A coconut shell-based foam carbon modified phosphate cathode material and its application

Abstract

The present invention discloses a coconut shell-based foamed carbon modified phosphate positive electrode material and its application. The positive electrode material comprises a phosphate positive electrode core and a coconut shell-based foamed carbon coated on the surface of the phosphate positive electrode core. The present invention coats the phosphate positive electrode core with a lightweight sponge-like structure of coconut shell-based foamed carbon, which can form a three-dimensional conductive network between the positive electrode material particles, thereby improving the electronic conductivity between the phosphate positive electrode material particles and accelerating the Li + / electron transfer, reducing electrode polarization effect and lowering the resistivity of the electrode. In addition, the carbon foam coated on the surface of the positive electrode particles can inhibit particle agglomeration and improve the Li + Diffusion dynamics greatly improves the rate performance of lithium-ion batteries. In addition, the coconut shell-based foam carbon on the surface of the main material can effectively inhibit the chemical reaction between the phosphate positive electrode material and the electrolyte, forming a stable interface, thereby increasing the cycle life of the battery and making the battery exhibit excellent electrochemical performance.

Description

Technical Field

[0001] The invention belongs to the technical field of batteries and relates to a coconut shell-based foam carbon modified phosphate positive electrode material and application thereof. Background Art

[0002] To meet the growing demand for environmentally friendly, efficient, and sustainable energy, the development of higher-performance lithium-ion batteries is gaining increasing attention. Since the discovery of olivine-structured LiMPO4 (M = at least one of Fe, Mn, Co, and Ni), a remarkable number of achievements have emerged in this field in recent years. Inspired by the successful commercialization of LiFePO4 (hereinafter referred to as LFP), LiMnPO4 has been extensively studied due to its higher electrode potential. Compared to LFP, LiMnPO4 has become one of the most promising cathode materials for power batteries due to its high theoretical capacity, high voltage platform, low cost, and environmental friendliness.

[0003] As one of the most promising cathode active materials, LiMnPO4 has attracted much attention due to its high energy density, good safety performance and low cost. However, the extremely low electronic conductivity of LiMnPO4 (2 to 3 orders of magnitude lower than that of LFP) and Li + Intrinsic problems such as slow diffusion of manganese ions have become the main factors limiting its development, resulting in poor rate performance of LiMnPO4 batteries. Moreover, during the electrochemical reaction, some manganese ions undergo disproportionation and dissolve in the electrolyte, resulting in poor battery cycle performance.

[0004] In order to improve the electrochemical performance of LiMnPO4, the LiMnPO4 particles can be refined to nanoscale to improve the + The kinetics of diffusion can be improved, thereby improving its rate performance. In addition, surface modification can prevent direct contact between LiMnPO4 and the electrolyte, inhibiting side reactions and thus extending its cycle life.

[0005] CN118136813A discloses a coated lithium iron manganese phosphate material, a preparation method thereof, and an application thereof. The coated lithium iron manganese phosphate material comprises lithium iron manganese phosphate nanoparticles, the surfaces of which are coated with fluorine-doped amorphous carbon and graphene. By coating the surfaces of the lithium iron manganese phosphate nanoparticles with fluorine-doped amorphous carbon and graphene, a "point-surface" combined dense contact conductive network is constructed. At the same time, the nanosized particles shorten the lithium ion migration path, thereby greatly improving the ion transmission rate of the material, making its structure stable and non-collapsed during repeated charge and discharge, and effectively alleviating the problem of manganese dissolution, thereby having both excellent structural stability and conductive properties. When applied to lithium-ion batteries, the cycle performance and rate performance of the lithium-ion battery can be effectively improved.

[0006] CN116706020A discloses a lithium iron manganese phosphate cathode material and its preparation method. The lithium iron manganese phosphate cathode material includes an ionic conductor LiAlO₂ bulk coating layer and a LaF₃ / carbon surface coating layer. The mass fraction of the ionic conductor LiAlO₂ bulk coating layer is 0.5% - 2%, the mass fraction of the carbon surface coating layer is 0.5% - 2%, and the mass fraction of the LaF₃ surface coating layer is 1% - 3%. By forming a carbon coating layer and a LaF₃ coating layer on the surface of the material, this patent can improve the electronic conductivity of the material, inhibit the side reaction between the electrolyte and the material, inhibit the dissolution of transition metals in the material, and inhibit the increase in impedance during the battery cycle.

[0007] CN118738326A discloses a lithium iron manganese phosphate co-coated with nano-scale lithium phosphate and a carbon layer, and its preparation and application. The preparation method includes the following steps: Step 1, weigh a lithium iron manganese phosphate LiMn x Fe 1-x PO₄ precursor, 1% - 8% of nano-scale lithium phosphate, 10% - 20% of a carbon source, and 1% - 5% of an antioxidant by mass percentage of the lithium iron manganese phosphate, and mix them evenly, where 0 < x < 1; Step 2, sinter the evenly mixed material obtained in Step 1 at 600°C - 800°C in an inert atmosphere, keep it warm for 4h - 12h, and then naturally cool to room temperature to obtain the lithium iron manganese phosphate co-coated with lithium phosphate and a carbon layer. This method combines a simple ball milling process with solid-phase sintering synthesis. By utilizing the complementary structural properties of nano-scale lithium phosphate and the carbon layer, the coating layer is made more complete to better protect the lithium iron manganese phosphate material, thereby avoiding contact with the electrolyte during its application and reducing side reactions; at the same time, a good coating effect can reduce the lattice distortion caused by the Jahn-Teller effect induced by Mn 3+ , and thus relieve the stress generated during the phase change of the cathode material during charge and discharge, improve the structural stability of the cathode material, and thereby improve the cycle stability and service life when applied in a battery; moreover, due to the good stability and ionic conductivity of lithium phosphate, the ionic and electronic conductivities of the cathode material are improved, and the lithium ion migration rate is increased.

[0008] However, when carbon coating is performed on phosphate cathode materials such as lithium iron manganese phosphate, carbon materials such as graphene and carbon nanotubes are often used. Their preparation cost is high, and it is easy to cause the problem of uneven coating, which limits the performance improvement and application of phosphate cathode materials. Summary of the Invention

[0009] Aiming at the above technical problems existing in the prior art, the purpose of the present invention is to provide a coconut shell-based foam carbon modified phosphate cathode material and its application.

