Lithium cobalt oxide battery positive electrode material coated with lithium-deficient coating layer and preparation method of lithium cobalt oxide battery positive electrode material

Abstract

The invention provides a lithium cobalt oxide battery positive electrode material coated with a lithium-deficient coating layer and a preparation method thereof, the material is of a core-shell structure, and comprises a doped lithium cobalt oxide core material in the core-shell structure and a coating layer formed on the surface of the doped lithium cobalt oxide core material; the chemical formula of the doped lithium cobalt oxide core material is LiCo (1-a) MMaO2, and a is greater than or equal to 0.001 and less than or equal to 0.005; the coating layer comprises LixCoO2, yttrium oxide and lanthanum oxide; and the content of the coating layer accounts for 2-6% of that of the lithium cobalt oxide battery positive electrode material coated by the lithium-deficient coating layer. Nanometer Co3O4 on the surface of the lithium cobalt oxide battery positive electrode material is coated with a lithium-deficient coating layer to react with excessive Li to generate lithium-deficient state LixCoO2 with a spinel structure, and the lithium-deficient state LixCoO2 (xlt; according to the present invention, with the composite metal oxide coating layer of 1, 1), yttrium oxide and lanthanum oxide, the stable mechanical property and the small volume change are provided, the precipitation of O atoms in the structure under the high voltage is inhibited, the direct contact between the material and the electrolyte is reduced by the coating layer, the side reaction and the electrolyte corrosion are inhibited, and the capacity and the cycle performance are significantly improved;

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, in particular to a lithium-deficient coating layer coated lithium cobalt oxide battery positive electrode material and a preparation method thereof. BACKGROUND

[0002] Lithium ion batteries are one of the most widely used energy storage devices at present, and compared with traditional lead-acid batteries, they have higher energy density, longer cycle life, no pollution and no memory effect during use, and are widely loved by consumers. Lithium cobalt oxide is mainly used in lithium ion batteries of small portable electronic devices, such as mobile phones, notebook computers, smart watches, Bluetooth earphones and other small electronic products.

[0003] The theoretical specific capacity of lithium cobalt oxide is relatively low, about 274 mAh / g, and the actual specific capacity in practical application is generally about 140-160 mAh / g. With the increasing demand for battery energy density, lithium cobalt oxide can only be improved by continuously increasing the charge and discharge voltage and further improving the specific capacity, but at a higher voltage, lithium cobalt oxide is prone to structural changes and thermal runaway, and the safety performance and cycle are greatly reduced. SUMMARY

[0004] The purpose of the present application is to overcome the deficiencies in the prior art and provide a lithium-deficient coating layer coated lithium cobalt oxide battery positive electrode material and a preparation method thereof.

[0005] To achieve the above purpose, the technical scheme adopted by the present application is as follows: a lithium-deficient coating layer coated lithium cobalt oxide battery positive electrode material, the lithium-deficient coating layer coated lithium cobalt oxide battery positive electrode material is a core-shell structure, including a doped lithium cobalt oxide core material inside the core-shell structure and a coating layer formed on the surface of the doped lithium cobalt oxide core material. The chemical formula of the doped lithium cobalt oxide core material is: LiCo 1-a Mg a O2, wherein 0.001≤a≤0.005; The coating layer includes Li x CoO2, yttrium oxide and lanthanum oxide; The content of the coating layer is 2%-6% of the lithium-deficient coating layer coated lithium cobalt oxide battery positive electrode material.

[0006] The above-mentioned lithium-deficient coating layer coated lithium cobalt oxide battery positive electrode material lithium cobalt oxide is a core-shell structure, including a doped lithium cobalt oxide core material inside the core-shell structure and a coating layer formed on the surface of the doped lithium cobalt oxide core material, and the lithium cobalt oxide is a single crystal structure, the surface nanometer Co3O4 reacts with excess Li to generate a spinel structure of lithium-deficient Li xCoO2, or lanthanum oxide, possesses good thermal stability. By improving the crystal structure of the cathode material, its structural stability during charge and discharge processes is enhanced, thereby extending the battery's lifespan. Furthermore, lanthanum ions have a large ionic radius and strong redox reactivity, which can improve the material's energy density and ionic conductivity. Yttrium oxide exhibits good thermal and chemical stability, effectively preventing oxidation and corrosion of the material at high temperatures. It improves the electronic conductivity of the cathode material by reducing the work function, and its high dielectric constant can increase the battery's energy density. The coating layer encapsulates lithium-deficient Li... x The composite metal oxide coating of CoO2 (x<1), yttrium oxide, and lanthanum oxide works together to improve the crystal structure of the cathode material, resulting in better thermal and chemical stability, more stable mechanical properties, and smaller volume changes. It also suppresses the precipitation of O atoms in the structure under high voltage. At the same time, the coating reduces the direct contact between the material and the electrolyte, suppressing side reactions and electrolyte corrosion, and significantly improving capacity and cycle performance.

