Positive electrode sheet and lithium-ion battery

Abstract

A positive electrode sheet and a lithium-ion battery. The positive electrode sheet comprises a current collector and a positive electrode material compounded on the current collector, wherein the positive electrode material comprises a positive electrode active material. The positive electrode active material is selected from lithium manganese iron phosphate, and the D90 particle size of the lithium manganese iron phosphate particles, the tensile strength σ of the positive electrode sheet, and the compacted density PD of the positive electrode sheet satisfy the relational expression as shown in inequality (I): 1600 μm·MPa / (g / cm3)＜D90 / PD×σ＜4800μm·MPa / (g / cm3)(I).

Description

A positive electrode and a lithium-ion battery Technical Field

[0001] This disclosure belongs to the field of lithium-ion battery cathode technology, and relates to a cathode and a lithium-ion battery, particularly to a lithium manganese iron phosphate cathode and a lithium-ion battery. Background Technology

[0002] In lithium manganese iron phosphate (LMP) materials, high electron and ion transport impedance leads to poor conductivity and kinetic properties. Solid-phase diffusion of lithium ions within LMP is a key factor limiting its kinetics. Therefore, reducing the primary particle size to shorten the lithium-ion diffusion path can help improve the kinetic performance of LMP materials. However, reducing the primary particle size to a certain extent results in excessively large particle surface areas. This increases the amount of solvent and binder required for uniform particle dispersion during the slurry mixing process, further reducing the solid content of the slurry. This makes the slurry prone to cracking during thick electrode coating, resulting in low coating surface density and hindering the improvement of cell energy density.

[0003] Therefore, how to design a more suitable lithium manganese iron phosphate cathode to solve the above-mentioned problems of existing lithium manganese iron phosphate cathodes has become one of the focuses of attention for many technical personnel in the industry. Summary of the Invention

[0004] The inventors unexpectedly discovered that by controlling the particle size of lithium manganese iron phosphate particles, the tensile strength of the cathode sheet, and the compaction density of the cathode sheet to satisfy a specific relationship, the performance of the lithium manganese iron phosphate cathode sheet can be improved.

[0005] Based on this discovery, this disclosure provides a positive electrode sheet and its preparation method, a lithium-ion battery, and in particular a lithium manganese iron phosphate positive electrode sheet.

[0006] The first aspect of this disclosure provides a positive electrode sheet, the positive electrode sheet comprising a current collector and a positive electrode material composited on the current collector;

[0007] The cathode material includes a cathode active material;

[0008] The positive electrode active material comprises lithium iron manganese phosphate particles;

[0009] The D90 particle size of the lithium manganese iron phosphate particles, the tensile strength σ of the positive electrode, and the compaction density PD of the positive electrode satisfy the relationship described in equation (I): 1600 μm·MPa / (g / cm³) 3 ) <D90 / PD×σ<4800μm·MPa / (g / cm 3 (I);

[0010] Where D90 is measured in μm, σ in MPa, and PD in g / cm³. 3 .

[0011] Preferably, in formula (I), 2000 μm·MPa / (g / cm) 3 ) <D90 / PD×σ<4000μm·MPa / (g / cm 3 )

[0012] Preferably, the lithium manganese iron phosphate particles specifically include secondary lithium manganese iron phosphate particles.

[0013] Preferably, the D90 particle size of the lithium manganese iron phosphate particles is 10–50 μm.

[0014] Preferably, the D90 particle size of the lithium manganese iron phosphate particles is 15–40 μm.

[0015] Preferably, the compaction density PD is 2.0–2.5 g / cm³. 3 .

[0016] Preferably, the tensile strength σ is 100–500 MPa.

[0017] Preferably, the compaction change rate ΔPD of the positive electrode sheet and the tensile strength σ of the positive electrode sheet satisfy the relationship described in equation (II): 0 MPa < σ × ΔPD < 95 MPa (II);

[0018] Where σ is in MPa, ΔPD = (PD max -PD) / PD, PD max This represents the maximum compaction density.

[0019] The maximum compaction density is the compaction density corresponding to the positive electrode sheet when it is folded in half to transmit light after secondary rolling or the compaction density corresponding to the lithium manganese iron phosphate particles being crushed after secondary rolling of the positive electrode sheet.

[0020] Preferably, the range of △PD is 4% to 25%.

[0021] A second aspect of this disclosure also provides a lithium-ion battery, including a positive electrode sheet;

[0022] The positive electrode sheet is any one of the positive electrode sheets described in the above technical solutions.

[0023] This disclosure provides a positive electrode sheet, which includes a current collector and a positive electrode material composited on the current collector; the positive electrode material includes a positive electrode active material; the positive electrode active material comprises or is selected from lithium manganese iron phosphate particles; the D90 particle size of the lithium manganese iron phosphate particles, the tensile strength σ of the positive electrode sheet, and the compaction density PD of the positive electrode sheet satisfy the relationship described in equation (I): 1600 μm·MPa / (g / cm³) 3 ) <D90 / PD×σ<4800μm·MPa / (g / cm 3 (I). Compared with the prior art, the positive electrode of this disclosure achieves both excellent fast charging performance and energy density by balancing the D90 particle size of lithium manganese iron phosphate particles, the tensile strength σ of the positive electrode, and the compaction density PD of the positive electrode. Attached Figure Description

[0024] Figure 1 is a 1000X SEM image of the lithium manganese iron phosphate electrode sheet after rolling provided in this disclosure;

[0025] Figure 2 is a 3000X SEM image of the lithium manganese iron phosphate electrode sheet after rolling provided in this disclosure. Detailed Implementation

[0026] To further understand this disclosure, preferred embodiments of the disclosure are described below in conjunction with examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the disclosure and not for limiting the claims of the disclosure.

[0027] All raw materials disclosed herein are not subject to any particular restriction on their source; they may be purchased from the market or prepared using conventional methods known to those skilled in the art.

[0028] All raw materials disclosed herein are not subject to any particular restrictions on their purity. This disclosure preferably adopts analytical grade or the conventional purity requirements in the field of lithium-ion battery cathode material preparation.

[0029] All raw materials disclosed herein are conventional in the art, and each designation and abbreviation is clear and distinct in the field of its relevant application. Those skilled in the art can purchase them from the market or prepare them by conventional methods based on the designation, abbreviation, and corresponding application.

