CN115692655A - Positive active material, preparation method and application thereof - Google Patents
Positive active material, preparation method and application thereof Download PDFInfo
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Abstract
The application provides a positive active material, and a preparation method and application thereof. The preparation method can prepare the phosphate-series positive electrode active material with high compacted density, high specific capacity and excellent cycle performance by combining primary high-temperature calcination, particle size refinement and secondary calcination. The positive active material comprises a core and a carbon coating layer, wherein the core is formedThe chemical general formula is LiMPO 4 M is a transition metal element, and the compacted density of the positive electrode active material is 2.6g/cm 3 ‑2.8g/cm 3 And the specific capacity of the positive electrode active material is in the range of 155mAh/g to 160 mAh/g.
Description
Technical Field
The application relates to the technical field of batteries, in particular to a positive active material and a preparation method and application thereof.
Background
The phosphate positive electrode material has good safety performance, long cycle life and good specific capacity, gradually replaces ternary materials, has larger and larger market share, and is representative of phosphate positive electrode materials such as lithium iron phosphate.
With the rapid development of new energy industries, the market has increasingly strict requirements on the energy density of lithium ion batteries, and the energy density of the lithium iron phosphate material can be improved to a certain extent by improving the pole piece compaction density of the lithium iron phosphate material. The powder compaction density is an important factor influencing the pole piece compaction density, but the discharge capacity of the positive active material with high compaction density is often lower, such as lithium manganese iron phosphate. Based on this, the development of a method with higher powder compaction density and high discharge capacity is an important direction for improving the energy density of the finished product.
Disclosure of Invention
In view of this, the present application provides a method of preparing a positive electrode active material. The preparation method can prepare the positive active material with high compacted density, high specific capacity and excellent cycle performance by combining the mode of primary high-temperature calcination, grain size refinement and secondary calcination.
A first aspect of the present application provides a method for preparing a positive electrode active material, comprising the steps of:
(1) Mixing a lithium source, a phosphorus source, an M transition metal element source, a first carbon source and a solvent to obtain a mixed material;
(2) Drying the mixed material and then carrying out primary calcination treatment under a protective atmosphere to obtain a primary calcined sample; wherein, the primary calcination treatment comprises heating from room temperature to a first temperature of 400-600 ℃, and then heating from the first temperature to a second temperature of 950-1200 ℃;
(3) Adding a lithium supplement agent and a second carbon source into the primary burning sample, thinning the particle size, and then placing the primary burning sample in a protective atmosphere to carry out secondary calcination treatment to obtain a positive electrode active material;
preferably, the temperature of the secondary calcination treatment is 400 ℃ to 550 ℃.
In the preparation method, the mixed material is heated to 400-600 ℃ at a slow heating rate, the raw material phases are dissolved and carry out chemical reaction, volatile matters in the raw materials are fully discharged, and LiMPO is formed at the same time 4 A crystal nucleus; subsequently, the temperature is increased to 950 ℃ to 1200 ℃ to further convert the unreacted reaction gel into LiMPO 4 Crystal, increase crystallinity of material, and decrease LiMPO 4 The crystal structure of the positive electrode active material tends to be completely beautified due to the crystal defects, so that the powder compaction density of the positive electrode active material is improved. The particle of the sample after the primary sintering is refined, and the lithium and carbon are supplemented for secondary sintering at low temperature, so that the problems of capacity reduction caused by the fact that the primary particle size of the positive active material is continuously increased along with the rise of the sintering temperature and the capacity reduction caused by the loss of lithium at high temperature can be well solved, and finally the positive active material with high compaction density and high specific capacity can be prepared. In addition, the preparation method has simple steps and high production efficiency, and can realize large-scale industrial preparation.
According to a second aspect of the present application, there is provided a positive electrode active material obtained by the preparation method provided in the first aspect of the present application, wherein the positive electrode active material comprises a core and a carbon coating layer, and the chemical formula of the core is LiMPO 4 M is a transition metal element; the positive electrode active material has a compacted density of 2.6g/cm 3 -2.8g/cm 3 And the specific capacity of the positive electrode active material is in the range of 155mAh/g to 160 mAh/g.
The positive active material has high compaction density and high specific capacity, can show higher energy density, and can be used for providing a battery with high energy density and high cycle performance.
The third aspect of the application provides a positive pole piece, which comprises the positive active material provided by the second aspect of the application. The battery assembled with the positive pole piece can realize higher energy density and better cycle performance.
The fourth aspect of the present application provides a secondary battery comprising the positive electrode sheet provided by the third aspect of the present application. The secondary battery has higher energy density and better cycle performance.
Drawings
Fig. 1 is an XRD spectrum of the cathode active material provided in example 1.
