Surface-modified lithium iron manganese phosphate and modification method thereof, positive electrode sheet and battery

Abstract

The application discloses surface modified lithium manganese iron phosphate and a modification method thereof, a positive pole piece and a battery, belongs to the technical field of surface modification of lithium manganese iron phosphate, and comprises: a core, which is lithium manganese iron phosphate; and a coating layer, raw materials of the coating layer comprising bacterial cellulose, lithium polyacrylic acid and PEDOT:PSS. The modification method comprises the following steps: step one, preparation of a cellulose dispersion liquid: after bacterial cellulose is treated by alkali washing, acid washing and neutralization, lithium polyacrylic acid aqueous solution is added and ultrasonic dispersion is carried out, so that a fiber dispersion liquid is obtained; step two, surface composite treatment: lithium manganese iron phosphate is added into the fiber dispersion liquid, PEDOT:PSS is further added, stirring and crosslinking are carried out, and a mixed slurry is obtained; and step three, granulation and heat treatment: the mixed slurry is spray dried and granulated, and gradient annealing treatment is carried out, so that the surface modified lithium manganese iron phosphate is obtained. The surface modified lithium manganese iron phosphate positive material has high conductivity and structural stability, and meets the demand of high energy density and long cycle life of a lithium ion battery.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of surface modification of lithium manganese iron phosphate and specifically relates to surface modified lithium manganese iron phosphate, a modification method thereof, a positive electrode sheet and a battery. BACKGROUND

[0002] With the rapid development of lithium ion batteries in the field of electric vehicles and energy storage, the energy density, cycle life and safety of the positive electrode material have become the key research direction. Lithium iron phosphate is widely used due to its stable structure and good safety, but its low working voltage limits the further improvement of the energy density. In contrast, lithium manganese iron phosphate combines the stability of lithium iron phosphate and the high voltage characteristics of lithium manganese phosphate, and exhibits more excellent performance potential. However, lithium manganese iron phosphate also has some problems to be solved, such as low ionic conductivity, easy dissolution of manganese element and frequent interface side reactions, so it is usually necessary to modify the surface thereof.

[0003] At present, common coating methods include carbon coating, conductive polymer coating and coating based on nanostructure design. However, although carbon coating can effectively improve the conductivity, if the carbon layer is too thick, it will hinder the transmission of lithium ions; the conductive polymer coating can build a uniform conductive network, but it is easy to agglomerate when used alone; the nanostructure design can effectively shorten the diffusion path of lithium ions, but its mechanical strength is relatively weak and is easy to break during the cycle process. SUMMARY

[0004] The application provides surface modified lithium manganese iron phosphate, a modification method thereof, a positive electrode sheet and a battery to overcome the above technical problems. The surface modified lithium manganese iron phosphate positive electrode material has high conductivity and structural stability, and meets the demand of high energy density and long cycle life lithium ion battery.

[0005] The application solves the above technical problems through the following technical solutions.

[0006] The surface modified lithium manganese iron phosphate comprises:

[0007] The core is lithium manganese iron phosphate;

[0008] The coating layer is made of bacterial cellulose, lithium polyacrylate and PEDOT:PSS (Chinese name: [poly (3, 4-ethylenedioxythiophene)-poly (styrene sulfonic acid)], CAS number: 155090-83-8).

[0009] The bacterial cellulose of the application can provide mechanical support for the core, and its hydroxyl structure provides interface bonding force for crosslinking with PEDOT:PSS.

[0010] PEDOT:PSS provides an electron conduction path to reduce interface impedance; lithium polyacrylate: as an ion conductor, promotes Li + transmission while compensating for lithium loss in the cycle.

[0011] According to some embodiments of the present application, the chemical composition of the lithium manganese iron phosphate is LiMn x Fe 1-x PO4; wherein x = 0.3-0.6, preferably x = 0.3-0.5.

[0012] According to some embodiments of the present application, the D50 particle size of the lithium manganese iron phosphate is 0.2-2 μm.

[0013] According to some embodiments of the present application, the raw material mass of the coating layer accounts for 3-12% of the mass of the lithium manganese iron phosphate, preferably 3-8%.

[0014] According to some embodiments of the present application, the mass ratio of the bacterial cellulose: the lithium polyacrylate: the PEDOT:PSS is 1-5:0.5-2:2-8, preferably 3-5:0.5-2:4-8.

[0015] According to some embodiments of the present application, the fiber diameter of the bacterial cellulose is 50-100 nm.

[0016] The present application discloses a modification method of surface modified lithium manganese iron phosphate, comprising the following steps:

[0017] Step one, cellulose dispersion liquid preparation: after alkaline washing, acid washing and neutralization treatment of bacterial cellulose, ultrasonic dispersion is performed in lithium polyacrylate aqueous solution to obtain a fiber dispersion liquid;

[0018] Step two, surface composite treatment: lithium manganese iron phosphate is added to the fiber dispersion liquid, and PEDOT:PSS is added, and stirring and crosslinking are performed to obtain a mixed slurry;

[0019] Step three, granulation and heat treatment: the mixed slurry is spray dried and granulated, and gradient annealing treatment is performed to obtain surface modified lithium manganese iron phosphate.