[0030] The processes used in this disclosure are all common abbreviations in the field. The specific steps and common parameters of each abbreviation are clear and unambiguous in their respective fields. Those skilled in the art can implement them using conventional methods based on the abbreviations.

[0031] Lithium manganese iron phosphate particle size D90: The particle size value corresponding to the cumulative distribution percentage of particles from smallest to largest reaching 90% in the particle size distribution.

[0032] Tensile strength (σ) of positive electrode: In a tensile test, the maximum tensile stress that the specimen experiences until it breaks is the tensile strength, and the result is expressed in MPa.

[0033] Compaction Change Rate ΔPD after Re-pressing of Positive Electrode Sheet: The degree of change in compaction of the disassembled positive electrode sheet after re-rolling relative to the compaction before rolling, expressed in percentage.

[0034] This disclosure provides a positive electrode sheet, the positive electrode sheet comprising a current collector and a positive electrode material composited on the current collector;

[0035] The positive electrode material includes a positive electrode active material;

[0036] The positive electrode active material is selected from lithium iron manganese phosphate, and the structural formula of lithium iron manganese phosphate particles is: LiMn x Fe y M z PO4, where x is greater than 0 and less than 1; y is greater than 0 and less than 1; z is greater than or equal to 0 and less than 1; M is a dopant element with a valence of n; satisfying: 2(x+y)+n*z=2.

[0037] The D90 particle size of the lithium manganese iron phosphate particles, the tensile strength σ of the positive electrode, and the compaction density PD of the positive electrode satisfy the relationship described in equation (I): 1600 μm·MPa / (g / cm³) 3 ) <D90 / PD×σ<4800μm·MPa / (g / cm 3 (I);

[0038] Where D90 is measured in μm, σ in MPa, and PD in g / cm³. 3 .

[0039] The inventors have discovered that while using an agglomerate structure can reduce the solvent and binder content in the slurry mixing process, achieving both primary particle size reduction and thick electrode coating, agglomeration results in larger particle sizes. This makes it difficult to achieve high compaction density during processing, thus hindering the improvement of cell energy density. Therefore, it is necessary to address the problem of improving the compaction density of lithium manganese iron phosphate (LFP) agglomerate electrodes. This disclosure specifically designs a LFP cathode with a unique structure, where the D90 particle size of the LFP particles, the tensile strength σ of the cathode, and the compaction density PD satisfy a specific relationship. By controlling the LFP particles, the tensile strength of the cathode, and the compaction density of the cathode, this disclosure further improves the performance of the LFP cathode.

[0040] This disclosure relates the tensile strength of lithium manganese iron phosphate (LMP) agglomerate electrodes to the particle size distribution and the electrode state corresponding to high compaction. The tensile strength is further related to the properties of the current collector (foil), the active material layer, and the processing technology / equipment. Under the condition that D90 and σ, PD satisfy the relationship (I), at a certain compaction density, using relatively large LMP agglomerates with suitable foil and processing technology results in electrodes with lower tensile strength; using relatively small LMP agglomerates with suitable foil and processing technology results in electrodes with higher tensile strength. With a fixed particle size, using a higher compaction density with suitable foil and processing technology results in electrodes with higher tensile strength; using a lower compaction density with suitable foil and processing technology results in electrodes with lower tensile strength. Furthermore, compared to LMP particle D50, which reflects the overall particle size distribution, D90 better reflects the size characteristics of large particles in the LMP material. If the particle size exceeds the upper limit of D90, it will be too large, which will easily cause scratches on the electrode during coating and require higher strength of the foil during rolling. If the particle size exceeds the lower limit of D90, it will be too small, requiring more solvent and binder during slurry mixing, resulting in poor processing performance of the slurry and making it unfavorable for thick electrode coating.

[0041] Furthermore, the tensile strength of the electrode is related to the properties of the current collector (foil) itself, the properties of the active material layer, and the processing technology, especially the foil, the amount of active material coating (areal density), and the adhesion between the active material layer and the foil. A low tensile strength σ indicates a deterioration in the processability of the electrode after rolling, which may lead to negative effects such as electrode breakage during rolling. In addition, there may be some microcracks inside the current collector, affecting electron transport and thus impacting cell performance.

[0042] In this disclosure, the relationship described in equation (I) can be 2000 μm·MPa / (g / cm²). 3 ) <D90 / PD×σ<4000μm·MPa / (g / cm 3 ), which can reach 2500 μm·MPa / (g / cm 3 ) <D90 / PD×σ<3500μm·MPa / (g / cm 3 ).

[0043] In this disclosure, the lithium manganese iron phosphate particles are preferably secondary lithium manganese iron phosphate particles.

[0044] In this disclosure, the D90 particle size of the lithium manganese iron phosphate particles is preferably 10-50 μm, more preferably 15-40 μm, more preferably 20-35 μm, and even more preferably 25-30 μm.

[0045] In this disclosure, the compaction density PD is preferably 2.0 to 2.5 g / cm³. 3 More preferably, it is 2.1–2.4 g / cm³. 3 More preferably, it is 2.2–2.3 g / cm³. 3 .

[0046] In this disclosure, the tensile strength σ is preferably 100-500 MPa, more preferably 150-450 MPa, even more preferably 200-400 MPa, and even more preferably 250-350 MPa.

[0047] In this disclosure, the compaction change rate ΔPD of the positive electrode sheet and the tensile strength σ of the positive electrode sheet preferably satisfy the relationship described in equation (II): 0 MPa < σ × ΔPD < 95 MPa (II);

[0048] Where σ is in MPa, ΔPD = (PD max -PD) / PD, PD max This represents the maximum compaction density.

[0049] In this disclosure, the maximum compaction density is preferably the compaction density corresponding to the time when the positive electrode sheet is folded in half to transmit light after secondary rolling or the compaction density corresponding to the time when the lithium manganese iron phosphate particles are crushed after secondary rolling of the positive electrode sheet.

[0050] In this disclosure, in formula (II), σ×△PD can also be 5~90MPa, or 15~80MPa, or 35~60MPa.

[0051] In this disclosure, the range of △PD is preferably 4% to 25%, more preferably 5% to 23%, more preferably 8% to 20%, and even more preferably 10% to 18%.