[0007] Preferably, the Li in the coating layer x The ratio of CoO2, yttrium oxide, and lanthanum oxide is 10:(0.5~2):(0.5~2).

[0008] Preferably, the coating layer comprises Li x CoO2, niobium oxide, yttrium oxide, and lanthanum oxide.

[0009] The inventors discovered through research that the coating layer encapsulates lithium-deficient Li x In the composite metal oxide coating layer of CoO2 (x<1) and niobium oxide, yttrium oxide, and lanthanum oxide, the lithium-deficient LixCoO2 (x<1) has a spinel structure, exhibits small volume changes during delithiation, and possesses better structural stability. It maintains the mechanical integrity of the core-shell cathode material structure during cycling. Furthermore, Li vacancies provide more ion transport channels, improving battery capacity and rate performance. Niobium oxide has a stable crystal structure and a unique lithium-ion intercalation mechanism (intercalation reaction), enabling rapid charge and discharge and improving rate performance; its volume expansion rate is less than 2%, significantly enhancing battery safety. Lanthanum oxide has good thermal stability; by improving the crystal structure of the cathode material, its structural stability during charge and discharge is enhanced, thereby extending battery life. Additionally, lanthanum ions have a large ionic radius and strong redox reactivity, which can improve the material's energy density and ionic conductivity. Yttrium oxide has good thermal and chemical stability, effectively preventing oxidation and corrosion at high temperatures. It improves the electronic conductivity of the cathode material by reducing the work function, and its high dielectric constant can increase the battery's energy density.

[0010] The present application adopts a lithium-deficient LixCoO2 (x<1) combined with a composite metal oxide coating layer, which firstly absorbs the surface Li through a nano Co3O4 to form a stable surface structure coating layer with more Li ion channels, and then improves the thermal stability of the material, and improves the ion and electron conductivity and energy density through the stable crystal structure of the composite metal. Through the synergistic effect of the multi-layer coating, on the one hand, the surface structure stability and chemical stability of the positive electrode material can be enhanced, the decomposition corrosion or structure damage of the material at high voltage can be prevented, the contact and compatibility between the positive electrode material and the electrolyte can be improved, so that the cycle life and safety performance of the battery are improved, on the other hand, the electronic conductivity and ion conductivity of the positive electrode material are improved through increasing the oxygen vacancy concentration and surface activity of the positive electrode material, so that the capacity and rate performance of the battery are improved. The ion and electron conductivity of the material is improved, the volume change is reduced, the crystal structure is more stable, the surface structure activity and stability are enhanced, and the coating layer reduces the direct contact between the material and the electrolyte, inhibits the side reaction and electrolyte corrosion, and the capacity and cycle performance are obviously improved.

[0011] Preferably, the content of Li x The ratio of CoO2, niobium oxide, yttrium oxide and lanthanum oxide is 10: (0.5-2): (0.5-2): (0.5-2).

[0012] Preferably, the content of Li x The source material of CoO2 is nano Co3O4.

[0013] Preferably, the content of the coating layer is 2.0%-4.5% of the lithium-deficient coated lithium cobalt oxide battery positive electrode material.

[0014] Preferably, the content of Li x The ratio of CoO2, niobium oxide, yttrium oxide and lanthanum oxide is 10: (0.8-1.2): (1.2-1.5): (1.0-1.5).

[0015] The inventors found through research that the coating layer coated with the lithium-deficient Li x When the content of the composite metal oxide coating layer of CoO2 (x<1) and niobium oxide, yttrium oxide and lanthanum oxide meets the above requirements, the cycle performance is more excellent.

