Composite lithium supplement agent and preparation method and application thereof

Abstract

The invention discloses a composite lithium supplement agent and a preparation method and application thereof, and belongs to the technical field of lithium ion batteries. The composite lithium supplement agent comprises a porous carbon material and lithium oxalate and lithium carbonate nanoparticles which grow on the surface of porous carbon and in a pore structure in situ. According to the composite lithium supplementing agent provided by the invention, lithium oxalate and lithium carbonate composite nanoparticles uniformly distributed in the porous carbon pore structure are obtained by limiting the growth of lithium oxalate and lithium carbonate through the porous carbon pore structure, so that the decomposition voltage of the composite lithium supplementing agent is greatly reduced, and the total lithium supplementing capacity and the continuous lithium supplementing capacity are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and in particular to a composite lithium supplement agent, a preparation method thereof, and an application thereof. Background Art

[0002] With the application of lithium-ion batteries in the fields of 3C, power, and energy storage, people's demand for long battery life and long cycle life of lithium-ion batteries is becoming more urgent. In lithium-ion batteries, the formation, rupture, and continuous repair of the SEI film on the negative electrode surface are the main reasons for the loss of active Li, + which further limits the initial Coulombic efficiency and cycle performance of lithium-ion batteries. To solve the above problems, adding a lithium supplement agent to the cathode material is an effective measure. Among the cathode lithium supplement agents, lithium-rich lithium ferrite and lithium-rich lithium nickelate have been preliminarily commercialized, but they have high alkalinity, are sensitive to moisture, and are prone to gelation during pulping after being added to the cathode, resulting in poor processability. In contrast, lithium oxalate has a high lithium supplement capacity, is stable to air and moisture, and has a low cost, and has great application potential as a cathode lithium supplement agent.

[0003] However, the decomposition potential of lithium oxalate is usually higher than 4.7V, and side reactions such as electrolyte decomposition are easily triggered at high potentials, which also limits the exertion of the lithium supplement capacity. In order to reduce the decomposition potential, previous studies have used transition metal oxides, carbon materials, etc. as catalysts and added them to lithium oxalate, and physically mixed them by solid-phase mixing methods such as ball milling. The improvement effect on the decomposition potential and actual lithium supplement capacity of lithium oxalate is very limited. Summary of the Invention

[0004] The purpose of the present invention is to provide a composite lithium supplement agent with a low decomposition potential, a high lithium supplement capacity, and the ability to continuously supplement lithium; another purpose of the present invention is to provide a preparation method of a composite lithium supplement agent with a low decomposition potential, a high lithium supplement capacity, and the ability to continuously supplement lithium.

[0005] The present invention discloses a composite lithium supplement agent, which includes a porous carbon material and lithium oxalate and lithium carbonate nanoparticles that grow in-situ on the surface and pore structure of the porous carbon.

[0006] Further, the pore structure of the porous carbon includes three types of pores: micropores, mesopores, and macropores; the average pore diameter of the porous carbon is 1.7 - 2 nm; the pore volume ratio of the micropores is 60 - 90%.

[0007] The porous carbon material with a multi-level pore structure contains micropores (pore size <2nm), mesopores (pore size 2~50nm) and macropores (pore size >50nm), which restrict the growth of lithium oxalate and lithium carbonate crystals, forming nano-scale particles with a multi-level particle size distribution, which are evenly distributed on the surface and internal pores of the porous carbon material. Porous carbon materials can not only serve as carriers for the confined growth of lithium oxalate and lithium carbonate, significantly reducing their particle size, but also serve as conductive agents and catalysts to promote the decomposition of lithium oxalate and lithium carbonate, reduce the decomposition potential, and increase the lithium replenishment capacity. Lithium oxalate and lithium carbonate nanoparticles with a multi-level particle size distribution can be decomposed and replenished in multiple times according to the type of lithium replenisher, particle size and distribution position. The lithium replenisher particles with smaller particle size and closer to the surface interface will decompose first, and lithium oxalate will decompose before lithium carbonate, solving the problem of difficulty in continuous lithium replenishment during the cycle due to the one-time decomposition of the lithium replenisher.

[0008] Furthermore, in the composite lithium supplement, the content of the porous carbon is 60-70wt%; the content of the lithium oxalate is 15-30wt%; and the content of the lithium carbonate is 5-20wt%.

[0009] Furthermore, the particle size distribution range of the lithium oxalate and lithium carbonate particles in the composite lithium supplement is 0.1-300 nm.

