Composite positive electrode material with gradient interface and preparation method thereof

Abstract

The invention discloses a composite positive electrode material with a gradient interface and a preparation method thereof, and belongs to the technical field of lithium ion battery preparation. The composite positive electrode material forms a gradient interface surface in the electrolyte; the composite positive electrode material is composed of a transition metal oxide positive electrode material and a phosphate material, a CEI coating layer with gradient distribution is formed on the surface of the composite positive electrode material, the CEI coating layer comprises lithium phosphate and a lithium salt, the lithium salt comprises LiF, F in the LiF comes from a decomposition product of an electrolyte, and the surface of the composite positive electrode material is coated with the CEI coating layer. And the content of the LiF is in a gradient decreasing trend in the extending direction from the surface to the inside. Phosphate and transition metal oxide are sequentially subjected to drying, mixing, heat treatment and electrochemical activation treatment, and the composite positive electrode material with the gradient interface is obtained. And the performance of the composite positive electrode material is effectively improved.

Description

Technical Field

[0001] The present invention relates to a composite cathode material with a gradient interface and a preparation method thereof, belonging to the technical field of lithium-ion battery preparation. Background Art

[0002] The increasing energy consumption in modern society has put forward higher and higher requirements for energy storage technologies. Lithium-ion batteries (LIBs) have always been the dominant energy storage technology for mobile applications and are also considered the most suitable technology for electric vehicles and stationary energy storage systems. LiNi x Co y Mn 1-x-y O2 (NCM)-based cathodes have attracted much attention due to their advantages such as high power / energy density, low cost, and low toxicity. Layered LiNi x Co y Mn 1-x-y O2 (NCM) has a relatively high energy density and is the dominant family of lithium-ion battery cathode materials. Among NCM cathode materials, NCM613 has a relatively good energy density. However, NCM613 is limited in practical applications by side reactions occurring at the electrolyte-cathode interface and related safety issues. Existing studies have shown that NCM cathode materials are prone to phase transformation from a layered structure to a spinel structure and ultimately to a rock salt phase at high temperatures. At the same time, the electrolyte is in direct contact with the surface of the cathode material, and the presence of the electrolyte during charge and discharge accelerates the phase transformation of NCM. The electrolyte will directly attack the surface of the cathode material, resulting in premature structural changes in the cathode material. Therefore, the erosion of the electrolyte on the cathode material will cause the cycling performance of the battery to decay and reduce the rate performance. In order to improve the cycling stability and rate performance of NCM613 materials, various strategies have been designed, including the use of flame-retardant electrolytes, oxide coatings, and / or temperature-responsive electrical (or ionic) barrier additives. In addition, the active NCM material is modified with different coatings to prevent its reaction with the electrolyte. The success of these methods is often accompanied by a price in the electrochemical performance of the battery. Lithium iron manganese phosphate (LMFP) has an olivine structure similar to that of lithium iron phosphate (LFP) and has high safety and stability. At the same time, LMFP can preferentially induce the formation of a gradient-distributed cathode electrolyte interface on the surface of NCM613. The formation of this gradient interface can effectively hinder the direct attack of the electrolyte on the cathode material. The gradient cathode electrolyte interface formed by compounding LMFP with NCM613 can effectively alleviate the problem of NCM613 cycling decay and improve its rate performance. Summary of the Invention

[0003] In order to improve the phenomenon that the direct erosion of the electrolyte on the cathode material leads to premature change of the structure of the cathode material, resulting in a significant reduction in the cycle stability performance and capacity retention rate of the battery. The present invention proposes a composite cathode material with a gradient interface and a method with a simple process that can induce the generation of a gradient interface.

[0004] To achieve the invention purpose, the present invention adopts the following technical solutions:

[0005] One of the purposes of the present invention is to provide a composite cathode material with a gradient interface, which is characterized in that the composite cathode material forms a gradient interface surface in the electrolyte; the composite cathode material is composed of a transition metal oxide cathode material and a phosphate material, and a CEI coating layer with a gradient distribution is formed on the surface of the composite cathode material. The CEI coating layer includes lithium phosphate and a lithium salt, the lithium salt includes LiF, the F in the LiF comes from the decomposition product of the electrolyte, and the content of the LiF shows a trend of decreasing in a gradient from the surface to the inward extension direction.

[0006] The thickness of the CEI coating layer is 1 - 500 nm.

[0007] After the first cycle activation of the composite cathode material in the electrolyte containing F element, since the surface layer phosphate will preferentially induce the generation of a gradient interface on the surface of the composite cathode material, this gradient structure is loose on the surface and dense in the deep layer. Therefore, this gradient structure can not only not affect the transmission of Li + , but also greatly reduce the side reaction between the electrolyte and the composite cathode material, protect the composite cathode material, inhibit the loss of electrons, and improve the battery cycle performance.

[0008] Furthermore, the CEI coating layer also contains a polymer, and the polymer generated by the decomposition of the electrolyte will also enter the CEI coating layer.

[0009] As a preferred technical solution of the present invention, the gradient interface is the contact surface between the electrolyte and the composite cathode material. The gradient interface can be composed of lithium phosphate, a polymer, and a lithium salt. The polymer is generated by the decomposition of the electrolyte, such as polyacrylonitrile, and the lithium salt is such as LiF.

