Double-site co-doped lithium iron manganese phosphate-based positive electrode material as well as preparation method and application of double-site co-doped lithium iron manganese phosphate-based positive electrode material

Abstract

The invention discloses a double-site co-doped lithium manganese iron phosphate-based positive electrode material as well as a preparation method and application thereof. The lithium manganese iron phosphate-based positive electrode material comprises sodium and lanthanum co-doped lithium manganese iron phosphate, in the lithium manganese iron phosphate-based positive electrode material, the molar ratio of lithium to sodium is (1-x): x, and the value range of x is 0.002-0.02. The double-site co-doped lithium manganese iron phosphate-based positive electrode material provided by the invention has relatively high electronic conductivity and lithium ion diffusion rate, has excellent rate capability and cycling stability, and still has excellent electrochemical performance even at a high temperature of 45 DEG C. The invention also provides a preparation method and application of the lithium iron manganese phosphate-based positive electrode material.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium ion battery positive electrode materials, and relates to a dual-site co-doped lithium manganese iron phosphate-based positive electrode material and a preparation method and application thereof. Background Art

[0002] Positive electrode materials are the main components of lithium-ion batteries, and their performance directly affects various battery performance indicators, such as safety, energy density, service life, charging time, high and low temperature performance, etc. With the continuous development and widespread application of lithium-ion battery technology, positive electrode materials have become a hot research field in industry and academia. Olivine-type phosphate-based positive electrode material LiMPO 4 Compared with layered cathode materials, lithium manganese iron phosphate (LiMnxFe 1-x PO 4 ) has attracted widespread attention because of its higher theoretical energy density than lithium iron phosphate (LFP), lower cost and higher safety than ternary materials (NCM). However, LMFP has low conductivity and slow ion diffusion rate, resulting in poor rate performance. When the current is large, both charging and discharging will produce significant polarization, resulting in reduced capacity, thus affecting energy density; and because Mn 3+ The John-Teller effect leads to a decrease in the cyclic stability of the material.

[0003] In order to improve the performance of LMFP materials so that they can be better developed and applied, relevant technical researchers have conducted a lot of research work and designed many new process synthesis routes and modification strategies. 4 Like the modification methods, researchers usually use single measures such as reducing particle size, surface coating, ion doping, material structure design and morphology control, or use the synergistic effect of multiple measures to improve the performance of LMFP. Among them, surface coating starts from the outside of the material to improve the material's conductivity and stability, but it does not fundamentally solve the problems of lithium manganese iron phosphate materials, and its performance improvement is limited; and the liquid phase method used for material structure design and morphology control requires high equipment conditions, high energy consumption, and high cost, which is not conducive to large-scale industrial production. Only ion doping can fundamentally solve the problems of low conductivity and poor stability, and single ion doping may also have little effect, so multi-element and multi-site doping is the only way to solve the problems of lithium manganese iron phosphate materials.

[0004] At present, there have been studies on the improvement of the performance of lithium manganese iron phosphate materials by multi-element doping. For example, some technologies use the three-stage ball milling method to prepare multi-site multi-element doped and doped coated modified lithium manganese iron phosphate materials. Although the electrochemical performance of the modified materials has been improved to a certain extent, the discharge capacity has not been fully released, resulting in a low 0.1C discharge capacity of only 150.9mAh / g, and the rate performance and cycle performance have yet to be verified, and the preparation process is relatively complicated. Some technologies use the solid phase method to prepare lithium manganese iron phosphate materials doped with different rare earth elements, but their 0.1C discharge capacity is only 120mAh / g, and the long cycle performance has not been verified, which has limited improvement on the performance of the material. Some technologies use the solid phase method to dope the purchased lithium manganese iron phosphate precursor with multiple elements at the manganese and iron positions, but the modified material has a discharge capacity of 145mAh / g at 45°C and 1C rate, and a capacity retention rate of 99.15% after 100 cycles. The rate performance and long cycle performance are not described. There is a technology that uses the co-precipitation method to dope multiple elements at the manganese iron site. Although its rate performance is good and the capacity is high, the cycle stability is general. At a rate of 1C, the capacity retention rate is only 98.8% after 80 cycles. There is a technology that uses the co-precipitation method combined with the solid-phase method to perform two-step doping to prepare multi-element doped lithium manganese iron phosphate, and the rate performance and long cycle performance are not mentioned. There is also a technology that uses the hydrothermal method to prepare lithium manganese iron phosphate materials co-doped with anions and cations at oxygen and manganese iron sites, but the rate performance and cycle performance are also unknown. In addition, these liquid phase preparation processes are relatively complicated and energy-intensive.

[0005] Therefore, there is an urgent need to provide a lithium manganese iron phosphate and a preparation method thereof to achieve high material conductivity, stable structure, simple synthesis process, environmental friendliness and pollution-free, and suitable for industrial production. Summary of the invention

[0006] The present invention aims to solve at least one of the technical problems existing in the prior art. To this end, the present invention proposes a dual-site co-doped lithium manganese iron phosphate-based positive electrode material, which can effectively overcome the problems of poor electronic conductivity, low lithium ion diffusion rate, poor rate performance, and poor cycle stability of the LMFP material in the prior art.

