Positive electrode material and preparation method therefor, and battery

Abstract

A positive electrode material and a preparation method therefor, and a battery. The positive electrode material is a lithium nickel cobalt oxide composite oxide, which comprises an inner core and a surface reconstruction layer located outside the inner core, wherein the surface reconstruction layer is a region etched by using 100-times diluted aqua regia to dissolve the positive electrode material for 30 minutes, with the environmental temperature being controlled to be 300°C during dissolution; and the inner core is the residual region of the positive electrode material after the surface reconstruction layer is etched. The molar content of Li in the inner core is m1, and the molar content of Li in the surface reconstruction layer is m2, wherein 1.3≤m2 / m1≤4.0. While the positive electrode material has a high capacity, the crystal structure stability of the positive electrode material is improved, thereby improving the cycling stability and withstand voltage of the positive electrode material.

Description

Positive electrode material and preparation method thereof, and battery Technical Field

[0001] The present invention relates to the technical field of positive electrode materials, and in particular to a positive electrode material and a preparation method thereof, and a battery. Background Art

[0002] In recent years, lithium-ion batteries have been widely used in modern portable electronic devices. Due to their high specific energy, high operating voltage, long cycle life, lack of memory effect, and environmental friendliness, they have a broad market in hybrid and electric vehicles. Cathode materials are often the bottleneck affecting battery energy density. Layered ternary cathode materials offer a promising combination of high discharge capacity, good cycle life, and low cost. Increasing the nickel content of cathode materials can improve their reversible specific capacity and cycling stability, making them more suitable for high-energy-density power batteries.

[0003] However, as the Ni content increases, its structural stability also deteriorates. Its particle phase structure undergoes an irreversible phase transition during the charge and discharge process, layered structure-spinel structure-inactive rock salt phase, which will cause the capacity and cycle performance to decay. In addition, during the cycle of the ternary positive electrode material, the charging process Li + The positive electrode material is removed, the interlayer distance increases, and the grains expand; during the discharge process, Li + When the positive electrode material is embedded, the interlayer spacing decreases and the grains shrink. As the degree of charge and discharge deepens, the expansion / contraction of the positive electrode material grains intensifies, the internal stress of the positive electrode material particles intensifies, the positive electrode material particles are prone to cracking and pulverization, and the battery cycle life and safety performance are affected.

[0004] Application Contents

[0005] The present application provides a positive electrode material, a preparation method thereof, and a battery. The positive electrode material can improve the stability of the crystal structure while having a high capacity, thereby improving the cycle stability of the positive electrode material and increasing the withstand voltage of the positive electrode material.

[0006] In a first aspect, the present application provides a positive electrode material, which is a lithium nickel cobalt oxide composite oxide; the lithium nickel cobalt oxide composite oxide includes a core and a surface reconstruction layer located outside the core; the surface reconstruction layer is the area etched away by diluting 0.3g of the positive electrode material 100 times with 0.4ml of aqua regia and dissolving it for 30 minutes, and the ambient temperature is controlled at 300°C during dissolution, and the core is the remaining area of ​​the positive electrode material after etching away the surface reconstruction layer; the molar content of Li in the core is m1; the molar content of Li in the surface reconstruction layer is m2; wherein, 1.3≤m2 / m1≤4.0.

[0007] In a second aspect, the present application also provides a positive electrode material, which is a lithium nickel cobalt oxide composite oxide; the lithium nickel cobalt oxide composite oxide includes a core and a surface reconstruction layer located outside the core; the surface reconstruction layer is the area etched away by diluting 0.3g of the positive electrode material 100 times with 0.4ml of aqua regia and dissolving it for 30 minutes, and the ambient temperature is controlled at 300°C during dissolution, and the core is the remaining area of ​​the positive electrode material after etching away the surface reconstruction layer; the molar content of Li in the surface reconstruction layer is m2, and the total molar content of Li in the positive electrode material is m Li总 , 1.1≤m2 / m Li总 ≤3.0.

[0008] In a third aspect, the present application provides a method for preparing a positive electrode material, the preparation method comprising:

[0009] The first mixture containing the cathode material precursor and the lithium-containing compound is subjected to a sintering treatment to obtain a core matrix material, wherein the core matrix material has a chemical formula of Li 1-δ Ni a Co b M c M1 1-a-b-c O2, wherein -0.05≤δ≤0.05, 0.3≤a<1.0, 0.01≤b≤0.3, 0≤c≤0.5, 0≤1-abc≤0.05, M is Mn and / or Al, and M1 is a metal element;

[0010] The second mixture comprising the surface reconstruction material and the core matrix material is subjected to a secondary sintering process to obtain a positive electrode material; wherein the surface reconstruction material comprises Ni r Co p M q M3 1-r-p-q (OH)2、Ni r Co p M q M3 1-r-p-q OOH and (Ni r Co p M q M3 1-r-p-q )O d At least one of the following, 0≤r≤0.5, 0.3≤p≤1, 0≤q≤0.5, 0≤1-rpq≤0.2, 1≤d≤2, M is Mn and / or Al, and M3 is a metal element.

[0011] In a fourth aspect, the present application provides a battery, comprising the positive electrode material described in the first aspect or the second aspect.

[0012] Compared with the prior art, this application has at least the following beneficial effects:

[0013] The positive electrode material proposed in this application has a molar content of Li in the core that is lower than the molar content of Li in the surface reconstruction layer, so that a lithium ion concentration gradient is formed in the inner and outer layers of the positive electrode material. During the charge and discharge process, the lithium ion deintercalation reaction activity in the core is lower than the lithium ion deintercalation reaction activity in the surface reconstruction layer, and lithium ions are preferentially deintercalated in the surface reconstruction layer. Since the degree of lithium ion deintercalation in the core is lower than that in the surface reconstruction layer, the lattice parameter change amplitude of the core is smaller than that of the surface reconstruction layer, which reduces the lattice expansion / contraction stress in the core, and the lattice expansion / contraction stress in the surface reconstruction layer is easily released to the outside of the positive electrode material particles, which can effectively reduce the breakage and crushing of the positive electrode material particles, improve the cycle life of the positive electrode material, and increase the withstand voltage of the positive electrode material. In addition, the higher lithium ion molar content in the surface reconstruction layer can also improve the lithium ion conductivity of the positive electrode material and improve the rate performance of the positive electrode material.

[0014] The positive electrode material proposed in this application has a molar content of Li in the surface reconstruction layer of the positive electrode material that is higher than the total molar content of Li in the positive electrode material, that is, the molar content of lithium ions in the surface reconstruction layer of the positive electrode material is higher than that in other regions of the positive electrode material. A lithium ion concentration gradient is formed between the inner and outer layers of the positive electrode material. During the charge and discharge process, the lithium ion deintercalation reaction activity in the core is lower than that in the surface reconstruction layer, and lithium ions are preferentially deintercalated in the surface reconstruction layer. Since the degree of lithium ion deintercalation in the core is lower than that in the surface reconstruction layer, the lattice parameter change of the core is smaller than that of the surface reconstruction layer, which reduces the lattice expansion / contraction stress in the core. The lattice expansion / contraction stress in the surface reconstruction layer is easily released to the outside of the positive electrode material particles, which can effectively reduce the breakage and crushing of the positive electrode material particles, improve the cycle life of the positive electrode material, and increase the withstand voltage of the positive electrode material. In addition, the higher molar content of lithium ions in the surface reconstruction layer can also improve the lithium ion conductivity of the positive electrode material and improve the rate performance of the positive electrode material.

