Ternary material precursor, ternary material and preparation method

Abstract

The invention discloses a ternary material precursor, a ternary material and a preparation method, and belongs to the field of lithium ion battery positive electrode materials. The ternary material precursor is of a multi-layer composite structure and sequentially comprises a core layer, a first transition layer and a second transition layer from inside to outside, and all the layers are prepared by adopting the same ternary (nickel-cobalt-aluminum) system. Through reasonable design of the proportion and thickness of the ternary system of the core layer and the transition layer, the ternary material precursor gives consideration to the high-capacity characteristic of the high-nickel core and the stability of the shell, and the stable structure of the shell can effectively inhibit the structural change and interface side reaction of the high-nickel material in the charging and discharging process. The phenomena of particle cracking and shell layer falling are reduced, and the material shows excellent specific discharge capacity, good cycle performance and thermal stability when being used as a lithium ion battery positive electrode material.

Description

A ternary material precursor, ternary material and preparation method Technical Field

[0001] This application belongs to the field of lithium-ion battery cathode material technology, specifically relating to a ternary material precursor, ternary material, and preparation method. Background Technology

[0002] As a core component of electric vehicles and energy storage systems, lithium-ion batteries require high energy density and long cycle life for their cathode materials, making this a key technological challenge. Among numerous candidate materials, the nickel-cobalt-manganese (NCM) and nickel-cobalt-aluminum (NCA) ternary systems stand out due to their superior specific capacity and voltage platform. Nickel is a major component in redox energy storage, and increasing its content can effectively improve the specific capacity of ternary materials. However, while meeting the demands of high-energy-density batteries, high-nickel ternary materials simultaneously face problems such as easy degradation of surface structure and side reactions at the electrolyte interface, which greatly restricts their commercialization process.

[0003] Currently, the main methods to alleviate the above problems are surface coating and doping modification. Among them, surface coating physically isolates the positive electrode active material from the electrolyte through the coating layer, which has a more significant effect on suppressing interfacial side reactions. However, although traditional single-layer coating can partially improve interfacial stability, it is difficult to balance the requirements of ionic conductivity and mechanical strength, and it does not significantly improve the stability of the internal structure and long-term cycling performance. On the other hand, multi-layer shell structures with core-shell / yolk-shell characteristics, with a high-capacity material as the core and a shell made of structurally stable or high-voltage resistant material, can significantly suppress particle breakage and transition metal dissolution during cycling through stress buffering and element isolation mechanisms, effectively ensuring the stability of the internal structure and long-term cycling performance. For example, Chinese invention patent application CN 103236537A discloses a gradient core-shell positive electrode material for lithium-ion batteries and its synthesis method, using ternary materials (LiNi) x Co y Mn z O2) as the core material, with binary material (LiNi) x Mn yUsing a core-shell structure with a single-element material (LiMn2O4 / LiMn2O4) as the shell material improves the cycle performance and rate performance of the material while ensuring structural stability. However, the inconsistency between the core and shell materials leads to weak interlayer bonding, and the shell material still risks peeling off after multiple cycles. Chinese invention patent application CN 102347483A discloses a multilayer composite ternary material and its precursor, which coats the core with multiple layers of the same multi-element material in different proportions, improving its cycle stability and rate performance. However, the shell thickness of the core-shell structure also affects the energy density, cycle performance, and safety of the ternary cathode material; too thin a shell results in insufficient protection, while too thick a shell hinders lithium-ion transport. Therefore, achieving a balance between "interface protection" and "kinetic performance" is crucial, a point that this patent does not explore.

[0004] In summary, there is an urgent need to develop a ternary material with a multi-layered shell structure that balances stability and dynamic performance. Summary of the Invention

[0005] 1. Problem to be solved: In existing technologies, the core and shell materials of ternary materials with core-shell structures are inconsistent, which may lead to insufficient interlayer bonding and easy peeling of the shell material. At the same time, the shell thickness also needs to be well balanced between the material structure stability and dynamic performance (capacity, charge and discharge rate, etc.). This application provides a ternary material precursor, ternary material and its preparation method. The precursor consists of a core layer, a first transition layer and a second transition layer from the inside to the outside. The core layer (core) and the transition layer (shell) are prepared using the same ternary system (nickel-cobalt-aluminum) material. Through the reasonable design of the ternary system ratio and thickness of the core layer and the transition layer, the prepared precursor takes into account the high capacity characteristics of the high-nickel core and the stability of the shell. As a result, the ternary material used as a cathode material of lithium batteries exhibits high discharge specific capacity and good cycle performance and thermal stability.

