RU2467434C1 - Active cathode material providing improved efficiency and energy density of electrode - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: invention relates to an active material for a positive electrode, having a composition of formula LiFe(P_1-xO₄), where P has molar ratio from 0.910 to 0.999. The disclosed active material for a positive electrode has operational efficiency levelled to operational efficiency of active material of a negative electrode, which is relatively low, which improves energy density of the active material of the positive electrode. The Fe²⁺ and Fe³⁺ ions contained in the disclosed material prevent structure destruction caused by shortage of phosphorus (P), resulting in improvement of ion conduction of the material and achieving excellent current properties and inhibiting voltage drop on the active resistance during charging/discharging, thus improving energy density.

EFFECT: material is obtained using a supercritical hydrothermal method and can be used in cathode material for lithium secondary batteries.

21 cl, 4 dwg, 2 ex

Description

Technical field

The present invention relates to an active cathode material with improved efficiency and energy density of the electrode. More specifically, the present invention relates to an active cathode material having a lithium iron phosphate (LiFe (P _1-x O ₄ )) structure, in which the mole fraction of (1-x) phosphorus (P) is in the range of 0.910 to 0.999.

This active cathode material allows you to level the efficiency of its work to a lower efficiency of the active anode material and demonstrates an increased energy density. In addition, the cathode active material, in which the mole fraction of (1-x) phosphorus (P) is brought to a level below 1, contains Fe ²⁺ and Fe ³⁺ , thus eliminating structural deformation resulting from phosphorus (P) deficiency. demonstrating increased ionic conductivity, and hence excellent current properties, and preventing the voltage drop across the active resistance during charge / discharge, thereby providing a high energy density.

BACKGROUND OF THE INVENTION

Technological development and increased demand for mobile equipment have led to a sharp increase in demand for secondary (rechargeable) batteries as energy sources. Among these secondary batteries, lithium secondary batteries are commercially available and widely used, having high energy density and voltage, long service life and low self-discharge.

In lithium secondary batteries, carbon material is usually used as the anode active material. The use of lithium metal, sulfur compounds, and the like was also considered. In this case, lithium-cobalt composite oxide (LiCoO ₂ ) is used as the cathode active material in lithium secondary batteries. In addition, the use of complex lithium-manganese oxides, such as LiMnO ₂ with a layered crystalline structure and LiMn ₂ O ₄ with a spinel crystal structure, as well as complex lithium-nickel oxide (LiNiO ₂ ), was considered as an active cathode material.

LiCoO ₂ is currently used due to its excellent physical properties, such as its exceptional cyclic resource, but it has disadvantages such as low stability, high cost due to the use of cobalt, which is disadvantageous due to limited natural resources, and limited mass application as a power source for electric cars. LiNiO ₂ is unsuitable for practical use in mass production at a reasonable cost due to some features associated with its receipt. A disadvantage of lithium manganese oxides such as LiMnO ₂ and LiMn ₂ O ₄ is their short cyclic life.

In recent years, methods for using lithium transition metal phosphate as an active cathode material have been investigated. Phosphates of lithium transition metal are in principle divided into Li _x M ₂ (PO ₄ ) ₃ with the structure NASICON (sodium superionic conductor) and LiMPO ₄ with the structure of olivine and exhibit, as it was found, excellent high-temperature stability compared to traditional LiCoO ₂ . Today Li ₃ V ₂ (PO ₄ ) ₃ with the NASICON structure is well known, and LiFePO ₄ and Li (Mn, Fe) PO ₄ are the most widely known compounds with the olivine structure.

Among compounds with olivine structure, LiFePO ₄ has a high output voltage of ~ 3.5 V and a high theoretical capacity of 170 mA ∙ h / g compared to lithium (Li) and shows excellent high temperature stability compared to cobalt (Co), and also uses a cheap Fe, thus being very suitable for use as an active cathode material for lithium secondary batteries. However, such a LiFePO ₄ type olivine has an efficiency of about 100%, thereby creating difficulties in controlling the efficiency of the anode.

In this regard, by giving equivalent efficiency to the cathode and anode in batteries, the inefficient use of electrodes can be minimized. For example, in the case where an anode with an efficiency of about 100% is used for a battery, the battery can operate with 100% efficiency, while in the case where a cathode with 100% efficiency and an anode with 90% are used in a battery With efficiency, a battery can only operate with 90% efficiency. As a result, 10% of the cathode efficiency is uselessly lost.

For example, in the case of commonly used carbon-based active anode materials, an initial charge / discharge, including a first charge, produces about 10-20% irreversible capacity, and its reversible capacity is only about 80-90%. Therefore, if a material with an efficiency of 100% is used as an active cathode material, then this electrode material is uselessly wasted in direct proportion to an irreversible capacity of about 10-20%. In addition, if active anode material with relatively low efficiency is used, the amount of active anode material should be increased, depending on the higher efficiency of the cathode, which disadvantageously entails an increase in manufacturing costs.

On the other hand, in order to give 100% efficiency to a battery that uses a cathode with 100% efficiency, an anode with about 100% efficiency should be used. In this case, the selection range of the active anode material is unfavorably narrowed.

However, to date, there is no technology that offers a way to control the effectiveness of LiFePO ₄ as an active cathode material.

In addition, there is an increasing need for a breakthrough that can significantly improve the conductivity of LiFePO ₄ and solve the problems of Li ⁺ diffusion in it by improving the initial voltage drop across the active resistance and the diffusion properties of Li ⁺ .

Moreover, in the case where LiFePO _{4 is} used as the cathode active material, the internal resistance of the batteries unfavorably increases due to its low electrical conductivity and the limitation of a sufficient increase in energy density due to the low density compared to traditional active cathode materials. In addition, the olivine crystal structures in which lithium is deintercalated are very unstable, thereby causing unfavorable blocking when passing through the region where lithium is deintercalated on the crystal surface and slowing down the rates of intercalation / deintercalation of lithium.

