High-power spherical lithium iron phosphate composite material and preparation method thereof

Abstract

The invention discloses a high-power spherical lithium iron phosphate composite material and a preparation method thereof.The high-power spherical lithium iron phosphate composite material is of a core-shell structure composed of an inner core, a first shell and a second shell, and the inner core, the first shell and the second shell are sequentially wrapped from inside to outside; the inner core is rare earth doped lithium iron phosphate, the first shell is a fast plasma conductor composite layer, and the second shell is a lithium sulfonate composite layer; when the high-power spherical lithium iron phosphate composite material provided by the invention is applied as a positive electrode active material of a lithium ion battery, the power performance and the energy density of the lithium ion battery can be improved at the same time.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery material preparation, specifically relating to a high-power spherical lithium iron phosphate composite material and its preparation method. Background Technology

[0002] As the requirements for energy density and power performance of lithium-ion batteries increase, the power performance of lithium iron phosphate cathode materials used in lithium-ion batteries should also be improved while increasing specific capacity and compaction density.

[0003] However, due to the olivine crystal structure of lithium iron phosphate, its electronic conductivity is low, and the kinetics of lithium ion diffusion along one-dimensional channels are slow, especially under low temperature conditions. The low temperature environment will exacerbate the lithium ion migration barrier in the lithium iron phosphate cathode material lattice, resulting in a significant slowdown in lithium ion insertion / extraction kinetics, increased interfacial polarization, and also an increase in the activation energy of charge transfer reaction and electrochemical impedance, thus severely limiting the power performance (including power output and rate performance) of lithium iron phosphate cathode materials.

[0004] To improve the power performance of lithium iron phosphate while maintaining energy density, researchers have tried various methods such as doping, surface coating, particle size distribution, and morphology control to enhance the power and other properties of lithium iron phosphate materials. However, these methods have limited effectiveness and fail to achieve both high power performance and minimal impact on energy density. Summary of the Invention

[0005] In view of this, the purpose of the present invention is to provide a high-power spherical lithium iron phosphate composite material and its preparation method, which can be used as a positive electrode active material for lithium-ion batteries to simultaneously improve the power performance and energy density of lithium-ion batteries.

[0006] The technical solution adopted in this invention is as follows: A high-power spherical lithium iron phosphate composite material adopts a core-shell structure consisting of a core, a first shell, and a second shell, wherein the core, the first shell, and the second shell are arranged in a sequential manner from the inside out; the core is rare earth-doped lithium iron phosphate, the first shell is a fast plasma conductor composite layer, and the second shell is a lithium sulfonate composite layer.

[0007] Preferably, based on a 100% mass ratio of high-power spherical lithium iron phosphate composite material, the mass ratio of the core, the first shell, and the second shell ranges from 95-99:0.5-2:0.5-3.

[0008] Preferably, the rare earth content in the core is 0.05-0.5% by mass.

[0009] Preferably, based on a first shell mass ratio of 100%, the first shell is composed of 80-95% fast ion conductors and 5-20% amorphous carbon.