[0006] 2. Technical Solution To solve the above problems, the technical solution adopted in this application is as follows: This application provides a ternary material precursor, which is a multilayer composite structure, comprising a core layer, a first transition layer, and a second transition layer from the inside out, wherein: the core layer has a Ni... x Co y Mn 1-x-y (OH)₂, 0.8≤x≤0.9, 0.05≤y≤0.1; the first transition layer structure is Ni x Co y Mn 1-x-y (OH)₂, 0.5≤x≤0.6, 0.2≤y≤0.3; the second transition layer structure is Ni x Co y Mn1-x-y (OH)2, 0.3≤x<0.5, 0.1≤y<0.2; It should be noted that the core layer and transition layer in this application are prepared using the same ternary system, which effectively enhances the interlayer bonding force of the material, thereby reducing the risk of easy peeling of the outer shell material during recycling; at the same time, the core layer is a high-nickel material, and the second transition layer is made manganese-rich, so that the precursor can take into account the high capacity characteristics of the high-nickel core and the stability of the shell. The stable structure of the shell is used to suppress the structural changes and interfacial side reactions of the high-nickel material during charging and discharging, reducing particle cracking and shell peeling.

[0007] Furthermore, the thickness ratio of the core layer, the first transition layer, and the second transition layer is (0.6~0.7):(0.2~0.3):(0~0.2). It should be noted that, in order to achieve a further balance between the structural stability and dynamic performance of the precursor material, this application tested the performance of precursors with different shell thicknesses. Based on the performance characteristics such as discharge specific capacity and charge / discharge efficiency, the above-mentioned core (core layer) and shell (transition layer) thickness ratio is preferred.

[0008] This application also provides a method for preparing the aforementioned ternary material precursor, which includes the following steps: S1, preparing a mixed salt solution A, a mixed salt solution B, and a mixed salt solution C containing Ni, Co, and Mn metal ions; S2, preparing the core layer material by pumping the mixed salt solution A from S1 into a reaction vessel, controlling the temperature in the reaction vessel to be 50~55℃ and the pH to be 11~12, and continuously stirring to obtain precipitated solid 1, which is the core layer material; S3, coating the first transition layer by pumping the mixed salt solution B from S1 into the reaction vessel... The mixture is introduced into reactor S2, and the temperature in the reactor is controlled at 50~55℃ and the pH is 11~12. After continuous stirring, precipitated solid 2 is obtained, which completes the coating of the first transition layer outside the core layer material of S1; S4, coating the second transition layer: the mixed salt solution C in S1 is pumped into reactor S3, and the temperature in the reactor is controlled at 50~55℃ and the pH is 11~12. After continuous stirring, precipitated solid 3 is obtained, which completes the coating of the second transition layer outside the first transition layer of S2, and the ternary material precursor is obtained.

[0009] Furthermore, the molar concentration ratio of Ni, Co, and Mn metal ions in the above mixed salt solution A is (8~9):(1~0.5):(1~0.5).

[0010] Furthermore, the molar concentration ratio of Ni, Co, and Mn metal ions in the above mixed salt solution B is (5~6):(2~3):(3~1).

[0011] Furthermore, the molar concentration ratio of Ni, Co, and Mn metal ions in the above mixed salt solution C is (3~5):(2~1):(5~4).

[0012] Furthermore, the metal ions Ni in the above-mentioned mixed salt solution A and / or mixed salt solution B and / or mixed salt solution C originate from any one or more of NiSO4·6H2O, NiCO4, Ni(C2H3O2)2, and Ni(NO3)2·6H2O. It should be noted that the Ni-containing metal salts in this application are not limited to the above-mentioned representative salts.

[0013] Furthermore, the metal ions Co in the above-mentioned mixed salt solution A and / or mixed salt solution B and / or mixed salt solution C originate from any one or more of CoSO4·7H2O, CoCO3, Co(C2H3O2)2, and Co(NO3)2·6H2O. It should be noted that the Co-containing metal salts in this application are not limited to the above-mentioned representative salts.