In response to this, it was proposed to reduce the size of olivine crystals to a nanometer scale in order to make the path of lithium ions shorter and thereby increase the discharge capacity (see Japanese Patent Application Publication Nos. 2002-15735 and 2004-259470).

However, the manufacture of electrodes using olivine particles with such a miniature diameter inevitably entails the use of a large amount of binder, thereby disadvantageously lengthening the mixing time of the suspension and worsening the efficiency of the process.

Therefore, there is an increasing demand for lithium iron phosphate, such as LiFePO ₄ , which exhibits excellent electrical conductivity and density, as well as process efficiency.

Disclosure of invention

Technical problem

Therefore, the present invention was created in order to solve the above problems and other technical problems that have not yet been resolved.

As a result of various wide and intensive studies and experiments to solve the problems described above, the inventors of the present invention found that controlling the mole fraction of (1-x) phosphorus (P) in high-efficiency lithium iron phosphate in the range from 0.910 to 0.999 makes it possible to level the effectiveness of the active cathode material to a lower efficiency of the active anode material, minimizing the useless loss of electrode efficiency and thus the resulting maximization of the efficiency and capacity of the electrodes and batteries, and that controlling the valence of Fe makes it possible to improve the voltage drop across the active resistance and current properties, to improve the potential of the charge / discharge plateau and thereby maximize the energy density.

Additionally, the authors of the present invention found that lithium iron phosphate composed of secondary particles with a given porosity formed by aggregation of primary particles with a small diameter can satisfy excellent electrical conductivity, a stable crystalline structure and high density, which are the advantages of smaller primary particles, as well as high process efficiency, which is the advantage of secondary particles, thereby maximizing the total capacity and energy density of the electrodes and battery Ray.

Based on these discoveries, the present invention was created.

Technical solution

In accordance with one aspect of the present invention, the above and other objectives can be achieved by providing an active cathode material with the composition represented by the following Formula I,

LiFe (P _(1-x) O ₄ ) (I)

where the molar fraction of (1-x) phosphorus (P) is in the range from 0.910 to 0.999.

Such an active cathode material with the composition of Formula I allows you to level the efficiency of the active cathode material to a lower efficiency of the active anode material and demonstrates improved energy density by controlling the molar fraction of (1-x) phosphorus (P) in the range from 0.910 to 0.999.

As mentioned above, LiFePO ₄ has an efficiency of approximately 100%. Therefore, when a relatively low efficiency active anode material is used as the active anode material, a variety of electrode materials is required in order to make the active anode material having a reversible capacity comparable to the active cathode material, thereby unprofitable causing an increase in manufacturing costs.

In this regard, the authors of the present invention have found that the initial operating efficiency can be relatively reduced by controlling the mole fraction of (1-x) phosphorus (P) in the range from 0.910 to 0.999. In accordance with this discovery, although an anode active material with lower work efficiency is used, the cathode active work efficiency can be leveled up to the work efficiency of the active anode material.

Therefore, the present invention makes it possible to minimize the waste of electrode material and thereby significantly reduce manufacturing costs and maintain the desired battery efficiency and capacity, thereby remaining highly beneficial from the point of view of manufacturing processes. In addition, mainly the present invention solves the problems associated with the irreversible capacity of the active anode materials, and expands the range of those active anode materials that can be used, given the efficiency of the battery.

Moreover, ordinary LiFePO ₄ contains Fe only with a valency of 2 ⁺ , while LiFeP _(1-x) O ₄ , where the mole fraction of (1-x) phosphorus (P) is in the range from 0.910 to 0.999 in accordance with this the invention, has a reduced molar fraction of phosphorus (P), and therefore contains both Fe ²⁺ and Fe ³⁺ . If the metal present in the structure of the active material has a mixed valency (for example, Fe ²⁺ / Fe ³⁺ ), then the electrical conductivity and diffusion-related Li ⁺ ion conductivity increase and thereby significantly improve the overall current properties compared with the case when the metal has a single valency (Fe ²⁺ ).

The inventors of the present invention found that the cathode active material of the present invention prevents the voltage drop across the active resistance during charge / discharge and improves the discharge profiles without causing any structural changes, and thereby ultimately increases the battery energy density.

The phrase “active anode material with relatively low work efficiency” as used herein refers to a material having a work efficiency lower than the compound of Formula (I) as an active cathode material, which includes all active anode materials with lower efficiency and active anode materials with reduced efficiency compared to active cathode materials due to the irreversible capacity arising in them during the initial charge / discharge, including the first charge, not Motrya the fact that they have theoretical capacity comparable to cathode active materials.

The active anode material has a work efficiency of less than 100%, preferably 90 to 98%, more preferably 90 to 95%.

For example, such an anode active material is preferably a carbon based material capable of exhibiting high discharge capacity.

Any carbon-based material can be used without special restrictions, provided that it allows reversible intercalation / deintercalation of lithium ions. The carbon-based material may be a carbon-based crystalline composition, a carbon-based amorphous composition, or a combination thereof. A typical representative of the carbon-based crystalline composition is graphite. Crystalline carbons (graphite-based composition) include artificial graphite in the form of potato-like or mesocarbon microbeads (MCMB), natural surface-treated graphite to make flat edges, and the like. In addition, a carbon-based amorphous composition is a material containing carbon atoms and having an amorphous crystalline structure, and examples thereof include non-graphitizable carbon (solid carbon) obtained by subjecting phenolic or furan resins to pyrolysis, and graphitizable carbon (soft carbon) obtained by carbonization coke, needle coke or pitch.