Fig. 2 is a voltage-discharge gram capacity curve of the positive electrode active material provided in example 1.
Detailed Description
The embodiment of the application provides a preparation method of a positive electrode active material, which comprises the following steps:
(1) Mixing a lithium source, a phosphorus source and an M transition metal element source with a first carbon source and a solvent to obtain a mixed material;
(2) Drying the mixed material and then carrying out primary calcination treatment under a protective atmosphere to obtain a primary calcined sample; wherein the primary calcination treatment comprises heating from room temperature to a first temperature of 400-600 ℃, and then heating from the first temperature to a second temperature of 950-1200 ℃;
(3) And adding a lithium supplement agent and a second carbon source into the primary burning sample, thinning the particle size, and then placing the primary burning sample in a protective atmosphere to carry out secondary calcination treatment to obtain the anode active material.
The mixed material is firstly sintered at the low temperature of 400-600 ℃ to fully discharge the volatile components in the raw materials, which is beneficial to the beautification of the crystal structure of the core material in the subsequent high-temperature calcination process; the element sources react with each other to generate a core material crystal nucleus, meanwhile, the first carbon source can be used as a reducing agent to provide reducing atmosphere, amorphous carbon formed after carbonization can be wrapped on the surface of a core precursor material, and a formed carbon coating layer is beneficial to limiting the size of crystal particles formed by the core precursor, inhibiting agglomeration among the crystal particles and avoiding overlarge crystal particle growth; then the temperature is raised to 950 ℃ to 1200 ℃ for high-temperature calcination, and the high-temperature calcination can further reduce the unreactedThe gel phase is converted into the crystal phase of the core material, and more importantly, the crystal defects of the core crystal can be reduced, and the unit cell volume of the core crystal is reduced, so that the compaction density of the core material is improved. The particle size of the primary sintered sample is refined and the lithium and carbon are supplemented, so that the primary particle size of the positive active material can be ensured to be smaller, and the Li is favorably ensured + The extraction path is short, the capacity of the anode active material is good, and the loss of lithium and carbon during high-temperature roasting is compensated by lithium and carbon supplementation. And finally, low-temperature secondary roasting is carried out, the newly supplemented carbon source is coated on the particle surface again to improve the coating uniformity of the carbon layer, so that the agglomeration among particles is prevented, the crystal particles are inhibited from being enlarged, and the sphericity of a calcined sample with refined particle size and possibly existing fragments can be improved, thereby preparing the anode active material with high compaction density and high specific capacity. In addition, the preparation method has the advantages of simple steps, strong process reliability and high production efficiency, and is suitable for large-scale industrial preparation.
In the present application, each raw material may be in an excess amount as appropriate, considering that a part of the raw materials may be lost during mixing, calcination and other processes. For example, the lithium source may be added in an excess of 10 wt.%.
In the present application, for example, in the one-time calcination treatment process in the step (2), the calcination temperature (i.e., the first temperature) in the low-temperature sintering stage may be 400 ℃, 450 ℃, 500 ℃, 550 ℃, 600 ℃, or the like. The high-temperature calcination step may be carried out at a calcination temperature (i.e., second temperature) of 950 ℃, 960 ℃, 970 ℃, 980 ℃, 990 ℃, 1000 ℃, 1010 ℃, 1020 ℃, 1030 ℃, 1040 ℃, 1050 ℃, 1060 ℃, 1070 ℃, 1080 ℃, 1090 ℃, 1100 ℃, 1120 ℃, 1140 ℃, 1150 ℃, 1160 ℃, 1180 ℃, 1200 ℃ or the like.
In some embodiments of the present application, in the step (1), the M transition metal doping source may be a salt, an oxide, or the like of a corresponding metal element. Illustratively, the M transition metal elements include Fe and Mn elements.
In some embodiments of the present application, the method of mixing comprises a liquid phase mixing method or a solvent-gel method. In some embodiments of the present application, a liquid phase mixing method is adopted, which specifically comprises the following steps: adding a lithium source, an iron source, a phosphorus source, a manganese source and a first carbon source into a solvent, fully dissolving, and then carrying out spray drying. Wherein the solvent includes, but is not limited to, water, ethanol, acetone, and the like.