[0007] The present invention also provides a method for preparing the lithium manganese iron phosphate-based positive electrode material.

[0008] The present invention also provides application of the lithium manganese iron phosphate-based positive electrode material.

[0009] According to an embodiment of the first aspect of the present invention, a dual-site co-doped lithium iron manganese phosphate-based positive electrode material is provided, wherein the lithium iron manganese phosphate-based positive electrode material comprises sodium and lanthanum co-doped lithium iron manganese phosphate;

[0010] In the lithium manganese iron phosphate-based positive electrode material, the molar ratio of lithium to sodium is (1-x):x, and the value range of x is 0.002 to 0.02.

[0011] The lithium manganese iron phosphate-based positive electrode material according to the embodiment of the present invention has at least the following beneficial effects:

[0012] Sodium and lithium are both alkali metal elements with the same outer electron configuration, so they are more inclined to occupy lithium sites, while lanthanum is more inclined to occupy transition metal sites. The synergy of the two can effectively widen the lithium layer spacing, widen the lithium ion diffusion channel, thereby increasing the lithium ion diffusion coefficient, and stabilize the crystal structure, inhibiting the Jan-Taylor effect; ultimately effectively improving the rate performance and cycle performance of the material.

[0013] Furthermore, due to the limitation of the type and amount of doping elements, the lithium manganese iron phosphate-based positive electrode material provided by the present invention still has excellent rate performance and cycle stability at a high temperature of 45°C. The stable structure brought by the dual-site co-doping can effectively inhibit the manganese dissolution of the material due to the Jan-Taylor effect at high temperature, thereby avoiding structural collapse and also avoiding the damage of the positive electrode material to the electrolyte, and has excellent high-temperature stability.

[0014] According to some embodiments of the present invention, in the lithium manganese iron phosphate-based positive electrode material, the value of x ranges from 0.005 to 0.01, for example, it may be about 0.007, 0.008 or about 0.009.

[0015] According to some embodiments of the present invention, in the lithium manganese iron phosphate-based positive electrode material, the molar ratio of manganese, iron and lanthanum is 0.5:(0.5-y):y, and the value range of y is 0.005 to 0.05. For example, it can be about 0.008, 0.01, 0.015, 0.02, 0.025, 0.03 or about 0.04.

[0016] According to some embodiments of the present invention, the lithium iron manganese phosphate-based positive electrode material further includes a carbon layer; the carbon layer wraps the lithium iron manganese phosphate.

[0017] According to an embodiment of the second aspect of the present invention, a method for preparing the lithium iron manganese phosphate-based positive electrode material in the embodiment of the first aspect of the present invention is provided, and the preparation method comprises the following steps:

[0018] S1. Take a lithium source, a sodium source, a manganese source, an iron source, a lanthanum source and a phosphorus source, and dry-mill them; add an organic solvent and wet-mill them, and remove the organic solvent to obtain a precursor; the molar ratio of lithium in the lithium source to sodium in the sodium source is (1-x):x; and the value range of x is 0.002 to 0.02;

[0019] S2. calcining the precursor at high temperature under the protection of an inert gas atmosphere.

[0020] Since the preparation method adopts all the technical solutions of the lithium manganese iron phosphate-based positive electrode material of the above embodiment, it at least has all the beneficial effects brought by the technical solutions of the above embodiment.

[0021] Furthermore, the preparation method provided by the present invention uses a solid phase method of dry-wet mixing and grinding, the preparation raw materials are added at one time, the process flow is simple, and the synthesis time is short;

[0022] Dry ball milling has high intensity, which can make the raw material powders fully collide and crush each other, and can obtain a smaller product particle size, but the sample will be coated on the zirconium beads and lumps will be generated. On the one hand, it is difficult to separate the sample from the zirconium beads after ball milling, and on the other hand, it makes it impossible to fully and evenly mix the raw materials; due to the presence of dispersants, wet ball milling has a small friction coefficient, and the interface energy of the particles in the dispersant is smaller than the surface energy in the air. Therefore, the time required for the crushing of the raw material powder is longer than dry grinding, and the product particle size will also be larger in the wet grinding environment; the dry-wet mixing method used in the present invention has a shorter ball milling time, and the material and zirconium beads are easier to separate. The product particle size is smaller and more uniform, and better electrochemical performance can be obtained.

[0023] Compared with traditional liquid phase methods such as hydrothermal method and solvent thermal method, the preparation method provided by the present invention has simple operation process, low cost and is suitable for large-scale industrial production.

[0024] According to some embodiments of the present invention, in step S1, the raw materials for dry ball milling also include a carbon source.

[0025] According to some embodiments of the present invention, the carbon source is at least one of glucose, sucrose, fructose, cellulose, starch, citric acid, polyacrylic acid, polyethylene glycol and dopamine.

[0026] According to some embodiments of the present invention, the amount of the carbon source added is 8% to 16% of the total mass of other raw materials (lithium source, sodium source, manganese source, iron source, lanthanum source and phosphorus source), for example, about 10%, 12% or about 15%.