[0010] To achieve the above object, the present invention adopts the following technical solutions:

[0011] In a first aspect, the present invention provides a coconut shell-based foamed carbon modified phosphate positive electrode material, wherein the positive electrode material comprises a phosphate positive electrode core and a coconut shell-based foamed carbon coated on the surface of the phosphate positive electrode core.

[0012] In the present invention, coconut shell-based foamed carbon refers to a foamed carbon material prepared from coconut shell as a raw material, which has the advantages of low density, high strength, electrical conductivity, thermal conductivity, thermal stability, chemical stability, etc. The present invention uses a lightweight sponge-like structure of coconut shell-based foamed carbon to coat the phosphate positive electrode core, which can form a three-dimensional conductive network between the positive electrode material particles, thereby improving the electronic conductivity between the phosphate positive electrode material (such as LiMnPO4) particles and accelerating the Li + / electron transfer, reducing electrode polarization effect and lowering the resistivity of the electrode. In addition, the carbon foam coated on the surface of the positive electrode particles can inhibit particle agglomeration and improve the Li + Diffusion dynamics greatly improves the rate performance of lithium-ion batteries. In addition, the coconut shell-based foam carbon on the surface of the main material can effectively inhibit the chemical reaction between the phosphate positive electrode material and the electrolyte, forming a stable interface, thereby increasing the cycle life of the battery and making the battery exhibit excellent electrochemical performance.

[0013] The following are preferred technical solutions of the present invention, but are not intended to limit the technical solutions provided by the present invention. Through the following preferred technical solutions, the technical objectives and beneficial effects of the present invention can be better achieved and realized.

[0014] Preferably, based on the total mass of the positive electrode material as 100%, the mass fraction of the coconut shell-based foamed carbon is 2%-3%, for example, it can be 2%, 2.1%, 2.2%, 2.3%, 2.5%, 2.6%, 2.7%, 2.8%, 2.9% or 3%, etc.

[0015] In a second aspect, the present invention provides a method for preparing the coconut shell-based foam carbon modified phosphate cathode material as described in the first aspect, the preparation method comprising the following steps:

[0016] (1) preparing coconut shell-based foamed carbon;

[0017] (2) The phosphate positive electrode material is formed into a core slurry, and the coconut shell-based foamed carbon is formed into a shell slurry. The core slurry and the shell slurry are coaxially electrospinned to obtain the coconut shell-based foamed carbon modified phosphate positive electrode material.

[0018] The method of the present invention uses coconut shell as a raw material to prepare coconut shell-based foamed carbon with a sponge-like structure as a carbon source. Coconut shell is widely available and can greatly reduce production costs. Moreover, the morphology and performance of the coconut shell-based foamed carbon with a sponge-like structure are very conducive to the coating of phosphate positive electrode materials. The core-shell structure is prepared by coaxial electrospinning, and the coconut shell-based foamed carbon is tightly and evenly coated on the surface of the phosphate positive electrode core. Compared with the method of directly mixing the carbon source with the phosphate positive electrode material and then sintering in the prior art, the method has the following advantages: the carbon coating layer is more uniform and complete, which is conducive to improving the processability of the positive electrode slurry, as well as the rate performance and cycle life of the battery.

[0019] The method of the invention is environmentally friendly, has a simple preparation process, a short preparation cycle, low preparation cost, and is suitable for industrial production.

[0020] As a preferred technical solution of the preparation method of the present invention, the preparation method of coconut shell-based foamed carbon in step (1) comprises the following steps:

[0021] (a) coconut shell is pre-treated to obtain coconut shell powder;

[0022] (b) after coconut shell powder is mixed with aldehyde substance and concentrated acid, obtain mixed slurry, described mixed slurry is placed in reactor, is filled with protective gas and is heated to react, obtains coconut shell resin after foaming;

[0023] (c) heat-treating the foamed coconut shell resin to obtain coconut shell-based foamed charcoal.

[0024] In the method of the present invention, aldehyde substances serve as carbon sources, and concentrated acid can play an activation role, promoting the cracking and activation of foamed carbon, thereby generating more micropores and mesopores, increasing the porosity of the material, and removing inorganic impurities in the foamed carbon, thereby improving the purity of the foamed carbon.

[0025] In the present invention, concentrated acid refers to an acid having a molar concentration greater than or equal to 10 mol / L, for example, 10 mol / L, 10.5 mol / L, 11 mol / L, 12 mol / L, 13 mol / L, 14 mol / L, 15 mol / L or 16 mol / L, etc. Preferably, the pretreatment in step (a) is crushing and screening.

[0026] Preferably, in step (b), the aldehyde substance includes at least one of formaldehyde, octanal, nonanal, decanal and undecanal.

[0027] Preferably, in step (b), the mass ratio of the coconut shell powder to the aldehyde substance is (5-15):1, for example, it can be 5:1, 5.5:1, 6:1, 6.5:1, 7:1, 7.5:1, 8:1, 8.5:1, 9:1, 9.5:1, 10:1, 10.5:1, 11:1, 11.5:1, 12:1, 12.5:1, 13:1, 13.5:1, 14:1 or 15:1, etc.

[0028] Preferably, the concentrated acid in step (b) comprises at least one of concentrated sulfuric acid, concentrated sulfuric acid and concentrated nitric acid.

[0029] Preferably, the concentration of the mixed slurry in step (b) is 1.5 mol / L-2 mol / L, for example, it can be 1.5 mol / L, 1.6 mol / L, 1.7 mol / L, 1.8 mol / L, 1.9 mol / L or 2 mol / L.

[0030] Preferably, the heating reaction in step (b) includes two stages, wherein the first stage is: heating to a first temperature and keeping it warm; the second stage is: continuing to heat to a second temperature and keeping it warm.

[0031] The purpose of carrying out hydrothermal treatment in two stages is to make the foaming more uniform, the particle size of the prepared foamed carbon more uniform, and to obtain foamed carbon of higher purity.

[0032] Preferably, the first temperature is 300°C-400°C, for example, it can be 300°C, 320°C, 340°C, 350°C, 360°C, 380°C or 400°C; the insulation time at the first temperature is 12h-36h, for example, it can be 12h, 13h, 14h, 15h, 16h, 17h, 18h, 19h, 20h, 21h, 22h, 23h, 24h, 25h, 26h, 27h, 28h, 29h, 30h, 31h, 32h, 33h, 34h, 35h or 36h, etc.