[0015] The present application also provides a method for preparing a positive electrode material, wherein a surface reconstruction material is mixed with a core matrix material and then sintered to obtain a positive electrode material. The constituent elements of the surface reconstruction material are highly consistent with the constituent elements of the core matrix material, and the lattice parameter difference is small. In addition, the surface reconstruction material of the present application has a stronger ability to bind to Li than the matrix core material, the molar content of Ni is lower than that of the core matrix material, the molar content of Co and / or M is higher than that of the core matrix material, and the binding ability of Co and M to Li is stronger than that of Ni to Li. Therefore, during the sintering process, the Co and M in the surface reconstruction material can pull out part of the Li in the core matrix material to participate in the surface reconstruction of the positive electrode material. The surface reconstruction material can play a role similar to "pumping", so that the molar content of Li in the surface reconstruction layer of the prepared positive electrode material is greater than the molar content of Li in the core. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The present application is further described below with reference to the accompanying drawings and examples.

[0017] FIG1 is a schematic structural diagram of a cathode material provided in an embodiment of the present application;

[0018] FIG2 is a schematic diagram of a discharge state of a battery provided in an embodiment of the present application;

[0019] FIG3a is a SEM image of the positive electrode material prepared in Example 1 of the present application;

[0020] FIG3 b is a SEM image of the base material prepared in Example 1 of the present application;

[0021] FIG4 is a comparison chart of the cycle performance of the positive electrode materials prepared in Example 1 of the present application and Comparative Example 1. DETAILED DESCRIPTION

[0022] In order to better understand the technical solution of the present application, the embodiments of the present application are described in detail below with reference to the accompanying drawings.

[0023] It should be clear that the embodiments described are only part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0024] The terms used in the embodiments of the present application are only for the purpose of describing specific embodiments and are not intended to limit the present application. The singular forms "a", "an", "the" and "the" used in the embodiments of the present application and the appended claims are also intended to include plural forms unless the context clearly indicates otherwise.

[0025] It should be understood that the term "and / or" as used herein is merely a description of the relationship between associated objects, indicating that three possible relationships exist. For example, "A and / or B" can represent: A exists alone, A and B exist simultaneously, or B exists alone. Furthermore, the character " / " in this document generally indicates that the associated objects are in an "or" relationship.

[0026] At present, in conventional multi-element positive electrode materials, the concentration distribution of various elements between the core and the outer layer of the positive electrode material particles is uniform, the degree of Li+ deintercalation in the core and the outside during charging and discharging is close, the lattice parameter change range is close, and the stress of the outer layer can be effectively released to the outside world. However, the stress inside the positive electrode material particles is easy to accumulate and difficult to be effectively released.

[0027] Furthermore, due to the differences in the lattice parameters of lithium nickelate, lithium cobaltate, and lithium manganate in the multi-element positive electrode materials, the difficulty of lithium ions being deintercalated in different crystal structures is different. Lithium ions are easiest to deintercalate in lithium nickelate, and most difficult to deintercalate in lithium manganate. Therefore, the inner and outer layers with different Ni, Co, and Mn contents in the conventional concentration gradient lead to different lithium ion deintercalation rates. The inner layer has a higher molar content of nickel, and lithium deintercalation is easier than the outer layer, causing greater changes in the core lattice size. The larger lattice expansion / contraction stress causes the particles of the multi-element positive electrode material to break and shatter, seriously affecting the life of the positive electrode material, and it is difficult to fully exert the advantages that the positive electrode material should have.

[0028] In the first aspect, the present application provides a positive electrode material, as shown in Figure 1, wherein the positive electrode material is a lithium nickel cobalt oxide composite oxide; the lithium nickel cobalt oxide composite oxide includes a core and a surface reconstruction layer located outside the core; the surface reconstruction layer is the area etched away by diluting 0.3g of the positive electrode material 100 times with 0.4ml of aqua regia and dissolving it for 30 minutes, and the ambient temperature is controlled at 300°C during dissolution. The core is the remaining area of ​​the positive electrode material after etching away the surface reconstruction layer; the molar content of Li in the core is m1; the molar content of Li in the surface reconstruction layer is m2; wherein, 1.3≤m2 / m1≤4.0.

[0029] The positive electrode material proposed in this application has a molar content of Li in the core that is lower than the molar content of Li in the surface reconstruction layer, so that a lithium ion concentration difference is formed between the inner and outer layers of the positive electrode material. During the charge and discharge process, the lithium ion deintercalation reaction activity in the core is lower than the lithium ion deintercalation reaction activity in the surface reconstruction layer, and lithium ions are preferentially deintercalated in the surface reconstruction layer. Since the degree of lithium ion deintercalation in the core is lower than that in the surface reconstruction layer, the lattice parameter change amplitude of the core is smaller than that of the surface reconstruction layer, which reduces the lattice expansion / contraction stress in the core, and the lattice expansion / contraction stress in the surface reconstruction layer is easily released to the outside of the positive electrode material particles, which can effectively reduce the breakage and crushing of the positive electrode material particles, improve the cycle life of the positive electrode material, and increase the withstand voltage of the positive electrode material. In addition, the higher lithium ion molar content in the surface reconstruction layer can also improve the lithium ion conductivity of the positive electrode material and improve the rate performance of the positive electrode material.

[0030] In some embodiments, m2 / m1 can specifically be 1.3, 1.5, 1.6, 1.8, 2.0, 2.5, 2.8, 2.9, 3.0, 3.2, 3.5, 3.8 or 4.0, etc., and of course it can also be other values ​​within the above range, which is not limited here. When m2 / m1 is too high, that is, the molar content of Li in the surface reconstruction layer is too high, the surface residual alkali content of the positive electrode material will increase significantly, which is not conducive to improving the first effect of the positive electrode material, and the processing performance of the material will also decrease. When m2 / m1 is too low, that is, the molar content of Li in the core is too high, the molar concentration of the overall lithium element of the positive electrode material will increase, the gas expansion rate of the battery cell prepared by the positive electrode material will increase significantly, and the safety performance will decrease. When both m2 and m1 are too low, it will affect the gram capacity and cycle life of the positive electrode material.

[0031] In some embodiments, the molar content of Li in the surface reconstruction layer is m2, and the total molar content of Li in the positive electrode material is m Li总 , 1.1≤m2 / m Li总 ≤3.0, m2 / m Li总 Specifically, it can be 1.1, 1.15, 1.2, 1.5, 1.8, 1.9, 2, 2.1, 2.2, 2.3, 2.5, 2.9 or 3.0, etc. Of course, it can also be other values ​​within the above range, which are not limited here. The molar content of Li in the surface reconstruction layer of the positive electrode material is higher than the total molar content of Li in the positive electrode material, that is, the molar content of lithium ions in the surface reconstruction layer of the positive electrode material is higher than the molar content of lithium ions in the core. A lithium ion concentration difference is formed in the inner and outer layers of the positive electrode material. During the charge and discharge process, the lithium ion deintercalation reaction activity in the core is lower than the lithium ion deintercalation reaction activity in the surface reconstruction layer. Lithium ions are preferentially deintercalated in the surface reconstruction layer, and the expansion / contraction stress generated by deintercalation can be effectively released.