[0020] According to some embodiments of the present application, the modification method of the lithium polyacrylate in the present application comprises the following steps: polyacrylic acid and lithium hydroxide are reacted at 40-60°C; preferably, the modification method of the lithium polyacrylate comprises the following steps: after polyacrylic acid is dispersed in water, lithium hydroxide is added dropwise, and the reaction is carried out at 40-60°C until the pH is 7.0±0.2, and then the mixture is placed in a dialysis bag and washed with deionized water until the conductivity is ≤10 μS / cm.

[0021] According to some embodiments of the present application, the alkali cleaning uses a 3-5wt% NaOH solution to treat for 5-30min.

[0022] According to some embodiments of the present application, the acid cleaning uses a hydrochloric acid solution or an acetic acid solution to wash until the pH is 6.5-7.5.

[0023] According to some embodiments of the present application, the mass concentration of the aqueous solution of lithium polyacrylate is 5-15wt%, preferably 7-12wt%.

[0024] According to some embodiments of the present application, the lithium manganese iron phosphate needs to be dispersed with a solvent before being added to the dispersion;

[0025] Further, the solvent is at least one of ethanol, water and N-methyl pyrrolidone;

[0026] Further, the mass-volume ratio of the lithium manganese iron phosphate to the solvent is 50-80g / L, preferably 50-65g / L.

[0027] According to some embodiments of the present application, the pH of the solution of PEDOT:PSS is 4.5-5.5, preferably 5.0-5.5.

[0028] According to some embodiments of the present application, 1-2wt% of citric acid is further added to the mixed slurry to stabilize the manganese ions.

[0029] According to some embodiments of the present application, the stirring and cross-linking time is 10-50min, preferably 15-40min.

[0030] According to some embodiments of the present application, the gradient annealing includes the following stages:

[0031] The low-temperature stage: 200-250℃ for 1-2h to remove residual solvent;

[0032] The medium-temperature stage: 350-450℃ for 2-3h in an Ar atmosphere to promote the solidification of PEDOT:PSS;

[0033] The high-temperature stage: 550-650℃ for 3-5h in an Ar / H2 atmosphere to enhance the crystallinity while avoiding the oxidation of manganese; wherein the volume ratio of Ar / H2 is 90-95 / 10-5.

[0034] The present application discloses a positive electrode tab, which comprises the aforementioned surface-modified lithium manganese iron phosphate, or is prepared by the aforementioned modification method.

[0035] The present application also discloses a battery comprising the aforementioned positive electrode tab.

[0036] The above-mentioned preferred conditions can be combined arbitrarily to obtain preferred examples of the present application.

[0037] Compared with the prior art, the present application has the following advantages:

[0038] 1. The present application adopts PEDOT:PSS and lithium polyacrylate to construct double conductive channels, and coats the lithium manganese iron phosphate particles with bacterial cellulose after cross-linking, thereby effectively reducing the resistivity and significantly improving the conductivity of the positive electrode material. In addition, the mechanical support provided by the bacterial cellulose can ensure that the surface-modified lithium manganese iron phosphate can maintain a high capacity retention rate after 200 cycles, and in some preferred embodiments, the capacity retention rate is ≥ 94%.

[0039] 2. To make the structure of the lithium manganese iron phosphate core combined with the coating layer more stable, the present application selects gradient annealing process. In the low-temperature stage, the residual solvent is removed, in the medium-temperature stage, the polymer is solidified, and in the high-temperature stage, the crystallization treatment is carried out, so as to avoid cracking caused by thermal stress. In addition, to effectively inhibit the precipitation of manganese, the present application adjusts the pH value of PEDOT:PSS and uses reducing atmosphere for treatment in the high-temperature stage of gradient annealing. BRIEF DESCRIPTION OF DRAWINGS

[0040] For the convenience of those skilled in the art, the present application will be further described below in conjunction with the drawings.

[0041] Figure 1 SEM image of surface-modified lithium manganese iron phosphate prepared for Example 1.

[0042] Figure 2 SEM image of surface-modified lithium manganese iron phosphate prepared for Example 7.

[0043] Figure 3 SEM image of surface-modified lithium manganese iron phosphate prepared for Comparative Example 2. DETAILED DESCRIPTION

[0044] For the convenience of understanding the present application, the following will describe the present application more fully and in detail in conjunction with preferred examples, but the protection scope of the present application is not limited to the following specific examples.

[0045] Unless otherwise defined, all professional terms used in the following have the same meaning as generally understood by those skilled in the art. The professional terms used in this paper are only for the purpose of describing specific examples and are not intended to limit the protection scope of the present application.