[0010] Preferably, the electrolyte - cathode interface is gradient lithium fluoride (LiF). From the direction close to the surface of the cathode material to the inward extension direction away from the surface of the cathode material, the content of LiF shows a trend of decreasing in a gradient. LiF is an inorganic component, and the electrolyte - cathode interface formed by LiF is dense, which can not only inhibit the loss of electrons but also inhibit the reaction between the electrolyte and the cathode material directly, achieving the purpose of stabilizing the interface. However, if there is too much LiF, it will affect the ion transmission. The gradient LiF structure is loose on the surface and dense inside. Therefore, the gradient LiF can not only protect the cathode material from the erosion of the electrolyte, but also will not affect the ion transmission rate.

[0011] As a preferred technical solution of the present invention, the transition metal oxide cathode material includes any one or a combination of at least two of a ternary cathode material, a lithium-rich cathode material, or a lithium nickel manganese oxide spinel material. The combination can be a ternary cathode material and a lithium-rich cathode material, a lithium-rich cathode material and a lithium nickel manganese oxide spinel material, or a ternary cathode material and a lithium nickel manganese oxide spinel material, etc., but not only the combinations listed herein. Other unlisted combinations within this combination range are equally applicable.

[0012] The phosphate is a phosphate-based substance with a carbon layer on its surface. The thickness of the carbon layer is generally 1 - 5 nm, and commercially available phosphate-based substances with a carbon layer can be used, etc. The phosphate-based substance is any one or a combination of at least two of lithium iron phosphate, lithium manganese phosphate, lithium manganese iron phosphate, lithium vanadium phosphate, lithium vanadate phosphate, lithium cobalt phosphate, and lithium nickel phosphate. The combination can be lithium iron phosphate and lithium manganese phosphate, lithium manganese iron phosphate and lithium vanadium phosphate, or lithium vanadate phosphate and lithium cobalt phosphate, etc., but not only the combinations listed herein. Other unlisted combinations within this combination range are equally applicable. Preferably, it is lithium manganese iron phosphate (LMFP) with a carbon layer coating on its surface. Lithium manganese iron phosphate (LMFP) has a higher energy density than lithium iron phosphate (LFP). At the same time, lithium manganese iron phosphate has a higher capacity and a higher voltage platform. In order to overcome the problems of poor safety performance and low cycle times of NCM materials and reduce costs, LMFP is selected as the composite material. As the doping ratio of LMFP increases, the first charge efficiency and cycle performance of the material are improved. However, if the proportion of LMFP is too high, the surface carbon layer of LMFP will be burned out during heat treatment, which will instead reduce the performance of the material.

[0013] In a preferred technical solution of the present invention, based on the mass of the composite cathode material being 100%, the mass fraction of the transition metal oxide cathode material is 0.5 - 99.5%. Among them, the mass fraction can be 0.5%, 1%, 5%, 8%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 99%, or 99.5%, etc., but not only the values listed herein. Other unlisted values within this value range are equally applicable. Preferably, it is 90 - 99.9%. If the content of the transition metal oxide is too large, the proportion of the phosphate in the composite cathode material will be too small to play a role in stabilizing the structure. If the content of the transition metal oxide is too small, the specific capacity of the composite cathode material will decrease.

[0014] As a preferred technical solution of the present invention, based on the mass of the composite cathode material being 100%, the mass percentage content of the phosphate in the composite material is 0.5-99.5%, and the mass fraction can be 0.5%, 1%, 5%, 8%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 99% or 99.5%, etc., but not only the values listed herein. Other unlisted values within this numerical range are equally applicable, preferably 0.5-10%. When the phosphate content is too large, the conductivity of the composite cathode material will decrease, and when it is too small, the structure of the transition metal oxide cannot be protected from the side reactions of the electrolyte.

[0015] The second object of the present invention is to provide a preparation method of the composite cathode material as described in the first object. The preparation method includes the following steps: drying, mixing, heat treatment, and electrochemical activation treatment of the phosphate and the transition metal oxide in sequence to obtain the composite cathode material with a gradient interface.

[0016] Pretreat the phosphate and the transition metal oxide, and the pretreatment includes drying.

[0017] Preferably, the temperature of the drying is 80-120°C;

[0018] Preferably, the drying time is 18-24h.

[0019] As a preferred technical solution of the present invention, the mechanical fusion refers to mixing the transition metal oxide and the phosphate in a high-speed mixer.

[0020] Preferably, the rotation speed is 500-6000rpm;

[0021] Preferably, the mixing time is 5-10min.

[0022] If the rotation speed of the mechanical fusion of the present invention is too large, the composite cathode material will be damaged, and if the rotation speed is too small, the phosphate cannot closely adhere to the transition metal oxide material.

[0023] As a preferred technical solution of the present invention, the heat treatment temperature is 100-300°C. Preferably, the heat treatment time is 3-5h;

[0024] Preferably, the atmosphere of the heat treatment is an inert atmosphere, preferably an argon atmosphere.