[0014] Furthermore, the metal ions Mn in the above-mentioned mixed salt solution A and / or mixed salt solution B and / or mixed salt solution C are derived from any one or more of MnSO4·H2O, MnCO3, Mn(C2H3O2)2, and Mn(NO3)2·6H2O. It should be noted that the Mn-containing metal salts in this application are not limited to the above-mentioned representative salts.

[0015] This application also provides the application of the above-mentioned precursor in the preparation of ternary materials for lithium-ion battery cathode materials.

[0016] This application also provides a ternary material comprising the aforementioned precursor.

[0017] This application also provides a method for preparing the above-mentioned ternary material, which includes: drying the above-mentioned precursor, mixing it with a lithium source and a boron source in a molar ratio of 1:(0.98~1.04):(0.01~0.05), and then sintering the mixture to obtain the final product.

[0018] Furthermore, the lithium source mentioned above includes lithium hydroxide and / or lithium carbonate. It should be noted that the lithium source in this application is not limited to the two compounds mentioned above, and a suitable lithium source compound can be selected according to actual needs.

[0019] Furthermore, the aforementioned metal oxides include any one or more of B2O3, Al2O3, MgO, CeO2, WO3, TiO2, ZrO2, Nb2O5, Ta2O5, La2O3, and Y2O3. It should be noted that, in this application, B doping can change the surface energy and partially alleviate the internal stress generated by deep charging; Al doping can form ion-conducting bodies such as LiAlO2 and AIF3, increasing lithium-ion diffusion; Zr, W, Ti, Al, and Mg doping can reduce lattice parameter changes and suppress crack formation; Nb doping helps stabilize the layered structure and suppress the transformation to spinel or rock salt phases; Ta doping can reduce resistance and suppress side reactions; Ce doping can change the particle size and morphology; and Y can improve the material stability.

[0020] Furthermore, the above sintering is carried out in an oxygen atmosphere and includes two stages: the first stage is carried out at 450~500℃ for 3~7 h, and the second stage is carried out at 600~800℃ for 12~16 h.

[0021] 3. Beneficial effects Compared with the prior art, the beneficial effects of this application are as follows: (1) This application provides a ternary material precursor, ternary material and preparation method. The ternary material precursor is a multi-layer composite structure. Each layer is prepared using the same ternary (nickel-cobalt-aluminum) system. By rationally designing the ratio and thickness of the ternary system of the core layer and the transition layer, the precursor can take into account the high capacity characteristics of the high nickel core and the stability of the shell. The stable structure of the shell can suppress the structural changes and interface side reactions of the high nickel material during the charging and discharging process, and reduce particle cracking and shell shedding.

[0022] (2) The ternary material precursor, ternary material and preparation method provided in this application, through optimization of the thickness of the core layer and the transition layer, the obtained ternary material, when used as a cathode material in a battery, exhibits excellent discharge specific capacity (up to 221.6 mAh g-1 at 0.2C discharge specific capacity) and good cycle retention rate (98.1% retention rate after 100 cycles at 1C current). At the same time, the thermal stability of the ternary material is also significantly improved, thereby reducing the risk of thermal runaway of the battery material.

[0023] (3) The present application provides a ternary material precursor, ternary material and preparation method. The preparation process of the precursor and ternary material is simple, highly repeatable and has low energy consumption, which is conducive to large-scale production and use. Attached Figure Description

[0024] Figure 1 shows the thermal stability test results of coin cells prepared from ternary materials in Example 2 and Comparative Example 1 of this application. Detailed Implementation

[0025] The present application will be further described below with reference to specific embodiments.

[0026] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the term “and / or” as used herein includes any and all combinations of one or more of the associated listed items.

