Method for preparing composite cathode active material, composite cathode active material, secondary battery and electronic device

Abstract

A method for preparing a composite cathode active material includes: adding an iron source, a manganese source, a phosphorus source, a lithium source, a carbon source and an optional transition metal element source into deionized water under stirring to obtain a first mixture, and spray-drying the first mixture to obtain a precursor material; mixing the precursor material with a polymer in a dry manner to obtain a second mixture, where an addition amount of the polymer is ranged from 3 to 18 g per mole of phosphorus element; compressing the second mixture to obtain a blank, and crushing the blank to obtain a third mixture, where a pressure for the compressing is from 0.5 to 10 MPa; and calcinating the third mixture in a nitrogen atmosphere, and performing airflow crushing on a calcinated product to obtain the composite cathode active material.

Description

[0001] P139682-FO

[0002] METHOD FOR PREPARING COMPOSITE CATHODE ACTIVE MATERIAL, COMPOSITE CATHODE ACTIVE MATERIAL, SECONDARY BATTERY AND ELECTRONIC DEVICE

[0003] FIELD

[0004] The present disclosure relates to the technical field of batteries, and more particularly to a method for preparing a composite cathode active material, a composite cathode active material, a secondary battery and an electronic device.

[0005] BACKGROUND

[0006] Lithium-ion batteries are widely used in electric vehicles, consumer electronics and energy storage systems. A cathode active material used for a positive electrode in the batteries will affect performances such as energy density of the lithium-ion batteries. However, with the existing preparation methods, the prepared lithium manganese iron phosphate usually has a low electron / ion conductivity, which leads to a low discharge capacity.

[0007] Therefore, there is a need to develop a method for preparing a cathode active material which is able to provide a cathode active material with improved electrochemical performances.

[0008] SUMMARY

[0009] Embodiments of the present disclosure seek to solve at least one of the problems existing in the related art to at least some extent. Objects of the present disclosure are to provide a method for preparing a composite cathode active material which is capable of providing a composite cathode active material with improved electrochemical performances such as a capacity and an electrical conductivity.

[0010] The finding of the present disclosure is to provide a method for preparing a composite cathode active material, a composite cathode active material, and a secondary battery and an electronic device.

[0011] According to a first aspect of the present disclosure, a method for preparing a composite cathode active material is provided. The composite cathode active material includes a lithium manganese iron phosphate material having a formula of LixMnyFezRaPO4, where R represents a transition metal element, 0.95<x<1.05, 0.3<y<0.8, 0.2<z<0.7, 0.95<y+z<l, and 0<a<0.05. The method includes: adding an iron source, a manganese source, a phosphorus source, a lithium source, a carbon source and an optional transition metal element source into deionized water under stirring to obtain a first mixture, and spray-drying the first mixture to obtain a precursor material; mixing the precursor material with a polymer in a dry manner to obtain a second mixture, where an addition amount of the polymer is ranged from 3 to 18 g per mole of phosphorus element; compressing the second mixture to obtain a blank, and crushing the blank to obtain a third mixture, where a pressure for the compressing is from 0.5 to 10 MPa; and calcinating the third mixture in a nitrogen atmosphere, and performing airflow crushing on a calcinated product to obtain the composite cathode active material. With the method provided in the present disclosure, the prepared cathode active material has a high compaction density, a low resistance, and a high capacity.

[0012] In some embodiments, before spray-drying the first mixture, the method further includes: performing grinding on the first mixture for 1-12 h.

[0013] In some embodiments, mixing the precursor material with the polymer includes: performing ball-milling on the precursor material and the polymer for 1 to 15 h.

[0014] In some embodiments, the crushing is selected from jaw crushing, cone crushing, and impact crushing.

[0015] In some embodiments, calcinating the third mixture includes: placing the third mixture in a furnace, increasing a temperature of the furnace to a first temperature ranging from 400 to 600 °C with a first heating rate of 1 to 5 °C / min, increasing the temperature of the furnace to a second temperature ranging from 600 to 800 °C with a second heating rate of 0.5 to 3 °C / min, and keeping the second temperature for 6 to 12 h.

[0016] In some embodiments, the polymer includes at least one selected from polyacrylonitrile, polyurethane, polyvinylpyrrolidone (PVP), poly(ethylene oxide) (PEO), polylactic acid (PLA), polyglycolic acid (PGA), poly(lactic-co-glycolic acid (PLGA), ethylene-vinyl acetate copolymer (EVA), acrylic resin, cellulose and derivatives thereof, starch and derivatives thereof, wax, polyethylene glycol, polyvinyl alcohol, and ethylene bis(stearamide).

[0017] In some embodiments, the cellulose and derivatives thereof include at least one selected from hydroxypropyl methylcellulose (HPMC), hydroxypropylcellulose (HPC), ethyecellulose (EC), methyl cellulose (MC) and viscose.

[0018] In some embodiments, the addition amount of the polymer is ranged from 6 to 15 g per mole of phosphorus element.

[0019] In some embodiments, the pressure for the compressing is from 1 to 5 MPa. In some embodiments, R is at least one element selected from a group including Mg, Ca, Sr, Ti, V, Cr, Co, Cu, Zn, Zr, Y, Mo, Nb, W, La and Sm.

[0020] In some embodiments, an addition amount of the carbon source is ranged from 6 to 12 g per mole of phosphorus element.

[0021] In some embodiments, the carbon source includes a first component served as a dispersing agent of 20 wt% to 80wt% based on a total weight of the carbon source.

