Positive electrode composite material and preparation method thereof, positive electrode plate and battery

Abstract

The invention discloses a positive electrode composite material and a preparation method thereof, a positive electrode plate and a battery, and belongs to the technical field of batteries. The positive electrode composite material provided by the embodiment of the invention comprises an inner core, a buffer layer and a shell which are sequentially coated from inside to outside, the inner core comprises lithium manganese iron phosphate, the shell comprises lithium nickel cobalt manganese oxide, the Fermi level of the material of the buffer layer is between the Fermi level of the lithium manganese iron phosphate and the Fermi level of the lithium nickel cobalt manganese oxide, and the molar ratio of the Mn element to the Fe element in the inner core conforms to the decreasing trend in the direction from inside to outside. The material can reduce the interface impedance and the interface side reaction degree, thereby improving the rate capability and cycle life of the battery.

Description

Technical Field

[0001] This application relates to the technical field of batteries, and particularly to a cathode composite material, a preparation method thereof, a cathode electrode sheet, and a battery. Background Art

[0002] The cathode composite material composed of LMFP (lithium iron manganese phosphate) and NCM (lithium nickel cobalt manganese oxide) combines the advantages of the two materials and has the advantages of high energy density and high safety performance.

[0003] In the related art, LMFP and NCM are mechanically mixed to prepare the cathode composite material. However, the interface impedance of the cathode composite material prepared in this way is relatively high, and the interfacial side reactions are relatively serious, which is not conducive to the rate performance and cycle life of the battery. Summary of the Invention

[0004] Embodiments of this application provide a cathode composite material, a preparation method thereof, a cathode electrode sheet, and a battery, which can reduce the interface impedance and the degree of interfacial side reactions, and improve the rate performance and cycle life of the battery. The technical solutions are as follows:

[0005] On the one hand, a cathode composite material is provided. The cathode composite material includes: a core, a buffer layer, and a shell coated in sequence from inside to outside;

[0006] The core includes lithium iron manganese phosphate, the shell includes lithium nickel cobalt manganese oxide, the Fermi level of the buffer layer material is between the Fermi levels of the lithium iron manganese phosphate and the lithium nickel cobalt manganese oxide, and along the direction from inside to outside, the molar ratio of Mn element to Fe element in the core conforms to a decreasing trend.

[0007] In a possible implementation, along the direction from inside to outside, the molar ratio of Mn element to Fe element in the core gradually decreases.

[0008] In another possible implementation, the core includes a core region and a surface region. The molar ratio of Mn element to Fe element in the core region is 9:1 to 7:3, and the molar ratio of Mn element to Fe element in the surface region is 6:4 to 5:5.

[0009] In another possible implementation, the buffer layer material includes lithium titanium aluminum phosphate.

[0010] In another possible implementation, the porosity of the shell is greater than the porosity of the buffer layer, and the porosity of the buffer layer is greater than the porosity of the core.

[0011] In another possible implementation, the porosity of the outer shell is 15-20%, the porosity of the inner core is less than 5%, and the porosity of the buffer layer is 5-15%.

[0012] In another possible implementation, the particle size of the inner core and the thickness of the outer shell are both greater than the thickness of the buffer layer.

[0013] In another possible implementation, the median particle size of the inner core is 1.5-2.5 μm;

[0014] The thickness of the buffer layer is 18-52 nm;

[0015] The thickness of the outer shell is 100-300 nm.

[0016] In another possible implementation, the outer shell further includes carbon nanotubes, and the carbon nanotubes are bonded to the surface of the lithium nickel cobalt manganese oxide.

[0017] On the other hand, a method for preparing a positive electrode composite material is provided. The positive electrode composite material is as described in any one of the above, and the preparation method includes:

[0018] Preparing the inner core;

[0019] Depositing the buffer layer material on the surface of the inner core to form the buffer layer on the surface of the inner core, obtaining a first precursor;

[0020] Depositing lithium nickel cobalt manganese oxide on the buffer layer surface of the first precursor to obtain the positive electrode composite material.

[0021] In a possible implementation, the preparing the inner core includes:

[0022] Mixing MnSO 4 and FeSO 4 in water according to a molar ratio of 9:1-7:3 to obtain a first mixed solution, and dropping the first mixed solution into the LiH 2 PO 4 solution;

[0023] Gradually adjusting the Fe 2+ concentration. During the process of adjusting the Fe 2+ concentration, continue to drop the adjusted first mixed solution into the LiH 2 PO 4 solution until the molar ratio of the Mn element to the Fe element is 6:4-5:5, obtaining a second mixed solution;

[0024] Performing spray drying on the second mixed solution to obtain a second precursor;

[0025] Sinter the second precursor to obtain the core.

[0026] In another possible implementation, depositing the buffer layer material on the surface of the core to form the buffer layer on the surface of the core to obtain the first precursor includes:

[0027] Deposit trimethylaluminum, lithium isopropoxide, titanium tetrachloride, and water on the surface of the core in sequence through atomic layer deposition process, and cycle multiple times to form a deposit;

[0028] Calcine the deposit to form the buffer layer on the surface of the core to obtain the first precursor.

[0029] In another possible implementation, depositing lithium nickel cobalt manganese oxide on the buffer layer surface of the first precursor to obtain the positive electrode composite material includes:

[0030] Mix a raw material solution containing a nickel source, a cobalt source, and a manganese source, a complexing agent, and the first precursor evenly to obtain a third precursor;

[0031] Calcine the mixture of the third precursor and lithium carbonate to obtain the positive electrode composite material.

[0032] On the other hand, a positive electrode sheet is provided, and the positive electrode sheet includes the positive electrode composite material described in any one of the above.

[0033] On the other hand, a battery is provided, and the battery includes: a housing, an electrolyte accommodated inside the housing, a negative electrode sheet, a positive electrode sheet, and a separator. Wherein, the positive electrode sheet is as described above, and the negative electrode sheet and the positive electrode sheet are separated by the separator.

[0034] The embodiment of the present application provides a positive electrode composite material. In this material, the buffer layer is located between the core and the outer shell, and the Fermi level of the buffer layer material is between the Fermi level of lithium iron manganese phosphate and the Fermi level of lithium nickel cobalt manganese oxide. Therefore, from the inside to the outside, an energy band gradient can be formed between the buffer layer material, lithium iron manganese phosphate, and lithium nickel cobalt manganese oxide to reduce the Fermi level difference between lithium iron manganese phosphate and lithium nickel cobalt manganese oxide, eliminate the interfacial potential barrier, thereby reducing the interfacial impedance and the degree of interfacial side reactions. And the molar ratio of Mn element to Fe element in the core conforms to a decreasing trend, that is, the proportion of Fe element conforms to an increasing trend, and the increase in the proportion of Fe element is beneficial to the outward diffusion of lithium ions, reduces the lithium ion diffusion potential barrier, thereby reducing the interfacial impedance and the degree of interfacial side reactions, and further improving the battery rate performance and cycle life. Description of the Drawings

[0035] Figure 1 is a flowchart of a preparation method of a positive electrode composite material provided by an embodiment of the present application;

[0036] Figure 2 It is a flowchart of another method for preparing a cathode composite material provided by an embodiment of the present application. Specific embodiments

[0037] To make the technical solutions and advantages of the present application clearer, the following further describes the embodiments of the present application in detail.

[0038] An embodiment of the present application provides a cathode composite material, which includes: a core, a buffer layer, and a shell sequentially coated from the inside to the outside.

[0039] The core includes lithium manganese iron phosphate (LMFP), the shell includes lithium nickel cobalt manganese oxide (NCM), the Fermi level of the buffer layer material is between the Fermi levels of lithium manganese iron phosphate and lithium nickel cobalt manganese oxide, and along the direction from the inside to the outside, the molar ratio of Mn element to Fe element in the core conforms to a decreasing trend.

