Positive electrode material, method for preparing the same, positive electrode sheet, method for preparing the same, lithium ion battery, and lithium ion battery pack

Abstract

To provide a positive electrode material, a preparation method for the same, and a positive electrode sheet.SOLUTION: A positive electrode material contains manganese iron lithium phosphate, and carbon that covers a surface of a particle of manganese iron lithium phosphate. Carbon constitutes 1.4% to 4.7% of the total mass of the positive electrode material. A preparation method for the positive electrode material includes the following steps of mixing and drying a carbon source, a manganese source, a lithium source, an iron source, a doping element, and a phosphorus source, then performing preliminary calcination and preliminary fusion at a first stage temperature, and subsequently increasing the temperature to a second stage temperature in order to continue the calcination so as to obtain the positive electrode material. A positive electrode sheet is further provided and a raw material includes the positive electrode material described above in order to prepare the positive electrode sheet.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and particularly to a cathode material, a method for preparing the same, and a cathode sheet.

Background Art

[0002] Since the development of lithium-ion batteries, they have been favored in various technical fields due to advantages such as high energy density, stable discharge voltage, and long service life. In addition, lithium-ion batteries are generally adopted as portable energy storage tools, especially in the new energy vehicle industry that is currently popular around the world. As an important part of lithium-ion batteries, the selection of cathode materials directly affects the performance of lithium-ion batteries. Cathode materials mainly include lithium cobaltate, lithium manganate, lithium iron phosphate, etc. Lithium iron phosphate has the advantages of good structural stability, good thermal stability, good safety performance, good cycle life, and rich raw material sources, and thus is more commonly adopted in the fields of energy storage batteries and electric vehicle batteries. However, the voltage plateau of lithium iron phosphate is only 3.4V vs. Li / Li+, which limits the energy density of lithium iron phosphate. Lithium manganese iron phosphate has the same olivine structure as lithium iron phosphate and has a higher voltage plateau than lithium iron phosphate.

[0003] However, lithium manganese iron phosphate has poor conductivity and high charge-discharge polarization. Currently, the conductivity of lithium manganese iron phosphate is mainly modified by coating with carbon materials to improve the rate performance of lithium manganese iron phosphate and improve polarization. However, there are several major technical problems in carbon coating. First, the carbon content of the coating is high, which reduces the compression of the material due to the high specific surface area of carbon. Second, the coating efficiency is limited, the material is not well protected, and the main performance shows that the high-temperature cycle life is limited. Therefore, the preparation of lithium manganese iron phosphate materials with low carbon content, good coating effect, and good stability is very important for the research and development of electrochemical devices.

Summary of the Invention

Problems to be Solved by the Invention

[0004] In view of the above-mentioned drawbacks of the related art, the object of the present invention will be described below.

Means for Solving the Problems

[0005] In a first aspect, a positive electrode material is provided. The main components include lithium iron manganese phosphate and carbon coating the surface of the particles of lithium iron manganese phosphate. The chemical formula of lithium iron manganese phosphate is Li a Mn x Fe 1-x-y M y PO4, where 0.9 ≦ a ≦ 1.10, 0 ≦ x ≦ 1.0, 0 ≦ y ≦ 0.02, 0.5 ≦ x / (1 - x - y) ≦ 0.9. M is a doping element selected from one or more of Ti, Mg, Ni, Co, Al, V, Cr, Zr, and Nb. Carbon accounts for 1.4% - 4.7% of the total mass of the positive electrode material.

[0006] In combination with the first aspect, in one implementation, carbon coats the surface of the particles of lithium iron manganese phosphate, and from the inside to the outside, a core body composed of lithium iron manganese phosphate, a fused layer of lithium iron manganese phosphate fused with carbon, and a coating layer composed of carbon are formed.

[0007] Referring to the first aspect, in one implementation, the thickness ratio of the fused layer to the coating layer is (10 - 15):(5 - 10).

[0008] In combination with the first aspect, in one implementation, the proportion of carbon at a first depth from the surface of the coating layer is 1.2% ≦ W1 ≦ 2.3%, and the first depth is within the range of the fused layer.

[0009] In combination with the first aspect, in one implementation, the carbon ratio at the second depth from the surface of the coating layer is 0.8% ≤ W2 ≤ 1.8%, the second depth is within the range of the fusion layer, and the second depth is deeper than the first depth.