[0033] Preferably, the second temperature is 500°C-650°C, for example, it can be 500°C, 525°C, 550°C, 560°C, 570°C, 580°C, 600°C, 620°C, 630°C or 650°C; the insulation time at the second temperature is 5h-10h, for example, it can be 5h, 6h, 7h, 8h, 9h or 10h.

[0034] Preferably, the pressure of the heating reaction in step (b) is 1 MPa-5 MPa, for example, it can be 1 MPa, 1.5 MPa, 2 MPa, 2.5 MPa, 3 MPa, 3.5 MPa, 4 MPa, 4.5 MPa or 5 MPa.

[0035] Preferably, the temperature of the heat treatment in step (c) is 700°C-800°C, for example, it can be 700°C, 720°C, 740°C, 760°C, 780°C or 800°C.

[0036] Preferably, the heat treatment time in step (c) is 3 h to 6 h, for example, 3 h, 3.5 h, 4 h, 4.5 h, 5 h, 5.5 h or 6 h.

[0037] In the present invention, the preparation method of the phosphate positive electrode material is not specifically limited, and those skilled in the art can prepare it according to the method disclosed in the prior art, for example, it can be a sol-gel-casting method.

[0038] As a preferred technical solution of the preparation method of the present invention, the core slurry in step (2) is obtained by dispersing a phosphate cathode material and a first surfactant in an organic solvent. The addition of the surfactant is beneficial for enhancing the bonding effect between the phosphate cathode material and the coconut shell-based foamed carbon in subsequent steps.

[0039] Preferably, the first surfactant comprises at least one of polyurethane and hexadecyltrimethylammonium bromide (CTAB).

[0040] Preferably, the mass ratio of the phosphate positive electrode material to the first surfactant is 90:(0.5-1.5), for example, it can be 90:0.5, 90:0.6, 90:0.7, 90:0.8, 90:1, 90:1.2, 90:1.3, 90:1.4 or 90:1.5, etc.; the concentration of the core slurry is 6mol / L-8mol / L, for example, it can be 6mol / L, 6.5mol / L, 7mol / L, 7.5mol / L or 8mol / L, etc.

[0041] Preferably, the shell slurry is obtained by dispersing coconut shell-based foamed carbon and a second surfactant in an organic solvent. The addition of the surfactant is beneficial for improving the bonding effect between the phosphate positive electrode material and the coconut shell-based foamed carbon in subsequent steps.

[0042] Preferably, the second surfactant comprises at least one of polyurethane and CTAB.

[0043] In the present invention, the "first" and "second" in "first surfactant" and "second surfactant" have no substantial meaning and are merely used to distinguish names. The first surfactant and the second surfactant may be of the same or different types.

[0044] Preferably, the mass ratio of the coconut shell-based foamed carbon and the second surfactant is 80:(0.5-1.5), for example, it can be 80:0.5, 80:0.6, 80:0.7, 80:0.8, 80:0.9, 80:1, 80:1.1, 80:1.2, 80:1.3, 80:1.4 or 80:1.5, etc.; the concentration of the shell slurry is 1.5mol / L-2mol / L, for example, it can be 1.5mol / L, 1.6mol / L, 1.7mol / L, 1.8mol / L, 1.9mol / L or 2mol / L, etc.

[0045] By regulating the composition of the core slurry and the shell slurry, the present invention can make the carbon coating of the phosphate positive electrode material more uniform and complete, which is beneficial to improving the processability of the positive electrode slurry, as well as the battery's rate performance, cycle life and other properties.

[0046] Preferably, the voltage of the coaxial electrospinning in step (2) is 20 kV-40 kV, for example, it can be 20 kV, 22 kV, 23 kV, 25 kV, 26 kV, 28 kV, 30 kV, 32 kV, 34 kV, 36 kV, 38 kV or 40 kV.

[0047] Preferably, the spinning rate of the coaxial electrospinning in step (2) is 5 mL / min-15 mL / min, for example, it can be 5 mL / min, 6 mL / min, 8 mL / min, 9 mL / min, 10 mL / min, 12 mL / min, 13 mL / min or 15 mL / min.

[0048] Preferably, the method further comprises the steps of washing and spray drying the product after the electrospinning in step (2).

[0049] In a third aspect, the present invention provides a positive electrode slurry, wherein the positive electrode slurry is obtained by homogenizing the coconut shell-based carbon foam modified phosphate positive electrode material described in the first aspect. Since the positive electrode slurry is prepared using the positive electrode material of the first aspect, the positive electrode slurry has better processing performance.

[0050] In a fourth aspect, the present invention provides a method for homogenizing the positive electrode slurry according to the third aspect, the homogenization method comprising the following steps:

[0051] After mixing the coconut shell-based foam carbon modified phosphate positive electrode material with a conductive agent and a binder, an organic solvent is added at least three times, and stirring is performed after each addition of the organic solvent to obtain a positive electrode slurry;

[0052] The amount of organic solvent added in the latter step is greater than that in the previous step.

[0053] By optimizing the homogenization process, this invention effectively increases the solid content of the slurry and significantly reduces the use of organic solvents (such as NMP), thereby achieving the goal of cost reduction. At the same time, this process improves the processability and dispersibility of the slurry, effectively reducing the resistivity of the electrode and the aging internal resistance of the battery, which is beneficial to battery performance.

[0054] In a fifth aspect, the present invention provides a positive electrode, which is prepared using the positive electrode slurry described in the third aspect.

[0055] In a sixth aspect, the present invention provides a battery comprising the positive electrode described in the fifth aspect.

[0056] The numerical range described in the present invention includes not only the point values ​​listed above, but also any point values ​​between the above numerical ranges that are not listed. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0057] Compared with the prior art, the present invention has the following beneficial effects:

[0058] (1) The present invention coats the phosphate positive electrode core with a lightweight, sponge-like structure of coconut shell-based foamed carbon, which can form a three-dimensional conductive network between the positive electrode material particles, thereby improving the electronic conductivity between the particles of the phosphate positive electrode material (such as LiMnPO4), accelerating the transfer of Li+ / electrons, reducing the electrode polarization effect, and lowering the resistivity of the electrode sheet. In addition, the foamed carbon coated on the surface of the positive electrode particles can inhibit particle agglomeration, improve the Li+ diffusion kinetics, and greatly improve the rate performance of the lithium-ion battery. Moreover, the coconut shell-based foamed carbon on the surface of the main material can effectively inhibit the chemical reaction between the phosphate positive electrode material and the electrolyte, forming a stable interface, thereby improving the cycle life of the battery and making the battery exhibit excellent electrochemical performance.

