High-nickel cobalt-free positive electrode material capable of dual residual alkali reduction and preparation method therefor

Abstract

A high-nickel cobalt-free positive electrode material capable of dual residual alkali reduction and a preparation method therefor. The chemical expression of the high-nickel cobalt-free positive electrode material is LixNiyMn1-yAzO2, where 0.75＜y≤0.95, 0.95≤x≤1.15, and 0.001＜z≤0.005. Primary residual alkali reduction is implemented by means of water washing. In-situ formation of perovskite on the surface of the material consumes residual alkali in the material so as to achieve secondary residual alkali reduction, and can simultaneously form a LaaLibCoO3 perovskite coating layer. Because of the formed perovskite coating layer, the positive electrode material has high lithium ionic conductivity, a good capacity retention capacity and rate capability, and high reversibility. The prepared high-nickel cobalt-free positive electrode material has low residual alkali, a high discharge capacity, a good rate capability and good battery cycle performance.

Description

A high-nickel cobalt-free cathode material with double reduced residual alkali and its preparation method

[0001] This application claims priority to Chinese patent application No. 202410015280.7, filed on January 5, 2024. This application incorporates the entire text of the aforementioned Chinese patent application. Technical Field

[0002] The present application relates to a cobalt-free positive electrode material for lithium batteries, and in particular to a high-nickel cobalt-free positive electrode material with double reduced residual alkali and a preparation method thereof. Background Art

[0003] As the automotive industry transitions to electrification, rechargeable batteries must also meet future demands. Although efforts to improve battery performance have achieved some success, they currently face high cost barriers. These challenges in battery costs are mainly related to the soaring prices and increased demand for transition metals (TMs), especially cobalt (Co), which is a core material component of widely used commercial cathodes such as LiCoO2, LiNi x Mn y Co 1-x-y O2 and LiNi 0.8 Co 0.15 Al 0.05 O2. In recent years, Co has lost its economic appeal. In response to these cost pressures, those skilled in the art are working hard to develop low-Co or even Co-free cathodes without sacrificing battery performance. Although including lithium-rich and manganese-rich cathodes, high voltage spinel LiNi 0.5 Mn 1.5 Several possible alternatives, including O4 and disordered rock salts, have been highlighted as viable replacements for cobalt-containing cathodes, but they suffer from difficult-to-obtain capacity and stability, preventing them from being used for large-scale commercial applications. Therefore, layered cathodes with high Ni content and low Co content, or even no Co content at all, have become a hot topic of research in the battery community.

[0004] However, the higher the nickel content, the more alkaline substances are formed on the cathode surface, especially LiOH and Li2CO3. This will accelerate capacity decay and intensify structural phase transitions, seriously affecting the air stability of the cathode material and its processing performance during slurry preparation and coating, affecting the safety and cyclability of the battery during use. Therefore, reducing the residual alkaline content on the surface of high-nickel materials is an urgent technical problem to be solved.

[0005] For example, CN110148728A adds LiNO3, La(NO3)3, and Ti[OCH(CH3)2]4 through a series of operations to create an LLTO-coated ternary material. This method yields a positive electrode material with excellent cycle stability and rate performance. However, the addition of an additional Li source prevents the reduction of residual alkalinity in the material.

[0006] For example, in CN112086638A, the cathode material of the battery is dispersed in an organic solvent containing a phosphorus-containing organic compound. Through liquid-phase mixing or in-situ polymerization reaction, the phosphorus-containing organic compound is uniformly distributed on the surface of the cathode material of the battery. During the subsequent calcination treatment process, phosphorus reacts with the residual lithium on the surface at high temperature to in-situ generate lithium phosphate, and the cathode material of the battery coated with lithium phosphate is obtained. Thereby, the adverse effects of water on the cathode material of the battery are avoided, the structural stability of the cathode material of the battery is maintained, the residual lithium on the surface is consumed, the alkalinity is reduced, a uniform lithium phosphate coating is formed on the surface of the cathode material of the battery, and the stability of the cathode material of the battery in air is improved. However, this method fails to effectively improve the electrochemical performance and the rate performance of the material, and the process is complex, which is not conducive to large-scale production. Therefore, the preparation method of the high-nickel and cobalt-free cathode material still needs to be improved.

[0007] Summary of the Invention

[0008] The main technical problem to be solved in this application is to overcome the deficiencies such as the ternary material coated with LLTO obtained by adding LiNO3, La(NO3)3, and Ti[OCH(CH3)2]4 in the above background technology that fails to reduce the residual alkali of the material and the surface coating of lithium phosphate that fails to effectively improve the electrochemical performance, and to provide a high-nickel and cobalt-free cathode material that doubles the reduction of residual alkali and its preparation method.

[0009] To achieve the above object, the technical solution adopted in this application is a preparation method of a high-nickel and cobalt-free cathode material that doubles the reduction of residual alkali, including the following steps:

[0010] S1.充分混合 the high-nickel and cobalt-free precursor with a lithium salt and a dopant A to obtain a mixture B; the general formula of the high-nickel and cobalt-free precursor is Ni y Mn 1-y (OH)2, and 0.75 < y ≤ 0.95; the molar ratio of Li element in the lithium salt to Ni y Mn 1-y (OH)2 is (0.95 - 1.15):1, and the dopant A accounts for 0.1 - 0.5 mol% of the total amount of the lithium salt and Ni y Mn 1-y (OH)2; the dopant A is a compound containing at least one of Al, Zr, W, Mo, and Nb;

[0011] S2.在氧气气氛下对混合物B进行第一温度曲线一次焙烧、冷却、破碎得到高镍无钴正极材料C;第一温度曲线为在氧气气氛下首先以3 - 10℃ / min的升温速率升温至800 - 850℃,到达所需温度后保温3 - 20h; <00001​

[0013] S4. Add the La source and Co source to the filter cake, and then put them into a drying equipment together and rotate to dry. After drying, put the dried material into a mixing equipment and mix for 10 - 30 min to make it evenly mixed, obtaining a high-nickel and cobalt-free cathode material D; after the cathode material is pulverized, the content range of matrix Li is 0.2500 - 0.4500 ppm, the added La source accounts for 0.5 - 3 wt% of the filter cake, and the molar ratio of La element in the La source to Co element in the Co source is 0.5 - 0.9:1;

