A coated lithium manganese oxide cathode material and its preparation method

Abstract

The present invention discloses a coated lithium manganese oxide cathode material and a preparation method thereof, belonging to the technical field of lithium ion batteries. The preparation method first obtains a primary sintered material by using a lithium compound, a manganese compound and a doping compound containing a doping element, wherein the molar ratio of lithium, manganese and the doping element is a: 2 - b: b, 1 ≤ a ≤ 1.2, 0 < b ≤ 0.1; then the primary sintered material is subjected to surface coating treatment with a sodium compound to obtain a coated product, wherein the ratio of the sodium compound to the primary sintered material makes the molar ratio of sodium to manganese be c: 2 - b - c, 0 < c ≤ 0.1; and then the coated product is sintered and post-treated in sequence to obtain the coated lithium manganese oxide cathode material, and its particle surface has a sodium manganate coating layer. By forming a sodium manganate Na2MnO3 coating layer on the surface of the lithium manganese oxide cathode material particles, the present invention can significantly improve the conductivity and structural stability of the lithium manganese oxide structure, and further improve the rate performance and cycle performance of the cathode material.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and particularly relates to a coated lithium manganese oxide cathode material and a preparation method thereof. Background Art

[0002] The lithium manganese oxide cathode material with a spinel structure is considered to be one of the most ideal cathode materials for power lithium ion batteries. However, its disadvantage is poor high-temperature (above 55 °C) cycling performance, and serious specific capacity attenuation occurs during the charge and discharge cycling process. After a large number of studies, it is confirmed that the main reasons are as follows: A small amount of water in the electrolyte reacts with LiPF6 in the electrolyte to generate HF, resulting in 3+ Mn to undergo disproportionation reaction to generate Mn 4+ and Mn 2+ . Mn 2+ will dissolve, and the dissolution rate is faster at high temperatures, destroying the spinel structure of lithium manganese oxide, resulting in poor high-temperature cycling performance. At the same time, when the average valence state of Mn is lower than +3.5, the Jahn-Teller effect will be induced, and the crystal structure will be distorted, changing from a cubic system to a tetragonal system, resulting in lattice distortion, enhancing the polarization effect of the electrode, and causing specific capacity attenuation.

[0003] Currently, the mainstream improvement methods in the industry are, firstly, through surface modification to reduce the direct contact between lithium manganese oxide and the electrolyte, thereby reducing the dissolution of Mn in the electrolyte and improving the high-temperature cycling performance; secondly, through bulk doping to improve the stability of the spinel structure during cycling to inhibit the occurrence of the Jahn-Teller effect and improve the specific capacity attenuation phenomenon. However, the resulting impurity phase Li2MnO3 will reduce the conductivity and structural stability of the lithium manganese oxide structure, and further reduce the rate performance and cycling performance of the lithium manganese oxide cathode material. Summary of the Invention

[0004] The main technical problem to be solved by the present invention is to provide a coated lithium manganese oxide cathode material and a preparation method thereof, which can improve the rate performance and cycling performance of the lithium manganese oxide cathode material.

[0005] To solve the above technical problem, a technical solution adopted by the present application is: to provide a preparation method of a coated lithium manganese oxide cathode material, including:

[0006] Obtaining a primary sintered material by using a lithium compound, a manganese compound, and a doping compound containing a doping element, wherein the ratio of the lithium compound, the manganese compound, and the doping compound makes the molar ratio of lithium, manganese, and the doping element a: 2 - b: b, 1 ≤ a ≤ 1.2, 0 < b ≤ 0.1;

[0007] The primary sintered material is surface-coated with a sodium compound to obtain a coated product, where the ratio of the sodium compound to the primary sintered material is such that the molar ratio of sodium to manganese is c:2 - b - c, and 0 < c ≤ 0.1;

[0008] The coated product is sintered and post-treated in sequence to obtain a coated lithium manganese oxide cathode material, and the surface of the particles of the coated lithium manganese oxide cathode material has a sodium manganate coating layer.