[0010] The present invention also discloses a method for preparing a composite lithium supplement, comprising the following steps:

[0011] S1: mixing lithium oxalate, lithium carbonate and a solvent to form a uniform solution;

[0012] S2: mixing the porous carbon and the solution evenly to fully wet the porous carbon to form a uniform suspension;

[0013] S3: heating the suspension to evaporate, drying, and removing the solvent to obtain the composite lithium supplement agent as described above.

[0014] The present invention provides a method for preparing a composite lithium supplement, wherein lithium oxalate and lithium carbonate are dissolved and then fully stirred and mixed with porous carbon, and during the drying process, lithium oxalate and lithium carbonate nucleate and grow in the multi-level pores of the porous carbon, and nanoparticles with multi-level particle size distribution are obtained by utilizing the multi-level pore confinement effect, and are evenly distributed in the multi-level pores of the porous carbon. The preparation process is simple and fast, has low energy consumption, low processing cost, and is easy to mass produce.

[0015] The drying method of the mixed solution in step (3) includes but is not limited to drying after centrifugation, drying while stirring, spray drying, freeze drying, etc.

[0016] Furthermore, in step S1, the solvent is water.

[0017] Further, in the step S1, the mass ratio of the lithium oxalate to the lithium carbonate is 1-5:1.

[0018] Further, in the step S2, the sufficient infiltration is: stirring at room temperature for 18-24 h.

[0019] The present invention also discloses a lithium-ion battery, which includes a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte. The positive electrode sheet includes a composite lithium supplement agent prepared by the preparation method described above.

[0020] The composite lithium supplement agent is added to the positive electrode material before or / and during the positive electrode pulping, and mixed evenly to form a lithium-supplemented positive electrode, which is assembled with the negative electrode and the electrolyte into a lithium-ion battery. When the lithium-ion battery is charged for the first time, it is charged to the decomposition voltage of the lithium supplement agent and above. The composite lithium supplement agent releases a part of active lithium to compensate for the loss of active lithium caused by the formation of SEI during the first charge; during the long-term cycle process of the lithium-ion battery, according to the degree of battery capacity attenuation, it is charged to the decomposition voltage of the lithium supplement agent and above multiple times. The composite lithium supplement agent releases active lithium multiple times to continuously compensate for the loss of active lithium during the battery cycle process.

[0021] Further, in the positive electrode sheet, the addition amount of the composite lithium supplement agent accounts for 1-10 wt% of the positive electrode active material.

[0022] Further, in the positive electrode sheet, the addition amount of the composite lithium supplement agent accounts for 1-5 wt% of the positive electrode active material.

[0023] A composite lithium supplement agent provided by the present invention restricts the growth of lithium oxalate and lithium carbonate through the pore structure of porous carbon, and obtains composite lithium oxalate and lithium carbonate nanoparticles uniformly distributed in the pore structure of porous carbon, greatly reducing the decomposition voltage of the composite lithium supplement agent and increasing the total lithium supplement capacity. The porous carbon has a developed pore structure and a high specific surface area, and can be used as a capacitive positive electrode material to store energy by forming an electric double layer at the interface with the electrolyte. Compared with other conventional catalysts, it can further improve the battery capacity, low-temperature performance, and rate performance, and avoid the problem of increased battery polarization caused by the addition of the positive electrode lithium supplement agent. Description of the Drawings

[0024] Figure 1 is the SEM image of the composite lithium supplement agent prepared in Example 1 of the present invention;

[0025] Figure 2 is the SEM image of the lithium supplement agent prepared in Comparative Example 3 of the present invention;

[0026] Figure 3 is the first three charge-discharge curves of the coin cell assembled in Example 1 of the present invention;

[0027] Figure 4It is the graph of the cycle performance test results of the button cell assembled in Example 1 of the present invention;

[0028] Figure 5 It is the graph of the cycle performance test results of the soft-pack battery cell assembled in Example 1 of the present invention and the soft-pack battery cell without adding the lithium supplement agent. Detailed implementation manners

[0029] To make the technical solution of the present invention clearer, the following further describes the present invention in detail with reference to the accompanying drawings and specific embodiments.