[0027] According to some embodiments of the present invention, in step S1, the lithium source is at least one of lithium carbonate, lithium hydroxide and lithium dihydrogen phosphate.

[0028] According to some embodiments of the present invention, in step S1, the sodium source is at least one of sodium carbonate, sodium chloride and sodium sulfate.

[0029] According to some embodiments of the present invention, in step S1, the molar ratio of lithium in the lithium source to sodium in the sodium source is (1-x):x; and the value range of x is 0.005 to 0.01. For example, it can be about 0.007, 0.008 or about 0.009.

[0030] According to some embodiments of the present invention, the manganese source is at least one of manganese sulfate, manganese chloride, manganese carbonate and manganese oxalate.

[0031] According to some embodiments of the present invention, the iron source is at least one of ferrous sulfate, ferrous chloride and ferrous oxalate. The iron source is selected from at least one of anhydrous compounds and hydrated compounds thereof, for example, ferrous oxalate includes at least one of anhydrous ferrous oxalate and dihydrated ferrous oxalate.

[0032] According to some embodiments of the present invention, the lanthanum source is at least one of lanthanum oxide, lanthanum chloride and lanthanum carbonate.

[0033] According to some embodiments of the present invention, the phosphorus source is at least one of diammonium hydrogen phosphate, ammonium dihydrogen phosphate, phosphoric acid, lithium dihydrogen phosphate and pyrophosphoric acid.

[0034] According to some embodiments of the present invention, in step S1, the molar ratio of manganese in the manganese source, iron in the iron source and lanthanum in the lanthanum source is 0.5:(0.5-y):y, and the value range of y is 0.005 to 0.05. For example, it can be about 0.008, 0.01, 0.015, 0.02, 0.025, 0.03 or about 0.04.

[0035] According to some embodiments of the present invention, in the lithium source, sodium source, manganese source, iron source, lanthanum source and phosphorus source, the ratio of the molar sum of sodium and lithium, the molar sum of manganese, iron and lanthanum, and the molar amount of phosphorus is 1:0.8~1.2:0.8~1.2; specifically, it can be about 1:1:1.

[0036] It should be noted that in step S1, the molar ratios in the raw materials are all atomic molar ratios. Therefore, if the lithium source contains phosphorus, the lithium source is also a phosphorus source, and the phosphorus content thereof needs to be considered when calculating phosphorus atoms. Similarly, if other raw materials contain lithium, they also serve as lithium sources.

[0037] According to some embodiments of the present invention, in step S1, the duration of the dry ball milling is 1 to 6 hours; for example, it can be about 1.5 hours, 2 hours, 2.5 hours, 3 hours, 4 hours or about 5 hours.

[0038] According to some embodiments of the present invention, in step S1, the organic solvent includes at least one of anhydrous ethanol, acetone and n-hexane. In actual production, as long as the organic solvent does not react with other raw materials and does not dissolve other raw materials, it is not limited to the above examples.

[0039] According to some embodiments of the present invention, in step S1, the solid content of the raw material prepared in the wet ball milling is 50% to 80%, for example, about 55%, 60%, 65%, 70% or about 75%.

[0040] According to some embodiments of the present invention, in step S1, the duration of the wet ball milling is 1 to 6 hours, for example, about 1.5 hours, 2 hours, 2.5 hours, 3 hours, 4 hours or about 5 hours.

[0041] According to some embodiments of the present invention, in step S1, the grinding media of the dry ball milling and / or wet ball milling includes zirconium beads.

[0042] According to some embodiments of the present invention, in step S1, the ball-to-material ratio of the dry ball milling and / or wet ball milling is 2 to 10:1 (the mass and ratio of zirconium beads to other preparation raw materials except organic solvent). For example, it can be about 5:1 or about 8:1.

[0043] According to some embodiments of the present invention, in step S1, the ball milling speed of the dry ball milling and / or wet ball milling is 300-900 rpm. For example, it can be about 500 rpm, 600 rpm, or about 800 rpm.

[0044] According to some embodiments of the present invention, in step S1, the method of removing the organic solvent includes at least one of filtering and drying.

[0045] The drying temperature is 60-120°C, for example, about 80°C, 90°C or about 100°C.

[0046] According to some embodiments of the present invention, in step S2, the inert atmosphere includes a nitrogen atmosphere and / or an argon atmosphere;

[0047] According to some embodiments of the present invention, in step S2, the high temperature calcination temperature is 500-800°C, for example, about 600°C, 700°C or about 750°C.

[0048] According to some embodiments of the present invention, in step S2, the high temperature calcination time is 4 to 10 hours, for example, about 5 hours, 6 hours, 7 hours, 8 hours or about 9 hours.

[0049] According to some embodiments of the present invention, in step S2, the heating rate of the high temperature calcination is 2-10°C / min.

[0050] According to an embodiment of the third aspect of the present invention, a lithium-ion battery is provided, wherein raw materials for preparing the lithium-ion battery include the lithium iron manganese phosphate-based positive electrode material provided by the embodiment of the first aspect of the present invention.