[0010] In combination with the first aspect, in one implementation, the ratio of the carbon ratio at the first depth to the carbon ratio at the second depth from the surface of the coating layer is 1.05 ≤ W1 / W2 ≤ 1.8.

[0011] In combination with the first aspect, in one implementation, the first depth is in the range of 5 nm to 20 nm from the surface of the coating layer.

[0012] In combination with the first aspect, in one implementation, the second depth is in the range of 90 nm to 100 nm from the surface of the coating layer.

[0013] In combination with the first aspect, in one implementation, the metal exposure rate P me after the particles of lithium iron manganese phosphate are coated with carbon me is 68% ≤ P me ≤ 88%, and the metal exposure rate P me = n tol / n me where n tol is the total molar amount of all metal elements on the surface of the particles of lithium iron manganese phosphate, and n

[0014] In the second aspect, a method for preparing a cathode material is provided. The key to preparing the cathode material is to mix a carbon source, a manganese source, a lithium source, an iron source, a doping element, and a phosphorus source and dry them, then perform preliminary calcination and preliminary fusion at the first-stage temperature, and then raise the temperature to the second-stage temperature and / or continue the calcination to obtain the cathode material, and / or the first-stage temperature is 300 °C to 500 °C, the preliminary calcination time is 10 to 20 hours, and / or the second-stage temperature is 500 °C to 650 °C, and the calcination time is 10 to 20 hours.

[0015] In combination with the second mode, in one implementation, the carbon source is selected from one or more of glucose, fructose, polyethylene glycol, and sucrose, and / or the manganese source is one or more of manganese sulfate, manganese nitrate, and manganese chloride, and / or the iron source is one or more of iron nitrate and iron chloride, and / or the phosphorus source is one or more of ammonium dihydrogen phosphate and diammonium hydrogen phosphate, and / or the doping element is selected from sulfates, nitrates, or chloride salts of the corresponding doping elements, and / or the lithium source is lithium carbonate.

[0016] In the third mode, a positive electrode sheet is provided. The key is that the raw materials for preparation include the positive electrode material described in either the first mode or the second mode above.

Advantages of the Invention

[0017] As described above, the positive electrode material and the electrochemical device of the present invention at least include the following beneficial effects. The carbon content of the positive electrode material is 1.4% - 4.7%, which is lower than that of general positive electrode materials, but the coating effect is good. The application of such a positive electrode material in the electrode sheet and the battery can exhibit improved electrical performance.

Brief Description of the Drawings

[0018]

Figure 1

Figure 2

Embodiments for Carrying out the Invention

[0019] The implementation of the present invention is represented below by specific embodiments. Those skilled in the art can easily understand the advantages and effects of the present invention from the disclosure of this specification. The present invention can also be implemented or applied by other different specific implementation methods, and various modifications or changes can be made to the details in this specification based on different viewpoints and applications without departing from the spirit of the present invention. It should be understood that the preferred embodiments are merely for explaining the present invention and not for limiting the protection scope of the present invention.

[0020] The term "coating" refers to using other materials to form a uniform and thin layer structure that can cover the active material on the surface of the active material. Also, the term "carbon coating" refers to covering the surface of the active material with a layer of carbon thin film, forming a carbide layer with high strength, high conductivity, and high corrosion resistance on the surface of the active material.

[0021] One part of an embodiment of the present invention provides a cathode material containing lithium iron manganese phosphate and carbon. The chemical formula of lithium iron manganese phosphate is Li a Mn x Fe 1-x-y M y PO4, where 0.9 ≤ a ≤ 1.10, 0 ≤ x ≤ 1.0, 0 ≤ y ≤ 0.02, 0.5 ≤ x / (1 - x - y) ≤ 0.9, and M is a doping element selected from one or more of Ti, Mg, Ni, Co, Al, V, Cr, Zr, Nb. Carbon coats the surface of the lithium iron manganese phosphate particles, and the carbon accounts for 1.4% - 4.7% of the total mass of the cathode material.

[0022] Referring to Figure 1, carbon coats the surface of the lithium iron manganese phosphate particles. From the inside to the outside, a core body d1 made of lithium iron manganese phosphate, a fusion layer d2 of lithium iron manganese phosphate fused with carbon, and a coating layer d3 made of carbon are formed. The thickness ratio of the fusion layer d2 to the coating layer d3 is (10 - 15):(5 - 10). Within the range of the fusion layer, the proportion of carbon in the fusion layer gradually increases along the thickness direction from the core body as the distance from the core body increases.