[0052] This disclosure provides a method for preparing a positive electrode sheet, comprising the following steps:

[0053] 1) After mixing the above-mentioned lithium manganese iron phosphate agglomerate secondary particles, binder, conductive agent and solvent, a positive electrode slurry is obtained;

[0054] 2) Coat the positive electrode slurry obtained in the above steps onto the current collector, and then roll it to obtain the positive electrode sheet.

[0055] This disclosure first involves mixing the aforementioned lithium manganese iron phosphate agglomerates, secondary particles, binder, conductive agent, and solvent to obtain a positive electrode slurry.

[0056] In this disclosure, the method for preparing the secondary particles of lithium manganese iron phosphate agglomerates preferably includes the following steps:

[0057] (1) After ball milling and mixing manganese source, iron source, carbon source, lithium source and phosphorus source, the precursor powder is obtained after one grinding, one spray drying and one sintering.

[0058] (2) After the precursor powder and carbon source obtained in the above steps are ball-milled and mixed again, they are then subjected to secondary grinding, secondary spraying and secondary sintering to obtain secondary particles of lithium manganese iron phosphate agglomerates.

[0059] In this disclosure, the sintering process of the secondary particles of lithium manganese iron phosphate agglomerates is specifically carried out under a protective atmosphere.

[0060] In this disclosure, the pressure of the primary spray is preferably 0.1 to 1.35 MPa, more preferably 0.5 to 1.3 MPa, and even more preferably 1 to 1.2 MPa.

[0061] In this disclosure, the carbon content of the single spray coating is preferably 0.1%wt to 0.8%wt, more preferably 0.2%wt to 0.7%wt, and even more preferably 0.3%wt to 0.6%wt.

[0062] In this disclosure, the sintering time for one sintering is preferably 1 to 10 hours, more preferably 3 to 8 hours, and even more preferably 5 to 6 hours.

[0063] In this disclosure, the temperature of the first sintering is preferably 200-800°C, more preferably 300-700°C, and even more preferably 400-600°C.

[0064] In this disclosure, the carbon content of the secondary spray coating is preferably 1.0%wt to 3.0%wt, more preferably 1.2%wt to 2.8%wt, and even more preferably 1.5%wt to 2.5%wt.

[0065] In this disclosure, the pressure of the secondary spray is preferably 0.1 to 1.35 MPa, more preferably 0.5 to 1.3 MPa, and even more preferably 1 to 1.2 MPa.

[0066] In this disclosure, the secondary sintering time is 1 to 10 hours, more preferably 3 to 8 hours, and even more preferably 5 to 6 hours.

[0067] In this disclosure, the temperature of the secondary sintering is 400–800°C, more preferably 450–750°C, and even more preferably 500–700°C.

[0068] In this disclosure, the secondary spraying is preferably carried out with the addition of a flux and a dispersant;

[0069] In this disclosure, the amount (mass content) of the fluxing agent added is preferably 0.1% to 1.5%, more preferably 0.3% to 1.2%, and even more preferably 0.5% to 1.0%.

[0070] In this disclosure, the amount (mass content) of the dispersant added is preferably 0.1% to 1.3%, more preferably 0.3% to 1.1%, and even more preferably 0.5% to 0.8%.

[0071] In this disclosure, the method for preparing the secondary particles of lithium manganese iron phosphate agglomerates preferably includes the following steps:

[0072] (1) Manganese source, iron source, carbon source, lithium source, phosphorus source and water are ground and mixed once, and then spray dried and sintered to obtain precursor powder;

[0073] (2) Under a protective atmosphere, the precursor powder obtained in the above steps is ball-milled and mixed with carbon source and water again. Under a protective atmosphere, it is then sprayed and sintered twice to obtain secondary particles of lithium manganese iron phosphate agglomerates.

[0074] In this disclosure, the sintering process of the secondary particles of lithium manganese iron phosphate agglomerates is specifically carried out under a protective atmosphere.

[0075] In this disclosure, the pressure of the primary spray is preferably 0.3 to 1.25 MPa, more preferably 0.5 to 1.0 MPa, and even more preferably 0.7 to 0.8 MPa.

[0076] In this disclosure, the carbon content of the single spray coating is preferably 0.2%wt to 0.7%wt, more preferably 0.3%wt to 0.6%wt, and even more preferably 0.4%wt to 0.5%wt.

[0077] In this disclosure, the sintering time for the first sintering is preferably 2 to 8 hours, more preferably 3 to 7 hours, and even more preferably 4 to 6 hours.

[0078] In this disclosure, the temperature of the first sintering is preferably 300-600°C, more preferably 350-550°C, and even more preferably 400-500°C.

[0079] In this disclosure, the carbon content of the secondary spray coating is preferably 1.2%wt to 3.0%wt, more preferably 1.5%wt to 2.8%wt, and even more preferably 1.8%wt to 2.5%wt.

[0080] In this disclosure, the pressure of the secondary spray is preferably 0.3 to 1.25 MPa, more preferably 0.5 to 1.05 MPa, and even more preferably 0.7 to 0.9 MPa.

[0081] In this disclosure, the secondary sintering time is 2 to 11 hours, more preferably 4 to 9 hours, and even more preferably 6 to 7 hours.

[0082] In this disclosure, the temperature of the secondary sintering is 500–700°C, more preferably 550–650°C.

[0083] In this disclosure, the secondary spraying is preferably carried out with the addition of a flux and a dispersant;

[0084] In this disclosure, the amount (mass content) of the fluxing agent added is preferably 0.05% to 1.4%, more preferably 0.3% to 1.2%, and even more preferably 0.5% to 1.0%.

[0085] In this disclosure, the amount (mass content) of the dispersant added is preferably 0.01% to 1.0%, more preferably 0.05% to 0.8%, and even more preferably 0.1% to 0.5%.

[0086] Finally, the positive electrode slurry obtained in the above steps is coated onto the current collector, and then rolled to obtain the positive electrode sheet.

[0087] This disclosure provides a lithium-ion battery, including a positive electrode;

[0088] The positive electrode sheet is the positive electrode sheet described in any one of the above technical solutions or the positive electrode sheet prepared by the preparation method described in any one of the above technical solutions.