[0010] Preferably, a method for preparing a high-power spherical lithium iron phosphate composite material according to the above description includes at least the following steps: S1. According to the molar ratio of phosphorus source, iron source, and lithium source of 1:0.95-1.05:0.95-1.05, add the iron source, phosphorus source, and lithium source to the solvent, mix evenly, and then add 1-3wt% organic pore-forming agent (referring to the weight ratio of the sum of the weights of phosphorus source, iron source, and lithium source), 1-3wt% dispersant (referring to the weight ratio of the sum of the weights of phosphorus source, iron source, and lithium source), and 0.05-0.5% rare earth compound (referring to the weight ratio of the sum of the weights of phosphorus source, iron source, and lithium source). After mixing evenly, transfer to a high-pressure reactor and react for 12-24 hours at a temperature of 180-220℃ and a pressure of 1.1-5.1 MPa. After filtration, washing, and freeze-drying, spherical lithium iron phosphate precursor (i.e., rare earth-doped lithium iron phosphate with the core) is obtained. S2. According to the mass ratio of fast ion conductor, lithium carboxymethyl cellulose, and lithium iron phosphate precursor of 1-5:1-5:100, lithium carboxymethyl cellulose is added to N-methylpyrrolidone solvent to prepare a solution with a mass concentration of 0.5-2wt%. Then, fast ion conductor and lithium iron phosphate precursor are added and dispersed evenly. The reaction is carried out at a temperature of 50-100℃ for 1-6 hours. The filter residue obtained after filtration is sintered at a temperature of 500-800℃ for 1-6 hours to obtain lithium iron phosphate coated with fast ion conductor (i.e., the first shell). S3. According to the mass ratio of lithium sulfonate derivative, ionic liquid, and fast ion conductor coated lithium iron phosphate of 1-5:50-100:100, the lithium sulfonate derivative is added to the ionic liquid and mixed to prepare a solution. The fast ion conductor coated lithium iron phosphate is pressed into a block structure as a working electrode. A lithium sulfonate outer layer (i.e., the second shell: lithium sulfonate composite layer) is formed on the surface of the fast ion conductor coated lithium iron phosphate by electrochemical deposition. After vacuum drying at 70-100℃ for at least 20 hours, the high-power spherical lithium iron phosphate composite material is obtained.

[0011] Preferably, in step S1, the lithium source is at least one of lithium oxalate, lithium nitrate, lithium phosphate, and lithium hydroxide; the iron source is at least one of ferric sulfate, ferric nitrate, ferric citrate, ferric phosphate, and ferric oxide; the phosphorus source is at least one of ferric phosphate, lithium phosphate, phosphoric acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate; the organic pore-forming agent is at least one of polystyrene, polyvinyl carbazole, polyvinyl acetate, polymethyl methacrylate, and polybutyl acrylate; the dispersant is at least one of polyvinylbenzenesulfonic acid, polyacrylic acid, and sodium tripolyphosphate; and the rare earth compound is one of yttrium oxide, cerium oxide, lanthanum oxide, praseodymium oxide, rubidium oxide, samarium oxide, europium oxide, gadolinium oxide, and terbium oxide.

[0012] Preferably, in step S2, the fast ion conductor is a fast ion conductor composite LiX. a PO4(MoO4) b , where X is one of Fe, Mn, Sn, Sr, Ce, 1≤a≤4, 1≤b≤4.

[0013] Preferably, in step S3, the lithium sulfonate derivative is at least one of lithium 4-methylbenzenesulfonate, lithium trifluoromethanesulfonate, lithium perfluorohexanesulfonate, lithium 4-vinylbenzenesulfonate, and lithium benzofuran-2-sulfinate; and the ionic liquid is at least one of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, 1-ethyl-3-methylimidazolium perchlorate, and 1-ethyl-3-methylimidazolium p-toluenesulfonate.

[0014] Preferably, in step S3, the electrochemical deposition process involves scanning for 10-100 cycles at a voltage range of -1.5 to 1.5V and a scan rate of 1-5mV / s.

[0015] The working principle and beneficial effects of this invention are as follows: This application improves the electronic conductivity and structural stability of lithium iron phosphate by doping it with rare earth elements as a core. The core exhibits a spherical structure, which allows lithium ions to be transported in multiple directions during charging and discharging, shortening the migration path and improving rate performance. At the same time, the fast ion conductor composite layer, which serves as the first outer shell, acts as another conductive network, accelerating the transfer speed of charge on the surface of the spherical lithium iron phosphate particles and reducing the impact of low temperature. Furthermore, since the ionic conductivity of the fast ion conductor is much higher than that of the lithium iron phosphate particles, it can help reduce the contact resistance between the cathode material particles, greatly alleviating the problem of increased polarization and decreased rate performance caused by electron transport at low temperatures. The lithium sulfonate composite layer (specifically, a lithium sulfonate derivative with strong solvation ability), which serves as the second outer shell (i.e., the outermost layer), enhances the lithium ion insertion and extraction rate, improving rate performance.

