Preparation method of composite positive electrode material with Na gradient interface and composite positive electrode material

Abstract

The invention discloses a preparation method of a composite positive electrode material with a Na gradient interface and the composite positive electrode material, and belongs to the technical field of lithium ion battery preparation. Sodium phosphate and transition metal oxide are sequentially subjected to drying, mixing, heat treatment and electrochemical activation treatment, the composite positive electrode material with the Na gradient interface is obtained, the NaF content is in the gradient decreasing trend from the surface to the inner extending direction, and meanwhile in the first-circle activation process, due to Li < + > removal of the composite positive electrode material, the Na < + > content of the composite positive electrode material is reduced, and the Na < + > content of the composite positive electrode material is reduced. And part of Na < + > occupies the position of Li < + >, so that the diffusion channel of Li < + > is increased.

Description

Technical Field

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

[0002] Lithium-ion batteries (LIBs) have been widely studied due to their extensive applications in energy storage devices of electric vehicles (EVs) and hybrid electric vehicles (HEVs). Layered transition metal oxide materials have attracted much attention due to their high capacity (>250 mAh g -1 ) and low cost. 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. At the same time, they have problems of poor cycle stability, voltage decay, and too slow Li + migration rate. Among NCM cathode materials, NCM613 has a relatively good energy density. Existing studies have shown that NCM cathode materials are prone to phase transformation from a layered structure to a spinel structure at high temperatures and finally transform into a rock salt phase. At the same time, the electrolyte is in direct contact with the surface of the cathode material. The presence of the electrolyte during the charge and discharge process accelerates the phase transformation of NCM. The electrolyte will directly attack the surface of the cathode material, resulting in premature structural changes of the cathode material and seriously affecting Li +The transmission rate. 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. To improve the cycling stability and rate performance of the NCM613 material, various strategies have been designed, including using flame-retardant electrolytes, oxide coatings, and / or temperature-responsive electrical (or ionic) barrier additives. Additionally, 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 the price of the battery's electrochemical performance. Phosphates have been a research hotspot in recent years. By compounding phosphates with transition metal oxides, the erosion of the electrolyte on the cathode material can be effectively alleviated, and the rate performance of the battery can be improved. At present, most phosphates use lithium iron manganese phosphate, however, lithium iron manganese phosphate is expensive, and lithium resources are gradually scarce. Compared with lithium resources, sodium resources are abundant and have obvious price advantages. Olivine-type sodium iron manganese phosphate (NMFP) has received the most attention because it not only has the consistent structural stability of olivine-type materials but also has excellent electrochemical curves and high capacity. Sodium iron manganese phosphate (NMFP) has an olivine structure similar to that of lithium iron manganese phosphate (LMFP) and has high safety and stability. Introducing NMFP on the surface of the cathode material will cause a certain degree of mixing of Na atoms and Li atoms. Since the atomic radius of Na is 186 picometers, which is larger than the atomic radius of lithium (152 picometers), when Na atoms occupy the Li atom positions, they expand the Li + diffusion channels and increase the Li + transmission rate. At the same time, NMFP 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 NMFP with NCM613 can effectively alleviate the problem of NCM613 cycling decay and improve its rate performance.

[0003] In the prior art, there is little doping of other ionic substances in the electrode materials of lithium-ion batteries. In the present invention, the performance of the cathode material is further improved by doping other ions, sodium. Summary of the Invention

[0004] To improve problems such as the direct erosion of the electrolyte on the cathode material and the low Li + migration rate. The present invention proposes a preparation method of a composite cathode material with a Na gradient interface and a composite cathode material with a Na gradient interface, which can effectively improve the phenomenon that the direct erosion of the electrolyte on the cathode material causes the structure of the cathode material to change prematurely, resulting in a significant reduction in the cycling stability performance and capacity retention rate of the battery. At the same time, it expands the Li + transmission channels and increases the Li + migration rate.