[0089] This disclosure aims to complete and refine the overall technical solution, and better ensure that the positive electrode sheet satisfies the relationship described in equation (I). Specifically, the aforementioned lithium manganese iron phosphate positive electrode sheet and its preparation method, as well as the lithium-ion battery, may include the following:

[0090] A battery includes a positive electrode sheet comprising a positive electrode material, wherein the positive electrode active material comprises lithium manganese iron phosphate particles, and the particle size D90 of the lithium manganese iron phosphate particles and the tensile strength (denoted as σ) and compaction density (denoted as PD) of the positive electrode sheet satisfy the following relationship (I): 1600 μm·MPa / (g / cm³) 3 ) <D90 / PD×σ<4800μm·MPa / (g / cm 3 (I)

[0091] In this disclosure, under a certain compaction density, the larger the D90, the more large the particles are present. Large particles can cause damage to foil materials during the rolling process. The tensile strength of the electrode sheet made under a certain compaction condition is low. Therefore, it is necessary to improve the tensile strength of the electrode sheet by taking into account other conditions to avoid the electrode sheet from breaking during subsequent processes such as winding.

[0092] In this disclosure, if the value exceeds the lower limit of formula (I), D90 is too small, resulting in an excessively large specific surface area, poor slurry processability, and excessive interfacial side reactions, which in turn leads to accelerated capacity decay and DCR growth during long-term cycling or storage, or the tensile strength of the electrode is too low to meet processing requirements, or the compaction of the electrode is too large, leading to cracking of the agglomerate material, poor conductivity between primary particles, and affecting the kinetic performance of the battery; if the value exceeds the upper limit of formula (I), D90 is too large, which makes it easy to cause scratches on the electrode during coating, and requires higher strength of the foil during rolling, or the compaction of the electrode is too low, resulting in lower battery energy density and poor ohmic contact between agglomerate particles, affecting the kinetic performance of the battery; or the tensile strength of the electrode is too large, requiring higher strength of the foil.

[0093] Specifically, the lithium manganese iron phosphate is an agglomerate comprising multiple primary particles.

[0094] Specifically, the D90 range of lithium manganese iron phosphate particles is 10–50 μm.

[0095] Specifically, the PD range is 2.0 to 2.5. The larger the PD, the higher the energy density of the battery. However, if the PD is too large, the agglomerate particles will be crushed, and the conductivity between the primary particles will be poor, which is not conducive to kinetic performance. At the same time, the current collector will be over-pressed, and the rolled electrode sheet will be prone to breakage, which is not conducive to processing. If the PD is too small, the energy density of the battery will be low. At the same time, the contact resistance between the agglomerate particles will be large, and the electronic conductivity will be low, which is also not conducive to kinetic performance.

[0096] Specifically, the tensile strength σ of the positive electrode sheet ranges from 100 to 500 MPa.

[0097] Specifically, the compaction change rate ΔPD (the degree of change in compaction after the positive electrode sheet is rolled again after disassembly, relative to the compaction before rolling) and the tensile strength σ of the positive electrode sheet satisfy the following relationship (II):

[0098] 0 MPa < σ × △PD < 95 MPa. Equation (II) reflects the matching of particle strength / agglomeration degree.

[0099] In this disclosure, the tensile strength σ is the tensile strength of the electrode under initial compaction conditions, in MPa, and ΔPD is the maximum compaction change rate of the electrode, ΔPD = (PD... max -PD) / PD, PD max This refers to the maximum compaction value corresponding to the breakage of light-transmitting or agglomerated particles after the electrode is rolled again. Particle breakage is defined as a decrease in the D90 change rate of the original positive electrode particles exceeding 20%.

[0100] The larger the ΔPD, the higher the particle strength of the electrode, and the higher the strength and toughness of the electrode / aluminum foil.

[0101] Specifically, the compaction change rate ΔPD after the positive electrode is repressurized ranges from 4% to 25%.

[0102] In this disclosure, PD, PD max The detection method for σ specifically includes the following steps:

[0103] PD:

[0104] 1. Measure the actual surface density 'a' of the electrode sheet before rolling.

[0105] 2. Based on the material powder compaction b, calculate the corresponding electrode thickness c = a / (b+0.1) + foil thickness under the corresponding compaction gradient according to the design compaction. After adjusting the appropriate pressure of the roller press machine according to the calculated electrode thickness under different compaction conditions, the electrode is rolled by the roller press.

[0106] With the same folding force, after folding the electrode sheet in both directions once, if no light passes through the fold seam when the fold is compared to a bright light source, then increase the number of folds. After two folds, if light passes through one point at the fold seam, this indicates that the electrode sheet is actually compacted. Actual compaction d = Electrode sheet compaction density after roller pressing / (Electrode sheet thickness after roller pressing - Foil thickness)

[0107] △PD=(PD max -PD) / PD, PD max This represents the maximum compaction density.

[0108] In this disclosure, the maximum compaction density PD max This refers to the compacted density of the positive electrode sheet after secondary rolling and folding to allow light to pass through, or the compacted density of the lithium manganese iron phosphate particles after secondary rolling and crushing. "Folded to allow light to pass through" refers to the light transmittance at the fold after the electrode sheet is folded in both directions. The actual areal density can be measured as follows: After the electrode sheet is coated and dried, take a 40mm small circular piece with corresponding mass m1, area s1, and foil mass m0. Then, the actual areal density = (m1 - m0) / s1.

[0109] The thickness of the electrode after roller compaction at the set limit is the thickness of the electrode when the compaction is 2.1-2.6. An exemplary test method is as follows: After roller compaction of the electrode, it is manually folded in both directions. With the fold facing the light source, if light passes through the fold, this compaction represents the maximum compaction density. Maximum compaction density = Actual surface density of the electrode / (Thickness of the electrode after roller compaction at the set limit - Thickness of the foil).

[0110] Specific test conditions: After the positive electrode sheet undergoes a normal single rolling process, the compacted PD of the positive electrode sheet under the single rolling process is obtained. Then, the positive electrode sheet is subjected to a second rolling process to obtain the maximum compacted PD of the positive electrode sheet under the second rolling process. max The specific constraint is that the positive electrode sheet must not break under the secondary rolling pressure.

[0111] The specific test conditions are shown in Table 1.

[0112] σ:

[0113] 1. Cut a 50cm positive electrode sheet using a utility knife.

[0114] 2. Place the positive electrode sheet on the thin film sample preparation machine and cut it into several strips with a length of 100mm and a width of 150mm along the MD and TD directions.