[0022] In some embodiments, the dispersing agent includes at least one selected from polyethylene glycol 400, polyethylene glycol 6000, polyvinyl alcohol, methyl cellulose and ethylene bis(stearamide).

[0023] In some embodiments, the carbon source includes a second component being at least one selected from glucose, sucrose, vitamin C, graphene, graphite and carbon black.

[0024] In some embodiments, the iron source includes at least one selected from iron phosphate, ferric oxide, ferroferric oxide, ferrous sulfate, ferric hydroxide, iron carbonate, ferrous oxalate, iron nitrate, and ferric chloride.

[0025] In some embodiments, the manganese source includes at least one selected from manganous-manganic oxide, manganese dioxide, manganese carbonate, manganese hydrogen phosphate, manganese anhydride, manganese sulfate, manganese hydroxide, manganese acetate, manganese nitrate, and manganese oxalate.

[0026] In some embodiments, the lithium source includes at least one selected from lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, lithium phosphate and lithium acetate.

[0027] In some embodiments, the phosphorus source includes at least one selected from orthophosphate, condensed phosphate and organophosphorus.

[0028] In some embodiments, the phosphorus source includes at least one selected from phosphoric acid, lithium dihydrogen phosphate, monoammonium phosphate, pyrophosphoric acid, metaphosphoric acid, hydroxyethylidene diphosphonic acid, 2-hydroxyphosphonoacetic acid and aminotrimethylene phosphonic acid.

[0029] According to a second aspect of the present disclosure, a composite cathode active material prepared by the method as described in the first aspect is provided. The prepared cathode active material has a high compaction density, a low resistance, and a high capacity.

[0030] According to a third aspect of the present disclosure, s secondary battery including a positive electrode including the composite cathode active material as described in the second aspect is provided. Due to the composite cathode active material, the secondary battery of the present disclosure has a high capacity and a long service life.

[0031] According to a fourth aspect of the present disclosure, an electronic device including the secondary battery of the third aspect is provided. The electronic device of the present disclosure is powered by the secondary battery and may be working stably.

[0032] It is to be understood that both the foregoing general description and the following detailed description are illustrative and explanatory only and shall not be construed to limit the present disclosure.

[0033] DETAILED DESCRIPTION

[0034] Reference will now be made in detail to embodiments. The implementations set forth in the following description of the embodiments do not represent all implementations consistent with the present disclosure.

[0035] Terms used herein in embodiments of the present disclosure are only for the purpose of describing specific embodiments, but should not be construed to limit the present disclosure. As used in the embodiments of the present disclosure and the appended claims, “a / an”, and “the” in singular forms are intended to include plural forms, unless clearly indicated in the context otherwise. It should also be understood that, the term “and / or” used herein represents and contains any or all possible combinations of one or more associated listed items.

[0036] When term “about” is used, this term may mean that there can be a variance in value of up to ±10%, of up to 5%, of up to 2%, of up to 1%, of up to 0.5%, of up to 0.1%, or up to 0.01%.

[0037] Term “range” disclosed in the present disclosure is defined in the form of a lower limit and an upper limit, a given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of a special range. The range defined in this way can be inclusive or exclusive, and can be arbitrarily combined, that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is understood that ranges of 60-110 and 80-120 are also obtained. In addition, if the listed minimum values are 1 and 2, and if the listed maximum values are 3, 4 and 5, the ranges of 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5 may be obtained. In the present disclosure, unless otherwise specified, the numerical range “a-b” means the abbreviated representation of any combination of real numbers between a and b, where a and b are both real numbers. For example, the numerical range “0-5” means that all the real numbers between “0-5” have been listed, and “0-5” is only the abbreviated representation of these numerical combinations. In addition, when a parameter is an integer >2, it is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0038] Lithium-ion batteries are widely used in electric vehicles, consumer electronics and energy storage systems. A cathode active material used for a positive electrode in the batteries will affect performances of the lithium-ion batteries. However, with the existing preparation methods, the prepared lithium manganese iron phosphate usually has a low electron / ion conductivity, which leads to a low discharge capacity. To improve the electrochemistry performance of lithium manganese iron phosphate, a carbon material is added for modification. A conductive carbon material is usually added to crystallized lithium manganese iron phosphate, and in order to achieve sufficient conductivity, a large amount of the carbon material is required, which will affect the energy density and the processability of the prepared cathode active material.

[0039] Therefore, there is a need to develop a method for preparing a cathode active material which is able to provide a cathode active material with improved electrochemical performances.

[0040] In the present disclosure, a method for preparing a composite cathode active material is provided. The composite cathode active material includes a lithium manganese iron phosphate material having a formula of LixMnyFezRaPO4, where R represents a transition metal element, 0.95<x<1.05, 0.3<y<0.8, 0.2<z<0.7, 0.95<y+z<l, and 0<a<0.05. It can be understood that in the formula, a sum of positive valences and negative valences is zero. For example, the lithium manganese iron phosphate material is LiMno.sFer PO^, LiMno.7Feo.3PO4, LiMno.oFeo.4PO4, or LiMno.5Feo.5PO4, LiMn .1Feo.6PO4, LiMno 3Feo.7PO4. Further, the lithium manganese iron phosphate material may be doped by the transition metal element, and may be for example LiMn oFeons T1002PO4.

