A zif-67 derived composite lithium oxalate lithium supplementing agent, a preparation method and application thereof

Abstract

The application belongs to the field of lithium ion batteries, and discloses a ZIF-67 derived lithium oxalate lithium supplementing agent, a preparation method and application thereof. The lithium supplementing agent comprises a lithium oxalate matrix and a ZIF-67 derived Co-based catalyst. The ZIF-67 derived Co-based catalyst is uniformly distributed on the surface or intercrystalline of the lithium oxalate particles to form a composite structure. The preparation adopts an aqueous phase synthesis method to convert the ZIF-67 into Co-based nanoparticles and form a composite structure with the lithium oxalate particles. The lithium supplementing agent can be applied to a lithium ion battery positive electrode as a positive electrode additive. In the first charging process of the battery, in-situ thermal decomposition / electrolysis reaction occurs to release Li+ to compensate for irreversible capacity loss. The decomposition potential can be reduced to below 4.0 V, the first charging efficiency is increased to above 90%, and the cycle stability is significantly enhanced.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, specifically relating to a ZIF-67-derived composite lithium oxalate supplement agent, its preparation method, and its application. Background Technology

[0002] In the field of lithium-ion batteries, irreversible lithium loss can occur during the first cycle due to factors such as the formation of the SEI film (solid electrolyte interface film), which in turn reduces the battery's energy density and cycle life. The key to solving this problem is to introduce lithium replenishment.

[0003] Lithium oxalate (Li₂C₂O₄), as a high-capacity, low-toxicity lithium replenisher, can directly compensate for lithium loss by releasing Li⁺ through decomposition. However, the decomposition potential of lithium oxalate is generally higher than 4.5 V, far exceeding the stability window of existing electrolytes, which easily leads to problems such as electrolyte decomposition, gas generation, and low initial efficiency, thus restricting its application in high-energy-density batteries.

[0004] To reduce the decomposition potential of lithium oxalate, existing technologies attempt to construct composite systems by combining catalytic materials such as composite metal oxides (e.g., Co3O4, MnO2). However, problems such as poor interfacial coupling, low conductivity, and complex composite processes still exist, making it difficult to scale up production.

[0005] In recent years, metal-organic frameworks (MOFs), especially ZIF-67, have shown great potential in electrocatalysis and electrode material modification due to their highly ordered pore structure, abundant Co active centers, and ability to form Co / C heterogeneous catalytic interfaces after thermal treatment. ZIF-67 can be converted into Co or CoOx nanoparticles with controllable particle size through medium- and low-temperature pyrolysis, and then coated within carbon layers or nitrogen-doped carbon layers, forming a triple functional structure of "catalysis-conductivity-structural support," providing a new direction for the composite modification of lithium oxalate.

[0006] Theoretically, if ZIF-67 is combined with lithium oxalate and a nanocomposite structure is constructed through in-situ synthesis and low-temperature calcination, it is expected that the electrochemical active sites of lithium oxalate can be activated by the catalytic effect of ZIF-67 pyrolysis products, thereby reducing its decomposition energy barrier. Simultaneously, the conductivity and interfacial compatibility of the carbon layer can improve the contact state between lithium oxalate and the electrolyte and electrodes. However, in practical research, the combination of ZIF-67 and lithium oxalate still faces the following technical challenges: mismatch between solubility and interfacial affinity—ZIF-67 easily aggregates in aqueous solution, making it difficult to achieve microscale uniform blending with lithium oxalate particles; unclear combination mechanism—the interaction mode between ZIF-67 and lithium oxalate, and the formation law of the catalytic phase during pyrolysis, are not yet clear; lack of decomposition regulation mechanism—the specific action pathway of ZIF-67-derived catalysts on the C-C bond breaking of lithium oxalate is unclear, making it impossible to precisely control the decomposition potential.

[0007] Therefore, there is an urgent need to develop a technical solution that can achieve efficient composite preparation of ZIF-67 and lithium oxalate with low energy consumption and significantly reduce the decomposition potential of lithium oxalate. Summary of the Invention

[0008] The purpose of this invention is to address the problems of high decomposition potential, low lithium replenishment efficiency, and high preparation cost of existing lithium oxalate replenishing agents by providing a ZIF-67-derived composite lithium oxalate replenishing agent, its preparation method, and its application. The method involves co-synthesizing lithium oxalate and ZIF-67 in an aqueous phase, followed by low-temperature calcination to form a composite structure. The Co-based nanoparticles formed after ZIF-67 calcination are uniformly distributed on the surface of lithium oxalate, constructing an effective catalytic interface, significantly reducing the decomposition voltage, and improving the first-charge efficiency.

[0009] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0010] A ZIF-67-derived composite lithium oxalate supplementary agent comprises a lithium oxalate matrix and a ZIF-67-derived Co-based catalyst, wherein the ZIF-67-derived Co-based catalyst is Co / C and / or CoO. X / NC nanoparticles are uniformly distributed on the surface or between the crystals of lithium oxalate particles, forming a "dot-like embedding" or "surface-like coating" composite structure.

