A positive electrode material and its preparation method and application

Abstract

The present invention discloses a positive electrode material and a preparation method and application thereof. The preparation method of the positive electrode material of the present invention is to immerse the NCM positive electrode material with lithium carbonate generated on the surface in lithium bromide solution and lithium sulfate solution in sequence, and heat to remove the solvent, so that lithium bromide and lithium sulfate will precipitate and adhere to the surface of the NCM positive electrode material, and then calcine at high temperature. Under the action of high-temperature melting, the lithium carbonate on the NCM surface that is not conducive to the electrochemical performance forms a solid electrolyte layer with higher lithium ion conductivity with lithium sulfate and lithium bromide, and the electrochemical performance of the nickel-rich ternary positive electrode material after long-term storage is restored. The solid electrolyte layer on the surface of the prepared NCM composite positive electrode material can effectively block the contact between the electrode material and the electrolyte, and improve the stability of the interface between the electrode and the electrolyte.

Description

Technical Field

[0001] The present invention relates to the technical field of cathode materials for lithium batteries, and particularly to a cathode material, a preparation method thereof, and an application thereof. Background Art

[0002] In the field of new energy pure electric vehicles, although lithium-ion battery systems have been widely used, how to further improve the endurance and how to minimize potential safety hazards have always been the focus issues in the field of new energy vehicles. The driving range is mainly determined by the total amount of electrical energy that the power battery system can provide itself. Therefore, the demand for lithium-ion batteries with high energy density is becoming increasingly urgent. For lithium-ion batteries, the specific capacity of the electrode material has a direct impact on performance such as energy density. An important way to improve its energy density is to develop electrode materials with high energy density.

[0003] As a cathode material for lithium-ion batteries, LiNiO2 material has a discharge specific capacity of 220 mAh / g, and the energy density can reach 800 Wh / kg. However, the synthesis process of LiNiO2 is complex, the cycle performance is poor, and the thermal stability is poor, so it cannot be used in practical applications. By using cobalt and manganese elements to replace part of Ni, the prepared LiNi x Co y Mn z O2, usually called ternary layered oxide cathode material (NCM). Among them, manganese and cobalt elements play a role in stabilizing the structure and increasing the conductivity; while the nickel content is closely related to the specific capacity, and the capacity of the ternary layered oxide cathode can be improved by increasing the nickel content. Therefore, NCM cathode materials with x > 0.5 (i.e., high-nickel type) have attracted much attention in the research and application field of lithium-ion batteries. When the nickel content in NCM rises to 0.6 < x < 0.9, the nickel-rich NCM can exhibit a high capacity of 160 - 200 mAh / g.

[0004] As is well known, carbonates will be formed on the surface of the cathode material when it is exposed to air. The formation of surface basic compounds on the surfaces of different types of cathode materials is different. In the synthesis of ternary materials, an excessive amount of lithium salt is required to ensure the stoichiometric ratio in the product. This excessive approach causes the remaining lithium salt in the product after high-temperature calcination (mainly oxides of Li) to react with H2O and CO2 in the air to form LiOH and Li2CO3 again, which remain on the material surface, making the pH value of the material relatively high. Especially for high-nickel ternary materials, the higher the nickel content, the lower the sintering temperature. When the molar ratio of lithium salt to transition metal ions remains unchanged, the decrease in sintering temperature leads to a decrease in the volatilization amount of lithium salt, and then an increase in the content of lithium salt remaining on the material surface, resulting in a greater alkalinity of the material.

[0005] Excessive residual alkali content on the surface of the positive electrode material will have many negative effects on the electrochemical performance. First, it will affect the coating process. The alkaline oxide content on the surface of the nickel-rich ternary material is too high, and it is easy to form a jelly-like gel during the homogenization process. Secondly, the excessive alkaline compounds on the surface of the positive electrode material increase the irreversible capacity loss of the battery and deteriorate the cycle performance. The decomposition of Li2CO3 on the surface of the positive electrode material under high voltage is one of the main causes of battery bloating, which brings hidden safety risks.

[0006] Usually, in order to solve the problem of residual alkaline substances on the surface of NCM, it is necessary to wash it with water after the first calcination to remove the excess alkali and then calcine it for the second time. This method can clean the residual alkali on the surface more thoroughly, but it will also lose some electrical properties. Summary of the invention

[0007] The purpose of the present invention is to provide a positive electrode material and a preparation method and application thereof in view of the deficiencies in the prior art.

