Gradient doped cobalt carbonate precursor and preparation method and application thereof

Abstract

The invention provides a gradient doped cobalt carbonate precursor and a preparation method and application thereof. The preparation method comprises the following steps: firstly, respectively and simultaneously introducing a first cobalt-aluminum-magnesium mixed salt solution and a precipitant solution into a base solution, and carrying out a first coprecipitation reaction until nucleation is completed; introducing a second cobalt-aluminum-magnesium mixed salt solution into the first cobalt-aluminum-magnesium mixed salt solution to obtain a transition solution; and continuously and simultaneously introducing the transition solution and the precipitant solution into the reaction system, respectively, and carrying out secondary coprecipitation reaction until crystal growth is completed, thereby obtaining the gradient doped cobalt carbonate precursor. According to the gradient-doped cobalt carbonate precursor and the preparation method thereof, the gradient real-time change of the two doping elements in the metal salt solution is controlled, so that the prepared gradient-doped cobalt carbonate precursor can simultaneously improve the conductivity and the structural stability of a lithium cobalt oxide positive electrode material, and the capacity, the cycle life and the rate capability of the lithium cobalt oxide positive electrode material under high voltage are remarkably improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of positive electrode precursors, and in particular relates to a gradient-doped cobalt carbonate precursor and a preparation method and application thereof. Background Art

[0002] Lithium cobalt oxide (LiCoO2) cathode material has advantages such as high volumetric energy density and mature process system. These characteristics make it dominate the consumer electronics field such as smartphones, laptops, tablets and smart wearable devices.

[0003] However, under high voltage conditions (>4.45V), on the one hand, the layered structure of lithium cobalt oxide materials is prone to irreversible phase transitions, resulting in lattice distortion and mechanical stress accumulation, which in turn causes capacity decay and a decrease in cycle life. On the other hand, the lattice oxygen of lithium cobalt oxide participates in the charge transfer process, resulting in a significant increase in oxygen activity. This high activity of oxygen not only accelerates the structural degradation of the material, but also undergoes oxidation reactions with the electrolyte, producing a large amount of by-products, such as gases and metal ions, which in turn cause the decomposition of the electrolyte and a decrease in battery safety.

[0004] In order to further obtain high-performance positive electrode materials, researchers mostly use element doping or surface coating to modify the lithium cobalt oxide material precursor. Among them, element doping is conducive to stabilizing the layered structure of lithium cobalt oxide, inhibiting phase change and volume expansion during charging and discharging, thereby improving the cycle life and safety of the battery. For example, the prior art discloses doping Al element in the preparation process of cobalt carbonate, and then sintering to obtain aluminum-doped cobalt tetroxide as a lithium cobalt oxide precursor, so that Al 3+ Partial replacement of Co 3+ Entering the crystal lattice, it enhances the stability of the Co-O bond, inhibits structural phase transitions, and thus broadens the battery's charge and discharge window.

[0005] In fact, the high Al doping content in the cobalt carbonate precursor can easily cause Al segregation, and the replacement of Co by Al in the crystal lattice can also affect the overall capacity of the cathode material. Therefore, there is an urgent need to develop a precursor material and its preparation method to improve the overall performance of lithium cobalt oxide cathode materials at high voltages. Summary of the Invention

[0006] In view of the shortcomings of the existing technology, the purpose of the present invention is to provide a gradient-doped cobalt carbonate precursor and its preparation method and application, which can simultaneously improve the conductivity and structural stability of lithium cobalt oxide positive electrode materials, and significantly improve their capacity, cycle life and rate performance at high voltage.