[0005] To achieve the object of the invention, the following technical solutions are adopted in the present invention:

[0006] One of the objects of the present invention is to provide a method for preparing a composite cathode material, and the preparation method includes the following steps: drying, mixing, heat treatment and electrochemical activation treatment of a sodium phosphate salt and a transition metal oxide in sequence to obtain the composite cathode material with a Na gradient interface. At the same time, during the first-cycle activation, due to the Li + extraction of the composite cathode material, part of the Na+ will occupy the position of Li + and increase the diffusion channel of Li + .

[0007] The sodium phosphate salt and the transition metal oxide are pretreated, and the pretreatment includes drying.

[0008] Preferably, the drying temperature is 80-120°C;

[0009] Preferably, the drying time is 18-24 h.

[0010] As a preferred technical solution of the present invention, the mechanical fusion means mixing the transition metal oxide and the phosphate in a high-speed mixer.

[0011] Preferably, the rotation speed is 500-6000 rpm;

[0012] Preferably, the mixing time is 5-10 min.

[0013] If the rotation speed of the mechanical fusion of the present invention is too high, the composite cathode material will be damaged, and if the rotation speed is too low, the phosphate cannot closely adhere to the transition metal oxide material.

[0014] As a preferred technical solution of the present invention, the heat treatment temperature is 100-300°C. Preferably, the heat treatment time is 3-5 h;

[0015] Preferably, the atmosphere of the heat treatment is an inert atmosphere, preferably an argon atmosphere.

[0016] If the heat treatment temperature of the present invention is too high, on the one hand, the original layered structure will transform into a bad spinel phase and the bonding strength between the coating layers will be greatly reduced. In addition, it will also affect the carbonization effect of NMFP (including carbon content and the order degree of amorphous carbon). Too high a heat treatment temperature will cause part of the amorphous carbon formed by carbonization to be oxidized, resulting in a decrease in the carbon content in the composite structure.

[0017] 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 positive electrode and an electrolyte for the electrochemical activation treatment, and the electrochemical activation treatment is at least one-cycle activation treatment.

[0018] Preferably, the rate of the electrochemical activation treatment is 0.05 to 0.5 C;

[0019] Preferably, the temperature of the electrochemical activation treatment is 25 to 45 °C.

[0020] The electrolyte described above 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.

[0021] The electrochemical activation treatment is directly carried out in the corresponding lithium-ion battery.

[0022] After the battery is prepared according to the present invention, the capacity and performance of the battery are relatively poor. Because there is an oxide film on the surface of the active material used on the electrode, which prevents the insertion and extraction of lithium ions, resulting in insufficient surface activity. It is necessary to use a very small current 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 positive electrode material, the decomposition products of the electrolyte will produce a gradient NaF interface on the surface of the positive electrode material, and the activation will be more complete at 45 °C.

[0023] The present invention provides a preparation method for inducing a Na gradient interface. The specific steps of this method include:

[0024] The first step: Preparation of the composite positive electrode material:

[0025] (1) Put the transition metal oxide material and sodium phosphate into a vacuum drying oven at 120 °C and dry for 24 h;

[0026] (2) Prepare the composite positive electrode material: Using a high-speed mixer or mechanical fusion method, put 400 g of the transition metal oxide and 2 g of the phosphate into the high-speed mixer. First, premix 3 times at 500 rpm, and then once at 2000 rpm and 4000 rpm, with each rotation time being 2 minutes, and the composite positive electrode material can be obtained;

[0027] (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 5 h.

[0028] The second step: Preparation of the composite positive electrode sheet:

[0029] (1) Prepare a slurry by mixing the composite positive electrode material, binder, and conductive agent in a certain proportion;

[0030] (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.

[0031] (3) Apply the mixed slurry onto the aluminum foil and scrape it flat with a scraper.

[0032] (4) Place the flattened electrode sheet in a vacuum drying oven at 120 °C and dry it for 12 h.

[0033] The third step is the assembly of the battery:

[0034] (1) Punch the dried electrode sheet into electrode sheets with a diameter of 12 mm using a punching machine.

