A positive electrode lithium supplementing pole piece slurry, a preparation method of a pole piece and a battery formation method

Abstract

The application provides a positive electrode lithium supplementing pole piece slurry, a preparation method of a pole piece and a battery formation method, and belongs to the technical field of lithium ion batteries. The application comprises the following steps: preparing a lithium oxalate dispersion liquid; mixing lithium iron phosphate powder and a conductive agent, and then adding the lithium oxalate dispersion liquid and a binder to prepare a slurry; the particle size of lithium oxalate in the lithium oxalate dispersion liquid and the lithium iron phosphate powder satisfies the following formula: D 50LFP :D 50LCO =100:(15-25), the D 50LFP range is 1.0-2.0 um, wherein D 50LFP is the D 50 particle size of lithium iron phosphate, and D 50LCO is the D 50 particle size of lithium oxalate. Based on the theoretical decomposition potential of lithium oxalate, a plurality of nano-micron scale corrosion battery models are constructed in the positive electrode pole piece, the chemical properties of phosphoric acid iron after lithium is removed from lithium iron phosphate are utilized, lithium oxalate is electrochemically / chemically catalytically decomposed, and the decomposition potential of lithium oxalate is reduced to 3.5 V.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion batteries, and particularly relates to a positive electrode lithium supplementing pole piece slurry, a preparation method of a pole piece and a battery formation method. BACKGROUND

[0002] Among many lithium batteries, lithium iron phosphate batteries have high safety, long cycle life, abundant resources and low cost. Among all commercial lithium ion battery positive electrode materials, lithium iron phosphate is the material system with the lowest comprehensive cost, which is suitable for large-scale use. However, due to the loss of negative active lithium, lithium iron phosphate batteries are difficult to achieve their best performance. The positive electrode lithium supplementing strategy has a short process flow and excellent industrial compatibility, which can significantly improve the capacity and cycle life of lithium iron phosphate batteries. Lithium oxalate as a sacrificial salt lithium supplementing agent has high specific capacity and no by-product decomposition characteristics, which has high practical application value. Lithium oxalate not only supplements the lost lithium ions of the battery, but also optimizes the composition and morphology of the solid electrolyte membrane on the negative side, further improving the cycle life of the battery. However, lithium oxalate has a very high decomposition potential (4.7V) and is not suitable for lithium iron phosphate battery systems. In order to prevent excessive reaction of the electrolyte caused by the high charging voltage, it is necessary to reduce the decomposition voltage of lithium oxalate. Therefore, reducing the decomposition voltage of lithium oxalate has become a hot spot for improving the capacity and cycle life of lithium ion batteries.

[0003] The reason for the high decomposition potential of lithium oxalate comes from the poor kinetic behavior. The conventional method to reduce the potential is surface modification or adding a catalyst to improve the conductivity of lithium oxalate and reduce the reaction activation energy. However, this method introduces too much non-active material into the battery, increasing the mass and volume of the battery. Compared with the improved capacity, the energy density of the battery is limited. Secondly, the preparation cost of this complex three-dimensional nanoparticle material is high, which cannot take advantage of the low cost of lithium iron phosphate batteries, and is more suitable for basic research, and is not suitable for actual production and application of batteries. SUMMARY

[0004] To solve the above problems, the application provides a positive electrode lithium supplementing pole piece slurry, a preparation method of a pole piece and a battery formation method. Starting from the theoretical decomposition potential of lithium oxalate, a plurality of nano-micro scale corrosion cell models are constructed in the positive electrode pole piece, and the chemical properties of lithium iron phosphate after lithium removal are used to electrochemically / chemically catalyze the decomposition of lithium oxalate, so as to reduce the decomposition potential of lithium oxalate to 3.5V.

[0005] In order to achieve the above purpose, the technical scheme adopted by the application is as follows:

[0006] In one aspect, the present application provides a preparation method of a positive electrode lithium supplement electrode piece slurry, comprising the following steps: preparing a lithium oxalate dispersion liquid; mixing lithium iron phosphate powder and a conductive agent, and then adding the lithium oxalate dispersion liquid and a binder to prepare a slurry; the particle size of lithium oxalate in the lithium oxalate dispersion liquid and the lithium iron phosphate powder satisfies: D 50 LFP :D 50 LCO = 100: (15-25), and the D 50 LFP range is 1.0-2.0um, wherein D 50 LFP is the D 50 particle size of lithium iron phosphate, and D 50 LCO is the D 50 particle size of lithium oxalate.