[0027] Unless otherwise specified in the examples, the procedures should be performed under standard conditions or conditions recommended by the manufacturer. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0028] As used herein, the term “about” is used to provide flexibility and imprecision in relation to a given term, measure, or value. Those skilled in the art can readily determine the degree of flexibility for a particular variable. As used herein, the term “at least one of…” is intended to be synonymous with “one or more of…”. For example, “at least one of A, B, and C” explicitly includes only A, only B, only C, and combinations thereof. Concentration, amount, and other numerical data may be presented herein in range format. It should be understood that such range format is used only for convenience and brevity and should be flexibly interpreted to include not only the numerical values ​​explicitly stated as the limits of the range, but also all individual numerical values ​​or subranges covered within the range, as if each numerical value and subrange were explicitly stated. For example, a numerical range of about 1 to about 4.5 should be interpreted to include not only the explicitly stated limits of 1 to about 4.5, but also individual numbers (such as 2, 3, 4) and subranges (such as 1 to 3, 2 to 4, etc.). The same principle applies to ranges that state only one numerical value, such as “less than about 4.5,” which should be interpreted to include all the values ​​and ranges described above. Furthermore, this interpretation should apply regardless of the scope or breadth of the features described.

[0029] Example 1 This example provides a ternary material precursor, a ternary material, and a preparation method thereof.

[0030] The precursor has a multi-layered composite structure, consisting of a core layer, a first transition layer, and a second transition layer from the inside out. The core layer has a Ni structure. x Co y Mn 1-x-y (OH)₂, x=0.85, y=0.08; the first transition layer structure is Ni x Co y Mn 1-x-y (OH)₂, x=0.55, y=0.25; the second transition layer structure is Ni x Co y Mn 1-x-y(OH)2, x=0.45, y=0.15; the thickness ratio of the core layer, the first transition layer, and the second transition layer is approximately 0.6:0.3:0.1.

[0031] The precursor preparation method includes the following steps: S1, preparing mixed salt solutions A, B, and C containing Ni, Co, and Mn metal ions; S2, preparing the core layer material: pumping mixed salt solution A from S1 into a reactor, controlling the temperature in the reactor at 52℃ and the pH at 11.1 (4 mol / L sodium hydroxide solution), and stirring continuously for 60 h to obtain precipitated solid 1, which is the core layer material; S3, coating the first transition layer: pumping mixed salt solution B from S1 into reactor S2, controlling the temperature in the reactor at 52℃ and the pH at 11.1, and stirring continuously for 60 h to obtain precipitated solid 2, which completes the coating of the first transition layer outside the core layer material from S1; S4, coating the second transition layer: pumping mixed salt solution C from S1 into reactor S3, controlling the temperature in the reactor at 52℃ and the pH at 11.1, and stirring continuously for 60 h to obtain precipitated solid 2, which completes the coating of the first transition layer outside the core layer material from S1; h, precipitated solid 3 is obtained, that is, the second transition layer is completed outside the first transition layer of S2, and the ternary material precursor is obtained.

[0032] In S1: the molar concentration ratio of Ni, Co, and Mn metal ions in mixed salt solution A is 8.5:0.8:0.7; the molar concentration ratio of Ni, Co, and Mn metal ions in mixed salt solution B is 5.5:2.5:2.0; and the molar concentration ratio of Ni, Co, and Mn metal ions in mixed salt solution C is 4.5:1.5:4.0. The concentration of the above mixed salt solutions A, B, and C in this embodiment is 1 mol / L. The Ni metal ions are all derived from NiSO4·6H2O; the Co metal ions are all derived from CoSO4·7H2O; and the Mn metal ions are all derived from MnSO4·H2O.

[0033] Meanwhile, by controlling the pumping volume of the above-mentioned mixed salt solutions A, B, and C, the thickness of the core layer, the first transition layer, and the second transition layer can be controlled. The pumping volume of mixed salt solution A accounts for 60% of the total salt solution volume, the pumping volume of mixed salt solution B accounts for 30% of the total salt solution volume, and the pumping volume of mixed salt solution C accounts for 10% of the total salt solution volume. Finally, a precursor material with a thickness ratio of approximately 0.6:0.3:0.1 for the core layer, the first transition layer, and the second transition layer is obtained.

[0034] The ternary material in this embodiment contains the precursor prepared above. The specific preparation process is as follows: the precursor prepared above is dried at 80°C for 16 h, and then mixed with LiOH·H2O and B2O3 at a molar ratio of 1:1.02:0.02. After mechanical grinding, the mixture is placed in a tube furnace and sintered in an oxygen atmosphere. The first stage is carried out at 500°C for 5 h, and the second stage is carried out at 790°C for 15 h. After cooling, the ternary material is obtained.

[0035] Example 2 This example provides a ternary material precursor, a ternary material, and a preparation method thereof.