[0007] In order to achieve the purpose of the invention, the present invention adopts the following technical solutions:

[0008] In a first aspect, the present invention provides a method for preparing a gradient-doped cobalt carbonate precursor, the preparation method comprising the following steps:

[0009] S1. The first cobalt-aluminum-magnesium mixed salt solution and the precipitant solution are simultaneously introduced into the bottom liquid for the first coprecipitation reaction until nucleation is completed;

[0010] S2. introducing a second cobalt, aluminum, and magnesium mixed salt solution into the first cobalt, aluminum, and magnesium mixed salt solution to obtain a transition solution; and continuing to simultaneously introduce the transition solution and the precipitant solution into the reaction system in step S1 in which nucleation is completed, performing a second coprecipitation reaction until crystal growth is completed, thereby obtaining the gradient-doped cobalt carbonate precursor;

[0011] Among them, the concentration of magnesium ions in the first cobalt-aluminum-magnesium mixed salt solution is higher than the concentration of magnesium ions in the second cobalt-aluminum-magnesium mixed salt solution, and the concentration of aluminum ions in the first cobalt-aluminum-magnesium mixed salt solution is lower than the concentration of aluminum ions in the second cobalt-aluminum-magnesium mixed salt solution.

[0012] The present invention provides a method for preparing a cobalt carbonate precursor with a magnesium-aluminum concentration gradient. By controlling the real-time change of the gradient of two doping elements in a metal salt solution, a dual-element gradient-doped cobalt carbonate with high internal Mg and high external Al is synthesized. The synergistic effect of the two can better regulate the comprehensive performance of the positive electrode material, not only enhancing the stability of the lithium cobalt oxide positive electrode material under high voltage, but also significantly improving its cycle performance and rate performance.

[0013] Specifically, the in-situ co-doping method of the present invention can fully combine the characteristics of Mg / Al incorporation at different sites to stabilize the material's lattice structure, reduce internal stress, and delay the formation of microcracks. As the particle size continues to increase, the increase in surface Al content can enhance the structural stability of the lithium cobalt oxide cathode material, while the moderate increase in internal Mg content can optimize ion diffusion performance, thereby improving the cycle life and rate performance of the lithium cobalt oxide cathode material in high-voltage systems.

[0014] Preferably, the concentration of magnesium ions in the first cobalt-aluminum-magnesium mixed salt solution is 0.3 g / L-0.6 g / L, for example, it can be 0.3 g / L, 0.32 g / L, 0.35 g / L, 0.38 g / L, 0.4 g / L, 0.42 g / L, 0.45 g / L, 0.48 g / L, 0.5 g / L, 0.52 g / L, 0.55 g / L, 0.58 g / L or 0.6 g / L, etc., but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0015] Preferably, the concentration of magnesium ions in the second cobalt-aluminum-magnesium mixed salt solution is 0.2 g / L-0.3 g / L, for example, it can be 0.2 g / L, 0.21 g / L, 0.22 g / L, 0.23 g / L, 0.24 g / L, 0.25 g / L, 0.26 g / L, 0.27 g / L, 0.28 g / L, 0.29 g / L or 0.3 g / L, etc., not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0016] Preferably, the concentration of aluminum ions in the first cobalt-aluminum-magnesium mixed salt solution is 0.2 g / L-0.4 g / L, for example, it can be 0.2 g / L, 0.22 g / L, 0.25 g / L, 0.28 g / L, 0.3 g / L, 0.32 g / L, 0.35 g / L, 0.38 g / L or 0.4 g / L, etc., and is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0017] Preferably, the concentration of aluminum ions in the second cobalt-aluminum-magnesium mixed salt solution is 0.4 g / L-0.6 g / L, for example, it can be 0.4 g / L, 0.42 g / L, 0.45 g / L, 0.48 g / L, 0.5 g / L, 0.52 g / L, 0.55 g / L, 0.58 g / L or 0.6 g / L, etc., and is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0018] Preferably, the concentration of cobalt ions in the first cobalt-aluminum-magnesium mixed salt solution and the second cobalt-aluminum-magnesium mixed salt solution is independently 95 g / L-105 g / L, for example, it can be 95 g / L, 98 g / L, 100 g / L, 102 g / L or 105 g / L, etc., and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0019] In the present invention, a Mg / Al in-situ double-gradient co-doped cobalt carbonate precursor material is prepared by dissolving a magnesium source and an aluminum source in two mixed salt solutions and further regulating the concentrations of the two doping elements in different mixed salt solutions.

[0020] Preferably, the first cobalt-aluminum-magnesium mixed salt solution and the second cobalt-aluminum-magnesium mixed salt solution also include a surfactant.