[0025] On the one hand, too high heat treatment temperature in the present invention will cause the original layered structure to transform into a bad spinel phase and the bonding strength between the coating layers will be greatly reduced. On the other hand, it will also affect the carbonization effect of LMFP (including carbon content and the order degree of amorphous carbon). Too high heat treatment temperature will cause partial oxidation of the amorphous carbon formed by carbonization, resulting in a decrease in the carbon content in the composite structure.

[0026] As a preferred technical solution of the present invention, the electrochemical activation treatment is to prepare the composite cathode material into a battery composed of a cathode and an electrolyte for the electrochemical activation treatment, and the electrochemical activation treatment is an activation treatment for at least one cycle.

[0027] Preferably, the rate of the electrochemical activation treatment is 0.05 - 0.5C;

[0028] Preferably, the temperature of the electrochemical activation treatment is 25 - 45°C.

[0029] The electrolyte includes an electrolyte containing F element (such as LiPF6, LiBF4), and the electrolyte may also contain polymers. For example, the electrolyte includes Guotai electrolyte, Yienke electrolyte, and high-nickel electrolyte.

[0030] The electrochemical activation treatment is directly carried out in the corresponding lithium-ion battery.

[0031] After the battery is prepared in the present invention, the capacity and performance of the battery are relatively poor because there will be an oxide film on the surface of the active material used on the electrode, which prevents the insertion and extraction process of lithium ions, resulting in insufficient activity on the surface. A very small current needs to be used for charging to reduce the oxide layer on the surface of the active material and improve the activity. At the same time, during the activation process, due to the decomposition of the electrolyte and the side reaction with the cathode material, the decomposition products of the electrolyte will generate a gradient LiF interface on the surface of the cathode material, and the activation will be more complete at 45°C.

[0032] The present invention provides a preparation method for inducing a gradient interface, and the specific steps of the method include:

[0033] The first step: Preparation of the composite cathode material:

[0034] (1) Put the transition metal oxide material and phosphate into a vacuum drying oven at 120°C for drying for 24h;

[0035] (2) Prepare the composite cathode material: Using a high-speed mixer or mechanical fusion method, put 400g of the transition metal oxide and 2g of phosphate into the high-speed mixer, pre-mix 3 times at 500rpm first, and then once at 2000rpm and 4000rpm respectively, with each rotation time being two minutes, and the composite cathode material can be obtained;

[0036] (3) The composite cathode material is placed in a tubular furnace under an argon atmosphere for heat treatment. The heat treatment temperature is 300 °C and the heat treatment time is 5 h.

[0037] The second step is the preparation of the composite cathode electrode:

[0038] (1) The composite cathode material, binder, and conductive agent are configured in a certain proportion to obtain a slurry.

[0039] (2) Take a certain amount of the above-prepared materials and put them into a degassing machine for mixing. Use the degassing machine to mix the materials three times to uniformly mix the composite cathode material, binder, and conductive agent together to obtain a uniformly mixed slurry.

[0040] (3) The mixed slurry is applied on the aluminum foil and leveled with a scraper.

[0041] (4) The leveled electrode is placed in a vacuum drying oven at 120 °C and dried for 12 h.

[0042] The third step is the assembly of the battery:

[0043] (1) The dried electrode is punched into an electrode with a diameter of 12 mm using a punching machine.

[0044] (2) The battery assembly operation is carried out in a glove box. The battery assembly sequence is the negative electrode case, spring piece, 1-mm-thick gasket, composite cathode electrode, separator, electrolyte, lithium piece, and positive electrode case. Finally, the battery is pressed tightly with a sealing machine.

[0045] (3) The assembled battery is activated at a rate of 0.1 C. During the cycling process, phosphate will induce the formation of a gradient interface on the surface of the positive electrode.

[0046] Compared with the prior art, the present invention has the following beneficial effects:

[0047] (1) In view of the problems of poor safety performance and poor cycle stability of ternary materials, the present invention designs and constructs a composite cathode material with a gradient interface. By using a mechanical fusion process, phosphate and transition metal oxide materials are proportionally fused through high-speed fusion processes such as high-speed blending, etc., so that part or all of the phosphate particles are coated on the surface of the transition metal oxide material. The composite material is then subjected to subsequent heat treatment. After the first-cycle activation, a composite cathode material with a gradient interface is formed on the surface of the composite cathode material: the innermost layer is the transition metal oxide material, the second-outer layer is the phosphate material, and the outermost layer is the gradient interface structure. Gradient lithium fluoride (LiF) has a trend of decreasing content from the surface of the cathode material to away from the surface of the cathode material. LiF is an inorganic component, and the electrolyte cathode interface formed by LiF is dense, which can not only inhibit the loss of electrons but also inhibit the reaction between the electrolyte and the cathode material directly, achieving the purpose of stabilizing the interface. Therefore, gradient LiF can not only protect the cathode material from being eroded by the electrolyte but also does not affect the ion transport rate, and can improve the rate performance of the composite material. The composite cathode material of the present invention simultaneously improves the cycle stability of the composite cathode material and effectively improves the performance of the composite cathode material.

[0048] (2) In the present invention, lithium iron manganese phosphate has the function of stabilizing the structure. As a coating layer, lithium iron manganese phosphate can effectively alleviate the problem of voltage attenuation during the cycle, especially improving the high-temperature cycle stability.