[0036] The ternary material precursor differs from the ternary material precursor in Example 1 in that the thickness ratio of its core layer, first transition layer, and second transition layer is approximately 0.6:0.25:0.15. Specifically, it is prepared by adjusting the pumped volume of mixed salt solution A to 60% of the total salt solution volume, the pumped volume of mixed salt solution B to 25% of the total salt solution volume, and the pumped volume of mixed salt solution C to 15% of the total salt solution volume during the precursor preparation process.

[0037] The ternary material and its preparation method are the same as in Example 1.

[0038] Example 3 This example provides a ternary material precursor, a ternary material, and a preparation method thereof.

[0039] The ternary material precursor differs from the ternary material precursor in Example 1 in that the thickness ratio of its core layer, first transition layer, and second transition layer is approximately 0.6:0.35:0.05. Specifically, it is prepared by adjusting the pumped volume of mixed salt solution A to 60% of the total salt solution volume, the pumped volume of mixed salt solution B to 35% of the total salt solution volume, and the pumped volume of mixed salt solution C to 5% of the total salt solution volume during the precursor preparation process.

[0040] The ternary material and its preparation method are the same as in Example 1.

[0041] Comparative Example 1 This comparative example provides a ternary material precursor, a ternary material, and a preparation method thereof.

[0042] Unlike the ternary material precursor in Example 1, the ternary material precursor in this comparative example has a single-layer structure with the structural formula Ni. x Co y Mn 1-x-y (OH)2, x=0.72, y=0.138.

[0043] The preparation process of the precursor is as follows: a mixed salt solution containing Ni, Co, and Mn metal ions is prepared, wherein the molar concentration ratio of Ni, Co, and Mn metal ions is 7.2:1.38:1.42, and the concentration of the mixed salt solution is 1 mol / L. The mixed salt solution is pumped into a reaction vessel, and the temperature in the reaction vessel is controlled at 52℃ and the pH is 11.1. After continuous stirring for 60 h, a precipitated solid is obtained, which is the single-layer ternary material precursor.

[0044] In this comparative example, the ternary material was prepared using the above-mentioned monolayer precursor, and the specific preparation process was the same as in Example 1.

[0045] Comparative Example 2: This comparative example provides a ternary material precursor, a ternary material, and a preparation method thereof.

[0046] In this comparative example, the ternary material precursor and its preparation method are the same as in Example 1; the difference between the ternary material in this comparative example and Example 1 is that in the preparation process, the dried precursor is only mixed with LiOH·H2O at a molar ratio of 1:1.02, and no metal oxide is added. All other preparation processes are the same as in Example 1.

[0047] Example 4 This example provides performance testing of the ternary materials prepared in Examples 1-3 and Comparative Examples 1-2.

[0048] Specifically, using the ternary material as the active material, conductive carbon black (SP) as the conductive agent, polyvinylidene fluoride (PVDF) as the binder, and N-methyl-2-pyrrolidone (NMP) as the dispersant, a slurry was prepared at a mass ratio of ternary material, SP, and PVDF of 90:5:5. This slurry was then coated onto aluminum foil to form the positive electrode. A lithium metal sheet was used as the negative electrode, a polypropylene microporous membrane as the separator, and 1 mol / L LiPF6 as the electrolyte. A CR2032 type button cell was fabricated in an argon-filled glove box. Constant current charge-discharge tests were conducted at room temperature at 0.2 C, with charge-discharge cutoff voltages ranging from 2.8 V to 4.35 V. The results are shown in Table 1.

[0049] Table 1

[0050] It can be seen that the ternary material prepared in Example 2 exhibits the best performance when used as a cathode material for lithium batteries, with an initial discharge specific capacity of 221.69 mAh g⁻¹ and an initial charge-discharge efficiency of 87.4%; at a 1C current, the cycle retention rate is 98.1% after 100 cycles. This indicates that the ternary material in this application can effectively improve the capacity, rate performance, and cycle stability of the cathode material.

[0051] Example 5 This example provides thermal stability tests on the ternary materials prepared in Example 2 and Comparative Example 1.

[0052] Specifically, the ternary material from Example 2 was used to fabricate a CR2032 type button cell (using the same method as in Example 4), and its thermal performance was analyzed using differential scanning calorimetry (DSC).