[0023] In some embodiments, the proportion of carbon increases gradually. Specifically, the proportion of carbon at the first depth from the surface of the coating layer is 1.2% ≤ W1 ≤ 2.3%, and the first depth is located within the range of the fusion layer. The proportion of carbon at the second depth from the surface of the coating layer is 0.8% ≤ W2 ≤ 1.8%, the second depth is located within the range of the fusion layer, and the second depth is deeper than the first depth. The proportion of carbon here refers to the local carbon content in a specific depth range measured by the etching XPS test method. The dispersion proportion of carbon in the fusion layer is reflected in the penetration depth of carbon within the particles of lithium iron manganese phosphate. The better the fusion effect of carbon and lithium iron manganese phosphate, the higher the electrical performance. Therefore, by controlling the distribution of the carbon content in the fusion layer, the electrical performance of the cathode material can be controlled.

[0024] In order to better the coating effect and increase the cycle life, in some embodiments, the ratio of the carbon proportions at the first depth and the second depth is further limited. Specifically, the ratio of the proportion of carbon at the first depth to the proportion of carbon at the second depth from the surface of the coating layer is 1.05 ≤ W1 / W2 ≤ 1.8.

[0025] In one specific implementation, based on the thickness ratio of the fusion layer d2 to the coating layer d3 being (10 - 15):(5 - 10), the first depth is in the range between 5 nm and 20 nm from the surface of the coating layer. The second depth is preferably in the range between 90 nm and 100 nm from the surface of the coating layer.

[0026] In addition, in some embodiments, in order to improve the performance of the cathode material, a further requirement for the effect of carbon coating is imposed. Specifically, the metal exposure rate P me after the particles of lithium iron manganese phosphate are coated with carbon is 68% ≤ P me ≤ 88%, and the metal exposure rate P me = n me / n tol where n me is the total molar amount of all metal elements on the surface of the particles of lithium iron manganese phosphate. The metal elements include transition metal elements, and n tolIt is the total molar amount of all elements on the surface of lithium iron manganese phosphate particles. The contents of C, Fe, and Mn on the surface of lithium iron manganese phosphate particles are measured by the EDS method. Note that the molar amount of surface (Fe + Mn) is divided by the total molar amounts of surface C, Fe, and Mn to obtain the metal exposure rate.

[0027] A part of one embodiment of the present invention provides a method for preparing a cathode material. A carbon source, a manganese source, a lithium source, an iron source, a doping element, and a phosphorus source are mixed and dried, and then pre-fired at a temperature of 300 °C to 500 °C for 10 to 20 hours for pre-fusion. Then, the temperature is raised to 500 to 650 °C, and the firing is continued for 10 to 20 hours to obtain the cathode material.

[0028] Note that the pre-firing temperature is very important for the pre-fusion of the carbon source. The pre-firing improves the quality of the carbon material coating and increases the penetration depth. If the pre-firing temperature is too low, the fusion effect between carbon and other raw materials becomes unfavorable. If the pre-firing temperature is too high, the carbon source will be carbonized in advance, and the pre-fusion effect cannot be achieved. In both cases, the electrical performance deteriorates. The firing temperature is very important for the carbon coating effect. If the firing temperature is too low, the cracks in the carbon source are not sufficient, and an amorphous carbon coating cannot be completely formed. If the firing temperature is too high, the carbon source will volatilize, and the fusion effect between the carbon source and the main material will deteriorate. In both cases, the electrical performance deteriorates. Therefore, it is necessary to select an appropriate firing temperature.

[0029] In some embodiments, the carbon source is selected from one or more of glucose, fructose, polyethylene glycol, and sucrose. Note that the carbon source is not limited to one or more of glucose, fructose, polyethylene glycol, and sucrose, and other organic carbon sources may be adopted. Compared with inorganic carbon sources, the pre-fusion effect of organic carbon sources is better.

[0030] In some embodiments, the manganese source is one or more of manganese sulfate, manganese nitrate, and manganese chloride.

[0031] In some embodiments, the iron source is one or more of iron nitrate and iron chloride.

[0032] In some embodiments, the phosphorus source is one or more of ammonium dihydrogen phosphate and diammonium hydrogen phosphate.