[0049] (3) The present invention provides a simple and effective method for preparing a gradient interface. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 Schematic diagram of the gradient interface formed in Example 1.

[0051] Figure 2 XPS diagram of the gradient interface in Example 6. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0052] The present invention will be further described below in conjunction with embodiments, but the present invention is not limited to the following embodiments.

[0053] Example 1

[0054] This example provides a method for inducing a gradient interface as follows:

[0055] First step, using NCM613 material as the substrate and LMFP as the composite material, both are used to prepare the composite cathode material:

[0056] (1) Put the NCM613 material and LMFP into a vacuum drying oven at 120 °C respectively and dry for 24 h;

[0057] (2) Preparation of the composite cathode material: Using a high-speed mixer or mechanical fusion method, put 400 g of NCM613 and 12 g of LMFP (with a layer of carbon on the surface) into the high-speed mixer. First, premix at 500 rpm three times, and then once at 2000 rpm, 4000 rpm, and 6000 rpm, with each rotation speed lasting for two minutes, and the LMFP@NCM613 composite cathode material can be obtained;

[0058] (3) Put the composite cathode material into a tubular furnace under an argon atmosphere for heat treatment. The heat treatment temperature is 200 °C, and the heat treatment time is 3 h.

[0059] The second step is the preparation of the composite cathode electrode:

[0060] (1) Prepare the composite cathode material (3% LMFP@NCM613), binder (PVDF), and conductive agent (C) in a ratio of 8:1:1 and put them into a mixing tank. 250 mg of the composite cathode material, 31.25 mg of PVDF, and 31.25 mg of C can be taken;

[0061] (2) Take the prepared materials and put them into a degassing machine for mixing. Use the degassing machine to mix the materials three times, each time for 12 min, so that the composite cathode material, binder, and conductive agent are evenly mixed together to obtain a uniformly mixed slurry;

[0062] (3) Apply the mixed slurry on the aluminum foil and scrape it flat with a 400-thickness scraper;

[0063] (4) Place the scraped electrode in a vacuum drying oven at 120 °C and dry it for 12 h.

[0064] The third step is the assembly of the battery:

[0065] (1) Punch the dried electrode into a circular electrode with a diameter of 12 mm using a punching machine;

[0066] (2) Roll press the electrode;

[0067] (3) Weigh the weight of the roll-pressed electrode, calculate and record the amount of active material. For example, if the mass of the electrode is 8.4 mg, then the amount of active material = (the mass of the electrode - the mass of the aluminum foil) * 0.8;

[0068] (4) Conduct the battery assembly operation in a glove box. The battery assembly sequence is the negative electrode shell, spring piece, 1-mm-thick gasket, composite cathode electrode, separator, ENCHEM electrolyte, lithium piece, and positive electrode shell. Finally, use a sealing machine to compress the battery, and the pressure is 50 MPa;

[0069] (5) Activate the assembled battery at a rate of 0.1C. During the cycling process, LMFP will induce the gradient formation of LiF on the surface of the positive electrode.

[0070] The gradient interface prepared in this example is as Figure 1 shown on the right. Figure 1 On the left is the positive electrode material without the formation of gradient LiF. The interface on its surface is thick and loose. This kind of interface cannot protect the positive electrode from the erosion of the electrolyte. Therefore, LiPF6 in the electrolyte will react with the positive electrode material, resulting in premature structural changes in the positive electrode material. Figure 1 On the right, due to the material composite of LMFP and NCM, during the cycling process, a thin, dense, uniform and firm gradient interface will be formed on the surface of the NCM positive electrode material due to the action of LMFP. The composition of this gradient interface shows a gradually increasing trend from the outside to the inside. The formation of this gradient interface can effectively alleviate the erosion of the electrolyte on the positive electrode material.

[0071] Its electrochemical performance is shown in Table 1. Its initial discharge specific capacity is 206.5 mAh / g. Under the conditions of 45°C and a rate of 1C, cyclic tests are carried out. Its capacity retention rate after 300 cycles is 89.8%, which has been significantly improved compared with Comparative Example 1.

[0072] Example 2

[0073] This example provides a method for inducing a gradient interface as follows:

[0074] The first step is to use NCM811 material as the substrate and LMFP as the composite material to prepare a composite positive electrode material:

[0075] (1) Put the NCM811 material and LMFP into a vacuum drying oven at 120°C and dry for 24 h.

[0076] (2) Prepare the composite positive electrode material: Using a high-speed mixer or mechanical fusion method, put 400 g of NCM811 and 2 g of LMFP (with a layer of carbon on the surface) into the high-speed mixer. First, premix 3 times at 500 rpm, and then once at 2000 rpm, 4000 rpm, and 6000 rpm, with each rotation time being 2 minutes, and the LMFP@NCM811 composite positive electrode material can be obtained.

[0077] (3) Put the composite positive electrode material into a tubular furnace under an argon atmosphere for heat treatment. The heat treatment temperature is 300°C and the heat treatment time is 3 h.