[0009] To solve the above technical problems, another technical solution adopted in this application is: to provide a coated lithium manganese oxide cathode material prepared by the preparation method according to the above technical solution, where the chemical general formula of the coated lithium manganese oxide cathode material is Li a Mn 2-b-c M b Na c O4, M represents a doping element, 1 ≤ a ≤ 1.2, 0 < b ≤ 0.1, 0 < c ≤ 0.1, and the surface of the particles of the coated lithium manganese oxide cathode material has a sodium manganate coating layer.

[0010] Compared with the prior art, the beneficial effects of the present invention are: in the preparation method provided by the present invention, a sodium manganate Na2MnO3 coating layer is formed on the surface of the particles of the lithium manganese oxide cathode material, preventing the formation of the impurity phase Li2MnO3. Na2MnO3 in this coating layer has a layered structure, including a transition metal (TM) layer and a Na layer. In the TM layer, Na, Mn, and O atoms are arranged, and only Na atoms are arranged in the Na layer. Li2MnO3 has the same structure, except that Na is replaced by Li. The bond lengths of the Na - O bonds in the TM layer of Li2MnO3 and the Na - O bonds between Na in the Na layer and O in the TM layer are not very different, while the Na - O bonds in the TM layer of Na2MnO3 are much shorter than the Na - O bonds between Na in the Na layer and O in the TM layer, that is, the Na - O interaction in the TM layer of Na2MnO3 is stronger than the Na - O interaction in the Na layer. This makes the Na in the TM layer have a strong capture effect and is not easily captured and leave its original position, which is beneficial to maintaining the layered structure during the charge and discharge cycle. Moreover, the band gap (1.18 eV) of Na2MnO3 is smaller than the band gap (1.62 eV) of Li2MnO3, and it can be expected that Na2MnO3 has higher conductivity. Therefore, by forming a sodium manganate Na2MnO3 coating layer on the surface of the particles of the lithium manganese oxide cathode material, the conductivity and structural stability of the lithium manganese oxide structure can be significantly improved, and further the rate performance and cycle performance of the cathode material can be improved. Description of the Drawings

[0011] To more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings. Among them:

[0012] Figure 1 It is a schematic flow chart of an embodiment of the preparation method of the present invention;

[0013] Figure 2 It is the SEM image corresponding to the comparative example;

[0014] Figure 3 It is the SEM image corresponding to Example 6;

[0015] Figure 4 It is the XRD pattern corresponding to the comparative example and Examples 1-6 respectively;

[0016] Figure 5 It is the comparison of the XPS spectra of the comparative example and Example 6. Specific embodiments

[0017] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the embodiments 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, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0018] Please refer to Figure 1 , Figure 1 It is a schematic flow chart of an embodiment of the preparation method of the coated lithium manganese oxide cathode material of the present invention. The preparation method includes the following steps S11-S13.

[0019] Step S11, obtaining a primary sintered material by using a lithium compound, a manganese compound, and a doping compound containing a doping element, wherein the ratio of the lithium compound, the manganese compound, and the doping compound makes the molar ratio of lithium, manganese, and the doping element be a:2-b:b, 1≤a≤1.2, 0<b≤0.1.

[0020] Among them, the lithium compound is at least one of lithium hydroxide, lithium carbonate, and lithium oxalate; the manganese compound is manganese dioxide and / or manganese tetroxide; the doping element is at least one of Li, Mg, Al, Ni, Co, Zr, Ti, Y, W, Mo, Sr, Ba, B, La, Ce, Er, and Nb; and the doping compound is an oxide, hydroxide, or carbonate of the doping element. The specific dosages of the corresponding compounds are calculated inversely according to the molar ratios of the required lithium, manganese, and doping elements, and then the lithium compound, manganese compound, and doping compound are mixed evenly according to the ratio, and sintered at 650-950 °C (such as 700 °C, 750 °C, 800 °C, 850 °C, 900 °C, etc.), and after cooling, it is subjected to a crushing treatment to obtain a primary sintered material. At this time, crystalline lithium manganese oxide particles have been formed.

[0021] Step S12: Surface-coat the primary sintered material with a sodium compound to obtain a coated product, where the ratio of the sodium compound to the primary sintered material is such that the molar ratio of sodium to manganese is c:2-b-c, and 0 < c ≤ 0.1.