[0016] This application also prepares high-power spherical lithium iron phosphate composite materials using a liquid-phase method, which has advantages such as better coating uniformity and more complete reaction compared to solid-phase coating. Specifically, in the preparation process, an organic pore-forming agent is added to the lithium iron phosphate precursor to generate a spherical structure through a hydrothermal reaction. During sintering, the organic pore-forming agent decomposes, leaving nanopores, increasing the specific surface area of ​​the material, improving the diffusion rate of lithium ions, reducing polarization, and enhancing the specific capacity of the material. Furthermore, a fast ion conductor and lithium carboxymethyl cellulose are coated onto the surface of the lithium iron phosphate precursor using a liquid-phase method. After carbonization and sintering, the lithium carboxymethyl cellulose generates lithium-doped amorphous carbon, which has advantages such as low impedance and good isotropy compared to other carbon-based materials (glucose, sucrose, etc.), thus improving its power performance in lithium-ion batteries. Attached Figure Description

[0017] Figure 1 This is a SEM image of the high-power spherical lithium iron phosphate composite material prepared in Example 1 of the present invention. Detailed Implementation

[0018] Example 1: A method for preparing a high-power spherical lithium iron phosphate composite material, comprising the following steps: S1. According to the molar ratio of phosphorus:iron:lithium = 1:1:1, 100g of ferric sulfate, ferric phosphate and lithium oxalate were added to 500g of ethanol solvent and mixed evenly. Then, 2g of polystyrene, 2g of polyvinylbenzenesulfonic acid and 0.2g of cerium oxide were added and mixed evenly. The mixture was then transferred to a high-pressure reactor and reacted for 18 hours at a temperature of 200℃ and a pressure of 3MPa. The reaction product was filtered, washed and freeze-dried at -40℃ for 48 hours to obtain spherical lithium iron phosphate precursor. S2. Add 3g of lithium carboxymethyl cellulose to 300g of N-methylpyrrolidone solvent to prepare a solution with a mass concentration of 1wt%. Then add 3g of LiFe2PO4(MoO4)2 and 100g of the lithium iron phosphate precursor obtained in step S1 above. After dispersing evenly, react at 80℃ for 3 hours. Filter and sinter the resulting filter residue at 650℃ for 3 hours to obtain fast ion conductor coated lithium iron phosphate. S3. Add 3g of lithium 4-methylbenzenesulfonate to 80g of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt to prepare a solution. Press 100g of the fast ion conductor-coated lithium iron phosphate obtained in step S2 into a block structure as the working electrode. Scan for 50 cycles using an electrochemical deposition method (specifically cyclic voltammetry) at a voltage range of -1.5 to 1.5V and a scan rate of 3mV / s to form a lithium sulfonate outer layer on the surface of the fast ion conductor-coated lithium iron phosphate. Then, vacuum dry at 80℃ for 24 hours to obtain a high-power spherical lithium iron phosphate composite material.

[0019] This application presents a SEM analysis of the high-power spherical lithium iron phosphate composite material prepared in Example 1. Please refer to [link to relevant documentation]. Figure 1 As shown, the lithium iron phosphate composite material is spherical, exists in particle size, has a suitable particle ratio, and the particle size is between 1-5µm.

[0020] Example 2: A method for preparing a high-power spherical lithium iron phosphate composite material, comprising the following steps: S1. According to the molar ratio of phosphorus:iron:lithium = 1:0.95:1.05, 100g of lithium nitrate, iron nitrate and ammonium dihydrogen phosphate were added to 500g of ethanol solvent and mixed evenly. Then, 1g of polyvinylcarbazole, 1g of polyacrylic acid and 0.05g of yttrium oxide were added and mixed evenly. The mixture was then transferred to a high-pressure reactor and reacted for 24 hours at a temperature of 180℃ and a pressure of 1.1MPa. The reaction product was filtered, washed and freeze-dried at -40℃ for 48 hours to obtain spherical lithium iron phosphate precursor. S2. Add 1g of lithium carboxymethyl cellulose to 200g of N-methylpyrrolidone solvent to prepare a solution with a mass concentration of 0.5wt%. Then add 1g of LiMn2PO4(MoO4)2 and 100g of the lithium iron phosphate precursor obtained in step S1 above. After uniform dispersion, react at 50℃ for 6 hours. Filter and sinter the resulting filter residue at 500℃ for 6 hours to obtain fast ion conductor coated lithium iron phosphate. S3. Add 1g of lithium trifluoromethanesulfonate to 50g of 1-ethyl-3-methylimidazolium perchlorate to prepare a solution. Press 100g of the fast ion conductor-coated lithium iron phosphate obtained in step S2 into a block structure as the working electrode. Scan for 10 weeks using an electrochemical deposition method (specifically cyclic voltammetry) at a voltage range of -1.5 to 1.5V and a scan rate of 1mV / s to form a lithium sulfonate outer layer on the surface of the fast ion conductor-coated lithium iron phosphate. Then, vacuum dry at 80℃ for 24 hours to obtain a high-power spherical lithium iron phosphate composite material.