[0059] (2) The method of the present invention uses coconut shell as a raw material to prepare a carbon source. Coconut shell is widely available and can greatly reduce production costs. Moreover, the morphology and performance of the coconut shell-based foamed carbon with a sponge-like structure are very conducive to the coating of phosphate positive electrode materials. By utilizing coaxial electrospinning to prepare a core-shell structure, the coconut shell-based foamed carbon is tightly and evenly coated on the surface of the phosphate positive electrode core. Compared with the method of directly mixing the carbon source with the phosphate positive electrode material and then sintering it in the prior art, the method has the following advantages: the carbon coating layer is more uniform and complete, which is conducive to improving the processability of the positive electrode slurry, as well as the rate performance and cycle life of the battery.

[0060] (3) The method of the present invention is environmentally friendly, has a simple preparation process, a short preparation cycle, low preparation cost, and is suitable for industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] Figure 1 Schematic diagram of the structure of a coconut shell-based foamed carbon modified LiMnPO4 material provided in one embodiment of the present invention, wherein 1 is the LiMnPO4 core and 2 is the coconut shell-based foamed carbon. DETAILED DESCRIPTION

[0062] The technical solution of the present invention will be further described below with reference to the accompanying drawings and through specific implementation methods.

[0063] The specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0064] Example 1

[0065] This embodiment provides a coconut shell-based foam carbon modified LiMnPO4 material (referred to as LiMnPO4@coconut shell-based foam carbon composite material), the structural diagram of which is shown in FIG. Figure 1 , including a LiMnPO4 core 1 and a coconut shell-based foam carbon 2 coated on the surface of the LiMnPO4 core 1. Based on the total mass of the coconut shell-based foam carbon modified LiMnPO4 material as 100%, the mass fraction of the coconut shell-based foam carbon 2 is 2.5%.

[0066] This embodiment also provides a method for preparing the above-mentioned coconut shell-based foam carbon modified LiMnPO4 material, comprising the following steps:

[0067] (1) LiMnPO4 was prepared by sol-gel-casting method. The specific preparation method is as follows:

[0068] ① Stir and mix 1 mol of Li2CO3, 2 mol of MnCl2, 2 mol of (NH4)3PO4 and an appropriate amount of dichloromethane, wherein the mass of dichloromethane is twice the sum of the masses of Li2CO3, MnCl2 and (NH4)3PO4. Then add 1 mmol of benzalkonium chloride and stir the mixture in an 80°C water bath for 4 hours at a stirring speed of 600 rpm / min. After stirring, cool it to room temperature and evenly apply it on a glass slide with a rubber-tipped dropper.

[0069] ② Move the coated slide into a vacuum drying oven and keep it at 100℃ for 10 hours;

[0070] ③ Peel the product obtained in ② off the glass slide and grind it;

[0071] ④ The ground powder was sieved with a 250-mesh sieve. The sieved powder was placed in a tube furnace and heated to 800°C at a heating rate of 10°C / min under a nitrogen atmosphere and kept warm for 6 hours. After the insulation, it was cooled to room temperature to obtain LiMnPO4.

[0072] (2) Preparation of coconut shell-based foamed carbon, the specific preparation method is:

[0073] ① The coconut shell was crushed and sieved (250 mesh sieve), and then the sieved powder and formaldehyde were placed in concentrated phosphoric acid (concentration 14.7 mol / L) at a mass ratio of 10:1 to obtain a mixed slurry. The concentration of the mixed slurry was 1.5 mol / L. The mixture was stirred evenly and then placed in an autoclave, filled with nitrogen protective atmosphere, heated to 350 ° C for 24 h, and the gas pressure in the autoclave was 2 MPa;

[0074] ② The product obtained in ① was placed in a high-temperature and high-pressure reactor, filled with nitrogen protective atmosphere, heated to 600°C and kept warm for 8 hours. The gas pressure in the high-pressure reactor was 2MPa. After the reaction was completed, it was cooled to room temperature to obtain foamed coconut shell resin;

[0075] ③ The product obtained in step ② was placed in a tubular furnace, nitrogen was introduced for protection, heated to 700°C and kept warm for 4 hours, the gas pressure in the high-pressure reactor was 2 MPa, and then naturally cooled to room temperature to obtain coconut shell-based foamed carbon.

[0076] (3) LiMnPO4@coconut shell-based foam carbon composites were prepared by coaxial electrospinning. The specific preparation method is as follows:

[0077] ① Ultrasonically uniformly disperse the coconut shell-based foamed carbon and polyurethane (the mass ratio of coconut shell-based foamed carbon to polyurethane is 80:1) obtained in step (2) in an ethylene glycol monomethyl ether solution to obtain a shell slurry with a concentration of 2 mol / L;

[0078] ② Ultrasonic dispersion of the LiMnPO4 nanoparticles and polyurethane (the mass ratio of LiMnPO4 nanoparticles to polyurethane is 90:1) obtained in step (1) in an ethylene glycol monomethyl ether solution to obtain a core slurry with a concentration of 7 mol / L;

[0079] ③ The electrospinning solutions (i.e., the shell slurry and the core slurry) obtained in steps ① and ② were placed in the two feeding ports of the electrospinning machine, respectively. The shell slurry and the core slurry flowed out from different channels of the coaxial nozzle, respectively. Under the action of a high-voltage electric field (voltage of 30 kV), the spinning rate was 10 mL / min, and the two solutions spontaneously formed particles at the same time, thereby forming a core-shell structure composite material;

[0080] ④ The product obtained in step ③ was washed with deionized water and anhydrous ethanol for 6 times, and spray-dried to obtain a LiMnPO4@coconut shell-based foam carbon composite material, that is, a coconut shell-based foam carbon-modified LiMnPO4 material.