[0115] 3. Clamp the positive electrode strip onto the pneumatic fixture of the universal testing machine. At this time, the gauge distance between the fixtures is 100mm.

[0116] 4. The universal testing machine performs tensile testing at a tensile speed of 100 mm / min.

[0117] After the 5-meter universal testing machine stops, open the pneumatic clamp, clean the diaphragm debris, and readjust the gauge length to the initial length.

[0118] 6. Record the tensile strength and elongation at break results displayed on the instrument program.

[0119] 7. Repeat steps 1 to 6, and take the arithmetic mean of the results, keeping one decimal place.

[0120] See Figure 1, which is a 1000X SEM image of the lithium manganese iron phosphate electrode sheet after rolling provided in this disclosure.

[0121] See Figure 2, which is a 3000X SEM image of the lithium manganese iron phosphate electrode sheet after rolling provided in this disclosure.

[0122] This disclosure also provides a method for preparing a lithium-ion battery made of lithium iron phosphate manganese phosphate, comprising the following steps:

[0123] 1) Preparation of cathode material 1: Manganese source, iron source, carbon source, lithium source and phosphorus source are mixed, ball milled evenly (the manganese-iron ratio of the agglomerates is adjusted according to the molar ratio of manganese source and iron source), and spray dried to obtain precursor powder;

[0124] Precursor powder, carbon source, dopant source (additives can also be added) are mixed and ball-milled twice. After the material is sprayed once, it is sintered at high temperature for a period of time, and then sprayed and sintered twice (the particle size is controlled by the sintering temperature) to obtain lithium manganese iron phosphate agglomerate particles.

[0125] Preparation of cathode material 2: Manganese source, iron source, carbon source, lithium source and phosphorus source are added to deionized water and mixed, dried and pulverized to obtain a precursor. The precursor is sprayed once and sintered under a nitrogen atmosphere to obtain a sintered sample. The sintered sample, carbon source and deionized water are mixed, ball-milled, dried and pulverized, sprayed a second time and sintered a second time under a nitrogen atmosphere. After pulverization, lithium manganese iron phosphate agglomerate particles are obtained.

[0126] 2) Preparation of positive electrode sheet: The above-mentioned lithium manganese iron phosphate agglomerates are used as the main material and mixed evenly with binder PVDF, conductive agent SP and conductive agent according to the mass ratio (94~97.5:1.5~4.0:0.8~1.2:0.2~0.8). The mixture is dispersed in a solvent to obtain a positive electrode slurry. The positive electrode slurry is coated on aluminum foil to obtain a double-sided coated positive electrode sheet. Then it is rolled and cut to obtain a positive electrode sheet.

[0127] 3) Negative electrode preparation: Natural graphite, artificial graphite, mesophase carbon microspheres, hard carbon, soft carbon, silicon, and SiO2 are used. x Silicon-carbon, Li4Ti5O 12 One or more of the ingredients are mixed evenly with conductive agent SP, binder CMC and binder in a certain mass ratio, and dispersed in deionized water to obtain a negative electrode slurry; the negative electrode slurry is coated on copper foil to obtain a single-sided coated electrode sheet; then it is rolled and cut to obtain a negative electrode sheet.

[0128] 4) Preparation of electrolyte

[0129] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 to obtain an organic solvent. Then, thoroughly dried lithium salt LiPF6 was dissolved in the mixed organic solvent to prepare an electrolyte with a concentration of 1 mol / L. (The specific electrolyte solvent may be adjusted according to actual conditions.)

[0130] 5) Preparation of the separating membrane

[0131] The separator is selected from PP, PE, or PP / PF. (The choice of separator can also be adjusted according to actual needs.)

[0132] 6) Assembly and formation

[0133] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator acting as a separator between the positive and negative electrodes. The cells are then wound to obtain a bare cell. The bare cell is placed in an outer packaging shell, dried, and then injected with electrolyte. After vacuum sealing, settling, formation, and shaping, a lithium-ion battery is obtained.

[0134] This disclosure provides a lithium manganese iron phosphate (LMP) cathode sheet, its preparation method, and a lithium-ion battery. The LMP cathode sheet with a specific structure is specially designed, wherein the D90 particle size of the LMP particles, the tensile strength σ of the cathode sheet, and the compaction density PD of the cathode sheet satisfy specific relationships. By controlling the LMP particles, the tensile strength of the cathode sheet, and the compaction density of the cathode sheet, this disclosure further improves the performance of the LMP cathode sheet.

[0135] This disclosure relates the tensile strength of lithium manganese iron phosphate (LFP) agglomerate electrodes to the particle size distribution and the electrode state corresponding to high compaction. The tensile strength is further related to the properties of the current collector (foil), the properties of the active material layer, and the processing technology / equipment. Under the condition that D90 and σ, PD satisfy the relationship (I), at a certain compaction density, using LFP agglomerates with relatively large particle sizes, combined with suitable foil and processing technology, results in electrodes with lower tensile strength; conversely, using LFP agglomerates with relatively small particle sizes, combined with suitable foil and processing technology, results in electrodes with higher tensile strength. With a fixed particle size, to obtain a higher compaction density, using suitable foil and processing technology results in electrodes with higher tensile strength; conversely, at a lower compaction density, using suitable foil and processing technology results in electrodes with lower tensile strength. However, when the lower limit of the relationship is exceeded, the tensile strength of the electrode is too low to meet processing requirements. Furthermore, compared to D50, which reflects the overall particle size of lithium manganese iron phosphate particles, D90 better reflects the size characteristics of large particles in lithium manganese iron phosphate materials. Exceeding the upper limit of D90 results in excessively large particle sizes, which can easily cause scratches on the electrode sheet during coating and places higher demands on the foil strength during rolling. Conversely, exceeding the lower limit of D90 results in excessively small particle sizes, requiring more solvent and binder during slurry preparation, leading to poorer slurry processing performance and hindering thick electrode coating.

[0136] Furthermore, the tensile strength of the electrode is related to the properties of the current collector (foil) itself, the properties of the active material layer, and the processing technology, especially the foil, the amount of active material coating (areal density), and the adhesion between the active material layer and the foil. A low tensile strength σ indicates a deterioration in the processability of the electrode after rolling, which may lead to negative effects such as electrode breakage during rolling. In addition, there may be some microcracks inside the current collector, affecting electron transport and thus impacting cell performance.