In a preferred embodiment, the carbon material may be natural or artificial graphite, which has a high capacity and high energy density due to its excellent density and conductivity and thereby exhibits excellent power and current properties. More preferably, the carbon material may be mesocarbon microbeads (MCMB from mesocarbon microbeads), which are optically anisotropic spherical particles obtained by heating coke, pitch, or the like. at approximately 400 ° C.

In addition, as examples of carbon materials that can be used in the present invention, mention may be made of complex metal oxides Li _y Fe ₂ O ₃ (0≤y≤1), Li _y WO ₂ (0≤y≤1), Sn _x Me _1-x Me ' _y O _z (Me: Mn, Fe, Pb, Ge; Me': Al, B, P, Si, elements of groups I, II and III of the Periodic table, halogens; 0 <x≤1; 1≤ y≤3; 1≤z≤8); lithium metal; lithium alloys; silicon-based alloys; tin-based alloys and metal oxides such as SnO, SnO ₂ , PbO, PbO ₂ , Pb ₂ O ₃ , Pb ₃ O ₄ , Sb ₂ O ₃ , Sb ₂ O ₄ , Sb ₂ O ₅ , GeO, GeO ₂ , Bi ₂ O ₃ , Bi ₂ O ₄ , Bi ₂ O ₅ , etc.

In the present invention, the molar fraction of phosphorus (P) is in the range from 0.910 to 0.999, preferably from 0.955 to 0.995. If the molar fraction of phosphorus (P) is 1, then the efficiency is close to 100%, and if the molar fraction is less than 0.910, then the crystal structure of LiFeP _(1-x) O _{4 is} deformed, making it difficult to maintain structural stability.

The efficiency of the active cathode material is almost proportional to the molar fraction of phosphorus (P). Therefore, the cathode active material according to the present invention, in which the molar fraction of phosphorus (P) is from 0.910 to 0.999, can be leveled to an efficiency of 90 to 99.9%, preferably 95 to 99%.

There are several ways to control the molar fraction of phosphorus (P) from 0.910 to 0.999. For example, the amount of phosphorus precursor (P) added is reduced during the preparation of LiFePO _4, or the pH is controlled during synthesis. According to the first method, if the amount of added phosphorus precursor (P) is reduced during the short-term reaction, the reaction product is obtained in the presence of a small phosphorus (P) deficiency, and thus the desired mole fraction range can be obtained. According to the latter method, a portion of the phosphorus (P) is washed out of the reaction product at a slightly reduced pH, thereby providing the desired molar fraction range.

If necessary, Fe can be partially replaced by at least one selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn and Y. In this In this case, the stability of crystalline structures and electrical conductivity can be advantageously improved. However, in the case where the amount of Fe to be replaced exceeds 0.5 mol, the capacity may be disadvantageously degraded.

In a preferred embodiment, the active cathode material of the present invention is composed of secondary particles with an average particle diameter (D50) of 5 to 100 μm formed by aggregation of primary particles with an average particle diameter (D50) of 50 to 550 nm, the primary and secondary particles having the composition represented by Formula I below, and the secondary particles have a porosity of 10% or more.

When secondary particles are formed from primary particles, and the pores between them are almost absent, they retain their shape, thereby lengthening the diffusion length Li ⁺ from the surface of the secondary particles to their center and worsening current properties. In addition, when a carbon coating is applied to such a secondary particle, if desired, it should not be applied to the inside of the secondary particle, thereby causing a deterioration in electrical conductivity.

The inventors of the present invention have found that if secondary particles with high porosity are prepared and used in the pressing process to make an electrode, then at least a portion of them can be deformed and thereby return to the primary particles, thereby causing a deterioration in the diffusion capacity of Li ⁺ and electrical conductivity.

That is, in a preferred embodiment of the present invention, the cathode active material takes the form of secondary particles in which the primary particles are aggregated and has high porosity, thereby exhibiting excellent electrical conductivity and high density, which are the advantages of smaller primary particles, as well as high process efficiency, which is the virtue of secondary particles. More specifically, the use of secondary particles in the preparation of the electrode mixture makes it possible to reduce the amount of binder and solvent used, reducing the periods of mixing and drying. Therefore, the capacitance and energy density of the electrodes and batteries can be maximized.

In the active cathode material, the secondary particles have a porosity of at least 10% and have a porosity of 15 to 40%, so they can be returned to the primary particles in the process of pressing the electrodes.

If the secondary particles have a porosity of less than 15%, then grinding the secondary particles requires the application of a higher pressure than in ordinary cases. On the other hand, if the porosity exceeds 40%, then the bond strength between the primary particles is low, making it difficult to handle. In addition, more preferably, the secondary particles have a porosity of from 20 to 30%, given the uniform dispersion and processing efficiency of the primary particles.

The pores present in the secondary particles can be closed or open pores. If we take into account the ease of preparation and the uniform dispersion of the primary particles, the primary particles have many small pores. Therefore, such pores preferably have a size of from 10 to 1000 nm, more preferably from 200 to 700 nm, when measured using a mercury (Hg) porosimeter.

In addition, primary particles should be used in crystalline form so that secondary particles are formed so that they can provide excellent electrical conductivity, a stable crystalline structure and high bulk density, despite the fact that they are transformed from deformed secondary particles in the manufacture of electrodes. That is, the primary particles, preferably each individually, have a crystalline olivine structure.

On the other hand, it is undesirable for secondary particles to be prepared by aggregation of primary particles and crystallization by sintering, since it is necessary to apply high pressure due to the high bond strength between the primary particles in order to allow the secondary particles to transfer to the primary particles, and the secondary particles lose their crystalline structure when deformed. This means that improving Li ⁺ diffusion and conductivity cannot be realized due to the small diameter.

