A cathode material, its preparation method and application

Abstract

The present invention discloses a cathode material, a preparation method thereof and an application thereof. The cathode material provided by the present invention comprises: a layered cathode material with the chemical formula LixMO2 and a coating substance, wherein the range of x is 0.95 to 1.1, M is a transition metal including Mn; the coating substance is disposed on the surface of the layered cathode material and is partially doped into the surface layer of the layered cathode material; the coating substance includes tetravalent manganese, lithium ions and phosphate ions. The cathode material provided by the present invention can effectively inhibit the dissolution of manganese in the manganese-containing cathode material and improve the cycling performance. The present invention also provides a preparation method and an application of the above-mentioned cathode material.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary batteries, and in particular to a cathode material, a preparation method thereof, and an application thereof. Background Art

[0002] Among the cathode materials for lithium-ion secondary batteries, layered cathode materials are an important type, mainly including single-element materials (lithium cobaltate, lithium nickelate, lithium manganate), binary materials (lithium nickel cobaltate, lithium nickel manganate, lithium cobalt manganate), and ternary materials (lithium nickel cobalt manganate). However, when the Ni content in the above-mentioned layered cathode material exceeds 90% (mole percentage of the total transition metals), residual alkali is likely to be generated on the surface of the cathode material. The surface residual alkali mainly refers to substances such as LiOH and Li2CO3 on the surface of the cathode material particles. The source of the residual alkali is mainly Li that has not been sintered into the sintering reaction, or the residual alkali generated by the decomposition of the material during high-temperature sintering. On the other hand, it is generated when the material is placed in the air for too long. Specifically, when the humidity content in the air is relatively high, lithium in the lattice tends to migrate to the surface of the cathode material and react with moisture and carbon dioxide in the environment to form residual alkali.

[0003] The higher the Ni content in the cathode material, the more stringent the sintering conditions, and it is more difficult to sinter into a material with a specific lithium metal ratio, resulting in more residual alkali in the sintered product. At the same time, the higher the Ni content, the more obvious the migration tendency of lithium from the lattice to the surface. Therefore, relatively speaking, the residual alkali content of high-nickel materials is higher than that of other cathode materials.

[0004] During the cycling process of the manganese-containing cathode material, Mn(III) undergoes a disproportionation reaction to form Mn(IV) and Mn(II). In addition, as the Ni content increases, manganese (especially Mn(II)) in the layered cathode material is likely to dissolve out during the cycling process, migrate and precipitate at the negative electrode, damaging the negative electrode SEI film; at the same time, as manganese dissolves out, the lattice of the layered cathode material is damaged, which may lead to the fragmentation of the cathode particles, further affecting the cycling performance of the cathode material.

[0005] In summary, it is very important to provide a cathode material that can effectively inhibit the dissolution of manganese, improve the cycling performance, and reduce the residual alkali content to a certain extent. Summary of the Invention

[0006] The present invention aims to solve at least one of the technical problems existing in the prior art. For this reason, the present invention provides a cathode material that can effectively inhibit the dissolution of manganese in the manganese-containing cathode material and reduce the residual alkali content to a certain extent.

[0007] The present invention also provides a preparation method for the above-mentioned cathode material.

[0008] The present invention also provides an application of the above-mentioned cathode material.

[0009] According to an embodiment of the first aspect of the present invention, a cathode material is provided, and the cathode material includes:

[0010] A layered cathode material, the chemical formula of the layered cathode material being Li x MO2, where the range of x is 0.95 to 1.1, M is a transition metal, including Mn;

[0011] A coating substance, part of the coating substance is provided on the surface of the layered cathode material, and part is doped in the surface layer of the layered cathode material; the coating substance includes tetravalent manganese, lithium ions and phosphate ions.

[0012] The cathode material provided by the embodiment of the present invention has at least the following beneficial effects:

[0013] (1) When the manganese-containing layered cathode material is charged and discharged, Mn(III) (trivalent manganese) inside is prone to disproportionation reaction to generate Mn(II) and Mn(IV), and the former is prone to dissolve out of the lattice of the layered cathode material.