[0021] Preferably, the surfactant comprises polyvinyl pyrrolidone. The addition of the surfactant to the dual-doped element mixed salt solution not only promotes uniform dispersion of metal ions and reduces enrichment, but also reduces the agglomeration rate of small particles, thereby obtaining a small-particle cobalt carbonate precursor with a more uniform particle size.

[0022] Preferably, the concentration of the surfactant in the first cobalt-aluminum-magnesium mixed salt solution and the second cobalt-aluminum-magnesium mixed salt solution is independently 1 g / L-2 g / L, for example, it can be 1 g / L, 1.2 g / L, 1.5 g / L, 1.8 g / L or 2 g / L, etc., not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0023] Preferably, in step S1, the feed rate of the first cobalt-aluminum-magnesium mixed salt solution is 10 L / H-20 L / H, for example, it can be 10 L / H, 12 L / H, 15 L / H, 18 L / H or 20 L / H, etc., not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0024] Preferably, in step S1, the concentration of the precipitant solution is 150 g / L-250 g / L, for example, it can be 150 g / L, 180 g / L, 200 g / L, 220 g / L or 250 g / L, etc., and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0025] Preferably, in step S1, the feed rate of the precipitant solution is 30 L / H-40 L / H, for example, it can be 30 L / H, 32 L / H, 35 L / H, 38 L / H or 40 L / H, etc., and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0026] Preferably, in step S1, the base liquid includes ammonium bicarbonate solution.

[0027] Preferably, in step S1, the concentration of the ammonium bicarbonate solution in the base liquid is 40 g / L-90 g / L, for example, 40 g / L, 50 g / L, 60 g / L, 70 g / L, 80 g / L or 90 g / L, etc., and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0028] Preferably, in step S1, the pH value of the first coprecipitation reaction is 7.1-7.6, for example, it can be 7.1, 7.2, 7.3, 7.4, 7.5 or 7.6, etc., and is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0029] Preferably, in step S1, the temperature of the first coprecipitation reaction is 40°C-50°C, for example, 40°C, 42°C, 45°C, 48°C or 50°C, etc., and is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0030] Preferably, in step S2, the feed rate of the second cobalt-aluminum-magnesium mixed salt solution is 8 L / H-12 L / H, for example, it can be 8 L / H, 9 L / H, 10 L / H, 11 L / H or 12 L / H, etc., and is not limited to the listed values. Other unlisted values ​​within this numerical range are also applicable.

[0031] Preferably, in step S2, the feed rate of the transition solution is 10 L / H-20 L / H, for example, it can be 10 L / H, 12 L / H, 15 L / H, 18 L / H or 20 L / H, etc., and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0032] In the present invention, by introducing the second cobalt-aluminum-magnesium mixed salt solution into the first cobalt-aluminum-magnesium mixed salt solution, and then introducing the formed transition solution into the reaction system, and further regulating the feed rates of the second cobalt-aluminum-magnesium mixed salt solution and the transition solution, the concentration of elemental aluminum gradually increases and the concentration of elemental magnesium gradually decreases from the center to the surface of the prepared cobalt carbonate precursor particles, thereby constructing an overall stable material structure inside and outside, and ensuring that the lithium cobalt oxide positive electrode material has excellent electrochemical properties.

[0033] Preferably, in step S2, the concentration of the precipitant solution is 150 g / L-250 g / L, for example, it can be 150 g / L, 180 g / L, 200 g / L, 220 g / L or 250 g / L, etc., and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0034] Preferably, in step S2, the feed rate of the precipitant solution is 30 L / H-40 L / H, for example, it can be 30 L / H, 32 L / H, 35 L / H, 38 L / H or 40 L / H, etc., and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0035] Preferably, in step S2, the pH value of the second coprecipitation reaction is 7.1-7.6, for example, it can be 7.1, 7.2, 7.3, 7.4, 7.5 or 7.6, etc., and is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0036] Preferably, in step S2, the temperature of the second coprecipitation reaction is 40°C-50°C, for example, 40°C, 42°C, 45°C, 48°C or 50°C, etc., and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0037] Preferably, the average particle size of the gradient-doped cobalt carbonate precursor is 4 μm-6 μm, for example, it can be 4 μm, 4.2 μm, 4.5 μm, 4.8 μm, 5 μm, 5.2 μm, 5.5 μm, 5.8 μm or 6 μm, etc., and is not limited to the listed values. Other unlisted values ​​within this numerical range are also applicable.

