CN115911381A - Electrode material and preparation method thereof - Google Patents

Abstract

The invention discloses an electrode material and a preparation method thereof, wherein the preparation method comprises the following steps: step 1: ascorbic acid and sulfate were added to deionized water with stirring to prepare solution a: step 2: adding lithium phosphate into the solution A, uniformly mixing, and then heating for reaction; and step 3: carrying out solid-liquid separation on the reaction slurry obtained after cooling in the step 2, and obtaining a filtrate and slurry after separation; and 5: cleaning the slurry by using a cleaning solution until the conductivity of the filtrate is less than 5mS/cm; step 6: and drying the washed slurry, adding the dried slurry into a high-speed mixing grinder, grinding, and adding a carbon source to sinter to obtain the carbon-coated anode material. The preparation method realizes the preparation of the anode material coated with the carbon layer, and has the advantages of simple process and easy large-scale production.

Description

Electrode material and preparation method thereof

Technical Field

The invention belongs to the technical field of lithium iron phosphate preparation processes, and particularly belongs to the technical field of lithium iron phosphate preparation processes.

Background

At present, lithium intercalation compounds as positive electrode materials of lithium ion batteries are mainly lithium cobaltate (LiCoO) ₂ LCO), lithium manganate (L)iMn ₂ O ₄ LMO), lithium nickelate (LiNiO) ₂ LNO), lithium iron phosphate (LiFePO) ₄ LFP), nickel cobalt manganese ternary material (LiNi) _x Co _y Mn _1-x-y O ₂ NCM) and nickel cobalt aluminum modified ternary materials (LiNi _x Co _y Al _1-x-y O ₂ NCA), etc. Since 1991, the lithium ion battery has taken transition metal oxides (such as LCO, LMO, LNO, NCM, NCA, etc.) as the main source of positive electrode materials, however, these materials release oxygen in the highly delithiated oxide, and have poor thermal stability and safety performance.

Olivine phosphate-based positive electrode material LiMPO ₄ (M = Fe, mn, co, ni and the like) has the advantages of stable structure, better safety and the like compared with the laminar positive electrode materials such as LCO, LMO, ternary and the like. Wherein, the charge-discharge platforms of cobalt lithium phosphate LCP (4.8V) and nickel lithium phosphate LNP (5.2V) are too high to exceed the maximum voltage (4.5V) which can be borne by the traditional electrolyte, and the commercial application of the electrolyte is limited; and the lithium iron phosphate LFP and the lithium manganese phosphate LMP can be adapted to the existing electrolyte system and have commercial application foundation.

Currently, LFP is widely used as a positive electrode material of a lithium ion battery due to its advantages of stable structure, good safety, long cycle life, simple and convenient synthesis method, low cost and the like, however, fe is widely used as a positive electrode material of a lithium ion battery ³⁺ /Fe ²⁺ Relative to Li ⁺ The electrode potential of/Li is only 3.4V, and the energy density is low, so that the application of the electrode potential in high-energy equipment is limited. The theoretical specific capacity of the LMP is similar to that of the LFP (170 mAh/g), the voltage platform is higher (4.1V), and the theoretical energy density (mass energy density = gram capacity of the battery multiplied by working voltage) is 20% higher than that of the LFP, so that the LMP attracts a great deal of attention. However, the energy gap of the LMP material is as high as 2eV, the electron transition energy gap is large, and the electron conductivity is highLi ⁺ The diffusion coefficient is lower than that of LFP, and the material is considered as an insulator, and has extremely poor electrochemical performance (low discharge capacity and poor rate performance); and Mn is dissolved during the cycle to make the capacity retention ratio poor, and Mn ³⁺ The occurrence of Jahn-Teller distortion limits the application of LMP.

In order to improve the electrochemical performance of the LMP cathode material, researchers have adopted methods such as reducing the crystal size, carbon coating, and doping. In addition, because the LMP and the LFP have the same olivine structure and the sizes of Fe and Mn ions are close, the Fe ions and the Mn ions can be mutually dissolved at any ratio to form a solid solution, and the solid solution can be understood that Fe replaces part of Mn in the LMP or Mn replaces part of Fe in the LFP, namely lithium manganese iron phosphate (LiFe) _x Mn _1-x PO ₄ LFMP). The working voltage is 3.5-4.1V, and the electrolyte is suitable for a stable electrochemical window of a traditional electrolyte system. The doping of Fe at Mn position increases the conductivity of the material, improves the transmission performance of the material, and reduces Mn ³⁺ The Jahn-Teller distortion can ensure the specific capacity and simultaneously improve the rate capability and the cycle performance of the material, so that the LFMP becomes one of the most promising positive electrode materials of the high-performance lithium ion battery. The ferromanganese ratio Mn/Fe in LFMP can directly influence the electrochemical performance of the material. The Mn content is too low, the voltage of a material platform is not obviously improved, and the energy density is improved in a limited way; the content of Mn is too high, the conductivity of the material is poor, so that the capacity cannot be exerted, the Jahn-Teller effect is obvious, and the stability and the cycle life of the material are influenced. The research proves that the Mn/Fe has application value in comparison between 6:4-8:2.