[0137] Furthermore, this disclosure also provides formula (II), where the compaction change rate ΔPD reflects the particle strength and agglomeration of lithium manganese iron phosphate particles. Simultaneously controlling ΔPD and tensile strength σ helps achieve the desired battery energy density while maintaining good processability. If the upper limit of the formula is exceeded, ΔPD is too large, indicating that the initial compaction is too low, which is detrimental to achieving high battery energy density, or that the electrode tensile strength is too high, placing excessive demands on the aluminum foil. Moreover, a certain ΔPD indicates that the lithium manganese iron phosphate particles have suitable agglomeration; too high a ΔPD indicates low particle strength, and under greater pressure, the agglomerates are easily crushed; too low a ΔPD indicates that the lithium manganese iron phosphate particles are close to their ultimate compaction limit, resulting in insufficient electrode flexibility, or that the particles are close to breakage.

[0138] To further illustrate this disclosure, the following describes in detail, in conjunction with embodiments, a positive electrode sheet and its preparation method, and a lithium-ion battery provided by this disclosure. However, it should be understood that these embodiments are implemented based on the technical solution of this disclosure, and provide detailed implementation methods and specific operating procedures. They are only for further illustrating the features and advantages of this disclosure, and are not intended to limit the scope of protection of the claims of this disclosure. The scope of protection of this disclosure is not limited to the following embodiments.

[0139] Example 1

[0140] 1) Preparation of cathode material: Manganese acetate, ferric acetate, sucrose / polyvinyl alcohol, lithium carbonate, and ammonium dihydrogen phosphate are mixed in a ratio of n(manganese acetate + ferric acetate), n(lithium carbonate), and n(ammonium dihydrogen phosphate) = 1:1:1, where the ratio of n(manganese acetate):n(ferric acetate) satisfies 3:1. The mixture is ground uniformly once, spray-dried once, and sintered once to obtain precursor powder. The precursor powder, sucrose / polyvinyl alcohol, magnesium chloride, and calcium chloride are mixed and ball-milled a second time. The material is then spray-dried a second time and sintered at high temperature for a period of time. Finally, a second spray-drying and second sintering process is performed to obtain lithium manganese iron phosphate agglomerate particles. The spray equipment pressure before the first and second sintering is 0.4 MPa. The amount of flux (potassium carbonate) added during the second spray drying before the second sintering is 0.1%, and the amount of dispersant (polyester polyether) added is 0.05%.

[0141] 2) Preparation of positive electrode sheet: The above-mentioned lithium manganese iron phosphate agglomerates are used as the main material and mixed evenly with binder PVDF, conductive agent SP and conductive agent according to the mass ratio (94:3:1:2). The mixture is dispersed in a solvent to obtain positive electrode slurry. The positive electrode slurry is coated on aluminum foil to obtain double-sided coated positive electrode sheet. Then it is rolled and cut to obtain positive electrode sheet.

[0142] 3) Preparation of negative electrode sheet: Artificial graphite is mixed evenly with conductive agent SP 4% and binder 4% at a mass ratio of 92%, and dispersed in deionized water to obtain negative electrode slurry; the negative electrode slurry is coated on copper foil to obtain single-sided coated electrode sheet; then it is rolled and cut to obtain negative electrode sheet.

[0143] 4) Preparation of electrolyte

[0144] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 to obtain an organic solvent. Then, fully dried lithium salt LiPF6 was dissolved in the mixed organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0145] 5) The separator is made of PP.

[0146] 6) Assembly and formation

[0147] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator acting as a separator between the positive and negative electrodes. The cells are then wound to obtain a bare cell. The bare cell is placed in an outer packaging shell, dried, and then injected with electrolyte. After vacuum sealing, settling, formation, and shaping, a lithium-ion battery is obtained.

[0148] Example 2

[0149] 1) Preparation of cathode material: Manganese acetate, ferric acetate, sucrose / polyvinyl alcohol, lithium carbonate, and ammonium dihydrogen phosphate were mixed in a ratio of n(manganese acetate + ferric acetate), n(lithium carbonate), and n(ammonium dihydrogen phosphate) = 1:1:1, where the ratio of n(manganese acetate):n(ferric acetate) satisfies 3:1. The mixture was ball-milled until homogeneous and then spray-dried to obtain precursor powder. The precursor powder, sucrose / polyvinyl alcohol, magnesium chloride, and calcium chloride were mixed and ball-milled a second time. After a first spraying, the material was sintered at high temperature for a period of time, followed by a second spraying and second sintering to obtain lithium manganese iron phosphate agglomerates. The spraying equipment pressure before the first and second sintering was 0.9 MPa. The amount of flux (potassium carbonate) added during the second spray drying before the second sintering was 1.32%, and the amount of dispersant (polyester polyether) added was 0.4%.

[0150] 2) The preparation of the positive electrode is the same as in Example 1.

[0151] 3) The negative electrode preparation is the same as in Example 1.

[0152] 4) The preparation of the electrolyte is the same as in Example 1.

[0153] 5) The separator is made of PE.

[0154] 6) Assemble and process as in Example 1.

[0155] Example 3

[0156] 1) Preparation of cathode material: Manganese acetate, ferric acetate, sucrose / polyvinyl alcohol, lithium carbonate, and ammonium dihydrogen phosphate were mixed in a ratio of n(manganese acetate + ferric acetate), n(lithium carbonate), and n(ammonium dihydrogen phosphate) = 1:1:1, where the ratio of n(manganese acetate):n(ferric acetate) satisfies 3:1. The mixture was ball-milled until homogeneous and then spray-dried to obtain precursor powder. The precursor powder, sucrose / polyvinyl alcohol, magnesium chloride, and calcium chloride were mixed and ball-milled a second time. After a first spraying, the material was sintered at high temperature for a period of time, followed by a second spraying and a second sintering to obtain lithium manganese iron phosphate agglomerates. The spraying equipment pressure before the first and second sintering was 0.65 MPa. The amount of flux (potassium carbonate) added during the second spray drying before the second sintering was 0.3%, and the amount of dispersant (polyester polyether) added was 0%.

[0157] 2) The preparation of the positive electrode is the same as in Example 1.

[0158] 3) The negative electrode preparation is the same as in Example 1.

[0159] 4) The preparation of the electrolyte is the same as in Example 1.

[0160] 5) The separator is selected from PP / PF.