[0033] In some embodiments, the doping element is selected from the sulfate, nitrate, or chloride salt of the corresponding doping element.

[0034] In some embodiments, the lithium source is lithium carbonate.

[0035] A part of one embodiment of the present invention provides a positive electrode sheet. The preparation of its raw materials includes the above positive electrode material.

[0036] In some embodiments, the method for preparing the positive electrode sheet includes mixing the positive electrode material of any part of the embodiment with the conductive agent Super P and the binder polyvinylidene fluoride (PVDF) based on a weight ratio of 97:1.5:1.5, adding the solvent N-methylpyrrolidone (NMP), stirring the mixture to completely mix it to obtain a positive electrode slurry, coating the positive electrode slurry on the aluminum foil of the positive electrode current collector, and finally obtaining the positive electrode sheet through drying, cold pressing, slitting, and other processes.

[0037] Examples 1 to 8 and Comparative Examples 1 to 10

[0038] I. Based on various embodiments in Table 1, the positive electrode sheets prepared using different parameters of the positive electrode material were respectively laminated with the same separator film and negative electrode sheet, and the separator film was placed between the positive electrode sheet and the negative electrode sheet to play the role of separation. Subsequently, the outside was covered with an aluminum-plastic film, and after drying, the same electrolyte was injected into each. After packaging, standing, and chemical treatment, soft pack batteries with a capacity of 1 Ah were finally completed for each.

[0039] Table 1: Parameters of the positive electrode materials adopted in Examples 1 - 8

[0040]

Table 1

[0041] II. Based on the comparative examples in Table 2, the positive electrode sheets prepared using different parameters of the positive electrode material were respectively laminated with the same separator film and negative electrode sheet, and the separator film was placed between the positive electrode sheet and the negative electrode sheet to play the role of separation. Subsequently, the outside was covered with an aluminum-plastic film, and after drying, the same electrolyte was injected into each. After packaging, standing, and chemical treatment, soft pack batteries with a capacity of 1 Ah were finally completed for each.

[0042] Table 2: Parameters of the positive electrode materials adopted in Comparative Examples 1 - 10

[0043]

Table 2

[0044] III. Test method: For the batteries obtained in the above examples and comparative examples, a gram capacity test, a DCR test, and a cycle test were respectively performed.

[0045] 1. Gram capacity test method: Within the range of 2.5 - 4.2 V, charge and discharge were performed at 0.33 C at 25°C for 3 cycles, and the discharge capacity of the third cycle was divided by the total mass of the positive electrode material to obtain the gram capacity of the positive electrode.

[0046] 2. DCR test method: Charge and discharge were performed at 0.33C for 3 cycles at 25°C, and the discharge capacity of the third cycle was 100% of the cell capacity. Then, the cell capacity was adjusted to 70% of the cell capacity, i.e., 70% SOC, at 0.33C. Discharge was performed at a current density of 4C for 30 s, and the voltage difference before and after discharge was divided by the discharge current. The result is the 70% SOC DCR.

[0047] 3. Cycle test method: Charge and discharge were performed at 1C at 45°C within the range of 2.5 - 4.2V until the remaining capacity reached 80% of the initial capacity. The number of cycles is the cycle life of the material.

[0048] IV. Test Results

[0049] The test results of various examples are shown in Table 3.

[0050] Table 3: Test Results of Examples 1 - 8

[0051]

Table 3

[0052] Explanation of Results:

[0053] 1. In Examples 1 - 8, the gram capacity exceeded 140 mAh / g. Most of the DCR results were lower than 120, and a few exceeded 120. The cycle life reached an average of over 800 times, most exceeded 900 times, and some exceeded 1000 times.

[0054] 2. Comparison between Comparative Examples 1 - 3 and Examples 1 - 8:

[0055] Figure 2 is a comparison diagram showing the comparison of the cycle life curves at 45°C between Example 1 and Comparative Examples 1 and 2. While the capacity of Example 1 decreased steadily, the capacities of Comparative Examples 1 and 2 decreased at an increasing rate.

[0056] In Comparative Examples 1 and 3, the metal exposure rate Pme The ratio of the carbon content at the first depth (hereinafter, W1), the carbon content at the second depth (hereinafter, W2), and the ratio of the carbon content at the first depth to the carbon content at the second depth (hereinafter, W1 / W2) is outside the preferred range of the present invention. The gram capacity does not exceed 130, the DCR is higher than 140, and the cycle life is lower than 500 times. Compared with Examples 1 to 8, there are significant differences among the test results of various parameters. When the parameters are within the preferred range, the performance is considerably optimized.