LFMP is prepared by a method similar to LFP, including a solid phase method and a liquid phase method. The solid phase method comprises the steps of ball milling and uniformly mixing a lithium source, a manganese source, an iron source, a phosphorus source and a carbon source, drying and calcining at a certain temperature; the method has simple process, is beneficial to batch production, but has high synthesis temperature, long synthesis time, difficult control of the shape and size of the crystal and poor uniformity. Therefore, it is difficult to obtain LFMP with high Mn/Fe content and good performance by the solid phase method. The liquid phase method includes a coprecipitation method, a hydrothermal/solvothermal method, a sol-gel method, and the like. The liquid phase method is easier to form uniform solid solution, but the process is more complicated and the cost is higher. Both the coprecipitation method and the sol-gel method are to obtain a precursor in a liquid phase and then to sinter the precursor at a high temperature. The LFMP is directly obtained by hydrothermal/solvothermal liquid phase reaction, and then carbon coating is carried out at high temperature. In order to synthesize LFMP with smaller particle size, a solvothermal method which uses an organic solvent or a mixture of the organic solvent and water as a medium is often adopted, so that primary particles with the size of less than 100nm can be obtained more favorably, and the solid content is limited due to solubility and the production cost is higher due to the recovery treatment of the organic solvent.

To improve Li ⁺ Diffusion and electron transmission, making LFMP granule little, but can lead to the compaction density lower, and then influence the energy density of whole material, the increase of ratio table makes the material in the technology of preparing battery size mixing, needs multi-purpose dispersant, binder and conductive agent, and solid content reduces and influences production efficiency, and the difficult stoving of pole piece just needs the longer energy consumption that increases of stoving time, leads to the cost to increase, and these all restrict LFMP's independent application.

Meanwhile, in the preparation of LFP, the hydrothermal/solvothermal synthesis method is to uniformly mix the raw materials on the molecular level, water-soluble impurities are not easy to enter the product, the particle size of the reaction product is uniform, the batch stability is good, the synthesis route is easy to adjust, and the selectable range of the reaction raw materials is wide; however, the method needs high-temperature and high-pressure resistant equipment, so that the manufacturing cost is high, the investment is large, the organic solvent used in the solvothermal method needs to be recycled, and the large-scale production is difficult to popularize due to the high cost, so that the application of the method is limited.

Disclosure of Invention

The invention aims to provide an electrode material and a preparation method thereof, which realize the preparation of the anode material coated with a carbon layer and have the advantages of simple process and easy large-scale production.

In order to solve the technical problems, the technical scheme adopted by the invention is as follows:

a preparation method of an electrode material comprises the following steps:

step 1: ascorbic acid and sulfate were added to deionized water with stirring to prepare a solution a:

and 2, step: adding lithium phosphate into the solution A, uniformly mixing, and then heating for reaction;

and 3, step 3: carrying out solid-liquid separation on the reaction slurry obtained after cooling in the step 2, and obtaining a filtrate and slurry after separation;

and 5: cleaning the slurry by using a cleaning solution until the conductivity of the filtrate is less than 5mS/cm;

and 6: and drying the washed slurry, adding the dried slurry into a high-speed mixing grinder, grinding the ground slurry, and adding a carbon source to sinter the ground slurry to obtain the carbon-coated anode material.

In the step 1, the phosphate is manganese sulfate, ferrous sulfate and nickel sulfate, and the mass ratio of the manganese sulfate to the ferrous sulfate to the nickel sulfate is Mn to Ni = 8; to obtain a catalyst containing Mn ²⁺ 、Fe ²⁺ And Ni ²⁺ The solution A of (1);

in the step 2, after being uniformly mixed, the mixture is heated to 110-200 ℃ to react for 1-8 hours.

Further optimizing, in the step 5, the method further comprises a measuring step and an adding step, wherein the measuring step comprises measuring the content of Li, fe, mn, ni and P, and the adding step is specifically to supplement a lithium source, an iron source, a manganese source or a phosphorus source according to the mass ratio of Li to Fe to Mn to Ni to P = 1.

And 6, uniformly mixing the slurry added with the carbon source, spray-drying, feeding the spray-dried material into a sintering furnace, heating to 680-800 ℃ from room temperature, keeping for 6-12h, cooling to room temperature, and thus obtaining the LFMP anode material coated with the carbon layer and doped with Ni.

Further optimized, the method also comprises a step 7 of mixing the reaction filtrate with the first washing liquid, adding phosphoric acid and sodium hydroxide (or ammonia water) to recycle Li in the reaction filtrate ⁺ And obtaining the precipitated lithium phosphate which can be reused in the step one.

As a preferable scheme, in practical use, in step 1, the sulfate is ferrous sulfate, and the prepared solution a is a ferrous sulfate solution;

in the step 2, adding lithium phosphate into ferrous sulfate solution, mixing uniformly, heating to 110-200 ℃, keeping for 1-8 hours, and cooling.

And 5, measuring the content of Li, fe and P in the slurry, adding at least one of a lithium source, an iron source and a phosphorus source to enable the mass ratio of the Li, fe and P to be 1.