[0041] The method for preparing the composite cathode active material includes: adding an iron source, a manganese source, a phosphorus source, a lithium source, a carbon source and an optional transition metal element source into deionized water under stirring to obtain a first mixture, and spray drying the first mixture to obtain a precursor material; mixing the precursor material with a polymer in a dry manner to obtain a second mixture, where an addition amount of the polymer is ranged from 3 to 18 g per mole of phosphorus element; compressing the second mixture to obtain a blank, and crushing the blank to obtain a third mixture, where a pressure for the compressing is from 0.5 to 10 MPa; and calcinating the third mixture in a nitrogen atmosphere, and performing airflow crushing on a calcinated product to obtain the composite cathode active material.

[0042] In the present disclosure, the lithium manganese iron phosphate precursor material is prepared by the spray drying, so that the different material sources are mixed uniformly. The polymer which is served as an additional carbon material is mixed with the precursor material in the dry manner to obtain the second mixture, the second mixture is compressed into the blank, the blank is crushed and calcinated. The pressure for the compressing is from 0.5 to 10 MPa, for example, from 1 to 8 MPa, such as 2 MPa, 3 MPa, 4 MPa, 5 MPa, 6 MPa and 7 MPa. If the pressure is too low, a lot of gaps exist among particles of the blank and the particles lack contact to each other, which may damage the effect of substance transfer caused by the calcination. If the pressure is too high, it is difficult to crush the blank. With the method provided in the present disclosure, the prepared cathode active material has a high compaction density, a low resistance, and a high capacity.

[0043] For example, the prepared cathode active material may have a compaction density measured at 3 tons of no less than 2.3 g / cm3, a resistivity of less than 50 (1 cm, and a 0.1 C discharge capacity of greater than 150 mAh / g.

[0044] In some embodiments, before spray drying the first mixture, the method further includes: performing grinding on the first mixture for 1-12 h. By grinding the first mixture, the size of particles of the first mixture is reduced and the different types of the sources in the first mixture may be mixed more uniformly.

[0045] In some embodiments, mixing the precursor material with the polymer includes: performing ball-milling on the precursor material and the polymer for 1 to 15 h. For example, a ball mill may be used to grind the precursor material. By ball-milling the precursor material, the size of particles of the precursor material is reduced and different substances in the precursor material may be mixed more uniformly.

[0046] In some embodiments, the crushing is selected from jaw crushing, cone crushing, and impact crushing.

[0047] In some embodiments, the calcinating the third mixture including: placing the third mixture in a furnace, increasing a temperature of the furnace to a first temperature ranging from 400 to 600 °C with a first heating rate of 1 to 5 °C / min, increasing the temperature of the furnace to a second temperature ranging from 600 to 800 °C with a second heating rate of 0.5 to 3 °C / min, and keeping the second temperature for 6 to 12 h. In the present disclosure, the second temperature is greater than the first temperature. In some embodiments, the first heating rate is greater than or equal to the second heating rate.

[0048] In some embodiments, R in the formula of the lithium manganese iron phosphate material is at least one element selected from a group including Mg, Ca, Sr, Ti, V, Cr, Co, Cu, Zn, Zr, Y, Mo, Nb, W, La and Sm, and 0.0002<a<0.05. For example, Ti is doped in lithium manganese iron phosphate. Without wishing to be bound by any theory, a bond energy of Ti-0 is greater than that of Fe-O, introduction of Ti can reduce a grain size of the lithium manganese iron phosphate material, shorten the lithium ion transmission path and improve the capacity of the composite cathode active material.

[0049] In some embodiments, the polymer includes at least one selected from polyacrylonitrile, polyurethane, polyvinylpyrrolidone (PVP), poly(ethylene oxide) (PEO), polylactic acid (PLA), polyglycolic acid (PGA), poly(lactic-co-glycolic acid (PLGA), ethylene-vinyl acetate copolymer (EVA), acrylic resin, cellulose and derivatives thereof, starch and derivatives thereof, wax, polyethylene glycol, polyvinyl alcohol, and ethylene bis(stearamide).

[0050] In some embodiments, the cellulose and derivatives thereof include at least one selected from hydroxypropyl methylcellulose (HPMC), hydroxypropylcellulose (HPC), ethyecellulose (EC), methyl cellulose (MC) and viscose.

[0051] In the present disclosure, the addition amount of the polymer is ranged from 3 to 18 g per mole of phosphorus element. In some embodiments, the addition amount of the polymer is ranged from 6 to 15 g per mole of phosphorus element, for example, 7, 8, 9, 10, 11, 12, 13, 14 g. When the addition amount of the polymer is too low, the resistivity of the composite cathode active material is high and the capacity of the composite cathode active material is low. When the addition amount of the polymer is too high, the resistivity is low but the capacity is also low because the carbon material itself has no electrochemical activity.

[0052] In some embodiments, an addition amount of the carbon source is ranged from 6 to 12 g per mole of phosphorus element. In some embodiments, the carbon source includes a first component served as a dispersing agent of 20 wt% to 80wt% based on a total weight of the carbon source. Further, the dispersing agent includes at least one selected from polyethylene glycol (for example, polyethylene glycol 400, polyethylene glycol 6000, or polyethylene glycol 20000), polyvinyl alcohol, methyl cellulose and ethylene bis(stearamide). In some embodiments, the carbon source includes a second component being at least one selected from glucose, sucrose, vitamin C, graphene, graphite and carbon black.