[0057] 3. Comparison between Comparative Examples 4 to 5 and Examples 1 to 3:

[0058] When W1 or W2 in Comparative Example 4 or 5 does not meet the preferred range, the metal exposure rate is relatively high, indicating that the penetration of carbon into lithium iron manganese phosphate is not uniform. Also, the gram capacity is lower than 140 mAh / g, the DCR is higher than 130, and the cycle life is only about 600 times. Compared with Examples 1 to 3, the test results of Comparative Examples 4 to 5 are relatively inferior.

[0059] 4. Comparison between Comparative Example 6 and Examples 3 to 5:

[0060] In Comparative Example 6, although the metal exposure rate, W1, and W2 are within the preferred range, when the ratio of W1 / W2 is outside the value range set by the present invention, the carbon penetration is not uniform, the gram capacity is lower than 140, the DCR is higher than 130, and the cycle life is lower than 700 times, indicating that the overall performance is relatively inferior. In Examples 3 to 5, by optimizing the ratio of W1 / W2, the gram capacity increases to 140 or more, the DCR is about 120, and the cycle life is maintained at 800 times or more, showing a significant improvement. With other parameters unchanged and adopting the ratio of W1 / W2 within the value range set by the present invention, the performance of the battery can be further improved.

[0061] 5. Comparison between Comparative Example 6 and other Comparative Examples:

[0062] The gram capacity, DCR, and cycle life of Comparative Example 6 are all better than the results of other comparative examples. This indirectly indicates that by adopting the value ranges set in the present invention, the metal exposure rate, W1, and W2 can further improve the performance of the battery.

[0063] 6. Comparison between Comparative Examples 7 - 9 and Examples 6 - 8:

[0064] When only one of the ratios of W1, W2, and W1 / W2 is within the preferred range, the metal exposure rate exceeds the preferred range, the carbon coating effect is relatively poor, and the performances of the gram capacity, DCR, and cycle life are all inferior.

[0065] 7. Comparison between Comparative Examples 4 - 9 and Comparative Examples 1 - 3, 10:

[0066] Among the metal exposure rate, W1, W2, and the ratio of W1 / W2 in Comparative Examples 4 - 9, at least one of the value ranges of the parameter conditions among these four parameters adopts the value ranges set in the present invention. In Comparative Examples 1 - 3, the value ranges of these four parameters are outside the value ranges set in the present invention. The gram capacity, DCR, and cycle life of Comparative Examples 4 - 9 are all more preferable than those of Comparative Examples 1 - 3, 10. It shows that when at least one of these four parameters adopts the value ranges set in the present invention among the metal exposure rate, W1, W2, and the ratio of W1 / W2, it is possible to improve the performance of the battery.

[0067] Comparison between Examples 9 - 11 and Comparative Examples 11 - 12:

[0068] Some of the examples and comparative examples of the present invention provide a method for preparing a positive electrode material. According to the parameter conditions shown in Table 4, first, polyethylene glycol, manganese sulfate, iron nitrate, lithium carbonate, a doping element, and ammonium dihydrogen phosphate were mixed and dried, and then pre-fired at a first temperature for 10 to 20 hours for pre-fusion. Then, to obtain the positive electrode material, the temperature was raised to a second temperature and firing was continued for 10 to 20 hours. To fabricate a 1 Ah soft pack battery based on the battery fabrication methods of Examples 1 to 8, the prepared positive electrode material was used. The gram capacity test, DCR test, and cycle test were respectively performed on the batteries obtained in the above-described examples and comparative examples. The results are shown in Table 4.

[0069] Table 4: Influence of pre-firing on carbon content, carbon coating effect, and battery performance in the positive electrode material

[0070]

Table 4

[0071] The results are as follows. In Comparative Examples 11 and 12, when the pre-firing temperature is lower than 300 °C or pre-firing is not performed, even if the firing temperature meets the requirements, the carbon coating effect of the positive electrode material is inferior, the carbon content and the ratio of carbon do not meet the requirements of the present invention, the gram capacity of the battery is relatively low, the cycle life is lower than 500 times, and the overall performance of the battery is relatively poor. In Examples 9 to 11, while other conditions remain unchanged, when the pre-firing temperature is increased while adopting the value range set by the present invention for the pre-firing temperature, the obtained positive electrode material has an improved carbon coating effect, and all of the carbon content and the carbon ratio meet the values set by the present invention, the gram capacity of the battery is improved, the cycle life increases to 800 times or more, and even 1000 times or more, and the overall performance of the battery is improved.