[0007] Further, the mass ratio of lithium oxalate to solvent in the lithium oxalate dispersion liquid is 0.15-0.25; and the solvent is selected from one or more of N-methyl pyrrolidone, N,N-dimethylformamide, ethanol, isopropanol, acetone, toluene or n-hexane.

[0008] Further, the mass percentage of the binder is 4%-6%, and the binder is selected from one or more of polyvinylidene fluoride, polyvinyl alcohol, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, and polypropylene.

[0009] Further, the mass ratio of the lithium iron phosphate powder to the conductive agent is 3.5-4.5; and the conductive agent is selected from one or more of porous carbon, SuperP, acetylene black, carbon nanotubes, carbon black, Ketjen black, graphene, and MXenes.

[0010] Further, the ratio of the addition amount of the lithium oxalate dispersion liquid to the mass of the lithium iron phosphate powder is 1:(1.3-5.3).

[0011] Further, the lithium iron phosphate powder and the conductive agent are mixed by using a centrifugal planetary mixer, and the rotation speed is kept at 300-600r / min for 5-10min; the binder and the lithium oxalate dispersion liquid are added, and the rotation speed is kept at 400-700r / min for 5-10min; and then the rotation speed is kept at 1200-2000r / min for 40-80min.

[0012] In another aspect, the present application provides a preparation method of a positive electrode lithium supplement electrode piece, which uses the above preparation method to prepare a positive electrode lithium supplement electrode piece slurry; the slurry is coated and dried to obtain a positive electrode blank, and the positive electrode blank is rolled to obtain the positive electrode lithium supplement electrode piece.

[0013] Further, the rolling reduction of the positive electrode blank is 30%-40%.

[0014] In another aspect, the application provides a positive electrode lithium supplement electrode plate, which is prepared by the above preparation method.

[0015] In another aspect, the application provides a formation method of a lithium ion battery, wherein the above positive electrode lithium supplement electrode plate is assembled as a positive electrode to form the lithium ion battery; and the formation system is as follows: constant current charging, the current size is 14-17 mA / g, the limiting voltage is 3.75-4.5 V; the constant current discharge current size is 14-17 mA / g, the terminal voltage is 2.7-2.0 V, and then exhaust and final sealing are performed.

[0016] The technical scheme provided by the embodiment of the application has the following beneficial effects:

[0017] The positive electrode lithium supplement electrode plate provided by the application is prepared into a battery, and in the formation process, the chemical properties of the lithium iron phosphate phase itself after lithium is removed from lithium iron phosphate are used to catalyze and decompose lithium oxalate to realize 3.5 V decomposition, and the product carbon dioxide is used to optimize the SEI on the negative electrode side in the first circle formation process of the battery. The method does not need to introduce additional inactive materials (such as a catalyst), and the battery has higher mass and volume energy densities. The decomposition of lithium oxalate at 3.5 V not only improves the capacity of the battery, but also improves the cycle life of the battery by optimizing the SEI on the negative electrode side with the product carbon dioxide. Similarly, the decomposition of lithium oxalate at 3.5 V reduces the side reactions of the electrolyte, reduces the amount of electrolyte, and reduces the energy consumption in the formation process with a lower limiting voltage, thereby reducing the cost of the battery. However, it should be noted that in order to realize the catalytic decomposition of lithium oxalate by the lithium iron phosphate phase generated in the subsequent formation, the particle size of the lithium oxalate and the lithium iron phosphate powder needs to meet the limited conditions of the application, otherwise, even if the lithium oxalate and the lithium iron phosphate are mixed to prepare the positive electrode lithium supplement electrode plate in the prior art, a catalyst needs to be added to realize the catalytic decomposition of lithium oxalate. However, in the application, the decomposition ratio of lithium oxalate is as high as at least 70% by controlling the particle size within the above range, and compared with the prior art, the lithium supplement effect can be realized without adding a catalyst. BRIEF DESCRIPTION OF DRAWINGS

[0018] In order to more clearly illustrate the technical solutions in the embodiments of the application, the following will briefly introduce the drawings needed to be used in the embodiment description. Obviously, the drawings in the following description are only some embodiments of the application, and other drawings can also be obtained by those skilled in the art without any creative effort on the basis of these drawings.