[0040] Among them, the chemical general formula of lithium manganese iron phosphate can be expressed as: LiMn x Fe 1-x PO 4 , lithium manganese iron phosphate is obtained by partially substituting the position of Fe element with Mn element, x represents the molar fraction of Mn element, 0 < x < 1. Lithium manganese iron phosphate represents a class of substances, not a single substance, and the ratio of manganese to iron can be changed by adjusting x. For example, when x is 0.1, LiMn x Fe 1-x PO 4 can be expressed as LiMn 0.1 Fe 0.9 PO 4 ; again, when x is 0.2, LiMn x Fe 1-x PO 4 can be expressed as LiMn 0.2 Fe 0.8 PO 4 .

[0041] The chemical general formula of lithium nickel cobalt manganese oxide can be expressed as: LiNi q Co y Mn z O 2 , where q + y + z = 1. Lithium nickel cobalt manganese oxide represents a class of substances, not a single substance, and its performance can be optimized by adjusting the ratio of Ni, Co, and Mn. For example, NCM111 (LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O 2 ), NCM523 (LiNi 0.5 Co0.2 Mn 0.3 O 2 )、NCM811(LiNi 0.8 Co 0.1 Mn 0.1 O 2 )。

[0042] An embodiment of the present application provides a cathode composite material. In this material, the buffer layer is located between the inner core and the outer shell. Since the Fermi level of the buffer layer material is between the Fermi levels of lithium iron manganese phosphate and lithium nickel cobalt manganese oxide, a band energy gradient can be formed among the buffer layer material, lithium iron manganese phosphate, and lithium nickel cobalt manganese oxide from the inside to the outside, so as to reduce the Fermi level difference between lithium iron manganese phosphate and lithium nickel cobalt manganese oxide and eliminate the interfacial potential barrier, thereby reducing the interfacial impedance and the degree of interfacial side reactions. Moreover, the molar ratio of Mn element to Fe element in the inner core conforms to a decreasing trend, that is, the proportion of Fe element conforms to an increasing trend. The increase in the proportion of Fe element helps to reduce the resistance to the outward migration of lithium ions, facilitates the outward diffusion of lithium ions, thereby reducing the interfacial impedance and the degree of interfacial side reactions, and further improving the rate performance and cycle life of the battery.

[0043] In a possible implementation manner, along the direction from the inside to the outside, the molar ratio of Mn element to Fe element in the inner core gradually decreases.

[0044] In this implementation manner, the molar ratio of Mn element to Fe element in the inner core can gradually decrease from the core to the surface. For example, the number of moles of Mn element in the core is greater than the number of moles of Fe element, and then along the direction from the inside to the outside, the number of moles of Mn element gradually decreases, and the number of moles of Fe element gradually increases.

[0045] In the embodiment of the present application, along the direction from the inside to the outside, the molar ratio of Mn element to Fe element in the inner core gradually decreases, which is beneficial to the outward diffusion of lithium ions, reduces the lithium ion diffusion potential barrier, thereby reducing the interfacial impedance and the degree of interfacial side reactions. Moreover, the number of moles of Mn element gradually decreases from the core to the surface, which can enrich Mn inside the inner core, inhibit the Jahn-Teller effect, and reduce the manganese dissolution rate. At the same time, the number of moles of Fe element gradually increases from the core to the surface, which can form a more stable interfacial phase on the surface of the inner core and improve the stability of the inner core.

[0046] In a possible implementation manner, the inner core includes a core region and a surface region. The molar ratio of Mn element to Fe element in the core region is 9:1 to 7:3, and the molar ratio of Mn element to Fe element in the surface region is 6:4 to 5:5.

[0047] Exemplarily, the molar ratio of Mn element to Fe element in the core region can be 9:1, 8.5:1.5, 8:2, 7.5:2.5, or 7:3, etc., and no specific limitation is made thereto. The molar ratio of Mn element to Fe element in the surface region can be 6:4, 5.8:4.2, 5.5:4.5, 5.2:4.8, or 5:5, etc., and no specific limitation is made thereto.

[0048] In the embodiment of the present application, along the direction from the inside to the outside, the molar ratio of Mn element to Fe element in the inner core shows a gradient change from the core region to the surface region. Such a gradient change can be a continuous change or a discontinuous change. Due to this gradient change, not only can the interfacial impedance and the degree of interfacial side reactions be reduced, but also the Jahn-Teller effect can be inhibited, making the dissolution rate of manganese less than 0.5%, thereby improving the stability of the cathode composite material.

[0049] In a possible implementation manner, the buffer layer material includes lithium aluminum titanium phosphate (LATP).

[0050] Lithium aluminum titanium phosphate can be expressed as: Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 , but the proportion of each element can also be adjusted by adjusting the synthesis process or the doping ratio of each element. For example, lithium aluminum titanium phosphate can also be expressed as Li 1.5 Al 0.5 Ti 1.5 (PO 4 ) 3 or Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 .

[0051] In the embodiment of the present application, the Fermi level of lithium aluminum titanium phosphate is located between the Fermi level of lithium iron manganese phosphate and the Fermi level of lithium nickel cobalt manganese oxide. Therefore, from the inside to the outside, a band gradient can be formed between lithium aluminum titanium phosphate, lithium iron manganese phosphate, and lithium nickel cobalt manganese oxide to reduce the Fermi level difference between lithium iron manganese phosphate and lithium nickel cobalt manganese oxide, eliminate the interfacial potential barrier, and thus reduce the interfacial impedance and the degree of interfacial side reactions.

[0052] In a possible implementation manner, the porosity of the outer shell is greater than the porosity of the buffer layer, and the porosity of the buffer layer is greater than the porosity of the inner core.

[0053] In the embodiments of the present application, the porosity of the core is small, which can keep the core in a dense structure and ensure the lithium-ion storage capacity. Compared with the core, the porosity of the buffer layer increases, which can guide the uniform diffusion of lithium ions. The porosity of the outer shell further increases, so that a continuous ion channel can be formed, which is beneficial to the rapid outward diffusion of lithium ions. By setting the above porosity gradient, a rapid lithium-ion diffusion channel can be formed, thereby improving the charge transfer efficiency.

[0054] In a possible implementation manner, the porosity of the outer shell is 15-20%, the porosity of the core is less than 5%, and the porosity of the buffer layer is 5-15%.

[0055] Exemplarily, the porosity of the outer shell can be 15%, 16%, 17%, 18%, 19%, 20%, etc., the porosity of the core can be 1%, 2%, 3%, 4%, etc., and the porosity of the buffer layer can be 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, etc.

[0056] In the embodiments of the present application, the porosity of the core is less than 5%, which makes the core form a dense structure and ensures the lithium-ion storage capacity. The porosity of the buffer layer is between the porosities of the core and the outer shell. As a bridge for ion transport between the core and the outer shell, it is beneficial to the diffusion of lithium ions. The porosity of the outer shell is between 15-20%, which makes the outer shell form a loose porous structure, which is beneficial to the rapid outward diffusion of lithium ions. Therefore, by setting the porosities of the core, the outer shell and the buffer layer in this way, a rapid lithium-ion diffusion channel can be formed from the inside to the outside, thereby improving the charge transfer efficiency.

[0057] In a possible implementation manner, both the particle size of the core and the thickness of the outer shell are greater than the thickness of the buffer layer.

[0058] In the embodiments of the present application, compared with the thickness of the buffer layer, the particle size of the core is larger, which can enhance the stability of the positive composite material and reduce the risk of particle breakage. And the outer shell is thicker, which can enhance the surface protection and withstand greater external impacts. And the outer shell is lithium nickel cobalt manganese oxide, which has a high energy density and good cycling performance. Therefore, the thicker thickness of the outer shell is also beneficial to improving the energy density and cycling performance of the battery. Compared with the core and the outer shell, the buffer layer is thinner. As a transition zone between the core and the outer shell, it can effectively absorb the phase change stress and prevent cracks from spreading from the core to the outer shell.