[0078] The second step is the preparation of the composite positive electrode sheet:

[0079] (1) Prepare the composite cathode material (0.5% LMFP@NCM811), binder (PVDF), and conductive agent (C) in a ratio of 8:1:1 and place them in a mixing tank. For example, 250 mg of the composite cathode material, 31.25 mg of PVDF, and 31.25 mg of C can be taken;

[0080] (2) Take the prepared materials and put them into a degassing machine for mixing. Use the degassing machine to mix the materials three times, each time for 12 minutes, so that the composite cathode material, binder, and conductive agent are evenly mixed together to obtain a uniformly mixed slurry;

[0081] (3) Apply the mixed slurry onto the aluminum foil and scrape it flat with a 400 - thick scraper;

[0082] (4) Place the flattened electrode sheet in a vacuum drying oven at 120 °C and dry it for 12 hours.

[0083] The third step, battery assembly:

[0084] (1) Punch the dried electrode sheet into a circular electrode sheet with a diameter of 12 mm using a punching machine;

[0085] (2) Roll press the electrode sheet;

[0086] (3) Weigh the rolled - pressed electrode sheet, calculate and record the amount of active material. For example, if the mass of the electrode sheet is 8.4 mg, then the amount of active material = (mass of the electrode sheet - mass of the aluminum foil) * 0.8;

[0087] (4) Conduct the battery assembly operation in a glove box. The battery assembly sequence is the negative electrode case, spring sheet, 1 - mm - thick gasket, composite positive electrode sheet, separator, Enchem - ENCHEM electrolyte, lithium sheet, and positive electrode case. Finally, use a sealer to compress the battery, and the pressure is 50 MPa;

[0088] (5) Activate the assembled battery at a rate of 0.2C. During the cycling process, LMFP will induce the gradient formation of LiF on the surface of the positive electrode.

[0089] Its electrochemical performance is shown in Table 1. Its initial discharge specific capacity is 205.3 mAh / g. Under the conditions of 45 °C and a rate of 1C, a cycle test is carried out, and its capacity retention rate after 300 cycles is 88.7%, which has been significantly improved compared with Comparative Example 3.

[0090] Example 3

[0091] This example provides a method for inducing a gradient interface as follows:

[0092] The first step, using NCM622 material as the substrate and LMFP as the composite material, the two are used to prepare the composite cathode material:

[0093] (1) Put the NCM622 material and LMFP into a vacuum drying oven at 120 °C and dry for 12 h;

[0094] (2) Prepare the composite cathode material: Using a high-speed mixer or mechanical fusion method, put 400 g of NCM622 and 2 g of LMFP (with a layer of carbon on the surface) into the high-speed mixer. First, premix 3 times at 500 rpm, then once at 2000 rpm and once at 4000 rpm, with each rotation time being 2 minutes, and the LMFP@NCM622 composite cathode material can be obtained;

[0095] (3) Put the composite cathode material into a tubular furnace in an argon atmosphere for heat treatment. The heat treatment temperature is 300 °C and the heat treatment time is 3 h.

[0096] The second step, preparation of the composite cathode plate:

[0097] (1) Configure the composite cathode material (0.5% LMFP@NCM622), binder (PVDF), and conductive agent (C) in a ratio of 8:1:1 and put them into a mixing tank. 250 mg of the composite cathode material, 31.25 mg of PVDF, and 31.25 mg of C can be taken;

[0098] (2) Take the above-configured materials and put them into a degassing machine for mixing. Use the degassing machine to mix the materials three times, each time for 12 min, so that the composite cathode material, binder, and conductive agent are evenly mixed together to obtain a uniformly mixed slurry;

[0099] (3) Apply the mixed slurry on the aluminum foil and scrape it flat with a 400-thick scraper;

[0100] (4) Place the scraped plate in a vacuum drying oven at 120 °C and dry for 12 h.

[0101] The third step, assembly of the battery:

[0102] (1) Punch the dried plate into a circular plate with a diameter of 12 mm using a punching machine;

[0103] (2) Roll press the plate;

[0104] (3) Weigh the weight of the roll-pressed plate, calculate and record the amount of active material. For example, if the mass of the plate is 8.4 mg, then the amount of active material = (the mass of the plate - the mass of the aluminum foil) * 0.8;

[0105] (4) Conduct the battery assembly operation in a glove box. The battery assembly sequence is the negative electrode case, spring piece, 1 mm thick gasket, composite cathode plate, separator, Enchem electrolyte, lithium piece, and positive electrode case. Finally, use a sealing machine to compress the battery, and the pressure is 50 MPa;

[0106] (5) Activate the assembled battery at a rate of 0.1C. During the cycling process, LMFP will induce the gradient formation of LiF on the surface of the positive electrode.

[0107] Its electrochemical performance is shown in Table 1. Its initial discharge specific capacity is 202.1 mAh / g. Under the conditions of 45°C and a rate of 1C, cyclic tests are carried out, and its capacity retention rate after 300 cycles is 86.8%, showing a significant improvement compared with Comparative Example 2.