[0053] For example, the carbon source includes 20 wt% of polyvinyl alcohol and 80 wt% of glucose, that is a content of the dispersing agent based on a total weight of the carbon source is 80 wt%. Without wishing to be bound by any theory, the lack of the dispersing agent affects distribution of the components and formation of the crystal structure. Uniformity of carbon coating will poor, and the conductive carbon network formed on the surface and inside of the final product will also be poor, resulting in poor electrochemical performance of the final product. On the other hand, when the carbon source is 100% dispersing agent, the carbon material tends to disperse inside the product, and carbon may be not enough to form the coating on the surface of the product, which leads to poor electrochemical performance of the final product.

[0054] In some embodiments, the iron source includes at least one selected from iron phosphate, ferric oxide, ferroferric oxide, ferrous sulfate, ferric hydroxide, iron carbonate, ferrous oxalate, iron nitrate, and ferric chloride.

[0055] In some embodiments, the manganese source includes at least one selected from manganous-manganic oxide, manganese dioxide, manganese carbonate, manganese hydrogen phosphate, manganese anhydride, manganese sulfate, manganese hydroxide, manganese acetate, manganese nitrate, and manganese oxalate.

[0056] In some embodiments, the lithium source includes at least one selected from lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, lithium phosphate and lithium acetate.

[0057] In some embodiments, the phosphorus source includes at least one selected from orthophosphate, condensed phosphate and organophosphorus. For example, the phosphorus source includes at least one selected from phosphoric acid, lithium dihydrogen phosphate, monoammonium phosphate, pyrophosphoric acid, metaphosphoric acid, hydroxyethylidene diphosphonic acid, 2- hydroxyphosphonoacetic acid and aminotrimethylene phosphonic acid.

[0058] It can be known that a substance may be used to provide one or more elements. For example, iron phosphate is served as both the iron source and the phosphorus source.

[0059] The present disclosure further provides in embodiments a composite cathode active material prepared by the method described in any one of the above embodiments.

[0060] With the method described above, the composite cathode active material is prepared and includes a lithium manganese iron phosphate material having a formula of LixMnyFezRaPO4, where R represents a transition metal element, 0.95<x<1.05, 0.3<y<0.8, 0.2<z<0.7, 0.95<y+z<l, and 0<a<0.05, and a carbon material. The prepared cathode active material has a high compaction density, a low resistance, and a high capacity.

[0061] It should be noted that details and effects of the composite cathode active material can be referred to the above embodiments of the method of the present disclosure, which will not be elaborated here.

[0062] The composite cathode active material may be used to preparing a positive electrode plate.

[0063] The positive electrode plate includes a positive electrode current collector and a positive electrode film layer arranged on at least one surface of the positive electrode current collector, and the positive electrode film layer includes the cathode active material, i.e., the composite material of the present disclosure.

[0064] As an example, the positive electrode current collector has two surfaces that are opposite in its thickness direction, and the positive electrode film layer is arranged on either or both of the two opposite surfaces of the positive electrode current collector. The positive electrode film layer includes the present material that is capable of absorbing and releasing lithium.

[0065] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, an aluminum foil is used as the metal foil. The composite current collector may include a polymer base layer and a metal layer formed on at least one surface of the polymer base layer. The composite current collector may be formed by forming a metal material (such as aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy) on the polymer material (such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE)).

[0066] The positive electrode film layer optionally includes a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, fluorinated acrylate resin, or any combination thereof.

[0067] In some embodiments, the positive electrode film layer includes the binder of 0.1 to 3.5%, optionally 0.5 to 2.5% by weight.

[0068] In some embodiments, the positive electrode plate can be prepared by dispersing the above-mentioned components for preparing the positive electrode plate, such as the cathode active material, the binder and any other components, in a solvent (such as N-methylpyrrolidone) to form a positive paste. The positive paste is coated on the positive electrode current collector, and after drying and cold pressing, the positive electrode plate is obtained.

[0069] A secondary battery may include the positive electrode plate of the present disclosure described above, a negative electrode plate, a separator and an electrolyte. The secondary battery may be a battery module or a battery pack, which may be applied in electronic devices, such as mobile terminals and vehicles. Due to the composite cathode active material, the secondary battery of the present disclosure has a high capacity and a long service life. The electronic device of the present disclosure is powered by the secondary battery and may be working stably.

[0070] The negative electrode plate includes a negative electrode current collector and a negative electrode film layer disposed on at least one surface of the negative electrode current collector. The negative electrode film layer includes an anode active material.

[0071] As an example, the negative electrode current collector has two surfaces along a thickness direction thereof and facing in opposite directions, and the negative electrode film layer is provided on either or both of the two surfaces.

[0072] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, as the metal foil, a copper foil may be used. The composite current collector may include a polymeric material substrate and a metal layer formed on at least one surface of the polymeric material substrate. The composite current collector may be formed by forming a metallic material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a substrate of a high molecular material such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0073] In some embodiments, the anode active material may be an anode active material known in the art. As an example, the anode active material may include at least one selected from artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials and lithium titanate. The silicon-based material may be at least one selected from elemental silicon, silicon-oxygen compounds, silicon-carbon complexes, silicon-nitrogen complexes, and silicon alloys. The tin-based material may be at least one selected from elemental tin, tin oxide compounds, and tin alloys. The present disclosure is not limited to these materials, and other materials that may be used as an anode active material for a battery may be used. These anode active materials may be used separately or in combination (for example two or more kinds of materials are used). In some embodiments, the negative electrode film layer optionally includes a binder. The binder may be at least one selected from styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA) and carboxymethyl chitosan (CMCS).