[0059] In a possible implementation manner, the median particle size (D50) of the core is 1.5-2.5 μm, the thickness of the buffer layer is 18-52 nm, and the thickness of the outer shell is 100-300 nm.

[0060] Exemplarily, the D50 of the core can be 1.5 μm, 1.8 μm, 2.0 μm, 2.2 μm, 2.3 μm, 2.5 μm, etc., the thickness of the buffer layer can be 18 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 52 nm, etc., and the thickness of the outer shell can be 100 nm, 120 nm, 150 nm, 180 nm, 200 nm, 220 nm, 250 nm, 280 nm, 300 nm, etc.

[0061] From the median particle size of the above-mentioned core, the thickness of the buffer layer, and the thickness of the outer shell, it can be seen that the thicknesses of the buffer layer and the outer shell are at the nanometer level, and the particle size of the core is at the micrometer level. The larger particle size of the core can enhance the stability of the positive electrode composite material and reduce the risk of particle breakage. The thickness of the outer shell is 5 - 6 times that of the buffer layer, and the relatively thick outer shell can enhance the surface protection of the positive electrode composite material, enabling the positive electrode composite material to withstand greater external impacts. Moreover, the outer shell is lithium nickel cobalt manganate, which has a high energy density and good cycling performance. Therefore, the relatively thick outer shell is also beneficial to improving the energy density and cycling performance of the battery. Compared with the core and the outer shell, the buffer layer is relatively thin. As a transition zone between the core and the outer shell, it can effectively absorb the phase change stress and prevent cracks from spreading from the core to the outer shell.

[0062] For the buffer layer, if the thickness of the buffer layer is too thick, such as greater than 52 nm, it will affect the ion transport efficiency. If the thickness of the buffer layer is too thin, such as less than 18 nm, it will lead to poor stability of the positive electrode composite material and affect the cycling performance of the battery. Therefore, in this application, the thickness of the buffer layer is controlled within the above range. The buffer layer within this thickness range can effectively adjust the Fermi level difference between the outer shell and the core, eliminate the interface barrier, thereby reducing the interface impedance and improving the charge transport efficiency.

[0063] In a possible implementation manner, the outer shell further includes carbon nanotubes, and the carbon nanotubes are bonded to the surface of the lithium nickel cobalt manganate.

[0064] Among them, the carbon nanotubes can be single-walled carbon nanotubes or multi-walled carbon nanotubes, and no specific limitation is made in this regard. For example, when acetylene is used as the carbon source of the carbon nanotubes and deposited by chemical vapor deposition, in this case, multi-walled carbon nanotubes are formed on the surface of the lithium nickel cobalt manganate. Among them, the diameter of the multi-walled carbon nanotubes can be adjusted by controlling reaction conditions such as temperature and catalyst type, and no specific limitation is made in this regard.

[0065] In the embodiments of the present application, carbon nanotubes have good electrical conductivity and mechanical properties. Therefore, depositing carbon nanotubes on the surface of lithium nickel cobalt manganese oxide can significantly improve the electrical conductivity and mechanical properties of the positive electrode composite material, optimize the lithium ion transport path, thereby increasing the energy density of the battery and enhancing the rate performance and cycle life of the battery.

[0066] In a possible implementation, the mass of lithium nickel cobalt manganese oxide is greater than the mass of carbon nanotubes.

[0067] In the embodiments of the present application, the mass of lithium nickel cobalt manganese oxide is greater than the mass of carbon nanotubes, and the carbon nanotubes are dispersed on the surface of lithium nickel cobalt manganese oxide, which can form a continuous conductive network on the surface of lithium nickel cobalt manganese oxide, thereby reducing the interfacial impedance and improving the charge transfer efficiency.

[0068] In the related art, there are solutions for carbon coating on LMFP and solutions for modifying the surface of NCM. However, carbon coating on the surface of LMFP will increase the difficulty of ion transport. This solution sacrifices the ion transport channels and cannot be compatible with high voltages. And the solution for modifying the surface of NCM has poor interfacial stability at high temperatures, and the battery capacity decay accelerates.

[0069] The positive electrode composite material provided by the present application includes a core, a buffer layer, and a shell coated in sequence from the inside to the outside. The core includes LMFP, the buffer layer includes LATP, the shell includes NCM, and along the direction from the inside to the outside, the molar ratio of Mn element to Fe element in the core gradually decreases. The Fermi level of LATP is between LMFP and NCM, forming an energy band gradient between LMFP and NCM. Along the direction from the inside to the outside, the molar ratio of Mn element to Fe element in the core gradually decreases, forming a gradient doping in the core. At the same time, along the direction from the inside to the outside, the porosity of the core, the buffer layer, and the shell gradually increases, forming a porosity gradient. Among them, the electron transport efficiency can be improved through gradient doping and energy band gradient, and the ion transport efficiency can be improved through the porosity gradient. The dual conductive network of ions and electrons coordinates with each other, which can effectively improve the kinetic performance of the positive electrode composite material, reduce the interfacial impedance and the degree of interfacial side reactions, improve the charge transfer efficiency, and at the same time reduce the manganese dissolution rate, thereby enhancing the rate performance and cycle life of the battery.

[0070] The embodiments of the present application also provide a preparation method for the positive electrode composite material. Refer to Figure 1 , and the preparation method includes the following steps:

[0071] Step 101: Prepare the core.

[0072] The core includes lithium iron manganese phosphate. When preparing the core, the raw material liquid containing a manganese source and an iron source can be first mixed, and then the raw material liquid containing the manganese source and the iron source is gradually dropped into the raw material liquid containing a phosphorus source and a lithium source, and the concentration of the iron source is adjusted gradient. During the process of adjusting the iron source concentration, the raw material liquid containing the manganese source and the iron source is continuously dropped into the raw material liquid containing the phosphorus source and the lithium source until the molar ratio of the Mn element to the Fe element meets the condition, obtaining a mixed liquid.

[0073] The mixed liquid is spray-dried. Through spray-drying, the moisture in the mixed liquid is quickly evaporated, and finally a powdery second precursor is obtained. Then the second precursor is sintered. Through this sintering, the second precursor is converted into lithium iron manganese phosphate, thereby obtaining the core.

[0074] Step 102: Deposit a buffer layer material on the surface of the core to form a buffer layer on the surface of the core, obtaining a first precursor.

[0075] In this step, a buffer layer material can be deposited on the surface of the core through atomic layer deposition technology, and then calcined, thereby forming a buffer layer on the surface of the core and obtaining a first precursor.

[0076] Step 103: Deposit lithium nickel cobalt manganese oxide on the buffer layer surface of the first precursor to obtain a positive electrode composite material.

[0077] In this step, the first precursor can be first mixed with the raw material liquid containing a nickel source, a cobalt source and a manganese source to obtain a third precursor, and then the third precursor is mixed with a lithium source and calcined, thereby realizing the deposition of lithium nickel cobalt manganese oxide on the buffer layer surface of the first precursor. After depositing lithium nickel cobalt manganese oxide on the buffer layer surface of the first precursor, C 2 H 2 can be used as a carbon source, and carbon nanotubes are deposited on the surface of lithium nickel cobalt manganese oxide by chemical vapor deposition method, thereby forming a positive electrode composite material.