[0016] The present application also provides a preparation method of the above-mentioned lithium-deficient coated lithium cobalt oxide battery positive electrode material, which comprises the following steps: (1) mixing cobalt trioxide, lithium carbonate and magnesium salt in a certain proportion, and sintering at 950-1100°C in an oxygen-containing atmosphere to obtain LiCo 1-a Mg a O2 material particles; (2) sintering the LiCoO2 material particles obtained in step (1) with nano Co3O4 and oxides of other materials forming the coating layer in an oxygen-containing atmosphere at 800-900°C to obtain the lithium-deficient coating layer coated lithium cobalt oxide battery cathode material. 1-a Mg a O2 material particles with nano Co3O4 and oxides of other materials forming the coating layer in an oxygen-containing atmosphere at 800-900°C to obtain the lithium-deficient coating layer coated lithium cobalt oxide battery cathode material.

[0017] The preparation method of the lithium-deficient coating layer coated lithium cobalt oxide battery cathode material is simple and practical, and can be prepared in large quantities, and provides a simple and short high-temperature sintering process for preparing high-voltage lithium cobalt oxide material.

[0018] Preferably, in step (1), the particle size Dv50 of the tricobalt tetraoxide is 9.5-11 μm, the Li / Co molar ratio of the tricobalt tetraoxide and lithium carbonate is 1.05-1.07:1, and the lithium carbonate is a single crystal structure.

[0019] Preferably, in step (2), the LiCoO2 material particles obtained in step (1) are sintered at a particle size Dv50 of 12-14 μm. 1-a Mg a O2 material particles with nano Co3O4 and oxides of other materials forming the coating layer in an oxygen-containing atmosphere at 800-900°C to obtain the lithium-deficient coating layer coated lithium cobalt oxide battery cathode material.

[0020] Preferably, the sintering time of step (1) is 6-8 hours, and the sintering time of step (2) is 6-9 hours.

[0021] The lithium-deficient coating layer coated lithium cobalt oxide battery cathode material provided by the present application has a core-shell structure, including a doped lithium cobalt oxide core material inside the core-shell structure and a coating layer formed on the surface of the doped lithium cobalt oxide core material, and the lithium cobalt oxide is a single crystal structure. x CoO2, the coating layer coats the lithium-deficient Li x CoO2 (x<1) and a composite metal oxide coating layer of yttrium oxide and lanthanum oxide, has more stable mechanical properties and smaller volume change, inhibits the release of O atoms in the structure at high voltage, and at the same time the coating layer reduces the direct contact of the material with the electrolyte, inhibits the side reaction and electrolyte corrosion, and the capacity and cycle performance are obviously improved. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 SEM image of the lithium-deficient coating layer coated lithium cobalt oxide battery cathode material of the embodiment of the present application.

[0023] Figure 2 TEM image of the lithium-deficient coating layer coated lithium cobalt oxide battery cathode material of the embodiment of the present application. DETAILED DESCRIPTION

[0024] For better illustrating the purpose, technical scheme and advantages of the present application, the present application will be further described in combination with specific examples.

[0025] Example 1

[0026] As a kind of lithium deficiency coating layer coated lithium cobaltate battery positive electrode material of the embodiment of the present application, lithium deficiency coating layer coated lithium cobaltate battery positive electrode material is core-shell structure, including doped lithium cobaltate core material in the core-shell structure and the coating layer formed on the surface of doped lithium cobaltate core material; The chemical formula of the doped lithium cobaltate core material is: LiCo 1-a Mg a O 2, wherein a=0.003; The coating layer includes Li x CoO 2, yttrium oxide and lanthanum oxide; The content of the coating layer is 3.0% of the lithium deficiency coating layer coated lithium cobaltate battery positive electrode material; Li x CoO 2, yttrium oxide and lanthanum oxide in the coating layer are 10:1.2:1.2.