In addition, in order to easily convert secondary particles to primary particles, secondary particles are preferably formed by aggregation of primary particles due to physical bonds, such as van der Waals attraction, rather than chemical bonds, such as covalent or ionic bonds.

The primary particles preferably have an average particle diameter (D50) of from 50 to 550 nm, more preferably from 100 to 300 nm, given the fact that when the average particle diameter of the primary particles is too large, the ionic conductivity cannot be improved to the desired level, and that particles with extremely small diameters are difficult to prepare.

In addition, if the secondary particles have an excessively large average particle diameter, then the porosity between them increases and the bulk density deteriorates, and if the secondary particles have an excessively small average particle diameter, then high processing efficiency cannot be obtained. Therefore, it is preferred that the secondary particles have an average particle diameter (D50) of 5 to 100 μm. Preferably, the secondary particles have an average particle diameter (D50) of 5 to 40 μm, taking into account the mixing of the suspension and the smoothness of the surface of the electrodes. It is not preferable that the average particle diameter (D50) be greater than 40 μm, since precipitation occurs when the suspension is mixed.

Preferably, the secondary particles have a specific surface area (BET) of 5 to 15 m ² / g.

In addition, the shape of the cathode active material is not particularly limited. Preferably, olivine-type lithium iron phosphate may be spherical in view of bulk density.

In a preferred embodiment, the cathode active material has a bulk density of from 0.5 to 1.5 g / ml. If the active cathode material has the bulk density indicated above, then the surface area in contact with the conductive material is increased, thereby making it possible to form an excellent conductive network having excellent electrical conductivity. More specifically, the cathode active material has a bulk density of from 0.8 to 1.3 g / ml.

If necessary, in order to improve conductivity, the cathode active material can be coated with at least one of the group consisting of carbon, noble metals and conductive polymers. In particular, in the case where a carbon coating is applied to the cathode active material, the conductivity can be advantageously increased without a significant increase in preparation costs and weights.

In another preferred embodiment, the cathode active material contains 0.02-5% by weight of Li ₃ PO ₄ based on the total weight and does not contain Li ₂ CO ₃ or contains Li ₂ CO ₃ in an amount of less than 0.25%.

The active cathode material contains an extremely small amount of lithium carbonate, thereby reducing gas formation and exhibiting excellent stability at high temperature and storage stability. In addition, the cathode active material contains Li ₃ PO ₄ , which has exceptionally excellent electrochemical stability, thermal stability and ionic conductivity, thereby advantageously demonstrating excellent current properties when used as lithium secondary batteries as cathode active material. In fact, the idea that the introduction of Li ₃ PO ₄ into lithium-iron phosphate will improve its electrical conductivity is new.

As mentioned above, Li ₂ CO ₃ can be formed by the reaction of a carbon material added to improve conductivity with lithium ions, or it can be an unreacted residual lithium precursor. Preferably, the content of Li ₂ CO _{3 is} as low as possible. In particular, a bloating phenomenon may increase if Li ₂ CO _{3 is} present in an amount of not less than 0.25%. Therefore, Li ₂ CO _{3 is} preferably present in an amount of less than 0.25% by weight, more preferably not more than 0.1% by weight.

Meanwhile, Li ₃ PO ₄ exhibits exceptionally excellent electrochemical stability and excellent thermal stability. Therefore, Li ₃ PO ₄ can improve the high temperature stability of the active cathode material of Formula I without causing side reactions in the batteries and without compromising their charge / discharge properties. In addition, Li ₃ PO ₄ can improve ionic conductivity, thereby advantageously compensating for the low conductivity of the active cathode material and improving the current properties of the batteries. If Li ₃ PO _{4 is} present in an amount exceeding 5% by weight, the battery capacity is disadvantageously degraded under equivalent other conditions. Therefore, it is required that Li ₃ PO ₄ be present in an amount of from 0.1 to 5% by weight.

Li ₃ PO ₄ can be added separately or formed during the preparation of the active cathode material of Formula I by a supercritical hydrothermal method.

The active cathode material of Formula I, containing an appropriate amount of Li ₃ PO ₄ and / or Li ₂ CO ₃ in accordance with the present invention, has a pH of from 8.5 to 11.5, preferably from 10.0 to 11.5.

In a preferred embodiment, the contents of Li ₃ PO ₄ and Li ₂ CO ₃ can be measured using the following methods.

The first method is x-ray diffraction of an element having a Ka of 1.6 to 2, used to confirm the presence of Li ₃ PO ₄ . An element having a Ka of 1.6 to 2 may be cobalt (Co) or iron (Fe).

Li ₃ PO _{4 is} characterized in that, due to interference, it is not easy to detect by X-ray diffraction of Cu or Ka. Accordingly, the authors of the present invention have confirmed through various experiments that the use of X-ray diffraction using an element having Ka from 1.6 to 2 makes it possible to efficiently analyze the presence of Li ₃ PO ₄ .

The second method is a method for studying the content of Li ₂ CO ₃ and / or Li ₃ PO ₄ , which includes mixing 10 g of the sample with 100 ml of distilled water, stirring the mixture for 5 to 10 minutes, filtering the reaction mixture, titrating the filtrate with acid and pH measurement of the resulting solution.

In these methods, re-impregnation and decanting can be performed on the samples so that the Li ₂ CO ₃ or Li ₃ PO ₄ present in the samples can be completely dissolved in distilled water. As a result, the accuracy of determining the content can be further improved. At the same time, parameters such as the total time the sample was added do not change much. The acid used for titration is not particularly limited and is preferably HCl.