[0078] An embodiment of the present application provides a method for preparing a cathode composite material. In this method, a core is first synthesized, and then a buffer layer material is deposited on the surface of the core to form a buffer layer. Then, lithium nickel cobalt manganese oxide is deposited on the surface of the buffer layer, and carbon nanotubes are deposited on the surface of the lithium nickel cobalt manganese oxide, thereby obtaining the cathode composite material. In this material, the buffer layer is located between the core and the outer shell, and the Fermi level of the buffer layer material is between the Fermi level of lithium iron manganese phosphate and the Fermi level of lithium nickel cobalt manganese oxide. Therefore, from the inside to the outside, a band gradient can be formed between the buffer layer material, lithium iron manganese phosphate, and lithium nickel cobalt manganese oxide to reduce the Fermi level difference between lithium iron manganese phosphate and lithium nickel cobalt manganese oxide, eliminate the interfacial potential barrier, and thus reduce the interfacial impedance and the degree of interfacial side reactions. Moreover, the molar ratio of Mn element to Fe element in the core conforms to a decreasing trend, which is beneficial to the outward diffusion of lithium ions, reduces the lithium ion diffusion barrier, and thus reduces the interfacial impedance and the degree of interfacial side reactions, and further improves the battery rate performance and cycle life.

[0079] An embodiment of the present application also provides a method for preparing a cathode composite material. Refer to Figure 2 , and this preparation method includes the following steps:

[0080] Step 201: Dissolve MnSO 4 and FeSO 4 in water according to a molar ratio of 9:1 to 7:3 to obtain a first mixed solution, and drop the first mixed solution into the LiH 2 PO 4 solution.

[0081] Prepare a 1.0 mol / L LiH 2 PO 4 solution in advance, adjust the pH with acetic acid so that the pH of the LiH 2 PO 4 solution is between 2.8 and 3.2.

[0082] Dissolve MnSO 4 and FeSO 4 in water according to a molar ratio of 9:1 to 7:3. The total concentration of Mn and Fe is 0.5 mol / L. Under a nitrogen atmosphere and in a constant temperature water bath, drop the first mixed solution into the above-mentioned LiH 2 PO 4 solution, and add a chelating agent and stir continuously.

[0083] Among them, LiH 2 PO 4The pH of the solution can be 2.8, 2.9, 3.0, 3.1, 3.2, etc. The temperature of the constant temperature water bath can be set and changed as needed, and no specific limitation is made thereto. For example, the temperature of the constant temperature water bath is 58°C, 60°C, 62°C, etc. The dropping speed of the first mixed solution can be set and changed as needed, and no specific limitation is made thereto. For example, the dropping speed of the first mixed solution is 9 mL / min, 10 mL / min, 11 mL / min, etc. The stirring rate can be 450 rpm to 650 rpm. For example, the stirring rate is 450 rpm, 500 rpm, 550 rpm, 600 rpm, 650 rpm, etc. The chelating agent can be set and changed as needed, and no specific limitation is made thereto. For example, the chelating agent is citric acid.

[0084] Step 202: Gradually adjust the Fe 2+ concentration. During the process of adjusting the Fe 2+ concentration, continue to drop the adjusted first mixed solution into the LiH 2 PO 4 solution until the molar ratio of Mn element to Fe element is 6:4 to 5:5, obtaining a second mixed solution.

[0085] FeSO can be continuously or discontinuously added through a peristaltic pump 4 , gradually adjust the Fe 2+ concentration, and continue to drop the adjusted first mixed solution into the LiH 2 PO 4 solution until the molar ratio of Mn element to Fe element is 6:4 to 5:5, and react for 5 to 7 h to obtain a second mixed solution.

[0086] Among them, the gradient growth rate can be set and changed as needed, and no specific limitation is made thereto. For example, the gradient growth rate is 0.09 mol / L·h, 0.1 mol / L·h, 0.12 mol / L·h, etc. The reaction time can be 5 h, 5.2 h, 5.5 h, 6 h, 6.5 h, 7 h, etc.

[0087] In the embodiment of the present application, during the process of gradually adjusting the Fe 2+ concentration, the core particles gradually increase, and the molar ratio of Mn element to Fe element gradually decreases, so as to realize a gradient distribution of the molar ratio of Mn element to Fe element.

[0088] Step 203: Perform spray drying on the second mixed solution to obtain a second precursor.

[0089] Add the second mixed solution into a spray dryer, and perform spray drying on the second mixed solution through the spray dryer to obtain a second precursor.

[0090] Among them, the inlet temperature of the spray dryer is 150 - 250 °C, the outlet temperature is 80 - 100 °C, and the atomization pressure is 0.25 - 0.35 MPa.

[0091] The inlet temperature can be 150 °C, 180 °C, 200 °C, 220 °C, 250 °C, etc., the outlet temperature can be 80 °C, 85 °C, 90 °C, 95 °C, 100 °C, etc., and the atomization pressure can be 0.25 MPa, 0.28 MPa, 0.3 MPa, 0.3 MPa, 0.35 MPa, etc.

[0092] In the embodiment of the present application, through the spray drying technology, the moisture in the second mixed solution can be quickly evaporated, which can not only keep the second precursor powder uniform, but also greatly shorten the drying time. Among them, by adjusting the inlet temperature and outlet temperature of the spray dryer, rapid drying can be achieved, and by adjusting the atomization pressure, the powder can be ensured to be uniform.

[0093] Step 204: Sinter the second precursor to obtain the core.

[0094] Step 204 can be realized through the following steps (1-1) to (1-2), including:

[0095] (1-1) In an argon atmosphere, heat the second precursor to 300 - 400 °C and keep it warm for 2 - 3 h.

[0096] In the embodiment of the present application, the second precursor can be placed in a tubular furnace or other equipment that meets the conditions for sintering.

[0097] The flow rate of argon in step (1-1) can be set and changed as needed, and no specific limitation is made here. For example, the flow rate of argon is 45 mL / min, 48 mL / min, 50 mL / min, 52 mL / min, 55 mL / min, etc. The heating temperature can be 300 °C, 350 °C, 400 °C, etc., and the holding time can be 2 h, 2.2 h, 2.5 h, 2.8 h, 3 h, etc.

[0098] When heating the second precursor, it can be heated to 300 - 400 °C at a rate of 3 - 5 °C / min and kept warm for 2 - 3 h.

[0099] The heating rate in step (1-1) can be 3 °C / min, 4 °C / min, 5 °C / min, etc.

[0100] In the embodiment of the present application, heating the second precursor to 300 - 400 °C and keeping it warm for 2 - 3 h can remove the residual moisture or volatile impurities in the second precursor, and avoid the increase in the porosity of the core or structural defects caused by the residue of volatiles during high-temperature sintering in step (1-2).

[0101] (1-2) Continuously raise the temperature to 600 - 800 °C and hold for 9 - 11 h to obtain the core.

[0102] The temperature in step (1-2) can be 600 °C, 650 °C, 700 °C, 750 °C, 800 °C, etc., and the holding time can be 9 h, 9.5 h, 10 h, 10.5 h, 11 h, etc.

[0103] After performing step (1-1), the temperature can be continuously raised to 600 - 800 °C and held for 9 - 11 h to obtain monodisperse LMFP microspheres, that is, the core.

[0104] The heating rate in step (1-2) can be the same as or different from the heating rate in step (1-1). For example, the heating rate in step (1-2) is less than the heating rate in step (1-1). Exemplarily, the heating rate in step (1-1) is 5 °C / min, and the heating rate in step (1-2) is 3 °C / min. By setting the heating rate in this way, rapid heating can be achieved in the low-temperature stage, and the heating rate can be avoided from being too fast in the high-temperature stage to prevent thermal runaway.

[0105] In the embodiment of the present application, residual impurities or moisture are removed in the low-temperature stage first, and then sintered at 600 - 800 °C for 9 - 11 h to form a core with porosity, particle size, and gradient doping, which is beneficial to the outward diffusion of lithium ions, reduces the lithium ion diffusion barrier, thereby reducing the interfacial impedance and the degree of interfacial side reactions, and further improving the battery rate performance and cycle life. And the gradient-doped core can also inhibit the Jahn-Teller effect, making the dissolution rate of manganese less than 0.5%, further reducing the degree of interfacial side reactions.