[0030] Example 1

[0031] Prepare the composite lithium supplement agent:

[0032] Weigh 0.8 g of lithium oxalate and 0.2 g of lithium carbonate, dissolve them in 22.5 mL of deionized water, and magnetically stir for 6 h at room temperature to obtain a uniform mixed solution of lithium oxalate and lithium carbonate. Weigh 2 g of resin-based porous carbon powder, add it to the above solution, and magnetically stir at room temperature for 24 h to fully mix evenly to obtain a mixed solution. The selected resin-based porous carbon material has an average pore diameter of 1.7 nm, the proportion of micropore volume is 83%, and the specific surface area reaches 1959 m 2 / g. Place the above mixed solution in an oven at 60 °C and dry for 24 h. After drying, grind it to obtain the composite lithium supplement agent.

[0033] Perform SEM testing on the obtained composite lithium supplement agent, and the results are as Figure 1 shown. Figure 1 The left side in Figure 1 is the overall characterization of the composite lithium supplement agent. It can be seen from the figure that the composite lithium supplement agent is in the shape of round particles. The right side in

[0034] is the enlarged view of the composite lithium supplement agent particles. It can be seen from the figure that the surface of the composite lithium supplement agent is relatively rough, and lithium oxalate / lithium carbonate nanoparticles with sizes ranging from 10 to 100 nm are attached. However, due to the too small pore size inside the porous carbon, smaller-scale particles cannot be clearly seen in the SEM photo. Figure 3As shown in the figure. The specific charge capacities of the composite lithium supplement agent in the first three charge cycles are 205.1 mAh / g, 234.62 mAh / g, and 118.6 mAh / g respectively. The average decomposition voltage is below 4V in all cases. The specific discharge capacities in the first three discharge cycles are all about 57 mAh / g. During the subsequent cycling process, the discharge specific capacity is maintained at about 59 mAh / g. The above-mentioned coin cell test results show that the confined catalytic composite lithium supplement agent prepared in Example 1 can decompose and supplement lithium multiple times, which helps to continuously compensate for the active lithium during the cycling process, and has a lower decomposition potential and a higher lithium supplement capacity. The total lithium supplement capacity of the first three decompositions reaches 387 mAh / g. In addition, the porous carbon as a capacitive cathode provides a reversible charge-discharge specific capacity of about 60 mAh / g.

[0035] Using lithium iron phosphate and graphite as the positive and negative active materials respectively, 3 wt% of the composite lithium supplement agent based on the mass of the positive active material was added during the positive electrode slurry preparation process to prepare a 3 Ah soft-pack lithium supplement battery cell. At the same time, a 3 Ah soft-pack battery cell without the lithium supplement agent was made as a control group. The design of the control group battery cell was the same as that of the lithium supplement battery cell except that the lithium supplement agent was not added. During the first formation charge, the two groups of battery cells were charged at a constant current of 0.1C to 4V to activate the decomposition of the lithium supplement agent and release active lithium. Subsequently, the two groups of battery cells were subjected to a constant current charge-discharge cycle test at 0.5C. The first discharge capacity of the lithium supplement group increased by 0.13 Ah compared with the control group; after 50 cycles, the capacity of the lithium supplement group hardly decayed, and the capacity retention rate of the control group was 98.7%, indicating that the use of the composite lithium supplement agent can improve the battery capacity and cycling stability. Subsequently, the two groups of battery cells were charged to 4V at 0.1C again and continued to be cycled at a constant current of 0.5C. The cycling performance is as Figure 5 shown. It can be seen that after recharging to 4V, the 0.5C discharge capacity of the lithium supplement battery cell increased by 0.14 Ah, indicating that the composite lithium supplement agent can achieve multiple lithium supplementations.

[0036] Example 2

[0037] Weigh 0.5 g of lithium oxalate and 0.5 g of lithium carbonate powder, dissolve them in 40 mL of deionized water, and stir magnetically at room temperature for 12 h to obtain a homogeneous mixed solution. Weigh 2 g of biomass-based porous carbon powder and add it to the above solution, stir magnetically at room temperature for 18 h, and mix well to obtain a mixed solution. The selected biomass-based porous carbon material has an average pore diameter of 1.9 nm, the proportion of micropore volume is 76%, and the specific surface area reaches 1707 m 2 / g. The above mixed solution was dried with stirring in a water bath at 80 °C, and then ground to obtain a composite lithium supplement agent.