[0027] The preparation method of the lithium deficiency coating layer coated lithium cobaltate battery positive electrode material of the embodiment includes the following steps: (1) the Dv50 is 10.5 μm three cobaltic tetroxide, lithium carbonate and magnesium salt are mixed in proportion, and then sintered at 1050 DEG C in oxygen-containing atmosphere for 7 hours to obtain LiCo 1-a Mg a O 2 Material particles; after airflow crushing, the Dv50 particle size is 13.63 μm; the Li / Co molar ratio of three cobaltic tetroxide and lithium carbonate is 1.05~1.07:1, and the lithium carbonate is single crystal structure; (2) the LiCo 1-a Mg a O 2 Material particles obtained in step (1) are mixed with nano Co 3 O 4 and other material oxides forming the coating layer, and then sintered at 880 DEG C in oxygen-containing atmosphere for 8 hours to obtain the lithium deficiency coating layer coated lithium cobaltate battery positive electrode material.

[0028] Example 2

[0029] As a kind of lithium deficiency coating layer coated lithium cobaltate battery positive electrode material of the embodiment of the present application, the only difference between the embodiment and example 1 is that: the coating layer includes Li x CoO 2, yttrium oxide and lanthanum oxide;The ratio of Li x CoO 2, yttrium oxide and lanthanum oxide in the coating layer is 10:0.8:1.5.

[0030] Example 3

[0031] As a kind of lithium deficiency coating layer coated lithium cobaltate battery positive electrode material of the embodiment of the present application, the only difference between this embodiment and embodiment 1 is that: the content of the coating layer is 2.0% of the lithium deficiency coating layer coated lithium cobaltate battery positive electrode material. x CoO2, niobium oxide, yttrium oxide and lanthanum oxide; the ratio of Li x CoO2, niobium oxide, yttrium oxide and lanthanum oxide is 10:0.8:1.2:1.5.

[0032] Example 4

[0033] As a kind of lithium deficiency coating layer coated lithium cobaltate battery positive electrode material of the embodiment of the present application, the only difference between this embodiment and embodiment 3 is that: the content of the coating layer is 2.0% of the lithium deficiency coating layer coated lithium cobaltate battery positive electrode material.

[0034] Example 5

[0035] As a kind of lithium deficiency coating layer coated lithium cobaltate battery positive electrode material of the embodiment of the present application, the only difference between this embodiment and embodiment 3 is that: the content of the coating layer is 2.5% of the lithium deficiency coating layer coated lithium cobaltate battery positive electrode material.

[0036] Example 6

[0037] As a kind of lithium deficiency coating layer coated lithium cobaltate battery positive electrode material of the embodiment of the present application, the only difference between this embodiment and embodiment 3 is that: the content of the coating layer is 4.5% of the lithium deficiency coating layer coated lithium cobaltate battery positive electrode material.

[0038] Example 7

[0039] As a kind of lithium deficiency coating layer coated lithium cobaltate battery positive electrode material of the embodiment of the present application, the only difference between this embodiment and embodiment 3 is that: the content of the coating layer is 5.5% of the lithium deficiency coating layer coated lithium cobaltate battery positive electrode material.

[0040] Example 8

[0041] As a kind of lithium deficiency coating layer coated lithium cobaltate battery positive electrode material of the embodiment of the present application, the only difference between this embodiment and embodiment 3 is that: the content of the coating layer is 2.0% of the lithium deficiency coating layer coated lithium cobaltate battery positive electrode material. x CoO2, niobium oxide, yttrium oxide and lanthanum oxide is 10:0.5:0.5:2.0.

[0042] Example 9

[0043] As a kind of lithium deficiency coating layer coated lithium cobaltate battery positive electrode material of the embodiment of the present application, the only difference between this embodiment and embodiment 3 is that: the content of the coating layer is 2.0% of the lithium deficiency coating layer coated lithium cobaltate battery positive electrode material. xCoO2, niobium oxide, yttrium oxide and lanthanum oxide are in a ratio of 10:0.5:0.5:0.5.

[0044] Example 10

[0045] As a lithium-deficient coated layer coated lithium cobalt oxide battery cathode material of an embodiment of the present invention, the only difference between this example and Example 3 is that Li x CoO2, niobium oxide, yttrium oxide and lanthanum oxide are in a ratio of 10:2.0:0.5:0.5.

[0046] Example 11

[0047] As a lithium-deficient coated layer coated lithium cobalt oxide battery cathode material of an embodiment of the present invention, the only difference between this example and Example 3 is that Li x CoO2, niobium oxide, yttrium oxide and lanthanum oxide are in a ratio of 10:1.0:1.2:1.0.