[0035] (2) 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, F-containing electrolyte, lithium sheet, and positive electrode case. Finally, use a sealing machine to tightly press the battery.

[0036] (3) Activate the assembled battery at a rate of 0.1C. During the cycling process, phosphate will induce the formation of a Na gradient interface on the surface of the positive electrode, and at the same time, Na + occupies the Li + vacancy phenomenon, expanding the Li + transport channels.

[0037] The second object of the present invention is to provide a composite cathode material as described in the first object, characterized in that a gradient interface surface is formed on the composite cathode material 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 sodium phosphate and sodium salts. The sodium salts include NaF. The F in the NaF comes from the decomposition products of the electrolyte, and the content of the NaF shows a gradient decreasing trend from the surface to the inward extension direction;

[0038] The thickness of the CEI coating layer is 1 - 500 nm.

[0039] After the first cycle activation of the composite cathode material in the F-containing electrolyte, since the surface layer phosphate will preferentially induce the formation 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 Li + transport, 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.

[0040] Furthermore, the CEI coating layer also contains polymers, and the polymers generated by the decomposition of the electrolyte will also enter the CEI coating layer.

[0041] As a preferred technical solution of the present invention, the Na gradient interface is the contact surface between the electrolyte and the composite cathode material. The Na gradient interface can be composed of sodium phosphate, polymer, and sodium salt. The polymer is generated by the decomposition of the electrolyte, such as polyacrylonitrile, and the sodium salt is such as NaF.

[0042] Preferably, the electrolyte-cathode interface is gradient sodium fluoride (NaF). The content of NaF shows a gradient decreasing trend from the surface of the cathode material to the inside away from the surface of the cathode material. NaF is an inorganic component, and the electrolyte-cathode interface formed by NaF 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 NaF, it will affect the ion transport. The gradient NaF structure is loose on the surface and dense inside. Therefore, the gradient NaF can not only protect the cathode material from being eroded by the electrolyte but also will not affect the ion transport rate.

[0043] As a preferred technical solution of the present invention, the transition metal oxide cathode material includes any one or at least two combinations of ternary cathode materials, lithium-rich cathode materials, or lithium nickel manganese oxide spinel materials. The combinations can be ternary cathode materials and lithium-rich cathode materials, lithium-rich cathode materials and lithium nickel manganese oxide spinel materials, or ternary cathode materials and lithium nickel manganese oxide spinel materials, etc., but not only the combinations listed herein. Other unlisted combinations within this combination range are also applicable.

[0044] The phosphate sodium salt is a phosphate sodium salt with a carbon layer on the surface. The thickness of the carbon layer is generally 1-5 nm, and commercially available phosphate substances with a carbon layer can be used, etc. The phosphate sodium salt is any one or at least two combinations of sodium iron phosphate, sodium manganese phosphate, sodium manganese iron phosphate, sodium vanadium phosphate, sodium oxovanadate phosphate, sodium cobalt phosphate, sodium nickel phosphate, etc. The combinations can be sodium iron phosphate and sodium manganese phosphate, sodium manganese iron phosphate and sodium vanadium phosphate, or sodium oxovanadate phosphate and sodium cobalt phosphate, etc., but not only the combinations listed herein. Other unlisted combinations within this combination range are also applicable. Preferably, it is sodium manganese iron phosphate (NMFP) with a carbon layer coating on the surface. Sodium manganese iron phosphate (LMFP) has a higher energy density than lithium manganese iron phosphate (LMFP). At the same time, sodium 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 existing in NCM materials and reduce costs at the same time, NMFP is selected as the composite material. With the increase of the doping ratio of NMFP, the first charge efficiency and cycle performance of the material are both improved. However, if the proportion of NMFP is too high, the surface carbon layer of NMFP will be burned out during heat treatment, which will instead reduce the performance of the material.