[0014] S5. Carry out secondary calcination, cooling and sieving of the high-nickel and cobalt-free cathode material D under an oxygen atmosphere according to a second temperature curve, and obtain a high-nickel and cobalt-free cathode material with a surface-coated LaLiCoO3 perovskite of the chemical formula LiNiMnO2, where 0.75 < y ≤ 0.95, 0.95 ≤ x ≤ 1.15, 0.001 < z ≤ 0.005; the second temperature curve is to first heat up at a heating rate of 3 - 7 °C / min to 300 - 450 °C under an oxygen atmosphere, and keep the temperature for 3 - 15 h after reaching the required temperature. x Ni y Mn 1-y A z O2, where 0.75 < y ≤ 0.95, 0.95 ≤ x ≤ 1.15, 0.001 < z ≤ 0.005; the second temperature curve is to first heat up at a heating rate of 3 - 7 °C / min to 300 - 450 °C under an oxygen atmosphere, and keep the temperature for 3 - 15 h after reaching the required temperature. a Li b CoO3 perovskite high-nickel and cobalt-free cathode material, where 0.75 < y ≤ 0.95, 0.95 ≤ x ≤ 1.15, 0.001 < z ≤ 0.005; the second temperature curve is to first heat up at a heating rate of 3 - 7 °C / min to 300 - 450 °C under an oxygen atmosphere, and keep the temperature for 3 - 15 h after reaching the required temperature.

[0015] Preferably, the lithium salt in S1 is LiOH.

[0016] Preferably, in S, the mass ratio of water to the high-nickel and cobalt-free cathode material C during the washing process is 1 - 3:1, the washing time is 10 - 30 min, and then solid-liquid separation is carried out in a solid-liquid separation equipment for 15 - 30 min. <000016)3>

[0017] Preferably, the added La source in S4 includes at least one of La2O3, La(NO3)3, La2(CO3)·8H2O, and the Co source includes at least one of Co2O3, Co(OH)2, Co(NO3)2, CoCO3, Co3O4; the particle size range of the La source is 10 - 400 nm, and the particle size range of the Co source is 20 - 300 nm.

[0018] Preferably, the drying method in S4 is a rotatable drying equipment such as a conical spiral ribbon vacuum mixer dryer, the drying temperature is 100 - 200 °C, and the drying time is 0.5 - 7 h.

[0019] Preferably, the mixing of the high-nickel and cobalt-free precursor, lithium salt and dopant A in S1 is carried out by a high-speed mixer or a planetary ball mill, etc. The mixing time of the high-speed mixer is 10 - 30 min, and the mixing time of the planetary ball mill is 3 - 5 h.

[0020] The chemical formula obtained by the preparation method of the high nickel cobalt-free cathode material with double reduction of residual alkali of the present application is Li x Ni y Mn 1-y A z O2 surface coated with La a Li b CoO3 perovskite high nickel cobalt-free cathode material, of which 0.75 <y≤0.95,0.95≤x≤1.15,0.001<z≤0.005,0.5≤a≤0.9,0.3≤b≤1.5。

[0021] The beneficial effects of the present invention are as follows: the present invention adopts a method of washing the high nickel cobalt-free positive electrode material matrix with water, separating the solid and liquid, and then adding additives to the filter cake to coat the surface of the material with La source and Co source, and then sintering to form a perovskite coating layer in situ on the surface of the material. This not only reduces the residual alkali once by washing, but also the Li (Li2CO3) required for the synthesis of perovskite by the positive electrode material is the residual lithium on the surface of the positive electrode material, which not only consumes the lithium in the material and reduces the residual alkali on the surface twice, but also forms a La a Li b The perovskite coating of CoO3 (LLCO) is used to form a cathode material with high lithium ion conductivity, excellent capacity retention and rate performance, and high reversibility. The high-nickel, cobalt-free cathode material prepared using this method exhibits low residual alkali, high discharge capacity, good rate performance, and excellent battery cycling performance, and has broad industrial application value. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The drawings that constitute part of this application are used to provide a further understanding of this application. The illustrative embodiments of this application and their descriptions are used to explain this application and do not constitute an improper limitation on this application. In the drawings:

[0023] FIG1 is a FESEM image of Example 2 of the present application;

[0024] FIG2 is an XRD pattern of the LLCO coating layer produced in Example 2 of the present application;

[0025] FIG3 is a comparison diagram of the first charge and discharge curves of the button-type half-cells of the embodiment of the present application and the comparative example;

[0026] FIG4 is a comparison chart of the capacity retention rates of button-type half-cells according to the embodiment of the present application and the comparative example;

[0027] FIG5 is a comparison chart of the discharge capacity retention rates of button-type half-cells at different rates according to the embodiment of the present application and the comparative example. DETAILED DESCRIPTION

[0028] It should be noted that, without conflict, the embodiments in this application and the features in the embodiments can be combined with each other. The following will describe this application in detail with reference to the accompanying drawings and in combination with the embodiments.

[0029] As described in the background art of this application, there are problems in the prior art such as that the ternary material coated with LLTO fails to reduce the residual alkali of the material and that the surface coating of lithium phosphate fails to effectively improve the electrochemical performance. To solve the above problems, in a typical implementation manner of this application, a preparation method of a high-nickel and cobalt-free cathode material with double reduction of residual alkali is provided, including the following steps: S1. Sufficiently mix a high-nickel and cobalt-free precursor with a lithium salt and a dopant A to obtain a mixture B; the general formula of the high-nickel and cobalt-free precursor is Ni y Mn 1-y (OH)2, and 0.75 < y ≤ 0.95; the molar ratio of Li element in the lithium salt to Ni y Mn 1-y (OH)2 is (0.95 - 1.15):1, and the dopant A accounts for 0.1 - 0.5 mol% of the total amount of the lithium salt and Ni y Mn 1-y (OH)2; the dopant A is a compound containing at least one of Al, Zr, W, Mo, and Nb; S2. Subject the mixture B to a first temperature curve for one-time roasting, cooling, and crushing in an oxygen atmosphere to obtain a high-nickel and cobalt-free cathode material C; the first temperature curve is that in an oxygen atmosphere, first heat up at a heating rate of 3 - 10 °C / min to 800 - 850 °C, and after reaching the required temperature, keep it warm for 3 - 20 h; S3. Wash the high-nickel and cobalt-free cathode material C with water, and perform solid-liquid separation to obtain a filter cake; S4. Add a La source and a Co source to the filter cake, then put them into a drying device together and rotate and dry them. After drying, put the dried material into a mixing device and mix it evenly to obtain a high-nickel and cobalt-free cathode material D; the La source accounts for 0.5 - 3 wt% of the filter cake, and the molar ratio of La element in the La source to Co element in the Co source is 0.5 - 0.9:1; S5. Subject the high-nickel and cobalt-free cathode material D to a second temperature curve for secondary roasting, cooling, and sieving in an oxygen atmosphere to obtain a high-nickel and cobalt-free cathode material with a surface-coated La x Ni y Mn 1-y A z O2 and La a Li b CoO3 perovskite, where 0.75 < y ≤ 0.95, 0.95 ≤ x ≤ 1.15, 0.001 < z ≤ 0.005; the second temperature curve is that in an oxygen atmosphere, first heat up at a heating rate of 3 - 7 °C / min to 300 - 450 °C, and after reaching the required temperature, keep it warm for 3 - 15 h.