[0014] In the cathode material provided by the present invention, tetravalent manganese is doped in the surface layer of the layered cathode material (equivalent to forming a surface layer rich in tetravalent manganese), thereby suppressing the disproportionation of Mn(III) in the layered cathode material, and further suppressing the generation and dissolution of Mn(II), and finally improving the cycle performance of the obtained cathode material.

[0015] (2) Lithium ions and phosphate ions in the coating substance may combine to form lithium phosphate, which, as a fast ion conductor, can effectively improve the rate performance of the obtained cathode material.

[0016] (3) In the coating substance, phosphate can also fix and protect tetravalent manganese in manganese dioxide, and inhibit the dissolution of manganese in the coating substance.

[0017] (4) The coating substance provided on the surface of the layered cathode material blocks the contact between the layered cathode material and the outside to a certain extent, so the generation of residual alkali can be reduced, and the comprehensive performance of the obtained cathode material can be improved.

[0018] According to some embodiments of the present invention, in Li x MO2, M further includes Ni.

[0019] According to some embodiments of the present invention, in Li x MO2, the molar percentage of Ni in M ≥ 75%.

[0020] According to some preferred embodiments of the present invention, in Li x MO2, the molar percentage of Ni in M is 80 to 99%.

[0021] According to some preferred embodiments of the present invention, in Li x MO2, the molar percentage of Ni in M is 90-95%.

[0022] At this nickel content, the cathode materials provided by the prior art usually include a relatively high residual alkali and have relatively poor further cycling performance; through the design of the structure and materials, the present invention can effectively improve the cycling performance of the obtained cathode materials.

[0023] According to some embodiments of the present invention, in Li x MO2, M further includes Co.

[0024] According to some embodiments of the present invention, in Li x MO2, M is Ni, Co and Mn.

[0025] According to some embodiments of the present invention, in Li x MO2, M is Ni, Co and Mn, and the molar ratio of the three is 1-19:1:1.

[0026] According to some embodiments of the present invention, in the coating material, the existing form of tetravalent manganese includes manganese dioxide.

[0027] According to the embodiments of the second aspect of the present invention, a method for preparing the cathode material is provided, including:

[0028] Mix the layered cathode material and an aqueous phosphoric acid solution, and sequentially add an alkaline potassium permanganate solution and a divalent manganese precursor to the obtained mixed system; after the reaction, dry and calcine the solid product.

[0029] The mechanism of the preparation method is as follows:

[0030] There is a certain amount of residual alkali on the surface of the layered cathode material. During the mixing with the aqueous phosphoric acid solution, on the one hand, the aqueous phosphoric acid solution reacts with the residual alkali to reduce the residual alkali content; on the other hand, lithium in the residual alkali can react with phosphate radicals to generate lithium phosphate precipitate, which is deposited on the surface of the layered cathode material; on the other hand, the acidity of phosphoric acid will also damage the surface structure of the layered cathode material to a certain extent, leaving defects on it and increasing the specific surface area of the layered cathode material.

[0031] Alkaline potassium permanganate and the divalent manganese precursor react to form manganese dioxide precipitate attached to the surface of the layered cathode material;

[0032] During the calcination process, manganese dioxide and lithium phosphate perform superficial doping on the layered cathode material.

[0033] The preparation method provided by the embodiments of the present invention has at least the following beneficial effects:

[0034] (1) During the calcination process, the defect positions formed on the surface of the layered cathode material by phosphoric acid etching are more likely to deposit manganese dioxide and are also more likely to serve as a pathway for the formation of surface doping with coating substances such as tetravalent manganese;

[0035] Thus, the steps of the present invention cooperate with each other, enabling the coating substance to more easily enter the lattice to form surface doping, and then play its role in improving the comprehensive performance of the obtained cathode material.

[0036] (2) Treating the layered cathode material with an aqueous phosphoric acid solution can convert the lithium in the residual alkali into lithium phosphate, which avoids the lithium loss caused by traditional pickling and alkali washing, thereby avoiding the capacity loss of the cathode material and improving the capacity of the cathode material to a certain extent.