[0046] The ranges disclosed herein are intended to include both endpoints and intervening ranges. For example, if a range of "60-120" and "80-110" is listed, it is intended that a range of "60-110" and "80-120" is also contemplated. Furthermore, if a minimum range value of 1 and 2 is listed, and if a maximum range value of 3, 4, and 5 is listed, then the following ranges are all contemplated: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In the present invention, unless otherwise stated, a numerical range "a-b" indicates a shorthand way of describing all of the real numbers between a and b, wherein a and b are both real numbers. For example, the numerical range "0-5" indicates that all of the real numbers between 0 and 5 have been listed herein, and "0-5" is merely a shorthand way of describing those numerical combinations. In addition, when a parameter is stated to be an integer ≥ 2, it is equivalent to disclose that the parameter is, for example, integer 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0047] All the embodiments and optional embodiments of the present invention can be combined with each other to form new technical solutions, unless otherwise specified.

[0048] All the technical features and optional technical features of the present invention can be combined with each other to form new technical solutions, unless otherwise specified.

[0049] All the steps of the present invention can be performed in sequence or randomly, preferably in sequence, unless otherwise specified. For example, the method comprises steps (a) and (b), which means that the method can comprise steps (a) and (b) in sequence, or steps (b) and (a) in sequence. For example, the method can further comprise step (c), which means that step (c) can be added to the method in any sequence. For example, the method can comprise steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0050] The terms "comprise" and "include" mentioned in the present invention are open-ended, and can also be closed-ended. For example, the terms "comprise" and "include" can mean that other components not listed can also be included, or only the listed components can be included.

[0051] If not specifically stated, in the present invention, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, any of the following satisfy the condition "A or B": A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0052] The raw material information used in the following examples is as follows:

[0053] The lithium iron phosphate is from Hunan Yunneng New Energy Battery Material Co., Ltd., D50 = 1.3 μm;

[0054] The bacterial cellulose is purchased from Nanjing Tianlu Nanometer Technology Co., Ltd., the fiber diameter is 50-100 nm, the crystal structure is cellulose I type, and the surface contains hydroxyl functional groups. The specific structure is:

[0055]

[0056] The PEDOT:PSS (1.5% aqueous solution) is purchased from Shanghai Yinn Chemical Technology Co., Ltd.

[0057] Including but not limited to the above manufacturers' models.

[0058] Preparation of lithium polyacrylate: The raw material polyacrylic acid Mw is 3000, the temperature is controlled at 50°C, the polyacrylic acid is dispersed in water, then lithium hydroxide is slowly added until the pH is 7.0±0.2, then the product is washed in a dialysis bag with deionized water until the conductivity is 10 μS / cm, and then the product is vacuum dried to obtain lithium polyacrylate.

[0059] Example 1

[0060] 1. The surface modified manganese iron phosphate in this example:

[0061] The core is manganese iron phosphate lithium phosphate, the chemical composition of the manganese iron phosphate lithium phosphate is LiMn 0.4 Fe 0.6 PO4;

[0062] The raw materials of the coating layer include bacterial cellulose, lithium polyacrylate and PEDOT:PSS; the mass ratio of bacterial cellulose, lithium polyacrylate and PEDOT:PSS is 4.5:1.5:6;

[0063] The mass of the raw materials of the coating layer accounts for 4.7% of the mass of the manganese iron phosphate lithium phosphate.

[0064] 2. The modification method process of the surface modified manganese iron phosphate lithium phosphate in this example is as follows:

[0065] Step one, preparation of cellulose dispersion:

[0066] Pre-treatment of bacterial cellulose: after crushing, the bacterial cellulose was first washed with 3wt% NaOH solution for 25min, then washed with 0.5mol / L dilute hydrochloric acid solution to neutralize to pH 7, and finally washed with water;

[0067] The above pre-treated bacterial cellulose was added with lithium polyacrylate aqueous solution and ultrasonically dispersed to obtain a fiber dispersion liquid;

[0068] Step two, surface composite treatment:

[0069] The mass / volume ratio of manganese iron phosphate and solvent was 60g / L, the solvent was selected as ethanol / water with a volume ratio of 1:1, the manganese iron phosphate lithium was dispersed with the solvent, then added into the fiber dispersion liquid, and finally added with a solution of PEDOT:PSS (ammonia was used to adjust the pH to 5.0), and stirred for crosslinking for 20min to obtain a mixed slurry;

[0070] Step three, granulation and heat treatment:

[0071] The mixed slurry was spray-dried and granulated, and gradient annealing treatment was performed to obtain the surface-modified manganese iron phosphate lithium;

[0072] The gradient annealing included the following stages:

[0073] Low-temperature stage: 250℃ for 1h;

[0074] Medium-temperature stage: 400℃ for 2.5h under Ar atmosphere;

[0075] High-temperature stage: 600℃ for 3h under Ar / H2 (volume ratio 95 / 5) atmosphere.