[0072] Examples 12 to 14 and Comparative Example 13:

[0073] Some of the examples and comparative examples of the present invention provide a method for preparing a positive electrode material. According to the parameter conditions shown in Table 5, first, a carbon source, manganese sulfate, iron chloride, lithium carbonate, a doping element, and ammonium dihydrogen phosphate were mixed and dried, and then pre-fired at a first temperature for 10 to 20 hours for pre-fusion. Next, in order to obtain the positive electrode material, the temperature was raised to a second temperature and the firing was continued for 10 to 20 hours. The prepared positive electrode material was used to fabricate a 1 Ah soft pack battery based on the battery fabrication methods of Examples 1 to 8. The gram capacity test, DCR test, and cycle test were respectively performed on the batteries obtained in the above-described examples and comparative examples. The results are shown in Table 5.

[0074] Table 5: Influence of Organic Carbon Source and Inorganic Carbon Source on Carbon Content, Carbon Coating Effect, and Battery Performance of Positive Electrode Material

[0075]

Table 5

[0076] The results are as follows. In the comparison between Examples 12 to 14 and Comparative Example 13, in the positive electrode material obtained by using an inorganic carbon source, the parameters regarding carbon coating and carbon content cannot fall within the value range set by the present invention. And the overall performance of the battery is inferior to that of the battery fabricated using an organic carbon source.

[0077] Examples 15 to 17 and Comparative Example 14:

[0078] Some of the examples and comparative examples of the present invention provide a method for preparing a positive electrode material. According to the parameter conditions shown in Table 6, first, fructose, manganese chloride, iron nitrate, lithium carbonate, a doping element, and ammonium dihydrogen phosphate were mixed and dried, and then pre-fired at a first temperature for 10 to 20 hours for pre-fusion. Then, to obtain the positive electrode material, the temperature was raised to a second temperature and the firing was continued for 10 to 20 hours. To fabricate a 1 Ah soft pack battery based on the battery fabrication methods of Examples 1 to 8, the prepared positive electrode material was used. The gram capacity test, DCR test, and cycle test were respectively performed on the batteries obtained in the above-described examples and comparative examples. The results are shown in Table 6.

[0079] Table 6: Influence of Firing Temperature on Carbon Content, Carbon Coating Effect, and Battery Performance of Positive Electrode Material

[0080]

Table 6

[0081] The results are as follows. Comparing Examples 15 to 17 with Comparative Example 14, when the firing temperature is within the value range set by the present invention, the higher the temperature, the better the battery performance. When the value range set by the present invention is not adopted for the firing temperature, the battery performance is relatively poor.

Industrial Applicability

[0082] Furthermore, the positive electrode material of the present invention, its preparation method, the positive electrode sheet containing the same, the lithium ion battery containing the same, and the lithium ion battery pack containing the same can be used in the electronics field, and thus have high industrial applicability.

[0083] It should be noted that the test results of the above examples are only for showing the obvious advantages of the positive electrode material of the present invention under the same conditions, and the battery test results do not represent the optimal test results of the battery fabricated with the positive electrode material of the present invention.

[0084] The above-described embodiments illustrate the principles and effects of the present invention and are not intended to limit the present invention. Those skilled in the art can modify or change the above-described embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or changes made by those skilled in the art without departing from the spirit and technical idea disclosed in the present invention shall also be included in the scope of the claims of the present invention.