In some embodiments of the present application, in the aforementioned step (1), the lithium source includes, but is not limited to, li 2 CO 3 、LiH 2 PO 4 、LiOH·H 2 O、CH 3 COOLi and LiNO 3 At least one of (1). The phosphorus source includes, but is not limited to (NH) 4 ) 3 PO 4 、LiH 2 PO 4 And H 3 PO 4 、FePO 4 At least one of (1). The manganese source includes, but is not limited to MnO 2 、Mn(NO 3 ) 2 、MnSO 4 And Mn 3 (PO) 4 ·3H 2 And O. The first carbon source includes, but is not limited to, at least one of sucrose, polyvinylidene fluoride, glucose, carbon black, polyethylene glycol, paraffin, graphite, and graphene. The iron source includes but is not limited to FePO 4 、FeCl 3 、Fe(NO 3 ) 3 、Fe 2 O 3 And FeSO 4 ·7H 2 At least one of O. When the selected iron source is or contains ferric ions, the first carbon source can also act as a reducing agent to convert Fe 3+ Reduction to Fe 2+ 。
In the present application, in the step (1), the amount of the first carbon source is not specifically limited, and those skilled in the art can determine the amount of the first carbon source according to actual production needs and the final coating state of the carbon layer.
In some embodiments of the present application, in the step (2), the temperature is increased from room temperature to the first temperature of 400 ℃ to 600 ℃ at a temperature increase rate of 1 ℃/min to 3 ℃/min. Illustratively, the above-mentioned temperature rise rate may be 1 ℃/min, 2 ℃/min, 3 ℃/min, or the like. In some embodiments of the present invention, in the above-mentioned heating process, the heating is preferably performed at 2 ℃/min. The heating rate in the low-temperature sintering stage is controlled to be slower, so that the volatile components in the raw materials can be sufficiently removed, and the crystal structure defects of the core material can be reduced. In some embodiments of the present application, in the step (2), the temperature is maintained at the first temperature for 0h to 2h. The low-temperature sintering is to sufficiently remove volatile components, and a person skilled in the art can automatically determine whether heat preservation is needed and specific time for heat preservation according to raw materials actually selected for production and the removal effect of the volatile components in the actual production situation.
In some embodiments of the present application, in the step (2), the temperature is increased from the first temperature to the second temperature at a temperature increase rate of 5 ℃/min to 10 ℃/min, and by controlling the temperature increase rate in the process within the above range, the size of the core particles can be prevented from being too large, the crystal structure of the core can be beautified, and the final compacted density of the cathode active material can be further improved. Illustratively, the above-mentioned temperature rise rate may be 5 ℃/min, 6 ℃/min, 7 ℃/min, 8 ℃/min, 9 ℃/min, 10 ℃/min, or the like. In some embodiments of the present application, the temperature is preferably raised from the first temperature to the second temperature at a temperature rise rate of 7 ℃/min, which not only can ensure better calcination effect and improved powder density of the core crystal, but also can shorten the process time.
In some embodiments of the present application, in the step (2), the heat preservation is performed at the second temperature for 0h to 1h, specifically, when the second temperature is 1200 ℃, the heat preservation may not be performed. When the second heat preservation temperature is less than 1200 ℃, the heat preservation time can be 30min, 40min, 50min, 60min and the like. Preferably, when the second heat preservation temperature is 950-1100 ℃, the heat preservation time is 60min.
In some embodiments of the present application, in the step (3), the temperature of the secondary calcination treatment is 400 ℃ to 550 ℃. Illustratively, the temperature of the secondary calcination treatment may be 400 ℃, 425 ℃, 450 ℃, 475 ℃, 500 ℃, 525 ℃, 550 ℃ or the like. Controlling the temperature of the secondary calcination treatment within the above range is beneficial to further improving the uniformity of the coating layer formed on the surface of the core material by the carbon source (especially the second carbon source), preventing the agglomeration among particles, inhibiting the crystal particles from becoming larger, and simultaneously further improving the sphericity of the one-shot sample after the particle size refinement and the fragments possibly existing.
In some embodiments of the present application, in the step (3), the particle size of the particle size-refined calcined sample is controlled to have a D99 particle size in a range of 0.1 μm to 0.2 μm. Illustratively, the D99 of the above-described fine particles may be 0.1 μm, 0.15 μm, 0.2 μm, or the like. Wherein D99 is the corresponding particle size when the cumulative particle size distribution of the particles reaches 99%. Controlling the D99 of the particles within the range can ensure that the average particle size of primary particles of the finally prepared anode active material is within the range of 50nm-200nm, shorten the de-intercalation path of active lithium ions, reduce the amount of dead lithium in the anode active material, accelerate the diffusion of the lithium ions, improve the rapid charge and discharge capacity of the anode active material and ensure the higher discharge capacity of the material.
It is understood that the particle size refinement process in step (3) includes, but is not limited to, pulverization, ball milling/sanding, spray drying, and the like. Illustratively, in some embodiments, a fired sample is pulverized, ball milled/sanded, and spray dried sequentially. In the ball milling/sanding process, a certain amount of solvent is added, and the solvent is mixed with a calcined sample to form ball milling/sanding slurry.