Further optimization is carried out, in step 2,

the mass ratio of lithium phosphate to ferrous sulfate is 0.9-1.1;

the amount concentration of the ferrous sulfate substance is more than 1mol/L;

the carbon-coated lithium iron phosphate comprises the following specific steps:

step S101: adding a carbon source into the filtered and washed slurry, uniformly mixing, and performing spray drying;

step S102: and (3) feeding the spray-dried material into a sintering furnace, heating the material from room temperature to 680-800 ℃ under the protection of inert atmosphere, keeping the temperature for 4-12h, cooling the material to room temperature, and thus obtaining the carbon-coated lithium iron phosphate.

In addition, the invention also discloses an electrode material, namely LFMP coated with the carbon layer doped with Ni or lithium iron phosphate coated with the carbon layer, which is prepared by the preparation method.

The lithium iron phosphate coated with the carbon layer prepared by the preparation method of the electrode material has the following electrical properties at 25 ℃ and normal temperature: 0.1C, 158-160mAh/g of discharge capacity, and more than 144mAh/g of 3C discharge capacity;

electrical properties at low temperature of-20 ℃:0.1C, the discharge capacity retention rate is more than 75 percent;

the compacted density of the powder is more than 2.45g/cm ³ 。

Compared with the prior art, the invention has the following beneficial effects:

according to the invention, lithium phosphate is used as a raw material, and is a Li source and a P source, so that the amount of additionally added P source is reduced compared with other Li sources; from the lithium salt production end, the lithium phosphate process for extracting lithium from the salt lake has higher yield than the lithium carbonate process, so that lithium phosphate with equal lithium amount is cheaper than lithium carbonate. Based on the two reasons, the raw material selection saves the production cost; the deionized water is adopted, so that the removal of water-soluble impurities in the raw materials is facilitated, no organic solvent is used, and the recycling of the organic solvent is not involved. Meanwhile, the grinding and spheroidizing technology improves the compaction density of the product, keeps good low-temperature multiplying power performance, obviously improves the compaction density and solves the problem that the compaction density and the low-temperature multiplying power performance of a solid phase method cannot be obtained simultaneously.

Detailed Description

In the following, only certain exemplary embodiments are briefly described. As those skilled in the art will recognize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the embodiments of the invention.

Example one

The embodiment discloses a preparation method of an electrode material, in particular to the preparation of an LFMP anode material coated with a carbon layer and doped with Ni;

the method specifically comprises the following steps:

a. ascorbic acid, manganese sulfate, ferrous sulfate, and nickel sulfate (substance amount ratio Mn: fe: ni =8: 0.05-0.50 ²⁺ 、Fe ²⁺ And Ni ²⁺ The solution of (1);

b. adding lithium phosphate into Mn-containing material prepared in step a ²⁺ 、Fe ²⁺ And Ni ²⁺ The solution is evenly mixed, and the temperature is raised to 110 to 200 ℃ for reaction for 1 to 8 hours;

c. carrying out solid-liquid separation on the cooled reaction slurry to obtain a separated filtrate and slurry, and cleaning the slurry by using a cleaning solution until the conductivity of the filtrate is less than 5mS/cm;

d. drying the washed slurry, respectively measuring the contents of Li, fe, mn, ni and P, supplementing a certain amount of a lithium source, an iron source, a manganese source or a phosphorus source according to the mass ratio of Li to Fe to Mn to Ni to P = 1;

e. adding a certain amount of carbon source into the sanded slurry, uniformly mixing, and then spray-drying;

f. sending the spray-dried material into a sintering furnace, heating to 680-800 ℃ from room temperature, keeping for 6-12h, cooling to room temperature to obtain LFMP coated with the carbon layer and doped with Ni;

g. mixing the reaction filtrate and the first washing liquid, adding phosphoric acid to recover Li ⁺ And obtaining precipitated lithium phosphate which can be reused in the step a.

In the step a, lithium phosphate is an insoluble substance, does not need to be dissolved, and is only dispersed by water; the manganese sulfate, the nickel sulfate and the ferrous sulfate have good water solubility and form a solution after being dissolved in water; the quantity proportion relation of lithium phosphate, manganese sulfate and ferrous sulfate is n [ Li ] ₃ PO ₄ ]:n[Mn+Fe+Ni]＝(0.9-1.1):1。

In the step b, because the solubility product constant of the lithium iron phosphate and the lithium manganese phosphate is smaller than that of the lithium phosphate, the lithium phosphate can be gradually dissolved due to the existence of ferrous ions, and a solid solution of the lithium iron phosphate and the lithium manganese phosphate which are more insoluble is generated.

The reaction formula is as follows: li ₃ PO ₄ +0.2FeSO ₄ +0.8MnSO ₄ →LiFe _0.2 Mn _0.8 PO ₄ ↓+Li ₂ SO ₄ The reaction can be carried out at normal temperature and normal pressure, only the reaction rate is slow, the time required for the reaction is long, the reaction rate can be accelerated by increasing the reaction temperature, the reaction can be carried out at a higher speed under the hydrothermal condition when the temperature is increased to more than 100 ℃, and the crystal growth is more complete. A certain amount of soluble Ni salt is added for doping LFMP and increasing Li ⁺ Mobility.