[0108] Example 4

[0109] First step, using NCM613 material as the substrate and LMFP as the composite material, the two are used to prepare the composite positive electrode material:

[0110] (1) Put the NCM613 material and LMFP into a vacuum drying oven at 120°C respectively and dry for 24 h;

[0111] (2) Prepare the composite positive electrode material: By using a high-speed mixer or mechanical fusion method, put 400 g of NCM613 and 4 g of LMFP (with a layer of carbon on the surface) into the high-speed mixer. First, premix 3 times at 500 rpm, and then once at 2000 rpm, 4000 rpm, and 6000 rpm respectively, with each rotation time being 2 minutes, and then the LMFP@NCM613 composite positive electrode material can be obtained;

[0112] (3) Put the composite positive electrode material into a tubular furnace in an argon atmosphere for heat treatment. The heat treatment temperature is 300°C and the heat treatment time is 4 h.

[0113] Second step, preparation of the composite positive electrode sheet:

[0114] (1) Configure the composite positive electrode material (1% LMFP@NCM613), binder (PVDF), and conductive agent (C) in a ratio of 8:1:1 and put them into a mixing tank. 250 mg of the composite positive electrode material, 31.25 mg of PVDF, and 31.25 mg of C can be taken;

[0115] (2) Take the above-configured materials and put them into a degassing machine for mixing. Use the degassing machine to mix the materials three times, each time for 12 min, so that the composite positive electrode material, binder, and conductive agent are evenly mixed together to obtain a uniformly mixed slurry;

[0116] (3) Apply the mixed feed on the aluminum foil and scrape it flat with a 400-thick scraper;

[0117] (4) Place the scraped electrode sheet in a vacuum drying oven at 120°C and dry for 12 h.

[0118] Third step, battery assembly:

[0119] (1) Punch the dried electrode sheet into a circular electrode sheet with a diameter of 12 mm using a punching machine;

[0120] (2) Roll press the electrode sheet;

[0121] (3) Weigh the weight of the roll-pressed electrode sheet, calculate and record the amount of active material. For example, if the mass of the electrode sheet is 8.4 mg, then the amount of active material = (mass of the electrode sheet - mass of the aluminum foil) * 0.8;

[0122] (4) Conduct the battery assembly operation in a glove box. The battery assembly sequence is the negative electrode case, spring piece, 1-mm-thick gasket, composite positive electrode sheet, separator, ENCHEM electrolyte, lithium sheet, and positive electrode case. Finally, use a sealing machine to tightly press the battery, with a pressure of 50 MPa;

[0123] (5) Activate the assembled battery at a rate of 0.1 C. During the cycling process, LMFP will induce the gradient formation of LiF on the surface of the positive electrode.

[0124] Its electrochemical performance is shown in Table 1. Under the conditions of 45 °C and a rate of 1 C, a cycle test is conducted, and its capacity retention rate after 300 cycles is 84.9%, showing a significant improvement compared to Comparative Example 1.

[0125] Example 5

[0126] The first step is to use NCM613 material as the substrate and LMFP as the composite material to prepare the composite positive electrode material:

[0127] (1) Put the NCM613 material and LMFP into a vacuum drying oven at 120 °C for drying for 24 h;

[0128] (2) Prepare the composite positive electrode material: Using a high-speed mixer or mechanical fusion method, put 400 g of NCM613 and 4 g of LMFP (with a layer of carbon on the surface) into the high-speed mixer. First, premix 3 times at 500 rpm, and then once each at 2000 rpm, 4000 rpm, and 6000 rpm, with each rotation time being two minutes, and the LMFP@NCM613 composite positive electrode material can be obtained.

[0129] The second step is the preparation of the composite positive electrode sheet:

[0130] (1) Prepare the composite positive electrode material (1% LMFP@NCM613), binder (PVDF), and conductive agent (C) in a ratio of 8:1:1 and put them into a mixing tank. 250 mg of the composite positive electrode material, 31.25 mg of PVDF, and 31.25 mg of C can be taken;

[0131] (2) Put the above-prepared materials into a degassing machine for mixing. Use the degassing machine to mix the materials three times, each time for 12 minutes, so that the composite cathode material, binder, and conductive agent are evenly mixed together to obtain a uniformly mixed slurry;

[0132] (3) Apply the mixed slurry on the aluminum foil and scrape it flat with a 400-thick scraper;

[0133] (4) Place the flattened electrode sheet in a vacuum drying oven at 120 °C and dry it for 12 hours.

[0134] The third step, battery assembly:

[0135] (1) Punch the dried electrode sheet into a circular electrode sheet with a diameter of 12 mm using a punching machine;

[0136] (2) Roll press the electrode sheet;

[0137] (3) Weigh the rolled electrode sheet, calculate and record the amount of active material. For example, if the mass of the electrode sheet is 8.4 mg, then the amount of active material = (mass of the electrode sheet - mass of the aluminum foil) * 0.8;

[0138] (4) Conduct the battery assembly operation in a glove box. The battery assembly sequence is the negative electrode shell, spring sheet, 1-mm-thick gasket, composite positive electrode sheet, separator, electrolyte ENCHEM electrolyte, lithium sheet, and positive electrode shell. Finally, use a sealing machine to compress the battery, and the pressure is 50 MPa;

[0139] (5) Test the assembled battery at a 1C rate. During the cycling process, LMFP will induce the gradient formation of LiF on the surface of the positive electrode.