[0022] Among them, the sodium compound includes at least one of sodium hydroxide, sodium carbonate, sodium bicarbonate, and sodium acetate.

[0023] Specifically, the surface coating treatment can be a dry coating, that is: the sodium compound and the primary sintered material are placed in a mixing tank according to the required ratio and mixed evenly to obtain a coated product.

[0024] The surface coating treatment can also be a wet coating, that is: the sodium compound and the primary sintered material are made into a solution and dried by spray drying to obtain a coated product.

[0025] Step S13: Sinter and post-treat the coated product in sequence to obtain a coated lithium manganese oxide cathode material, and the particle surface of the coated lithium manganese oxide cathode material has a sodium manganate coating layer.

[0026] After surface-coating the primary sintered material with a sodium compound, first sinter the coated product to form a sodium manganate coating layer on the surface of the crystalline lithium manganese oxide particles, and then perform post-treatment processes such as crushing and sieving to obtain a coated lithium manganese oxide cathode material. When sintering the coated product, the sintering temperature is 300-800 °C, such as 350 °C, 400 °C, 450 °C, 500 °C, 550 °C, 600 °C, 650 °C, 700 °C, 750 °C, etc.

[0027] The surface of the particles of the coated lithium manganate cathode material obtained by the preparation method provided in this embodiment has a sodium manganate Na2MnO3 coating layer, which prevents the formation of the impurity phase Li2MnO3. Na2MnO3 in this coating layer has a layered structure, including a transition metal (TM) layer and a Na layer. In the TM layer, Na, Mn, and O atoms are arranged, and only Na atoms are arranged in the Na layer. Li2MnO3 has the same structure, except that Na is replaced by Li. The bond lengths of the Na-O bonds in the TM layer of Li2MnO3 and the Na-O bonds between Na in the Na layer and O in the TM layer are not very different, while the Na-O bonds in the TM layer of Na2MnO3 are much shorter than the Na-O bonds between Na in the Na layer and O in the TM layer. That is, the Na-O interaction in the TM layer of Na2MnO3 is stronger than the Na-O interaction in the Na layer, which makes Na in the TM layer have a stronger capture effect and is not easily captured and leave its original position, which is beneficial to maintaining the layered structure during the charge and discharge cycle. Moreover, first-principles calculations show that the band gap (1.18 eV) of Na2MnO3 is smaller than the band gap (1.62 eV) of Li2MnO3, and it can be expected that Na2MnO3 has higher conductivity. Therefore, by forming a sodium manganate Na2MnO3 coating layer on the surface of the lithium manganate cathode material particles, the conductivity and structural stability of the lithium manganate structure can be significantly improved, and further the rate performance and cycle performance of the cathode material can be improved during the charge and discharge cycle.

[0028] Based on the same inventive concept, the present invention also provides a coated lithium manganate cathode material prepared by the preparation method according to the above embodiment. The chemical general formula of this cathode material is Li a Mn 2-b-c M b Na c O4, where M represents a doping element, 1≤a≤1.2, 0<b≤0.1, 0<c≤0.1, and the surface of the particles of this coated lithium manganate cathode material has a sodium manganate coating layer. This embodiment has the same beneficial effects as the above embodiment, which will not be elaborated here.

[0029] The following combines specific examples and comparative examples to illustrate the performance improvement brought by the technical solution of the present invention.

[0030] Comparative example:

[0031] Weigh a certain amount of lithium carbonate, manganese tetroxide with a D50 of 9 μm, and the doping substance MgCO3 and place them in a mixing tank for mixing. The molar ratio of Li, Mn, and Mg is 1.15:1.96:0.04. Sinter the uniformly mixed material at 850 °C, naturally cool it to room temperature, crush it, and screen it to obtain the primary sintered material Li 1.15 Mn 1.96 Mg 0.04 O4, which is the comparative example. Its SEM photo is asFigure 2 as shown

[0032] Example 1 was prepared by the following steps:

[0033] Step S1: Weigh a certain amount of lithium carbonate, manganese tetroxide with D50 of 9 μm, and doping substance MgCO3 and place them in a mixing tank for mixing. The molar ratio of Li, Mn, and Mg is 1.15:1.96:0.04. Sinter the uniformly mixed material at 850 °C, naturally cool it to room temperature, crush it, and sieve it to obtain the first sintered material;

[0034] Step S2: Mix the first sintered material in Step S1 with the sodium compound NaOH in a mixing tank to obtain a coated product, where the molar ratio of Mn to Na is 1.92:0.04;

[0035] Step S3: Sinter the coated product in Step S2 at 550 °C, naturally cool it to room temperature, and then perform mechanical crushing to obtain the coated lithium manganese oxide cathode material Li 1.15 Mn 1.92 Mg 0.04 Na 0.04 O4.

[0036] Example 2 was prepared by the following steps:

[0037] Step S1: Weigh a certain amount of lithium carbonate, manganese tetroxide with D50 of 9 μm, and doping substance MgCO3 and place them in a mixing tank for mixing. The molar ratio of Li, Mn, and Mg is 1.15:1.96:0.04. Sinter the uniformly mixed material at 850 °C, naturally cool it to room temperature, crush it, and sieve it to obtain the first sintered material;

[0038] Step S2: Mix the first sintered material in Step S1 with the sodium compound NaOH in a mixing tank to obtain a coated product, where the molar ratio of Mn to Na is 1.88:0.08;

[0039] Step S3: Sinter the coated product in Step S2 at 550 °C, naturally cool it to room temperature, and then perform mechanical crushing to obtain the coated lithium manganese oxide cathode material Li 1.15 Mn 1.88 Mg 0.04 Na 0.08 O4.

[0040] That is, the difference between Example 2 and Example 1 is that in Step S2, the molar ratio of Mn to Na is 1.88:0.08, and the finally obtained coated lithium manganese oxide cathode material is Li 1.15 Mn 1.88 Mg 0.04 Na 0.08 O4.

[0041] Example 3 was prepared by the following steps:

[0042] Step S1: Weigh a certain amount of lithium carbonate, manganese tetroxide with a D50 of 9 μm, and the doping substance MgCO3, and place them in a mixing tank for mixing. The molar ratio of Li, Mn, and Mg is 1.15:1.96:0.04. Sinter the uniformly mixed material at 850 °C, naturally cool it to room temperature, crush it, and screen it to obtain the first sintered material.

[0043] Step S2: Mix the first sintered material in Step S1 with the sodium compound Na2CO3 in a mixing tank to obtain a coated product, where the molar ratio of Mn to Na is 1.88:0.08.

[0044] Step S3: Sinter the coated product in Step S2 at 550 °C, naturally cool it to room temperature, and then perform mechanical crushing to obtain the coated lithium manganese oxide cathode material Li 1.15 Mn 1.88 Mg 0.04 Na 0.08 O4.

[0045] That is, the difference between Example 3 and Example 2 is that: the sodium compound in Step S2 is Na2CO3.

[0046] Example 4 was prepared by the following steps:

[0047] Step S1: Weigh a certain amount of lithium carbonate, manganese tetroxide with a D50 of 9 μm, and the doping substance MgCO3, and place them in a mixing tank for mixing. The molar ratio of Li, Mn, and Mg is 1.15:1.96:0.04. Sinter the uniformly mixed material at 850 °C, naturally cool it to room temperature, crush it, and screen it to obtain the first sintered material.

[0048] Step S2: Mix the first sintered material in Step S1 with the sodium compound NaHCO3 in a mixing tank to obtain a coated product, where the molar ratio of Mn to Na is 1.88:0.08.

[0049] Step S3: Sinter the coated product in Step S2 at 550 °C, naturally cool it to room temperature, and then perform mechanical crushing to obtain the coated lithium manganese oxide cathode material Li 1.15 Mn 1.88 Mg 0.04 Na 0.08 O4.

[0050] That is, the difference between Example 4 and Example 2 is that: the sodium compound in Step S2 is NaHCO3.