[0106] Step 205: Sequentially deposit trimethylaluminum, lithium isopropoxide, titanium tetrachloride, and water on the surface of the core through atomic layer deposition technology for multiple cycles to form a deposit.

[0107] Among them, depositing trimethylaluminum (TMA), lithium isopropoxide (LiOPr), titanium tetrachloride (TiCl 4 ) and water is one cycle.

[0108] Through atomic layer deposition technology (ALD), under the conditions of an argon atmosphere, 200 - 300 °C, and 0.01 Pa, TMA, LiOPr, TiCl 4 and water can be sequentially deposited on the surface of the core.

[0109] The pulse time of TMA is 0.1 to 0.3 s, for example, the pulse time of TMA is 0.1 s, 0.15 s, 0.2 s, 0.25 s, 0.3 s, etc. The pulse time of LiOPr is 0.2 to 0.4 s, for example, the pulse time of LiOPr is 0.2 s, 0.25 s, 0.3 s, 0.35 s, 0.4 s, etc. TiCl 4 The pulse time is 0.15s to 0.3s. For example, TiCl 4 The pulse time of is 0.15s, 0.2s, 0.25s, 0.3s, etc. The pulse time of water is 0.1-0.3s, for example, the pulse time of water is 0.1s, 0.15s, 0.2s, 0.25s, 0.3s, etc. The number of cycles is 40-55 times, for example, the number of cycles is 40 times, 42 times, 45 times, 48 ​​times, 50 times, 52 times, 55 times, etc.

[0110] The single cycle time is 30 seconds. In one cycle, after depositing each substance, it is left for 5 to 10 seconds before the next substance is deposited. The total time is 30 seconds.

[0111] In the embodiment of the present application, by setting the pulse time of TMA, it is possible to ensure that TMA forms an aluminum oxide film layer of appropriate thickness on the surface of the core, avoiding excessive deposition leading to excessive thickness or agglomeration of the film layer. By setting the pulse time of LiOPr, it is possible to ensure that the lithium source is fully diffused, so that LiOPr and the aluminum oxide film layer are fully combined to form a Li-Al-O interface layer, thereby enhancing the lithium ion transmission efficiency. By setting the pulse time of TiCl 4 The pulse time can make the titanium base layer uniformly generated and balance the TiCl 4 The adsorption and reaction rate of TMA, LiOPr, TiCl 4 and water, multi-level modification of the inner core surface can be achieved, laying the foundation for the next calcination to form a buffer layer.

[0112] Step 206: calcining the deposit to form a buffer layer on the surface of the inner core to obtain a first precursor.

[0113] The deposit is placed in a tubular furnace or other equipment that meets the conditions for calcination, and a mixed gas of argon and oxygen is introduced. The calcination temperature is 500-700°C, and the calcination time is 2-3 hours. A buffer layer is formed on the surface of the inner core to obtain a first precursor.

[0114] Among them, the volume ratio of argon to oxygen can be set and changed as needed, and no specific limitation is made thereto. For example, the volume ratio of argon to oxygen is 9:1. The calcination temperature can be 500°C, 550°C, 600°C, 650°C, 700°C, etc., and the calcination time can be 2h, 2.2h, 2.5h, 2.8h, 3h, etc.

[0115] In the embodiment of the present application, through calcination, the sediment can be converted into stable lithium titanium phosphate-aluminum, so as to form a buffer layer on the surface of the inner core, so as to adjust the Fermi level difference between LMFP and the subsequent formed NCM shell, eliminate the interface barrier, thereby reducing the interface impedance and improving the charge transfer efficiency. Among them, calcining the sediment at 500-700°C for 2-3h can promote the chemical bonding between LATP and LMFP, making the combination of LATP and LMFP more stable.

[0116] Step 207: Mix the raw material liquid containing nickel source, cobalt source, manganese source, complexing agent and the first precursor evenly to obtain a third precursor.

[0117] The raw material liquid containing nickel source, cobalt source, manganese source can be a mixed solution of their nitrates, that is, a mixed solution of nickel nitrate, cobalt nitrate and manganese nitrate.

[0118] Among them, the mass ratio of Ni:Co:Mn in the nitrate mixed solution is (60-80):(20-10):(20-10). For example, the mass ratio of Ni:Co:Mn is 60:20:20, 70:15:15, 80:10:10, etc. The concentration of the nitrate mixed solution can be set and changed as needed, and no specific limitation is made thereto. For example, the concentration of the nitrate mixed solution is 1.5mol / L. The complexing agent can be set and changed as needed, and no specific limitation is made thereto. For example, the complexing agent is ammonia water, and the pH of the ammonia water is 11.0. The complexing ratio of the nitrate mixed solution to the complexing agent can be set and changed as needed. For example, the complexing ratio of the nitrate mixed solution to the complexing agent is 1:1, 1:2, 1:3, etc. Correspondingly, the complexing ratio of the first precursor to the complexing agent can also be set and changed as needed. For example, the complexing ratio of the first precursor to the complexing agent is 1:1, 1:2, 1:3, etc.

[0119] After adding the complexing agent and the first precursor to the nitrate mixed solution containing nickel source, cobalt source, manganese source, stir continuously, and react at a certain temperature for a certain time to obtain a third precursor.

[0120] Among them, the stirring rate can be set and changed as needed. For example, the stirring rate is 250 rpm, 300 rpm, 350 rpm, etc. The reaction temperature can be set and changed as needed. For example, the reaction temperature is 70 °C, 75 °C, 80 °C, 85 °C, 90 °C, etc. The reaction time can be set and changed as needed. For example, the reaction time is 3.5 h, 4 h, 4.5 h, 5 h, etc.

[0121] In the embodiment of the present application, the raw material liquid containing a nickel source, a cobalt source, and a manganese source is mixed uniformly with the first precursor to obtain an NCM precursor coated with the first precursor, so as to facilitate the introduction of a lithium source in the next step, thereby obtaining NCM particles coated with the first precursor.

[0122] Step 208: Calcinate the mixture of the third precursor and lithium carbonate to obtain a positive electrode composite material.

[0123] Step 208 can be implemented through the following steps (2-1) to (2-3), including:

[0124] (2-1) Mix the third precursor and lithium carbonate to obtain a mixture, and then calcinate the mixture in an oxygen atmosphere, heat it up to 600-800 °C, keep it warm for 6-8 h, take it out after cooling, and obtain NCM coated with the first precursor.

[0125] During calcination, first heat it up to 600-800 °C at a certain rate, keep it warm for 6-8 h, and take it out after cooling to a preset cooling temperature to obtain NCM coated with the first precursor.

[0126] The heating rate in step (2-1) can be 2 °C / min, 3 °C / min, 4 °C / min, etc., the heating temperature can be 600 °C, 650 °C, 700 °C, 750 °C, 800 °C, etc., the heat preservation time can be 6 h, 6.5 h, 7 h, 7.5 h, 8 h, etc. The preset cooling temperature can be set and changed as needed. For example, the preset cooling temperature is 180 °C, 190 °C, 200 °C, 210 °C, 220 °C, etc.

[0127] In the embodiment of the present application, by setting the calcination temperature and calcination time in this way, the NCM precursor can be fully converted into NCM, and thus deposited on the surface of the buffer layer.

[0128] (2-2) Through chemical vapor deposition, using C 2 H 2 as a carbon source, deposit carbon nanotubes on the surface of the NCM coated with the first precursor to obtain composite particles.

[0129] Through chemical vapor deposition, at 700 °C and normal pressure, using C 2 H 2 as a carbon source, N2 is used as the carrier gas, and Fe / Mo / Al 2 O 3 (loading amount 1 wt%) is used as the catalyst, and carbon nanotubes are deposited on the surface of NCM coated with the first precursor, and after reacting for a certain time, composite particles are obtained.