[0019] Figure 1 SEM image of the positive electrode lithium supplement electrode plate prepared in the embodiment 1 of the application;

[0020] Figure 2 The first cycle charge-discharge curve of the positive electrode lithium supplementing tab of the present application example 1 under 4.2V limiting voltage and the positive electrode tab prepared in the comparative example 1 under 4.2V limiting voltage;

[0021] Figure 3 The cycle capacity curve of the positive electrode lithium supplementing tab of the present application example 1 under 4.2V limiting voltage and the positive electrode tab prepared in the comparative example 1 under 4.2V limiting voltage;

[0022] Figure 4 The first cycle charge-discharge curve of the positive electrode lithium supplementing tab of the present application example 1 under 4.5V limiting voltage and the positive electrode tab prepared in the comparative example 1 under 4.2V limiting voltage;

[0023] Figure 5 The cycle capacity curve of the positive electrode lithium supplementing tab of the present application example 1 under 4.5V limiting voltage and the positive electrode tab prepared in the comparative example 1 under 4.2V limiting voltage. DETAILED DESCRIPTION

[0024] In order to make the purpose, technical scheme and advantages of the present application more clear, the present application will be further described in detail below with the aid of the accompanying drawings and specific examples.

[0025] The present application example provides a preparation method of positive electrode lithium supplementing tab slurry, comprising the following steps: preparing lithium oxalate dispersion liquid; mixing lithium iron phosphate powder and conductive agent, then adding into the lithium oxalate dispersion liquid and binder to prepare slurry; the particle size of lithium oxalate in the lithium oxalate dispersion liquid and the lithium iron phosphate powder satisfies: D 50 LFP :D 50 LCO =100:(15-25), the D 50 LFP range is 1.0-2.0um, wherein D 50 LFP is the D 50 particle size of lithium iron phosphate, D 50 LCO is the D 50 particle size of lithium oxalate. Preferably, D 50 LFP :D 50 LCO =100:20, D 50 LFP is 1.5um.

[0026] The positive electrode lithium supplement pole piece slurry prepared by the preparation method is prepared into a battery, and in the formation process, lithium is removed from lithium iron phosphate, and the chemical properties of the iron phosphate phase itself are used to catalytically decompose lithium oxalate to realize 3.5V decomposition, and the product carbon dioxide optimizes the SEI of the negative electrode side in the first circle formation process of the battery. This method does not need to introduce additional inactive materials (such as catalysts), and the battery has higher mass and volume energy density. The decomposition of lithium oxalate at 3.5V not only improves the capacity of the battery, but also optimizes the SEI of the negative electrode side by the product carbon dioxide, thereby improving the cycle life of the battery. Similarly, the decomposition of lithium oxalate at 3.5V reduces the side reaction of the electrolyte, reduces the amount of electrolyte, and reduces the energy consumption in the formation process by lowering the limit voltage, thereby reducing the cost of the battery. However, it should be clear that in order to achieve the subsequent catalytic decomposition of lithium oxalate by the iron phosphate phase generated by formation, the particle size of lithium oxalate and lithium iron phosphate powder needs to meet the above conditions, otherwise even if lithium oxalate and lithium iron phosphate are mixed to prepare a positive electrode lithium supplement pole piece in the prior art, a catalyst needs to be added to achieve catalytic decomposition of lithium oxalate. However, by controlling the particle size of lithium oxalate within the above range, the decomposition rate of lithium oxalate is as high as at least 70%, compared with the prior art, without the need to add a catalyst to achieve lithium supplement.

[0027] The mass ratio of lithium oxalate to solvent in the lithium oxalate dispersion is 0.15-0.25; the solvent is selected from one or more of N-methyl pyrrolidone, N,N-dimethylformamide, ethanol, isopropanol, acetone, toluene or n-hexane. Specifically, the lithium oxalate powder is mixed with the solvent by ball milling, preferably using a planetary ball mill, and the ball milling process is as follows: ball milling at a speed of 1300-1500 r / min for 8-12 h to reduce the particle size of the lithium oxalate powder. For illustration, N-methyl pyrrolidone (NMP) is selected as the solvent in the embodiments of the present application, and the final product lithium oxalate dispersion has suitable fluidity and uniform dispersion.

[0028] The mass percentage of the binder is 4%-6%, and the binder is selected from one or more of polyvinylidene fluoride, polyvinyl alcohol, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene and polypropylene. Its main function is to control the viscosity of the slurry on the one hand, and to facilitate the formation of a continuous, dense and uniform positive electrode material film layer on the surface of the current collector after mixing with the positive electrode material on the other hand. The solvent is selected from one or more of N-methyl pyrrolidone, N,N-dimethylformamide, ethanol, isopropanol, acetone, toluene or n-hexane. For illustration, polyvinylidene fluoride (PVDF) is selected as the binder and N-methyl pyrrolidone (NMP) is selected as the solvent in the embodiments of the present application. The corresponding binder and solvent are weighed according to the mass percentage, and mixed by magnetic stirring, preferably for 8-12 h.