[0076] The SEM image of the surface-modified manganese iron phosphate lithium prepared in this example is shown in Figure 1 .

[0077] Example 2

[0078] 1. The surface-modified manganese iron phosphate lithium of this example:

[0079] The core is manganese iron phosphate lithium, and the chemical composition of the manganese iron phosphate lithium is LiMn 0.4 Fe 0.6 PO4;

[0080] The coating layer is made of bacterial cellulose, lithium polyacrylate and PEDOT:PSS; the mass ratio of bacterial cellulose, lithium polyacrylate and PEDOT:PSS is 4.5:1.5:6;

[0081] The raw material mass of the coating layer accounts for 7.8% of the mass of the manganese iron phosphate lithium.

[0082] 2. The modification method process of the surface-modified manganese iron phosphate lithium of this example is as follows:

[0083] Step one, cellulose dispersion liquid preparation:

[0084] Pretreatment of bacterial cellulose: after crushing the bacterial cellulose, first wash with 3wt% NaOH solution for 25min, then wash with 0.5mol / L dilute hydrochloric acid solution to neutralize to pH 7, and finally wash with water;

[0085] Add lithium polyacrylate aqueous solution to the above pretreated bacterial cellulose after ultrasonic dispersion to obtain a fiber dispersion liquid;

[0086] Step two, surface composite treatment:

[0087] The mass-volume ratio of manganese iron phosphate and solvent is 60g / L, the solvent is selected as 1:1 ethanol / water, the lithium manganese iron phosphate is dispersed with the solvent, then added to the fiber dispersion liquid, and finally added with a solution of PEDOT:PSS (ammonia water is used to adjust the pH to 5.0), stirred and crosslinked for 20min to obtain a mixed slurry;

[0088] Step three, granulation and heat treatment:

[0089] Spray drying and granulation of the mixed slurry, and gradient annealing treatment to obtain surface modified lithium manganese iron phosphate;

[0090] The gradient annealing includes the following stages:

[0091] Low temperature stage: 250℃ for 1h;

[0092] Medium temperature stage: 400℃ for 2.5h under Ar atmosphere;

[0093] High temperature stage: 600℃ for 3h under Ar / H2 (volume ratio 95 / 5) atmosphere.

[0094] Example 3

[0095] 1. The surface modified lithium manganese iron phosphate of this example:

[0096] The core is lithium manganese iron phosphate, and the chemical composition of the lithium manganese iron phosphate is LiMn 0.4 Fe 0.6 PO4;

[0097] The coating layer is made of bacterial cellulose, lithium polyacrylate and PEDOT:PSS; the mass ratio of bacterial cellulose, lithium polyacrylate and PEDOT:PSS is 4.5:1.5:6;

[0098] The raw material mass of the coating layer accounts for 15% of the mass of the lithium manganese iron phosphate.

[0099] 2. The modification method process of the surface modified lithium manganese iron phosphate of this example is as follows:

[0100] Step one, cellulose dispersion liquid preparation:

[0101] Pretreatment of bacterial cellulose: after crushing the bacterial cellulose, first wash with 3wt% NaOH solution for 25min, then wash with 0.5mol / L dilute hydrochloric acid solution to neutralize to pH 7, and finally wash with water;

[0102] Add lithium polyacrylate aqueous solution to the above pretreated bacterial cellulose after ultrasonic dispersion to obtain a fiber dispersion liquid;

[0103] Step two, surface composite treatment:

[0104] The mass-volume ratio of manganese iron phosphate and solvent is 60g / L, the solvent is selected as 1:1 ethanol / water, the lithium manganese iron phosphate is dispersed with the solvent, then added to the fiber dispersion liquid, and finally added with a solution of PEDOT:PSS (ammonia water is used to adjust the pH to 5.0), stirred and crosslinked for 20min to obtain a mixed slurry;

[0105] Step three, granulation and heat treatment:

[0106] Spray dry granulation of the mixed slurry and gradient annealing treatment to obtain surface modified lithium manganese iron phosphate;

[0107] The gradient annealing includes the following stages:

[0108] Low temperature stage: 250℃ for 1h;

[0109] Medium temperature stage: 400℃ for 2.5h under Ar atmosphere;

[0110] High temperature stage: 600℃ for 3h under Ar / H2 (volume ratio 95 / 5) atmosphere.

[0111] Example 4

[0112] 1. The surface modified lithium manganese iron phosphate of this example:

[0113] The core is lithium manganese iron phosphate, and the chemical composition of the lithium manganese iron phosphate is LiMn 0.4 Fe 0.6 PO4;

[0114] The coating layer is made of bacterial cellulose, lithium polyacrylate and PEDOT:PSS; the mass ratio of bacterial cellulose, lithium polyacrylate and PEDOT:PSS is 4.5:1.2:7.5; the raw material mass of the coating layer accounts for 5% of the mass of lithium manganese iron phosphate.