[0038] In a second aspect, the present invention provides a gradient-doped cobalt carbonate precursor, which is prepared by the preparation method of the gradient-doped cobalt carbonate precursor as described in the first aspect.

[0039] In a third aspect, the present invention provides a gradient-doped cobalt tetroxide precursor, which is obtained by calcining the gradient-doped cobalt carbonate precursor as described in the second aspect.

[0040] In the present invention, the calcination process comprises: heating to 750° C. at a rate of 5° C. / min in an air atmosphere, and calcining the gradient-doped cobalt carbonate precursor as described in the second aspect for 2 h to obtain a gradient-doped cobalt trioxide precursor.

[0041] In a fourth aspect, the present invention provides a lithium cobalt oxide positive electrode material, wherein the lithium cobalt oxide positive electrode material is prepared from the gradient-doped cobalt trioxide precursor as described in the third aspect.

[0042] In the present invention, the preparation method of the lithium cobalt oxide positive electrode material comprises the following steps: sintering the gradient-doped cobalt trioxide precursor and a lithium source as described in the third aspect to obtain the lithium cobalt oxide positive electrode material.

[0043] In the present invention, the lithium source includes lithium hydroxide or lithium carbonate.

[0044] In the present invention, the sintering temperature is 900°C-1100°C, for example, it can be 900°C, 1000°C or 1100°C; the sintering time is 10h-12h, for example, it can be 10h, 11h or 12h, etc., and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0045] In a fifth aspect, the present invention provides a secondary battery comprising a positive electrode, a negative electrode and an electrolyte, wherein the active material of the positive electrode comprises the lithium cobalt oxide positive electrode material as described in the fourth aspect.

[0046] In the present invention, the electrolyte includes a liquid electrolyte, a gel electrolyte or a solid electrolyte.

[0047] The numerical range described in the present invention includes not only the point values ​​listed above, but also any point values ​​between the above numerical ranges that are not listed. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0048] Compared with the prior art, the present invention has the following beneficial effects:

[0049] The present invention provides a method for preparing a gradient-doped cobalt carbonate precursor. By controlling the real-time change of the gradient of two doping elements in a metal salt solution, a dual-element gradient-doped cobalt carbonate with high internal Mg and high external Al is synthesized. The synergistic effect of the two can better regulate the comprehensive performance of the positive electrode material, not only enhancing the stability of the lithium cobalt oxide positive electrode material under high voltage, but also significantly improving its cycle performance and rate performance.

[0050] Specifically, the in-situ co-doping method of the present invention can fully combine the characteristics of Mg / Al incorporation at different sites to stabilize the material's lattice structure, reduce internal stress, and delay the formation of microcracks. As the particle size continues to increase, the increase in surface Al content can enhance the structural stability of the lithium cobalt oxide cathode material, while the moderate increase in internal Mg content can optimize ion diffusion performance, thereby improving the cycle life and rate performance of the lithium cobalt oxide cathode material in high-voltage systems. DETAILED DESCRIPTION

[0051] The technical solution of the present invention is further described below by way of specific embodiments. It should be understood by those skilled in the art that the embodiments are merely to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0052] Example 1

[0053] This embodiment provides a gradient-doped cobalt carbonate precursor and a preparation method thereof, the preparation method comprising the following steps:

[0054] S1. Prepare a first cobalt-aluminum-magnesium mixed salt solution, a second cobalt-aluminum-magnesium mixed salt solution, and a base solution. The first cobalt-aluminum-magnesium mixed salt solution comprises: a cobalt ion concentration of 100 g / L, an aluminum ion concentration of 0.3 g / L, a magnesium ion concentration of 0.5 g / L, and a polyvinyl pyrrolidone surfactant concentration of 1.5 g / L. The second cobalt-aluminum-magnesium mixed salt solution comprises: a cobalt ion concentration of 100 g / L, an aluminum ion concentration of 0.5 g / L, a magnesium ion concentration of 0.2 g / L, and a polyvinyl pyrrolidone surfactant concentration of 1.5 g / L. The base solution is a 45 g / L ammonium bicarbonate solution.