Preferably, most of Li ₂ CO ₃ and Li ₃ PO ₄ are present on the surface of the active cathode materials. This is because in the case where Li ₃ PO ₄ is on the surface of the particles, Li ₃ PO ₄ can advantageously effectively improve the ionic conductivity, while in the case when Li ₂ CO _{3 is} present in the active cathode materials, it is difficult to remove particles . Examples of methods for preparing primary particles include, but are not limited to, the solid phase method, co-precipitation, hydrothermal synthesis, and supercritical hydrothermal synthesis.

In a preferred embodiment, the preparation of the cathode active material using supercritical hydrothermal synthesis is carried out in accordance with the following steps (a) to (c):

(a) primary mixing of the starting materials with an alkalizing agent to precipitate a transition metal hydroxide;

(b) secondary mixing supercritical or supercritical water with the mixture obtained in stage (a) to synthesize complex lithium metal oxide and drying it; and

(c) calcining a complex lithium metal oxide.

In step (a), one of the ingredients, Li ₂ CO ₃ , Li (OH), Li (OH) · H ₂ O, LiNO _3, or the like can be used as a lithium precursor. As a precursor of iron (Fe), a ferrous iron-containing compound such as FeSO ₄ , FeC ₂ O ₄ · 2H ₂ O or FeCl _{2 can be used} . As the phosphorus precursor (P), an ammonium salt such as H ₃ PO ₄ , NH ₄ H ₂ PO ₄ , (NH ₄ ) ₂ HPO ₄ or P ₂ O _{5 can be used} .

In addition, the alkalizing agent may be an alkali metal hydroxide, an alkaline earth metal hydroxide or an ammonia compound.

In step (b), supercritical or supercritical water may have a temperature of from 200 to 700 ° C at a pressure of from 180 to 550 bar. In step (c), the calcination temperature may be from 600 to 1200 ° C.

In addition, a complex lithium metal oxide composed of secondary particles can be prepared by drying a mixture of primary particles with a given diameter, a binder and a solvent, followed by aggregation. In this mixture, the primary particles and the binder are present in the mixture in an amount of 5 to 20% by weight and 5 to 20% by weight, respectively, relative to the weight of the solvent. The porosity of the secondary particles can be controlled by changing the ratio of primary particles to solvent. The solvent used in this step may include all organic solvents, including polar solvents such as water, and non-polar solvents. Examples of the binder used in this step include, but are not limited to, sucrose and lactose-based sugars, PVDF and PE-based polymers and bitumen, which are soluble in a polar solvent.

Drying and preparation of the secondary particles can be carried out at the same time by various methods known in the art, including spray drying, fluidized bed drying, vibration drying, etc. In particular, rotary spray drying is preferred since it allows the preparation of secondary particles in the form of spheres and thereby improves bulk density.

Drying can be carried out at a temperature of from 120 to 200 ° C in an inert gas atmosphere (for example, Ar, N ₂ ).

The present invention also provides a cathode mixture containing cathode active material.

In addition to the active cathode material, the cathode mixture may optionally contain a conductive material, a binder, a filler, and the like.

The conductive material is usually added in an amount of 1 to 50% by weight, based on the total weight of the mixture, including the cathode active material. Any conductive material can be used without special restrictions, provided that it has suitable conductivity without causing adverse chemical changes in the batteries. As examples of the conductive material that can be used in the present invention, mention may be made of conductive materials including graphite, such as natural or artificial graphite; carbon blacks such as carbon black, acetylene black, Ketjen black, duct gas black, furnace black, lamp black and thermal black; conductive fibers such as carbon fibers and metal fibers; metal powders such as carbon fluoride powder, aluminum powder and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and polyphenylene derivatives.

A binder is a component that helps to bind the active material with a conductive material and a current collector. The binder is usually added in an amount of from 1 to 50% by weight, based on the total weight of the composition, including the cathode active material. Examples of a binder include polyvinylidene, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, terpolymer ethylene propylene diene monomer, various ethylene propylene monomer, butyrene monomer,

A filler is a component optionally used to restrain anode expansion. There are no particular restrictions on the filler, provided that it does not cause adverse chemical changes in the fabricated battery and is a fibrous material. Olefin polymers such as polyethylene and polypropylene can be used as an example of a filler; and fibrous materials such as glass fibers and carbon fibers.

In addition, the present invention provides a cathode for secondary batteries, in which the cathode mixture is deposited on a current collector.

The cathode for secondary batteries can be prepared by applying a suspension obtained by mixing the cathode mixture with a solvent, such as IMF, on the cathode current collector, with the following drying and pressing by rolling.

The cathode current collector is usually made having a thickness of 3 to 500 microns. There are no specific restrictions on the cathode current collector, provided that it has suitable conductivity without causing adverse chemical changes in the fabricated battery. As examples of a cathode current collector, mention may be made of stainless steel, aluminum, nickel, titanium, sintered carbon, and also aluminum or stainless steel with a surface treated with carbon, nickel, titanium or silver. If necessary, such current collectors can also be processed to form thin irregularities on their surface in order to increase the adhesion strength to the active cathode materials. In addition, cathode current collectors can be used in various forms, including films, sheets, foil, nets, porous structures, foams and non-woven materials.

In addition, the present invention provides a lithium secondary battery comprising a cathode, an anode, a separator and a non-aqueous electrolyte containing lithium salt. LiFe (P _1-x O ₄ ) is used as the active cathode material of the secondary lithium battery, where the molar fraction of (1-x) phosphorus (P) is in the range from 0.910 to 0.999, thereby leveling the efficiency of the active cathode material to work efficiency active anode material and advantageously realizing maximization of battery efficiency and demonstrating excellent current properties and improved energy density due to high electrical conductivity and ionic conductivity.

For example, the anode is prepared by applying an anode mixture, including the active anode material, to the anode current collector, followed by drying. The anode mixture may contain the aforementioned ingredients, i.e. conductive material, binder and filler.

