KR20210006252A - Positive electrode active material for lithium secondary battery and positive electrode for lithium secondary battery and lithium secondary battery including the same - Google Patents

Abstract

The present invention relates to a lithium secondary battery which includes a positive electrode including a positive electrode active material and a negative electrode including a silicon-based negative electrode active material, wherein the positive electrode active material of which the ratio of the thickness of the shell part to the radius of the positive electrode active material is formed to be less than 1 %. The positive electrode active material includes a core part including a first lithium transition metal oxide represented by following chemical formula 1; and a shell part formed on the surface of the core part and including a second lithium transition metal oxide represented by following chemical formula 2. [Chemical formula 1] Li_(1+d1)(Ni_(a1)Co_(1-(a1+b1))M^1_(b1))_(1-d1)O_2 [Chemical formula 2] Li_(1+d2)(Ni_(a2)Co_(1-(a1+b2))M^1_(b2))_(1-d2)O_2 In Chemical formulas 1 and 2, M^1 is at least one of Mn and Al, 0 <= d1 <= 0.5, 0.5 <= a1 < 0.9, 0.1 <= b1 <= 0.5, a1+b1 <= 1, 0 <= d2 <= 0.5, 0.9 < a2 <= 1.0, 0 <= b2 <= 0.1, and a1 < a2.

Description

Positive electrode active material for lithium secondary battery and positive electrode and lithium secondary battery for lithium secondary battery including the same {POSITIVE ELECTRODE ACTIVE MATERIAL FOR LITHIUM SECONDARY BATTERY AND POSITIVE ELECTRODE FOR LITHIUM SECONDARY BATTERY AND LITHIUM SECONDARY BATTERY INCLUDING THE SAME}

The present invention relates to a positive electrode for a lithium secondary battery and a lithium secondary battery including a positive electrode active material for a lithium secondary battery and the positive electrode active material for a lithium secondary battery.

As technology development and demand for mobile devices increase, the demand for secondary batteries as an energy source is rapidly increasing. Among these secondary batteries, lithium secondary batteries having a high energy density and voltage, a long cycle life, and a low self-discharge rate have been commercialized and widely used.

A lithium secondary battery is generally composed of a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, a separator, and an electrolyte, and is charged and discharged by intercalation-decalation of lithium ions. Lithium secondary batteries are being applied in various fields because they have high energy density, high electromotive force, and high capacity.

Various methods have been studied in order to realize a higher capacity of such a lithium secondary battery. Specifically, a method of realizing a high capacity of a lithium secondary battery has been attempted by using one or two or more materials such as LCO, LNMCO, and LMO as a positive electrode active material included in a positive electrode for a lithium secondary battery. However, in order to actually increase the capacity of a lithium secondary battery, not only the capacity of the positive electrode but also the capacity of the negative electrode must be improved. To this end, a method of using a silicon-based negative active material having a high capacity as a negative electrode has also been attempted. However, in the case of such a silicon-based negative active material, since the irreversible capacity is also large, there is a problem that the charging/discharging efficiency is low.

Thus, conventionally, in a lithium secondary battery including a silicon-based negative electrode, due to the decrease in the efficiency of the negative electrode, not only the above-described positive electrode active material but also a compound capable of lowering the efficiency of the positive electrode such as LiNiO ₂ was added when manufacturing the positive electrode. In addition, a method of designing a battery by forcibly lowering the efficiency of the anode was studied. That is, conventionally, when a silicon-based negative electrode is applied to realize a high-capacity secondary battery, a compound that can forcibly lower the efficiency of the positive electrode must be added, which deteriorates the profitability and requires additional compound input. there was.

Therefore, it is possible to lower the efficiency of the positive electrode during charging and discharging without the addition of a compound that can forcibly lower the efficiency of the positive electrode, and there is a need to develop a positive electrode active material that can be applied with a negative electrode including a silicon-based negative active material. have.

Republic of Korea Patent Publication No. 10-2014-0085347

In order to solve the above problems, in the positive electrode active material having a core-shell structure, a nickel oxide layer on the surface of the positive electrode active material after the initial charging and discharging of a secondary battery including the same is used by using a positive electrode active material containing an excess of nickel in the shell portion. Is formed, and the nickel oxide layer does not participate in the subsequent reaction, and thus a positive electrode active material capable of lowering the efficiency of the positive electrode upon application thereof is provided.

In addition, a second technical problem of the present invention is to provide a positive electrode including the positive electrode active material.

In addition, a third technical problem of the present invention is to provide a lithium secondary battery comprising the positive electrode and the silicon-based negative electrode active material as a negative electrode active material, and having high capacity characteristics and capable of improving the irreversible phenomenon of the silicon-based negative electrode.

The present invention includes a core portion including a first lithium transition metal oxide represented by the following formula (1); And

In the positive electrode active material including; a shell portion formed on the surface of the core portion and including a second lithium transition metal oxide represented by the following Formula 2, wherein the thickness ratio of the shell portion to the radius of the positive electrode active material is less than 1%. It provides a positive active material.