[0055] The first cobalt-aluminum-magnesium mixed salt solution and a 200 g / L ammonium bicarbonate solution were simultaneously introduced into a reactor containing a bottom solution to carry out a first coprecipitation reaction. The feed rate of the first cobalt-aluminum-magnesium mixed salt solution was controlled to be 15 L / H, the feed rate of the ammonium bicarbonate solution was controlled to be 35 L / H, the stirring rate was controlled to be 400 rpm, the reaction temperature of the reactor was set to be 45° C., and the pH value was set to be 7.2-7.5, until nucleation was completed;

[0056] S2. The second cobalt-aluminum-magnesium mixed salt solution is passed into the first cobalt-aluminum-magnesium mixed salt solution at a feed rate of 10 L / H to obtain a transition solution; the transition solution and the ammonium bicarbonate solution are continued to be passed into the above-mentioned reaction system in parallel to carry out a second coprecipitation reaction, and the feed rate of the transition solution is controlled to be 15 L / H, the feed rate of the ammonium bicarbonate solution is 35 L / H, the stirring rate is 400 rpm, the reaction temperature of the reactor is set to 45°C, and the pH value is 7.2-7.5, until crystal growth is completed to obtain the gradient-doped cobalt carbonate precursor with an average particle size of 4 μm.

[0057] Example 2

[0058] This embodiment provides a gradient-doped cobalt carbonate precursor and a preparation method thereof, the preparation method comprising the following steps:

[0059] S1. Prepare a first cobalt-aluminum-magnesium mixed salt solution, a second cobalt-aluminum-magnesium mixed salt solution, and a base solution. The first cobalt-aluminum-magnesium mixed salt solution comprises: a cobalt ion concentration of 95 g / L, an aluminum ion concentration of 0.2 g / L, a magnesium ion concentration of 0.3 g / L, and a polyvinyl pyrrolidone surfactant concentration of 1 g / L. The second cobalt-aluminum-magnesium mixed salt solution comprises: a cobalt ion concentration of 95 g / L, an aluminum ion concentration of 0.4 g / L, a magnesium ion concentration of 0.2 g / L, and a polyvinyl pyrrolidone surfactant concentration of 1 g / L. The base solution is a 40 g / L ammonium bicarbonate solution.

[0060] The first cobalt-aluminum-magnesium mixed salt solution and a 200 g / L ammonium bicarbonate solution were simultaneously introduced into a reactor containing a bottom solution to carry out a first coprecipitation reaction. The feed rate of the first cobalt-aluminum-magnesium mixed salt solution was controlled to be 10 L / H, the feed rate of the ammonium bicarbonate solution was controlled to be 30 L / H, the stirring rate was controlled to be 400 rpm, the reaction temperature of the reactor was set to be 40° C., and the pH value was set to be 7.2-7.5 until nucleation was completed;

[0061] S2. The second cobalt-aluminum-magnesium mixed salt solution is introduced into the first cobalt-aluminum-magnesium mixed salt solution at a feed rate of 8 L / H to obtain a transition solution; the transition solution and the ammonium bicarbonate solution are continued to be introduced into the above-mentioned reaction system in parallel to carry out a second coprecipitation reaction, and the feed rate of the transition solution is controlled to be 10 L / H, the feed rate of the ammonium bicarbonate solution is 30 L / H, the stirring rate is 400 rpm, the reaction temperature of the reactor is set to 40°C, and the pH value is 7.2-7.5, until crystal growth is completed to obtain the gradient-doped cobalt carbonate precursor with an average particle size of 5 μm.