[0051] Since the lithium-ion battery adopts all the technical solutions of the lithium iron manganese phosphate-based positive electrode material of the above embodiment, it at least has all the beneficial effects brought by the technical solutions of the above embodiment.

[0052] According to some embodiments of the present invention, the lithium-ion battery includes at least one of a button battery, a soft-pack battery, a cylindrical battery, and a square battery.

[0053] According to some embodiments of the present invention, the lithium-ion battery includes one of a full cell and a half cell.

[0054] If there is no special explanation, the actual meaning of “about” in the present invention is that the error is allowed to be within the range of ±2%, for example, about 100 is actually 100±2%×100.

[0055] Unless otherwise specified, “between” in the present invention includes the number itself, for example, “between 2 and 3” includes the endpoint values ​​2 and 3.

[0056] Other features and advantages of the present invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] The above and / or additional aspects and advantages of the present invention will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0058] Figure 1 This is a low-magnification scanning electron microscope image of the lithium iron manganese phosphate-based positive electrode material obtained in Comparative Example 1 of the present invention.

[0059] Figure 2 This is a high-magnification scanning electron microscope image of the lithium iron manganese phosphate-based positive electrode material obtained in Comparative Example 1 of the present invention.

[0060] Figure 3 This is a rate performance diagram of the lithium iron manganese phosphate-based positive electrode materials obtained in Example 2 of the present invention and Comparative Example 1, Comparative Example 5 and Comparative Example 13.

[0061] Figure 4 This is a 1C cycle performance diagram of the lithium iron manganese phosphate-based positive electrode materials obtained in Example 2 of the present invention and Comparative Example 1, Comparative Example 5 and Comparative Example 13.

[0062] Figure 5 This is a 10C cycle performance diagram of the lithium iron manganese phosphate-based positive electrode materials obtained in Example 2 of the present invention and Comparative Example 1, Comparative Example 5 and Comparative Example 13.

[0063] Figure 6 This is a rate performance diagram of the lithium iron manganese phosphate-based positive electrode materials obtained in Example 2 of the present invention and Comparative Examples 1, 5 and 13 at a high temperature of 45°C.

[0064] Figure 7 This is a 1C cycle performance diagram of the lithium iron manganese phosphate-based positive electrode materials obtained in Example 2 of the present invention and Comparative Example 1, Comparative Example 5 and Comparative Example 13 at a high temperature of 45°C. DETAILED DESCRIPTION

[0065] The following will be combined with the embodiments to clearly and completely describe the concept of the present invention and the technical effects produced, so as to fully understand the purpose, characteristics and effects of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, other embodiments obtained by those skilled in the art without paying creative labor all belong to the scope of protection of the present invention. In the following embodiments, if there is no special description of the raw materials or processing techniques, it is indicated that they are all conventional commercially available raw materials or conventional processing techniques in the art.

[0066] In the following examples, the electrolyte used (1 mol / L LiPF 6 / EC+DMC+EMC (volume ratio 1:1:1)) was purchased from Suzhou Duoduo Chemical Technology Co., Ltd.

[0067] Example 1

[0068] This embodiment prepared a designed chemical formula Li 0.992 Na 0.008 Mn 0.5 Fe 0.495 La 0.005 PO 4 / C dual-site co-doped lithium manganese iron phosphate-based positive electrode material, the specific process is:

[0069] S1. Lithium carbonate, sodium carbonate, manganese carbonate, ferrous oxalate dihydrate, lanthanum oxide and ammonium dihydrogen phosphate were weighed in a 100 mL ball mill according to the molar ratio of lithium: sodium: manganese: iron: lanthanum: phosphorus = 0.992: 0.008: 0.5: 0.495: 0.005: 1 (the remaining embodiments also use the molar ratio to calculate the amount of raw materials added) and added 12% of the total mass of the raw materials to glucose;

[0070] The obtained mixture was dry-milled in a planetary ball mill at a speed of 900 rpm for 2 h; during the ball milling process, the grinding medium was zirconium beads, and the ball-to-material ratio was 8:1;

[0071] Then, an appropriate amount of anhydrous ethanol was added to the mixture to obtain a mixture with a solid content of 60%, and the mixture was further wet-milled in a planetary ball mill for 2 h to obtain a precursor slurry;

[0072] The precursor slurry was filtered and dried in an oven at 90° C. for 8 h to obtain a precursor powder;

[0073] S2. The precursor powder was calcined at 700°C for 8 h in an argon atmosphere; during the calcination process, the heating rate was 5°C / min.

[0074] Example 2

[0075] This embodiment prepared a designed chemical formula Li 0.992 Na 0.008 Mn 0.5 Fe 0.49 La 0.01 PO 4 The dual-site co-doped lithium iron manganese phosphate positive electrode material of / C is different from that of Example 1 in that:

[0076] In step S1, the addition amount of the raw materials is lithium: sodium: manganese: iron: lanthanum: phosphorus = 0.992: 0.008: 0.5: 0.49: 0.01: 1.