[0038] The composite lithium supplement agent, conductive carbon black, and PVDF binder were made into a positive electrode sheet at a mass ratio of 8:1:1, and assembled into a button cell together with a lithium sheet, separator, and electrolyte in a glove box. The assembled button cell was tested for charge-discharge cycle performance at 0.025C, and the charge-discharge voltage range was 2.5 - 4.5V. The results showed that the first three charge specific capacities of the composite lithium supplement agent were 166.5 mAh / g, 223.6 mAh / g, and 178.8 mAh / g respectively, the average decomposition voltage was about 4.1V, and the first three discharge specific capacities were all about 50 mAh / g. During the subsequent cycling process, the discharge specific capacity was maintained at about 55 mAh / g. The test results of the button cell indicated that the composite lithium supplement agent in Example 2 could decompose and supplement lithium multiple times, which was helpful for the continuous compensation of active lithium during the cycling process, and had a lower decomposition potential and a higher lithium supplement capacity. The total lithium supplement capacity of the first three decompositions reached 418.9 mAh / g. In addition, the porous carbon, as a capacitive positive electrode, provided a reversible charge-discharge specific capacity of about 55 mAh / g.

[0039] Using lithium iron phosphate and graphite as the positive and negative active materials respectively, 1 wt% of the composite lithium supplement agent based on the mass of the positive active material was added during the positive electrode pulping process to prepare a 3Ah soft-pack lithium-supplemented battery cell. At the same time, a 3Ah soft-pack battery cell without the lithium supplement agent was made as a control group. The design of the control group cell was the same as that of the lithium-supplemented battery cell except for the absence of the lithium supplement agent. During the first formation charge, the two groups of cells were charged at a constant current of 0.1C to 4.2V to activate the decomposition of the lithium supplement agent and release active lithium. Subsequently, the two groups of cells were tested for constant current charge-discharge cycling at 0.5C. The first discharge capacity of the lithium-supplemented group increased by 0.1 Ah compared with the control group; after 50 cycles, the capacity of the lithium-supplemented group hardly decayed, and the capacity retention rate of the control group was 97.8%, indicating that the use of the composite lithium supplement agent could improve the battery capacity and cycling stability. Subsequently, the two groups of cells were charged to 4.2V at 0.1C again and continued to be cycled at a constant current of 0.5C. After being charged to 4.2V again, the 0.5C discharge capacity of the lithium-supplemented battery cell increased by 0.13 Ah, indicating that the composite lithium supplement agent could achieve multiple lithium supplements.

[0040] Example 3

[0041] Weigh 0.8g of lithium oxalate and 0.2g of lithium carbonate, dissolve them in 22.5 mL of deionized water, and stir magnetically at room temperature for 6h to obtain a homogeneous mixed solution. Weigh 2g of porous carbon powder and add it to the above solution, stir magnetically at room temperature for 24h, and mix well to obtain a mixed solution. The selected porous carbon material has a unimodal pore size distribution, an average pore size of 1.2nm, a micropore volume ratio of 95%, and a specific surface area of 2536m 2 / g. The mixed solution was placed in an oven at 60°C, dried and ground to obtain the composite lithium supplement agent.

[0042] The composite lithium supplement agent, conductive carbon black, and PVDF binder are made into a positive electrode sheet at a mass ratio of 7:2:1, and assembled into a button cell together with a lithium sheet, separator, and electrolyte in a glove box. The assembled button cell is subjected to a 0.025C charge-discharge cycle performance test, and the charge-discharge voltage range is 2.5 - 4.5V. The first three charge specific capacities of the composite lithium supplement agent are 389.2 mAh / g, 88.5 mAh / g, and 82.6 mAh / g respectively, the average decomposition voltage is less than 4V, the average discharge specific capacity of the first three times is about 55 mAh / g, and the total lithium supplement capacity of the first three times is about 395.3 mAh / g. During the subsequent cycle process, the discharge specific capacity is maintained at about 56 mAh / g.

[0043] Comparative Example 1

[0044] The preparation method of the lithium supplement agent is as follows: Weigh 1g of lithium oxalate, dissolve it in 22.5 mL of deionized water, and magnetically stir for 6h at room temperature to obtain a uniform lithium oxalate solution. Weigh 2g of resin-based porous carbon powder, add it to the above solution, and magnetically stir at room temperature for 24h to mix evenly to obtain a mixed solution. The resin-based porous carbon material selected has an average pore diameter of 1.7nm, the proportion of micropore volume is 83%, and the specific surface area reaches 1959m 2 / g. Place the above mixed solution in an oven at 60°C and dry for 24h, grind it after drying to obtain the lithium supplement agent.