[0062] Example 3

[0063] This embodiment provides a gradient-doped cobalt carbonate precursor and a preparation method thereof, the preparation method comprising the following steps:

[0064] S1. Prepare a first cobalt-aluminum-magnesium mixed salt solution, a second cobalt-aluminum-magnesium mixed salt solution, and a base solution. The first cobalt-aluminum-magnesium mixed salt solution comprises: a cobalt ion concentration of 105 g / L, an aluminum ion concentration of 0.4 g / L, a magnesium ion concentration of 0.6 g / L, and a polyvinyl pyrrolidone surfactant concentration of 2 g / L. The second cobalt-aluminum-magnesium mixed salt solution comprises: a cobalt ion concentration of 105 g / L, an aluminum ion concentration of 0.6 g / L, a magnesium ion concentration of 0.3 g / L, and a polyvinyl pyrrolidone surfactant concentration of 2 g / L. The base solution is a 90 g / L ammonium bicarbonate solution.

[0065] The first cobalt-aluminum-magnesium mixed salt solution and a 200 g / L ammonium bicarbonate solution were simultaneously introduced into a reactor containing a bottom solution to carry out a first coprecipitation reaction. The feed rate of the first cobalt-aluminum-magnesium mixed salt solution was controlled to be 20 L / H, the feed rate of the ammonium bicarbonate solution was controlled to be 40 L / H, the stirring rate was controlled to be 400 rpm, the reaction temperature of the reactor was set to be 50° C., and the pH value was set to be 7.2-7.5, until nucleation was completed;

[0066] S2. The second cobalt-aluminum-magnesium mixed salt solution is introduced into the first cobalt-aluminum-magnesium mixed salt solution at a feed rate of 12 L / H to obtain a transition solution; the transition solution and the ammonium bicarbonate solution are continued to be introduced into the above-mentioned reaction system in parallel to carry out a second coprecipitation reaction, and the feed rate of the transition solution is controlled to be 20 L / H, the feed rate of the ammonium bicarbonate solution is 40 L / H, the stirring rate is 400 rpm, the reaction temperature of the reactor is set to 50°C, and the pH value is 7.2-7.5, until crystal growth is completed to obtain the gradient-doped cobalt carbonate precursor with an average particle size of 6 μm.

[0067] Example 4

[0068] The difference between this embodiment and embodiment 1 is that in step S1, the concentration of magnesium ions in the first cobalt-aluminum-magnesium mixed salt solution is adjusted to 0.9 g / L, and the concentration of magnesium ions in the second cobalt-aluminum-magnesium mixed salt solution is adjusted to 0.1 g / L. The rest is the same as embodiment 1.

[0069] Example 5

[0070] The difference between this embodiment and embodiment 1 is that in step S1, the concentration of aluminum ions in the first cobalt-aluminum-magnesium mixed salt solution is adjusted to 0.1 g / L, and the concentration of aluminum ions in the second cobalt-aluminum-magnesium mixed salt solution is adjusted to 0.9 g / L. The rest is the same as embodiment 1.

[0071] Example 6

[0072] The difference between this embodiment and embodiment 1 is that in step S1, the feed rate of the first cobalt-aluminum-magnesium mixed salt solution is 5 L / H, and the rest is the same as embodiment 1.

[0073] Example 7

[0074] The difference between this embodiment and embodiment 1 is that in step S1, the feed rate of the first cobalt-aluminum-magnesium mixed salt solution is 25 L / H, and the rest is the same as embodiment 1.

[0075] Example 8

[0076] The difference between this embodiment and embodiment 1 is that in step S2, the feed rate of the second cobalt-aluminum-magnesium mixed salt solution is 5 L / H, and the rest is the same as embodiment 1.

[0077] Example 9

[0078] The difference between this embodiment and embodiment 1 is that in step S2, the feed rate of the second cobalt-aluminum-magnesium mixed salt solution is 18 L / H, and the rest is the same as embodiment 1.

[0079] Comparative Example 1

[0080] The difference between this comparative example and Example 1 is that in step S1, the concentration of magnesium ions in the first cobalt-aluminum-magnesium mixed salt solution is adjusted to 0.2 g / L, and the concentration of magnesium ions in the second cobalt-aluminum-magnesium mixed salt solution is adjusted to 0.5 g / L, and the rest are the same as Example 1.