And c, filtering and washing in the steps c and d, wherein ceramic membrane filtration or plate-and-frame filter pressing can be adopted, and the conductivity of the filtrate is required to be less than 5mS/cm in order to ensure that the filtrate is cleanly washed.

In the step, the supplemented lithium source is preferably lithium carbonate, the supplemented phosphorus source is preferably phosphoric acid, and the supplemented lithium source and the supplemented phosphorus source can be mixed in advance or directly added into a sand mill;

in the step e, the carbon source can be selected from glucose, sucrose, starch, phenolic resin, furfural resin, polyethylene glycol, polyvinyl alcohol and the like, and the adding amount of the carbon source is 8-20% by LFP.

In step f, the sintering furnace is preferably a rotary furnace.

In step g, a large amount of lithium sulfate (Li) is formed due to the reaction ₂ SO ₄ ) To save cost, phosphoric acid (H) is added ₃ PO ₄ ) And sodium hydroxide (NaOH) or ammonia (NH) ₃ ·H ₂ O) recovery of Li therein ⁺ Lithium phosphate (Li) formed ₃ PO ₄ ) Recycling; the amount of phosphoric acid added is calculated according to the measured Li content; by controlling Li ₂ SO ₄ And H ₃ PO ₄ Concentration preparation of Li of suitable grain size ₃ PO ₄ (Bie table 15-35 m) ² /g)。

In the embodiment, lithium phosphate is used as a raw material, and is a Li source and a P source, so that the amount of additionally added P sources is reduced compared with other Li sources; from the lithium salt production end, the lithium phosphate process for extracting lithium from the salt lake has higher yield than the lithium carbonate process, so that the lithium phosphate with equal lithium amount is cheaper than lithium carbonate, and the raw material selection saves the production cost based on the two reasons.

The grain size of lithium phosphate is found to influence the particle size of the LFMP produced, and the LFMP with the ideal size is prepared by controlling the size of the lithium phosphate.

In the invention, lithium phosphate does not need to be completely dissolved, and the process route is simple.

In the invention, the solvent is only deionized water, which is beneficial to removing water-soluble impurities in the raw materials; no organic solvent is used, and the recycling of the organic solvent is not involved.

The invention utilizes the advantage of uniform molecular level mixing of a liquid phase method, and measures the content of Li, fe and P in the reacted materials after liquid phase reaction according to LiFe _0.2 Mn _0.8 PO ₄ Supplementing Li source, iron source, manganese source or phosphorus source according to a certain theoretical proportion, and sintering the coated carbon layer after the materials are fully mixed by sanding.

The sanding of the invention has more important function of shaping besides uniform mixing, can obtain the LFMP primary particles which are approximately spherical, and in addition, the sanding can break part of the LFMP particles to obtain the particles with size gradation, thereby improving the compaction density of the product.

The method controls the particle size from the raw material end, combines Ni doping and carbon coating, and has the combined action of improving the ionic conduction and electronic transmission of the LMP, improving the electrical property and fully playing the advantages of the LMP high-voltage platform.

The carbon coated LiFe obtained by the invention _0.2-x Mn _0.8 Ni _x PO ₄ (x = 0.005-0.050), electric property at normal temperature (25 deg.C) 0.1C discharge capacity of 150-155mAh/g, and powder compacted density of more than 2.25g/cm ³ 。

That is, in the present invention, lithium phosphate is used as a lithium source and a phosphorus source, ferrous sulfate is used as an iron source, manganese sulfate is used as a manganese source, nickel sulfate is used as a nickel source, and the stoichiometric ratio of the first charge is Li ₃ PO ₄ (Fe + Mn + Ni) =0.9-1.1, mn;

according to the invention, lithium and phosphorus are supplemented by sanding, the nonstoichiometric ratio of each element in the product is compensated, and the LFMP has higher purity after high-temperature sintering.

The invention is shaped by sanding to obtain primary particles which are approximately spherical and have size gradation, thereby improving the compaction density.

The material of the invention is superior to the solid phase product in electrical property, and has high compaction density, thereby being beneficial to improving the energy density of the battery and improving the processing property of the material.

Lithium sulfate generated after the reaction is converted into lithium phosphate for recycling by using lithium resources in the recovered lithium phosphate.

In addition, the embodiment also discloses an LFMP cathode material coated with a carbon layer and doped with Ni, which is mainly prepared by the method described in the embodiment.

The specific preparation of the LFMP coated with the carbon layer and doped with Ni is as follows:

LFMP concrete preparation case 1

Adding 25g of ascorbic acid, 4.335kg of manganese sulfate monohydrate, 1.763kg of ferrous sulfate heptahydrate and 3.712kg of lithium phosphate into 50L of deionized water under the condition of stirring, and stirringAfter being uniform, the temperature is raised to 180 ℃, and the mixture is continuously stirred for 4 hours at 180 ℃; cooling the reacted slurry to below 40 ℃, carrying out solid-liquid separation, and washing the solid slurry obtained by separation until the conductivity of the cleaning liquid is less than 5mS/cm. After the washed slurry is dried, the content of Li, mn, fe and P is respectively measured to be 4.21%, 31.38%, 6.94% and 19.15%, 34.92g of lithium carbonate and 79.31g of 85% phosphoric acid are supplemented to 1kg of dry material, a high-speed mixing grinder is added for sanding, 75g of glucose and 75g of polyethylene glycol are added into the slurry, the mixture is uniformly stirred, the mixture is spray-dried and sintered at 720 ℃ for 8 hours, and the lithium manganese iron phosphate LFMP coated with the carbon layer is obtained, and the detection is carried out: is LiFe _0.2 Mn _0.8 PO ₄ Specific surface area 15.6m ² (g), powder compaction density 2.21g/cm ³ Carbon content 1.65%,25 ℃ 0.1C discharge capacity 145mAh/g.