[Formula 1]

Li _1+d1 (Ni _a1 Co _1-(a1+b1) M ¹ _b1 ) _1-d1 O ₂

[Formula 2]

Li _1+d2 (Ni _a2 Co _1-(a+b2) M ¹ _b2 ) _1-d2 O ₂

In Formulas 1 and 2,

M ¹ is at least one of Mn and Al,

0≤d1≤0.5, 0.5≤a1<0.9, 0.1≤b1≤0.5, a1+b1≤1, 0≤d2≤0.5, 0.9<a2≤1.0, 0≤b2≤0.1, and a1<a2.

In addition, it provides a positive electrode for a lithium secondary battery according to the present invention.

In addition, the anode according to the present invention; A negative electrode including a silicon-based negative active material; And a separator interposed between the positive electrode and the negative electrode, wherein when the lithium secondary battery is charged and discharged at a driving voltage of 2.5V to 4.2V, nickel oxide on the surface of the positive electrode active material included in the positive electrode A layer is formed, and the thickness ratio of the nickel oxide layer to the radius of the positive electrode active material is less than 1%, providing a lithium secondary battery.

According to the present invention, a high-capacity battery is manufactured when applying this to a battery by using a positive electrode active material having a core-shell structure containing Ni in a larger amount in the surface part (shell) than the central part (core) of the particle as the positive electrode active material. For this purpose, it is possible to solve the irreversible capacity problem that occurred when the silicon-based negative electrode is applied, thereby improving efficiency characteristics and providing a secondary battery exhibiting high capacity characteristics.

Hereinafter, the present invention will be described in more detail.

The terms or words used in this specification and claims should not be construed as being limited to their usual or dictionary meanings, and the inventor may appropriately define the concept of terms in order to describe his own invention in the best way. It should be interpreted as a meaning and concept consistent with the technical idea of the present invention based on the principle that there is.

The terms used in the present specification are only used to describe exemplary embodiments, and are not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly indicates otherwise.

In the present specification, terms such as "comprise", "include" or "have" are intended to designate the presence of implemented features, numbers, steps, components, or a combination thereof, and one or more other features or It is to be understood that the possibility of the presence or addition of numbers, steps, elements, or combinations thereof is not preliminarily excluded.

In this specification, "%" means% by weight unless otherwise indicated.

Positive electrode active material

Hereinafter, the positive active material according to the present invention will be described in detail.

The positive electrode active material according to the present invention includes a core portion including a first lithium transition metal oxide represented by Formula 1 below; And a shell portion formed on the surface of the core portion and including a second lithium transition metal oxide represented by Formula 2 below, wherein the thickness ratio of the shell portion to the radius of the positive electrode active material is less than 1% It is formed by

[Formula 1]

Li _1+d1 (Ni _a1 Co _1-(a1+b1) M ¹ _b1 ) _1-d1 O ₂

[Formula 2]

Li _1+d2 (Ni _a2 Co _1-(a+b2) M ¹ _b2 ) _1-d2 O ₂

In Formulas 1 and 2,

M ¹ is at least one of Mn and Al,

0≤d1≤0.5, 0.5≤a1≤0.9, 0.1≤b1≤0.5, a1+b1≤1, 0≤d2≤0.5, 0.9<a2≤1.0, 0≤b2≤0.1, a1<a2, preferably Is 0.75≤a1≤0.85, 0.01≤b1≤0.15, 0.95≤a2≤1, 0≤b2≤0.05.

As in the present invention, when a difference in nickel content occurs between the first lithium transition metal oxide and the second lithium transition metal oxide contained in the positive electrode active material, when manufacturing a secondary battery using the same, the initial charging and discharging of the secondary battery As a result, Ni ions are desorbed from the shell portion containing an excessive amount of nickel, and the Ni ions desorbed from the shell portion and oxygen in the air react to form a nickel oxide layer on the surface of the positive electrode active material. That is, in the case of the present invention, the thickness ratio of the shell portion to the radius of the positive electrode active material and the thickness ratio of the nickel oxide layer to the radius of the positive electrode active material measured after initial charging and discharging of the secondary battery are formed at the same level. By controlling the thickness of the shell portion, the amount of the nickel oxide layer produced can also be easily controlled.

On the other hand, in the case of a positive electrode active material having a concentration gradient in which the content of nickel gradually changes from the center of the particle to the surface, it is not easy to control the amount of formation of the nickel oxide layer, so it may be formed beyond the scope of the present invention. There is a disadvantage in that there is a risk of cost increase due to the complexity of the product.

For example, the shell portion may contain nickel in an amount of more than 90 mol% and 100 mol% or less, preferably 95 mol% to 100 mol% with respect to the total number of moles of the transition metal excluding lithium. When the number of moles of nickel contained in the shell portion is within the above range, the nickel oxide layer may be uniformly formed on the surface of the positive electrode active material since Ni ^{2 +} may be easily removed from the shell portion during charging and discharging of the secondary battery.

The positive active material may have a layered structure.