[0074] In some embodiments, the negative electrode film layer optionally includes a conductive agent. The conductive agent may be at least one selected from superconducting carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0075] In some embodiments, the negative electrode film layer optionally includes other adjuvants, such as thickeners (e.g. sodium carboxymethylcellulose (CMC -Na)).

[0076] In some embodiments, the negative electrode plate may be prepared by: dispersing the above-mentioned components for preparing the negative electrode plate, such as the anode active material, the conductive agent, the binder and any other components in a solvent (such as deionized water) to form a negative electrode slurry; coating the negative electrode slurry on the negative electrode current collector, and obtaining the negative electrode plate after drying, cold pressing and other processes.

[0077] The electrolyte serves to conduct ions between the positive electrode plate and the negative electrode plate. The kind of the electrolyte is not particularly limited in the present disclosure, and may be selected according to requirements. For example, the electrolyte may be liquid, gel, or solid.

[0078] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte includes an electrolyte salt and a solvent.

[0079] In some embodiments, the electrolyte salt may include at least one selected from lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonimide, lithium bis-trifluoromethane sulfonimide, lithium triflate, lithium difluorophosphate, lithium difluorooxalato borate, lithium dioxalato borate, lithium difluorooxalato phosphate, and lithium tetrafluorooxalato phosphate.

[0080] In some embodiments, the solvent may include at least one selected from ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0081] In some embodiments, the electrolyte optionally includes an additive. For example, the additive may include a negative electrode film-forming additive, a positive electrode film-forming additive, and may further include an additive capable of improving properties of the battery, such as an additive for improving overcharge properties of the battery, and an additive for improving high-temperature or low-temperature properties of the battery.

[0082] In some embodiments, the separator is further included in the secondary battery. The type of the separator is not particularly limited in the present disclosure, and any known separator having a porous structure and good chemical and mechanical stability may be used.

[0083] In some embodiments, the material of the separator may be at least one selected from glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator may be a single-layer film or a multi-layer composite film, which is not limited in the present disclosure. In a case where the separator is a multilayer composite film, the materials of individual layers may be the same or different.

[0084] In some embodiments, the electrolyte is a lithium ion solid electrolyte. In some embodiments, the positive electrode plate, the negative electrode plate, and the separator may be prepared into an electrode assembly by a winding process or a lamination process.

[0085] In some embodiments, the secondary battery includes an outer package. The outer package is used to package the electrodes and the electrolyte.

[0086] In some embodiments, the outer package of the secondary battery may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell and the like. Alternatively, the outer package of the secondary battery may be a soft package, such as a soft bag. The soft bag may be made of a polymer material such as plastics, polypropylene, polybutylene terephthalate and polybutylene succinate.

[0087] The shape of the secondary battery may be cylindrical, square or any other shape, which is not limited in the present disclosure.

[0088] Experimental Section

[0089] The following Examples are included to demonstrate aspects and embodiments of the present disclosure. It should be appreciated that the following description is explanatory only and should not be taken in any way as a restriction of the present disclosure.

[0090] Test methods Resistivity of the composite cathode active material is measured with a powder resistivity tester according to GB / T 30835-2014 at 25 °C.

[0091] Compaction density is measured by a compaction density instrument at a testing pressure of 3 tons according to GB / T 30835-2014.

[0092] Initial discharge capacity at a rate of 0.1C is tested on a coin half-cell. In the half-cell, a weight ratio LMFP:SP:PVDF = 95.8%:2%:2.2%, where LMFP represents the composite cathode active material of the present disclosure, SP represents SUPER P conductive carbon black and PVDF represents polyvinylidene fluoride. Materials represented by LMFP, SP and PVDF are added into a solvent and a solid content is 50 wt%. A prepared electrode plate has a compaction density of 2.40 g / cm3, and a surface density of 15 mg / cm2. An aluminum foil of 13 pm is used. An electrolyte is 1 mol / L LiPFe / EC+DMC (that is, a solute of the electrolyte is LiPFe, and a solvent is a mixed solvent of ethylene carbonate and dimethyl carbonate with a mass ratio of 1 : 1). A test voltage is from 2.0 to 4.5 V and a test rate is 0.1 C to obtain an initial discharge capacity of the composite cathode active material of the present disclosure.

[0093] Examples

[0094] Inventive Example 1 (IE1)

[0095] (1) 0.5 mol of lithium carbonate, 0.2 mol of manganous-manganic oxide, 0.19 mol of ferric oxide, 0.02 mol of titanium dioxide, 1 mol of phosphoric acid, 9 g of glucose and 3 g of polyvinyl alcohol were added into 5L of deionized water under stirring to obtain a first mixture, the first mixture was grinded for 1 hour, and then spray-dried to obtain a precursor material.

[0096] (2) The precursor material and 6g of polyvinylpyrrolidone were added into a ball mill for ball milling and mixing in a dry manner for 8 hours to obtain a second mixture.

[0097] (3) The second mixture was compressed into a blank with a pressure of 2 MPa, and the blank was crushed to obtain a third mixture.

[0098] (4) The third mixture was transferred to a furnace, and calcinated in a nitrogen atmosphere. In the calcination, a temperature of the furnace was raised to 500 °C at a heating rate of 5 °C / min, and then to 680 °C at a heating rate of 2 °C / min, and the temperature was kept for 12 hours. After cooled to a room temperature, the calcinated product was crushed by airflow to obtain a composite cathode active material.