[0045] 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%. 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 90 - 99.9%. If the content of the transition metal oxide is too large, the proportion of phosphate in the composite cathode material will be too small to play the role of stabilizing the structure. If the content of the transition metal oxide is too small, the specific capacity of the composite cathode material will decrease.

[0046] 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%. 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 content of the phosphate is too large, the conductivity of the composite cathode material will decrease. When it is too small, it cannot protect the structure of the transition metal oxide from the damage of side reactions of the electrolyte.

[0047] Compared with the prior art, the present invention has the following beneficial effects:

[0048] (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. At the same time, by introducing Na, the Li +The transmission channel adopts a mechanical fusion process. According to a proportion, sodium phosphate salts and transition metal oxide materials are fused through high-speed fusion and other fusion processes, so that part or all of the sodium phosphate salt particles are coated on the surface of the transition metal oxide materials. The composite material after combination is subjected to subsequent heat treatment. After the first-cycle activation, a composite cathode material with a Na gradient interface is formed on the surface of the composite cathode material: the innermost layer is the transition metal oxide material, the second outermost layer is the sodium phosphate salt, and the outermost layer is the Na gradient interface structure. Along with the introduction of Na during the first-cycle activation process, Na occupies the position of Li, expanding the Li transmission channel. Gradient sodium fluoride (NaF) shows a trend of decreasing content from the surface of the cathode material to away from the surface of the cathode material. NaF is an inorganic component, and the electrolyte cathode interface formed by NaF 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 NaF can not only protect the cathode material from being eroded by the electrolyte, but also, due to the presence of Na, improve the + transmission rate of Li, and can improve the rate performance of the composite material. The composite cathode material described in the present invention improves the cycle stability of the composite cathode material at the same time, and effectively improves the performance of the composite cathode material.

[0049] (2) In the present invention, sodium manganese iron phosphate has the function of stabilizing the structure. As a coating layer, sodium manganese iron phosphate can effectively alleviate the problem of voltage decay during the cycle, especially improve the high-temperature cycle stability. Compared with traditional lithium phosphate salts, the sodium manganese iron phosphate used in the present invention has a lower cost, and the formed electrolyte cathode interface is more stable.

[0050] (3) The present invention provides a simple and effective preparation method for the gradient interface. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 is the TEM image of NMFP composite on the surface of the transition metal oxide.

[0052] Figure 2 is the + Na + occupying the + Li position to expand the Li transmission channel schematic diagram.

[0053] Figure 3 is the XPS diagram of NMFP preferentially inducing the Na gradient interface in Example 3.

[0054] Figure 4 is the leakage current image of Example 2 and Comparative Example 2.

[0055] Figure 5 is the GITT test image of Example 2 and Comparative Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0056] The present invention will be further described below in conjunction with embodiments, but the present invention is not limited to the following embodiments.

[0057] Embodiment 1

[0058] This embodiment provides a method for inducing a Na gradient interface and expanding the Li + transmission channel, as follows:

[0059] In the first step, using NCM613 material as the substrate and NMFP as the composite material, the two are used to prepare a composite cathode material:

[0060] (1) Put the NCM613 material and NMFP into a vacuum drying oven at 120 °C and dry for 24 h;

[0061] (2) Prepare the composite cathode material: Using a high-speed mixer or mechanical fusion method, put 400 g of NCM613 and 12 g of NMFP (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 2 minutes, to obtain the NMFP@NCM613 composite cathode material. As Figure 1 shown, Figure 1 the larger particles in it are NCM613 materials, and there is a layer of small particles on the surface of the material, and the small particles are sodium iron manganese phosphate;

[0062] (3) Put the composite cathode material into a tubular furnace in an argon atmosphere for heat treatment. The heat treatment temperature is 200 °C and the heat treatment time is 3 h.