[0011] The lithium replenishing agent of the present invention forms a composite structure by combining a lithium oxalate matrix with a ZIF-67-derived Co-based catalyst. The Co-based catalyst is uniformly distributed on the surface or between the crystals of the lithium oxalate particles, which can give full play to the synergistic effect of catalysis and conductivity. It can reduce the decomposition potential of lithium oxalate to match the electrolyte stability window and reduce side reactions, improve the first charging efficiency to effectively compensate for irreversible lithium loss, and enhance charge transfer and structural stability, thereby improving the battery cycle performance.

[0012] As a further description of the above technical solution: it also includes conductive carbon material, wherein the mass ratio of lithium oxalate, ZIF-67-derived Co-based catalyst to conductive carbon material is (12-15):(1.5-2.5):(0.8-1.5). This ensures that lithium oxalate, as the main lithium supplement, accounts for a sufficient proportion to provide a sufficient lithium source, while maintaining an effective amount of ZIF-67-derived Co-based catalyst to reduce the decomposition potential of lithium oxalate. At the same time, by supplementing with an appropriate amount of conductive carbon material, a continuous electron transport network is constructed, which, in conjunction with the conductivity of the Co-based catalyst, further reduces the internal resistance of the electrode, improves the charge transfer efficiency, avoids catalytic failure or uneven lithium supplementation due to insufficient local conductivity, and is also compatible with electrode slurry preparation and coating processes to ensure that the three components are uniformly dispersed in the electrode. Ultimately, this synergistically enhances the capacity release efficiency, first-charge efficiency, and long-term cycle stability of the battery.

[0013] As a further description of the above technical solution: the ZIF-67-derived Co-based catalyst has a particle size of 50–200 nm and a surface coating layer (C layer or NC layer) thickness of <20 nm. This nanoscale size ensures sufficient specific surface area, allowing the catalytic active sites (Co or CoOx) to be fully exposed for efficient catalytic decomposition of lithium oxalate and reduction of its decomposition potential. It also avoids particle agglomeration caused by excessively small particle size, preventing the active sites from being blocked and ineffective, or uneven dispersion caused by excessively large particle size, ensuring full contact with the lithium oxalate particles. Furthermore, the relatively small surface coating layer thickness allows for the construction of continuous electron transport channels based on the ultrathin structure, synergistically improving charge transfer efficiency and avoiding localized insufficient conductivity. It also protects the catalytic active center, reducing side reactions with the electrolyte, while not hindering Li… + The catalyst is released and migrates from lithium oxalate, thereby forming an integrated interface structure on the lithium oxalate surface that is "highly efficient in catalysis, stable in conductivity, and allows for smooth ion flow," further ensuring the capacity release efficiency, initial charge efficiency, and long-term cycle stability of the lithium replenishment agent.

[0014] As a further description of the above technical solution: the purity of the lithium oxalate is ≥99.5%, which can minimize the introduction of impurities, avoid side reactions caused by impurities from interfering with the lithium replenishment process, and ensure the effective release and utilization of the lithium source; the D50 particle size is between 3 and 8 μm, which can provide a suitable attachment substrate for the ZIF-67 derived Co-based catalyst, ensuring that the Co-based catalyst is uniformly distributed to form a stable "dot-like embedding" or "area-like coating" composite structure to enhance the interfacial interaction, and can also avoid particle agglomeration caused by too small a particle size or uneven dispersion caused by too large a particle size. It is suitable for electrode slurry preparation and coating processes, so that the lithium replenishment agent is uniformly dispersed in the electrode, further synergistically improving the lithium replenishment efficiency, first charge performance and battery cycle stability.

[0015] As a further description of the above technical solution: the conductive carbon material is selected from carbon nanotubes (diameter 5-20 nm, aspect ratio >100) or conductive carbon black (Super P or Ketjenblack). Carbon nanotubes can construct continuous electron transport channels based on their one-dimensional tubular structure, while conductive carbon black achieves efficient charge transfer due to its small particle size and high specific surface area. It can work synergistically with ZIF-67-derived Co-based catalysts to further optimize the internal conductive network of the lithium replenisher, avoiding catalytic failure or uneven lithium replenishment caused by insufficient local conductivity. At the same time, both materials are easy to uniformly disperse with lithium oxalate and Co-based catalysts in aqueous phase synthesis and subsequent electrode slurry preparation, without causing component agglomeration or stratification. They are compatible with existing industrial preparation processes, and both are low-cost conductive materials that are maturely applied in the lithium battery field. This ensures the stability of the conductivity of the lithium replenisher while controlling the cost of raw materials, ultimately synergistically improving the capacity release efficiency, charge transfer rate, and long-term cycle stability of the battery, avoiding the impact of improper selection of conductive materials on the overall function.

[0016] This invention also provides a method for preparing the above-mentioned ZIF-67-derived composite lithium oxalate supplement, comprising the following steps:

[0017] (1) Cobalt salt (Co 2+ When 2-methylimidazole (Hmim) is added to deionized water, a coordination precipitation occurs to form the ZIF-67 precursor.