[0081] Example 2

[0082] This embodiment provides a coconut shell-based foam carbon modified LiMnPO4 material (referred to as LiMnPO4@coconut shell-based foam carbon composite material), including a LiMnPO4 core and a coconut shell-based foam carbon coated on the surface of the LiMnPO4 core. Based on the total mass of the coconut shell-based foam carbon modified LiMnPO4 material as 100%, the mass fraction of the coconut shell-based foam carbon is 2.5%.

[0083] This embodiment also provides a method for preparing the above-mentioned coconut shell-based foam carbon modified LiMnPO4 material, comprising the following steps:

[0084] (1) LiMnPO4 was prepared by the sol-gel-casting method, the preparation method was the same as that in Example 1.

[0085] (2) Preparation of coconut shell-based foamed carbon, the specific preparation method is:

[0086] ① The coconut shell was crushed and sieved (250 mesh sieve), and then the sieved powder and formaldehyde were placed in concentrated phosphoric acid (concentration 14.7 mol / L) at a mass ratio of 5:1 to obtain a mixed slurry. The concentration of the mixed slurry was 2 mol / L. After stirring, it was placed in an autoclave filled with nitrogen protective atmosphere, heated to 300 ° C for 36 h, and the gas pressure in the autoclave was 2 MPa;

[0087] ② Place the product obtained in ① into a high-temperature and high-pressure reactor, fill it with nitrogen protective atmosphere, heat it to 650℃ and keep it for 5 hours. The gas pressure in the high-pressure reactor is 2MPa. After the reaction is completed, cool it to room temperature to obtain foamed coconut shell resin;

[0088] ③ The product obtained in step ② was placed in a tubular furnace, nitrogen was introduced for protection, and the temperature was heated to 750°C and kept warm for 4 hours. The gas pressure in the high-pressure reactor was 2 MPa, and then naturally cooled to room temperature to obtain coconut shell-based foamed carbon.

[0089] (3) LiMnPO4@coconut shell-based foam carbon composites were prepared by coaxial electrospinning. The specific preparation method is as follows:

[0090] ① Ultrasonically uniformly disperse the coconut shell-based foamed carbon and polyurethane (the mass ratio of coconut shell-based foamed carbon to polyurethane is 80:1.5) obtained in step (2) in an ethylene glycol monomethyl ether solution to obtain a shell slurry with a concentration of 1.5 mol / L;

[0091] ② The LiMnPO4 nanoparticles and polyurethane obtained in step (1) (the mass ratio of LiMnPO4 nanoparticles to polyurethane is 90:1.5) are ultrasonically dispersed in an ethylene glycol monomethyl ether solution to obtain a core slurry with a concentration of 6 mol / L;

[0092] ③ The electrospinning solutions (i.e., the shell slurry and the core slurry) obtained in steps ① and ② were placed in the two feeding ports of the electrospinning machine, respectively. The shell slurry and the core slurry flowed out from different channels of the coaxial nozzle, respectively. Under the action of a high-voltage electric field (voltage of 20 kV), the spinning rate was 5 mL / min, and the two solutions spontaneously formed particles at the same time, thereby forming a core-shell structure composite material;

[0093] ④ The product obtained in step ③ was washed with deionized water and anhydrous ethanol for 6 times, and spray-dried to obtain a LiMnPO4@coconut shell-based foam carbon composite material, that is, a coconut shell-based foam carbon-modified LiMnPO4 material.

[0094] Example 3

[0095] This embodiment provides a coconut shell-based foam carbon modified LiMnPO4 material (referred to as LiMnPO4@coconut shell-based foam carbon composite material), including a LiMnPO4 core and a coconut shell-based foam carbon coated on the surface of the LiMnPO4 core. Based on the total mass of the coconut shell-based foam carbon modified LiMnPO4 material as 100%, the mass fraction of the coconut shell-based foam carbon is 2.5%.

[0096] This embodiment also provides a method for preparing the above-mentioned coconut shell-based foam carbon modified LiMnPO4 material, comprising the following steps:

[0097] (1) LiMnPO4 was prepared by the sol-gel-casting method, the preparation method was the same as that in Example 1.

[0098] (2) Preparation of coconut shell-based foamed carbon, the specific preparation method is:

[0099] ① The coconut shell was crushed and sieved (250 mesh sieve), and then the sieved powder and formaldehyde were placed in concentrated phosphoric acid (concentration 14.7 mol / L) at a mass ratio of 15:1 to obtain a mixed slurry. The concentration of the mixed slurry was 1.7 mol / L. After stirring, it was placed in an autoclave filled with nitrogen protective atmosphere, heated to 400 ° C for 12 h, and the gas pressure in the autoclave was 2 MPa;

[0100] ② The product obtained in ① was placed in a high-temperature and high-pressure reactor, filled with nitrogen protective atmosphere, heated to 500°C and kept warm for 10 hours. The gas pressure in the high-pressure reactor was 2 MPa. After the reaction was completed, it was cooled to room temperature to obtain foamed coconut shell resin;

[0101] ③ The product obtained in step ② was placed in a tubular furnace, nitrogen was introduced for protection, heated to 800°C and kept warm for 3 hours, and then naturally cooled to room temperature to obtain coconut shell-based foamed carbon.

[0102] (3) LiMnPO4@coconut shell-based foam carbon composites were prepared by coaxial electrospinning. The specific preparation method is as follows:

[0103] ① Ultrasonically uniformly disperse the coconut shell-based foamed carbon and polyurethane (the mass ratio of coconut shell-based foamed carbon to polyurethane is 80:0.5) obtained in step (2) in an ethylene glycol monomethyl ether solution to obtain a shell slurry with a concentration of 1.8 mol / L;

[0104] ② The LiMnPO4 nanoparticles and polyurethane obtained in step (1) (the mass ratio of LiMnPO4 nanoparticles to polyurethane is 90:0.5) are ultrasonically dispersed in an ethylene glycol monomethyl ether solution to obtain a core slurry with a concentration of 6.5 mol / L;

[0105] ③ The electrospinning solutions (i.e., the shell slurry and the core slurry) obtained in steps ① and ② were placed in the two feeding ports of the electrospinning machine, respectively. The shell slurry and the core slurry flowed out from different channels of the coaxial nozzle, respectively. Under the action of a high-voltage electric field (voltage of 40 kV), the spinning rate was 15 mL / min, and the two solutions spontaneously formed particles at the same time, thereby forming a core-shell structure composite material;

[0106] ④ The product obtained in step ③ was washed with deionized water and anhydrous ethanol 5 times, and spray-dried to obtain a LiMnPO4@coconut shell-based foam carbon composite material, that is, a coconut shell-based foam carbon-modified LiMnPO4 material.