[0063] In the second step, preparation of the composite cathode electrode sheet:

[0064] (1) Prepare the composite cathode material (3% NMFP@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;

[0065] (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, to uniformly mix the composite cathode material, binder, and conductive agent together to obtain a uniformly mixed slurry;

[0066] (3) Apply the mixed slurry on the aluminum foil and scrape it flat with a 400-thick scraper;

[0067] (4) Place the scraped electrode sheet in a vacuum drying oven at 120 °C and dry for 12 h.

[0068] Step 3: Assembly of the battery:

[0069] (1) Punch the dried electrode sheet into a circular electrode sheet with a diameter of 12 mm using a punching machine;

[0070] (2) Roll press the electrode sheet;

[0071] (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;

[0072] (4) Conduct the battery assembly operation in a glove box. The assembly sequence of the battery is the negative electrode case, spring sheet, 1 mm thick gasket, composite positive electrode sheet, separator, Enkor-ENCHEM electrolyte, lithium sheet, and positive electrode case. Finally, use a sealing machine to compress the battery with a pressure of 50 MPa;

[0073] (5) Activate the assembled battery at a rate of 0.1C. During the cycling process, NMFP will preferentially induce the gradient formation of NaF on the surface of the positive electrode. At the same time, during the first-cycle activation process, Na occupies the position of Li, expanding the migration path of Li + as shown Figure 2 in the figure.

[0074] The composite positive electrode material prepared in this example is as shown Figure 1 in the figure, Figure 1 where the larger particles are NCM613 materials, and there is a layer of small particles on the surface of the material. These small particles are sodium iron manganese phosphate, proving that the NMFP material is uniformly composite on the surface of the NCM613 positive electrode material.

[0075] Its electrochemical performance is shown in Table 1. Its first-cycle discharge specific capacity is 208.3 mAh / g. Under the conditions of 45 °C and a rate of 1C, a cycle test is carried out, and its 300-cycle capacity retention rate is 88.8%, which has been significantly improved compared with Comparative Example 1.

[0076] As shown Figure 2 in the schematic diagram, during the charging process of the first-cycle activation, Li + will be released. At the same time, Na + in NMFP on the surface of the composite positive electrode material or Na + forming a NaF gradient interface will enter the position of Li + on the surface of the positive electrode material. Since the radius of Na is much larger than that of Li, the migration path of Li + is widened, enabling Li + to be better inserted and released.

[0077] Example 2

[0078] This embodiment provides a method for inducing a Na gradient interface and expanding the Li + transmission channel, as follows:

[0079] First step, using NCM811 material as the substrate and NMFP as the composite material, both are used to prepare the composite cathode material:

[0080] (1) Put the NCM811 material and NMFP into a vacuum drying oven at 120 °C and dry for 24 h;

[0081] (2) Prepare the composite cathode material: Using a high-speed mixer or mechanical fusion method, put 400 g of NCM811 and 2 g of NMFP (with a layer of carbon on the surface) into the high-speed mixer. First, premix 3 times at 500 rpm, then 2000 rpm, 4000 rpm, and 6000 rpm once each, with each rotation time being 2 minutes, and the NMFP@NCM811 composite cathode material can be obtained;

[0082] (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.

[0083] Second step, preparation of the composite cathode electrode:

[0084] (1) Prepare the composite cathode material (0.5% NMFP@NCM811), 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;

[0085] (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;

[0086] (3) Apply the mixed slurry on the aluminum foil and scrape it flat with a 400-thick scraper;

[0087] (4) Place the scraped electrode in a vacuum drying oven at 120 °C and dry for 12 h.

[0088] Third step, battery assembly:

[0089] (1) Punch the dried electrode into a circular electrode with a diameter of 12 mm using a punching machine;

[0090] (2) Roll press the electrode;

[0091] (3) Weigh the weight of the rolled 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 = (mass of the electrode - mass of the aluminum foil) * 0.8;

[0092] (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, separator, ENCHEM electrolyte, lithium sheet, and positive electrode case. Finally, use a sealing machine to compress the battery with a pressure of 50 MPa;

[0093] (5) Activate the assembled battery at a rate of 0.2C. During the cycling process, NMFP will induce the gradient formation of NaF on the surface of the positive electrode. At the same time, during the first cycle activation process, Na occupies the position of Li, expanding the migration path of Li+.