In some embodiments of the present application, in the step (3), in the step of refining the particle size, the mass of the lithium supplement agent is 0.1% to 20% of the mass of the lithium source in the step (1), and the amount of the lithium supplement agent may be, for example, 0.1%, 0.5%, 1.0%, 3.0%, 5.0%, 7.0%, 9.0%, 12.0%, 14.0%, 16.0%, 18.0%, 20.0%, or the like of the lithium source. The mass of the second carbon source is 2% -5% of the mass of the first burned sample. Illustratively, the amount of the second carbon source used may be 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, etc. of the mass of the one-shot sample. In the sintering process and the sanding process in the step (2), lithium loss and damage of the carbon coating layer are easy to occur, and the addition of a proper amount of a lithium supplement agent and a second carbon source in the step (3) is more favorable for ensuring the electrochemical performance of the final positive active material.
In some embodiments of the present application, the lithium supplement agent comprises Li 2 O、Li 5 FeO 4 、Li 2 NiO 2 At least one of (1).
In some embodiments of the present application, the second carbon source comprises at least one of sucrose, polyvinylidene fluoride, glucose, carbon black, polyethylene glycol, paraffin, graphite, and graphene. The second carbon source may be selected to be the same as the first carbon source or may be different from the second carbon source.
In the present application, in step (2) and step (3), the protective atmosphere includes, but is not limited to, nitrogen, helium, and the like.
In some embodiments of the present application, the holding time for the second calcination is 4h to 6h. Illustratively, the holding time for the secondary calcination may be 4h, 5h, 6h, etc. The control of the heat preservation time within the range is favorable for ensuring better uniformity of a carbon coating layer on the surface of the final anode active material and further improving the electrochemical performance of the final anode active material.
In some embodiments of the present application, in step (3), the temperature increase rate during the secondary calcination treatment is 4 ℃/min to 6 ℃/min. Illustratively, the temperature ramp rate of the above process can be 4 deg.C/min, 4.5 deg.C/min, 5 deg.C/min, 5.5 deg.C/min, 6 deg.C/min, etc. In some embodiments, the rate of temperature increase during the secondary calcination treatment is preferably 5 deg.C/min. The appropriate heating rate is beneficial to improving the quality of the carbon coating layer, and further beneficial to further improving the electrochemical performance of the positive active material.
Correspondingly, the embodiment of the application also provides a positive active material prepared by the preparation method provided by the embodiment of the application, the positive active material comprises a core and a carbon coating layer, and the chemical general formula of the core is LiMPO 4 M is a transition metal element, and the compacted density of the positive electrode active material is 2.6g/cm 3 -2.8g/cm 3 Within the range of (1).
The positive active material has high compaction density and high specific capacity, can show higher energy density, and can be used for providing a battery with high energy density and high cycle performance. In addition, the carbon coating layer can obviously improve the electronic conductivity of the positive active material and further improve the electrochemical performance of the positive active material.
Illustratively, the compacted density of the above-described cathode active material may be 2.60g/cm 3 、2.62g/cm 3 、2.64g/cm 3 、2.65g/cm 3 、2.66g/cm 3 、2.68g/cm 3 、2.70g/cm 3 、2.72g/cm 3 、2.74g/cm 3 、2.75g/cm 3 、2.78g/cm 3 、2.80g/cm 3 And so on. Illustratively, the specific capacity of the above-described positive electrode active material may be 155mAh/g, 156mAh/g, 157mAh/g, 158mAh/g, 159mAh/g, 159.5mAh/g, 160mAh/g, or the like.
In some embodiments of the present application, the transition metal element includes, but is not limited to, one or more of Fe, mn, co, ni, cr, V, and Ti. In some embodiments, the transition metal element is Fe, and in this case, the cathode active material is LiFePO with a carbon coating layer 4 . In other embodiments, the transition metal elements are Fe and Mn, and in this case, the positive electrode active material is LiMn with a carbon coating layer x Fe 1-x PO 4 ，0<x<1. In some embodiments, the M element represents three or more transition metal elements, and the transition metal element M is Fe, mn, and one or more of Co, ni, cr, V, and Ti, for example, in which case the cathode active material is LiMn with a carbon coating layer x Fe 1-x-y M y PO 4 ，0<x+y<1，0<y≤0.02。
In some embodiments of the present application, the material of the inner core is LiMn x Fe 1-x PO 4 . At this time, the relative crystallinity of the core is greater than or equal to 99%. Illustratively, liMn x Fe 1-x PO 4 The relative crystallinity of (a) may be 99%, 99.1%, 99.2%, 99.3%, 99.4%, 99.5%, 99.6%, 99.7%, 99.8%, 99.9%, etc. LiMn x Fe 1-x PO 4 Has a unit cell volume of less than or equal to
In some embodiments, liMn x Fe 1-x PO 4 Has a unit cell volume of
Within the range of (1). Illustratively, liMn x Fe 1-x PO 4 May have a cell volume of
And so on.