The anode current collector is usually made having a thickness of 3 to 500 microns. There are no specific restrictions on the anode current collector, provided that it has suitable conductivity without causing adverse chemical changes in the fabricated battery. As examples of the anode current collector, mention may be made of copper, stainless steel, aluminum, nickel, titanium, sintered carbon, as well as copper or stainless steel with a surface treated with carbon, nickel, titanium or silver, and aluminum-cadmium alloys. Similarly to the cathode current collector, if necessary, such current collectors can also be processed to form thin irregularities on their surface in order to increase the adhesion strength to the active cathode materials. In addition, current collectors can be used in various forms, including films, sheets, foil, nets, porous structures, foams and non-woven materials.

The separator is located between the cathode and the anode. An insulating thin film having high ion permeability and mechanical strength is used as a separator. The separator usually has pores with a diameter of from 0.01 to 10 microns and a thickness of from 5 to 300 microns. Sheets or non-woven materials made of an olefin polymer, such as polypropylene, and / or fiberglass or polyethylene, which are chemically resistant and hydrophobic, are used as a separator. If a solid electrolyte, such as a polymer, is used as the electrolyte, then the solid electrolyte can also serve as a separator and an electrolyte.

A non-aqueous electrolyte containing a lithium salt consists of a non-aqueous electrolyte and a lithium salt. As the non-aqueous electrolyte, a non-aqueous organic solvent, a solid electrolyte and an inorganic solid electrolyte can be used.

Examples of non-aqueous organic solvents that can be used in the present invention include aprotic organic solvents such as n-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma butyrolactone, 1,2-dimethoxyethane, tetrahydroxyfran methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid ester, trimethoxymethane, dioxole derivatives ana, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ether, methyl propionate and ethyl propionate.

Examples of organic solid electrolytes that can be used in the present invention include derivatives of polyethylene, derivatives of polyethylene oxide, derivatives of polypropylene oxide, polymers of phosphoric acid esters, mixed polylysine (poly agitation lysine), polyester sulfide, polyvinyl alcohols, polyvinylidene fluoride and polymers containing ionic groups dissociation.

As examples of the inorganic solid electrolyte that can be used in the present invention, mention may be made of lithium nitrides, halides and sulfates, such as Li ₃ N, LiI, Li ₅ NI ₂ , Li ₃ N-LiI-LiOH, LiSiO ₄ , LiSiO ₄ - LiI-LiOH, Li ₂ SiS ₃ , Li ₄ SiO ₄ , Li ₄ SiO ₄ -LiI-LiOH and Li ₃ PO ₄ -Li ₂ S-SiS ₂ .

Lithium salt is a substance that readily dissolves in the aforementioned non-aqueous electrolyte and may include, for example, LiCl, LiBr, LiI, LiClO ₄ , LiBF ₄ , LiB ₁₀ Cl ₁₀ , LiPF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiAsF ₆ , LiSbF ₆ , LiAlCl ₄ , CH ₃ SO ₃ Li, CF ₃ SO ₃ Li, (CF ₃ SO ₂ ) ₂ NLi, lithium chloroborane, lower aliphatic carboxylic acid lithium salt, tetraphenyl borate and lithium imide.

Additionally, in order to improve charge / discharge characteristics and fire resistance, for example, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphoric triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N can be added to a non-aqueous electrolyte -substituted oxazolidinone, N, N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxyethanol, aluminum trichloride or the like. If necessary, in order to impart fire resistance, the non-aqueous electrolyte may further include halogen-containing solvents such as carbon tetrachloride or trifluoroethylene. Additionally, in order to improve storage performance at high temperatures, the non-aqueous electrolyte may further include carbon dioxide gas.

Brief Description of the Drawings

The above and other objectives, features and other advantages of the present invention will be more clearly understood from the following detailed description, given in combination with the accompanying drawings, in which:

FIG. 1 is a graph showing the results of XRD / ND analysis in Experimental Example 2;

FIG. 2 is a view showing the results of a HRTEM structural analysis in Experimental Example 2;

FIG. 3 is a graph showing the results of an analysis of Fe valency using the Mossbauer effects in Experimental Example 2;

FIG. 4 is a graph showing discharge results in Experimental Example 2.

The best way

Now the present invention will be described in more detail with reference to the following examples. These examples are provided only to illustrate the present invention and cannot be construed as limiting the scope and essence of the present invention.

Preparation Examples 1-4

Active cathode materials (LiFeP _1-x O ₄ ) in which the phosphorus (P) content is 0.94 (preparation example 1), 0.96 (preparation example 2), 0.97 (preparation example 3) and 0.985 (example preparations 4) were prepared by means of a hydrothermal process, well known as a method of preparing LiFePO _4, in accordance with the following method. A more specific description will be given below.

A solution of lithium hydroxide (LiOH · H ₂ O) and iron sulfate (FeSO ₄ · 6H ₂ O) in distilled water as sources of Li and Fe and phosphoric acid (H ₃ PO ₄ ) as source P were placed in the reaction chamber. The molar ranges of these substances placed in the reaction chamber were as follows: FeSO ₄ : 0.5 mol, LiOH · H ₂ O: 0.5 mol-1.5 mol, H ₃ PO ₄ : 0.5 mol-0.6 the mole.

The reaction was carried out in a reaction chamber at 380 ° C for 15 seconds. The reaction pressure was maintained at 270 bar using a pressure regulator. If an excess of Li and P is introduced into hydrothermal reactions, usually having a low reaction rate, impurities are usually easily released at high temperature under high pressure. Therefore, in this example, the release of impurities was inhibited by maintaining a high reaction rate. The pH of the solution was adjusted to 6 by adding a small amount of aqueous ammonia. Thus prepared LiFePO ₄ particles were washed and then dried under vacuum at 90 ° C. The dried particles were carbon-coated with sucrose and heat treated at 700 ° C. for 10 hours.