[0099] Inventive Example 2 (IE2)

[0100] (1) 0.33 mol of lithium phosphate, 0.5 mol of manganese carbonate, 0.49 mol of ferrous oxalate, 0.005 mol of niobium pentoxide, 0.67 mol of monoammonium phosphate, 6 g of fatty acid and 6 g of polyethylene glycol 20000 were added into 5L of deionized water under stirring to obtain a first mixture, the first mixture was grinded for 5 hours, and then spray-dried to obtain a precursor material.

[0101] (2) The precursor material and 4.5 g of poly(ethylene oxide) were added into a ball mill for ball milling and mixing in a dry manner for 2 hours to obtain a second mixture.

[0102] (3) The second mixture was compressed into a blank with a pressure of 5 MPa, and the blank was crushed to obtain a third mixture.

[0103] (4) The third mixture was transferred to a furnace, and calcinated in a nitrogen atmosphere. In the calcination, a temperature of the furnace was raised to 600 °C at a heating rate of 3 °C / min, and then to 740 °C at a heating rate of 3 °C / min, and the temperature was kept for 10 hours. After cooled to a room temperature, the calcinated product was crushed by airflow to obtain a composite cathode active material.

[0104] Inventive Example 3 (IE3)

[0105] (1) 1.05 mol of lithium hydroxide, 0.7 mol of manganese oxalate, 0.285 mol of iron phosphate, 0.015 mol of magnesium oxide, 0.715 mol of 2-hydroxyphosphonoacetic acid, 3 g of vitamin C and 9 g of methylcellulose were added into 5L of deionized water under stirring to obtain a first mixture, the first mixture was grinded for 9 hours, and then spray-dried to obtain a precursor material.

[0106] (2) The precursor material and 12 g of ethylene bis(stearamide) were added into a ball mill for ball milling and mixing in a dry manner for 5 hours to obtain a second mixture.

[0107] (3) The second mixture was compressed into a blank with a pressure of 1 MPa, and the blank was crushed to obtain a third mixture.

[0108] (4) The third mixture was transferred to a furnace, and calcinated in a nitrogen atmosphere. In the calcination, a temperature of the furnace was raised to 550 °C at a heating rate of 1 °C / min, and then to 720 °C at a heating rate of 0.5 °C / min, and the temperature was kept for 6 hours. After cooled to a room temperature, the calcinated product was crushed by airflow to obtain a composite cathode active material.

[0109] Inventive Example 4 (IE4)

[0110] A same preparation process as that of IE1 was performed expect that titanium dioxide was not added and 0.2 mol of ferric oxide was added for obtaining the first mixture.

[0111] Inventive Example 5 (IE5) A same preparation process as that of IE1 was performed expect that 3g of polyvinylpyrrolidone was added into a ball mill.

[0112] Inventive Example 6 (IE6)

[0113] A same preparation process as that of IE1 was performed expect that 9g of polyvinylpyrrolidone was added into a ball mill.

[0114] Inventive Example 7 (IE7)

[0115] A same preparation process as that of IE1 was performed expect that 12g of polyvinylpyrrolidone was added into a ball mill.

[0116] Inventive Example 8 (IE8)

[0117] A same preparation process as that of IE1 was performed expect that 15g of polyvinylpyrrolidone was added into a ball mill.

[0118] Inventive Example 9 (IE9)

[0119] A same preparation process as that of IE1 was performed expect that 18g of polyvinylpyrrolidone was added into a ball mill.

[0120] Inventive Example 10 (IE 10)

[0121] A same preparation process as that of IE1 was performed expect that the second mixture was compressed into a blank with a pressure of 0.5 MPa.

[0122] Inventive Example 11 (IE11)

[0123] A same preparation process as that of IE1 was performed expect that the second mixture was compressed into a blank with a pressure of 1 MPa.

[0124] Inventive Example 12 (IE 12)

[0125] A same preparation process as that of IE1 was performed expect that the second mixture was compressed into a blank with a pressure of 5 MPa.

[0126] Inventive Example 13 (IE 13)

[0127] A same preparation process as that of IE1 was performed expect that the second mixture was compressed into a blank with a pressure of 8 MPa.

[0128] Inventive Example 14 (IE 14)

[0129] A same preparation process as that of IE1 was performed expect that the second mixture was compressed into a blank with a pressure of 10 MPa.

[0130] Inventive Example 15 (IE15)

[0131] A same preparation process as that of IE1 was performed expect that 4.5 g of glucose and 1.5 g of polyvinyl alcohol were added in operation (1), and 12 g of polyvinylpyrrolidone was added in operation (2).

[0132] Inventive Example 16 (IE 16)

[0133] A same preparation process as that of IE1 was performed expect that 6.75 g of glucose and 2.25 g of polyvinyl alcohol were added in operation (1), and 9 g of polyvinylpyrrolidone was added in operation (2).

[0134] Inventive Example 17 (IE17)

[0135] A same preparation process as that of IE1 was performed expect that 2.4 g of glucose and 9.6 g of polyvinyl alcohol were added.

[0136] Inventive Example 18 (IE18)

[0137] A same preparation process as that of IE1 was performed expect that 6 g of glucose and 6 g of polyvinyl alcohol were added.

[0138] Comparative Example 1 (CE1)

[0139] A same preparation process as that of IE1 was performed expect that polyvinylpyrrolidone was not added.

[0140] Comparative Example 2 (CE2)

[0141] A same preparation process as that of IE1 was performed expect that operations (2) and (3) were not performed. That is, the precursor material of CE2 was calcinated directly.