[0018] (2) Add ZIF-67 precursor to lithium oxalate aqueous solution (ultrasonic dispersion or wet mixing can be performed before blending with ZIF-67 to improve the interfacial contact area), and lithium oxalate and ZIF-67 precursor are co-deposited in situ to form ZIF-67@Li2C2O4 composite precursor.

[0019] (3) After vacuum drying, the composite precursor is placed in a tube furnace and calcined at 300°C for 2 hours in an argon atmosphere at a heating rate of 2-3°C / min. This allows the organic ligand (2-methylimidazole) of ZIF-67 to carbonize in an inert atmosphere to form a carbon layer (C) or a nitrogen-doped carbon layer (NC), which contains Co. 2+ It is then converted into metallic Co or low-valent Co oxide (CoOx) and encapsulated by a carbon layer (C) or a nitrogen-doped carbon layer (NC), realizing the transformation of ZIF-67 into a Co / C or CoOx / NC structure, and forming a catalytically active phase in situ on the surface of lithium oxalate particles. Finally, after natural cooling, it is ground and sieved (≤400 mesh) to obtain the ZIF-67-derived composite lithium oxalate supplement.

[0020] This preparation method involves in-situ reaction in an aqueous phase to form a ZIF-67 precursor (a purple metal-organic framework crystal with a rhombic dodecahedral structure, a particle size range of 100–300 nm, and a specific surface area >800 m²). 2 ZIF-67 nanocrystals are then co-deposited with lithium oxalate to achieve uniform blending at the microscale, ensuring a uniform coating distribution of ZIF-67 nanocrystals on or between the lithium oxalate surface. Vacuum drying combined with low-temperature calcination (300℃) under argon atmosphere protection and precise heating rate (2–3℃ / min) gently converts ZIF-67 into a Co / C or CoOx / NC-rich Co-based catalyst, improving interfacial electron transport efficiency and providing active sites for lithium oxalate decomposition, achieving a three-in-one functional integration of conductivity, catalysis, and lithium donation. This also avoids premature decomposition of lithium oxalate due to high temperatures, reducing oxidation side reactions and ensuring catalyst structural stability. Natural cooling followed by grinding and sieving further controls particle uniformity, adapting to subsequent electrode slurry preparation processes. The entire process requires no organic solvents, has low energy consumption and minimal pollution, and the synergistic effect of each step ensures a tight and uniform interfacial bond between the Co-based catalyst and lithium oxalate in the product. The resulting lithium replenishment agent exhibits stable performance, is easily scalable, and is compatible with existing battery fabrication processes.

[0021] As a further description of the above technical solution: the cobalt salt in step (1) is cobalt nitrate (Co(NO3)2·6H2O), and the molar ratio of cobalt salt to 2-methylimidazole is 1:(8-12). Cobalt nitrate has good solubility in deionized water, its purity is easy to control, and it can stably provide Co. 2+ To ensure the smooth in-situ synthesis of the ZIF-67 precursor, and to ensure that it contains only N, O, and Co elements, minimizing the introduction of impurities that could interfere with the subsequent formation of the Co-based catalyst; simultaneously, to control the molar ratio of 2-methylimidazole to cobalt salt, an excess of 2-methylimidazole can ensure the formation of Co. 2+ Sufficient coordination leads to the generation of ZIF-67 precursors with complete structure, uniform particle size, and rhombic dodecahedral structure, avoiding structural defects in the precursors due to insufficient coordination. This provides a high-quality foundation for subsequent calcination and conversion into highly active Co-based catalysts, ultimately ensuring the catalytic efficiency of Co-based catalysts in lithium supplementation agents. It is also compatible with aqueous synthesis processes, taking into account both the availability of raw materials and the stability of product performance.

[0022] As a further description of the above technical solution: the mass ratio of lithium oxalate to ZIF-67 precursor in step (2) is (10-15):(1-3). This ensures that lithium oxalate has a sufficient proportion as the main lithium supplement, providing enough lithium source to compensate for irreversible lithium loss during the first cycle of the battery. It also ensures that the amount of ZIF-67 precursor is reasonable, so that it can be converted into sufficient Co / C or CoOx / NC nanoparticles after calcination. This avoids insignificant catalytic effect and high decomposition potential due to insufficient catalyst, or excessive catalyst proportion crowding out lithium oxalate space and reducing the overall lithium capacity of the lithium supplement (specific capacity < 400). (mAh / g), with large conductivity fluctuations; at the same time, it can also achieve microscale uniform mixing of the two during the co-deposition process, providing a basis for the formation of a tight "dot-like embedding" or "area-like coating" composite structure in the subsequent calcination, enhancing the interfacial interaction between the catalyst and lithium oxalate, thereby synergistically improving lithium replenishment efficiency, first charge efficiency and battery cycle stability, and can also be adapted to the subsequent electrode slurry preparation process to avoid particle agglomeration or uneven dispersion caused by excessive amount of a certain component.