[0077] Example 3

[0078] This embodiment prepared a designed chemical formula Li 0.992 Na 0.008 Mn 0.5 Fe 0.48 La 0.02 PO 4 The dual-site co-doped lithium iron manganese phosphate positive electrode material of / C is different from that of Example 1 in that:

[0079] In step S1, the addition amount of the raw materials is lithium: sodium: manganese: iron: lanthanum: phosphorus = 0.992: 0.008: 0.5: 0.48: 0.02: 1.

[0080] Example 4

[0081] This embodiment prepared a designed chemical formula Li 0.992 Na 0.008 Mn 0.5 Fe 0.47 La 0.03 PO 4 The dual-site co-doped lithium iron manganese phosphate positive electrode material of / C is different from that of Example 1 in that:

[0082] In step S1, the addition amount of the raw materials is lithium: sodium: manganese: iron: lanthanum: phosphorus = 0.992: 0.008: 0.5: 0.47: 0.03: 1.

[0083] Comparative Example 1

[0084] This comparative example prepared a LiMn 0.5 Fe 0.5 PO 4The lithium iron manganese phosphate-based positive electrode material of / C is different from that of Example 1 in that:

[0085] In step S1, the addition amount of the raw materials is lithium: sodium: manganese: iron: lanthanum: phosphorus = 1.0:0:0.5:0.5:0:1. That is, no sodium source and lanthanum source are added.

[0086] Comparative Example 2

[0087] This comparative example prepared a designed chemical formula of Li 0.998 Na 0.002 Mn 0.5 Fe 5 PO 4 The lithium iron manganese phosphate-based positive electrode material of / C is different from that of Example 1 in that:

[0088] In step S1, the addition amount of the raw materials is lithium: sodium: manganese: iron: lanthanum: phosphorus = 0.998:0.002:0.5:0.5:0:1.

[0089] Comparative Example 3

[0090] This comparative example prepared a designed chemical formula of Li 0.996 Na 0.004 Mn 0.5 Fe 5 PO 4 The lithium iron manganese phosphate-based positive electrode material of / C is different from that of Comparative Example 2 in that:

[0091] In step S1, the addition amount of the raw materials is lithium: sodium: manganese: iron: lanthanum: phosphorus = 0.996: 0.004: 0.5: 0.5: 0: 1.

[0092] Comparative Example 4

[0093] This comparative example prepared a designed chemical formula of Li 0.994 Na 0.006 Mn 0.5 Fe 5 PO 4 The lithium iron manganese phosphate-based positive electrode material of / C is different from that of Comparative Example 2 in that:

[0094] In step S1, the addition amount of the raw materials is lithium: sodium: manganese: iron: lanthanum: phosphorus = 0.994: 0.006: 0.5: 0.5: 0: 1.

[0095] Comparative Example 5

[0096] This comparative example prepared a designed chemical formula of Li 0.992 Na 0.008 Mn 0.5 Fe 5 PO4 The lithium iron manganese phosphate-based positive electrode material of / C is different from that of Comparative Example 2 in that:

[0097] In step S1, the addition amount of the raw materials is lithium: sodium: manganese: iron: lanthanum: phosphorus = 0.992: 0.008: 0.5: 0.5: 0: 1.

[0098] Comparative Example 6

[0099] This comparative example prepared a designed chemical formula of Li 0.99 Na 0.01 Mn 0.5 Fe 5 PO 4 The lithium iron manganese phosphate-based positive electrode material of / C is different from that of Comparative Example 2 in that:

[0100] In step S1, the addition amount of the raw materials is lithium: sodium: manganese: iron: lanthanum: phosphorus = 0.99: 0.01: 0.5: 0.5: 0: 1.

[0101] Comparative Example 7

[0102] This comparative example prepared a designed chemical formula of Li 0.98 Na 0.02 Mn 0.5 Fe 5 PO 4 The lithium iron manganese phosphate-based positive electrode material of / C is different from that of Comparative Example 2 in that:

[0103] In step S1, the addition amount of the raw materials is lithium: sodium: manganese: iron: lanthanum: phosphorus = 0.98:0.02:0.5:0.5:0:1.

[0104] Comparative Example 8

[0105] This comparative example prepared a designed chemical formula of Li 0.97 Na 0.03 Mn 0.5 Fe 5 PO 4 The lithium iron manganese phosphate-based positive electrode material of / C is different from that of Comparative Example 2 in that:

[0106] In step S1, the addition amount of the raw materials is lithium: sodium: manganese: iron: lanthanum: phosphorus = 0.97: 0.03: 0.5: 0.5: 0: 1.

[0107] Comparative Example 9

[0108] This comparative example prepared a designed chemical formula of Li 0.96 Na 0.04 Mn 0.5 Fe 5 PO4 The lithium iron manganese phosphate-based positive electrode material of / C is different from that of Comparative Example 2 in that:

[0109] In step S1, the addition amount of the raw materials is lithium: sodium: manganese: iron: lanthanum: phosphorus = 0.96: 0.04: 0.5: 0.5: 0: 1.