[0130] Among them, C 2 H 2 The flow rate of and the flow rate of N 2 can both be set and changed as needed. For example, the flow rate of C 2 H 2 is 45 sccm, 48 sccm, 50 sccm, 52 sccm, etc., and the flow rate of N 2 is 180 sccm, 190 sccm, 200 sccm, 210 sccm, 220 sccm, etc. The reaction time can be set and changed as needed. For example, the reaction time is 25 min, 28 min, 30 min, 32 min, 35 min, 40 min, etc.

[0131] Among them, the mass of NCM is greater than the mass of carbon nanotubes. Among them, the volume of the nitrate mixture containing nickel source, cobalt source, and manganese source can be set and changed as needed, and no specific limitation is made thereto. For example, if the volume of the nitrate mixture is 1 L, then the mass of NCM is 150 - 180 g, and correspondingly, the mass of carbon nanotubes is 0.3 - 0.8 g.

[0132] In the embodiments of the present application, by setting the flow rate of acetylene, continuous and stable gas supply can be achieved. Combining with the conditions of chemical vapor deposition, carbon nanotubes can be evenly dispersed on the surface of NCM to form a continuous conductive network, thereby improving the conductivity and mechanical properties of the positive electrode composite material, optimizing the lithium ion transmission path, and further increasing the energy density of the battery, enhancing the battery rate performance and cycle life.

[0133] (2 - 3) Crush and screen the composite particles to obtain the positive electrode composite material.

[0134] In this step, the composite particles can be ball - milled and then screened through a standard sieve to obtain the positive electrode composite material.

[0135] Among them, the particle size of the positive electrode composite material obtained by screening satisfies the following conditions: D10 is 0.55 - 0.95 μm, D50 is 1.5 - 2.5 μm, D90 ≤ 15 μm, D99 ≤ 25 μm.

[0136] In the embodiments of the present application, by controlling the particle size of the positive composite material to meet the above conditions, not only can the proportion of large particles be reduced, the surface roughness of the subsequently prepared positive electrode sheet be decreased, but also the over-wide particle size distribution can be avoided, which affects the coating uniformity on the surface of the positive electrode sheet subsequently.

[0137] The embodiments of the present application provide a preparation method of a positive composite material. In this method, the core is first synthesized, and then lithium titanium phosphate is deposited on the surface of the core to form a buffer layer. Then, lithium nickel cobalt manganate is deposited on the surface of the buffer layer, and carbon nanotubes are deposited on the surface of the lithium nickel cobalt manganate, thereby obtaining the positive composite material. In this material, lithium titanium phosphate is used as the buffer layer between the core and the shell, and the Fermi level of lithium titanium phosphate is between the Fermi level of lithium iron manganese phosphate and the Fermi level of lithium nickel cobalt manganate. Therefore, from the inside to the outside, an energy band gradient can be formed between lithium titanium phosphate, lithium iron manganese phosphate, and lithium nickel cobalt manganate to reduce the Fermi level difference between lithium iron manganese phosphate and lithium nickel cobalt manganate, eliminate the interface barrier, thereby reducing the interface impedance and the degree of interface side reactions. And the molar ratio of Mn element to Fe element in the core conforms to a decreasing trend, which is beneficial to the outward diffusion of lithium ions, reduces the lithium ion diffusion barrier, thereby reducing the interface impedance and the degree of interface side reactions, and further improving the battery rate performance and cycle life.

[0138] The embodiments of the present application also provide a positive electrode sheet, which includes the positive composite material introduced above, and this positive electrode sheet has all the advantages of the positive composite material.

[0139] This positive electrode sheet may further include a positive current collector, a first conductive agent, and a first binder. Among them, the positive composite material, the first conductive agent, and the first binder are uniformly coated on the positive current collector in the form of a mixed slurry. The coating method can be double-sided coating or single-sided coating, and no specific limitation is made thereto.

[0140] Among them, the first conductive agent can be selected from at least one of SP (carbon black), CNT (carbon nanotubes), and conductive graphite, and the first binder can be selected from at least one of PVDF (polyvinylidene fluoride), polytetrafluoroethylene, and carboxymethyl cellulose.

[0141] The mass fraction of the positive composite material can be 94.5-97%, the mass fraction of the first conductive agent can be 1.5-3%, and the mass fraction of the first binder can be 1.5-2.5%.

[0142] Exemplarily, the mass fraction of the positive composite material is 94.5%, 95%, 95.5%, 96%, 96.5%, 97%, etc., the mass fraction of the first conductive agent is 1.5%, 2%, 2.5%, 3%, etc., and the mass fraction of the first binder is 1.5%, 2%, 2.5%, etc.

[0143] When preparing the positive electrode plate, the positive electrode composite material, the first conductive agent, and the first binder can be dispersed in the solvent NMP (N-methylpyrrolidone), and after stirring evenly, a slurry is obtained. The slurry is coated on the positive electrode current collector, and after processes such as drying and cold pressing, the positive electrode plate is obtained. The coating method can be single-sided coating or double-sided coating, and the single-sided surface density is 210 g / m 2 ~230 g / m 2 .

[0144] An embodiment of the present application also provides a battery, which includes: a housing, an electrolyte accommodated inside the housing, a negative electrode plate, a positive electrode plate, and a separator. The negative electrode plate and the positive electrode plate are separated by the separator. This battery has all the advantages of the positive electrode composite material.

[0145] Among them, the negative electrode plate may include a negative electrode current collector, a negative electrode material, a second conductive agent, and a second binder. The second conductive agent may be selected from at least one of SP (carbon black), carbon fiber, and graphene, and the second binder may be selected from at least one of CMC (sodium carboxymethyl cellulose), SBR (styrene-butadiene rubber), and lithium hydroxymethyl cellulose. The negative electrode material may be selected from at least one of graphite materials and silicon-based materials.

[0146] In a possible implementation manner, when preparing the negative electrode plate, the negative electrode material, the second conductive agent, and the second binder are dispersed in a solvent, and after stirring evenly, a slurry is obtained, and then it is coated on the negative electrode current collector, and after processes such as drying and cold pressing, the negative electrode plate is obtained. The coating method can be single-sided coating or double-sided coating. This solvent can be NMP or other solvents, and no specific limitation is made thereto.

[0147] Among them, the mass fraction of the negative electrode material can be 94.5 to 97%, the mass fraction of the second conductive agent can be 1.5 to 3%, and the mass fraction of the second binder can be 1.5 to 2.5%.

[0148] Exemplarily, the mass fraction of the negative electrode material is 94.5%, 95%, 95.5%, 96%, 96.5%, 97%, etc., the mass fraction of the second conductive agent is 1.5%, 2%, 2.5%, 3%, etc., and the mass fraction of the second binder is 1.5%, 2%, 2.5%, etc.

[0149] In a possible implementation manner, when preparing the battery, the positive and negative electrode plates and the separator after vacuum drying are subjected to processes such as cutting, die-cutting, and slitting, and then the electrolyte is injected into the housing under high temperature and negative pressure to assemble the battery.

[0150] Among them, the type of the housing can be set and changed as needed, and no specific limitation is imposed thereon. For example, the type of the housing can be an aluminum alloy housing, a steel housing or a housing made of other materials. In addition, the shape of the housing can also be set and changed as needed, and no specific limitation is imposed thereon. For example, the shape of the housing can be square, cylindrical or other shapes, and no specific limitation is imposed thereon. The diaphragm can be a diaphragm purchased on the market or a diaphragm prepared by itself. The material of the diaphragm can be PP (polypropylene), can be PE (polyethylene), can also be a three-layer material composed of PP and PE composites, or can be other materials, and no specific limitation is imposed thereon.

[0151] The technical solution of the present application will be described in detail below through specific embodiments.