Example 1

90% by weight of LiFeP _0.94 O ₄ prepared in Preparation Example 1 as cathode active material, 5% by weight of Super-P as a conductive material and 5% by weight of PVDF as a binder were added to n-methyl-2 -pyrrolidinone (NMP) in order to prepare a suspension of the cathode mixture. To manufacture the cathode, this suspension of the cathode mixture was applied onto one surface of an aluminum foil, followed by drying and pressing.

95% by weight of carbon as the active anode material, 1.5% by weight of Super-P as the conductive material and 3.5% by weight of PVDF as the binder were added to the NMP as a solvent in order to prepare a suspension of the anode mixture. To make the anode, this suspension of the anode mixture was applied onto one surface of an aluminum foil, followed by drying and pressing.

The electrode kit was made by layering the cathode and anode using cellguard ™ as a separator, and a non-aqueous lithium electrolyte containing 1M LiPF ₆ in a mixed solvent of cyclic and linear carbonates was added to the electrode kit to make the battery.

Example 2

The battery was manufactured in the same manner as in Example 1, except that LiFeP _0.96 O ₄ prepared in Example 2 was used as the cathode active material.

Example 3

The battery was manufactured in the same manner as in Example 1, except that LiFeP _0.97 O ₄ prepared in Example 3 was used as the cathode active material.

Example 4

The battery was manufactured in the same manner as in Example 1, except that LiFeP _0.985 O ₄ prepared in Example 4 was used as the cathode active material.

Comparative Example 1

The battery was manufactured in the same manner as in Example 1, except that LiFePO _{4 was} used as the active cathode material.

Experimental Example 1

The performance was measured for the batteries prepared in example 1 and comparative example 1, and the results thus obtained are shown in the following Table 1

Table 1 Li: Fe: P Efficiency Example 1 1: 1: 0.94 95.8% Example 2 1: 1: 0.96 96.8% Example 3 1: 1: 0.97 97.6% Example 4 1: 1: 0.985 99.6% Comp. example 1 1: 1: 1 one hundred%

As can be seen from Table 1 below, the charge / discharge efficiency in each cycle can be adjusted to a level below 100% by controlling the amount of P present in LiFePO ₄ to a level below 1.

Experimental Example 2

The active cathode material obtained in Preparation Example 4 was subjected to X-ray diffraction (XRD) analysis, and the batteries prepared in Example 4 and Comparative Example 1 were examined by precision neutron diffraction analysis (ND), high resolution transmission electron microscopy (HRTEM) analysis. and Fe valency analysis using Mossbauer effects. The results thus obtained are shown in FIG. 1-3.

As can be seen in these figures, the active cathode material in accordance with the present invention does not undergo any structural changes and retains its single crystal olivine structure free of impurities, despite the fact that the molar fraction of P is below 1.

In addition, the change in voltage during the discharge with a current of 0.5 ° C was measured, and the results thus obtained are shown in FIG. 4. As can be seen in FIG. 4, the battery (LiFe (P _(1-x) O ₄ ); x = 0.015) of the present invention experienced a lower initial voltage drop across the resistance and showed discharge profiles at high voltage compared to the batteries (LiFePO ₄ ) of Comparative Example 1. This means a significant improvement in ionic conductivity and electrical conductivity and thereby a significant improvement in energy density.

It is believed that this behavior is due to the fact that Fe ^{+ 2} and Fe ⁺³ coexist in the active cathode material, given that the dominant amount of Fe ^{+ 2} and a small amount of Fe ^{+3 were} measured in the analysis of Fe valency using Mossbauer effects shown in FIG. 3. Moreover, as is obvious from FIG. 1 and 2, in the study of XRD / ND and HRTEM, a single phase was observed containing no impurities, which indicates the coexistence of Fe ^{+ 2} / ⁺³ in the olivine structure.

Experimental Example 2

The primary particles were prepared in Preparation Example 1 and added together with sucrose to water, followed by stirring to prepare a mixture, and this mixture was aggregated and dried by rotary spray drying to prepare secondary particles having a porosity of 28%. The secondary particles were dried at 120 ° C. using a spray dryer, and calcined at 700 ° C. under nitrogen for 10 hours.

90% by weight of the secondary particles thus prepared, 5% by weight of Super-P as a conductive material and 5% PVDF as a binder were added to n-methyl-2-pyrrolidinone (NMP) to prepare a suspension of the cathode mixture. A suspension of the cathode mixture was applied to one surface of an aluminum foil, followed by drying. After that, a suspension of the cathode mixture was pressed to make the cathode. It was confirmed that many of the secondary particles were ground and thereby returned to the primary particles.

On the other hand, secondary particles in which there are no pores were prepared by co-precipitation. A cathode was manufactured using these secondary particles in the same manner as described above. It was confirmed that most of the secondary particles were applied to the metal foil in an unmilled state.

After that, an electrode set was made by laying layers of cathodes and anodes and placing cellguard ™ between them as a separator, and a non-aqueous lithium electrolyte containing 1 M LiPF ₆ in a mixed solvent of cyclic and linear carbonates was added to the electrode set to make a battery.

The current coefficient of the batteries was changed to 0.1 ° C, 0.5 ° C, 1.0 ° C, 2.0 ° C, and 5.0 ° C, and their current discharge properties were measured. It was confirmed that the batteries of the present invention, in which the secondary particles were returned to the primary particles during the battery manufacturing process, showed a high total discharge capacity (in particular, a capacitance difference of about 50% at a high speed discharge of 5.0C) compared to the batteries, in which left secondary particles.