[0032] It should be noted that the molar content of Li in this application refers to the molar content of the positive electrode material excluding the free lithium Li on the surface. + The ratio of the amount of Li in all lattice structures except Li to the sum of the amounts of all metal elements except Li, that is, m Li =n(Li):n(Ni+Co+Mn+M1). Surface free lithium can specifically include lithium ions in substances such as Li2CO3 and LiOH remaining on the surface of the positive electrode material. Furthermore, the lithium nickel cobalt oxide composite oxide, i.e., the lithium nickel cobalt oxide positive electrode material, in this application can be a lithium nickel cobalt manganese oxide (NCM) positive electrode material or a lithium nickel cobalt aluminum oxide (NCA) positive electrode material.

[0033] In some embodiments, the chemical formula of the surface reconstruction layer is Li 1+λ Ni x Co y M z M1 1-x-y-z O2, wherein 0.01≤λ≤2, 0≤x≤0.5, 0.3≤y≤1, 0≤z≤0.5, 0≤1-xyz≤0.2, M is Mn and / or Al, and M1 is a metal element.

[0034] In this application, 0.3g of positive electrode material was weighed, 0.4mL of aqua regia was diluted 100 times and added to the positive electrode material, heated and dissolved, the ambient temperature was controlled to 300°C, the dissolution time was 30min, and the obtained solution was filtered, washed, fixed to volume, and diluted 100 times before ICP testing to measure the molar content of Li, Ni, Co, M, and M1, where the molar content of Li m2 is the molar content of Li in the surface reconstruction layer. It can be understood that since the concentration and amount of aqua regia in 0.4mL of aqua regia are low after being diluted 100 times, it can only dissolve the surface reconstruction layer of the positive electrode material particles.

[0035] Weigh 0.3g of positive electrode material, dilute 8mL of aqua regia 5 times and add it to the positive electrode material, heat and dissolve it, control the ambient temperature to 300℃, and dissolve it for 30min. The obtained solution is filtered, washed, fixed to volume, diluted 100 times and then tested by ICP. The molar content of Li, Ni, Co, M and M1 is measured, among which the molar content of Li is m Li总 That is, the total molar content of Li in the positive electrode material, and the molar content of Li in the core is m1=(m Li总 *w1-m2*w2) / (w1-w2), where w1 is the total mass of 0.3 g of cathode material and w2 is the mass of the surface reconstruction layer. It can be understood that since 8 mL of aqua regia is diluted 5-fold, the concentration and mass of the aqua regia are high, and it can completely dissolve the cathode material particles.

[0036] In some embodiments, the chemical formula of the surface reconstruction layer is Li1+λ Ni x Co y M z M1 1-x-y-z The value of λ can be 0.01, 0.02, 0.05, 0.08, 0.1, 0.15, 0.18, 0.2, 0.25, 0.5, 0.8, 1, 1.25, 1.5, 1.8 or 2. The value of x can be 0, 0.1, 0.12, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45 or 0.5, etc., the value of y can be 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 or 1, etc., the value of z can be 0, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45 or 0.5, etc., and the molar content of M1 can be 0, 0.001, 0.002, 0.005, 0.01, 0.05, 0.1, 0.15, 0.18 or 0.2, etc.

[0037] In some embodiments, the metal element M1 includes at least one of Al, Ti, Y, Zr, Mg, Sr, W, Nb, Ce, La, Mo, and Dy. Specifically, the metal element M1 can be present in the positive electrode material as a metal element or as a coating element.

[0038] It should be noted that the content of each element in the positive electrode material can be measured by a well-known instrument for qualitative analysis and / or quantitative analysis of each element, such as ICP and ICP-MS.

[0039] In some embodiments, the molar content of Ni in the core is greater than the molar content of Ni in the surface reconstruction layer. It can be understood that the molar content of Ni in the surface reconstruction layer is low, which can improve the side reaction between the positive electrode material and the electrolyte and improve the cycle life of the positive electrode material. The molar content of Li in the core of the positive electrode material of the present application is low, and the molar content of Ni is high. The expansion / contraction effect of the core of the positive electrode material is weakened during the lithium insertion and extraction process, which is beneficial to improve the stability of the particle structure; at the same time, the molar content of Li in the surface reconstruction layer of the positive electrode material is high, and the molar content of Ni is low. The expansion / contraction stress of the positive electrode material can be well released, and the side reaction between the positive electrode material and the electrolyte is reduced, which is beneficial to improve the cycle stability of the positive electrode material; thereby, the positive electrode material has high voltage, high rate and good cycle performance.

[0040] In some embodiments, the molar content of all metal elements other than Li and Ni in the core is less than the molar content of all metal elements other than Li and Ni in the surface reconstruction layer. It can be understood that the molar content of Co and M in the core is less than the molar content of Co and M in the surface reconstruction layer. The higher molar content of Co and M in the surface reconstruction layer can inhibit the transformation of the crystal phase of the positive electrode material from H2 to H3, and the crystal phase structure stability of the surface reconstruction layer of the positive electrode material is better. In addition, more metal elements M1 in the surface reconstruction layer can reduce the reactivity of the positive electrode material with the electrolyte. The molar content of multiple elements in the positive electrode material of the present application varies in a gradient, and the elements cooperate with each other, so that the electrochemical performance of the positive electrode material can be further improved, and the performance of the positive electrode material can be better exerted.

[0041] In some embodiments, 0.6≤m1<1, m1 can be 0.6, 0.7, 0.8, 0.85, 0.9, 0.95 or 0.99, etc., which are not limited here. Experiments have found that when the molar content of Li in the core is controlled within the above range, the electrochemical performance of the positive electrode material is better.

[0042] In some embodiments, the chemical formula of the surface reconstruction layer is Li 1+λ Ni x Co y M z M1 1-x-y-z In O2, 0.1≤λ≤2, that is, the molar ratio of n(Li):n(Ni+Co+M+M1) is 1.1~3.

[0043] In some embodiments, the molar ratio of lithium to all other metal elements (Me) in the positive electrode material is (0.95-1.05):1. Specifically, the n(Li) / n(Me) ratio can be 0.95:1, 0.96:1, 0.98:1, 1.0:1, 1.02:1, 1.03:1 or 1.05:1, etc., where Me represents the molar content of all metals in the positive electrode material except Li.

[0044] By controlling the value of n(Li):n(Ni+Co+M+M1) in the surface reconstruction layer to 1.1-3 and the ratio of n(Li) / n(Me) of the overall particles of the positive electrode material to (0.95-1.05):1, the stress caused by the change in the lattice parameters of the positive electrode material can be transferred from the core to the surface reconstruction layer to the maximum extent and then released to the outside world, which is beneficial to the comprehensive improvement of the electrochemical performance of the positive electrode material.