[0152] In the following specific embodiments, operations not specified in conditions are carried out according to conventional conditions or conditions recommended by the manufacturer.

[0153] Example 1

[0154] Example 1 provides a positive electrode composite material, which is prepared by the following method:

[0155] Step 1: Dissolve MnSO 4 and FeSO 4 in water at a concentration of 0.5 mol / L to obtain a first mixed solution. The first mixed solution is dropped into the LiH 2 PO 4 solution at a rate of 10 mL / min, and citric acid is added, and stirring is carried out at a rate of 500 rpm. Among them, the concentration of the LiH 2 PO 4 solution is 1.0 mol / L, and the pH is 3.0.

[0156] Step 2: Continuously or discontinuously add FeSO 4 through a peristaltic pump to dynamically adjust the Fe 2+ concentration, and the gradient growth rate is 0.1 mol / L·h. During the process of adjusting the Fe 2+ concentration, the adjusted first mixed solution is continuously dropped into the LiH 2 PO 4 solution until the molar ratio of Mn element to Fe element is 5:5, and the reaction is carried out for 6 h to obtain a second mixed solution.

[0157] During the process of adjusting the Fe 2+ concentration, the core particles gradually increase, and the molar ratio of Mn element to Fe element shows a gradient distribution.

[0158] Step 3: Add the second mixture into a spray dryer, and perform spray drying on the second mixture through the spray dryer to obtain a second precursor.

[0159] Among them, the inlet temperature of the spray dryer is 200 °C, the outlet temperature is 90 °C, and the atomization pressure is 0.3 MPa.

[0160] Step 4: In an argon atmosphere (flow rate of 50 mL / min), heat the second precursor to 350 °C at a rate of 5 °C / min and hold for 2 h; then heat it to 600 °C at a rate of 3 °C / min and hold for 10 h to obtain LMFP microspheres.

[0161] Step 5: Through atomic layer deposition (ALD), under the conditions of an argon atmosphere, 250 °C, and 0.01 Pa, deposit TMA, LiOPr, TiCl 4 and water on the surface of the inner core in sequence, cycle 50 times, and the single cycle time is 30 s.

[0162] Among them, the pulse time of TMA is 0.1 s, the pulse time of LiOPr is 0.2 s, and the pulse time of TiCl 4 is 0.15 s, and the pulse time of water is 0.1 s.

[0163] Step 6: Introduce a mixed gas of argon and oxygen with a volume ratio of 9:1, and calcine at 500 °C for 2 h to obtain a first precursor.

[0164] Step 7: Add a complexing agent NH 3 ·H 2 O and the first precursor to the mixed solution of nitrates of Ni, Co, and Mn, and react at 80 °C with a stirring rate of 300 rpm for 4 h to obtain a third precursor.

[0165] Among them, the mass ratio of Ni:Co:Mn is 70:15:15, and the total concentration of the nitrate mixed solution is 1.5 mol / L.

[0166] Step 8: Mix the third precursor with lithium carbonate to obtain a mixture, and calcine the mixture in an oxygen atmosphere, heat it to 750 °C at a rate of 2 °C / min, hold for 8 h, cool to 200 °C and then take it out to obtain NCM coated with the first precursor.

[0167] Step 9: Through chemical vapor deposition, at 700 °C and atmospheric pressure, using C 2 H 2 as the carbon source (flow rate of 50 sccm), N 2 as the carrier gas (flow rate of 200 sccm), Fe / Mo / Al 2 O 3(Loading of 1 wt%) was used as the catalyst to deposit carbon nanotubes on the surface of NCM coated with the first precursor, and the reaction was carried out for 30 min to obtain composite particles.

[0168] Step 9: Crush and screen the composite particles to obtain the cathode composite material.

[0169] Example 2

[0170] The difference between Example 2 and Example 1 is that the molar ratio of Mn element to Fe element in Step 1 is 9:1, and the molar ratio of Mn element to Fe element in Step 2 is 5:5. That is, in the core, in the direction from the inside to the outside, the molar ratio of Mn element to Fe element changes from 9:1 to 5:5, and the rest of the steps are the same.

[0171] Example 3

[0172] The difference between Example 3 and Example 1 is that the molar ratio of Mn element to Fe element in Step 1 is 7:3, and the molar ratio of Mn element to Fe element in Step 2 is 5:5. That is, in the core, in the direction from the inside to the outside, the molar ratio of Mn element to Fe element changes from 7:3 to 5:5, and the rest of the steps are the same.

[0173] Comparative Example 1

[0174] The difference between Comparative Example 1 and Example 1 is that when preparing the core, in the direction from the inside to the outside, the molar ratio of Mn element to Fe element is 5:5, and the rest of the steps are the same.

[0175] Comparative Example 2

[0176] The difference between Comparative Example 2 and Example 1 is that when depositing LATP on the core surface by atomic layer deposition process, the number of cycles is 60 times, and the thickness of the formed buffer layer is 60 nm, and the rest of the steps are the same.

[0177] Comparative Example 3

[0178] The difference between Comparative Example 3 and Example 1 is that when depositing LATP on the core surface by atomic layer deposition process, the number of cycles is 20 times, and the thickness of the LATP buffer layer is 10 nm, and the rest of the steps are the same.

[0179] Comparative Example 4

[0180] The difference between Comparative Example 4 and Example 1 is that when calcining the mixture of the third precursor and lithium carbonate in Step 8, the heat preservation time is extended to 10 h, and the rest of the steps are the same.

[0181] Comparative Example 5

[0182] The LMFP and NCM are mechanically mixed to prepare a cathode composite material. Among them, the mass fraction of LMFP is 80%, and the mass fraction of NVM is 20%.

[0183] Test example

[0184] Preparation of the cathode electrode sheet:

[0185] SP and CNT are used as the first conductive agents, and the mass ratio of SP to CNT is 1:1. PVDF is used as the first binder.

[0186] The first conductive agents, the first binder, and the cathode composite material prepared in Example 1 are dispersed in NMP according to the mass fraction ratio of 2.5%:2.5%:95%. After stirring evenly, a slurry is obtained. The slurry is double-sided coated on the cathode current collector, and after processes such as drying and cold pressing, the cathode electrode sheet 1 is prepared.

[0187] The process of preparing the cathode electrode sheets using the cathode composite materials prepared in Examples 2 to 3 and Comparative Examples 1 to 5 is the same as the process of preparing the cathode electrode sheet using the cathode composite material prepared in Example 1. The cathode electrode sheets 2 to 3 and the comparative electrode sheets 1 to 5 are obtained respectively.

[0188] Preparation of the anode electrode sheet:

[0189] SP is used as the second conductive agent, SBR and CMC are used as the second binders, and the mass ratio of SBR to CMC is 1:1. Graphite is used as the anode material.

[0190] The anode material, the second conductive agent, and the second binder are dispersed in NMP according to the mass fraction ratio of 95%:2.5%:2.5%. After stirring evenly, a slurry is obtained. The slurry is double-sided coated on the anode current collector, and after processes such as drying and cold pressing, the anode electrode sheet is prepared.

[0191] Preparation of the symmetric battery:

[0192] Electrode sheet cutting: The cathode electrode sheet is cut into circular pieces with a diameter of 12 mm, and after cleaning, it is reserved for use; the anode electrode sheet is cut into circular pieces with a diameter of 12 mm, and after cleaning, it is reserved for use;

[0193] Symmetric assembly: The cathode electrode sheet and the anode electrode sheet are assembled into a button battery in the structure of "cathode / separator / anode", electrolyte is injected and sealed to obtain a symmetric battery.

[0194] Preparation of the battery:

[0195] The vacuum-dried positive and negative electrode sheets and the separator are subjected to processes such as cutting, die-cutting, slitting, and stacking, and then electrolyte is injected, sealed, and assembled into a battery. Among them, the separator is a PP separator.