The layered structure refers to a structure in which atoms are strongly bonded according to covalent bonds and densely arranged surfaces overlapped in parallel by weak bonding forces such as van der Waals force. In the lithium transition metal oxide having a layered structure, lithium ions, transition metal ions, and oxygen ions are densely arranged, specifically a metal oxide layer composed of a transition metal and oxygen and an oxygen octahedral layer surrounding lithium are alternately arranged with each other, Intercalation and desorption of lithium ions is possible because a Coulomb repulsive force acts between the metal oxide layers, and since the lithium ions diffuse along a two-dimensional plane, ionic conductivity is high. Therefore, in the case of the positive electrode active material having a layered structure, since it is possible to quickly and smoothly move lithium ions in the particles, and it is easy to insert and desorb lithium ions, it is possible to reduce the initial internal resistance of the battery to reduce the rate characteristics and the initial capacity characteristics. The discharge capacity and life characteristics can be further improved without concern.

anode

In addition, the present invention provides a positive electrode for a lithium secondary battery including the positive electrode active material. Specifically, the positive electrode for a secondary battery includes a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector, and the positive electrode active material layer includes the positive electrode active material according to the present invention.

In this case, since the positive active material is the same as described above, detailed descriptions are omitted, and only the remaining components will be described in detail below.

The positive electrode current collector is not particularly limited as long as it has conductivity without causing chemical changes to the battery, for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or carbon, nickel, titanium on the surface of aluminum or stainless steel. , A surface-treated with silver or the like may be used. In addition, the positive electrode current collector may generally have a thickness of 3 to 500 μm, and fine unevenness may be formed on the surface of the current collector to increase the adhesion of the positive electrode active material. For example, it can be used in various forms such as films, sheets, foils, nets, porous bodies, foams, and nonwoven fabrics.

The positive electrode active material layer may include a conductive material and optionally a binder, together with the positive electrode active material.

In this case, the positive electrode active material may be included in an amount of 80 to 99% by weight, more specifically 85 to 98.5% by weight, based on the total weight of the positive electrode active material layer. When included in the above content range, excellent capacity characteristics may be exhibited.

The conductive material is used to impart conductivity to the electrode, and in the battery to be configured, it may be used without particular limitation as long as it does not cause chemical change and has electronic conductivity. Specific examples include graphite such as natural graphite and artificial graphite; Carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; Metal powder or metal fibers such as copper, nickel, aluminum, and silver; Conductive whiskey such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Alternatively, a conductive polymer such as a polyphenylene derivative may be used, and one of them alone or a mixture of two or more may be used. The conductive material may be included in an amount of 0.1 to 15% by weight based on the total weight of the positive active material layer.

The binder serves to improve adhesion between positive electrode active material particles and adhesion between the positive electrode active material and the current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC). ), starch, hydroxypropylcellulose, recycled cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene butadiene rubber (SBR), fluororubber, or various copolymers thereof, and the like, and one of them alone or a mixture of two or more may be used. The binder may be included in an amount of 0.1 to 15% by weight based on the total weight of the positive active material layer.

The positive electrode may be manufactured according to a conventional positive electrode manufacturing method except for using the positive electrode active material described above. Specifically, the positive electrode active material and optionally, a composition for forming a positive electrode active material layer prepared by dissolving or dispersing a binder and a conductive material in a solvent may be coated on a positive electrode current collector, followed by drying and rolling.

The solvent may be a solvent generally used in the art, dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or Water, and the like, and one of them alone or a mixture of two or more may be used. The amount of the solvent is sufficient to dissolve or disperse the positive electrode active material, the conductive material, and the binder in consideration of the coating thickness and production yield of the slurry, and then have a viscosity that can exhibit excellent thickness uniformity when applied for the production of the positive electrode. Do.

In addition, as another method, the positive electrode may be prepared by casting the composition for forming a positive electrode active material layer on a separate support and then laminating a film obtained by peeling from the support on a positive electrode current collector.

Lithium secondary battery

In addition, the present invention can manufacture an electrochemical device including the anode. The electrochemical device may specifically be a battery, a capacitor, or the like, and more specifically, a lithium secondary battery.

Specifically, the lithium secondary battery includes a positive electrode including the above-described positive electrode active material, a negative electrode disposed to face the positive electrode and including a silicon-based negative electrode active material, and a separator and an electrolyte interposed between the positive electrode and the negative electrode. In the secondary battery, when the lithium secondary battery is initially charged and discharged at a driving voltage of 2.5V to 4.2V, a nickel oxide layer is formed on the surface of the positive electrode active material included in the positive electrode, and the radius of the positive electrode active material Including that the thickness ratio of the nickel oxide layer is less than 1%. At this time, since the positive electrode active material and the positive electrode are the same as described above, detailed descriptions are omitted, and only the remaining components will be described in detail below.

In addition, the lithium secondary battery may optionally further include a battery container for accommodating the electrode assembly of the positive electrode, the negative electrode, and the separator, and a sealing member for sealing the battery container.

In the lithium secondary battery, the negative electrode including the negative electrode active material includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector.

The negative active material layer optionally includes a binder and a conductive material together with the negative active material.