[0030] Specifically, first, the high-nickel and cobalt-free precursor Ni yMn 1-y (OH)2 is fully mixed with lithium salt and dopant A to obtain mixture B. During this process, the mixture is mixed evenly to ensure that the ratio of any part of the high nickel cobalt-free precursor to the lithium salt and dopant A is consistent with the theory, so that the performance and index of the sintered material are consistent; then the mixture B is subjected to a first temperature curve in an oxygen atmosphere for one time calcination, cooling, and crushing to obtain a high nickel cobalt-free positive electrode material C. In this process, the mixture B is calcined at a high temperature in an oxygen atmosphere to generate a high nickel cobalt-free positive electrode material Li x Ni y Mn 1-y A z O2; the high-nickel cobalt-free positive electrode material C is then washed with water and subjected to solid-liquid separation to obtain a filter cake. The residual lithium on the surface of the high-nickel cobalt-free positive electrode material is effectively removed by water washing to reduce the impact of the residual lithium on the material, improve conductivity, accelerate lithium ion migration, and improve battery safety performance.

[0031] La source and Co source are added to the filter cake, and then put into a drying equipment together to rotate and dry it. After drying, the dried material is put into a mixing equipment for uniform mixing to obtain a high-nickel cobalt-free positive electrode material D. In this process, the filter cake, La source and Co source are evenly mixed to ensure that the ratio of the filter cake, La source and Co source in any part is consistent with the theory, so that the performance and indicators of the sintered material are consistent; finally, the high-nickel cobalt-free positive electrode material D is subjected to a second calcination at a second temperature curve under an oxygen atmosphere, cooled, and sieved to obtain a chemical formula of Li x Ni y Mn 1-y A z O2 surface coated with La a Li b The high-nickel, cobalt-free positive electrode material of CoO3 perovskite uses the energy generated during high-speed rotation and mixing before the secondary calcination to cause a chemical reaction between the solid material particles, so perovskite can be obtained without high-temperature calcination. In this process, a perovskite coating layer is formed in situ on the surface of the high-nickel, cobalt-free positive electrode material D after secondary calcination in an oxygen atmosphere, which reduces the residual alkali of the high-nickel, cobalt-free positive electrode material, and has high discharge capacity, good rate performance and good battery cycle performance.

[0032] The present application adopts a method of washing the high nickel cobalt-free positive electrode material matrix with water, separating the solid and liquid, and then adding additives to the filter cake to coat the surface of the material with La source and Co source, and then sintering to form a perovskite coating layer in situ on the surface of the material. This not only reduces the residual alkali once by washing, but also the Li (Li2CO3) required for the synthesis of perovskite by the positive electrode material is the residual lithium on the surface of the positive electrode material, which not only consumes the lithium in the material and achieves a secondary reduction in the surface residual alkali, but also forms a La a Li bThe perovskite coating of CoO3 (LLCO) is used to form a cathode material with high lithium ion conductivity, good capacity retention and rate performance, and high reversibility. The high-nickel, cobalt-free cathode material prepared using this method has low residual alkali, high discharge capacity, good rate performance, and good battery cycling performance.

[0033] In a preferred embodiment, the lithium salt in S1 is LiOH. Under the above conditions, the molten lithium hydroxide can be more evenly and fully mixed with the high nickel precursor during the sintering process, thereby reducing surface lithium residue and improving the discharge specific capacity of the raw material.

[0034] In order to further reduce the residual alkali and reduce the deterioration of the cycle performance and capacity of the positive electrode material due to the excessively high water-to-material ratio and washing time, in a preferred embodiment, the mass ratio of water to high-nickel cobalt-free positive electrode material C during the washing process in S3 is 1-3:1, the washing time is 10-30 minutes, and then the material enters the solid-liquid separation equipment for solid-liquid separation for 15-30 minutes.

[0035] In a preferred embodiment, the La source in S4 comprises at least one of La2O3, La(NO3)3, and La2(CO3)·8H2O, and the Co source comprises at least one of Co2O3, Co(OH)2, Co(NO3)2, CoCO3, and Co3O4. The particle size of the La source is in the range of 10-400 nm, and the particle size of the Co source is in the range of 20-300 nm. This appropriate particle size range enables the La source and the Co source to be effectively mixed into the filter cake, thereby obtaining a uniformly mixed high-nickel, cobalt-free cathode material D.

[0036] In order to further dry the filter cake and reduce damage to the morphology and structure of the positive electrode material, in a preferred embodiment, the drying temperature in S4 is 100-200° C. and the drying time is 0.5-7 h.

[0037] In a preferred embodiment, the high-nickel-free cobalt precursor in S1 is mixed with the lithium salt and the dopant A using a high-speed mixer or a planetary ball mill. The mixing time of the high-speed mixer is 10-30 minutes, and the mixing time of the planetary ball mill is 3-5 hours. Under the above conditions, the high-nickel-free cobalt precursor can be mixed more evenly with the lithium salt and the dopant A, further ensuring that the ratio of any part of the high-nickel-free cobalt precursor to the lithium salt and the dopant A is consistent with the theory, so that the performance and indicators of the material after sintering are consistent.

[0038] In order to optimize the performance and cost, in a preferred embodiment, the Li element in the lithium salt in S1 and the Ni y Mn 1-y The molar ratio of (OH)2 is (1.03-1.08):1.