[0094] Its electrochemical performance is shown in Table 1. Its first-cycle discharge specific capacity is 206.2 mAh / g. Under the conditions of 45 °C and a rate of 1C, a cycle test is carried out. Its 300-cycle capacity retention rate is 84.7%, which has been significantly improved compared with Comparative Example 2.

[0095] The leakage current test was carried out on the materials described in Example 2 and Comparative Example 2. The results are as Figure 4 shown. The smaller the area enclosed by the curve in the figure, the more stable the electrolyte positive electrode interface. Figure 4 The leakage current of Example 2 and Comparative Example 2 at the 200th cycle was respectively tested. The results show that the leakage current of the material with composite NMFP is smaller, indicating that the positive electrode electrolyte interface of Example 2 is more stable.

[0096] To prove that the present invention can improve the diffusion rate of Li+, GITT tests were carried out on Example 2 and Comparative Example 2. The results are as Figure 5 shown. The light-colored line is the diffusion rate of Li in Example 2. + As can be seen from the figure, the diffusion rate of Li in Example 2 is greater. Therefore, the present invention can expand the diffusion channel of Li by making Na occupy the Li position. + +

[0097] Example 3

[0098] The present embodiment provides a method for inducing a Na gradient interface and expanding the Li + transmission channel, as follows:

[0099] In the first step, using NCM622 material as the substrate and NMFP as the composite material, the two are used to prepare the composite positive electrode material:

[0100] (1) Put the NCM622 material and NMFP into a vacuum drying oven at 120 °C and dry for 12 h;

[0101] (2) Preparation of the composite cathode material: By using a high-speed mixer or mechanical fusion method, 400 g of NCM622 and 2 g of NMFP (with a layer of carbon on the surface) are placed in the high-speed mixer. First, pre-mix at 500 rpm three times, and then mix at 2000 rpm and 4000 rpm once each, with a mixing time of two minutes for each rotation speed, to obtain the NMFP@NCM622 composite cathode material;

[0102] (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 3 h.

[0103] The second step is the preparation of the composite cathode electrode:

[0104] (1) The composite cathode material (0.5% NMFP@NCM622), 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;

[0105] (2) Take the above-configured materials and place them in a degassing machine for mixing. Use the degassing machine to mix the materials three times, each time for 12 min, to uniformly mix the composite cathode material, binder, and conductive agent together to obtain a uniformly mixed slurry;

[0106] (3) The mixed slurry is coated on the aluminum foil and leveled with a 400-thickness scraper;

[0107] (4) The leveled electrode is placed in a vacuum drying oven at 120 °C and dried for 12 h.

[0108] The third step is the assembly of the battery:

[0109] (1) The dried electrode is punched into a circular electrode with a diameter of 12 mm using a punching machine;

[0110] (2) Roll press the electrode;

[0111] (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;

[0112] (4) The battery assembly operation is carried out 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 sheet, and positive electrode shell. Finally, use a sealer to tightly press the battery, with a pressure of 50 MPa;

[0113] (5) Activate the assembled battery at a rate of 0.1C. During the cycling process, NMFP induces the gradient formation of NaF on the surface of the positive electrode. Meanwhile, during the activation process of the first cycle, Na occupies the position of Li, expanding the migration path of Li+.

[0114] Its electrochemical performance is shown in Table 1. Its discharge specific capacity in the first cycle is 203.1 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 3.

[0115] The gradient interface prepared in this example is as Figure 3 shown. Figure 3 To etch the activated material for different times, the etching depth increases from top to bottom. The intensity of the rightmost peak represents the content of NaF. The stronger the peak intensity, the more the content of NaF. Therefore, it can be seen that the content of NaF is less on the outer layer and more on the inner layer, showing a gradient change trend. The formation of this gradient interface can effectively alleviate the erosion of the electrolyte on the positive electrode material.