[0115] 2. The modification method process of the surface modified lithium manganese iron phosphate of this example is as follows:

[0116] Step one, cellulose dispersion liquid preparation:

[0117] Pretreatment of bacterial cellulose: after crushing the bacterial cellulose, first wash it with 3wt% NaOH solution for 25min, then wash it with 0.5mol / L dilute hydrochloric acid solution to neutralize to pH 7, and finally wash it with water;

[0118] Add lithium polyacrylate aqueous solution to the pretreated bacterial cellulose and ultrasonically disperse to obtain a fiber dispersion liquid;

[0119] Step two, surface composite treatment:

[0120] The mass-volume ratio of manganese iron phosphate and solvent is 60g / L, the solvent is selected as a volume ratio of 1:1 ethanol / water, the lithium manganese iron phosphate is dispersed with the solvent, then added to the fiber dispersion liquid, and finally added with a solution of PEDOT:PSS (ammonia water is used to adjust the pH to 5.0), stirred and crosslinked for 20min to obtain a mixed slurry;

[0121] Step three, granulation and heat treatment:

[0122] Spray dry granulation of the mixed slurry and gradient annealing treatment to obtain surface modified lithium manganese iron phosphate;

[0123] The gradient annealing includes the following stages:

[0124] Low temperature stage: 250℃ for 1h;

[0125] Medium temperature stage: 400℃ for 2.5h under Ar atmosphere;

[0126] High temperature stage: 600℃ for 3h under Ar / H2 (volume ratio 95 / 5) atmosphere.

[0127] Example 5

[0128] 1. The surface modified lithium manganese iron phosphate of this example:

[0129] The core is lithium manganese iron phosphate, and the chemical composition of the lithium manganese iron phosphate is LiMn 0.4 Fe 0.6 PO4;

[0130] The coating layer is made of bacterial cellulose, lithium polyacrylate and PEDOT:PSS; the mass ratio of bacterial cellulose, lithium polyacrylate and PEDOT:PSS is 5:2:3;

[0131] The raw material mass of the coating layer accounts for 5% of the mass of the lithium manganese iron phosphate.

[0132] 2. The modification method process of the surface modified lithium manganese iron phosphate of this example is as follows:

[0133] Step one, cellulose dispersion liquid preparation:

[0134] Pre-treatment of bacterial cellulose: after crushing the bacterial cellulose, first wash it with 3wt% NaOH solution for 25min, then wash it with 0.5mol / L dilute hydrochloric acid solution to neutralize to pH 7, and finally wash it with water;

[0135] Add the lithium polyacrylate aqueous solution to the above pre-treated bacterial cellulose after ultrasonic dispersion to obtain a fiber dispersion liquid;

[0136] Step two, surface composite treatment:

[0137] The mass-volume ratio of manganese iron phosphate and solvent is 60g / L, the solvent is selected as 1:1 ethanol / water, the manganese iron phosphate lithium is dispersed with the solvent, then added to the fiber dispersion liquid, and finally added with a solution of PEDOT:PSS (ammonia water is used to adjust the pH to 5.0), stirred and crosslinked for 20min to obtain a mixed slurry;

[0138] Step three, granulation and heat treatment:

[0139] Spray dry granulation of the mixed slurry and gradient annealing treatment to obtain the surface modified manganese iron phosphate lithium;

[0140] The gradient annealing includes the following stages:

[0141] Low temperature stage: 250℃ for 1h;

[0142] Medium temperature stage: 400℃ for 2.5h under Ar atmosphere;

[0143] High temperature stage: 600℃ for 3h under Ar / H2(volumetric ratio 95 / 5) atmosphere.

[0144] Example 6

[0145] 1. The surface modified manganese iron phosphate lithium of this example:

[0146] The core is manganese iron phosphate lithium, and the chemical composition of the manganese iron phosphate lithium is LiMn 0.4 Fe 0.6 PO4;

[0147] The coating layer is made of bacterial cellulose, lithium polyacrylate and PEDOT:PSS; the mass ratio of bacterial cellulose, lithium polyacrylate and PEDOT:PSS is 4.5:1.5:6;

[0148] The raw material mass of the coating layer accounts for 5% of the mass of the manganese iron phosphate lithium.

[0149] 2. The modification method process of the surface modified manganese iron phosphate lithium of this example is as follows:

[0150] Step one, preparation of cellulose dispersion liquid:

[0151] Pre-treatment of bacterial cellulose: after crushing, the bacterial cellulose was washed with 3wt% NaOH solution for 25 min, then washed with 0.5mol / L dilute hydrochloric acid solution to neutralize to pH 7, and finally washed with water;

[0152] The above pre-treated bacterial cellulose was added with lithium polyacrylate aqueous solution to obtain a fiber dispersion liquid after ultrasonic dispersion;

[0153] Step two, surface composite treatment:

[0154] The mass-volume ratio of manganese iron phosphate and solvent was 60g / L, and the solvent was selected as ethanol / water with a volume ratio of 1:1. The manganese iron phosphate lithium was dispersed with the solvent, and then added to the fiber dispersion liquid. Finally, the solution of PEDOT:PSS (ammonia water was used to adjust the pH to 5.0) was added, and the mixture was stirred and crosslinked for 20 min to obtain a mixed slurry;

[0155] Step three, granulation and heat treatment:

[0156] The mixed slurry was spray dried and granulated, and then gradient annealing treatment was performed to obtain the surface modified manganese iron phosphate lithium;

[0157] The gradient annealing included the following stages:

[0158] Low temperature stage: 250℃ for 1h;

[0159] Medium temperature stage: 400℃ for 2.5h under Ar atmosphere;

[0160] High temperature stage: 600℃ for 3h under Ar / H2 (volume ratio 95 / 5) atmosphere.

[0161] Example 7

[0162] 1. The surface modified manganese iron phosphate lithium of this example:

[0163] The core was manganese iron phosphate lithium, and the chemical composition of the manganese iron phosphate lithium was LiMn 0.4 Fe 0.6 PO4;

[0164] The coating layer was prepared from bacterial cellulose, lithium polyacrylate and PEDOT:PSS. The mass ratio of bacterial cellulose, lithium polyacrylate and PEDOT:PSS was 4.5:1.5:6;

[0165] The mass of the raw materials of the coating layer accounted for 4.7% of the mass of the manganese iron phosphate lithium.

[0166] 2. The modification method process of the surface modified manganese iron phosphate lithium of this example was as follows:

[0167] Step one, preparation of cellulose dispersion liquid:

[0168] Pre-treatment of bacterial cellulose: after crushing the bacterial cellulose, the bacterial cellulose was first washed with 3wt% NaOH solution for 25min, then washed with 0.5mol / L dilute hydrochloric acid solution to neutralize to pH 7, and finally washed with water;

[0169] The above pre-treated bacterial cellulose was added with lithium polyacrylate aqueous solution after ultrasonic dispersion to obtain a fiber dispersion liquid;

[0170] Step two, surface composite treatment:

[0171] The mass-volume ratio of manganese iron phosphate and solvent was 60g / L, the solvent was selected as ethanol / water with a volume ratio of 1:1, the manganese iron phosphate lithium was dispersed with the solvent, and then added to the fiber dispersion liquid, and finally the solution of PEDOT:PSS (ammonia water was used to adjust the pH to 5.0) was added, and stirred for 20min to obtain a mixed slurry;

[0172] Step three, granulation and heat treatment:

[0173] The mixed slurry was spray dried and granulated, and gradient annealing treatment was performed to obtain the surface modified manganese iron phosphate lithium;

[0174] The gradient annealing includes the following stages:

[0175] Low temperature stage: 200℃ for 2h;

[0176] Medium temperature stage: 450℃ for 2h under Ar atmosphere;

[0177] High temperature stage: 550℃ for 4h under Ar / H2(volumetric ratio 90 / 10) atmosphere.

[0178] Example 8

[0179] 1. The surface modified manganese iron phosphate lithium of this example:

[0180] The core is manganese iron phosphate lithium, and the chemical composition of the manganese iron phosphate lithium is LiMn 0.4 Fe 0.6 PO4;

[0181] The coating layer is made of bacterial cellulose, lithium polyacrylate and PEDOT:PSS; the mass ratio of bacterial cellulose, lithium polyacrylate and PEDOT:PSS is 4.5:1.5:6;

[0182] The raw material mass of the coating layer accounts for 4.7% of the mass of the manganese iron phosphate lithium.

[0183] 2. The modification method process of the surface modified manganese iron phosphate lithium of this example is as follows:

[0184] Step one, preparation of cellulose dispersion liquid:

[0185] Pre-treatment of bacterial cellulose: after crushing the bacterial cellulose, the bacterial cellulose was first washed with 3wt% NaOH solution for 25min, then washed with 0.5mol / L dilute hydrochloric acid solution to neutralize to pH 7, and finally washed with water;

[0186] The above pre-treated bacterial cellulose was added with lithium polyacrylate aqueous solution to obtain a fiber dispersion liquid after ultrasonic dispersion;

[0187] Step two, surface composite treatment:

[0188] The mass-volume ratio of manganese iron phosphate and solvent was 60g / L, and the solvent was selected as ethanol / water with a volume ratio of 1:1. The manganese iron phosphate lithium was dispersed with the solvent, and then added to the fiber dispersion liquid. Finally, the solution of PEDOT:PSS (ammonia water was used to adjust the pH to 5.0) was added, and the mixture was stirred and crosslinked for 20min to obtain a mixed slurry;

[0189] Step three, granulation and heat treatment:

[0190] The mixed slurry was spray dried and granulated, and then gradient annealing treatment was performed to obtain the surface modified manganese iron phosphate lithium;

[0191] The gradient annealing includes the following stages:

[0192] Medium temperature stage: 300℃ for 2h under Ar atmosphere;

[0193] High temperature stage: 700℃ for 2h under Ar atmosphere.