[0037] (3) In the preparation method, if the divalent manganese precursor is added first and then alkaline potassium permanganate is added, the divalent manganese tends to form manganese phosphate precipitate with phosphate ions, and the probability of forming tetravalent manganese decreases. The present invention limits the addition sequence of the materials, further ensuring the consistency and high quality of the performance of the obtained cathode material.

[0038] (4) In the cathode material prepared by the preparation method, the coating substance is doped in the surface layer, thereby forming a manganese-rich surface layer, inhibiting the dissolution of manganese in the cathode material, and improving the cycle performance of the obtained cathode material.

[0039] According to some embodiments of the present invention, the concentration of the aqueous phosphoric acid solution is 1-5 wt%.

[0040] According to some embodiments of the present invention, the solid-liquid ratio of the layered cathode material to the aqueous phosphoric acid solution is 0.5-1 g / mL.

[0041] According to some embodiments of the present invention, the mixing duration is 5-30 min.

[0042] According to some embodiments of the present invention, the mass ratio of the solute potassium permanganate in the alkaline potassium permanganate to the layered cathode material is 4-16 g:500 g.

[0043] According to some preferred embodiments of the present invention, the mass ratio of the solute potassium permanganate in the alkaline potassium permanganate to the layered cathode material is 7.8-8.1 g:500 g.

[0044] According to some embodiments of the present invention, the pH of the alkaline potassium permanganate solution is 7-13.

[0045] According to some preferred embodiments of the present invention, the pH of the alkaline potassium permanganate solution is about 12.

[0046] According to some embodiments of the present invention, the concentration of the alkaline potassium permanganate solution is 0.1-2 mol / L.

[0047] According to some embodiments of the present invention, the concentration of the alkaline potassium permanganate solution is about 1 mol / L.

[0048] According to some embodiments of the present invention, the molar ratio of the alkaline potassium permanganate to the divalent manganese precursor is 0.8-1.2:1.

[0049] According to some embodiments of the present invention, the divalent manganese precursor includes at least one of manganese hydroxide, manganese sulfate, and manganese chloride.

[0050] According to some preferred embodiments of the present invention, the divalent manganese precursor is selected from manganese hydroxide. Thereby, impurity components introduced via the divalent manganese precursor can be avoided as much as possible.

[0051] According to some embodiments of the present invention, the duration of the reaction is 0.5-2 h; preferably, the reaction method is a static reaction.

[0052] According to some embodiments of the present invention, the drying temperature is 80-200 °C.

[0053] According to some embodiments of the present invention, the drying duration is 4-20 h.

[0054] According to some embodiments of the present invention, the drying duration is 8-10 h.

[0055] According to some embodiments of the present invention, the calcination temperature is 450-550 °C.

[0056] According to some embodiments of the present invention, the calcination duration is 6-8 h.

[0057] According to some embodiments of the present invention, the calcination atmosphere is an oxygen atmosphere.

[0058] According to the embodiments of the third aspect of the present invention, a secondary battery is provided, and the preparation raw materials of the secondary battery include the positive electrode material described above.

[0059] The secondary battery provided by the embodiments of the present invention has at least the following beneficial effects:

[0060] Since the secondary battery adopts all the technical solutions of the positive electrode material in the above embodiments, it has at least all the beneficial effects brought by the technical solutions in the above embodiments.

[0061] Other features and advantages of the present invention will be described in the subsequent specification, and some of them will become obvious from the specification or be understood by implementing the present invention. Description of the Drawings

[0062] The above and / or additional aspects and advantages of the present invention will become apparent and be readily understood from the following description of embodiments in conjunction with the accompanying drawings, in which:

[0063] Figure 1 It is a scanning electron microscope image of the positive electrode material obtained in Example 1 of the present invention.

[0064] Figure 2 It is a scanning electron microscope image of the positive electrode material obtained in Example 1 of the present invention. Detailed Description of the Invention

[0065] The embodiments of the present invention will be described in detail below. Examples of the embodiments are shown in the accompanying drawings, where the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described by referring to the accompanying drawings are exemplary only for explaining the present invention and should not be construed as limiting the present invention.