[0039] In a preferred embodiment, the first temperature curve in S2 is that, in an oxygen atmosphere, the temperature is first raised at a heating rate of 3 - 5 °C / min to 800 - 820 °C, and after reaching the required temperature, it is kept warm for 10 - 20 h. Under the above conditions, the high-nickel and cobalt-free precursor, lithium salt, and dopant A can react at the optimal temperature, making the material properties and indicators better. Because after the calcination temperature is higher than 820 °C, the crystallinity of the raw material will increase, the grain size will increase, and the specific surface area will become smaller, which is not conducive to the deintercalation and intercalation of lithium ions during the charge and discharge process; in addition, too high sintering temperature will also cause the phenomenon of lithium-nickel mixing, which is not conducive to calcining the high-nickel layered raw material with the required stoichiometric ratio, and may further cause the decline of the diffusion ability of lithium ions and the decline of specific capacity.

[0040] For the purpose of making the reaction temperature and performance optimal, in a preferred embodiment, the second temperature curve in S5 is that, in an oxygen atmosphere, the temperature is first raised at a heating rate of 3 - 6 °C / min to 300 - 350 °C, and after reaching the required temperature, it is kept warm for 6 - 15 h.

[0041] In another typical embodiment of the present application, a high-nickel and cobalt-free cathode material is further provided, which is obtained by using the preparation method of the high-nickel and cobalt-free cathode material with double reduction of residual alkali in the present application, and its chemical formula is Li x Ni y Mn 1-y A z O2 and surface-coated with La a Li b CoO3 perovskite, where 0.75 < y ≤ 0.95, 0.95 ≤ x ≤ 1.15, 0.001 < z ≤ 0.005, 0.5 ≤ a ≤ 0.9, 0.3 ≤ b ≤ 1.5. Due to the use of the preparation method of the present application, this high-nickel and cobalt-free cathode material has significantly reduced residual alkali and significantly improved discharge capacity, rate performance, and battery cycle performance.

[0042] Next, the technical solutions in the embodiments of the present application will be described clearly and completely. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present application.

[0043] Example 1

[0044] A preparation method of a high-nickel and cobalt-free cathode material with double reduction of residual alkali, comprising the following steps:

[0045] Weigh the high-nickel and cobalt-free precursor (Ni 0.90 Mn 0.10(OH)2) with lithium salt and additive A, wherein the lithium salt is LiOH, the additive A is ZrO2, LiOH and Ni 0.90 Mn 0.10 The molar ratio of (OH)2 is 1.03:1, and ZrO2 accounts for 1.03% of LiOH and Ni 0.90 Mn 0.10 0.1 mol% of the total (OH)2 was added to a high-pressure mixer and mixed for 15 minutes. The uniformly mixed material was first sintered in an oxygen atmosphere, heating from room temperature to 800°C at a rate of 3°C / min and then held at the desired temperature for 10 hours. The sintered material was cooled, crushed, and pulverized to obtain a pulverized material. This was washed with pure water at a mass ratio of 1:1 for 15 minutes, and then placed in a centrifuge for solid-liquid separation for 20 minutes. La2O3 with a particle size range of 10-400nm and Co2O3 with a particle size range of 20-300nm were added to the filter cake, with La2O3 accounting for 1wt% of the filter cake and a molar ratio of La2O3 to Co2O3 of 0.5:1. The mixture was then placed in a conical spiral ribbon vacuum mixer dryer for drying at 150°C for 3 hours, with the equipment rotating. After drying, the dried material was put into a high-speed mixer and mixed for 15 minutes to make it uniform. It was then sintered in an oxygen atmosphere at a heating rate of 3°C / min from room temperature to 300°C and kept warm for 6 hours. Finally, it was naturally cooled to room temperature and sieved to obtain a coating layer of La. 0.5 Li 1.5 Example 1 sample of CoO3.

[0046] Example 2

[0047] A method for preparing a high-nickel cobalt-free cathode material with double reduced residual alkali comprises the following steps:

[0048] Weigh the high nickel cobalt-free precursor (Ni 0.90 Mn 0.10 (OH)2) with lithium salt and additive A, wherein the lithium salt is LiOH, the additive A is ZrO2, LiOH and Ni 0.90 Mn 0.10 The molar ratio of (OH)2 is 1.05:1, and ZrO2 accounts for 1.05% of LiOH and Ni 0.90 Mn 0.100.3 mol% of the total (OH)2 was added to a high-pressure mixer and mixed for 20 minutes. The uniformly mixed material was first sintered in an oxygen atmosphere, heating from room temperature to 820°C at a rate of 5°C / min and then held at the desired temperature for 10 hours. The sintered material was cooled, crushed, and pulverized to obtain a pulverized material. This was washed with pure water at a mass ratio of 1.5:1 for 20 minutes, and then placed in a centrifuge for solid-liquid separation for 20 minutes. La2O3 with a particle size range of 10-400nm and Co2O3 with a particle size range of 20-300nm were added to the filter cake, with La2O3 accounting for 2wt% of the filter cake and a molar ratio of La2O3 to Co2O3 of 0.7:1. The mixture was then placed in a conical spiral ribbon vacuum mixer dryer for drying at 150°C for 3 hours, with the equipment rotating. After drying, the dried material was put into a high-speed mixer and mixed for 15 minutes to make it uniformly mixed. It was then sintered in an oxygen atmosphere at a heating rate of 6°C / min from room temperature to 350°C and kept warm for 6 hours. Finally, it was naturally cooled to room temperature and sieved to obtain a coating layer of La. 0.7 Li 0.9 Example 2 sample of CoO3.