In some embodiments of the present application, the average particle diameter of the positive electrode active material is in a range of 50nm to 200 nm. Illustratively, the average particle size of the positive electrode active material may be 50nm, 60nm, 70nm, 80nm, 90nm, 100nm, 110nm, 120nm, 130nm, 140nm, 150nm, 160nm, 170nm, 180nm, 190nm, 200nm, or the like. The above average particle diameter refers to the particle diameter of the primary particles of the positive electrode active material, and in some specific embodiments, secondary particles in which a plurality of primary particles are aggregated are also present in the positive electrode active material. The particle size of the positive active material is controlled within the range, the lithium ion extraction path is short, lithium in the positive active material crystal can be extracted better, the diffusion rate of the lithium ions in the positive active material is accelerated, and the rate capability of the positive active material can be facilitated.
In some embodiments of the present application, the carbon coating layer has a thickness in the range of 2nm to 4 nm. Illustratively, the thickness of the carbon coating layer may be 2nm, 2.5nm, 3nm, 3.5nm, or the like. Controlling the thickness of the carbon coating layer within the range is beneficial to improving the electronic conductivity of the anode active material and ensuring that the inner core in the anode active material has enough mass ratio, thereby being beneficial to the normal exertion of the electrochemical performance of the anode active material.
The embodiment of the application also provides a positive pole piece which comprises the positive active material provided by the embodiment of the application or the positive active material prepared by the preparation method provided by the embodiment of the application. The battery assembled with the positive pole piece can realize higher energy density and better cycle performance.
In some embodiments of the present disclosure, the positive electrode sheet includes a current collector and a positive electrode active material layer disposed on at least one side surface of the current collector, and the positive electrode active material layer includes the positive electrode active material, a binder, and an optional conductive agent.
In some embodiments of the present application, the positive electrode sheet can be prepared by the following steps: adding the positive electrode active material provided by the embodiment of the application, a binder and an optional conductive agent into a certain amount of solvent (such as N-methylpyrrolidone), and uniformly mixing to obtain positive electrode slurry; and coating the anode current collector with the coating, drying, rolling and cutting to obtain the anode piece.
In some embodiments of the present application, the positive electrode current collector includes, but is not limited to, an aluminum foil and the like. The binder includes, but is not limited to, at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl alcohol (PVA), styrene-butadiene rubber (SBR), polyacrylonitrile (PAN), polyimide (PI), polyacrylic acid (PAA), polyacrylate, polyolefin (e.g., polyethylene, polypropylene, polystyrene, etc.), sodium carboxymethylcellulose (CMC), and sodium alginate.
In some embodiments of the present application, the conductive agent is a material known to those skilled in the art. Illustratively, the conductive agent may be Carbon Nanotubes (CNTs), carbon fiber conductive agents (HV), carbon black (e.g., acetylene black, ketjen black), furnace black, graphene, or the like.
The embodiment of the application also provides a secondary battery which comprises the positive pole piece provided by the embodiment of the application. The secondary battery has higher energy density and excellent cycle performance.
Among them, the secondary battery may be a liquid battery using a liquid electrolyte, or a semi-solid or solid battery using a semi-solid electrolyte or a solid electrolyte. In some embodiments, a secondary battery may include the positive electrode tab, the negative electrode tab described above, and a separator and an electrolyte disposed between the positive electrode tab and the negative electrode tab. In other embodiments, a secondary battery may include a positive electrode tab, a negative electrode tab, and a semi-solid electrolyte or a solid electrolyte disposed between the positive electrode tab and the negative electrode tab. In addition, when a semi-solid electrolyte or a solid electrolyte is used, the positive electrode plate and the negative electrode plate can also contain a semi-solid electrolyte material or a solid electrolyte material.
The negative electrode sheet may be a negative electrode current collector (including but not limited to a copper foil) coated with a negative electrode active material layer. Among them, the negative active material is a material well known to those skilled in the art, and may be, for example, graphene, hard carbon, soft carbon, carbon nanotube, silicon carbon composite, or the like.
The technical solution of the present application is described in detail below with reference to a number of examples.
Example 1
(1) Taking 1888.14g of LiNO 3 1269.09g of FeCl 3 3000g of NH 4 H 2 PO 4 3266.93g Mn (NO) 3 ) 2 5363 and 482.27g of cane sugar are added into 15071.09g of water together to be uniformly mixed to obtain slurry, and the slurry is transferred into a spray dryer to be spray-dried to obtain a dry raw material.