[0066] Example 1

[0067] In this example, a positive electrode material was prepared. The specific process is as follows:

[0068] S1. Take 500 g of LiNi 0.9 Co 0.05 Mn 0.05 O2 and immerse it in a phosphoric acid aqueous solution with a mass fraction of 5% and a volume of 500 ml for 5 minutes;

[0069] S2. Add 50 ml of 1 mol / L alkaline potassium permanganate (pH = 12) and 5 g of manganese hydroxide to the mixture obtained in step S1 in sequence. After stirring evenly, let it stand for reaction for 0.5 h. The manganese dioxide generated by the reaction precipitates on the surface of LiNi 0.9 Co 0.05 Mn 0.05 O2;

[0070] S3. Separate the solid and liquid of the product obtained in step S2, and place the obtained solid in a drying oven to dry at 80 °C for 8 h;

[0071] S4. Transfer the product obtained in step S3 to a muffle furnace and calcine it in an oxygen atmosphere. The sintering temperature is 450 °C and the time is 8 h.

[0072] The SEM image of the positive electrode material obtained in this example is as Figures 1 to 2 shown. The results show that there are certain depressions on the surface of the positive electrode material, and coating substances are deposited in the pits and on the surface. This shows that the coating substances have been successfully deposited on the surface of the layered positive electrode material.

[0073] Example 2

[0074] In this embodiment, a cathode material was prepared. The specific difference from Embodiment 1 is as follows:

[0075] In step S1, the mass concentration of the phosphoric acid aqueous solution was 1%.

[0076] Embodiment 3

[0077] In this embodiment, a cathode material was prepared. The specific difference from Embodiment 1 is as follows:

[0078] In step S4, the sintering temperature was 550 °C and the time was 6 h.

[0079] Comparative Example 1

[0080] In this comparative example, a cathode material was prepared. The specific difference from Embodiment 1 is as follows:

[0081] In step S1, the phosphoric acid aqueous solution was replaced with an oxalic acid aqueous solution of the same concentration.

[0082] Comparative Example 2

[0083] In this comparative example, a cathode material was prepared. The specific difference from Embodiment 1 is as follows:

[0084] (1) In step S1, the phosphoric acid aqueous solution was replaced with water of the same volume.

[0085] (2) Step S2 was not included.

[0086] Comparative Example 3

[0087] In this comparative example, a cathode material was prepared. The specific difference from Embodiment 1 is as follows:

[0088] In step S1, the phosphoric acid aqueous solution was replaced with water of the same volume.

[0089] Comparative Example 4

[0090] In this comparative example, a cathode material was prepared. The specific difference from Embodiment 1 is as follows:

[0091] Step S2 was not included.

[0092] Experimental Example

[0093] In this test example, the cathode materials obtained in Embodiments 1 to 3 and Comparative Examples 1 to 4 were used as the cathode active materials to prepare coin cells, and the electrochemical performance of the coin cells was tested. Specifically:

[0094] Using N-methylpyrrolidone as the solvent, the cathode material, acetylene black, and PVDF were mixed uniformly in a mass ratio of 9.2:0.5:0.3 to form a slurry. Then, the slurry was coated on aluminum foil, dried in a blast oven at 80 °C for 8 h, and then dried in vacuo at 120 °C for 12 h to obtain the cathode.

[0095] The battery was assembled in an argon - protected glove box. The negative electrode was a lithium metal sheet, the separator was a polypropylene film, the electrolyte was 1 M LiPF6 - EC / DMC (1:1, v / v), and a 2032 - type button battery case was used.

[0096] The obtained button battery was subjected to an electrochemical performance test at 25 °C with a current of 0.1C in the voltage range of 3.0 - 4.5V.

[0097] The test results are shown in Table 1.