[0110] Comparative Example 10

[0111] This comparative example prepared a LiMn 0.5 Fe 0.495 La 0.005 PO 4 / C manganese iron phosphate lithium-based positive electrode material, compared with Example 1, the difference is:

[0112] In step S1, the addition amount of the raw materials is lithium: sodium: manganese: iron: lanthanum: phosphorus = 0.1:0:0.5:0.495:0.005:1.

[0113] Comparative Example 11

[0114] This comparative example prepared a LiMn 0.5 Fe 0.49 La 0.01 PO 4 / C manganese iron phosphate lithium-based positive electrode material, compared with comparative example 10, the difference is:

[0115] In step S1, the addition amount of the raw materials is lithium: sodium: manganese: iron: lanthanum: phosphorus = 0.1:0:0.5:0.49:0.01:1.

[0116] Comparative Example 12

[0117] This comparative example prepared a LiMn 0.5 Fe 0.48 La 0.02 PO 4 / C manganese iron phosphate lithium-based positive electrode material, compared with comparative example 10, the difference is:

[0118] In step S1, the addition amount of the raw materials is lithium: sodium: manganese: iron: lanthanum: phosphorus = 0.1:0:0.5:0.48:0.02:1.

[0119] Comparative Example 13

[0120] This comparative example prepared a LiMn 0.5 Fe 0.47 La 0.03 PO 4 / C manganese iron phosphate lithium-based positive electrode material, compared with comparative example 10, the difference is:

[0121] In step S1, the addition amount of the raw materials is lithium: sodium: manganese: iron: lanthanum: phosphorus = 0.1:0:0.5:0.47:0.03:1.

[0122] Comparative Example 14

[0123] This comparative example prepared a LiMn 0.5 Fe 0.46 La 0.04 PO 4 / C manganese iron phosphate lithium-based positive electrode material, compared with comparative example 10, the difference is:

[0124] In step S1, the addition amount of the raw materials is lithium: sodium: manganese: iron: lanthanum: phosphorus = 0.1:0:0.5:0.46:0.04:1.

[0125] Comparative Example 15

[0126] This comparative example prepared a LiMn 0.5 Fe 0.45 La 0.05 PO 4 / C manganese iron phosphate lithium-based positive electrode material, compared with comparative example 10, the difference is:

[0127] In step S1, the addition amount of the raw materials is lithium: sodium: manganese: iron: lanthanum: phosphorus = 0.1:0:0.5:0.45:0.05:1.

[0128] Comparative Example 16

[0129] This comparative example prepared a LiMn 0.5 Fe 0.5 PO 4 The lithium iron manganese phosphate-based positive electrode material of / C is different from that of Comparative Example 1 in that:

[0130] In step S1, dry grinding is not performed; the duration of wet grinding is 4 hours.

[0131] Application Examples

[0132] This example provides a lithium-ion battery, the positive electrode material of which is the lithium iron manganese phosphate-based positive electrode material obtained in Examples 1 to 4 or Comparative Examples 1 to 16. Specifically:

[0133] (1) Preparation of positive electrode slurry: The lithium manganese iron phosphate-based positive electrode material obtained in the embodiment and the comparative example is used as the positive electrode active material, superconducting carbon black is used as the conductive agent, and PVDF is used as the binder, and the mixture is mixed in a mass ratio of 8:1:1, and a certain amount of organic solvent NMP is added and mixed and stirred evenly to prepare a positive electrode slurry;

[0134] (2) Coating: The stirred positive electrode slurry is evenly coated on the carbon-coated aluminum foil, dried at 60°C, and finally cut into discs with a diameter of 12 mm using a cutting machine to obtain positive electrode sheets.

[0135] (3) Assembly: The above-mentioned positive electrode sheet was used as the positive electrode and the metal lithium sheet was used as the negative electrode in a glove box filled with inert gas. Celgard 2500 was used as the separator and 1M LiPF 6 (using a mixture of EC, EMC and DMC in a volume ratio of 1:1:1 as a solvent) as an electrolyte, assembling a half-cell to prepare a lithium manganese iron phosphate button cell.

[0136] Test example

[0137] In the first aspect of this test example, undoped LiMn 0.5 Fe 0.5 PO 4 The microscopic morphology of the / C material (Comparative Example 1) is shown in Figure 1. The results show that the prepared material is in the form of nano-blocks with irregular morphology and a particle size range of 50nm to 200nm. Figure 1 and Figure 2 shown.

[0138] In the second aspect of this test example, the XRD of the lithium iron manganese phosphate-based positive electrode materials obtained in Examples 1 to 4, Comparative Examples 1 to 7 and Comparative Examples 10 to 14 was tested and refined. The refinement results are shown in Table 1.

[0139] Table 1 XRD refinement results of lithium manganese iron phosphate positive electrode materials obtained in some examples and comparative examples

[0140]

[0141]

[0142] The results in Table 1 show that Na does tend to be doped in the lithium layer, and La tends to be doped in the transition metal layer. The doping of Na and La can effectively increase the unit cell volume of the lithium iron manganese phosphate-based positive electrode material, and thus may effectively improve the rate performance of the obtained lithium iron manganese phosphate-based positive electrode material.