[0021] Example 3: A method for preparing a high-power spherical lithium iron phosphate composite material, comprising the following steps: S1. According to the molar ratio of phosphorus:iron:lithium = 1:1.05:0.95, 100g of ferric citrate, diammonium hydrogen phosphate and lithium phosphate were added to 500g of ethanol solvent and mixed evenly. Then, 3g of polyvinyl acetate, 3g of sodium tripolyphosphate and 0.5g of lanthanum oxide were added and mixed evenly. The mixture was then transferred to a high-pressure reactor and reacted for 12 hours at a temperature of 220℃ and a pressure of 5.1MPa. The reaction product was filtered, washed and freeze-dried at -40℃ for 48 hours to obtain spherical lithium iron phosphate precursor. S2. Add 5g of lithium carboxymethyl cellulose to 250g of N-methylpyrrolidone solvent to prepare a solution with a mass concentration of 2wt%. Then add 5g of LiSn2PO4(MoO4)2 and 100g of the lithium iron phosphate precursor obtained in step S1 above. After dispersing evenly, react at 100℃ for 1 hour. Filter and sinter the resulting filter residue at 800℃ for 1 hour to obtain fast ion conductor coated lithium iron phosphate. S3. Add 5g of lithium perfluorohexane sulfonate to 100g of 1-ethyl-3-methylimidazolium p-toluenesulfonate to prepare a solution. Press 100g of the fast ion conductor-coated lithium iron phosphate obtained in step S2 into a block structure as the working electrode. Scan for 100 cycles using an electrochemical deposition method (specifically cyclic voltammetry) at a voltage range of -1.5 to 1.5V and a scan rate of 5mV / s to form a lithium sulfonate outer layer on the surface of the fast ion conductor-coated lithium iron phosphate. Then, vacuum dry at 80℃ for 24 hours to obtain a high-power spherical lithium iron phosphate composite material.

[0022] Comparative Example 1: The remaining technical solutions of Comparative Example 1 are the same as those of Example 1, except that polystyrene and cerium oxide are not added in step S1 of Comparative Example 1.

[0023] Comparative Example 2: The remaining technical solutions of Comparative Example 2 are the same as those of Example 1, except that in Comparative Example 2, step S3 is not performed, and the fast ion conductor obtained in step S2 is directly coated with lithium iron phosphate as the material provided in Comparative Example 2.

[0024] Comparative Example 3: The remaining technical solutions of Comparative Example 3 are the same as those of Example 1, except that step S2 is omitted, and the spherical lithium iron phosphate precursor obtained in step S1 is used to directly replace the fast ion conductor coating lithium iron phosphate in step S3, and a lithium sulfonate outer layer is directly formed on the core surface of the lithium iron phosphate precursor.