[0045] In some embodiments, the surface reconstruction layer comprises 5 wt% to 50 wt% of the positive electrode material, specifically 5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%, 30 wt%, 35 wt%, 40 wt%, or 50 wt%, and may be other values ​​within the above range, which are not limited herein. When the surface reconstruction layer comprises too little of the positive electrode material, the surface reconstruction layer of the positive electrode material cannot effectively protect the core, and it is difficult for the core and the surface reconstruction layer to form an effective lithium element concentration difference. When the surface reconstruction layer comprises too much of the positive electrode material, the thickness of the surface reconstruction layer of the positive electrode material increases. The excessively thick surface reconstruction layer makes it difficult to effectively transfer the expansion stress of the core, making it difficult to effectively release the expansion stress of the core, and thus reducing the cycle performance of the positive electrode material. By controlling the surface reconstruction layer within the range of 5 wt% to 40 wt%, the expansion / contraction stress inside the positive electrode material can be effectively released, and a suitable lithium ion concentration gradient is beneficial to improving the cycle structural stability of the positive electrode material. More preferably, the mass percentage of the surface reconstruction layer in the positive electrode material is 7 wt % to 40 wt %.

[0046] In some embodiments, the mass percentage of free lithium in the positive electrode material is 200ppm to 1000ppm, specifically 200ppm, 300ppm, 400ppm, 500ppm, 600ppm, 800ppm or 1000ppm, etc., and of course it can also be other values ​​within the above range, which is not limited here. The free lithium in the positive electrode material mainly comes from the alkaline impurities on the surface of the positive electrode material, such as Li2CO3, LiOH, etc. Controlling the content of free lithium in the positive electrode material can reduce the corrosive effect of alkaline impurities on the positive electrode material, protect the structural stability of the positive electrode material, and help improve the cycle stability of the positive electrode material. The mass percentage of free lithium in the positive electrode material will increase with the increase of Ni content. Preferably, the mass percentage of free lithium is 200ppm to 600ppm.

[0047] In some embodiments, the specific surface area of ​​the positive electrode material is 0.2 m 2 / g~2.0m 2 / g. Specifically, it can be 0.2m 2 / g, 0.5m 2 / g, 0.8m 2 / g, 1.0m 2 / g, 1.5m 2 / g, 1.8m 2 / g or 2.0m 2 / g, etc., and of course other values ​​within the above range are also possible and are not limited here. Controlling the specific surface area of ​​the positive electrode material within the above range can reduce the occurrence of side reactions between the positive electrode material and the electrolyte, and the positive electrode material can exhibit better cycle stability.

[0048] In some embodiments, the true density of the positive electrode material is 4.6 g / cm 3 ~4.8g / cm 3 , specifically 4.6g / cm 3 , 4.65g / cm 3 , 4.68g / cm 3 , 4.70g / cm 3 , 4.72g / cm 3 , 4.75g / cm 3 , 4.78g / cm 3 , 4.79g / cm 3 or 4.8g / cm 3 etc., not limited here.

[0049] In a second aspect, the present application provides a method for preparing a positive electrode material, the method comprising:

[0050] Step S10: sintering the first mixture containing the cathode material precursor and the lithium-containing compound to obtain a core matrix material, wherein the core matrix material has a chemical formula of Li 1-δ Ni a Co b M c M1 1-a-b-c O2, wherein -0.05≤δ≤0.05, 0.3≤a<1.0, 0.01≤b≤0.3, 0≤c≤0.5, 0≤1-abc≤0.05, M is Mn and / or Al, and M1 is a metal element;

[0051] Step S20, performing a secondary sintering process on the second mixture comprising the surface reconstruction material and the core matrix material to obtain a positive electrode material; wherein the surface reconstruction material comprises Ni r Co p M q M3 1-r-p-q (OH)2、Ni r Co p M q M3 1-r-p-q OOH and (Ni r Co p M q M3 1-r-p-q )O dAt least one of the following, 0≤r≤0.5, 0.3≤p≤1, 0≤q≤0.5, 0≤1-rpq≤0.2, 1≤d≤2, M is Mn and / or Al, and M3 is a metal element.

[0052] In the above technical solution, a secondary sintering process is performed using a surface reconstruction material and a core matrix material. Since the constituent elements of the surface reconstruction material are highly consistent with those of the core matrix material, the difference in lattice parameters is small. In addition, the surface reconstruction material of the present application has a strong binding ability with the Li element, the molar content of Ni is lower than that of the core matrix material, and the molar content of Co and / or M is higher than that of the core matrix material. The binding ability of Co and M with Li is stronger than that of Ni with Li. Therefore, during the secondary sintering process, the Co and M in the surface reconstruction material can pull out part of the Li in the core matrix material to participate in the surface reconstruction of the positive electrode material. The surface reconstruction material can play a role similar to "pumping", so that the molar content of Li in the surface reconstruction layer of the prepared positive electrode material is greater than the molar content of Li in the core.

[0053] The molar content of Li in the core of the positive electrode material prepared in the present application is lower than the molar content of Li in the core matrix material (preparation raw material). This is because the Li in the core matrix material is partially extracted to participate in the surface reconstruction of the positive electrode material. It has been verified that when the molar content of Li in the core of the final positive electrode material m1 is in the range of 0.6 to 1.0, the electrochemical performance of the positive electrode material is better, and the surface reconstruction material combines with the Li extracted from the core matrix material to form a Li-containing compound (i.e., a surface reconstruction layer). The molar content of Li in the surface reconstruction layer of the prepared positive electrode material is higher than the molar content of Li in the core.

[0054] Surface reconstruction layer In addition, the positive electrode material prepared in the present application has a Li ion concentration in the surface reconstruction layer that is greater than the Li ion concentration in the core. This difference in Li ion concentration is not formed in the precursor preparation stage, nor is it formed in the stage where the precursor and the lithium-containing compound undergo a solid-phase reaction after a single sintering to generate a layered structure material. Instead, it is formed by surface reconstruction of the core matrix material. Therefore, the surface reconstruction process of the matrix material of the present application is not limited by the type of matrix material or the primary sintering temperature, and can be applied to polycrystalline matrix materials or single crystal matrix materials. The concentrations of the constituent elements in the prepared positive electrode material are different in the core and the surface, which further improves the rate performance, high voltage resistance, cycle life, material impedance and other properties of the positive electrode material.

[0055] The preparation method of the present application is described in detail below with reference to the examples:

[0056] Step S10: sintering the mixture containing the cathode material precursor and the lithium-containing compound to obtain a core matrix material, wherein the core matrix material has the general chemical formula of Li 1-δ Ni a Co b M c M1 1-a-b-c O2, wherein -0.05≤δ≤0.05, 0.3≤a<1.0, 0.01≤b≤0.3, 0≤c≤0.5, 0≤1-abc≤0.05, M is Mn and / or Al, and M1 is a metal element.

[0057] In some embodiments, the chemical formula of the cathode material precursor is Ni a1 Co b1 M1 c1 (OH)2, where 0.3≤a1<1.0, 0.01≤b1≤0.3, and 0≤c1≤0.5.

[0058] In some embodiments, the lithium-containing compound includes at least one of lithium carbonate, lithium hydroxide, lithium nitrate, lithium fluoride, and lithium acetate. Preferably, the lithium-containing compound includes lithium hydroxide, specifically, the lithium hydroxide includes at least one of anhydrous lithium hydroxide and lithium hydroxide monohydrate.

[0059] In some embodiments, the molar ratio of metal Me in the positive electrode material precursor to Li in the lithium-containing compound is 1:1≤Li / Me≤1:1.3, and can be specifically 1:1, 1:1.02, 1:1.05, 1:1.08, 1:1.09, 1:1.1, 1:1.2, 1:1.25 or 1:1.3, etc., which is not limited here.