[0143] The second aspect of this test example tests the rate performance of the button cell obtained in the application example. The specific test results are shown in Table 2 and Figure 3 As shown in Table 2, from left to right are the rates used in the tests, and each rate was tested for 5 weeks and the average value was taken. In all electrochemical tests, 1C was set to 170mA / g.

[0144] Table 2 Rate performance test results of batteries assembled in the embodiment and comparative example at room temperature

[0145]

[0146]

[0147] From Table 2 and Figure 3 It can be seen that compared with Comparative Example 1, in Comparative Examples 2 to 9, the doping of a single sodium can effectively improve the rate performance of the material, and Comparative Example 5 has the best rate performance. This is because the ion radius of sodium is larger than that of lithium ions. After sodium occupies the lithium site, it widens the lithium interlayer spacing, thereby improving the lithium ion diffusion coefficient. If the amount of sodium source doping is too low, the lithium site is occupied too little, the lithium interlayer spacing is limited, and the lithium ion diffusion is limited. If the amount of sodium source doping is too high, the lithium site is occupied excessively, which leads to a reduction in the amount of lithium ions that can be transferred, resulting in a decrease in the discharge capacity of the material.

[0148] Compared with Comparative Example 1, in Comparative Examples 10-15, the doping of a single lanthanum can also effectively improve the rate performance of the material, and Comparative Example 13 has the best rate performance, and is better than Comparative Example 5. This is because the ion radius of lanthanum is larger than that of sodium ions. After doping at the manganese iron site, it can also broaden the lithium ion diffusion channel, and the effect is better than sodium doping. If the doping amount of lanthanum source is too high, it is easy to cause the material structure to collapse, reducing the discharge capacity and cycle stability of the material.

[0149] Compared with Examples 1 to 4 and Comparative Examples 1 to 15, dual-site co-doping can effectively improve the rate performance of lithium manganese iron phosphate materials, and the improved rate performance is between Comparative Example 5 and Comparative Example 13.

[0150] The third aspect of this test example tests the 1C cycle performance of Example 2 and Comparative Example 1, Comparative Example 5 and Comparative Example 13. The specific test results are shown in Table 3 and Figure 4 shown.

[0151] Table 3 1C cycle performance test results of batteries assembled in some embodiments and comparative examples at room temperature

[0152]

[0153] From Table 3 and Figure 4 It can be seen that although the first discharge specific capacity of Example 2 at 1C rate is slightly lower than that of Comparative Example 5 and Comparative Example 13, its discharge specific capacity after 500 cycles is much higher than that of Comparative Example 5 and Comparative Example 13, and the capacity retention rate is 89.06%. Although Comparative Example 13 has a higher initial capacity, which is due to the improved rate performance brought by lanthanum doping, the lanthanum element with a larger ion radius doped at the manganese iron site is likely to cause the structural collapse of the material during the cycle, so the cycle stability is poor.

[0154] The fourth aspect of this test example tests the 10C cycle performance of Example 2 and Comparative Examples 1, 5 and 13. The specific test results are shown in Table 4 and Figure 5 shown.

[0155] Table 4 10C cycle performance test results of batteries assembled in some embodiments and comparative examples at room temperature

[0156]

[0157] From Table 4 and Figure 5 It can be seen that the first discharge specific capacity of Example 2 at 10C rate is also lower than that of Comparative Example 5 and Comparative Example 13, but its discharge specific capacity after 1000 cycles is much higher than that of Comparative Example 5 and Comparative Example 13, and the capacity retention rate is 89.74%. The excellent cycle stability exhibited by Example 2 is due to the ion synergistic effect caused by the dual doping of sodium and lanthanum at the lithium site and manganese iron site, which stabilizes the crystal structure and prevents the structural collapse caused by the material during the cycle, thereby improving the cycle stability.

[0158] The fifth aspect of this test example tests the rate performance of Example 2 and Comparative Examples 1, 5 and 13 at a high temperature of 45°C. The specific test results are shown in Tables 5 and Figure 6 shown.

[0159] Table 5 Rate performance test results of batteries assembled in some embodiments and comparative examples at 45°C

[0160]

[0161] The sixth aspect of this test example tests the 1C cycle performance of Example 2 and Comparative Example 13 at a high temperature of 45°C. The specific test results are shown in Tables 6 and Figure 7 shown.

[0162] Table 6 1C cycle performance test results of batteries assembled in some embodiments and comparative examples at 45°C

[0163]

[0164] From Table 5, Table 6, Figure 6 and Figure 7 It can be seen that Example 2 has the best rate performance and the best cycle stability at a high temperature of 45°C. The first discharge specific capacity at a rate of 1C is 147.9 mAh / g, and the capacity retention rate after 200 cycles is as high as 98.24%, indicating that the excellent performance brought by dual-site co-doping is also applicable at high temperatures.