Experimental Example 4

10 g of the active cathode material prepared in the Preparation Example was stirred in 100 ml of distilled water for 5 minutes, followed by filtration. Then, a 0.1 M HCl solution was added to the obtained filtrate, the mixture was subjected to pH meter titration with stirring, and the pH was recorded as a function of time. This experiment was carried out until the pH reached 3 or lower, and the flow rate was respectively determined so that the titration took about 20 to 30 minutes. The content of the water-soluble base was calculated based on the amount of acid used, until the pH reached 5 or lower, and the water-soluble base was characterized by pH profiles.

The results showed that the initial pH was about 11, the amount of Li ₃ PO ₄ was about 0.2% by weight and extremely small amounts of Li ₂ PO ₃ were present.

The battery was manufactured using the cathode active material in the same manner as in Example 1. On the other hand, the battery was manufactured using the cathode active material containing about 0.27% Li ₂ PO ₃ in the same manner as in Example 1. For these batteries, tests were carried out to preserve the capacitance while increasing the current coefficient of capacitance (C-rate), discharge capacity when increasing the number of cycles and storage properties at high temperature. Storage properties at high temperature were measured by holding fully charged batteries in a high-temperature chamber at 90 ° C for 4 hours and measuring the change in battery thickness at room temperature.

As a result, the batteries of the present invention demonstrated significantly superior properties in respective tests (in particular, high capacity retention of about 40% or higher with a high speed discharge current of 5 ° C and a slight change in thickness of about 30% or higher).

Industrial applicability

As is evident from the above description, in accordance with the present invention, the molar fraction of (1-x) phosphorus (P) in LiFePO ₄ as a highly effective cathode active material is controlled in the range from 0.910 to 0.999, thereby allowing to level the operating efficiency of the active cathode material to the efficiency of the active anode material, maximizing the efficiency of the batteries, minimizing the waste of the electrode and thereby reducing the cost of manufacturing batteries. In addition, controlling the valence of Fe leads to an improvement in the voltage drop across the active resistance and current properties, as well as to the potential of the charge / discharge plateau, thereby realizing the manufacture of excellent batteries with increased energy density.

Additionally, the active cathode material in accordance with the present invention is composed of secondary particles with high porosity, thereby facilitating circulation and demonstrating the effectiveness of the process when used as an active electrode material for the manufacture of an electrode. Moreover, primary particles are present in the manufactured batteries, thereby demonstrating high electrical conductivity and bulk density, and giving the batteries improved capacitance and current properties.

Additionally, the active cathode material in accordance with the present invention contains an appropriate amount of Li ₃ PO ₄ and an extremely small amount of Li ₂ CO ₃ , thereby advantageously giving stability during high-temperature storage, and also improving the stability and current properties of secondary lithium batteries when used in as an active cathode material for secondary lithium batteries.

Although the preferred embodiments of the present invention have been disclosed above for purposes of illustration, those skilled in the art will recognize that various modifications, additions, and substitutions are possible without departing from the scope and spirit of the invention, as disclosed in the attached claims.

Claims (21)

1. The active cathode material with the composition represented by the following Formula I:
LiFe (P _(1-x) O ₄ ), (I)
where the molar fraction of (1) phosphorus (P) is in the range from 0.910 to 0.999. 2. The active cathode material according to claim 1, wherein the molar fraction of phosphorus (P) is in the range from 0.955 to 0.995. 3. The active cathode material according to claim 1, wherein the active cathode material is composed of secondary particles with an average particle diameter (D50) of 5 to 100 μm formed by aggregation of primary particles with an average particle diameter (D50) of 50 to 550 nm, wherein the primary and secondary particles, respectively, have the composition represented by Formula I, and the secondary particles have a porosity of 10% or more. 4. The active cathode material according to claim 3, while the porosity is from 15 to 40%. 5. The active cathode material according to claim 3, wherein the primary particles are aggregated through physical bonding with the formation of secondary particles. 6. The active cathode material according to claim 3, wherein the pore size present in the secondary particles is from 10 to 1000 nm. 7. The active cathode material according to claim 3, wherein the primary particles have an average particle diameter (D50) of 100 to 300 nm, and the secondary particles have an average particle diameter (D50) of 5 to 40 μm. 8. The active cathode material according to claim 3, wherein the secondary particles are spherical in shape. 9. The active cathode material according to claim 3, wherein the secondary particles have a specific surface area (BET) of 5 to 15 m ² / g. 10. The active cathode material according to claim 3, wherein the active cathode material has a bulk density of from 0.5 to 1.5 g / ml. 11. The active cathode material of claim 10, wherein the active cathode material has a bulk density of 0.8 to 1.3 g / ml. 12. The active cathode material according to claim 1, wherein the active cathode material contains from 0.02 to 5% by weight of Li ₃ PO ₄ and does not contain Li ₂ CO ₃ or contains Li ₂ CO ₃ in an amount of less than 0.25% . 13. The active cathode material according to item 12, while Li ₃ PO _{4 is} present in an amount of from 0.1 to 5% by weight. 14. The active cathode material of claim 12, wherein Li ₂ CO _{3 is} present in an amount of 0.1% by weight or less. 15. The active cathode material according to item 12, while the active cathode material has a pH from 8.5 to 11.5. 16. The active cathode material according to item 12, while the active cathode material has a pH from 10.0 to 11.5. 17. The active cathode material according to item 12, with most of Li ₂ CO ₃ or Li ₃ PO ₄ present on the surface of the particles. 18. The active cathode material according to claim 1, wherein the active cathode material is obtained by a supercritical hydrothermal method. 19. The cathode mixture containing the active cathode material according to any one of claims 1 to 18. 20. The cathode for secondary batteries, in which the cathode mixture according to claim 19 is applied to the current collector. 21. A lithium secondary battery containing a cathode according to claim 20.

Images