[0023] This invention also provides the application of the aforementioned ZIF-67-derived composite lithium oxalate supplement as a positive electrode additive in lithium-ion batteries. It directly addresses the irreversible lithium loss caused by SEI film formation during the first charging process of lithium-ion battery positive electrodes by releasing Li through the decomposition of lithium oxalate. +This lithium replenishment agent achieves precise compensation and, relying on a Co-based catalyst derived from ZIF-67, reduces the lithium oxalate decomposition potential to a range suitable for the electrolyte's stability window, avoiding side reactions such as electrolyte oxidation and gas generation at high potentials. Simultaneously, conductive carbon materials can be added to the lithium replenishment agent, synergistically constructing a continuous electron transport network with the Co-based catalyst to optimize the charge transfer efficiency within the cathode, preventing uneven lithium replenishment or catalytic failure. Furthermore, this lithium replenishment agent exhibits good compatibility with cathode active materials, conductive agents, and binders, allowing direct adaptation to existing cathode slurry and coating processes without production line adjustments. This not only improves the battery's initial charging efficiency and long-term cycle stability but also ensures convenience and economy for industrial applications, further unlocking the performance potential of high-energy-density lithium-ion batteries.

[0024] As a further description of the above technical solution: the amount of lithium replenishing agent added to the positive electrode of the lithium-ion battery is 8-12% of the mass of the positive electrode active material. It is mixed with the positive electrode material, conductive agent, and binder in a mass ratio of (90-92):(2-5):(2-5) to form a slurry for coating. This ensures that sufficient lithium oxalate decomposes and releases Li. + This method effectively compensates for irreversible lithium loss caused by SEI film formation during the first charge of the cathode, and also maintains the ZIF-67-derived Co-based catalyst at a dosage that can efficiently reduce the lithium oxalate decomposition potential. This avoids the problem of insufficient lithium replenishment and insignificant improvement in first-charge efficiency due to insufficient addition (below 8%) (when the addition is below 8%, the coulombic efficiency improvement in the first week is <5%). At the same time, it prevents the lithium replenishment agent from crowding out the space of the cathode active material, causing electrode expansion and cracking (thickness change exceeding 15%), or affecting the electrode compaction density when the addition is too high (above 12%), thus benefiting cycle stability. Moreover, this addition ratio can also be adapted to the cathode slurry preparation process, ensuring that the lithium replenishment agent is uniformly mixed with the cathode active material, conductive agent, and binder, without slurry agglomeration or stratification. Ultimately, while ensuring the original energy density of the cathode, it synergistically improves the battery's first-charge efficiency, capacity release efficiency, and long-term cycle stability.

[0025] In terms of materials design, this invention introduces a ZIF-67 precursor (made from Co) into an aqueous system. 2 (⁺ Coordination with 2-methylimidazole to form a synergistic reaction with lithium oxalate solution) achieves uniform blending of the two precursors at the microscale, ensuring a uniform coating distribution of ZIF-67 nanocrystals on or between the lithium oxalate surface. During subsequent heat treatment, ZIF-67 can be converted into Co / C or CoO-rich compounds. X / NC's Co-based catalyst not only improves the efficiency of interfacial electron transport, but also provides active sites for the decomposition reaction of lithium oxalate, achieving a three-in-one functional integration of "conductivity-catalysis-lithium supply".

[0026] In terms of structural control, this invention employs a low-temperature calcination process (300°C, Ar atmosphere, 2 h) to control the carbonization degree of ZIF-67 and suppress the thermal decomposition of lithium oxalate. The nanoscale Co-based particles formed after calcination can be embedded or coated onto the surface of the lithium oxalate matrix, establishing a strong interfacial coupling relationship with the oxalate structure. Microstructurally, this manifests as lithium oxalate particles being coated with multiphase Co / C and / or CoO. X / NC nanoparticles are embedded in dots or coated in a planar manner, which effectively reduces the reaction energy barrier required for their oxidative decomposition, while constructing continuous electronic pathways, significantly improving the charge transfer rate and structural stability.

[0027] In terms of electrochemical performance, the ZIF-67-derived composite lithium oxalate supplement constructed in this invention can stably release Li⁺ within the range of <4.0 V during the first charge, improving the first charge efficiency to over 90%. Adding 5–10 wt% of the supplement to the high areal capacity cathode electrode improves the first-cycle coulombic efficiency of the full cell to over 91%. After 100 cycles, the electrode maintains structural integrity (SEM confirmed no pulverization or cracking), and the capacity retention rate is improved by 7–18 percentage points, demonstrating excellent long-term stability and practical potential. Compared to traditional Co₃O₄ / lithium oxalate composites, the strategy of this invention significantly reduces the addition ratio (≤10%), improves lithium supplementation efficiency, simplifies the composite process, and effectively suppresses electrolyte side reactions.

[0028] In summary, compared with the prior art, the present invention has the following beneficial effects:

[0029] 1. Significantly reduced decomposition potential and greatly improved first-charge efficiency: This invention introduces a ZIF-67 precursor, and the Co-based catalyst formed by in-situ pyrolysis can effectively catalyze the decomposition of lithium oxalate. In linear sweep voltammetry (LSV) tests at a scan rate of 0.1 mV / s, the decomposition potential is reduced to 3.32–3.82 V (vs. Li⁺ / Li), which is fully compatible with the existing electrolyte stability window. After adding 10 wt% lithium replenishing agent to the high areal capacity electrode, the first-charge efficiency is increased from 22% to over 91%, effectively alleviating lithium depletion and improving battery energy output.