As the negative active material, a compound capable of reversible intercalation and deintercalation of lithium may be used. Preferably, the negative electrode may include a silicon-based negative active material exhibiting high capacity characteristics.

In addition, the negative active material may further include a carbon-based negative active material as well as a silicon-based negative active material. For example, when the anode active material includes a silicon-based anode active material and a carbon-based anode active material, the irreversible capacity may be reduced compared to the case where only the silicon-based anode active material is included while having high capacity characteristics.

In the case of the silicon-based negative active material and the carbon-based negative active material, it may be used by mixing in a weight ratio of 1:99 to 20:80.

The negative active material may be included in an amount of 80% to 99% by weight based on the total weight of the negative active material layer.

In addition, the negative electrode current collector is not particularly limited as long as it has high conductivity without causing a chemical change in the battery, for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel. Surface treatment with carbon, nickel, titanium, silver, etc., aluminum-cadmium alloy, etc. may be used. In addition, the negative electrode current collector may generally have a thickness of 3 μm to 500 μm, and, like the positive electrode current collector, microscopic irregularities may be formed on the surface of the current collector to enhance the bonding strength of the negative electrode active material. For example, it may be used in various forms such as films, sheets, foils, nets, porous bodies, foams, and nonwoven fabrics.

The binder is a component that aids in bonding between the conductive material, the active material, and the current collector, and is typically added in an amount of 0.1% to 10% by weight based on the total weight of the negative active material layer. Examples of such binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoro. Ethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber, nitrile-butadiene rubber, fluorine rubber, and various copolymers thereof.

The conductive material is a component for further improving the conductivity of the negative electrode active material, and may be added in an amount of 10% by weight or less, preferably 5% by weight or less based on the total weight of the negative electrode active material layer. Such a conductive material is not particularly limited as long as it has conductivity without causing a chemical change in the battery, and examples thereof include graphite such as natural graphite or artificial graphite; Carbon blacks such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; Conductive fibers such as carbon fibers and metal fibers; Metal powders such as carbon fluoride, aluminum, and nickel powder; Conductive whiskey such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives may be used.

For example, the negative electrode active material layer is prepared by applying and drying a negative electrode active material, and a composition for forming a negative electrode active material layer prepared by dissolving or dispersing a binder and a conductive material in a solvent on a negative electrode current collector, or It can be prepared by casting the composition for forming an active material layer on a separate support, and then laminating a film obtained by peeling from the support on a negative electrode current collector.

The negative active material layer is, for example, coated with a composition for forming a negative active material layer prepared by dissolving or dispersing a negative active material, and optionally a binder and a conductive material in a solvent on a negative current collector and drying, or for forming the negative active material layer It can also be prepared by casting the composition on a separate support and then laminating a film obtained by peeling from the support on a negative electrode current collector.

On the other hand, as in the present invention, when the silicon-based negative active material is included as the negative electrode, high capacity characteristics can be achieved, but the efficiency of the secondary battery is lowered due to the high irreversible capacity of the silicon-based negative active material, and the energy density is rather There was a drawback of being lowered.

Thus, as described above, the core portion including the first lithium transition metal oxide represented by the following formula (1); And a shell part formed on the surface of the core part and including a second lithium transition metal oxide represented by Formula 2 below, wherein the thickness ratio of the shell part to the radius of the positive electrode active material is less than 1%. By manufacturing a positive electrode using the formed positive electrode active material and applying it to a lithium secondary battery, the positive electrode active material contained in the positive electrode generated during the initial charging and discharging of the secondary battery, specifically, due to the structural change of the shell portion of the positive electrode active material, By lowering the efficiency according to the negative electrode and using it, it is possible to significantly improve the efficiency decrease of the battery due to the irreversible phenomenon of the negative electrode and to manufacture a secondary battery with improved energy density.

[Formula 1]

Li _1+d1 (Ni _a1 Co _1-(a1+b1) M ¹ _b1 ) _1-d1 O ₂

[Formula 2]

Li _1+d2 (Ni _a2 Co _1-(a+b2) M ¹ _b2 ) _1-d2 O ₂

In Formulas 1 and 2, M ¹ is at least one of Mn and Al, and 0≤d1≤0.5, 0.5≤a1<0.9, 0.1≤b1≤0.5, a1+b1≤1, 0≤d2≤0.5, 0.9 <a2≤1.0, 0≤b2≤0.1, and a1<a2.

Specifically, in the case of the lithium secondary battery according to the present invention, when charging and discharging at a driving voltage of 2.5V to 4.2V, due to excessive nickel content in the shell part included in the positive electrode active material, the surface of the positive electrode active material As the structure becomes unstable, as the lithium secondary battery is charged and discharged, nickel desorbed from the surface of the positive electrode active material having a layered structure reacts with oxygen in the air to form a nickel oxide layer on the surface of the positive electrode active material. I can.

When a nickel oxide layer is formed on the surface of the positive electrode active material, the nickel oxide has an electrochemically stable rock salt structure, and does not participate in the reaction even if charging and discharging of the lithium secondary battery proceeds thereafter. Accordingly, when the nickel oxide layer is uniformly formed on the surface of the positive electrode active material according to the charging and discharging of the lithium secondary battery, the efficiency of the positive electrode is also lowered.