[0194] The SEM image of the surface modified manganese iron phosphate lithium prepared in this example is shown in Figure 2 .

[0195] Comparative Example 1

[0196] 1. The surface modified manganese iron phosphate lithium of this comparative example:

[0197] Core, which is manganese iron phosphate lithium, the chemical composition of the manganese iron phosphate lithium is LiMn 0.4 Fe 0.6 PO4;

[0198] Coating layer, which is made of bacterial cellulose and lithium polyacrylate; the mass ratio of bacterial cellulose to lithium polyacrylate is 5:2;

[0199] The mass of the raw materials of the coating layer accounts for 5% of the mass of the manganese iron phosphate lithium.

[0200] 2. The modification method process of the surface modified manganese iron phosphate lithium of this example is as follows:

[0201] Step one, preparation of cellulose dispersion liquid:

[0202] Pre-treatment of bacterial cellulose: after crushing, the bacterial cellulose was first washed with 3wt% NaOH solution for 25 min, then washed with 0.5mol / L dilute hydrochloric acid solution to neutralize to pH 7, and finally washed with water;

[0203] The above pre-treated bacterial cellulose was added with lithium polyacrylate aqueous solution and ultrasonically dispersed to obtain a fiber dispersion liquid;

[0204] Step two, surface composite treatment:

[0205] The mass-volume ratio of manganese iron phosphate and solvent was 60g / L, the solvent was selected as ethanol / water in a volume ratio of 1:1, the manganese iron phosphate lithium was dispersed with the solvent, and then added to the fiber dispersion liquid for stirring and crosslinking for 15 min to obtain a mixed slurry;

[0206] Step three, granulation and heat treatment:

[0207] The mixed slurry was spray dried and granulated, and gradient annealing treatment was performed to obtain surface modified manganese iron phosphate lithium;

[0208] The gradient annealing included the following stages:

[0209] Low temperature stage: 250℃ for 1h;

[0210] Medium temperature stage: 400℃ for 2.5h under Ar atmosphere;

[0211] High temperature stage: 600℃ for 3h under Ar / H2 (volume ratio 95 / 5) atmosphere.

[0212] Comparative Example 2

[0213] 1. The surface modified manganese iron phosphate lithium of the present comparative example:

[0214] The core was manganese iron phosphate lithium, and the chemical composition of the manganese iron phosphate lithium was LiMn 0.4 Fe 0.6 PO4;

[0215] The coating layer was prepared from bacterial cellulose, lithium polyacrylate and PEDOT:PSS; the mass ratio of bacterial cellulose, lithium polyacrylate and PEDOT:PSS was 4.5:1.5:6;

[0216] The mass of the raw materials of the coating layer accounted for 4.7% of the mass of the manganese iron phosphate lithium.

[0217] 2. The modification method of the surface modified manganese iron phosphate lithium of the present example was as follows:

[0218] Step one, preparation of cellulose dispersion liquid:

[0219] The bacterial cellulose was added to the lithium polyacrylate aqueous solution and ultrasonically dispersed to obtain a fiber dispersion liquid;

[0220] Step two, surface complex treatment:

[0221] The mass-volume ratio of manganese iron phosphate and solvent is 60 g / L, the solvent is selected as ethanol / water with a volume ratio of 1:1, the lithium manganese iron phosphate is dispersed with the solvent, then added into the fiber dispersion solution, finally the solution of PEDOT:PSS (ammonia is used to adjust the pH to 5.0) is added, and stirring is performed for 20 min for crosslinking to obtain a mixed slurry;

[0222] Step three, granulation and heat treatment:

[0223] The mixed slurry is spray-dried and granulated, and gradient annealing treatment is performed to obtain surface-modified lithium manganese iron phosphate;

[0224] The gradient annealing includes the following stages:

[0225] Low-temperature stage: 250℃ for 1h;

[0226] Medium-temperature stage: 400℃ for 2.5h under Ar atmosphere;

[0227] High-temperature stage: 600℃ for 3h under Ar / H2 (volume ratio 95 / 5) atmosphere.

[0228] The SEM image of the surface-modified lithium manganese iron phosphate prepared in the present comparative example is shown in Figure 3 .

[0229] Test example

[0230] The polymer composite lithium iron phosphate material prepared in the above examples and comparative examples is prepared into a button-type half cell according to the following preparation steps:

[0231] (1) The polymer composite lithium iron phosphate material: acetylene black: polyvinylidene fluoride are mixed according to a mass ratio of 8:1:1, and a solvent N-methyl pyrrolidone (NMP) is added to adjust the viscosity to 4000-8000 mPa·s, to prepare a positive electrode slurry;

[0232] (2) The positive electrode slurry is uniformly coated on one surface (perpendicular to the thickness direction) of an aluminum foil (positive electrode current collector) and dried, then roll-pressed and sheared to obtain a positive electrode sheet containing a positive electrode film layer.