Another processing method is as follows: and (2) measuring the solid content of the washed slurry to be 30% without supplementing a lithium source and a phosphorus source or sanding, adding 75g of glucose and 75g of polyethylene glycol into 1kg of dry material, uniformly stirring, spray-drying, sintering at 720 ℃ for 8 hours to obtain lithium manganese iron phosphate LFMP coated with a carbon layer, and detecting: is Li _0.90 Fe _0.2 Mn _0.8 PO ₄ Specific surface area 14.3m ² Per g, powder compacted density 2.01g/cm ³ Carbon content 1.67%, discharge capacity 133mAh/g at 25 ℃ and 0.1C.

LFMP concrete preparation case 2

Under the condition of stirring, adding 25g of ascorbic acid, 4.335kg of manganese sulfate monohydrate, 1.455kg of ferrous sulfate heptahydrate, 0.292kg of nickel sulfate hexahydrate and 3.712kg of lithium phosphate into 50L of deionized water, uniformly stirring, heating to 180 ℃, and continuously stirring for 4 hours at 180 ℃; cooling the reacted slurry to below 40 ℃, carrying out solid-liquid separation, and washing the solid slurry obtained by separation until the conductivity of the cleaning liquid is less than 5mS/cm. After the washed slurry was dried, the contents of Li, mn, fe, ni and P were measured to be 4.16%, 30.46%, 4.97%, 0.93% and 19.32%, respectively, 1kg of the dried material was supplemented with 39.71g of lithium carbonate, 29.68g of ferrous carbonate and 85% of phosphoric acid 79.50g, and the mixture was added to a high-speed mixing mill for sanding, and 70g of each of glucose and polyethylene glycol was added to the slurry, followed by stirring uniformly, spray drying, and sintering at 720 ℃ for 8 hours to obtain a coated carbon layerThe lithium ferric manganese phosphate LFMP is detected as follows: is LiFe _0.17 Mn _0.8 Ni _0.03 PO ₄ Specific surface area of 15.3m ² (iv) g, powder compacted density 2.23g/cm ³ Carbon content 1.61%, discharge capacity 151mAh/g at 25 ℃ and 0.1C.

Example two

The embodiment discloses a preparation method of an electrode material, in particular to the preparation of lithium iron phosphate coated with a carbon layer.

The method comprises the following specific steps:

step 1: under the condition of stirring, adding ascorbic acid and ferrous sulfate into deionized water to prepare a ferrous sulfate solution;

step 2: adding lithium phosphate into ferrous sulfate solution, mixing uniformly, heating to 110-200 ℃, keeping for 1-8 hours, and cooling;

and step 3: carrying out solid-liquid separation on the cooled slurry to obtain a filtrate and the slurry after separation;

and 4, step 4: cleaning the slurry by using a cleaning solution until the conductivity of the cleaning solution is less than 5mS/cm;

and 5: adding a carbon source into the cleaned slurry to prepare coated lithium iron phosphate;

step 6: and mixing the filtrate after the solid-liquid separation with the cleaning solution, adding phosphoric acid and sodium hydroxide or ammonia water to obtain precipitated lithium phosphate, and returning the lithium phosphate to the previous use.

In the step 2, the mass ratio of the lithium phosphate to the ferrous sulfate is 0.9-1.1.

Further defined, the ferrous sulfate species is present at a concentration greater than 1mol/L.

Further optimization, the carbon-coated lithium iron phosphate comprises the following specific steps:

step S101: adding a carbon source into the filtered and washed slurry, uniformly mixing, and performing spray drying;

step S102: and (3) feeding the spray-dried material into a sintering furnace, heating the material from room temperature to 680-800 ℃ under the protection of inert atmosphere, keeping the temperature for 4-12h, cooling the material to room temperature, and thus obtaining the carbon-coated lithium iron phosphate.

Wherein the carbon source is at least one of glucose, sucrose, starch, phenolic resin, furfural resin, polyethylene glycol and polyvinyl alcohol.

Further optimizing, the adding amount of the carbon source is 8-16% of the weight of the ground material.

The invention adopts lithium phosphate as raw material, thus saving cost. Lithium phosphate is a phosphorus source and a lithium source, the addition amount of the single phosphorus source is reduced by one third, and from the lithium salt production end, the lithium phosphate process for extracting lithium from the salt lake has higher yield than the lithium carbonate process, so the lithium phosphate with the same lithium amount is cheaper than lithium carbonate. In the process, lithium in the mother liquor is recovered in the form of lithium phosphate, the recovery efficiency is high, and the cost of a hydrothermal/solvothermal method is reduced. Solvents of raw materials lithium phosphate and ferrous sulfate are deionized water, which is beneficial to removing water-soluble impurities in the raw materials; no organic solvent is used, and the recycling of the organic solvent is not involved.