[0048] Example 12

[0049] As a lithium-deficient coated layer coated lithium cobalt oxide battery cathode material of an embodiment of the present invention, the only difference between this example and Example 3 is that Li x CoO2, niobium oxide, yttrium oxide and lanthanum oxide are in a ratio of 10:1.2:1.2:1.5.

[0050] Example 13

[0051] As a lithium-deficient coated layer coated lithium cobalt oxide battery cathode material of an embodiment of the present invention, the only difference between this example and Example 3 is that Li x CoO2, niobium oxide, yttrium oxide and lanthanum oxide are in a ratio of 10:0.8:1.2:1.0.

[0052] Comparative Example 1 As a lithium-deficient coated layer coated lithium cobalt oxide battery cathode material of a comparative example of the present invention, the only difference between this comparative example and Example 3 is that: the coated layer includes Li x CoO2, yttrium oxide.

[0053] Comparative Example 2 As a lithium-deficient coated layer coated lithium cobalt oxide battery cathode material of a comparative example of the present invention, the only difference between this comparative example and Example 3 is that: the coated layer includes Li x CoO2, niobium oxide.

[0054] Comparative Example 3 As a comparative example of the present invention, a lithium-deficient coating layer for a lithium cobalt oxide battery cathode material differs from Example 3 only in that: The coating layer includes Li x CoO2, Lanthanum oxide.

[0055] Comparative Example 4 As a comparative example of the present invention, a lithium-deficient coating layer for a lithium cobalt oxide battery cathode material differs from Example 3 only in that: The coating layer includes niobium oxide, lanthanum oxide, and yttrium oxide.

[0056] Experimental methods I. Material Characterization (a) Types of covering layers ICP tests were performed on samples from Examples 1, 3, and Comparative Examples 1-4. The test results are shown in Table 1 below: Table 1. Main elemental contents of lithium cobalt oxide battery cathode materials with lithium-deficient coating. Figure 1 The image shown is a SEM image of Example 3, illustrating that the core material has a single-crystal morphology. Figure 2 The image shown is a high-magnification transmission electron microscope (TEM) image from Example 3 of this invention. The results indicate that a dense coating layer has formed on the surface of the core structure. Table 1 shows the elemental analysis of the main elements present in the lithium-deficient coating layer covering the cathode material of the lithium cobalt oxide battery. Figure 2 It can be seen that the samples of Example 1, Example 3, and Comparative Examples 1-4 formed different types of coating layers.

[0057] II. Material Performance Testing The positive electrode material, conductive agent acetylene black, and binder polyvinylidene fluoride were weighed in a mass ratio of 90%:5%:5%. An appropriate amount of N-methylpyrrolidone was added and the mixture was stirred into a slurry by centrifugation. The slurry was then uniformly coated onto a 20 μm thick aluminum foil to form a 150 μm thick electrode sheet. After drying the electrode sheet at 150℃ for 8 hours, it was pressed to a thickness of 80 μm with a pressure of 5 MPa and then cut into circular positive electrode sheets with a diameter of Φ = 13.5 mm.

[0058] Using a prepared circular positive electrode as the positive electrode and a lithium metal sheet as the negative electrode, and 1 mol / L LiPF6 / EC+DMC (volume ratio 1:1, produced in Zhangjiagang, battery grade) as the electrolyte, and a Celgard 2400 membrane as the separator, the cells were assembled and sealed into CR2032 coin cell half-cells in a vacuum glove box filled with dry argon gas. The battery's electrical performance was tested at voltages of 3.0–4.48 V.

[0059] The experimental results are shown in Table 2. As shown in Table 2, the lithium cobalt oxide cathode material coated with the lithium-deficient coating layer of the present invention has a core-shell structure, including a doped lithium cobalt oxide core material inside the core-shell structure and a coating layer formed on the surface of the doped lithium cobalt oxide core material. The lithium cobalt oxide has a single crystal structure, and its surface nano-Co3O4 reacts with excess Li to generate a spinel-structured lithium-deficient Li. x CoO2, coating layer encapsulating lithium-deficient Li x The composite metal oxide coating of CoO2 (x<1), yttrium oxide, and lanthanum oxide can enhance the surface structural stability and chemical stability of the cathode material, improve the contact and compatibility between the cathode material and the electrolyte, thereby improving the cycle life and safety performance of the battery. On the other hand, it can improve the electronic conductivity and ionic conductivity of the cathode material, thereby improving the capacity and rate performance of the battery.