[0045] The lithium supplement agent, conductive carbon black, and PVDF binder are made into a positive electrode sheet at a mass ratio of 7:2:1, and assembled into a button cell together with a lithium sheet, separator, and electrolyte in a glove box. The assembled button cell is subjected to a 0.025C charge-discharge cycle performance test, and the charge-discharge voltage range is 2.5 - 4.5V. The first three charge specific capacities of the composite lithium supplement agent are 382.4 mAh / g, 86.3 mAh / g, and 75.6 mAh / g respectively, the average decomposition voltage is about 3.9V, and the discharge specific capacity of the first three times is about 55 mAh / g. During the subsequent cycle process, the discharge specific capacity is maintained at about 55 mAh / g. The above button cell test results show that the lithium oxalate lithium supplement agent can decompose and supplement lithium multiple times, which helps to continuously compensate for active lithium during the cycle process, and has a lower decomposition potential and a higher lithium supplement capacity. The total lithium supplement capacity of the first three decompositions reaches 379.3 mAh / g. In addition, the porous carbon, as a capacitive positive electrode, provides a reversible charge-discharge specific capacity of about 55 mAh / g.

[0046] Comparative Example 2

[0047] The preparation method of the lithium supplement agent is as follows: Weigh 0.5 g of lithium carbonate and dissolve it in 40 mL of deionized water. Stir magnetically at room temperature for 6 h to obtain a homogeneous lithium carbonate solution. Weigh 1 g of resin-based porous carbon powder and add it to the above solution. Stir magnetically at room temperature for 24 h to mix evenly and obtain a mixed solution. The resin-based porous carbon material selected has an average pore diameter of 1.7 nm, a micropore volume ratio of 83%, and a specific surface area of 1959 m 2 / g. Place the above mixed solution in an oven at 60 °C and dry for 24 h. After drying, grind it to obtain the lithium supplement agent.

[0048] The lithium supplement agent, conductive carbon black, and PVDF binder are made into a positive electrode sheet at a mass ratio of 7:2:1, and assembled into a button cell in a glove box together with a lithium sheet, a separator, and an electrolyte. Perform a 0.025C charge-discharge cycle performance test on the assembled button cell, and the charge-discharge voltage range is 2.5 - 4.5 V. The first three charge specific capacities of the composite lithium supplement agent are 126.7 mAh / g, 256.5 mAh / g, and 207.2 mAh / g respectively, the average decomposition voltage is about 4.2 V, and the first three discharge specific capacities are all about 53 mAh / g. In the subsequent cycling process, the discharge specific capacity remains at about 58 mAh / g. The above button cell test results show that the lithium carbonate lithium supplement agent can decompose and supplement lithium multiple times, which helps to continuously compensate for the active lithium during the cycling process, and has a relatively high lithium supplement capacity. The total lithium supplement capacity of the first three decompositions reaches 431.4 mAh / g, but the decomposition potential is relatively high. In addition, the porous carbon as a capacitive positive electrode provides a reversible charge-discharge specific capacity of about 58 mAh / g.

[0049] Comparative Example 3

[0050] Weigh 1 g of lithium oxalate and dissolve it in 22.5 mL of deionized water. Stir magnetically at room temperature for 6 h to obtain a homogeneous lithium oxalate solution. Place the lithium oxalate solution in an oven at 60 °C and dry it. After lithium oxalate precipitates, grind it to obtain the lithium oxalate lithium supplement agent. As Figure 2 shown, the particles of the lithium oxalate lithium supplement agent are relatively large, about 5 - 50 μm.

[0051] The lithium oxalate lithium supplement agent, conductive carbon black, and PVDF binder are made into a positive electrode sheet at a mass ratio of 7:2:1, and assembled into a button cell in a glove box together with a lithium sheet, a separator, and an electrolyte. Perform a 0.025C charge-discharge cycle performance test on the assembled button cell, and the charge-discharge voltage range is 2.5 - 4.5 V. The first charge and discharge specific capacities of the lithium supplement agent are 427.4 mAh / g and 8.2 mAh / g respectively, and the average decomposition voltage is about 4.7 V. In the subsequent cycling process, the charge-discharge specific capacity is almost 0.