[0140] Its electrochemical performance is shown in Table 1. Its initial discharge specific capacity is 196.2 mAh / g. Under the conditions of 45 °C and 1C rate, cyclic testing is carried out, and its 300-cycle capacity retention rate is 85.2%, showing a significant improvement compared with Comparative Example 1.

[0141] Example 6

[0142] The first step, using NCM613 material as the substrate and LMFP as the composite material, the two are used to prepare the composite cathode material:

[0143] (1) Put the NCM613 material and LMFP into a vacuum drying oven at 120 °C respectively and dry them for 24 hours;

[0144] (2) Preparation of the composite cathode material: By using a high-speed mixer or mechanical fusion method, 400 g of NCM613 and 2 g of LMFP (with a carbon layer on the surface) are placed in the high-speed mixer. First, pre-mix 3 times at 500 rpm, and then mix once each at 2000 rpm, 4000 rpm, and 6000 rpm, with each rotation time being 2 minutes, to obtain the LMFP@NCM613 composite cathode material;

[0145] (3) The composite cathode material is placed in a tubular furnace under an argon atmosphere for heat treatment. The heat treatment temperature is 200 °C, and the heat treatment time is 5 h.

[0146] The second step is the preparation of the composite cathode electrode:

[0147] (1) The composite cathode material (0.5% LMFP@NCM613), binder (PVDF), and conductive agent (C) are configured in a ratio of 8:1:1 and placed in a mixing tank. 250 mg of the composite cathode material, 31.25 mg of PVDF, and 31.25 mg of C can be taken;

[0148] (2) Take the above-configured materials and put them into a degassing machine for mixing. Use the degassing machine to mix the materials three times, each time for 12 min, so that the composite cathode material, binder, and conductive agent are evenly mixed together to obtain a uniformly mixed slurry;

[0149] (3) Apply the mixed slurry onto the aluminum foil and scrape it flat with a 300-thick scraper;

[0150] (4) Place the scraped electrode in a vacuum drying oven at 120 °C and dry it for 12 h.

[0151] The third step is the assembly of the battery:

[0152] (1) Punch the dried electrode into a circular electrode with a diameter of 12 mm using a punching machine;

[0153] (2) Roll press the electrode;

[0154] (3) Weigh the weight of the roll-pressed electrode, calculate and record the amount of active material. For example, if the mass of the electrode is 8.4 mg, then the amount of active material = (the mass of the electrode - the mass of the aluminum foil) * 0.8;

[0155] (4) Conduct the battery assembly operation in a glove box. The battery assembly sequence is the negative electrode shell, spring piece, 1-mm-thick gasket, composite cathode electrode, separator, ENCHEM electrolyte, lithium piece, and positive electrode shell. Finally, use a sealer to tightly press the battery, with a pressure of 50 MPa;

[0156] (5) Activate the assembled battery at a rate of 0.1C. During the cycling process, LMFP will induce the gradient formation of LiF on the surface of the positive electrode.

[0157] Its electrochemical performance is as Figure 2 shown in and Table 1. Its initial discharge specific capacity is 197.2 mAh / g, and the initial Coulombic efficiency is 98.3%. Under the conditions of 45 °C and a rate of 1C, a cycling test is carried out, and its capacity retention rate after 300 cycles is 90.8%, which has been significantly improved compared with Comparative Example 1.

[0158] Select the battery prepared in the above example for electrochemical performance testing and testing of the composition analysis of the gradient interface: The battery with a gradient interface can exhibit better rate performance at different rates. The present invention has a higher discharge specific capacity at 0.1C, 0.2C, 0.5C, 1C, 2C, and 5C.

[0159] Figure 2 It is the XPS etching of the composite positive electrode material. Figure 2 The rightmost peak in is the peak of the LiF. The greater the peak intensity, the more the content. Figure 2 From top to bottom is from the place far from the surface of the positive electrode material to the place close to the surface of the positive electrode material. From Figure 2 it can be seen that the peak intensity of LiF increases from outside to inside. Therefore, it can be concluded that the LiF interface increases gradually in a gradient from outside to inside.

[0160] Table 1. Comparison of battery performance of different modified samples assembled into coin cells

[0161]

[0162] Example 1 constructs a gradient interface by compounding 3% LMFP with NCM613 and heat-treating at 200 °C for 3 h. Example 2 constructs a CEI layer by compounding 0.5% LMFP with NCM811 material. Example 3 constructs a CEI layer by compounding 0.5% LMFP with NCM622. Both Example 4 and Example 5 construct a CEI layer by compounding 1% LMFP with NCM613. The difference between the two is that Example 4 heat-treats the composite positive electrode material, while Example 5 does not. Example 6 constructs a CEI layer by compounding 0.5% LMFP with NCM613. The difference between Comparative Examples 1-3 and Examples 1-6 is only that the comparative columns only use transition metal oxides and do not use phosphates for compounding.