[0060] In some embodiments, the first mixture further includes a first dopant, and the metal element M1 in the first dopant includes at least one of Zr, Sr, Nb, Mo, Y, Co, Ti, Zr, Mg, B, and W.

[0061] In some embodiments, the amount of the first dopant added to the first mixture is controlled to be 0.001 wt% to 2 wt%, specifically 0.001 wt%, 0.005 wt%, 0.007 wt%, 0.009 wt%, 0.01 wt%, 0.05 wt%, 0.08 wt%, 0.1 wt%, 0.5 wt%, 0.8 wt%, 1 wt%, 1.2 wt%, 1.5 wt%, 1.8 wt%, or 2 wt%, and other values ​​within the above range are also possible and are not limited herein.

[0062] In some embodiments, the mixing speed of the first mixture is ≥300 rpm / min, and the mixing time is ≥15 min.

[0063] In some embodiments, the temperature of the primary sintering treatment is 700°C to 1000°C. Specifically, the temperature of the primary sintering treatment is 700°C, 750°C, 800°C, 850°C, 900°C, 950°C or 1000°C, etc. Of course, it can also be other values ​​within the above range, which is not limited here.

[0064] In some embodiments, the time for one sintering treatment is 4 hours to 10 hours. Specifically, the time for one sintering treatment is 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours or 10 hours, etc. Of course, it can also be other values ​​within the above range, which is not limited here.

[0065] In some embodiments, the primary sintering process is performed in an oxygen-containing atmosphere, and the oxygen content of the oxygen-containing gas is greater than or equal to 90%. Specifically, the oxygen content of the oxygen-containing gas can be 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, or 100%, etc., and of course, other values ​​within the above range are also possible, and are not limited herein. Preferably, the oxygen content of the oxygen-containing gas is greater than or equal to 95%.

[0066] In some embodiments, the equipment for the primary sintering process includes a stationary box furnace, a continuous roller kiln, and the like.

[0067] In some embodiments, the product after the primary sintering process can be crushed, screened, and demagnetized to obtain a core matrix material with a suitable particle size.

[0068] Step S20 , performing a secondary sintering process on the second mixture including the surface reconstruction material and the core matrix material to obtain a positive electrode material.

[0069] In some embodiments, the core matrix material and the surface reconstruction material are mixed in a mixed atmosphere containing CO2 to produce the second mixture, wherein the concentration of CO2 in the mixed atmosphere is ≥80%. The second mixture is mixed in a mixed atmosphere containing CO2 because carbon dioxide gas readily reacts with lithium-containing compounds, such as Li2O or LiOH, on the surface of the core matrix material in the second mixture, forming Li2CO3 that adheres to the surface of the core matrix material particles. This makes it easier for the subsequently added surface reconstruction material to react with the core matrix material to form lithium-containing compounds, thereby reconstructing the surface of the core matrix material.

[0070] In some embodiments, the CO2 atmosphere contains 5 vol% to 10 vol% water, which further increases the rate and extent of the reaction.

[0071] It should be noted that the present application controls the mixing conditions and mixing atmosphere of the second mixture in order to make the surface reconstruction material in the second mixture more evenly and firmly adhere to the surface of the core matrix material. In the actual mixing process, corresponding adjustments can also be made according to actual conditions.

[0072] It can be understood that during the secondary sintering process, the second mixture of the surface reconstruction material and the core matrix material is sintered. Since the constituent elements of the surface reconstruction material are highly consistent with the constituent elements of the core matrix material, the lattice parameter difference is small. In addition, the surface reconstruction material described in the present application has a stronger binding ability with the Li element than the core matrix material, the molar content of Ni is lower than that of the core matrix material, and the molar content of Co and / or M is higher than that of the core matrix material. The binding ability of Co and M with Li is stronger than that of Ni with Li. Therefore, during the secondary sintering process, the Co and M in the surface reconstruction material can extract part of the Li in the core matrix material to participate in the surface reconstruction of the positive electrode material. The surface reconstruction material can play a role similar to "pumping", so that the molar content of Li in the surface reconstruction layer of the prepared positive electrode material is greater than the molar content of Li in the core.

[0073] In some embodiments, the surface reconstruction material comprises Ni r Co p M q M3 1-r-p-q (OH)2、Ni r Co p M q M3 1-r-p-q OOH and (Ni r Co p M q M3 1-r-p-q )O d At least one of the following, 0≤r≤0.5, 0.3≤p≤1, 0≤q≤0.5, 0≤1-rpq≤0.2, 1≤d≤2, M is Mn and / or Al, and M3 is a metal element. Co and M in the surface reconstruction material have a stronger binding ability with Li ions than Ni in the core matrix material.

[0074] In some embodiments, the metal element M3 includes at least one of Al, Ti, Y, Zr, Mg, Sr, W, Nb, Ce, La, Mo, and Dy.

[0075] In some embodiments, the median particle size of the surface reconstruction material is 10 nm to 1000 nm, specifically 10 nm, 20 nm, 30 nm, 40 nm, 50 nm, 80 nm, 100 nm, 300 nm, 500 nm, 800 nm or 1000 nm, etc. Of course, it can also be other values ​​within the above range, which is not limited here.

[0076] In some embodiments, the bulk density of the surface reconstruction material is ≤0.6 g / cm 3 , specifically 0.6g / cm 3 , 0.5g / cm 3 , 0.4g / cm 3 , 0.3g / cm 3 , 0.35g / cm 3 , 0.25g / cm 3 , 0.2g / cm 3 , 0.15g / cm 3 , 0.1g / cm 3 Of course, it can also be other values ​​within the above range, which is not limited here.

[0077] In some embodiments, the specific surface area of ​​the surface reconstruction material is ≥20m 2 / g, specifically 20m 2 / g、30m 2 / g, 50m 2 / g、80m 2 / g、100m 2 / g, or 120m 2 / g, etc., and of course, it can also be other values ​​within the above range, which is not limited here.

[0078] In the present application, by controlling the particle size, apparent density, specific surface area, etc. of the surface reconstruction material, the size of the surface reconstruction material can be controlled at the nanometer level. The fine particles of the surface reconstruction material can easily and evenly adhere to the surface of the core matrix material and improve the activity of the surface reconstruction reaction, which is conducive to forming a surface reconstruction layer with a higher molar content of lithium in the positive electrode material, thereby improving the cycle stability of the positive electrode material.

[0079] In some embodiments, the molar ratio of the core matrix material to the surface reconstruction material is 1:(0.01-0.1), specifically 1:0.01, 1:0.03, 1:0.05, 1:0.08, 1:0.09, 1:0.1, etc. Of course, it can also be other values ​​within the above range, which is not limited here. The introduction of the surface reconstruction material can effectively form a surface reconstruction layer of the positive electrode material, so that the Li concentration difference between the core and the surface can be effectively formed. If the addition amount is too large, the surface reconstruction material will be too thick, which will greatly increase the time for Li+ to diffuse from the core to the surface and reduce the effect of the Li concentration difference between the core and the surface. At the same time, due to the excessive thickness of the surface reconstruction layer, the stress in the charge and discharge process of the matrix core cannot be effectively transferred to the surface. Preferably, the molar ratio of the core matrix material to the surface reconstruction material is 1:(0.02-0.05).