[0116] Comparative Example 1

[0117] The difference from Example 1 is only that: only NCM613 material is used, and NMFP is not used for compounding.

[0118] A preparation method of a positive electrode sheet slurry is as follows:

[0119] (1) Put the NCM613 material into a vacuum drying oven at 120°C and dry it for 24h;

[0120] (2) Prepare the positive electrode material (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;

[0121] (3) Take the above-prepared materials and put them into a defoamer for mixing. Use the defoamer to mix the slurry three times, 12 minutes each time, to uniformly mix the composite positive electrode material, binder, and conductive agent together to obtain a uniformly mixed slurry.

[0122] A preparation method of a positive electrode sheet is as follows:

[0123] (1) Apply the mixed feed on the aluminum foil and scrape it flat with a 400-thick scraper;

[0124] (2) Place the scraped electrode sheet in a vacuum drying oven at 120°C and dry it for 12h.

[0125] Assembly of the battery:

[0126] (1) Punch the dried electrode sheet into a circular electrode sheet with a diameter of 12 mm using a punching machine;

[0127] (2) Roll press the electrode sheet;

[0128] (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;

[0129] (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, electrolyte, lithium sheet, and positive electrode case. Finally, use a sealing machine to compress the battery with a pressure of 50 MPa;

[0130] (5) Activate the assembled battery at a rate of 0.1C.

[0131] Comparative Example 2

[0132] The difference from Example 2 is only that: only NCM822 material is used, and NMFP is not used for compounding.

[0133] A method for preparing a positive electrode sheet slurry is as follows:

[0134] (1) Put the NCM613 material into a vacuum drying oven at 120 °C and dry it for 24 h;

[0135] (2) Prepare the positive electrode material (NCM613), binder (PVDF), and conductive agent (C) in a ratio of 8:1:1 and put them into a mixing tank. 250 mg of composite positive electrode material, 31.25 mg of PVDF, and 31.25 mg of C can be taken;

[0136] (3) Take the above-prepared materials and put them into a degassing machine for mixing. Use the degassing machine to mix the slurry three times, 12 min each time, so that the composite positive electrode material, binder, and conductive agent are evenly mixed together to obtain a uniformly mixed slurry.

[0137] A method for preparing a positive electrode sheet is as follows:

[0138] (1) Apply the mixed slurry on the aluminum foil and scrape it flat with a 400-thick scraper;

[0139] (2) Place the scraped electrode sheet in a vacuum drying oven at 120 °C and dry it for 12 h.

[0140] Battery assembly:

[0141] (1) Punch the dried electrode sheet into a circular electrode sheet with a diameter of 12 mm using a punching machine;

[0142] (2) Roll press the electrode sheet;

[0143] (3) Weigh the weight of 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 = (the mass of the electrode sheet - the mass of the aluminum foil) * 0.8;

[0144] (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, electrolyte, lithium sheet, and positive electrode case. Finally, use a sealing machine to press the battery tightly with a pressure of 50 MPa;

[0145] (5) Activate the assembled battery at a rate of 0.1c.

[0146] Comparative Example 3

[0147] The difference from Example 3 is only that: only NCM622 material is used and NMFP is not used for compounding.

[0148] A method for preparing a positive electrode sheet slurry is as follows:

[0149] (1) Put the NCM622 material into a vacuum drying oven at 120 °C and dry it for 12 h;

[0150] (2) Prepare the positive electrode material (NCM622), binder (PVDF), and conductive agent (C) in a ratio of 8:1:1 and put them into a mixing tank. 250 mg of composite positive electrode material, 31.25 mg of PVDF, and 31.25 mg of C can be taken;

[0151] (3) Take the above-prepared materials and put them into a degassing machine for mixing. Use the degassing machine to mix the slurry three times, 12 min each time, to uniformly mix the composite positive electrode material, binder, and conductive agent together to obtain a uniformly mixed slurry.