[0029] The mass ratio of lithium iron phosphate powder to conductive agent is 3.5-4.5; the conductive agent is selected from one or more of porous carbon, SuperP, acetylene black, carbon nanotubes, carbon black, Ketjen black, graphene, and MXenes. Preferably, Super P-Li and Ketjen black (KB) are used as conductive agents in the same mass ratio. MXenes are a class of transition metal compounds, represented by the chemical formula M. n+1 X n T x Where M is selected from one or more transition metal elements, X is selected from one, two or three of carbon, nitrogen or boron elements, n is between 1 and 4, and T x It is a functional group. Adding a conductive agent to the lithium replenishment slurry serves two purposes: first, it improves conductivity, thereby enhancing the conductivity of the positive electrode material layer in the positive electrode sheet; second, it adsorbs the byproduct S of the lithium replenishment agent. x 2- This is to avoid the loss of lithium ions.

[0030] The ratio of the amount of lithium oxalate dispersion added to the mass of lithium iron phosphate powder is 1:(1.3-5.3). Preferably, the amount of binder added is 1.8-2.2 times the total mass of lithium iron phosphate and conductive agent.

[0031] The lithium iron phosphate powder and conductive agent are mixed using a centrifugal planetary mixer at a speed of 300-600 rpm for 5-10 minutes. A binder and lithium oxalate dispersion are then added, and the mixture is kept at 400-700 rpm for 5-10 minutes, followed by a speed of 1200-2000 rpm for 40-80 minutes. It should be noted that the mixing method significantly affects the mixing effect of lithium oxalate and lithium iron phosphate. Initially, the components are mixed at a lower speed and for a shorter time to improve their uniformity. At this stage, lithium oxalate exhibits slight agglomeration. Subsequently, high-energy ball milling is performed at a higher speed and for a longer time to allow the agglomerated lithium oxalate to interact with the lithium iron phosphate, thereby adhering lithium oxalate to the surface of the lithium iron phosphate. This improves the contact between the smaller lithium oxalate and the larger lithium iron phosphate. After delithiation, the lithium iron phosphate can effectively contact the lithium oxalate, enhancing the catalytic decomposition ability of the lithium oxalate.

[0032] This invention also provides a method for preparing a positive lithium-ion electrode sheet, comprising: preparing a positive lithium-ion electrode sheet slurry using the above-described preparation method; coating the slurry and drying it to obtain a positive electrode sheet blank; and rolling the positive electrode sheet blank to obtain the positive lithium-ion electrode sheet.

[0033] Specifically, the aluminum foil is coated by the way of flat plate coating and then dried, the thickness of the pole piece coating is 100-250 um; the drying process is to keep the temperature at 100-125 DEG C for 20-50 min, and then to dry in a vacuum oven at 60-85 DEG C for 6-12 h, which can effectively prevent the pole piece from cracking caused by too fast drying speed, and also can prevent the lithium oxalate from local segregation caused by too slow drying speed, so that the surface density of the pole piece is uniform, especially for the case that the catalytic decomposition of lithium oxalate by iron phosphate is difficult in the application. The pole piece after drying is rolled by a roller machine to obtain a lithium oxalate 3.5V decomposition positive electrode lithium supplement pole piece. The thickness of the positive pole blank after rolling is 60%-70% of the thickness before rolling, that is, the reduction rate is 30%-40%, and the particles of the pole piece after rolling are connected closely, which is conducive to the catalytic effect of iron phosphate on lithium oxalate.

[0034] The application also provides a positive electrode lithium supplement pole piece prepared by the above preparation method.

[0035] The application also provides a formation method of a lithium ion battery, which comprises assembling the lithium ion battery by using the above positive electrode lithium supplement pole piece; and the formation system is as follows: constant current charging, the current size is 14-17 mA / g, the limit voltage is 3.75-4.5 V; the constant current discharge current size is 14-17 mA / g, and the terminal voltage is 2.7-2.0 V, and then the exhaust is terminated. The formation system is very critical, and the formation system affects the lithium extraction effect of lithium iron phosphate, and the lithium extraction effect of lithium iron phosphate affects the catalytic decomposition of lithium oxalate, such as the uniformity and the amount of iron phosphate generated. If the amount of iron phosphate generated is small, the catalytic decomposition of lithium oxalate is weak, and lithium cannot be effectively supplemented. If the amount of iron phosphate generated is large, the electrode activity is affected. In addition, the formation system has the advantages that the exhaust is performed after the first complete cycle, and the carbon dioxide in the battery is conducive to the formation of a stable negative electrode surface SEI, thereby improving the cycle performance of the battery.

