionized water are added to the beaker in a mass ratio of 1%, 1%, and 98%. The pH value of the cleaning solution is 8.9, and all other aspects remain the same.

[0092] Comparative Example 1

[0093] The only difference between this comparative example and Example 1 is that steps S1 to S3 are not included, that is, the positive electrode material is not cleaned and is directly used in the manufacture of secondary batteries.

[0094] Comparative Example 2

[0095] The only difference between this comparative example and Example 1 is that step S1 is omitted, and the cleaning solution in step S2 is replaced with a 1wt% citric acid solution for cleaning; all other aspects are the same.

[0096] Performance testing

[0097] 1. Material surface morphology

[0098] Samples of the cleaned cathode materials from Examples 1-11, Comparative Example 1, and Comparative Example 2 were taken and SEM images were taken using a scanning electron microscope. Figure 1 This is a surface morphology image of the cleaned cathode material from Example 1. Figure 2 This is a surface morphology diagram of the cathode material in Comparative Example 1. Figure 3 This is a surface morphology diagram of the cleaned cathode material in Comparative Example 2.

[0099] Depend on Figures 1-3 The comparison shows that, Figure 2 In Comparative Example 1, the surface of the unwashed cathode material had many tiny residual alkali particles remaining, while Figure 1 , Figure 3In both Example 1 and Comparative Example 2, a considerable amount of residual alkali was removed after cleaning, and the surface of the cleaned cathode material was relatively neat and clean.

[0100] 2. Material residual alkali content test

[0101] Samples of the cleaned cathode materials in Examples 1-11, Comparative Example 1, and Comparative Example 2 were taken and tested and verified for residual alkali content by acid-base titration and XPS analysis, including tests for total alkali, LiOH, and Li2CO3.

[0102] The results of Example 1, Comparative Examples 1 and 2 are as follows: Figure 4 As shown, Figure 4 As can be seen, Comparative Example 1 did not clean the cathode material, and its residual alkali content was extremely high. In contrast, Example 1 and Comparative Example 2 were both cleaned, and the residual alkali content decreased significantly. Moreover, the residual alkali content of Example 1 was even lower, which can be compared with the above SEM results.

[0103] 3. Cyclic performance test

[0104] The secondary batteries from Examples 1-11 and Comparative Examples 1 and 2 were subjected to charge-discharge cycle performance tests. The test conditions were 25°C, a charging cut-off voltage of 4.6 V, a discharging cut-off voltage of 3 V, a rate of increase of 0.5C, and at least 100 cycles. The cycle performance of Examples 1-7 and Comparative Examples 1 and 2 was compared to... Figure 5 As shown, it can be seen that the cathode material in Comparative Example 1 was not cleaned, and its discharge specific capacity decayed most significantly during charge-discharge cycles. Although Comparative Example 2 underwent the above cleaning treatment, and the residual alkali content of the cathode material decreased significantly, the cleaning method also led to a significant capacity decay in the secondary battery prepared from the material. In contrast, the cycle performance of all embodiments was relatively good, and the capacity decay was small.

[0105] 4. Ratio Performance Test

[0106] The secondary batteries from Examples 1-11 and Comparative Examples 1 and 2 were subjected to rate performance tests. Constant current and constant voltage charging at the same rate were used, followed by constant current discharging tests at different rates to evaluate performance at different discharge rates. Each case was tested following the same procedures. The rate performance comparisons for Examples 1 and Comparative Examples 1 and 2 are as follows: Figure 6 As shown, from Figure 6 As can be seen, the rate performance of Example 1 is better than that of Comparative Example 2, and even better than that of Comparative Example 1. Especially at high rates, the discharge capacity of the secondary batteries of Comparative Example 1 and Comparative Example 2 decreases significantly.

[0107] 5. Performance stability test at high temperature

[0108] The secondary batteries from Examples 1-11 and Comparative Examples 1 and 2 were subjected to charge-discharge cycle performance tests. The test conditions were 45°C, a charging cut-off voltage of 4.6 V, a discharging cut-off voltage of 3 V, and a rate of 0.5C. The cycle performance of Examples 1 and Comparative Examples 1 and 2 was compared to... Figure 7 As shown, it can be seen that the secondary battery of Example 1 has the best cycle stability at high temperature and the lowest discharge capacity decay, while Comparative Examples 1 and 2 have poor high temperature resistance and rapid discharge capacity decay.

[0109] In summary, existing technologies can lead to problems such as battery capacity decay, poor cycle performance, poor rate performance, and poor performance at high temperatures after cleaning residual alkali. However, the cleaning method in this application can not only clean the residual alkali on the surface of the positive electrode material, but also make the battery's performance more stable.

[0110] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method for cleaning a positive electrode material, characterized in that, Includes the following steps: Prepare a cleaning solution by mixing raw materials, including water-soluble electrolytes, with water; The positive electrode material to be cleaned is mixed with the cleaning solution. The slurry obtained from the mixing process is filtered, and the filter residue is dried to obtain the cleaned positive electrode material. The general chemical formula of the cathode material is LiNi. x Co y M z O2, wherein 0.6≤x<1, 0<y<0.4, z=1-xy, and M includes at least one of Al, Mn, and Mg; The mass ratio of the positive electrode material to the cleaning solution is 1:(5-25). The water-soluble electrolyte contains at least two hydrophilic groups, and the pKa values ​​of the hydrophilic groups are either the same or different, ranging from 6 to 10. The pH value of the cleaning solution is 7-9; a pH adjuster is also added to the cleaning solution, the pH adjuster including hydroxide and / or ammonia; The drying temperature is 80–120°C; The water-soluble electrolyte includes at least one of polyphosphoric acid, 4,5-dihydroxy-1,3-benzenedisulfonic acid, 1,3,5-benzenedisulfonic acid, cyclohexanehexaphosphate, aminotrimethylenephosphonic acid, and hydroxyethylidene diphosphonic acid. The water-soluble electrolyte has a mass fraction of 0.25% to 5% in the cleaning solution; The cleaned cathode material includes a cathode material core and a coating layer, wherein the coating layer covers the cathode material core, the coating layer contains the water-soluble electrolyte, and the thickness of the coating layer is 3 to 10 nm.

2. The cleaning method for the positive electrode material according to claim 1, characterized in that: The cleaning solution also contains lithium ions, and the concentration of lithium ions is 0.0004 to 0.002 mol / mL.

3. The cleaning method for the positive electrode material according to claim 1, characterized in that: The Dv50 particle size of the cathode material is 2–4 μm.

4. The cleaning method for the positive electrode material according to claim 1, characterized in that: The mixing process is performed at a temperature of 5–28°C; and / or The mixing process includes stirring, with a stirring speed of 300–1000 rpm and a stirring time of 2–15 min.

5. A modified cathode material, characterized in that: The modified cathode material is the cathode material cleaned by the cleaning method described in any one of claims 1 to 4.

6. A secondary battery, characterized in that: The positive electrode of the secondary battery contains the cleaned positive electrode material obtained by the cleaning method as described in any one of claims 1 to 4, or contains the modified positive electrode material as described in claim 5.

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