[0025] To verify the implementation effects achieved by the embodiments of this application, the applicant tested the specific surface area and compaction density of the materials provided in Examples 1-3 and Comparative Examples 1-3 in accordance with GB / T 30835-2014 "Carbon Composite Lithium Iron Phosphate Cathode Materials for Lithium-ion Batteries"; and tested their powder resistivity using a four-probe tester. The applicant assembled the button cells according to the following procedure and conducted performance tests on each button cell: According to the mass ratio of positive electrode active material: carbon nanotubes: PVDF: NMP = 96:2:3:300, high-power spherical lithium iron phosphate composite materials (as positive electrode active materials) prepared in Examples 1-3 and Comparative Examples 1-3, carbon nanotubes, and PVDF were weighed and mixed. Then, NMP (N-methylpyrrolidone) organic solvent was added and mixed thoroughly. Then, a film with a thickness of 140 micrometers was coated on aluminum foil, vacuum dried at 120°C for 2 hours, punched into 5 mm discs, and pressed into sheets at 10 MPa using a tablet press. The sheets were then vacuum-insulated at 120°C for 12 hours, and the weight of the positive electrode sheets was measured. Then, a coin cell was assembled in an argon-protected glove box, with lithium metal sheets as the counter electrode and an electrolyte consisting of EC (ethylene carbonate) and DMC (1,2-dimethyl carbonate) in a volume ratio of 1:1. Solvent, electrolyte LiPF6, separator Celgard 2400 microporous polyethylene membrane; coin cells corresponding to Examples 1-3 and Comparative Examples 1-3 were assembled respectively. The electrical performance of each coin cell was tested on a Blue Electric Tester; the first discharge specific capacity and first efficiency were tested by constant current charging / discharging at 0.2C within a voltage range of 2.75V-4.25V; at the same time, its charge DCR (50% SOC) and its cycle performance (0.2C / 0.2C, 100 cycles) were tested. Please refer to Table 1 below for the test results of the above test items:

[0026] As can be seen from Table 1 above, the coin cells made using the high-power spherical lithium iron phosphate composite materials provided in Examples 1-3 have significantly higher initial discharge capacity and initial efficiency than those in Comparative Examples 1-3. The main reason for this is that the materials provided in the embodiments of this application have excellent lithium-ion diffusion rate and lattice structure stability, which can significantly improve cycle performance, thereby enabling the battery to have both excellent power performance and energy density performance.

[0027] This application also assembles pouch cells according to the following process and performs performance tests on each pouch cell: Using the materials provided in Examples 1-3 and Comparative Examples 1-3 as positive electrode materials, graphite as negative electrode material, LiPF6 / EC+DEC (EC and DEC volume ratio 1:1) as electrolyte, and Celgard 2400 membrane as separator, two sets of 5AH soft-pack batteries corresponding to Examples 1-3 and Comparative Examples 1-3 were prepared respectively. Cyclic performance tests were conducted on each of the pouch cells in one group. Specifically, the cycle performance of each pouch cell was tested at a charge / discharge rate of 1C / 1C, a voltage range of 2.5V-3.65V, and a temperature of 25±3℃. HPPC performance tests were also conducted on the pouch batteries in another group, specifically: charging DCR at different SOCs (10%, 30%, 50%, 70%, 90%) using 3C charging / 4C discharging rates. Please refer to Table 2 below for the test results:

[0028] As can be seen from Table 2 above, the rate performance and cycle performance of the soft-pack batteries made using the high-power spherical lithium iron phosphate composite materials provided in Examples 1-3 are better than those of Comparative Examples 1-3. The main reason is that the materials provided in this application are coated with a fast-ion conductor composite layer, which can significantly improve the structural stability of the material and improve the cycle performance. In addition, a lithium sulfonate composite layer with strong solvation ability (the main component is lithium sulfonate derivative) is also coated on the surface of the fast-ion conductor composite layer, thereby improving the lithium ion insertion and extraction rate during the charging and discharging process of the battery and improving the rate performance. This makes the battery have excellent power performance and energy density performance at the same time.

[0029] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0030] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A high power spherical lithium iron phosphate composite material, characterized in that, The core-shell structure is composed of a core, a first shell and a second shell, wherein the core, the first shell and the second shell are sequentially coated from inside to outside; the core is a rare earth doped lithium iron phosphate, the first shell is a fast ion conductor composite layer, and the second shell is a lithium sulfonate composite layer.

2. The high power spherical lithium iron phosphate composite material of claim 1, wherein, According to the mass ratio of 100% of the high-power spherical lithium iron phosphate composite material, the mass ratio of the core, the first shell and the second shell ranges from 95-99:0.5-2:0.5-3.