[0036] Specifically, the lithium oxalate 3.5V decomposition positive electrode lithium supplement pole piece is used as a positive electrode, a graphite pole piece is used as a negative electrode, 1 mol / L lithium hexafluorophosphate electrolyte, and the solvent is a mixture of dimethyl carbonate (DMC), ethylene carbonate (EC) and methyl ethyl carbonate (EMC) in a volume ratio of 1:1:1 to assemble a battery. The assembled battery is placed in a constant temperature box at 30 DEG C for 24 h to allow the electrolyte to be fully soaked, and the constant current charging and discharging method is used in the constant temperature box at 30 DEG C to perform charging and discharging. The battery is vacuumed at a vacuum degree of-20 to-50 Kpa, and the vacuum degree is-70 to-100 Kpa when sealed, and the advantage is that the negative pressure injection is conducive to the soaking between the electrolyte and the positive electrode particles, and reduces the impedance of lithium ion diffusion.

[0037] In order to better illustrate the embodiments of the application, the application will be further described in detail through specific examples.

[0038] Example 1

[0039] The embodiment of the present application provides a positive electrode lithium supplementing pole piece slurry, a preparation method of a pole piece and a battery formation method, which comprises the following steps:

[0040] S1, preparing a lithium oxalate dispersion solution. Lithium oxalate powder and NMP are weighed according to the mass ratio of 0.25, sealed in a ball mill jar and taken out after being ball milled on a planetary ball mill for 10 hours.

[0041] S2, mixing lithium iron phosphate powder and a conductive agent, adding the lithium oxalate dispersion solution and a binder to prepare a slurry. The lithium iron phosphate D 50 is 1.5 um, the conductive agent is added according to the mass ratio of 4 of the lithium iron phosphate powder and the conductive agent, the D 50 LCO is 0.23 um, the conductive agent is Super P Li and Ketjen black with the same mass, which are uniformly mixed in a mixing jar; then the binder is added according to 2 times of the total mass of the lithium iron phosphate and the conductive agent, the binder is PVDF powder mixed with NMP to prepare a binder with a mass fraction of 5wt%, the lithium oxalate dispersion solution is added according to the mass ratio of 1:2.2 of the lithium iron phosphate powder, and the rotation speed is kept at 500r / min for 5min and then kept at 1200r / min for 60min. The slurry is obtained.

[0042] S3, using a flat automatic coating machine to coat the prepared slurry on an aluminum foil, the coating thickness is 100um, the drying temperature is 120℃, the heating time is 40min after reaching the temperature, then the aluminum foil is placed in a vacuum oven at 80℃ for 12h. Then the pole piece is rolled, the thickness before rolling is 80um, the rolling rate is 40%, and the pole piece is rolled multiple times until the thickness of the pole piece is 48um(±5um), and the prepared positive electrode lithium supplementing pole piece is as shown in Figure 1 .

[0043] S4, preparing two same lithium ion batteries, selecting a graphite negative electrode according to the surface capacity after the prepared positive electrode, laminating, packaging with an aluminum plastic film, moving into a glove box, and injecting liquid. The injection vacuum degree is-45Kpa, and the packaging vacuum degree is-90Kpa. The packaged battery is placed in a constant temperature box at 30℃ for 24h. Then the two lithium ion batteries are charged at a current of 15mA / g to a limiting voltage of 4.2V and 4.5V respectively, and are discharged at the same current to 2.0V after standing for 2min.

[0044] As Figure 2 and 3As shown, the prepared lithium ion battery has a first cycle charge specific capacity of 180.74 mAh / g at 2.0-4.2 V and a 0.1 C rate, a first cycle discharge specific capacity of 131.23 mAh / g, a delithiation potential platform of about 3.5 V, and a 200 cycle efficiency of 82.42% at 1 C.