Further optimizing, in this embodiment, in step 4, determining the content of Li, fe, and P in the cleaned slurry, and adding at least one of a lithium source, an iron source, and a phosphorus source so that the mass ratio of Li, fe, and P is 1; therefore, li, fe and P can be supplemented by measuring the content of Li, fe and P after reaction, the non-stoichiometric ratio of each element in the product is made up, and the LFP has higher purity after high-temperature sintering. The grinding process can uniformly mix the materials on one hand, and on the other hand, the grinding process is a key step of shaping the LFP to finally obtain LFP primary particles with size gradation and approximate spherical shape, thereby improving the compaction density of the product.

In addition, the embodiment also discloses a lithium iron phosphate anode material coated with a carbon layer, which is mainly prepared by the method in the embodiment.

To facilitate a further understanding of the invention by those skilled in the art, the invention is further described below in connection with specific embodiments.

Case one

A method for preparing lithium iron phosphate, comprising the following steps S001 to S007.

Step S001:

sequentially adding ascorbic acid and ferrous sulfate into deionized water to prepare a ferrous sulfate solution; the ferrous sulfate has good water solubility, ferrous ions are easy to be oxidized, and ascorbic acid is added as a reducing agent;

step S002:

adding lithium phosphate into ferrous sulfate solution, stirring and mixing uniformly, heating to 110-200 ℃, keeping for 1-8 hours, and cooling.

In the step, the mass ratio of lithium phosphate to ferrous sulfate is (0.9-1.1): 1.

Preferably, the ratio of 0.98:1.

in actual use, the lithium phosphate is an insoluble substance, does not need to be dissolved, and only needs to be uniformly dispersed in a material system.

Meanwhile, the solubility product constant of the lithium iron phosphate is smaller than that of the lithium phosphate, and the lithium phosphate can be gradually dissolved due to the existence of ferrous ions, so that the more insoluble lithium iron phosphate is generated.

The reaction formula is as follows:

Li ₃ PO ₄ +FeSO ₄ →LiFePO ₄ ↓+Li ₂ SO ₄ ，

the reaction can occur at normal temperature and normal pressure, only the reaction rate is slow, the time required for the reaction is long, the reaction rate can be accelerated by increasing the reaction temperature, and the reaction can occur more quickly under the hydrothermal condition when the temperature is increased to more than 100 ℃;

the reaction temperature is preferably controlled at 180 ℃, so that Li/Fe mixed row is reduced, and crystal growth is more perfect.

Step S003:

carrying out solid-liquid separation on the cooled slurry to obtain a filtrate and slurry after separation, wherein the solid contained in the slurry is LiFePO ₄ 。

Step S004:

and cleaning the slurry by using a cleaning solution until the conductivity of the cleaning solution is less than 5mS/cm.

In actual use, the steps adopt ceramic membrane filtration or plate-and-frame filter pressing for filtration and washing; and the conductivity of the cleaning solution is controlled to be 5mS/cm so as to ensure LiFePO ₄ The cleanliness of the cleaning.

Step S005:

and adding a carbon source into the cleaned slurry, uniformly mixing, spray-drying, and sintering at 680-800 ℃ for 4-12 hours under the protection of inert atmosphere to obtain the carbon-coated lithium iron phosphate.

It should be noted that the sintering furnace may be a rotary furnace;

the carbon source is at least one of glucose, sucrose, starch, phenolic resin, furfural resin, polyethylene glycol and polyvinyl alcohol, and the addition amount of the carbon source is 8-20% of the weight of the ground material.

Step S006:

and mixing the separated filtrate with the cleaning solution, and adding phosphoric acid and sodium hydroxide to obtain precipitated lithium phosphate.

Lithium phosphate and ferrous sulfate generate a large amount of lithium sulfate Li in the reaction process ₂ SO ₄ To save cost, phosphoric acid H is added ₃ PO ₄ Recovering Li therein ⁺ Formation of lithium phosphate Li ₃ PO ₄ Recycling;

the amount of phosphoric acid added is calculated from the measured Li content.

It should be noted that this step may be performed synchronously with any step after step S004, and steps S001 to S007 in this embodiment do not limit the order of the steps, and are only for convenience of understanding and explanation.

The carbon-coated LFP prepared by the above method,

electrical properties at 25 ℃ at room temperature: 0.1C discharge capacity 158-160mAh/g; the 3C discharge capacity is 144mAh/g;

the retention rate of 0.1C discharge capacity at-20 ℃ at low temperature is more than 75 percent;

the compacted density of the powder is more than 2.45g/cm ³ 。

The invention adopts lithium phosphate as raw material, thus saving the cost of raw material. Lithium phosphate is a phosphorus source and a lithium source, the addition amount of the single phosphorus source is reduced by one third, and from the lithium salt production end, the lithium phosphate process for extracting lithium from the salt lake has higher yield than the lithium carbonate process, so the lithium phosphate with the same lithium amount is cheaper than lithium carbonate. In the process, lithium in the mother liquor is recovered in the form of lithium phosphate, the recovery efficiency is high, and the cost of a hydrothermal/solvothermal method is reduced.