[0030] 2. Enhanced interfacial synergy and suppression of side reactions: The Co-based catalyst and lithium oxalate matrix are nanoscale composite coated through low-temperature calcination to construct an integrated "conductive-catalytic" interface, forming a highly contacted and conductive continuous composite structure, which improves the interfacial charge transfer rate. At the same time, the catalyst can also shield the direct contact between lithium oxalate and electrolyte, avoiding the oxidative decomposition of electrolyte at high potential. The stability of the electrode structure is significantly enhanced during cycling. After 100 cycles, there is no obvious pulverization or cracking on the electrode surface (SEM verification), and the stability of the SEI film is significantly enhanced.

[0031] 3. The process is green and simple, with optimized energy consumption and cost: This invention adopts in-situ synthesis in aqueous phase and low-temperature calcination at 300℃, without the need for any organic solvents, freeze drying or high-temperature reduction steps. The overall energy consumption is controlled at 1.0~1.5 kWh / kg, which is lower than the traditional wet composite process (> 5 kWh / kg). The raw material cost is low (ZIF-67 is prepared from inexpensive cobalt nitrate and 2-methylimidazole), and the overall raw material cost is reduced by more than 30% compared with the traditional composite lithium oxalate system, making it suitable for industrial expansion.

[0032] 4. Improved battery performance and strong practicality: Batteries using this lithium replenishment agent have a capacity release efficiency of >90%, a capacity retention rate of 7-18 percentage points after 100 cycles, and a capacity decay rate of <0.14% / week, which is better than traditional lithium replenishment systems (more than 0.5% / week), making them suitable for the demanding use scenarios of high energy density batteries.

[0033] 5. High industrial adaptability and strong compatibility: The lithium supplement agent of this invention is directly compatible with existing positive electrode slurry preparation processes (NMP solvent system) and does not cause slurry agglomeration or delamination; after rolling, the electrode surface is uniform and dense, with a thickness deviation between sheets <5%, and has extremely high inter-sheet stability and coating uniformity. It can be adapted to existing lithium battery electrode production lines without the need for additional equipment or changes to the process route. Attached Figure Description

[0034] Figure 1 The image shows a SEM image of the ZIF-67-derived carbon-composite lithium oxalate supplement from Example 1.

[0035] Figure 2 This is a TEM image of the ZIF-67-derived carbon-composite lithium oxalate supplement from Example 1.

[0036] Figure 3 XPS spectrum of the ZIF-67-derived composite lithium oxalate supplement from Example 1.

[0037] Figure 4 SEM images of the comparative Co3O4 composite lithium oxalate supplement.

[0038] Figure 5 The first charge-discharge curve of the full battery corresponding to experimental sample 1 is shown. Detailed Implementation

[0039] The following specific embodiments and comparative examples further illustrate the technical effects of the present invention. The embodiments given are only for explaining the present invention and do not constitute any limitation on the present invention. Any limited modifications made by any person within the scope of protection of the claims of the present invention are still within the scope of protection of the claims of the present invention.

[0040] All raw materials used in the examples are commercially available, and all operations were performed at room temperature (25°C) unless otherwise specified.

[0041] Example 1

[0042] This embodiment provides a ZIF-67-derived composite lithium oxalate supplement, comprising a lithium oxalate matrix, a ZIF-67-derived Co-based catalyst, and multi-walled carbon nanotubes. Its preparation process includes the following steps:

[0043] (1) Cobalt nitrate (Co(NO3)2・6H2O) and 2-methylimidazole (Hmim) were added to deionized water at a molar ratio of 1:8 and stirred at room temperature for 30 min to form a ZIF-67 precursor (particle size of about 200 nm).

[0044] (2) Add ZIF-67 precursor to an aqueous solution of lithium oxalate (Li2C2O4, commercial grade, purity ≥99.5%, D50 particle size = 5 μm) and multi-walled carbon nanotubes (diameter 20 nm, length 1-5 μm) at the same time. The mass ratio of lithium oxalate, ZIF-67 precursor to multi-walled carbon nanotubes is 12:2:1. Continue stirring for 40 min to form ZIF-67@Li2C2O4 composite precursor.

[0045] (3) The ZIF-67@Li2C2O4 composite precursor was vacuum dried at 70℃ for 20 h, and then placed in a tube furnace. Argon gas was introduced into the protective atmosphere (to avoid excessive oxidation of Co). The argon gas flow rate was 80 mL / min, and the temperature was increased to 300℃ at 2℃ / min. The temperature was kept constant for 2 h to convert the ZIF-67 precursor into Co / C or CoOx / NC nanoparticles and uniformly distributed on the surface or between the crystals of lithium oxalate particles. Then, it was naturally cooled to room temperature, ground, and passed through a 400-mesh sieve to obtain the ZIF-67-derived composite lithium oxalate supplement.