[0080] In some embodiments, the second mixture further includes a second dopant, and the metal element M1 in the second dopant includes at least one of Zr, Sr, Nb, Mo, Y, Co, Ti, Zr, Mg, B, and W.

[0081] In some embodiments, the amount of the second dopant added to the second mixture is controlled to be 0.01 wt% to 2 wt%, specifically 0.01 wt%, 0.05 wt%, 0.08 wt%, 0.1 wt%, 0.5 wt%, 0.8 wt%, 1 wt%, 1.2 wt%, 1.5 wt%, 1.8 wt%, or 2 wt%, and other values ​​within the above range are also possible and are not limited herein.

[0082] In some embodiments, the mixing speed of the second mixture is ≥300 rpm / min, and the mixing time is ≥15 min.

[0083] It should be noted that the present application controls the mixing conditions of the second mixture in order to achieve a uniform mixing of the components in the second mixture. In the actual mixing process, corresponding adjustments may also be made according to actual conditions.

[0084] In some embodiments, the temperature of the secondary sintering treatment is 600°C to 900°C. Specifically, the temperature of the secondary sintering treatment is 600°C, 650°C, 700°C, 750°C, 800°C, 850°C or 900°C, etc. Of course, it can also be other values ​​within the above range, which is not limited here. As the temperature of the secondary sintering treatment increases, the concentration of lithium elements at the grain boundaries of the surface of the substrate material near the coating layer will increase. However, when too much lithium elements are pulled out to the surface, it will affect the layered structure of the core positive electrode material to transform into spinel and / or rock salt phase, resulting in structural changes in the positive electrode material and affecting the discharge capacity of the positive electrode material.

[0085] In some embodiments, the secondary sintering treatment time is 4 hours to 10 hours. Specifically, the secondary sintering treatment time is 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours or 10 hours, etc. Of course, it can also be other values ​​within the above range, which is not limited here.

[0086] In some embodiments, the secondary sintering process is performed in an oxygen-containing atmosphere, and the oxygen content of the oxygen-containing gas is greater than or equal to 20%. Specifically, the oxygen content of the oxygen-containing gas can be 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, or 100%, etc., and of course other values ​​within the above range are also possible, and are not limited herein. Preferably, the oxygen content of the oxygen-containing gas is greater than or equal to 40%.

[0087] In some embodiments, the equipment for the secondary sintering process includes a stationary box furnace, a continuous roller kiln, and the like.

[0088] The present application also provides a battery. FIG2 is a schematic diagram of the discharge state of the battery provided in the present application. As shown in FIG2 , the battery includes a housing and an electrode assembly. The electrode assembly includes a positive electrode sheet 1, a negative electrode sheet 2, and a separator 3. The separator 3 is provided between the positive electrode sheet 1 and the negative electrode sheet 2. The electrode assembly can be a laminated structure, which is formed by alternatingly stacking the positive electrode sheet 1, the separator 3, and the negative electrode sheet 2. In other embodiments, the electrode assembly can also be a wound structure, which is formed by stacking the positive electrode sheet, the separator, and the negative electrode sheet in sequence and then winding them.

[0089] In some embodiments, the positive electrode sheet 1 includes a positive electrode current collector 101 and a positive electrode active layer 102 disposed on at least one surface of the positive electrode current collector 101 .

[0090] In some embodiments, the positive electrode current collector 101 may be aluminum foil or nickel foil, or any composite current collector disclosed in the prior art, such as, but not limited to, a current collector formed by combining the aforementioned conductive foil (aluminum foil or nickel foil, etc.) and a polymer substrate. The positive electrode active layer 102 includes a positive electrode active material, which includes the positive electrode materials described above.

[0091] In some embodiments, the negative electrode sheet 2 includes a negative electrode current collector 201 and a negative electrode active material layer 202 disposed on at least one surface of the negative electrode current collector.

[0092] In some embodiments, the negative electrode current collector 201 may be made of at least one of copper foil, nickel foil, stainless steel foil, titanium foil, or a carbon-based current collector. It may also be any composite current collector disclosed in the prior art, such as, but not limited to, a current collector formed by combining the aforementioned conductive foil and a polymer substrate. The negative electrode active material layer 202 comprises a negative electrode material, which may include at least one of the following: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate. Silicon-based materials may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. Tin-based materials may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to these materials; other conventional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination. The batteries provided in the embodiments of this application have the advantages of high capacity, high initial efficiency, long cycle life, excellent rate capability, and low expansion. The batteries may be lithium-ion batteries, sodium-ion batteries, solid-state electrolyte batteries, and the like, without limitation.

[0093] The above are only preferred embodiments of the present application and are not intended to limit the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present application should be included in the scope of protection of the present application.

[0094] Test method:

[0095] 1) Specific surface area test of positive electrode material:

[0096] The nitrogen adsorption specific surface area analysis test method is used for testing and the BET (Brunauer Emmett Teller) method is used for calculation. The nitrogen adsorption specific surface area analysis test can be performed using a Tri Star II surface and pore analyzer produced by Micromeritics of the United States.

[0097] 2) SEM test:

[0098] A Hitachi Regulus 8100 cold-field emission scanning electron microscope (CFSEM) was used to obtain the surface morphology of the material.

[0099] 3) Molar content test of each element of the positive electrode material:

[0100] The content of each element in the positive electrode material can be measured by a well-known instrument for qualitative analysis and / or quantitative analysis of each element, such as ICP and ICP-MS.

[0101] 4) A method for determining the molar content of Li in the core and surface reconstruction layer of a positive electrode material, comprising the following steps:

[0102] First, weigh 0.3g of positive electrode material, dilute 8mL of aqua regia 5 times and add it to the positive electrode material, heat and dissolve, control the ambient temperature to 300℃, and dissolve for 30min. The obtained solution is filtered, washed, fixed to volume, diluted 100 times, and then ICP tested to measure the molar content of Li, Ni, Co, M, and M1. Aqua regia is prepared by a volume ratio of 3:1 of concentrated hydrochloric acid: concentrated nitric acid. The concentration of concentrated hydrochloric acid used is 12mol / L, and the concentration of concentrated nitric acid is 14.5mol / L. Calculate the molar content of each element in the fully dissolved state of the positive electrode material:

[0103] ①The total amount of Ni, Co, M, and M1 is w1, which is normalized to obtain n(Ni), n(Co), and n(M);

[0104] ②[n(Li):n(Ni+Co+Mn+M1)]=m Li总 ;

[0105] ③[n(N):n(Ni+Co+M+M1)]=b1N, N=Ni or Co or M;

[0106] ④[n(M1+M2):n(Ni+Co+M+M1)]=c1;

[0107] Second, weigh 0.3 g of the positive electrode material, dilute 0.4 mL of aqua regia 100 times, and add it to the positive electrode material. Heat and dissolve it. Control the ambient temperature to 300°C and the dissolution time to 30 minutes. The obtained solution is filtered, washed, fixed to volume, diluted 100 times, and then ICP tested to measure the molar content of Li, Ni, Co, M, and M1 in the surface reconstruction layer.