Deionized water is adopted as solvents of raw materials lithium phosphate and ferrous sulfate, so that water-soluble impurities in the raw materials can be removed; no organic solvent is used, and the recycling of the organic solvent is not involved.

The grinding and spheroidizing technology improves the compaction density of the product, and obviously improves the compaction density while keeping good low-temperature rate capability. The problem that the compacted density and the low-temperature rate performance can not be achieved simultaneously by a solid phase method is solved.

In order to facilitate further understanding of the present invention for those skilled in the art, the present invention will be further described with reference to specific preparation examples.

Preparation example 1

Weighing 25g of ascorbic acid, 5363 kg of ferrous sulfate, 8.599kg and 3.788kg of lithium phosphate ₃ PO ₄ 、FeSO ₄ The mass ratio was 1:1.

Under the condition of stirring, adding ascorbic acid, ferrous sulfate and lithium phosphate into 45L of deionized water, uniformly stirring, heating to 180 ℃, and continuously stirring for 4 hours at 180 ℃; and cooling the slurry after the reaction to below 40 ℃, centrifuging to perform solid-liquid separation, and washing the solid LFP obtained by separation until the conductivity of the cleaning solution is less than 5mS/cm.

Drying the washed solid LFP, determining the contents of Li, fe and P to be 4.071%, 35.24% and 18.63% respectively, supplementing 16.46g of lithium carbonate and 85% of phosphoric acid 33.84g by 1kg LFP, adding a high-speed mixing mill, and sanding to a specific table of 13.5m ² And/g, adding 60g of glucose and 60g of polyethylene glycol into the slurry, uniformly stirring, spray-drying, sintering at 720 ℃ for 8 hours to obtain lithium iron phosphate LFP coated with the carbon layer, and detecting:

it is in table 13.8m ² (g), powder compaction density 2.49g/cm ³ The carbon content is 1.45 percent, the discharge capacity at 25 ℃ is 0.1C and 159mAh/g, the discharge capacity at 1C is 153mAh/g, the discharge capacity at 3C is 145mAh/g, and the capacity retention rate at-20 ℃ and 0.1C is 78 percent.

Preparation example 2

Weighing 25g of ascorbic acid, 8.599kg of ferrous sulfate and 3.977kg of lithium phosphate ₃ PO ₄ 、FeSO ₄ The mass ratio of the substances is 1.05.

Under the condition of stirring, sequentially adding ascorbic acid, ferrous sulfate and lithium phosphate into 45L of deionized water, uniformly stirring, heating to 160 ℃, and continuously stirring for 6 hours at 160 ℃; and cooling the slurry after the reaction to below 40 ℃, centrifuging to perform solid-liquid separation, and washing the LFP obtained by separation until the conductivity of the cleaning solution is less than 5mS/cm.

Drying the washed solid LFP, determining the contents of Li, fe and P to be 4.082%, 35.19% and 18.70%, respectively, supplementing 15.54g of lithium carbonate and 30.23g of 85% phosphoric acid to 1kg of LFP, adding into a high-speed mixing and grinding machine, and grinding to a specific surface of 13.6m ² Adding 65g of glucose and 65g of polyethylene glycol into the slurry, uniformly stirring, spray-drying, sintering at 740 ℃ for 6 hours to obtain the LFP coated with the carbon layer, and detecting:

TABLE 13.9m ² (g), powder compaction density 2.48g/cm ³ Carbon content 1.53%, 0.1C discharge capacity 158mAh/g at 25 ℃,1C discharge capacity 152mAh/g, and 3C discharge capacity 144mAh/g.

Preparation example 3

Weighing 25g of ascorbic acid, 8978 kg of ferrous sulfate, 8978 kg of zxft 8978 kg and 3.600kg of lithium phosphate ₃ PO ₄ 、FeSO ₄ The mass ratio of the substances is 0.95.

Under the condition of stirring, sequentially adding ascorbic acid, ferrous sulfate and lithium phosphate into 45L of deionized water, uniformly stirring, heating to 170 ℃, and continuously stirring for 5 hours at 170 ℃; and cooling the slurry after the reaction to below 40 ℃, centrifuging to separate solid from liquid, and washing the solid LFP obtained by separation until the conductivity of washing liquid is less than 5mS/cm.

After drying, the Li, fe and P contents are respectively determined to be 4.025%, 35.28% and 18.49%,1kg of LFP is supplemented with lithium carbonate 19.17g and 85% phosphoric acid 39.84g, a high-speed mixing grinder is added for sanding until the ratio table is 13.8m2/g, glucose and polyethylene glycol are added into the slurry for 70g respectively, the mixture is stirred uniformly, spray-dried and sintered for 5 hours at 760 ℃, so that the LFP coated with the carbon layer is obtained, and the detection shows that:

ratiometric Table 14.1m ² /g，The compacted density of the powder is 2.46g/cm ³ Carbon content 1.61%, 0.1C discharge capacity 158mAh/g at 25 ℃,1C discharge capacity 153mAh/g, and 3C discharge capacity 144mAh/g.