[0008] To achieve the above object, the technical solution adopted by the present invention is:

[0009] The first aspect of the present invention is to provide a method for preparing a positive electrode material, comprising the following steps:

[0010] Step 1: Determination of the ternary layered oxide cathode material LiNi x Co y Mn z The content of lithium carbonate on the surface of O2;

[0011] Step 2: The ternary layered oxide positive electrode material LiNi x Co y Mn z O2 is placed in a lithium bromide acetone solution and stirred for 1-6 hours until the surface is fully wetted; then a lithium sulfate aqueous solution is added and stirred for 1-6 hours; the solvent is removed by heating, and lithium bromide and lithium sulfate are precipitated and attached to the ternary layered oxide positive electrode material LiNi x Co y Mn z O2 surface; calcined in a muffle furnace, and after natural cooling, the positive electrode material was obtained, which was quickly transferred to a dryer and stored for later use.

[0012] Furthermore, in step 2, the calcination temperature is 580-720° C., and the calcination time is 1-2 h.

[0013] Furthermore, the ternary layered oxide positive electrode material LiNi x Co y Mn zThe total mass of lithium carbonate, lithium sulfate and lithium bromide on the surface of O2 does not exceed the total mass of the ternary layered oxide positive electrode material LiNi x Co y Mn z 5% of O2's own mass.

[0014] Further preferably, the mass ratio of lithium carbonate, lithium sulfate and lithium bromide is 30:45:15.

[0015] The second aspect of the present invention is to provide a positive electrode material prepared by the above preparation method.

[0016] The third aspect of the present invention is to provide the use of the above-mentioned positive electrode material in a solid-state lithium battery, wherein the solid-state lithium battery comprises a composite positive electrode sheet, an electrolyte membrane and metallic lithium.

[0017] Furthermore, the preparation process of the composite positive electrode sheet is specifically as follows: after the positive electrode material described in claim 5 is fully stirred and mixed with the conductive carbon black powder and the PEO powder, hot pressing is carried out at 80-85°C and 8-10MPa to form a positive electrode sheet with a thickness of 10 to 20um; the positive electrode sheet is placed on an aluminum foil, pressed at 80-85°C and 8-10MPa for 1 to 10 minutes, cut to the required size, and then vacuum dried at 75-80°C for 10-12 hours to obtain the composite positive electrode sheet.

[0018] Furthermore, the preparation process of the electrolyte membrane is specifically as follows: under the protection of inert gas, PEO, oxide solid electrolyte powder, LiTFSI, and LiBOB are ball-milled and mixed in an agate jar for 2 to 6 hours, and then the mixture is hot-pressed into a film at 55-60° C. on a substrate under the protection of inert gas, and then cut into required sizes after cooling, and then vacuum dried at 40-45° C. for 10-12 hours to obtain the electrolyte membrane.

[0019] The present invention adopts the above technical solution, and has the following technical effects compared with the prior art:

[0020] The present invention sequentially immerses the NCM positive electrode material with lithium carbonate generated on the surface into a lithium bromide solution and a lithium sulfate solution, and heats to remove the solvent, so that lithium bromide and lithium sulfate will precipitate and adhere to the surface of the NCM positive electrode material, and then calcines at high temperature. Under the action of high-temperature melting, the lithium carbonate on the NCM surface that is not conducive to electrochemical performance forms a solid electrolyte layer with higher lithium ion conductivity with lithium sulfate and lithium bromide, and the electrochemical performance of the nickel-rich ternary positive electrode material after long-term storage is restored. The solid electrolyte layer on the surface of the prepared NCM composite positive electrode material can effectively block the contact between the electrode material and the electrolyte, and improve the stability of the interface between the electrode and the electrolyte. DETAILED DESCRIPTION

[0021] The present invention is further described below in conjunction with specific embodiments, but is not intended to be limiting of the present invention. It should be noted that the embodiments and features in the embodiments of the present invention may be combined with each other without conflict.