[0142] Comparative Example 3 (CE3)

[0143] A same preparation process as that of IE1 was performed expect for operation (1), in which 0.5 mol of lithium carbonate, 0.2 mol of manganous-manganic oxide, 0.19 mol of ferric oxide, 0.02 mol of titanium dioxide, 1 mol of phosphoric acid, 9 g of glucose and 3 g of polyvinyl alcohol were mixed to obtain a precursor material.

[0144] Comparative Example 4 (CE4)

[0145] A same preparation process as that of IE1 was performed expect that 6g of polyvinylpyrrolidone was added into 5L of deionized water in operation (1) instead of being added in operation (2), and the precursor material obtained in operation (1) was subjected to operation (3).

[0146] Comparative Example 5 (CE5)

[0147] A same preparation process as that of IE1 was performed expect that 21g of polyvinylpyrrolidone were added into a ball mill.

[0148] Comparative Example 6 (CE6)

[0149] A same preparation process as that of IE1 was performed expect that the second mixture was compressed into a blank with a pressure of 0.1 MPa.

[0150] Comparative Example 7 (CE7)

[0151] A same preparation process as that of IE1 was performed expect that the second mixture was compressed into a blank with a pressure of 12 MPa.

[0152] Comparative Example 8 (CE8)

[0153] A same preparation process as that of IE1 was performed expect that 12 g of glucose was added and no polyvinyl alcohol was added.

[0154] Comparative Example 9 (CE9)

[0155] A same preparation process as that of IE1 was performed expect that 12 g of polyvinyl alcohol was added and no glucose was added.

[0156] Performances of the composite cathode active materials of the inventive examples and comparative examples are tested and recorded in tables as follows.

[0157] Table 1

[0158] Table 1 shows that the prepared composite cathode active materials of IE1-4 of the present disclosure exhibit higher compaction density, lower resistance, and higher capacity compared with the materials of CE1-CE4. IE1 to IE3 show that the composite cathode active material can be obtained at various conditions. IE4 show that the composite cathode active material without the doping element (i.e., Ti) can still achieve the good electrochemical performance.

[0159] The material of CE1 is prepared without adding the polymer material (e.g., polyvinylpyrrolidone). Compared with IE1, CE1 shows a higher resistivity and an inferior discharge capacity. The material of CE2 is prepared without addition of the polymer material and the compression. It can be seen that the compaction density drops significantly and the discharge capacity is poor. The material of CE3 is prepared in a dry manner and exhibits undesired high resistivity and low discharge capacity. This may be because, without wishing to be bound by any theory, dry mixing is not enough to evenly distribute the components for preparing the composite cathode active material. For CE4, the polymer is added in water in operation (1), but it still shows an inferior discharge capacity.

[0160] Table 2

[0161] It can be known from Table 2 that when the addition amount of the polymer is too low, the resistivity of the composite cathode active material is high and the capacity of the composite cathode active material is low. When the addition amount of the polymer is too high, the resistivity is low but the capacity is also low because the carbon material itself has no electrochemical activity. It can be seen that compared with IES, the composite cathode active material of CE5 has a low resistivity due to the large amount of the polymer added, but the discharge capacity is still not satisfactory.

[0162] Table 3

[0163] It can be known from Table 3 that when the pressure for compressing the second mixture into the blank is in a range of 0.5 to 10 MPa, the properties of the composite cathode active materials of the present disclosure are at a good balance. When the pressure is low and the compaction density is low (e.g., for CE6), a lot of gaps exist among particles of the blank and the particles lack contact to each other, which may damage the effect of substance transfer caused by the calcination. When the pressure is too high, it is difficult to crush the blank, which will lead to the increase of the resistivity and the decrease of the capacity.

[0164] Table 4

[0165] It can be known from Table 4 that the ratio of the carbon source (i.e., the weight sum of glucose and polyvinyl alcohol in the examples) to the polymer (i.e., polyvinylpyrrolidone in the examples) may affect the properties of the composite cathode active material. According to Table 4, the material prepared under a condition of a ratio of 1 : 1 exhibits the optimum discharge capacity, indicating that at this ratio, a uniform carbon network can be formed.

[0166] Table 5

[0167] Table 5 shows effect of a content of a dispersing agent based on a total weight of the carbon source added in operation (1) on the properties of the composite cathode active material. It can be seen from Table 5, CE8 and CE9 show poor properties. Without wishing to be bound by any theory, this may be because, the lack of the dispersing agent affects the distribution of components and formation of the crystal structure. Uniformity of carbon coating is poor, and the conductive network formed by the carbon material on the surface and inside of the product is also poor, resulting in poor electrochemical performance of the final product. When the carbon source is 100% dispersing agent, the carbon material tends to disperse inside the product, and there is not enough carbon to form the coating on the surface of the product, which leads to poor electrochemical performance of the final product.

[0168] Reference throughout this specification to “an embodiment,” “some embodiments,” “one embodiment”, “another example,” “an example,” “a specific example,” or “some examples,” means that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the present disclosure. Thus, the appearances of the phrases such as “in some embodiments,” “in one embodiment”, “in an embodiment”, “in another example,” “in an example,” “in a specific example,” or “in some examples,” in various places throughout this specification are not necessarily referring to the same embodiment or example of the present disclosure. Furthermore, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments or examples.