While preferred embodiments of the present invention have been described, additional variations and modifications in those embodiments may occur to those skilled in the art once they learn of the basic inventive concepts. Therefore, it is intended that the appended claims be interpreted as including preferred embodiments and all such alterations and modifications as fall within the scope of the invention.

The present invention should be considered as limited only by the preferred embodiments and not by the specific details, but rather as limited only by the accompanying drawings, and as used herein, is intended to cover all modifications, equivalents and improvements falling within the spirit and scope of the invention.

Claims (10)

1. The preparation method of the electrode material is characterized by comprising the following steps of:

step 1: ascorbic acid and sulfate were added to deionized water with stirring to prepare solution a:

step 2: adding lithium phosphate into the solution A, uniformly mixing, and then heating for reaction;

and step 3: carrying out solid-liquid separation on the reaction slurry obtained after cooling in the step 2, and obtaining a filtrate and slurry after separation;

and 5: cleaning the slurry by using a cleaning solution until the conductivity of the filtrate is less than 5mS/cm;

step 6: and drying the washed slurry, adding the dried slurry into a high-speed mixing grinder, grinding the ground slurry, and adding a carbon source to sinter the ground slurry to obtain the carbon-coated anode material.

2. The method for preparing an electrode material according to claim 1, wherein:

in the step 1, the phosphate is manganese sulfate, ferrous sulfate and nickel sulfate, and the mass ratio of the manganese sulfate to the ferrous sulfate to the nickel sulfate is Mn: fe: ni = 8; to obtain a catalyst containing Mn ²⁺ 、Fe ²⁺ And Ni ²⁺ The solution A of (1);

in the step 2, after being uniformly mixed, the mixture is heated to 110-200 ℃ to react for 1-8 hours.

3. The method for preparing an electrode material according to claim 2, wherein: in the step 5, a measuring step and an adding step are further included, wherein the measuring step comprises the steps of measuring the content of Li, fe, mn, ni and P, and the adding step is specifically that according to the measuring result, a lithium source, an iron source, a manganese source or a phosphorus source is supplemented according to the mass ratio of Li to Fe to Mn to Ni to P = 1.

4. The method for preparing an electrode material according to claim 3, wherein: and 6, uniformly mixing the slurry added with the carbon source, spray-drying, feeding the spray-dried material into a sintering furnace, heating to 680-800 ℃ from room temperature, keeping for 6-12h, cooling to room temperature, and thus obtaining the LFMP anode material coated with the carbon layer and doped with Ni.

5. The method for preparing an electrode material according to claim 4, wherein: further comprising step 7 of mixing the reaction filtrate with the first washing solution, adding phosphoric acid and sodium hydroxide (or ammonia water) to recover Li therein ⁺ And obtaining the precipitated lithium phosphate which can be reused in the step one.

6. The method for preparing an electrode material according to claim 1, wherein:

in the step 1, the sulfate is ferrous sulfate, and the prepared solution A is a ferrous sulfate solution;

in the step 2, adding lithium phosphate into ferrous sulfate solution, mixing uniformly, heating to 110-200 ℃, keeping for 1-8 hours, and cooling.

7. The method for preparing an electrode material according to claim 6, wherein: and step 5, determining the content of Li, fe and P in the slurry, adding at least one of a lithium source, an iron source and a phosphorus source so that the mass ratio of the Li, fe and P is 1.

8. The method for preparing an electrode material according to claim 7, wherein: in the step 2, the step of the method is carried out,

the mass ratio of lithium phosphate to ferrous sulfate is 0.9-1.1;

the amount concentration of the ferrous sulfate substance is more than 1mol/L;

the carbon-coated lithium iron phosphate comprises the following specific steps:

step S101: adding a carbon source into the filtered and washed slurry, uniformly mixing, and performing spray drying;

step S102: and (3) feeding the spray-dried material into a sintering furnace, heating the material from room temperature to 680-800 ℃ under the protection of inert atmosphere, keeping the temperature for 4-12h, cooling the material to room temperature, and thus obtaining the carbon-coated lithium iron phosphate.

9. An electrode material, characterized in that: comprising LFMP obtained by doping Ni on a carbon-coated layer by the method for preparing an electrode material according to claim 4;

the molecular formula is as follows: liFe _0.2-x Mn _0.8 Ni _x PO ₄ (x＝0.005-0.050)；

The electrical property at 25 ℃ is 0.1C, the discharge capacity is 150-155mAh/g, and the powder compaction density is more than 2.25g/cm ³ 。

10. An electrode material characterized by comprising lithium iron phosphate coated with a carbon layer produced by the method for producing an electrode material according to claim 8;

the lithium iron phosphate has the following electrical properties at the normal temperature of 25 ℃:0.1C, discharge capacity of 158-160mAh/g,3C discharge capacity of more than 144mAh/g;

electrical properties at low temperature of-20 ℃:0.1C, the discharge capacity retention rate is more than 75 percent;

the compacted density of the powder is more than 2.45g/cm ³ 。