[0161] 6) Assemble and process as in Example 1.

[0162] Example 4

[0163] 1) Preparation of cathode material: Manganese acetate, ferric acetate, sucrose / polyvinyl alcohol, lithium carbonate, and ammonium dihydrogen phosphate were mixed with deionized water at a ratio of n(manganese acetate + ferric acetate), n(lithium carbonate), n(ammonium dihydrogen phosphate) = 1:1:1, where the ratio of n(manganese acetate):n(ferric acetate) satisfies 3:1. The mixture was dried and pulverized to obtain a precursor. After a single spraying, the precursor was sintered under a nitrogen atmosphere to obtain a first-burned sample. The first-burned sample, carbon source, and deionized water were mixed, ball-milled, dried, pulverized, and then subjected to a second spraying and a second sintering under a nitrogen atmosphere. After pulverization, lithium manganese iron phosphate agglomerates were obtained. The spraying equipment pressure before the first and second burns was 1.05 MPa. The amount of flux (potassium carbonate) added during the second spray drying before the second burn was 1.28%, and the amount of dispersant (polyester polyether) added was 0.3%. The amounts of flux and dispersant added were based on the mass of the lithium manganese iron phosphate powder after the first burn.

[0164] 2) The preparation of the positive electrode is the same as in Example 1.

[0165] 3) The negative electrode preparation is the same as in Example 1.

[0166] 4) The preparation of the electrolyte is the same as in Example 1.

[0167] 5) The separator is made of PP.

[0168] 6) Assemble and process as in Example 1.

[0169] Example 5

[0170] Cathode material preparation: Manganese acetate, ferric acetate, sucrose / polyvinyl alcohol, lithium carbonate, and ammonium dihydrogen phosphate were mixed with deionized water in a ratio of n(manganese acetate + ferric acetate), n(lithium carbonate), n(ammonium dihydrogen phosphate) = 1:1:1, where the ratio of n(manganese acetate):n(ferric acetate) was 3:1. The mixture was dried and pulverized to obtain a precursor. The precursor was then sprayed once and sintered under a nitrogen atmosphere to obtain a first-burned sample. The first-burned sample, carbon source, and deionized water were mixed, ball-milled, dried, pulverized, sprayed a second time, and then sintered a second time under a nitrogen atmosphere. After pulverization, lithium manganese iron phosphate agglomerates were obtained. The spraying equipment pressure before the first and second burns was 0.82 MPa. The amount of flux (potassium carbonate) added during the second spray drying before the second burn was 1.4%, and the amount of dispersant (polyester polyether) added was 0.8%.

[0171] 2) The preparation of the positive electrode is the same as in Example 1.

[0172] 3) The negative electrode preparation is the same as in Example 1.

[0173] 4) The preparation of the electrolyte is the same as in Example 1.

[0174] 5) The separator is made of PE.

[0175] 6) Assemble and process as in Example 1.

[0176] Example 6

[0177] 1) Manganese acetate, ferric acetate, sucrose / polyvinyl alcohol, lithium carbonate, and ammonium dihydrogen phosphate were mixed with deionized water at a ratio of n(manganese acetate + ferric acetate), n(lithium carbonate), n(ammonium dihydrogen phosphate) = 1:1:1, where the ratio of n(manganese acetate):n(ferric acetate) satisfies 3:1. The mixture was dried and pulverized to obtain a precursor. The precursor was sprayed once and then sintered under a nitrogen atmosphere to obtain a first-burned sample. The first-burned sample, carbon source, and deionized water were mixed, ball-milled, dried, pulverized, sprayed a second time, and then sintered a second time under a nitrogen atmosphere. After pulverization, lithium manganese iron phosphate agglomerate particles were obtained. The spraying equipment pressure before the first and second burns was 0.6 MPa. The amount of flux (potassium carbonate) added during the second spray drying before the second burn was 0.4%, and the amount of dispersant (polyester polyether) added was 0.25%.

[0178] 2) The preparation of the positive electrode is the same as in Example 1.

[0179] 3) The negative electrode preparation is the same as in Example 1.

[0180] 4) The preparation of the electrolyte is the same as in Example 1.

[0181] 5) The separator is selected from PP / PF.

[0182] 6) Assemble and process as in Example 1.

[0183] Comparative Example 1

[0184] 1) Preparation of cathode material: Manganese acetate, ferric acetate, sucrose / polyvinyl alcohol, lithium carbonate, and ammonium dihydrogen phosphate were mixed with deionized water in a ratio of n(manganese acetate + ferric acetate), n(lithium carbonate), n(ammonium dihydrogen phosphate) = 1:1:1, where the ratio of n(manganese acetate):n(ferric acetate) was 3:1. The mixture was dried and pulverized to obtain a precursor. After a first spraying, the precursor was sintered under a nitrogen atmosphere to obtain a first-burned sample. The first-burned sample, carbon source, and deionized water were mixed, ball-milled, dried, pulverized, and then subjected to a second spraying and a second sintering under a nitrogen atmosphere. After pulverization, lithium manganese iron phosphate agglomerates were obtained. The spraying equipment pressure before the first and second burns was 1.12 MPa. The amount of flux (potassium carbonate) added during the second spray drying before the second burn was 0.25%, and the amount of dispersant (polyester polyether) added was 0.15%. The first and second firing temperatures were 570 and 720℃, respectively, and the sintering times for the first and second firings were 9 and 10 hours, respectively.

[0185] 2) The preparation of the positive electrode is the same as in Example 1.

[0186] 3) The negative electrode preparation is the same as in Example 1.

[0187] 4) The preparation of the electrolyte is the same as in Example 1.

[0188] 5) The separator is made of PE.

[0189] 6) Assemble and process as in Example 1.

[0190] Comparative Example 2

[0191] 1) Preparation of cathode material: Manganese acetate, ferric acetate, sucrose / polyvinyl alcohol, lithium carbonate, and ammonium dihydrogen phosphate were mixed with deionized water in a ratio of n(manganese acetate + ferric acetate), n(lithium carbonate), n(ammonium dihydrogen phosphate) = 1:1:1, where the ratio of n(manganese acetate):n(ferric acetate) satisfies 3:1. The mixture was dried and pulverized to obtain a precursor. After a single spraying, the precursor was sintered under a nitrogen atmosphere to obtain a first-burned sample. The first-burned sample, carbon source, and deionized water were mixed, ball-milled, dried, pulverized, and then subjected to a second spraying and a second sintering under a nitrogen atmosphere. After pulverization, lithium manganese iron phosphate agglomerates were obtained. The spraying equipment pressure before the first and second burns was 0.35 MPa. The amount of flux (potassium carbonate) added during the second spray drying before the second burn was 0.13%, and the amount of dispersant (polyester polyether) added was 0.2%. The first and second firing temperatures were 450 and 665℃, respectively, and the sintering times for the first and second firings were 10 and 9 hours, respectively.