(2) Transferring the dried raw material to a tubular furnace, heating to 500 ℃ from room temperature at the speed of 2 ℃/min in a nitrogen atmosphere, heating to 950 ℃ from 500 ℃ at the speed of 7 ℃/min, and keeping the temperature at 950 ℃ for 1h to obtain a calcined sample;
(3) After the burned sample was cooled to room temperature, it was pulverized, and 12.06g of Li was added 2 O,180.85g of sucrose, 9042.65g of water are added together, ball milling is carried out until the particle size D99 of the slurry is 0.15 mu m, the slurry is transferred to a tube furnace after spray drying, the temperature is raised from room temperature to 500 ℃ in nitrogen atmosphere, the temperature is kept for 4h, the mixture is cooled to room temperature and taken out, secondary crushing is carried out, and the positive electrode active material is obtained. Wherein the positive active material is LiMn with a carbon layer coated on the surface 0.7 Fe 0.3 PO 4 The average primary particle diameter thereof was 110nm.
Example 2
The only difference from example 1 is that in step (2), the dried starting material was transferred to a tube furnace, raised from room temperature to 500 ℃ at 2 ℃/min, further raised from 500 ℃ to 1000 ℃ at 7 ℃/min in a nitrogen atmosphere, and kept at 1000 ℃ for 1 hour to obtain a calcined sample. The finally prepared positive active material is LiMn with the surface coated with a carbon layer 0.7 Fe 0.3 PO 4 The average primary particle diameter thereof was 120nm.
Example 3
Only the difference from example 1In the step (2), the dried raw material is transferred to a tube furnace, and is heated from room temperature to 500 ℃ at a speed of 2 ℃/min in a nitrogen atmosphere, and is heated from 500 ℃ to 1200 ℃ at a speed of 7 ℃/min, so that a calcined sample can be obtained. The finally prepared positive active material is LiMn with the surface coated with a carbon layer 0.7 Fe 0.3 PO 4 The average primary particle diameter was 105nm.
Example 4
The only difference from example 1 is that in step (3), the calcined sample was cooled to room temperature, then pulverized, and 12.06g of Li was added 2 O,180.85g of cane sugar and 9042.65g of water are added together to be uniformly mixed, ball milling is carried out until the grain diameter D99 of the slurry is 0.2 mu m, the dried slurry is transferred to a tube furnace, the temperature is raised from room temperature to 500 ℃ in nitrogen atmosphere, the temperature is kept for 4 hours, the mixture is cooled to room temperature and taken out, secondary crushing is carried out, and the positive electrode active material is obtained. Wherein the positive active material is LiMn with a carbon layer coated on the surface 0.7 Fe 0.3 PO 4 The average primary particle diameter thereof was 128nm.
Example 5
The only difference from example 1 is that in step (3), the calcined sample was cooled to room temperature, then pulverized, and 12.06g of Li was added 2 O,180.85g of sucrose, 9042.65g of water are added together, ball milling is carried out until the particle size D99 of the slurry is 0.15 mu m, the slurry is transferred to a tube furnace after drying, the temperature is raised from the room temperature to 400 ℃ at the speed of 5 ℃/min in the nitrogen atmosphere, the temperature is kept for 4h, the mixture is cooled to the room temperature and taken out, and secondary crushing is carried out, so that the positive electrode active material is obtained. Wherein the positive active material is LiMn with a carbon layer coated on the surface 0.7 Fe 0.3 PO 4 The average primary particle diameter was 108nm.
Example 6
The only difference from example 1 is that in step (3), a calcined sample was cooled to room temperature, then subjected to pulverization treatment, and 12.06g of Li was added 2 O,180.85g of sucrose, 9042.65g of water are added together, ball milling is carried out until the particle size D99 of the slurry is 0.15 mu m, the slurry is transferred to a tube furnace after drying, the temperature is raised from room temperature to 550 ℃ at the speed of 5 ℃/min in the nitrogen atmosphere, the temperature is kept for 4h, the mixture is taken out after being cooled to room temperature and is crushed for the second time, and the positive electrode active material is obtained. Wherein the positive active material is LiMn with a carbon layer coated on the surface 0.7 Fe 0.3 PO 4 The average particle size was 115nm.
Example 7
The only difference from example 1 is that in step (3), the calcined sample was cooled to room temperature, then pulverized, and 12.06g of Li was added 2 O,180.85g of sucrose, 9042.65g of water are added together, ball milling is carried out until the particle size D99 of the slurry is 0.15 mu m, the slurry is transferred to a tube furnace after drying, the temperature is raised from room temperature to 650 ℃ at the speed of 5 ℃/min in a nitrogen atmosphere, the temperature is kept for 4h, the mixture is cooled to room temperature and taken out, and secondary crushing is carried out, so that the positive electrode active material is obtained. Wherein the positive active material is LiMn with a carbon layer coated on the surface 0.7 Fe 0.3 PO 4 The average particle size was 135nm.