[0049] Example 3

[0050] A method for preparing a high-nickel cobalt-free cathode material with double reduced residual alkali comprises the following steps:

[0051] Weigh the high nickel cobalt-free precursor (Ni 0.90 Mn 0.10 (OH)2) with lithium salt and additive A, wherein the lithium salt is LiOH, the additive A is ZrO2, LiOH and Ni 0.90 Mn 0.10 The molar ratio of (OH)2 is 1.08:1, and ZrO2 accounts for 1.08% of LiOH and Ni 0.90 Mn 0.100.5 mol% of the total (OH)2 was added to a high-pressure mixer and mixed for 20 minutes. The mixed material was first sintered in an oxygen atmosphere, heating from room temperature to 800°C at a rate of 3°C / min and then held at the desired temperature for 10 hours. The sintered material was cooled, crushed, and pulverized to obtain a pulverized material. This was washed with pure water at a mass ratio of 1:1 for 15 minutes, and then placed in a centrifuge for solid-liquid separation for 20 minutes. La2O3 with a particle size range of 10-400nm and Co2O3 with a particle size range of 20-300nm were added to the filter cake, with La2O3 accounting for 3wt% of the filter cake and a molar ratio of La2O3 to Co2O3 of 0.9:1. The mixture was then placed in a conical spiral ribbon vacuum mixer dryer for drying at 150°C for 3 hours, with the equipment rotating. After drying, the dried material was put into a high-speed mixer and mixed for 15 minutes to make it uniform. It was then sintered in an oxygen atmosphere at a heating rate of 3°C / min from room temperature to 300°C and kept warm for 6 hours. Finally, it was naturally cooled to room temperature and sieved to obtain a coating layer of La. 0.9 Li 0.3 Example 3 sample of CoO3.

[0052] Example 4

[0053] A method for preparing a high-nickel cobalt-free cathode material with double reduced residual alkali comprises the following steps:

[0054] S1. Weigh the high nickel cobalt-free precursor (Ni 0.90 Mn 0.10 (OH)2) with lithium salt and additive A, wherein the lithium salt is LiOH, the additive A is ZrO2, LiOH and Ni 0.90 Mn 0.10 The molar ratio of (OH)2 is 0.95:1, and ZrO2 accounts for 1% of LiOH and Ni 0.90 Mn 0.10 0.1 mol% of the total amount of (OH)2 was added into a high-speed mixer and mixed for 30 minutes.

[0055] S2. The mixed materials are sintered for the first time in an oxygen atmosphere, heating from room temperature to 800°C at a heating rate of 3°C / min, and then kept at the desired temperature for 20 hours.

[0056] S3. The sintered material is cooled, crushed and pulverized to obtain a pulverized material, which is washed with pure water in a mass ratio of 1:1 for 10 minutes, and then placed in a centrifuge for solid-liquid separation for 15 minutes.

[0057] S4. Add La2O3 with a particle size range of 10-400nm and Co2O3 with a particle size range of 20-300nm to the filter cake, with La2O3 accounting for 0.5wt% of the filter cake and the molar ratio of La2O3 to Co2O3 being 0.5:1. Then put them together into a conical screw ribbon vacuum mixing dryer to enter the drying process. The drying temperature is 100°C and the drying time is 7h. The equipment rotates. After drying, the dried materials are put into a high-pressure mixer and mixed for 15 minutes to make them evenly mixed.

[0058] S5. Sintering the sample in an oxygen atmosphere at a heating rate of 3°C / min from room temperature to 300°C and keeping the temperature for 15 hours; finally, naturally cooling the sample to room temperature and sieving the sample to obtain the sample of Example 4.

[0059] Example 5

[0060] A method for preparing a high-nickel cobalt-free cathode material with double reduced residual alkali comprises the following steps:

[0061] S1. Weigh the high nickel cobalt-free precursor (Ni 0.90 Mn 0.10 (OH)2) with lithium salt and additive A, wherein the lithium salt is LiOH, the additive A is ZrO2, LiOH and Ni 0.90 Mn 0.10 The molar ratio of (OH)2 is 1.15:1, and ZrO2 accounts for 1.5% of LiOH and Ni 0.90 Mn 0.10 0.5 mol% of the total amount of (OH)2 was added into a planetary ball mill and mixed for 5 hours.

[0062] S2. The mixed materials are sintered for the first time in an oxygen atmosphere, with the temperature rising from room temperature to 850°C at a heating rate of 10°C / min, and kept at the desired temperature for 3 hours.

[0063] S3. The sintered material is cooled, crushed and pulverized to obtain a pulverized material, which is washed with pure water in a mass ratio of 3:1 for 30 minutes, and then placed in a centrifuge for solid-liquid separation for 30 minutes.

[0064] S4. Add La2O3 with a particle size range of 10-400nm and Co2O3 with a particle size range of 20-300nm to the filter cake, with La2O3 accounting for 3wt% of the filter cake and the molar ratio of La2O3 to Co2O3 being 0.9:1. Then put them together into a conical screw ribbon vacuum mixing dryer to enter the drying process. The drying temperature is 200℃ and the drying time is 0.5h. The equipment rotates. After drying, the dried materials are put into a high-pressure mixer and mixed for 15 minutes to make them evenly mixed.

[0065] S5. Sintering the mixture in an oxygen atmosphere, heating the temperature from room temperature to 450° C. at a heating rate of 7° C. / min, and keeping the temperature for 3 h; finally, naturally cooling the mixture to room temperature, and sieving the mixture to obtain the sample of Example 5.

[0066] Comparative Example 1

[0067] A method for preparing a high-nickel cobalt-free positive electrode material comprises the following steps:

[0068] Weigh the high nickel cobalt-free precursor (Ni 0.90 Mn 0.10 (OH)2) with lithium salt and additive A, wherein the lithium salt is LiOH, the additive A is ZrO2, LiOH and Ni 0.90 Mn 0.10 The molar ratio of (OH)2 is 1.03:1, and ZrO2 accounts for 1.03% of LiOH and Ni 0.90 Mn 0.10 0.1 mol% of the total (OH)2 was added to a high-speed mixer and mixed for 15 minutes. The mixed material was first sintered in an oxygen atmosphere, heating from room temperature to 800°C at a rate of 3°C / min and holding at the desired temperature for 10 hours. The sintered material was cooled, crushed, and pulverized to obtain a pulverized material. The pulverized material was washed with pure water at a mass ratio of 1:1 for 15 minutes and then centrifuged for solid-liquid separation for 20 minutes. The filter cake was directly fed into a conical spiral ribbon vacuum mixer dryer for drying at 150°C for 3 hours with the equipment rotating. WO3 additive was added to the dried material, with the WO3 accounting for 1wt% of the dried material, and the mixture was placed in a high-speed mixer and mixed for 15 minutes to achieve uniform mixing. The coated material was then sintered in an oxygen atmosphere, heating from room temperature to 300°C at a rate of 3°C / min and holding at that temperature for 6 hours. Finally, the mixture was naturally cooled to room temperature and sieved to obtain the sample of Comparative Example 1.