Description of Reference Numerals

[0085] 1: Core body 2: Fusion layer 3: Coating layer d2: Thickness of the fusion layer d3: Thickness of the coating layer

Claims

1. Lithium iron manganese phosphate, and carbon coating the surface of the particles of the lithium iron manganese phosphate are included, The chemical formula of the lithium iron manganese phosphate is Li ａ Mn ｘ Fe １－ｘ－ｙ M ｙ PO ４ where 0.9 ≤ a ≤ 1.10, 0 ≤ x ≤ 1.0, 0 ≤ y ≤ 0.02, 0.5 ≤ x / (1 - x - y) ≤ 0.9, M is a doping element selected from one or more of Ti, Mg, Ni, Co, Al, V, Cr, Zr, and Nb, the carbon occupies 1.4% to 4.7% of the total mass of the positive electrode material, the carbon coats the surface of the particles of the lithium iron manganese phosphate, and from the inside out, a core body composed of the lithium iron manganese phosphate, a fusion layer of the lithium iron manganese phosphate fused with the carbon, and a coating layer composed of the carbon are formed, the ratio of the carbon at a first depth from the surface of the coating layer is 1.2% ≤ W1 ≤ 2.3%, the first depth is within the range of the fusion layer and is an interval between 5 nm and 20 nm from the surface of the coating layer, the ratio of the carbon at a second depth from the surface of the coating layer is 0.8% ≤ W2 ≤ 1.8%, the second depth is within the range of the fusion layer and is an interval between 90 nm and 100 nm from the surface of the coating layer, the ratio of the ratio of the carbon at the first depth to the ratio of the carbon at the second depth from the surface of the coating layer is 1.05 ≤ W1 / W2 ≤ 1.8, The metal exposure rate P after the particles of the iron lithium manganese phosphate are coated with the carbon ｍｅ is 68% ≤ P ｍｅ ≤ 88%, and the metal exposure rate P ｍｅ = n ｍｅ / n ｔｏｌ where n ｍｅ is the total molar amount of all metal elements on the surface of the particles of the iron lithium manganese phosphate, and n ｔｏｌ is the total molar amount of all elements on the surface of the particles of the iron lithium manganese phosphate A positive electrode material.

2. The thickness ratio of the fusion layer to the coating layer is (10 - 15):(5 - 10), The positive electrode material according to Claim 1.

3. After mixing an organic carbon source, a manganese source, a lithium source, an iron source, a doping element, and a phosphorus source and drying, pre-sintering and pre-fusion are performed at a first-stage temperature, and then the temperature is raised from the first-stage temperature to a second-stage temperature in order to continue sintering to obtain a positive electrode material, (1) The first-stage temperature is 300°C to 500°C, and the pre-sintering time is 10 to 20 hours, and (2) The second-stage temperature is 500°C to 650°C, and the sintering time is 10 to 20 hours, satisfying the two conditions of A method for preparing a positive electrode material.

4. (a) The organic carbon source is selected from one or more of glucose, fructose, polyethylene glycol, and sucrose (b) The manganese source is one or more of manganese sulfate, manganese nitrate, and manganese chloride, (c) The iron source is one or more of iron nitrate and iron chloride, (d) The phosphorus source is one or more of ammonium dihydrogen phosphate and diammonium hydrogen phosphate, (e) The doping element is selected from sulfates, nitrates, or chloride salts of the corresponding doping element, and (f) The lithium source is lithium carbonate, A method for preparing a positive electrode material according to claim 3, further satisfying at least one of the following conditions: A method for preparing the positive electrode material according to claim 3.

5. A positive electrode sheet having a raw material containing the positive electrode material according to claim 1. Positive electrode sheet.

6. After mixing an organic carbon source, a manganese source, a lithium source, an iron source, a doping element, and a phosphorus source and drying them, pre-sintering and pre-fusing are performed at a first-stage temperature, and then the temperature is raised from the first-stage temperature to a second-stage temperature to obtain a positive electrode material in order to continue firing to obtain the positive electrode material. (1) The first-stage temperature is 300°C to 500°C, and the pre-sintering time is 10 to 20 hours, and (2) The second-stage temperature is 500°C to 650°C, and the firing time is 10 to 20 hours. A positive electrode material preparation process that satisfies the following two conditions: A positive electrode slurry preparation process of mixing the positive electrode material, a conductive agent, a binder, and a solvent to obtain a positive electrode slurry. A positive electrode sheet preparation process of applying the positive electrode slurry onto an aluminum foil of a positive electrode current collector and obtaining a positive electrode sheet by post-treatment including drying, cold pressing, and slitting. A method for preparing a positive electrode sheet, comprising:

7. A lithium-ion battery having the positive electrode sheet according to claim 5. Lithium-ion battery.

8. A lithium-ion battery pack having the lithium-ion battery according to claim 7. Lithium-ion battery pack.