[0052] Table 1 Lithium Supplement Performance Test Results

[0053] 1st Charge Specific Capacity (mAh / g) 2nd Charge Specific Capacity (mAh / g) 3rd Charge Specific Capacity (mAh / g) Average Discharge Specific Capacity (mAh / g) Average Decomposition Voltage Total Lithium Compensation Specific Capacity for the First Three Cycles (mAh / g) Example 1 205.1 234.6 118.6 57 < 4.0V 387 Example 2 166.5 223.6 178.8 50 ≈4.1V 418.9 Example 3 389.2 88.5 82.6 55 < 4.0V 395.3 Comparative Example 1 382.4 86.3 75.6 55 ≈3.9V 379.3 Comparative Example 2 126.7 256.5 207.2 53 ≈4.2V 431.4 Comparative Example 3 427.4 0 0 0~8 4.7V 419.2

[0054] As shown in Table 1, the experimental results of Example 3 and Example 1 show that using porous carbon with single-stage pores can increase the initial lithium supplementation capacity, but the continuous lithium supplementation ability is weak. While using porous carbon with a multi-stage pore structure has a higher continuous lithium supplementation capacity.

[0055] Based on the result analysis of Comparative Example 1, Comparative Example 2 and Example 1, lithium carbonate has a low initial lithium supplementation capacity, high lithium supplementation capacities in the second and third times, and a relatively high decomposition potential; lithium oxalate has a high initial lithium supplementation capacity, weak continuous lithium supplementation ability, and a low decomposition potential. The synergistic use of lithium oxalate and lithium carbonate can obtain a high lithium supplementation capacity, continuous lithium supplementation ability and a low decomposition potential, with excellent comprehensive performance.

[0056] The experimental results of Comparative Example 3 and Comparative Example 1 show that when no porous carbon is added, the lithium oxalate lithium supplement agent has large particles and a high decomposition potential. Adding porous carbon can inhibit the growth of lithium oxalate crystals and at the same time improve the conductivity, thereby reducing polarization and lowering the decomposition potential of lithium oxalate.

[0057] Compared with Example 1, Comparative Example 1 has a higher initial lithium supplementation capacity, but the lithium supplementation capacities in the second and third times decrease, indicating that compared with lithium carbonate, lithium oxalate can increase the initial and total lithium supplementation capacities, but the continuous lithium supplementation ability is poor.

[0058] Comparing Example 1 and Example 2 with Comparative Example 1 and Comparative Example 2, it can be seen that Example 1 and Example 2 have the effects of high initial lithium supplementation capacity and continuous lithium supplementation. While in Comparative Example 1, only lithium oxalate is used, the continuous lithium supplementation capacity is not high, and in Comparative Example 2, only lithium carbonate is used, the initial lithium supplementation capacity is relatively low.

[0059] The above-described embodiments only represent several implementation manners of the present invention. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the patent of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention should be subject to the appended claims.

Claims

1. A composite lithium supplement, characterized in that, It includes a porous carbon material, as well as lithium oxalate and lithium carbonate nanoparticles grown in-situ on the surface and pore structure of the porous carbon.

2. The composite lithium supplement agent according to claim 1, wherein The pore structure of the porous carbon includes three types of pores: micropores, mesopores, and macropores; the average pore diameter of the porous carbon is 1.7 - 2 nm; the pore volume ratio of the micropores is 60 - 90%.

3. The composite lithium supplement according to claim 2, wherein, In the composite lithium supplement, the content of the porous carbon is 60 - 70 wt%; the content of the lithium oxalate is 15 - 30 wt%; the content of the lithium carbonate is 5 - 20 wt%.

4. The composite lithium supplement agent according to claim 1, characterized in that, In the composite lithium supplement, the particle size distribution range of the lithium oxalate and lithium carbonate particles is 0.1 - 300 nm.

5. A preparation method of a composite lithium supplement, characterized in that, It includes the following steps: S1: Mix lithium oxalate, lithium carbonate, and a solvent to form a homogeneous solution; S2: Mix the porous carbon with the solution evenly and fully infiltrate to form a uniform suspension; S3: Heat and evaporate the suspension, dry it, and remove the solvent to obtain the composite lithium supplement as described in any one of claims 1 - 4.

6. The preparation method of the composite lithium supplement agent according to claim 1, characterized in that In step S1, the solvent is water.

7. The preparation method of the composite lithium supplement agent according to claim 1, wherein, In step S1, the mass ratio of the lithium oxalate to the lithium carbonate is 1 - 5:

1.

8. The preparation method of the composite lithium supplement agent according to claim 1, wherein, In step S2, the full infiltration means: stir at room temperature for 18 - 24 h.

9. A lithium-ion battery, comprising a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte, characterized in that, The positive electrode sheet includes the composite lithium supplement prepared by the preparation method described in any one of claims 6 - 8.

10. A lithium-ion battery according to claim 9, characterized in that, In the positive electrode sheet, the addition amount of the composite lithium supplement accounts for 1 - 10 wt% of the positive electrode active material.

Images