[0022] Example 1

[0023] This embodiment provides a method for preparing a positive electrode material, comprising the following steps:

[0024] Step 1: Determination of the ternary layered oxide cathode material LiNi x Co y Mn z The content of lithium carbonate on the surface of O2(NCM) was measured to be 1.2%;

[0025] Step 2: The above-mentioned ternary layered oxide positive electrode material LiNi x Co y Mn z O2 is placed in a lithium bromide acetone solution (0.006 g lithium bromide dissolved in 18 mL acetone) and stirred for 1 hour until the surface is fully wetted; then lithium sulfate aqueous solution (0.018 g lithium sulfate dissolved in 2 mL deionized water) is added and stirred for another 1 hour; the solvent is removed by heating, and lithium bromide and lithium sulfate will precipitate and adhere to the above-mentioned ternary layered oxide positive electrode material LiNi x Co y Mn z O2 surface; calcine in a muffle furnace at a temperature of 600°C for 2 hours. After natural cooling, the positive electrode material is obtained and quickly transferred to a dryer for storage.

[0026] In this embodiment, the ternary layered oxide positive electrode material LiNi x Co y Mn z The total mass of lithium carbonate, lithium sulfate and lithium bromide on the surface of O2 is the ternary layered oxide composite positive electrode material LiNi x Co y Mn z 5% of the mass of O2 itself. Among them, the mass ratio of lithium carbonate, lithium sulfate and lithium bromide is 30:45:15.

[0027] Example 2

[0028] This embodiment provides a method for preparing a positive electrode material, comprising the following steps:

[0029] Step 1: Determination of the ternary layered oxide cathode material LiNi x Co y Mn z The content of lithium carbonate on the surface of O2(NCM) was measured to be 1.1%;

[0030] Step 2: The above-mentioned ternary layered oxide positive electrode material LiNi x Co y Mn z O2 is placed in a lithium bromide acetone solution (0.0055 g lithium bromide dissolved in 18 mL acetone) and stirred for 1 h until the surface is fully wetted; then lithium sulfate aqueous solution (0.0165 g lithium sulfate dissolved in 2 mL deionized water) is added and stirred for another 1 h; the solvent is removed by heating, and lithium bromide and lithium sulfate will precipitate and adhere to the above-mentioned ternary layered oxide positive electrode material LiNi x Co y Mn z O2 surface; calcine in a muffle furnace at a temperature of 600°C for 2 hours. After natural cooling, the positive electrode material is obtained and quickly transferred to a dryer for storage.

[0031] Example 3

[0032] This embodiment provides a method for preparing a positive electrode material, comprising the following steps:

[0033] Step 1: Determination of the ternary layered oxide cathode material LiNi x Co y Mn z The content of lithium carbonate on the surface of O2(NCM) was measured to be 0.9%;

[0034] Step 2: The above-mentioned ternary layered oxide positive electrode material LiNi x Co y Mn z O2 is placed in a lithium bromide acetone solution (0.0045 g lithium bromide dissolved in 18 mL acetone) and stirred for 1 hour until the surface is fully wetted; then lithium sulfate aqueous solution (0.0135 g lithium sulfate dissolved in 2 mL deionized water) is added and stirred for another 1 hour; the solvent is removed by heating, and lithium bromide and lithium sulfate will precipitate and adhere to the above-mentioned ternary layered oxide positive electrode material LiNi x Co y Mn z O2 surface; calcine in a muffle furnace at a temperature of 600°C for 2 hours. After natural cooling, the positive electrode material is obtained and quickly transferred to a dryer for storage.

[0035] Example 4

[0036] The positive electrode materials prepared in the above-mentioned Examples 1-3 are respectively used to prepare solid-state lithium batteries. The solid-state lithium batteries include a composite positive electrode sheet, an electrolyte membrane and metallic lithium. The specific preparation process is as follows:

[0037] The preparation process of the composite positive electrode sheet is as follows: the positive electrode materials prepared in the above embodiments 1-3 are fully stirred and mixed with the conductive carbon black powder (super P) and the PEO powder, and then hot pressed in a mold at 80°C and 10MPa to form a positive electrode sheet with a thickness of 10um; the positive electrode sheet is placed on an aluminum foil, pressed at 80°C and 10MPa for 10min, cut to the required size, and then vacuum dried at 80°C for 12h to obtain a composite positive electrode sheet. The mass ratio of the positive electrode material, the conductive carbon black powder and the PEO powder is 80:10:10.