[0192] 2) The preparation of the positive electrode is the same as in Example 1.

[0193] 3) The negative electrode preparation is the same as in Example 1.

[0194] 4) The preparation of the electrolyte is the same as in Example 1.

[0195] 5) The separator is made of PE.

[0196] 6) Assemble and process as in Example 1.

[0197] Other testing methods:

[0198] D90:

[0199] Preparation of positive electrode powder: The disassembled positive electrode (10cm×10cm) is washed three times with dimethyl carbonate (DMC) and dried at room temperature. It is then fired at 400℃ for 4 hours. After gently tapping the electrode, the positive electrode powder can be collected and obtained.

[0200] Place the positive electrode powder into a self-sealing bag, turn the sample upside down 2 to 3 times, and then weigh out about 0.04 to 0.06 g of the sample and put it into a 50 ml beaker.

[0201] Add 10 ml of 1% NP-40 sample dispersant to a beaker, stirring with a glass rod during the addition process. Then, ultrasonically disperse the sample at 40 kHz for 3 minutes. Quickly pour the dispersed sample into the sample cell of the particle size analyzer. Rinse the beaker with a wash bottle and pour all the rinsing solution into the sample cell. Set the sample refractive index and absorptivity in the instrument, and the laser intensity ≥75%. Start the test. After the test is completed, the computer connected to the particle size analysis software will automatically output the particle size D90 data.

[0202] 2C constant current ratio:

[0203] Assemble a positive electrode half-cell using fresh positive electrode sheets. The assembled positive electrode half-cell is then filled with electrolyte, sealed, and subjected to room temperature capacitance testing. The capacitated half-cell is then placed in a room temperature chamber to complete a double-charge test. The double-charge test (charge rate is 0.33C / 1 / 2C, constant voltage charging at 4.3V, cutoff current 0.05C, discharge rate is 0.33C) is performed. The 2C charging procedure is as follows: charge at a constant 2C rate to 4.3V, then charge at a constant voltage of 4.3V, cutoff current 0.05C, and then discharge at 0.33C. The 2C constant current ratio = 2C constant rate charging capacity / total 2C charging capacity. The capacity in the current is taken as the design capacity.

[0204] Energy density: Battery energy density = cell capacity × discharge plateau ÷ cell volume

[0205] See Table 1, which lists the Product Design Process (PD) and Product Design Process (PD) of this disclosure. max Specific test conditions

[0206] Table 1

[0207] See Table 2, which contains some experimental parameters from the embodiments and comparative examples of this disclosure.

[0208] Table 2

[0209] See Table 3, which shows the detection effect data in the embodiments and comparative examples of this disclosure.

[0210] Table 3

[0211] The foregoing has provided a detailed description of a lithium manganese iron phosphate cathode sheet, its preparation method, and a lithium-ion battery. Specific examples have been used to illustrate the principles and implementation methods of this disclosure. The descriptions of these embodiments are merely for the purpose of helping to understand the methods and core ideas of this disclosure, including the best mode, and also to enable any person skilled in the art to practice this disclosure, including manufacturing and using any device or system, and implementing any combined method. It should be noted that those skilled in the art can make several improvements and modifications to this disclosure without departing from its principles, and these improvements and modifications also fall within the scope of protection of the claims of this disclosure. The scope of patent protection of this disclosure is defined by the claims and may include other embodiments that can be conceived by those skilled in the art. If these other embodiments have structural elements that are not different from the wording of the claims, or if they include equivalent structural elements that are not substantially different from the wording of the claims, then these other embodiments should also be included within the scope of the claims.

Claims

1. A positive electrode sheet, comprising a current collector and a positive electrode material composited on the current collector; The cathode material includes a cathode active material; The positive electrode active material comprises lithium iron manganese phosphate particles; The D90 particle size of the lithium manganese iron phosphate particles, the tensile strength σ of the positive electrode, and the compaction density PD of the positive electrode satisfy the relationship described in equation (I): 1600 μm·MPa / (g / cm 3 )<D90 / PD x σ < 4800 μm·MPa / (g / cm 3 ) (I); wherein D90 is in μm, σ is in MPa, PD is in g / cm 3 .

2. The positive electrode according to claim 1, wherein in formula (I), 2000 <D90 / PD×σ<4000。 3. The positive electrode sheet according to claim 1 or 2, wherein the lithium manganese iron phosphate particles specifically include secondary lithium manganese iron phosphate particles.

4. The positive electrode sheet according to any one of claims 1 to 3, wherein the D90 particle size of the lithium manganese iron phosphate particles is 10 to 50 μm.

5. The positive electrode sheet according to claim 4, wherein the D90 particle size of the lithium manganese iron phosphate particles is 15-40 μm.

6. The positive electrode sheet according to any one of claims 1 to 5, wherein the compact density PD is 2.0 to 2.5 g / cm3. 3 .

7. The positive electrode sheet according to any one of claims 1 to 6, wherein the tensile strength σ is 100 to 500 MPa.

8. The positive electrode sheet according to any one of claims 1 to 7, wherein the compaction change rate ΔPD of the positive electrode sheet and the tensile strength σ of the positive electrode sheet satisfy the relationship described in equation (II): 0 MPa < σ × ΔPD < 95 MPa (II); wherein σ is in MPa, ΔPD = (PD max -PD) / PD, PD max is the maximum compaction density; The maximum compaction density is the compaction density corresponding to the positive electrode sheet when it is folded in half to transmit light after secondary rolling or the compaction density corresponding to the lithium manganese iron phosphate particles being crushed after secondary rolling of the positive electrode sheet.

9. The positive electrode according to claim 8, wherein the range of ΔPD is 4% to 25%.

10. A lithium-ion battery, comprising a positive electrode; The positive electrode is the positive electrode according to any one of claims 1 to 9.

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