[0163] The foregoing basic example of the present invention and its various further alternative examples can be freely combined to form multiple embodiments, all of which are embodiments that can be adopted and claimed by the present invention. In the solution of the present invention, each alternative example can be arbitrarily combined with any basic example and alternative example. Those skilled in the art will know that there are numerous combinations. The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A composite cathode material having a gradient interface, characterized in that: The composite positive electrode material forms a gradient interface surface in the electrolyte; the composite positive electrode material is composed of a transition metal oxide positive electrode material and a phosphate material, and a CEI coating layer with a gradient distribution is formed on the surface of the composite positive electrode material, the CEI coating layer includes lithium phosphate and a lithium salt, the lithium salt includes LiF, the F in the LiF comes from the decomposition product of the electrolyte, and the content of the LiF shows a gradient decreasing trend in the direction extending from the surface to the inside.

2. A composite positive electrode material with a gradient interface according to claim 1, characterized in that: The CEI coating layer also contains a polymer; the gradient interface can be composed of lithium phosphate, polymer and lithium salt.

3. A composite positive electrode material with a gradient interface according to claim 1 or 2, characterized in that: The thickness of the CEI coating layer is 1 to 500 nm.

4. A composite positive electrode material with a gradient interface according to claim 1 or 2, characterized in that: The transition metal oxide positive electrode material includes any one of a ternary positive electrode material, a lithium-rich positive electrode material or a lithium nickel manganese oxide spinel material or a combination of at least two thereof; Preferably, the phosphate is a phosphate-based substance containing a carbon layer on the surface, and the thickness of the carbon layer is generally 1-5 nm. The phosphate-based substance includes any one of lithium iron phosphate, lithium manganese phosphate, lithium manganese iron phosphate, lithium vanadium phosphate, lithium vanadium phosphate, lithium cobalt phosphate, and lithium nickel phosphate, or a combination of at least two thereof, preferably lithium manganese iron phosphate.

5. A composite positive electrode material with a gradient interface according to claim 1 or 2, characterized in that: Taking the mass of the composite positive electrode material as 100%, the mass fraction of the transition metal oxide is 0.5-99.5%, preferably 90-99.9%; the mass percentage content of the phosphate in the composite material is 0.1-99.5%, preferably 0.1-10%.

6. A method for preparing a composite positive electrode material with a gradient interface as claimed in any one of claims 1 to 5, characterized in that: The preparation method comprises: drying, mixing, heat treating and electrochemically activating phosphate and transition metal oxide in sequence to obtain the composite positive electrode material with a gradient interface.

7. The preparation method according to claim 6, characterized in that: The drying temperature is 80-120°C; Preferably, the drying time is 18 to 24 hours; Preferably, the mixing comprises mechanical fusion; Preferably, the rotation speed of the mechanical fusion is 500-6000 rpm; Preferably, the mechanical mixing time is 5 to 10 minutes; The temperature of the heat treatment is 100 to 300°C; Preferably, the heat treatment time is 3 to 5 hours; Preferably, the atmosphere of the heat treatment is an inert atmosphere, preferably an argon atmosphere.

8. The preparation method according to claim 6, characterized in that: The electrochemical activation treatment is to prepare the composite positive electrode material into a positive electrode and an electrolyte to form a battery for the electrochemical activation treatment, and the electrochemical activation treatment is at least one cycle of activation treatment; Preferably, the electrochemical activation treatment has a rate of 0.05 to 0.5C; Preferably, the temperature of the electrochemical activation treatment is 25 to 45°C; The electrolyte contains F elements.

9. The preparation method according to claim 6, characterized in that: The specific steps of the method include the following: The first step is the preparation of composite positive electrode materials: (1) placing the transition metal oxide material and the phosphate in a vacuum drying oven at 120° C. for 24 hours; (2) Preparation of composite positive electrode material: 400 g of transition metal oxide and 2 g of phosphate were placed in a high-speed mixer by means of a high-speed mixer or mechanical fusion, and pre-mixed three times at 500 rpm, and then once at 2000 rpm and 4000 rpm, each speed for two minutes, to obtain a composite positive electrode material; (3) placing the composite cathode material in a tubular furnace in an argon atmosphere for heat treatment at a temperature of 300° C. for a heat treatment time of 5 h; The second step is the preparation of composite positive electrode sheet: (1) preparing a slurry by mixing a composite positive electrode material, a binder, and a conductive agent in a certain ratio; (2) taking a certain amount of the above-prepared materials and putting them into a degassing machine for mixing, and using the degassing machine to mix the materials three times, so that the composite positive electrode material, the binder, and the conductive agent are evenly mixed together to obtain a uniformly mixed slurry; (3) Apply the mixed slurry on aluminum foil and smooth it with a scraper; (4) Place the flattened electrode in a vacuum drying oven at 120°C and dry for 12 hours. Step 3: Battery assembly: (1) Punch the dried electrode into a electrode with a diameter of 12 mm using a punching machine; (2) The battery is assembled in a glove box. The assembly order of the battery is the negative electrode shell, spring sheet, 1 mm thick gasket, composite positive electrode sheet, diaphragm, electrolyte, lithium sheet, positive electrode shell, and finally the battery is pressed with a sealing machine; (3) The assembled battery is activated at a rate of 0.1C. During the cycle, phosphate induces the formation of a gradient interface on the positive electrode surface.

10. The use of the composite positive electrode material with gradient interface according to any one of claims 1 to 5, characterized in that: The composite positive electrode material is used for preparing lithium ion batteries.

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