[0046] The prepared ZIF-67-derived composite lithium oxalate supplement was tested by scanning electron microscopy (SEM), and the results are as follows: Figure 1 As shown in the figure, several Co-based nanoparticles with a diameter of 50–200 nm are uniformly distributed on the surface of the lithium oxalate particles (the larger blocky particles in the figure), forming a "dot-like embedded" or "area-like coated" composite structure. No obvious aggregation is observed, proving that the ZIF-67-derived Co-based nanoparticles have good interfacial bonding with lithium oxalate. Furthermore, from… Figure 2 It can be seen that the particle size of the ZIF-67-derived Co-based catalyst is in the range of 50–200 nm, and the thickness of the surface coating layer is <20 nm.

[0047] The X-ray photoelectron spectroscopy (XPS) results of the O1s orbitals of pure lithium oxalate and the ZIF-67-derived lithium oxalate composite lithium supplement prepared in this example are as follows: Figure 3 As shown, the O1s peak intensity of pure lithium oxalate (A) is low, indicating a low oxygen content and a single peak shape, corresponding to the chemical environment of the C=O bond in lithium oxalate. This proves that the oxygen in the material mainly comes from lithium oxalate itself. In this embodiment, the peak intensity of the ZIF-67 derived composite lithium oxalate supplement (B) is significantly higher than that of pure lithium oxalate, indicating a higher oxygen content. The peak shape corresponds to various chemical environments such as the C=O bond of lithium oxalate, the CO bond of the carbon layer of the ZIF-67 derived catalyst, and the NO bond of the nitrogen-doped carbon layer. This proves that the ZIF-67 derived catalyst and lithium oxalate form a strong interfacial coupling through chemical bonds (such as Co-OC) rather than physical mixing. This is the key structural guarantee for the supplement to efficiently reduce the decomposition potential of lithium oxalate.

[0048] Example 2

[0049] This embodiment provides a ZIF-67-derived composite lithium oxalate supplementary lithium agent, comprising a lithium oxalate matrix, a ZIF-67-derived Co-based catalyst, and conductive carbon black (Super P). Its preparation process includes the following steps:

[0050] (1) Cobalt nitrate (Co(NO3)2・6H2O) and 2-methylimidazole (Hmim) were added to deionized water at a molar ratio of 1:12 and stirred at room temperature for 30 min to form a ZIF-67 precursor (particle size of about 200 nm).

[0051] (2) Add ZIF-67 precursor to lithium oxalate aqueous solution and simultaneously add conductive carbon black (Super P). The mass ratio of lithium oxalate, ZIF-67 precursor and conductive carbon black is 13:2.2:1.2. Continue stirring for 45 min to form ZIF-67@Li2C2O4 composite precursor.

[0052] (3) The ZIF-67@Li2C2O4 composite precursor was vacuum dried at 75℃ for 16 h, and then placed in a tube furnace with an argon protective atmosphere. The argon flow rate was 90 mL / min, and the temperature was increased to 300℃ at 3℃ / min. The calcination was carried out at a constant temperature for 2 h to convert the ZIF-67 precursor into Co / C or CoOx / NC nanoparticles and uniformly distributed on the surface or between the crystals of lithium oxalate particles. Then, it was naturally cooled to room temperature, ground, and passed through a 400-mesh sieve to obtain the ZIF-67-derived composite lithium oxalate supplement.

[0053] Comparative Example

[0054] Catalyst: Co3O4 nanoparticles (prepared by precipitation method, particle size 100 nm);

[0055] Composite process: 2 g Co3O4, 1 g multi-walled carbon nanotubes, and 12 g lithium oxalate were added to an ethanol / water mixed solvent (volume ratio 1:1), and reacted solvothermally at 120℃ for 6 h, followed by freeze-drying for 24 h to obtain a Co3O4 composite lithium oxalate lithium supplement. Its SEM image is shown below. Figure 4 As shown, the Co3O4 particles and lithium oxalate particles are loosely bonded at the interface, and the lithium oxalate exhibits a blocky stacking characteristic. The fine particles (Co3O4 or carbon nanotubes) are unevenly distributed on the surface, and there is obvious local agglomeration. This proves that the traditional composite system of metal oxide and lithium oxalate cannot form a uniform composite structure due to solvothermal process and material compatibility issues.

[0056] Battery assembly and performance testing:

[0057] 1. Battery assembly

[0058] Positive electrode sheet: First, prepare the slurry by mixing NCM811 (lithium nickel cobalt manganese oxide), lithium supplementer, PVDF (polyvinylidene fluoride), and conductive carbon black (Super P) in a ratio of 88:10:1:1 in NMP (N-methylpyrrolidone) solvent to achieve a solid content of 60 wt% and an apparent viscosity target of 3000 mPa·s at 25 °C. Use a degassing-stirring machine (stirring at 2200 rpm for 10 min, then degassing at 2000 rpm for 10 min) to form a uniform slurry. Then, use an automatic coating machine to evenly coat the slurry onto aluminum foil, controlling the active material loading to 40 mg / cm³. 2 The lithium supplements used were ZIF-67-derived composite lithium oxalate supplements prepared in Examples 1 and 2 (as experimental sample 1 and experimental sample 2, respectively), Co3O4 composite lithium oxalate supplements prepared in Comparative Example 1 (as control sample), and commercial grade lithium oxalate (purity 99.5%, D50=5 μm, as blank sample).