[0045] As shown, the prepared lithium ion battery has a first cycle charge specific capacity of 180.74 mAh / g at 2.0-4.2 V and a 0.1 C rate, a first cycle discharge specific capacity of 131.23 mAh / g, a delithiation potential platform of about 3.5 V, and a 200 cycle efficiency of 82.42% at 1 C. Figure 4 5 As shown, the prepared lithium ion battery has a first cycle charge specific capacity of 180.74 mAh / g at 2.0-4.2 V and a 0.1 C rate, a first cycle discharge specific capacity of 131.23 mAh / g, a delithiation potential platform of about 3.5 V, and a 200 cycle efficiency of 82.42% at 1 C.

[0046] Example 2

[0047] The present application provides a positive electrode lithium supplementing electrode piece slurry, a preparation method of the electrode piece, and a battery formation method, comprising:

[0048] S1: preparing a lithium oxalate dispersion solution. Lithium oxalate powder and NMP are weighed according to a mass ratio of 0.15, sealed in a ball mill jar, and then ball milled on a planetary ball mill for 8 hours.

[0049] S2: mixing lithium iron phosphate powder and a conductive agent, and then adding the lithium oxalate dispersion solution and a binder to prepare a slurry. The lithium iron phosphate D 50 is 1 um, and the conductive agent is added according to a mass ratio of 3.5 between the lithium iron phosphate powder and the conductive agent. The D 50 LCO is 0.2 um, and the conductive agent is Super P Li and Ketjen black with the same mass, which are mixed in a mixing jar. Then, the binder is added according to 1.8 times the total mass of the lithium iron phosphate and the conductive agent. The binder is a PVDF powder mixed with NMP to prepare a binder with a mass fraction of 4 wt%. The lithium oxalate dispersion solution is added according to a mass ratio of 1:1.3 between the lithium iron phosphate powder and the lithium oxalate dispersion solution. The rotation speed is maintained at 400 r / min for 5 min, and then the rotation speed is maintained at 1200 r / min for 40 min. The slurry is obtained.

[0050] S3: using a flat automatic coating machine to coat the prepared slurry on an aluminum foil. The drying temperature is 100 DEG C, the heating time is 20 min after reaching the temperature, and then the electrode piece is placed in a vacuum oven at 60 DEG C for 6 h. Then, the electrode piece is rolled at a rolling reduction rate of 30% to obtain the positive electrode lithium supplementing electrode piece.

[0051] ​S4 prepared positive electrode according to the surface capacity of the graphite negative electrode, then laminated, aluminum plastic film packaging, moved into the glove box, and then injected. The injection vacuum degree is -20Kpa, and the packaging vacuum degree is -70Kpa. The packaged battery is placed in a constant temperature box at 30℃ for 24h. Then, using a current of 14mA / g, the battery is charged to a limit voltage of 4.2V, and then discharged to 2.0V at the same current after 2min of standing.

[0052] The first cycle charge specific capacity of the prepared lithium ion battery at 2.0-4.2V and 0.1C rate is 175.25mAh / g, the first cycle discharge specific capacity is 126.45mAh / g, the delithiation potential platform is about 3.5V, and the cycle efficiency at 1C for 200 cycles is 79.40%.

[0053] Example 3

[0054] The present application provides a kind of positive electrode lithium supplement pole piece slurry, the preparation method of pole piece and battery formation method, comprising:

[0055] S1 prepares lithium oxalate dispersion. Lithium oxalate powder and NMP are weighed according to the mass ratio of lithium oxalate to solvent of 0.25, sealed in a ball mill jar, and then taken out after ball milling on a planetary ball mill for 10h.

[0056] S2 mixes lithium iron phosphate powder and conductive agent, then adds the lithium oxalate dispersion and binder to prepare a slurry. The lithium iron phosphate D 50 is 2.0um, and the conductive agent is added according to the mass ratio of lithium iron phosphate powder to conductive agent of 4.5, D 50 LCO is 0.4um, and the conductive agent uses Super P Li and Ketjen black with the same mass, which are mixed uniformly in a mixing jar; then the binder is added in an amount of 2.2 times the total mass of lithium iron phosphate and conductive agent, the binder is PVDF powder mixed with NMP solution to prepare a binder with a mass fraction of 6wt%, and the lithium oxalate dispersion is added in a mass ratio of 1:5.3 to the lithium iron phosphate powder, at a rotation speed of 700r / min for 10min, and then at a rotation speed of 2000r / min for 80min. The slurry is obtained.

[0057] S3 uses a flat automatic coating machine to coat the prepared slurry on an aluminum foil, with a coating thickness of 250um, a drying temperature of 125℃, a heating time of 50min after reaching the temperature, and then placed in a vacuum oven at 85℃ for 12h. Then the pole piece is rolled at a reduction rate of 40% to obtain the positive electrode lithium supplement pole piece.