[0107] Example 4

[0108] This embodiment provides a coconut shell-based foam carbon modified LiMnPO4 material and a preparation method thereof. The difference between the preparation method and Example 1 is that by changing the concentration of the shell slurry in step (3), the mass fraction of the coconut shell-based foam carbon in the coconut shell-based foam carbon modified LiMnPO4 material is 1.2%.

[0109] Example 5

[0110] This embodiment provides a coconut shell-based foamed carbon modified LiMnPO4 material and a preparation method thereof. The difference between the preparation method and that of Example 1 is that the mass fraction of the coconut shell-based foamed carbon in the coconut shell-based foamed carbon modified LiMnPO4 material is 3.6% by changing the concentration of the shell slurry in step (3).

[0111] Example 6

[0112] This embodiment provides a coconut shell-based foam carbon modified LiMnPO4 material and a preparation method thereof. The difference between the preparation method and that of Example 1 is that in step (2), steps ①-② are adjusted to:

[0113] The coconut shell was crushed and sieved (250 mesh sieve), and then the sieved powder and formaldehyde were placed in concentrated phosphoric acid (concentration 14.7 mol / L) at a mass ratio of 10:1 to obtain a mixed slurry. The concentration of the mixed slurry was 1.5 mol / L. The mixture was stirred evenly and then placed in a high-pressure reactor filled with nitrogen protective atmosphere. The mixture was heated to 350°C and kept warm for 24 hours. The gas pressure in the high-pressure reactor was 2 MPa. After the reaction was completed, the mixture was cooled to room temperature to obtain foamed coconut shell resin.

[0114] Example 7

[0115] This embodiment provides a coconut shell-based foam carbon modified LiMnPO4 material and a preparation method thereof. The difference between the preparation method and that of Example 1 is that in step (2), steps ①-② are adjusted to:

[0116] ① The coconut shell was crushed and sieved (250 mesh sieve), and then the sieved powder and formaldehyde were placed in concentrated phosphoric acid (concentration 14.7 mol / L) at a mass ratio of 10:1 to obtain a mixed slurry. The concentration of the mixed slurry was 1.5 mol / L. It was stirred evenly and then placed in a high-temperature and high-pressure reactor, filled with nitrogen protective atmosphere, heated to 600°C and kept warm for 8 hours. The gas pressure in the high-pressure reactor was 2 MPa. After the reaction was completed, it was cooled to room temperature to obtain foamed coconut shell resin.

[0117] Comparative Example 1

[0118] This comparative example provides unmodified LiMnPO4 material.

[0119] Comparative Example 2

[0120] This comparative example provides a carbon-coated LiMnPO4 material (LiMnPO4 / C material), which is carbon-coated with glucose as a carbon source. The specific preparation method is as follows:

[0121] 1g of glucose was mixed with 100g of DMF and heated at 60°C for 8h to form a transparent solution; 2g of LiMnPO4 was then added to the transparent solution of glucose and DMF and magnetically stirred at room temperature for 12h to form a uniform dispersion; the mixture was then heated and stirred at 150°C to evaporate the solvent to obtain a powder uniformly mixed with the two; the resulting powder was placed in an argon atmosphere tubular furnace, heated to 600°C at a rate of 2°C / min, and calcined for 6h. After cooling, the product LiMnPO4 / C material was obtained.

[0122] Comparative Example 3

[0123] This comparative example provides a carbon-coated LiMnPO4 material. The difference between its preparation method and that of Example 1 is that the coconut shell-based foam carbon is replaced with a peanut shell-based carbon material. The preparation method of the peanut shell-based carbon material is as follows:

[0124] The peanut shells were crushed and passed through a 250-mesh sieve. The sieved powder was then placed in a 14 mol / L concentrated sulfuric acid solution, kept at 80°C for 1 hour, washed and dried, and carbonized at 350°C for 24 hours under nitrogen protection to obtain a peanut shell-based carbon material.

[0125] In other embodiments, the present invention provides a positive electrode slurry and a homogenization process thereof, wherein the homogenization process comprises: mixing a coconut shell-based foam carbon-modified LiMnPO4 material with a conductive agent and a binder, and then adding an organic solvent thereto in three times, stirring after each addition of the organic solvent to obtain a positive electrode slurry; wherein the amount of organic solvent added in the latter addition is greater than the amount of organic solvent added in the previous addition.

[0126] Examples 8-14

[0127] A positive electrode slurry is provided, and its slurrying process comprises: mixing the coconut shell-based foam carbon modified LiMnPO4 material prepared in Examples 1-7 with a conductive agent (Super P) and a binder (PVDF) in a mass ratio of 8:1:1, then adding an organic solvent NMP thereto three times, stirring after each addition of the organic solvent, to obtain a positive electrode slurry; wherein the NMP added sequentially accounts for 1 / 4, 1 / 3 and 5 / 12 of the total NMP mass.

[0128] Comparative Example 1

[0129] A positive electrode slurry is provided, and its homogenization process is a conventional homogenization process, specifically: the coconut shell-based foam carbon modified LiMnPO4 material prepared in Example 1 is mixed with a conductive agent (Super P), a binder (PVDF) and NMP, wherein the mass ratio of the coconut shell-based foam carbon modified LiMnPO4 material to Super P and PVDF is 8:1:1, and then NMP is added to adjust the slurry viscosity to obtain a positive electrode slurry.

[0130] Comparative Example 2

[0131] A positive electrode slurry is provided, which differs from Example 8 in that the coconut shell-based foam carbon modified LiMnPO4 material of Example 1 is replaced by the unmodified LiMnPO4 material of Comparative Example 1.

[0132] The processing performance test was conducted on the positive electrode slurries of Example 8, Control Example 1, and Control Example 2:

[0133] (1) The viscosity was tested using a rotational viscometer at 25°C, a 64# rotor, and a rotation speed of 60 rpm. Each slurry was tested three times and the average value was taken to record the viscosity of the slurry.

[0134] (2) Use a solid content meter to test the solid content. Perform three tests on each slurry and take the average value. Record the viscosity of the slurry.