[0059] Negative electrode sheet: Pure graphite is used as the active material, and the preparation process is similar to that of the positive electrode, without the addition of lithium supplementation agent.

[0060] Lithium oxalate cathode sheet: To evaluate catalytic performance and decomposition efficiency, a cathode sheet using lithium oxalate as the active material was also prepared. A slurry was prepared with a lithium oxalate:Super P:PVDF mass ratio of 7:2:1, and the active material loading was controlled at 4 mg / cm³. 2 .

[0061] Electrolyte: Commercial ester electrolyte is used: 1M LiPF6 dissolved in EC / DMC / EMC (volume ratio 1:1:1), and 10% FEC film-forming additive is added, which can quickly form a stable lithium fluoride SEI layer during the first charge and discharge, inhibiting the dissolution of transition metals and oxygen evolution, and improving battery stability.

[0062] 2. Battery performance test

[0063] Decomposition potential and peak current testing: A three-electrode system was used: the target positive electrode material was used as the working electrode (WE); a platinum sheet was used as the counter electrode (CE); and a saturated calomel electrode (SCE) was used as the reference electrode. Before use, the liquid level and the presence of air bubbles were checked to ensure the salt bridge was unobstructed. The oxidation potential was tested at a scan rate of 0.1 mV / s within the voltage range of 2.5–4.5 V vs. Li⁺ / Li, and the peak current was recorded. Simultaneously, a half-cell was assembled using a lithium metal sheet as the reference / counter electrode and a positive electrode sheet as the working electrode. Charge-discharge tests were conducted under 0.1C (1C=200 mA / g) conditions, and the changes in the charge-discharge plateau were recorded to evaluate the decomposition potential. The first charge efficiency of lithium oxalate (the ratio of the actual charge capacity to the standard capacity of 525 mAh / g) was calculated using the charge-discharge data of the lithium oxalate positive electrode.

[0064] Charge-discharge and cycle performance testing: The full battery was tested at 25℃, with a charge-discharge range of 2.8 to 3.8V. The positive and negative electrode capacity ratio (N / P ratio) was controlled to approximately 1 to avoid overcharge interference. Long-term cycle tests were conducted at different rates, and the capacity changes and coulombic efficiency were recorded.

[0065] 3. Test Results and Analysis

[0066] The test results are shown in Table 1: The lithium oxalate decomposition potential of test sample 1 was as low as 3.32 V (vs. Li). + The reaction energy level of the lithium oxide compound (Li₂O₄ composite lithium oxalate supplement, 4.22 V) was 0.9 V lower than that of the control sample (Co₃O₄ composite lithium oxalate supplement, 4.22 V) and 1.38 V lower than that of the blank sample (pure lithium oxalate, 4.70 V). Its reaction energy barrier was only 1.3 eV (44.8% of that of the blank sample), which can avoid electrolyte oxidation and gas generation at high potentials; at the same time, the peak current density reached 0.99 mA / cm². 2 This is the control sample (0.22 mA / cm). 2 The concentration was 4.5 times that of the blank sample (0.09 mA / cm). 2 The efficiency is 11 times that of the "Co-based catalyst-multi-walled carbon nanotube" conductive network, which proves that the charge transfer efficiency can be significantly improved.

[0067] Regarding lithium replenishment efficiency and cycle stability, sample 1 achieved an initial charge efficiency of 94%, and after mixing with the positive electrode, the first-cycle coulombic efficiency increased to 95%. After 100 cycles, the capacity retention rate reached 95%, representing improvements of 74%, 17%, and 15% respectively compared to the blank sample (initial charge efficiency of 22%, first-cycle coulombic efficiency of 78%, and cycle retention rate of 80%). This verifies that the ZIF-67-derived composite lithium oxalate replenisher of Example 1 can compensate for irreversible lithium loss through efficient lithium release and inhibit the loss of active material during cycling. In contrast, sample 2, due to the replacement of the conductive carbon material with Super P (whose dispersibility is slightly inferior to multi-walled carbon nanotubes), showed a decomposition potential increase to 3.82 V and a peak current density decrease to 0.63 V. The first-week coulombic efficiency and cycle retention rate were 91% and 86%, respectively, which were slightly lower than those of the experimental sample 1, but still significantly better than the Co3O4 composite lithium oxalate supplement and pure lithium oxalate. This further proves the universality and superiority of the ZIF-67-derived composite lithium oxalate supplement of this invention. Moreover, the selection of different conductive carbon materials can achieve differentiated control of the performance of the supplement and adapt to different scenario requirements.