[0233] (3) A button-type half cell is prepared by using lithium hexafluorophosphate-methyl ethyl carbonate as an electrolyte, polypropylene as a separator film, and combining the prepared positive electrode sheet and a negative lithium sheet.

[0234] The button-type half cell prepared is tested by using a blue light equipment in a voltage range of 2.0-4.35V, including 1.0C and 5.0C discharge specific capacity, and capacity retention rate after 200 cycles at 1C, and the test results are shown in Table 1.

[0235] Table 1

[0236]

[0237] According to the test results in the above table and figure, the annealing process of Example 8 is different from that of Example 1, and the dispersibility and uniformity of the surface-modified lithium iron manganese phosphate material prepared in Example 8 are poor. In Comparative Example 1, the coating layer does not contain PEDOT:PSS, lacks a conductive network, and the performance failure leads to low capacity of the surface-modified lithium iron manganese phosphate material and fast cycle attenuation. In Comparative Example 2, the bacterial cellulose is not pretreated, the surface coating effect is poor, and the organic matter is aggregated on the surface of small particles, and the film formation is uneven.

[0238] Unless otherwise specifically indicated, all materials, reagents, solvents, and the like used in the present application are commercially available or are prepared by known methods. The above specific examples further illustrate the objects, technical solutions, and advantages of the present application. It should be understood that the above examples are merely specific embodiments of the present application and are not intended to limit the present application. Any modifications, equivalent replacements, improvements, and the like made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for modifying surface-modified lithium manganese iron phosphate, characterized in that: The following steps are involved: Step 1: Preparation of cellulose dispersion: After alkali washing, acid washing and neutralization treatment of bacterial cellulose, add it into lithium polyacrylate aqueous solution and ultrasonically disperse it to obtain fiber dispersion; Step 2: Surface composite treatment: adding lithium manganese iron phosphate to the fiber dispersion, then adding PEDOT:PSS, stirring and cross-linking to obtain a mixed slurry; Step 3: Granulation and heat treatment: spray drying the mixed slurry to granulate it, and then performing gradient annealing treatment to obtain surface-modified lithium manganese iron phosphate; The gradient annealing includes the following stages: Low temperature stage: 200~250℃ insulation for 1~2h; Medium temperature stage: in Ar atmosphere, keep at 350~450℃ for 2~3h; High temperature stage: in Ar / H2 atmosphere, keep warm at 550~650℃ for 3~5h.

2. The method for modifying the surface-modified lithium manganese iron phosphate according to claim 1, wherein: The alkali washing is carried out using a 3-5 wt% NaOH solution for 5-30 min; And / or, the acid washing is performed using a hydrochloric acid solution or an acetic acid solution until the pH value reaches 6.5 to 7.5; And / or, the mass concentration of the aqueous solution of lithium polyacrylate is 5-15wt%.

3. The method for modifying the surface-modified lithium iron manganese phosphate according to claim 1, wherein: Before the lithium manganese iron phosphate is added to the dispersion, it needs to be dispersed with a solvent; wherein the solvent is at least two of ethanol, water and N-methylpyrrolidone; and / or, the pH of the PEDOT:PSS solution is 4.5-5.5; And / or, 1-2 wt% of citric acid is further added to the mixed slurry; And / or, the stirring and cross-linking time is 10 to 50 minutes.

4. Surface modified lithium manganese iron phosphate, characterized in that: Prepared by the modification method of surface-modified lithium manganese iron phosphate according to any one of claims 1 to 3; The surface-modified lithium manganese iron phosphate comprises: a core, which is lithium manganese iron phosphate; The coating layer comprises bacterial cellulose, lithium polyacrylate and PEDOT:PSS.

5. The surface-modified lithium manganese iron phosphate according to claim 4, characterized in that: The chemical composition of the lithium manganese iron phosphate is LiMn x Fe 1-x PO4; where x = 0.3 to 0.6; And / or, the D50 particle size of the lithium manganese iron phosphate is 0.2-2 μm.

6. The surface-modified lithium manganese iron phosphate according to claim 4, characterized in that: The mass of the raw materials of the coating layer accounts for 3-12% of the mass of lithium manganese iron phosphate; And / or, the mass ratio of the bacterial cellulose: the lithium polyacrylate: the PEDOT:PSS is 1-5:0.5-2:2-8.

7. The surface-modified lithium manganese iron phosphate according to claim 4, characterized in that: The fiber diameter of the bacterial cellulose is 50-100 nm.

8. A positive electrode plate, characterized in that: The invention comprises the surface-modified lithium manganese iron phosphate according to any one of claims 4 to 7, or the surface-modified lithium manganese iron phosphate prepared by the modification method according to any one of claims 1 to 3.

9. A battery, characterized in that Comprising the positive electrode sheet as claimed in claim 8.

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