According to the present invention, in the nickel oxide layer generated during charging and discharging of the secondary battery, the thickness ratio of the nickel oxide layer to the radius of the positive electrode active material is less than 1%, preferably 0.1% to 0.99%, more preferably 0.1% It may be formed in a thickness ratio of 0.9%. For example, when the thickness ratio of the nickel oxide layer to the radius of the positive electrode active material is 1% or more, a problem of reducing the discharge capacity may occur when applying this to a battery, and the nickel oxide layer does not participate in the reaction during charging and discharging. Therefore, when it is applied to a battery, lifespan characteristics may be rapidly deteriorated as the cycle progresses.

Accordingly, according to the present invention, in order to implement a conventional high-capacity secondary battery, a compound capable of forcibly lowering the efficiency of the positive electrode when applying a silicon-based negative electrode, that is, a sacrificial positive electrode such as LiNiO _{2 in} addition to the positive electrode active material had to be added. In this case, since the efficiency of the positive electrode is lowered due to the unstable structure of the positive electrode active material according to the charging and discharging of the lithium secondary battery, high irreversibility of the silicon-based negative electrode can be eliminated even if the sacrificial positive electrode is not added.

On the other hand, if the nickel oxide layer formed on the surface of the positive electrode active material as described above, Ni ⁺ ions having a similar size and Li ⁺ is entering the space of the Li layer by the cation mixing (cation mixing) occurs the surface of the positive electrode active material It may change from a layered structure to a rock salt structure.

The rock salt structure refers to a structure having a face centered cubic structure in which metal atoms are coordinated by six oxygen atoms located in an octahedral shape. A compound having such a rock salt crystal structure has high structural stability, particularly at high temperatures.

That is, in the lithium secondary battery according to the present invention, due to the unstable structure of the surface of the positive electrode active material as charging and discharging proceeds, the surface of the positive electrode active material having a layered structure may change into a rock salt structure by initial charging and discharging. Since the rock salt structure does not participate in the reaction during charging and discharging due to its stable structure, the efficiency of the anode may be lowered when a rock salt structure is formed on the surface, and the efficiency of the anode is also regulated even if a separate sacrificial anode is not added. Since it can be lowered to suit the negative electrode including the sub-type negative active material, the reduction in efficiency of the secondary battery due to the irreversible characteristic of the negative electrode can be improved.

On the other hand, in the lithium secondary battery, the separator separates the negative electrode and the positive electrode and provides a passage for lithium ions, and can be used without particular limitation as long as it is used as a separator in a general lithium secondary battery. On the other hand, it is preferable to have low resistance and excellent electrolyte-moisturizing ability. Specifically, a porous polymer film, for example, a porous polymer film made of polyolefin-based polymers such as ethylene homopolymer, propylene homopolymer, ethylene/butene copolymer, ethylene/hexene copolymer, and ethylene/methacrylate copolymer, or these A stacked structure of two or more layers of may be used. In addition, a conventional porous nonwoven fabric, for example, a nonwoven fabric made of a high melting point glass fiber, polyethylene terephthalate fiber, or the like may be used. In addition, a coated separator containing a ceramic component or a polymer material may be used to secure heat resistance or mechanical strength, and may be optionally used in a single layer or multilayer structure.

In addition, electrolytes used in the present invention include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes, molten inorganic electrolytes, etc. that can be used in the manufacture of lithium secondary batteries. It does not become.

Specifically, the electrolyte may include an organic solvent and a lithium salt.

The organic solvent may be used without particular limitation as long as it can serve as a medium through which ions involved in the electrochemical reaction of a battery can move. Specifically, examples of the organic solvent include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; Ether solvents such as dibutyl ether or tetrahydrofuran; Ketone solvents such as cyclohexanone; Aromatic hydrocarbon solvents such as benzene and fluorobenzene; Dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), propylene carbonate, Carbonate-based solvents such as PC); Alcohol solvents such as ethyl alcohol and isopropyl alcohol; Nitriles, such as R-CN (R is a C2-C20 linear, branched or cyclic hydrocarbon group, and may contain a double bonded aromatic ring or an ether bond); Amides such as dimethylformamide; Dioxolanes such as 1,3-dioxolane; Alternatively, sulfolanes or the like may be used. Among them, carbonate-based solvents are preferable, and cyclic carbonates having high ionic conductivity and high dielectric constant (e.g., ethylene carbonate or propylene carbonate, etc.), which can increase the charging/discharging performance of the battery, and low-viscosity linear carbonate-based compounds ( For example, a mixture of ethylmethyl carbonate, dimethyl carbonate or diethyl carbonate) is more preferable. In this case, when the cyclic carbonate and the chain carbonate are mixed and used in a volume ratio of about 1:1 to about 1:9, the electrolyte may exhibit excellent performance.