[0058] S4 will be prepared according to the surface capacity selection of graphite negative electrode, then laminating, aluminum plastic film packaging, moving into the glove box, and then injecting liquid. The injection vacuum degree is-50Kpa, and the packaging vacuum degree is-100Kpa. The packaged battery is placed in a constant temperature box at 30℃ for 24h. Then, using a current of 17mA / g, it is charged to a limit voltage of 4.2V, and then discharged to 2.0V at the same current after 2min.

[0059] The prepared lithium ion battery has a first cycle charge specific capacity of 178.56mAh / g at 0.1C rate in the range of 2.0-4.2V, a first cycle discharge specific capacity of 129.43mAh / g, and a delithiation potential platform of about 3.5V. The cycle efficiency at 1C for 200 cycles is 80.21%.

[0060] Comparative Example 1

[0061] Different from Example 1, no lithium oxalate is added in this comparative example.

[0062] In step S4, the lithium ion battery is charged to a limit voltage of 4.2V at a current of 15mA / g, and then discharged to 2.0V at the same current after 2min.

[0063] As shown in Figures 2-5 , the prepared lithium ion battery has a first cycle charge specific capacity of 167.14mAh / g at 0.1C rate in the range of 2.0-4.2V, a first cycle discharge specific capacity of 124.07mAh / g, and a cycle efficiency of 68.98% at 1C for 200 cycles.

[0064] Comparative Example 2

[0065] Different from Example 1, in step S2 of this comparative example, the lithium iron phosphate D 50 particle size is 3um, and the D 50 particle size of lithium oxalate is 0.5um.

[0066] The prepared lithium ion battery has a first cycle charge specific capacity of 172.25mAh / g at 0.1C rate in the range of 2.0-4.2V, a first cycle discharge specific capacity of 127.51mAh / g, and a delithiation potential platform of about 3.5V. The cycle efficiency at 1C for 200 cycles is 69.25%.

[0067] Comparative Example 3

[0068] Different from Example 1, in step S2 of this comparative example, the lithium iron phosphate D 50 particle size is 1.5um, and the D 50 particle size of lithium oxalate is 0.2um.

[0069] The prepared lithium ion battery has a first cycle charge specific capacity of 178.56 mAh / g at 0.1C rate in 2.0-4.2V, a first cycle discharge specific capacity of 129.45 mAh / g, a de-lithiation potential platform of about 3.5V, and a 200 cycle efficiency of 79.50% at 1C.

[0070] Comparative Example 4

[0071] Different from Example 1, in step S2 of the present comparative example, the lithium iron phosphate D 50 is 1.5um, and the D 50 of lithium oxalate is 0.4um.

[0072] The prepared lithium ion battery has a first cycle charge specific capacity of 175.56 mAh / g at 0.1C rate in 2.0-4.2V, a first cycle discharge specific capacity of 127.50 mAh / g, a de-lithiation potential platform of about 3.5V, and a 200 cycle efficiency of 76.45% at 1C.

[0073] Comparative Example 5

[0074] Different from Example 1, in step S2 of the present comparative example, the rotation speed is kept at 500r / min for 65min.

[0075] The prepared lithium ion battery has a first cycle charge specific capacity of 172.45 mAh / g at 0.1C rate in 2.0-4.2V, a first cycle discharge specific capacity of 127.05 mAh / g, a de-lithiation potential platform of about 3.5V, and a 200 cycle efficiency of 70.25% at 1C.

[0076] Comparative Example 6

[0077] Different from Example 1, in step S2 of the present comparative example, the rotation speed is kept at 1200r / min for 65min.

[0078] The prepared lithium ion battery has a first cycle charge specific capacity of 170.45 mAh / g at 0.1C rate in 2.0-4.2V, a first cycle discharge specific capacity of 124.98 mAh / g, a de-lithiation potential platform of about 3.5V, and a 200 cycle efficiency of 68.06% at 1C.