In order to highlight the advantageous effects of the examples of the present application, the following comparative examples were provided.
Comparative example 1
(1) Taking 1888.14g of LiNO 3 1269.09g of FeCl 3 3000g of NH 4 H 2 PO 4 3266.93g Mn (NO) 3 ) 2 5363 and 482.27g of cane sugar are added into 15071.09g of water together to be uniformly mixed to obtain slurry, and the slurry is transferred into a spray dryer to be spray-dried to obtain a dry raw material.
(2) Transferring the dried raw material into a tubular furnace, heating the dried raw material to 500 ℃ from room temperature at the speed of 5 ℃/min under the nitrogen atmosphere, and preserving the heat for 4 hours to obtain a primary combustion sample;
(3) A calcined sample was cooled and pulverized, and 12.06g of Li was added 2 Adding 180.85g of cane sugar and 9042.65g of water together, uniformly mixing, ball-milling until the particle size D99 of the slurry is 0.15 mu m, drying, transferring to a tube furnace, heating from room temperature to 800 ℃ at the speed of 5 ℃/min in a nitrogen atmosphere, keeping the temperature for 4 hours, cooling to room temperature, taking out, and performing secondary crushing to obtain the cathode active material. Wherein the positive active material is LiMn with a carbon layer coated on the surface 0.7 Fe 0.3 PO 4 The average particle size was 2 μm.
Comparative example 2
(1) Taking 1888.14g of LiNO 3 1269.09g of FeCl 3 3000g of NH 4 H 2 PO 4 3266.93g Mn (NO) 3 ) 2 5363 and 482.27g of cane sugar are added into 15071.09g of water together to be uniformly mixed to obtain slurry, and the slurry is transferred into a spray dryer to be spray-dried to obtain a dry raw material.
(2) The dried material was transferred to a tube furnace, heated from room temperature to 800 ℃ at 7 ℃/min in a nitrogen atmosphere, and kept at the temperature of 800 ℃ for 1 hour to obtain a calcined sample.
(3) A calcined sample was cooled and pulverized, and 12.06g of Li was added 2 O,180.85g of cane sugar and 9042.65g of water are added together to be uniformly mixed, ball milling is carried out until the grain diameter D99 of the slurry is 0.15 mu m, the dried slurry is transferred to a tube furnace, the temperature is increased from room temperature to 400 ℃ in nitrogen atmosphere, the constant temperature is kept for 4 hours, the mixture is cooled to room temperature and taken out, secondary crushing is carried out, and the positive electrode active material is obtained. Wherein the positive active material is LiMn with a carbon layer coated on the surface 0.7 Fe 0.3 PO 4 The average particle size was 105nm.
Performance testing
(1) Compacted density test of positive electrode active material: the positive electrode active materials prepared in the examples and comparative examples were taken in appropriate amounts, and the compacted density of each positive electrode active material was measured using an electronic pressure tester. The results are summarized in table 1.
(2) XRD test of positive electrode active material: an X-ray diffraction (XRD) spectrum of the positive active material of example 1 was obtained by performing an XRD test on a certain amount of the positive active material, as shown in fig. 1.
Calculation of relative crystallinity: the ratio of the sum of the intensities of the stronger characteristic peaks in the XRD spectrums of the sample and the standard substance. For example, the positive electrode material in example 3 is taken as a standard sample, and the ratio of the sum of the intensities of the characteristic peaks a, b, c, d and e in the XRD spectrum of the positive electrode material in example 1 to the sum of the intensities of the corresponding characteristic peaks in example 3 is the relative crystallinity.
Calculating the unit cell volume: and refining and calculating an XRD spectrogram of the sample by GSAS software to obtain unit cell parameters.
(3) Specific capacity test of the positive electrode active material:
a) Adding the positive electrode active material, the conductive agent (specifically acetylene black) and the binder (specifically polyvinylidene fluoride) prepared in each example and comparative example into a proper amount of solvent (specifically N-methylpyrrolidone, abbreviated as NMP) according to a mass ratio of 8; uniformly coating the positive active material on the surface of a positive current collector (specifically aluminum foil), drying for 12h in a vacuum drying oven at 120 ℃, and rolling and slitting to obtain a circular positive pole piece with the diameter of 14 mm.
b) Punching a lithium metal sheet into a round sheet with the size of 14.5mm, and taking the round sheet as a negative pole piece;
c) Stacking the positive pole piece, the diaphragm (specifically a microporous polypropylene film, the trademark of Celgard 2400) and the negative pole piece in sequence in a glove box to obtain a dry battery core; then injecting a proper amount of electrolyte into the dry cell, wherein the electrolyte is LiPF 6 The organic solvent (concretely, the ethylene carbonate and the dimethyl carbonate with the volume ratio of 1:1) in the electrolyte, and LiPF 6 The molar concentration of (A) is 1.0mol/L. Button cells of the CR2025 type with the positive active materials of the examples and comparative examples were prepared.
d) And (3) placing each button cell at 25 +/-0.5 ℃, and carrying out charge-discharge cycle test on the cell at 1C by using a lithium ion battery charge-discharge test system. The method comprises the following steps: constant voltage charging to 4.3.0V/0.05C cutoff; standing for 10min; constant current discharge to 2.0V is a cycle. The procedure was repeated, and the cycle was 2000 times, to measure the specific capacity of each positive electrode active material. The results are summarized in Table 1, and the voltage-discharged gram-capacity curve of example 1 is shown in FIG. 2.