The lithium salt may be used without particular limitation as long as it is a compound capable of providing lithium ions used in a lithium secondary battery. Specifically, the lithium salt is LiPF ₆ , LiClO ₄ , LiAsF ₆ , LiBF ₄ , LiSbF ₆ , LiAl0 ₄ , LiAlCl ₄ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiN(C ₂ F ₅ SO ₃ ) ₂ , LiN(C ₂ F ₅ SO ₂ ) ₂ , LiN(CF ₃ SO ₂ ) _{2 .} LiCl, LiI, or LiB(C ₂ O ₄ ) ₂ or the like may be used. The concentration of the lithium salt is preferably used within the range of 0.1 to 2.0M. When the concentration of the lithium salt is within the above range, since the electrolyte has an appropriate conductivity and viscosity, excellent electrolyte performance can be exhibited, and lithium ions can effectively move.

In addition to the electrolyte constituents, the electrolyte includes, for example, haloalkylene carbonate-based compounds such as difluoroethylene carbonate, pyridine, and trivalent for the purpose of improving battery life characteristics, suppressing reduction in battery capacity, and improving battery discharge capacity. Ethyl phosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, hexaphosphate triamide, nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazolidinone, N,N-substituted imida One or more additives, such as zolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxy ethanol, or aluminum trichloride, may be further included. At this time, the additive may be included in an amount of 0.1 to 5% by weight based on the total weight of the electrolyte.

As described above, since the lithium secondary battery including the positive electrode active material according to the present invention stably exhibits excellent discharge capacity, output characteristics, and life characteristics, portable devices such as mobile phones, notebook computers, digital cameras, and hybrid electric vehicles ( It is useful in electric vehicle fields such as hybrid electric vehicle, HEV).

Accordingly, according to another embodiment of the present invention, a battery module including the lithium secondary battery as a unit cell and a battery pack including the same are provided.

The battery module or battery pack may include a power tool; Electric vehicles including electric vehicles (EV), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEV); Alternatively, it may be used as a power source for any one or more medium and large-sized devices in a power storage system.

The external shape of the lithium secondary battery of the present invention is not particularly limited, but may be a cylindrical shape using a can, a square shape, a pouch type, or a coin type.

The lithium secondary battery according to the present invention can be used not only for a battery cell used as a power source for a small device, but also can be preferably used as a unit cell for a medium or large battery module including a plurality of battery cells.

Hereinafter, examples will be described in detail to illustrate the present invention in detail. However, the embodiments according to the present invention may be modified in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. Embodiments of the present invention are provided to more completely describe the present invention to those of ordinary skill in the art.

Example

Example One

In a batch type 4L reactor set at 40°C, NiSO ₄ , CoSO ₄ and MnSO ₄ are N-methylpyrrolidone (NMP) in an amount such that the molar ratio of nickel: cobalt: manganese is 80:10:10. Mixing in a solvent to prepare a 3M concentration of a first transition metal-containing solution, and NiSO ₄ was added to the NMP solvent to prepare a 3M concentration of a second transition metal-containing solution.

The vessel containing the first transition metal-containing solution and the vessel containing the second transition metal-containing solution were connected to the batch reactor. In addition, 4M NaOH solution and NH ₄ OH aqueous solution were prepared and connected to the batch reactor, respectively.

After adding deionized water to the co-precipitation reactor (capacity 4L), nitrogen gas was purged into the reactor at a rate of 0.005L/min to remove dissolved oxygen in the water, and the reactor was made into a non-oxidizing atmosphere. After 4M NaOH was added, the mixture was stirred at a stirring speed of 1,000 rpm, and the pH in the coprecipitation reactor was maintained at pH 10.5.

Thereafter, the first transition metal-containing solution was introduced into the coprecipitation reactor at a rate of 300 mL/hr, NaOH aqueous solution was added at a rate of 300 mL/hr, and NH ₄ OH aqueous solution was added at a rate of 300 mL/hr, respectively, followed by co-precipitation reaction for 6 hours. Ni _{0 . 8} Co _{0 . 1} to form the anode active material particles represented by _{0 .1} Mn (OH) _2.

Thereafter, the addition of the first transition metal-containing solution was stopped, the second transition metal-containing solution was added at a rate of 300 mL/hr, NaOH aqueous solution was added at a rate of 300 mL/hr, and NH ₄ OH aqueous solution was added at a rate of 300 mL/hr. Then, a co-precipitation reaction was performed for 1 hour, and a compound represented by Ni(OH) ₂ was grown on the surface of the positive electrode active material particles to prepare a core-shell structured positive electrode active material precursor.

The cathode active material precursor and LiOH were mixed (1.2 mol of lithium source per 1 mol of precursor metal), and fired at 700° C. for 8 hours in an oxygen atmosphere to prepare a core-shell structured cathode active material.

Example 2

As the first transition metal-containing solution, NiSO ₄ , CoSO ₄ and MnSO ₄ were mixed in an NMP solvent in an amount such that the molar ratio of nickel: cobalt: manganese was 83:6:11 to prepare a solution containing a 3M concentration of the first transition metal. And, except for using it, a positive electrode active material was prepared in the same manner as in Example 1.