[0108] Calculate the molar content of each element in the dissolved state on the surface of the positive electrode material:

[0109] ①The total amount of Ni, Co, M, and M1, w2, is normalized to obtain n(Ni), n(Co), and n(M);

[0110] ② In the surface reconstruction layer [n(Li):n(Ni+Co+M+M1)]=m2;

[0111] ③[n(N):n(Ni+Co+M+M1)]=b2N, N=Ni or Co or M;

[0112] ④[n(M1+M2):n(Ni+Co+M+M1)]=c2;

[0113] ⑤ The mass percentage of the surface reconstruction layer is η = w2 / w1.

[0114] The molar content of each element in the core of the positive electrode material is calculated by the molar content of each element corresponding to the entire material and the surface reconstruction layer. The calculation process is as follows:

[0115] Third, calculate the molar content of Li, Ni, Co, M, and M1 in the core:

[0116] ① The total molar content of Ni, Co, M, and M1 in the core is w3 = w1-w2, which is normalized to obtain n(Ni), n(Co), and n(M);

[0117] ②[n(Li):n(Ni+Co+M+M1)]=[(m Li总 *w1-m2*w2) / w3]=m1,

[0118] ③[n(N):n(Ni+Co+M+M1)]=[(b1N*w1-b2N*w2) / w3]=vN, N=Ni or Co or M;

[0119] ④[n(M1+M2):n(Ni+Co+M+M1)]=[(c1*w1-c2*w2) / w3]=x;

[0120] When |m2-m1|≥0.01, it can be determined that the tested positive electrode material has a Li concentration difference between the surface reconstruction layer and the core. The larger the calculated |m2-m1| value, the more significant the change in the Li molar content of the corresponding positive electrode material.

[0121] When |b1N-b2N|≥0.01, it can be determined that the tested positive electrode material has a Ni and / or Co and / or M concentration difference between the surface reconstruction layer and the core. The larger the calculated |b1N-b2N| value, the more significant the difference in the molar content of Ni and / or Co and / or M in the corresponding positive electrode material.

[0122] When |c1-c2|≥0.001wt%, it can be determined that the tested positive electrode material has a metal element concentration difference between the surface reconstruction layer and the core.

[0123] 5) Free lithium mass content test:

[0124] 5 g of the positive electrode material was dispersed in 100 ml of deionized water, and magnetic stirring was performed for 10 minutes while maintaining the water temperature at 25°C to obtain a solid-liquid mixture of dissolved residual lithium carbonate and lithium hydroxide. The solid-liquid mixture was filtered to obtain a filtrate, and hydrochloric acid solution was added to an automatic potentiometric titrator for titration using an equivalent drop method. The lithium content in the positive electrode materials lithium carbonate and lithium hydroxide was calculated based on the breakthrough point and the degree of hydrochloric acid solution consumption.

[0125] Example 1

[0126] A method for preparing a positive electrode material comprises the following steps:

[0127] (1) Ni 0.80 Co 0.01 Mn 0.098 The (OH)2 precursor, LiOH·H2O and dopant ZrO2 were added to a high-speed mixer in a molar ratio of 1:1.02:0.002 and mixed evenly to obtain a first mixture; wherein the stirring speed was 1000 rpm and the stirring time was 30 min.

[0128] (2) The mixture was calcined in a pure oxygen atmosphere at a temperature of 880°C for 8 hours. The sintered product was cooled, rolled, and air flow crushed, and then passed through a 400-mesh sieve and demagnetized to obtain the matrix material Li 1.02 Ni 0.8 Co 0.1 Mn 0.098 Zr 0.002 O2.

[0129] (3) The core matrix material Li 1.02 Ni 0.8 Co 0.1 Mn 0.098 Zr 0.002 O2 and surface reconstruction material Ni 0.08 Co 0.8 Mn 0.08 Al 0.04 OOH was uniformly mixed in a high-speed mixer at a molar ratio of 1:0.02, wherein the stirring speed was 1000 rpm and the stirring time was 30 minutes. The stirring was performed under a mixed atmosphere comprising 95% by volume of carbon dioxide gas and 0.05% by volume of water vapor to obtain a second mixture;

[0130] (4) The second mixture was sintered at 730° C. for 8 h in a pure oxygen atmosphere, and then cooled, roller crushed, 400-mesh screened, and demagnetized to obtain a positive electrode material.

[0131] The chemical composition of the positive electrode material is Li 0.986 Ni 0.8 Co 0.1 Mn 0.098 Zr 0.002 O2·0.02Li 1.70 Ni 0.08 Co 0.8 Mn 0.08 Al 0.04 O2.

[0132] Examples 2 to 17 (abbreviated as S1 to S17) and comparative examples (abbreviated as D1 to D2) were prepared according to the steps of Example 1. The differences in the preparation processes are shown in Table 1. The properties of the prepared positive electrode materials are shown in Table 2:

[0133] Table 1. Parameters of cathode material preparation process

[0134] Table 2. Physicochemical parameters of cathode materials

[0135] Preparation of positive electrode

[0136] The positive electrode material prepared in the embodiment or comparative example, the conductive agent acetylene black, and the binder polyvinylidene fluoride (PVDF) were dissolved in a solvent N-methylpyrrolidone (NMP) in a weight ratio of 93:5:2. After thorough stirring, the obtained positive electrode slurry was coated on an aluminum foil. After drying and sheeting, a positive electrode sheet was obtained. A metal lithium sheet was used as the negative electrode sheet, a polyethylene film was used as the separator, and lithium hexafluorophosphate with a concentration of 1 mol / L and a diethyl carbonate and ethylene carbonate solution with a volume ratio of 1:1 were used as the electrolyte to assemble into a 2032 button battery.

[0137] The charge and discharge test uses the Blue Electric test system CT2001A, and the capacity, first efficiency and rate performance tests are carried out in the voltage range of 2.5V to 4.3V.

[0138] The button batteries prepared in the above examples and comparative examples were cycled at room temperature (25°C) at a charge and discharge rate of 0.1C and a charge and discharge range of 2.5V to 4.3V. The initial discharge capacity and initial coulombic efficiency were measured. The ratio of the capacity after 50 cycles to the initial discharge capacity was recorded as the room temperature cycle performance.

[0139] The button cell assembled above was cycled at room temperature (25°C) at a charge and discharge rate of 0.1C and a charge and discharge range of 2.5V to 4.3V for 2 weeks, and then charged to a fully charged state of 4.3V. The electrochemical impedance spectroscopy (EIS) test was performed using a Shanghai Chenhua CHI600E electrochemical workstation with a test frequency of 100kHz to 10mHz and an amplitude of 5mV to obtain the electrochemical impedance performance.

[0140] Table 3. Performance parameters of positive electrode materials and battery performance parameters

[0141] The test data from Examples 1-4 show that as the molar ratio of the surface reconstruction material to the base material increases, the molar content of Li (m2) within the surface reconstruction layer of the cathode material decreases, while the molar content of Li (m1) within the core increases. This is because the increasing thickness of the surface reconstruction material weakens the Li extraction capability. In Examples 2 and 4, the addition of too much or too little surface reconstruction material reduced the specific capacity of the cathode material. In Example 2, when less surface reconstruction material was added, the cycle retention rate of the cathode material decreased, and the swelling rate increased.