Table 1 summary of the results parameters of the positive electrode active materials of the examples and comparative examples
As can be seen from the data in table 1, the compacted density of the positive active material provided in the examples of the present application is significantly higher than that of the comparative examples, and the specific capacity of the materials of the examples can be maintained at a higher level, which is partly attributed to the higher relative crystallinity, less crystal defects, and relatively smaller unit cell volume of the core material of the examples. When the secondary calcination temperature was outside the preferred range of the present application (example 7), however, the battery capacity retention rate after 2000 cycles of the material was greatly affected, which is probably because the capacity exertion was weakened because the primary particle size of the finished positive electrode active material was relatively large.
The foregoing is illustrative of the present application and it will be appreciated that modifications and variations may be made by those skilled in the art without departing from the principles of the application and are considered to be within the scope of the application.
Claims (10)
1. A method for preparing a positive electrode active material, comprising the steps of:
(1) Mixing a lithium source, a phosphorus source and an M transition metal element source with a first carbon source and a solvent to obtain a mixed material;
(2) Drying the mixed material and then carrying out primary calcination treatment under a protective atmosphere to obtain a primary calcined sample; wherein, the primary calcination treatment comprises heating from room temperature to a first temperature of 400-600 ℃, and then heating from the first temperature to a second temperature of 950-1200 ℃;
(3) Adding a lithium supplement agent and a second carbon source into the primary sintered sample, carrying out particle size refinement, and then carrying out secondary calcination treatment in a protective atmosphere to obtain a positive active material;
preferably, the temperature of the secondary calcination treatment is 400 ℃ to 550 ℃.
2. The production method according to claim 1, wherein in the step (3), the grain diameter of the grain-diameter-refined calcined sample has a D99 grain diameter in the range of 0.1 μm to 0.2 μm.
3. The production method according to claim 1, wherein in the step (2), the temperature is raised from the first temperature to the second temperature at a temperature raising rate of 5 ℃/min to 10 ℃/min;
preferably, the temperature is increased from the first temperature to the second temperature at a temperature increase rate of 7 ℃/min;
preferably, in the step (2), the temperature is increased from the room temperature to the first temperature at the temperature increasing rate of 1-3 ℃/min;
preferably, in the step (2), the temperature is kept at the first temperature for 0h-2h;
preferably, in step (2), the temperature is kept at the second temperature for 0h-1h.
4. The preparation method according to claim 1, wherein in the step (3), the holding time of the secondary calcination is 4h-6h;
the temperature rise rate of the secondary calcination is 4-6 ℃/min;
preferably, the temperature increase rate of the secondary calcination is 5 ℃/min.
5. The preparation method according to claim 1, wherein the mass of the lithium supplement agent is 0.1-20% of the mass of the lithium source, and the mass of the second carbon source is 2-5% of the mass of the primary sintered sample;
preferably, the transition metal element is one or more of Fe, mn, co, ni, cr, V, and Ti.
6. A positive electrode active material produced by the production method according to any one of claims 1 to 5, wherein the positive electrode active material comprises a core and a carbon coating layer, and the core has a chemical formula of LiMPO 4 M is a transition metal element; the positive electrode active material has a compacted density of 2.6g/cm 3 -2.8g/cm 3 And the specific capacity of the positive electrode active material is in the range of 155mAh/g to 160 mAh/g.
7. The positive electrode active material according to claim 6, wherein the transition metal element comprises one or more of Fe, mn, co, ni, cr, V, and Ti.
8. The positive electrode active material according to claim 6, wherein the chemical formula of the core is LiMn x Fe 1- x PO 4 ，0<x<1; the LiMn x Fe 1-x PO 4 Is greater than or equal to 99%; the LiMn x Fe 1-x PO 4 Has a unit cell volume of less than or equal to
9. A positive electrode plate, characterized in that the positive electrode plate comprises the positive electrode active material prepared by the preparation method of any one of claims 1 to 5 or comprises the positive electrode active material of any one of claims 6 to 8.
10. A secondary battery having the positive electrode tab according to claim 9.
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