[0051] Example 5 was prepared by the following steps:

[0052] Step S1: Weigh a certain amount of lithium carbonate, manganese tetroxide with a D50 of 9 μm, and doping substance MgCO3, and place them in a mixing tank for mixing. The molar ratio of Li, Mn, and Mg is 1.15:1.96:0.04. Sinter the uniformly mixed material at 850 °C, naturally cool it to room temperature, crush it, and screen it to obtain the first sintered material.

[0053] Step S2: Mix the first sintered material in Step S1 with the sodium compound CH3COONa in a mixing tank to obtain a coated product, where the molar ratio of Mn to Na is 1.88:0.08.

[0054] Step S3: Sinter the coated product in Step S2 at 550 °C, naturally cool it to room temperature, and then perform mechanical crushing to obtain the coated lithium manganese oxide cathode material Li 1.15 Mn 1.88 Mg 0.04 Na 0.08 O4.

[0055] That is, the difference between Example 5 and Example 2 is that: in Step S2, the sodium compound is CH3COONa.

[0056] Example 6 is prepared by the following steps:

[0057] Step S1: Weigh a certain amount of lithium carbonate, manganese tetroxide with a D50 of 9 μm, and doping substance MgCO3, and place them in a mixing tank for mixing. The molar ratio of Li, Mn, and Mg is 1.15:1.96:0.04. Sinter the uniformly mixed material at 850 °C, naturally cool it to room temperature, crush it, and screen it to obtain the first sintered material.

[0058] Step S2: Mix the first sintered material in Step S1 with the sodium compound CH3COONa in water to form a suspension, and then spray-dry the suspension to obtain a coated product, where the molar ratio of Mn to Na is 1.88:0.08.

[0059] Step S3: Sinter the coated product in Step S2 at 550 °C, naturally cool it to room temperature, and then perform mechanical crushing to obtain the coated lithium manganese oxide cathode material Li 1.15 Mn 1.88 Mg 0.04 Na 0.08 O4, and its SEM photograph is as Figure 3 shown.

[0060] That is, the difference between Example 6 and Example 5 is that: in Step S2, the coated product is obtained by wet coating.

[0061] The chemical compositions and main process conditions of the comparative example and Examples 1-6 are shown in Table 1, and the corresponding XRD patterns are as Figure 4 shown.

[0062] From Figure 2 and Figure 3 it can be seen that the final product morphologies of the comparative example and Example 6 are polycrystalline particles with good uniform sphericity. From Figure 4 it can be seen that the lithium manganate phases without impurity peaks are formed in both the comparative example and Examples 1-6. Among them, after coating in Examples 1-6, lithium manganate materials coated with sodium manganate are formed. The XRD peaks circled by the left dashed box correspond to the sodium manganate phases of Examples 1-6.

[0063]

[0064] The materials of the comparative example and Examples 1-6 were evaluated as follows: CR2025 coin cells were made according to the ratio of SP:PVDF:active material = 5:3:92, and the tap density of the electrode was 3.0 - 3.2 g / cm 3 , and the specific test process was a working voltage of 3 - 4.3 V, charge and discharge at 0.1C / 0.1C and 0.5C / 2C, and the CV cut-off current for charging was 0.01C. The room temperature capacity at 0.1C and 2C and the 50-cycle 0.5C cycle retention rate at 45°C were investigated. The test for manganese dissolution was to test the amount of Mn dissolved after the lithium manganate powder was placed in the electrolyte at 60°C for 7 days. XPS analysis was used to analyze the element contents on the surface and at different etched depths of the comparative example and each example. Table 2 contains the test results of the comparative example and Examples 1-6.

[0065]

[0066] As can be seen from Tables 1-2, by comparing Examples 1-6 with the comparative example, it can be seen that by forming a sodium manganate coating layer on the surface of the lithium manganate cathode material particles, the high-temperature cycle performance of lithium manganate can be improved, manganese dissolution can be reduced, and the rate performance can be improved.

[0067] By comparing Example 2 with Example 1, it can be seen that increasing the amount of sodium manganate coating can improve the high-temperature cycle performance of lithium manganate. As the amount of sodium manganate coating increases, more sodium manganate with good conductivity and structural stability is formed on the particle surface, which further improves the high-temperature cycle performance of lithium manganate, reduces manganese dissolution, and improves the rate performance.