[0142] According to the test data of Examples 5 to 13, the effects of core matrix materials with different Ni, Co and M contents on the performance of the positive electrode material are explored. As the Ni content decreases, the gram capacity of the positive electrode material gradually decreases, the cycle retention rate gradually increases, and the swelling rate gradually decreases.

[0143] According to Examples 14 to 15 and the test data, different sintering temperatures and times have little effect on the electrochemical performance of the positive electrode material.

[0144] According to the test data of Examples 16 and 17, different surface reconstruction materials have little effect on the electrochemical performance of the positive electrode material.

[0145] According to the test data of the embodiment and comparative example 1, it can be seen that the comparative example does not use the surface reconstruction material for secondary sintering treatment, and the electrochemical performance of the positive electrode material is significantly reduced compared with all the embodiments. Figure 3a and Figure 3b are SEM images of the positive electrode material and the base material of Example 1, respectively. It can be found that after the surface reconstruction, the particles of the positive electrode material of Example 1 become more rounded and can withstand higher stress without breaking; while the particles of the base material without surface reconstruction present an irregular structure with clear edges and corners, low particle strength, and are easy to break and shatter during the cycle. Figure 4 is a comparison of the cycle performance of the positive electrode materials of Example 1 and Comparative Example 1. It can be found that the cycle performance of the positive electrode material of Example 1 is better than that of the positive electrode material of Comparative Example 1.

[0146] According to the test data of the embodiment and comparative example 2, comparative example 2 uses B2O3 to coat the surface of the base material. Although the performance is improved compared with comparative example 1, the molar content of various elements in the positive electrode material particles is not distributed in a stepped state, and the cycle performance of the positive electrode material is inferior to all the embodiments.

[0147] The preferred embodiments of the present application are described in detail above, but the present application is not limited thereto. Within the technical scope of the present application, the technical solution of the present application may be subjected to various simple modifications, including combining the various technical features in any other suitable manner. These simple modifications and combinations shall also be regarded as the contents disclosed in the present application and shall fall within the scope of protection of the present application.

Claims

1. A positive electrode material, characterized in that The positive electrode material is a lithium nickel cobalt oxide composite oxide; the lithium nickel cobalt oxide composite oxide includes a core and a surface reconstruction layer located outside the core; the surface reconstruction layer is the area etched away by diluting 0.3g of the positive electrode material 100 times with 0.4ml of aqua regia and dissolving it for 30 minutes, and the ambient temperature is controlled at 300°C during dissolution. The core is the remaining area of the positive electrode material after etching away the surface reconstruction layer; the molar content of Li in the core is m1; the molar content of Li in the surface reconstruction layer is m2; wherein, 1.3≤m2 / m1≤4.

0.

2. A positive electrode material, characterized in that The positive electrode material is a lithium nickel cobalt oxide composite oxide; the lithium nickel cobalt oxide composite oxide includes a core and a surface reconstruction layer located outside the core; the surface reconstruction layer is the area etched away by diluting 0.3g of the positive electrode material with 0.4ml of aqua regia 100 times and then dissolving it for 30 minutes, and the ambient temperature is controlled at 300°C during dissolution. The core is the remaining area of the positive electrode material after etching away the surface reconstruction layer; the molar content of Li in the surface reconstruction layer is m2, and the total molar content of Li in the positive electrode material is m Li总 , 1.1≤m2 / m Li总 ≤3.

0.

3. The positive electrode material according to claim 1 or 2, characterized in that The molar content of Ni in the core is greater than the molar content of Ni in the surface reconstruction layer.

4. The positive electrode material according to claim 1 or 2, characterized in that The total molar content of all metal elements except Li and Ni in the core is less than the total molar content of all metal elements except Li and Ni in the surface reconstruction layer.

5. The positive electrode material according to claim 1 or 2, characterized in that The chemical formula of the surface reconstruction layer is Li 1+λ Ni x Co y M z M1 1-x-y-z O2, wherein 0.01≤λ≤2, 0≤x≤0.5, 0.3≤y≤1, 0≤z≤0.5, 0≤1-xyz≤0.2, M is Mn and / or Al, and M1 is a metal element.

6. The positive electrode material according to claim 5, characterized in that 0.6≤m1<1, 0.1≤λ≤2; and / or, the molar ratio of lithium element to all other metal elements in the positive electrode material is (0.95~1.05):

1.

7. The positive electrode material according to claim 5, characterized in that The metal element M1 includes at least one of Al, Ti, Y, Zr, Mg, Sr, W, Nb, Ce, La, Mo, and Dy.

8. The positive electrode material according to claim 1 or 2, characterized in that The mass percentage of the surface reconstruction layer in the positive electrode material is 5 wt % to 50 wt %.

9. The positive electrode material according to claim 1 or 2, characterized in that The mass content of free lithium in the positive electrode material is 200ppm to 1000ppm.

10. The positive electrode material according to claim 1 or 2, characterized in that The specific surface area of the positive electrode material is 0.2 m 2 / g~2.0m 2 / g; and / or, the true density of the positive electrode material is 4.6g / cm 3 ~4.8g / cm 3 .

11. A method for preparing a positive electrode material, characterized in that: The preparation method comprises: The first mixture containing the cathode material precursor and the lithium-containing compound is subjected to a sintering treatment to obtain a core matrix material, wherein the core matrix material has a chemical formula of Li 1-δ Ni a Co b M c M1 1-a-b-c O2, wherein -0.05≤δ≤0.05, 0.3≤a<1.0, 0.01≤b≤0.3, 0≤c≤0.5, 0≤1-abc≤0.05, M is Mn and / or Al, and M1 is a metal element; The second mixture comprising the surface reconstruction material and the core matrix material is subjected to a secondary sintering process to obtain a positive electrode material; wherein the surface reconstruction material comprises Ni r Co p M q M3 1-r-p-q (OH)2、Ni r Co p M q M3 1-r-p-q OOH and (Ni r Co p M q M3 1-r-p-q )O d At least one of the following, 0≤r≤0.5, 0.3≤p≤1, 0≤q≤0.5, 0≤1-rpq≤0.2, 1≤d≤2, M is Mn and / or Al, and M3 is a metal element.

12. The preparation method according to claim 11, characterized in that The surface reconstruction material satisfies at least one of the following characteristics: (1) The median particle size of the surface reconstruction material is 10 nm to 1000 nm; (2) The bulk density of the surface reconstruction material is ≤0.6 g / cm 3 ; (3) The specific surface area of the surface reconstruction material is ≥ 20m 2 / g.

13. The preparation method according to claim 11, characterized in that The metal elements M1 and M3 each independently include at least one of Al, Ti, Y, Zr, Mg, Sr, W, Nb, Ce, La, Mo and Dy.

14. The preparation method according to claim 11 or 12, characterized in that: It meets at least one of the following characteristics: (1) The molar ratio of the core matrix material to the surface reconstruction material is 1:(0.01-0.1); (2) mixing the core matrix material and the surface reconstruction material in a mixed atmosphere containing carbon dioxide to obtain the second mixture; (3) The temperature of the secondary sintering treatment is 600°C to 900°C, and the time of the secondary sintering treatment is 4h to 10h.

15. A battery, characterized in that: The battery comprises the positive electrode material according to any one of claims 1 to 10 or the positive electrode material prepared by the preparation method according to any one of claims 11 to 14.

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