[0069] Comparative Example 2

[0070] A method for preparing a high-nickel cobalt-free positive electrode material comprises the following steps:

[0071] Weigh the high nickel cobalt-free precursor (Ni 0.90 Mn 0.10 (OH)2) with lithium salt and additive A, wherein the lithium salt is LiOH, the additive A is ZrO2, LiOH and Ni 0.90 Mn 0.10 The molar ratio of (OH)2 is 1.03:1, and ZrO2 accounts for 1.03% of LiOH and Ni 0.90 Mn 0.100.1 mol% of the total (OH)2 was added to a high-pressure mixer and mixed for 15 minutes. The uniformly mixed material was first sintered in an oxygen atmosphere, heating from room temperature to 800°C at a rate of 3°C / min and then held at the desired temperature for 10 hours. The sintered material was cooled, crushed, and pulverized to obtain a pulverized material. This was washed with pure water at a mass ratio of 1:1 for 15 minutes, followed by solid-liquid separation in a centrifuge for 20 minutes. The filter cake was then directly fed into a conical screw-ribbon vacuum mixer dryer for drying at 150°C for 3 hours, with the equipment rotating. La2O3 and Co2O3 were added to the dried material, with La2O3 accounting for 1wt% of the dried material and the molar ratio of La2O3 to Co2O3 being 0.9:1. The materials were put into a high-speed mixer and mixed for 15 minutes to make them evenly mixed. The coated material was then sintered in an oxygen atmosphere at a heating rate of 3°C / min from room temperature to 300°C and kept warm for 6 hours. The material was naturally cooled to room temperature and sieved to obtain the comparative example 2 sample.

[0072] Comparative Example 3

[0073] A method for preparing a high-nickel cobalt-free positive electrode material comprises the following steps:

[0074] Weigh the high nickel cobalt-free precursor (Ni 0.90 Mn 0.10 (OH)2) with lithium salt and additive A, wherein the lithium salt is LiOH, the additive A is ZrO2, LiOH and Ni 0.90 Mn 0.10 The molar ratio of (OH)2 is 1.03:1, and ZrO2 accounts for 1.03% of LiOH and Ni 0.90 Mn 0.10 0.1 mol% of the total amount of (OH)2 was added to a high-pressure mixer and mixed for 15 minutes. The mixed material was then subjected to a first sintering process in an oxygen atmosphere, heating from room temperature to 800°C at a rate of 3°C / min and holding at the desired temperature for 10 hours. The sintered material was cooled, crushed, and pulverized to obtain a pulverized material. This was washed with pure water at a mass ratio of 1:1 for 15 minutes and then centrifuged for solid-liquid separation for 20 minutes. A WO3 additive was added to the filter cake, with the WO3 accounting for 1wt% of the filter cake. The mixture was then placed in a conical ribbon vacuum mixer dryer for drying at 150°C for 3 hours with the equipment rotating. After drying, the dried material was placed in a high-pressure mixer and mixed for 15 minutes to achieve uniform mixing. The material was then sintered in an oxygen atmosphere, heating from room temperature to 300°C at a rate of 3°C / min and holding at the desired temperature for 6 hours. Finally, the material was naturally cooled to room temperature and sieved to obtain the sample for Comparative Example 3.

[0075] Comparative Example 4

[0076] A method for preparing a high-nickel cobalt-free positive electrode material comprises the following steps:

[0077] S1. Weigh the high nickel cobalt-free precursor (Ni 0.90 Mn 0.10 (OH)2) with lithium salt and additive A, wherein the lithium salt is LiOH, the additive A is ZrO2, LiOH and Ni 0.90 Mn 0.10 The molar ratio of (OH)2 is 0.9:1, and ZrO2 accounts for 1% of LiOH and Ni 0.90 Mn 0.10 0.05 mol% of the total amount of (OH)2 was added into a high speed mixer and mixed for 5 minutes.

[0078] S2. The mixed materials are sintered for the first time in an oxygen atmosphere, with the temperature rising from room temperature to 750°C at a heating rate of 2°C / min, and kept at the desired temperature for 22 hours.

[0079] S3. The sintered material is cooled, crushed and pulverized to obtain a pulverized material, which is washed with pure water at a mass ratio of 0.5:1 for 5 minutes, and then placed in a centrifuge for solid-liquid separation for 10 minutes.

[0080] S4. Add La2O3 with a particle size range of 10-400nm and Co2O3 with a particle size range of 20-300nm to the filter cake, with La2O3 accounting for 0.3wt% of the filter cake and the molar ratio of La2O3 to Co2O3 being 0.4:1. Then put them into a conical screw ribbon vacuum mixing dryer to enter the drying process. The drying temperature is 80°C and the drying time is 8h. The equipment rotates. After drying, the dried materials are put into a high-pressure mixer and mixed for 15 minutes to make them evenly mixed.

[0081] S5. Sintering the sample in an oxygen atmosphere, heating the temperature from room temperature to 250° C. at a heating rate of 2° C. / min, and keeping the temperature for 18 h; finally, naturally cooling the sample to room temperature, and obtaining a comparative example 4 sample after sieving.

[0082] Comparative Example 5

[0083] A method for preparing a high-nickel cobalt-free positive electrode material comprises the following steps:

[0084] S1. Weigh the high nickel cobalt-free precursor (Ni 0.90 Mn 0.10 (OH)2) with lithium salt and additive A, wherein the lithium salt is LiOH, the additive A is ZrO2, LiOH and Ni 0.90Mn 0.10 The molar ratio of (OH)2 is 1.2:1, and ZrO2 accounts for 1.2% of LiOH and Ni 0.90 Mn 0.10 0.6 mol% of the total amount of (OH)2 was added into a high-speed mixer and mixed for 35 minutes.

[0085] S2. The mixed materials are sintered for the first time in an oxygen atmosphere, with the temperature rising from room temperature to 900°C at a heating rate of 12°C / min, and kept at the desired temperature for 2 hours.

[0086] S3. The sintered material is cooled, crushed and pulverized to obtain a pulverized material, which is washed with pure water in a mass ratio of 4:1 for 35 minutes, and then placed in a centrifuge for solid-liquid separation for 35 minutes.

[0087] S4. Add La2O3 with a particle size range of 10-400nm and Co2O3 with a particle size range of 20-300nm to the filter cake, with La2O3 accounting for 4wt% of the filter cake and the molar ratio of La2O3 to Co2O3 being 1:1. Then put them into a conical screw ribbon vacuum mixing dryer to enter the drying process. The drying temperature is 220℃, the drying time is 0.2h, and the equipment rotates. After drying, the dried materials are put into a high-pressure mixer and mixed for 15min to make them evenly mixed.