[0060] Meanwhile, the coating layer encapsulates lithium-deficient Li. x The composite metal oxide coating of CoO2 (x<1) with niobium oxide, yttrium oxide and lanthanum oxide, in addition to the above performance advantages, further improves rate performance through the unique lithium-ion intercalation mechanism of niobium oxide.

[0061] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A lithium-deficient coating layer for lithium cobalt oxide battery cathode material, characterized in that, The lithium-deficient coating layer covers the lithium cobalt oxide battery cathode material with a core-shell structure, including a doped lithium cobalt oxide core material inside the core-shell structure and a coating layer formed on the surface of the doped lithium cobalt oxide core material. The chemical formula of the doped lithium cobalt oxide core material is: LiCo 1-a Mg a O2, where 0.001≤a≤0.005; The coating layer includes Li x CoO2, yttrium oxide, and lanthanum oxide; The content of the coating layer is 2% to 6% of the lithium-deficient coating layer coating lithium cobalt oxide battery cathode material.

2. The lithium-deficient coating layer for lithium cobalt oxide battery cathode material according to claim 1, characterized in that, The coating layer includes Li x CoO2, niobium oxide, yttrium oxide, and lanthanum oxide.

3. The lithium-deficient coating layer for lithium cobalt oxide battery cathode material according to claim 1, characterized in that, Li in the coating layer x The ratio of CoO2, niobium oxide, yttrium oxide and lanthanum oxide is 10:(0.5~2):(0.5~2):(0.5~2).

4. The lithium-deficient coating layer for lithium cobalt oxide battery cathode material according to claim 1, characterized in that, The Li in the coating layer x The source material for CoO2 is nano-Co3O4.

5. The lithium-deficient coating layer for lithium cobalt oxide battery cathode material according to claim 1, characterized in that, The content of the coating layer is 2.0% to 4.5% of the lithium-deficient coating layer coating of the lithium cobalt oxide battery cathode material.

6. The lithium-deficient coating layer for lithium cobalt oxide battery cathode material according to claim 1, characterized in that, Li in the coating layer x The ratio of CoO2, niobium oxide, yttrium oxide and lanthanum oxide is 10: (0.8~1.2): (1.2~1.5): (1.0~1.5).

7. The method for preparing the lithium cobalt oxide battery cathode material coated with a lithium-deficient coating layer as described in any one of claims 1 to 6, characterized in that, The method includes the following steps: (1) LiCo was obtained by mixing cobalt tetroxide, lithium carbonate and magnesium salt in a certain proportion and sintering at 950~1100℃ in an oxygen-containing atmosphere. 1-a Mg a O2 material particles; (2) The LiCo obtained in step (1) 1-a Mg a O2 material particles are mixed with nano-Co3O4 and oxides of other materials forming the coating layer, and then sintered at 800~900℃ in an oxygen-containing atmosphere to obtain lithium-deficient coating layer coated lithium cobalt oxide battery cathode material.

8. The method for preparing the lithium-deficient coating layer of the lithium cobalt oxide battery cathode material according to claim 7, characterized in that, In step (1), the particle size Dv50 of cobalt tetroxide is 9.5~11 μm, the Li / Co molar ratio of cobalt tetroxide to lithium carbonate is 1.05~1.07:1, and the lithium carbonate is a single crystal structure.

9. The method for preparing the lithium-deficient coating layer of the lithium cobalt oxide battery cathode material according to claim 7, characterized in that, In step (2), the LiCo obtained in step (1) 1-a Mg a The O2 material particles were controlled to have a Dv50 of 12~14μm for sintering in step (2).

10. The method for preparing the lithium-deficient coating layer of the lithium cobalt oxide battery cathode material according to claim 7, characterized in that, The sintering time for step (1) is 6 to 8 hours, and the sintering time for step (2) is 6 to 9 hours.

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