[0038] The specific preparation process of the electrolyte membrane is as follows: under the protection of inert gas, PEO, oxide solid electrolyte powder (LATP), LiTFSI, and LiBOB are ball-milled in an agate jar for 6 hours, and then the mixture is hot-pressed into a film at 55°C on a substrate under the protection of inert gas. The average thickness of the film is 50um. After cooling, it is cut into required sizes and then vacuum-dried at 40°C for 12 hours to obtain the electrolyte membrane. In the above electrolyte membrane, the mass percentage of LATP is 10%, the molar ratio of LiTFSI to LiOB is 1:1, and the PEO monomer (C2H4O) and Li + The molar ratio is 19:1.

[0039] The composite positive electrode sheet, electrolyte membrane and metallic lithium are assembled into a solid-state battery.

[0040] Comparative Example 1

[0041] The solid-state battery is assembled by the method of the above-mentioned embodiment 4, except that the positive electrode material prepared in embodiments 1-3 is replaced by a ternary layered oxide positive electrode material LiNi with a surface lithium carbonate content of 1.1%. x Co y Mn z O2(NCM).

[0042] result

[0043] The solid-state batteries assembled in Example 4 and Comparative Example 1 were tested, and the results are shown in Table 1:

[0044] Table 1

[0045]

[0046] The test data showed that without solid electrolyte coating on the surface of NCM electrode material, NCM electrode could not match PEO-based solid electrolyte. The assembled battery failed after the first week of charge and discharge.

[0047] The above description is only a preferred embodiment of the present invention, and does not limit the implementation mode and protection scope of the present invention. For those skilled in the art, it should be aware that all solutions obtained by equivalent substitutions and obvious changes made using the contents of the specification of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing a positive electrode material, characterized in that: The steps include: Step 1: Determination of the ternary layered oxide cathode material LiNi x Co y Mn z The content of lithium carbonate on the surface of O2; Step 2: The ternary layered oxide positive electrode material LiNi x Co y Mn z O2 is placed in a lithium bromide acetone solution and stirred for 1-6 hours until the surface is fully wetted; then a lithium sulfate aqueous solution is added and stirred for 1-6 hours; the solvent is removed by heating, and lithium bromide and lithium sulfate are precipitated and attached to the ternary layered oxide positive electrode material LiNi x Co y Mn z O2 surface; calcined in a muffle furnace, and after natural cooling, the positive electrode material was obtained, which was quickly transferred to a dryer and stored for later use.

2. The preparation method according to claim 1, characterized in that: In step 2, the calcination temperature is 580-720° C., and the calcination time is 1-2 hours.

3. The preparation method according to claim 1, characterized in that: The ternary layered oxide positive electrode material LiNi x Co y Mn z The total mass of lithium carbonate, lithium sulfate and lithium bromide on the surface of O2 does not exceed the total mass of the ternary layered oxide positive electrode material LiNi x Co y Mn z 5% of O2's own mass.

4. The preparation method according to claim 3, characterized in that: The mass ratio of the lithium carbonate, lithium sulfate and lithium bromide is 30:45:

15.

5. A positive electrode material prepared by the preparation method according to any one of claims 1 to 4.

6. Use of the positive electrode material according to claim 5 in a solid-state lithium battery, characterized in that: The solid-state lithium battery comprises a composite positive electrode sheet, an electrolyte membrane and metallic lithium.

7. The use according to claim 6, characterized in that: The preparation process of the composite positive electrode sheet is specifically as follows: the positive electrode material described in claim 5 is fully stirred and mixed with the conductive carbon black powder and the PEO powder, and then hot-pressed into a positive electrode sheet with a thickness of 10 to 20 um at 80-85°C and 8-10 MPa; the positive electrode sheet is placed on an aluminum foil, pressed at 80-85°C and 8-10 MPa for 1 to 10 minutes, cut to the required size, and then vacuum-dried at 75-80°C for 10-12 hours to obtain the composite positive electrode sheet.

8. The use according to claim 6, characterized in that: The preparation process of the electrolyte membrane is specifically as follows: Under the protection of inert gas, PEO, oxide solid electrolyte powder, LiTFSI and LiBOB are ball-milled and mixed in an agate jar for 2 to 6 hours, and then the mixture is hot-pressed into a film at 55-60° C. on a substrate under the protection of inert gas, and cut into required sizes after cooling, and then vacuum-dried at 40-45° C. for 10-12 hours to obtain the electrolyte membrane.

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