[0135] See Table 1 for the results.

[0136] Table 1

[0137]

[0138] In conjunction with Table 1, a comparison between Example 8 and Comparative Example 1 shows that, compared to conventional homogenization processes, the homogenization process of the present invention can reduce the amount of NMP used, effectively increasing the solid content of the positive electrode slurry made from the coconut shell-based foam carbon modified LiMnPO4 material by 3%. A comparison between Example 1 and Comparative Example 2 shows that the positive electrode slurry using the coconut shell-based foam carbon modified LiMnPO4 material as the positive electrode material has a lower slurry viscosity and a 6.2% increase in solid content compared to unmodified LiMnPO4.

[0139] Application Example 1

[0140] A battery is provided, and a method for manufacturing the battery is as follows:

[0141] (I) Preparation of positive electrode sheet: The positive electrode slurry of Example 8 was evenly coated on an aluminum foil current collector, dried, and then rolled and die-cut into positive electrode sheets for small soft-pack batteries.

[0142] (II) Preparation of negative electrode sheet: Graphite, carbon black, sodium carboxymethyl cellulose and styrene-butadiene rubber were mixed evenly in a mass ratio of 8:1:0.6:0.4 and then coated on the surface of copper foil. After drying, the negative electrode sheet was prepared by roller pressing and die cutting.

[0143] (III) Glass fiber was used as the separator and 1 mol / L LiPF6 / EC-DEC-DMC (volume ratio of 1:1:1) was used as the electrolyte.

[0144] Finally, the negative electrode sheet, separator, positive electrode sheet, and separator are stacked in the order of top to bottom to form an electrode assembly. The bare battery cell is placed in an aluminum-plastic film outer packaging shell, dried, and then injected with electrolyte. After vacuum packaging, welding, static standing, and formation, the test battery is obtained.

[0145] Application Example 2-7

[0146] A battery is provided, the manufacturing method of which is different from that of Application Example 1 in that the positive electrode slurry of Example 8 is replaced by the positive electrode slurries of Examples 9 to 14 respectively.

[0147] Application Comparative Examples 1-3

[0148] A battery is provided, the manufacturing method of which is different from that of Application Example 1 in that the positive electrode slurry of Example 8 is replaced by the positive electrode slurries of Comparative Examples 1-3 respectively.

[0149] The following tests are performed on Use Case 1 and Comparative Example 1:

[0150] (1) Test the surface resistance of the positive electrode sheet prepared in step (I). The test method is as follows: first, measure the thickness of the positive electrode sheet with a micrometer, then gently press the probes of the four-probe tester vertically on the surface of the positive electrode sheet, apply uniform pressure, enter the instrument test parameters (current, four-probe spacing), and then start the test. Finally, the test results of the electrode sheet surface resistance are obtained. The test results are shown in Table 2.

[0151] (2) The internal resistance of the test battery after formation is recorded as the aging internal resistance, see Table 2.

[0152] Table 2

[0153]

[0154] Combined with Table 2, by comparison, it can be seen that when the coconut shell-based foam carbon modified LiMnPO4 material is selected as the positive electrode material, the surface resistance of the prepared positive electrode sheet is reduced by 30.8% compared with the unmodified LiMnPO4.

[0155] At the same time, the internal resistance of the battery manufactured using this positive electrode sheet decreased by 24.5% after formation.

[0156] The following tests were performed on the batteries corresponding to Use Cases 1-7 and Comparative Examples 1-3:

[0157] (1) First-effect test: After full charging at 0.33C constant current and constant voltage, record the charge capacity. Then, discharge at 0.33C constant current to 2.5V and record the discharge capacity. The first-effect is the ratio of discharge capacity to charge capacity.

[0158] (2) 3C constant current charging ratio test: The battery is charged to 3.65V at 0.33C constant current and constant voltage, then discharged at 0.33C, and the discharge capacity C0 is recorded. After that, it is fully charged at 3C constant current and constant voltage, and the charge capacity C1 is recorded. C1 / C0 is the constant current charging ratio.

[0159] (3) 3C / 0.33C ratio test: The battery is charged at 0.33C constant current and constant voltage to 3.65V, then discharged at 0.33C, and the discharge capacity C0 is recorded. After that, the battery is fully charged at 0.33C constant current and constant voltage. After full charge, it is discharged at 0.33C to 2.5V, and the discharge capacity C1 is recorded. C1 / C0 is the rate discharge ratio.

[0160] (4) 25℃@80% SOH cycle test: Charge the battery to 3.65V at 0.33C constant current and constant voltage, then discharge at 0.33C, record the discharge capacity C0, then fully charge at 0.33C constant current and constant voltage, and discharge at 0.33C after full charge, record the discharge capacity C1, and continue the cycle of 0.33C charging and 0.33C discharging until the discharge capacity Cn / C0 = 70%, record the cycle number n.

[0161] See Table 3 for the results.

[0162] Table 3

[0163]

[0164] As shown in Table 3, the present invention effectively improves the electrochemical performance of the battery by coating the phosphate positive electrode core with a lightweight, sponge-like coconut shell-based foam carbon. Compared to Example 1, Example 1 shows a 5% increase in first efficiency, a 16.4% increase in 3C constant current charge ratio, a 12.6% increase in 3C rate discharge capacity, and a 41% increase in 3C / 1C room temperature cycle times.

[0165] By comparing Application Example 1 with Application Examples 4-5, it can be seen that too little or too much coconut shell-based foam carbon content will lead to poor first efficiency and rate performance of the battery, and the cycle performance will also be slightly deteriorated.

[0166] By comparing Application Example 1 with Application Examples 6-7, it can be seen that the two-stage hydrothermal treatment is conducive to obtaining foam carbon with more uniform particle size and higher purity, which helps to improve the rate and cycle performance.

[0167] By comparing Application Example 1 with Comparative Application Examples 1-2, it can be seen that in Example 1, the coconut shell-based foam carbon-coated LiMnPO4 main material has less free carbon, which is beneficial to improving electrical performance. In Comparative Example 1, LiMnPO4 is not carbon-coated, resulting in a significant decrease in the first efficiency, rate performance and cycle performance of the battery. In Comparative Example 2, LiMnPO4 coated with a carbon coating layer formed by glucose results in a significant decrease in rate performance and cycle performance. In Comparative Example 2, peanut-based foam carbon has more impurities, more free carbon, and uneven coating, resulting in its electrical performance being inferior to that of coconut shell-based foam carbon-coated LiMnPO4.