[0098] Table 1 Electrochemical performance results of the button batteries corresponding to the positive electrode materials obtained in Examples 1 - 3 and Comparative Examples 1 - 4

[0099]

[0100] It can be seen from the results in Table 1 that for the positive electrode material prepared by the preparation method provided by the present invention, due to the superficial doping of the coating material and the synergistic effect between manganese and phosphate in the coating material, excellent cycle performance can be maintained on the basis of ensuring the first - cycle discharge specific capacity (Examples 1 - 3).

[0101] Comparing Example 1 and Example 2, it can be seen that if the concentration of phosphoric acid is reduced, the surface defects formed on the surface of LiNi 0.9 Co 0.05 Mn 0.05 O2 are reduced, and the corresponding loss of lithium is reduced, so the specific capacity is slightly higher; however, the effect of superficial doping of the coating material is reduced, the manganese content in the formed manganese - rich layer is reduced, and the cycle performance becomes worse.

[0102] Comparing Example 1 and Example 3, it can be seen that a relatively high calcination temperature may cause a certain degree of loss of cycle performance; the surface defects formed by phosphoric acid in the present invention can significantly reduce the temperature required for forming superficial doping, thereby improving the cycle performance of the obtained positive electrode material to a certain extent.

[0103] Comparing Example 1 and Comparative Example 1, it can be seen that if phosphoric acid is replaced by oxalic acid, due to the too - strong acidity of oxalic acid, the structure of the positive electrode material is severely damaged, and at the same time, the capacity and cycle performance are lost.

[0104] Comparing Example 1 and Comparative Example 2, it can be seen that if only ordinary water washing is used, compared with phosphoric acid washing, the loss of lithium is less, so the capacity is improved to a certain extent; however, due to the lack of deposition and superficial doping of the coating material, the cycle performance of the obtained positive electrode material is significantly worse.

[0105] Comparing Example 1 with Comparative Example 3, it can be seen that if the phosphoric acid washing is replaced by water washing, since the coating does not contain phosphate radicals, during the preparation process, defects cannot be formed on the surface of the cathode material, and thus manganese cannot be effectively promoted to enter the lattice of the cathode material to form superficial doping. As a result, the disproportionation and dissolution of manganese in the cathode material cannot be inhibited, and finally the cycle performance decreases significantly.

[0106] Comparing Example 1 with Comparative Example 4, it can be seen that if only phosphoric acid washing is used, the coating does not contain manganese, and the dissolution of manganese in the cathode material cannot be inhibited either. Therefore, the cycle performance also decreases significantly.

[0107] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those of ordinary skill in the art to which the present invention pertains, various changes can be made without departing from the gist of the present invention.

Claims

1. A method for preparing a cathode material, characterized in that, Comprising: Mixing a layered cathode material and an aqueous phosphoric acid solution, and successively adding an alkaline potassium permanganate solution and a divalent manganese precursor to the obtained mixed system; After the reaction, drying and calcining the solid product; The obtained cathode material comprises: Layered cathode material, the chemical formula of the layered cathode material is Li x MO2, where the range of x is 0.95 to 1.1, M is a transition metal, including Mn and Ni, and the molar percentage of Ni in M ≥ 75%; A coating substance, part of which is disposed on the surface of the layered cathode material and part of which is doped in the surface layer of the layered cathode material; the coating substance includes tetravalent manganese, lithium ions and phosphate ions.

2. The preparation method according to claim 1, characterized in that, The concentration of the aqueous phosphoric acid solution is 1-5 wt%.

3. The preparation method according to claim 1, characterized in that, The solid-liquid ratio of the layered cathode material to the aqueous phosphoric acid solution is 0.5-1 g / mL.

4. The preparation method according to claim 1, characterized in that, The molar ratio of the alkaline potassium permanganate to the divalent manganese precursor is 0.8-1.2:

1.

5. The preparation method according to claim 1, characterized in that, The temperature of the calcination is 450-550 °C.

6. The preparation method according to claim 1, characterized in that, The duration of the calcination is 6-8 h.

7. The preparation method according to claim 1, characterized in that, In the coating substance, the existence form of tetravalent manganese includes manganese dioxide.

8. A secondary battery, characterized in that, The preparation raw materials include the cathode material prepared by the preparation method according to any one of claims 1-7.

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