[0068] By comparing Examples 2-5, it can be seen that CH3COONa has the best effect as the coating source of sodium manganate, which can improve the high-temperature cycle performance of lithium manganate, reduce manganese dissolution, and improve the rate performance.

[0069] Comparing Comparative Example 6 with Example 5, it can be seen that the wet coating method has a better coating effect than the dry coating method. During the wet coating process compared with the dry coating process, the coating effect of the Na source on the particle surface is more uniform, forming a more uniform sodium manganate coating layer, which has the effects of improving the high-temperature cycling performance of lithium manganate, reducing manganese dissolution, and improving the rate performance.

[0070] Furthermore, the Na content analysis of the XPS tests with different depths of etching corresponding to the comparative example and Example 6 is shown in Figure 5 . It can be seen from Figure 5 that the surface Na content of Example 6 is very high, mainly existing in the depth range of 0 to 480 nm, that is to say, the formed sodium manganate is mainly coated on the surface of the material.

[0071] The above is only the implementation mode of the present invention, and does not limit the patent scope of the present invention accordingly. Any equivalent structure or equivalent process transformation made by using the content of the specification and drawings of the present invention, or directly or indirectly applied to other related technical fields, shall be equally included in the patent protection scope of the present invention.

Claims

1. A preparation method of a coated lithium manganese oxide cathode material, characterized in that, Comprising: Obtaining a primary sintered material by using a lithium compound, a manganese compound and a doping compound containing a doping element, wherein the ratio of the lithium compound, the manganese compound and the doping compound is such that the molar ratio of lithium, manganese and the doping element is a:2-b:b, 1≤a≤1.2, 0<b≤0.1; Performing a surface coating treatment on the primary sintered material with a sodium compound to obtain a coated product, wherein the ratio of the sodium compound to the primary sintered material is such that the molar ratio of sodium to manganese is c:2-b-c, 0<c≤0.1; Successively sintering and post-treating the coated product to obtain a coated lithium manganese oxide cathode material, and the particle surface of the coated lithium manganese oxide cathode material has a sodium manganate Na2MnO3 coating layer.

2. The preparation method according to claim 1, wherein, The sodium compound includes at least one of sodium hydroxide, sodium carbonate, sodium bicarbonate, and sodium acetate.

3. The preparation method according to claim 2, characterized in that, The surface coating treatment is a dry coating, and the dry coating includes: Placing the sodium compound and the primary sintered material in a mixing tank and mixing them evenly to obtain the coated product.

4. The preparation method according to claim 2, wherein The surface coating treatment is a wet coating, and the wet coating includes: Preparing a solution of the sodium compound and the primary sintered material, and drying it by spray drying to obtain the coated product.

5. The preparation method according to claim 1, wherein The lithium compound is at least one of lithium hydroxide, lithium carbonate, and lithium oxalate, and the manganese compound is manganese dioxide and / or manganese tetroxide.

6. The preparation method according to claim 1, wherein, The doping element is at least one of Li, Mg, Al, Ni, Co, Zr, Ti, Y, W, Mo, Sr, Ba, B, La, Ce, Er, and Nb, and the doping compound is an oxide, hydroxide, or carbonate of the doping element.

7. The preparation method according to claim 5 or 6, characterized in that, The step of obtaining the primary sintered material by using the lithium compound, the manganese compound and the doping compound containing the doping element includes: Mixing the lithium compound, the manganese compound and the doping compound evenly according to the ratio, sintering at 650-950°C, and performing a crushing treatment after cooling.

8. The preparation method according to claim 1, characterized in that, When sintering the coated product, the sintering temperature is 300-800°C.

9. A coated lithium manganese oxide cathode material prepared by the preparation method according to any one of claims 1-8, characterized in that, The chemical general formula of the coated lithium manganese oxide cathode material is Li a Mn 2-b-c M b Na c O4, where M represents a doping element, 1 ≤ a ≤ 1.2, 0 < b ≤ 0.1, 0 < c ≤ 0.1, and the surface of the particles of the coated lithium manganese oxide cathode material has a sodium manganate Na2MnO3 coating layer.

Images