[0196] (1) Tests of ionic conductivity and interfacial impedance

[0197] According to the preparation process of the symmetric cell, the positive electrode plates 1-3 and the comparative electrode plates 1-5 were respectively assembled with the negative electrode plate into symmetric cells, and the ionic conductivity and interfacial impedance were measured on an electrochemical workstation.

[0198] (2) Tests of electronic conductivity

[0199] Four-probe method test: Four probes were contacted with the surface of the positive electrode plate 1 at equal intervals (1-2 mm), a current (I) was applied, and the voltage drop (V) was measured.

[0200] The same method was used to test the positive electrode plates 2-3 and the comparative electrode plates 1-5.

[0201] (3) Cycling performance test

[0202] The positive electrode plates 1-3 and the comparative electrode plates 1-5 were respectively assembled with the negative electrode plate into cells. After the cells were left standing at 25 °C for 1 h, they were charged at a constant current and constant voltage of 1C to 4.2V, the cut-off current was 0.05C, then left standing for 30 min, and discharged at a constant current of 1C to 2.5V. After 500 cycles, the stability of the cells was evaluated by the cycling capacity retention rate.

[0203] The test results are shown in Table 1 below.

[0204] Table 1

[0205]

[0206] It can be seen from the above test results that: compared with Comparative Example 1, in Examples 1-3, the Mn element and the Fe element were gradient-doped. By gradient doping, the electronic conductivity can be improved, and it has higher cycling performance. While in Comparative Example 1, no gradient doping was carried out, so the electronic conductivity is low, and at the same time the cycling performance is poor.

[0207] Among them, the molar ratio of the Mn element to the Fe element in Example 3 is 7:3. Compared with Examples 1 and 2, the molar ratio of the Mn element is reduced, which better inhibits the Jahn-Teller effect. At the same time, the molar ratio of the Fe element is increased, which improves the electronic conductivity.

[0208] Compared with Comparative Example 2 and Comparative Example 3, the buffer layer thickness in Examples 1-3 is 50 nm, and this thickness is appropriate. Therefore, it has both high ionic conductivity and good cycling performance. While in Comparative Example 2, the buffer layer thickness is too thick, resulting in a slow ion transport rate, and in Comparative Example 3, the buffer layer thickness is too thin, resulting in poor material stability and poor cycling performance of the cells.

[0209] In Comparative Example 4, the calcination time of the mixture of the third precursor and lithium carbonate was too long. Since the outermost layer of the material is NCM and carbon nanotubes, a too long calcination time will result in a reduction in the pores on the material surface, thereby reducing the ionic conductivity and affecting the cycle performance of the battery.

[0210] In Comparative Example 5, LMFP and NCM were mechanically mixed, resulting in a relatively high interfacial impedance, small ionic conductivity and electronic conductivity, and poor cycle performance of the battery.

[0211] In summary, for the positive electrode composite material prepared in this application, the electron transport efficiency can be improved through gradient doping and energy band gradient, and the ionic transport efficiency can be improved through pore gradient. The ionic and electronic double-conductive networks coordinate with each other, which can effectively improve the kinetic performance of the positive electrode composite material, reduce the interfacial impedance, improve the charge transport efficiency, simultaneously reduce the manganese dissolution rate, and reduce the degree of interfacial side reactions, thereby improving the battery rate performance and cycle life.

[0212] The above description is only for the convenience of those skilled in the art to understand the technical solution of this application, and is not intended to limit this application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of this application shall be included in the protection scope of this application.

Claims

1. A positive electrode composite material, characterized in that: The positive electrode composite material comprises: a core, a buffer layer and a shell which are sequentially coated from the inside to the outside; The core includes lithium iron manganese phosphate, the shell includes lithium nickel cobalt manganese oxide, the Fermi level of the buffer layer material is between the Fermi level of the lithium iron manganese phosphate and the Fermi level of the lithium nickel cobalt manganese oxide, and along the direction from the inside to the outside, the molar ratio of the Mn element to the Fe element in the core follows a decreasing trend.

2. The positive electrode composite material according to claim 1, characterized in that: From the inside to the outside, the molar ratio of the Mn element to the Fe element in the core gradually decreases.

3. The positive electrode composite material according to claim 2, characterized in that: The inner core includes a core region and a surface region, wherein the molar ratio of Mn element to Fe element in the core region is 9:1 to 7:3, and the molar ratio of Mn element to Fe element in the surface region is 6:4 to 5:

5.

4. The positive electrode composite material according to claim 1, characterized in that: The buffer layer material includes lithium aluminum titanium phosphate.

5. The positive electrode composite material according to claim 1, characterized in that: The porosity of the outer shell is greater than the porosity of the buffer layer, and the porosity of the buffer layer is greater than the porosity of the inner core.

6. The positive electrode composite material according to claim 5, characterized in that: The porosity of the shell is 15-20%, the porosity of the core is less than 5%, and the porosity of the buffer layer is 5-15%.

7. The positive electrode composite material according to claim 1, characterized in that: The particle size of the core and the thickness of the shell are both greater than the thickness of the buffer layer.

8. The positive electrode composite material according to claim 7, characterized in that: The median particle size of the core is 1.5 to 2.5 μm; The thickness of the buffer layer is 18 to 52 nm; The thickness of the shell is 100-300 nm.

9. The positive electrode composite material according to any one of claims 1 to 8, characterized in that: The shell also includes carbon nanotubes, which are bonded to the surface of the lithium nickel cobalt manganese oxide.

10. A method for preparing a positive electrode composite material, characterized in that: The positive electrode composite material is as claimed in any one of claims 1 to 9, wherein the preparation method comprises: preparing the core; Depositing the buffer layer material on the surface of the core to form the buffer layer on the surface of the core to obtain a first precursor; Lithium nickel cobalt manganese oxide is deposited on the surface of the buffer layer of the first precursor to obtain the positive electrode composite material.

11. The preparation method according to claim 10, characterized in that: The preparation of the kernel comprises: Dissolve MnSO4 and FeSO4 in water at a molar ratio of 9:1 to 7:3 to obtain a first mixed solution, and drop the first mixed solution into the LiH2PO4 solution; Gradient regulation of Fe 2+ concentration, in adjusting Fe 2+ During the concentration adjustment process, the adjusted first mixed solution is continuously dripped into the LiH2PO4 solution until the molar ratio of the Mn element to the Fe element is 6:4 to 5:5, thereby obtaining a second mixed solution; spray drying the second mixed liquid to obtain a second precursor; The second precursor is sintered to obtain the core.

12. The preparation method according to claim 10, characterized in that: Depositing the buffer layer material on the surface of the core to form the buffer layer on the surface of the core to obtain a first precursor includes: By atomic layer deposition, trimethylaluminum, lithium isopropoxide, titanium tetrachloride and water are sequentially deposited on the surface of the core for multiple cycles to form a deposit; The deposit is calcined to form the buffer layer on the surface of the inner core, thereby obtaining the first precursor.

13. The preparation method according to claim 10, characterized in that: The step of depositing lithium nickel cobalt manganese oxide on the surface of the buffer layer of the first precursor to obtain the positive electrode composite material comprises: uniformly mixing a raw material solution containing a nickel source, a cobalt source, and a manganese source, a complexing agent, and the first precursor to obtain a third precursor; The mixture of the third precursor and lithium carbonate is calcined to obtain the positive electrode composite material.

14. A positive electrode sheet, characterized in that: The positive electrode sheet comprises the positive electrode composite material according to any one of claims 1 to 9.

15. A battery, characterized in that: The battery comprises: a shell, an electrolyte contained in the shell, a negative electrode sheet, a positive electrode sheet and a separator, wherein the positive electrode sheet is as described in claim 14, and the negative electrode sheet and the positive electrode sheet are separated by the separator.

Images