[0088] S5. Sintering the sample in an oxygen atmosphere at a heating rate of 8°C / min from room temperature to 500°C and keeping the temperature for 2 hours; finally, naturally cooling the sample to room temperature and sieving the sample to obtain the comparative example 5.

[0089] The physicochemical indexes and electrical performance of the positive electrode materials obtained in the above examples and comparative examples were evaluated under the same conditions.

[0090] 1. The surface residual alkali content of the positive electrode materials obtained in the examples and comparative examples was tested.

[0091] The test process is as follows: weigh a certain amount of sample, add a certain amount of deionized water, stir for a period of time, filter, and titrate directly with hydrochloric acid standard titration solution until the color of the indicator changes, indicating that the acid and base have been completely neutralized. Record the volume of the acid solution used for titration, and calculate the residual alkali content in the sample according to the chemical equation of the acid-base reaction.

[0092] Among them, the residual alkali content test results are shown in Table 1. According to the data, the surface residual alkali of the example samples is significantly lower than that of the comparative example samples. And the analysis data shows that the residual alkali of the comparative example 1 sample in which other additives are added to the drying material is the highest, and the residual alkali of the comparative example 2 and 3 samples in which other additives are added to the filter cake and La source and Co source are added to the drying material is slightly lower than that of the comparative example 1 sample, but the effect is not obvious, while the residual alkali is significantly reduced by using the above-mentioned embodiments of the present application. Therefore, the method for reducing the surface residual alkali content of the above-mentioned embodiments of the present application is effective. This shows that the positive electrode material obtained by the double modification method for reducing residual alkali in the above-mentioned embodiments of the present application not only achieves a one-time reduction in residual alkali during the washing process, but also consumes the residual alkali on the surface of the material in the subsequent process due to the addition of additives to the filter cake and the formation of the LLCO coating layer. This is a secondary reduction in residual alkali.

[0093] Table 1

[0094] 2. The positive electrode materials obtained in the examples and comparative examples were subjected to button-type half-cell electrical performance tests.

[0095] The testing process is as follows: In an argon-filled glove box, button cells were assembled using a lithium metal sheet as the negative electrode. The positive electrode sheet consisted of 92 wt% active material, 4 wt% Super-P (superconducting carbon black) conductive agent, and 4 wt% PVDF (polyvinylidene fluoride) binder. Cyclic charge and discharge testing was performed over a voltage range of 3.0-4.4 V at charge and discharge rates of 0.2C, 0.5C, 1C, 2C, 3C, and 0.2C for three cycles each; the battery was then cycled 50 times at a charge and discharge rate of 1C.

[0096] A comparison of the initial charge and discharge curves of coin-type half-cells prepared from the positive electrode materials obtained from Examples 1 and 2 and Comparative Examples 2 and 3 is shown in FIG3 . As can be seen from FIG3 , in the corresponding initial charge specific capacity of the coin-type half-cells, the order is Example 2 sample > Example 1 sample > Comparative Example 3 sample > Comparative Example 2 sample; in the corresponding initial discharge specific capacity of the coin-type half-cells, the order is Example 2 sample > Example 1 sample > Comparative Example 2 sample > Comparative Example 3 sample; and in the corresponding initial efficiency of the coin-type half-cells, the order is Example 2 sample > Example 1 sample > Comparative Example 2 sample > Comparative Example 3 sample. This comparison shows that the coin-type half-cells prepared using the positive electrode materials obtained by the double modification method for reducing residual alkali according to the above embodiments of the present application have improved initial charge specific capacity, initial discharge specific capacity, and initial efficiency compared to the comparative examples.

[0097] Among them, the corresponding button-type half-cells prepared from the positive electrode materials obtained in Examples 1-3 and Comparative Examples 1-3 after 50 charge-discharge cycles under the same test conditions are shown in Figure 4. As can be seen from Figure 4, the capacity retention rates of the samples in Examples 1, 2, and 3 are superior to those of the samples in Comparative Examples 1, 2, and 3. At the same time, combined with the FESEM image in Figure 1, it can be seen that the surface of the sample material in Example 2 in Figure 1 is evenly coated with a coating layer, and the surface dispersion after coating is good, the coating is relatively uniform, and there is no agglomeration. Combined with the XRD pattern of LLCO generated in Figure 2 and the EDS test results of the sample in Example 2, as shown in Table 2, it can be seen that the measured coating layer contains La, Li, and Co elements, indicating that a perovskite coating layer has been generated. Combined with the above electrical performance test results, it can be seen that the LLCO coating layer formed in situ on the surface of the positive electrode material after the perovskite is coated in the embodiment of the present application can effectively improve the ionic conductivity of the positive electrode material, improve the efficiency of lithium ion insertion / extraction, effectively inhibit the direct contact between the electrolyte and the positive electrode material, reduce the dissolution of metal ions, and thus improve the discharge capacity, charge and discharge efficiency and cycle performance of the material. This shows that the positive electrode material obtained by the double modification method of reducing residual alkali in the above embodiment of the present application has a significantly improved cycle performance of the positive electrode material relative to the comparative example after the perovskite coating layer is formed due to the high lithium ion conductivity of the LLCO material.

[0098] Table 2

[0099] Among them, the capacity retention rate comparison of the corresponding button-type half-cells prepared from the positive electrode materials obtained in Examples 1-3 and Comparative Examples 1-3 at different rates is shown in Figure 5. As can be seen from Figure 5, compared with the positive electrode material with an LLCO coating layer prepared in Example 2, the discharge capacity and capacity retention rate of the positive electrode material prepared in Comparative Example 2 at different rates are significantly lower, especially at high rates, the rate performance of the positive electrode material prepared in Example 2 is significantly better, because adding La source and Co source to the filter cake is more conducive to the performance of capacity and rate performance than adding La source and Co source to the dried material. And compared with the positive electrode material with an LLCO coating layer prepared in Example 2, the discharge capacity and capacity retention rate of the positive electrode material prepared in Comparative Example 3 at different rates are significantly lower, especially at high rates, the rate performance of the positive electrode material prepared in Example 2 is significantly better, because the LLCO coating layer generated by the positive electrode material coated with La source and Co source is a fast ion conductor, which reduces the resistance of the positive electrode material and is conducive to the performance of capacity and rate performance.