[0081] Comparative Example 2

[0082] The difference between this comparative example and Example 1 is that in step S1, the concentration of aluminum ions in the first cobalt-aluminum-magnesium mixed salt solution is adjusted to 0.5 g / L, and the concentration of aluminum ions in the second cobalt-aluminum-magnesium mixed salt solution is adjusted to 0.3 g / L, and the rest are the same as Example 1.

[0083] The doped cobalt carbonate precursors provided in Examples 1 to 9 and Comparative Examples 1 to 2 were used to prepare lithium cobalt oxide positive electrode materials, which were then assembled to obtain lithium ion batteries. The preparation method is as follows:

[0084] The gradient-doped cobalt carbonate precursors provided in the above examples and comparative examples were calcined in an air atmosphere at a rate of 5°C / min to 750°C for 2 hours to obtain a gradient-doped cobalt tetroxide precursor. The gradient-doped cobalt tetroxide precursor and lithium hydroxide were sintered at 1000°C for 11 hours to obtain a lithium cobalt oxide positive electrode material.

[0085] The above-mentioned lithium cobalt oxide positive electrode material is mixed with conductive carbon black and polyvinylidene fluoride binder in a mass ratio of 8:1:1, and then N-methylpyrrolidone solvent is added to prepare a positive electrode slurry. The positive electrode slurry is scraped onto aluminum foil, and then cut into discs with a diameter of 12 mm as the positive electrode sheet after drying and roller pressing. The metal lithium sheet is used as the negative electrode sheet, and a polypropylene separator is used to separate the positive and negative electrode sheets. The solute of the electrolyte is lithium hexafluorophosphate with a concentration of 1 mol / L, and the solvent is a mixture of ethylene carbonate, dimethyl carbonate and diethyl carbonate with a volume ratio of 1:1:1. The negative electrode shell, negative electrode sheet, electrolyte, separator, electrolyte, positive electrode sheet, current collector and positive electrode shell are stacked and pressed in this order to assemble into a button lithium-ion battery.

[0086] The prepared button-type lithium-ion battery was subjected to electrochemical performance tests at a test voltage of 3.0V-4.5V to test its discharge specific capacity at room temperature and 0.2C, as well as the capacity retention rate after 200 charge and discharge cycles at a rate of 1C.

[0087] The test results are shown in Table 1:

[0088] Table 1

[0089]

[0090]

[0091] As can be seen from Table 1, compared with Comparative Examples 1-2, Examples 1-3 provided by the present invention synthesize dual-element gradient-doped cobalt carbonate with high internal Mg and high external Al by controlling the real-time change of the gradient of the two doping elements in the metal salt solution. The synergistic effect of the two can better regulate the comprehensive performance of the positive electrode material, which not only enhances the stability of the lithium cobalt oxide positive electrode material under high voltage, but also significantly improves its cycle performance.

[0092] Comparing Example 1 and Example 4 to Example 9, it can be seen that, in general, lithium cobalt oxide positive electrode materials with high internal Mg and high external Al have higher capacity and better capacity retention. Furthermore, the present invention preferably has a concentration range of doped metal elements within a specific range, and the feed rate of different mixed salt solutions can ensure a slow change in the concentration of aluminum and magnesium in the transition solution, which is more conducive to the above advantages. In contrast, lithium cobalt oxide positive electrode materials made with high internal Al and high external Mg have lower capacity and worse capacity retention, which is not conducive to the comprehensive improvement of battery performance.

[0093] The applicant declares that the above is only a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention fall within the scope of protection and disclosure of the present invention.

Claims

1. A method for preparing a gradient-doped cobalt carbonate precursor, characterized in that: The preparation method comprises the following steps: S1. The first cobalt-aluminum-magnesium mixed salt solution and the precipitant solution are simultaneously introduced into the bottom liquid for the first coprecipitation reaction until nucleation is completed; S2. introducing a second cobalt, aluminum, and magnesium mixed salt solution into the first cobalt, aluminum, and magnesium mixed salt solution to obtain a transition solution; and continuing to simultaneously introduce the transition solution and the precipitant solution into the reaction system in step S1 in which nucleation is completed, performing a second coprecipitation reaction until crystal growth is completed, thereby obtaining the gradient-doped cobalt carbonate precursor; Among them, the concentration of magnesium ions in the first cobalt-aluminum-magnesium mixed salt solution is higher than the concentration of magnesium ions in the second cobalt-aluminum-magnesium mixed salt solution, and the concentration of aluminum ions in the first cobalt-aluminum-magnesium mixed salt solution is lower than the concentration of aluminum ions in the second cobalt-aluminum-magnesium mixed salt solution.