[0068] The initial charge-discharge curve of test sample 1 is as follows: Figure 5 As shown, during charging, the voltage slowly increases with increasing specific capacity, eventually stabilizing at around 4.2 V. The corresponding initial charging capacity is as high as 250 mAh / g. This is because the lithium oxalate in the lithium replenisher decomposes to release active lithium, replenishing the irreversible lithium consumed in SEI film formation. Therefore, the charging capacity is significantly higher than the theoretical capacity of the positive electrode (approximately 200 mAh / g). During discharging, the voltage gradually decreases with increasing specific capacity, and drops sharply when the specific capacity approaches 200 mAh / g. This discharge capacity is close to the theoretical capacity of the positive electrode, proving that the active lithium released by the lithium replenisher can effectively compensate for the lithium loss due to SEI formation, allowing the capacity of the positive electrode active material to be fully utilized. This verifies the lithium replenishment effect of the ZIF-67 derived composite lithium oxalate lithium replenisher of this invention, solving the pain points of traditional lithium oxalate lithium replenishers, namely "low initial efficiency and insufficient lithium compensation."

[0069] Table 1 Battery performance test results

[0070]

[0071] Mechanism analysis:

[0072] The superior battery performance of test samples 1 and 2 compared to the control and blank samples may be due to the Co / C and / or CoO formed during the ZIF-67 heat treatment. X / NC nanoparticles possess abundant metallic d-band state density, exhibiting a high-energy electron density region near the Fermi level. They can strongly couple with the LUMO orbitals of lithium oxalate, effectively reducing the oxalate C-band bond breaking barrier (from 2.9 eV to 1.3 eV in pure lithium oxalate), and significantly lowering the decomposition potential. Simultaneously, the continuous conductive pathways constructed by the carbon layer or nitrogen-doped carbon layer enhance the charge transfer rate, with a peak current density reaching 0.99 mA / cm². 2 The efficiency was significantly higher than that of the control sample, ultimately achieving high first-efficiency and high cycle stability. Theoretical calculations show that this composite structure can reduce the dissociation barrier of oxalate by about 42%, thereby achieving low-voltage lithium release. In contrast, Co3O4 has poor conductivity and low activity; pure lithium oxalate has a high energy barrier and the lowest activity.

[0073] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them; those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions will not cause the essence of the corresponding technical solutions to deviate from the protection scope of the present invention.

Claims

1. A ZIF-67-derived composite lithium oxalate supplementary lithium agent, comprising a lithium oxalate matrix and a ZIF-67-derived Co-based catalyst, wherein the ZIF-67-derived Co-based catalyst is Co / C and / or CoO. X / NC nanoparticles, with ZIF-67-derived Co-based catalysts uniformly distributed on the surface or between the crystals of lithium oxalate particles, forming a "dot-like embedded" or "planar coated" composite structure; characterized in that: It also includes conductive carbon materials, wherein the mass ratio of the lithium oxalate, the ZIF-67-derived Co-based catalyst to the conductive carbon materials is (12-15):(1.5-2.5):(0.8-1.5).

2. The ZIF-67-derived composite lithium oxalate supplementary lithium agent according to claim 1, characterized in that: The ZIF-67-derived Co-based catalyst has a particle size of 50–200 nm and a surface coating thickness of < 20 nm.

3. The ZIF-67-derived composite lithium oxalate supplementary lithium agent according to claim 1, characterized in that: The lithium oxalate has a purity of ≥99.5% and a D50 particle size between 3 and 8 μm.

4. The ZIF-67-derived composite lithium oxalate supplementary lithium agent according to claim 1, characterized in that: The conductive carbon material is selected from carbon nanotubes or conductive carbon black.

5. A method for preparing a ZIF-67-derived composite lithium oxalate supplement as described in any one of claims 1 to 4, characterized in that, Includes the following steps: (1) Cobalt salt and 2-methylimidazole were added to deionized water, and coordination precipitation was formed to form ZIF-67 precursor; (2) Add ZIF-67 precursor to lithium oxalate aqueous solution, and add conductive carbon material at the same time. Lithium oxalate and ZIF-67 precursor are co-deposited in situ to form ZIF-67@Li2C2O4 composite precursor. (3) After vacuum drying of the composite precursor, it is sent into a tube furnace and heated to 300℃ in an argon atmosphere at a heating rate of 2-3℃ / min for 2 hours. After natural cooling, it is ground and sieved to obtain the ZIF-67-derived composite lithium oxalate supplement.

6. The preparation method according to claim 5, characterized in that: The cobalt salt mentioned in step (1) is cobalt nitrate, and the molar ratio of cobalt salt to 2-methylimidazole is 1:(8-12).

7. The preparation method according to claim 5, characterized in that: In step (2), the mass ratio of lithium oxalate to ZIF-67 precursor is (10-15):(1-3).

8. The application of the ZIF-67-derived composite lithium oxalate supplement as a lithium-ion battery cathode additive according to any one of claims 1 to 4 in the cathode of a lithium-ion battery.

9. The application according to claim 8, characterized in that: The ZIF-67-derived composite lithium oxalate supplement is added to the positive electrode of a lithium-ion battery at an amount of 8-12% of the mass of the positive electrode active material.

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