[0100] As can be seen from the above, compared with the comparative example, each embodiment of the present application adopts La source and Co source coated on the surface of the material, and then forms a perovskite coating layer in situ on the surface of the material after sintering. This not only reduces the residual alkali once by washing with water, but also generates a perovskite coating layer by reacting with the residual lithium of the material, thereby reducing the residual lithium content on the surface of the high-nickel material and achieving a secondary reduction in residual alkali. The positive electrode material has high lithium ion conductivity, good capacity retention and rate performance due to the formation of the perovskite coating, and has high reversibility. The prepared high-nickel cobalt-free positive electrode material has low residual alkali, high discharge capacity, good rate performance, and good battery cycle performance. In addition, it can be seen that when each process parameter is within the preferred range of this application, the comprehensive performance of the material is better.

[0101] The above description is merely a preferred embodiment of the present application and is not intended to limit the present application. Various modifications and variations are possible for those skilled in the art. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present application shall be included within the scope of protection of the present application.

Claims

1. A preparation method of a high-nickel cobalt-free cathode material with double reduction of residual alkali, characterized in that, It includes the following steps: S1. Thoroughly mix the high-nickel and cobalt-free precursor with a lithium salt and dopant A to obtain mixture B; the general formula of the high-nickel and cobalt-free precursor is Ni y Mn 1-y (OH)2, and 0.75 < y ≤ 0.95; the molar ratio of Li element in the lithium salt to Ni y Mn 1-y (OH)2 is (0.95 - 1.15):1, and dopant A accounts for 0.1 - 0.5 mol% of the total amount of the lithium salt and Ni y Mn 1-y (OH)2; the dopant A is a compound containing at least one of Al, Zr, W, Mo, and Nb; S2. Subject the mixture B to a first temperature curve for one-time roasting, cooling, and crushing in an oxygen atmosphere to obtain a high-nickel cobalt-free cathode material C; the first temperature curve is to first heat up to 800-850°C at a heating rate of 3-10°C / min in an oxygen atmosphere, and keep the temperature for 3-20 h after reaching the required temperature; S3. Wash the high-nickel cobalt-free cathode material C with water, and perform solid-liquid separation to obtain a filter cake; S4. Add a La source and a Co source to the filter cake, then put them into a drying device and rotate them for drying. After drying, put the dried material into a mixing device and mix evenly to obtain a high-nickel cobalt-free cathode material D; the La source accounts for 0.5-3 wt% of the filter cake, and the molar ratio of La element in the La source to Co element in the Co source is 0.5-0.9:1; S5. The high-nickel and cobalt-free cathode material D is subjected to secondary calcination, cooling, and sieving under an oxygen atmosphere according to a second temperature curve to obtain a high-nickel and cobalt-free cathode material with a perovskite of La x Ni y Mn 1-y A z O2 surface-coated with La a Li b CoO3, where 0.75 < y ≤ 0.95, 0.95 ≤ x ≤ 1.15, and 0.001 < z ≤ 0.005; the second temperature curve is to first heat up to 300 - 450°C at a heating rate of 3 - 7°C / min under an oxygen atmosphere, and keep the temperature for 3 - 15 h after reaching the required temperature.

2. The preparation method of the high-nickel cobalt-free cathode material with double reduced residual alkali according to claim 1, characterized in that The lithium salt in S1 is LiOH.

3. The preparation method of the high-nickel and cobalt-free cathode material with double reduced residual alkali according to claim 1 or 2, characterized in that, In S3, the mass ratio of water to the high-nickel cobalt-free cathode material C during the washing process is 1-3:1, the washing time is 10-30 min, and then enter a solid-liquid separation device for solid-liquid separation for 15-30 min.

4. The preparation method of the high-nickel and cobalt-free cathode material with double reduced residual alkali according to any one of claims 1 to 3, characterized in that, In S4, the La source includes at least one of La2O3, La(NO3)3, and La2(CO3)·8H2O, and the Co source includes at least one of Co2O3, Co(OH)2, Co(NO3)2, CoCO3, and Co3O4; the particle size range of the La source is 10-400 nm, and the particle size range of the Co source is 20-300 nm.

5. The preparation method of the high-nickel cobalt-free cathode material with double reduced residual alkali according to any one of claims 1 to 4, characterized in that In S4, the drying temperature is 100-200°C, and the drying time is 0.5-7 h.

6. The preparation method of the high-nickel cobalt-free cathode material with double reduced residual alkali according to any one of claims 1 to 5, characterized in that, In S1, the mixing of the high-nickel cobalt-free precursor, the lithium salt, and the dopant A is carried out by a high-speed mixer or a planetary ball mill. The mixing time of the high-speed mixer is 10-30 min, and the mixing time of the planetary ball mill is 3-5 h.

7. The preparation method of the high-nickel cobalt-free cathode material with double reduced residual alkali according to any one of claims 1 to 6, characterized in that, The molar ratio of Li element to Ni in the lithium salt described in S1 y Mn 1-y (OH)2 is (1.03 - 1.08):

1.

8. The preparation method of the high-nickel cobalt-free cathode material with double reduced residual alkali according to any one of claims 1 to 7, characterized in that, The first temperature curve in S2 is to first heat up to 800-820°C at a heating rate of 3-5°C / min in an oxygen atmosphere, and keep the temperature for 10-20 h after reaching the required temperature.

9. The preparation method of the high-nickel cobalt-free cathode material with double reduced residual alkali according to any one of claims 1 to 8, characterized in that, The second temperature curve in S5 is to first heat up to 300-350°C at a heating rate of 3-6°C / min in an oxygen atmosphere, and keep the temperature for 6-15 h after reaching the required temperature.

10. A high-nickel and cobalt-free cathode material, characterized in that, Obtained by using the preparation method of the high-nickel cobalt-free cathode material with double reduced residual alkali according to any one of claims 1 to 9, with the chemical formula Li x Ni y Mn 1-y A z O2 and surface-coated with La a Li b CoO3 perovskite, where 0.75 < y ≤ 0.95, 0.95 ≤ x ≤ 1.15, 0.001 < z ≤ 0.005, 0.5 ≤ a ≤ 0.9, 0.3 ≤ b ≤ 1.5.

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