3. The high power spherical lithium iron phosphate composite material of claim 1, wherein, The mass ratio of the rare earth in the core is 0.05-0.5%.

4. The high power spherical lithium iron phosphate composite material of claim 1, wherein, According to the mass ratio of 100% of the first shell, the first shell is composed of 80-95% of a fast ion conductor and 5-20% of amorphous carbon.

5. A method of producing the high power spherical lithium iron phosphate composite material according to any one of claims 1 to 4, characterized in that, At least the following operation steps are included: S1, according to the molar ratio of phosphorus source, iron source and lithium source being 1:0.95-1.05:0.95-1.05, the iron source, the phosphorus source and the lithium source are added to the solvent, after mixing uniformly, 1-3wt% organic pore forming agent, 1-3wt% dispersing agent and 0.05-0.5% rare earth compound are added, after mixing uniformly, it is transferred to a high-pressure reaction kettle, under the condition of temperature being 180-220℃ and pressure being 1.1-5.1Mpa, it is reacted for 12-24 hours to obtain a spherical lithium iron phosphate precursor; S2, according to the mass ratio of fast ion conductor, lithium carboxymethyl cellulose and lithium iron phosphate precursor being 1-5:1-5:100, the lithium carboxymethyl cellulose is added to N-methyl pyrrolidone solvent to configure a solution with a mass concentration of 0.5-2wt%, then the fast ion conductor and the lithium iron phosphate precursor are added, after being dispersed uniformly, it is reacted for 1-6 hours under the condition of temperature being 50-100℃, the filter residue obtained after filtration is sintered for 1-6 hours under the condition of temperature being 500-800℃ to obtain a fast ion conductor coated lithium iron phosphate; S3, according to the mass ratio of lithium sulfonate derivative, ionic liquid and fast ion conductor coated lithium iron phosphate being 1-5:50-100:100, the lithium sulfonate derivative is mixed with the ionic liquid to configure a solution, the fast ion conductor coated lithium iron phosphate is pressed into a block structure as a working electrode, a lithium sulfonate outer layer is formed on the surface of the fast ion conductor coated lithium iron phosphate by electrochemical deposition to obtain the high-power spherical lithium iron phosphate composite material.

6. The high power spherical lithium iron phosphate composite material of claim 5, wherein, In step S1, the lithium source is at least one of lithium oxalate, lithium nitrate, lithium phosphate and lithium hydroxide; the iron source is at least one of iron sulfate, iron nitrate, iron citrate, iron phosphate, and diiron trioxide; the phosphorus source is at least one of iron phosphate, lithium phosphate, phosphoric acid, ammonium dihydrogen phosphate and diammonium hydrogen phosphate; the organic pore forming agent is at least one of polystyrene, polyvinyl carbazole, polyvinyl acetate, polymethyl acrylate and polybutyl acrylate; the dispersing agent is at least one of polyvinyl benzene sulfonic acid, polyacrylic acid and sodium tripolyphosphate; and the rare earth compound is one of yttrium oxide, cerium oxide, lanthanum oxide, praseodymium oxide, rubidium oxide, samarium oxide, europium oxide, gadolinium oxide and terbium oxide.

7. The high power spherical lithium iron phosphate composite material of claim 5, wherein, In step S2, the fast ion conductor adopts a fast ion conductor composite LiX a PO4(MoO4) b , wherein X is one of Fe, Mn, Sn, Sr, Ce, 1≤a≤4, 1≤b≤4.

8. The high power spherical lithium iron phosphate composite material of claim 5, wherein, In step S3, the lithium sulfonate derivative is at least one of lithium 4-methylbenzenesulfonate, lithium triflate, lithium perfluorohexanesulfonate, lithium 4-vinylbenzenesulfonate, lithium benzofuran-2-sulfinate; and the ionic liquid is at least one of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1-ethyl-3-methylimidazolium perchlorate, 1-ethyl-3-methylimidazolium p-toluenesulfonate.

9. The high power spherical lithium iron phosphate composite material of claim 5, wherein, In step S3, the electrochemical deposition method is performed by scanning 10-100 times at a voltage range of -1.5 to 1.5 V and a scanning rate of 1-5 mV / s.

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