Comparative example One

The first transition metal-containing solution and the second transition metal-containing solution prepared in Example 1 were mixed in a ratio of 100% by volume: 0% by volume to 0% by volume: 100% by volume. The resulting mixed metal solution was continuously introduced into the coprecipitation reactor at a rate of 300 mL/hr through a pipe for a mixed solution, and NaOH aqueous solution was added at a rate of 300 mL/hr, and NH ₄ OH aqueous solution was added at a rate of 300 mL/hr for 6 hours. The cathode active material precursor particles were precipitated by co-precipitation reaction. The precipitated positive electrode active material precursor particles are separated and dried in an oven at 700° C. for 8 hours after washing with water to have a concentration gradient gradually changing from the center of the particles to the surface, and the average composition is Ni _{0 . 8} Co _{0 . 1} Mn _{0 . A} positive electrode active material having a concentration gradient was prepared in the same manner as in Example 1, except that the co-precipitation reaction time was adjusted to be ₁ (OH) ₂ to prepare a precursor for the positive electrode active material.

Comparative example 2

Using only the second transition metal-containing solution prepared in Example 1, a co-precipitation reaction was performed for 6 hours to form a positive electrode active material precursor represented by Ni(OH) ₂ , and the same method as in Example 1 was used. To prepare a positive electrode active material.

Comparative example 3

Instead of the first transition metal-containing solution prepared in Example 1, the first transition of 3M concentration prepared by mixing NiSO ₄ , CoSO ₄ and MnSO ₄ in an amount such that the molar ratio of nickel: cobalt: manganese is 50:20:30 A positive electrode active material was prepared in the same manner as in Example 1, except for using a metal-containing solution.

Experimental example 1: Confirmation of capacity characteristics of lithium secondary battery

A lithium secondary battery was manufactured using the positive electrode active material each prepared in Examples 1 to 2 and Comparative Examples 1 to 3, and the lifespan characteristics thereof were measured. At this time, the lithium secondary battery was manufactured using the same method as described below, except for using the positive active material prepared in Examples 1 to 2 and Comparative Examples 1 to 3, respectively.

Specifically, the positive electrode active material, carbon conductive material, and polyvinylidene fluoride (PVDF) binder each prepared in Examples 1 to 2 and Comparative Examples 1 to 3 were mixed in a weight ratio of 90:10:10, and this was N -Methylpyrrolidone (NMP) was mixed in a solvent to prepare a composition for forming a positive electrode. The composition for forming a positive electrode was applied to an aluminum current collector having a thickness of 15 μm, dried, and roll pressed to prepare a positive electrode. Subsequently, a mixed negative active material, a carbon conductive material, and a PVDF binder were mixed at a weight ratio of 90:10:10 and added to deionized water as a solvent to prepare a negative electrode active material slurry. This was coated on a copper current collector having a thickness of 15 μm, dried, and then roll pressed to prepare a negative electrode. A polymer-type battery was prepared by a conventional method by laminating the positive electrode and negative electrode prepared above with a polyethylene separator, and then placed in a battery case and 1.0 M in a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC). By injecting an electrolyte solution in which LiFP ₆ was dissolved, lithium secondary batteries according to Examples 1 to 2 and Comparative Examples 1 to 3 were prepared.

Each of the lithium secondary batteries of Examples 1 to 2 and Comparative Examples 1 to 3 prepared above was charged at a rate of 0.05C cut off to 4.2V at a constant current of 0.2C at 25°C. Thereafter, discharge was performed at a constant current of 0.2C until it became 2.5V. The charging and discharging capacity and charging/discharging efficiency at this time were measured, and are shown in Table 1 below.

Charging capacity (mAh/g) Discharge capacity (mAh/g) efficiency(%) Example 1 222.1 196.8 88.6 Example 2 225.8 199.4 88.3 Comparative Example 1 222.4 193.9 87.2 Comparative Example 2 232.2 181.3 78.1 Comparative Example 3 172.2 148.3 86.1

As shown in Table 1, in the case of Examples 1 and 2, it was confirmed that the efficiency was improved compared to Comparative Examples 1 to 3. In particular, in the case of the secondary batteries prepared in Comparative Examples 1 and 2, it was confirmed that both the capacity characteristics and efficiency were lowered compared to the secondary batteries prepared in Examples 1 to 2 due to the formation of an excessive nickel oxide layer. On the other hand, in the case of the secondary battery prepared in Comparative Example 3, it was confirmed that the capacity was lower than that of the secondary batteries prepared in Examples 1 to 2 due to the decrease in the content of nickel in the positive electrode active material.

Experimental example 2: Analysis of the surface phase of the positive electrode active material

After extracting the positive electrode active material completed charging and discharging, the cross section of the positive electrode active material was cut to a thickness of 1 nm, and the surface of the positive electrode active material was observed using TEM (FE-STEM, TITAN G2 80-100 ChemiSTEM), and the image of the positive electrode active material ( phase) was measured through a small angle diffraction pattern (SADP).