[0152] A method for preparing a positive electrode sheet is as follows:

[0153] (1) Apply the mixed slurry on the aluminum foil and scrape it flat with a 400-thick scraper;

[0154] (2) Place the scraped electrode sheet in a vacuum drying oven at 120 °C and dry it for 12 h.

[0155] Battery assembly:

[0156] (1) Punch the dried electrode sheet into a circular electrode sheet with a diameter of 10 mm using a punching machine;

[0157] (2) Roll the electrode sheet;

[0158] (3) Weigh the weight of the rolled 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 = (mass of the electrode - mass of the aluminum foil) * 0.8;

[0159] (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, separator, electrolyte, lithium piece, and positive electrode case. Finally, use a sealing machine to compress the battery with a pressure of 50 MPa;

[0160] (5) Activate the assembled battery at a rate of 0.1C.

[0161] Table 1. Comparison of battery performance of button cells assembled with different modified samples

[0162]

[0163] Example 1 is to construct a gradient interface by compounding 3% NMFP with NCM613 and heat-treating it at 200 °C for 3 h, and during the first-cycle activation process, the Li + transmission channel is expanded. Example 2 is to construct a CEI layer by compounding 0.5% NMFP with NCM811 material, and during the first-cycle activation process, the Li + transmission channel is expanded. Example 3 is to construct a CEI layer by compounding 0.5% NMFP with NCM622, and during the first-cycle activation process, the Li + transmission channel is expanded. The only difference between Comparative Examples 1-3 and Examples 1-3 is that the comparative examples only use transition metal oxides and do not use phosphates for compounding.

[0164] The foregoing basic examples of the present invention and their respective further selected 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 selected example can be arbitrarily combined with any basic example and selected example. Those skilled in the art 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, and improvements made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing a composite cathode material having a Na gradient interface, characterized in that: The preparation method comprises: drying, mixing, heat treating and electrochemically activating the sodium phosphate salt and the transition metal oxide positive electrode material in sequence to obtain the composite positive electrode material with a Na gradient interface.

2. The preparation method according to claim 1, 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.

3. The preparation method according to claim 1, characterized in that: The electrochemical activation treatment is to prepare a positive electrode from a sodium phosphate and a transition metal oxide positive electrode material 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.

4. The preparation method according to claim 1, 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 positive electrode material and the sodium 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 sodium phosphate are placed in a high-speed mixer by means of a high-speed mixer or mechanical fusion, and premixed three times at 500 rpm, then once at 2000 rpm and once at 4000 rpm, and each speed is 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, F-containing electrolyte, lithium sheet, positive electrode shell, and finally the battery is pressed tightly 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 Na gradient interface on the positive electrode surface.

5. A composite cathode material having a Na 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 sodium phosphate and sodium salt, the sodium salt includes NaF, the F in the NaF comes from the decomposition product of the electrolyte, and the content of the NaF shows a gradient decreasing trend in the direction extending from the surface to the inside.

6. A composite cathode material with a Na gradient interface according to claim 5, characterized in that: The CEI coating layer also contains a polymer; the gradient interface can be composed of sodium phosphate, polymer and sodium salt.

7. A composite cathode material with a Na gradient interface according to claim 5 or 6, characterized in that: The thickness of the CEI coating layer is 1 to 500 nm.

8. A composite cathode material with a Na gradient interface according to claim 5 or 6, 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 to 5 nm. The phosphate-based substance includes any one of sodium iron phosphate, sodium manganese phosphate, sodium ferromanganese phosphate, sodium vanadium phosphate, sodium vanadium phosphate, sodium cobalt phosphate, and sodium nickel phosphate, or a combination of at least two thereof, preferably sodium ferromanganese phosphate.

9. A composite cathode material with a Na gradient interface according to claim 5 or 6, 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%.

10. The use of the composite cathode material with a Na gradient interface according to any one of claims 6 to 9, characterized in that: The composite positive electrode material is used for preparing lithium ion batteries.

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