[0079] From the embodiment 1 and the comparative example 1, it can be seen that the first charge specific capacity and the first circle discharge specific capacity are greatly increased, compared with the comparative example 1 in which only lithium iron phosphate is used as the positive electrode, the lithium ion battery prepared in the embodiment 1 has the first circle discharge specific capacity increased by 7.2 mAh / g and the capacity retention rate increased by 13.44% when the limiting voltage is 4.2 V; the first circle discharge specific capacity is increased by 15.93 mAh / g and the capacity is increased by 23.85% when the limiting voltage is 4.5 V, and the above effects are realized by the lithium supplement by lithium oxalate decomposition, so it can be seen that the technical scheme provided by the present application can effectively catalyze and decompose lithium oxalate by using the phosphorus iron of lithium iron phosphate after lithium extraction. From the embodiment 1 and the comparative examples 2-4, it can be seen that the D 50 particle size of the lithium iron phosphate and the ratio of the D 50 particle size of the lithium iron phosphate and the lithium oxalate are particularly important, if the particle size of the lithium iron phosphate is too large or the ratio of the two is not matched, the phosphorus iron after lithium extraction cannot effectively catalyze and decompose the lithium oxalate. From the embodiment 1 and the comparative examples 5-6, it can be seen that the mixing system greatly affects the prepared lithium ion battery, the reason is that the size of the lithium oxalate is small, under low speed, the lithium oxalate has micron-scale agglomeration and cannot effectively contact the surface of the phosphorus iron; under high speed, the lithium oxalate is seriously segregated, and the amount of the lithium oxalate enriched on the surface of part of the phosphorus iron is very small, which leads to poor catalytic decomposition effect of the phosphorus iron on the lithium oxalate, only the cooperation of low speed and high speed and the corresponding speed maintaining time can realize excellent mixing effect to improve the performance of the prepared battery.

[0080] The above only describes the preferred embodiments of the present application and is not intended to limit the present application, any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for forming a lithium-ion battery, characterized in that, Includes the following steps: A lithium-ion battery is assembled using a positive electrode supplemented with lithium as the positive electrode. The formation process is as follows: constant current charging, with a current of 14-17 mA / g and a limiting voltage of 3.75-4.5V; constant current discharging, with a current of 14-17 mA / g and a termination voltage of 2.7V-2.0V, followed by exhaust and final sealing. The method for preparing the positive lithium-filled electrode includes: Preparation of positive lithium supplementation electrode slurry; The slurry is coated and dried to obtain a positive electrode blank. The positive electrode blank is then rolled to obtain the positive lithium supplement electrode. The preparation method of the positive lithium supplementation electrode slurry includes: Preparation of lithium oxalate dispersion; Lithium iron phosphate powder is mixed with a conductive agent and then added to the lithium oxalate dispersion and binder to form a slurry; The lithium oxalate dispersion contains lithium oxalate, and the lithium iron phosphate powder has particle sizes that satisfy: D 50 LFP :D 50 LCO =100:(15-25)The D 50 LFP The range is 1.0-2.0 μm, where D 50 LFP D of lithium iron phosphate 50 Particle size, D 50 LCO D of lithium oxalate 50 Particle size; During the formation process, the chemical properties of the iron phosphate phase after lithium iron phosphate delithiation are utilized to catalyze the decomposition of lithium oxalate to achieve 3.5V decomposition.

2. The formation method according to claim 1, characterized in that, The mass ratio of lithium oxalate to solvent in the lithium oxalate dispersion is 0.15-0.25; The solvent is selected from one or more of N-methylpyrrolidone, N,N-dimethylformamide, ethanol, isopropanol, acetone, toluene, or n-hexane.

3. The formation method according to claim 1, characterized in that, The adhesive has a mass percentage of 4%-6%, and the adhesive is selected from one or more of polyvinylidene fluoride, polyvinyl alcohol, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, and polypropylene.

4. The formation method according to claim 1, characterized in that, The mass ratio of lithium iron phosphate powder to conductive agent is 3.5-4.5; The conductive agent is selected from one or more of porous carbon, SuperP, acetylene black, carbon nanotubes, Ketjen black, graphene, and MXenes.

5. The formation method according to claim 1, characterized in that, The conductive agent is carbon black.

6. The formation method according to claim 1, characterized in that, The ratio of the amount of lithium oxalate dispersion added to the mass of lithium iron phosphate powder is 1:(1.3-5.3).

7. The formation method according to claim 1, characterized in that, The lithium iron phosphate powder and conductive agent are mixed using a centrifugal planetary mixer at a speed of 300-600 r / min for 5-10 min. Then, the binder and lithium oxalate dispersion are added and the mixture is kept at a speed of 400-700 r / min for 5-10 min, followed by a speed of 1200-2000 r / min for 40-80 min.

8. The formation method according to claim 7, characterized in that, The reduction rate of the positive electrode blank roll is 30%-40%.

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