[0168] The applicant states that the present invention is intended to illustrate the detailed methods of the present invention through the above-described embodiments, but the present invention is not limited to the above-described detailed methods, that is, it does not mean that the present invention must rely on the above-described detailed methods in order to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions for various raw materials in the products of the present invention, addition of auxiliary ingredients, and selection of specific methods, etc., are all within the scope of protection and disclosure of the present invention.

Claims

1. A coconut shell-based foam carbon modified phosphate cathode material, characterized in that: The positive electrode material comprises a phosphate positive electrode core and a coconut shell-based foamed carbon coated on the surface of the phosphate positive electrode core; The coconut shell-based foam carbon modified phosphate positive electrode material is prepared by the following method, which comprises the following steps: (1) Preparation of coconut shell-based foamed carbon; (2) forming a core slurry of the phosphate cathode material and forming a shell slurry of the coconut shell-based foamed carbon, and coaxially electrospinning the core slurry and the shell slurry to obtain the coconut shell-based foamed carbon-modified phosphate cathode material; The preparation method of the coconut shell-based foamed carbon in step (1) comprises the following steps: (a) pre-treating coconut shell to obtain coconut shell powder; (b) mixing coconut shell powder with an aldehyde substance and concentrated acid to obtain a mixed slurry, placing the mixed slurry in a reaction kettle, filling it with a protective gas and heating it to react, thereby obtaining a foamed coconut shell resin; (c) heat-treating the foamed coconut shell resin to obtain coconut shell-based foamed carbon.

2. The coconut shell-based foamed carbon modified phosphate cathode material according to claim 1, characterized in that: Based on the total mass of the positive electrode material being 100%, the mass fraction of the coconut shell-based foamed carbon is 2%-3%.

3. A method for preparing the coconut shell-based foam carbon modified phosphate cathode material according to claim 1 or 2, characterized in that: The preparation method comprises the following steps: (1) Preparation of coconut shell-based foamed carbon; (2) forming a core slurry of the phosphate cathode material and forming a shell slurry of the coconut shell-based foamed carbon, and coaxially electrospinning the core slurry and the shell slurry to obtain the coconut shell-based foamed carbon-modified phosphate cathode material; The preparation method of the coconut shell-based foamed carbon in step (1) comprises the following steps: (a) pre-treating coconut shell to obtain coconut shell powder; (b) mixing coconut shell powder with an aldehyde substance and concentrated acid to obtain a mixed slurry, placing the mixed slurry in a reaction kettle, filling it with a protective gas and heating it to react, thereby obtaining a foamed coconut shell resin; (c) heat-treating the foamed coconut shell resin to obtain coconut shell-based foamed carbon.

4. The preparation method according to claim 3, characterized in that The pretreatment in step (a) is crushing and screening.

5. The preparation method according to claim 3, characterized in that In step (b), the aldehyde substance includes at least one of formaldehyde, octanal, nonanal, decanal and undecanal.

6. The preparation method according to claim 3, characterized in that In step (b), the mass ratio of the coconut shell powder to the aldehyde substance is (5-15):

1.

7. The preparation method according to claim 3, characterized in that The concentrated acid in step (b) includes one of concentrated sulfuric acid, concentrated phosphoric acid and concentrated nitric acid.

8. The preparation method according to claim 3, characterized in that The heating reaction in step (b) includes two stages, wherein the first stage is heating to a first temperature and keeping the temperature; the second stage is continuing to heat to a second temperature and keeping the temperature.

9. The preparation method according to claim 8, characterized in that The first temperature is 300° C.-400° C., and the insulation time at the first temperature is 12 h-36 h.

10. The preparation method according to claim 8, characterized in that The second temperature is 500° C.-650° C., and the holding time at the second temperature is 5 h-10 h.

11. The preparation method according to claim 3, characterized in that The pressure of the heating reaction in step (b) is 1 MPa-5 MPa.

12. The preparation method according to claim 3, characterized in that The temperature of the heat treatment in step (c) is 700°C-800°C.

13. The preparation method according to claim 3, characterized in that The heat treatment time in step (c) is 3h-6h.

14. The preparation method according to claim 3, characterized in that The core slurry in step (2) is obtained by dispersing a phosphate positive electrode material and a first surfactant in an organic solvent.

15. The preparation method according to claim 14, characterized in that The first surfactant includes at least one of polyurethane and CTAB.

16. The preparation method according to claim 14, characterized in that The mass ratio of the phosphate positive electrode material to the first surfactant is 90:(0.5-1.5).

17. The preparation method according to claim 3, characterized in that The shell slurry is obtained by dispersing coconut shell-based foamed carbon and a second surfactant in an organic solvent.

18. The preparation method according to claim 17, characterized in that: The second surfactant includes at least one of polyurethane and CTAB.

19. The preparation method according to claim 17, characterized in that The mass ratio of the coconut shell-based foamed carbon to the second surfactant is 80: (0.5-1.5).

20. The preparation method according to claim 3, characterized in that The voltage of the coaxial electrospinning in step (2) is 20 kV-40 kV.

21. The preparation method according to claim 3, characterized in that The spinning rate of the coaxial electrospinning in step (2) is 5 mL / min-15 mL / min.

22. The preparation method according to claim 3, characterized in that The method further comprises the steps of washing and spray drying the product after the electrospinning in step (2).

23. A positive electrode slurry, characterized in that: The positive electrode slurry is obtained by homogenizing the coconut shell-based foamed carbon modified phosphate positive electrode material according to claim 1 or 2.

24. A method for homogenizing a positive electrode slurry according to claim 23, characterized in that: The homogenization method comprises the following steps: After mixing the coconut shell-based foam carbon modified phosphate positive electrode material with a conductive agent and a binder, an organic solvent is added at least three times, and stirring is performed after each addition of the organic solvent to obtain a positive electrode slurry; The amount of the organic solvent added in the latter step is greater than that in the previous step.

25. A positive electrode, characterized in that The positive electrode is prepared using the positive electrode slurry according to claim 23.

26. A battery, characterized in that: The battery includes the positive electrode according to claim 25.

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