[0165] The present invention has a great influence on the electrochemical properties of lithium iron manganese phosphate materials through dual-site co-doping. and the ionic radius of lanthanum The ionic radius of lithium Therefore, after sodium is doped at the lithium site, the lattice spacing of the lithium layer can be increased, the lithium ion transport path can be widened, and thus the lithium ion diffusion coefficient can be improved; after lanthanum is doped at the manganese iron site, the transition metal layer spacing is increased, the lithium ion transport path is further widened, and the lithium ion diffusion coefficient is further increased; after co-doping at the double site, the synergistic effect between ions can be used to stabilize the crystal structure and improve the cycle stability. The electrochemical test results show that not only the rate performance of Example 2 is effectively improved, but also the cycle stability is significantly improved. After 500 cycles at a rate of 1C, Example 2 still has a capacity retention rate of 89.06%; even after 1000 cycles at a high rate of 10C, Example 2 still has a capacity retention rate of 89.74%; it also has better electrochemical performance at a high temperature of 45°C, and the 1C first discharge specific capacity can reach 147.9mAh / g, and after 200 cycles, it can still have a capacity retention rate of 98.24%. The results show that the present invention provides a simple and effective method for improving the electrochemical properties of lithium manganese iron phosphate positive electrode materials.

[0166] The above description of the embodiments is to facilitate the understanding and use of the invention by those skilled in the art. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments, and improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the present invention should be within the scope of protection of the present invention.

Claims

1. A dual-site co-doped lithium manganese iron phosphate-based positive electrode material, characterized in that: The lithium iron manganese phosphate-based positive electrode material includes lithium iron manganese phosphate co-doped with sodium and lanthanum; In the lithium manganese iron phosphate-based positive electrode material, the molar ratio of lithium to sodium is (1-x):x, and the value range of x is 0.002 to 0.

02.

2. The lithium iron manganese phosphate-based positive electrode material according to claim 1, characterized in that: The lithium manganese iron phosphate-based positive electrode material also includes a carbon layer; the carbon layer wraps the lithium manganese iron phosphate.

3. A method for preparing the lithium iron manganese phosphate-based positive electrode material according to claim 1 or 2, characterized in that: The preparation method comprises the following steps: S1. Take a lithium source, a sodium source, a manganese source, an iron source, a lanthanum source and a phosphorus source, and dry-mill them; add an organic solvent and wet-mill them, and remove the organic solvent to obtain a precursor; the molar ratio of lithium in the lithium source to sodium in the sodium source is (1-x):x; and the value range of x is 0.002 to 0.02; S2. calcining the precursor at high temperature under the protection of an inert gas atmosphere.

4. The preparation method according to claim 3, characterized in that: In step S1, the raw materials for dry ball milling also include a carbon source; Preferably, the carbon source is at least one of glucose, sucrose, fructose, cellulose, starch, citric acid, polyacrylic acid, polyethylene glycol and dopamine; Preferably, the added amount of the carbon source is 8% to 16% of the total mass of other preparation raw materials.

5. The preparation method according to claim 3, characterized in that: In step S1, the lithium source is at least one of lithium carbonate, lithium hydroxide and lithium dihydrogen phosphate; Preferably, in step S1, the sodium source is at least one of sodium carbonate, sodium chloride and sodium sulfate; Preferably, the manganese source is at least one of manganese sulfate, manganese chloride, manganese carbonate and manganese oxalate; Preferably, the iron source is at least one of ferrous sulfate, ferrous chloride and ferrous oxalate; Preferably, the lanthanum source is at least one of lanthanum oxide, lanthanum chloride and lanthanum carbonate; Preferably, the phosphorus source is at least one of diammonium hydrogen phosphate, ammonium dihydrogen phosphate, phosphoric acid, lithium dihydrogen phosphate and pyrophosphoric acid.

6. The preparation method according to any one of claims 3 to 5, characterized in that: In step S1, the molar ratio of manganese in the manganese source, iron in the iron source and lanthanum in the lanthanum source is 0.5:(0.5-y):y, and the value range of y is 0.005 to 0.05; Preferably, in the lithium source, sodium source, manganese source, iron source, lanthanum source and phosphorus source, the molar sum of sodium and lithium, the molar sum of manganese, iron and lanthanum, and the molar amount ratio of phosphorus is 1:0.8-1.2:0.8-1.

2.

7. The preparation method according to any one of claims 3 to 5, characterized in that: In step S1, in the wet ball milling, the solid content of the raw material prepared is 50% to 80%.

8. The preparation method according to any one of claims 3 to 5, characterized in that: In step S1, the dry ball milling time is 1 to 6 hours; Preferably, in step S1, the duration of the wet ball milling is 1 to 6 hours.

9. The preparation method according to any one of claims 3 to 5, characterized in that: In step S2, the inert atmosphere includes a nitrogen atmosphere and / or an argon atmosphere; Preferably, in step S2, the high temperature calcination temperature is 500-800°C; Preferably, in step S2, the high temperature calcination time is 4 to 10 hours; Preferably, in step S2, the heating rate of the high temperature calcination is 2-10°C / min.

10. A lithium ion battery, characterized in that: The raw materials for preparing the lithium-ion battery include the lithium iron manganese phosphate-based positive electrode material according to claim 1 or 2.

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