[0169] Other embodiments of the present disclosure will be apparent to those skilled in the art from consideration of the specification and practice of the disclosure disclosed here. This application is intended to cover any variations, uses, or adaptations of the disclosure following the general principles thereof and including such departures from the present disclosure as come within known or customary practice in the art. It is intended that the specification and examples be considered as illustrative only, with a true scope and spirit of the disclosure being indicated by the following claims.

Claims

CLAIMS1. A method for preparing a composite cathode active material, wherein the composite cathode active material comprises a lithium manganese iron phosphate material having a formula of LixMnyFezRaPO4, where R represents a transition metal element, 0.95<x<1.05, 0.3<y<0.8, 0.2<z<0.7, 0.95<y+z<l, and 0<a<0.05, and wherein the method comprises: adding an iron source, a manganese source, a phosphorus source, a lithium source, a carbon source and an optional transition metal element source into deionized water under stirring to obtain a first mixture, and spray-drying the first mixture to obtain a precursor material; mixing the precursor material with a polymer in a dry manner to obtain a second mixture, wherein an addition amount of the polymer is ranged from 3 to 18 g per mole of phosphorus element; compressing the second mixture to obtain a blank, and crushing the blank to obtain a third mixture, wherein a pressure for the compressing is from 0.5 to 10 MPa; and calcinating the third mixture in a nitrogen atmosphere, and performing airflow crushing on a calcinated product to obtain the composite cathode active material.

2. The method according to claim 1, wherein before spray-drying the first mixture, the method further comprises: performing grinding on the first mixture for 1-12 h.

3. The method according to claim 1 or 2, wherein mixing the precursor material with the polymer comprises: performing ball-milling on the precursor material and the polymer for 1 to 15 h.

4. The method according to any one of claims 1 to 3, wherein the crushing is selected from jaw crushing, cone crushing, and impact crushing.

5. The method according to any one of claims 1 to 4, wherein calcinating the third mixture comprises: placing the third mixture in a furnace, increasing a temperature of the furnace to a first temperature ranging from 400 to 600 °C with a first heating rate of 1 to 5 °C / min, increasing thetemperature of the furnace to a second temperature ranging from 600 to 800 °C with a second heating rate of 0.5 to 3 °C / min, and keeping the second temperature for 6 to 12 h.

6. The method according to any one of claims 1 to 5, wherein the polymer comprises at least one selected from polyacrylonitrile, polyurethane, polyvinylpyrrolidone (PVP), poly(ethylene oxide) (PEO), polylactic acid (PLA), polyglycolic acid (PGA), poly(lactic-co-glycolic acid (PLGA), ethylene-vinyl acetate copolymer (EVA), acrylic resin, cellulose and derivatives thereof, starch and derivatives thereof, wax, polyethylene glycol, polyvinyl alcohol, and ethylene bis(stearamide).

7. The method according to claim 6, wherein the cellulose and derivatives thereof comprise at least one selected from hydroxypropyl methylcellulose (HPMC), hydroxypropylcellulose (HPC), ethyecellulose (EC), methyl cellulose (MC) and viscose.

8. The method according to any one of claims 1 to 7, wherein the addition amount of the polymer is ranged from 6 to 15 g per mole of phosphorus element.

9. The method according to any one of claims 1 to 8, wherein the pressure for the compressing is from 1 to 5 MPa.

10. The method according to any one of claims 1 to 9, wherein R is at least one element selected from a group comprising Mg, Ca, Sr, Ti, V, Cr, Co, Cu, Zn, Zr, Y, Mo, Nb, W, La and Sm.

11. The method according to any one of claims 1 to 10, wherein an addition amount of the carbon source is ranged from 6 to 12 g per mole of phosphorus element.

12. The method according to any one of claims 1 to 11, wherein the carbon source comprises a first component served as a dispersing agent of 20 wt% to 80wt% based on a total weight of the carbon source.

13. The method according to claim 12, wherein the dispersing agent comprises at least oneselected from polyethylene glycol 400, polyethylene glycol 6000, polyvinyl alcohol, methyl cellulose and ethylene bis(stearamide).

14. The method according to 12 or 13, wherein the carbon source comprises a second component being at least one selected from glucose, sucrose, vitamin C, graphene, graphite and carbon black.

15. The method according to any one of claims 1 to 14, wherein the iron source comprises at least one selected from iron phosphate, ferric oxide, ferroferric oxide, ferrous sulfate, ferric hydroxide, iron carbonate, ferrous oxalate, iron nitrate, and ferric chloride; wherein the manganese source comprises at least one selected from manganous-manganic oxide, manganese dioxide, manganese carbonate, manganese hydrogen phosphate, manganese anhydride, manganese sulfate, manganese hydroxide, manganese acetate, manganese nitrate, and manganese oxalate; wherein the lithium source comprises at least one selected from lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, lithium phosphate and lithium acetate; and / or wherein the phosphorus source comprises at least one selected from orthophosphate, condensed phosphate and organophosphorus.

16. The method according to any one of claims 1 to 15, wherein the phosphorus source comprises at least one selected from phosphoric acid, lithium dihydrogen phosphate, monoammonium phosphate, pyrophosphoric acid, metaphosphoric acid, hydroxyethylidene diphosphonic acid, 2-hydroxyphosphonoacetic acid and aminotrimethylene phosphonic acid.

17. A composite cathode active material prepared by the method according to any one of claims 1 to 16.

18. A secondary battery, comprising: a positive electrode comprising the composite cathode active material according to claim 17.

19. An electronic device, comprising: the secondary battery according to claim 18.

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