It was confirmed whether a rock salt structure was formed from the surface of the positive electrode active material particle to the center direction, and it is shown in Table 1 below. When the rock salt structure was formed, it was marked as ○, and when the rock salt structure was not formed, it was marked as x.

In addition, the ratio of the thickness in which the rock salt structure is formed to the radius of the positive electrode active material particles, that is, the thickness ratio of the nickel oxide layer to the radius of the positive electrode active material particles, is also shown in Table 2 below.

Whether rock salt structure is formed Ratio of the thickness of the nickel oxide layer to the radius of the positive electrode active material (%) Example 1 ○ 0.9 Example 2 ○ 0.7 Comparative Example 1 ○ 5.3 Comparative Example 2 ○ 15.7 Comparative Example 3 ○ 0.9

As shown in Table 2, in the case of the positive electrode active material prepared in Examples 1 to 2, it was confirmed that the thickness ratio of the nickel oxide layer to the radius of the positive electrode active material was less than 1%. On the other hand, in Comparative Examples 1 and 2, it was confirmed that the thickness ratio of the nickel oxide layer to the radius of the positive electrode active material was increased compared to Examples 1 and 2. In particular, in the case of Comparative Example 1, it was confirmed that the thickness ratio of the nickel oxide layer to the radius of the positive electrode active material was increased compared to Examples 1 and 2 as a positive electrode active material having a concentration gradient gradually changing from the center of the particle was applied.

Experimental example 3: Capacity retention measurement

The capacity retention rate of the secondary batteries according to Examples 1 to 2 and Comparative Examples 1 to 3 prepared in Experimental Example 1 was measured.

Specifically, each of the lithium secondary batteries of Examples 1 to 2 and Comparative Examples 1 to 3 prepared in Experimental Example 1 was charged at a rate of 0.05C cut off to 4.2V at a constant current of 0.2C at 25°C. Thereafter, discharge was performed at a constant current of 0.2C until it became 2.5V, and the charging and discharging were repeated 50 times with 1 cycle, and the capacity retention rate at the 50th time was measured, which is shown in Table 3 .

Capacity retention rate (%) Example 1 97.5 Example 2 97.1 Comparative Example 1 95.3 Comparative Example 2 87.1 Comparative Example 3 97.1

As shown in Table 3, it was confirmed that the capacity retention rate of the secondary batteries prepared in Examples 1 to 2 was superior to that of the secondary batteries prepared in Comparative Examples 1 to 2. In particular, in the case of the secondary battery manufactured in Comparative Example 2, it was confirmed that the capacity retention rate was the most inferior due to the formation of an excessive nickel oxide layer.

Claims (11)

A core portion including a first lithium transition metal oxide represented by Formula 1 below; And
In the positive electrode active material comprising; a shell portion formed on the surface of the core portion and including a second lithium transition metal oxide represented by Formula 2 below,
A positive electrode active material in which a thickness ratio of the shell portion to a radius of the positive electrode active material is less than 1%.
[Formula 1]
Li _1+d1 (Ni _a1 Co _1-(a1+b1) M ¹ _b1 ) _1-d1 O ₂
[Formula 2]
Li _1+d2 (Ni _a2 Co _1-(a+b2) M ¹ _b2 ) _1-d2 O ₂
In Formulas 1 and 2,
M ¹ is at least one of Mn and Al,
0≤d1≤0.5, 0.5≤a1≤0.9, 0.1≤b1≤0.5, a1+b1≤1,
0≤d2≤0.5, 0.9<a2≤1.0, 0≤b2≤0.1, and a1<a2.
The method of claim 1,
In Formula 2,
0≤d2≤0.5, 0.95≤a2≤1.0, 0≤b2≤0.05 positive active material.
The method of claim 1,
The positive electrode active material is a positive active material having a layered structure.
The method of claim 1,
A positive active material having a thickness ratio of the shell portion to a radius of the positive active material of 0.1% to 0.9%.
A positive electrode comprising the positive electrode active material according to claim 1.
The anode according to claim 5;
A negative electrode including a silicon-based negative active material; And
In the lithium secondary battery comprising a separator interposed between the positive electrode and the negative electrode,
When charging and discharging the lithium secondary battery at a driving voltage of 2.5V to 4.2V, a nickel oxide layer is formed on the surface of the positive electrode active material included in the positive electrode,
A lithium secondary battery in which a thickness ratio of a nickel oxide layer to a radius of the positive electrode active material is less than 1%.
The method of claim 6,
When charging and discharging at a driving voltage of 2.5V to 4.2V, a nickel oxide layer is formed by reacting an excess of nickel contained in the shell portion of the positive electrode active material with oxygen in the air.
The method of claim 6,
The positive electrode active material is a lithium secondary battery having a layered structure and a rock salt structure.
The method of claim 6,
The nickel oxide layer is a lithium secondary battery having a rock salt structure.
The method of claim 6,
A lithium secondary battery in which a thickness ratio of the nickel oxide layer to a radius of the positive active material is 0.1% to 0.9%.
The method of claim 6,
The negative electrode is a lithium secondary battery further comprising a carbon-based negative active material.