2. The preparation method according to claim 1, characterized in that The concentration of magnesium ions in the first cobalt-aluminum-magnesium mixed salt solution is 0.3 g / L-0.6 g / L; Preferably, the concentration of magnesium ions in the second cobalt-aluminum-magnesium mixed salt solution is 0.2 g / L-0.3 g / L; Preferably, the concentration of aluminum ions in the first cobalt-aluminum-magnesium mixed salt solution is 0.2 g / L-0.4 g / L; Preferably, the concentration of aluminum ions in the second cobalt-aluminum-magnesium mixed salt solution is 0.4 g / L-0.6 g / L; Preferably, the concentration of cobalt ions in the first cobalt-aluminum-magnesium mixed salt solution and the second cobalt-aluminum-magnesium mixed salt solution is independently 95 g / L-105 g / L.

3. The preparation method according to claim 1 or 2, characterized in that The first cobalt-aluminum-magnesium mixed salt solution and the second cobalt-aluminum-magnesium mixed salt solution both further include a surfactant; Preferably, the surfactant comprises polyvinylpyrrolidone; Preferably, the concentration of the surfactant in the first cobalt-aluminum-magnesium mixed salt solution and the second cobalt-aluminum-magnesium mixed salt solution is independently 1 g / L-2 g / L.

4. The preparation method according to any one of claims 1 to 3, characterized in that In step S1, the feeding rate of the first cobalt-aluminum-magnesium mixed salt solution is 10 L / H-20 L / H; Preferably, in step S1, the concentration of the precipitant solution is 150 g / L-250 g / L; Preferably, in step S1, the feed rate of the precipitant solution is 30 L / H-40 L / H; Preferably, in step S1, the base liquid comprises an ammonium bicarbonate solution; Preferably, in step S1, the concentration of the ammonium bicarbonate solution in the base liquid is 40 g / L-90 g / L; Preferably, in step S1, the pH value of the first coprecipitation reaction is 7.1-7.6; Preferably, in step S1, the temperature of the first coprecipitation reaction is 40°C-50°C.

5. The preparation method according to any one of claims 1 to 4, characterized in that In step S2, the feeding rate of the second cobalt-aluminum-magnesium mixed salt solution is 8 L / H-12 L / H; In step S2, the feeding rate of the transition solution is 10 L / H-20 L / H.

6. The preparation method according to any one of claims 1 to 5, characterized in that In step S2, the concentration of the precipitant solution is 150 g / L-250 g / L; Preferably, in step S2, the feed rate of the precipitant solution is 30 L / H-40 L / H; Preferably, in step S2, the pH value of the second coprecipitation reaction is 7.1-7.6; Preferably, in step S2, the temperature of the second coprecipitation reaction is 40°C-50°C; Preferably, the average particle size of the gradient-doped cobalt carbonate precursor is 4 μm-6 μm.

7. A gradient-doped cobalt carbonate precursor, characterized in that: The gradient-doped cobalt carbonate precursor is prepared by the preparation method of the gradient-doped cobalt carbonate precursor according to any one of claims 1 to 6.

8. A gradient-doped cobalt trioxide precursor, characterized in that: The gradient-doped cobalt trioxide precursor is obtained by calcining the gradient-doped cobalt carbonate precursor as claimed in claim 7.

9. A lithium cobalt oxide positive electrode material, characterized in that: The lithium cobalt oxide positive electrode material is prepared from the gradient-doped cobalt trioxide precursor as claimed in claim 8.

10. A secondary battery, characterized in that: The secondary battery includes a positive electrode, a negative electrode, and an electrolyte, and the active material of the positive electrode includes the lithium cobalt oxide positive electrode material according to claim 9.

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