[0053] Results Analysis: As shown in Figure 1, it can be seen that compared with the ternary material in Comparative Example 1, the ternary material prepared in Example 2 exhibits a shift in the exothermic reaction peak, and its thermal stability temperature increases by about 11°C. This confirms that the ternary material in this application has good thermal stability when used as a cathode material, which can effectively improve the safety performance of the battery.

Claims

1. A ternary material precursor, characterized in that, The precursor is a multi-layered composite structure, comprising, from the inside out, a core layer, a first transition layer, and a second transition layer, wherein: the core layer has a Ni structure. x Co y Mn 1-x-y (OH)₂, 0.8≤x≤0.9, 0.05≤y≤0.1; the first transition layer structure is Ni x Co y Mn 1-x-y (OH)₂, 0.5≤x≤0.6, 0.2≤y≤0.3; the second transition layer structure is Ni x Co y Mn 1-x-y (OH)2, 0.3≤x<0.5, 0.1≤y<0.

2.

2. The precursor according to claim 1, characterized in that, The thickness ratio of the core layer, the first transition layer, and the second transition layer is (0.6~0.7):(0.2~0.3):(0~0.2).

3. The method for preparing the precursor according to claim 1 or 2, characterized in that, The method includes the following steps: S1, preparing mixed salt solutions A, B, and C containing Ni, Co, and Mn metal ions; S2, preparing the core layer material: pumping mixed salt solution A from S1 into a reactor, controlling the temperature in the reactor at 50-55℃ and the pH at 11-12, and continuously stirring to obtain precipitated solid 1, which is the core layer material; S3, coating the first transition layer: pumping mixed salt solution B from S1 into reactor S2, controlling the temperature in the reactor at 50-55℃ and the pH at 11-12, and continuously stirring to obtain precipitated solid 2, which completes the coating of the first transition layer outside the core layer material from S1; S4, coating the second transition layer: pumping mixed salt solution C from S1 into reactor S3, controlling the temperature in the reactor at 50-55℃ and the pH at 11-12, and continuously stirring to obtain precipitated solid 3, which completes the coating of the second transition layer outside the first transition layer from S2, thus obtaining the ternary material precursor.

4. The method according to claim 3, characterized in that, The molar concentration ratio of Ni, Co, and Mn metal ions in the mixed salt solution A is (8~9):(1~0.5):(1~0.5); and / or the molar concentration ratio of Ni, Co, and Mn metal ions in the mixed salt solution B is (5~6):(2~3):(3~1); and / or the molar concentration ratio of Ni, Co, and Mn metal ions in the mixed salt solution C is (3~5):(2~1):(5~4).

5. The method according to claim 3 or 4, characterized in that, The metal ions Ni in the mixed salt solution A and / or mixed salt solution B and / or mixed salt solution C are derived from any one or more of NiSO4·6H2O, NiCO3, Ni(C2H3O2)2, and Ni(NO3)2·6H2O; and / or the metal ions Co are derived from any one or more of CoSO4·7H2O, CoCO3, Co(C2H3O2)2, and Co(NO3)2·6H2O; and / or the metal ions Mn are derived from any one or more of MnSO4·H2O, MnCO3, Mn(C2H3O2)2, and Mn(NO3)2·6H2O.

6. The use of the precursor according to claim 1 or 2 in the preparation of ternary materials for lithium-ion battery cathode materials.

7. A ternary material, characterized in that, The ternary material comprises the precursor as described in claim 1 or 2.

8. The method for preparing the ternary material according to claim 7, characterized in that, The method includes: drying the precursor according to claim 1 or 2, mixing it with a lithium source and a metal oxide in a molar ratio of 1:(0.98~1.04):(0.01~0.05), and then sintering it to obtain the product.

9. The method according to claim 8, characterized in that, The lithium source includes lithium hydroxide and / or lithium carbonate; and / or the metal oxide includes any one or more of B2O3, Al2O3, MgO, CeO2, WO3, TiO2, ZrO2, Nb2O5, Ta2O5, La2O3, and Y2O3.

10. The method according to claim 9, characterized in that, The sintering is carried out in an oxygen atmosphere and includes two stages: the first stage is carried out at 450~500℃ for 3~7 h, and the second stage is carried out at 600~800℃ for 12~16 h.

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